Thermal energy storage based on chemical reactions is a prospective technology for the reduction of fossil-fuel consumption by storing and using waste heat. For widespread application, a critical challenge is to identify appropriate reversible reactions that occur below 250 °C, where abundant low-grade waste heat and solar energy might be available. Here, it is shown that lanthanum sulfate monohydrate La (SO ) ⋅H O undergoes rapid and reversible dehydration/hydration reactions in the temperature range from 50 to 250 °C upon heating/cooling with remarkably small thermal hysteresis (<50 °C), and thus it emerges as a new candidate system for thermal energy storage. Thermogravimetry and X-ray diffraction analyses reveal that the reactions proceed through an unusual mechanism for sulfates: water is removed from, or inserted in La (SO ) ⋅H O with progressive change in hydration number x without phase change. It is also revealed that only a specific structural modification of La (SO ) exhibits this reversible dehydration/hydration behavior.

The fast hydration/dehydration (water insertion/deinsertion) reaction of β-La2(SO4)3 is expected to be applied to thermochemical heat storage, but its kinetics have not been well understood.

The stable structure and diffusion mechanism of water in lanthanum sulfate La2(SO4)3 have been theoretically analyzed in a first-principles manner based on the density functional theory (DFT).

To evaluate rhombohedral Y2(SO4)3 as a new potential material for low-temperature thermochemical energy storage, its thermal behavior, phase changes, and hydration/dehydration reaction mechanisms are investigated.

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.