p-Block metal atom-induced spin state transition of Fe–N–C catalysts for efficient oxygen reduction

A deep understanding of the role of spin configurations of Fe-N-C catalysts in the adsorption and desorption of oxygen intermediates during ORRs is critical for the development of new catalysts for the ORR. Herein, we successfully implanted p-block metal single sites (SnN₄, SbN₄) into the Fe-N-C system to vary the spin states of Fe species and investigated the ORR performance of active metal centers with varying effective magnetic moments. Through a combination of zero-field cooling (ZFC) temperature-dependent magnetic susceptibility measurements and DFT calculations, we successfully established correlations between the spin state and ORR activity. Magnetic analysis reveals that the p-block metal catalytic sites can effectively induce a low-to-high (or medium) spin state transition of Fe centers. Consequently, the 3d orbital electrons in Fe,M-N-C catalysts penetrate the antibonding π-orbitals of oxygen more easily, thus optimizing the adsorption/desorption of key oxygen intermediates on Fe-N-C catalysts. As a result, the optimized Fe,M-N-C catalyst exhibits a half-wave potential of 0.97 V in a 0.1 M KOH electrolyte, as well as higher durability than conventional Pt/C catalysts. Moreover, the Fe,M-N-C catalysts show encouraging performance in a rechargeable Zn-air battery with high power density and long-term cyclability, indicating the practical applicability of these Fe,M-N-C catalysts.