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) materials have been suggested as the most promising replacement for Pt-based catalysts in the oxygen reduction reaction (ORR) owing to the FeN₄ active moiety. Based on the relationship between the oxygen binding energy and the catalytic activity, Fe–N–C has a very strong oxygen binding energy; hence, hard to desorb the final reaction intermediate of *OH. Herein, we provide an effective method of tuning the active moiety using a phosphine-gas treatment for Fe–N–C. Combined analyses of experimental and computational results reveal that the conventional FeN₄ moiety is transformed into FeN₃PO through the P-doping post-treatment. Furthermore, we propose an ORR mechanism on the unique FeN₃PO moiety based on a microkinetic model, in which *OH intermediates are considered. Compared to the FeN₄ moiety, the FeN₃PO moiety facilitates *OH desorption, thereby enhancing the ORR activity in both alkaline and acidic electrolytes. The effects of P-doping on the ORR performance are also validated in both anion exchange membrane fuel cells (AEMFCs) and proton exchange membrane fuel cells (PEMFCs).