Free Hydroxyl-Boosted Electrocatalytic Oxygen Evolution through Reorganizing the Interfacial Water Structure

The structure and dynamics of interfacial water are pivotal in governing the kinetics and stability of electrocatalytic reactions, yet their targeted manipulation to resolve the persistent activity‒stability trade-off in acidic oxygen evolution reactions (OERs) remains a formidable challenge because of the elusive relationships among water networks, intermediates, and metal-site states. Herein, we developed a rational template-mediated Cr-promoted oxidation method to synthesize ultrafine Cr-doped RuO 2 nanoparticles with precision. Cr dopants anomalously reside in an octahedral environment analogous to that of corundum-type Cr 2 O 3 within the host rutile-type RuO 2, a configuration stabilized by its favorable oxidation state and well-matched ionic radius. By use of advanced multimodal in situ spectroscopy, we decoupled the co-evolution of metallic active sites and the interfacial aqueous environment. We reveal that the surface high-valence Lewis-acidic Cr sites deliberately reorient interfacial water into an oxygen-down configuration, generating an abundance of weakly hydrogen-bonded, pre-activated free hydroxyl (Free-OH) water—an interfacial motif in oxygen electrocatalysis. This Free-OH-rich environment supplies ample *OH intermediates for direct O−O coupling via the oxide path mechanism. Importantly, the potential-driven heterolytic cleavage of water at hydroxyl-trapping Cr sites dynamically supplies electrons to adjacent Ru sites, preventing their over-oxidation and ensuring structural stability. This work demonstrates interfacial water engineering as a transformative strategy to transcend conventional catalyst-centric design for stable and efficient acidic OER.