A systematic deliquescent additive selection approach for enhancement of reaction kinetics of thermochemical heat storage materials

A series of 16 salt mixtures consisting of potassium carbonate (K2CO3) as the base salt and a secondary salt hydrate in a 20 : 1 mol ratio of anhydrous K2CO3 to anhydrous additive were investigated as potential thermochemical heat storage (TCHS) composites. Those materials were evaluated based on their (de)hydration temperatures and reaction kinetics to find a suitable secondary salt mixture with enhanced phase change behaviour. The improved performance is expected to come from the enhancement of ionic mobility due to the deliquescence of the secondary salt. Based on the screening, we found that deliquescent and highly soluble salts are prone to reacting with K2CO3, forming new compounds that do not impact the behaviour of the base salt. Therefore, the most promising additives were salts that share a common ion with K2CO3 (KF, Cs2CO3) or salts that can react with K2CO3 forming a highly deliquescent salt (CsF, Cs2SO4). Based on the findings, we were able to design a selection procedure that can be applied to other salt hydrates considered for TCHS applications that suffer from poor kinetics and large reaction hysteresis.