Self-activation of nitrogen and sulfur dual-doping hierarchical porous carbons for asymmetric supercapacitors with high energy densities

Carbon 153 (2019) 225e233

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Self-activation of potassium gluconate derived nitrogen and sulfur dual-doping hierarchical porous carbons for asymmetric supercapacitors with high energy densities Xiaoliang Wu a, b, Bing Ding a, Chenguang Zhang a, Bin Li a, b, **, Zhuangjun Fan c, * a b c

Northeast Forestry University, College of Science, Heilongjiang Key Lab Mol Design & Preparat Flame, Harbin, 150040, PR China Northeast Forestry University, Postdoctoral Mobile Res Stn Forestry Engn, Harbin, 150040, PR China China University of Petroleum, College of Material Science and Engineering, Qingdao, 266580, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 June 2019 Received in revised form 29 June 2019 Accepted 5 July 2019 Available online 5 July 2019

Porous carbon obtained by the conventional two-step method (chemical activation after carbonization) is confronted with the problems of complex preparation process, excess of strong corrosive chemical agents and poor ion-transport kinetics. Herein, we report a convenient way for the preparation of porous carbons by a self-activation process using potassium gluconate as the precursor, and then followed by effective nitrogen/sulfur dual-doping. The obtained carbon materials have hierarchical porous framework with moderate specific surface area, numerous mesopores and high content of heteroatoms doping. Benefitting from their synergistic effect, the nitrogen and sulfur dual-doped hierarchical porous carbons electrode displays a high specific capacitance of 320 F g
1 at 0.5 A g
1, superior rate characteristic and excellent electrochemical stabilization. More interestingly, an asymmetric supercapacitor using the nitrogen/sulfur dual-doped hierarchical porous carbons as negative electrode and the hierarchical porous carbons/manganese dioxide composite as positive electrode exhibits a high energy density of 42.5 Wh kg
1 and good electrochemical stabilization (90.3% capacitance retention after 5000 cycles). Contrast with the traditional preparation way for porous carbons, this method can be easily executed, avoiding complex process, hard/soft template and excess activation, and thus highlights a novel, convenient and efficient way to synthesize high performance porous carbons for supercapacitor. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Self-activation Potassium gluconate Porous carbon Heteroatoms doping Asymmetric supercapacitor

1. Introduction Supercapacitors have recently drawn numerous attentions owing to their distinctive features, for instance, rapid chargedischarge rate, ultrahigh power density and outstanding cycle durability [1e5]. In various electrode materials, carbon materials, especially activated carbons have drawn extensive interests owing to their massive raw materials, low cost, large specific surface area and excellent chemical stabilization, which make it regard as the most suitable electrode material for commercial supercapacitors [6,7]. However, the activated carbons-based supercapacitors are confronted with low energy density (<10 Wh kg
1), which limits their extensive applications. The pore structure of activated carbons

* Corresponding author. ** Corresponding author. Northeast Forestry University, College of Science, Heilongjiang Key Lab Mol Design & Preparat Flame, Harbin, 150040, PR China. E-mail addresses: [email protected] (B. Li), [email protected] (Z. Fan). https://doi.org/10.1016/j.carbon.2019.07.020 0008-6223/© 2019 Elsevier Ltd. All rights reserved.

mostly consist of micropores (<2 nm). Such pore structure usually suffers from long diffusion pathway and large ion diffusion resistance, and thus all access to the whole electrode/electrolyte interface area is not achieved, which results in an unsatisfactory specific capacitance and poor rate performance [8,9]. Besides, activated carbon obtained by the conventional two-step method (chemical activation after carbonization) is confronted with the problems of complex preparation process, excess of strong corrosive chemical agents and poor ion-transport kinetics [10]. Hierarchical porous carbons (HPCs) with controllable porosity, high specific surface area and interconnected pore structure have drawn extensive attentions [11e14]. Micropores (<2 nm) provide large surface area for ion adsorption and thus enhance the capacitive capability, while mesopores (2e50 nm) can act as high speed pathway to reduce ion diffusion resistance and achieve excellent rate capability. Besides, macropores (>50 nm) act as ion-buffering reservoirs and effectively shorten ion transport route. Generally, HPCs can be obtained by carbonization of organic precursor with

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hard/soft template and post-activation approach. Template way is powerful, but involves tedious and time-consuming process with preparation and removal of templates, extra harsh chemical postactivation, which are not conducive to large-scale production [15,16]. More recently, synthesis of HPCs derived from biomass materials is highly promising, which can simplify the synthetic process, but remain face with poor tailoring of pore structure because of the inherent microstructure of biomass precursors [17,18]. Therefore, development of a facile and large-scale production way to synthesize HPCs is highly attractive, but still a huge challenge. Besides tailoring of the microstructure of porous carbons, doping heteroatoms (N, S, B, O atoms) in carbon skeleton is another effective strategy to boost the electrochemical capability of porous carbons [19e24]. Particularly, nitrogen doping has been reported to be the most attractive strategy for improving capacitance significantly by changing the electrical conductivity, electrochemical activity and also by generating Faraday redox reaction [25e27]. Compared with monoatom doping that improves merely one aspect of performance, dual-doping with other heteroatoms can improve overall characteristic of materials owing to their synergetic effect [28e30]. Herein, we report a convenient way to prepare hierarchical porous carbons by a self-activation process using potassium gluconate as the precursor, and then followed by effective nitrogen/ sulfur dual-doping. Benefitting from the hierarchical porous framework with moderate specific surface area, numerous mesopores and high content of heteroatoms doping, the nitrogen/sulfur dual-doped hierarchical porous carbons (NSHPC) shows high specific capacitance, superior rate characteristic and outstanding cycle durability. More interestingly, an asymmetric supercapacitor using NSHPC materials as negative electrode and hierarchical porous carbons/manganese dioxide (HPC-700/MnO2) composite as positive electrode (as seen in Scheme 1) shows high energy density and superior cycle durability.

2. Experimental section 2.1. Construction of hierarchical porous carbons 3 g potassium gluconate was directly carbonized at various temperatures (600, 700, 800  C) for 120 min under N2, and the prepared materials were bathed by dilute HCl and deionized water. The products were dried in oven. The prepared materials were denoted as HPC-x, which x relates to the temperature of heating. For comparison, glucose was directly carbonized at 700  C for 120 min in N2 and the products were denoted as GC. Then GC and KOH with a mass proportion of 1:1 were further activated at 700  C for 120 min in N2. The products bathed by dilute HCl and deionized water and the products were denoted as AGC.

2.2. Construction of nitrogen/sulfur dual-doped hierarchical porous carbons 50 mL thiourea solution (12 mg mL
1) was mixed with 200 mg HPC-700 under stirring. The mixture was dried in oven. Afterwards the mixture was pyrolyzed at 750  C for 2 h under N2. The obtained samples were named as NSHPC.

2.3. Construction of hierarchical porous carbons/manganese bioxide 100 mL potassium hypermanganate solution (4 mg mL
1) was mixed with 150 mg HPC-700 with vigorous stirring for 5 h. Afterwards the mixture was heated in a domestic microwave oven for 10 min. At last, the obtained materials were filtered, bathed three times using deionized water, and dried in oven. The prepared materials were denoted as HPC-700/MnO2.

Scheme 1. Schematic diagram illustrating the construction of the electrode materials using potassium gluconate as the precursor for asymmetric supercapacitor device.

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2.4. Material characterizations The prepared materials of scanning/transmission electron microscopy (SEM/TEM) images were conducted by JEOL JSM-6480 and JEM-2010. Thermogravimetric analysis (TGA) was performed from 30 to 1000  C in N2 by a thermogravimetric analyzer TG/STDA 851e Mettler Toledo. The structure characterization of the prepared samples was performed by X-ray diffractometer and Raman spectrometer. Nitrogen adsorption/desorption curves were checked by Quantachrome NOVA 2000. X-ray photoelectron spectroscope (XPS) tests were investigated by a PHI5700ESCA spectrometer. 2.5. Electrochemical measurements Electroactive material, carbon black and tetrafluoroethylene were mixed according to the mass proportion of 75:20:5 in the ethanol to generate slurry, then pressed on Ni foam current collector. The three-electrode system with electroactive materials coated Ni foam, Pt foil and Hg/HgO electrodes were acted as the working, counter and reference electrodes, respectively. The asymmetrical supercapacitor device was assembled using NSHPC as negative electrode and HPC-700/MnO2 as positive electrode and separated by a glassy fibrous separator. The asymmetric supercapacitor was measured in 1 M Na2SO4 aqueous electrolyte. 3. Results and discussion To reveal the process of structural evolution of hierarchical porous carbons, potassium gluconate was carbonized at various temperatures (600e800  C) and the obtained materials were detected by scanning electron microscope (SEM). As observed in

227

Fig. 1aec, All HPC samples show porous structure with massive mesopores and with the increase of pyrolysis temperature, the pore size increases. The formation process of porous structure was checked by thermogravimetric analysis (TGA). As seen in Fig. 1d, in the TGA curve of potassium gluconate, the original slight weight loss from 30  C to 180  C can be ascribed to the dewatering of the crystalline water. Moreover, two distinct weight lessness at 200 to 500  C and 700 to 850  C are attributed to the pyrolysis of the organic moiety and the activation process of carbon, respectively. At 600  C, potassium gluconate precursor decomposes to generate potassium carbonate and the resulting potassium carbonate was embedded into the carbon skeleton as template, thus the porous structure is formed after washing with hydrochloric acid (Fig. 1a). With the increase of pyrolysis temperature, potassium carbonate decomposed and generated K2O and CO2, the porous structure was developed and the pore size further increased to 30 nm at 700  C (Fig. 1b). When the pyrolysis temperature increase to 800  C, the molten salt effect may promote the dissolution of carbon samples in K2O melt and further strengthen the reaction between K2O and carbon, resulting in that the pore sizes were further increased and carbon walls were partly tattered (Fig. 1c). These results confirm that the size-controlled preparation of pore carbon can be obtained by changing the pyrolysis temperature. For comparison, the direct carbonized glucose (GC, Figs. S1f and g) and KOH-activated GC (AGC, Figs. S1h and i) show rather dense structure without porous structure. The porous structures of the carbon materials were also demonstrated by nitrogen adsorption/desorption isotherm. All samples display the combined characteristics of I and IV-type nitrogen adsorption/desorption isotherm profiles (Fig. 1e), demonstrating the existence of micropores and mesopores structure. The

Fig. 1. SEM images of (a) HPC-600, (b) HPC-700, (c) HPC-800. (d) TGA curve of potassium gluconate. (e) Nitrogen adsorption-desorption isotherms of the HPC-600, HPC-700 and HPC-800 samples. (f) Pore size distribution of the HPC-600, HPC-700 and HPC-800 samples. (A colour version of this figure can be viewed online.)

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detailed porous characteristic of the carbon materials are recorded in Table S1. With the increase of pyrolysis temperature, the Brunauer-Emmett-Teller (BET) specific surface area progressively increases from 370 m2 g
1 (HPC-600) to 709 m2 g
1 (HPC-700) and finally to 920 m2 g
1 (HPC-800) because of high temperature is in favor of activation. Moreover, the pore size distributions of the carbon samples were computed by DFT method. As seen in Fig. 1f, the pore size distributions become broader as the increasing of pyrolysis temperature, which is in accord with the SEM observation. Similar with HPC-700, after nitrogen and sulfur dual-doping, the NSHPC sample still remains the similar porous structure with massive mesopores (Fig. 2a). The well-distribution of C, O, N, and S elements can be observed from the element mapping images of NSHPC (Figs. S1bee). The porous structure of NSHPC was further characterized through transmission electron microscopy (TEM). As observed in Fig. 2b, NSHPC is composed of substantial highly interconnected mesopores to form three-dimensional interconnected porous framework, which could facilitate ion transport to the interior of the bulk materials and reduce ion diffusion resistance. Furthermore, high-resolution TEM confirms numerous micropores in the surface of carbon skeleton (Fig. 2c), which is propitious to energy storage for supercapacitor. The structure properties of the carbon samples were performed by X-ray diffraction (XRD) and Raman spectroscopy. As seen in Fig. S2a, all samples show two wide characteristic peaks with low intensity at 23.4 and 43.5 , which are attributed to the (002) and (100) plane of carbon, indicating that the obtained samples have

massive defects and disordered carbons. The results are also confirmed by Raman spectroscopy. As seen in Fig. S2b, the D peak (cm
1) and G peak (cm
1) are ascribed to the defective graphitic structure and the characteristic of graphitic layer, respectively. After nitrogen/sulfur dual-doping, the ratio of ID/IG increases from 0.99 to 1.01, meaning the presence of more defects and disordered structure in NSHPC [31]. The surface features of the carbon samples were tested by XPS analysis (Fig. 2d) and the relevant data is summarized in Table S2. The NSHPC samples exhibit four strong peaks at 164.7, 285.7, 400.5, and 533.3 eV, which are attributed to S2p, C1s, N1s and O1s peaks, indicating successful doping of N and S heteroatoms into porous carbons. The N and S contents evaluated from the elemental analysis are 4.3 and 1.2 at.% in NSHPC, respectively. Furthermore, the spectrum of N1s peak can be approximately split into four peaks at around 399.0, 400.1, 401.2 and 402.3 eV (Fig. 3a), which are attributed to pyridinic-N (N-6), pyrrolic-N (N-5), quaternary-N (NQ) and oxidized-N (NeO) [32e34]. Generally, the N-6 and N-5 can offer extra free electron or delocalized electron, which take part in pseudocapacitive faradaic reactions. Moreover, the N-Q can improve the conductivity of carbon by serving as electron donors and/or by attracting protons [35,36]. For the S2p peak (Fig. 3b), the S2p can be divided into three peaks at 164.7, 166.1 and 168.1 eV, which are ascribed to S2p 3/2, S2p 1/2 and SOx groups with characteristic of thiophene-S because of their spine-orbit coupling [29,37]. The porous characteristic of the NSHPC materials was also conducted by nitrogen adsorption/desorption isotherm (Fig. 3c).

Fig. 2. (a) SEM image of NSHPC. (b) TEM image of NSHPC. (c) High-resolution TEM image of NSHPC. (d) XPS spectra survey of the HPC-600, HPC-700, HPC-800 and NSHPC samples. (A colour version of this figure can be viewed online.)

X. Wu et al. / Carbon 153 (2019) 225e233

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Fig. 3. (a) High-resolution N 1s spectra of the NSHPC samples. (b) High-resolution S2p spectra of the NSHPC samples. (c) Nitrogen adsorption-desorption isotherms of the NSHPC samples. (d) Pore size distribution of the NSHPC samples. (A colour version of this figure can be viewed online.)

The NSHPC samples have a similar nitrogen adsorption/desorption isotherm curves with HPC-700 and the specific surface area slightly increased to 721 m2 g
1. Moreover, as seen in the pore size distribution curves (Fig. 3d), the NSHPC samples have substantial micropores between 0.6 and 2 nm and mesopores between 5 and 30 nm. The well-developed micropores and mesopores structure with suitable surface area and fast ion diffusion channel can effectively improve charge storage and ion diffusion kinetics. As expected, the NSHPC samples have the well-developed hierarchical porous framework with suitable surface area and substantial heteroatom doping, which would be in favor of high capability supercapacitor. The electrochemical capabilities of the carbon materials were checked through a three-electrode system in 6 M KOH aqueous electrolyte. Cyclic voltammetry (CV) were firstly performed in the voltage window of
1 to 0 V. CV profile of the NSHPC electrode displays a relatively rectangular shape in contrast with the GC, AGC and HPC electrodes, indicating the NSHPC electrode has a better capacitive characteristic (Figs. 4a and S3a). The NSHPC electrode shows higher current response, indicating that it has a higher specific capacitance than other samples. Notably, even at 200 mV s
1, the NSHPC electrode remains a nearly rectangle-like contour, meaning outstanding rate characteristic (Fig. 4b). Moreover, the charge/discharge profiles of NSHPC electrode (Fig. 4c) exhibit slightly distorted triangular contour, further confirming the electrode material has a good capacitive characteristic. As seen in Fig. 4d, the NSHPC electrode shows a specific capacitance of 320 F g
1 at 0.5 A g
1, which is comparable to those of commercial activated carbon (YP-50), GC, AGC, HPC-700 electrodes and other published porous carbons in literatures (Table 1). Significantly, the

specific capacitance of NSHPC still keeps 200 F g
1 at 50 A g
1, confirming outstanding rate characteristic. The kinetics of ion and charge transport was performed by electrochemical impedance spectroscopy. As seen in Fig. 4e, The NSHPC electrode shows a relatively lower intrinsic resistance (0.41 U) than those of GC (0.47 U), AGC (0.48 U) and HPC-700 (0.43 U). Moreover, in contrast with GC, AGC and HPC-700, the NSHPC electrode displays a smaller semicircle in the high frequency area, meaning a rapider charge transfer characteristic. Furthermore, the NSHPC electrode shows a nearly vertical slope at the low frequency area, demonstrating a small ion diffusion resistance and an ideal capacitive behavior. The electrochemical stability of NSHPC was conducted by repeating CV tests at 200 mV s
1 for 10,000 cycles (Fig. 4d). The specific capacitance of NSHPC can keep 99.4% capacitance retention after 10,000 cycles, confirming outstanding electrochemical stabilization. In recent years, asymmetric supercapacitors can effectively combine positive and negative voltage ranges, and further enhance the energy density, which have attracted extensive attentions [38e42]. Additionally, reasonable matching of positive and negative electrode materials with suitable mass loading is crucial to optimize the energy density of the full cell. Among different positive materials, MnO2 has drawn numerous attentions owing to its large theory capacitance, broad potential window and low cost [43e45]. Moreover, the carbon materials can act as sacrificial substrate for the reduction of KMnO4 to MnO2 based on the redox reaction in neutral solution [22]: 2
2
4MnO
4 þ 3C þ H2O / 4MnO2 þ CO3 þ 2HCO3

(1)

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Fig. 4. (a) CV profiles of the GC, AGC, HPC-700 and NSHPC electrodes at 50 mV s
1. (b) CV profiles of the NSHPC electrodes at various scan rates. (c) Galvanostatic charge-discharge profiles of the NSHPC electrode at different current densities. (d) Specific capacitance of the GC, AGC, HPC-700 and NSHPC electrodes at various current densities. (e) Nyquist plots of the GC, AGC, HPC-700 and NSHPC electrodes. (f) Electrochemical stabilization of the NSHPC electrode. (A colour version of this figure can be viewed online.)

Table 1 Summary of electrochemical capabilities of the carbon materials. Sample

C (F g
1)

Electrolyte

Ref

Hierarchical porous carbon Hierarchical porous carbon N-doped porous carbon N-doped mesoporous carbon N-doped porous graphitic carbon N, S co-doped porous carbon nanosheets N, S co-doped hierarchical porous carbon NSHPC

318.2(0.5 A g
1) 294.0(2 mV s
1) 245.0(0.5 A g
1) 190.2(0.5 A g
1) 293.0(1.0 A g
1) 281.0(0.5 A g
1) 272.0(1.0 A g
1) 320.0(0.5 A g
1)

6M 6M 6M 6M 6M 6M 6M 6M

8 17 21 24 26 29 38 This work

KOH KOH KOH KOH KOH KOH KOH KOH

X. Wu et al. / Carbon 153 (2019) 225e233

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Fig. 5. (a) XRD pattern of the HPC-700/MnO2 composite. (b) TEM image of the HPC-700/MnO2 composite. (c) CV profiles of the HPC-700/MnO2 electrode at various scan rates. (d) Specific capacitance of the HPC-700/MnO2 electrode at different scan rates. (A colour version of this figure can be viewed online.)

The XRD pattern (Fig. 5a) confirms that the characteristic peaks of MnO2 is in accord with birnessite-type MnO2 (JCPDS 42e1317). The high-resolution TEM image demonstrates that the obtained MnO2 nanoflakes can be tightly decorated on the surface of porous carbon (Fig. 5b), which ensures rapid electron transfer and thus improves electrochemical utilization of MnO2. The d-spacing of MnO2 is 0.72 nm (Fig. S4), corresponding to (001) diffraction peak of MnO2. As seen in Fig. S5a, the XPS spectrum of the HPC-700/ MnO2 sample exhibits three strong peaks at 296.4, 543.4 and 658.4 eV corresponding to C 1s, O 1s and Mn 2p peaks, which further confirms successful synthesis of HPC-700/MnO2 composite. High-resolution spectrum of Mn 2p (Fig. S5b) can be fitted into two peaks at binding energies of 642.2 and 653.8 eV corresponding to Mn 2p 3/2 and Mn 2p 1/2 [46,47]. The electrochemical characteristics of the HPC-700/MnO2 composite were measured in 1 M Na2SO4 aqueous eletrolyte in a three-electrode system by CV. As seen in Fig. 5c, CV profile of the HPC-700/MnO2 composite exhibits quasi-rectangular contour at 100 mV s
1, meaning superior rate characteristic. The HPC-700/MnO2 composite delivers a specific capacitance of 225 F g
1 at 2 mV s
1 and superior rate characteristic (Fig. 5d). An asymmetric supercapacitor was assembled using the NSHPC samples as negative electrode and the HPC-700/MnO2 samples as positive electrode in 1 M Na2SO4. As seen in Fig. 6a, the steady voltage ranges are from
1.1 to
0.1 V for NSHPC and from
0.1e0.9 V for HPC-700/MnO2. As a result, the total potential of the cell can broaden to 2.0 V in 1 M Na2SO4 aqueous electrolyte. The charge balance for asymmetric supercapacitor follows the

relationship qþ ¼ q- (Eq. S(1) and S(2)). Fig. S6a displays the CV curves of the NSHPC//HPC-700/MnO2 asymmetric supercapacitor at various scan rates. The NSHPC//HPC-700/MnO2 asymmetric supercapacitor shows a quasi-rectangular contour even at 200 mV s
1, meaning a rapid charge/discharge performance. As seen in Fig. 6b, the charge/discharge profiles of the asymmetric supercapacitor exhibit slightly distorted triangular contour, confirming a good capacitive characteristic. The NSHPC//HPC-700/ MnO2 asymmetrical supercapacitor shows a high specific capacitance of 76.5 F g
1 at 0.5 A g
1 (Fig. S6b). Due to its high specific capacitance and broad voltage range, the NSHPC//HPC-700/MnO2 asymmetrical supercapacitor shows a high energy density of 42.5 Wh kg
1 at 666.7 W kg
1 (according to the gross mass of the active materials of both electrodes), which is comparable with other reported Mn-based asymmetrical supercapacitors in aqueous electrolyte (Fig. 6c), for instance hierarchically porous carbon//hierarchically porous carbon/MnO2 (25.93 Wh kg
1 at 199.9 W kg
1) [45], carbon fiber fabric/MnO2//carbon fiber fabric/MnO2 (20 Wh kg
1 at 175 W kg
1) [46], activated carbon//MnO2/CNTs (13.3 Wh kg
1 at 600 W kg
1) [47], activated carbon//NaMnO2 (19.5 Wh kg
1 at 130 W kg
1) [48], graphene//MnO2 (25.2 Wh kg
1 at 100 W kg
1) [49], hollow carbon spheres//hollow carbon spheres/ MnO2 (18.04 Wh kg
1 at 8.97 W kg
1) [50]. Electrochemical impedance was further investigated the electrode kinetics of the asymmetrical supercapacitor. As seen in Fig. S5c, the ohmic resistance of the electrolyte and cell components (R (e)) of the asymmetrical supercapacitor is 0.73 U, indicating a good conductivity. Furthermore, the NSHPC//HPC-700/MnO2 asymmetrical

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Fig. 6. (a) CV curves of the individual NSHPC and HPC-700/MnO2 electrodes in 1 M Na2SO4 at 10 mV s
1. (b) CV curves of the NSHPC//HPC-700/MnO2 asymmetrical supercapacitor at various scan rates. (c) Ragone plot of the NSHPC//HPC-700/MnO2 and other previously reported Mn-based asymmetrical supercapacitors. (d) Electrochemical stabilization of the NSHPC//HPC-700/MnO2 asymmetrical supercapacitor. (A colour version of this figure can be viewed online.)

supercapacitor can keep 90.3% capacitance retention after 5000 cycles at 5 A g
1 (Fig. 6d), confirming superior electrochemical stabilization. 4. Conclusions In summary, we report a convenient way for the construction of hierarchical porous carbon by a self-activation process using potassium gluconate as the precursor, and then followed by effective nitrogen and sulfur co-doping. Due to the hierarchical porous framework with moderate specific surface area, numerous mesopores and high content of heteroatoms doping, the NSHPC materials show high specific capacitance, superior rate characteristic and good electrochemical stabilization. Remarkably, an asymmetric supercapacitor using NSHPC material as negative electrode and HPC-700/MnO2 composite as positive electrode can achieve high energy density and superior electrochemical stabilization. Acknowledgements This work was supported by National Natural Science Foundation of China (51702043), Taishan Scholars Program and Heilongjiang Postdoctoral Foundation (LBH-Z18008). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2019.07.020.

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