Nitrogen-Doped Carbon Membrane Derived from Polyimide as Free-Standing Electrodes for Flexible Supercapacitors.

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Supercapacitors

Nitrogen-Doped Carbon Membrane Derived from Polyimide as Free-Standing Electrodes for Flexible Supercapacitors Yingzhi Li, Jie Dong, Junxian Zhang, Xin Zhao, Pingping Yu, Lei Jin, and Qinghua Zhang*

Nitrogen-doped carbon materials have attracted great interest in the energy storage due to the better electrochemical performances than the pristine carbon materials. In this work, a heterocyclic polyimide containing benzopyrrole and benzimidazole rings is carbonized to fabricate the free-standing and flexible carbon membrane (CarbonPI) with a high packing density (0.89 cm−3), in which the location of nitrogen atoms in the doped configurations is easily controlled. XPS analysis indicates that quaternary nitrogen is the predominant nitrogen-doped configurations. The high content of nitrogen effectively improves the wettability of the electrode materials. The CarbonPI membrane exhibits excellent volumetric capacitance (159.3 F cm−3 at 1 A g−1), high rate capability (127.5 F cm−3 at 7 A g−1), and long cycle life. TEM images reveal the very slight change of the microstructure of graphitic nanosheet of CarbonPI during the long charge/discharge cycles.

1. Introduction Currently, the development of portable electronics promotes the increasing demands for high-performance energy-storage systems, such as lightweight, ultrathin, flexible, wearable, and even foldable features.[1–6] Among various storage devices, supercapacitors are promising alternatives as they can provide higher power densities, faster charge/discharge rates, and longer lifetimes. However, a key challenge is to meet the practical requirements for portable devices without compromising other electrochemical characteristics such as high energy density and long cycle life. In a novel approach, the

Dr. Y. Li, Dr. J. Dong, Dr. J. Zhang, Dr. X. Zhao, Dr. P. Yu, L. Jin, Prof. Q. Zhang State Key Laboratory for Modiﬁcation of Chemical Fibers and Polymer Materials College of Materials Science and Engineering Donghua University Shanghai 201620, P. R. China E-mail: [email protected] DOI: 10.1002/smll.201403575 small 2015, DOI: 10.1002/smll.201403575

free-standing electrodes serve as both electrodes and current collectors, and no need for extra binder or conductive additives, which significantly improve the volumetric energy densities. For instance, carbon nanotube (CNT) or reduced graphene oxide (rGO) are assembled on the surface of yarn or cellulose fibers to obtain lightweight, flexible, and foldable electrodes.[7–11] However, the insulating skeletons contribute nothing to capacity but increase the impedance of the devices. In order to overcome these disadvantages, various porous carbon monoliths including graphene, CNT or activated carbon-based films were prepared by templateinduced methods.[12] The electrode materials with high electrical conductivity and accessible surface area can achieve high electrical double layer (EDL) capacitances, which can further enhance the pseudocapacitance by loading transition metal oxides and/or electronically conducting polymers.[13,14] However, the excess micropores of electrodes decrease the volumetric capacity, and pseudocapacitive components suffer from the short cyclic stability and the low stability of devices.[15–19] Recently, thin film and fiber-shaped supercapacitors with high packing density electrodes have attracted great attentions for relatively high volumetric energy density and long lifetimes, based on rGO and/or CNT films by

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fibers.[20–22]

filtration, CNT fibers, rGO fibers, carbon These porous and compact active materials can maximize the utilization of limited volume of storage devices. For carbon materials, particularly thin film and fibershaped electrodes, the surface wettability is the critical factor.[23] Nitrogen doping is an effective way to tailor the electronic properties and improve the interfacial interaction of carbon materials.[24–26] A general strategy is in-situ preparation of N-doped graphene and CNT used ammonia,[25] acetonitrile,[27] dicyandiamide,[28] melamine,[29] or complex molecular as precursor via a chemical vapor deposition (CVD) method.[30,31] The N-doped electrodes show higher capacitance than the pristine graphene or CNT.[32,33] Another widely used approach is to etch or to anneal carbon materials in the nitrogen-containing atmosphere by the plasma or thermal treatment, respectively.[33–35] The carbon materials with high nitrogen concentrations facilitate to anchor metal oxides and improve electrical conductivity of the compounds, which can further enhance the energy and power densities.[36,37] The third way is the pyrolysis of nitrogen heterocyclic compounds, nitrogen-containing polymers or biomass derivatives (chitosan, glucosamine or crawfish shell) to keep nitrogen element on the basal plane of carbon materials, increasing rich defects, and improving electrode/electrolyte wettability.[38,39] The theoretical calculations demonstrates that the doping with nitrogen significantly enhances the electronic density of states near the Fermi level of N-doped materials and thus greatly enhances the interfacial capacity.[40] The above-mentioned and many other reports on N-doped methods do not allow controlling of the exact N-doping configurations. However, the high temperature annealing and the precursor with nitrogen heterocyclic facilitate the adjustment of N-doped species. In this work, the

heterocyclic polyimide (PI) containing benzopyrrole and benzimidazole rings was used as a precursor to control the location of nitrogen atoms in the doped configurations. We fabricated the precursor polyimide membranes by electrospinning, subsequently, carbonized them via the carbonization technologies of applied pretension and preoxidization to obtain the free-standing and flexible carbon membranes (CarbonPI). N-doped configurations with quaternary N as the predominant type was achieved probably due to the diamine monomers of PI with benzopyrrole and benzimidazole rings whose excellent thermal stability contributed to remain nitrogen atom in the heterocyclic ring during the high temperature. The high concentration of nitrogen promotes the wettability of electrode and facilitates its double layer capacity and the ultracyclic stability of supercapacitors.[41,42] In view of the advantage of no binder and no conductive additive, the microstructure of electrode is further investigated by HR-TEM/-SEM after charge/discharge cycles.

2. Results and Discussion 2.1. Morphology and Structure Aromatic polyimide (PI) has attracted considerable interests for applications in aerospace and microelectronics industry owing to its high-strength and high-modulus mechanical properties, excellent thermal stability, and high dielectrics.[43] These unique performances are due to the introduction of aromatic heterocyclic structure into the molecular backbone of PI. In this work, we synthesized the PI containing with benzopyrrole and benzimidazole rings (Figure 1d), and subsequently fabricated PI membranes by electrospinning

Figure 1. Schematic of preparation of CarbonPI membrane. a) Electronspinning PI membrane; b) carbonization of CarbonPI membrane in alumina furnace at 1000 °C; c) the photograph of CarbonPI membrane and schematic types of nitrogen doping; d) the chemical structure of PI polymer.

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Figure 2. Microstructures of the CarbonPI membrane. a,b) SEM image of CarbonPI membrane and nanofibers surface; c,d) cross-section SEM image of CarbonPI membrane; e) SEM image of CarbonPI membrane and the corresponding EDS mapping f) of carbon; and g) nitrogen, respectively. The contact angle of CarbonPI membrane (Figure 2e, inset).

(Figure 1a). PI membranes are smooth, compacted and flexible (Figure S1a, Supporting Information), exhibiting the high mechanical and thermal properties. After carbonization (Figure 1b), the PI membrane translated into the freestanding and flexible CarbonPI membrane (Figure 1c). During the carbonization process, the as-prepared PI membranes were applied pretension and preoxidization. These carbonization technologies derived from the fabrication technologies of carbon fibers are benefit to enhance the mechanical properties of CarbonPI membrane. As the precursor of CarbonPI membrane, the porous PI membranes composed of the random fibers with the diameters of in the range from 1 to 2 µm (Figure S1b, Supporting Information). High-magnification SEM image shows the rough surfaces of fibers (Figure S1d, Supporting Information), mainly due to the solvent evaporation. After carbonization, the yellow PI membrane converts into the black CarbonPI membrane. As shown in SEM images (Figure 2a), CarbonPI membrane is composed of the smooth and uniform carbon fibers, which form a continuous network structure. However, nitrogen adsorption isotherm reveals a specific surface area of 442 m2 g−1, and the pore-size distribution indicates the mesoporous feature (Figure S2, Supporting Information). The range of diameter of carbon fibers is about 0.5–1 µm, thinner small 2015, DOI: 10.1002/smll.201403575

than the pristine PI fibers (Figure S1, Supporting Information). The results are consistent with the previous reports: the carbonized materials generally lost more than their half mass after high temperature carbonization.[44] The thickness of CarbonPI membrane is about 30–40 µm (Figure 2c). Highmagnification SEM images of surface and across-section indicate the carbon fibers are compact without obviously pore structures, which facilitate the stability of geometric structure of CarbonPI membrane (Figure 2b,d). The images of EDS elemental mapping reveal that the N atoms are homogeneously distributed over the whole carbon fibers (Figure 2e–g). The doping with N atoms improves electronic density of the active materials, and tailors their chemical performances. The four-point probe electrical conductivity of CarbonPI membrane reaches up to 2500 S m−1. As the bond energy of C−N is higher than that of C−C, the incorporation of N atoms into carbon materials improve the surface energy, increasing the wettability of electrode. The contact angle test shows that the angle of CarbonPI membrane is 78.8° (Figure 2e), implicating CarbonPI membrane with a hydrophilic surface.[45,46] In order to further investigate the surface compositions of CarbonPI membrane and the bonding configurations of N atoms, the high resolution XPS measurements were carried out (Figure 3). The spectrum shows strong signals of carbon,



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Figure 3. a) XPS spectra of the N-doped CarbonPI membrane: survey spectrum; b) C1s spectrum; c) N1s spectrum; d) Raman spectra.

nitrogen, and oxygen elements (Figure 3a). In the C1s spectrum (Figure 3b), the sharp peak at around 284.5 eV corresponds to the sp2 carbon with C = C bonds; a second weak peak at higher energy (285.8 eV) is attributed to the sp2 carbon with C = N bonds; a third small peak near at 289 eV is ascribed to the sp3 carbon.[32] High density sp2 carbons indicate the CarbonPI membrane with high graphitic structure, along with the good conductivity. The results are agreed with XRD patterns (Figure S3, Supporting Information) and the following HR-TEM images (Figure S6, Supporting Information). The fitting of the N 1s core level peaks shows two contributions at the same binding energies; however, with different relative contributions. The weak peak is near 398.3 eV, corresponding to pyridinic N (Figure 3c)[47,48] and the predominant peak is near 401.1 eV (Figure 3c),[49] which can be indexed to quaternary N. N atoms substitutes carbon atoms in the graphitic structure and bond to three carbon atoms (Figure 1c). Dai and co-workers[50] reported that the higher temperature annealing of GO (above ≈900 °C) afforded more quaternary N incorporated into the carbon network of graphene. For the fabrication of CarbonPI membrane with high conductivity, PI membrane is carbonized at 1000 °C, which leads to quaternary N-configurations becoming predominant N-doping species in CarbonPI membrane. Generally, high temperature is not beneficial to remain the high nitrogen content of N-doped materials. Fortunately, elemental analysis demonstrates the content of nitrogen in CarbonPI membrane, reaches up to 5.8 wt%, comparing with 7.85 wt% value of the pristine PI membrane. Raman spectroscopy is a direct and nondestructive technique to characterize the structure and quality of carbon


materials, particularly to determine the defects structure and doping level.[51] Figure 3d shows the Raman spectra of the initial CarbonPI membrane (red) and after 100 000 cycles (blue). Two typical peaks at around 1328 and 1580 cm−1 (the red plot) are attributed to the well-defined D band and G band, respectively.[52] The intensity of the D band is strongly associated with the disorder degree of graphitic plane while the G band corresponds to the first-order scattering of the stretching vibration mode E2g observed for sp2 carbon domains.[22] The intensity ratio of the G peak to the D peak (IG/ID) demonstrates the ordered extent of graphitic structure. Here, the IG/ID ratio of CarbonPI membrane is 0.95. Compared with previous reported doped-graphitic materials, CarbonPI membrane exhibits the relatively high graphitization.[30] The value of IG/ID ratio reduces to 0.90 after 100 000 cycle times, indicating the EDL capacitance derived from electrostatic adsorption during the galvanostatic charge/discharge process still slightly affects the microstructures of CarbonPI membrane. Meanwhile, the upshift of the G-band of ≈8 cm−1 is observed, further confirming the increase of defects after 100 000 cycles.[36,53] These phenomena are consistent with the following TEM results of electrode with various cycle times.

2.2. Electrochemical Performances The electrochemical performances were analyzed in the symmetric two-electrode and three-electrode system with two CarbonPI membranes. Cyclic voltammetry (CV) curves in three-electrode system exhibit a perfect shape, no redox reactions in the N-doped materials (Figure S4a, Supporting


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Figure 4. Electrochemical performances of CarbonPI membrane. a,b) Cyclic voltammetry curves of CarbonPI membrane at various scan rates from 20 to 1000 mV s−1 in 1 M NaOH, respectively; c) the galvanostatic charge–discharge plots at current densities from 0.75 to 7 A g−1; d) capacitance at various current densities. e) Nyquist plots of CarbonPI membrane electrodes in the frequency range of 100 kHz to 0.01 Hz; f) capacitance retention after 100 000 cycles; the corresponding galvanostatic charge–discharge plots at various current densities during the cyclic procedure (Figure 4f, inset).

Information), implicating that the capacitive response of CarbonPI membrane does not derive from pseudocapacitance but from the EDL capacitance. The CV curves from the twoelectrode configuration have a similar rectangle-like shape, confirmed the EDL capacitance of CarbonPI membrane (Figure 4a,b). The results are different from the previous reports,[29,38,54] maybe attributing to the fact that the most of N-doping species belong to quaternary N and it is less apt to take place redox reaction, similar to graphitic structure. The area under CV curves enlarges in proportion with the increase of scan rates from 20 to 1000 mV s−1, revealing the ideal capacitor behavior and fast charge/discharge property of CarbonPI membrane electrode. The galvanostatic charge/discharge curves have a symmetrical triangular shape (Figure 4c), and the slopes of curves keep constant during the charge/discharge process, further demonstrating that the storage energy arises from electrical double layer adsorption instead of Faradic reaction. No small 2015, DOI: 10.1002/smll.201403575

voltage drop indicates the good electrical conductivity of the electrode materials. The Coulombic efficiency increases from 94.9% at 0.75 A g−1 to 99.7% at 7 A g−1, implying the excellent electrochemical reversibility of CarbonPI membrane. The specific and volumetric capacitance are shown in Figure 4d. Significantly, the discharge capacitance does not display obvious reduction with the increase of current densities. The capacitance of CarbonPI membrane is 179 F g−1 at 0.75 A g−1 (corresponding 159.3 F cm−3 at 0.67 A cm−3), while slightly decreases to 143.3 F g−1 at 7 A g−1 (127.5 F cm−3 at 6.23 A cm−3). High rate capability is attributed to the stable charge double layer formation at the high accessible specific surface area (Figure S4, Supporting Information) and fluent ion transport through open porous structure. The free-standing electrode with no binders and no conductive additives effectively improves the utilization of the interface of electrode/electrolyte.[12] As a thin-film supercapacitor, CarbonPI membrane possess a relatively high packing density (0.89 g cm−3) and



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high specific capacitance. So it exhibits larger volumetric results are due to the double layer energy storage mechacapacitances (e.g., 127.5 F cm−3 at 6.23 A cm−3 or 7 A g−1) nism, the excellent physicochemical properties and the stable and higher rate capability than those previous free-standing intrinsic structure of the electrode. or flexible electrodes of 100.4 F cm−3 at 800 mA cm−3 or To further investigate the structure features of the 9.7–67.5 F cm−3 at 0.1–1.33 A g−1.[1,2] The areal capacitance supercapacitor with ultra long cycle life, the morphologies of CarbonPI membrane is 957.6 mF cm−2 at 5.6 mA cm−2 of the CarbonPI membrane electrode after 100 000 cycles (171 F cm−3 at 1 A g−1), which is far better than those of pre- were observed using the SEM again. As shown in Figure 5, vious carbon film materials such as bulk graphene gels with the morphologies of the CarbonPI membrane are almost random network morphology.[18] Ragone plot shows the rela- unchanged after long cycles. The carbon fibers still remain the tionship between the volumetric energy density and power smooth surface, the uniform diameters and the continuous density of a supercapacitor based on CarbonPI membrane. The network, presenting no fracture or deformation (Figure 5a,b). specific power and specific energy are calculated by the galva- High-magnification images of the across-section maintain nostatic charge/discharge tests at various rates. The CarbonPI the fracture without clearly porous structure (Figure 5d). membrane displays the relatively high energy densities at dif- We suppose just the excellent stable structure leading to the ferent power capabilities, possessing the good energy density ultralong cycle life. However, the contact angle of electrode at high power capability (Figure S5, Supporting Information). materials reduces to 27° after long cycle times, demonstrating The observed large volumetric capacitances of CarbonPI the increase of wettability. membrane can be attributed to the synergistic effects associThe investigation of microstructure of electrode materials ated with physicochemical properties and the porous struc- after the long charge/discharge cycles is rare because of the ture, in which the good wettability of electrodes facilitates conventional electrodes containing binder and conductive ions adsorption on the interface of active materials, and additive, which disturbs the observation of intrinsic texture. the porous and continuous network structures improve the As a free-standing electrode, we observed the detailed morcharge transport. Electrochemical impedance spectroscopic phology of CarbonPI membrane after different cycles using measurements show that the CarbonPI membrane has much HRTEM. Figure 6 shows nanofibers with a cylinder shape small equivalent series resistances (Rs from the X-intercept of and a range of diameters of 0.5–1 µm, consisting with the the Nyquist plot, Figure 4e), and a nearly vertical line at the above SEM images. The results demonstrate the galvanoend of the semicircular (Figure 4e). Rs comprises the resist- static charge/discharge process does not change the apparent ance of the electrolyte solution, the intrinsic resistance of the structure of CarbonPI membrane. However, all of HR-TEM active material, and the contact resistance at the interface images exhibit a number of orientation stripe structures, indiactive material/electrolyte.[55] The N-doping improve the wet- cating a certain degree of graphitization. The structure facilitability of the electrode material contributing to the reduc- tates the high conductivity of CarbonPI membrane. Notably, tion of Rs (Figures 4e and S4b). When the cycles increase some interesting slight changes present in the different cycle from 1 to 100 000, the value of Rs slightly decreases from 0.20 stages. The cross-sectional TEM images of the initial CarbonPI to 0.13 Ω, that means, the adsorption/desorption of electrolyte ions on the electrode lead to the reduction of contact resistance. The result is in agreement with the contact angle decreasing after 100 000 cycles. By contrast, the charge-transfer resistance (Rct) mildly enhances upon the increasing cycles. Maybe the ions insertion/deinsertion into the external tens of nanometers of active materials slightly reduces the ordered extent of them during the cyclic test, which is confirmed by the following TEM images. More importantly, the values of Rct are small, indicating the charge/ discharge cycles are attributed to mass transfer control, not kinetic control.[56] As a double layer capacitor, CarbonPI membrane displays the outstanding cycling life (Figure 4f), similar to the other N-doped carbon materials.[26,57] The specific capacitance barely decreases at 1, 3, and 5 A g−1 after 100 000 cycles. Moreover, the corresponding charge–discharge plots Figure 5. Microstructures of the CarbonPI membrane after 100 000 cycles. a) SEM image at various current densities during the of Carbon membrane surface; b) HR-image of nanofibers of Carbon membrane; c,d) the PI PI cyclic process keep almost isosceles trian- cross-section SEM image of CarbonPI membrane. The contact angle of CarbonPI membrane gles (inset Figure 4f). Those remarkable after 100 000 cycles (Figure 5c, inset).



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long cycle life. The results are attributed to the well-designed structure. CarbonPI membrane possesses the high graphitic structures and the doping of quaternary N in the plane structure, which facilitates the fast transfer of electrons. The high content of N atom enhances the wettability of active materials, helping the adsorption of ions on the electrode. The open porous structure favors the fast diffusion of ions. The factors improve the specific capacitance and power capability of CarbonPI membrane. The stable structure of carbon nanofibers leads to the long cyclability.

4. Experimental Section

Figure 6. TEM images of the CarbonPI membrane during different galvanostatic charge discharge cycles: a) initial; b) after 20 000 cycles; c) after 60 000 cycles; and d) after 100 000 cycles, respectively. Inset: the corresponding low-resolution TEM images.

nanofibers display the most long and ordered orientation stripe structures (Figure 6a). With the increase of charge/discharge cycles the long-range order stripe structures gradually disappear. After 100 000 cycles, the individual lamellae still remain parallel texture, and the overall morphologies gradually turn into the more disordered structure (Figure 6d). On the base of the storage mechanism of EDL capacitance, electrolyte ions are adsorbed on the interface of electrode forming the double layers charge for charge storage.[22] The previous research reported electrolyte ions can diffuse into the micropore structure of electrodes to achieve a significant special capacitance.[58,59] Based on the above results, it is assumed that the electrolyte ions diffuse into the external loose layer-to-layer space of the nanofibers during the charge/ discharge process. The ions of long-term periodical insertion and deinsertion destroy the long-range order stripe structures of CarbonPI membrane (further detail in supporting information Figure S6, Supporting Information).

3. Conclusion The PI membranes with benzopyrrole and benzimidazole rings were fabricated by electrospinning process. Subsequently, the as-prepared PI membranes were carbonized via the carbonization technologies of applied pretension and preoxidization, to obtain the free-standing and flexible CarbonPI membrane. SEM images show that the N-doped CarbonPI membrane consists of the nanofibers with uniform diameter, and presents a high packing density (0.89 cm−3). XPS analysis indicates that quaternary N is the predominant N-doped configurations. The electrode of CarbonPI membrane exhibits excellent large volumetric capacitance (159.3 F cm−3 at 1 A g−1), high rate capability (127.5 F cm−3 at 7 A g−1) and a small 2015, DOI: 10.1002/smll.201403575

Materials: 3,3′,4,4′-Benzophenonetetracarboxylicdianhydride (BTDA) was obtained from Beijing Multi Technology co., Ltd., and dried in vacuo at 120 °C for 24 h prior to use. 2,2′-Bis(tri-fluoromethyl)-4,4′diaminobiphenyl (TFMB) and 2-(4-aminophenyl)-5-aminobenzimidazole (BIA) were purchased from Changzhou Sunlight Pharmaceutical Co., Ltd. N-methyl-2-pyrrolidone (NMP) was purchased from Sinopharm Chemical Reagent Co., Ltd. and stirred in the presence of phosphorus pentoxide (P2O5) overnight and then distilled under reduced pressure. Sodium hydroxide was purchased from Shanghai Chemical Co. Other commercially available reagent grade chemicals were used without further purification. Preparation of CarbonPI Membrane: PI solution in NMP was synthesized according to our previous report.[60] In brief, ODA was firstly dispersed in anhydrous NMP. A three-necked flask equipped with a nitrogen inlet and a mechanical stirrer was charged with distilled NMP, TFMB, BIA, and appropriate amount of ODA/NMP colloid solution. After the diamines were dissolved, equimolar dianhydride BTDA was added. The solution was stirred at room temperature for 3 h, and isoquinolin was added and further stirred for 3 h at 120 °C. Then the mixture reacted at 195 °C for 10 h, and the water produced during the imidization was continuously removed with a stream of nitrogen. PI membranes was fabricated by electrospinning using a syringe with a diameter of 0.5 mm spinner at an applied voltage of 17 kV, fed at the rate of 8 µL min−1, with a distance of 15 cm between the tip of needle and the collector. The electrostatic spinning solution of PI contained the solid content of 9%. The thickness of PI membranes was controlled by the deposition time of the electrospinning process. All of PI membranes were overnight dried at room temperature, further removed the residual NNP solvent in vacuum oven at 60 °C. The carbonization process of PI membranes in aluminum oxide furnace is divided into three stages. First, PI membranes were fixed on an Al2O3 plate by carbon fibers, and heated at the rate of 3 °C min–1 from room temperature to 290 °C under a high purity argon atmosphere, from 290 to 450 °C under an air atmosphere (preoxidization), and from 450 to 1000 °C under a high purity argon atmosphere. Second, PI membranes was annealed at 1000 °C for 30 min. Third, the sample cool down at the same rate of 3 °C min–1 from 1000 to 50 °C. Whole carbonization process was under a high purity argon atmosphere except preoxidization process. Materials Characterization and Electrochemical Measurements: The morphology and structure of the as-spun precursor nanofibers (PI membranes) and the final CarbonPI membrane were characterized by field emission scanning electron microscopy (FESEM, Hitachi S-4800), energy-dispersive X-ray spectroscopy



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(EDS), elemental mapping, and transmission electron microscopy (TEM, JEOL JEM-2100) at an accelerating voltage of 100 kV. X-ray photoelectron spectroscopy (XPS) experiments were carried out on a RBD upgraded PHI-5000C ESCA system (Perkin Elmer) with Mg Kα radiation (hν = 1253.6 eV). The crystalline structure of CarbonPI membrane was characterized by X-ray diffraction (XRD) measurement on a Rigaku D-max-2500 diffractometer with nickelfiltered Cu–Kα radiation with λ = 1.5406 Å. The conductivity of CarbonPI membrane at room temperature is measured by a typical four-probe method (LORESTA-EP MCP-T360). Two-electrode cell configurations were employed to measure the electrochemical performances of the N-doped CarbonPI membrane as supercapacitor electrodes. First, CarbonPI membranes were punched into wafer electrodes (the diameter of 0.8 cm). Second, two nearly identical (by weight of ≈3 mg) electrodes were assembled in a test cell. 1 M NaOH, solutions were used as the electrolytes. Electrochemical evaluations were performed on an atuolab electrochemical working station. The cyclic voltammetry (CV) and galvanostatic charging/discharging techniques were measured, with an applied potential window ranged from 0 to 1 V in a 1 M NaOH electrolyte. Electrochemical impedance spectroscopy (EIS) was conducted in the frequency ranging between 100 kHz and 10 mHz with a perturbation amplitude of 5 mV versus the open-circuit potential. The cyclability of the products was carried out by LAND testing system over 100 000 cycles.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements Financial support of this work is provided by China Postdoctoral Science Foundation (Grant No. 13B10624), NSFC (Grant Nos. 51233001 and 51173024), SRFDP (Grant No. 20110075110009) and 111 Project (111–2–04).

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Received: December 1, 2014 Revised: January 29, 2015 Published online:


9

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