Hydrogen storage eased
Technology Research News
One of the biggest challenges to using
hydrogen as a fuel is finding a way to store it. The lighter-than-air
gas makes the perfect fuel -- it contains three times the energy of liquid
hydrocarbons and when it reacts with oxygen to produce energy the only
byproduct is water -- but it isn't easy to contain.
Today's hydrogen storage materials hold 2 to 4 percent of their
weight in hydrogen, short of the 6.5 percent Department of Energy goal
for using hydrogen as automobile fuel.
Researchers from the University of Michigan, the University of
California at Santa Barbara, the University of South Florida and Arizona
State University have discovered a new class of materials, dubbed metal-organic
frameworks, that are relatively inexpensive to make and have the potential
to reach the 6.5 percent mark. "We are in sight of the DOE goal," said
Omar Yaghi, a chemistry professor at the University of Michigan.
The discovery promises to remove the principal stumbling block
to hydrogen-powered cars, and the method could be ready for production
use within five years.
Hydrogen storage materials act like sponges, capable of filling
up with certain gases and later releasing them. The challenge is developing
materials that hold useful amounts of hydrogen, and that store and release
the hydrogen easily.
Current hydrogen storage systems chemically bind powdered metal
hydrides to hydrogen at high temperatures. In November, researchers in
Singapore developed a metal material that holds more than 11 percent of
its weight in hydrogen, but requires high temperatures and pressures.
Researchers are also exploring carbon-based approaches, including carbon
nanotubes, but these require very low temperatures.
It is easy to store and retrieve hydrogen using metal-organic
frameworks materials, said Yaghi. "Hydrogen can be inserted into the material
and then removed reversibly with no change to the storage medium," he
said. When the materials are exposed to hydrogen at room temperature and
under modest pressure, they take it up immediately, he said.
This is possible because hydrogen is adsorbed by rather than chemically
bound to the storage material, said Yaghi. Adsorption is the process of
gas or vapor atoms sticking to a surface. "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."
Metal-organic frameworks are exceptionally porous at the molecular
scale, with surface areas of more than 3,000 square meters per gram, according
to Yaghi. They are "basically scaffolds of linked rods," he said.
The materials have several other advantages, said Yaghi. They're
made from low-cost starting materials including zinc oxide, which is used
in sunscreen lotion, and terephthalate, which is a component of plastic
soda bottles. They are simple to make, and manufacturing yields are high,
The researchers previously showed that metal-organic frameworks
can absorb voluminous quantities of nitrogen and organic vapors, said
Yaghi. "Given the importance of hydrogen as a fuel, we sought to examine
the hydrogen storage capabilities," he said.
The researchers' showed that it is possible to design metal-organic
frameworks materials that absorb incrementally more hydrogen. Their best
prototypes store two percent of their weight in hydrogen, but the materials
have the potential to store much more, said Yaghi. "We have shown that
we can systematically increase the hydrogen storage capacity of these
materials, thus identifying a clear path toward achieving the DOE hydrogen
storage goal," he said
The researchers are currently working on increasing the hydrogen
capacity of the materials and also on better understanding the reasons
the materials are able to absorb so much hydrogen, said Yaghi.
The researchers are also collaborating with BASF Corporation to
use the materials in practical applications. It will take from two to
five years of development before the material can be used in practical
applications, Yaghi said.
Yaghi's research colleagues were Nathaniel Rosi, David T. Vodak
and Jaheon Kim from the University of Michigan, Jurgen Eckart from the
University of California at Santa Barbara and the Los Alamos National
Laboratory, Muhamed Eddaoudi from the University of South Florida, and
Michael O'Keefe from Arizona State University. The work appeared in the
May 16, 2003 issue of Science. The research was funded by the National
Science Foundation (NSF), the Department of Energy (DOE) and BASF Corporation.
Timeline: 2-5 years
Funding: Corporate, Government
TRN Categories: Energy; Materials Science and Engineering
Story Type: News
Related Elements: Technical paper, "Hydrogen Storage in
Microporous Metal-Organic Frameworks," Science, May 16, 2003.
May 21/28, 2003
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