Molecular patterns in the thermophysical properties of pyridinium ionic liquids as phase change materials for energy storage in the intermediate temperature range

Phase change materials (PCMs) that store and release thermal energy via a reversible phase transition in the intermediate temperature range are a promising solution for renewable energy storage as they can be durable and inexpensive. Towards the development and understanding of new intermediate PCMs, this work describes a family of pyridinium ionic liquids and their thermophysical properties that show the potential of protic ionic liquids in the PCM field. Various pyridine structural isomers were used to explore the molecular patterns that affect the enthalpy of fusion and melting. The results show that small structural variations in the cation can change the thermophysical properties drastically; for example, melting temperatures varied between 357 ± 1 K and 499 ± 1 K, and enthalpies of fusion covered a wide spectrum from 38 to 190 J g⁻¹ ± 5%. The most promising results in terms of PCM application, and one of the best among all protic ionic liquids reported thus far, were obtained for 2-hydroxypyridinium methanesulfonate [2-OHpyH][CH₃SO₃] (T_m = 433 K and ΔH_f = 190 J g⁻¹).