High-performance thermochemical energy storage composites using CaCl2 in mesoporous silicas

Thermochemical materials that exhibit reversible solid-gas reactions are promising for compact energy storage for thermal loads in buildings. Inorganic salt hydrates exhibit high reaction enthalpies and minimal self-discharge; however, hygrothermal instabilities (deliquescence and agglomeration) diminish their energy storage capacity and reaction kinetics during repeated cycling. Here, we develop a salt-in-matrix composite based on hierarchical silica with a bimodal pore system that nanoconfines a hygroscopic salt (CaCl 2 ) within its mesopores while allowing water vapor diffusion through macropores. This composite accommodates salt loadings of 60 wt % and demonstrates stable hydration at 25°C and a range of relative humidity up to 80% by altering the phase transition and preventing salt leakage (capillary attraction). Additionally, the composite achieves a consistently high water uptake (>1.1 g water /g) and energy density (>2,300 J/g) over 100 charge-discharge cycles, with faster hydration kinetics compared with commercial fumed silica-based composites. This highlights the potential of thermochemical materials for load shifting/shaving buildings.