Catalytic properties of transition metal-N₄ moieties in graphene for the oxygen reduction reaction: Evidence of spin-dependent mechanisms

The O₂ dissociation after the chemisorption on the metal center of M-N₄ moieties in graphene (with M = Mn, Fe, and Co) is addressed by density functional theory calculations. Both minimum energy paths and saddle points for the oxygen reduction reaction (ORR) in the allowed spin states have been identified. Our calculations indicate that ORR can evolve through different spin states, those where the M-O₂ adducts are stable. We find that Mn-N₄ and Fe-N₄ centers in graphene exhibit the lowest O₂ dissociation energies of ∼0.7 and 1.1 eV, respectively, over three spin channels, while for Co-N₄ we find two spin channels with the same dissociation energy of ∼1.6 eV. The O₂ dissociation barriers on the Mn-N₄ and Fe-N₄ centers are comparable to that found on Pt(111), suggesting similar ORR catalytic activity, in agreement with experimental results.