Beyond the Volcano: Kinetic Insights into the Hydrogen Evolution Reaction on Platinum

The hydrogen evolution reaction (HER) is a cornerstone of electrochemical water splitting. Platinum’s exceptional activity is conventionally attributed to its optimal hydrogen adsorption free energy (ΔGH ∼ 0), which places it at the apex of the theoretical volcano plot. While this thermodynamic descriptor successfully identifies promising catalyst families, it often fails to distinguish top performers, as numerous candidates with similar ΔGH values significantly underperform Pt. Herein, we employ constant-potential density functional theory calculations to unravel the atomic-scale origin of Pt’s unique performance. We demonstrate that Pt, along with Ir and Au, exhibits distinct potential energy surfaces for hydrogen adsorption, enabling stable binding at on-top sites due to relativistic effect─a feature less pronounced in 3d and 4d metals. This distinct chemistry leads to a breakdown of the Brønsted–Evans–Polanyi relationship for the Volmer, Heyrovsky, and Tafel steps. Crucially, the stable on-top site drastically lowers the activation barrier for the Volmer step and facilitates high hydrogen coverages, which subsequently accelerate H–H bond formation via the Tafel reaction. Consequently, Pt follows an efficient Volmer–Tafel pathway with uniquely low kinetic barriers. Our work reveals that Pt’s supreme activity is not fully captured by thermodynamic scaling relations but is ultimately governed by its distinctive kinetic profile. These findings suggest that complementing thermodynamic descriptors with kinetic insights can provide a more complete blueprint for the rational design of next-generation electrocatalysts.