Probing the catalytic activity of M-N4-xOx embedded graphene for the oxygen reduction reaction by density functional theory

In this work, the detailed oxygen reduction reaction (ORR) catalytic performance of M-N4-xOx (M = Fe, Co, and Ni; x =1-4) has been explored via the detailed density functional theory method. The results suggest that the formation energy of M-N4-xOx shows a good linear relationship with the number of doped O atoms. The adsorption manner of O-2 on M-N4-xOx changed from end-on (x = 1 and 2) to side-on (x = 3 and 4), and the adsorption strength gradually increased. Based on the results for binding strength of ORR intermediates and the Gibbs free energy of ORR steps on the studied catalysts, we screened out two highly active ORR catalysts, namely Co-N3O1 and Ni-N2O2, which possess very small overpotentials of 0.27 and 0.32 V, respectively. Such activities are higher than the precious Pt catalyst. Electronic structure analysis reveals one of the reasons for the higher activity of Co-N3O1 and Ni-N2O2 is that they have small energy gaps and moderate highest occupied molecular orbital energy levels. Furthermore, the results of the density of states reveal that the O doping can improve the electronic structure of the original catalyst to tune the adsorption of the ORR intermediates.