Electrocatalytic oxygen reduction kinetics on Fe-center of nitrogen-doped graphene

Abstract

The Fe/N/C catalysts have emerged recently as a representative class of non-Pt catalysts for oxygen electrocatalytic reduction, which could have a competitive catalytic performance to Pt. However, the nature of the catalyst remains elusive, especially on the active site structure and the electrocatalytic kinetics. Here we examine two kinds of Fe/N active sites for Fe/N/C catalysts, namely, the four-coordinated FeN₄ and the five-coordinated Fe(CN)N₄ centers embedded in graphene layers. By using large-scale first principles calculations with a periodic continuum solvation model based on the Modified-Poisson–Boltzmann equation (CM-MPB), we identified the four (4e) and two electron (2e) oxygen reduction pathways under acidic conditions. We find that both 4e and 2e pathways involves the formation of an OOH intermediate, which breaks its O–OH bond in the 4e pathway but is reduced to H₂O₂ in the 2e pathway. We show that at 0.8 V vs. SHE, the 4e pathway is preferred at both FeN₄ and Fe(CN)N₄ centers, but the 2e pathway is kinetically also likely on the Fe(CN)N₄ center. The O–OH bond breaking of OOH is the key kinetic step, which has a similar free energy barrier to the OH reduction on the FeN₄ center, and is the rate-determining step on the Fe(CN)N₄ center. Due to the high adsorption energy of Fe towards the fifth ligand, such as OH and CN, we expect that the active site of the real Fe/N/C catalyst is the five coordinated Fe center. We found that the barrier of the O–OH bond breaking step is not sensitive to potential and a Tafel slope of 60 mV is predicted for the ORR on the Fe(CN)N₄ center, which is consistent with experimental observation.