Theoretical insights on the oxygen-reduction reaction mechanism of LaN4-embedded graphene.

We studied the structural stability, adsorption behavior of the key intermediates in the oxygen-reduction reaction (ORR) process, and ORR mechanism on LaN4-embedded graphene (LaN4-Gra) theoretically in an acid environment. The geometry optimization and energy calculations in this study were performed using the generalized gradient approximation (GGA) within the Perdew-Burke-Ernzerhof (PBE) functional. The thermodynamic stability of LaN4-Gra is confirmed by its negative formation energy. The sequential hydrogenation of O2 to generate H2O molecules is the most favorable pathway and it is a four-electron process. The last hydrogenation of OH species to form H2O molecules is the rate-determining step in the whole ORR process. Compared with the energy barrier of pure Pt electrocatalyst (0.8 eV), the close energy barrier value for LaN4-Gra (0.89 eV) is quite favorable. This implies that LaN4-embedded graphene could be a potential ORR catalyst. Finally, the ORR free energy of LaN4-Gra electrocatalyst was also analyzed.