Materials that soak up and release heat as they melt and solidify are used as heating pads in high-performance clothing and to protect food or electronics from temperature changes. These phase change materials, typically waxes or fatty acids, cannot stay liquid in freezing temperatures nor release their stored heat on demand. Now researchers have made a phase-change material that has a switch: it can store heat below 0 °C and discharge it when triggered by light (J. Am. Chem. Soc. 2020, DOI: 10.1021/jacs.0c00374).
The ability to store heat at subzero temperatures makes the new material valuable for frigid environments, where it could quickly defrost water boilers, engine oils, and machine parts when needed. “The goal is to heat up frozen water or oil to temperatures that are usable in that extreme cold,” says Grace G. D. Han, a chemist at Brandeis University who led the work.
Han, Matthew J. Fuchter of Imperial College London, and their colleagues started with a family of photoswitchable arylazopyrazole compounds that can store heat. At room temperature, the E isomers of these molecules undergo a phase transition between solid and liquid, while the Z isomers form a more stable liquid. Researchers have shown that arylazopyrazoles can store heat for a few days in the liquid Z state at room temperature and then be optically triggered to change back to the E state, which causes the compounds to crystallize into a solid and thus release their stored heat. The problem is that below room temperature, the Z isomers start to crystallize and can’t reliably store heat as a liquid.
So Han and her colleagues sought to change the temperature range of this transition. They added a dodecanoate group to four different compounds’ pyrazole rings. The E isomers of the new materials melt like wax between 60 and 90 °C, storing the latent heat of melting. This heat storage can then be locked in place by shining ultraviolet light on them to transform them into the Z isomers, which can remain stable liquids for 2 weeks at temperatures as low as –30 °C.
To unlock the stored heat, researchers shine visible light on the materials, triggering the arylazopyrazoles to flip back to their original E isomers, which readily solidify, releasing heat. The materials can withstand thousands of phase-changing cycles, Han says.
The team has demonstrated the concept with a few hundred milligrams of the material. Real-world applications would require hundreds of grams or more to store enough heat. That will bring some challenges, Han says, because UV light cannot penetrate deep into the material. Also, the crystallization can sometimes be incomplete and leave heat unreleased. But, she says, the challenges can be addressed by changing the chemical structure of the molecules to optimize the absorption spectrum and improve crystallization.
Mogens Brøndsted Nielsen, a chemist at the University of Copenhagen, calls this work elegant and says it uses “clever molecular engineering to overcome the important challenge of keeping the liquid below 0 °C.” Heat storage for over 2 weeks is remarkably long, but it would be convenient to extend the storage time even longer, he says.