Image: Grossman/Kolpak |
A
novel application of carbon nanotubes, developed by Massachusetts Institute of
Technology (MIT) researchers, shows promise as an innovative approach to
storing solar energy for use whenever it’s needed.
Storing
the sun’s heat in chemical form—rather than converting it to electricity or storing
the heat itself in a heavily insulated container—has significant advantages,
since in principle the chemical material can be stored for long periods of time
without losing any of its stored energy. The problem with that approach has
been that until now the chemicals needed to perform this conversion and storage
either degraded within a few cycles, or included the element ruthenium, which
is rare and expensive.
Last
year (2010), MIT associate professor Jeffrey Grossman and four coauthors
figured out exactly how fulvalene diruthenium—known to scientists as the best
chemical for reversibly storing solar energy, since it did not degrade—was able
to accomplish this feat. Grossman says at the time that better understanding
this process could make it easier to search for other compounds, made of
abundant and inexpensive materials, which could be used in the same way.
Now,
he and postdoc Alexie Kolpak have succeeded in doing just that. A paper describing
their new findings has been published online in Nano Letters.
The
new material found by Grossman and Kolpak is made using carbon nanotubes in
combination with a compound called azobenzene. The resulting molecules,
produced using nanoscale templates to shape and constrain their physical
structure, gain “new properties that aren’t available” in the separate
materials, says Grossman, the Carl Richard Soderberg Associate Professor of
Power Engineering.
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Not
only is this new chemical system less expensive than the earlier
ruthenium-containing compound, but it also is vastly more efficient at storing
energy in a given amount of space—about 10,000 times higher in volumetric
energy density, Kolpak says—making its energy density comparable to lithium-ion
batteries. By using nanofabrication methods, “you can control [the molecules’]
interactions, increasing the amount of energy they can store and the length of
time for which they can store it—and most importantly, you can control both
independently,” she says.
Thermo-chemical
storage of solar energy uses a molecule whose structure changes when exposed to
sunlight, and can remain stable in that form indefinitely. Then, when nudged by
a stimulus—a catalyst, a small temperature change, a flash of light—it can
quickly snap back to its other form, releasing its stored energy in a burst of
heat. Grossman describes it as creating a rechargeable heat battery with a long
shelf life, like a conventional battery.
One
of the great advantages of the new approach to harnessing solar energy,
Grossman says, is that it simplifies the process by combining energy harvesting
and storage into a single step. “You’ve got a material that both converts and
stores energy,” he says. “It’s robust, it doesn’t degrade, and it’s cheap.” One
limitation, however, is that while this process is useful for heating
applications, to produce electricity would require another conversion step,
using thermoelectric devices or producing steam to run a generator.
While
the new work shows the energy-storage capability of a specific type of
molecule—azobenzene-functionalized carbon nanotubes—Grossman says the way the
material was designed involves “a general concept that can be applied to many new
materials.” Many of these have already been synthesized by other researchers
for different applications, and would simply need to have their properties
fine-tuned for solar thermal storage.
The
key to controlling solar thermal storage is an energy barrier separating the
two stable states the molecule can adopt; the detailed understanding of that
barrier was central to Grossman’s earlier research on fulvalene dirunthenium,
accounting for its long-term stability. Too low a barrier, and the molecule
would return too easily to its “uncharged” state, failing to store energy for
long periods; if the barrier were too high, it would not be able to easily
release its energy when needed. “The barrier has to be optimized,” Grossman
says.
Already,
the team is “very actively looking at a range of new materials,” he says. While
they have already identified the one very promising material described in this
paper, he says, “I see this as the tip of the iceberg. We’re pretty jazzed up
about it.”
