Facile synthesis of ZnCo₂O₄ mesoporous structures with enhanced electrocatalytic oxygen evolution reaction properties

Abstract

Recently, as one kind of ternary spinel metal oxide, ZnCo₂O₄ has attracted extensive attention in the energy storage and conversion field due to it being cost-effective, scalable, and environmentally friendly. Herein, porous ZnCo₂O₄ micro-spindles and truncated drums were synthesized by a solvent thermal route first and then with an annealing treatment with the precursors. The morphology and structure of ZnCo₂O₄ were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution TEM (HRTEM). It is worth noting that ZnCo₂O₄ porous micro-spindles show a high surface area of 65.85 m² g⁻¹ with an average pore diameter of ∼6.34 nm. The as-prepared ZnCo₂O₄ materials, especially micro-spindles, exhibited good electrocatalytic water-splitting performance with excellent activity and stability. The ZnCo₂O₄ micro-spindle catalyst exhibits an overpotential of 0.389 V at the benchmark of a current density of 10 mA cm⁻². The Tafel slope is 59.54 mV dec⁻¹ and the OER overpotential of the catalyst shows no obvious increase during 7200 s. The high activity, low Tafel slope, and good stability will pave the way for the use of ZnCo₂O₄ micro-spindles in practical applications.

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Introduction

With the rapid depletion of traditional fossil energy and growing demand for utilization of clean renewable energy, energy conversion and storage technologies have become one of the great challenges in the future.^1–5 Recently, many efforts have been devoted to the exploration of efficient and cheap catalysts for clean energy generation and conversion.^6–14 Electrochemically splitting water to form H₂ and O₂ provides an effective way to clean renewable energy.^15,16 As is known to all, electrocatalytic decomposition of water involves two half reactions with anodic oxygen evolution and cathodic hydrogen evolution reactions. The anodic oxygen evolution reaction (OER) is a critical problem in developing efficient electrolysis of water owing to the inherent sluggish kinetics.¹⁷ This OER process is related to a four-electron transfer pathways in acid or alkaline media associated with O–H bond breaking and O–O bond formation, thus usually needs a high overpotential and leads to low energy efficiency. Precious-metal oxides IrO₂ and RuO₂ are the best OER electrocatalysts,^18,19 but their high cost and low abundance have severely hampered their large-scale practical application. Therefore, it is highly desirable and more challenging to develop inexpensive and efficient OER electro-catalysts with earth-abundant non-precious elements. Indeed, many nano-structured earth-abundant transition-metal oxides,^20–24 chalcogenides,^25–31 and layered double hydroxide^32–34 have been reported and showed good stability and low overpotentials during the OER process, nevertheless, most of them exhibited low catalytic activities due to their poor electrical conductivity and low exposed active sites. Therefore, to exploit suitable catalyst materials with high conductivity and rich active sites is still a challenging research topic in water electrolysis filed.

In recent years, spinel ZnCo₂O₄ have attracted special attention because of their potential application in various fields, such as lithium-ion batteries,^35–39 photocatalytic CO₂ reduction,⁴⁰ field emission properties,⁴¹ gas sensing,^42,43 biosensing,⁴⁴ catalytic CO oxidation,⁴⁵ supercapacitors,^46,47 and oxygen-reduction reaction.⁴⁸ So far, some ZnCo₂O₄ with different morphologies and sizes have been reported, including nanoparticles,^49,50 nanosheet,⁵¹ and nanowire arrays,^52,53 nanotubes,⁵⁴ nanoflakes,⁵⁵ nanosheets,^56–58 nanorod,⁵⁹ nanoflakes,⁶⁰ microspheres,^61–63 and nanocomposites,^64,65 to the best of our knowledge, nevertheless, there have been no reports up now on the formation of porous micro-spindles and truncated drums-like structure self-assembled with nanoparticles and nanosheets, respectively. We think spinel ZnCo₂O₄ with mesoporous structures could improve conductivity and provide massive catalytic active sites. Herein, we present the rational design of ZnCo₂O₄ mesoporous spindle-like and truncated drums-like structure for OER electrocatalysts in alkaline media.

Experimental

Synthesis of ZnCo₂O₄ precursors and porous structures

In a typical procedure, Co(Ac)₂·4H₂O (0.0498 g, 0.2 mmol), Zn(Ac)₂ (0.0220 g, 0.1 mmol), and polyvinylpyrrolidone K30 (PVP, 0.200 g) were dispersed in the mixture of 0.5 mL H₂O, 4.0 mL N,N-dimethylacetamide (DMA) and 1.0 mL ethylene glycol. And then, 0.1 mL hexylamine was added to the mixed solution. The mixture was stirred for a few minutes, sealed and heated to 180 °C for 12 h. The obtained products were separated by centrifugation and washed by ethanol and water alternately, and then dried in vacuum at 60 °C for 1 h. ZnCo₂O₄ truncated drum shaped precursor were prepared by altering the mixture solution to H₂O (0.5 mL), N,N-dimethylacetamide (4.0 mL) and cyclohexane (3.0 mL), the amount of PVP (0.05 g), the type of amine (triethylamine, 0.4 mL), and keep other reaction parameters unchanged.

The as-synthesized two precursors were calcinated at 400 °C for 2 h in air (5 °C min⁻¹) to obtain the porous ZnCo₂O₄ spindle and porous ZnCo₂O₄ truncated drum.

Materials characterization

The resultant phase of the porous ZnCo₂O₄ spindle and porous ZnCo₂O₄ truncated drum were characterized by X-ray diffraction (XRD) on a Philips X'pert Pro X-ray diffractometer with Cu_Ka radiation (λ = 1.5418 Å) and operated at 40 kV and 40 mA. The surface morphology and microstructure were determined by a field scanning electron microscopy on Hitachi SU8010 and transmission electron microscopy on a FEI Tecnai G2 s-twin F20. The composition of the samples was studied by energy-dispersive X-ray spectroscope (EDX) attached to the FSEM instrument. The specific surface areas of the samples were measured by N₂ adsorption measured using Gemini VII 2390 Analyzer at 77 K by using the volumetric method. X-ray photoelectron spectroscopy (XPS) was performed on the Thermo Scientific ESCALab 250Xi using 200 W monochromatic Al Kα radiation.

OER electrocatalytic electrode preparation and measurement

5 mg catalyst powder was dispersed in a mixture of water and ethanol (v/v = 3 : 1, 1 mL) with 50 μL Nafion solution under continuous ultrasonication for 0.5 h to obtain a homogenous ink. Then, 10 μL of the dispersive catalyst ink was casted to a polished glass carbon electrode of 5 mm diameter (0.196 cm²), with a loading catalyst density of 0.24 mg cm⁻² and dried at room temperature. All the electrochemical tests were carried out by CHI 760E electrochemical workstation, using O₂ pre-saturated 1 M KOH solution as the electrolyte. Platinum wire was used as the counter and Ag/AgCl electrode was used as the reference electrode. The linear sweep voltammetry (LSV) curves were obtained at a sweep rate of 5 mV s⁻¹. Electrochemical impedance spectroscopy (EIS) were carried out at open circuit potential, the frequency scan range was from 100 kHz to 0.1 Hz. The pH value of 1 M KOH is ∼14. All the OER catalytic measurements for the ZnCo₂O₄ samples were conducted with iR compensation unable.

Results and discussion

Fig. 1 shows typical field-emission scanning electron microscopy (FESEM) images of the as-prepared ZnCo₂O₄ precursors and 3D porous ZnCo₂O₄ spindle. The as-prepared spindle-like ZnCo₂O₄ precursors with an average size of 4.96 μm in length and 1.78 μm in width determined from the statistics of ∼100 particles (Fig. S1†). Some reports suggested that the spindle morphology is similar to spherical particles and is easy to get high stacking density in the surface of the electrode. Fig. 1b and c show that “spindle-like” ZnCo₂O₄ precursors with a compact surface structure. Fig. S2† shows the XRD pattern of the intermediate precursor, which can be indexed to the crystalline CoCO₃ (PDF no. 11-0692) and ZnCO₃ (PDF no. 08-0499). We investigated the formation mechanism of the “spindle-like” ZnCo₂O₄ precursors by a series of control experiments, the corresponding SEM images demonstrated in Fig. S3–S7.† Compared with the dosage of PVP, hexylamine and initial reactant concentration (Fig. S3–S5), the effect of reaction temperature (Fig. S6) and time (Fig. S7†) are dramatic on the formation of spindle morphology. From the images in Fig. S3–S7,† we can see the reaction time and reaction temperature are main influencing factors, the amount of PVP, hexylamine, and reactant concentrations also play important roles on the morphology of high quality. Thus, we obtained the optimal reaction conditions in the Experiment section.

Fig. 1 The SEM images of spindle-like ZnCo₂O₄ precursors (a–c) and porous ZnCo₂O₄ micro-spindles obtained by an annealing process (d–f).

Fig. S8† shows the isotherm and pore distribution plot of the spindle precursor, which indicated the low surface area (3.23 m² g⁻¹) and nonporous structure of the zinc doped cobalt carbonate. In order to get porous ZnCo₂O₄ micro-crystals, a followed annealing process is conducted at 400 °C for 2 h in air. The obtained ZnCo₂O₄ particles could maintain their initial spindle morphology (Fig. 1d and e) after the calcination treatment, which enriched the sample with open porous architecture, which is clearly illustrated in Fig. 1f.

Fig. 2 shows typical XRD patterns of the as-prepared ZnCo₂O₄ sample. The peaks present in the spectra well match with those in the standard crystallographic spectrum of ZnCo₂O₄ (JCPDS 23-1390). The analysis of N₂ sorption isotherms based on the Brunauer–Emmett–Teller (BET) theory further indicates ZnCo₂O₄ mesoporous spindle-like structure shows high surface area of 65.85 m² g⁻¹. The average pore diameters of the mesoporous microspindle are ∼6.34 nm (inset in Fig. 2b). According to STEM studies in Fig. 2c, we can find the as-obtained ZnCo₂O₄ spindle contains highly porous structure, which is consisted with the BJH result. Fig. 2d shows the HRTEM image of a further magnified top of the spindle, the clear fingers indicated the highly crystallinity of the porous ZnCo₂O₄ micro-spindle. The marked blue and red squire in Fig. 2d shows the lattice distance of 0.234 and 0.244 nm, which respectively correspond to the (222) and (311) plane of cubic spinel ZnCo₂O₄ crystal (inset in Fig. 2d and e).

Fig. 2 The character of porous spindle-like ZnCo₂O₄ (a) XRD pattern of porous ZnCo₂O₄ micro-spindles obtained by an annealing process. (b) The isotherm plot and the corresponding pore distribution curve of porous spindle-like ZnCo₂O₄. (c) The STEM image of an individual ZnCo₂O₄ micro-spindle. (d) HRTEM of a single particle (inset is magnified lattice finger circled in blue squire). (e) Magnification of the lattice finger highlighted by a red rectangle in (d).

Energy-dispersive X-ray (EDX) analysis confirms the even existence of Co, Zn, O elements in the ZnCo₂O₄ porous spindle (Fig. 3). The atomic ratio of Zn and Co is ∼1 : 1.87, which is consist with the initial raw material ratio. The EDX elements mapping show the homogenous distribution of Co and Zn elements in the porous spindle. Furthermore, the EDX linear scan profile reveals higher intensity of Co and Zn elements in inner than the external region along the belly of a micro-spindle (Fig. S9†), which is in accord with the thickness variation across the ZnCo₂O₄ micro-spindle. This micro-spindle provides an abundant porous surface area and conductive network for contacting between the electrode and electrolyte, which facilitates the electrolyte diffusion and electron transport during the electrochemical reaction.

Fig. 3 Energy-dispersive X-ray (EDX) analysis and the EDX elemental mapping (inset) of porous ZnCo₂O₄ micro-spindles.

For comparison, when changing the kind of amine during the precursor formation procedure, we can obtain another precursor with the different truncated drum morphology. The SEM image in Fig. 4 display the as-prepared truncated drum shaped ZnCo₂O₄ precursor and the corresponding ZnCo₂O₄ micro-truncated drum with porous and lamellar structure obtained by a calcination process. The size of the truncated drum shaped precursor is smaller than the spindle shaped, there are three groups of truncated section parallel to each other in axis direction. The typical SEM image in Fig. 4b also exhibits the rough surface and clear edge of the three intersected plane. The loose plates in Fig. 4c and d demonstrate the truncated drum may be the self-assembly of stacked nanosheets, which are ultrathin and consisted of numerous pores.

Fig. 4 The SEM images of truncated drum shaped ZnCo₂O₄ precursors (a and b) and porous ZnCo₂O₄ truncated drums obtained by an annealing process at 400 °C for 2 h (c and d).

XRD analysis was also performed to identify the phase structures of the truncated drum shaped ZnCo₂O₄ precursors (Fig. S10†) and the corresponding calcinated ZnCo₂O₄. As shown in Fig. 5a, the annealed sample consisted with pure cubic phase of ZnCo₂O₄ as well. The pore distribution and surface area of truncated drum-like ZnCo₂O₄ derived from the desorption data and calculated by BJH and BET method, respectively, shows the major pore size of 14.16 nm and the relatively high surface area of 28.43 m² g⁻¹, which is lower than the spindle-like sample. Similarly, Zn and Co is also detected by EDX with an automatic ratio of 1 : 1.9 and equally distributed in the truncated drum-like ZnCo₂O₄ (Fig. 5c and d).

Fig. 5 The character of porous ZnCo₂O₄ truncated drums (a) XRD pattern of porous ZnCo₂O₄ truncated drums obtained by calcination treatment. (b) Adsorption isotherm plot (the pore distribution inset) of ZnCo₂O₄ truncated drums. (c) EDX result of porous ZnCo₂O₄ truncated drums (Cu is from the copper grid). (d) EDX elemental maps of an individual truncated drum.

To evaluate the potential application of the porous ZnCo₂O₄ particles as OER electrocatalysts, the as-synthesized two samples with different morphology are all drop casting onto the 5 mm glass carbon electrode, the loading density is 0.255 g cm⁻², their OER curves were recorded in an O₂-saturated 1 M KOH solution. The linear sweep voltammogram (LSV) present in Fig. 6a obtained at a scan rate of 5 mV s⁻¹ without iR compensation, the overpotential of ZnCo₂O₄ micro-spindle is 0.389 V, which is smaller than the truncated drum counterpart (0.419 V) at the benchmark of a current density of 10 mA cm⁻². This performance is consist with the BET result, the spindle-like sample have a higher surface area of 65.85 m² g⁻¹ than that of truncated drum (28.43 m² g⁻¹), which benefit the electrocatalysis process by exposing more active sites. The Tafel slope is 59.54 and 70.26 mV dec⁻¹ for ZnCo₂O₄ micro-spindle and ZnCo₂O₄ truncated drum, respectively (Fig. 6b). The small Tafel slope of porous spindle sample further implied its favorable OER reaction kinetics. The electrochemical impendence spectroscopies were further measured in order to investigate the OER reaction kinetically. In Fig. 6c, the Nyquist plot of spindle-like ZnCo₂O₄ show the smaller semicircle than the truncated drum sample at the low frequent region, which reveal the smaller charge transfer resistance and faster charge transfer of the porous ZnCo₂O₄ micro-spindle during OER processes. Furthermore, the chronoamperometry test also conducted to confirm the stability of the two samples at the current density 10 mA cm⁻². As shown in Fig. 6d, the OER overpotential of porous ZnCo₂O₄ spindle shows no obvious increase during 7200 s, while porous ZnCo₂O₄ truncated drum shows a little scope rise of the overpotential to deliver a current density of 10 mA cm⁻², indicating the excellent durability of porous micro-spindle in alkaline condition, meanwhile, which can explain the degrading activity of porous ZnCo₂O₄ truncated drum. All of these features of the porous ZnCo₂O₄ spindle promise an application of a cheap but efficient electrocatalyst, which is equivalent and better than the reported transition metals oxides and sulfide catalysts^65–70 (Table 1 in ESI†).

Fig. 6 The catalytic OER process of porous ZnCo₂O₄ spindle and truncated drum samples: (a) LSV curves obtained at a sweep rate of 5 mV s⁻¹, (b) Tafel plots of OER currents in (a). (c) Nyquist plots of the two samples measured at open circuit potential. (d) Comparison of the chronoamperometry curves at the current density of 10 mA cm⁻².

The OER mechanistic of Co-based nanocatalysts in alkaline electrolyte have been proposed as follows:^71,72

Co²⁺ + 3OH⁻ ↔ CoOOH + H₂O + e⁻ (1)

CoOOH + OH⁻ ↔ CoO(OH)₂ + e⁻ (2)

CoO(OH)₂ + 2OH⁻ ↔ CoOO₂ + 2H₂O + 2e⁻ (3)

CoOO₂ + OH⁻ → CoOOH + O₂ + e⁻ (4)

Summary OER: 4OH⁻ → O₂ + 2H₂O + 4e⁻

Our ZnCo₂O₄ porous micro-spindle structure may share a similar above mechanism. Co atoms on the porous ZnCo₂O₄ surface are partially oxidized into CoOOH at first and form CoOOH/ZnCo₂O₄ as the real surface catalytic sites. At higher potential, the CoOOH/ZnCo₂O₄ intermediate can be further oxidized into CoOO₂/ZnCo₂O₄, and with further electro-oxidation, the O₂ is evolution and CoOO₂/ZnCo₂O₄ is reduced to CoOOH/ZnCo₂O₄.

The XPS tests of the above two morphology of ZnCo₂O₄ samples have been conducted and shown in Fig. S11.† Compared with the truncated drums, the spindle-like sample possess a higher content of Co³⁺(Fig. S11B and Table 2†), which is believed to be easier to oxide to Co⁴⁺ and served as catalytic center of oxygen evolution reaction.

The good conductivity of ZnCo₂O₄ micro-spindle provides dramatically more efficient electron transportation between electrode and CoOOH/ZnCo₂O₄ surface active sites compared with ZnCo₂O₄ truncated drum, which benefits OER on ZnCo₂O₄ micro-spindle structure with more effective reaction rate.

Conclusion

Herein, we demonstrated porous ZnCo₂O₄ spindle-like structure and ZnCo₂O₄ truncated drum can be synthesized by solvothermal method and then a calcination process in air. In addition, the as-prepared ZnCo₂O₄ porous spindle-like structure exhibited enhanced electrocatalytic activity for OER than the compared truncated drum-like sample, with a small overpotential and high robust in alkaline medium. This impressive result could attribute to the higher surface area, more exposed active sites and higher conductivity of the porous spindle-like microstructure. This work shows a great potential in designing a new type of porous structure in the future for promising applications in the high performance electrocatalyst.

Acknowledgements

Financial supports from the National Science Foundation of China (No. 21501006, 21603004, 21405005) and Foundation of Henan Educational Committee (No. 15A150002, 15A150031) are gratefully acknowledged.

Notes and references

1. N. S. Choi, Z. H. Chen, S. A. Freunberger, X. L. Ji, Y. K. Sun, K. Amine, G. Yushin, L. F. Nazar, J. Cho and P. G. Bruce, Angew. Chem., Int. Ed., 2012, 51, 9994–10024.
2. H. L. Wang and H. J. Dai, Chem. Soc. Rev., 2013, 42, 3088–3113.
3. Z. Liu, J. Xu, D. Chen and G. Z. Shen, Chem. Soc. Rev., 2015, 44, 161–192.
4. D. Larcher and J.-M. Tarascon, Nat. Chem., 2015, 7, 19–29.
5. X. L. Hu, W. Zhang, X. X. Liu, Y. N. Mei and Y. H. Huang, Chem. Soc. Rev., 2015, 44, 2376–2404.
6. F. Y. Cheng and J. Chen, Chem. Soc. Rev., 2012, 41, 2172–2192.
7. F. E. Osterloh, Chem. Soc. Rev., 2013, 42, 2294–2320.
8. G. Wu and P. Zeleney, Acc. Chem. Res., 2013, 46, 1878–1889.
9. Z. L. Wang, D. Xu, J. J. Xu and X. B. Zhang, Chem. Soc. Rev., 2014, 43, 7746–7786.
10. T. Wang, Z. B. Luo, C. C. Li and J. L. Gong, Chem. Soc. Rev., 2014, 43, 7469–7484.
11. T. Hisatomi, J. Kubota and K. Domen, Chem. Soc. Rev., 2014, 43, 7520–7535.
12. Y. Jiao, Y. Zheng, M. Jaroniec and S. Z. Qiao, Chem. Soc. Rev., 2015, 44, 2060–2086.
13. X. X. Zou and Y. Zhang, Chem. Soc. Rev., 2015, 44, 5148–5180.
14. J. H. Wang, W. Cui, Q. Liu, Z. C. Xing, A. M. Asiri and X. P. Sun, Adv. Mater., 2016, 28, 215–230.
15. Z. B. Wu, Y. R. Zhu and X. B. Ji, J. Mater. Chem. A, 2014, 2, 14759–14772.
16. C. Z. Yuan, H. B. Wu, Y. Xie and X. W. (David) Lou, Angew. Chem., Int. Ed., 2014, 53, 1488–1504.
17. C. C. L. McCrory, S. Jung, J. C. Peters and T. F. Jaramillo, J. Am. Chem. Soc., 2013, 135, 16977–16987.
18. J. Wang, H. X. Zhong, Y. L. Qin and X. B. Zhang, Angew. Chem., Int. Ed., 2013, 52, 5248–5253.
19. 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.
20. F. Jiao and H. Frei, Angew. Chem., Int. Ed., 2009, 48, 1841–1844.
21. Y. Y. Liang, Y. G. Li, H. L. Wang, J. G. Zhou, J. Wang, T. Regier and H. J. Dai, Nat. Mater., 2011, 10, 780–786.
22. J. Bao, X. D. Zhang, B. Fan, J. J. Zhang, M. Zhou, W. L. Yang, X. Hu, H. Wang, B. C. Pan and Y. Xie, Angew. Chem., Int. Ed., 2015, 54, 7399–7404.
23. T. Y. Ma, S. Dai, M. Jaroniec and S. Z. Qiao, J. Am. Chem. Soc., 2014, 136, 13925–13931.
24. L. Xu, Q. Q. Jiang, Z. H. Xiao, X. Y. Li, J. Huo, S. Y. Wang and L. M. Dai, Angew. Chem., Int. Ed., 2016, 55, 5277–5281.
25. M. R. Gao, Y. F. Xu, J. Jiang, Y. R. Zheng and S. H. Yu, J. Am. Chem. Soc., 2012, 134, 2930–2933.
26. Y. W. Liu, H. Cheng, M. J. Lyu, S. J. Fan, Q. H. Liu, W. S. Zhang, Y. D. Zhi, C. M. Wang, C. Xiao, S. Q. Wei, B. J. Ye and Y. Xie, J. Am. Chem. Soc., 2014, 136, 15670–15675.
27. Y. R. Zheng, M. R. Gao, Q. Gao, H. H. Li, J. Xu, Z. Y. Wu and S. H. Yu, Small, 2015, 11, 182–188.
28. K. Xu, H. Ding, K. C. Jia, X. L. Lu, P. Z. Chen, T. P. Zhou, H. Cheng, S. Liu, C. Z. Wu and Y. Xie, Angew. Chem., Int. Ed., 2016, 55, 1710–1713.
29. L. Liang, H. Cheng, F. C. Lei, J. Han, S. Gao, C. M. Wang, Y. F. Sun, S. Qamar, S. Q. Wei and Y. Xie, Angew. Chem., Int. Ed., 2015, 54, 12004–12008.
30. C. Tang, N. Y. Cheng, Z. H. Pu, W. Xing and X. P. Sun, Angew. Chem., Int. Ed., 2015, 54, 9351–9355.
31. M. R. Gao, X. Cao, Q. Gao, Y. F. Xu, Y. R. Zheng, J. Jiang and S. H. Yu, ACS Nano, 2014, 8, 3970–3978.
32. F. Song and X. L. Hu, J. Am. Chem. Soc., 2014, 136, 16481–16484.
33. M. R. Gao, W. C. Sheng, Z. B. Zhuang, Q. R. Fang, S. Gu, J. Jiang and Y. S. Yan, J. Am. Chem. Soc., 2014, 136, 7077–7084.
34. M. Gong, Y. G. Li, H. L. Wang, Y. Y. Liang, J. Z. Wu, J. G. Zhou, J. Wang, T. Regier, F. Wei and H. J. Dai, J. Am. Chem. Soc., 2013, 135, 8452–8455.
35. B. Liu, J. Zhang, X. F. Wang, G. Chen, D. Chen, C. W. Zhou and G. Z. Shen, Nano Lett., 2012, 12, 3005–3011.
36. L. L. Hu, B. H. Qu, C. C. Li, Y. J. Chen, L. Mei, D. N. Lei, L. B. Chen, Q. H. Li and T. H. Wang, J. Mater. Chem. A, 2013, 1, 5596–5602.
37. H. Long, T. L. Shi, S. L. Jiang, S. Xi, R. Chen, S. Y. Liu, G. L. Liao and Z. R. Tang, J. Mater. Chem. A, 2014, 2, 3741–3748.
38. H. Yu, C. Guan, X. H. Rui, B. Ouyang, B. L. Yadian, Y. Z. Huang, H. Zhang, H. E. Hoster, H. J. Fan and Q. Y. Yan, Nanoscale, 2014, 6, 10556–10561.
39. L. Y. Guo, Q. Ru, X. Song, S. J. Hu and Y. D. Mo, J. Mater. Chem. A, 2015, 3, 8683–8692.
40. S. B. Wang, Z. X. Ding and X. C. Wang, Chem. Commun., 2015, 51, 1517–1519.
41. S. Ratha, R. T. Khare, M. A. More, R. Thapa, D. J. Late and C. S. Rout, RSC Adv., 2015, 5, 5372–5378.
42. X. Zhou, W. Feng, C. Wang, X. L. long Hu, X. W. Li, P. Sun, K. Shimanoe, N. Yamazoe and G. Y. Lu, J. Mater. Chem. A, 2014, 2, 17683–17690.
43. K. B. Gawande, S. B. Gawande, S. R. Thakare, V. R. Mate, S. R. Kadam, B. B. Kale and M. V. Kulkarni, RSC Adv., 2015, 5, 40429–40436.
44. K. K. Naik and C. S. Rout, RSC Adv., 2015, 5, 79397–79404.
45. F. Wang, X. Wang, D. P. Liu, J. M. Zhen, J. Q. Li, Y. H. Wang and H. J. Zhang, ACS Appl. Mater. Interfaces, 2014, 6, 22216–22223.
46. B. K. Guan, D. Guo, L. L. Hu, G. H. Zhang, T. Fu, W. J. Ren, J. D. Li and Q. H. Li, J. Mater. Chem. A, 2014, 2, 16116–16123.
47. W. Q. Ma, H. H. Nan, Z. X. Gu, B. Y. Geng and X. J. Zhang, J. Mater. Chem. A, 2015, 3, 5442–5448.
48. Y. P. Huang, Y. E. Miao, H. Y. Lu and T. X. Liu, Chem.–Eur. J., 2015, 21, 10100–10108.
49. S. R. Chen, M. Xue, Y. Q. Li, Y. Pan, L. K. Zhu, D. L. Zhang, Q. R. Fang and S. L. Qiu, Inorg. Chem. Front., 2015, 2, 177–183.
50. C. R. Mariappan, R. Kumara and G. V. Prakash, RSC Adv., 2015, 5, 26843–26849.
51. N. Du, Y. F. Xu, H. Zhang, J. X. Yu, C. X. Zhai and D. R. Yang, Inorg. Chem., 2011, 50, 3320–3324.
52. Z. P. Sun, W. Ai, J. L. Liu, X. Y. Qi, Y. L. Wang, J. H. Zhu, H. Zhang and T. Yu, Nanoscale, 2014, 6, 6563–6568.
53. S. B. Wang, J. Pu, Y. Tong, Y. Y. Cheng, Y. Gao and Z. H. Wang, J. Mater. Chem. A, 2014, 2, 5434–5440.
54. W. Luo, X. L. Hu, Y. M. Sun and Y. H. Huang, J. Mater. Chem., 2012, 22, 8916–8921.
55. T. F. Hung, S. G. Mohamed, C. C. Shen, Y. Q. Tsai, W. S. Chang and R. S. Liu, Nanoscale, 2013, 5, 12115–12119.
56. F. X. Bao, X. F. Wang, X. D. Zhao, Y. Wang, Y. Ji, H. D. Zhang and X. Y. Liu, RSC Adv., 2014, 4, 2393–2397.
57. I. K. Moon and K.-Y. Chun, RSC Adv., 2015, 5, 807–811.
58. Y. Q. Zhu, C. B. Cao, J. T. Zhang and X. Y. Xu, J. Mater. Chem. A, 2015, 3, 9556–9564.
59. H. Wu, Z. Lou, H. Yanga and G. Z. Shen, Nanoscale, 2015, 7, 1921–1926.
60. J. B. Cheng, Y. Lu, K. W. Qiu, H. L. Yan, X. Y. Hou, J. Y. Xu, L. Han, X. M. Liu, J. K. Kime and Y. S. Luo, Phys. Chem. Chem. Phys., 2015, 17, 17016–17022.
61. L. Y. Guo, Q. Ru, X. Song, S. J. Hu and Y. D. Mo, RSC Adv., 2015, 5, 19241–19247.
62. Q. H. Wang, L. X. Zhu, L. Q. Sun, Y. C. Liu and L. F. Jiao, J. Mater. Chem. A, 2015, 3, 982–985.
63. Q. H. Wang, J. L. Du, Y. X. Zhu, J. Q. Yang, J. Chen, C. Wang, L. Li and L. F. Jiao, J. Power Sources, 2015, 284, 138–145.
64. T. T. Chen, Y. Fan, G. N. Wang, Q. Yang and R. X. Yang, RSC Adv., 2015, 5, 74523–74530.
65. X. L. Ge, Z. Q. Li, C. X. Wang and L. W. Yin, ACS Appl. Mater. Interfaces, 2015, 7, 26633–26642.
66. Y. Xiao, L. G. Feng, C. Q. Hu, V. Fateev, C. P. Liu and W. Xing, RSC Adv., 2015, 5, 61900–61905.
67. S. Hirai, S. Yagi, A. Seno, M. Fujioka, T. Ohno and T. Matsudaa, RSC Adv., 2016, 6, 2019–2023.
68. X. X. Yu, Z. J. Sun, Z. P. Yan, B. Xiang, X. Liu and P. W. Du, J. Mater. Chem. A, 2014, 2, 20823–20831.
69. Z. Wang, M. Li, C. G. Liang, L. Q. Fan, J. N. Han and Y. P. Xiong, RSC Adv., 2016, 6, 69251–69256.
70. B. L. Chen, R. Li, G. P. Ma, X. L. Gou, Y. Q. Zhua and Y. D. Xia, Nanoscale, 2015, 7, 20674–20684.
71. N. H. Chou, P. N. Ross, A. T. Bell and T. Don Tilley, ChemSusChem, 2011, 4, 1566–1569.
72. P. Z. Chen, K. Xu, Z. W. Fang, Y. Tong, J. C. Wu, X. L. Lu, X. Peng, H. Ding, C. Z. Wu and Y. Xie, Angew. Chem., Int. Ed., 2015, 54, 14710–14714.

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Footnote

† Electronic supplementary information (ESI) available: XRD pattern and SEM of ZnCo₂O₄ precursors obtained at different reaction parameters, line-scan, XPS, comparison of OER activity and so on. See DOI: 10.1039/c6ra14191a

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