The atomistic origin of the extraordinary oxygen reduction activity of Pt₃Ni₇ fuel cell catalysts

Recently Debe et al. reported that Pt₃Ni₇ leads to extraordinary Oxygen Reduction Reaction (ORR) activity. However, several reports show that hardly any Ni remains in the layers of the catalysts close to the surface ("Pt-skin effect"). This paradox that Ni is essential to the high catalytic activity with the peak ORR activity at Pt₃Ni₇ while little or no Ni remains close to the surface is explained here using large-scale first-principles-based simulations. We make the radical assumption that processing Pt-Ni catalysts under ORR conditions would leach out all Ni accessible to the solvent. To simulate this process we use the ReaxFF reactive force field, starting with random alloy particles ranging from 50% Ni to 90% Ni and containing up to ∼300000 atoms, deleting the Ni atoms, and equilibrating the resulting structures. We find that the Pt₃Ni₇ case and a final particle radius around 7.5 nm lead to internal voids in communication with the exterior, doubling the external surface footprint, in fair agreement with experiment. Then we examine the surface character of these nanoporous systems and find that a prominent feature in the surface of the de-alloyed particles is a rhombic structure involving 4 surface atoms which is crystalline-like but under-coordinated. Using density-functional theory, we calculate the energy barriers of ORR steps on Pt nanoporous catalysts, focusing on the O_ad-hydration reaction (O_ad + H₂O_ad → OH_ad + OH_ad) but including the barriers of O₂ dissociation (O_2ad → O_ad + O_ad) and water formation (OH_ad + H_ad → H₂O_ad). We find that the reaction barrier for the O_ad-hydration rate-determining-step is reduced significantly on the de-alloyed surface sites compared to Pt(111). Moreover we find that these active sites are prevalent on the surface of particles de-alloyed from a Pt-Ni 30:70 initial composition. These simulations explain the peak in surface reactivity at Pt₃Ni₇, and provide a rational guide to use for further optimization of improved catalytic and nanoporous materials.