Mixed magnesium, cobalt, nickel, copper, and zinc sulfates as thermochemical heat storage materials

Abstract

Thermochemical energy storage is an emerging technology being researched for harvesting waste heat and promoting integration of renewable energy in order to combat climate change. While many simple salts such as MgSO₄⋅7H₂O have been investigated thoroughly, there remains much work to be done in the domain of materials that take advantage of synergetic effects of multiple different cations located in the same crystal. To this end, a solid solution library of divalent metal sulfates of the formula M₁₋ₓM²ₓSO₄⋅nH₂O (M, M² = Mg, Co, Ni, Cu, Zn) has been synthesized. Following X-ray powder diffraction to confirm phase purity, scanning electron microscopy provided insight into particle morphology. One of the most conspicuous features was the presence of star-shaped cracks in some of the materials, which may contribute to increased surface area and enhance reaction kinetics. The simultaneous thermal analysis of the mixed salt sulfates led to several conclusions. Corresponding to the high initial dehydration barrier of NiSO₄⋅6H₂O, incorporation of nickel into other sulfates led to lower degrees of dehydration at low temperatures. The opposite effect was observed with the addition of copper. Of great interest was the surprisingly facile dehydration of hydrated Mg₀.₂₅Zn₀.₇₅SO₄, which exceeded that of both pure MgSO₄⋅7H₂O and ZnSO₄⋅7H₂O. This promising compound is one representative of three different compounds with 75 % zinc which all have the highest dehydration activity up to 100 ◦C of all compounds in the series of hydrates of M1-xZnxSO₄⋅nH₂O (M = Mg, Ni, Cu).