Is iron nitride or carbide highly active for oxygen reduction reaction in acidic medium?

Abstract

Currently, iron nitride (Fe₂N) and iron carbide (Fe₃C), which are regarded as the new non-precious metal electrocatalysts for the oxygen reduction reaction (ORR) with high activity and stability in acidic medium, are attracting increasing attention. Herein, by systematic comparison of the ORR activities for Fe₂N- and Fe₃C-based catalysts designed with or without a solid nitrogen source, we found that only the former is highly active for ORR while the latter is quite poorly active despite their similar crystalline phases. This result indicates that the Fe–N related species are responsible for the high ORR activities of the Fe-based catalysts, similar to the case of Fe/N/C catalysts. Density functional theory calculations demonstrate that the Fe–N₄/C moiety has a far superior ORR activity to that of Fe₂N and Fe₃C. The experimental and theoretical results mutually support that the high activities of the Fe-based catalysts originate from Fe–N_x/C moieties (x ≥ 4) rather than Fe₂N or Fe₃C phases, which is significant for exploring advanced Fe-based electrocatalysts.