Strategic Engineering of Axially Coordinated Ligands in Fe–N–C Catalysts for Enhanced Oxygen Reduction Electrocatalysis

The single-atom Fe-N-C electrocatalyst is considered one of the most promising alternatives to the expensive and scarce Pt-based catalysts for promoting oxygen reduction reaction in fuel cells. Regulating the coordination environment of the Fe center is a feasible strategy to improve its stability and catalytic activity. Recently, the introduction of axial ligands to Fe-N-C has attracted extensive research interest, providing a new dimension for coordination environment regulation compared with the common approaches of in-plane doping or defect construction. This review focuses on discussing the contribution of axial ligand decoration to the activity and stability of the Fe-N-C catalyst, evaluating different types of axial ligands that have been introduced in the recent literature. Through summarizing the progress in decorating axial ligands to the Fe-N-C system, this review provides profound insights into the design and preparation of axially coordinated Fe-N-C catalysts.