Physicochemical Characterisation of a Na-H-F Thermal Battery Ma- terial Terry D. Humphries,* † Aditya Rawal, ‡ Matthew R. Rowles, †∫ Christopher R. Prause, † Julianne E. Bird, † Mark Paskevicius, † M. Veronica Sofianos, † Craig. E. Buckley † † Physics and Astronomy, Fuels and Energy Technology Institute, Curtin University, GPO Box U1987, Perth, WA 6845, Australia. ‡ Nuclear Magnetic Resonance Facility, Mark Wainwright Analytical Centre, The University of New South Wales, Sydney, NSW 2052, Australia. ∫ John De Laeter Centre, Curtin University, GPO Box U1987, Perth, WA 6845, Australia. ABSTRACT: Fluorine-substituted sodium hydride is investigated for application as a thermal energy storage material inside ther- mal batteries. A range of compositions of NaHxF1−x (x = 0, 0.5, 0.7, 0.85, 0.95, 1) have been studied using synchrotron radiation powder X-ray diffraction (SR-XRD), near edge X-ray absorption fine structure spectroscopy (NEXAFS), and nuclear magnetic resonance spectroscopy (NMR), with the thermal conductivity and melting points also being determined. SR-XRD and NMR spec- troscopy studies identified that the solid solutions formed during synthesis contain multiple phases rather than a single stoichio- metric compound, despite the materials exhibiting a single melting point. As the fluorine content of the materials increases, the Na−H(F) bond length decreases, increasing the stability of the compound. This trend is also observed during the melting point analysis where increasing the fluorine content increases the melting point of the material, i.e. x < 0.3 (i.e. F − > 0.7) enables melting at temperatures above 750 °C. 1. INTRODUCTION The future of global energy supply and demand is consist- ently on the political and economic agenda. At the same time, governments are realising that hydrogen must be in the frame due to its high energy density, and relative ease of production and storage compared to alternative energy storage media. 1 Currently, high-density storage is one of the key challenges to overcome for the hydrogen economy, although metal hydrides are able to reversibly store hydrogen at an ever decreasing cost. The reaction of metal with hydrogen is an exothermic reaction and much research has been conducted to use metal hydrides as thermal energy stores, also known as thermal bat- teries. 2-6 Metal hydrides are materials that have potential for multiple technological applications, including as a fuel for vehicles and infrastructure, thermal and electrical batteries, and smart windows. 4, 6-14 These applications require drastically diverse physical properties from the hydride, although the prevailing characteristic is that hydrogen must be stored re- versibly to be cost effective. Thermal batteries can be designed to store waste heat pro- duced during industrial processes (e.g. smelting plants or re- fineries) or from concentrating solar power, where the stored heat can be released upon demand to produce electricity. 4, 6, 15- 16 To optimise thermal-to-electrical conversion efficiency, the thermal batteries should have high operating temperatures of > 600 °C, although low-temperature applications, such as cen- tralised heating/cooling, solar cooking, or greenhouse heating have also been identified. 15-16 Three methods of thermal ener- gy storage have been identified: (i) sensible, (ii) latent, and (iii) thermochemical heat storage. A full review of these meth- ods has been previously undertaken, 17-18 identifying thermo- chemical heat storage as the most energy dense. During ther- mochemical heat storage the heat is stored by way of an endo- thermic process of breaking chemical bonds between elements (e.g Na−H), whilst heat is released during the exothermic pro- cess of forming bonds. An example of this process is the re- versible hydrogenation of metals, of which a host of metal hydrides have been reported to be potential materials for high- temperature thermal batteries, including NaH, MgH2, NaMgH3, FeTiHx, CaH2, CaH2/2Al and Mg2FeH6. Lab-scale prototypes have also been developed to illustrate the potential of thermal batteries. 2-6, 11-12, 19-25 An inherent problem with employing Na or Mg as the base metal in a thermal battery is their low vapour pressure, which causes the metal to segregate during cycling at elevated oper- ating temperatures. 19, 24 A recent method to inhibit metal seg- regation, and to also increase the operating temperatures and/or reduce the hydrogen operating pressure, is to stoichio- metrically substitute F − for H − . A variety of these systems have been explored and physically characterised including NaMgH2F, 26 NaHxF1−x, and Mg(HxF1−x)2 (x = 0.5, 0.7, 0.85, 0.95, 1). 19, 24 In these systems, segregation is inhibited by the formation of the metal fluoride during hydrogen release (e.g. NaMgF3, NaF or MgF2, respectively). Some segregation is reported to occur at higher temperatures, although further in- vestigations have shown that encapsulation of the powder in iron tubes is beneficial. 19 Despite the evaporative problems associated with NaHxF1−x upon dehydrogenation, the physical properties displayed by this material still make it attractive as a thermal battery mate-