A significant portion of energy input in industrial processes, about 20–50%, is lost as waste heat, and about 63% of that wasted energy is released at temperatures below 100 oC. This massive amount of low-grade (&lt;100 oC) waste heat is not currently recovered by economically viable means. The state-of-the-art materials that enable this heat storage are known as phase change materials (PCMs) that undergo ordered-disordered phase transition to reversibly store and release latent heat. The conventional PCMs such as organic wax and salts, however, have an intrinsic limitation, i.e. the lack of active control over heat absorption/release temperatures and heat storage time. We have discovered novel functional organic material systems that exhibit solid-liquid phase transitions controlled by optical stimulation. The functional PCMs are designed to have an aliphatic group and a photoswitch unit such as an azobenzene which isomerizes between planar E and bent Z configurations upon light irradiations (panel a). The ground-state E isomers initially form a crystalline solid phase, absorb heat to melt, then are UV-activated to switch to Z isomers that are sterically bulkier and more polar than E counterpart. The Z isomers, therefore, form a liquid phase that is preserved over a wide range of temperatures, storing both latent heat and the isomerization energy. The stored energy is later released by the visible-light irradiation that generates E isomers and crystallizes them (panel b). This novel design of PCMs enables the storage of thermal energy for a few weeks without any crystallization under various temperature conditions, which addresses the major challenge of the conventional PCMs. In this talk, various novel molecular designs that are recently discovered to exhibit the optically-controlled phase transitions will be presented, and their thermal energy storage properties will be discussed.

Optically-controlled thermal energy storage in photoswitchable phase change materials

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ACS Fall 2022

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