Effect of Solvation on the Oxygen Reduction Reaction on Pt Catalyst

Previous models of PEM fuel cells have not explicitly considered the eflect of a bulk water layer on the mechanism of the oxygen reduction reaction. The dominant center dot O2H3O++e(-) -> HO2 -> HO + O + H2O PtO2H -> PtO + PtOH, z PtO + H+ -> e(-) -> PtOH and center dot 2H(2)O + O-2 -> H2O + OH + HO2 -> 2OH + H2O2 PtOH + H+ + e(-) -> PtOH2. Using the B3LYP functional with the LANL2DZ basis set and effective core potential on Pt and the CC-pVDZ basis set on O and H with two clusters, the first a planar Pt-10 cluster and the second a three layer Pt-20 cluster, we have considered the effect of a solvation model consisting of two stacked hexagonal water structures with another water in the center above a hexagon of surface pt atoms. We find that dissociation of O2H is much less favorable on the water covered surface as compared to the bare surface unless at least one water molecule is removed to create two adjacent surface Pt atoms. We find an H2O center dot O-2 complex, where one H2O has been turned so as to form a hydrogen bond with the O-2, and this complex can be viewed as the branch point in the mechanism O-2 can be reduced to HO2, which can dissociate to OH plus O if at least one water is removed from the surface. Alternatively, we find a new low energy pathway: H2O + O-2 -> OH + HO2, HO2 + H2O -> OH, and H2O2 -> OH + OH. This reaction sequence is found to be all downhill or thermoneutral. We believe both of these pathways are involved in the oxygen reduction reduction in PEM fuel cells