Mechanism for Oxygen Reduction Reaction on Pt_3Ni Alloy Fuel Cell Cathode

We use quantum mechanics, density functional theory at the PBE level, to predict the binding-site preferences and reaction barriers for all intermediates involved in the oxygen reduction reaction (ORR) on the low energy surface of Pt_3Ni alloy. Here we calculate that the surface layer is Ni depleted (100% Pt) while the second layer is Ni enriched (50% Pt) as shown by experiment. Even though the top layer is pure Pt, we find that the sublayer Ni imposes strong preferences in binding sites for most intermediates, which in turn strongly influences the reaction barriers. This strong preference leads to a strong site dependence of the barriers. Considering water as the solvent, we predict that, at low coverage of O_(ad) and OH_(ad), the barrier for the rate-determining step is 0.81 eV, whereas, at high coverage, this barrier decreases to 0.43 eV. It can be compared to a barrier of 0.50 eV for pure Pt, explaining the improved ORR rate for the Pt_3Ni alloy. We report the results both for gas phase and for aqueous phase environments.