June 1, 2005

'Decorated' nanotubes can aid fuel cell storage

Carbon nanotubes "decorated" with titanium or other transition metals can latch on to hydrogen molecules for efficient hydrogen storage, a capability key to long-term efforts to develop fuel cells.

Using established quantum physics theory, National Institute of Standards and Technology theorist Taner Yildirim and physicist Salim Ciraci of Turkey's Bilkent University predict hydrogen can amass in amounts equivalent to 8% of the weight of "titanium-decorated" singled walled carbon nanotubes. That's one-third better than the 6% minimum storage-capacity requirement set by the FreedomCar Research Partnership involving the Department of Energy and the nation's "Big 3" automakers.

As important, the four hydrogen molecules (two atoms each) that link to a titanium atom relinquish readily when heated. Such reversible desorption is another requirement for practical hydrogen storage.

Resembling exceedingly small cylinders of chicken wire, single-walled carbon nanotubes are among several candidate materials eyed for hydrogen storage. Reaching the 6% target, however, has proven difficult. Positioning a titanium atom above the center of hexagonally arranged carbon atoms (the repeating geometric pattern characteristic of carbon nanotubes) appears to resolve the impasse, according to the study.

The new results, obtained with a method for calculating the electronic structure of materials, surprised the researchers. Interactions among carbon, titanium, and hydrogen seem to give rise to unusual attractive forces. The upshot is four hydrogen molecules can dock on a titanium atom, apparently by means of a unique chemical bond of modest strength. Several forces at work within the geometric arrangement appear to play a role in the reversible tethering of hydrogen, Yildirim said.

Yildirim and Ciraci report their findings "suggest a possible method of engineering new nanostructures for high-capacity storage and catalyst materials."