Sais de tutton triplos CM2CD(SO4)2(H2O)6 (CM = K⁺, Rb⁺, NH4⁺; CD = Mg2+, Co2+ e Ni2+) para aplicações em dispositivos termoquímicos: síntese, estrutura e propriedades térmicas.

Abstract

The development of new materials for thermochemical heat storage (TCHS) is essential for harnessing intermittent renewable energy. Although hydrated salts offer high energy densities, their performance is often limited by issues such as poor cyclability, necessitating the production and study of new compositions. This work introduces a new subclass of Tutton's salts, in which three distinct divalent cations (Mg2+, Co2+ e Ni2+) simultaneously occupy a single crystallographic site, with the general formula CM2CD(SO4)2(H2O)6 (CM = K⁺, Rb⁺, NH4⁺; CD = Mg2+, Co2+ e Ni2+). These materials were synthesized via slow solvent evaporation, and their structures were determined using powder X-ray diffraction data, confirming crystallization in monoclinic symmetry with space group P21/a or P21/c. X-ray fluorescence spectroscopy revealed non-equimolar incorporation of the divalent cations, indicating a significant influence of the monovalent cation on site occupancy. Comprehensive structural analysis, including Hirshfeld surfaces and crystal voids, demonstrated that the NH₄⁺-based salt possesses a more porous structure with elongated void channels, contrasting with the compact structures of the K+ and Rb+ analogues. Raman spectroscopy provided insights into the local coordination environment and the hydrogen-bonding network. Thermal analyses revealed tunable dehydration temperatures (onset between 69 and 97 °C) and the release of all six water molecules in a single step. Dehydration enthalpies (ΔH) ranged from 299.64 to 426.83 kJ/mol, resulting in promising volumetric energy storage densities of up to 1.88 GJ/m3. The Rb+-based compound emerged as the most promising candidate for a thermochemical material (TCM), exhibiting the highest ΔH and a theoretical thermal efficiency (η) of 42.83%. Thus, this study establishes the first triple Tutton salts as versatile compounds for tuning thermochemical properties through cationic composition, paving the way for tunable, high-performance TCHS materials.