Fabrication of a snail shell-like structured MnO₂@CoNiO₂ composite electrode for high performance supercapacitors

Abstract

For the first time, we develop a cost-effective and facile one-step hydrothermal approach to directly grow MnO₂@CoNiO₂ snail shell-like, CoNiO₂ nanoflake-like, and MnO₂ nanoflower-like structures on nickel foam as electrode for supercapacitor applications. Cyclic voltammetry, galvanostatic charge–discharge cycling, and electrochemical impedance spectroscopy are used to investigate the electrochemical responses of the electrodes. The MnO₂@CoNiO₂ composite electrode exhibits a high specific capacitance and energy density of 1605.4 F g⁻¹ and 51.37 W h kg⁻¹, respectively, at 20 mA cm⁻² in 3 M KOH aqueous solution, as well as an attractive cycling ability (98.87% even after 3000 cycles at 40 mA cm⁻²), which are much better than those of the CoNiO₂ and MnO₂ electrodes. Such superior electrochemical performance of the MnO₂@CoNiO₂ electrode is attributed to the combination of two active materials with an improved surface morphology, which can offer more pathways for electron transport and enhance the utilization of the electrode materials. These results show that the MnO₂@CoNiO₂ composites are promising positive electrode materials for practical supercapacitors.

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1. Introduction

Lightweight, high performance, excellent reversibility, stability, safety, and environmentally friendly energy storage devices, such as supercapacitors (SCs) and lithium-ion batteries, are required in the present modern electronic industry.^1,2 Lithium-ion batteries (LIBS) and supercapacitors (SCs) have attracted considerable worldwide attention. SCs, also called ultra-capacitors or electrochemical capacitors (ECs), are considered as a new class of electrochemical energy storage devices because they can achieve higher energy density and power density when compared to traditional capacitors and batteries.^3–8 Supercapacitors (SCs) have attracted increasing attention due to their simple principle, fast recharge ability, long cycle life, high power performance, and low maintenance cost,^9,10 which allows applications for a range of power and energy necessities, such as short-term power sources for mobile electronic devices, hybrid electric vehicles, etc.^11,12

SCs can be classified into two categories, pseudo capacitors and electrical double-layer capacitors (EDLCs), according to the electrochemical storage mechanism. Typically, pseudo capacitors have a much larger energy density and higher specific capacitance than EDLCs due to the reversible faradaic redox reactions at the electrode surface.^13–15 Many different materials and its systems have been studied widely as active electrode materials for SCs because of their higher theoretical specific capacitance.^16–18 Transition metal oxides/hydroxides, carbonaceous materials, and conducting polymers are commonly used materials for supercapacitor applications.^19–26 Among them, transition metal oxides usually offer good electrical performance because of their increased conductivity and their synergic effects.^27,28 Transition metal oxides, such as RuO₂, V₂O₅, NiO, MoO₃, NiCo₂O₄, Co₃O₄, TiO₂, SnO₂, and MnO₂,^27–40 have been studied as an attractive electrode material for SCs. Moreover, the electrochemical performance, natural abundance, low cost, and environmental friendliness of MnO₂ makes it a promising candidate for supercapacitor electrodes.^41–44 Recently, many literatures about the fabrication and its super capacitive behavior of NiCo₂O₄ have been investigated, such as nanoflakes, nanoparticles, nanowires and flowers.^45–48 The use of both metals in this way enhances electronic conductivity with a reduction in the ternary metal oxides, and is beneficial to electrochemical applications.⁴⁹ However, there have been very few studies on the synthesis and supercapacitor evaluation of CoNiO₂ due to the lack of feasible fabrication methods without the use of poisonous and noxious reagents to realize mass production.^50,51 Therefore, simple and scalable preparation routes for MnO₂ and CoNiO₂ should be investigated to achieve various structure designs of MnO₂ and CoNiO₂ nanomaterials with different morphologies for practical applications and supercapacitor performance is unsatisfactory.^52,53 Therefore composites were developed to improve the electrochemical performance of supercapacitors.

Currently, composites containing oxides have been studied most widely. First-rate supercapacitor electrode materials, such as CNT@MnO₂, CuO@MnO₂, Co₃O₄/SnO₂@MnO₂, TiO₂@MnO₂, ZnCo₂O₄@Ni, ZnO@Ni₃S₂, ZnO@MnCo₂O₄, Fe₃O₄@Fe₂O₃, Co₃O₄@MnO₂, Fe₃O₄@MnO₂, and ZnCoO₄@MnO₂,^54–64 have attracted considerable attention. Recently, Yu et al. developed NiCo₂O₄@MnO₂ integrated electrode for high-performance supercapacitors.⁶⁵ Impressively, the Ni foam-supported core–shell heterostructured NW array electrode (NiCo₂O₄@MnO₂) delivers higher area specific capacitance of 3.31 F cm⁻² at current density of 2 mA cm⁻². On the other hand, there are no reports on the electrochemical capacitance of their integrated materials with the aim of elevating the supercapacitor performance by rationally combining CoNiO₂ and MnO₂, even though the individual capacitive behavior of both has been demonstrated.

In this study, a low cost and facile strategy was developed for the design and fabrication of novel hierarchical MnO₂@CoNiO₂ nanotube arrays on nickel foam using a facile hydrothermal pathway for supercapacitor applications. This hybrid nanotube arrays delivered a high specific capacitance of 1605.4 F g⁻¹, and operated at a high power density of 1142.75 W kg⁻¹ with an energy density of 51.37 W h kg⁻¹ at a current density of 20 mA cm⁻². These interesting results highlights the potential of MnO₂@CoNiO₂ supercapacitors as a high performance energy storage system for practical applications.

2. Experimental section

2.1 Materials

All chemicals, such as manganese(II) nitrate tetrahydrate [Mn(NO₃)₂·4H₂O], cobalt(II) nitrate hexahydrate [Co(NO₃)₂·6H₂O], nickel(II) sulfate hexahydrate [NiSO₄·6H₂O], ammonium fluoride [NH₄F], urea [CH₄N₂O], potassium hydroxide [KOH], and nickel (Ni) foam were purchased from Sigma-Aldrich and used as received.

2.2 Synthesis of MnO₂, CoNiO₂ and MnO₂@CoNiO₂ electrodes

In a typical procedure, a piece of Ni foam was cut into 1 × 1 cm² squares and cleaned ultrasonically in a 2 M HCl solution for 30 minutes. The Ni foams were then cleaned sequentially in acetone, ethanol, and DI water for 15 min each. For the synthesis of the MnO₂@CoNiO₂ electrode, 0.702 g (0.04 M) of Mn(NO₃)₂·4H₂O, 0.814 g (0.04 M) of Co(NO₃)₂·6H₂O, and 0.735 g (0.04 M) of NiSO₄·6H₂O were dispersed in 70 ml of DI water and stirred vigorously for 30 min to form a homogeneous solution. Subsequently, 0.622 g (0.24 M) of NH₄F and 2.018 g (0.48 M) of CH₄N₂O were added sequentially to the above homogeneous solution. After stirring for 30 minutes, the solution was poured into a 100 ml polytetrafluoroethylene (PTFE) Teflon-lined stainless steel autoclave and the pretreated Ni foam was dipped into the reaction mixture. The autoclave was sealed and maintained at 100 °C for 6 h, and cooled naturally to 27 °C. The as-gained Ni foam was removed, washed with DI water, and dried in the oven. Finally, MnO₂@CoNiO₂ electrode was obtained by annealing the sample at 200 °C for 2 h in air. CoNiO₂ and MnO₂ electrodes were also prepared using the above procedure with the relevant chemicals. The loading of the MnO₂@CoNiO₂, CoNiO_2, and MnO₂ samples was approximately 4.2, 3.6, and 3.0 mg cm⁻², respectively.

2.3 Materials characterization

The morphology of the films was analyzed by field emission scanning electron microscopy (FE-SEM, S-2400, Hitachi) equipped with energy-dispersive X-ray spectroscopy (EDX) operated at 15 kV. The crystalline nature and structure of the electrodes were studied by X-ray diffraction (XRD) analysis on a D8 ADVANCE with a diffractometer using a Cu Kα radiation source operated at 40 kV and 40 mA in the 2θ range of 20–80°. X-ray photon electron spectroscopy (XPS, VG Scientific ESCALAB 250) was performed using monochromatic Al-K_α radiation at 1486.6 eV. Brunauer–Emmett–Teller (BET) analysis was used to measure the specific surface area of the samples using a BELsorp-Max (BEL, Japan) instrument at 77 K.

2.4 Electrochemical measurements

All electrochemical characterizations were conducted in a three-electrode configuration consisting of MnO₂ (or CoNiO₂ or MnO₂@CoNiO₂), Pt wire, and Ag/AgCl as the working electrode, counter electrode, and reference electrode, respectively. The supporting electrolyte consisted of a 3 M KOH aqueous solution. Cyclic voltammetry (CV), galvanostatic charge–discharge tests, and electrochemical impedance spectroscopy (EIS) were measured using a Biologic-SP150 workstation. CV was studied at a potential range from +0.0 V to +0.5 V at 5 mV s⁻¹ to 100 mV s⁻¹ scan rates. The galvanostatic charge–discharge tests were studied by chronopotentiometry at a voltage window of +0.0 V to +0.48 V at different current densities (20–100) mA cm⁻². Electrochemical impedance spectroscopy (EIS) was conducted over a frequency region from 0.1 Hz to 500 kHz. The specific capacitance (C_s, F g⁻¹), energy density (E, W h kg⁻¹), and power density (P, W kg⁻¹) were calculated from the galvanostatic charge–discharge curves using the following equations:^66,67where I is the discharge current, t is the discharge time, V is the potential window and m is the mass of the active material.

3. Results and discussion

The morphology and composition of the precursor samples were analyzed by low and high magnification FE-SEM. Fig. 1 presents the morphology of the MnO₂, CoNiO₂, and MnO₂@CoNiO₂ on the Ni-foam substrates. Fig. 1(a1 and a2) show FE-SEM images of the MnO₂ electrode, which had a nanoflower morphology with a uniform deposition. Fig. 1(b1 and b2) present low and high magnification SEM images of the as-synthesized nanoflakes, such as the CoNiO₂ sample. A low magnification FE-SEM of MnO₂@CoNiO₂ (Fig. 1(c1)) revealed uniform snail shell-like structure deposited on the Ni-foam. On the other hand, the high magnification FESEM image of MnO₂@CoNiO₂ (Fig. 1(c2)) showed the dense CoNiO₂ nanoneedles attached to the MnO₂. These novel nanostructured films provided an abundant porous surface area for contact between the electrolyte and electrode, which enhanced the performance of supercapacitors.

Fig. 1 (a1, b1 and c1) high magnification and (a2, b2 and c2) low magnification typical FE-SEM images of the (a1 and a2) MnO₂, (b1 and b2) CoNiO₂ and (c1 and c2) MnO₂@CoNiO₂ electrodes.

EDX was performed to determine the elemental distribution of the prepared electrodes. Fig. 2a–c present the EDX spectra of MnO₂, CoNiO₂ and MnO₂@CoNiO₂ electrodes, respectively. The EDX spectrum of MnO₂ revealed Mn and O, whereas CoNiO₂ exhibited Co, Ni, and O. On the other hand, the EDX spectrum of MnO₂@CoNiO₂ contained Co, Ni, Mn, and O, confirming that the MnO₂@CoNiO₂ had been deposited successfully on the surface of the Ni-foam. The crystal structures of the MnO₂@CoNiO₂ precursor was further studied using XRD analysis, as shown in Fig. 3. Besides the three strong peaks from the nickel foam substrate, the other diffraction peaks can be indexed to the (121) and (211) planes of the MnO₂ (JCPDS no. 14-0644) and the (111), (220) and (311) planes of the CoNiO₂ (JCPDS no. 10-0188), which is consistent with the previous literatures.^68,69

Fig. 2 EDX spectrum of (a) MnO₂, (b) CoNiO₂ and (c) MnO₂@CoNiO₂ electrodes on Ni-foam.

Fig. 3 XRD pattern of the MnO₂@CoNiO₂ on the surface of nickel foam substrate.

XPS is one of the most useful and effective techniques for analyzing the surface composition and oxidation states of the elements. Fig. 4a presents the survey spectrum of the MnO₂@CoNiO₂ composite, indicating the existence of Co, Ni, O, C, and Mn. The high-resolution Co 2p spectrum (Fig. 4b) exhibited peaks at 781.8 eV and 797.15 eV for Co 2p_3/2 and Co 2p_1/2, respectively.⁷⁰ These two oxidation states were separated by a 15.35 eV spin energy, which indicates Co⁺². The Ni 2p spectrum (Fig. 4c), showed two main spin–orbit peaks of Ni 2p_3/2 at 856.24 eV and Ni 2p_1/2 at 873.31 eV with a separation of approximately 17 eV, which is characteristic of Ni²⁺.⁷¹ The high resolution scan of Mn 2p of the composite (Fig. 4d) showed two peaks at 642.48 and 653.32 eV, which were assigned to Mn 2p_3/2 and Mn 2p_1/2, respectively, and the binding energy peaks of Mn 2p were separated by 10.84 eV. XPS confirmed that the MnO₂@CoNiO₂ composite consisted of Co²⁺, Ni²⁺, and Mn²⁺.

Fig. 4 XPS spectrum of MnO₂@CoNiO₂ on Ni-foam; (a) survey spectrum, high-resolution spectra for (b) Co 2p, (c) Ni 2p, and (d) Mn 2p.

To investigate the specific areas and the porous nature of the MnO₂, CoNiO₂ and MnO₂@CoNiO₂, BET gas sorption measurements were performed. The nitrogen adsorption/desorption isotherms and the pore size distribution plots of the three samples are shown in Fig. 5. Type IV isotherms were clearly exhibited, denoting the existence of mesopores in the three samples. The BET surface area of the prepared MnO₂@CoNiO₂ composite was obtained to be 40.6 m² g⁻¹, which was higher than that of the CoNiO₂ (27.3 m² g⁻¹) and MnO₂ (22.6 m² g⁻¹) samples. The pore size distribution curves from the N₂ adsorption isotherm of the Fe-SNC and SNC are shown in the inset of Fig. 5, it can be seen that MnO₂@CoNiO₂ has the pore size distribution of 20.1 nm, whereas MnO₂ and CoNiO₂ show pore sizes of 20.4 and 20.8 nm. These results indicate that the three samples are having mesoporous structures with large surface area that is much needed for the electro-catalytic applications.

Fig. 5 The Brunauer–Emmett–Teller (BET) surface area measured from nitrogen adsorption–desorption isotherms and the insets show the corresponding pore-size distribution of the (a) MnO₂, (b) CoNiO₂ and (c) MnO₂@CoNiO₂ on Ni-foam substrate.

CV was conducted in a 3 M KOH electrolyte over the potential range, 0.00 to 0.50 V at different scan rates of 5 to 100 mV s⁻¹ for the MnO₂, CoNiO₂ and MnO₂@CoNiO₂ electrode materials, as shown in Fig. 6. Fig. 6a shows the CV curves of the three electrodes at a scan rate of 20 mV s⁻¹ in a 3 M KOH electrolyte solution. The CV curve shape is different from the electric double-layer capacitance, showing that the capacitance was due mainly to the pseudo capacitive capacitance. Moreover, from the CV curve shapes, the integrated area of the MnO₂@CoNiO₂ electrode was higher than that of the other two electrode materials, showing that the MnO₂@CoNiO₂ electrode has the largest specific capacitance compared to the CoNiO₂ and MnO₂ electrodes. Fig. 6b and c shows a smaller surrounded area of CoNiO₂ and MnO₂ than that of the MnO₂@CoNiO₂ (Fig. 6d), suggesting the low electrochemical performance of CoNiO₂ and MnO₂.

Fig. 6 (a) A comparison CV curves of the MnO₂, CoNiO₂ and MnO₂@CoNiO₂ electrodes at the scan rate of 20 mV s⁻¹. CV curves of the (b) MnO₂, (c) CoNiO₂ and (d) MnO₂@CoNiO₂ electrodes at different scan rates (5–100 mV s⁻¹) in 3 M KOH solution.

Furthermore, the electrochemical performance of the electrodes was examined by galvanostatic charge–discharge (GCD) tests. The GCD measurements were studied over the potential range, +0.0 V to +0.48 V, at different current densities (20 to 100 mA cm⁻²). Fig. 7a–c show GCD plots of the MnO₂, CoNiO₂, and MnO₂@CoNiO₂ electrodes, respectively. The extracted specific capacitance of MnO₂@CoNiO₂ (Fig. 7d) were 1605.4, 1487.7, 1338.2, 1201.3, 1060.7, and 881.4 F g⁻¹ at current densities of 20, 30, 40, 50, 60, and 100 mA cm⁻², respectively. The measured specific capacitances of CoNiO₂ were 1338.4, 1134.4, 1028.1, 970.8, 867.9, and 759.7 F g⁻¹ at different current densities. Those of MnO₂ were 1069.0, 953.8, 810.1, 713.5, 593.7, and 482.0 F g⁻¹ at different current densities. The specific capacitance increased gradually with decreasing current density. The MnO₂@CoNiO₂ electrode showed a high specific capacitance of 1605.4 F g⁻¹ at a current density of 20 mA cm⁻², whereas that of the CoNiO₂ and MnO₂ electrodes were only 1338.4 and 1069.0 F g⁻¹ at the same current density. The excellent and superior specific capacitance of MnO₂@CoNiO₂ may be due to its higher surface area with the improved morphology of nanostructures. The power density and energy density are two essential parameters to examine the performance of supercapacitors. Fig. 8 shows the Ragone plots of MnO₂, CoNiO₂, and MnO₂@CoNiO₂. The MnO₂@CoNiO₂ exhibited a high energy density (51.37 W h kg⁻¹) and power density (1142.75 W kg⁻¹), which were much higher than those of the CoNiO₂ (42.83 W h kg⁻¹ and 1599.95 W kg⁻¹) and MnO₂ (34.21 W h kg⁻¹ and 1333.29 W kg⁻¹) at a current density of 20 mA cm⁻².

Fig. 7 Charge and discharge curves of the (a) MnO₂, (b) CoNiO₂ and (c) MnO₂@CoNiO₂ electrodes at different current densities (20–100 mA cm⁻²) in 3 M KOH solution; (d) the measured specific capacitance as a function of current density.

Fig. 8 Ragone plot of the MnO₂, CoNiO₂ and MnO₂@CoNiO₂ electrodes.

EIS is an efficient way to examine the electrochemical properties of electrodes; the impedance was measured over the frequency range, 100 mHz to 500 kHz, at a bias of 0 V, and the plots are shown in Fig. 9. All plots displayed a depressed semicircle in the high-frequency region and a straight line in the low-frequency region. At high frequencies, the Z′-axis intercept indicated the equivalent series resistance (ESR) and the semicircle diameter was assigned to the charge transfer resistance (R_ct) at the interface of electrode/electrolyte. The ESR value of the MnO₂@CoNiO₂ electrode was smaller than that of the other two electrodes (MnO₂, CoNiO₂), which reflects the higher conductivity obtained in the MnO₂@CoNiO₂ electrode. On the other hand, as shown in the figure, a significant change in R_ct was observed after MnO₂@CoNiO₂ was added to the electrode material. According to the diameter of the semicircle, the R_ct of MnO₂, CoNiO₂, and MnO₂@CoNiO₂ electrodes were 27.3 Ω, 25.1 Ω, and 3.8 Ω, respectively, which shows that the MnO₂@CoNiO₂ composite can enhance the electronic conductivity and develop electron transfer at the interface of electrode material compared to MnO₂ and CoNiO₂ electrodes. The low ESR and R_ct of the MnO₂@CoNiO₂ electrode plays an important role in improving the performance of supercapacitors.

Fig. 9 Impedance Nyquist plots of the MnO₂, CoNiO₂ and MnO₂@CoNiO₂ electrodes.

The stability of the electrode materials is one of the most important requirements for supercapacitor applications. The cycling stability test of the MnO₂, CoNiO₂, and MnO₂@CoNiO₂ electrodes were evaluated at a constant charge–discharge current density of 40 mA cm⁻² for 3000 cycles, as shown in Fig. 10. The MnO₂ electrode for the 1^st cycle showed a specific capacitance of 800.32 F g⁻¹, which decreased slowly to 762.36 F g⁻¹ after 3000 cycles, showing 4.94% loss after 3000 cycles. The CoNiO₂ electrode showed specific capacitances of 1021.45 F g⁻¹ and 998.56 F g⁻¹ at the 1^st cycle and after 3000 cycles, respectively, showing 2.30% loss after 3000 cycles. On the other hand, the MnO₂@CoNiO₂ electrode exhibited only 1.13% loss of a specific capacitance after more than 3000 cycles, highlighting the good long-term stability of the MnO₂@CoNiO₂ electrode.

Fig. 10 Cyclic performance of the MnO₂, CoNiO₂ and MnO₂@CoNiO₂ electrodes at a current density of 40 mA cm⁻² for 3000 cycles.

4. Conclusions

In summary, novel MnO₂@CoNiO₂ snail shell-like structures were designed and synthesized using a simple hydrothermal method followed by annealing. The MnO₂@CoNiO₂ composite electrode exhibited superior specific capacitance (1605.4 F g⁻¹) and energy density (51.37 W h kg⁻¹), which were much higher than those of the MnO₂ (1069.0 F g⁻¹; 34.21 W h kg⁻¹) and CoNiO₂ (1338.4 F g⁻¹; 42.83 W h kg⁻¹) electrodes at a current density of 20 mA cm⁻². The elevated performance of the composite electrode was attributed to the improved surface morphology with a high surface area, which facilitates electron diffusion at the contact between the electrode and electrolyte. Moreover, the MnO₂@CoNiO₂ electrode delivered outstanding cycling stability with a capacity retention of 98.87% after 3000 cycles (97.7% and 95.06% for the CoNiO₂ and MnO₂ electrodes after 3000 cycles, respectively). As a result, the MnO₂@CoNiO₂ composite electrode provides a path for high-performance supercapacitors because of its low-cost, facile preparation, and environmental friendliness.

Acknowledgements

This research was supported by Basic Research Laboratory through the National Research Foundations of Korea funded by the Ministry of Science, ICT and Future Planning (NRF-2015R1A4A1041584).

References

1. B. Liu, D. Tan, X. Wang, D. Chen and G. Shen, Small, 2013, 9, 1998–2004.
2. T. Chen, L. Qiu, Z. Yang, Z. Cai, J. Ren, H. Li, H. Lin, X. Sun and H. Peng, Angew. Chem., Int. Ed., 2012, 51, 11977–11980.
3. X. Lu, G. Wang, T. Zhai, M. Yu, S. Xie, Y. Ling, C. Liang, Y. Tong and Y. Li, Nano Lett., 2012, 12, 5376–5381.
4. B. Liu, B. Liu, Q. Wang, X. Wang, Q. Xiang, D. Chen and G. Shen, ACS Appl. Mater. Interfaces, 2013, 5, 10011–10017.
5. T. Liu, Y. Ling, Y. Yang, L. Finn, E. Collazo, T. Zhai, Y. Tong and Y. Li, Nano Energy, 2015, 12, 169–177.
6. C. Zhou, Y. Zhang, Y. Li and J. Liu, Nano Lett., 2013, 13, 2078–2085.
7. X. Wu, L. Jiang, C. Long, T. Wei and Z. Fan, Adv. Funct. Mater., 2015, 25, 1648–1655.
8. L. Yuan, X. H. Lu, X. Xiao, T. Zhai, J. Dai, F. Zhang, B. Hu, X. Wang, L. Gong, J. Chen, C. Hu, Y. Tong, J. Zhou and Z. L. Wang, ACS Nano, 2011, 6, 656–661.
9. P. Simon and Y. Gogotsi, Nat. Mater., 2008, 7, 845–854.
10. C. Meng, C. Liu, L. Chen, C. Hu and S. Fan, Nano Lett., 2010, 10, 4025–4031.
11. T. Stimpfling and F. Leroux, Chem. Mater., 2010, 22, 974–987.
12. A. Burke, J. Power Sources, 2000, 91, 37–50.
13. L. Yang, S. Cheng, Y. Ding, X. B. Zhu, Z. L. Wang and M. L. Liu, Nano Lett., 2012, 12, 321–325.
14. W. Zhou, X. Cao, Z. Zeng, W. Shi, Y. Zhu, Q. Yan, H. Liu, J. Wang and H. Zhang, Energy Environ. Sci., 2013, 6, 2216–2221.
15. L. Huang, D. Chen, Y. Ding, S. Feng, Z. L. Wang and M. Liu, Nano Lett., 2013, 13, 3135–3139.
16. T. Zhu, J. S. Chen and X. W. Lou, J. Mater. Chem., 2010, 20, 7015–7020.
17. B. Wang, T. Zhu, H. B. Wu, R. Xu, J. S. Chen and X. W. Lou, Nanoscale, 2012, 4, 2145–2149.
18. Q. Wang, X. Wang, B. Liu, G. Yu, X. Hou, D. Chen and G. Shen, J. Mater. Chem. A, 2013, 1, 2468–2473.
19. M. Huang, Y. Zhang, F. Li, L. Zhang, R. S. Ruoff, Z. Wen and Q. Liu, Sci. Rep., 2014, 4, 3878.
20. M. Huang, R. Mi, H. Liu, F. Li, X. L. Zhao, W. Zhang, S. X. He and Y. X. Zhang, J. Power Sources, 2014, 269, 760–767.
21. T. Brezesinski, J. Wang, S. H. Tolbert and B. Dunn, Nat. Mater., 2010, 9, 146–151.
22. Z. Lu, Z. Chang, W. Zhu and X. Sun, Chem. Commun., 2011, 47, 9651–9653.
23. M. Huang, Y. Zhang, F. Li, L. Zhang, Z. Wen and Q. Liu, J. Power Sources, 2014, 252, 98–106.
24. L. Yu, B. Guan, W. Xiao and X. W. Lou, Adv. Energy Mater., 2015, 5, 1500981.
25. P. M. DiCarmine, T. B. Schon, T. M. McCormick, P. P. Klein and D. S. Seferos, J. Phys. Chem. C, 2014, 118, 8295–8307.
26. N. Kundakarla, S. Lindeman, M. H. Rahman and M. D. Ryan, Inorg. Chem., 2016, 55, 2070–2075.
27. Y. Zhao, L. Hu, S. Zhao and L. Wu, Adv. Funct. Mater., 2016, 26, 4085–4093.
28. D. Cai, D. Wang, B. Liu, L. Wang, Y. Liu, H. Li, Y. Wang, Q. Li and T. Wang, ACS Appl. Mater. Interfaces, 2014, 6, 5050–5055.
29. J. Xu, Q. Wang, X. Wang, Q. Xiang, B. Liang, D. Chen and G. Shen, ACS Nano, 2013, 7, 5453–5462.
30. Y. Wu, G. Gao and G. Wu, J. Mater. Chem. A, 2015, 3, 1828–1832.
31. W. Zhou, D. Kong, X. Jia, C. Ding, C. Cheng and G. Wen, J. Mater. Chem. A, 2014, 2, 6310–6315.
32. W. Tang, L. Liu, S. Tian, L. Li, Y. Yue, Y. Wu and K. Zhu, Chem. Commun., 2011, 47, 10058–10060.
33. L. Hou, C. Yuan, L. Yang, L. Shen, F. Zhang and X. Zhang, RSC Adv., 2011, 1, 1521–1526.
34. L. Zhu, W. Wu, Y. Zhu, W. Tang and Y. Wu, J. Phys. Chem. C, 2015, 119, 7069–7075.
35. X. Lu, G. Wang, T. Zhai, M. Yu, J. Gan, Y. Tong and Y. Li, Nano Lett., 2012, 12, 1690–1696.
36. R. Li, X. Ren, F. Zhang, C. Du and J. Liu, Chem. Commun., 2012, 48, 5010–5012.
37. L. Zhu, Z. Chang, Y. Wang, B. Chen, Y. Zhu, W. Tang and Y. Wu, J. Mater. Chem. A, 2015, 3, 22066–22072.
38. R. Tummala, R. K. Guduru and P. S. Mohanty, J. Power Sources, 2012, 209, 44–51.
39. F. Li, Y. X. Zhang, M. Huang, Y. Xing and L. L. Zhang, Electrochim. Acta, 2015, 154, 329–337.
40. S. J. Zhu, J. Zhang, J. J. Ma, Y. X. Zhang and K. X. Yao, J. Power Sources, 2015, 278, 555–561.
41. M. Huang, F. Li, F. Dong, Y. X. Zhang and L. L. Zhang, J. Mater. Chem. A, 2015, 3, 21380–21423.
42. J. G. Wang, F. Kang and B. Wei, Mater. Sci.-Pol., 2015, 74, 51–124.
43. J. Zhu, S. Tang, H. Xie, Y. Dai and X. Meng, ACS Appl. Mater. Interfaces, 2014, 6, 17637–17646.
44. Y. He, W. Chen, X. Li, Z. Zhang, J. Fu, C. Zhao and E. Xie, ACS Nano, 2013, 7, 174–182.
45. G. Q. Zhang and X. W. Lou, Adv. Mater., 2013, 25, 976–979.
46. T. Y. Wei, C. H. Chen, H. C. Chien, S. Y. Lu and C. C. Hu, Adv. Mater., 2010, 22, 347–351.
47. C. Z. Yuan, J. Y. Li, L. R. Hou, L. Yang, L. F. Shen and X. G. Zhang, J. Mater. Chem., 2012, 22, 16084–16090.
48. L. L. Li, Y. L. Cheah, Y. Ko, P. Teh, G. Wee, C. L. Wong, S. J. Peng and M. Srinivasan, J. Mater. Chem. A, 2013, 1, 10935–10941.
49. Y. Liu, Y. Zhao, Y. Yu, M. Ahmad and H. Sun, Electrochim. Acta, 2014, 132, 404–409.
50. Z. J. Fan, J. Yan, L. J. Zhi, Q. Zhang, T. Wei, J. Feng, M. Zhang, W. Qian and F. Wei, Adv. Mater., 2010, 22, 3723–3728.
51. B. G. Choi, M. Yang, W. H. Hong, J. W. Choi and Y. S. Huh, ACS Nano, 2012, 6, 4020–4028.
52. J. Han, M. Wang, S. Cao, P. Fang, S. Lu, R. Chen and R. Guo, J. Mater. Chem. A, 2013, 1, 13197–13202.
53. J. Zhi, O. Reiser, Y. Wang and A. Hu, Nanoscale, 2016, 8, 11976–11983.
54. J. Zhou, H. Zhao, X. Mu, J. Chen, P. Zhang, Y. Wang, Y. He, Z. Zhang, X. Pan and E. Xie, Nanoscale, 2015, 7, 14697–14706.
55. H. Chen, M. Zhou, T. Wang, F. Li and Y. X. Zhang, J. Mater. Chem. A, 2016, 4, 10786–10793.
56. M. Huang, X. L. Zhao, F. Li, W. Li, B. Zhang and Y. X. Zhang, J. Mater. Chem. A, 2015, 3, 12852–12857.
57. A. Ramadoss and S. J. Kim, Int. J. Hydrogen Energy, 2014, 39, 12201–12212.
58. Z. Sun, W. Ai, J. Liu, X. Qi, Y. Wang, J. Zhu, H. Zhang and T. Yu, Nanoscale, 2014, 6, 6563–6568.
59. Z. C. Xing, Q. X. Chu, X. B. Ren, C. J. Ge, A. H. Qusti, A. M. Asiri, A. O. Al-Youbi and X. P. Sun, J. Power Sources, 2014, 245, 463–467.
60. C. V. V. M. Gopi, M. V. Haritha, S. K. Kim, K. Prabakara and H. J. Kim, RSC Adv., 2016, 6, 102961–102967.
61. X. Tang, R. Jia, T. Zhai and H. Xia, ACS Appl. Mater. Interfaces, 2015, 7, 27518–27525.
62. J. Liu, J. Jiang, C. Cheng, H. Li, J. Zhang, H. Gong and H. J. Fan, Adv. Mater., 2011, 23, 2076–2081.
63. S. Zhang, S. Wang and S. Li, Nanotechnology, 2010, 10, 5612–5617.
64. W. Ma, H. Nan, Z. Gu, B. Geng and X. Zhang, J. Mater. Chem. A, 2015, 3, 5442–5448.
65. L. Yu, G. Zhang, C. Yuan and X. W. Lou, Chem. Commun., 2013, 49, 137–139.
66. Z. Gu, R. Wang, H. Nan, B. Geng and X. Zhang, J. Mater. Chem. A, 2015, 3, 14578–14584.
67. S. Yang, P. Yan, Y. Li, K. Cheng, K. Ye, C. Zhang, D. Cao, G. Wang and Q. Li, RSC Adv., 2015, 5, 87521–87527.
68. P. Sun, H. Yi, T. Peng, Y. Jing, R. Wang, H. Wang and X. Wang, J. Power Sources, 2017, 341, 27–35.
69. J. Zhang, Z. Chen, Y. Wang and H. Li, Energy, 2016, 113, 943–948.
70. D. Li, Y. Gong and C. Pan, Sci. Rep., 2016, 6, 29788.
71. Y. Liu, Y. Zhao, Y. Yu, J. Li, M. Ahmad and H. Sun, New J. Chem., 2014, 38, 3084–3091.

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