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Superior supercapacitors based on nitrogen and sulfur co-doped hierarchical porous carbon: Excellent rate capability and cycle stability
Journal of Power Sources 358 (2017) 112e120
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Superior supercapacitors based on nitrogen and sulfur co-doped hierarchical porous carbon: Excellent rate capability and cycle stability Deyi Zhang a, b, c, *, Mei Han b, Bing Wang b, Yubing Li b, Longyan Lei b, Kunjie Wang b, Yi Wang b, Liang Zhang b, Huixia Feng b a b c
State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals, Lanzhou University of Technology, Lanzhou 730050, China College of Petrochemical Technology, Lanzhou University of Technology, Lanzhou 730050, China Key Laboratory of Eco-Environment-Related Polymer Materials of Ministry of Education, Northwest Normal University, Lanzhou 730070, China
h i g h l i g h t s An N and S co-doped hierarchical porous carbon was successfully fabricated. The fabricated NSHPC material exhibited high SBET with short opened micropores. High capacitance, excellent rate capability and cycle stability was observed. Large and highly utilized surface area response for superior performance.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 January 2017 Received in revised form 12 April 2017 Accepted 11 May 2017
Vastly improving the charge storage capability of supercapacitors without sacrificing their high power density and cycle performance would bring bright application prospect. Herein, we report a nitrogen and sulfur co-doped hierarchical porous carbon (NSHPC) with very superior capacitance performance fabricated by KOH activation of nitrogen and sulfur co-doped ordered mesoporous carbon (NSOMC). A high electrochemical double-layer (EDL) capacitance of 351 F g1 was observed for the reported NSHPC electrodes, and the capacitance remains at 288 F g1 even under a large current density of 20 A g1. Besides the high specific capacitance and outstanding rate capability, symmetrical supercapacitor cell based on the NSHPC electrodes also exhibits an excellent cycling performance with 95.61% capacitance retention after 5000 times charge/discharge cycles. The large surface area caused by KOH activation (2056 m2 g1) and high utilized surface area owing to the ideal micro/mesopores ratio (2.88), large micropores diameter (1.38 nm) and short opened micropores structure as well as the enhanced surface wettability induced by N and S heteroatoms doping and improved conductivity induced by KOH activation was found to be responsible for the very superior capacitance performance. © 2017 Elsevier B.V. All rights reserved.
Keywords: Supercapacitor Hierarchical porous carbon Heteroatoms doping Rate capability Superior capacitance performance
1. Introduction For overcoming the challenge provoked by the mismatch between the accelerating energy consumption and depletable fossil fuels, exploring sustainable energy and efficient energy storage devices is a pressing issue for researchers. Supercapacitors have been considered as a most promising energy device which could be
* Corresponding author. State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals, Lanzhou University of Technology, Lanzhou 730050, China. E-mail address: [email protected] (D. Zhang). http://dx.doi.org/10.1016/j.jpowsour.2017.05.031 0378-7753/© 2017 Elsevier B.V. All rights reserved.
used for portable electronic devices, hybrid electric vehicles and short-term power sources for mobile electronic devices owning to their high power density, low equivalent series resistance (ESR), long cycle life and fast charge-discharge rates [1e4]. Although supercapacitors demonstrate their distinctive merits, the much lower energy density than batteries (1e2 orders of magnitude) severely restricts its practical application [4,5]. Therefore, recent attention has been focused on increasing the energy density of supercapacitors without sacrificing their high power density and cycle performance [1]. The energy density of the supercapacitor is defined by the following equation: E ¼ CV2/2 [3]. Thus, improving the specific capacitance of electrodes or working voltage of the cells is a
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reasonable option for enhancing the energy density of supercapacitor. In a general sense, the electrodes with high specific area, suitable pore architecture, good wettability and conductivity tend to harvest high charge storage capability [5,6]. Thus, various carbon materials with porous morphology are ideal choices [1]. For instance, the porous carbon nanosheets prepared by Xiaoming Fan et al. exhibit a high specific capacitance of up to 228 F g1 [7]. However, the pristine porous carbons hardly bring a prominent enhancement for energy density of supercapacitors. For improving the specific capacitance of supercapacitors based on porous carbon electrodes, KOH activation is considered as a well-established method [8e10]. For example, the porous graphene activated by KOH exhibits an extra high specific surface area of up to 3100 m2 g1, which specific capacitance reaches up to 166 F g1 in organic electrolyte [9]. Even so, the extra-high specific surface area of KOH activated porous carbons is mainly contributed by the highly-developed micropores [10]. Due to the inherently hydrophobic nature of carbon matrix surface, the availability of the micropores always is unsatisfied, thus it is hard to achieve an expected high specific capacitance [11e13]. Heteroatoms doping is considered as an effective method to manipulate surface physical and chemical activity, improve wettability and conductivity of porous carbon materials [2,14,15]. Especially for nitrogen or sulfur heteroatoms, the difference in electronegativity between the N, S and C atoms provides a more polarized surface, consequentially promoting the wettability of the carbon surface [14,15], which will ensure a fast transfer rate of electrolytic ions in microporous and small mesoporous, improve the availability of surface area and thus enhancing supercapacitance performance of porous carbons [16]. Meanwhile, the pdoping effect of the N and S heteroatoms for graphene lattice of carbon matrix enhances conductivity of porous carbon [17,18]. Actually, the enhanced capacitance performance of KOH-activated nitrogen and sulfur monoacidic or dual doped porous carbon has been reported [16,19e21]. Very recently, Wang et al. reported the KOH-activated nitrogen doped porous carbon nanowires, which exhibits a specific capacitance of 291 F g1. However, the rate capability of these porous materials is unsatisfied, which capacitance retention only reaches over 50% as the current density increasing from 1 to 10 A g1 [19]. Hierarchical porous structure is considered as the key to obtain excellent rate capability for porous carbon [11,22e24]. The nitrogen-doped hierarchical porous carbons derived from nitrogenous dopamine reported by Jing Tang et al. exhibits a specific capacitance of 252 F g1 at a current density of 2 A g1, which capacitance retention reaches 75.7% at a current density of 20 A g1 [22]. An extra high capacitances of 280 F g1 at a current density of 100 A g1 for hierarchical porous carbons derived from carbonization of seaweed also was reported by Danmiao Kang et al. [23]. The capacitance performance of hierarchical porous carbon depends on the specific surface area, ratio of pores with different pore size and connectivity of the hierarchical pores [24]. The suitable hierarchical pores structure and good pore connectivity would promote the diffusion rate of electrolyte ions in pores, and thus further enhances the capacitance performance of porous carbon [11]. Herein, we demonstrate a successful fabrication of a nitrogen and sulfur co-doped hierarchical porous carbon (NSHPC) prepared by KOH activation of nitrogen and sulfur co-doped ordered mesoporous carbon (NSOMC) reported in our previous work [25]. The ordered and opened mesoporous structure of NSOMC provides an ideal matrix for obtaining hierarchical porous carbon with high specific area, suitable hierarchical pores structure and perfect pore connectivity. The resultant NSHPC materials demonstrate a large specific surface area (2056 m2 g1), high specific capacitance
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(351 F g1 at the current density of 0.2 A g1), good cycle stability (the capacitance retention of 95.61% over 5000 cycles) and excellent rate capability, which capacitance remains up to 288 F g1 at the current density of 20 A g1. 2. Experiment 2.1. Fabrication of NSOMC The NSOMC was fabricated according to our previous work [25]. Briefly, a mixture solution of 1.0 mL of pyrrole (Aldrich, 99%), 0.1 mL of sulphuric acid (98%), and 0.6 mL of absolute alcohol was slowly dropped onto the surface of 1.0 g of SBA-15 template, and then heated at 80 C for 2 h under vacuum to evaporate redundant solvent. A brown powder was obtained by pre-carbonizing above mixture under 160 C for 6 h, and then was carbonized under Ar atmosphere at 350 C for 3 h, and completely carbonized at 650 C for 2 h. Finally, the silica template was removed by 10% HF solution to obtain the NSOMC materials. 2.2. Fabrication of the NSHPC The NSHPC sample was fabricated by KOH-activation of NSOMC under Ar atmosphere. The general fabrication process is summarized and illustrated in Fig. 1. Briefly, the KOH and NSOMC powders were ground together with a mass ratio of 2.0 in an agate mortar for about 10 min. After heating at 400 C for 2 h under an Ar atmosphere, the above mixture was calcined at 700 C for 2 h under an Ar atmosphere, and then followed by washing with HCl and deionized water until the pH of the filtrate was 7.0 and drying at 80 C for 12 h. For comparison, a pure carbon CMK-K with the exclusion of S and N was fabricated by employing the same method using an ordered mesoporous carbon CMK-3 with the similar morphology and pores structure as the fabricated NSHPC as raw material. 2.3. Electrochemical measurements A symmetric supercapacitor cell was assembled using the fabricated NSHPC as electrode materials and 6 mol L1 KOH solution as electrolyte. To prepare the electrodes, NSHPC was ground with acetylene black (10 wt%) and polytetrauoroethylene (PTFE, 10 wt%), and then pressed onto nickel foam which served as a current collector. The total mass of each electrode was between 4.0 and 4.5 mg, and two electrodes with almost identical weight were selected for supercapacitor assembling. The NSHPC electrodes fitted with the separator (PP/PE complex film) and electrolyte solution were symmetrically assembled into electrode/separator/ electrode construction (sandwich-type cells). Electrochemical measurements, including cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), were performed on a CHI 660E electrochemical analyzer (Shanghai, China). The galvanostatic charge/discharge and cycle-life stability were tested using a computer controlled supercapacitor testing system (NEWARE 5V0.1A, Shenzhen China). The symmetric supercapacitor cells using CMK-K, NSOMC and commercial activated carbon (AC) as electrode materials also were assembled for comparing the electrochemical properties with the reported NSHPC. 2.4. Characterization The structures and morphologies of the samples were examined by a high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2010) and a field emission scanning electron microscope (FESEM, Hitachi S-4800). The small-angle X-ray scattering (SAXS)
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Fig. 1. Scheme for the fabrication of the NSHPC.
and X-ray diffraction patterns (XRD) were collected on a X'Pert Xray defractometer (Phillips) using Cu Ka radiation. Raman spectra were recorded via a All the Raman spectrometer (JY-HR800 micro Raman) with an argon ion laser (l ¼ 532 nm). The nitrogen adsorptionedesorption isotherms were measured at 77 K using a Micromeritics ASAP 2020 nitrogen adsorption apparatus (U.S.A.). The SBET (specific surface area) was calculated from the N2 isotherms at relative pressures (P/P0) of 0.06e0.20 via the BET (Brunauere-Emmette-Teller) method. The pore volume was calculated using the BET plot from the amount of nitrogen gas adsorbed at the last adsorption point (P/P0 ¼ 0.95), and the pore size distribution using the BarretteJoynereHalenda (BJH) method. X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Multilab 2000 intrument, using Monochrome Al Ka as the excitation source. 3. Results and discussion NSHPC was fabricated by KOH-treatment of NSOMC under Ar atmosphere. As shown in Fig. 1, the opened and well-ordered mesopores, large specific surface, and high heteroatoms doping content of NSOMC provides an ideal matrix for preparation of N and S co-doped hierarchical porous carbon. Under 400 C, the melted KOH was filled into the ordered mesoporous of NSOMC due to the capillarity, and then the carbon wall reacts with the melted KOH under 700 C to generate developed micropores. The difference of electronegativity and atoms size between C framework atoms and N or S heteroatoms cause lots of edge defects and others structural defects at graphene plan of carbon matrix [15,16]. The C atoms bonded to the N or S heteroatoms located at the defects exhibits high chemical activity [14,26], and tend to prior react with KOH. The prior evolution of micropores around the defects provide a protection to avoid complete collapse of ordered mesopores wall while generate developed micropores in the mesopores wall as the ordered mesoporous structure is partially remained after KOH activation. The very thin pore wall of NSOMC (ca. 6 nm) contributes to form developed short opened micropores while the still remained ordered mesoporous structure ensures the connectivity of the micro/mesopores hierarchical porous structure, and thus ensure high availability of the large surface area. Therefore, the reaction equation between NSOMC and KOH in this work is proposed as follow [27]: The effect of the N and S heteroatoms on the evolution of hierarchical porous structure during the KOH activation process is implied by the XPS results. As shown in Table 1, the KOH activation exhibits a significant effect on the elements composition and their chemical circumstances of the produced NSHPC. The NSOMC exhibits high heteroatoms doping content, which N doping content detected by XPS reaches up to 10.0 at% while the S content reaches 0.69 at%. A sharp decrease is observed for N and S content after treatment by KOH under 700 C, the N and S doping content just
reaches 2.32 and 0.13 at% respectively for NSHPC. During the KOH activation process, the reaction between the C and KOH at high temperature under inert atmosphere responses for the formation of developed micropores [8,10]. The sharp decrease for the N and S heteroatoms content indicates the prior reaction of the C atoms bonded to heteroatoms with the KOH. The incorporation of N and S heteroatoms into the graphene lattice of carbon matrix causes a sharp increase of defects content, the C atoms located at these defects with higher chemical activity tend to prior react with KOH, thus the sharp decrease of the N and S doping content during the KOH activation is comprehensible. Beside the doping content, the activation process also affects the chemical bonds configuration of N and S heteroatoms. As shown in Fig. 2, the N1s orbits of both NSOMC and NSHPC exhibit three XPS peaks around 398.4, 400.4 and 401.0 eV, which are attributed to pyridinic-N, pyrrole-N and graphite-N species, respectively [6]. Among that, the first peak about 398.4 eV is attributed to the pyridinic-N species, which is bound to two carbon atoms and donates one p electron to the conjugation with the aromatic p-conjugated rings, and thus improving the carbon specific capacitance performances. The second peak about 400.4 eV can refer to the pyrrole-N species with a good electron-donor characteristics and higher charge mobility, which contributes to reinforce carbon surface activity in electrontransfer reactions and effectively improves electrochemical behaviors. And the last peak around 401.0 eV can be assigned to graphite-N species, which is incorporated into the graphene lattice by replacing a carbon atom [18,28e30]. The high pyridinic-N and pyrrole-N species content contributes to improve the defects content and thus enhance chemical activity of carbon matrix [15,16]. As shown in Fig. 2 and Table 1, the relativity content of the pyridinic-N and pyrrole-N species apparently decreases after KOH treatment while that of the graphite-N species signally improves from 30.95 to 35.89%, these results also illuminate that the C atoms around pyridinic-N and pyrrole-N species which mainly locate at the defects tend to prior react with KOH, and give priority to form microporous structure. For S2p, the XPS peaks locate at 164.7 and 165.6 eV which refer to CeS and CeSeN binding motifs disappear after KOH activation while the relativity content of the thiophenelike sulfur (163.9 eV) and sulfur oxides (168.4 eV) apparently increases [31,32]. Introduction of sulfide bridges may serve as an effective route to depress the shrinkage of the smallest pores to afford rapid ion transport without inducing unwanted redox reactions [33]. Beside N and S heteroatoms, abundant O element also are detected on both NSOMC and NSHPC. It should be noted that the O content markedly increases from 8.7 to 13.1 at.% after KOH activation, the O element in KOH molecules may be in charge for the increase of surface O content. The increased O content would result the corresponding improvement of Oecontained functional groups on surface of carbon matrix, which would be helpful for the enhancement of surface wettability and thus improving the
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Table 1 Summary of XPS peak analysis on NSOMC and NSHPC. Samples O1s
N1s
S2p
NSOMC
NSHPC
Content
8.7 at. %
13.1 at. %
531.2 eV 532.4 eV 533.9 eV
21.93% 29.03% 49.04%
14.61% 23.33% 62.06%
Content
10.0 at. %
2.32 at. %
398.4 eV 400.4 eV 401.0 eV
26.75% 42.29% 30.95%
25.67% 38.43% 35.89%
Content
0.69 at. %
0.13 at. %
163.9 164.7 165.6 168.4
21.41% 22.23% 4.56% 51.80%
40.68% 0 0 59.32%
eV eV eV eV
Fig. 2. XPS spectra of N1s, S2p and O1s for NSHPC and NSOMC.
availability of micropores and small mesopores. For O1s orbits, the binding energy around 531.2 eV corresponds to the oxygen double bonded to carbon (C¼O) in quinone, and the peak at 532.4 eV is attributed to the singly bonded oxygen (eOe) in CeO groups while the binding energy of 533.9 eV is ascribed to carboxylic groups (COOH) [34,35]. After KOH activation, the relativity content of the quinone and singly bonded oxygen groups sharply decrease while that of the carboxylic groups correspondingly increase from 49.04 to 62.06%. The quinone groups on the surface of carbon matrix are reported in response to the unstable pseudo-capacitance, which causes unsatisfied cycle stability and rate capability [34]. Thus, the decrease of quinone groups would suppress the unstable pseudocapacitance, and effectively improve the cycle stability and rate capability of carbon based supercapacitor. The reserved ordered mesoporous structure of NSOMC after KOH activation is confirmed by the SAXS results. As shown in Fig. 3, the strong (100), (110) and (200) characteristic diffraction peaks of two-dimensional (2-D) hexagonal symmetry (p6mm) mesoporous structure indicate the well-ordered mesoporous structure of NSOMC [25]. After KOH activation, the intensity of the (100), (110) and (200) diffraction peaks apparently decline. Although the (110) and (200) diffraction peaks cannot be identified, the (100) diffraction peak still is distinguishable for the produced NSHPC, indicating that the NSHPC still partly remains ordered mesoporous structure of NSOMC after KOH treatment. The more direct proof of the
Fig. 3. Small angle XRD pattern of NSHPC and NSOMC.
reserved ordered mesoporous structure from the TEM images. As shown in Fig. 4, the pristine NSOMC shows highly ordered stripelike and hexagonally arranged images (Fig. 4 a, b). Although the
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Fig. 4. HRTEM patterns of NSOMC (a and b) and NSHPC (c and d).
formation of KOH generated micro/mesopores in the wall is obvious in the activated sample after KOH activation, the partly ordered stripe-like and hexagonally arranged images still can be distinguished for the produced NSHPC (Fig. 4 c, d). Beside the ordered mesoporous structure, the wheat-like morphology of NSOMC also is partly maintained after activated by KOH. As shown in Fig. 5, the SEM images of NSHPC also exhibit similar morphology like that of NSOMC, illuminating that the generation of micropores in the pore wall due to the KOH activation does not completely destroy the ordered mesoporous structure. Raman spectra was employed to confirm the graphitization degree of the investigated samples. As shown in Fig. 6, the spectra of all samples display two obvious peaks, the G band with E2g symmetry ascribed to ordered sp2 carbon at around 1591 cm1 and the D band with A1g symmetry at about 1364 cm1 ascribed to disordered carbon, edge defect, and other defects [36]. So, the area ratio between G and D bands (IG/ID) indicates the graphitization degree of the samples. Obviously, the NSHPC exhibits an increased graphitization degree than that of the pristine NSOMC, which IG/ID data closes to that of the commercial AC. The increasing IG/ID data after KOH activation indicate that this activation process tends to enhance graphitization degree of samples. The carbon atoms located at the edge and structural defects exhibit higher chemical activity than those carbon atoms on graphene plan, and tend to prior react with KOH. The decrease of defects content on graphene lattice thus increasing the graphitization degree of the sample. The
improved graphitization degree after KOH activation also is affirmed by the wide angle XRD patterns. As shown in Fig. S1, the similar XRD patterns with that of the commercial AC indicate the partly graphitic nature of the NSHPC and NSOMC. The intensity of the peak around 22 of NSOMC which refers to the inter-plane (002) reflections of graphitic carbon apparently increases after KOH activation [37], illuminating the enhanced graphitization degree of the produced NSHPC. The direct proof of hierarchical porous structure of the produced NSHPC from the nitrogen adsorption-desorption experiments. As shown in Fig. 7, the adsorption-desorption isotherms of pristine NSOMC exhibit typical type-IV isotherms with H2 hysteresis loops in the range of ca. 0.4e0.9 P/P0, which suggest mesoporous nature of the NSOMC [25,38,39]. After KOH-activating, the adsorptiondesorption isotherms of the produced NSHPC and CMK-K all exhibit a type-I isotherms with H4 hysteresis loops caused by slitlike pores with uniform shape and size [39]. The pore size distribution of NSOMC, CMK-K and NSHPC is shown in the inset illustration of Fig. 7. The NSOMC shows a uniform mesoporous structure with a narrow pore size distribution centered ca. 3.24 nm. After KOH activation, the produced NSHPC also displays uniform mesoporous structure with a pore size of 3.88 nm, indicates that the ordered mesoporous structure is partly reserved. The increase pore size from 3.24 to 3.88 nm maybe due to the reaction between the pore wall and KOH under high temperature, but which reaction does not completely destroy the mesopores wall, and not induces
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Fig. 7. Nitrogen adsorption-desorption isotherms and the pore size distribution (insert) for NSHPC, CMK-K and NSOMC.
Fig. 5. SEM patterns of NSOMC (a) and NSHPC (b).
the thorough collapse of pore wall. After KOH activation, the SBET of the produced NSHPC increases form 978 m2 g1 to 2056 m2 g1 while the mesopore volume decreases from 1.11 to 0.26 cm3 g1 (as shown in Table 2), the proportion of mesopores declines from 92.5% to 25.7% after KOH activation while the micropore volume dramatically increases from 0.09 to 0.75 cm3 g1. The ratio of micro/ mesopores volume for the produced NSHPC reaches 2.88, which is an ideal ratio for improving the diffusion rate of electrolyte in hierarchical porous structure, and thus tends to get enhanced electrochemical performance. A similar result was also described in a KOH-activated pure carbon CMK-K with ordered mesoporous carbon CMK-3 (as shown in Table 2). Above results demonstrate that the large specific surface area of NSHPC is mainly contributed by micropores which generated from the KOH activation process. The average micropores diameter of the produced NSHPC is calculated according to the below equation:
Dmi
Fig. 6. Raman spectrum of NSHPC, NSOMC and commercial AC.
Vmi Vme þ Dme ¼ Dav Vt Vt Vt
(1)
where Dmi refers to the micripore diameter, Dme and Dav represent the mesoproe and average pore diameter, which are 3.88 and 2.0 nm (as shown in the inset illustration of Fig. 7 and Table 2) respectively, Vmi, Vme and Vt represent the micropores, mesopores and total pore volume of NSHPC, which are 0.75, 0.26 and 1.01 cm3 g1 (as shown in Table 2), respectively. The micropore diameter calculated from equation (1) is about 1.38 nm, the large micropore diameter would be helpful for the fast diffusion of electrolyte ions in micropores. Meanwhile, the pristine NSOMC exhibits a thin pore wall of ca. 6 nm (as shown in Fig. 4), as the KOH is filled into the mesopores and corrodes the pore wall from both sides, the generation of short opened micropores is reasonably comprehensible. The short opened porous structure also would be helpful for rapid diffusion of electrolyte ions in micropores. Over all, the ideal micro/mesopores ratio, large micropores diameter and short opened micropores structure would dramatically raise the availability of micropores, and then improve the surface utilization rate of the produced NSHPC, thus the high charge storage capability is expectable. The capacitive performance of the two-electrode symmetrical supercapacitor cell based on the produced NSHPC electrodes (SCNSHPC) was evaluated using best-practice methods with 6 M KOH
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Table 2 Textural parameters of the NSOMC, CMK-K and NSHPC. Sample ID
Dav/nm
SBET/m2 g1
Vt/cm3 g1
Vme/cm3 g1
Vmi/cm3 g1
Vmi/Vt
NSOMC CMK-K NSHPC
3.24 2.48 2.00
978 1809 2056
1.20 1.12 1.01
1.11 0.96 0.26
0.09 0.16 0.75
0.075 0.14 0.74
Dav: average pore diameter, Vt: total pore volume, Vme: mesoporous volume, Vmi: microporous volume.
as electrolyte. As shown in Fig. 8a, the cyclic voltammetry (CV) curve of SC-NSHPC exhibits a good symmetrically rectangular shape in the potential range from 0 to 1 V under a scan rate of 5 mV s1, illuminates the ideal electrochemical double-layer (EDL) capacitance nature. The similar CV curves are observed for the symmetrical supercapacitor cells based on the pristine pure carbon CMK-K (SC-CMK-K), NSOMC (SC-NSOMC) and commercial AC electrodes (SC-AC), but the area of CV curves of the SC-NSHPC is far higher than that of SC-CMK-K, SC-NSOMC and SC-AC, indicating the more large charge storage capability of SC-NSHPC. As the scan rate increases from 0.5 to 20 mV s1, only a little distortion for CV curves of SC-NSHPC is observed (as presented in Fig. S2 in the Supporting Information) while a little decrease for CV curves area is presented when using the ratio of current intensity vs scan rate as the Y-axis, implying the good ions transport ability and rate capability of the NSHPC electrodes. The galvanostatic chargeedischarge curves of the SC-NSHPC, SC-CMK-K, SC-NSOMC and SC-AC are displayed on Fig. 8b. As shown in Fig 8b and Fig. S3, all tested cells exhibit a triangular chargeedischarge curve with a coulombic efficiency of nearly 100% under a current density of 1 A g1, which illuminate the excellent EDL capacitance performance for the tested cells again. No apparent IR drop (voltage drop) is observed as the current density is lower than 5 A g1 for SC-NSHPC cell, and then the IR drop increases with the increasing current density. Even so, the IR
drop is ca. 0.04 V under a current of 5 A g1 and 0.22 V under 20 A g1 (as shown in Fig. S4), suggesting a low ESR in the test cell. The specific discharge capacitance (F g1) for a single electrode can be calculated using the following formula:
Cm ¼
4ðI DtÞ DV m
(2)
where I refers the applied current density (A), Dt and DV present the discharging time (s) and potential range after the IR drop (V), respectively, and m (g) is the total mass of electrodes. The specific capacitance values calculated from the linear part of discharge curves for all tested electrodes under a lower current density (0.2 A g1) and high current density (20 A g1) are presented in Table 3, and the specific capacitance values of others similar porous carbon electrodes recently reported in literatures also are presented in Table 3. As shown in Table 3, the specific capacitance value of the produced NSHPC electrodes reaches 351 F g1 under a current density of 0.2 A g1, which value is improved 1.7, 1.9 and 2.2 times than that of the pure carbon CMK-K, pristine NSOMC and commercial AC electrodes respectively, indicating an efficiently enhanced charge storage capability for the produced NSHPC electrodes. Meanwhile, more competitive charge storage capability than the similar porous carbon electrodes reported in literatures
Fig. 8. The CV curves of SC-NSHPC, SC-CMK-K, SC-NSOMC and SC-AC at a scan rate of 5 mV s1 (a); galvanostatic chargeedischarge curves of SC-NSHPC, SC-CMK-K, SC-NSOMC and SC-AC under a current density of 1 A g1 (b); Rate capability of the SC-NSHPC, SC-CMK-K, SC-NSOMC and SC-AC (c); The cycling performance of SC-NSHPC, SC-CMK-K, SC-NSOMC and SC-AC at the current density of 2 A g1, and the inset illustration is the charge/discharge curves of the 1st, 1000st, 2000st, 3000st,4000st and the 5000st cycles, respectively (d).
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Table 3 Comparison of specific capacitance with various porous carbon electrode tested by two-electrode configuration. Electrode materials
Electrolyte
Specific capacitance (F g1)
Ref.
2D porous carbon nanosheets KOH-activated nitrogen doped porous carbon nanowires Orientational graphene hydrogel films Graphene-based nitrogen self-doped HPC aerogels 3D HPC derived from KOH activation of GO Bio-inspired beehive-like HPC HPC by KOH activation of shiitake mushroom N-doped HPC derived from KOH activation of aminophenol formaldehyde resin Commercial AC NSOMC CMK-K NSHPC
6M 1M 6M 6M 6M 6M 6M 6M 6M 6M 6M 6M
228 F g1/1 A g1, 189 F g1/40 A g1 291/1 A g1, 150 F g1/10 A g1 183 F g1/1 A g1, 160 F g1/10 A g1 197 F g1/0.2 A g1, 108 F g1/10 A g1 188 F g1/1 A g1 301 F g1/0.1 A g1; 192 F g1/100 A g1 238 F g1/0.2 A g1, 178 F g1/30 A g1 260 F g1/0.2 A g1, 114 F g1/40 A g1 162 F g1/0.2 A g1, 140 F g1/20 A g1 186 F g1/0.2 A g1, 155 F g1/20 A g1 209 F g1/0.2 A g1, 184 F g1/20 A g1 351 F g1/0.2 A g1, 288 F g1/20 A g1
[7] [19] [40] [41] [42] [43] [44] [45] This This This This
also is observed from Table 3, especially under high current density. The large surface area caused by KOH activation and high surface area utilization rate contributed by the ideal micro/mesopores ratio, large micropores diameter and short opened micropores structure as well as the enhanced surface wettability induced by N and S heteroatoms doping response for the superior capacitance performance of the produced NSHPC electrodes. The excellent rate capability of the produced NSHPC electrodes is demonstrated from Fig. 8c. As shown in Fig. 8c, a little decline of capacitance with the increasing current density for NSHPC electrodes is observed, the capacitance even remains at 288 F g1 under a current density of 20 A g1, and more than 82% of the capacitance is remained as the current density increases from 0.2 to 20 A g1. Moreover, the capacitance of the produced NSHPC electrodes return to their original value when the current density turns back from 20A g1 to 0.2 A g1, suggesting a good structure stability and pure EDL capacitance nature of as-prepared materials. A very satisfying cycle stability of the SC-NSHPC cell is revealed by Fig. 8d. After 5000 times charge/discharge cycles, the capacitance retention of the SCNSHPC cell reached up to 95.61% while that for CMK-K, SC-NSOMC and SC-AC cells reached 97.55%, 92.05 and 94.96%, respectively. Although the specific capacitance of the SC-NSHPC cell was dramatically increased after KOH activation, the excellent cycle performances still was maintained. The good structure stability and pure EDL capacitance nature of the produced NSHPC electrodes response for this excellent cycle stability (as shown in the inset curves in Fig. 8d). The high cycling stability can also be confirmed by the CVs before and after 5000 charging-discharging cycles (as presented in Fig. S5 in the Supporting Information). Both curves are almost fully overlapped except a faint decay in plateau current for the latter, further evidencing the excellent cycling stability of NSHPC supercapacitor. KOH activation enhances the conductivity of the produced NSHPC electrodes. As shown in Fig. S6, a lower ESR of 0.93 U for SC-NSHPC cell is obtained from the Nyquist plots of the tested cells while that for SC-CMK-K, SC-NSOMC and SC-AC cells reaches 0.42, 1.15 and 0.96 U. The improved graphitization degree due to the reduction of defects content on graphene lattice caused by KOH activation which has been certified by the results of Raman spectra and XRD characterization contributes to the enhanced conductivity of the produced NSHPC electrodes. For practical application in electronics, it is important to evaluate the leakage current and selfdischarge characteristics of SC-NSHPC (Fig. S7 in the Supporting Information). The leakage current of the device quickly stabilized at ~0.156 mA, indicating a small leakage current and high stability. After being charged at 1.0 V for 30 min, the open-circuit voltage of the device exhibited a rapid decrease in the first hour and gradually reached ~0.744 V after 24 h, revealing a low self-discharge characteristic.
KOH H2SO4 KOH KOH KOH KOH KOH KOH KOH KOH KOH KOH
work work work work
4. Conclusions A NSHPC electrodes materials for supercapacitor was successfully fabricated by KOH treating a NSOMC material. The supercapacitor cell based on NSHPC electrodes exhibits large EDL capacitance, excellent rate capability and very satisfying cycle stability. The capacitance of the produce NSHPC electrodes reaches up to 351 F g1, and remains at 288 F g1 under a large current density of 20 A g1, which value is improved 1.7, 1.9 and 2.2 times than that of the pure carbon CMK-K, pristine NSOMC and commercial AC electrodes respectively, and higher than most of the porous carbon electrodes recently reported in literatures. The large surface area caused by KOH activation and high surface area utilization rate contributed by the ideal micro/mesopores ratio, large micropores diameter and short opened micropores structure as well as the enhanced surface wettability induced by N and S heteroatoms doping and improved conductivity induced by KOH activation result in the very superior capacitance performance of the reported NSHPC electrodes. Meanwhile, the SC-NSHPC cell also exhibits an extraordinary cycling performance with 95.61% capacitance retention after 5000 times charge/discharge cycles at 2.0 A g1, the good structure stability and pure EDL capacitance nature is found to be response for this excellent cycle stability of the produced NSHPC electrodes. Acknowledgment This work was supported by the National Natural Science Foundation of China (Grant No. 51462020 and 201664009), the Key Laboratory of Eco-Environment-Related Polymer Materials of the Ministry of Education Program (Grant No. KF-13-01), the Foundation for Innovation Groups of Basic Research in Gansu Province (Grant No. 1606RJIA322), the Hongliu young teacher cultivate project of Lanzhou University of Technology (Grant No. Q201112). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.05.031. References [1] C. Zhong, Y. Deng, W. Hu, J. Qiao, L. Zhang, J. Zhang, Chem. Soc. Rev. 44 (2015) 7484e7539. [2] Y. Deng, Y. Xie, K. Zou, X. Ji, J. Mater. Chem. A 4 (2016) 1144e1173. lez, E. Goikolea, J.A. Barrena, R. Mysyk, Renew. Sust. Energy Rev. 58 [3] A. Gonza (2016) 1189e1206. [4] E. Lim, C. Jo, J. Lee, Nanoscale 8 (2016) 7827e7833. [5] T. Kim, G. Jung, S. Yoo, K.S. Suh, R.S. Ruoff, ACS Nano 7 (2013) 6899e6905. [6] L. Qie, W. Chen, H. Xu, X. Xiong, Y. Jiang, F. Zou, X. Hu, Y. Xin, Z. Zhang, Y. Huang, Energy. Environ. Sci. 6 (2013) 2497e2504.
120
D. Zhang et al. / Journal of Power Sources 358 (2017) 112e120
[7] X. Fan, C. Yu, J. Yang, Z. Ling, C. Hu, M. Zhang, J. Qiu, Adv. Energy. Mater 5 (2015), 1401761-n/a. [8] M. Sevilla, R. Mokaya, Energy. Environ. Sci. 7 (2014) 1250e1280. [9] Y. Zhu, S. Murali, M.D. Stoller, K.J. Ganesh, W. Cai, P.J. Ferreira, A. Pirkle, R.M. Wallace, K.A. Cychosz, M. Thommes, D. Su, E.A. Stach, R.S. Ruoff, Science 332 (2011) 1537e1541. [10] J. Wang, S. Kaskel, J. Mater. Chem. 22 (2012) 23710e23725. [11] A.J. Amali, J.-K. Sun, Q. Xu, Chem. Commun. 50 (2014) 1519e1522. [12] M. Biswal, A. Banerjee, M. Deo, S. Ogale, Energy. Environ. Sci. 6 (2013) 1249e1259. [13] L. Li, Q. Zhong, N.D. Kim, G. Ruan, Y. Yang, C. Gao, H. Fei, Y. Li, Y. Ji, J.M. Tour, Carbon 105 (2016) 260e267. [14] J.P. Paraknowitsch, A. Thomas, Energy. Environ. Sci. 6 (2013) 2839e2855. ski, M. Szala, M. Bystrzejewski, Carbon 68 (2014) 1e32. [15] W. Kicin [16] M. Zhou, F. Pu, Z. Wang, S. Guan, Carbon 68 (2014) 185e194. [17] D. Zhang, L. Zheng, Y. Ma, L. Lei, Q. Li, Y. Li, H. Luo, H. Feng, Y. Hao, ACS Appl. Mater. Interfaces 6 (2014) 2657e2665. [18] X. Wang, G. Sun, P. Routh, D.-H. Kim, W. Huang, P. Chen, Chem. Soc. Rev. 43 (2014) 7067e7098. [19] B. Wang, J. Qiu, H. Feng, E. Sakai, T. Komiyama, Electrochim. Acta 190 (2016) 229e239. [20] L. Sun, C. Tian, Y. Fu, Y. Yang, J. Yin, L. Wang, H. Fu, ChemeEur. J. 20 (2014) 564e574. [21] Y. Li, G. Wang, T. Wei, Z. Fan, P. Yan, Nano Energy 19 (2016) 165e175. [22] J. Tang, T. Wang, R.R. Salunkhe, S.M. Alshehri, V. Malgras, Y. Yamauchi, ChemeEur. J. 21 (2015) 17293e17298. [23] D. Kang, Q. Liu, J. Gu, Y. Su, W. Zhang, D. Zhang, ACS Nano 9 (2015) 11225e11233. [24] X. Wei, X. Jiang, J. Wei, S. Gao, Chem. Mater 28 (2016) 445e458. [25] D. Zhang, Y. Hao, L. Zheng, Y. Ma, H. Feng, H. Luo, J. Mater. Chem. A 1 (2013) 7584e7591. [26] Y. Ito, W. Cong, T. Fujita, Z. Tang, M. Chen, Angew. Chem. Int. Ed. 54 (2015) 2131e2136. [27] M.A. Lillo-Rodenas, D.C. Amoros, A.L. Solano, Carbon 41 (2003) 267e275.
[28] M. Wahid, G. Parte, D. Phase, S. Ogale, J. Mater. Chem. A 3 (2015) 1208e1215. [29] J. Liu, X. Wang, Q. Lu, R. Yu, M. Chen, S. Cai, X. Wang, J. Electrochem. Soc. 163 (2016) 2991e2998. [30] D. Liu, C. Zeng, D. Qu, H. Tang, Y. Li, B.-L. Su, D. Qu, J. Power Sources 321 (2016) 143e154. [31] W. Deng, Y. Zhang, L. Yang, Y. Tan, M. Ma, Q. Xie, RSC Adv. 5 (2015) 13046e13051. [32] X. Yu, Y. Kang, H.S. Park, Carbon 101 (2016) 49e56. [33] W. Gu, M. Sevilla, A. Magasinski, A.B. Fuertes, G. Yushin, Energy. Environ. Sci. 6 (2013) 2465e2476. [34] U.B. Nasini, V.G. Bairi, S.K. Ramasahayam, S.E. Bourdo, T. Viswanathan, A.U. Shaikh, J. Power. Sources 250 (2014) 257e265. [35] C. Wang, Y. Zhou, L. Sun, P. Wan, X. Zhang, J. Qiu, J. Power. Sources 239 (2013) 81e88. [36] B. Li, F. Dai, Q. Xiao, L. Yang, J. Shen, C. Zhang, M. Cai, Energy. Environ. Sci. 9 (2016) 102e106. [37] S. Zhong, C. Zhan, D. Cao, Carbon 85 (2015) 51e59. [38] J. Joseph, A. Paravannoor, S.V. Nair, Z.J. Han, K. Ostrikov, A. Balakrishnan, J. Mater. Chem. A 3 (2015) 14105e14108. [39] K.A. Cychosz, R. Guillet-Nicolas, J. Garcia-Martinez, M. Thommes, Chem. Soc. Rev. (2017), http://dx.doi.org/10.1039/C6CS00391E. [40] L. Kou, Z. Liu, T. Huang, B. Zheng, Z. Tian, Z. Deng, C. Gao, Nanoscale 7 (2015) 4080e4087. [41] P. Hao, Z. Zhao, Y. Leng, J. Tian, Y. Sang, R.I. Boughton, C.P. Wong, H. Liu, B. Yang, Nano Energy 15 (2015) 9e23. [42] J. Xu, Z. Tan, W. Zeng, G. Chen, S. Wu, Y. Zhao, K. Ni, Z. Tao, M. Ikram, H. Ji, Y. Zhu, Adv. Mater 28 (2016) 5222e5228. [43] W. Tian, Q. Gao, Y. Tan, K. Yang, L. Zhu, C. Yang, H. Zhang, J. Mater. Chem. A 3 (2015) 5656e5664. [44] P. Cheng, S. Gao, P. Zang, X. Yang, Y. Bai, H. Xu, Z. Liu, Z. Lei, Carbon 93 (2015) 315e324. [45] J. Zhou, Z. Zhang, W. Xing, J. Yu, G. Han, W. Si, S. Zhuo, Electrochim. Acta 153 (2015) 68e75.
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Superior supercapacitors based on nitrogen and sulfur co-doped hierarchical porous carbon: Excellent rate capability and cycle stability Deyi Zhang a, b, c, *, Mei Han b, Bing Wang b, Yubing Li b, Longyan Lei b, Kunjie Wang b, Yi Wang b, Liang Zhang b, Huixia Feng b a b c
State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals, Lanzhou University of Technology, Lanzhou 730050, China College of Petrochemical Technology, Lanzhou University of Technology, Lanzhou 730050, China Key Laboratory of Eco-Environment-Related Polymer Materials of Ministry of Education, Northwest Normal University, Lanzhou 730070, China
h i g h l i g h t s An N and S co-doped hierarchical porous carbon was successfully fabricated. The fabricated NSHPC material exhibited high SBET with short opened micropores. High capacitance, excellent rate capability and cycle stability was observed. Large and highly utilized surface area response for superior performance.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 January 2017 Received in revised form 12 April 2017 Accepted 11 May 2017
Vastly improving the charge storage capability of supercapacitors without sacrificing their high power density and cycle performance would bring bright application prospect. Herein, we report a nitrogen and sulfur co-doped hierarchical porous carbon (NSHPC) with very superior capacitance performance fabricated by KOH activation of nitrogen and sulfur co-doped ordered mesoporous carbon (NSOMC). A high electrochemical double-layer (EDL) capacitance of 351 F g1 was observed for the reported NSHPC electrodes, and the capacitance remains at 288 F g1 even under a large current density of 20 A g1. Besides the high specific capacitance and outstanding rate capability, symmetrical supercapacitor cell based on the NSHPC electrodes also exhibits an excellent cycling performance with 95.61% capacitance retention after 5000 times charge/discharge cycles. The large surface area caused by KOH activation (2056 m2 g1) and high utilized surface area owing to the ideal micro/mesopores ratio (2.88), large micropores diameter (1.38 nm) and short opened micropores structure as well as the enhanced surface wettability induced by N and S heteroatoms doping and improved conductivity induced by KOH activation was found to be responsible for the very superior capacitance performance. © 2017 Elsevier B.V. All rights reserved.
Keywords: Supercapacitor Hierarchical porous carbon Heteroatoms doping Rate capability Superior capacitance performance
1. Introduction For overcoming the challenge provoked by the mismatch between the accelerating energy consumption and depletable fossil fuels, exploring sustainable energy and efficient energy storage devices is a pressing issue for researchers. Supercapacitors have been considered as a most promising energy device which could be
* Corresponding author. State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals, Lanzhou University of Technology, Lanzhou 730050, China. E-mail address: [email protected] (D. Zhang). http://dx.doi.org/10.1016/j.jpowsour.2017.05.031 0378-7753/© 2017 Elsevier B.V. All rights reserved.
used for portable electronic devices, hybrid electric vehicles and short-term power sources for mobile electronic devices owning to their high power density, low equivalent series resistance (ESR), long cycle life and fast charge-discharge rates [1e4]. Although supercapacitors demonstrate their distinctive merits, the much lower energy density than batteries (1e2 orders of magnitude) severely restricts its practical application [4,5]. Therefore, recent attention has been focused on increasing the energy density of supercapacitors without sacrificing their high power density and cycle performance [1]. The energy density of the supercapacitor is defined by the following equation: E ¼ CV2/2 [3]. Thus, improving the specific capacitance of electrodes or working voltage of the cells is a
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reasonable option for enhancing the energy density of supercapacitor. In a general sense, the electrodes with high specific area, suitable pore architecture, good wettability and conductivity tend to harvest high charge storage capability [5,6]. Thus, various carbon materials with porous morphology are ideal choices [1]. For instance, the porous carbon nanosheets prepared by Xiaoming Fan et al. exhibit a high specific capacitance of up to 228 F g1 [7]. However, the pristine porous carbons hardly bring a prominent enhancement for energy density of supercapacitors. For improving the specific capacitance of supercapacitors based on porous carbon electrodes, KOH activation is considered as a well-established method [8e10]. For example, the porous graphene activated by KOH exhibits an extra high specific surface area of up to 3100 m2 g1, which specific capacitance reaches up to 166 F g1 in organic electrolyte [9]. Even so, the extra-high specific surface area of KOH activated porous carbons is mainly contributed by the highly-developed micropores [10]. Due to the inherently hydrophobic nature of carbon matrix surface, the availability of the micropores always is unsatisfied, thus it is hard to achieve an expected high specific capacitance [11e13]. Heteroatoms doping is considered as an effective method to manipulate surface physical and chemical activity, improve wettability and conductivity of porous carbon materials [2,14,15]. Especially for nitrogen or sulfur heteroatoms, the difference in electronegativity between the N, S and C atoms provides a more polarized surface, consequentially promoting the wettability of the carbon surface [14,15], which will ensure a fast transfer rate of electrolytic ions in microporous and small mesoporous, improve the availability of surface area and thus enhancing supercapacitance performance of porous carbons [16]. Meanwhile, the pdoping effect of the N and S heteroatoms for graphene lattice of carbon matrix enhances conductivity of porous carbon [17,18]. Actually, the enhanced capacitance performance of KOH-activated nitrogen and sulfur monoacidic or dual doped porous carbon has been reported [16,19e21]. Very recently, Wang et al. reported the KOH-activated nitrogen doped porous carbon nanowires, which exhibits a specific capacitance of 291 F g1. However, the rate capability of these porous materials is unsatisfied, which capacitance retention only reaches over 50% as the current density increasing from 1 to 10 A g1 [19]. Hierarchical porous structure is considered as the key to obtain excellent rate capability for porous carbon [11,22e24]. The nitrogen-doped hierarchical porous carbons derived from nitrogenous dopamine reported by Jing Tang et al. exhibits a specific capacitance of 252 F g1 at a current density of 2 A g1, which capacitance retention reaches 75.7% at a current density of 20 A g1 [22]. An extra high capacitances of 280 F g1 at a current density of 100 A g1 for hierarchical porous carbons derived from carbonization of seaweed also was reported by Danmiao Kang et al. [23]. The capacitance performance of hierarchical porous carbon depends on the specific surface area, ratio of pores with different pore size and connectivity of the hierarchical pores [24]. The suitable hierarchical pores structure and good pore connectivity would promote the diffusion rate of electrolyte ions in pores, and thus further enhances the capacitance performance of porous carbon [11]. Herein, we demonstrate a successful fabrication of a nitrogen and sulfur co-doped hierarchical porous carbon (NSHPC) prepared by KOH activation of nitrogen and sulfur co-doped ordered mesoporous carbon (NSOMC) reported in our previous work [25]. The ordered and opened mesoporous structure of NSOMC provides an ideal matrix for obtaining hierarchical porous carbon with high specific area, suitable hierarchical pores structure and perfect pore connectivity. The resultant NSHPC materials demonstrate a large specific surface area (2056 m2 g1), high specific capacitance
113
(351 F g1 at the current density of 0.2 A g1), good cycle stability (the capacitance retention of 95.61% over 5000 cycles) and excellent rate capability, which capacitance remains up to 288 F g1 at the current density of 20 A g1. 2. Experiment 2.1. Fabrication of NSOMC The NSOMC was fabricated according to our previous work [25]. Briefly, a mixture solution of 1.0 mL of pyrrole (Aldrich, 99%), 0.1 mL of sulphuric acid (98%), and 0.6 mL of absolute alcohol was slowly dropped onto the surface of 1.0 g of SBA-15 template, and then heated at 80 C for 2 h under vacuum to evaporate redundant solvent. A brown powder was obtained by pre-carbonizing above mixture under 160 C for 6 h, and then was carbonized under Ar atmosphere at 350 C for 3 h, and completely carbonized at 650 C for 2 h. Finally, the silica template was removed by 10% HF solution to obtain the NSOMC materials. 2.2. Fabrication of the NSHPC The NSHPC sample was fabricated by KOH-activation of NSOMC under Ar atmosphere. The general fabrication process is summarized and illustrated in Fig. 1. Briefly, the KOH and NSOMC powders were ground together with a mass ratio of 2.0 in an agate mortar for about 10 min. After heating at 400 C for 2 h under an Ar atmosphere, the above mixture was calcined at 700 C for 2 h under an Ar atmosphere, and then followed by washing with HCl and deionized water until the pH of the filtrate was 7.0 and drying at 80 C for 12 h. For comparison, a pure carbon CMK-K with the exclusion of S and N was fabricated by employing the same method using an ordered mesoporous carbon CMK-3 with the similar morphology and pores structure as the fabricated NSHPC as raw material. 2.3. Electrochemical measurements A symmetric supercapacitor cell was assembled using the fabricated NSHPC as electrode materials and 6 mol L1 KOH solution as electrolyte. To prepare the electrodes, NSHPC was ground with acetylene black (10 wt%) and polytetrauoroethylene (PTFE, 10 wt%), and then pressed onto nickel foam which served as a current collector. The total mass of each electrode was between 4.0 and 4.5 mg, and two electrodes with almost identical weight were selected for supercapacitor assembling. The NSHPC electrodes fitted with the separator (PP/PE complex film) and electrolyte solution were symmetrically assembled into electrode/separator/ electrode construction (sandwich-type cells). Electrochemical measurements, including cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), were performed on a CHI 660E electrochemical analyzer (Shanghai, China). The galvanostatic charge/discharge and cycle-life stability were tested using a computer controlled supercapacitor testing system (NEWARE 5V0.1A, Shenzhen China). The symmetric supercapacitor cells using CMK-K, NSOMC and commercial activated carbon (AC) as electrode materials also were assembled for comparing the electrochemical properties with the reported NSHPC. 2.4. Characterization The structures and morphologies of the samples were examined by a high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2010) and a field emission scanning electron microscope (FESEM, Hitachi S-4800). The small-angle X-ray scattering (SAXS)
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Fig. 1. Scheme for the fabrication of the NSHPC.
and X-ray diffraction patterns (XRD) were collected on a X'Pert Xray defractometer (Phillips) using Cu Ka radiation. Raman spectra were recorded via a All the Raman spectrometer (JY-HR800 micro Raman) with an argon ion laser (l ¼ 532 nm). The nitrogen adsorptionedesorption isotherms were measured at 77 K using a Micromeritics ASAP 2020 nitrogen adsorption apparatus (U.S.A.). The SBET (specific surface area) was calculated from the N2 isotherms at relative pressures (P/P0) of 0.06e0.20 via the BET (Brunauere-Emmette-Teller) method. The pore volume was calculated using the BET plot from the amount of nitrogen gas adsorbed at the last adsorption point (P/P0 ¼ 0.95), and the pore size distribution using the BarretteJoynereHalenda (BJH) method. X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Multilab 2000 intrument, using Monochrome Al Ka as the excitation source. 3. Results and discussion NSHPC was fabricated by KOH-treatment of NSOMC under Ar atmosphere. As shown in Fig. 1, the opened and well-ordered mesopores, large specific surface, and high heteroatoms doping content of NSOMC provides an ideal matrix for preparation of N and S co-doped hierarchical porous carbon. Under 400 C, the melted KOH was filled into the ordered mesoporous of NSOMC due to the capillarity, and then the carbon wall reacts with the melted KOH under 700 C to generate developed micropores. The difference of electronegativity and atoms size between C framework atoms and N or S heteroatoms cause lots of edge defects and others structural defects at graphene plan of carbon matrix [15,16]. The C atoms bonded to the N or S heteroatoms located at the defects exhibits high chemical activity [14,26], and tend to prior react with KOH. The prior evolution of micropores around the defects provide a protection to avoid complete collapse of ordered mesopores wall while generate developed micropores in the mesopores wall as the ordered mesoporous structure is partially remained after KOH activation. The very thin pore wall of NSOMC (ca. 6 nm) contributes to form developed short opened micropores while the still remained ordered mesoporous structure ensures the connectivity of the micro/mesopores hierarchical porous structure, and thus ensure high availability of the large surface area. Therefore, the reaction equation between NSOMC and KOH in this work is proposed as follow [27]: The effect of the N and S heteroatoms on the evolution of hierarchical porous structure during the KOH activation process is implied by the XPS results. As shown in Table 1, the KOH activation exhibits a significant effect on the elements composition and their chemical circumstances of the produced NSHPC. The NSOMC exhibits high heteroatoms doping content, which N doping content detected by XPS reaches up to 10.0 at% while the S content reaches 0.69 at%. A sharp decrease is observed for N and S content after treatment by KOH under 700 C, the N and S doping content just
reaches 2.32 and 0.13 at% respectively for NSHPC. During the KOH activation process, the reaction between the C and KOH at high temperature under inert atmosphere responses for the formation of developed micropores [8,10]. The sharp decrease for the N and S heteroatoms content indicates the prior reaction of the C atoms bonded to heteroatoms with the KOH. The incorporation of N and S heteroatoms into the graphene lattice of carbon matrix causes a sharp increase of defects content, the C atoms located at these defects with higher chemical activity tend to prior react with KOH, thus the sharp decrease of the N and S doping content during the KOH activation is comprehensible. Beside the doping content, the activation process also affects the chemical bonds configuration of N and S heteroatoms. As shown in Fig. 2, the N1s orbits of both NSOMC and NSHPC exhibit three XPS peaks around 398.4, 400.4 and 401.0 eV, which are attributed to pyridinic-N, pyrrole-N and graphite-N species, respectively [6]. Among that, the first peak about 398.4 eV is attributed to the pyridinic-N species, which is bound to two carbon atoms and donates one p electron to the conjugation with the aromatic p-conjugated rings, and thus improving the carbon specific capacitance performances. The second peak about 400.4 eV can refer to the pyrrole-N species with a good electron-donor characteristics and higher charge mobility, which contributes to reinforce carbon surface activity in electrontransfer reactions and effectively improves electrochemical behaviors. And the last peak around 401.0 eV can be assigned to graphite-N species, which is incorporated into the graphene lattice by replacing a carbon atom [18,28e30]. The high pyridinic-N and pyrrole-N species content contributes to improve the defects content and thus enhance chemical activity of carbon matrix [15,16]. As shown in Fig. 2 and Table 1, the relativity content of the pyridinic-N and pyrrole-N species apparently decreases after KOH treatment while that of the graphite-N species signally improves from 30.95 to 35.89%, these results also illuminate that the C atoms around pyridinic-N and pyrrole-N species which mainly locate at the defects tend to prior react with KOH, and give priority to form microporous structure. For S2p, the XPS peaks locate at 164.7 and 165.6 eV which refer to CeS and CeSeN binding motifs disappear after KOH activation while the relativity content of the thiophenelike sulfur (163.9 eV) and sulfur oxides (168.4 eV) apparently increases [31,32]. Introduction of sulfide bridges may serve as an effective route to depress the shrinkage of the smallest pores to afford rapid ion transport without inducing unwanted redox reactions [33]. Beside N and S heteroatoms, abundant O element also are detected on both NSOMC and NSHPC. It should be noted that the O content markedly increases from 8.7 to 13.1 at.% after KOH activation, the O element in KOH molecules may be in charge for the increase of surface O content. The increased O content would result the corresponding improvement of Oecontained functional groups on surface of carbon matrix, which would be helpful for the enhancement of surface wettability and thus improving the
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Table 1 Summary of XPS peak analysis on NSOMC and NSHPC. Samples O1s
N1s
S2p
NSOMC
NSHPC
Content
8.7 at. %
13.1 at. %
531.2 eV 532.4 eV 533.9 eV
21.93% 29.03% 49.04%
14.61% 23.33% 62.06%
Content
10.0 at. %
2.32 at. %
398.4 eV 400.4 eV 401.0 eV
26.75% 42.29% 30.95%
25.67% 38.43% 35.89%
Content
0.69 at. %
0.13 at. %
163.9 164.7 165.6 168.4
21.41% 22.23% 4.56% 51.80%
40.68% 0 0 59.32%
eV eV eV eV
Fig. 2. XPS spectra of N1s, S2p and O1s for NSHPC and NSOMC.
availability of micropores and small mesopores. For O1s orbits, the binding energy around 531.2 eV corresponds to the oxygen double bonded to carbon (C¼O) in quinone, and the peak at 532.4 eV is attributed to the singly bonded oxygen (eOe) in CeO groups while the binding energy of 533.9 eV is ascribed to carboxylic groups (COOH) [34,35]. After KOH activation, the relativity content of the quinone and singly bonded oxygen groups sharply decrease while that of the carboxylic groups correspondingly increase from 49.04 to 62.06%. The quinone groups on the surface of carbon matrix are reported in response to the unstable pseudo-capacitance, which causes unsatisfied cycle stability and rate capability [34]. Thus, the decrease of quinone groups would suppress the unstable pseudocapacitance, and effectively improve the cycle stability and rate capability of carbon based supercapacitor. The reserved ordered mesoporous structure of NSOMC after KOH activation is confirmed by the SAXS results. As shown in Fig. 3, the strong (100), (110) and (200) characteristic diffraction peaks of two-dimensional (2-D) hexagonal symmetry (p6mm) mesoporous structure indicate the well-ordered mesoporous structure of NSOMC [25]. After KOH activation, the intensity of the (100), (110) and (200) diffraction peaks apparently decline. Although the (110) and (200) diffraction peaks cannot be identified, the (100) diffraction peak still is distinguishable for the produced NSHPC, indicating that the NSHPC still partly remains ordered mesoporous structure of NSOMC after KOH treatment. The more direct proof of the
Fig. 3. Small angle XRD pattern of NSHPC and NSOMC.
reserved ordered mesoporous structure from the TEM images. As shown in Fig. 4, the pristine NSOMC shows highly ordered stripelike and hexagonally arranged images (Fig. 4 a, b). Although the
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Fig. 4. HRTEM patterns of NSOMC (a and b) and NSHPC (c and d).
formation of KOH generated micro/mesopores in the wall is obvious in the activated sample after KOH activation, the partly ordered stripe-like and hexagonally arranged images still can be distinguished for the produced NSHPC (Fig. 4 c, d). Beside the ordered mesoporous structure, the wheat-like morphology of NSOMC also is partly maintained after activated by KOH. As shown in Fig. 5, the SEM images of NSHPC also exhibit similar morphology like that of NSOMC, illuminating that the generation of micropores in the pore wall due to the KOH activation does not completely destroy the ordered mesoporous structure. Raman spectra was employed to confirm the graphitization degree of the investigated samples. As shown in Fig. 6, the spectra of all samples display two obvious peaks, the G band with E2g symmetry ascribed to ordered sp2 carbon at around 1591 cm1 and the D band with A1g symmetry at about 1364 cm1 ascribed to disordered carbon, edge defect, and other defects [36]. So, the area ratio between G and D bands (IG/ID) indicates the graphitization degree of the samples. Obviously, the NSHPC exhibits an increased graphitization degree than that of the pristine NSOMC, which IG/ID data closes to that of the commercial AC. The increasing IG/ID data after KOH activation indicate that this activation process tends to enhance graphitization degree of samples. The carbon atoms located at the edge and structural defects exhibit higher chemical activity than those carbon atoms on graphene plan, and tend to prior react with KOH. The decrease of defects content on graphene lattice thus increasing the graphitization degree of the sample. The
improved graphitization degree after KOH activation also is affirmed by the wide angle XRD patterns. As shown in Fig. S1, the similar XRD patterns with that of the commercial AC indicate the partly graphitic nature of the NSHPC and NSOMC. The intensity of the peak around 22 of NSOMC which refers to the inter-plane (002) reflections of graphitic carbon apparently increases after KOH activation [37], illuminating the enhanced graphitization degree of the produced NSHPC. The direct proof of hierarchical porous structure of the produced NSHPC from the nitrogen adsorption-desorption experiments. As shown in Fig. 7, the adsorption-desorption isotherms of pristine NSOMC exhibit typical type-IV isotherms with H2 hysteresis loops in the range of ca. 0.4e0.9 P/P0, which suggest mesoporous nature of the NSOMC [25,38,39]. After KOH-activating, the adsorptiondesorption isotherms of the produced NSHPC and CMK-K all exhibit a type-I isotherms with H4 hysteresis loops caused by slitlike pores with uniform shape and size [39]. The pore size distribution of NSOMC, CMK-K and NSHPC is shown in the inset illustration of Fig. 7. The NSOMC shows a uniform mesoporous structure with a narrow pore size distribution centered ca. 3.24 nm. After KOH activation, the produced NSHPC also displays uniform mesoporous structure with a pore size of 3.88 nm, indicates that the ordered mesoporous structure is partly reserved. The increase pore size from 3.24 to 3.88 nm maybe due to the reaction between the pore wall and KOH under high temperature, but which reaction does not completely destroy the mesopores wall, and not induces
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Fig. 7. Nitrogen adsorption-desorption isotherms and the pore size distribution (insert) for NSHPC, CMK-K and NSOMC.
Fig. 5. SEM patterns of NSOMC (a) and NSHPC (b).
the thorough collapse of pore wall. After KOH activation, the SBET of the produced NSHPC increases form 978 m2 g1 to 2056 m2 g1 while the mesopore volume decreases from 1.11 to 0.26 cm3 g1 (as shown in Table 2), the proportion of mesopores declines from 92.5% to 25.7% after KOH activation while the micropore volume dramatically increases from 0.09 to 0.75 cm3 g1. The ratio of micro/ mesopores volume for the produced NSHPC reaches 2.88, which is an ideal ratio for improving the diffusion rate of electrolyte in hierarchical porous structure, and thus tends to get enhanced electrochemical performance. A similar result was also described in a KOH-activated pure carbon CMK-K with ordered mesoporous carbon CMK-3 (as shown in Table 2). Above results demonstrate that the large specific surface area of NSHPC is mainly contributed by micropores which generated from the KOH activation process. The average micropores diameter of the produced NSHPC is calculated according to the below equation:
Dmi
Fig. 6. Raman spectrum of NSHPC, NSOMC and commercial AC.
Vmi Vme þ Dme ¼ Dav Vt Vt Vt
(1)
where Dmi refers to the micripore diameter, Dme and Dav represent the mesoproe and average pore diameter, which are 3.88 and 2.0 nm (as shown in the inset illustration of Fig. 7 and Table 2) respectively, Vmi, Vme and Vt represent the micropores, mesopores and total pore volume of NSHPC, which are 0.75, 0.26 and 1.01 cm3 g1 (as shown in Table 2), respectively. The micropore diameter calculated from equation (1) is about 1.38 nm, the large micropore diameter would be helpful for the fast diffusion of electrolyte ions in micropores. Meanwhile, the pristine NSOMC exhibits a thin pore wall of ca. 6 nm (as shown in Fig. 4), as the KOH is filled into the mesopores and corrodes the pore wall from both sides, the generation of short opened micropores is reasonably comprehensible. The short opened porous structure also would be helpful for rapid diffusion of electrolyte ions in micropores. Over all, the ideal micro/mesopores ratio, large micropores diameter and short opened micropores structure would dramatically raise the availability of micropores, and then improve the surface utilization rate of the produced NSHPC, thus the high charge storage capability is expectable. The capacitive performance of the two-electrode symmetrical supercapacitor cell based on the produced NSHPC electrodes (SCNSHPC) was evaluated using best-practice methods with 6 M KOH
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Table 2 Textural parameters of the NSOMC, CMK-K and NSHPC. Sample ID
Dav/nm
SBET/m2 g1
Vt/cm3 g1
Vme/cm3 g1
Vmi/cm3 g1
Vmi/Vt
NSOMC CMK-K NSHPC
3.24 2.48 2.00
978 1809 2056
1.20 1.12 1.01
1.11 0.96 0.26
0.09 0.16 0.75
0.075 0.14 0.74
Dav: average pore diameter, Vt: total pore volume, Vme: mesoporous volume, Vmi: microporous volume.
as electrolyte. As shown in Fig. 8a, the cyclic voltammetry (CV) curve of SC-NSHPC exhibits a good symmetrically rectangular shape in the potential range from 0 to 1 V under a scan rate of 5 mV s1, illuminates the ideal electrochemical double-layer (EDL) capacitance nature. The similar CV curves are observed for the symmetrical supercapacitor cells based on the pristine pure carbon CMK-K (SC-CMK-K), NSOMC (SC-NSOMC) and commercial AC electrodes (SC-AC), but the area of CV curves of the SC-NSHPC is far higher than that of SC-CMK-K, SC-NSOMC and SC-AC, indicating the more large charge storage capability of SC-NSHPC. As the scan rate increases from 0.5 to 20 mV s1, only a little distortion for CV curves of SC-NSHPC is observed (as presented in Fig. S2 in the Supporting Information) while a little decrease for CV curves area is presented when using the ratio of current intensity vs scan rate as the Y-axis, implying the good ions transport ability and rate capability of the NSHPC electrodes. The galvanostatic chargeedischarge curves of the SC-NSHPC, SC-CMK-K, SC-NSOMC and SC-AC are displayed on Fig. 8b. As shown in Fig 8b and Fig. S3, all tested cells exhibit a triangular chargeedischarge curve with a coulombic efficiency of nearly 100% under a current density of 1 A g1, which illuminate the excellent EDL capacitance performance for the tested cells again. No apparent IR drop (voltage drop) is observed as the current density is lower than 5 A g1 for SC-NSHPC cell, and then the IR drop increases with the increasing current density. Even so, the IR
drop is ca. 0.04 V under a current of 5 A g1 and 0.22 V under 20 A g1 (as shown in Fig. S4), suggesting a low ESR in the test cell. The specific discharge capacitance (F g1) for a single electrode can be calculated using the following formula:
Cm ¼
4ðI DtÞ DV m
(2)
where I refers the applied current density (A), Dt and DV present the discharging time (s) and potential range after the IR drop (V), respectively, and m (g) is the total mass of electrodes. The specific capacitance values calculated from the linear part of discharge curves for all tested electrodes under a lower current density (0.2 A g1) and high current density (20 A g1) are presented in Table 3, and the specific capacitance values of others similar porous carbon electrodes recently reported in literatures also are presented in Table 3. As shown in Table 3, the specific capacitance value of the produced NSHPC electrodes reaches 351 F g1 under a current density of 0.2 A g1, which value is improved 1.7, 1.9 and 2.2 times than that of the pure carbon CMK-K, pristine NSOMC and commercial AC electrodes respectively, indicating an efficiently enhanced charge storage capability for the produced NSHPC electrodes. Meanwhile, more competitive charge storage capability than the similar porous carbon electrodes reported in literatures
Fig. 8. The CV curves of SC-NSHPC, SC-CMK-K, SC-NSOMC and SC-AC at a scan rate of 5 mV s1 (a); galvanostatic chargeedischarge curves of SC-NSHPC, SC-CMK-K, SC-NSOMC and SC-AC under a current density of 1 A g1 (b); Rate capability of the SC-NSHPC, SC-CMK-K, SC-NSOMC and SC-AC (c); The cycling performance of SC-NSHPC, SC-CMK-K, SC-NSOMC and SC-AC at the current density of 2 A g1, and the inset illustration is the charge/discharge curves of the 1st, 1000st, 2000st, 3000st,4000st and the 5000st cycles, respectively (d).
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Table 3 Comparison of specific capacitance with various porous carbon electrode tested by two-electrode configuration. Electrode materials
Electrolyte
Specific capacitance (F g1)
Ref.
2D porous carbon nanosheets KOH-activated nitrogen doped porous carbon nanowires Orientational graphene hydrogel films Graphene-based nitrogen self-doped HPC aerogels 3D HPC derived from KOH activation of GO Bio-inspired beehive-like HPC HPC by KOH activation of shiitake mushroom N-doped HPC derived from KOH activation of aminophenol formaldehyde resin Commercial AC NSOMC CMK-K NSHPC
6M 1M 6M 6M 6M 6M 6M 6M 6M 6M 6M 6M
228 F g1/1 A g1, 189 F g1/40 A g1 291/1 A g1, 150 F g1/10 A g1 183 F g1/1 A g1, 160 F g1/10 A g1 197 F g1/0.2 A g1, 108 F g1/10 A g1 188 F g1/1 A g1 301 F g1/0.1 A g1; 192 F g1/100 A g1 238 F g1/0.2 A g1, 178 F g1/30 A g1 260 F g1/0.2 A g1, 114 F g1/40 A g1 162 F g1/0.2 A g1, 140 F g1/20 A g1 186 F g1/0.2 A g1, 155 F g1/20 A g1 209 F g1/0.2 A g1, 184 F g1/20 A g1 351 F g1/0.2 A g1, 288 F g1/20 A g1
[7] [19] [40] [41] [42] [43] [44] [45] This This This This
also is observed from Table 3, especially under high current density. The large surface area caused by KOH activation and high surface area utilization rate contributed by the ideal micro/mesopores ratio, large micropores diameter and short opened micropores structure as well as the enhanced surface wettability induced by N and S heteroatoms doping response for the superior capacitance performance of the produced NSHPC electrodes. The excellent rate capability of the produced NSHPC electrodes is demonstrated from Fig. 8c. As shown in Fig. 8c, a little decline of capacitance with the increasing current density for NSHPC electrodes is observed, the capacitance even remains at 288 F g1 under a current density of 20 A g1, and more than 82% of the capacitance is remained as the current density increases from 0.2 to 20 A g1. Moreover, the capacitance of the produced NSHPC electrodes return to their original value when the current density turns back from 20A g1 to 0.2 A g1, suggesting a good structure stability and pure EDL capacitance nature of as-prepared materials. A very satisfying cycle stability of the SC-NSHPC cell is revealed by Fig. 8d. After 5000 times charge/discharge cycles, the capacitance retention of the SCNSHPC cell reached up to 95.61% while that for CMK-K, SC-NSOMC and SC-AC cells reached 97.55%, 92.05 and 94.96%, respectively. Although the specific capacitance of the SC-NSHPC cell was dramatically increased after KOH activation, the excellent cycle performances still was maintained. The good structure stability and pure EDL capacitance nature of the produced NSHPC electrodes response for this excellent cycle stability (as shown in the inset curves in Fig. 8d). The high cycling stability can also be confirmed by the CVs before and after 5000 charging-discharging cycles (as presented in Fig. S5 in the Supporting Information). Both curves are almost fully overlapped except a faint decay in plateau current for the latter, further evidencing the excellent cycling stability of NSHPC supercapacitor. KOH activation enhances the conductivity of the produced NSHPC electrodes. As shown in Fig. S6, a lower ESR of 0.93 U for SC-NSHPC cell is obtained from the Nyquist plots of the tested cells while that for SC-CMK-K, SC-NSOMC and SC-AC cells reaches 0.42, 1.15 and 0.96 U. The improved graphitization degree due to the reduction of defects content on graphene lattice caused by KOH activation which has been certified by the results of Raman spectra and XRD characterization contributes to the enhanced conductivity of the produced NSHPC electrodes. For practical application in electronics, it is important to evaluate the leakage current and selfdischarge characteristics of SC-NSHPC (Fig. S7 in the Supporting Information). The leakage current of the device quickly stabilized at ~0.156 mA, indicating a small leakage current and high stability. After being charged at 1.0 V for 30 min, the open-circuit voltage of the device exhibited a rapid decrease in the first hour and gradually reached ~0.744 V after 24 h, revealing a low self-discharge characteristic.
KOH H2SO4 KOH KOH KOH KOH KOH KOH KOH KOH KOH KOH
work work work work
4. Conclusions A NSHPC electrodes materials for supercapacitor was successfully fabricated by KOH treating a NSOMC material. The supercapacitor cell based on NSHPC electrodes exhibits large EDL capacitance, excellent rate capability and very satisfying cycle stability. The capacitance of the produce NSHPC electrodes reaches up to 351 F g1, and remains at 288 F g1 under a large current density of 20 A g1, which value is improved 1.7, 1.9 and 2.2 times than that of the pure carbon CMK-K, pristine NSOMC and commercial AC electrodes respectively, and higher than most of the porous carbon electrodes recently reported in literatures. The large surface area caused by KOH activation and high surface area utilization rate contributed by the ideal micro/mesopores ratio, large micropores diameter and short opened micropores structure as well as the enhanced surface wettability induced by N and S heteroatoms doping and improved conductivity induced by KOH activation result in the very superior capacitance performance of the reported NSHPC electrodes. Meanwhile, the SC-NSHPC cell also exhibits an extraordinary cycling performance with 95.61% capacitance retention after 5000 times charge/discharge cycles at 2.0 A g1, the good structure stability and pure EDL capacitance nature is found to be response for this excellent cycle stability of the produced NSHPC electrodes. Acknowledgment This work was supported by the National Natural Science Foundation of China (Grant No. 51462020 and 201664009), the Key Laboratory of Eco-Environment-Related Polymer Materials of the Ministry of Education Program (Grant No. KF-13-01), the Foundation for Innovation Groups of Basic Research in Gansu Province (Grant No. 1606RJIA322), the Hongliu young teacher cultivate project of Lanzhou University of Technology (Grant No. Q201112). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.05.031. References [1] C. Zhong, Y. Deng, W. Hu, J. Qiao, L. Zhang, J. Zhang, Chem. Soc. Rev. 44 (2015) 7484e7539. [2] Y. Deng, Y. Xie, K. Zou, X. Ji, J. Mater. Chem. A 4 (2016) 1144e1173. lez, E. Goikolea, J.A. Barrena, R. Mysyk, Renew. Sust. Energy Rev. 58 [3] A. Gonza (2016) 1189e1206. [4] E. Lim, C. Jo, J. Lee, Nanoscale 8 (2016) 7827e7833. [5] T. Kim, G. Jung, S. Yoo, K.S. Suh, R.S. Ruoff, ACS Nano 7 (2013) 6899e6905. [6] L. Qie, W. Chen, H. Xu, X. Xiong, Y. Jiang, F. Zou, X. Hu, Y. Xin, Z. Zhang, Y. Huang, Energy. Environ. Sci. 6 (2013) 2497e2504.
120
D. Zhang et al. / Journal of Power Sources 358 (2017) 112e120
[7] X. Fan, C. Yu, J. Yang, Z. Ling, C. Hu, M. Zhang, J. Qiu, Adv. Energy. Mater 5 (2015), 1401761-n/a. [8] M. Sevilla, R. Mokaya, Energy. Environ. Sci. 7 (2014) 1250e1280. [9] Y. Zhu, S. Murali, M.D. Stoller, K.J. Ganesh, W. Cai, P.J. Ferreira, A. Pirkle, R.M. Wallace, K.A. Cychosz, M. Thommes, D. Su, E.A. Stach, R.S. Ruoff, Science 332 (2011) 1537e1541. [10] J. Wang, S. Kaskel, J. Mater. Chem. 22 (2012) 23710e23725. [11] A.J. Amali, J.-K. Sun, Q. Xu, Chem. Commun. 50 (2014) 1519e1522. [12] M. Biswal, A. Banerjee, M. Deo, S. Ogale, Energy. Environ. Sci. 6 (2013) 1249e1259. [13] L. Li, Q. Zhong, N.D. Kim, G. Ruan, Y. Yang, C. Gao, H. Fei, Y. Li, Y. Ji, J.M. Tour, Carbon 105 (2016) 260e267. [14] J.P. Paraknowitsch, A. Thomas, Energy. Environ. Sci. 6 (2013) 2839e2855. ski, M. Szala, M. Bystrzejewski, Carbon 68 (2014) 1e32. [15] W. Kicin [16] M. Zhou, F. Pu, Z. Wang, S. Guan, Carbon 68 (2014) 185e194. [17] D. Zhang, L. Zheng, Y. Ma, L. Lei, Q. Li, Y. Li, H. Luo, H. Feng, Y. Hao, ACS Appl. Mater. Interfaces 6 (2014) 2657e2665. [18] X. Wang, G. Sun, P. Routh, D.-H. Kim, W. Huang, P. Chen, Chem. Soc. Rev. 43 (2014) 7067e7098. [19] B. Wang, J. Qiu, H. Feng, E. Sakai, T. Komiyama, Electrochim. Acta 190 (2016) 229e239. [20] L. Sun, C. Tian, Y. Fu, Y. Yang, J. Yin, L. Wang, H. Fu, ChemeEur. J. 20 (2014) 564e574. [21] Y. Li, G. Wang, T. Wei, Z. Fan, P. Yan, Nano Energy 19 (2016) 165e175. [22] J. Tang, T. Wang, R.R. Salunkhe, S.M. Alshehri, V. Malgras, Y. Yamauchi, ChemeEur. J. 21 (2015) 17293e17298. [23] D. Kang, Q. Liu, J. Gu, Y. Su, W. Zhang, D. Zhang, ACS Nano 9 (2015) 11225e11233. [24] X. Wei, X. Jiang, J. Wei, S. Gao, Chem. Mater 28 (2016) 445e458. [25] D. Zhang, Y. Hao, L. Zheng, Y. Ma, H. Feng, H. Luo, J. Mater. Chem. A 1 (2013) 7584e7591. [26] Y. Ito, W. Cong, T. Fujita, Z. Tang, M. Chen, Angew. Chem. Int. Ed. 54 (2015) 2131e2136. [27] M.A. Lillo-Rodenas, D.C. Amoros, A.L. Solano, Carbon 41 (2003) 267e275.
[28] M. Wahid, G. Parte, D. Phase, S. Ogale, J. Mater. Chem. A 3 (2015) 1208e1215. [29] J. Liu, X. Wang, Q. Lu, R. Yu, M. Chen, S. Cai, X. Wang, J. Electrochem. Soc. 163 (2016) 2991e2998. [30] D. Liu, C. Zeng, D. Qu, H. Tang, Y. Li, B.-L. Su, D. Qu, J. Power Sources 321 (2016) 143e154. [31] W. Deng, Y. Zhang, L. Yang, Y. Tan, M. Ma, Q. Xie, RSC Adv. 5 (2015) 13046e13051. [32] X. Yu, Y. Kang, H.S. Park, Carbon 101 (2016) 49e56. [33] W. Gu, M. Sevilla, A. Magasinski, A.B. Fuertes, G. Yushin, Energy. Environ. Sci. 6 (2013) 2465e2476. [34] U.B. Nasini, V.G. Bairi, S.K. Ramasahayam, S.E. Bourdo, T. Viswanathan, A.U. Shaikh, J. Power. Sources 250 (2014) 257e265. [35] C. Wang, Y. Zhou, L. Sun, P. Wan, X. Zhang, J. Qiu, J. Power. Sources 239 (2013) 81e88. [36] B. Li, F. Dai, Q. Xiao, L. Yang, J. Shen, C. Zhang, M. Cai, Energy. Environ. Sci. 9 (2016) 102e106. [37] S. Zhong, C. Zhan, D. Cao, Carbon 85 (2015) 51e59. [38] J. Joseph, A. Paravannoor, S.V. Nair, Z.J. Han, K. Ostrikov, A. Balakrishnan, J. Mater. Chem. A 3 (2015) 14105e14108. [39] K.A. Cychosz, R. Guillet-Nicolas, J. Garcia-Martinez, M. Thommes, Chem. Soc. Rev. (2017), http://dx.doi.org/10.1039/C6CS00391E. [40] L. Kou, Z. Liu, T. Huang, B. Zheng, Z. Tian, Z. Deng, C. Gao, Nanoscale 7 (2015) 4080e4087. [41] P. Hao, Z. Zhao, Y. Leng, J. Tian, Y. Sang, R.I. Boughton, C.P. Wong, H. Liu, B. Yang, Nano Energy 15 (2015) 9e23. [42] J. Xu, Z. Tan, W. Zeng, G. Chen, S. Wu, Y. Zhao, K. Ni, Z. Tao, M. Ikram, H. Ji, Y. Zhu, Adv. Mater 28 (2016) 5222e5228. [43] W. Tian, Q. Gao, Y. Tan, K. Yang, L. Zhu, C. Yang, H. Zhang, J. Mater. Chem. A 3 (2015) 5656e5664. [44] P. Cheng, S. Gao, P. Zang, X. Yang, Y. Bai, H. Xu, Z. Liu, Z. Lei, Carbon 93 (2015) 315e324. [45] J. Zhou, Z. Zhang, W. Xing, J. Yu, G. Han, W. Si, S. Zhuo, Electrochim. Acta 153 (2015) 68e75.