Is Platinum a Proton Blocking Catalyst?

Platinum is the benchmark electrocatalyst for the hydrogen evolution reaction (HER) in acidic environments, largely due to its near-optimal adsorption energetics. Historically, it has been regarded as a proton-blocking material, operating under the assumption that catalyst-hydrogen interactions are strictly confined to the surface. Here, we challenge this surface-centric paradigm by investigating the bulk interactions of Pt with hydrogen (H) and its heavier isotope, deuterium (D). By employing operando electrochemical quartz crystal microbalance, we monitored real-time mass evolution of the Pt electrode during galvanostatic water and heavy- water splitting. Unexpectedly, we observed a significant and irreversible temporal mass gain alongside distinct shifts in the reaction’s overpotential. We show that this arises from deep diffusion of H and D into the bulk Pt lattice, corroborated by atom probe tomography and thermal desorption spectroscopy. We observed a sub-surface concentration of at least 15 at. % of D inside Pt, extending to depths exceeding 10 nm. We quantified the diffusion coefficient of D in Pt at (3.2±0.05)×10−18 cm2∙s−1. Density functional theory calculations support these observations, predicting formation of interstitial hydrogen solid solutions with sub-surface concentrations peaking near 33 at. % under cathodic polarization. These findings challenge the long-held assumption of Pt-proton interaction being limited to the surface, necessitating the expansion of catalyst design strategies to account for property-modifying bulk diffusion of H/D in the Pt matrix.