Catalytic trends of nitrogen doped carbon nanotubes for oxygen reduction reaction

Replacing the state-of-the-art fuel cell catalyst platinum for a cheaper and abundant alternative would make the hydrogen economy viable. Both nitrogen-doped graphene and nitrogen-doped carbon nanotubes (N-CNT) have been shown to be capable of acting as a metal-free catalyst for the oxygen reduction reaction (ORR). Until now, most of the research has been focused on the nitrogen doping and less on the structure of the nanotubes. Here, density functional theory calculations are used to calculate trends in ORR catalytic activity of graphitic-N-doped CNTs of different sizes and chirality of selected tubes between (4,0) and (20,10). This includes 13 armchair tubes, 17 zig-zag tubes and 42 chiral tubes, or 72 N-CNTs in total. 22 tubes are predicted to have a lower overpotential than the platinum catalyst and 46 tubes have lower overpotential than nitrogen doped graphene. The most active tubes are (14,7), (12,6), and (8,8), and display an overpotential of around 0.35 V, or 0.1 V lower overpotential than predicted on Pt(111) with the same level of theory.

N-Doped carbon nanotubes enriched with graphitic nitrogen in a buckypaper configuration as efficient 3D electrodes for oxygen reduction to H2O2

Herein, a series of N-doped carbon nanotube (CNx) samples were obtained by modifying the synthesis temperature. Consequently, the proportion of graphitic nitrogen (Ngraph) in the samples was systematically increased as a function of temperature. This allowed evaluation of the role of the CNx graphitic nitrogen in the oxygen reduction reaction (ORR). A correlation between the Ngraph content and the ORR onset potential was observed, which shifted to more positive potentials with an increase in kinetic current density (jk); this showed that Ngraph played a significant catalytic role in the ORR. The samples with high Ngraph content favored the two-electron pathway for the ORR not only in basic media (pH = 13) but also in neutral media (pH = 7), representing an attractive alternative for wastewater remediation through the on-site generation of H2O2. The energetic calculations showed that the formation of H2O2 must be favorable in the presence of graphitic nitrogen sites. Finally, the performance of the buckypaper arrangement was evaluated, and the CNx buckypaper showed a higher cathodic current peak as compared to CNx traditional ink dispersions. Overall, this study not only sheds light on the role of Ngraph in the ORR, but also demonstrates that CNx buckypaper is an efficient 3D electrode for electrocatalytic applications.