Dehydration kinetics and thermodynamics of magnesium chloride hexahydrate for thermal energy storage

Reversible thermal dehydration reaction of MgCl2·6H2O has been studied as a potential working way for thermochemical heat storage with high energy density. Understanding its complex multistep dehydration behavior is significant for guiding practical applications; however, there is a lack of deep understanding about the phase transition of MgCl2·6H2O during its dehydration reaction process. In this work, the reaction mechanism, dehydration kinetics, and equilibrium behaviors of MgCl2·6H2O are studied in detail with the help of thermal analytical measurements and morphological observations. The thermogravimetric analysis coupled with differential scanning calorimetry (TG-DSC) shows the dehydration behavior and reaction pathways varying with gas atmosphere, heating rate, water vapor pressure, and sample mass. The formation of intermediate molten MgCl2·6H2O is identified that it makes strong effects on the dehydration kinetics and reaction equilibrium. Futhermore, a revised Clapeyron equilibrium equation is developed and experimentally verified for describing the reaction equilibrium of molten MgCl2·6H2O. It is expected that the findings in this study pave paths for efficient thermal energy storage application based on MgCl2·6H2O.