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."