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Gas tank of the future takes a step closer

Chemists have taken us a little further along the road to a hydrogen economy with a fuel-tank material that might one day replace the automobile petrol tank.

Researchers at the University of Nottingham in the UK and General Motors in Warren, Michigan, have come up with a sponge-like material that can hold 10% of its own weight in hydrogen gas.

The need for effective hydrogen fuel tanks is a major stumbling block for the dream of a hydrogen economy. A litre of liquid hydrogen contains just a quarter of the energy of a litre of petrol, and so treating it like a standard fuel is not an option. But the energy density can be increased if hydrogen gas is squeezed into a porous material able to hold hydrogen like a sponge does water.

So far, though, such materials have not been able to store enough energy to provide a realistic alternative to a car's petrol tank - the previous best attempts, using a carbon structure, can hold around 6 to 7.5% of their weight in hydrogen.

The latest candidate material is a combination of copper atoms and organic molecules called a "metal-organic framework" by chemists.

With this setup, each copper atom is surrounded by a polyhedral "cage" of the organic molecules. These cages slot, or tessellate, together to form a highly porous material with exceptional hydrogen storage capacity, explains Martin Schröder from Nottingham, who led the research.

Cool achievement

"The [10%] figure is a real number - this has been experimentally achieved," Schröder told New Scientist. A number of other hydrogen storage materials are theoretical models rather than practical realities.

However, the material can only achieve that figure at 77 times atmospheric pressure and -196 °C, which limits its real-world possibilities.

"The pressure is actually not so extreme," says Schröder. Standard internal combustion engines already compress petrol to around 12 times atmospheric pressure, and some specialised engines operate at 230 times atmospheric pressure.

"It is the temperature that is the real issue, since we need to be able to achieve these uptakes at higher, more compatible temperatures," he says.

Low temperatures are needed because the weak inter-molecular forces that hold hydrogen within the material are only strong enough to latch onto hydrogen molecules made sluggish by the cold.

New approach

All hydrogen-storage materials face some version of this temperature challenge. One way to avoid it is to use a material that chemically bonds to the hydrogen instead. That so-called "chemisorption" approach has yielded a material that holds up to 14% hydrogen by weight.

But forming and then breaking the chemical bonds requires heating up the material, says Schröder, who thinks the ultimate answer might be a halfway house. "Most people are working towards something intermediate between physical adsorption and chemisorption," he adds.

That could increase the binding energy enough for the hydrogen to be stored at room temperature, while leaving the hydrogen in a gaseous form for easy retrieval.

Schröder's material hints at this intermediate approach: hydrogen molecules cluster around the copper atoms, and appear to be attracted to the metal. Enhancing that effect could raise the sponge's working temperature and potentially produce a real-world hydrogen fuel tank.

The US Department of Energy has set a target to produce a fuel tank that can hold 6% of its weight in hydrogen by 2010. But that figure includes the weight of any devices to pressurise or cool the gas as well as the hydrogen storage material itself, says Schröder, and it has yet to be reached.

Journal reference: Chemical Communications (DOI: 10.1039/b900013e)



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