Surface Atomic Arrangements on High-Entropy-Alloy Nanocrystals Govern Alkaline Hydrogen Oxidation via Symmetry-Selected Multi-Atom Ensembles

Abstract High-entropy alloys (HEAs) provide compositionally diverse surfaces for electrocatalysis, yet how their surface atomic arrangements dictate active-site ensembles and catalytic activity remains unclear. Here, we develop a dropwise deposition strategy to synthesize Pd@Pd0.2Pt0.2Ir0.2Ru0.2Rh0.2 core–shell nanocrystals with comparable shell compositions but distinct surface atomic arrangements, including square {100} facets, hexagonal {111} facets, and twin-interrupted {111} facets. Among them, {111}-enclosed nanooctahedra exhibit the highest alkaline hydrogen oxidation reaction (HOR) activity, delivering a 4.18-fold enhancement over commercial Pt/C at 0.10 VRHE. In situ synchrotron X-ray absorption spectroscopy and ambient-pressure X-ray photoelectron spectroscopy reveal potential-dependent interactions between Ru and OH*, accompanied by electronic-structure coupling among Pd, Pt, Ir, and Rh during HOR. Density functional theory and climbing-image nudged elastic band calculations show that the hexagonal {111} symmetry optimizes H*/OH* adsorption, H* spillover, and H2O formation through symmetry-selected multi-atom ensembles. These elementary processes lower the Volmer-step barrier, supporting a Volmer-influenced pathway. Experimental and computational insights demonstrate that HEA surface atomic arrangements govern alkaline HOR kinetics. Unlike conventional mono- or bimetallic catalysts with limited active-site motifs, the HEA {111} surface provides a distributed ensemble network with varied adsorption strengths, establishing surface atomic arrangement and active multi-atom ensembles as key descriptors for high-performance HEA electrocatalysts.