Interfacial Water‐Induced Reaction Pathway Reorientation via Support Engineering in Acidic Oxygen Evolution Reaction

The activity–stability trade‐off of acidic oxygen evolution reaction (OER) restricts proton exchange membrane water electrolysis (PEMWE). Lattice oxygen mechanism (LOM) delivers high activity yet causes catalyst dissolution via oxygen loss, so switching to adsorbate evolution mechanism (AEM) is critical for long‐term stability. While extensive studies tune electronic/coordination structures to adjust intermediate kinetics, the structural effect of interfacial water as a reactant is rarely studied. Herein, we anchor RuO 2 nanoparticles on MnO 2 (MnO 2 /RuO 2 ) and 0.29 wt.% trace‐Cr‐doped MnO 2 (Cr‐MnO 2 /RuO 2 ), yielding Ru sites with nearly identical electronic structures but different support‐regulated microenvironments. With similar active site density and surface area, Cr‐MnO 2 /RuO 2 exhibits 193 mV lower overpotential, 65.5‐fold mass activity (406 A g Ru −1 ) vs commercial RuO 2 , and exceptional cell durability (>1000 h @ 1 A cm −2 ) at only 0.478 mg cm −2 Ru loading. Theoretical calculations and in situ characterizations verify that Cr‐doping‐derived oxygen vacancies create localized positive potential traps on supports, enriching O‐down‐oriented water and loosening rigid hydrogen‐bond networks to boost proton transfer. Hence, the O‐down water promotes nucleophilic attack to generate abundant * OOH, driving a LOM‐to‐AEM shift to improve the durability. This work underscores interfacial water's dominant role in governing electrocatalytic pathways, offering a strategy for stable catalyst design via support microenvironment regulation.