|Illustration of charge transfer process and oxygen reduction reaction on PDDA-CNT [poly(diallyldimethylammonium chloride)-carbon nanotube]. Credit: ACS, Wang et al. Click to enlarge.|
Researchers at Case Western University have developed catalysts made of carbon nanotubes dipped in a polymer solution that equal the energy output and otherwise outperform platinum catalysts in fuel cells. A paper on their work is published in the Journal of the American Chemical Society.
The team led by Liming Dai, a professor of chemical engineering, is certain they can boost the power output and maintain the other advantages by matching the best nanotube layout and type of polymer. They’ve already shown the simple technique can significantly reduce fuel cell cost. While platinum, which represents at least a quarter of the cost of fuel cells, currently sells for about $65,000 per kilogram, the researchers say their activated carbon nanotubes cost about $100 per kilogram.
Dai and research associates Shuangyin Wang and Dingshan Yu found that by soaking carbon nanotubes in a water solution of the polymer poly(diallyldimethylammoniumn chloride) (PDDA) for a couple of hours, the polymer coats the nanotube surface and pulls an electron partially from the carbon, creating a net positive charge.
Having a strong electron-withdrawing ability, poly(diallyldimethylammonium chloride) (PDDA) was used to create net positive charge for carbon atoms in the nanotube carbon plane via intermolecular charge transfer. The resultant PDDA functionalized/adsorbed carbon nanotubes (CNTs), either in an aligned or nonaligned form, were demonstrated to act as metal-free catalysts for oxygen reduction reaction (ORR) in fuel cells with similar performance as Pt catalysts.—Wang et al.
They placed the nanotubes on the cathode of an alkaline fuel cell. There, the charged material acts as a catalyst for the oxygen-reduction reaction (ORR) that produces electricity while electrochemically combining hydrogen and oxygen. In testing, the fuel cell produced as much power as an identical cell using a platinum catalyst.
The activated nanotubes also last longer and are more stable, the researchers said. Unlike platinum, the carbon-based catalyst doesn’t lose catalytic activity and, therefore, efficiency, over time; isn’t fouled by carbon monoxide poising; and is free from the crossover effect with methanol. Methanol, a liquid fuel that’s easier to store and transport than hydrogen, reduces activity of a platinum catalyst when the fuel crosses over from the anode to the cathode in a fuel cell.
...we have demonstrated that certain polyelectrolyte (e.g., PDDA) functionalized carbon nanotubes, either in an aligned or nonaligned form, could act as metal-free electrocatalysts for ORR. It is notable that the PDDA adsorbed vertically aligned CNT electrode possesses remarkable electrocatalytic properties for ORR—similar to that of commercially available Pt/C electrode but a better fuel selectivity and long-term durability. Furthermore, this work clearly indicates that the important role of intermolecular charge transfer to ORR for nitrogen-free carbon nanotubes can be applied to other carbon materials for the development of various other metal-free efficient ORR catalysts for fuel cell applications, even new catalytic materials for applications beyond fuel cells. Therefore, the methodology developed in this study could serve as a general approach to the development of various metal-free catalysts.—Wang et al.
The new process builds on the Dai lab’s earlier work using nitrogen-doped carbon nanotubes as a catalyst. In that process, nitrogen, which was chemically bonded to the carbon, pulled electron partially from the carbon to create a charge. Testing showed the doped tubes tripled the energy output of platinum.
Dai said the new process is far simpler and cheaper than using nitrogen-doped carbon nanotubes and he’s confident his lab will increase the energy output as well.
Shuangyin Wang, Dingshan Yu, Liming Dai (2011) Polyelectrolyte Functionalized Carbon Nanotubes as Efficient Metal-free Electrocatalysts for Oxygen Reduction. Journal of the American Chemical Society doi: 10.1021/ja1112904