Multi-step hydration/dehydration mechanisms of rhombohedral Y2(SO4)3: a candidate material for low-temperature thermochemical heat storage

Abstract

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. Rhombohedral Y₂(SO₄)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 Y₂(SO₄)₃ host structure to form rhombohedral Y₂(SO₄)₃·xH₂O (x = ∼1). For the low-temperature reaction, rhombohedral Y₂(SO₄)₃·xH₂O accommodates additional water molecules (x > 1) and is eventually hydrated to Y₂(SO₄)₃·8H₂O (monoclinic) with changes in the host structure. At a water vapor pressure above 0.08 atm, intermediate Y₂(SO₄)₃·3H₂O appears. A phase stability diagram of the hydrates is constructed and the potential usage of Y₂(SO₄)₃ for thermal energy upgrades is assessed. The high-temperature reaction may act similarly to an existing candidate, CaSO₄·0.5H₂O, in terms of reaction temperature and water vapor pressure. Additionally, the hydration of rhombohedral Y₂(SO₄)₃·xH₂O to Y₂(SO₄)₃·3H₂O should exhibit a larger heat storage capacity. With respect to the reaction kinetics, the initial dehydration of Y₂(SO₄)₃·8H₂O to rhombohedral Y₂(SO₄)₃ introduces a microstructure with pores on the micron order, which might enhance the reaction rate.