Preparation And Oxygen Reduction Performances Of Nitrogen-Doped Carbon Nanotubes/Carbon Composites

Due to their unique structure as well as synergistic effect, nitrogen-doped carbon nanocomposites have exhibited enhanced catalytic activity towards oxygen reduction reaction(ORR) and have also been considered as the promising metal-free cathodic catalysts in fuel cells. In this thesis, two types of nitrogen-doped carbon nanotubes/carbon composite catalysts were prepared by in situ polymerization followed by high temperature pyrolysis. The electrochemical performances of these catalysts towards ORR in alkaline medium were also investigated. The main research contents are shown as follows:(1) Preparation and ORR performance of the nitrogen-doped carbon nanotubes/carbon composite(Ⅰ). Firstly, melamine-functionalized carbon nanotubes(CNT-MAM)was prepared using a noncovalent functionalization approach. Then the composite of CNTs and poly(melamine-formaldehyde)(MCPMF) was synthesized by the in situ polymerization of melamine and formaldehyde in the presence of CNT-MAM in DMSO. Finally, the nitrogen-doped CNTs/carbon composite catalyst(NCNT-m-900)was prepared by directly pyrolyzing the MCPMF precursor at 900 °C in nitrogen atmosphere. It was found that the carbon nanoparticle aggregates were attached to the surface of CNTs. The as-synthesized composite catalyst showed very high nitrogen content up to 10 atom%. The catalytic activity of the NCNT-m-900 towards ORR in alkaline medium was evaluated using rotating ring-disk electrode voltammetry. The NCNT-m-900 exhibited excellent electrochemical performance towards ORR with an onset potential of –0.029 V(vs Ag/AgCl), a high kinetic current density of 25.3 mA cm–2 at –0.35 V, a dominant four-electron transfer mechanism(n = 3.83 at –0.35 V),as well as excellent methanol tolerance and durability.(2) Preparation and ORR performance of the nitrogen-doped carbon nanotubes/carbon composite(Ⅱ). Firstly, CNT-MAM complex was synthesized by noncovalent functionalization of CNTs with melamine. Secondly, the composite of CNTs and poly(melamine-benzaldehyde)(MCPMB) was synthesized by the polymerization of melamine and benzaldehyde in the presence of CNT-MAM in DMSO. Finally, the nitrogen-doped CNTs/carbon composite catalyst(NCNT-NC-900) was prepared by directly pyrolyzing the MCPMB precursor at 900 °C under nitrogen atmosphere. The catalytic activity of NCNT-NC-900 towards ORR was evaluated using rotating disk electrode voltammetry. The as-synthesized NCNT-NC-900 exhibited superiorelectrochemical performance towards ORR with an onset potential of –0.035 V(vs Ag/AgCl), a high kinetic current density of 26.7 mA cm–2 at –0.35 V, a dominant four-electron transfer mechanism(n = 3.77 at –0.35 V), as well as excellent durability.