Can new reactor improve sunlight fuel production efficiency?

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Ceria Solar Thermochemical Reactor focuses concentrated sunlight into a cerium oxide redox cycle to split CO2 and H2O, generating syngas, hydrogen, and methane for synthetic fuels, pioneered by Caltech and PSI.

The Core Facts

A high-temperature cerium oxide system using concentrated sunlight to split CO2 and H2O, yielding hydrogen, CO, syngas.

Uses cerium oxide redox cycle to release and uptake oxygen.
Splits CO2/H2O into CO and H2 for syngas, fuels, methane.
Achieved record CO2 dissociation rates at PSI solar simulator.
Operates near 3,000 F; targets lower temps with new formulations.
Potential 15% efficiency; scalable for round-the-clock solar fuels.

Using a common metal most famously found in selfcleaning ovens, Sossina Haile hopes to change our energy future. The metal is cerium oxide or ceria and it is the centerpiece of a promising new technology developed by Haile and her colleagues that concentrates solar energy and uses it to efficiently convert carbon dioxide and water into fuels.

Solar energy has long been touted as the solution to our energy woes, but while it is plentiful and free, it can't be bottled up and transported from sunny locations to the drearier -- but more energy-hungry -- parts of the world. The process developed by Haile -- a professor of materials science and chemical engineering at the California Institute of Technology Caltech -- and her colleagues could make that possible and move us toward energy self-sufficiency across regions today.

The researchers designed and built a two-foot-tall prototype reactor that has a quartz window similar to a new window on solar energy demonstrated in related research today 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 -- a metal oxide that is commonly embedded in the walls of self-cleaning ovens, where it catalyzes reactions that decompose food and other stuck-on gunk -- propels the solar-driven reactions, echoing a recent solar power advance in high-temperature chemistry today. 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, including a new fuel cell that addresses storage challenges in renewables, and 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 Fahrenheit. 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, while some labs are also exploring virus-based power concepts for nanoscale energy harvesting today.

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," which points toward solar profitability as systems scale today. 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, in fact, is the most common of the rare earth metals -- about as abundant as copper.

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, providing grid flexibility that has helped eliminate utility bills for some U.S. homes today. The CO2 emitted by vehicles could be collected and converted to fuel, "but that is difficult," she says. 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 explains. Alternatively, she 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.

The work was funded by the National Science Foundation, the State of Minnesota Initiative for Renewable Energy and the Environment, and the Swiss National Science Foundation.