Using a common metal found in self-cleaning ovens, it may soon be possible to concentrate solar energy and use it to efficiently convert carbon dioxide and water into fuel.
Solar energy is always in the forefront of being the solution for energy problems, but while it is plentiful and free, you can’t take it from the sunny desert to an energy-needy environment. At least not yet.
A new process developed by Professor of Materials Science and Chemical Engineering at the California Institute of Technology (Caltech) Sossina Haile, and her team, could make that possible.
The researchers created 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, Haile said.
At the heart of the reactor is a cylindrical lining of ceria. Ceria is a metal oxide commonly embedded in the walls of self-cleaning ovens, where it catalyzes reactions that decompose food and other stuck-on gunk. Ceria helps propel 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 said. “When we cool it back down, the material’s thermodynamically preferred state is to pull oxygen back into the structure.”
Specifically, the inhaled oxygen loses its carbon dioxide (CO2) and/or water (H2O) gas molecules pumped into the reactor, producing carbon monoxide (CO) and/or hydrogen gas (H2). H2 can then fuel hydrogen fuel cells; CO, combined with H2, can 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 heat 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 Fahrenheit. At Caltech, Haile and her students achieved such temperatures using electrical furnaces. But for a real-world test, she said, “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 said. The efficiency of the reactor was uncommonly high for CO2 splitting, in part, she said, “because we’re using the whole solar spectrum, and not just particular wavelengths.” And unlike in electrolysis, the low solubility of CO2 in water does not limit the rate. Furthermore, the high operating temperatures of the reactor mean that fast catalysis is possible, without the need for expensive and rare metal catalysts (cerium, in fact, is the most common of the rare earth metals—about as abundant as copper), Haile said.
In the short term, Haile and her colleagues plan to tinker with the ceria formulation so they can lower the reaction temperature, and they also want to re-engineer the reactor, to improve its efficiency. Currently, the system 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,” Haile said. Thermodynamic modeling by lead author and former Caltech graduate student William Chueh suggests that efficiencies of 15% or higher are possible.
Ultimately, the process could work in large-scale energy plants, allowing solar-derived power to be reliably available during the day and night, Haile said. The CO2 emitted by vehicles could be collected and converted to fuel, “but that is difficult,” she said. A more realistic scenario might be to take the CO2 emissions from coal-powered electric plants and convert them to transportation fuels. “You’d effectively be using the carbon twice,” Haile said. Alternatively, the reactor could work in a “zero CO2 emissions” cycle: H2O and CO2 would convert to methane, and would fuel electricity-producing power plants that generate more CO2 and H2O, to keep the process going, she said.