Manipulating the spin state to activate the atomically dispersed Fe–N–C catalyst for oxygen reduction

Abstract

Atomically dispersed metal–nitrogen carbon (M–N–C) catalysts with e¹_g are believed to be very active for the oxygen reduction reaction; however, the activity origin of spin state manipulation is unclear. Here, using Fe–N–C as a model catalyst, we have developed a facile approach to manipulate its spin state from the low-spin to medium-spin state (Fe_SAC–NC, e¹_g) by utilizing the secondary coordination sphere effect of adjacent NC–N moieties. The presence of NC–N configurations with high electronegativity modifies the degree of hybridization between Fe 3d_z², 3d_xz (3d_yz), and oxygen-containing intermediate π* orbitals, leading to an ideal balance between O₂ activation and *OH desorption. More impressively, we found that the catalysts with e_g = 1 in their metal centres may show very different activity, and the magnetic moment of 3d_xz + 3d_yz as a descriptor can accurately predict the ORR activity. Benefiting from the regulated coordination and electronic structures, the designed Fe_SAC–NC catalyst exhibited 3-fold mass activity and 7-fold specific activity than Pt/C.