Power cells find uses for fossil fuel - Brief Article

Science News, March 18, 2000 by C. Wu

A new fuel cell that runs on hydrocarbons such as natural gas, butane, and diesel could be an efficient, practical way to generate power without pollution.

Unlike typical fuel cells, which run on hydrogen, this new device oxidizes fossil fuels to produce electricity. Hydrogen fuel cells produce only water as a by-product, making them an attractive power source for electric cars. Storing volatile hydrogen onboard a vehicle raises worries about safety, however.

Some existing fuel cells do make use of hydrocarbons like gasoline, but they work by reforming the fuel first--that is, by stripping off the hydrogen gas (SN: 11/1/97, p. 279). This extra step reduces the fuel cell's efficiency.

"We can run the [new] fuel cell directly on hydrocarbon fuels, avoiding the reforming step," says Raymond J. Gorte of the University of Pennsylvania in Philadelphia. He and his colleagues John M. Vohs and Seungdoo Park describe their work in the March 16 NATURE.

Like all fuel cells, the device generates electricity by means of an electrochemical reaction. At the cathode, oxygen picks up electrons to form negatively charged oxygen ions. The ions diffuse through a membrane made of a compound known as yttria-stabilized zirconia. At the anode, the oxygen ions react with hydrocarbons to generate carbon dioxide, water, and electrons at a higher potential energy. Fuel cells with this design are known as solid-oxide fuel cells.

The researchers were able to solve a problem that has limited the usefulness of these cells. Graphite from the partial oxidation of hydrocarbons tends to build up on the anode, which is usually made of a nickel and yttria-stabilized zirconia composite.

"We've developed a material that doesn't coke up the anode," says Gorte. His team made an anode of copper and ceria, a rare earth oxide used widely as a catalyst in automobile exhaust systems. In this anode, ceria more efficiently catalyzes the oxidation of hydrocarbons to carbon dioxide and water, and the copper conducts the electrons produced.

This fuel cell runs at about 700 [degrees] C, much lower than the typical temperatures, 1,000 [degrees] C or above, of other solid-oxide fuel cells. High operating temperatures become impractical, says Brant A. Peppley of the Royal Military College of Canada in Kingston, Ontario. "If you have to connect one fuel cell to the next, you can't use copper wires. They're going to melt," he explains. Researchers now resort to expensive ceramic connectors.

The only waste products generated by the new cell are carbon dioxide and water. Even if some unused hydrocarbon comes out in the exhaust, the fuel cell could take it up again for reuse, Gorte says. The efficiency of the new fuel cell could reach 50 percent, he adds, which is significantly higher than that of an internal combustion engine burning hydrocarbons.

With this new development, Peppley says, researchers can "seriously look at practical applications for [local] power systems." In other words, fuel cells could be coming soon to an electric power plant or automobile near you.