polyvinylpyrrolidone blending membranes: Preparation, characterization and electrochemical capacitive performance

Journal of Energy Chemistry 23(2014)684–693

A hierarchical porous carbon membrane from polyacrylonitrile/ polyvinylpyrrolidone blending membranes: Preparation, characterization and electrochemical capacitive performance Huili Fanb ,

Fen Rana∗ , Xuanxuan Zhangb , Haiming Songb , Wenxia Jingb , Kuiwen Shenb , Lingbin Konga, Long Kanga

a. State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, Lanzhou University of Technology, Lanzhou 730050, Gansu, China; b. School of Materials Science and Engineering, Lanzhou University of Technology, Lanzhou 730050, Gansu, China [ Manuscript received June 2, 2014; revised July 23, 2014 ]

Abstract Novel hierarchical porous carbon membranes were fabricated through a simple carbonization procedure of well-defined blending polymer membrane precursors containing the source of carbon polyacrylonitrile (PAN) and an additive of polyvinylpyrrolidone (PVP), which was prepared using phase inversion method. The as-fabricated materials were further used as the active electrode materials for supercapacitors. The effects of PVP concentration in the casting solution on structure feature and electrochemical capacitive performance of the as-prepared carbon membranes were also studied in detail. As the electrode material for supercapacitor, a high specific capacitance of 278.0 F/g could be attained at a current of 5 mA/cm2 and about 92.90% capacity retention could be maintained after 2000 charge/discharge cycles in 2 mol/L KOH solution with a PVP concentration of 0.3 wt% in the casting solution. The facile hierarchical pore structure preparation method and the good electrochemical capacitive performance make the prepared carbon membrane particularly promising for use in supercapacitor. Key words porous carbon; supercapacitors; phase separation; electrode materials; additive

1. Introduction Energy consumption is forecasted to have severe problems in future for economic development and ecology protection of the world. Thus, the development of new inexpensive, flexible, light-weight and environmentally friendly energy storage devices is currently a strong demand [1−5]. Supercapacitors are under serious consideration as an alternative energy/power source, because this kind of energy consumption is designed to be more sustainable and more environmentally friendly. The electrode material plays a key role in supercapacitors. Hence for the capacitive performance of supercapacitors, the nature of electrode material is very important [6]. Normally, the electrode materials of supercapacitors can be categorized into three types as follows [7]: carbon materials, conducting polymers and metal oxides [8−10]. It is well known that a high specific surface area is the ba-

sic requirement for the carbon-based electrode materials since supercapacitor stores energy physically using reversible adsorption and desorption of the electrolyte ions onto the surface of active materials [11]. Among these carbon materials, activated carbons with a surface area of 3000 m2 /g have been successfully prepared as electrodes for commercial supercapacitor devices. However, a poor rate performance has been observed for such a kind of high specific surface area porous carbon materials, mainly due to the high ion-transport resistance and insufficient ionic diffusion within the micropores, which in turn limit the pore accessibility of the electrolyte ions at high scanning rates [12]. In order to develop high-performance energy-storage carbon-based electrode materials, the challenge is how to achieve the desirable properties such as large specific surface area, high conductivity and efficient porosities. Obviously, the microstructure and synthesis method of carbon materials are particularly crucial.

Corresponding author. Tel: +86-931-2976579; Fax: +86-931-2976578; E-mail: [email protected], [email protected] This work was supported by the National Natural Science Foundation of China (51203071, 51363014 and 51362018), China Postdoctoral Science Foundation (2014M552509), the Key Project of Chinese Ministry of Education (212183) and the Natural Science Funds for Distinguished Young Scholars of Gansu Province (1111RJDA012). ∗

Copyright©2014, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi: 10.1016/S2095-4956(14)60200-X

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The phase-inversion process is induced by immersion precipitation, which is a well-known technique for preparing asymmetric polymeric membranes [13,14]. The porous polymer scaffolds can be fabricated into an asymmetric membrane by the phase separation process. The fast exchange between solvent and non-solvent leads to the generation of porous tunnels within the matrix during the phase separation process, forming a continuous and interconnected porous structure. Furthermore, the as-prepared composite can be heat-treated to carbonize the carbon precursor. During the carbonization procedure, both the molecular reaction as well as volume shrinking can happen. Thus, 3D nanoscale architecture porous structure is produced. The desirable morphology and good performance of the resulting membrane can be easily controlled by changing the compositions of constituents and/or additives, being set off by exchange between solvent and non-solvent, and heat transfer through temperature variation [15−19]. Recently, introduction of various additives as a third component, such as NaCl, LiCl, ethanol, ethylene glycol, PVP, poly (ethylene glycol) (PEG), etc., to a casting solution to enhance the desired properties of the membranes has been received more and more attention. As mentioned in the reported study [20], using additives as a pore-forming agent will enhance the permeation properties of membranes. We combine these processes as a simple and economical method for synthesizing porous carbon materials. In this work, we attempted to construct a porous carbon with interconnected and hierarchical 3D nano-architecture by phase inversion with PVP-added in casting solutions. The effects of PVP added in casting solutions on structure feature and electrochemical performance of the as-prepared carbon membranes were also studied. High capacitance, excellent rate performance and long-term stability of the prepared electrochemical materials were achieved in 2 mol/L KOH aqueous solution. 2. Experimental 2.1. Materials Analytical grade 2,2-azobisisobutyronitrile (AIBN), polyvinylpyrrolidone (PVP-30), N,N-dimethylformamide (DMF) and absolute alcohol were purchased from Sinopharm Chemical Reagent Co. Ltd. and used as received without any further purification. Vinylcyanide (AN) purchased from Sinopharm Chemical Reagent Co. Ltd. was purified via distillation before use. 2.2. Synthesis of polyacrylonitrile (PAN) precursor PAN was synthesized as follows: AN (9.6 g) and AIBN (0.028 g) were dissolved in 30 mL DMF with magnetic stirring at 70 ◦ C for 6 h, and then the sample was precipitated by ethanol. And the resulting PAN precipitate was washed thoroughly with distilled water several times. Finally, the product was dried under vacuum at 30 ◦ C.

2.3. Preparation of PAN membranes (PM) PAN membranes were prepared by a phase-inversion technique. The percents of components in solutions were calculated as follows P (%) =

MP × 100% MP + MS + MA

(1)

S (%) =

MS × 100% MP + MS + MA

(2)

A (%) =

MA × 100% MP + MS + MA

(3)

where, P (%) and S (%) are the concentrations of PAN (15 wt%) and solvent (DMF) in solutions, respectively. Meanwhile, A (%) means the concentration of the additive (PVP) in solutions. First, these prepared solutions were stirring at 70 ◦ C until a clear homogeneous solution was obtained. The bubbles in solutions were removed by vacuum degassing. Then, the membranes were prepared with the casting solutions by spin coating coupled with a liquid-liquid phase separation technique at room temperature. And subsequently the solution films were immediately immersed in a deionized water bath for 24 h. After complete coagulation, the membranes were peeled off and then dried for 24 h at room temperature before characterization. The morphology and nanostructure of PM can be tuned by adjusting the additive concentrations in the casting solutions of 0, 0.1 wt%, 0.3 wt%, 0.7 wt%, 1.0 wt% and 5 wt%, which were named as PM-0, PM-1, PM-2, PM-3, PM-4 and PM-5, respectively. 2.4. Preparation and modif ication of hierarchical porous carbon membrane Hierarchical porous carbon membranes (HPCMs) were further prepared through a simple polymer carbonization method involving two steps as follows: (1) preoxidization, the synthesized PAN membranes were heated and preoxidized at 200 ◦ C in air for 5 h; (2) carbonation, the members were pyrolyzed at 800 ◦ C under a flowing nitrogen for 2 h and then the samples were cooled down to room temperature. To introduce oxygen-containing functional groups on carbon surface, the carbon membranes were oxidized by nitric acid. 0.1 g of dried carbon membrane was treated with 10 mL HNO3 solution at 80 ◦ C for 6 h through refluxing process. After being treated with acid, the samples were recovered and washed thoroughly with distilled water until the pH value was close to 7, and the resulting products were further dried at 60 ◦ C for 16 h. The HPCMs fabricated by PM-0, PM-1, PM-2, PM-3, PM-4 and PM-5, were named as HPCM-0, HPCM-1, HPCM-2, HPCM3, HPCM-4 and HPCM-5, respectively. 2.5. Structure characterization The microstructure and morphology of the as-prepared HPCMs were characterized by field emission scanning elec-

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tron microscope (SEM, JEOL, JSM-6701F, Japan). Functional groups on HPCMs surface were characterized by FT-IR technique. FT-IR spectra were recorded on a Nicolet Nexus 670 Fourier transform infrared spectrometer using KBr tabletting technique. Pore structures of the samples were characterized by Nitrogen adsorption/desorption experiments at 77 K (Micromeritics, ASAP 2010M, USA). The surface area was calculated using the Brunauer-Emmett-Teller (BET) equation. Pore size distribution was calculated by the Barrett-JoynerHalenda (BJH) method using the adsorption branch of the isotherm. Thermo gravimetric analysis (TGA) and differential scanning calorimetry (DSC) of the samples were performed in a nitrogen atmosphere on a NETZSCH STA 449F3 instrument with a heating rate of 10 ◦ C/min. The surface chemical compositions of the samples were analyzed by X-ray photoelectron spectroscopy (XPS, physical Electronics UK). 2.6. Electrode preparation and electrochemical measurements The working electrodes were prepared as follows: 80 wt% HPCM was mixed with 7.5 wt% acetylene black and 7.5 wt% conducting graphite in an agate mortar until a homogeneous black powder was obtained. During electrode preparation and electrochemical measurements process, a suspension of polytetrafluoroethylene was used as a fibrous binder. And into this mixture 5 wt% poly(tetrafluoroethylene) was added together with a few drops of ethanol. The resulting paste was pressed at 10 MPa into nickel foam (Changsha Lyrun New Material Co. Ltd., 90 PPI, 2 mm) then dried at 80 ◦ C for 12 h. Each carbon electrode contained approximately 8 mg electroactive material and had a geometric surface area of about 1 cm2 . A typical three-electrode glass cell equipped with a working electrode, a platinum foil counter electrode, and a saturated calomel reference electrode (SCE) was used for electrochemical measurements of the as-prepared working electrodes. All electrochemical measurements were performed using an electrochemical working station (CHI660C, Shanghai, China) in 2 mol/L KOH aqueous solution at 25 ◦ C. The corresponding specific capacitance was calculated from the following equation: I ×D t C = Cm = m m×D V

(4)

where, Cm (F/g) is the specific capacitance, I (A) is the discharge current, D t (s) is the discharge time, D V (V) represents the potential drop during the discharge process, and m (g) is the mass of the active material [21−23]. 3. Results and discussion 3.1. Preparation and characterization of materials Preparation of hierarchical porous carbon membranes (HPCMs) from phase-separated blending polymer membrane

containing a carbon source (PAN) and a sacrificial additive (PVP) is illustrated in Scheme 1. The basic steps were as follows. First of all, casting solutions including PVP and PAN were prepared into membranes by spin coating technique, which were immediately immersed into deionized water coupled with the phase separation process. After immersed in deionized water for several hours, PAN membranes (PM) were obtained. During PM preparation process, PVP was used as a hydrophilic additive with non-solvent properties (additive that similar to non-solvent has high and low affinity to the solvent (DMF) and the polymer (PAN), respectively) [24], to adjust pore size of membranes (suppression or enlargement) [25]. During the phase separation process, the fast exchanges among solvent, additive and non-solvent leads to the generation of porous tunnels within the matrix, thus forming a continuous and interconnected porous structure. Then, the synthesized polymer membranes was stabilized and pyrolyzed for carbonization. Because of molecular reaction and volume shrinking during the carbonization procedure, hierarchical porous structure was produced. Therefore, the specific surface area and pore volume of the obtained hierarchical porous carbon can be readily mediated by changing the additive amount in casting solution.

Scheme 1. Illustration of HPCM fabrication process

Fourier transform infrared spectroscopy (FT-IR) was employed to analyze the surface groups of the prepared HPCM, and the recorded full FT-IR spectrum is shown in Figure 1. The broad band observed around 3432.0 cm−1 is mainly attributed to the O−H stretching vibration of the adsorbed water molecule. The weak bands at 1722.1 cm−1 is assigned to the absorption peak of C = O stretching vibration of –COOH radical. In addition, the band appeared at 1384.6 cm−1 is assigned to the stretching vibration and bending vibration of – NO2 on the surface of the material. Obviously, FT-IR characterization indicated that these functional groups containing oxygen and nitrogen could be found on the surface of HPCM, which should play important roles in improving the capacitance properties of HPCM. The surface chemical composition of HPCM-2 was also

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characterized by XPS and the orresponding spectrum is presented in Figure 2(a). It manifests that all the carbon surface were composed of C, N, O with the binding energy peaks appeared at 284.6, 400.0 and 530.0 eV, respectively. The highresolution spectrum of O 1s for HPCM-2 and its deconvolution results are also shown in Figure 2(b). The strong O 1s signal suggested that the substratum surface was covered with O-containing functional groups. The presence of two dominant peak components in O 1s core-level spectrum with binding energies (BEs) at 533.2 and 533.8 eV, could be attributable to the C–OH and O–C = O species, respectively [26]. These results confirmed the existence of N, C and O elements and Ocontaining functional groups on HPCM surface. Indeed, these O-containing functional groups binding to the material surface could have certain effects on their electrochemical capacitive behaviors.

Figure 1. FT-IR spectrum of HPCM

Figure 2. XPS profiles of HPCM-2. (a) General XPS profile ranging from 0−1500 eV, (b) O 1s XPS profile and its deconvoluted profiles

The weight loss of the sample associated with corresponding thermal treatment was studied by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) in the temperature range of 25∼900 ◦ C. The results of TGA and DSC measurements are shown in Figure 3. There were three distinct weight-loss stages in the TGA profile. First, the sample weight decreased gradually up to ∼270 ◦ C. Second, a sharp sample weight reduction occurred between the temperature range of 270−470 ◦ C. And third, an additional step in weight loss could be observed at ∼700 ◦ C. Meanwhile, the DSC data showed that there were two broad exothermic peaks in ∼270 ◦ C and ∼700 ◦ C. The significant weight loss stage below ∼270 ◦ C is mainly due to the loss of crystal water and partial dehydrogenation and cross-linking. The weight loss in the temperature range of 270−470 ◦ C can be attributed to the decomposition of PVP in the membrane. At temperature exceeding ∼470 ◦ C the weight loss can be assigned to the carbonization of PAN accompanying with further dehydrogenation and partial denitrogenation.

Figure 3. TGA and DSC curves of HPCM-2

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Figure 4. SEM images of cross-sectional view (1) and zoom-in views of top-layer (2) and finger pore (3) of (a) PM-2, (b) HPCM-2, (c) HPCM-0, (d) HPCM-1, (e) HPCM-3, (f) HPCM-4 and (g) HPCM-5

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The morphologies and microstructures of PM and the resulting HPCM were examined by scanning electron microscope (SEM) analysis. The corresponding SEM crosssectional images of the prepared membranes are presented in Figure 4. It can be seen from the images that the microstructures of PAN membranes were mainly maintained in the carbon membranes after the pyrolysis process, which contains the characteristic morphology of asymmetric membranes with a dense bottom layer and finger-like pore layer. The zoom-in views of finger pore for the membranes indicated that a large number of pores existed on the surface of finger pore walls in carbon membrane, resulting from the hydrophilic additive loss, the molecular reaction and volume shrinking during the carbonization procedure [27,28]. SEM images were also used to investigate the effect of PVP concentration in casting solution on membrane morphologies. During the phase separation process, the exchange rate between solvent and non-solvent plays a crucial role in controlling the resulting membrane morphology [29−32]. In the casting solution system, the presence of PVP makes

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the blending system have a better thermodynamic instability and consequently lead to instantaneous demixing in the coagulation bath, which can further lead to the formation of macrovoids in the membrane structure [33]. So, compared with the non-PVP membrane HPCM-0, the addition of PVP in the casing solution results in the formation of macropores and more porous structures. It can be observed that the interconnected pores on the surface of carbon membrane surface were formed by fast solvents exchange during phase-conversion procedure, and molecular reaction and volume shrinking during carbonization procedure. Comparing the structure features of PVP-containing membranes, the most obvious change is the zoom-in views of finger pore for membranes. With increasing PVP concentration from 1.0 wt% to 5.0 wt% in the casting solution (i.e., for HPCM-4 and HPCM-5 samples), a large number of new macropores (>500.0 nm) could be observed on the surface of finger pores, resulting from the hydrophilic additive loss. Meanwhile the mesopores also existed on the surface of enlarged macrovoid surface areas.

Figure 5. Nitrogen adsorption/desorption isotherms and corresponding pore size distribution of (a) HPCM-0, (b) HPCM-1, (c) HPCM-2 and (d) HPCM-5

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Nitrogen sorption experiments were performed to evaluate the overall porosity of the carbon samples and the results are shown in Figure 5. Generally, the N2 uptake at pressure range of 0

100 nm), high BET surface area (332.9 m2 /g) and big total pore volume (0.233 m3 /g). It should be expected that maintaining such a hierarchical pore structure can potentially offer enhanced electrolyte access to the high interfacial area, which may improve the charge transport and power capability. Meanwhile, ion-buffering reservoirs are formed in the larger macropores (above 100 nm), which can reduce the diffusion distances to the interior surfaces. Table 1. Pore structure parameters of different HPCM samples Samples HPCM-0 HPCM-1 HPCM-2 HPCM-5

SBET (m2 /g) 178 83 333 432

Vtotal (cm3 /g) 0.141 0.066 0.233 0.232

Vmic (cm3 /g) 0.044 0.060 0.111 0.150

d (nm) 3∼35 10∼60 3∼60 3∼5

3.2. Electrochemical performance Figure 6 is the cyclic voltammograms (CV) of the asprepared carbon materials in 2 mol/L KOH solution at scan rates ranging from 5 to 50 mV/s. It shows that the CV curves of HPCM-2 electrode deviated from the idealized doublelayer behavior. In addition, pseudo-capacitance from the N and/or O containing functional groups was also observed, resulting from a pair of broad, superimposed and reversible faradaic surface redox reactions [41]. These N and/or O containing groups can improve the wettability and hence, maximize the electroactive surface area of materials [42]. With the increase of sweep rates, the shape of CV curves was not significantly influenced, suggesting that it is a highly reversible system. However, the distortion in CV curves at high scan rates was observed, which is due to the limited diffusion and migration of electrolyte ions in the bulk of the carbon materials. Actually, this is a common disadvantage of carbon monoliths needing further improvement.

Figure 6. Cyclic voltammograms of HPCM-2 in 2 mol/L KOH solution at different scan rates

Figure 7 shows the charge-discharge curves of HPCM-2 electrode within a potential window of −1.0 to 0 V at various current densities. It is clear that the charge-discharge curves had a shape like an isosceles triangle, indicating its electric double-layer capacitive behavior, which is in accordance with CV curves. It is well known that the N and/or O containing functional groups are much essential for increasing the specific capacitance of HPCM due to its active feature in alkaline electrolyte [43]. The as-prepared HPCM exhibited excellent capacitors behavior at different current densities, and the specific capacitance values calculated from discharge curves were 278, 261, 245, 232 and 206 F/g at current densities of 5, 10, 20, 30, 40 and 50 mA/cm2 , respectively. When the current density increased to 50 mA/cm2 , the specific capacitance could still retain 74.1% of the initial specific capacitance at a current density of 5 mA/cm2 , revealing good retention capability of the materials. The slightly decrement of the specific capacitance at higher current densities is due to the insufficient mass transportation and the voltage drop at high current density.

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Figure 7. Galvanostatic charge-discharge curves of HPCM-2 in 2 mol/L KOH solution at different scan rates

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diameter for the sample was small, indicating low impedance on the electrode/electrolyte interfaces (Rct ). The spectrum of the electrode had an angle higher than 45o , suggesting lower Warburg impedance of HPCM-2, resulting from the effective nanoporous structure which facilitates the transport of electrolyte ions. To optimize of the pore structures, the concentration of PVP on specific capacitance of HPCM was systematically investigated. From the above results, it is found that when the content of PVP in the composite was 0.3 wt%, well-connected hierarchical porous structure of porous carbon could be formed. The hierarchical pore structure can potentially offer enhanced electrolyte access to the high interfacial area, which may improve the charge transport and power capability. Meanwhile, ion-buffering reservoirs were formed in the larger macropores, which can reduce the diffusion distances to the interior surfaces. The specific capacitances of the prepared carbon materials modified at different conditions are shown in Figure 9. The specific capacitance increased to 278 F/g and the capacitance retention ratios relative to the current density of 50 mA/cm2 were 74.1%, indicating the suitability of the prepared porous carbon as electrode materials for supercapacitors. However, with the PVP content increased, the specific capacitance decreased. The result could be attributed to lower size distribution and the structure of the pore, which are in well agreement with the above characterization of samples. These results indicate that the additive has an important role in the electrochemistry process of the composite materials.

Figure 8. Complex-plane impedance plot of HPCM-2

To further investigate the capacitive behavior of HPCM, EIS test was also carried out over a frequency range from 10 kHz to 10 mHz (Figure 8). It can be seen from the figure that a semi-circle intersecting the real axis in high frequency range was observed and then the plot transformed to a vertical line with decreasing frequency. The internal resistance (Rb ) of the electrode material involves the intrinsic resistance of active material, the total resistances of the ionic resistance of the electrolyte, and the contact resistance at the active material/current collector interface, can be estimated from the point intersecting with the real axis in the range of high frequency. Only the electrochemical process occurring on the exterior surface of electrodes can be sensed at high frequencies, so the semicircle is always suggested to represent the faradic charge transfer resistance (Rct ) at the interface between current collector and HPCM, as well as that within HPCM material. Thus, the semicircle may be result from faradaic reaction as well as powder-like structure of the electrode. At lower frequencies, a straight sloping line was observed, representing the diffusive resistance (W) of the electrolyte in electrode pores and the proton diffusion in host material [44,45]. As shown in Figure 8, the short x-intercept (1.10 W ) reveals that the electrodes of HPCM had relatively low Rb . The semicircle

Figure 9. Specific capacitances of HPCMs modified at different conditions

The stability and reversibility of an electrode material are very important for its practical use in a supercapacitor. So the cyclic performance of the prepared carbon material was further examined by galvanostatic charge-discharge tests for 2000 cycles at discharge current density of 10 mA/cm2 and the results are shown in Figure 10. It is found that the prepared carbon electrode exhibited excellent long cycle life with 92.9% specific capacitance retained after 2000 cycles, indicating its excellent cycle stability and a very high degree of

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reversibility in the repetitive charge-discharge cycling. The above results demonstrate that the superior electrochemical performance of porous carbon materials can be attributed to its high specific surface area, unique hierarchical porous nanostructure, and it is very promising as an advanced electrode for supercapacitors.

Figure 10. Cycle life of HPCM-2 at a current density of 10 mA/cm2

4. Conclusions Various hierarchical porous carbon membranes were prepared by different amount of PVP in the casting solution, the pore structure of the as-prepared membranes were characterized through SEM and BET methods, and the electrochemical properties were examined using CV, Galvanostatic charge-discharge and EIS tests. The results showed that the high specific surface area, high pore volume and reasonable pore distribution were readily obtained. Interestingly enough, the pore size and distribution could be tuned easily just by changing the PVP concentration in the casting solution. The unique well-defined interconnected hierarchical porous structure feature of the prepared carbon materials endowed the carbon materials with good capacitive performance. A high electrochemical capacitance of 278.0 F/g at the current density of 5 mA/cm2 with good retention capability of 74.1% at current densities up to 50 mA/cm2 and good cycling stability of 92.9% capacity retention after 2000 cycles were obtained. The facile hierarchical pore structure preparation method and the good electrochemical capacitive performance make the prepared carbon membrane particularly promising for use in supercapacitor. References [1] Tarascon J M, Armand M. Nature, 2001, 414(6861): 359 [2] Nam K T, Kim D W, Yoo P J, Chiang C Y, Meethong N, Hammond P T, Chiang Y M, Belcher A M. Science, 2006, 312(5775): 885 [3] Nishide H, Oyaizu K. Science, 2008, 319(5864): 737 [4] Rogers J A, Someya T, Huang Y G. Science, 2010, 327(5973): 1603

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