Roles of Fe−Nx and Fe−Fe3C@C Species in Fe−N/C Electrocatalysts for Oxygen Reduction Reaction

Iron and nitrogen codoped carbons (Fe-N/C) have emerged as promising nonprecious metal catalysts for the oxygen reduction reaction (ORR). While Fe-N_x sites have been widely considered as active species for Fe-N/C catalysts, very recently, iron and/or iron carbide encased with carbon shells (Fe-Fe₃C@C) has been suggested as a new active site for the ORR. However, most of synthetic routes to Fe-N/C catalysts involve high-temperature pyrolysis, which unavoidably yield both Fe-N_x and Fe-Fe₃C@C species, hampering the identification of exclusive role of each species. Herein, in order to establish the respective roles of Fe-N_x and Fe-Fe₃C@C sites we rationally designed model catalysts via the phase conversion reactions of Fe₃O₄ nanoparticles supported on carbon nanotubes. The resulting catalysts selectively contained Fe-N_x, Fe-Fe₃C@C, and N-doped carbon (C-N_x) sites. It was revealed that Fe-N_x sites dominantly catalyze ORR via 4-electron (4 e^-) pathway, exerting a major role for high ORR activity, whereas Fe-Fe₃C@C sites mainly promote 2 e^- reduction of oxygen followed by 2 e^- peroxide reduction, playing an auxiliary role.