14 Externe wissenschaftliche Einrichtungen
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Item Open Access Effects of oxide incorporation in proton conducting organic electrolytes(2009) Sörgel, Seniz; Maier, Joachim (Prof. Dr.)In this work, the effects of incorporation of various types of oxide particles (e.g. ZrO2, TiO2, Al2O3) into proton conducting organic electrolytes is investigated. As a weak liquid model electrolyte, moderately proton conducting imidazole is chosen. As a highly proton conducting strong polymer electrolyte, and simultaneously practically very important electrolyte, Nafion® is selected for the second part of the work. In the first part of this work, for the first time, the applicability of the concept of heterogeneous doping to imidazole is demonstrated. Imidazole exhibits moderate proton conductivity due to low intrinsic charge carrier concentration. Therefore, a perceptible conductivity increase by heterogeneously doping imidazole is expected. Ac-impedance spectroscopy measurements of composites of imidazole with various types of nanometer sized oxide particles, which were performed as a function of temperature and oxide concentration show that the composites exhibit significantly enhanced ionic conductivities compared to the pure imidazole. The highest measured composite ionic conductivity is observed for the composite with heated sZrO2, viz. 1.66x10-2 -1 cm-1 at 90 °C corresponding to an enhancement by a factor of 10 compared to the pure ImiH at the same temperature. The composites prepared with the oxides having the highest activity and density of the acidic sites on the surface show the most pronounced improvement in conductivity. These results were quantitatively analyzed in light of the concept of heterogeneous doping. The proton conductivities calculated according to the heterogeneous doping concept are consistent with the experimentally observed conductivities. The results of zeta potential measurements show that the surface charge of the inorganic oxides becomes strongly more negative on the addition of imidazole. This is consistent with the formation of a space-charge layer on the oxide surface as a consequence of an adsorptive interaction: trapping of imidazolate anions (Imi-) on the oxide surface results in an increased concentration of imidazolium cations (ImiH2+) in the space charge region at the interface of oxide and conductor. The second part of this work focuses on the investigation of the effects of inorganic oxide admixture on proton conductivity, microstructure and mechanical properties of a strong polymer electrolyte, namely Nafion®. Various composite and respective bare membranes were investigated for which performance improvements had been proven in literature before. Thermal and hydrothermal treatments were applied to the membranes in order to get an insight into the properties of the materials at high temperature and low humidity conditions. According to the attenuated total reflection infrared (ATR-IR) spectroscopy results, upon hydrothermal treatments a condensation reaction and consequently an anhydride formation (R-O2S-O-SO2-R) is suggested to occur in the membranes. The thermal treatment above Tg may also lead to the same kind of products. In addition, sulphur formation (aging) is proposed to occur in such conditions which can be derived by X-ray powder diffractometry and energy dispersive microanalysis. These reactions (condensation and sulphur formation) result in an increase of the equivalent weight (EW) and local ordering between polymer crystallites which were detected by acid-base titrimetry and small-angle X-ray scattering (SAXS) measurements, respectively. The conductivity of the membranes is observed to decrease upon thermal and hydrothermal treatments. At high water contents, the decay of conductivity can be explained by the equivalent weight increase. However, at low water contents the mobility of the charge carriers is observed to be slightly suppressed which can explain the conductivity behavior. The lower mobility at low water contents can be due to the less favorable microstructure of the membranes for proton conduction. The proposed condensation reaction and/or sulphur formation (aging) lead to a decrease of hydrophilicity of the side chains. This negatively affects the nanophase separated morphology since hydration of the ionic clusters decreases. Thereby, the water content in the membranes decreases. It is observed by dynamic mechanical analysis (DMA) measurements that the lower amount of water in the membranes is unfavorable for the mechanical properties of the membranes at high temperatures as water acts as a stiffener in such conditions. The above explained effects of thermal and hydrothermal treatments on EW, proton conductivity, activation enthalpy, mobility and microstructure of the membranes without oxide particles are more severe than they are for the composite membranes. A probable condensation reaction and/or aging and therefore changes in microstructure and transport properties of the material are suppressed in the presence of oxide particles. DMA measurement results show that the composite membranes also keep a higher amount of water at elevated conditions and they are thermally and mechanically slightly more stable compared to the respective bare membranes. The incorporation of the oxide particles also increases the glass transition temperature about 10 °C which indicates that the composites have slightly higher thermal stability. In conclusion, in this work it is shown that the oxide incorporation has a positive effect on both weak and strong proton conducting electrolytes: while in the former the proton conductivity is improved by charge carrier concentration increase in the space charge layer, in the latter one it is the structural, thermal and mechanical stability of the material that is beneficially affected at elevated conditions. This study may encourage further developments of electrolyte materials for alternative energy conversion devices.Item Open Access Anion exchange membranes for fuel cells and flow batteries : transport and stability of model systems(2015) Marino, Michael G.; Maier, Joachim (Prof. Dr.)Polymeric anion exchange materials in membrane form can be key components in emerging energy storage and conversions systems such as the alkaline fuel cell and the RedOx flow battery. For these applications the membrane properties need to include good ionic conductivity and sufficient chemical stability, two aspects, that are not sufficiently understood in terms of materials science. Materials fulfilling both criteria are currently not available. The transport of ions and water in a model anion exchange membrane (AEM) as well as the alkaline stability of their quaternary ammonium functional groups is therefore investigated in this thesis from a basic point of view but with the aim to bring these technologies one step closer to large scale application, as they have several advantages compared to existing energy storage and conversion systems. The hydroxide exchanging alkaline fuel cell (AFC), for example, is in principle more cost-effective than the more common acidic proton exchange fuel cell (PEMFC). Unfortunately AFCs suffer from base induced decomposition of the membrane. Especially the quaternary ammonium (QA) functional groups are easily attacked by the nucleophilic hydroxide. QAs with higher alkaline stability are required but there is considerable disagreement regarding which QAs are suitable, with widely varying and partially contradicting results reported in the literature. In this thesis, the decay of QA salts was investigated under controlled accelerated aging conditions (up to 10 M NaOH and 160 °C). This allowed a stability comparison based solely on the molecular structure of the QAs. A number of different approaches to stabilize the QAs which potentially inhibit degradation reactions such as β-elimination, substitution and rearrangements were compared. These include β-proton removal, charge delocalization, spacer-chains, electron-inducing groups and conformational confinement. Heterocylic 6-membered QAs based on the piperidine structure proved to be by far the most stable cations at the chosen conditions. This was not readily apparent from their structure since they contain β-protons in anti-periplanar positions, which generally cause rapid decomposition in other types of QAs. The geometry of the cyclic structure probably exerts strain on the reaction transition states, kinetically inhibiting the degradation reactions. Other stabilization approaches resulted in markedly less stable compounds. Noticeably the benzylic group, which is the current standard covalent tether between QA and polymer, degrades very fast compared to almost all aliphatic QAs. The results of this stability study suggest that hydroxide exchange membranes for alkaline fuel cells, which are significantly more stable than current materials are achievable. Besides stability, the transport of anions and water in AEMs was investigated in this Hydroxide exchange membranes (HEM) have been reported to exhibit surprisingly low ionic conductivities compared to their proton exchange membrane (PEM) counterparts. This is partially because hydroxide charge carriers are rapidly converted to carbonates when a HEM comes into contact with ambient air. Careful exclusion of CO2 was required to investigate pure hydroxide form membranes. For this purpose a custom glove box was designed and built that allowed preparation and measurements of HEM samples in a humidified CO 2 -free atmosphere. It was found that the conductivity reduction of a carbonate contaminated HEM is not only due to the reduced ionic mobility of carbonate charge carriers compared to hydroxide, but also because of reduced water absorption of the corresponding membrane which decreases conductivity even further. Pure HEMs can in fact achieve conductivities within a factor of two of PEMs at equal ion exchange capacity at sufficient hydration, according to the differences in the ionic mobility of hydroxide and hydronium. At lower water contents though, the hydroxide mobility decreases faster than that of hydronium in comparable PEMs due to reduced dissociation and percolation as well as a break-down of structural diffusion Apart from the HEM, membranes in other ionic forms were investigated. Generally, all investigated AEM properties were found to change if the type of anion was exchanged. This comprises the degree of dissociation, conductivity, membrane morphology and sometimes even water diffusion. Remarkably, at low water contents, the ionic conductivity of the HEM sank below that of the halides, despite the much higher hydroxide mobility in aqueous solution. A gradual break-down of the hydroxide structural diffusion is probably responsible. Another noticeable observation was that the degree of dissociation for at least the bromide and chloride form membranes remains almost constant over a considerable water content range, suggesting the formation of associates consisting of several ions, which probably also exists in other ionic forms.Item Open Access Einfluß relativistischer Effekte auf die Chemie von Platin und Thallium(2006) Karpov, Andrey; Jansen, Martin (Prof. Dr. Dr. h. c.)Im Rahmen der vorliegenden Arbeit wurden zum ersten Mal Verbindungen mit Platinid-Anionen dargestellt und charakterisiert. Die negativen Valenzzustände der Platinatome wurden sowohl theoretisch durch quantenchemische Analyse als auch experimentell mittels der Photoelektronenspektroskopie für die chemische Analyse (ESCA) bestätigt. Das Vorliegen der Platinid-Ionen liefert einen weiteren eindrucksvollen Beweis für die Bedeutung der relativistischen Effekte in der Chemie der schweren Elemente. Bemerkenswerte Parallelen der chemischen Eigenschaften des Edelmetalls Platins zu der Chemie der Hauptgruppenelemente (16. Gruppe) werden auf die relativistische Kontraktion des 6s-Orbitals zurückgeführt. Des Weiteren wurden Versuche zur Darstellung der isolierten, vermutlich diamagnetischen (Tl-)-Anionen durchgeführt, um den Einfluß des zweiten relativistischen Effekts, der Spin-Bahn-Aufspaltung, als ein chemisch relevantes Phänomen nachzuweisen. Mit dieser Zielsetzung wurden ternäre Systeme Alkalimetall-Thallium-Sauerstoff untersucht und dabei Verbindungen mit neuartigen Kristallstrukturen und Bindungsverhältnissen entdeckt.Item Open Access Electrochemical studies of MBE-grown CaF2/BaF2 heterolayers(2007) Matei, Ion; Maier, Joachim (Prof. Dr.)Ionic conductors, materials in which specific ions can migrate preferentially with high mobility, are of prime importance for electrochemical measurements, and for devices such as high-temperature batteries and fuel cells, chemical filters and sensors. This research study is focused on the dynamics of ion-conducting superlattices synthesized by molecular beam epitaxy (MBE) in which the interfaces are artificially tuned, with the aim of designing superior ionic conductors by controlling their interfaces. The interface is also expected to introduce lattice strain due to lattice mismatches and/or to change space charge distribution at the interfaces when superlattices of different ionic conductors are fabricated with a period of a few to a few hundred nanometres. Since the superlattice structure enables to tune the crystal structure to some extent, the ionic conductivity dependence on the structural parameters will also be investigated in this study. A qualitatively different conductivity behaviour is expected when the interface spacing is comparable to or smaller than the width of the space charge regions in comparatively large crystals: single layers lose their individuality and an artificial ionically conducting material with anomalous transport properties is generated. These results demonstrate mesoscopic ion conductivity effect in nanosystems (extremely thin films, nanocrystalline materials). In order to obtain more fundamental insight into the conductivity effects, some points still need to be clarified and are addressed in this study: (1) the detailed understanding of the defect chemical situation and the conductivity effects in parallel and in perpendicular direction to the interfaces; (2) the annealing effects; (3) theoretical model and numerical evaluation in periods of the mesoscopic situation (thinner than 50nm). To understand these effects in depth, electrical measurements on parallel (along the interfaces) and perpendicular (to the interfaces) configuration of the heterostructures as well as thermodynamic modelling are performed. Multilayers of CaF2/BaF2 have been prepared by molecular beam epitaxy on different substrates (Al2O3, Si, Nb-doped SrTiO3), with highly defined geometry, periodicity, interfacial spacings and layer sequence. The measured effective parallel conductivity (i.e. derived from the measurement of parallel conductance via the total thickness ~400nm) progressively increases with interfacial density. The purpose of the annealing experiment is to determine the anomalous decrease of the parallel conductivity of the sample as the annealing temperature increases. This can be associated with the existence of unstable dislocations not only at the interface, but also inside the layers that can be annealed out by thermal treatment. This results in a clear picture: in annealed samples there is a fluorite ( -ions) transfer from one phase to the other. In a non-annealed samples this is superimposed by charging of dislocations leaving vacancies in the vicinity. The heterostructures on conductive substrates were also prepared and allow us to carry out the conductivity measurement in the perpendicular direction to the interfaces. Mesoscopic size effects predict a decrease in the difference between parallel and perpendicular conductivity with the increase in the number of interfaces. This is very satisfactory as a parallel conductivity pronounces the highly conductive regions, while the perpendicular one emphasizes the less conductive regions. In this study, the thickness dependence of the layer conductivities is numerically calculated using both the Gouy-Chapman and the Mott-Schottky modes. The calculated concentration profile turns only out to be consistent with the charge density of the Mott-Schottky model if the frozen-in impurity profile is assumed to be moderately increased. In summary: 1. Heterolayers of CaF2/BaF2 have been prepared by molecular beam epitaxy (MBE) on different substrates (Al2O3, Nb-doped SrTiO3), with highly defined geometry, periodicity, interfacial spacings and layer sequence. 2. XRD and AFM measurements demonstrate that defined highly oriented heterostructures of CaF2/BaF2… can be prepared on different substrates. 3. The conductivity effects can be understood in terms of ionic space charge effects occurring as a consequence of a thermodynamic redistribution at equilibrium. 4. The influence of annealing effects on the resistance of the sample has been studied and analysed. Unstable dislocations appear to be charged by adsorption. 5. In this study, the thickness dependence of the layer conductivities is numerically calculated using both the Gouy-Chapman, and the Mott-Schottky models. In direct comparison to the experimental data, the modified Mott-Schottky mode (impurity profile with a gradient close to the interface) can reproduce the features of the experiments even in the mesoscopic range.Item Open Access Advances in the modelling of in-situ powder diffraction data(2013) Müller, Melanie; Dinnebier, Robert E. (Prof. Dr.)X-ray powder diffraction is a well-established technique to analyse structural and microstructural properties of materials. The possibility to record in-situ powder diffraction data allows studying changes within the structure and microstructure of a sample that occur in dependence on the applied external conditions (e.g. temperature, pressure). In the present thesis, in-situ X-ray powder diffraction was used to study structural and microstructural changes of different samples occurring at elevated temperature or upon UV illumination. Several structural phase transitions were studied using the approach of parametric Rietveld refinement. In parametric Rietveld refinement a set of powder diffraction pattern is refined simultaneously, constraining the evolution of some parameters using mathematical models, so that only the variables of the model need to be refined. In order to model and analyse the behaviour of structural parameters, Landau theory and its corresponding equations were used, owing to the fact that structural parameters (e.g. lattice strain, changes in atomic positions or occupancy) comprise an order parameter as defined in Landau theory. For description of the crystal structure of materials, several different approaches were tested, e.g. atomic coordinates, symmetry modes, rigid body rotations or rigid body symmetry modes. The dependence of preparation conditions on the properties of nanomaterials and their growth kinetics was studied using Whole Powder Pattern Modelling. This method allows modelling X-ray powder diffraction pattern using the microstructure of the sample without the use of arbitrary profile functions. The Fourier transforms of frequently observed effects as crystallite shape and size distribution or density of various defects, like dislocations and stacking faults, are utilised in order to get the resulting diffraction profile. Two different systems with industrial application, CeO2 and Cu2ZnSnS4, which were produced using a sol-gel approach, were investigated.Item Open Access Simulation of diffusion in nanocrystalline materials : continuum approach(2006) Grjaznovs, Deniss; Maier, Joachim (Prof. Dr.)Nowadays diffusion profiles can be measured on the scale of hundred nanometers, allowing one to analyze diffusion processes very close to interfaces. It is also obliged by the existence of nanocrystalline materials which are characterized by grain sizes of the order of hundred nanometers. For diffusion measurements in nanocrystalline materials short diffusion times and/or small temperatures are necessary in order to apply standard procedures for determining grain boundary diffusivities. This is particularly important when the diffusion profiles are supposed to be measured in the type-B kinetics. However, the standard procedures are based on different assumptions and, thus, can lead to erroneous results when used under extreme conditions. In the present work classical grain boundary diffusion models are re-examined with respect to the following effects: 1) short diffusion times 2) realistic microstructures in the type-B as well as -A kinetics, including segregation effects 3) space charge layers typically present in ionic materials and their role in both the B- and A-diffusion regimes. Accordingly, each effect is discussed in a separate chapter of the present dissertation. Errors in determining the grain boundary diffusivities due to each effect are given together with suggestions and improved procedures which allow one to better find the grain boundary diffusivities or substantially reduce the errors. Some of the procedures discussed are unique in the sense that they are based on pure mathematics and provide exact values of the grain boundary diffusivities. Also, numerical errors arising in the course of integrations used in the present study are seriously discussed. For example, the procedure to simulate grain boundary diffusion under conditions of long and short diffusion times by using the finite element method was established.Item Open Access Thermoelectric properties of oxide heterostructures(2013) Heinze, Stefan; Keimer, Bernhard (Prof. Dr.)The thermoelectric properties and the electrical conductivity of thin films of the high temperature superconductor YBa2Cu3O7-d and the ferromagnet/paramagnet La2/3Ca1/3MnO3 as well as heterostructures and superlattices of both materials were investigated in this thesis. Since the properties of YBa2Cu3O7-d and La2/3Ca1/3MnO3 are antagonistic, the simultaneous occurrence of both properties in heterostructures and superlattices and the mutual interactions between both materials are in the focus of interest. The investigations should gain insight into the nature of these interactions and can be seen complementary to the investigations using Hall coefficient and normal resistivity measurements. Also, the studies should lead to information about the properties of the interface between both materials. The thin films used in this thesis exhibit mainly a total thickness of 2000 Å. They were grown on different substrates (SrTiO3, (La0.27Sr0.73)(Al0.65Ta0.35)O3, NdGaO3) by pulsed laser deposition (PLD) techniques and studied in different temperature ranges (20 K to 390 K or 320 K to 750 K). The concept was to investigate at first the physical properties of single layer YBa2Cu3O7-d and La2/3Ca1/3MnO3. In a second step, heterostructures and superlattices of both materials, also with a total thickness of 2000 Å, were fabricated and studied. Using systems with constant total thickness, but with different thicknesses of the single layer materials in the structures and thus varying number of interfaces should lead to knowledge about the mutual influences between both materials and the properties of the interface between both materials. The investigations of single layer YBa2Cu3O7-d and La2/3Ca1/3MnO3 thin films lead to findings which were in a good agreement with the results published by different groups before. The measured Seebeck-coefficients of optimally doped YBa2Cu3O7 d (d = 0.05) are zero in the superconducting state and nearly zero (S(T) = ± 2 µV/K) up to temperatures of 400 K. The Seebeck-coefficients determined for La2/3Ca1/3MnO3 are nearly zero (S(T) = ± 2 µV/K) in the ferromagnetic regime and negative (S(T) = 10 µV/K to 17 µV/K) in the paramagnetic state. Both materials were combined in heterostructures and superlattices. The investigated Seebeck-coefficients result always in positive values between 20 K and 390 K. The findings are completely unexpected since they are strongly deviating from a theoretical combination of the physical properties of the single layer materials according to Kirchhoff’s law. The calculated values are nearly matching with the Seebeck-coefficients investigated for YBCO. Finally, the findings were addressed to long range interactions nucleated at an electronic and magnetic reconstructed interface between YBa2Cu3O7-d and La2/3Ca1/3MnO3. Information about the physical properties of the interface could not be gathered directly, because of its high resistivity.Item Open Access Size and morphology effects on the cell voltage of Li-batteries : case study of RuO2(2009) Delmer, Olga; Maier, Joachim (Prof. Dr.)In the present work RuO2 as a positive model electrode in a cell RuO2/Li+- electrolyte/Li was investigated with the focus on the influence of the particle size and morphology on the cell voltage of the system. The motivation of these investigations is to contribute to the understanding of that barely studied specific field of size effects and to make a step towards the improvement of the positive electrodes in the rechargeable Li-battery technology. As a model electrode, RuO2 is a particularly suitable material. It shows all the relevant modes of Li storage which are known in the Li batteries. Additionally, it exhibits excellent electronic conductivity, a good Li diffusion and offers a unique combination of high capacity and high Coulombic efficiency in the first couple of discharge/charge cycles. Not all possible reactions were investigated here in detail. The examinations were focused on the storage of Li in the single phase regime followed by a two phase reaction, namely transformation of Li saturated RuO2 into LiRuO2 by further Li addition. Furthermore, interfacial storage, which occurs after the full reduction of RuO2 to Ru, was one of the points of interest. The considered materials are crystalline powders with the average particle sizes of 10 μm, 60 nm, 30 nm and 1.5 - 3 nm and amorphous powders which could be obtained from the crystalline powders (60 nm and 30 nm) electrochemically. For the electrochemical investigations, pure powders were used directly as electrodes and in a parallel study the electrodes were prepared in a common way, namely using a binder (here polyvinylene difluoride (PVDF)). Electrochemical experiments, i.e., measurements of the open-circuit voltage (OCV), galvanostatic intermittent titration technique (GITT) and discharge and charge measurements, were systematically performed on Li/RuO2 systems delivering information about electrochemical potential against Li, Li diffusion coefficient, Li+ conductivity, discharge behavior and capacity. Li storage experiments on pure Ru, for better understanding of the interfacial storage, were performed on three Ru powders with different particle size distributions. The most important results obtained by the described investigations are following: - an excess in the electrochemical potential of ca. 0.5 V in the case of the amorphous phase in comparison to the crystalline could be obtained and qualitatively understood in terms of melting free energy. Additionally, a detailed quantitative analysis of the chemical potentials delivered striking differences in the single and two phase regime. A model, based on the mixture thermodynamics, was developed and correlated with the experimentally obtained Li solubility limits and OCVs. Furthermore, this model could be also applied to the nano-scaled RuO2 (1.5 - 3 nm). - an excess in the electrochemical potential up to 440 mV could be obtained for nano-particles (1.5 - 3 nm) of RuO2. The excess voltage could be explained by the surface contribution term which delivered the capillary term being 1 Jm-2. - the coating of the particles with binder led to the raised electrochemical potential up to 340 mV. Furthermore, - a sensitive method, based on the evaluation of the Li+-conductivity, for estimation of the Li solubility limits in amorphous and nano-scaled materials could be established. - an increase in apparently reversible capacity up to 1.5Li by addition of binder could be achieved. This effect might be attributed to the storage in the particle-binder-interface. - the attempts to store Li in pure Ru with different particle sizes were successful and showed a clear correlation between the surface area and the amount of stored Li. The most storage could be attributed to the storage in the solid electrolyte interface (SEI), whereas no evidences for storage in the bulk could be found.