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Solar Array Turns CO2 into Useful Chemicals

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Efforts abound in the interest of shrinking our carbon footprint and making Earth a healthier place. Recycling paper, plastic and other materials helps, but what if we could take it a step farther and repurpose carbon dioxide emissions? Scientists with the U.S. Department of Energy’s Lawrence Berkeley National Laboratory and the University of California, Berkeley, have discovered how to do it.

With funding from the DOE Office of Science, the team of researchers has developed an artificial photosynthesis process that, with help from sunlight, could ultimately capture CO2 emissions and convert them into plastics, drugs and fuels.

“In natural photosynthesis, leaves harvest solar energy, and carbon dioxide is reduced and combined with water for the synthesis of molecular products that form biomass,” said co-lead researcher Christopher Chang, an expert in catalysts for carbon-neutral energy conversions. “In our system, nanowires harvest solar energy and deliver electrons to bacteria, where carbon dioxide is reduced and combined with water for the synthesis of a variety of targeted, value-added chemical products.”

A new artificial photosynthesis process captures CO<sub>2</sub> emissions and converts them into useful chemical products.
A new artificial photosynthesis process captures CO2 emissions and converts them into useful chemical products.

A hybrid system, the semiconducting nanowires and bacteria mimic 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.

“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,” said co-lead researcher Michelle Chang, an expert in biosynthesis who holds appointments with UC Berkeley and Berkeley Lab.

The system starts with an “artificial forest” of heterostructures consisting of silicon and titanium oxide nanowires.

“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,” said Berkeley Lab chemist Peidong Yang. “The photogenerated electrons in the silicon will be passed on to bacteria for the CO2 reduction, while the photogenerated 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. As soon as the carbon dioxide has been reduced by S. ovata to acetate, genetically engineered E. coli bacteria are used to synthesize targeted chemical products.

The Berkeley team achieved solar energy conversion efficiency of up to 0.38 percent for about 200 hours under simulated sunlight, which is comparable to the efficiency of a leaf. The researchers now are working to increase the solar-to-chemical conversion efficiency.

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 antimalarial drug artemisinin; and 52 percent for the renewable, biodegradable plastic PHB.

“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,” Yang said. “Once we can reach a conversion efficiency of 10 percent in a cost-effective manner, the technology should be commercially viable.”

Photonics Spectra
Jun 2015
GreenLightenergychemicalsAmericascarbon footprintcarbon dioxide emissionsDOE Office of Scienceartificial photosynthesisacetatenanowire arraySporomusa ovatasolar-to-chemical conversion efficiencybutanolamorphadieneartemisininPHBChristopher ChangMichelle ChangLawrence Berkeley National LaboratoryUniversity of CaliforniaBerkeley California

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