Hierarchically porous nitrogen-doped carbon nanotubes derived from core-shell ZnO@zeolitic imidazolate framework nanorods for highly efficient oxygen reduction reactions

The development of effective nitrogen-doped (N-doped) metal-free carbon nanotube electrocatalysts with high activity and stability to replace the scarce Pt-based catalysts for the oxygen reduction reaction (ORR) is of great importance in energy conversion devices. The hierarchical porosity and content of nitrogen atoms in carbon nanotubes play an important role in the determination of active sites. The use of metal-organic frameworks (MOFs) as templates and/or precursors to fabricate porous carbon could endow the resulting products with their merits including highly doped heteroatoms, large surface areas and controllable structures. Herein, core-shell ZnO@ZIF-8 nanorods were employed as templates and precursors to fabricate hierarchically porous N-doped carbon nanotubes (NCNTs) using a self-sacrifice template approach. The high level of graphitization and nitrogen content of the NCNTs could be tuned by adjusting the core-shell ZnO@ZIF-8 nanorod precursors and pyrolysis temperature. The obtained hierarchically porous NCNTs exhibit superior performances for the ORR in alkaline media. Impressively, NCNT-24-800, which was pyrolyzed at 800 degrees C, exhibits an excellent performance with a positive onset potential of 0.06 V (vs. Ag/AgCl), a more positive half-wave potential at -0.103 V (vs. Ag/AgCl, a positive shift of 27 mV compared with Pt/C) and a high diffusion-limited current density of 5.68 mA cm(-2) at -0.8 V, which are comparable to the benchmark commercial 20 wt% Pt/C catalyst. Furthermore, compared with the traditional Pt/C, the NCNTs show excellent long term stability and methanol tolerance. The self-sacrifice template approach could open up a new avenue towards the fabrication of materials with unique porous structures from MOFs as highly efficient metal-free electrocatalysts for the ORR.