Enhanced electrochemical performance of cobalt oxide nanocube intercalated reduced graphene oxide for supercapacitor application

Abstract

We investigated different molar concentrations of cobalt precursor intercalated reduced graphene oxide (rGO) as possible electrode materials for supercapacitors. Cobalt oxide (Co₃O₄) nanocubes intercalated reduced graphene oxides (rGO) were synthesized via a facile hydrothermal method. It has been found that the Co₃O₄ particles with a cubical shape are decorated on rGO matrix with an average size of ∼45 nm. The structural crystallinity of rGO–Co₃O₄ composites was examined by X-ray diffraction (XRD). Raman spectroscopy confirmed the successful reduction of GO to rGO and effective interaction between Co₃O₄ and the rGO matrix. The electrochemical performances of rGO–Co₃O₄ electrodes were examined using cyclic voltammetry and charge–discharge techniques. The maximum specific capacitance (278 F g⁻¹) is observed at current density of 200 mA g⁻¹ in the C2 electrode resulting from effective ion transfer and less particle aggregation of Co₃O₄ on the rGO matrix than in the other electrodes. C2 exhibits good rate capability and excellent long-term cyclic stability of 91.6% for 2000 cycles. The enhanced electrochemical performance may result from uniform intercalation of cobalt oxide over the rGO. These results suggest that the Co₃O₄ intercalated rGO matrix could play a role in improved energy storage capability.

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1. Introduction

The imminent depletion of fossil fuels and undesirable consequences of air pollution are alarming significant environmental concerns. These issues have motivated researchers to develop green, sustainable, and highly efficient alternative energy resources (solar energy, wind energy, etc.) and energy storages (batteries, supercapacitors).¹ Among the various types of energy devices, the supercapacitor (SC, also known as an electrochemical capacitor) has emerged as an attractive energy storage device because of its extremely high power density, rapid dynamics of charge propagation, excellent cyclic retention, good energy density, and minimum charge separation compared with conventional capacitors and batteries.^2,3

Generally, SCs can be classified into three different types according to the charge storage phenomenon: the electric double layer capacitor (EDLC), the pseudocapacitor, and the hybrid supercapacitor. In EDLC, the charges are stored in the electrode and electrolyte interfaces, whereas in the pseudocapacitor, charges are stored electrostatically via reversible adsorption of ions from the electrolyte into the electrode surface, resulting in higher specific capacitance than that of EDLC.^4–6 Hybrid supercapacitors consist of asymmetric EDLC and pseudo (battery)-type electrode materials, hybridizing the advantages of EDLC and pseudocapacitors.

Usually, pseudocapacitors employ noble/transition metal oxides or conducting polymers as electrode materials.⁷ From the library of possible transition metal oxides, cobalt oxide (Co₃O₄) has emerged as a leader. It has high surface to volume ratio, simple preparation method, outstanding chemical durability, a promising ratio of surface atoms, and diverse morphology.⁸ However, the practical applications of such metal oxide-based pseudocapacitors may still be limited because of sluggish electron kinetics and rapid capacity decay arising from highly corrosive electrolyte and easily damaged morphologies of the materials during the faradic reactions.⁹

Many approaches have been explored for obtaining desirable electrode materials with different structures, to obtain high power and energy density.^10–13 Incorporation of transition metal oxides with carbon materials can enhance electrochemical performance because of the enormous surface area and high electrical conductivity. Herein, graphene (as a two-dimensional (2D) monolayer of carbon atoms with hexagonal honeycomb lattice structure) has attracted tremendous attention because of its unique physicochemical properties, especially high surface area (2630 m² g⁻¹), ballistic conductivity (106 S cm⁻¹), good electronic configuration,¹⁴ and a wide electrochemical window, suggesting it as a potential candidate for good electrochemical performance.^15–17 The synergistic effect of rGO with cobalt oxide nanocomposite can enhance electrochemical redox activity, reversibility, electrical conductivity, and cycling charge discharge performance.^18,19

Various techniques have been investigated for fabrication of cobalt oxide nanostructures, such as an unaided nano-casting method for preparation of Co₃O₄ nanowires,²⁰ a surfactant-aided approach for fabrication of Co₃O₄ nanoboxes,²¹ low temperature synthesis of single-crystalline Co₃O₄ nanorods on silicon substrate,²² thermal oxidative decomposition growth of Co₃O₄ nanorods,²³ nanotubes,²⁴ nanowalls,²⁵ gamma radiolysis-assisted formation of Co₃O₄ nanoparticles.²⁶ Besides these, a hydrothermal method also has been employed to synthesize different structural morphologies.²⁷ Of all these techniques, hydrothermal is the most preferred wet chemical route to fabricate nanostructured metal oxides because of its simplicity and the ability to tune various synthesis parameters (pH, temperature, time, and concentration of regents or precursors).²⁸ Therefore, in this work, we report synthesis of a cobalt oxide nanocube intercalated rGO matrix by a facile one-pot hydrothermal route for energy storage applications.²⁹

2. Experimental methods

2.1. Materials

Graphite flakes were purchased from Asbury Inc. (USA). Potassium permanganate (KMnO₄, >99%), sulfuric acid (H₂SO₄ ∼98%), hydrochloric acid (HCl ∼35%), phosphoric acid (H₃PO₄ ∼85%), and ammonia solution (NH₃, 25%) were purchased from R & M Chemicals. Cobalt acetate tetrahydrate (Co(CH₃COO)₂·4H₂O), absolute ethanol (C₂H₆O ∼99.8%), and hydrogen peroxide (H₂O₂, 35%) were obtained from Sigma-Aldrich and Merck. All experiments were performed using doubly distilled water.

2.2. Synthesis of rGO–Co₃O₄ composites

In a typical synthesis of rGO–Co₃O₄ composite, 20 mL of freshly prepared³⁰ exfoliated GO solution (1 mg mL⁻¹) was added dropwise to 25 mL of ethanol under stirring. Ten milliliters of 0.25 mmol Co(CH₃COO)₂·4H₂O was added drop by drop to the solution under vigorous stirring at 60 °C. Then, 15 mL of ammonia (6%) solution was added slowly to the mixture under constant stirring. Subsequently, the complete reaction mixture was transferred to a 100 mL Teflon lined stainless steel autoclave and subjected to hydrothermal treatment at 150 °C for 5 h. Then, the obtained precipitates of rGO–Co₃O₄ nanocomposites were washed with DI water and ethanol several times followed by drying at 90 °C in a hot air oven. The entire experiment was repeated to synthesize rGO–Co₃O₄ composites with other mole ratios (0.5, 0.75, and 1 mmol) of Co(CH₃COO)₂·4H₂O. The synthesized hybrid Co₃O₄ nanocubes/rGO composites with different molar ratios of cobalt precursor 0.25, 0.5, 0.75, and 1 mmol were denoted as C1, C2, C3, and C4, respectively.

2.3. Electrochemical measurements

The working electrode was fabricated by mixing electroactive materials, polyvinylidene fluoride (PVDF), and carbon black with mass percentage ratio of 75 : 15 : 10 in 1-methyl-2-pyrrolidone (NMP) medium under ultrasonication. The viscous slurry was subsequently drop-casted and compressed on nickel foam (area of 1 cm²). The mass loading of prepared working electrodes (C1, C2, C3, and C4) without carbon black and PVDF was ∼5.1 mg. The working electrodes were immersed in 1 M KOH aqueous solution for 2 h before taking the electrochemical measurements, carried out in a three-electrode cell. The active material-coated Ni foam was used as the working electrode, with platinum wire as the counter electrode and Ag/AgCl as the reference electrode in a 1 M KOH aqueous solution at room temperature. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge discharge were performed with Gamry (Reference 3000 instrument). CV measurements of synthesized samples were performed in the potential range of 0 to 0.55 V at different scan rates of 1, 3, 5, 10, 20, 30, 40, and 50 mV s⁻¹. EIS measurements were recorded in a frequency range from 0.01 Hz to 100 kHz under AC amplitude of 5 mV at open circuit voltage.

2.4. Characterization techniques

The structural crystallinity of rGO–Co₃O₄ nanocomposite was studied using Philips X'pert X-ray diffractometer with copper Kα radiation (λ = 1.5418 nm) at a scan rate of 0.02 degree per second. Raman spectra were obtained using the Renishaw inVia 2000 system green laser emitting at 514 nm. The surface morphologies of nanocomposites were studied using field emission scanning electron microscopy (JEOL JSM-7600F) and high-resolution transmission electron microscopy (JEOL JEM-2100F).

3. Results and discussion

3.1. Mechanism of Co₃O₄ intercalated rGO matrix

A schematic representation of formation of the rGO–Co₃O₄ nanocubes is shown in Fig. 1. The Co₃O₄ intercalated reduced graphene oxide was synthesized via a hydrothermal method. In the first step, GO solution (containing functional groups, such as hydroxyl (–OH), epoxy (C–O–C), carboxyl (–COOH), and carbonyl group) was sonicated for 1 hour to obtain exfoliated graphene oxide sheets. In the second step, addition of Co(CH₃COO)₂ in exfoliated GO solution led to adsorption of Co²⁺ ions on graphene oxide because of the electrostatic force of attraction between Co²⁺ ions and oxygen-based functional groups, resulting in the local creation of a bridge as Co–O–C bonding.^31,32 In the third step, the reaction medium was changed to basic conditions using a NH₃ solution followed by hydrothermal process. During the hydrothermal process, the GO–Co(OH)₂ transformed into rGO–Co₃O₄ nanocubes. Restacking of rGO sheets was controlled by incorporation of Co₃O₄ nanocubes during reduction of GO to rGO.¹⁸

Fig. 1 Schematic illustration of steps for the formation of rGO–Co₃O₄ hybrid: (a) exfoliated graphene oxide (GO) sheets, (b) intercalation of cobalt oxide, (c) ​cobalt nanocubes intercalated reduced graphene oxide (rGO).

3.2. X-ray diffraction analysis

The crystalline structures of synthesized GO, rGO, Co₃O₄, and rGO–Co₃O₄ were studied by recording the XRD patterns (Fig. 2). GO (Fig. 2a) showed a sharp intensity peak at 2θ value of 10° corresponding to the (001) plane.^33,34 The sharpness of the peak indicated a larger interlayer distance because of the presence of functional groups on the graphene basal plane.¹⁸ Under the hydrothermal process, the diffraction peak of GO at 2θ value of 10° disappeared and two new broader peaks were generated at 2θ values of 26.8° and 42.7°, which correspond to the (002) and (100) planes of rGO (Fig. 2b), respectively.³³ The two diffraction peaks observed revealed the disordered stacking of graphene sheets, indicating efficient transformation from GO to rGO.^34,35 Fig. 2c and d shows the XRD patterns of pure Co₃O₄ and rGO–Co₃O₄ composite. The diffraction pattern of pure Co₃O₄ exhibited strong crystalline peaks at 2θ = 31.2°, 36.8°, 44.7°, 55.5°, 59.2°, and 65.1° corresponding to the (220), (311), (400), (422), (511), and (440) planes of a face centered cubic structure of nanostructured Co₃O₄ (JCPDS card no. 42-1467). The rGO–Co₃O₄ composite showed the diffraction peaks of Co₃O₄, which was intercalated with rGO matrix. No other characteristic peaks were observed, suggesting the purity of the Co₃O₄ embedded rGO matrix.³⁶

Fig. 2 XRD patterns for (a) GO, (b) rGO, (c) Co₃O₄, and (d) rGO–Co₃O₄.

3.3. Raman analysis

Raman spectroscopy is a suitable and non-destructive tool to monitor the functional groups and structural defects in materials, especially carbon-based graphene/graphene oxide/reduced graphene oxides. Fig. 3a–d shows the Raman spectra of GO, rGO, and the Co₃O₄–rGO composite. The spectra of GO and rGO (Fig. 3a and b) exhibited well-referred D and G bands at 1355 cm⁻¹ and 1588 cm⁻¹, respectively. The D band is ascribed to the lattice defect induced phonon mode. The G band corresponds to the in-plane vibrations of C–C atoms and a doubly degenerated phonon mode (E_2g symmetry) at the Brillouin zone center.^37–39 In general, there was a slight increase in I_D/I_G ratio from 0.95 to 1.36, suggesting formation of partially ordered crystal structures and decreased size of in-plane sp² domains during reduction of GO to rGO.³⁸ However, in the rGO–Co₃O₄ hybrid, the I_D/I_G ratio was decreased to 0.90, because of decreasing the sp² domain size of carbon atoms and the reduction of sp³ to sp² carbon.⁴⁰ Additionally, defects were observed in the rGO matrix because of the removal of functional groups from GO layers.³² This confirmed successful reduction of GO to rGO. The 2D band or second order zone boundary phonon is extremely important for differentiation between monolayer and multilayer sheets in graphene-based material. Here, the 2D band was observed at 2901 cm⁻¹ in GO, and retained at the same position in the rGO–Co₃O₄ composite (Fig. 3d).^41,42 The inset of Fig. 3d shows intensity peaks at ∼194, 482, 525, 615, and 686 cm⁻¹ corresponding to the B_1g, E_g, F_2g, F_2g, and A_1g modes of Co₃O₄, respectively.

Fig. 3 Raman spectra for (a) GO, (b) rGO, (c) Co₃O₄, and (d) the rGO–Co₃O₄ composite.

3.4. Surface morphology

Surface morphologies of synthesized pure Co₃O₄ and rGO–Co₃O₄ composite were investigated via FE-SEM and HR-TEM analyses. Fig. 4a and b shows the clear distinction between pure cobalt oxide and composite of cobalt oxide with rGO matrix (C2). The FE-SEM image (Fig. 4a) revealed the cubical surface morphology of Co₃O₄ with particle aggregation, which led to reduction of the electrochemical surface area. This can be overcome by introducing rGO matrix as shown in Fig. 4b. The rGO matrix provided a platform to anchor cobalt oxide nanocubes, which were well distributed and intercalated with rGO sheets (C2 composite). However, C3 and C4 composite exhibited Co₃O₄ aggregation on rGO matrix because of a high concentration of cobalt precursor. Particle aggregation was increased with increasing concentration of cobalt precursor, as clearly shown in Fig. S1a and b.†

Fig. 4 Surface morphology: (a) FE-SEM image of pure Co₃O₄, (b) FE-SEM image of rGO–Co₃O₄ composite, (c) HR-TEM images of pure Co₃O₄, (d) HR-TEM image of rGO–Co₃O₄ composite.

Fig. 4c and d shows the HR-TEM image of pure Co₃O₄ and rGO–Co₃O₄ composite (C2). Fig. 4c shows the cubical shape of pure Co₃O₄ with an average particle size of ∼45 nm. The cubical shape of Co₃O₄ was well distributed on transparent rGO matrix (Fig. 4d). This result suggested that the rGO matrix provides a platform for improved electrochemical performance over that of pure Co₃O₄ because of effective intercalation and less particle aggregation of Co₃O₄ on rGO matrix.

3.5. Electrochemical performance

3.5.1. Cyclic voltammetry (CV). The electrochemical performance of pure Co₃O₄ and rGO–Co₃O₄ electrodes was evaluated using CV analysis. Cyclic voltammograms were performed for C1, C2, C3, and C4 with a potential window from 0 to +0.55 V at various scan rates. The measured currents were normalized with the electrode mass.

Fig. 5a–d shows the CV curves of C1, C2, C3, and C4 at different scan rates. Herein, the well-defined redox peaks show the pseudocapacitive behavior of composites, resulting from interaction of hydroxyl ions with the respective electrode.⁴³ While increasing the scan rates, redox peak intensities increase with a slight peak shift towards higher potential, evidence of fast redox reactions occurring at the interface between the active material and electrolyte.⁴⁴ Generally, the rate capability was mainly dependent on three processes: (i) the diffusion of electrolyte ions, (ii) the adsorption of ions on the electrode surface, and (iii) the charge transfer in the electrode. However, all of these processes were relatively slow at higher scan rate, leading to a reduction in specific capacitance.⁴⁵ In addition, the characteristic shape of the CV curve was not significantly changed, which was an indication that the rGO–Co₃O₄ composites have outstanding rate capability.⁴⁶

Fig. 5 CV curves of (a) C1, (b) C2, (c) C3, and (d) C4 electrodes at different scan rates in 1 M KOH electrolyte.

The charge storage mechanism of Co₃O₄ for a pseudocapacitor electrode in alkaline medium can be represented as a simple OH⁻ entering reaction, described in the following equation.³⁴

The CV curves of pure Co₃O₄ nanocubes at different scan rates are depicted in Fig. S2.† Well-defined redox peaks were not observed, perhaps because of aggregation of Co₃O₄ nanocubes resulting in reduction of the active electrochemical surface area. In addition, with increasing scan rates, the cathodic and anodic peaks rapidly shift to the low and high potentials, respectively. This is mainly because of diffusion of OH⁻ ions having less time to interact with respective electrode at high scan rates.^47,48 However, in the composites (rGO–Co₃O₄ nanocubes), the rGO matrix provides a highly conductive platform for the Co₃O₄ nanocubes because of the uniform dispersion and intercalation of the nanocubes on the matrix. This leads to an increase in electrochemical surface area by enhancing the active sites. A comparison of CV results of pure Co₃O₄ and composite (rGO–Co₃O₄) is shown in Fig. S3,† which suggests that the composite has well-defined redox peaks at lower potential than that of pure Co₃O₄ because of non-aggregation of Co₃O₄ nanocubes on the rGO matrix. Therefore, we confirmed that the rGO matrix not only provided a highly conductive platform for the Co₃O₄ nanocubes, but also facilitated non-aggregated Co₃O₄ nanocubes.

3.5.2. Galvanostatic charge discharge study. The cyclic stability of Co₃O₄ decorated rGO matrix was examined by galvanostatic charge–discharge analysis. Fig. 6 shows the galvanostatic charge/discharge curve of rGO–Co₃O₄ composite electrodes at various current densities (from 200 to 500 mA g⁻¹) under a potential range from 0 to 0.45 V (vs. Ag/AgCl).

Fig. 6 Galvanostatic charge/discharge curves for (a) C1, (b) C2, (c) C3, and (d) C4.

The specific capacitance can be calculated by the following equation:

where “I” is the discharge current (A), “t” is the discharge time (s), “m” is the mass of active material (g), and V is the discharge potential (V). The obtained specific capacitances of 216, 278, 136, and 123 F g⁻¹ correspond to C1, C2, C3, and C4, respectively, at the current density of 200 mA g⁻¹.

Fig. 7a shows that specific capacitance varies with respect to molar concentration. Lower specific capacitance was observed in pure cobalt oxide, from aggregation of Co₃O₄ nanocubes, resulting in poor electrochemical performance. Introducing the rGO caused the specific capacitance to increase significantly from 118.8 F g⁻¹ to 216 F g⁻¹ (C1), as shown in Fig. 7a. The improved specific capacitance resulted from high dispersion and intercalation of Co₃O₄ on rGO matrix. Further increases in molar concentration of cobalt precursor resulted in the specific capacitances attaining a maximum value of 278 F g⁻¹ (C2). However, beyond this addition of cobalt concentration, the specific capacitance began to decrease (C3 and C4) because of aggregation of cobalt oxide nanocubes on the rGO, resulting in limiting of OH⁻ ion diffusion into the electrode surface.

Fig. 7 (a) Variation in specific capacitance at different current densities for C1, C2, C3, and C4 samples. (b) Comparison of SC with respect to molar concentration of Co₃O₄ precursor for C1, C2, C3, and C4 samples.

Fig. 7b shows the specific capacitance versus current density plot. The values of specific capacitance decrease with increasing current density because of asynchronous movement of electric charges with respect to current rates. At higher current densities, the diffusion/migration of charges through the electrodes was very slow.⁴⁹

3.5.3. Electrochemical impedance spectroscopy. To further elucidate the origin of high electrochemical performance, electrochemical impedance spectroscopy (EIS) was carried out to examine rGO–Co₃O₄ composites. The equivalent series resistance (ESR) was obtained from the intersection point of the curves with the axis of real impedance. The difference in ESR of electrodes is attributed to the different conductance of electrode materials. ESRs of the rGO–Co₃O₄ electrodes (Fig. 8) were found to be 1.25, 1.3, 1.37, and 1.57 Ω corresponding to C1, C2, C3, and C4, respectively. The ESR of rGO–Co₃O₄ composites increased with increasing cobalt precursor concentration, indicating poor conductivity of material at high molar concentration. This may be because of aggregation of cobalt oxide and weak interaction of Co₃O₄ with the rGO matrix. At the high frequency region, the observed larger semicircle indicated poor electrical conductivity because of high interfacial charge transfer resistance.⁵⁰

Fig. 8 Nyquist plots for C1, C2, C3, and C4. Inset: magnified view of Nyquist plot at higher frequencies.

The long-term cycling stability test is a key factor to evaluate electrodes for practical applications.⁵¹ Long-term cycling performance over 2000 cycles of electrode material was carried out by repeating the charge/discharge test at a current density of 3 A g⁻¹. The cyclic stability of composites C1, C2, C3, and C4 corresponds to 86%, 91.6%, 94%, and 97.5% (Fig. 9), respectively. These results confirmed that the cyclic stability increased with increasing molar concentration of cobalt precursor. However, the specific capacitance was decreased beyond some limitation of molar concentration because of particle aggregation. The maximum specific capacitance (278 F g⁻¹) with longer cyclic stability (91.6% after 2000 cycles) was observed at C2 as compared with other electrodes. Therefore, these results indicate that C2 is a promising candidate for high performance energy storage systems.

Fig. 9 Variation of specific capacitance as a function of cycle number from 5^th to 2000^th.

4. Conclusion

In this work, the effect of different molar concentrations of cobalt precursor-based cobalt oxide–reduced graphene oxide was studied on electrochemical performance. Different molar concentrations of Co₃O₄ intercalated rGO were synthesized using a hydrothermal method. Surface morphology of the composite materials revealed uniform distribution of Co₃O₄ on the rGO matrix with an average particle size of ∼45 nm. The maximum specific capacitance (278 F g⁻¹) was observed in C2, with lower charge transfer resistance than that of other electrodes because of effective ion transfer and fewer particle aggregations on rGO matrix. Moreover, C2 exhibited long-term cyclic stability of 91.6% over 2000 cycles with good reversibility. These results demonstrate that controlling the molar concentration of cobalt precursor-based Co₃O₄ with rGO composite has potential in development of electrode materials for energy storage applications.

Acknowledgements

This work was supported by the High Impact Research Grant (H-21001-F000046) from Ministry of Education, Malaysia and University of Malaya Research Grant (RP025A-14AFR).

References

1. C. Yu, J. Yang, C. Zhao, X. Fan, G. Wang and J. Qiu, Nanoscale, 2014, 6, 3097.
2. J. K. Change, C. H. Huang, W. T. Tsai, M. J. Deng and I. W. Sun, J. Power Sources, 2008, 179, 435–440.
3. L. L. Zhang, R. Zhaou and X. S. Zhao, J. Mater. Chem., 2010, 20, 5983–5992.
4. J. J. Yoo, K. Balakrishnan, J. Huang, V. Meunier, B. G. Sumpter, A. Srivastava, M. Conway, A. L. M. Reddy, J. Yu, R. Vajtai and P. M. Ajayan, Nano Lett., 2011, 11, 1423–1427.
5. Y. Zhu, S. Murali, M. D. Stoller, K. J. Ganesh, W. Cai, P. J. Ferreira, A. Pirkle, R. M. Wallace, K. A. Cychosz, M. Thommes, D. Su, E. A. Stach and R. S. Ruoff, Science, 2011, 332, 1537–1541.
6. X. Fan, C. Yu, J. Yang, Z. Ling and J. Qiu, Carbon, 2014, 7, 130–141.
7. K. R. Prasad, K. Koga and N. Miura, Chem. Mater., 2004, 16, 1845–1847.
8. M. Jafarian, M. G. Mahjani, H. Heli and F. Gobal, Electrochim. Acta, 2003, 48, 3423–3429.
9. Y. Huang, J. Liang and Y. Chen, Small, 2012, 8, 1805–1834.
10. M. N. Hyder, S. W. Lee, F. C. Cebeci, D. J. Schmidt, Y. Shao-Horn and P. T. Hammond, ACS Nano, 2011, 5, 8552–8561.
11. A. Izadi-Najafabadi, Adv. Mater., 2010, 22, E235–E241.
12. P. C. Chen, G. Shen, Y. Shi, H. Chen and C. Zhou, ACS Nano, 2010, 4, 4403–4411.
13. X. Tian, M. Shi, X. Xu, M. Yan, L. Xu, A. M. Khan, C. Han, L. He and L. Mai, Adv. Mater., 2015, 27, 7476–7482.
14. H. Wang, L. Zhi, K. Liu, L. Dang, Z. Liu, Z. Lei, C. Yu and J. Qiu, Adv. Funct. Mater., 2015, 25, 5420–5427.
15. Y. Zhu, S. Murali, W. Cai, X. Li, J. W. Suk, J. R. Potts and R. S. Ruoff, Adv. Mater., 2010, 22, 3906–3924.
16. H. Chang and H. Wu, Energy Environ. Sci., 2013, 6, 3483.
17. J. Yang, C. Yu, X. Fan, S. Liang, S. Li, H. Huang, Z. Ling, C. Hao and J. Qiu, Energy Environ. Sci., 2016 10.1039/c5ee03633j.
18. Z. S. Wu, Y. Sun, Y. Z. Tan, S. Yang, X. Feng and K. Mullen, J. Am. Chem. Soc., 2012, 134, 19532–19535.
19. C. Xiang, M. Li, M. Zhi, A. Manivannan and N. Wu, J. Power Sources, 2013, 226, 65–70.
20. Z. Wang, X. Chen, M. Zhang and Y. Qian, Solid State Sci., 2005, 7, 13–15.
21. T. He, D. Chen, X. Jiao and Y. Wang, Adv. Mater., 2006, 8, 1078–1082.
22. L. He, Z. Li and Z. Zhang, Nanotechnology, 2008, 15, 155606.
23. E. L. Salabas, A. Rumplecker, F. Kleitz, F. Radu and F. Schüth, Nano Lett., 2006, 12, 2977–2981.
24. T. Li, S. Yang, L. Huang, B. Gu and Y. Du, Nanotechnology, 2004, 15, 1479–1482.
25. T. Yu, Y. W. Zhu, X. J. Xu, Z. X. Shen, P. Chen, C. T. Lim and C. H. Sow, Adv. Mater., 2005, 13, 1595–1598.
26. L. M. Alrehaily, J. M. Joseph, M. C. Biesinger, D. A. Guzonasc and J. C. Wren, Phys. Chem. Chem. Phys., 2013, 15, 1014–1024.
27. X. Liu, Q. Long, C. Jiang, B. Zhan, C. Li, S. Liu and X. Dong, Nanoscale, 2013, 14, 6525–6529.
28. M. M. Rahmana, J. Z. Wanga, X. L. Deng, Y. Li and H. K. Liu, Electrochim. Acta, 2009, 55, 504–510.
29. M. M. Lencka, M. Oledzka and R. E. Riman, Chem. Mater., 2000, 12, 1323–1330.
30. N. M. Huang, H. N. Lim, C. H. Chia, M. A. Yarmo and M. R. Muhamad, Int. J. Nanomed., 2011, 6, 3443–3448.
31. R. Kumar, H. J. Kim, S. Park, A. Srivastava and I. Oh, Carbon, 2014, 79, 92–202.
32. K. H. An, W. S. Kim, Y. S. Park, J. Mi Moon, D. J. Bae, S. C. Lim, Y. S. Lee and Y. H. Lee, Adv. Funct. Mater., 2001, 11, 387–392.
33. C. M. Chen, Q. Zhang, M.-G. Yang, C.-H. Huang, Y.-G. Yang and M.-Z. Wang, Carbon, 2012, 50, 3572–3584.
34. M. M. Shahid, A. Pandikumar, A. M. Golsheikh, N. M. Huang and H. N. Lim, RSC Adv., 2014, 4, 62793–62801.
35. R. Kanemoto, A. Anas, Y. Matsumoto, R. Ueji, T. Itoh, Y. Baba, S. Nakanishi, M. Ishikawa and V. Biju, J. Phys. Chem. C, 2008, 112, 8184–8191.
36. Z.-S. Wu, W. Ren, L. Wen, L. Gao, J. Zhao, Z. Chen, G. Zhou, F. Li and H.-M. Cheng, ACS Nano, 2010, 4, 3187–3194.
37. A. C. Ferrari, Solid State Commun., 2007, 143, 47–57.
38. D. Naumenko, V. Snitka, B. Snopok, S. Arpiainen and H. Lipsanen, Nanotechnology, 2012, 23, 465703.
39. S. S. Li, J. N. Zheng, X. Ma, Y. Y. Hu, A. J. Wang, J. R. Chen and J. J. Feng, Nanoscale, 2014, 6, 5708–5713.
40. Y. Kim, Y. Noh, E. J. Lim, S. Lee, S. M. Choi and W. B. Kim, J. Mater. Chem. A, 2014, 2, 6976.
41. G. T. S. How, A. Pandikumar, H. N. Ming and L. H. Ngee, Sci. Rep., 2014, 4, 5044.
42. S. Stankovich, D. A. Dikin, R. D. Piner, K. A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S. T. Nguyen and R. S. Ruoff, Carbon, 2007, 45, 1558–1565.
43. Q. Zheng, B. Zhang, X. Lin, X. Shen, N. Yousefi, Z. D. Huang, Z. Li and J. K. Kim, J. Mater. Chem., 2012, 22, 25072.
44. N. Duraisamy, A. Numan, K. Ramesh, K. H. Choi, S. Ramesh and S. Ramesh, Mater. Lett., 2015, 161, 694–697.
45. J. Li, W. Zhao, F. Huang, A. Manivannan and N. Wu, Nanoscale, 2011, 3, 5103.
46. H. W. Wang, Z. Ai Hu, Y. Q. Chang, Y. L. Chen, Z. Y. Zhang, Y. Y. Yang and H. Y. Wu, Mater. Chem. Phys., 2011, 130, 672–679.
47. C. Xu, B. Li, H. Du, F. Kang and Y. Zeng, J. Power Sources, 2008, 180, 664–670.
48. V. Subramanian, H. Zhu, R. Vajtai, P. M. Ajayan and B. Wei, J. Phys. Chem. B, 2005, 109, 20207–20214.
49. Q. Fu, F. Tietz and D. Stöver, J. Electrochem. Soc., 2006, 153, D74–D83.
50. A. Pendashteh, M. Mousavi and M. S. Rahmanifar, Electrochim. Acta, 2013, 88, 347–357.
51. S. Yoon, E. Kang, J. K. Kim, C. W. Lee and J. Lee, Chem. Commun., 2011, 47, 1021–1030.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra00160b

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