Omnivorous Fuel Cells

A prototype fuel cell runs on a wide range of fuels without turning up the heat.

Fuel cells are the most efficient way to convert chemical energy into electricity. But most either operate at high temperatures or require very pure hydrogen fuel. Superprotonic, a startup company in Pasadena, CA, is developing a fuel cell that can handle dirty hydrogen at relatively low temperatures. It could thus use hydrogen produced from other fuels--such as natural gas or ethanol--by a simple device called a "reformer."

Smokeless stack: This prototype stack of solid-acid fuel cells made by Superprotonic puts out 50 watts of power--enough to recharge a battery or power small electronic devices. The cells can run on a range of fuels, including natural gas and biofuels. Credit: Superprotonic

In a fuel cell, an electrolyte is sandwiched between an anode and a cathode. A catalyst at the anode splits hydrogen into electrons and protons. The protons can pass through the electrolyte, but the electrons can't. So in order to reach the cathode, the electrons travel through an external electrical circuit, where they can be used to recharge a battery or power a device. At the cathode, another catalyst helps the protons and electrons combine with oxygen sucked from the air to form water--the fuel cell's only waste product.

Superprotonic's fuel cells rely on a material called a solid acid, first tested as an electrolyte in 2001 by Caltech materials-science and chemical-engineering professor Sossina Haile. "What makes our fuel cell special is the nature of this electrolyte," she says. Solid-acid fuel cells operate at what Haile calls a Goldilocks temperature: not too hot, not too cold.

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Electrolytes made from polymer membranes provide a higher power output per unit area, but they require water to facilitate proton conduction. Consequently, polymer-electrolyte fuel cells require heat exchangers to keep the electrolyte temperature below 100 °C. At that temperature, the catalysts can be poisoned by carbon monoxide and other impurities at levels as low as ten parts per million. The cells thus require very pure fuel.

High-temperature fuel cells can run on other fuels when hooked up to a simple device called a reformer, which turns the fuels into hydrogen. But it takes a while for the cells to heat up, and their high operating temperatures (above 500 °C) cause wear and tear and limit where they can be used.