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).

Intrinsically Functionalized Fe/C3N4 Nanotubes Display pH-Dependent Oxygen Reduction Overpotentials

“…The experimental results indicate higher activity and lower overpotentials in alkaline than in acidic media across different catalytic systems. ,,,− Some theoretical literatures proposed that different pH or ORR potentials lead to different surface functionalizations, ,,,, which are summarized in Scheme a. During the operation of the ORR, the major groups present on the surfaces can change from O* to OH*, and then react or desorb to form the bare surface. ,,, The pH affects the duration of different species on the surfaces when the potential changes along the reaction pathway. Some theoretical models have included the functionalization effect by considering the O, OH, ,− ,,− or even the additional O 2 preadsorbed surfaces.…”

“…The theoretical explorations on different CNNTs have also been largely focused on photocatalytic applications. ,,− ,, In principle, the potential applications of the CNNTs could be as versatile as the 2D g-C 3 N 4 but with varied performances due to the curvature effect or the chirality. The well-known FeN x active center on graphene has shown comparable performance with Pt in ORR experimentally, ,,− and have also attracted many theoretical discussions. ,− While the modified or decorated 2D g-C 3 N 4 has been explored in the battery applications, ,− the potentials of Fe-decorated CNNTs , for ORR operations have never been evaluated. Previous researches have indicated that when the metal atom is adsorbed on the 2D C 3 N 4 surface, it can be bent to an indefinite extent, which has been reported to affect the catalytic performance .…”

“…It is reported that the ORR activity could be affected by the concentration or the diffusion capability of O 2 in the solutions, , as well as the pH value even with the same catalysts. Depending on the applications of the fuel cells or batteries, e.g., the proton or the anion exchange membrane fuel cells (PEMFC or AEMFC) and the metal–air batteries, − , the operating pH could be different. The experimental results indicate higher activity and lower overpotentials in alkaline than in acidic media across different catalytic systems. ,,,− Some theoretical literatures proposed that different pH or ORR potentials lead to different surface functionalizations, ,,,, which are summarized in Scheme a.…”

“…Depending on the applications of the fuel cells or batteries, e.g., the proton or the anion exchange membrane fuel cells (PEMFC or AEMFC) and the metal–air batteries, − , the operating pH could be different. The experimental results indicate higher activity and lower overpotentials in alkaline than in acidic media across different catalytic systems. ,,,− Some theoretical literatures proposed that different pH or ORR potentials lead to different surface functionalizations, ,,,, which are summarized in Scheme a. During the operation of the ORR, the major groups present on the surfaces can change from O* to OH*, and then react or desorb to form the bare surface. ,,, The pH affects the duration of different species on the surfaces when the potential changes along the reaction pathway.…”

“…During the operation of the ORR, the major groups present on the surfaces can change from O* to OH*, and then react or desorb to form the bare surface. ,,, The pH affects the duration of different species on the surfaces when the potential changes along the reaction pathway. Some theoretical models have included the functionalization effect by considering the O, OH, ,− ,,− or even the additional O 2 preadsorbed surfaces. In addition, the mechanisms or pathways of ORR may change due to the different pH environments of the materials. , These observations may be unified by the mechanism proposed in Scheme b.…”