With the move away from fossil fuels and the switch to renewable energies to combat climate change, the need for new ways to collect and store energy is becoming increasingly important.
Lancaster University researchers studying a crystalline material found that it has properties that allow it to trap energy from the sun. The energy can be stored for several months at room temperature and released in the form of heat when required.
As these types of materials evolve, they could offer exciting potential for capturing solar energy in the summer months and storing it for use in winter – where less solar energy is available.
This would be invaluable for applications such as heating systems in off-grid systems or in remote locations, or as an environmentally friendly addition to conventional heating in homes and offices. It could also potentially be made as a thin coating and applied to the surface of buildings or used on car windshields where the stored heat could be used to de-ice the glass on freezing winter mornings.
The material is based on a kind of “organometallic framework” (MOF). These consist of a network of metal ions that are connected to form 3D structures by carbon-based molecules. A key property of MOFs is that they are porous, which means that they can form composite materials by incorporating other small molecules into their structures.
The Lancaster research team wanted to see if a composite MOF, previously made by a separate research team at Kyoto University in Japan, known as “DMOF1”, could be used to store energy – something that has not been explored before .
The MOF pores were loaded with azobenzene molecules – a compound that strongly absorbs light. These molecules act as photoswitches, a type of “molecular machine” that can change shape when an external stimulus such as light or heat is applied.
In tests, the researchers exposed the material to UV light, which caused the azobenzene molecules within the MOF pores to change their shape into a taut configuration. This process stores the energy in a similar way to the potential energy of a bent spring. It is important that the narrow MOF pores capture the azobenzene molecules in their strained form, which means that the potential energy can be stored for long periods of time at room temperature.
The energy is released again when external heat is used as a trigger to “switch” the state. This release can be very quick – a bit like a spring just snapping back. This provides a boost of heat that can be used to heat other device materials.
Further tests showed that the material could store the energy for at least four months. This is an exciting aspect of the discovery, as many photosensitive materials switch back within hours or days. The long duration of the stored energy opens up possibilities for seasonal storage.
The concept of storing solar energy in photoswitches has been explored, but most previous examples required that the photoswitches be in a liquid. Because the MOF composite is a solid, not a liquid, fuel, it is chemically stable and easy to contain. This makes it much easier to develop into coatings or stand-alone devices.
Dr. John Griffin, Lecturer in Materials Chemistry at Lancaster University and joint principal investigator on the study, said, “The material works a bit like phase change materials that are used to provide heat in hand warmers. Hand warmers must, however.” To be heated to recharge them. The nice thing about this material is that it intercepts “free” energy directly from the sun. It also contains no moving or electronic parts and there are no losses in the storage and release of sunlight energy. We hope that with further development we can make other materials that store even more energy. “
These proof-of-concept results open new research opportunities to find out what other porous materials could have good energy storage properties using the concept of limited photoswitches.
The joint investigator Dr. Nathan Halcovitch added, “Our approach means that there are a number of ways to optimize these materials, either by changing the photoswitch itself or by changing the porous host framework.”
Other possible applications for crystalline materials containing photoswitch molecules include data storage – the well-defined arrangement of photoswitches in the crystal structure means that in principle they can be switched individually with a precise light source and therefore save data like on a CD or DVD, however at the molecular level. They also have the potential for drug delivery – drugs can be trapped in a material using photo switches and released into the body when needed using a light or heat trigger.
Although the results were promising for the ability of this material to store energy over long periods of time, its energy density was modest. The next steps are to explore other MOF structures, as well as alternative types of crystalline materials with greater energy storage potential.
The research, supported by the Leverhulme Trust, is described in the article “Long-Term Storage of Solar Energy Under Ambient Conditions in a MOF-Based Solid-Solid Phase Change Material” published by the journal Chemistry of Materials.
The researchers are John Griffin, Kieran Griffiths, and Nathan Halcovitch, all from the Department of Chemistry at Lancaster University.