Solar Nanowire-Bacteria Array Converts CO₂ into Useful Chemicals

A new artificial photosynthesis process could be used to capture carbon dioxide emissions and convert them into plastics, drugs and fuels with help from sunlight.

Scientists with the U.S. Department of Energy’s Lawrence Berkeley National Laboratory and the University of California, Berkeley, have created a hybrid system of semiconducting nanowires and bacteria that mimics the natural photosynthetic process. But instead of forming carbohydrates, the system synthesizes CO₂ and water into acetate, a versatile chemical intermediate that can be used to create an array of useful chemicals.

A SEM cross section image of the nanowire/bacteria hybrid array. Images courtesy of Berkeley Lab.

"Our system represents an emerging alliance between the fields of materials sciences and biology, where opportunities to make new functional devices can mix and match components of each discipline," said Michelle Chang, an expert in biosynthesis who holds appointments with UC Berkeley and Berkeley Lab and is a lead researcher on the project.

"For example, the morphology of the nanowire array protects the bacteria like Easter eggs buried in tall grass so that these usually oxygen-sensitive organisms can survive in environmental carbon dioxide sources such as flue gases."

The system starts with an "artificial forest" of nanowire heterostructures, consisting of silicon and titanium oxide nanowires, which Berkeley Lab chemist Peidong Yang compared to chloroplasts in green plants.

"When sunlight is absorbed, photoexcited electron-hole pairs are generated in the silicon and titanium oxide nanowires, which absorb different regions of the solar spectrum," he said. "The photogenerated electrons in the silicon will be passed onto bacteria for the CO₂ reduction while the photo-generated holes in the titanium oxide split water molecules to make oxygen."

Once the forest of nanowire arrays is established, it is colonized with microbial populations that produce enzymes known to selectively catalyze the reduction of CO₂. For this study, the Berkeley team used Sporomusa ovata, an anaerobic bacterium that readily accepts electrons directly from the surrounding environment and uses them to reduce CO₂. The bacteria are held in a solution of buffered brackish water with trace vitamins.

Once the carbon dioxide has been reduced by S. ovata to acetate, genetically engineered E. coli bacteria are used to synthesize targeted chemical products.

To improve yields of targeted chemical products, S. ovata and E. coli were kept separate for this study. In the future, their two activities – catalyzation and synthesis, respectively – could be combined into a single-step process.

With this approach, the Berkeley team achieved solar energy conversion efficiency of up to 0.38 percent for about 200 hours under simulated sunlight – about the same efficiency as a leaf.

The yields of target chemical molecules produced from the acetate were also encouraging: as high as 26 percent for butanol, a fuel comparable to gasoline; 25 percent for amorphadiene, a precursor to the antimalaria drug artemisinin; and 52 percent for the renewable, biodegradable plastic PHB.

Improved performances are anticipated with further refinements of the technology. Yang said the team is now working toward 3 percent solar-to-chemical conversion efficiency.

"Our system has the potential to fundamentally change the chemical and oil industry, in that we can produce chemicals and fuels in a totally renewable way, rather than extracting them from deep below the ground," he said. "Once we can reach a conversion efficiency of 10 percent in a cost effective manner, the technology should be commercially viable."

Funding was provided primarily by the Office of Science, U.S. Department of Energy.

The research was published in Nano Letters (doi: 10.1021/acs.nanolett.5b01254).

For more information, visit www.lbl.gov.

The artificial photosynthesis system has four main functions: (1) harvesting solar energy, (2) generating reducing equivalents, (3) reducing CO₂ to biosynthetic intermediates and (4) producing value-added chemicals.

Solar Nanowire-Bacteria Array Converts CO2 into Useful Chemicals

Solar Nanowire-Bacteria Array Converts CO₂ into Useful Chemicals