Transformation mechanism of high-valence metal sites for the optimization of Co- and Ni-based OER catalysts in an alkaline environment: recent progress and perspectives.

As an important semi-reaction process in electrocatalysis, oxygen evolution reaction (OER) is closely associated with electrochemical hydrogen production, CO 2 electroreduction, electrochemical ammonia synthesis and other reactions, which provide electrons and protons for the related applications. Considering their fundamental mechanism, metastable high-valence metal sites have been identified as real, efficient OER catalytic sites from the recent observation by in situ characterization technology. Herein, we review the transformation mechanism of high-valence metal sites in the OER process, particularly transition metal materials (Co- and Ni-based). In particular, research progress in the transformation process and role of high-valence metal sites to optimize OER performance is summarized. The key challenges and prospects of the design of high-efficiency OER catalysts based on the above-mentioned mechanism and some new in situ characterizations are also discussed.