Compact fuel cells could oust batteries

March 03, 2004

HOT on the heels of a warning about the mounting energy demands of smart cellphones comes a ray of hope.

Cellphone giant Nokia last week warned that battery technology is not keeping pace with advanced phone functions - but a trick that boosts the power of miniature hydrogen fuel cells by up to 50 per cent could help keep energy-hungry gadgets up and running.

By shrinking the channels that deliver fuel to the cell's heart, mechanical engineers Suk Won Cha and Fritz Prinz at Stanford University in California have found they can dramatically increase the cell's efficiency.

The apparent downside is that the effect only works with hydrogen fuel cells, whereas liquid methanol is currently the fuel of choice for consumer electronics firms like Motorola and NEC that are developing fuel-cell-powered cellphones and laptops.

They favour methanol because it releases more energy than hydrogen, volume for volume, so methanol-powered gadgets would be able to have smaller "fuel tanks".

However, methanol is toxic, and the fuel cells that use it produce the greenhouse gas carbon dioxide as a waste product.

Hydrogen produces only water, and this, along with Cha's efficiency-boosting trick, could make it a strong contender for fuelling mobile devices.

Fuel cells work by combining the fuel with oxygen from the air and using the energy liberated to drive an electrical current. Cha's fuel cell contains a polymer-based "proton exchange membrane" sandwiched between an anode and cathode layer, each containing a platinum catalyst.

Hydrogen travels to the anode through a polymer block bored with channels 500 micrometres wide.

At the anode, the platinum helps break the hydrogen down into protons and electrons. The protons cross the membrane and react with oxygen and electrons from the cathode, and this drives the electrons left at the anode around an electrical circuit to the cathode.

The Stanford team decided to see what would happen if they made the channels smaller and more numerous.

They used a microchip etching process to bore channels just 20 micrometres wide. The effect was to increase the speed at which the hydrogen is delivered and prevent the anode being flooded with fuel.

This boosted the rate of proton exchange and increased the fuel cell's power by half as much again.

Standard laptop batteries can usually run for 2 to 4 hours without a recharge.

Some hydrogen fuel cell companies hope to produce cells that run up to 20 hours, says Cha, and he claims that his technique should increase the running time by as much as 50 per cent on top of that.

Or you could achieve the same running time for just 70 per cent of the fuel.

David Hart, head of fuel cell research at Imperial College London, says it should be possible to scale up the Stanford team's technique to build much larger fuel cells out of many smaller cells.

But Manfred Stefener, head of Smart Fuel Cells in Germany, is worried about waste water clogging the microchannels.

"The smaller you make your channels, the higher the risk of water getting stuck in that channel," he says. "This can be disastrous." Cha acknowledges the design will have to take account of this.
-end-
New Scientist issue: 6 March 2004

Written by DUNCAN GRAHAM-ROWE

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