Preparation and enhanced supercapacitance performance of porous carbon spheres with a high degree of graphitization

Abstract

This paper describes the preparation of graphitic porous carbon spheres (GPCS) from spherical resorcinol/formaldehyde resin by Fe-catalysis at 900 °C. The GPCS were characterized by their highly graphitized structures, uniform spherical morphology with an average diameter of ∼450 nm, pore size of 1–4 nm and relatively large surface area of ∼1100 m² g⁻¹. Their electrochemical performance was studied using cyclic voltammetry and galvanostatic charge–discharge measurements, and the results showed an enhanced charge storage capacity, with a specific capacitance of 127.4 F g⁻¹ in 2 M KOH at a current density of 0.2 A g⁻¹ that was nearly 3 times larger than that of amorphous porous carbon spheres. Moreover, electrochemical impedance spectroscopy tests demonstrated the low electrical resistance and ion transfer resistance of the GPCS, which resulted in the high retention of specific capacitance at a 10 A g⁻¹ current density. The recycling experiments indicated their superior stability, and 96% of their initial specific capacitance was maintained after 5000 cycles.

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1. Introduction

With the rapid worldwide economic expansion and the increasing human reliance on energy-based appliances, global energy consumption has been accelerating at an alarming speed. In this regard, the development of highly efficient and sustainable energy sources, as well as advanced technologies for converting and storing energy are of fundamental importance. Supercapacitors are a preferred energy storage device, because of their high power density, simple principles, long cycle life, excellent pulse charge–discharge capability and environmental friendliness.^1,2 Based on the energy storage mechanism, supercapacitors can be divided into pseudocapacitors and electrical double-layer capacitors (EDLCs). In spite of possessing high capacitance values and energy densities, pseudocapacitors suffer from a relatively low power density, poor cycling stability and inferior electrical conductivity, which drastically restrict their practical applications.³ On the contrary, EDLCs are characterized by their fast charge and discharge rates, high power density and long cycle life, but their capacitance and energy density are unsatisfactory.⁴ Hence, one of the most challenging aspects in the development of EDLCs is to enhance their specific capacitance and energy density without deteriorating their high power density and long cycle life.

The most studied electrodes for EDLCs are fabricated using carbonaceous materials, such as activated carbon (AC), mesoporous carbon (MC), carbon black, carbon nanotubes, carbon nanofibers, graphene and so on.^5–9 Undoubtedly, ACs are currently preferred as electrode materials in commercial production, however, the small size of their pore channels is unfavorable for the transport of electrolyte ions. Taking advantage of their nanoscale pore sizes, ions can rapidly transfer in the pore channels of MC materials, and therefore superior charge–discharge rates and high capacitance are expected, especially under a big loading current density. Actually, the capacitance values do not linearly increase with the improvement of the specific surface area and porosity, which could be ascribed to the poor electronic conductivity of these porous carbon materials.¹⁰ Though a high degree of graphitization is achieved in carbon black or carbon nanotubes, their specific surface areas are relatively low and porosity is poor.¹¹ Therefore, graphitic porous carbon materials with excellent electrical conductivity, as well as superior porosity and a large surface area, have attracted extensive attention for application in EDLC electrodes.

Currently, there are several major routes to prepare graphitic porous carbon materials from various carbon sources, including chemical vapor deposition, high temperature or high pressure treatment and catalytic graphitization.^12–17 Among them, catalytic graphitization has been a dominating strategy, not only for its convenience, energy-saving features and low graphitization temperature, but also because it provides access to porous structures with large surface areas. For example, graphitic carbon with a hierarchically ordered porous structure, synthesized using Ni-catalysis, reported by Huang et al. exhibited improved electrochemical properties.¹⁸ Xie et al. synthesized nanoporous graphitic carbon using Fe catalysis at a temperature of <900 °C, and this material was characterized by its large surface area, high degree of graphitization and enhanced capacitive performance.¹⁹

Besides the electric conductivity of carbon materials, the ion diffusion in pore channels is also an important factor that influences their performance as supercapacitors. Porous structures with nanosized morphologies, such as porous nanospheres, are favorable for providing short distance ion transport in channels.^20–22 Wang et al. reported the fabrication of porous carbon spheres with meso/micropore structures using sodium molybdate as a porogen and catalyst, which exhibited a superior specific capacitance of 260 F g⁻¹ with long-term cycling stability.²³ In this paper, we describe the successful synthesis of uniform and dispersed graphitic porous carbon spheres (GPCS) with superior porosity and high crystallinity by the convenient catalytic graphitization of resorcinol/formaldehyde (RF) resin spheres. These GPCS exhibit large surface areas and remarkable crystal structures. Meanwhile, the porous structures and degree of graphitization can be adjusted by varying the heat-treatment temperature. Furthermore, the GPCS has been evaluated as an EDLC electrode material in aqueous KOH solution, and it exhibits improved capacitive performance. More importantly, more than 96% of the specific capacitance of GPCS can be retained after 5000 cycles, showing good long-term cycling stability.

2. Experimental

2.1 Preparation of the GPCS materials

Synthesis RF resin spheres. Monodisperse RF resin spheres were synthesized via the Stöber method using resorcinol and formaldehyde solution as precursors.²⁴ In a typical synthesis, 0.1 mL of ammonia aqueous solution, 8 mL of absolute ethanol and 20 mL of deionized water were mixed and stirred for more than 1 h. Then, 0.2 g of resorcinol was added and continually stirred for 30 min. Subsequently, 0.28 mL of formaldehyde solution was added to the reaction solution and stirred for 24 h at 30 °C, and then the solution was placed in a Teflon-sealed autoclave and heated for 24 h at 100 °C. The products were obtained by centrifugation and then washed repeatedly with distilled water and oven-dried at 100 °C for more than 10 h.

Synthesis of graphitic porous carbon spheres. Typically, 1.0 g of the obtained RF resin spheres was dispersed in 20 mL of Fe(NO₃)₃·9H₂O (0.83 g) solution, which corresponds to a RF/Fe mass ratio of 2. After stirring for 4 h, the mixture was placed in an oven at 100 °C until the water was completely evaporated. Then the dried material was calcined in a N₂ atmosphere at different temperatures (700, 800 and 900 °C) for 2 h. The calcined products were washed with 3.0 M of HCl solution and deionized water to remove the iron catalyst. Finally, the materials were dried at 100 °C for 8 h to obtain the final products of graphitic porous carbon spheres, which were denoted as GPCS-x (x = 7, 8 and 9, referring to the different processing temperatures of 700, 800 and 900 °C).

For comparison, mesoporous carbon (CMK-3) and porous carbon spheres (PCS) were used as reference materials. CMK-3 was synthesized using SBA-15 as the hard template and furfuryl alcohol as the carbon source and by carbonizing at 900 °C under a N₂ atmosphere. PCS were prepared using RF resin spheres as the precursor and by calcining at 900 °C under a N₂ atmosphere.

2.2 Characterization

X-ray diffraction (XRD) patterns were recorded using a DX-2700 diffractometer (Dandong Haoyuan Instrument Co. Ltd., China) using Cu Kα radiation (λ = 0.15418 nm) as an X-ray source. Nitrogen adsorption–desorption isotherms were measured at −196 °C using a micromeritics ASAP 2020 analyzer. Before adsorption, the samples were outgassed at 200 °C for 10 h. The specific surface area (S_BET) was evaluated using the Brunauer–Emmett–Teller (BET) method, and the mesopore volume and micropore volume were calculated according to the Barrett–Joyner–Halenda (BJH) formula and t-plot method, respectively. The pore size distributions were calculated according to Density Functional Theory (DFT). The morphology and graphitized structure were observed using transmission electron microscopy (TEM), performed on a JEOL JEM-2100 with an accelerating voltage of 200 kV, and scanning electron microscopy (SEM, Hitachi S-4800). The thermogravimetric (TG) analysis curves were obtained on a Mettler analyzer (TGA/SDTA851e/5FL1100). Raman spectra were recorded on a Raman spectrometer (WITEC Spectra Pro 2300I) with a 532 nm laser. X-ray photoelectron spectroscopy (XPS) was carried out on a VG ESCALAB MK II X-ray photoelectron spectrometer with excitation from a Mg Kα source (1253.6 eV).

2.3 Electrochemical measurements

The products were tested using a conventional three-electrode system in a 2 M KOH electrolyte solution, which was performed on a CHI660D electrochemical workstation at room temperature. Platinum foil and an Ag/AgCl electrode in saturated KCl were used as the counter and reference electrodes, respectively. The working electrodes were prepared by mixing the active material, acetylene black and polytetrafluorene polytetrafluoreneethylene (PTFE) binder with a weight ratio of 8 : 1 : 1. After coating the above slurries on foamed Ni grids (1 cm × 1 cm), the electrode was dried overnight at 100 °C before pressing under a pressure of 20 MPa. Cyclic voltammetry curves were obtained in the potential range of −1.0–0 V vs. Ag/AgCl by varying the scan rate from 1 to 50 mV s⁻¹. Charge–discharge measurements were obtained galvanostatically at 0.2–10 A g⁻¹ over a voltage range of −1.0–0 V vs. Ag/AgCl. Electrochemical impedance spectroscopy (EIS) was performed over a frequency range of 10 kHz to 0.01 Hz at open circuit voltage with an alternating current amplitude of 5 mV.

3. Results and discussion

XRD patterns were recorded to reveal the degree of graphitization of the carbon materials (Fig. 1). Before acid washing, the strong diffraction peaks at 2θ angles of 44.7°, 64.9° and 82.3° are assigned to Fe (JCPDS no. 06-0696), which indicates that Fe³⁺ ions have been reduced during the carbonization process. After removing the Fe contents, the samples obtained at 700 and 800 °C exhibit some widened diffraction peaks that are related to an amorphous carbon structure. Upon increasing the temperature to 900 °C, these peaks in GPCS-9 become strong and sharp, which indicates the presence of a graphitic structure after catalysis using Fe. The diffraction peaks at 24°, 43° and 54° can be respectively indexed as (002), (100) and (004) of the graphitized structure, and the appearance of a peak for (004) demonstrates the high degree of graphitization .²⁵ To prove the effect of the catalyst, the patterns of PCS and CMK-3, which were carbonized at 900 °C for 2 h without the Fe catalyst, are also shown in Fig. 1b, and the broad and weak diffraction peaks at about 24° and 43° suggest their amorphous carbon frameworks.²⁶

Fig. 1 The XRD patterns of GPCS-x before acid treatment (a); the XRD patterns of GPCS-x after acid treatment, as well as those of PCS and CMK-3 (b).

The Raman spectra of GPCS-x are shown in Fig. 2a. It can be clearly observed that there are two bands for all GPCS-x materials. The peak at about 1350 cm⁻¹, designated as the D-band, is associated with the vibration of carbon atoms with dangling bonds of disorder carbon.²⁷ Another peak at around 1570 cm⁻¹, denoted as the G-band, is ascribed to the vibration of inter-plane sp²-bonded C=C stretching, which is the characteristic feature of ordered graphite carbon.²⁸ According to the ratio of the relative intensities for the D-band and G-band (I_D/I_G), the level of graphitization of the carbon structure can be revealed. As expected, the I_D/I_G value of GPCS-9 (0.84) is obviously decreased in comparison with those of GPCS-7 (0.96) and GPCS (0.94), which implies that GPCS-9 possesses the highest degree of graphitization, and this result is consistent with that of the XRD analysis.

Fig. 2 The Raman spectra of GPCS-7, GPCS-8 and GPCS-9 (a); the XPS spectra of GPCS-9: (b) survey spectrum; (c) C1s spectra; and (d) O1s spectra.

The elemental composition and chemical state of GPCS-9 was estimated using the XPS technique. The survey spectrum (Fig. 2b) with a binding energy ranging from 0 to 1200 eV suggests the existence of only carbon and oxygen elements in the GPCS-9 sample, indicating the complete removal of Fe. The oxygen content from surface oxygen-containing groups was found to be 4.6%, which reveals the high carbonization level of GPCS-9. To illustrate the chemical state of the carbon and oxygen elements in detail, the deconvolution of XPS peaks was performed using XPS PEAK Software. Fig. 2c shows the C1s spectrum, which can primarily be divided into three peaks located at ∼284.4, 285.9 and 288.4 eV, respectively. The peak centered at 284.4 eV is attributed to the sp² C=C bond, and the 285.9 eV peak can be assigned to sp³ C–C bond of the graphitic structure.²⁹ With respect to the peak located at 288.4 eV, it could be derived from the contribution of –C=O.³⁰ However, it is undetectable in the GPCS-9 sample, which also implies its high carbonization level. Fig. 2d presents the O1s spectra with three fitted peaks centered at about 531.7, 532.4 and 533.5 eV, respectively. In detail, the peak centered at 531.7 eV represents the contribution of oxygen in carboxyl groups, the one at 532.4 eV is ascribed to the oxygen in esters and the signal at 533.5 eV originates from the ether oxygen.³⁰

To estimate the carbon and Fe catalyst content in the GPCS-x samples, thermogravimetric analysis was employed, to determine the changes in weight loss under an oxygen atmosphere (Fig. 3). All GPCS-x samples exhibit two steps of weight loss, where the first is the removal of adsorbed water and the second is the combustion of carbon. The residue after treatment at 800 °C is identified as Fe₂O₃, and therefore the carbon/Fe ratio in GPCS is calculated to be 3.9, 3.5 and 3.2 for GPCS-7, GPCS-8 and GPCS-9, respectively. The gradual increase in Fe content demonstrates that GPCS-9 has the highest carbonization level.

Fig. 3 The TG curves of GPCS-7, GPCS-8 and GPCS-9 before acid treatment.

Fig. 4 shows the SEM images of CMK-3 and GPCS-9. In Fig. 4a and b, we can clearly observe worm-like mesoporous carbon, indicating that CMK-3 completely replicated the morphology of the mesoporous silica template. Fig. 4c and d depict the maintenance of the monodisperse spherical morphology of RF resins after the graphitization process catalyzed by Fe and no apparent fragments are observed in the image, which is commonly the case in the thermal treatment of spherical nanoparticles. Meanwhile, the smooth surface of graphitic carbon spheres in GPCS-9 is revealed by Fig. 4d, and a uniform diameter of 400–500 nm was calculated. Fig. 5 presents the TEM and HRTEM images of the CMK-3 and GPCS-9 samples. A tube-like morphology and highly ordered mesoporous channels could be clearly observed in the CMK-3 sample (Fig. 5a and b). The uniform spherical shapes are found in GPCS-9 (Fig. 5c), demonstrating that the graphitization process catalyzed by Fe maintains the original morphology and structure of the RF resins, and the discernible pores could also be seen in the inset of Fig. 5c. Fig. 5d presents the formation of clear crystalline lattices with a lattice distance of ∼0.336 nm, assigned to the (002) plane of graphite, suggesting a highly graphitized carbon structure.

Fig. 4 The SEM images of CMK-3 (a and b) and GPCS-9 (c and d).

Fig. 5 The TEM images of CMK-3 (a and b); the TEM (c) and HRTEM (d) images of the GPCS-9 sample; and the inset is the image of a single GPCS-9 sphere.

N₂ sorption experiments were conducted to investigate the porosity of all the electrode materials (Fig. 6). As shown in Fig. 6a, the CMK-3 material shows a typical type IV curve with a clear hysteresis loop at relative pressures of 0.40 to 0.70, suggesting the existence of uniform mesoporous channels. Then, the PCS sample exhibits a typical type I isotherm that implies a microporous structure, which comes from the vacancies created by eliminating O- and H- functional groups during the hydrolysis process, as well as framework shrinkage.³¹ With Fe catalysis, the isotherms of the GPCS samples gradually change from type I to type IV with increasing carbonization temperature, and the GPCS-9 sample exhibits a marked H4 type hysteresis loop at relative pressures of 0.50 to 0.95, indicating the formation of a mesoporous structure.³² The pore size distribution (Fig. 6b) also reveals the evolution of the pore structure, where the pore sizes of PCS and GPCS-7 are microporous, and the distributions of GPCS-8 (1–3 nm) and GPCS-9 (1–4 nm) indicate their enlargement to mesoporous sizes. In contrast to the wide pore size distribution in the GPCS materials, uniform mesopores of ∼3.71 nm exist in the CMK-3 sample. The pore structure parameters are listed in Table 1. In accordance with their isotherms and pore size distributions, the surface areas of PCS and GPCS-7 are mainly from their micropores. As the temperature increases to 800 °C, the degree of hydrolysis of the carbon structure proceeds further, and more micropores evolve and some micropores combine to form mesopores, which results in a large surface area. When the temperature is further increased to 900 °C, framework shrinkage progresses with graphitization, and thus the pore size becomes larger and the carbon structure becomes much more compact, which incurs the decrease in specific surface area in comparison with that of GPCS-8.³³ Nevertheless, the GPCS-9 sample still has a high BET surface area of 1103.4 m² g⁻¹ and a large pore volume of 0.71 cm³ g⁻¹, which could be comparable with mesoporous carbon CMK-3.

Fig. 6 N₂ adsorption–desorption isotherms (a) and pore size distribution (b) of GPCS-x, PCS and CMK-3.

Table 1 Textural parameters of all samples

Sample	SBET^a (m^2 g^−1)	Smicro^b (m^2 g^−1)	Smeso^c (m^2 g^−1)	Vtotal^d (cm^3 g^−1)	Vmicro^e (cm^3 g^−1)
a BET surface area.b Micropore surface area calculated using the V–t plot method.c Mesopore surface area calculated using the V–t plot method.d The total pore volume calculated by single point adsorption at P/Po = 0.976.e The micropore volume calculated using the V–t plot method.
GPCS-7	934.6	813.5	121.1	0.49	0.38
GPCS-8	1808.6	852.5	956.1	0.89	0.38
GPCS-9	1103.4	403.3	700.1	0.71	0.36
PCS	935.9	789.3	146.6	0.47	0.37
CMK-3	1070.4	7.4	1063.0	1.01	—

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To investigate the electrochemical performance of the GPCS-9 material as a supercapacitor electrode, the sample was characterized using cyclic voltammetry and galvanostatic charge–discharge measurements. Fig. 7a shows the CV curves of GPCS-9 at various scan rates ranging from 1 to 100 mV s⁻¹. The specific capacitance of the electrode at various scan rates was calculated based on the CV curves according to the following equation:

C = ∫IdV/mVν

where I (A) is the response current density, V (V) is the potential, ν is the potential scan rate, and m (g) is the mass of the electroactive material in the electrode. As shown in Fig. 7a, all the CV curves exhibit quasi rectangular shapes in a potential range of −1.0–0 V at different scan rates, suggesting an approximately ideal EDLC nature in the charge–discharge process and the fast diffusion of electrolyte ions into/out of the electrode material.³⁴ On the other hand, as the scan rate is gradually increased, a deformation can be found in the rectangular curves that may be attributed to the slow charge–discharge kinetics of the electrode.³⁵ That is to say, the electrolyte ions could be unable to adequately access the surface of GPCS-9 by diffusion in such a short time at high scan rates, which in the meantime results in the diminishment of the specific capacitance with an increase in scan rate. Despite this, the specific capacitance of the GPCS-9 electrode at a high scan rate of 100 mV s⁻¹ still retains approximately 76% of its initial capacitance at the low scan rate of 1 mV s⁻¹, suggesting excellent power capability. These results should be attributed to its significantly graphitic carbon structure and prominently porous structure, which endow it with remarkable conductivity and the accelerated kinetic process of ion transfer within the electrode materials. Fig. 7b compares the CV curves of the GPCS-9, CMK-3 and PCS materials at a scan rate of 50 mV s⁻¹. Apparently, the curve of GPCS-9 encircles a much larger area than those of PCS and CMK-3, implying a much larger specific capacitance value. Given the limitations placed upon ion diffusion in its microporous structure, PCS exhibits a small capacitance value of 38.6 F g⁻¹, in spite of its large surface area of 935.9 m² g⁻¹. Taking advantage of its mesoporous channels and large surface area, the mesoporous carbon of CMK-3 gives an enhanced specific capacitance of 75.6 F g⁻¹, which is no match for the value of 113.9 F g⁻¹ for GPCS-9 at the same scan rate, and can be ascribed to the poor electrical conductivity of the amorphous framework of CMK-3.

Fig. 7 (a) CV curves of GPCS-9 at different scan rates; (b) CV curves of the GPCS-9, CMK-3 and PCS electrode materials at the scan rate of 50 mV s⁻¹; (c) charge–discharge curves of the GPCS-9 sample at different current densities; (d) specific capacitances of the GPCS-9, CMK-3 and PCS electrode materials at different current densities.

Fig. 7c displays typical galvanostatic charge–discharge curves of the GPCS-9 material at different current densities from 0.2 to 10 A g⁻¹. The specific capacitances of the electrodes are calculated by the following equation:

C = IΔt/mΔV

where I (A) is the current loaded, m (g) is the mass of the active material, Δt (s) is the discharge time and ΔV(V) is the potential range. From Fig. 7c, it can be clearly seen that all the charge and discharge curves present highly symmetric triangular shapes, indicating almost ideal EDLC behavior and excellent electrochemical reversibility. Meanwhile, its quasi-linear nature and the absence of any obvious internal resistance (IR) drop, even at a high current density of 10 A g⁻¹, imply that this GPCS-9 material has a low internal resistance value, which is contributed by the high degree of graphitization and prominent conductivity.^36,37 The specific capacitance values are calculated at different current densities, ranging from 0.2 to 10 A g⁻¹ (Fig. 7d). Over the different current densities, GPCS-9 exhibits a significantly enhanced specific capacitance value. At a current density of 0.2 A g⁻¹, a specific capacitance of 127.4 F g⁻¹ is obtained for GPCS-9, which is much higher than those of CMK-3 (108.2 F g⁻¹) and PCS (40.5 F g⁻¹), and also considerably larger than those of other porous carbon materials reported in the literature (Table S1†). Even at a higher current density of 10 A g⁻¹, GPCS-9 still maintains a specific capacitance of 105.5 F g⁻¹, which is a slighter decrease than occurs in other electrode materials. These results could be ascribed to its uniform porous spherical structure: (1) the suitable pore structure and large surface area are favorable for ion transport and accumulation; (2) the uniform nanospherical morphology provides a short diffusion distance for electrolyte ions; (3) the high level of graphitization results in rapid electron transfer in the carbon framework.

To further evaluate the capacitive behavior of the graphitized porous carbon structure, electrochemical impedance spectroscopy (EIS) was carried out. EIS is one of the most useful methods for evaluating the electrochemical properties of electrode materials, such as their resistivity, capacitive performance and accessibility for electrolyte ions. The Nyquist plots of PCS, CMK-3 and GPCS-9 are displayed in Fig. 8. In all plots, a semicircle and a straight line can be seen in the high frequency region and low frequency region, respectively (inset Fig. 8). The semicircle in the high frequency region is evidence to suggest the existence of charge transfer resistance (R_ct), and a smaller semicircle means a smaller R_ct. The short slope of about 45° in the intermediate frequency corresponds to a semi-infinite Warburg diffusion process, which is called the Warburg resistance (W), and is based on the frequency dependence of ion diffusion from the electrolyte to the electrode interface inside the small pores.^38,39 At low frequencies, a nearly vertical line is exhibited, which represents the dominance of electrical double-layer capacitors.⁴⁰ In addition, the high frequency intercept at the Z′ axis (the insert) corresponds to the solution resistance (R_s),⁴¹ which is a combination of ionic and electronic resistances, the intrinsic resistance of the active materials, and the resistance of the electrolyte, as well as the contact resistance at the interface of the electrode with the nickel current collector.⁴² The inset clearly shows that the R_ct and W values of GPCS-9 are much lower than those of CMK-3 and PCS, and this result suggests that the GPCS-9 electrode has a smaller electric resistance and ion transfer resistance, which could be related to its highly graphitic carbon framework and the development of its porous spherical structure.

Fig. 8 Electrochemical impedance spectra of the GPCS-9, CMK-3 and PCS electrode materials under the influence of an ac voltage of 5 mV; the inset shows detail in the high-frequency range.

To validate the practical application efficiency of the GPCS-9 electrode material, the cycling stability was estimated by means of galvanostatic charge–discharge cycling at a current density of 2 A g⁻¹. Fig. 9a shows only a slight variation in the specific capacitance with an increase in cycle number. As shown in Fig. 9b, after 5000 cycles the long term performance was maintained at about 96% of the initial specific capacitance. This result obviously demonstrates that GPCS-9 possesses excellent electrochemical cycling stability, which is ascribed to its highly graphitic framework and well-developed porosity.

Fig. 9 Cycling stability of the GPCS-9 sample at a current density of 2 A g⁻¹ for 5000 cycles in the 2 M KOH electrolyte (a); the comparison of the 1st and 5000th charge–discharge curves in assessing the cycling stability of the GPCS-9 sample (b). The inset image shows the galvanostatic charge–discharge curves.

4. Conclusions

In summary, highly graphitic porous carbon materials with a uniform spherical morphology and prominent porosity were successfully prepared using RF resin spheres as the precursor by the catalytic graphitization route. The graphitic structure could be formed by thermal treatment at 900 °C with Fe catalysis. Meanwhile, the GPCS-9 material also possessed a mesoporous structure with a relatively high surface area and a large pore volume. With its highly graphitic carbon framework and uniform porous spherical structure, the GPCS-9 material results in a noteworthy improvement in capacitive performance. The specific capacitance of GPCS-9 reaches 127.4 F g⁻¹ at a current density of 0.2 A g⁻¹, which is about 3 times as high as that of PCS and even much larger than that of traditional mesoporous carbon material. More importantly, it exhibits excellent long-term cycling stability, and the specific capacitance is only reduced by 4% at a current density of 2 A g⁻¹ after 5000 cycles. Therefore, the GPCS-9 material could be a promising and competitive electrode material for high performance EDLCs.

Acknowledgements

The authors gratefully acknowledge the financial support from the 521 talent project of ZSTU, the program for innovative research team of ZSTU (13060052-Y), Program for New Century Excellent Talents in University (NCET-12-0696), the Leading Talents for Zhengzhou Science and Technology Bureau (Grant no. 131PLJRC649), the program for University Innovative Talents of Science and Technology in Henan Province (Grant no. 2012HASTIT03) and National Natural Science Foundation of China (51472102).

References

1. G. P. Wang, L. Zhang and J. J. Zhang, Chem. Soc. Rev., 2012, 41, 797–828.
2. Z. Niu, H. Dong, B. Zhu, J. Li, H. H. Hng, W. Zhou, X. Chen and S. Xie, Adv. Mater., 2013, 25, 1058–1064.
3. X. Dong, X. Wang, L. Wang, H. Song, X. Li, M. B. Chan-Park, C. M. Li and P. Chen, Carbon, 2012, 50, 4865–4870.
4. M. Inagakia, H. Konnoa and O. Tanaike, J. Power Sources, 2010, 195, 7880–7903.
5. Z. D. Huang, B. Zhang, S. W. Oh, Q. B. Zheng, X. Y. Lin and N. Yousefi, et al., J. Mater. Chem., 2012, 22, 3591–3599.
6. T. Kim, G. Jung, S. Yoo, K. S. Suh and R. S. Ruoff, ACS Nano, 2013, 7, 6899–6905.
7. D. D. Zhou, H. J. Liu, Y. G. Wang, C. X. Wang and Y. Y. Xia, J. Mater. Chem., 2012, 22, 1937–1943.
8. S. Inagaki, Y. Yokoo, T. Miki and Y. Kubota, Microporous Mesoporous Mater., 2013, 179, 136–143.
9. A. Ghosh and Y. H. Lee, ChemSusChem, 2012, 5, 480–499.
10. K. Zhang, L. L. Zhang, X. S. Zhao and J. Wu, Chem. Mater., 2010, 22, 1392–1401.
11. D. S. Yuan, F. L. Zeng, J. Yan, X. L. Yuan, X. J. Huang and W. J. Zou, RSC Adv., 2013, 3, 5570–5576.
12. Z. Wu, W. Li, Y. Xia, P. Webley and D. Zhao, J. Mater. Chem., 2012, 22, 8835–8845.
13. S. B. Yoon, G. S. Chai, S. K. Kang, J. S. Yu, K. P. Gierszal and M. Jaroniec, J. Am. Chem. Soc., 2005, 127, 4188–4189.
14. P. Su, L. Jiang, J. Zhao, J. Yan, C. Li and Q. Yang, Chem. Commun., 2012, 48, 8769–8771.
15. Y. Hanzawa, H. Hatori, N. Yoshizawa and Y. Yamada, Carbon, 2002, 40, 575–581.
16. M. Sevilla, C. Sanchis, T. Valdes-Solis, E. Morallon and A. B. Fuertes, J. Phys. Chem. C, 2007, 111, 9749–9756.
17. Z. L. Wang, X. B. Zhang, X. J. Liu, M. F. Lv, K. Y. Yang and J. A. Meng, Carbon, 2011, 49, 161–169.
18. C. H. Huang, Q. Zhang, T. C. Chou, C. M. Chen, D. S. Su and R. A. Doong, ChemSusChem, 2012, 5, 563–571.
19. M. J. Xie, J. Yang, J. Y. Liang, X. F. Guo and W. P. Ding, Carbon, 2014, 77, 215–225.
20. L. Zhang, M. J. Zhang, Y. H. Wang, Z. L. Zhang, G. W. Kan, C. G. Wang, Z. Y. Zhong and F. B. Su, J. Mater. Chem. A, 2014, 2, 10161–10168.
21. M. X. Liu, L. H. Gan, W. Xiong, Z. J. Xu, D. Z. Zhu and L. W. Chen, J. Mater. Chem. A, 2014, 2, 2555–2562.
22. H. Zhang, X. Yu and P. V. Braun, Nat. Nanotechnol., 2011, 6, 277–281.
23. J. Wang, L. F. Shen, B. Ding, P. Nie, H. F. Deng, H. Dou and X. G. Zhang, RSC Adv., 2014, 4, 7538–7544.
24. J. Liu, S. Z. Qiao, H. Liu, J. Chen, A. Orpe, D. Y. Zhao and G. Q. Lu, Angew. Chem., Int. Ed., 2011, 50, 5947–5951.
25. Z. J. Zhang, P. Cui, C. Chen, X. Y. Chen and J. W. Liu, J. Solid State Electrochem., 2014, 18, 59–67.
26. M. Sevilla and A. B. Fuertes, Chem. Phys. Lett., 2010, 490, 63–68.
27. F. Su, X. Zhao, Y. Wang, J. Zeng, Z. Zhou and J. Y. Lee, J. Phys. Chem. B, 2005, 109, 20200–20206.
28. L. Ji and X. Zhang, Carbon, 2009, 47, 3219–3226.
29. T. I. T. Okpalugo, P. Papakonstantinou, H. Murphy, J. McLaughlin and N. M. D. Brown, Carbon, 2005, 43, 153–161.
30. V. Datsyuk, M. Kalyva, K. Papagelis, J. Parthenios, D. Tasis, A. Siokou, I. Kallitsis and C. Galiotis, Carbon, 2008, 46, 833–840.
31. R. L. Liu, Y. F. Shi, Y. Wan, Y. Meng, F. Q. Zhang, D. Gu, Z. X. Chen, B. Tu and D. Y. Zhao, J. Am. Chem. Soc., 2006, 128, 11652–11662.
32. J. Hu, P. P. Tao, S. Wang, Y. Liu, Y. G. Tang, H. Zhang and Z. G. Lu, J. Mater. Chem. A, 2013, 1, 6558–6562.
33. A. H. Lu, W. C. Li, E. L. Salabas, B. Spliethoff and F. Schuth, Chem. Mater., 2006, 18, 2086–2094.
34. R. Demir-Cakan, N. Baccile, M. Antonietti and M. M. Titirici, Chem. Mater., 2009, 21, 484–490.
35. J. Wang, L. F. Shen, B. Ding, P. Nie, H. F. Deng, H. Dou and X. G. Zhang, RSC Adv., 2014, 4, 7538–7544.
36. H. Zhu, X. Wang, X. Liu and X. Yang, Adv. Mater., 2012, 24, 6524–6529.
37. Q. Wang, J. Yan, Y. B. Wang, T. Wei, M. L. Zhang, X. Y. Jing and Z. J. Fang, Carbon, 2014, 67, 119–127.
38. M. J. Li, C. M. Liu, H. B. Cao, H. Zhao, Y. Zhang and Z. J. Fan, J. Mater. Chem. A, 2014, 2, 14844–14851.
39. Y. Chen, X. Zhang, D. Zhang, P. Yu and Y. Ma, Carbon, 2011, 49, 573–580.
40. L. Yang, S. Cheng, Y. Ding, X. Zhu, Z. L. Wang and M. Liu, Nano Lett., 2011, 12, 321–325.
41. Q. L. Zhao, X. Y. Wang, C. Wu, J. Liu, H. Wang, J. Gao, Y. W. Zhang and H. B. Shu, J. Power Sources, 2014, 254, 10–17.
42. S. Vijayakumar, S. Nagamuthu and G. Muralidharan, ACS Sustainable Chem. Eng., 2013, 1, 1110–1118.

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Footnote

† Electronic supplementary information (ESI) available: The comparison of the specific capacitance of GPCS-9 with those of other porous carbon materials is shown in Table S1. See DOI: 10.1039/c4ra09204j

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