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.