Covalency competition dominates the water oxidation structure-activity relationship on spinel oxides

Spinel oxides have attracted growing interest over the years for catalysing the oxygen evolution reaction (OER) due to their efficiency and cost-effectiveness, but fundamental understanding of their structure–property relationships remains elusive. Here we demonstrate that the OER activity on spinel oxides is intrinsically dominated by the covalency competition between tetrahedral and octahedral sites. The competition fabricates an asymmetric MT−O−MO backbone where the bond with weaker metal–oxygen covalency determines the exposure of cation sites and therefore the activity. Driven by this finding, a dataset with more than 300 spinel oxides is computed and used to train a machine-learning model for screening the covalency competition in spinel oxides, with a mean absolute error of 0.05 eV. [Mn]T[Al0.5Mn1.5]OO4 is predicted to be a highly active OER catalyst and subsequent experimental results confirm its superior activity. This work sets mechanistic principles of spinel oxides for water oxidation, which may be extendable to other applications.Ministry of Education (MOE)National Research Foundation (NRF)Accepted versionThis work was supported by Singapore Ministry of Education Tier 2 Grant (MOE2018-T2-2-027) and the Singapore National Research Foundation under its Campus for Research Excellence And Technological Enterprise (CREATE) programme. We thank the Facility for Analysis, Characterization, Testing, and Simulation (FACTS) in Nanyang Technological University. This research used resources of the National Synchrotron Light Source II, a US Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under contract no. DE-SC0012704. We also appreciate the XAS measurements from SSLS, soft X-ray and ultraviolet beamline. Y.S. and Z.X. thank A. Lapkin (University of Cambridge) for helpful discussion on machine-learning concepts and thank L. Zeng (Southern University of Science and Technology) for helpful discussion on catalyst performance. H.Z. gives thanks for the support from ITC via the Hong Kong Branch of National Precious Metals Material (NPMM) Engineering Research Center, and the start-up grant (project no. 9380100) and grants (project no. 9610478 and 1886921) in City University of Hong Kon