Theoretical Study of Electrochemical Processes on Pt-Ni Alloys

We have carried out an extensive computational study using periodic density functional theory of the structure, reactivity, and stability of three different Pt-Ni alloys, Pt3Ni, PtNi, and PtNi3, with the aim of determining the effect of the subsurface layer composition on the catalytic activity of the platinum surface. The segregation effect was taken into account by modeling slabs with a platinum surface monolayer supported on a second layer containing 50%, 100%, and 75% of nickel, respectively, with a bulk layer below. Calculated equilibrium adsorption potentials for the oxygen reduction reaction (ORR) intermediates and construction of free energy diagrams for the ORR dissociative mechanism are used to gauge the catalytic activity. The critical question of the stability of these materials in an aqueous environment is also assessed in terms of the relative shifts in electrochemical dissolution energies and by the identification of the most stable state of the surface as a function of pH and potential as illustrated in Pourbaix diagrams. The (111) surface of all three models of Pt-Ni alloys is found to exhibit improved oxygen reduction activity compared with that of pure Pt(111). The ORR overpotential was calculated to decrease in the order Pt (0.55 V) > Pt3Ni (0.24 V) > PtNi3 (0.19 V) > PtNi (0.15 V). We can therefore conclude that the catalytic activity for ORR will increase as Pt < Pt3Ni < PtNi3 < PtNi and find that the largest improvement occurs for a PtNi alloy with 100% nickel in the second layer. We also predict that PtNi is the least susceptible to corrosion at similar pH and cell potentials based on the calculated shifts of the electrochemical dissolution potentials for the Pt-Ni alloys relative to platinum with values of -0.27 V for PtNi3, +0.13 V for Pt3Ni, and +0.30 V for PtNi.