The mechanism change by switching the reactants from water to hydroxyl ions for electrocatalytic water oxidation: a case study of copper oxide microspheres.

Developing noble metal-free water oxidation catalysts is essential for many energy conversion/storage processes (e.g., water splitting). Herein, we report a facile synthesis of CuO microspheres composed of ultrathin, single-crystal-like nanosheets via a simple solution method. The as-obtained CuO microspheres can serve as an active and stable water oxidation catalyst under electrochemical reaction conditions, owing to their unique structural features. In electrochemical water oxidation, this catalyst affords a current density of 10 mA cm-2 (a value related to practical relevance) at an overpotential of ∼0.48 V. Pure CuO was reported as a water oxidation catalyst (WOC) from near-neutral conditions to alkalescent conditions. Electrochemistry values agree with the Nernstian behavior, suggesting ne-/nH+ transfer prior to a chemical rate-determining step. Our results suggest that the delicate nanostructure can offer unique advantages for developing efficient water oxidation catalysts.