Preparation and Characterization of Nanoscopic Solid State Hydrogen Storage Materials

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 75Storing 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 7

Local Band Gap Measurements by VEELS of Thin Film Solar Cells

This work presents a systematic study that evaluates the feasibility and reliability of local band gap measurements of Cu(In,Ga)Se2 thin films by valence electron energy-loss spectroscopy (VEELS). The compositional gradients across the Cu(In,Ga)Se2 layer cause variations in the band gap energy, which are experimentally determined using a monochromated scanning transmission electron microscope (STEM). The results reveal the expected band gap variation across the Cu(In,Ga)Se2 layer and therefore confirm the feasibility of local band gap measurements of Cu(In,Ga)Se2 by VEELS. The precision and accuracy of the results are discussed based on the analysis of individual error sources, which leads to the conclusion that the precision of our measurements is most limited by the acquisition reproducibility, if the signal-to-noise ratio of the spectrum is high enough. Furthermore, we simulate the impact of radiation losses on the measured band gap value and propose a thickness-dependent correction. In future work, localized band gap variations will be measured on a more localized length scale to investigate, e.g., the influence of chemical inhomogeneities and dopant accumulations at grain boundarie

ROS-mediated EB1 phosphorylation through Akt/GSK3β pathway: implication in cancer cell response to microtubule-targeting agents

International audienceMicrotubule-targeting agents (MTAs) are largely administered in adults and children cancers. Better deciphering their mechanism of action is of prime importance to develop more convenient therapy strategies. Here, we addressed the question of how reactive oxygen species (ROS) generation by mitochondria can be necessary for MTA efficacy. We showed for the first time that EB1 associates with microtubules in a phosphorylation-dependent manner, under control of ROS. By using phospho-defective mutants, we further characterized the Serine 155 residue as critical for EB1 accumulation at microtubule plus-ends, and both cancer cell migration and proliferation. Phosphorylation of EB1 on the Threonine 166 residue triggered opposite effects, and was identified as a requisite molecular switch in MTA activities. We then showed that GSK3β activation was responsible for MTA-triggered EB1 phosphorylation, resulting from ROS-mediated inhibition of upstream Akt. We thus disclosed here a novel pathway by which generation of mitochondrial ROS modulates microtubule dynamics through phosphorylation of EB1, improving our fundamental knowledge about this oncogenic protein, and pointing out the need to reexamine the current dogma of microtubule targeting by MTAs. The present work also provides a strong mechanistic rational to the promising therapeutic strategies that currently combine MTAs with anti-Akt targeted therapies

Pattern formation in active model C with anchoring: bands, aster networks, and foams

We study the dynamics of pattern formation in a minimal model for active mixtures made of microtubules and molecular motors. We monitor the evolution of the (conserved) microtubule density and of the (non-conserved) nematic order parameter, focusing on the effects of an "anchoring" term that provides a direct coupling between the preferred microtubule direction and their density gradient. The key control parameter is the ratio between activity and elasticity. When elasticity dominates, the interplay between activity and anchoring leads to formation of banded structures that can undergo additional bending or rotational instabilities. When activity dominates, the nature of anchoring instead gives rise to a range of active cellular solids, including aster-like networks, disordered foams and spindle-like patterns. We speculate that the introduced "active model C" with anchoring is a minimal model to describe pattern formation in a biomimetic analogue of the microtubule cytoskeleton

Inhibition of Neurogenic Inflammatory Pathways Associated with the Reduction in Discogenic Back Pain

Study Design Retrospective cohort study. Purpose This study aimed to determine whether the initiation of anti-calcitonin gene-related peptide (CGRP inhibitor) medication therapy for migraines was also associated with improvements in back/neck pain, mobility, and function in a patient population with comorbid degenerative spinal disease and migraine. Overview of Literature CGRP upregulates pro-inflammatory cytokines such as tumor necrosis factor-α, interleukin-6, brain-derived neurotrophic factor, and nerve growth factor in spinal spondylotic disease, which results in disc degeneration and sensitization of nociceptive neurons. Although CGRP inhibitors can quell neurogenic inflammation in migraines, their off-site efficacy as a therapeutic target for discogenic back/neck pain conditions remains unknown. Methods All adult patients diagnosed with spinal spondylosis and migraine treated with CGRP inhibitors at a single academic institution between 2017 and 2020 were retrospectively identified. Patient demographic and medical data, follow-up duration, migraine severity and frequency, spinal pain, functional status, and mobility before and after the administration of CGRP inhibitors were collected. Paired univariate analysis was conducted to determine significant changes in spinal pain, headache severity, and headache frequency before and after the administration of CGRP inhibitors. The correlation between changes in the spinal pain score and functional or mobility improvement was assessed with Spearman’s rho. Results In total, 56 patients were included. The mean follow-up time after the administration of CGRP inhibitors was 123 days for spinal pain visits and 129 days for migraine visits. Back/neck pain decreased significantly (p<0.001) from 6.30 to 4.36 after starting CGRP inhibitor therapy for migraine control. As recorded in the spine follow-up notes, 25% of patients experienced a functional improvement in the activities of daily living, and 17.5% experienced mobility improvement while taking CGRP inhibitors. Change in back/neck pain moderately correlated (ρ=−0.430) with functional improvement but was not correlated with mobility improvement (ρ=−0.052). Conclusions Patients taking CGRP inhibitors for chronic migraines with comorbid degenerative spinal conditions experienced significant off-target reduction of back/neck pain

Mortality and pulmonary complications in patients undergoing surgery with perioperative SARS-CoV-2 infection: an international cohort study

Background: The impact of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) on postoperative recovery needs to be understood to inform clinical decision making during and after the COVID-19 pandemic. This study reports 30-day mortality and pulmonary complication rates in patients with perioperative SARS-CoV-2 infection. Methods: This international, multicentre, cohort study at 235 hospitals in 24 countries included all patients undergoing surgery who had SARS-CoV-2 infection confirmed within 7 days before or 30 days after surgery. The primary outcome measure was 30-day postoperative mortality and was assessed in all enrolled patients. The main secondary outcome measure was pulmonary complications, defined as pneumonia, acute respiratory distress syndrome, or unexpected postoperative ventilation. Findings: This analysis includes 1128 patients who had surgery between Jan 1 and March 31, 2020, of whom 835 (74·0%) had emergency surgery and 280 (24·8%) had elective surgery. SARS-CoV-2 infection was confirmed preoperatively in 294 (26·1%) patients. 30-day mortality was 23·8% (268 of 1128). Pulmonary complications occurred in 577 (51·2%) of 1128 patients; 30-day mortality in these patients was 38·0% (219 of 577), accounting for 81·7% (219 of 268) of all deaths. In adjusted analyses, 30-day mortality was associated with male sex (odds ratio 1·75 [95% CI 1·28–2·40], p\textless0·0001), age 70 years or older versus younger than 70 years (2·30 [1·65–3·22], p\textless0·0001), American Society of Anesthesiologists grades 3–5 versus grades 1–2 (2·35 [1·57–3·53], p\textless0·0001), malignant versus benign or obstetric diagnosis (1·55 [1·01–2·39], p=0·046), emergency versus elective surgery (1·67 [1·06–2·63], p=0·026), and major versus minor surgery (1·52 [1·01–2·31], p=0·047). Interpretation: Postoperative pulmonary complications occur in half of patients with perioperative SARS-CoV-2 infection and are associated with high mortality. Thresholds for surgery during the COVID-19 pandemic should be higher than during normal practice, particularly in men aged 70 years and older. Consideration should be given for postponing non-urgent procedures and promoting non-operative treatment to delay or avoid the need for surgery. Funding: National Institute for Health Research (NIHR), Association of Coloproctology of Great Britain and Ireland, Bowel and Cancer Research, Bowel Disease Research Foundation, Association of Upper Gastrointestinal Surgeons, British Association of Surgical Oncology, British Gynaecological Cancer Society, European Society of Coloproctology, NIHR Academy, Sarcoma UK, Vascular Society for Great Britain and Ireland, and Yorkshire Cancer Research

Preparation and Characterization of Nanoscopic Solid State Hydrogen Storage Materials

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 75Storing 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 7

Preparation and Characterization of Nanoscopic Solid State Hydrogen Storage Materials

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 75Storing 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 7