Electrochemical oxygen reduction mechanism on FeN₂-graphene

Metal-coordinated nitrogen-doped carbons are highly active in promoting electrochemical oxygen reduction reactions (ORR). The detailed kinetic and thermodynamic ORR behavior on three different FeN₂-graphene [FeN₂-G (A), (B) and (C)] structures was investigated in this work. The results show that formation of these FeN₂-G configurations is energetically favorable; however, not all of them are effective for ORR. The higher HOMO energy and smaller HOMO-LUMO gap of FeN₂-G (A) and (C) make them have strong adsorption strengths to ORR intermediates, which leads to occupation the active sites on the catalysts during ORR, and thus loss of catalytic activity. Examination of the results of ∆G of each reduction step also drew the same conclusion. The ∆G of the elementary steps of the ORR at zero electrode potential vs. standard hydrogen electrode are downhill only on FeN₂-G (B). Throughout the entire four-electron ORR, the reduction of O to OH displays the highest reaction barrier. When the potential is >0.19 V, the reduction of OH species into water is uphill. Therefore, ORR activity is limited by two rate-determining steps on FeN₂-G (B) at high potential: O and OH reduction steps.