The process of purifying hydrogen gas using pressure swing adsorption columns heavily relies on highly efficient adsorbents. Such materials must be able to selectively adsorb a large amount of impurities, and must also be regenerated with ease. The work presented in this thesis focuses on a novel class of porous solids, metal-organic frameworks (MOFs), and their potential for use as adsorbents in hydrogen purification processes. MOFs are tuneable structures, a property that can be exploited in order to achieve the desired characteristics that are beneficial for a specific application. The design or selection of MOFs for any separation process however, relies on a thorough understanding of the relationship between a framework’s characteristics and its adsorption and selective properties. In order to identify favourable MOF characteristics for the separation of hydrogen from typical impurities a systematic molecular simulation study is performed on a large group of MOFs. Features such as the presence of short linkers, amine groups and additional aromatic rings, and a high density of linker groups are found to increase the adsorbate - framework interaction strength, and reduce the free volume available inside the pores. Both of these effects are shown to enhance MOF selectivity for impurities. Two promising materials, exhibiting desirable features, Mn MIL-53 and MIL-47, are studied further through a variety of approaches. A combination of experimental work and molecular simulations are employed in order to assess the level of flexibility in Mn MIL-53 on uptake of CO2 and CH4. An investigation of the experimental and simulation adsorption and characterization data indicates that the framework undergoes structural changes, in order to accommodate CO2 molecules, but not CH4. The form of the framework during CO2 uptake is also shown to be strongly influenced by temperature. In the case of MIL-47, adsorption isotherms simulated for a wide range of gases overpredict experimental adsorption data, leading to an in-depth investigation of non-porous effects, force field suitability, and framework rigidity. Ab initio molecular dynamics studies of MIL-47 indicate that the benzene dicarboxylate linkers rotate about their symmetry axis to reach more energetically favourable configurations, an effect responsible for the discrepancies between simulated and experimental isotherms. The effect of MOF flexibility on adsorption is further highlighted in a study of Sc2BDC3, a material able to undergo structural changes in order to accommodate a variety of adsorbates. Molecular simulations show that structural changes in the framework are responsible for the creation of additional CO2 adsorption sites as pressure is increased, whereas methanol adsorption sites occupied at extreme pressure are stabilized by the formation of hydrogen bonds. Finally, the exceptionally robust UiO-66(Zr) and UiO-67(Zr) families of MOFs are analysed using a multi-scale simulation study combining molecular level and process-scale computational work, seeking to compare the materials to commercial adsorbents, and assess whether they are suitable for H2 purification through pressure swing adsorption (PSA). Of the four MOFs studied, UiO-66(Zr)-Br is the most promising, as it significantly outperforms commercial zeolites and activated carbons in H2 purification from steam methane reformer offgas.

Metal membranes play a vital role in hydrogen purification. Defect-free membranes can exhibit effectively infinite selectivity for hydrogen. Membranes must meet multiple objectives, including providing high fluxes, resistance to poisoning, long operational standards, and be cost effective. Alloys offer an alternate route in improving upon membranes based on pure metal such as Pd. Development of new membranes is hampered by the large effort and time required not only to experimentally develop these membranes but also to properly test these materials. We show how first principle calculations combined with coarse-grained modeling can accurately predict H2 fluxes through binary and ternary alloy membranes as a function of alloy composition, temperature and hydrogen pressures. Our methods require no experimental input apart from the knowledge of the bulk crystal structure. Our approach is demonstrated for pure Pd, Pd-rich binary alloys, PdCu binary alloys, and PdCu-based ternary alloys. PdCu alloys have experimentally shown to have potential for resistance to sulfur poisoning. First, we used plane wave Density Functional Theory to study the binding and local motion of hydrogen for the alloys of interest. This data was used in combination with a Cluster Expansion Method along with the Leave-One-Out analysis to generate comprehensive models to predict hydrogen behavior in the interstitial binding sites within the bulk of the alloys of interest. These models not only were required to correctly fit our calculated data, but they were also required to properly predict behaviors for local conditions for which we had not collected information. These models were then used to predict hydrogen solubility and diffusivity at elevated temperatures. Although we are capable of combining first principle theory calculations with coarse grain modeling, we have explored a pre-screening method in order to determine which a particular material are worth performing additional calculations. Our heuristic lattice model is a simplified model involving as few factors as possible. It is by no means intended to predict the exact macroscopic H properties in the bulk of fcc materials, but it is intended as a guide in determining which materials merit additional characterization.

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First- principles modeling is used to predict hydrogen permeability through Palladium (Pd)-rich binary alloy membranes as a function of temperature and H2 pressure. We introduce a simplified model that incorporates only a few factors and yields quantitative prediction. This model is used to predict hydrogen permeability in a wide range of binary alloy membranes and to find promising alloys that have high hydrogen permeability. We show how our efficient Density Functional Theory (DFT)-based model predicts the chemical stability and proton conductivity of doped barium zirconate (BaZrO3), barium stannate (BaSnO3), and barium hafnate (BaHfO3). Our data is also used to explore the physical origins of the trends in chemical stability and proton conductivity among different dopants. We also study potassium tantalate (KTaO3), which is a prototype perovskite, to examine the characteristics of undoped perovskites. Specifically, we study the impacts of isotope effects, tunneling effects, and native point defects on proton mobility in KTaO3. It is important to find and develop solid-state Li-ion electrolyte materials that are chemically stable and have high ionic conductivities for high performance Li-ion batteries. We show how we predict the chemical stability of Li7La3Zr2O12, Li7La3Sn2O12, and Li7La3Hf2O12 with respect to carbonate and hydroxide formation reactions.

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Um eine homogene und ausreichende Versorgung des Brennstoffzellen (BZ)-Stapels zu gewährleisten, wird in einem Brennstoffzellen-Fahrzeug dem Stapel mehr Wasserstoff zugeführt, als für die Reaktion benötigt wird. Daher wird nicht verbrauchter Wasserstoff mit einer Strahlpumpe oder einem Rezirkulationsgebläse zum Anodeneingang des BZ-Stapels rezirkuliert. Aufgrund der Diffusion durch die Membran enthält das Anodenabgas neben dem Wasserstoff auch andere Bestandteile wie z.B. Stickstoff. Die Anreicherung des Stickstoffes im Anodenkreislauf führt zu einer ungleichmäßigen Stromdichteverteilung. Um dem entgegenzuwirken, wird in das System ein Spülventil eingebaut, das periodisch Gas ablässt um Stickstoff aus dem Anodenkreislauf abzuführen. Dabei lässt sich ein Wasser­stoffverbrauch nicht vermeiden. Diese Arbeit zielt darauf ab, die Rentabilität des Brennstoffzellensystems im automobilen Einsatz durch Reduzierung des Wasserstoffverbrauchs zu steigern. Hierfür wird die Verwendung eines elektrochemischen Wasserstoffkompressors (EHC) zur Wasserstoffumwälzung vorgeschlagen. Ein EHC ist eine innovative H2-Fördertechnologie, wobei der Wasserstoff gleichzeitig verdichtet und gereinigt werden kann. Im Vergleich zu mechanischen Kompressoren sind elektrochemische Wasserstoffkompressoren aufgrund der nahezu isothermen Bedingungen sehr effizient. Darüber hinaus können Wasserstoffkompressoren aufgrund ihres modularen Aufbaus sehr flexibel und kompakt gebaut werden.:1. EINLEITUNG 2. STAND DER TECHNIK 3. GRUNDLAGEN DER BRENNSTOFFZELLEN 4. TRANSPORTVORGÄNGE IN DER BRENNSTOFFZELLE 5. ELEKTROCHEMISCHER WASSERSTOFFKOMPRESSOR IM REZIRKULATIONSKREISLAUF 6. SIMULATION 7. EXPERIMENTELLE UNTERSUCHUNGEN 8. EINSATZ DES ELEKTROCHEMISCHEN WASSERSTOFFKOMPRESSORS IM BRENNSTOFFZELLENFAHRZEUG 9. ZUSAMMENFASSUNG 10. AUSBLICK / In order to ensure a homogeneous and sufficient supply of the fuel cell (FC) stack, more hydrogen is supplied to the stack in a fuel cell vehicle than required for the reaction. Therefore, unused hydrogen is recirculated to the anode inlet of the FC stack with an ejector or recirculation fan. Due to the diffusion through the membrane, the anode exhaust gas contains not only hydrogen but also other components such as nitrogen. The accumulation of nitrogen in the anode circuit leads to an uneven current density distribution. To counteract this, a purge valve is built into the system that periodically vents gas to purge nitrogen from the anode circuit. Hydrogen consumption cannot be avoided here. This work aims to increase the profitability of the fuel cell system in automotive application by reducing hydrogen consumption. For this purpose, the use of an electrochemical hydrogen compressor (EHC) for hydrogen circulation is proposed. An EHC is an innovative H2 delivery technology, whereby the hydrogen can be compressed and cleaned at the same time. Compared to mechanical compressors, electrochemical hydrogen compressors are very efficient due to the almost isothermal conditions. In addition, hydrogen compressors can be built very flexibly and compactly due to their modular design.:1. EINLEITUNG 2. STAND DER TECHNIK 3. GRUNDLAGEN DER BRENNSTOFFZELLEN 4. TRANSPORTVORGÄNGE IN DER BRENNSTOFFZELLE 5. ELEKTROCHEMISCHER WASSERSTOFFKOMPRESSOR IM REZIRKULATIONSKREISLAUF 6. SIMULATION 7. EXPERIMENTELLE UNTERSUCHUNGEN 8. EINSATZ DES ELEKTROCHEMISCHEN WASSERSTOFFKOMPRESSORS IM BRENNSTOFFZELLENFAHRZEUG 9. ZUSAMMENFASSUNG 10. AUSBLICK