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

Abstract

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.