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polypyrrole nanotube papers for flexible all-solid-state supercapacitors with excellent rate capability and high energy density
Journal of Power Sources 302 (2016) 39e45
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Reduced graphene oxide/polypyrrole nanotube papers for flexible all-solid-state supercapacitors with excellent rate capability and high energy density Chao Yang, Liling Zhang, Nantao Hu*, Zhi Yang, Hao Wei, Yafei Zhang** Key Laboratory for Thin Film and Microfabrication Technology of the Ministry of Education, School of Electronics, Information and Electrical Engineering, Shanghai Jiao Tong University, Dong Chuan Road No.800, Shanghai, 200240, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The structure of the rGO/PPy NT paper is incompact and hierarchical. The addition of rGO can improve the cycling stability of PPy NT paper electrode. The ASSSC device exhibits 86.3% capacitance retention from 1 to 10 mA/cm2. The device has an areal energy density of 61.4 mWh/cm2 at 10 mW/cm2. The device provides a volumetric energy density of 7.18 mWh/cm3 at 1.17 W/cm3.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 May 2015 Received in revised form 2 October 2015 Accepted 10 October 2015 Available online xxx
Pseudocapacitive materials are known to suffer from severe capacitance loss during charging/discharging cycling. Here we report flexible all-solid-state supercapacitors (ASSSCs) based on reduced graphene oxide (rGO)/polypyrrole nanotube (PPy NT) papers prepared by a facile vacuum filtration method. It is revealed that the incorporation of rGO nanosheets can improve the electrochemical stability of PPy NT paper electrodes for pseudocapacitors. The hybrid paper electrode shows a high areal specific capacitance of 807 mF/cm2 at 1 mA/cm2 and a large volumetric specific capacitance of 94.3 F/cm3 at 0.1 A/cm3. The assembled ASSSC possesses a maximum areal specific capacitance of 512 mF/cm2 at 1 mA/cm2 and a maximum volumetric specific capacitance of 59.9 F/cm3 at 0.1 A/cm3. Moreover, it also exhibits excellent rate capability (86.3% capacitance retention from 1 to 10 mA/cm2) and cycling stability, little capacitance deviation under different bending states, a small leakage current and a low self-discharge characteristic. The device can provide an areal energy density of 61.4 mWh/cm2 at 10 mW/cm2 and a volumetric energy density of 7.18 mWh/cm3 at 1.17 W/cm3, indicating this high-performance ASSSC is a promising candidate for flexible high-power supply devices. © 2015 Elsevier B.V. All rights reserved.
Keywords: Graphene Polypyrrole nanotube Rate capability All-solid-state Supercapacitor
1. Introduction * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Zhang).
(N.
http://dx.doi.org/10.1016/j.jpowsour.2015.10.035 0378-7753/© 2015 Elsevier B.V. All rights reserved.
Hu),
[email protected]
Graphene has aroused increasing interest and brought up a wide range of applications, such as field-effect transistors [1], selective hydrogen separation [2], transparent electrodes for electrochromic
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devices [3] and supercapacitors [4,5]. Graphene's flexibility, large surface area and chemical stability, combined with its excellent electrical conductivity [6], make it promising as excellent electrode materials for flexible all-solid-state supercapacitors (ASSSCs), which can address the emerging needs of portable and wearable energy conversion and storage devices. Compared with common supercapacitors (SCs) based on aqueous electrolyte, ASSSCs generally use solid-state electrolyte such as polyvinyl alcohol (PVA)-based gel electrolyte, taking the advantages of avoiding possible electrolyte leakage and reducing cost of packaging materials [7]. The employment of solid-state electrolyte has been reported to improve the electrochemical stability of pseudocapacitor electrodes [8] and the rate capability of electrochemical capacitors [9]. The electrode materials for flexible ASSSCs need to possess not only high electrochemical performances, but also high mechanical integrity upon bending or folding [10]. The emerging 3D graphene hydrogel films have been designed as ASSSC electrodes, but with a low bending angle of only 30 e40 under bending states [11,12]. Graphene papers, obtained by a vacuum filtration method [10,13,14], have shown better flexibility than graphene hydrogels but usually exhibit a rather low capacitance due to the parallel restacking of graphene sheets, which greatly reduces accessible surface area and diffusion rate of solid-state electrolytes [4]. In order to destroy the strong pep interaction between graphene sheets, pseudocapacitive transition metal oxides and conducting polymers have been incorporated into graphene papers, such as RuO2 [14], MnO2 [15,16], polyaniline [16] and polypyrrole [17]. In addition, pseudocapacitive materials hold the promise of achieving battery-level energy density combined with extremely high power density [18]. However, pseudocapacitors usually suffer from bad cycle life and a low retention rate of specific capacitance at a high current density, which are unsatisfactory for practical applications. Herein, we fabricate a flexible ASSSC based on reduced graphene oxide (rGO)/polypyrrole nanotube (PPy NT) hybrid paper, using PVA-H2SO4 gel as the solid-state electrolyte. Compared with the pure PPy NT paper electrode, the hybrid paper electrode shows a better cycling stability, without sacrificing the pseudocapacitance of PPy NTs. The obtained ASSSC possesses high rate capability, accompanied with other excellent electrochemical performances, such as high areal and volumetric specific capacitance, a small leakage current and a low self-discharge characteristic. With a high energy density at a large power density, this ASSSC device is greatly promising for flexible energy storage devices. 2. Experimental 2.1. Materials Graphite powder (500 meshes) was purchased from Shangdong Jinrilai Co. Ltd (China). Sodium dodecyl benzene sulfonate (SDBS), hydrazine hydrate aqueous solution (85%), Triton X-100, methyl orange (MO), pyrrole, FeCl3$6H2O and polyvinyl alcohol (PVA) with analytic grades were purchased from Shanghai Chemical Reagents Co. Ltd (China) and used without further purification. 2.2. rGO/PPy NT paper GO was prepared by the modified Hummers' method [19], which had been mentioned in our previous report [20]. The reduction of GO was performed as follows: 600 mg GO and 1.5 g SDBS were dispersed in 600 mL deionized water. 3 mL hydrazine hydrate aqueous solution (85%) was added, while keeping stirring and refluxing at 100 C for 14 h. After cooling down, the product was filtered and washed with deionized water. Finally, the asprepared rGO was dispersed in 1 vol.% Triton X-100 solution for
further use. PPy NTs were chemically polymerized as follows [21]: 0.676 g FeCl3$6H2O was dissolved in 50 mL 5 mM MO solution. Then 0.1677 g pyrrole monomer was added and stirred at room temperature for 24 h. The PPy precipitate was washed with deionized water and ethanol several times, and finally dried at 60 C for 12 h. Then the obtained PPy NTs were added into the rGO suspension with a concentration of 0.6 mg/mL. After sonication, the rGO/PPy NT suspension was vacuum filtered through nitrocellulose membrane. By adjusting the volume of the rGO suspension and the amount of PPy NTs, the final rGO/PPy NT papers with different PPy NT contents were obtained after drying at room temperature. 2.3. Flexible ASSSCs 1 g H2SO4 and 1 g PVA were added into 10 mL deionized water, and then heated to 85 C under stirring until the mixture became clear. The as-prepared rGO/PPy NT paper was cut into rectangular strips and pressed on carbon paper with PET substrate to form ASSSC electrodes. Then PVA-H2SO4 gel electrolyte was slowly poured on two ASSSC electrodes and air-dried at room temperature to evaporate excess water. The two electrodes were pressed together to form an integrated ASSSC device. 2.4. Characterization The morphologies of the samples were observed by fieldemission scanning electron microscope (FE-SEM, Carl Zeiss Ultra 55). Transmission electron microscopy (TEM) was carried out on JEM-2100 (JEOL Ltd., Japan). Fourier Transform Infrared Spectroscopy (FTIR) was recorded on a Bruker (Germany) VERTEX 70 spectrometer (KBr pellets). Raman scattering was performed on a Renishaw inVia Reflex Raman spectrometer using a 532 nm laser source. X-ray photoelectron spectrometry (XPS) was carried out on a Kratos Axis Ultra DLD using monochromated Al Ka X-ray beams as the excitation source (1486.6 eV). Conductivity was recorded using RTS-8 four-point probes resistivity measurement system. Thermal Gravity Analysis (TGA, PerkinElmer Pyris 1) was carried out by heating the samples from 30 C to 900 C. All of the electrochemical experiments were carried out on an electrochemical workstation (CHI 760E). Cyclic voltammetry (CV) and galvanostatic charge/ discharge measurements were conducted from 0.2 V to 0.8 V in 1 M H2SO4. The electrochemical impedance spectroscopy (EIS) measurements were performed over a frequency range from 105 to 102 Hz. For three-electrode tests, the areal specific capacitance (Ca) and volumetric specific capacitance (Cv) were calculated as follows: Ca ¼ (IDt)/(SDV) and Cv ¼ Ca/d, where I is the discharge current, Dt is the discharge time, S is the area of one electrode, DV represents the voltage window and d is the film thickness. For the ASSSCs, the gravimetric specific capacitance (Csm), areal specific capacitance (Csa) and volumetric specific capacitance (Csv) of single electrode were calculated according to the following equations [11,22]: Csm ¼ 2(IDt)/(mDV), Csa ¼ Csmm/S and Csv ¼ Csa/d, where m is the mass of one electrode. The areal energy density (Ea) and areal power density (Pa) were calculated by using the equations [22]: Pa ¼ (IDV)/S and Ea ¼ PaDt. The volumetric energy density (Ev) and volumetric power density (Pv) were calculated by using the equations: Pv ¼ Pa/d and Ev ¼ Ea/d. 3. Results and discussion PPy NTs were fabricated though a facile self-degraded template polymerization method. In Fig. 1a, the obtained PPy NT had a closed end with the inner diameter of ~62 nm and the wall thickness of ~38 nm. Then the micro-scale length nanotubes were combined with rGO nanosheets to form freestanding papers. Compared with
C. Yang et al. / Journal of Power Sources 302 (2016) 39e45
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Fig. 1. (a) TEM image of PPy NTs (inset: the high-magnification TEM image). Top-view SEM images of (b) pure rGO paper, (c) pure PPy NT paper and (d) the rGO/PPy-90% paper (inset: a photograph of the rGO/PPy-90% paper obtained by a vacuum filtration method). The red circle areas indicate the existence of rGO nanosheets. Cross-section SEM images of the rGO/PPy-90% paper at (e) low- and (f) high-magnification. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
pure rGO paper (Fig. 1b) and PPy NT paper (Fig. 1c), the hybrid rGO/ PPy NT paper with the PPy NT content of 90% (denoted as rGO/PPy90%), maintained the morphology of PPy NT paper not as compact as pure rGO paper, and clearly showed the combination of PPy NTs and rGO nanosheets which was circled in Fig. 1d. The film thickness of the rGO/PPy-90% paper was obtained from the cross-section SEM image (Fig. 1e). From the high-magnification cross-section SEM image (Fig. 1f), the rGO/PPy-90% paper was observed to be incompact and hierarchical. The morphology of pure PPy NT paper was shown in Figure S1 in the Supporting Information. The film thickness and areal density were also listed in Table S1 in the Supporting Information. FTIR spectra of rGO paper, PPy NT paper and the hybrid papers
were shown in Fig. 2a. The PPy NT paper exhibited characteristic peaks of PPy [23], such as the C]C and CeN stretching vibration peaks at 1547 and 1470 cm1, the CeH in-plane vibration at 1313 and 1174 cm1, the CeH in-plane bending at 1038 cm1, and the ring deformation at 900 cm1. The rGO paper showed the peak at 1083 cm1 ascribing to the CeO stretching band. It was indicated that the hybrid paper with low PPy NT content (such as rGO/PPy10%) showed the characteristics of rGO, while the hybrid paper with high PPy NT content (such as rGO/PPy-90%) exhibited the characteristic peaks of PPy. Raman spectroscopic analysis was shown in Fig. 2b. The hybrid paper also had the characteristic peaks of PPy, such as 920 and 975 cm1 attributed to the quinoid polaronic and bipolaronic structure, 1045 cm1 attributed to the CeH in-
Fig. 2. (a) FTIR, (b) Raman and (c) XPS spectra of pure rGO paper, pure PPy NT paper and the hybrid papers. High resolution C 1s spectra of (d) pure rGO paper, (e) pure PPy NT paper and (f) the rGO/PPy-90% paper.
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plane deformation, 1385 and 1584 cm1 attributed to ring stretching mode of the polymer backbone and the p-conjugation structure [24]. The peaks at 1110, 1140, 1188 and 1490 cm1 indicated the existence of residual MO in the PPy NTs [25]. Moreover, the D band at 1340 cm1 and G band at 1580 cm1 of rGO were overlapped by PPy in the hybrid composite. The atomic composition of rGO paper, PPy NT paper and the rGO/PPy-90% paper was analyzed by XPS (Fig. 2c). The PPy NT paper had a higher N content (12.4%) than those of rGO paper and the hybrid paper. Compared with rGO paper, the higher N content of the rGO/PPy-90% paper (8.7%) probably indicated the presence of PPy NTs in the hybrid composite. The N content in rGO paper (1.5%) might be incorporated through the reaction of hydrazine with the carbonyl groups of GO [26]. The curve fitting of C 1s spectra were shown in Fig. 2d, e and f. CeC bonding at 284.5 eV and CeN bonding at 285.5 eV were dominating in the spectra of rGO paper and PPy NT paper respectively. The spectrum of the rGO/PPy-90% paper had the characteristic peak of CeN backbone bonding with the binding energy of 285.5 eV, demonstrating the existence of PPy NTs. Conductivity was obtained by a four-point probe method and shown in Table S1 in the Supporting Information. The conductivity of the rGO/PPy-90% paper was ~52.9 S/m, higher than that of the pure rGO paper (~47.5 S/m). TGA curves were shown in Figure S2 in the Supporting Information. The char yields of the hybrid composites decreased with the increase of PPy NT content, and were consistent with the PPy NT content of the hybrid papers. A three-electrode test was used to evaluate the electrochemical performance of the flexible paper electrodes. With increasing the scan rate, the CV curves of the rGO/PPy-90% paper electrode were deviated from rectangular shape at the low scan rate, attributed to the contact resistance (Fig. 3a). The galvanostatic charge/discharge curves of the rGO/PPy-90% and pure PPy NT paper electrodes were shown in Fig. 3b and c. In addition, the galvanostatic charge/discharge curves of other rGO/PPy hybrid paper electrodes with different ratios were also shown in the Supporting Information as Figure S3. Areal and volumetric specific capacitances were calculated from galvanostatic charge/ discharge tests. In Fig. 3d, rGO/PPy-90% hybrid paper electrode had a higher areal specific capacitance than those of pure PPy NT paper electrode and other rGO/PPy hybrid paper electrodes (rGO/ PPy-10% and rGO/PPy-50%) shown in Figure S4a, from 807 mF/
cm2 at 1 mA/cm2 to 348 mF/cm2 at 10 mA/cm2. However, the volumetric specific capacitance was lower than that of pure PPy NT paper electrode, but higher than those of other rGO/PPy-10% and rGO/PPy-50% paper electrodes shown in Figure S4b, from 94.3 F/cm3 at 0.1 A/cm3 to 40.6 F/cm3 at 1 A/cm3, due to the larger film thickness (85.6 mm vs 63.1 mm). Electrochemical impedance spectroscopy (EIS) was tested, with the Nyquist plots shown in Fig. 3e. The rGO/PPy-90% and PPy NT paper electrodes all had a small equivalent series resistance (ESR) of ~1.1 U and a low charge transfer resistance (Rct), indicating fast ion transport within the electrodes. In the low frequency region, compared with the rGO/PPy-90% paper electrode, the impedance curve of PPy NT paper electrode was more parallel to the imaginary axis, indicating the addition of rGO was bad for the infiltration of electrolyte ion to some extent. Electrochemical stability was examined under continuous galvanostatic charge/discharge tests at 20 mA/cm2, as shown in Fig. 3f. The hybrid paper electrode exhibited excellent capacitance retention rate of about 78.0% after 2000 cycles, much better than the value of pure PPy NT paper electrode (about 48.0%). It was indicated that the addition of rGO nanosheets could improve the electrochemical stability of the PPy NT paper electrode. Symmetric ASSSC device was assembled in order to further investigate the electrochemical performance of the hybrid paper electrode, as shown in Fig. 4a. The photograph in Fig. 4b reflected that the thin device was highly flexible under a bending state. The nearly rectangular CV curves (Fig. 4c) and the small voltage drop at the start of the discharge curves (Fig. 4d) of the ASSSC device indicated a low contact resistance. Specific capacitance was calculated from galvanostatic charge/discharge curves. The gravimetric specific capacitance reached to 197.6 F/g at 0.5 A/g and decreased to 84.4 F/g at 15 A/g (Figure S5 in the Supporting Information). In Fig. 4e, the device had a large areal specific capacitance, from 512 mF/cm2 at 1 mA/cm2 to 442 mF/cm2 at 10 mA/cm2, and a high volumetric specific capacitance, from 59.9 F/cm3 at 0.1 A/cm3 to 51.7 F/cm3 at 1 A/cm3. More excitingly, it showed high rate capability, with 86.3% capacitance retention from 1 to 10 mA/cm2, better than the previously-reported ILCMG//Ru2O-IL-CMG (79.4% from 1 to 10 A/g) [14], MnO2/graphene (78.0% from 0.5 to 10 A/g) [15], rGO/CNT//CFP/PPy (69.3% from 0.5 to 10 A/g) [17] and graphene/cellulose nanofiber (66.0%
Fig. 3. (a) CV curves of the rGO/PPy-90% paper electrode at different scan rates. Galvanostatic charge/discharge curves of (b) the rGO/PPy-90% and (c) pure PPy NT paper electrodes at different current densities. (d) Areal and volumetric specific capacitance of the rGO/PPy-90% (stars) and pure PPy NT (spheres) paper electrodes obtained by galvanostatic charge/ discharge curves at different current densities. (e) Nyquist plots and (f) cycling stability of the rGO/PPy-90% and pure PPy NT paper electrodes in 1 M H2SO4 electrolyte. The cycling stability was obtained by galvanostatic charge/discharge curves at 20 mA/cm2.
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Fig. 4. (a) A schematic diagram of the ASSSC device. (b) A digital photograph of the rGO/PPy-90% ASSSC under a bending state. (c) CV and (d) galvanostatic charge/discharge curves of the rGO/PPy-90% ASSSC at different scan rates and current densities. (e) Areal and volumetric specific capacitance of the rGO/PPy-90% ASSSC obtained by galvanostatic charge/ discharge curves at different current densities. (f) Nyquist plots of the rGO/PPy-90% ASSSC compared with the rGO ASSSC. (g) Cycling stability of the rGO/PPy-90% ASSSC compared with the paper electrode in aqueous electrolyte by galvanostatic charge/discharge curves at 20 mA/cm2 (inset: galvanostatic charge/discharge curves of the 1st, 5 000th and 10 000th cycle for the rGO/PPy-90% ASSSC). (h) CV curves of the rGO/PPy-90% ASSSC under different bending angles at a scan rate of 50 mV/s.
from 0.7 to 11.2 mA/cm2) [27]. EIS measurement was performed in order to further investigate the relationship between capacitive performance and interior resistance (Fig. 4f). In the high frequency region, the rGO/PPy-90% ASSSC showed nearly the same ESR of ~0.68 U as the pure rGO ASSSC, and a small Rct. In the low frequency region, the impedance curve of the rGO/PPy90% ASSSC was more parallel to the imaginary axis than the rGO ASSSC, indicating the hierarchical PPy NT framework facilitated the infiltration of gel electrolyte. Electrochemical stability of the ASSSC was examined by galvanostatic charge/discharge tests at 20 mA/cm2 for 10,000 cycles, and compared with the hybrid paper electrode tested in aqueous electrolyte as shown in Fig. 4g. As a result, the employment of solid-state electrolyte (PVA-H2SO4) greatly improved the capacitance retention rate from 56.1% to 82.7%. This result had been proved by other previous literature [8]. The flexible ASSSC also exhibited excellent capacitance stability under different bending angles (0e180 ). The CV curves displayed almost the same shape at 50 mV/s, indicating the negligible variation of the capacitive behavior (Fig. 4h). Ragone plots of the rGO/PPy-90% ASSSC were shown in Fig. 5a and b. Areal and volumetric capacitive performances were compared with other reports as shown in Table S2 and S3 in the Supporting Information. The ASSSC showed a high areal energy density, from 71.2 mWh/cm2 at 1 mW/cm2 to 61.4 mWh/cm2 at 10 mW/cm2, much better than the reported ASSSCs based on graphene/cellulose paper (15 mWh/cm2 at 0.04 mW/cm2) [13], rGO/ MnO2 (from 35.1 mWh/cm2 at 0.0375 mW/cm2 to 11.5 mWh/cm2 at 3.8 mW/cm2) [22] and graphene/cellulose nanofiber (20 mWh/cm2 at 15.5 mW/cm2) [27]. Moreover, it also had a high volumetric energy density, from 8.32 mWh/cm3 at 0.12 W/cm3 to 7.18 mWh/ cm3 at 1.17 W/cm3, better than the reported laser-scribed graphene
(from 1.36 mWh/cm3 at 0.8 W/cm3 to 0.8 mWh/cm3 at 20 W/cm3) [5], graphene (from 2.5 mWh/cm3 at 0.1 W/cm3 to 0.14 mWh/cm3 at 0.495 W/cm3) [28] and MnO2//Fe2O3 (from 0.41 mWh/cm3 at 0.06 W/cm3 to 0.35 mWh/cm3 at 0.1 W/cm3) [29]. With the increase of power density, energy density of the ASSSC showed an excellent retention rate, resulting from the high rate capability of specific capacitance. For practical application in electronics, it is important to evaluate the leakage current and self-discharge characteristics of the ASSSCs (Figure S6 in the Supporting Information). The leakage current of the device quickly stabilized at ~32 mA, indicating a small leakage current and high stability. After being charged at 1.0 V for 15 min, the open-circuit voltage of the device exhibited a rapid decrease in the first hour and gradually reached ~0.22 V after 24 h, revealing a low self-discharge characteristic. Moreover, three ASSSC units with the same mass of the rGO/PPy-90% paper, were connected in series to further evaluate the performance of the solidstate devices. Fig. 5c showed the potential window was extended to 3.0 V for the tandem device based on three ASSSCs. The charge/ discharge time at the same current density was tinily changed for the tandem device based on two or three ASSSCs, indicating the capacitive performance of each ASSSC unit was well maintained in series. Then the tandem device was used to light up heterochromatic LEDs. As shown in Fig. 5d, the blue LED could be easily powered by the tandem device based on three ASSSCs. It also could power a LED with other colours, such as red, yellow and green (Figure S7 in the Supporting Information). 4. Conclusions In this work, we fabricated rGO/PPy NT papers for flexible ASSSC electrodes. The incorporation of rGO nanosheets could improve the
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Fig. 5. (a) Areal and (b) volumetric Ragone plots of the rGO/PPy-90% ASSSC compared with other reports in previous literature. (c) CV and (d) galvanostatic charge/discharge curves (inset: a photograph of a blue LED powered by three rGO/PPy-90% ASSSCs connected in series) of two and three rGO/PPy-90% ASSSCs connected in series at 100 mV/s and 2 mA. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
cycling stability of PPy NT paper electrode, without sacrificing the pseudocapacitance. The hybrid paper could be used as excellent electrode materials for ASSSC devices without using other binders or conducting additives. The assembled ASSSC had a large areal and volumetric specific capacitance, high rate capability, excellent capacitance stability under different bending states as well as cycling stability, a small leakage current and a low self-discharge characteristic. In addition, it showed a high areal energy density of 61.4 mWh/cm2 at 10 mW/cm2 and a large volumetric energy density of 7.18 mWh/cm3 at 1.17 W/cm3. Based on the high energy density with excellent retention rate, this flexible ASSSC holds great potential for high-performance energy storage devices. Acknowledgments The authors gratefully acknowledge financial supports by the National Natural Science Foundation of China (no. 51102164, no. 61376003 and no. 51302179), Medical-Engineering (Science) crossResearch Fund of Shanghai Jiao Tong University (no. YG 2014QN01 and no. YG2012MS37), the Natural Science Foundation of Jiangsu Province (no. BK2012184), and the Foundation for SMC Excellent Young Teacher in Shanghai Jiao Tong University. We also acknowledge the analysis supports from Instrumental Analysis Center of Shanghai Jiao Tong University and the Center for Advanced Electronic Materials and Devices of Shanghai Jiao Tong University. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.10.035.
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Reduced graphene oxide/polypyrrole nanotube papers for flexible all-solid-state supercapacitors with excellent rate capability and high energy density Chao Yang, Liling Zhang, Nantao Hu*, Zhi Yang, Hao Wei, Yafei Zhang** Key Laboratory for Thin Film and Microfabrication Technology of the Ministry of Education, School of Electronics, Information and Electrical Engineering, Shanghai Jiao Tong University, Dong Chuan Road No.800, Shanghai, 200240, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The structure of the rGO/PPy NT paper is incompact and hierarchical. The addition of rGO can improve the cycling stability of PPy NT paper electrode. The ASSSC device exhibits 86.3% capacitance retention from 1 to 10 mA/cm2. The device has an areal energy density of 61.4 mWh/cm2 at 10 mW/cm2. The device provides a volumetric energy density of 7.18 mWh/cm3 at 1.17 W/cm3.
a r t i c l e i n f o
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Article history: Received 16 May 2015 Received in revised form 2 October 2015 Accepted 10 October 2015 Available online xxx
Pseudocapacitive materials are known to suffer from severe capacitance loss during charging/discharging cycling. Here we report flexible all-solid-state supercapacitors (ASSSCs) based on reduced graphene oxide (rGO)/polypyrrole nanotube (PPy NT) papers prepared by a facile vacuum filtration method. It is revealed that the incorporation of rGO nanosheets can improve the electrochemical stability of PPy NT paper electrodes for pseudocapacitors. The hybrid paper electrode shows a high areal specific capacitance of 807 mF/cm2 at 1 mA/cm2 and a large volumetric specific capacitance of 94.3 F/cm3 at 0.1 A/cm3. The assembled ASSSC possesses a maximum areal specific capacitance of 512 mF/cm2 at 1 mA/cm2 and a maximum volumetric specific capacitance of 59.9 F/cm3 at 0.1 A/cm3. Moreover, it also exhibits excellent rate capability (86.3% capacitance retention from 1 to 10 mA/cm2) and cycling stability, little capacitance deviation under different bending states, a small leakage current and a low self-discharge characteristic. The device can provide an areal energy density of 61.4 mWh/cm2 at 10 mW/cm2 and a volumetric energy density of 7.18 mWh/cm3 at 1.17 W/cm3, indicating this high-performance ASSSC is a promising candidate for flexible high-power supply devices. © 2015 Elsevier B.V. All rights reserved.
Keywords: Graphene Polypyrrole nanotube Rate capability All-solid-state Supercapacitor
1. Introduction * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Zhang).
(N.
http://dx.doi.org/10.1016/j.jpowsour.2015.10.035 0378-7753/© 2015 Elsevier B.V. All rights reserved.
Hu),
[email protected]
Graphene has aroused increasing interest and brought up a wide range of applications, such as field-effect transistors [1], selective hydrogen separation [2], transparent electrodes for electrochromic
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devices [3] and supercapacitors [4,5]. Graphene's flexibility, large surface area and chemical stability, combined with its excellent electrical conductivity [6], make it promising as excellent electrode materials for flexible all-solid-state supercapacitors (ASSSCs), which can address the emerging needs of portable and wearable energy conversion and storage devices. Compared with common supercapacitors (SCs) based on aqueous electrolyte, ASSSCs generally use solid-state electrolyte such as polyvinyl alcohol (PVA)-based gel electrolyte, taking the advantages of avoiding possible electrolyte leakage and reducing cost of packaging materials [7]. The employment of solid-state electrolyte has been reported to improve the electrochemical stability of pseudocapacitor electrodes [8] and the rate capability of electrochemical capacitors [9]. The electrode materials for flexible ASSSCs need to possess not only high electrochemical performances, but also high mechanical integrity upon bending or folding [10]. The emerging 3D graphene hydrogel films have been designed as ASSSC electrodes, but with a low bending angle of only 30 e40 under bending states [11,12]. Graphene papers, obtained by a vacuum filtration method [10,13,14], have shown better flexibility than graphene hydrogels but usually exhibit a rather low capacitance due to the parallel restacking of graphene sheets, which greatly reduces accessible surface area and diffusion rate of solid-state electrolytes [4]. In order to destroy the strong pep interaction between graphene sheets, pseudocapacitive transition metal oxides and conducting polymers have been incorporated into graphene papers, such as RuO2 [14], MnO2 [15,16], polyaniline [16] and polypyrrole [17]. In addition, pseudocapacitive materials hold the promise of achieving battery-level energy density combined with extremely high power density [18]. However, pseudocapacitors usually suffer from bad cycle life and a low retention rate of specific capacitance at a high current density, which are unsatisfactory for practical applications. Herein, we fabricate a flexible ASSSC based on reduced graphene oxide (rGO)/polypyrrole nanotube (PPy NT) hybrid paper, using PVA-H2SO4 gel as the solid-state electrolyte. Compared with the pure PPy NT paper electrode, the hybrid paper electrode shows a better cycling stability, without sacrificing the pseudocapacitance of PPy NTs. The obtained ASSSC possesses high rate capability, accompanied with other excellent electrochemical performances, such as high areal and volumetric specific capacitance, a small leakage current and a low self-discharge characteristic. With a high energy density at a large power density, this ASSSC device is greatly promising for flexible energy storage devices. 2. Experimental 2.1. Materials Graphite powder (500 meshes) was purchased from Shangdong Jinrilai Co. Ltd (China). Sodium dodecyl benzene sulfonate (SDBS), hydrazine hydrate aqueous solution (85%), Triton X-100, methyl orange (MO), pyrrole, FeCl3$6H2O and polyvinyl alcohol (PVA) with analytic grades were purchased from Shanghai Chemical Reagents Co. Ltd (China) and used without further purification. 2.2. rGO/PPy NT paper GO was prepared by the modified Hummers' method [19], which had been mentioned in our previous report [20]. The reduction of GO was performed as follows: 600 mg GO and 1.5 g SDBS were dispersed in 600 mL deionized water. 3 mL hydrazine hydrate aqueous solution (85%) was added, while keeping stirring and refluxing at 100 C for 14 h. After cooling down, the product was filtered and washed with deionized water. Finally, the asprepared rGO was dispersed in 1 vol.% Triton X-100 solution for
further use. PPy NTs were chemically polymerized as follows [21]: 0.676 g FeCl3$6H2O was dissolved in 50 mL 5 mM MO solution. Then 0.1677 g pyrrole monomer was added and stirred at room temperature for 24 h. The PPy precipitate was washed with deionized water and ethanol several times, and finally dried at 60 C for 12 h. Then the obtained PPy NTs were added into the rGO suspension with a concentration of 0.6 mg/mL. After sonication, the rGO/PPy NT suspension was vacuum filtered through nitrocellulose membrane. By adjusting the volume of the rGO suspension and the amount of PPy NTs, the final rGO/PPy NT papers with different PPy NT contents were obtained after drying at room temperature. 2.3. Flexible ASSSCs 1 g H2SO4 and 1 g PVA were added into 10 mL deionized water, and then heated to 85 C under stirring until the mixture became clear. The as-prepared rGO/PPy NT paper was cut into rectangular strips and pressed on carbon paper with PET substrate to form ASSSC electrodes. Then PVA-H2SO4 gel electrolyte was slowly poured on two ASSSC electrodes and air-dried at room temperature to evaporate excess water. The two electrodes were pressed together to form an integrated ASSSC device. 2.4. Characterization The morphologies of the samples were observed by fieldemission scanning electron microscope (FE-SEM, Carl Zeiss Ultra 55). Transmission electron microscopy (TEM) was carried out on JEM-2100 (JEOL Ltd., Japan). Fourier Transform Infrared Spectroscopy (FTIR) was recorded on a Bruker (Germany) VERTEX 70 spectrometer (KBr pellets). Raman scattering was performed on a Renishaw inVia Reflex Raman spectrometer using a 532 nm laser source. X-ray photoelectron spectrometry (XPS) was carried out on a Kratos Axis Ultra DLD using monochromated Al Ka X-ray beams as the excitation source (1486.6 eV). Conductivity was recorded using RTS-8 four-point probes resistivity measurement system. Thermal Gravity Analysis (TGA, PerkinElmer Pyris 1) was carried out by heating the samples from 30 C to 900 C. All of the electrochemical experiments were carried out on an electrochemical workstation (CHI 760E). Cyclic voltammetry (CV) and galvanostatic charge/ discharge measurements were conducted from 0.2 V to 0.8 V in 1 M H2SO4. The electrochemical impedance spectroscopy (EIS) measurements were performed over a frequency range from 105 to 102 Hz. For three-electrode tests, the areal specific capacitance (Ca) and volumetric specific capacitance (Cv) were calculated as follows: Ca ¼ (IDt)/(SDV) and Cv ¼ Ca/d, where I is the discharge current, Dt is the discharge time, S is the area of one electrode, DV represents the voltage window and d is the film thickness. For the ASSSCs, the gravimetric specific capacitance (Csm), areal specific capacitance (Csa) and volumetric specific capacitance (Csv) of single electrode were calculated according to the following equations [11,22]: Csm ¼ 2(IDt)/(mDV), Csa ¼ Csmm/S and Csv ¼ Csa/d, where m is the mass of one electrode. The areal energy density (Ea) and areal power density (Pa) were calculated by using the equations [22]: Pa ¼ (IDV)/S and Ea ¼ PaDt. The volumetric energy density (Ev) and volumetric power density (Pv) were calculated by using the equations: Pv ¼ Pa/d and Ev ¼ Ea/d. 3. Results and discussion PPy NTs were fabricated though a facile self-degraded template polymerization method. In Fig. 1a, the obtained PPy NT had a closed end with the inner diameter of ~62 nm and the wall thickness of ~38 nm. Then the micro-scale length nanotubes were combined with rGO nanosheets to form freestanding papers. Compared with
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Fig. 1. (a) TEM image of PPy NTs (inset: the high-magnification TEM image). Top-view SEM images of (b) pure rGO paper, (c) pure PPy NT paper and (d) the rGO/PPy-90% paper (inset: a photograph of the rGO/PPy-90% paper obtained by a vacuum filtration method). The red circle areas indicate the existence of rGO nanosheets. Cross-section SEM images of the rGO/PPy-90% paper at (e) low- and (f) high-magnification. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
pure rGO paper (Fig. 1b) and PPy NT paper (Fig. 1c), the hybrid rGO/ PPy NT paper with the PPy NT content of 90% (denoted as rGO/PPy90%), maintained the morphology of PPy NT paper not as compact as pure rGO paper, and clearly showed the combination of PPy NTs and rGO nanosheets which was circled in Fig. 1d. The film thickness of the rGO/PPy-90% paper was obtained from the cross-section SEM image (Fig. 1e). From the high-magnification cross-section SEM image (Fig. 1f), the rGO/PPy-90% paper was observed to be incompact and hierarchical. The morphology of pure PPy NT paper was shown in Figure S1 in the Supporting Information. The film thickness and areal density were also listed in Table S1 in the Supporting Information. FTIR spectra of rGO paper, PPy NT paper and the hybrid papers
were shown in Fig. 2a. The PPy NT paper exhibited characteristic peaks of PPy [23], such as the C]C and CeN stretching vibration peaks at 1547 and 1470 cm1, the CeH in-plane vibration at 1313 and 1174 cm1, the CeH in-plane bending at 1038 cm1, and the ring deformation at 900 cm1. The rGO paper showed the peak at 1083 cm1 ascribing to the CeO stretching band. It was indicated that the hybrid paper with low PPy NT content (such as rGO/PPy10%) showed the characteristics of rGO, while the hybrid paper with high PPy NT content (such as rGO/PPy-90%) exhibited the characteristic peaks of PPy. Raman spectroscopic analysis was shown in Fig. 2b. The hybrid paper also had the characteristic peaks of PPy, such as 920 and 975 cm1 attributed to the quinoid polaronic and bipolaronic structure, 1045 cm1 attributed to the CeH in-
Fig. 2. (a) FTIR, (b) Raman and (c) XPS spectra of pure rGO paper, pure PPy NT paper and the hybrid papers. High resolution C 1s spectra of (d) pure rGO paper, (e) pure PPy NT paper and (f) the rGO/PPy-90% paper.
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plane deformation, 1385 and 1584 cm1 attributed to ring stretching mode of the polymer backbone and the p-conjugation structure [24]. The peaks at 1110, 1140, 1188 and 1490 cm1 indicated the existence of residual MO in the PPy NTs [25]. Moreover, the D band at 1340 cm1 and G band at 1580 cm1 of rGO were overlapped by PPy in the hybrid composite. The atomic composition of rGO paper, PPy NT paper and the rGO/PPy-90% paper was analyzed by XPS (Fig. 2c). The PPy NT paper had a higher N content (12.4%) than those of rGO paper and the hybrid paper. Compared with rGO paper, the higher N content of the rGO/PPy-90% paper (8.7%) probably indicated the presence of PPy NTs in the hybrid composite. The N content in rGO paper (1.5%) might be incorporated through the reaction of hydrazine with the carbonyl groups of GO [26]. The curve fitting of C 1s spectra were shown in Fig. 2d, e and f. CeC bonding at 284.5 eV and CeN bonding at 285.5 eV were dominating in the spectra of rGO paper and PPy NT paper respectively. The spectrum of the rGO/PPy-90% paper had the characteristic peak of CeN backbone bonding with the binding energy of 285.5 eV, demonstrating the existence of PPy NTs. Conductivity was obtained by a four-point probe method and shown in Table S1 in the Supporting Information. The conductivity of the rGO/PPy-90% paper was ~52.9 S/m, higher than that of the pure rGO paper (~47.5 S/m). TGA curves were shown in Figure S2 in the Supporting Information. The char yields of the hybrid composites decreased with the increase of PPy NT content, and were consistent with the PPy NT content of the hybrid papers. A three-electrode test was used to evaluate the electrochemical performance of the flexible paper electrodes. With increasing the scan rate, the CV curves of the rGO/PPy-90% paper electrode were deviated from rectangular shape at the low scan rate, attributed to the contact resistance (Fig. 3a). The galvanostatic charge/discharge curves of the rGO/PPy-90% and pure PPy NT paper electrodes were shown in Fig. 3b and c. In addition, the galvanostatic charge/discharge curves of other rGO/PPy hybrid paper electrodes with different ratios were also shown in the Supporting Information as Figure S3. Areal and volumetric specific capacitances were calculated from galvanostatic charge/ discharge tests. In Fig. 3d, rGO/PPy-90% hybrid paper electrode had a higher areal specific capacitance than those of pure PPy NT paper electrode and other rGO/PPy hybrid paper electrodes (rGO/ PPy-10% and rGO/PPy-50%) shown in Figure S4a, from 807 mF/
cm2 at 1 mA/cm2 to 348 mF/cm2 at 10 mA/cm2. However, the volumetric specific capacitance was lower than that of pure PPy NT paper electrode, but higher than those of other rGO/PPy-10% and rGO/PPy-50% paper electrodes shown in Figure S4b, from 94.3 F/cm3 at 0.1 A/cm3 to 40.6 F/cm3 at 1 A/cm3, due to the larger film thickness (85.6 mm vs 63.1 mm). Electrochemical impedance spectroscopy (EIS) was tested, with the Nyquist plots shown in Fig. 3e. The rGO/PPy-90% and PPy NT paper electrodes all had a small equivalent series resistance (ESR) of ~1.1 U and a low charge transfer resistance (Rct), indicating fast ion transport within the electrodes. In the low frequency region, compared with the rGO/PPy-90% paper electrode, the impedance curve of PPy NT paper electrode was more parallel to the imaginary axis, indicating the addition of rGO was bad for the infiltration of electrolyte ion to some extent. Electrochemical stability was examined under continuous galvanostatic charge/discharge tests at 20 mA/cm2, as shown in Fig. 3f. The hybrid paper electrode exhibited excellent capacitance retention rate of about 78.0% after 2000 cycles, much better than the value of pure PPy NT paper electrode (about 48.0%). It was indicated that the addition of rGO nanosheets could improve the electrochemical stability of the PPy NT paper electrode. Symmetric ASSSC device was assembled in order to further investigate the electrochemical performance of the hybrid paper electrode, as shown in Fig. 4a. The photograph in Fig. 4b reflected that the thin device was highly flexible under a bending state. The nearly rectangular CV curves (Fig. 4c) and the small voltage drop at the start of the discharge curves (Fig. 4d) of the ASSSC device indicated a low contact resistance. Specific capacitance was calculated from galvanostatic charge/discharge curves. The gravimetric specific capacitance reached to 197.6 F/g at 0.5 A/g and decreased to 84.4 F/g at 15 A/g (Figure S5 in the Supporting Information). In Fig. 4e, the device had a large areal specific capacitance, from 512 mF/cm2 at 1 mA/cm2 to 442 mF/cm2 at 10 mA/cm2, and a high volumetric specific capacitance, from 59.9 F/cm3 at 0.1 A/cm3 to 51.7 F/cm3 at 1 A/cm3. More excitingly, it showed high rate capability, with 86.3% capacitance retention from 1 to 10 mA/cm2, better than the previously-reported ILCMG//Ru2O-IL-CMG (79.4% from 1 to 10 A/g) [14], MnO2/graphene (78.0% from 0.5 to 10 A/g) [15], rGO/CNT//CFP/PPy (69.3% from 0.5 to 10 A/g) [17] and graphene/cellulose nanofiber (66.0%
Fig. 3. (a) CV curves of the rGO/PPy-90% paper electrode at different scan rates. Galvanostatic charge/discharge curves of (b) the rGO/PPy-90% and (c) pure PPy NT paper electrodes at different current densities. (d) Areal and volumetric specific capacitance of the rGO/PPy-90% (stars) and pure PPy NT (spheres) paper electrodes obtained by galvanostatic charge/ discharge curves at different current densities. (e) Nyquist plots and (f) cycling stability of the rGO/PPy-90% and pure PPy NT paper electrodes in 1 M H2SO4 electrolyte. The cycling stability was obtained by galvanostatic charge/discharge curves at 20 mA/cm2.
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Fig. 4. (a) A schematic diagram of the ASSSC device. (b) A digital photograph of the rGO/PPy-90% ASSSC under a bending state. (c) CV and (d) galvanostatic charge/discharge curves of the rGO/PPy-90% ASSSC at different scan rates and current densities. (e) Areal and volumetric specific capacitance of the rGO/PPy-90% ASSSC obtained by galvanostatic charge/ discharge curves at different current densities. (f) Nyquist plots of the rGO/PPy-90% ASSSC compared with the rGO ASSSC. (g) Cycling stability of the rGO/PPy-90% ASSSC compared with the paper electrode in aqueous electrolyte by galvanostatic charge/discharge curves at 20 mA/cm2 (inset: galvanostatic charge/discharge curves of the 1st, 5 000th and 10 000th cycle for the rGO/PPy-90% ASSSC). (h) CV curves of the rGO/PPy-90% ASSSC under different bending angles at a scan rate of 50 mV/s.
from 0.7 to 11.2 mA/cm2) [27]. EIS measurement was performed in order to further investigate the relationship between capacitive performance and interior resistance (Fig. 4f). In the high frequency region, the rGO/PPy-90% ASSSC showed nearly the same ESR of ~0.68 U as the pure rGO ASSSC, and a small Rct. In the low frequency region, the impedance curve of the rGO/PPy90% ASSSC was more parallel to the imaginary axis than the rGO ASSSC, indicating the hierarchical PPy NT framework facilitated the infiltration of gel electrolyte. Electrochemical stability of the ASSSC was examined by galvanostatic charge/discharge tests at 20 mA/cm2 for 10,000 cycles, and compared with the hybrid paper electrode tested in aqueous electrolyte as shown in Fig. 4g. As a result, the employment of solid-state electrolyte (PVA-H2SO4) greatly improved the capacitance retention rate from 56.1% to 82.7%. This result had been proved by other previous literature [8]. The flexible ASSSC also exhibited excellent capacitance stability under different bending angles (0e180 ). The CV curves displayed almost the same shape at 50 mV/s, indicating the negligible variation of the capacitive behavior (Fig. 4h). Ragone plots of the rGO/PPy-90% ASSSC were shown in Fig. 5a and b. Areal and volumetric capacitive performances were compared with other reports as shown in Table S2 and S3 in the Supporting Information. The ASSSC showed a high areal energy density, from 71.2 mWh/cm2 at 1 mW/cm2 to 61.4 mWh/cm2 at 10 mW/cm2, much better than the reported ASSSCs based on graphene/cellulose paper (15 mWh/cm2 at 0.04 mW/cm2) [13], rGO/ MnO2 (from 35.1 mWh/cm2 at 0.0375 mW/cm2 to 11.5 mWh/cm2 at 3.8 mW/cm2) [22] and graphene/cellulose nanofiber (20 mWh/cm2 at 15.5 mW/cm2) [27]. Moreover, it also had a high volumetric energy density, from 8.32 mWh/cm3 at 0.12 W/cm3 to 7.18 mWh/ cm3 at 1.17 W/cm3, better than the reported laser-scribed graphene
(from 1.36 mWh/cm3 at 0.8 W/cm3 to 0.8 mWh/cm3 at 20 W/cm3) [5], graphene (from 2.5 mWh/cm3 at 0.1 W/cm3 to 0.14 mWh/cm3 at 0.495 W/cm3) [28] and MnO2//Fe2O3 (from 0.41 mWh/cm3 at 0.06 W/cm3 to 0.35 mWh/cm3 at 0.1 W/cm3) [29]. With the increase of power density, energy density of the ASSSC showed an excellent retention rate, resulting from the high rate capability of specific capacitance. For practical application in electronics, it is important to evaluate the leakage current and self-discharge characteristics of the ASSSCs (Figure S6 in the Supporting Information). The leakage current of the device quickly stabilized at ~32 mA, indicating a small leakage current and high stability. After being charged at 1.0 V for 15 min, the open-circuit voltage of the device exhibited a rapid decrease in the first hour and gradually reached ~0.22 V after 24 h, revealing a low self-discharge characteristic. Moreover, three ASSSC units with the same mass of the rGO/PPy-90% paper, were connected in series to further evaluate the performance of the solidstate devices. Fig. 5c showed the potential window was extended to 3.0 V for the tandem device based on three ASSSCs. The charge/ discharge time at the same current density was tinily changed for the tandem device based on two or three ASSSCs, indicating the capacitive performance of each ASSSC unit was well maintained in series. Then the tandem device was used to light up heterochromatic LEDs. As shown in Fig. 5d, the blue LED could be easily powered by the tandem device based on three ASSSCs. It also could power a LED with other colours, such as red, yellow and green (Figure S7 in the Supporting Information). 4. Conclusions In this work, we fabricated rGO/PPy NT papers for flexible ASSSC electrodes. The incorporation of rGO nanosheets could improve the
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Fig. 5. (a) Areal and (b) volumetric Ragone plots of the rGO/PPy-90% ASSSC compared with other reports in previous literature. (c) CV and (d) galvanostatic charge/discharge curves (inset: a photograph of a blue LED powered by three rGO/PPy-90% ASSSCs connected in series) of two and three rGO/PPy-90% ASSSCs connected in series at 100 mV/s and 2 mA. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
cycling stability of PPy NT paper electrode, without sacrificing the pseudocapacitance. The hybrid paper could be used as excellent electrode materials for ASSSC devices without using other binders or conducting additives. The assembled ASSSC had a large areal and volumetric specific capacitance, high rate capability, excellent capacitance stability under different bending states as well as cycling stability, a small leakage current and a low self-discharge characteristic. In addition, it showed a high areal energy density of 61.4 mWh/cm2 at 10 mW/cm2 and a large volumetric energy density of 7.18 mWh/cm3 at 1.17 W/cm3. Based on the high energy density with excellent retention rate, this flexible ASSSC holds great potential for high-performance energy storage devices. Acknowledgments The authors gratefully acknowledge financial supports by the National Natural Science Foundation of China (no. 51102164, no. 61376003 and no. 51302179), Medical-Engineering (Science) crossResearch Fund of Shanghai Jiao Tong University (no. YG 2014QN01 and no. YG2012MS37), the Natural Science Foundation of Jiangsu Province (no. BK2012184), and the Foundation for SMC Excellent Young Teacher in Shanghai Jiao Tong University. We also acknowledge the analysis supports from Instrumental Analysis Center of Shanghai Jiao Tong University and the Center for Advanced Electronic Materials and Devices of Shanghai Jiao Tong University. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.10.035.
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