Abstract Electrocatalytic hydrogen oxidation reaction (HOR) on Pt depends strongly on pH of electrolyte; its kinetics is orders of magnitude slower in alkaline solutions referring to that in acid solutions. Recent studies reveal that interfacial water plays an important role in HOR on Pt, whether in acid or alkaline media. In this study, by measuring the ratio of mass change to charge change involved in oxidation of Hupd/H2/D2 on Pt (and also on PtRu) in acid and alkaline media, we discovered that the oxidative desorption of adsorbed hydrogen (Had) off Pt is coupled with adsorption of water molecules on Pt in acid solutions, whereas it occurs directly without obvious ad/desorption of H2O or OH- up to ca. 0.15 V (vs. RHE) in alkaline solutions. AIMD calculations further reveal that the dynamic processes of interfacial water reorientation and adsorption critically modulate the energy barrier of oxidative desorption of Had. In acidic environments the interfacial water demonstrates rotational flexibility, which actively interact with Had on the Pt surface, providing a favorable energy motif for Had stripping off Pt. In comparison, in alkaline solutions, owing to dielectric saturation, re-orientation and interaction of the interfacial water with Had suffers from higher energy barrier, constituting a key mechanistic origin of the sluggish alkaline HOR kinetics on Pt electrode.
Zhou et al. (Wed,) studied this question.