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Inorganic salt hydrates are promising materials for thermochemical energy storage as they undergo reversible solid-gas chemical reactions with water vapor to yield high energy densities with negligible self-discharge. However, material-level challenges such as structural and hygrothermal instabilities during the dehydration (charging) and hydration (discharging) reaction have limited their practical application in the buildings sector. The objective of this study is to address these irreversibilities in SrCl2 and MgCl2 by establishing a fabrication procedure that minimizes vapor diffusion resistance and lowers kinetic barriers for nucleation via particle size reduction. Furthermore, the distinct phase behavior of these two salts is leveraged to demonstrate a new binary salt mixture. Characterization of these materials was done using simultaneous thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) with a humidity generator. The results demonstrate that ball milling to particle sizes <50 μm yields a structurally stable material with improved hydration kinetics, while a 50/50 mass ratio of the binary mixture extends the range of conditions for the hydration reaction. Importantly, the salt mixture achieves a high specific energy density of 1100 J g-1 and peak thermal power output of 1.4 W g-1 under conditions at which the individual salts either deliquesce (MgCl2) or do not fully/rapidly hydrate (SrCl2). This work provides a procedure for the standardized fabrication and rational design of thermochemical salt mixtures with complementary phase behavior. 

Thermochemical energy storage is promising for building applications as it offers high energy density and near-lossless storage. For example, inorganic salt hydrates that undergo reversible solid-gas thermochemical reactions can be used for thermal load shifting and/or shedding in buildings. However, this technology is still in early stages of development and drawbacks need to be addressed to make such a thermal battery viable. As salt hydrates differ in their morphology, crystal and/or particle size, and hygrothermal stability, it is critical to characterize thermochemical reactions accurately under specific operating conditions. Not only is the amount of heat delivered important, but so is the rate at which this heat is extracted for thermal end-use in buildings. However, the latter is not well reported in the literature, which is largely focused on energy storage rather than power density during the hydration reaction (battery discharge). To address this gap and the lack of standardized measurement methods, this work lays out a systematic simultaneous thermal analysis (STA) method for characterizing five different salt hydrate thermochemical materials (TCM). The effects of particle size, temperature and vapor pressure are analyzed to obtain the energy storage density and thermal power density across a full hydration-dehydration cycle under controlled conditions.