03 Fakultät Chemie
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Item Open Access Interphases between alkali metals (Li, Na) and battery electrolytes : ion transport and growth behavior(2022) Lim, Kyungmi; Maier, Joachim (Prof. Dr.)Item Open Access First-principles thermodynamic study of oxygen vacancies in ABO3-type perovskites(2017) Arrigoni, Marco; Maier, Joachim (Prof. Dr.)Item Open Access Interfacial effects in solid-liquid glyme-based electrolytes for lithium batteries(2018) Nojabaee, Seyedeh Maryam; Maier, Joachim (Prof. Dr.)Item Open Access Thin film growth and structural investigation of DyBa2Cu3O7-δ(2020) Putzky, Daniel; Keimer, Bernhard (Prof. Dr.)Item Open Access Defect chemistry of mixed conducting perovskites : interplay of protonic defects, oxygen vacancies and electron holes(2022) Raimondi, Giulia; Maier, Joachim (Prof. Dr.)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 Spin-orbital entanglement and molecular orbital formation in 4d and 5d transition metal oxides(2020) Krajewska, Aleksandra; Takagi, Hidenori (Prof. Dr.)Item Open Access Impact of point defects on reaction kinetics of systematically doped ceria(2020) Schaube, Maximilian; Maier, Joachim (Prof. Dr.)This thesis investigates the interplay between ionic and electronic point defects such as oxygen vacancies, electrons or holes, and the catalytic activity for heterogeneous reactions, in particular oxygen exchange, carbon monoxide and methane oxidation. The importance of point defects for reaction kinetics is demonstrated for more than 30 differently doped ceria, strontium titanate and zirconia samples, whereby the focus is set in doped ceria. Fundamental relationships between catalytic activity and defect chemistry are elucidated emphasizing the importance of defect chemistry in heterogeneous catalysis.Item Open Access Electronic structure and defect chemistry in iron perovskites(2021) Hoedl, Maximilian F.; Maier, Joachim (Prof. Dr.)This thesis systematically investigates the electronic structure and defect chemistry of BaxSr1-xFeO3-d through first-principles density functional theory (DFT) calculations. First, the electronic structure of defect-free, cubic BaFeO3 was calculated using DFT and analyzed in terms of local atomic orbitals. The calculations revealed BaFeO3 to be a negative charge transfer material with a dominating d5L (L = ligand hole) configuration. A detailed chemical bonding analysis further showed that the Fe-O bond has a mixed ionic-covalent character, and that the frontier orbitals at the Fermi level (and ligand holes) have an anti-bonding pdsigma* character. The susceptibility of the ideal cubic perovskite structure towards phase transformations was evaluated on the basis of first-principles phonon calculations. The phonon dispersion revealed distinct dynamically unstable modes which are isostructural to Jahn-Teller type distortions. The distortion is able to lift the orbital degeneracy of O 2p dominated ligand holes inherent to the cubic phase, thereby alleviating stresses in the electronic structure. The defect chemistry of BaxSr1-xFeO3-d was explored with respect to two different types of point defects: oxygen vacancies and protonic defects. The energy of oxygen vacancy formation, i.e. the release of neutral oxygen at the expense of electron holes, increases with increasing Sr-content and increasing oxygen vacancy concentration. Both compositional variations correlate with an increasing Fermi level at which electrons from the removed oxygen have to be accommodated. With increasing oxygen vacancy concentration, the Fe-O bond is weakened which facilitates oxygen excorporation and should decrease the vacancy formation energy. However, this contribution is effectively outweighed by the concomitant increase in Fermi level, rendering the vacancy formation energy to experience a net increase. In solid oxides containing oxygen vacancies, protons can be incorporated via the hydration reaction, i.e. the absorption of water vapor in dissociated form (H+, OH-), with the proton being attached to a regular oxygen ion and the hydroxide ion filling an oxygen vacancy. A thermodynamic formalism was developed that allows quantifying the energy changes during the two partial reactions - the proton- and hydroxide affinities - from first-principles DFT calculations. The new formalism was applied to a wide range of solid oxides, ranging from binary oxides such as MgO to various perovskite oxides, including BaZrO3 and BaFeO3. The study revealed an intriguing correlation between proton- and hydroxide affinities and the ionization potential (IP, position of O 2p band relative to the vacuum level) of the materials across the various structure families investigated. In the series of compositions BaxSr1-xFeO3-d, the hydration energy becomes more negative with increasing Ba-content and increasing concentration of oxygen vacancies. Evaluation of the proton and hydroxide affinities in oxygen non-stoichiometric BaFeO3-d showed that the trend with oxygen vacancy concentration largely reflects an underlying trend of increasingly more negative hydroxide affinities. This is suggested to stem from the annihilation of delocalized ligand holes during oxygen vacancy formation; lattice oxygen ions (and incorporated OH-) become subsequently more negatively charged, and thus experience a stronger electrostatic interaction with their ionic environment.Item Open Access Lithium intercalation studies in 2D materials using electrolyte gating(2022) Fecher, Sven; Smet, Jurgen H. (Dr.)The aim of this thesis was to investigate lithium intercalation in the single van der Waals gallery of a bilayer of two-dimensional materials driven by electrochemistry. Lithium intercalation is the process of lithium ions being incorporated between the layers of a layered host material. Since these ions carry a charge, energy can be stored this way. The most known application are batteries, where upon charging, lithium is removed from the cathode, a lithium compound, and incorporated at the anode. The commercially most frequently used material for this purpose is a compound consisting primarily of graphite, the layered allotrope of carbon. The elemental building block of graphite is graphene, the two-dimensional, one atom thick allotrope of carbon. It was first isolated in experiments in 2004 by A. Geim and K. Novoselov. They were awarded the Nobel prize in 2010 for their findings. Graphene opened up a wide variety of research due to the ease of production. When thinning down graphite, which is a random process, bilayer graphene can also be isolated. It provides the thinnest possible material suitable for lithium intercalation by offering only one interlayer gap. Thus, it can be considered the most basic building unit of a battery anode. In previous studies, this material has been successfully intercalated with lithium ions using on-chip electrochemistry. By doing so, charged ions in the electrolyte are separated and they accumulate at the interfaces. In the case of a lithium containing electrolyte, lithium ions can additionally intercalate into the material when a voltage above a certain threshold is applied, which leads to doping of the host material. This method is applied in this thesis as well in order to further investigate the process of lithium intercalation into bilayer graphene. The investigation tool of choice here was the SALVE TEM, which enables in-situ recording on the atomic scale due to its outstanding resolution. To perform these TEM studies, modifications of the sample layout became necessary, which are explained in this thesis. The starting point for investigation of lithium intercalation is the standard sample layout developed earlier in the group of Jurgen Smet. It consists of a bilayer graphene flake, patterned into a Hallbar shape and contacted using metal leads. A lithium counter-electrode is used as cathode and an electrolyte is placed connecting the bilayer graphene device with the cathode. The electrolyte only covers a part of the sample in order to exclude the influence of the electrolyte on the investigated portion of the sample. It was possible to see a reversible change of the longitudinal resistance R_xx upon lithiation as well as a change in the Hall resistance R_xy, due to charge induced by the lithium ion intercalates. During measurements, some device degradation was observed after prolonged experiments resulting finally in device failure. We identified reaction of lithium metal used as counter-electrode with residual oxygen as the most likely source of this sample degradation. Therefore, lithium metal was excluded for use as counter-electrode during the sample fabrication process for the remainder of this thesis. Instead a bare Ti or Pt metal counter-electrode was used. This has the disadvantage that the voltage drop applied across the bilayer graphene electrode and the electrolyte droplet is not known, however, empirically this uncertainty can be lived with. Further samples were prepared and transferred into the TEM column in order to test if images with atomic resolution can be acquired. During these early investigations, two main challenges became obvious: The electrolyte droplet, containing hydrocarbons, seemed to outgas in the UHV atmosphere of the sample space and amorphous carbon formed in the beam illuminated regions, preventing proper imaging. The second challenge was that the bilayer graphene lattice developed defects with time because of the co-existence of the high energy electron beam and the carbon contaminants. Extremely clean sample surfaces are required for prolonged imaging. We were able to solve these issues by encapsulating the electrolyte droplet with a 200 nm thick layer of SiO_x, thermally evaporated on top of the drop. This was done with the help of a shadow mask to keep most of the bilayer graphene uncovered. This layer successfully prevented the electrolyte from outgasing and no additional amorphous carbon formed. The carbon residues from processing were removed by current-annealing the sample directly inside the TEM column. A high current is sent through the device which causes strong local heating. This reliably cleans the sample in essence by burning off carbon residuals. After these modifications to the sample fabrication procedure we were able to show that the bilayer graphene samples could still be intercalated with lithium ions and the intercalation was to a large extent reversible. Samples with the modified device layout were then used inside the TEM column to record image series with atomic scale resolution to monitor the intercalation and de-intercalation process in-situ. The images revealed that an additional lattice appears and grows with time. An analysis of the chemical elements present using EELS indicated that only the elements lithium and carbon are available. Since the graphene lattice was also feasible, we conclude that the extra crystalline lattice must consist of pure lithium. The Fourier transform of a real space image yields its image in reciprocal space and thus the lattice information can be extracted from such an image. In the specific sample three lithium crystals with slightly different rotation angles and a lattice constant of 0.31 nm, which is slightly larger than the lattice constant of bilayer graphene 0.246 nm) were identified. By masking the graphene reflexes in the FFT image and back-transforming to real space, the underlying bilayer graphene lattice can be removed to improve the visibility. Additionally, the three different crystal grain rotations can be coloured to further enhance the visibility of the growth process of these lithium crystals. Furthermore, we observed that even within one grain, regions with different contrast appear. This was attributed to a varying thickness of the lithium crystal grain. Hence, the lithium crystal grains are thicker than just one atomic layer. Attempts to measure this thickness by using low-loss EELS suggested a total thickness of 3 - 4 nm. However, the applicability of the usual expressions to convert the signal strength to thickness is questionable since for a two layer thick sample, surface plasmons dominate, normally not considered. Hence, this value only is an upper limit for the thickness. Unfortunately, no other possibility existed to determine the thickness from TEM data. Finally, we were also able to record image series during delithiation, i. e. the removal of lithium from the bilayer graphene host. Hence, lithium crystal growth is a reversible process. We focussed on the nucleation and the time development of the lithium crystal inside the bilayer graphene host. Nucleation starts at defect sites or residuals from fabrication. Typically, these are regions where the lattice is distorted and where the surface energy is higher. The crystal grains exhibit a triangular shape initially. After some time, they still are characterized by sharp edges at the growth front. The rotation angle of a lithium crystal seems to be independent from the orientation of the underlying bilayer graphene lattice. Hence, there appears to be no preferred growth direction and the FFT images show diffraction spots that form a ring. When different grains grow towards each other, they will eventually meet. Three possible scenarios can be distinguished. (1) The crystal grains overlap leading to an increase in thickness visible by a darker appearance in the real space TEM image. (2) One of the grains changes orientation in order to match the other grains orientation and therefore the grains merge into a single grain. This seems to happen only for very small grains, presumably because reorientation of a large grain is energetically too costly. (3) The crystal grains form a grain boundary between them and growth continues in different directions since the interlayer gap is already occupied. The growth rate was determined by a series of images in which the lithium crystal grains fill the entire field of view after about 6 minutes. This led to an estimated rate of 2.45 nm^2/s. TEM images were also taken on triple graphene layer samples. Lithium intercalation basically appears similar and the same mechanism seems to be active. Nucleation again occurs primarily at defects or amorphous residuals on the sample surface. The lithium crystal grains also exhibit a triangular shape and sharp edges after continued growth. Growth itself occurs in every direction without any preferred growth directions. Regions with increased contrast within one grain are visible as well pointing to regions with increased thickness.