Mechanism of Oxygen Reduction Reaction on Pt(111) in Alkaline Solution: Importance of Chemisorbed Water on Surface

We report a detailed mechanistic study of the oxygen reduction reaction (ORR) on Pt(111) in alkaline solution, combining density functional theory and kinetic Monte Carlo simulations. A complex reaction network including four possible pathways via either 2e^- or 4e^- transfer is established and is able to reproduce the experimental measured polarization curve at both low- and high-potential regions. Our results show that it is essential to account for solvation by water and the dynamic coverage of OH to describe the reaction kinetics well. In addition, a chemisorbed water (∗H₂O)-mediated mechanism including 4e^- transfers is identified, where the reduction steps via ∗H₂O on the surface are potential-independent and only the final removal of ∗OH from the surface in the form of OH^-(aq) contributes to the current. For the ORR in alkaline solutions, such a mechanism is more competitive than the associative and dissociative mechanisms typically used to describe the ORR in acid solution. Finally, ∗OH and ∗ ∗O₂ intermediates are found to be critically important for tuning the ORR activity of Pt in alkaline solution. To enhance the activity, the binding of Pt should be tuned in such a way that ∗OH binding is weak enough to release more surface sites under working conditions, while ∗ ∗O₂ binding is strong enough to enable the ORR via the 4e^- transfer mechanism.