Water sorption properties of functionalized MIL-101(Cr)-X (X = -NH2, -SO3H, H, -CH3, -F) based composites as thermochemical heat storage materials

Thermochemical heat storage based on inorganic salt hydrates is a promising approach to relieve the heat energy supply and demand conflicts, but there exist lots of defects of pure salts when applied to the thermochemical. This paper attempted to use functionalized Metal-Organic Frameworks (MOFs) as the porous matrices to improve the heat storage performance of inorganic salt hydrates. A series of MOFs/CaCl2 composites based on MIL-101(Cr)-X (X = -NH2, -SO3H, H, -CH3, -F) were successfully synthesized and fully characterized by combining multiple techniques (i.e. powder X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray analysis, N-2 adsorption-desorption, Fourier transform infrared spectroscopy, thermogravimetric and differential scanning calorimetry). The water sorption properties of the series of MOFs/CaCl2 composites were studied during the hydration process, and a continuous adsorption/desorption experiment was also performed to investigate the cycle stability of the MOFs/CaCl2 composites. The results show that the hydrophilic MOFs based composites have higher water sorption capacity (0.6 g(H2O)/g(sample) for MIL-101(Cr)-SO3H/CaCl2) accompanied with higher heat storage density (1274 J/g for MIL-101(Cr)-SO3H/CaCl2). Moreover, the series of MOFs all presented excellent cycle performance with little loss after continuous 17 adsorption/desorption cycles.