Rational design of nitrogen‒doped carbon nanotubes by defect engineering for Zn‒air batteries with high performance

The rational design of N‒doped multi‒walled carbon nanotubes (MWCNT) by surface engineering was performed by changing the synthetic method, the nitrogen source, and the addition of a surface defect promoter. The solvothermal (S), and the thermal treatment (TT) were used employing melamine or urea as nitrogen precursors. It was found that the activity for ORR through the TT increased in contrast to pure MWCNT and MWCNT modified by the S method. In addition, melamine allowed to obtain better activity than urea as N precursor. However, as the activity of N‒doped MWCNT was not as close to Pt/C, hydrogen peroxide as a surface‒defect promoter was added before the TT. The material obtained using melamine, the TT method, and H2O2 was employed as a highly active material for ORR, presenting onset (Eo) and half‒wave (E1/2) potentials of 0.9 and 0.77 V vs. reversible hydrogen electrode (RHE) respectively, having an activity loss of only 17% after 24 h. In the Zn‒air battery (ZAB), a cell voltage of 1.30 V was achieved, displaying a power density of 101 mW cm−2 and a specific capacity of 650 mA h g−1, being this capacity near to that obtained using Pt/C. Finally, Density Functional Theory allowed to explain how the incorporation of engineering defects and heteroatoms allowed to create a free energy state that increase the electrocatalytic activity.