PASADENA, Calif. - Using a common metal most famously found in
self-cleaning 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.
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 - 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.
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 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.
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, in fact, is the most
common of the rare earth metals - about as abundant as copper).
In the short term, Haile and her colleagues plan to tinker with
the ceria formulation so that the reaction temperature can be
lowered, and 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," says Haile. Thermodynamic modeling by lead author and
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. 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.
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