Roughening of windmill-shaped spinel CoO microcrystals grown on a flexible metal substrate by a facile surface treatment to enhance their performance in the oxidation of water

High-efficiency and Earth-abundant electrocatalysts for the oxidation of water are required in the production of clean energy from the electrolysis or photolysis of water. Spinel CoO microcrystals with a windmill shape were grown on a flexible metal substrate. The microcrystals were then roughened by a surface impregnation treatment. A secondary nanostructure grew out of the blades of the windmills to form a micro/nano hierarchical structure. The as-grown micro/nano CoO had an excellent electrochemical performance in the oxidation of water. The onset overpotential of the micro/nano CoO electrocatalyst for the oxidation of water was about 0.29 V in alkaline solution and the overpotential of the optimum CoO electrocatalyst was 0.41 V at a current density of 10 mA cm. These results suggest that the electrochemical performance is associated with the roughness and active surface of the CoO electrodes. The turnover frequency of the optimized CoO reached 0.39 s at an overpotential of 0.6 V, about 1.4 times higher than that for the pristine CoO microcrystals. The turnover frequency of micro/nano CoO is higher than, or comparable to, that previously reported for high-efficiency nanosized CoO in alkaline solution. Stability tests indicated that these micro/nano CoO electrocatalysts were highly durable towards the oxidation of water, with no structural change and no decrease in noticeable activity after operating for 12 h in oxygen-evolving reactions. This work verifies the contribution of surface roughness and an active surface to the electrochemical oxidation of water by the design of an optimum micro/nano CoO electrocatalyst. Our current understanding of the catalytic role of a specific micro/nano structure is also strengthened. This journal is