Hydrogen Evolution on FTO-Supported Ptn Clusters: Ensemble of Hydride States Governs the Size Dependent Reactivity

We report the size-dependent activity and stability of FTO-supported Pt1,4,7,8 for electrocatalytic hydrogen evolution reaction, and show that clusters are significantly more active than polycrystalline Pt, but also have stability under HER conditions that is size-dependent. To understand the mechanistic origin of the size effects, we carried out detailed DFT-based theory, accounting for the structural fluxionality under varying potentials. We show that, even under coverage of the simplest adsorbate H, the clusters can undergo drastic changes and populate a grand canonical ensemble of hydride states with diverse distributions of stoichiometry, structure, and thus reactivity. Both experiment and theory find that electrocatalysis mainly results from heavily hydrogenated states of the clusters (~2 H/Pt), and an ensemble-based kinetic model reproduces the experimental activity trend and provides detailed insights into the contributions from metastable configurations. The size-dependent stability trend is rationalized by chemical bonding analysis. The comparison of experiment and theory demonstrates the potential- and adsorbate-coverage-dependent fluxionality of subnano clusters of different sizes and offers a systematic modeling strategy to tackle the complexities.