Novel fuel cell gets hot, but not by a lot

Fuel cells convert hydrogen and oxygen to water and energy with minimal pollution. Although often touted as the power source of the future, the leading devices developed so far will have to cool down or warm up before they can become a really hot technology, fuel-cell specialists say. Now, a team of materials scientists reports developing a new type of fuel cell that operates in a coveted midtemperature range.

Sossina M. Haile and her colleagues at the California Institute of Technology (Caltech) in Pasadena have made a prototype cell that operates at 160C. Among the most promising types of fuel cells already well along in development, one known as a solid-oxide fuel cell operates at a scorching 600 to 1,000C (SN: 3/18/00, p. 181). That makes it impractical for powering cars or portable electronic devices. Another popular design, the proton-exchange-membrane fuel cell, runs at such a tepid temperature–around 80C–that it faces problems, including failure of its catalyst.

The Caltech prototype, described in the April 19 Nature, uses cesium hydrogen sulfate as its electrolyte, the fuel cell component that conducts charged atoms, or ions, between electrodes. The prototype’s electrolyte comes from an odd family of chemicals that are both acids and solids, Haile notes.

The new device is one of a class of fuel cells in which protons pass through the electrolyte. In such cells, two electrodes enclose the electrolyte (SN: 11/13/93, p. 314). One of them is exposed to hydrogen-rich gaseous fuel. At that electrode, hydrogen atoms ionize into protons by giving up an electron. Then, these ions traverse the electrolyte to the other electrode, where they bind with oxygen to form water. These transformations generate electricity.

Creating a proton-conducting electrolyte that can operate above 100C is a high priority, says chemist JoAnn Milliken, a manager in the Department of Energy’s fuel cell research program. The Caltech findings are “an important first step” to that goal, she adds.

In proton-exchange-membrane cells, protons hitch rides on water molecules. That’s why the cells must stay below water’s 100C boiling point. These low-temperature designs require catalysts such as platinum.

In the new, warmer cell, any catalyst used is less prone to fouling by carbon monoxide molecules, Haile says. At the same time, the device doesn’t have to become so hot that it takes a long time to reach operating temperature–a drawback of solid-oxide fuel cells.

As is, the Caltech cell currently generates too little power to challenge existing designs. What’s more, its performance may falter as hydrogen-sulfur reactions degrade the electrolyte. Still, Haile and her colleagues “have proved the concept” of using solid proton conductors in dry fuel cells, comments Truls Norby of the University of Oslo in the same issue of Nature. There are many other solids of that type to be explored as fuel cell electrolytes, he adds.

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