Unraveling the Common Nature of O and S Doping in Improving Electrochemical O₂ Reduction Reaction Performance of FeN₄C

Heteroatom-doped Fe-N-C catalysts have emerged as promising alternatives to noble metals for the oxygen reduction reaction (ORR) due to their lower cost. However, the underlying mechanisms responsible for their enhanced performance, particularly electrochemical stability, remain a subject of debate. This study leverages density functional theory calculations coupled with a constant potential and implicit solvent model to investigate the electrochemical stabilities and activities of pyridinic (PD-) and pyrrolic FeN4C (PL-FeN4C) catalysts. Our findings reveal that the hydrogenation susceptibility of coordinating nitrogen atoms is a critical determinant of electrochemical stability within FeN4C catalysts. Moreover, we demonstrate that oxygen and sulfur doping exerts similar effects on enhancing the overall ORR performance of PD-FeN4C catalysts: (1) by reducing the p-band center of the coordinating nitrogen, thereby improving their resistance to hydrogenation, and (2) by increasing the valence electrons of iron, leading to stronger adsorption of reaction intermediates and consequently enhanced ORR activity. Finally, our predictions suggest that O/S-doped PL-FeN4C catalysts could achieve significantly improved electrochemical stability and superior ORR performance in both acidic and alkaline environments. These insights contribute to a deeper understanding of microenvironment engineering in single-atom catalysts (SACs) and offer valuable guidelines for the development of unprecedented M-N-C catalysts.