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

Abstract

The development of active and durable non-precious metal-based oxygen reduction reaction (ORR) catalysts has been of prime importance for renewable-energy technologies including fuel cells and metal-air batteries. Iron and nitrogen co-doped carbons (Fe＆ndash;N/C) have been considered as the most promising candidates among non-precious metal ORR catalysts, owing to their outstanding ORR activity, but the catalytic role of active species in Fe＆ndash;N/C catalysts is still a subject of debates. Although there has been a consensus that Fe＆ndash;Nx sites play a significant role in the ORR, recently iron and/or iron carbide encased within carbon shells (Fe＆ndash;Fe3C@C) has also been suggested as an active species for the ORR. However, a broad spectrum of possible roles has been proposed for the Fe＆ndash;Fe3C@C species. Some groups reported that high ORR activity could be achieved with catalysts containing only Fe＆ndash;Fe3C@C sites. Others suggested that the Fe＆ndash;Fe3C@C sites play a synergistic role in conjunction with the Fe＆ndash;Nx sites. Another viewpoint is that Fe＆ndash;Fe3C@C sites are merely an impurity phase. Such a discrepancy is stem from similarity of synthetic route between Fe＆ndash;Nx and Fe＆ndash;Fe3C@C sites, hampering the identification of exclusive role of each species. In this presentation, to establish the respective roles of Fe＆ndash;Nx and Fe＆ndash;Fe3C@C sites we rationally designed model catalysts via the phase conversion reactions of Fe3O4 nanoparticles supported on carbon nanotubes. The resulting three catalysts selectively contained Fe＆ndash;Nx, Fe＆ndash;Fe3C@C, and N-doped carbon (C＆ndash;Nx) sites. The catalysts containing Fe＆ndash;Nx sites exhibited superior ORR activity with low HO2＆minus; yield, compared to other two catalysts. When Fe＆ndash;Nx sites were etched by an acid or poisoned by CN＆ndash;, the catalytic activity decreased drastically, confirming that Fe＆ndash;Nx sites play a major role for high ORR activity via 4-electron (4 e＆minus;) pathway. On the other hand, the catalysts only with Fe＆ndash;Fe3C@C sites showed inferior ORR performance and high HO2＆minus; yield. Through additional peroxide reduction experiment, encapsulated form of Fe＆ndash;Fe3C@C sites could facilitate sequential peroxide reduction, concluding that Fe＆ndash;Fe3C@C sites play an auxiliary role for the ORR via 2 e＆minus; ＆times; 2 e＆minus; pathway.