Constant Potential Insights into Axially Coordinated Fe–N–C Catalysts for the Oxygen Reduction Reaction

Axial ligand modification is an effective strategy to tune the oxygen reduction reaction (ORR) activity of FeN₄ single-atom catalysts. Density functional theory calculations were performed on unmodified FeN₄ and its axial ligand-modified derivatives, FeN₄-L (L = F, Cl, Br, I, OH, H, SH). Neutral charge model analysis suggested that FeN₄-SH and FeN₄-I exhibit a high catalytic efficiency. However, comparison with the constant potential model, which allows the surface charge to self-adjust under applied voltage, revealed slight differences in limiting potentials, showing that constant potential modeling is essential for accurate evaluation. Under this model, FeN₄-I emerges as the most promising catalyst, followed by FeN₄-Br and FeN₄-Cl. Electronic structure analysis indicates that hybridization between the Fe 3dz² orbital and the axial ligand weakens OH* adsorption, facilitating protonation in the final ORR step and enhancing activity. These results provide guidance for improving Fe-N-C catalysts and highlight the importance of realistic surface charge modeling.