Paris–As part of a research collaboration on miniaturized energy sources, the French Atomic Energy Agency (CEA) and STMicroelectronics NV (Geneva) have prototyped a hydrogen fuel cell for mobile phones that aims to reduce dependency on the use of electrical power supplies to recharge batteries.

The miniature fuel cell is made using microfluidic structures etched into the back surface of silicon, with a reaction interface at the silicon surface. The silicon is paired with a hydrogen-filled cartridge the size of a small cigarette lighter. The final product is due to be commercialized by the end of 2009 or early 2010.

"Five years ago, a cell phone used 1 watt of power; [today] a 3G phone uses 3 to 5 W. With the incoming generation of cell phones, if we do not change the battery, the autonomy will be reduced by one-third, limiting access to multimedia services," said Didier Marsacq, director of CEA-Liten, the Laboratory for Innovation in New Energy Technologies and Nanomaterials. Marsacq made the comments at a press conference held at CEA headquarters here.

The proliferation of mobile devices has generated 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.

Marsacq said the hydrogen-based fuel cell core unit (FCCU) used in CEA's energy source was developed jointly by CEA-Liten and ST; the energy management unit (EMU) by ST; and the cartridge by Bic (Clichy, France).

CEA-Liten went with a proton-exchange-membrane fuel cell (PEMFC) structure, which uses a solid electrolyte (the internal component between the anode and the cathode in which the H+ ions [protons] circulate). Its electrolyte is composed of a polymer and takes the form of a membrane.

The cathode, protected by a pierced coating that lets in air (oxygen), tops the electrolyte membrane. Below, the anode is accessed via channels that allow the influx of gaseous hydrogen. All of the structures are built on a silicon substrate using microelectronics processes.

The CEA-Liten design stores sodium boro-hydride (NaBH) in powder form in a small cartridge that also contains water. Projecting the liquid onto the powder initiates hydrolysis, yielding a 500-Wh/liter power-to-weight ratio.

The lab said the disposable hydrogen cartridge provides a 500-Wh/kg energy density. The energy is available on demand via electronic control of the chemical reaction. The system can recharge a cell phone battery up to five times before the disposable storage cartridge is spent, the researchers said.

The system produces a small amount of water as a by-product of the reaction. The water cools the system as it evaporates, said Frédéric Gaillard, lab manager for micropower sources at CEA.

CEA-Liten claims the micro fuel cell can provide up to 400 mW per square centimeter of silicon and generally stabilizes at 150 mW/cm² for several hundreds of hours. The team aims to reach 1 W/cm² in about four years; at that level, only a few square centimeters of silicon would be necessary to supply a 3- to 5-W 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 that under the terms of the agreement signed with CEA-Liten in 2007, a 3,500-square-meter laboratory and production facility will be established by year-end 2008, paving the way for industrialization of the hydrogen fuel cell.

"At the end of 2009 or early 2010, our first products will hit the shelves in the form of an external accessory--either a charger or an add-on-battery extender," said Bimbaud. "For the time being, we have designed a belt pouch containing the hydrogen cartridge and micro fuel cell." The goal is to improve the run-time of handsets by a factor of five.

Plans are also under way to provide a micro fuel cell that would be built into the device and would be complemented by a small Li-ion battery, which would handle power peaks when starting the handset or implementing new functionality.

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