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issue: May 2004 APPLIANCE Magazine

Electronics Report
Micro Fuel Cells


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The need to regularly recharge batteries in mobile phones, laptops, and other portable terminals may be eliminated in the future with the use of fuel cells.

Pictured is cross-section analysis of a fuel cell showing a buried gas channel surrounded by a nanoporous silicon layer - with a very high surface area. (The four small dark areas below the gas channel are artifacts and not part of the process.

STMicroelectronics (Geneva, Switzerland) has developed fuel cells that are reportedly small enough to fit inside a mobile handset and are able to generate all the electrical energy needed to power the phone from liquid methanol, a low-cost and easily available organic fuel.

A fuel cell, a device that generates energy using electrochemical reactions instead of fuel combustion, contains a pair of electrodes—the anode and the cathode—pressed against either side of an electrolyte, explains Savlo Coffa, director, STMicroelectronics R&D. “Fuel such as methanol, for instance, is fed into the anode side of the cell, and the hydrogen in the fuel splits into a proton and an electron,” Mr. Coffa says. “At the same time, oxygen in the form of air enters through the cathode side. The proton from the fuel then passes through the membrane to the cathode, where the oxygen from the air is. The electron, on the other hand, creates an electric current, which can be used as such before entering the cathode where it is reunited with oxygen molecules to produce water as a harmless by-product. Carbon dioxide is also produced at the anode.”

Although the fuel cells are small, they are able to provide power because the 3-D structure of the cells maximizes the active area between the gas channel’s circulation, the electrode membrane, and the catalysts while minimizing overall size and volume, Mr. Coffa says. “Using fuel cells instead of batteries would make mobile phones lighter and much more convenient to use, as they could simply be topped up with fuel whenever necessary,” he points out. Additionally, emissions of this type are much smaller than combustion emissions. The by-products are mainly water and a low level of carbon dioxide—much lower than that of fuel combustion, Mr. Coffa says.

“Potentially, any portable device could benefit from the use of the fuel cell to recharge a main battery—in particular, any device that manages power by maintaining a standby mode for periods of times,” Mr. Coffa remarks. Devices such as personal digital assistants (PDAs) and laptop computers might ultimately benefit, but other hand-held devices such as electric shavers, portable CD players, camcorders, cordless tools, and electric mixers may benefit as well, he explains. “It is important to take into account that use in mobile phones is one of the most demanding applications since it requires high power to be generated in a very small volume,” Mr. Coffa notes. “Specifications for applications for a laptop, for example, are much more relaxed since more space is available.”

However, there are still some challenges with applying fuel cell concepts. With mobile phones, the power source—battery or fuel cell—must be able to deliver approximately 300mA of current at 3.6 V. It also must not occupy a volume of more than about 12 cu cm. “However, the output current of a fuel cell is directly related to the common surface area between the electrodes and the membrane,” Mr. Coffa points out. “To obtain 300mA of current using conventional fuel cell technologies would require a surface area of around 60 cm, much larger than the space available in a mobile phone.”

Although this remains a challenge, he says, STMicroelectronics has made significant progress in overcoming this problem by developing new technologies in which the fuel cell could be implemented as a 3D structure containing thousands of buried microchannels that maximize the contact area between the gases, the catalysts, and the electrodes. “The small pore sizes give the layer a very large effective surface area, thus increasing the efficiency of the catalysis,” Mr. Coffa explains.

STMicroelectronics

 

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