The secrets of storing hydrogen

These ultra-lightweight materials, Li4BN3H10 (left) and LiNH2 (right

Ultra-lightweight materials, Li4BN3H10 (left) and LiNH2 (right), offer significant hydrogen storage potential. Amazingly, they contain a higher density of hydrogen (illustrated as white spheres) than liquid hydrogen itself.

Concern over climate change and a dwindling supply of fossil fuels mean that there is increasing interest in alternative forms of energy. Hydrogen shows much promise as a clean, green fuel. However, storing hydrogen is not easy. ISIS has enabled studies of materials that have the potential to store hydrogen cheaply and efficiently.

We have a plentiful supply of hydrogen on Earth: over three-quarters of our planet is covered in water. Hydrogen can be used as a very clean fuel because the only by-product is water. Fuel cells, discovered by William Groves almost 180 years ago, are the most environmentally friendly way of utilising hydrogen – the chemical combination of hydrogen and oxygen within the fuel cell produces a large amount of electrical energy, far greater than that provided by equivalent-sized batteries.
Finding the right kind of material to store hydrogen is proving difficult. Fuel cells have power levels that match internal combustion engines. However, the best solid-state hydrogen stores at present weigh hundreds of kilograms – too heavy for practical use in cars. To travel 300 km, a standard car would require about 5 kg of hydrogen. To come close to matching the weight of a petrol tank, scientists have to find materials that store and cycle almost ten times more hydrogen than is currently attainable.
To tackle this challenge, chemists at the University of Oxford have been synthesising hundreds of new materials.Those that are the most promising are then analysed using neutron diffraction at ISIS. Professor Bill David from RAL,who leads this part of the project, says, “We investigate our best hydrogen storage materials by mimicking their performance in a car. We cycle hydrogen in and out of these stores and monitor the hydrogen weight loss and gain. The unique extra dimension to our experiments is that we do all these measurements in the neutron beam.This allows us to see what is happening at atomic level and enables us to build up a complete understanding of these promising hydrogen storage materials."
One of the most exciting materials they have looked at so far is Li4BN3H10. This material can store hydrogen at a much higher weight percent – around 12%. Using neutron diffraction, David and his colleagues were able to analyse the structure of the material and see exactly where the hydrogen is stored and how mobile it is. There is, however, one drawback – Li4BN3H10 is a ‘oneshot’ store and is not easily recycleable. Nonetheless it could still be useful as a disposable store for a fuel-cell rather like the AA battery. Perhaps, thanks to ISIS, we are one step closer to using hydrogen instead of fossil fuels.

B David (ISIS), P Edwards, M Jones (University of Oxford)

Research date: Feb/Mar 2006

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