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Flexible Transparent Supercapacitors based on Hierarchical Nanocomposite Films fanhong chen, Pengbo Wan, Haijun Xu, and Xiaoming Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 11 May 2017 Downloaded from http://pubs.acs.org on May 12, 2017

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ACS Applied Materials & Interfaces

Flexible Transparent Supercapacitors based on Hierarchical Nanocomposite Films Fanhong Chen,† Pengbo Wan,*,† Haijun Xu,‡ and Xiaoming Sun‡ †

Center of Advanced Elastomer Materials, State Key Laboratory of Organic-Inorganic

Composites, Beijing University of Chemical Technology, Beijing 100029, P. R. China. ‡

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical

Technology, Beijing 100029, P. R. China.

KEYWORDS: flexible electronic devices, nanocomposites, transparent conducting films, all-solid state supercapacitors, graphene

ABSTRACT Flexible transparent electronic devices have recently gained immense popularity in smart wearable electronics and touch screen devices, which accelerates the development of the portable power sources with reliable flexibility, robust transparency and integration to couple these electronic devices. For potentially coupled as energy storage modules in various flexible, transparent and portable electronics, the flexible transparent supercapacitors are developed and assembled from hierarchical nanocomposite films of reduced graphene oxide (rGO) and aligned polyaniline (PANI) nanoarrays upon their synergistic advantages. The nanocomposite films are fabricated from in situ PANI nanoarrays preparation in a blended solution of aniline monomers

ACS Paragon Plus Environment

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and rGO onto the flexible, transparent, and stably conducting film (FTCF) substrate, which is obtained by coating silver nanowires (Ag NWs) layer with Meyer rod and then coating of rGO layer on polyethylene terephthalate (PET) substrate. By optimizing the transparency, the specific capacitance, and the flexibility, the obtained all-solid state nanocomposite supercapacitors exhibit enhanced capacitance performance, good cycling stability, excellent flexibility, and superior transparency. It provides promising application prospects for exploiting flexible, low-cost, transparent, and high-performance energy storage devices to be coupled into various flexible, transparent and wearable electronic devices.

1. INTRODUCTION Flexible transparent electronic devices, featured with robust wearability, reliable flexibility, high transparency, and compatibility on flexible substrates, have recently attracted enormous research attention in various wearable and integrated electronics, typically including flexible transparent touch screens, displays, sensors, transistors, electronic books, generators, and actuators.1-9 The rapid growth of flexible transparent electronics prompts the development of the portable power sources with corresponding flexibility, transparency, and sufficient integration to couple these electronic devices.10,11 Meanwhile, supercapacitor, as one kind of energy storage devices, is well established with the benefits of excellent specific capacitance, low cost, high power density, and superiors charge/discharge rate and cycle stability.12-14 Various transparent electrodes and flexible electrodes for all-solid state supercapacitors have been established, respectively.


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However, the transparent and flexible electrodes for supercapacitors have rarely been achieved by combining the flexibility and transparency together in the supercapacitors. For example, the rigid transparent electrodes were obtained from the rigid, transparent and conductive glass substrates without flexibility.15 The flexible electrodes for supercapacitors were successfully prepared from the flexible carbon cloth or hydrogels without the corresponding transparency.16,17 Many electrochemical properties of the electrodes for all-solid state supercapacitors were tested in the liquid electrolytes (three electrodes system), which is lack of real-time portability.18 For potentially working as integrated energy storage modules in various flexible, transparent and portable electronics, it is highly desirable to prepare supercapacitors with reliable flexibility, transparency and compatibility, in contrast to the developed supercapacitor electrodes with electrochemical properties tested in electrolyte solutions. Among the various supercapacitor electrode materials, polyaniline (PANI) and reduced graphene oxide (rGO), are excellent competitive capacitive materials for pseudo-capacitor and electrical double-layer capacitor (EDLCs), respectively. PANI has been comprehensively investigated as capacitive materials in flexible supercapacitors due to the high specific capacitance, low monomer cost, and relative flexibility.19-22 However, the poor cycling stability largely restricted their practical applications due to the volume variation during the continuous doping/de-doping of ions in the charge and discharge process. Meanwhile, enormous research works have confirmed rGO as the competitive material candidates for preparing deformable supercapacitor electrodes because of the conjugated 2D structural characteristics, high theoretical


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specific area, excellent electrical conductivity, high specific surface area, facile scale-up production, intriguing charge/discharge cycling stability, and high mechanical strength.23-26 However, the disadvantage of the relatively low specific capacitance hinders its practical applications in supercapacitors. To overcome above challenges, nanocomposites of rGO and PANI with their synergistic advantages have been employed for preparing supercapacitor devices. Various supercapacitors have been well fabricated from rGO and PANI nanocomposites with their synergistic advantages. However, flexible transparent supercapacitors based on nanocomposites of rGO and PANI, have rarely been realized with efficient charge transport, enhanced specific capacitance, reliable flexibility, and optimum charge/discharge cycling stability for promising integrated applications in portable and coupled electronic equipment.24 The nanocomposites of rGO and PANI are lack of enough solubility and could not be uniformly coated onto the flexible substrates. The effective preparation of nanocomposites of rGO and PANI and the uniform coating of nanocomposites onto flexible substrates, remain to be addressed for fabricating flexible transparent nanocomposites films in supercapacitor assembly.

Scheme 1. Schematic illustration for preparing the flexible transparent all-solid state supercapacitor devices. Firstly, the FTCF substrate obtained by coating Ag NWs with Meyer rod, and subsequently rGO coating on


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PET, was immersed in a blended suspension of aniline and rGO to deposit rGO/PANI nanocomposite layer via in situ polymerization. Then, the flexible transparent rGO/PANI films are assembled as supercapacitors.

Herein, we prepare flexible and transparent supercapacitors from hierarchical rGO/PANI nanocomposite conducting films (Scheme 1). Firstly, a conducting substrate was obtained by coating silver nanowires (Ag NWs) layer with Meyer rod on a flexible transparent polyethylene terephthalate (PET) film (PET/Ag NWs) with Meyer-rod and subsequently coating of rGO onto the Ag NWs layer (PET/Ag NWs/rGO).27,28 The resulted PET/Ag NWs/rGO film was regarded as FTCF substrate for further electrode fabrication. Next, the FTCF substrate was immersed in a blended suspension of aniline monomers and rGO to deposit rGO/PANI nanocomposite layer (FTCF/rGO/PANI) via in situ polymerization. Finally, the prepared flexible transparent rGO/PANI films could be assembled into flexible transparent all-solid state supercapacitors. By balancing the transparency and the specific capacitance with optimized amount of active electrode materials, the obtained flexible transparent all-solid state rGO/PANI nanocomposite supercapacitors exhibit enhanced capacitance performance, superior transparency at visible light region, good cycling stability, and excellent flexibility. They created potential opportunities for working as integrated flexible transparent power sources in various flexible, transparent and wearable electronic devices.

2. EXPERIMENTAL SECTION 2.1. Materials


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Poly(vinyl alcohol) (PVA), ammonium persulfate, HClO4, HNO3, H2SO4, natural graphite flakes, and hydrazine (80%, N2H4⋅H2O) were purchased from Beijing Chemical Works. Aniline monomer was purchased from J&K Chemical. The conductive silver glue (05002-AB) was purchased from SPI supplies and the Meyer-rod (thread thickness at 0.02 mm) was purchased from Shanghai Pushen Chemical Machinery Co, Ltd.

Through a modified Hummer method with natural graphite flakes as starting materials, graphene oxide was obtained from.29 A graphene oxide suspension was firstly reduced by N2H4⋅H2O under hydrothermal treatment at 95 °C to obtain rGO. Then the 0.05 mg/mL rGO suspension was obtained via rGO redispersion into aqueous solution through ultrasonication.30 Polyethylene terephthalate (PET, 125 µm in thickness) were purchased from HuiZhixing Company. Ag NWs suspension (5 mg/mL) was purchased from Coldstones Tech. Company. 2.2. Electrode preparation A transparent PET film (30 mm × 20 mm) was treated by O2 plasma for 5 min to enhance hydrophilic properties of PET film. Then Ag NWs suspension was dropped on PET film and spread by coating with Meyer rod.5 Then the obtained PET/Ag NWs film was heated at 120 °C for 2 min. After that, rGO aqueous solution was spin-coated onto the PET/Ag NWs film to ensure Ag NWs from corrosion.27,28 After covering rGO onto Ag NWs layer, the flexible, transparent and stably conductive films (FTCF) were obtained. The hierarchical rGO/PANI nanocomposite layer was deposited on the FTCF substrate by dipping the FTCF film into a


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blended rGO suspension with 0.01 M aniline monomers ([APS]/[aniline]=1:1.5 molar ratio) through polymerization. The FTCF substrate was dipped into the 40 mL 1 M HClO4 solution containing 4.08 mL rGO aqueous solution (0.05 mg/mL) and 60 µL aniline monomer (aniline to rGO at 300:1 weight ratio). 99.95 mg ammonium persulfate was added to 1 M HClO4 solution (10 mL) and was dissolved by stirring. After that, the ammonium persulfate-contained initiator suspention was poured into the FTCF substrate-contained rGO and aniline monomer solution for polymerization for 8, 16, 24 and 32 h at 5 °C, respectively.16 Finally, the prepared film was washed with ethanol and then dried for electrochemical test. For comparison, the PANI nanorod film was obtained by similar method without graphene. The graphene films were coated onto the FTCF substrate by spinning 200 µL graphene suspension. 2.3. All-solid state supercapacitor device assembly Firstly, 3 g PVA powder and 1.631 mL concentrated H2SO4 (ρ=1.84 g/mL) were added into 30 m water with vigorous stirring and subsequently heated to 95 °C until the suspension became clear. The above-prepared film electrode was cut into two symmetrical electrodes (30 mm × 10 mm). The H2SO4-PVA gel electrolyte layer was then coated onto one film electrode and solidified for 2 h at room temperature. Next, the two electrodes were assembled face to face with H2SO4-PVA gel electrolyte layer as separator.31 Finally, silver glue was coated at the end of the electrodes to generate excellent electrical contact with the instrument. Therefore, an all-solid state supercapacitor device was successfully achieved. 2.4. Capacitive performance measurement


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The electrochemical performance of supercapacitor devices were performed by electrochemical workstation (CHI 660C, Chenhua, Shanghai, China). The cyclic voltammetry curves, the charge and discharge curves, and the electrochemical impedance spectroscopy (frequency range: 0.01-100000 Hz) were conducted to assess the capacitance performance of supercapacitor devices. All the electrochemical tests were performed using two electrodes system in electrochemical workstation. 2.5. General characterization techniques SEM images, FTIR spectra, TEM images, Raman spectra, and transmittance spectra of the samples were collected from Zeiss Supra 55 instrument with 20 kV current voltage, Nicolet6700 FT-IR spectrometer, JEOL 2100F equipment, JobinYvon Lab RAM HR Raman microscope, and Shimadzu UV-3150 UV spectrophotometer, respectively. The sheet resistance and photographs of samples were obtained from four-probe tester (RTS-8) and Nikon digital camera D5200, respectively.

3. RESULTS AND DISCUSSION

Figure 1. SEM images of (a) hierarchical nanostructured rGO/PANI nanocomposite film (24 h sample) (Inset:


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the enlarged SEM image for rGO/PANI nanocomposite film) and (b) PANI nanorod film.

Figure 2. (a) The transmittance spectra for PET/Ag NWs film, FTCF substrate, FTCF/rGO, FTCF/rGO/PANI, and FTCF/PANI, respectively. (b) The corresponding photographs (from left to right) for PET/Ag NWs film, FTCF substrate, FTCF/rGO, FTCF/rGO/PANI, FTCF/PANI, and the flexible FTCF/rGO/PANI nanocomposite film.

Typically, the FTCF substrates were prepared and employed as the flexible, transparent and stably conducting current collectors and electrode substrates. The Ag NWs coated films (PET/Ag NWs) with conductivity, excellent transparency, reliable flexibility and network structures (Figure S1(a)) were prepared by coating Ag NWs ink on PET substrate with Meyer rod and dried at 120 oC in an oven for 2 min. The film sheet resistance and transparency of PET/Ag NWs were 12 Ω/□ and 93.67%, respectively. After covering rGO onto Ag NWs films to ensure Ag NWs from corrosion,27,28 the PET/Ag NWs/rGO (FTCF) substrates were obtained with the resistance


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at 50 Ω/□ and transparency at 92.96% for further electrode fabrication (Figure S1(b)). The TEM images and the Raman spectra for rGO and GO were presented in Figure S2. Then, by dipping FTCF substrates into a blended rGO suspension with the presence of 0.01 M aniline monomers ([APS]/[aniline]=1:1.5 molar ratio), different morphological rGO/PANI nanocomposites with different PANI nanoarray sizes, could be achieved onto the FTCF substrates under different polymerization time (Figure 1(a) and Figure S3). The remaining oxygen-containing functional groups of rGO from inadequate reduction of graphene oxide, could work as active moiety for attaching aniline cations for aniline polymerization growth via hydrogen bonding, electrostatic interaction, and π–π interaction.18 With the continuous polymerization for 8, 16, 24, and 32 h, the gradually enlarged sizes of PANI nanoarray were observed in the nanocomposites (Figure S3 and Figure 1(a)), demonstrating the largest aligned PANI nanoarray size on rGO surfaces with the width at ca. 44 nm, and the length at ca. 174 nm for 24 h polymerization. The uniformly aligned PANI nanoarray in the nanocomposites would benefit the ion diffusion and enhance the specific areas of electroactive materials for the improved capacitance of supercapacitor devices. For comparison, PANI nanorod films were prepared by the same methods without rGO, as displayed in Figure 1(b) and Figure S4. Moreover, the corresponding film transparency varied simultaneously during the FTCF substrate preparation and the further film electrode fabrication. As shown in Figure 2(a), the transparency of the relevant films changed from 93.67% for PET/Ag NWs film, 92.96% for FTCF substrate, 89.67% for rGO-coated FTCF film (FTCF/rGO), 78.76% for FTCF/rGO/PANI to 72.92% for PANI-coated FTCF film


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(FTCF/PANI) at 550 nm, respectively. The related pictures were presented in Figure 2(b) for the corresponding films. As displayed in Figure 2(b), the FTCF/rGO/PANI nanocomposite film has excellent bendability, indicating reliable flexibility because of the substrate and nanocomposite layer with robust substrate adhesion. The transmittance of the FTCF/rGO/PANI nanocomposite films decreased from 81.28% (8 h), 80.49% (16 h), 78.76% (24 h) to 76.22% (32 h) with the prolonged polymerization (Figure S5).


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Figure 3. (a) The photograph of a flexible transparent rGO/PANI nanocomposite film supercapacitor. (b) The CV plots of the FTCF/rGO/PANI film device, FTCF/rGO film device and FTCF/PANI film device (100 mV/s). (c) The detailed CV plots of FTCF/rGO/PANI film device (from 5 to 100 mV/s). (d) The charge and discharge plots for FTCF/rGO film device, FTCF/PANI film device, and FTCF/rGO/PANI film device (0.1 mA/cm2). (e) The charge and discharge plots for the FTCF/rGO/PANI nanocomposite film supercapacitor (0.08-0.3 mA/cm2). (f) Nyquist curves of FTCF/rGO film device, FTCF/PANI film device, and FTCF/rGO/PANI film device (Inset: magnified Nyquist curves of FTCF/rGO/PANI film device).

The formation of the hierarchical rGO/PANI nanocomposites was certified by Raman and FTIR spectra. As displayed in Figure S6(a), the peak for C=C vibration of rGO is centered at 1626 cm−1. The characteristic bands for PANI (C=C stretching of the quinone ring and benzene ring) are demonstrated at 1570 and 1495 cm−1. The peaks at 1626 and 1502 cm−1 are observed for the rGO/PANI nanocomposites, displaying the existence of both rGO and PANI in the nanocomposites. As shown in Figure S6(b), the Raman spectrum of rGO for the D band at 1333 cm−1 and G band at 1593 cm−1 were observed. The two main modes located at 1561 and 1300 cm−1 in the PANI spectrum are ascribed to the C-C stretching and C–N•+ stretching of the situated benzoid, respectively. Meanwhile, the modes at 1312 and 1563 cm−1 were observed for the nanocomposites, demonstrating the formation of the rGO/PANI nanocomposites.32,15 The flexible transparent supercapacitors were directly assembled from the obtained FTCF/rGO/PANI nanocomposite films with poly(vinyl alcohol) (PVA)/H2SO4 gel electrolyte as


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separator (Scheme 1).33 Figure 3(a) is the photograph of a flexible transparent FTCF/rGO/PANI nanocomposite film supercapacitor (24 h sample) with a transmittance of 57.45% at 550 nm (Figure

S7).34

Characterizations

including,

galvanostatic

charge/discharge

(GCD),

electrochemical impedance spectroscopy (EIS), and cycling stability measurement, were fully conducted to further confirm the merits of rGO/PANI nanocomposite-incorporated flexible transparent supercapacitor devices. Due to the characteristic of the electrical double-layer capacitance,17 no obvious peaks were detected in the cyclic voltammetry (CV) curve of FTCF/rGO film devices, as exhibited in Figure 3(b). Redox peaks for FTCF/rGO/PANI films devices (24 h sample) appeared in the CV curve, attributing to the redox state transitions of PANI and the integration of the pseudocapacitance behavior of PANI in the nanocomposites. The integral area for the CV curve of FTCF/rGO/PANI film devices was much greater than that for FTCF/rGO film devices and FTCF/PANI film devices, revealing the higher specific capacitance for rGO/PANI nanocomposite film devices due to the additional pseudocapacitance. Meanwhile, the charge and discharge behaviors, and the CV behaviors for FTCF/rGO/PANI nanocomposite supercapacitors from FTCF/rGO/PANI nanocomposite films with different polymerization time were demonstrated in Figure S8, implying the relatively larger integrated area and higher specific capacitance for rGO/PANI film (24 h sample). Therefore, by balancing the transparency, the integral area of CV plots and the capacitance for FTCF/rGO/PANI film devices in different polymerization time, the rGO/PANI film devices for 24 h polymerization with better transparency and higher specific capacitance could be the excellent candidates for the


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supercapacitor assembly. Figure 3(c) illustrated the detailed CV plots of FTCF/rGO/PANI nanocomposite supercapacitor (5 to 100 mV/s), displaying the increased current densities of peaks with increased sweep speeds and the well-kept CV shape within the related potential range and high scan rates for beneficially rapid current response on voltage reversal. As demonstrated in Figure 3(d), the charge and discharge plots were measured (0.1 mA/cm2) to clarify the electrochemical capacitance behavior of the assembled supercapacitors. The almost triangular shapes with deviations from ideal straight line were observed for all charge and discharge plots, indicative of the faradic reaction process. The specific capacitance (Cs), energy density (E) and power density (P) were obtained from the discharge plots with the equation Cs=I/(Ae(∆V/∆t)), E= 3.6(C∆V2)/2 and P=(3600E)/∆t, where Ae (cm2), I (A), ∆V (V), C (F/g) and ∆t (s) represent the area of the electrode, the current, the potential window, the capacitance and the discharge time, respectively.35,36 The calculated capacitance (Cs) of the FTCF/rGO/PANI nanocomposite film supercapacitor was 4.50 mF/cm2, much better than that of rGO film supercapacitor (0.32 mF/cm2) and the PANI film supercapacitor (1.94 mF/cm2), indicating the synergistic effect of rGO and PANI nanoarray for enhanced capacitance behavior. Figure 3(e) displayed the charge/discharge curves with maintaining almost the similar shape in 0-0.8 V for the rGO/PANI nanocomposite film supercapacitor at different current densities of 0.08-0.3 mA/cm2, suggesting the sustainable capacitance behavior of rGO/PANI nanocomposite film supercapacitor in a broad current range. Figure S9 demonstrated the specific capacitance variations with the varied current densities for rGO/PANI nanocomposite film supercapacitor and PANI film supercapacitor. For


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rGO/PANI nanocomposite film supercapacitor, the specific capacitance decreased from 6.4 mF/cm2 to 4.84 mF/cm2 with the increased current density from 0.08 to 0.09 mA/cm2, indicating the capacitive retention at 75.63%. Even when the current density increased to 0.3 mA/cm2, 43.91% of the specific capacitance was maintained (2.81 mF/cm2). However, for PANI film supercapacitor, when the current densities varied from 0.08 to 0.3 mA/cm2, the corresponding specific capacitance altered from 3.0 to 0.1 mF/cm22 with only 3.33% capacitive retention. Meanwhile, when the current density was 0.1 mA/cm2, E and P for the FTCF/rGO/PANI nanocomposites films supercapacitor were 7.07 Wh/kg and 707 W/kg, in comparison with 2.23 Wh/kg and 517.27 W/kg for PANI film supercapacitor, demonstrating that rGO/PANI nanocomposite film supercapacitor has better electrochemical performance. Therefore, it revealed that the rGO/PANI nanocomposite film supercapacitor has weaker capacitive performance attenuation with the increasing current density, compared to that for PANI film supercapacitor, probably ascribing to the optimized PANI nanoarrays in the hierarchical nanocomposite and the better ion transport channel for leading to an enhanced capacitance performance.37 Electrochemical impedance spectroscopy (EIS) was conducted to study the electronic conductivity and the charge transport property of supercapacitor devices during the redox process. As shown in Figure 3(f), the x-axis intercept of the Nyquist plot in the high-frequency region for rGO/PANI nanocomposites devices exhibited a lower value of 123 Ω, compared to that for rGO film devices (289.3 Ω) and PANI film devices (467.4 Ω), representing the lower


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equivalent series resistance (Rs) for rGO/PANI nanocomposite devices. Furthermore, the steeper slope of the nearly straight line in the low frequency region for rGO/PANI nanocomposite device was observed, in comparison with rGO film devices and PANI film devices, presenting the excellent capacitance behavior for facilitating ions diffusion to the inner electrode from the vertical PANI nanoarray. In the high frequency region, the diameter of semicircle for rGO/PANI nanocomposite devices was the smallest one, displaying the lower charge transfer resistance (Rct) of nanocomposite devices among these devices.38 Therefore, it implied that the nanocomposite devices had better conductivity and diffusion behavior of ions, resulting in the improved capacitance of the FTCF/rGO/PANI nanocomposite devices. The cycling stability for supercapacitor is a key factor for its practical application, which was implied from the capacitance under the number of cycles from galvanostatic charge and discharge method with the current density at 0.1 mA/cm2 (Figure S10). The specific capacitance was maintained at 75% after 1000 galvanostatic charge and discharge cycles. After 1000 cycles, the capacitive performance was retained stably. There was still 72% capacity retention after 2000 galvanostatic charge/discharge cycles, demonstrating 3% decrease compared to the specific capacitance at 1000 cycles and indicating good cycling stability for rGO/PANI nanocomposite film supercapacitor. The cycling life of the supercapacitor devices is mainly attributed to the well-defined PANI nanoarrays, the good cycle stability of rGO and gel electrolyte separator with the accommodation and reduction of the swelling/shrinking during the charge/discharge process.39,40 In order to assess the flexible characteristics of the assembled supercapacitors, the electrochemical performance under varied bendable cycles were assessed by employing


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galvanostatic charge/discharge test. Figure 4 represents that there is no significant effect on electrochemical capacitance performance of the assembled supercapacitors under different bending cycles. The specific capacitance of supercapacitor remained 2.76 mF/cm2 at current density of 0.3 mA/cm2 after 600 bending cycles, compared to the original specific capacitance of 2.81 mF/cm2 with the retention at about 98.22%. It indicated that the assembled supercapacitors had superior flexibility for potentially integrated into various wearable electronic devices.41 For the specific application, we have lighted a small LED bulb by connecting four charged supercapacitor devices in series (Figure S11).

Figure 4. The flexible characteristics of the assembled supercapacitors from the electrochemical performance under varied bendable cycles were assessed by galvanostatic charge/discharge test (the current density at 0.3

mA/cm2).

4. CONCLUSIONS In conclusion, a transparent flexible supercapacitor based on hierarchical rGO/PANI nanocomposite films was successfully assembled. The hierarchical rGO/PANI conducting films


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were fabricated via polymerization of rGO/PANI nanocomposite on FTCF substrates, which were obtained by firstly Meyer-rod coating of Ag NWs layer and then coating of rGO layer on PET substrates. The assembled supercapacitor exhibited enhanced capacitance performance, superior transparency at visible light region, good cycling stability, and excellent flexibility. It may be thanks to the synergistic effects of PANI and graphene, the enhanced electron transport from the incorporated highly conductive graphene, and the large surface area from nanocomposites with aligned PANI nanoarrays for facilitating ion diffusion. The better transparency, excellent capacitive performance, improved flexibility of the supercapacitor device, may benefit potential opportunities for working as integrated flexible transparent power sources in various flexible, transparent and wearable electronic devices.

ASSOCIATED CONTENT Supporting Information Supplementary material (additional data of SEM and TEM images, optical images, FTIR spectra, Raman spectra, CV curves) is available in the online version of this article at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *

Pengbo Wan. E-mail: [email protected].


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ACKNOWLEDGMENTS This work was financially supported by Beijing Natural Science Foundation (2152023), National Key Research and Development Project (2016YFC0801302), National Natural Science Foundation of China, and the Fundamental Research Funds for the Central Universities.

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