Significant improvement of adsorption thermal energy storage of zeolite by simple cation exchange, inducing strong polarization of water

A series of zeolite 13X with various cations was tested as a candidate for water-adsorption-based thermal storage. In the case of pristine commercial zeolite 13X pellet, >99.9 % of cation in the zeolite is confirmed to be Na⁺. Via conventional cation-exchange method, the Na⁺ could be almost completely exchanged to Li⁺, K⁺, Cs⁺, Mg²⁺, Ca²⁺, and Ba²⁺ to the thermodynamical levels. The improvement of adsorption thermal storage requires both the increases of water adsorption capacity and heat of desorption. Thermogravimetric analysis (TGA) and Differential Scanning Calorimetry (DSC) measurements were performed for the zeolites, showing that Li⁺-, and Mg²⁺-ion form zeolite samples exhibited 26 % and 29 % higher water adsorption capacities at room temperature under fully saturation condition than pristine sample, respectively, and that especially Mg²⁺-ion form shows 18 % higher heat of desorption than pristine Na⁺-form. Although Li⁺-ion form shows notably high water adsorption capacity, its desorption occurs mostly at low temperature (<388 K). The increase of thermal storage as exchanging the cation to Mg²⁺ and Li⁺ was fundamentally explained by density functional theory calculation. The cations induce the strong polarization of the adsorbed water molecules, which leads to strong interactions between water molecules and an increase in the amount of water stored.