A Tankful of Sunshine

Taking cues from plants to produce fuel for the future.

What if we could wring liquid fuel from sunlight? What if chemists could capture energy from our nearly inexhaustible star and store it in the chemical bonds of hydrocarbon fuels, like gasoline?

Plants have been doing something like this for billions of years through photosynthesis. Plants harvest solar energy and use it to power a chemical chain reaction that makes sugars, or in other words, their “fuel.”

Inspired by those leafy green powerhouses, UC San Diego scientists are working to develop a system they call “artificial photosynthesis.” The process will likewise use sunlight to create a chemical reaction that creates energy, only instead of plant‑powering sugars, the product will be high-density fuel.

The effort is led by Cliff Kubiak, UC San Diego distinguished professor of chemistry and biochemistry, and a member of the Joint Center for Artificial Photosynthesis, a multi-university innovation hub funded by the Department of Energy to create alternative fuels.

Kubiak and colleagues are working on ways to loosen the tenacious bonds in carbon dioxide molecules so that the atoms therein can be reassembled into liquid fuels. Plants do this naturally via enzymes, yet these enzymes are high-maintenance proteins that must continually be replenished—not so easy to recreate artificially.

Kubiak and colleagues are developing a more efficient system that replaces those unwieldy proteins with chemical structures that serve like scaffolding. Kubiak likens these “metal-organic frameworks,” or MOFs, to the mix-and-match children’s building kits we all know as Tinkertoys.

“They’re modular—we can put whatever catalyst we want in there and then it’s nailed down into this robust, solid-state structure,” he says. “We decided to use the MOF as a platform for making things that are like enzymes but aren’t floppy peptides that can degrade.”

Along with Seth Cohen, another chemistry professor at UC San Diego, the team created an MOF that allowed them to infuse the “scaffolding” struts with a photosensitive chemical, akin to chlorophyll and other pigments that nature uses for photosynthesis. When light shines on the setup, the electrons shuffle and carbon dioxide molecules recombine with hydrogen to form nascent molecules of fuel.

This particular MOF is just one possibility in the progress toward achieving viable products that create fuel via artificial photosynthesis. “There’s going to be a lot more,” Kubiak says. “We’re doing such new things that there are still broad strokes to contribute.”