Axial ligand promoted phosphate tolerance of an atomically dispersed Fe catalyst towards the oxygen reduction reaction

Developing active, stable and phosphate anion resistant catalysts is critical for high-temperature proton exchange membrane fuel cells based on phosphoric acid-doped polybenzimidazole (PA-PBI) membranes. Herein, an iron catalyst with a five-coordinated Fe active center is elaborately designed. The experimental and theoretical studies show that the planar Fe-N-4 moiety with an axial O ligand, benefiting from the optimized charge redistribution, weakens phosphate anion adsorption on Fe active centers and simultaneously promotes oxygen molecule dissociation, resulting in excellent phosphate anion tolerance and ORR activity with the half-wave potential remaining at 0.81 V. The axial-ligand promoted Fe-N-C catalyst is assembled for the first time in a PA-PBI fuel cell, delivering a decent performance. This study sheds light on the intrinsic cause for the phosphate anion tolerance of Fe-N-C catalysts at a molecular level, which provides guidance for designing highly active and stable electrocatalysts for PA-PBI fuel cells.