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ST, CEA concoct tiny fuel cell for wireless phones

Posted: 05 Jun 2008 ?? ?Print Version ?Bookmark and Share

Keywords:mobile? battery? fuel cell?

As part of research collaboration on miniaturized energy sources, the French Atomic Energy Agency (CEA) and STMicroelectronics NV unveiled a hydrogen fuel cell for mobile phones aiming to reduce the use of electrical power supplies to recharge batteries.

The miniature fuel cell is made using microfluidic structures etched at the back surface of silicon and a reaction interface at the silicon surface. This works together with a hydrogen-filled cartridge having the size of a small cigarette lighter. Currently at its prototype stage, the final product will be out in the markets by end of 2009 or early 2010.

"Five years ago, cellphones used 1W of power while 3G phones now use from 3W to 5W. With the next-generation cellphones, if we will not change the battery, the autonomy will be reduced by one-third limiting access to multimedia services," said Didier Marsacq, director of CEA-Laboratory for Innovation in New Energy Technologies and Nanomaterials (Liten), during a press conference at CEA headquarters in Paris.

The proliferation of mobile devices has generated an exceptional demand for rechargeable batteries, but the existing solutions leave much to be desired in terms of performance, operating life and environmental impact, according to CEA-Liten.

Architecture of a silicon-based micro fuel cell

The growing battery market
Li-ion technology represents the fastest growing rechargeable battery market, according to the Santa Clara Consulting Group and Japanese and Korean firms such as Sony, Sanyo and Matsushita, LG and Samsung, holding about 70 percent of the market.

Marsacq said CEA's energy source comprises a hydrogen-based fuel cell core unit developed jointly by CEA-Liten and ST; an energy management unit developed by ST; and a hydrogen cartridge developed by Bic, manufacturer of cigarette lighters, ballpoint pens and shaving razors.

The cathode, protected by a pierced coating that lets oxygen in tops the polymer membrane used as electrolyte. Below, the anode is accessed via channels with gaseous hydrogen coming in. All is built on a silicon substrate using microelectronics process technologies.

CEA-Liten said it has selected the proton exchange membrane fuel cell technique. In this case, the electrolyte, that is to say the internal component between the anode and the cathode, where H+ ions (protons) circulate, is solid. It is composed of a polymer and takes the form of a membrane.

For the hydrogen storage, CEA-Liten added that it has adopted a chemical compound named sodium borohydride (NaBH). NaBH, in the form of white powder, is stored in a small cartridge that contains water. Hydrolysis, a chemical reaction, takes place by projecting the liquid on the powder. The hydrogen cartridge then offers a 500Wh/l power-weight ratio.

The disposable hydrogen cartridge, noted CEA-Liten, offers a 500Wh/kg energy density. The energy is available on demand via electronic control of the chemical reaction. "We have developed a concept where we put inert chemical components in the tank. Hydrogen is transformed into electricity on the silicon chip and can recharge up to five times the cellphone battery," the group noted.

"The system produces water but in limited quantity. The water evaporates and is used to cool the system," noted Fr�d�ric Gaillard, lab manager for micropower sources, CEA.

The disposable hydrogen cartridge offers a 500Wh/kg energy density.

Micro fuel cell
CEA-Liten claimed the micro fuel cell can provide up to 400mW/cm2 of silicon and generally stabilizes at 150mW/cm2 for several hundreds of hours. The team of researchers said it aims to reach, in about four years, 1W/cm2, which means only a few square centimeters of silicon will be necessary to supply a 3W to 5W mobile phone.

"Our main concern is the cost of material. Today, we are already well above Li-ion batteries in terms of energy density, while we are still at the prototype stage," said Marsacq.

Igor Bimbaud, manager of ST's energy sources business unit, said under the terms of the collaboration agreement signed with CEA-Liten in 2007, a 3500sqm laboratory and production unit would be established by the end of 2008. This would pave the way towards industrialization of the hydrogen fuel cell.

"In 2007, we presented a 1W demonstrator. We are now in the proof-of-concept phase," said Bimbaud. At the end of 2009 or early 2010, he added, the company's first products will hit the shelves in the form of an external accessory, either as a charger or an add-on battery extender. Presently, it has designed a belt pouch containing a hydrogen cartridge and micro fuel cell.

"The idea is to improve the runtime of handsets by a factor of five," Bimbaud said. Other plans are set to provide a micro fuel cell built in the device. It will be complemented by a small Li-ion battery to take care of power peaks when starting the handset or implementing new functionalities. The micro fuel cell will operate when the cellphone is in sleep mode or only performing basic functions. "The price of the fuel cell is yet to be determined," added Bimbaud.

- Anne-Francoise Pele
EE Times Europe

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