Fe-, S-, and N-Doped Carbon Nanotube Networks as Electrocatalysts for the Oxygen Reduction Reaction

It remains a challenge to develop an efficient and facile method to synthesize nonprecious metal electrocatalysts with controlled structure and desired properties. This study reports a Acid two-step pyrolysis method plus acid leaching between the two 900 degrees C pyrolysis stages to synthesize Fe-, S-, and N-doped carbon nanotubes (CNTs) with a controlled size as efficient electrocatalysts for the oxygen reduction reaction (ORR). The synthesis involves a first-step pyrolysis at 700 degrees C to form small-diameter CNTs, a subsequent acid-etching to remove the majority of the metal nanoparticles, and a second-step pyrolysis at 900 degrees C to improve the crystallinity but retain the small diameter of the CNTs. The method is proved to be effective in achieving a large specific surface area and stabilizing the S species of the catalyst. Owing to the synergistic effect between the S functionalities and Fe-N-x sites, the high specific surface area, and the 3D interconnect network of small-diameter CNTs, the catalyst exhibits superior activity toward the ORR with a half-wave potential of 0.854 V and a limiting current density of 5.62 mA cm(-2) in alkaline medium, better than those prepared by one-step pyrolysis and the commercial Pt/C. The present simple synthetic strategy for small-diameter CNTs may have potentials for diverse applications.