Challenges and strategies for imidazolium ionic liquids as novel phase change materials for low and medium temperature thermal energy storage: A critical review

This review aims to provide an insight into the imidazolium ionic liquids (ILs) as novel phase change materials (PCMs) for low and medium temperature thermal energy storage, with a focus on their multi-dimensional thermophysical/nucleation features within encapsulation for defect regulation during solid-liquid transition. Imidazolium ILs have been emerging as novel phase transition-based energy storage material due to unique properties of wide liquidous range, rich crystallization behavior, small thermal volumetric expansion and environmental-friendly properties, but suffer from defects of low enthalpy and large supercooling. To meet the challenge acting as PCMs, the work first gave a brief overview on supercooling regulation and enthalpy elevation of imidazolium ILs, and proposed micro-encapsulation with crystallization-promoting porous shell to regulate their defects. Then discussion regarding multi-dimensional thermophysical features of imidazolium ILs were given, including features within micro-confined capsule core (phase transition, melting point, thermal stability, specific heat capacity and thermal conductivity), nano-confined nucleation within the mesoporous shell, and mesoporous interfacial nucleation. Finally, the future applications of imidazolium ILs and microencapsulation in the fields of infrared stealthy, solar thermal utilization, thermal management in extreme environment and green energy-saving building were highlighted. The study provides a timely review of the imidazolium ILs acting as thermal energy storage (TES) materials, and future suggestion like functional ILs with more hydrogen bonds or supercooling utilization for seasonal TES may help concentrate efforts on solving the key issues in urgent need.