Polyindole and Activated Carbon

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The Bamboo-Like Composites of V2O5/Polyindole and Activated Carbon Cloth as Electrodes for All-Solid-State Flexible Asymmetric Supercapacitors Xi Zhou, Qiang Chen, Anqi Wang, Jian Xu, Shishan Wu, and Jian Shen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 22 Jan 2016 Downloaded from http://pubs.acs.org on January 22, 2016

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ACS Applied Materials & Interfaces

The Bamboo-Like Composites of V2O5/Polyindole and Activated Carbon Cloth as Electrodes for AllSolid-State Flexible Asymmetric Supercapacitors Xi Zhou, Qiang Chen, Anqi Wang, Jian Xu, Shishan Wu* and Jian Shen* School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, P. R.China. KEYWORDS: activated carbon cloth, V2O5, polyindole, flexible, asymmetric supercapacitors

ABSTRACT: A bamboo-like nanomaterial composed of V2O5/Polyindole (V2O5/PIn) decorated onto the activated carbon cloth was fabricated for supercapacitors. The PIn could effectively enhance the electronic conductivity and prevent the dissolution of vanadium. And the activation of carbon cloth with functional groups is conducive to anchoring the V2O5 and improving surface area, which results in an enhancement of electrochemical performance and leads to a high specific capacitance of 535.5 F/g. Moreover, an asymmetric flexible supercapacitor based on V2O5/PIn@activate carbon cloth and reduced graphene oxide (rGO)@activate carbon cloth exhibits a high energy density (38.7 W h/kg) at a power density of 900 W/kg and good cyclic stability (capacitance retention of 91.1% after 5000 cycles). And the prepared device is shown to power the light-emitting diode bulbs efficiently.

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INTRODUCTION Driven by surging demands for renewable energy vehicles, electronic devices, and digital communications, tremendous attention have been attracted to the field of energy storage systems during the past decades.1-5 Among various energy storage devices, flexible all-solid-state supercapacitors have drawn huge interest because of their superiorities of excellent power output, exceptional cycling life, light weight and ease of handing.6-8 These unique properties make them available as power sources for next-generation flexible and portable electronics such as miniature biomedical devices, roll-up displays and wearable devices.9-11 However, for practical applications, the operating voltage of supercapacitors is very limited, resulting in low energy density, which has restricted their widespread use.12 According to the formula of E = 0.5CV2, the improvement of the energy density (E) could be realized by the following ways: to maximize the operation potential windows (V) and the specific capacitance (C).13-15 Developing asymmetric supercapacitors is a promising strategy to enlarge potential windows. It is because such a capacitor can fully exploit the advantages of different potential windows of the two electrodes, leading to a greatly extended operation voltage and a significantly improved energy density.16-18 On the other hand, considerable research efforts have been focused on developing nanostructured active materials with high specific surface area and specific capacitance.19-21 Among these materials, Carbon-based nanomaterials such as carbon nanotube and graphene are commonly employed as electrode materials in supercapacitors owing to their excellent electrical conductivity, large surface area, and high power density.22,23 In addition, transition metal oxides as faradaic pseudocapacitance electrode materials have been extensively studied because of their


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high theoretical storage capacity and superior reversibility.24-26 Particularly, V2O5 has multiple valence states and unique layered structure, which plays a major role in delivering high energy density, allowing efficient ion diffusion.27 Nonetheless, the poor electrical conductivity and structural unstability prevent its application in high performance supercapacitors.28 To tackle this issue, intensive efforts have been devoted to preparing supercapacitors made of binder-free active materials grown on flexible conductive substrates. Carbon cloth (CC) is a promising candidate for the current collector and substrate due to excellent flexibility and high conductivity.29-31 Furthermore, it can provide more channels for rapid ion transport, thus benefiting the diffusion of electrolyte in the electrode material.32 Polyindole (PIn) is an intrinsically conductive polymer as a pseudocapacitor material because of its superior conductivity, excellent thermal stability, electrochemical reversibility and storage ability.33 In this work, a simple strategy was firstly employed to activate carbon cloth, which led to large amounts of functional groups brought onto the surface of carbon cloth, improving the properties of immobilizing foreign materials. Then V2O5 nanostructures were constructed on the activated carbon cloth (ACC) through an ion-exchange column, and the prepared V2O5 promoted the in situ oxidation polymerization process of indole monomer, resulting in the fabrication of the bamboo-like V2O5/polyindole@activated carbon cloth (V2O5/PIn@ACC) materials. The introduction of PIn shell with intimate contact with V2O5 is beneficial for not only preventing V2O5 dissolution into the electrolyte by avoiding the direct contact between them during cycling, but also affording a facile electron transport to ensure electrochemical activity. By matching the V2O5/PIn@ACC with the negative electrode of reduced graphene oxide@activated carbon cloth (rGO@ACC), we assembled an asymmetric supercapacitors device, which obtained a maximum energy density of 38.7 W h/kg at a stable potential window of 1.8 V. Besides, the device


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exhibited a superior cycling stability with capacitance retention of 91.1% after 5000 chargingdischarging cycles.

RESULTS AND DISCUSSION Due to the inertness of carbon fibers, the foreign materials coated on them are less solid. Therefore, surface activation is proposed to increase surface area and generate functional groups.30,31 As shown in Figure 1A and Figure S1A, the untreated carbon cloth (CC) consists of interlaced carbon fibers with a smooth surface. After activation, the carbon fibers have a rough surface with pores on them and are restrained together without being destroyed (Figure 1B, Figure S1B), implying the increasing of surface area and the surface activation of the carbon cloth. Moreover, to demonstrate the enhancement of the surface area after chemical activation, we carried out N2-adsorption/desorption measurements on CC and ACC (Figure S2A). As measured by Brunauer–Emmett–Teller (BET) method, the specific surface area of ACC (65.2 m2/g) is much higher than that of CC (5.8 m2/g). And from the pore-width distribution of CC and ACC (inset), we can find that the activation process lead to a great amount of mesoporous pores decorating on the carbon cloth, which again supports the result of SEM image. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) were further employed to characterize the activated carbon cloth surface. The XPS spectra of the ACC show a slight enhancement of O1s compared with untreated carbon cloth (Figure 1C). The C 1s spectrum of ACC shows a broader signal than that of untreated carbon cloth (Figure S3A), which may have been caused by other chemical states of carbon on the ACC surface after activation.30 And the high-resolution C 1s spectrum (Figure S3B) of ACC further confirms four characteristic carbon


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states of C−C (284.5 eV), C−OH (285.3 eV), C=O (286.3 eV) and C(O)O (289.2 eV), suggesting the producing of oxygen-containing functional groups during the activation process. Moreover, the XRD patterns of the untreated carbon cloth and ACC are presented in Figure 1D. The weak diffraction peak around 43.5° is due to the (101) planes of graphite. And the diffraction peak located at 25° is assigned to the (002) planes of graphite.30,31

Figure 1. Typical SEM images of (A) untreated carbon cloth and (B) activated carbon cloth. Insets in (A) and (B) are the magnified images of untreated carbon cloth and activated carbon cloth, respectively. (C) XPS spectra of the prepared materials. (D) XRD patterns of prepared CC, ACC and rGO@ACC. Morphologies of the prepared composites based on carbon cloth are displayed in Figure 2. The SEM image of rGO@ACC shows that the carbon fibers are entirely covered by the thin rGO layers with a rough surface, implying an increasing of surface area (Figure 2A). Furthermore, from Figure 2B it is found that V2O5/PIn shows a nanowire structure decoration on the carbon fibers, showing a similar morphology of bamboo, which is further proved by the magnified SEM image (the inset of Figure 2B). Figure S1C is a TEM image of V2O5 nanowire with a diameter of


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around 80 nm. The high-resolution TEM (HRTEM) image (inset) indicates the nanowire contains a crystalline structure with a lattice spacing of 0.35 nm.14 For comparison, the morphology and structure of V2O5/rGO were characterized by TEM (Figure S1D) and SEM (Figure S1E). As shown in SEM image, the composite demonstrates a continuous fibrous structure. More details are revealed by TEM, in which the V2O5 shows uniform nanowire morphology, coated on the rGO nanosheets completely. From careful inspection of XPS analysis, the V 2p spectrum of V2O5/PIn@ACC (Figure 2C) exhibits two peaks at 516.5 and 523.8 eV, which are ascribed to the binding energy of V 2p3/2 and V 2p1/2, respectively, indicating the formation of the V2O5 phase in the nanocomposite matrix.34,35 And the peak at 400 eV corresponding to N 1s is obvious observed, which justifies the presence of PIn (Figure 1C).33 Additionally, the N2adsorption/desorption measurement on V2O5/PIn@ACC (Figure S2B) reveals a specific surface area of 57.7 m2/g, which is also much larger than untreated CC. The as-prepared V2O5 and V2O5/PIn@ACC were characterized by XRD to identify the crystallinity, and the typical diffraction patterns are exhibited in Figure 2D. It is shown that the characteristic peaks for V2O5 at 2θ = 10.5°, 25.6°, 29.0°, 31.75°, 34.5°, 45.5°, 50.7°, 57.3° and 67.1°, which are corresponding to (001), (110), (012), (003), (310), (005), (020), (021), and (300) planes, respectively. Besides, the V2O5/PIn@ACC exhibits another characteristic peaks which are assigned to (111) planes of PIn and (002) planes of ACC, respectively.36,37 The sharp peaks with strong intensity suggest the high crystallinity of the prepared materials. Also the rGO@ACC displays another characteristic XRD peak at 23° for rGO (Figure 1D), which is in good agreement with the reported data.38 All these results demonstrate that the V2O5/PIn and rGO are successfully coating on the carbon cloth.


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Figure 2. SEM images of (A) rGO@ACC and (B) V2O5/PIn@ACC. Insets in (A) and (B) are the magnified images of rGO@ACC and V2O5/PIn@ACC, respectively. (C) High-resolution XPS spectra of V 2p of V2O5/PIn@ACC. (D) XRD patterns of V2O5 and V2O5/PIn@ACC. As is known to all, due to their complementary operating voltage windows with various faradic positive electrodes, carbon materials are commonly used as negative electrodes. At the same time, pseudocapacitive materials such as transition metal oxides or conducting polymers, could exhibit quite large energy density and specific capacitance, making them as the ideal positive electrode materials.8-11 The electrochemical activity of rGO@ACC and V2O5/PIn@ACC electrodes were investigated by cyclic voltammetry (CV) and galvanostatic charge–discharge tests in 5 M LiNO3 solution as shown in Figure 3.


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Figure 3. CVs of rGO@ACC (A) and V2O5/PIn@ACC (C) at different scan rates. Galvanostatic charge/discharge curves of rGO@ACC (B) and V2O5/PIn@ACC (D) with different current densities. From the CV curves of rGO@ACC electrode at different scan rates from 5-50 mV/s (Figure 3A), we can find that they exhibit a quasi-rectangular shape without noticeable redox peaks, implying excellent double layer charge storage performance. Additionally, the charge/discharge curves are symmetric and linear in the potential range of -1.0 to 0 V, which further confirms the remarkable capacitive property (Figure 3B). The specific capacitance of rGO@ACC electrode is calculated to be 156.3 F/g at a current density of 1 A/g and it decreases with increasing current density and attains a value of 133.2 F/g at 15 A/g. At the same time, the V2O5/PIn@ACC shows the CV curves (Figure 3C) in distorted rectangular shapes with the scan rates from 5-50 mV/s, which is due to the pseudocapacitive behavior of PIn and V2O5. Galvanostatic charge–discharge measurements (Figure 3D) are performed at different current densities to estimate the specific capacitances. The profiles of charge–discharge curves display a non-linear profile, owing to the pseudocapacitive electrode. The charge–discharge curves are symmetric, suggesting the


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remarkable reversibility. The specific capacitance is calculated to be 535.3 F/g at 1 A/g, which is much higher than that of V2O5@ACC (304.7 F/g) under the same measure conditions (Figure S4). These results are probably caused by the improvement of electronic conductivity derived from PIn, and the intimate combination with V2O5 accessible for fast ion transportation. Besides, we have noted that these specific capacitance values are higher than many of those reported so far: V2O5 nanosheets (253 F/g, 1 A/g),34 rGO/V2O5 xerogels (195.4 F/g, 1 A/g)39 and V2O5PPy/RGO composites (434.7 F/g, 1 A/g).40 In order to study the role of PIn in the electrochemical measurements, CV of the prepared materials were carried out. As displayed in Figure S5A, the enclosed area of CV curve for V2O5@ACC is slightly larger than that of ACC because of the poor electrical conductivity. A SEM image of V2O5@ACC is illustrated in Figure S5B, in which region I is immersed into 5 M LiNO3 solution for 12 h. Region II is above the liquid surface. We can obviously find that the V2O5 on the ACC has been eroded largely after immersing in LiNO3 solution for 12 h. This is the reason that the introduction of V2O5 could not improve the electrochemical performance of ACC effectively.40 Furthermore, CV curves obtained from ACC, PIn@ACC and V2O5/PIn@ACC at the same scan rate of 30 mV S-1, respectively, are shown in Figure S5C. It indicates that the area of the CV curve of V2O5/PIn@ACC is much larger not only than that of the ACC, but also than that of PIn@ACC. These results are probably attributed to the excellent electronic conductivity and the remarkable corrosion resistance of PIn, which could protect V2O5 against dissolution in the electrolyte.41 Figure S5D is a SEM image of the V2O5/PIn@ACC after being immersed into LiNO3 solution of the same concentration for 24 h. It manifests that V2O5/PIn on the ACC has not been eroded by electrolyte solution, which further confirms the role of PIn preventing the dissolution of V2O5.


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The electrochemical impedance spectroscopy (EIS) analysis has been recognized as one of the principal methods for examining the fundamental behavior of the electrodes. For further comparison, EIS of different electrodes including ACC, V2O5@ACC and V2O5/PIn@ACC were measured in the frequency range of 0.01 Hz to 100000 Hz. In Figure S6, the Nyquist curves show the form with a semicircle in the higher frequency region and spike in the lower frequency region. The intersections of the curves on the real axis indicate the ohmic resistance (RS) of the electrolyte and the electrode materials.42 As for the above three electrodes, the small RS values of V2O5/PIn@ACC (1.1 Ω), V2O5@ACC (2.2 Ω) and ACC (2.3 Ω) manifest the good conductivity of the electrolyte and the very low internal resistance of the electrodes. Moreover, the charge transfer resistance (RCT), which is deduced by the diameter of the semicircle in the high frequency region. In the case of V2O5/PIn@ACC electrode, the RCT is ~7.6 Ω, which is lower than that of V2O5@ACC (~10.8 Ω) and ACC (~16.3 Ω) electrodes, clearly revealing the high conductivity after PIn decorating. We also investigated the performance of our electrodes in an all-solid-state asymmetric supercapacitors device, where V2O5/PIn@ACC was employed as the positive electrode while rGO@ACC was used as the negative electrodes (denoted as V2O5/PIn//rGO). To achieve the maximum device capacitance, the charge balance (q+ = q-) of different electrodes should be carefully maintained. Since the operation potential window and specific capacitances of the prepared materials are different, their charge storage capacities are balanced by adjusting the mass loading between these two electrodes, which is adjusted to be 1.4:1. For device testing, the potential windows are extended up from 0-1.0 V to 0-1.8 V (Figure 4A). It is noted that the distorted rectangular CV curves are due to the contribution from the V2O5 pseudocapacitive material. The large potential window indicates a high power density. This is a crucial superiority


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compared to common symmetric supercapacitors and a significant factor to satisfy the demand of applications.9-11 And the CV curves at different scan rates in the potential window of 0-1.8 V are shown in Figure 4B, further confirming the excellent electrochemical behavior of the asassembled device. Furthermore, the galvanostatic charge–discharge curves suggest that the device can not only work at a high voltages of 1.8 V (Figure 4C), but also the specific capacitance has a certain improvement, achieving a max cell capacitance of 275.5 F/g with a broadening of potential windows (Figure S7), which is attributed to the fact that more pseudocapacitance of V2O5/PIn is utilized with an increasing of cell voltages. Good linear proles of the galvanostatic charge–discharge curves with various current densities in the range of 1-15 A/g are displayed in Figure 4D, we can find that with the current density up to 15 A/g, it still maintains 236.3 F/g, with a high capacitance retention of 86.5% compared with that obtained at 1 A/g (Figure S7).

Figure 4. CVs of V2O5/PIn//rGO at different potential windows at a scan rate of 50 mV s-1 (A) and CVs at different scan rates of V2O5/PIn//rGO (B). Galvanostatic charge/discharge curves of


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V2O5/PIn//rGO measured at different potential windows (C) and different current densities (D). Ragone plots of the device revealing the relationship between energy density and power density are achieved. Figure 5A shows that the assembled device possesses a high energy density of 38.7 W h/kg at an exceptional power density of 0.9 kW/kg. And the device exhibits satisfactory stability with the energy density of 32.6 W h/kg at a power density of 18 kW/kg. Designed for flexible device, we further assess its flexibility under different bending angles. Intriguingly, the CV curves obtained at different bending angles reveal similar capacitive performance, indicating outstanding electrochemical and mechanical stability (Figure 5B). To display the application potential of this supercapacitor, we assembled two devices in series, which could power a light-emitting diode (LED) as shown in Figure 5C.

Figure 5. (A) Ragone plots of the asymmetric supercapacitors device. The values reported for other devices are added for comparison.41-44 (B) CVs of the device at 30 mV s-1 under different bending angles. (C) A picture displaying that a yellow LED can be powered by the devices. (D) Cycling performance of V2O5/PIn//rGO and V2O5//rGO devices. To further research the cycling stability of the device, the cycling performance within 0-1.8 V


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at a current density of 10 A/g were recorded by galvanostatic charge/discharge for 5000 cycles. Figure 5D reveals that the specific capacitance exhibits a smooth degradation stage with a high retention rate of 91.1% after 5000 cycles, which is much higher than that of V2O5//rGO (55.5%, V2O5@ACC was employed as the positive electrode while rGO@ACC was used as the negative electrodes). It may be due to the instability of V2O5 during cycling, tending to form soluble complexes. The PIn could prevent V2O5 dissolution into the electrolyte by avoiding the direct contact between them during cycling. Additionally, the high intimate contact between V2O5/PIn and ACC enhances the mechanical adhesion between the V2O5/PIn and the substrate, and insures the good electrical contact with the highly conductive substrate in this self-supporting electrode. These results demonstrate V2O5/PIn//rGO as a promising material for practical application in supercapacitors.

CONCLUSIONS In summary, a novel asymmetric device based on V2O5/PIn composite growing on activated carbon cloth as the positive electrode and reduced graphene oxide deposited on carbon cloth as the negative electrodes has been prepared successfully. The activation of carbon cloth offers the advantages of immobilizing the V2O5 with improving electrochemical activity and the PIn could effectively enhance the electronic conductivity and prevent the dissolution of vanadium. The assembled V2O5/PIn//rGO asymmetric supercapacitor shows a stable potential window of 1.8 V with reversible charging and discharging procedure. Thus, a satisfying energy density of 38.7 Wh/kg at the power density of 900 W/kg is achieved. Moreover, because of the unique structure and composition of V2O5/PIn as well as the high intimate contact between the V2O5/PIn and the


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ACC, the device exhibits quite a superior cycling performance with a high retention rate of 91.1% after 5000 cycles, which is encouraging for the application of self-supporting asymmetric supercapacitors.

EXPERIMENTAL SECTION Activation of Carbon Cloth Prior to the activation, carbon cloth (WOS 1002 CeTech CO., Ltd., China, 125 g m-2, the thickness is 360 mm) was cleaned by acetone, ethanol and deionized water in sequence. In a typical method, the cleaned carbon cloth was soaked in a mixture containing 20 ml concentrated HNO3 and 40 ml concentrated H2SO4 under stirring at room temperature. After stirring for 30 min, 6 g KMnO4 was added into the solution slowly, keeping stirring for 1 h at 35 ℃. Then 100 mL distilled water was added to the mixture and stirred remained for 3 h. After that H2O2 solution was added into the mixed solution until gas bubbles weren't evolving and the solution became clear. The obtained carbon cloth was then washed with distilled water and air-dried. Finally, the activated carbon cloth (ACC) was obtained by reducing in H2/N2 (5%/95%) atmosphere at 1000 ℃ for 3 h in a tubular furnace at a heating rate of 10 ℃ min-1. Synthesis of V2O5/PIn@ACC 60 mL 1 M sodium metavanadate solution was dropped onto an activated carbon cloth through an ion-exchange column, and then allowed to stand overnight. The V2O5 nanostructures were obtained by crystallization at 400 ℃ for 2 h. V2O5/PIn@ACC was initiated with the addition of 10 mL H2O2 into the mixture of 50 mg as-synthesized V2O5 nanostructures, 5 mg indole, and 50


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mL deionized water. After 12 h of vigorous stirring, the products were obtained by washing with distilled water. Synthesis of rGO@ACC GO was synthesized using a modified Hummer’s method and dispersed in distilled water with a concentration of 1.0 mg/1.0 mL by ultrasonication. Then the GO was deposited on activated carbon cloth by a dip-coating method. Finally, the conversion of GO to the reduced form rGO on the flexible carbon cloth was achieved in H2/N2 (5%/95%) atmosphere at 400 ℃ for 3 h in a tubular furnace. Apparatus The morphology of the nanostructures was investigated by emission scanning electron microscopy (SEM, SU8010, Japan). High-resolution transmission electron microscope (HRTEM) images were taken in a Tecnai F30 (PEI, USA) operated at 200 kV. Surface elemental analysis was performed on an X-ray photoelectron spectrometer (XPS, Kratos Axis Ultra Dld, Japan). X-ray powder diffraction (XRD) patterns were obtained by XRD-6000 X-ray Diffractometer (Shimadzu, Japan). Nitrogen adsorption and desorption isotherms analysis was carried out at 77 K on an ASAP 2020 accelerated surface area and porosimetry instrument (Micromeritics, USA), using Brunauer-Emmett-Teller (BET) calculations for the surface area. The pore size distribution plot was recorded from the adsorption branch of the isotherm based on the Barrett-Joyner-Halenda (BJH) model.

Electrochemical characterization Electrochemical tests were carried out using a CHI 660D (Chenhua Shanghai, China) electrochemical workstation. For the three-electrode measurements, the as-prepared


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V2O5/PIn@ACC and rGO@ACC served as the working electrodes using 5 M LiNO3 aqueous solution as the electrolyte, with Ag/AgCl as the reference electrode and a Pt mesh as the counter electrode. Solid-state asymmetry capacitors were tested in a two electrode system, with LiNO3/PVA as the gel electrolyte. In detail, the LiNO3/PVA gel electrolyte was prepared as follows: 3 g PVA was added into LiNO3 (5 M) aqueous solution and heated at 85 °C under vigorous stirring until the solution became clear. To assemble an all-solid-state asymmetric supercapacitor, two pieces of the prepared electrodes were assembled with a filter paper saturated with LiNO3/PVA gel as the separator, sandwiched in between (Figure 6). The specific capacitances (C) of the electrodes are calculated based on their galvanostatic charge/discharge curves, according to following equation: C = I∆t/m∆E

(1)

where ∆E is the potential window during the discharge process, m is the electrode mass, I is the discharge current, and ∆t is the discharge time. The energy density (E) and power density (P) were calculated according to the following equation: P = E/∆t

(2)


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Figure 6. Schematic illustration of the asymmetric supercapacitor configuration.

ASSOCIATED CONTENT Supporting Information. SEM images of untreated CC and V2O5/rGO, TEM images of V2O5 and V2O5/rGO, HRTEM image of V2O5, photograph of ACC. Nitrogen adsorption isotherms of CC, ACC and V2O5/PIn@ACC. C 1s spectrum of XPS collected for untreated and activated CC and highresolution XPS spectra of C 1s of ACC, the specific capacitances of ACC, rGO@ACC, V2O5@ACC and V2O5/PIn@ACC at different current densities. SEM images of V2O5@ACC and V2O5/PIn@ACC, CV curves of ACC, V2O5@ACC, V2O5/PIn@ACC and PIn@ACC. Nyquist plots of the three prepared different material electrodes. The specific capacitance of asymmetric supercapacitors device as a function of different potential windows and current densities. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (Shishan Wu)


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* E-mail: [email protected] (Jian Shen) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Nos. 51272100 and 51273073).

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Table of Contents Graphic


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Polyindole and Activated Carbon

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