1,720,984 research outputs found
Hydrogen Storage in Porous Materials and Magnesium Hydrides
No full textIn this thesis representatives of two different types of materials for potential hydrogen storage application are presented. Usage of either nanoporous materials or metal hydrides has both operational advantages and disadvantages. A main objective of this thesis is to characterize the hydrogen storage mechanism of selected Metal-Organic Framework (MOF) materials. Such knowledge may provide information in which direction improvements of the materials may be possible. Detailed analysis of the hydrogen storage mechanism was performed using experimental techniques like powder X-ray diffraction, FT-IR, TEM, hydrogen sorption experiments and solid state NMR. Apart from the hydrogen sorption characteristics also knowledge about the stability of the frameworks themselves was obtained. Additional insight on the MOFs structure and on hydrogen sorption kinetics and thermodynamic factors was gained from modeling methods which includes Density Functional Theory and force field calculations. Apart from that new information on the microstructure of the MgH2-TiF3 complex were obtained and subsequently the models of the dehydrogenated and re-hydrogenated structures were proposed.Radiation Science & TechnologyApplied Science
Charge carrier transport at the nanoscale: Electron and hole transport in self-assembled discotic liquid crystals: Mobile ionic charges in nanocomposite solid electrolytes
No full textThis thesis explores some fundamental aspects of charge carrier transport at the nanoscale. The study is divided in two parts. In the first part, the structural, dynamical and vibrational properties of discotic liquid crystals are studied in relation to the potential of these self-assembled ‘mesophases’ to form molecular conducting wires. Although the study is fundamental in nature, a direct link will be made to the potential of discotic liquid crystals for opto-electronic applications such as solar cells. The second part presents a study on the interfacial defect chemistry in nano-structured solid acids. This part addresses the issue of interface-dominated charge transport in nano-sized materials. The heart of the work is a theoretical framework that explains the strong enhancement of proton conductivities observed when solid acids such as CsHSO4 or CsH2PO4 are blended with TiO2 or SiO2 nanoparticles. The results are of fundamental interest for the development of solid state ionic conductors that can be used as electrolytes in batteries and fuel cells.Radiation, Radionuclides & ReactorsApplied Science
Kinetic and thermodynamic aspects of Mg and Mg-Ti hydride nanomaterials
No full textReliable and affordable energy storage represents a bottleneck in a scenario where renewable energy sources become prominent on the energy market. With its high potential energy density and the natural abundance of the element, hydrogen gas is an attractive energy storage medium. But its gaseous nature, its high explosive potential and permeability of gaseous hydrogen through materials pose important limitations on its usability. In this thesis, Mg based nanomaterials with improved potential for applications as hydrogen stores are developed and analyzed. We have addressed the key issues of slow sorption kinetics, high stability and air sensitivity of MgH2 and provided viable solution directions to these challenges.Reactor Institute DelftApplied Science
Lithium insertion in nanostructured titanates
No full textUpon nano-sizing of insertion compounds several significant changes in Li-insertion behavior have been observed for sizes below approximately 50 nm. Although the origins of the phenomena are interrelated, the changes can be divided in three main observations. (1) The formation of new phases, leading to enhanced reactivity and extended capacities, which is most likely due to the lesser role of kinetic restrictions, and easier accommodation of strain in nanoscale compounds. (2) Thermodynamic changes due to the relatively increased impact of both surface and interface energy, and (3), the excess Li-storage on the surface of the particles in the form of Li2O. These three phenomena have a positive effect on the performance of the electrode material in a Li-ion battery application. The higher Li-capacities provide the electrode material with a higher energy density. Furthermore, the enhanced extend of the solid solution regime observed at lower Li-capacities, due to changes in the particle’s thermodynamics, supports better ionic mobility in the absence of a rate-limiting phase boundary. However, the relatively increase op the surface also enhances the negative effects, such as the formation of a solid electrolyte interface (SEI). Also, a Li-rich surface layer inherently shows poor Li-ion mobility, and effectively acts as a block for further intercalation of the bulk of the particle. The detailed insight in the nano-size related surface storage mechanisms discussed in this thesis forms an important basis for understanding the properties of nano-sized insertion materials. This knowledge supports future design of nano-sized electrode materials for Li-ion batteries and H-storage devices, potentially paving the way for the manufacturing of environmentally clean and highly efficient full-electrical cars.Radiation, Radionuclides & ReactorsApplied Science
Structure and dynamics of hydrogen in nanocomposite solid acids for fuel cell applications
No full textThe transition to sustainable energy sources is inevitable. A possible future scenario could be the hydrogen economy, where the fuel cell plays an important role in the conversion of hydrogen back to electricity. The technology behind the fuel cell however, still has significant room for improvement. A next step would be the realization of so called intermediate temperature fuel cells. Solid acids have been shown to be promising candidates for fuel cell electrolytes as they possess high proton conductivity in the intermediate temperature range, from ambient up to 250 °C. One major problem of the solid acids was the low proton conduction at temperatures below their superprotonic phase transition. This has been solved by nanostructuring the materials, where the solid acid is mixed with e.g. TiO2 or SiO2 nanoparticles of 40 nm in size and smaller. The conductivity increases with orders of magnitude. Here a multi-technique approach is used to study this matter. Using Neutron Diffraction, direct experimental proof of the occurrence of space charge effects in these nanocomposites was shown in the form of deuterium ion intercalation in TiO2 nanoparticles together with deuterium depletion in the solid acid CsHSO4 phase. Very high, particle size dependent proton densities, up to 10% in the 7 nm TiO2 particles were found. Quasi elastic neutron scattering experiments showed fractions of up to 25% of the hydrogen ions at temperatures below the superprotonic phase transition to possess mobilities similar to the protons above this transition. Nuclear magnetic resonance spectroscopy showed similar fractions as well as T1 relaxation times of roughly 2 orders of magnitude shorter compared to the bulk crystalline solid acids. These results lead to the conclusion that because of the space charge effect, vacancies are created in the solid acid at temperatures below the superprotonic phase transition temperature. These empty sites allow a large fraction of the hydrogen ions to move, to such an extent that they become almost as mobile as in the superprotonic phase. Furthermore a composite electrolyte of CsHSO4 and Nafion was synthesized. The composite electrolyte membrane showed good mechanical strength resulting from the Nafion polymer matrix. It exhibited similar proton conductivity to pure Nafion at low temperatures in a humid environment and showed high conductivity of 10-3 S/cm at intermediate temperatures around 140 °C, where Nafion filled with water is inoperable. Finally the investigation was extended to another solid acid: CsH2PO4. Using the multi-technique approach, these CsH2PO4 composites with nanoparticulate TiO2 or SiO2 were studied and compared to the CsHSO4 composites. Acidity was found to be indicative for the amount of space charge occurring. The results indicate a reduced space charge effect in CsH2PO4-TiO2 composites consistent with the reduced acidity of CsH2PO4 and lower proton accepting capacity of TiO2 compared to SiO2. Using the acidity combined with computer calculations might be a useful way to predict the extent of space charge in future research towards the optimal combination for electrolyte membrane material.Applied physics, R3, FAMEApplied Science
Discotic liquid crystals: From dynamics to conductivity
No full textThe dynamics and conductivity of the discotic liquid-crystal, hexakis(n-hexylox) triphenylene (HAT6), and charge-transfer complex that it forms with 2,4,7trinitro-9-fluorenone (TNF) are studied using quasielastic neutron-scattering (QENS) and Pulse-Radiolysis Time resolved Conductivity. These two techniques measure the molecular dynamics and conductivity decay, respectively, are interpreted in terms of a single model for the relaxation kinetics for a correlating environment. Use of a single model supposes that the molecular and charge dynamics are described by the same kinetic equation, which allows description of the system behaviour in space and time when combined with fractional diffusion equation. This supposition is substantiated by relating the two systems through the dispersion parameter, I2. This enables the range of the charge mobility to be linked to the length-scale of the molecular motion from which a coupling parameter is derived. The conductivity and molecular dynamics of HAT6 and HAT6-TNF are analysed in terms of the dynamical and coupling parameters. The thermal motion of a short column of HAT6 is simulated by a molecular dynamics simulation in order to provide the correct distribution of distorted molecular units, including the effects of the alkoxy tails. Ab-initio calculations are then used to determine the electronic structure of the distorted triphenylene cores and the time dependence of the interaction between these. A compact model for the vibrational dynamics of HAT6 is derived from the combined use of inelastic neutron scattering spectroscopy and first-principles calculations. It reproduces the essential features of the vibrational dynamics and electronic structure on the aromatic core of HAT6.Applied Science
Hydrogen Storage in Nanostructured Light Metal Hydrides
No full textThe global energy issues can be solved by the abundantly available hydrogen on earth. Light metals are a compact and safe medium for storing hydrogen. This makes them attractive for vehicular use. Unfortunately, hydrogen uptake and release is slow in light metals at practical temperature and pressure conditions. Catalysts are known to accelerate both processes. This research presents three mechanisms for catalyst actions: as grain refining agents, as a hydrogen vacancy formation facilitator, and (in its conventional role) as a hydrogen molecule splitter. Moreover, plentiful hydrogen vacancies are reported from experimental observations in nano-structured light metal hydrides. For specific storage systems, guidelines to select the optimal catalyst are developed based on the exploitable mechanisms.Radiation, Radionuclides & ReactorsApplied Science
In operando phase transitions and Lithium ion transport in LiFePO4
No full textChemical energy storage in Li-ion batteries is a key technology for the future renewable society. Their energy and power density is largely determined by electrode materials that are able to host lithium in their crystal structure. Aiming at faster and more efficient energy storage, one of the key objectives in Li-ion batteries is to improve the charge transport through the complex heterogeneous electrode morphology. The complex transport phenomena and phase behavior are timely topics and subject of many studies and intense debates. Remarkably, our current knowledge is mostly based on ex-situ techniques or techniques that do not have sufficient time and space resolution to reveal the actual phase nucleation and growth in individual grains. On the electrode length scale the absence of experimental probes that allow direct observation of Li-ion transport in electrodes under realistic in-operando conditions hinders fundamental understanding and the development of rational strategies towards improved electrodes. For LiFePO4, a state of the art cathode material for today’s Li-ion batteries, such direct insights are of high fundamental and practical impact, potentially creating new perspectives in the working and improvements of electrode materials in general. This is the main object of this thesis, revealing the cycling rate dependent phase transition behavior and Li-ion transport throughout the electrodes in LiFePO4 under realistic in-operando conditions.RST/Radiation, Science and TechnologyApplied Science
A Template for Enhanced Lithium Ion Battery Electrodes
No full textDictated by exhausting sources of conventional fossil fuels, their irreversible damage on environment, international conflicts for oil resources – distressing global economy, worldwide research has focused towards developing alternate, but sustainable, sources of energy generation and storage to meet global energy demands and emission-free mobility applications. In recent years, electric cars have become a contemporary notion of sustainable mobility. This has been enabled by the progress in electrochemical energy storage and conversion in batteries. Among the various types of batteries, lithium ion batteries are recognized for their high energy and power density, prerequisites for a long-range and fast charging of electric vehicles. Despite recent developments lithium ion batteries at present are not able to provide the desired energy and power density output for electric vehicles to compete with conventional fossil fuels. The research presented in this thesis explores various fundamental and technical facets of lithium ion batteries aiming at improvement of the energy and power performanceRadiation Science & Technology - Reactor Institute DelftApplied Science
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