New materials show promise for hydrogen fuel storage

May 15, 2003
Written By:
Nancy Ross-Flanigan
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ANN ARBOR, Mich.—In the push to develop hydrogen fuel cells for powering automobiles, cell phones, laptop computers and other devices, one of the biggest challenges has been finding ways of storing large amounts of hydrogen at normally encountered temperatures and pressures.

A new class of materials achieves that aim without the problems associated with other approaches, researchers report in the May 16 issue of the journal Science. Their work also points to ways of making the materials hold even more hydrogen.

“Our idea was to create a material with pores that attract hydrogen,” said Yaghi. “That makes it possible to ‘stuff’ more hydrogen molecules into a small area without resorting to high pressure or low temperature.” The class of materials, called metal-organic frameworks (MOFs), can be made from low-cost ingredients, such as zinc oxide—a common component of sunscreen—and terephthalate, which is used in plastic soda bottles. Sometimes called crystal sponges, MOFs are essentially scaffolds made up of linked rods—a structure that makes for maximum surface area. Just one gram of a MOF, in fact, has the surface area of a football field.

The researchers found that they can increase the material’s storage capacity by modifying the rods in various ways. “The material that we’re reporting on takes up two percent of its weight in hydrogen,” Yaghi said. “The U.S. Department of Energy (DOE) standard for use of hydrogen in automobile applications is about six percent. The exciting thing about this report is not only that we’ve discovered a new material that takes up hydrogen, but also that we’ve identified a clear path for how to get to six percent.” In work published in Science last year, the researchers found that MOFs can also store large amounts of methane. “We now have materials that exceed the DOE requirements for methane, and we think we can apply the same sort of strategy for hydrogen storage.”

MOFs should prove superior to metal hydride alloys, which also are being explored for hydrogen storage, said Yaghi. “One of the problems with metal hydride is that the stored hydrogen is chemically bound to the metal. That means that you have to pressurize the material to charge it with hydrogen, and you have to heat the material to high temperatures to discharge the hydrogen. The process of charging and discharging under these extreme conditions ends up contaminating the metal and breaking the whole process down, so these materials have a limited lifetime. With MOFs, the hydrogen is physically absorbed, not chemically absorbed, so it’s easier to take the hydrogen out and put it back in without much energy cost.”

Yaghi’s group collaborated on the work with researchers from the University of California, Santa Barbara; Los Alamos National Laboratory; and Arizona State University. The research was funded by the National Science Foundation, DOE, Nalco Chemical Company, and BASF.

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Professor Omar Yaghi