Dual-site oxygen reduction reaction mechanism on CoN4 and CoN2 embedded graphene: Theoretical insights

As one of the potential candidates of electrocatalysts, non-precious transition metal and nitrogen embedded graphene has attracted extensive attention in recent years. Deep understanding of the oxygen reduction reaction (ORR) mechanism including the specific active sites and reaction pathways will contribute to the further enhancement of their catalytic activity. In this work, density functional study is conducted on the ORR process of CoN2 and CoN4 embedded graphene in acid medium. The calculated formation energy shows that both CoN2 and CoN4 embedded graphenes are thermodynamically stable configurations. On the CoN4 site, the reaction pathway to form HOOH is the most favorable pathway. However, CoN4 site does not promote complete ORR and HOOH is the final product. Meanwhile, for CoN2 embedded graphene, the reaction pathway of HOOH dissociation is also the most favorable pathway and the energy barrier is 0.58 eV in the rate-determining step. This implies that CoN2 site serves as the second site for the complete ORR on the CoNx embedded graphene. Therefore, the HOOH formed on the CoN4 site can be dissociated on the CoN2 site, leading to a dual-site 2 x 2e(-) ORR mechanism. Finally, the effect of different electrode potentials on the ORR is discussed. (C) 2016 Elsevier Ltd. All rights reserved.