The influence of carboxyl group on nitrogen doping for defective carbon nanotubes toward oxygen reduction reaction

To investigate the effect of different oxygen functional groups (OFGs) on nitrogen doping for defective multi-walled carbon nanotube (DMWNT), the DMWNTs were annealed at various temperatures (300 °C, 500 °C, 700 °C, 900 °C) by rapid thermal annealing (RTA) to remove parts of OFGs before nitrogen doping. After the OFGs were removed, the oxygen reduction reaction (ORR) activity in 0.1 M HClO4 decreased. The greatest decrease was observed between 300 °C and 500 °C annealed samples, with onset potential shifting from approximately 0.86 V–0.82 V vs. reversible hydrogen electrode (RHE), indicating paired carboxyl groups contribute to the formation of active sites. The overall nitrogen content, FeNx content, and electron transfer number all followed the same pattern. These findings support the importance of carboxyl groups in nitrogen doping and the formation of FeNx structure in DMWNTs.