Porous host hydrophobicity affects hydration-dehydration cyclability of thermochemical energy storage composites

High-energy-density thermochemical materials (TCMs), such as salt hydrates are ideal for low- and medium-temperature thermal energy storage applications. However, pure salt hydrates face challenges like poor thermal cyclability and control of reaction kinetics. Composite TCMs, created by integrating salt hydrates into support matrices, can improve the performance. However, condensation of water vapor in the pores of the composite can hinder hydration-dehydration reactions, leading to poor cyclability and reaction kinetics. In this work, we study the role of matrix hydrophobicity in the thermal cyclability of composite TCMs based on MgSO₄·7H₂O. Two silica matrices were employed: hydrophobic silica grafted with C8 chains and hydrophilic silica. Hydrophobic composites demonstrated excellent structural stability during 20 cycles with minimal morphological changes in silica and progressive salt particle size reduction. In contrast, hydrophilic matrix-based composites rapidly degraded, liquefying after only five cycles and showing complete loss of energy density. The improved performance of hydrophobic composites is attributed to the prevention of liquid water formation during thermal cycling, which otherwise accelerates salt dissolution and structural failure. These findings emphasize the critical importance of hydrophobic host materials in developing stable, high-performance thermochemical energy storage systems using salt hydrates.