Synthesis, structure and characterisation of novel lightweight energy materials based on group I & II metal compounds

Reardon, Hazel (2014) Synthesis, structure and characterisation of novel lightweight energy materials based on group I & II metal compounds. PhD thesis, University of Glasgow.

Abstract

The need for light-weight, high capacity energy stores is driven by the necessity for a more sustainable approach to reducing the global dependency on fossil fuels. Storing hydrogen in the solid state is an attractive method in which the safety, sustainability and performance requirements for the automotive and aviation sectors may be met.
Mechanochemical methods have been exploited in this work to modify and synthesise inorganic materials for hydrogen storage based on Group I and Group II metal compounds. The properties of un-milled and milled commercial MgH2 have been examined and milling conditions optimised to obtain desirable hydrogen desorption characteristics. Subsequently, inexpensive, non-toxic, non-oxide catalyst materials were considered for enhancing the hydrogen release properties and three catalysed hydride systems were examined; MgH2-xSiC, MgH2-xgraphite and MgH2-xSiC:graphite (x = 1-20 wt%). The hydrogen desorption properties of the 1:1 molar SiC:graphite doped MgH2 system are shown to exhibit improved hydrogen release properties relative to the carbide and graphite systems alone, suggesting a synergistic effect. The Ea for hydrogen desorption from MgH2 could be decreased from 144±5 kJ/mol to 84±5 kJ/mol in the MgH2-10 wt% SiC:graphite system, maintaining a desirable hydrogen capacity >5 wt%. A recurring artefact of thermal analysis profiles for MgH2, in this work and in literature, indicates a two-step decomposition process under relatively mild milling conditions. Therefore, beyond the investigations described for optimisation of hydrogen release conditions, the effect that the aforementioned catalysts have on the two-step desorption anomaly using milder milling has also been investigated. This has given insight in to how the tuning of MgH2 may be made possible by selection of catalysts which have a more prominent effect on the low temperature desorption step relative to the higher temperature feature.
Direct synthesis of ternary hydrides from their corresponding binary hydrides has been investigated by mechanical alloying of stoichiometric and non-stoichiometric binary hydride mixtures. High purity NaMgH3 powder (Orthorhombic space group Pnma, a = 5.437(2) Å, b = 7.705(5) Å, c = 5.477(2) Å; Z = 4) was prepared in 5 h at high ball:powder ratios using a stoichiometric mixture of the respective binary hydrides. The dehydrogenation behaviour of the sub-micron (crystallites typically 200 – 400 nm in size) ternary hydride was investigated by thermal analysis. The nanostructured hydride releases hydrogen in two-steps with an onset temperature for the first step of 240 °C.
ii
Using a range of initial binary hydride stoichiometries, a series of potentially new cubic ternary (Ca1-xMgxH2)n hydride phases has been proposed, such that the initial stoichiometry of Ca:Mg results in (non-)stoichiometric Ca-Mg-H phases relative to the known Ca19Mg8H54 phase. The crystallographic properties of the (Ca1-xMgxH2)n series have been examined by both lab and in-situ synchrotron X-ray diffraction experiments, and the Rietveld method employed to establish detailed structure information. The thermal properties of the (Ca1-xMgxH2)n hydrides have also been determined and their relative hydrogen desorption and gravimetric capacities compared. This work demonstrates that as the proportion of Mg increases, the thermal stability of the Ca-Mg-H system is lowered and higher hydrogen capacities are obtained. The effect of small alkali metal vs. larger alkaline earth metal inclusion on the Mg-H system is explored through this work.
With a focus on new solid state synthesis routes to hydrides, mechanochemical metathesis reactions have been examined. Complex and ternary halides were selected as halide precursors, towards the synthesis of complex and ternary hydrides. The halides; LiAlCl4, NaMgCl3 and NaAlCl4, were synthesised using mechanochemical alloying of stoichiometric mixtures their respective binary metal halides. Their structures and thermal properties were determined and comparisons drawn between conventional synthesis in literature and the mechanochemical method employed in this work. The halides were then milled in appropriate stoichiometric ratios with alkali metal hydrides to determine whether a proposed metathesis reaction may result in the formation of the respective ternary/complex hydride. The products of the mechanochemical metathesis reactions were evaluated using powder diffraction and then thermal analysis, where low temperature hydrogen release corresponding to the desired hydride product was found. One metathesis route in particular highlights the potential of this approach, where analysis of the product suggests that the elusive “LiMgH3” hydride has been formed with hydrogen release at 316.6 ºC. This work illustrates that the solid state metathesis route is a suitable means for materials synthesis and design, where tailored reactions can yield exciting results.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Energy materials, Mechanochemistry, Metal Hydrides, Powder X-ray diffraction, Synchrotron X-ray diffraction, Rietveld refinement, Thermal analysis
• Subjects: Q Science > QD Chemistry
• Colleges/Schools: College of Science and Engineering > School of Chemistry
• Supervisor's Name: Gregory, Professor Duncan H.
• Date of Award: 2014
• Depositing User: Miss Hazel Reardon
• Unique ID: glathesis:2014-5372
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 08 Aug 2014 11:31
• Last Modified: 08 Aug 2014 11:52
• URI: https://theses.gla.ac.uk/id/eprint/5372