Flexible and ultralong-life cuprous oxide microsphere-nanosheets with superior pseudocapacitive properties

Abstract

Nanostructured transition metal oxides have been investigated extensively for supercapacitor electrodes due to their high theoretical specific capacitance, low-cost, environment benignity and abundance. However, pristine transition metal oxides suffer from difficulty of synthesis, poor electronic conductivity and mechanical flexibility. In this work, we report a facile, low-cost and high-throughput synthesis of hierarchical structure which consists of cuprous oxide (Cu₂O) microsphere-nanosheets on the surface of flexible Cu foil (namely Cu₂O@Cu) via a two-step electrochemical method (anodization and electro-oxidation). The influence of the anodization parameters on surface roughness of Cu foil has been investigated, and the optimum anodization procedure was determined to be 50 V for 4 cycles. This Cu₂O@Cu electrode exhibits excellent capacitance properties, such as up to 390.9 mF cm⁻² at 2 mA cm⁻² in areal capacitance, and high flexibility, as observed by cyclic voltammetry measurement under various deformation (bending and folding) situations. Furthermore, the Cu₂O@Cu electrode presents superior long-term cycling stability over 100 000 cycles, with the capacitance retention of over 80%. The present binder-free Cu₂O@Cu microsphere-nanosheets electrode is highly promising for future applications in flexible supercapacitors.

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Introduction

The increasing demand for next-generation portable electronics with roll-up and wearable characteristics has inspired intensive efforts to explore high-performance energy storage devices that are flexible, lightweight, and environmentally friendly.^1–3 Among these, supercapacitors have been widely investigated as one of the most important and advanced candidates due to their high power density, ultra-fast charge/discharge rate, superlong cycling life, and then enable highly efficient, safe, green use of energy.^4–7 Among various supercapacitor electrode materials, nanostructured transition metal oxides, such as RuO₂,⁸ MnO₂,⁹ Co₃O₄,¹⁰ TiO₂,¹¹ NiO,¹² Fe₃O₄,¹³ V₂O₅,¹⁴ SnO₂¹⁵ which possess multiple oxidation states that enable rich redox reactions for pseudocapacitance generation, can provide higher specific capacitance and energy density than that delivered by carbon-based active materials, and better cycling stability than conductive polymer materials.¹⁶ However, the undesirably poor electronic conductivity of metal oxides significantly impedes them from wide use in supercapacitor devices.¹⁷ One of major strategies to enhance charge transport in electrodes is to design composite materials by combining metal oxides with a conductive substance.¹⁷ Carbon in various forms^18–20 and conductive polymers^21–23 have been explored to serve as conductive pathways of metal oxides. Although these composites could improve electronic conductivity, the assembled electrodes still have to face the deficiency of high contact resistance between the metal oxides and the current collector, greatly hindering their practical applications.²⁴

Besides these issues, conventional supercapacitors with current collectors, insulating binder and conductive additives are still too thick, bulky and rigid to meet the practical requirement for flexible energy storage devices.^25,26 A considerable part of these electrodes are susceptible to structure fracture and the active materials are apt to peel off under the situation of bending or during charge/discharge process, giving rise to abundant waste, inconvenience, as well as safety problems.²⁷ Thus, there is an urgent need to develop flexible or even foldable supercapacitors which can accommodate large levels of deformation without sacrificing electrochemical performance.²⁸ Recently, flexible supercapacitor electrodes were mainly fabricated by integrating active capacitive materials including metal oxides, conductive polymers, and carbon materials, into various flexible substrates, such as cellulose paper, textiles, plastics and carbon fiber paper, exhibiting remarkable flexibility.^29–33 However, cellulose paper, textiles and plastics are typical insulators and much effort should be dedicated to enhance the conductivity of them.³⁴ Moreover, cellulose paper is easy to be torn up, resulting in the whole scale breakdown of the supercapacitor. As for carbon fiber paper, its specific capacitance is much lower when comparing to metal oxides.

In the past several years, cuprous oxide (Cu₂O) has been synthesized through various routes and extensively studied for applications in Li-ion batteries.^35,36 Xiang et al.³⁷ fabricated core–shell Cu₂O/Cu composites towards Li-ion batteries application by a special reduction process of CuSO₄. Park et al.³⁸ synthesized uniform Cu₂O nanocubes in a gram scale by a one-pot polyol process. Besides, many other groups have synthesized Cu₂O for Li-ion batteries by using γ-irradiation,³⁹ thermal decomposition,⁴⁰ electrochemical oxidation,⁴¹ and chemical bath deposition⁴² etc., showing remarkable charge storage capacity. These existing results well suggest promising charge-storage performance of Cu₂O for pseudocapacitors.

Based on the above considerations, in this study, we report a facile electrochemical route to prepare cuprous oxide (Cu₂O) microsphere-nanosheets which in situ grow on the surface of Cu foil substrate, and exhibit superior pseudocapacitive performance. The aims of designing such an electrode are listed as follows: (i) Cu foil is quite abundant, flexible, thin, cheap and environmentally friendly; (ii) Cu₂O in situ grows on the surface of Cu foil without any binders and conductive additives, which can effectively ensure fine interface contact between the active material and substrate, as well as excellent conductivity of the whole electrode; (iii) Cu foil can not only serve as the support for the active material but serve as the current collector, which greatly reduces the size of supercapacitors; (iv) compared with various reported flexible composite electrodes, the fabricating method of the Cu₂O@Cu electrode is facile, safe, environmentally friendly and high-throughput. Here, the flexible Cu₂O@Cu electrode exhibits high areal capacitance and superior long-term cycling stability, suggesting promising future in practical applications for supercapacitors.

Experimental section

The Cu₂O@Cu microsphere-nanosheets electrode was fabricated through a two-step electrochemical process, namely the anodization of Cu foil to prepare nanostructured Cu followed by electro-oxidation to form Cu₂O microsphere-nanosheets. The anodization procedure was operated in a two-electrode electrochemical cell with a commercial Cu foil and a graphite plate as the anode and cathode electrode, respectively. Both the two electrodes were sealed to ensure that the geometric working area was 1.0 cm × 1.0 cm. Anodization was carried out in an aqueous solution containing 0.4 M H₂C₂O₄ and a little amount of HF at a constant potential (10, 20, 30, 40 and 50 V) for 3 min, and then −10 V for 30 s at room temperature. This was regarded as 1 cycle, while the anodization needed to cycle several times. A potentiostat (CHI660E, Shanghai, Chenhua) with a three-electrode configuration was used to perform electro-oxidation. The as-prepared nanostructured Cu foil was used as the working electrode, a Pt foil (1 cm × 1 cm) acted as the counter electrode and an Ag/AgCl electrode served as the reference electrode. The Cu₂O microsphere-nanosheets were fabricated by employing cyclic voltammetry (CV) in the potential range from −0.4 to 0.4 V vs. Ag/AgCl at the scan rate of 5 mV s⁻¹ for 1 segment in a 1 M KOH aqueous solution. The obtained electrodes were designated as Cu₂O@Cu for simplicity.

The phase formation of the as-synthesized samples was characterized using X-ray diffraction (XRD, Rigaku D/max-rB) with Cu Kα radiation. The size and morphology of the samples were characterized using a scanning electron microscope (SEM, LEO 1530 VP). The resistivity of Cu foil and the Cu₂O@Cu electrode was measured by a constant current four-electrode method, and was recorded every 2 s by a computer connected to the measurement system. Nitrogen adsorption/desorption isotherms were measured at 77 K with a V-Sorb 2800P Surface Area and Pore Size Analyzer. The specific surface area was calculated using the Brunauer–Emmett–Teller (BET) method.^43–45 The pore size distribution was obtained by the Barrett–Joyner–Halenda (BJH) method.^10,46,47

The mass of Cu₂O was calculated by means of the following steps. Firstly, the mass of the completely clean and dried Cu₂O@Cu electrode was weighed, and named m₁. Secondly, this electrode was immersed into a N₂-saturated 0.1 M HCl solution about 10 min until no obvious color change occurred. During this step, the Cu₂O layer was removed. The mass of the Cu foil, named m₂, was weighed again after careful washing with highly pure de-ionized water and complete drying. Finally, the mass of Cu₂O attached on Cu foil was calculated through (m₁ − m₂).

To evaluate the pseudocapacitive performance of the Cu₂O@Cu electrode, all electrochemical measurements were carried out using the potentiostat (CHI660E, Shanghai, Chenhua) in a three-electrode electrochemical cell. The Cu₂O@Cu electrode was used as the working electrode, with the Pt foil as the counter electrode and the Ag/AgCl electrode as the reference electrode. All electrochemical tests (CV and galvanostatic charge/discharge) were performed in a 1 M KOH aqueous solution at room temperature.

Results and discussion

The Cu₂O@Cu electrode was developed through the anodization of Cu foil followed by electro-oxidation to form Cu₂O microsphere-nanosheets. Compared to common methods, including template-directed,^9,48,49 hydrothermal,^50,51 or solvothermal,^52,53 as well as dealloying preparation^54–56 for nanomaterials, the proposed electrochemistry strategy does not involve specific templates, high temperatures, reducing agents, and separation/purification procedures. To study the effect of anodizing voltage on the electrochemical performance and further explore the optimum voltage for anodization, the Cu foil samples were anodized at 10 V, 20 V, 30 V, 40 V and 50 V for 1 cycle respectively, and then were electro-oxidized to Cu₂O. The pseudocapacitive performance of these Cu₂O@Cu electrodes were measured by calculating the loop area recorded using cyclic voltammetry in the potential range from 0.3 to 0.7 V (vs. Ag/AgCl). Here for comparison, the surface increasing factor (SIF) for Cu₂O@Cu electrode gained only by electro-oxidation (without anodization) was defined to be 1. Fig. 1a shows the SIFs for the Cu₂O@Cu electrodes subjected to anodization under different voltages for one cycle and the same electro-oxidation process. As we can see, the value of SIF increases with increasing anodizing voltage. It is worth noting that the increasing degree of SIF is not significant as the anodizing voltage varies from 10 V to 40 V, while the value of SIF increases sharply as the voltage further reaches up to 50 V. The SIF at 50 V is 43.2, indicating that the surface of Cu foil was roughened to a large extent after anodization at 50 V and the loading of Cu₂O would increase dramatically after the subsequent electro-oxidation. Similarly, the Cu₂O@Cu electrodes fabricated through anodization at 50 V for various cycles followed by the same electro-oxidation were used to explore the optimal preparation process. Fig. 1b displays the relationship between the SIF values and the number of anodizing cycles. The SIF is enhanced obviously with the increasing number of anodizing cycles at first (less than 4 cycles), while too many cycles will weaken the surface roughness of Cu foil. It is mainly due to the roughed Cu surface layer will peel off when cycling too many times. As expected, the SIF at 50 V for 4 cycles reaches up to 304. Finally, the optimum anodization procedure was determined to be 50 V for 4 cycles.

Fig. 1 (a) Surface increasing factors (SIFs) for the Cu₂O@Cu electrodes subjected to anodization for 1 cycle under different anodizing voltages and the same electro-oxidation. (b) SIFs for the Cu₂O@Cu electrodes subjected to anodization under 50 V for various cycles and the same electro-oxidation.

Fig. 2 depicts the SEM images of the Cu foil electrode after anodization at 50 V for 4 cycles. It can be seen that irregular-shaped Cu particles disperse well on the surface of the Cu foil, which gives rise to the rough island-like surface and remarkable enhanced specific surface area. The size of the Cu particles is submicron. Fig. 3 illustrates the low- and high-magnification SEM images of the Cu₂O@Cu electrode obtained through the anodization (50 V/4 cycles) and subsequent electro-oxidation. The surface of the electrode (Fig. 3a) is ravine-like and is further roughened remarkably compared to that of the anodized Cu foil. Careful observation (Fig. 3b and c) can find that the surface is covered by microspheres consisting of nanosheets stretching along different directions. The nanosheets with about 20 nm in thickness are interconnected with each other, and one nanosheet is highlighted by an arrow in the inset of Fig. 3c. There exist micron-sized channels among the microspheres as well as nano-sized channels among the nanosheets, which could allow easy diffusion of the electrolyte onto the nanosheets surface. Fig. 4 shows the corresponding X-ray diffraction (XRD) pattern of the Cu₂O@Cu electrode. The diffraction peaks at 2θ of 36.4° and 42.4° are attributed to the (111) and (200) reflections of Cu₂O (JCPDS no. 05-0667) and the three characteristic peaks at 2θ of 43.3°, 50.5° and 74.2° correspond to the (111), (200) and (220) planes of Cu (JCPDS no. 04-0836).^36,37,39 The weak (200) peak of Cu₂O is highlighted as the inset in Fig. 4. The present results well confirm the formation of the Cu₂O microsphere-nanosheets on the Cu foil substrate.

Fig. 2 Low- and high-magnification SEM images of the Cu foil electrode after anodization at 50 V for 4 cycles.

Fig. 3 Low- and high-magnification SEM images of the Cu₂O@Cu electrode.

Fig. 4 XRD pattern of the Cu₂O@Cu nanosheets electrode.

The specific surface area and porous structure of the as-prepared Cu₂O@Cu sample was investigated by N₂ adsorption/desorption measurements. The N₂ adsorption/desorption isotherm of the sample (Fig. 5a) can be classified as a type IV isotherm with a H₃-type hysteresis loop, indicating the existence of mesopores possibly formed by the loose stacking of Cu₂O nanosheets.^57,58 On the basis of the adsorption/desorption isotherm, the BET surface area of the Cu₂O@Cu electrode was calculated to be 0.5997 m² g⁻¹ (considering the whole mass of Cu₂O active material and Cu foil substrate), which is 250 times that of raw Cu foil (only 0.0024 m² g⁻¹). Fig. 5b presents the pore size distribution calculated from the N₂ adsorption/desorption isotherm using the BJH method. The average pore diameter of the Cu₂O@Cu electrode is in the mesoporous region, with multimodal pore size distributions. The pore size distribution maximums of the sample mainly center in the range of 2–10 nm. Combined with the above results, it suggests that such type of hierarchical surface morphologies with well-developed pore structures possess the advantages of both microsized structures and nanostructures, which can not only offer a large electrochemical surface area, but also provide good electrolyte access.^59,60 The electrolyte could come into contact with more active surfaces through abundant pores and channels, which can enable excellent electrochemical behaviors of the Cu₂O@Cu microsphere-nanosheets electrode.

Fig. 5 (a) N₂ adsorption–desorption isotherm and (b) BJH adsorption pore size distribution of the Cu₂O@Cu nanosheets electrode.

To explore the electrochemical performance of the binder-free Cu₂O@Cu microsphere-nanosheets electrode for supercapacitors, cyclic voltammetry (CV), galvanostatic charge/discharge and cyclic stability measurements were performed in a three-electrode cell. Fig. 6a shows the representative CV curves of the Cu₂O@Cu microsphere-nanosheets electrode in the voltage window of 0.3 to 0.7 V (vs. Ag/AgCl) at various scan rates. All the curves present strong redox peaks and quite different from the ideal rectangular shape for electric double layer capacitance, indicating that the capacitance of the electrode primarily derives from faradic redox reactions. The anodic current flow is associated with oxidation of Cu⁺ to Cu²⁺ during the positive-going scan. Similarly, the cathodic peak is related to its reverse process during the negative-going scan. The cathodic peak current increases linearly with the increment of the scan rate (inset of Fig. 6a), suggesting that the rates of electronic and ionic transportation are rapid enough with respect to the scan rates.⁵¹ It should be noted that the cathodic peak position shifts in the more negative direction with the increase of the scan rate from 1 to 100 mV s⁻¹. But the shape of the series CV curves has no significant change, indicative of excellent electronic conductivity of the Cu₂O@Cu electrode. The conductivity of the Cu foil and Cu₂O@Cu electrode was measured by a constant current four-electrode method. The calculated conductivity of the Cu foil and Cu₂O@Cu electrode are 0.556 × 10⁶ S cm⁻¹ and 0.513 × 10⁶ S cm⁻¹ respectively, confirming the high conductivity of the Cu₂O@Cu electrode. The areal capacitance could be calculated from CV curves by the formula,⁶¹

where C is the areal capacitance (F cm⁻²), S is the geometric area of the electrode (cm²), V_c and V_a represent the highest and lowest voltages (V), respectively, v is the potential scan rate (mV s⁻¹), I(V) is the response current density (A), and V is the potential (V). Fig. 6b presents the calculated areal capacitance as a function of the scan rate. The areal capacitance values are 350.5, 280.9, 274.5, 255.4, 243.8, 234.9 and 225.5 mF cm⁻², corresponding to the scan rate of 1, 5, 10, 25, 50, 75 and 100 mV s⁻¹, respectively. Clearly, the areal capacitance of the Cu₂O@Cu microsphere-nanosheets electrode is higher than that for the 3D Si/Au/RuO₂·xH₂O (99.3 mF cm⁻² at 5 mV s⁻¹),⁶¹ amorphous Ni(OH)₂ nanospheres (263 mF cm⁻² at 1 mV s⁻¹),⁶² MnO₂/Ni/graphite/paper electrode (175 mF cm⁻² at 5 mV s⁻¹),³⁴ and TiO₂@C nanowires (31.3 mF cm⁻² at 10 mV s⁻¹).⁶³ And the corresponding specific capacitances are 1402.0, 1123.6, 1098, 1021.6, 975.2, 939.6 and 902.0 F g⁻¹, showing excellent charge storage performance. On the other hand, the prepared Cu₂O@Cu electrode has an excellent flexibility and good mechanical properties and can undergo different bending and fold states, as shown in Fig. 7a. More significantly, the electrochemical performance of the electrode almost has no change under various distortion situations (Fig. 7b), further confirming that the electrode has a remarkable mechanical flexibility.

Fig. 6 (a) CV curves of the Cu₂O@Cu nanosheets electrode at various scan rates (1–100 mV s⁻¹) recorded in the 1 M KOH aqueous solution. The inset presents plot of cathodic peak currents versus the scan rate. (b) Specific capacitance of the electrode as a function of scan rate based on the CVs.

Fig. 7 (a) Photos of the fabricated Cu₂O@Cu nanosheets electrode in the flat (left), bending (middle) and fold (right) states. (b) CV curves of the Cu₂O@Cu nanosheets electrode in various bending states (different L/L₀ values and fold states), where L₀ is the length of the electrode in the flat state, and L is the distance between two ends of the electrode in different bending states (inset). Scan rate: 50 mV s⁻¹.

Fig. 8a shows the galvanostatic charge/discharge profiles of the Cu₂O@Cu microsphere-nanosheets electrode. An increase in the IR drop with an increase in the discharge current density can be clearly observed. The nonlinear feature of charge/discharge behavior further demonstrates the pseudocapacitive nature of the Cu₂O@Cu microsphere-nanosheets electrode. The areal capacitance of the electrode can be calculated from the galvanostatic charge/discharge curves by the following equation:⁴

where C is the specific capacitance (F cm⁻²), I is the constant discharge current (A), Δt is the discharge time (s), S is the geometric area of the electrode (cm²), and ΔV is the discharge potential range (V). The calculated areal capacitances as a function of the discharge current density were plotted in Fig. 8b. The capacitance reaches up to 390.9 mF cm⁻² (1563.6 F g⁻¹) at 2 mA cm⁻². With increasing current density to 25 mA cm⁻², the capacitance decreases to 177.3 mF cm⁻² (709.2 F g⁻¹). This scenario is due to the incremental IR drop and insufficient active material involved in the redox reaction at high current densities. The present capacitance values of the electrode are higher than those for 3D porous gear-like CuO (69.6 mF cm⁻² at 0.2 mA cm⁻²),²⁴ CuO/CNT (300 mF cm⁻² at 2 mA cm⁻²),⁶⁴ α-MnO₂ nanowires (150 mF cm⁻² at 1 mA cm⁻²),⁴ and Fe₂O₃ nanotubes/carbon fabric (180.4 mF cm⁻² at 1 mA cm⁻²).⁴

Fig. 8 (a) Galvanostatic charge/discharge profiles of the Cu₂O@Cu nanosheets electrode at different current densities. (b) The specific capacitance of the Cu₂O@Cu nanosheets electrode versus the current density, based on the charge/discharge curves.

The cycling stability of electrode materials is a critical requirement for practical applications. The cycling stability of the Cu₂O@Cu electrode was investigated in the range of 0.3–0.7 V (vs. Ag/AgCl) in the 1 M KOH aqueous solution by repeated CV processes at the scan rate of 100 mV s⁻¹ (Fig. 9). The areal capacitance still remains 95.8% and 80.4% of the initial value each after 50 000 and 100 000 cycles, respectively. To the best of our knowledge, such superior cycling stability has not been reported in the literature for metal oxides till now. After 100 000 cycles, the microstructure of the electrode was observed using SEM (inset of Fig. 9). The microsphere-nanosheets morphology of the electrode still remained, but the nanosheets became thicker after 100 000 cycles, which could well explain the 19.6% loss of the specific capacitance.

Fig. 9 Cycling performance of the Cu₂O@Cu nanosheets electrode at the scan rate of 100 mV s⁻¹. The insets present low- and high-magnification SEM images of the Cu₂O electrode after 100 000 cycles.

Conclusions

In summary, a Cu₂O@Cu microsphere-nanosheets electrode has been developed through a facile two-step electrochemical method. The Cu₂O microsphere-nanosheets in situ grow on the surface of Cu foil without any binders and conductive additives, which not only keeps fine interface contact between the active material and substrate, but also ensures excellent conductivity of the electrode. The as-prepared Cu₂O@Cu electrode exhibits remarkable mechanical flexibility and high specific capacitance of 390.9 mF cm⁻² at 2 mA cm⁻². Moreover, the excellent cycling stability was confirmed to be about 80.4% retention of the initial capacitance after 100 000 cycles. All these points suggest that the binder-free Cu₂O@Cu microsphere-nanosheets electrode is promising for flexible high-performance supercapacitors.

Acknowledgements

The authors gratefully acknowledge financial support by National Natural Science Foundation of China (51371106), Specialized Research Found for the Doctoral Program of Higher Education of China (20120131110017), and Young Tip-top Talent Support Project (the Organization Department of the Central Committee of the CPC). We also acknowledge the experimental assistance from Ruhr University Bochum, Germany. Y. L. Gao acknowledges the support by Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning (no. TP2014042).

References

1. Y. Qin, X. Wang and Z. L. Wang, Nature, 2008, 451, 809–813.
2. K. Wang, Q. Meng, Y. Zhang, Z. Wei and M. Miao, Adv. Mater., 2013, 25, 1494–1498.
3. L. Yuan, B. Yao, B. Hu, K. Huo, W. Chen and J. Zhou, Energy Environ. Sci., 2013, 6, 470–476.
4. P. Yang, Y. Ding, Z. Lin, Z. Chen, Y. Li, P. Qiang, M. Ebrahimi, W. Mai, C. P. Wong and Z. L. Wang, Nano Lett., 2014, 14, 731–736.
5. P. Simon and Y. Gogotsi, Nat. Mater., 2008, 7, 845–854.
6. X. Wang, C. Yan, A. Sumboja and P. S. Lee, Nano Energy, 2014, 3, 119–126.
7. Y.-Z. Su, K. Xiao, N. Li, Z.-Q. Liu and S.-Z. Qiao, J. Mater. Chem. A, 2014, 2, 13845–13853.
8. R.-R. Bi, X.-L. Wu, F.-F. Cao, L.-Y. Jiang, Y.-G. Guo and L.-J. Wan, J. Phys. Chem. C, 2010, 114, 2448–2451.
9. Z. Yu, B. Duong, D. Abbitt and J. Thomas, Adv. Mater., 2013, 25, 3302–3306.
10. R. B. Rakhi, W. Chen, M. N. Hedhili, D. Cha and H. N. Alshareef, ACS Appl. Mater. Interfaces, 2014, 6, 4196–4206.
11. X. Lu, G. Wang, T. Zhai, M. Yu, J. Gan, Y. Tong and Y. Li, Nano Lett., 2012, 12, 1690–1696.
12. G. Zhang, L. Yu, H. E. Hoster and X. W. Lou, Nanoscale, 2013, 5, 877–881.
13. J. Mu, B. Chen, Z. Guo, M. Zhang, Z. Zhang, P. Zhang, C. Shao and Y. Liu, Nanoscale, 2011, 3, 5034–5040.
14. B. Saravanakumar, K. K. Purushothaman and G. Muralidharan, ACS Appl. Mater. Interfaces, 2012, 4, 4484–4490.
15. W. Wang, Q. Hao, W. Lei, X. Xia and X. Wang, RSC Adv., 2012, 2, 10268–10274.
16. F. Wang, S. Xiao, Y. Hou, C. Hu, L. Liu and Y. Wu, RSC Adv., 2013, 3, 13059–13084.
17. M. Zhi, C. Xiang, J. Li, M. Li and N. Wu, Nanoscale, 2013, 5, 72–88.
18. C. Wu, S. Deng, H. Wang, Y. Sun, J. Liu and H. Yan, ACS Appl. Mater. Interfaces, 2014, 6, 1106–1112.
19. Q. Li, X.-F. Lu, H. Xu, Y.-X. Tong and G.-R. Li, ACS Appl. Mater. Interfaces, 2014, 6, 2726–2733.
20. P. Li, Y. Yang, E. Shi, Q. Shen, Y. Shang, S. Wu, J. Wei, K. Wang, H. Zhu, Q. Yuan, A. Cao and D. Wu, ACS Appl. Mater. Interfaces, 2014, 6, 5228–5234.
21. C. Zhou, Y. Zhang, Y. Li and J. Liu, Nano Lett., 2013, 13, 2078–2085.
22. Q. Qu, Y. Zhu, X. Gao and Y. Wu, Adv. Energy Mater., 2012, 2, 950–955.
23. Y. Hou, L. Chen, P. Liu, J. Kang, T. Fujita and M. Chen, J. Mater. Chem. A, 2014, 2, 10910–10916.
24. L. Yu, Y. Jin, L. Li, J. Ma, G. Wang, B. Geng and X. Zhang, CrystEngComm, 2013, 15, 7657–7662.
25. J. P. Cheng, X. Chen, J.-S. Wu, F. Liu, X. B. Zhang and V. P. Dravid, CrystEngComm, 2012, 14, 6702–6709.
26. L. Fan, L. Tang, H. Gong, Z. Yao and R. Guo, J. Mater. Chem., 2012, 22, 16376–16381.
27. H. Wang, B. Zhu, W. Jiang, Y. Yang, W. R. Leow, H. Wang and X. Chen, Adv. Mater., 2014, 26, 3638–3643.
28. X. Dong, Z. Guo, Y. Song, M. Hou, J. Wang, Y. Wang and Y. Xia, Adv. Funct. Mater., 2014, 24, 3405–3412.
29. Y.-C. Chen, Y.-K. Hsu, Y.-G. Lin, Y.-K. Lin, Y.-Y. Horng, L.-C. Chen and K.-H. Chen, Electrochim. Acta, 2011, 56, 7124–7130.
30. S.-L. Chou, J.-Z. Wang, S.-Y. Chew, H.-K. Liu and S.-X. Dou, Electrochem. Commun., 2008, 10, 1724–1727.
31. Y. Zhao, Y. Meng and P. Jiang, J. Power Sources, 2014, 259, 219–226.
32. I. Shakir, Z. Ali, J. Bae, J. Park and D. J. Kang, Nanoscale, 2014, 6, 4125–4130.
33. C. Ma, Y. Li, J. Shi, Y. Song and L. Liu, Chem. Eng. J., 2014, 249, 216–225.
34. J.-X. Feng, Q. Li, X.-F. Lu, Y.-X. Tong and G.-R. Li, J. Mater. Chem. A, 2014, 2, 2985–2992.
35. K. Chen and D. Xue, Funct. Mater. Lett., 2014, 7, 1430001 (9 pages).
36. Q. Pan and M. Wang, J. Electrochem. Soc., 2008, 155, A452–A457.
37. J. Y. Xiang, J. P. Tu, Y. F. Yuan, X. H. Huang, Y. Zhou and L. Zhang, Electrochem. Commun., 2009, 11, 262–265.
38. J. C. Park, J. Kim, H. Kwon and H. Song, Adv. Mater., 2009, 21, 803–807.
39. W. Liu, G. Chen, G. He and W. Zhang, J. Nanopart. Res., 2011, 13, 2705–2713.
40. L. Hu, Y. Huang, F. Zhang and Q. Chen, Nanoscale, 2013, 5, 4186–4190.
41. J. Y. Xiang, J. P. Tu, X. H. Huang and Y. Z. Yang, J. Solid State Electrochem., 2008, 12, 941–945.
42. Q. Pan, M. Wang and Z. Wang, Electrochem. Solid-State Lett., 2009, 12, A50–A53.
43. A. Seri-Levy and D. Avnir, Langmuir, 1993, 9, 2523–2529.
44. S. Brunauer, P. H. Emmett and E. Teller, J. Am. Chem. Soc., 1938, 60, 309–319.
45. K. Sing, Colloids Surf., A, 2001, 187–188, 3–9.
46. E. P. Barrett, L. G. Joyner and P. P. Halenda, J. Am. Chem. Soc., 1951, 73, 373–380.
47. J. Choma, M. Jaroniec, W. Burakiewicz-Mortka and M. Kloske, Appl. Surf. Sci., 2002, 196, 216–223.
48. C. Wang, G. Zhou, D. Xu, B. Sun, Y. Zhang and F. Chen, RSC Adv., 2014, 4, 37270–37273.
49. N. Du, H. Zhang, B. Chen, X. Ma and D. Yang, Chem. Commun., 2008, 3028–3030.
50. X. Feng, N. Chen, Y. Zhang, Z. Yan, X. Liu, Y. Ma, Q. Shen, L. Wang and W. Huang, J. Mater. Chem. A, 2014, 2, 9178–9184.
51. X. Yu, B. Lu and Z. Xu, Adv. Mater., 2014, 26, 1044–1051.
52. L. Hu, B. Qiu, Y. Xia, Z. Qin, L. Qin, X. Zhou and Z. Liu, J. Power Sources, 2014, 248, 246–252.
53. H. Akram, C. Mateos-Pedrero, E. Gallegos-Suárez, A. Guerrero-Ruíz, T. Chafik and I. Rodríguez-Ramos, Appl. Surf. Sci., 2014, 307, 319–326.
54. X. Chen, Y. Jiang, J. Sun, C. Jin and Z. Zhang, J. Power Sources, 2014, 267, 212–218.
55. T. Kou, D. Li, C. Zhang, Z. Zhang and H. Yang, J. Mol. Catal. A: Chem., 2014, 382, 55–63.
56. C. Zhang, X. Wang, J. Sun, T. Kou and Z. Zhang, CrystEngComm, 2013, 15, 3965–3973.
57. K. S. W. Sing, D. H. Everett, R. A. W. Haul, L. Moscou, R. A. Pierotti, J. Rouquerol and T. Siemieniewska, Pure Appl. Chem., 1985, 57, 603–619.
58. D. P. Dubal, G. S. Gund, R. Holze and C. D. Lokhande, J. Power Sources, 2013, 242, 687–698.
59. Y. Li, G. Duan, G. Liu and W. Cai, Chem. Soc. Rev., 2013, 42, 3614–3627.
60. Y. Li, N. Koshizaki, H. Wang and Y. Shimizu, ACS Nano, 2011, 5, 9403–9412.
61. X. Wang, Y. Yin, X. Li and Z. You, J. Power Sources, 2014, 252, 64–72.
62. H. B. Li, M. H. Yu, F. X. Wang, P. Liu, Y. Liang, J. Xiao, C. X. Wang, Y. X. Tong and G. W. Yang, Nat. Commun., 2013, 4, 1894.
63. H. Zheng, T. Zhai, M. Yu, S. Xie, C. Liang, W. Zhao, S. C. I. Wang, Z. Zhang and X. Lu, J. Mater. Chem. C, 2013, 1, 225–229.
64. X. Zhang, W. Shi, J. Zhu, D. J. Kharistal, W. Zhao, B. S. Lalia, H. H. Hng and Q. Yan, ACS Nano, 2011, 5, 2013–2019.

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