A Density Functional Theory Study on Mechanism of Electrochemical Oxygen Reduction on FeN₃-Graphene

Fe/N/C catalysts are proven to be highly active for oxygen reduction reaction (ORR). The detailed kinetic and thermodynamic ORR behavior on FeN3-G (three-coordinated FeN3 center embedded in graphene) based on density functional theory is investigated in this work. The results show that O2 dissociation mechanism with a high active barrier is unfavorable. All ORR intermediates except H2O2 can be chemisorbed stably on the surface. H2O2 is easily dissociated, which indicates that a two-electron pathway is impossible. This conclusion is further supported by the calculations of OOH dissociation reaction barriers, which are extremely low. The results prove that FeN3-G catalysts promote four-electron ORR. Given the high adsorption energy of the surface toward O2, the O2(ads)-to-OOH(ads) reaction has the highest reaction barrier in the whole reduction steps, which functions as the kinetic rate-determining step. However, free energy diagrams show that reduction of OH into H2O remains uphill for all positive electrode potential vs. the normal hydrogen electrode. The high endothermic ΔG value of H2O formation indicates this step is the most sluggish one and implies the highest resistance for the whole ORR. The FeN3-G is not a well catalyst for ORR.