Multi-step hydration/dehydration mechanisms of rhombohedral Y 2 (SO 4 ) 3 : a candidate material for low-temperature thermochemical heat storage.

To evaluate rhombohedral Y 2 (SO 4 ) 3 as a new potential material for low-temperature thermochemical energy storage, its thermal behavior, phase changes, and hydration/dehydration reaction mechanisms are investigated. Rhombohedral Y 2 (SO 4 ) 3 exhibits reversible hydration/dehydration below 130 °C with relatively small thermal hysteresis (less than 50 °C). The reactions proceed via two reaction steps in approximately 0.02 atm of water vapor pressure, i.e. "high-temperature reaction" at 80-130 °C and "low-temperature reaction" at 30-100 °C. The high-temperature reaction proceeds by water insertion into the rhombohedral Y 2 (SO 4 ) 3 host structure to form rhombohedral Y 2 (SO 4 ) 3 · x H 2 O ( x = ∼1). For the low-temperature reaction, rhombohedral Y 2 (SO 4 ) 3 · x H 2 O accommodates additional water molecules ( x > 1) and is eventually hydrated to Y 2 (SO 4 ) 3 ·8H 2 O (monoclinic) with changes in the host structure. At a water vapor pressure above 0.08 atm, intermediate Y 2 (SO 4 ) 3 ·3H 2 O appears. A phase stability diagram of the hydrates is constructed and the potential usage of Y 2 (SO 4 ) 3 for thermal energy upgrades is assessed. The high-temperature reaction may act similarly to an existing candidate, CaSO 4 ·0.5H 2 O, in terms of reaction temperature and water vapor pressure. Additionally, the hydration of rhombohedral Y 2 (SO 4 ) 3 · x H 2 O to Y 2 (SO 4 ) 3 ·3H 2 O should exhibit a larger heat storage capacity. With respect to the reaction kinetics, the initial dehydration of Y 2 (SO 4 ) 3 ·8H 2 O to rhombohedral Y 2 (SO 4 ) 3 introduces a microstructure with pores on the micron order, which might enhance the reaction rate.