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

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 (Delta Hf) 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 Delta Hf, 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 Delta Hf (28 kJ mol-1). Conversely, all chloride salts (imidazolium chloride, pyrazolium chloride and 1,2,4-triazolium chloride) display the lowest values of Delta Hf due to very weak H-bonding. Interestingly, strong H-bonding in the bisulfate salts has not resulted in high Delta Hf. 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 Delta Hf. This study thereby shows that azolium salts can be used to design intermediate temperature PCMs for renewable thermal energy storage. Azolium salts are potential intermediate-temperature phase change materials. Crystallographic analysis shows that H-bonding along with other intermolecular interactions strongly influence the melting enthalpy of azolium salts.