Second-Shell Sulfur Coordination Engineering of Iron Single-Atom Catalysts for Enhanced Oxygen Reduction Reaction

Precisely modulating the local coordination microenvironment of metal centers to synthesize high-efficiency Fe–N–C single-atom catalysts remains critical yet challenging for optimizing the oxygen reduction reaction (ORR) performance. Especially, the influence and mechanism of second-shell coordination engineering on Fe centers remain ambiguous and often overlooked. Herein, a facile strategy is proposed to synthesize Fe–N4 single atoms with precisely controlled second-shell sulfur coordination on hierarchical porous carbon (Fe–N, S-HPC). Density functional calculations first reveal that second-shell sulfur coordination can redistribute charge density, upshift the d-band center, and strengthen the adsorption of oxygen-containing intermediates, thereby significantly reducing energy barriers and accelerating ORR kinetics. As expected, the Fe–N, S-HPC catalyst demonstrates a half-wave potential of 0.908 V and an enhanced turnover frequency of 1.68 s–1 for ORR. Furthermore, the assembled Zn-air battery delivers a high specific capacity of 702.7 mAh g–1 and a large energy density of 886.1 Wh kgZn–1 along with long-term charge/discharge stability. This work reveals the correlation between second-shell sulfur coordination and enhanced ORR activity and offers a valuable design principle for advanced single-atom catalysts.