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The key to the device is a new polymer that protects the fragile enzymes used to break down the ethanol fuel, scientists told the American Chemical Society's annual meeting in New Orleans on Monday. Enzyme-based batteries have the potential to be cheaper than fuel cells that rely on expensive platinum or ruthenium catalysts. Pure hydrogen is an explosive gas and difficult to store, so fuel cells often use a chemical source. Ethanol is used in Minteer's cell, and the enzymes strip off the hydrogen. But the enzymes are sensitive to slight changes in pH and temperature and can rapidly degrade and become inactive. Until now no bio-battery had enzymes that lasted for more than a few days. Specially tailored pores The typical approach to solving this problem has been to immobilise the enzymes by attaching them to the fuel cell's electrodes, but they still tend to decay too quickly to be useful. More on this story 36 Subscribe to New Scientist for more news and features Related Stories 37 Food scraps could help power homes 10 October 2002 38 Sewage turned into hydrogen fuel 29 April 2002 39 Soon you could plug your palmtop into your clothes 11 April 2001 For more related stories search the print edition 40 Archive Weblinks 41 Toshiba fuel cells 42 Energy Related Devices 43 American Chemical Society meeting So Shelley Minteer and her colleagues at St Louis University in Missouri coated the electrodes with a polymer that has specially tailored pores. These maintain a neutral pH, while being small enough to trap the enzymes yet big enough to let the alcohol pass through. Thanks to the polymer, the new bio-batteries have power densities 32 times greater than those of other groups, the team claim. Toshiba has just unveiled its first miniature fuel cell, which uses a metal catalyst and runs on methanol. Minteer says: "The main advantage of ethanol over methanol is that it is simply more readily available. However, unlike the Toshiba prototype, the cell is still too large for portable use. The group is currently working to shrink the technology, perhaps by tweaking the polymer-enzyme matrix in order to increase its surface area further.
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