Solid-oxide fuel cells run efficiently on a wide variety of conventional fuels and biofuels, but their high operating temperatures have limited their applications. Many researchers are working on this problem, developing new electrode and electrolyte materials that operate at lower temperatures without compromising performance. Now researchers in Japan have demonstrated a high-performance micro fuel cell that operates at lower temperatures, thanks to a restructured electrode.

"The cell is suitable for portable power sources, which require quick start-up," as well as auxiliary power for automotives, says Toshio Suzuki, a research scientist at Japan's National Institute of Advanced Industrial Science and Technology. Suzuki led the development of the new fuel cell, which is described today in the journal Science. The cell is tube shaped and about two millimeters in diameter; its power output is about one watt at 600 degrees Celsius. Conventional solid-oxide fuel cells operate at temperatures above 700 degrees.

Solid-oxide fuel cells generate an electrical current by pulling oxygen from the air and using it to oxidize fuel. Oxygen comes through the cathode side, fuel through the anode side; the two react in the electrolyte and form water and carbon dioxide as a waste products, depending on the fuel type. This reaction is more efficient than conventional generators. Solid-oxide fuel cells are also more efficient than the other predominant fuel-cell type, which uses expensive platinum catalysts and a polymer membrane that can become contaminated, and runs only on hydrogen fuel.

Solid-oxide fuel cells are "more flexible, more powerful, and don't have the problem of getting contaminated," says Eric Wachsman, director of the Florida Institute for Sustainable Energy and chair of materials science and engineering at the University of Florida. The problem with these devices, says Wachsman, is the operating temperatures. This means a long warm-up time can be required, and you can't use one in a cellular phone. The high temperatures also cause the battery cell to wear out.

Suzuki's group created a power source with a lower operating temperature by improving the structure of the anode, where the fuel comes in. The Japanese group used conventional techniques including lithography and etching to make anodes with varying degrees of porosity. The best-performing anode was a very porous structure based on nickel oxide, a conventional material for these electrodes. Suzuki says they chose to use existing materials because their performance over time has been proven. "These are reliable materials for long-term stability, and have a cost advantage compared with other new materials for low-temperature solid-oxide fuel cells," he explains.

"The performance is no doubt quite good," says Harry Tuller, professor of ceramics and electronic materials at MIT. "This is a nice systematic study showing the evolutionary impact of demonstrated improvements" in the electrode, he says. However, Tuller cautions that the electrodes and the electrolyte are doped with small amounts of expensive materials, which could add expense to the cells. The anode contains, in addition to nickel oxide, a small amount of the rare element scandium.

Wachsman says that it's difficult to bring down the operating temperature of these cells without compromising on power output. He's also working on new solid-oxide fuel-cell electrode structures. Using a different set of materials and a similar approach, Wachsman recently demonstrated a fuel cell with a restructured anode and a new electrolyte for a power output of two watts per square centimeter at 650 degrees. This work is described in the journal Electrochemistry Communications.

Suzuki says his group is in discussions with several companies about commercializing the cells.