Insights into the activity of single-atom Fe-N-C catalysts for oxygen reduction reaction

Single-atom Fe-N-C catalysts has attracted widespread attentions in the oxygen reduction reaction (ORR). However, the origin of ORR activity on Fe-N-C catalysts is still unclear, which hinder the further improvement of Fe-N-C catalysts. Herein, we provide a model to understand the ORR activity of Fe-N(4) site from the spatial structure and energy level of the frontier orbitals by density functional theory calculations. Taking the regulation of divacancy defects on Fe-N(4) site ORR activity as examples, we demonstrate that the hybridization between Fe 3dz(2), 3dyz (3dxz) and O(2) π* orbitals is the origin of Fe-N(4) ORR activity. We found that the Fe–O bond length, the d-band center gap of spin states, the magnetic moment of Fe site and *O(2) as descriptors can accurately predict the ORR activity of Fe-N(4) site. Furthermore, these descriptors and ORR activity of Fe-N(4) site are mainly distributed in two regions with obvious difference, which greatly relate to the height of Fe 3d projected orbital in the Z direction. This work provides a new insight into the ORR activity of single-atom M-N-C catalysts.