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

Abstract

To identify potential low-temperature thermochemical heat storage (TCHS) materials, hydration/dehydration reactions of M 2 (SO 4 ) 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 (SO 4 ) 3 , has been recently proposed as a promising material. All M 2 (SO 4 ) 3 •xH 2 O hydrate/dehydrate reversibly between 30 and 200 °C at a relatively low p H 2 O (=0.02 atm). Among them, rare-earth (RE) sulfates RE 2 (SO 4 ) 3 •xH 2 O (RE = Sc, Yb, Y, Dy) show narrower thermal hystereses (less than 50 °C), indicating that they have faster reaction rates than the other sulfates M 2 (SO 4 ) 3 •xH 2 O (M = Al, Ga, Fe, In). As for the heat storage density, Y 2 (SO 4 ) 3 •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 CaSO 4 •0.5H 2 O (6.6 mass % anhydrous basis). Regarding the reaction temperature of the water insertion into rhombohedral RE 2 (SO 4 ) 3 (RE = Yb, Y, Dy) to form RE 2 (SO 4 ) 3 •H 2 O, it increases as the ionic radius of RE 3+ becomes larger. Since such a relationship is also observed in β-RE 2 (SO 4 ) 3 •xH 2 O, RE(OH) 3, and REPO 4 •xH 2 O, this empirical knowledge should be useful to expect the dehydration/hydration reaction temperatures of the RE compounds.

Fourier-Transform Infrared Analysis of the Dehydration Mechanism of Mg(OH)2 and Chemically Modified Mg(OH)2

“…To date, several reports on CHS materials have been published. Salt hydrates, such as MgCl 2 ·6H 2 O, , LiOH·H 2 O, and metal sulfates, ,− have been investigated as CHS materials in the relatively low temperature range of 100–250 °C. In contrast, Mg­(OH) 2 , − Ca­(OH) 2 , ,− and MgCO 3 are suitable for thermal energy storage at 200–500 °C and have been explored owing to their high energy density and reversibility.…”