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."
Specifically, the inhaled oxygen is stripped off of carbon dioxide (CO2) and/or water (H2O) gas molecules that are pumped into the reactor, producing carbon monoxide (CO) and/or hydrogen gas (H2). H2 can be used to fuel hydrogen fuel cells; CO, combined with H2, can be used to create synthetic gas, or "syngas," which is the precursor to liquid hydrocarbon fuels. Adding other catalysts to the gas mixture, meanwhile, produces methane. And once the ceria is oxygenated to full capacity, it can be heated back up again, and the cycle can begin anew.
For all of this to work, the temperatures in the reactor have to be very high — nearly 3,000 degrees F. At Caltech, Haile and her students achieved such temperatures using electrical furnaces. But for a real-world test, she says, "we needed to use photons, so we went to Switzerland." At the Paul Scherrer Institute's High-Flux Solar Simulator, the researchers and their collaborators — led by Aldo Steinfeld of the institute's Solar Technology Laboratory — installed the reactor on a large solar simulator capable of delivering the heat of 1,500 suns.
In experiments conducted last spring, Haile and her colleagues achieved the best rates for CO2 dissociation ever achieved, "by orders of magnitude," she says. The efficiency of the reactor was uncommonly high for CO2 splitting, in part, she says, "because we're using the whole solar spectrum, and not just particular wavelengths." And unlike in electrolysis, the rate is not limited by the low solubility of CO2 in water. Furthermore, Haile says, the high operating temperatures of the reactor mean that fast catalysis is possible, without the need for expensive and rare metal catalysts (cerium is the most common of the rare earth metals and about as abundant as copper).
In the short term, Haile and her colleagues plan to work on the ceria formulation so that the reaction temperature can be lowered, and to re-engineer the reactor to improve its efficiency. The system currently harnesses less than 1% of the solar energy it receives, with most of the energy lost as heat through the reactor's walls or by re-radiation through the quartz window. "When we designed the reactor, we didn’t do much to control these losses," says Haile. Thermodynamic modeling by former Caltech graduate student William Chueh suggests that efficiencies of 15% or higher are possible.
Ultimately, Haile says, the process could be adopted in large-scale energy plants, allowing solar-derived power to be reliably available during the day and night. One scenario might be to take the CO2 emissions from coal-powered electric plants and convert them to transportation fuels. Alternatively, Haile says, the reactor could be used in a "zero CO2 emissions" cycle: H2O and CO2 would be converted to methane, would fuel electricity-producing power plants that generate more CO2 and H2O, to keep the process going.