Doped, Defect-Enriched Carbon Nanotubes as an Efficient Oxygen Reduction Catalyst for Anion Exchange Membrane Fuel Cells

A fuel cell basically converts chemical potential energy into electrical energy. At present, it stands as one of the best and most applicable green energy sources. Specifically, proton exchange membrane fuel cells based on hydrogen stand out as they are the most developed and widely used cell type, owing to their high power density and compact system design. Research has shown that bond polarization of dopants and carbon and lattice defects are important aspects in the catalytic mechanisms of oxygen reduction reaction on heteroatom-doped carbon catalysts. Unfortunately, for the aforementioned proton exchange-based fuel cells, the acidic conditions induced by the proton exchange membrane is not favorable for the oxygen reduction reaction. Platinum based catalysts are the best but the prohibitive cost of platinum hinders its wide scale utilization. Various metal-free heteroatom-doped carbon materials have been found to be promising alternative catalysts. Unfortunately, a great deal of the available literature on metal-free catalysts mainly focuses on either bond polarization or lattice defects.

Recently, University of Freiburg researchers: Dr. Chuyen Van Pham, Thomas Böhm, Prof. Simon Thiele in collaboration with Benjamin Britton and Professor Steven Holdcroft at Simon Fraser University successfully designed multi-heteroatom doped defect-enriched carbon nanotubes (MH-DCNTs) that combine both effects to enhance oxygen reduction reaction activity. They achieved this by enriching the lattice defects in the MH-DCNTs by unzipping and length-shortening of carbon nanotubes, and also by creating carbon vacancies via defluorination of fluorine-doped nanotubes. Their work is currently published in the research journal, Advanced Material Interfaces.

In brief, the research method employed commenced with the synthesis of the multi-heteroatom doped defect-enriched carbon nanotubes, in which the lattice defect density was enriched by unzipping and length-shortening of pristine carbon nanotubes, and also by creating carbon vacancies via decomposition of doping fluorine atoms. The researchers then examined the oxygen reduction reaction activity of MH-DCNT by half-cell electrochemical characterization in basic media. Lastly, MH-DCNTs were implemented as cathode catalyst layer in complete Anion exchange membrane fuel cells (AEMFCs) so as to fully evaluate their potential practical application.

The authors observed that the lattice defect density was diminished for pyrolysis temperature at 1000 °C due to annealing of defects. In addition, they noted that rotating disc electrode voltammetry showed the oxygen reduction reaction kinetic current density of MH-DCNT to increase with its lattice-defect density, while the onset potential increased with annealing temperatures.

In summary, the study presented the design and fabrication of doped, defect-enriched carbon nanotubes, and their successful utilization as a cathode catalyst in fuel cells. In general, the optimal MH-DCNT was demonstrated to be one of the most active oxygen reduction reaction metal-free catalysts reported to date and is comparable in performance with conventional platinum-carbon electrocatalysts in both half-cell electrochemical analyses in alkaline media and AEMFCs. Altogether, MH-DCNT is a promising alternative to platinum as a low-cost oxygen reduction reaction catalyst for alkaline fuel cells.