Tunable LiCl@UiO-66 composites for water sorption-based heat transformation applications

Porous composite materials are potential candidates for water-based adsorptive heat transformation (AHT) applications. Here, the solid adsorbent LiCl@UiO-66 as composite ‘salt inside porous matrix’ (CSPM) has been prepared by incorporating hygroscopic lithium chloride into the microporous metal-organic framework (MOF) UiO-66 as host matrix through the wet impregnation method. In our wet impregnation we did not let dry the excess salt solution to prevent salt precipitation on the matrix surface. This yielded a true salt@MOF composite with no deliquescence of LiCl and strongly enhanced the water adsorption capacity of UiO-66 through the salt content. At p/p0 = 0.1 the water vapor sorption isotherms show a hydration state of LiCl inside the MOF of LiCl·2-4H2O which is much higher than for neat LiCl with 0.5H2O, due to the dispersion of a small particle size inside the matrix. LiCl@UiO-66 with 30 wt% LiCl content (LiCl@UiO-66_30) has a 3 to 8 times higher water uptake over neat UiO-66 (depending on relative pressure) and could reach a volumetric and gravimetric water uptake of over 2.15 g/g at p/p0 = 0.9, which outperforms the so far known UiO-66-based composites. Cycling tests confirmed the hydrothermal stability of the LiCl@UiO-66 composites. Kinetic evalution of the gravimetric water uptake (at 90% relative humidity) over time yielded rate coefficients up to 2.0(1) × 10-4 s–1 which is slower than in neat UiO-66 (6.7(6) × 10-4 s–1) but faster than for salt@silica gel composites. The coefficient of performance for heat pumping mode (at Tdes/Tads/Tevap set to 90/40/10 °C) of 1.64 for LiCl@UiO-66_30 exceeds those of other MOFs, salt@MOF or salt@silica gel composites. For thermal battery applications the heat storage capacity (CHS) for LiCl@UiO-66_30 is 900 kJ/kg (= 0.25 kWh/kg), which can reach the Department of Energy (DOE) value of 2.5 kWh/35 kg with just 10 kg of material and outperforms CaCl2@UiO-66_38 with a CHS value of 367 kJ/kg.