Ionics https://doi.org/10.1007/s11581-017-2428-8

ORIGINAL PAPER

Biomass carbon/polyaniline composite and WO3 nanowire-based asymmetric supercapacitor with superior performance Haiping Wang 1 & Guofu Ma 2 & Yongchun Tong 1 & Zirong Yang 1 Received: 25 July 2017 / Revised: 16 December 2017 / Accepted: 26 December 2017 # Springer-Verlag GmbH Germany, part of Springer Nature 2018

Abstract The demand for advanced energy storage devices such as supercapacitors and lithium-ion batteries has been increasing to meet the application requirements of hybrid vehicles and renewable energy systems. Here, high energy density aqueous asymmetric supercapacitor (ASC) is assembled based on chestnut shell-based activated carbon (CAC)/PANI composite positive electrode and tungsten trioxide (WO3) nanowires negative electrode. The CAC/PANI composite and WO3 nanowires were synthesized through an interfacial polymerization method and a simple sodium sulfate assisted hydrothermal process, respectively. The CAC/ PANI//WO3 ASC device operates with a voltage of 1.5 V in 1 M H2SO4 electrolyte and achieved a high energy density of 15.4 Wh kg−1 at a power density of 252 W kg−1. Furthermore, the device shows an excellent cycling performance with capacitance retention of 83% after 1500 cycles. Keywords Chestnut shell . Polyaniline . Tungsten trioxide . Asymmetric supercapacitor

Introduction Supercapacitors or electrochemical capacitors have attracted much attention because they can instantaneously provide higher power density than batteries, and higher energy density than conventional dielectric capacitors. However, they deliver lower energy density, which greatly restricts their application [1–4]. An effective way is to develop hierarchical nanostructured or nanoporous materials to promote the specific capacitance of electrodes [5–8] and another way is to explore asymmetric supercapacitor (ASC), which integrates the different potential windows of capacitive and pseudocapacitive electrodes to increase the cell voltage [9–12]. The keys to achieve h i g h e n e rg y a n d p o w e r d e n s i t i e s o f as y m m e t r i c supercapacitors are to use appropriate materials, usually carbon materials, as a capacitive electrode; to design and * Haiping Wang [email protected] 1

College of Chemistry and Chemical Engineering, Hexi University, Zhangye 734000, China

2

Key Laboratory of Eco-Environment-Related Polymer Materials of Ministry of Education, Key Laboratory of Polymer Materials of Gansu Province, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, China

synthesize suitable materials, especially nanostructured materials, as a Faradic electrode; and to select a suitable electrolyte. Currently, activated carbon (AC) with a high specific surface area and moderate cost is the most widely used material for the capacitive electrode of asymmetric supercapacitors. Conducting polymers have been studied for their potential application as electrodes in energy storage devices [13–15]. Among conducting polymers, polyaniline (PANI) has attracted interest due to its high specific capacitance, good environmental stability, electroactivity, and doping-dedoping chemistry [16]. However, PANI exhibits the disadvantage of a low cycle life because swelling and shrinkage may occur during doping/dedoping processes, thus leading to mechanical degradation of the electrodes and fading of electrochemical performance. Interfacing other carbon materials with PANI is one effective way to improve the electrochemical stability of the composite. Previous reports have described that coupling nanostructured PANI to carbonaceous materials can improve the utilization of PANI and provides a short transport path for ion and electron. Therefore, composites based on the appropriate stoichiometric combination of carbonaceous materials and conducting polymers have been proven to be a potential breakthrough for a new generation of efficient supercapacitors [17–19]. Hence, the research on the development of high-performance supercapacitors originating from the synergistic combination of outstanding conducting

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properties of carbonaceous materials and high pseudocapacitance of conducting polymers is an ongoing interest in nanotechnology [20–22]. Tungsten trioxide (WO3) attracts extensive attention because of its distinctive physical and chemical properties, making it suitable for applications in electrochromic (EC) devices, photocatalysis, and gas sensing [23–26]. In the studies, WO3 is also a promising electrode material due to its low cost, simple synthesis, outstanding conductivity, and high theoretical capacity [27]. Recently, Samu et al. reported tungsten trioxide with nanoporous structures and studied their application as supercapacitor electrodes [28]. Wang et al. demonstrated WO3@polypyrrole core-shell nanowires arrays with excellent supercapacitive performance [29]. CNT-WO3 hybrid electrodes have also been recognized as a potential candidate for flexible asymmetric supercapacitor applications [30]. Here, we developed a novel asymmetric supercapacitor device using chestnut shell-based activated carbon (CAC)/ PANI composite and WO3 nanowires as the positive and negative electrodes, respectively. The as-fabricated CAC/PANI// WO3 asymmetric supercapacitor exhibits superior electrochemical properties in 1 M H2SO4 aqueous electrolyte, including wider operating voltage window (1.5 V), high energy density (15.4 W h kg−1), and outstanding cycle stability.

Experimental Materials Aniline (ANI, Shanghai Chemical Works, China) was distilled under reduced pressure before used. Ammonium persulfate (APS, Tianjing Damao Chemical Co., China) and sodium tungstate dihydrate (Na2WO4∙2H2O, Aladdin Ltd. Shanghai China) solutions were prepared using deionized water. All other chemical reagents were in analytical grade.

Synthesis of CAC/PANI composite The chestnut shell-based activated carbon (CAC) was first prepared from chestnut shells, which was performed by mixing the dried chestnut shells with KOH solution at an impregnation ratio of 5:2 (mass KOH/mass chestnut shells) and stirring at 60 °C for 3 h. The resulting slurry was dried in an oven for at least 24 h at 100 °C. The dried sample was carbonized at 800 °C using a temperature ramp rate of 5 °C min−1 and then kept for 2.5 h under a flowing nitrogen atmosphere in a tubular furnace. After being cooled to room temperature, the carbide sample was washed repeatedly with 0.5 M HCl until the pH of the washing solution reached 6.5. Finally, the sample was washed with distilled water and dried at 60 °C in ambient conditions for 24 h.

The synthesis procedure of CAC/PANI composite is similar to our previously reported [31]. Briefly, CAC (0.03 g) was added into a mixed solution of 50 mL of isopropyl alcohol and 50 mL of 1 M H2SO4, and the mixture was sonicated for 30 min to obtain a well-dispersed suspension. Then, ammonium persulfate (APS, 0.76 g) was dissolved in the above solution to form a water phase. After that, the resulting solution was cooled to 0 °C in an ice bath and the oil phase (aniline monomes (0.50 g) was dissolved in 100 mL of dichloromethane) cooled in advance was added drop-by-drop into the above solution. The reaction was carried out with magnetic stirring at 0 °C for 24 h; then, the resulting precipitate was washed several times with deionized water and ethanol, respectively. Finally, the product was dried at 60 °C for 12 h to obtain a dark green powder.

Synthesis of tungsten trioxide nanowires network Tungsten trioxide (WO3) nanowire arrays were synthesized by a sulfate-assisted hydrothermal method. Briefly, 3.29 g sodium tungstate powder was dissolved in 76 mL deionized water, and a 3-M HCl aqueous solution was used to adjust the pH value to 2.0. Afterwards, ammonium sulfate (2.64 g) was added to the reaction precursor to control the morphology of the WO3 product. After stirring for 1 h, the clear solution obtained was transferred into a 100-mL Teflon-lined stainless steel autoclave and heated at 180 °C for 24 h. After cooled to room temperature naturally, the resulting precipitates of WO3 were collected by filtration, washed with distilled water and absolute ethanol for several times to remove the residue of reactants, and then dried in vacuum at 60 °C for 12 h.

Characterizations The as-prepared electrode materials were characterized by scanning electron microscopy (SEM, JSM-6701F, Japan) at an accelerating voltage of 5.0 kV. X-ray diffraction (XRD) of samples was performed on a diffractometer (D/Max-2400, Rigaku) advance instrument using Cu Kα radiation (k = 1.5418 Å) at 40 kV, 100 mA. The 2θ range used in the measurements was from 5 to 80°.

Three-electrode cell fabrication For a conventional three-electrode system, a 3-mm-diameter glassy carbon electrode was used as the working electrode. The working electrodes were fabricated refers to ref. [32]. Typically, 4 mg of electroactive material was ultrasonically dispersed in 400 μL of deionized water, and 4 μL of the polytetrafluoroethylene (PTFE) emulsion (60 wt%) was added to this dispersion. Two microliters of the above suspension was dropped onto the glassy carbon electrode using a pipet gun and dried at room temperature. The three-electrode

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system was tested in 1 M H2SO4 electrolyte; Platinum electrode served as the counter electrode and saturated calomel electrode (SCE) as the reference electrode, respectively.

Two-electrode cell fabrication The CAC/PANI//WO3 asymmetric supercapacitor was assembled in 1 M H2SO4 aqueous electrolyte using two electrodes into sandwich-type cells construction. The positive and negative electrodes were pressed together and separated by a thin polypropylene film. The working electrode was prepared by mixing the electroactive material with polyvinylidene fluoride (PVDF) and commercial carbon black (8:1:1) in N-methyl-2pyrrolidone (NMP) until it forms a homogeneous slurry. The obtained slurry was coated on stainless steel nets with a working area of 1.0 cm2 and the electrodes were dried at 100 °C for 24 h and then weighted and pressed into sheets under 15 MPa. To construct an ASC, the loading mass ratio of active materials (m(CAC/PANI)/m(WO3)) was estimated to be 1.6 depends on their specific capacitance and potential range for the charge/discharge process.

Results and discussions Figure 1a shows the typical morphology of the as-synthesized CAC/PANI composite nanocones. Apparently, the nanocones composite appears as random aggregation to form a reticular structure and such a structure with a porous network is favorable in supercapacitor applications [32]. The XRD pattern of CAC/PANI composite shows three characteristic peaks (Fig. 1b). The peaks at 2θ of 14.9 and 25.7° resulted from the periodicity both perpendicular and parallel to the polymer chain, respectively. The peak at 2θ of 20.9° is caused by the layers of polymer chains at alternating distances [33]. Meanwhile, the diffraction peaks of chestnut shell-based activated carbons at 44.3° cannot be obviously observed; maybe the CAC particles interacted with PANI molecules and were almost completely covered by PANI. The as-obtained WO3 nanostructures are mainly nanowires with a random arrangement (Fig. 1c). The phase purity and crystal structure of the WO3 nanowires were confirmed by XRD. As shown in Fig. 1d, all the diffraction peaks can be exclusively indexed to a pure hexagonal crystalline phase of WO3 with lattice constants of a = 7.284 Å and c = 3.906 Å, which agrees well with the reported values of a = 7.298 Å, c = 3.899 Å, space group P6/mm from the JCPDS No. 33-1387 [34]. The strong and sharp diffraction peaks indicate good crystallinity of the as-synthesized WO3 products. Figure 2a shows CV curves obtained in a three-electrode cell for the CAC, pure PANI, and CAC/PANI composite electrodes under a scan rate of 5 mV s−1 in the potential window of − 0.2–0.8 V. As can be seen, there is no peak originating from

the CAC electrode, which indicates the CAC possesses an electrical double-layer capacitance. However, the capacitance characteristic of pure PANI or CAC/PANI composite is distinct from that of the CAC electrode close to the ideal rectangular shape with pseudocapacitance characteristics. PANI or CAC/PANI composite has three pairs of redox peaks: the first couple of peaks (about 0.23 V/0.10 V) are attributed to the redox transition of PANI between a semi-conducting state (leucoemeraldine form) and a conducting state (polaronicemeraldine form); the second couple of peaks (about 0.47 V/0.43 V) are due to the transformation between the p-benzoquinone/hydroquinone couple because of the attack by water; the third couple of peaks (about 0.73 V/0.70 V) are due to the formation/reduction of bipolaronic pernigraniline and protonated quinonediimine [32]. However, the electrochemical performance of the CAC/ PANI composite is different from the pure PANI. The CV curve area of CAC/PANI is larger than that of pure PANI. Furthermore, the first and the second couples of peaks of CAC/PANI composite are obviously higher than pure PANI at the same scan rate in 1 M H2SO4, and this verifies that the CAC/PANI composite has a higher specific capacitance than pure PANI, because of the linear relation between specific capacitance and CV curve area. Figure 2b gives the galvanostatic charge/discharge curves of the CAC, pure PANI, and CAC/PANI composite electrode at a current density of 1 A g−1. The specific capacitance (Cm) can be calculated according to the following equation: Cm = IΔt/mΔV, where I is the current of discharge (A), Δt is the discharge time (s), ΔV is the potential range during the discharge process, and m is the mass of active material within the electrode (g). According to the above equation, the gravimetric capacitances are 207 F g−1 (CAC), 251 F g−1 (PANI), and 597 F g−1 (CAC/PANI), respectively, which indicate that the specific capacitance of the composite material (CAC/PANI) is obviously enhanced compared with the corresponding CAC and pure PANI. The larger capacitance for CAC/PANI may be caused by the combination of the electric double-layer capacitance of CAC and Faradaic pseudocapacitance of PANI. Moreover, the specific capacitance of the CAC/PANI composite is superior to other composite electrode materials, such as graphene oxide/polyaniline (227 F g−1 at 2 A g−1) [35], grapheme/polyaniline nanofiber (480 F g−1 at 0.1 A g−1) [20], and polyaniline/activated carbon/TiO2 (286 F g−1 at 1 A g−1) [36]. The electrochemical studies for the CAC/PANI composite and WO3 nanowires were performed in a three-electrode cell using aqueous 1 M H2SO4 electrolyte. The performances of electrode materials were analyzed using cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) measurements (Fig. 3). CV curves of the CAC/PANI composites at scan rates from 5 to 50 mV s−1 with the potential range of − 0.2 to 0.8 V are shown in Fig. 3a. The CAC/PANI composite presents a couple of redox peaks attributed to the redox

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Fig. 1 a SEM images of as-synthesized CAC/PANI. b XRD pattern of CAC/PANI composites. c SEM images of as-synthesized WO3. d XRD pattern of WO3 nanowires

transition of PANI between a semiconducting state (leucoemeraldine form) and a conducting state (polaronic emeraldine form) [37], which results in the redox capacitance. Figure 3b shows the CV curves of WO3 nanowires, which were measured from − 0.7 to 0 V with a change of scan rate of 5 to 50 mV s−1. Characteristic redox peaks of WO3 electrodes were observed at about − 0.35 V of anodic scan and about − 0.4 V of cathodic scan, respectively, which corresponds to the reversible intercalation/deintercalation of H+ ions into/out of the WO3 host [38]. The electrochemical mechanism of WO3 in H2SO4 electrolyte can be expressed as the H+ insertion process takes place during the charge process, and consequently the extraction process happens during the discharge process: WO3 + xe− + xH+ ↔ HxWO3. In addition, the current density became larger, and anode/cathode peak potentials moved to higher/lower voltage according to the increase in scan rate, indicating the excellent electrochemical behavior. In addition, the obvious increase of current with scan rates means that the CAC/PANI composite and WO3 nanowires electrode materials have good capacitance retention rates as the current density increases. Galvanostatic charge/discharge (GCD) curves at different current densities were measured to evaluate the specific capacitance of the CAC/PANI composite and WO3 nanowires, as shown in

Fig. 3c, d, respectively. The GCD curves of the CAC/PANI composite consist of two clear voltage stages: a fast potential drop (from 0.8 to 0.65 V) and a slow potential decay (from 0.65 to 0 V) (Fig. 3c). The former, shorter discharge results from the electric double-layer capacitance of the electrode, while the latter, longer discharge is ascribed to a combination of electric double-layer capacitance and Faradaic capacitance, representing the pseudocapacitive feature of the electrode. Similarly, it should be noted that a characteristic pseudocapacitive behavior was observed for the WO3 electrode (Fig. 3d). The steep increase in the potential above − 0.1 V correlates with the observed decrease in the current in the CV curves of Fig. 3b. The specific capacitances were measured from the discharging curves at different current densities shown in Fig. 3e–f. For the discharging current densities from 1 to 5 A g−1, the specific capacitance values decreased from 597 to 394 F g−1 (retained about 66%) for the CAC/ PANI composite and decreased from 423 to 334 F g−1 (retained about 79%) for WO3 nanowires, respectively. Considering the high capacitance of the redox character over the CAC/PANI composite and WO3 nanowires material, an asymmetric supercapacitor was fabricated using these materials as the positive and negative electrodes, respectively. To further evaluate the electrochemical properties and estimate

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Fig. 2 a CV curves of the CAC, pure PANI, and CAC/PANI composite at a scan rate of 5 mV s−1. b GCD curves of CAC, pure PANI, and CAC/PANI composite at a current density of 1 A g−1

the stable potential windows of the CAC/PANI composite and WO3 nanowires, we performed CV measurements on the two electrode materials in 1 M H2SO4 aqueous solution before evaluating the asymmetric cell, using a three-electrode system with platinum as an auxiliary electrode and a saturated calomel electrode (SCE) as a reference electrode. The CAC/PANI electrode was measured within a potential window of 0 to 0.8 V (vs. SCE), while WO3 was measured within a potential window of − 0.7 to 0 V (vs. SCE) at a scan rate of 5 mV s −1 (Fig. 4). Additionally, it can be observed that these two

materials are quite stable in a different range of potentials. Consequently, if the total cell voltage can be expressed as the sum of the potential range for the CAC/PANI composite and WO3, it is possible to conclude that the cell potential can be extended up to 1.5 V in 1 M H2SO4 aqueous solution for an asymmetric supercapacitor. In order to obtain a capacitor operating in a 1.5-V potential window, it is crucial to control the experimental conditions for the CAC/PANI to work in the potential range from 0 to 0.8 V and the AC electrode in the range from 0 to − 0.7 V.

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the charge/discharge process (ΔV), and the mass of the electrode (m) following the Eq. [39]: q ¼ C m  ΔV  m and in order to get q+ = q−, the mass balancing will follow the Eq. [40]: mþ =m− ¼ C m‐  ΔV − =C mþ  ΔV þ :

Fig. 3 a, b CV curves of the CAC/PANI and WO3 electrodes at various scan rates performed in three electrode cells in 1 M H2SO4 electrolyte, respectively. c, d Galvanostatic charge/discharge curves of CAC/PANI and WO3 electrodes at different current densities performed in three electrode cells, respectively. e, f Specific capacitance of the CAC/PANI and WO3 electrodes at different current densities, respectively

As for a supercapacitor, the charge balance will follow the relationship q+ = q−. The charge stored by each electrode depends on the specific capacitance (Cm), the potential range for

The mass ratio of m(CAC/PANI)/m(WO3) was estimated to be 1.6 from the specific capacitance calculated from their galvanostatic charge/discharge curves. Figure 5a shows the CV curves of the CAC/PANI//WO3 asymmetric cell measured at various scan rates of 15~50 mV s−1. The fabricated ASC demonstrates a typical capacitive behavior with distorted rectangular CV curves even with the high voltage of 1.5 V, indicating ideal capacitive behavior and good reversibility. Meanwhile, when the scan rates are 15, 20, 40, and 50 mV s−1, the values of specific capacitance are calculated to be 300, 224, 107, and 82 F g−1. To further evaluate the performance of the asymmetric cell, we measured galvanostatic charge/discharge curves at various current densities (Fig. 5b). According to the GCD curves of the CAC/PANI//WO3 ASC were recorded with

Fig. 4 Comparative CV curves of CAC/PANI and WO3 electrodes performed in three electrode cells in 1 M H2SO4 electrolyte at a scan rate of 5 mV s−1

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Fig. 5 a CV curves of the CAC/PANI//WO3 ASC at different scan rates. b GCD curves of the CAC/PANI//WO3 ASC at different current densities. c GCD curves of the CAC/PANI//WO3 ASC at a current density of 0.5 A g−1. d Potential drop associated with CAC/PANI//WO3 ASC internal resistance (IR loss) vs. different discharge current densities. e Ragone plot related to energy and power densities of the CAC/PANI// WO3 ASC. f Cycling stability of the CAC/PANI//WO3 ASC test at a current density of 5 A g−1

various current densities, the specific capacitances of 274, 229, 203, and 173 F g−1 are obtained at current densities of 0.5, 1, 1.5, and 2 A g−1. From the galvanostatic charge/ discharge curve of the asymmetric supercapacitor at a current density of 0.5 A g−1 (Fig. 5c), the discharge curve is almost symmetric with its corresponding charge counterparts. In addition, a good linear relation of charge/discharge potentials versus time is observed in the H2SO4 electrolyte, indicating a rapid I-V response, small equivalent series resistance, and ideal capacitive characteristic, which are also confirmed by CV curves (Fig. 5a). The internal resistance values were also determined from the slope and the initial voltage drops of the discharge curves at different discharge current densities (Fig. 5d). The CAC/PANI//WO3 ASC has very little internal resistance (IRdrop[V] = 0.0965 + 0.5126 I) from a linear fit to the IR drop values, which favors high discharge power delivery in practical applications [40]. The Ragone plot of the device describing the relationship between energy density and power density was obtained and is shown in Fig. 5e. The energy and power densities were calculated from the discharge curves at different current densities. The specific energy density (E, Wh kg−1) and power density (P, W kg−1) for a supercapacitor cell can be calculated using the following equations: E = 1/2 CΔV2 and P = E/t, where C is the specific capacitance of the supercapacitor cell, ΔV is the scanned potential window (excluding IR drop at the beginning of the discharge process) in V, and t is the discharge time. It is obvious that the CAC/PANI//WO3 ASC exhibits the highest energy density of 15.4 Wh kg −1 with a power density of 252 W kg−1. Figure 5f shows the capacitance retention ratio of the asymmetric capacitor charged at 1.5 V as a function of

the cycle number. The asymmetric cell exhibits excellent electrochemical stability with only 5.6% deterioration of the initial available specific capacitance after 600 cycles. It can be seen that the asymmetric cell exhibits excellent cycling stability with 83% capacitance of its initial value after 1500 cycles. To further understand the resistive and capacitive behavior of the CAC/PANI//WO3 ASC device, electrochemical impedance spectroscopy (EIS) measurement was conducted with a frequency range from 100 kHz to 0.01 Hz. As shown in Fig. 6, the Nyquist plot of the CAC/PANI//WO3 ASC shows a small semicircle in the high-frequency region and a greater than 45° vertical curve in the low-frequency region, which results indicating a low charge-transfer resistance in the electrochemical system and a pronounced capacitive behavior with small diffusion resistance, respectively [41, 42]. The obtained Nyquist plot was further modeled and analyzed by the software of ZSimpWin on the basis of the electrical equivalent circuit (the inset of Fig. 6) [43], where Rs stands for a combined resistance of ionic resistance of the electrolyte, intrinsic resistance of the substrate, and contact resistance at the active material/current collector interface [44–46]. Rct is the charge transfer resistance ascribed to the process occurring at the electrode|electrolyte interface [44, 46–48]. Cdl is the electric double-layer capacitance, and CL is the limit capacitance [43]. The slope of the 45° portion of the curve is called the Warburg resistance (W) and is a result of the frequency dependence of ion diffusion/transport in the electrolyte to the electrode surface [49]. Based on the fitted impedance parameter values, the CAC/PANI//WO3 ASC not only has a low inner resistance (Rs, 8.5 Ω cm2), but also possesses a small interfacial charge transfer resistance (Rct, 25.3 Ω cm2). Simultaneously, the small Warburg coefficient (Aw, 0.8 Ω s-1/2 cm2) suggests that a faster ion diffusion/transport to the electrode surface existed in the CAC/PANI//WO3 ASC system [50].

Fig. 6 Nyquist plot of CAC/PANI//WO3 asymmetric supercapacitor for the two-electrode system; the inset shows an equivalent circuit used to fit the Nyquist spectra

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Conclusions A novel ASC device based on the CAC/PANI composite with a porous structure as the anode and WO3 with an interpenetrating network as the cathode has been successfully developed in an aqueous H2SO4 solution. Surprisingly, the asfabricated supercapacitor with a wide voltage range of 0~1.5 V exhibits outstanding electrochemical performance such as a large specific capacitance (274.3 F g−1), a high energy density (15.4 Wh kg−1), and good cycling stability (along with 83% specific capacitance retained after 1500 cycles). These encouraging findings can open up the possibility of cheap conductive polymers and metal oxides for applications in an asymmetric supercapacitor with low cost and high energy density to meet the diverse demands for nextgeneration energy storage systems. Acknowledgements The research was financially supported by the S c i e n c e a n d Te c h n o l o g y P r o g r a m o f G a n s u P r o v i n c e (NO.1308RJZA295, 1308RJZA265), the National Science Foundation of China (NO.21164009, 21174114), the program for Changjiang Scholars and Innovative Research Team in University (IRT1177), Key Laboratory of Eco-Environment-Related Polymer Materials (Northwest Normal University) of Ministry of Education, and Key Laboratory of Polymer Materials of Gansu Province, College of Chemistry and Environmental Science, Lanzhou City University.

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