Investigating the Roles of Fe-Nx and Fe-Fe3C@C Species in Fe-N/C Oxygen Reduction Electrocatalysts

Abstract

Iron and nitrogen co-doped carbons (Fe-N/C) have emerged as promising non-precious metal catalysts for replacing Pt-based catalysts for oxygen reduction reaction (ORR). While Fe-Nx sites have been widely accepted as active sites 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. In this work, in order to establish the respective roles of Fe-Nx and Fe-Fe3C@C sites we rationally designed new model catalysts via phase conversion method from Fe3O4 nanoparticles supported on carbon nanotubes. It was revealed that Fe-Nx sites dominantly catalyze the ORR with 4-electron pathway, exerting a major role for the high ORR performance, whereas Fe-Fe3C@C sites mainly promote 2-electron reduction of oxygen and sequential peroxide reduction (2 × 2 oxygen reduction), playing an auxiliary role for the ORR.