Iron and nitrogen codoped carbons (Fe-N/C) have emerged as promising nonprecious metal catalysts for the oxygen reduction reaction (ORR). While Fe-Nx 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-Nx and Fe-Fe3C@C species, hampering the identification of exclusive role of each species. Herein, in order to establish the respective roles of Fe-Nx 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-Nx, Fe-Fe3C@C, and N-doped carbon (C-Nx) sites. It was revealed that Fe-Nx 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.
Keywords: Fe−N/C catalyst; active site; electrocatalysis; model system; oxygen reduction reaction.