The warnings are becoming more dire. The burning of fossil fuels
is increasing carbon dioxide concentrations in the atmosphere and
altering the global climate. Now UCSD chemists have demonstrated
the feasibility of transforming the greenhouse gas into a useful
product.
Clifford Kubiak, a professor of chemistry and biochemistry, and
his graduate student Aaron Sathrum have developed a prototype device
that can capture energy from the sun, convert it to electrical
energy and “split” carbon dioxide into carbon monoxide
(CO) and oxygen (O).
Because their device is not yet optimized, they still need to
input additional energy for the process to work. However, they
hope that their results, which they presented at a conference of
the American Chemical Society, will draw attention to the promise
of the approach.
“For every mention of CO2 splitting, there are more than
100 articles on splitting water to produce hydrogen, yet CO2 splitting
uses up more of what you want to get rid of,” explains Kubiak. “It
also produces CO, an important industrial chemical, which is normally
produced from natural gas. So with CO2 splitting you can save fuel,
produce a useful chemical and reduce a greenhouse gas.”
Carbon monoxide is highly sought after, even though it is poisonous.
Millions of pounds of it are used each year to manufacture chemicals
including detergents and plastics and it can also be converted
into liquid fuel.
“The United States was very interested in the technology
during the 1970s energy crisis, but when the crisis ended people
lost interest,” says Kubiak. “Now rising fuel prices
make it economically competitive to convert CO into fuel.”
Kubiak and Sathrum’s device utilizes
a semiconductor with two thin layers of catalysts, and splits carbon
dioxide in a three-step process. In the first step, the semiconductor
captures solar energy. Next the semiconductor converts it into
electrical energy. In the final step, the electrical energy is
used by the catalysts, which convert carbon dioxide to carbon
monoxide on one side of the device and to oxygen on the other
side. Kubiak and Sathrum initially used a silicon semiconductor
to test their device. However, silicon cannot absorb enough solar
energy to drive the reactions, so researchers had to supply the
additional energy needed.
They are now using a gallium-phosphide semiconductor, which has
the capacity to absorb about twice as much solar energy as silicon,
and should absorb sufficient energy from the sun to drive the catalytic
splitting of carbon dioxide.
“This project brings together many scientific puzzle pieces,” says
Sathrum. “Quite a bit of work has been done on each piece,
but it takes more science to mesh them all together.”
— Sherry Seethaler
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