Supercapacitors

Highly Flexible and Adaptable, All-Solid-State Supercapacitors Based on Graphene Woven-Fabric Film Electrodes Xiaobei Zang, Qiao Chen, Peixu Li, Yijia He, Xiao Li, Miao Zhu, Xinming Li, Kunlin Wang, Minlin Zhong, Dehai Wu, and Hongwei Zhu* Recently, the applications of portable and flexible devices have come to fruition, for instance, devices like flexible touch screens[1] and flexible solar cells[2] have become daily essentials. Such breakthroughs have dramatically stimulated the development of related technologies, such as the design and construction of energy-storage devices, one of the most important devices for human prosperity and health. Supercapacitors, which are also called electrochemical capacitors, are fast but high-power energy-storage devices.[3–5] In such devices, charges are stored in the interface of electrolyte and electrode material through the rapid and reversible adsorption/desorption of ions.[6,7] Much effort has also been made to develop thin-film supercapacitors,[8,9] which are expected to possess high capacity while maintaining light weight and flexibility. Theoretically, the two-dimensional (2D) extension of thin-film supercapacitors could substantially reduce the deformation resistance from the vertical direction, which ultimately would make the entire device thin, flexible, and easy to fold, twist, or reshape. The travel distance of electrolyte ions in thin-film supercapacitors is also shorter than their counterparts.[10] Consequently, all-solid-state supercapacitors, X. B. Zang, Q. Chen, Y. J. He, X. Li, M. Zhu, Prof. K. L. Wang, Prof. M. L. Zhong, Prof. H. W. Zhu School of Materials Science and Engineering State Key Laboratory of New Ceramics and Fine Processing Key Laboratory of Materials Processing Technology of MOE Tsinghua University Beijing 100084, China E-mail: [email protected] Q. Chen, Y. J. He, X. Li, M. Zhu, Prof. H. W. Zhu Center for Nano and Micro Mechanics Tsinghua University Beijing 100084, China P. X. Li, Prof. D. H. Wu Department of Mechanical Engineering Tsinghua University Beijing 100084, China Dr. X. M. Li National Center for Nanoscience and Technology Zhongguancun, Beijing 100190, China DOI: 10.1002/smll.201303738 small 2014, DOI: 10.1002/smll.201303738

which do not employ liquid electrolytes, have attracted tremendous attention because of their simplicity, flexibility, efficiency,[11–16] and ease of assembly and usage. Graphene is a 2D crystalline form of carbon, in which the carbon atoms are arranged in a honeycomb lattice. Graphene possesses unique properties, such as excellent electrical conductivity, high surface-to-volume ratio, and specific surface area.[17–19] The intrinsic capacitance of single-layer graphene reaches ca. 21 μF cm–2 when the entire surface area is used.[20] So far, film graphene materials derived from graphite oxide (GO) and other carbon-based materials have exhibited excellent properties in various aspects, including high specific capacitance, life-cycle stability, energy density, and power density. However, recent efforts have predominately focused on developing novel electrode materials for thin-film supercapacitors,[21–24] including a reduced graphene oxide film (462 μF cm–2),[11] microporous carbons (2–15 μF cm–2),[25–27] polystyrene-based hierarchical porous carbon (28.7 μF cm–2),[28] active carbon (10 μF cm–2),[29] and carbon fabric (1 mF cm–2).[30,31] Therefore, studies aiming to further improve the area capacitance of thin-film supercapacitors are rare but highly desirable. Herein, we report the construction of thin-film supercapacitors that use graphene woven-fabric (GWF) films as electrode materials. The GWF films were synthesized by using direct chemical vapor deposition (CVD) on copper mesh,[32–34] which can be effectively removed with aqueous FeCl3/HCl solution, eventually affording GWFs deposited on the desired substrates. Compared with graphene films, GWFs possess superior flexibility and strength, presumably because they retain the network configuration of the copper mesh. Figure 1a shows images of GWFs deposited on different substrates, in which GWFs and substrates have formed integrated electrodes. The construction process of supercapacitors is schematically illustrated in Figure 1b. Silver wires were first attached to electrodes with silver sol, which should allow effective charge transfer; the finished supercapacitor is composed of two symmetrical electrodes and a poly(vinyl alcohol) (PVA)-H3PO4 polymer gel electrolyte, and no packaging is needed. In these devices, the PVA-H3PO4 polymer serves as both electrolyte and separator. Results obtained from the scanning electron microscopy (SEM) analysis of GWFs are given in Figure 1c, in which the woven fabric

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Figure 1. a) Digital photographs of GWF films deposited on various substrates. Scale bars: 0.5 cm. b) Schematic illustration of the fabrication of GWF thin-film supercapacitors. c) SEM of GWF films. d) Optical images of GWF films on various substrates: polishing cloth, poly(ethylene) (PE) cling film, polyethylene terephthalate (PET), and filter paper. Scale bars: 100 μm.

structure is clearly evident. Moreover, the internal surface in GWFs appears to be more abundant than regular graphene films. A detailed description of the microscopic structure of GWFs was reported previously.[32–34] A series of optical images of GWFs deposited on different substrates is presented in Figure 1d, showing their morphological features. Due to the distinct surface characteristics of the substrates, the surface appearance of the GWFs varies, however, all GWFs should maintain the fundamental structure of woven fabrics. To evaluate the electrochemical performance of the GWF-based thin-film supercapacitor, cyclic voltammetry (CV), galvanostatic charge-discharge (CD), electrochemical impedance spectroscopy (EIS), and cycle-stability studies were performed, by using a two-electrode method with a CHI 660B electrochemical workstation. The electrochemical performances of GWF supercapacitors on four different substrates are illustrated in Figure 2. CV curves were acquired at various scanning rates, ranging from 20–200 mV s–1, with a voltage window between 0–0.8 V. The shape of these CV curves is close to rectangular, and no redox peak is evident, which clearly indicates that the double-layer capacitive behavior should be ideal.[35] For GWF supercapacitors based on different substrates, the area of CV curves seems to be different. Particularly, the area of the CV curve shown in Figure 2a appears to be much larger than those illustrated in Figure 2b–d, which suggests that the device on polishing cloth should possess the highest specific capacitance, and the

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device on PET should have the lowest one. At a scan rate of 60 mV s–1, the area capacitance for devices based on GWFs deposited on different substrates were determined: 9 mF cm–2 for polishing cloth, 3 mF cm–2 for filter paper, 2 mF cm–2 for PE cling film, and 1 mF cm–2 for PET. The electrode is only nanometers thick (ca. 1–7 nm) and the device thickness is less than 1 mm. The mass loading of GWF film is 0.03 mg cm–2, and the mass specific capacitance is obtained (267 F g–1 at 60 mV s–1). The capacitance is also dependent on the electrode thickness. In PET-based devices, for example, the area capacitance increases with the layer number of GWF films and reaches 120% for a five-layer GWF device (Figure S1). Additionally, the profile of voltage versus time was obtained by galvanostatic charge–discharge tests at a current density of 0.2 mA cm–2. Theoretically, devices with decent doublelayer capacitive behavior should render highly symmetric curves. Under the same current density, the charge–discharge time for the device on filter paper appears to be the longest, however, its curve symmetry seems to be inferior (Figure S2a). In addition, these curves show only a small iR (current×resistance) drop of 60 (PET), 55 (polishing cloth), 35 (PE cling film), and 25 mV (filter paper), which indicates that these devices have low equivalent series resistance (ESR). To further evaluate the electrochemical response and suitability of these supercapacitors, electrochemical impedance spectroscopy was employed, to provide valuable information about their dielectric and transport properties. For instance, the phase response of frequency for the GWF

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Figure 2. Electrochemical performance of GWF film supercapacitors. CV curves at different scan rates, galvanostatic charge–discharge curves at a current density of 0.2 mA cm–2, Nyquist plots with magnification for the high-frequency region in the inset, and the cycling stability of supercapacitors using a) polishing cloth, b) PET, c) PE cling film, and d) filter paper as substrates.

supercapacitor on polishing cloth was obtained, as shown in Figure S3. Notably, the phase angle of this supercapacitor is close to –90° at low frequency (0.01 Hz), however, it changes to 0° when the frequency is increased to 100 KHz. These results undoubtedly indicate that this device switches from capacitor to resistance when the frequency is increased from 0.01 to 100 KHz. Different slopes are present in the Nyquist plots of samples (Figure S2b), and the slope of these curves is in the order of PE cling film > polishing cloth > filter paper > PET. Particularly, it has been found that the internal resistance is 8 (filter paper), 8 (PE cling film), 16 (polishing cloth), and 50 Ω (PET). Another critical electrochemical property that needs to be taken into account is the area capacitance retention, for which the results are summarized in Figure 2. Specifically, after 1000 cycles, the area-capacitance retention is 145% for filter paper, 100% for polishing cloth, 99% for PET, and 90% for PE cling film. During the charge–discharge processes, the stability of the entire system, which is composed of electrodes and electrolyte, is adequately increased. As illustrated in Figure 3a, the relationship between area capacitance and scan rate was closely examined, and the rate capability of the supercapacitor on filter paper appears to be small 2014, DOI: 10.1002/smll.201303738

superior. The capacitance retention values of devices on filter paper, polishing cloth, PE cling film and PET are 45%, 40%, 40% and 7%, respectively. To investigate the overall performance of these thin-film supercapacitors, the corresponding Ragone plots were established, as shown in Figure 3b, in which the energy density and power density of all the devices were examined. The device on polishing cloth exhibited the highest energy density, at 0.001 mWh cm–2. This value is substantially higher than the energy density of regular graphene films derived from chemical vapor deposition.[36] Theoretically, the supercapacitors reported in this study can be reshaped into multiple forms, depending on the flexibility of the substrates. Such transformation often leads to significant changes of CV curves and area capacitance, as illustrated in Figure 4. Various shapes of supercapacitors, such as folded or twisted ones, have been reported in previous studies, in which the GWFs were obtained in reasonable sizes that allowed sophisticated manipulation. The maximum size for a GWF single sheet is about 8 cm2, which can be readily transformed into an origami ship or paper airplane. GWFs generally do not come off the substrates during the course of deformation, presumably due to the favorable van der Waals

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Figure 3. a) Area capacitance at scan rates from 0.02–0.5 V s–1. b) Ragone plot.

force between films and substrates. As demonstrated in the CV curves and other related analyses, the area of most CV curves was augmented by proper reshaping; as a result, the area capacitance of the devices after deformation is the same as, or higher than before deformation. The percentages of area-capacitance increase for devices based on different substrates were determined, and the highest increase was for the device on polishing cloth, 160%; the increases for the devices on filter paper and PET were 150% and 120%, respectively; the device on PE cling film retains the original value. The area capacitance could not be less than the original situation, regardless of any deformation. As shown in Figure S4a, all the GWF-based devices can be deformed more than 20 times, while still maintaining a respectable area capacitance. After 20 deformations the area capacitance seems to be higher than the original value, presumably due to the tight contact between electrode materials and polymer gel electrolyte. As shown in Figure S4b, after 300 deformations the area capacitance of the PET-based device was still 100%. The devices could deform many times without obvious performance degradation. Consequently, both external and internal surfaces are fully employed, which significantly improves the surface utilization factor. The deformation process could also decrease the internal resistance, as shown in Figure S5. Therefore, we conclude that the ultimate outcome of deformation is equivalent to increasing weight. For instance, for the supercapacitor on PET, when the weight of the sample plus substrate increased from 20 to 500 g, its area capacitance increased accordingly, as shown in Figure S6. Even though the CV curves retain the same shape as the weights increase, the area of the CV curves significantly increases. The most appropriate weight appears to be 20 g, with an area capacitance rising up to 150%; addition of more weight did not increase the area of CV curves. On the other hand, the surface conditions of polishing cloth and filter paper are different from the ones of PET and PE cling film, and more space is available for compressing. Both PET and PE cling film have a flat surface, thus they generally exhibit less shape change upon slight deformation, compared with polishing cloth or filter paper. Because PET is the most resilient material used, the GWF supercapacitor based on PET substrate often curls, thus GWFs and PET substrates should tightly integrate with each other, and a higher degree of curling should induce rebound. As a result, the area capacitance of this curled supercapacitor is 150%,

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and removing external force can restore the area capacitance back to its original state. In contrast, PE cling film is a very soft substrate, thus the force generated during the course of deformation is fairly limited, affording little influence on the area capacitance, which consequently retains its original value. We have constructed a series of supercapacitors using graphene woven-fabric films deposited on different substrates. The supercapacitor based on polishing cloth exhibited remarkable electrochemical performance, affording an area specific capacitance as high as 8 mF cm–2 (267 F g–1). The electrode thickness is only ca. 1–7 nm and device thickness is

Highly flexible and adaptable, all-solid-state supercapacitors based on graphene woven-fabric film electrodes.

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