Fossil fuels constitute a significant fraction of the world’s energy demand. The burning of fossil fuels emits huge amounts of carbon dioxide into the atmosphere. Therefore, the limited availability of fossil fuel resources and the environmental impact of their use require a change to alternative energy sources or carriers (such as hydrogen) in the foreseeable future. The development of methods to mitigate carbon dioxide emission into the atmosphere is equally important. Hence, extensive research has been carried out on the development of cost-effective technologies for carbon dioxide capture and techniques to establish hydrogen economy. Hydrogen is a clean energy fuel with a very high specific energy content of about 120MJ/kg and an energy density of 10Wh/kg. However, its potential is limited by the lack of environment-friendly production methods and a suitable storage medium. Conventional hydrogen production methods such as Steam-methane-reformation and Coal-gasification were modified by the inclusion of NaOH. The modified methods are thermodynamically more favorable and can be regarded as near-zero emission production routes. Further, suitable catalysts were employed to accelerate the proposed NaOH-assisted reactions and a relation between reaction yield and catalyst size has been established. A 1:1:1 molar mixture of LiAlH4, NaNH2 and MgH2 were investigated as a potential hydrogen storage medium. The hydrogen desorption mechanism was explored using in-situ XRD and Raman Spectroscopy. Mesoporous metal oxides were assessed for CO2 capture at both power and non-power sectors. A 96.96% of mesoporous MgO (325 mesh size, surface area = 95.08 ± 1.5 m2/g) was converted to MgCO3 at 350°C and 10 bars CO2. But the absorption capacity of 1h ball milled zinc oxide was low, 0.198 gCO2 /gZnO at 75°C and 10 bars CO2. Interestingly, 57% mass conversion of Fe and Fe3O4 mixture to FeCO3 was observed at 200°C and 10 bars CO2. MgO, ZnO and Fe3O4 could be completely regenerated at 550°C, 250°C and 350°C respectively. Furthermore, the possible retrofit of MgO and a mixture of Fe and Fe3O4 to a 300 MWe coal-fired power plant and iron making industry were also evaluated.

Orientadores: Munir Salomão Skaf, Vitor Rafael Coluci / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Química / Made available in DSpace on 2018-08-26T20:42:42Z (GMT). No. of bitstreams: 1 Faro_TatianaMellodaCosta_D.pdf: 8054394 bytes, checksum: ce0d79df42ce453ffc39b51bf0ad1094 (MD5) Previous issue date: 2015 / Resumo: Atualmente, a economia mundial depende do uso de combustíveis fósseis para a geração de energia. Esse modelo apresenta problemas ambientais graves, uma vez que o petróleo é um material não-renovável e muito poluente. O gás hidrogênio apresenta-se como uma alternativa promissora para substituir os combustíveis utilizados atualmente devido a um conjunto de características positivas: ele é atóxico, tem uma alta densidade energética gravimétrica e gera apenas água como produto de sua combustão. Apesar de tais vantagens, ele ainda não é utilizado comercialmente em larga escala. O maior empecilho tecnológico para que o hidrogênio possa substituir os combustíveis fósseis está no seu armazenamento. Existem diversas propostas para armazenar o hidrogênio, como tanques contendo o hidrogênio nas formas de gás pressurizado ou de líquido, além de sistemas sólidos que permitam a sua adsorção. Todavia, nenhum sistema construído até então foi capaz de armazenar o hidrogênio de forma tão barata, segura e eficaz quanto seria necessário. Nanoestruturas de carbono são vistas como uma boa alternativa para construir dispositivos de armazenamento de hidrogênio baseados na fisissorção. Os nanopapiros de carbono, formados por folhas de grafeno enroladas no formato de um papiro, são considerados particularmente promissores para armazenar o hidrogênio, uma vez que possuem uma alta área superficial, extremidades abertas e distâncias intercamadas facilmente controláveis. Na primeira etapa deste trabalho, realizamos simulações de Dinâmica Molecular (MD) para estudar a dinâmica e a estabilidade de diversos nanopapiros em função de alguns dos seus parâmetros estruturais. Posteriormente, aplicamos o método de Monte Carlo Grand-Canônico (GCMC) para estudar o processo de adsorção de hidrogênio em nanopapiros selecionados, de forma a caracterizar quantitativamente e qualitativamente as fases adsorvidas / Abstract: Presently, the world economy depends on the use of fossil fuels to generate energy. This model presents serious environmental problems, since petroleum is a non-renewable and very pollutant material. Hydrogen gas presents itself as a promising alternative to substitute the fuels currently used due to a few positive characteristics: it is non-toxic, possesses a high gravimetric energetic density and only generates water as a combustion byproduct. In spite of all these advantages, hydrogen still isn't used commercially in a large scale. The biggest technological drawback for hydrogen to substitute fossil fuels is in its storage. There are many proposed ways to store hydrogen, such as tanks containing highly pressurized or liquid hydrogen, or solid systems that allow its adsorption. However, no system built up to the date had been able to store hydrogen as cheap, safe and efficiently as necessary. Carbon nanostructures are seen as a good alternative to build hydrogen storage devices based on physisorption. Carbon nanoscrolls, formed by graphene sheets scrolled in a papirus-like shape, are considered as particularly promising adsorption materials, since they possess a high surface area, open edges and easily controllable interlayer distances. In the first step of this work, we made Molecular Dynamics (MD) simulations to study the dynamics and the stability of several nanoscrolls as a function of their structural parameters. Subsequently, we used the Grand-Canonical Monte Carlo (GCMC) method to study the hydrogen adsorption process in selected nanoscrolls, as to characterize the adsorbed phases quantitatively and qualitatively / Doutorado / Físico-Química / Doutora em Ciências

The present study focuses on the development of Al-based systems and their examination as a medium for reversible hydrogen uptake. The first part of this thesis is dedicated to the chemistry and properties of Al-N-based materials. The synthesis, characterization, and detailed thermal decomposition studies of several aminoalanes have been described. As a result, single-crystal X-ray diffraction analyses revealed two new crystal structures of piperidinoalanes. The perspective approach employing activated aluminum and piperidine for reversible hydrogen uptake has been established. The second part of this work was focused on the modification of the properties of NaAlH4-based systems in order to generate the material with the high dissociation pressure suitable for high-pressure tank technologies. Considerable progress has been achieved in improving the hydrogen sorption properties by adding the extra aluminum powder to the Ti-catalysed NaAlH4-based system. Thus, the present study contributes to the understanding of the hydrogen sorption behavior of Al-based systems with perspectives being applicable to other related materials.:DECLARATION ACKNOWLEDGEMENTS DEFINITIONS AND ABBREVIATIONS ABSTRACT CONTENTS LIST OF TABLES LIST OF FIGURES MOTIVATION AND GOALS 1 INTRODUCTION 1.1 The prospects for hydrogen-based energy systems 1.2 Requirements for the hydrogen storage system 1.3 An overview of hydrogen storage strategies 1.4 Complex hydrides as a promising hydrogen storage materials 1.4.1 Metal borohydride systems 1.4.2 Alanate-based systems 1.4.3 Nitrogen-containing complex hydrides 1.5 Summary 2 GENERAL CHARACTERIZATION METHODS 2.1 X-ray crystallography 2.1.1 X-ray powder diffraction (XRPD) 2.1.2 Single-crystal structure analysis 2.2 Thermal analysis 2.3 Quantitative chemical analysis 2.3.1 Elemental analysis 2.3.2 Inductively coupled plasma optical emission spectrometry (ICP-OES) 2.4 Nuclear magnetic resonance spectroscopy (NMR) 3 LIQUID-STATE HYDROGEN STORAGE 3.1 State of the art 3.1.1 Liquid-state hydrogen storage materials 3.1.2 Al-N-based compounds as potential materials for hydrogen storage 3.1.3 Summary 3.2 Materials preparation and experimental details 3.2.1 Chemicals and sample handling 3.2.2 Synthesis of aminoalane in diethyl ether solution with aluminum hydride 3.2.3 Preparation of activated aluminum 3.2.4 Direct hydrogenation of activated aluminum supported by amine 3.3 Results and discussion 3.3.1 Is the solid-state hydrogen storage in aminoalanes possible? 3.3.2 Optimization of the direct hydrogenation of activated aluminum supported by amine 3.3.2.1 Synthesis and characterization of triethylenediamine alane complex 3.3.2.2 Synthesis of aminoalanes via direct hydrogenation of activated aluminum and N-heterocyclic amine 3.3.3 Investigation of piperidinoalanes for reversible hydrogen uptake 3.3.3.1 Crystal structure determination of piperidinoalanes 3.3.3.2 Influence of the initial reaction parameters on the piperidinoalane formation 3.3.3.3 Reversible hydrogenation in piperidinoalane system 3.3.4 Conclusions 4 SOLID-STATE HYDROGEN STORAGE 4.1 State of the art 4.1.1 Thermodynamic tuning of the hydrides 4.1.2 Features of the sodium alanate system 4.1.3 Catalytic enhancement of reversible hydrogenation in sodium alanate 4.1.4 The relevance of the Al-TM species in doped sodium alanate 4.1.5 Summary 4.2 Materials preparation and experimental details 4.2.1 Chemicals and purification procedure 4.2.2 Activation procedure of sodium alanate via mechanochemical treatment 4.2.3 Pressure-composition-isotherm measurements with a Sieverts-apparatus 4.2.4 High-pressure differential scanning calorimetry investigation of sodium alanate samples 4.3 Results and discussion 4.3.1 Tailoring the properties of sodium alanate-based system with the help of Ti-additive 4.3.2 Influence of the aluminum addition on the sorption behavior of Ti-doped sodium alanate 4.3.3 High-pressure DSC study of hydrogen sorption properties of doped sodium alanate system 4.3.4 Conclusions 5 SUMMARY AND CONCLUSIONS RECOMMENDATIONS AND OUTLOOK REFERENCES SUPPORTING INFORMATION Appendix A Appendix B Appendix C Publications

Dans le cadre de la recherche de vecteurs d'énergie “propres”, le borazane et ses dérivés amine-boranes sont devenus des candidats intéressants en tant que matériaux de stockage de l'hydrogène en raison de leur pourcentage massique relativement élevé en hydrogène (19,6% pour borazane) et de la réversibilité potentielle de la réaction de déshydrogénation. Pour des applications réelles, le contrôle des réactions se produisant à la température ambiante est fondamental. Dans ce contexte, la compréhension du processus de la déshydrogénation/déshydrocouplage catalytique de l'amine-borane apparaît comme un élément important. Dans cette thèse, les catalyseurs de types métallocènes du Groupe IV (Cp2M, M = Ti, Zr et Hf) sont étudiés en détail. Le déshydrocouplage de HMe2N·BH3 catalysé par le titanocène a été étudié à la fois expérimentalement et théoriquement mais aucun accord n'avait été atteint auparavant. Dans ce travail, les caractérisations systématiques des interactions 3-centre 2-électron M···H-B impliquées dans les intermédiaires réactionnels ont été réalisées avec des approches topologiques QTAIM et ELF. Par la suite, des mécanismes réactionnels détaillés ont été étudiés. Les résultats théoriques ont démontré que la méthode DFT corrigée avec la dispersion (DFT-D) étaient nécessaire et suffisantes pour une description énergétique correcte des chemins réactionnels. Mon travail a également permis l'identification d'un complexe de van der Waals jouant un rôle clé dans le mécanisme réactionnel en accord avec les observations expérimentales. / With the increasing demand of clean energy carriers, ammonia borane and its related amine-borane compounds have emerged as attractive candidates for hydrogen storage materials due to their relatively high weight percentage of available hydrogen (19.6% for ammonia borane) as well as the potential reversibility for the hydrogen release reactions. Actual applications would benefit from controlled reactions occurring close to room-temperature. In this context, catalytic dehydrogenation/dehydrocoupling of amine-borane appears as a promising solution. In this thesis the Group IV metallocene (Cp2M, M = Ti, Zr and Hf) are mainly discussed. The dehydrocoupling of HMe2N·BH3 catalyzed by titanocene was investigated both experimentally and theoretically but no agreement were reached. In this work, systematic characterization of M···H-B 3-center 2-electron interactions involved in reaction intermediates were carried out with QTAIM and ELF topological approaches. Afterwards, detailed mechanisms were further studied. Computational results have demonstrated that the dispersion corrected DFT (DFT-D) method was indispensable for a correct enegetic prediction for reaction pathways. The identification of a van der Waals complexe also plays a central role for a reaction mechanism with good agreement with experimental observations.

Le développement de l’hydrogène en tant que nouveau vecteur d’énergie demande de pouvoir le stocker à grande échelle, dans des conditions d’encombrement, de coût énergétique et de sécurité acceptables. Le stockage sous forme solide dans des hydrures métalliques réversibles, constitue une solution particulièrement sûre et intéressante pour des applications dans le secteur des transports. Parmi de nombreux matériaux possibles, le système Mg/MgH2, constitue l’un des meilleurs candidats : abondant, bon marché, capacité de stockage réversible et élevée (7,6 % H2 en masse). Son utilisation à l’échelle industrielle est néanmoins limitée par les cinétiques de sorption très lentes et la stabilité thermodynamique importante (enthalpie de formation élevée) nécessitant des températures de fonctionnement supérieure à 300 °C. Ce projet vise au développement de composites à base de MgH2 et d’additifs avec des propriétés de stockage améliorées. L’originalité des travaux menés repose sur le type d’additifs choisi, les complexes de métaux de transition (centre métallique : Ni et Ru, ligands organiques : phosphines). En effet, ces derniers ne sont pour le moment que très peu utilisés dans la littérature. L’objectif de ce travail de thèse est donc d’explorer leur potentiel et leur efficacité pour améliorer les propriétés de stockage du système Mg/MgH2. Différents composites "MgH2 + complexe" ont été préparés par broyage et imprégnation et les cinétiques de sorption des systèmes obtenus ainsi que leurs paramètres thermodynamiques ont été déterminés par analyse thermique (DTP, DSC, PCT). Enfin, de nombreuses techniques de caractérisation physico-chimiques (DRX, RMN, XPS, MEB, MET) ont été utilisées afin de comprendre les phénomènes se déroulant lors de l’hydrogénation et la déshydrogénation des composites préparés. Le meilleur système « MgH2 + complexe » préparé durant ce travail (MgH2 dopé avec 20 % du complexe NiHCl(PCy3)2) est capable d’absorber 6 % en masse d’H2 à 100 °C en 30 min et de libérer son hydrogène sous vide à 200 °C. Les énergies d’activation apparentes et enthalpies de formation de ce composite sont respectivement de 22 et –65 kJ/mol H2 pour l’hydrogénation (contre 200 et –74,7 kJ/mol H2 pour du Mg commercial) et de 127 et 63 kJ/mol H2 pour la déshydrogénation (contre 239 et 74,7 kJ/mol H2 pour du MgH2 commercial) / Although hydrogen is widely recognized as a promising energy carrier, its widespread adoption as alternative to fossil fuels depends critically on the ability to store hydrogen at adequate densities, cost and security. Application devices are far from a valuable technology, and fundamental research is still required. In this regard, solid-state systems present the advantage of denser and safer hydrogen storage. Among them, Mg/MgH2 is considered as a highly promising material in terms of reversibility, cost, gravimetric and volumetric capacity. However, high thermodynamic stability (high formation enthalpy) and slow hydrogen sorption kinetics limits its practical applications.This project aims to develop Mg/MgH2-based systems with improved hydrogen storage properties thanks to the use of additives. The originality of this work is brought by the kind of additive chosen, transition metal complexes (Ni and Ru based, with phosphine ligands). Indeed, they are, for now, only very little used in the literature. The objective of this work is therefore to study their potential and their efficiency to improve the hydrogen storage properties of the Mg/MgH2 system. Different “MgH2 + complex” composites were prepared by ball milling and impregnation method and the sorption kinetics and thermodynamic parameters of the formed systems were studied by TPD, DSC and PCT analyses. Finally, XRD, NMR, XPS, SEM and TEM techniques were used to understand the phenomena taking place during the hydrogenation and the dehydrogenation of the prepared composites.The best “MgH2 + complex” system prepared during this work (MgH2 doped with 20 wt% of NiHCl(PCy3)2 complex) is able to absorb 6 wt% of H2 at 100 °C in 30 min, and to release the stored hydrogen at 200 °C under vacuum. The apparent activation energies and the formation enthalpies of the composite are respectively of 22 and –65 kJ/mol H2 for the hydrogenation reaction (against 200 and –74,7 kJ/mol H2 for commercial Mg) and of 127 and 63 kJ/mol H2 for dehydrogenation (against 239 and 74,7 kJ/mol H2 for commercial MgH2).

A magnetic functionalization of microcrystalline MOF particles was realized using magnetic iron oxide particles. Such magnetic MOFs can be separated using a static magnetic field after use in catalytic processes and heated by an external alternating magnetic field to trigger desorption of encaged drug molecules. / Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich.

Im Rahmen dieser Arbeit wurden neuartige, gemischte Tetrahydridoborate (Borhydride), die für die Wasserstoffspeicherung im Festkörper für die mobile Anwendung geeignet sein könnten, synthetisiert und vollständig charakterisiert. Entscheidende Materialanforderungen für die Kombination mit einer Tieftemperaturbrennstoffzelle sind die hohe Wasserstoffspeicherkapazität von min. 6 m% bei einer Wasserstoffdesorption unterhalb von 100°C. Um beide dieser Hauptkriterien zu erfüllen, wurden Li-Al- und Na-Al-Borhydrid entsprechend dem Konzept von Nakamori u.a. ausgewählt. Beide Borhydride desorbieren unterhalb von 100°C, wobei das synthetisierte Li-Al-Borhydrid aufgrund des hohen Wasserstoffgehalts (17,2 m% H2) die vielversprechendsten Eigenschaften zeigte. Beide Systeme wurden mittels Pulverdiffraktometrie am Synchrotron hinsichtlich Ihrer Struktur aufgeklärt, wobei die Struktur der einzelnen komplexen Ionen anhand von Schwingungsspektroskopie (Infrarot-, Ramanspektroskopie) ebenfalls bestätigt werden konnte. Mit Hilfe verschiedener kombinierter Desorptionsanalysen war es möglich den Zersetzungspfad, insbesondere die Bildung instabiler Desorptionsprodukte, aufzuklären. So erfolgt die Zersetzung des Li-Al-Borhydrids über die Bildung von Lithiumborhydrid in der Festphase, das mittels in-situ Ramanspektroskopie in einer speziellen Ramanzelle beobachtet werden konnte. Die Infrarotspektroskopie des Desorptionsgases zeigte zunächst die Freisetzung von Aluminiumborhydrid, dass wiederrum Diboran und Wasserstoff desorbiert. Weiterhin wurden verschiedene Möglichkeiten verfolgt, wie der Zusatz von Kohlenstoff oder das Nanoconfinement von Lithiumalanat, um den Zersetzungsweg hinsichtlich ausschließlicher Wasserstofffreisetzung zu modifizieren und somit Reversibilität zu ermöglichen. Es konnte jedoch kein reversibles System mit hoher gravimetrischer Wasserstoffspeicherdichte und Desorption unterhalb von 100°C erzeugt werden. / Aim of the work was the synthesis and characterisation of novel mixed tetrahydroborates (borohydrides) for solid state hydrogen storage suitable for mobile applications. The combination with a PEM fuel cell requires a material with at least 6 wt% hydrogen combined with hydrogen desorption below 100°C. To fulfill both criteria, Li-Al- und Na-Al-borohydride were selected according to Nakamori’s concept. Both mixed borohydrides desorb well below 100°C whereas the mixed Li-Al-borohydride showed the most promising properties due to its high gravimetric hydrogen content (17.2 wt% H2). The crystal structures were examined by powder diffraction with a synchrotron source. The symmetry of the containing complex cations and anions was confirmed with vibrational spectroscopy (infrared, raman spectroscopy). The desorption pathway was clarified using a variety of combined thermal analysis techniques. Especially the desorption of unstable products of the most promising Li-Al-borohydride was possible via spectroscopy. Hence the desorption of Li-Al-borohydride leads to the formation of lithium borohydride in the solid state which was monitored via in-situ raman spectroscopy in a special raman cell. Infrared spectroscopy of the desorbed gas showed the initial desorption of aluminium borohydride which desorbs diborane and hydrogen in the following. Different options were examined in order to modify this desorption pathway by carbon addition or nanoconfinement of lithium alanate. However, none of the materials showed high hydrogen content combined with exclusive hydrogen desorption below 100°C and reversibility.

In dieser Arbeit werden kompaktierte Verbundwerkstoffe aus verschiedenen Speichermaterialien mit expandiertem Naturgraphit (ENG) in Hinblick auf die Anwendung als dynamische Wasserstofffeststoffspeichermaterialien untersucht. Pulverförmige hydridbildende Ausgangsmaterialien wurden mit bis zu 25 Masse-% ENG vermischt und bei Pressdrücken bis 600 MPa kompaktiert. Um einen weiten Anwendungsbereich abzudecken wurden ein Niedrigtemperaturmaterial (Ti-Mn-basierte Legierung; 0°C bis 100°C), zwei Mitteltemperaturmaterialien (Amid- und Alanatsystem; 100°C bis 200°C) und ein Hochtemperaturmaterial (Magnesium-Nickel-Legierung; 250°C bis 400°C) basierend auf einer umfangreichen Literaturrecherche gewählt. Die Verbundwerkstoffe weisen eine erhöhte radiale Wärmeleitfähigkeit auf und zeichnen sich im Vergleich zu herkömmlich verwendeten Pulverschüttungen durch höhere volumetrische Wasserstoffspeicherdichten aus. Im Fokus der Untersuchungen stehen vor allem die im Hinblick auf Anwendungstauglichkeit wesentlichen Eigenschaften der Verbundwerkstoffe. So wurde z.B. der Wasserstoffdruck während der Dehydrierung variiert, um sicher zu stellen, Verbraucher mit üblichen Überdrücken versorgen zu können. Darüber hinaus wurde die Stabilität, Gaspermeabilität, Wärmeleitfähigkeit und Porosität der Presslinge im Verlauf zyklischer Hydrierung evaluiert und diskutiert. Insgesamt zeichnet sich ein hohes Potenzial ab, derartige Presslinge als Wasserstoffspeichermaterial für verschiedene Anwendungen entsprechend der jeweiligen Arbeitstemperaturen und weiteren Randbedingungen (z.B. Systemmasse, Tankvolumen etc.) zu verwenden. Diesbezüglich konnte mittels eines Tankdemonstrators basierend auf dem Ti-Mn-System ein Wasserstofffahrzeug erfolgreich betrieben und somit auch die Praxistauglichkeit der Hydrid-Graphit-Verbundmaterialien gezeigt werden. / Compacted composites of solid-state hydrogen storage materials and expanded natural graphite (ENG) are investigated in view of their potential for hydrogen storage applications. Powdery hydride-forming materials were blended with up to 25 weight-% ENG and compacted with up to 600 MPa compaction pressure. In order to cover a wide range of possible applications one low-temperature material (Ti-Mn-based alloy; 0°C to 100°C), two mid-temperature materials (amide and alante system; 100°C to 200°C) and one high-temperature material (magnesium-nickel alloy; 250°C to 400°C) were chosen based on a thorough literature review. The composites result in an increased radial thermal conductivity and are superior in their volumetric hydrogen storage density compared to commonly used loose powder beds. The research is focused on the applicability of suchlike prepared composites. In this regard, the dehydrogenation back-pressures were varied to ensure a sufficient supply pressure of common consumer loads. Furthermore, the stability, gas permeability, thermal conductivity and porosity throughout cyclic hydrogenation were evaluated and discussed. Overall, a high potential to use suchlike composite materials for hydrogen storage applications regarding the specific working conditions (temperature, system mass, available volume etc.) is found. In this regard, a demonstrator tank equipped with Ti-Mn-based system was successfully supplying a hydrogen driven vehicle, which proves the feasibility of these hydride-graphite composite materials.

Hydrogen is an efficient, carbon-free and safe energy carrier. However, its compact and weight-efficient storage is an ongoing subject for research and development. Among the intensively investigated hydrogen storage materials, lithium aluminum hydride (LiAlH4) is an attractive candidate because of its high theoretical hydrogen density (volumetric: 96.7g H2/l material; gravimetric: 10.6 wt.%-H2) in combination with rather low decomposition temperatures (onset temperature <100°C after doping). Although the reversible dehydrogenation of LiAlH4 must be carried out with the help of organic solvent, LiAlH4 can serve as single-use hydrogen storage material for various special applications, for example, hydrogen fuel cell systems. This thesis deals with transition metal (TM)-doped LiAlH4 aiming at tailored dehydrogenation properties. The crystal structure and morphology of TM-doped LiAlH4 is characterized by XRD and SEM respectively. The positive effects of four dopants (NiCl2, TiCl3, ZrCl4 and TiCl4) on promoting the dehydrogenation kinetics of LiAlH4 are systematically studied by thermal analysis. Based on the state of each TM chloride (solid or liquid), three low-energy-input doping methods (1. ball-milling at low rotation speed; 2. manual grinding or magnetic stirring; 3. magnetic stirring in ethyl ether) are compared in order to prepare LiAlH4 with the maximum amount of hydrogen release in combination with fast dehydrogenation kinetics. The dehydrogenation properties of the TM-doped LiAlH4 powders are measured under isothermal conditions at 80°C at a H2 pressure of 1 bar, which is within the operating temperature range of proton exchange membrane (PEM) fuel cells, aiming at applications where the exhaust heat of the fuel cell is used to trigger the dehydrogenation of the hydrogen storage material. Furthermore, the mid-term dehydrogenation behavior of TM-doped LiAlH4 was monitored up to a few months in order to test its mid-term storability. In addition, the pelletization of TM-doped LiAlH4 is investigated aiming at a higher volumetric hydrogen storage capacity. The effects of compaction pressure, temperature and the H2 back-pressure on the dehydrogenation properties of TM-doped LiAlH4 pellets are systematically studied. Moreover, the volume change through dehydrogenation and the short-term storage of the TM-doped LiAlH4 pellets are discussed in view of practical applications for PEM fuel cell systems. / Wasserstoff ist ein effizienter, kohlenstofffreier und sicherer Energieträger. Jedoch die kompakte und gewichtseffiziente Speicherung ist ein permanentes Forschungs- und Entwicklungsthema. Unter den intensiv untersuchten Materialien für die Wasserstoffspeicherung ist aufgrund der hohen theoretischen Speicherdichte (volumetrisch: 96,7 g H2/L, gravimetrisch: 10.6 Gew.%-H2) in Kombination mit sehr niedrigen Zersetzungstemperaturen (Anfangstemperatur < 100°C nach Dotierung) Lithium Aluminiumhydrid (LiAlH4) ein vielversprechender Kandidat. Obwohl die reversible Dehydrierung von LiAlH4 mit Hilfe von organischen Lösungsmitteln durchgeführt werden muss, kann LiAlH4-Pulver als Einweg-Speichermaterial für verschiedene Anwendungen dienen, beispielsweise für Wasserstoff/Brennstoffzellensysteme. Diese Doktorarbeit beschäftigt sich mit LiAlH4 dotiert mit Übergangsmetall, mit dem Ziel maßgeschneiderte Dehydrierungseigenschaften zu erreichen. Die Kristallstruktur und die Morphologie der mit Übergangsmetallen dotierten LiAlH4-Pulver wurden mit Röntgenbeugung (XRD) und Rasterelektronenmikroskopie (REM) charakterisiert. Weiterhin wurde der positive Effekt der Dotanden auf die reaktionsfördernde Dehydrierung von LiAlH4 systematisch mit Hilfe thermoanalytischer Methoden untersucht. Für jedes Übergangsmetall, welches in Form von Übergangsmetallchloriden vorlag, wurden drei Dotierungsmethoden mit niedrigem Energieeintrag (Kugelmahlen mit geringer Rotations-geschwindigkeit, manuelles Schleifen/Magnetrühren, Magnetrühren mit Ethylether) verglichen, um LiAlH4-Pulver mit einer maximalen Wasserstofffreisetzungsmenge in Kombination mit einer schnellen Dehydrierungskinetik zu erzielen. Die Dehydrierung des dotierten LiAlH4-Pulvers wurde unter isothermen Bedingungen bei 80°C und einem H2-Druck von 1 bar gemessen, was im Bereich der Betriebstemperatur von PEM-Brennstoffzellen (Proton Exchange Membran) liegt. Dadurch sollen Anwendungen anvisiert werden, bei denen die entstehende Abwärme der Brennstoffzelle genutzt wird, um die Dehydrierung des Wasserstoffspeichermaterials auszulösen. Zudem wurde das Dehydrierungsverhalten des dotierten LiAlH4 bis zu einigen Monaten kontrolliert, um die mittelfristige Haltbarkeit zu testen. Weiterhin wurde die Pelletierung des mit Übergangsmetallen dotierten LiAlH4 mit dem Ziel untersucht, eine hohe volumetrische Speicherkapazität zu erreichen. Der Einfluss des Pressdrucks, der Dehydrierungstemperatur und des H2-Gegendrucks auf die Dehydrierungseigenschaften der mit Übergangsmetallen dotierten LiAlH4-Presslinge wurde systematisch analysiert. Außerdem wird die Volumenveränderung durch die Dehydrierung und die Kurzzeitspeicherung der mit Übergangsmetallen dotierten LiAlH4-Presslinge im Hinblick auf praktische Anwendungen unter Nutzung der Brennstoffzelle diskutiert.

Die Speicherung von Wasserstoff in Form von Hydriden im festen Aggregatzustand hat den Vorteil einer hohen volumetrischen und gravimetrischen Wasserstoffspeicherdichte, die sowohl für die stationäre als auch die mobile Anwendung nötig ist. Um die Anforderungen dieser Anwendungen erfüllen zu können, müssen die Speichereigenschaften dieser Materialien weiter verbessert werden. Als zentrales Konzept dieser Dissertation wird die Nanostrukturierung verfolgt, die eine vielversprechende Strategie zur Modifizierung der thermodynamischen und kinetischen Eigenschaften von Hydriden darstellt. Die Transmissionselektronenmikroskopie (TEM) stellt dabei eine unverzichtbare Untersuchungsmethode solch nanoskopischer Materialien dar. Als problematisch erweist sich dabei die durch Radiolyse hervorgerufene Zersetzung der meisten Hydride bei der Beleuchtung mit dem abbildenden Elektronenstrahl. Im ersten Teil dieser Arbeit wird eine Methodik entwickelt um dieses Phänomen quantitativ mit Hilfe von Valenzelektronenenergieverlustspektroskopie zu untersuchen. Hierzu kommt kugelgemahlenes MgH2 als Modellsystem zum Einsatz. Die Dehydrierung kann quantitativ durch die inelastische Streuung der hochenergetischen Elektronen am MgH2-Plasmon erklärt werden. Eine Lösung dieses grundlegenden Problems wird theoretisch an Hand von Multislice TEM-Kontrastsimulationen untersucht. Hierbei wird ein TEM Experiment unter Wasserstoff bei Umgebungsdruck anstatt unter Vakuum simuliert, was mit Hilfe eines speziellen TEM Halters, in dem das Gas durch elektronentransparente Fenster eingeschlossen ist, realisiert werden kann. Im zweiten Teil wird der Einfluss des Nanoconfinements (Nanoeinschließung), einer speziellen Form der Nanostrukturierung, des komplexen Hydrids LiBH4 auf dessen Wasserstoffspeichereigenschaften untersucht, wofür eine neuartige nanoporöse aerogel-ähnliche Kohlenstoff-Gerüststruktur zum Einsatz kommt. Diese wird durch Salt Templating synthetisiert - einer simplen und nachhaltigen Methode zur Herstellung nanoporöser kohlenstoffbasierter Materialien mit großen Porenvolumina. Es wird gezeigt, dass durch das Nanoconfinement die Wasserstoffdesorptionstemperatur, die für makroskopisches LiBH4 bei über 400 °C liegt, auf 310 °C sinkt und die Desorption bereits bei 200 °C einsetzt. Eine teilweise Rehydrierung ist unter moderaten Bedingungen (100 bar und 300 °C) möglich, wobei die Reversibilität durch eine partielle Oxidation des amorphen Bor gehemmt ist. Im Gegensatz zu Beobachtungen einer aktuellen Veröffentlichung von in hoch geordnetem, nanoporösen Kohlenstoff eingebetteten LiBH4 deuten die in-situ TEM-Heizexperimente der vorliegenden Arbeit darauf hin, dass beide Reaktionsprodukte (B und LiH) in den Poren des aerogel-ähnlichen Kohlenstoffs verbleiben.:List of Figures vi 1. Introduction 1 2. Fundamentals 5 2.1. Solid state hydrogen storage 5 2.2. Thermodynamics 7 2.3. Magnesiumhydride, MgH2 9 2.4. Lithiumborohydride, LiBH4 10 2.5. Nanoconfinement 12 2.5.1. Nanoconfinement of MgH2 13 2.5.2. Nanoconfinement of LiBH4 15 2.6. Radiation damage of hydrides in the TEM 17 3. Theoretical and Experimental Methods 19 3.1. Ball milling 19 3.2. X-ray diffraction analysis 19 3.3. Thermal Characterization 20 3.3.1. Differential Scanning Calorimetry 20 3.3.2. Coupled Thermogravimetry and Mass Spectroscopy 21 3.4. Melt infiltration of LiBH4 21 3.5. Solid State Nuclear Magnetic Resonance 21 3.6. Transmission Electron Microscopy 23 3.6.1. In-situ TEM Heating 25 3.6.2. Environmental TEM 26 3.6.3. Electron Energy Loss Spectroscopy 28 3.6.4. Multislice Simulations 33 4. Electron Beam induced Dehydrogenation of MgH2 38 4.1. Microscopic Model of Hydrogen Release 38 4.2. Determination of Characteristic Electron Doses 39 4.3. Beam Damage Mechanism 42 4.4. Thickness Dependence of the Electron Dose 43 4.5. Conclusions for the Electron Beam Induced Dehydrogenation 47 4.6. Multislice Simulations for ETEM Studies 47 4.6.1. Methods of the Multislice Simulations 48 4.6.2. Results and Discussion of the Multislice Simulations 50 4.6.3. Conclusions of the Multislice Simulations 54 5. Nanoconfinement of LiBH4 in Aerogel-Like Carbon 56 5.1. Nanoporous Carbon Scaffolds 56 5.2. DSC Analysis of Melt Infiltration 58 5.3. XRD Analysis 59 5.4. Thermogravimetry and Mass Spectrometry Analysis 60 5.4.1. Hydrogen Desorption Properties of Nanoconfined LiBH4 60 5.4.2. Rehydrogenation of Nanoconfined LiBH4 63 5.5. In-situ STEM Analysis 63 5.6. Solid State 11B NMR 67 6. Conclusions 69 A. Appendix 71 A.1. Derivation of the Fourier-Log Deconvolution 71 A.2. Derivation of Equation 4.2 73 Bibliography 75 / Storing hydrogen in solid hydrides has the advantage of high volumetric and gravimetric hydrogen densities, which are needed for both stationary and mobile applications. However, the hydrogen storage properties of these materials must be further improved in order to meet the requirements of these applications. Nanostructuring, which represents one of the central approaches of this thesis, is a promising strategy to tailor the thermodynamic and kinetic properties of hydrides. Transmission electron microscopy (TEM) is an indispensable tool for the structural characterization of such nanosized materials, however, most hydrides degrade fast upon irradiation with the imaging electron beam due to radiolysis. In the first part of this work, a methodology is developed to quantitatively investigate this phenomenon using valence electron energy loss spectroscopy on ball milled MgH2 as a model system. The dehydrogenation can be quantitatively explained by the inelastic scattering of the incident high energy electrons by the MgH2 plasmon. A solution to this fundamental problem is theoretically studied by virtue of multislice TEM contrast simulations of a windowed environmental TEM experiment, which allows for performing the TEM analysis in hydrogen at ambient pressure rather than vacuum. In the second part, the effect of the nanoconfinement of the complex hydride LiBH4 on its hydrogen storage properties is investigated. For this, a novel nanoporous aerogel-like carbon scaffold is used, which is synthesized by salt templating - a facile and sustainable technique for the production of nanoporous carbon-based materials with large pore volumes. It is shown that the hydrogen desorption temperature, which is above 400 °C for bulk LiBH4, is reduced to 310 °C upon this nanoconfinement with an onset temperature as low as 200 °C. Partial rehydrogenation can be achieved under moderate conditions (100 bar and 300 °C), whereby the reversibility is hindered by the partial oxidation of amorphous boron. In contrast to recent reports on LiBH4 nanoconfined in highly ordered nanoporous carbon, in-situ heating in the TEM indicates that both decomposition products (B and LiH) remain within the pores of the aerogel-like carbon.:List of Figures vi 1. Introduction 1 2. Fundamentals 5 2.1. Solid state hydrogen storage 5 2.2. Thermodynamics 7 2.3. Magnesiumhydride, MgH2 9 2.4. Lithiumborohydride, LiBH4 10 2.5. Nanoconfinement 12 2.5.1. Nanoconfinement of MgH2 13 2.5.2. Nanoconfinement of LiBH4 15 2.6. Radiation damage of hydrides in the TEM 17 3. Theoretical and Experimental Methods 19 3.1. Ball milling 19 3.2. X-ray diffraction analysis 19 3.3. Thermal Characterization 20 3.3.1. Differential Scanning Calorimetry 20 3.3.2. Coupled Thermogravimetry and Mass Spectroscopy 21 3.4. Melt infiltration of LiBH4 21 3.5. Solid State Nuclear Magnetic Resonance 21 3.6. Transmission Electron Microscopy 23 3.6.1. In-situ TEM Heating 25 3.6.2. Environmental TEM 26 3.6.3. Electron Energy Loss Spectroscopy 28 3.6.4. Multislice Simulations 33 4. Electron Beam induced Dehydrogenation of MgH2 38 4.1. Microscopic Model of Hydrogen Release 38 4.2. Determination of Characteristic Electron Doses 39 4.3. Beam Damage Mechanism 42 4.4. Thickness Dependence of the Electron Dose 43 4.5. Conclusions for the Electron Beam Induced Dehydrogenation 47 4.6. Multislice Simulations for ETEM Studies 47 4.6.1. Methods of the Multislice Simulations 48 4.6.2. Results and Discussion of the Multislice Simulations 50 4.6.3. Conclusions of the Multislice Simulations 54 5. Nanoconfinement of LiBH4 in Aerogel-Like Carbon 56 5.1. Nanoporous Carbon Scaffolds 56 5.2. DSC Analysis of Melt Infiltration 58 5.3. XRD Analysis 59 5.4. Thermogravimetry and Mass Spectrometry Analysis 60 5.4.1. Hydrogen Desorption Properties of Nanoconfined LiBH4 60 5.4.2. Rehydrogenation of Nanoconfined LiBH4 63 5.5. In-situ STEM Analysis 63 5.6. Solid State 11B NMR 67 6. Conclusions 69 A. Appendix 71 A.1. Derivation of the Fourier-Log Deconvolution 71 A.2. Derivation of Equation 4.2 73 Bibliography 75