Interfacial roles of electrolytes in oxygen reduction reactions.

Electrocatalytic oxygen reduction reactions (ORR) are central to energy conversion technologies, including fuel cells and metal-air batteries. The interfacial microenvironment, particularly shaped by electrolytes, profoundly influences ORR activity, selectivity, and reaction pathways. In this comprehensive Review, we systematically summarize the roles of aqueous and polymer electrolytes in modulating ORR performance, highlighting key mechanisms such as ion-specific effects, solvent and water structuring, and the adsorption of anionic/cationic species at the catalyst interface. We further discuss the impact of polymer electrolytes and ionomer-catalyst interactions on local oxygen transport, active site accessibility, and catalyst utilization in catalyst layers. By integrating insights from fundamental electrochemistry, materials design, and interface engineering, we provide a critical perspective on current strategies for tuning electrolyte-catalyst interfaces. Finally, we identify existing challenges and propose future research directions for developing advanced electrolyte systems and catalyst layer architectures to achieve high-performance, durable ORR catalysis in next-generation energy devices.