Computationally Guided Design of Facet-Defined Multielement Active Sites on Hexagonal Close-Packed High-Entropy-Alloy Nanosheets for Alkaline Hydrogen Evolution

Nonconventional hexagonal close-packed (HCP) high-entropy alloys (HEAs) provide distinct catalytic surfaces, but their synthesis with well-defined facets remains challenging, leaving facet-dependent multielement ensemble effects largely unexplored. Here, we report the computationally guided realization of atomically thin RuRhIrPtPd HEA nanosheets with predominantly exposed HCP {001} facets for the alkaline hydrogen evolution reaction (HER). Density functional theory (DFT) calculations identify HCP {001} as the most favorable facet over {100} and {110}, where near-thermoneutral hydrogen adsorption, narrow energy distribution, and balanced hydrogen/hydroxyl binding create an optimized alkaline HER landscape. Guided by this prediction, RuRhIrPtPd HEA atomic layers are epitaxially grown on HCP Ru nanosheets to maximize {001} exposure, with atomic-resolution electron microscopy and synchrotron X-ray absorption spectroscopy (XAS) confirming atomically mixed HEA layers. Electrochemical measurements demonstrate that the {001}-faceted HEA nanosheets exhibit higher alkaline HER activity and durability than irregular-faceted nanocrystal counterparts and commercial Pt/C. Climbing-image nudged elastic band (CI-NEB) calculations, operando XAS, and CO-stripping measurements identify Ru-centered ensembles as favorable Volmer sites, showing the lowest water-dissociation barrier of 1.04 eV compared with 1.13–1.47 eV for other metal sites, while neighboring multimetallic sites balance hydrogen and hydroxyl adsorption. This work provides a structural blueprint for harnessing facet-defined multielement ensembles to design high-entropy electrocatalysts.