A novel synthesis of hierarchical porous carbons from interpenetrating polymer networks for high performance supercapacitor electrodes

Carbon 111 (2017) 667e674

Contents lists available at ScienceDirect

Carbon journal homepage: www.elsevier.com/locate/carbon

A novel synthesis of hierarchical porous carbons from interpenetrating polymer networks for high performance supercapacitor electrodes Dazhang Zhu a, 1, Yawei Wang a, 1, Wenjing Lu a, Hong Zhang a, Ziyang Song a, Dong Luo a, Lihua Gan a, Mingxian Liu a, **, Dongmei Sun b, * a Shanghai Key Lab of Chemical Assessment and Sustainability, School of Chemical Science and Engineering, Tongji University, 1239 Siping Road, Shanghai, 200092, China b School of Life Science & Technology, Tongji University, 1239 Siping Road, Shanghai 200092, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 22 February 2016 Received in revised form 7 October 2016 Accepted 9 October 2016 Available online 12 October 2016

A novel strategy for the synthesis of hierarchical porous carbons (HPCs) from interpenetrating polymer networks (IPNs) for advanced supercapacitor electrodes was reported. There is hydrogen-bonding interaction between resorcinol/formaldehyde (R/F) resol and zinc tartrate, and they were introduced into the inter space of sodium polyacrylate (PAAS) to form IPNs. HPCs with foam-like macropores, uniform mesopores (~3.8 nm), and abundant micropores were fabricated by direct carbonization of the IPNs. The macropores come from the pyrolysis of PAAS, and the uniform mesopores are ascribed to the synergistic effect of PAAS and zinc tartrate, while the decomposition of IPNs and zinc tartrate and the carbothermal reduction process generate abundant micropores. The resultant HPCs with a high specific surface area up to 1371 m2 g
1 as a supercapacitor electrode exhibit a high specific capacitance of 283 F g
1 at 1.0 A g
1. Besides, the electrode shows high rate capability in which a high current density of 20 A g
1 for charge/discharge operation is available (182 F g
1). Moreover, HPC-1.5 electrode shows excellent electrochemical stability up to 10000 cycles at 2.0 A g
1 with 95.86% retention. This finding highlights new opportunities for well-structured porous carbons derived from IPNs to achieve advanced supercapacitor devices. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction Over the past several years, the continuous increasing energy demand has been accelerating at an alarming speed due to the global economic expansion and worsening pollution [1,2]. Therefore, developing energy conversion and storage devices with high efficiency and sustainability becomes a very important task. Supercapacitors (or electrochemical capacitors) are one of the most promising energy storage devices due to their high charge/ discharge efficiency, high cycle life and high power density, which meet the requirements of the application for hybrid electric

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (M. Liu), [email protected] (D. Sun). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.carbon.2016.10.016 0008-6223/© 2016 Elsevier Ltd. All rights reserved.

vehicles, cell phones, cameras, cordless tools and pulse laser [3,4]. According to different energy storage mechanisms, supercapacitors can be divided into two types: electrical double layer capacitors (EDLCs) and pseudocapacitors [5]. EDLCs store electrical energy by electrostatic accumulation of ionic charges at the double-layer interfaces between electrolyte and electrode, such as carbon materials [6,7], while pseudocapacitors store the electrical energy by fast and reversible faradic redox reactions occurring at the surface of electrodes, such as transition metal oxides and conducting polymers [8,9]. Electrode material is one of the important factors that influence the electrochemical performance of supercapacitors. Transition metal oxides and conducting polymers used as supercapacitor electrodes possess high specific capacitance, but suffer from poor cyclability and rate capability which limit their further applications [10,11]. On the contrary, porous carbon materials are the most promising candidates for commercial electrodes due to their large specific surface area, high conductivity, excellent chemical stability,

668

D. Zhu et al. / Carbon 111 (2017) 667e674

non-toxicity and low cost [12e14]. The specific capacitance of carbon based supercapacitor is influenced by the specific surface area and the pore structure of carbons importantly [15,16]. Micropores can significantly enhance the specific surface area of carbon materials and increase the capacitance of electrodes [17]. Mesopores provide channel for ion diffusion to improved electrochemical performance, while macropores serve as the ion reservoir and shorten the ion diffusion distance [18,19]. Consequently, the optimum pore structure for carbon materials used as supercapacitor electrode is supposed to possess smaller pores interconnected with larger sets of pores, which could achieve both high surface area and efficient ion diffusion pathways [20,21]. Recently, hierarchical porous carbons (HPCs) with a combination of micropores, mesopores and macropores have attracted broad attention to be used in supercapacitors [22e24]. Up to now, there have been several methods, including activation process and dual template synthesis, developed to the preparation of HPCs to achieve a better capacitive performance for supercapacitors [19,25e28]. The activation process often forms more micropores in macroporous or mesoporous carbon materials. However, the resulting carbon materials have non-uniform pore sizes and isolated non-interconnected pores. In contrast, dual template techniques have been widely used to obtain porous carbons with hierarchical porous structures and uniform pore sizes due to different sizes of templates. The templates commonly involve SiO2, ZnO, zeolite as the hard template and surfactant such as P123 and F127 as the soft template [18,29e32]. For example, Wang et al. prepared hierarchical porous graphitic carbons (HPGCs) by using phenolic resin as carbon source and Ni(OH)2 as template [33]. The typical HPGC shows a high specific surface area of 970 m2 g
1 and a specific capacitance of 198 F g
1 at a current density of 1.0 A g
1 in 6 M KOH. Qie et al. reported the preparation of three-dimensional hierarchical porous carbons (THPCs) through a facile modified chemical activation route with polypyrrole microsheets as precursor and KOH as activating agent [34]. The typical THPC has a large specific surface area of 2870 m2 g
1 and a large specific capacitance of 318.2 F g
1 at a current density of 0.5 A g
1. Guo et al. reported the preparation of HPCs through the self-assembly of poly(benzoxazine-co-resol) with ionic liquid C16mimBF4 and a carbonization process [35]. The typical sample shows a specific surface area of 376 m2 g
1 with a high skeleton density and a high specific capacitance of 247 F g
1 at a current density of 0.5 A g
1. Polymer blend carbonization has been regarded as a feasible method for the preparation of porous carbon materials. The polymer composite composes of a carbon precursor polymer and a decomposable polymer that is pyrolyzed to generate pores [36e38]. However, there is generally the macro-phase separation between carbon precursor and decomposable polymer which often generates broad distribution, small pore volume and low surface area in the resultant carbon materials [37,39]. For another, interpenetrating polymer networks (IPNs), defined as a combination of two polymeric networks at least one of which is synthesized and/or cross-linked in the immediate presence of the other, are special way to polymer blend [40]. The different polymers do not have chemical link, but physical contact with cross structure on a polymer molecular scale to overcome the negative influence resulted from the macro-phase separation. Therefore, IPNs are potential precursors for the preparation of porous carbons. Herein, we demonstrate a novel strategy for synthesis of HPCs based on IPNs to achieve high performance supercapacitor electrodes. Resorcinol/formaldehyde (R/F) resol, which has H-bonding interaction with zinc tartrate, was introduced into the interspace of sodium polyacrylate (PAAS) to form IPNs by swelling process, but there is no chemical link between zinc tartrate and PAAS. During the carbonization, PAAS was pyrolyzed to generate foam-like

macropores, and the synergistic effect of PAAS and zinc tartrate brings uniform mesopores (~3.8 nm). While the decomposition of IPNs and zinc tartrate and the carbothermal reduction process generate abundant micropores, endowing a hierarchical pore structure and a high specific surface area (up to 1371 m2 g
1) of the carbons. When used as supercapacitor electrodes, HPCs show a high specific capacitance of 283 F g
1 at 1.0 A g
1 and 182 F g
1 at a high current density of 20 A g
1 in 6 M KOH electrolyte solution. Besides, the electrodes maintain 95.86% retention of the original electrochemical capacitance after 10000 cycles, exhibiting excellent cycle stability. This finding opens up a new window for the synthesis of well-developed hierarchical porous carbons from IPNs, which are synthesized by swelling process, to support high performance supercapacitor application. 2. Experimental section 2.1. Materials Resorcinol, tartaric acid, zinc chloride was purchased from Sigma-Aldrich. Formaldehyde (37 wt%), graphite, sodium carbonate and concentrated hydrochloric acid (36.5 wt%) were obtained from Sinopharm Chemical Reagent Co. Ltd., China. Sodium polyacrylate (PAAS, a main component of a commercial super absorbant polymer, and hereafter PAAS was used to denote it) was obtained from LINYIWOHUA Co. Ltd., China. Polytetrafluoroethylene (PTFE, catalog number FR301B) was obtained from Shanghai 3F New Materials Co. Ltd. Nickel foil was provided by Shanghai Hongxiang Plant. Pure nitrogen was purchased from BOC Special Gases Sales Service Co. Ltd. All the chemicals were used as received and without further purification. 2.2. Synthesis of HPCs In a typical synthesis process, 0.5 g (4.5 mmol) resorcinol was dissolved in 10 mL sodium carbonate solution (0.34 mmol L
1), then 0.7 mL formaldehyde solution was added into the above mixture and stirred for 10 min. R/F was prepolymerized at 85  C for 3 h, then 30 mL zinc tartrate solution composed 1.03 g tartrate acid (6.8 mmol), 0.93 g zinc chloride (6.8 mmol), 3.8 mL of concentrated hydrochloric acid and 26.2 mL deionized water were added into R/F resol solution. After stirring for 30 min, the solution was transferred into a Teflon-lined stainless steel autoclave. PAAS was gradually added until the mixed solution changed into red gel completely. The autoclave was heated at 115  C for 24 h to obtain zinc tartrate and PAAS containing R/F polymer (R/F-PAAS-Zn). After washing with ethanol, the polymer was dried at 60  C in vacuum for 24 h, followed by carbonization under pure N2 atmosphere according to the following heating procedure: firstly heated to 650  C with a heating rate of 5  C min
1 (maintained for 2 h), and then heated to 950  C at 2  C min
1 (maintained for 6 h) to prepare HPCs (denoted as HPC-x, where x represents the mole ratio of zinc tartrate to resorcinol). For comparison, samples were prepared according to the HPC-1.5 recipe without zinc tartrate or PAAS, and were denoted as C-R/F-PAAS or C-R/F-Zn. 2.3. Characterization X-ray diffraction (XRD) patterns were recorded using a Focus D8 Advance diffractometer (Bruker, Germany) equipped with Cu-Ka radiation (l ¼ 0.154 nm) source at scanning speed of 10 min
1 in the range of 10e80 . The elemental compositions were investigated employing Energy Disperse Spectroscopy (EDS, 200 kV). Scanning electron microscopy (SEM) characterization was conducted on Hitachi S-4800 equipment. Transmission electron microscopy

D. Zhu et al. / Carbon 111 (2017) 667e674

669

(TEM) observation was performed using a JEM-2100 instrument operating at 200 kV. Before observation, the carbon sample was dispersed in ethanol, and the suspension was added drop wise on a carbon-coated copper grid. Nitrogen sorption and textural properties of all samples were determined at
196  C using nitrogen on a Micromeritics Tristar 3000 analyser. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) model based on adsorption data in the partial pressure (P/P0) ranging from 0.02 to 0.25 and the total pore volume was determined from the amount of nitrogen adsorbed at relative pressure of 1. The pore size distribution was calculated using the Barrett-Joyner-Halenda (BJH) model based on the desorption branch. All of the samples were degassed at 200  C for 2 h prior to the measurements. 2.4. Electrochemical measurement The test electrode was prepared by mixing as-prepared carbon samples (2.0 mg), graphite and PTFE (with a mass ratio of 8:1:1) on a nickel foam. The electrode was placed in a drying oven at 80  C for 24 h. All electrochemical behaviors of the working electrodes was performed by cyclic voltammetry (CV), galvanostatic chargedischarge (GCD) and electrochemical impedance spectroscopy (EIS) on a CHI660D electrochemical instrument with a typical three-electrode system in which Pt and Hg/HgO electrode was used as counter and reference electrode, respectively. The voltage range for CV test varied from
1.0 to 0 V with the different scan rates of 10e500 mV s
1. While, the voltage range for the galvanostatic measurement varied from
1.0 to 0 V at the current density from 1.0 to 20 A g
1. All measurements were performed in 6 M KOH electrolyte. The capacitance was calculated from CV curves according to the following equation (1):

Fig. 1. (a) XRD patterns of HPC-1.5 (carbonized at 950  C) and PAAS (carbonized at 600  C), (b) EDS spectrum of HPC-1.5 (carbonized at 950  C). (A colour version of this figure can be viewed online.)

Z IdV C¼

2  DV  m  r

(1)

where C (F g
1) is the specific capacitance, I (A) is the response current, V (V) is the potential, DV (V) is the potential window, m (g) is the mass of the active material in the working electrode and r (V s
1) is the potential scan rate. The specific capacitance was calculated from the GCD curves according to the following equation (2):

C¼

I  Dt DV  m

(2)

where C (F g
1) is the specific capacitance, I (A) is the constant discharging current, Dt (s) is the discharging time, DV (V) is the potential window and m (g) is the mass of the active material in the working electrode. 3. Results and discussion XRD patterns of HPC-1.5 (carbonized at 950  C) and PAAS (carbonized at 600  C) are given in Fig. 1a. For HPC-1.5, two broad peaks at 2q ¼ 24 and 43 are observed, corresponding to the (002) and (100) facets of amorphous carbon. There is no peak which is ascribed to zinc (or zinc oxide) or sodium (or sodium oxide) appeared in the pattern, implying the complete sublimation and removal of zinc and sodium species during the carbonization of R/ F-PAAS-Zn. On the one hand, zinc tartrate was decomposed to ZnO, and then ZnO could be reduced to gaseous metal Zn by the freshly produced carbon via ZnO þ C /Zn (g) þ CO at ~907  C [30,41]. On the other hand, the peak positions of PAAS carbonized at 600  C correspond to standard pattern of the Na2CO3 (PDF 19e1130), indicating Na2CO3 was generated from pyrolysis of PAAS. With

temperature increased, Na2CO3 was further decomposed into sodium oxide, which was further reduced into Na via Na2O þ C / 2Na þ CO and evaporated at 883  C [42,43]. The elemental composition of the HPC-1.5 was studied by EDS spectrum (Fig. 1b) to further certify the evaporation of Zn and Na. As a result, pure carbon materials were obtained by the carbonization process. Fig. 2 shows SEM images of C-R/F-PAAS, R/F-Zn and HPCs. C-R/FPAAS shows foam-like structure with abundant and open macropores, as shown in Fig. 2a. In the IPNs of R/F-PAAS, carbonization of R/F (thermosetting resins) provides carbon skeleton for the resultant HPCs, while decomposition of PAAS (thermoplastic resins) results in the formation of well-connected macropores. There is no macropores in C-R/F-Zn without the addition of PAAS, as shown in Fig. 2b. HPCs (Fig. 2c) retain similar foam-like structure compared with C-R/F-PAAS. The decomposition of zinc tartrate, the reduction of metal oxides and the evaporation of metal create pores for the HPCs [44]. Therefore, increase of the amount of zinc tartrate brings more abundant pore structure (Fig. 2d). However, when further increase the mole ratio of zinc tartrate/resorcinol to 2, the framework of the HPCs collapses due to the excessive pores generated by zinc tartrate, as shown in Fig. 2e. N2 adsorption-desorption isotherms and pore size distribution curves of C-R/F-PAAS, C-R/F-Zn and HPCs are shown in Fig. 3. C-R/FPAAS and C-R/F-Zn exhibit type I curve according to IUPAC classification (Fig. 3a) [45]. These curves increase rapidly at low pressure (<0.3 P/P0), indicating the existence of micropores, and the micropores were also confirmed by the pore size distribution curves shown in Fig. 3b. While HPCs exhibit type IV curves, the obvious hysteresis loop at high pressure (0.4e1.0 P/P0) reveals the existence of mesopores. Their pore size distribution curves indicate that the most probable pore sizes are around 3.8 nm. A typical TEM image of

670

D. Zhu et al. / Carbon 111 (2017) 667e674

Fig. 2. SEM images of C-R/F-PAAS (a), C-R/F-Zn (b), HPC-1(c), HPC-1.5 (d) and HPC-2 (e).

Fig. 3. (a) Nitrogen adsorption-desorption isotherms and (b) the pore size distribution curves of C-R/F-PAAS, C-R/F-Zn and HPCs. (A colour version of this figure can be viewed online.)

HPC-1.5 shown in Fig. 4a exhibits foam-liked macropores connected with each other, and the Fig. 4b shows uniform mesopores with a pore size similar with that of Fig. 3b. The uniform mesopores in HPCs should be ascribed to the synergistic effect of PAAS and zinc tartrate in R/F precursor. The schematic synthesis of HPCs is illustrated in Fig. 5. Resorcinol and formaldehyde was prepolymerized to obtain R/F resol solution [46].

There is hydrogen bond interaction between the hydroxyl of R/F resol and that of zinc tartrate, and the hydroxyl among zinc tartrate also could form H-bonding, leading to the formation of assemblies (Fig. 5A) [47]. The R/F resol containing zinc tartrate was absorbed into PAAS through a swelling process (Fig. 5B). During the hydrothermal reaction, R/F resol was further polymerized to obtain polymer, and it formed IPNs with PAAS. The zinc tartrate assemblies

Fig. 4. TEM images of HPC-1.5 in different size.

D. Zhu et al. / Carbon 111 (2017) 667e674

671

Fig. 5. Schematic illustration for the synthesis of HPCs. (A colour version of this figure can be viewed online.)

were trapped into the interspace of the R/F-PAAS IPNs (Fig. 5C). It is the removal of the zinc tartrate assemblies and PAAS that bring uniform mesopores (~3.8 nm) after the carbonization procedure. PAAS was pyrolyzed to form foam-like macropores, and the R/F decomposition and the evaporation of Zn (decomposed from zinc tartrate) generate abundant micropores. In addition, the sodium ions should contribute to the generation of micropores via a chemical activation process like KOH. All of the above process endow a hierarchical pore structure of the carbons (Fig. 5D). However, there is not such a limited interspace in C-R/F-PAAS and C-R/F-Zn for zinc tartrate molecules, and finally only macro- and/or microporous carbons were obtained. Table 1 shows the pore structure parameters of C-R/F-PAAS, C-R/ F-Zn and HPCs. C-R/F-PAAS shows a specific surface area of 336 m2 g
1 and total pore volume of 0.19 cm3 g
1. In C-R/F-Zn, besides carbonization of R/F, the removal of zinc tartrate from R/F polymer also generated micropores. As a result, C-R/F-Zn shows a higher surface area (629 m2 g
1) and total pore volume (0.32 cm3 g
1). The specific surface areas and pore volume of HPCs are higher than those of C-R/F-PAAS and C-R/F-Zn due to the

combination of micro-, meso- and macropores. As we have discussed before, introduction of zinc tartrate brings micro- and mesopores for the HPCs, which is beneficial to the fabrication of high surface porous carbon materials. When the mole ratio of zinc tartrate/resorcinol reaches 1.5, the HPCs show an optimum pore structure with a highest surface area up to 1371 m2 g
1 and a highest pore volume of 1.23 cm3 g
1. Decrease of the zinc tartrate results to low surface area of the HPCs, while excessive zinc tartrate makes the carbon framework collapse. As a result, the surface area of HPC-2 is decreased from 1371 m2 g
1 to 976 m2 g
1. Fig. 6a gives CV curves of C-R/F-PAAS, C-R/F-Zn and HPCs at a scan rate of 10 mV s
1 in 6 M KOH electrolyte solution. All electrodes show quasi-rectangular voltammogram shape in a potential window between 0 and
1.0 V, suggesting typical electric doublelayer capacitive energy storage. Generally, the specific capacitance of an electrode is in proportion to the integrated area of its CV profile under the same scan rate and voltage window; i.e., the larger the integrated area is, the higher the specific capacitance is [48]. The capacitances of the electrodes calculated from CV curves decrease in the following order: HPC-1.5 (288 F g
1) > HPC-2

Table 1 Pore structure parameters of C-R/F-PAAS, C-R/F-Zn and HPCs. Samples

SBET (m2 g
1)

Smicro (m2 g
1)

D (nm)

Vmicro (cm3 g
1)

Vmeso (cm3 g
1)

Vtotal (cm3 g
1)

C-R/F-PAAS C-R/F-Zn HPC-1 HPC-1.5 HPC-2

336 629 727 1371 976

273 623 556 1108 849

e e 3.7 3.8 3.7

0.16 0.29 0.13 0.47 0.43

0.03 0.03 0.26 0.76 0.46

0.19 0.32 0.39 1.23 0.89

SBET, specific surface area; Smicro, specific surface area of micropores; D, the most probable size of pore; Vmicro, the micropores volume; Vmeso, the mesopores volume; Vtotal, the total pore volume.

672

D. Zhu et al. / Carbon 111 (2017) 667e674

Fig. 6. CV curves of (a) C-R/F-PAAS, C-R/F-Zn, HPC-1, HPC-1.5 and HPC-2 electrodes at the scan rate of 10 mV s
1 and (b) HPC-1.5 electrode at different scan rates in 6 M KOH electrolyte solution. (A colour version of this figure can be viewed online.)

(218 F g
1) > HPC-1 (108 F g
1) > C-R/F-Zn (103 F g
1) > C-R/F-PAAS (97 F g
1). The small humps in the CV curves at low scan rates are derived from oxygen groups (determined by EDS in Fig. 1b). Among the carbon samples, HPC-1.5 shows the highest specific capacitance due to its uniform mesopores and highest specific surface area. The CV curves of HPC-1.5 shown in Fig. 6b retain identical basic shapes at a scanning rate of 50 mV s
1, indicating stable and reproducible capacitance behavior. When the scanning rate was increased to 100 mV s
1 and above, the shapes exhibited an increased distortion from the typical rectangular shape, which is mainly resulted from the inherent inner resistance in the electrode materials. GCD curves of C-R/F-PAAS, C-R/F-Zn and HPCs at a current density of 1.0 A g
1 are shown in Fig. 7a. These curves show regular triangular shapes between 0 and
1.0 V versus Hg/HgO electrode,

suggesting high coulombic efficiency and good reversibility. Fig. 7b shows GCD curves of the HPC-1.5 electrode under different current densities. It still retains a triangular shape even at a high current density of 20 A g
1, suggesting that the electrode is suitable for application in supercapacitor devices in which fast charge/ discharge is needed. Fig. 7c gives the specific capacitances of carbon electrodes under different loading current densities. The calculated specific capacitances of HPC-1.5 is 283 F g
1 at 1 A g
1 and 182 F g
1 at 20 A g
1, much higher than those of C-R/F-PAAS, C-R/FZn, HPC-1 and HPC-2. The excellent electrochemical performance of HPC-1.5 electrodes could be ascribed to following aspects: (a) foam-like macropores act as the ion reservoir and shorten the ion diffusion distance of electrolyte ions to the interior carbon surfaces, (b) uniform mesopores allow electrolyte ions to transfer rapidly

Fig. 7. GCD curves of (a) C-R/F-PAAS, C-R/F-Zn, HPC-1, HPC-1.5 and HPC-2 electrodes at 1.0 A g
1, (b) HPC-1.5 electrode at different current densities, (c) the relationship between the specific capacitances of carbon electrodes and current density, (d) cycle stability of HPC-1.5 electrode at 2.0 A g
1 in 6 M KOH electrolyte solution. (A colour version of this figure can be viewed online.)

D. Zhu et al. / Carbon 111 (2017) 667e674

during the charge/discharge process, and (c) abundant micropores could significantly increase the specific surface area of carbon materials and thus enhance the electric double-layer capacitance of electrodes. In addition, the specific capacitance of HPC-1.5 electrode is much larger than those of other HPCs, such as K700, CS15A6, CN2 and THPC-5 [18,31,49,50]. K700, CS15A6 and THPC-5 take the advantages of higher specific surface area. However, the micropores of these HPCs are generated by KOH activation (similar to the preparation of activated carbons) and are extremely small. Poor accessibility of electrolyte ions into the pore surface causes much difficulty to achieve proportionality between their electrochemical capacitances and surface areas [51]. The introduction of zinc tartrate into R/F derived carbons leads to relatively large micropores [44], and the synergistic effect of zinc tartrate and PAAS in R/F precursor produced uniform mesopores. Therefore, HPC-1.5 provide channel for electrolyte ions diffusion during the charge/ discharge process effectively and improve utilization percentage of specific surface area. As a result, the Cspec/SBET (the specific capacitance per unit surface area) of HPC-1.5 electrode is higher than those of above mentioned HPCs. The electrochemical capacitance and the Cspec/SBET of HPC-1.5 electrode are also higher than those of other carbon electrodes like carbon spheres [52e55], carbon nanotubes [56], carbon nanosheets [56], carbon fibers [57] and activated carbons [42,58e60]. The long-term cycle stability of HPC1.5 electrode was also evaluated by GCD test at a current density of 2.0 A g
1 for 10000 cycles, as shown in Fig. 7d. After 10000 cycles, the specific capacitance of HPC-1.5 electrode is 255 F g
1 and 95.86% retention of initial capacitance (266 F g
1), indicating excellent cycling stability. Electrochemical impedance spectroscopy (EIS) was conducted to understand the conductive and diffusive behavior of HPCs. Fig. 8 shows the Nyquist plot of all electrodes measured in 6 M KOH solution in the frequency range of 100 kHz to 0.01 Hz. In the low frequency region, vertical line (HPC-1.5) is the most parallel to the imaginary axis, indicating the good performance of HPC-1.5 as electrode materials for supercapacitors. The intermediate frequency region is the 45 line in all electrodes, which denotes the feature of ion diffusion into the electrode materials. The quasisemicircle at high frequency observed in HPCs electrodes corresponds to polarization resistance or charge transfer resistance of the electrode. The first intersection point in the semicircle with the real axis gives the equivalent series resistance (ESR), which

Fig. 8. Nyquist plot based on HPC-1, HPC-1.5 and HPC-2 electrodes with frequency range of 105 to 10
2 Hz. (A colour version of this figure can be viewed online.)

673

comprises electrolyte resistance, the intrinsic resistance of the active material, and the interfacial contact resistance of the active material/current collector. In this study, the ESR values of HPC-1.5 is 0.7 U, much lower than that of HPC-1 (1.2 U) and HPC-2 (0.9 U), which depicts the good transport of the HPC-1.5 electrolyte to the electrolyte/electrode interface of electrode materials. 4. Conclusions In conclusion, we demonstrate a novel synthesis of HPCs based on IPNs of PAAS and R/F-zinc tartrate. The resultant HPCs have a high specific surface area of 1371 m2 g
1 and exhibits foam-like macropores, uniform mesopores (~3.8 nm) and abundant micropores, which benefit ion transportation and diffusion and contribute to excellent electrochemical performance. HPCs as a supercapacitor electrode show a high specific capacitance of 283 F g
1 at 1.0 A g
1 in 6 M KOH electrolyte solution. Besides, the electrode maintains 182 F g
1 at a high current density of 20 A g
1. Furthermore, HPCs also exhibit excellent electrochemical stability up to 10000 cycles at 2 A g
1, with 95.86% retention of the original electrochemical capacitance. Therefore, we believe that the methodology presented in this work provides new opportunities for the synthesis of well-designed HPCs from IPNs to achieve high performance electrochemical energy storage. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (21207099, 21273162, 21473122), the Science and Technology Commission of Shanghai Municipality, China (11NM0501000, 12ZR1451100, 14DZ2261100), the Fundamental Research Funds for the Central Universities and the Large Equipment Test Foundation of Tongji University (0002014021, 0002014022). References [1] K.S. Xia, Q.M. Gao, J.H. Jiang, J. Hu, Hierarchical porous carbons with controlled micropores and mesopores for supercapacitor electrode materials, Carbon 46 (2008) 1718e1726. [2] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nat. Mater. 7 (2008) 845e854. [3] E. Frackowiak, F. Beguin, Carbon materials for the electrochemical storage of energy in capacitors, Carbon 39 (2001) 937e950. [4] D.Z. Zhu, Y.W. Wang, L.H. Gan, M.X. Liu, K. Cheng, Y.H. Zhao, et al., Nitrogencontaining carbon microspheres for supercapacitor electrodes, Electrochim. Acta 158 (2015) 166e174. [5] R. Kotz, M. Carlen, Principles and applications of electrochemical capacitors, Electrochim. Acta 45 (2000) 2483e2498. [6] Y.H. Zhao, M.X. Liu, X.X. Deng, L. Miao, P.K. Tripathi, X.M. Ma, et al., Nitrogenfunctionalized microporous carbon nanoparticles for high performance supercapacitor electrode, Electrochim. Acta 153 (2015) 448e455. [7] M.X. Liu, L.H. Gan, W. Xiong, Z.J. Xu, D.Z. Zhu, L.W. Chen, Development of MnO2/porous carbon microspheres with a partially graphitic structure for high performance supercapacitor electrodes, J. Mater. Chem. A 2 (2014) 2555e2562. [8] H.L. Li, L.X. Jiang, Q.L. Cheng, Y. He, V. Pavlinek, P. Saha, et al., MnO2 nanoflakes/hierarchical porous carbon nanocomposites for high-performance supercapacitor electrodes, Electrochim. Acta 164 (2015) 252e259. [9] O. Ghodbane, J.L. Pascal, F. Favier, Microstructural effects on charge-storage properties in MnO2-based electrochemical supercapacitors, ACS Appl. Mater. Interface 1 (2009) 1130e1139. [10] H. Jiang, J. Ma, C.Z. Li, Mesoporous carbon incorporated metal oxide nanomaterials as supercapacitor electrodes, Adv. Mater. 24 (2012) 4197e4202. [11] W.J. Lu, M.X. Liu, L. Miao, D.Z. Zhu, X. Wang, H. Duan, et al., Nitrogen-containing ultramicroporous carbon nanospheres for high performance supercapacitor electrodes, Electrochim. Acta 205 (2016) 132e141. [12] M. Inagaki, H. Konno, O. Tanaike, Carbon materials for electrochemical capacitors, J. Power Sources 195 (2010) 7880e7903. [13] S. Wang, B. Pei, X. Zhao, R.A.W. Fe, Highly porous graphene on carbon cloth as advanced electrodes for flexible all-solid-state supercapacitors, Nano Energy 2 (2013) 530e536. [14] Q. Shi, R.Y. Zhang, Y.Y. Lu, Y.H. Deng, A.A. Elzatahrya, D.Y. Zhao, Nitrogen-

674

[15] [16]

[17] [18]

[19]

[20]

[21] [22]

[23]

[24]

[25]

[26]

[27]

[28]

[29]

[30]

[31]

[32]

[33]

[34]

[35]

[36]

[37]

D. Zhu et al. / Carbon 111 (2017) 667e674 doped ordered mesoporous carbons based on cyanamide as the dopant for supercapacitor, Carbon 84 (2015) 335e346. P. Sharma, T.S. Bhatti, A review on electrochemical double-layer capacitors, Energy Convers. Manag. 51 (2010) 2901e2912. L. Wang, Z.Y. Gao, J.L. Chang, X. Liu, D.P. Wu, F. Xu, et al., Nitrogen-doped porous carbons as electrode materials for high-performance supercapacitor and dye-sensitized solar cell, ACS Appl. Mater. Interface 7 (2015) 20234e20244. L.L. Zhang, Y. Gu, X.S. Zhao, Advanced porous carbon electrodes for electrochemical capacitors, J. Mater. Chem. A 1 (2013) 9395e9408. W. Xing, C.C. Huang, S.P. Zhuo, X. Yuan, G.Q. Wang, D. Hulicova-Jurcakova, et al., Hierarchical porous carbons with high performance for supercapacitor electrodes, Carbon 47 (2009) 1715e1722. D.W. Wang, F. Li, M. Liu, G.Q. Lu, H.M. Cheng, 3D aperiodic hierarchical porous graphitic carbon material for high-rate electrochemical capacitive energy storage, Angew. Chem. Int. Ed. 47 (2008) 373e376. H.J. Liu, J. Wang, C.X. Wang, Y.Y. Xia, Ordered hierarchical mesoporous/ microporous carbon derived from mesoporous titanium-carbide/carbon composites and its electrochemical performance in supercapacitor, Adv. Energy Mater. 1 (2011) 1101e1108. B. You, J.H. Jiang, S.J. Fan, Three-dimensional hierarchically porous all-carbon foams for supercapacitor, ACS Appl. Mater. Interface 6 (2014) 15302e15308. X.T. Hong, K.S. Hui, Z. Zeng, K.N. Hui, L.J. Zhang, M.Y. Mo, et al., Hierarchical nitrogen-doped porous carbon with high surface area derived from endothelium corneum gigeriae galli for high-performance supercapacitor, Electrochim. Acta 130 (2014) 464e469. D.F. Zheng, M.Q. Jia, B. Xu, H. Zhang, G.P. Cao, Y.S. Yang, The simple preparation of a hierarchical porous carbon with high surface area for high performance supercapacitors, New Carbon Mater. 28 (2013) 151e155. J.X. Wang, S.S. Feng, Y.F. Song, W. Li, W.J. Gao, A.A. Elzatahry, et al., Synthesis of hierarchically porous carbon spheres with yolk-shell structure for high performance supercapacitors, Catal. Today 243 (2015) 199e208. Y.K. Lv, L.H. Gan, M.X. Liu, W. Xiong, Z.J. Xu, D.Z. Zhu, et al., A self-template synthesis of hierarchical porous carbon foams based on banana peel for supercapacitor electrodes, J. Power Sources 209 (2012) 152e157. J.G. Jiang, H. Chen, Z. Wang, L.K. Bao, Y.W. Qiang, S.Y. Guan, et al., Nitrogendoped hierarchical porous carbon microsphere through KOH activation for supercapacitors, J. Colloid Interface Sci. 452 (2015) 54e61. Y.F. Song, S. Hu, X.L. Dong, Y.G. Wang, C.X. Wang, Y.Y. Xia, A nitrogen-doped hierarchical mesoporous/microporous carbon for supercapacitors, Electrochim. Acta 146 (2014) 485e494. X.H. Liu, L. Zhou, Y.Q. Zhao, L. Bian, X.T. Feng, Q.S. Pu, Hollow, spherical nitrogen-rich porous carbon shells obtained from a porous organic framework for the supercapacitor, ACS Appl. Mater. Interface 5 (2013) 10280e10287. X.M. Ma, L.H. Gan, M.X. Liu, P.K. Tripathi, Y.H. Zhao, Z.J. Xu, et al., Mesoporous size controllable carbon microspheres and their electrochemical performances for supercapacitor electrodes, J. Mater. Chem. A 2 (2014) 8407e8415. X.Y. Chen, C. Chen, Z.J. Zhang, D.H. Xie, High performance porous carbon through hard-soft dual templates for supercapacitor electrodes, J. Mater. Chem. A 1 (2013) 7379e7383. H.A. Xu, Q.M. Gao, H.L. Guo, H.L. Wang, Hierarchical porous carbon obtained using the template of NaOH-treated zeolite beta and its high performance as supercapacitor, Microporous Mesoporous Mater. 133 (2010) 106e114. R.L. Liu, W.J. Ji, T. He, Z.Q. Zhang, J. Zhang, F.Q. Dang, Fabrication of nitrogendoped hierarchically porous carbons through a hybrid dual-template route or CO2 capture and haemoperfusion, Carbon 76 (2014) 84e95. D.W. Wang, F. Li, M. Liu, G.Q. Lu, H.M. Cheng, 3D aperiodic hierarchical porous graphitic carbon material for high-rate electrochemical capacitive energy storage, Angew. Chem. Int. Ed. 48 (2009), 1525e1525. L. Qie, W.M. Chen, H.H. Xu, X.Q. Xiong, Y. Jiang, F. Zou, et al., Synthesis of functionalized 3D hierarchical porous carbon for high-performance supercapacitors, Energy Environ. Sci. 6 (2013) 2497e2504. D.C. Guo, J. Mi, G.P. Hao, W. Dong, G. Xiong, W.C. Li, et al., Ionic liquid C16mimBF4 assisted synthesis of poly (benzoxazine-co-resol)-based hierarchically porous carbons with superior performance in supercapacitors, Energy Environ. Sci. 6 (2013) 652e659. J. Ozaki, N. Endo, W. Ohizumi, K. Igarashi, M. Nakahara, A. Oya, et al., Novel preparation method for the production of mesoporous carbon fiber from a polymer blend, Carbon 35 (1997) 1031e1033. J. Zhang, X.F. Zhong, H.B. Chen, Y. Gao, H.M. Li, Synthesis and electrochemical performance of porous carbon by carbonizing PF/PMMA interpenetrating

polymer networks, Electrochim. Acta 148 (2014) 203e210. [38] T. Horikawa, K. Ogawa, K. Mizuno, J. Hayashi, K. Muroyama, Preparation and characterization of the carbonized material of phenol-formaldehyde resin with addition of various organic substances, Carbon 41 (2003) 465e472. [39] Z.P. Li, E.H. Liu, Y.H. Zhu, T.T. Hu, Z.Y. Luo, T.T. Liu, Enhanced supercapacitive performance of porous activated carbon derived from polyaniline prepared by electrochemical synthesis, Mater. Res. Bull. 64 (2015) 6e11. [40] S. Xia, Y. Liu, F. Pei, L. Zhang, Q. Gao, W. Zou, et al., Identical steady tribological performance of graphene-oxide-strengthened polyurethane/epoxy interpenetrating polymer networks derived from graphene nanosheet, Polymer 64 (2015) 62e68. [41] G. Srinivas, V. Krungleviciute, Z.X. Guo, T. Yildirim, Exceptional CO2 capture in a hierarchically porous carbon with simultaneous high surface area and pore volume, Energy Environ. Sci. 7 (2014) 335e342. [42] J.F. Li, Q.S. Wu, Water bamboo-derived porous carbons as electrode materials for supercapacitors, New J. Chem. 39 (2015) 3859e3864. [43] J.C. Wang, S. Kaskel, KOH activation of carbon-based materials for energy storage, J. Mater. Chem. 22 (2012) 23710e23725. [44] J. Qian, M. Liu, L. Gan, Y. Lu, L. Chen, R. Ye, et al., Synthesis and electrochemical performance of microporous carbon using a zinc(II)-organic coordination polymer, Acta Phys. Chim. Sin. 29 (2013) 1494e1500. [45] A.B. Fuertes, M. Sevilla, Hierarchical microporous/mesoporous carbon nanosheets for high-performance supercapacitors, ACS Appl. Mater. Interface 7 (2015) 4344e4353. [46] Y. Meng, D. Gu, F.Q. Zhang, Y.F. Shi, H.F. Yang, Z. Li, et al., Ordered mesoporous polymers and homologous carbon frameworks: amphiphilic surfactant templating and direct transformation, Angew. Chem. Int. Ed. 44 (2005) 7053e7059. [47] X.Y. Chen, Y.Y. He, H. Song, Z.J. Zhang, A multi-template carbonization approach to hierarchically nanoporous carbon for high-performance supercapacitors, J. Solid State Electrochem. 19 (2015) 179e186. [48] B. Xu, F. Wu, R.J. Chen, G.P. Cao, S. Chen, Y.S. Yang, Mesoporous activated carbon fiber as electrode material for high-performance electrochemical double layer capacitors with ionic liquid electrolyte, J. Power Sources 195 (2010) 2118e2124. [49] P. Hao, Z.H. Zhao, J. Tian, H.D. Li, Y.H. Sang, G.W. Yu, et al., Hierarchical porous carbon aerogel derived from bagasse for high performance supercapacitor electrode, Nanoscale 6 (2014) 12120e12129. [50] Z.J. Zheng, Q.M. Gao, Hierarchical porous carbons prepared by an easy onestep carbonization and activation of phenol-formaldehyde resins with high performance for supercapacitors, J. Power Sources 196 (2011) 1615e1619. [51] M. Liu, X. Wang, D. Zhu, L. Li, H. Duan, Z. Xu, Z. Wang, L. Gan, Encapsulation of NiO nanoparticles in mesoporous carbon nanospheres for advanced energy storage, Chem. Eng. J. 308 (2017) 240e247. [52] W. Yang, Y.Y. Feng, D. Xiao, H.Y. Yuan, Fabrication of microporous and mesoporous carbon spheres for high-performance supercapacitor electrode materials, Int. J. Energy Res. 39 (2015) 805e811. [53] B.B. Chang, Y.L. Wang, K.M. Pei, S.M. Yang, X.P. Dong, ZnCl2-activated porous carbon spheres with high surface area and superior mesoporous structure as an efficient supercapacitor electrode, RSC Adv. 4 (2014) 40546e40552. [54] X.M. Ma, M.X. Liu, L.H. Gan, Y.H. Zhao, L.W. Chen, Synthesis of micro- and mesoporous carbon spheres for supercapacitor electrode, J. Solid State Electrochem. 17 (2013) 2293e2301. [55] Y.H. Zhao, M.X. Liu, L.H. Gan, X.M. Ma, D.Z. Zhu, Z.J. Xu, et al., Ultramicroporous carbon nanoparticles for the high-performance electrical double-layer capacitor electrode, Energy Fuels 28 (2014) 1561e1568. [56] W. Chen, R.B. Rakhi, M.N. Hedhili, H.N. Alshareef, Shape-controlled porous nanocarbons for high performance supercapacitors, J. Mater. Chem. A 2 (2014) 5236e5243. [57] Q. Xu, X.L. Yu, Q.H. Liang, Y. Bai, Z.H. Huang, F.Y. Kang, Nitrogen-doped hollow activated carbon nanofibers as high performance supercapacitor electrodes, J. Electroanal. Chem. 739 (2015) 84e88. [58] W.H. Qu, Y.Y. Xu, A.H. Lu, X.Q. Zhang, W.C. Li, Converting biowaste corncob residue into high value added porous carbon for supercapacitor electrodes, Bioresour. Technol. 189 (2015) 285e291. [59] J.F. Li, S.W. Ma, L.Y. Cheng, Q.S. Wu, Egg yolk-derived three-dimensional porous carbon for stable electrochemical supercapacitors, Mater. Lett. 139 (2015) 429e432. [60] L. Mao, Y. Zhang, Y. Hu, K.H. Ho, Q. Ke, H. Liu, et al., Activation of sucrosederived carbon spheres for high-performance supercapacitor electrodes, RSC Adv. 5 (2015) 9307e9313.