Mechanistic Insight into the Oxygen Reduction Reaction on the Mn-N4/C Single-Atom Catalyst: The Role of the Solvent Environment

The design of platinum group metal (PGM)-free catalysts is crucial to the application of energy conversion due to their excellent catalytic capability. Recently, M-N (M = Mn, Fe, Co, etc.) single-atom catalysts embedded in graphene have been extensively studied with respect to catalyzing the oxygen reduction reaction (ORR). Although the ORR is operated in the liquid phase, few mechanistic studies have taken the solvation effect into consideration. In the present work, we have performed ab initio molecular dynamics (AIMD) simulations as well as density functional theory (DFT) calculations to investigate the influence of the solvation effect on the mechanisms of ORR on MnN-graphene by using explicit water molecules. It is found that the solvent environment can effectively promote the charge transfer from the substrate to O, leading to the transformation from superoxide species to peroxide species. This also makes the ORR preferably proceed via a dissociative pathway, where O can be easily adsorbed on the single Mn site in the form of a "side-on" type, leading to the probable rupture of the O-O bond before being protonated. Furthermore, the solvent water molecules also raise the reactivity of protonation steps for *O and *OH intermediates by the elongation of the Mn-O bond with the assistance of the surrounding hydrogen bonds. Finally, on the basis of the calculated free-energy pathway, the liquid-phase model gives a more correct estimation for the overpotential than the gas-phase model, which is consistent with the experimental observation. The present work provides detailed information for understanding the reaction mechanisms of ORR at the surface-liquid interface on the M-Nx/C single-atom catalyst.