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

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