Facile fabrication of N/S-doped carbon nanotubes with Fe3O4 nanocrystals enchased for lasting synergy as efficient oxygen reduction catalysts

Transition metal-doped carbon materials are regarded as a promising replacement of commercial Pt/C catalysts for the oxygen reduction reaction (ORR) in polymer-electrolyte-membrane fuel cells and metal-air batteries. The current fabrication methods are generally very complex and involve the introduction of foreign species onto the surface or into the voids of carbon nanostructures; this leads to loose attachment and severe aggregation over long term usage, weakening the synergetic effects between the host and guest species. Herein, we report a facile and scalable method to fabricate Fe, N, and S co-doped carbon nanotubes (Fe-NSCNT). Specifically, iron species were precipitated in situ and further converted to Fe3O4 nanoparticles enchased in the wall structures of N/S-doped CNTs (NSCNTs), resulting in a greatly reinforced synergistic effect. The Fe-NSCNT catalysts thus obtained showed excellent ORR performance, with a four-electron selectivity, high methanol tolerance, enhanced stability (no significant loss after 6 h, cf. 19% loss for 20% Pt/C), and high diffusion-limited current density (6.01 mA cm(-2), higher than 5.79 mA cm(-2) of the commercial Pt/C), comparable to that of the state-of-theart Pt/C catalyst in alkaline media. Furthermore, when used as Zn-air battery cathode materials, the Fe-NSCNT catalyst enabled the same voltage (1.17 V at 20 mA cm(-2)) and specific capacity comparable (similar to 720 mA h gZn(-1) at 10 mA cm(-2)) to that of the commercial Pt/C (similar to 735 mA h gZn(-1) at 10 mA cm(-2)), indicating its great potential in replacing Pt/C for the practical applications in noble metal-free Zn-air batteries.