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.
A graphene-oxide framework (GOF), formed of layers of graphene connected by boron-carboxylic “pillars.” GOFs such as this one are just beginning to be explored as a potential storage medium for hydrogen and other gases. Credit: NIST
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
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,
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