Theoretical Study of Early Steps in Corrosion of Pt and Pt/Co Alloy Electrodes

Theoretical studies of Pt and PtCo alloy electrode dissolutions in PEMFCs were made using adsorption energies from VASP slab band calculations on the metal surfaces in a linear Gibbs energy relationship. The reversible potential for Co-OH formation from oxidation of H2O on the Pt3Co(111) surface was calculated to be 0.50 V, which is significantly lower than the calculated 0.70 V reversible potential for Pt-OH formation on the Pt(111) surface. Co(OH)(2) formation from oxidation of H2O on the Pt3Co(111) surface was calculated to take place at a lower potential than Pt(OH)2 formation on the Pt(111) surface, with the respective reversible potentials 0.58 and 1.92 V. These results for what may be the initial steps of anodic dissolution indicate that Co will dissolve at lower potential than Pt in the alloy surface and leave behind a Pt surface skin when the potential is > similar to 0.50 V. In addition, the reversible potential for OH formation on a step of edge atom on Pt(111) surface from oxidation of H2O was calculated to be 0.60 V, which is 0.10 V lower than reversible potential on the terrace sites, which implies that edge atoms are removed first. The final step of dissolution is believed to be the hydrolysis of the Pt2+ hydroxide, which considering the small equilibrium constant reported by Pourbaix, should proceed slowly.