Morphological Observation of LiCl Deliquescence in PDMS-Based Composite Foams

The LiCl-based heat storage system exhibits a high-energy density, making it an attractive and one of the most investigated candidates for low-temperature heat storage applications. Nevertheless, lithium chloride, due to its hygroscopic nature, incurs the phenomenon of deliquescence, which causes some operational challenges, such as agglomeration, corrosion, and swelling problems during hydration/dehydration cycles. Here, we propose a composite material based on silicone vapor-permeable foam filled with the salt hydrate, hereafter named LiCl-PDMS, aiming at confining the salt in a matrix to prevent deliquescence-related issues but without inhibiting the vapour flow. In particular, the structural and morphological modification during hydration/dehydration cycles is investigated on the composite foam, which is prepared with a salt content of 40 wt.%. A characterization protocol coupling temperature scanned X-ray diffraction (XRD) and environmental scanning electron microscopy (ESEM) analysis is established. The operando conditions of the dehydration/hydration cycle were reproduced while structural and morphological characterizations were performed, allowing for the evaluation of the interaction between the salt and the water vapor environment in the confined silicon matrix. The material energy density was also measured with a customized coupled thermogravimetric/differential scanning calorimetric analysis (TG/DSC). The results show an effective embedding of the material, which limits the salt solution release when overhydrated. Additionally, the flexibility of the matrix allows for the volume shrinkage/expansion of the salt caused by the cyclic dehydration/hydration reactions without any damages to the foam structure. The LiCl-PDMS foam has an energy density of 1854 kJ/kg or 323 kWh/m, thus making it a competitive candidate among other LiCl salt hydrate composites.