Azolium salts as potential phase change materials for thermal energy storage.
Saliha SAHER1, Samantha L. PIPER1, Mega KAR2, Douglas R. MACFARLANE1, Jenny PRINGLE2, Karolina MATUSZEK1
1Monash University, Melbourne, Australia
2Institute for Frontier Materials, Deakin University, Melbourne, Australia
Renewable energy sources remain largely underutilised due to their intermittent nature. One potential solution to that issue is to store renewable thermal energy in phase change materials (PCMs), that absorb and release energy in a reversible phase transition. PCMs, particularly those melting in the intermediate temperature region 100 – 200 °C, offer the opportunity to collect and store thermal energy from solar collectors and are also compatible with organic Rankine cycle engines for converting heat into electricity.[Matuszek, K., et al. Chemical Reviews (2022) doi.org/10.1021/acs.chemrev.2c00407] While traditional intermediate PCMs suffer from various drawbacks like thermal instability, high supercooling, low cycling ability, or phase separation, ionic liquid PCMs are anticipated to have thermal properties that may mitigate these problems. It is estimated that there are 106 possible combinations of cations and anions that will yield ionic liquids. However, a clear understanding of the molecular origins of high melting enthalpy (ΔHf), which indicates high energy storage capacity, is required to make a smart selection of ionic liquid PCMs.[Piper, Samantha L., et al. Green Chemistry 24.1 (2022): 102-117 ] In this work, we investigated the thermal properties including melting point (Tm), melting enthalpy (ΔHf), thermal stability and cycling ability of imidazolium, and 1,2,4-triazolium salts with various anions. These salts have suitable melting points (Tm = 93 °C to 195 °C), and promising melting enthalpies (ΔHf = 8 to 28 kJ. mol-1). Triazolium benzenesulfonate (Tm = 142 °C, ΔHf = 28 kJ.mol-1) has the highest energy storage capacity among the tested salts and is also stable upon cycling. Single crystal X-ray crystallographic structures and Hirshfeld surface analysis showed that a mix of traditional and bifurcated interionic H-bonds are found in imidazolium and triazolium salts (Figure 1). Even though varying crystal packing efficiency, and unexpected solid-solid transitions in some salts influence the ΔHf and Tm, H-bonds play a significant role in driving thermal behavior PCMs.