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 H-2 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.