N-doped Carbon Nanotubes for the Oxygen Reduction Reaction in Alkaline Medium: Synergistic Relationship between Pyridinic and Quaternary Nitrogen

Abstract

N-doped carbon nanotubes (N-CNTs) prepared using a sustainable methodology based on a ball milling process were applied as catalysts for Oxygen Reduction Reaction (ORR). From two different N-doping methodologies using melamine and urea as N-enriched precursors, resulted in N-CNTs with surface areas in the range 350-364 m(2) g(-1), and surface nitrogen contents from 0.2% to 4.8%, corresponding to three N-types: pyridinic, pyrrolic and quarternary. Irreversible cathodic peaks were observed in the cyclic voltammograms of all catalysts in the presence of an O-2-saturated KOH solution, proving their ORR electrocatalytic activity. Onset potentials (E-on) varied from -0.159 to -0.128 V for N-CNTs, approaching that of Pt/C (-0.124 V). Furthermore, the two best catalysts also exhibited an average number of electrons transferred per O-2 of 3.3 and 3.1 e, respectively. These results suggest an O-2-reduction mechanism which is half controlled by the indirect pathway and half determined by the direct pathway. Moreover, the best N-CNT catalyst exhibited higher durability and methanol tolerance than Pt/C. The correlation between ORR performance data and N-CNT catalyst composition suggests that the N-pyridinic/N-quaternary ratio is more relevant role than the total N content. In fact, the catalysts with the best performance (in terms of E-on and n(O2)) exhibited the highest N-pyridinic/N-quaternary ratio, 5.6 and 3.9 respectively, while their nitrogen contents were 3.1 and 0.8%.