Nitrogen-doped carbon coatings on carbon nanotubes as efficient oxygen reduction catalysts

Polypyrrole-coated carbon nanotubes (PPy-CNTs) synthesized by an in-situ chemical polymerization approach were heat-treated at 600, 800 and 1 000℃ in N₂ to prepare N-doped carbon coatings on CNTs (NC-CNTs). The specific surface area (SSA) and pore volume of the NC-CNTs increase significantly with heat treatment temperature and the nitrogen content decreases. Also, as the temperature increases the content of pyrrolic-N decreases while that of the graphitic-N increases. The SSA and pore volume of the NC-CNT1000 are 3 and 1.7 times higher than those of the NC-CNT600. As metal free electrocatalysts, the NC-CNTs show significant ORR activity in a basic electrolyte, but it is not proportional to their total nitrogen content. The ORR mechanism of the NC-CNT600 and NC-CNT800 belongs to a two-electron transfer reaction while that of the NC-CNT1000 to a mixture of the two-electron and four-electron transfer reactions. The NC-CNT1000 exhibits the best ORR activity and durability among the samples owing to its highest SSA and pore volume as well as having the largest amount of graphitic-N.