Binary Phase Behavior of Thermally Robust Ionic Liquids and Their Mixtures for Use in Solar Thermal Energy Production & Storage

The need to pivot away from fossil-fuel based energy production is more pressing than ever due to limited fossil-fuels and climate change. Concentrated solar power (CST) with thermal energy storage provides a potential opportunity to support this pivot but suffers from high costs and extended downtimes. Current heat transfer fluids often consist of mixture of alkali metal nitrate with melting points above 200°C. For CST and thermal storage to be viable, a thermally stable, low melting fluid is needed. Ionic liquids (ILs) provide a very feasible form of thermal energy storage due to their high heat capacities and low melting points. Even though ILs have favorable properties for a thermal storage fluid, most of the common cation and anion pairs only survive for short periods of time at elevated temperatures. Thermally robust ILs can be synthesized by restricting functionality on the cation to peraryl groups and coupling these cations with a thermally stable anion. However, these synthetic restrictions result in salts with melting points at or above 100°C. Mixtures of these salts, with each other or with structurally similar molecular species can yield fluids with melting temperatures low enough to be effective thermal storage fluids. In this project, the phase equilibrium of a binary mixture of tetraphenyl phosphonium bistriflimide (TPP) and benzanilide was examined to provide valuable information on eutectic behavior for organic ionic liquids with aromatic compounds. The use of digital scanning calorimetry (DSC) (Solid/Liquid Equilibrium, SLE) along with cloud-point analysis (Liquid/Liquid Equilibrium, LLE) was used to study this behavior. The phase transitions, including eutectic behavior, are expressed on a binary T-x diagram and show how the phase behavior varies with temperature and composition. Solid/liquid equilibrium behavior is compared to the ideal solution model to aid in understanding molecular-level interactions in the mixture. The study showed a minimum eutectic melting temperature of below 100 °C and liquid-liquid interactions in compositions having high amounts of TPP compared to benzanilide. The findings of this study can be used to further develop peraryl ILs and their mixtures as viable thermal energy storage fluids that could dramatically reduce the cost and difficulty of operating CST at scale.