The main focus of this thesis is the characterization of defects and microstructure in high-energy ball milled magnesium hydride powder and magnesium-based multilayered composites. Enhancement in kinetics of hydrogen cycling in magnesium can be achieved by applying severe plastic deformation. A literature survey reveals that, due to extreme instability of -MgH2 in transmission electron microscope (TEM), the physical parameters that researchers have studied are limited to particle size and grain size. By utilizing a cryogenic TEM sample holder, we extended the stability time of the hydride phase during TEM characterization. Milling for only 30 minutes resulted in a significant enhancement in desorption kinetics. A subsequent annealing cycle under pressurized hydrogen reverted the kinetics to its initial sluggish state. Cryo-TEM analysis of the milled hydride revealed that mechanical milling induces deformation twinning in the hydride microstructure. Milling did not alter the thermodynamics of desorption. Twins can enhance the kinetics by acting as preferential locations for the heterogeneous nucleation of metallic magnesium. We also looked at the phase transformation characteristics of desorption in MgH2. By using energy-filtered TEM, we investigated the morphology of the phases in a partially desorbed state. Our observations prove that desorption phase transformation in MgH2 is of nucleation and growth type, with a substantial energy barrier for nucleation. This is contrary to the generally assumed core-shell structure in most of the simulation models for this system. We also tested the hydrogen storage cycling behavior of bulk centimeter-scale Mg-Ti and Mg-SS multilayer composites synthesized by accumulative roll-bonding. Addition of either phase (Ti or SS) allows the reversible hydrogen sorption at 350C, whereas identically roll-bonded pure magnesium cannot be absorbed. In the composites the first cycle of absorption (also called activation) kinetics improve with increased number of fold and roll (FR) operations. With increasing FR operations the distribution of the Ti phase is progressively refined, and the shape of the absorption curve no longer remains sigmoidal. Up to a point, increasing the loading amount of the second phase also accelerates the kinetics. Microscopy analysis performed on 1-2 wt.% hydrogen absorbed composites demonstrates that MgH2 formed exclusively on various heterogeneous nucleation sites. During activation, MgH2 nucleation occurred at the Mg-hard phase interfaces. On the subsequent absorption cycles, heterogeneous nucleation primarily occurred in the vicinity of internal free surfaces such as cracks. / Materials Engineering

We have designed and engineered bi-catalyst magnesium hydride composites with superior sorption performance to that of ball milled magnesium hydride catalyzed with the individual baseline catalysts. We have examined the effect of single-walled carbon nanotube (SWCNT)-metallic nanoparticle additions on the hydrogen desorption behavior of MgH2 after high-energy co-milling. We showed the synergy between SWCNT's and metallic nanoparticles in catalyzing the sorption of magnesium hydride. The optimum microstructure for sorption, obtained after 1 h of co-milling, consists of highly defective SWCNTs in intimate contact with metallic nanoparticles and with the hydride. This microstructure is optimum, presumably because of the dense and uniform coverage of the defective SWCNTs on the MgH2 surface. Cryo-stage transmission electron microscopy (TEM) analysis of the hydride powders revealed that they are nanocrystalline and in some cases multiply twinned. Since defects are an integral component of hydride-to-metal phase transformations, such analysis sheds new insight regarding the fundamental microstructural origins of the sorption enhancement due to mechanical milling. The nanocomposite shows markedly improved cycling as well. Activation energy analysis demonstrates that any catalytic effect due to the metallic nanoparticles is lost during cycling. Improved cycling performance is instead achieved as a result of the carbon allotropes preventing MgH2 particle agglomeration and sintering. The nanocomposite received over 100 sorption cycles with fairly minor kinetic degradation. We investigated the catalytic effect of Fe + Ti bi-metallic catalyst on the desorption kinetics of magnesium hydride. Sub-micron dimensions for MgH2 particles and excellent nanoscale catalyst dispersion was achieved by high-energy milling. The composites containing Fe shows DSC desorption temperature of 170 °C lower than as-received MgH2 powder, which makes it suitable to be cycled at relatively low temperature of 250 °C. The low cycling temperature also prevents the formation of Mg2FeH6. The ternary Mg-Fe-Ti composite shows best performance when compared to baseline ball milled magnesium hydride with only one catalytic addition. With a very high BET surface area it also shows much less degradation during cycling. The synergy between Fe and Ti is demonstrated through use of TEM and by carefully measuring the activation energies of the baseline and the ternary composites. / Materials Engineering

Hydrogen is considered one of the best alternatives to fossil-fuels for the transportation sector because hydrogen can be burnt cleanly and efficiently in a fuel cell to drive an electric motor. However, due to the low density of H2 at ambient conditions the conventional H2 storage technologies (cryogenic liquid and compressed gas) cannot achieve energy densities comparable to to gasoline and diesel. A second energy storage challenge onboard electric fuel cell vehicles is fuel cell power management at peak current loads, which requires an auxiliary power source like a battery or supercapacitor. The development of efficient onboard energy storage systems for H2 and auxiliary power is critical to realisation of a hydrogen economy. Mesoporous carbons were investigated as H2 storage materials in composites with magnesium hydride (MgH2),and as electrode materials for electrochemical double-later capacitors. The mesoporous carbons were prepared by two methods: (1) from porous silica and alumina templates, and (2) by chemical activation of a waste carbon source (waste coffee grounds). The experimental approach targeted reducing the cost of mesoporous carbon preparation by using a cheaper template, where the cost of alumina template was one-fifth the cost of the silica template (at the laboratory scale), or by using a waste material as a carbon source. The alumina template was found to be suitable to produce a mesoporous carbon with an average pore size of 4.8 nm. Chemical activation of coffee grounds with ZnCl2 produced activated carbons with BET surface areas up to 1280 m2/g. Mesopore volume increased with ZnCl2 impregnation ratio, with mesopore size distributions in the range 2 - 20 nm. The theoretical H2 capacity of MgH2 is 7.6 % but MgH2 application in fuel cell vehicles is limited by slow hydrogenation kinetics and high temperatures (> 573 K) for H2 release. Magnesium was impregnated on activated carbon fibres (ACF) and mesoporous carbon (prepared from silica and alumina templates) to improve H2 storage kinetics and thermodynamics by reducing the magnesium hydride particle size. Thermal gravimetric analysis (TGA) and temperature programmed desorption (TPD) studies showed that thermal decomposition of MgCl2 supported on ACF at 1173 K in N2 and H2 can produce a Mg-ACF composite. At 573 K and 2 MPa H2 pressure a Mg-ACF composite, containing 11.2 %wt Mg, showed improved H2 adsorption kinetics compared to bulk Mg powder, but the total capacity of the Mg-ACF composite was only 0.4 % wt H2. To achieve a target of 6 %wt for onboard H2 storage higher Mg loadings are required. Attempts to impregnate Mg in mesoporous carbon via the MgCl2 thermal decomposition process highlighted the difficulties of avoiding MgO formation, and show that MgH2 loaded carbon is unlikely to be a practical high density onboard H2 storage technology. Activated carbons from waste coffee grounds (CGCs) were used as electrode materials in electrochemical double-layer capacitors. The specific capacitance of CGCs was as high as 368 F/g in 1 mol/L H2SO4, with good capacitance retention at fast charge rates and stable cycling performance. The good electrochemical performance of CGCs is attributed to a porous structure featuring both micropores 0.5 - 1.0 nm wide, which are effective for double-layer formation, and small mesopores, which facilitate electrolyte transport at fast charge rates. The capacitance of CGCs is enhanced by pseudo-Faradaic reactions involving nitrogen and oxygen functional groups. At fast charge-discharge rates the CGCs had higher energy density and better stability than a commercial benchmark activated carbon (Maxsorb). The ZnCl2 activation process can be optimised to develop mesopores for improved capacitance at fast charge rates and capacitance in organic electrolytes. In 1 mol/L tetra ethyl ammonium tetrafluoroborate (TEABF4) / acetonitrile the CGC with the most mesopores, which was prepared with a ZnCl2 to coffee ratio of 5:1, has the highest capacitance at high power density. CGCs with greater mesopore content retained higher specific capacitance at fast charge-discharge rates as the mesopores acts as channels or reservoirs for electrolyte transport. An improved model for evaluation of contributions to capacitance from micropore surfaces and mesopore surfaces is proposed. From this model the double-layer capacitance of mesopores surface area was found to be about 14 μF/cm2 and did not change considerably with increasing current load. The contribution of micropores to capacitance is dependent on the accessibility of ions to the micropores, and this accessibility is proportional to the mesopore surface area. An exponential function was found to describe the contribution of mesopores and micropore surfaces to capacitance. The effective double-layer capacitance of the micropore surface area drops at fast charge-discharge rates as a result of restricted ion transport, and this result highlights the importance of mesopores to retain energy density for high power supercapacitor applications.

Hydrogen has been considered as a potentially efficient and environmentally friendly alternative energy solution. However, one of the most important scientific and technical challenges that the “hydrogen economy” faces is the development of safe and economically viable on-board hydrogen storage for fuel cell applications, especially to the transportation sector. Ammonia borane (BH3NH3), a solid state hydrogen storage material, possesses exceptionally high hydrogen content (19.6 wt%).However, a fairly high temperature is required to release all the hydrogen atoms, along with the emission of toxic borazine. Recently research interests are focusing on the improvement of H2 discharge from ammonia borane (AB) including lowering the dehydrogenation temperature and enhancing hydrogen release rate using different techniques. Till now the detailed information about the bonding characteristics of AB is not sufficient to understand details about its phases and structures. Elemental substitution of ammonia borane produces metal amidoboranes. Introduction of metal atoms to the ammonia borane structure may alter the bonding characteristics. Lithium amidoborane is synthesized by ball milling of ammonia borane and lithium hydride. High pressure study of molecular crystal provides unique insight into the intermolecular bonding forces and phase stability. During this dissertation, Raman spectroscopic study of lithium amidoborane has been carried out at high pressure in a diamond anvil cell. It has been identified that there is no dihydrogen bond in the lithium amidoborane structure, whereas dihydrogen bond is the characteristic bond of the parent compound ammonia borane. It has also been identified that the B-H bond becomes weaker, whereas B-N and N-H bonds become stronger than those in the parent compound ammonia borane. At high pressure up to 15 GPa, Raman spectroscopic study indicates two phase transformations of lithium amidoborane, whereas synchrotron X-ray diffraction data indicates only one phase transformation of this material. Pressure and temperature has a significant effect on the structural stability of ammonia borane. This dissertation explored the phase transformation behavior of ammonia borane at high pressure and low temperature using in situ Raman spectroscopy. The P-T phase boundary between the tetragonal (I4mm) and orthorhombic (Pmn21) phases of ammonia borane has been determined. The transition has a positive Clapeyron slope which indicates the transition is of exothermic in nature. Influence of nanoconfinemment on the I4mm to Pmn21 phase transition of ammonia borane was also investigated. Mesoporus silica scaffolds SBA-15 with pore size of ~8 nm and MCM-41 with pore size of 2.1-2.7 nm, were used to nanoconfine ammonia borane. During cooling down, the I4mm to Pmn21 phase transition was not observed in MCM-41 nanoconfined ammonia borane, whereas the SBA-15 nanocondfined ammonia borane shows the phase transition at ~195 K. Four new phases of ammonia borane were also identified at high pressure up to 15 GPa and low temperature down to 90 K.

Cette thèse est consacrée à l'étude des métaux-N-H des matériaux pour le stockage d'hydrogène de solide. Le but est de caractériser la synthèse mechanochemical, structurelle et les propriétés d'hydrogénation de Li-N-H, Li-Mg-N-H et des systèmes Li-Mg-B-N-H. Premièrement, l'assimilation hydrogène pendant mechanochemistry de Li3N sous 9 MPA de H2 a été analysée au moyen de l'absorption solide-à-gaz in situ et la Diffraction de Radiographie d'ex-situ (XRD) des mesures. Deux étapes de H-sorption menant à une assimilation hydrogène globale de 9.8wt le % ont été obtenus. La première étape de réaction comprend la transformation de polymorphe-li3n (S.G.P6/mmm) dans li3n (S.G.P63/mmc) métastable la phase et la réaction du dernier avec l'hydrogène pour former lithium imide :-li3n + H2 Li2NH + LiH. La deuxième étape absorbant est lithium imide des convertis à lithium amide / This thesis is dedicated to the study of novel metal-N-H materials for solid state hydrogen storage. The aim is to characterize the mechanochemical synthesis, structural and hydrogenation properties of Li-N-H, Li-Mg-N-H and Li-Mg-B-N-H systems. Firstly, hydrogen uptake during mechanochemistry of Li3N under 9 MPa of H2 has been analyzed by means of in-situ solid-gas absorption and ex-situ X-Ray Diffraction (XRD) measurements. Two H-sorption steps leading to an overall hydrogen uptake of 9.8wt% was obtained. The first reaction step comprises the transformation of polymorph -Li3N (S.G.P6/mmm) into -Li3N (S.G.P63/mmc) metastable phase and the reaction of the latter with hydrogen to form lithium imide: -Li3N + H2 Li2NH + LiH. The second absorption step is lithium imide converts to lithium amide following the reaction scheme Li2NH + H2 LiNH2 + LiH. The assessment of reaction paths in this system as well as of the appraisal of the underlying reaction mechanisms was under taken. Secondly, reactive ball milling (RBM) under H2 of Li3N and Mg powder with a molar ratio of 2:1 was taken on to destabilize Li-N-H system and accelerate its sorption kinetics. The onset dehydrogenation temperature of the as-milled 2Li3N+Mg mixture was detected at 125°C, which is about 75°C lower than that of the Li-N-H system. The structural and phases evolution of the Li-Mg-N-H system during both the synthesis and subsequent hydrogenation/dehydrogenation cycling were characterized by combined analysis of in-situ XRD and neutron powder diffraction (NPD) measurements. It was found step wised for the both processes depending on mainly the temperature and hydrogen pressure to the system. Finally, the effect of the addition of Co-based compounds, lithium borohydides and the combination of them to Li-Mg-N-H system were systematically investigated by XRD, scanning electron microscopy (SEM), fourier transform infra-red (FTIR), differential scanning calorimetry (DSC) and hydrogen storage properties measurements with the aim to overcome the kinetic barriers and further decrease the dehydrogenation temperature. The Li-Mg-B-N-H/3wt% ZrCoH3 composite synthesized by RBM has the best hydrogen storage properties. It is shown that the activation energy was decreased and the N-H bonds were weakened, which could be the main reasons for improving the hydrogen storage properties of Li-Mg-N-H system

Hydrogen can be an unlimited source of clean energy for future because of its very high energy density compared to the conventional fuels like gasoline. An efficient and safer way of storing hydrogen is in metals and alloys as hydrides. Light metal hydrides, alanates and borohydrides have very good hydrogen storage capacity, but high operation temperatures hinder their application. Improvement of thermodynamic properties of these hydrides is important for their commercial use as a source of energy. Application of pressure on materials can have influence on their properties favoring hydrogen storage. Hydrogen desorption in many complex hydrides occurs above the transition temperature. Therefore, it is important to study the physical properties of the hydride compounds at ambient and high pressure and/or high temperature conditions, which can assist in the design of suitable storage materials with desired thermodynamic properties. The high pressure-temperature phase diagram, thermal expansion and compressibility have only been evaluated for a limited number of hydrides so far. This situation serves as a main motivation for studying such properties of a number of technologically important hydrides. Focus of this dissertation was on X-ray diffraction and Raman spectroscopy studies of Mg2FeH6, Ca(BH4)2, Mg(BH4)2, NaBH4, NaAlH4, LiAlH4, LiNH2BH3 and mixture of MgH2 with AlH3 or Si, at different conditions of pressure and temperature, to obtain their bulk modulus and thermal expansion coefficient. These data are potential source of information regarding inter-atomic forces and also serve as a basis for developing theoretical models. Some high pressure phases were identified for the complex hydrides in this study which may have better hydrogen storage properties than the ambient phase. The results showed that the highly compressible B-H or Al-H bonds and the associated bond disordering under pressure is responsible for phase transitions observed in brorohydrides or alanates. Complex hydrides exhibited very high compressibility suggesting possibility to destabilize them with pressure. With high capacity and favorable thermodynamics, complex hydrides are suitable for reversible storage. Further studies are required to overcome the kinetic barriers in complex hydrides by catalytic addition. A comparative study of the hydride properties with that of the constituting metal, and their inter relationships were carried out with many interesting features.

Recently, ammonia borane has increasingly attracted researchers’ attention because of its merging applications, such as organic synthesis, boron nitride compounds synthesis, and hydrogen storage. This dissertation presents the results from several studies related to ammonia borane. The pressure-induced tetragonal to orthorhombic phase transition in ammonia borane was studied in a diamond anvil cell using in situ Raman spectroscopy. We found a positive Clapeyron-slope for this phase transformation in the experiment, which implies that the phase transition from tetragonal to orthorhombic is exothermic. The result of this study indicates that the rehydrogenation of the high pressure orthorhombic phase is expected to be easier than that of the ambient pressure tetragonal phase due to its lower enthalpy. The high pressure behavior of ammonia borane after thermal decomposition was studied by in situ Raman spectroscopy at high pressures up to 10 GPa. The sample of ammonia borane was first decomposed at ~140 degree Celcius and ~0.7 GPa and then compessed step wise in an isolated sample chamber of a diamond anvil cell for Raman spectroscopy measurement. We did not observe the characteristic shift of Raman mode under high pressure due to dihydrogen bonding, indicating that the dihydrogen bonding disappears in the decomposed ammonia borane. Although no chemical rehydrogenation was detected in this study, the decomposed ammonia borane could store extra hydrogen by physical absorption. The effect of nanoconfinement on ammonia borane at high pressures and different temperatures was studied. Ammonia borane was mixed with a type of mesoporous silica, SBA-15, and restricted within a small space of nanometer scale. The nano-scale ammonia borane was decomposed at ~125 degree Celcius in a diamond anvil cell and rehydrogenated after applying high pressures up to ~13 GPa at room temperature. The successful rehydrogenation of decomposed nano-scale ammonia borane gives guidance to further investigations on hydrogen storage. In addition, the high pressure behavior of lithium amidoborane, one derivative of ammonia borane, was studied at different temperatures. Lithium amidoborane (LAB) was decomposed and recompressed in a diamond anvil cell. After applying high pressures on the decomposed lithium amidoborane, its recovery peaks were discovered by Raman spectroscopy. This result suggests that the decomposition of LAB is reversible at high pressures.

This thesis studies the development of a new method for photochemical overall water splitting using a small organic shuttle. In Section 2, BiVO4, was studied to determine the CO2 reduction mechanism and how catalytic activity decays. BiVO4 catalysts were capable of producing a maximum of 200 μmol of methanol per gram of catalyst from CO2 in basic media, and later decomposed by BiVO4. The decay of BiVO4¬ was studied by x-ray diffraction and scanning electron microscopy, demonstrating reversible decomposition of BiVO4. BiVO4 is etched, leeching vanadium into solution, while nanoparticles of bismuth oxide are deposited on the surface of BiVO4. In Section 3, ferrocyanide salts, an aqueous, cheap, and abundant photocatalyst was used for the first time to dehydrogenate aqueous formaldehyde selectively into formate and hydrogen. The catalyst is capable of record turnovers and turnover frequencies for formaldehyde dehydrogenation catalysts. A preliminary mechanism was proposed from experimental and computational data.

La thèse a concerné l'élaboration de matériau composite NH3BH3@BN pour le stockage chimique de l'hydrogène et a été divisée en 3 axes majeurs : la synthèse de NH3BH3 mais aussi celle d'un dérivé, l'hydrazine borane N2H4BH3 (publié dans /Phys.Chem.Chem.Phys/.: 2012, 14, 1768; "Hydrazine borane: synthesis,characterization, and application prospects in chemical hydrogen storage"), optimisation du processus de confinement en utilisant un matériau hôte commercial et abondant comme le charbon actif (NORIT SX1 700 m^2 /g) (publié dans /Int. J. Hydrogen Energy/:2012, 37, 13437 ; "Room-temperature hydrogen release from activated carbon-confined ammonia borane"), synthèse de nanostructures à base de BN, sous forme de nanocapsules creuses et de répliques de poreux à base de charbon actif pour le nanoconfinement de NH3BH3. (à paraitre dans /J. Mater.//Chem/.) / The thesis concerned the development of composite NH3BH3@BN for chemical hydrogen storage material and it has been divided into three major axes: the synthesis of NH3BH3 but also a derivative, hydrazine borane N2H4BH3 (published in / Phys Chem Chem Phys /: 2012, 14, 1768 "Hydrazine borane: synthesis, characterization, and application of prospects in chemical hydrogen storage"....), optimization of the confinement process using a commercial and abundant host material such as activated carbon (Norit SX1 700 m^2/g) (published / Int J. Hydrogen Energy /: 2012, 37, 13437, "Room-temperature hydrogen release from activated carbon-ammonia borane confined.") synthesis BN-based nanostructures in the form of hollow nanocapsules and replica of porous activated carbon for nanoconfinement of NH3BH3. (to be published in / J. Mater.Chem /.)

Philosophiae Doctor - PhD / The application of porous materials into industrial hydrogen (H2) storage systems is based on their use in combination with high-pressure cylinders. The processing of metal-organic frameworks (MOF) powders into shaped forms is therefore imperative in order to counteract the adverse effects of poor packing of powders in cylinders. The fabrication of shaped MOFs has, however, been shown to be accompanied by compromised properties such as surface areas, gravimetric and volumetric H2 capacities, and also the working/deliverable H2 capacities in comparison to MOF powders.