If we are to excel at running a planet with renewable energy, we must excel at finding ways to effectively store that energy until it is needed – and scientists have identified a specific object.
The material is called a metal-organic structure (MOF), in which carbon-based molecules form structures by binding metal ions. Essentially, MOFs are microscopic, so they can form composite materials with other small molecules.
This is what the team did here, adding light-absorbing compound azobenzene molecules. The finished composite material could be stored in UV light again for up to four months at room temperature before re-releasing it – a major improvement over the days or weeks that most light-responsive materials can manage.
“Materials act like modifiers that are used to deliver heat in hand heating,” says John Griffin, a chemist at the University of Lancaster in the UK.
“However, although you have to heat the hand warmers to recharge them, the good thing about this material is that it captures free energy directly from the sun.”
Azobenzene acts as a photosensitive – a molecular machine that responds to external stimuli such as light or heat. Under ultraviolet light, the molecules change shape and effectively store energy while in the MOF aperture structure.
The application of heat to the composite MOF material triggers a rapid release of energy, which gives off heat that can be used to heat other materials or devices.
While the material still needs some work to make it commercially viable, it could eventually be used for ice car windscreens, or provide additional heat to homes and offices, or be used as a heating source for off-grid locations. Photoshows like this also have applications in data storage and drug distribution.
“It has no moving or electronic components, so there is no loss in storing and releasing solar energy,” Griffin says. “We hope to be able to create other products that will save even more energy with further development.”
Although past research has also observed the storage of solar energy in photoswitches, they should generally be kept in liquids. Switching to a MOF composite solid is easy for the system to control and has high chemical stability.
Right now, more work is needed to get this MOF material ready for widespread use. Although experiments have shown that it can hold energy for several months at a time, the energy density of the material is relatively low, an area that researchers expect to improve.
The good news is that there is a lot to be said about the system used in this research, which seeks to make changes and improve results – leading to another cost-effective and reliable way of storing the energy we can trust.
“Our approach is that there are many ways to try to improve these products, either by changing the photoshoot switch or by changing the micro-host structure,” says Nathan Halkovitch, an X-ray technician at the University of Lancaster.
Research has been published Chemistry of materials.