Device uses solar energy to convert CO2 into fuel

Device uses solar energy to convert CO2 into fuel

mongabay.com

April 18, 2007

Chemists at the University of California, San Diego, (UCSD) have devised a device that uses solar energy to convert carbon dioxide into fuel. While the machine is only a prototype and not yet optimized, the researchers hope that their work will attract attention to their 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 put a dent into,” said Clifford Kubiak, professor of chemistry and biochemistry at UCSD. “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.”

“The technology to convert carbon monoxide into liquid fuel has been around a long time,” said Kubiak. “It was invented in Germany in the 1920s. The U.S. was very interested in the technology during the 1970s energy crisis, but when the energy crisis ended people lost interest. Now things have come full circle because rising fuel prices make it economically competitive to convert CO into fuel.”

A news release from UCSD explains the process:

The device designed by Kubiak and Sathrum to split carbon dioxide utilizes a semiconductor and two thin layers of catalysts. It splits carbon dioxide to generate carbon monoxide and oxygen in a three-step process. The first step is the capture of solar energy photons by the semiconductor. The second step is the conversion of optical energy into electrical energy by the semiconductor. The third step is the deployment of electrical energy to the catalysts. The catalysts convert carbon dioxide to carbon monoxide on one side of the device and to oxygen on the other side.

Because electrons are passed around in these reactions, a special type of catalyst that can convert electrical energy to chemical energy is required Researchers in Kubiak’s laboratory have created a large molecule with three nickel atoms at its heart that has proven to be an effective catalyst for this process.

Choosing the right semiconductor is also critical to making carbon dioxide splitting practical say the researchers. Semiconductors have bands of energy to which electrons are confined. Sunlight causes the electrons to leap from one band to the next creating an electrical energy potential The energy difference between the bands—the band gap—determines how much solar energy will be absorbed and how much electrical energy is generated.

Kubiak and Sathrum initially used a silicon semiconductor to test the merits of their device because silicon is well-studied. However, silicon absorbs in the infrared range and the researchers say it is “too wimpy” to supply enough energy. The conversion of sunlight by silicon supplied about half of the energy needed to split carbon dioxide, and the reaction worked if the researchers supplied the other half of the energy needed.

They are now building the device using a gallium-phosphide semiconductor. It has twice the band gap of silicon and absorbs more energetic visible light. Therefore, they predict that it will absorb the optimal amount of energy from the sun to drive the catalytic splitting of carbon dioxide.