Experimental studies on thermophysical properties of protic ionic liquids for thermal energy storage systems

Abstract

Energy storage chemicals play an important role in the design of thermal energy storage systems due to their thermal and chemical properties. In this regard, ionic liquids can be used as a potential for thermal energy storage owing to their remarkable thermophysical properties. At present, little research has been done in this field. In this project, protic ionic liquids 2-hydroxyethylammonium lactate [HEA]La, bis(2-hydroxyethylammonium) lactate [BHEA]La and tris(2-hydroxyethylammonium) lactate [THEA]La were synthesized. The synthesized ionic liquids have low toxicity, high thermal stability, easy and economical synthesis with suitable physical and chemical properties for thermal energy storage systems. In the following, thermophysical measurements such as thermal conductivity, heat capacity, surface tension, density, speed of sound, and electrochemical potential windows for the pure ionic liquids at different temperatures were performed to evaluate the efficiency of these systems as thermal energy storage. The differential scanning calorimetry analysis shows that the ionic liquid 2-hydroxyethylammonium lactate has a higher heat capacity at 1.800 J·g−1·K−1 at T = 298.15 K than the other two ionic liquids due to its small cation and structure. Heat capacity also increases with increasing temperature. The electrochemical potential window values for the ILs [HEA]La, [BHEA]La and [THEA]La were obtained at 2.5 V and 2.2 V, and 1.7 V, respectively. Efficient thermal energy storage systems have a sufficiently high heat capacity and thermal energy density with beneficial thermal conductivity. The ionic liquid 2-hydroxyethylammonium lactate with a maximum thermal conductivity of 0.255 W·m−1·K−1 compared to the other two ionic liquids is recommended as an appropriate candidate for thermal energy storage.