Transformation of the Active Moiety in Phosphorus-Doped Fe-N-C for Highly Efficient Oxygen Reduction Reaction

Abstract

Iron- and nitrogen-doped carbon (Fe-N-C)materialshave been suggested as the most promising replacement for Pt-basedcatalysts in the oxygen reduction reaction (ORR) owing to the FeN4 active moiety. Based on the relationship between the oxygenbinding energy and the catalytic activity, Fe-N-C hasa very strong oxygen binding energy; hence, hard to desorb the finalreaction intermediate of *OH. Herein, we provide an effective methodof tuning the active moiety using a phosphine-gas treatment for Fe-N-C.Combined analyses of experimental and computational results revealthat the conventional FeN4 moiety is transformed into FeN3PO through the P-doping post-treatment. Furthermore, we proposean ORR mechanism on the unique FeN3PO moiety based on amicrokinetic model, in which *OH intermediates are considered. Comparedto the FeN4 moiety, the FeN3PO moiety facilitates*OH desorption, thereby enhancing the ORR activity in both alkalineand acidic electrolytes. The effects of P-doping on the ORR performanceare also validated in both anion exchange membrane fuel cells (AEMFCs)and proton exchange membrane fuel cells (PEMFCs).