Mixed cation sulfates as thermochemical energy storage materials

Abstract

Energy security is a topic of ever-increasing importance. The harmful effects of greenhouse gas emissions and the resulting dramatic global warming have long been an impetus for reducing fossil fuel dependence, and for many countries, the urgency of the matter has become even more tangible with the war in Ukraine. A key step in the process of decreasing energy dependency is increasing energy efficiency via energy storage and waste heat capture; thermochemical energy storage is a promising solution for these applications, as it offers potentially lossless storage and higher storage densities than sensible or even latent heat storage. Thermochemical energy storage reactions can utilize the reversible thermal decomposition reaction of a material such as a salt hydrate into easily separable products to store energy indefinitely. Salt hydrates that have been extensively researched include readily available salts like the hydrates of magnesium chloride, which can decompose, thereby releasing corrosive hydrogen chloride, and magnesium sulfate, which suffers from sluggish rehydration [1]. The Vienna TCES database inspired us to turn to lesser-known salts, such as the Tutton salts, of which we synthesized and analyzed a solid solution library of 40 mixed Tutton salts K2Zn1-xMx(SO4)2⋅6H2O (M = Mg, Co, Ni, Cu) [2]. The compound library displayed several interesting trends: the reaction temperature can be increased nearly linearly by increasing the nickel content, whereas copper decreases the reaction onset temperature. Further work has been conducted on 30 mixed sulfates of the form M1-xM'xSO4⋅nH2O, and scanning electron microscopy has been conducted to shed light on particle morphology and elemental distribution. Ongoing projects aim at incorporating successful candidate materials in reactors not only for energy storage, but also for use as heat transformers.