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An Investigation Into the Synthesis and Characterisation of Metal An Investigation into the Synthesis and Characterisation of Metal Borohydrides for Hydrogen Storage by Daniel Thomas Reed A thesis submitted to the University of Birmingham For the degree of Doctor of Philosophy School of Metallurgy and Materials College of Engineering and Physical Sciences University of Birmingham October 2009 University of Birmingham Research Archive e-theses repository This unpublished thesis/dissertation is copyright of the author and/or third parties. The intellectual property rights of the author or third parties in respect of this work are as defined by The Copyright Designs and Patents Act 1988 or as modified by any successor legislation. Any use made of information contained in this thesis/dissertation must be in accordance with that legislation and must be properly acknowledged. Further distribution or reproduction in any format is prohibited without the permission of the copyright holder. SYNOPSIS With relatively high gravimetric and volumetric hydrogen storage capacities, borohydride compounds are being investigated for their potential use as hydrogen storage media. A study has been made into the mechanical milling of metal chlorides with sodium borohydride to try to form homoleptic borohydrides. Various characterisation techniques have been used to characterise the composition and microstructure of the samples, and to monitor in-situ the thermal decomposition processes. It was found that rather than homoleptic borohydrides (such as Zn(BH4)2 or Mg(BH4)2), complex borohydrides of the form AM2(BH4)5 and AM(BH4)3 (where A = Li or Na and M = Zn, Mg or Ca) tend to form. Mechanical milling of zinc chloride with sodium borohydride resulted in the formation of a covalent complex NaZn2(BH4)5. Thermal decomposition occurred at 80°C with a mass change of 12 wt.%, associated with the evolution of hydrogen and diborane. A composite mixture with magnesium hydride exhibited a large exotherm (DSC) around 90°C with a corresponding weight loss of 7.6 wt.% (TGA) corresponding to the reaction between diborane and magnesium hydride to form magnesium borohydride. The subsequent decomposition of magnesium borohydride between 260 and 360°C occurred with a weight loss of 1.5 wt.%. Mechanical milling of calcium chloride or magnesium chloride with sodium borohydride did not produce calcium borohydride and magnesium borohydride, but rather resulted in solid solutions where chlorine ions substitute for borohydride ions within the cubic sodium borohydride lattice. Thermal decomposition of milled calcium chloride and sodium borohydride occurs at a similar temperature (325°C) to that of Ca(BH4)2 (from Sigma-Aldrich), yielding the same decomposition products of CaH2, NaCl and CaB6. Milled magnesium chloride and sodium borohydride thermally decomposes via several unknown phases with a weight loss of 4.4 wt.% between 290 and 400°C to 2- yield Mg, MgB2, B, and [B12H12] , similar to the decomposition reaction reported for homoleptic magnesium borohydride. Lithium borohydride has a maximum theoretical gravimetric hydrogen storage density of 18.4 wt.%, and after decomposing at 600°C, has been shown by Orimo et al [1] to reform LiBH4 when heated to 600°C in 150 bar hydrogen. After heating lithium borohydride through its orthorhombic to hexagonal phase change (118°C) and melting point (280°C), shifts in Raman peak position and peak width were measured as a function of temperature, allowing the calculation of the energy barrier to reorientation, which agreed with other studies. A sample was also heated to 600°C (under 1 bar Ar) to decompose the sample. In the literature, a number of intermediate phases and reaction products have been predicted and observed ex-situ. This work shows the in- situ formation of lithium dodecaborane (Li2B12H12) at 340°C and amorphous boron from liquid lithium borohydride. During some measurements, an unknown phase was observed between 385 and 400°C; it is still unclear what structure the intermediate has. It is hoped that a greater understanding of the decomposition and ultimately the reformation mechanisms may lead to the development of LiBH4-based materials that can absorb and desorb hydrogen under less extreme conditions. Future work will look at the recombination of the complex borohydrides formed, with a view to reversible hydrogen storage media. This work has given a clearer understanding of the synthesis and decomposition of the complex borohydrides formed by mechanical milling. ACKNOWLEDGEMENTS Firstly, I am extremely grateful for the support and guidance of my supervisor Dr. David Book during my time here. He provided me with many helpful suggestions and important advice during the course of this work. My thanks to all the members of the hydrogen materials group past and present for your friendship, support and guidance during my 5 years as part of the group. My thanks also go to Dr Paul Anderson and Phil Chater and other members of floor 5 in chemistry for their help, especially with XRD. I also gratefully acknowledge the financial support from EPSRC to fund my project, to UKSHEC for financial support for consumables and chemicals and the opportunity to learn more about the UK hydrogen storage research program and AWM Science City for helping provide the research group with superb facilities. Gemma Nicholson, I could never have completed this PhD without your understanding, patience and love, for this I will be eternally grateful. To Jo, Barry and Amy thanks for your support. Finally to my parents, brother and sister for all the years of support and encouragement you have given me. CONTENTS 1 INTRODUCTION ...................................................................................................................... 1 1.1 Aims and Objectives of the Project ....................................................................................... 8 1.2 Scope of Thesis ...................................................................................................................... 9 2 HYDROGEN STORAGE REVIEW........................................................................................ 10 2.1 Storage of Hydrogen as a Compressed Gas ........................................................................ 14 2.2 Storage of Liquefied Hydrogen ........................................................................................... 15 2.3 Storage of Hydrogen through a Reaction with Water ........................................................ 16 2.4 Physisorption of Hydrogen on High Surface Area Materials ............................................... 18 2.5 Hydrogen Rich Chemical Compounds ................................................................................. 19 2.6 Metal Hydride Systems ....................................................................................................... 20 3 BOROHYDRIDES................................................................................................................... 23 3.1 Group 1................................................................................................................................ 26 3.1.1 LiBH4 ........................................................................................................................... 26 3.1.2 NaBH4.......................................................................................................................... 31 3.1.3 KBH4 ............................................................................................................................ 32 3.1.4 RbBH4 and CsBH4 ........................................................................................................ 33 3.2 Group 2................................................................................................................................ 34 3.2.1 Mg(BH4)2 ..................................................................................................................... 34 3.2.2 Ca(BH4)2 ...................................................................................................................... 41 3.3 Group 3................................................................................................................................ 44 3.3.1 Sc(BH4)3 ....................................................................................................................... 44 3.3.2 Y(BH4) .......................................................................................................................... 45 3.4 Group 4................................................................................................................................ 46 3.4.1 Ti(BH4)3 ....................................................................................................................... 46 3.4.2 Zr(BH4)4 ....................................................................................................................... 47 3.4.3 Hf(BH4)4 ....................................................................................................................... 48 3.5 Group 5................................................................................................................................ 48 i 3.5.1 V(BH4)3 ........................................................................................................................ 48 3.5.2 Nb(BH4) ....................................................................................................................... 49 3.6 Group 6...............................................................................................................................
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