Anion-exchange reaction synthesized CoNi₂S₄ nanowires for superior electrochemical performances

Abstract

In this paper, we report CoNi₂S₄ nanowire arrays (NWAs) on 3D nickel foams based on an anion-exchange reaction which involved the pseudo Kirkendall effect. Due to the low electronegativity of sulfur, CoNi₂S₄ NWAs exhibit higher conductivity compared with Ni–Co oxide NWAs when used as active materials in supercapacitors. The electrochemistry tests show that these self-supported electrodes are able to deliver ultrahigh specific capacitance (1250 F g⁻¹ at a current density of 5 mA cm⁻²), together with a considerable areal capacitance (2.5 F cm⁻² at a current density of 5 mA cm⁻²), Furthermore, a capacitance retention of 72% after 5000 charge–discharge cycles at 5 mA cm⁻² is obtained, indicating the excellent cycling stability of the CoNi₂S₄ NWA/nickel foam electrode. The superior electrochemistry capacity demonstrates that CoNi₂S₄ NWAs are promising electrode materials for supercapacitor applications.

---

1. Introduction

Recently, urgent and increasing demands for clean, efficient, and sustainable energy have greatly stimulated substantial research on alternative energy storage and conversion. Among the effective and practical technologies for energy storage and conversion, supercapacitors, also called electrochemical capacitors, have been considered as one of the most promising energy-storage devices and wildly investigated for applications in portable electronic devices and electric vehicles because of their advantages such as fast charge−discharge process, high power density, and long cycle lifetime.^1–4 In general, supercapacitors can be classified into electric double layer capacitors (EDLCs) and pseudocapacitors on the basis of their charge storage modes.^5,6 The capacitances of EDLCs rely on charge separation at the interface between the electrolyte and electrodes. Carbon-based materials such as activated carbon, carbon nanocages and graphene have been widely used as electrodes for EDLCs due to their high specific-areas, good electrical conductivities, long cycle lifetimes and low costs.^7–9 However, their specific capacitances are still limited. The other one, in contrast, is a pseudocapacitor, which is mainly dominated by reversible fast surface faradaic redox reactions and can provide much higher specific capacitance than EDLC.^10,11

Transition-metal oxides, hydroxides, and conducting polymers are demonstrated in supercapacitor applications based on their pseudocapacitive properties.^10–14 More recently, metal sulfides have attracted extensive attention owing to their excellent potential applications in optics, catalysis, sensing, solar energy, and batteries.¹⁵ Furthermore, metal sulfides are also especially notable candidates for pseudocapacitors because of their low electronegativity and high electrochemical activity.¹⁶ In comparison to the single phase of metal sulfides, these metal sulfides, such as CoNi₂S₄,¹⁷ Co₉S₈ nanotube,¹⁸ NiS, CoS, CoS/NiS nanostructures,¹⁹ NiCo₂S₄,²⁰ exhibit higher specific capacitance due to the synergy effect of the two metallic cations, were also began to concerned.

In particular, it has been proven that Ni–Co sulfides have much lower optical band-gap energy and higher conductivity compared to Ni–Co oxides.²¹ The substitution of oxygen with sulfur could create a more flexible structure because the electronegativity of sulfur is lower than that of oxygen, which prevents the disintegration of the structure by the elongation between layers and makes it easy for electrons to transport in the structure.²² To date, most successful attempts have been focused on fabrication of various morphologies and structures of CoNi₂S₄, such as CoNi₂S₄ nanoparticles,²³ CoNi₂S₄/graphene nanocomposite²⁴ with additive binder and CoNi₂S₄ mushroom-like arrays²⁵ as supercapacitor electrodes. However, there is no report on simply synthesize CoNi₂S₄ nanowire arrays directly deposited on nickel foams for supercapacitor electrode material.

Herein, we report the interesting formation of CoNi₂S₄ nanowire arrays (NWAs), the CoNi₂S₄ nanowire arrays were fabricated through S²⁻ ion exchange using our previously synthesized Ni–Co oxide NWAs as precursor. The electrochemistry tests showed that this self-supported electrode was able to deliver ultrahigh specific capacitance (1250 F g⁻¹ at a current density of 5 mA cm⁻²), together with a considerable areal capacitance (2.5 F cm⁻² at a current density of 5 mA cm⁻²). Furthermore, a capacitance retention of 72% after 5000 charge–discharge cycles at 5 mA cm⁻² is obtained, indicating the excellent cycling stability of the CoNi₂S₄ NWAs/nickel foam electrode. The superior electrochemistry capacity demonstrates that CoNi₂S₄ NWAs are promising electrode materials for supercapacitor applications.

2. Experimental section

2.1 Preparation of Ni–Co oxide precursor

All of the reagents used in the experiment were of analytical grade and were used without further purification. Before the deposition of Ni–Co precursor on nickel foam, the nickel foam was immersed in a 1 M HCl solution for 10 min to remove the possible surface oxide layer and cleaned sequentially in ethanol and deionized water for 15 min, respectively.

Typically, 0.4 mmol NiCl₂·6H₂O, 0.8 mmol CoCl₂·6H₂O, 0.8 mmol NH₄F and 2 mmol urea were dissolved in 40 mL of deionized water by magnetic stirring for 30 min. Then, the solution was transferred into a 100 mL Teflon-lined stainless steel autoclave. A piece of nickel foam was immersed in the solution followed by heating of the autoclave at 120 °C for 8 h, and the top side of the nickel foam was uniformly covered with poly(tetra fluoroethylene) tape to prevent the solution contamination. After the autoclave cooled to room temperature, the Ni–Co precursor on nickel foam was washed with ethanol and deionized water several times and dried at 60 °C for 2 h.

2.2 Preparation of Ni–Co sulfide NWAs

The Ni–Co precursor on nickel foam was treated in a hydrothermal environment with sodium sulfide. In brief, 20 mM Na₂S·9H₂O was dissolved in 40 mL of deionized water, and then the Ni–Co oxide precursor were transferred into a 100 mL Teflon-lined stainless steel autoclave. The autoclave was heated to 120 °C for 3 h. After the autoclave cooled to room temperature, the sample was washed with ethanol and deionized water, and dried at 60 °C for 2 h. The Ni–Co sulfide nanosheet-like arrays on nickel foam were obtained. The average mass loadings of Ni–Co oxide, and sulfide nanowire arrays on nickel foam were approximately 1.5, and 2.0 mg cm⁻², respectively.

2.3 Material characterization

The morphologies and structures of samples were characterized by field-emission scanning electron microscopy (FESEM, Hitachi S-4800) equipped with an energy dispersive X-ray spectroscopy detector (EDS), transmission electron microscopy (TEM, Tecnai G2 F20 U-TWIN), X-ray diffraction (XRD, Rigaku Max-2200, Cu Kα radiation, λ = 0.15406 nm) and X-ray photoelectron spectroscopy (XPS; ESCALAB, 250).

2.4 Electrochemical measurements

All the three-electrode electrochemical measurements were carried out at ambient temperature in 3 M KOH aqueous solution as the electrolyte with a Pt wire and Ag/AgCl (saturated KCl) electrode used as the counter electrode and reference electrode, respectively. For comparison, nickel foam under the same pretreatment and the prepared Ni–Co oxide and sulfide NWAs grown on nickel foam (1 cm × 1 cm) were directly used as the working electrode independently. All potentials were referred to the reference electrode. The electrochemical performances of the samples were performed on a CHI760D (Chenhua, Shanghai) workstation for cyclic voltammetry (CV), galvanostatic charge–discharge measurements and electrochemical impedance spectroscopy (EIS) tests.

3. Results and discussion

3.1 Morphology and structure characterization

Scheme 1 illustrates the fabrication process of Ni–Co sulfide nanowire arrays on nickel foam through a two-step method. First, the Ni–Co oxide nanowire was directly grown on nickel foam by the hydrothermal process. Then the Ni–Co sulfide nanowire arrays were converted from the Ni–Co oxide nanowire subsequently by hydrothermal anion-exchange reaction processes. The formation mechanism of Ni–Co sulfides is based on an anion-exchange reaction between S²⁻the sodium sulfide solution and O²⁻ in the Ni–Co oxide nanowire at hydrothermal environment.

Scheme 1 Schematic illustration of the fabrication process for Ni–Co sulfide nanowire arrays.

Fig. 1 shows the typical SEM images of the Ni–Co oxide (Fig. 1a, b and e), and Ni–Co sulfide (Fig. 1c, d and f) nanowire arrays supported on nickel foam. Obviously, as shown in Fig. 1a and b, Ni–Co oxide NWAs with high density are grown uniformly on macroporous nickel foam and the nanowires lie perpendicular to the substrate. Meanwhile, in Fig. 1c and d, we can see that the Ni–Co sulfide nanowire arrays also retained the primary morphology well after the hydrothermal anion-exchange reaction process. However, from the high magnification SEM images in Fig. 1e and f, there is a little difference in the morphologies between the Ni–Co oxide and Ni–Co sulfide nanowire arrays. The surface of Ni–Co sulfide with a nanobelt-like feature is larger than that of the Ni–Co oxide nanowire arrays. In addition, Ni–Co sulfide nanowire arrays still lie perpendicular to the substrate and are separated apart adequately. The overall alignment results in better charge transfer kinetics, as well as easier ion diffusion.

Fig. 1 Typical FESEM images at different magnifications of (a and b) Ni–Co oxide nanowire arrays, (c and d) Ni–Co sulfide nanowire arrays supported on nickel foam and (e and f) Ni–Co oxide and Ni–Co sulfide nanowire arrays on nickel foam with high magnification, respectively.

The successful preparation of Ni–Co sulfides nanowire is also suggested by XRD analysis as shown in Fig. 2a. The three typical peaks, marked with black pentacle, originate from nickel foam. Although the peaks of Ni–Co oxide nanowire arrays are relatively weak and broad, the XRD peaks at 20° and 36°, corresponding to the (111) and (220) planes, respectively. This result is consistent with the previous report on NiCo₂O₄ nanowire arrays.²⁶ The diffraction peaks of Ni–Co sulfide nanowire arrays are also weak, indicating the low crystallinity. Four diffraction peaks at 31.4°, 38.2°, 50.1°, 55.1°, 64° and 77.9°, corresponding to the (311), (400), (511), (440), (533) and (731) diffraction planes, respectively, can be indexed to the cubic phase of CoNi₂S₄ (JCPDS 24-0334) or NiCo₂S₄ (JCPDS 43-1477).

Fig. 2 (a) XRD patterns of Ni–Co oxide and sulfide nanowire arrays on nickel foam. (b) EDS spectra of a Ni–Co sulfide nanowire array. (c) Typical TEM images of CoNi₂S₄ nanowire arrays. (d) HRTEM image of CoNi₂S₄ nanowire arrays.

In addition, EDS analysis was conducted to examine the composition of the sample. The sample contains mostly Co, Ni and S with only a trace presence of O, arising from the incomplete sulfuration, and the element ratio of Co : Ni : S is approximately 1 : 1.85 : 3.5, in Ni–Co sulfide nanowire arrays, as shown in Fig. 2b. On the basis of analysis of the XRD and EDS results, it can be identified that the materials we fabricated in this study are mainly composed of CoNi₂S₄ and a small amount of Ni–Co oxides instead of pure phases. For the sake of simplicity, we still name them as CoNi₂S₄ nanowire arrays. To better illustrate the structure, CoNi₂S₄ nanowire arrays are further investigated by TEM characterization. As shown in Fig. 2c, the surface of the CoNi₂S₄ nanowire arrays are consist of numerous interconnected nanofilm and the planar size is about 50–150 nm. The corresponding high-resolution TEM (HRTEM) of the Fig. 2c. Fig. 2d shows that the interplanar spacing is about 0.284 nm, corresponding to the (311) planes of CoNi₂S₄ phase.

The elemental composition and chemical state of the CoNi₂S₄ nanowire arrays have been investigated by XPS measurements, and the corresponding results are presented in Fig. 3a–d. As shown in Fig. 3a, the intensity of the Co 2p peaks is relatively weak. The binding energies at around 777.62 and 794.13 eV of the Co 2p peaks are assigned to Co³⁺ and the binding energies at 783.25 and 798.42 eV to Co²⁺. Likewise, in the Ni 2p spectra shown in Fig. 3c, the peaks at 855.88 and 873.50 eV indicated Ni²⁺ and the peaks at 857.40 and 875.18 eV denoted Ni³⁺. In the S 2p spectra (Fig. 3d), the binding energy centered at 168.83 eV can be fitted by one main peak and one shakeup satellite peak. The peak centered at about 163.22 eV agrees with the binding energies of metal−sulfur bonding (Ni–S and Co–S bonding).^27–29 Therefore, the surface of the CoNi₂S₄ nanowire arrays is composed of Co²⁺, Co³⁺, Ni²⁺, Ni³⁺ and S²⁻.

Fig. 3 XPS spectra of CoNi₂S₄ nanowire arrays. (a) Survey spectrum (b) Co 2p, (c) Ni 2p, and (d) S 2p.

3.2 Electrochemistry measurements

To explore the potential applications in supercapacitors, we investigated the electrochemical performance of Ni–Co oxide, and CoNi₂S₄ nanowire arrays on nickel foam. Fig. 4a shows the typical cyclic voltammetry (CV) curves of CoNi₂S₄ nanowire arrays supported on nickel foam with the potential window from 0 to 0.6 V (vs. Ag/AgCl) at various sweep rates ranging from 5 to 100 mV s⁻¹. It is clear that a pair of redox peaks is visible in all CV curves, revealing distinct pseudocapacitive characteristics which are associated with the faradaic redox reactions related to M–O/M–O–OH, where M refers to Ni or Co. With an increase of the sweep rates, the anodic peak current density increases and the cathodic peak current density decreases, suggesting a relatively low resistance of the electrode and fast redox reactions at the interface of the electrode and electrolyte.³⁰

Fig. 4 (a) CV curves at various scan rates and (b) GCD plots at various current densities of CoNi₂S₄ nanowire arrays supported on nickel foam. (c) Comparative CV curves at a scan rate of 5 mV s⁻¹. (d) GCD curves at a current density of 5 mA cm⁻² of Ni–Co oxide, and CoNi₂S₄ nanowire arrays. (e) Specific capacitance and areal capacitance of Ni–Co oxide, and sulfide CoNi₂S₄ nanowire arrays. (f) Cycling performance and coulombic efficiency of CoNi₂S₄ nanowire arrays at a current density of 5 mA cm⁻².

Fig. 4b shows the galvanostatic charge−discharge (GCD) curves of CoNi₂S₄ nanowire arrays on nickel foam with a potential window of 0–0.6 V (vs. Ag/AgCl) at various current densities ranging from 5 to 50 mA cm⁻². The symmetric shape of the GCD curves implies good electrochemical capacitive characteristics and excellent reversibility of faradaic redox reactions. The distinct plateau regions in the GCD curves demonstrate the pseudocapacitive behaviors, caused by the charge-transfer reaction and electrochemical adsorption–desorption process at the electrode/electrolyte interface, which is well consistent with the redox peaks in the CV curves. For comparison, the CV curves at a sweep rate of 100 mV s⁻¹ and GCD curves at a current density of 50 mA cm⁻² of Ni–Co oxide, and sulfide nanowire arrays are shown in parts (c) and (d) of Fig. 4, respectively. Fig. S3† show the detailed CV and GCD curves of Ni–Co oxide nanowire arrays. The CV integral area of CoNi₂S₄ is larger than that of Ni–Co oxide nanowire arrays, and the GCD time of CoNi₂S₄ is above 1.3 times that of Ni–Co oxide nanowire arrays at a current density of 5 mA cm⁻². As for the capacitance contribution of conductive substrates, the electrochemical performance of nickel foam has been investigated, as shown in Fig. S4 in the ESI.† It is clearly that the capacitance contribution from nickel foam can be negligible.

The specific capacitance (F g⁻¹) and areal capacitance (F cm⁻²) of the electrode in a three-electrode system can be calculated from the galvanostatic discharge curves according to the eqn (1) in ESI,† the calculated specific and areal capacitance values as a function of the applied current density for Ni–Co oxide, and CoNi₂S₄ nanowire arrays are shown in Fig. 4e. Impressively, the CoNi₂S₄ NWAs electrode delivers high areal capacitances of 2.50, 2.22, 2.04, 1.98, 1.90 and 1.80 F cm⁻² at current densities of 5, 10, 15, 20, 30 and 50 mA cm⁻², respectively. And the specific capacitances are 1250, 1108, 1020, 990, 950 and 900 F g⁻¹ at the corresponding current densities mentioned above. It is well-known that preparation of electrodes with high mass loading of active materials plays a crucial role in practical applications for supercapacitors. However, it is found that electrodes with high specific capacitance often suffer from drawbacks of extremely low mass loadings of the active material, along with the low areal capacitance.^31–33 But our electrodes afford trade-off between them, which result in high capacitive performance and exhibit both excellent areal capacitance and specific capacitance with high mass loading of active materials. Even at a high current density of 50 mA cm⁻², the electrodes still reveal a quite considerable capacitance of 1.80 F cm⁻² (900 F g⁻¹). To the best of our knowledge, this performance of CoNi₂S₄ obtained here is superior to the relevant data about cobalt/nickel sulfides^34–37 reported in the literature and some hybrid oxide/sulfide^38–44 nanocomposite, except the NiCo₂S₄ nanotube arrays⁴⁵ reported most recently. To evaluate the durability of the CoNi₂S₄ NWAs, charge–discharge cycling tests at a constant current density of 5 mA cm⁻² were employed to characterize their cycling performance. As shown in Fig. 4f, the electrode retains a capacitance of 900 F g⁻¹ with 72% of the initial values after 5000 cycles, indicating the excellent cycling stability of the CoNi₂S₄ NWAs/nickel foam electrode.

Nyquist impedance spectral measurements were also carried out to evaluate the charge transfer and electrolyte diffusion in the electrode/electrolyte interface, as shown in Fig. S5 in the ESI.† Obviously, the Nyquist impedance plots of the Ni–Co oxide, and CoNi₂S₄ electrodes are composed of a semicircle at the high-frequency region and a straight line at the low-frequency region. The internal and charge-transfer resistance values of these samples are almost the same at the high-frequency region. However, according to the high slope of the straight line, the CoNi₂S₄ electrode exhibits lower diffusive resistance than the Ni–Co oxide electrodes at the low-frequency region, which could improve the diffusion and transportation of the electrolyte ions into the electrode materials.

Furthermore, the capacitance performance of the CoNi₂S₄ nanowire array loaded nickel foam electrode was also evaluated using a two-electrode system for reference. Fig. 5a depicts the CV curves at scan rates of 5–100 mV s⁻¹. Differing from the CV curves obtained in the three-electrode, here the CV curves show rectangular-like shapes, revealing that the electrode stores electrochemical charge by way of an electrical double-layer. It can also be seen that increasing the scan rate leads to further augmentation of the CV curves, in agreement with the trend revealed by the three-electrode system. Galvanostatic charge–discharge measurements designated to different current densities were conducted and the resultant profiles are given in Fig. 5b. The specific capacitances are obtained from formula (2) in ESI.† According to the results, a high specific capacitance of 560 F g⁻¹ (1.12 F cm⁻²) can be achieved at a low current density of 5 mA cm⁻² and 455 F g⁻¹ (0.91 F cm⁻²) at a high current density of 30 mA cm⁻². The energy density and power density are obtained from formulas (3) and (4) respectively. Fig. S6† shows the Ragone plot concerning energy vs. power density. In the case of a low power density of 1.134 kW kg⁻¹, a high energy density of 15.75 W h kg⁻¹ can be obtained. While increasing the power density up to 5.76 kW kg⁻¹, the energy density decreases to 12.8 W h kg⁻¹, accordingly.

Fig. 5 (a) CV curves of the CoNi₂S₄ nanowire array/nickel foam electrode at different scan rates in a two electrode system; (b) galvanostatic charge–discharge curves measured at various current densities in the two electrode system.

The superior supercapacitive performance of CoNi₂S₄ nanowire arrays can be attributed to the following aspects: (1) the sheet-like nanowire structure of CoNi₂S₄ nanowire arrays is directly grown on nickel foam with good mechanical and electrical contact. (2) The sheet-like nanowire structure not only provides numerous electroactive sites for the adsorption of ions but also contribute efficient pathways for charge transportation. (3) CoNi₂S₄ nanowire arrays exhibit low crystallinity and good wettability without an annealing process, which are beneficial for improving the supercapacitive performance. (4) CoNi₂S₄ nanowire arrays possess higher conductivity and richer redox reaction compared to Ni–Co oxide nanowire arrays.

4. Conclusions

In summary, we have successfully in situ grown CoNi₂S₄ nanowire arrays on nickel foams through a facile two-step hydrothermal method for electrochemical energy storage. The efficient and low-cost two-step fabrication method involves the hydrothermal method and anion-exchange reaction. The electrochemistry tests show that these self-supported electrodes are able to deliver ultrahigh specific capacitance (1250 F g⁻¹ at a current density of 5 mA cm⁻²), together with a considerable areal capacitance (2.50 F cm⁻² at a current density of 5 mA cm⁻²). Furthermore, a capacitance retention of 72% after 5000 charge–discharge cycles at 5 mA cm⁻² is obtained, indicating the excellent cycling stability of the CoNi₂S₄ NWAs/nickel foam electrode. The superior electrochemistry capacity demonstrates that CoNi₂S₄ NWAs are promising electrode materials for supercapacitor applications.

Acknowledgements

This work was financially supported by the projects (21371007) from National Natural Science Foundation of China, Anhui Provincial Natural Science Foundation (1208085QB28), Anhui Provincial Natural Science Foundation for Distinguished Youth (1408085J03), Natural Science Foundation of Anhui (KJ2012A139) and the Program for Innovative Research Team at Anhui Normal University.

Notes and references

1. P. Simon, Y. Gogotsi and B. Dunn, Science, 2014, 343, 1210–1211.
2. P. Simon and Y. Gogotsi, Nat. Mater., 2008, 7, 845–854.
3. G. P. Wang, L. Zhang and J. Zhang, Chem. Soc. Rev., 2012, 41, 797–828.
4. L. L. Zhang and X. S. Zhao, Chem. Soc. Rev., 2009, 38, 2520–2531.
5. P. Simon and Y. Gogotsi, Nat. Mater., 2008, 7, 845.
6. Y. Q. Wu, X. Y. Chen, P. T. Ji and Q. Q. Zhou, Electrochim. Acta, 2011, 56, 7517.
7. Q. Q. Zhou, X. Y. Chen and B. Wang, Microporous Mesoporous Mater., 2012, 158, 155.
8. Z. J. Fan, Q. K. Zhao, T. Y. Li, J. Yan, Y. M. Ren, J. Feng and T. Wei, Carbon, 2012, 50, 1699.
9. K. Xie, X. Qin, X. Wang, Y. Wang, H. Tao, Q. Wu, L. Yang and Z. Hu, Adv. Mater., 2012, 24, 347.
10. H. Chen, S. X. Zhou and L. M. Wu, ACS Appl. Mater. Interfaces, 2014, 6, 8621–8630.
11. H. Chen, L. F. Hu, Y. Yan, R. C. Che, M. Chen and L. M. Wu, Adv. Energy Mater., 2013, 3, 1636–1646.
12. C. Z. Yuan, H. B. Wu, Y. Xie and X. W. Lou, Angew. Chem., Int. Ed., 2014, 53, 1488–1504.
13. H. Chen, L. F. Hu, M. Chen, Y. Yan and L. M. Wu, Adv. Funct. Mater., 2014, 24, 934–942.
14. G. A. Snook, P. Kao and A. S. Best, J. Power Sources, 2011, 196, 1–12.
15. C. H. Lai, M. Y. Lu and L. J. Chen, J. Mater. Chem., 2012, 22, 19–30.
16. J. Pu, Z. H. Wang, K. L. Wu, N. Yu and E. H. Sheng, Phys. Chem. Chem. Phys., 2014, 16, 785–791.
17. W. Hu, R. Q. Chen, W. Xie, L. L. Zou, N. Qin and D. H. Bao, ACS Appl. Mater. Interfaces, 2014, 21, 19318–19326.
18. J. Pu, Z. H. Wang, K. L. Wu, N. Yu and E. H. Sheng, Phys. Chem. Chem. Phys., 2014, 16, 785–791.
19. X. H. Xia, C. R. Zhu, J. S. Luo, Z. Y. Zeng, C. Guan, C. F. Ng, H. Zhang and H. J. Fan, Small, 2014, 10, 766–773.
20. H. C. Chen, J. J. Jiang, Y. D. Zhao, L. Zhang, D. Q. Guo and D. D. Xia, J. Mater. Chem. A, 2015, 1, 428–437.
21. H. C. Chen, J. J. Jiang, L. Zhang, H. Z. Wan, T. Qi and D. D. Xia, Nanoscale, 2013, 5, 8879–8883.
22. S. H. Park, Y. K. Sun, K. S. Park, K. S. Nahm and Y. S. Lee, Electrochim. Acta, 2002, 47, 1721–1726.
23. W. M. Du, Z. Q. Zhu, Y. B. Wang, J. N. Liu, W. J. Yang, X. F. Qian and H. Pang, RSC Adv., 2014, 4, 6998–7002.
24. W. M. Du, Z. Y. Wang, Z. Q. Zhu, S. Hu, X. Y. Zhu, Y. F. Shi, H. Pang and X. F. Qian, J. Mater. Chem. A, 2014, 2, 9613–9619.
25. L. Mei, T. Yang, C. Xu, M. Zhang, L. B. Chen, Q. H. Li and T. H. Wang, Nano Energy, 2014, 3, 36–45.
26. G. Q. Zhang and X. W. Lou, Adv. Mater., 2013, 25, 976–979.
27. H. C. Chen, J. J. Jiang, L. Zhang, H. Z. Wan, T. Qi and D. D. Xia, Nanoscale, 2013, 5, 8879–8883.
28. W. M. Du, Z. Y. Wang, Z. Q. Zhu, S. Hu, X. Y. Zhu, Y. F. Shi, H. Pang and X. F. Qian, J. Mater. Chem. A, 2014, 2, 9613–9619.
29. J. Pu, F. L. Cui, S. B. Chu, T. T. Wang, E. R. Sheng and Z. H. Wang, ACS Sustainable Chem. Eng., 2014, 2, 809–815.
30. C. Z. Yuan, J. Y. Li, L. R. Hou, X. G. Zhang, L. F. Shen and X. W. Lou, Adv. Mater., 2012, 22, 4592–4597.
31. T. Y. Wei, C. H. Chen, H. C. Chien, S. Y. Lu and C. C. Hu, Adv. Mater., 2010, 22, 347.
32. C. Z. Yuan, J. Y. Li, L. R. Hou, X. G. Zhang, L. F. Shen and X. W. Lou, Adv. Funct. Mater., 2012, 22, 4592.
33. J. K. Chang, C. M. Wu and I. W. Sun, J. Mater. Chem., 2010, 20, 3729.
34. J. Xu, Q. F. Wang, X. W. Wang, Q. Y. Xiang, B. Liang, D. Chen and G. Z. Shen, ACS Nano, 2013, 7, 5453.
35. Q. H. Wang, L. F. Jiao, H. M. Du, Y. C. Si, Y. J. Wang and H. T. Yuan, J. Mater. Chem., 2012, 22, 21387.
36. T. Zhu, Z. Y. Wang, S. J. Ding, J. S. Chen and X. W. Lou, RSC Adv., 2011, 1, 397.
37. S. W. Chou and J. Y. Lin, J. Electrochem. Soc., 2013, 160, D178.
38. H. B. Wu, H. Pang and X. W. Lou, Energy Environ. Sci., 2013, 6, 3619.
39. H. Jiang, J. Ma and C. Z. Li, Chem. Commun., 2012, 48, 4465.
40. C. Z. Yuan, J. Y. Li, L. R. Hou, X. G. Zhang, L. F. Shen and X. W. Lou, Adv. Funct. Mater., 2012, 22, 4592.
41. G. Q. Zhang, H. B. Wu, H. E. Hoster, M. B. Chan-Park and X. W. Lou, Energy Environ. Sci., 2012, 5, 9453.
42. S. J. Peng, L. L. Li, C. C. Li, H. T. Tan, R. Cai, H. Yu, S. Mhaisalkar, M. Srinivasan, S. Ramakrishna and Q. Y. Yan, Chem. Commun., 2013, 49, 10178.
43. M. C. Liu, L. B. Kong, C. Lu, X. M. Li, Y. C. Luo and L. Kang, ACS Appl. Mater. Interfaces, 2012, 4, 4631.
44. J. H. Zhong, A. L. Wang, G. R. Li, J. W. Wang, Y. N. Ou and Y. X. Tong, J. Mater. Chem., 2012, 22, 5656.
45. H. C. Chen, J. J. Jiang, L. Zhang, D. D. Xia, Y. D. Zhao, D. Q. Guo, T. Qi and H. Z. Wan, J. Power Sources, 2014, 254, 249.

---

Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra18625k

‡ Q. Q. Hu and W. Q. Ma are cofirst authors and contributed equally.

---