Optically-Controlled Variable-Temperature Storage and Upgrade of Thermal Energy by Photoswitchable Phase Change Materials

Phase change materials (PCMs) show great promise for thermal energy storage and thermal management. However, some critical challenges remain due to the difficulty in tuning solid–liquid phase transition behaviors of PCMs. Here we present optically-controlled tunability of solid–liquid transitions in photoswitchable PCMs (ps-PCMs) synthesized by decorating the molecular structure of long-chain alkanes with a photo-induced isomerization unit. The dual and reversible solid–liquid phase transitions have remarkable differences in melting/crystallization points, which can be optically switched. This unique characteristic of ps-PCMs enables unconventional thermal energy storage, including variable-temperature thermal storage and optically-controlled thermal upgrade. The experimental results show that the synthesized PCMs can adapt to the variable thermal charging temperatures by photo-induced switching of the phase transition behaviors and can also store heat at below 75 °C and release it at above 100 °C. Besides, the ps-PCMs possess a high gravimetric energy storage density of up to 0.36 MJ kg–1 due to the sum of phase-change enthalpy and molecular isomerization enthalpy. We believe that this work provides a new avenue for versatile PCM-based thermal energy storage and thermal management.