On the pH-Dependence of the H_upd Peak of Pt-Group Nanoparticles

Understanding the electrochemical behavior of hydrogen adsorption at Pt-group metal surfaces, particularly in the context of non-well-defined nanoparticle surfaces, is crucial for advancing electrocatalytic applications such as the hydrogen evolution reaction (HER). This study investigates the non-Nernstian pH shifts observed for underpotential deposited H upd -like cyclic voltammetry peaks on Pt, Ir, Pd, and Rh nanoparticles. Utilizing density functional theory calculations, we explore the potential-dependent stability of H and OH adsorbates at undercoordinated surface sites, emphasizing the role of non-ideal electrosorption valencies in these shifts. Our results support that the peaks arise predominantly from a direct H-OH replacement process and suggest the primary influence of partial charge transfer. The theoretical predictions show good agreement with experimental observations across various Pt-group metals, even on non-well-defined surfaces, and provide insights into cation-specific effects at Pt across the entire pH scale. This work not only clarifies the origin of the H upd -like peak within the water stability region but also offers a foundation for understanding cation effects in HER kinetics, paving the way for more detailed analyses of cation type, concentration, and interfacial solvent structure.