Impact of Atmospheric CO₂ on Thermochemical Heat Storage Capabilities of K₂CO₃

This work investigates the reactions occurring in K₂CO₃–H₂O–CO₂ under ambient CO₂ pressures in temperature and vapor pressure ranges applicable for domestic thermochemical heat storage. The investigation shows that depending on reaction conditions, the primary product of a reaction is K₂CO₃·1.5H₂O, K₂CO₃·2KHCO₃·1.5H₂O, or a mixture of both. The formation of K₂CO₃·1.5H₂O is preferred far above the equilibrium conditions for the hydration reaction. On the other hand, the formation of double salt is preferred at conditions where hydration reaction is inhibited or impossible, as the thermogravimetric measurements identified a new phase transition line below the hydration equilibrium line. The combined X-ray diffraction, thermogravimetric analysis, and Fourier-transform infrared spectroscopy study indicates that this transition line corresponds to the formation of K₂CO₃·2KHCO₃, which was not observed in any earlier study. In view of thermochemical heat storage, the formation of K₂CO₃·2KHCO₃·(1.5H₂O) increases the minimum charging temperature by approximately 40 °C. Nevertheless, the energy density and cyclability of the storage material can be preserved if the double salt is decomposed after each cycle.