Lateral Size Governs Hydrogen Evolution Reaction Mechanisms and Kinetics of Ultrathin Pd Nanosheets in Acidic and Alkaline Electrolytes.

Understanding the influence of nanoscale geometry on hydrogen evolution reaction (HER) mechanisms and kinetics is crucial for rational catalyst design. Here, we construct a well-defined model system of ultrathin Pd nanosheets with controlled lateral sizes (ca. 11 to 44 nm) and comparable thickness to systematically probe lateral size effects on HER kinetics in different electrolytes. Electrochemical impedance spectroscopy (EIS) was employed to analyze size-dependent interfacial kinetics and reaction mechanisms. In an acidic electrolyte, HER proceeds through a Volmer-Heyrovsky-dominated mechanism with a parallel Volmer-Tafel pathway, where lateral size influences both the intrinsic kinetics and the relative kinetic contribution. In alkaline electrolytes, HER kinetics are dominated by adsorption-related interfacial processes, and the lateral size affects the interfacial adsorption resistance. These results demonstrate that lateral size, a key geometric parameter, regulates HER mechanisms and kinetics under both acidic and alkaline conditions.