High performance triboelectric nanogenerators based

Nano Energy (2015) 11, 304–322

Available online at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/nanoenergy

REVIEW

High performance triboelectric nanogenerators based on large-scale mass-fabrication technologies Xiao-Sheng Zhang, Meng-Di Han, Bo Meng, Hai-Xia Zhangn Science and Technology on Micro/Nano Fabrication Laboratory, Institute of Microelectronics, Peking University, Beijing 100871, China Received 27 September 2014; received in revised form 4 November 2014; accepted 6 November 2014 Available online 15 November 2014

KEYWORDS

Abstract

Triboelectric nanogenerator; Energy harvesting; Mass-fabrication; Low-powerconsumption applications

Recently, people’s daily lives are surrounded by microchips, small devices, consumer electronics, smart products, and portable electronics etc. (i.e., microsystem), which significantly improve the quality of human life but also simultaneously bring the energy supply problem. Although the power consumption of microsystem has been reduced dramatically to several Watts, even at mW or μW levels, the total consumed energy amount is huge from the worldwide macro view point. Meanwhile, this part of power is major supplied by batteries which also caused serious problem of pollution. Therefore, as an essential aspect of the global increasing urgent energy crisis, sustainably powering the low-power-consumption microsystems has attracted so much attention in the past decades. Harvesting energy from the living environment is a promising environment-friendly approach to meet the energy demand of these microsystems, and shows the attractive potential to realize the whole integrated self-powered microsystem. Triboelectric nanogenerator (TENG) harvesting living environmental energy based on the triboelectrification effect has been demonstrated to be a reliable energy source since it was firstly reported in 2012, and its unique properties, such as high-output performance, clean, sustainability, etc., result in the explosive growth of TENG research in the past three years. This paper focuses on three main developments of TENG: fabrication technology, performance enhancement and applications, which is illustrated in details based on our research work. Soft lithography process and flexible printed circuit board (i.e., FPCB) process are emphasized as the mass-fabrication technologies to realize the flexible TENG. The large-scale techniques of enhancing the output performance of TENG are summarized as three points, including surface roughening, plasma treatment and folded structure. Moreover, the applications of TENG for the low-power-consumption devices, including portable electronics, self-powered sensor and biomedical microsystems, are introduced. As a sustainable and green energy source, TENG

n

Corresponding author. Tel: +86 10 6276 6570. E-mail address: [email protected] (H.-X. Zhang).

http://dx.doi.org/10.1016/j.nanoen.2014.11.012 2211-2855/& 2014 Elsevier Ltd. All rights reserved.

High performance triboelectric nanogenerators based on large-scale mass-fabrication technologies

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shows extensively attractive potential in many fields, not only for the academic research but also for the industrial applications. & 2014 Elsevier Ltd. All rights reserved.

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305 Mass-fabrication technologies for TENG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306 2.1. Soft lithography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306 2.2. Flexible printed circuit board (FPCB) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307 3. Large-scale enhancement technology for TENG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310 3.1. Surface morphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310 3.2. Fluorocarbon plasma treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311 3.3. Multiple friction surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312 4. Novel TENG based on large-scale mass-fabrication technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312 4.1. Single-friction-surface TENG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312 4.2. Hybrid TENG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314 4.2.1. Piezoelectric–triboelectic effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314 4.2.2. Electromagnetic–triboelectric effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315 5. Applications for low-power-consumption devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316 5.1. Portable electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316 5.2. Self-powered active sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317 5.3. Biomedical microsystems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317 6. Conclusions and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 318 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321

1.

Introduction

Harvesting energy from our living environment has been considered to be a promising approach to response the worldwide energy crisis [1–3]. Many techniques have been developed by previous research work, such as photoelectric conversion [4,5], piezoelectric effect [6,7], pyroelectric effect [8,9], biochemical effect [10], and so on. These techniques can be employed to harvest the ambient energy with various forms, such as light, mechanical change, temperature difference, variation of electromagnetic field, etc. Among the above energy forms, mechanical energy may be the most widely distributed energy form, which exists all over our living environment. Walking, running, talking, touching smart phones, typing keyboards, and so on, even blinking, breathing and heart beating, all of these activities occur every day even every moment, and contain adequate mechanical energy. Several techniques have been developed to capture this mechanical energy from our living environment to the electrical energy, especially nanogenerators based on the piezoelectric effect [11–13]. When the piezoelectric device made of nanoscale materials, such as ZnO nanowires, is applied with an external force, the potential difference occurs between the two endpoints and the current will form with the external load. Piezoelectric nanogenerators have been demonstrated to be a clean energy source and the self-powered active sensor [14–17]. However, the power generated by piezoelectric nanogenerators is normally at nW or μW level, which cannot meet the actual power requirement of electronic devices. In January 2012, the research group led by Prof. Zhong Lin Wang in Georgia Institute of Technology firstly reported

a novel triboelectric nanogenerator (TENG) based on the triboelectrification [18]. Equal but opposite charges will be generated by the contact and separation of two different materials, such as fur and rubber stick, which is named as triboelectrification [19,20]. By the combination of triboelectric effect and electrostatic induction, TENG can be utilized to transform the ambient mechanical energy to the electrical power [21–25]. Due to its unique properties, such as high-output, pollution free, small size, low-cost, etc., TENG has attracted so much attentions during the past three years [21,26–30].

Fig. 1 Schematic view of the road map of triboelectric nanogenerator.

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When we look into the massive developments of TENG, three major lines can be summarized, as is shown in Fig. 1. The first line is the fabrication of TENG, which is generally based on the microfabrication technology, such as soft lithography [31–33], chemical or physical etching [34,35], deposition [36,37], etc. The second line is enhancing the output performance of TENG, including structural optimization (especially arch-shaped structure [31]) [38–40], roughening surface [31–33,41–43], developing materials [44,45], and hybrid with other mechanisms [46–48], and so on. And the third line is applying TENG in various fields not only as power source, but also as self-powered system, such as portable electronics [31,49–51], active sensors [52–54], microsystems [55,56], and so on. Besides the above lines, there are other essential aspects of TENG development, i.e., innovative working mechanism and their theoretical analysis. Both of contact-mode TENGs [31,57] and slidingmode TENGs [58–60] are developed, theoretically analyzed and systematically modeled [61–64]. Several aspects of the TENG development have been summarized by the review article of Ref. [21], such as the fundamentals and working mechanisms, and also including several typical applications. Herein, this review article is to give a summary about the latest results of fabrication technologies of flexible TENGs, the approaches to enhance the output performance and TENG’s applications in several fields, especially for low-powerconsumption device. There are plenty of remarkable research work intended for the above scope, which has been performed by different groups all over the world. Unfortunately, as a limited-length paper, it is hard to emphasize all of these extensive literatures to form a comprehensive review. As one of the major research group in TENGs, we started to work in this field at 2012 fall at Peking University, our research covered fabrication, structure design, performance enhancement, applications, etc. Therefore, this review article will attempt to mainly focus on the research work about TENGs from our research group in last two years [33,39,40,48,51,54,60,64–74].

2.

Mass-fabrication technologies for TENG

Fabrication technology is always the footstone to build a scientific skyscraper, and also the backbone to support a

new research conception. For TENG, from the perspective of easily harvesting the ambient energy and realizing the whole integrated system, flexible TENG is a promising choice. Therefore, developing fabrication techniques of flexible TENG have become a research hot spot in the past years. Moreover, in order to drive TENG to be closer to the practical application, the mass-fabrication technology has been developed to reduce the cost and enlarge the efficiency, which is the essential requirement of industrial productions and practical applications. Thus, this review paper is to give the summary about soft lithography process and flexible printed circuit board process which have been demonstrated as universal mass-fabrication techniques to realize flexible TENGs.

2.1.

Soft lithography

Replication process is a powerful technique to pattern the surface by replicating structures from the mold, which has been widely used for hundreds years. As the rapid development of micro/nano science and technology, researchers introduced the traditional replication process into micro/ nano fabrication field to realize the mass-fabrication of surface-patterned soft materials. Generally, a kind of soft material (liquid or solid) is utilized to cover a pre-patterned substrate (i.e., mold), then this soft material will change to be solid under a specific external treatment (lightening, heating, radiation, etc.). Finally, this solid-phase soft material is removed from the mold, and the patterns is successfully transferred to its surface. In micro/nanoscience field, this replication process is also named soft lithography. Thus, the essential benefit of this technique is massproduction of surface-patterned materials due to the reusability of the mold, and the cost of this process is very low by avoiding the photolithography. In Ref. [33], we proposed a flexible TENG fabricated by using this soft lithography process, as is shown in Fig. 2. A silicon mold with micro/nano hierarchical structures was prepared by the combination of conventional microfabrication process and the improved deep reactive ion etching (i.e., DRIE) process [65–68]. The conventional microfabrication processes, including low-pressure chemical vapor

Fig. 2 Schematic view of (a) the 3D structure of the sandwich-shaped (i.e., three-layer) TENG, (b) the process flow of soft lithography (i.e., replication process) and (c) the high-magnification view of the soft lithography. Reproduced with permission from [33]. Copyright 2013 American Chemical Society.

High performance triboelectric nanogenerators based on large-scale mass-fabrication technologies deposition (LPCVD), reactive ion etching (RIE), photolithography and KOH wet etching, were employed to fabricate microstructure arrays on the silicon substrate, as is shown in Fig. 2b(i–ii). And the improved DRIE process was used to fabricate nano-pillar forest atop microstructure arrays to form micro/nano hierarchical structures, as is shown in Fig. 2b(iii). Then, this silicon mold was dipped into the liquid polydimethylsiloxane (PDMS) mixture prepared by the base solution and the curing agent with the quantity ratio of 10:1. After heated at a high temperature, this liquid mixture changed to be solid PDMS film, and then the PDMS film with micro/nano hierarchical structures was peeled off from the silicon mold, as is shown in Fig. 2b(iv–vi) [69,70]. These surface-textured PDMS films are widely used to build the high-performance TENG, which serve as the triboelectrification surface [31–33]. The key point of the above soft lithography process is employing the improved DRIE process, which can significantly reduce the surface energy of silicon mold due to both of minimizing the solid–liquid contact area and depositing a thin fluorocarbon polymer [69–71]. Therefore, an ultra-low-surface-energy silicon mold can be realized, and the replication process is single-step and surfactant free, which actually reduces the cost, enhances the pattern transfer precision and avoids the surface pollution [69]. Thus, wafer-level surface-textured PDMS film was easily fabricated and the micro/nano structures were accurately replicated from silicon mold to PDMS film, as is shown in Fig. 3. Furthermore, this soft lithography process based on the improved DRIE process is also suitable to fabricate other soft materials with micro/nano-textured surface.

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By using this soft lithography technique, a sandwich-shaped flexible TENG was fabricated, as is shown in Fig. 4 [33]. Due to the two triboelectrification effects among the three layers, this sandwich-shaped TENG generated two triboelectric outputs under the external force in one cycle. The working principle of this flexible TENG was systematically investigated by finite element analysis (i.e., FEA) simulation and experimental measurement shown in Fig. 4(a) and (c), respectively. According to the measurement results, the effect of the frequency of external force on the output performance of TENG was obtained. As the external force frequency increased from 1 to 5 Hz, the open-circuit voltage increased from 120 to 320 V due to external electrons flowing to reach equilibrium in a shorter time [31,33]. Then, the open-circuit voltage kept constant while the frequency increased from 5 to 7 Hz. However, the open-circuit voltage decreased to 218 V under 10 Hz external force due to the under-releasing of the sandwich-shape nanogenerator, which means that the TENG cannot recover to the original state due to the long recovery time.

2.2.

Flexible printed circuit board (FPCB)

As a mature industrial technology, printed circuit board (i.e., PCB) process is widely used in almost all fields, such as consumer electronics, automotive manufacturing, aviation, etc. This technology provides a cheap but reliable approach to orderly connect all the electronic components to realize an integrated system, such as smart phone, computer,

Fig. 3 The photos and scanning electronic microscope (SEM) images of the samples fabricated by using the soft lithography process, including four inch silicon mold and PDMS film shown on the right side. Reproduced with permission from [69]. Copyright 2013 American Chemical Society.

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Fig. 4 Analysis and measurement of the sandwich-shaped TENG, including (a) the finite element simulation of the TENG working principle, (b) the photo of the TENG device and (c) the output performance of the TENG under different frequency external applied forces. Reproduced with permission from [33]. Copyright 2013 American Chemical Society.

Fig. 5 The schematic view of 3D structure of zigzag TENG (a) and the FPCB fabrication process flow (b). Reproduced with permission from [40]. Copyright 2013 Elsevier.

laptop, household electric equipment, etc. As one kind of PCB technologies, flexible printed circuit board (i.e., FPCB) technique not only maintains the common advantages of PCB, but also possesses the unique properties of flexibility, which has attractive applications in many fields, such as flexible electronics, wearable electronics, implantable device, etc. Herein, we utilized FPCB technique into TENG to realize the mass fabrication of flexible TENGs, as is shown in Fig. 5 [40]. The 3D schematic view of this TENG based on FPCB is shown

in Fig. 5(a), including front-side view and back-side view. The thin epoxy layer attached on the polyimide substrate serves as one friction surface. While gold serves as the other opposite friction surface, which is well-designed as micro-cubic arrays to enhance the triboelectrification effect. Additionally, these independent micro-cubic arrays also play the role as an insulator friction surface, despite that the surface charge would redistribute on the surface of the conductive cubic. The fabrication process flow is also illustrated in the following Fig. 5(b). The two surfaces of each tooth of the zigzag

High performance triboelectric nanogenerators based on large-scale mass-fabrication technologies structure form one friction pair, and multiple friction pairs are integrated to realize the whole TENG device (Fig. 5(b–v)). The photographs and SEM images of this TENG are shown in Fig. 6, which demonstrate its remarkable flexibility. According to the electrical measurement, the output performance of the TENG with 10 integrated friction pairs was outstanding, and its voltage and current density achieved 620 V and 45.1 μA/cm2, respectively (Fig. 6(c)).

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Besides the above contact-mode TENG, this FPCB technique was also applied to fabricate the sliding-mode TENG, as is shown in Fig. 7 [60]. The friction surface of this slidingmode TENG was designed to be grid shown in Fig. 7(a). When the other friction surface slides atop this grid-shaped friction surface, the triboelectric charge is generated on the surface of each comb, and then the transferred charge will be generated on the backside comb due to the induction

Fig. 6 The photo of the flexible zigzag TENG based on the FPCB (a), the SEM image of the gold coated micro-cubic patterns (b) and the measured output performance of the flexible zigzag TENG with 10 pairs. Reproduced with permission from [40]. Copyright 2013 Elsevier.

Fig. 7 The photograph of sliding-mode TENG based on FPCB (a), the working principle (b) and the electrical measurement results (c)–(e). Reproduced with permission from [60]. Copyright 2014 Elsevier.

310 effect, as is shown in Fig. 7(b). The output performance was systematically investigated by experimental test, as is shown in Fig. 7(c)–(e). The peak output voltage achieved 342.8 V, and the power reached the maximum value of 2.523 mW with the load resistance adjusted to 33 MΩ. The amount of generated charge of the sliding-mode TENG with different objects sliding on in one cycle is shown in Fig. 7 (e), which reveals the effect of different friction materials. When a TENG slides on another, more than 1000 nC charges (25 nC/cm2) are transferred in each sliding cycle.

3. Large-scale enhancement technology for TENG Although the output performance of TENG is more attractive compared with other energy harvesting mechanisms, the output energy is still not large enough to supply the traditional electronic device directly. Therefore, enhancing energy density has become one of the most important research topics. This section is to summarize three approaches to strengthen the energy density of TENG, including roughening surface, fluorocarbon plasma treatment and multiple friction surfaces. All of the above approaches have

X.-S. Zhang et al. two common points, i.e., large scale (wafer level, even bigger size) and universality (be suitable to various materials), which make them suitable for low-cost high-efficiency mass production.

3.1.

Surface morphology

There are many factors affecting the output performance of TENG, and one of them is the surface morphology (i.e., surface roughness). Increasing the surface roughness increase means enlarging the effective area of friction surface, then the triboelectrification effect is more sufficient. In previous research work, researchers have employed microstructures and nanostructures to increase the surface roughness [31,32]. Furthermore, we introduced micro/nano hierarchical structures into TENG, and studied the surface roughness effect systematically, as is shown in Fig. 8 [33]. According to Fig. 8, generally, the voltage and current are enhanced by increasing the friction surfaces roughness. The friction surface with micro/nano hierarchical structures (Fig. 8(e and g)) strengthened the TENG performance further compared with pure microstructures (Fig. 8(d and f)) or

Fig. 8 Characterization of the output performance of the sandwich-shape triboelectric nanogenerator with different structures, including (a) flat PET film: (a1) 2 cm 2 cm or (a2) 2 cm 4 cm, (b) flat PDMS film, (c) surface-nanostructured PDMS film, (d) PDMS film with micro V-shape grooves, (e) PDMS film with micro/nano dual-scale V-shape grooves, (f) PDMS film with micro pyramid arrays, (e) PDMS film with micro/nano dual-scale pyramid arrays. Reproduced with permission from [33]. Copyright 2013 American Chemical Society.

High performance triboelectric nanogenerators based on large-scale mass-fabrication technologies nanostructures (Fig. 8(c)). Additionally, compared with the micro/nano hierarchical structure with V-shaped grooves (Fig. 8e), the one with inverted pyramids (Fig. 8g) showed more attractive property to enlarged the voltage and the current by 100% and 157% than the flat PDMS film (Fig. 8b), respectively. Moreover, the microscale morphology is the dominant factor according to the small difference of output performance compared with micro/nano hierarchical structure.

3.2.

Fluorocarbon plasma treatment

Obviously, the two friction surfaces made of different materials are the core of TENG. Although the triboelectrification effect is a complex process where the material composition, the operation mode and the environmental factors contribute together, the ability difference of bounding electrons between these two friction materials is the essential point. Larger the triboelectric difference is, higher the TENG output performance is. According to this phenomenon, researchers arranged materials in order and obtained the “triboelectric series” [75]. Therefore, the output performance of TENG can be enhanced by changing the triboelectric difference between its two friction surfaces. Based on the above conception, we proposed the fluorocarbon plasma treatment as a universal approach to enhance the power density of TENG, as is shown in Fig. 9(i) [72]. After the optimization of plasma treatment cycles, the maximum instantaneous energy area density of the TENG with micro/nano hierarchical structures is enhanced

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by 278% to 4.85 mW/cm2, with a peak output voltage of 265 V and current density of 18.3 μA/cm2, as is shown in Fig. 9(ii). The stability and reliability of this fluorocarbon plasma treatment were systematically studied by continuously working experiments and comparative experiments. The TENG enhanced by the fluorocarbon plasma treatment stably worked for about 2000 cycles (Fig. 9(iii)), and the TENG output performance almost kept constant (Fig. 9(iv)). Additionally, the universality of this fluorocarbon plasma treatment was studied from three aspects, including noble gas comparison, different plasma generation equipment, and different substrates, as is shown in Fig. 9(v). The noble gas of argon was employed to verify the contribution of fluorocarbon polymer deposited by the plasma treatment, in other words, the effect from plasma itself may be ignored. According to the similar output performances of TENGs treated by DRIE and RIE, respectively, this fluorocarbon plasma treatment approach is suitable for different plasma generation equipment. Finally, this plasma treatment approach was demonstrated to be worked for TENGs with different friction materials, such as PET and Kapton. As is known, the triboelectric charge density of σ0 is tightly related to the output performance of TENG. Moreover, σ0 is related to the difference of bound ability for electrons, which can be evaluated by the electron binding energy. Therefore, it is believed that the fluorocarbon polymer layer possesses larger electron binding energy than PDMS, which is the main reason for the enhancement of power density. Thus, first-principle calculations (i.e., density functional theory) were employed to calculate the vertical ionization energy of these two materials, in other

Fig. 9 The analysis and measurement of TENG enhanced by fluorocarbon plasma treatment. (i) schematic view of the fluorocarbon plasma treatment process; (ii) characterization of the output performance of the TENG treated with different plasma treatment cycles under the external force with frequency of 5 Hz; (iii)–(v) investigations of the stability and reliability of the fluorocarbon plasma treatment. Reproduced with permission from [72]. Copyright 2014 Elsevier.

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Fig. 10 The theoretical calculation results of vertical ionization energy of model complexes of (a) PDMS and (b) fluorocarbon layer deposited by the C4F8 plasma treatment; (c) Fourier Transform Infrared (FTIR) spectra of the fluorocarbon layer. Reproduced with permission from [72]. Copyright 2014 Elsevier.

words, electron binding energy, as is shown in Fig. 10. Obviously, the model of complex C4F8 shows larger ionization energy than the model of complex PDMS, which are 12.31 eV vs 8.98 eV, respectively. This clearly reveals that the former is easier to obtain one electron than the latter, consistent with the observed experimental results of enhancing the TENG output performance by C4F8 plasma treatment. The reliability of this calculation model of the fluorocarbon polymeric layer was also demonstrated by the Fourier Transform Infrared (FTIR) spectra, and the calculated spectrum is highly consistent with the measured one, as is shown in Fig. 10(b).

represented in the former sections. Besides the above ones, some other novel TENGs with unique properties and functions were also developed by using the large-scale massfabrication technology. Herein, this section is to introduce the single-friction-surface TENG and the hybrid TENG fabricated by either the soft lithography process or the FPCB process.

4.1.

Single-friction-surface TENG

4. Novel TENG based on large-scale massfabrication technology

In the middle of 2013 year, a novel structure of TENG was fabricated by soft lithography, which is named singlefriction-surface triboelectric nanogenerator (i.e., STEG) [51]. The structure of STEG is simpler, and moreover, it is very convenient to be integrated with other devices. STEG drives the attractive vision of realizing self-powered portable electronics without battery, moving closer to practical applications. In order to develop STEG further, we also studied its working principle by theoretical analysis [64], as is shown in Fig. 12. This STEG shows remarkable transparence and flexibility, which can be placed atop the screen of a smart phone to harvest the energy generated by finger typing or touching. The effect of different friction materials on the output performance of STEG was studied, as is shown in Fig. 12(b) and (c). After the optimization of surface geometry, the output performance of STEG was significantly strengthened. When tapped with a finger, the STEG with micro-patterned PDMS surface achieved an open-circuit voltage over 130 V with a short-circuit current density of about 1 μA/cm2. The theoretical model of STEG was also developed. The equivalent circuit of STEG with the electrode grounded is shown in Fig. 13(a). Thus, the transient equation is obtained based on Kirchhoff’s law,

Based on the large-scale mass-production technology, many high-performance TENGs have been proposed as is

Q Q i ðtÞ Q ðtÞΔQ Q i ðtÞ dQ i ðtÞ ¼ i þ þ ðRB þRL Þ CG ðtÞ CB C1 dt

3.3.

Multiple friction surface

Another useful method to strengthen the TENG output performance is manufacturing folded zigzag structures, which means that the TENG consists of multiple friction surfaces. Based on the FPCB technique, this zigzag-shaped TENG was successfully built up and showed remarkable output performance, as is shown in Fig. 11 [40]. The TENGs with 2, 6, 10 integrated friction pairs were fabricated and compared. According to Fig. 11(b)–(d), the output voltage and current of TENG significantly increase as the number of friction pairs enlarges. The comparison of charging ability of these three TENGs also demonstrated the above trend, as is shown in Fig. 11(e). However, it is worth mentioning that the average charge generated by each friction pair was almost equal, which implied that this enhancement is linear to the number of friction pairs. The voltage and current density of the TENG with 10 integrated friction pairs achieved 620 V and 45.1 μA/cm2, respectively.

ð1Þ


313

Fig. 11 Characterization of the TENGs based on FPCB process with different numbers of integrated friction pairs. (a) Schematic of 3 TEGs with 2, 6, and 10 integrated friction pairs, respectively; (b) voltage and (c) short-circuit current of the 3 TEGs under the same impact; (d) comparison of the voltage, short-circuit current and (e) the generated charge of the 3 TEGs. Reproduced with permission from [40]. Copyright 2013 Elsevier.

where Q is the charge on friction surface, Qi(t) is the charge on induction electrode and ΔQ is the leaked charge due to the good conductivity of human body. At the opencircuit condition, Qi(t) is 0 because there is no charge transferred. So the open-circuit voltage is obtained from Eq.(1), V OC ¼

Q ΔQ þ CG ð tÞ CB

ð2Þ

At the short-circuit condition, in regardless of the influence of body resistance and assuming that the charge leaks very quick, the short-circuit current is given by, ΔQ C21 CB Q C21 CB Q C2B C1 dCG ðtÞ ISC ¼ ð3Þ ðCB C1 þC1 CG ðtÞ þCB CG ðtÞÞ2

Then, the transferred charge during one cycle is obtained by, ΔQ C1 Q T ¼ 2ðQ imax Q imin Þ ¼ 2 Q ð4Þ C1 þCB Assuming RB =RB0 , then the equivalent circuit in Fig. 13(b) can be considered as a special case of that in Fig. 13(a) with an infinitely large body capacitance CB. Thus, for the STEG with human body conduit as is shown in Fig. 13(b), it can be calculated from Eqs. (2) to (4), V OC ¼ ISC ¼

Q CG ðtÞ

Q C1 dCG ðtÞ ðC1 þCG ðtÞÞ2

Q T ¼ 2Q

ð5Þ ð6Þ ð7Þ

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Fig. 12 Analysis and measurement of the single-friction-surface triboelectric generator (STEG). (a) SEM image of the PDMS fabricated by using soft lithography process, and photos of PDMS, PET and STEG; (b) the open-circuit voltage, short-circuit current and charge transferred in a half cycle of the STEG with the micro-patterned PDMS surface (b) when tapped with a bare finger and (c) when tapped with a finger covered in a PE glove. Reproduced with permission from [51]. Copyright 2013 Royal Society of Chemistry.

Theoretically, the STEG with grounded electrode (Fig. 13(a)) shows a small offset of ΔQ/CB in the open-circuit voltage compared to the case with human body conduit. While the optimized STEG with human body conduit (Fig. 13(b)) shows a better performance in short-circuit current and the amount of transferred charge than the case with grounded electrode.

4.2.

Hybrid TENG

Hybrid TENG means the nanogenerator based on the combination of triboelectric effect and other energy harvesting mechanisms, which can also be considered as a universal technique to strengthen the output performance of TENG. Different from the above three single-mechanism enhancement techniques based on the internal improvement in Largescale enhancement technology for TENG section, the hybrid TENG is multi-mechanism technique which employed the

external method to improve the TENG output performance. More importantly, these hybrid TENGs are fabricated by the mass-production technology, i.e., soft lithograph and FPCB. Thus, the introduction of hybrid TENG is separated from other enhancement techniques and formed this section.

4.2.1. Piezoelectric–triboelectic effect As is mentioned in the introduction section, the piezoelectricity is another essential approach to realize nanogenerator besides the triboelectricity. When a hybrid TENG is designed to integrate piezoelectric effect and triboelectric effect together, its output performance should be enhanced due to harvesting energy by two different mechanisms simultaneously. Herein, based on this concept, a piezoelectric–triboelectric hybrid nanogenerator was proposed, as is shown in Fig. 14 [48]. A specific r-shaped cantilever was well-designed by using the piezoelectric polymer of PVDF, and a PDMS film


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with micro/nano hierarchical structures was placed underneath. When the external force is applied to the r-shaped PVDF cantilever, it will be deflected and impact to the PDMS surface, which causes the piezoelectric effect and the triboelectric effect, respectively. Fig. 14(a) shows the schematic view and SEM images of this r-shaped hybrid TENG, and Fig. 14(b) shows the electrical measurement results. Voltage at 100 MΩ of the piezoelectric part and the triboelectric part were 52.8 V and 240 V, respectively. The output currents at 10 kΩ of the piezoelectric part and the triboelectric part were 166 μA and 27.2 μA, respectively. Transferred charge in 600 cycles of the piezoelectric part and the triboelectric part were 16.6 μC and 13.0 μC, respectively.

Fig. 13 Working principle and equivalent circuit of (a) the STEG with grounded electrode and (b) the STEG with human body conduit when finger taps the friction surface and showing a tendency to donate electrons. (For (a): RB—body resistance, and CB—body capacitance; for (b): RL—load resistance, R0 B—body resistance between two hands, CG—capacitance between finger skin and the friction surface, C1—capacitance between the friction surface and the induction electrode, and CIB—capacitance between the induction electrode and the body contact electrode. Since the gap between the two electrodes is fairly large, CIB will be very small which is not be considered in the following analysis.) Reproduced with permission from [64]. Copyright 2014 AIP Publishing LLC.

4.2.2. Electromagnetic–triboelectric effect Recently, another hybrid TENG based on electromagnetic– triboelectric effect was developed, as is shown in Fig. 15 [54]. Fig. 15(a)–(c) shows the schematic view, photograph and SEM image of this hybrid TENG. It is made of a polytetrafluoroethene (PTFE) cylinder container, and the other components are placed in order, including the top part (steel mass, top NdFeB permanent magnet, and silica layer) and the bottom part (spiral shaped electrode wrapped by polyimide, and the bottom NdFeB permanent magnet). The opposite magnet pairs play two important roles. One is to provide magnetic repulsive force, which is essential for the separation of two friction materials. The other is to induce voltage in the copper electrodes, thus combining electromagnetic output together with the triboelectric output. Thus, when the top magnet moves up and down inside the cylinder and impacts the bottom part frequently, the mechanical energy will be harvested and transformed by both of electromagnetic effect and triboelectrification effect. This hybrid TENG shows remarkable output performance, and the maximum power

Fig. 14 Photograph and measurement results of the piezoelectric–triboelectric hybrid nanogenerator. (a) Schematic view and photograph of the hybrid nanogenerator, and (b) its output characterization. Output voltages of (i) the piezoelectric part and (iv) the triboelectric part, and the insets show the enlarged voltage curve in one cycle; output currents of (ii) the piezoelectric part and (v) the triboelectric part; voltages of a 1 μF capacitor charged by (iii) the piezoelectric part and (vi) the triboelectric part, respectively. Reproduced with permission from [48]. Copyright 2013 American Chemical Society.

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Fig. 15 Photograph and measurement results of the electromagnetic–triboelectric hybrid nanogenerator. (a) Schematic diagram of the magnetic-assisted TENG; (b) photo and (c) SEM image of the spiral-shaped copper electrode; output performance of (d) the triboelectric part and (e) the electromagnetic part: (i) output voltage, (ii) output current and (iii) peak voltage and peak power under different external load resistances. Reproduced with permission from [54]. Copyright 2014 Nature Publishing Group.

density of 541.1 mW/m2 was obtained at 16.67 MΩ for the triboelectric part, while the electromagnetic part generated power density of 649.4 mW/m2 at 16 Ω.

5. Applications for low-power-consumption devices During the past three years, TENGs have attracted so much attentions due to its strong potential in many application fields. Researchers have done plenty of demonstrations to show how TENGs contribute to our daily life and benefit people. This section is to give the summary about the TENG applications for low-power-consumption devices, including three fields, i.e., portable electronics, self-powered active sensors and biomedical microsystems.

5.1.

Portable electronics

In modern times, people’s life is surrounded by various portable electronics, such as computer, laptop, smart phone, iPad, etc. And the battery is the major obstacle to

realize the miniaturization of portable electronics, which is also the major problem for realizing environmental friendly. Therefore, as an attractive vision for the future, manufacturing self-powered portable electronics without battery or with a self-charged battery is the guide direction for researchers. TENG provides a promising approach to realize this target, which is capable to be integrated with portable electronics. Ref. [40] showed a demonstration that 100 red LEDs mounted on FPCB were simultaneously lit up by finger pressing, which shows the potential of TENG applied in integrated portable system and device, as is shown in Fig. 16(a). Another demonstration was presented in Ref. [48], the r-shaped hybrid TENG was successfully integrated inside the keyboard, as is shown in Fig. 16(b). Thus, when the fingers type the keyboard, the r-shaped piezoelectric– triboelectric hybrid TENG will be pressed and then the mechanical energy will be transformed to electrical output. Furthermore, the innovation of STEG opens the chapter of TENG impacting the personal electronics due to its outstanding transparence, simple structure and high output performance. Ref. [51] released two application


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Fig. 16 The applications of nanogenerator for portable electronics. (a) 100 red LEDs mounted on the self-powered FPC board are simultaneously lit up by finger pressing. Reproduced with permission from [40]. Copyright 2013 Elsevier. (b) Electricity generated by pressing a keyboard integrated with an r-shaped piezoelectric–triboelectric hybrid nanogenerator. Reproduced with permission from [48]. Copyright 2013 American Chemical Society.

demonstrations by integrating STEG with cell phone and liquid crystal display (LCD), as is shown in Fig. 17. When your fingers touch/release on the surface of STEG placed atop the screen of cell phone, the energy will be harvested and transformed to electrical output. In other words, as an attractive vision for future, cell phone can be charged by its daily use.

5.2.

Self-powered active sensors

The output performance of TENG is related with the external factors, in other words, the variation of TENG output reflects the change of external factor. Based on this relationship, TENGs can be designed to self-powered active sensors, which power itself and also indicates the change of a specific factor. Fig. 18(a) shows a visualized tilt selfpowered senor based on the electromagnetic–triboelectric hybrid TENG [54]. At different tilt angles, the output voltage of the hybrid TENG changes, which leads to different voltages applied to the LCD, thus affecting the final display of the LCD. As shown in Fig. 18(a-iii), when operating at the tilt angle of 601, only the first square displays on the LCD screen. Then, as the tilt angle increases to 701 and 801, the second and third squares on the LCD screen are lightened. This self-powered system directly converts the tilt angle information to visual display, without the treatment of recording and analyzing the output signal.

Therefore, complex measurement device can be removed in some cases, and this self-powered system is completely electric-free. Ref. [72] developed another self-powered active sensor based on TENG, i.e., humidity sensor, as is shown in Fig. 18(b). According to the experimental results, the output performance of TENG is tightly related with the environmental humidity, and the output voltage decreases as the relative humidity increases. It is believed that this phenomenon results in the discharging effect of the water molecules adsorbed on the friction surface. A commercial light emitting diode (LED) lamp with a matching variable resistor was used as an indicator to replace the expensive electrometer to monitor the change of environmental humidity, which shows the possibility to simplify the humidity detection system. The two working states of the LED, turn-off and turn-on, semiquantitatively indicate the humidity range of 450% and o50%, respectively.

5.3.

Biomedical microsystems

Micro/nano science and technology bring an innovative storm to the biomedical field in decades, which induce plenty of miniaturized and implantable devices. But the power supply limits the rapid development and the wider applications of biomedical microsystems. Herein, we demonstrated the capability of TENG driving the

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Fig. 17 The applications of nanogenerator for portable electronics. (a) A STEG applied as a transparent cover on the screen of a smartphone: (i) a monochrome LCD was powered and (ii) 3 LEDs connected in series were illuminated by the STEG when the smartphone touchscreen was operated in the normal way; (b) self-powered visualized touch sensor based on the STEG: (i–iv) the LCD displays which touch pad was touched when a STEG touch pad was tapped with a finger. Reproduced with permission from [51]. Copyright 2013 Royal Society of Chemistry.

implantable biomedical microsystems [33,72]. In 2013, the first demonstration of TENG directly driving the neural prosthesis was reported, as is shown in Fig. 19 [33]. The neural prosthesis consists of well-designed microelectrode arrays, as is shown in Fig. 19(i) and (ii). This neural prosthesis dipped into the PBS solution was electrically connected to the TENG, and then the circuit current was measured, which achieved 88 μA (Fig. 19(iii)), which is large enough to stimulate the damaged nerve system. The high-output TENG is also successfully applied to drive an implantable microneedle electrode array to stimulate a frog’s sciatic nerve [72]. A 3D microneedle electrode array (i.e., MEA) consists of 9 9 Si-based high-aspect-ratio tips covered by a 4 μm gold layer. The whole device was covered by an 8 μm parylene-C membrane to realize flexibility and bio-compatibility, while the top of tips was exposed by patterning parylene-C. In the experiment, this MEA was implanted into real frog tissue such that the high-aspectratio tips pricked the frog’s sciatic nerve. The two electrodes of the MEA were connected to the TENG directly without

any extra circuit. When the external force is applied to the TENG, the instantaneous output voltage of the TENG induces the loop current among the microneedle tips via the sciatic nerve. Therefore, the sciatic nerve is stimulated by the loop current and actuated the leg muscle of the frog, as is shown in Fig. 20. This experiment represents the first application of a TENG for sustainably powering a biomedical microsystem implanted in real biological tissue, moving closer to practical biomedical applications of TENGs.

6.

Conclusions and outlook

Based on the last three years productive world-wide research work, the developments of TENG can be summarized into three major lines, including fabrication technologies, output enhancement techniques, and various applications. Each of the above lines is illustrated in details by our group work from several aspects, such as background, significance, approaches, samples, and so on. At the


Fig. 18 The applications of nanogenerator for active selfpowered sensors. (a) Demonstration of the visualized tilt selfpowered sensing system: (i) measurement system of the selfpowered sensor, (ii) electric circuit of the self-powered visible system, and (ii) photos of the self-powered visible sensor at different operation states. Reproduced with permission from [54]. Copyright 2014 Nature Publishing Group. (b) The measured relation of environmental humidity and TENG output voltage: the output voltage is normalized based on the TENG voltage under the humidity of 10%, and a commercial light emitting diode (LED) lamp with a matching variable resistor is used as the indicator to replace the expensive electrometer to monitor the change of environmental humidity, which shows the possibility to simplify the detection system. Reproduced with permission from [72]. Copyright 2014 Elsevier.

beginning of the TENG development, most research work focused on the fabrication technologies and the output enhancement by plenty of experimental explorations, such as arch-shaped structure, surface-textured friction material, well-designed operation mechanisms, etc. Additionally, there is one driven force throughout the development history of TENG, i.e., applications. From stable energy source to self-powered active sensor, the capability of TENG to impact human’s life has been demonstrated in multidisciplinary fields. Above achievements provide a bright future for TENGs, give confidence to many researchers in related fields, therefore, attract more researcher’s attention in the world. Though it has many advantages and reasonable demonstrations, however, as a brand

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new research field, TENG is still far from the practical applications, there is plenty of room for researcher to dig and develop deeply, especially in following three directions. First, more attentions should be paid to the theoretical study and principle measurement of TENG, especially the fundamental chemical–physical mechanism at molecular even atomic level. Although triboelectrification effect is known for thousands years, there is still lack of knowledge of its substantial mechanism several aspects, such as the polarity reversal phenomenon. The fundamental principle of TENG and output enhancement mechanism hid behind measurement results and complicated experimental phenomena. Thus, at the subsequent development stage, theoretical analysis, modeling calculation and principle measurement should be investigated deeply and widely. Thus, it is believed that the theoretical achievement will promote TENG into a new rapid developing times. Moreover, the theoretical development is very important to the further evolution of TENG, which will establish the footstone of promoting TENG as an independent discipline branch. Second, the power density enhancement, which is always the key point of TENG development, and the TENG based on hybrid mechanisms may lead TENGs to the higher level. Although the TENG shows remarkable output performance, the power density is not large enough to feed the traditional electronic system directly yet. And we believe that the energy conversion efficiency can be enhanced significantly by exploring new materials, surface modifications (either chemical, physical, biological, or other methods), and especially combining different working mechanisms to form novel hybrid TENGs. Especially, hybrid TENGs, which efficiently harvest ambient energy via two or more working principles simultaneously, can not only realize high performance as power generator, also provide potential specific functions resulting from the combined mechanisms. Finally, the self-powered smart microsystem will become another key word for TENG in the future. As is mentioned in the introduction section, which is also shown in Fig. 1, the applications of TENG contain two main fields, i.e., stable power source and self-powered smart system. Therefore, as an attractive vision for the future, self-powered smart microsystems based on the combination of the above two unique properties will open a new chapter of TENG practical applications. Several examples have involved self-powered active sensors in previous work, but most of them were just the original demonstrations, where the stability and performance of the whole device needs to be upgraded and strengthened. More importantly, for the smart system application, we couldn’t miss the packaging issue. Appropriate package will be very useful to reduce the damage risk of TENG resulting from mechanical fatigue and environmental effect. Additionally, the suitable package of TENG is significant for some specific application fields, such as implantable biomedical devices, chemical erosion and high temperature harsh environments, etc. In summary, the research and development of TENG is just started, next 10 year there must be an outbreak of TENG both in academy and commercialization, not only remarkable achievements of research will be done for understanding and developing of powerful generators, the self-powered smart system also will be utilized in real applications to benefit our daily life.

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Fig. 19 The applications of the sandwich-shape triboelectric nanogenerator for electric device and biomedical system. The implantable 3-D microelectrode array (i and ii) for neural prosthesis was directly driven, and the current reached 88 μA (iii). Reproduced with permission from [33]. Copyright 2013 American Chemical Society.

Fig. 20 Frog’s sciatic nerve stimulation by the instantaneous output of the high-performance TENG. (a–i) Photo of the nerve stimulation test system, including the power source of TENG and the microneedle electrode array implanted into frog tissue and stimulating sciatic nerve. (a-ii) Photo of the flexible implantable microneedle electrode array. (a-iii) SEM image of the microneedle electrode array. (a-iv) SEM image of the tip of the microneedle electrode array. (b,i–iv) Photos that illustrate the real-time response of frog’s leg by the stimulation of the microneedle electrode array driven by the TENG. Reproduced with permission from [72]. Copyright 2014 Elsevier.


Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant nos. 61176103, 91023045 and 91323304), the National Hi-Tech Research and Development Program of China (“863” Project) (Grant no. 2013AA041102), and the Beijing Natural Science Foundation of China (Grant no. 4141002).

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[68] X.S. Zhang, Q.L. Di, F.Y. Zhu, G.Y. Sun, H.X. Zhang, J. Nanosci. Nanotechnol. 13 (2013) 1539–1542. [69] X.S. Zhang, F.Y. Zhu, M.D. Han, X.M. Sun, X.H. Peng, H.X. Zhang, Langmuir 29 (2013) 10769–10775. [70] N.J. Peter, X.S. Zhang, S.G. Chu, F.Y. Zhu, H. Seidel, H.X. Zhang, Appl. Phys. Lett. 101 (2012) 221601. [71] X.S. Zhang, B. Meng, F.Y. Zhu, W. Tang, H.X. Zhang, Sens. Actuators, A 208 (2014) 166–173. [72] X.S. Zhang, M.D. Han, R.X. Wang, B. Meng, F.Y. Zhu, X.M. Sun, W. Hu, W. Wang, Z.H. Li, H.X. Zhang, Nano Energy 4 (2014) 123–131. [73] M.D. Han, X.S. Zhang, W. Liu, X.M. Sun, X.H. Peng, H.X. Zhang, Sci. China Technol. Sci. 56 (2013) 1835–1841. [74] M.D. Han, W. Liu, X.S. Zhang, B. Meng, H.X. Zhang, Sci. China Technol. Sci. 56 (2013) 2636–2641. [75] A.F. Diaz, R.M. Felix-Navarro, J. Electrostat. 62 (2004) 277–290. Xiao-Sheng Zhang received the Ph.D. degree from Peking University, Beijing, China, in July 2014. He is currently working as a postdoctoral researcher at École polytechnique fédérale de Lausanne (EPFL), Switzerland, and as a joint researcher at the University of Tokyo. His research interests mainly include three areas: (1) micro/ nano integrated fabrication technology; (2) micro/nano energy technology; (3) smart drug delivery system. Meng-Di Han received the B.S. degree in Electronic Science & Technology from Huazhong University of Science and Technology, China, in 2012. He is currently pursuing the Ph.D. degree at the National Key Laboratory of Nano/Micro Fabrication Technology, Peking University, Beijing, China. His research work is focusing on nanogenerator and active sensors.

Bo Meng received the B.S. degree in Electronic Science & Technology from Huazhong University of Science and Technology, China in 2011. He is a Ph.D. candidate in National Key Lab of Nano/Micro Fabrication Technology at Peking University, China. He majors in MEMS and his research interests are energy harvester and SiC MEMS.

Hai-Xia (Alice) Zhang (SM’10) received the Ph.D. degree in mechanical engineering from Huazhong University of Science and Technology, Wuhan, China, 1998. She joined Peking University at 2001 after finishing her post-doctoral research in Tsinghua University. She is currently a Professor with the Institute of Microelectronics, Peking University, Beijing, China. Her research interests include micro-nano design, fabrication and energy technology. She has served on the General Chair of the IEEE NEMS 2013 Conference, the Organizing Chair of Transducers’11. As the Founder of the International Contest of Applications in Nano-Micro Technologies (iCAN), she has been organizing this event since 2007. In 2006, she received the National Invention Award of Science and Technology.