Towards hydrogen as fuel for cars and electronic devices
Chemists at UCLA and the University of Michigan report an advance toward the goal of cars that run on
hydrogen rather than gasoline. While the U.S. Department of Energy estimates that practical hydrogen
fuel will require concentrations of at least 6.5 percent, the chemists have achieved concentrations of 7.5
percent -- nearly three times as much as has been reported previously -- but at a very low temperature
(77 degrees Kelvin). The research, scheduled to be published in late March in the Journal of the
American Chemical Society, could lead to a hydrogen fuel that powers not only cars, but laptop
computers, cellular phones, digital cameras and other electronic devices as well.
"We have a class of materials in which we can change the components nearly at will," said Omar Yaghi,
UCLA professor of chemistry, who conducted the research with colleagues at the University of
Michigan. "There is no other class of materials where one can do that. The exciting discovery we are
reporting is that, using a new material, we have identified a clear path for how to get above seven
percent of the material's weight in hydrogen."
The materials, which Yaghi invented in the early 1990s, are called metal-organic frameworks (MOFs),
pronounced "moffs," which are like scaffolds made of linked rods -- a structure that maximizes the
surface area. MOFs, which have been described as crystal sponges, have
pores, openings on the nanoscale in which Yaghi and his colleagues can store gases that are usually
difficult to store and transport. MOFs can be made highly porous to increase their storage capacity; one
gram of a MOF has the surface area of a football field! Yaghi's laboratory has made more than 500
MOFs, with a variety of properties and structures.
"We have achieved 7.5 percent hydrogen; we want to achieve this percent at ambient temperatures,"
said Yaghi, a member of the California NanoSystems Institute. "We can store significantly more
hydrogen with the MOF material than without the MOF."
MOFs can be made from low-cost ingredients, such as zinc oxide -- a common ingredient in sunscreen -- and
terephthalate, which is found in plastic soda bottles.
"MOFs will have many applications. Molecules can go in and out of them unobstructed. We can make
polymers inside the pores with well-defined and predictable properties. There is no limit to what structures we
can get, and thus no limit to the applications."
In the push to develop hydrogen fuel cells to power cars, cell phones and other devices, one of the biggest
challenges has been finding ways to store large amounts of hydrogen at the right temperatures and
pressures. Yaghi and his colleagues have now demonstrated the ability to store large amounts of hydrogen
at the right pressure; in addition, Yaghi has ideas about how to modify the rod-like components to store
hydrogen at ambient temperatures.
"Instead of a battery, one would have a medium such as MOF that stores hydrogen and releases it into a
fuel cell," he said.
Yaghi, whose research overlaps chemistry, materials science and engineering, has long been interested
in making materials in a rational way.
"When I started out in chemistry, I always thought it should be possible to take two well defined molecules
as building blocks and stitch them together into a predetermined chemical structure -- almost like you
produce a blueprint of the structure ahead of time and then find the right building blocks
necessary to build it. In this way, one can control the structure and the composition. This approach was
difficult to implement at the beginning, but is not so difficult at this stage."
Hydrogen, when burned, produces only 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.
"The challenge has been, how do you store enough hydrogen for an automobile to run for 300 to 400 miles
without refueling?" Yaghi asked. "You have to concentrate the hydrogen into a small volume without using
high pressure of very low temperature.
"Our idea was to create a material with pores that attract hydrogen, making it possible to stuff more
hydrogen molecules into a small volume," he said.
In previous research, Yaghi and colleagues reported that MOFs also can store large amounts of methane
"We have materials that exceed the DOE requirements for methane, and we think we can apply the same
sort of strategy for hydrogen storage," 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