Understanding the Hydration Process of Salts: The Impact of a Nucleation Barrier

Abstract

The solid-state hydration of salts has gained particular interest within the frame of thermochemical energy storage. In this work, the water vapor pressure−temperature (p−T) phase diagram of the following thermochemical salts was constructed by combining equilibrium and nonequilibrium hydration experiments: CuCl 2 , K 2 CO 3 , MgCl 2 •4H 2 O, and LiCl. The hydration of CuCl 2 and K 2 CO 3 involves a metastable zone of ca. 10 K, and the induction times preceding hydration are well-described by classical homogeneous nucleation theory. It is further shown for K 2 CO 3 (metastable) and MgCl 2 •4H 2 O (not metastable) through solubility calculations that the phase transition is not mediated by bulk dissolution. We conclude that the hydration proceeds as a solid− solid phase transition, mobilized by a wetting layer, where the mobility of the wetting layer increases with increasing vapor pressure. In view of heat storage application, the finding of metastability in thermochemical salts reveals the impact of nucleation and growth processes on the thermochemical performance and demonstrates that practical aspects like the output temperature of a thermochemical salt are defined by its metastable zone width (MZW) rather than its equilibrium phase diagram. Manipulation of the MZW by e.g. prenucleation or heterogeneous nucleation is a potential way to raise the output temperature and power on material level in thermochemical applications.

Molten Chloride Thermophysical Properties, Chemical Optimization, and Purification

“…The pre-dried AC resting in ambient, however, slowly grew and had a visible volume increase (or a popping behavior) above its original height (about the height of the petri dish shown in Figure 4b) because of the formation of the snowflake-like "arms" on the surface of the salt grains. Although we didn't characterize these "arms," this behavior seems to closely resemble the growth of salt hydrates by nucleation [60,61]. HCl release data (discussed below) also corroborate the hypothesis that pre-drying does remove some hydrates (which makes rehydration a possibility).…”

“…The DSC traces can be obtained upon request. 61 We are confident that the vapor pressure data from the University of Arizona are free of similar effects because a full quartz set was used as salt containment in the vapor pressure measurement apparatus.…”

“…Therefore, it is very likely that the contamination was due to "clean vapor"-from evaporation of purified salt-in contact with the SS316 vessel wall in the furnace hot zone that generated Fe-containing chloride salt with high vapor pressure. 61 Table 13 shows the elemental analysis from ICP-AES of the collected vapor condensate and the remaining salt in the quartz crucible. It can be seen that the vapor condensate is indeed very different from either the baseline salt or the remaining salt in the quartz crucible after the evaporation test.…”

Materials Engineering for Atmospheric Water Harvesting: Progress and Perspectives

“…And the adsorption has been proved to have negligible contributions to the overall sorption process. [52][53][54] Although the high hygroscopicity of salts is attractive for AWH, some critical drawbacks restrict their practical use. For example, the deliquescence can cause salt particle aggregations and suppress the absorption kinetics, leading to a decreased performance in the following cycles.…”

“…The moisture harvesting process of hygroscopic salts is based on the hydration process. [ 52 ] As the water uptake proceeds, the hygroscopic salts will eventually dissolve in the captured water, indicating the transition from hydration to deliquescence. Thermodynamically, the whole process will occur when the relative air humidity (RH) exceeds the hygroscopic point ( h g.p. )…”