Illustration of 'artificial photosynthesis,' a process by which sunlight, CO2, and water are converted in labs to useful fuels. This is the main goal of the Joint Center for Artificial Photosynthesis (JCAP), a US Department of Energy (DOE) Energy Innovation Hub, which seeks to 'secure energy supplies for future generations.' (JCAP)

A research team from Caltech has discovered a more efficient way to create carbon-based fuels from carbon dioxide (CO2). The team identified a new additive that helps convert CO2 into fuels with multiple carbon atoms, a major step toward making renewable liquid fuels not derived from oil or coal.

"The results were quite shocking," says Jonas Peters, Bren Professor of Chemistry at Caltech and director of the Resnick Sustainability Institute, who jointly led the research in collaboration with Theodor Agapie, professor of chemistry at Caltech. "Usually, in these types of reactions with CO2, you see a lot of by-products like methane and hydrogen. In this case, the reaction was highly selective for the more desirable fuels that contain multiple carbons -- such as ethylene, ethanol and propanol. We saw an 80 percent conversion to these multi-carbon fuel products, with only 20 percent or so going into hydrogen and methane."

Fuels with multiple carbon atoms are desirable because they are usually liquid, and liquid fuels store more energy per volume than gaseous fuels. Propanol, a good example, is liquid and contains three carbon atoms. It stores more energy than methane, which is a gas with only one carbon atom.

Chemists from the Joint Center for Artificial Photosynthesis (JCAP) aim to artificially create multi-carbon liquid transportation fuels with widely available ingredients like sunlight, water and CO2. The study’s research was conducted by Caltech postdoctoral scholars Rudd Kortiever and Hsiang-Yun Chen and former postdoc Zhiji Han and was funded by JCAP.

In order to find the right combination for making multi-carbon fuels, the research team experimented in the lab with a few different chemicals. They used an aqueous solution and a copper electrode that served as a catalyst and a source of energy in place of the sun. The researchers added CO2 to the solution, along with a class of organic molecules called N-substituted arylpyridniums. Those chemicals formed a thin deposit on the electrode. The film dramatically improved the fuel-making reaction, but why the film improved is not yet understood. The film selectively produced the desirable chemicals: ethanol, ethylene and propanol.

"It's easy to make hydrogen under these conditions, so usually we see a lot of it," says Agapie. "But we want to disfavor the hydrogen production and favor high-energy density liquid fuels with carbon-carbon bonds, which are exactly what we get in our experiments."

The next step is to figure out how the additives enhance the reaction. The researchers plan to test similar additives to find out if they can improve the selectivity for desired fuels.

A study written on this research was published on July 21st in the ACS Central Science.