A Density Functional Theory Study on Mechanism of Electrochemical Oxygen Reduction on FeN4-Graphene

Abstract

Metal-coordinated nitrogen-doped carbons are catalytically active for oxygen reduction reaction (ORR). The present study describes in detail the ORR on Fe-coordinated, pyrrolic nitrogen-doped graphene (FeN4-G) based on a density functional theory calculation. On this model, pathways of four- and two-electron reductions are investigated for ORR. The most feasible four-electron reduction pathways are arranged in the following order: O2(ads)→OOH(ads)→O(ads)+H2O(ads) (or 2OH(ads))→OH(ads)+H2O(ads) →2H2O(ads). Free energy diagrams show that the elementary steps of the ORR along the four-electron pathway are downhill at a low electrode potential (up to 0.41 V vs. standard hydrogen electrode). The rate-determining step appears at the OH(ads)-to-H2O(ads)reduction, with a reaction barrier of 1.02 eV. The two-electron reduction product H2O2 can chemisorb on the surface. However, the free energy diagrams show that the reduction of OOH into H2O2 remains uphill for all positive electrode potential vs. the normal hydrogen electrode. The high endothermic ΔG value of H2O2 formation indicates that the two-electron ORR pathway is unfavorable on FeN4-G catalysts.