Three Cations, One Site: Structural and Thermal Design of the First Triple Tutton Salts as Thermochemical Heat Storage Materials

The strategic design of advanced thermochemical heat storage (TCHS) materials is crucial for harnessing intermittent renewable energy. While salt hydrates offer high energy densities, their performance is often limited by issues like poor cyclability. This work pioneers the crystal engineering of Tutton salts, a novel subclass of materials where three distinct divalent cations (Mg2+, Co2+, and Ni2+) simultaneously occupy a single crystallographic site, with the general formula MC2DC(SO4)2(H2O)6 (MC = K+, Rb+, NH4+; DC = Mg2+, Co2+, and Ni2+). These materials were synthesized via slow evaporation, and their structures were solved from powder X-ray diffraction data, confirming crystallization in the monoclinic symmetry with space group P21/a or P21/c. X-ray fluorescence spectroscopy revealed nonequimolar cation incorporation, indicating a significant influence of the monovalent cation on the site occupancy. A comprehensive structural analysis, including Hirshfeld surfaces and crystal voids, demonstrated that the NH4+-based salt features a more open and porous framework with elongated void channels, contrasting with the compact structures of the K+ and Rb+ analogues. Raman spectroscopy confirmed the successful integration of all cations and provided insights into the local coordination environment and hydrogen-bonding lattice. Thermogravimetric and differential scanning calorimetry analyses revealed tunable dehydration temperatures (onset 342–370 K) and a single-step release of all six water molecules. The dehydration enthalpies (ΔH) ranged from 299.64 to 426.83 kJ/mol, translating promising volumetric energy storage densities of up to 1.88 GJ/m3. The Rb-based compound emerged as the most promising candidate, exhibiting the highest ΔH and a theoretical thermal efficiency of 42.83%. This study establishes the first triple Tutton salts as a versatile platform for tailoring thermochemical properties through cationic composition, paving the way for high-performance, tunable TCHS materials.