Ammonia borane and ammonium borohydride have been considered extensively as potential hydrogen storage materials. This thesis reports their structure and functional properties, emphasising the key role that dihydrogen bonding plays in both materials. The formation of a 'mobile phase' is considered to be the preliminary step in the decomposition of ammonia borane. The formation of this mobile phase has been studied using neutron diffraction, inelastic neutron spectroscopy and NMR. It has been found that in the mobile phase, 'end-to-end' flipping of the ammonia borane molecule occurs. This is an important precursor to the next step in the decomposition: the formation of the diammoniate of diborane. The dihydrogen bonding networks which occur in both the orthorhombic and the tetragonal phases of ammonia borane, and are the controlling factor in the decomposition process, were investigated using Density Functional Theory Molecular Dynamics (DFT-MD) simulations. It was hence shown that in the high-temperature tetragonal phase of ammonia borane, dihydrogen bonding is still an important stabilising interaction and there is little to distinguish between the three crystallographically distinct dihydrogen bonds. A closely related hydrogen storage material, ammonium borohydride, was also studied using the same techniques. Its low temperature phase progression was examined using variable temperature neutron diffraction. The vibrational modes of ammonium borohydride were assigned by comparing vibrational spectra determined using inelastic neutron spectroscopy with the results of DFT-MD simulations. Quasielastic neutron spectroscopy was used to show that both the ammonium and borohydride groups in ammonium borohydride perform discrete 'hopping' reorientational motions at a wide range of temperatures, and that the ammonium group has a mean residence time approximately 100 times less than that of the borohydride group. Hydrogen atom densities in the ammonium group were determined from DFT-MD simulations, and from refinements of high-resolution neutron diffraction data using cubic harmonic basis functions.

<p>Hydrogen is abundant, uniformly distributed throughout the Earth's surface and its oxidation product (water) is environmentally benign. Owing to these features, it is considered as an ideal synthetic fuel for a new world energetic matrix (renewable, secure and environmentally friendly) that could allow a sustainable future development. However, for this prospect to become a reality, efficient ways to produce, transport and store hydrogen still need to be developed. In the present thesis, theoretical studies of a number of potential hydrogen storage materials have been performed using density functional theory. In NaAlH₄ doped with 3d transition metals (TM), the hypothesis of the formation of Ti-Al intermetallic alloy as the main catalytic mechanism for the hydrogen sorption reaction is supported. The gateway hypothesis for the catalysis mechanism in TM-doped MgH₂ is confirmed through the investigation of MgH₂ nano-clusters. Thermodynamics of Li-Mg-N-H systems are analyzed with good agreement between theory and experiments. Besides chemical hydrides, the metal-organic frameworks (MOFs) have also been investigated. Li-decorated MOF-5 is demonstrated to possess enhanced hydrogen gas uptake properties with a theoretically predicted storage capacity of 2 wt% at 300 K and low pressure.</p><p>The metal-hydrogen systems undergo many structural and electronic phase transitions induced by changes in pressure and/or temperature and/or H-concentration. It is important both from a fundamental and applied viewpoint to understand the underlying physics of these phenomena. Here, the pressure-induced structural phase transformations of NaBH₄ and ErH₃ were investigated. In the latter, an electronic transition is shown to accompany the structural modification. The electronic and optical properties of the low and high-pressure phases of crystalline MgH₂ were calculated. The temperature-induced order-disorder transition in Li₂NH is demonstrated to be triggered by Li sub-lattice melting. This result may contribute to a better understanding of the important solid-solid hydrogen storage reactions that involve this compound. </p>

This thesis describes the activation of small molecules using frustrated Lewis pairs, in particular investigating their use to reduce CO₂ to methanol, thus producing a new route towards a renewable fuel. Chapter One summarises the requirement for a renewable fuel source, the alternative methods currently available and previous research conducted into converting CO₂ to methanol using FLPs and other reducing agents. Chapter Two describes the synthesis of a new family of electron-deficient tris(aryl)boranes, B(C₆F₅)_3-x(C₆Cl₅)_x (x = 1-3), allowing the electronic effects, resulting from the gradual replacement of C₆F₅ with C₆Cl₅ ligands, to be studied. The novel Lewis acids have been fully characterised and their Lewis acidities have been determined using NMR spectroscopy, electrochemistry and DFT studies. Chapter Three discusses the synthesis of nine novel FLPs and their use to successfully split H2. Each borohydride salt has been spectroscopically fully characterised and five of the salts have been characterised using single crystal X-ray diffraction. To determine the exact positions of the H atoms, single crystal neutron diffraction and DFT experiments were carried out on [1-H][H-TMP]. Chapter Four details attempts to use the borohydride salts, synthesised in Chapter Three, to reduce CO₂ to methanol. Each experiment was been fully investigated and their catalytic viability was determined. The X-ray crystal structure of [1-OCHO][H-TMP] is described and each formatoborate and methoxyborate salt were fully characterised. Chapter Five describes experimental procedures and characterisation data.

A supply of clean, carbon neutral and sustainable energy is the most scientific and technical challenge that humanity is facing in the 21st century. Though there is enough fossil fuels available for a few centuries, their use would increase the level of CO2 in the atmosphere. This would lead to global warming and may pose serious threats such as rising of sea level, change in hydrological cycle, etc. Hence there is a need for an alternative source of fuel that is clean and sustainable. Among the many resources considered as an alternative power source, hydrogen is considered one of the most promising candidates. To use hydrogen commercially, appropriate hydrogen storage system is required. Various options to store hydrogen for onboard use include gaseous form in high-pressure tanks, liquid form in cryogenic conditions, solid form in chemical or metal hydrides, or by physisorption of hydrogen on porous materials. One of the emerging porous materials are metal-organic frameworks (MOFs) which provide several advantages over zeolites and carbon materials because the MOFs can be designed to possess variable pore size, dimensions, and metrics. In general, MOFs adsorb hydrogen through weak interactions such as London dispersion and electrostatic potential which lead to low binding enthalpies in the range of 4 to 10 kJ/mol. As a result, cryogenic conditions are required to store sufficient amounts of hydrogen inside MOFs. Up to date several MOFs have been designed and tested for hydrogen storage at variable temperature and pressure levels. The overall results thus far suggest that the use of MOFs for hydrogen storage without chemical and electronic modifications such as doping with electropositive metals or incorporating low density elements such as boron in the MOFs backbone will not yield practical storage media. Such modifications are required to meet gravimetric and volumetric constraints. With these considerations in mind, we have selected a Cr-based MOF (MIL-101; Cr(F,OH)-(H2O)2O[(O2C)-C6H4-(CO2)]3•nH2O (n ≈ 25)) to investigate the impact of nickel inclusion inside the pores of MIL-101 on its performance in hydrogen storage. MIL-101 has a very high Langmuir surface area (5900 m2/g) and two types of mesoporous cavities (2.7 and 3.4 nm) and exhibits exceptional chemical and thermal stabilities. Without any modifications, MIL-101 can store hydrogen reversibly with adsorption enthalpy of 10 kJ/mol which is the highest ever reported among MOFs. At 298 K and 86 bar, MIL-101 can store only 0.36 wt% of hydrogen. Further improvement of hydrogen storage to 5.5 wt% at 40 bar was achieved only at low temperatures (77.3 K). As reported in the literature, hydrogen storage could be improved by doping metals such as Pt. Doping is known to improve hydrogen storage by spillover mechanism and Kubas interaction. Hence we proposed that doping MIL-101 with a relatively light metal possessing large electron density could improve hydrogen adsorption. Preferential Ni doping of the MIL-101’s large cavities which usually do not contribute to hydrogen uptake is believed to improve hydrogen uptake by increasing the potential surface in those cavities. We have used incipient wetness impregnation method to dope MIL-101 with Ni nanoparticles (NPs) and investigated their effect on hydrogen uptake at 77.3 K and 298 K, at 1 bar. In addition, the impact of metal doping on the surface area and pore size distribution of the parent MIL-101 was addressed. Metal content and NPs size was investigated by ICP and TEM, respectively. Furthermore, crystallinity of the resulting doped samples was confirmed by Powder X-ray Diffraction (PXRD) technique. The results of our studies on the successful doping with Ni NPs and their impact on hydrogen adsorption are discussed.

A new class of porous polymers has been designed and successfully synthesized by thermal dehydrogenation of several polytopic arylamine-borane adducts and has been designated Borazine-Linked Polymers (BLPs). The polymers reported are constructed of linear, triangular, and tetrahedral amine building units to form 2D and 3D frameworks. The boron sites of the pores are aligned with hydrogen atoms contrasted with the recently reported halogenated BLPs which consist of pore channels aligned with bromine or chlorine atoms. One of the reported BLPs, BLP-2(H), was proven to be crystalline by PXRD, matching the experimental pattern to theoretical patterns calculated from modeled structures. BLPs were found to be thermally stable by thermogravimetric analysis, decomposing at temperatures ~450 ºC. Infrared spectroscopy and 11B MAS NMR spectra confirm the formation of borazine as reported in previous borazine-containing polymers and 13C CP MAS spectra confirmed that the structural integrity of the amine building units were maintained and incorporated in the framework of BLPs. Nitrogen isotherms revealed that BLPs exhibit high surface areas ranging from 1132-2866 m2/g (Langmuir) and 400-2200 m2/g (Brunauer-Emett-Teller, BET) with pore sizes from 7-14 Å. Hydrogen, methane, and carbon dioxide measurements were performed at low pressure (up to 1 atm) and were found to be among the best of organic polymers. High pressure isotherms (up to 40 bar) were also taken at various temperatures ranging from 77-298 K. Isosteric heats of adsorption were calculated using the virial method at low pressures. Gas storage performance of BLPs at 40 bar were found to be: 14.7-42.5 mg/g for H2 uptake at 77 K; 348.9-717.4 mg/g for CO2 uptake at 298 K; and 40.8-116.1 mg/g for CH4 uptake at 298 K. The CO2/CH4 selectivity of BLPs at 298 K up to 40 bar was calculated using the Ideal Adsorbed Solution Theory (IAST) to determine their performance as carbon capture and sequestration materials. Additionally, non-borazine containing nanoporous organic polymers (NPOFs) consisting of all carbon and hydrogen atoms were also synthesized and subjected to low pressure hydrogen storage measurements. The results show that though NPOFs generally exhibit higher surface areas (SALang = 2423-4227 m2/g), the H2 storage capacity of BLPs is superior.

Les matériaux à base de bore et d'azote présentent un grand potentiel et donc un grand intérêt pour des applications énergétique et en particulier dans le domaine du stockage de l'hydrogène. L'ammoniaborane (NH3BH3) s'est révélé, au milieu des années 2000, comme un matériau avec une grande capacité gravimétrique (19,6%m) et volumétrique (140 g.L-1) en hydrogène. Au cours de l'analyse de la bibliographie nous nous sommes aperçus que tous les travaux sur l'ammoniaborane portés sur sa déstabilisation thermique, nous avons donc choisi une approche originale en nous concentrant sur la stabilisation thermique de l'ammoniaborane. Mon travail de thèse a consisté à revisiter la synthèse de l'ammoniaborane pour en dégager les meilleurs paramètres de synthèse (précurseurs de bore et d'azote, solvant et température) possible en vue d'obtenir une température de début de déshydrogénation la plus haute possible. En effet, en faisant varier certains précurseurs nous avons pu observer une modification de la température de début de déshydrogénation et donc de la stabilité thermique de l'ammoniaborane. Après avoir déterminé les meilleurs paramètres de synthèses nous avons entrepris une étude thermique et thermolytique afin de comprendre quel(s) facteur(s) étai(en)t à l'origine de cette différence de stabilisation. Pour cela nous avons effectué une étude d'analyse thermogravimétrique couplée à un spectromètre de masse afin de déterminer le mécanisme de déshydrogénation et une étude en conditions isotherme afin de vérifier la stabilité des ammoniaboranes que nous avons synthétisés. Dans un troisième temps nous avons effectué une étude spectroscopique de surface, grâce à l'XPS et du matériau dans son ensemble, grâce à la RMN-MAS à l'état solide des noyaux de bore 11 et d'azote 15. Ces études nous ont permis de déterminer un nouveau mécanisme de déshydrogénation de l'ammoniaborane pour des expériences en conditions isotherme. / Boron and nitrogen based-materials offer a great potential and interest in energy applications and in particular in the field of hydrogen storage. The ammonia borane (NH3BH3) was revealed, in the mid 2000s, as a material with high gravimetric (19.6%m) and volumetric (140 g.L-1) capacities in hydrogen. During the analysis of the literature we realized that all studies on ammonia borane treated on its thermal destabilization, so we chose an original approach by focusing our work on the thermal stabilization of ammonia borane. My thesis work focused on the synthesis of ammonia borane to identify the best synthesis parameters (boron and nitrogen precursors, solvent, and temperature) for the highest possible onset temperature. Indeed, by varying some precursors we observed a change in the onset temperature and therefore in the thermal stability of the ammonia borane. After determining the best synthesis parameters we undertook thermal and thermolytic studies to understand which factor(s) is(are) responsible for the stabilization's differences. For this, we performed thermogravimetric analysis coupled to mass spectrometer studies to determine the dehydrogenation mechanism and studies in isothermal conditions to verify the stability of our ammonia boranes. Thirdly we performed a spectroscopic study by XPS and solid state MAS-NMR of boron 11 and nitrogen 15. These studies allowed us to identify a new mechanism of dehydrogenation of ammonia borane for experiments in isothermal conditions.

This master thesis investigated how a hydrogen energy storage could be used anddimensioned to reduce the problem of power shortage in the local distributiongrid in Uppsala, Sweden. By implementing such a storage system in mobilityhouses, which are parking garages with integrated charging stations for electric vehicles and smart renewable energy solutions for power generation, the problem with power shortage could be decreased. The results showed that by integrating a hydrogen storage together with battery packs, it was possible to reduce power peaks in mobility houses. Further, it was clear that more power peaks facilitated the dimensioning of these type of systems. It was also shown that due to today's initial cost of hydrogen storages, the total savings related to a limited purchase of electricity from the grid were insignificant. It was therefore found that this type of hydrogen storage would not reduce costs in the short term for the mobility houses considered in this study. However, implementing a smaller kW storage could generate and improve knowledge in the hydrogen/hybrid field, which could facilitate the implementation of larger systems in the future. Furthermore, the results showed that it could be interesting to implement hydrogen storages on a bigger scale for municipalities or actors, who would want to reduce the power shortage in the local distribution grid.

Hydrogen storage is a developing technology that can be used as an energy vector for sustainable energy applications such as fuel cells for transport applications or for supplying power to the grid in moments of high demand. However, before hydrogen can be used as a practical energy vector, hydrogen storage issues, such as low gravimetric storage density, need to be addressed. One possible solution could be using nanoporous materials to physically adsorb hydrogen at low temperatures and moderate pressures. Hydrogen adsorption excess isotherms in solid-state porous materials can be obtained experimentally. However, the total amount stored in them, a quantity of more practical interest, cannot be measured by experimental techniques. Therefore, a model developed at the University of Bath is used to predict the total amount of hydrogen contained in nanoporous materials from their experimentally derived excess isotherm data. According to inelastic neutron scattering experiments (TOSCA, ISIS, RAL, Oxfordshire), solid-like hydrogen is likely to exist within the pores. The model is applied in this work in order to search for relationships between intrinsic properties of the materials (BET surface area, pore volume and pore size) and the predicted total hydrogen capacity of the materials. The model assumes adsorbed hydrogen at a constant density within the pore (defined as the absolute), also taking bulk hydrogen in the pore (amount that is not considered to be adsorbed by the adsorbent), into account. Several MOF datasets have been used to search for these relations, since they are the materials that have the highest hydrogen uptake in solid-state adsorption. Different MOFs and MOF families have been tested in order to widen the range of the correlations. Also, different strategies, such as fixing the pore volume when applying the fittings, relying on experimental data, or using high pressure hydrogen isotherm data to increase the robustness of the model have been researched. These MOFs have been either synthesized and characterized at the University of Bath or their datasets obtained from literature. Some of these MOFs with zeolitic structure exhibited unreported flexibility, being their structures further characterized. Changes on accessible pore size for hydrogen storage were also investigated using C60 in IRMOF-1. The final aim of this work is to find possible correlations between BET surface area, pore volume and pore size to find out what the values of these parameters have to be in a specific material to fulfil the DOE hydrogen storage requirements.

Les matériaux nanostructurés multifonctionnels à base de terre rare (R) et métal de transition (T) présentent un intérêt croissant dans la recherche scientifique. Le développement de cet axe est basé sur la maitrise de la structure fondamentale et le comportement de la matière à l’échelle nanométrique. Dans ce travail, nous nous sommes intéressés aux alliages Pr5Co19, leurs dérivés carburés et hydrurés. Ce système cristallise dans la structure rhomboédrique de type Ce5Co19 de groupe d’espace R-3m. Le composé Pr5Co19 présente une transition magnétique de l’état ferromagnétique à l’état paramagnétique à 690 K. Une anisotropie uniaxiale avec un champ coercitif de l’ordre de 1.5 T ont été enregistrés à la température ambiante.Par ailleurs, Nous avons observé que l’insertion d’un élément léger tel que le carbone ou l’hydrogène est un moyen efficace permettant d’augmenter la température de Curie par rapport au composé parent. Les nanomatériaux, de formule générale Pr5Co19Hx, présentent des cycles d’absorption et désorption réversibles, avec une capacité d’absorption de l’hydrogène égale à 12H/f.u, soit 0.5 hydrogène par maille (H/M) au total.Parallèlement, nous nous sommes intéressés à l’étude de l’effet magnétocalorique des intermétalliques de type Pr-Co. Le composé Pr5Co19 présente un effet magnétocalorique géant de l’ordre de 5.2 J/kg.K pour un faible champ appliqué.Les nanomatériaux intermétalliques de type Pr5Co19 peuvent être ainsi considérés comme des composés multifonctionnels. Grâce à leurs propriétés structurales, magnétiques et magnétocaloriques, ils s’avèrent être de bons candidats dans le domaine des aimants permanents, mais aussi pour la réfrigération magnétique à haute température et pour le stockage de l’hydrogène, vu le besoin croissant en énergie alternative moins polluante / Multifunctional nanomaterials based on rare-earth (R) and transition metal (T) present a major interest in scientific research. We are interrested in the Pr5Co19 alloy. This system crystallizes in the rhombohedral Ce5Co19-type structure with space group R-3m. The Curie temperature Tc is about 690 K. We determined the value of the magnetization at saturation MS = 83 Am2 / kg using the approach law to saturation. A uniaxial anisotropy with a coercive field equal to 1.5 T at room temperature were obtained.Moreover, we have observed that the insertion of a light element such as carbon or hydrogen, allows to increase the Curie temperature of the system. The Pr5Co19Hx hydrides present a reversible cycle of absorption/desorption, with a hydrogen absorption capacity equal to 12H / f.u, or 0.5H / M in total.We are also interested in the study of the magnetocaloric effect of the intermetallics Pr-Co type. We have shown that Pr5Co19 compound has a giant magnetocaloric effect, of about 5.2 J / kg.K at low field.The intermetallic nanomaterials Pr5Co19 could be used as a multifunctional compound. These results indicate that it is an attractive alloy due to its structural, magnetic and magnetocaloric properties. It could be good candidates for permanent magnets, but also for magnetic refrigeration at high temperature and for hydrogen storage

Cette thèse est consacrée à l'étude de composés riches en magnésium innovants destinés au stockage solide de l'hydrogène. Le but est de déstabiliser l'hydrure de Mg et d'accélérer sa cinétique de sorption par des effets d'alliage et de nano-structuration. La première famille de composés concerne les phases pseudo-binaires Mg6Pd1-xMTx (MT = Ni, Ag, Cu). Leurs propriétés structurales et les effets de substitution du Pd ont été étudiés par diffraction des rayons X, microscopie électronique à balayage et microsonde de Castaing. Les propriétés thermodynamiques et cinétiques d'hydrogénation de ces matériaux ont ensuite été déterminées par réaction solide-gaz. Différents mécanismes d'hydrogénation sont mis en jeu en fonction de l'élément de substitution. La nature des phases formées lors de la réaction d'hydrogénation modifie la stabilité des systèmes métal-hydrogène. Ainsi, la transformation de métal à hydrure est caractérisée par au moins deux plateaux de pression. Le premier plateau a lieu à une pression proche de celle de Mg/MgH2, alors que le second se produit à pression plus élevée. La détermination des valeurs d'enthalpie et d'entropie de réaction ont permis de quantifier la déstabilisation atteinte. Les meilleures cinétiques de désorption sont obtenues pour l'alliage au Ni, grâce à l'effet catalytique de la phase Mg2NiH4 formée lors de l'hydrogénation. La seconde approche vise à combiner les effets d'alliage et de nano-structuration. Des nanoparticules de Mg6Pd atteignant des tailles aussi petites que 3 nm sont confinées dans des matrices carbonées nano-poreuses. En comparant leurs propriétés d'hydrogénation à celles de l'alliage massif équivalent, on démontre non seulement que la cinétique de (dés)hydrogénation des nanoparticules est bien plus rapide, mais aussi que leur état hydrogéné est déstabilisé. Enfin, des nano-composites MgH2-TiH2 ont été synthétisés par broyage mécanique sous atmosphère réactive. L'ajout d'un catalyseur (TiH2) et la nano-structuration du Mg permettent de considérablement accélérer les cinétiques d'absorption et désorption d'hydrogène dans le Mg. Afin de comprendre le rôle de la phase TiH2 sur les propriétés cinétiques remarquables de ces nano-composites, leurs propriétés structurales ont été déterminées par diffraction des rayons X et des neutrons. L'existence d'une interface cohérente entre les phases Mg et TiH2 est d'importance majeure pour faciliter la mobilité de H au sein du nano-composite. De plus, il est démontré que les inclusions de TiH2 freinent la croissance de grain de Mg/MgH2, permettant ainsi de maintenir la nano-structuration des composés lors de leur cyclage / This thesis is dedicated to the study of novel magnesium-rich compounds for solid state hydrogen storage. The aim is to destabilize Mg hydride and accelerate its sorption kinetics by alloying and nanostructuration. The first family of compounds concerns the Mg6Pd1-xTMx (TM = Ni, Ag, Cu) pseudo-binary phases. Their structural properties and the effects of Pd substitution have been studied by X-ray diffraction, scanning electron microscopy and electron microprobe analyses. Their thermodynamics and kinetics of hydrogenation have been determined by solid-gas reaction. Different hydrogenation mechanisms take place depending on the substituting element. The stability of the metal-hydrogen system is altered by the nature of the phases formed during hydrogenation reaction. Thus, metal to hydride transformation is characterized by at least two absorption plateau pressures. The pressure of the first plateau is similar to that of Mg/MgH2 while the second one occurs at higher pressure. The enthalpy and entropy of reaction are determined to quantify the destabilizing effect of Pd by TM substitution. Best desorption kinetics are found for the Ni containing alloy thanks to the catalytic effect of the Mg2NiH4 phase formed on hydrogenation. The second approach aims to combine alloying with nanostructuration effects. Nanoparticles of Mg6Pd as small as 3 nm are confined into nanoporous carbon matrix. By comparing their hydrogenation properties with those of the bulk alloy, we demonstrate that not only the (de)hydrogenation kinetics are much faster for the nanoparticles, but also that their hydrided state is destabilized. Finally, MgH2-TiH2 nanocomposites were synthesized by mechanical milling under reactive atmosphere. The addition of a catalyst (TiH2) and Mg nanostructuration allow strongly accelerating the sorption kinetics of hydrogen in Mg. To understand the role of the TiH2 phase on the outstanding kinetics of these nanocomposites, their structural properties have been determined by X-ray and neutron diffraction. The existence of a coherent interface between Mg and TiH2 phases is of major importance to facilitate H-mobility within the nanocomposite. Furthermore, it is shown that the TiH2 inclusions inhibit the Mg/MgH2 grain growth, thus maintaining the composites nanostructure during their cycling