Researchers from Lancaster University studied a crystalline material and found that it could capture energy from the sun and store it for months at room temperature. The energy could be released on demand as heat.
The new material is based on a type of metal-organic framework (MOF). MOFs are made of a network of metal ions linked by carbon-based molecules to form 3D structures. A key property of MOFs is that they are porous. This means they can form small composite materials by hosting other small molecules within the structures. The MOF composite used in the study, called DMOF1, was previously prepared by a separate research group at Kyoto University in Japan. The Lancaster team explored the material's energy storage potential, something that had yet to be examined.
The team's MOF pores were loaded with molecules of azobenzene, a compound that strongly absorbs light. The molecules act as photoswitches, a type of molecular machine. Photoswitches can change shape when an external stimulus is applied.
During testing, the team exposed the material to UV light, which caused the azobenzene molecules to change shape into a strained configuration in the MOF pores. The process stores energy in the same way as the potential energy of a bent spring. The narrow MOF pores trap azobenzene molecules in a strained shape. The stored energy is released again when external heat is applied as a trigger in the switch state. The release is quick, like a spring snapping straight. The heat boost could be used to warm materials in other devices.
The MOF composite is solid, doesn’t require liquid fuel, is chemically stable and easily contained, which makes it easier to develop coatings or standalone devices. There are no moving parts in the new material, so there are no losses involved in the storage and release of solar energy.
Further testing proved that the material could store energy for at least four months. Other light-responsive materials are only able to store energy within a few hours or a few days. This long duration of stored energy opens new doors for cross-seasonal storage.
With more development, the new material could offer a way to capture energy in the summer and store it for use in the winter. There are many ways that this material could be used in off-grid systems and remote locations. It could even be used as an environmentally friendly way to supplement conventional heating in homes or commercial offices. If produced as a thin coating, the material could be applied to the surface of buildings or windscreens of cars to store heat and de-ice glass in the winter.
There is even potential for the material in drug delivery. Drugs could be loaded into the material with photoswitches and released on-demand in the body with light or a heat trigger. The next step is to research other MOF structures as well as alternative types of crystalline materials with greater energy storage potential.
This research was published in Chemistry of Materials.
