Density Functional Theory Study of Anode Reactions on Pt-Based Alloy Electrodes

DFT calculations have been applied to investigate reaction mechanisms occurring on anode catalysts of polymer electrolyte fuel cells. This Article consists of three parts. First, CO and H2 adsorption was investigated using Pt-based alloy electrodes modeled by Pt5M5 (M = V, Cr, Fe, Co, Ni, Mo, Ru, Rh, Pd, Ag, Sn, W, and Au) clusters. The most durable catalysts for CO poisoning were searched using two criteria: (1) adsorption of CO should be weakened as compared to pure Pt, and (2) the transition state (TS) energy for H−H bond fission of adsorbed H2 should not be higher than that for pure Pt. Pt−Ru alloy and a few candidates were found. Second, periodic DFT calculations were carried out to explain the surface segregation tendency of one component element over the other for the nine alloys, Pt−Fe, Pt−Co, Pt−Ni, Pt−Mo, Pt−Ru, Pt−Rh, Pt−Pd, Pt−Ag, and Pt−Au. For example, for Pt−Ru alloy, Pt atom was shown to have higher potential for surface segregation, but for Pt−Ag alloy, Ag atom had higher potential. Finally, in relation to the CO elimination mechanism, the energetics of four typical reactions were investigated using extended cluster models and including the solvation effects: (1) COad + H2Oad + H2O → COOHad + H3O+, (2) H2Oad + H2O → OHad + H3O+, (3) COOHad + OHad → (CO2)ad + H2Oad, and (4) COad + OHad + H2O → COOHad + H2O → (CO2)ad + H3O+. Individual reactions were compared among Pt metal and Pt and Ru sites of Pt−Ru alloy. Reaction 1 is most preferable on Pt sites of Pt−Ru alloy, whereas the disproportionation of water (reaction 2) is most favorable for pure Pt and then Ru sites of the alloy, which is consistent with the so-called bifunctional mechanism. Reactions 3 and 4 proceed without the activation energy, and the difference in metals was less significant.