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."
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