Spin-Orientation-Dependent\nO<sub>2</sub> Adsorption\nMechanism in Fe Single-Atom Catalysts

Iron and nitrogen codoped carbon (Fe–N–C)\ncatalysts\nare promising non-noble catalysts for the electrochemical oxygen reduction\nreaction (ORR) that is technologically important for fuel cells. As\nthe initial and presumably rate-determining step for the ORR on Fe–N–C\ncatalysts, O<sub>2</sub> adsorption on FeN<sub>4</sub> sites attracts\nextensive interest. Provided that both Fe ions and O<sub>2</sub> molecules\nare magnetic, strong spin effects are expected in the O<sub>2</sub> adsorption process, yet a comprehensive understanding of the involved\nspin effects is still lacking. We here present a theoretical study\nbased on density functional theory on O<sub>2</sub> adsorption over\nFe–N–C catalysts, focusing on the effects of Fe spin\nstates, spin orientations, and spin selection rules on the O<sub>2</sub> adsorption process. Our findings show that O<sub>2</sub> adsorption\nenergy strongly depends on the spin states of FeN<sub>4</sub> sites\nand the spin orientations of Fe and O<sub>2</sub>. Notably, regardless\nof Fe spin states, the mechanism of O<sub>2</sub> adsorption is spin-orientation\ndependent: when the O<sub>2</sub> spin is oriented parallel with the\nspin of Fe, the charge transfer mechanism is preferred, whereas antiparallel\nspin orientation favors the hybridization mechanism. The spin-orientation-dependent\nO<sub>2</sub> adsorption mechanism originates from the spin selection\nrule. These insights are of use not only for fundamental understanding\nbut also for providing guidance on enhancing electrochemical reactions\nthrough manipulating spin states and spin coupling by applying an\nexternal magnetic field.