The shift away from fossil fuels toward renewable energy to mitigate climate change has made the requirement for novel techniques to harness and store energy highly crucial.
Researchers from Lancaster University have analyzed a crystalline material and found it exhibits properties that enable it to harness energy from the sun. This energy can be stored at room temperature for many months and can be used on demand in the form of heat.
Further tuning of such materials could enable them to offer exciting potential to harness solar energy during the summer months and store it for use in winter when less solar energy is available.
This would be indispensable for applications like heating systems in remote locations or off-grid systems, or as an eco-friendly alternative to traditional heating in offices and houses. It could even be formed as a thin coating and attached to the surface of buildings, or used on the windscreens of cars where the stored heat could be useful for de-icing of the glass in freezing winter mornings.
A type of "metal-organic framework" (MOF) forms the base of the material. MOFs include a network of metal ions connected by carbon-based molecules to produce 3D structures. A major characteristic of MOFs is that they are porous, that is, they can produce composite materials by enabling the addition of other small molecules within their structures.
The Lancaster researchers intended to find whether a MOF composite, prepared earlier by a separate research team at Kyoto University in Japan and called “DMOF1,” can be employed for energy storage, which has not been studied previously.
Molecules of azobenzene—a compound with strong light absorption property—were loaded into the MOF pores. These molecules serve as photoswitches—a kind of “molecular machine” that can change shape in response to an external stimulus, such as heat or light.
As part of the tests, the material was exposed to UV light, which leads to a shape change of the azobenzene molecules to a strained configuration within the MOF pores. This process results in the storage of energy similar to the potential energy of a bent spring.
Most significantly, the azobenzene molecules are trapped by the narrow MOF pores in their strained shape, which means that it is possible to store the potential energy at room temperature for long periods.
Again, the energy is released upon applying external heat as a trigger to “switch” its state. This release can be very fast—somewhat like a spring snapping back straight. This offers a heat boost that can be used to warm other materials or devices.
Additional investigations revealed the material could store the energy for a minimum of four months. This is a fascinating aspect of the finding as several light-responsive materials switch back in a few hours or a few days. The longer duration of the stored energy paves way for potential cross-seasonal storage.
Although the concept of solar energy storage in photoswitches has been investigated earlier, a majority of the earlier examples have necessitated the photoswitches to be stored in a liquid. The MOF composite is a solid, and not a liquid fuel; thus, it is easily contained and chemically stable. This renders it considerably easier to form into standalone devices or coatings.
The material functions a bit like phase change materials, which are used to supply heat in hand warmers. However, while hand warmers need to be heated in order to recharge them, the nice thing about this material is that it captures ‘free’ energy directly from the sun. It also has no moving or electronic parts and so there are no losses involved in the storage and release of the solar energy.
Dr John Griffin, Senior Lecturer in Materials Chemistry, Lancaster University
“We hope that with further development we will be able to make other materials which store even more energy,” added Dr Griffin, who is the joint Principal Investigator of the study.
These proof-of-principle results pave the way for new avenues of research to analyze other porous materials that might exhibit ideal energy storage properties, with the help of the concept of confined photoswitches.
Our approach means that there are a number of ways to try to optimise these materials either by changing the photoswitch itself, or the porous host framework.
Dr Nathan Halcovitch, Joint Investigator of the Study, Lancaster University
Other promising applications for crystalline materials hosting photoswitch molecules are data storage—the clear-cut arrangement of photoswitches in the crystal structure enables switching them one-by-one with the help of a precise light source and thus store data similar to a CD or DVD, but at a molecular level.
They also exhibit the potential for drug delivery—it would be possible to lock drugs within a material using photoswitches and then released them when needed within the body using a heat or light trigger.
The outcomes for the new material’s potential to store energy for longer periods were promising, but its energy density was modest. For the team, the next stage would be to study other MOF structures and supplementary crystalline materials that exhibit higher energy storage potential.
Griffiths, K., et al. (2020) Long-Term Solar Energy Storage under Ambient Conditions in a MOF-Based Solid–Solid Phase-Change Material. Chemistry of Materials. doi.org/10.1021/acs.chemmater.0c02708.