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Issue 4, 2018
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Thermodynamics and performance of the Mg–H–F system for thermochemical energy storage applications

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Abstract

Magnesium hydride (MgH2) is a hydrogen storage material that operates at temperatures above 300 °C. Unfortunately, magnesium sintering occurs above 420 °C, inhibiting its application as a thermal energy storage material. In this study, the substitution of fluorine for hydrogen in MgH2 to form a range of Mg(HxF1−x)2 (x = 1, 0.95, 0.85, 0.70, 0.50, 0) composites has been utilised to thermodynamically stabilise the material, so it can be used as a thermochemical energy storage material that can replace molten salts in concentrating solar thermal plants. These materials have been studied by in situ synchrotron X-ray diffraction, differential scanning calorimetry, thermogravimetric analysis, temperature-programmed-desorption mass spectrometry and Pressure–Composition–Isothermal (PCI) analysis. Thermal analysis has determined that the thermal stability of Mg–H–F solid solutions increases proportionally with fluorine content, with Mg(H0.85F0.15)2 having a maximum rate of H2 desorption at 434 °C, with a practical hydrogen capacity of 4.6 ± 0.2 wt% H2 (theoretical 5.4 wt% H2). An extremely stable Mg(H0.43F0.57)2 phase is formed upon the decomposition of each Mg–H–F composition of which the remaining H2 is not released until above 505 °C. PCI measurements of Mg(H0.85F0.15)2 have determined the enthalpy (ΔHdes) to be 73.6 ± 0.2 kJ mol−1 H2 and entropy (ΔSdes) to be 131.2 ± 0.2 J K−1 mol−1 H2, which is slightly lower than MgH2 with ΔHdes of 74.06 kJ mol−1 H2 and ΔSdes = 133.4 J K−1 mol−1 H2. Cycling studies of Mg(H0.85F0.15)2 over six absorption/desorption cycles between 425 and 480 °C show an increased usable cycling temperature of ∼80 °C compared to bulk MgH2, increasing the thermal operating temperatures for technological applications.

Graphical abstract: Thermodynamics and performance of the Mg–H–F system for thermochemical energy storage applications

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Publication details

The article was received on 03 Nov 2017, accepted on 19 Dec 2017 and first published on 19 Dec 2017


Article type: Paper
DOI: 10.1039/C7CP07433F
Citation: Phys. Chem. Chem. Phys., 2018,20, 2274-2283
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    Thermodynamics and performance of the Mg–H–F system for thermochemical energy storage applications

    M. S. Tortoza, T. D. Humphries, D. A. Sheppard, M. Paskevicius, M. R. Rowles, M. V. Sofianos, K. Aguey-Zinsou and C. E. Buckley, Phys. Chem. Chem. Phys., 2018, 20, 2274
    DOI: 10.1039/C7CP07433F

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