Rate and Mechanism of Electrochemical Formation of Surface-Bound Hydrogen on Pt(111) Single Crystals.

The formation of surface-bound hydrogen from one proton and one electron plays an enabling role in renewable hydrogen production. Quantifying the surface-bound hydrogen formation, however, requires decoupling the delicate interplay of numerous processes. We study cyclic voltammetry (CV) at fast scan rates to characterize the rate constant for the surface-bound hydrogen formation (also known as underpotential deposition hydrogen, UPD H). We find that the formation of H on Pt(111) single crystals is ∼100× faster in acid than in base. Reaction-order analysis indicates that the formation of H occurs as a standard proton-coupled electron transfer (PCET) reaction in acid, whereas in base, it displays a pH-independent rate constant, indicating the presence of a chemical step such as the reorganization of interfacial water. Our results provide a methodology for quantifying the interfacial PCET kinetics and reveal the mechanistic nature of the UPD H formation as the reason the hydrogen evolution electrocatalysis on Pt is faster in acid than in base.