03 Fakultät Chemie
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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 Influencing the ionic space charge potential in grain boundaries of oxide ceramics(2017) Weissmayer, Michael Patrick; Maier, Joachim (Prof. Dr. rer. nat.)Item Open Access In situ characterization of phase evolution in LiFePO4(2015) Ohmer, Nils; Maier, Joachim (Prof. Dr.)Among the candidates for electrodes in future Li-based batteries, LiFePO4 (LFP) is one of the most important and most frequently studied materials, undergoing a phase transformation upon delithiation to FePO4 (FP). In spite of the great scientific and practical interest in this material, there is still an extensive debate on the mechanism of this phase transformation and the underlying factors of influence. Within the framework of this thesis, first studies are carried out ex situ on multi-particle, full electrode LFP materials, being electrochemically cycled and analyzed at various states of charge by a combination of highly spatially resolved methods (high-resolution transmission electron microscopy and electron energy loss spectroscopy (HRTEM, EELS)) and integral measurement techniques (analyzing the X-ray diffraction and X-ray absorption near edge structure (XRD, XANES)). This combination of characterization techniques allows one to distinguish between the cycling behaviour of differently sized crystallites within the same electrode. It is found that for electrodes with hydrothermally grown LFP as active material, a particle size dependent cycling behaviour exists, with nanosized particles apparently not participating in the charging process at all. A turbostratic stacking of layers in these nanosized particles is found and identified to be responsible for sluggish lithium insertion and extraction. These higher dimensional defects prevent the small particles from participating in the charging process, most likely by disturbing the lithium diffusion along the 1-dimensional channels, as well as impair the transport along the other directions in the LFP host structure and thus blocking the lithium transport, resulting in a comparibly lower practical capacity during electrochemical cycling. To study the lithium exchange mechanism upon charging a LFP thin film cathode, an all-solid-state thin film battery cell with a lateral design concept is developed and realized by pulsed laser deposition (PLD) and thermal evaporation techniques. Using PLD and shadow masks LFP cathode, Li2O-V2O5-SiO2 (LVSO) electrolyte and LiAl anode thin films are deposited sequentially in a way that the Li transport pathway in the resulting battery is along the X-ray transparent commercial Si3N4 membrane substrate. This enables the usability of synchrotron-based energy resolved scanning transmission X-ray microscopy (STXM) with its high chemical and spatial resolution to perform in situ absorption measurements at the Fe L3 edge. Upon delithiation, a shift in the main absorption feature from 708 to 710 eV is used to fingerprint the change in the local state of charge, identifying areas containing Fe2+ (lithiated) and Fe3+ (delithiated), respectively. The initial lithiation process of a LFP thin film cathode material has been followed by in situ STXM, with a lateral resolution of 30 nm, during electrochemical charging of the thin film battery. The observed initial lithiation process does not follow the classical particle by particle mechanism, typical for multi-particle LFP cathodes, but instead a rather simultaneous, although inhomogeneous, lithiation is observed. The reason for this change in mechanism, compared to multi-particle powder electrodes, is found in mechanical interactions within the thin film upon lithiation, i.e. in the corresponding volume expansion and formation of high energy surfaces, changing the shape of the single-particle chemical potential to a monotone form upon lithiation. This has far-reaching consequences: not only the many-particle mechanism is changed to a concurrent lithiation, but also the single-particle mechanism is changed from a two-phase to a single-phase mechanism upon lithiation. Furthermore, a vanishing hysteresis loop and the disappearing of the memory effect is predicted. These findings are rather general and applicable to all kind of thin films of phase separating intercalation materials, undergoing a volume change upon lithium exchange. To fill the gap in literature on in situ observations of the (L)FP phase evolution on a single-particle level with appreciable space and time resolution, a micrometer-sized all-solid-state thin film battery is built with a defect-chemically well characterized LFP single crystal as cathode material with dimensions of 16x1x0.2 micrometer. Using STXM, the phase evolution along the fast (010) orientation is followed during in situ electrochemical (de)lithiation on a micro-meter scale with a lateral resolution of 30 nm and with minutes of time resolution. Furthermore, the STXM measurements performed on this sample are one of the few experiments ever taken on LFP materials with a well defined defect chemistry, even though fundamentally necessary for an overall understanding of the materials behaviour. This combination discloses not only the mechanism of LFP transformation on a single-particle level, but also the significance of elastic effects on the (de)lithiation process. Using a defect chemical analysis, the position of phase formation is found to be determined by the defect chemical situation, while the growth pattern of both LFP and FP is found to be dominated by elastic effects.Item Open Access Mixed-conducting perovskites as cathodes in protonic ceramic fuel cells : defect chemistry and transport properties(2018) Zohourian, Reihaneh; Maier, Joachim (Prof. Dr. rer. nat.)