Revealing the Kinetic-Driven Oxygen Reduction Reaction Using Grand Canonical Ensemble Modeling

As one of the most crucial electrochemical reactions, the oxygen reduction reaction (ORR) follows two possible pathways after the formation of the OOH* intermediate. However, the origin of the H2O2/H2O selectivity remains elusive. Herein, in this work, the grand canonical density functional theory (GC-DFT)-based fixed potential method was employed to investigate such discrepancy between experimental and theoretical results. The impact from solvent and applied potential conditions has been fully considered, with Pourbaix diagrams demonstrating the H-preoccupying state for the Fe1N4-graphene surface over the pH range of 0.0–14.0 under the cathodic negative potential of −1.23 V to −0.74 V vs standard hydrogen electrode. Electronic structure analysis further demonstrates that additional H chemisorption on the Fe site donates electrons, leading to the occupation of the Fe-3dz2-β orbital and resulting in an Fe frontier orbital configuration of 4s03d7 with a valence state of +1. A proton-exchange mechanism for the *O1O2H intermediate transformation was proposed. Despite the thermodynamic preference for O1–O2H dissociation, kinetic barriers for protonation at the O1 site to form H2O2 are calculated to be 0.75–0.89 eV lower than those for H2O formation, with a rate constant 14 orders of magnitude higher. Bonding analysis reveals that the Fe–O bond primarily arises from interactions between the Fe-3dz2 orbital and the O-2pz orbital below the Fermi level, while the O–O bond in the OOH* adsorbate is formed through the hybridization of 2py–2py and 2py–2pz orbitals. The presence of H at the Fe site effectively weakens the Fe–O bond while strengthening the covalent O–O bond, thereby preventing *O1–O2H dissociation and promoting H2O2 selectivity. This work provides new insights into the ORR mechanism by explicitly incorporating solvent effects and applied potential considerations.