A new method of artificial photosynthesis can convert waste CO2 into valuable chemical products such as biodegradable plastics, pharmaceuticals and liquid fuels.
Using a hybrid system of semiconducting nanowires and bacteria, which mimics the photosynthetic process, the system can synthesise CO2 and water into acetate – a versatile building block in both chemical and biological systems that can then be synthesised into more complex molecules.
The technology could be combined with carbon capture methods aiming to remove the greenhouse gas CO2 from the exhausts of power stations and provide an alternative to storing the gas deep underground. Instead, it could be used to create useful chemicals.
"We believe our system is a revolutionary leap forward in the field of artificial photosynthesis," said Peidong Yang, a chemist at the University of California (UC) Berkeley, one of the leaders of the study.
"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."
The system uses an "artificial forest" of silicon and titanium oxide nanowires that mimics the function of chloroplasts in plants populated with microbes that produce enzymes known to selectively catalyse the reduction of carbon dioxide.
Yang said: "When sunlight is absorbed, photo-excited electron hole pairs are generated in the silicon and titanium oxide nanowires, which absorb different regions of the solar spectrum. The photo-generated 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."
For the groups latest study, published in the journal Nano Letters, the Berkeley team used sporomusa ovata – an anaerobic bacteria that readily accepts electrons directly from the surrounding environment and uses them to reduce carbon dioxide.
"S. ovata is a great carbon dioxide catalyst as it makes acetate, a versatile chemical intermediate that can be used to manufacture a diverse array of useful chemicals," said Berkley chemist Michelle Chang, another of the study's leaders.
"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.
"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."
Once S. ovata has reduced the CO2 to acetate or another biosynthetic intermediate, genetically engineered E.coli bacteria kept separately are then used to synthesise targeted chemical products form the intermediate. In the future, the team plan to combine the two procedures into a single step process.
The Berkeley team achieved a solar energy conversion efficiency of up to 0.38-per cent for about 200 hours under simulated sunlight – roughly the same as that of a leaf.
Yields of target chemical molecules produced from the acetate were also encouraging – as high as 26 per cent for butanol, a fuel comparable to gasoline, 25 per cent for amorphadiene, a precursor to the anti-malaria drug artemisinin, and 52 per cent for the renewable and biodegradable plastic PHB.
According to the researchers, key to the success of the new system has been the separation of the demanding requirements for light-capture efficiency and catalytic activity that is made possible by the nanowire/bacteria hybrid technology.
"We are currently working on our second-generation system which has a solar-to-chemical conversion efficiency of three per cent," Yang said. "Once we can reach a conversion efficiency of 10 per cent in a cost-effective manner, the technology should be commercially viable."