A new method of artificial photosynthesis could get humans one step closer to using the machinery of plants to make fuels.
The new system is 10 times more efficient than previous synthetic photosynthesis methods. While natural photosynthesis allows plants to turn carbon dioxide (CO2) and water into carbohydrates using the power of the sun, the artificial method can turn carbon dioxide and water into energy-dense fuels like methane and ethanol. This could provide an alternative to fossil fuels drilled out of ancient rock.
"The biggest challenge many people don’t realize is that even nature has no solution for the amount of energy we use," University of Chicago chemist Wenbin Lin, one of the authors of the new study, said in a statement. Natural photosynthesis, while sufficient for plants to feed themselves, falls short of providing the quantity of energy required to fuel our homes, cities and nations. "We will have to do better than nature, and that’s scary," he said.
Researchers have been working to borrow the machinery of photosynthesis to create their own desired chemicals for years, but tweaking photosynthesis to serve human needs is not easy. The process is complicated and involves two steps: First, breaking apart water and CO2, and second, reconnecting the atoms into carbohydrates. Lin and his team had to create a system that would instead produce methane, or CH4, which is a carbon surrounded by four hydrogen molecules.
Though combusting this synthetic methane would still lead to greenhouse gas emissions, researchers are also working on using artificial photosynthesis to make hydrogen fuels, which release only water vapor and warm air.
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To do this, they began with a metal-organic framework — a web made of charged metal atoms linked by organic molecules. (Organic molecules contain carbon.) They submerged single layers of this metal-organic framework in a cobalt solution; this element is good at picking up electrons and moving them around during chemical reactions.
Then the researchers did something that hadn't been tried before. They added amino acids, the molecular building blocks of proteins, to the mix. These amino acids boosted the efficiency of both sides of the reaction, breaking down CO2 and water and rebuilding them as methane. The resulting system was 10 times more efficient than previous artificial photosynthesis methods, the team reported in the journal Nature Catalysis on Nov. 10.
However, that's still not efficient enough to make enough methane for human fuel use.
"Where we are now, it would need to scale up by many orders of magnitude to make a sufficient amount of methane for our consumption," Lin said. But, he said, the team was able to determine how the system works on a molecular level, which had never been fully understood before. Understanding the process is a crucial step before they can scale up the process.
If the system isn't currently efficient enough to fuel cars or heat homes, it may already be feasible for other uses that don't require such a high volume of product. For example, Lin said, a similar method could be used to produce basic chemicals for pharmaceuticals.
"So many of these fundamental processes are the same,” said Lin. "If you develop good chemistries, they can be plugged into many systems."
