Open-system thermal characterization of NH2-MIL-125(Ti)/salt composites (CaCl2, SrBr2) for solar-driven low-temperature sorption heat storage

Sorption heat storage allows solar thermal energy to be stored for weeks or months with negligible losses, making it a promising route to decarbonize space heating in buildings, yet most sorbent data are acquired under closed-system conditions that poorly represent the open, atmospheric-pressure operation of solar-driven building-scale stores. Here, we report the thermal characterization, under open-system boundary conditions (T des = 80 °C, T ads = 20 °C, p ads = 11.6 mbar), of the amino-functionalized titanium framework NH 2 -MIL-125(Ti) and of two composites, namely, NH 2 -MIL-125(Ti)+45 wt% CaCl 2 and NH 2 -MIL-125(Ti)+45 wt% SrBr 2 . To the best of our knowledge, NH 2 -MIL-125(Ti)/SrBr 2 is a new MOF/salt pairing, and no NH 2 -MIL-125(Ti)/salt composite has previously been characterized under open-system conditions. Materials were characterized by X-ray powder diffraction, N 2 physisorption (BET) and simultaneous thermogravimetry–differential scanning calorimetry (TG–DSC) coupled to a controlled-humidity generator. The pristine framework delivered a water uptake of 0.21 g g −1 and an energy storage capacity of 163 Wh kg −1 , whereas the salt loading increased these values to 0.56 g g −1 and 342 Wh kg −1 (CaCl 2 ) and 0.40 g g −1 and 313 Wh kg −1 (SrBr 2 ), respectively, at the expense of slower kinetics. All the materials retained their water uptake over six adsorption/desorption cycles (uptake loss ≤5%; energy storage capacity loss 8–12%, reflecting a slight decrease in the measured heat of adsorption). A quantitative kinetic analysis (characteristic times t50/t90 and linear-driving-force constants) is provided. The results establish NH 2 -MIL-125(Ti)/salt composites as promising sorbents for low-temperature solar heat storage, the 80 °C regeneration temperature being directly compatible with common flat-plate solar collectors, and provide a rare open-system dataset for this material class.