Flower-shaped cobalt oxide nano-structures as an efficient, flexible and stable electrocatalyst for the oxygen evolution reaction

Abstract

The industrial application of water splitting for oxygen evolution requires low cost, high performance and stable electrocatalysts which can operate at low overpotential. Here, we develop a high performance and stable electrocatalyst for the oxygen evolution reaction (OER) using earth abundant materials. A binder free approach for the synthesis of flower-shaped cobalt oxide (Co₃O₄) composed of nanosheets showed high OER catalytic activity. The Co₃O₄ electrode requires a low overpotential of 356 mV to achieve a current density of 10 mA cm⁻² with a low onset potential of 284 mV. The electrode showed outstanding flexibility and stability. The catalytic activity of the Co₃O₄ electrode was very stable up to the 2000th cycle of the polarization study. The high catalytic activity and structural stability arise due to efficient and fast charge transportation through the nanosheets of Co₃O₄ which are in direct contact with the conducting nickel of the electrode. The porous structure of Co₃O₄ allows easy access of the electrolyte and escape of generated oxygen without damaging the structure. Collectively, the flower-shaped nanostructured Co₃O₄ electrode can be used as a flexible and high performance electrode for the OER in an industrial setup.

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The oxygen evolution reaction (OER) is a key step in many electrochemical processes, such as water electrolysis, solar to energy generation and metal–air batteries.^1–4 Among these processes, water electrolysis is attracting considerable research interest as it can provide hydrogen and oxygen as fuels in a sustainable way. However, oxygen evolution through water electrolysis requires a high overpotential, which is due to slow kinetics and the involvement of several steps.^5,6 Therefore, efficient electrocatalysts need to be developed which can accelerate the kinetics and lower the overpotential. To date, IrO₂ and RuO₂ are considered to be the ultimate electrocatalysts for the oxygen evolution reaction; however their applications are limited due to high cost and limited availability of the precious metals.^7–9

Recently, several non-precious metal-based electrocatalysts have been synthesized and used to reduce the oxygen overpotential in water splitting processes. Among the various non-precious metal based electrocatalysts, transition metal based materials and perovskites have been considered to be most promising.^10–20 Multi-shelled hollow structures have been synthesized via various techniques for energy applications.^21–27 Lv et al. synthesized hollow mesoporous NiCo₂O₄ nanocages and studied their electrochemical properties for the oxygen evolution reaction.²⁸ The hollow nanostructured NiCo₂O₄ showed an overpotential of about 0.34 V at a current density of 10 mA cm⁻² with a Tafel slope of 75 mV per decade. Further, Chen et al. improved the catalytic properties of NiCo₂O₄ by growing a hierarchically porous nitrogen-doped graphene (PNG)–NiCo₂O₄ composite.²⁹ The OER kinetics were also observed to improve as Tafel slopes of 156, 249 and 254 mV per decade were reported for PNG–NiCo₂O₄, NG–NiCo₂O₄ and PNG, respectively. Tang et al. used stainless steel containing Fe, Ni, Cr and Mn for OER catalysts.³⁰ They grew in situ thin films of Fe(Ni)OOH by immersing stainless steel in an alkaline oxidant solution containing NaOH and (NH₄)₂S₂O₈. The electrode showed an overpotential of 300 mV at 10 mA cm⁻² with a low Tafel slope of 34 mV per decade. Other groups have also used stainless steel for water oxidation due to the presence of Fe, Ni, Cr in it.^31–35

Among other transition metal based electrocatalysts, Co₃O₄ shows promising behaviour for OER.⁶ The catalytic activities of crystalline and amorphous Co₃O₄ films were compared, and it was observed that amorphous Co₃O₄ films are more catalytically active than crystalline Co₃O₄ films.⁶ The crystalline and amorphous Co₃O₄ films showed Tafel slopes of 49 and 36 mV per decade, respectively. It was observed that the exchange current density also depends on the nature of the films. Exchange current densities of 2.0 × 10⁻¹⁰ and 5.4 × 10⁻¹² A cm⁻² were observed for the crystalline and amorphous Co₃O₄ films, respectively. The effect of the size of Co₃O₄ on the catalytic activity for the OER was also examined.³⁶ It was also observed that the catalytic properties depend on the size of the synthesized Co₃O₄. A current density of 10 mA cm⁻² was observed at overpotentials of 328, 363 and 382 mV for anodes loaded with 1 mg cm⁻² of 5.9, 21.1 and 46.9 nm sized Co₃O₄ nanoparticles, respectively. The observed OER activities were directly correlated with the surface area of the Co₃O₄ nanoparticles. The electrocatalytic activity of Co₃O₄ was improved by synthesizing Co₃O₄/NiCo₂O₄ double-shelled nanocage structures.¹⁰ An overpotential of 410 and 340 mV was required for Co₃O₄ and Co₃O₄/NiCo₂O₄ double-shelled nanocages, respectively, to provide a current density of 10 mA cm⁻². The OER kinetics of Co₃O₄ were also improved by making Co₃O₄/NiCo₂O₄ double-shelled nanocages as they showed Tafel slopes of 110 and 88 mV per decade, respectively. In general, the electrocatalytic activities of the synthesized materials were tested by loading the materials over a current collector using a polymer binder. These polymeric binders increase the contact resistance and block the active site of the catalyst and thus reduce the catalytic activities.³⁷ Schäfer et al. used various ways to oxidize different types of stainless steel in a binder free approach for OER applications.^33,38–40 For example, they oxidized the surface of steel S235 by Cl₂ gas and observed its catalytic activity. They reported an overpotential of 462 mV (at 1 mA cm⁻²) at pH 7, which decreased to 347 mV (at 1 mA cm⁻²) at pH 13. They also used a facile anodization process to convert AISI Ni42 steel into a bifunctional (OER and HER) electrocatalyst. The performance of the prepared catalyst was very stable even under standard industrial operation conditions. Lu et al. used a binder free approach to synthesize Co₃O₄ and studied the catalytic activities of the deposited Co₃O₄ film.⁴¹ They observed a Tafel slope of 74 mV per decade, which was better than those of Co₃O₄ (110 mV per decade) and Co₃O₄/NiCo₂O₄ (88 mV per decade) double-shelled nanocages tested using a polymer binder.¹⁰

Herein, we report a facile binder free approach to synthesize flower-shaped Co₃O₄ nanostructures which are composed of nanosheets for the oxygen evolution reaction (details in the ESI†). The binder free Co₃O₄ electrode showed a low overpotential of 356 mV to gain a current density of 10 mA cm⁻² in an alkaline medium. Furthermore, stability and flexibility tests confirmed no obvious decay in the current density of the Co₃O₄ electrode during the OER process. The porous nanosheets of Co₃O₄ over nickel foam allow fast electron and ion transportation through the nanosheets to the electrode with excellent structural stability, and thus high catalytic performance.

The phase purity and crystallinity of the synthesized cobalt oxide were studied using X-ray diffraction analysis (Fig. 1a). All the diffraction peaks observed in the synthesized sample match with those of the cubic spinel phase of Co₃O₄ (JCPDS: 74-2120). No peaks other than those due to Co₃O₄ were observed, suggesting the phase purity and high crystallinity of the synthesized cobalt oxide. The lattice parameter of the synthesized Co₃O₄ was calculated using the most intense peak (311), and it was found to be 0.808 nm. The observed lattice parameter of Co₃O₄ matches well with its theoretical value of 0.808 nm. The chemical composition of the synthesized cobalt oxide was further examined using X-ray photoelectron spectroscopy. Fig. 1b shows the survey-scan spectrum of the cobalt oxide. The deconvolution of the complex Co 2p spectrum (Fig. 1c) confirms the existence of two chemically distinct species: Co²⁺ and Co³⁺. The peaks at around 794.8 and 779.6 eV correspond to the Co 2p_1/2 and Co 2p_3/2 spin–orbit peaks of Co₃O₄. The presence of Co₃O₄ can be further confirmed by the O 1s XPS spectrum (Fig. 1d). The peak at 529.7 eV is corresponding to the lattice oxygen in the spinel Co₃O₄.⁴² The other observed peaks are associated with the oxygen of OH⁻ and the H₂O adsorbed onto the surface of Co₃O₄.^43,44

Fig. 1 (a) XRD powder pattern, (b) XPS survey scan spectrum, (c) XPS spectra of Co 2p, and (d) XPS spectra of O 1s of the synthesized Co₃O₄.

Fig. 2 shows the SEM images of Co₃O₄ at various magnifications. Co₃O₄ densely grows on the nickel foam with a flower-shaped morphology. The higher magnification images show that these flower-shaped structures are made of nanosheets. These individual flower-shaped nanostructures are very porous in nature, which will allow easy access of the electrolyte to the entire surface of the Co₃O₄. These porous nanosheets in direct contact with the nickel foam will also allow fast electron and ion transportation through the nanosheets to the electrode with excellent structural stability and thus could provide high catalytic activity.

Fig. 2 SEM images of synthesized Co₃O₄ at various magnifications.

The electrocatalytic activities of Co₃O₄ were studied in alkaline solution for the oxygen evolution reaction. Fig. 3a shows the OER polarization curves of the nickel foam, platinum foil and Co₃O₄ grown over nickel foam. The Co₃O₄ electrode showed the lowest overpotential and the highest current density compared with those of nickel foam and platinum. The onset potentials for Co₃O₄, nickel foam and platinum were found to be 284, 327 and 328 mV, respectively (Fig. S1, ESI†). The overpotentials of 356, 441 and 459 mV were observed at 10 mA cm⁻² for Co₃O₄, nickel foam and platinum, respectively. The Tafel slopes of Co₃O₄, nickel foam and platinum in 1 M NaOH were calculated to be 68, 142 and 220 mV per decade, respectively. As seen in Fig. 3b, the slope for the Co₃O₄ electrode remains linear even at higher current densities, indicating fast electron and mass transfer between the surface of Co₃O₄ and the electrolyte.⁴⁵ Jeon et al. observed an overpotential of 377 mV at 10 mA cm⁻² for a chemical solution deposited Co₃O₄ thin film.⁴⁶ Chou et al. synthesized different phases of cobalt oxide such as CoO and Co₃O₄ for OER applications.⁴⁷ Overpotentials of 495 and 496 mV at 10 mA cm⁻² for CoO and Co₃O₄, respectively, were reported. Mesoporous Co₃O₄ has been synthesized using porous silica as a hard template via a nanocasting route as a catalyst for OER activity.⁴⁸ They observed that the aging temperature of the silica hard template for Co₃O₄ affects the overpotential. They observed overpotentials from 525 to 636 mV at 10 mA cm⁻² for Co₃O₄ with different aging temperatures. The electrochemical activities of a series of Co₃O₄ structures and their nanocomposites were investigated as oxygen evolution catalysts.⁴⁹ It was observed that among the ordered mesoporous composites, Co₃O₄–CuCo₂O₄ showed the best performance as an OER catalyst. Co₃O₄ and Co₃O₄–CuCo₂O₄ showed overpotentials of 528 and 498 mV at 10 mA cm⁻², respectively.

Fig. 3 (a) OER polarization curves, (b) Tafel slope, (c) roughness factor and current density (at 1.65 V, RHE) for platinum, nickel foam and Co₃O₄, and (d) flexibility testing of the Co₃O₄ electrode (inset of (d) shows flexibility of Co₃O₄ electrode).

Our synthesized Co₃O₄ showed an overpotential of 356 mV at 10 mA cm⁻² and a Tafel slope of 68 mV per decade. The better performance of our Co₃O₄ electrode could be due to the highly porous and nanostructured morphology, which allows easy access of the electrolyte to the entire surface of Co₃O₄ and easy escape of the generated oxygen from the electrode. In addition, the direct growth of Co₃O₄ on nickel foam reduces the series resistance and the binder free approach does not block the active sites for the OER reaction. The specific surface area and roughness factor of nickel foam, platinum foil and Co₃O₄ grown over nickel foam were estimated using cyclic voltammetry measurements in non-faradaic regions. The CV curves show typical rectangular curves with no oxidation or reduction peaks, suggesting regular features of an electrical double layer capacitor (Fig. S2(a), ESI†). A linear relationship was observed between the current density and the scan rate of the samples as seen in Fig. S2(b) (ESI†). The slope of these curves provides the value of the double layer capacitance [i = C(dE/dt)]. Double layer capacitances of 0.48, 5.75 and 573 mF cm⁻² were calculated for platinum, nickel foam and the Co₃O₄ electrode, respectively. The roughness factor (R_f) was determined using the following equation:^41,50

R_f = C_dl/C_dl* (1)

where C_dl is the double layer capacitance of the synthesized Co₃O₄ and C_dl* is the double layer capacitance of the smooth surface. The double layer capacitance of the smooth surface was used as 60 μF cm⁻².⁴¹ The roughness factor and current density for nickel foam, platinum and cobalt oxide in 1 M NaOH are given in Fig. 3c. A good correlation between the roughness factor and the OER current density was observed.

The effect of bending on the OER current density of Co₃O₄ was studied to investigate its applicability as a flexible electrode. The linear sweep voltammogram curves of the Co₃O₄ electrode at 0° and 60° bending show almost identical behaviors (Fig. 3d). As seen in Fig. 3d, a difference of 1 mV in overpotential at 10 mA cm⁻² was observed between 0° and 60° bending. The long term stability of the Co₃O₄ electrode was studied. Fig. 4a shows the linear sweep voltammetry curves of cobalt oxide at various cycles. The performance of the Co₃O₄ electrode was very stable up to the 2000th cycle. A small difference (∼4 mV) in overpotential at 10 mA cm⁻² was observed between the 2000th cycle and the first cycle. Our results suggest that the Co₃O₄ electrode could be used as a high performance and flexible OER electrode. The stability of the Co₃O₄ electrode was further examined using chronoamperometry. As seen in Fig. 4b, the current density was almost constant for over 20 h of study. Our results confirmed that the method adopted in this work could be used to fabricate highly efficient and durable electrocatalysts for the oxygen evolution reaction in the electrolysis of water.

Fig. 4 (a) Linear sweep voltammetry curves and (b) time-dependent current density curve for the synthesized Co₃O₄ (inset of (b) shows oxygen evolution during experiment).

The performance of the Co₃O₄ electrode was also studied using electrochemical impedance spectroscopy. Fig. S3 (ESI†) shows the Nyquist plots of the Co₃O₄ electrode at different potentials. The behavior of the Nyquist plots depends on the potential. At lower potential, the Nyquist plot shows a depressed semicircle near the origin (at high frequency), followed by a straight line at higher impedance (at low frequency). The straight line in the low frequency region starts to convert into a semicircle with an increase in the potential. It is further noted that the semicircle in the high frequency region is almost independent of the potential, whereas the semicircle in the low frequency region depends on the overpotential. The semicircle at low frequency is related to the OER process as its diameter reduces with increasing overpotential, which is due to a faster reaction at high overpotential. EIS measurements of the Co₃O₄ electrode before and after the long-term stability test were performed. Fig. S4(a) (ESI†) shows the Nyquist plot of the Co₃O₄ electrode before the stability test. The EIS data fit well with the equivalent circuit given in the inset of Fig. S4(a) (ESI†), where R_s is the electrolyte resistance, C_dl is the double layer capacitance at the Co₃O₄/electrolyte interface, R_ct, C_pc and R₁ are related to the OER process, and C_ox and R_ox could be due to additional contribution from the underlying Co₃O₄ passive film.⁵¹ The Nyquist plots before and after the stability test are shown in Fig. S4(b) (ESI†). A small increase in the impedance of the Co₃O₄ electrode was seen after the stability test (inset of Fig. S4(b) (ESI†)). A small increment in R_s, R_ct, R₁ and R_ox was observed after the stability test (Table S1, ESI†). The electrochemical investigations on the Co₃O₄ electrode confirm that our binder free approach can be used to fabricate low-cost, high performance, durable and flexible electrodes for the oxygen evolution reaction which can operate at low overpotentials.

In summary, we have used a facile method to synthesise a binder free Co₃O₄ nanostructured electrode. The as-synthesized Co₃O₄ nanostructure electrode was tested for the oxygen evolution reaction in alkaline solution. Co₃O₄ showed a highly promising electrocatalytic behaviour. In alkaline solution, the Co₃O₄ electrode showed a low overpotential of 356 mV (at 10 mA cm⁻²) with great electrochemical stability and flexibility. The Co₃O₄ electrode showed almost identical oxygen evolution curves on bending (1 mV difference in overpotential before and after bending). The high catalytic activity of the Co₃O₄ electrode was due to the highly porous structure and the direct contact of the Co₃O₄ nanosheets with the nickel foam. Thus, it is anticipated that this binder free approach can be used to synthesize low cost and high performance electrocatalysts for industrial water splitting processes.

Author contributions

RKG conceived the project and designed the experiments. RKG also performed some structural and electrochemical characterizations, interpreted and analyzed the data, and prepared the manuscript. CR, CZ and SB synthesized and performed some electrochemical characterizations. MG and SM recorded the SEM images. FP performed the XPS measurements. All the authors reviewed and commented on the manuscript.

Competing financial interests

The authors declare no competing financial interests. Requests for materials should be addressed to RKG.

Acknowledgements

Dr Ram K. Gupta expresses his sincere acknowledgment to the Polymer Chemistry Initiative, Pittsburg State University for providing financial and research support. SM thanks funding from FIT-DRONES at the University of Memphis.

References

1. M. Gong, Y. Li, H. Wang, Y. Liang, J. Z. Wu, J. Zhou, J. Wang, T. Regier, F. Wei and H. Dai, J. Am. Chem. Soc., 2013, 135, 8452–8455.
2. Y.-C. Lu, Z. Xu, H. A. Gasteiger, S. Chen, K. Hamad-Schifferli and Y. Shao-Horn, J. Am. Chem. Soc., 2010, 132, 12170–12171.
3. A. Kudo and Y. Miseki, Chem. Soc. Rev., 2009, 38, 253–278.
4. M. G. Walter, E. L. Warren, J. R. McKone, S. W. Boettcher, Q. Mi, E. A. Santori and N. S. Lewis, Chem. Rev., 2010, 110, 6446–6473.
5. T. J. Meyer, Nature, 2008, 451, 778–779.
6. J. A. Koza, Z. He, A. S. Miller and J. A. Switzer, Chem. Mater., 2012, 24, 3567–3573.
7. S. Trasatti, Electrochim. Acta, 1984, 29, 1503–1512.
8. S. Cherevko, T. Reier, A. R. Zeradjanin, Z. Pawolek, P. Strasser and K. J. J. Mayrhofer, Electrochem. Commun., 2014, 48, 81–85.
9. C. C. L. McCrory, S. Jung, J. C. Peters and T. F. Jaramillo, J. Am. Chem. Soc., 2013, 135, 16977–16987.
10. H. Hu, B. Guan, B. Xia and X. W. Lou, J. Am. Chem. Soc., 2015, 137, 5590–5595.
11. J. Yu, Y. Zhong, W. Zhou and Z. Shao, J. Power Sources, 2017, 338, 26–33.
12. X. Wang, Y. Zheng, J. Yuan, J. Shen, A.-J. Wang, L. Niu and S. Huang, Electrochim. Acta, 2016, 212, 890–897.
13. H. Osgood, S. V. Devaguptapu, H. Xu, J. Cho and G. Wu, Nano Today, 2016, 11, 601–625.
14. Y. Xu, A. Tsou, Y. Fu, J. Wang, J.-H. Tian and R. Yang, Electrochim. Acta, 2015, 174, 551–556.
15. X. Xu, Y. Pan, W. Zhou, Y. Chen, Z. Zhang and Z. Shao, Electrochim. Acta, 2016, 219, 553–559.
16. J. Wang, Y. Fu, Y. Xu, J. Wu, J.-H. Tian and R. Yang, Int. J. Hydrogen Energy, 2016, 41, 8847–8854.
17. C. Jin, X. Cao, L. Zhang, C. Zhang and R. Yang, J. Power Sources, 2013, 241, 225–230.
18. K. Elumeeva, J. Masa, J. Sierau, F. Tietz, M. Muhler and W. Schuhmann, Electrochim. Acta, 2016, 208, 25–32.
19. D. S. Bick, A. Kindsmüller, G. Staikov, F. Gunkel, D. Müller, T. Schneller, R. Waser and I. Valov, Electrochim. Acta, 2016, 218, 156–162.
20. H. A. Bandal, A. R. Jadhav, A. A. Chaugule, W. J. Chung and H. Kim, Electrochim. Acta, 2016, 222, 1316–1325.
21. J. Wang, H. Tang, L. Zhang, H. Ren, R. Yu, Q. Jin, J. Qi, D. Mao, M. Yang, Y. Wang, P. Liu, Y. Zhang, Y. Wen, L. Gu, G. Ma, Z. Su, Z. Tang, H. Zhao and D. Wang, Nat. Energy, 2016, 1, 16050.
22. J. Wang, H. Tang, H. Ren, R. Yu, J. Qi, D. Mao, H. Zhao and D. Wang, Adv. Sci., 2014, 1, 1400011.
23. J. Zhang, H. Ren, J. Wang, J. Qi, R. Yu, D. Wang and Y. Liu, J. Mater. Chem. A, 2016, 4, 17673–17677.
24. H. Ren, R. Yu, J. Wang, Q. Jin, M. Yang, D. Mao, D. Kisailus, H. Zhao and D. Wang, Nano Lett., 2014, 14, 6679–6684.
25. J. Wang, N. Yang, H. Tang, Z. Dong, Q. Jin, M. Yang, D. Kisailus, H. Zhao, Z. Tang and D. Wang, Angew. Chem., Int. Ed., 2013, 52, 6417–6420.
26. J. Wang, H. Tang, H. Wang, R. Yu and D. Wang, Materials Chemistry Frontiers, 2017, 1, 414–430.
27. J. Qi, X. Lai, J. Wang, H. Tang, H. Ren, Y. Yang, Q. Jin, L. Zhang, R. Yu, G. Ma, Z. Su, H. Zhao and D. Wang, Chem. Soc. Rev., 2015, 44, 6749–6773.
28. X. Lv, Y. Zhu, H. Jiang, X. Yang, Y. Liu, Y. Su, J. Huang, Y. Yao and C. Li, Dalton Trans., 2015, 44, 4148–4154.
29. S. Chen and S.-Z. Qiao, ACS Nano, 2013, 7, 10190–10196.
30. D. Tang, O. Mabayoje, Y. Lai, Y. Liu and C. B. Mullins, ChemistrySelect, 2017, 2, 2230–2234.
31. F. Yu, F. Li and L. Sun, Int. J. Hydrogen Energy, 2016, 41, 5230–5233.
32. H. Schäfer, S. M. Beladi-Mousavi, L. Walder, J. Wollschläger, O. Kuschel, S. Ichilmann, S. Sadaf, M. Steinhart, K. Küpper and L. Schneider, ACS Catal., 2015, 5, 2671–2680.
33. H. Schafer, S. Sadaf, L. Walder, K. Kuepper, S. Dinklage, J. Wollschlager, L. Schneider, M. Steinhart, J. Hardege and D. Daum, Energy Environ. Sci., 2015, 8, 2685–2697.
34. F. Moureaux, P. Stevens, G. Toussaint and M. Chatenet, J. Power Sources, 2013, 229, 123–132.
35. J. S. Chen, J. Ren, M. Shalom, T. Fellinger and M. Antonietti, ACS Appl. Mater. Interfaces, 2016, 8, 5509–5516.
36. A. J. Esswein, M. J. McMurdo, P. N. Ross, A. T. Bell and T. D. Tilley, J. Phys. Chem. C, 2009, 113, 15068–15072.
37. Y. Luo, J. Jiang, W. Zhou, H. Yang, J. Luo, X. Qi, H. Zhang, D. Y. W. Yu, C. M. Li and T. Yu, J. Mater. Chem., 2012, 22, 8634–8640.
38. H. Schafer, D. M. Chevrier, K. Kuepper, P. Zhang, J. Wollschlaeger, D. Daum, M. Steinhart, C. He, U. Krupp, K. Muller-Buschbaum, J. Stangl and M. Schmidt, Energy Environ. Sci., 2016, 9, 2609–2622.
39. H. Schäfer, K. Küpper, J. Wollschläger, N. Kashaev, J. Hardege, L. Walder, S. Mohsen Beladi-Mousavi, B. Hartmann-Azanza, M. Steinhart, S. Sadaf and F. Dorn, ChemSusChem, 2015, 8, 3099–3110.
40. H. Schäfer, D. M. Chevrier, P. Zhang, J. Stangl, K. Müller-Buschbaum, J. D. Hardege, K. Kuepper, J. Wollschläger, U. Krupp, S. Dühnen, M. Steinhart, L. Walder, S. Sadaf and M. Schmidt, Adv. Funct. Mater., 2016, 26, 6402–6417.
41. B. Lu, D. Cao, P. Wang, G. Wang and Y. Gao, Int. J. Hydrogen Energy, 2011, 36, 72–78.
42. Y. Tan, Q. Gao, C. Yang, K. Yang, W. Tian and L. Zhu, Sci. Rep., 2015, 5, 12382.
43. S. Xiong, J. S. Chen, X. W. Lou and H. C. Zeng, Adv. Funct. Mater., 2012, 22, 861–871.
44. C. Shang, S. Dong, P. Hu, J. Guan, D. Xiao, X. Chen, L. Zhang, L. Gu, G. Cui and L. Chen, Sci. Rep., 2015, 5, 8335.
45. X. Lu and C. Zhao, Nat. Commun., 2015, 6, 6616.
46. H. S. Jeon, M. S. Jee, H. Kim, S. J. Ahn, Y. J. Hwang and B. K. Min, ACS Appl. Mater. Interfaces, 2015, 7, 24550–24555.
47. N. H. Chou, P. N. Ross, A. T. Bell and T. D. Tilley, ChemSusChem, 2011, 4, 1566–1569.
48. H. Tüysüz, Y. J. Hwang, S. B. Khan, A. M. Asiri and P. Yang, Nano Res., 2013, 6, 47–54.
49. T. Grewe, X. Deng, C. Weidenthaler, F. Schüth and H. Tüysüz, Chem. Mater., 2013, 25, 4926–4935.
50. L. M. Da Silva, L. A. De Faria and J. F. C. Boodts, Electrochim. Acta, 2001, 47, 395–403.
51. E. B. Castro, C. A. Gervasi and J. R. Vilche, J. Appl. Electrochem., 1998, 28, 835–841.

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Footnotes

† Electronic supplementary information (ESI) available: Synthesis, characterization and some additional figures. See DOI: 10.1039/c7qm00108h

‡ C. K. Ranaweera and C. Zhang contributed equally.

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