Artificial Photosynthesis Yields Valuable Chemicals

photo credit: Berkeley Lab. These nanowire superconductors don't look like much even under a scanning electron microscope, but in combination with bacteria, they could turn carbon dioxide into useful products

Tiny semiconductors and bacteria have been combined to create a system that uses sunlight to turn carbon dioxide into valuable chemicals.

Photosynthesis forms the basis of most life on Earth. However, it cannot draw carbon dioxide out of the atmosphere fast enough to match the rate at which we are releasing what was stored over millions of years. This has led to a quest to produce an artificial and more efficient version – ideally one that would turn the carbon into something we can easily use.

Recently, there has been a lot of work based around the idea of combining bacteria with manufactured materials. The Lawrence Berkeley National Laboratory has announced what team leader Professor Peidong Yang calls “a revolutionary leap forward” in this area.

"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 Yang.

The work, described in Nano Letters, combines an array of semiconductor nanowires with Sporomusa ovata to turn carbon dioxide into acetate (C2H3O2−) using just sunlight and water.

The silicon and titanium dioxide wires use sunlight to produce a flow of electrons and have a large surface area for the bacteria to colonize. Using the electron's flow, the bacteria turn carbon dioxide to acetate. Genetically engineered E. coli exist that can turn the acetate into a variety of valuable products, including the fuel butanol, the pharmaceutical precursor amorphadiene and the biodegradable plastic PHB. 

"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," says co-author Chris Chang. "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."

Credit: Berkeley Lab. Schematic of the four-step process to turn waste carbon dioxide into useful products through artificial photosynthesis.

The carbon dioxide would be sourced from the exhaust of coal or gas-fired power stations. Unlike some plans for making use of power station waste, the wires offer protection to the normally oxygen-phobic bacteria, removing the need to separate the waste carbon dioxide from oxygen.

The water is slightly salty and contains trace vitamins for the bacteria, both of which are not in short supply. Likewise, the use of readily available raw materials for the wires indicates that the process should be able to be conducted very cheaply once mass production is under way. The authors add that by combining the two bacterial species, costs could be reduced further.

The conversion efficiency of the acetate to valuable chemicals is already between 25 and 52%, but the wires are currently only turning 0.38% of sunlight to electric charge, a 50th of good commercial solar cells.

"We are currently working on our second generation system which has a solar-to-chemical conversion efficiency of three-percent," Yang says. "Once we can reach a conversion efficiency of 10-percent in a cost effective manner, the technology should be commercially viable."

ORIGINAL: IFL Science

by Stephen Luntz

April 20, 2015