MECHANICAL SYNTHESIS OF MAGNESIUM ALLOYS FOR HYDROGEN STORAGE by Luis Felipe Contreras Vásquez 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Sciences University of Birmingham September 2017 University of Birmingham Research Archive e-theses repository This unpublished thesis/dissertation is copyright of the author and/or third parties.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 Synthesis, characterisation and hydrogen sorption properties of a variety of magnesium based hydrides were investigated in this work. The structure of these composites was studied using X-ray diffraction (XRD) and Raman spectroscopy. The thermal stability and decomposition reactions of the Mg-based hydrides was studied using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), coupled with a mass spectrometer (MS) to determine the gaseous products released during heating. Compositional changes and reversibility were investigated in detail using in-situ XRD under Ar and H2. Mechanical milling of magnesium hydride (MgH2) under Ar and H2 resulted in a reduction of the crystallite size from 207 nm for the as received to 18 nm for 10 h milled MgH2. For the first time was reported the effect of Raman spectroscopy on milled MgH2 demonstrating that milled samples were Raman active. Hydrogen desorption temperatures were decreased ~120 °C (DSC) with increasing milling time (10 h), however, hydrogen capacity was decreased ~6.55 wt. % (TGA). Hydrogenation of Li-Mg alloy was investigated via reactive milling in 100 bar H2, after 1 h milling Li-Mg phase was hydrogenated forming LiH and MgH2. Hydrogen desorption was observed at 250 °C (DSC), releasing 0.19 wt. % (TGA). Although, the desorption temperature was decreased, the amount of hydrogen released is insignificant and is hard to consider for on- board applications. Mechanical milling of sodium hydride (NaH) and magnesium hydride (MgH2) under Ar and H2 lead to the formation of sodium magnesium ternary hydride (NaMgH3). Thermal decomposition occurred at ~ 325 °C with a mass change of 5 wt. %, associated with the evolution of hydrogen. Hydrogen desorption occurred in two-step reactions. Rehydrogenation of the NaMgH3 hydride was observed at 250 °C and 10 bar H2. Mechanical milling of lithium hydride (LiH) substituted into NaMgH3 hydride resulted in the formation of a quaternary LixNa1-xMgH3 hydride with molar compositions (x=0, 0.2, 0.5, 0.8). Thermal decomposition started at 250 °C, releasing a total amount of 5 wt% of H2. Decomposition reactions occurred in two and three steps. Furthermore, reversibility of the main phase was achieved at 250 °C and 10 bar H2. Milling calcium hydride (CaH2) and MgH2 lead to the formation of calcium magnesium (Ca- Mg-H) ternary hydride. Hydrogen sorption characteristics showed a dehydrogenation temperature of 325 °C (DSC) with a total amount of 2.24 wt.% H2 evolution up to 500 °C. However, dehydrogenation of CaH2 was not completed even at 500 °C. Thermal decomposition suggested two steps reactions. Reversibility was successfully achieved at 365 °C and 10 bar H2. LiH and NaH were substituted into the Ca-Mg-H to form quaternary hydrides with composition MxCa1-xMgH4. Hydrogen sorption properties showed desorption temperatures between 300 °C and 385°C with a maximum of 3.5 wt.% H2 released. Thermal decomposition proceeded in three-step reaction. Nonetheless, complete dehydrogenation was not achieved. Overall, this investigation has demonstrated for a variety of Mg-based hydrides that reducing crystallite size has a positive effect in the sorption properties, unfortunately, none of the materials and composites investigated in this work meet the targets for light-duty vehicles set out by the Department of Energy (DoE). However, other applications such as heat storage might be of interest. ACKNOWLEDGEMENTS First of all, I would like to thank Almighty GOD for the opportunity to be at this stage of my life, successfully accomplishing one more goal, embarking into a new dream. Thank you for the wisdom, perseverance, love and faith given through Jesus Christ and the Holy Spirit to walk firmly and not tumble. All the Glory Must be to The Lord. Special thanks dedicated to you my beloved wife, for your patience, love, support, and sacrifices. For leaving everything and everyone behind, to help me to accomplish this degree. I can certainly say that without you by my side, I would not be able to be where I am now. Thank you for giving me the best gift one can dream of, a beautiful angel that completely changed my perception of what life is. I love you both, and I am grateful for the time, love and laughs shared with me during these years. I am blessed to have you. Thank you to my family for the love, support, and prayers. You encouraged me to keep going forward no matter what, backed me up as a family and showed me the importance of having GOD in my life. Thank you to my supervisor Professor David Book for his guidance and support on every stage of this work and for the opportunity to be part of the Hydrogen group at the University of Birmingham. Thank you to Dr. Reed, Mr. Simon Cannon, Dr. Joshua Vines, Dr. Luke Hughes, Dr. Sheng Guo and all my fellows at hydrogen group, for their help and share knowledge of the equipment. Thank you for all the food shared during our social hours. I also would like to extend my thanks to the Government of Ecuador through the SENESCYT for the grant awarded to study my postgraduate degree at the University of Birmingham. CONTENTS 1. INTRODUCTION ................................................................................................................ 1 1.1 Introduction ..................................................................................................................... 1 1.2 Hydrogen as Energy vector ............................................................................................. 3 1.3 Hydrogen Storage ............................................................................................................ 8 1.3.1 Physical-based storage ................................................................................................. 13 1.3.1.1 Compressed Gas Hydrogen Storage ...................................................................... 13 1.3.2 Material-based Storage ................................................................................................ 15 1.3.2.1 Physisorption ......................................................................................................... 15 1.3.2.3 Complex Hydrides................................................................................................. 16 1.3.2.4 Metal hydrides ....................................................................................................... 17 1.4 Summary ............................................................................................................................. 21 2. Mg-BASED HYDRIDES ..................................................................................................... 24 2.1 Introduction ........................................................................................................................ 24 2.2 Mg/MgH2 Structures ........................................................................................................... 25 2.3 Hydrogen Storage Properties .............................................................................................. 27 2.3.1 Kinetics ........................................................................................................................ 27 2.3.1.1 Effect of Microstructural Modification ................................................................. 27 2.3.1.2 Catalysis and additives .......................................................................................... 28 2.3.2 Thermodynamic behaviour .......................................................................................... 30 2.3.3 Ternary Hydrides ......................................................................................................... 34 2.3.3.1 Na-Mg-H (Sodium Magnesium Hydride) ............................................................. 35 2.3.3.2 Ca-Mg-H (Calcium Magnesium Hydrides)........................................................... 37 2.3.3.3 Li-Mg-H (Lithium Magnesium Hydrides) ............................................................ 38 2.3.4 Destabilisation of ternary hydrides by light-weight metals substitution. .................... 40 2.3.4.1 Li substitution into Na-Mg-H hydride .................................................................. 40 2.3.4.2 Li and Na substitution into Ca-Mg-H hydrides ..................................................... 42 2.4 Magnesium-based hydrides applications. ........................................................................... 42 2.5 Summary ............................................................................................................................. 44 2.6 Aims and Objectives ..........................................................................................................