Spin-dominated oxygen activation and reduction mechanism on a Ru-N-C single-atom-catalyst

M-N-C-type single atom catalysts (SACs) have exhibited promising performance in the electrocatalytic oxygen reduction reaction (ORR). The spin states of the active center were found to dominate the ORR activity and mechanism in Fe/Co/Ni-based M-N-C, while this effect was seldom considered in other M-N-C families, leading to insufficient exploration of their ORR mechanism. Here, taking the Ru-N₄-C as an example, we employ first-principles calculations to study its ORR mechanism with various spin states. Our simulation reveals that the evolution of the spin is synergistically governed by potential and ligand change, emphasizing the significance of the spin crossover effect in the ORR. We further found that the O₂ activation mechanism is significantly altered by the spin state of the Ru center. Three active moieties were identified to dominate the ORR kinetics at different potentials: RuNIS4, RuNLS4*OH, and RuNIS4*O, jointly leading to the half-wave potential of 0.6 V vs. SHE. Our work provides fundamental insights into the effect of spin state on the ORR mechanism of Ru-N-C SACs, which could benefit the spin-involved mechanistic study of other Ru-based electrocatalysts.