Revealing the Catalytic Heart of Brownmillerites for the Oxygen Evolution Reaction.

Brownmillerite oxides are a family of metal oxides structurally related to perovskite oxides that have ordered oxygen vacancies. Despite their structural similarity, unlike perovskite oxide, they remain relatively unexplored as oxygen evolution reaction (OER) electrocatalysts. In this study, we systematically examined two representative brownmillerite compounds, Ca₂Fe₂O₅ and CaSrFe₂O₅, to evaluate their OER mechanism and determine the dominant active center. Our findings reveal that both compounds predominantly follow the lattice-oxygen-mediated (LOM) mechanism, with lattice oxygen bridging the two FeO₆ octahedra identified as the principal active site. Our combined experimental and computational study indicates that the B-site atom forming the BO₆ octahedra has a larger role in determining the OER activity in both compounds. These insights not only highlight the OER potential of brownmillerite oxides but also provide a foundation for the rational design of next-generation brownmillerite-based electrocatalysts.