Spin-State Engineering of Fe–N ₃ S ₁ Sites for High-Performance Oxygen Reduction

The advancement of high-performance iron-based single-atom catalysts (Fe-SACs) for the oxygen reduction reaction (ORR) presents great promise for sustainable electrochemical energy technologies. However, the precise regulation of the spin state of the Fe center, a critical factor governing catalytic activity, remains a fundamental challenge. Here, we synthesize a sulfur-doped Fe-SAC with an asymmetric Fe–N3S1 coordination structure, which effectively modulates the local electronic environment compared with the conventional Fe–N4 configuration. Using synchrotron-based X-ray and infrared spectroscopy, we reveal that sulfur incorporation optimizes the electron occupancy of Fe 3d orbitals, including a favorable transition of the Fe spin state from low-spin to high-spin. This engineered electronic structure enhances the adsorption and activation of oxygen intermediates, substantially accelerating the kinetics of the rate-determining *OOH formation and conversion steps. Thus, the catalyst displays superior ORR performance, achieving an excellent catalytic efficiency characterized by a high half-wave potential of 0.904 V. This work provides a robust pathway for designing advanced electrocatalysts.