Scientists at the Lawrence Berkeley National Laboratory and the University of California, Berkeley have created a hybrid system of bacteria and semiconducting nanowires that mimics photosynthesis. According to the researchers, their versatile, high-yield system can take water, sunlight and carbon dioxide and turn them into the building blocks of biodegradable plastics, pharmaceutical drugs and even biofuel.
Although renewable energy is making up a growing portion of the world’s energy production, scientists have suggested that the current trends of CO2 buildup in our atmosphere are still likely to lead to serious consequences, and do so sooner than we had anticipated.
One way to keep harmful emissions under control could be to trap the CO2 coming out of smokestacks using materials like polymers or sponges. Some scientists are even going one step further, working on technology that can convert carbon dioxide into useful byproducts like calcium carbonate or biofuels such as methanol and isobutanol. However, these systems are still either very low-yield or in an early experimental phase.
Taking inspiration from Mother Nature, scientists have now devised a system that uses sunlight and water to convert carbon dioxide into a wide range of useful chemicals. Artificial photosynthesis is not a new concept – it’s been used to split water into hydrogen and oxygen and synthesize formic acid – but this new approach could be a game changer because of its versatility and the high yields it produces.
"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," says Peidong Yang, who led the study along with Christopher and Michelle Chang.
Their invention uses two different types of bacteria interspersed within arrays of silicon and titanium nanowires. The silicon nanowires act like a miniature solar cell, capturing incoming light and releasing electrons. These electrons are then absorbed by Sporomusa ovata, an anaerobic bacterium that combines them with water and turns carbon dioxide into acetate, a versatile chemical precursor. Meanwhile, the titanium portion of the structure takes the positive charge left in place of the electron and uses it to extract oxygen from water. The oxygen is used by genetically engineered E. Coli bacteria to synthesize the desired chemicals.
The nanowire array also acts as a layer of protection for the bacteria, burying them in something akin to tall grass so that these usually-oxygen sensitive organisms can survive in adverse environmental conditions like flue gases.
As a proof of principle, the scientists showed that their system can reduce CO2 to chemicals including fuels, polymers and pharmaceutical precursors. The yields were up to 26 percent for butanol, 25 percent for amorphadiene, a precursor to the antimalarial drug artemisinin, and 52 percent for PHB, a renewable and biodegradable plastic, although these figures could rise even further with future optimizations.
Solar energy conversion efficiency was at 0.38 percent after 200 hours under simulated sunlight, which the researchers say is about the same as an actual leaf. But the team is already working to improve on this.
"We are currently working on our second generation system which has a solar-to-chemical conversion efficiency of three-percent," says Yang. "Once we can reach a conversion efficiency of 10 percent in a cost effective manner, the technology should be commercially viable."
The team's research appears on the latest issue of the journal Nano Letters.
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