▎ 摘　　要

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 O-2 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 O-2, the O-2(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 Delta G 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. (C) 2015 The Electrochemical Society. All rights reserved.