Carbon nanotubes (CNTs) have received much research interest for energy conversion and storage due to their inherent extraordinary electrical and mechanical properties. Modified CNTs could be used as electrocatalyst supports for the oxygen reduction reaction (ORR) in fuel cells and for the oxygen evolution reaction (OER) for water electrolyzers. Here we present metal oxides supported on CNTs as bifunctional catalysts for reversible alkaline membrane fuel cells.

In order to introduce more active sites and further optimize both the ORR and OER activity in alkaline media, growing transition metal oxides, such as cobalt and nickel oxides, along pre-oxidized CNTs is a state-of-the-art technique. However, in this study, we have discovered that the oxidation degree of the support CNTs (i.e., different oxidizing agents and length of oxidation time) may lead to variable proportions of oxygen functional groups or defective sites, which can further influence the size of metal oxides and their distribution on the CNTs.

As shown in Fig. 1a and b, the support CNTs were pre-oxidized by KMnO₄ and concentrated HNO₃, respectively, resulting in different densities of metal oxide particles along the tubes. Comparing different length of oxidation time in Fig. 1b and c, the CNTs oxidized by concentrated HNO₃ for 0.5 h (Fig. 1c) have much fewer particles anchored than the ones with 6 h oxidation (Fig. 1b).  All of these feature changes would have a huge impact on the activity and durability of the bifunctional catalysts. 

This work may provide novel perspectives for design of CNTs as support materials in electrochemical applications.

Acknowledgement: The project is financially supported by the Department of Energy’s Fuel Cell Technology Office under the Grant DE-EE0006960.

Figure 1.  Metal oxide particle growth along CNTs based on various oxidation degrees