Metal-free and electrocatalytically active nitrogen-doped carbon nanotubes synthesized by coating with polyaniline

Abstract

Nitrogen doping of multi-walled carbon nanotubes (CNTs) was achieved by the carbonization of a polyaniline (PANI) coating. First, the CNTs were partially oxidized with KMnO₄ to obtain oxygen-containing functional groups. Depending on the KMnO₄ loading, thin layers of birnessite-type MnO₂ (10 wt% and 30 wt%) were obtained by subsequent thermal decomposition. CNT-supported MnO₂ was then used for the oxidative polymerization of aniline in acidic solution, and the resulting PANI-coated CNTs were finally heated at 550 °C and 850 °C in inert gas. The samples were characterized by transmission electron microscopy and X-ray photoelectron spectroscopy. A thin layer of carbonized PANI was observed on the CNT surface, and the surface nitrogen concentration of samples prepared from 30% MnO₂ was found to amount to 7.6 at% and 3.8 at% after carbonization at 550 °C and 850 °C, respectively. These CNTs with nitrogen-containing shell were further studied by electrochemical impedance spectroscopy and used as catalysts for the oxygen reduction reaction. The sample synthesized from 30 wt% MnO₂ followed by carbonization at 850 °C showed the best electrochemical performance indicating efficient nitrogen doping.