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-the-art 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 (∼720 mA h g_Zn⁻¹ at 10 mA cm⁻²) to that of the commercial Pt/C (∼735 mA h g_Zn⁻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.