Cation‐Engineered Hydration Microenvironments in Magnesium Chloride Composites for Enhanced Low‐Humidity Vapor Adsorption and Thermochemical Heat Storage

This study investigates the thermochemical energy storage performance of magnesium chloride (MgCl 2 )‐based composite hydrated salts physically mixed with eight chloride salts (LiCl, KCl, CaCl 2 , LaCl 3 , CuCl 2 , CrCl 3 , SrCl 2 , and SrBr 2 ). Through low‐field nuclear magnetic resonance (NMR), X‐ray diffraction (XRD), water vapor adsorption isotherms, and thermogravimetric analysis (TGA), we reveal how cation types govern water states (bonded/immobilized/free water) and influence dehydration kinetics, hygroscopicity, and energy storage density. Key findings include: NMR T 2 spectra demonstrate that KCl and CrCl 3 composites predominantly contain bonded water (BW, T 2 < 10 ms), enhancing hydrolysis resistance, while LiCl/CaCl 2 composites exhibit free water (FW, T 2 > 100 ms). XRD confirms new crystalline phases (e.g., KMgCl 3 ·6H 2 O in MgCl 2 –KCl) formed during mixing, increasing the latent heat density of MgCl 2 –CaCl 2 to 377.0 J g −1 . All composites surpass pure MgCl 2 in low‐humidity adsorption (30°C, P/P 0 < 0.6), with MgCl 2 –SrCl 2 achieving a storage density of 978.8 J g −1 at 60% RH. The LaCl 3 composite shows the highest hydration rate ( k eff = 0.0097 min −1 ), attributed to optimized ion dissolution and crystallization kinetics.