Conspectus
Renewable energy resources are mostly intermittent and not evenly
distributed geographically; for this reason, the development of new
technologies for energy storage is in high demand.
Molecules
that undergo photoinduced isomerization reactions that
are capable of absorbing light, storing it as chemical energy, and
releasing it as thermal energy on demand are referred to as molecular
solar thermal energy storage (MOST) or solar thermal fuels (STF).
Such molecules offer a promising solution for solar energy storage
applications. Different molecular systems have been investigated for
MOST applications, such as norbornadienes, azobenzenes, stilbenes,
ruthenium derivatives, anthracenes, and dihydroazulenes. The polycyclic
strained molecule norbornadiene (NBD), which photoconverts to quadricyclane
(QC), is of great interest because it has a high energy storage density
and the potential to store energy for a very long time. Unsubstituted
norbornadiene has some limitations in this regard, such as poor solar
spectrum match and low quantum yield. In the past decade, our group
has developed and tested new NBD systems with improved characteristics.
Moreover, we have demonstrated their function in laboratory-scale
test devices for solar energy harnessing, storage, and release.
This Account describes the most impactful recent findings on how
to engineer key properties of the NBD/QC system (photochemistry, energy
storage, heat release, stability, and synthesis) as well as examples
of test devices for solar energy capture and heat release. While it
was known that introducing donor–acceptor groups allows for
a red-shifted absorption that better matches the solar spectrum, we
managed to introduce donor and acceptor groups with very low molecular
weight, which allowed for an unprecedented solar spectrum match combined
with high energy density. Strategic steric hindrance in some of these
systems dramatically increases the storage time of the photoisomer
QC, and dimeric systems have independent energies barriers that lead
to an improved solar spectrum match, prolonged storage times, and
higher energy densities. These discoveries offer a toolbox of possible
chemical modifications that can be used to tune the properties of
NBD/QC systems and make them suitable for the desired applications,
which can be useful for anyone wanting to take on the challenge of
designing efficient MOST systems.
Several test devices have
been built, for example, a hybrid MOST
device that stores sunlight energy and heat water at the same time.
Moreover, we developed a device for monitoring catalyzed QC to NBD
conversion resulting in the possibility to quantify a significant
macroscopic heat generation. Finally, we tested different formulations
of polymeric composites that can absorb light during the day and release
the energy as heat during the night for possible use in future window
coating applications. These lab-scale realizations are formative and
contribute to pushing the field forward towar...
