Effect of wall number on the electro-catalytic activity of nitrogen-doped carbon nanotubes for oxygen reduction reaction

Nitrogen-doped carbon nanotubes (NCNT) were prepared by introducing nitrogen-containing groups into surface of carbon nanotubes of different wall numbers using the pre-oxidation treatment and the ammonia hydrothermal method. Effect of carbon nanotube wall number on the electro-catalytic activity of NCNT for oxygen reduction reaction (ORR) was investigated. X-ray photoelectronic spectroscopy showed that the nitrogen content and the types of nitrogen-containing groups of each NCNT was similar. On the contrary, the content of different nitrogen-containing groups in each NCNT was quite different. Among them, the NCNT sample with an average wall number of 2.5 exhibited the lowest contents of pyridine nitrogen and graphite nitrogen, whereas it exhibited the highest ORR activity in terms of the most positive half-wave potential and the highest limiting current density. This indicated that the ORR activity of NCNT was decided by the number of walls of NCNT, rather than that of pyridine nitrogen and graphite nitrogen active groups when the samples had the similar nitrogen content. For this sample, the inner layer provided an effective conductive path to transfer electrons from the inner layer to the outer layer through the tunneling effect, and then the active sites of the nitrogen-containing groups on the outer wall converted O2 to OH-. Nevertheless, the tunneling effect became weaker with increasing the wall number of NCNT, leading to the decrease of the ORR activity of NCNT. © 2022, Chemical Industry Press Co., Ltd. All right reserved.