Metal Oxides in Supercapacitors

MetaleOrganic Framework (MOF)eDerived Metal Oxides for Supercapacitors

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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

Metal Oxides in Supercapacitors. http://dx.doi.org/10.1016/B978-0-12-810464-4.00007-3 Copyright © 2017 Elsevier Inc. All rights reserved.

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Metal Oxides in Supercapacitors

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.


7.2.2.1

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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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