Inspired by photosynthesis in plants, scientists have built two systems that use sunlight and water to produce methane, the main component of natural gas, and butanol, which can be used as vehicle fuel.
The processes both use nanomaterials that absorb solar energy and microbes that use the energy, in addition to carbon dioxide, to produce useful chemicals. Materials scientist Peidong Yang (University of California, Berkeley), a senior fellow of CIFAR’s program in Bio-inspired Solar Energy, has been developing the concept with his collaborators for more than a decade.
“This is something we learned from nature,” Yang says. It mimics the same system plants use in photosynthesis, absorbing sunlight and converting it to chemical energy in carbohydrate molecules such as sugar.
They made their first major breakthrough in April, with a system that used semi-conducting nanowires to harvest sunlight and turn it into electrons, and then feed the electrons to bacteria that produced acetate. By feeding the acetate to synthetically-engineered E.coli, they could produce chemicals such as butanol, a liquid fuel. The process was described in a Nanoletters paper.
A diagram showing how solar energy can be used to produce methane and other useful chemicals. Solar energy is captured and then used to generate hydrogen from water, the hydrogen is fed to microbes that use it to produce the final product. Credit: Proceedings of the National Academy of Sciences
“You can consider solar generated butanol as a new sort of gasoline,” Yang says. Butanol can be used in a standard vehicle without modifying the engine and it releases less carbon dioxide into the atmosphere than standard gas when burned, but it is currently expensive and difficult to produce. This is the first system that produces butanol from solar energy using CO2 and water.
Most recently, Yang was a corresponding author on a Proceedings of the National Academy of Sciences paper about a new artificial photosynthesis system that produces methane. Sunlight and nickel sulfide nanoparticles split the hydrogen from water (H2O), and then a microbe called Methanosarcina barkeri takes in the hydrogen and CO2 and produces methane (CH4). Renewable methane is a useful product, but Yang hopes to improve the approach to produce even more complex chemical compounds.
He says one of the biggest challenges ahead will be replacing the microbes with synthetic alternatives that are easier to work with.
“Bacteria is very, very good at producing these chemical compounds with great selectivity,” Yang says. “The real challenge here is how do we mimic these bacteria to make catalysts with similar activity and selectivity?”
He says overcoming that challenge requires experts in many fields, such as biologists to study how nature works, materials scientists to work with nanomaterials that harvest sunlight, and chemists to produce synthetic catalysts that can take over the microbes’ jobs.
“The interdisciplinary research is the key here, and that’s the central feature in CIFAR,” Yang says.
The research is still in its early days, but he says the future could involve a much bigger role for solar power.
“It really is possible in my mind that we can make liquid fuel from CO2 in the environment and use solar energy, and then we can use this synthetic fuel to power our planet.”
This research was funded in part by the Department of Energy Office of Science, the National Science Foundation and the National Institutes of Health.