Potential-Dependent Oxygenated Surface Phases and Interfacial Water Layers Underlie the High Overpotential and...

Precisely deciphering the intrinsic origin of the high overpotential for oxygen evolution reaction (OER), even on the most active RuO₂ catalysts, remains a long-standing challenge in electrocatalysis. Herein, by meticulously elucidating the electrode charging behavior, oxygenated surface phases and interfacial double-layer structures under OER-relevant potentials on RuO₂(110), together with their impact on reaction pathways and elementary-step energetics through ab-initio molecular dynamics simulations, we reveal that the high overpotential jointly arises from the pronounced surface negative charge, due to the unusually high potential of zero charge, and the excessive protonation of surface-active *O at coordinatively unsaturated Ru sites (*O_CUS) at low potentials (<1.60 V). This, on one hand, severely depletes active *O_CUS intermediate, thereby suppressing the rate-determining step (RDS) of oxide pathway mechanism (OPM), necessarily involving surface O─O coupling between two *O_CUS via Langmuir-Hinshelwood mechanism. On the other hand, it induces the dense, strongly hydrogen-bonded interfacial water layer that, together with electrostatic repulsion, obstructs the essential water reorientation and approach for the RDS of adsorbate evolution mechanism (AEM), featuring incoming interfacial water to reorient and react with *O_CUS via Eley-Rideal-like mechanism. Furthermore, a potential-dependent mechanistic switching between AEM and OPM is identified, dictated by their distinct RDS natures and kinetic sensitivities.