Discovery of Rapid and Reversible Water Insertion in Rare Earth Sulfates: A New Process for Thermochemical Heat Storage

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 La₂(SO₄)₃⋅H₂O 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 La₂(SO₄)₃⋅H₂O with progressive change in hydration number x without phase change. It is also revealed that only a specific structural modification of La₂(SO₄)₃ exhibits this reversible dehydration/hydration behavior.

Water molecules can be inserted in crystalline rare earth sulfates such as La₂(SO₄)₃ without phase change, accompanied by heat release. Since the reaction proceeds rapidly and reversibly upon changing temperature and water vapor pressure in the temperature range 50–250 °C, it will be a promising process for thermal energy storage, especially for low-grade waste heat and solar energy.

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