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Some Like It Medium Hot

18 April 2001 7:00 pm
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Just right. Fuel cells might someday power automobiles cleanly and efficiently, but only if they can run within a reasonable temperature range.

Fuel cells have the potential to generate energy without pollution. But some run too cold to be efficient, while the others are too hot to be practical. But a newly tested type of fuel cell might be just right, though it's still a long way from appearing in automobile showrooms.

The beauty of fuel cells is that they directly generate electricity from the chemical energy stored in pure hydrogen or other fuels, bypassing the mess and waste of combustion (Science, 16 June 2000, p. 1955). One very efficient design operates at temperatures above 600ºC, so it must be constructed of expensive, heat-resistant materials. A low-temperature variety of fuel cell works best at a mere 80ºC, but it is less efficient, relies on expensive catalysts, and feeds on a smaller repertoire of fuels than its high-temperature cousin. Researchers have been on the quest for a fuel cell that works between these two temperatures, in a range that would be suitable for vehicles.

The key fuel cell component that determines its operating temperature is the electrolyte, a layer of material that conducts charged atoms from one side of the cell to the other. Chemists have known for decades that a class of compounds called solid acids conduct hydrogen ions efficiently at moderate temperatures, but these materials had never been put to work in a fuel cell.

Now materials scientist Sossina Haile and colleagues at the California Institute of Technology in Pasadena have constructed the first fuel cell based on a solid acid. They sandwiched a thin layer of a solid acid called cesium hydrogen sulfate between electrodes and pressed it tight for good contact. The fuel cell ran on hydrogen for several days at a favorable 160ºC, although it produced just 1% as much energy as existing fuel cells. Haile says that tinkering with the electrolyte composition and thinning the solid acid layer will improve the efficiency.

The result is "one of the more important discoveries lately" in the study of fuel cell electrolytes, says Jack Brouwer, associate director of the National Fuel Cell Research Center at the University of California, Irvine. But many caution that it's a long way from the laboratory to a working product. Says Subhash Singhal, director of fuel cell research at the Pacific Northwest National Laboratory in Richland, Washington, "no one is going to start using them on a large scale immediately."

Related sites

Sossina Haile's faculty page
A diagram of how fuel cells work
National Center for Fuel Cell Research
Fuel Cells 2000

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