A new "bioreactor" could store electricity as liquid fuel with the help of a genetically engineered microbe and copious carbon dioxide. The idea—dubbed "electrofuels" by a federal agency funding the research—could offer electricity storage that would have the energy density of fuels such as gasoline. If it works, the hybrid bioelectric system would also offer a more efficient way of turning sunlight to fuel than growing plants and converting them into biofuel.
"The method provides a way to store electrical energy in a form that can be readily used as a transportation fuel," chemical engineer James Liao of the University of California, Los Angeles, explains. Liao and his colleagues report on their "integrated electro-microbial bioreactor" in Science on March 30.
To convert electricity into liquid fuel, Liao and his colleagues focused on Ralstonia eutropha, a soil microbe that can use hydrogen as an energy source to build CO2 into more microbial growth. Already, the microbe's biological machinery is being harnessed for industrial purposes—for example, to churn out plastic instead of proteins. By tweaking the industrial microorganism's genetics, the team now has coaxed it to churn out various butanols—a liquid fuel. "If one speaks with combustion engineers, then they will tell you that the simplest real fuel is butanol," says chemist Andrew Bocarsly of Princeton University, who is not involved in the electrofuel project.
Liao's bioreactor gets its electricity from a solar panel. The current flows into an electrode in the bioreactor, which is full of water, CO2 and R. eutropha. The electricity starts a chemical reaction that uses the CO2 to make formate—carbon dioxide with a hydrogen atom attached, which is an ion (electrically charged) that substitutes for insoluble hydrogen as an energy source for the microbe. The genetically engineered R. eutropha then consumes the formate, yielding butanols, plus more CO2 as a waste product—the latter of which is recycled back through the biochemical process.
R. eutropha doesn't particularly like to be shocked, however, so Liao's team built a "porous ceramic cup" to shield the microbe from the electrical current. Powered by its photovoltaic panel, the bioreactor produced 140 milligrams per liter of butanol fuel over 80 hours, although it then stopped working. "In principle, we can use the same approach to produce other kinds of fuels or chemicals," Liao says.
The approach combines the appeal of energy-dense liquid fuels—packing 50-times or more the energy per kilogram of even the best batteries—with the potential to produce more fuel in a limited area than plants. Photosynthesis achieves the same thing, absorbing sunlight and storing its energy in the bonds of carbohydrate molecules—otherwise known as food and, nowadays, fuel. But photosynthesis is inefficient. For example, corn converted to ethanol captures less than 0.2 percent of the original energy in the sunlight as fuel. A photovoltaic cell can convert 15 percent of incoming photons into electricity, but such solar electricity is hard to store. Using solar power in an electrofuel bioreactor such as Liao's could theoretically convert as much as 9 percent of the incoming sunlight into the final and storable fuel. "By combining a man-made device, which has a great potential for improvement, with biological CO2 fixation, we get the best of both worlds," Liao argues, although that kind of efficiency has yet to be demonstrated. Even this demonstration process turns more sunlight into liquid fuel, however, than biofuels such as corn ethanol or even photosynthetic microbes genetically altered to make butanols. Plus, Liao adds, "it is possible to increase the productivity much higher, since Ralstonia is an industrial microorganism."
Of course, chemists can also build liquid fuels directly, either via electricity or the application of high heat and pressure. For example, Bocarsly's lab has created an electrochemical cell that uses electricity to knit CO2 and hydrogen into methanol, the simplest liquid hydrocarbon. And chemist Nate Lewis of the U.S. Department of Energy's (DoE) Joint Center for Artificial Photosynthesis in Pasadena, Calif., is trying to create an entirely man-made version of a plant's food-making process. It remains to be seen whether bio-based systems like Liao's can deliver a more efficient method of storing electricity as liquid fuel. "If the authors had provided information on the currents used and the voltage dropped across the cell, one could calculate an energy-conversion efficiency," Bocarsly notes.
But the novel bioreactor and its electrofuel demonstrate a proof of principle—one that is also being demonstrated with microbes that can use electricity directly. "We now know it's going to work," says Eric Toone, deputy director of the DoE's Advanced Research Projects Agency–Energy, which funded the research in the hopes of displacing fossil fuels with such electrofuels. "Now we have to ask the harder question: Will it matter?" In fact, the electrofuel process will only matter if it can efficiently deliver liquid fuels from electricity on a large scale at low cost. As it stands, Liao's hybrid process breaks down after about 80 hours, perhaps due to R. eutropha's genetic instability, susceptibility to butanol poisoning or other factors. As to whether electrofuels will ultimately have an impact, Toone says, "We don't know the answer."