As promising as hydrogen fuel cells are for automotive and mobile electronic applications, a barrier to their practical use has been the inability to store enough hydrogen fuel at the proper pressure, temperature, and concentration. Now, chemists at UCLA and the University of Michigan are reporting progress on two of those fronts, and they’ve got some ideas for tackling the third.
Using a class of materials called metal-organic frameworks (MOFs) as a storage medium, the researchers have demonstrated the ability to achieve hydrogen concentrations of more than 7%. The U.S. Department of Energy estimates that practical hydrogen fuels will require concentrations of at least 6.5%. In addition, the researchers have shown that they can store the fuel at the proper pressures. Only the temperature frontier remains to be conquered.
According to Omar Yaghi, a professor of chemistry at UCLA and the inventor of MOFs, similar materials have been used to solve problems associated with storing methane gas. Now Yaghi is turning his attention to MOFs and hydrogen.
MOFs can be likened to “scaffolds made of linked rods,” says Yaghi, resulting in a structure with maximum surface area. One gram of a MOF material (Yaghi’s lab has made more than 500 variations of MOFs) has a surface area equivalent to a football field. Another way to think of MOFs is "crystal sponges" with nanoscale pores that can absorb very large quantities of difficult-to-store gases such as hydrogen. MOFs can be made from very low-cost materials such as zinc oxide—a common ingredient in sunscreen—and terephthalate, which is found in plastic soda bottles.
According to Yaghi, his team has shown that it can store significantly more hydrogen with MOFs than without. The goal is to store enough of the fuel in the MOF-based storage medium for an automobile to be driven 300 to 400 miles between fuel-ups. The challenge, says Yaghi, is to concentrate the hydrogen into a small volume without using high pressure or very low temperature.
Temperature is the final challenge. To date, Yaghi's successes have required cooling the fuels to very low temperatures of 77 K (-196.15°C). But Yaghi has ideas about how to modify the rod-like components to allow storage of hydrogen at ambient temperatures (0°C to 45°C).
When burned, hydrogen only produces water, which is harmless to the environment, Yaghi noted. With MOFs, hydrogen is physically absorbed, and it is easy to take the hydrogen out and put it back in without much energy cost, he said. Additionally, Yaghi has shown that MOFs store prodigious amounts of carbon dioxide at ambient conditions, a development relevant to preventing carbon-dioxide emissions from power plants and automobile tailpipes from reaching the atmosphere.
Yaghi's research is funded by the National Science Foundation, the U.S. Department of Energy, and BASF.