Universität Stuttgart
Permanent URI for this communityhttps://elib.uni-stuttgart.de/handle/11682/1
Browse
20 results
Search Results
Item Open Access Growth and characterization of transition metal oxide heterostructures with a tailored work function(2018) Rastegar Alam, Iman; Mannhart, Jochen (Prof. Dr.)Item Open Access X-ray and Raman scattering studies of novel phases in 3d and 4d transition metal oxides(2020) Fürsich, Katrin; Keimer, Bernhard (Prof. Dr.)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 Miscibility, viscoelastic reinforcement, and transport properties of blend membranes based on sulfonated poly(phenylene sulfone)s(2021) Saatkamp, Torben; Maier, Joachim (Prof. Dr.)Chemical energy that hydrogen may generate during combustion and the corresponding electrical energy are interconvertible by means of a fuel cell (FC) and by the electrolysis of water (WE), which allows for the utilization of the complementary nature of these two key energy vectors towards energy sustainability. A proton exchange membrane (PEM) made from an ionomer is commonly employed as the electrolyte in mobile fuel cell applications and in water electrolyzers that require dynamic operability and pressurized product gases. New PEM materials are needed to increase performance, reduce environmental impact, and allow for a more targeted design of PEMFC and PEMWE systems, all of which is in some way limited by the use of the established perfluorosulfonic acid (PFSA) type ionomers. This work’s focus lies on sulfonated poly(phenylene sulfone)s (sPPS), a unique group of fluorine-free cation conducting ionomers. They are unique in terms of their chemical stability and transport properties, however, typical in terms of their salt-like brittleness in the dry state and extensive swelling at high humidity and in water. To make the unique properties of sPPS available in application, the goal of this work is to take a comprehensive approach to their viscoelastic reinforcement. Therefore, the structure of this thesis entails three related aspects along the process from pure materials to the optimization of robust PEMs for application. The first chapter focuses on the optimization of the intrinsic viscoelastic properties of a particularly suited sPPS (termed S360, with IEC 2.78 meq g-1, EW 360 g mol-1) which lays the groundwork for reliable and systematic further development. To achieve this, relevant properties of S360 are first characterized and viscoelastic shortcomings as seen in water uptake measurements and tensile tests under dry conditions (≤ 30% relative humidity, RH) discussed. The step-growth polymerization of S360 is optimized after finding significant inorganic contamination retained in the established purification process of the widely used monomer sulfonated difluorodiphenyl sulfone (sDFDPS), allowing for the preparation of the ionomer in reproducible high molecular weight. Relevant properties of high molecular weight S360 are characterized and an enhancement of mechanical properties at 30% RH as well as when submerged in water is found. Access to reproducible high quality of S360 enables its first-time use and study as a PEM in a completely fluorine-free WE cell. At 80 °C, record performance amongst fluorine free electrolytes in PEMWEs of 3.48 A cm-2 at 1.8 V is achieved, showcasing the potential of sPPS for application. The second chapter entails the identification and better understanding of a suitable and versatile reinforcement concept for creating robust membranes based on sPPS. To achieve this, the established homogeneously miscible acid-base polymer blends of sulfonated ionomers with poly(benzimidazole) (PBI, and its derivatives PBIO and PBIOO) are discussed in-depth and chosen for later systematic optimization in combination with sPPS. Since the origin of miscibility in PBI blends with sulfonated ionomers is insufficiently described in literature and could facilitate targeted design of new blend components, a model acid-base polymer blend system comprising pyridine-functionalized poly(sulfone) (PSU) is created. Pyridine groups of different basicity tethered to PSU in varying concentration are used to investigate the effect that interpolymer acid-base interaction strength and concentration have on miscibility in blends with 80 wt% S360, as derived from the blend membranes’ cross-sectional SEMs. High mutual compatibility is achieved at high concentration of weak interpolymer interaction, which is interpreted with regards to the observed miscibility in PBI blends. Based on the derived role that hydrogen bonds may play in PBI blends, the difference of interpolymer interaction in solution (during membrane formation) and in the dry membrane is described. This could enable the development of new blend concepts in the future. An exemplary miscible blend that comprises interpolymer hydrogen bonds only in solution but not in the final membrane is shown. The third chapter describes the optimization and balance of properties in the previously described polymer blends with PBIO, following the goal to prepare membranes which can be evaluated in fuel cells and fabricated on a wider scale in order to bring the attractive properties of sPPS into application. To achieve this, S360-blend membranes of varying PBIO content are characterized with regard to conductivity and mechanical properties in various conditions. High mechanical robustness is achieved in S360 blends with 30 wt% PBIO but is accompanied by dramatic reduction of conductivity, due to the charge-consuming acid-base interaction. The findings are translated into blends with fully sulfonated sPPS (termed S220, with IEC 4.54 meq g-1, EW 220 g mol-1) which allows for the creation of membranes that combine mechanical toughness with high conductivity at a ratio of 25 wt% PBIO in S220, making the material suited for production on a commercial casting line and fuel cell testing. Membranes based on S360 that comprise 15 wt% PBIO are designated for further studies in PEMWEs, where membrane requirements differ significantly from that in PEMFCs, highlighting the versatility of the reinforcement approach chosen in this work. Finally, first fuel cell tests of thin spray coated PBIO blend membranes are conducted, and initial durability testing of sPPS-based membranes in fuel cells is possible. Overall, the results presented in this work are strongly interrelated which underlines the importance of comprehensiveness in the successful viscoelastic reinforcement of sulfonated poly(phenylene sulfone)s. Ultimately, the blend membranes resulting from this work can be used as a platform for further development of sPPS-based PEMs in the future.Item Open Access Two-dimensional X-ray powder diffraction(2007) Hinrichsen, Bernd; Dinnebier, Robert E. (Prof. Dr.)The combination two-dimensional detectors, powder diffraction and synchrotron light sources has been staggeringly successful, opening doors to many new experiments. The great advantages of such data collection lie in the short exposure times as well as in the huge redundancy. A large angular region of the Bragg cone is recorded in a single exposure; indeed most detectors are set up perpendicular and centrally to the primary beam, intercepting the Bragg cone over the entire azimuthal range. The standard practice is to integrate the image along the ellipses described by the intersection of the cone with the planar detector to a conventional powder pattern. This commonly reduces the amount of information by the square root of the number of pixels. Does this represent the gamut of information contained in a powder diffraction image? A glance at an image from a calibration standard might lend itself to such a conclusion. Less perfect samples, as well as sample environments leave distinctive artefacts on images. How can they be extracted, filtered or interpreted? Methods offering answers to these questions are introduced. The origins of powder diffraction were based on diffraction images, however with the onset of equatorial electronic point detectors all high quality powder diffraction experiments switched to this method. It has remained the experimental doctrine to this day. Only recently have powder diffraction scientists rediscovered the allure of diffraction images. Indeed high pressure powder diffraction experiments are unthinkable without two-dimensional detectors. What seems like such a positive development does, on closer inspection have its problems. Two dimensional correction factors effectively do not exist for powder diffraction experiments. All commonly used Lorentz and polarization (LP) corrections are meaningless outside the thin equatorial strip for which they were determined. Furthermore various other detector and geometry dependent factors have to be considered should a high quality powder diffraction pattern be extracted from the image. The first chapter of this thesis takes on this challenge and presents all applicable two-dimensional correction factors, as well as the basis for their application: the experimental set-up. Determining the geometry to the highest possible precision is paramount to the quality of the experiment. How can one achieve this goal, without losing oneself in diverging refinements and renitent analysis software? Pattern recognition methods and whole image refinement have been used to solve the two main problems of calibration and are presented in the second chapter. The first global search gives sensible starting values for what is probably the most extreme refinement single pattern powder diffraction has to offer: whole image refinement. Here the entire two-dimensional image is rebuilt, based on the initial values, and subtracted from the experimental image. This residual is then minimized using a Levenberg-Marquardt non-linear least squares refinement algorithm. This method leads to calibrations that are at least one order of magnitude more precise than traditional calibration routines. This is of fundamental importance for the effective use of future high resolution area detectors. A perfect calibration does not suffice to ensure a successful data reduction. Especially in situ experiments - the forte of two-dimensional detectors cause intensity aberrations that need to be removed before the image can successfully be integrated to a conventional powder diffractogram. The source of deviations can be sorted into two camps: those originating from the sample environment and those emanating from the sample itself. Of course the former is both more easily recognized visually and also removed more simply by the fractile filters presented in the third chapter. When intensity deviations originate from the sample the matter becomes far more complex. A new distribution function, the normal Pareto function, has been shown to describe the intensity distribution that results from small sample amounts without substantial sample rotation, as is the case in high pressure powder diffraction. The great benefit of this function is that it opens the possibility of extracting a fractional filtering setting which ultimately leads to normally distributed intensities. Structural analysis from diffraction data is always connected to a plethora of reliability values, describing the raw data as well as the refinement quality. Powder diffraction images completely lack any numerical estimation of their quality. Functions giving universally comparable, detector independent reliability values for images can be found in chapter four.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 Influencing the ionic space charge potential in grain boundaries of oxide ceramics(2017) Weissmayer, Michael Patrick; Maier, Joachim (Prof. Dr. rer. nat.)