Atomically precise layer-by-layer titration of perovskite oxides reveals the termination-specific reactivity in oxygen electrocatalysis

Understanding how atomic-scale surface structures govern catalytic pathways is central to advancing electrocatalysis yet remains poorly resolved in complex oxides. Here we develop an in situ titration platform integrating atomic layer-by-layer titration with reactivity quantification to dissect elevated-temperature oxygen incorporation reactions on (La 0.5 Sr 0.5 )FeO 3− δ . We reveal a volcano-shaped correlation between oxygen incorporation reaction activity and SrO termination layers, and find that a single-layer SrO termination maximizes performance. Microkinetic modelling and theoretical calculations reveal a rate-determining step shift with surface termination. The single-layer SrO termination optimally balances oxygen dissociation, incorporation and subsurface diffusion by modulating charge transfer and steric constraints. This platform paves the way to quantitatively correlate catalytic activity with surface atomic structures, offering atomically precise surface engineering methodologies for designing high-performance electrocatalysts in energy and environmental applications.