Efficiency and long-term durability of a nitrogen-doped single-walled carbon nanotube electrocatalyst synthesized by defluorination-assisted nanotube-substitution for oxygen reduction reaction

Nitrogen-doped carbon nanomaterials are known to be excellent electrocatalysts for the oxygen reduction reaction (ORR) in polymer electrolyte fuel cells. In this study, we used a new and simple post-doping synthesis method to prepare nitrogen-doped single-walled carbon nanotubes (SWCNTs) by reacting fluorinated SWCNTs with ammonia gas at 300-600 °C. The structure and morphology of the N_x°C-doped SWCNTs (x: reaction temperature) synthesized by the defluorination-assisted nanotube-substitution reaction were characterized. Their levels of nitrogen doping (1.38-3.04 at%) are fairly high, with enriched pyridinic- and pyrrolic-nitrogen species. Their electrochemical catalytic activity for ORR in 0.5 M H₂SO₄ was evaluated by cyclic voltammetry and linear sweep voltammetry, and their catalytic durability was assessed in load-potential cycle tests. For the N_x°C-doped SWCNTs, the peak and the onset potential (E_peak and E_onset, respectively) shifted towards the positive and the current density (j@E_1/2) at the half-wave potential (E_1/2) increased with increasing reaction temperature. The E_onset values of N_500°C- and N_600°C-doped SWCNTs were +0.51 V while that of the non-fluorinated hc-SWCNTs was +0.16 V. The number of electrons transferred per oxygen molecule (n) in ORR was determined to be 2.32-3.64 at the potential of -0.3 V. The ORR catalytic activity was evaluated comprehensively through the measured parameters E_peak, E_onset, n, and j@E_1/2. The N_500°C-doped SWCNTs possessed the highest ORR catalytic activity. After 11000 cycles in the load-potential test, its current density remained at 93% of the initial value, indicating better durability than platinum nanoparticles supported on carbon black (Pt-C).