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The material, based on a type of metal-organic framework (MOF), could capture solar energy during the summer months and store it for winter, the team from Lancaster University said.
Stored at room temperature, the material's captured energy could be invaluable for off-grid heating, or a sustainable supplement to conventional heating in houses and offices. The team said it could potentially also be produced as a thin coating and applied to the surface of buildings, or used on car windscreens for de-icing.
MOFs consist of a network of metal ions, linked by carbon-based molecules to form 3D structures. A key property is that they are porous, meaning they can form composite materials by hosting other small molecules within their structures.
The Lancaster team set out to discover if a composite, previously prepared by a separate team at Kyoto University in Japan and known as DMOF1, could be used to store energy – something not previously researched.
The MOF pores were loaded with molecules of azobenzene, a compound that strongly absorbs 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.
The researchers exposed the material to UV light, which caused the azobenzene molecules to change shape to a strained configuration inside the MOF pores. This process stored the energy in a similar way to the potential energy of a bent spring. Importantly, the narrow MOF pores trapped the azobenzene molecules in their strained shape, meaning that the potential energy could be stored for long periods of time at room temperature.
The energy is released again when external heat is applied as a trigger to 'switch' the material’s state. The release can reportedly be very quick, like a spring snapping back.
Further tests showed the material was able to store the energy for at least four months, unlike other light-responsive materials that switch back within hours or a few days.
The concept of storing solar energy in photoswitches has been studied before, but most previous examples have required them to be in a liquid. Because the MOF composite is solid, it is chemically stable and easily contained. This makes it much easier to develop into coatings or standalone devices, the researchers said.
“The material functions a bit like phase change materials, which are used to supply heat in hand warmers,” said materials chemist Dr John Griffin, joint principal investigator of the study.
“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. We hope that with further development we will be able to make other materials which store even more energy.”
Other potential applications include data storage – the well-defined arrangement of photoswitches in the crystal structure means they could in principle be switched one-by-one using a precise light source, storing data like a CD or DVD but on a molecular level.
The researchers said they also have potential for drug delivery. Medicines could be locked inside a material using photoswitches and then released on demand inside the body using a light or heat trigger.
Although the team said the results were promising, the material’s energy density was ‘modest’. They now plan to research other MOF structures, as well as alternative types of crystalline materials with greater energy storage potential.
The work was published in Chemistry of Materials.
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