Metal Oxides in Supercapacitors

Metal Oxides in Supercapacitors

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Metal Oxides Series

Metal Oxides in Supercapacitors Series Editor

Ghenadii Korotcenkov

Editors

Deepak P. Dubal Pedro Gomez-Romero

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2017 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-12-811169-7 (print) ISBN: 978-0-12-810465-1 (online) For information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

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Contents

List of Contributors Editors’ Biography Series Editor’s Biography Preface Preface to the Series 1

2

3

Capacitive and Pseudocapacitive Electrodes for Electrochemical Capacitors and Hybrid Devices Thierry Brousse, Olivier Crosnier, Daniel Bélanger, Jeffrey W. Long 1.1 Introduction 1.2 Devices 1.3 Electrodes for Electrochemical Capacitors and for Hybrid Capacitors 1.4 Conclusions Acknowledgments References Features of Design and Fabrication of Metal OxideeBased Supercapacitors Y. Liu, Y. Zhang, X.W. Wu, Y.S. Zhu, Y.P. Wu 2.1 Introduction 2.2 Fundamentals of Symmetric and Asymmetric Supercapacitors 2.3 Configuration Design of Metal OxideeBased Supercapacitors 2.4 Conclusions and Outlook Acknowledgment References Electrolytes in Metal Oxide Supercapacitors Maria J. Carmezim, Catarina F. Santos 3.1 Introduction 3.2 Supercapacitors and Interaction With Electrolytes 3.3 Electrolytes for Metal Oxide Supercapacitors 3.4 Conclusions and Outlooks Acknowledgments References

ix xi xiii xv xvii 1 1 1 9 20 21 21 25 25 27 33 43 44 44 49 49 51 53 72 72 73

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4

5

6

7

8

Contents

Fundamentals of Binary Metal OxideeBased Supercapacitors Deepak P. Dubal, Nilesh R. Chodankar, Pedro Gomez-Romero, Do-Heyoung Kim 4.1 Introduction 4.2 Binary Metal Oxides in Supercapacitors References Structure and Basic Properties of Ternary Metal Oxides and Their Prospects for Application in Supercapacitors Rongming Wang, Jian Wu 5.1 Introduction 5.2 Several Types of Ternary Metal Oxides 5.3 Synthesis Routes 5.4 Nanostructures 5.5 Concluding Remarks References Polyoxometalates: Molecular Metal Oxide Clusters for Supercapacitors Matthew Genovese, Keryn Lian 6.1 Introduction 6.2 Polyoxometalate Structure and Electrochemistry 6.3 Fabricating Polyoxometalate Composites for Supercapacitor Electrodes 6.4 Application of Polyoxometalate Electrodes in Supercapacitor Devices 6.5 Conclusions and Future Perspectives References MetaleOrganic Framework (MOF)eDerived Metal Oxides for Supercapacitors Nayarassery N. Adarsh 7.1 Introduction 7.2 MetaleOrganic FrameworkeDerived Metal Oxides for Supercapacitors 7.3 Conclusion and Future Perspectives References Metal OxideeCarbon Hybrid Materials for Application in Supercapacitors Dongfang Yang, Mihnea I. Ionescu 8.1 Introduction 8.2 Porous CarboneMetal Oxide Hybrids 8.3 Carbon NanofibereMetal Oxide Nanocomposites 8.4 GrapheneeMetal Oxide Nanocomposites 8.5 Conclusion and Future Directions References

79 79 81 95 99 99 99 109 111 124 124 133 133 134 136 147 157 158 165 165 166 187 188 193 193 193 207 212 214 215

Contents

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10

Metal Oxide/Conducting Polymer Hybrids for Application in Supercapacitors Rudolf Holze 9.1 Introduction 9.2 Conclusions References Enhanced Hybrid Supercapacitors Utilizing Nanostructured Metal Oxides Etsuro Iwama, Kazuaki Kisu, Wako Naoi, Patrice Simon, Katsuhiko Naoi 10.1 Introduction 10.2 Li4Ti5O12: Dimension-Controlled Nanosheet/Nanobook, Highly Dispersed on the Carbon Nanotube Surface 10.3 TiO2(B): Dimension Control and Hyperdispersion of Nano Metal Oxides Within a Nanocarbon Matrix 10.4 Li3VO4: Electrochemical Activation; Control of Crystal Structure of Nano Metal Oxides for Liþ Diffusion Enhancement via the Electrochemical Method 10.5 LiFePO4: Defective (Crystalline/Amorphous) Control of Nano Metal Oxides Within the Peculiar CoreeShell LiFePO4/Graphitic Carbon Structure 10.6 Li3V2(PO4)3: Nano Entanglement of Metal Oxides in Carbon Nanotube Matrix 10.7 Conclusions and Remarks Acknowledgments References

Index

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219 219 239 240 247 247 249 249 251 254 256 259 260 260 265


List of Contributors

Nayarassery N. Adarsh Catalan Institute of Nanoscience and Nanotechnology (ICN2-CSIC), Bellaterra, Spain Daniel Bélanger Université du Québec a Montréal, Montréal, QC, Canada Thierry Brousse Université de Nantes, CNRS, Nantes, France; Réseau sur le Stockage Electrochimique de l’Energie (RS2E), FR CNRS, Amiens, France Maria J. Carmezim ESTSetubal, Instituto Politécnico de Setubal, Setubal, Portugal; CQE-IST, Universidade de Lisboa, Lisboa, Portugal Nilesh R. Chodankar Chonnam National University, Gwangju, South Korea Olivier Crosnier Université de Nantes, CNRS, Nantes, France; Réseau sur le Stockage Electrochimique de l’Energie (RS2E), FR CNRS, Amiens, France Deepak P. Dubal School of Chemical Engineering, The University of Adelaide, Adelaide, South Australia 5005, Australia; Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC, The Barcelona Institute of Science and Technology, Barcelona, Spain Matthew Genovese University of Toronto, Toronto, ON, Canada Pedro Gomez-Romero Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC, The Barcelona Institute of Science and Technology, Barcelona, Spain Rudolf Holze

Technische Universit€at Chemnitz, Chemnitz, Germany

Mihnea I. Ionescu

National Research Council Canada, London, ON, Canada

Etsuro Iwama Tokyo University of Agriculture and Technology, Tokyo, Japan; Institute of Global Innovation Research, Tokyo, Japan Do-Heyoung Kim

Chonnam National University, Gwangju, South Korea

Kazuaki Kisu Tokyo University of Agriculture and Technology, Tokyo, Japan; Institute of Global Innovation Research, Tokyo, Japan Keryn Lian

University of Toronto, Toronto, ON, Canada

Y. Liu Nanjing Tech University, Nanjing, China; Technische Universit€at Chemnitz, Chemnitz, Germany

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List of Contributors

Jeffrey W. Long

U.S. Naval Research Laboratory, Washington, DC, United States

Katsuhiko Naoi Tokyo University of Agriculture and Technology, Tokyo, Japan; K & W Inc, Tokyo, Japan; Institute of Global Innovation Research, Tokyo, Japan Wako Naoi

K & W Inc, Tokyo, Japan

Catarina F. Santos ESTSetubal, Instituto Politécnico de Setubal, Setubal, Portugal; CQE-IST, Universidade de Lisboa, Lisboa, Portugal Patrice Simon Tokyo University of Agriculture and Technology, Tokyo, Japan; Université Paul Sabatier, Toulouse, France; Institute of Global Innovation Research, Tokyo, Japan Rongming Wang Beijing Key Laboratory for Magneto-Photoelectrical Composite and Interface Science, School of Mathematics and Physics, University of Science and Technology Beijing, Beijing, People’s Republic of China Jian Wu Beijing Key Laboratory for Magneto-Photoelectrical Composite and Interface Science, School of Mathematics and Physics, University of Science and Technology Beijing, Beijing, People’s Republic of China X.W. Wu

Hunan Agricultural University, Changsha, China

Y.P. Wu Nanjing Tech University, Nanjing, China; Hunan Agricultural University, Changsha, China Dongfang Yang

National Research Council Canada, London, ON, Canada

Y. Zhang

Nanjing Tech University, Nanjing, China

Y.S. Zhu

Nanjing Tech University, Nanjing, China

Editors’ Biography

Deepak P. Dubal, PhD, is currently working as senior researcher at the University of Adelaide, Australia. He worked as a Marie-Curie Fellow (BP-DGR) at the Catalan Institute of Nanoscience and Nanotechnology, Spain (2014), and a Alexander von Humboldt Fellow (2012) at the Chemnitz University of Technology, Germany. Dr. Dubal received his PhD in Physics from the Shivaji University, Kolhapur, India, in 2011, and since then, he has been awarded at international and national (India) conferences for his research excellence. He is a member of the Editorial Board for Scientific Reports, Nature Publishing Group, and Electrochemical energy technology (De Gruyder publications). Dr. Dubal is the author of over 120 research articles and book chapters and has filed seven patents. His research interest is focused on the chemical synthesis of nanostructured materials and hybrid nanomaterials and their applications in energy storage devices, with special emphasis on Li-ion batteries, supercapacitors, electrochemical flow cells, and Li-ion capacitor. Pedro Gomez-Romero, PhD, is a full professor at the NEO-Energy Group Leader at the Catalan Institute of Nanoscience and Nanotechnology, Spain. Prof. GomezRomero received his PhD in Chemistry from the Georgetown University, United States, in 1987 with distinction; was a CSIC Researcher at ICMAB from 1990 to 2007; and was a sabbatical at the National Renewable Energy Laboratory, United States (1998e99). Dr. Gomez-Romero is the leading scientist of five main national (Spanish) research projects and several international research projects. His research has focused on energy storage and conversion, advanced functional materials and nanocomposites, new oxides, polyoxometalates, and polymers. He is the editor of Functional Hybrid Materials (Wiley-VCH) and two award-winning books in the area of popular science. Dr. Gomez-Romero has published over 200 articles, book chapters, and conference proceedings, as well as filed six patents.


Series Editor’s Biography

Ghenadii Korotcenkov received his PhD in Material Sciences from the Technical University of Moldova in 1976 and his Doctor of Science degree in Physics from the Academy of Science of Moldova in 1990 (Highest Qualification Committee of the USSR, Moscow). He has more than 40 years of experience as a scientific researcher. For a long time, he was the leader of the gas sensor group and manager of various national and international projects carried out in the Laboratory of Micro- and Optoelectronics, Technical University of Moldova. His research had financial support from international foundations and programs such as CRDF, MRDA, ICTP, INTAS, INCO-COPERNICUS, COST, and NATO. From 2007 to 2008, he was an invited scientist in the Korea Institute of Energy Research (Daejeon). After which, and until now, Dr. G. Korotcenkov is a research professor at the School of Materials Science and Engineering at the Gwangju Institute of Science and Technology in Korea. Scientific interests of G. Korotcenkov, starting from 1995, include material sciences, focusing on metal oxide film deposition and characterization, surface science, and the design of thin film gas sensors and thermoelectric convertors. G. Korotcenkov is the author or editor of 35 books, including the 11-volume “Chemical Sensors” series published by Momentum Press (United States), 15-volume “Chemical Sensors” series published by Harbin Institute of Technology Press (China), 3-volume “Porous Silicon: From Formation to Application” published by CRC Press (United States), and 2-volume “Handbook of Gas Sensor Materials” published by Springer (United States). G. Korotcenkov is author and coauthor of more than 550 scientific publications, including 20 review papers, 35 book chapters, more than 250 articles published in peer-reviewed scientific journals [h-factor ¼ 37 (Scopus) and h ¼ 44 (Google Scholar citation)]. He is a holder of 17 patents. He has presented more than 200 reports at national and international conferences, including 15 invited talks. G. Korotcenkov was coorganizer of several international conferences. His research activities are honored by an award of the Supreme Council of Science and Advanced Technology of the Republic of Moldova (2004); prize of the Presidents of the Ukrainian, Belarus, and Moldovan Academies of Sciences (2003); the National Youth Prize of the Republic of Moldova in the field of science and technology (1980); among others. G. Korotcenkov also received a fellowship from the International Research Exchange Board (IREX, United States, 1998), Brain Korea 21 Program (2008e12), and Brainpool Program (Korea, 2015e17).


Preface

As long as silicates remain a mineral category in themselves, oxides will not be the most abundant minerals on the Earth crust. They are however the richest source of chemical and physical variety available to humans on the mesmerizing interphase between our small rocky-planet geosphere and our oxygen-rich atmosphere. Indeed, the multiple combinations between a few dozen metallic elements and oxygen lead to an unexhausted list of fascinating and useful properties from natural magnetism to human-made superconductivity, from a colorful palette of natural pigments to an arsenal of energy-storing materials. Speaking of energy storage, batteries were the first electrochemical energy storage devices making use of the versatility of oxides by harnessing their rich redox chemistry. Reversible lithium-ion cells reached the highest performances among batteries by refining the concept of Liþ insertion/deinsertion to compensate for stored charges, thus leading to relatively high energy densities at the expense of the low power imposed by slow ion diffusion. Supercapacitors, on the other hand, were low-energy but highpower devices conceptually derived from the electrophysical world of capacitors and originated from the concept of double-layer capacitance accumulated at the interface between an electrolyte and a high-surface-area electrode, which originally was always a carbon electrode. Not anymore. Oxides are catching up as supercapacitor active materials, providing enhanced energy densities and blurring the boundaries between supercapacitors and batteries. Supercapacitors used to be secondary actors in the energy storage scene, providing support to batteries or occupying small niche applications when high power was momentarily needed. This is quickly changing, and supercapacitors are increasingly contributing to the recent energy storage device booming, reaching the world of portable electronics, sustainable mobility (from tramways to cars), and grid applications and renewable energy sources management. The current success of supercapacitors is in large part due to the use of transition metal oxides in one or both electrodes. Our book is concerned with supercapacitor electrode materials based on transition metal oxides and their composites. In these materials, capacitance arises through reversible redox reactions taking place at or near the surface of an electrode material that is in contact with an electrolyte, or when these reactions are not limited by solidstate ion diffusion. The most significant practical difference between battery and supercapacitor materials (in particular, metal oxides) is that the charging and discharging processes of supercapacitors occur in the order of seconds and minutes. Thus, a strong

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Preface

motivation for investigating and developing metal oxideebased supercapacitors is that it can lead to both high energy and high power densities in the same material. To provide the directions for further research and development, we believe a book discussing both the fundamentals and applications of Metal OxideeBased Supercapacitors is highly needed. Indeed, the famous book in the field by B.E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, published in 1999 provided the first broad coverage of the development of supercapacitors in the past century. However, in the last 17 years, the field has made significant advancements with new ideas such as better explanation of pseudocapacitance, introduction of new metal oxides, and new device configurations such as hybrid and asymmetric capacitors that require a further comprehensive review. In this respect, we have taken the efforts to write a book in collaboration with leading scientists in the field of energy storage materials and devices with special focus on supercapacitors. Our book is structured in a total of 10 chapters, which are well organized and provide detailed explanation about the respective titles with adequate examples. Moreover, the content of our book is multidisciplinary, covering different science and engineering disciplines. The first two introductory chapters give a deep knowledge about the pseudocapacitance involved in the metal oxides and the device configurations. In the next chapter, the electrolytes used for metal oxideebased supercapacitors are discussed. Further chapters are focused on the structural properties of different metal oxides, molecular clusters of metal oxides, and metal organic framework (MOF)-derived metal oxides and their contribution to the electrochemical properties. We believe that the content of this book will provide a clear understanding of the fundamental as well as practical aspects and will facilitate the final application of this technology in the industry, while also providing a prospective view of the new advanced materials developed by experts in the field, which may surface in the future. We are sure that both industrial and academic scientists and engineers, along with undergraduate and graduate students, in the field will benefit by the knowledge gathered in this book and also will help foster ideas for new devices that will help further the technology. Moreover, the readers will find this logical evolution highly appealing, as it introduces a didactic element to the reading of the book apart from the joy of grasping the essentials of an important subject. Finally, we sincerely acknowledge and thank all our contributors who devoted their valuable time for preparing and developing this wonderful book. We also would like to thank the series editor Prof. Korotcenkov Ghenadii and Elsevier Publishers for inviting us to lead this book project, especially Anna Valutkevich, for their patience and support in smoothing out the book preparation process. Finally, I (DPD) personally would like to dedicate this book to all my supervisors with whom I worked so far, including Prof. Pedro Gomez-Romero (coeditor), for empowering me to fly high in this fascinating field. Deepak P. Dubal and Pedro Gomez-Romero

Preface to the Series

The field of synthesis, study, and application of metal oxides is one of the most rapidly progressing areas of science and technology. Metal oxides are one of the most ubiquitous compound groups on Earth, which has a large variety of chemical compositions, atomic structures, and crystalline shapes. In addition, metal oxides are known to possess unique functionalities that are absent or inferior in other solid materials. In particular, metal oxides represent an assorted and appealing class of materials, properties of which exhibit a full spectrum of electronic properties, i.e., from insulating to semiconducting, metallic, and superconducting. Moreover, almost all the known effects, including superconductivity, thermoelectric effects, photoelectrical effects, luminescence, and magnetism, can be observed in metal oxides. Therefore, metal oxides have emerged as an important class of multifunctional materials with a rich collection of properties, which have great potential for numerous device applications. Specific properties of the metal oxides, such as the wide variety of materials with different electrophysical, optical, and chemical characteristics, their high thermal and temporal stability, and their ability to function in harsh environments, make metal oxides very suitable materials for designing transparent electrodes, high-mobility transistors, gas sensors, actuators, acoustical transducers, photovoltaic and photonic devices, photoand heterogeneous catalysts, solid-state coolers, high-frequency and micromechanical devices, energy harvesting and storage devices, nonvolatile memories, and many others in the electronics, energy, and health sectors. In these devices, metal oxides can be successfully used as sensing or active layers, substrates, electrodes, promoters, structure modifiers, membranes, and fibers; that is, they can be used as active and passive components. Among the other advantages of metal oxides are the low fabrication cost and robustness in practical applications. Metal oxides can be prepared in various forms such as ceramics, thick films, and thin films. Moreover, for thin-film deposition, techniques that are compatible with standard microelectronic technology can be used. The last factor is very important for large-scale production because the microelectronic approach promotes low cost for mass production, offers the possibility of manufacturing devices on a chip, and guarantees good reproducibility. Various metal oxide nanostructures, including nanowires, nanotubes, nanofibers, coreeshell structures, and hollow nanostructures, can also be synthesized. As it is known, the field of metal-oxide nanostructured morphologies (e.g., nanowires, nanorods, nanotubes) has become one of the most active research areas within the nanoscience community.

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The ability to create a variety of metal oxideebased composites and to synthesize various multicomponent compounds significantly expand the range of properties that metal oxideebased materials can have, making metal oxides a truly versatile multifunctional material for widespread use. As it is known, small changes in their chemical composition and atomic structure can be accompanied by the spectacular variation in properties and behavior of metal oxides. Even now, advances in synthesizing and characterizing techniques are revealing numerous new functions of metal oxides. Taking into account the importance of metal oxides for progress in microelectronics, optoelectronics, photonics, energy conversion, sensor, and catalysis, a large number of various books devoted to this class of materials have been published. However, one should note that some books from this list are too general, some books are collections of various original works without any generalizations, and other ones were published many years ago. But during the past decade great progress has been made on the synthesis as well as on the structural, physical, and chemical characterization and the application of metal oxides in various devices, and a large number of papers have been published on metal oxides. In addition, till now many important topics related to metal oxide study and application have not been discussed. To remedy this situation, we decided to generalize and systematize the results of research in this direction and to publish a series of books devoted to metal oxides. The proposed book series “Metal Oxides” is the first one devoted to the consideration of metal oxides only. We believe that combining books on metal oxides in a series could help readers in searching required information on the subject. In particular, we plan that the books from our series, which have a clear specialization by its content, will provide interdisciplinary discussion for various oxide materials with a wide range of topics, from material synthesis and deposition to characterizations, processing, and then to device fabrications and applications. This book series is prepared by a team of highly qualified experts, which guarantees it a high quality. I hope that our books will be useful and comfortable. I would also like to hope that readers will consider this “Metal Oxides” book series like an encyclopedia of metal oxides that enables to understand the present status of metal oxides, to estimate the role of multifunctional metal oxides in the design of advanced devices, and then based on the observed knowledge, to formulate new goals for further research. The intended audience of this book series is scientists and researchers who are working or planning to work in the field of materials related to metal oxides, i.e., scientists and researchers whose activities are related to electronics, optoelectronics, energy, catalysis, sensors, electrical engineering, ceramics, biomedical designs, etc. I believe that this “Metal Oxides” book series will also be interesting for practicing engineers or project managers in industries and national laboratories, which would like to design metal oxideebased devices, but do not know how to do it and how to select optimal metal oxides for specific applications. With many references to the vast resource of the recently published literature on the subject, this book series will be serving as a significant and insightful source of valuable information, providing scientists and engineers with new insights for understanding and improving existing metal oxideebased devices and for designing new metal oxideebased materials with new and unexpected properties.


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I believe that this “Metal Oxides” book series would be very helpful for university students, postdocs, and professors. The structure of these books offers a basis for courses in the field of material sciences, chemical engineering, electronics, electrical engineering, optoelectronics, energy technologies, environmental control, and many others. Graduate students could also find the book series to be very useful in their research and in understanding features of metal oxide synthesis, study, and application of this multifunctional material in various devices. We are sure that all of them will find the information useful for their activity. Finally, I thank all the contributing authors and book editors who have been involved in the creation of these books. I am thankful that they agreed to participate in this project and for their efforts in the preparation of these books. Without their participation, this project would have not been possible. I also express my gratitude to Elsevier for giving us the opportunity to publish this series. I especially thank the team of editorial office at Elsevier for their patience during the development of this project and for encouraging us during the various stages of preparation. Ghenadii Korotcenkov


Capacitive and Pseudocapacitive Electrodes for Electrochemical Capacitors and Hybrid Devices

1

Thierry Brousse 1,2 , Olivier Crosnier 1, 2 , Daniel Bélanger 3 , Jeffrey W. Long 4 1 Université de Nantes, CNRS, Nantes, France; 2Réseau sur le Stockage Electrochimique de l’Energie (RS2E), FR CNRS, Amiens, France; 3Université du Québec a Montréal, Montréal, QC, Canada; 4U.S. Naval Research Laboratory, Washington, DC, United States

1.1

Introduction

Electrochemical capacitors (ECs, also sometimes denoted as “supercapacitors” or “ultracapacitors”) are energy storage devices that bridge the performance gap between the high energy density provided by batteries and the high power density (but very limited energy density) derived from dielectric capacitors. Commercially available ECs exhibit gravimetric energy density up to 8.5 Wh kg1 and usable power density up to 9.0 kW kg1 [1]. In the field of ECs, there is often confusion between the electric parameters of a full device and the electrochemical properties of the individual electrodes that comprise the cell. The aim of this chapter is to describe the distinctions between these various devices and their constituents, starting with a comparison of dielectric capacitors and ECs, followed by discussion of other electrochemical energy storage devices with regard to their electric properties. The electrochemical behavior of common electrode materials used in ECs and related devices will be discussed in terms of capacitive, pseudocapacitive, and faradaic charge storage mechanisms, as well as recommended methods with which such electrodes should be characterized. We highlight the distinctions between carbon-based capacitive electrodes that are commonly found in commercial ECs and pseudocapacitive electrodes [2,3], such as RuO2 [4,5], or MnO2 [6,7], that have the electrochemical signature of a capacitive electrode but express different charge storage mechanisms. In the last part of the chapter, we describe the important distinctions between high-power battery-type electrodes and pseudocapacitive electrodes.

1.2 1.2.1

Devices Dielectric Capacitors

A dielectric is an electronically insulating material that can be polarized by an applied electric field where electric charges do not flow through the material as they do in an Metal Oxides in Supercapacitors. http://dx.doi.org/10.1016/B978-0-12-810464-4.00001-2 Copyright © 2017 Elsevier Inc. All rights reserved.

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electronic conductor, such as metals, but are only slightly shifted from their equilibrium positions. Positive charges are displaced in the direction of the field and negative charges shift in the opposite direction. This separation of charge creates an internal electric field that reduces the overall field within the dielectric itself [8]. In a dielectric capacitor (e.g., conventional polymer film or ceramic capacitor), the dielectric material is thin and sandwiched between two current collectors that are usually metals. When a voltage is applied to the dielectric capacitor, an electric field is created within the dielectric, and charges (electrons and holes) accumulate in the metallic current collectors at the interface with the dielectric material. Thus, dielectric capacitors store charges through electrostatic interactions but at levels that are much lower than for standard batteries, and they are usually not designed or used where high energy density is required. Capacitance is the ability of a body to store an electric charge. This capacitance is constant over a given voltage window and can be used to calculate the charge stored using Eq. (1.1), DQ ¼ C DU

(1.1)

where DQ is the charge stored (expressed in coulombs, C) and DU is the width of the voltage window (V). In this case the capacitance, C, is the amount of charge stored when a 1-V window is used. For a given voltage window, there is a direct and simple access to the charge stored. The SI unit of capacitance is farad (symbol: F), named after the English physicist, Michael Faraday. For conventional dielectric capacitors, charges can be stored over a wide voltage window, sometimes reaching several hundred volts. Thus, capacitance was introduced to compare the performance of dielectric materials: the higher the capacitance, the more charge stored within a given voltage. In a conventional dielectric capacitor where a thin dielectric layer separates the two metallic current collectors, the capacitance is proportional to the surface S of the metallic plates and inversely proportional to the thickness of the dielectric, e; the thinner it is, the larger the capacitance (Eq. 1.2). C ¼ ðε0 εr SÞ=e

(1.2)

where ε0 is the electric permittivity of vacuum and εr is the relative permittivity of the given dielectric material. However, thinner dielectric are more susceptible to breakdown. Indeed, the concept of capacitance is accompanied in dielectric materials by that of breakdown voltage. This is characteristic of an insulator that defines the maximum voltage difference that can be applied across a given dielectric material before it collapses and conducts charges. In solid dielectric materials, the breakdown is usually due to the creation of a weakened pathway within the material that enables charge transfer from one electrode to the other. Thus, from relation (1.1), high capacitance values and high breakdown voltage lead to high charge storage. Furthermore, the energy stored in the dielectric capacitor is related to capacitance and cell voltage by relation (1.3): DE ¼ 1=2C ðDUÞ2

(1.3)

Capacitive and Pseudocapacitive Electrodes

3

5

Voltage / V

4 3 2 1 0 2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

Time / s

Figure 1.1 Constant-current charge galvanostatic cycling of a 4.7-nF dielectric capacitor (charge current ¼ 100 nA, discharge current ¼ 100 nA).

Thus, a 1-nF 400-V/DC dielectric capacitor available from any electronic component supplier stores an energy of 80 mJ, i.e., 22 n Wh when translated into units familiar to battery users. Similarly, a 100-mF 6.3-V/DC stores 2 mJ, i.e., 0.55 m Wh. If a constant-current chargeedischarge test is performed between 0 V and a positive voltage value, the voltageetime (or chargeetime) response of the dielectric capacitor will be triangular, as shown, for example, in Fig. 1.1. The capacitance is inversely proportional to the slope DU/Dt (during charge or discharge) according to Eqs. (1.4) and (1.5): DQ ¼ I Dt ¼ C DU

(1.4)

C ¼ I=ðDU=DtÞ

(1.5)

Because Fig. 1.1 clearly shows a linear slope, a constant capacitance can be defined no matter the voltage window used. The cell voltage for a dielectric capacitor can be cycled over a much broader range than accessible with an electrochemical device, but the time scale (microseconds or milliseconds) shows that the amount of charge stored is relatively low, and consequently, energy density is also orders of magnitude lower than that for a battery. This is the main reason why dielectric capacitors are not used to store energy, but instead used as passive components in electronic circuits/devices. Although Fig. 1.1 presents valuable information, it does not identify which kind of dielectric material is involved in the capacitor or the nature of the metallic plates sandwiching this dielectric. This sounds like an obvious remark but many researchers become confused with the concept of two-terminal devices. In most cases, dielectric capacitors do not have a polarity, such that any terminal can be used as the positive polarity and the other one as the negative (the exception being electrolytic capacitors). Dielectric capacitors are based on a very robust technology enabling billions of chargeedischarge cycles without significant fade in their electric properties.

4

1.2.2


Electrochemical Capacitors

ECs [2,9], also called supercapacitors, ultracapacitors, electric double-layer capacitors, or electrochemical double-layer capacitors, are now commercial devices that are manufactured and sold by many companies all around the world. A wide range of devices are available with capacitance ranging from a few farads to thousands of farads. Unlike dielectric capacitors, the maximum operating voltage of individual cells is usually in the range of 0 V to 2.5e3 V. These two main distinctions from dielectric capacitors, in the maximum operating cell voltage and in the order of magnitude of the capacitance values, suggest that the underlying chargeestorage processes are quite different. Indeed, ECs are typically made of two carbon electrodes, each of them coated on an aluminum current collector. Commonly used materials for commercial ECs mainly consist of various activated carbons. These carbons have a very high specific surface area, typically 1000e2000 m2 g1. They are usually mixed with a carbon additive such as carbon black, which improves the electronic conductivity of the electrode (activated carbons are not inherently good electronic conductors). A polymeric binder is also necessary to provide mechanical stability to the electrode, i.e., between the grains of carbons and also between carbon and the current collector. The two electrodes are sandwiched on either side of a polymeric separator (or a paper), which prevents the electrodes from contacting while also supporting an infiltrated electrolyte solution. Most commercial ECs use tetraethylammonium tetrafluoroborate, ðC2 H5 Þ4Nþ BF4 , as the electrolyte salt, acetonitrile (CH3CN) and propylene carbonate (C4H6O3) being the two most commonly used solvents. Commercial ECs are commonly considered as symmetric devices using two identical electrodes. However, commercially available cells possess a negative and a positive terminal. This is due to the charge storage mechanism that occurs upon polarization of each electrode. Ions from the electrolyte are attracted by each charged carbon surface. Briefly, cations preferentially migrate toward the negatively polarized electrode and anions in the opposite direction toward the positive one, thus giving rise to a double-layer capacitance at each electrodeeelectrolyte interface (Fig. 1.2). Although Electrolyte

Separator

Current Collector (Al)

+

-

Porous carbon electrode

Figure 1.2 (Left) Schematic representation of a double-layer capacitor and (right) a model of the electrochemical double layer. Rreproduced with the kind permission of Patrice Simon and Pierre-Louis Taberna, CIRIMAT, Toulouse, France.


5

it is beyond the scope of this chapter to discuss the mechanistic nuances of double-layer capacitance and their implications for performance, it is important to realize that the charge separation occurs across a narrow distance at the electrode surfaces, typically less than 2 nm. Additionally, unlike the metal plates in a conventional dielectric capacitor, ions carrying charges occupy the extensive porosity of activated carbon grains. Thus, the effective surface area of the capacitor is much higher than the geometric footprint of a given electrode, but rather correspond reasonably well with the specific surface area of the carbon electrode. Considering these two parameters in relation to Eq. (1.2), the very low thickness of the double layer and the large surface area of the carbon electrode, together with the dielectric constant of the electrolyte, give rise to a very high capacitance compared to dielectric capacitors, typically several orders of magnitude more for the same volume of device. Another distinction is that the capacitance of an EC, Ccell, is the result of two capacitances in series, one at the positive electrodeeelectrolyte interface (Cþ) and another at the negative electrodeeelectrolyte interface (C), according to Eq. (1.6): 1=Ccell ¼ 1=Cþ þ 1=C

(1.6)

It can be noted that all the capacitances in Eq. (1.6) are expressed in farad, not gravimetric capacitance, areal capacitance, or volumetric capacitance. Thus, Ccell has to be divided by the mass, surface, or volume of the device to obtain a technologically relevant unit of charge storage performance. Eq. (1.6) does not necessarily reflect the complex charge storage mechanisms that occur at the electrodeeelectrolyte interface; for a more thorough understanding one must consider modern theory involving the influence of pore size distribution and partial desolvation of electrolyte ions when entering the pores [10e13]. Because carbon electrodes do not store anions and cations in the same way, the two electrodes do not have to be necessarily identical in terms of mass loading. Thus it is important to keep the right polarity when connecting the EC to another device. Despite the high capacitance that can be obtained with ECs, they exhibit a cell voltage that is one or two orders of magnitude smaller than the operating voltage of most dielectric capacitors. Thus the gain in energy density for ECs, compared to dielectric capacitors, due to their high capacitance is somewhat mitigated by the low cell voltage. For ECs, the cell voltage is dictated by electrochemical considerations such as electrolyte decomposition on carbon electrodes and subsequent gas evolution reactions, as well as carbon electrode oxidation. Organic electrolytes help achieve a 3 V maximum operating voltage for modern ECs, whereas aqueous electrolytes typically yield a much lower operating voltage. Even though ECs are based on entirely different materials and charge storage mechanisms, the typical constant-current chargeedischarge plot of ECs is similar to that of dielectric capacitors (Fig. 1.3). Eqs. (1.4) and (1.5) still apply, and again one can define a constant capacitance irrespective of the cell voltage used. The main differences between Figs. 1.1 and 1.3 are that the cell voltage is lower for ECs than for dielectric capacitors and the timescale is much higher for ECs, on the order of a few seconds or even minutes.


Voltage / V

6

4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0

50

100

150 200 Time / s

250

300

350

Figure 1.3 Constant-current galvanostatic cycling of a 22-F Nichicon electrochemical capacitor (charge current ¼ 1 A, discharge current ¼ 1 A).

According to Eq. (1.3), a 1-F EC over a 3-V voltage window can deliver an energy of 4.5 J (or 1.3 m Wh). A 3000-F cell operated under the same voltage reaches 3.75 Wh. These values are much higher than those calculated for dielectric capacitor, and in this case the stored energy can be used to power a device. It must be noted that the maximum energy density can be determined by operating the EC between 0 V and the positive cell voltage limit indicated by the manufacturer. Operating the capacitor between a negative cell voltage and a positive one does not provide any additional energy (but can severely damage the EC). Thus, studies claiming outstanding energy density by operating a cell between 3V and þ3V are wrong and mislead the reader and the potential users. As for dielectric capacitors, the capacitance and the maximum operating cell voltage of an EC are electric parameters that give pertinent information neither on the electrodes and electrolyte inside the cell nor on the specific charge storage mechanisms involved. Furthermore, the safe electrochemical potential window of a single carbon electrode as well as the specific capacitance of this electrode cannot be deduced from electric parameters listed for the full cell and must be separately determined using particular electrochemical techniques. It can be noted that apart from the capacitance and the rated cell voltage, EC manufacturers also provide the equivalent series resistance of the cell (named ESR, in milliohms), which is of crucial importance for estimating the power density. Similar to dielectric capacitors, ECs can exhibit long-term stability and excellent cycle life (many thousands to millions of cycles) when properly operated within their manufacturers’ specifications.

1.2.3

Secondary (Rechargeable) Batteries

Although it is beyond the scope of this book to detail all the available battery technologies, it is important to focus on the electric parameter usually provided for rechargeable batteries. Unlike capacitors, both dielectric capacitors and ECs, the constant-current chargeedischarge plots of batteries do not exhibit a triangular shape


7

5

Voltage / V

4 3 2 1 0 5

10

15

20

25

30

Time / h

Figure 1.4 Constant-current galvanostatic cycling of a 18650 Li-ion cell (ENIX) between 2 and 3.6 V at C/5 rate (charge current ¼ 300 mA, discharge current ¼ 300 mA).

with linear slopes, but show flat or sloping plateaus at a given voltage or range of voltage, respectively, as shown in Fig. 1.4. Such behavior is typical for faradaic reactions occurring at well-defined potentials at each electrode of the battery. Thus rechargeable batteries typically charge and discharge within narrow voltage windows, which can be quite advantageous when powering an external device that required a specific voltage. As with ECs, a battery has negative and positive terminals that are not interchangeable when powering a device or upon charging. Unlike capacitors, Eqs. (1.3)e(1.5) do not apply to batteries and subsequently a constant capacitance cannot be defined. For example, by considering an ideal battery with a flat discharge plateau, Eq. (1.5), the slope DU/Dt is close to zero and thus the capacitance has an infinite value. At the end of the charge or discharge of a battery, the voltage drastically increases or decreases, respectively (Fig. 1.4). In this voltage window, the slope DU/Dt reaches a very high value, almost infinite, and consequently the capacitance is very small. Thus, it makes no sense to report capacitance as an electric parameter for a battery. Hence, the electric parameters usually provided for a single cell are the average operating voltage and the cell capacity, expressed in C or milliampere hours (mAh). The first approximation is that the energy stored in a cell is the product of the capacity and the average discharge voltage. Unlike ECs, the electrochemical reactions in battery-type electrodes often result in stresses that lead to a steady decay in performance and consequently batteries can only sustain a few thousands of chargee discharge cycles (or less) at 100% nominal depth of discharge.

1.2.4

Asymmetric or Hybrid Device

The past 2 decades have seen the rise of new devices called asymmetric or hybrid capacitors. Multiple versions of these emerging energy storage devices are now commercially

8


available [14] and, similar to secondary batteries, they exploit a large variety of chemistries for the electrode that cannot be described in great detail here. As a representative example, the lithium-ion capacitor (LIC) is presented in the following [15]. Unlike batteries, LICs include a battery-type electrode (as the negative terminal) and a capacitive electrode (as the positive terminal). More precisely, the negative side is usually a graphite electrode (lithium-ion intercalation compound) and the positive one is activated carbon. On first inspection, the chargeedischarge profile of such hybrid cell, shown in Fig. 1.5A, looks similar to that of an EC. The presence of a capacitive electrode (as the positive electrode in LIC) provides a linear slope to the device when coupled with the flat discharge plot of the faradaic negative electrode. As can be seen from Fig. 1.5B, this slope is the same as that of the capacitive electrode. As the capacitance of a device can be determined from Eq. (1.5), and is inversely proportional to the slope of the discharge plot, DU/Dt, the capacitance of the LIC will be almost the same as the carbon electrode. In an EC, the slope of the full device combines the discharge profile of the two carbon electrodes, resulting in a cell capacitance divided by two compared to a single carbon electrode. Thus, for LICs, (1) a capacitance can be determined, (2) the presence of a battery-type electrode provides a higher capacitance to the LIC than the one of an EC device that uses two capacitive carbon electrodes, (3) the faradaic electrode also determines the minimum voltage at which the cell can be operated, and (4) the maximum operating cell voltage is determined by the faradaic electrode and the electrochemical window in which the capacitive carbon electrode is operated. In summary, the electric parameters of an LIC, and more generally speaking of a hybrid capacitor, can be given as the cell capacitance, the minimum and maximum cell operating voltages, and the ESR. In this example, it can be clearly seen that the electric parameters reflect neither the chemistry nor the electrochemistry of the electrodes. The cycling ability of LICs is usually less than a conventional EC but much better than rechargeable batteries.

(B)

Charge

4.0 Hybrid capacitor -1 157 F g

4 3.5 3 2.5 2 1.5 1 0.5 0

Symmetric capacitor 98 F g-1

0.0

0

100

200

300 400 t/s

500

600

700

Discharge 5.0

AC

Potential (V vs Li/Li+)

U/V

4.5

Cell Graphite

0

100 Time (s)

Cell voltage (V)

(A)

0.0 200

Figure 1.5 (A) Comparison of the galvanostatic chargeedischarge profiles for symmetric activated carbon (AC)/AC electrochemical capacitor and hybrid AC/graphite (SLP-30) hybrid capacitors using 2 mol L1 LiTFSI (lithium bis(trifluoromethanesulfonyl) imide) electrolyte [16]. (B) Schematic galvanostatic chargeedischarge behavior of a lithium-ion capacitor: positive electrode, AC; negative electrode, graphite; electrolyte, LiPF6 in ethylene carbonate diethyl carbonate.


9

For all such devices, it is not possible to discern the electrochemical behavior of a single electrode when only the electric parameters of a device are known. Thus, authors should refrain from calling a device “pseudocapacitive,” a term related to the electrochemical behavior of particular electrode materials, which will be described later in this chapter. The same remark applies for authors applying the term pseudocapacitive to battery-type electrodes inside a hybrid capacitor. In the following, we will focus on the electrodes used in ECs and hybrid capacitors as well as on their electrochemical properties. Contrary to what was presented earlier sections, which mainly focused on commercially available devices, more prospective materials will be discussed in the following section. The term “hybrid” supercapacitor should be preferentially used when pairing two electrodes, one capacitive and one faradaic. The term “asymmetric” supercapacitor encompasses a wider range of electrode combinations because it can be used for supercapacitors using electrodes of the same nature but with different mass loading, or two electrodes using different materials [17e19].

1.3 1.3.1

Electrodes for Electrochemical Capacitors and for Hybrid Capacitors Capacitive Electrodes

A wide variety of carbons have been proposed as capacitive electrodes for ECs [20]. Not all of them are pertinent in terms of capacitance, electronic conductivity, density, specific surface area, pore size distribution, and many other characteristics. The investigation of the carbon electrode itself without the target electrolyte in which it might be operated is useless. Because the capacitance of a single carbon electrode is related to its ability to attract as many ions as possible close to its surface when polarized, the surface accessible to ions should be one of the major parameters for selecting a carbon. This accessible surface is strongly related to the pore size distribution of the carbon. Practically, to obtain a high capacitance, most of the surface measured by gas adsorption method should be available to the electrolyte infused into the pore structure when the material is tested as an electrode or in a device. Even if the porosity is accessible to ions but the tortuosity of the carbon and the electrode is unfavorable to fast ion diffusion, the electrode surface will only be accessible at low cycling rates, which will impact the power capability of the electrode. These important properties can only be investigated by preparing an electrode and testing this electrode in a three-electrode cell, with an adequate reference electrode (e.g., Ag/AgCl in saturated KCl solution, Hg/HgO in KOH) and a high-surface-area counterelectrode. With such a cell, cyclic voltammetry can be used to determine (1) the useful electrochemical potential window of a given carbon electrode (not the voltage window, as too often presented in the literature), (2) the capacitance of the carbon electrode (usually the gravimetric capacitance), and (3) the rate capability of the electrode by varying the potential scan rate. Other electroanalytical methods

10


may also be used to investigate the behavior and the performance of an electrode, but voltammetry is the most commonly used method in the literature, and thus, we will also use such results for comparison in the discussions of this chapter Interested researchers could also use more powerful techniques, such as electrochemical impedance spectroscopy, as presented elsewhere [21]. Because carbon electrodes are mainly capacitive, i.e., storing charges in the electrochemical double layer, they should exhibit a linear dependence on the width of the investigated potential window with the amount of charge stored. This results in quasi-rectangular cyclic voltammograms (CVs), which means that while scanning the potential at a constant sweeping rate, the measured current is constant. When a potential limit, imposed by the user, is reached, the sweeping direction is reversed and the current should immediately shift to the same current value but of opposite sign. As mentioned earlier, the electrode itself must be correctly prepared to determine its important characteristics. If the electrode is too thick, this will introduce some extrinsic tortuosity, which is not related to the intrinsic tortuosity of the carbon, and then the electrode engineering will become predominant in the electrochemical evaluation of the electrode performance. Thus, low mass loading should be preferred (typically a few micrograms to a few milligrams per square centimeter), i.e., the amount of the investigated carbon per footprint area of the electrode. However, most of the authors who are following this advice also tend to extrapolate the gravimetric values they measure to a device-type carbon electrode, which is definitively not correct [22]. Thus, evaluating the intrinsic performance of a carbon in a three-electrode cell and evaluating the performance of the same carbon in an electrode for a full device are two different things that require different experiments. In the latter case, the electrode preparation is very important and likely requires some additional optimization to determine the most suitable conductive additive, binder, current collector, and electrode porosity when preparing a 100-mm-thick electrode designed for evaluating its performance in a full cell. This engineering process will not be further discussed in the following paragraph but one should keep in mind that the electrode preparation should be suitable to determine the desired parameters. To avoid intrinsic tortuosity, carbons with large geometric surface and low porosity are preferred [20]. Indeed carbons such as zero-dimensional (0D) carbons [carbon quantum dots, onion-like carbons (OLCs)], one-dimensional (1D) carbons (carbon nanotubes or nanofibers), and two-dimensional (2D) carbons (graphene) theoretically provide an easily accessible surface to the ions, thus strongly limiting the problem of tortuosity. OLC-based thin-film electrodes, for example, have demonstrated very high rate capability and they still exhibit a quasi-rectangular shape (Fig. 1.6A) [23]. The same behavior can be found for graphene (Fig. 1.6B) [24] but to a lesser degree because graphene sheets tend to restack, thus rebuilding a porous architecture. Carbide-derived carbons (CDCs) [10,25] have been proposed as three-dimensional (3D) carbon electrodes with tailored porosity that can drastically improve the gravimetric capacitance when used with an adequate supply of electrolyte ions. However, their synthesis requires the use of specific techniques that are not exempt of hazards, such as chlorination of transition metal carbide precursors. 3D carbons such as CDCs (Fig. 1.6C) [25], activated carbons (Fig. 1.6D) [26], and templated mesoporous


(A)

C1350-Tair-H2 C1350-2Tair C1350-Tair C1350

60

11

(B)

40 Capacitance (F/g)

40 20

Capacitance / F g–1

20

0

–20

(a)

0 –20 –1

20 mV s

–40

50 mV s–1 150 mV s–1

–60 –40 0.0

0.5

1.0

1.5

250 mV s–1

–80 –1.0

2.0

–0.8

E(V)

(D)

0.0

0.2

80

(C)

60 40

200

20 C am / F g–1

C m;cv / F g–1

–0.6 –0.4 –0.2 Potential vs. (Hg/HgO) / V

ΔV=3.2 V

100

ΔV=3.0 V

0

ΔV=2.7 V

0 –20 –40 –60

–100

–80 –100

–200

0

2

1 ΔV / V

3

–120 –3

–2

–1 0 E / V vs Ag

1

2

Figure 1.6 Cyclic voltammogram (CV) of different carbon electrodes prepared with (A) cyclic voltammetric curves of onion-like carbons modified by different treatments at scan rate 20 mV s1 using Swagelok assembling (two electrodes) in organic electrolyte [1.5 M NEt4BF4/AN (tetraethylammonium tetrafluoroborate/acetonitrile) [23], (B) graphene nanosheets in KOH aqueous solution (30 wt%) as electrolyte at different cycling rates [24], (C) carbide-derived carbon (derived from Mo2C) symmetric cell at potential scan rate of 1 mV s1 at different cell voltages in 1 M (C2H5)3CH3NBF4 electrolyte in AN [25], and (D) activated carbon electrode in PYR14TFSI electrolyte at 60 C, obtained from CV at 20 mV s1 [26]. inset: capacitance vs. scan rate.

carbons [27] mainly exhibit surface area related to their intrinsic porosity. Their high surface area and corresponding high specific capacitance are technologically interesting, but performance at high rate may be compromised, and their low density (0.8 g cm3) limits volume-normalized capacitance. Most of the carbon investigated as electrodes in ECs are not pure carbon materials, especially with regard to their surface. Indeed, many chemical moieties such as quinoic or carboxylic radicals are usually found by X-ray photoelectron spectroscopic (XPS) analysis. These functional groups provide additional charge storage, which comes in addition to the double-layer capacitance of the carbon. Thus, sometimes CVs deviate

12


(A)

A 3

T1

(a)

4

2

2

1

0 –2

5 V/s 4 V/s 2 V/s 1.5 V/s 0.5 V/s

–4 –6 –0.2

0.0

0.2

0.4

Potential (V vs. Ag/AgCI)

0.6

j / μA.cm–2

Current density (mA/cm2)

6

(B) a) p-SiNWs b) n-SiNWs a’) p-Si b’) n-Si

0 –1 –2 –3 –4 –1,4

–1,2

–1,0 –0,8 E / V vs Ag+/Ag

–0,6

–0,4

Figure 1.7 Cyclic voltammogram of (A) TiN thin film in 0.5 M K2SO4 electrolyte [28] and (B) silicon nanowires (Si-NWs) in 1 M NEt4BF4 in propylene carbonate (PC) (tetraethylammonium tetrafluoroborate/propylene carbonate) electrolyte [33].

from a quasi-rectangular shape by the presence of bumps assigned to the redox activity of the related moieties. Heteroatoms such as nitrogen or sulfur can also give rise to faradaic contributions, but they can also modify other properties such as the improvement of the electronic conductivity of N-doped carbons. Apart from carbon electrodes, a few materials have been proposed as capacitive electrodes suitable for ECs. Titanium nitride (TiN) thin films have been described as capacitive electrodes [28] (Fig. 1.7A). Because of their good electronic conductivity, they can be used as both active materials and current collectors. However, the lack of in situ or operando experimental results makes it difficult to clearly demonstrate a purely capacitive behavior and the role of titanium cations cannot be definitely ruled out [29]. Silicon-based nanomaterials have also attracted substantial attention owing to their capacitive properties [30], large voltage window [31], and very high cyclability [32] (Fig. 1.7B). Doped silicon is preferred for its higher electronic conductivity. Different morphologies have been proposed, such as bottom-up nanowires [silicon nanowires (Si-NWs)] [33e35] and nanotrees [silicon nanotrees (Si-NTrs)] [32], top-down metal-assisted etched Si-NWs [36] or porous silicon [37e39]. Electrodes based on Si-NWs have also been characterized when coated with silicon carbide (SiC) [40] or gold [38]. In all the cases, the capacitance reported by specific surface area is in the range of a few microfarads per square centimeter and the typical rectangular shape of the related CVs strongly suggests a capacitive behavior similar to that of carbon electrodes. The main interest in capacitive electrodes resides in their ability to store charge at their surface and not in the bulk of the material. Of course, this severely limits the accessible capacity but provides a very long durability to the electrode because it does not suffer from chemical or mechanical changes on cycling. Indeed, when implemented in a full cell, millions of chargeedischarge cycles can be achieved without much fade in capacitance, or significant increase in the ESR.


1.3.2

13

Pseudocapacitive Electrodes

As described earlier, capacitive carbon-based electrodes have many advantages when used as the charge-storing material within ECs: (1) compatibility with organic electrolytes that provide wide voltage windows; (2) a large range of material morphologies (nanotubes, graphene sheets, nanoparticles) that can be selected for either power- or energycentric cell designs; (3) good gravimetric capacitance, particularly with activated carbons; and (4) outstanding long-term cycling ability. However, carbon-based electrodes have nearly reached their limit in terms of specific capacitance. Typical double-layer capacitance at a carbon surface is 15 mF cm2, which is a maximum of z400 F g1 when considering that the theoretical specific surface area of graphene is 2630 m2 g1. Such specific capacitance values have never been reported for practical carbon-based electrodes, especially when organic electrolytes are used. Furthermore, the apparent density of such carbon is usually less than 1 g cm3, which also strongly limits the volumetric energy density of corresponding devices. Ultimately, charge storage at carbon electrodes is constrained by their reliance on double-layer capacitance mechanisms. Some metal oxides, such as RuO2 [4,5] or MnO2 [6,7], have demonstrated much higher gravimetric capacitance than carbon electrodes. Because of their higher solid density than carbon, even their apparent density in practical electrodes, the volumetric capacitance is also much higher than carbon. Although double-layer capacitance also occurs at such materials, many of which are prepared in high-surface-area forms, their main advantage is that charge storage proceeds through fast and reversible redox reactions occurring at the surface and subsurface of the material. These redox reactions typically involve changes in the oxidation state of the metal sites within the oxide. The term “pseudocapacitance” [2] has been proposed to describe the electrochemical behavior of such electrode materials (typically RuO2 or MnO2) because they exhibit the electrochemical signature of a capacitive electrode (Fig. 1.8A and B), i.e., a linear dependence of the charge stored on changing potential within the window of interest. However, charge storage does not originate strictly from capacitive phenomena but rather by electron-transfer mechanisms. The origin of the word “pseudocapacitance” is interesting to trace, constructed by the association of the Greek prefix “pseudo” and the term capacitance. Although this prefix can take two different meanings, the one that applies in this case is “almost, approaching, or trying to be,” as noted in the English dictionary [42]. For example, “pseudomyxoma” in medicine is a gelatinous mass resembling a myxoma but composed of epithelial mucus. Similarly, the term “pseudocapacitance” was created to describe the properties of an electrode that shows the electrochemical signature of a capacitive electrode, but whose charge storage mechanism is primarily faradaic [4e7]. In Conway’s influential book, “Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications,” [2] it is stated, “Regular double-layer capacitance arises from the potential-dependence of the surface density of charges stored electrostatically (i.e., non-Faradaically) at the interfaces of the capacitor electrodes. (.) Pseudocapacitance arises at the electrode surfaces where a completely different charge-storage mechanism applies. It is Faradaic in origin, involving the passage of charge across the double-layer, as in battery charging or discharging, but capacitance arises in account of the special

14


(A)

(B) Specific Capacitance (F g–1 RuO2)

3

I / mA cm2

2 1 0

–1

1500 1000 500 0 –500

–1000

–2 0.4

0.9 E (SCE) / V

1.4

–1500 0.1

0.2

0.3 0.4 0.5 0.6 0.7 Potential (V vs SCE)

0.8 0.9

Figure 1.8 Cyclic voltammogram of (A) RuO2 electrode at 20 mV sl in 1 M solutions of HCIO4 (dashed line) and KOH (plain line) [4]. (B) Cyclic voltammogram of RuO2 electrode in 0.5 M H2SO4 at 2 mV s1 versus saturated calomel electrode (SCE) as a function of calcination temperature: 100 C [blue], 150 C [red], 200 C [green], 250 C (black), and 300 C [purple] [41].

relation that can originate for thermodynamic reasons between the extent of charge acceptance (Dq) and the change of potential (DV), so that the derivative d(Dq)/ d(DV) or dq/dV, which is equivalent to a capacitance, can be formulated and experimentally measured by dc, ac, or transient techniques. (.).” Since the first claim of pseudocapacitive charge storage for RuO2 electrodes, many materials have been proposed as pseudocapacitive electrodes for ECs. Both pseudocapacitance and battery-like charge storage are due to faradaic processes, which is a source of confusion for many authors in the literature. This misunderstanding arises in part from the similarity between the electric behavior of ECs and hybrid capacitors. It must be emphasized once again that the electric behavior of a given two-electrode device does not presuppose anything about the nature or the electrochemical behavior of the individual electrodes inside the cell. In some cases, material compositions that would normally exhibit battery-type charge storage behavior may appear capacitive when expressed particularly in nanoscale forms. Indeed, nanosized or ultrathin films of certain faradaic electrodes show such pseudocapacitive behavior, but as a consequence of their morphology or electrode construction [43] rather than any intrinsic properties of the material itself. Thus we may distinguish between “intrinsic pseudocapacitance” and “extrinsic pseudocapacitance,” although both types of electrodes still exhibit a capacitive-like electrochemical signature. The two types of behaviors are described in more detail in the following sections.

1.3.2.1

Intrinsic Pseudocapacitance

Intrinsic pseudocapacitance is related to the chemical nature of the material itself and does not depend on its microstructure. Returning to the example of RuO2 and MnO2


15

Figure 1.9 Cyclic voltammograms of (A) an MnO2 thin film in 2 M KCl electrolyte at 25 C with a scan rate of 20 mV s1 [44] and (B) a thick powder-based electrode in 0.1 M K2SO4 at 25 C with a scan rate of 2 mV s1 [45].

electrodes, thin films (100 nm thick) as well as bulk powder-composite electrodes (100 mm thick) have demonstrated exactly the same electrochemical signature, even though the gravimetric capacitance that can be calculated from the CV experiments is usually much larger for thin-film electrodes. Fig. 1.9 compares the CVs of an MnO2 thin film to that of a thick powder-based electrode. The pseudocapacitive behavior of RuO2 or MnO2 electrodes has been investigated by various electrochemical and spectroscopic techniques, sometimes in a coupled fashion, to follow the change in the mean oxidation state of the metal oxide upon cycling the electrode. Fig. 1.10 provides an example of X-ray absorption spectroscopy used in fluorescence mode [44]. This in situ experiment clearly demonstrated a change in the mean oxidation state of Mn from þ3.2 when the electrode was polarized at the lower potential limit of the electrochemical window [0.0 V vs. saturated calomel electrode (SCE)] up to þ3.95 when polarized to þ1.0 V versus SCE. Despite some hysteresis, the process is largely reversible. The same conclusions were reached by another group using in situ Mn K-edge X-ray absorption spectroscopic studies of electrodeposited manganese dioxide thin films [46]. Other techniques such as XPS [47] also led to the same conclusions for MnO2 powderdpseudocapacitance is due to fast and reversible faradaic reactions that involve the electrochemical interconversion between Mn4þand Mn3þ in the solid and the concomitant insertion of cations, Hþ or Cþ (Cþ ¼ Naþ, Kþ, .), from the electrolyte [44,46,48]. Similar studies have been performed on RuO2 electrodes. In this case, proton insertion was ascribed as the main cause of pseudocapacitance, concomitant with a change in Ru oxidation state over the potential window [49,50]. The electrochemical behavior during both charge and discharge of MnO2-based ECs has been analyzed and a 1D model adapted from the transmission line model was proposed, taking into account partial cation diffusion in the solid oxide [51]. A linear relationship between the variation of the mean Mn oxidation state and the

16


Figure 1.10 (A) Schematic diagram of the spectroelectrochemical cell for in situ X-ray absorption spectroscopic study in the fluorescence mode, and (B) dependence of the Mn oxidation state and the E0 on the applied potential during the electrochemical redox cycle [44].

potential on cycling was used to implement the model (Fig. 1.11A), as experimentally established through previous in situ XPS experiments. As depicted in Fig. 1.11B, the CVs simulated from the model fit well with the experimental CV.

1.3.2.2

Extrinsic Pseudocapacitance

The concept of extrinsic pseudocapacitance can be used to describe materials that exhibit capacitor-like behavior, but only when expressed in particular morphologies


(A)

17

1.0

(B)

0.6 OCP

2

0.4 0.2 0.0 3.75

20 mV.s–1

3

I / A g–1

Eeq vs (Ag/AgCI)/V

0.8

1 0 –1 –2

3.80

3.85

3.90

3.95

–3 –0.1

–0.2 Mn oxidation state

0.1

0.3

0.5

0.7

0.9

U/V

Figure 1.11 (A) Relation between the equilibrium potential of the couple Mn3þ/Mn4þ and its oxidation state, and (B) cyclic voltammograms, as measured (solid line) and as computed (dashed line). Potential is reported versus Ag/AgCl electrode Ref. [51]. OCP, open circuit potential.

(ultrathin films) or nanoscale forms, for example, when the electrode material (e.g., powder, thin film) reaches a critical size [52,53] where diffusion occurs through very limited timescales. But in such cases, capacitorlike behavior is only due to electrode design or architecture and not an intrinsic property of the parent material, as observed with LiCoO2 thin films. LiCoO2 is known as a lithium-intercalation compound that is commonly used as positive electrode in lithium-ion batteries and does not show “pseudocapacitive” behavior in such devices. Yet when synthesized in thin-film form (6 nm thick)[53], LiCoO2 yields a capacitorlike response (Fig. 1.12). MnO2

0.0

nanoscale

bulk

0

I Q/Qmax

LiCoO2

4.2

bulk

Potential (V vs Li/Li+)

Potenal (V vs Ag/AgCl)

0.9

1

3.0

nanoscale

0

I Q/Qmax

1

Figure 1.12 Schematic constant-current galvanostatic discharge of an intrinsic pseudocapacitive electrode and an extrinsic one in the case of a bulk electrode and a nanoscale electrode. Q and Qmax are the charge stored and the maximum charge that can be stored, respectively. Adapted from P. Simon, Y. Gogotsi, B. Dunn, Materials science. Where do batteries end and supercapacitors begin? Science 343 (2014) 1210e1211.

18


Dunn et al. denoted that such behavior is “extrinsic” pseudocapacitance, as opposed to MnO2, which exhibits capacitive behavior whether in thin-film or thick powdercomposite electrodes and is thus intrinsically pseudocapacitive (Fig. 1.12) [53].

1.3.3

High-Power Battery Electrodes

The electric behavior of a given device does not presume the processes that occur at each electrode inside the device, for example, whether the individual electrodes are capacitive, pseudocapacitive, faradaic, or some combination thereof in nature. Thus, some confusion may arise when considering the so-called asymmetric or hybrid devices that have been discussed in the literature and are also commercially available. Such devices are designed with a battery-type negative [e.g., graphite, Li4Ti5O12, TiO2(B), .] [15,16,54e56] or positive electrode [e.g., PbO2, Ni(OH)2, .] [57,58] and a complementary capacitive electrode, typically activated carbon. These devices exhibit interesting energy and power densities compared to standard ECs (Fig. 1.13). The first report of an energy storage device that combined an electric double-layer capacitor negative electrode with a positive nickel-oxide battery electrode was a patent by Varakin et al. in the mid 90s [59]. The chargeedischarge plots of such

Figure 1.13 Ragone plots of hybrid capacitor systems [Li4Ti5O12/carbon nanofiber (CNF) nanocomposite/activated carbon] and conventional electric double-layer capacity or system (activated carbon/activated carbon). The hybrid capacitor systems were assembled using two types of the composites with weight ratio of Li4Ti5O12/CNFs ¼ 50/50 or 70/30. The power and energy densities were calculated on the basis of the electrode volume. inset: schematic illustration for the two-step formation procedure of the Li4Ti5O12/CNFs nanocomposite [55].


19

devices appear capacitive as a combination of a capacitive electrode (triangular shape) and a faradaic electrode (plateau shape). Activated carbon/Ni(OH)2 hybrid devices have been extensively investigated, and calculations of projected energy densities are as high as 50 Wh kg1 based on a 1.65 V working voltage in 6.25 M KOH [57,58], i.e., six to seven times higher than an electric double-layer capacitor operated at 3 V in a standard organic electrolyte. Although the energy density of these hybrid devices is promising, the power capability is strongly dependent on the kinetics of charge storage at the battery-type electrode. Thus, many investigations have focused on improving the electronic conductivity and ionic transport within such electrodes tuning their nano-/micro-/macroarchitecture. Several strategies have been proposed to circumvent power/rate limitations, focusing on (1) new synthesis routes for Ni(OH)2 nanoparticles [60] and (2) fabrication of composite materials comprising Ni(OH)2 particles that are incorporated with a conductive carbonaceous material such as activated carbon [61] nanotubes or graphene sheets [62,63] to improve electronic conductivity within the electrode.

1.3.4

Capacity Versus Capacitance

Despite historic precedence for a working definition of “pseudocapacitance,” many faradaic electrodes based on Ni or cobalt-based oxides, hydroxides, or even sulfides [64e68] or composite materials involving these compounds [69,70] have been presented in the literature as pseudocapacitive materials; however, these materials exhibit well-defined redox peaks when examined by voltammetry (Fig. 1.14). This misunderstanding confuses the readers because the concept of “capacitance” (F) cannot apply to faradaic behavior; the metric of “capacity” [(coulomb, C, or milliampere hours (mAh)] is the most appropriate and meaningful option in such cases. Indeed, a constant capacitance cannot be determined from the CVs presented in Fig. 1.14 because they are not rectangular and correspondingly have no linear dependence of the charge stored on changing potential within the electrochemical window of interest. Misleading

(A)

CoS NiS Co1.5Ni1.5S4

40

(B) 6 Current density (Ag–1)

Current density / A g–1

30 20 10 0 –10

4

Ni(OH)2 nanosheets Ni(OH)2-GS composite Ni(OH)2-GS-CNT composite

2 0 –2

–20 –30

–4 0.0

0.1

0.2 0.3 0.4 Potential / V vs.HgO/Hg

0.5

0.6

–0.1 0.0 0.1 0.2 0.3 0.4 0.5 Potential vs. Hg/HgO (V)

0.6

Figure 1.14 Cyclic voltammograms of (A) CoS, NiS, and Co1.5Ni1.5S4 electrodes at 5 mV s1 in 2 M KOH and (B) Ni(OH)2 [68] and different Ni(OH)2/carbon nanocomposites at 5 mV s1 in 6 M KOH that exhibit pure faradaic behavior [70].

20


comments when describing such CVs, for example, “A pair of redox peaks can be observed in each CV curves which indicates a pseudocapacitance characteristic,” [64] must not be used in the scientific literature. Furthermore, a representative example of the misuse of capacitance is presented. Park et al. [61] stated that, “In order to enhance energy density, a hybrid type pseudocapacitor/electric double-layer capacitor (EDLC) was considered and its electrochemical properties were investigated. At various current densities, stable charge/discharge behaviors were observed with much higher specific capacitance values of 530 F g1 compared with that of EDLC (230 F g1), by introducing Ni(OH)2 as a cathode material.” As detailed earlier, it is not appropriate to compare these two values of capacitance because the value of 530 F g1 was determined over a limited potential window and corresponds to an “average” value. For example, if a wider or narrower potential window is arbitrarily chosen, the calculated specific capacitance could decrease or increase, respectively. For an electrode material such as Ni(OH)2, only the capacity in coulomb per gram (C g1) or milliampere hours per gram (mAh g1) provides a useful metric to be used for comparison against other materials [3,71]. Furthermore, charge storage properties could be compared with those of carbon electrode by transforming the constant capacitance of the carbon electrode into C g1 using the width of the potential window over which the device is cycled. Thus a direct comparison of the total charge stored at each electrode can be made. Similar treatment can be applied to an MnO2 pseudocapacitive electrode, which also exhibits a rectangular CV. The signature of a faradaic electrode such as Ni(OH)2 is entirely different.

1.4

Conclusions

The electrochemical behavior of faradaic electrodes are fundamentally different from that of pseudocapacitive electrodes. We propose to use the term “pseudocapacitive” only to describe electrode materials, such as MnO2, that display an electrochemical behavior typical of that observed for a capacitive carbon electrode. Using the same term for materials such as Ni(OH)2 or cobalt oxides that exhibit an electrochemical signature of a “battery” electrode should be avoided. Obviously, many EC-relevant materials display more complex behaviors and exhibit both mechanisms with a pseudocapacitive contribution coming from the surface properties and faradaic contribution coming from intercalation mechanisms, for example, as is the case for birnessite-type MnO2 [48,72]. Such materials must be described in such a way that the readers can understand which mechanisms are involved. Additionally, many references to pseudocapacitive materials can be found in Conway’s book [2], and new pseudocapacitive materials, such as FeWO4, are also proposed in the recent literature [73]. These include hydrous oxides such as RuO2 underpotential-deposition reactions, intercalation in TiS2, and conversion of an oxidized species to a reduced species in a redox system in solution. These three examples obviously do not match the definition given later on in the book: “pseudocapacitance arises when the extent of reaction, Q, is some continuous function of potential, V, so that the derivative, dQ/dV, arises that has the properties of a capacitance.” We hope our explanations will shed some light on this topic and help authors to correctly address the description of their electrodes.


21

Acknowledgments The authors would like to acknowledge Anne-Lise Brisse, Dr. Annaïg Le Comte, Dr. Christophe Aucher, and Alban Morel. J.W. Long acknowledges the U.S. Office of Naval Research.

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Features of Design and Fabrication of Metal OxideeBased Supercapacitors

2

Y. Liu 1,2 , Y. Zhang 1 , X.W. Wu 3, *, Y.S. Zhu 1, *, Y.P. Wu 1,3, * 1 Nanjing Tech University, Nanjing, China; 2Technische Universit€at Chemnitz, Chemnitz, Germany; 3Hunan Agricultural University, Changsha, China

2.1

Introduction

Supercapacitors (SCs), also named as electrochemical capacitors (ECs), can complement or replace batteries in electronic energy storage and harvesting applications, when high-power uptake or delivery is needed [1]. In general, SCs could be classified by several criteria such as the energy storage mechanism, electrode material, electrolyte, or configuration design. With respect to the energy storage mechanism, there are three main types: electric double-layer capacitive (EDLC) storage (also called nonfaradaic capacitive storage), pseudocapacitive storage (capacitive faradaic storage), and noncapacitive faradaic storage [2]. Meanwhile, symmetric or asymmetric SCs can also be discerned, which is shown in Fig. 2.1. In symmetric SCs, both the electrodes are composed of EDLC storage or pseudocapacitive storage. The electrode materials frequently employed in symmetric configuration are carbon materials, conducting polymers, and redox oxides [1,3,4]. The electric double-layer (EDL) capacitor is normally composed of carbon material that possesses a pure electrostatic charge storage mechanism, which is the most representative design (see Fig. 2.1). Symmetric configurations fabricated with a conducting polymer or metal oxide as electrode material are referred to as “pseudocapacitor” in Fig. 2.1. Asymmetric ECs or SCs are defined as follows. To be specific, the negative (or positive) electrode uses EDL to store charges; then the other electrode must store charges with pseudocapacitive storage mechanism. Therefore, asymmetric SCs can be built with different materials in each electrode to increase the energy density of asymmetric ECs by extending the cell voltage range. The most representative configuration is the activated carbon (AC)//MnO2 SC, although other devices combing one electrode based on AC and another based on electronically conducting polymer or different transition metal oxide have also been broadly investigated [5]. Recently, the novel term “supercapattery” was introduced, which means the combination of the term “supercapacitor” and “battery” or the hybridization of

*

Corresponding author

Metal Oxides in Supercapacitors. http://dx.doi.org/10.1016/B978-0-12-810464-4.00002-4 Copyright © 2017 Elsevier Inc. All rights reserved.

26


EDLC electrode + EDLC electrode

Electrochemical capacitor

Symmetric EC

Pesudocapacitive electrode + Pesudocapacitive electrode EDLC electrode + Pesudocapacitive electrode

Asymmetric EC

EDLC electrode + Faradaic electrode Pesudocapacitive electrode + Faradaic electrode

Figure 2.1 Classification of electrochemical capacitor (EC) in accordance with energy storage mechanism.

capacitor-type (EDLC storage or pseudocapacitive storage) and battery-type (noncapacitive faradaic storage) electrode materials (see Fig. 2.1). Anyway, it is also a kind of SC [6]. Moreover, this design has attracted considerable attention because it constitutes a promising strategy for enhancing the energy density of ECs. In brief, it consists of Li-intercalation electrode (e.g., Li4Ti5O12 nanostructure) as the negative electrode, in conjunction with lithiated nonaqueous electrolyte [7]. The positive electrode of these configurations is fabricated with AC, electronically conducting polymer, or Li-intercalation metal oxide (e.g., LiMn2O4 or LiCoO2) as battery-type material [5]. According to the difference of electrolyte or the configuration design, SCs could be classified as all-solid-state SCs, quasi-solid-state SCs, or liquid-electrolyte-based SCs, which have been studied substantially during the past several decades. Regarding the increase of energy density of symmetric or asymmetric ECs, broadening the potential window of electrolyte solution, in other words, increasing the cell voltage, can effectively enhance the energy density according to the equation of E ¼ 1/2CV2. For instance, the working voltage range of aqueous asymmetric ECs has been reported to be about 2 V by using neutral aqueous electrolytes [8]. A wide variety of novel organic electrolytes with wider electrochemical windows and less toxicities have been investigated for ECs when compared to the commercial ECs. For ionic liquid (IL) electrolytes, the working voltage of the corresponding ECs was increased to 4 V, even though they had high viscosity and low ionic conductivity [9]. Furthermore, the development of all-solid-state or quasi-solid-state electrolytes


27

has led to the invention of flexible, wearable, or solid-state ECs (symmetric or asymmetric), which have no potential leakage problem unlike the liquid-electrolyte-based ECs [10,11]. In this chapter, the main goal is to summarize some features of design and fabrication of symmetric and asymmetric ECs. It includes the basics of symmetric and asymmetric ECs and configuration design of full SCs. Finally, future developments of symmetric and asymmetric SCs are discussed.

2.2 2.2.1

Fundamentals of Symmetric and Asymmetric Supercapacitors Symmetric and Asymmetric Supercapacitors With Aqueous Electrolyte

As mentioned earlier, the electrolyte and electrode material directly influence the electrochemical performance of ECs, such as energy density and power density. A symmetric SC with an aqueous electrolyte is illustrated as the potential profile for the charging state with a single electrode system in Fig. 2.2, which is helpful to compare and discuss the amount of energy stored in each configuration. The capacitance is an intrinsic property of the electrode material related to the applied electrolyte.

(A)

(B) C1, qs, ∆Vs

C1, qs, ∆Vs

q1, ∆Vmax, C1 Cs, qs, ∆Vmax

C1 C1

∆ Vs

∆Vmax E1 q1

∆Vmax

Cs C2 Es

∆Vs

qs

Figure 2.2 Schematic representation of (A) single-electrode system and (B) symmetric supercapacitor with aqueous electrolyte according to the chargeepotential profiles. Modified from H.S. Choi, C.R. Park, Theoretical guidelines to designing high performance energy storage device based on hybridization of lithium-ion battery and supercapacitor, J. Power Sources 259 (2014) 1e14, copyright 2014, with permission from Elsevier.

28


This parameter, denoted as capacitance C, is defined as amount of charge (q), stored in electrode material per potential difference (DV). Thus, C is the slope of the line in the qeDV profile of Fig. 2.2, which is calculated as the following equation [12]: C¼

q 1 Fg or q ¼ C $ DV DV

(2.1)

The amount of electrochemical energy (E) is determined by the integration of stored charge over the electrostatic potential difference, representing the energy of each charge. Thus, E is equal to the value of the qeDV plot area in Fig. 2.2, calculated with the following equation: 1 1 E ¼ $ q $ DV ¼ $ C $ DV2 ðWÞ 2 2

(2.2)

The specific energy density (ED) refers to the storable electrochemical energy per unit mass of the electrode and unit time, as defined by 1 q $ DV 1 1 C $ DV2 1 $ ¼ $ $ ED ¼ $ Whg1 2 m 3600 s 2 3600 s m

(2.3)

where m is the mass of the electrode and 3600 s is 1 h. When a single capacitor-type electrode stores energy in an aqueous electrolyte with a maximum working voltage range of DVmax, the capacitance of single capacitor-type electrode (C1) is calculated by the equation (C1 ¼ q1/DVmax), where q1 is the storable charge for single electrode with aqueous electrolyte, as shown in Fig. 2.2A. Also, E1 and ED1, which are the amount of electrochemical energy and the specific energy density of single capacitor-type electrode in aqueous electrolyte, respectively, are determined by the following equation: 1 E1 ¼ $ q1 $ DVmax 2

(2.4)

1 q $ DVmax 1 ED1 ¼ $ 1 $ 2 3600 s m1

(2.5)

where m1 represents the mass of single capacitor-type electrode. The symmetric SC in aqueous electrolyte shown in Fig. 2.2B has both positive and negative electrodes. Assuming that symmetric ECs in aqueous electrolyte include the same electrode material and electrolyte as the single capacitor-type electrode in Fig. 2.2A, the specific capacitance of both systems will have the same value. Moreover, the energy stored in this symmetric SC (Es) is calculated by 1 1 1 Es ¼ $ qs $ DVmax ¼ $ $ q1 $ DVmax 2 2 2

(2.6)


Cc, qc, ∆Vc

Qf, ∆Vb

29

Vc Electronic force

Cc

∆Vc ∆Vmax

Ec qas, ∆Vmax

Redox Reaction Vb

Eb qs

∆Vb

Figure 2.3 Schematic representation of asymmetric supercapacitor with aqueous electrolyte according to the chargeepotential profile. Modified from H.S. Choi, C.R. Park, Theoretical guidelines to designing high performance energy storage device based on hybridization of lithium-ion battery and supercapacitor, J. Power Sources 259 (2014) 1e14, copyright 2014, with permission from Elsevier.

when qs represents the amount of charge stored in both the electrodes and the entire symmetric ECs. The same amount of charge is simultaneously stored both outside and inside each electrode surface through an increase in potential difference to DVmax due to the identical capacitance values with single capacitor-type electrode. Thus the total storable charge for symmetric EC with aqueous electrolyte (qs), will be 1/2q1. Also, the entire capacitance of symmetric EC (Cs) is calculated as half of C1, that is, Cs ¼ 1/2C1, in Fig. 2.2B. Consequently, the specific energy density of symmetric EC can be determined by the following equation: 1 1 q $ DVmax 1 1 q $ DVmax 1 ¼ $ 1 EDs ¼ $ $ 1 $ $ 2 2 3600 s 4 3600 s ms ms

(2.7)

where ms is the total mass of symmetric EC consisting of the same capacitor-type electrodes. Because each electrode can store an amount of charge equal to half of q1, the mass of each electrode will be 1/2m1 such that ms ¼ 1/2m1 þ 1/2m1 ¼ m1. In contrast to symmetric EC, asymmetric SC is assembled with different types of positive and negative capacitor-type electrodes. However, an asymmetric EC with aqueous electrolyte (Fig. 2.3) consisting of different types of electrodes (e.g., capacitor-type electrode and battery-type electrode) is considered, with a totally different electrochemical reaction mechanism. We also assume that this asymmetric configuration applies the same aqueous electrolyte and capacitor-type electrode utilized in the previous symmetric EC with aqueous electrolyte. Thus, the energy stored in this asymmetric SC (Eas) is expressed as follows [12]: 1 Eas ¼ qas $ DVb þ $ qas $ DVc 2

(2.8)

when qas is the amount of charge stored in both the electrodes and the asymmetric SC and DVb and DVc are the working voltage range of battery- and capacitor-type

30


electrodes, respectively. Because the mass of battery-type electrode is mb and the mass of capacitor-type electrode is mc, the specific energy density (EDas) of asymmetric SC is presented as 1 qas $ DVb þ $ qas $ DVc Eb þ Ec 1 1 2 EDas ¼ ¼ $ $ 3600 s mb þ mc 3600 s mb þ mc q 1 1 ¼ as $ DVb þ $ DVc $ 2 3600 s mas

(2.9)

These equations for the specific capacitance can be regenerated to consider the performance relative to symmetric EC using the mass and working voltage range ratio factors of the battery-type electrode, k1 and k2, respectively: k1 ¼

mb or mb ¼ k1 $ mas ð0 < k1 < 1Þ mas

(2.10)

k2 ¼

DVb or DVb ¼ k2 $ DVmax ð0 < k2 < 1Þ DVmax

(2.11)

Therefore, EDas is rearranged as the following equation: qas 1 1 $ DVb þ $ DVc $ EDas ¼ 2 3600 s mas qas $ DVmax 1 1 ¼ $ ð1 k1 Þ $ $ ð1 þ k2 Þ $ 2 3600 s mc 1 q $ DV 1 max ¼ ð1 k1 Þ $ 1 k22 $ $ 1 $ 2 3600 s m1

(2.12)

According to Eqs. (2.7) and (2.12), the specific energy density of symmetric and asymmetric ECs with aqueous electrolyte, EDs and EDas, respectively, can be compared in terms of battery-type material-related coefficient (KBM), depending on the ratio factors of k1 and k2: KBM ¼ ð1 k1 Þ $ 1 k22 1 q $ DVmax 1 $ EDas ¼ KBM $ $ 1 2 3600 s m1

(2.13) (2.14)

According to Eqs. (2.13) and (2.14), the smaller the values of k1 and k2, the higher the KBM will be. It is also apparent that k1 has more effect on the value of KBM than k2 owing to the substitution of the square value of k2 in the equation for KBM.


31

As shown in Eq. (2.7), KBM for symmetric EC with aqueous electrolyte is 1/2; thus, EDas is greater than Es when the value of KBM is higher than 1/2, that is, as in the following condition: rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 $ k1 1 k2 < 2 $ k1 2

(2.15)

k1 should be lower than 0.5 and k2 should be lower than 1/20.5 for a value of KBM higher than 1/2. In other words, the mass of battery-type electrode should be less than half of the total mass of asymmetric EC and the working voltage range of battery-type electrode cannot exceed 1/20.5 of the whole working voltage range for asymmetric SC in aqueous electrolyte. For specific maximum power density (PD), based on the same assumption used in the energy density calculation, the PD for each capacitor system in aqueous electrolyte can be expressed by the following equations: PD1 ¼

Imax $ DVmax m1

(2.16)

PDs ¼

Imax $ DVmax ¼ PD1 ms

(2.17)

PDas ¼

Imax $ DVmax ¼ mas

1 k1 $ PD1 1 k2

(2.18)

where Imax is the maximum current for each electrode, PD1 is the PD for single electrode, PDs is the power density of symmetric EC, and PDas is the power density of asymmetric EC in aqueous electrolyte. Moreover, PDas can be higher than PDs for the condition k1 < k2. Consequently, according to Eqs. (2.15)e(2.18), the specified condition of better electrochemical performance for asymmetric EC can be predicted when compared to symmetric EC in aqueous electrolyte.

2.2.2

Symmetric and Asymmetric Supercapacitors With Organic Electrolyte

One of the effective strategies to increase energy density and power density of SCs is to employ organic solvent as electrolyte because its working voltage range is broader than that of water [9]. Regarding the similar process of calculation for energy density and power density of symmetric and asymmetric SCs between organic electrolyte and aqueous electrolyte, the results are listed in Table 2.1. More details on the calculation is available in Ref. [12].

Table 2.1

Theoretical Equations to Describe Electrochemical Performance of Supercapacitors Specific Energy Density

Specific Power Density

Symmetric EC with aqueous electrolyte

1 1 q $DVmax 1 EDs ¼ $ $ 1 $ 2 2 3600 s ms

PDs ¼

Asymmetric EC with aqueous electrolyte

1 q $DVmax 1 $ ¼ $ 1 4 3600 s ms q 1 1 EDas ¼ as $ DVb þ $DVc $ 2 3600 s mas ¼

Asymmetric EC with organic electrolyte

1 q $DVmax 1 ¼ KBM $ $ 1 $ 2 3600 s m1 ð1 k1 Þ$ 1 k22 q $DVo max 1 EDo as ¼ $ o $ 3600 s 2 mo 1 1 q $DV k4 1 max ¼ 2 $ð1 k1 Þ$ 1 k22 $ $ 1 $ m1 2 3600 s k3 1 q $DVmax 1 $ ¼ KE $KBM $ $ 1 2 3600 s m1 1 1 $qB c $DVB c þ $qB c $DVB c þ qB b $DVB b 1 2 2 $ EDB ¼ 3600 s mB c þ mB B 1 k2 $k5 þ 2$k2 $k02 $k5 1 q $DVo max 1 ¼ $KBM $ $ o b $ 2 3600 s 1 þ k2 mo 1 1 q $DVmax 1 ¼ KBC $KE $KBM $ $ 1 $ 2 3600 s m1

EC, electrochemical capacitor.

Imax $DVmax ms

¼ PD1 Imax $DVmax mas 1 k1 $PD1 ¼ 1 k2

PDas ¼

PDo

as

PDB ¼

¼

Io

max $DVo max

mo as 1 k1 $PDo ¼ 1 k2

IB

max $DVB max

mB 1 k1 $PDo ¼ 1 k2

1

1


Asymmetric supercapattery with organic electrolyte

qas $DVmax 1 1 $ð1 k1 Þ$ $ð1 þ k2 Þ$ 2 3600 s mc

32

Energy Storage Cell


2.3

33

Configuration Design of Metal OxideeBased Supercapacitors

Recently, the configuration design of metal oxideebased SCs has caused great interest because of the appearance of special abilities, such as flexible and wearable. In this section, all-solid-state, quasi-solid-state, and liquid electrolyteebased SCs would be classified and introduced based on the difference in the electrolyte’s state or configuration design, which are listed in Table 2.2.

2.3.1

All-Solid-State Supercapacitors

Solid-state electrolytes can serve not only as the ionic conducting media but also as the electrode separators. So far, most types of the solid-state electrolytes developed for SCs have been focused on polymer electrolytes, but there is no report on inorganic solid materials, such as ceramic electrolytes. Hence, the discussed all-solid-state SCs are mainly based on solid polymer electrolytes (also known as dry polymer electrolytes) [34]. Interestingly, solid-state SCs can be further grouped into linear and planar types depending on the different configuration designs. Furthermore, flexible and lightweight wire-shaped supercapacitors (WSSCs) have recently attracted more interest because of their versatility in the device design and application potentials in portable or wearable electronics. To demonstrate the potential of CuO@CoFeelayered double hydroxide (LDH) grafted on copper wire electrode for the WSSC application, a wire-shaped asymmetric all-solid-state micro-SC was fabricated by using CuO@LDH nanowire arrays as the positive electrode, a Cu wire supported AC as the negative electrode, and poly(vinyl alcohol) (PVA)/KOH as the solid electrolyte. The configuration of the all-solid-state flexible WSSC described herein is schematically shown in Fig. 2.4. The whole surface area of the fabricated WSSCs is 6.28 cm2 and the volume of the whole cell is 0.314 cm3. Owing to the hierarchical coreeshell nanostructure and the efficient pseudocapacitance properties of CuO and CoFeeLDH, the as-fabricated asymmetric all-solid-state WSSC device shows high energy density and excellent cycling stability [13]. Moreover, the related energy storage mechanisms can be expressed as follows: Negative electrode : Carbon þ Kþ þ e 4 Carbon=K

(2.19)

Positive electrode : CuO þ dOH 4 CuOðOHÞd þ de

(2.20)

CoFeðOHÞx þ dOH 4 CoFeðOHÞxþd þ de

(2.21)

Total reaction: dCarbon þ CuO þ dKþ þ dOH 4 dCarbon=K þ CuOðOHÞd (2.22) dCarbon þ CoFeðOHÞx þ dKþ þ dOH 4 dCarbon=K þ CoFeðOHÞxþd (2.23)

Table 2.2

Some Features of Design and Mechanisms of Supercapacitors

Negative Materials

Positive Materials

Mechanisms

Symmetric Characteristic

Electrolytes

AC

Types of Design

CuO/LDH nanowire

EDLC þ pseudocapacitive

Asymmetric

PVA/KOH

All-solid-state (linear)

[13]

AC

Co3O4/C/Ni3S2 nanoneedle

EDLC þ noncapacitive faradaic

Asymmetric

PVA/KOH

All-solid-state (planar)

[14]

RCFs/MnO2/ PEDOT

RCFs/MnO2/ PEDOT

Pseudocapacitive þ pseudocapacitive

Symmetric

PVA/KCl

All-solid-state (linear)

[15]

NiFe2O4/ carbon textile

NiFe2O4/carbon textile


Symmetric

PVA/LiCl


[16]

MoO3/CuO

MoO3/CuO


Symmetric

PVA/LiOH


[17]

CNT

CNT/MnO2


Asymmetric

PVA/H3PO4

Quasi-solidstate (linear)

[18]

Graphene/ Bi2O3

Graphene/Bi2O3


Symmetric

PVA/H3PO4

Quasi-solidstate (linear)

[19]

MnO2/ graphene

MnO2/graphene


Symmetric

PVA/H3PO4

Quasi-solidstate (planar)

[20]

pMeT/ RuO2$xH2O

pMeT/ RuO2$xH2O


Symmetric

PVdF-HFP/EMITf/ NH4Tf

Quasi-solidstate (planar)

[21]

MnO2/CNT

LiMn2O4

Pseudocapacitive þ noncapacitive faradaic

Asymmetric

1 M LiClO4/propylene carbonate

Organic liquid

[22]

References

AC

MnO2


Asymmetric

EMIMBF4

Organic liquid

[23]

V2O5/CNT

AC


Asymmetric

1 M NaClO4/propylene carbonate

Organic liquid

[24]

Nb2O5/C/rGO

AC


Asymmetric

1 M NaPF6/EC/DMC

Organic liquid

[25]

MnO2/CNT

MnO2/CNT


Symmetric

1.2 M LiPF6/EC/EMC

Organic liquid

[26]

AC

PbO2 nanowire

EDLC þ noncapacitive faradaic

Asymmetric

0.1 M CH3SO3H/ Pb(NO3)2/4 M NaNO3

Aqueous

[27]

N-doped carbon

PPy/MnO2


Asymmetric

1 M Na2SO4

Aqueous

[28]

MoO2/C nanofilm

NiCo2O4

Pseudocapacitive þ noncapacitive faradaic

Asymmetric

1 M LiOH

Aqueous

[29]

AC

Co2Ni3ZnO8


Asymmetric

2 M KOH

Aqueous

[30]

RuO2/rGO

RuO2/rGO


Symmetric

1 M H2SO4

Aqueous

[31]

Mn5O8

Mn5O8


Symmetric

1 M Na2SO4

Aqueous

[32]

NiMnO3/rGO

NiMnO3/rGO


Symmetric

6 M KOH

Aqueous

[33]

AC, activated carbon; CNT, carbon nanotube; DMC, dimethyl carbonate; EC, ethylene carbonate; EDLC, electric double-layer capacitive; EMC, ethyl methyl carbonate; EMIMBF4, 1-ethyl-3-methylimidazolium tetrafluoroborate; EMITf, 1-ethyl-3-methylimidazolium trifluoromethanesulfonate; HFP, hexafluoropropylene; LDH, layered double hydroxide; PEDOT, poly[3,4-ethylenedioxythiophene]; pMeT, poly-3-methyl thiophene; PPy, polypyrrole; PVA, poly(vinyl alcohol); PVdF, poly(vinylidene fluoride); RCFs, ramie-derived carbon fibers; rGO, reduced graphene oxide.

36


Figure 2.4 Schematic representation of the flexible asymmetric wire-shaped all-solid-state supercapacitor based on CuO@CoFeelayered double hydroxide (LDH), activated carbon (AC) electrode, and poly(vinyl alcohol) (PVA)/KOH electrolyte [the inset shows the photograph of the as-prepared wire-shaped supercapacitor (WSSC)]. Modified from Z. Li, M. Shao, L. Zhou, R. Zhang, C. Zhang, J. Han, M. Wei, D.G. Evans, X. Duan, A flexible all-solid-state micro-supercapacitor based on hierarchical CuO@layered double hydroxide coreeshell nanoarrays, Nano Energy 20 (2016) 294e304, copyright 2016, with permission from Elsevier.

where “/” represent EDLC that comes from the pure electrostatic charge accumulated at the electrodeeelectrolyte interface. In the case of planar SCs, an all-solid-state asymmetric SC was assembled using the Co3O4@C@Ni3S2 nanoneedle array electrode as the positive electrode and the AC electrode as the negative electrode with PVA/KOH as the gel electrolyte (denoted as Co3O4@C@Ni3S2//AC asymmetric SC). A schematic diagram of the asymmetric SC is shown in Fig. 2.5A. As shown in Fig. 2.5B and C, the as-prepared SC can endure the folding without destruction of its structure, indicating high flexibility. Therefore, solid-state devices could have several advantages such as flexibility, ease of fabrication, wide range of working temperature, and improved safety compared to liquid-based SCs [14]. Moreover, another asymmetric flexible SC based on V2O5/polyindole@activated carbon cloth and reduced graphene oxide (rGO) @activated carbon cloth with LiNO3/PVA as the gel electrolyte was also investigated [35]. In addition, the as-fabricated flexible symmetric SC based on the ramie-derived carbon fibers/MnO2/ poly[3,4-ethylenedioxythiophene] (PEDOT) electrode was studied by a two-electrode configuration in which PVA/KCl gel was utilized as the solidstate electrolyte [15]. Porous MoO3@CuO electrode was also assembled into a symmetric all-solid-state SC with PVAeLiOH gel polymer, which demonstrates good electrochemical performance [17]. The charge-transfer mechanism following this capacitive faradaic behavior can be described by the following plausible process: Negative electrode : MoO3 þ xLiþ þ xe 4 Lix MoO3

(2.24)

Positive electrode: CuO þ xOH 4 CuOðOHÞx þ xe

(2.25)


37

Figure 2.5 (A) Schematic diagram of the all-solid asymmetric supercapacitor configuration. (B,C) Photographs of the device in the normal and bending states, and the rectangle indicates the active region. NNAs, nanoneedle arrays; PET, polyethylene terephthalate; PVA, poly(vinyl alcohol). Modified from D. Kong, C. Cheng, Y. Wang, J.I. Wong, Y. Yang, H.Y. Yang, Threedimensional Co3O4@C@Ni3S2 sandwich structured nanoneedle arrays: towards high performance flexible all-solid-state asymmetric supercapacitors, J. Mater. Chem. A 3 (2015) 16150e16161, copyright 2015, with permission from RSC.

Total reaction: MoO3 þ CuO þ xLiþ þ xOH 4 Lix MoO3 þ CuOðOHÞx (2.26) It should be pointed out that these PVA-based polymer electrolytes and the assembled solid-state SCs claimed by the authors should be classified into the following quasi-solid-state SCs. However, to respect the authors’ original work, we classified these kinds of reports into this section as claimed by the authors.

2.3.2

Quasi-Solid-State Supercapacitors

Because of the presence of a liquid phase in gel polymer electrolytes (GPEs), some studies named them as quasi-solid-state electrolytes [34]. Thus, the corresponding electrochemical SCs can be addressed as quasi-solid-state SCs. In contrast to the solid polymer electrolytes, which are composed of a polymer (e.g., PVA) and a salt (e.g., KCl), the GPE consists of a polymer host and an aqueous electrolyte (e.g., H3PO4) or a conducting salt dissolved in a solvent [34]. For instance, a flexible quasi-solidstate configuration of the redox pseudocapacitors assembled with an ILebased proton-conducting nonaqueous GPE and composite electrodes of a conducting polymer (poly-3-methyl thiophene) and hydrous ruthenium dioxide (RuO2$xH2O) was reported [21]. The gel film has been found to be suitable as separator/electrolyte to fabricate SCs. Similar to all-solid-state SCs, there are also linear and planar types in quasi-solidstate SCs. Specifically, a high-performance asymmetric two-ply yarn SC with as-spun carbon nanotube (CNT) yarn as the negative electrode and CNT@MnO2 composite yarn as the positive electrode in PVA/H3PO4 gel aqueous electrolyte was produced. A schematic illustration of the preparation procedures for the two-ply yarn asymmetric SC is shown in Fig. 2.6A. The resulting asymmetric SC has a diameter of about

38


(A)

(B)

(C)

Figure 2.6 (A) Preparation procedures for the two-ply yarn supercapacitors. (B) Scanning electron microscopic micrograph of the asymmetric two-ply yarn supercapacitor carbon nanotube (CNT)@MnO2@poly(vinyl alcohol) (PVA)//CNT@PVA, and (C) optical micrograph of the asymmetric two-ply yarn supercapacitor. Modified from F. Su, M. Miao, Asymmetric carbon nanotubeeMnO2 two-ply yarn supercapacitors for wearable electronics, Nanotechnology 25 (2014) 1e8, copyright 2014, with permission from IOP.

50e60 mm, as shown in Fig. 2.6B and C. It retains the strength and flexibility of the CNT yarn so that the SC can be processed into textile fabrics using conventional textile processes, such as weaving, knitting, and braiding [18,36]. The spaces between the SCs in the fabric structure allow air and moisture to pass through, providing comfort to the wearers. For comparison, this asymmetric architecture allows the operating potential window to be extended from 1.0 to 2.0 V and results in much higher energy and power densities than the symmetric two-ply yarn SCs [18]. Furthermore, the associated energy storage mechanism for asymmetric SCs is proposed in Eqs. (2.27)e(2.29): Negative electrode : Carbon þ Hþ þ e 4 Carbon=H

(2.27)

Positive electrode: MnO2 þ dPO3 4 4 MnO2 /ðPO4 Þd þ 3de

(2.28)

Total electrode : 3dCarbon þ MnO2 þ 3dHþ þ dPO3 4 4 3dCarbon=H þ MnO2 /ðPO4 Þd

(2.29)

where “/” represents pseudocapacitance produced by adsorption/desorption process (fast redox reaction on electrode surface).


39

Besides, grapheneebismuth oxide nanotube fiber as the electrode material for constituting flexible SCs using a PVA/H3PO4 gel electrolyte was reported. The asfabricated fiber-based SC is flexible and compact with a reduced space compared to the other types, such as film and composite SCs [19]. Planar SCs as a new emerging branch of ECs could make the entire device much thin and flexible [37]. For instance, a schematic illustration of fabrication procedures for ultraflexible planar quasi-solid-state SCs is shown in Fig. 2.7. Briefly, a rectangle strip of the two-dimensional (2D) d-MnO2/graphene hybrid thin film obtained by vacuum filtration was rolled with a glass rod onto the flexible polyethylene terephthalate substrate, followed by tightly pressing and peeling off the cellulose acetate membrane. This procedure led to the transfer of freestanding 2D hybrid thin film on the target substrate, as shown in Fig. 2.7C. By scraping along the dashed line as shown in Fig. 2.7C and D, slim strips of the transferred thin film were obtained as the working electrodes for the planar SC. After repeated scrapings for construction of an array of parallel lines as primary units of the working electrodes, two columns of gold current collectors were thermally evaporated on each side of the working electrodes and integrated the primary units into a typical planar SC device. The quasi-solid-state planar SC was finally established after filling the channel between two working electrodes with gel electrolyte of PVA/H3PO4, as demonstrated in Fig. 2.7F. Moreover, the asfabricated planar SCs exhibit extraordinary mechanical flexibility and stability under various bending states from being curved, folded to rolled, with more than 90% capacitance retention after thousands of folding/unfolding steps [20]. In the case of symmetric SCs, the pseudocapacitive mechanism is suggested as follows: Negative electrode : MnO2 þ dHþ þ de 4 MnO2 /Hd

(2.30)

Figure 2.7 Schematic of the fabrication procedures for ultraflexible planar supercapacitors. PET, polyethylene terephthalate; PVA, poly(vinyl alcohol). Modified from L. Peng, X. Peng, B. Liu, C. Wu, Y. Xie, G. Yu, Ultrathin two-dimensional MnO2/graphene hybrid nanostructures for high-performance, flexible planar supercapacitors, Nano Lett. 13 (2013) 2151e2157, copyright 2013, with permission from ACS.

40

Metal Oxides in Supercapacitors Positive electrode: MnO2 þ dPO3 4 4 MnO2 /ðPO4 Þd þ 3de

(2.31)

Total reaction: ð3MnO2 Þneg þ ðMnO2 Þpos þ 3dHþ þ dPO3 4 4 3MnO2 /H3d þ MnO2 /ðPO4 Þd (2.32)

2.3.3 2.3.3.1

Liquid ElectrolyteeBased Supercapacitors Organic Electrolytes

In liquid electrolyteebased SCs, the electrolytes are composed of conducting salts dissolved in organic or aqueous solvents. The organic electrolyteebased SCs, which are currently dominating the commercial market because of their high operation voltage window typically in the range of 2.5e2.8 V, were less investigated in academic research, especially for metal oxideebased SCs [34]. To the best of our knowledge, the configuration design for commercial SCs is commonly cylindrical or packaged. However, the configuration for academic research is normally shown as threeelectrode or coin cells. For example, a beaker-type, three-electrode cell is used to measure the electrochemical properties of the hybrid capacitor, which is composed of LiMn2O4 as the positive electrode and manganese oxide (MnO2)/CNT nanocomposite as the negative electrode using 1 M LiClO4 in propylene carbonate (PC) as the electrolyte [22]. The pseudocapacitive and noncapacitive faradaic mechanisms are shown as Negative electrode : MnO2 þ xLiþ þ xe 4 MnO2 /Lix

(2.33)

Positive electrode: LiMn2 O4 4 Li1x Mn2 O4 þ xLiþ þ xe

(2.34)

Total reaction: MnO2 þ LiMn2 O4 4 Li1x Mn2 O4 þ MnO2 /Lix

(2.35)

A symmetric capacitor containing two identical pure MnO2/CNT electrodes was fabricated with Celgard 2300 separator and 1.2 M LiPF6 in ethylene carbonate/ethyl methyl carbonate (3:7 by volume) as the electrolyte [26]. In the case of asymmetric SCs, two-electrode asymmetric SC coin cells (2032 type) were assembled with a hollow spherical nanostructured MnO2 electrode and an AC electrode separated by a Celgard 2300 membrane (thickness 20 mm) using 1-ethyl3-methylimidazolium tetrafluoroborate as the electrolyte. Good results could be partly attributed to the unique porous structure of the high surface area of the MnO2 hollow spheres where the mesopores enable the IL ions to get access and wet electrode surface for efficient ion transport. Moreover, a broader potential window can offer higher energy density [23]. Besides, a hybrid asymmetric capacitor incorporating the V2O5/ CNT nanowire composites as the negative electrode and AC as the positive electrode was also studied. To make 2032-type coin cells, glass fiber (GF/D) was used as the


41

separator and the electrolyte was 1 M NaClO4 in PC solution. The layer-structured V2O5/CNT nanowires provide facile sodium insertion/extraction and fast electron transfer, enabling the fabrication of high-performance Na-ion SCs with an organic electrolyte [24]. Therefore, the correlated pseudocapacitive and EDL capacitive mechanism is expressed in Eqs. (2.36)e(2.38): Negative electrode : V2 O5 þ xNaþ þ xe 4 Nax V2 O5

(2.36)

Positive electrode : Carbon þ ClO 4 4 Carbon=ClO4 þ e

(2.37)

Total reaction: xCarbon þ V2 O5 þ xNaþ þ xClO 4 4 xCarbon=ClO4 þ Nax V2 O5

2.3.3.2

(2.38)

Aqueous Electrolytes

Besides the organic electrolyteebased SCs mentioned earlier, aqueous SCs have been assembled and investigated extensively because they are safe, are inexpensive, and can be easily handled in the laboratory without requiring special conditions. Generally, aqueous electrolytes can be further categorized into acid, alkaline, and neutral solutions in which H2SO4, KOH, and K2SO4, respectively, are representatives and also the most frequently used electrolytes [34]. As to the configuration of the assembled SCs, almost all the researchers would like to use three-electrode or two-electrode cells. The schematic of aqueous metal oxideebased SCs is shown in Fig. 2.8. In the case of acid electrolytes for SCs, RuO2 is one of the most widely studied metal oxide materials in H2SO4 electrolytes. The capacitance of RuO2/rGO

Figure 2.8 Schematic of aqueous metal oxideebased supercapacitors. Modified from F.X. Wang, S.Y. Xiao, Y.Y. Hou, C.L. Hu, L.L. Liu, Y.P. Wu, Electrode materials for aqueous asymmetric supercapacitors, RSC Adv. 3 (2013) 13059e13084, copyright 2013, with permission from RSC.

42


nanocomposites can be up to 1099 F g1 [31]. Other metal oxide materials such as MoO3 were also investigated in such strong acidic electrolytes [39]. In addition, a new asymmetric EC based on an AC negative electrode, PbO2 thin film, and nanowire array positive electrode with an electrolyte made of lead salt dissolved in methane sulfonic acid was reported [27]. The noncapacitive faradaic and EDL capacitive mechanisms are listed as follows: Negative electrode : Carbon þ Naþ þ e 4 Carbon=Na

(2.39)

Positive electrode: Pb2þ þ 2H2 O 4 PbO2 þ 4Hþ þ 2e

(2.40)

Total reaction: 2Carbon þ 2Naþ þ Pb2þ þ 2H2 O 4 2Carbon=Na þ PbO2 þ 4Hþ

(2.41)

In alkaline electrolytes, NiOx and CoOx have been extensively explored because of their high theoretical capacitances. For instance, an asymmetric SC has been fabricated using carbonized kapok fiber/NiO composites as the positive electrode and commercial AC as the negative electrode with 1 M KOH as aqueous electrolytes. Moreover, the as-fabricated SC exhibits a superior long cycle life and the good capacitance retention with no evident degradation after 4000 cycles [40]. Other asymmetric SCs based on Co3O4 nanowires as the positive electrode and AC as the negative electrode in 6 M KOH aqueous solution was also assembled [41]. Additionally, asymmetric SCs were also developed based on TiO2 atomic layer deposition filmecoated CNT samples as the positive electrode and uncoated CNT samples as the negative electrode in 1 M KOH electrolyte, which present high energy density and excellent stability over 1000 cycles [42]. Composites of Fe3O4/graphene have been tested as the negative electrode in 6 M KOH aqueous asymmetric SCs comprising AC as the positive electrode. The results show that the presence of graphene causes an increase in the capacitance of magnetite as an individual electrode as well as the capacitance of the entire asymmetric SCs [43]. The redox reaction and EDL capacitive mechanism can be expressed by the following equations: Negative electrode : Fe3 O4 þ 4H2 O þ 2e 4 3FeðOHÞ2 þ 2OH

(2.42)

Positive electrode: Carbon þ OH 4 Carbon=OH þ e

(2.43)

Total reaction: 2Carbon þ Fe3 O4 þ 4H2 O 4 2Carbon=OH þ 3FeðOHÞ2 (2.44) Besides acidic and alkaline electrolytes, neutral electrolytes have also been extensively studied for their larger working voltage windows, less corrosion, and greater safety [34]. To be specific, the asymmetric SCs were built based on the polypyrrole@MoO3 nanocomposite as the negative electrode, AC as the positive electrode,


43

and 0.5 M K2SO4 aqueous solution as the electrolyte. The charge and discharge voltage range for the asymmetric SCs is from 0 to 1.5 V. Better rate capability as well as excellent cycling performance were achieved [44]. An asymmetric SC based on AC as the negative electrode and V2O5$0.6H2O nanoribbons as the positive electrode in 0.5 M K2SO4 aqueous electrolyte were also successfully assembled, which could be cycled reversibly in the voltage region of 0e1.8 V. This SC presents an energy density of 29.0 Wh kg1 and also a rather good cycling performance [45]. In addition, the symmetric SCs with three-dimensional (3D) Fe2O3/m-rGO as the negative electrode and 3D MnO2/m-rGO film as the positive electrode in 2 M LiCl aqueous solution were evaluated in a two-electrode configuration. The gap in working voltage windows of Fe2O3 and MnO2 enables the stable expansion of the cell voltage up to 1.8 V, which is responsible for the high energy density [46]. Moreover, the pseudocapacitive mechanisms can be exhibited as follows: Negative electrode : Fe2 O3 þ dLiþ þ de 4 Fe2 O3 /Lid

(2.45)

Positive electrode : MnO2 þ dCl 4 MnO2 /Cld þ de

(2.46)

Total reaction: Fe2 O3 þ MnO2 þ dLiþ þ dCl 4 Fe2 O3 /Lid þ MnO2 /Cld (2.47) Likewise, the ZnO/graphene nanoribbon nanocomposite electrode in an asymmetric SC with lacey rGO nanoribbons as a negative electrode exhibits a voltage window of 2.0 V in 0.5 M Na2SO4 aqueous electrolyte. The high cycling stability of asymmetric SCs is very good, with 96.7% capacitance retention after 5000 cycles [47].

2.4

Conclusions and Outlook

In this chapter the features of design and fabrication for metal oxideebased SCs are summarized. As described in the second part, theoretical equations for energy and power densities for different configurations, which can be classified in accordance with the symmetry of electrodes and types of electrolyte, are deduced successively. These calculations apparently show that the introduction of organic electrolyte and asymmetric electrode is effective to improve electrochemical performance of SCs. Subsequently, three main configurations of metal oxideebased SCs are also concluded. Among them, all-solid-state and quasi-solid-state SCs have gained attention in the recent years because of their flexible and wearable properties in smart devices. In terms of liquid electrolyteebased SCs, aqueous devices possess the dominant advantage in academic research because they are safe, are inexpensive, and can be easily handled in the laboratory without critical conditions. However, organic electrolyteebased SCs are less investigated in the academic research, especially for metal oxideebased SCs. Fortunately, they are currently dominating the commercial market because of their high operation voltage window.

44


In the future, research directions on the design and fabrication of metal oxidee based SCs would be as in the following: 1. Not only the existing model of SCs should be inherited but also novel and versatile SCs to meet different requirements in various fields should be developed. More efforts are needed to make SC devices more compact and miniaturized, which could guide us to facilitate space sufficiently and prepare metal oxideebased materials with high volume specific capacitance. 2. Although progress has shown successful cases such as symmetric EDL capacitors and asymmetric SCs, a wide range of other asymmetric pseudocapacitors is highly pursued for enhancing the energy density and broadening the application of metal oxideebased SCs. 3. More theoretical investigations on the electrochemical properties and thermodynamics as well as kinetics effects of metal oxideebased SCs should be made based on first-principle calculations, which will greatly reduce the development process of metal oxideebased SCs from the laboratory to commercial products.

Acknowledgment Financial support from the MOST (2016YFB0700600), Distinguished Young Scientists Program of NSFC (51425301, 21374021, 51673096, and U1601214), and Sanyo Chemical Co. Ltd. is gratefully appreciated.

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47

[44] Y. Liu, B. Zhang, Y. Yang, Z. Chang, Z. Wen, Y.P. Wu, Polypyrrole-coated MoO3 nanobelts with good electrochemical performance as anode materials for aqueous supercapacitors, J. Mater. Chem. A 1 (2013) 13582e13587. [45] Q.T. Qu, Y. Shi, L.L. Li, W.L. Guo, Y.P. Wu, H.P. Zhang, S.Y. Guan, R. Holze, V2O5$0.6H2O nanoribbons as positive electrode material for asymmetric supercapacitor in K2SO4 solution, Electrochem. Commun. 11 (2009) 1325e1328. [46] M.H. Yang, K.G. Lee, S.J. Lee, S.B. Lee, Y.K. Han, B.G. Choi, Three-dimensional expanded graphenemetal oxide film via solid state microwave irradiation for aqueous asymmetric supercapacitors, ACS Appl. Mater. Interfaces 7 (2015) 22364e22371. [47] V. Sahu, S. Goel, R.K. Sharma, G. Singh, Zinc oxide nanoring embedded lacey graphene nanoribbons in symmetric/asymmetric electrochemical capacitive energy storage, Nanoscale 7 (2015) 20642e20651.


Electrolytes in Metal Oxide Supercapacitors

3

Maria J. Carmezim 1, 2 , Catarina F. Santos 1, 2 1 ESTSetubal, Instituto Politécnico de Setubal, Setubal, Portugal; 2CQE-IST, Universidade de Lisboa, Lisboa, Portugal

3.1

Introduction

As other engineering systems that have reached an advanced stage of development, the electrochemical energy storage devices such as supercapacitors (SCs) put forward an urgent need of optimizing their performance. To achieve this upgrade, it is necessary not only to optimize the metal oxide material of the electrodes but also to promote an optimal device design. In the recent years, the performance of SCs has greatly improved, enabling more efficient storage mechanisms because of the development of new electrode materials at the nanoscale, development of new cell configurations, and the use of new electrolyte systems [1,2]. It is not uncommonly believed that the effect of electrolytes on the performance of the SCs is nowadays considered as one of the major challenges for researchers. Broadly, the SC devices are composed of an electrochemical supercapacitor (ES) of the electrolyte (Fig. 3.1), two electrodes, and a separator. The electrolyte in SC devices is maintained between the separator and active material layers to allow charge storage by several processes involving electrolyte ions on the electrodeeelectrolyte interface. The electrolytes to afford a high performance of SCs should guarantee wide voltage window, high electrochemical stability, high ionic conductivity, high wettability, a wide operational temperature range and low solvated ionic radius, low resistivity,

Figure 3.1 Schematic representation of a supercapacitor and the main electrolyte properties. Metal Oxides in Supercapacitors. http://dx.doi.org/10.1016/B978-0-12-810464-4.00003-6 Copyright © 2017 Elsevier Inc. All rights reserved.

50


low volatility, low toxicity, low cost, noninflammable property, and availability at high purity [3]. The most common electrolyte classification used in SCs includes three groups: the aqueous electrolytes, the organic electrolytes, and the liquid salts [usually called ionic liquids (ILs)] [3,4]. Although ILs have been extensively studied for carbon-based devices [4], usually they are scarcely applied in metal oxideebased SCs. The most relevant and advanced types of electrolytes for metal oxideebased SCs will be described in the following [5,6]. Aqueous electrolytes present many unquestionable advantages, namely, low price, low toxicity, environmental friendliness, safety, and more important they allow higher power capabilities [7e9]. However, they have intrinsic drawbacks such as low energy density and narrow operating voltage (w1 V) [4,9]. The aqueous electrolytes used in metal oxide SCs can be classified into three types depending on the solution pH: alkaline, acidic, and neutral. KOH is the most common alkaline electrolyte used in metal oxide SCs, whereas H2SO4 is the acid electrolyte usually applied. These alkaline and acidic electrolytes are able to minimize internal resistance and maximize power capability, as they have excellent ionic conductivity or lower resistivity [2]. The high ionic conductivity can be explained by the great number of proton (Hþ) and hydroxide (OH) involved in proton hopping or proton transport [2]. Nevertheless, the acid electrolytes are not so commonly used as the neutral electrolytes, despite their much higher conductivity, because they can cause corrosion of the device and/or dissolution of the electrodes. To mitigate the corrosive character of the acidic and alkaline electrolytes and to increase the working potential window and the cycle life, neutral electrolytes have been investigated [2,8]. The widely studied neutral electrolytes in metal oxide SCs are the compounds X2SO4, where X ¼ Li, Na or K, with concentration ranges between 0.1 and 2 M [1,8]. An electrolyte with high ionic conductivity is a critical requirement that prevails to other properties. However, in the recent years, there has been a tremendous effort focusing on the development of novel devices with both flexible electrodeeelectrolyte and optimal electrochemical and mechanical properties. Among which, polymer gel electrolytes encourage the design of flexible, thin, lightweight, and small-volume novel energy storage devices [10e12]. On the other hand, gel electrolytes have been demonstrated as an adequate substitute for aqueous electrolytes for solid-state SCs, being safer (liquid-leakage free and less nontoxic) and serving not only as ionic conducting media but also as electrode separator. It is well known that electrolyte composition plays a critical role in electrochemical performance of metal oxide electrodes. Accordingly, faradaic reactions at the electrodee electrolyte interface can be facilitated by introducing reversible redox species as additives or mediators (e.g., halide ions, phenylamide, and quinones) into conventional electrolytes. This electrolyte optimization allows enhancing redox activity and simultaneously improves ionic conductivity. As it will be shown in this chapter, the performance of the metal oxide SCs is limited by electrochemical processes that occur at the electrodeeelectrolyte interface and the transport path length for both electrons and ions. For this reason, it will also be unavoidable to point out the recent status of metal oxides in SCs.


3.2

51

Supercapacitors and Interaction With Electrolytes

Supercapacitors will be the next-generation centerpiece of power devices because of their excellent properties such as high power density, fast chargeedischarge rate, excellent reversibility, and long cycle life. For a long time, the SCs were named into different categories in light of their predominant electrochemical storage behavior: pseudocapacitance and electric double-layer capacitance. But recently the pseudocapacitance behavior has generated some controversy and fundamental discussion in the scientific community [5]. Charge storage in the electric double-layer SCs occurs at the electrodeeelectrolyte interface because of the formation of the Helmholtz double layer, and the storage mechanism is purely of electrostatic nature [13,14]. It means that the storage of electric charge and energy involves no chemical changes of the solid-phase electrode, usually called a nonfaradaic process [14,15]. In such case the electrolyte must be electrochemically inert. The electric double-layer SCs are considered as capacitor-type materials [13]. The cyclic voltammogram (CV) of double-layer SCs features a characteristic rectangular shape, with voltage-independent current, whereas the chargeedischarge galvanostatic plots display a linear triangular shape typical of a capacitive behavior and are completely reversible. Typically, the electrode materials that charged through a capacitive double-layer nonfaradaic process are carbon, carbon-based materials, and carbon allotropes, especially for their highly porous surface area. They are extensively studied because they have an extremely long cycle life, are cheap, are relatively environment friendly, and can operate over a wide range of temperatures. Nevertheless, they have low energy densities (5e10 W h L1) and they are not able to fully meet the various performances required nowadays in the market (20e30 W h L1) [9]. A completely different charge storage behavior called pseudocapacitance was introduced by B.E. Conway [13]. In the pseudocapacitance behavior the charge could be stored by a faradaic process, which means electrosoption of ions accompanied by surface redox reactions in which electron transfer occurs by crossing the interface of the current collector and active material [15]. According to B.E. Conway the pseudocapacitance behavior can result from mainly three mechanisms: (1) redox, (2) intercalation, and (3) underpotential deposition. Although the mentioned mechanisms are considered the classics, Qian-Long Lu and coworkers [16] found that mesoporous MoO3x electrodes in H2SO4 electrolyte comprise both redox and intercalation mechanisms, hence the name new redox/intercalation mechanism. Numerous reports ascribe the pseudocapacitance behavior to binary and ternary metal oxides but a timely paper has contributed to clarify this misleading topic [5]. Indeed, the pseudocapacitance behavior is only valid for capacitorlike electrochemical metal oxide electrodes, such as RuO2, Nb2O5, and MnO2 in mild aqueous electrolyte. Despite a faradaic process that occurs at the electrodeeelectrolyte interface, these oxide electrodes exhibit a linear dependence of charge storage with potential giving rise to an almost rectangular currentevoltage profile curve [13]. Chargeedischarge curves display a linear triangular shape (Fig. 3.2, left side) similar to that observed in carbon electrodes.

52


Figure 3.2 Metal oxide supercapacitor behavior and schematic electrochemical response.

Moreover, most binary and ternary metal oxide electrodes that also undergo a faradaic process at the electrodeeelectrolyte interface, i.e., redox reactions, exhibit a nonlinear dependence of charge storage with potential and a faradaic plateau in chargeedischarge curves, also called batterylike behavior (Fig. 3.2, right side). However, capacitorlike behavior can be the result of electrode design, i.e., nanoparticle size, thin film thickness, and not an intrinsic property of the material. In case of use of bulk LiCoO2 as positive electrode in lithium-ion batteries, a faradaic behavior is observed but a capacitorlike response occurs with LiCoO2 thin films when reaching a critical 6 nm film thickness [17]. Actually the validity of using capacitance to describe the charge storage performance of SCs where there is direct electron transfer between the oxide electrode material and the electrolyte redox species, originating a batterylike response, is convincingly brought into question [5]. In fact, the charge-to-potential ratio of an electrode (i.e., NiO2, Co2O3, and spinel oxide materials) in a redox electrolyte is significantly dependent on the applied voltage, resulting in an overestimated specific energy. Moreover, the noninclusion of the redox additive mass in the overall active mass could lead to overestimations of the specific charge and energy storage capacity of the device. The total mass of the redox species can be easily derived from the volume and concentration of the redox species in the electrolyte and if the redox species is ionic, the mass of the charge balancing the counterions should also be considered. Akinwolemiwa and coauthors [6] propose that true performance evaluation and comparison between batterylike SCs, with and without added redox electrolyte, should be based on energy capacity and power capability, instead of capacitance or pseudocapacitance.


3.3

53

Electrolytes for Metal Oxide Supercapacitors

To improve the performance of SCs, various types of electrolytes have been developed and reported in the literature. Despite the emergence of new electrolytes, an ideal electrolyte has not been developed yet, and all electrolytes have pros and cons. Taking into account the last published developments and applications, a possible categorization of electrolytes for metal oxide SCs is proposed: liquid, redox additive, and solid and/or gel state, as shown in Fig. 3.3. All liquid electrolytes are aqueous and can be subdivided into alkaline, acidic, and neutral depending on the pH. The solid can be divided into gel polymer and inorganic electrolytes and the redox additive into aqueous and polymer gel electrolytes. From the beginning, aqueous electrolytes have attracted special attention owing to their high conductivity (up to w1 S cm2) and capacitance, but their working voltage is limited (about 1.0 V) because of the narrow electrochemical stability window of water (1.23 V theoretical value), which further limits the energy stored in the device [4,18]. It is worthy to note that power is affected by the square value of the working voltage (P ¼ V2/4R). The specific energy (E) of SCs E ¼ ½CV2 can be enhanced by increasing the operating voltage window (V) and/or capacitance (C) [8,19,20]. It is known that the working voltage depends mainly on the stability of the electrolyte. Moreover, the specific capacitance is dependent on the electrolyte conductivity and the size of electrolyte ions that must penetrate into and out of the electrode pores [2,8,21]. Although it has been found that the working voltage of aqueous electrolytes

Figure 3.3 Classification of electrolytes used in metal oxideebased supercapacitors.

54


is restricted by the splitting of water, which could cause the rupture of the SC cells, the performance of a metal oxideebased SC with an aqueous electrolyte is consistently 40%e50% higher than that with the organic electrolytes [22]. Additionally, the aqueous electrolytes can be prepared and utilized without stringently controlling the preparing processes and conditions, which is an uncontested benefit, whereas the organic ones need controlled processes and conditions to get electrolytes with high purity [3]. The organic electrolytes can operate at higher voltages (w2.7e3.5 V) when compared with aqueous electrolytes (w1 V), which is an advantage and for this reason they are often an option [2,3]. Furthermore, the organic electrolytes allow the use of cheaper materials for the current collectors and packages [8], but they have low ionic conductivity, have higher cost, have smaller specific capacitance, are less environment friendly, and could have problems such as flammability, volatility, and toxicity [8,18]. The use of solid inorganic electrolytes is less common when compared with aqueous or organic electrolytes. Although solid inorganic electrolytes are usually mechanically robust, they can be produced in thin film form, which allows the miniaturization. For these reasons, inorganic electrolytes do not have problems with electrolyte leakage. Moreover, they are inflammable and thermally stable [8,23]. However, only one example of inorganic electrolytes used with metal oxide SCs as solid SCs can be found in the literature and correspond to a mixture of oxides (e.g., 0.4 LiClO4 0.6 Al2O3 [23]). This electrolyte operates at high temperature (100e300 C) [23], but usually the specific capacitance (29 F g1 at 0.05 A g1) is lower than that of liquid electrolytes [8,23].

3.3.1 3.3.1.1

Liquid Electrolytes Aqueous Electrolytes

3.3.1.1.1 Alkaline Electrolytes The alkaline electrolytes are the most widely reported aqueous electrolytes in the literature [1,4,8] for metal oxide SC. In many studies the selection of the alkaline electrolyte takes into account the performance of metal oxide SCs and the predominant storage mechanism. Among the numerous basic electrolytes, KOH has been the most extensively used because of its high ionic conductivity (Table 3.1); however, other basic electrolytes, such as NaOH and LiOH, have also been investigated. Several properties of electrolytes could affect the performance of metal oxide SCs, such as the operating temperature, ion nature, and concentration. For example, it was observed for NiO electrode that the concentration of alkaline electrolyte (2, 4, and 6 M KOH) can affect the value of specific capacitance, i.e., the specific capacitance increases with increasing concentration of electrolyte [24,25]. This is due to the increased conductivity of an electrolyte generally with the concentration in an aqueous solution. Thus, 6 M KOH will provide higher conductivity and higher concentration of OH species than 2 and 4 M solutions, which facilitates charge transfer in both bulk electrolyte and electrode [24]. Additionally, according to Zhao et al. [24] the cycling stability is excellent in high-concentration electrolyte (6 M KOH). Furthermore, using


Table 3.1

55

The Ion Size and Ionic Conductivity Values [8,19]

Ion

Ion Size (Å)

Hydrated Ion Size (Å)

Ionic Conductivity (S cm2 molL1)

Hþ

1.15

2.80

350.1

1.00

4.12

119

1.33

3.31

73.5

0.95

3.58

50.11

0.60

3.82

38.69

OH

1.76

3.00

198

SO4 2

2.90

3.79

160

Br

1.95

3.30

78.40

2.16

3.31

76.80

1.81

3.32

76.31

2.64

3.35

71.42

2þ

Ca

þ

K

þ

Na

þ

Li

I

C1

NO3

higher concentration of electrolyte the peaks in the CVs become broader and the peak area is larger in comparison with the sharp redox peak for low concentration, which is indicative of a better capacitive performance [20,24]. However, high concentration of electrolyte usually causes corrosion at the electrode surface and/or current collector, which affects the device performance. As mentioned earlier the operating temperature also has a strong effect on the efficiency of the metal oxide SCs. It is well known that an increase in the electrolyte temperature (0e60 C) can result in an increase in the specific capacitance and a decrease in the equivalent series resistance (ESR) of the device, as observed by R. Gupta et al. [26] for NiCo2O4 electrode material. A decrease in the ESR is usually attributed to the enhanced conductivity of the electrolyte due to the increase in the mobility of the ions [26,27]. Another important aspect is the cycle stability of the electrode after a repeated heating and cooling in the temperature. W. Li et al. [27] reported that the MnO2 nanobelt electrode had 91.3% of retention after 5000 cycles with repeating heating and cooling in the temperature range of 0e50 C, showing a good high-temperature-resistive long-term cycle stability. Another important issue to the overall metal oxide SC performance is the type of alkali metal ion of electrolytes. For example, I. Mismon et al. prepared d-MnO2 electrode [28] and R. Gupta et al. prepared NiCo2O4 electrode [26] and both observed that the specific capacity as well as the specific energy density increase with increase in the ion size (ion size: Liþ > Naþ > Kþ) [26,28]. The authors attributed this results to the Liþ ions in aqueous solution, which have the highest hydrated radius (Table 3.1). This contributes to reduce the mobility and consequently the redox current when compared

56


with Naþ and Kþ ions [26]. Similar results for other different metal oxide electrodes such as MnFe2O4 [29] and Bi2WO6 [30] have been reported by other researchers. As generally observed, for alkaline electrolytes the electrochemical reaction between the electrolyte and the electrode materials is well established, as demonstrated by the data present in Table 3.2.

3.3.1.1.2 Acidic Electrolytes For practical applications, the metal oxide SCs should exhibit many desirable properties, such as high power density, good pulse chargeedischarge characteristics, and superior cycling stability. Aqueous electrolytes, with acid groups, have also been studied in systems with metal oxideebased materials [49e51]. The most commonly used acidic electrolyte is H2SO4, mainly because of its very high ionic conductivity (0.8 S cm2 for 1 M at 25 C). The ionic conductivity of the aqueous acid electrolyte is strongly dependent on the concentration and has a strong influence on the specific capacitance [21]. Although the acidic electrolytes have demonstrated advantages (high specific capacity) when used in SCs, they have also caused problems, especially in the electrodes. In acidic aqueous electrolytes the metal oxide electrodes present poor stability, mainly because of their sensitivity to the type and pH of the electrolytes [35]. S.-K. Chang et al. [35] found that the stability of NiCo2O4 was poor in acidic electrolyte (1 M HCl) because of the dissolution of spinel oxide. To overcome this limitation, other electrolytes (alkaline or neutral) are being targeted as the final choice by most of the researchers. In the past few years, one of the widely studied electrode materials in acid aqueous electrolyte was RuO2. The reason for this interest is related to the high stability, chemical resistance, and specific capacitance obtained for RuO2. This high value of specific capacitance, which could reach 1000 F g1 for amorphous RuO2, is most of the times associated to the surface reaction between ruthenium ions present in electrode and Hþ ions existing in the electrolyte. The RuO2 is an expensive material and the sources of Ru are limited, which have restricted their commercial usage. To solve these issues, other alternative oxide electrodes, for example, WO3 [50], NiFe2O4 [51], and IrO2 [49], have been searched and tested in such acidic electrolytes (Table 3.3). Most recently, a new combination of stable nanometric oxide clusters also known as polyoxometalates (POMs) with reversible redox activities and working window in an extended voltage range (1.6 V) in acidic (H2SO4) aqueous electrolyte has been reported [52,53]. Most of the POMs are anchored to conducting polymers or carbon-based materials to increase the faradaic (POMs) and capacitive (polymer or carbon-based) charge storage in a hybrid nanocomposite electrode. Two examples of POMs, H3PW12O403$H2O [53] and H3PMO12O40 [52], are reported by SuarezGuevara et al. and V. Ruiz et al. respectively. Table 3.3 also depicted the energy storage/delivery process between electrolyte and some recent metal oxideebased electrodes. There are also different acidic aqueous electrolytes, for instance, perchloric acid, hexafluorosilicic acid, and tetrafluoroboric acid, that are used in SCs. As far as known, until today, none of them were used in SCs with metal oxideebased electrodes.

Alkaline Aqueous Electrolyte Used in Metal Oxide Supercapacitors, the Reaction Between Electrolyte and Electrodes, and Electrochemical Supercapacitor Performance

Table 3.2

Aqueous Electrolyte/ Concentration

Electrode

Electrode Setup

Specific Capacitance (F gL1)

Potential Window (V)/ Reference 1

Cycling Stability

Power Density

Reaction Between Electrolyte and Electrode þ

References

KOH/0.5 M

SnO2 quantum dots

3

10 at 20 mV s

0.2 to 0.5

98% after 1000 cycles

e

SnO2 þ Ka þ e a 4 SnOOKa

[31]

KOH/1 M

Networked NiCo2O4/ MnO2

3

1891 at 100 mA cm2

0e0.6

98.4% after 3000 cycles

e

Faradaic redox reaction MeO/MeOeON, where M ¼ Mn, Co, Ni and N ¼ K or H ions

[32]

KOH/1 M

LaMnO3.09

2/3

400 at 10 mV s1

1.2 to 0

e

220.4 W kg1

i h La Mn2d 2þ ; Mnð12dÞ 3þ Oð3dÞ þ 2dOH

1

r-LaMnO2.91

KOH/1 M

KOH/1 M

Fe3O4

500 at 10 mV s

2/3

379.8 at 2 A g1

Co2AlO4

575.2 at 2 A g1

Co2AlO4@ MnO2

915.1 at 2 A g1

294.6 at 2 mV s1

MnO2 nanoflower

3

4214 W kg

4 LaMn3þ O3 þ 2de þ dH2 O h LaMn3þ O3 þ 2dOH 4 La Mn2d 4þ ; i Mnð12dÞ 3þ Oð3dÞ þ 2de þ H2 O

800 W kg1

Fe0eFe3þ

1

0e0.4

94.23% after 2000 cycles 93.3% after 2000 cycles

Co2þeCo4þ associated with OH

1 to 0


MnO2 adsorption Kþ cations

0.45 to 0.45


e

EDLC mechanism

[20]

[33]

[28]

ðMnO2 Þsurface þ Cþ þ e 4 MnO2 Cþ surface Cþ ¼ Hþ, Liþ, Naþ, Kþ Pseudocapacitance mechanism (MnO2)surface þ Cþ þ e 4 (MnOOC)

Continued

Alkaline Aqueous Electrolyte Used in Metal Oxide Supercapacitors, the Reaction Between Electrolyte and Electrodes, and Electrochemical Supercapacitor Performancedcont’d

Table 3.2


Electrode

Electrode Setup



Cycling Stability

Power Density 1

Reaction Between Electrolyte and Electrode

References

KOH/1 M

RuO2

2/3

124.8 at 50 mV s

1 to 0

e

900 W kg

Not shown

[34]

KOH/1 M

NiCo2O4

3

249.8 at 0.5 A g1

0.2 to 0.55

w100% after 1000 cycles

e

Reversible process:

[35]

OH bonded to Co2þ of Co3O4

Irreversible process: Co3O4 þ H2O þ OH 4 3CoOOH þ e Co(OH)2 þ OH 4 CoOOH þ H2O þ e CoOOH þ OH 4 CoO2 þ H2O þ e KOH/2 M

NiO nanospheres

3

612.5 at 0.5 A g1

0e0.7


e

NiO þ OH 4 NiOOH þ e

[36]

KOH/2 M

Porous NiCo2O4 nanotubes

2/3

1647.6 at 1 A g1

0e0.41


205 W kg1

Faradaic redox reaction MeO/MeOeOH, where M ¼ Co or Ni

[37]

KOH/2 M

NiCo2O4@ NiCo2O4 nanocatus

3

1264 at 2 A g1

0e0.7


e

NiCo2O4 þ OH þ H2O 4 NiOOH þ 2CoOOH þ e

[38]

KOH/2 M

NiCo2O4

3

1118.6 at 5.56 mA cm2

0.2 to 0.6


e

Redox reaction related to MeO/MeOeOH, where M refers to Ni or Co

[39]

KOH/2 M

Ni0.61Co0.39

2/3

1523.0 at 2 A g1

0.05 to 0.7


142 W kg1

Redox reaction related to MeO/MeOeOH, where M refers to Ni or Co

[40]

KOH/2 M

MnFe2O4

3

97.1 at 0.1 A g1

e


e

Not shown

[29]

CoOOH þ OH 4 CoO2 þ H2O þ e

KOH/2 M

LiCoO2

3

814.5 at 1 A g1

0e0.6


e

Not shown

[41]

KOH/3 M

NiOeIn2O3 foam

2/3

1096.8 at 5 A g1

0.2 to 0.6


1752.8 W kg1

NiO þ OH 4 NiOOH þ e

[42]

The In3þ creates the oxidation of Ni2þ into Ni3þ, followed by holes migration based on the following reaction: ½O2 4O0 þ V0Ni þ h$ V0Ni refers to a singly ionized nickel vacancy, O0, to an oxygen ion on an oxygen site, and h$ to an electron hole.

KOH/6 M

Bi2O2.33 microspheres

3

893 at 0.1 A g1

1 to 0.2


e

Not shown

LiOH/1 M

MnO2 nanoflower

3

363 at 2 mV s1

0.45 to 0.45


e

EDLC mechanism

[43]

[28]

ðMnO2 Þsurface þ Cþ þ e 4 MnO2 C

þ

surface

Cþ ¼ Hþ, Liþ, Naþ, Kþ Pseudocapacitance mechanism (MnO2)surface þ Cþ þ e 4 (MnOOC) LiOH/1 M

Co3O4@MnO2

3

400 at 10 mA cm2

0.2 to 0.6


e

Reversible reactions of Co3þ/Co4þ associated with anions OH ðMnO2 Þsurface þ Liþ þ e 4 MnO2 Liþ surface

[44]

LiOH/2 M

MnFe2O4

2/3

74.2 at 0.1 A g1

e


14.5 kW kg1

Not shown

[29]

NaOH/1 M

Ni(OH)2 @a-Fe2O3

2/3

908 at 21.8 A g1

0e0.6


16.4 kW kg1

Reversible reaction of Ni2þ/Ni3þ associated with anions OH

[45]

Continued

Alkaline Aqueous Electrolyte Used in Metal Oxide Supercapacitors, the Reaction Between Electrolyte and Electrodes, and Electrochemical Supercapacitor Performancedcont’d

Table 3.2

Aqueous Electrolyte/ Concentration NaOH/1 M

Electrode MnO2 nanoflower

Electrode Setup 3



312 at 2 mV s

0.45 to 0.45

Cycling Stability 93% after 2000 cycles

Power Density


References

e

EDLC mechanism

[28]

þ

ðMnO2 Þsurface þ C þ

e 4

þ

MnO2 C

surface

Cþ ¼ Hþ, Liþ, Naþ, Kþ Pseudocapacitance mechanism (MnO2)surface þ Cþ þ e 4 (MnOOC) NaOH/1 M

NaOH/2 M

CoO@NiOOH

MnMoO4/ CoMoO4

3

3

798.3 at w1.67 A g1

0.01e0.52

187.1 at 1 A g1

0.1e0.4


e


e

CoO þ OH 4 CoOOH þ e

[46]

CoOOH þ OH 4 CoO2 þ H2O þ e 2[Mn(OH)3] 4 Mn2O3 þ 3H2O þ 2e

[47]

3[Co(OH)3] 4 Co3O4 þ 4H2O þ OH þ 2e Co3O4 H2O þ OH 4 3CoOOH þ e Mn2O3 þ 2OH 4 2MnO2 þ H2O þ 2e CoOOH þ OH 4 CoO2 þ H2O þ e

NaOH/2 M

MnFe2O4

2

93.9 at 0.1 A g1

e


e

Not shown

NaOH/5 M

3D CuO flowerlike

3

1462.8 at 5 mA cm2

0e0.6

79% after 10,000 cycles

e

CuO þ H2O þ 2e 4 Cu2O þ 2OH

[29]

Cu2O þ H2O þ 2OH 4 2Cu(OH)2 þ 2e CuOH þ OH 4 2Cu(OH)2 þ e

EDLC, electric double-layer capacitor.

[48]

Acid Aqueous Electrolyte Used in Metal Oxide Supercapacitors, the Reaction Between Electrolyte and Electrodes, and Electrochemical Supercapacitor Performance

Table 3.3

Aqueous Electrolyte/ Concentration H2SO4/0.5 M

Electrode

Electrode Setup


Potential Window (V)/ Reference

Cycling Stability

Power Density

Ti/IrO2/WO3

3

46 at 50 mV s1

0.2e1.2

e

e

Reaction Between Electrolyte and Electrode WO3 þ xHþ þ xe 4 HxWO3 (0 < x < 1)

References [49]

IrO2 þ 2Hþ þ 2e 4 Ir2O3 þ H2O H2SO4/1 M

RuO2

3

120.4 at 50 mV s1 1

0e0.9

e

e

Not shown

[34]

H2SO4/1 M

H3PMO12O40

3

160 at 2 A g 183 at 2 A g1

0e1


e

PMoðVIÞO40 3 þ 6e þ 6Hþ 4H6 PMoðVÞ6 MoðVIÞ6 O4 3

[52]

H2SO4/1 M

H3PW12 O403.H2O

2/3

254 at 6 A g1

0e1.7


115 kW kg1

PW12 O40 3 þ e 4PW12 O40 4

[53]

PW12 O40 4 þ e 4PW12 O40 5 PW12 O40 5 þ 2e þ Hþ 4PW12 O40 6

H2SO4/1 M

WO3 nanoflowers

2/3

196 at 10 mV s1

0.5e0


229.3 mW cm3

Redox reactions from W6þ to W5þ

[50]

(W6þ þ e/W5þ) Hþ insertion/extraction WO3 þ e þ Hþ 4 HWO3

H2SO4/1 M

NiFe2O4

3

454 at 2.5 A g1

0e0.6


e

Redox reactions (Ni2þ/3þ and/or Fe3þ/2þ)

[51]

HCl/1 M

NiCo2O4

3

3.8 at 0.5 A g1

0.35e1


e

Not shown

[35]

62


It may also be noted that the acid aqueous electrolytes are extremely corrosive for current collectors and packaging materials. To solve this problem, gold (Au) and indium tin oxide (ITO) have been used in current collectors [8]. However, this solution is less used in part due to the increased prices.

3.3.1.1.3 Neutral Electrolytes In addition to the acidic and alkaline electrolytes, neutral electrolytes have also been widely studied for metal oxide SCs (Table 3.4). This is because of their advantages, such as: larger working potential windows (up to 2 V), environmentally friendly, low cost and greater safety [4,54]. However, the neutral electrolytes also have disadvantages, the power performance of the device is still limited due to their relatively low ionic conductivity at room temperatures (Table 3.1) [55]. Moreover, devices with neutral electrolytes can suffer crevice corrosion, due to cell design, like in coin cell. Among the various neutral electrolytes, Na2SO4 is the most commonly used neutral electrolyte [56e63]. But there are various other neutral electrolytes, such as LiCl [64], Li2SO4 [65], K2SO4 [20] and LiNO3 [66], Na2SO3 [67], Ca(NO3)2 [68], and KCl salt [35], which could be used in metal oxideebased SCs. To date the MnO2 electrode material is the most studied in neutral electrolytes because neutral electrolyte ions play a significant role in the pseudocapacitive performance of the MnO2, as they are directly involved in the chargingedischarging process [69]. But the list of metal oxide electrode materials studied to optimize the performance of SCs in neutral electrolytes is already quite long (see Table 3.4). Recently, some efforts have been made to the use of neutral electrolytes in SCs. However, it should be emphasized that for neutral electrolytes the energy storage/delivery processes are not well understood as demonstrated by the absence of equations in the Table 3.4. This is due to the complexity of the electrochemical mechanisms, which are involved in the charge storage process. Like in alkaline or acidic electrolytes, various aspects of the neutral electrolytes, for instance, the types of cation and anion species, additives, and solution temperatures, have been found to have influence on the SC performance. Another weakness of the neutral electrolytes (e.g., K2SO4) is the inability to reach high molar concentration as the alkaline electrolytes at lower temperatures, which has a strong impact on the SC performance. For example, a study carried out by Li and coworkers [70] using comparatively three neutral electrolytes (Na2SO4, K2SO4, Li2SO4) on mesoporous MnO2 electrode has shown that the specific capacitance increases and the resultant energy and powder densities increase in the order Li2SO4 > Na2SO4 > K2SO4 for low scan rate, whereas at high scan rate the specific capacitance values overlap [70]. This trend was explained by the fact that in the charge storage, intercalation/ deintercalation of smaller alkaline metal cations occurs and the radius of the hydrated ions follows the order: Liþ > Naþ > SO4 þ [70]. Q. Qu et al. [71] also reported that V2O5$0.6H20 nanoribbons showed the largest specific capacitance value in the 0.5 M K2SO4 electrolyte compared with the two other neutral electrolytes (Li2SO4 and Na2SO4) with same concentration because of the most facile intercalation/ deintercalation of Kþ into/from the interlayers of V2O5$0.6H20. Results similar to

Neutral Aqueous Electrolytes Used in Metal Oxide Supercapacitors, the Reaction Between Electrolyte and Electrodes, and Electrochemical Supercapacitor Performance Table 3.4


Electrode

Electrode Setup

Specific Capacitance (F gL1) 1

Potential Window (V)/Reference

Cycling Stability

Powder Density


References

Na2SO4/0.1 M

ZnO nanorod/NiO shell composite

3

305 at 10 mV s

0.1 to 0.5


e

Not shown

[63]

Na2SO4/0.5 M

Wo3x/MoO3x

2/3

190 at 1.5 A g1

0e1.9


0.73 W cm3

Not shown

[56]

Na2SO4/1 M

NiOx composite

2/3

150 at 1 A g1

1.2 to 1.0


4 kW kg1

Not shown

[73]

Na2SO4/1 M

3D V2O5 architecture

2/3

521 at 5 mV s1

1 to 1


9.4 kW kg1

Not shown

[58]

Na2SO4/1 M

MnO2 mesoporous

3

278.8 at 1 mV s

0e1


e

Not shown

[70]

Na2SO4/1 M

Co3O4@Pt@MnO2 nanowires

2/3

497 at 10 mV s1

0e1


19.6 kW kg1

Not shown

[59]

Na2SO4/1 M

ZnO@MoO3 nanocable

3

241 at 5 mV s1

1.3 to 0.2


e

Not shown

[60]

Na2SO4/1 M

MnO2 nanotubes

3

245 at 10 A g1

0e0.8


e

Not shown

[61]

Na2SO4/1 M

MnO2 nanopillars

2/3

603 at 5 mV s1

0e0.8


3.57 kW kg1

Not shown

[62]

Na2SO4/1 M

RuO2

2/3

117.6 at 50 mV s

1 to 0.8

e

e

Not shown

[34]

Na2SO4/1 M

Bi2O3@MnO2

3

93.1 at 1 A g1

0.2 to 0.8


e

Not shown

[74]

1

1

Continued

Neutral Aqueous Electrolytes Used in Metal Oxide Supercapacitors, the Reaction Between Electrolyte and Electrodes, and Electrochemical Supercapacitor Performancedcont’d

Table 3.4


Electrode

Electrode Setup


Potential Window (V)/Reference

Cycling Stability

Powder Density


References

Na2SO4/1 M

CuO@MnO2

2/3

49.2 at 0.25 A g

0.2 to 0.8


85.6 kW kg1

Not shown

[75]

Na2SO4/1 M

MoO3 microrods

2/3

194 at 5 mV s1

1.2 to 0.5


1200 W kg1

Not shown

[76]

Na2SO4/1 M

Mn3O4 nanorods

3

258 at 5 mV s1

0e1


e

Mn3O4 (spinel) 4 NadMnOxnH2O (birnessite)

[77]

1

NadMnOxnH2O þ yHþ þ zNaþ(y þ Z)e 4 Nad þ zMnOxnH2O Na2SO4/2 M

MnFe2O4

2

47.4 at 0.1 A g1

e


e

Not shown

[29]

K2SO4/0.65 M

MnO2 mesoporous

3

224.9 at 1 mV s

0e1


e

Not shown

[70]

Li2SO4/1 M

MnO2 mesoporous

3

284.24 at 1 mV s

0e1


e

Not shown

[70]

Li2SO4/1 M

MnO2 amorphous

3

139 at 5 mV s1

0e0.9

e

e

MnO2 þ (x þ y) e þ yCþ 4 MnOOHxCy (C: cation from electrolyte)

[65]

1

1

1

MnO2 birnessite

200 at 5 mV s

MnO2 cryptomelane

102 at 5 mV s1

MnO2 spinel

78 at 5 mV s1

LiNO3/5 M

FeWO4

3

35 at 10 mV s1

0.6 to 0


e

Reversible redox reactions (Fe3þ/Fe2þ)

[66]

LiCl/5 M

V6O13x

3

1050 at 10 mV s

1 to 0


e

Reversible redox reactions of V3þ/V4þ and V5þ states

[64]

LiCl/5 M

MnO2/Fe2O3

2/3

1.2 F cm3 at 10 mV s1

0.8 to 0


0.1 W cm3

Not shown

[78]

Na2SO3/1 M

Fe3O4 nanoparticles

3

207.7 at 0.4 A g

0.9 to 0.1


e

2SO3 2 þ 3H2 O þ 4e 4S2 O3 2 þ 6OH

[67]

1

1

S2 O3 2 þ 3H2 O þ 8e 42S2 þ 6OH Ca(NO3)2/2 M

(2D) MnO2

3

587.3 at 2 mV s

0e1


e

Not shown

KCl/1 M

NiCo2O4

3

41.9 at 0.5 A g1

0e1


e

MOH2 þ þCl 4MOH2 þ Cl

1

þ

[68]

MO þ K 4 MO K

þ

[35]

66


that K2SO4 resulted in a fast chargeedischarge process and a superior power compared to the Li2SO4 and Na2SO4 electrolytes were reported for KxMnO2$nH2O electrodes [72]. The authors attributed this to slight structural expansion/contraction degree during the chargeedischarge process [72]. Some comparative studies between aqueous electrolytes have been done. Using the (LaMnO3.09) metal oxide electrode, Mefford et al. [20] have shown that the specific capacitance is lower in neutral electrolytes (Li2SO4, Na2SO4, K2SO4) compared to alkaline electrolytes (LiOH, NaOH, KOH) because of the lower ionic conductivity of the neutral electrolytes. Although the neutral electrolytes have been extensively studied in metal oxidee based SCs, they have lower Hþ and OH concentration compared with alkaline or acid electrolytes, which is in several systems a disadvantage. With this in mind, S.-K. Chang et al. [35] explored the influence of different types of electrolyte systems (acidic 1 M HCl, alkaline 1 M KOH, and neutral 1 M KCl) on pure-phase NiCo2O4 electrode performance. They showed that in acidic electrolyte (1 M HCl), there is dissolution of spinel oxide (NiCo2O4), whereas the nanosized NiCo2O4 electrode exhibits excellent discharge capability in alkaline (1 M KOH) and neutral (1 M KCl) electrolytes [35].

3.3.1.2

Nonaqueous Electrolytes

3.3.1.2.1 Organic Electrolytes Nowadays, organic electrolytes dominate the ES world market and are extensively applied in double-layer SCs mostly for their high operative cell voltage (2.5e2.8 V). But organic electrolytes have disadvantages already conveyed and require a special handling procedure to avoid impurities, such as water, that implies high cost compared to the aqueous electrolytes. Concomitantly, research reports of organic electrolytes in metal oxide SCs are scarce and typically the developed organic electrolytes contain salts with lithium ions to facilitate the mechanism of ion intercalation/deintercalation for Li battery applications. The most common salts found in the literature include LiClO4 [79] for MoO3 and LiPF6 [80e82] for electrode oxide materials such as SnO2 and TiO2. The most used organic solvents are propylene carbonate (PC) [79] and a mixture of different solvents such as ethylene carbonate (EC)edimethyl carbonate (DMC)eethyl methyl carbonate (EMC) [81] and ECeDMCediethyl carbonate (DEC) [80]. For instance, the 1 M LiPF6/PCeDMC electrolyte enables the MnO2 electrodes from reaching higher specific capacitance (455 F g1) when compared to that showed in the 1 M NaOH aqueous electrolyte (342 F g1) [83]. Otherwise, the Li-containing organic electrolytes such as LiClO4/PC and lithium bis(trifluoromethanesulfonyl)imide/acetonitrile (LiTFSI/ACN) have a wider application, for example, with V2O5, because they also permit higher specific capacitance than the aqueous electrolyte KCl [84,85]. Moreover, the 1 M LiClO4/PC electrolyte on an MnO2eCNT SC allows a specific energy density about six times higher than that obtained with 1 M KCl electrolyte because of the resulting larger potential window, but it shows a lower cycling stability than KCl electrolyte [86].


67

Organic electrolytes used for asymmetric ESs are also reported, such as in asymmetric carbon/MnO2 [86], carbon/V2O5 [85], and carbon/TiO2 [82], which exhibit high energy density because of the resulting large working potential window (up to 3e4 V).

3.3.1.2.2 Ionic Liquids ILs are known as low-temperature or room-temperature molten salts and those salts are composed solely of cations and anions. Normally the ILs present several advantages including high thermal, chemical, and electrochemical stability and low volatility [87]. However, they also present some drawbacks such as high viscosity, low ionic conductivity, and high cost, which limit their use in metal oxide SCs. As mentioned earlier, a few reports could be found in the literature that explore metal oxide SCs as electrodes and ILs as electrolyte. Nonetheless, Rochefort and Pont [88] evaluate the pseudocapacitance of RuO2 electrodes in two ILs. The authors concluded that RuO2 present pseudocapacitance in a protic IL composed of 2-methylpyridine (a-picoline) and trifluoroacetic acid (TFA), whereas no obvious pseudocapacitance was observed in 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]) IL [88]. Another study reported by Mayrand-Provencher et al. [89] in which RuO2 was used as the electrode and different pyridinium-based protic ILs were used as electrolytes showed that the alkyl chain length of a cation’s substituent and the substituent position had an effect on the electrolyte conductivity and viscosity and the specific capacitance and cycling stability of the electrode material. Mn oxide electrode was also studied with IL electrolytes. It was reported that the Mn oxide has a pseudocapacitive performance in an aprotic 1-ethyl-3-methylimidazoliumdicyanamide IL electrolyte [90]. The same researchers, found that in the 1-butyl-1methylpyrrolidinium dicyanamide ([BMP][DCA]) IL electrolyte, smaller [DCA] anions, instead of [BMP]þ cations, could reversibly insert into or desert out of the MnO6 structure, compensating the Mn3þ/Mn4þ valency state variation during the chargee discharge process [91]. Other metal oxide SC-IL electrolyte systems, such as Ru-doped Cu oxide in 1-butyl-3-methylimidazolium hydrogen sulfate ([BMIM] [HSO4]) [92], TiO2 in [EMIM][TFSI] [93], and Fe2O3 in [EMIM][BF4] [94], have also been studied. One of the problems of ILs it their high viscosity. In order to reduce the viscosity and increase the conductivity, some organic solvents have been added and the mixtures were tested as electrolytes; however, just few were explored with metal oxide electrodes. Zhang et al. [95] found that the addition of N,N-dimethylformamide to 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) IL increased the capacitance and also decreased the internal resistance of the asymmetric AC/MnO2 SC when compared to pure IL. This improvement was attributed to the improved electrolyte penetration and ion mobility.

3.3.2

Redox Additives

Actually the electrochemical behavior of metal oxide electrode materials is strongly dependent on the nature of electrolytes. Consequently, the foremost goal is that both electrolyte and electrode can contribute synergistically to an optimal pseudocapacitance

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and\or faradaic response, driving redox reactions that can enhance the charge storage capacity of SCs. Redox aqueous electrolytes (e.g., H2SO4, Na2SO4, and KOH) are intrinsically able to undertake fast electron-transfer reactions at the electrodeeelectrolyte interface but can be successfully upgraded with redox additives, in which redox-active species are added to the electrolyte to improve electron-transfer reactions [96]. The actual developments in acid, neutral, and alkaline aqueous redox additive electrolytes benefit from the background of previous studies on redox-active species (e.g., KI) for activated carbonecarbon SC devices [97,98]. Nowadays an emergent strategy is reinforced because of intense research demonstrating the advantage of using redox additive aqueous electrolytes to improve the performance of metal oxide asymmetric supercapacitors (ASCs). As a matter of fact, the addition of K3Fe(CN)6 as redox additive electrolyte to 2 M KOH solution provokes about three times increment in the charge storage performance of binder-free CoMoO4 nanoplate array (NPA)/activated carbon ASC [99]. Similarly, addition of 0.02 M K3Fe(CN)6 redox species revealed a maximum areal capacity of 603.5 mA h cm2 at 2.5 mA cm2, with an extended operating voltage window of 1.5 V. A mechanism has been proposed based on the charging process, in which the presence of this redox additive accelerated the transition of Co(II)/Co(III) by the redox reaction FeðCNÞ6 3 FeðCNÞ6 4 facilitated by the high electrochemical reversibility nature of FeðCNÞ6 3 FeðCNÞ6 4 ions. The FeðCNÞ6 3 ion accepts the electron from the Co element of the ternary metal oxide and gives FeðCNÞ6 4 ion. When the electrode is discharged, the FeðCNÞ6 4 loses one electron (oxidation reaction) giving FeðCNÞ6 3 in hexacyanoferrate for the reduction of Co(III) to Co(II) in CoMoO4. In this circumstance, the FeðCNÞ6 4 ion acts as an electron donor and Co(III) is an electron acceptor. Enhancement in the redox reaction is attributed to hexacyanoferrate ions that play the role of “electron shuttle” in the chargeedischarge processes and accelerate the rate of the reaction, leading to the higher charge capacity. The CoMoO4 NPA/activated carbon ASC device exhibited a high energy density of 125 mW h cm2 at a power density of 1507 mW cm2 and excellent cycling stability even after 2000 continuous cycles. Research reveals that a redox electrolyte system consisting of sodium persulfate added conventional KOH electrolyte is adequate not only for metal oxide electrodes but also for metal sulfide, vanadate, and phosphate electrodes. Additionally, when Na2S2O8 is added into KOH electrolyte, fast charge and slow discharge is observed, which means that the redox-active electrolyte system can be applied in the batterytype SC [100]. Nickel oxide electrodes in Na2S2O8/KOH redox-active aqueous electrolyte exhibit a specific capacitance threefold higher than without the additive, reaching 6317.5 F g1 at 0.5 A g1. Moreover, the specific capacitance showed a decreasing trend, and the reason is when the Na2S2O8 concentration is lower, the contribution of the redox reaction from Na2S2O8 is relatively lower, resulting in low specific capacitance. If the electrolyte is a pure KOH aqueous solution, the chargeedischarge process corresponds to the reversible redox reaction of Ni(II)/Ni(III), and the chargeedischarge time is similar. When Na2S2O8 is added to the KOH electrolyte,


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S2 O8 2 will take part in the reaction. During charging process, the Ni(II) is oxidized to Ni(III), though Ni(II) can also reacts with the S2 O8 2 due to the two steps reaction, and the charging time is shortened. During the discharging process, Ni(III) is reduced to Ni(II) and then it will be oxidized to Ni(III) by S2 O8 2 and the Ni(III) will repeat the backward reaction, thus the discharging time is extended. Furthermore, galvanostatic chargeedischarge curves show that the effect of addition of Na2S2O8 disappears for higher concentration (>0.05 M) probably because of the concentration polarization phenomenon. For temperatures above 60 C, sodium persulfate may decompose and generate sodium sulfate. The effect of various concentrations (0.010, 0.025, 0.050, 0.075, and 0.100 g) of p-phenylenediamine added to 1.78 M KOH has been investigated for MnO2 electrodes [101]. A maximum capacitance of 325.24 F g1 (1 A g1) was found for 0.050 g of p-phenylenediamine added to KOH. This maximum capacitance was nearly 6.25 fold higher than the value observed for pure KOH electrolyte and the energy density also increased from 1.29 to 10.12 W h kg1. These increases in capacitance and energy density are attributed to the redox reaction between p-phenylenediamine/ p-phenylenediimine without neglecting that p-phenylenediamine also modifies the conductivity mechanism of electrolyte. The system capacity retention attains 75% in redox additive electrolyte for 5000 cycles. Redox-active aqueous electrolyte is also explored to evaluate the electrochemical properties of a monoclinic WO3 thin film in 1 M H2SO4 and in a mixture of H2SO4e hydroquinone (HQ) redox electrolytes [102]. Various concentrations (from 0.1 to 0.4 M) of HQ are added to 1 M H2SO4 electrolyte to assess the ionic conductivity of the resulting redox electrolytes. The authors found that electrolyte ionic conductivity increases with the concentration of HQ from 0.1 to 0.2 M and decreases for higher HQ concentration in 1 M H2SO4. The redox electrolyte with 0.2 M HQ in H2SO4 attends a maximal ionic conductivity of 129.4 mS cm1. For higher HQ concentrations, there is interaction of HQ molecules resulting in aggregation of free ions and crystallization of HQ in H2SO4, with expected slow diffusion of ions that decreases the ionic conductivity. The specific capacitance and energy density of WO3 electrode in conventional H2SO4 electrolyte are 352 F g1 and 12.25 W h kg1, respectively, which increase to 725 F g1 and 25.18 W h kg1 in H2SO4 þ 0.2 M HQ redox electrolyte, at the same power density of 1166 W kg1. However, the WO3 film electrode shows low capacity retention (81%) in redox additive electrolyte as compared to pure H2SO4 electrolyte (89%) for 1000 cycles. Authors attribute the slight loss in capacitance to faster degradation of electrode material in the presence of redox electrolyte. KI redox additive has been proposed as an economically favorable and environment-friendly redox additive aqueous electrolyte contrasting with toxic CAN or expensive ILs usually applied in ASCs [103]. Neutral aqueous electrolyte of 1 M Li2SO4 with varying concentration of KI redox additive achieved a significant enhancement in the specific energy while maintaining the specific power for various transition metal oxide (TMO)/multiwalled carbon nanotube (MWCNT) composite ASCs. Indeed, along with w105% increase in specific energy at an optimized concentration (7.5 mmol KI concentration), iodine-based redox reactions can also ensure good cycling stability and high specific power of mesoporous MWCNT/ZrO2 (WO3) composite ASCs. The proposed charge storage mechanism for a system with

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1 M Li2SO4 electrolyte was a convolution effect originating from the intercalation/ deintercalation and surface absorption/desorption of the lithium electrolyte cations. Moreover, the improved faradaic capacitance observed in 1 M Li2SO4 and KI mixture electrolyte is attributed to various redox reactions of the iodine/iodide redox pairs, which can form a variety of negatively charged polyiodides (In ), such as I3 , I5 and IO3 through dissolved I2. The iodine/iodide pairs produce redox peaks in CVs and a faradaic plateau in chargeedischarge curves. In this study, for high KI concentrations (15, 30, 45, 75 mmol KI concentration) the galvanostatic chargeedischarge curves lose linearity and the stable potential window of the positive electrode show a trend toward narrowing, resulting in H2/O2 evolution at a potential lower than 2.2 V (stability window provided by the electrolyte). The literature proposes that oxidation/reduction reactions can occur at the electrodeeelectrolyte interface because of the presence of these iodide/iodine redox pairs [97,103].

3.3.3

Solid and/or Gel State

Solid-state SCs have emerged because of new challenges in transportation field (space and automotive), modern electronics, and bioimplantable systems that require high performance in extreme environmental conditions and most of all, electrolyte nonleakage. Adequate solid- or gel-state electrolytes have been proposed to address those requirements in new configuration devices as well as in microscale energy storage devices [8,104]. Accordingly, this chapter mainly focuses on the latest new developments of gel electrolytes applied on metal oxide SCs and hybrid, symmetric, and asymmetric configuration devices. The most commonly used ionic conducting hydrated gel electrolyte is poly(vinyl alcohol) (PVA) based, which is prepared by blending the polymer with common alkaline, acid, and neutral aqueous solutions to satisfy a specific condition. Gel electrolytes such as PVA/H3PO4, PVA/H2SO4, PVA/KOH, PVA/NaNO3, and PVA/LiCl are examples of solid electrolytes used in macro- and micro-SCs. The alkaline PVA/gel electrolyte (e.g., PVA/KOH, PVA/NaOH, and PVA/LiOH) has been extensively employed with the pseudocapacitive electrode material MnO2 [105], other TMOs such as spinel NiCo2O4 [106], and polyporous MoO3@CuO composite [107]. PVAeLiOH solid-state gel was used both as electrolyte and separator to assemble NiCo2O4/carbon cloth//porous graphene paper ASC that exhibits a maximum energy density (60.9 W h kg1), maximum power density (11.36 kW kg1), and cycling stability (96.8% capacitance retention ratio after 5000 cycles under mechanical bending) and an exceptionally high operating potential of 1.8 V [108]. On the other hand, acid PVA-based gel electrolytes (e.g., PVA/H2SO4 and PVA/ H3PO4) have been frequently used with the traditional pseudocapacitive electrode materials RuO2 [109] and MnO2 [110]. A solid-state symmetric SC is also fabricated using PVA/H2SO4 as gel electrolyte and one-dimensional (1D) nanofiber network of spinel CoMn2O4, having interconnected nanoparticles [111]. It reached a high energy density of 75 W h kg1 at a power density of 2 kW kg1, which was better than that of many battery systems (Pb-acid and Ni-MH batteries). The energy density and 2 V output voltage of this device was also confirmed by red light-emitting diode light at


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1.8 V and 20 mA for 5 min. This high performance is attributed to 1D nanofibers consisting of voids/gaps with minimum interparticle resistance that facilitates smoother transportation of electrons/ions. The authors propose that these voids/gaps can act as intercalation/deintercalation sites for electrolyte. Mainly, the neutral PVA-based gel electrolytes (e.g., PVA/LiCl, PVA/Na2SO4, and PVA/NaNO3) have been studied with electrode materials such as MnO2 [112], Mn3O4 [113], V2O5 [10], and ZnO [114]. Generally, the neutral PVA/LiCl electrolyte has a wider application, namely, with V2O5, an amphoteric material that can dissolve in both basic and acidic media. Moreover, LiCl can be dissolved in PVA gel without crystallization during the gel drying process and preserves excellent electrochemical and mechanical properties without the need for periodical wetting. A study replaces an aqueous electrolyte that provokes vanadium oxide dissolution and poor structural stability during chargingedischarging cycling, with neutral PVA/LiCl gel electrolyte allowing a vanadium oxide nanowire SC to achieve a maximum areal capacitance of 236 mF cm2 at a current density of 0.2 mA cm2 and an excellent capacitance retention rate of more than 85% after chargeedischarge cycling of 5000 cycles [115]. Another trend in recent research explores symmetric and asymmetric neutral hydrogel-based devices to fabric new flexible solid-state SCs in an attempt to combine high power and high energy density. For instance, a PVA/Na2SO4 gel electrolyte permits the assembling of a flexible and lightweight all-solid-state asymmetric supercapacitors (AASCs) using 3D coated MnOx nanosheets on nanoporous current collectors (3D MnOx@Ni@CC) and a chemically converted graphene as the negative electrode [116].The polymer gel electrolyte under a pressure of w1 MPa and during 10 min penetrates into each electrode and also acts as a thin separator. A bending test was performed to evaluate the flexibility performance of the device and the influence of the bending-induced mechanical stress on specific capacitance. The flexible AASC exhibits minimal change in the chargeedischarge behavior on bending to various angles, being the change in capacitance with increasing bending angle (0 degree to 180 degrees) less than 5%. It also presents an excellent performance with energy density of 1.16 mW h cm3 at 1 mA cm2, with 81.5% capacitance retention after 10,000 cycles of charginge discharging at 2 mA cm2 and 85.7% capacitance retention after 200 bending cycles. Carboxymethyl celluloseeNa2SO4 (CMC-Na2SO4) gel electrolyte was used to fabricate flexible all-solid-state thin-film symmetric supercapacitors (FASSTF-SSCs) of identical electrodes MnO2 thin films over stainless steel substrate. The 0.7-mmthick FASSTF cell exhibits a specific capacitance of 145 F g1 with specific energy of 16 W h kg1 and excellent cycling stability after 2500 cycles [117]. The same inexpensive polymer gel CMC-1 M Na2SO4 that acts simultaneously as separator and electrolyte allows the assembling of a thin and flexible symmetric MnO2/ MnO2 SC solid device. Also an asymmetric SC device based on compiled MnO2 nanosheets as positive electrode and Fe2O3 nanoparticles as negative electrode was developed. The asymmetric configuration exhibits twofold high energy density compared to symmetric SC, a higher window voltage of 2 V and excellent mechanical flexibility associated to good cycling stability [118].

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Three ionic conducting polymer gel electrolytes were prepared using PVA, CMC, and polyethylene oxide with lithium perchlorate (LiClO4) to fabric a solid-state MnO2 SC device [119]. Among others, the device of (PVA)elithium perchlorate (LiClO4) gel electrolyte assembled with MnO2 thin film electrodes on flexible stainless substrate provided better performance, with an operating potential window of 1.2 V, a specific capacitance of 112 F g1, and an energy density of 15 W h kg1 with extended cycling stability up to 2500 CV cycles. The better electrochemical performance of PVA gel electrolyteebased SC device is attributed to the electrolyte’s higher ionic conductivity (48 mS cm1) and higher wettability that allows maximum interfacial contact with electrodes.

3.4

Conclusions and Outlooks

In the past years, many efforts have been focused on low-cost aqueous electrolytes with and without redox additives for application in binary and ternary metal oxide SCs. Nevertheless, it is imperative to increase the working potential window and the corrosion resistance of those SC devices as well as to control the high risk of electrolyte leakage. Concomitantly, the state of the art reveals a tremendous effort focusing on developing novel electrolytes used in metal oxide SCs that allow production of miniature, flexible, safe, and lightweight energy storage devices. As a result, the next generation of SCs based on metal oxides demands the development of innovative electrolytes and electrode materials, as well as novel electrodee electrolyte configurations, to overcome their disadvantages and simultaneously raise their limited energy density. To improve the energy storage of SCs, future studies should explore novel electrolytes that guarantee adequate properties that fit the specificities of emergent metal oxideebased electrodes. The new strategies focus on the development of solid-electrolyte systems, namely, the solid-state gel electrolytes. These electrolytes must meet the requirements of high conductivity to increase the operating voltage, consequently leading to an increase in energy density and high chemical and physical stability to provide excellent cycling stability and longer-lasting cells. Another critical factor in the further development of electrolytes is to establish the charge storage mechanisms of neutral electrolyte on the electrodeemetal oxide interface and intensify theoretical kinetic studies on redox reactions. To obviate electrolyte degradation, it is also important to reach the fundamental understanding of degradation mechanisms.

Acknowledgments The authors would like to thank Fundaç~ao para a Ciência e Tecnologia (FCT) for the funding under the contract CQE (UID/QUI/00100/2013).


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Fundamentals of Binary Metal OxideeBased Supercapacitors

4

Deepak P. Dubal 1,2 , Nilesh R. Chodankar 3 , Pedro Gomez-Romero 2 , Do-Heyoung Kim 3 1 School of Chemical Engineering, The University of Adelaide, Adelaide, South Australia 5005, Australia; 2Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC, The Barcelona Institute of Science and Technology, Barcelona, Spain; 3Chonnam National University, Gwangju, South Korea

4.1

Introduction

The accumulation of charges on solids is known since ancient times (dating back to the Greek word for amber, i.e., electron [1]); however, the first patent for an electrochemical capacitor was filed in 1957 by General Electric [2]. The actual phenomenon of charge storing was not clear at that time and hence an interesting statement was mentioned in the patent: “it is not positively known exactly what takes place when the devices are used as energy storing devices.” [2] Later the concept was explained with the name of electric double-layer capacitors (EDLCs) where the charges are stored at the interface between a carbon electrode and an electrolyte through the formation of a Helmholtz double layer on polarization [1]. This increased capacitance results from the large active area of those carbon electrodes and the minimal magnitude of charge separation down to a few angstroms. The storage mechanism in this case is purely physical. The energy is accumulated because of the presence of an electric field resulting from a charge separation at the electrodeeelectrolyte interface. In 1971, a new type of electrochemical capacitance was discovered using the RuO2 electrode and was termed as pseudocapacitance because it involved faradaic charge-transfer reactions [3]. The storage of protons from the electrolyte resulted in a faradaic charge-transfer reaction on the RuO2 thin-film electrode. This invention of pseudocapacitance was the clearest transition of supercapacitors from purely electrophysical (EDLC) to electrochemical devices and a truly major step forward in the field. Later Conway [1] identified several faradaic mechanisms that can result in capacitive electrochemical features: (1) underpotential deposition, (2) redox pseudocapacitance (as in RuO2$nH2O), and (3) intercalation pseudocapacitance. These processes are illustrated in Fig. 4.1. Underpotential deposition occurs when metal ions form an adsorbed monolayer at a different metal’s surface well above their redox potential. One classic example of underpotential deposition is that of lead on the surface of a gold electrode. Redox pseudocapacitance occurs when ions are electrochemically adsorbed onto the surface or near the surface of a material with a


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Figure 4.1 Different types of reversible redox mechanisms that give rise to pseudocapacitance: (A) underpotential deposition, (B) redox pseudocapacitance, and (C) intercalation pseudocapacitance. Reproduced from V. Augustyn, P. Simon, B. Dunn, Energy Environ. Sci. 7 (2014) 1597, with the permission from Royal Society of Chemistry.

concomitant faradaic charge transfer. Intercalation pseudocapacitance occurs when ions intercalate into the tunnels or layers of a redox-active material accompanied by a faradaic charge transfer with no crystallographic phase change. In between the regimes where EDLCs and lithium-ion batteries exhibit their best performance is a time domain (10 s to 10 min) that appears well suited for pseudocapacitive materials. Such high-rate electrochemical energy storage is desirable for numerous applications where a large amount of energy needs to be either stored or delivered quickly. These include kinetic energy harvesting in seaports or with regenerative braking, pulse power in communication devices, and power quality applications in the power grid. In addition, shorter charging times would be very convenient for portable devices and especially for electric vehicles. A wide variety of electroactive oxides and conducting polymers have been applied as active materials for pseudocapacitors. Among oxides, hydrated RuO2 constitutes a paradigmatic example, but in principle any intercalation phase featuring solide solution behavior could be used, and various oxides such as MnO2, IrO2, Nb2O5, Fe3O4, and V2O5, as well as their mixed oxides, have been studied as alternatives to RuO2, which despite its high specific capacitance (SC) (theoretical value 1360 F g1) is too expensive for practical applications [5]. However, the theoretical capacitances of these metal oxides have rarely been achieved in experiments mainly because of their poor electronic conductivity, which limits the rate capability for high power performance and thus hinders their widespread application in energy storage systems. This chapter provides fundamentals of charge-storing mechanisms in different binary metal oxides with a comprehensive summary of recent advancements in metal oxideebased supercapacitors.


4.2

81

Binary Metal Oxides in Supercapacitors

4.2.1

Ruthenium Oxide

Ruthenium oxide (RuO2) is one of the well-known supercapacitive materials because of its high theoretical SC (w2000 F g1), long cycle life, wide potential window, high electric conductivity, high rate capability, and good electrochemical reversibility. It occurs in both crystalline and amorphous hydrous forms with capacitance values exceeding 900 F g1, which is due to the underpotential hydrogenation and oxygenation in acidic and alkaline electrolytes, respectively [6,7]. The higher value of capacitance is due to the pseudocapacitance from the surface reaction between Ru ions and Hþ ions [6]. The cyclic voltammetry (CV) curve of RuO2 in an H2SO4 electrolyte is mirrorlike and mainly featureless within a potential range of 0.9 V (Fig. 4.2). Mainly, RuO2 obeys the following equation for the charge transformation during electrochemical reactions: RuO2 þ dHþ þ de 4RuO2d dðOHÞd

with 0 < d < 2

(4.1)

In the literature, different theories of charge storage were proposed to determine the supercapacitive performance of RuOx$nH2O, including the electron hopping within RuOx$nH2O particles, the electron hopping between electrode materials and current collectors, and the proton diffusion within RuOx$nH2O particles [8]. The charge storage is prominently dependent on electron hopping; therefore, it is essential to reduce the intraparticle electron hopping resistance of RuOx$nH2O, which can be reduced by forming crystalline RuO2. But it will create the diffusion barrier for proton insertion/desertion process within crystalline RuO2 and there will

Figure 4.2 The typical cyclic voltammograms of ruthenium oxide electrode annealed at (A) different temperatures and (B) different scanning rates in 0.5 M H2SO4 electrolyte. SCE, saturated calomel electrode. Reproduced from P.J. Mahon, C.J. Drummond, Aust. J. Chem. 54 (2001) 473; E. Seo, T. Lee, K.T. Lee, H.K. Song, B.S. Kim, J. Mater. Chem. 22 (2012) 11598, with the permission from Royal Society of Chemistry.

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be loss of electroactive sites [9]. It is observed that the supercapacitive performance of the RuO2 electrode is usually lower than its theoretical value, for which a number of factors are responsible. The hydrophilic nature, crystallinity, annealing temperature, and particle size of RuO2 effectively modify the capacitive performance. It is well known that RuO2 shows better electrochemical performance in acidic electrolytes. For storing the charges, the Hþ ions take part in the insertion and extraction process and are good proton conductors [10]. So RuO2 with the water content (hydrous) can accelerate the diffusion of Hþ in the electrode material. The hydrous RuO2 exhibits high SC of 900 F g1, which is much larger than that of anhydrous RuO2 electrode [11,12]. The crystallinity of RuO2 strongly affects the supercapacitive performance. The crystalline RuO2 material strongly affects the electrochemical performances because it has limits on the insertion and extraction process, which leads to an increase in the charge-transfer resistance and decreases the supercapacitive performances. On other hand, the redox reactions of the amorphous RuO2 take place not only on the surface but also in the bulk of the electrode materials [13]. Hence, there are numerous articles reporting the superior performance of amorphous RuO2 materials compared with the crystalline ones [14]. In addition, the amorphous ruthenium oxide achieved a maximum potential range of 1.35 V that is greater than that measured for crystalline ruthenium oxides, i.e., about 1.05 V in aqueous electrolytes. The annealing temperature and particle size also alter the electrochemical performance of the RuO2 electrode. When the annealed temperature is high, RuO2$nH2O would possess good crystallinity and low water content [15], resulting in low electrochemical performance. Furthermore, small-sized particles have very short diffusion and transport pathways for electrolyte ions, as well as possess high specific surface area that provides larger electroactive sites for electrochemical reactions. As a result, the energy-storing capacity of RuO2 will be improved. Despite all the advantages, still RuO2 is not suitable for commercial application because of its high cost, toxic nature, and scarce source. To reduce the cost of the supercapacitors, it is necessary to develop low-cost and environment-friendly electrode materials.

4.2.2

Manganese Oxide

The pseudocapacitive behavior of MnO2 was first investigated in 1999 by Lee and Goodenough as they studied the properties of amorphous MnO2$nH2O in KCl aqueous electrolyte [16]. Manganese oxide is a highly explored electrode material for supercapacitor application because of its high theoretical SC (1370 F g1), natural abundance, environment friendliness, low cost, and better chemical and thermal stability, as well as because it is the alternative for ideal RuO2. The different factors that can affect the electrochemical properties of MnO2 are its crystallinity, crystal structure, morphology, conductivity, and mass loading. The MnO2-based electrode stores the electric charges by pseudocapacitive mechanism. The charge storage arises from the


83

III/IV oxidation state change at or near the surface of MnO2 nanostructures [17,18]. The charge storage mechanism for MnO2 can be described by following equation: MnO2 þ Cþ þ e 4MnOOC

(4.2)

where C represents electrolyte captions such as Hþ, Kþ, Naþ, and Liþ. To store the electric charges, the surface and bulk faradaic reactions occur in MnO2. The surface faradaic reactions are due to the adsorption/desorption of electrolyte ions and/or protons on the surface of MnO2, whereas the bulk pseudocapacitive reaction depends on the intercalation/deintercalation process of either protons or cations into the bulk of MnO2. During electrochemical reactions, MnO2 changes its oxidation between Mn(III)/Mn(II), Mn(IV)/Mn(III), and Mn(VI)/Mn(IV) within the potential window of water decomposition. The typical CV curve for MnO2 thin film in 1 M Na2SO4 electrolyte is shown in Fig. 4.3. The CV curve exhibits a nearly rectangular shape that involves electron transfer across the electrodeeelectrolyte interface, suggesting actually the faradaic mechanism. MnO2 limits the bulk pseudocapacitive electrochemical reactions due to the poor electric conductivity and slower proton/cation insertion in solid. In the literature, many studies have been done on the supercapacitive behavior of MnO2 with amorphous and nanocrystalline MnO2 compounds. Xue et al. [20] prepared the different polymorphs of a-, b-, g-, and d-MnO2 by a simple redox reaction between KMnO4 and NaNO2. The capacitive performance for each phase decreased with increasing crystalline nature. Higher capacitance is observed for poorly crystalline a-MnO2 (200 F g1 at current density of 1 a g1). But logically it is not acceptable because poorly crystallized MnO2 contains the random growth

Figure 4.3 Cyclic voltammetry (CV) curves of MnO2 at different scan rates in 1 M Na2SO4 electrolyte. Reproduced from D.P. Dubal, D.S. Dhawale, R.R. Salunkhe, C.D. Lokhande, J. Electroanal. Chem. 647 (2010) 60; C. Ye, Z.M. Lin, S.Z. Hu, J. Electrochem. Soc. 152 (2005) 1272, with permission from Elsevier.

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of tunnels that will restrict the intercalation process by increasing the charge-transfer resistance. Therefore, for amorphous MnO2, it is assumed that capacitance comes from the surface or subsurface pseudocapacitive reactions. Therefore, in the literature, several attempts have been reported for the preparation of crystallized MnO2 materials with different crystal structure and their effect on electrochemical properties. For example, the different polymorphs of MnO2 have been prepared by Brousse and coworkers [21] with the aim of investigating the relationship between the crystallinity and electrochemical properties of MnO2 material. Prominently, they studied the effect of tunnel size on the intercalation/deintercalation process [21]. In MnO2 the capacitance mainly originates from the proton/cation intercalation/deintercalation and only some crystallographic structures allow sufficient gaps for this process. They observed that the capacitive performance of MnO2 is clearly relevant on the tunnel sizes of different crystalline structures. The wide tunnel sizes with large interlayer separation of the crystalline structure are favorable to achieve high electrochemical performance in MnO2 electrode. On the basis of the tunnel size, manganese oxide can be classified into several crystalline structures, including a-, b-, g-, d-, and l-MnO2; the different structures are described in Figs. 4.4 and 4.5 and in Table 4.1 [22]. The a-MnO2 contains double chains of edge-sharing MnO6 octahedra, which are linked at the corners to form

Figure 4.4 Schematic representation of the crystal structure of manganese oxides: (A) rock salt, (B) spinel (Mn3O4), (C) bixbyite (Mn2O3), (D) pyrolusite b-MnO2 (rutile type) (note the single chains of edge-sharing octahedra), (E) ramsdellite (diaspore type; MnO6 octahedra form infinite double layers), and (F) phyllomanganate (birnessiteebuserite family of layered MnO2). Reproduced from W. Wei, X. Cui, W. Chen, D.G. Ivey, Chem. Soc. Rev. 40 (2011) 1697, with the permission from Royal Society of Chemistry.


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Figure 4.5 Polyhedral representation of the eight Mn oxides reported herein: (a) b-MnO2, (b) R-MnO2, (c) a-MnO2, (d) d-MnO2, (e) l- MnO2, (f) LiMn2O4, (g) Mn2O3, and (h) Mn3O4. þ þ The light, dark, and black polyhedra represent Mnþ 2 tetrahedra, Mn3 and Mn4 octahedra, and 1þ þ Li tetrahedra, respectively. Black spheres represent K ions. Reproduced from D.M. Robinson, Y.B. Go, M. Mui, G. Gardner, Z. Zhang, D. Mastrogiovanni, E. Garfunkel, J. Li, M. Greenblatt, G.C. Dismukes, J. Am. Chem. Soc. 135 (2013) 3494, with the permission from American Chemical Society.

one-dimensional (1D) (2 2) and (1 1) tunnels in the tetragonal unit cell. The size of the (2 2) tunnel is w4.6 Å, which is a larger tunnel for insertion/extraction of alkali cations. The b-MnO2 forms 1D (1 1) tunnel that is composed of single strands of edge-shared MnO6 octahedra. The narrow tunnel size of (w1.89 Å) b-MnO2 cannot accommodate easy insertion/extraction of cations. The g-MnO2 is the random intergrowths of ramsdellite (1 2) and pyrolusite (1 1) domains, whereas the d-MnO2 is a two-dimensional layered structure with an interlayer separation of w7 Å. The l-MnO2 is a three-dimensional spinel structure [23e26]. As the energy-storing capacity of MnO2 depends on the insertion/extraction of protons or cations in MnO2, the crystal structures with sufficient tunnel space play a crucial role in determining the electrochemical properties of MnO2 material. The surface morphology is another critical factor that decides the SC of MnO2 material. There have been a few systematic studies on the morphology-dependent electrochemical properties of MnO2. The different surface morphologies result in different specific surface areas and surface to volume ratios, which may lead to

86

Table 4.1


Crystal Structure of Manganese Oxides

Type

Crystal Structure

MnO

Description

References

Rock salt, Fm3m

fcc lattice with a 6 : 6 octahedral coordination

[34]

Mn3O4

Tetragonal spinel, I41/amd

Metal cations occupy one-eighth of the tetrahedral sites and half of the octahedral sites, and there are 32 oxygen anions in the fcc unit cell

[35]

Mn2O3

bcc, Ia

bcc unit cell with 16 formula units per unit cell

[36]

a-MnO2

Monoclinic, A2/m

Cross-linking of double or triple chains of the [MnO6] octahedra, resulting in two-dimensional tunnels within the lattice

[37]

b-MnO2

Rutile structure, P42/mnm

Rutile structure with an infinite chain of [MnO6] octahedra sharing opposite edges; each chain is corner-linked with four similar chains

[38]

b-MnO2

Pbnm

Closely related to rutile except that the single chains of edge-sharing octahedra are replaced by double chains

[39]

An irregular intergrowth of layers of pyrolusite and ramsdellite

[40,41]

Layered structure containing infinite two-dimensional sheets of edge-shared [MnO6] octahedra

[42,43]

g-MnO2 d-MnO2

Birnessite, R3m

bcc, body-centered cubic; fcc, face-centered cubic. Reproduced from W. Wei, X. Cui, W. Chen, D.G. Ivey, Chem. Soc. Rev. 40 (2011) 1697, with the permission of Royal Society of Chemistry.

different electrochemical performances. Many different morphologies of MnO2 have been synthesized and the corresponding electrochemical properties are investigated. Depending on the morphology, the obtained specific surface areas of the MnO2 nanostructures are in the range from 20 to more than 200 m2 g1, thus altering the resultant electrochemical performance of various MnO2 nanostructures. Appropriate mass loading is very essential for MnO2 to get a better electrochemical performance. In general, the resultant electrochemical performance of MnO2 electrode will decrease with increasing mass loading because a large mass loading may damage the electric conductivity by increasing the series resistance. The higher mass loading of MnO2 may reduce the porosity by forming a dense electrode. Thus optimization of mass loading is essential to maintain the easy electrochemical interactions between


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MnO2 and electrolyte ions. Furthermore, conductivity is another issue that limits the electrochemical performance of the MnO2-based electrode. MnO2 has very low electric conductivity in the range of 104 to 106 S1, which results in the localized electron-transfer process in a limited volume near the current collector, leading to low electrochemical performance. The formation of a very thin layer of MnO2 or incorporation of different elements such as Au, Ag, Cu, Ni, Co, Fe, Al, Zn, Mo, and Sn into the MnO2 structure is the effective way to improve the electric conductivity and the resultant electrochemical performance of the MnO2 electrode [21,27e32].

4.2.3

Nickel Oxide

Nickel oxide (NiO) is the most promising battery-type electrode material for assembling the high-performance supercapacitor device. NiO has an ultrahigh theoretical SC of 3750 F g1, is of low cost, and is environment friendly, making it a suitable candidate in supercapacitor applications [44e47]. In general, to store the electric charges, NiO follows the following electrochemical reactions: NiO þ OH 4NiOOH þ e

(4.3)

NiO þ H2 O4NiOOH þ Hþ þ e

(4.4)

NiðOHÞ2 4NiOOH þ Hþ þ e

(4.5)

NiðOHÞ2 þ OH 4NiOOH þ H2 O þ e

(4.6)

or

The supercapacitive mechanism of NiO and its oxidation state alteration during faradaic reactions are still provocative. Basically there are two theories regarding the electrochemical reactions of NiO. One is that the charge-transfer process occurs between NiO and NiOOH (Eqs. 4.3 and 4.4). Another theory suggests that first NiO is converted to Ni(OH)2 [48] in alkaline electrolyte and then the electrochemical reactions occur between Ni(OH)2 and NiOOH (Eqs. 4.5 and 4.6) [49e54]. Both theories suggest that during electrochemical reactions, NiO is oxidized to NiOOH by losing an electron. Most researchers accept the first theory as reasonable for NiO. But the second theory is also correct because in the alkaline electrolyte, NiO combines with OH to form Ni(OH)2, which will take part in the electrochemical reactions. In comparison with the bulk material, the nanostructured NiO can provide much large specific surface area as well as very short diffusion pathway for electrons to perform electrochemical reactions [49,55e61]. In the literature, different nanostructures of NiO, such as nanobelts, nanowires, nanorods (NRs), nanoplatelets, and nanoflowers, have been prepared and tested for supercapacitor applications [62e67]. Fig. 4.6 shows the typical CV curve for NiO, and it contains a strong redox

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Figure 4.6 Cyclic voltammetry curves for NiO thin film in 1 M KOH electrolyte. Reproduced from A.K. Singh, D. Sarkar, G.G.K.M. Khan, ACS Appl. Mater. Interfaces 6 (2014) 4684, with the permission from American Chemical Society.

peak (one oxidation and one reduction peak) that corresponds to the surface oxidation/ reduction reactions. It confirms that NiO exhibits battery-like faradaic charge-storing mechanism. There are some major issues that constrain the use of NiO in supercapacitor applications. The first one is that NiO is a p-type semiconductor having low electric conductivity. Second, the cycling stability and operating potential of NiO is very low, thus limiting the resultant electrochemical performance. In the literature, many reports are available on NiO-based supercapacitors. Nickel hydroxide [Ni(OH)2] is also utilized as a supercapacitive electrode because of its high theoretical SC (3650 F g1) as compared to that of NiO [69e72]. Ni(OH)2 has a hexagonal layered structure and has two polymorphs, a- and b-Ni(OH)2. a-Ni(OH)2 contains water molecules and anions in interlayered structure, whereas b-Ni(OH)2 possesses a brucite structure without water molecules. a-Ni(OH)2 shows better SC value but it is not very stable because it can easily transform to b-Ni(OH)2 by aging in an alkaline electrolyte [73e75]. The better supercapacitive performance of Ni(OH)2 as compared to NiO attributed to its loosely packed crystal structure, which allows for easy insertion/extraction of ions, and the more hydrophilic nature makes the conditions favorable for electrochemical reactions [76].

4.2.4

Cobalt Oxide

Although the resources of cobalt are very rare as compared to other metal oxides, several attempts have been taken to prepare cobalt-based active electrodes for supercapacitors. Typically, Co3O4 is used as an active electrode in supercapacitor having the AB2O4 spinel structure belonging to the cubic system. The calculated theoretical charge density for Co3O4 is approximately 3560 F g1. Co3O4 is a p-type semiconductor having low electronic and ionic conductivity, which leads to poor rate capability. But it has higher electric conductivity and corrosion resistance as compared to NiO, making it better for supercapacitor applications [77e85]. During


89

the chargeedischarge cycles, pulverization and volume expansion occur, resulting in a short cycle life. To store the electric charges the following electrochemical reactions are noticed for Co3O4 [86]: Co3 O4 þ OH þ H2 O43CoOOH þ e

(4.7)

CoOOH þ OH 4CoO2 þ H2 O þ e

(4.8)

The nanoscale Co3O4 shows very high SC as compared to their bulk counterpart. The Co3O4 nanosheet arrays on Ni foam exhibit a very high SC of 2735 F g1 prepared by the electrodeposition of Co(OH)2 and then thermally transferred to Co3O4 [87]. Fig. 4.7 shows typical CV curves of Co3O4 in the potential range of 0.2 to 0.5 V at different scan rates. The multiple peaks at different potentials correspond to the formation of a number of cobalt oxide phases with different oxidation states, which indicated the battery-like faradaic behavior of Co3O4. Cobalt hydroxide [Co(OH)2] has some properties and structure similar to those of Ni(OH)2. It has a hexagonal layered structure and is divided into two polymorphs, namely, a- and b-Co(OH)2. a-Co(OH)2 has better supercapacitive performance than b-Co(OH)2. The supercapacitive contribution occurs by the following electrochemical reaction [88e94]: CoðOHÞ2 þ OH 4CoOOHþ þ H2 O

(4.9)

However, poor rate capability, poor cycle stability, and lower operating potential limit its application at practical level.

Figure 4.7 Cyclic voltammetry (CV) of Co3O4 at different scan rates in 1 M KOH. The inset shows the linearity of anodic current density with scan rate. Reproduced from S.K. Meher, G.R. Rao, J. Phys. Chem. C 115 (2011) 15646, with the permission from American Chemical Society.

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4.2.5

Tin Oxide

Tin dioxide (SnO2) is an n-type wide-band-gap semiconductor material that has received much attention for various applications, including in lithium batteries [96,97], supercapacitors [98,99], gas sensors [100,101], and catalysis [102], because of its tunable physicochemical properties. Mainly the effective functioning of SnO2 depends on the morphologies and structural features. Therefore, in the literature, considerable efforts have been made to prepare nanostructured SnO2 with different surface morphologies such as NRs [103,104], nanowires [105,106], and nanosheets [107]. The higher electric conductivity and the nanostructured nature of SnO2 could be beneficial for supercapacitor application, as it attributes to a large surface for electrolyte cation hosting and contributes to higher SC. In comparison with other metal oxides the capacitance value of SnO2 is smaller, which is in the range of 50e400 F g1. To store the electric charges, it follows Eqs. (4.10)e(4.12). A mechanism based on the surface adsorption of electrolyte cations (Cþ) such as Kþ on SnO2 and simultaneously, the intercalation of Hþ or alkali metal cations (Cþ) in the bulk of the electrode upon reduction followed by deintercalation upon oxidation. SnO2 surface þ Cþ þ e 4 SnO2 Cþ

surface

(4.10)

SnO2 surface þ Hþ þ e 4SnO2 H

(4.11)

SnO2 surface þ Cþ þ e 4SnO2 C

(4.12)

Many researchers have made efforts to manipulate the structures and morphologies of SnO2 to improve their performances and widen applications for supercapacitors. But the SC of bare SnO2 is not high enough for use in supercapacitor applications. Therefore, to fulfill the application demand and to improve the capacitance value of the SnO2-based electrode, different strategies have been adopted, including the doping of some elements and preparation of hybrid or composite electrodes. Yan et al. [108] synthesized SnO2/MnO2 composite for the application of supercapacitors and reported the SC of 800 F g1. Fig. 4.8A shows the CV curves of the SnO2/MnO2 composites at different scan rates in 1 M Na2SO4 electrolyte, whereas Fig. 4.8B shows the CV curve of SnO2 grown on the SS substrate at a scan rate of 2 mV s1. The SnO2/RuO2 composite prepared by the Kuo and Wu [109] was reported to have a maximum SC of 930 F g1.

4.2.6

Vanadium Oxide

In the literature, vanadium oxides (VOx) have been intensely investigated as active electrode materials for supercapacitors because of the multiple stable oxidation states (IIIeV) with typical layered structure, such as the graphene shown in Fig. 4.9. These features are useful for obtaining better electrochemical parameters, as the layered structure allows for very easy and fast electron transportation. Additionally, during


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Figure 4.8 (A) Cyclic voltammetry (CV) curves of the SnO2/MnO2 composites at different scan rates in 1 M Na2SO4 aqueous solution. (B) CV curve of SnO2 grown on stainless steel (SS) substrate at a scan rate of 2 mV s1. Reproduced from J. Yan, E. Khoo, A. Sumboja, P.S. Lee, ACS Nano 4 (2010) 4247, with the permission from American Chemical Society.

Figure 4.9 Two types of layered sheet structures of V2O5: (A) the structure of orthorhombic V2O5 and (B) the layered structure of V2O5 xerogels. (A) Reproduced from A. Venkatesan, N.R.K. Chandar, A. Kandasamy, M.K. Chinnu, K.N. Marimuthu, R.M. Kumar, R. Jayavel, RSC Adv. 5 (2015) 21778, with permission from Royal Society of Chemistry. (B) Reprinted from J. Livage, Nat. Mater. 2 (2003) 297, by permission from Macmillan Publishers Ltd., Copyright 2003.

the electrochemical reactions the conducting electrode materials would give a quick response for electrochemical processes, and it also helps improve the utilization ratio of active materials because of the presence of the conducting tunnel networks in the active electrode material [110]. In case of energy storage devices such as supercapacitors, the tunable structure is the main barrier to further improvements in energy-storing capacity. As is known, the supercapacitive features are all significantly dependent on the intrinsic electric and structural properties of the material [113,114]. For vanadium, optimization of the particular structure and valence state is essential to obtain higher SC and

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electrochemical stability. Particularly, 1D VO2 does not show better supercapacitive performance because its smaller tunnel size with rutile structure creates a barrier for the insertion of protons and/or cations. This will restrict the utilization of inner material for electrochemical reactions [115]. In the literature, many attempts have been made to prepare the VOx for supercapacitor applications. Yu et al. [115] developed a novel method to achieve long cycling performance for vanadium oxideebased supercapacitor electrodes by tuning the valence state of vanadium. They applied a potential of 0.7 V to V2O3 to form the amorphous VOx thin film, which contains V3þ, V4þ, and V5þ inside the electrode. They obtained a remarkable areal capacitance of 356.8 mF cm2 (corresponding gravimetric capacitance of 637.1 F g1) at 0.5 mA cm2 and an excellent capacitance retention of 77.1%, with the current density increasing 20-fold. Fig. 4.10 shows the electrochemical features of the VOx thin film. For V2O3 a steep slope appears from about 0.5 V, which reflects a violent oxidation reaction and suggests that the V2O3 electrode can hardly fulfill the efficient chargeedischarge demands. Meanwhile, the CV profile of VOx electrode is extremely symmetric and nearly rectangular, revealing its superior charge storage ability and high efficiency. The amorphous V2O5 prepared by quenching V2O5 powder was heated at 1183 K and its supercapacitive behavior was reported in KCl aqueous electrolyte, with an SC of 350 F g1 [116]. They proposed that the Kþ ions in the KCl aqueous electrolyte were responsible for the supercapacitive redox reaction of amorphous V2O5.

4.2.7

Other Metal Oxides

Other metal oxides including Fe2O3 [117], MoO3 [118], Bi2O3 [119], In2O3 [120], CuO [121], and TiO2 [122] are also utilized for supercapacitor applications but with less success. Its low SC and cycling stability may limit their practical application. Among

Figure 4.10 Cyclic voltammetry (CV) curves of V2O5 prepared at (A) different temperatures and (B) different scanning rates. Reproduced from M. Yu, Y. Zeng, Y. Han, X. Cheng, W. Zhao, C. Liang, Y. Tong, H. Tang, X. Lu, Adv. Funct. Mater. 25 (2015) 3534; G. Wee, H.Z. Soh, Y.L. Cheah, S.G. Mhaisalkar, M. Srinivasan, J. Mater. Chem. 20 (2010) 6720, with the permission from Royal Society of Chemistry.


93

these materials, Fe2O3 (magnetite) was prominently used as a negative electrode for the supercapacitor application. Prominently, the Fe2O3-based electrode materials are used in the alkali sulfite and sulfate solutions (pH < 11), which gave a moderate value for capacitances. The previous studies of Fe2O3-based electrode indicated a charge storage mechanism that was different from that of RuO2 or MnO2. The highly ordered Fe2O3 nanotube arrays show an average SC of 138 F g1 with 89% capacitance retention after 500 cycles, and an octadecahedron Fe3O4 thin film exhibits an SC of 118.2 F g1 with 88.75% capacitance retention after 500 cycles [122,123]. The surface oxidation reactions for Fe2O3 create the insulating layer that might lower the SC. Additionally, Wang et al. [124] prepared Fe2O3 by the electrodeposition method and tested its electrochemical performance in three different electrolytes, namely, 1 M Na2SO3, Na2SO4, and KOH. The Fe2O3 electrode exhibits an SC of 170, 25, and 3 F g1 in the Na2SO3, Na2SO4, and KOH electrolytes, respectively. Higher value of capacitance is observed in the Na2SO3 electrolyte, which is explained by the author on the basis of both EDLC and pseudocapacitive mechanisms that occur for charge storage. On the other hand, in the Na2SO4 electrolyte, current is contributed only from the EDLC mechanism. However, the formation of an insulating layer on the surface of Fe2O3 electrode in KOH electrolyte showed comparatively low capacitance values. Bismuth oxide (Bi2O3) is another transition metal oxide that has been intensively studied for its unique thermoelectric transport properties, nontoxic nature, excellent chemical inertness, and biocompatibility [125,126]. Polycrystalline monoclinic thin Bi2O3 was prepared on copper substrates at room temperature by the electrodeposition method, which shows an average SC of 98 F g1 in 1 M NaOH electrolyte at the scan rate of 20 mV s1 [125]. As the scan rate increased from 20 to 100 mV s1, a moderate decrement in the capacitance value is observed from 98 to the 60 F g1. To improve the SC of Bi2O3-based electrode, Wang et al. [127] prepared the composites of activated carbon (AC) and Bi2O3. The ACeBi2O3 composite was synthesized by vacuum impregnation and roasting process. The prepared ACeBi2O3 composite shows a moderate SC of 332.6 F g1 in 6 M KOH electrolyte at current density of 1 A g1. Xu et al. [128] assembled the flexible asymmetric microsupercapacitor using Bi2O3 and MnO2 as the negative and positive electrode, respectively. Thus the optimal MnO2/Bi2O3 asymmetric device works with an operation voltage of 1.8 V, can deliver a high energy density of 43.4 m Wh cm2 (11.3 W h kg1, based on the total electrodes), and a maximum power density of 12.9 mW cm2 (3370 W kg1). Indium oxide (In2O3) is a well-known transparent conducting metal oxide that finds many applications in electroluminescent devices, solar cells, antireflection coatings, and optoelectronic devices to gas sensors. In2O3 in supercapacitor applications is rarely discussed in the literature. According to the literature, In2O3 was prepared on indium tin oxide substrates by the chemical deposition method to get two different morphologies: nanospheres (NSs) and NRs [119]. The NR In2O3 electrode shows a higher SC of 104.9 F g1 in Na2SO4 electrolyte, which is much higher than that of the NS In2O3 electrode (7.6 F g1). As the NR electrode offers a higher surface area and nanosized pores and voids, the inner and outer charge contribution increased. Prasad et al. [129] reported the maximum SC of 198 F g1 at a scan rate of the 10 mV s1 for In2O3 thin film.

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More recently, molybdenum oxideebased electrode materials have drawn much attention in supercapacitor applications because of their low cost and higher electronic conductivity from the strong MoeMo metallic bonds. Molybdenum oxideebased electrode materials especially store electric charges by the pseudocapacitance mechanism in which redox transitions occur between the Mo4þ and Mo6þ states [130]. However, owing to the complicated synthesis process, very few reports are available on the MoOx-based supercapacitors. The 1D MoO2 NRs synthesized by thermal decomposition of tetrabutylammonium hexamolybdate [((C4H9)4N)2Mo6O19] in an inert atmosphere show an SC of 140 F g1 [131]. Tang et al. [118] reported an SC of 280 F g1 in 0.5 M Li2SO4 aqueous electrolyte. Furthermore, the asymmetric supercapacitor was assembled using the MoO3 nanoplates as the anode material. They can deliver a high energy density of 45 Wh kg1 at 450 W kg1 and even maintain 29 Wh kg1 at 2 kW kg1 in 0.5 M Li2SO4 aqueous electrolyte. Mendoza-Sanchez et al. [132] prepared MoO3 nanobelts using a hydrothermal method, and thin-film electrodes were manufactured by spray deposition. Electrochemical testing is carried out in several aqueous electrolytes, with significant charge storage in 1 M H2SO4 with a complex electrochemical activity that was further investigated by X-ray photoelectron spectroscopy and various electrochemical characterization methods. In a 0e1 V (vs. Ag/AgCl) electrochemical window, MoO3 was reduced to a mixture of lowervalence oxides with concentrations varying as Mo (5þ) > Mo (4þ) > Mo (6þ) and MoO2 as the main component at potentials below 0.185 V (vs. Ag/AgCl). Also some reports were available on CuO-based material for supercapacitor applications. Dubal et al. [120] prepared the cauliflower-like CuO thin film on the stainless steel electrode by the electrodeposition method. They reported an SC of 179 F g1 in 1 M Na2SO4 electrolyte. However, the lower SC and cyclic stability of these materials make them unsuitable for supercapacitor applications. More importantly, still more efforts are necessary to improve the electrochemical performance of these metal oxideebased electrodes. In conclusion, to explore the metal oxideebased electrodes for supercapacitor applications, it is essential to carry out extensive research work to overcome some limitations such as poor conductivity, low SC, small operating potential, and low cycling stability. Making hybrid electrodes of metal oxides with some other material (such as conducting polymers and carbon-based materials) is the best way to overcome these issues. For example, the hybrid of metal oxide and carbon-based materials can effectively enhance both the conductivity and SC of the electrode. Additionally, carbonbased materials can contribute capacitance by the electric double-layer mechanism. It is also essential to optimize the proper electrolyte for each metal oxide having proper chemical interaction with the active electrode material. To use these electrodes in some advanced applications, lightweight and mechanically flexible electrodes are needed, and to control the cost of the supercapacitor device, it is necessary to select the proper electrode material and the synthesis route. Developing facile solution-based methods and printing techniques is the best solution for preparing uniform large-scale metal oxideebased electrodes. Lastly, to completely exploit the possible applicability of metal oxideebased electrode materials for supercapacitors, it is essential to optimize the experimental conditions to maintain the desired material properties. Engineering


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the surface of an active electrode, optimization of electrolytes, and selection of proper separators, current collectors, counter electrodes, and packaging materials are not well discussed or discovered in the literature; therefore, more research is needed to optimize these aspects. However, it is believed that in the future, metal oxideebased supercapacitors will be available in the consumer market for practical purposes.

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Structure and Basic Properties of Ternary Metal Oxides and Their Prospects for Application in Supercapacitors

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Rongming Wang, Jian Wu Beijing Key Laboratory for Magneto-Photoelectrical Composite and Interface Science, School of Mathematics and Physics, University of Science and Technology Beijing, Beijing, People’s Republic of China

5.1

Introduction

Recently, advanced functional materials have been extensively explored to satisfy the urgent requirement of faster and more efficient energy storage systems. Among the various electrode materials for supercapacitors, ternary metal oxides (TMOs) represented by AxByOz with two metal elements of A, B ¼ Co, Ni, Zn, Mn, Cu, Fe, Mo, etc. have attracted much attention because of their low cost, environmental friendliness, high theoretical specific capacitance, and multiple oxidation states. There are different forms of TMOs. According to the ratio of A/B/O, TMOs can be mainly categorized into three groups: AB2O4, ABO2/3/4, and A3B2O8. Generally, the crystal structures of the different types of TMOs are not the same, a difference which results in their diverse physical and chemical properties, and thus their supercapacitive performances. On the other hand, to fully utilize electrode materials and realize a high-performance supercapacitor electrode, nanoscale TMOs can also have various electrode designs, ranging from zero-dimensional (0D) to three-dimensional (3D) nanostructures, all of which have been effectively manipulated via methods such as hydrothermal, electrospinning, electrodeposition, and solegel methods. This chapter gives a brief overview of the nanostructured TMOs including AB2O4, ABO2/3/4, and A3B2O8 used as electrode materials for supercapacitors. The focus is mainly on their crystal structure, electronic property, synthetic method, and morphology and corresponding electrochemical performance.

5.2

Several Types of Ternary Metal Oxides

TMOs with two metal elements can be represented by AxByOz (A, B ¼ Co, Ni, Zn, Mn, Cu, Fe, Mo, etc.), and both A and B play significant roles in electrochemical energy storage. According to the ratio of A/B/O, TMOs can be mainly categorized into three groups: AB2O4, ABO2/3/4, and A3B2O8. Generally, the crystal structures of the Metal Oxides in Supercapacitors. http://dx.doi.org/10.1016/B978-0-12-810464-4.00005-X Copyright © 2017 Elsevier Inc. All rights reserved.

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different types of TMOs are not the same. The difference will result in their diverse physical and chemical properties, and thus will make some extensive influence on the electrochemical performance of the TMOs during the process of electrochemical energy storage. To understand the structures and basic properties of AB2O4, ABO2/ 3/4, and A3B2O8, we will look at the following schematic representation.

5.2.1

AB2O4

Spinel structured oxides, such as Co3O4 [2], Mn3O4 [3], and Fe3O4 [4], possess 3D diffusion pathways as well as numerous active sites for the redox reaction. As a type spinel structure (space group Fd3m), the primitive cell of Co3O4 has two distinct Co sites: a Co2þ ion in a tetrahedral field and a Co3þ ion in an octahedral field (Fig. 5.1) [1]. The spinel Co3O4 is a magnetic semiconductor with a GGA þ U band gap of 1.96 eV by density functional theory (DFT) calculations [5]. Besides, the electrical conductivity of Co3O4 is as low as 103 to 104 S cm1, resulting in poor supercapacitive performance with low rate capability [6]. As is well known, the Co2þ and Co3þ ions can be partially or wholly replaced by A2þ and B3þ metal ions, respectively, forming mixed-metal spinel of AB2O4. The more diversified valence between A2þ/A3þ and B2þ/B3þ may provide an enhanced electrochemical activity [7,8]. Spinel AB2O4 of TMOs as electrode materials for supercapacitors can be

Figure 5.1 (A) Face-centered cubic unit cell of Co3O4 with two nonequivalent Co ions: Co2þ with tetrahedrally coordinated oxygens (CoT) and Co3þ with octahedrally coordinated oxygens (CoO). (B) Model magnetic structure of Co3O4, where CoT [blue spheres and yellow spheres] located in the [111] plane interact antiferromagnetically. (bottom) Schematic diagram showing the expected crystal field splittings of (C) a Co2þ ion in a tetrahedral field and (D) a Co3þ ion in an octahedral field. Reproduced with permission from V. Singh, M. Kosa, K. Majhi, D.T. Major, Putting DFT to the test: a first-principles study of electronic, magnetic, and optical properties of Co3O4, J. Chem. Theory Computat. 11 (2015) 64e72. Copyright 2014 American Chemical Society.

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simply classified into three categories: spinel metal-cobalt oxides (ACo2O4, A ¼ Ni, Zn, Cu, etc.), spinel metal-ferrum oxides (AFe2O4, A ¼ Ni, Co, Mn, etc.), and spinel metal-manganese oxides (AMn2O4, A ¼ Ni, Co, Zn, etc.). Spinel metal-cobalt oxides (ACo2O4, A ¼ Ni, Zn, Cu, Mn, etc.) have been extensively investigated as supercapacitor materials because of their ultrahigh specific capacitance and rate capabilities [10]. Among the spinel cobalties, NiCo2O4 is a very promising material for supercapacitors and has captivated considerable attentions [11,12]. Note that NiCo2O4 is an inverse spinel with the nickel ions occupying the octahedral sites and half of the cobalt ions occupying the tetrahedral sites and the other half ions occupy the remaining octahedral sites (Fig. 5.2) [10,13]. Interestingly, Co2þ, Co3þ, Ni2þ, and Ni3þ can exist in the as-prepared NiCo2O4 samples, and the general formula of NiCo2O4 3þ 3þ 3þ 3þ can be expressed as Co2þ 1x Cox Co Nix Ni1x O4 (0 x 1) [14e16]. Meanwhile, three kinds of photoexcitation of electrons can be found (Fig. 5.3), in agreement with the diverse valence states of Ni and Co ions [9]. Moreover, NiCo2O4 exhibits a p-type conductivity, with its band gap estimated to be about 2.1 eV from the UVevis absorption spectra [9] and possesses much higher electronic conductivity than binary nickel oxide and cobalt oxide because of the diverse redox behavior form both cobalt and nickel [17]. Used as supercapacitor material, reversible faradaic reactions and diverse valence state changes between Ni2þ/Ni3þ, Co2þ/Co3þ, and Co3þ/Co4þ will appear on the surface of NiCo2O4. Thus in alkaline electrolyte, the redox processes are illustrated as follows [12,18,19]: NiCo2 O4 þ OH þ H2 O 4 NiOOH þ 2CoOOH þ e

(5.1)

CoOOH þ OH 4 CoO2 þ H2 O þ e

(5.2)

As shown in Fig. 5.4, one NiCo2O4 can get three electrons from the current collector, implying that the theoretical capacity of NiCo2O4 is as high as 2005 F g1,

Figure 5.2 Spinel crystal structure of NiCo2O4. From J. Wu, P. Guo, R. Mi, X. Liu, H. Zhang, J. Mei, H. Liu, W.-M. Lau, L.-M. Liu, Ultrathin NiCo2O4 nanosheets grown on three-dimensional interwoven nitrogen-doped carbon nanotubes as binder-free electrodes for high-performance supercapacitors, J. Mater. Chem. A 3 (2015) 15331e15338. http://pubs.rsc.org/en/content/articlelanding/2015/ta/c5ta02719e#!divAbstract.

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Figure 5.3 Schematic illustration of the electronic band structure of NiCo2O4. Different photoexcitations of electrons: (1) from O 2p to Co 3d-eg (or Ni 3d-eg), (2) from O 2p to Co 3d-t2g (or Ni 3d-t2g), and (3) from Co 3d-t2g to Co 3d-eg (or from Ni 3d-t2g to Ni 3d-eg). They are induced by UV and visible light radiation accordingly. Reproduced with permission from B. Cui, H. Lin, Y.Z. Liu, J.B. Li, P. Sun, X.C. Zhao, C.J. Liu, Photophysical and photocatalytic properties of core-ring structured NiCo2O4 nanoplatelets, J. Phys. Chem. C 113 (2009) 14083e14087. Copyright 2009 American Chemical Society.

Figure 5.4 A schematic of the charge storage mechanism of NiCo2O4. Reproduced with permission from Z. Wu, Y. Zhu, X. Ji, NiCo2O4-based materials for electrochemical supercapacitors, J. Mater. Chem. A 2 (2014) 14759e14772. Copyright 2014 The Royal Society of Chemistry.


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assuming a potential window of 0e0.6 V [20]. It should be noted that the spinel materials possess both double-layer capacitance and batterylike faradaic contribution, which accounts for the major contribution [12,20]. Different from NiCo2O4, ZnCo2O4 is a “normal” p-type spinel with Zn2þ ions located at the tetrahedral sites and Co3þ ions located at the octahedral sites [21]. Dekkers et al. [22] revealed that the band gap of ZnCo2O4 is 2.26 eV and its conductivity is 0.39 and 0.61 S cm1 for polycrystalline and epitaxial films, respectively. The lower conductivity of ZnCo2O4 compared to that of NiCo2O4 was also reported [21]. As shown in Fig. 5.5, Ndione et al. confirmed that densities of states and electrical conductivity of both ZnCo2O4 and NiCo2O4 are related to their cation distribution (disorder) and pointed out that the conductivity of ZnCo2O4 can be improved by increasing the degree of the inversion l. Moreover, the number of antisite spinel structure in

Figure 5.5 Calculated spin-resolved densities of states (DOSs, black curves) for normal (l ¼ 0), half-inverse (l ¼ 0.5), and P4122 ordered-inverse (l ¼ 1) spinel structure of (A) Co2ZnO4 and (B) Co2NiO4. The half-inverse and ordered-inverse structures are constructed by making one and two inversions on the normal spinel 14-atom primitive unit cell, respectively. Blue and indigo curves are the DOS averaged over the two spin channels and three random configurations with the same l values, except the middle box in (B) in which the indigo curve represents the averaged DOS for l ¼ 0.75. The arrows denote the directions in which the structures transform in terms of the l value due to annealing. Reproduced with permission from P.F. Ndione, Y. Shi, V. Stevanovic, S. Lany, A. Zakutayev, P.A. Parilla, J.D. Perkins, J.J. Berry, D.S. Ginley, M.F. Toney, Control of the electrical properties in spinel oxides by manipulating the cation disorder, Adv. Funct. Mater. 24 (2014) 610e618. Copyright 2013 WILEY-VCH Verlag GmbH %26 Co. KGaA, Weinheim.

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ZnCo2O4 would increase when the sample is sintered in air at a temperature above 500 C, becoming a disordered structure that is heavily Co nonstoichiometric [23]. Based on first-principle defect calculation, Paudel et al. [24] proved that the appeared antisite defects in AB2O4 spinels can create donors and acceptors and thus improve their electrical conductivity. In alkaline electrolyte, the redox reactions appearing in ZnCo2O4 systems are mainly related to the faradic redox of CoeO/CoeOeOH based on the following equations [25,26]: Co2 O2 4 þ 2H2 O 4 2CoOOH þ 2OH

(5.3)

CoOOH þ H2 O þ e 4 CoðOHÞ2 þ OH

(5.4)

CuCo2O4 is an inverse spinel with a band gap of 2.0 eV, calculated by diffused reflectance UVevis measurement [27]. Interestingly, when the copper content x is greater than 0.2 in the CuxCo3xO4 system, a transform from a normal to an inverse spinel could occur [28]. When compared with NiCo2O4 and ZnCo2O4, CuOH can be formed on the surface of CuCo2O4 electrode in alkaline electrolyte and takes part in faradaic reactions of the Cu2þ/Cuþ redox pair as follows [29,30]: CuCo2 O4 þ H2 O þ e 4 2CoOOH þ CuOH

(5.5)


(5.6)

CuOH þ OH 4 CuðOHÞ2 þ e

(5.7)

FeCo2O4 is also an inverse spinel with a magnetite structure. The inverse FeCo2O4 is a semiconductor with a small band gap, whereas the normal one is a half-metal [31]. So far, research on FeCo2O4 as electrode materials for supercapacitors is limited [32e34]. However, it is known that the diverse valance of Fe could also bring extra capacitance contribution, and the energy storage mechanism of FeCo2O4 may be illustrated as follows [35]: FeCo2 O4 þ H2 O þ OH 42CoOOH þ FeOOH þ e

(5.8)


(5.9)

FeOOH þ H2 O4FeðOHÞ3 4FeO2 4 þ 3e

(5.10)

MnCo2O4 is a “normal” spinel with a band gap of 2.1 eV by DFT calculations [36]. Experimental results demonstrated that the octahedral Co3þ ions are usually replaced by Mn3þ ions, a phenomenon that is related to the preparation method, the calcination temperature, and the Mn/Co ratio [37]. Because of the larger ionic radius of Mn3þ than Co3þ and the breaking symmetry of the “normal” spinel, the octahedral Mn3þ ions


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exhibit a strong JahneTeller effect and a magnetic moment of 3.85 mB. [36]. The redox reaction of MnCo2O4 is mostly described by the following equations [38,39]: MnCo2 O4 þ H2 O þ OH 42CoOOH þ MnOOH þ e

(5.11)

MnOOH þ OH 4 MnO2 þ H2 O þ e

(5.12)

Based on the abovementioned literature about redox of other spinel metalecobalt oxide in alkaline electrolyte, the faradaic reaction of CoOOH could also appear. Because of its abundance, environmental friendliness, and low cost, iron is an interesting element for substitution of Co-based ACo2O4 to form spinel metal-ferric oxide (AFe2O4, A ¼ Ni, Co, Mn, etc.). Another benefit is that the ionic radius of Fe3þ (0.79 Å) is larger than that of Co3þ (0.75 Å) [40]. The differences between Fe3þ and Co3þ could result in changes in the physical and chemical properties. For instance, like NiCo2O4, NiFe2O4 is an inverse spinel with Fe3þ ions substituted by Co3þ ions equally located at the tetrahedral and octahedral sites [41], but NiFe2O4 exhibits a larger magnetic moment [42,43] and a lower band gap [9,44] than NiCo2O4. Compared to NiFe2O4, CoFe2O4 possesses a larger unit cell (8.39 vs. 8.34 Å) and a lower band gap because of the larger ionic radius of Co2þ than Ni2þ [45]. Hou et al. demonstrated that a varying degree of the inversion l for the spinel CoFe2O4 can be obtained depending on the heat treatments. Meanwhile, the lattice parameters, magnetic properties, and electronic structures for various l had been systematically investigated [46]. Compared to NiFe2O4 and CoFe2O4, MnFe2O4 possesses a normal spinel with the lowest band gap [47]. ZnFe2O4 is also a normal spinel with a band gap of about 1.9 eV, larger than that of NiFe2O4 [48,49]. Therefore, the band gap decreases in the order: ZnFe2O4 > NiFe2O4 > CoFe2O4 > MnFe2O4 [47,49]. CuFe2O4 has tetragonal (low-temperature phase) or cubic (high-temperature phase) unit cells and is thus usually prepared in the form of an inverse spinel structure with a low indirect band gap of 1.54 eV [49,50]. Manganese, compared to cobalt, is also much cheaper, more abundant, and environmentally benign. Thus the spinel metal-manganese oxides (AMn2O4, A ¼ Ni, Co, Zn, etc.) have been attracting increasing attention as electrode materials for supercapacitors. The larger radius of Mn3þ (0.79 Å) replacing Co3þ (0.75 Å) may significantly affect the properties of the material [40]. For example, ZnMn2O4, just like ZnCo2O4, is a normal spinel structure with Zn2þ ions occupying the tetrahedral sites and Mn3þ ions replacing Co3þ ions occupying the octahedral sites [51]. However, the band gap of ZnMn2O4 (1.23 eV) is much lower than that of ZnCo2O4 [52]. Compared to ZnMn2O4, CuMn2O4 is an inverted cubic spinel and shows a larger band gap (1.40 vs. 1.23 eV) and a higher conductivity (6 105 vs. 2.5 105 U cm) [53,54]. Generally, CoMn2O4 and NiMn2O4 show a tetrahedral structure and a cubic spinel structure, respectively. Furthermore, the band gap decreases in the order: CuMn2O4 > NiMn2O4 > CoMn2O4 [55]. The calculation of the inversion energy per formula unit for the normal and inverse cation distribution structures of FeMn2O4 is similar, implying these two structures can be obtained at a similar temperature [31]. On the other hand, the inverse cation distribution of FeMn2O4 can only be synthesized above 1150 K and the system shows half-semiconductor with a negligible band gap [31].

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Figure 5.6 Rock salt structure of CoO crystal. Permission from J. Wu, W-J. Yin, W-W. Liu, P. Guo, G. Liu, X. Liu, D. Geng, W-M. Lau, H. Liu, L-M. Liu, High performance NiO nanosheets anchored on three-dimensional nitrogen-doped carbon nanotubes as a binder-free anode for lithium ion batteries, J. Mater. Chem. A 4 (2016) 10940e10947. http://pubs.rsc.org/en/content/articlelanding/2016/ta/c6ta03137d#!divAbstract.

5.2.2

ABO2/3/4

CoO is a typical NaCl-type crystal structure [56], as shown in Fig. 5.6. Compared with NiCo2O4, NiCoO2 (classified as ABO2) shows a rock salt structure with all the octahedral sites embedded with both Ni2þ and Co2þ ions and all the tetrahedral sites unoccupied. The empty tetrahedral sites in NiCoO2 form a 3D lattice, which is beneficial for ion and electron transition [57]. The lower band gap of NiCoO2 than NiCo2O4 is estimated by solid UVevis absorption spectrum [58]. Furthermore, NiCoO2 possesses higher conductivity than NiCo2O4 [59]. In alkaline electrolyte, the redox processes of NiCoO2 like that of NiCo2O4 are shown as follows [60]: NiCoO2 þ 2OH 4NiOOH þ CoOOH þ e

(5.13)


(5.14)

The family of perovskite phase ABO3 (such as BiFeO3, ZnSnO3, SrRuO3) has gained attention as the electrode materials for supercapacitors. The most stable phase of BiFeO3 is R3c, as shown in Fig. 5.7. The band gap of BiFeO3 is calculated to be 2.8 eV by the density functionalebased screened exchange method [61]. Among the various aqueous electrolytes used in supercapacitors, BiFeO3 exhibits the best performance in NaOH electrolyte than others [62]. The band gap of perovskite phase ZnSnO3 is estimated to be 3.9 eV by the absorption spectra [63], and ZnSnO3 also shows good capacitor behaviors [64]. Compared to BiFeO3 and ZnSnO3, the perovskite SrRuO3 is a conductive metallic oxide [65]. Furthermore, Sr sites and Ru sites in SrRuO3 can be doped by Mn, La, etc., and the doped SrRuO3 exhibits higher capacitances [66]. ABO4 (A ¼ Ni, Co, Mn, etc., B ¼ Mo, W, etc.) is a new class of TMOs and has been investigated as supercapacitor electrode materials. Although the Mo element


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Figure 5.7 Structure of BiFeO3 with the R3c phase: (A) rhombohedral cell and (B) pseudocubic cell. Oxygen, red; Bi, green; and Fe, blue. Reproduced with permission from S.J. Clark, J. Robertson, Band gap and Schottky barrier heights of multiferroic BiFeO3, Appl. Phys. Lett. 90 (2007) 132903e132905. Copyright 2014 The Royal Society of Chemistry.

has no contribution to the measured capacitance of AMoO4, it can improve the conductivity and the electrochemical performances [68,69]. On the other hand, the reversible redox reaction of AMoO4 is based on the diverse valence state change of A ions, mediated by OH ions in the alkaline electrolyte, such as Ni2þ/Ni3þ in NiMoO4 [70], Co2þ/ Co3þ/Co4þ in CoMoO4, and Mn2þ/Mn3þ/Mn4þ in MnMoO4 [68]. As a representative structure of ABO4, NiMoO4 has two phases: a low-temperature phase of a-NiMoO4 and a high-temperature phase of b-NiMoO4 [71,72]. Both phases are monoclinic, as shown in Fig. 5.8. When Mo ions occupy the octahedral sites, NiMoO4 exhibits

Figure 5.8 Crystal structure of NiMoO4. Reproduced with permission from S. Peng, L. Li, H.B. Wu, S. Madhavi, X.W.D. Lou. Controlled growth of NiMoO4 nanosheet and nanorod arrays on various conductive substrates as advanced electrodes for asymmetric supercapacitors, Adv. Energy Mater. 5 (2015) 1401172e1401178. Copyright 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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a phase. However, if the tetrahedral sites are embedded by Mo ions, b-NiMoO4 will be obtained [73]. The band gap decreases in the order: NiMoO4 > CoMoO4 > MnMoO4 [74,75], and their capacitances exhibit the same order [76]. However, the unordered relationship is also reported and the electrochemical performances depend on the morphology and the synthesis methods [77]. Similar to AMoO4, W ions replacing Mo ions with the formula AWO4 have also been investigated for supercapacitors, and scheelite (a-phase) and wolframite (b-phase) can be formed related to the calcination temperature and the synthesis methods [78]. It was reported that NiWO4 exhibits an enhanced conductivity than NiMoO4 [79]. Compared to CoWO4, NiWO4 possesses superior electrochemical performance, and the CoWO4@NiWO4 nanocomposites show the best supercapacitive performance compared to each of CoWO4 and NiWO4 [80]. The band gap of AWO4 decreases in the order: ZnWO4 > NiWO4 > CoWO4 > CuWO4 [81]. SnWO4 is also reported for the supercapacitor study [78], and the band gaps of a-SnWO4 and b-SnWO4 is 1.64 and 2.68 eV, respectively [82].

5.2.3

A3B2O8

A3B2O8 (A ¼ Ni, Co, etc., B ¼ V) is a Kagomé staircase compound with the orthorhombic space group Cmca [84]. The typical crystal structure of Ni3V2O8 is that Ni ions occupy the octahedral field and V ions occupy the tetrahedral field, as shown in Fig. 5.9. Both Co3V2O8 and Ni3V2O8 exhibit similar crystal structure and insulating character with small band gap but demonstrate different orbital correlation effects and energy magnetodielectric contrast [85]. A3B2O8 possesses a high capacitance because of its high A/B ratio, in which the element A plays a dominate capacitance

Figure 5.9 Projection of the structure of Ni3V2O8 onto (A) the bec plane and (B) the aec plane. A theoretical morphology of Ni3V2O8 established on the basis of its structure is seen. Reproduced with permission from Z. He, J.I. Yamaura, Y. Ueda, Morphologies of Ni3V2O8 single crystals, Cryst. Growth Des. 8 (2008) 799e801. Copyright 2008 American Chemical Society.


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contribution [86]. Liu et al. [86] reported that Co3V2O8 shows higher rate capability and longer cycling stability than Ni3V2O8, while displaying lower specific capacitance. In addition, the hybrid Co3V2O8/Ni3V2O8 composite possesses the best electrochemical performance than each of Co3V2O8 and Ni3V2O8. Similar to ABO4, the B element of A3B2O8 does not participate in the redox reaction. Therefore, in alkaline electrolyte, the redox processes of Co3V2O8 is shown as follows [87]: Co2þ þ 3OH 4 CoOOH þ H2 O þ e

(5.15)


(5.16)

5.3

Synthesis Routes

Nanostructured TMOs have been proven to exhibit excellent electrochemical performance because of their decreased size and unique shape. Up to now, lots of methods (such as hydrothermal method, electrospinning, electrodeposition, and solegel method) have been developed to prepare nanoscale TMOs for supercapacitor applications. Different synthetic approaches have their own advantages and disadvantages, and the characteristics of the major methods are described in the following.

5.3.1

Hydrothermal/Solvothermal Method

Hydrothermal procedure is one of the most popular procedures to prepare TMO nanostructures with controllable specific size and morphology. Hydrothermal method is conducted in a sealed container using water-soluble metal precursors and the reaction temperature is about 100 C. Meanwhile, a high pressure will self-develop and is related to the reaction temperature, the percentage of the liquid filled, and any dissolved salts. If the water is replaced by an organic solvent, the method is called solvothermal synthesis. A serious of TMOs with various structures have been synthesized, such as CuCo2O4 nanobelts [88], ultrathin mesoporous NiMoO4 nanosheets [89], and Ni3(VO4)2&NiO nanohybrid with two-dimensional (2D) nanoflakes on Ni foam [90] based on the hydrothermal method and 3D-network-like mesoporous NiCo2O4 nanosheets [91] and MnFe2O4/carbon black/polyaniline [92] based on the solvothermal method.

5.3.2

Chemical Precipitation Method

Chemical precipitation is a facile method and suitable for large-scale production of power samples. Two conditions should be satisfied when chemical precipitation occurs: the concentration of one solid is over the solubility limit and the temperature is high enough to prompt segregation into precipitates. However, the prepared morphology is difficult to control precisely because of its fast precipitation. When this method is

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used, TMOs with various morphologies for supercapacitors were reported, such as FeVO4 nanoparticles [93], Ni2P/Co3V2O8 nanocomposite [94], and NiCo2O4 nanosheets@halloysite nanotubes [95].

5.3.3

Electrodeposition Method

The electrodeposition method is widely adopted to prepare nanostructured TMOs [such as MneNi oxide thin film [96], NiCo2O4 nanosheets on Ni foam/nitrogendoped carbon nanotube (N-CNT) substrate [97], and NiCo2O4@CoxNi1x(OH)2 coreeshell nanosheets [98]]. Nanostructured films with different mass loading and morphologies can be effectively and easily controlled by the careful choice of deposition solution and conductions. The process is involved with electron transfer and phase change and can be classified into two types: anodic electrodeposition and cathodic electrodeposition based on the reaction mechanism.

5.3.4

SoleGel Method

The mechanism of solegel technique is a method in which microparticles/molecules will be agglomerated and linked together in a solution (sols) to form a coherent network (gel) of nanostructures with high purity, homogeneity, and porosity. The method is appropriate for large-scale commercial production because of its simplicity, low cost, and high yield, but the synthetic process is relatively complex and time consuming. Thus, TMOs (such as NiCo2O4 [17], MnCo2O4 [99]) synthesized using the solegel method have been rarely reported.

5.3.5

Microwave Synthesis Method

Compared to conventional heating routes, microwave synthesis is beneficial for its higher reaction rates, shorter reaction time, better yields, and enhanced product purity because of its high penetration and concentrated power. Therefore, TMOs with good size distribution can be obtained (such as SnWO4 [78], NiCo2O4/NiO [100] composite).

5.3.6

Electrospinning Synthesis Method

Electrospinning has been widely employed to prepare one-dimensional (1D) nanostructures (nanofibers and nanotubes). The as-obtained 1D samples or their assembled membranes can be further used as a substrate for the hybrid of electrode materials [such as porous ZnCo2O4 nanotubes [101], NiCo2O4-doped carbon nanofiber (CNF) @MnO2 nanosheets, and nanorod hybrid membranes [102]]. The diameters of the 1D nanostructure can be controlled via the precise management of the metal precursor concentrations, the viscosity of the precursor solution, the electrospinning parameters, and so on.


5.3.7

111

Other Synthesis Methods

Several other synthesis methods were reported to prepare TMOs as supercapacitor materials, such as molten salt method (MSM) for MgCo2O4, MnCo2O4, and CuCo2O4 [103]; hot injection synthesis of Co3xMnxO4 nanoparticles [104]; solution combustion method for MnFe2O4 particles [105]; and microemulsion technique for NiCo2O4 nanowires [106].

5.4

Nanostructures

A high-performance supercapacitor electrode should possess high specific capacitance, large rate capability, and a long cycle life, the characteristics which are mainly determined by the surface area, electronic and ionic conductivity, and the mechanical and chemical stability of the electrode, etc. Therefore, to fully utilize the active materials and realize a high-performance supercapacitor electrode, various electrode designs have been effectively manipulated. In the following, the TMO electrodes from 0D to 3D nanostructures used in supercapacitors are presented.

5.4.1

Zero Dimension

TMOs with 0D nanostructures including solid and hollow nanoparticles benefit from their small size and have been widely investigated as electrode materials in supercapacitors. As shown in Fig. 5.10, Perera et al. synthesized colloidal Co3xMnxO4 nanoparticles with different Co/Mn concentration ratios by a scalable hot-injection reaction, in which the nanoparticles were further assembled into the carbon- and additive-free nanoparticle electrodes by electrophoretic deposition (EPD) [104]. A two-electrode system was employed to study the supercapacitive performance, and the Co3xMnxO4 (a 1 : 1 ratio of Co to Mn) nanoparticles exhibit the best electrochemical properties with a specific capacitance of 173.6 F g1, an energy density of 26.6 Wh kg1, and a power density of 3.8 kW kg1. Krishnan et al. [103] prepared MCo2O4 (M ¼ Mg, Mn, Cu) powers via a MSM and found that MgCo2O4 electrode displays the highest specific capacitance. Hollow nanostructures benefit from high surface areas and open spaces, and thus possess numerous active sites, which help the structure to endure volume change and reduce diffusion pathways. Wang et al. reported a simple “ion adsorptioneannealing” method for the preparation of uniform porous ZnCo2O4, NiFe2O4 and ZnSnO3 hollow spheres [64]. The size and shell thickness of the hollow spheres could be easily controlled by the carbohydrate sphere templates and the solution concentration. These TMOs hollow spheres, especially ZnCo2O4 sample, exhibit remarkable supercapacitive properties because of their ultrathin shells and high porosity. Furthermore, it is evidenced that double-shelled NiCo2O4 hollow spheres benefit from higher surface area and specific capacitance compared to single-shelled NiCo2O4 (115.2 vs. 76.6 m2 g1) and (718 vs. 445 F g1 at 1 A g1), respectively [108]. Lou et al. [107] developed an efficient

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Figure 5.10 (A) Schematic diagram showing the synthesis of CoeMn nanoparticles and the preparation of carbon- and additive-free electrodes using the electrophoretic deposition (EPD) technique. (B) Scanning electron microscopic image of a Co3xMnxO4 (x ¼ 1.49) electrode assembled through EPD. Inset shows clusters of nanoparticles. (C) Transmission electron microscopic image of nanoparticles in the Co3xMnxO4 (x ¼ 1.49) sample. DPE, diphenyl ether; TOPO, trioctylphosphine oxide. Reproduced with permission from S.D. Perera, X. Ding, A. Bhargava, R. Hovden, A. Nelson, L.F. Kourkoutis, R.D. Robinson, Enhanced supercapacitor performance for equal CoeMn stoichiometry in colloidal Co3xMnxO4 nanoparticles, in additive-free electrodes, Chem. Mater. 27 (2015) 7861e7873. Copyright 2015 American Chemical Society.

self-templated strategy for the synthesis of NiCo2O4 hollow spheres with a core-indouble-shell interior structure (Fig. 5.11). Employed as electrode materials for supercapacitors, the hollow NiCo2O4 demonstrates a high capacitance of 1141 F g1 at a current density of 1 A g1 and excellent cycling stability with only 5.3% loss after 4000 cycles at a high current density of 5 A g1 [107]. In addition, Li et al. [109] reported the preparation of multishelled NieCoeO hollow microspheres with various Ni/Co ratios and controlled their shell porosity, numbers, and thickness by a new and flexible strategy. The triple-shelled NieCo1.5eO electrode displays a remarkable capacitance (1884 F g1 at 3 A g1) and excellent rate capability (77.7% capacitance retention from 3 to 30 A g1). Moreover, an asymmetric supercapacitor (ASC) was constructed by using the NieCo1.5eO as the positive electrode and RGO (reduced graphene oxide)@Fe3O4 as the negative electrode. The ASC device delivers high energy density of 41.5 Wh kg1 at 505 W kg1 and high cycling stability of 79.4% retention of its initial capacitance after 10,000 cycles.


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Figure 5.11 (A) Schematic illustration of the formation process of NiCo2O4 core-in-doubleshell hollow spheres. Typical field-emission scanning electron microscopic (B, D) and transmission electron microscopic (C, E) images of the NiCoeglycolate precursor spheres (B, C) and NiCo2O4 core-in-double-shell hollow spheres (D, E). Reproduced with permission from L. Shen, L. Yu, X.Y. Yu, X. Zhang, X.W. Lou, Selftemplated formation of uniform NiCo2O4 hollow spheres with complex interior structures for lithium-ion batteries and supercapacitors, Angew. Chem. Int. Ed. 54 (2015) 1868e1872. Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

5.4.2

One Dimension

The longitudinal axes of 1D nanostructures (such as nanotubes, nanowires, nanobelts) not only possess the advantages of 0D nanostructure but also can offer an efficient transport path for electrons and ions [110]. Zhou and coworkers [101] have successfully prepared 1D ZnCo2O4 porous nanotubes (PNTs) via a facile electrospinning method followed by calcination in air (Fig. 5.12A). The scanning electron microscopic (SEM) images (Fig. 5.12B and C) show that the homogeneous ZnCo2O4 nanofibers display rough surfaces composed of abundant nanoparticles, which can be further evidenced from the transmission electron microscopic (TEM) image (Fig. 5.12C).

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Figure 5.12 (A) Schematic representation for the preparation of the ZnCo2O4 porous nanotubes (PNTs) by electrospinning. (B, C) scanning electron microscopic images of ZnCo2O4 nanotubes after annealing. (D) Transmission electron microscopic and high-resolution transmission electron microscopic images of the ZnCo2O4 PNTs. (E) Cycling performance of ZnCo2O4 PNT electrode [red spots] and ZnCo2O4 nanoparticle electrode (black spots) at a current density of 10 A g1. (F) Cycling performance of ZnCo2O4 PNT electrode [blue spots] and ZnCo2O4 nanoparticle electrode [red spots] at gradual increasing current densities. Reproduced with permission from G. Zhou, J. Zhu, Y. Chen, L. Mei, X. Duan, G. Zhang, L. Chen, T. Wang, B. Lu, Simple method for the preparation of highly porous ZnCo2O4 nanotubes with enhanced electrochemical property for supercapacitor, Electrochim. Acta 123 (2014) 450e455. Copyright 2014 Elsevier Ltd. All rights reserved.

Compared with the ZnCo2O4 nanoparticles, the as-prepared ZnCo2O4 PNTs exhibit improved supercapacitive performance with a specific capacitance of 770 F g1 at 10 A g1, high rate property (84% capacitance retention at 60 A g1), and excellent cycling stability (10.5% loss of its initial capacitance after 3000 cycles) (Fig. 5.12E and F). In addition, 1D hierarchical CuCo2O4 nanobelts (length: 500 nme1 mm;


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width: 18e40 nm; thickness: 20 nm) were prepared by Ryu’s group via a hydrothermal method [88]. The as-prepared CuCo2O4 nanobelts present areal capacitance of 2.42 F cm2 or specific capacitance of 809 F g1 at a current density of 2 mA cm2. An et al. reported porous NiCo2O4 nanowires by a simple microemulsion technique followed by a thermal treatment. The nanowires thus produced possess a large surface area, pore volume, and pore size [106]. Employed for supercapacitor applications, the NiCo2O4 nanowire electrode demonstrates a high specific capacitance of 1197 F g1 at 1 A g1 and superior cycling stability with 91.4% retention of its highest value after 2000 cycles because of its well-aligned 1D nanowire microstructure. With the assistance of various 1D substrates [such as CNFs, carbon nanotubes (CNTs), polymeric nanotubes], 1D TMOs can be easily prepared. For instance, multiwalled carbon nanotubes (MWCNTs) are excellent host substrates for electrode material coating because of their high aspect ratio, good conducting path for charge transfer, and large activated surface area. As shown in Fig. 5.13A, the surface of acid-treated CNTs possesses numerous negatively charged functional groups, including hydroxyl and carboxyl groups, which are liable to bond with electrode materials. TEM images (Fig. 5.13B and C) demonstrate that NiCo2O4 nanoparticles are uniformly anchored onto MWCNTs and the thickness of the coated layer is about 5 nm. The supercapacitive performance of the NiCo2O4@CNT electrode is superior than that of the neat NiCo2O4 or NiCo2O4/CNT physical mixture system [111,112]. 1D polymeric nanotubes were also prepared by cationic polymerization of divinylbenzene and directly employed as hard templates for the growth of NiCoO2 nanosheet nanotubes (Fig. 5.13D). After the templates were removed, hollow internal structure with 2e4 nm thickness of external NiCoO2 nanosheets was synthesized, as shown in Fig. 5.13E and F. Among nanotubes decorated with NiCoO2, NiO, and Co3O4 nanosheets and NiCo2O4 flakes (Fig. 5.13G), the unique NiCoO2 nanosheetedecorated nanotubes display the best supercapacitive performance with a high specific capacitance of 1468 F g1 at a current density of 2 A g1 and an excellent cycling stability of 99.2% retention of its highest value after 3000 cycles at 10 A g1. Niu et al. [114] developed a simple hydrothermal method followed by heat treatment for the synthesis of ZnCo2O4 nanoneedles on porous carbon nanofibers (PCFs) forming coreeshell heterostructures. The as-prepared PCF@ZnCo2O4 electrode exhibits a high capacitance of 1384 F g1 at 2 mV s1. In addition, an all-solid-state ASC is fabricated by using the PCF@ZnCo2O4 as the positive electrode and PCFs directly as the negative electrode. The assembled ASC device displays a high energy density of 49.5 Wh kg1 at a power density of 222.7 W kg1 and outstanding cycling stability of 90% capacitance retention at 50 mV s1 over 3000 cycles. The abovementioned 1D TMOs cannot be directly used as supercapacitor electrodes, and utilization of polymer binder and conductive additives is inevitable when configured as the working electrode, the manufactured electrode, which would result in capacitance loss and poor cycling performance. Therefore, the rational design of the TMOs with 1D nanostructure directly on substrates [Ni foam, carbon cloth (CC), stainless steel mesh, etc.] has attracted extensive attention. Such binder-free electrodes not only can increase the active surface area but also can enhance the electronic conductivity and the ion diffusion. Ni foam is the most used as conductive substrate to

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(A)

(B)

(C)

(D)

(E)

(G)

(F)

Figure 5.13 (A) Schematic illustration of the formation of strong bonds between nickel cobaltite nanoparticles and functionalized carbon nanotubes (CNTs) after hydrothermal treatment and annealing. (B, C) Transmission electron microscopic (TEM) images of NiCo2O4-anchored multiwalled carbon nanotubes, whereas the inset in (C) shows the selected area electron diffraction pattern. (D) Schematic illustration of the synthetic procedure of NiCoO2 nanosheet nanotubes. (E, F) TEM images of hierarchical NiCoO2 nanosheet nanotubes. (G) Average specific capacitance versus cycle number of NiCoO2, NiO, and Co3O4 nanosheet nanotubes and NiCo2O4 flakes at a current density of 10 A g1. (A) Reproduced with permission from S. Abouali, M. Akbari Garakani, Z.L. Xu, J.K. Kim, NiCo2O4/CNT nanocomposites as bi-functional electrodes for Li ion batteries and supercapacitors, Carbon 102 (2016) 262e272. Copyright 2016 Elsevier Ltd. All rights reserved. (B, C) Reproduced with permission from I. Shakir, High performance flexible pseudocapacitor based on nano-architectured spinel nickel cobaltite anchored multiwall carbon nanotubes, Electrochim. Acta 132 (2014) 490e495. Copyright 2014 Elsevier Ltd. All rights reserved. (DeG) Reproduced with permission from X. Xu, H. Zhou, S. Ding, J. Li, B. Li, D. Yu, The facile synthesis of hierarchical NiCoO2 nanotubes comprised ultrathin nanosheets for supercapacitors, J. Power Sour. 267 (2014) 641e647. Copyright 2014 Elsevier B.V. All rights reserved.


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grow TMOs. For example, Chen and Shen’s group developed a polyol refluxing method followed by a heat treatment for the preparation of ZnCo2O4 nanorods on Ni foam [25]. The ZnCo2O4/Ni foam exhibits a high specific capacitance of 1400 F g1 at 1 A g1, outstanding rate capability of 72.5% capacity retention at 20 A g1, and excellent cycling stability of 97% capacity retention after 1000 cycles at 6 A g1. In addition, they also prepared NiCo2O4 nanowire arrays on various kinds of flexible substrates [such as Ni foam, Ti foil, CC, and polytetrafluorethylene (PET) tape] through a facile hydrothermal process with successive calcination in air [115]. The nanowire arrays on Ni foam show ultrahigh specific capacitances of 2681, 2524, and 2305 F g1 at 2, 3, and 8 A g1, respectively. Furthermore, when assembled as all-solid-state symmetric supercapacitors, the sample displays outstanding electrochemical performance with areal capacitance of 161 mF cm2 at 1 mA cm2 and excellent cycling stability over 3000 cycles at 3 mA cm2 even under twisted and bent conditions. As shown in Fig. 5.14, Wang et al. [70] synthesized NiMoO4 nanowires on CC, and the as-prepared electrode yields high specific capacitances

Figure 5.14 (A) Optical images of carbon cloth substrate and NiMoO4 nanowires on carbon cloth. (B) Scanning electron microscopic (SEM) image of the bare carbon cloth. (C, D) Lowand high-magnification SEM images of NiMoO4 nanowires on the carbon cloth. (E) Schematic illustration. (FeH) Electrochemical characterizations of NiMoO4 nanowires. Reproduced with permission from D. Guo, Y. Luo, X. Yu, Q.Li, T. Wang, High performance NiMoO4 nanowires supported on carbon cloth as advanced electrodes for symmetric supercapacitors, Nano Energy 8 (2014) 174e182. Copyright 2014 Elsevier Ltd. All rights reserved.

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of 1.27 F cm2 (1587 F g1) and 0.76 F cm2 (951 F g1) at 5 mA cm2 and 30 mA cm2, respectively. The voltage of the fabricated ASC device is as high as 1.7 V, and the device delivers both high energy density of 70.7 Wh kg1 and power density of 16,000 W kg1 at 14.1 Wh kg1. These results demonstrate that the binder-free 1D TMO architectures could be a good choice for the next-generation high-performance supercapacitors.

5.4.3

Two Dimension

TMOs with 2D nanostructures, especially those with ultrathin nanosheet morphology and interconnected network structure, possess superior electrochemical cycling performance when compared to 1D nanostructures because of their faster ion/electron transport and better accommodation of volume variation [116e118]. For example, using different solvents, the growth of NiMoO4 nanosheets or nanorods on different conductive substrates could be controlled via a facile hydrothermal approach coupled with heat treatment [67]. The NiMoO4 nanosheetebased electrodes show more outstanding supercapacitor performance than NiMoO4 nanorodebased electrodes. In addition, Lou and Zhang [119] synthesized NiCo2O4 nanosheets or nanorods on CNFs and also found that the CNF@NiCo2O4 nanosheets display higher capacitance and better cycling stability than CNF@NiCo2O4 nanorods. Therefore, TMOs with 2D nanosheet nanostructure grown on various substrates have been extensively prepared and studied for supercapacitor application. Hui and coworkers [122] used graphite paper as a substrate to grow CuCo2O4 nanosheets via a hydrothermal method with postannealing treatment. The electrode delivers specific capacitances of 1131 F g1 at 1 A g1 and 409 F g1 at 50 A g1 and a longterm cycling stability of 79.7% capacitance retention over 5000 cycles at 10 A g1. Cross-linked ZnFe2O4 nanoflakes were grown on a flexible stainless steel mesh (FSSM) substrate by the chemical bath deposition method [123]. The effect of mesh size on the morphology and the corresponding supercapacitive properties were also investigated. Among the different 200, 250, and 300 mesh sizes, the ZnFe2O4/ FSSM-300 electrode shows the best supercapacitive performance, with a high specific capacitance of 1625 F g1 at 1 mA cm2 and good cycling stability of 97% capacitance retention up to 8000 cycles. Hu et al. [77] synthesized MnMoO4$4H2O nanoplates on Ni foam by a simple hydrothermal method, and the as-prepared sample displays outstanding electrochemical performance with a high specific capacitance of 1.15 F cm2 (2300 F g1) at 4 mA cm2 and excellent cycling ability with 92% of its initial specific capacitance over 3000 cycles. Besides TMOs grown on the bare 2D nanosheets on various substrates, the TMOs based on the 2D nanosheets with coreeshell structures have also been widely explored for supercapacitors. For example, Zhang et al. [90] reported the fabrication of the Ni3(VO4)2&NiO nanohybrid with 2D nanoflakes on Ni foam by a three-step process. Remarkably, the as-obtained electrode shows high specific capacitances of 2068 and 1540 F g1 at 0.5 and 8 A g1, respectively. As shown in Fig. 5.15A and B, Tu et al. [120] carried out a hydrothermal method combined with chemical bath

Figure 5.15 Scanning electron microscopic (SEM) images of (A) the bare NiCo2O4 nanoflake array and (B) the NiCo2O4@NiCo2O4 coreeshell nanoflake array grown on Ni foam substrate. SEM images of (C, D) NiCo2O4 nanosheets on carbon cloth (CC) substrate. (E) Cyclic voltammetry curves obtained from NiCo2O4/CC/porous graphene paper (PGP) asymmetric supercapacitor with different cell voltages of 1, 1.2, 1.4, 1.6, and 1.8 V at a scan rate of 50 mV s1 (inset is the illustration of the assembled asymmetric supercapacitor). (F) Chargeedischarge curves for asymmetric supercapacitor at different current densities. (G) Long-term cycling performance of NiCo2O4/CC//PGP asymmetric supercapacitor under normal, twisted, and bent states with a constant current density of 100 mA cm2 over 5000 cycles. The inset shows a red light-emitting diode lighted by two asymmetric supercapacitors connected in series and the chargeedischarge curve during the cyclic test. (A, B) Reproduced with permission from X. Liu, S. Shi, Q. Xiong, L. Li, Y. Zhang, H. Tang, C. Gu, X. Wang, J. Tu, Hierarchical NiCo2O4@NiCo2O4 core/shell nanoflake arrays as high-performance supercapacitor materials, ACS Appl. Mater. Inter. 5 (2013) 8790e8795. Copyright 2013 American Chemical Society. (CeG) Reproduced with permission from Z. Gao, W. Yang, J. Wang, N. Song, X. Li, Flexible all-solid-state hierarchical NiCo2O4/porous graphene paper asymmetric supercapacitors with an exceptional combination of electrochemical properties, Nano Energy 13 (2015) 306e317. Copyright 2015 Elsevier Ltd. All rights reserved.

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deposition to fabricate hierarchical NiCo2O4@NiCo2O4 coreeshell nanoflakes on Ni foam, and the as-prepared electrode demonstrates better capacitive behaviors than bare NiCo2O4 nanoflakes. The coreeshell electrode displays a maximum areal specific capacitance of 2.20 F cm2 at 5 mA cm2 and excellent cycling stability (98.6% capacitance retention up to 4000 cycles at 5 mA cm2). In addition, Wang et al. [124] prepared CoMoO4 nanoplates on NiCo2O4 nanowires by a simple two-step hydrothermal method followed by an annealing treatment, and the hybrid electrode exhibits several times larger areal capacitances than the pristine NiCo2O4 nanowire electrode. As shown in Fig. 5.15C and D, Li et al. [121] prepared hierarchical NiCo2O4 nanostructures with vertical NiCo2O4 nanoarrays on flexible CC and then the surface of the as-obtained samples were further covered with airy organdy NiCo2O4 nanosheets via a simple template-free approach. Flexible all-solid-state ASC was assembled with NiCo2O4/ CC as the positive electrode, the porous graphene papers (PGP) as the negative electrode, and the polyvinyl alcohol (PVA)eLiOH gel as both the solid-state electrolyte and separator. The working potential can reach as high as 1.8 V (Fig. 5.15E and F). The as-synthesized electrode displays a maximum energy of 60.9 Wh kg1, maximum power density of 11.36 kW kg1, and excellent cycling stability of 96.8% capacitance retention after 5000 cycles even under mechanical bending. These results indicate that the TMOs based on the 2D nanosheets with coreeshell structures could have potential for the high-performance supercapacitors.

5.4.4

Three Dimension

TMOs with 3D porous structures have gained considerable attention as supercapacitors because of their large surface area, high density of defects, and well-defined pathways to electrolyte access [110,125,126]. Generally, 3D TMO electrodes are fabricated by using 3D substrates as templates or TMO materials self-assembled into 3D nanostructures. As shown in Fig. 5.16A and B, 3D flowerlike NiCo2O4 architectures assembled by nanosheets radically were grown from the center via a simple solvothermal approach with a postannealing treatment [127]. Both the polyvinylpyrrolidone (PVP) soft templates and the formation of metal glycolate have significant effects on the formation of the 3D nanoflower hierarchitectures. The as-prepared 3D NiCo2O4 nanoflowers exhibit a high specific capacitance of 1191.2 F g1 at 1 A g1. Chen et al. [128] demonstrated a facile synthesized porous NiCo2O4 flowerlike nanostructure without templates (Fig. 5.16C and D). Compared to the aforementioned NiCo2O4 nanoflowers, the NiCo2O4 sample is composed of numerous lamellar nanodiscs with thinner nanosheets and a smoother surface. The sample shows excellent supercapacitor performance and especially ultralong cycling stability (no capacitance loss even after 10,000 cycles). Zhang et al. [129] presented a 3D flowerlike NiCo2O4 architecture with petals assembled by numerous nanorods, the diameter of which is about 100 nm in average (Fig. 5.16E and F). Remarkably, the as-obtained NiCo2O4 presents a high specific capacitance of 1619.1 F g1 at 2 A g1. Meanwhile, 3D NiCo2O4


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Figure 5.16 Low- and high-magnification scanning electron microscopic images of NiCo2O4 with different morphologies. (A, B) Reproduced with permission from C. An, Y. Wang, Y. Huang, Y. Xu, C. Xu, L. Jiao, H. Yuan, Novel three-dimensional NiCo2O4 hierarchitectures: solvothermal synthesis and electrochemical properties. CrystEngComm 16 (2014) 385e392. Copyright The Royal Society of Chemistry 2014, (C, D) Reproduced with permission from H. Chen, J. Jiang, L. Zhang, T. Qi, D. Xia, H. Wan, Facilely synthesized porous NiCo2O4 flowerlike nanostructure for high-rate supercapacitors, J. Power Sour. 248 (2014) 28e36. Copyright 2013 Elsevier B.V. All rights reserved. (E, F) Reproduced with permission from Y. Zhang, M. Ma, J. Yang, H. Su, W. Huang, X. Dong, Selective synthesis of hierarchical mesoporous spinel NiCo2O4 for high-performance supercapacitors, Nanoscale 6 (2014) 4303e4308. Copyright The Royal Society of Chemistry 2014, and (G, H) Reproduced with permission from R. Zou, K. Xu, T. Wang, G. He, Q. Liu, X. Liu, Z. Zhang, J. Hu. Chain-like NiCo2O4 nanowires with different exposed reactive planes for high-performance supercapacitors, J. Mater. Chem. A 1 (2013) 8560e8566. Copyright The Royal Society of Chemistry 2013.

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microspheres assembled by radial chainlike nanowires with different exposed reactive planes have been successfully synthesized by Hu’s group (Fig. 5.16G and H) [130]. The nanowires are long and thin and possess high aspect ratios with diameters ranging from 5 to 15 nm and lengths of up to several micrometers. In situ electrical properties reveal that the as-prepared chainlike nanowires exhibit an enhanced electronic conductivity, which makes the NiCo2O4 nanowires beneficial for supercapacitor application. In addition, a new starfish-shaped porous Co3O4/ZnFe2O4 hollow composite was synthesized by the assistance of metal-organic frameworks (MOFs) as precursors and sacrificial templates, resulting in materials that exhibit potential applications in both supercapacitors and magnetic devices [131]. On the other hand, TMO materials are widely grown on 3D substrates forming 3D electrodes, in which the substrates not only serve as stable skeletons to improve the specific surface area of the TMOs but also enhance their conductivity. Ye et al. [132] reported a facile approach for the synthesis of MnFe2O4/graphene hybrid inks onto flexible graphite sheets followed by drying under an infrared lamp. When assembled into all-solid-state supercapacitors, the MnFe2O4/graphene hybrid exhibits a high specific capacitance of 120 F g1 at 0.1 A g1 and an outstanding cycling stability of 105% capacitance retention after 5000 cycles. Freestanding 3D graphene foam was fabricated with the sacrifice of the template of Ni foam by Xu’s group (Fig. 5.17AeD) [125]. The 3D graphene films were first prepared on 3D Ni foam via chemical vapor deposition forming Ni foamegraphene hybrids. Then the Ni substrate was etched away with HCl and the freestanding 3D graphene foam was obtained. Finally, honeycomb-like CoMoO4 nanosheets were grown on the 3D graphene by a simple hydrothermal approach. The CoMoO4/3D graphene honeycomb-like electrode shows ultrahigh specific capacitances of 2741 F g1 and 1101 F g1 at 1.43 A g1 and 85.71 A g1, respectively. Even over 100,000 cycles, at an ultrahigh current density of 400 A g1, the hybrid electrode could maintain 96.36% of its initial specific capacitance. Moreover, an ASC device, constructed by using the CoMoO4/3D graphene as the positive electrode and active carbon as the negative electrode, delivers superior supercapacitor performance. As shown in Fig. 5.17EeG, dense and entangled N-CNTs were grown on Ni foam forming a 3D mesh network [97]. The introduction of nitrogen into CNTs results in the surface being a bamboolike structure because of the shorter length of the CeN bond than the CeC bond. The N-CNTs possess enhanced surface reactivity, strong mechanical strength, and high electrical conductivity. Ultrathin NiCo2O4 nanosheets were further grown on the skeleton of the 3D Ni foam/N-CNT substrate forming 3D Ni foam/N-CNT/NiCo2O4 electrode, which displays a high specific capacitance of 1472 F g1 at 1 A g1 and excellent cycling stability of less than 1% capacitance loss after 3000 cycles at 10 A g1. The attractive supercapacitor performance of the as-prepared electrode could be attributed to the 3D Ni foam/N-CNT substrate and the ultrathin and interconnected NiCo2O4 nanosheets.


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Figure 5.17 (A) The typical synthesis procedure of honeycomb (NHC)-like CoMoO4e3D graphene hybrid electrodes. Scanning electron microscopic (SEM) images of (B) 3D graphene and (C) NHC-like strongly coupled CoMoO4e3D graphene hybrid at various magnifications. (D) Cross-sectional SEM image of NHC-like CoMoO4e3D graphene hybrid. (E, F) Low- and high-magnification SEM images of nitrogen-doped carbon nanotubes (N-CNTs) on Ni foam. (G) SEM image of NiCo2O4 nanosheets on the 3D Ni foam/N-CNT substrate. (AeD) Reproduced with permission from X. Yu, B. Lu, Z. Xu, Super long-life supercapacitors based on the construction of nanohoneycomb-like strongly coupled CoMoO4-3D graphene hybrid electrodes, Adv. Mater. 26 (2014) 1044e1051. Copyright 2013 WILET-VCH Verlag GmbH & Co. KGaA, Weinheim. (EeG) Reproduced with permission from J. Wu, P. Guo, R. Mi, X. Liu, H. Zhang, J. Mei, H. Liu, W.-M. Lau, L.-M. Liu, Ultrathin NiCo2O4 nanosheets grown on 3D interwoven nitrogen-doped carbon nanotubes as binder-free electrodes for highperformance supercapacitors, J. Mater. Chem. A 3 (2015) 15331e15338. Copyright The Royal Society of Chemistry 2015.

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5.5


Concluding Remarks

In this chapter, we have systematically summarized the current advances in theory calculations, synthetic methods, and nanostructures of TMOs. The theoretical calculations of the crystal structure and electronic property of TMOs promote the understanding of their physical and chemical properties, which are closely related to their electrochemical performances. To improve supercapacitive performances, diverse synthetic routes have been employed to prepare advanced TMO electrodes with the designation of special nanostructures. Although in the recent years great progress has been made in designing rational TMO electrodes with excellent supercapacitive performances, several practical challenges still need to be overcome and more efforts should focus on a few areas: precise control in the morphologies and dimensions, new synthetic strategies for designing electrodes with high surface area and special structures, new methods to manufacture electrodes with high mass loading of active materials for commercial applications, new technology for optimizing the engineering factors of electrodes (such as electrolyte, current collectors, and packing), and a deep understanding of the mechanism during the electrochemical process. Altogether, we look forward to the future with confidence that the TMO-based supercapacitors will have practical applications in electrochemical energy storage.

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Polyoxometalates: Molecular Metal Oxide Clusters for Supercapacitors

6

Matthew Genovese, Keryn Lian University of Toronto, Toronto, ON, Canada

6.1

Introduction

Polyoxometalates (POMs), a large class of metaleoxygen clusters of the early transition elements, display an extraordinary range of physicochemical properties and are some of the most promising building blocks for functional nanomaterials [1,2]. As of 2010, there were over 500 annual publications on POM chemistry and this number continues to increase as interest in the field steadily expands [1]. This interest stems from the seemingly endless variety of POM structures, sizes, and compositions, which can be tuned to create a diverse range of functionalities. POMs show tremendous catalytic activity [1], as well as ionic conductivity [3,4], photo- and electrochromism [5,6], magnetism [1], and antiviral activity [1,7]. However, the most important functionality of POMs, which this chapter will focus on, involves their unique electrochemical behavior. POMs demonstrate high stability of their redox states and can participate in fast reversible multielectron-transfer reactions [8]. These properties have led some authors to refer to POMs as electron reservoirs or sponges, an ideal characteristic for energy storage applications [9,10]. The reversible nature of POM electron transfer is particularly valuable for the design of redox-active electrodes, as the fast electrontransfer kinetics are well suited to the high-power operation required for supercapacitors (SCs). POMs, however, usually exist as salts and solid acids with negligible electronic conductivity and high solubility in water and many organic solvents, making them unsuitable for direct use as electrode materials [11]. To leverage the strong redox activity of POMs for SC applications, these molecules must be immobilized on a highly conductive and insoluble support. Nanostructured carbon materials are excellent candidates for POM supports owing to their stability, large surface areas, and high electronic conductivity. This combination is particularly effective for applications in SCs and other energy storage devices, as the carbon support contributes doublelayer capacitive effects, while the POM provides faradaic charge storage [12e16]. The POMecarbon composites also allow for considerable versatility because of the wide variety of carbon allotropes that can be leveraged, including activated carbon (AC), carbon nanotubes, and graphene [11,17]. In addition to carbon materials, other organic substrates such as conductive organic polymers (COPs) are also common and effective POM supports [18e20]. COPs alone have been used extensively as Metal Oxides in Supercapacitors. http://dx.doi.org/10.1016/B978-0-12-810464-4.00006-1 Copyright © 2017 Elsevier Inc. All rights reserved.

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pseudocapacitive electrodes; therefore, in addition to providing structural integrity for the POM, these substrates have the added benefit of contributing their own intrinsic redox activity [18]. The fabrication of these POMecarbon or POMeCOP composites requires robust methodologies for immobilizing the POM cluster on the carbon surface or embedding these molecules within a polymer matrix. This chapter will discuss the most common and promising methods for the fabrication of POM composite SC electrodes, including the underlying mechanisms for POM deposition and the design parameters that can be used to control electrode morphology and functionality. The use of these POM composites in SC applications will also be discussed, along with an evaluation of electrode performance from early works to recent concepts and new directions. This discussion of POM-based SC electrodes will be divided into three sections based on three different categories of electrodes: (1) electrodes using a single POM chemistry, (2) POMeCOP hybrid electrodes, and (3) electrodes utilizing multiple POM chemistries. The first two sections will describe SC electrodes that show large faradaic contributions, but do not meet the strict definition of pseudocapacitance [21]. The third section will show how the use of multiple POM chemistries with an optimized design methodology allows one to engineer POM coatings that are more capacitive. The chapter will conclude with a brief commentary on current challenges and future perspectives.

6.2

Polyoxometalate Structure and Electrochemistry

The POM category is a very broad classification that encompasses a wide variety of molecules with different shapes, sizes, and structures. In general, the term POM refers to compounds based on metaleoxygen building blocks with the general formula [MOx]n, where M is a transition metal such as Mo, W, V, or (sometimes) Nb and x is between 4 and 7 [1]. Under aqueous acidic conditions, the simple [MOx]n building blocks undergo a unique self-assembly process in which they aggregate to form large compounds with specific structures. The resulting POM clusters are generally anionic in nature, with molecular dimensions of approximately 1e5 nm. These compounds can contain up to 368 metallic centers, with a rich structural diversity afforded by the thousands of possible combinations of the basic metal oxide building blocks [1,20,22]. This huge variety of POM species can be broadly classified into three main categories: 1. Heteropolyanions: In addition to oxygen, these molecules contain a high atomic proportion of one type of transition metal atom known as the addenda atom and a much smaller proportion of another atom known as the heteroatom. The addenda atom is usually Mo, W, or V in a high oxidation state, whereas over 60 elements including most nonmetals and transition metals can function as the heteroatom [6]. Heteropolyanions are the most widely studied class of POMs, particularly the classic Keggin (XM12O40) and WellseDawson (X2M18O62) anions (Fig. 6.1), where M is the addenda atom and X is the heteroatom [1]. 2. Isopolyanions: These molecules contain the same metaleoxygen framework as the heteropolyanions but without the central heteroatom. Compared to heteropolyanions, isopolyanions are often more highly charged and unstable, making them more attractive as intermediate building blocks.


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Figure 6.1 Schematic representation of (A) Keggin and (B) Dawson polyoxometalate anion structures. The central {XO4} heteroatom units (shown in orange) are surrounded by a {MOx}n addenda atom cage. From S. Herrmann, C. Ritchie, C. Streb, Polyoxometalate e conductive polymer composites for energy conversion, energy storage and nanostructured sensors, Dalton Trans. 44 (16) (2015) 7092e7104, copyright 2015, with permission from RSC.

3. Nanosized Polymolybdate Clusters: These are the most recently characterized class of POMs and consist of molybdenum blue and molybdenum brown reduced POM clusters. These are high nuclearity [Mo154] metaleoxygen cluster anions that can be prepared in symmetric ring or sphere conformations [1,6].

Although there is a tremendous variety of POM chemistries, this chapter will focus almost exclusively on the Keggin and WellseDawson heteropolyanions, as these are by far the most widely studied for SC applications. These molecules demonstrate high stability of their redox states and can participate in reversible and multielectrontransfer reactions. The electrons are accepted by the addenda atoms of the POM cluster, and if these atoms are identical, the electrons become delocalized on the addenda atom oxide framework [8]. This mechanism helps the Keggin and Dawson molecules undergo multielectron uptake without subsequent decomposition. For instance, in acidic aqueous solutions the phosphomolybdate Keggin anion PMo12O3 40 (PMo12) demonstrates the following three reversible multielectron-transfer reactions: þ 3 PMo12O3 40 þ 2e þ 2H 4 H2PMo12O40 þ 3 PMo12O3 40 þ 2e þ 2H 4 H4PMo12O40 þ 3 PMo12O3 40 þ 2e þ 2H 4 H6PMo12O40

This ability of the Keggin molecule to act as an electron reservoir is well documented; Wang et al. [10] have even reported the reversible uptake of 24 electrons per PMo12O3 40 cluster, measured via X-ray adsorption near-edge structure analysis. The exact electrochemical properties of POM anions, such as the number of electrons transferred and the redox potential, can be tuned by adjusting the heteroatom or

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addenda atom chemistry. For instance, the redox potential of the Keggin electrontransfer reactions has been shown to decrease linearly with the valence of the central heteroatom [8]. Similar trends have been established for addenda atom chemistry that can be organized by decreasing oxidizing ability as follows: V(V) > Mo(VI) > W(VI). For example, the substitution of two Mo(VI) atoms on the phosphomolybdate cluster with V(V) to create PMo10V2O5 40 was shown to significantly change the redox properties of the molecule and increase the redox potential of the first electron-transfer reaction [14,23]. The fine tuning of the POM redox behavior is aided by the fact that these molecules are hydrolytically unstable in alkaline media. This behavior can be exploited to promote an “opening” of the addenda atom cage followed by substitution of different metal complexes to impart the desired electrochemical properties [24]. This tunability of the POM redox states is very important for the design of pseudocapacitive POM electrodes. Although individual POM chemistries show abundant redox activity, it is usually concentrated in voltage regions that do not overlap. Consequently, these POM molecules do not demonstrate the rectangular cyclic voltammograms (CVs) that mimic electric double-layer capacitor (EDLC) and thus do not meet the definition of pseudocapacitance [21]. However, by adjusting the POM chemistry to tune their electrochemical properties, one can design combinations of POMs that together demonstrate the overlapping redox features required for pseudocapacitance. Therefore, the capability of POMs for rapid and stable faradaic processes makes these molecules excellent candidates for use in redox-active SC electrodes, while the tunability of these redox processes enables the design of electrodes that are more ideally pseudocapacitive.

6.3

Fabricating Polyoxometalate Composites for Supercapacitor Electrodes

There are a variety of different methods that can be used to immobilize POMs on organic supports to fabricate SC electrodes, but this chapter will focus on three of the most promising: (1) chemisorption on carbon substrates, (2) immobilization in a polymer matrix, and (3) layer-by-layer (LbL) assembly and electrostatic interactions.

6.3.1

Chemisorption on Carbon Substrates

POM chemisorption on carbon substrates has been widely reported and the POMe carbon interaction is often described as strong and irreversible [25e27]. Synthesizing these chemisorbed POMecarbon composites usually follows a very straightforward procedure: the carbon substrate is first oxidized with a strong acid to create oxygen surface functional groups that act as binding sites for POM modification. The carbon material is then dispersed in an aqueous or organic POM solution and agitated by sonication or stirring. The resulting solid is recovered by filtration or centrifugation and rinsed repeatedly to remove loosely bound species to yield the surface-modified carbonePOM composite [25e28].


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The general procedure described earlier has been extended to a variety of different carbon substrates, including carbon nanotubes [25,27e31], carbon nanofibers [32], microporous AC [13,33,34], and graphene [35e38]. For the fabrication of carbone POM SC electrodes, much of the early work focused on multiwalled carbon nanotube (MWCNT) substrates because of their high electronic conductivity and ease of surface modification [27,28]. For instance, Cuentas-Gallegos [31] investigated the fabrication of these MWCNTePOM composites prepared by chemisorption, with focus on the role of carbon oxidation. In this study, MWCNT was oxidized to varying degrees by refluxing for 7 h in 2 M HNO3, 2 M HNO3 þ 0.5 M H2SO4, or 2 M H2SO4. The oxidized tubes were then sonicated in aqueous solutions of the PMo12O3 40 (PMo12) Keggin ion for modification. Fourier transform infrared (FTIR) analysis confirmed that the oxidation procedure imparted several functional groups to the carbon surface, including hydroxide, carbonyl, and carboxylic acid. It was found that the samples treated in 2 M H2SO4 showed the highest concentration of carboxylic acid functionalities and resulted in the composite electrodes with the highest loading and most homogeneous well-dispersed arrangement of PMo12 on the carbon surface. The FTIR spectra of the MWCNTePOM composite provided evidence of interaction between the POM anion and the carboxylic sites on the carbon surface. These results led Cuentas-Gallegos to propose the following mechanism in which the carbonePOM chemisorption occurs through electron transfer and POM reduction at the hydroxyl portion of the carboxylic group [31].

A study by the same author explored how the chemisorption of PMo12 on Vulcan carbon (VC) was affected by the pregrafting of specific oxygen (-OH) or nitrogen (-NH2) functional groups to the carbon surface [39]. The unmodified carbon matrix (VC) as well as the pregrafted samples (VC-OH and VC-NH2) were sonicated in either 1 mM aqueous or ethanol PMo12 solutions for 1 h. The authors found that modification in the aqueous POM solution generally resulted in composites with higher POM loading, a likely result of the superior dissociation of the PMo12 ion in aqueous solutions as compared to ethanol. Furthermore, it was demonstrated that the pregrafted carbon materials showed much higher amounts of POM modification than the carbon samples without grafting, providing further evidence for the importance of surface functional groups in the chemisorption process. Among the pregrafted samples, the VC-OH matrix showed both the largest amount of POM modification and strongest POM retention on repeated electrochemical cycling. These results indicate that the OH functionalities may promote the largest concentration of strong covalently bonded POM clusters. These experimental findings were validated by a computational study performed by Mu~ niz et al. [40] in which density functional theory (DFT) was used to model the atomic arrangements of the PMo12 anion on a functionalized graphene surface. The

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Figure 6.2 Functionalized graphene (FG) with a PMo12 cluster in the (A) top interaction and (B) side interaction. From J. Mu~niz, A.K. Cuentas-Gallegos, M. Robles, M. Valdéz, Bond formation, electronic structure, and energy storage properties on polyoxometalateecarbon nanocomposites, Theor. Chem. Acc. 135 (4) (2016) 1e17, copyright 2016, with permission from Springer.

authors concluded that some noncovalent electrostatic bonding occurs between PMo12 and the carbon surface because of p-p stacking interactions. However, NH2 and OH functional groups on the carbon surface are able to form covalent bonds with PMo12 in both the top and side configurations (Fig. 6.2), greatly enhancing the POM adsorption on the carbon surface. This covalent bonding mechanism is the most desirable for the design of SC electrodes, as it leads to superior charge-transfer properties and greater retention of the redox-active POMs on repeated voltage cycling. In addition to fundamental studies on the POMecarbon adsorption mechanism, a great deal of work has focused on exploring alternative carbon substrates for chemisorption modification. Graphene, in particular, has been studied extensively for its abundant and highly accessible two-dimensional (2D) surface area [36,38]. Treatment with POM solutions has been combined with conventional methods for the reduction of graphite oxide (GO) such as thermal, chemical, and UV-assisted procedures [35,36,41,42]. The strong POMecarbon interaction has been shown to enhance the reduction process resulting in single-layer reduced graphene oxide (rGO) sheets decorated with POM nanoparticles [41]. Dubal et al. has prepared SC electrodes by sonication of thermally rGO in aqueous PMo12 solutions ranging from 1 to 15 mM concentrations [43]. The POM loading increased with solution concentration but reached saturation at 10 mM. Fig. 6.3 shows the surface morphology of the PMo12e rGO composites, with the high-resolution transmission electron microscopic (Fig. 6.3B) and scanning transmission electron microscopic (Fig. 6.3C) images clearly showing carbon nanosheets abundantly decorated with nanometer-sized PMo12 clusters, which appear well dispersed and free of aggregation. This type of homogeneous molecular dispersion of PMo12 is very important for SC applications to overcome the intrinsically poor conductivity of the metal oxide and maximize electron transfer within the material.


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Figure 6.3 (A) Scanning electron microscopic, (B) high-resolution transmission electron microscopic, and (C) scanning transmission electron microscopic dark-field images of polyoxometalateereduced graphene oxide (POMerGO) composites illustrating the dispersed homogeneous arrangement of POM clusters on rGO nanosheets. From D.P. Dubal, J. Suarez-Guevara, D. Tonti, E. Enciso, P. Gomez-Romero, A high voltage solid state symmetric supercapacitor based on graphene-polyoxometalate hybrid electrodes with a hydroquinone doped hybrid gel-electrolyte, J. Mater. Chem. A 3 (46) (2015) 23483e23492, copyright 2015, with permission from RSC.

Although one-dimensional (1D) and 2D nanomaterials such as carbon nanotube and graphene are well suited as POM substrates, AC remains the most common commercial EDLC material, making the recent studies on POMeAC composite electrodes quite relevant. ACs usually possess a highly disordered graphitic microstructure and lack the intrinsically high conductivity of the low-dimension nanomaterials. However, AC materials have a very large surface area and high adsorption capacity; they can also be produced in larger quantities and at a lower cost than designer nanocarbons [44]. Highly porous AC substrates can pose interesting challenges in terms of the accessible surface area for adsorption of the large POM cluster. An early study by Alca~niz-Monge et al. [45] found that PMo12 adsorbed preferentially to the microporous walls of AC. The amount of PMo12 adsorbed to the AC followed a linear correlation with the volume of supermicropores (micropores > 0.8 nm), whereas adsorption to the smallest micropores was not possible because of the large size of PMo12 anion. GomezRomero et al. [13,33] reported the successful synthesis of SC electrode materials based on the chemisorption of PMo12 and PW12O3 40 (PW12) Keggin ions on the commercial Norit DLC Super 30 AC. These materials were prepared by simply sonicating 1 g of AC in a 10 mM POM solution for 24 h. The high surface area of the AC substrate resulted in abundant loading of POM active species, as the composite product showed a 50% increase in mass compared to the bare carbon material. This simple modification strategy of porous carbon supports represents a promising approach for the design of practical and cost-effective redox-active SC electrodes.

6.3.2

Immobilization in a Conductive Polymer Matrix

The conductivity, easy processability, and relatively low cost of COPs make these materials attractive substrates for POM modification. The inherent electrical

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conductivity and redox activity of COP substrates are particularly useful for SC applications. Many different COPs have been used to create hybrid materials with POMs, but the most common polymers are polypyrrole (PPy) [46e48] and polyaniline (PANI) [18,19,49,50]. Cationic polymers are best suited for this application, as they can more effectively immobilize the anionic POM cluster. The preparation of POMe COP hybrid materials can be achieved by two general approaches summarized in Fig. 6.4 [6]. The postpolymerization strategy involves first depositing a polymer film on a substrate through spin-coating or electropolymerization. The polymer film is then dipped in a POM solution allowing for diffusion and incorporation of the POM into the COP matrix. The prepolymerization approach is the most common and involves combining the POM with the monomer before performing chemical or electrochemical oxidation to synthesize the polymer film. The strong oxidizing power and acidic character of heteropolyacids provides the ideal environment for the polymerization of monomers such as aniline and PPy [18,20]. This chemical or electrochemically aided polymerization results in the deposition of conductive polymer films that are highly doped with POM molecules and resistant to leaching [20,51]. These basic techniques described have been adapted and modified to optimize the immobilization procedure and create novel COPePOM hybrid electrodes. For instance, G omez-Romero et al. [52] prepared PANIePMo12 composite electrodes by comparing a direct electrochemical polymerization procedure with a combined chemicaleelectrochemical polymerization procedure. In the combined approach, aniline monomer was first reacted with PMo12. The resulting product was dispersed in a 0.5 M H2SO4 bath and used for electrodeposition of a hybrid polymer film on a carbon foil substrate. Conversely, the direct electrochemical method involved electrochemical polymerization of a hybrid film from a 0.5 M H2SO4 bath containing the pure aniline and PMo12 reagents. This direct method resulted in thicker, more porous polymer films showing higher faradaic currents and superior charge transfer compared to those prepared with the combined approach. Although POMs can be easily immobilized in a COP matrix, much effort has focused on controlling and refining the structure of these composites to improve their electrochemical properties. Yang [53] has reported the synthesis of PANIePMo12 hybrid nanofibers via a chemical interfacial polymerization method. An aqueous solution of the external oxidant ammonium persulfate was added to an anilineePMO12 mixture dispersed in organic solvent. Polymerization occurred at the interface between the aqueous and organic phases, resulting in hybrid nanofibers with 100 nm diameter consisting of PMo12 anions embedded in a PANI matrix. The 1D geometry of this composite allowed for more effective material utilization and improved electrochemical properties. Hydrogen peroxide (H2O2) was also used as an external oxidant for posttreatment to change the morphology of chemically polymerized PANIePMo12 composites. In this study the control sample of PANIePMo12 prepared by conventional chemical polymerization appeared as agglomerates with particle sizes around 2 mm (Fig. 6.5A), whereas the PANIePMo12 sample with posttreatment in H2O2 appeared as needlelike nanostructures that are 400 nm long and 50 nm in diameter (Fig. 6.5B) [50].


Figure 6.4 Schematic of the preparation of a polyoxometalateeconductive organic polymer (POMeCOP) matrix via (A) postpolymerization and (B) prepolymerization methods. (C) Schematic illustrating the structure of a polypyrrole (PPy)ePMo12 hybrid. From S. Liu, Z. Tang, Polyoxometalate-based functional nanostructured films: current progress and future prospects, Nano Today 5 (4) (2010) 267e281, copyright 2010, with permission from Elsevier.

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Figure 6.5 Scanning electron microscopic images of the morphology of polyaniline (PANI)ePMo12 hybrids prepared (A) without and (B) with the aid of the H2O2 external oxidant. From J. Vaillant, M. Lira-Cantu, K. Cuentas-Gallegos, N. Casa~ n-Pastor, P. G omez-Romero, Chemical synthesis of hybrid materials based on PAni and PEDOT with polyoxometalates for electrochemical supercapacitors, Prog. Solid State Chem. 34 (2e4) (2006) 147e159, copyright 2006, with permission from Elsevier.

In addition to the specific structure of the polymerePOM composites, controlling the porosity of these hybrid films is an important consideration. Porous PPyePMo12 composite films have been prepared by chemical polymerization with the addition of sodium sulfate (Na2SO4) as a porogen [48]. By adjusting the Na2SO4-tomonomer ratio, the porosity of the resulting hybrid film could be readily controlled. Increasing this porosity was found to improve the ionic conductivity in the composite and in turn its performance as an SC electrode material. Vapor transport of the monomer is another adaptation that has been made to the traditional chemical polymerization procedure. White and Slade [54] have demonstrated that the vapor growth of pyrrole on carbon paper coated with an aqueous POM oxidant is a facile method for the synthesis of PPyePMo12 films, which show smooth and complete surface coverage as well as strong adhesion on repeated electrochemical cycling. The synthesis of “ternary hybrids” combining polymerePOM composites with high-surface-area carbon substrates such as graphene, has emerged as a promising approach for the design of electrode materials[55,56]. Chen et al. [55] has reported a novel redox relay strategy for the synthesis of PPy-PMo12-rGO ternary nanohybrids. In this process a pyrrole monomer solution is added to an aqueous dispersion of GO nanosheets and PMo12 and the reaction is allowed to proceed for 24 h. The PMo12 acts as an oxidant for the polymerization of pyrrole, while the resulting reduced heteropoly blue species aids in the simultaneous reduction of GO to rGO. The result is PPye PMo12 nanoparticles uniformly anchored on the surface of rGO nanosheets. The authors found that the intercalation of the PPyePMo12 nanoparticles helped reduce the restacking of the rGO sheets and create a large amount of mesopores, which aid in fast ion diffusion. Furthermore, as both polymerization and reduction are facilitated by only one component, namely, PMo12, this synthesis process represents a more efficient, economical, and environmentally benign approach.


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While PPy and PANI are clearly the most common COPs used for immobilization of POMs, recent studies have explored new hybrid structures based on alternative polymer matrices. The cationic COPs poly(luminol) and poly(new fuchsine) have been used to synthesize composite films with both PMo12 and SiMo12O4 40 (SiMo12) via electrochemical polymerization [57,58]. Raj [59] has reported the synthesis of spin-coated supramolecular architectures consisting of PW12 embedded in a polyampholyte copolymer matrix. The structure of these composites could be controlled by the pH of the spin-coating solution. High-pH solutions resulted in extended, long polymer chains decorated with PW12 molecules, whereas lower-pH solutions led to globular structures with a PW12 core and polymer shell. These supramolecular POM structures have yet to be used for energy storage applications; however, their good conductivity and tunable structure show potential value for the design of pseudocapacitive electrode materials.

6.3.3

Layer-by-Layer Assembly and Electrostatic Interactions

LbL deposition popularized by Decher [60] in the 1990s involves the alternate adsorption of positive and negative layers on a support surface to fabricate multilayer films stabilized by electrostatic attraction. LbL is a wet-chemical procedure, as it usually involves aqueous solutions of two oppositely charged species being sequentially coated on a substrate support. Strong electrostatic attractions are the primary stabilization force for LbL films, but other interactions such as hydrogen bonding may also be present [61]. LbL assembly is a popular technique for the fabrication of composite electrodes because it is cost effective, can be applied to a wide variety of materials, and is simple while still allowing for precise control over structure and functionality [17]. The high anionic charge of POM molecules makes them particularly well suited for LbL assembly, and since the pioneering work by Ingersoll, Kulesza, and Faulkner in 1994 [62], there have been numerous additional reports of LbL-assembled POM composite electrodes [14,63e65]. The LbL process for the fabrication of composite POM films is illustrated in Fig. 6.6. These composites can be assembled on a variety of different supports, but EDLC carbon materials are the substrate of choice for SC applications. The carbon substrate is first oxidized with a strong acid to enhance the surface negative charge. The material is then dipped in a solution of cationic or polycationic species and subsequently rinsed to remove loose connections. The positively charged material is then dipped in a solution of the POM anion followed by another rinsing step. Because of the strong electrostatic attraction, each deposition step takes only a few minutes and the LbL process can be repeated multiple times to quickly build up electrostatically stabilized multilayer thin films [17]. A number of different polycation species have been studied for the LbL deposition of POMs, including polyallylamine hydrochloride [66], polyethyleneimine [67], poly(sodium styrene sulfonate) [66,68], chitosan [69], and poly(diallyldimethylammonium chloride) (PDDA) [14,63,65,70]. In addition to the polycation species, monomer cationic linkers have also been explored, such as methyl viologen [71], ruthenium bipyridine [72], and osmium bipyridine [72]. Kuhn et al. have further explored the

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Figure 6.6 Schematic of the layer-by-layer process for self-assembly of a polyoxometalate (POM) thin-film composite with alternating polyelectrolyte and POM layers. From M. Genovese, K. Lian, Ionic liquid-derived imidazolium cation linkers for the layer-bylayer assembly of polyoxometalate-MWCNT composite electrodes with high power capability, ACS Appl. Mater. Interfaces 8 (29) (2016) 19100e19109, copyright 2016, with permission from ASC.

incorporation of metal cations into LbL-assembled POM thin films. Both Agþ and 6 Hg2þ 2 ions were used to build multilayer films of P2Mo18O62 (P2Mo18) on glassy carbon (GC) substrates [73]. This strategy can be valuable in adding additional redox activity to the composite film; however, the stability of the metallic layer on repeated cycling can vary greatly depending on the choice of electrolyte. Cationic linkers derived from the imidazolium units of monomer and polymer ionic liquids (ILs) have been used to assemble highly conductive POM composites on nanocarbon substrates [74]. In addition to the choice of cation linker, there are a number of other parameters that can affect the structure of LbL-assembled electrodes, such as anion charge, deposition time, ionic strength, pH, and concentration of the coating solutions [6,75,76]. For instance, the ionic strength of the polycation solution can have a dramatic effect on the structure and thickness of the resulting LbL composite. Wang et al. [76] have reported on the LbL deposition of Keggin POMs using a PDDA polycation solution in which the ionic strength was adjusted with NaCl. At low ionic strength, repulsion between the charged segments of the PDDA chain caused the polymer to adopt relatively long and flat profile with very few large loops. The more effective stacking of the polymer chains in this flat configuration results in less free volume for POMs and as such, these molecules adsorb at submonolayer coverage. However, in solutions of high ionic strength, the polycation adopted a nonlinear configuration with many loops, creating more free volume for POM adsorption and above-monolayer coverage.


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Figure 6.7 Cyclic voltammograms of glassy carbon/multiwalled carbon nanotube electrodes modified with one to eight layers of SiMo12 via the layer-by-layer process (scan rate of 20 mV s1 in a pH 1 buffer solution). From A. Salimi, A. Korani, R. Hallaj, S. Soltanian, H. Hadadzadeh, Deposition of aeSiMo12O4 40 -[Ru(bipyridine)(terpyridine)Cl]þ multilayer film on single wall carbon nanotube modified glassy carbon electrode: improvement of the electrochemical properties and chemical stability, Thin Solid Films 518 (18) (2010) 5304e5310, copyright 2010, with permission from Elsevier.

The structure of POMeLbL composites can have a strong impact on their electrochemical performance. Salimi et al. [64] have prepared multilayer SiMo12 composites on a GC/MWCNT substrate with Ru(bpy) complex as the cation linker. The CVs of these composite electrodes shown in Fig. 6.7 illustrate that the current density of the three SiMo12 redox processes (I, II, and III) and the redox process contributed by Ru(bpy) increase almost linearly with the number of layers deposited. This result has been shown in other studies [62,71] and is an effective demonstration of how to easily achieve precise electrochemical control using the LbL technique. The faradaic current that increases as more layers are deposited will eventually plateau, but the number of layers at which this occurs can vary greatly based on the substrate, polycation, and POM properties as well as the deposition conditions [67]. Genovese et al. [74] have shown that the type and structure of the LbL polycation can have a large impact on the capacitive properties of a POM composite electrode. This study investigated the use of ILederived imidazolium cation linkers for the LbL assembly of GeMo12 thin films on MWCNT. Composites fabricated with a polymerized imidazolium linker (PIL) resulted in significantly higher POM loading than those constructed with the same imidazolium cation in the monomer form. This

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implies that there exists a relationship between POM surface coverage and the molecular weight of the cation species. Furthermore, this study also showed that electrodes fabricated with both monomer and polymer imidazolium cation linkers demonstrated thinner and more uniform POM coatings compared to those fabricated with the conventional polycation PDDA. These thin uniform surface coatings helped improve ion diffusion to the electrode surface as well as charge transfer between the POM active layer and carbon substrate. Thus the imidazolium cations resulted in higher electrode conductivity and superior performance at fast chargeedischarge conditions. The PIL was shown to offer the best compromise between POM loading and thin uniform coating morphology, enabling composites with both high faradaic capacity and good conductivity. The versatility of the LbL technique enables the preparation of multilayer POM composite electrodes on a variety of carbon substrates including GC [62], MWCNT [14,65], single-walled carbon nanotube (SWCNT) [64], and graphene [77]. LbL deposition has also been used along with COPs in a process that incorporates the polymer immobilization technique discussed previously. Kulesza et al. [78] have reported the fabrication of multilayer PANIePMo12 composites on GC electrodes. A monolayer of PMo12 was first adsorbed on a GC electrode, which was then transferred to an aniline solution, where electrochemical polymerization was used to deposit a PANI layer. This process was repeated to build up multilayer composites of alternative PMo12 and ultrathin PANI layers demonstrating good electrical contact in the vertical direction and combined redox activity from both components. The efficient assembly of multiple redox-active layers with the LbL process is particularly advantageous for the design of pseudocapacitive electrodes. This approach makes it possible to incorporate multiple layers of different POM chemistries on the same substrate. As the redox properties of POM molecules can be tuned by adjusting their molecular chemistry, incorporating multiple POM chemistries enables the design of electrodes demonstrating several different and overlapping redox features. Engineering this type of electrochemical response is necessary to achieve true pseudocapacitive electrodes. Lian et al. have used LbL assembly to fabricate dual-layer composites that combined multiple POM chemistries on an MWCNT substrate via the PDDA cationic linker. These investigations have explored a number of different dual-layer POM combinations including PMo12/SiMo12, PMo12/PMo10V2, SiMo12/ GeMo12, and PMo12/GeMo12 [14,65,79]. These dual-layer composites demonstrated an electrochemical response that was an additive combination of both constituent POM layers. The surface morphology of the dual-layer electrodes was also analyzed in Fig. 6.8 with the scanning electron micrographs of MWCNT electrodes that are bare, GeMo12 modified, and dual-layer PMo12eGeMo12 modified [79]. The bare MWCNT showed an average tube diameter of around 14 nm, but after modification with single and dual PDDAePOM bilayers, the tube diameter increased by 5 and 10 nm, respectively. The coated tubes also maintain a relatively smooth surface profile free from large aggregates, a necessary condition to enable good electrode conductivity. Thus the LbL technique proves quite effective for the assembly of nanometer-scale multilayer thin films for the addition of multiple redox-active POM chemistries to EDLC carbon substrates.


(A)

(B)

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(C)

Figure 6.8 Scanning electron microscopic images of (A) bare multiwalled carbon nanotube (MWCNT), (B) single-layer GeMo12-modified MWCNT, and (C) dual-layer PMo12eGeMo12 modified MWCNT. From M. Genovese, Y.W. Foong, K. Lian, Germanomolybdate (GeMo12O4 40 ) modified carbon nanotube composites for electrochemical capacitors, Electrochimica Acta 117 (2014) 153e158, copyright 2013, with permission from Elsevier.

6.4 6.4.1

Application of Polyoxometalate Electrodes in Supercapacitor Devices Single Polyoxometalate Chemistries for Faradaic Charge Storage

The most commonly reported SC applications involve electrodes in which a single POM chemistry is used to modify an EDLC carbon substrate. In such situations, POMs have been shown to significantly enhance the charge storage capacity of the electrode because of their faradaic contributions. However, as the currentevoltage profile of individual POM chemistries does not demonstrate a rectangular capacitive shape, these electrodes cannot be defined as truly pseudocapacitive but rather should be referred to as redox active or faradaic. Early patents by Calahan et al. [16] and Li et al. [15] first recognized the potential of POMecarbon composites for SC applications; however, one of the earliest detailed literature studies of these POMecarbon SCs was reported by CuentasGallegos et al. MWCNTs functionalized through acid oxidation were modified with PMo12 and characterized in symmetric solid-state SC cells [28]. A commercial NAFION membrane activated with sulfuric acid was used as the electrolyte and separator. In the device, the hybrid electrodes demonstrated a high capacitance of 285 F g1 at a slow 0.2 A g1 discharge and a stable capacity of 30 F g1 at 1 A g1 discharge rate. This sharp drop in capacity with increasing discharge rate was attributed to the poor ionic conductivity of the solid electrolyte, which becomes limiting at fast discharge conditions. However, even at high current densities the capacity of the hybrid electrode was significantly larger than that of the unmodified MWCNT. The device also demonstrated minimal capacity fade after 500 respective chargeedischarge cycles. This promising electrochemical behavior was also shown by similarly prepared solid-state devices based on PMo12-modified carbon nanofibers [32].

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Since these initial studies, there have been numerous additional reports detailing the benefits of POM-modified carbon substrates for SC electrodes [13,33,36,38,43,80]. More recently, POM-modified graphene composites have received attention [36,38,43]. Gomez-Romero et al. [43] haves reported PMo12erGO hybrids prepared by hydrothermal treatment of GO in a PMo12 solution leading to the simultaneous reduction of GO and incorporation of the POM clusters. Tested in a three-electrode configuration with 1 M H2SO4 electrolyte, these PMo12erGO composites showed a capacity of 276 F g1 at a scan rate of 10 mV s1, a 30% improvement over the capacity of unmodified rGO. This composite material was also quite stable, exhibiting a capacity decline of only 4% after 10,000 cycles. PMo12erGO hybrids have also been demonstrated in symmetric all-solid-state SC devices using a H2SO4ePVA gel electrolyte. In the solid-state configuration, the cell with composite electrodes showed a capacity of 51.2 F g1 compared to 36.8 F g1 for unmodified rGO. Note that the gravimetric capacity of POM hybrid electrodes will always be somewhat limited by the large heavy nature of the POM cluster. In terms of volumetric capacity, a more practical performance metric, the hybrid electrodes showed close to a threefold improvement over unmodified carbon with a capacity of 3.18 F cm3 for the hybrid compared to only 1.10 F cm3 for rGO. This enormous increase in specific capacity was attributed to the significant redox activity of the POM clusters on the graphene surface. Furthermore, the hybrid material led to a 300 mV extension in the operating voltage window resulting in a volumetric energy density of 1.07 mWh cm3 for the PMo12erGO device, over four times greater than that of the symmetric rGO device (0.25 mWh cm3). Yang et al. [81] have also developed graphene-based POM hybrid electrodes, following a slightly different approach involving the use of a polymerized ionic liquid (p-IL) as an interfacial linker to anchor PMo12 clusters on the graphene surface. Fig. 6.9A shows the CV in a three-electrode cell of the PMo12-p-IL-rGO hybrid at 5 mV s1, which shows a capacity of 456 F g1 that is over three times larger than that of a PMo12erGO hybrid prepared without the interfacial linker. The p-IL-linked hybrid also demonstrated excellent electrode conductivity and high rate performance as illustrated in Fig. 6.9B, which shows minimal distortion in CV profile and a linear increase in current up to a high rate of 1 V s1. Analysis of impedance spectroscopy and first-principle DFT calculations revealed that the p-IL enhanced the redox activity of the POM active layer by providing efficient ion diffusion channels and improving electron transfer within the hybrid [81]. In an effort to test the versatility of POM redox-active coatings while exploring more cost-effective fabrication strategies, researchers have investigated SC devices based on POM-modified AC substrates [13,33,82]. The first of these studies was performed by Ruiz et al. [13] who demonstrated an electrode consisting of PMo12 anchored on a commercial AC, which had a capacity of 186 F g1 compared to only 136 F g1 for the unmodified carbon. In a subsequent study, the authors expanded on this work by investigating a commercial AC substrate modified with the tungsten-based PW12 cluster [33]. The addition of PW12 to the AC not only resulted in a capacity increase from 185 F g1 for AC to 254 F g1 for PW12eAC but also led to an increase in the overpotential for the hydrogen evolution reaction. In a 1 M H2SO4 electrolyte, the ACePW12 device showed a 1.6 V


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Figure 6.9 (A) Cyclic voltammograms (CVs) of PMo12e reduced graphene oxide (rGO), PMo12epolymerized ionic liquid (p-IL), and PMo12-p-IL-rGO electrodes in 0.5 M H2SO4 at 50 mV s1. (B) CV of PMo12-p-IL-rGO electrode at increasing scan rates from 10 to 1000 mV s1. Inset shows the peak current of the second redox wave as a function of scan rate. From M. Yang, B.G. Choi, S.C. Jung, Y.-K. Han, Y.S. Huh, S.B. Lee, Polyoxometalate-coupled graphene via polymeric ionic liquid linker for supercapacitors, Adv. Funct. Mater. 24 (46) (2014) 7301e7309, copyright 2014 with permission from Wiley.

operating window compared to only 1 V for unmodified AC. This combination of the POM faradaic activity and the increased operating voltage resulted in a 50% improvement in energy density and a substantial 155% improvement in specific power compared to the unmodified AC device. Similarly, promising results were reported by Mu et al. [82] who used the Dawson P2Mo18 chemistry to modify a commercial AC substrate. The capacity of the AC electrode increased from 182 to 275 F g1 with POM modification, and the hybrid device maintained 89% of this capacity at a fast 6 A g1 discharge.

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POM modification can significantly enhance the energy and power density of SC electrodes. However, because of the high solubility of these molecules, the cycling stability of POMecarbon hybrids can be a concern. Cycle life stability is of utmost importance for SC devices and thus recent reports have explored some novel solutions to improve the stability of POM-modified electrodes. Chen et al. [83] have reported an interesting nanohybrid material consisting of SWCNTs combined with the POM (TBA)5[PV2Mo10O40] (TBA: [(CH3(CH2)3)4N]þ, tetra-n-butyl ammonium). Instead of directly modifying the carbon surface with the PV2Mo10 anion, this anion is combined with the organic TBA cation and grafted to the SWCNT substrate. A symmetric SWCNT-TBA-PV2Mo10 SC operating in 1 M H2SO4 showed a 39% increase in capacity compared to a device with only SWCNTs. More importantly, the hybrid device showed excellent cycling stability retaining 95% of its capacity after 6500 cycles. This high-capacity retention is attributed to the organic TBA cation, which makes the POM insoluble in aqueous species, greatly improving the stability of the composite in the acidic electrolyte. A different approach to improve cycling stability was offered by Hu et al. [84], who focused on the electrolyte in which the POMecarbon electrodes were operating. The vast majority of SC devices based on POMs utilize sulfuric acid electrolytes. Hu’s study investigated PMo12-modified microporous carbons operating in aqueous solutions of a protic IL and compared the performance to conventional sulfuric acid solutions. The PMo12ecarbon electrodes operating in 0.5 M H2SO4 and 1 M 1-butyl-3-methylimidazolium hydrogen sulfate {BMIM} HSO4 showed very similar capacity. However, the electrodes in the acid electrolyte lost over half of their capacity after 3000 cycles, whereas the electrodes in the aqueous IL showed minimal capacity decline. Furthermore, asymmetric cells consisting of an AC negative electrode and a POM composite positive electrode showed virtually no capacity decline after 10,000 cycles in the aqueous IL electrolyte. This stability was attributed to the bulkier IL cation, which helps prevent the dissolution of PMo12 [84]. Another limitation of POM-modified SC electrodes results from the instability of the POM cluster in alkaline solutions. At pH values above 5, most Keggin and Dawson POM clusters undergo hydrolysis. Therefore, although POM composite electrodes work well in acidic electrolytes, their use in alkaline or even neutral media is a challenge. To solve this problem, Chinnathambi et al. have investigated the stabilization of POM clusters by combining them with organic moieties. The P2Mo18 Dawson cluster was combined with Ru(bpy)3Cl2$6H2O (Ru(bpy)) in the presence of a reducing agent to synthesize the molecular hybrid [Ru(bpy)3]3.33PMo18O62$mH2O. In a 0.25 M salt solution at a pH of 7, a GC electrode modified with this POM molecular hybrid showed not only stable operation but also a high capacity of 125 F g1. The electrode also exhibited 87% capacity retention after 500 cycles and most of that loss was attributed to delamination from the GC rather than alkaline hydrolysis. This type of stability at high pH would not be possible with the parent P2Mo18 cluster. These findings are very encouraging, as they show the potential for POM-based SCs in neutral electrolytes, which can help in the design of more environmentally benign devices and expand the use of POM energy storage electrodes to important physiological applications.


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151

PolyoxometalateeConductive Polymer Hybrid Supercapacitors

SC devices based on hybrid electrodes combining conductive polymers with POMs can take advantage of the reversible redox chemistry of both the COP and POM. Gomez-Romero was among the first to realize the potential of COPePOM composites for energy storage applications [52], and since then, there have been numerous reports of SC devices based on POMs immobilized in a conducting polymer matrix [19,48,50,54]. Early reports by G omez-Romero et al. [52] detail devices based on electrochemically grown PANIePMo12 hybrids deposited on carbon foil current collectors. The PANIePMo12 device operating with phosphoric acid electrolyte showed a steady increase in capacitance over the first 300 cycles because of the progressive impregnation of the electrode with electrolyte. After this initial activation the capacitance stabilized at a maximum value of 120 F g1. This result represented a promising proof-of-concept study, as good performance was achieved with no attempt to optimize the thickness or morphology of the polymer hybrid [19,52]. Subsequent studies reported on chemically grown PANIePMo12 electrodes in which the morphology was more precisely controlled. The PANIePMo12 material was compared to a new poly[3,4-ethylenedioxythiophene] (PEDOT)ePMo12 hybrid in terms of performance in symmetric SC devices [50]. The chemically grown PANIePMo12 was shown to be the superior SC material, as it achieved a maximum capacity of 168 F g1 compared to 130 F g1 for the PEDOT composite. Suppes et al. [48] have shown that tailoring the structure and porosity of the polymerePOM hybrid film can significantly improve its performance as an energy storage electrode. PPyePMo12 hybrids prepared with varying degrees of porosity via a Na2SO4 porogen were electrochemically characterized in 0.5 M H2SO4. The hybrid films with increased porosity showed excellent capacitive characteristics, demonstrating a linear relationship between peak current and scan rate up to 20 V s1. For comparison, the nonporous films deviated from the ideal linear behavior already at 500 mV s1. The superior performance of the porous hybrid was attributed to solvent-containing channels within the bulk polymer, which facilitate rapid ion transport. Unfortunately, there is a limit to this effect; if the film becomes too porous, electronic conductivity begins to decline, outweighing improvements to ion transport and degrading the capacitive performance. Impedance analysis revealed that adding the optimized porosity to the polymer films increased the ionic conductivity by over an order of magnitude with minimal decline and with limited effects on electronic conductivity compared to the unmodified polymer matrix. The porous hybrid film displayed a reduced resistorecapacitor time constant and a maximum specific capacity of 210 F g1, compared to only 130 F g1 for a previously reported nonporous PPyePMo12 composite. Capacity retention of the porous electrodes was also quite good, showing less than 10% reduction after 4000 cycles. Devices based on “ternary hybrids” in which COPePOM matrices are combined with high-surface-area carbon substrates have demonstrated promising performance [55,56]. Cui et al. [56] used PMo12 for the in situ polymerization of aniline on commercial graphene sheets (GSs) to synthesize PMo12ePANI/GS hybrids. The PMo12ePANI

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Figure 6.10 Cyclic voltammograms of unmodified graphene sheet (GS), polyaniline (PANI)/GS hybrid, and PMo12ePANI/GS ternary hybrid electrodes at 20 mV s1. SCE, saturated calomel electrode. From Z. Cui, C.X. Guo, W. Yuan, C.M. Li, In situ synthesized heteropoly acid/polyaniline/ graphene nanocomposites to simultaneously boost both double layer- and pseudo-capacitance for supercapacitors, Phys. Chem. Chem. Phys. 14 (37) (2012) 12823e12828, copyright 2012, with permission from RSC.

functionalization of graphene was found to prevent sheet aggregation due to the hydrophilic nature of PMo12. Thus the ternary composite displayed a morphology of thinner sheets as compared to the unmodified graphene resulting in more accessible surface area for double-layer capacitance. The combined electrochemical contribution of all three components of the hybrid is illustrated in Fig. 6.10. The CV of graphene demonstrates a fairly rectangular profile indicative of EDLC. With the addition of PANI, the current density increases because of the two pairs of peaks A1/C1 and A2/C2 attributed to the PANI redox processes. The PMo12ePANI/GS hybrid combines the EDLC capacity of graphene with the redox contributions of both PANI and PMo12. The A10 /C10 and A30 / C30 peaks correspond to the characteristic redox processes of PMo12, A40 /C40 corresponds to PANI, and the large A20 /C20 feature represents the overlapping of PANI and PMo12 redox peaks. At 20 mV s1 the ternary hybrid displays a capacity of 363.5 F g1, significantly higher than that of PANI/GS (238.8 F g1) and the unmodified GS (89.6 F g1). Cui also showed that this approach could be extended to hybrids containing the mixed addenda PMo12-xVx POM, which showed an additional redox process because of the vanadium species. Although the capacity of these hybrids was impressive, the cycling stability requires improvement because they showed a 20% capacity reduction after only 1000 cycles. Chen et al. [55] have prepared similar POM-COP-graphene hybrids, but instead of using commercial graphene, they synthesized graphene oxide sheets and used PMo12 for the simultaneous reduction of GO and polymerization of pyrrole to create PMo12PPy-rGO ternary nanohybrids. In SC applications, these hybrids also showed excellent performance with a maximum capacity of 360 F g1 at a current density of 0.5 A g1 compared to only 70 F g1 for unmodified rGO. To develop flexible energy storage


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devices, the PMo12ePPy/rGO active material was used to fabricate stacked symmetric all-solid-state SCs. The ternary hybrid active material was deposited on Au-coated polyethylene terephthalate current collectors that were used to sandwich a H2SO4ePVA solid polymer electrolyte. These cells demonstrated an areal capacity of 2.61 mF cm2 at a current density of 150 mA cm2, superior to both pure graphene and SnSe2 nanodiskebased solid-state EC devices. Furthermore, the cells showed little to no deviation in electrochemical response, as they were bent at angles from 0 to 120 degrees. This study demonstrates the potential of POM-based redox-active electrodes as promising candidates for advanced flexible/wearable energy storage devices.

6.4.3

Multiple Polyoxometalate Chemistries: Toward Ideal Pseudocapacitance

POMs are clearly very promising materials for adding abundant redox activity to SC electrodes. However, when only one POM chemistry is utilized (as described in the previous two sections) the resulting electrode is not truly pseudocapacitive; it does not display the constant chargingedischarging current densities and the rectangular CV, which mimics double-layer capacitance. Fig. 6.11A shows the CV of an MWCNT electrode modified with PMo12, the most commonly reported POM for SC applications. Although the MWCNTePMo12 electrode shows a much greater capacity than the unmodified carbon, this charge storage is concentrated into relatively narrow redox peaks that do not overlap. As a result, the current density of the POM composite electrode varies dramatically with voltage. This limitation of POM electrodes can be overcome by exploiting the redox tunability of the POM cluster along with the LbL modification technique. This allows

Figure 6.11 (A) Cyclic voltammograms (CVs) of bare and PMo12-modified multiwalled carbon nanotube (MWCNT) electrodes. (B) CVs of bare, PMo12-modified, GeMo12-modified, and dual-layer PMo12eGeMo12 modified MWCNT electrodes. 1 M H2SO4 electrolyte at 50 mV s1. From M. Genovese, Y.W. Foong, K. Lian, Germanomolybdate (GeMo12O4 40 ) modified carbon nanotube composites for electrochemical capacitors, Electrochimica Acta 117 (2014) 153e158, copyright 2013, with permission from Elsevier.

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for multiple POM chemistries with different redox peak potentials to be combined to achieve overlapping redox features and a more ideally capacitive CV profile. This technique is demonstrated in Fig. 6.11B, which shows the CV of a dual-layer electrode modified with both PMo12 and GeMo12 along with the CVs of single-layer PMo12- and GeMo12-modified electrodes for comparison [79]. The single-layer CVs illustrate that simply changing the Keggin heteroatom from P to Ge results in a significant change in electrochemical properties. All three redox features shift to different potentials creating a series of partially overlapping peaks. When PMo12 and GeMo12 are superimposed on the same electrode, there is an additive combination of the constituent layers leading to broader redox features and a more ideally capacitive profile, with current densities that vary less dramatically with voltage. The dual-layer electrode also showed a volumetric capacity almost twice as large as the single-layer modified MWCNT. Although the results of this study were promising, there was also room for optimization; the dual-layer electrode moved toward the ideal behavior but was still not truly pseudocapacitive. To expand on the multi-POM modification approach, Genovese and Lian demonstrated an alternative technique that involves the deposition of a mixed aqueous POM solution in which multiple POM chemistries are deposited simultaneously on a carbon surface in a single active layer [85]. This approach made it even easier to combine the contributions of different POMs with complementary redox behavior. Even more interesting was the discovery that when certain POMs were combined in aqueous solutions, the resulting mixtures would demonstrate a new and unique electrochemical behavior different from the original components. This phenomena occurred with mixtures of PMo12 and PW12, Keggin POMs with different addenda atoms [85,86]. Fig. 6.12 shows the CVs of bare, pure PMo12 coated, pure PW12 coated, and equimolar mixed PMo12ePW12 coated MWCNT electrodes. The figure also includes a predicted CV, which consists of an equally weighted combination of the CVs of pure PMo12 and PW12, calculated and plotted along with the experimental data, a reasonable prediction of the combination of electrochemical properties in a physical mixture. However, the actual PMo12ePW12 mixture displayed its own unique electrochemical behavior, with broad redox features shifted to different peak potentials, different from both this prediction and the pure components. The mixture displayed a much more ideally capacitive profile than either of the pure Keggin POMs. Genovese et al. [86] further explored this mixture phenomena with a series of PMo12ePW12 mixed electrodes of varying composition. When PW12 was incrementally introduced to a pure PMo12-active layer a consistent trend developed: the redox peaks of PMo12 progressively broadened and shifted to lower peak potentials. Simply combining PMo12 and PW12 in different ratios produced a range of mixtures with unique and controllable redox properties. Liquid NMR analysis on these mixtures revealed that PMo12 and PW12 were not physically mixing but reacted spontaneously to form PMo12-xWx mixed addenda chemistries [87]. For the equimolar PMo12ePW12 mixture in Fig. 6.13, the pure PMo12 and PW12 components were completely consumed reacting to form a complex mixture of mixed addenda molecules. The underlying mechanism of this reaction was discovered to be the hydrolysis of the POM cluster to its basic building blocks and


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Figure 6.12 Cyclic voltammograms (CVs) of bare, pure PW12-modified, pure PMo12-modified, and equimolar mixed PMo12ePW12 modified multiwalled carbon nanotube electrodes in 1 M H2SO4 at 100 mV s1. An equally weighted combination of the PMo12 and PW12 curves was calculated and plotted (gray line) as a prediction of the mixed-layer CV shape. From M. Genovese, Y.W. Foong, K. Lian, Designing polyoxometalate based layer-by-layer thin films on carbon nanomaterials for pseudocapacitive electrodes, J. Electrochem. Soc. 162 (5) (2015) A5041eA5046, copyright 2015, with permission from ECS.

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Figure 6.13 (A) Cyclic voltammograms (CVs) of bare, pure PMo12-modified, and GeMo12eSiMo12 (1:1) mixed modified MWCNT electrodes in 1 M H2SO4 at 100 mV s1. A simple model representing the direct addition of the GeMo12eSiMo12 and PMo12 curves is also plotted (gray line) as a prediction of the dual-layer CV. (B) CVs of bare, GeMo12eSiMo12 (1:1) mixed modified, PMo12ePW12 (3:1) mixed modified, and dual-layer GeMo12eSiMo12 (1:1)//PMo12ePW12 (3:1) modified MWCNT electrodes in 1 M H2SO4 at 100 mV s1. From M. Genovese, Y.W. Foong, K. Lian, Designing polyoxometalate based layer-by-layer thin films on carbon nanomaterials for pseudocapacitive electrodes, J. Electrochem. Soc. 162 (5) (2015) A5041eA5046, copyright 2015, with permission from ECS.

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subsequent recombination to more stable mixed addenda species. This hydrolysis reaction occurs spontaneously for PMo12ePW12 in dilute aqueous POM solutions. It can also be replicated with more hydrolytically stable POMs such as SiMo12eSiW12 and GeMo12eGeW12 by simply increasing the pH to initiate hydrolysis [87]. This discovery enabled simple yet precise control over POM redox functionality resulting in tremendous potential for the design of electrodes with desired electrochemical properties, such as pseudocapacitance. Fig. 6.13 illustrates how Genovese et al. [86] used the POM mixture technique combined with LbL modification to engineer pseudocapacitive POM molecular coatings. Fig. 6.13A shows the CV of a SiMo12eGeMo12 mixture (red) overlaid with that of PMo12, another molybdenum-based POM. The authors also predicted the shape of a dual-layer CV superimposing a PMo12 layer on a SiMo12eGeMo12 layer with a simple model showing the direct addition of the SiMo12eGeMo12 and PMo12 CVs. As the redox features of the constituent layers are not complementary, the duallayer CV model retains a shape that is no more rectangular than the single layers, still showing prominent gaps in charge storage at 0.1 and þ0.15 V. The mixing technique can be used to create more complementary charge storage. Fig. 6.13B shows the CVs of the same GeMo12eSiMo12 mixture (red), but this time overlaid with the CV of a 75-25 PMo12ePW12 mixture (blue). The addition of PW12 stimulates PMo12-xWx mixed addenda formation and the second and third redox peaks of PMo12 shift to lower potentials under the influence of the added tungsten. This creates a series of almost perfectly overlapping peaks when overlaid with the GeMo12e SiMo12 CV, filling in the charge storage gaps shown in Fig. 6.13A. A fabricated dual-layer 50-50 GeMo12eSiMo12//75-25 PMo12ePW12 electrode exhibited a CV (black) that looked very similar to the prediction (gray), showing no large peaks or gaps as well as relatively constant charging and discharging currents. The resulting CV profile is very close to the ideal rectangular shape of true pseudocapacitance. Furthermore, the optimized dual-layer electrode demonstrated a volumetric capacity of 181.2 F cm3 at 50 mV s1, almost twice as large as the single-layer modified electrodes and over 10 times larger than the unmodified MWCNT [86]. Genovese et al. obtained the above results using a cavity microelectrode utilizing very small powder volumes. However, the authors have also demonstrated the same multilayer POM molecular coatings on larger, more conventional electrode films with 1 cm2 active area and loadings of 3e5 mg. In this case the 50-50 GeMo12eSiMo12//75-25 PMo12ePW12 electrode showed a gravimetric capacity of 161.5 F g1, over five times larger than the 31.5 F g1 of the unmodified MWCNT, still a very significant improvement over the EDLC [88]. These results are significant, as they demonstrate that with multiple chemistries and optimized electrode design, it is possible to achieve pseudocapacitive coatings based on POMs. Although these multi-POM electrodes are quite promising, they still suffer from performance limitations. The majority of the pseudocapacitance for these electrodes is achieved over a relatively narrow 0.7 V potential window [17,86]. Bajwa et al. [14] have shown that tuning of the POM chemistry to include metals of higher redox potential, such as vanadium, may be a valuable approach to help extend the pseudocapacitive potential window. Additionally, the multiple POM active layers required


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for pseudocapacitive coatings must be anchored by polycation species that generally show limited electronic conductivity. This leads to thick nonconductive surface coatings that limit SC performance at fast chargeedischarge conditions. Genovese and Lian [74] have shown that novel polycation species can help improve the morphology and conductivity of multilayer POM electrodes. IL-derived PILs were shown to result in thin and uniform POM surface coatings, leading to multilayer composites that retained 72% of their capacity at a fast 2 V s1 rate, compared to only 50% with electrodes fabricated with the conventional polycation PDDA. Exploring novel cation linkers will be an important next step in optimizing the performance of multilayer POM-based pseudocapacitive coatings.

6.5

Conclusions and Future Perspectives

POMs, with their abundant and controllable electrochemical activity, have emerged as promising candidates for the design of faradaic SC electrodes. The inherently poor electronic conductivity of these materials along with their high solubility present a challenge for their direct use as energy storage electrodes. However, by combining these molecules with conductive and stable supports such as carbon or conducting polymers, one can realize the full potential of POMs in SC applications. This chapter outlined three of the most promising methods for the fabrication of POM composite SC electrodes: (1) POMs can be grafted onto a variety of different high-surface-area carbon allotropes by strong covalent interactions with oxygen or nitrogen surface functionalities; (2) the oxidizing nature of heteropolyacids can be used to polymerize monomers, such as aniline or pyrrole, to fabricate hybrids consisting of POM clusters embedded in a conducting polymer matrix; and (3) the strong negative charge of POM anions can enable their coupling with positively charged polyelectrolytes to fabricate electrostatically stabilized LbL thin films on carbon substrates. When individual POM chemistries such as PMo12 are immobilized on a carbon substrate or embedded within a polymer matrix, the resulting composite electrode demonstrates significantly enhanced capacity over that of the substrate because of the abundant redox processes of the POM cluster. Increases in electrode specific energy and power are consistently reported with the composite material, in addition to good capacity retention at fast chargeedischarge conditions. Although these electrodes show large faradaic contributions, they cannot be defined as truly pseudocapacitive because they do not display the ideal rectangular CV profile. However, mixed-layer or multilayer modification techniques can be used to combine multiple different POM chemistries on the support surface to engineer a more ideally capacitive response. These techniques leverage the POM compositional diversity and tunable redox chemistry to design molecular combinations with complementary charge storage to achieve a rectangular CV profile. The development of novel mixtures combining POM molecules with different addenda atoms is a particularly valuable tool for controlling POM redox behavior and designing pseudocapacitive electrodes. The application of POMs for SC electrodes is still an emerging field and there is tremendous potential for future research. One area, in particular, involves developing

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a more fundamental understanding of the interaction between the POM cluster and the substrate support: how does the orientation of the POM molecule change on different support surfaces and what is the nature of the charge transfer between the POM and the substrate? Answering these questions will be an important step in advancing the design of POM composite electrodes. Computational methods using DFT models will be an important part of such investigations [40,89]. More work will also be needed on how to best incorporate POM composite electrodes into SC devices. POMs are most active in acidic aqueous electrolytes that have a limited stability window. Furthermore, the redox active potential window of POMs in these electrolytes is usually even smaller (0.5e0.7 V). This presents challenges to fully capitalize on POM faradaic activity while designing SC devices with high energy and power density. There are many potential research areas that could aid in solving this issue, such as further tuning of the POM chemistry to expand the potential window of the redox activity. Combining POMs with organic moieties to improve their stability in neutral electrolytes with larger potential windows is another promising approach. Additionally, exploring the combination of POM composites with different counterelectrodes in asymmetric configurations may be an interesting method to create high-voltage devices. In addition to the optimization of POM-based devices, continued fundamental research on POM chemistry and composite morphology will remain important. There are thousands of POM molecules and with the ever-expanding field of POM inorganic chemistry, new and unconventional POMs are continuously being discovered. The vast majority of studies on POM SC electrodes focus only on simple Keggin POMs, especially PMo12. Expanding these investigations to include other lessconventional POM chemistries will help with the discovery of more POM composites with unique functionality that can be used to enhance SC electrode performance. In addition to exploring new POM chemistries, future efforts should also focus on developing better control over the organization of POM molecules on the substrate support. Hu et al. [90] have reported an interesting method for “wiring” POMs onto an SWCNT substrate, in which they were able to achieve periodic patterned POM crystals with controllable size and shape deposited on the carbon surface. These composites have yet to be used for SC electrodes, but this type of monodispersed morphology is ideal for high-power devices. Future efforts should focus not only on screening new POM chemistries but also on optimizing their organization across length scales to engineer the electrode surface for high-performance pseudocapacitive applications.

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MetaleOrganic Framework (MOF)eDerived Metal Oxides for Supercapacitors

7

Nayarassery N. Adarsh Catalan Institute of Nanoscience and Nanotechnology (ICN2-CSIC), Bellaterra, Spain

7.1

Introduction

Owing to their long life cycle, high power density, and significant energy density, supercapacitors have attracted the interest of materials scientists as a versatile key to meet the increasing demands of energy storage, more particularly for its crucial roles in power sources for hybrid electric vehicles, mobile electronic devices, etc. [1,2]. One kind of supercapacitor is called electric double-layer supercapacitor (EDLC) that usually uses carbon materials with high surface area and porosity [3]. However, the charges physically stored and the energy densities of EDLCs are quite low. Pseudocapacitive with electrochemically active materials such as transition metal oxide and sulfides with increased energy density are more attractive in the recent years [4,5]. In this case, metaleorganic frameworks (MOFs) and MOF-derived nanomaterials indicate great potential and advantages in the application of supercapacitors because of their high surface areas and controllable pore sizes and nanostructures. In another approach, MOFs can be excellent template and precursors for the preparation of porous carbon and metal oxide or sulfide, which are treated as the electrode for EDLCs and pseudocapacitors. MOFs [6a] or coordination polymers [6b] are an important class of compounds because of their various potential applications, especially in energy storage [7]. MOFs are the result of a spontaneous supramolecular self-assembly process of ligands and metal centers or metal-containing units [secondary building units (SBUs)] via metaleligand coordination (MLC) and many other nonbonded interactions. Such porous materials have attracted enormous attention because of their high surface areas, controllable structures and tunable pore sizes, and potential applications. Various research groups have contributed significantly toward the size- and shape-controlled synthesis of MOFs in nano- and microscales and also explore the opportunity of MOF crystals (nano- and microcrystals) as template for the synthesis of various functional micro- and nanostructures [8]. Because of their porous structure, MOFs were explored as templates for the synthesis of porous metal oxides, porous carbon, etc., and such large-surface-area pores can be utilized for interfacial transport and short diffusion paths for the electrolyte [9]. Moreover, among the various parameters that play a crucial role in controlling the outcome of the supercapacitance property (such


166


as the specific capacitance, rate capability, and cycle stability) of MOF-derived metal oxides, the surface area of their nanostructure is important. As charges are stored on the surface of the supercapacitor electrodes, an electrode with a higher surface area leads to an improved specific capacitance. In other words, large surface area and high conductivity are two important properties of an electrode material for high capacitance. The design and synthesis of functional nanomaterials, such as porous metal oxides, porous metal oxideecarbon composite, and porous carbon derived from MOFs, for the application of supercapacitors have attracted many researchers because of their unprecedented porous nanostructure and high surface area, which is absent in ordinary metal oxides and carbon. This chapter will discuss the promising methods for the fabrication of porous metal oxides, porous metal oxideecarbon composite, and porous carbon derived from MOFs. Thus the discussion of MOF-derived porous nanomaterials will be divided into three sections based on three different categories of electrodes: (1) binary metal oxides derived from MOF, (2) ternary or mixed-metal oxides derived from MOF, and (3) a combination of metal oxide and carbon electrode derived from a single MOF. the chapter will conclude with a brief commentary on current challenges and future perspectives of these nanomaterials.

7.2

MetaleOrganic FrameworkeDerived Metal Oxides for Supercapacitors

Transition metal oxides have been explored by various groups as the best candidate for supercapacitor applications because of their multiple oxidation states/structures that enable rich redox reactions. For example, transition metal oxides such as Co3O4, RuO2, Fe3O4, MnO2, CeO2, and NiO showed excellent supercapacitance. Owing to the presence of intrinsic surface area of the MOF-derived transition metal oxides, obtained by the thermal decomposition of MOF nanostructures, the ions can easily diffuse through the pores of their nanostructures. Such free diffusion of ions is one of the crucial factors for the successful electrochemical activity of the electrodes.

7.2.1 7.2.1.1

MetaleOrganic FrameworkeDerived Binary Metal Oxides Co3O4

Co3O4 is a well-known electrode material candidate for supercapacitors because of its novel nanostructures, high specific capacitance with long cycle life, and normal spinel crystal structure (space group, cubic Fd3m) with Co2þ ions in tetrahedral interstices and Co3þ ions in the octahedral interstices of the cubic close-packed lattice of oxide anions. In fact, nanoscaled Co3O4 materials have received much research attention in the past years because their novel nanostructures are dependent on physicochemical properties for many potential applications [10]. Therefore, several efforts have been explored for the controlled synthesis of nanostructures of Co3O4, such as cubes, rods, wires, tubes, and sheets [11]. Moreover, Co3O4 is considered by the researchers


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as one of the best electrode material for supercapacitors because of its high reversibility and theoretical specific capacitance (3560 F g1) and it might be a potential alternate to expensive RuO2 [12]. One of the first examples of Co3O4 nanostructures derived from Co(II) MOF precursors was reported by Zhang et al. [13]Co(II) MOF was synthesized from CoCl2$6H2O and p-benzenedicarboxylic acid (BDC) ligand by the solvothermal method and was characterized by Fourier transform infrared spectroscopy and thermogravimetric analysis/differential scanning calorimetry, and the authors proposed the formula as [Co(BDC)2]n. Solid-state annealing of this Co(II) MOF at 450 C resulted in porous Co3O4 nano-/microsuperstructures; the nanostructure was characterized by electron microscopy and X-ray diffraction (XRD). Electrochemistry measurement of the as-synthesized porous Co3O4 superstructures showed specific capacitance of 208 F g1 at a current density of 1 A g1 and a specific capacitance retention of c.97% after 1000 continuous chargeedischarge cycles in 6.0 M aqueous KOH solution (vs. saturated calomel electrode), indicating their supercapacitor property. Later, Lu et al. [14] synthesized a dendritic porous Co3O4 nanostructure consisting of many nanorods, with 15e20 nm diameter and 2e3 mm length, from a Co(II) MOF synthesized from 8-hydroquinoline, urea, and Co(NO3)2 by the solvothermal method. Calcination of the resultant Co(II) MOF at 450 C for 1 h in air led to the formation of black Co3O4 porous nanostructure having the same morphology as the precursor. The XRD data confirmed the formation of Co3O4, which indicated the transformation of Co3O4 from the Co(II) MOF precursor. The constant-current galvanostatic charginge discharging curves of the as-synthesized Co3O4 showed their symmetric nature, indicating their characteristic good electrochemical capacity. Interestingly it retains the symmetric nature even at a low density of 0.5 A g1. At a current density of 0.5 A g1 the Co3O4 electrodes exhibit a specific capacitance of 207.8 F g1 (much higher than that of commercial Co3O4 electrode, which is 77.0 F g1) and maintain 97.5% of the specific capacitance after 1000 cycles (Fig. 7.1). Although welldefined nanosized pores were observed in t heCo3O4 material by scanning electron microscopy (SEM), no sorption property was reported. Guo et al. [15] reported another porous Co3O4 electrode material synthesized from a Co(II) MOF by thermolysis via two-step calcination. The single-crystal X-ray diffraction experiment of the Co(II) MOF revealed that the Co-MOF was formed via the extended coordination of a carboxylic ligand azobenzene-3,5,40 -tricarboxylic acid (H3ABTC) and auxiliary ligand 4,40 -bipyridine. Such coordination polymerization results in the formation of two-dimensional (2D) bilayer structural intermediates, which further extend to three-dimensional (3D) polycatenated supramolecular architecture sustained by p-p stacking and hydrogen-bonding interactions. The XRD data of the porous Co3O4 confirmed the crystalline phase purity and assigned the face-centered cubic phase. Transmission electron microscopic (TEM) study indicates the morphology of Co3O4 as irregular porous agglomerates composed of several nanoparticles with a mean size of 10 nm. The specific Brunauer-Emmett-Teller (BET) surface area measurement of the as-synthesized Co3O4 confirmed their porous structure having a surface area of 47.12 m2g1. Galvanostatic chargeedischarge measurements of the as-synthesized porous Co3O4 in the presence of 2 M KOH electrolyte between

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Figure 7.1 (A) Schematic representation of the synthesis of parent Co(II) metaleorganic framework and Co3O4, (B) powder X-ray diffraction data of the as-synthesized Co3O4, and (C) chargingedischarging curves of the Co3O4 dendrite-electrode at various potentials and current densities. Copyright 2015, The Royal Society of Chemistry. These figures are reprinted from H. Pang, F. Gao, Q. Chen, R. Liua, Q. Lu, Dendrite-like Co3O4 nanostructure and its applications in sensors, supercapacitors and catalysis, Dalton Trans. 41 (2012) 5862e5868 with permission. Copyright 2012, The Royal Society of Chemistry.

0.0 and 0.5 V (vs. Ag/AgCl) at various current densities (from 0.5 to 3 Ag1) revealed that the Co3O4 electrode has a high chargeedischarge coulombic efficiency and low polarization. Chronopotentiometric (CP) data of the as-synthesized porous Co3O4 electrode exhibits a specific capacitance of 150 F g1 and displayed excellent recycling stability over 3400 cycles at 1 A g1 (Fig. 7.2). Porous hollow Co3O4 with rhombic dodecahedral nanostructures was synthesized by Huang and coworkers [16] from a well-known MOF compound, namely, zeolitic imidazolate framework (ZIF)-67. Calcination of ZIF-67 [17] rhombic dodecahedral microcrystals was performed by heating in furnace in air from room temperature (rt) to 450 C at a heating rate of 1 C min1 and maintained at 450 C for 30 min. The SEM image of the resultant black powder material revealed a porous rhombic dodecahedral structure. The powder X-ray diffraction (PXRD) data of the as-synthesized material further confirmed the formation of Co3O4 from the precursor compound ZIF-67. The BET surface area measurement of Co3O4 revealed its porous structure having a surface area of 128 m2g1. CP measurement of Co3O4 electrode showed an unprecedented specific capacitance of 1100 F g1 and the electrode retained more than 95.1% of the specific capacitance after 6000 continuous chargeedischarge cycles. Interestingly,


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Figure 7.2 (A) Two-dimensional layered structure of Co(II) metaleorganic framework (MOF), which is the primary supramolecular architecture. (B) Three-dimensional polycatenation array of Co(II) MOF. (C) X-ray diffraction pattern of Co3O4 derived from Co(II) MOF. (D) Transmission electron microscopic image of porous Co3O4. (E) Cyclic voltammetry of porous Co3O4 electrode in 2 M KOH electrolyte at various scan rates. (F) Galvanostatic chargee discharge curves of porous Co3O4 electrode material at various chargeedischarge current densities. (AeF) are reprinted from F. Meng, Z. Fang, Z. Li, W. Xu, M. Wang, Y. Liu, J. Zhang, W. Wang, D. Zhao, X. Guo, Porous Co3O4 materials prepared by solid-state thermolysis of a novel Co-MOF crystal and their superior energy storage performances for supercapacitors, J. Mater. Chem. A 1 (2013) 7235e7241 with permission. Copyright 2014, The Royal Society of Chemistry.

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Figure 7.3 (A) Crystal structure of ZIF-67. (BeD) Scanning electron microscopic image of Co3O4 derived from ZIF-67, displaying the porous rhombic dodecahedral structure. (E) X-ray diffraction pattern of Co3O4 derived from ZIF-67. (F) The specific capacitances measured by the chronopotentiometric curves and current densities of the porous hollow Co3O4 electrode. (A) is reprinted from D. Zhang, H. Shi, R. Zhang, Z. Zhang, N. Wang, J. Li, B. Yuan, H. Bai, J. Zhang, Quick synthesis of zeolitic imidazolate framework microflowers with enhanced supercapacitor and electrocatalytic performances, RSC Adv. 5 (2015) 58772e58776 with permission. Copyright 2015, The Royal Society of Chemistry. (BeF) are reprinted from Y-Z. Zhang, Y. Wang, Y-L. Xie, T. Cheng, W-Y. Lai, H. Pang, W. Huang, Porous hollow Co3O4 with rhombic dodecahedral structures for high-performance supercapacitors, Nanoscale 6 (2014) 14354e14359 with permission. Copyright 2014, The Royal Society of Chemistry.

this porous rhombic dodecahedral Co3O4 exhibits higher specific capacitance than the other reported Co3O4 nanostructures (Fig. 7.3). A previously reported porous Co(II) MOF [18] having nicotinic acid as linker was resynthesized by Han and coworkers [19] as nanoscale crystals. SEM image of the assynthesized nanoscale Co(II)enicotinate MOF revealed its “microflower” morphology having a uniform size and shape with a length of 5e6 mm and width of 3e4 mm, and XRD data indicates its crystalline phase purity and uniqueness toward the reported Co(II) MOF structure [18]. The Co(II) MOF exhibits robust, thermally stable open framework structure with an effective channel (along crystallographic axis “a”) having a dimension of 10.8 4.5 Å. Calcination of this porous Co(II) MOF at 500 C for 20 min in air led to the formation of black Co3O4 having identical morphology as


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Co(II) MOF. Thus, it was confirmed from the SEM image that the precursor MOF retains its morphology even after calcination in air, despite the size of the Co3O4 microflowers has shrunk a little and the surface has changed roughness. Moreover, the SEM image of Co3O4 further revealed the formation new nanopores, which could be formed as a result of escape of gas from the MOF during the calcination process. The BET surface area analysis (N2 gas) of Co3O4 showed the typical type IV curve indicating the presence of a mesoporous structure in the sample with a surface area of 16.7 m2g1. Moreover, the Barrett-Joyner-Halenda measurement of the Co3O4 sample revealed a mean pore size diameter of 20 nm, which is the ideal size for the transfer of ions and electrons at the electrodeeelectrolyte interface and to afford sufficient active sites for the Faraday reaction. CP of the porous Co3O4 electrode material in 3 M KOH aqueous solution showed a specific capacitance of 240.2 F g1 at a current density of 0.625 A g1, with more than 96.3% capacitance remaining after 2000 cycles. Interestingly, with the increase in the current densities from 0.625 to 6.25 A g1, the specific capacitance decreased to 202.1 F g1 and remained at 84.1% of capacitance. This result showed the good rate capability of the porous Co3O4 microflower electrodes (Fig. 7.4). Sui et al. [20], synthesized Co3O4 in two different ways: (1) Co3O4-TH, direct thermal decomposition of ZIF-67 at a temperature of 600 C for 5 h with a heating rate of 2 C min1 in air and (2) Co3O4-ZIF, by using simple liquid phase method. Co3O4-TH and Co3O4-ZIF showed spherical morphology having an average size of 100 and 30 nm, respectively. Electrochemistry results of Co3O4-ZIF showed predominant specific capacitance (189.1 F g1 at a current density of 0.2 A g1) than that of Co3O4-TH (67.9 F g1 at a current density of 0.2 A g1). It is further confirmed from the SEM image (100 nm) and BET surface area measurement of Co3O4-TH (11.32 m2 g1) that the Co3O4 nanoparticles are more aggregated than that of Co3O4-ZIF (surface area, 50.12 m2 g1). This result supports the proof of the concept, the importance of guest ZIF-8 framework to host the Co(II) precursor within their cavities, which restricted their movement and aggregation and allows the formation of Co3O4 upon heating. Nanoscale Co-MOF-74 [21] is one of the successful MOF candidates because of its potential applications. It exhibits thermal and chemical stability and a high surface area with an average cross-sectional channel dimension of 11.08 11.08 Å2. Wang et al. [22] explored the possibility of porous hexagonal cuboid Co-MOF-74 nanocrsytals to synthesize porous Co3O4 by the calcination method at three distinct temperatures, that is, 350, 400, and 500 C, to optimize the best temperature, to synthesize best quality Co3O4, and to demonstrate the effect of specific surface area of Co3O4 on supercapacitance. The as-synthesized Co3O4 were characterized by various methods such as SEM, TEM, PXRD, and nitrogen sorption isotherm. The SEM image of the porous Co3O4 synthesized at 350 C revealed its nanotubular nanostructure having hexagonal cuboid morphology, identical to the precursor MOF. Thus, even after the calcination process at 350 C, the morphology was successfully maintained. PXRD data confirmed the formation and crystalline phase purity of Co3O4 from the Co-MOF-74 precursor at all three distinct temperatures (350, 400, and 500 C), even though the SEM image showed different morphologies. While nanotubes started to rupture after the

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Figure 7.4 (A) Crystal structure of the Co(II) metaleorganic framework precursor. (B) Scanning electron microscopic image of the as-synthesized Co3O4, displaying the “microflower” morphology. (C) N2 adsorption/desorption isotherm and the corresponding pore size distribution (inset) of the porous Co3O4 microflowers. (D) Chronopotentiometic curves of porous Co3O4 microflower electrode at various current densities. Reprinted from G-C. Li, X-N. Hua, P-F. Liu, Y-X. Xie, L. Han, Porous Co3O4 microflowers prepared by thermolysis of metal-organic framework for supercapacitor, Mater. Chem. Phys. 168 (2015) 127e131 with permission. Copyright © 2015 Elsevier B.V

calcination process at a temperature of 400 C, the morphology transformed to nanoparticle aggregates once the temperature reached 500 C. The N2 isotherm of the -as-synthesized Co3O4 (at 350 C) showed H3-type hysteresis loop, which indicated the presence of a mesoporous structure having a BET specific surface area of w45.9 m2 g1. In contrast to Co3O4 (at 350 C), the sample synthesized at 400 and 500 C exhibits less specific surface area, i.e., 36.2 and 21.6 mm2 g1, respectively. The electrochemical measurements of the as-synthesized Co3O4 materials at three distinct temperatures revealed that the optimum temperature to synthesize is 350 C, because of the significantly high capacitive property with a specific capacitance of 647 F g1 at current density of 1 Ag1 at this temperature when compared to the other two temperatures, 400 and 500 C (specific capacitances were 439.5 and 129 F g1, respectively). Moreover, Co3O4 nanotubes also showed excellent cycling stability


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without any decrease in specific capacitance after 1500 cycles at 2 A g1, which indicated that it can be a promising electroactive material for supercapacitors (Fig. 7.5). Co3O4 polyhedrons with a porous structure were synthesized by Chen and Hu [23] from a Prussian blue analog, namely, Co3[Co(CN)6]2, by thermal decomposition at 400 C in air. The as-synthesized porous Co3O4 material showed polyhedron morphology. The porous structure of Co3O4 is further revealed by their BET gassorption measurement, which showed a normal physical adsorption isotherm curve with a pore size distribution of 2e50 nm. PXRD data of the prepared Co3O4 revealed the crystalline phase purity as the pure face-centered-cubic phase of spinel Co3O4 with the lattice constant a ¼ 8.072 Å, which corroborated with the reported structure of Co3O4, indicating the completion of transformation from Co3[Co(CN)6]2 to Co3O4. The Co3O4 polyhedrons as an electrode showed an excellent specific capacitance of 110 F g1. A 3D Co(II) MOF, namely, [Co3(L-6H)(H2O)6]n, derived from the ligand L ¼ 1,2,3,4,5,6-cyclohexanehexacarboxylate (linker) is a highly symmetric (space group R-1) MOF having planar and chair-shaped cyclic water hexamers [24]. In fact this ligand (L1) is utilized as a linker in MOF chemistry because of the multiple binding sites and pH-dependent versatile coordination modes of the linker [25]. When the 3D MOF was heated to 380 C at a ramping rate of 1 C mine1 from rt and stabilized at the same temperature for 4 h under airflow and finally cooled to rt, it resulted in Co3O4 nanoparticles [26]. The formation of Co3O4 nanoparticles from the precursor MOF was confirmed by PXRD; the PXRD pattern of the as-synthesized Co3O4 indexed to the face-centered cubic phase Co3O4 [Joint Committee on Powder Diffraction Standards (JCPDS) card no. 43-1003]. SEM image of Co3O4 revealed the spindle-shaped microcrystals with length 10e15 mm and diameter 2e4 mm, and they maintained the morphology of the parent MOF after the calcination process. In fact the surfaces of such microcrystals were split into layers composed of several nanoparticles; this is because of the decomposition of the organic part of MOF during the process of thermolysis. BET surface area measurement of the Co3O4 revealed its typical type IV adsorption isotherm and porous structure having a surface area of 34.8 m2 g1. The pore size distribution had a relatively wide peak centered at 15.8 nm. Such porous Co3O4 nanostructures with moderate surface area and large pore size are crucial for the fast electronic and ionic conducting channels and they allow flexibility in change in volume during the chargeedischarge cycling process. The electrochemistry data of the porous Co3O4 nanostructure showed a specific capacitance of 148 F ge1 at a current density of 1 A ge1. In fact the Co3O4 material maintains the specific capacitance and showed a slight enhancement after 2500 continuous chargeedischarge cycles (Fig. 7.6). Wang et al. [29] synthesized two Co(II) MOFs {[Co(H2IDC)2(H2O)2]2DMF}n (where DMF is N,N0 -dimethylformamide) CoeIm [27] and {[Co(H2PDC)(PDC)] 3H2O}n CoePy [28] in nanoscale using a poor solvent via a precipitation method. The nanoscale synthesized Co(II) MOFs CoeIm and CoePy were shown to have identical PXRD patterns with the reported crystal structures [28,29], thus indicating their crystalline phase purity. High-temperature calcination of these Co(II) MOFs resulted in the formation of porous Co3O4 materials, which was confirmed by their PXRD data. SEM image of both samples of Co3O4 revealed the spherical nanoparticles. The electrochemical measurement data of the as-prepared Co3O4 material revealed a

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Figure 7.5 (A) Crystal structure of 2,5-dioxidoterephthalate (DOT), M2O2(CO2)2, and M-MOF-74 (M ¼ Zn, Co, Ni, Mg; MOF is metaleorganic framework), displaying the formation of the M-MOF-74 and their porous structure. (B) Scanning electron microscopic images of Co-MOF-74; insets display the hexagonal cuboid morphology. (C) N2 adsorptionedesorption isotherm of Co3O4 synthesized from Co-MOF-74 by thermal


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Figure 7.6 (A) Crystal structure of a three-dimensional Co(II) metaleorganic framework, namely, [Co3(L-6H)(H2O)6]n, displaying their hexagonal packing. (B) Scanning electron microscopic image of the Co3O4 porous material. Insetdmagnified image, displaying the porous structure. (C) Chargeedischarge curves of the porous Co3O4 electrode at various current densities. (D) Cycle performance of the porous Co3O4 electrode at the current density of 1 A g1 after 2500 cycles. (AeD) are reprinted from W. Xu, T-T. Li, Y-Q. Zheng, Porous Co3O4 nanoparticles derived from a Co(II) cyclohexanehexacarboxylate metaleorganic framework and used in a supercapacitor with good cycling stability, RSC Adv. 6 (2016) 86447e86454 with permission. Copyright 2016, The Royal Society of Chemistry.

=treatment at 350 C. (D) Chargeedischarge curves at various current densities of Co O 3

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synthesized from Co-MOF-74 by thermal treatment at 350 C. (E) Average specific capacitance at various current densities of Co3O4 synthesized at various temperatures. (A) is reprinted from T. Glover, G. Peterson, B. Shindler, D. Britt, O. Yaghi, MOF-74 building unit has a direct impact on toxic gas adsorption, Chem. Eng. Sci. 66(2) (2011) 163e170 with permission. Copyright© 2011 Elsevier B.V. (BeE) are reprinted from H. Li, F. Yue, C. Yang, P. Qiu, P. Xue, Q. Xu, J. Wang, Porous nanotubes derived from a metal-organic framework as high-performance supercapacitor electrodes, Ceram. Int. 42 (2016) 3121e3129 with permission. Copyright© 2016 Elsevier B.V.

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maximum specific capacitance of 233 F ge1, good stability over 1500 cycles with the capacitance retention of 89.8%, and low charge-transfer resistance of 1.22 U. Among various porous nanostructures of Co3O4 reported so far, a hollow nanostructure with diverse morphologies, such as hollow nanospheres, hollow nanotubes, and hollow nanopolyhedrons, is very important. Moreover, deliberate synthesis of hollow structures with complexity on the surface will give additional functional properties and potential applications [30]. Nonetheless, relatively low space utilization efficiency and energy density due to the inner space of the hollow structure are the main obstacles in exploring the functional properties. To address this problem, Zhang and coworkers [31] designed and synthesized an unprecedented self-supported array in which Co3O4 nanowires self-penetrated- Co3O4 nanocages in Ni foam. The design of such a combination of two nanostructures such as one-dimensional (1D) nanowires and hollow nanostructures (nanocage) is based on the fact that such combination is expected to enhance the synergistic effect and will help improve the supercapacitance. The

Figure 7.7 (A) Schematic illustration of the fabrication of self-supported Co3O4 wirepenetrated-cage hybrid arrays. (B) Transmission electron microscopic image of the Co3O4 wire-penetrated-cage displaying the nanocage and nanowire morphologies. (C) Comparison of the chronopotentiometric curve plot of the Co3O4 self-penetrated arrays and Co3O4 nanowire arrays at a current density of 2 mA cm2. (D) The capacitance versus current density plot of Co3O4 self-penetrated arrays and Co3O4 alone. PAA, polyacrylic acid. (AeD) are reprinted from J. Li, X. Wang, S. Song, S. Zhao, F. Wang, J. Pan, J. Feng, H. Zhang, Self-supported Co3O4 wire-penetrated-cage hybrid arrays with enhanced supercapacitance properties, CrystEngComm (2017), http://dx.doi.org/10.1039/C6CE02616H with permission. Copyright 2017, The Royal Society of Chemistry.


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synthesis of such hybrid nanostructures involves various steps as shown in Fig. 7.7A. In the first step, Co(OH)F/Ni foam arrays were fabricated on the Ni foams by following a literature method [32]. In the second step, Co(OH)F/Ni foam arrays were annealed leading to the formation of Co3O4 nanowire arrays on Ni foams, and the nanowires were treated with polyacrylic acid (PAA) to modify the surface. In the next step, the surface-modified nanowire, namely, Co3O4-PAA/Ni foam, was reacted with Co(NO3)2 and 2-methylimidazole in methanol resulting in the formation of Co3O4 nanowire-penetrated ZIF-67 precursors. Finally, annealing of the precursor in air at 400 C led to the formation of the hybrid nanostructure, wire-penetrated-cage mixed Co3O4, which was confirmed by TEM (Fig. 7.7B) and PXRD. Electrochemical measurement of the as-synthesized hybrid nanostructure of Co3O4 revealed the unprecedented enhancement in the area capacitance of 3.7 F cm2, which is 3.5-fold more than that of Co3O4 nanowires at a current density of 2 mA cm2. Thus, the importance of the hybrid nanostructure of Co3O4 and the exclusive penetrating features of nanowires onto the nanocage for the successful design of a supercapacitor derived from Co3O4 are clearly demonstrated (Fig. 7.7C and D).

7.2.1.2

Fe3O4

To the best of our knowledge, there is no report of single Fe3O4 electrode nanostructures derived from Fe-MOFs, as it has not been explored in the context of supercapacitor, in contrast to Co3O4. However, Zhi and coworkers [33] reported a porous Fe3O4/ carbon composite supercapacitor electrode material prepared via calcination of FeMIL-88B-NH2 under N2.

7.2.1.3

CeO2

Mahanty and coworkers [34] synthesized CeO2 from a Ce-BTC (benzene-1,3,5tricarboxylate) MOF by thermolysis at 650 C. The resultant oxide material showed a brick-upon-tile morphological feature. In fact the rapid redox-active reaction between Ce(III) and Ce(IV) and its close potentials significantly affect the supercapacitive performance of the CeO2 electrode, which is confirmed by the redox peaks at around 0.23/0.35 V. The electrochemistry data (galvanostatic chargeedischarge scans) of the porous CeO2 nanostructure showed a specific capacitance of 1204 F ge1 at a current density of 0.2 A ge1. Moreover, it shows excellent stability up to 5000 chargeedischarge cycles with nearly 100% capacity retention. They also demonstrated the contribution of both the complementary electron buffer source 4 [Fe(CN)4 6 /Fe(CN)6 ] and the brick-upon-tile morphological feature to the highperformance metal oxide supercapacitors. Thus, in this section, we highlighted the importance of MOFs and their use as a sacrificial template for the synthesis of high-quality porous metal oxides for excellent performance of supercapacitors. The metal oxides derived from MOFs by thermolysis usually have a high surface area and high conductivity, properties that are highly desirable for supercapacitors. Surface faradaic processes are strongly affected by the diffusion of ions and the pores developed in the metal oxide from MOFs support such diffusion. Therefore, the construction of highly porous metal oxides having suitable

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ion transport channels and accessible surface area is important in designing new MOFs, especially manganese and nickel MOFs because there are no reports on MnO2 and NiO so far.

7.2.2

MetaleOrganic FrameworkeDerived Ternary or Mixed Transition Metal Oxides

Doping additional transition-metal atoms into binary metal oxides is an ideal method to improve the cycle stability as well as capacitance. Inspired by this idea, transitionmetal mixed oxides (TMMOs) were developed from bulk crystals to nanoscale [35]. TMMOs (designated as AxB3xO4; A or B ¼ Co, Ni, Zn, Mn, Fe, etc.) [36] are promising materials with stoichiometric or nonstoichiometric compositions, characteristically in a spinel crystal structure, that have attracted many researchers because of their various potential applications such as multiferroics [37], thermoelectrics [38], superconductivity [39], colossal magnetoresistivity [40] etc. The cations in the spinel structure of TMMOs generally occupy the tetrahedral (A cations) or octahedral (B cations) voids created by the cubic close-packed arrangement of the oxide ions [41]. Moreover, TMMOs exhibit higher electrical conductivity than binary metal oxides because of the relatively low activation energy for electron transfer between cations [42]. Precursors from a single source are very crucial for the synthesis of TMMOs because only then the stoichiometry of the resultant TMMOs can be more precisely controlled. Heterometallic MOFs are used as a precursor for the synthesis of TMMOs, more particularly, in nanoscale [43]. Mixed MOFs or heterometallic MOFs are considered as a potential precursor candidate for the synthesis of TMMOs for supercapacitor application. The general approach toward heterometallic MOFs involves the use of a metalloligand (metal porphyrin-based ligands) [44], postsynthetic modification [45] and direct and partial replacement of the transition metal in a homometallic MOF by controlling the stoichiometry of the reaction mixture. Caskey and Matzger [46] exploited the geometrical preference of the metal ions in an MOF for selective incorporation of a transition metal within a homometallic MOF. By following the “escapeby-crafty-scheme” a series of spinel mixed-metal oxides MMn2O4 (M ¼ Co, Ni, Zn) were synthesized from a heterometallic MOF derived from perylene-3,4,9,10tetracarboxylic dianhydride [43]. For example, the nanoscale heterometallic precursor, ZnMn2eptcda (perylene-3,4,9,10-tetracarboxylic dianhydride), containing Zn2þ and Mn2þ was prepared by the soft chemical assembly of mixed-metal ions and organic ligands at a molecular scale. The synthesis of metal oxides from MOFs showed more advantages than the other methods because of their high surface area and unique structure. The resulting metal oxides usually have a high surface area and high conductivity, properties that are highly required for supercapacitors. Although many binary metal oxides have been developed using this method, single metal oxides are the most reported and the synthesis of mixedmetal oxides from mixed-metal MOFs is rare. To the best of our knowledge, there are only three reports of using MOFs to synthesize a mixed-metal oxide for supercapacitors. This section deals with this three TMMOs such as ZnCo2O4 [53], MnCo2O4 [55], and NixCo3xO4 [59] derived from the corresponding heterometallic MOFs.


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ZnCo2O4

Zinc cobaltite (ZnCo2O4) is one of the successful TMMOs because of its advantages such as improved reversible capacities, enhanced cycling stability, and good environmental benignity [47]. The spinel crystal structure of ZnCo2O4, in which Zn2þ occupies the tetrahedral sites and Co3þ occupies the octahedral sites, has been widely investigated as a high-performance material for supercapacitors [48], and ZnCo2O4 has emerged as a promising alternative because of its higher conductivity and richer redox reactions, caused by the coupling of the two metal species [42]. Nanoscaled ZnCo2O4 have received much research attention because of their novel nanostructures dependent on physicochemical properties for many potential applications including supercapacitors [49]. Various synthetic strategies were explored by the researchers to synthesize ZnCo2O4 in nanoscale, such as coprecipitation, the solegel method, and synthesis on a particular support [50e52]. Qiu and coworkers [53] self-assembled a new mixed MOF ZnCo2O(BTC)2(DMF)$ H2O, namely, JUC-155, by reacting a mixture of ZnCl2, CoCl2$6H2O, and BTC in a stoichiometric ratio of 1:2:5 in DMF via the traditional solvent thermal method. Single-crystal XRD data revealed that the heterometallic MOF JUC-155 crystallized in a tetragonal space group I4cm. The asymmetric unit is composed of a halfoccupied Zn(II) metal center [site occupancy factor (sof) ¼ 0.5], one fully occupied Co(II) metal (sof ¼ 1), one molecule of BTC, one half-occupied DMF (sof ¼ 0.5) and one half oxide (O2), and a lattice-included water molecule (Fig. 7.8A). BTC and O2 are coordinated to both metal centers, whereas DMF is coordinated to Zn(II). Zn(II) exhibits distorted octahedral coordination geometries with six oxygen atoms from four different BTC ligands, one coordinated DMF molecule, and one central bridging oxygen atom. On the other hand, the Co(II) center showed distorted tetrahedral geometry in which all coordination sites are occupied by O atoms of BTC and O2. The coordination of such ligands with Co(II) and Zn(II) resulted in a trinuclear metal cluster ZnCoO2, which is the SBU. The extended coordination of BTC with such SBUs led to the formation of 3D porous MOFs having 1D channels along the crystallographic axis “c” (Fig. 7.8B). The resultant MOF precursor was then calcined at three distinct temperatures (400, 450, and 500 C) to optimize the best temperature for the preparation of quality ZnCo2O4 electrode material for the best performance of supercapacitors. PXRD data analysis of these three samples (1, 2, and 3) from three distinct temperatures showed that all of them transformed to the pure spinel phase of ZnCo2O4 (JCPDS card no. 23-1390) from the parent MOF precursor (Fig. 7.8C). TEM image of the as-synthesized three distinct samples 1, 2, and 3 of ZnCo2O2 revealed the spherical morphology of nanoparticles with a diameter less than 20 nm and these nanoparticles further aggregate to form a porous structure (Fig. 7.8D). N2 adsorptionedesorption analysis of all three samples revealed that the BET surface area decreased (55.0, 45.9, and 20.4 cm2 g1 for ZnCo2O2 synthesized at 400, 450, and 500 C, respectively) with increase in the thermolysis temperature. The cyclic voltammetry (CV) measurement of the as-synthesized three samples, 1, 2, and 3, of ZnCo2O2 revealed their ideal capacitive behavior. The effect of surface area on the supercapacitance behavior of electrode is well demonstrated here; from the comparison

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Figure 7.8 (A) Asymmetric unit of JUC-155. (B) The three-dimensional structure of JUC-155 displayed along the crystallographic axis “c.” (C) PXRD comparison plot of ZnCo2O4 synthesized at various temperatures: 400 C (green), 450 C (red), and 500 C (black). (D) Transmission electron microscopic image of the nanoparticles of ZnCo2O4. (E) Galvanostatic chargeedischarge at a current density of 1 A g1 for three electrodes. (F) Specific capacitance derived from the discharging curves of the three electrodes. (A and B) are generated using the crystallographic coordinates retrieved from CSD, version 5.31, November 2010. (CeF) are reprinted from S. Chen, M. Xue, Y. Li, Y. Pan, L. Zhu, D. Zhang, Q. Fang, S. Qiu, Porous ZnCo2O4 nanoparticles derived from a new mixed-metal organic framework for supercapacitors, Inorg. Chem. Front. 2 (2015) 177e183 with permission. Copyright 2016, © the Partner Organizations 2015.


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plot of the CV curves of the sample electrodes 1, 2, and 3, it can be seen that sample 1 has the largest CV internal and the electrode 3 has the least, which corroborated well with the higher surface area of 1. The large surface area of the electrode enables the transfer of ions and electrons at the electrodeeelectrolyte interface and affords sufficient active sites for the Faraday reaction. Moreover, the specific capacitance values of the three samples at a low scan rate of 5 mV s1 are 451, 330, and 234 F g1 for the electrodes prepared from samples 1, 2, and 3, respectively, and are maintained at 97.9%, 96.8%, and 94.6% (1, 2, and 3 respectively) of the initial values after 1500 cycles.

7.2.2.2

MnCo2O4

MnCo2O4 is one of the successful TMMOs that showed excellent pseudocapacitive property because of the synergistic effects of Mn2þ and Co2þ cations, structural stability, and reversibility of the electrodes. Co2þ shows a higher oxidation potential, whereas Mn2þ can transport more electrons and achieve a higher capacitance [54]. Thus, MnCo2O4 exhibits higher electronic conductivity and electrochemical activity than the binary oxides MnO2 and Co3O4. Wang et al. [55] demonstrated the synthesis of MnCo2O4 from a dual metal ZIF (Mn-Co-ZIF) [17a] as both precursor and template. As shown in Fig. 7.9, initially the purple powder of Mn-Co-ZIF was synthesized by mixing Mn(NO3)2$4H2O and Co(NO3)2$4H2O [1:2 stoichiometric ratio of Mn(II) and Co(II) salts] with the linker 2-methylimidazole. PXRD data of the as-synthesized Mn-Co-ZIF showed a pattern identical to ZIF-67 [17a], indicating their isomorphous nature. Thermolysis of Mn-Co-ZIF at 400 C resulted in a black powder. The PXRD data confirmed that the black powder is MnCo2O4 because all its diffraction peaks indexed well with the characteristic spinel structure (standard JCPDS no. 23-1237), which also indicates its crystalline phase purity and the complete conversion of MOF precursor to MnCo2O4 (Fig. 7.10A and B). X-ray photoelectron spectroscopic studies further support the presence of Mn2þ, Mn3þ, Co2þ, and Co3þ. The nanostructure of the as-synthesized MnCo2O4 was characterized by SEM and TEM. The SEM image of

Figure 7.9 Schematic view of the different steps in the synthesis of Mn-Co-zeolitic imidazolate frameworks (ZIFs). Reprinted from Y. Dong, Y. Wang, Y. Xu, C. Chen, Y. Wang, L. Jiao, H. Yuan, Facile synthesis of hierarchical nanocage MnCo2O4 for high performance supercapacitor, Electrochimica Acta 225 (2017) 39e46 with permission. Copyright© 2017 Elsevier B.V.

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the MnCo2O4 showed a hollow polyhedral nanocage morphology (average size, 200 nm), and the polyhedral structure of the Mn-Co-ZIF precursor (average size, 400 nm) was retained without much breakdown (Fig. 7.10C). The decrease in the size of the nanocage during the transformation of the Mn-Co-ZIF precursor to MnCo2O4 is due to the decomposition of the organic part of the precursor. The TEM image further revealed the presence of interconnected nanoparticles (5e20 nm) on the surface of nanocage, which led to porous nanostructure (Fig. 7.10D). Such porous structures support the ion diffusion pathway easier and allow collaborative electronic transmission. The porous structure of MnCo2O4 cages were further confirmed by N2 isotherm data. The BET surface area measurement of MnCo2O4 revealed its typical type Ⅳ hysteresis loop at relative pressure (P/P0) range from 0.4 to 1.0, characteristic to a mesoporous structure having a surface area of 117 m2 g1. To investigate the advantage of the porous hollow nanostructure, the authors studied the electrochemical performance of the as-synthesized nanocage MnCo2O4. They also synthesized Co3O4 nanocage to compare the electrochemical performance with nanocage MnCo2O4. The CV data of the as-synthesized MnCo2O4 and Co3O4 nanocages were measured separately with various scan rates from 2 to 50 mV s1, which revealed the characteristic redox peaks. From the CV curves, the average specific capacitance of the MnCo2O4 and Co3O4 nanocage electrodes were determined and plotted at various scan rates (Fig. 7.10E). From the figure, it is clear that the electrochemical performance of MnCo2O4 is more predominant than Co3O4; the MnCo2O4 and Co3O4 electrodes exhibit the maximum specific capacitances of 1763 and 1209 F g-1 at a current density of 1 A g-1, respectively. Moreover, after 4500 cycles at 1 A g-1, the MnCo2O4 electrode shows a capacitance retention of 95%, which establishes its greater cycle stability over Co3O4 (81.2%).

7.2.2.3

NixCo3xO4

NiCo2O4 is another example of a TMMO having spinel crystal structure, which exhibits superior electronic conductivity and higher electrochemical performance than NiO and Co3O4 [56]. NixCo3xO4 with different stoichiometric ratio of Ni and Co having mesoporous hierarchical structures attracted the attention of researchers because of its “stoichiometric ratio”-dependent electrochemical property [57]. In other words, the systematic evaluation of the contribution of Co and Ni (in terms of their stoichiometry) in NiCo2O4 toward the capacitance behavior is very important [58]. However, the synthesis of NixCo3xO4 is a great challenge to material chemists because of the difficulty to control the precise metal ratio (Ni:Co). Qiu et al. [59] demonstrated a new strategy for the synthesis of NixCo3xO4 based on a well-known MOF, the so-called MOF-74. Thus, five isostructural MOF-74 having Co, Ni, and a combination of Ni and Co (in various ratios of Ni:Co ¼ 1:1, 1:2, 1:4), namely, MOF-74-Co, MOF-74-Ni, MOF74-NiCo1, MOF-74-NiCo2, and MOF-74-NiCo4, were reported. The authors deliberately synthesized the mixed MOFs such as MOF-74-NiCo1, MOF-74-NiCo2, and MOF-74-NiCo4 to study the effect of the amount of Ni and Co in these MOFs on their electrochemical properties. A good match of the PXRD of all the five MOF-74s with


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Figure 7.10 (A) Crystal structure of MnCo2O4. (B) Bulk (red) and simulated (black) powder X-ray diffraction comparison plot of MnCo2O4. (C,D) Scanning and transmission electron microscopic images of the as-synthesized MnCo2O, displaying the hollow polyhedral nanocage. (E) Dependence of the current on scan rate 1/2 plot for the MnCo2O4 and Co3O4 electrodes. (F) Lighted by two supercapacitor devices (inset, a green light-emitting diode indicator). Reprinted from Y. Dong, Y. Wang, Y. Xu, C. Chen, Y. Wang, L. Jiao, H. Yuan, Facile synthesis of hierarchical nanocage MnCo2O4 for high performance supercapacitor, Electrochimica Acta 225 (2017) 39e46 with permission. Copyright© 2017 Elsevier B.V.

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that simulated establishes their isomorphous crystalline phase. The stoichiometric ratios between Ni:Co present in MOF-74-NiCo1, MOF-74-NiCo2, and MOF-74-NiCo4 were confirmed as 1:1, 1:2, and 1:4, respectively, by inductively coupled plasma and energy-dispersive spectrometry. The calcination of all the five MOF-74s at 400 C resulted in the formation of corresponding oxides such as Co3O4 and NiO, as well as three kinds of NixCo3xO4 mixed-metal oxide nanoparticles NixCo3xO4-1, NixCo3xO4-2, and NixCo3xO4-4 (Fig. 7.11). PXRD confirmed the formation of pure crystalline phase of all these five oxides, and TEM revealed porous “coarse” morphology (Fig. 7.12A). The corresponding crystalline diffraction rings of (220), (311), (400), (422), and (511) in the selected area electron diffraction pattern indicated the polycrystalline nature of NixCo3xO4-1 (Fig. 7.12B). N2 adsorptionedesorption analysis (77K) of all the five as-synthesized oxides revealed the ideal type I isotherm with a steep increase in the low-pressure range and the BET surface area in the range 64e117m2 g1, which is very high compared to other reported oxides. Nix Co3x O4 þ OH þ H2 O 4 xNiOOH þ ð3 xÞCoOOH þ ð3 xÞe CoOOH þ OH 4 CoO2 þ H2 O þ e

Figure 7.11 Schematic view of the synthesis of various MOF-74 crystals [color code: Ni, cyan; Co, pink; C, gray; O, red]. Reprinted from S. Chen, M. Xue, Y. Li, Y. Pan, L. Zhu, S. Qiu, Rational design and synthesis of NixCo3xO4 nanoparticles derived from multivariate MOF-74 for supercapacitors, J. Mater. Chem. A 3 (2015) 20145e20152 with permission. Copyright 2015, The Royal Society of Chemistry.


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Figure 7.12 (A) High-resolution transmission electron microscopic image and (B) selected area electron diffraction of nanoparticles of NixCo3xO4-1. (C) Comparative cyclic voltammetric curves recorded at a scan rate of 5 mV s1 of NixCo3xO4-1 and other oxides. (D) Chargee discharge at a current density of 1 A g1 for NixCo3xO4-1 and other oxides. (E) Specific capacitance derived from the discharging curves of NixCo3xO4-1 and other oxides. (F) The Ragone plots of the estimated specific energy and specific power at various chargeedischarge rates for NixCo3xO4-1 and other oxides. Reprinted from S. Chen, M. Xue, Y. Li, Y. Pan, L. Zhu, S. Qiu, Rational design and synthesis of NixCo3xO4 nanoparticles derived from multivariate MOF-74 for supercapacitors, J. Mater. Chem. A 3 (2015) 20145e20152 with permission. Copyright 2015, The Royal Society of Chemistry.

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CV data of the as-synthesized five samples of oxides revealed the greater capacitance (six-fold higher) of NixCo3xO4 samples compared to the binary metal oxides Co3O4 and NiO, thus demonstrating the advantage of TMMOs over binary metal oxides (Fig. 7.12C). Moreover, among the three kinds of NixCo3xO4 mixed-metal oxide electrode materials, NixCo3xO4-1, where Ni:Co ¼ 1:1, showed the best performance with a specific capacitance of 789 F g1 (Fig. 7.12D and E) and excellent cycling stability (after 10,000 chargeedischarge cycles, the specific capacitances still remained at 557 F g1). The reasons behind such high specific capacitance of NixCo3xO4-1 compared to other compounds are (1) its high surface area, which makes ion/electron transfer in the electrode more easier and (2) the presence of abundant amounts of Ni3þ (half of the compound is Ni) compared to other compounds; the Ni3þ-rich surface of the electrode enhances the formation of active NiOOH (see later discussion), which is proved to be an important redox site [60]. From the Ragone plots shown in Fig. 7.12F, it is confirmed that the mixed-metal oxides have higher energy density than the binary metal oxides. This is due to their higher capacitance and lower resistance. Thus, in this section, we have discussed three important TMMOs synthesized from a sacrificial MOF template. Because of the porous nanostructure formed from the MOF, these TMMOs showed exclusive supercapacitance.

7.2.3

MetaleOrganic FrameworkeDerived Metal Oxide/Carbon Composite

Porous carbons are the most extensively used electrode materials for supercapacitors, especially EDLCs, because of their extremely high surface area and excellent stability [61,62]. Poddar et al. [63] were the first to demonstrate a general strategy for the synthesis of carbon-supported metal or metal-oxide nanoparticles from MOFs. Nonetheless, most of the research work on MOF-based materials has targeted the fabrication of a single electrode (MOF-derived metal oxide or TMMO as the cathode). Thus, to complete the full electrochemical cell, we have to depend on other materials for anode, for example, carbon. The synthesis of an asymmetric supercapacitor having anode and cathode electrode nanomaterials from a single precursor is very important to maintain the morphology, surface area, and flexibility of the resultant electrode nanomaterials (both anode and cathode). Construction of such kind of asymmetric supercapacitors also provides possible synergetic effects with different physicochemical properties. Yamauchi et al. [64] were the first to demonstrate the construction of an asymmetric supercapacitor having nanoporous carbon and nanoporous cobalt oxides (Co3O4) from a single MOF ZIF-67. When the ZIF-67 precursor is heated under the flow of N2, it resulted in nanoporous carbon, but when heated in air, it leads to the decomposition of organic species, resulting in nanoporous Co3O4. In both cases, it retains the original polyhedral morphology of the parent ZIF-67 (Fig. 7.13). Several nanopores were observed on the surface of nanoporous carbon. On the other hand, nanoporous Co3O4 consists of randomly aggregated granular nanocrystals with an average size of 15e20 nm. The surface area measurements of the as-synthesized nanoporous carbon and nanoporous Co3O4 were 350 and 148 m2 g1, respectively. The asymmetric


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Figure 7.13 Schematic representation of the synthesis of the nanoporous carbon and nanoporous Co3O4 from ZIF-67 precursor, displaying retention of the polyhedral morphology of the parent ZIF-67, as shown in the scanning electron microscopic and transmission electron microscopic images.

supercapacitor (Co3O4//carbon) showed exclusively high specific energy of 36 Wh kg1 and a high specific power of 8000 W kg1 compared to their symmetric counterpart (carbon//carbon (w7 Wh kg1) Co3O4//Co3O4 (w8 Wh kg1)). Wang and coworkers [65] build an asymmetric supercapacitor from a single 2D MOF precursor. A 2D sheetlike Co-MOF was synthesized by reacting Co2þ and 2-methylimidazole in water and grown on the surface of carbon (CC@Co-MOF). Pawley refinement of the PXRD data of the as-synthesized Co-MOF confirmed its crystal structure as 2D layered structure. Later, the as-synthesized 2D Co-MOF precursor was transformed to Co3O4 nanosheets (CC@Co3O4) or N-doped porous carbon nanosheets (CC@NC) by thermolysis. Finally, the flexible asymmetric supercapacitor was assembled using the CC@Co3O4 cathode and the CC@NC anode with a poly(vinyl alcohol)eKOH gel electrolyte. From the Ragone plot, it is clear that the CC@Co3O4//CC@NC device showed excellent performance compared with the previously reported MOF-based supercapacitors, demonstrating the importance of such an unprecedented asymmetric supercapacitor.

7.3

Conclusion and Future Perspectives

The achievements of MOF-derived binary metal, ternary metal, or mixed-metal oxides and a combination of binary metal oxide and carbon have been highlighted in this chapter. First, we discussed the binary metal oxides derived from MOF. With examples, we discussed the synthesis of metal oxides from MOFs and the advantages over other methods, owing to their high surface area and unique crystal structure. The resulting

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metal oxides showed high surface area and high conductivity, which are highly desirable properties for supercapacitors. Among the binary metal oxides obtained from the MOFs, Co3O4 is exclusively explored by the researchers for supercapacitor application. Co3O4 nanostructures discussed here showed porous structure and retained the morphology of the parent MOF precursor. While the decomposition of organic species present in the MOF and the subsequent reassembly of the metal oxide facilitated the nanostructure of Co3O4, the gas molecules produced during the thermal treatment led to the porous structure. Such porous structure is one of the crucial factors in designing functional supercapacitors because it makes possible the transfer of ions and electrons at the electrodeeelectrolyte interface and gives sufficient active sites for the Faraday reaction. Second, we discussed the TMMO, an alternative to the binary oxides. Precursors from a single source are very crucial for the synthesis of TMMOs because only then the final stoichiometry of the resultant TMMOs can be controlled more precisely. The resultant nanostructures of TMMOs showed porous structure, and such pores are responsible for the free motion of electrons or ions and facilitate electrodeeelectrolyte dynamics. We also emphasized the importance of hollow porous nanostructure of TMMO compared to the binary metal oxide Co3O4. Finally, we highlighted the importance of a single precursor MOF for the synthesis of asymmetric supercapacitors having nanoporous metal oxide (cathode) and nanoporous carbon (anode) as the electrodes. Such a system provides almost identical morphology, surface area, and flexibility to the electrode and resulted in synergism and best electrochemical performance. Although reports on nanoporous metal oxides obtained from MOFs are ever increasing, it is important to recognize that controlling the outcome in terms of the resultant nanostructure of metal oxides is indeed a daunting task. This is because formation of nanoporous metal oxides having a particular morphology (for example, hollow nanostructure) is a result of the thermodynamic stability of the product (metal oxide), which is influenced by various factors such as reaction condition (temperature, rate of heating) and the style of thermal treatment. The quality of electrode materials (metal oxides, TMMOs, and carbon) still needs to be improved by tuning their morphology and porous structure, so that new materials with high specific capacity and high rate performance can be developed, which are two crucial factors to progress the performance of supercapacitors.

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Metal OxideeCarbon Hybrid Materials for Application in Supercapacitors

8

Dongfang Yang, Mihnea I. Ionescu National Research Council Canada, London, ON, Canada

8.1

Introduction

Supercapacitors are energy storage devices that store energy as charge on the electrode surface, rather than in the bulk material as in batteries; therefore, they can provide higher power than batteries by their ability to release energy more easily from the surface than from the bulk of active materials. Carbon-based materials such as activated carbon (AC), mesoporous carbon, aerogel carbon, carbon nanotubes (CNTs), graphene, and carbon nanofibers (CNFs) are used as electrode active materials in electrochemical double-layer capacitors (EDLCs). EDLCs depend only on the surface area to storage charge, and therefore, they often exhibit very higher power output and excellent cycling ability. However, the small specific capacitance of carbon-based materials limits their applications in high-energy-density devices. Metal oxides, such as RuOx, NiOx, CoOx, FeOx, and MnOx, have higher capacitance than carbon materials, as they possess faradaic or pseudocapacitance originated from fast redox reactions occurring at the interface between metal oxides and electrolytes in addition to double-layer capacitance. However, low electrical conductivity, severe agglomeration, and poor cycle life of metal oxides restrict their applications in high-power-storage devices. Using hybrid active materials consisting of metal oxide with a carbon host is a promising approach to address those challenges. To date, various nano-/microstructural carbon materials are composited with metal oxides. Those hybrids include metal oxides composited with AC, carbon aerogels, ordered mesoporous carbon, CNTs, CNFs, and graphene. This chapter will summarize the most recent development of metal oxideecarbon hybrid materials for supercapacitor applications, including the synthesis methods, the structural and electrochemical characterization, chargestoring mechanism, and the structureeproperty relationships.

8.2

Porous CarboneMetal Oxide Hybrids

Porous carbon materials can be classified according to their pore diameters as microporous (pore size < 2 nm), mesoporous (pore size, 2 e 50 nm), and macroporous (pore size > 50 nm). Microporous carbon materials afford high specific surface areas


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(SSAs), large pore volumes, and large capacities for gaseous or liquid adsorption. However, microporous carbon materials suffer some limitations when they are used as active materials for supercapacitors. The key drawbacks are (1) slow mass transport of electrolyte ions because of space confinement imposed by small pore sizes, (2) low conductivity because of the presence of enormous surface functional groups and defects due to activated process, and (3) collapse of porous structures during hightemperature treatments. Macroporous carbon has limited surface area; therefore, they will not allow the storage of large quantity of charges. Mesoporous carbon possesses both high surface area for absorbing charges and a relatively large and narrow pore size distribution (between 2 and 50 nm), allowing high-rate electrolyte ion transportation. Conventional porous carbon materials, such as AC, are synthesized by pyrolysis and physical or chemical activation of organic precursors, such as coal, wood, or polymers, at elevated temperatures. These carbon materials normally have relatively broad pore-size distributions in both the microporous and mesoporous ranges. When porous carbon materials are composited with metal oxides to form electrode active materials, each type of porous carbon material has its own unique advantages and disadvantages as the host. Effective carbon support is expected to provide good electrical conduction with metal oxides, large surface area to disperse them, and suitable pore channels for their deposition and for electrolyte ion transportation. In this section, the fabrication and electrochemical performance of electrode active materials using metal oxides composited with different types of porous carbons, such as AC, mesoporous carbon, macroporous carbon, and aerogel carbon, will be reviewed and compared. Microporous carbon materials will not be included in this section because of their small pore channels for metal oxide deposition and for electrolyte ion transportation.

8.2.1

Mesoporous Carbon

The synthesis of carbon materials with mesoporous structure was done typically by the templating method using spherical solid gel as the template. The approach includes the following steps: (1) preparation of silica gel with controlled pore structure, (2) impregnation/infiltration of the silica template with monomer or polymer precursors, (3) cross-linking and carbonization of the organic precursors, and (4) dissolution of the silica template. The space once occupied by the host silica materials is thus transferred into the pores in the resulting carbon materials, and the carbon in the pores of the host silica becomes the continuous carbon framework. Lei et al. [1] synthesized mesoporous carbon materials with high surface areas (reaching up to 900 m2 g 1) using three-dimensional (3D) assembles of silica spheres as hard template. Fig. 8.1A(a) shows the 3D hexagonal packing of silica spheres obtained by slow solvent evaporation and heating at 800 C for 6 h. Scanning electron microscopic (SEM) micrograph in Fig. 8.1A(b) was taken after NaOH etching and it shows the presence of silica spheres embedded in the carbon matrix indicating partial dissolution of silica spheres. After a week in contact with the etching solution, the material porosity is fully opened confirming the interconnectivity of the porous structure. Fig. 8.1A(c) reveals the ordered porous arrangement of the resulting carbon as a perfect replica of the initial silica


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Right Figure 8.1 (A) Scanning electron microscopic images of (a) assembled silica spheres, (b) SiO2/C composite after partial etching, (c) mesoporous carbon after complete template removal, and (d) final MnO2/C composite. (B) Cyclic voltammograms for (a) pure carbon, (b) pure MnO2ebirnessite, and (c) 10 wt% MnO2 in carbon/MnO2 composite. Reproduced from Y. Lei, C. Fournier, J.L. Pascal, F. Favier, Mesoporous carbonemanganese oxide composite as negative electrode material for supercapacitors, Microporous Mesoporous Mater. 110 (1) (2008) 167e176.

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sphere 3D assemble. Birnessite-type MnO2 was then deposited by a coprecipitation method in the porous carbon network as shown in Fig. 8.1A(d). The MnO2/C hybrid material was used in supercapacitors and its electrochemical performances were evaluated. Fig. 8.1B shows the cyclic voltammograms (CVs) measured for as-prepared mesoporous carbon (a), birnessite MnO2 (b), and MnO2/C composite with 10 wt% MnO2 (c). By comparing the integrals of each curve, incorporation of birnessite into the porous carbon structure results in a strong improvement of the electrode capacitance. Kiani et al. [2] prepared an ordered mesoporous carbon (CMK-3) matrix using mesoporous silica as a hard template and sucrose as carbon source. They deposited manganese dioxide (MnO2) nanoparticles in the mesoporous carbon (CMK-3) matrix by impregnating the CMK-3 with a Mn(NO3)2$4H2O solution followed by annealing in nitrogen. The use of the hybrid MnO2eCMK3 is to overcome the drawback of low electrical conductivity (105e106 S cm 1) of bulk MnO2 and to fully utilize its pseudocapacitance (theoretical pseudocapacitance up to 1370 F g 1). A symmetric supercapacitor based on the 40% MnO2/CMK-3 composite electrode was built and shows a high operation voltage of 2.0 V and specific capacitances of 640 and 440 F g 1 at discharge current densities of 1 and 10 A g 1, respectively. Almost no significant decrease in specific capacity is observed after 10,000 cycles at different chargeedischarge rates. Zhao et al. [3] synthesized well-dispersed cobalt oxide nanoparticles supported on ordered mesoporous carbon also by an impregnation method combined with a posttreatment with ammonia. The hybrids have not only retained the ordered mesoporous structure of the carbon support with high SSA of over 1000 m2 g 1 but also shown a good control of cobalt oxide nanoparticle dispersion with uniform size. The well-retained mesoporous structure facilitates the ion diffusion and then contributes to the electrical double-layer capacitance, while the cobalt oxide nanoparticle provides the faradaic behavior. A composite with 12.7 wt% cobalt oxide nanoparticles loading demonstrated the high specific value of 524.8 F g 1, which is due to faradaic contribution of cobalt oxides and the unique mesoporous structure of carbon. Such porous structure facilitates the ion diffusion and contributes to EDLC. Zhang et al. [4] incorporated nanosized manganese oxide (Mn2O3) in templated mesoporous carbon and used the Mn2O3ecarbon composites as active materials in supercapacitors. With a well-controlled and homogeneous deposition of Mn2O3 nanoparticles in the carbon host, the electrochemical performance of the composite material was significantly better than carbon-only materials because of pseudocapacitance of the metal oxide. A specific capacitance of over 600 F g 1 and a volumetric specific capacitance of 253 F cm 3 was obtained. Good capacity retention of over 85% was achieved after 800 chargeedischarge cycles. Results from Refs. [1e4] clearly show that ordered mesoporous carbon holds a great promise for high-energy and high-power supercapacitor applications. The greatly enhanced energy storage and high rate capability of the mesoporous carbonemetal oxide hybrids are attributed to the unique ordered mesoporous structure that helps facilitate ion diffusion and the relatively large surface areas that contribute the electrical double-layer capacitance as well as pseudocapacitance of metal oxides due to their faradaic reactions.


8.2.2

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Aerogel Carbon Nanoparticles

Aerogel is a synthetic porous ultralight material derived from a gel, in which the liquid component of the gel has been replaced with a gas. The result is a solid with extremely low density and low thermal conductivity. Carbon aerogel was made by heating organic aerogels to several hundred degrees Celsius in an inert atmosphere (such as argon). The polymer that makes up the aerogel can be dehydrated to leave behind an aerogel made of carbon. Organic aerogels are aerogels with a framework primarily composed of organic polymers. Organic aerogels can be made from resorcinol formaldehyde, phenol formaldehyde, melamine formaldehyde, epoxies, and many others [5]. At the nanoscale, carbon aerogels are composed of nanoparticles of carbon, with diameters approximately 1e2 nm. Like other aerogels, carbon aerogels are primarily mesoporous with a mean pore diameter of approximately 7e10 nm. Most carbon aerogels have a surface area ranging from 500 to 800 m2 g 1. The surface area of a carbon aerogel can be easily increased after production by placing it under a flow of steam or hydrogen at elevated temperatures (400e1000 C). At these temperatures, water and hydrogen will react with carbon in the aerogel to form gaseous products. Water and hydrogen also etch micropores (pores < 2 nm in diameter) throughout the interior of the aerogel, thereby increasing the surface area. Carbon aerogels have surface areas ranging from about 500 to 2500 m2 g 1 and they are also electrically conductive. These two properties make them valuable as electrode active materials for applications in supercapacitors. Hao et al. [6] synthesized carbon aerogeleMnCo2O4.5 nanoneedle hybrid nanostructures by depositing MnCo2O4.5 nanoneedles on walls of hierarchical porous carbon aerogels derived from chitosan. The hybrid nanostructure was formed by first growing MneCo precursors on carbon aerogel in a hydrothermal process with the help of hydroxyl groups and carboxyl groups on the surface of carbon aerogels. The MneCo precursors were then calcinated into MnCo2O4.5 nanoneedles. The resulting MnCo2O4.5 nanoneedle/carbon aerogel hybrid nanostructures have macro-, meso-, and microhierarchical porous structures and possess a very high SSA of 888.6 m2 g 1. The microstructures of the hybrid with different mass ratios of MnCo2O4.5 to carbon aerogel were characterized by SEM as shown in Fig. 8.2. Samples GMC1, GMC2, and GMC3 correspond to the mass ratio of MnCo2O4.5 to carbon aerogel of 1:8, 1:3.3, and 1:1.2, respectively. The carbon aerogel exhibits a squashed 3D porous structure with interconnected carbon nanosheets of uniform thickness with the pore size of 30e150 mm. After a chemical precipitation and then calcination process, MnCo2O4.5 nanoneedles are grown directly on the surface of the 3D carbon aerogels, which facilitates charge transfer between nanoneedle arrays and carbon aerogels. For the composite with low MnCo2O4.5 loading (GMC1) (Fig. 8.2AeC), MnCo2O4.5 nanoneedles with a diameter of w30 nm did not cover the whole carbon surface. With an increase in the mass ratio of MnCo2O4.5 to carbon aerogel (GMC2), the entire surface of carbon aerogels is covered by MnCo2O4.5 nanoneedle arrays having morphology similar to that in GMC1 (Fig. 8.2F). With further increase in the mass ratio of MnCo2O4.5 (GMC3), the porous structure of carbon aerogel was blocked by MnCo2O4.5 nanoneedle arrays (Fig. 8.2G), which show a different morphology. Electrochemical performance of the carbon

198 Metal Oxides in Supercapacitors

Figure 8.2 Scanning electron microscopic images of MnCo2O4.5/carbon aerogel hybrid nanostructures: (AeC) GMC1, (DeF) GMC2, and (GeI) GMC3. Images on the left of (A), (D), and (G) are the corresponding schematic drawings. Reproduced from P. Hao, Z. Zhao, L. Li, C.C. Tuan, H. Li, Y. Sang, H. Jiang, C.P. Wong, H. Liu, The hybrid nanostructure of MnCo2O4.5 nanoneedle/carbon aerogel for symmetric supercapacitors with high energy density, Nanoscale 7 (2015) 14401e14412.


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aerogeleMnCo2O4.5 hybrids with different metal oxide mass loading was evaluated by CV in a symmetric supercapacitor cell in an aqueous electrolyte. The electrochemical evaluation results indicate that too high mass loading of MnCo2O4.5 can block the porous structure of carbon aerogels and reduce the electrical conduction between the metal oxide and carbon host, which will result in lower current density. The specific capacitance of optimized loading (GMC2) reaches a value of 385.6 F g 1, which is higher than the reported values for commercial carbon-based electrodes (w200 F g 1) and is a result of the redox reactions of MnCo2O4.5. The MnCo2O4.5 nanoneedle/carbon aerogel hybrid nanostructure exhibits a high energy density of about 84.3 Wh kg 1 at a power density of 600 W kg 1. The voltage window is as high as 1.5 V in neutral Na2SO4 aqueous electrolytes. Owing to the unique nanostructure of the electrodes, the capacitance retention reaches 86% over 5000 cycles. Lee et al. [7] prepared nanosized Ni-doped carbon aerogel by a precipitation method in an ethanol solvent. To understand the effect of Ni content on electrochemical properties, hybrid materials with different Ni content (21, 35, 60, and 82 wt%) were prepared and their performance for supercapacitor electrode was investigated by CV and chargeedischarge test in 6 M KOH electrolyte. Similar to Hao’s work, Lee found that the hybrid with middle loading of Ni performed the best: 35 wt% nanosized Ni-doped carbon aerogel showed the highest capacitance of 120 F g 1 and excellent chargeedischarge behavior. To compare the performance of different metal oxides, 35 wt% Co-, Cu-, Fe-, and Mn-doped carbon aerogels were also prepared and evaluated. The results show that Mn-doped carbon aerogel has the highest capacitance (141 F g 1) among all the nanosized metal-doped carbon aerogels investigated.

8.2.3

Macroporous Carbon

Porous carbon materials with ordered pores bigger than 50 nm are also good candidates for supporting metal oxides and producing electrode materials for supercapacitors. The ordered feature of relatively large channels greatly decreases the resistance of electrolyte ions to transfer through the channels and allows for a higher degree of metal oxide dispersion when compared with the disordered porous carbon. Yang et al. [8] prepared MnO2/carbon composites by depositing ultrathin MnO2 nanofibers (diameter of 5e10 nm) on three-dimensional ordered macroporous (3DOM) carbon frameworks using a self-limiting redox process followed by a direct redox reaction between KMnO4 solution and carbon at 50 C. The synthetic procedure and structural illustration of the MnO2e3DOM carbon nanocomposites is described in Fig. 8.3A. The morphology of the synthesized MnO2/C nanocomposites is shown in Fig. 8.3B. Ultrathin MnO2 nanofibers with lengths of w30 to 300 nm were uniformly deposited on the interior surface of the hollow macropores. The diameter of these MnO2 nanofibers was found to be weakly dependent on the electroless deposition time. When the deposition time gradually increased from 90 to 150 min, the structure collapsed as observed by SEM (Fig. 8.3B(d)). In the composite, MnO2 nanofibers provide a large surface area for charge storage, whereas the 3DOM carbon serves as a desirable supporting material providing rapid ion and electron transport through the composite electrodes. Optimized composition with 57 wt% MnO2 content gives both a high

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Figure 8.3 Synthetic procedure and structural illustration of the MnO2e3DOM (threedimensional ordered macroporous) carbon composites. Scanning electron microscopic images of MnO2/C composites: (A) MnO2/C, 10 min; (B) MnO2/C, 40 min; (C) MnO2/C, 90 min; and (D) MnO2/C, 150 min. Reproduced from C. Yang, M. Zhou, Q. Xu, Three-dimensional ordered macroporous MnO2/ carbon nanocomposites as high-performance electrodes for asymmetric supercapacitors, Phys. Chem. Chem. Phys. 15 (2013) 19730e19740.


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specific capacitance of 234 F g 1 at a discharge current of 0.1 A g 1 and good cycle life of 52% retention of the capacitance at 5 A g 1. An asymmetric supercapacitor was fabricated by assembling the optimized MnO2/carbon composite as the positive electrode and 3DOM carbon as the negative electrode as shown in Fig. 8.4A. The CV curves were measured in the potential range from 1.0 to 0.0 V for the 3DOM carbon and from 0.0 to 1.0 V for the 3DOM MnO2/C composite independently in a threeelectrode configuration as shown in Fig. 8.4B. This indicated that the asymmetric supercapacitor using carbon as the negative electrode and MnO2/carbon composite as the positive electrode can be operated up to 2.0 V in aqueous electrolyte. Fig. 8.4C shows the CV curves of the asymmetric supercapacitor, with different working voltages extending from 0e1 V to 0e2 V suggesting a good electrochemical stability of the asymmetric supercapacitor up to 2 V. CV curves at different scan rates were presented in Fig. 8.4D showing rectangular CV curves up to 100 mV s 1, which is an indication of fast chargeedischarge of the hybrid materials. Fig. 8.4E shows the galvanostatic chargeedischarge curves. Energy density and power density calculated from the discharge process and plotted in the Ragone diagram is shown in Fig. 8.4F. Compared to the symmetric supercapacitor based on 3DOM carbon, the asymmetric supercapacitor (3DOM MnO2/carbon// 3DOM carbon) delivered a much higher energy density and power density because of the enhanced specific capacitance of the MnO2/carbon composite electrodes and the enlarged cell voltage (from 1.0 to 2.0 V). This improved performance can be understood as an effect of the 3DOM structure of both the positive and negative electrodes, which provides 3D interconnected networks for the fast transport of electrons and ions. Liu et al. [9] reported a composite consisting of MnO2 nanoparticles supported by 3DOM carbon (MnO2/3DOM carbon nanocomposites) fabricated by means of a simple multicomponent infiltration of 3D templates. MnO2 nanoparticles with size between 2 and 6 nm are observed to be highly dispersed on the 3DOM carbon scaffolds. The specific capacitance of the nanocomposite electrode can reach as high as 347 F g 1 at a current density of 0.5 A g 1. Moreover, the electrode exhibits excellent chargeedischarge rate and good cycling stability, retaining over 92% of its initial charge after 5500 cycles at a current density of 2.5 A g 1. Chou et al. [10] developed MnO2/hierarchically porous carbon (HPC) nanocomposites in which MnO2 crystals with a particle size of 3e5 nm and mass loadings of 7.3e10.8 wt% are homogeneously distributed onto the HPCs, and the utilization efficiency of MnO2 was enhanced to 94%e96%. By combining the ultrahigh utilization of MnO2 and the conductive and ion transport advantages of HPCs, MnO2/HPC electrodes can achieve higher specific capacitance values (196 F g 1) than pure carbon electrodes (60.8 F g 1) and maintain their superior rate capability in neutral electrolyte solutions. The asymmetric supercapacitor consisting of an MnO2/HPC cathode and an HPC anode shows an excellent performance at a cell voltage of 2 V, with energy and power densities of 15.3 Wh kg 1 and 19.8 kW kg 1, respectively.

8.2.4

Mixed Porous (Micro, Meso, and Macro) Carbon

Porous carbon materials that consist of microporous, mesoporous, and macroporous structures are also interesting as the support for metal oxides. The most common mixed

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Figure 8.4 (A) Schematic illustration of the asymmetric supercapacitor, (B) cyclic voltammograms (CVs) of three-dimensional ordered macroporous (3DOM) carbon and MnO2/ C-90 min electrodes, (C) CVs at different potential windows, (D) CVs at different scan rates with a potential window of 2.0 V, (E) galvanostatic chargeedischarge curves at different current densities, and (F) the Ragone plot of the asymmetric supercapacitor. Reproduced from C. Yang, M. Zhou, Q. Xu, Three-dimensional ordered macroporous MnO2/ carbon nanocomposites as high-performance electrodes for asymmetric supercapacitors, Phys. Chem. Chem. Phys. 15 (2013) 19730e19740.


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porous carbon is the AC. Significant amount of research and development on active materials for supercapacitors was focused on ACemetal oxide hybrids. The most recent results will be overviewed in the following subsection.

8.2.4.1

Activated Carbon

AC is the carbon produced from carbonaceous source materials such as nutshells, coal, and petroleum pitch. It can be produced by physical activation or chemical activation. Physical activation follows carbonization at temperatures in the range 600e900 C in an inert atmosphere and oxidation activation at temperatures in the range 600e1200 C in oxidizing atmospheres. Chemical activation is done by impregnating source materials with certain chemicals such as an acid, a strong base, or a salt. Then the raw material is carbonized at lower temperatures (450e900 C). Chemical activation is preferred to physical activation owing to the lower temperatures and shorter time needed for the AC material. Owing to their low cost, renewability, recyclability, nontoxicity, large SSA, and easy accessibility, AC materials are the most widely used active materials for supercapacitors. However, most solvated electrolyte ions cannot access the micropores of AC because of their small size. Therefore, the specific capacitance of AC is limited to values less than 200 F g 1, which is not high enough for many applications such as energy storage for electrical vehicles. Similar to other carbon materials, AC can also be used as carbon support for metal oxides to develop practical hybrid electrode active materials. However, to achieve high capacitance and high power performance in ACemetal oxide based hybrid materials, some issues of the AC support, such as comparatively tortuous pore channels and complex surface physicochemical property, have to be addressed. The tortuous pore channels are easily blocked up by even a very low metal oxide loading, leading to a sharp decrease in SSA and thus can neither be accessed by electrolyte ions nor slow down the diffusion speed of the ions. The complex surface increases the difficulty of forming a solid and intimate contact at the carboneoxide interface, which is crucial for increasing the chargetransfer ability. It is, therefore, very important to control the metal oxide loading and how the metal oxide interfaces with AC. Forming an ultrathin layer or ultrafine particles of metal oxides on the AC surface is one example to achieve high surface utilization of electroactive metal oxides during redox reactions. Additionally, a solid binding between the ultrafine metal oxides and carbon supports is necessary to achieve a long and stable cycle life, as electrons produced during the redox reactions should be transferred across the carbonemetal oxide interfaces. Co3O4 has a high theoretical capacitance of 3560 F g 1 with good reversibility and it is also environment friendly, which made it a faradaic material of choice for supercapacitors. By compositing Co3O4 with AC, the challenge of low electrical conductivity of Co3O4 can be addressed. Zhou et al. [11] used a microwave-assisted deposition/precipitation method to prepare uniformly distributed Co3O4 nanoparticles on the surface of AC for supercapacitors. In their work, the AC was first preoxidized in air or acid solutions to functionalize its surface with oxygen-containing groups to provide active sites to anchor Co3O4 nanoparticles. Then the oxide nanoparticles were deposited on the pretreated AC support using urea as a precipitating agent. The fabrication process is schematically

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Figure 8.5 (A) Schematic drawing showing the effect of Co3O4/activated carbon (AC) hybrid microstructures on capacitive performance. (B) Cyclic voltammogram curves of AC (black), Co3O4/AC-A (red), Co3O4/AC-A3 (blue), and Co3O4/AC-A6 (olive) at 100 mV s 1. TMO, transition metal oxide. Reproduced from F. Zhou, Q. Liu, J. Gu, W. Zhang, D. Zhang, A facile low-temperature synthesis of highly distributed and size-tunable cobalt oxide nanoparticles anchored on activated carbon for supercapacitors, J. Power Sourc. 273 (2015) 945e953.

shown in Fig. 8.5A. With a moderate Co3O4 loading, Co3O4/AC-A3 (16.4 wt% Co3O4) hybrid exhibits the largest specific capacitance and best rate performance because of a comparatively large SSA and pore volume. With a high Co3O4 loading, Co3O4/AC-A6 (30.9 wt% Co3O4) hybrid had a much severe decrease in SSA after hybridization (Fig. 8.5B). In addition, large nanoparticles formed in Co3O4/AC-A6 can block the porous channels rather than the small ones in the Co3O4/AC-A3. In this study, AC was treated with concentrated HNO3 (68%) for 3 and 6 h to introduce different levels of surface oxygen-containing groups and marked as AC-A3 and AC-A6, respectively. The oxygen-containing groups could effectively capture cobalt ions from solutions and prevent the agglomeration of oxide nanoparticles during the low-temperature synthesis. As a result, a highly uniform distribution of Co3O4 nanoparticles was obtained on the surface of AC support. The hybrid material with 7 nm average size of Co3O4 nanoparticles with 16.4 wt% loading can provide high specific capacitance up to 491 F g 1 at 0.1 A g 1 in a 6 M KOH electrolyte because of the synergistic effects of EDLC from the AC support and faradaic contribution from the anchored Co3O4 nanoparticles. In addition, an excellent electrochemical stability is achieved with only 11% capacitance degradation after 5000 chargeedischarge cycles at a large current density of 5 A g 1, demonstrating that solid bindings between the Co3O4 nanoparticles and AC support have been achieved. Fe3O4 also is of low cost, is environment friendly, and is a potential material for supercapacitors. By compositing Fe3O4 with AC, the challenges of low electrical


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conductivity and severe agglomeration can also be addressed. Wang et al. [12] prepared AC by chemical activation of coal tar residues and then they fabricated nanostructured Fe3O4/AC composites by simple chemical coprecipitation method, with AC as an additive. The oxygen groups on AC formed during activation served as the anchoring sites for Fe3O4 nanoparticles, consequently preventing serious agglomeration of the Fe3O4 nanoparticles. The Fe3O4/AC composites were tested in a supercapacitor in 1.0 M Na2SO3 electrolyte. It was found that the specific capacitance of Fe3O4/AC nanocomposites reached 150 F g 1 at a current density of 3.0 A g 1 and was a great improvement over Fe3O4 or AC alone. Furthermore, the as-prepared Fe3O4/AC nanocomposites exhibited long cycle life without obvious capacitance fading even after 1000 chargeedischarge cycles. It was suggested that 3D network of AC increased the electrical conductivity and the cycling stability of Fe3O4. In addition, Fe3O4 nanoparticles anchored on the surface of AC increased the electrochemical utilization of Fe3O4 and facilitated the rapid transport of the electrolyte ions. ZnO is another well-known active material used in batteries and it is expected that zinc oxide can also be a promising electrode material for supercapacitors. Selvakumar et al. [13] prepared nanostructured ZnO/AC composites by mixing AC with ZnO in three different ratios (1:1e1:3) in N-methylpyrrolidone along with a binder and polyvinylidene fluoride. The ZnO/AC composite electrode showed a specific capacitance of 160 F g 1 for 1:1 composition in 0.1 M Na2SO4 electrolyte. The specific capacitance of the electrodes decreased with increase in zinc oxide content. Galvanostatic chargee discharge measurements have shown that the specific capacitance is constant up to 500 cycles at current densities of 2, 4, 6, and 7 mA cm 2. Faraji and Ani [14] also prepared ZnO/AC composites by the electroless deposition method. The AC was obtained from the carbonization and activation of coconut shells, with a surface area of 980 m2 g 1. A specific capacitance of 187 F g 1 at a scan rate of 5 mV s 1 was obtained in 0.1 M Na2SO4 electrolyte by CV. The supercapacitor was quite stable during chargee discharge cycling and exhibited constant capacitance during long-term cycling. It also yielded a specific capacitance of 171 F g 1 at 5 mA cm 2 with an energy density of 21.9 Wh kg 1 and a power density of 4.2 kW kg 1. The ZnO/AC and TiO2/AC composite electrodes were found to achieve the specific capacitance in aqueous Na2SO3 electrolyte at fairly low oxide loading onto AC by Ho and Khiew [15]. High-surface-area, guava-leaf-derived, heteroatom-containing activated carbon materials were synthesized by Madhu et al. [16] by the chemical activation method. The AC was used as carbon supports to disperse nickel oxide (NiO) nanocrystals through a hydrothermal process. The as-synthesized NiO/AC nanocomposites exhibited high specific capacitance up to 461 F g 1 at a current density of 2.3 A g 1 and remarkable cycling stability.

8.2.4.2

Other Mixed Ordered Porous Carbon

Mixed porous carbon with ordered pore channels are developed to address the challenge of pore channel blockage by metal oxide particles. Chaudhari et al. [17] synthesized well-dispersed cubelike iron oxide (a-Fe2O3) nanoparticles supported on ordered multimodal porous carbon (OMPC). The OMPC produced by the glycine-assisted

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hydrothermal route has a unique combination of macropores and mesopores in the framework. The a-Fe2O3/OMPC composites reveal significant improvement in the performance as electrode active material for supercapacitors. Compared to the bare Fe2O3 and OMPC, the composite exhibits excellent cycling stability, rate capability, and enhanced specific capacitances of 294 F g 1 at 1.5 A g 1, which is twice as that of OMPC (145 F g 1) and about four times higher than that of bare Fe2O3 (85 F g 1). The improved electrochemical performance of the composite can be attributed to the well-defined structure, high conductivity, and hierarchical porosity of OMPC, as well as the unique a-Fe2O3 nanoparticles with cubelike morphology that are well-anchored on the OMPC support. Geng et al. [18] synthesized MnO2/carbon nanocomposites with hierarchical pore structure and controllable MnO2 loading using a self-limiting growth method. This was achieved by the redox reactions of KMnO4 with sacrificed carbon substrates that contain hierarchical pores. The unique pore structure allows the synthesis of nanocomposites with tunable MnO2 loading up to 83 wt%. The specific capacitance of nanocomposites increased with MnO2 loading, whereas conductivity decreased with increasing MnO2 loading. Optimization of the MnO2 loading resulted in nanocomposites with high specific capacitance and excellent rate capability. In summary, various types of porous carbon materials such as mesoporous carbon, aerogel carbon, macroporous carbon, AC, and ordered mixed porous carbon are used to composite with metal oxides to form hybrid active materials for supercapacitors. Ordered mesoporous carbon with a controlled range of pore size has the following advantages: a relatively large surface area (w1000 m2 g 1) that contributes to double-layer capacitance and helps in dispersing the metal oxide and a suitable size of pore channels for metal oxide deposition and for electrolyte ion transportation. Carbon aerogels have a surface area ranging from 500 to 800 m2 g 1, which is slightly lower than that of ordered mesoporous carbon. Carbon aerogels also have a relatively small pore size that is not favorable for the deposition of metal oxide and electrolyte ion transportation. Macroporous carbon has a relatively low surface area (w600 m2 g 1) but allows deposition of enormous metal oxide particles and has less chance of blocking the electrolyte ion transportation. Mixed porous carbon such as AC with micro-, meso-, and macropores has the largest surface area (w3000 m2 g 1) among all types of porous carbons. After deposition of metal oxide particles, micropores in AC can be easily blocked by metal oxide particles and thus cannot be accessed by electrolyte ions, resulting in low contribution to double-layer capacitance. Although there is not enough data to compare the performance of different types of porous carbonemetal oxide hybrids, generally speaking, ordered mesoporous carbonemetal oxide hybrids have the highest specific capacitance. Carbon aerogelemetal hybrids do not show any advantages over ordered mesoporous carbonemetal oxide hybrids, which is may be due to lower surface area and smaller pore size. Mixed porous carbonemetal oxide and macroporous carbonemetal oxide hybrids have similar specific capacitance values. The loading of metal oxides is a very important factor to fully utilize the synergistic effect of metal oxide and porous carbon. Optimized loading of metal oxide allows the contribution of double-layer capacitance of porous carbon and pseudocapacitance


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or faradaic contribution from metal oxides. With small loading, the pseudocapacitance of metal oxides becomes unnoticeable, whereas with too high loading, metal oxide particles could block the pore channels so that they become inaccessible to the electrolyte ions. The electrochemical double-layer capacitance contributed from porous carbon, thereafter, will be reduced. In addition, electrical conduction between carbon support and metal oxides will also be affected at high metal oxide loadings.

8.3 8.3.1

Carbon NanofibereMetal Oxide Nanocomposites Carbon Nanofibers

CNFs have been used as the support material for metal oxides in the composite electrodes because of their excellent mechanical and electrical properties. CNFs have lengths in the order of micrometers and diameters between tens of nanometers and several hundreds of nanometers. Their cylindrical nanostructure is formed by stacking the graphene sheets in a stacked platelet, ribbon, or herringbone way [19]. Several techniques are used for CNF synthesis, such as chemical vapor deposition [20], template synthesis [21], self-assembly [22], wet-chemical method [23], and electrospinning [24]. Among these techniques, electrospinning is an efficient method and can produce CNFs at reduced cost. The fiber size can be controlled by modifying the electrostatic force during the synthesis and transition metal oxides can be added as reinforcements during or after fiber growth. A typical example of CNFs fabricated by adding metal oxide contents during deposition is electrospinning a solution of polyacrylonitrile in dimethylformamide, which contains small amounts of MnO2 powders. MnO2 is one of the most studied electrode material because it is environment friendly and abundant [25]. The resulted composite material was followed by carbonization and activation processes and used to fabricate electrodes for supercapacitor applications. The CNFs showed the highest value of electrical conductivity of 2.82 10 3 S cm 1 and also the highest specific capacitance of 186.28 F g 1 when MnO2 was used with a mass load of 20 wt% in polyacrylonitrile. A different approach to prepare the composite is to coat the CNF with MnO2 after the electrospinning process by a redox reaction between permanganate ions and CNFs [26]. The obtained composite nanofibers had a specific capacitance of 539 F g 1 at a low scan rate of 2 mV s 1 and 213 F g 1 at a scan rate of 100 mV s 1. By increasing the coating time and the metal oxide thickness a much lower specific capacitance was obtained even at low scan rates, i.e., 188 F g 1 at 2 mV s 1. Zhou et al. [27] studied the effect of the mass loading of MnO2 in the composite by modifying the weight ratio of potassium permanganate and CNFs. Weight ratios between MnO2 and nanofibers were modified from 1:5 to 1:9. For a weight ratio of 1:5, the electrode presented best performances with a maximum specific capacitance of 520 F g 1 at 0.5 A g 1 current density and 220 F g 1 at 20 A g 1. The cycling stability indicated 92.3% retention of the initial capacitance after 4000 cycles. A comparative study that uses fibers obtained with and without incorporating an organometallic compound into the electrospun precursor was also conducted. After electrospinning, the fibers were coated with MnO2. Using iron acetylacetonate

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as the organometallic compound and cleaning the fibers in acid, the porosity and SSA and the electronic conductivity of CNFs were improved [28]. After immersing the fibers in potassium permanganate solution, a porous MnO2 layer was obtained on fibers, which increased their thickness by increasing the weight of the permanganate in solution. The nanomaterial had a porous and coreeshell structure. Composites obtained without using iron acetylacetonate and with an MnO2 load of 39% presented a maximum specific capacitance of 120 F g 1, which was obtained at a scan rate of 2 mV s 1. At a scan rate of 200 mV s 1, the maximum capacitance was around 40 F g 1, obtained for composites with an MnO2 load of 14%. For composites obtained using iron acetylacetonate in the electrospun precursor, the best electrochemical performances were obtained for samples produced using an MnO2 mass loading of 14%. The specific capacitance was changed from 140 to 106 F g 1 when the scan rate increased from 2 to 200 mV s 1, which represents 76% rate capability. A material obtained by using a combination of both synthesis methods described earlier was able to perform at much higher current densities. First, the composite of carbon fibers and MnOx was prepared by electrospinning of a mixture of polyacrylonitrile with manganese acetate tetrahydrate. In the second step the material was used as a conductive support for MnO2, which was grown by the hydrothermal method. The produced composite material had a hollow structure with the outer diameter in the range of 300e400 nm and an inner diameter of about 100 nm. The specific capacitance was decreased from 151.1 to 115.2 F g 1 when the current density was increased from 1 to 8 A g 1, which represents 76% rate capability. In addition to the capacitance contribution of MnO2, the hollow structure and high SSA of the composite provide active sites and internal space for accessing the electrolyte ions, which is beneficial for the pseudosupercapacitor performance [29]. Although CNFs have been used as the conductive backbone material with manganese oxides, RuO2 was combined with CNFs to obtain better charge storage performances. Hydrous RuO2 have been deposited on both the inner and outer walls of commercial CNFs by a simple solegel process. The specific capacitance of the electrodes was 645 F g 1 obtained at a scan rate of 5 mV s 1 [30]. In a different study, RuO2 nanoparticles were uniformly deposited on commercial CNFs with an average diameter of 100 nm by using conventional hydrothermal deposition. The hydrothermal process created nanosized pores on the fiber surfaces and RuO2 nanoparticles with an average diameter of 2 nm. At the highest potential scan rate of 200 mV s 1, the composite material presented a capacitance value of 155 F g 1 [31]. Similar to MnO2 and RuO2, other metal oxides such as SnO2 [32], V2O5 [33], and ZnO [34] have been used with CNFs for electrode fabrication. The electrochemical performance of nanocomposites used for supercapacitor fabrication can be increased by using even thinner carbon materials, such as CNTs, which are able to increase the total surface area of composites and to better expose metal oxide nanoparticles to the electrolyte.

8.3.2

Carbon Nanotubes

CNTs have been widely studied as electrode material for supercapacitors because of their high electrical conductivity, mechanical strength, specific area, and dimensional


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ratio. Structurally, the arrangement of graphite sheets distinguishes nanotubes from nanofibers. For CNTs, the graphite sheets are curled into cylinders that represent the morphology of single-walled and multiwalled CNTs [35]. It has been reported that pure CNT electrodes present the combined characteristic of EDLC and pseudocapacitance and could produce a specific capacitance of approximately 100 F g 1 [36]. The open porous network of CNTs coupled with metal oxides provides an easy access for ions from electrolyte to the active surface of the composite and, consequently, lowers the equivalent series resistance of the device. Composite materials of MnO2 and single-walled CNTs with weight ratios between 5 and 40 wt% were prepared by the precipitation of potassium permanganate on the surface of CNTs. All composite materials showed good cycling capability. The composite having 20 wt% CNTs shows a coulombic efficiency of 75% and a specific capacitance of 110 F g 1 at current densities of 2 A g 1 [37]. MnO2 and multiwalled nanotube composites were also used for electrode fabrication in a similar procedure [38]. The specific capacitance of the composite electrode reached 250 F g 1 for a scan rate of 2 mV s 1. Such an important difference of specific capacitance between single-walled and multi-walled CNT composites indicates that the high surface area specific for single-walled CNTs could not be exploited because of the effect of van der Waals forces that hold single-walled tubes together. MnO2 is deposited over these tube bundles in clusters that are not well exposed to electrolyte ions [39]. Most of the methods implemented to obtain CNT and metal oxide composites deposit the metal oxide on the outside walls of nanotubes. In a different approach, the composite of CNTs and MnO2 was obtained by depositing the metal oxide inside the tubes following a wet-chemical method and an ultrasonic treatment [40]. For comparison of results, CNTs with MnO2 deposited outside the tube walls were obtained by the precipitation of potassium permanganate. Electrodes were prepared with an MnO2 loading of 15 wt% for both materials. The capacitance of CNTs with encapsulated MnO2 was 225 F g 1, which represents an important improvement than the capacitance of 144 F g 1 obtained for composites with MnO2 deposited on the outside walls of the tubes. The enhancement of the pseudocapacitance of MnO2 particles encapsulated within CNTs is the result of obtaining improved contact between CNTs and MnO2, which increase the electrical conductivity of the composite material. Ruthenium oxide nanoparticles were dispersed on the surface of CNTs for improving the material utilization and reducing the mass of the noble metal. A thin and uniform deposit of RuO2 on CNT films was obtained by the potential cycling method. Films were obtained using commercial CNTs and the electrostatic spray deposition technique. The composite electrodes showed a maximum specific capacitance of 1170 F g 1 at 10 mV s 1, which decreased to 965 F g 1 at high scan rates of 400 mV s 1 [41]. In a different study, RuO2 nanoparticles were deposited on pristine and nitrogen-doped CNTs to examine the N-doping effect on composite materials used for supercapacitor applications. The CNTs were grown vertically aligned on silicon substrates by chemical vapor deposition technique assisted by microwave plasma. Nitrogen was introduced in the deposition chamber during the synthesis process to produce N-doped CNTs. RuO2 nanoparticles were deposited on pristine and N-doped CNTs using sputtering technique [42]. Doping with nitrogen created

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preferential sites on CNTs for a uniform coverage with RuO2 nanoparticles. The electrochemical performances of RuO2 on doped CNTs are enhanced as compared with those on pristine CNTs. Even at high scan rates of 2000 mV s 1, the areal capacitance increased from less than 5 to more than 30 mF cm 2. Effect of the RuO2 mass loading was also studied in this investigation. Different RuO2 mass loadings have been obtained by changing the plasma power during the sputtering process. It was shown that the electrical properties of the supercapacitor were improved by increasing the oxide mass loading on the electrode material. Vanadium oxide can be used as a commercially viable strategy because of its relatively low cost as compared to ruthenium oxide. Vanadium oxide nanospheres were uniformly deposited on vertically aligned CNTs using chemical vapor deposition. The nanocomposite material was obtained directly on metallic substrates and exhibited areal capacitances ranging from 89 mF cm 2, obtained at a scan rate of 2 mV s 1, to 14 mF cm 2, obtained at 200 mV s 1 [43]. An alternative solution for obtaining thin and uniform deposition of oxides on porous substrate surfaces, including CNTs, is atomic layer deposition (ALD). This vapor coating technique consists of heating the substrate and sequentially introducing precursor vapors in the deposition chamber. Vanadium tri-n-propoxide oxide and deionized water were used as precursors to obtain vanadium oxide films. Freestanding electrodes composed of entangled and wellbonded CNTs covered by oxide films with different thicknesses were fabricated. The obtained composites presented specific capacitances of up to 600 F g 1, considering the mass of the entire electrode. The thickness of oxide layers had a strong effect on the electrochemical performance and an oxide thickness of around 10 nm produced the best electrochemical performance [44]. The ALD technique was used in a different study to obtain electrodes composed of CNTs deposited on carbon cloth and covered by cobalt oxide nanoparticles. Oxide nanoparticles were obtained using dicarbonylcyclopentadienylcobalt as precursor and oxygen plasma. Modifying the number of cycles during the coating process controls the oxide loading on electrodes. Fig. 8.8 presented SEM and transmission electron microscopic images of CNTs covered by Co3O4 after 800, 1600, and 2400 cycles. Nanoparticles are uniformly distributed on the surface of CNTs and their size increases with increasing number of ALD cycles, i.e., from 5 nm obtained after 800 ALD cycles to 20 nm after 2400 cycles. The relationship between the amount of Co3O4 and the electrochemical performance was investigated. Areal capacitance increases with the number of ALD cycles, and the highest capacitance of 416.7 mF cm 2 was obtained after 2400 cycles (Fig. 8.6E,F) [45]. Although ALD gives the possibility to create well-distributed and uniform structures at nanometer level, it is a complex, expensive, and time-consuming technique. Alternatively, cobalt oxide nanoparticles have been obtained on CNTs using in situ decomposition at 140 C of a solution composed of cobalt nitrate dissolved in organic alcohol. The composite material displayed capacitive behavior with a specific capacitance around 200 F g 1. In a different approach, simple hydrothermal precipitation followed by thermal annealing was used to fabricate composites formed by CNTs and nickel oxide. With the addition of surfactant in the solution, the NiO nanoparticles were dispersed well on CNT surface and specific capacitances of up to 1329 F g 1 were obtained at a current density of 84 A g 1 [46]. Although composites of CNTs NiO or Co3O4


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Figure 8.6 Scanning electron microscopic and transmission electron microscopic images of nanocomposites obtained after (A,D) 800, (B,E) 1600, and (C,F) 2400 atomic layer deposition (ALD) cycles. (G) Chargeedischarge curves and (H) areal capacitance for composites obtained after different ALD cycles. CC, carbon cloth. Reproduced from C. Guan et al., Atomic layer deposition of Co3O4 on carbon nanotubes/carbon cloth for high-capacitance and ultrastable supercapacitor electrode. Nanotechnology 26 (2015) 94001.

provided promising results, combining nickel cobalt oxide(NiCo2O4) was also experimented, as it has better electron conductivity than pure NiO or Co3O4 [47]. NiCo2O4 nanocrystals with diameters less than 10 nm were deposited on functionalized bundles of single-walled CNTs by hydrolysis process using a mixture of water and ethanol as a solvent. Electrodes with composite mass loading of around 3.0 mg cm 2 presented a specific capacitance of 1642 F g 1 within 0.45 V potential range and a cycling stability of 94.1% retention after 2000 cycles [48]. In a comparative study, different metal oxides have been deposited on CNTs and the electrochemical properties were determined for electrodes with a total mass of 10 mg. The average specific capacitances were 93, 138, and 160 F g 1 for composites of CNTs with SnO2, RuO2, and TiO2, respectively.

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All composites showed increased specific capacitance compared with that of 67 F g 1 obtained for pristine CNTs. This study indicated that TiO2eCNT composite is a good material for supercapacitors [49].

8.4

GrapheneeMetal Oxide Nanocomposites

Since the discovery of graphene, they tend to replace CNTs in the fabrication of electrode materials for energy storage applications, as a strategy for improving the device performances because of their excellent physical properties. Extensive research has been conducted to combine the advantages of both graphene and metal oxides for supercapacitor applications. The unique planar structure, the large SSA, high flexibility, and high electrical conductivity of materials from the graphene family, such as graphene, graphene oxide (GO), and reduced graphene oxide [50], make them attractive candidates for conductive supports of metal oxides. The formation of composites using graphene and metal oxides can be realized using various methods including hydrothermal processing [51], coprecipitation [52], electrodeposition [53], spray pyrolysis [54], ALD [55], etc. The metal oxides deposited on the surface of graphene keep the graphene sheets separated and facilitate the interaction with the electrolyte. For supercapacitor fabrication, the active material based on MnO2 nanosheets integrated on graphene sheets has been obtained based on the electrostatic interaction between reduced graphene oxide and MnO2 sheets. The composite material was used in realizing supercapacitors with planar architecture, which yielded specific capacitances between 254 and 208 F g 1 when the current density increased from 0.5 to 10 A g 1. The capacitance retention was 92% after 7000 chargee discharge cycles [56]. Two-dimensional nanomaterials represent a promising material platform to realize thin, robust, and flexible energy storage devices. However, electrodes fabricated using low material loadings hinder the practical application when high power and high energy density are needed. A 3D macroporous material, with a pore size of around 2 mm, was obtained by vacuum filtration of a mixed aqueous colloidal suspension of chemically modified graphenes and polymer particles [57]. Polystyrene was used as sacrificial template material that was selectively removed using toluene (Fig. 8.7). Using filtration, the porous graphene-based material was combined with MnO2 and the final product presented a surface area of 142 m2 g 1. The composite with a well-defined network of interconnected pores showed an electrical conductivity of 1204 S m 1. Electrochemical characterizations indicated a specific capacitance of 389 F g 1 at 1 A g 1 and 97% capacitance retention after increasing the specific current to 35 A g 1 (Fig. 8.7E,F). Composites of GO or graphene sheets with nickel hydroxide (Ni(OH)2) were obtained by hydrolysis of nickel acetate [58]. For comparison, a third type of composite material was obtained by mixing graphene sheets with Ni(OH)2 nanoplates. For electrochemical measurements, 1 mg of the obtained composites was compressed into a Ni foam support without using any additives or binders. The electrode fabricated using the composite of graphene sheets and Ni(OH)2 synthesized by hydrolysis showed superior electrochemical performance compared to composites


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Figure 8.7 Cross-sectional (A,B) scanning electron microscopic images and (C,D) transmission electron microscopic images of porous graphene-based material at different magnifications. (E) Chargeedischarge curves and (F) areal capacitance for composites. From B.G. Choi, M. Yang, W.H. Hong, J.W. Choi, Y.S. Huh, 3D macroporous graphene frameworks for supercapacitors with high energy and power densities. ACS Nano. 6 (2012) 4020e4028.

of Ni(OH)2 and GO and compared to composites obtained by physically mixing Ni(OH)2 with graphene sheets. The average specific capacitance of Ni(OH)2 grown on graphite sheets was 887 F g 1 at a scan rate of 5 mV s 1 and 877 F g 1 at a scan rate of 40 mV s 1 (based on total mass of the sample). Galvanostatic measurements showed a specific capacitance of 935 F g 1 at a current density of 1.4 A g 1, which remained high even at a high current density of 45.7 A g 1.

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The material delivered an energy density of 37 Wh kg 1 at a power density of 10 kW kg 1. The study indicates that the degree of oxidation of graphene and the morphology and crystallinity of metal oxide play important roles in realizing electrode materials with good redox activity and scan rate capability. Various other metal oxides such as RuO2 [59], SnO2 [60], TiO2 [55], NiO [61], and Co3O4 [62] have been coupled with graphene-based materials for the fabrication of electrodes for supercapacitors. Another approach was to realize asymmetric configurations for the fabrication of macroporous electrodes for supercapacitors using for each electrode different metal oxide nanoparticles incorporated into graphene layers [63]. Although these results are promising, the loss of performance observed at high scan rate or at high metal oxide loading suggests the need to find strategies to increase both high power and energy densities for practical applications.

8.5

Conclusion and Future Directions

Metal oxideecarbon hybrid materials have been developed and used as electrode active materials for supercapacitor applications. It has been shown that significant improvement in terms of surface utilization of electroactive metal oxide, electrical and ionic conductions, available potential window, specific capacitance, cyclic stability, and energy and power density in supercapacitors can be achieved by using the hybrid electroactive materials, in comparison with electrode active materials using metal oxides or carbon materials alone. This can be attributed to the complementary and synergy behaviors of electrochemical double-layer capacitance of carbon and pseudocapacitance/faradaic of metal oxide, to the unique interface characteristics between the two materials, to the significant increase in surface areas and electrical conduction, and to nanoscale dimensional effects. Different types of carbon materials, such as porous carbon, CNTs, carbon fibers, and graphene, were used to composite with metal oxides including RuOx, NiOx, CoOx, FeOx, and MnOx. These hybrids can be fabricated by various processes such as mechanical mixing, chemical coprecipitation, hydrothermal, electrochemical anodic deposition, solegel, and other wetchemical synthesis methods. Although metal oxideecarbon hybrid materials have demonstrated their great potential for supercapacitor applications, several challenges still remain. Synthesis of the carbonemetal oxide hybrid electroactive materials, while precisely controlling their chemical composition ratio, micro-/nanostructures, phases, surface area, and interfacial characteristics, is still challenging. Depending on the preparation techniques and process parameters, the property and behaviors of the composite electroactive materials can vary significantly; therefore, the ability to synthesize composite materials with consistent properties is very important for their wide use in supercapacitors. The degradation of the composite electroactive materials stemming from the aggregation of the nanoscale components because of the relatively strong forces between them, the change in structure and chemical composition due to chargeedischarge cycling process, and material contamination due to impurity introduced during synthesis


215

processes and during reactions with electrolyte, etc. have to be resolved before the large-scale adoption and reliable use by the industry. In addition, the costs of materials and their synthesis processes have to be reduced significantly. With the increased demand of energy storage devices with both high energy and high power, it is expected that metal oxideecarbon hybrid electroactive materials will attract more and more attention by the scientific community.

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Metal Oxide/Conducting Polymer Hybrids for Application in Supercapacitors

9

Rudolf Holze Technische Universit€at Chemnitz, Chemnitz, Germany

9.1 9.1.1

Introduction The Components

Direct (i.e., without any chemical, mechanical, or other conversion) storage of electric energy is possible only with capacitors and inductances [1,2]. Although the latter principle is applied in electric and electronic devices in huge numbers (transformers, inductivities), it is unsuitable for energy storage on a larger scale and beyond very short times. The former principle is as widespread in use as the latter (see, e.g., Refs. [3,4]), but again large-scale storage has been limited by available low energy density, high self-discharge, and high costs of capacitors. The use of large-surface-area materials, such as activated carbons, employing the capacitive properties of the electrochemical double layer has resulted in capacitors showing substantially higher storage capabilities and thus higher energy densities [5], the resulting devices employing two electrodes of the same material were the first representatives of a class of devices generally called supercapacitors.1 Because charge 1

The term supercapacitorTM (as well as ultracap/ultracapacitor) and the abbreviation supercap (SC) seemingly lack a generally accepted proper definition. At first glance, it appears sufficient to assume that capacitors based on the capacitive properties of the electrochemical double layer instead of a dielectric material, such as Al2O3 or Ta2O5, showing huge capacities are correctly called supercapacitors. Temporarily the latter term was trademarked (from August 1978) to the NEC Corporation, but currently this protection has apparently expired. The acronym SC seems to be too short to enable immediate identification. Acronyms such as ES for electrochemical supercapacitor or FS for faradaic supercapacitor further increase the confusion. Recently this device, in which purely electrostatic charge storage in the double layer is operative, has been frequently called EDLC (electrochemical double-layer capacitor). Thus devices in which charge storage is based on both electrostatic charge separation (like in an EDLC) and faradaic redox processes (pseudocapacity) can be referred to as SCs. Because of the combination of these fundamentally different charge storage mechanisms, these devices are also sometimes called hybrids, adding further to the confusion. A device in which two effects or mechanisms are utilized is not necessarily a hybrid until the effects, as in most SCs are, act in addition to each other. In the present report, SCs are such “hybrid devices,” the term ultracapacitor is not used at all. Its use to designate only those devices employing pseudocapacitances seems to be a losing proposition [170]. The statement, that Conway coined the term supercapacitor in 1991 is apparently erroneous. The rich collection of terms, some of them presumably protected by trademarks, does not help really: APowerCap, BestCap, BoostCap, CAP-XX, DLCAP, EneCapTen, EVerCAP, DynaCap, Faradcap, GreenCap, Goldcap, HY-CAP, Kapton capacitor, Super capacitor, SuperCap, PAS Capacitor, PowerStor, PseudoCap, etc.


220


storage proceeds exclusively by accumulation and/or adsorption of charged species (ions) in the electrochemical double layer at the interface between the electronically conducting phase (carbon, commonly and imprecisely called electrode) and the ionically conducting phase (electrolyte solution), these devices are more precisely called electrochemical double-layer capacitors (EDLCs) [6]. Although these devices represented a substantial progress when compared with, e.g., electrolytic capacitors, they were (and still) are no match to secondary batteries in terms of energy density. The utilization of superficial redox processes at electrode surfaces as proposed first by Conway and Gileadi [7] yielded electrode materials with weight-related charge storage capabilities 10e100 orders of magnitude larger than those of EDLC materials discussed earlier (for an overview see Refs. [1,2,8]). Because of the current response observed when changing the electrode potential of such a material, the behavior was designated as being pseudocapacitive [9]. This term has a widespread use, so it is commonly applied to practically all redox-active materials possibly useful in a supercapacitor electrode. Because many materials do not show the specified current response (constant current with electrode potential/cell voltage changing linearly with time) but behave instead more like a battery electrode material, the term pseudocapacitive and its imprecise use has met some scrutiny recently [10]. Reasons for the surprising differences in current response between similar metal oxides are still a subject of ongoing research (for a preliminary discussion of some findings see Ref. [2]). Even these materials still do not satisfy in terms of charge storage capability of the single materials and energy density2 of the complete devices. Beyond fundamental properties (equivalent weight), morphology of the materials controlling the rate capability (i.e., available current per gram of material) and utilization is important. Although spurious statements in reports implying that highest numbers are frequently observed with very thin films suggest that poor data frequently reported with thicker films and layers may simply be due to insufficient utilization, this aspect has not been studied sufficiently. This gap is matched by a respective gap in electrochemical modeling: on the atomic and molecular levels the structure and dynamics at electrochemical interfaces can be modeled with an impressive degree of precision and sophistication, basically the same applies to batteries and battery packs/modules. Modeling at the electrode level applicable to, e.g., highly porous materials is still quite insufficient (this has been addressed [11,12]). Optimization of porous structures aiming at improved utilization still proceeds (despite infrequent claims of rational design) in a rather empirical manner, for a review see Ref. [13]. To remedy the often-insufficient electronic conductance of employed metal oxides (e.g., MnO2 is a wide-bandgap semiconductor), almost generally conducting additives such as acetylene black or carbon, in some of its modifications such as carbon nanotubes (CNTs), are employed by, e.g., simple mechanical mixing or pyrolytic deposition from dissolved precursors (e.g., sugar) and subsequent thermal treatment.

2

Frequently an energy is attributed to a single electrode. This is apparently incorrect. In most cases the tacit assumption is made that the values refer to the standard hydrogen electrode or another mostly unspecified point of reference.


221

Contrary to the ion accumulation/depletion process with EDLC materials, which inherently does not fix an electrode potential, these metal oxideebased materials operate based on a redox process (in many cases, e.g., RuO2, several redox transitions can be utilized), which determines the working potential of the electrode and implicitly the supercapacitor cell voltage. Intrinsically conducting polymers (ICPs) [14,15] that can be prepared and, even more important, oxidized and reduced electrochemically have been suggested as active electrode masses initially for primary and secondary batteries (numerous reviews are available, see Refs. [16e20]). Their gravimetric and volumetric charge storage densities are not overly impressive even when compared with heavy metal (e.g., PbO2) oxides as active species (Table 9.1). Despite the almost generic claims for high chemical stability, the rather reactive state of both the oxidized and reduced forms [starting with the neutral form, the former is a radical cationic state reached by electrooxidation and the latter (although rarely encountered, e.g., polythiophene) is a radical anionic state reached by electroreduction] makes them susceptible for nucleophilic (in the former case) or electrophilic (in the latter one) reactions involving mostly electrolyte solution constituents. Subsequently the molecular structure will deteriorate, resulting in e.g., diminished electronic conductance because of chain and thus conjugation distortion. Nevertheless, research has been and is still intense for various reasons, particularly because metal species in the electrode mass can be avoided. A major drawback of all ICPs is their variable conductance (Fig. 9.1). In the neutral state, they are very poor electronic conductors (or semiconductors, proper assignment appears to be a question of purity, mode of preparation, and even terminology [22]), whereas in the oxidized (as well as in the reduced) state, they are electronic conductors.

Table 9.1

Theoretical and Experimental Capacitance Data of ICPs [21]

ICP

Molecular Weight of Repeat Unit (g)

Dopant Level

Theoretical Capacitance (F gL1)

Measured Capacitance (F gL1)

PANI

93

0.5

750

240

PPy

67

0.33

620

530

PTh

84

0.33

485

e

PEDOT

142

0.33

210

92

For comparison PbO2

239.20

2

224

MnO2

86.94

2

616

ICP, intrinsically conducting polymer; PANI, polyaniline; PEDOT, poly[3,4-ethylenedioxythiophene]; PPy, polypyrrole; PTh, polythiophene.

222


Figure 9.1 Change of conductance of a polyaniline film as a function of electrode potential in different electrolyte solutions (for details see Refs. [23,24]).

The achieved conductances (specific conductance values, i.e., conductivities) are difficult to measure because of the mostly porous nature of the obtained products, they depend on the concentration of charge carriers (in turn depending on the degree of oxidation, i.e., the amount of charge injected into or withdrawn from when leaving the neutral state by electrochemically means) and their mobility. The mobility is strongly influenced by the molecular structure, the degree of conjugation, the length of conjugated segments, the ease of charge transfer between molecular chains (sometimes called hopping), and further factors. Because charge and discharge of an ICP used as active mass in an electrode will always imply a redox process, the conductance of the active material will change widely (up to several orders of magnitude, see Fig. 9.1) during operation. This is not acceptable for an application because a poorly conducting active material will seriously limit the rate capability of an electrode. Consequently, many reports have mentioned the addition of electronically conducting ingredients (again, mostly carbon in its various modifications [25,26]), as reported earlier for metal oxides. The added amounts appear to be the result of some guesswork, and until today, no well-defined and carefully executed study of optimized amounts of such additives has been reported. Because these additives hardly contribute to the charge storage capability, they diminish the available charge density. The same applies to added binders. The charge storage processes in an ICP, although a faradaic redox process just as the processes occurring in metal oxides, is fundamentally different from those in metal oxides. They involve electron-transfer reactions at repeat units of a polymer chain. This can be envisaged starting with a typical cyclic voltammogram (CV) of a polyaniline (PANI)-coated stainless steel grid as shown in Fig. 9.2. In the positive-going scan, two oxidation peaks can be discerned, which are caused by redox transformation of the ICP, and they slowly grow in terms of current and thus in stored charge with growing


223

2.0 1.5

l / mA

1.0 0.5 0.0 -0.5 -1.0 -1.5 0.0

0.2

0.4

0.6 ERHE / V

0.8

1.0

Figure 9.2 Cyclic voltammogram of a polyaniline-coated (300 electrode potential cycles within the potential range as indicated) stainless steel grid electrode (1 cm2) in an aqueous electrolyte solution of 0.1 M aniline þ 1 M HClO4, dE/dt ¼ 100 mV s1, nitrogen purged.

number of electrode potential cycles. The first is related to the transition from the leucoemeraldine salt state to the electronically highly conducting emeraldine salt (protonated PANI because of solution acidity) state; the second, to the transition into the poorly conducting pernigraniline state. In the reverse scan, these processes are going backward. Poorly prepared PANI sometimes shows an additional peak between these major peaks that is attributed to degradation products formed at electrode potentials positive to the second peak [27,28]. For electrochemical charge (i.e., energy) storage, only the first transition can be utilized, as use of the second will result in rapid degradation. The corresponding molecular transformations have been studied extensively using electrochemical and, in particular, spectroelectrochemical methods [14,15]. Because of the particular effect of protons on the behavior of PANI, Fig. 9.3 shows the scheme that is the current understanding of the rather complicated pathways. With a thicker film and with an electrode potential range limited to the first transition, an anodic current peak as shown in the CV in Fig. 9.5 is seen still reminding very much of the typical CV in Fig. 9.2. The positive-going peak potential shift attributed to slow charge transfer in case of CVs with dissolved species is most likely caused by ohmic resistance within the polymer film, which changes from a poorly conducting material in the neutral3 to a highly conducting one in the course of the scan. Peak broadening caused by a wide distribution of conjugation lengths (this is the important and behavior-controlling aspect, the overall length of polymer chains is 3

This state is frequently and erroneously called the reduced state. A reduced state would correctly be reached by the reduction of the neutral stated (the leucoemeraldine state). This has not been observed so far with PANI.

224


Figure 9.3 Simplified schematic representation of the molecular transformations during redox transitions of PANI [14,15].

only the upper limit of conjugation length) cannot be the reason, the increased thickness of the film as compared to the one in Figs. 9.2 and 9.4 cannot cause a widening of the distribution of effective lengths during or after electropolymerization. In the negative-going scan, no peak is observed and the response is almost flat with a current growing toward the negative limit. Based on the current knowledge of the behavior of ICPs, this can be assigned to the reduction of even longer oligomer units [27e30]. Besides this process, diffusiondin this case, egressdof ions needed for charge compensation (i.e., acting as counterions) may also be effective. When again compared to the shape of the CV in Fig. 9.5 the higher thickness of the film, although highly porous, may indeed cause a larger effect of slow diffusion, i.e., egress of counterions. Certainly, the shape does not suggest the current in the electrode potential range of the current wave to be called pseudocapacitive, because only at electrode potentials


0.5

225

D B

0.4 0.3

C

I / mA

0.2 A

0.1 0.0 -0.1 -0.2 -0.3 0.0

0.2

0.4

0.6 0.8 ERHE / V

1.0

1.2

Figure 9.4 Cyclic voltammogram of a polyaniline-coated (50 electrode potential cycles within the potential range as indicated) stainless steel grid electrode (1 cm2) in an aqueous electrolyte solution of 0.1 M aniline þ 1 M HClO4, dE/dt ¼ 100 mV s1, nitrogen purged.

beyond the wave, this description may be applicable. The observed specific shape of the current response has been noticed earlier in numerous studies of ICPs. A complete explanation of the shape of the curve appears to be important. In extensive reviews on ICPs, Heinze [31,32] has collected the various explanations of the particular shape of CVs of thin films (as in Figs. 9.2 and 9.4) starting with the common feature of a pronounced anodic peak followed by a current plateau. The discussion is/was strongly focused on peak shape and its relation to influences of

I / mA

2

dE/dt =

0

-1

1 mV·s 5 50 100

-2

0.2

0.3

0.4

0.5 0.6 ERHE / V

0.7

0.8

0.9

Figure 9.5 Cyclic voltammogram of a polyaniline-coated (7.47 mg cm2, deposition charge 0.108 C) stainless steel grid electrode (1 cm2) in contact with an aqueous electrolyte solution of 1 M HClO4, at different scan rates as indicated, nitrogen purged.

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counterion diffusion and charge propagation in the film and between the electroreaction sites. The same limitation applies to peak separation and shift of peak positions as a function of, e.g., scan rate. The fairly large current at electrode potentials positive to the said peak has attracted minor attention only. This current was tentatively called capacitive [33e36], and it was even claimed [35] that faradaic processes associated with current peaks proceed only in the potential range of the peak, whereas the current plateau is associated with double-layer capacity charging. Impedance measurements seemed to support this [33,36], but measurements reported elsewhere provided a clear separation between true double-layer capacity and redox capacity [37]. Further consideration of both experimental evidence with oligomers of specified chain lengths as well as of theoretical evidence related to charged species (e.g., polarons and bipolarons) should be considered. Multiple charge transfers to chain segments at electrode potentials growing with the number of charges per chain segment result in multiple peaks already with oligomers. Studies of an aniline tetramer loaded onto graphene oxide [38] might have provided evidence confirming or refuting this model; with aniline tetramers, the current positive to the first oxidation peak should be much smaller. Unfortunately, no CVs of thin films of aniline tetramers only without graphene oxide were provided. The CVs shown instead have two weak peaks as observed with thin films of PANI. Whether this implies that in thick films these peaks are overlapped and whether this is caused by the mentioned ohmic effects or by the wide distribution of conjugation lengths remains unanswered, as mentioned earlier. Adding to this the distribution of chain lengths in an actual polymer, numerous presumably overlapping peaks will follow. Because the relationship between oxidation potential and chain length, presumably and more precisely conjugation length, shows an asymptotic approach to a limiting value [30], the observed current peak is due exactly to this large number of available states/sites. At more positive potentials the number of sites is less. Digital simulations support this model [39]. These considerations may not explain sufficiently and convincingly the change from the pronounced peak in Figs. 9.2 and 9.4 to the broad wave in Fig. 9.5. In addition the explanations given for metal oxides [40] can be taken into account. Whether electronic interactions between sites have a significant effect on oxidation potentials remains an open question.

9.1.2

The Combination

As reviewed elsewhere [17,18,20,21,41,42], ICPs can be employed in electrodes of electrochemical energy conversion and storage devices for various purposes besides being the active ingredients themselves, i.e., as binder and as conductance enhancer combined with a metal oxide. In the former use, an ICP will keep the active mass together and may add to the charge storage capability, whereas in the latter case, conductance will be improved and again storage may be enlarged. The morphology of ICPs (for an overview see Ref. [43]) may even provide a mechanical buffer ameliorating volume and shape change of many electrode materials during charge and discharge. The reduced state of an ICP will not be considered in the following discussion because it is observed only with a few ICPs and appears unlikely to result in a


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useful combination with a metal oxide. If such combination is ever discovered, the same arguments provided here for the oxidized case can be applied with properly exchanged signs. But the already mentioned significant change of conductance poses a problem when the ICP serves as the conductance enhancer. Various combinations have to be distinguished: 1. The redox process (or the processes) of the metal oxide proceeds around a formal potential positive to the (first) oxidation potential of the ICP (range C in Fig. 9.4). The ICP will always stay in the highly conductive state and execute its task. Its redox capability and thus its charge storage possibility are only partially exploited. In the oxidized state the degree of oxidation still changes with electrode potential because even at an electrode potential positive to the first oxidation peak, there are still neutral conjugated segments available for oxidation. Depending on the type of polymer, the onset of further oxidation occurs, i.e., removal of a second electron from a conjugated segment (corresponding to the polaronebipolaron transition in case of PANI). Further details have been discussed elsewhere in greater detail [40]. This first transition is still visible, although less pronounced, in Fig. 9.5, with a thick deposit of PANI measured under the same conditions. The reduction is not evident as a current wave anymore. 2. The redox transition of the metal oxide proceeds close to the electrode potential where the ICP shows its redox process (range B in Fig. 9.4). Now both materials contribute to charge storage. As discussed elsewhere, the current peak implying a much larger redox charge in range B than in range C is caused by the large, dominating fraction of shorter segments (more precisely segments of a particular conjugation length) with a lower oxidation potential corresponding to the peak potential than longer segments oxidized in range C. Unfortunately, the changing conductance of the ICP will be a possible reason of rate limitation particularly when the ICP is oxidized. Accordingly, the rate limitation will be particularly pronounced during charging. So far, ICPs studied only in the neutral and oxidized (the term p-doped should be avoided because it is inappropriate) states are suitable for use as a positive electrode. 3. The redox transition of the metal oxide proceeds in a range of electrode potentials significantly below the first oxidation peak. The ICP will stay in its poorly conducting/semiconducting/ insulating state. Hence, the double-layer capacity will contribute only partially because of the large ohmic interference and no redox capacitance will be effective. Only mechanical contributions (shape stabilization) can be expected. In case of redox transition of the metal oxide in range D, PANI will be poorly conducting again and the same arguments as for range A apply. Other ICPs not showing this second oxidation (this is the majority of ICPs) will stay conducting (as far as data have reported). The stability of ICPs versus chemical degradation generally tends to decrease with more positive potentials.

9.1.3

The Applications4

The publications available when preparing this report (in August 2016) are reviewed. Various organizations of the material have been considered: according to the identity 4

In the following text, no specific numbers (gravimetric or volumetric ones pertaining to a single electrode, to active masses only, to a device, etc.) are reported because of the rather limited use, given the very poorly defined measurement conditions and lack of standards far away from any reasonable comparability.

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of the metal oxide, the ICP, its use (as mechanical support, conductance enhancer, storage material), or the mechanical arrangement (coreeshell, metal oxide embedded in ICP, layer-by-layer, etc.). Although previous authors [44] covering part of this overview have followed the first ordering, none of them was adopted here primarily. Whether explicitly stated or not, the prepared materials show some specific structure on the microscopic level. Numerous authors suggest the formation of hybrid materials without addressing the reasons for this designation, frequently composite might be the more appropriate term. In any case, controlled formation of nanostructures may provide beneficial effects [45]. Subsequently first binary combinations will be considered, and then the ternary ones. Even this distinction is imprecise because some authors specify the support used for handling their material and treat it as a component of the active mass (this constitutes a ternary material), whereas other authors barely mention it (a binary material). Within these major sections, reports are grouped according to the employed ICP. As far as possible, oxides of a given element are the next sorting criterion. Of particular interest are interactions between the various constituents. Better understanding will help in interpreting the observed behavior, and even more importantly, it might guide further research and development. However, the vast majority of publications do not even touch this most important aspect, frequently unspecified synergistic aspects are invoked only.

9.1.4 9.1.4.1

Binary Materials Polyaniline

Polyoxymetalates being highly soluble in most solvents because of their large charge density can be immobilized with an ICP [e.g., PANI or polypyrrole (PPy)]; in the hybrid material, redox storage of both the ICP and the polyoxymetalate is utilized [46e49]. Nanoparticles of ZrO2 were incorporated into PANI by adding the particles to the polymerization solution [50]. The material showed a capacity retention of 88% after 500 cycles. PANI formed by chemical oxidation in the presence of colloidal SnO particles yielded a composite material with particularly high capacity retention after more than 10,000 cycles [51]. The SnO2 formed during the procedure was found on the PANI fibers, and further polymer growth embedded them into PANI (it remains surprising to notice the author’s statement that SnO2 acts as a backbone for PANI). PANI improves the electronic conductance and may also provide some own charge storage capability. SnO2 might stabilize PANI by keeping the electrode potential safely away from the region of overoxidation. Perhaps the redox potential of the SnO2 electrode material is conveniently close to the potential of PANI oxidation. A PANI/SnO2 prepared by chemical polymerization with simultaneous hydrolysis of SnCl4 by Li et al. [52] showed a higher capacitance than of PANI alone. A pore size distribution more favorable for ion transport in case of the hybrid as compared to PANI was noticed. Rao et al. [53] prepared an SnO2/PANI composite by exposing PANI


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microparticles and nanofibers to an aqueous solution of SnCl2 and by chemical polymerization of aniline in the presence of SnCl2 in the polymerization solution. In the former case, SnO2 particles are deposited on the surface of PANI, whereas in the latter, these particles are found on the surface and embedded in the polymer. An enhanced electronic conductance (of PANI presumably) and improved electrochemical properties, in particular, capacitance, were reported. Patil et al. [54] prepared a PANI/ SnO2 composite by chemical polymerization of aniline in the presence of SnO2 nanoparticles dispersed in the polymerization solution. The observed capacitance values were higher than those of the single components. To utilize the attractive storage capability of Ni(OH)2 generally hampered by lack of stability and poor electronic conductance, flowerlike Ni(OH)2 has been decorated onto fibrous PANI [55]. The less impressive stability of about 20% capacity fading during 2500 cycles may be due to the direct exposure of the Ni(OH)2 to the electrolyte solution. This may in turn be responsible for the high capacity and rate capability. PANI appears to modify the crystallinity of Ni(OH)2. PANI coated by a double surfactant method on MnO2 nanowires yielded a material with high capacitance, rate capability, and stability [56]. Although the thin coating presumably did not contribute much to the capacity, it improved electronic conductance thus increasing utilized capacity and high rate performance, presumably it also contributed to the enhanced stability. MnO2 nanorods were coated with PANI by chemical polymerization of aniline [57]. An increased capacitance of the composite when compared with plain MnO2 was found. Contributions from redox and double-layer capacitance could be separated by impedance measurements at a ratio of 10,800:1. Electrochemical codeposition of PANI (from aniline monomers) and MnO2 (from KMnO4 in solution) yielded films with fibrous morphology [58] and a capacity retention of 76% after 1200 cycles. A similar approach was pursued by Zou et al. [59]. With a nonaqueous electrolyte solution a capacity retention of 82% after 1500 cycles was found. PANI nanofibers prepared with MnO2 nanofibers as seeds have been synthesized by Iranagh et al. [60]. In the hardly comprehensible report, it remains unclear how manganese metal is formed and how this metal, if present, will enhance PANI conductance. PANI formed electrochemically in the presence of MnO2 nanofibers dispersed in small amounts in the electrolyte solution has been described by Siadat [61]. Increase of capacitance when compared with plain PANI was observed and attributed to unspecified synergistic effects. PANI nanofibers were coated with a layer of floccular K-birnessite MnO2 [62]. The capacitance is attributed to both the constituents; the performance, to the particular morphology. Ansari et al. [63] prepared a composite of fibrous PANI and MnO2 by simultaneous deposition from a solution containing aniline, KMnO4, and an added chemical oxidant. No explanation was provided why permanganate ions were reduced under these conditions. A layered material with PANI coated onto a layer of MnO2 with a further layer of MnO2 on top was reported by Sun et al. [64]. The performance improvement in comparison to a bilayer system without PANI was attributed to the uniform mesoporous structure facilitating ion transport and to the increased electronic conductance. Prasad and Miura [65] electrodeposited MnO2 on PANI. After optimization of electrodeposition parameters for both

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constituents, improved stability and capacitance were found. Hu et al. prepared a copolymer [66] of aniline and o-aminophenol [67]. Subsequently the ICP was treated with an aqueous solution of KMnO4. Formation of MnO2 was claimed based on an infrared band found subsequently. The actual amount of MnO2 in the composite was not determined. A slight stability improvement was observed in comparison to the plain copolymer and it was attributed unspecifically to the presence of MnO2. A composite of PANI and MnO2 was prepared in a simple one-step procedure by Meng et al. [68]. Selvaraj et al. [69] prepared a PANI/MnO2 composite by chemical mixing of PANI prepared by chemical polymerization and MnO2 obtained by reduction of KMnO4. Best data were obtained at a content of 5% PANI with further 30% of Ketjenblack added for improved conductance. Capacitance increase was attributed to the presence of PANI, and a capacity retention of 77% after 1000 cycles was reported. Zhou et al. [70] prepared PANI from aniline adsorbed on a carbon-powder-coated titanium substrate by chemical oxidation in an aqueous solution saturated with KMnO4. CVs recorded in acidic solution yielded redox peaks typical of PANI, but in neutral solution the pseudocapacitive behavior of MnO2 prevailed. A capacity retention of 61% after 5000 cycles was observed. A composite of MnO2 and PANI was prepared by utilizing the oxidative property of MnO2 to oxidize aniline monomers [71]. No data pertaining to stability were provided. Zhang et al. [72] prepared MnO2 intercalated with PANI by an exchange reaction. Increased capacitance with respect to the data for both constituents and only a slightly improved stability during the first 1000 cycles were noted. A PANI/RuO2 composite was prepared via Ru(II)-mediated chemical synthesis [73]. From cyclic voltammetry of the product, interactions between the RuO2 particles of unspecified size and PANI have been concluded. RuO2 was grown by atomic layer deposition on PANI nanofibers by Xia et al. [74]. Improved stability of PANI was attributed to the RuO2 layer, and this layer also contributed to the capacitance. Layered PANI/RuO2 with different sequences of the layers has been prepared [75]. A more flat top layer of RuO2 resulted in better stability, whereas a rougher top layer resulted in higher rate capability. Chemical polymerization of aniline in the presence of dispersed CuO particles yielded a composite with twice the storage capacity of CuO and better rate capability [76]. Although not stated explicitly, the improvements are mostly due to the increased overall electronic conductance. Cycling stability may be improved. A CuO/PANI composite was prepared and compared with CuO/poly[3,4-ethylenedioxythiophene] (PEDOT) and CuO/PPy [77], all were synthesized by electropolymerization with small amounts of CuO particles in the electrolyte solution. Stability was monitored during 500 cycles and CuO/PANI performed best. An electronically highly conducting composite of MoS2 sheets with intercalated PANI was prepared by Wang et al. [78]. Depending on the composition, doublelayer capacity contributions from MoS2 and redox contributions from PANI were identified. Intercalation of PANI (one to two monolayers) into layered V2O5 by direct reaction of aniline with the metal oxide has been observed [79], with the electronic conductance depending on the PANIemetal oxide ratio. A smaller PANI content resulted in higher electronic conductance.


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A composite of PANI sulfate and TiO2 was prepared by chemical polymerization of aniline in the presence of TiO2 [80]. Particular attention was paid to the action of sodium lauryl sulfate added as a surfactant to the polymerization solution. Although the performance data are not exceptional, the role of the surfactant/counterion has been identified: It seems to accelerate ingress/egress of electrolyte. Given the provided evidence, this conclusion is mysterious. Gottam and Srinivasan [81] obtained a PANI/ TiO2 composite in a one-step procedure using peroxotitanium acid as the oxidant for aniline. High capacity retention of 83.5% after 30,000 cycles proves the significant stability. PANI was attached to TiO2 nanotubes by graft polymerization [82], yielding an electrode material superior in performance than that prepared by simple polymerization. Xu et al. [83] observed TiO2 particles dispersed on the internal netlike structure of PANI prepared by chemical polymerization in the presence of neutral red. A capacitance improvement of 22% for the polymer with TiO2 when compared to the one without TiO2 was attributed to the presence of TiO2, without giving further details. Fibriform PANI prepared by chemical polymerization in the presence of TiO2 nanoparticles yielding a PANI/TiO2 composite has been reported. Negligible capacity fading after 12,000 cycles and high rate capability are attributed to the favorable morphology accommodating, among other possible benefits, volume changes during cycling. On the chemically functionalized surface of urchinlike mesoporous TiO2, PANI was coated by graft polymerization [84]. Large active surface area caused by the TiO2 morphology and mechanical stability contributed to almost 100% capacity retention after 500 cycles. Hollow hexagonal MoO3 nanorods were decorated with a shell of PANI [85]. In comparison to undecorated nanorods and plain PANI, enhanced capacity and stability were observed and attributed to the coating with PANI. A composite of PANI and MoS2 showed very high electronic conductance [86]. This enabled extremely high utilization of the redox capacitance of both the constituents with a still insufficient stability. A composite of PANI and tantalum(IV)oxide prepared by Njomo et al. [87] showed no evidence of any electrochemical activity of the metal oxide; its purpose was nowhere addressed or revealed. A nanocomposite of PANI and WoO3 was prepared by chemical deposition of PANI on WO3 nanograins deposited on a support [88]. The claimed purely pseudocapacitive behavior is in striking contradiction with the displayed CV rich in several peaks, at least an overall low capacitance was concluded. Excellent but unspecified properties of a composite of Eu2O3 nanoparticles and poly-o-aminophenol have been mentioned [89].

9.1.4.2

Poly[3,4-ethylenedioxythiophene]

SnO2 nanoparticles dispersed in PEDOTepolystyrene sulfonate (PSS) [90,91] caused a rather unsurprising increase in capacitance [92]. Presumably the larger surface area of the composite in comparison to plain PEDOTePSS evidenced from the BrunauerEmmett-Teller (BET) measurements also contributed to the increased capacity. A coreeshell structured material with hexagonal ZnO nanowires and a PEDOT coating was reported by Wang et al. [93]. An intermediate layer of poly(trifluoroethyl

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methacrylate)-block-poly(sodium styrene sulfonate) (PTFEMA-b-PSSNa) was grafted onto the nanowires, and finally PEDOT was precipitated onto this layer by interaction with PSSNa. Stability and other advantages of the procedure and the obtained products were not reported. Coreeshell particles of a-Fe2O3 coated with PEDOT have been prepared by Park et al. [94]. The coating stabilized the otherwise poorly performing a-Fe2O3. MnO2 nanoparticles were incorporated into porous PEDOT by Yang et al. [95]. The reported performance was attributed to the highly porous and electronically conducting host morphology of PEDOT. In addition, PEDOT prevented agglomeration of the MnO2 nanoparticles, thus enabling high utilization. Whether PEDOT contributed to charge storage was not studied. PEDOT nanowires were prepared using an alumina template [96]. Subsequent soaking in a solution of KMnO4 resulted in the formation of MnO2 nanocrystals. The reduction of permanganate ions proceeded via oxidation of sulfur in PEDOT. The material has so far been studied only for lithium-ion batteries. Spontaneous reduction of KMnO4 at PEDOT yielding MnO2 particles incorporated into the ICP has been observed [97]. Performance was ascribed to the fine dispersion of the metal oxide particles in an electronically conducting highly porous ICP matrix with large active surface area. About 82% capacity retention after 2200 cycles was observed. PEDOT was used as a coating on MnO2 nanoflakes electrodeposited on ramie-derived carbon nanofibers [98]. The high electronic conductance of the nanofibers and the ICP was stressed, but synergistic effects between the three components were not specified. A PEDOT-PSS-MnO2 composite was obtained by freeze-drying a solution of the ICP with dispersed MnO2 particles [99]. Only 5% capacity fading after 2000 cycles was observed. Rate performance and mass utilization were attributed to the favorable spongy morphology caused by the formation procedure. MnO2 nanorods were coated with PEDOT by chemical polymerization [57]. An increased capacitance of the composite when compared with plain MnO2 was found. Contributions from redox and double-layer capacitance could be separated by impedance measurements at a ratio of 96,400:1. A sandwich-like layered arrangement of PEDOT/MnO2/PEDOT has been reported by Tang et al. [100]. The observed capacitance was ascribed exclusively to the metal oxide. About 99.5% capacity retention after 1000 cycles was observed. PEDOTePSS with incorporated small RuO2 particles prepared by Huang et al. [101] showed increased capacitance of RuO2, and this was attributed to the small particle size and the homogeneous distribution in the conducting polymer. PEDOT was coated with RuO2 by Hong et al. [102]. Based on impedance measurements, contributions to overall capacitance from the electrochemical double layer and the redox processes of PEDOT and RuO2 were identified. Poly(3,6-dithien-2-yl-9H-carbazol-9-yl acetic acid) prepared by electropolymerization of the monomer on a stainless steel electrode subsequently modified by strongly adsorbed TiO2 particles of various sizes was used in a symmetric capacitor (the naming pseudocapacitor appears to be an unnecessary extension of terminology) utilizing the fairly rare capability of this material for both p- and n-doping [103]. Improved capacity of the device was attributed to an effect of the TiO2 not further specified and to the redox capacitance contribution of TiO2 (it remains unclear how this should work in


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both electrodes), and the increased high rate performance and stability evidenced from figure (4f) in Ref. [103] was attributed to the inhibition of deformation of the ICP surface. A layered MoS2ePEDOT composite was prepared by Wang et al. [104]. Compared to that of PEDOT alone, the capacitance increased fourfold. PANI intercalated into MoO3 yielded an electronically highly conducting and stable sandwichlike structure [105]. Rios et al. [106] electrodeposited MnO2 onto titanium coated with poly(3methylthiophene). In comparison to the uncoated metal oxide, an increased capacitance was found. Contribution of the ICP to the capacitance was considered minor, with the major improvement of capacitance assigned to the metal oxide, particularly to its morphology. The improvement was more pronounced for a thin film, but this sample showed a much poorer rate capability.

9.1.4.3

Polypyrrole

PPy prepared by chemical polymerization in the presence of phosphomolybdate yielding a hybrid material with controlled porosity (provided by addition of controlled amounts of porogens) was studied by Suppes et al. [107]. Again the ICP immobilized the phosphomolybdate, and an increase in porosity improved the high rate performance. The overall capacitance was attributed to both the constituents. PPy nanotubes yielded a composite by chemical reaction in an aqueous solution with KMnO4 [108]. MnO2 was found outside the nanotubes. High capacity and stabil selj et al. [109] preity (as compared to plain PPy) were attributed to this coating. Se pared MnO2/PPy composites by chemical oxidation of pyrrole with KMnO4 or by oxidation with FeCl3 in the presence of MnO2 particles. Applied electrode potentials below ESCE ¼ 0 V resulted in capacity loss of the former composite, whereas the latter was more stable even at low electrode potentials. This was attributed to PPy acting as a protector for embedded MnO2 particles. In a barely comprehensible report, formation of a composite apparently composed of MnO2 nanowires coated with PPy has been addressed [110]. PPy prepared by electropolymerization in the presence of MnSO4 in the solution contained nanoparticles of the metal oxide [111]. The porous polymer matrix with large active surface area was found to be beneficial, and the enhanced electronic conductance was ascribed to the metal oxide nanoparticles, which also improved stability by interlinking polymer chains. Initial capacity loss was attributed to release of poorly attached particles. PPy was deposited electrochemically on TiO2 nanotubes by Gao et al. [112,113]. The high surface area of the nanotubes and the electronic conductance of the ICP could be utilized; however, the observed capacity retention of 92.6% after 1000 cycles needs improvement. Nanocones of PPy prepared by electrodeposition were coated with a thin layer of RuO2 [114]. A capacitance triple as that of uncoated nanocones was observed and attributed to the particular morphology, yielding a large interfacial contact area. PPy was chemically deposited onto surface-functionalized CeO2 particles by Wang et al. [115]. Improved performance and higher electronic conductance when compared with plain PPy were observed and attributed to specifics of the interaction

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between PPy and CeO2, and details like rate capability and stability depended on the chemical identity of the functionalization. Honeycomblike Fe2O3 (hematite) nanoflakes deposited on a nickel foam as support were decorated with branched PPy nanoleaves [116]; high capacity and significant stability (97.1% retention after 3000 cycles) were reported. Nanowires of nickel cobaltite NiCo2O4 were deposited onto nickel foam and subsequently coated with a shell of PPy [117]. The PPy coating kept the nickel cobaltite in place during volume changes when cycled and in steady electric contact improving the stability (94.8% capacity retention after 5000 cycles). By cathodic deposition of Co3O4 and polyindole a composite was obtained [118] with the metal oxide anchored by the ICP. Its superior performance when compared with the single components was attributed to unspecified synergistic effects. Metal ion supercapacitors employ a carbon-based electrode (as used in EDLC supercapacitors) and an electrode as used in a metal ion battery. Because the electrode reaction at the latter electrode is metal ion ingress/egress, the electrode potential is determined by this process, i.e., rather negative. This results in a larger cell voltage and a larger energy density. Because the total release of metal ions during discharge must be limited (the metal concentration established by the “predoping” with said metal must not decrease to zero), the actually useful energy density understood as the difference in energy content between the charged and the discharged state is smaller; but given that charge and discharge proceed at higher voltages, the energy density is larger than the number for a similar cell without such processes at the (mostly negative) metal ion electrode. Research on electrode materials for both types of electrodes is progressing. For safety reasons, aqueous electrolyte solutions are preferred. Because of the large number of electrons involved in the electrode reaction and the low density, aluminum is particularly attractive [119]. Hollow MoO3 nanotubes coated with PPy prepared by chemical polymerization showed a capacity retention of 93% after 1800 cycles [119]. The beneficial effect of PPy was explained by assuming that it helped in maintaining the structure possibly affected by volume change during cycling and buffered the effects of hydrolysis of the Al2(SO4)3 electrolyte, which might otherwise result in acid etching of MoO3.

9.1.5 9.1.5.1

Ternary Materials Polyaniline

A ternary material of PANI, a metal oxide (SnO2, Fe3O4, TiO2) and multiwalled carbon nanotubes (MWCNTs) was wet-spun into fibers [120]. Strong effects of anions on the reported capacitances were noted. Only a rather general improvement of capacitance performance of PANI was concluded. A nanocomposite of SnO2 and reduced graphene oxide (rGO) finally coated with PANI was prepared by Nguyen and Shim [121]. Results imply a lower capacitance when the electrodes were studied in sulfuric acid electrolyte solution but a better rate capability than in a solution of Na2SO4. The latter observation agrees with the higher redox activity of PANI in acidic solution, the former requires further investigation. In acidic solution, coulombic efficiency seems to be disappointing, in particular, at low rates, but this also requires further work. Wang


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et al. prepared a composite material based on nitrogen-doped graphene with nickel ferrite (NiFe2O4) and PANI [122]. The nickel ferrite was deposited first on graphene hydrothermally, subsequently coating and embedding by PANI was afforded by chemical reaction yielding a material with a capacity retention of 90% after 10,000 cycles at 5 A g1. Graphene oxide reduced chemically with an organometallic molybdenum precursor yielded a rGO/MoO3 composite that was finally coated with PANI by chemical polymerization [123]. Simple preparation and high energy density were claimed as advantages, but nothing specific is stated about the function of the various components. A capacity retention after 200 cycles of 86.6% in acidic electrolyte solution and of 73.4% in neutral electrolyte solution was reported. A ternary composite of MoO3, PANI, and graphene nanoplatelets was prepared by chemical polymerization in the presence of the other constituents [124]. A capacity retention of 92.4% after 1000 cycles was reported. Arrays of TiO2 nanotubes coated with PANI and graphene (the details are difficult to follow in the report) were studied by Huang et al. [125] as supercapacitor electrodes. The good performance was attributed to the three-dimensional structure, which prevents mechanical deformation during chargeedischarge and enables efficient ion transport. Nanowires of TiO2 and activated carbon were prepared by a hydrothermal method, subsequently a coating of PANI was applied by chemical polymerization [126]. The presence of TiO2 improved stability significantly. Graphene was coated with a composite of Ag2O and PANI in a chemical oxidation procedure, with both aniline and AgNO3 in the reaction solution [127]. Based on the stability (capacity retention of 85% after 3000 cycles) and the large capacitance, unspecified synergistic effects were claimed. Small PANI particle size and a claimed increase in electronic conductance of the composite were suggested as possible explanations. Copper nanowires were oxidized yielding CuO-coated nanowires. Then aniline and graphene oxide were mixed in the presence of the nanowires with an oxidant yielding a coating of PANI, and in a following hydrothermal procedure, rGO was formed [128]. A high stability with 97.4% capacity retention after 10,000 cycles was noticed, and the high energy density was attributed to efficient mass utilization supported by a suitable porous structure and high electronic conductance. Graphene oxide nanosheets were functionalized with PANI, and subsequent exposure to solutions of the respective metal salts yielded the respective metal oxide/hydroxide (Co3O4, Fe2O3, and Ni(OH)3) [129]. Improved performance (95% capacity retention after 2500 cycles) when compared with that of a material with only two of the three constituents was reported. Graphene oxide reduced hydrothermally by Fe2þ ions yielded rGO coated with finely dispersed a-Fe2O3 [130]. Subsequently chemical polymerization of aniline provided a coating of PANI. A capacity decay of 8% after 5000 cycles was observed. The thin coating prevented agglomeration and dissolution during cycling and mitigated the effects of volume change. PANI and cobalt ferrite (CoFe2O4) were deposited onto graphene oxide by reaction of Co(NO3)2 and Fe(NO3)3 in a hydrothermal reaction, subsequently a PANI coating was applied by chemical polymerization [131]. A capacity retention of 96% after 5000 cycles was reported, and a significant improvement in comparison to composites

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employing only two of the three constituents was noted. Similar observations with a slightly inferior stability were reported by the same authors [132] for manganese ferrite. MnO2 nanoparticles were deposited first on textile, subsequently they were coated with single-walled carbon nanotubes, and finally a top layer of PEDOTePSS (or PANI-Cl) was applied by dip coating [133]. Only limited stability was noticed, and the use of the material in disposable power sources for textiles was suggested. In a different approach on PANI-coated exfoliated graphene oxide sheets, MnO2 nanorods were self-assembled, finally a second coating of PANI was applied [134]. High capacitance and fair stability were attributed to the favorable porosity enabling fast ion transport to the relatively large surface area. Graphene deposited on/in a nickel foam subsequently decorated with MnO2 and finally coated with PANI has been studied [135]. Slow penetration of electrolyte solution into the porous morphology despite the apparently suitable morphology featuring PANI nanorods with intercalated MnO2 caused a slow growth of capacitance followed by a moderate decline up to the 5000th cycle. A nickel foam coated with graphene was used for deposition of an MnO2/PANI composite prepared by chemical oxidation of aniline by KMnO4 [136]. The simultaneous formation was claimed as a reason for good contact between the constituents, yielding in turn good rate capability and 99% capacity retention during 2000 cycles. Various procedures for the preparation of PANI/MnO2 composites were compared by Huang et al. [137]. Best performance was observed with a rough material prepared hydrothermally in a solution of 8 M NaOH at 120 C with a capacity retention of almost 100% after 6000 cycles. Vertically aligned PANI nanofibers on carbon cloth used as a highly electronically conducting support were prepared by chemical polymerization. By subsequent reaction with KMnO4, deposits of MnO2 were obtained [138]. Improved utilization of the material was attributed to the particular morphology. A mechanically particularly sturdy material was prepared by Liang et al. [139]. To a solution of MnSO4 and aniline, ammonium persulfate was added. It oxidized manganese ions from Mn(II) to Mn(IV) and aniline yielding PANI. The composite was directly deposited onto carbon cloth, further processing steps provided an aerogel with only 4% capacity loss after 20,000 cycles and appealing rate capability. Yan et al. [140] deposited about 12% MnO2 into mesoporous carbon by reductive reaction of the carbon with KMnO4. Subsequently PANI was formed by chemical polymerization. MnO2 was found inside the pores of the carbon material. Interaction between carbon and PANI was claimed to be enhanced by MnO2. PANI protected the composite in acidic electrolyte solution and decreased diffusion and charge-transfer resistance. A ternary composite of PANI, Ag and MnO2 was prepared by various electrochemical deposition procedures [141]. Silver was apparently added because of its electronic conductance, and an improvement of stability in comparison to plain PANI was stated. MnO2 deposited on graphene subsequently wrapped with CNTs or an ICP showed improved capacitance and cycling stability attributed to the conducting coating [142]. Yuan et al. [143] deposited MnO2 on MWCNTs, wrapped them with PSS, and finally applied a coating of PANI. An improved capacity retention of 79.9% after 1000 cycles was attributed to the protection afforded by PANI against dissolution of


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the manganese dioxide in acidic electrolyte solutions. Li et al. [144] prepared a coaxial composite by depositing PANI (thickness of the coating was varied by changing the concentration of aniline) on MWCNTs in the presence of PSS by chemical oxidation. Subsequently MnO2 was deposited by exposing the initially formed material to an aqueous solution of manganese acetate and potassium permanganate. About 77% capacity retention after 1000 cycles and microscopically visible substantial changes in morphology were noticed. MnO2 nanorods were electrodeposited onto graphene, subsequently a coaxial coating of PANI was applied [145]. Large active surface area, high electronic conductance, and effective prevention of agglomeration provided a stable material with high capacitance. Wang et al. [146] coated sulfonated graphene nanosheets with a composite of MnO2 and PANI by reaction of aniline with KMnO4. Interactions between the constituents by p-p stacking and hydrogen bonding as well as improved electronic conductance provided by the graphene were noted. A composite of MWCNTs with deposited NiO subsequently coated with PANI has been reported [147]. Within 1000 cycles (only) the composite showed stability superior to that of PANI and NiO/PANI. Giri et al. [148] prepared a composite of ZrO2 and graphene and onto this they deposited vertically aligned PANI. The final product showed high electronic conductance, large BET surface area, and high capacitance.

9.1.5.2

Poly[3,4-ethylenedioxythiophene]

A hierarchical ternary nanocomposite of MnO2 nanospheres (for better mass utilization and higher surface/volume ratio) deposited on CNTs was finally covered with PEDOTePSS [149]. CNTs provided electronic conductance5 not present in MnO2, and their functionalized surface enhanced adherence and controlled distribution (and presumably prevented agglomeration). PEDOTePSS (at about 10 wt%) kept the CNTeMnO2 structure finely dispersed and also acts as a mechanical binder. Its contribution to overall charge storage capacity was not reported. MWCNTs first coated with MnO2 by direct redox reaction and finally with PEDOT have been described [150]. Improved rate capability was attributed to the electronically highly conducting coating. Xia et al. prepared a graphene foam by chemical vapor deposition onto a highly porous nickel film. Subsequently Co3O4 was attached to this foam [151]. By codeposition a shell of PEDOT and MnO2 was applied. In the neutral electrolyte solution employed in this study the capacity contribution of Co3O4 was negligible, the oxide serves only as a “template.” Enhanced rate capability and stability (more than 20,000 cycles with very small capacitance loss) were attributed to the presence of PEDOT. A sandwich structure of layered V2O5, PEDOT, and layered MnO2 was prepared by Guo et al. [152]. The “green” reaction conditions and high energy density were highlighted; a moderate stability (93.5% capacity retention after 3000 cycles) was observed. PEDOTePSS was modified by chemical polymerization of PANI into it [153]. An increased electronic conductance was claimed without evidence but explained with decreased distances for electron transport. The remarkable change in microscopically 5

The absent electronic conductance of RuO2 claimed in the abstract is presumably a misunderstanding.

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observed morphology was not explained. Finally, MnO2 was electrodeposited yielding a stable material with a high capacitance. Li et al. [154] grew MnO2 nanoflakes on rGO paper, subsequently PANI nanorods were deposited on top/between the flakes by chemical polymerization. The excellent rate capability was attributed to the morphology. Plain graphite (pencil lead) was applied to a commercial porous supercapacitor separator serving as a porous scaffold [155]. Subsequently PEDOT and MnO2 were deposited by chemical and electrochemical procedures, respectively. The cheap approach yielded an electrode with limited stability only. For applications in microdevices a layered material composed of PEDOTePSS with embedded MnO2 coated finally by vapor phase polymerization with PEDOT has been developed [156]. Using instead rGO coated with PEDOTePSS as a support for porous a-Fe2O3 nanorods yielded a layer-by-layer structure [157]. The good rate capability and improved stability were attributed to the high electronic conductance of the support, the strong p-p interaction between rGO and PEDOTePSS, and an unspecified noncovalent interaction between PEDOTePSS and the porous a-Fe2O3 nanorods. rGO coated with PEDOTePSS and finally decorated with RuO2 nanoparticles can be processed by screen printing [158]. PSS provided a stable dispersion of the constituents PEDOT and RuO2, enabling better utilization of the materials. Strong p-p stacking interactions between graphene and PEDOT enhanced charge transport in the solid. Beyond a significant capacitance, only moderate stability was observed. In a one-pot procedure, Wang et al. [159] prepared a composite of graphene, SnO2, and PEDOT, with 100% capacity retention in acidic electrolyte solution and 70% in neutral electrolyte solution after 5000 cycles. The limitations of Ni(OH)2 and its low electronic conductance and low stability, hindering its simple utilization in a supercapacitor electrode, were tackled by Jiang et al. [160]. Ni(OH)2 was deposited onto MWCNTs, subsequently a coating of PEDOTePSS was applied. About 86% capacity retention after 30,000 cycles imply efficient stabilization of the Ni(OH)2, the very high rate capability attests to the beneficial effect of the MWCNTs. WO3 fibers were deposited on fluoride-doped tin oxideecoated glass, subsequently gold nanoparticle and PEDOT were coated on top [161]. The composite was suggested for an electrochromic supercapacitor.

9.1.5.3

Polypyrrole

Structures based on an electronically conducting scaffold of CNTs coated first with PPy and finally with MnO2 have been prepared [162], for comparison the reverse sequence of coatings was studied and they performed less good. CNTs provided electronic transport and a flexible backbone. The statement “good interface” provided by PPy (without specific evidence) may be related to the claimed strong adhesion of PPy to CNT and the chemical connection between PPy and MnO2. Finally the spongelike structure enabled easy access to electrolyte ions. Co3O4 nanoparticles deposited on carbon paper and subsequently coated with PPy by electropolymerization have been studied by Wei et al. [163]. The beneficial effect of the highly conducting substrate was noticed for both morphologies (flower- and


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ball-like) of the metal oxide. de Oliveira [164] prepared a composite of PPy/MWCNT/ TiO2 by chemical polymerization with MWCNTs and TiO2 present in the polymerization solution. Strong interactions between the constituents were claimed but not specified; its use in supercapacitors was suggested. Beyond the scope of this report, a nanocomposite of aniline-pyrrole-copolymer with copper chloride (CuCl2) has been reported by Dhibar et al. [165]. A slightly (about 10%) increased electronic conductance when compared with the copper-free copolymer was reported, nothing is stated regarding the role of the various constituents or the advantage of a copolymer versus a homopolymer. A strong interaction between copper ions and the polymer chain was deduced from spectroscopic results, and this was claimed as the reason for increased electronic conductance without providing any explanation. Basically the same approach was already pursued (and the same evidence regarding Cu2þePANI interactions was obtained) by these authors using PANI instead of PPy earlier [166]. Best performance was found at a copper ion content of 2 wt% of CuCl2. In a further variation of this approach, PPy was deposited on MWCNTs with CuCl2 present in the polymerization solution [167]. Because only one copper content (2 wt% of CuCl2 in the polymerization solution, the actual composite composition was not examined) was studied, no concentration dependency was established. The function of the copper ions was not discussed, only its interaction with PPy was inferred from vibrational spectra. Yang and Liu [168] prepared a composite of PPy and conductive mica (SnO2coated mica SiO2). The electronic conductance strongly depended on the fraction of PPy in the composite. Nevertheless, in CVs the composites performed better, and this may be due to a different morphology of PPy coated onto mica particles enabling faster ion ingress/egress. Without details of the conductance measurements the apparent contradiction between decrease in electronic conductance in the presence of mica and improved electrode performance cannot be resolved. Further combinations of, e.g., ICPs with graphene [169] are not discussed here.

9.2

Conclusions

Given the vast number of combinations, compositions, recipes, and procedures, an ideal material is currently impossible to identify. Even a trend is hard to find. A general model or structure cannot be definitely deduced because the function of the constituents, besides providing charge storage capability, remains unclear in most cases and the reported evidence and claims based on them are frequently ambiguous or even contradictory as with ICPs acting as a protective coating and metal oxide deposits acting as protective coating. It appears safe to say that highly porous, more precisely mesoporous, structures providing ample access to whatever redox-active sites are welcome. Depending on the selected electrolyte solution a protective coating by an ICP seems to be more reasonable than the inverse arrangement. Intimate mixing seems to be better than layered structures or other arrangements providing a more segregated distribution of the constituent materials. A relatively large weight fraction of nanoparticles of a

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metal oxide embedded in an ICP bulk with a thin topcoat of an ICP may be a simple approach, possibly even commercially attractive. In any case, stability measurements, i.e., galvanostatic cycling, under carefully defined and fully disclosed conditions, presumably best at high current densities, should help identify suitable materials faster. Evaluating details like the most suitable cation for a given metal oxide should be and can be studied first with the single compound in question. For both metal oxides and ICPs the extensive literature should be consulted more frequently.

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Enhanced Hybrid Supercapacitors Utilizing Nanostructured Metal Oxides

10

Etsuro Iwama 1, 4 , Kazuaki Kisu 1,4 , Wako Naoi 2 , Patrice Simon 1,3, 4 , Katsuhiko Naoi 1,2,4 1 Tokyo University of Agriculture and Technology, Tokyo, Japan; 2K & W Inc, Tokyo, Japan; 3 Université Paul Sabatier, Toulouse, France; 4Institute of Global Innovation Research, Tokyo, Japan

10.1

Introduction

Conventional symmetrical carbon supercapacitors, also called electric double-layer capacitors (EDLCs), are energy storage devices showing extremely fast charginge discharging characteristics, remarkable stability, and long cycle life [1,2]. Thus, EDLCs are used mainly in applications in which instant power is required for short periods (a few seconds). The energy density of supercapacitors is low and needs to be enhanced to further expand their range of applications. A promising option to fulfill this goal is to design hybrid supercapacitors in which an activated carbon electrode is combined with a large-capacity faradic (pseudocapacitive or batterylike) electrode, thus increasing the energy density compared to that of conventional EDLCs [3]. Pseudocapacitive materials, especially metal oxides such as RuO2, MnO2, V2O5, Nb2O5, and TiO2, have been explored as electrode active materials for hybrid capacitors [4e8]. One of the promising hybrid supercapacitor systems is based on the use of high-rate Li4Ti5O12 (LTO) negative electrode, a lithium-insertion-type transition metal oxide [9]. This material delivers a capacity of 120 mAh g1 at a chargeedischarge rate of 30C (full discharge in 2 min) [10]. Moreover, in contrast to MnO2 and RuO2, which operate in aqueous electrolytes within a limited potential window (about 1 V), LTO works in nonaqueous electrolytes, in which the Li-ion intercalation is achieved at a constant potential of 1.55 V compared to Li/Liþ [11,12]. The energy density and cell voltage of hybrid capacitors can be further enhanced, for example, by replacing the LTO electrode with other negative electrodes operating at a lower redox potential and similar capacity such as bronze-type TiO2 [TiO2(B)] and Li3VO4 (LVO) (Fig. 10.1). The synthesis of active materials by using an ultracentrifugation (UC) process has been explored to further extend the performance of high C-rate and pseudocapacitive materials such as RuO2 [13]. Such an UC process, called UC treatment, enables the preparation of nanosized and dimension-controlled [one-dimensional (1D) or twodimensional (2D)] materials directly bonded on high-surface-area conducting carbons such as carbon nanotubes (CNTs) or disordered porous carbons (Fig. 10.2) [3,14]. Metal Oxides in Supercapacitors. http://dx.doi.org/10.1016/B978-0-12-810464-4.00010-3 Copyright © 2017 Elsevier Inc. All rights reserved.

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Figure 10.1 (Left) Chargeedischarge curves for an electric double-layer capacitor (EDLC) [activated carbon (AC)/AC] and hybrid supercapacitors composed of Li4Ti5O12 (LTO) (175 mAh g1 at 1.55 V vs. Li/Liþ), bronze-type TiO2 [TiO2(B)] (250 mAh g1 down to 1.4 V vs. Li/Liþ), and Li3VO4 (LVO) (197 mAh g1 down to 0.65 V vs. Li/Liþ) negative electrodes and AC positive electrode (40 mAh g1). (Right) Ratio of energy density for three hybrid capacitors [LTO/AC, TiO2(B)/AC, and LVO/AC] compared to an EDLC (AC/AC system). Energy densities were calculated from the area framed by the chargeedischarge curves.

Figure 10.2 Illustrated concept of the ultracentrifugation process (UC treatment). The UC treatment simultaneously realizes a nanofabrication of active material (nanosizing) and nanohybridization (hyperdispersion) with coexisting nanocarbons such as single-/multiwalled carbon nanotubes (MWCNTs) and the hollow-structured carbon [Ketjenblack (KB)], which result in the peculiar structure of the synthesized composites, such as entanglement and encapsulation, depending on the nature of the active material and nanocarbons.

Depending on the nature of nanocarbons such as CNT and hollow-structured carbon (Ketjenblack), various morphologies of active material nanoparticles/nanocarbon composites can be achieved by UC treatment, such as entanglement and encapsulation. By combining the established material criteria for batteries and capacitors (reaction


249

potential, theoretical capacity, electronic/ionic conductivity, etc.), the UC treatment enhances the cohesion between the active materials and the carbon substrate to form a new composite material capable of meeting and even exceeding the current energy [15]. Initially developed with RuO2 nanoparticles, we further expanded the UC treatment process to Liþ insertion materials such as TiO2(B), LVO, LiFePO4 (LFP), and Li3V2(PO4)3 (LVP) to achieve high power and high energy density compared to the activated carbon used in EDLCs [16e23]. This chapter presents examples of active material nanoparticles/carbon nanocomposites prepared by the UC treatment. We will show how the specific structures of the prepared composites contribute to achieve ultrafast electrochemical performance with high stability, which are needed to develop the next generation of supercapacitors.

10.2

Li4Ti5O12: Dimension-Controlled Nanosheet/ Nanobook, Highly Dispersed on the Carbon Nanotube Surface

Dimension-controlled 2D-LTO (nanosheets, or “nanobooks,” which are two interconnected LTO nanosheets) is shown in Fig. 10.3A and B. High-resolution transmission electron microscopic images clearly show that 2D-LTO nanosheets nucleate on the CNT surface and are firmly attached to it. Fast-Fourier transform analyses revealed a d-spacing between lattice fringes of 4.81 Å, corresponding to the (111) plane of the spinel structure of LTO [14]. Such particular crystal morphology originates from the presence of the surrounding interstitial carbon in the growth direction of the 2DLTO crystals. These dimension-controlled LTOs show ultrafast chargeedischarge behavior with operation at 300C (full discharge in 12 s) and even at 1200C (full discharge in 3 s) as shown in Fig. 10.3C [15]. Using such ultrafast 2D-LTO, a new hybrid supercapacitor system (2D-LTO/ LiBF4 in PC/activated carbon) called “nanohybrid capacitor” (NHC) was assembled with threefold energy density improvement compared to EDLCs. The NHC can achieve fast charging and discharging at a rate up to 300C for more than 20,000 cycles.

10.3

TiO2(B): Dimension Control and Hyperdispersion of Nano Metal Oxides Within a Nanocarbon Matrix

TiO2(B) exhibits higher electric conductivity (z102 S cm1) than other TiO2 polymorphs such as anatase and rutile (1014e1013 S cm1) [24]. TiO2(B) has a theoretical capacity of 335 mAh g1 during Liþ-ion intercalation, where Liþ diffusion proceeds along the b-axis tunnel resulting in a poor Liþ diffusion coefficient of 1014e1016 cm2 s1 [25]. Conventional TiO2(B) synthesis via hydrothermal treatment from alkaline titanates leads to cylindrical morphology with long b-axis,

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Figure 10.3 (A, B) High-resolution transmission electron microscopic images of nanocrystalline Li4Ti5O12 prepared as a (A) nanosheet and/or a (B) nanobook, which were prepared with the coexisting nanocarbons (multiwalled carbon nanotube) by ultracentrifugation (w75,000g). (C) Discharge curves at various C-rates from 10 to 1200C after charging at a fixed rate of 10C. AC, activated carbon; CNT, carbon nanotube; NHC, nanohybrid capacitor. Reproduced from K. Naoi, S. Ishimoto, Y. Isobe, S. Aoyagi, High-rate nano-crystalline Li4Ti5O12 attached on carbon nano-fibers for hybrid supercapacitors, J. Power Sources 195 (2010) 6250e6254; K. Naoi, W. Naoi, S. Aoyagi, J-i. Miyamoto, T. Kamino, New generation “nanohybrid supercapacitor”, Acc. Chem. Res. 46 (2012) 1075e1083. American Chemical Society.

resulting in poor C-rate capability up to 6C [26]. TiO2(B)/MWCNT or graphene composites exhibit a decent capacity (110e200 mAh g1) because of the conducting MWCNT or graphene, yet their operated C-rate is limited to the range 4.5e18C [27,28]. Downsizing the particle size (3e10 nm) enabled a C-rate operation up to 60C with more than 50% capacity retention obtained at low C-rate [29,30]. Operation at over 100C, however, has been hampered because of the inevitable agglomeration of TiO2(B) nanoparticles [29,30] that limits the accessibility of Liþ from the bulk electrolyte. Nano-TiO2(B)/carbon composites using activated carbon fiber or reduced graphene oxide show less TiO2(B) reagglomeration but often produce the undesirable growth of TiO2(B) in b-axis dimension [31,32].


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Hyperdispersed single nano TiO2(B) crystals were uniformly formed in an MWCNT matrix using UC treatment combined with a follow-up hydrothermal treatment. These TiO2(B)/MWCNT composites have size-controlled crystalline TiO2(B) particles (5 nm in average) and anisotropic crystal growth (ultrashort along b-axis) limiting the agglomeration of the TiO2(B) nanoparticles, described as short-TiO2(B) [S-TiO2(B)] in Fig. 10.4A (top) [16]. The image shows representative TiO2(B) single nanoparticles with clear lattice fringes of d ¼ 0.65 nm corresponding to the (001) plane of TiO2(B) structure. For comparison purposes, we prepared rod-type TiO2(B) crystals with long b-axis [L-TiO2(B) Fig. 10.4A, bottom], which contains the same amount of MWCNT (30 wt%) as the S-TiO2(B). Transmission electron microscopic (TEM) image comparison between S- and L-TiO2(B) highlights a significant difference in the b-axis length [eight times shorter for S-TiO2(B) compared to L-TiO2(B)]. The broadened (020) peak for S-TiO2(B) compared to L-TiO2(B) confirms the shortened length of b-axis in S-TiO2(B). Such ultrashort b-axis length and hyperdispersion of TiO2(B) within the MWCNT matrix improve the power capability of TiO2(B) by enabling ultrafast Liþ deintercalation (235 mAh g1 at 300C, 1C ¼ 335 mA g1), which is clearly superior to that of L-TiO2(B) as shown in Fig. 10.4B and C. These UC-treated TiO2(B)/ MWCNT nanocomposites with controlled (ultrashort) b-axis length can be used to prepare hybrid supercapacitor with higher energy density.

10.4

Li3VO4: Electrochemical Activation; Control of Crystal Structure of Nano Metal Oxides for LiD Diffusion Enhancement via the Electrochemical Method

One way to further increase the energy density is to replace the high-potential LTO or TiO2 (B) electrode (1.55 V vs. Li/Liþ) with other negative electrodes operating at a lower redox potential. LVO has been reported to reversibly intercalate up to two Li per LVO at a low potential (0.1e1.0 V vs. Li), leading to a capacity of 394 mAh g1 [33,34]. However, LVO exhibits a very low electronic conductivity (120 mV s1). The LFP/graphitic carbon composites have an extremely high rate capability both in charge and discharged89, 60, 36, and 24 mAh g1 at 1, 100, 300, and 480C, respectively (Fig. 10.8A). Such a linear relationship means that the composites can offer a high-power capability of the material in both discharge and charge,

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(A)

(B)

100

30C 100C

60

0

180C 300C

40 480C

20 20

40

Capacity / mAh g-1 50 75 100 25

480C

LFP/graphitic carbon composite

100

1C

4.0 3.5 3.0

Capacity retention / %

80

Potential / V vs. Li/Li

Discharge capacity / mAh g-1

1C

80 60 40 20 Typical commerical LiFePO powder

2.5

60

80

Charge capacity / mAh g-1

100

0

0

20

40

60

80

100

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Discharge rate / -

Figure 10.8 (A) Plots of discharge capacity versus charge capacity of a half-cell consisting of Li/1 M LiPF6 in EC þ DEC/(LFP/graphitic carbon composites) as a function of C-rate. Inset: chargeedischarge profiles at different charge C-rates from 1 to 480C. (B) The capacity retention versus discharge C-rate for the LFP/graphitic carbon composites, compared with the reported literature and typical commercial C-rates. DEC, diethyl carbonate; EC, ethylene carbonate. Reproduced from K. Naoi, K. Kisu, E. Iwama, S. Nakashima, Y. Sakai, Y. Orikasa, P. Leone, N. Dupre, T. Brousse, P. Rozier, W. Naoi, P. Simon, Ultrafast charge-discharge characteristics of a nanosized core-shell structured LiFePO4 material for hybrid supercapacitor applications, Energy Environ. Sci. 9 (2016) 2143e2151. The Royal Society of Chemistry.

as expected for the practical use of hybrid supercapacitors. The electrochemical properties and performance of the LFP/graphitic carbon material outperform the reported data in the literature (Fig. 10.8B) [49e60]. Such results pave the way for designing highenergy and high-power materials to be used in hybrid supercapacitors.

10.6

Li3V2(PO4)3: Nano Entanglement of Metal Oxides in Carbon Nanotube Matrix

Monoclinic LVP has attracted much attention because of its high specific capacity of 131 mAh g1 with two-electron reactions at a potential up to 4.2 V versus Li/Liþ and 196 mAh g1 with three-electron reactions at a potential up to 4.8 V versus Li/Liþ [61,62]. In the past 5 years, approximately two-thirds of the published papers dealing with monoclinic LVP focused on a three-electron reaction [63,64]. However, Li-ion intercalation in LVP is limited to two per LVP formula (two-electron reaction) because of the strong repulsion between the VOx layers and the destabilization of its crystalline structure after the three-electron (Liþ) extraction and the possible oxidation of the electrolyte at such a high reaction potential, which results in poor cyclability (1000 cycles), although the capacity is limited


257

Figure 10.9 Plots of the estimated particle size of Li3V2(PO4)3 (LVP) (orange), Li3VO4 (LVO) (blue), and LFP (green), versus the estimated chargeedischarge time based on their Liþ diffusion coefficients and the equation: d ¼ (4Dt)1/2, where d ¼ diffusion length (corresponding to the particle size), D ¼ diffusion coefficient, and t ¼ diffusion time (corresponding to the chargeedischarge time).

when compared to that of the three-electron reaction mechanism. Thus, limiting the Li intercalation process to a two-electron reaction (limited capacity) is a more realistic strategy to use LVP as a cathode material for ultrafast hybrid supercapacitors. Owing to its three-dimensional (3D) diffusion path, LVP also possesses a higher lithium diffusion coefficient (>1010 cm2 s1) than other lithium metal phosphates, such as LFP, which has a b-axis oriented diffusion path [65,66]. Such high Liþ diffusion coefficient of LVP means that the particle size down to a few hundred nanometers is enough to achieve its fast electrochemical performance, unlike other compounds such as LVO and LFP, as shown in Fig. 10.9. Use of such submicrometer-sized LVP particles has an advantage in terms of the cyclability, by avoiding the undesirable oxidation of the electrolyte catalyzed by nanoparticles measuring less than 10 nm in size even at such high reaction potential. Still one of the challenges associated with the use of the LVP cathodes is circumventing the limitation of their intrinsically low electronic conductivity (108e109 S cm1) [67]. Wang et al. reported nitrogen-doped carbon-coated LVP obtained from a facile in situ fabrication. The N-doped carbon-coated LVP, with approximately 1- to 2-mm particles, achieved an excellent performance rate (80 mAh g1 at 100C) [68]. This result indicates that 100C rates can be achieved by even using large LVP particle sizes (>1 mm) because of the fast 3D Liþ diffusion, but only if an efficient electron path is achieved through the conductive carbon network. However, to enable ultrafast LVP chargeedischarge over 100C, better attachment of the LVP particles on the surface of the conductive carbon has to be achieved. To achieve this, nanostructured LVP particles dispersed into an MWCNT matrix were synthesized by the combination of a UC treatment and the subsequent short-duration heat treatment [21]. Most LVP nanocrystals in the synthesized UC-LVP/MWCNT composites with optimized condition of the heat treatment [21] have a platelet shape (w200 nm side length and w20 nm thickness) and are well entangled within the MWCNT matrix (30 wt%), as shown in the SEM images in Fig. 10.10A and B. Such a morphology suggests that the electron path between the LVP nanocrystals

258


Figure 10.10 (A, B) Scanning electron microscopic images with different magnifications of Li3V2(PO4)3/multiwalled carbon nanotube (LVP/MWCNT) nanocomposites. (C) Discharge capacity versus discharge C-rate for the LVP/MWCNT composites. (D) Plots of capacity versus cycle number for the LVP/MWCNT composites showing excellent cyclability over 10,000 cycles. Reproduced from K. Naoi, K. Kisu, E. Iwama, Y. Sato, M. Shinoda, N. Okita, W. Naoi, Ultrafast cathode characteristics of nanocrystalline-Li3V2(PO4)3/carbon nanofiber composites, J. Electrochem. Soc. 162 (2015) A827eA833. Journal of the Electrochemical Society.


259

and MWCNT became shorter, and the lithium diffusion path also became shorter because of the platelet shape and 3D diffusion of LVP. Owing to such a smooth Liþ diffusion and good electron path between the LVP nanocrystals and the MWCNT, the LVP/MWCNT composites exhibited an excellent rate capability (83 mAh g1 at 480C) as shown in Fig. 10.10C. Furthermore, a combination of the controlled size of LVP nanocrystals (w200 nm) and limited operation voltage range (up to 4.2 V vs. Li, two-electron reaction) assured the stability of the LVP/electrolyte interface, showing a long life stability (100 mAh g1, corresponding to 85% of the initial capacity after 10,000 cycles at 10C).

10.7

Conclusions and Remarks

The UC-treated transition metal oxides/nanocarbon composites described in this chapter are newly synthesized materials, which may be the candidates as electrode active materials for the next-generation hybrid supercapacitors. Nanosized and dimensioncontrolled materials directly bonded on high-surface-area conducting carbons through the UC treatment showed ultrafast electrochemical performance with high stability. Dimension-controlled 2D-LTO nanosheets highly dispersed and firmly attached on the CNT surface show ultrafast chargeedischarge behavior as a negative electrode with operation at 300C and in the potential range from 1.0 to 3.0 V versus Li/Liþ. Hyperdispersed single nano TiO2(B) crystals with anisotropic crystal growth (ultrashort along b-axis) were uniformly formed in an MWCNT matrix using UC treatment combined with a follow-up hydrothermal treatment. The ultrashort b-axis length and hyperdispersion of UC-TiO2(B) overcome the problem of poor Liþ diffusion coefficient and improve the power capability of TiO2(B) by enabling ultrafast Liþ deintercalation (235 mAh g1 at 300C). To further increase the energy density of the high-potential negative electrodes such as LTO and TiO2(B), LVO was chosen, whose operation potential is below 1.0 V (down to 0.1 V vs. Li/Liþ). The combination of the activation process of UC-LVO/MWCNT composites and limited operation voltage range (2.5 V down to 0.76 V vs. Li) improved the Liþ intercalation and deintercalation with a small voltage hysteresis (below 0.1 V) because of the fast solidesolution process of the LVO after activation. Highly dispersed defective (crystalline/amorphous) LFP nanoparticles encapsulated within hollow-structured graphitic carbon enabled ultrafast discharge rates (60 mAh g1 at 100C, 36 mAh g1 at 300C) and ultrafast charge rates (60 mAh g1 at 100C, 36 mAh g1 at 300C), showing promising characteristics as a positive electrode for the next-generation hybrid supercapacitors. LVP nanoplates well entangled within the MWCNT matrix were prepared as another candidate of a positive electrode for hybrid supercapacitors. The composite morphology enabled the ultrafast rate capability (83 mAh g1 at 480C), while the combination of the controlled particle size of LVP (w200 nm) and limited operation voltage range (up to 4.2 V vs. Li/Liþ, limited within the 2-electron reaction) assured the stability of the LVP nanoparticleeelectrolyte interface, showing a long life stability over 10,000 cycles. The specific structures of the composites prepared by the UC treatment contribute to achieve ultrafast electrochemical performance with high stability, which are needed to develop the next generation of supercapacitors.

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Acknowledgments This study was supported by the Global Innovation Research Organization in TUAT. This study was also supported by JSPS Grant-in Aid for Scientific Research (KAKENHI) A under Grant No. JP25249140, KAKENHI Grand-in-Aid for Young Scientists B Grant No. JP16K17970, and Research Activity Start-up Grant No. JP15H06193.

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Index ‘Note: Page numbers followed by “f ” indicate figures, “t” indicate tables.’ A Acidic electrolytes, 56e62 Activated carbon (AC), 33e36, 133e134, 139 chemical activation, 203e204 Co3O4 loading, 203e204 electrochemical stability, 204 Fe3O4, 204e205 oxygen-containing groups, 203e204 physical activation, 203e204 tortuous pore channels, 203e204 ultrathin layer/ultrafine particles, 203e204 ZnO, 204e205 Aerogel carbon nanoparticles, 197e199 Alkaline electrolytes, 54e56 All-solid-state supercapacitors advantages, 36e37 charge-transfer mechanism, 36e37 WSSC, 33e36 Aqueous electrolytes acidic electrolytes, 56e62 alkaline electrolytes, 54e56 liquid electrolyteebased supercapacitors acid electrolytes, 41e42 nanoribbons, 43 noncapacitive faradaic and EDL capacitive mechanisms, 41e42 pseudocapacitive mechanisms, 42e43 redox reaction and EDL capacitive mechanism, 42 neutral electrolytes, 62e66 symmetric and asymmetric, fundamentals of, 27e31 battery- and capacitor-type electrodes, 29e30 ED, 28e29 electrodes, types of, 29e30 KBM, 30e31 PD, 31

single capacitor-type electrode, 28 single electrode system, 27e28, 27f types, 50 Asymmetric supercapacitors (ASCs), 9, 68, 111e112 Asymmetric/hybrid devices, 18 Atomic layer deposition (ALD), 210e212 B Band gap, 105 Battery-type electrode, 29e30 Binary metal oxideebased supercapacitors Bi2O3, 93 capacitive electrochemical features, 79e80 Co3O4, 88e89 electrochemical testing, 94 Fe2O3, 92e93 In2O3, 93 MnO2 capacitive performance, 83e84 charge storage mechanism for, 82e83 crystal structure of, 84, 85f, 87t CV curve, 82e83, 83f electrochemical performance of, 84e85 electrolyte captions, 82e83 pseudocapacitive behavior of, 82e83 surface morphology, 84 tunnel size, 84 molybdenum oxideebased electrode materials, 94 NiO battery-type electrode material, 85e86 electric charges, 85e86 nanostructures of, 87e88 Ni(OH)2, 88 supercapacitive mechanism of, 86e87 redox pseudocapacitance, 79e80 RuO2

266

Binary metal oxideebased supercapacitors (Continued) annealing temperature and particle size, 82 charge transformation, 81 crystallinity of, 82 cyclic voltammetry (CV) curve, 81 tin oxide, 90 VOx, 90e92 Bismuth oxide (Bi2O3), 93 Brunauer-Emmett-Teller (BET), 167e168, 231e232 C Capacitor-type electrode, 29e30 Carbide-derived carbons (CDCs), 10e11 Carbon hybrid materials EDLCs, 193 grapheme, 212e214 metal oxide nanocomposites CNFs, 207e208 CNTs, 208e212 porous carbonemetal oxide hybrids aerogel carbon nanoparticles, 197e199 drawbacks, 193e194 macroporous carbon, 199e201 mesoporous carbon, 194e196 microporous carbon materials, 193e194 mixed porous carbon, 201e207 Carbon nanofibers (CNFs), 110, 207e208 Carbon nanotubes (CNTs), 37e38, 115 ALD technique, 210e212 MnO2, 209 NiCo2O4, 210e212 oxide nanoparticles, 210e212 ruthenium oxide nanoparticles, 209e210 vanadium oxide, 210e212 vapor coating technique, 210e212 CeO2, 177e178 Charge-storing mechanisms, 87e88 Chemical precipitation method, 109e110 Chronopotentiometric (CP), 167e168 Cobalt oxide (Co3O4), 88e89 Co(II) MOF, 170e171 constant-current galvanostatic chargingedischarging curves, 167 CP, 167e168 electrochemical measurements, 167, 171e173

Index

formation of, 173 liquid phase method, 171 nanoscale Co-MOF-74, 171e173 nanostructures of, 166e167 porous hollow, 168e170 porous structure of, 173 SEM, 170e171, 170f solid-state annealing, 167 TEM, 167e168 Conductive organic polymers (COPs), 133e134 Cyclic voltammograms (CVs), 10, 11f, 81, 136 D Density functional theory (DFT), 100e101, 137e138 Dielectric capacitors cell voltage, 3 charge stored, 2 constant-current charge, 3, 3f Dimension control, 249e251 Dry polymer electrolytes. See Solid polymer electrolytes E Electric double-layer capacitors (EDLCs), 19e20, 79, 136, 165 Electrochemical capacitors (ECs) asymmetric/hybrid device, 7e9 capacitive electrodes carbon electrodes, 10 CDCs, 10e11 CVs, 11e12 electroanalytical methods, 9e10 grapheme, 10e11 silicon-based nanomaterials, 12 Si-NWs, 12 titanium nitride (TiN), 12 capacity vs. capacitance, 19e20 Ccell, 4e5 cell voltage, 5e6 classification of, 26f dielectric capacitors cell voltage, 3 charge stored, 2 constant-current charge, 3, 3f high-power battery electrodes, 18e19 ions, 4e5

Index

organic electrolytes, 5 polymeric binder, 4 pristine materials, 4 pseudocapacitive electrodes advantages, 12e13 carbon-based electrodes, 12e13 double-layer capacitance, 13 electric behavior, 14 extrinsic pseudocapacitance, 14, 16e17 intrinsic pseudocapacitance, 14e15 metal oxides, 13 origin of, 13 secondary (rechargeable) batteries, 6e7 Electrochemical supercapacitor (ES), 49 Electrodeposition method, 110 Electron shuttle, 68 Electrophoretic deposition (EPD), 111 Electrospinning synthesis method, 110 Energy density (ED), 28 Energy storage, 151e153 Equivalent series resistance (ESR), 55 Extrinsic pseudocapacitance, 14, 16e17 F Fabricate SC electrodes carbon substrates, chemisorption on AC, 139 covalent bonding mechanism, 137e138 FTIR, 137 MWCNT, 137 PMo12, 137e138 rGO, 138 conductive polymer matrix, immobilization in COP, 139e140, 143 direct electrochemical method, 140 H2O2, 140 postpolymerization strategy, 139e140 PPyePMo12, 142 prepolymerization approach, 139e140 PW12, 143 ternary hybrids, 142 LbL assembly and electrostatic interactions cationic linkers, 143e144 CVs, 145, 145f MWCNT, 146 polycation species, 143e144 redox-active layers, 146

267

thin uniform surface coatings, 145e146 wet-chemical procedure, 143 Faradaic charge storage, single polyoxometalate chemistries for high-capacity retention, 150 MWCNTs, 147 P2Mo18, 150 pH values, 150 PMo12erGO hybrids, 148 PW12, 148e149 volumetric capacity, 148 Fe3O4, 177 Flexible stainless steel mesh (FSSM), 118 Fourier transform infrared (FTIR), 137 G Gel polymer electrolytes (GPEs), 37 Graphene sheets (GSs), 151e152 Graphene, 138, 148 Graphite oxide (GO), 138 H Helmholtz double layer, 79 Heteroatoms, 11e12 Heteropolyanions, 134 High-power battery electrodes, 18e19 Hybrid, 148 Hybrid capacitors, 7e9 Hybrid materials, 139e140, 148 Hydrogen peroxide (H2O2), 140 Hydrothermal/solvothermal method, 109 Hyperdispersion, 249e251 I Indium oxide (In2O3), 93 Indium tin oxide (ITO), 62 Intrinsic pseudocapacitance, 14e15 Intrinsically conducting polymers (ICPs), 221 Ion adsorptioneannealing method, 111e112 Ionic liquids (ILs) electrolytes, 26e27, 49e50, 67 Isopolyanions, 134 ITO. See Indium tin oxide (ITO) L Layer-by-layer (LBL) assembly, 143e146 Layered double hydroxide (LDH), 33e36

268

Liquid electrolytes aqueous electrolytes acidic electrolytes, 56e62 alkaline electrolytes, 54e56 neutral electrolytes, 62e66 nonaqueous electrolytes ILs, 67 organic electrolytes, 66e67 Lithium iron phosphate (LFP), 254 Lithium perchlorate (LiClO4), 72 Lithium-intercalation compound, 16e17 Lithium-ion capacitor (LIC), 7e8 M Macroporous carbon, 199e201 Manganese oxide (MnO2) capacitive performance, 83e84 charge storage mechanism for, 82e83 crystal structure of, 84, 85f CV curve, 82e83, 83f electrochemical performance of, 84e85 electrolyte captions, 82e83 pseudocapacitive behavior of, 82e83 surface morphology, 84 tunnel size, 84 Mesoporous carbon, 194e196 Metal oxide nanocomposites CNFs, 207e208 CNTs, 208e212 Metal oxide supercapacitors, electrolytes in aqueous electrolytes, 50, 53e54 polymer gel electrolytes, 50 types, 50 charge storage, 51 electrolyte classification, 49e50, 53, 53f faradaic process, 51e52 liquid electrolytes, 54e67 organic electrolytes, 54 pseudocapacitance behavior, 51 electrochemical metal oxide electrodes, 51 mechanisms, 51 redox additives electron shuttle, 68 H2SO4 electrolyte, 69 iodine/iodide pairs, 69e70 KI, 69e70 KOH electrolyte, 68e69 Na2S2O8, 68e69

Index

nickel oxide electrodes, 68e69 p-phenylenediamine, 69 redox species, 52 solid/gel state alkaline PVA/gel electrolyte, 70 carboxymethyl cellulose, 71 ionic conducting polymer gel electrolytes, 72 neutral PVA-based gel electrolytes, 71 PVA/Na2SO4 gel electrolyte, 71 solid inorganic electrolytes, use of, 54 Metal oxide/conducting polymer hybrids applications, 227e228 binary materials poly[3,4-ethylenedioxythiophene], 231e233 polyaniline, 228e231 PPy, 233e234 combination ICPs, 226e227 redox process, 227 redox transition, 227 components accumulation/depletion process, 221 achieved conductance, 222 charge storage processes, 222e223 CVs, 226 EDLC materials, 219e220 generic claims, 221 ICPs, 221, 225e226 large-surface-area materials, 219e220 PANI, 222e223 porous structures aiming, 219e220 pseudocapacitive, 219e220 thicker film, 223 ternary materials polyaniline, 234e237 poly[3,4-ethylenedioxythiophene], 237e238 PPy, 238e239 Metaleligand coordination (MLC), 165e166 Metaleorganic frameworks (MOFs), 120e122 derived binary metal oxides CeO2, 177e178 Co3O4, 166e177 Fe3O4, 177

Index

derived metal oxide/carbon composite, 186e187 derived ternary/mixed transition metal oxides MnCo2O4, 181e182 NixCo3_xO4, 182e186 synthesis of, 178 ZnCo2O4, 179e181 Microwave synthesis method, 110 Mixed porous carbon, 201e207 AC chemical activation, 203e204 Co3O4 loading, 203e204 electrochemical stability, 204 Fe3O4, 204e205 oxygen-containing groups, 203e204 physical activation, 203e204 tortuous pore channels, 203e204 ultrathin layer/ultrafine particles, 203e204 ZnO, 204e205 carbon aerogels, 206e207 Fe2O3, 205e206 macroporous carbon, 206e207 MnO2 loading, 205e206 OMPC, 205e206 MnO2, 14e15 metal oxides, 13 pseudocapacitive behavior, 15 Multiple polyoxometalate chemistries CV, 153 limitation of, 153e154 PMo12 and GeMo12, 153e154 PMo12ePW12 mixture, 154e156 SiMo12eGeMo12, 156 Multiwalled carbon nanotubes (MWCNTs), 69e70, 115, 137 N Nanocomposites, 152f Nanohybrid capacitor (NHC), 249 Nanosized polymolybdate clusters, 135 Nanostructured metal oxides Li3V2(PO4)3, 256e259 Li3VO4, 251e253 Li4Ti5O12, 249 LiFePO4, 254e256 Li-ion intercalation, 247 TiO2(B), 249e251

269

Nanostructures one dimension CNTs, 115 CuCo2O4, 113e115 longitudinal axis of, 113e115 Ni foam, 115e118 NiCo2O4 nanowire, 113e115 PCFs, 115 PNT, 115 specific capacitance, 113e118 ZnCo2O4 nanoparticles, 113e115 three dimension, 120e122 two dimension, 118e120 zero dimension, 111e112 Neutral electrolytes, 62e66 Ni(OH)2, 19e20 Nickel oxide (NiO) battery-type electrode material, 85e86 electric charges, 85e86 nanostructures of, 87e88 Ni(OH)2, 88 supercapacitive mechanism of, 86e87 Nonaqueous electrolytes ILs, 67 organic electrolytes, 66e67 O Ordered multimodal porous carbon (OMPC), 205e206 Organic electrolyte, 31 liquid electrolyteebased supercapacitors, 40e41 symmetric and asymmetric, fundamentals of, 31 P PC. See Propylene carbonate (PC) Poly(vinyl alcohol) (PVA), 33e36 Poly[3,4-ethylenedioxythiophene] (PEDOT), 36e37, 151 binary materials, 231e233 ternary materials, 237e238 Polyacrylic acid (PAA), 176e177 Polyaniline (PANI), 222e223 binary materials chemical polymerization, 230 Eu2O3, 231 MnO2, 229e230 MoS2, 230

270

Polyaniline (PANI) (Continued) Ni(OH)2, 229 PANI, 228e230 polyoxymetalates, 228 RuO2, 230 SnCl2, 228e229 TiO2, 231 WoO3, 231 ternary materials cobalt ferrite, 235e236 copper nanowires, 235 graphene oxide, 235 MnO2 nanoparticles, 236e237 MWCNTs, 236e237 PANI, 234e235 TiO2, 235 Polymeric nanotubes (PNTs), 115 Polymerized imidazolium linker (PIL), 145e146 Polyoxometalates (POMs), 56 COPs, 133e134 electrodes, categories of, 134 fabricate SC electrodes carbon substrates, chemisorption on, 136e139 conductive polymer matrix, immobilization in, 139e143 LbL assembly and electrostatic interactions, 143e146 fabrication of, 134 nanostructured carbon materials, 133e134 structure and electrochemistry categories, 134e135 electrochemical properties of, 135e136 heteropolyanions, 134 isopolyanions, 134 Keggin molecule, 135 nanosized polymolybdate clusters, 135 reversible multielectron-transfer reactions, 135 supercapacitor devices faradaic charge storage, single polyoxometalate chemistries for, 147e150 polyoxometalateeconductive polymer hybrid supercapacitors, 151e153 toward ideal pseudocapacitance, 153e157

Index

Polypyrrole (PPy), 233e234 Polystyrene sulfonate (PSS), 231e232 Polythiophene, 221 Polyvinyl alcohol (PVA), 118e120 Polyvinylpyrrolidone (PVP), 120e122 Porous carbon nanofibers (PCFs), 115 Porous graphene papers (PGP), 118e120 Porous hybrid film, 151 Power density (PD), 31 p-Phenylenediamine, 69 Propylene carbonate (PC), 66 Pseudocapacitive electrodes advantages, 12e13 carbon-based electrodes, 12e13 double-layer capacitance, 13 electric behavior, 14 extrinsic pseudocapacitance, 14, 16e17 intrinsic pseudocapacitance, 14e15 metal oxides, 13 origin of, 13 Pseudocapacitive, 9, 219e220 PVP. See Polyvinylpyrrolidone (PVP) Q Quasi-solid-state supercapacitors asymmetric SCs, energy storage mechanism for, 37e38 CNT, 37e38 conventional textile processes, 37e38 GPE, 37 pseudocapacitive mechanism, 39e40 ultraflexible planar supercapacitors, fabrication procedures for, 39e40, 39f R Reduced graphene oxide (rGO), 138 Ruthenium oxide (RuO2), 14e15, 56 annealing temperature and particle size, 82 charge transformation, 81 crystallinity of, 82 cyclic voltammetry (CV) curve, 81 metal oxides, 13 pseudocapacitive behavior, 15 S Saturated calomel electrode (SCE), 15 Silicon carbide (SiC), 12 Silicon nanowires (Si-NWs), 12

Index

Single-walled carbon nanotube (SWCNT), 146 Solegel method, 110 Solid polymer electrolytes, 33e36 Solid/gel state alkaline PVA/gel electrolyte, 70 carboxymethyl cellulose, 71 ionic conducting polymer gel electrolytes, 72 neutral PVA-based gel electrolytes, 71 PVA/Na2SO4 gel electrolyte, 71 Solidesolution process, 251e253 Specific surface areas (SSAs), 193e194 Stable nanometric oxide clusters, 56 Supercapacitors (SCs). See also Electrochemical capacitors (ECs) binary metal oxides in. See Binary metal oxideebased supercapacitors carbon hybrid materials. See Carbon hybrid materials fabricating polyoxometalate composites for carbon substrates, chemisorption on, 136e139 conductive polymer matrix, immobilization in, 139e143 LbL assembly and electrostatic interactions, 143e146 MOFs. See Metaleorganic frameworks (MOFs) metal oxideebased SCs, design and fabrication of, 44 metal oxideebased supercapacitors, configuration design of all-solid-state supercapacitors, 33e37 design and mechanisms, features of, 33, 34te35t liquid electrolyteebased supercapacitors, 40e43 quasi-solid-state supercapacitors, 37e40 metal oxide. See Metal oxide supercapacitors, electrolytes in negative electrode, 25e26 polyoxometalate electrodes, application of faradaic charge storage, single polyoxometalate chemistries for, 147e150 polyoxometalateeconductive polymer hybrid supercapacitors, 151e153 toward ideal pseudocapacitance, 153e157

271

positive electrode, 25e26 symmetric and asymmetric, fundamentals of aqueous electrolyte, 27e31 organic electrolyte, 31 TMOs. See Ternary metal oxides (TMOs) types, 25 T Ternary hybrids, 151e152 Ternary metal oxides (TMOs) AB2O4 alkaline electrolyte, 104 Co3O4, 100e101 CoFe2O4, 105 CuCo2O4, 104 CuFe2O4, 105 FeCo2O4, 104 MnCo2O4, 104e105 NiCo2O4, 101e104, 101f NiFe2O4, 105 ZnCo2O4, 103e104 ZnFe2O4, 105 ZnMn2O4, band gap of, 105 ABO2/3/4 AMoO4, 106e108 BiFeO3, band gap of, 106 CoO, 106 NiCo2O4 and NiCoO2, 106 NiMoO4, 106e108 SnWO4, 106e108 ZnSnO3, 106 A3B2O8, 108e109 nanostructures one dimension, 113e118 three dimension, 120e122 two dimension, 118e120 zero dimension, 111e112 synthesis routes, 111 chemical precipitation method, 109e110 electrodeposition method, 110 electrospinning synthesis method, 110 hydrothermal/solvothermal method, 109 microwave synthesis method, 110 solegel method, 110 Tetragonal unit cell, 84 Tin oxide, 90 Titanium nitride (TiN), 12 Transition metal oxide (TMO), 69e70

272

Transitionmetal mixed oxides (TMMOs), 178 Transmission electron microscopic (TEM), 167e168, 251 U UC treatment, 247 Li3V2(PO4)3, 256e259 Li3VO4, 251e253 Li4Ti5O12, 249 LiFePO4, 254e256 Li-ion intercalation, 247 TiO2(B), 249e251 Ultracapacitors. See Electrochemical capacitors (ECs) Ultracentrifugation (UC). See UC treatment Ultrafast performance, 259e260

Index

V Vanadium oxides (VOx), 90e92 Vulcan carbon (VC), 137 W Wire-shaped supercapacitors (WSSCs), 33e36 X X-ray diffraction (XRD), 167 X-ray photoelectron spectroscopic (XPS), 11e12