How Does the Surface Structure of Pt-Ni Alloys Control Water and Hydrogen Peroxide Formation?

Polymer electrolyte membrane fuel cells have been studied for more than three decades as promising clean energy converters for automotive applications. However, improving their durability and reducing the catalyst cost (platinum loading) are ongoing challenges. Alloys are often proposed as an alternative solution which combines a good catalytic activity toward the oxygen reduction reaction and a decreased platinum content. In this work, we address density functional theory calculations for the formation of water and hydrogen peroxide on three different Pt3Ni(111) alloy model surfaces: bulk-truncated, Pt-skin, and Pt-skeleton terminations, in comparison with Pt(111). From a low-coverage Gibbs free energy analysis, the prediction of the activity order between all the catalysts agrees with electrochemical measurements: Pt-skin > Pt-skeleton > Pt(111) > bulk-truncated Pt3Ni(111). The superior activity of Pt-skin has been explained in terms of thermodynamic and kinetic properties and through an energy decomposition analysis. A strong loss of stability for atomic oxygen and a significant decrease of the barrier to form hydroxyl induce more competitive transformation routes on this surface. The intermediate activity of Pt-skeleton has been linked to similar thermodynamic properties (at least for OH formation) and a rather moderate lowering of the activation barriers of oxygen dissociation (morphology effect) and OH formation. Hence, the combined effects of coordination loss for surface Pt atoms (Pt skeleton) and the indirect effect of Ni (Pt-skin) are promoting the activity, with respect to Pt(111). In contrast, the direct effect from surface Ni atoms in the bulk-truncated surface is rather inhibiting. The expected selectivity to water is preferential, Pt skeleton being more selective than the three other surfaces.