Green Design

Solar Reactor Converts Carbon Dioxide and Water Into Fuel

Cerium oxide — or ceria — is a common metal most famously used in self-cleaning ovens, and it is the centerpiece of a new technology from California Institute of Technology that concentrates solar energy and uses it to efficiently convert carbon dioxide and water into fuels.

The process was developed by Sossina Haile, a professor of materials science and chemical engineering at Caltech, and colleagues. The researchers designed and built a two-foot-tall prototype reactor that has a quartz window and a cavity that absorbs concentrated sunlight. The concentrator works "like the magnifying glass you used as a kid" to focus the sun's rays, says Haile.

At the heart of the reactor is a cylindrical lining of ceria. Ceria, commonly used to catalyze reactions that decompose food and other stuck-on gunk in ovens, propels the solar-driven reactions. The reactor takes advantage of ceria's ability to "exhale" oxygen from its crystalline framework at very high temperatures and then "inhale" oxygen back in at lower temperatures.

"What is special about the material is that it doesn't release all of the oxygen. That helps to leave the framework of the material intact as oxygen leaves," Haile explains. "When we cool it back down, the material's thermodynamically preferred state is to pull oxygen back into the structure."