The world’s transition from fossil fuels to renewables to tackle climate change will require energy storage techniques because the sun isn’t always shining, and the wind isn’t always blowing. It’s a hot topic among research teams nowadays – from studying energy-storing materials to developing batteries.
One of the latest discoveries comes from scientists at Lancaster University. While studying a crystalline material commonly used for filtering or desalinating water, they found it could capture solar energy and store it at room temperature for at least four months. When needed, the power could be released on demand in the form of heat. The process works without electronic components.
The team suggests the material could be used as a building coating to store summer energy and release it in the winter as heat or car windshields to de-ice the glass in freezing winter mornings. It would be especially useful for heating systems in remote locations or off-grid systems. It’s also an environmentally-friendly supplement to conventional heating in houses.
The material is based on a kind of ‘metal-organic framework’ (MOF) known as ‘DMOF1,’ previously prepared by a separate research team at Japan’s Kyoto University. MOFs consist of a 3D network of metal ions linked by carbon-based molecules. MOFs are porous, so they can host other small molecules within their structure. In this case, they were loaded with azobenzene molecules – a compound that strongly absorbs light. Azobenzene also acts as photoswitches – a type of ‘molecular machine’ that can shape-shift when an external stimulus is applied, such as light or heat.
When the team exposed the new material to UV light, it caused the azobenzene molecules to change shape. They were stuck in a strained configuration inside the MOF pores, storing the energy like a compressed spring stores potential energy. In their strained condition, the azobenzene molecules were stuck within the MOF because the pores are too narrow for them to escape, meaning the power remains stored for later use. To release the energy, external heat is applied as a trigger that switches the molecules’ state. The release provides a heat boost. The process is swift, like a spring snapping back straight.
Dr. John Griffin, Co-Principal Investigator of the study and Senior Lecturer in Materials Chemistry at Lancaster University, said:
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 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, so there are no losses involved in solar energy storage and release. We hope that with further development we will be able to make other materials which store even more energy.
This study opens up a new category of research focused on the concept of confining photoswitches within porous materials that have good energy storing properties. The researchers could store solar energy for four months but estimate it could hold a charge for up to 54 months. However, the energy density was modest, so now the team wants to research other MOF structures or material alternatives that may hold more significant energy storage potential.
Joint investigator Dr. Nathan Halcovitch added:
Our approach means that there are many ways to try to optimize these materials either by changing the photoswitch itself, or the porous host framework.
Other potential applications for this form of energy storage include data storage and drug delivery. For data storage, the organized arrangement of photoswitches within the crystal structure could theoretically be switched on one at a time using a precise light source. Such a system could store data like on a DVD but at a molecular level. For drug delivery, medicines could be contained within a material using photoswitches then released inside the body on demand using light or heat triggers.
