Microchip power from a shrunken fuel cell.

Microchip power from a shrunken fuel cell

Placing miniature power supplies right where they're needed on integrated-circuit chips is a quick and efficient way of getting electrical power to a circuit's microscopic components. This goal now seems within reach with the construction of a tiny, thin-film fuel cell that generates electricity when one of its electrodes is exposed to a mixture of air and hydrogen.

"It's probably the smallest electrochemical device that anyone has ever built," says Christopher K. Dyer of Bell Communications Research in morristown, N.J. Such a device could have a broad range of applications, from low-cost portable power supplies to information processing. Dyer describes his unconventional fuel cell in the Feb. 8 NATURE.

Just as a solar cell converts light energy directly into electrical energy, Dyer's fuel cell converts chemical energy, from the reaction between hydrogen and oxygen, directly into electrical energy. The device consists of a porous, aluminum-oxide membrane only 2,000 to 5,000 angstroms thick, sandwiched between two thin platinum films that serve as electrodes (see diagram).

When exposed to a mixture of air and hydrogen at room temperature, the device develops a potential difference of roughly 1 volt between its electrodes and generates a few milliwatts of power per square centimeter. "There's never been anything in electrochemistry quite this small that can give such a high voltage with mixed gases," Dyer says.

But why the device works remains a mystery. "It's not the sort of thing that you, as an electrochemist, would predict could happen," he says. Conventional fuel cells cannot operate with gas mixtures.

"Regardless of the precise mechanism involved," he adds, "the facility with which the pehnomena can be reproduced with a variety of different membrane materials should lead to rapid duplication of these results."

The possibility of using relatively simple manufacturing techniques to fabricate these miniature, thin-film fuel cells at a low cost suggests a variety of applications. Dyer and his colleagues are investigating how to deposit these devices on integrated circuits, especially for supplying power to tightly packed, high-speed switches. They are also looking into the development of lightweight, portable power supplies fed by a stream of methanol vapor and air, which could be used instead of batteries.

One barrier to such applications is the fuel cell's low power output. But researchers may solve that problem by depositing the fuel cell's thin layers on a rough rather than a smooth surface, furnishing a greater surface area on which the necessary electrochemical reactions can occur.

The fuel cells also has a relatively low energy-conversion efficiency. "You wouldn't see this in a huge power-generating plant," Dyer says. "It's purely a convenience power supply, comparable to a small battery, where the cost of the fuel is not important."