“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”Nathan Halcovitch
A team from Lancaster University have demonstrated a material that can conserve solar energy for long periods at room temperature before releasing it on demand as heat. With the transition from fossil fuels to renewable energy becoming an ever-greater priority, this approach for capturing and storing energy during the summer months for use in winter will help combat climate change, as well as being used for heating systems in off-grid systems or at remote locations, or to complement traditional heating approaches.
As reported in Chemistry of Materials [Griffiths et al. Chem. Mater. (2020) DOI: 10.1021/acs.chemmater.0c02708], the crystalline material, which is based on a type of metal-organic framework (MOF), consists of a network of metal ions linked by carbon-based molecules to form 3D structures. To identify whether a MOF composite could be employed to store energy, the MOF pores were loaded with molecules of azobenzene, a compound that is very good at absorbing light. The molecules act as photoswitches, a type of 'molecular machine' that can change shape when an external stimulus, such as light or heat, is applied.
Exposing the material to UV light resulted in the molecules altering shape to a strained configuration inside the narrow MOF pores, a process that stores the energy at room temperature. This energy is quickly re-released on external heat being applied as a trigger to change its state. As well as storing energy, the material could act as a thin coating on the surface of buildings or car windscreens where the stored heat could de-ice the glass in winter.
As the material has no moving or electronic parts, there are no losses from the storage and release of the solar energy, so there is potential to develop other porous materials to identify if they offer effective energy storage properties using this approach of confined photoswitches. As joint investigator Nathan Halcovitch said, “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”.
Other possible applications for crystalline materials that contain photoswitch molecules include for data storage, as the specific arrangement of photoswitches in the crystal structure means that they could be individually switched based on a precise light source and therefore store data at a molecular level. The material could also one day be used for drug delivery, with medicines trapped inside a material using photoswitches before being released inside the body on demand by means of a light or heat trigger.