Revealing Dynamic Structural Changes in Platinum-Group-Metal Nanoparticles during the Catalytic Hydrogen Evolution Reaction.

Despite the crucial role of structural dynamics in catalytic reactions, the influence of nanoparticle (NP) size and elemental composition on these dynamics under electrochemical conditions is often overlooked. Herein, we present the first systematic operando investigation of the size- and element-dependent structural and electronic dynamics of platinum-group-metal (PGM) NPs (Pt, Pd, Rh, and Ru) during the hydrogen evolution reaction (HER) under both acidic and alkaline conditions. By combining operando powder X-ray diffraction and X-ray absorption spectroscopy, we revealed that the smaller the NPs, the more significant the structural changes, indicating surface adsorbate-induced dynamics. Notably, we also found that the constant lattice expansion in Pt, Pd, and Rh NPs occurred on a slow time scale (several tens of seconds). The electronic states of these NPs were more reductive than their standard zerovalent states owing to interactions with hydrogen. However, the Ru NPs remained oxidized even during the HER, indicating distinct electronic changes in the PGMs. These findings reveal previously unrecognized dynamics, providing a comprehensive understanding of how size and composition influence the structural and electronic changes of PGM NPs in realistic electrochemical reaction environments. This study establishes a foundational framework for operando catalyst investigations and opens new avenues for the rational design of size- and element-optimized HER catalysts.