Carbon tubes pumped up with hydrogen.

To some people, hydrogen has the potential to be a clean, abundant fuel for the 21st century. One obstacle to achieving this goal, however, lies in the difficulty of storing and transporting the fuel safely. Researchers have suggested that carbon nanotubes might serve as ideal containers, acting as tiny vials to encapsulate molecules of hydrogen gas.

Now, by using electricity to draw hydrogen ions into pressed pellets of carbon nanotubes, scientists at the University of Freiburg, Switzerland, have demonstrated a way to package useful amounts of hydrogen. Christoph Natzenadel, Andreas Zuttel, Daniel Chartouni, and Louis Schlapbach report their findings in the January ELECTROCHEMICAL AND SOLID STATE LETTERS.

The Freiburg team compacts nanotubes with either gold or copper powder to make a pellet. The researchers then immerse the product in a solution of potassium hydroxide and run a current through it. By following this procedure, they can pack the pellets with up to 1.95 percent hydrogen by weight, says Nutzenadel. He and his colleagues still aren't sure whether the hydrogen ends up inside the nanotubes, on their surfaces, or between them.

Researchers at the National Renewable Energy Laboratory in Golden, Colo., have employed pressurized gas to get hydrogen into nanotubes, but the hydrogen there represents only 0.01 percent of the weight. A practical fuel cell for a vehicle that runs on hydrogen would probably require nanotubes to reach a storage efficiency of about 6.5 percent, they say.

So far, the scientists have been able to use the electrochemical method only on single-walled nanotubes. Multiwalled nanotubes, consisting of many concentric graphite tubes, crumbled into a powder instead of forming a solid pellet. "I have the feeling that the storage capacity of multiwalled nanotubes could be excellent, but [we have] no results so far," Nutzenadel says. "There are some tricks left we can try."