Investigation of the intermolecular origins of high and low heats of fusion in azolium salt phase change materials for thermal energy storage

Abstract

Renewable energy sources remain largely underutilised due to their intermittent nature. One potential solution is to store renewable thermal energy in phase change materials (PCMs), which absorb and release energy in a reversible phase transition. A clear understanding of the molecular origins of heat of fusion (ΔH_f) is required to design efficient PCMs. In the present work, the thermal energy storage potential of imidazolium and 1,2,4-triazolium organic salts is investigated and compared with analogous pyrazolium salts. The studied azolium salts have melting points in the intermediate temperature range, which is ideal for renewable energy storage. To probe the origins of the heat of fusion, comparative single crystal X-ray diffraction and Hirshfeld surface analysis with pyrazolium salts showed that a mix of canonical and bifurcated H-bonds exist in the solid state structures of the azolium salts. The H-bonds play a crucial role in driving high ΔH_f, as shown by 1,2,4-triazolium benzenesulfonate [Tri][C₆H₅SO₃] and 1,2,4-triazolium trifluoromethansulfonate [Tri][CF₃SO₃] which have strong canonical H-bonds and high ΔH_f (28 kJ mol⁻¹). Conversely, all chloride salts (imidazolium chloride, pyrazolium chloride and 1,2,4-triazolium chloride) display the lowest values of ΔH_f due to very weak H-bonding. Interestingly, strong H-bonding in the bisulfate salts has not resulted in high ΔH_f. This is probably due to relatively little disruption of the H-bonds during melting and highlights the need for studies of the molten states to determine the whole picture. Additional factors including efficient crystal packing, solid–solid phase transitions, and repulsive interactions between anions also significantly influence ΔH_f. This study thereby shows that azolium salts can be used to design intermediate temperature PCMs for renewable thermal energy storage.