Miniature fuel cell could power electronic devices

Scientists at Motorola labs have reached another milestone in their development of a new, miniature fuel cell that may one day replace the traditional batteries that now power everything from cellular phones and laptop computers to portable cameras and electronic games.

They have demonstrated a prototype of a ceramic-based microfluidic fuel delivery system for a miniature direct methanol fuel cell (DMFC).

“Portable electronics are becoming more essential to daily life and increasingly we all want them to have new capabilities,” said Jerry Hallmark, manager of Motorola’s’ energy technology lab, “but adding features increases the demand on energy sources and systems. We need to develop new energy solutions – and fuel cells could be the breakthrough technology.

“Our challenge is to make these systems small, light and easy for consumers to use. Eventually, these fuel cells could enable what people just dream of today – a lightweight energy source that would safely power a cellular phone for a month.”

To produce energy, the new fuel cell uses a reservoir of inexpensive methanol that, when combined with the oxygen in the air, produces electricity at room temperature. Motorola’s initial strategy is to develop a hybrid energy source, which combines a miniature fuel cell with a rechargeable battery for peak power demands.

The key to successfully miniaturizing a DMFC system is scaling down the system components surrounding the actual fuel cell device. Previous DMFC systems have used discrete tubes to mix the methanol fuel with water and deliver it to the fuel cell. The company has successfully demonstrated the use of multi-layer ceramic technology for processing and delivering fuel and air to the fuel cell membrane electrode assembly (MEA). This fuel delivery system can be built into a miniature fuel cell.

The prototype, shown at the Power 2000 Conference in San Diego, California, combines fuel mixing and microchannels for delivery, substrate for MEA mounting, and electrical contact in just two ceramic pieces. The lower ceramic piece handles the liquid fuel processing while the upper piece provides for passive air delivery (air-breathing). The MEA is sandwiched between the two ceramic layers, making for simple assembly.

This ceramic technology also simplifies the interconnection of multiple fuel cells. In this implementation, they are arranged in a planar layout rather than a standard vertical stack. This simplifies the design of the fuel cell system and eliminates the need for an air fan or pump since all of the fuel cells are exposed to air. Several cells are connected in series electrically to increase the output voltage of the system. This simplifies the interface to the actual electronic system.

While the research work is expected to continue for a few more years before being brought to market, the ceramic fluid-delivery technology will be used to build an integrated 100mW DMFC system, with the goal of five times the energy density of conventional Li-Ion rechargeable batteries.

In laboratory testing, the ceramic fuel cell assembly, measuring about two inches (5cm) on a side and less than one-half of an inch (1cm) in thickness, produces over 100mW continuously, when combined with an external fluid pumping system. It can output up to 180mW depending on the load. The fuel cell prototype has been operated for several weeks with very little degradation in performance.

The ‘air breathing’ fuel cell was developed in cooperation with Los Alamos National Laboratory. The highly simplified and miniaturised design eliminates the need for air pumps, heat exchangers and other complex devices that previous fuel cells required, and which made them unsuitable for use in today’s small, portable electronic products.

The methanol required to run electronic devices could be packaged in small, inexpensive cartridges, similar in size to fountain pen ink cartridges. Thus, the technology could have the same consumer-friendliness as batteries.

Motorola Labs, Motorola’s Energy Systems Group and Los Alamos National Lab are developing a center of excellence to implement this new technology as quickly as possible.

A fuel cell converts chemical energy directly to electrical energy. The basic concept of a fuel cell originated in 1839, but practical applications came from NASA in the early days of space flight.

Many kinds of fuel could be used in a fuel cell. The most active fuel for fuel cells is hydrogen. However, hydrogen is a flammable/explosive gas and is typically pressurised, which causes practical concerns for transportation and storage. Recently, methanol has been considered as an alternative fuel. Methanol has more energy for a given size and weight compared to hydrogen.

A direct methanol fuel cell converts the energy in methanol directly to electricity and operates at normal room temperatures. A catalyst (typically a mixture of platinum and ruthenium) is used to react a dilute mixture of methanol and water to form protons and electrons, which provide the electrical current, and the by-product carbon dioxide. The electrons are collected and used to power external loads such as portable electronics.

The protons are conducted through a proton-conducting organic membrane to another platinum catalyst where the protons combine with electrons and oxygen from the ambient air to form pure water. The generation of electrons during the methanol-water reaction and their consumption during the proton-oxygen reaction complete the electrical power generation cycle of the fuel cell. Some of the water is recycled back to mix with the methanol, and the excess water evaporates as water vapour in the air.

The key technical challenges of fuel cells are to make them at a cost lower than rechargeable batteries. Also, the entire fuel cell system needs to be miniaturised to fit into today’s small portable electronic equipment. Of course, one wants very high efficiency in the conversion of methanol to electricity and a very long operating life for the system. For more detailed information on the general topic of fuel cells, a booklet is available from Los Alamos National Lab entitled ‘Fuel Cells – Green Power’.

The advanced direct methanol fuel cell technology at Los Alamos National Laboratory was created with support from the Office of Advanced Automotive Technologies of the US Department of Energy and the Defense Science Office of the Defense Advanced Research Projects Agency. The University of California operates Los Alamos National Laboratory for the US Department of Energy.

Motorola provides integrated communications solutions and embedded electronic solutions. Sales in 1999 were $33.1 billion. Motorola Labs serves as the advanced research arm of the company, focusing on technologies that may lead to future products and enhancements. Motorola Labs then works closely with researchers and engineers in the product businesses to transfer the technology into product development. Motorola also licenses technologies developed in the labs to external customers.">Los Alamos National Laboratory

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