From Proximity to Energetics: Unveiling the Hidden Compass of Hydrogen Evolution Reaction

The Volmer–Tafel (VT) and Volmer–Heyrovsky (VH) mechanisms were believed to be determined solely by the proximity of adsorbed hydrogen atoms on metallic surfaces for the hydrogen evolution reaction (HER). However, recent investigations challenge this notion, particularly with catalysts such as Au and Ag, where VH pathways are observed despite the close hydrogen atom distance. This study investigates the influence of free energy on hydrogen adsorption (Δ G H * ) and active site density on HER pathways, incorporating the consideration of the rate-determining step (RDS). Contrary to previous assumptions, it is found that Δ G H * plays a pivotal role, with VT pathways favored when Δ G H * approaches zero, and while VH pathways occurs in other cases, irrespective of active site density. This inclusive analysis, integrating both thermodynamic and energetic considerations with experimental and theoretical support, sheds new light on the mechanistic intricacies of the HER, challenging conventional paradigms and providing insights that are crucial for catalyst design.