First Principles Tafel Kinetics for Resolving Key Parameters in Optimizing Oxygen Electrocatalytic Reduction Catalyst

Oxygen reduction is a critical reaction in the global energy cycle and a vital catalytic process in fuel cells. To date, the atomic level picture on how oxygen is electrocatalytically reduced on the traditional Pt catalyst is not established yet and the design of both active and economic catalysts remains a great challenge. Here first principles based theoretical methods can for the first time resolve the Tafel behavior and the polarization kinetics for oxygen reduction reaction (ORR) on Pt in aqueous soundings and reveal the origin of some key problems, mainly associated with the low intrinsic activity and the rapid poisoning of the electrocatalyst. The atomic level mechanism of ORR on Pt at the concerned potentials (similar to 0.8 V) is established, in which the critical surface coverage to achieve the reaction equilibrium is identified to be 0.25 ML O coverage. From the computed Tafel curve, the proton-coupled O-O bond breaking, i.e. H+ + e + O-2ad -> O + OH, is assigned to be the major O-2 reduction channel on Pt; and the reaction is quenched at the high potentials due to the presence of the surface O/OH/H2O network that prevents the adsorption of bidentate O-2. We predict that a qualified ORR catalyst must allow bidentate O-2 adsorption under the equilibrium between adsorbed O and H2O in solution at the concerned potential.