Compositional Uniformity and Thermal Properties of Tetra- N -butyl Ammonium Bromide and Tetra- N -butyl Ammonium Chloride Mixed Semiclathrate Hydrates

Abstract

Mixed semiclathrate hydrates (SCHs) incorporating various guest molecules exhibit high latent heat and allow precise control of the melting temperatures, making them promising materials for cold energy storage applications. The compositional uniformity and thermal properties of tetra-n-butylammonium bromide (TBAB) + tetra-n-butylammonium chloride (TBAC) mixed SCHs were evaluated via dual-energy X-ray computer tomography (DECT), microdifferential scanning calorimetry (μDSC), digital microscopy, and ion chromatography. DECT indicated that the tetragonal mixed SCHs are homogeneous solids, whereas the orthorhombic mixed SCHs with high or low-Br content are heterogeneous. The DSC curve of the tetragonal mixed SCHs exhibited a single endothermic peak, whereas those of the orthorhombic mixed SCHs showed split peaks. The orthorhombic mixed SCH dissociated in two stages, consistent with the split DSC peaks. The Br– content of the orthorhombic mixed SCH was higher than that of tetragonal SCH. The orthorhombic mixed SCHs exhibit high latent heats (227–231 J g–1) as phase change materials, with melting temperatures around 283–285 K. These values are higher than other SCHs formed from a single guest molecule with melting temperatures below 289 K, making them suitable for cold energy storage in air-conditioning systems.