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

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

Ammonia uptake and release in the MnX₂–NH₃ (X = Cl, Br) systems and structure of the Mn(NH₃)nX₂ (n = 6, 2) ammines

Hexa-ammine complexes, Mn(NH<sub>3</sub>)<sub>6</sub>X<sub>2</sub> (X = Cl, Br), have been synthesized by ammoniation of the corresponding transition metal halide and characterized by Powder X-ray diffraction (PXRD) and Raman spectroscopy. The hexa-ammine complexes are isostructural (Cubic,Fm-3m, Z = 4; a = 10.2742(6) Å and 10.527(1) Å for X = Cl, Br respectively). Temperature programmed desorption (TPD) demonstrated that ammonia release from Mn(NH<sub>3</sub>)<sub>6</sub>X<sub>2</sub> complexes occurred in three stages corresponding to the release of 4, 1 and 1 NH<sub>3</sub> equivalents respectively. The chloride and bromide both exhibit a deammoniation onset temperature below 323 K. The di-ammoniates from the first desorption step were isolated during TPD measurements and their crystal structures determined by Rietveld refinement against PXRD data (X = Cl: orthorhombicCmmm, a = 8.1991(9) Å, b = 8.2498(7) Å, c = 3.8212(4) Å, Z = 2; X = Br: orthorhombic Pbam, a = 6.0109(5) Å, b = 12.022(1) Å, c = 4.0230(2) Å, Z= 2)

Facile synthesis of nanosized sodium magnesium hydride, NaMgH₃

The ternary magnesium hydride NaMgH3 has been synthesised via reactive milling techniques. The method employed neither a reactive H2 atmosphere nor high pressure sintering or other post-treatment processes. The formation of the ternary hydride was studied as a function of milling time and ball:powder ratio. 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 and characterised by powder X-ray diffraction (PXD), Raman spectroscopy and scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDX). The products formed sub-micron scale (typically 200–400 nm in size) crystallites that were approximately isotropic in shape. The dehydrogenation behaviour of the ternary hydride was investigated by temperature programmed desorption (TPD). The nanostructured hydride releases hydrogen in two steps with an onset temperature for the first step of 513 K

The chemistry of ZnWO₄ nanoparticle formation

The need for a change away from classical nucleation and growth models for the description of nanoparticle formation is highlighted. By the use of in situ total X-ray scattering experiments the transformation of an aqueous polyoxometalate precursor mixture to crystalline ZnWO$_{4}$ nanoparticles under hydrothermal conditions was followed. The precursor solution is shown to consist of specific Tourné-type sandwich complexes. The formation of pristine ZnWO$_{4}$ within seconds is understood on the basis of local restructuring and three-dimensional reordering preceding the emergence of long range order in ZnWO$_{4}$ nanoparticles. An observed temperature dependent trend in defect concentration can be rationalized based on the proposed formation mechanism. Following nucleation the individual crystallites were found to grow into prolate morphology with elongation along the unit cell c-direction. Extensive electron microscopy characterization provided evidence for particle growth by oriented attachment; a notion supported by sudden particle size increases observed in the in situ total scattering experiments. A simple continuous hydrothermal flow method was devised to synthesize highly crystalline monoclinic zinc tungstate (ZnWO$_{4}$) nanoparticles in large scale in less than one minute. The present results highlight the profound influence of structural similarities in local structure between reactants and final materials in determining the specific nucleation of nanostructures and thus explains the potential success of a given synthesis procedure in producing nanocrystals. It demonstrates the need for abolishing outdated nucleation models, which ignore subtle yet highly important system dependent differences in the chemistry of the forming nanocrystals

Mechanochemical synthesis and structure of lithium tetrahaloaluminates, LiAlX₄ (X = Cl, Br, I); a family of Li-ion conducting ternary halides

State-of-the-art oxides and sulfides with high Li-ion conductivity and good electrochemical stability are among the most promising candidates for solid-state electrolytes in secondary batteries. Yet emerging halides offer promising alternatives because of their intrinsic low Li+ migration energy barriers, high electrochemical oxidative stability, and beneficial mechanical properties. Mechanochemical synthesis has enabled the characterization of LiAlX4 compounds to be extended and the iodide, LiAlI4, to be synthesized for the first time (monoclinic P21/c, Z = 4; a = 8.0846(1) Å; b = 7.4369(1) Å; c = 14.8890(2) Å; β = 93.0457(8)°). Of the tetrahaloaluminates, LiAlBr4 exhibited the highest ionic conductivity at room temperature (0.033 mS cm–1), while LiAlCl4 showed a conductivity of 0.17 mS cm–1 at 333 K, coupled with the highest thermal and oxidative stability. Modeling of the diffusion pathways suggests that the Li-ion transport mechanism in each tetrahaloaluminate is closely related and mediated by both halide polarizability and concerted complex anion motions

Facile synthesis of nanosized sodium magnesium hydride, NaMgH3

The ternary magnesium hydride NaMgH3 has been synthesised via reactive milling techniques. The method employed neither a reactive H2 atmosphere nor high pressure sintering or other post-treatment processes. The formation of the ternary hydride was studied as a function of milling time and ball:powder ratio. 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 and characterised by powder X-ray diffraction (PXD), Raman spectroscopy and scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDX). The products formed sub-micron scale (typically 200–400 nm in size) crystallites that were approximately isotropic in shape. The dehydrogenation behaviour of the ternary hydride was investigated by temperature programmed desorption (TPD). The nanostructured hydride releases hydrogen in two steps with an onset temperature for the first step of 513 K

Atomic Scale Design of Spinel ZnAl2O4ZnAl_{2}O_{4} Nanocrystal Synthesis

The chemistry of ZnAl$_2$O$_4$ nanocrystal nucleation and growth is examined by X-ray scattering methods, and the results challenge the conventional understanding of its preparation by hydrothermal methods. The common assumption that a specific metal to hydroxide ion (M/OH) ratio is necessary to achieve a phase-pure product is shown to be inadequate. Pair distribution function analysis is used to identify distinct precursor structures, providing an understanding of why particular impurity phases are observed under certain M/OH ratios as heating is applied. In situ X-ray diffraction studies then probe the ZnAl$_2$O$_4$ growth in real time, from which optimal synthesis conditions and the influence of impurities is established. It is found that the heating rate plays a dominant role in impurity formation and dissolution. This observation is explored in three different hydrothermal synthesis methods (microwave, autoclave, and supercritical flow) having different intrinsic heating rates, and methodologies to prepare phase-pure ZnAl$_2$O$_4$ were successfully developed in each case. Ultimately, the atomic scale X-ray scattering information provides concrete guidance to tune the crystallite size, band gap, morphology, and defects of ZnAl$_2$O$_4$ nanocrystals in hydrothermal synthesis establishing a bottom up nonempirical approach to synthesis design

Influence of Phase Separation and Spinodal Decomposition on Microstructure of Mg2Si1- xSnx Alloys

Mg2Si1-xSnx alloys with nominal values of x [0.03:0.18] were synthesized at 780 \ub0C by solid-state reaction from Mg2Si and Mg2Sn and subsequently annealed at either 680 or 580 \ub0C. Their microstructure was investigated by X-ray diffraction using the Rietveld method. Depending on the treatment temperature and the nominal composition, the solid solutions split into different Si- and/or Sn-rich Mg2Si1-xSnx phases. Traces of spinodal decomposition were observed for the samples with a low Sn content independent of treatment temperature due to the limited diffusion kinetics when entering the miscibility gap. A similar effect was observed when applying a higher cooling rate to the samples with higher Sn concentration. In this case, the samples experience thermodynamic spinodal decomposition being located in the spinodal region sufficiently long time at higher temperatures. Samples treated in the miscibility gap showed an agreement of the Si-rich binodal line with calculated phase diagrams. However, the Sn-rich binodal line stays undefined, perhaps due to grain boundary pinning of diffusing atoms. The study elucidates the possibility of tailoring the microstructure of magnesium silicide-stannide alloys utilizing merely judiciously designed heat treatment protocols. A particular attention is brought to spinodal decomposition, which has the potential to reduce the lattice thermal conductivity

The challenge of storage in the hydrogen energy cycle:nanostructured hydrides as a potential solution

Hydrogen has the capacity to provide society with the means to carry ‘green’ energy between the point of generation and the point of use. A sustainable energy society in which a hydrogen economy predominates will require renewable generation provided, for example, by artificial photosynthesis and clean, efficient energy conversion effected, for example, by hydrogen fuel cells. Vital in the hydrogen cycle is the ability to store hydrogen safely and effectively. Solid-state storage in hydrides enables this but no material yet satisfies all the demands associated with storage density and hydrogen release and uptake; particularly for mobile power. Nanochemical design methods present potential routes to overcome the thermodynamic and kinetic hurdles associated with solid state storage in hydrides. In this review we discuss strategies of nanosizing, nanoconfinement, morphological/dimensional control, and application of nanoadditives on the hydrogen storage performance of metal hydrides. We present recent examples of how such approaches can begin to address the challenges and an evaluation of prospects for further development

Revealing the Slow Decomposition Kinetics of Type-I clathrate Ba8Ga16Ge30

Inconsistencies in high temperature thermoelectric property measurements of Ba8Ga16Ge30 have prompted our study on the thermal stability of this heavily studied inorganic clathrate. Using X-ray diffraction, thermal analysis, and imaging techniques on both powder and spark plasma sintered pelletized samples, we probe the structure and decomposition characteristics of this important high temperature thermoelectric material. We demonstrate that the decomposition of Ba8Ga16Ge30 is extremely dependent on the heating conditions employed and, as a result of the slow decomposition kinetics of the clathrate, reveal that the true stability of this system has been overlooked in the extensive literature available. Loss of Ga and Ge from the clathrate cage is evident in all high temperature experiments under both air and inert environments. This study serves to highlight that the underlying structural chemistry and stability of thermoelectric materials at high temperature needs to be considered in parallel with the thermoelectric properties which constitute the figure of merit. Only then will reliable thermoelectric modules for real applications be realized