Nature

news@nature.com - the best science journalism on the web

Published online: 24 August 2007; | doi:10.1038/news070820-14

Hydrogen fuel goes liquid

Nitrogen unlocks the possibility of convenient clean fuels.

Katharine Sanderson

Forget trying to shove gaseous hydrogen into porous materials for safe storage: the future of the clean-fuel economy lies in carrying hydrogen in a liquid, argues Robert Crabtree of Yale University, New Haven.

This means that cars running on fuel cells, which run on hydrogen and oxygen and produce only water as a byproduct, could fill up at stations using roughly the same liquid-fuel infrastructure that already exists. High-pressure gaseous hydrogen, which is potentially dangerous, could be taken completely out of the public sphere. And there would be no need for totally new distribution networks and fuel-delivery systems.

"By using a liquid, we simplify the engineering," says Crabtree, who told the American Chemical Society meeting in Boston, Massachusetts, this week about his work developing nitrogen-packed organic liquids that hold and release hydrogen.

Action packed

Most research on hydrogen storage and transport has focused on materials called metal hydrides and, recently, on metal-organic-frameworks (MOFs) - incredibly porous materials that can be stuffed full of gas. But getting enough hydrogen into these frameworks to make a fuel tank of reasonable size and weight is problematic1, and getting the fuel in and out would require novel fuelling systems.

Instead, Crabtree envisages a system that uses a standard petrol tank containing an organic liquid. This liquid would be passed through a heated module containing a catalyst, which would unlock hydrogen and release it a little at a time to be used as fuel. The remaining dehydrogenated liquid would then be removed at a filling station and whisked away to be reprocessed - the liquid can be hydrogenated and rehydrogenated repeatedly, making it re-usable. Meanwhile the tank would be quickly refilled with fresh, hydrogenated liquid.

The main problem with such liquids is that it usually requires high temperatures (an increase of about 600 degrees Celsius) to unlock the hydrogen - not very practical in a car. Crabtree proposes getting around this by incorporating nitrogen into his organic liquids.

Nitrogen binds to hydrogen less strongly than carbon does, and the presence of nitrogen within a carbon-based ring weakens the remaining C-H bonds. These weakened bonds make it easier to get hydrogen out as the liquid passes over a catalyst, and lower operating temperatures would be needed - the material only needs to be raised by 50 degrees Celsius.

Not hard to handle

The world's largest hydrogen producer, Air Products and Chemicals, based in Allentown, Pennsylvania, is investing serious cash in a similar liquid-based system. "In any future hydrogen economy you would expect it to be much easier to move liquids around rather than gas," says Alan Cooper, a research chemist at Air Products. With the work that's going on now, he says, "consumers of the future will never have to handle hydrogen gas." He is confident that their liquids are close to meeting the US Department of Energy target of creating a fuel tank holding 6% hydrogen by weight by 2010.

Air Products says that public concerns about the safety of high-pressure gas is one of the reasons for their interest in liquids. But Peter Edwards, a hydrogen-storage expert at Oxford University, UK, is less convinced that we need to worry about gaseous hydrogen storage. "High-pressure containment technology advances have been remarkable," he says; he has seen a full high-pressure hydrogen tank sit on a bonfire without incident.

But this doesn't mean liquid technologies should be dismissed. "All potential innovative chemical routes to effective hydrogen storage materials - solid or liquid - are highly important," says Edwards.

References

Story from news@nature.com:

http://news.nature.com//news/2007/070820/070820-14.html

Nature Publishing Group, publisher of Nature, and other science journals and reference works © 2006 Nature Publishing Group

posted to ClimateConcern