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−Fe 3 C@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−Fe 3 C@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−Fe 3 C@C sites we rationally designed model catalysts via the phase conversion reactions of Fe 3 O 4 nanoparticles supported on carbon nanotubes. The resulting catalysts selectively contained Fe−N x, Fe−Fe 3 C@C, and N-doped carbon (C−N x ) sites. It was rev…