Effects of catalyst aging on the growth morphology and oxygen reduction activity of nitrogen-doped carbon nanotubes

Fuel cells for low temperature applications are the subject of intensive research because they could reduce consumption of primary fossil fuels and greenhouse gas emissions. Difficulties come from the sluggish kinetic behavior of cathodic oxygen reduction reaction (ORR). Platinum and Pt-alloys are currently the best catalysts for this reaction. Nevertheless, due to the natural scarcity and cost of platinum, finding alternative non-precious materials is a necessary and very attractive challenge. In the last years, many efforts have been devoted to demonstrate that nitrogen-doped carbon nanostructures, like graphene and nanotubes (N-CNTs), have great potential as Pt substitutes in ORR. Their specific electronic properties are extraordinarily improved by the presence of nitrogen species which disturb the uniform -cloud of carbon nanotubes and increase the localized density of states at the Fermi level. In N-CNTs these modifications impart a n-type dopant activity and an enhancement of the electrocatalytic activity. Furthermore, many papers show that the ORR activity of N-CNTs can be modified by varying preparation method, precursor nature and composition of the catalyst used in the nanotube synthesis. In this work we present some unexpected data about the effect of the aging time of the catalyst used in the nanotubes synthesis, particularly concerning morphology, surface composition and electrocatalytic properties of the obtained N-CNTs. A possible explanation of these results in terms of modification of catalyst composition and morphology is done.