Free-standing graphene films prepared via foam film method for great capacitive flexible supercapacitors

Accepted Manuscript Title: Free-standing graphene films prepared via foam film method for great capacitive flexible supercapacitors Authors: Yucan Zhu, Xingke Ye, Zhonghua Tang, Zhongquan Wan, Chunyang Jia PII: DOI: Reference:

S0169-4332(17)31729-4 http://dx.doi.org/doi:10.1016/j.apsusc.2017.06.074 APSUSC 36274

To appear in:

APSUSC

Received date: Revised date: Accepted date:

6-3-2017 31-5-2017 6-6-2017

Please cite this article as: Yucan Zhu, Xingke Ye, Zhonghua Tang, Zhongquan Wan, Chunyang Jia, Free-standing graphene films prepared via foam film method for great capacitive flexible supercapacitors, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.06.074 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Free-standing graphene films prepared via foam film method for great capacitive flexible supercapacitors Yucan Zhu, Xingke Ye, Zhonghua Tang, Zhongquan Wan, Chunyang Jia

State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Microelectronics and Solid-State Electronics, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China.

Free-standing graphene films prepared via foam film method for great capacitive flexible supercapacitors Yucan Zhu, Xingke Ye, Zhonghua Tang, Zhongquan Wan, Chunyang Jia

State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Microelectronics and Solid-State Electronics, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China. Graphical abstract

Free-standing graphene films prepared via foam film method for great capacitive flexible supercapacitors Yucan Zhu, Xingke Ye, Zhonghua Tang, Zhongquan Wan, Chunyang Jia

State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Microelectronics and Solid-State Electronics, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China. Highlights 

Free-standing GO films were prepared by foam film method.

 Corresponding author. Tel.: +86 28 83201991; Fax: +86 28 83202569. Email: [email protected] (C. Y. Jia)  Corresponding author. Tel.: +86 28 83201991; Fax: +86 28 83202569. Email: [email protected] (C. Y. Jia)  Corresponding author. Tel.: +86 28 83201991; Fax: +86 28 83202569. Email: [email protected] (C. Y. Jia) 1



Thickness of the GO films was regulated by changing concentration of CTAB.



The prepared GO films have excellent mechanical property.



Supercapacitors with the rGO films show excellent volumetric capacitance and flexibility.

Abstract Recently, graphene films have always attracted attention due to their excellent characteristics in energy storage. In this work, a novel graphene oxide (GO) film with excellent mechanical properties, whose thickness was regulated simply via changing the concentration of the surfactant, was successfully prepared by foam film method. After chemical reduction, the reduced GO (rGO) films have excellent electrical conductivity of ~172 S cm-1. Moreover, the supercapacitors based on the rGO films exhibit satisfied capacitive performance of ~56 mF cm-2 at 0.2 mA cm-2 in 6 M KOH aqueous solution. Meanwhile, the flexible all solid state supercapacitors (FSSCs) based on the rGO films also show great volumetric capacitance of ~2810 mF cm-3 at 12 mA cm-3 (~1607 mF cm-3 at 613 mA cm-3) with polyvinyl alcohol-KOH gel electrolyte. Besides, after 10000 cycles and continuously bent to 180° for 300 times, the volumetric capacitance of the FSSC remains at 81.4% and 90.4% of its initial capacitance value, respectively. Therefore, the free-standing rGO films prepared via foam film method could be considered as promising electrode materials for high performance flexible supercapacitors. Keywords: foam film; ; ; , graphene oxide films, surfactant, flexible all solid state supercapacitors 1. Introduction

2

Wearable electronic devices, such as pedometers, heat-rate monitor, military garment devices and smartwatch, have attracted increasing attention in the last few years, due to their applications in fitness, medicare, military, information and so on.[1-6] However, the development of wearable electronics is always restricted by the conventional power systems (e.g., batteries) which have some drawbacks of small lifespan and low charge/discharge rates. Even worse, they are too bulky and heavy to be considered as power sources of flexible electronic devices.[7, 8] Therefore, fabricating the flexible power system with light weight, long lifespan and high charge/discharge rates is critical for the development of wearable electronics. Electrochemical capacitors, which are also called supercapacitors (SCs), store energy by ion adsorption/desorption at the interface of electrode and electrolyte or by redox reactions at surface/near-surface of electrode materials.[9, 10] In comparison to conventional Li-ion batteries, SCs have higher security, longer cycle life, higher power density and faster charge/discharge rates. Thus SCs are more suitable for wearable electronics as power system than batteries. In SCs, electrode materials are researched extensively, since it is one of the major factors that determines capacitance performance.[11, 12] Among the electrode materials, carbon materials (e.g., activated carbon, CNTs, black carbon, graphene, ect.) have become typical promising electrode materials due to their merits of high conductivity, widespread, stable electrochemical property and so on.[13] In particular, graphene possesses excellent properties of intrinsic physics and chemistry.[14, 15] For example, it has an excellent theoretical specific surface area (2360 m2 g-1),[16] high intrinsic mobility (200000 cm2 v -1 s 3

1

),[17] as well as wonderful mechanical strength. What is more, the intrinsic

properties of graphene also endow graphene films with outstanding mechanical strength and chemical stability, indicating that their potential applications as flexible electrode materials.[18, 19] Effective efforts for preparing graphene films were also reported in recent years. Chen et al. reported the chemical vapor deposition (CVD) prepared graphene films, which possess high specific capacitance and electrical conductivity.[20] However, CVD method requires harsh conditions and high-cost to be scaled up. Many methods for preparing graphene films are therefore proposed to prepare graphene oxide (GO) films. The typical methods include vacuum filtration,[21] spray-coating,[22] spin-coating,[23] blade-casting technique.[24] The prepared GO films were further reduced to the reduced GO (rGO) films by the reduction process. These approaches for constructing free-standing ultrathin GO films, however, have the cumbersome process for practical application, which have to undergo the delamination of the films from the substrate, and sometimes even need to apply electrochemical delamination or bubble delamination methods to obtain highintegrity films.[25-27] In this work, ultrathin free-standing GO films were prepared by the method of foam films without any substrates. This method not only simplifies the process of fabricating free-standing films, but also is a low-cost and facile operation. Furthermore, thickness of the GO films also can be regulated by simply changing the concentration of the surfactant. The prepared GO films have excellent mechanical properties. The rGO films, obtained from the GO films by chemical reduction 4

process, also have extremely high electrical conductivity. Meanwhile, the SCs based on the rGO films exhibit excellent specific capacitance, and the flexible all solid state supercapacitors (FSSCs) based on the rGO films also have impressive capacitance performance, great electrochemical stability and excellent mechanical flexibility. 2. Materials and experimental 2.1. Materials Natural graphite was purchased from Shanghai Hua Yi Group Hua Yuan Chemical Industry Co. Ltd. (Shanghai, China). Hexadecyl trimethyl ammonium bromide (CTAB, 99%) was purchased from Aladdin Industrial Corporation. Polyvinyl alcohol (PVA, 98-99% hydrolyzed, medium molecular weight) was purchased from Alfa Aesar (USA). Other chemical reagents were purchased from China as reagent grades and used without further purification. 2.2. Preparation of the GO films The graphite oxide was synthesized from natural graphite via modified Hummers method.[28] Then graphite oxide was dispersed into deionized water and sonicated for 30 minutes at room temperature to form GO suspension of 20 mg mL-1. Subsequently, 3 mL of CTAB (0.1 mg mL-1) was poured into 20 mL GO suspension (20 mg mL-1). After the suspension was stirred for 5 minutes, a small amount of GO suspension was spread onto Teflon substrate by using dropper. Then copper circle was placed on Teflon, followed by slowly move of copper circle parallel to the substrate. After copper circle was moved from the suspension, a small amount 5

of GO suspension was captured to the template of copper circle. Finally, the GO films were obtained by evaporating the water of captured GO suspension in air at room temperature (the preparation process as shown in Figure 1a). Additionally, the GO films with different thicknesses can be obtained by adding different concentrations of CTAB into GO suspension in the above process. 2.3. Preparation of the rGO films 2.3.1. Vapor-phase reduction of the GO films The GO films were putted into 300 mL sealed jar containing 3 mL HI (57 wt% in H2O) and 6 mL CH3COOH (≧99.5 wt%) (HI/AcOH) and then the sealed jar was placed into oven with 60°C for 10 h.[29] At this point, the color of the GO films changed from golden yellow to black, indicating that the GO films were reduced. Subsequently, the rGO films were rinsed with anhydrous ethanol for three times. Finally vapor-phase reduced GO films (V-rGO) were dried at room temperature. 2.3.2. Liquid-phase reduction of the GO films The GO films were transferred into the mixture solution comprised of 3mL HI and 6mL AcOH. Then the reaction system was sealed and placed into oven with 60 °C for 2 h. Subsequently, the rGO films were rinsed with anhydrous ethanol for three times. The final liquid-phase reduced GO films (L-rGO) were dried at room temperature. 2.4. Preparation of the FSSCs

6

PVA-KOH gel electrolyte was synthesized simply by a physical mixing method. At first, 1 g PVA particles was added into 10 mL deionized water. Subsequently, the mixture was stirred continuously for 1 h at 85°C to form a clear solution. Then 1 g KOH aqueous solution (6 M) was added into PVA solution and stirred 30 mins at room temperature to form PVA-KOH gel electrolyte. The electrodes of devices were prepared through the rGO films with 10 mm Χ 10 mm pressed on nickel foam by using pressure of 20 MPa. Finally, PVA-KOH gel electrolyte was spread uniformly on the electrodes. After evaporating for 1 h at room temperature, two electrodes were completely overlapped and the system was packaged by using polyethylene terephthalate (PET). 2.5. Material characterization Surface morphologies of the samples were investigated by scanning electron microscope (SEM, JEOL JSM-7600F). Analysis of element content was carried out by XPS measurements (Kratos XSAM800). Structure of the samples was investigated via Raman spectroscope (WITec Alpha 300) using 532 nm laser and X-ray diffraction (SHIMADZU XRD-7000) using Cu Ka radiation. The surface roughness and microstructure of samples were characterized by atomic force microscope (AFM, Bruker instruments Dimension Edge) with a silicon tip in tapping mode. Sheet resistance of the samples was tested by a standard four-point probe (MODEL 280) method, and volume conductivity was calculated based on Equation (1). Mechanical strength of the samples with rectangular strips of 5 mm Χ 20 mm was tested by using

7

an INSTRON Universal Testing Machine with a constant loading rate of 0.5 mm min1

.

σ=

1 𝑅𝑠 𝑡

(𝑆 𝑐𝑚−1 )

(1)

Where Rs is sheet resistance (Ω sq-1), t is thickness of the samples (cm). 2.6. Electrochemical measurements Electrochemical performance of the supercapacitors, such as cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS), were tested via electrochemical working station (CHI660E). Gravimetric capacitance (Cm) of the rGO films was obtained from GCD curves by calculating based on Equation (2). Areal capacitance (CA) and volumetric capacitance (Cv) of the supercapacitors were obtained from GCD curves by calculating based on Equation (3) and (4), respectively. Energy density (ED) and power density (PD) of the supercapacitors were obtained via calculating based on Equation (5) and (6), respectively.

𝐶𝑚 = 𝐶𝐴 = 𝐶𝑣 = 𝐸𝐷 = 𝑃𝐷 =

4𝐼∆𝑡

(𝐹 𝑔−1 )

(2)

(𝐹 𝑐𝑚−2 )

(3)

(𝐹 𝑐𝑚−3 )

(4)

(𝑊ℎ 𝑐𝑚−3 )

(5)

𝑚∆𝑉 𝐼∆𝑡 𝐴∆𝑉 𝐼∆𝑡 𝑉∆𝑉

2 𝐶𝑣 𝑉𝑚𝑎𝑥

7200

3600𝐸𝐷 ∆𝑡

(𝑊 𝑐𝑚−3 )

(6)

8

Wherem is total mass of active material (g), A is area of device (cm2), V is total volume of device (cm3), I is galvanostatic discharging current (A), Δt is discharging time (s), ΔV is voltage window (V), Vmax is operation potential (V). 3. Results and discussion 3.1. Preparation of the GO films In this paper, the GO films were fabricated via foam film method.[30, 31] First, thin foam films, which are the sandwich structure composed of the water core between amphipathic molecules with hydrophobic groups outward, are obtained via the holes template.[31] Then, the embedded water in the thin foam films is removed via the evaporation in air, followed by the self-assembly of the amphipathic molecules to form the dried foam films. However, for GO flakes, which are 2D nanosheets with the amphiphilic character, are different from conventional surfactant molecule composed of hydrophilic and hydrophobic groups.[32] Thus a small amount of surfactant is still added into GO suspension in order to boost the stability of the bubble wall.[33] As shown in Figure 1b, the GO films are formed via the evaporation caused self-assembly of the GO flakes. The foam film method has some distinct advantages of low-cost, low-energy consumption and facile operation. Meanwhile, we can also prepare the GO films directly with the arbitrary shapes to satisfy practical application (Figure 1c and Figure S1), and the holes templates can be reused for preparation of the same shape films. More interestingly, the thickness of GO films were regulated by changing the concentration of CTAB ranging from 0.1 to 0.5 mg 9

mL-1. As shown in Figure 1d-f, the GO films fabricated with concentrations of CTAB of 0.1, 0.3, and 0.5 mg mL-1 have the corresponding thicknesses of ~4.5, ~1.5 and ~1.0 µm, respectively (areal densities of the corresponding rGO films are presented in Table 1). This result strongly suggests that the thickness of the GO films reduces with the increasing concentration of CTAB within a certain range. In general, surface tension of the solution decreases and the viscosity of the solution increases with the increasing concentration of CTAB in the studied range.[34] For this reason, the holes template captured GO suspension reduced with the increasing concentration of CTAB in a certain range, which eventually results in the decrease of the GO film thickness. Figure 1 is here Table 1 is here 3.2. Microstructure characterizations Chemical reduction is a low-cost and effective method to reduce the GO films. In this paper, the rGO films were obtained via reducing GO films with HI/AcOH. The cross-sectional SEM images (Figure 2a-c) show that L-rGO and V-rGO films compared with GO films possess obvious hierarchical structure, which is beneficial to solution diffusion, since the diffusion layer thickness range from several micrometers to dozens of micrometers in electrochemical reactions.[35] Figure 2d-f shows that the surface of GO film has numerous wrinkles, but the surface of rGO film emerges several ripples and slight wrinkles. Meanwhile, the surface roughness of GO and rGO films were also characterized by AFM, and the characterization results show that the 10

surface roughness of GO film is obvious higher than that of rGO film (corresponding data and discussion were shown in the Supplementary Materials). This phenomenon may be caused by the decomposition of oxygen functional groups and the formation of the structural defects.[24, 36-38] Figure 2 is here XPS technique also provide the information of element content, which is helpful for studying the reduction degree of the GO films. Figure 3a shows that intensity of C1s peak in the GO films, which has distinctly increased while intensity of O1s peak decreased after the reduction process. The ratio of carbon and oxygen (C/O) content was obtained via calculating the ratio of C1s and O1s peak area, as shown in Table 2. For the GO films, the ratio of C/O increases from 1.77 to 4.70 after the vapor-phase reduction process, indicating that some oxygen functional groups were removed.[39] Relative to vapor-phase reduction, the ratio of C/O increased to 7.24 via the liquidphase reduction process, suggesting that the reduction degree of the GO films by liquid-phase process is higher than that of vapor-phase process. In addition, Figure 3b shows that high resolution C1s peak of the GO films consist of two main peaks at 284.5 eV and 286.8 eV, corresponding to C=C/C-C and C-O-C/C-OH, respectively.[40] After the reduction process, the peak of C=C/C-C becomes the only main peak, suggesting the elimination of some oxygen functional groups and the restoration of the most of graphene networks. Moreover, the intensity of C=C/C-C peak in L-rGO films is also higher than that of V-rGO films, demonstrating that the reduction degree of liquid-phase process is higher than that of vapor-phase process. 11

Table 2 is here Because XPS technique can only detect a few nanometers depth of the samples, the information of element content reveals only the change of GO film surface.[41] Thus it is necessary to use XRD technique to investigate the internal micro-structure of the samples. Figure 3c shows that XRD pattern of the GO films presents an obvious (001) peak at 8.96° corresponding to d-spacing of ~9.86 Å, whose interlayer spacing is distinctly larger than that of graphite (d-spacing: ~3.34 Å) due to the presence of the abundant oxygen functional groups and the intercalated water molecules between layers.[29, 42] XRD pattern of V-rGO films not only has a (001) peak at 10.18° (d-spacing: ~8.68 Å), but also has an broad (002) peak at 24.56° (dspacing: ~3.62 Å), which indicating that the part decomposition of oxygen functional groups of the GO films and results in the different interlayer distances.[41] L-rGO films have only a broad (002) peak at 24.68° corresponding to d-spacing of ~3.6 Å, suggesting that the GO films were deeply reduced. And the d-spacing of L-rGO films is slightly larger than that of graphite due to the residual oxygen functional groups or the structural defects.[39] In addition, structural characterization of the GO and rGO films was also investigated via Raman spectroscopy. Raman spectra (Figure 3d) of the GO films present two main peaks at ~1344 and ~1577 cm-1, corresponding to D and G bands, respectively. After the chemical reduction, the intensity ratio of D and G bands (ID/IG) did increase notably, suggesting that the increase of structural defects or degree of disordered sp2 domains in the rGO films.[35] Figure 3 is here 12

3.3. Mechanical strength and electrical conductivity With the concentration of CTAB aqueous solution is 0.1 mg mL-1, the GO films exhibit outstanding mechanical properties, which can support a one-yuan coin (Figure 4c). Figure 4a-b shows that stress-strain curves of the GO films and the corresponding L-rGO films. The GO films possess greater Young’s modulus (~5.8 GPa) and tensile strength (~96.4 MPa) than those of the corresponding L-rGO films (Young’s modulus and tensile strength of L-rGO films are ~2.8 GPa and ~54.7 MPa, respectively). The reason of degraded mechanical properties of L-rGO films due to the functional groups on the GO nanosheets can bind individual sheets together, which leads that GO films are stiffer, denser and stronger than rGO films.[43] In addition, the increase of the structural defects also results in the decline of mechanical properties of rGO films.[44] Figure 4 is here L-rGO films have excellent electrical conductivity of ~172 S cm-1, which is equivalent to the reported reduced graphene oxide films.[24, 45, 46] For example, Kumar et al. reported that reduced graphene oxide thin films exhibit electrical conductivity of ~152 S cm-1[45] and flexible porous graphene film shows electrical conductivity of 1.12 S cm-1.[46] 3.4. Electrochemical properties 3.4.1. Electrochemical properties of the SCs

13

Electrochemical properties of the rGO films in symmetrical SCs were investigated using 6 M KOH aqueous solution as electrolyte. First, capacitance performance of V-rGO films was investigated with different thicknesses. Figure 5a shows CA of the SCs based on V-rGO films, which decreases with the increase of concentration of CTAB. However, Cm of V-rGO films almost remain unchanged (Figure 5b). This phenomenon reveals that the thickness of the films could be regulated by changing the concentrations of CTAB (Figure 1d-f). Meanwhile, there are almost no “dead weight” in electrode materials for the SCs when thickness of the film increases in this studied range. Based on the above results, it is clearly demonstrated that V-rGO films with 0.1 mg mL-1 CTAB possess the best capacitance performance among the different concentration of CTAB. In addition, the GO films with 0.1 mg mL-1 CTAB possess excellent mechanical strength that can withstand the liquid phase reduction process. Thus we further investigated capacitance performance of the SCs based on L-rGO films with 0.1 mg mL-1 CTAB. Figure 5c shows that the shape of CV curves of device based on L-rGO films is better than that of the device based on V-rGO films, which is due to the higher reduction degree of liquid-phase reduction compared to that of vapor-phase reduction. Moreover, the CV curves are the near-rectangular shapes with scan rates of 10, 20, 50, 100 and 200 mV s-1 at the potential window ranging from 0 to 1 V, indicating L-rGO films possess the capacitance characteristic of electronic double layer. In addition, the excellent current response rate to voltage reversal at end potential of CV curves demonstrates that the low internal resistance of the device, 14

which can be attributed to the high conductivity of the L-rGO films and the fast ion diffusion in electrode material.[47, 48] The shape of GCD curves is nearly symmetrical triangle, as shown in Figure 5d, also suggesting that L-rGO films exhibit ideal capacitive properties and high charge mobility. In addition, EIS of the SCs based L-rGO films is also important for in-depth research of electrochemical properties. Nyquist impedance curve of the SCs based L-rGO films is comprised of a small semicircle in the high frequency region and a vertical line in the low frequency region (Figure 5e). The inset in figure 5e shows that the intercept of real axis (Z’), corresponding to internal resistance of the SCs, is ~0.55 Ω, which also demonstrates that L-rGO films possess excellent electrical conductivity. The Warburg region of 0.68 to 12.1 Hz is relatively short, indicating the fast ions diffusion in electrode materials, which is attributed to the presence of hierarchical structure. As shown in Figure 5f, CA of the SCs based on L-rGO films achieves ~56 mF cm-2 at 0.2 mA cm-2, and still possesses ~40 mF cm-2 at 10 mA cm-2 (Figure 5f). It suggests that the devices possesses an impressive rate capability. In addition, Cm of L-rGO films is also up to ~111 F g-1 at 0.1 A g-1 (~80 F g-1 at 5 A g-1), which is even comparable with the reported literature for graphene-based composite materials.[35, 49, 50] Figure 5 is here 3.4.2. Electrochemical properties of the FSSCs For the reason of encapsulating difficulty and possible electrolyte leakage of SCs with liquid electrolyte, the FSSCs were packaged by using PVA-KOH gel electrolyte 15

in this work. The CV curves of the FSSC with scan rates ranging from 10 to 200 mV s-1 at the potential window of 0~1 V are quasi-rectangular, as shown in Figure 6a. Compared with the shape of CV curves at low scan rates, the shapes of CV curves at high scan rates such as 200 mV s-1 distorts slightly, which is due to the ions from the interface of the electrode and electrolyte have sufficient time to infiltrate into the interlayers of L-rGO films when scan rate is low. But when scan rates is rising, the diffusion time of the ions was shortened, which causes that some ions can’t migrate to the effective interlayers of the L-rGO films, eventually resulting in the distortion of CV curves shape and the slightly decrease of the device capacitance.[47, 48, 51] For the GCD curves of the devices (Figure 6b), the charging curves and the discharging curves are almost symmetrical, exhibiting excellent capacitive characteristics. In addition, according to EIS of FSSCs (Figure 6c), the intercept of Nyquist impedance curves in real axis shows that the internal resistance of the device is ~1 Ω, which is higher than the internal resistance of the SCs with liquid electrolyte. This can be attributed to the tardy diffusion rate of the electrolyte in the solid-state device.[51] Moreover, the Warburg region (from 0.46 to 12.1 Hz) is slightly longer than that of the SCs with liquid electrolyte, also demonstrating that slower ions diffusion in the device with gel electrolyte. Figure 6d shows Cv of the device at different current densities, which exhibits impressive specific capacitance of ~2810 mF cm-3 at current density of 12 mA cm-3, and Cv of the FSSCs still remain ~1607 mF cm-3 at current density of 613 mA cm-3, indicating that the device possesses high rate capability. Ragone plot is also great important for estimating overall performance of the FSSCs. 16

As shown in Figure 6e, ED of the FSSCs is up to 0.39 mWh cm-3 with PD of 6.13 mW cm-3, which is even superior to the values reported for some solid-state SCs, such as graphene paper-based FSSCs (0.12 mWh cm-3),[24] graphite/PANI paper-based SCs (0.32 mWh cm-3),[52] carbon/MnO2-based FSSCs (0.22 mWh cm-3)[53] and HTiO2@MnO2//H-TiO2@C-based FASCs (0.3 mWh cm-3).[54] What’s more, when PD of the FSSCs achieves 307 mW cm-3, ED still remains at 0.22 mWh cm-3. In addition, the FSSCs has also excellent cycle stability performance and high degree reversibility according to the CV measurement with potential range of 0~1 V at scan rate of 500 mV s-1, which still possesses high capacitance retention of 81.4% after 10000 cycles (Figure 6f). Figure 6 is here Considering the practicability of flexible devices, it is essential that electrochemical properties of the devices remain unchanged at the different bending states. Figure 7a shows the CV curves of the FSSCs under different bending states at scan rate of 20 mV s-1, the shape of CV curves under different bending states exhibits no obvious change. Interestingly, capacitance of the FSSCs has slightly increased at bending angle of 100° (Figure 7b), which may be attributed to the convenient charge transfer because of the decreased distance between two electrodes at bending state. Furthermore, the flexibility of the FSSCs was further tested via measuring capacitance retention of the FSSCs after repeated bending with bending angle of 180°. Figure 7c shows that the shape of CV curves is still basically superposition after different bending times. Moreover, capacitance retention of the FSSCs is up to 90.4% after 17

continuous 300 times bending cycles (Figure 7d), which is slightly better than the values reported for flexible supercapacitor, such as the CNT/PPy/HQ-based FSSC (90% after 100 cycles),[55] graphene paper-based FSSCs (84.6% after 100 cycles),[24] conductive graphene-cotton fabric-based FSSCs (90.5% after 100 cycles).[48] These results demonstrate that the FSSCs possess great electrochemical stability and excellent mechanical flexibility. Figure 7 is here 4. Conclusion In summary, foam film technique was applied to prepare GO films, whose thickness can be regulated by changing the concentration of CTAB. The prepared GO films possess excellent mechanical strength (Young’s modulus and tensile strength are 5.8 GPa and 96.4 MPa, respectively). After liquid-phase reduction process, L-rGO films exhibit electrical conductivity of ~172 S cm-1. In addition, the SCs based on LrGO films exhibit CA of ~56 mF cm-2 at 0.2 mA cm-2, and Cm of L-rGO films is up to ~111 F g-1 at 0.1 A g-1. The FSSCs based on L-rGO films also show impressive capacitance performance of ~2810 mF cm-3 at 12 mA cm-3. More importantly, the capacitance value of FSSC still maintains 90.4 % of its initial capacity after bending 300 times bending cycles with bending angle of 180°, demonstrating that the device possesses strong practicability and reliability. Therefore, the FSSCs based on the prepared rGO films via foam film method are well suited for applications in wearable electronic as power system.

18

Acknowledgements We are grateful to the National Natural Science Foundation of China (Grant Nos. 21572030, 21272033, 21402023) and Technology Innovative Research Team of Sichuan Province of China (No.2015TD0005) for financial support.

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21

Figure Caption

22

Figr-1

Figure 1

23

Figr-2

Figure 2

24

(a) O1s

Intensity (a.u.)

C1s GO

V-rGO

L-rGO

1000

Figr-3

800 600 400 Binding energy (eV)

200

0

V-rGO L-rGO GO

Intensity (CPS)

(b)

280

282

284

286

288

290

292

Binding energy (eV)

(c)

V-rGO L-rGO GO

V-rGO L-rGO GO

Intensity (a.u.)

Intensity (a.u.)

(d)

5

10

15

20

25

30

35

40

1200

1400

1600

1800

Wavenumber (cm-1)

2(angle)

Figure 3

25

100

(a)

Stress (Mpa)

80 60 40 20 0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

Strain (%)

Figr-4

60

(b)

Stress (Mpa)

50 40 30 20 10 0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

Strain (%)

Figure 4

26

8

(a)

Current (mA cm-2)

6 4 2 0 -2 -4 -6

-1

0.1 mg mL

-8

-1

0.3 mg mL

-1

-10

0.5 mg mL

1.0

0.8

0.4

0.2

0.0

(b)

2

0

-2 -1

0.1 mg mL

-1

-4

0.3 mg mL

-1

0.5 mg mL

1.0

0.8

0.6

0.4

0.2

0.0

Potential (V)

(c) 1.0

-2

(d)

0.2 mA cm

-2

0.4 mA cm

8

-2

1 mA cm

0.8

4

Potential (V)

Current desity (mA cm-2)

12

0 -1

10 mV s

-4

-1

20 mV s

-2

2 mA cm

0.6 0.4

-1

50 mV s

-8

0.2

-1

100 mV s

-1

200 mV s

-12 1.0

0.8

0.6

0.4

0.2

0.0

0.0

0

100

200

Potential (V)

-2

)

60

30 25

300 -Z'' (ohms)

20

200

15 10

-1

0 0

5

10

15

20

25

30

Z' (ohms)

0 0

100

200

300

400

500

600

700

800

(f)

40 30 20 10 0

0

2

0

1

120

5

100

50

Capacitance (mF cm

(e)

300

Time (s)

Capacitance (F g )

400

-Z'' (ohms)

Current density (A g-1)

4

0.6

Potentional (V)

Figr-5

400

500

Z' (ohms)

4

6

2

3

-2

Current density (mA cm

)

8

10

12

4

5

6

100 80 60 40 20 0

-1

Current density (A g )

Figure 5

27

Figr-6

-3

(b)

1.0

12 mA cm

-3

25 mA cm

-3

60 mA cm

0.4

-3

120 mA cm

0.8

0.2

Potential (V)

Current density (mA cm-3)

0.6 (a)

0.0 -0.2

-1

10 mV s

0.6 0.4

-1

20 mV s

-0.4

-1

0.2

50 mV s

-1

100 mV s

-0.6

-1

200 mV s

1.0

0.8

0.6

0.4

0.2

0.0

0.0

0

100

200

Potential (V)

3000

400

(c)

Specific capacitance (mF cm-3)

30 25

-Z'' (ohms)

-Z'' (ohms)

300

200

20 15 10 5 0

100

0

5

10

15

20

25

30

Z' (ohms)

0 0

100

200

300

400

(d)

1500 1000 500 0

0

100

200

300

400

500

600

700

Current density (mA cm-3)

0

(f)

(e)

100

Capacitance retention (%)

Energy density (mWh cm-3)

600

2000

500

This work Carbon fiber-FASC ref.54 Carbon/MnO2-FSSC ref.53

10

500

2500

Z' (ohms)

10

300 400 Time (s)

-1

Graphene-FSSC ref.24

80 60 40 20 0

10

1

10

2

Power density (mW cm-3)

10

3

0

2000

4000

6000

8000

10000

Cycle numbers

Figure 6

28

Current density (mA cm-3)

0.04

(a) 0.02

0.00

-0.02

0

0 0 150

-0.04 1.0

0.8

0.6

0.4

0

0

50 100 0 180

0.2

Capacitance retention (%)

Figr-7

(b) 100 80 60 40 20 0

0

0.0

20

40

Potential (V)

(c) 0.02

0.00

-0.02

-0.04

original 200 times

1.0

0.8

0.6

0.4

0.2

100 times 300 times

0.0

Potential (V)

Capacitance retention (%)

Current density (mA cm-3)

0.04

100

60

80 100 120 140 160 180 200

Bending angle (o)

(d)

80 60 40 20 0

0

50

100

150

200

250

300

Bending numbers

Figure 7

29

Figr-23Figure captions

Figure 1. (a) Preparing procedure of the GO films. (b) Formation process of the freestanding ultrathin GO films. (c) Different shapes of the GO films. (d-f) Crosssectional SEM images of the GO films with different thicknesses. Figure 2. (a-c) Cross-sectional SEM image of the GO, L-rGO and V-rGO films. (d-f) SEM image of the GO, L-rGO and V-rGO films. Figure 3. (a) XPS spectra of the GO, V-rGO and L-rGO films. (b) High-resolution C1s spectra of the GO, V-rGO and L-rGO films. (c) XRD patterns of the GO, V-rGO and L-rGO films. (d) Raman spectra of the GO, V-rGO and L-rGO films. Figure 4. (a-b) Stress-strain curves of the GO and L-rGO films. (c) Optical image of a one-yuan coin supported by the prepared GO films. Figure 5. (a-b) CV curves at 100mV s-1 of the SC based on V-rGO films with different concentrations of CTAB. (c) CV curves of the SCs based on L-rGO films with potential window ranging from 0~1 V at different scan rates. (d) GCD curves of the SC based on L-rGO films at different current densities. (e) Nyquist plot of the SC based on L-rGO films. (f) CA and the Cm of the SC at different current densities. Figure 6. (a) CV curves of the FSSC based on L-rGO films at different scan rates. (b) GCD curves of the FSSC based on L-rGO films at different current densities. (c) Nyquist plot of the FSSC based on L-rGO films. (d) Cv of the FSSC at different current densities. (e) Ragone plot of the FSSC. (f) Cycling performance of the FSSC. Figure 7. (a) CV curves at 20mV s-1 of the FSSC under the different bending angles. (b) Capacitance retention of the FSSC at different bending angles. (c) CV curves of the FSSC after different bending cycles. (d) Capacitance retention of the FSSC after different bending cycles.

30

Table 1. Different concentrations of CTAB and the corresponding areal density of the rGO films. CTAB concentration (mg mL-1)

Areal density (mg cm-2)

0.1 0.3 0.5

1.07 0.84 0.45

31

Table 2. The element content and the ratio of carbon (C) and oxygen (O). Samples GO V-rGO L-rGO

C (%) 63.94 82.46 87.86

O (%) 36.06 17.54 12.14

C/O 1.77 4.70 7.24

32