The biggest challenge with hydrogen-powered fuel
cells lies in the storage of hydrogen: how to store enough of it, in a safe and
cost-effective manner, to power a vehicle for 300 miles? Lawrence Berkeley
National Laboratory (Berkeley Lab) is aiming to solve this problem by
synthesizing novel materials with high hydrogen adsorption capacities.
The U.S. Department of Energy (DOE) recently
awarded Berkeley Lab a three-year, $2.1 million grant for the project, which
will also include contributions by the National Institute of Standards and
Technology (NIST) and General Motors (GM). The grant was part of more than $7
million awarded by DOE last month for hydrogen storage technologies in fuel
cell electric vehicles.
"We're working on materials called metal-organic
frameworks to increase the capacity of hydrogen gas in a pressure cylinder,
which would be the fuel tank," said Jeffrey Long, a Berkeley Lab scientist who
co-leads the project along with Berkeley Lab chemist Martin Head-Gordon. "With
these materials, we're working on storing the hydrogen without the use of very
high pressures, which will be safer and also more efficient without the
significant compression energy losses."
Metal-organic frameworks (MOFs) are 3D sponge-like
framework structures that are composed primarily of carbon atoms and are
extremely lightweight. "What's very special about these materials is that you
can use synthetic chemistry to modify the surfaces within the materials and
make it attractive for hydrogen to stick on the surface," Long explained.
Separately, Long is also using MOFs in a carbon
capture project, in which the material would selectively absorb carbon dioxide
over nitrogen. For the fuel cell project, the trick lies not in getting the MOF
to select hydrogen out of a mixture but to store as much hydrogen as possible.
Currently, vehicles using hydrogen fuel cells can
achieve a range of close to 300 miles—but only if the hydrogen is stored at
extremely high pressures (600 to 700 bar), which is expensive and potentially
unsafe. It is also energy intensive to pressurize the hydrogen.
So far Long has succeeded in more than doubling
hydrogen capacity, but only at very low temperatures (around 77 K, or -321 F). "It's still very much basic research on how to create revolutionary new
materials that would boost the capacity by a factor of four or five at room
temperature," he said. "We have an idea of what kinds of frameworks we might
make to do this."
Long's approach is to create frameworks with
lightweight metal sites on the surface, making it attractive for hydrogen
molecules to bind to the sites. "Our approach has been to make some of the
first metal-organic frameworks that have exposed metal cations on the surface,"
he said. "Now we need to figure out ways of synthesizing the materials so that
instead of one hydrogen molecule we can get two or three or even four hydrogen
molecules per metal site. Nobody's done that."
This is where Head-Gordon, a computational chemist,
comes in. He will work on theoretical understanding of MOFs so that he can try
to predict their hydrogen storage properties and then instruct Long's team as
to what kind of material to synthesize. "He can do calculations on a lot of
different target structures and say, here's the best one for you guys to spend
time trying to make, because synthetic chemistry is very cost and labor
intensive," Long said.
The scientist at GM will aid in providing accurate
high-pressure measurements. The NIST scientist is an expert in neutron
diffraction and neutron spectroscopy, which will allow Long and his team to
pinpoint where exactly the hydrogen is going and verify that it is binding to
the metals.
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