(506a) Salt Hydrate Eutectics: Tailored Design of Equilibria, Reversibility, and Stability in Complex Phase Change Material Systems

Salt hydrate phase change materials (PCMs) represent a promising class of materials for thermal energy storage in buildings and environmental climate control systems, but are currently limited by the lack of compounds with reversible melting-solidifying behavior within particular temperature ranges of practical use for building thermal energy storage applications (approximately 10 to 30 °C). Here, I will present a general strategy, harnessing the ability of salt hydrates to form pseudo-binary or higher-order eutectics, to refine the melting point of low-cost stable eutectic salt hydrate PCMs for building thermal energy storage applications. While this approach utilizes a well-documented strategy of relying on a thermodynamically invariant point (i.e., the eutectic point) to result in well-defined and robust melting behavior, it introduces a number of unique challenges, including 1) efficient exploration of multi-component systems and identification of eutectic compositions, 2) nucleation in multicomponent salt hydrate systems, including the potential impact of metastable liquids on the stability of such eutectics, and 3) complex interactions in PCM materials systems, which generally include multiple distinct phases or chemical constituents added to the system to address nucleation, heat transfer, and to encapsulate the salt hydrates. In such systems, interactions between different components can serve to aid or to deteriorate the overall performance of the material system. Throughout, examples will be provided based on nitrate and chloride hydrate systems and their eutectics.

Efficient exploration of salt hydrate compositions is required because of the vast compositional space that these systems can span. Here, we utilized the modified Brunauer–Emmett–Teller (BET) method as an efficient predictive method, which requires minimal initial knowledge on material systems. We use this method to evaluate pseudo-binary and ternary eutectics formed between five nitrate hydrates (of Li, Mg, Ca, Mn, and Zn), and three anhydrous nitrates (of Na, K, and NH₄). We demonstrate the overall success of the method, while highlighting some of its limitations, including the inability to predict isomorphous solid solutions, and challenges faced in describing anhydrous nitrate phases.

A number of salt hydrates are known to experience nucleation-limited undercooling, although the degree of this phenomenon varies widely, even across chemically similar salt hydrates. Here we address fundamental questions related to the occurrence of undercooling in multi-component salt hydrates. In particular, we demonstrate the intrinsic variability of undercooling observed in different related salt hydrate phases, which illustrates the difficulty in predicting nucleation behavior in multi-component systems. Furthermore, while inclusion of a single class of nucleation particles (targeting a single crystalline phase) can serve to promote complete solidification of multiple crystalline solids in a pseudo-binary eutectic, they do not necessarily initiate crystallization simultaneously. This can result in a potential for phase segregation, even in a system that (in equilibrium) solidifies in a manner that would not be susceptible to phase segregation. Finally, we illustrate the challenge in identifying active nucleation particles in reactive salt hydrate systems, drawing from a pathological example in the CaCl₂⸱6H₂O system, where Ba-based compounds undergo multiple cation-substitution and precipitation reactions, obscuring the nature of the active nucleation particle.

Finally, I will address the topic of PCMs as systems of materials, and will introduce some of the interactions that can occur between different constituents of that system, including 1) the PCM, 2) nucleation particles, 3) encapsulating materials, 4) high thermal conductivity graphitic additives, and 5) polymer molecules introduced to thicken or form a gel. Specifically, some of these components can serve multiple functions (e.g., encapsulating materials which also nucleate solidification), while other of these components can interact in a negative destructive manner (e.g., polymer gel molecules, which can reduce the efficacy of nucleation particles in some cases). Additional examples include the potential effect of shape stabilization on the cyclic degradation observed in thermal conductivity due to cycles of expansion and contraction, as well as the effect of polymer networks to limit complete compositional mixing during melting cycles, which can result in formation of bimodal or trimodal characteristic melting peaks.

Overall, this presentation seeks to emphasize the salient features of salt hydrate eutectic systems, while providing illustrative examples from nitrate and chloride eutectics. We will conclude by identifying remaining challenges in the field, as well as instructive strategies to continue to advance low cost salt hydrate thermal energy storage materials.