pH-dependent interaction of interfacial water with Pt in the electrocatalytic hydrogen oxidation reaction

Abstract

The electrocatalytic hydrogen oxidation reaction on Pt is orders of magnitude slower in alkaline solutions than in acidic solutions. Here, by measuring the ratio of mass change to charge change for the oxidation of adsorbed hydrogen (Had) in Ar/H2/D2, we show that the oxidative desorption of Had off Pt is coupled with the adsorption of water molecules on Pt in acidic solutions. Conversely, it occurs directly without obvious ad/desorption of H2O or OH− up to approximately 0.12 V versus the reversible hydrogen electrode in alkaline solutions. Ab initio molecular dynamics simulations reveal that the interfacial water in acidic environments demonstrates rotational flexibility, which can serve as a proton acceptor assisting in the stripping of Had from Pt. By contrast, in alkaline solutions, owing to dielectric saturation, the reorientation of interfacial water and its interaction with Had suffer from a higher energy barrier, constituting a key mechanistic origin of the sluggish alkaline hydrogen oxidation reaction kinetics on Pt electrodes. The electrocatalytic hydrogen oxidation reaction (HOR) on Pt is orders of magnitude slower in alkaline media compared with acidic media. Here, by measuring the mass–charge change at the electrode during the HOR, the simultaneous adsorption of water molecules during desorption of hydrogen is observed only under acidic conditions.