(506f) Degradation and Recovery of the Efficacy of Strontium Chloride Hexahydrate As a Nucleation Particle for Calcium Chloride Hexahydrate

As alternative renewable energy sources, which tend to be intermittent and in some cases unpredictable in nature, become more widespread, the need for technologies to store surplus energy for use at a later time will only grow. Thus, to move towards full decarbonization of the energy grid, effective thermal energy storage systems could play an important role in this process. Specifically, phase change materials (PCMs) can passively store and release building process heat, either assisting with cooling or heating of internal building air, thereby introducing a means of displaced peak energy load for buildings. However, robust and economical phase change materials that are tailorable to a specific desired energy storage temperature remain limited, and still face challenges with robust long-term thermal behavior. Salt hydrates represent one class of PCMs of interest due to their high volumetric energy densities, low cost, and the ability to tailor the thermal energy storage temperature through the design of custom salt hydrate eutectics. However, they can also experience undercooling, a nucleation-limited phenomena which results in the existence of a metastable liquid below the melting point. This phenomena can lead to undesired issues, including incongruent melting and phase segregation. To combat these issues, nucleation particles are often added to systems to induce nucleation and suppress formation of metastable phases, thus mitigating the chance of phase segregation occurring.

While compounds that are isostructural to the desired PCM tend to have low interfacial energy with the target phase, resulting in low degrees of undercooling, these phases (which also tend to be salt hydrates) may not be stable under all conditions, reacting to form lower hydrates, or even anhydrous solids. In this study, we investigated the efficacy of strontium chloride hexahydrate (SrCl₂·6H₂O; SCH) as a nucleation particle to reduce undercooling in calcium chloride hexahydrate (CaCl₂·6H₂O; CCH) over a range of temperatures and concentrations. Calcium chloride hexahydrate is a stoichiometric salt hydrate of interest for use in thermal energy storage systems because it is a low-cost salt hydrate that melts at 29°C [1, 2]. However, calcium chloride hexahydrate is also prone to undercooling and phase segregation due to the presence of a peritectic point at 29.45°C, where the metastable state, calcium chloride tetrahydrate (CaCl₂·4H₂O) can also be present. Therefore, nucleation particles are often added to CCH in order to induce nucleation, suppress the formation of a metastable state, and to reduce undercooling. Throughout many different studies, strontium chloride hexahydrate is often noted as an effective nucleation particle for CCH due to their isostructural relationship [3, 4].

Here, we demonstrate that the effectiveness of SCH as a nucleation particle is dependent on the time-temperature history of the system, suggesting a reaction pathway that results in the degradation of the desired SCH phase. For example, increasing the temperature of a system of CCH mixed with SCH to greater than ~65°C for a minimum of two minutes increases the degree of undercooling of the system, indicating that SCH can lose efficacy. Previous solubility studies explain this behavior, as it has been previously reported that CCH and SCH cannot exist in equilibrium with each other at specific temperatures and concentrations, and other hydrate phases of SCH will instead be present [5]. Surprisingly, cooling below ~ -30°C recovers this efficacy, but only if the system is cooled for at least 5 minutes at this cold temperature prior to holding at 0°C. Thus, if the system is allowed to solidify completely, we are able to regain the phase that is responsible for nucleation of CCH, which indicates that the reactivity of SCH can affect its efficacy as a nucleation particle.

In contrast, reactivity between a salt hydrate PCM and a nucleation particle can be harnessed in a positive way, resulting in in-situ reactions with products that are both stable, and active nucleators. We will introduce an example of this mechanism describing recent results from Ba-based compounds which undergo in-situ reactions upon exposure to liquid CCH, resulting in a decrease in undercooling of ~10°C. However, the Ba-based nucleation particles have no apparent structural relationship to CCH, instead we hypothesize that they are effective nucleators due to the existence of atomic-scale defects on the surface of a solid solution phase. Therefore, in the case of strontium chloride hexahydrate, the strategy of isostructural nucleation particles does present a path towards effective nucleation, however the reactivity of the system must also be considered and evaluated.

References:

[1] G. A. Lane, Solar Heat Storage: Latent Heat Materials Volume II: Technology. CRC Press, 1986.

[2] B. Carlsson, H. Stymne, and G. Wettermark, "An Incongruent Heat-of-Fusion System - CaCl2.6H2O - Made Congruent through Modification of the Chemical Composition of the System," Solar Energy, vol. 23, pp. 343-350, 1979.

[3] L. Yan, J. Charles, C. E. Romero, S. Neti, and G. Balasubramanian, "Optimizing supercooling and phase stability by additives in calcium chloride hexahydrate for cyclical latent heat storage," International Communications in Heat and Mass Transfer, vol. 149, 2023, doi: 10.1016/j.icheatmasstransfer.2023.107119.

[4] N. Kumar and D. Banerjee, "Thermal Cycling of Calcium Chloride Hexahydrate With Strontium Chloride as a Phase Change Material for Latent Heat Thermal Energy Storage Applications in a Nondifferential Scanning Calorimeter Set-Up," Journal of Thermal Science and Engineering Applications, vol. 11, no. 5, 2019, doi: 10.1115/1.4042859.

[5] G. O. Assarsson and A. Balder, "Equilibria between 18 and 114 in the aqueous ternary system containing Ca2+, Sr2+, and Cl-," 1953.