Zinc Nitrate Hexahydrate Pseudobinary Eutectics for Near-Room-Temperature Thermal Energy Storage

Stoichiometric salt hydrates can be inexpensive and provide higher volumetric energy density relative to other near-room-temperature phase change materials (PCMs), but few salt hydrates exhibit congruent melting behavior between 0 and 30 °C. Eutectic salt hydrates offer a strategy to design bespoke PCMs with tailored application-specific eutectic melting temperatures. However, the general solidification behavior and stability of eutectic salt hydrate systems remain unclear, as metastable solidification in eutectic salt hydrates may introduce opportunities for phase segregation. Here, we present a new family of low-cost zinc-nitrate-hexahydrate-based eutectics: Zn(NO₃)₂·6(H₂O)-NaNO₃ (T_eu = 32.7 ± 0.3 °C; ΔH_eu = 151 ± 6 J·g^-1), Zn(NO₃)₂·6(H₂O)-KNO₃ (T_eu = 22.1 ± 0.3 °C; ΔH_eu = 140 ± 6 J·g^-1), Zn(NO₃)₂·6(H₂O)-NH₄NO₃ (T_eu = 11.2 ± 0.3 °C; ΔH_eu = 137 ± 5 J·g^-1). While the tendency to undercool varies greatly between different eutectics in the family, the geologic mineral talc has been identified as an active and stable phase that dramatically reduces undercooling in Zn(NO₃)₂·6(H₂O) and all related eutectics. Zn(NO₃)₂·6(H₂O) and its related eutectics have shown stability for over a hundred thermal cycles in mL scale volumes, suggesting that they are capable of serving as robust and stable media for near-room-temperature thermal energy storage applications in buildings.