Unveiling the Axial Hydroxyl Ligand on Fe-N₄-C Electrocatalysts and Its Impact on the pH-Dependent Oxygen Reduction Activities and Poisoning Kinetics

Fe-N-C materials have shown a promising nonprecious oxygen reduction reaction (ORR) electrocatalyst yet their active site structure remains elusive. Several previous works suggest the existence of a mysterious axial ligand on the Fe center, which, however, is still unclarified. In this study, the mysterious axial ligand is identified as a hydroxyl ligand on the Fe centers and selectively promotes the ORR activities depending on different Fe-N₄-C configurations, on which the adsorption free energy of the hydroxyl ligand also differs greatly. The selective formation of hydroxyl ligand on specific Fe-N-C configurations can resolve contradictories between previous theoretical and experimental results regarding the ORR activities and associated active configurations of Fe-N-C catalysts. It also explains the pH-dependent ORR activities and, moreover, a previously unreported pH-dependent poisoning kinetics of the Fe-N-C catalysts.