A Better Way to Make Hydrogen?
A new process for using aluminum alloys to generate hydrogen from water could make fuel-cell vehicles more practical, says Jerry Woodall, a professor of electrical and computer engineering at Purdue.
Hydrogen fuel cells are attractive because they produce no harmful emissions, but hydrogen gas is hard to transport, and hydrogen vehicles have a limited range because it’s difficult to store large amounts of hydrogen onboard. Many researchers are developing methods for storing more hydrogen, including packing it into carbon nanotubes or temporarily storing it in chemical compounds. Woodall’s solution is to store hydrogen as water, splitting hydrogen from oxygen only when it’s needed to power the vehicle.
Earlier this year, Woodall reported successfully generating significant amounts of hydrogen using a combination of aluminum and gallium. In those experiments, however, the alloy contained mostly gallium, which both limited the hydrogen-generating capacity of the material and kept costs high. At a nanotechnology conference on Friday, Woodall will present new work that shows that the process succeeds with an alloy containing 80 percent aluminum. This could make the system far more practical by reducing the amount of expensive gallium while increasing the amount of active material.
Woodall’s process works because of aluminum’s strong affinity for oxygen, which causes the metal to break water apart, forming aluminum oxide and releasing hydrogen. This basic chemical process is, of course, well known, but the problem has been that as soon as aluminum is exposed to air, it quickly forms a thin layer of aluminum oxide that seals off the bulk of the aluminum and prevents it from reacting with water. Woodall’s insight, says Sunita Satyapal, who heads the Department of Energy’s (DOE) hydrogen-storage program, is to use gallium to prevent this layer from completely sealing off the aluminum. Although the molecular mechanisms are still not understood, it’s known that the gallium causes gaps in the oxide layer that allow the aluminum to react quickly with the oxygen in water, but not with the oxygen in air.
Woodall envisions a system in which aluminum pellets would be delivered to fueling stations where drivers would load about 50 kilograms of pellets and 20 kilograms of water into separate containers, with the two mixed as needed to generate hydrogen and aluminum oxide. (This would provide the equivalent of about 60 kilograms of gasoline, Woodall says.) The aluminum oxide can be recycled employing the same process used for aluminum cans, and the gallium can be easily separated from the aluminum oxide and used again.
But the electricity needed to recycle the aluminum could be a problem, since it would be a major source of pollution unless it comes from clean sources such as solar or wind. Also, Satyapal says that the energy efficiency of the process falls short of DOE goals.
The DOE, together with oil and car companies, has set goals for the amount of hydrogen that should be stored onboard a vehicle, aiming to provide the same range as gasoline-powered cars without changing vehicle designs or reducing cargo and passenger space. Woodall says that he can meet the goals for cars and other light vehicles, in part by recycling water produced by the fuel cells. The DOE, however, estimates that Woodall’s process would take up too much room because, among other reasons, recycling water will likely not be practical, Satyapal says.
Woodall is working with AlGalCo, a startup based in West Lafayette, IN, to commercialize the process. The company’s initial products will be fuel-cell generators that run on hydrogen produced with a version of his aluminum alloy.
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