Enhanced electrochemical performance of NiO by addition of sulfonated graphene for supercapacitors

Abstract

A facile approach for the fabrication of NiO/sulfonated graphene composites (NiO/sGNS) using ammonium carbamate as a precipitating agent was developed. With an enhanced specific surface area and a desirable surface chemical environment, the NiO/sulfonated graphene composites exhibit excellent electrochemical performance. The results from cyclic voltammetry (CV) demonstrate that traditional electrical double-layer capacitive processes contribute to improving the storage energy capabilities of the NiO/sGNS composites. Galvanostatic charge–discharge tests show that the specific capacitance of the NiO/sulfonated graphene composites increased to 530 F g⁻¹ at a current density of 1.0 A g⁻¹ in comparison with specific capacitances of 382 F g⁻¹ for NiO/thermal reduced graphene composites (NiO/TRG) and 238 F g⁻¹ for pristine NiO. Moreover, electrochemical impedance spectroscopy (EIS) indicates the considerable effect of sulfonated graphene on building electron and ion transport channels for faradaic reaction processes of the NiO/sGNS composites.

---

1. Introduction

To lower the environmental pollution arising from the industrial, urban and vehicular combustion of fossil fuels, the development of renewable and sustainable energy imposes new challenges to find high power and energy storage devices.^1–3 Supercapacitors have attracted a great deal of attention, as these can bridge the gap between conventional capacitors and rechargeable batteries for electronic devices.^4–8 The novel characteristics of electrode materials is a crucial issue in the competition of various energy storage devices. Among all of the candidate materials, considerable efforts have been devoted to the research of NiO as a pseudocapacitive material because of its high theoretical specific capacitance, low cost, and high chemical stability.^9–12

However, the commercial application of NiO is hindered by its low electrical conductivity and inferior electrolyte compatibility. To tackle these problems, carbon-based materials (carbon nanotubes, activated carbon, mesoporous carbons and so on) are very popularly employed to improve the electrochemical performance of this metal oxide.^13–16 Mesoporous carbons, which were obtained from the carbonization of plant-derived lignin precursors, could embed highly dispersed NiO nanoparticles using a liquid-crystalline phase-templating approach.¹⁷ Meanwhile, the high surface area, uniform pore sizes, various porous distributions and large pore volumes of NiO-containing mesoporous carbon materials ensured that they had high specific capacitance and good cycle stability. The study on the capacitive properties of the core–shell structure carbon aerogel microbead–nanowhisker-like NiO composites showed that the improvement in the electrochemical behavior for these composite materials was attributed to the combination of the electrical double-layer capacitance of the carbon aerogel microbeads and the pseudo-capacitance based on the redox reaction of NiO.¹⁸

Moreover, graphene, with a two-dimensional honeycomb lattice structure of graphite, could also improve the electrochemical performance of metal oxide due to its unique properties with its ultra large surface area and extremely high electrical conductivity.^19,20 Hence, the application of NiO/graphene composites for supercapacitors has been enthusiastically attempted. P. Q. Cao et al. successfully synthesized mesoporous NiO/RGO composites through a facile hydrothermal method.²¹ The as-prepared composites had a unique 3D network structure consisting of graphene sheets and anchored NiO particles, which could increase the contact between electrolytes and the active materials, as well as shorten the ion diffusion paths. Y. M. Chen et al. synthesized a hybrid material composed of high density graphene/NiO using a microwave-assisted in situ synthesis of NiO at the defects of the graphene.²² The composite electrodes exhibited good cycle performance because of the uniform dispersion of the NiO nanoparticles on the graphene sheets and the larger distance between the neighboring graphene sheets provided enough space to buffer the volume change of the NiO nanoparticles during the charge–discharge redox reaction.

However, graphene lacks hydrophilic character and tends to undergo irreversible agglomeration, limiting the formation of uniform graphene-based composites by conventional process methods. To overcome these drawbacks, sulfonated graphene with strong hydrophilic properties and a high specific surface area has been introduced. Recently, many researchers have demonstrated that the electrical conductivity and electrochemical performance of sulfonated graphene are superior compared to those of pristine graphene.²³ Importantly, sulfonated graphene is highly effective in strengthening the chemical adsorption of various ions on carbon nanosheets, which is conducive to assembling hierarchical graphene-based composites with enhanced electrochemical performance.^24–26

In our previous work,²⁷ we fabricated NiO/sulfonated graphene composites via hydrothermal methods. The porous structure of the NiO/sulfonated graphene composites, resulting from the adsorption of urea on the sulfonated graphene via ionic bonding, improves the electrochemical properties relative to the NiO/thermal reduced graphene composites. However, how to consolidate the effects of sulfonated graphene on the electrochemical behavior of NiO is still an interesting issue for designing supercapacitors. In this work, NiO/sulfonated graphene composites are assembled using ammonium carbamate as a precipitating agent. Moreover, the presence of absolute alcohol as a solvent in the fabrication process weakens the hydrogen bonding between the solvent molecules and sulfonated graphene, strengthening the positive effects of the sulfonated graphene on the microstructure and electrochemical properties of NiO.

2. Experimental section

2.1 Reagents

Reagents were commercially obtained and used without further purification. Graphite oxide (GO) was synthesized from natural graphite powders using a modified Hummer’s method.²⁸ Thermal reduced graphene (TRG) was obtained after graphite oxide was put into a muffle oven preheated to 800 °C for 60 s.²⁹

2.2 Synthesis of sulfonated graphene

Sulfonated graphene was synthesized from thermal reduced graphene following procedures described in the literature.³⁰ Of the thermal reduced graphene, 80 mg was dispersed in 30 mL of deionized water using mild sonication. The above sample was sulfonated with 2.1 g of an aryl diazonium salt of sulfanilic acid in an ice bath for 1 h. Subsequently, 100 mL of absolute alcohol was added, and the mixture was stirred for 1 h. The obtained sulfonated graphene was washed with a 3 wt% HCl aqueous solution and deionized water, and then dried at 60 °C overnight.

2.3 Synthesis of the NiO/sGNS composites

The NiO/sGNS composites were synthesized through a two-step synthetic method. The typical route was as follows: 20 mg of the sulfonated graphene and 230 mg of nickel nitrate hexahydrate were added into 50 mL of absolute alcohol. The mixture was treated with ultrasonic waves for 30 min, and then was stirred at 70 °C for 12 h. Then, 120 mg of ammonium carbamate was added into the above solution, and the mixture was stirred at 70 °C for an additional 2 h. Afterwards, the resulting product was filtered, washed with distilled water and absolute alcohol several times, and then dried at 60 °C for 12 h in a vacuum oven. Subsequently, the NiO/sGNS composites were obtained through the thermal treatment of the above product at 350 °C in air for 3 h. For comparison, pristine NiO and NiO/TRG composites were prepared by a similar method.

2.4 Material characterization

Powder X-ray diffraction (XRD) analysis was performed on a Rigaku D/MAX-RC X-ray diffractometer in order to identify the crystalline phases of the materials. The morphologies of the as prepared products were observed with transmission electron microscopy (TEM, Hitachi, H-7700, 100 kV) and field emission scanning electron microscopy (FESEM, Hitachi, S-4800, 15 kV). X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Kratos Axis Ultra-dld spectrometer equipped with a monochromatic Al-K radiation source (hν = 1486.6 eV). The carbonaceous C 1s line (284.6 eV) was used as the reference to calibrate the binding energies (BEs). The nitrogen adsorption–desorption spectra of the samples were obtained from Brunauer–Emmett–Teller (BET) measurements using a Quantachrome Autosorb-1C-VP at 77 K. The specific surface area was calculated via the BET method in the relative pressure range of 0.01–0.3, and the pore size distribution was calculated using the adsorption branches of the nitrogen adsorption–desorption isotherms via the BJH method.

2.5 Electrochemical measurements

Cyclic voltammetry curves and galvanostatic charge–discharge tests of the as prepared electrodes were investigated using a conventional three-electrode cell in the potential range of 0 to 0.5 V (vs. Hg/HgO) at room temperature with a 6 mol L⁻¹ KOH aqueous solution as the electrolyte. The working electrode was fabricated by mixing the as prepared powders with 20 wt% acetylene black and a 10 wt% polytetrafluorene ethylene (PTFE) binder. A small amount of distilled water was added to the mixture to produce a homogeneous paste. The mixture was pressed onto nickel foam current collectors (1.0 cm × 1.0 cm) to make electrodes. The mass of the active material in a single electrode was around 2 mg. A platinum foil and a Hg/HgO electrode served as the counter and reference electrode, respectively. The CV curves were conducted on a CHI 660E electrochemical workstation (Shanghai CH Instrument Company, China). The galvanostatic charge–discharge tests were investigated by a Neware battery testing workstation.

3. Results and discussion

The samples obtained were characterized using XRD, as shown in Fig. 1. It is clearly found that the peaks appearing at 37.1, 43.2 and 62.8° correspond to the (111), (200) and (220) crystal planes of cubic NiO (JCPDS 65-2901). Both of the composites exhibit a similar profile to the pristine NiO due to the disordered stacking of graphene in the composites and the quite uniform dispersion of the metal oxide on the surface of the two-dimensional carbon materials. Remarkably, the peak half-width is gradually increased in the sequence pristine NiO, the NiO/TRG composites and then the NiO/sGNS composites, suggesting that the growth of NiO grains is obviously inhibited due to the fact that the hydrophilic configurations of sGNS enhance the adsorption of nickel cations on the carbon nanosheets in absolute alcohol. The average sizes of the NiO crystals calculated by the Scherrer equation (D = 0.89/λ cos θ) based on the (200) reflection are 8.7, 5.8 and 5.3 nm for NiO, the NiO/TRG composites and the NiO/sGNS composites.

Fig. 1 XRD patterns of pristine NiO, the NiO/TRG composites and the NiO/sGNS composites.

The microstructures and morphologies of pristine NiO, the NiO/TRG composites and the NiO/sGNS composites are observed using SEM and TEM in Fig. 2. The wavy nanosheets of sGNS are ultrathin as few layers are overlapping each other in Fig. 2(a), demonstrating the similar hierarchical structure of the sGNS to conventional graphene obtained using the chemical method.^31–34 As seen in Fig. 2(b), the size of the pristine NiO nanoparticles with irregular spheres is in the range of around 8 to 11 nm, due to the lack of external pressure stress. In the presence of TRG, Fig. 2(c) shows that the NiO nanoparticles are reduced in size to around 5 nm, and dispersed on the surface of TRG as they form slight aggregates. The changes in the NiO nanoparticles resulting from the addition of sGNS are revealed in Fig. 2(d). Surprisingly, the NiO nanoparticles in the SEM image of the NiO/sGNS composites exhibit a more uniform distribution on the surface of sGNS, as well as a maximum size of 3 nm from the TEM image. The morphological transformation of NiO is closely related to the chemical structure of the graphene-based materials. During the assembly process of the NiO/TRG composites, the metal ions are imbibed into the empty space between the carbon nanosheets of TRG by capillary forces due to the poor polarity of absolute alcohol. Hence, the agglomeration of the smaller sized NiO is retarded depending on the toughness properties of the carbon nanosheets. Moreover, the chemical adsorption of nickel cations on the surface of the carbon nanosheets is based on bonding interactions and also plays a vitally important role in curtailing the further dimensional expansion of NiO. The sulfonic groups anchored on the surface of sGNS are capable of trapping nickel cations by forming coordinate bonds, and hence provide the extra stress to limit the growth of NiO nanoparticles and ensure the uniform dispersion on the surface of carbon nanosheets.

Fig. 2 SEM and TEM images of (a) sGNS, (b) pristine NiO, (c) the NiO/TRG composites and (d) the NiO/sGNS composites.

The N₂ sorption isotherms in Fig. 3 illustrate the considerable influence of sGNS on the porous structure of the graphene-based composites. As seen above, all of the samples exhibit a type IV isotherm, corresponding to the presence of a mesoporous structure. The relative pressure range of the NiO/sGNS composites for the hysteresis region is broadened in comparison with those of other samples, implying that the chemical adsorption of nickel cations onto sGNS in the fabrication process serves to diversify the pore sizes in the composites. Among all of the samples, the NiO/sGNS composites exhibit the highest specific surface area due to the enhanced specific surface area of sGNS compared with TRG.³⁵ The pore size distribution curve of the NiO/sGNS composites reveals a series of peaks in the pore diameter range of 2 to 10 nm, whereas the peaks of the pristine NiO sample are at around 5 nm and the NiO/TRG composites at around 3 nm, demonstrating that the addition of sGNS leads to a hierarchical porous structure of the composites and hence benefits the accessibility to electrolytes. Moreover, the pore volumes of the pristine NiO, NiO/TRG composites and NiO/sGNS composites are 0.32, 0.88 and 0.78 cm³ g⁻¹, respectively, corresponding to pore lengths of 4.5, 4.1 and 2.9 nm, respectively. This result suggests that the unique structure of the NiO/sGNS composites, caused by the enhanced coordination bonding between nickel cations and sGNS in the absence of water, can obviously shorten the diffusion path of hydroxide ions in the composites during faradaic reaction processes.

Fig. 3 N₂ adsorption–desorption isotherms of (a) pristine NiO, (c) NiO/TRG composites and (e) NiO/sGNS composites, as well as the pore size distribution curves of (b) pristine NiO, (d) NiO/TRG composites and (f) NiO/sGNS composites.

To further clarify the differences in all of the samples in the surface chemical environment, X-ray photoelectron spectroscopy experiments were carried out. Fig. 4(a–c) shows that the Ni 2p_3/2 peaks observed in pristine NiO and its composites appear at around 853.7 and 855.5 eV. The two components are ascribed to Ni–O bonds and Ni–OH bonds, respectively, which is consistent with previous reports.^36,37 However, all of the samples have distinct differences in the relative intensity between these two peaks, due to the potential effects of the two-dimensional carbon materials on the chemical structure of the metal oxide. Fig. 4(d) shows that the ratio of peak 1 to peak 2 is drastically increased in both of the composites compared to pristine NiO, which suggests a higher degree of surface hydroxylation of NiO in the composites which contributes to the improvement of the compatibility between the electrodes and electrolytes. Moreover, the contribution of the NiO/sGNS composites at 856.1 eV is most prevalent, confirming that the sulfonic groups anchored on the surface of sGNS are in favor of the surface activated process of NiO. Fig. 5 shows the S 2p region for sGNS and the NiO/sGNS composites. The peaks at around 169.0, 166.8 and 164.1 eV are assigned to sulfonic acid, sulfoxide and thioether groups, respectively.³⁸ The appearance of the sulfonic acid groups in the NiO/sGNS composites confirms that the chemical structure of sGNS possesses moderate thermostability, as these are partially oxidized to sulfoxide groups during the thermal treatment process.

Fig. 4 Ni 2p XPS spectra of (a) pristine NiO, (b) the NiO/TRG composites and (c) the NiO/sGNS composites, as well as (d) the ratio of peak 1 to peak 2 for all the samples.

Fig. 5 S 2p XPS spectra of (a) sGNS and (b) the NiO/sGNS composites.

Considering the effect of the unique structural features on the electrochemical performance of the NiO/sGNS composites as supercapacitors, CV curves of the three samples were collected at different scan rates of 10, 20 and 30 mV s⁻¹ to investigate their electrochemical behavior. As indicated in Fig. 6, a pair of current peaks can be clearly identified during the cathodic and anodic sweep processes, which corresponds to the reversible reaction of Ni²⁺ to Ni³⁺ that occurs at the surface of the NiO contained in the electrode and can be expressed as follows:^9,11

NiO + OH⁻ → NiOOH + e⁻

Fig. 6 CV curves of (a) pristine NiO, (b) the NiO/TRG composites and (c) the NiO/sGNS composites at different scan rates of 10, 20 and 30 mV s⁻¹.

In addition, the peak intensity increases with an increase in the scan rate, which is consistent with a type of pseudocapacitive characteristic. It should be noted that the NiO/TRG composites exhibit a higher cathodic current density at a peak potential of around 0.35 V with respect to the Hg/HgO electrode compared to pristine NiO, due to the insulating NiO having the rich electrical wiring of the conductive carbon nanosheets and achieving a greatly enhanced electrical contact in the composites. However, the electrochemical behavior of the NiO/sGNS composites differs from that of the NiO/TRG composites. Despite the NiO/sGNS composites exhibiting similar redox peak current densities to the NiO/TRG composites, the excess response currents of the NiO/sGNS composites originating from electrical double-layer capacitive processes contribute to amplifying the integral area of the capacitive loop. The improvement in the capacitive properties for the NiO/sGNS composites is attributed to the fact that the porous structure of the NiO/sGNS composites facilitates the adsorption of ions on the surface of the electrodes and the diffusion of ions into the hierarchical transport channels of the electrodes.

The galvanostatic charge–discharge tests of all of the electrodes were performed within a stable potential window of 0–0.5 V at different current densities ranging between 1.0 and 5.0 A g⁻¹, shown in Fig. 7, to demonstrate the improved electrochemical performance of the composite electrodes. The potential–time plots show mainly the characteristic curves of a pseudocapacitor, rather than an electrical double-layer capacitor. The values of C_s were calculated from the charge–discharge curves using the equation as follow:

where C_s, I, t, V and m are the specific capacitance (F g⁻¹), the constant current (A), the discharge time (s), the total potential difference (V) and the mass of the active material in the electrode (g) respectively.

Fig. 7 Galvanostatic charge–discharge curves of (a) pristine NiO, (b) the NiO/TRG composites, and (c) the NiO/sGNS composites at current densities of 1.0, 2.0, 3.0, 4.0 and 5.0 A g⁻¹, (d) the plots of the specific capacitances for all of the samples following the increase in the current densities.

The maximum values of C_s were found to be 238, 382 and 530 F g⁻¹ for pristine NiO, the NiO/TRG composites and the NiO/sGNS composites at a current density of 1.0 A g⁻¹. Regarding the significance of a fast charge–discharge process in pseudocapacitive materials, the specific capacitances were observed at increased current densities. The specific capacitances of pristine NiO, the NiO/TRG composites and the NiO/sGNS composites were maintained at 152, 244 and 340 F g⁻¹ at a current density of 5.0 A g⁻¹. As is well known, ions in the electrolyte can penetrate into the inner-structure of the electrode materials at low current densities and hence have access to almost all of the available material of the electrode. However, effective utilization of the material is limited to only the outer-surface of the electrodes in a fast charge–discharge process. In this regard, the improved capacitance of the composites is ascribed to the fact that the porous structure caused by the addition of sGNS provides adequate transport channels for the diffusion of ions through the rough surface of the electrode during faradaic reaction processes at different current densities. The cycle stability of the NiO/sGNS composites is analysed at a current density of 5.0 A g⁻¹ in Fig. 8(a). The cyclability test indicates the good stability of the NiO/sGNS composites after 1000 cycles when the specific capacitance is stabilized at 327 F g⁻¹, resulting in only 3.9% capacitance loss. This result demonstrates that the presence of sGNS is beneficial to maintain the structural integrity of the composites.

Fig. 8 (a) Cycle life of the NiO/sGNS composites at a current density of 5.0 A g⁻¹, (b) Nyquist plots of pristine NiO, the NiO/TRG composites and the NiO/sGNS composites.

The electrical conductivity and ion transfer of the supercapacitive electrodes have been further investigated by EIS. Fig. 8(b) shows that the Nyquist plots of all of the samples consist of two distinct regions following the order of decreasing frequencies. The semicircle in the high frequency region is related to the charge transfer resistance (R_ct) caused by the faradaic reactions. The diameters of the semicircles for pristine NiO, the NiO/TRG composites and the NiO/sGNS composites are 24.8, 9.9 and 7.1 Ohm, indicating that the electrical interconnection lead by the combination of NiO and sGNS in the composites contributes to the diffusion of electric charge at the surface of the electrode during the faradaic reaction processes. The inclined line in the low frequency region corresponds to the diffusive resistance of the electrolyte in the electrode (Warburg impedance, W). The phase angles of the impedance plots for pristine NiO, the NiO/TRG composites and the NiO/sGNS composites are found to be sequentially increased, due to the fact that the porous structure of the NiO/sGNS composites provides abundant variations in the ion diffusion paths and benefits the movement of ions within the pores.

4. Conclusion

In summary, porous NiO/sGNS composites are successfully fabricated utilizing ammonium carbamate as a precipitating agent. In the absence of water, sulfonated graphene reveals the potential ability to control the specific surface area of the composites and the surface chemical environment. The addition of sGNS not only enhances the specific surface area of the composites and broadens the pore size distribution, but also strengthens the surface hydroxylation of NiO in the composites. The peculiar structure of NiO/sGNS is capable of providing sufficient channels for the transfer of electrons in the electrode and favors the movement of ions to the inner-structure of the composite electrodes. As expected, the NiO/sGNS electrode among all of the samples exhibits the highest specific capacitance of 530 F g⁻¹ and excellent cycling stability with 3.9% capacitance loss after 1000 cycles.

Acknowledgements

The above work was supported by the China Postdoctoral Science Foundation (No. 2014M551047), the National Natural Science Foundation of China (No. 21403189), the Hebei province natural science foundation of China (No. B2012203005, E2015203089), and the Key Technology Research and Development Program of Qinhuangdao (No. 201401A004).

Notes and references

1. H. L. Wang, H. S. Casalongue, Y. Y. Liang and H. J. Dai, J. Am. Chem. Soc., 2010, 132, 7472–7477.
2. S. Teng, G. Siegel, M. C. Prestgard, W. Wang and A. Tiwari, Electrochim. Acta, 2015, 161, 343–350.
3. L. O’Neill, C. Johnston and P. S. Grant, J. Power Sources, 2015, 274, 907–915.
4. M. Jana, S. Saha, P. Khanra, P. Samanta, H. Koo, N. C. Murmua and T. Kuila, J. Mater. Chem. A, 2015, 3, 7323–7331.
5. P. Hao, Z. H. Zhao, J. Tian, H. D. Li, Y. H. Sang, G. W. Yu, H. Q. Cai, H. Liu, C. P. Wong and A. Umar, Nanoscale, 2014, 6, 12120–12129.
6. X. H. Liu, L. Zhou, Y. Q. Zhao, L. Bian, X. T. Feng and Q. S. Pu, ACS Appl. Mater. Interfaces, 2013, 5, 10280–10287.
7. I. Nam, G. P. Kim, S. Park, J. W. Han and J. Yi, Energy Environ. Sci., 2014, 7, 1095–1102.
8. Y. Wang, Z. Q. Shi, Y. Huang, Y. F. Ma, C. Y. Wang, M. M. Chen and Y. S. Chen, J. Phys. Chem. C, 2009, 113, 13103–13107.
9. K. K. Purushothaman, I. M. Babu, B. Sethuraman and G. Muralidharan, ACS Appl. Mater. Interfaces, 2013, 5, 10767–10773.
10. G. A. Babu, G. Ravi, T. Mahalingam, M. Kumaresavanji and Y. Hayakawad, Dalton Trans., 2015, 44, 4485–4497.
11. S. Vijayakumar, S. Nagamuthu and G. Muralidharan, ACS Appl. Mater. Interfaces, 2013, 5, 2188–2196.
12. J. T. Li, W. Zhao, F. Q. Huang, A. Manivannan and N. Q. Wu, Nanoscale, 2011, 3, 5103–5109.
13. S. Vijayakumar, S. Nagamuthu and G. Muralidharan, ACS Sustainable Chem. Eng., 2013, 1, 1110–1118.
14. N. Chopra, W. W. Shi and A. Bansal, Carbon, 2011, 49, 3645–3662.
15. Y. Cheng, P. K. Shen and S. P. Jiang, Int. J. Hydrogen Energy, 2014, 39, 20662–20670.
16. X. F. Lu, J. Lin, Z. X. Huang and G. R. Li, Electrochim. Acta, 2015, 161, 236–244.
17. F. Chen, W. J. Zhou, H. F. Yao, P. Fan, J. T. Yang, Z. D. Fei and M. Q. Zhong, Green Chem., 2013, 15, 3057–3063.
18. X. Y. Wang, X. Y. Wang, L. H. Yi, L. Liu, Y. Z. Dai and H. Wu, J. Power Sources, 2013, 224, 317–323.
19. C. G. Liu, Z. N. Yu, D. Neff, A. Zhamu and B. Z. Jang, Nano Lett., 2010, 10, 4863–4868.
20. J. L. Huang, J. Y. Wang, C. W. Wang, H. N. Zhang, C. X. Lu and J. Z. Wang, Chem. Mater., 2015, 27, 2107–2113.
21. P. Q. Cao, L. C. Wang, Y. J. Xu, Y. B. Fu and X. H. Ma, Electrochim. Acta, 2015, 157, 359–368.
22. Y. M. Chen, Z. D. Huang, H. Y. Zhang, Y. T. Chen, Z. D. Cheng, Y. B. Zhong, Y. P. Ye and X. L. Lei, Int. J. Hydrogen Energy, 2014, 39, 16171–16178.
23. C. Bora, J. Sharma and S. Dolui, J. Phys. Chem. C, 2014, 18, 29688–29694.
24. J. L. Lu, W. S. Liu, H. Ling, J. H. Kong, G. Q. Ding, D. Zhou and X. H. Lu, RSC Adv., 2012, 2, 10537–10543.
25. Q. Wu, Y. Q. Sun, H. Bai and G. Q. Shi, Phys. Chem. Chem. Phys., 2011, 13, 11193–11198.
26. B. Ma, X. Zhou, H. Bao, X. W. Li and G. C. Wang, J. Power Sources, 2012, 215, 36–42.
27. L. Wang, H. Tian, D. L. Wang, X. J. Qin and G. J. Shao, Electrochim. Acta, 2015, 151, 407–414.
28. W. S. Hummers and R. E. Offeman, J. Am. Chem. Soc., 1958, 80, 1339.
29. Q. L. Du, M. B. Zheng, L. F. Zhang, Y. W. Wang, J. H. Chen, L. P. Xue, W. J. Dai, G. B. Ji and J. M. Cao, Electrochim. Acta, 2010, 55, 3897–3903.
30. Y. C. Si and E. T. Samulski, Nano Lett., 2008, 8, 1679–1682.
31. M. D. Stoller, S. J. Park, Y. W. Zhu, J. H. An and R. S. Ruoff, Nano Lett., 2008, 8, 3498–3502.
32. D. W. Wang, F. Li, Z. S. Wu, W. C. Ren and H. M. Cheng, Electrochem. Commun., 2009, 11, 1729–1732.
33. J. Yan, Z. J. Fan, T. Wei, W. Z. Qian, M. L. Zhang and F. Wei, Carbon, 2010, 48, 3825–3833.
34. M. S. Park, J. S. Yu, K. J. Kim, G. J. Jeong, J. H. Kim, Y. N. Jo, U. Hwang, S. Kang, T. Woob and Y. J. Kim, Phys. Chem. Chem. Phys., 2012, 14, 6796–6804.
35. H. B. Li, Y. Wang, Y. M. Shi, J. Li, L. J. He and H. Y. Yang, RSC Adv., 2013, 3, 14954–14959.
36. K. B. Wang, Z. Y. Zhang, X. B. Shi, H. J. Wang, Y. N. Lu and X. Y. Ma, RSC Adv., 2015, 5, 1943–1948.
37. A. K. Singh, D. Sarkar, G. G. Khan and K. Mandal, ACS Appl. Mater. Interfaces, 2014, 6, 4684–4692.
38. A. J. Crisci, M. H. Tucker, M. Y. Lee, S. G. Jang, J. A. Dumesic and S. L. Scott, ACS Catal., 2011, 1, 719–728.

---