Effect of carbon nanotube diameter for the synthesis of Fe/N/multiwall carbon nanotubes and repercussions for the oxygen reduction reaction

The effect of the diameter of multiwalled carbon nanotubes for the incorporation of N and Fe and the consequences for the oxygen reduction reaction in acid medium has been studied. For this, a series of multiwalled carbon nanotubes with mean diameters of 10, 20 and 60 nm have been thermally treated in acid media and modified by addition of N- and Fe-groups by means of thermal treatments under inert atmosphere. The chemically treated nanotubes and the FeN-CNTs have been thoroughly characterized by N2 adsorption/desorption isotherms, Raman spectroscopy, X-ray photoelectron spectroscopy, elemental analysis, thermogravimetric analysis and X-ray diffraction. The performance of the FeN-CNTs for the oxygen reduction reaction (ORR) in acid medium has been evaluated by means of electrochemical techniques. We have found that the amount of nitrogen actually incorporated onto the multiwalled carbon nanotubes, which ranges between 2 and 3 wt.%, can be directly related with the number of defects of the chemically treated multiwalled nanotubes. On the other hand, the BET specific surface area of the FeN-CNTs increases with the decreasing diameter of the CNTs. A direct relationship between the nitrogen external surface area and the ORR performance has been observed. [Display omitted] •Fe/N centres have been created onto chemically treated MWCNT with different mean diameters.•The amount of N incorporated onto the MWCNT increases with their mean diameter.•Fe/N/CNTS are very active electrocatalysts for the ORR in acid medium.•N-sites located at the external surface area are the most actives for the ORR.