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

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, O2 adsorption on FeN4 sites attracts extensive interest. Provided that both Fe ions and O2 molecules are magnetic, strong spin effects are expected in the O2 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 O2 adsorption over Fe-N-C catalysts, focusing on the effects of Fe spin states, spin orientations, and spin selection rules on the O2 adsorption process. Our findings show that O2 adsorption energy strongly depends on the spin states of FeN4 sites and the spin orientations of Fe and O2. Notably, regardless of Fe spin states, the mechanism of O2 adsorption is spin-orientation dependent: when the O2 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 O2 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.