Here's a new way to make fuel from sunlight: starve a microbe nearly to death, then feed it carbon dioxide and hydrogen produced with the help of voltage from a solar panel. A newly developed bioreactor feeds microbes with hydrogen from water split by special catalysts connected in a circuit with photovoltaics. Such a batterylike system may beat either purely biological or purely technological systems at turning sunlight into fuels and other useful molecules, the researchers now claim.
"We think we can do better than plants," says Joseph Torella of Boston Consulting Group, who helped lead the work published February 9 in Proceedings of the National Academy of Sciences.
The process started in 2009 with the cheap, water-splitting catalysts developed by chemist Daniel Nocera, now at Harvard University. These cobalt–phosphate catalysts use electricity to make hydrogen out of ordinary water. But hydrogen has not caught on as an alternative fuel. So when Nocera arrived at Harvard, he partnered with biochemist Pamela Silver of Harvard Medical School, her then-graduate student Torella and others to build a hybrid system that could make a more useful fuel.
By pairing machine and microbe, this new "bionic leaf" gains the best features of both. Photovoltaics can turn much more incoming sunlight into electric current than the photosynthesis employed by bacteria or plants—and the new catalysts can split ordinary water, even the dirty stuff from the Charles River in Boston. But microbes, photosynthetic or otherwise, are good at turning incoming energy into useful molecules, whether food, fuel or even pharmaceuticals. So Torella and the rest of the team paired the photovoltaic water-splitting wafer with Ralstonia eutropha, a soil bacteria that can use the split hydrogen to power the building of molecules out of carbon, in a jar. Using a genetically engineered variant of R. eutropha, the team made isopropanol (C3H8O), an alcohol molecule that can be used as fuel like ethanol or gasoline and can be easily separated from water with salt.
The bionic leaf can pump out 216 milligrams of isopropanol per liter of water—an efficiency that rivals that of a corn plant making starch-rich kernels out of sunlight. The key is using the specially tweaked R. eutropha and putting them in a sealed jar filled with nutrient-free liquid plus hydrogen and dissolved CO2. A few transfers from jar to jar mixed with vigorous stirrings plus time cause the R. eutropha to switch from normal growth to panic mode, inducing the microbes to feed directly on the hydrogen. The resulting colony was placed in the jar with the water splitter and a stainless steel electrode connected to a photovoltaic array to provide current and, after a lag of a few days, the new bionic leaf began to grow—and spit out isopropanol.
This is not the first time R. eutropha has been used to make fuel from solar electricitybut the new work is the first to put the unique microbe in the same chamber as the water-splitting, electrically driven chemistry rather than separating the living from the nonliving to prevent the nonliving chemistry from killing off the life. The new work also heralds engineering progress for the dream of electrofuels—liquid fuels made using electricity, an innovative program that ran from 2008 to 2012 as part of the Advanced Research Projects Agency for Energy, or ARPA–E, which helped inspire this work.
The idea is to reverse combustion and use the waste product of fossil-fuel burning—CO2—to build fuels as well, just as plants do. "Oil and gas are not sustainable sources of fuel, plastic, fertilizer or the myriad other chemicals produced with them," Torella says. "The next best answer after oil and gas is biology, which in global numbers produce[s] 100 times more carbon per year via photosynthesis than humans consume from oil."
Improved, the bionic leaf could enable production of fuel, pharmaceuticals or other useful molecules wherever there is sunlight and CO2. "Imagine a system that can be created in a glass of water to produce new and useful chemicals," Silver says. "Efficiency will be our primary goal for the bionic leaf."
That improvement could come in the form of mutant R. eutropha that might be better at this job or more tolerant of harsh conditions, which could help produce more fuel. Or an entirely different microbe might more easily divert most of the CO2 to useful molecules. Or, conversely, the electrode materials could be tweaked to minimize or remove the challenges they present to the microbe.
The trick to making the bionic leaf work best is to operate at the high voltages that help the R. eutropha cells thrive while also producing lots of the desired molecule. But low voltages enable extra production of the desired molecules, with the signal disadvantage of killing the cells via toxic oxygen by-products from unwanted reactions at the electrode. Oxygen also poses challenges to life in photosynthesis and that may ultimately mean that the bionic leaf is surpassed by nonliving chemistry. "What if we took that hydrogen and thermally reacted it with [carbon monoxide] or CO2 itself?" asks chemist Andrew Bocarsly of Princeton University, who has worked on electrochemical cells that can turn CO2 into fuels. Building molecules out of such syngas using heat is already used in industry so "how do the energy efficiencies now compare? I don't know the answer."
Regardless of which method wins, reversing combustion could help solve the problem of global warming. In fact, the final product of the bionic leaf need not be isopropanol but could be many different carbon-based molecules in principle—even, perhaps someday, the hydrocarbons more commonly known as oil or natural gas. "The pathway that was modified to create isopropanol is one with tremendous carbon flux," Silver notes of the bionic leaf. "In theory other fuel molecules can be made.""