Revealing Geometric-Site-Dependent Activity of Cobalt Spinel Oxides for Oxygen Evolution Reaction

Abstract Co-based spinel oxides are promising oxygen evolution reaction (OER) catalysts, but the intrinsic active site, tetrahedral (CoTd2+) or octahedral (CoOh3+), remains debated due to interference from defects and reconstruction in nanostructured catalysts. Here, single crystal ZnCo2O4 (mainly CoOh3+), CoAl2O4 (mainly CoTd2+), and Co3O4 (both sites) epitaxial thin films with well-defined structures serve as model electrocatalysts to eliminate extrinsic effects. Our results reveal that CoOh3+-rich motifs exhibit higher OER activity than CoTd2+-rich motifs. Electronic structure analysis shows that stronger CoOh 3d-O 2p hybridization enhances electron transfer to oxygen-containing intermediates involved in the OER, thereby governing the activity. Furthermore, tetrahedral cations modulate this hybridization, with ZnTd2+ strengthening CoOh-O interaction compared to CoTd2+. This work identifies CoOh as the dominant active site and suggests that OER performance can be optimized by maximizing CoOh exposure and tuning tetrahedral ions to enhance CoOh 3d-O 2p hybridization.