Decoding the Oxygen Reduction Reaction: Mechanistic Insights from Transition Metal Heterostructures

The oxygen reduction reaction (ORR) plays a central role in determining the efficiency and durability of a broad range of electrochemical energy conversion technologies, while its intrinsically slow kinetics and multistep reaction nature remain key challenges. Transition metal-based heterogeneous electrocatalysts, particularly those incorporating Fe, Mn, Co, Ni, and Cu, have been extensively investigated as ORR active systems owing to their earth abundance, diverse coordination environments, and tunable electronic structures. Recent studies show that ORR on these catalysts can diverge into two-electron and four-electron pathways from a common O₂ reactant, with pathway selectivity governed by atomic-scale coordination and electronic effects. This review summarizes current understanding of ORR mechanisms on transition metal-based catalysts, with a focus on how atomic-scale structural features are related to reaction activity, selectivity, and stability under different electrolyte conditions. Representative applications in metal-air batteries, fuel cells, and electrochemical hydrogen peroxide production are also discussed to illustrate how mechanistic insights are reflected in practical electrochemical systems. By distilling shared mechanistic features across different systems, this review provides a coherent framework for understanding and guiding ORR catalyst design.