Defect Engineering of Brownmillerites for Oxygen Evolution and Reduction Electrocatalysis

The slow kinetics of the oxygen evolution (OER) and reduction (ORR) reactions, combined with the high costs of noble metals, remain major barriers to a sustainable hydrogen economy. Earth-abundant brownmillerite-type oxides (A2B2O5) offer a promising alternative, yet their catalytic mechanism is frequently misunderstood. Because the highest activities emerge following electrochemical surface amorphization (OER) or order–disorder transitions (ORR), the pristine vacancy-ordered framework is rarely the static active phase. Herein, we reframe brownmillerites as highly specific, compositionally addressable precatalysts whose initial crystal chemistry acts as a structural template that governs the depth, composition, and stability of the operando-reconstructed active phase. Despite their potential, existing literature often treats them as perovskite subsets, neglecting the mechanistic impact of their vacancy-ordered structure and operando surface reconstruction. This review provides a unified framework linking brownmillerite defect chemistry to electrocatalytic performance. By evaluating synthesis, structural, and anionic modifications along with a comprehensive benchmarking table for this material class, we establish the mechanistic depth and structure–property correlations necessary for designing next-generation, noble-metal-free catalysts.