Spin-Orientation-Dependent O2 Adsorption Mechanism in Fe Single-Atom Catalysts

Abstract

Iron and nitrogen codoped carbon (Fe–N–C) catalysts are promising non-noble catalysts for the electrochemical oxygen reduction reaction (ORR) that is technologically important for fuel cells. As the initial and presumably rate-determining step for the ORR on Fe–N–C catalysts, O 2 adsorption on FeN 4 sites attracts extensive interest. Provided that both Fe ions and O 2 molecules are magnetic, strong spin effects are expected in the O 2 adsorption process, yet a comprehensive understanding of the involved spin effects is still lacking. We here present a theoretical study based on density functional theory on O 2 adsorption over Fe–N–C catalysts, focusing on the effects of Fe spin states, spin orientations, and spin selection rules on the O 2 adsorption process. Our findings show that O 2 adsorption energy strongly depends on the spin states of FeN 4 sites and the spin orientations of Fe and O 2 . Notably, regardless of Fe spin states, the mechanism of O 2 adsorption is spin-orientation dependent: when the O 2 spin is oriented parallel with the spin of Fe, the charge transfer mechanism is preferred, whereas antiparallel spin orientation favors the hybridization mechanism. The spin-orientation-dependent O 2 adsorption mechanism originates from the spin selection rule. These insights are of use not only for fundamental understanding but also for providing guidance on enhancing electrochemical reactions through manipulating spin states and spin coupling by applying an external magnetic field.