Thermal properties of novel phase change materials based on protic ionic liquids containing ethanolamines and stearic acid for efficient thermal energy storage

Abstract

In the field of energy harvesting, phase change materials (PCMs) hold great promise. 2-hydroxyethylammonium stearate ([HEA]Ste), bis(2-hydroxyethyl)ammonium stearate ([DHEA]Ste), and tris(2-hydroxyethyl)ammonium stearate ([THEA]Ste) ionic liquids (ILs) demonstrate promising capabilities to enhance thermal energy storage (TES) performance within the 30–100 °C temperature range. This research presents these ILs as PCMs for the first time, emphasizing their environmentally friendly characteristics, safety profile, and cost-effectiveness. The chemical composition and microstructure of these PCMs were investigated using scanning electron microscopy (SEM), and fourier transform infrared spectroscopy (FT-IR), while differential scanning calorimetry (DSC) was employed to assess their latent heat of fusion and specific heat capacity. Furthermore, thermal gravimetric analysis (TGA) was utilized to evaluate the thermal stability of these ILs. In addition, valuable insights into the surface properties and behavior of PCMs at the nanoscale are provided using atomic force microscopy (AFM). Results show that the latent heats of fusion for [HEA]Ste, [DHEA]Ste, and [THEA]Ste are about 171.12, 152.58, and 136.55 kJ kg⁻¹, respectively. Also, thermal stability analysis shows the maximum stability (99.5%) for [HEA]Ste. Ultimately, a custom-built setup featuring a cell containing a PCM derived from synthesized ILs and a commercially available thermoelectric generator (TEG) was employed to measure live voltage (V) in the conversion of heat energy into electrical power. On the other hand, specifically in this measurement we recorded the output voltage (open circuit case) of the TEG device versus time and demonstrated that after turning off the thermal energy source, the proposed system provides the electrical energy for a while (more than 2 hours).