Reversible redox reaction and water configuration on a positively charged platinum surface: first principles molecular dynamics simulation

The water dissociation reaction and water molecule configuration on a positively charged platinum (111) surface were investigated by means of first principles molecular dynamics under periodic boundary conditions. Water molecules on the Pt surface were mostly in the O-down orientation but some H-down structures were also found. OH− ion, generated by removing H from H2O in the bulk region, moved to the Pt surface, on which a positive charge is induced, by a Grotthuss-like proton-relay mechanism and adsorbed on it as OH(Pt). Hydrogen atom exchange between OH(Pt) and a near-by water molecule frequently occurred on the Pt surface and had a low activation energy of the same order as room temperature energy. When a positive charge (7 μC cm−2) was added to the Pt surface, H3O+ and OH(Pt) were generated from 2H2O on the Pt. This may be coupled with an electron transfer to the Pt electrode [2H2O → H3O+ + OH(Pt) + e−]. The opposite reaction was also observed on the same charged surface during a simulation of duration about 10 ps; it is a reversible redox reaction. When further positive charge (14 μC cm−2) was added, the reaction shifted to the right hand side completely. Thus, this one-electron transfer reaction, which is a part of the oxygen electrode reaction in fuel cells and water electrolysis, was confirmed to be a low activation energy process.