Researchers Use New Additive to Selectively Convert CO2 to Multicarbon Fuels

A Caltech team of Chemists have discovered a new, more efficient method to produce carbon-based fuels from carbon dioxide (CO₂). In chemical reactions done in the lab, the team has identified a new additive that helps selectively change CO₂ into fuels possessing multiple carbon atoms — a step toward eventually producing renewable liquid fuels that are not derived from coal or oil.

The results were quite shockingUsually, 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.

Jonas Peters, Bren Professor of Chemistry, Caltech and Director of the Resnick Sustainability Institute

Fuels with several carbon atoms are preferred more because they tend to be liquid—and liquid fuels stock more energy per volume compared to gaseous fuels. For example, propanol, which is liquid and has three carbon atoms, stocks more energy compared to methane, which is a gas and only contains one carbon atom.

The goal of Chemists like Peters, Agapie and their colleagues working at the Joint Center for Artificial Photosynthesis (JCAP), a U. S. Department of Energy (DOE) Energy Innovation Hub, is to artificially develop multi-carbon liquid transportation fuels using the commonly available ingredients of water, sunlight and CO₂. The new research, published on July 21^st in ACS Central Science and funded by JCAP, is a step in that direction.

The research was conducted by Caltech Postdoctoral Scholars Ruud Kortlever and Hsiang-Yun Chen and Former Postdoc Zhiji Han.

To identify the perfect combination for manufacturing the multi-carbon fuels, the team tested a mix of different chemicals in the lab. They used an aqueous solution and a copper electrode, which acted as a catalyst as well as a source of energy instead of the sun. The team incorporated both CO₂ and a class of organic molecules called N-substituted arylpyridiniums to the solution, which created a very thin deposit on the electrode. This film, for reasons that are not understood yet, greatly enhanced the fuel-making reaction, selectively creating the necessary chemicals ethylene, ethanol and propanol.

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

Theodor Agapie, Professor of Chemistry, Caltech

One subsequent step is to find out how the additives are improving the reaction. The Researchers also plan to examine similar additives to check if they can further enhance the selectivity for the desired fuels. Finally, this information may help arrive at alternate fuels made efficiently from CO₂, sunlight and water—instead of oil.

Nature has stored solar energy in the form of oil over a long period of earth history via a process that takes millions of years. Chemists would like to figure out how to do this much faster.

Jonas Peters, Bren Professor of Chemistry, Caltech and Director of the Resnick Sustainability Institute

The ACS Central Science research paper is titled, "CO₂ Reduction Selective for C≥2 Products on Polycrystalline Copper with N-substituted Pyridinium Additives”.