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    Thermodynamics and performance of the Mg-H-F system for thermochemical energy storage applications

    Access Status
    Fulltext not available
    Authors
    Tortoza, M.
    Humphries, Terry
    Sheppard, Drew
    Paskevicius, Mark
    Rowles, Matthew
    Sofianos, M.
    Aguey-Zinsou, K.
    Buckley, Craig
    Date
    2018
    Collection
    • Curtin Research Publications
    Type
    Journal Article
    Metadata
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    Abstract

    © 2018 the Owner Societies. Magnesium hydride (MgH 2 ) 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 MgH 2 to form a range of Mg(H x F 1-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(H 0.85 F 0.15 ) 2 having a maximum rate of H 2 desorption at 434 °C, with a practical hydrogen capacity of 4.6 ± 0.2 wt% H 2 (theoretical 5.4 wt% H 2 ). An extremely stable Mg(H 0.43 F 0.57 ) 2 phase is formed upon the decomposition of each Mg-H-F composition of which the remaining H 2 is not released until above 505 °C. PCI measurements of Mg(H 0.85 F 0.15 ) 2 have determined the enthalpy (?H des ) to be 73.6 ± 0.2 kJ mol -1 H 2 and entropy (?S des ) to be 131.2 ± 0.2 J K -1 mol -1 H 2 , which is slightly lower than MgH 2 with ?H des of 74.06 kJ mol -1 H 2 and ?S des = 133.4 J K -1 mol -1 H 2 . Cycling studies of Mg(H 0.85 F 0.15 ) 2 over six absorption/desorption cycles between 425 and 480 °C show an increased usable cycling temperature of ~80 °C compared to bulk MgH 2 , increasing the thermal operating temperatures for technological applications.

    Citation
    Tortoza, M. and Humphries, T. and Sheppard, D. and Paskevicius, M. and Rowles, M. and Sofianos, M. and Aguey-Zinsou, K. et al. 2018. Thermodynamics and performance of the Mg-H-F system for thermochemical energy storage applications. Physical Chemistry Chemical Physics. 20 (4): pp. 2274-2283.
    Source Title
    Physical Chemistry Chemical Physics
    URI
    http://hdl.handle.net/20.500.11937/65723
    DOI
    10.1039/c7cp07433f
    Department
    School of Electrical Engineering, Computing and Mathematical Science (EECMS)

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