Hydrogen-evolution and -oxidation on Ag(1 1 1) covered by Pt

Physical vapour deposited Pt on Ag(1 1 1) forms inhomogeneous films, leaving access to the Ag(1 1 1) surface, as proven by electroreflectance. The overpotential of the hydrogen evolution reaction at Ag(1 1 1) by water splitting in neutral perchlorate and sulphate electrolytes and by H3O+ discharge in acidic perchlorate and sulphate electrolytes is considerably reduced by the Pt coverage, but does not vanish. The transfer coefficient for water splitting is more than 50% lower than for H3O+ discharge. In H2-saturated neutral electrolytes, the potential gap between hydrogen evolution by water splitting and H2-oxidation decreases with the increasing number of Pt monolayers. In acidic electrolytes, this gap does not exist. Here, the absolute current densities of H+-reduction and H2-oxidation grow with the number of Pt monolayers. In the neutral electrolytes, sulphate strongly inhibits the H2-oxidation (by a factor of about 25 with respect to perchlorate).