Layered proton-type zirconium phosphates as candidate heat storage materials based on water intercalation

Abstract

Utilization of waste heat is essential for energy conservation and CO₂ reduction, driving demand for the development of efficient thermal storage materials. Although various types of thermal storage materials have been proposed, few meet the requirements of reversibility, high energy density, long life, and ease of handling. This study reports that α-Zr(HPO₄)₂·H₂O and γ-Zr(PO₄)(H₂PO₄)·2H₂O, which possesses a layered structure with one and two interlayer water molecules, exhibits water intercalation at temperatures below 160 and 80 °C and is an excellent candidate for heat storage applications. The apparent activation energy for interlayer water desorption was 91 kJ·mol⁻¹ for both samples. TG and DSC measurements revealed an endothermic reaction during heating, corresponding to the weight loss from desorption of water molecules, and an exothermic reaction during cooling, corresponding to the weight gain from insertion of water molecules. α-Zr(HPO₄)₂·H₂O and γ-Zr(PO₄)(H₂PO₄)·2H₂O also demonstrated excellent cycling performance, with calorific values obtained from the endothermic and exothermic peak areas in the third cycle of −147 and 203 J·g⁻¹, and −82 and 121 J·g⁻¹, respectively. These findings highlight the potential of α-Zr(HPO₄)₂·H₂O and γ-Zr(PO₄)(H₂PO₄)·2H₂O as sustainable heat storage materials for energy conservation and thermal management applications.