Exploring the Electronic Structure of New Doped Salt Hydrates, Mg1–xCaxCl2·nH2O, for Thermochemical Energy Storage

Koen Heijmans, Ionut C. Tranca (Corresponding author), Silvia V. Gaastra-Nedea (Corresponding author), David M.J. Smeulders

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Abstract

Chloride-based salt hydrates, MgCl2·nH2O and CaCl2·nH2O (n = 0,1,2,4,6), are promising materials for thermochemical heat storage systems due to their high sorption energy capacity. However, both salts have their own shortcoming characteristics within the operational temperature of the thermochemical heat storage applications. While the higher hydrates of CaCl2·nH2O (n = 4,6) have a low melting point, the lower hydrates of MgCl2·nH2O (n = 0,1,2) can form the highly toxic and corrosive HCl gas. Both shortcomings cap the individual use of these salts to a restricted range of the available hydrates. A combination of these two salts showed to have the potential to overcome these shortcomings. The present study focuses on finding stable configurations of potential superior salt hydrate combinations using the evolutionary algorithm USPEX as well as manual mutations of known pristine structures. The newly found structures are less stable than the pure salts, but stable enough to be combined. Extensive electronic density-derived tools, like the Density Derived Electronic and Chemical (DDEC6) bond orders and net atomic charges, as well as Bader topological analysis, are used to predict the HCl gas formation based on the chemical environment in the new metastable structures. We find that doping MgCl2·nH2O with calcium considerably reduces HCl formation compared to its pure form, caused by a combination of the stronger Ca-Cl interaction than Mg-Cl and a less polar H2O molecule in a calcium environment than in a magnesium environment. This provides the possibility to shift the p, T-equilibrium curve of HCl outside the thermal storage operational window.

Original languageEnglish
Pages (from-to)24580-24951
JournalJournal of Physical Chemistry C
Volume124
Issue number45
DOIs
Publication statusPublished - 29 Oct 2020

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