Synthesis, Characterization and Thermochemical Energy Storage Potential of Tetraamminecopper(II) Sulfate Monohydrate

ABSTRACT: Tetraamminecopper(II) sulfate monohydrate, [Cu(NH₃)₄]SO₄·H₂O, can be used as a thermochemical energy storage material. When heated, [Cu(NH₃)₄]SO₄·H₂O releases ammonia gas and water, leaving behind CuSO₄. When CuSO₄ is cooled and exposed to ammonia, the reverse reaction occurs, forming [Cu(NH₃)₄]SO₄ and releasing the stored heat. The reaction occurs at medium temperatures, can store a significant amount of thermal energy, and is highly reversible, allowing repeated cycles of heat storage and release without significant material degradation. This type of thermochemical energy storage can be used in various applications, particularly industrial waste heat recovery and solar thermal energy storage. In this study, tetraamminecopper(II) sulfate monohydrate was synthesized by chemical precipitation and thoroughly characterized via various techniques. Phase identification was performed by powder X-ray diffraction (PXRD) and Fourier transformed infrared spectroscopy (FTIR). The morphology of the sample was examined by scanning electron microscopy (SEM), and its chemical composition and elemental distribution were analyzed by energy-dispersive X-ray spectroscopy (EDS). Thermal properties were investigated via differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). UV-Vis diffuse reflectance spectroscopy of the solid sample revealed a broad absorption band characteristic of [Cu(NH₃)₄]SO₄·H₂O, consistent with its dark blue color. XRD and FTIR analyses confirmed that the obtained sample is [Cu(NH₃)₄]SO₄·H₂O. SEM investigation showed that the prepared material consists of agglomerated particles of varying sizes. The process of thermal decomposition of the examined tetraamine copper(II) sulfate monohydrate takes place in three steps below 350 °C, followed by two additional steps at higher temperatures. Thermochemical energy storage potential of the prepared material is assessed on the basis of operating temperature range (20–200 °C), water elimination during the initial cycle, and volume changes in the course of charging/discharging process, yielding volumetric energy storage density estimation of 382 MJ·m⁻³.