Experimental Study of Hydration/Dehydration Behaviors of Metal Sulfates M2(SO4)3 (M = Sc, Yb, Y, Dy, Al, Ga, Fe, In) in Search of New Low-Temperature Thermochemical Heat Storage Materials

To identify potential low-temperature thermochemical heat storage (TCHS) materials, hydration/dehydration reactions of M-2(SO4)(3) (M = Sc, Yb, Y, Dy, Al, Ga, Fe, In) are investigated by thermogravimetry (TG). These materials have the same rhombohedral crystal structure, and one of them, rhombohedral Y-2(SO4)(3), has been recently proposed as a promising material. All M-2(SO4)(3)center dot xH(2)O hydrate/dehydrate reversibly between 30 and 200 degrees C at a relatively low p(H2O) (=0.02 atm). Among them, rare-earth (RE) sulfates RE2(SO4)(3)center dot xH(2)O (RE = Sc, Yb, Y, Dy) show narrower thermal hystereses (less than 50 degrees C), indicating that they have faster reaction rates than the other sulfates M-2(SO4)(3)center dot xH(2)O (M = Al, Ga, Fe, In). As for the heat storage density, Y-2(SO4)(3)center dot xH(2)O is most promising due to the largest mass change (>10 mass % anhydrous basis) during the reactions. This is larger than that of the existing candidate CaSO4 center dot 0.5H(2)O (6.6 mass % anhydrous basis). Regarding the reaction temperature of the water insertion into rhombohedral RE2(SO4)(3) (RE = Yb, Y, Dy) to form RE2(SO4)(3)center dot H2O, it increases as the ionic radius of RE3+ becomes larger. Since such a relationship is also observed in beta-RE2(SO4)(3)center dot xH(2)O, RE(OH)(3), and REPO4 center dot xH(2)O, this empirical knowledge should be useful to expect the dehydration/hydration reaction temperatures of the RE compounds.