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-3d_z²-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-3d_z² orbital and the O-2p_z orbital below the Fermi level, while the O–O bond in the OOH* adsorbate is formed through the hybridization of 2p_y–2p_y and 2p_y–2p_z 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.