Nanotube Catalysts

Nitrogen-doped carbon nanotubes could make fuel cells more affordable
Bethany Halford

CARBON NANOTUBES doped with nitrogen have the potential to replace the pricey platinum catalysts used to reduce oxygen in fuel cells, according to research from scientists in Ohio (Science 2009, 323, 760). The finding could bring down the cost of fuel cells, a promising energy technology that's had trouble breaking into large-scale applications—such as automobiles—because of costly catalysts and durability issues.
Doped up: Nanotubes made from carbon (yellow) and doped with nitrogen (blue) could replace platinum in fuel cells.© 2009 Science
Doped up Nanotubes made from carbon (yellow) and doped with nitrogen (blue) could replace platinum in fuel cells.

A team led by Liming Dai of the University of Dayton discovered that a forest of vertically aligned carbon nanotubes, in which some of the carbon atoms have been swapped with nitrogen, can reduce oxygen in alkaline solution more effectively than the platinum catalysts that have been favored in fuel cells since the 1960s. Furthermore, the nanotube catalysts aren't susceptible to the carbon monoxide poisoning that's known to deactivate platinum catalysts.

Dai attributes the high activity of the N-doped nanotube catalyst to the electron-accepting ability of the nitrogen atoms, which creates net positive charge on adjacent carbon atoms. This charge readily attracts electrons from the anode and drives the oxygen reduction reaction. "The demonstration of this new role of nitrogen doping in this paper is important and could be applied to the design and development of various other metal-free, efficient, oxygen-reducing catalysts—even new catalyst materials for applications beyond fuel cells," he says. Dai's group is currently working to incorporate the nanotube catalyst into a working fuel cell.

"This new discovery could have a fundamental impact on the commercial viability of the fuel-cell technology," says Yushan Yan, a chemical engineering professor at the University of California, Riverside. He notes that the results would have been more compelling if Dai's team had carried out the experiments in an acidic medium, where platinum seems to be necessary, instead of in an alkaline medium, where nonprecious metals are known to be as effective as platinum. "Nonetheless, seeing anything that is free of platinum rival platinum in activity and durability is absolutely exciting," Yan adds.