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

Uncovering atomic-scale structure-activity relationships in electrocatalysis demands capability of simultaneously manipulating and probing surfaces under operando conditions. To this end, we developed an in situ electrochemical platform integrating atomic layer-by-layer titration with reactivity quantification, which we applied to dissect elevated-temperature oxygen incorporation reactions (OIR) on (La0.5Sr0.5)FeO3-δ surfaces. We reveal a volcano-shaped correlation between OIR activity and SrO termination layers, where a single-layer SrO termination was visually confrimed to maximizes performance. Microkinetic modeling and theoretical calculations reveal a rate-determining step (RDS) shift with surface termination. The single-layer SrO termination optimally balances oxygen dissociation, incorporation, and subsurface diffusion by modulating interfacial charge transfer and steric constrains. This platform paves the way to directly correlate catalytic activity with surface atomic-level structures, offering atomically surface engineering methodologies for designing high-performance electrocatalysts in energy conversion systems.