title: Roles of Fe-Nxand Fe-Fe3C@C Species in Fe-N/C Electrocatalysts for Oxygen Reduction Reaction.
abstract: Iron and nitrogen codoped carbons (Fe-N/C) have emerged as promising nonprecious metal catalysts for the oxygen reduction reaction (ORR). While Fe-Nxsites 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-Nxand Fe-Fe3C@C species, hampering the identification of exclusive role of each species. Herein, in order to establish the respective roles of Fe-Nxand Fe-Fe3C@C sites we rationally designed model catalysts via the phase conversion reactions of Fe3O4nanoparticles 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-Nxsites 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.
authors.affiliation: School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST) , 50 UNIST-gil, Ulsan 44919, Republic of Korea.
authors.forename: Jae Hyung
authors.initials: JH
authors.lastname: Kim
authors.affiliation: Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST) , 50 UNIST-gil, Ulsan 44919, Republic of Korea.
authors.forename: Young Jin
authors.initials: YJ
authors.lastname: Sa
authors.affiliation: UNIST Central Research Facilities (UCRF), Ulsan National Institute of Science and Technology (UNIST) , 50 UNIST-gil, Ulsan 44919, Republic of Korea.
authors.forename: Hu Young
authors.initials: HY
authors.lastname: Jeong
authors.affiliation: School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST) , 50 UNIST-gil, Ulsan 44919, Republic of Korea.|Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST) , 50 UNIST-gil, Ulsan 44919, Republic of Korea.
authors.forename: Sang Hoon
authors.identifier: http://orcid.org/0000-0002-8941-9662
authors.initials: SH
authors.lastname: Joo
keywords: Fe−N/C catalyst; active site; electrocatalysis; model system; oxygen reduction reaction