The roles of interfacial water in electrocatalytic water splitting across diverse electrolyte environments

Abstract Electrocatalytic water splitting is central to green hydrogen production, yet its efficiency is increasingly governed by both active-site energetics and the nanoscale water layer at the electrode-electrolyte interface. Interfacial water directly influences water dissociation, proton and hydroxide transport, and catalyst stability. Its mechanistic roles, however, are fundamentally electrolyte-dependent, varying across alkaline, acidic, and neutral conditions and further diverging between the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), with seawater serving as a representative case in which additional ionic perturbations are superimposed on the alkaline and neutral baselines. In this review, we systematically examine how the distinct functions of interfacial water vary with the electrolyte environment. For each, we analyze how the electrolyte chemistry dictates the core function of interfacial water, survey the corresponding catalyst design strategies, and identify the mechanistic principles linking interfacial water structure to catalytic performance. We emphasize that an effective strategy in one environment may be irrelevant in another, and that catalyst design must target the electrolyte-specific roles of interfacial water rather than following a universal approach. We conclude by outlining a forward-looking agenda that spans standardized in situ characterization, electrolyte-specific quantitative descriptors, and device-level translation, and by distilling three prioritized research directions for converting these electrolyte-specific principles into practical catalyst design.