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Fill her up with caged hydrogen
24 May 2003
From New Scientist Print Edition.
Nicola Jones

Enlarge A safe hydrogen store

Engineers are a step closer to developing safer, low-pressure fuel tanks for the next generation of electric cars powered by hydrogen fuel cells. They have developed a new material that locks hydrogen gas into cage-like molecules at room temperature, but releases it easily when the fuel is needed. Better still, the material could be customised to store vast amounts of hydrogen safely.

Hydrogen-powered electric vehicles could cut pollution, but a host of problems will have to be overcome before they become commonplace. One of the biggest challenges - apart from making the hydrogen without creating greenhouse gases - is working out how to store the gas.

Today, most vehicles that run on fuel cells store their hydrogen in heavy cylinders at more than 300 times atmospheric pressure. To make them safe they need to be made of layers of carbon fibre, aluminium and steel, and the cylinders weigh between 7 and 20 times as much as the fuel they can hold. "This is the main problem," says Maria Maack, environmental manager at Icelandic New Energy, a company aiming to make Iceland the first country to kick the fossil-fuel habit.

So engineers around the world have been striving to develop new ways to store the gas. Last October, an American team worked out a way to use an icy material called a clathrate (New Scientist, 5 October 2002, p 14) but it has to be cooled with liquid nitrogen.

Most other research has tried to use either metals - like lanthanum and nickel - or organic compounds - like carbon nanotubes - to trap the hydrogen. Some metals will chemically bond hydrogen on their surface. But the metals that hold the most hydrogen also bind it very strongly, so it takes more heat to get the hydrogen out.

And while carbon nanotubes are light and hold a lot of hydrogen, it is hard to mass-produce them in a uniform shape and size, making it tough to control gas capacity.

Now a team of materials chemists led by Omar Yaghi at the University of Michigan has found that cubic, cage-like molecules called metal organic frameworks (MOFs) would combine the best qualities of metals and organics for hydrogen storage (Science, vol 300, p 1127). The MOFs the team is using are made of zinc and oxygen-based metal complexes linked by benzene-based organic molecules.

Instead of binding the hydrogen tightly as metals do, the MOFs only lightly adsorb hydrogen molecules on the metal complexes and their organic connectors. This makes it easy to get them in and out at room temperature and at low pressure.

Twenty times atmospheric pressure is enough to do the trick - about twice the pressure in a cigarette lighter's fuel reservoir.

When the team used a benzene-based compound called 1,4-benzenedicarboxylate as a link between metal complexes, the cubes held about nine hydrogen molecules each. But the researchers doubled and quadrupled that figure just by slotting extra carbon rings into the connectors.

So far, MOFs can't beat the hydrogen capacity of metals, but further tinkering of the organic links could improve them dramatically.

"It's not ready to go commercial," says Michael Ward, a chemical engineer at the University of Minnesota in Minneapolis. But he is impressed by how easy it would be to customise the new material. Both Maack and a spokesperson from Honda - which has some early hydrogen-powered cars on the market - say they are intrigued by the results and are keen to look at new research into low-pressure storage.

From issue 2396 of New Scientist magazine, 24 May 2003, page 18

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