Synthesis and thermophysical property evaluation of ionic semiclathrate hydrate formed with tetrabutylphosphonium glycine as a chilling temperature phase change material

Ionic semiclathrate hydrate, which is formed under atmospheric pressure with sustainably safe properties, has a potential for solid-liquid phase change materials(PCMs). In this study, the thermophysical property of tetrabutylphosphonium glycine(TBPGly) hydrate containing no halogen guest compound, glycine (aminoacetic acid), was investigated to evaluate the availability as an eco-efficient thermal energy storage medium. The phase equilibrium temperature was obtained through visual observation method. The heat flow rate curve behavior and dissociation heat were measured via differential scanning calorimetry(DSC). In the dissociation process, a single hydrate dissociation peak emerged, indicating TBPGly hydrate has one type of crystallographic structure. The concentration dependency of the TBPGly aqueous solution on the equilibrium temperature and the dissociation heat was confirmed. The highest equilibrium temperature of 5.8 °C was confirmed at the fraction range w_TBPGly =0.368 or x_TBPGly =0.0306, and the largest dissociation heat was 177.7 kJ·kg⁻¹ at the fraction range w_TBPGly =0.377or x_TBPGly =0.0318. The estimated hydration number in the congruent composition was found to be in 31.2 to 32.5. In addition, based on the measured physical property, the optimum operating temperature and volume fraction of TBPGly hydrate was evaluated to determine the concentration for the practical cooling system. These data revealed TBPGly hydrate is available as a solid or slurry PCM under the operating temperature range 2 °C to 5.8 °C. The experimental data as well as the illustrative calculations contribute to accelerate the expansion in the application range, and provide the prospect in selecting the suitable composition toward the actual hydrate-based cooling system.