In Search of the Active Site in Nitrogen-Doped Carbon Nanotube Electrodes for the Oxygen Reduction Reaction

Nitrogen-doped carbon nanomaterials are known to exhibit good electrocatalytic activity for the oxygen reduction reaction (ORR). However, the structure of the ORR active site and optimum content of nitrogen in the carbon lattice for ORR activity remains unknown. In this study, a series of vertically aligned carbon nanotubes (VA-CNTs) with a surface nitrogen concentration of 0, 4.3, 5.6, 8.4, and 10.7 atom % is prepared by the alumina template technique and characterized with XRD, Raman spectroscopy, SEM, and XPS. Electrocatalytic ORR activity is investigated by rotating disk electrode (RDE) voltammetry. Among them, VA-CNTs with a nitrogen concentration of 8.4 atom % exhibited the best ORR performance. This is ascribed to a greater number of pyridinic-type nitrogen sites. The good performance of less expensive nitrogen-doped CNTs makes the ORR electrodes a viable alternative to platinum for energy conversion device applications.