Characterization of Ca-Dicarboxylate Salt Hydrates as Thermochemical Energy Storage Materials

Salt hydrates are highly promising materials for thermochemical energy storage applications to store waste heat below 200 °C. Although highly researched and theoretically promising, in practical applications salt hydrates often cannot fulfill expectations. Based on the promising results of the Ca-oxalate monohydrate/Ca-oxalate system, other Ca-dicarboxylate salt hydrates were investigated to determine whether potential materials for heat storage can be found amongst them. A simultaneous thermal analysis showed that all candidates are applicable in the temperature range of 100–200 °C, and thermally stable up to 220 °C. Calcium malonate dihydrate (637 J/g), calcium terephthalate trihydrate (695 J/g), and tetrafluoro calcium terephthalate tetrahydrate (657 J/g) have shown higher enthalpies of dehydration than Ca-oxalate monohydrate. Due to the investigation of derivatives of Ca-terephthalate, it is possible to report the crystal structure of 2-fluoro calcium terephthalate. In single crystals, it forms a trihydrate and crystallizes in the Pmna space group (Z = 4, Z’ = ½) forming infinite chains of Ca atoms. De- and rehydration reactions of the most promising candidates were studied in situ with powder X-ray diffraction showing the structural changes between the hydrate and anhydrate states.