Unraveling the mechanism of pyrrole and N-defect regulating CoN4 single atom catalysts as a pH-universal bifunctional electrocatalyst for OER and ORR

The development of bifunctional electrocatalysts for fuel cells and rechargeable metal-air cells was very urgent, but still faced enormous challenges. Single atom catalysts had achieved satisfactory performance in this research fields, however, the coordination environment and catalytic mechanism of the active center were still unclear. Herein, density functional theory calculations were used to systematically investigated the effects of heterolateral pyridine or pyrrole ligand and/or N-defect on the catalytic oxygen evolution reaction (OER) and oxygen reduction reaction (OER) of CoN4 supported on graphene both in acidic and alkaline media. Compared with CoN4, the N-defect and axial pyrrole or pyridine ligand jointly modified CoN4 catalysts had improve activities for OER, the N-defect and heterolateral pyrrole ligand modified CoN4 catalysts had improve activities for ORR. Additionally, CoN3-pyrrole also exhibited moderate intrinsic barriers for the kinetics rate-limiting step (KRLS) in both acidic and alkaline media for OER and ORR. In conclusion, CoN3-pyrrole was considered as the optimal candidate for bifunctional catalysts due to its relatively superior bifunctional theoretical overpotential and reaction barrier of KRLS in acidic and alkaline media. The theoretical overpotential was not the only evaluation criteria, the intrinsic barrier of KRLS must be considered as another evaluation criteria of electrocatalytic performance.