LIGHT METAL BOROHYDRIDES AND Mg-BASED HYDRIDES FOR HYDROGEN STORAGE by SHENG GUO 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Sciences University of Birmingham December 2014 University of Birmingham Research Archive e-theses repository This unpublished thesis/dissertation is copyright of the author and/or third parties.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 This work has investigated structural and compositional changes in LiBH4, Mg(BH4)2, Ca(BH4)2, LiBH4-Ca(BH4)2 during heating. The crystal and vibrational structures of these borohydrides/composites were characterized using lab-based X-ray diffraction (XRD) and Raman spectroscopy, with particular attention to the frequency/width changes of Raman vibrations of different polymorphs of borohydrides. The thermal stability and decomposition pathway of the borohydrides was studied in great detail mainly using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), in/ex situ XRD and Raman measurements, whilst the gaseous products during heating were monitored using a mass spectrometry (MS). Hydrogen is the main decomposition gaseous product from all of these compounds, but in some cases a very small amount of diborane release was also detected. These studies suggest that the thermal decomposition of the metal borohydrides occurs via a wide range of reaction pathways, often in several steps, which may involve simultaneous, and in some cases coupled, competing reactions. This can include the formation of stable borane intermediates/by-products which largely precludes the possibility of reversibility. Furthermore, the role of diborane in the decomposition and formation of borohydrides, was later studied by heating metal borohydrides (or hydrides) to various temperatures in a gaseous diborane-hydrogen atmosphere; and different types of borane products were observed. Attempts to synthesise Mg-B-Ti-H ternary hydrides which can store up to 10.6 wt% H2 in a bulk powder form, were made in this work, by ball-milling mixtures of 0.4MgH2+0.6B, and 0.36MgH2+0.58B+0.06TiX (TiX = Ti, TiH2 or TiCl3) in 100 bar H2. As a result, a single-phase material has not yet been produced, as the composition and ball-milling conditions need to be further optimised. One of the MgH2-B-Ti samples exhibited a 110 °C reduction in hydrogen desorption temperature and accelerated hydrogen sorption kinetics. Finally, high-energy reactive milling of a Li-Mg ribbon alloy was carried out in 100 bar H2, to attempt to directly insert H atoms into the sites of the ordered Li-Mg alloy phase. After milling, the majority of the Li-Mg phase was hydrogenated to form LiH and Mg(1-x)LixH2 (x = ~0.04) phases. In addition, MS shows a small release of H2 at around 425 °C, which might suggest the formation of a minor Li-Mg-H phase. Although this dehydrogenation temperature is too high for mobile hydrogen storage, it may have properties that are of interest for other applications such as heat storage and electrodes in lithium batteries. In summary, this investigation has showed for a number of light metal borohydrides and Mg-based hydride materials with good hydrogen storage capacities, that the hydrogen sorption process(es) is very complex. A better understanding of constituent phases and reaction pathways for sorption reversibility is required, before practical hydrogen media can be produced. Acknowledgements I owe a great deal of thanks to many people for their kind help during my PhD studies. First and foremost many thanks to my supervisor Professor David Book for his invaluable guidance and support on everything from experimental work to writing thesis. He is always generous with his time providing me with many inspiring discussions and much advice. I highly appreciate that he has given me the opportunities to participate in international scientific conferences, allowing me to learn a lot from other world-leading researchers and develop network for potential collaborations. Thanks to Dr Daniel Reed and Mr Simon Cannon for technical assistances. This work would not be completed without their help. Thanks to Dr Ruixia Liu for the kindest support in the first place and offering me a place to live for over three years. I would like to thank to all other colleagues in the Hydrogen Materials Group, both past and present, for their help, contributions and friendships. I would also like to thank to University of Birmingham and Ilika Ltd., for providing funding and equipment in order to make this work possible. Special thanks go to Dr Jean-Philippe Soulié for helpful suggestions and encouraging comments on our collaboration projects. Finally, I would like to thank to my family and friends for their continued support and encouragement! List of Abbreviations ATR Attenuated Total Reflectance BEV Battery-electric Vehicle CCD Charge-Coupled Device CcH2 Cryo-compressed Hydrogen CGH2 Compressed Gaseous Hydrogen DFT Density Functional Theory DOE US Department of Energy DSC Differential Scanning Calorimetry FCEV Fuel Cell Electric Vehicle FTIR Fourier Transform Infrared ICE Internal Combustion Engine ICSD Inorganic Crystal Structure Database IEA International Energy Agency LH2 Liquid Hydrogen MOF Metal-Organic Framework MS Mass Spectrometry NMR Nuclear Magnetic Resonance PCT Pressure-Composition-Temperature PEC Photo-electrochemical PEMFC Proton Exchange Membrane Fuel Cell PLD Pulsed Laser Deposition PSD Position Sensitive Detector SEM Scanning Electron Microscope SMR Steam Methane Reformation TEM-EELS Transmission Electron Microscopy - Electron Energy Loss Spectroscopy TGA Thermogravimetric Analysis TPD Temperature Programmed Desorption XRD X-ray Diffraction Contents 1 Introduction ....................................................................................................... 1 1.1 Hydrogen production ............................................................................................... 2 1.1.1 Natural gas reformation ................................................................................ 2 1.1.2 Water electrolysis ......................................................................................... 3 1.1.3 Emerging processes ..................................................................................... 4 1.2 Hydrogen as a fuel ................................................................................................... 5 1.2.1 Hydrogen internal combustion engines (HICE) ............................................ 5 1.2.2 Hydrogen fuel cells ....................................................................................... 5 1.2.3 Fuel cell or battery electric vehicles at a glance ........................................... 7 1.3 Hydrogen storage .................................................................................................... 8 1.3.1 Gaseous and liquid storage .......................................................................... 9 1.3.2 Materials-based hydrogen storage ............................................................. 10 1.3.3 Summary .................................................................................................... 15 2 Metal borohydrides and Mg-based hydrides ................................................ 17 2.1 Metal borohydrides ................................................................................................ 17 2.1.1 Structures ................................................................................................... 17 2.1.2 Thermodynamics and decomposition process ........................................... 20 2.2 Alkali metal borohydrides ....................................................................................... 23 2.2.1 LiBH4 ........................................................................................................... 23 2.2.2 NaBH4 ......................................................................................................... 32 2.2.3 KBH4 ........................................................................................................... 34 2.3 Alkaline earth metal borohydrides ......................................................................... 35 2.3.1 Mg(BH4)2 ..................................................................................................... 35 2.3.2 Ca(BH4)2 ..................................................................................................... 43 2.4 Other borohydride systems .................................................................................... 50 2.4.1 Single-cation borohydrides ......................................................................... 50 2.4.2 Mixed-cations borohydrides ........................................................................ 51 2.4.3 Eutectic borohydride system ....................................................................... 54 2.5 Role of diborane in the H2 cycling of borohydrides ................................................ 58 2.5.1 Diborane, B2H6 ...........................................................................................