Hydrogen storage

With technologies for fuel cells now well established, one of the main stumbling blocks to their acceptance is in how to store large quantities of hydrogen.

The answer may be to absorb the hydrogen into a solid material - and researchers in the US believe they have found a way to achieve this.

The team, from the US Department of Energy's Brookhaven National Laboratory and the New Jersey Institute of Technology, is studying sodium aluminium hydride, also known as sodium alanate, which is one of a group of solid metal hydrides that can reversibly absorb hydrogen molecules into its structure.

The effect has been known for some years, but sodium alanate, and the other hydrides, are rather heavy materials and can only store around 5% hydrogen by weight; not enough to be an effective storage medium. Also, high temperatures are generally needed to force the hydrogen into the solid lattice of the hydrides. However, unlike pressurisation or cooling, they don't pose a risk of explosion or freezing, so they are seen as a promising solution.

The team, led by Jason Graetz of Brookhaven, explains that doping the alanate with titanium makes the absorption and release of hydrogen much more efficient. The titanium appears to act as a catalyst for the process, but its mechanism is unexpected, says Graetz: 'We found that the titanium resides on the surface of the sodium alanate as a titanium aluminium compound called titanium aluminide, rather than entering the bulk material and replacing other atoms, or occupying empty spots within the lattice,' he says.

The next stage of Graetz's research is to study the reaction between the hydrogen, the alanate lattice and the titanium aluminide in more detail.

'Understanding that mechanism may help us identify better catalysts for the sodium alanate system and help us find dopants for new compounds that are currently impractical energy storage materials, due to the high temperatures and pressures required for the release and re-absorption of hydrogen.'