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Tuesday, January 6, 2009

Omnivorous Fuel Cells

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

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."

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.

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.

Solid acids, however, are conductive at about 250 °C, which is hot enough that the catalyst can withstand impurities in the fuel. Superprotonic's prototype 50-watt fuel-cell stack can run on any fuel that can be reformed into hydrogen, says Calum Chisholm, the company's vice president and founder. Because a catalyst's activity increases with increased temperature, future solid-acid fuel cells may operate with much less catalyst, or with less-expensive catalysts that are not active below 100 °C. "Other materials become active at this temperature, including nickel, cobalt, and iron," says Chisholm.

However, the technology is young and has a way to go before it reaches its potential. "There's been a lot of work on the structure of electrodes to improve the performance of other types of fuel cells," says Robert Savinell, a chemical engineer at Case Western Reserve University, who is not affiliated with Superprotonic. Haile says that the solid-acid technology is nowhere near as efficient as it can be, and that it's merely a matter of time before it catches up with polymer-based systems. Superprotonic is still making its cells one at a time and by hand, using more platinum catalyst than other kinds of fuel cells require. Haile says that, in the meantime, she and her Caltech colleagues are "trying to see if we can come up with better catalysts with higher surface areas. That's a milestone the company needs to reach."

Eventually, Superprotonic hopes to develop fuel-cell-based residential and commercial "cogeneration" systems, which would use natural gas or other common fuels to generate electricity and waste heat to heat water. However, given the current economy and the disappointment that has surrounded fuel cells over the past 20 years or so, the company is proceeding cautiously. "What we can give somebody right now," says Chisholm, is a 50-to-250-watt battery recharger or electrical generator for camping or military use that employs what he calls "real-world fuels." Superprotonic has funding from the military to develop a battery charger and is talking with other companies about commercializing a civilian version of it.