Using a stew of enzymes culled from several organisms, researchers have developed a way to convert starch, available from numerous sources including corn and potatoes, into hydrogen gas at low temperatures and pressures. The method produces three times more hydrogen than an older enzymatic method does, suggesting that it might be practical to use such enzymes to produce hydrogen for fuel-cell vehicles.
While fuel-cell vehicles are appealing because they emit no pollutants, it’s been a challenge to find clean and affordable ways to produce, transport, and store hydrogen to fuel them. Most commonly, hydrogen is extracted from fossil fuels. Making hydrogen by electrolyzing water is energy intensive and can be expensive. The new system improves on other experimental methods for creating hydrogen from biomass by using low temperatures, making it potentially more convenient and energy efficient.
The researchers–from Virginia Tech, in Blacksburg, VA; Oak Ridge National Laboratory; and the University of Georgia, in Athens–combined 13 commercially available enzymes isolated from yeast, bacteria, spinach, and rabbit muscle. The work is available online in PLoS ONE, a journal published by the Public Library of Science. The hydrogen comes from two sources: the starch and the water used to oxidize the starch. The enzymes facilitate chemical reactions in which the water and starch can be completely converted into hydrogen and carbon dioxide, says Y. Percival Zhang, professor of biological systems at Virginia Tech. (The carbon dioxide released is offset by the carbon dioxide captured by plants that provide the starch.)
The new system produces a higher yield of hydrogen than previous experimental systems that used enzymes for converting sugars into hydrogen. But while the yield of hydrogen is high, so far the rates at which the gas is produced are extremely low. That’s in part because the researchers used off-the-shelf enzymes and have not optimized the system, Zhang says. The scientists’ next project will include analyzing each stage of the process in detail to find the rate-limiting steps.
For example, one of the enzymes may be producing a by-product that slows down later steps, says Michael Adams, professor of biochemistry and molecular biology at the University of Georgia. The researchers would then look for other enzymes, or modify current ones, to minimize the by-product. They will also look for enzymes that can operate at higher temperatures. “If you increase the temperature by 10 degrees, most times you can increase the reaction rate twofold,” Zhang says.
One of the first applications of the system, Zhang says, could be generating hydrogen for fuel cells in portable electronics. The starch could be a safer way of storing energy than using methanol, a current leading option for such small fuel-cell systems. He estimates that it will take about six to eight years to improve the rates enough for such applications. Eventually, he hopes to use his process to solve one of the biggest current problems with hydrogen fuel-cell vehicles: fitting enough hydrogen on board to compete with gasoline-powered vehicles.
But some Department of Energy (DOE) officials doubt that the entire system will be light enough for onboard use. Sunita Satyapal, the hydrogen-storage team leader at DOE, notes that the researchers’ estimates do not include the weight of the water or the other equipment needed to produce the hydrogen. These things could more than double the weight of the system, she says, even if water produced by the fuel cell is recycled. The system will probably be too heavy to give the vehicle a driving range competitive with gasoline engines, suggests Satyapal.
She also notes that the rate of hydrogen production is now orders of magnitude lower than it would need to be for use in vehicles, and it will be very difficult, if not impossible, to sufficiently improve the rate.
But even if the new system is not useful as a way of producing hydrogen in a car, it eventually could prove useful for producing hydrogen at fueling stations. One of the challenges with hydrogen production is the cost of compressing and transporting hydrogen from central locations. On-site production using enzymes at filling stations, or even in people’s homes, could get around these issues. In such applications, the hydrogen production rate can be lower than it is aboard a vehicle, as the hydrogen can be produced around the clock in relatively large tanks.
Still, some are skeptical of the basic concept of using starch to create fuel. “Making food into hydrogen is not such a great idea,” says John Deutch, a chemistry professor at MIT. Indeed, demand for corn to make ethanol is already increasing food prices. Using corn starch to make hydrogen could exacerbate the problem.
But Zhang notes that employing starch to make hydrogen would be a much better use of the available corn than turning it into ethanol: fuel cells can be three times more efficient than ethanol-burning internal combustion engines. Nevertheless, he sees starch as a temporary solution. Zhang is also developing a version of the process that starts with cellulose, found primarily in the nonfood parts of plants.