Magnesium and calcium hydroxides are promising for thermochemical heat storage (THS) and chemical heat pumping since they undergo a reversible reaction of de-/rehydration. Pure Mg(OH)2 and Ca(OH)2 lose water with the rate acceptable for THS at T > 300oC and 400oC, respectively, which restricts a range of possible sources of surplus heat to be stored. The dehydration reactivity enhancement is desirable because it can increase the TES efficiency.
In the present work, the effect of inorganic salts of alkali metals on de-/rehydration of Mg(OH)2 and Ca(OH)2 is studied. Mixtures of Mg(OH)2 and Ca(OH)2 with various inorganic salts (XCl, XNO3 and XOAc, X2SO4, where X = Li, Na, K) were prepared and studied. The salt screening showed that modification with nitrates, acetates and LiCl results in considerable (30-80oC) decrease in the Mg(OH)2 dehydration temperature (Td). Lithium nitrate (LiNO3) was found to have the most profound effect. Potassium nitrate (KNO3) had the maximal effect on Ca(OH)2 (~35oC). The study of LiNO3/Mg(OH)2 and KNO3/Ca(OH)2 with the salt content Y = 0.5-20 wt. % reveals that the Td-decrease depends on the salt content and can be intently varied.
The dehydration kinetics was measured at P(H2O) = 24 mbar that is typical for closed THS cycle. The study showed that the dehydration of XNO3/Mg(OH)2 (X = Li, Na) at T = 230-280oC and KNO3/Ca(OH)2 at T = 380-400oC is considerably faster as compared to the pure hydroxides. The rehydration kinetics study showed lower half-conversion times for salt-modified materials.
The de- and rehydration were studied by XRD, vibrational spectroscopy, HRTEM and NMR methods. The pathways of the salts effect on the dehydration of the hydroxides were discussed. One of the possible explanations of dehydration temperature depression is a drastic decrease of the surface area of the resulting oxides which brings the system closer to the calculated equilibrium of bulk phases.
Thus, the screening of various salts showed the maximal effect of LiNO3 on Mg(OH)2 (~80oC) and and KNO3 on Ca(OH)2 (~35oC) dehydration. The study of de- and rehydration kinetics showed that the new composites may be promising for THS. The mechanism of the salt effect is discussed.