Improvement in store

General Motors this week announced a $10m research programme to find better ways of storing hydrogen fuel in cars using metal hydrides, and a new hydride blend that punches well above its weight.

General Motors this week announced a $10m (£5.3m) research programme to find better ways of storing hydrogen fuel in cars using metal hydrides, and a new hydride blend that punches well above its weight.

Solid metal hydrides can absorb hydrogen molecules into their structure and release them to power a fuel cell without the danger, bulk and inefficiency of compressed or cooled tanks. The process is well known, but researchers have yet to find a blend that holds sufficient hydrogen and releases it fast enough at low temperatures.

GM and Sandia National Laboratories announced a four-year joint effort on hydrogen storage. The Sandia team will look at the hydride storage hardware needed to optimise size, heat exchange and energy flow without sacrificing efficiency. The researchers will draft several storage designs and materials over the next year, with the aim of building a working prototype by mid-2007.

The team will use sodium aluminium hydride (sodium alanate), which is the current strongest contender as a hydride storage medium. Today it can only store around five per cent hydrogen by weight, however, which is too low to be effective, and requires high temperatures to refuel at a large cost. A number of groups — for example Brookhaven National Laboratory and New Jersey Institute of Technology — are working to improve sodium alanate’s performance.

But Jim Spearot, director of GM’s Advanced Hydrogen Storage programme, said new hydrides other than sodium alanate will need to be developed. ‘Sodium alanate can hold up to six per cent hydrogen that drops to around five per cent, and when you add the “parasitic” weight of the tank storage system it drops further,’ he said. ‘So sodium alanate may not be the final answer.’

GM’s researchers revealed a new blend of hydrides at a Materials Research Society meeting last month. Lithium boro-hydride contains up to 16 per cent hydrogen by weight and is very stable, said Spearot, but in its basic form it is reluctant to release hydrogen. The researchers mixed in other simple hydrides, which diluted the hydrogen weight but also destabilised the lithium boro-hydride enough to release hydrogen at 10 per cent weight.

The lithium boro-hydride blend only yields hydrogen at 280ËšC however, and takes up to 30 minutes to refuel. US Department of Energy targets for 2010 call for a hydride that takes under 10 minutes to charge and releases hydrogen at around 100ËšC.

‘We think there will be other blends that will get us to the desired range. We have high hopes for solid state storage,’ said Spearot. ‘Although even at 280ËšC if you have 10 per cent weight you might be able to use some of the hydrogen to raise the temperature of the bed to help with the release process.’