Metal-Organic Frameworks show promise as a way to solve the hydrogen storage problem for vehicles.

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.

"Hydrogen is an ideal fuel, because when burned it produces only water, which is quite harmless," said University of Michigan chemistry professor Omar Yaghi, whose work over the past 12 years led to the new materials. "But the problem has been, how do you store enough hydrogen for an automobile to run for 300 to 400 miles without refueling? You can't just put a huge tank of hydrogen on the back of an automobile; you have to concentrate the hydrogen into a small volume." That can be done by cooling hydrogen to an extremely low temperature or by compressing it under very high pressure, but neither option would be practical in a car or electronic gadget.

"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."

A solution to the hydrogen storage problem would not by itself reduce the demand for fossil fuels. There would still be the need for alternative energy sources to use to generate the hydrogen in the first place. Still, the ability to easily and cheaply store and retrieve hydrogen with minimal energy loss would be a great enabling technology for the use of other energy sources.

The key point to keep in mind is that fossil fuels are both fuel sources and great forms of fuel storage (though natural gas is less easy to store). To move to a different source of energy (e.g. wind or solar or nuclear) for, say, transportation applications we need both that alternative source of energy and a way to put that energy into a form that is easy to put into vehicles. Many alternative sources of energy are made into electricity but existing types of batteries weigh too much and cost too much. However, existing methods for storing hydrogen are all quite unsatisfactory for transportation applications as well.

Whether a new battery technology or a new hydrogen storage technology will become the first viable non-fossil fuel energy storage technology for vehicles remains to be seen. A major commitment to shift to hydrogen as an energy storage form remains premature as long as there is not a great way to store it in vehicles.

Update: Yaghi explains why the MOFs do not need large temperature or pressure changes to store and retrieve the hydrogen.

"The hydrogen is physically attracted to the walls of the [material's] pores," he said. "This attraction makes it possible to stuff more hydrogen molecules into a small area without requiring either low temperatures or high pressures."