Potential- and Rate-Determining Step for Oxygen Reduction on Pt(111)

Using density functional theory calculations, we study reaction thermodynamics and kinetics for the oxygen reduction reaction (ORR) on surface coverages that develop in the 0-1.0 V potential range on Pt(111), with special emphasis on the role of water. At low potentials, water affects adsorption energies of hydrophilic *O-2 and *OOH intermediates but displays a limited effect on the transition state energies for their dissociation. We calculate the O-2 and OOH adsorption and dissociation free energies at most stable surfaces in the investigated potential range and arrive at two important conclusions: (1) the ORR proceeds through the associative reaction mechanism in the diffusion and the mixed kinetic-diffusion region, and (2) moderate O-2 and OOH activation energies support the notion that the reaction is predominantly controlled by the rate prefactor. We associate the rate prefactor with the probability for an O-2 molecule to replace a water molelecule on hydrophilic (OH-H2O covered) surfaces and the inability of O-2 to adsorb and dissociate on hydrophobic (O covered) surfaces that develop at higher potentials. Finally, in light of new results, we discuss activities of Pt alloys that lie close to the top of the volcano.