Computational Screening of Hydration Reactions for Thermal Energy Storage: New Materials and Design Rules

The implementation of thermal energy storage (TES) can improve the efficiency of existing industrial processes, and enable new applications that require the uptake/release of heat on-demand. Among the myriad strategies for TES, thermochemical hydration/dehydration reactions are arguably the most promising due to their high energy densities, simplicity, cost effectiveness, and potential for reversibility at moderate temperatures. The present study uses first-principles calculations to identify TES materials that can out-perform known compounds. High-throughput density functional theory calculations were performed on metal halide hydrates and hydroxides mined from the Inorganic Crystal Structure Database. In total, 265 hydration reactions were characterized with respect to their thermodynamic properties, gravimetric and volumetric energy densities, and operating temperatures. Promising reactions were identified for three temperature ranges: low (300 °C). Several high-energy-density reactions were identified, including the dehydration of CrF3·9H2O, a compound that appears to be unexplored for TES. Correlations linking TES performance with dozens of chemical features for hydrates and hydroxides were quantified using a Pearson correlation matrix. These analyses reveal property–performance relationships involving energy densities and the thermodynamics of hydration. In salt hydrates, the thermodynamics depend strongly on the water capacity of the hydrate. In hydroxides, thermodynamic properties are largely determined by the ionicity of the cation–hydroxide bond, which is in turn influenced by the cation’s electronegativity and polarizing power. Based on these correlations, design rules for hydration-based TES systems are proposed.