(452b) A First-Principles Analysis of Electrocatalytic Oxidation of Co at the Dmfc Anode

A key factor limiting the anode performance in the direct methanol fuel cell is the poisoning of Pt by CO, an abundant surface intermediate that forms in methanol oxidation. The oxidation of CO from pure Pt requires a substantial overpotential. Alloying the surface with Ru reduces the overpotential by 170-260 mV.[1] This reduction is typically attributed to ligand and/or bifunctional effects. While this has been well-established experimentally, the electrocatalytic environment has presented challenges to theoretical modeling. Herein, we present density functional theory results examining the energetics of CO oxidation over Pt and different Pt-Ru alloys. We explicitly examine the influences of solution and electrochemical potential on the overall reaction energies and activation barriers. This is performed by using an approach that we have recently developed to simulate constant potential systems. The addition of ruthenium to a Pt surface significantly reduces the equilibrium potential of water activation, thus indicating that the bifunctional mechanism contributes substantially to the oxidation of CO. The introduction of Ru into Pt, however, also acts to increase the equilibrium potential for the overall reaction of CO and H2O to form CO2, two protons and two electrons. This is the result of the enhanced binding of water to the surface upon alloying with Ru; however the overall oxidation potential shift is smaller than the effect on water activation. Under some conditions, platinum can preferentially segregate to the surface of the Pt-Ru alloys. Therefore the energetics for CO oxidation were also explored using an overlayer of Pt atoms on a Ru substrate. The equilibrium overpotential for the overall oxidation of CO with H2O is substantially reduced in the overlayer system due to decreased binding energies of both species. A combined alloy system, with a mixed Pt/Ru layer over a Ru bulk substrate is shown to combine both the bifunctional and ligand effects. The activation barriers for the combination of CO and OH on the electrode surface to form COOH were also calculated as a function of electrode potential over the Pt and Ru-Pt surfaces. The activation barrier was found to decrease at more positive potentials. The effect of the surface alloy configuration on this activation barrier is discussed, and activation barriers calculated at a constant electrode potential are compared to those determined for a constant system charge. Following detailed consideration of CO oxidation over Pt/Ru alloys, a simpler model examining only gas-phase reaction energetics over various alloy metal surfaces was employed for rapid screening of catalyst materials.

References 1. P. Waszczuk, G. Q. Lu, A Wieckowski, C. Lu, C. Rice, R. I. Masel, Electrochim. Acta, 47, 3637 (2002).