Size-Dependent Dynamic Surface States of Ru Clusters: Modulating Interfacial Water Structure for Enhanced Alkaline Hydrogen Evolution Reaction

Abstract

Constructing well-defined atomic structures is a pivotal strategy for enhancing catalytic performance and metal utilization. In this context, sub-nanometer clusters, bridging the gap between single atoms and nanoparticles, have attracted considerable interest by offering distinct electronic structures with efficient metal utilization. However, the dynamic surface states of active sites under electrochemical conditions have often been overlooked. This is particularly critical for sub-nanometer clusters, where the abundance of low-coordinated metal atoms leads to distinct surface behavior. Herein, we elucidate the dynamic surface behavior of Ru catalysts by probing size-dependent oxidation states under alkaline hydrogen evolution reaction (HER) conditions, ranging from atomically dispersed structures to sub-nanometer and larger Ru clusters. By combining in situ X-ray absorption spectroscopy and in situ attenuated total reflectance surface-enhanced infrared absorption spectroscopy, we reveal that sub-nanometer Ru clusters sustain partially oxidized surface states even under reductive reaction conditions, accompanied by pronounced modulation of the interfacial water structure. Benefiting from the dynamically stable oxidized surface states, ~1 nm Ru sub-nanometer clusters achieve an industrially relevant current density of 5.34 A cm-2 at 2.0 V even at a low operating temperature of 50 ℃ in an anion exchange membrane water electrolyzer. Notably, even with the Department of Energy (DOE) ultimate target loading, the designed catalyst exceeds the 2026 DOE performance target. These findings highlight the critical role of dynamic surface states in cluster catalysts and demonstrate that tuning the atomic structure of Ru provides an effective strategy to optimize interfacial water structure for advanced HER electrocatalysts.