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Journal of Power Sources 319 (2016) 262e270 Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour A melamine-assisted chemical blowing synthesis of N-doped activated carbon sheets for supercapacitor application Yiliang Wang a, Huaqin...

Journal of Power Sources 319 (2016) 262e270 Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour A melamine-assisted chemical blowing synthesis of N-doped activated carbon sheets for supercapacitor application Yiliang Wang a, Huaqing Xuan a, Gaoxin Lin a, Fan Wang a, Zhi Chen b, Xiaoping Dong a, * a b Department of Chemistry, School of Sciences, Zhejiang Sci-Tech University, 928 Second Avenue, Xiasha Higher Education Zone, Hangzhou 310018, China College of Materials Science and Engineering, China Jiliang University, 258 Xueyuan Street, Xiasha Higher Education Zone, Hangzhou 310018, China h i g h l i g h t s g r a p h i c a l a b s t r a c t
N-doped activated carbon sheets (NACSs) are prepared by a chemical blowing method.
This synthetic method is novel and convenient than other traditional routes.
NACS material possesses hierarchical porous structure and high nitrogen content.
It shows a high specific capacitance of 312 F g1 and good rate performance.
The energy density of NACS supercapacitor reaches 20.2 Wh kg1 at 448 W kg1. a r t i c l e i n f o a b s t r a c t Article history: Received 22 December 2015 Received in revised form 12 April 2016 Accepted 14 April 2016 Available online 22 April 2016 N-doped activated carbon sheets (NACS) have been successfully synthesized using glucose as carbon source via melamine-assisted chemical blowing and sequent KOH-activation method. The obtained carbon material possesses a sheet-like morphology with ultrathin thickness, hierarchical micro/mesoporous structure, high specific surface area (up to 1997.5 m2 g1) and high pore volume (0.94 cm3 g1). Besides, NACS material with a nitrogen content of 3.06 wt% presents a maximum specific capacitance of 312 F g1 at a current density of 0.5 A g1 in 6 M KOH aqueous electrolyte due to the cocontribution of double layer capacitance and pseudocapacitance. It also displays good rate performance (246 F g1 at 30 A g1) and cycle stability (~91.3% retention after 4000 galvanostatic charge-discharge cycles). The assembled NACS-based symmetric capacitor exhibits a maximum energy density of 20.2 Wh kg1 at a power density of 448 W kg1 within a voltage range of 0e1.8 V in 0.5 M Na2SO4 aqueous electrolyte. Thus, the unique porous sheet structure and nitrogen-doping characteristic endue the electrode material a potential application for high-performance supercapacitors. © 2016 Elsevier B.V. All rights reserved. Keywords: Melamine Chemical blowing Carbon sheets Nitrogen doping Supercapacitor 1. Introduction * Corresponding author. E-mail address: [email protected] (X. Dong). http://dx.doi.org/10.1016/j.jpowsour.2016.04.069 0378-7753/© 2016 Elsevier B.V. All rights reserved. Supercapacitors, which have been extensively applied in mobile electrical systems, memory back-up systems, consumer electronics and energy management, are supposed to be a kind of promising candidate for alternative energy storage/conversion devices for Y. Wang et al. / Journal of Power Sources 319 (2016) 262e270 their high energy/power densities [1e3]. Electrochemical doublelayer capacitors (EDLCs) storing charge on the interfacial double layer between electrode and electrolyte and pseudocapacitors associated with rapid surface redox reaction are the two categories [4,5]. The former requires carbon materials with high specific surface area and suitable pore channels to achieve satisfied specific capacitance and long cycling life [6,7]. The latter, which is consist of transition metal oxide, conducting polymers and some carbons with abundant oxygen- and nitrogen-containing surface functional groups, provides extremely higher capacitance value by 3e4 times of EDLCs. Whereas their poor conductivity and cycleability still can not be ignored [8,9]. Therefore, preparing a carbon material containing parts of pesudocapacitance should be an efficient way to improve the energy density without sacrificing the power density and cycle life [10,11]. Porous carbon materials, especially activated carbons (ACs), are the most widely used electrode material of EDLCs for their chemical stability, less weight, low cost, good electric conductivity and environment-friendly advantage [12e15]. The developed micro/ mesporous channels and ultrahigh specific surface area of ACs do not only accelerate the kinetic process of ion migration but also provide abundant surface active sites that is essential to high specific capacitance and outstanding cycleability of supercapacitors [16]. Generally, ACs are prepared using various activating agents, such as air, steam, KOH, NaOH, ZnCl2, H3PO4 and K2CO3 [17]. While various activating agents may generate different porous networks due to varied activation mechanisms. Among them, KOH is widely used since it can result in ACs with dramatically increased specific surface area (up to 3000 m2 g1) and high micropore volume, defined micropore size distribution and controllable surface functional groups, depending on the activation condition and carbon sources used [18]. However, ACs with low graphitization degree always suffer poor electric conductivity that severely prohibits their fast charge/discharge ability, particularly at high current density [19]. It has been demonstrated that modifying carbon materials with heteroatom species could not only modulate the conductivity of carbon matrix, but also endow carbon with acid/ base characters (oxygen and nitrogen functionalities) that helps to increase the wettability between electrodes and electrolytes and introduce additional pseudocapacitance via redox reaction, consequently enhancing the whole capacitive performance [20e22]. Nitrogen-doped carbons are usually prepared by two methods including treating carbon sources (organic polymers, biomass materials) with chemical agents (amines, melamine, urea) and in situ doping using nitrogen-containing precursors [23]. Gao et al. reported a porous and nitrogen-rich carbon material using hydrochar as precursor, KOH as activating agent and melamine as nitrogen source and it displayed a high specific capacitance of 279 F g1 at a current density of 0.1 A g1 in 6 M KOH electrolyte [24]. Ma et al. presented a direct pyrolysis of solid melamine-formaldehyde (MF) resin spheres to prepare nitrogen-doped hollow carbon microspheres with graphitic carbons shells, which showed a high specific capacitance of 306 F g1 at a current density of 0.1 A g1 in 2 M H2SO4 electrolyte [25]. As a common nitrogen source, melamine is widely used for their high nitrogen content of 66.7%. Besides, electrochemical properties of carbon electrode materials can also be tuned by adjusting their morphology and size. Carbon materials with different morphologies, including nanosphere, nanosheet, nanoflower and nanofiber, exhibit quite distinct electrochemical properties as supercapacitors [26e28]. Among them, two-dimensional (2D) carbon materials such as graphene and carbon nanosheets possess numerous remarkable characteristics of high specific surface area, short electrolyte ion diffusion distance and mechanical stability. They are usually prepared by mechanical exfoliation, templating, self-assembly, chemical vapor 263 deposition and solvothermal synthesis [29]. However, these methods are limited in industrial field since they are timeconsuming and complexly operating, high-cost and even harmful to environment. Recently, a facile and rapid synthesis strategy to fabricate carbon sheets via a chemical blowing process has drawn more and more attentions. Lei et al. reported a NH4Cl-assisted chemical blowing method to prepare graphene-like carbon nanosheets for capacitive deionization [30]. Jiang et al. reported a highthroughput fabrication of strutted graphene by ammoniumassisted chemical blowing for high-performance supercapacitors [31]. Peng et al. synthesized a highly crumpled nitrogen-doped graphene-like nanosheets with excellent electrochemical properties using urea as a nitrogen source and an expanding agent [32]. Herein, we report the synthesis of nitrogen-doped activated carbon sheets using glucose as carbon precursor, melamine as nitrogen source and blowing agent and KOH as activating agent. The resulting carbon sheets possess hierarchical micro/mesoporous structure, high specific surface area of 1997.5 m2 g1, high pore volume of 0.94 cm3 g1 and a high nitrogen content of 3.06 wt%. Electrochemical measurements demonstrate that NACS electrode has a high specific capacitance of 312 F g1, good rate capability and cycling stability in 6 M KOH electrolyte. It also displays attractive energy density and power density when assembled into a symmetrical supercapacitor. 2. Experimental 2.1. Materials preparation All the chemical reagents in this work were of analytical grade purity and used without any further purification. Deionized water was used in all of the processes. 2.1.1. Synthesis of N-doped carbon sheets (NCS) N-doped carbon sheet material was synthesized through a facile melamine-assisted chemical blowing method using glucose as carbon precursor, melamine as both blowing agent and nitrogen source. In a typical synthesis, glucose was directly mixed with melamine with a mass ratio of 1:1 and heated in a tube furnace at 700  C for 2 h at a heating rate of 4  C min1 under N2 atmosphere. 2.1.2. Preparation of N-doped activated carbon sheets NCS was added into 20 mL KOH aqueous solution with a KOH/ NCS mass ratio of 2:1. After stirring at room temperature for 30 min, the mixture was air-dried and then annealed at 700  C for 2 h under N2 atmosphere with a heating rate of 3  C min1. After cooling to room temperature, the resulting sample was rinsed several times with 1 M HCl and deionized water respectively to neutral pH and finally the pure NACS was obtained. For comparison, glucose was firstly carbonized under the same condition of NCS without melamine, followed by KOH-activation under the same condition of NACS. The resulting product was named as activated carbon (AC) material. 2.2. Characterization X-ray diffraction (XRD) analysis was performed by a DX-2700 diffractometer (Dandong Haoyuan Instrument Co. Ltd., China) using Cu Ka radiation (l ¼ 0.15418 nm). Raman spectrum was collected on a Renishaw inVia Raman microscope. Nitrogen adsorption-desorption isotherm measurements were carried out at 196  C using a micromeritics ASAP 2020 surface area analyzer. Before adsorption, the samples were out-gassed at 150  C for 6 h. The specific surface area was evaluated using the BrunauerEmmett-Teller (BET) method and the total pore volume was 264 Y. Wang et al. / Journal of Power Sources 319 (2016) 262e270 calculated according to single point method at relative pressure (P/ P0) ¼ 0.975. The pore size distributions were estimated according to the density functional theory (DFT) method. The morphologies were observed using a FEI Tecnai G2 20 transmission electron microscope (TEM) with an accelerating voltage of 200 kV and a scanning electron microscope (SEM, Quanta 250 FEG). Energydispersive X-ray spectroscopy (EDX) and element mapping measurements were performed using the same SEM system equipped with an energy-dispersive X-ray spectrometer. X-ray photoelectron spectra (XPS) were obtained on a VG ESCALAB MK Ⅱ X-ray photoelectron spectrometer with an excitation source of Mg Ka (1253.6 eV). The specific energy density (E, Wh kg1) for a symmetric supercapacitor can be estimated using the following formula: E¼ 1 2 CV 2 (4) The specific power density (P, W kg1) was calculated according to the following equation: P¼ E t (5) where C is the specific capacitance of the total symmetric system, V is voltage change during discharge process after IR drop in V-t curve and t is the discharge time. 2.3. Electrochemical analysis 3. Results and discussion All electrochemical measurements were carried out on a CHI660E electrochemical workstation (Chenghua, Shanghai, China) at room temperature. In the three-electrode system, a platinum slice was used as the counter electrode and Ag/AgCl electrode was served as the reference electrode. The working electrode was prepared by coating a mixture of 75 wt% active material, 10 wt% acetylene black and 15 wt% polyvinylidene fluoride (PVDF) binder onto a piece of Ni foam (1  1 cm) and then dried at 100  C overnight before pressing under a pressure of 20 MPa. The mass loading of active material in each working electrode is about 1.8 mg. The electrolyte was 6 M KOH aqueous solution. Cyclic voltammetry (CV) was performed at scan rates of 5e100 mV s1 in the voltage range between 1.1 and 0.1 V. The galvanostatic charge/discharge (GCD) curves were tested at current densities ranging from 0.5 A g1 to 30 A g1. Electrochemical impedance spectroscopy (EIS) was measured at open circuit potential over a frequency range from 10 mHz to 100 kHz with an AC amplitude of 5 mV. In the twoelectrode configuration, two nearly identical working electrodes were prepared using the abovementioned method and a cellulose acetate membrane was used as separator. The electrochemical performance was measured in the potential range of 0e1.8 V in 0.5 M Na2SO4 aqueous solution. Before assembling the symmetric two-electrode configuration, the working electrodes and separator were immersed in the electrolyte for 3 h to assure electrolyte solutions sufficiently contacted with them. In three-electrode system, the gravimetric specific capacitance was calculated from CV curves and galvanostatic charge/discharge curves by the following equations: Z IdV C¼ C¼ mVv I Dt mDV (1) (2) where I (A) is the constant current, Dt (s) is the discharge time, m (g) is the mass of electrode material, v (mV s1) is the scan rate, V and DV (V) are the potential window. In the two-electrode setup, the gravimetric specific capacitance for a single electrode Cs (F g1), was determined using the following equation: Cs ¼ 4I Dt mDV (3) where I (A) is the charge/discharge current, Dt (s) is the discharge time, m (g) is the total mass of active material in both electrodes and DV (V) is the voltage change excluding the IR drop during the discharge process. 3.1. Characterization of carbon materials The morphology and microstructure of samples were detected by electron microscope observation. The SEM image (Fig. 1a) of AC shows an irregular shape of massive bulk with a diameter of tens of micrometers, and thus a low specific surface area of AC material may be speculated. In contrast, the NCS sample prepared by melamine-assisted chemical blowing displays a sheet-like morphology with smooth surface and the lateral size is in the range of several to several 10 mm (Fig. 1b). From the highmagnification SEM image of NCS (Fig. 1c), the thickness of carbon sheets before activation is determined to 100e200 nm. Because of the relatively large thickness, the sheets of NCS lack flexibility. The chemical blowing process can be described as follows: (1) when the temperature reaches at ~300  C the sublimation and decomposition of melamine occurs and the subsequently decomposed gas blows the melted glucose to form sheet-like precursors, together with doping nitrogen element; (2) with the continuing increase of temperature, the sheet-like precursors are further carbonized to finally form NCS. Fig. 1def reveals the SEM images of NACS in different magnifications. After the KOH chemical activation, it is apparent that the sheet-like morphology is well retained in NACS from NCS. Furthermore, the sheets in NACS become much more flexible in comparison with those in NCS, and numerous wrinkles are found on NACS. This may be ascribed to the decrease in thickness of sheets as a large amount of carbon is removed via the reaction with KOH. The thickness of NACS is evaluated to several 10 nm. Fig. 2 illustrates the TEM images of NACS in different magnifications. From Fig. 2a the ultrathin sheets with a lateral size of several micrometers are observed, and the flexibleness of NACS is demonstrated by the crumpled sheets in Fig. 2b. We can estimate the thickness of NACS from the curled edge of sheets. As shown in Fig. 2c using red arrows, the thickness of sheets is 10e20 nm that is in accordance with the evaluated value in the SEM result. Moreover, an apparent porous structure can be observed in Fig. 2c, which is resulted from the KOH chemical activation. The surface area and porous nature of the obtained materials were investigated by means of nitrogen sorption technique. The Nitrogen adsorption-desorption isotherms and corresponding pore size distribution curves are shown in Fig. 3. It can be found that both NCS and AC samples exhibit a combined curve of type Ⅰ and Ⅳisotherms, indicating a transmitted porous structure from micropores to mesopores. However, the isotherm of NACS material should belong to type Ⅰ because of the huge N2 adsorption quantity in low pressure region. The pore size of all samples are focused on 1e3 nm from the inset pore size distribution curves. Detailed textural parameters of carbon samples, including specific surface Y. Wang et al. / Journal of Power Sources 319 (2016) 262e270 265 Fig. 1. SEM images of (a) AC, (b, c) NCS and (e, d, f) NACS. Fig. 2. TEM images of NACS in different magnifications. Fig. 3. Nitrogen adsorption-desorption isotherms and corresponding pore size distribution curves of (a) NCS, (b) AC and (c) NACS samples. area (SBET), total pore volume (Vtotal) and average pore diameter (Dp) are listed in Table 1 below. The NCS sample from chemical blowing with poor porosity exhibits a low specific surface area of 125.8 m2 g1, which mainly comes from the surface area of carbon sheets. This low specific surface area may badly restrict its doublelayer capacitance. AC prepared by direct KOH activation exhibits a relatively high specific surface area of 1120.9 m2 g1, which is higher than NCS but still lower than NACS. The maximum specific surface area (1997.5 m2 g1) of NACS should be attributed to the synergetic effect of sheet-like morphology as well as the porous structure from KOH activation treatment. The reaction equation between C and KOH is as followed [24]: 6KOH þ 2C ¼ 2K þ 3H2 þ2K2 CO3 (6) 266 Y. Wang et al. / Journal of Power Sources 319 (2016) 262e270 Table 1 Textural parameters of carbon materials. Sample SBETa (m2 g1) Smicroporeb (m2 g1) Smesoporec (m2 g1) Vtotald (cm3 g1) Dpe (nm) NCS AC NACS 125.8 1120.9 1997.5 57.8 624.7 1220.9 68.0 496.2 776.6 0.077 0.56 0.94 1.18/2.73 1.59/2.16 1.27/2.00 a b c d e Specific surface area estimated using BET method. Micropore surface area calculated using the V-t plot method. Mesopore surface area calculated using the V-t plot method. Total pore volume calculated using single point method at P/P0 ¼ 0.975. Pore size calculated using DFT method. In addition, NACS also presents a hierarchical micro/mesoporous network, high micropore specific surface area (Smicopore) of 1220.9 m2 g1, high Smicopore/Smesopore ratio of 1.57 and total pore volume of 0.94 cm3 g1, which is essential to high capacity for supercapacitors. Fig. 4a shows the XRD pattern of NACS material. Two broad diffraction peaks at 2q values of around 25 and 42 reflects the amorphous carbon features of NACS, meaning that no pronounced graphitization occurred under our carbonization/activation process [7,17]. The graphitization degree can also be monitored by Raman measurement. From Fig. 4b, a characteristic peak at approximate 1356 cm1 in Raman spectra can be clearly observed, which corresponds to the D-band and suggests the presence of disordered carbon structure. And the peak at about 1596 cm1 is related to the G-band denoting the ordered graphite lattice of carbon material [12,22,32]. The intensity ratio of D and G bands (ID/IG) is calculated to be 0.92, indicating a low crystallinity of the obtained NACS. Moreover, the components of NACS were firstly estimated by EDX spectra. Three chemical elements of carbon, nitrogen and oxygen can be detected from Fig. S1. The element mapping images (Fig. S2) of C, N and O are also carried out to observe the elemental distribution. The uniform distribution of blue dots reveals that nitrogen element has been uniformly doped in the carbon skeleton. XPS technology was taken to further analyze the surface element compositions of NACS material, as depicted in Fig. 4c. It can be clearly seen that two pronounced peaks locate at around 284.8 and 531.9 eV, corresponding to the C1s and O1s signals, respectively. And a weak peak centered at around 401.6 eV should be assigned to the N1s excitation. Both high heteroatom contents of 12.33 wt% for O and 3.06 wt% for N should be ascribed to the lower carbonization/activation temperature of 700  C. The high resolution N1s spectrum was taken to investigate the bonding configurations of N-atoms for NACS. And it can be deconvoluted into four different types of nitrogen functionalities, including pyridinic-N (N6) at 398.3 eV, pyrrolic-N (N-5) at 400.2 eV, quaternary-N (N-Q) at 401.4 eV and pyridine-N-oxide (N-X) at 403.0 eV, respectively [15,32]. Just as previously reported, the pseudocapacitive interactions are mainly induced by the negatively charged N-5 and N6 groups, while the positive charge on N-Q and N-X may facilitate the electron transfer in the carbon skeleton, enhancing the conductivity of carbon materials [33]. Fig. 4. (a) XRD pattern, (b) Raman spectrum, (c) XPS survey spectra of NACS material; (d) High resolution XPS spectra of N1s peaks for NACS. Y. Wang et al. / Journal of Power Sources 319 (2016) 262e270 3.2. Electrochemical performance of carbon materials Electrochemical performance of the prepared carbon materials as supercapacitor electrodes was firstly characterized by a threeelectrode system in 6 M KOH aqueous solution. The CV curves were measured at 20 mV s1 over the potential range of 1.1 to 0.1 V (Fig. 5a). The curves of both AC and NACS electrodes exhibit approximately rectangular shape, suggesting typical double-layer capacitive behavior. The appearance of humps within the potential window between 0.8 and 0.6 V on NACS curve indicates the existence of pseudocapacitance from surface nitrogen functional groups [34]. The pseudocapacitance of NACS may come from the following faradic redox reactions [35]: C H  NH2 þ 2OH ⇔C ¼ NH þ 2H2 O þ 2e (7) C  NH2 þ 2OH ⇔C  NHOH þ H2 O þ 2e (8) where C* stands for the carbon network. In contrast, the CV curve of NCS prepared only by melamine chemical blowing shows a small irregular triangular shape due to the small pore dimensions of electrode material that may hamper electrolyte ion diffusion thus leading to an ion sieving effect [36]. Besides, the maximum closed area of CV curve of NACS implies the highest specific capacitance compared with that of other samples. The calculated specific capacitances from CV curves for NACS, AC and NCS are 288.1, 135.5 and 42.4 F g1, respectively. Fig. 5b shows the GCD profiles, which are a more accurate approach to calculate specific capacitance. The GCD curve of NACS displays a nearly isosceles triangular shape, indicating high charge/discharge efficiency (nearly 100%) [37]. Specific capacitances of NACS, AC and NCS from discharge curves are 294.1, 143.4 and 61.7 F g1, respectively, which are well consistent with those from CV curves. Besides, the values of IR drop evaluated from discharge curves are 6, 12 and 28 mV with an increasing trend for NACS, AC and NCS, respectively. The negligible IR value of NACS electrode reveals a low internal resistance due to the good electric conductivity and superb porous texture, which is beneficial to supercapacitors since a less energy will be converted to the unwanted heat during charge/ discharge process [38]. The plot of specific capacitance calculated from the discharge curves at various current densities is presented in Fig. 5c. In the case of NACS, the specific capacitance reaches up to 312 F g1 at 0.5 A g1 that is significantly higher than those of AC and NCS samples. More importantly, it still retains 246 F g1 even at a high current density of 30 A g1, demonstrating its excellent rate performance. Meanwhile, it is worth noting that the specific capacitances of NACS at any current densities are much higher than those of AC and NCS materials. The high specific capacitance and 267 excellent rate performance of NACS are mainly attributed to the following reasons: (1) the sheet-like morphology provides a large exposed area to electrolytes and a short diffusion distance for electrolyte ions into the inner pore walls; (2) hierarchical micro/ mesoporous structure with high specific surface area, microporous volume and uniform pore size distribution could accelerate ion transport process within the pore channels and provide abundant inner surface regions for ion accumulation; (3) doping nitrogen groups don't only increase the surface wetting characters, but also improve the conductivity of carbon materials and introduce extra pseudocapacitance, thus further enhancing the capacity performance [3e5,12,15,21,22,28,32,37]. The effect of the chemical blowing agent percentage on the electrochemical performance was investigated. In view of the yield of NACS and the experimental operability (the high melamine/ glucose ratio would result in production of abundant gas to blow mixture out from vessel), we studied the melamine/glucose mass ratio of 0:1, 0.5:1 and 1:1. As shown in Fig. S3, the redox peaks of N functional groups become significant, indicating much more N doping content with the enhancement of melamine/glucose mass ratio. The capacitance values calculated from discharge curves are 143.4, 233.0 and 294.1 F g1 for the carbon materials respectively prepared from the 0:1, 0.5:1 and 1:1 precursor ratio. The improved capacitance should be related to the much more pseudocapacitance of surface N species as well as the distinct sheet-like morphology from the high melamine/glucose ratio. Electrochemical properties of the optimal electrode material of NACS were further investigated. It can be observed from Fig. 6a that the CV curves of NACS still maintain a nearly rectangular shape at a high scan rate of 100 mV s1. GCD curves (Fig. 6b) of NACS were tested at different current densities ranging from 0.5 A g1 to 30 A g1, reflecting good charge/discharge ability. Fig. 6c depicts the specific capacitance vs. cycle number at a current load of 5 A g1 for 4000 cycles and the inset is the last 10 cycles. It can be seen that the specific capacitance decreases slightly during the whole cycling process and finally retains 91.3% of the initial capacitance after 4000 cycles, suggesting its long cycling life. The high-performance NACS electrode was also studied by electrochemical impedance spectroscopy (Fig. 6d). The Nyquist plot could be divided into three parts: (1) a depressed semicycle in middle and high frequency regions that may be related to some faradic interactions between the nitrogen species on NACS and ion in solution and the diameter representing the charge-transfer resistance (Rct) [39]; (2) a nearly straight line with 45 slope corresponding to the Warburg impedance (W) that describes the diffusion resistance of ions to porous structure; (3) a vertical line at low frequency region reflecting the ideal capacitive behavior of carbons. (4) the intercept of EIS curve at Z0 axis representing to the solution resistance (Rs), which results Fig. 5. (a) Cyclic voltammetry curves of NCS, AC and NACS at a scan rate of 20 mV s1; (b) galvanostatic charge/discharge curves of NCS, AC and NACS at a current density of 1 A g1; (c) plot of specific capacitance calculated from the discharge curves at various current densities. 268 Y. Wang et al. / Journal of Power Sources 319 (2016) 262e270 Fig. 6. Electrochemical capacitive behaviors of NACS: (a) Cyclic voltammetry curves at various scan rates; (b) galvanostatic charge/discharge curves of NACS at different current densities; (c) specific capacitance vs. cycle number measured at a current load of 5 A g1 for 4000 cycles (the inset is the last 10 cycles); (d) Nyquist plots of NACS electrode before and after 4000 cycles. Fig. 7. Electrochemical behaviors measured in a two-electrode system in 0.5 M Na2SO4 aqueous electrolyte: (a) CV curves at a scan rate of 20 mV s1 in various voltage windows; (b) CV curves at different scan rates in the potential range from 0 to 1.8 V; (c) galvanostatic charge/discharge curves at different current densities; (d) plot of specific capacitance for a single electrode at various current densities. Y. Wang et al. / Journal of Power Sources 319 (2016) 262e270 269 Fig. 8. (a) Ragone plot of the symmetric supercapacitor, (b) Nyquist plot of the symmetric supercapacitor in 0.5 M Na2SO4 aqueous electrolyte. from the intrinsic resistance of the active materials, the resistance of the electrolyte and the contact resistance in NACS/current collector interface. A low internal resistance is of great importance to energy storage and conversion for supercapacitors. The Rct and Rs values are estimated to be 0.33 and 0.82 U for NACS electrode before cycling. After 4000 cycles, both Rct and Rs increase to 0.54 and 1.21 U, respectively. This increased internal resistance may be responsible for the capacitance decline during long-term cycles. Considering the practical application of NACS material, a twoelectrode symmetric supercapacitor was fabricated in neutral aqueous electrolyte. Operating voltage, which strongly depends on the type of electrolyte used, is a key parameter for the energy density of supercapacitors according to Equation (4). Na2SO4 solution has been reported to be a favorable selection among various aqueous electrolytes for its higher operating voltage than those of acid and basic electrolyte, low toxic, inexpensive and easily available features [40]. Here, a 0.5 M Na2SO4 aqueous electrolyte was used and CV curves (Fig. 7a) at different voltage windows were tested to determine the optimal potential range. A slight variation in the CV curves can be observed when the voltage range is lower than 1.8 V. However, a rapid current increase appears when the potential increases to 2.0 V due to the electrolyte decomposition with hydrogen and/or oxygen evolution [32]. In addition, a stable charge/discharge behavior below a maximum voltage of 1.8 V can also be found from Fig. S4. Therefore, a voltage window of 0e1.8 V was chosen for the later two-electrode measurements. Fig. 7b demonstrates the CV curves of symmetric cell at different scan rates and an almost rectangular shape is retained even at a high scan rate of 300 mV s1, which well meets the industrial application requirements. In Fig. 7c, the nearly symmetric linear GCD curves reveal its good charge/discharge reversibility. What is more, the specific capacitance for a single electrode of the symmetric supercapacitor at various current densities is depicted in Fig. 7d. The symmetric configuration still keeps a retention of 81.1% of initial capacitance at a current density of 10 A g1. This result is in conformity with that measured in three-electrode system. Ragone plot of NACS-based symmetric supercapacitor calculated from discharge curves at different current densities is shown in Fig. 8a. It presents a maximum energy density of up to 20.2 Wh kg1 at a power density of 448 W kg1. As the current density increases the energy density is suppressed since limited pores on the surface are accessed by electrolyte ions for fast discharging at high current densities, whereas almost all the pore channels (no matter inner or near the surface) can be utilized at a low current density [41]. And it still remains 13.6 Wh kg1 at a power density of 8160 W kg1. The data is higher than those of commercially available supercapacitors (3e5 Wh kg1) and many other carbon-based/nitrogen-doped carbonaceous supercapacitors as previously reported (Table S1) [42e51]. Fig. 8b describes the Nyquist plot of symmetric supercapacitor in 0.5 M Na2SO4 aqueous electrolyte. It was measured in a frequency range from 10 mHz to 100 kHz with an AC amplitude of 5 mV at open circuit potential. A low internal resistance of 1.09 U suggests good electric conductivity of NACS symmetric supercapacitor. The inset of Fig. 8b is the equivalent circuit for the fitting of EIS data by Zview software. The fitted curve almost overlaps the original EIS curve, indicating a good fit is obtained. The equivalent circuit consists of the following parts: Rct, Rs, W, Cdl (the double-layer capacitance) and CL (the limit pseudocapacitance stemming from redox reaction of heteroatomcontaining groups). 4. Conclusions In summary, we developed a novel and low-cost method to synthesize NACS material with sheet-like morphology and high specific surface area. The sheet-like structure and nitrogen-doping are simultaneously achieved via a facile melamine-assisted chemical blowing method. KOH-activation effectively introduces porous structure including high specific surface area and narrow pore size distribution. The obtained NACS materials show a high charge storage capacity with a specific capacitance of 312 F g1 in 6 M KOH solution at a current density of 0.5 A g1 and excellent long-term stability over 4000 cycles. More importantly, the assembled symmetric supercapacitor also exhibits remarkable energy density and power density in neutral electrolyte. The results demonstrate that the prepared NACS electrode material may be a promising candidate for high performance supercapacitor. Acknowledgements The authors gratefully acknowledge the financial support from 521 talent project of ZSTU, the National Undergraduate Training Program for Innovation and Entrepreneurship (201510338001) and the project-sponsored by the Scientific Research Foundation (SRF) for the Returned Overseas Chinese Scholars (ROCS), State Education Ministry (SEM). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2016.04.069. References [1] T. Kim, G. Jung, S. Yoo, K.S. Suh, R.S. Ruoff, Activated graphene-based carbons as supercapacitor electrodes with macro- and mesopores, ACS Nano 7 (2013) 270 Y. 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