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A graphene-oxide framework Credit: NIST |
Graphene—carbon formed into
sheets a single atom thick—now appears to be a promising base material
for capturing hydrogen, according to recent research* at the National
Institute of Standards and Technology (NIST) and the University of
Pennsylvania. The findings suggest stacks of graphene layers could
potentially store hydrogen safely for use in fuel cells and other
applications.
Graphene has become something of a celebrity material
in recent years due to its conductive, thermal and optical properties,
which could make it useful in a range of sensors and semiconductor
devices. The material does not store hydrogen well in its original form,
according to a team of scientists studying it at the NIST Center for
Neutron Research. But if oxidized graphene sheets are stacked atop one
another like the decks of a multilevel parking lot, connected by
molecules that both link the layers to one another and maintain space
between them, the resulting graphene-oxide framework (GOF) can
accumulate hydrogen in greater quantities.
Inspired to create GOFs by the metal-organic
frameworks that are also under scrutiny for hydrogen storage, the team
is just beginning to uncover the new structures’ properties. “No one
else has ever made GOFs, to the best of our knowledge,” says NIST
theorist Taner Yildirim. “What we have found so far, though, indicates
GOFs can hold at least a hundred times more hydrogen molecules than
ordinary graphene oxide does. The easy synthesis, low cost and
non-toxicity of graphene make this material a promising candidate for
gas storage applications.”
The GOFs can retain 1 percent of their weight in
hydrogen at a temperature of 77 degrees Kelvin and ordinary atmospheric
pressure—roughly comparable to the 1.2 percent that some well-studied
metal-organic frameworks can hold, Yildirim says.
Another of the team’s potentially useful discoveries
is the unusual relationship that GOFs exhibit between temperature and
hydrogen absorption. In most storage materials, the lower the
temperature, the more hydrogen uptake normally occurs. However, the team
discovered that GOFs behave quite differently. Although a GOF can
absorb hydrogen, it does not take in significant amounts at below 50
Kelvin (-223 degrees Celsius). Moreover, it does not release any
hydrogen below this “blocking temperature”—suggesting that, with further
research, GOFs might be used both to store hydrogen and to release it
when it is needed, a fundamental requirement in fuel cell applications.
Some of the GOFs’ capabilities are due to the linking
molecules themselves. The molecules the team used are all
benzene-boronic acids that interact strongly with hydrogen in their own
right. But by keeping several angstroms of space between the graphene
layers—akin to the way pillars hold up a ceiling—they also increase the
available surface area of each layer, giving it more spots for the
hydrogen to latch on.
According to the team, GOFs will likely perform even
better once the team explores their parameters in more detail. “We are
going to try to optimize the performance of the GOFs and explore other
linking molecules as well,” says Jacob Burress, also of NIST. “We want
to explore the unusual temperature dependence of absorption kinetics,
as well as whether they might be useful for capturing greenhouse gases
such as carbon dioxide and toxins like ammonia.”
The research is funded in part by the Department of
Energy.
* J. Burress, J. Simmons, J. Ford and T.Yildirim. "Gas
adsorption properties of graphene-oxide-frameworks and nanoporous
benzene-boronic acid polymers." To be presented at the March meeting of
the American Physical Society (APS) in Portland, Ore., March 18, 2010.
An abstract is available at http://meetings.aps.org/Meeting/MAR10/Event/122133
Media Contact: Chad Boutin, boutin@nist.gov, (301) 975-4261
