Catalytic Activity for Oxygen Reduction Reaction on CoN2-Graphene: A Density Functional Theory Study

Abstract

The detailed catalytic mechanism of oxygen reduction reaction (ORR) on CoN2 embedded graphene (CoN2-G) has been investigated based on a density functional theory calculation. Pathways of four- and two-electron reductions are investigated for ORR. The most feasible four-electron reduction pathways are arranged in the following order: the reductions of O2(ads) into OOH(ads), OOH(ads) into O(ads) and H2O, then O(ads) into OH(ads), and finally OH(ads) into H2O. Free energy diagrams show that the elementary steps of ORR along the four-electron pathway are downhill at low electrode potential (up to 0.62 V vs. standard hydrogen electrode). The first reduction step, O2(ads)-to-OOH(ads) reaction has the highest reaction barrier (0.53 eV), acting as the kinetic rate-determining step. However, at a higher potential, the last reduction step, OH(ads)-to-H2O(ads) becomes uphill, and is the thermodynamic rate-determining step. Therefore, the first and the last reduction steps restrict the ORR performance together at high potential on CoN2-G catalysts. The two-electron reduction product H2O2 can be chemisorbed on the surface. However, the H2O2(ads) is unstable and easy to dissociate into OH species. Therefore, the two-electron ORR pathway is unfavorable on CoN2-G.