US scientists have developed a technique to produce 'hydrogen on demand' to act as a pollution-free energy source for vehicles and other engine-powered devices.

The technique developed at Purdue University produces hydrogen by adding water to an alloy of aluminium and gallium. The aluminium splits water by attracting oxygen and liberating hydrogen in the process.

The Purdue researchers are developing a method to create particles of the alloy that could be placed in a tank to react with water and produce "hydrogen on demand".

The gallium is a critical component because it hinders the formation of an aluminium oxide skin normally created on aluminium's surface after bonding with oxygen, a process called oxidation.

This skin usually acts as a barrier and prevents oxygen from reacting with aluminium.

Reducing the skin's protective properties allows the reaction to continue until all of the aluminium is used to generate hydrogen, explained Jerry Woodall, a distinguished professor of electrical and computer engineering at Purdue, who invented the process.

The gallium component is inert, which means that it can be recovered and reused.

"This is especially important because of the currently much higher cost of gallium compared with aluminium," said Professor Woodall.

"Because gallium can be recovered, this makes the process economically viable and more attractive for large-scale use.

"Also, since the gallium can be of low purity, the cost of impure gallium is ultimately expected to be many times lower than the high-purity gallium used in the electronics industry."

As the alloy reacts with water the aluminium turns into aluminium oxide, also called alumina, which can be recycled back into aluminium.

The recycled aluminium would be less expensive than mining the metal, making the technology more competitive with other forms of energy production, Woodall claimed.

The research findings are detailed in the first research paper about the work, which will be presented on 7 September during the second Energy Nanotechnology International Conference in Santa Clara.

The paper was written by Woodall, Charles Allen and Jeffrey Ziebarth, both doctoral students in Purdue's School of Electrical and Computer Engineering.