Fuel cell hydrogen promises portability

Engineers at Purdue University have developed a way of producing hydrogen for fuel cells to automatically recharge batteries in portable electronics, such as notebook computers, and eliminate the need to use a wall outlet.

The researchers developed the method earlier this year and envision a future system in which pellets of hydrogen-releasing material would be contained in disposable credit-card-size cartridges.

Once the pellets were used up, a new cartridge would be inserted into devices such as mobile phones, personal digital assistants, notebook computers, digital cameras, handheld medical diagnostic devices and defibrillators.

The method also might have military applications in portable electronics for soldiers and for equipment in spacecraft and submarines.

The technique combines two previously known methods for producing hydrogen. The previous methods have limitations making them impractical when used alone, but those drawbacks are overcome when the methods are combined.

One of the methods was invented by Herbert C Brown, a chemist and Nobel laureate from Purdue who discovered a compound called sodium borohydride during World War II. The compound contains sodium, boron and hydrogen.

He later developed a technique for producing hydrogen by combining sodium borohydride with water and a catalyst. The method, however, has a major drawback because it requires expensive catalysts such as ruthenium.

The other method involves a chemical reaction in which tiny particles of aluminium are combined with water in such a way that the aluminium ignites, releasing hydrogen during the combustion process.

This method does not require an expensive catalyst, but it yields insufficient quantities of hydrogen to be practical for fuel cell applications.

The latest solution is to combine both methods by using what they call a triple borohydride-metal-water mixture, which does not require a catalyst and has a high enough hydrogen yield to make the method promising for fuel cell applications.

So far experiments have shown that it can convert 6.7% of the mixture to hydrogen, which means that for every 100 g of mixture it can produce nearly 7 g of hydrogen and that yield is already better than alternative methods.

The researchers hope to increase the yield to about 10% through additional experiments.

Hydrogen produced by the method could be used to drive a fuel cell, which then would produce electricity to charge a battery. A computer chip would automatically detect when the battery needed to be recharged, activating a new pellet until all the pellets on the cartridge were consumed.

By-products from the reaction are environmentally benign and can either be safely discarded or recycled.

In addition to its potential use in portable electronics, the technology offers promise as an energy source to power hardware in spacecraft.

The Apollo 13 accident was caused by an explosion involving liquid oxygen, which is needed along with liquid hydrogen to feed a fuel cell in spacecraft. Use of chemical mixtures, such as this, for generating hydrogen and oxygen would eliminate the possibility of such an explosion.

A key step in the hydrogen-producing reaction is the use of tiny particles of aluminium only about as wide as 100 nanometres, or 100 billionths of a metre.

You don't want to use large lumps of aluminium because then you only get reactions on the outer surfaces of those lumps, so not enough hydrogen is produced. Tiny particles have a high surface area, which enables them to completely react, leaving no waste and producing more hydrogen.

Another crucial component is a special gel created by combining water with a material called polyacrylamide.

If you want to ignite a mixture of aluminium with water, the problem is that water boils at 100°C and aluminium ignites at a much higher temperature. If you try to ignite the mixture you just vaporise water and the aluminium doesn't ignite.

When the gel is used, water boils at a much higher temperature and the nanoscale powder also decreases the ignition temperature of aluminium. So you are both increasing the boiling point of water and decreasing the ignition temperature of aluminium, making the reaction possible.

The researchers believe they will be able to safely dissipate the heat produced by the reaction, making the technology practical for portable electronics.