(506g) Development of Shape-Stabilized Salt Hydrates with Polymer Components for Low-Cost Thermal Energy Storage

Salt hydrates are a class of phase change materials (PCMs) that are of interest due to their lower cost-energy density and higher volumetric energy density than other PCMs such as paraffins. This PCM also has the ability to form eutectics, like other PCMs, which allows for the chemical specification of bespoke PCMs designed to store thermal energy at specific invariant temperatures. Unfortunately, the use of salt hydrates in thermal energy storage (TES) is hindered by fluid flow and overall redistribution when the PCM is in its liquid state, as well as their susceptibility to form a metastable liquid state during cooling. Additionally, salt hydrate eutectics are potentially susceptible to phase segregation if solidification occurs in a metastable manner, as density-driven differences preferentially causes one phase or more to separate out. This segregation would result in inhomogeneity throughout the material. Here, we investigate the use of polymer gel networks, referred to as salogels due to the high salt concentrations, where a small amount (<5 wt%) of a compatible polymer is incorporated in a salt hydrate system, resulting in a matrix formed through hydrogen-bonding and polymer-ion interactions. The resultant polymer network enables the PCM to retain its shape above its melting temperature, T_m, by preventing its flow, allows the solid-to-liquid phase transition of the salt hydrate that provides the desired high latent heat, and can potentially mitigate phase segregation by trapping all eutectic components in place even as the PCM becomes metastable below T_m. In addition, these salogels are thermo-reversible as they are physical gels, meaning the salogel can flow above its gel-to-sol transition temperature (T_gel) allowing processability, and it exists as a stiff solid below T_gel. However, limited data is available on the thermal cycling behavior of salogels made with stoichiometric salt hydrates and their eutectics, including the effect of the salogel polymer framework on the efficacy of nucleation particle (NP), which serve to nucleate the crystalline salt hydrate from their exposed solid surfaces. Our previous work investigated the cycling and nucleation behaviors of pure salt hydrates and their family of eutectics with and without any NPs in the absence of polymers. Now, our work seeks to determine how a polymer component can affect the cycling and nucleation behavior in a system, especially when it is expected that the polymer plays no active role in the melting behavior of the PCM.

Here we present a family of zinc nitrate hexahydrate (Zn(NO₃)₂∙6(H₂O)) salogels, including those made with pseudo-binary eutectics (Zn(NO₃)₂∙6(H₂O)-NaNO₃, Zn(NO₃)₂∙6(H₂O)-KNO₃, and Zn(NO₃)₂∙6(H₂O)-NH₄NO₃). Through the addition of a low concentration of polyacrylamide (PAAm), we observe the effects of this polymer addition on the cycling and nucleation behavior of these systems, as both have plagued salt hydrate eutectics. In prior work, the base, polymer-absent Zn(NO₃)₂∙6(H₂O) systems did not undergo phase segregation as a result of metastability upon cycling; the presence of a polymer should not change this at all. From our work, we have seen that salogel systems with little to no anhydrous salt concentration (Zn(NO₃)₂∙6(H₂O) and Zn(NO₃)₂∙6(H₂O)-NaNO₃) show consistent, uni-modal melting behavior across >150 thermal cycles even as they were cycled to maximum temperatures near T_gel. As anhydrous salt concentration increases in a eutectic system (Zn(NO₃)₂∙6(H₂O)-KNO₃ and Zn(NO₃)₂∙6(H₂O)-NH₄NO₃), the melting behavior devolves into a multi-modal (bi- or tri-modal) shaped melting curve across >150 cycles if the maximum cycling temperature is in proximity of T­_gel. However, this shape appears to stabilize after a number of cycles. This is exceptionally seen in the Zn(NO₃)₂∙6(H₂O)-NH₄NO₃ salogel as bimodal melting curves are observed as the system is cycled close to its T_gel of ~30 ËšC, show that the loading of the salogel into any PCM module above T_gel can have potentially negative long-term impact on its melting behavior. The bimodal behavior is potentially due to the PAAm component swelling more at higher temperatures, resulting in compositional changes to the PCM. This is seen in polarized light microscopy studies as it was observed that precipitates formed at higher temperatures.

For the base Zn(NO₃)₂∙6(H₂O) system, talc has been identified as an effective NP. In neat salogel systems, undercooling decreased to <15 ËšC even in the absence of an NP when compared to the polymer-absent form, which had undercooling values <25 ËšC, showing that the polymer aids in nucleation. However, talc usage is warranted as it makes undercooling low and consistent across cycles. The work presented here showcases the effects a polymer component can have on the cycling and nucleation ability of salt hydrate systems, especially as its need arises when trying to make shape-stable TES media. In terms of undercooling, the polymer reduces it in salt hydrate systems, maximizing more volume available for PCMs. For melting, the polymer component in some systems does not hinder its melting behavior when compared to its polymer-absent form; some systems did experience changes at higher temperatures after polymer was added, posing a potential hinderance in implementation as a TES media.