Abstract
Nitrogen (N)-doped carbon materials (NCMs) are a promising catalyst for oxygen reduction reaction (ORR). However, the formation process of N species in NCMs has never been discussed and which type of N species created ORR active sites is still under debate, which restricts the exploitation of high performance NCMs catalysts. Herein, we firstly prepared a variety of N-doped carbon nanotubes (NCNTs-t) with well-controlled N species derived from oxidized carbon nanotubes (OCNTs-t) with well-defined oxidation degree, to investigate the transformation from oxygen-containing groups on OCNTs-t to consequential N species in NCNTs-t and the ORR catalytic role of different types of N species. A linear relationship existed between the oxygen content on OCNTs-t and the N content in NCNTs-t. Meanwhile, the ketone (CO) and carboxyl (COOH) groups were favorable to form the pyridinic N, while the hydroxyl (OH) and epoxy (C(O)C) groups tended to produce graphitic N. Furthermore, the pyridinic N was demonstrated to play the major role on the ORR activity, in which the active sites are the carbon atoms bonded to the pyridinic N.