Modeling Water Transport in Prototype Thermal Energy Storage Materials: Potassium Carbonate and Potassium Carbonate Sesquihydrate

Abstract K2CO3 is a promising thermochemical energy storage material due to its ability to absorb and release heat through reversible hydration reactions. Nevertheless, the implementation of K2CO3 and other salts that undergo hydration is constrained by their slow reaction kinetics. Experimentally, these sluggish kinetics are reflected in the existence of a metastable zone near the hydration/dehydration equilibrium line. However, the atomistic mechanisms that limit reaction rates in these materials are incompletely understood. Here, first-principles calculations are employed to investigate the thermodynamics and kinetics of hydration and dehydration in bulk K2CO3 and its hydrate, K2CO3·1.5H2O. The energetics and site preferences for water insertion into K2CO3 at the early stages of hydration are predicted, as are the formation energies for water vacancies at the outset of K2CO3·1.5H2O dehydration. The energetics for water desorption in the hydrate are consistent with experimental observations of a two-step dehydration process. Several water diffusion pathways are explored in the bulk for both the anhydrous and hydrated phases. The calculated free-energy migration barriers reveal that in anhydrous K2CO3 diffusion occurs preferentially along [1̄2̄0] and [12̄0] directions, parallel to the carbonate planes. In the hydrate, vacancy-mediated water migration is also anisotropic, with the lowest barriers observed for migration within water planes in the [001] direction. The calculated diffusion coefficients suggest that particle sizes on the order tens of microns are necessary to achieve full (de)hydration under expected (dis)charging times. In sum, this study provides insights into the atomic scale energetics and mechanisms underlying hydration reactions for thermal energy storage.