Electron Transfer Induced by the Change of Spin States as a Catalytic Descriptor on C₂N–TM Single-Atom Catalysts

The catalytic activity and selectivity of metal single-atom catalysts strongly depend upon their spin states. However, their intrinsic connections are not yet clear. In this work, we evaluate the catalytic activity and selectivity of oxygen reduction reactions (ORRs) on C₂N-supporting 3d transition metal (TM = Mn/Co/Ni/Cu) single-atom catalysts (SACs) using the density functional theory calculations. It is found that all of the SACs with different spin states tend to follow the 2e^- H₂O₂ pathway, except for C₂N-Mn (S = 1/2), which takes the 4e^- OOH pathway. Interestingly, we found that the sum of the changes in the electron spin moments of the metal active centers and the reaction intermediate OOH affects the OOH electron transfer, and the electron transfer promotes the catalytic activity of the 2e^- H₂O₂ pathway on C₂N-TM SACs. Moreover, there is a strong linear relationship between the OOH electron transfer and the catalytic activity of the 2e^- H₂O₂ pathway on C₂N-TM SACs. These findings indicate that electron transfer induced by the change of spin states serves as a descriptor of the catalytic activity of the 2e^- H₂O₂ pathway on C₂N-TM SACs, which is very helpful for designing more powerful SACs.