Experimental and simulation study of two-stage water adsorption in salt porous composites for advanced thermochemical heat storage

The 'two-stage adsorption mechanism' within different Composite salt porous matrix (CSPM) materials is studied for the enhancement in water sorption performance and heat storage capacities of thermochemical heat storage materials. Several Metal-Organic Frameworks (MOFs) in combination with SrCl2 were selected as examples to study the mechanism. The simulation and experimental results indicate that within this two-stage adsorption mechanism, SrCl2 salts function as 'moisture pumps,' rapidly capturing water from ambient and increasing humidity levels inside MOF pores, while MOFs act as 'water reservoirs,' efficiently storing water within a short timeframe. Grand Canonical Monte Carlo (GCMC) simulation confirms the two-stage sorption process, demonstrating that salts within the MOFs exhibit a stronger affinity for water molecules and the impregnation of salt in MOF composites results in additional water sorption in MOF pores, therefore enhanced water sorption performance of the composites. Experimental results also reveal that composites exhibit enhanced water sorption as the result of the two-stage adsorption, with SrCl2@MIL-101(Cr) showing the highest sorption enhancement, reaching 0.825 g/g within 120 min. The two-stage adsorption not only enables rapid sorption but also achieves remarkable heat storage enthalpy, where SrCl2@MIL-101(Cr) reaches 1462 kJ/kg with lower dehydration temperatures. Experimental results also demonstrated that the composites have a lower desorption temperature in comparison with their single components, which may be related to the two-stage desorption facilitated by the impregnated salt. The two-stage adsorption mechanism demonstrated in this study offers valuable insights for the design and optimization of porous matrices of composites for advanced thermochemical heat storage applications.