SYNTHESIS, CHARACTERISATION AND DECOMPOSITION PROPERTIES OF MANGANESE-BASED BOROHYDRIDES FOR HYDROGEN STORAGE by RUIXIA LIU 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Physical Sciences University of Birmingham July 2012 University of Birmingham Research Archive e-theses repository This unpublished thesis/dissertation is copyright of the author and/or third parties.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 Manganese borohydride has a theoretical hydrogen content of 9.5 wt% and relatively low decomposition temperature of 130-180 °C , making it of interest for hydrogen storage. Mechanochemical milling is a viable synthesis route where alkali metal borohydrides react via a metathesis reaction with manganese chloride. In this work, a series of manganese-based borohydrides were synthesized by mechanochemical milling of the xABH4-MnCl2 (A = Li, Na, K; x = 2 or 3) samples under argon. The structural and thermal decomposition properties of the borohydride compounds were investigated using XRD, Raman spectroscopy, FTIR, TGA-MS, DSC, in-situ XRD and in-situ Raman spectroscopy. Samples prepared by ball-milling xLiBH4-MnCl2 (x = 2 or 3) resulted in the formation of Mn(BH4)2 with a space group symmetry P3112 and LiCl in a stoichiometric ratio, with excess LiBH4 observed when x = 3. Thermal decomposition of Mn(BH4)2 was observed between 130 and 180 °C (in 1 bar flowing Ar) for milled 2LiBH4-MnCl2 with mass loss of 8.4 wt %, and 105 and 145 °C for milled 3LiBH4-MnCl2 with mass loss of 8.1 wt %, and both with the concurrent evolution of hydrogen and diborane (molar ratio of 9:1). Analysis of the decomposition products suggested the presence of amorphous boron, manganese metal and LiCl for both milled samples, and the formation of three as yet unidentified phases (320-400 °C) for the milled 3LiBH4-MnCl2 sample. When sodium borohydride was used as a reagent, Mn(BH4)2 was also observed, however instead of NaCl a solid solution of NaClx(BH4)1-x and the ternary chloride Na6MnCl8 were formed. After heating, Mn(BH4)2 decomposed between 118 and 170 °C with a 6.4 wt% loss for milled 2NaBH4-MnCl2, and between 110 and 160 °C with a 6.0 wt% loss for milled i 3NaBH4-MnCl2; both occurred with the concurrent evolution of hydrogen and diborane. As part of the milled products, solid solutions of NaClx(BH4)1-x were formed where x = 0.77 and 0.68 for milled 2NaBH4-MnCl2 and 3NaBH4-MnCl2, respectively. Following hydrogen desorption from Mn(BH4)2, thermal decomposition of these solid solutions by the interaction with the ternary chloride Na6MnCl8 occurred between 250 and 425 ºC (2NaBH4-MnCl2) and between 150 and 225 °C (3NaBH4-MnCl2) with the release of hydrogen. Analysis of both milled samples heated under flowing Ar showed that amorphous B and possibly Mn formed (110 to 170 °C); the reaction of the solid solutions NaClx(BH4)1-x with the ternary chloride Na6MnCl8 (150 to 425 °C) resulted in the formation of NaCl; and, in the case of milled 3NaBH4-MnCl2, unknown phases were observed after heating to above 380 °C. When potassium borohydride was used as a reagent, Mn(BH4)2 was not observed (under the milling conditions investigated). Rather, mixed cation borohydride K2Mn(BH4)4, ternary chloride KMnCl3 and unreacted KBH4 were observed. The mass losses for the milled sample were from 100 to 160 °C with a 1.6 wt% loss (with a release of majority hydrogen and trace diborane), associated with the decomposition of K2Mn(BH4)4 to form KBH4, boron, and finely dispersed manganese and from 165 to 260 °C with a 1.9 wt% loss (only hydrogen release), associated with the reaction of KBH4 with ternary chloride to give KCl, boron, finely dispersed manganese. Simultaneously, the formed KCl can be dissolved into KBH4 to yield a K(BH4)xCl1-x solid solution, and react with KMnCl3 to form a new ternary chloride K4MnCl6 phase. The potential higher borane species decomposed to release a small amount of hydrogen between 270 and 380 °C. This study has given a clearer understanding of the synthetic routes, structural characteristics and decomposition properties of manganese-based borohydrides formed by mechanical ball ii milling of ABH4 (A = Li, Na, K) and MnCl2 (with 2:1 and 3:1 molar ratios) for 360 minutes under argon. The dehydrogenation of the manganese-based borohydrides was irreversible under experimental conditions (100-150 bar H2, 100-250 ºC ). iii Acknowledgments Firstly, I express my sincere thanks to Dr. David Book for his help and advice during the supervision of this project. He provided me with many helpful suggestions and important advice during the course of this work. I am grateful for the assistance offered by Dr. Dan Reed and other members of the hydrogen materials group, both past and present, for their useful discussions, contributions and friendship over the last 4 years. I also gratefully acknowledge the financial support from EPSRC to fund my project, to MEGS for the conference travel funding. Finally, a special thank to all my family members. Without their understanding, support and encouragement I could never have completed this PhD. iv List of Abbreviations ACs Activated Carbons ATR Attenuated Total Reflectance CCD Charge-Coupled Device CNTs Carbon Nanotubes DFT Density Functional Theory DME Dimethylether DOE Department of Energy DSC Differential Scanning Calorimetry ESDs Estimated Standard Deviations FTIR Fourier Transform Infrared GNFs Graphite Nanofibers IEA International Energy Agency MAS- NMR Magic Angle Spinning-Nuclear Magnetic Resonance MCAS Mechano-Chemical Activation Synthesis MOFs Metal of Frameworks MS Mass Spectrometry MWCNTs Multiwalled Carbon Nanotubes PCT Pressure-Composition-Temperature PEMFC Proton Exchange Membrane Fuel Cell PSD Position Sensitive Detector SR-PXD Synchrotron Radiation Powder X-ray Diffraction SWCNTs Single-Walled Carbon Nanotubes TGA Thermogravimetric Analysis THF Tetrahydrofuran TPD Temperature Programmed Desorption XRD X-ray Diffraction v Contents Chapter 1 Introduction ……………………………………………………………………...1 Chapter 2 Literature Review: Hydrogen storage………………………………………….7 2.1 Introduction…………………………………………………………………………...7 2.2 Gaseous storage…………………………………………………………………….....9 2.3 Liquid storage……………………………………………………………………….10 2.4 Solid-state storage…………………………………………………………………...12 2.4.1 Introduction…………………………………………………………………...12 2.4.2 Porous materials………………………………………………………………13 2.4.3 Metal hydrides……………………………………………….. ………………20 2.4.4 Complex hydrides…………………………………………………………….23 Chapter 3 Literature Review: Borohydrides……………………………………………...31 3.1 Introduction………………………………………………………………………….31 3.2 Alkali metal borohydride systems…………………………………………………...35 3.2.1 LiBH4 ………………………………………………………………………...35 3.2.1.1 Synthesis……………………………………………………………….35 3.2.1.2 Structure………………………………………………………………..39 3.2.1.3 Dehydrogenation and rehydrogenation ………………………………..40 3.2.1.4 Destabilization and kinetics modification……………………………..42 3.2.2 NaBH4………………………………………………………………………..46 3.2.2.1 Synthesis……………………………………………………………….46 3.2.2.2 Structure………………………………………………………………..48 3.2.2.3 Solid-state dehydrogenation and rehydrogenation……………………..49 3.2.2.4 Aqueous dehydrogenation and regeneration……………….. …………50 3.2.3 KBH4………………………………………………………………………….51 3.2.3.1 Synthesis and structure…………………………………………………51 3.2.3.2 Dehydrogenation……………………………………………………….52 3.2.3.3 K-based bialkali borohydrides…………………………………………53 3.2.4 Other alkali metal borohydrides………………………………………………53 vi 3.3 Alkaline earth metal borohydride systems………………………………..................54 3.3.1 Mg(BH4)2 …………………………………………………………..................54 3.3.1.1 Synthesis……………………………………………………………….54 3.3.1.2 Structure………………………………………………………………..56 3.3.1.3 Dehydrogenation and rehydrogenation……………………...................57 3.3.1.4 Destabilization and kinetics modification……………………………...60 3.3.2 Ca(BH4)2……………………………………………………………………...61 3.3.2.1 Synthesis……………………………………………………………….61 3.3.2.2 Structure………………………………………………………………..62 3.3.2.3 Dehydrogenation and rehydrogenation……………………...................63 3.3.2.4 Destabilization and kinetics modification……………………………...64 3.4 Typical transition metal borohydrides………………………………………………65 3.4.1 Introduction…………………………………………………………………..65 3.4.2 Scandium-based borohydride………………………………………………...66 3.4.3 Yttrium-based borohydride…………………………………………………..68 3.4.4 Titanium-based borohydride…………………………………………………70 3.4.5 Zirconium-based borohydride………………………………………………..70 3.4.6 Hafnium-based borohydride………………………………………………….71 3.4.7 Zinc-based borohydride………………………………………………………72 3.5 Manganese-based borohydride……………………………………………………...74 3.6 Aims and objectives of the project………………………………………………......78 Chapter 4 Experimental Techniques………………………………………………………80 4.1 Synthesis of Mn-based borohydrides………………………………………………..81 4.2 Characterization of milled samples………………………………………………….84 4.2.1 X-ray diffraction……………………………………………………. ……….84 4.2.2 Raman and FTIR……………………………………………………………..86 4.3 Thermogravimetric and calorimetric analysis………………………………………91 4.3.1 Thermogravimetric analysis …………………………………………………91 4.3.2 Differential scanning calorimetry…………………………………………….94