Efficient Hydroxyl Diffusion Triggers Surface Hydronium Enrichment and Bulk Proton Participation to Boost Alkaline Hydrogen‐Evolving Reaction

ABSTRACT Deciphering the diffusion, induction, and reaction processes of key chemical species in the catalyst lattice is critical for solution‐phase electrochemical applications, yet remains underexplored. Taking the alkaline hydrogen‐evolving reaction (HER) in an anion‐exchange‐membrane (AEM) electrolyzer as an example, prior efforts were devoted to optimizing water dissociation and proton recombination steps on catalyst surfaces, neglecting the important role of bulk electrochemistry induced by abundant OH − in the electrolyte. As a proof‐of‐concept, we design oxygen‐vacancy‐ordered and oxygen‐vacancy‐disordered model oxides to explore the bulk electrochemistry triggered by OH − diffusion. Combined systematic experiments and computations reveal that the ordered and high‐concentration features of oxygen vacancies improve the mobility and flux of OH − diffusion into the bulk lattice, respectively. Multiple operando characterizations demonstrate that efficient bulk OH − diffusion lowers surface OH − concentration and thus drives the water ionization equilibrium toward products (2H 2 O ↔ H 3 O + + OH − ) following Le Chatelier's principle, contributing to enriched surface H 3 O + and enhanced surface HER kinetics. Interestingly, due to the electrostatic interactions, bulk OH − diffusion behavior triggers surface‐to‐bulk proton migration and participation, extending HER regions from the surface to the bulk and thus boosting HER activity. The high OH − diffusion capability of the cathode also greatly improves the AEM‐electrolyzer performance. Our work offers new insights into the long‐overlooked bulk electrochemistry.