Universität Stuttgart
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Item Open Access Ab initio thermodynamic study of defective strontium titanate(2013) Blokhin, Evgeny; Maier, Joachim (Prof. Dr.)In the presented thesis the perfect and defective SrTiO3 bulk crystals and their (001) surfaces are considered on ab initio level. Since the experimental study of the complex defective systems is comparatively expensive and difficult, and the computer performance has been greatly increased in the last years, the ab initio modeling became very efficient tool to be applied in this field. Additionally, there is a significant industrial demand for the investigation and improvements of the performance of the perovskite-based electrochemical devices, e.g. solid oxide fuel cells and permeation membranes. Finally, there is a lack of methodological studies on defect thermodynamics. The oxygen vacancies and iron impurities, as well as their complexes, are the characteristic defects in SrTiO3 perovskite material. The understanding of basic defect properties and defect-induced phenomena under realistic external conditions requires calculation of thermodynamic properties (the free energy and entropy effects). Thus, thesis is focused on atomic vibrations, i.e. phonons, which are necessary to go beyond standard 0°K approximation and to provide a link to the thermodynamic properties at finite temperatures. This is necessary for a realistic treatment of electrochemical devices. The chosen ab initio modeling scheme is found to ensure the most accurate description simultaneously for the structural, electron and phonon properties of the perfect SrTiO3. Namely, the splitting of the phonon frequencies due to the antiferrodistortive phase transition at 105°K is confirmed to be very small (2–12 cm-1). The experimental temperature dependence of the SrTiO3 heat capacity is also successfully reproduced. Further, the modeling scheme is applied for thermodynamic treatment of oxygen vacancies and iron impurities in SrTiO3 at finite temperatures. The calculated phonon densities of states and group-theoretical study of the defect-induced phonon frequencies are used for the experiment analysis. Several defect-induced local phonon modes are identified, and the experimental Raman- and IR-spectroscopy data are interpreted. The Jahn-Teller-type local lattice distortion around both Fe4+ impurity and oxygen vacancy VO is shown to result in Raman- and IR-active phonons. In particular, the experimentally observed Raman frequency near 700 cm-1 is shown to arise for both defects due to a local O ion stretching vibration nearby the Jahn-Teller defect. However, an absence of such a frequency in an experimental phonon spectrum is found to be a manifestation of formation of Fe3+–VO complexes with oxygen vacancies in the first coordination sphere of iron impurities. The Gibbs formation energy calculated for the neutral oxygen vacancies in bulk SrTiO3 taking into account the phonon contribution is found to be in excellent agreement with the experiment. The phonon contribution to the formation energy is shown to increase with temperature, to about 5% above 1000°K. The predicted relative stability of several structural complexes of oxygen vacancy and iron impurity in SrTiO3 is confirmed by known experimental measurements. Several structural models of such Fe3+–VO complexes in SrTiO3 are discriminated according to the XANES and EXAFS experiments. On an example of SrO-terminated SrTiO3 ultrathin films, the one-dimensional confinement effect on the vacancy formation energy is found to be inconsiderable at 0°K. The phonon contribution to the Gibbs free energy of VO formation in such ultrathin films at finite temperatures is shown to be minor. This suggests the further account of anharmonic effects is required. The workflow developed in thesis is proposed for the modeling of wide class of defects in non-metallic solids. Several auxiliary computer tools were designed in order to simplify such possible studies.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 Novel theoretical methods for transition metal chemistry(2025) Safari, Arta A.; Alavi, Ali (Prof. Dr.)Item Open Access Automatic generation of coupled cluster algorithms with application to transcorrelated Hamiltonians(2025) Schraivogel, Thomas; Kats, Daniel (PD Dr.)We have used automatic code generation techniques to derive and implement full transcorrelated coupled and distinguishable cluster methods for accelerated basis set convergence and wave function compactification, leading to increased accuracy with manageable cost. The methods were benchmarked on thermochemical properties and two integral approximations based on normal ordering have been explored. The integral approximations have been shown to reduce the cost of the calculations without significant sacrifices in accuracy. They have paved the way for the very versatile transcorrelated method via exclusion of explicit three-body components (xTC). It has been demonstrated that the distinguishable cluster approximation improves the accuracy of transcorrelated coupled cluster methods. The transcorrelated coupled and distinguishable cluster methods with single and double excitations have been shown to outperform their F12 counterparts and approach the accuracy of CCSD(T)-F12. A two determinant distinguishable cluster method, alongside the traditional version, has been implemented in a new Julia package for electronic structure methods, called ElemCo.jl. The methods have been benchmarked on singlet and triplet excited states of closed-shell molecules and singlet-triplet gaps of diradicals using state-specific orbitals in the delta coupled cluster framework. The distinguishable cluster approximation improved the accuracy of the two determinant coupled cluster method considerably and the two determinant distinguishable cluster method has been shown to provide comparable and sometimes better accuracy than the equation-of-motion coupled cluster methods.Item Open Access Polymer electrolyte membrane degradation and mobility in fuel cells : a solid-state NMR investigation(2010) Ghassemzadeh Khoshkroodi, Lida; Müller, Klaus (Prof. Dr.)It is generally believed that fuel cells will play an important role in energy technology already in the near future. Operating polymer electrolyte membrane fuel cells (PEMFCs) at temperatures higher than 100 °C and reduced humidity is anticipated to avoid most of the shortcomings associated with the low-temperature fuel cell operation, such as CO poisoning of the electrode catalysts, slow electrode kinetics of the oxygen reduction reaction and expensive water/thermal management. To date, the operation temperature of PEMFCs is limited to about 90 °C, and this limit is given by the properties of the perfluorosulfonic acid (PFSA) ionomer, Nafion, which is commonly used as a separator material. Apart from the proton conductivity decay at higher temperature and lower humidification, it is also the limited stability of Nafion preventing it from long term operation. Despite the high stability of the PTFE backbone in Nafion, severe deterioration is observed during fuel cell operation. Formation of pinholes and cracks, thinning of the membranes and decrease of ion exchange capacity were reported. The fluorine release indicated that the bond cleavage process takes place under fuel cell operating conditions. Bond cleavage was initially believed to proceed from radical attacks to the carboxyl groups terminating the PTFE backbone of Nafion, and it was claimed to be controlled by the endcapping of the polymer backbone with a CF3 group. However, the release of fluoride was reported even after endcapping of the materials. The observations proved that bond cleavage limits the stability of PFSA membranes, but the elementary reactions and consequences on the membrane microstructure are not fully understood yet. In this work, it has been tried to get new insights into the problems of long term stability of polymer electrolytes for low temperature fuel cells. The aim was to identify the changes in the chemical structure of the membrane after operating in a fuel cell. This understanding is essential for extending the operation limit of PFSA-type membranes by either improving the membrane properties or adjusting the conditions within the running fuel cell. In the present work, therefore the changes taking place in PFSA membranes after applying in-situ and ex-situ aging protocols have been investigated. While the in-situ experiments provide a global picture, the analysis of membranes after ex-situ tests, with various conditions, allows the separation of different types of reactions. In previous studies the degradation changes were mainly monitored by analyzing the released water of the fuel cell or by using the liquid ionomers. In this work with the help of solid-state NMR spectroscopy, the direct study of the chemical structure and dynamics of the polymer membranes before and after the degradation tests became possible. The structural changes in different parts of the PFSA membranes were first inspected after an in-situ aging test. These examined membranes (Nafion and Hyflon Ion) differed by the length of the side chains. The comparison of the solid-state 13C and 19F NMR data of polymers before and after the in-situ degradation test showed that changes can take place not only in the main chain of the polymer, but also within the polymer side chains, as reflected by changes of NMR signals associated with CFSO3, CF3, OCF2 and CF groups. The degree of degradation is found to decrease with increasing membrane thickness while for a given thickness the short side chain polymer, Hyflon Ion, appears to degrade less than Nafion. In order to understand the reason for these observations, a new ex-situ method has been developed to mimic the degradation of polymer electrolyte membranes in PEM fuel cells (caused by the cross-leakage of H2 and O2). In this ex-situ setup, it was possible to expose membranes to flows of different gases with controlled temperature and humidity. H+-form Nafion films with and without electrode layer (Pt) have been treated in the presence of different gases in order to simulate the anode and cathode side of a PEMFC. The changes of the chemical structure occurring during the degradation tests were primarily examined by solid-state 19F NMR spectroscopy. For completion, liquid-state NMR studies and ion exchange capacity measurements were performed. It was found that degradation occurs only when both H2 and O2 are present (condition of gas cross-leakage), and when the membrane is coated with Pt catalyst. The chemical degradation rate is found to be highest for H2-rich mixtures of H2 and O2, which corresponds to the conditions at the anode under OCV. It is further shown that side chain disintegration is very important for chemical degradation, although backbone decomposition also might take place. The fact that in-situ degradation effects were reproduced by the present ex-situ experiments, suggest that membrane degradation in a running fuel cell is mainly the consequence of chemical aging. Detecting the degradation for the membranes coated with Pt in the presence of both gases, H2 and O2, points toward the importance of radicals in the degradation process, which in a running fuel cell (in-situ conditions) may only form in the presence of some gas cross-over, allowing H2 and O2 to react at the Pt catalyst of the anode or cathode structure. Since the gas cross-over increases for the thinner Nafion membrane, these results indirectly explain the higher degradation rate of thin Nafion in the in-situ degradation test. The chemical degradation and stability of PFSA membranes against radical attacks was also investigated in a Fenton ex-situ degradation test. Liquid and solid-state NMR as well as ATR-FTIR spectroscopy were applied to the samples before and after the Fenton reaction. A Comparison of the degradation rate of Nafion and Hyflon Ion in the ex-situ Fenton test again proved that the Hyflon Ion membrane is more stable than Nafion. Comparing the degradation rate of the side chain in these two polymers showed that the stability of Hyflon Ion is mainly due to the shortening of the side chain in this polymer. Hence, the absence of one ether group and the tertiary carbon reduces the degradation rate of the side chain and makes this polymer less sensitive to the radical attacks than Nafion. For the performance of a membrane not only the chemical structure but also the polymer dynamics is important. Therefore the molecular mobility of the ionomer was investigated by variable temperature 19F NMR lineshape, T1 and T1ρ relaxation experiments. The decrease of the temperature dependent linewidth was explained by the reduction of static disorder in the Nafion membrane. From the relaxation data there was evidence for structural annealing, which is independent of the chemical degradation. Chemical degradation is considered to reduce the chain flexibility (i.e. the motional amplitudes), which may be explained by chain cross-linking and condensation reaction for the side chains. To overcome the problem of Nafion's low conductivity at temperatures above 100 °C and low relative humidity, also composite membranes were introduced. These membranes consist of Nafion modified by inorganic oxide additives. It has been reported that under dry conditions, these membranes show enhanced water uptake and water diffusion when compared with filler-free Nafion. In order to understand the reason for the better performance of these polymers, the impact of the oxide particles on the polymer dynamics has been investigated. [Nafion/(SiO2)x] composite membranes in the dry and wet state with x ranging from 0 to 15 w/w% were investigated by variable temperature solid-state 19F NMR spectroscopy. 19F T1 and T1ρ relaxation times and NMR lineshapes were analyzed in order to get details about the polymer mobility. It is concluded that solid oxide SiO2 particles play an important role in stabilizing the chemical structure and morphology of the polymer especially in the dry state. The filler particles lead to higher mobility of polymer chains, if the filler content has an optimized value of about 9 w/w%. The results were further supported by comparing the sideband intensity as well as the linewidth in 19F NMR and recording the 19F{1H} CP/MAS NMR spectra. Furthermore, it has been shown that the structure of composite membranes is more stable after dehydration and possible condensation reactions are less likely in these membranes. The presence of filler particles decrease the chance for morphology changes and close packing of polymer chains in the dry state. Also the decrease of ionic exchange capacity after dehydration is less severe for the composite membrane as compared to filler-free Nafion. In conclusion, the present results provide a complete picture of solid membrane before and after degradation and of possible mechanisms for radical formation and radical attacks to the polymer. In addition, it is shown which changes can occur in the morphology of polymer chains in low humidification and high temperature. Some general suggestions for the better performance of polymer electrolyte membrane are therefore: For improving the performance of polymer electrode membrane, the sources for the radical formation in the fuel cell should be controlled. This can be possible to some extend by avoiding the use of iron end plates in the fuel cells. Also the chance for the gas crossover through the membrane should be decreased. Thicker membranes show less gas cross-over. By taking into account the higher resistivity of thicker membranes, an optimized membrane thickness should be selected. Hydrocarbon sulfonated polyetherketones possess narrower hydrophilic channels which significantly reduce electroosmotic drag, water permeation as well as gas cross-over. Also the short side chain perfluorinated polymer, Hyflon Ion, with lower electroosmotic drag of water should possess a reduced gas cross-over though the membrane. The more efficient way for decreasing degradation is to use membranes which are stable against radical attacks. At this point the perfluorinated polymers are still the best available membranes. Endcapping of the backbone in these polymers and decreasing the concentration of reactive end groups like COOH during the polymer manufacturing process can significantly decrease degradation. To minimize degradation of the side chains in perfluorinated polymers, short side chain polymers are suggested because of less reactive groups for the radical attacks and higher concentration of acidic groups. When higher operation temperatures are required, composite Nafion membranes might be used. The higher stability of these membranes makes them advantageous for operating at evaluated temperatures and low relative humidity. The novel results from the present work lead to a better understanding of membrane degradation, which still represents a serious problem for fuel cells under operation conditions, and provide important indications for future developments of membranes with improved performance for alternative energy conversion devices.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 Fluorine-promoted intramolecular aryl-aryl coupling : toward the isomer-specific fullerene synthesis(2013) Kabdulov, Mikhail; Jansen, Martin (Prof. Dr. Dr. h. c.)Direct synthesis of fullerenes is of considerable interest as a method to access new fullerenes which cannot be obtained in the uncontrolled process of graphite evaporation or form in low yields as a “hard-to-isolate” mixture. The general strategy of the direct approach to fullerenes is based on the synthesis of polycyclic aromatic hydrocarbons (PAH) that already contain the required carbon framework. Such “unrolled” molecules can be “rolled up” to form fullerenes through intramolecular Aryl-Aryl condensation under flash vacuum pyrolysis (FVP) conditions. The presence of chlorine or bromine in the initial precursor is essential for effective Aryl-Aryl condensation via free radical mechanism. On the other hand the use of chlorine and bromine functionalizations reaches its limits in the case of large molecules such as fullerene precursors. Availability of alternative promoters which do not have these disadvantages is a key prerequisite for successful direct fullerene synthesis. In this work various functional groups have been tested as alternative promoters of Aryl-Aryl intramolecular condensation under FVP conditions. Methyl and fluorine functionalization has been found to be promising approaches. Unexpected high selectivity in cyclization was observed for fluorine derivatives. It was found that HF elimination is a synchronous process leading directly to the target molecule without any intermediates, thus producing no side products. The small size and low molecular weight of fluorine as well as high thermostability of the C-F bond, makes fluorine a “perfect” activating group for rational fullerene synthesis. Since fluorine can promote the desired ring closure only if hydrogen is placed neighboring in space in the precursor structure, full control in the direction of the condensation can be achieved. It was shown that the use of fluorine, as an activating group, solves the problem of selectivity in FVP and provides an effective conversion of the respective PAH precursors. Several fullerene precursors containing fluorine atoms in key positions have been synthesized and investigated as a precursor for direct fullerene synthesis. Furthermore optimization has led to the discovery of a highly effective alternative solid-state strategy for intramolecular Aryl-Aryl coupling via HF elimination. The efficiency of the approach has been demonstrated by quantitative transformation of the precursor molecules to the desired PAHs and buckybowl structures. The quantitative conversion to the extended C46 buckybowl, representing more than 75% of the C60 fullerene connectivity, demonstrates the high potential of the technique for construction of extended non-planar carbon based nanostructures, including higher fullerenes, giant buckybowls and nanotubes. The results obtained point the way to the fabrication fullerenes as well as other carbon based nanostructures such as single walled nanotubes and nanoribbons in a fully controllable manner.Item Open Access Multi-phase La-Sr-Co-O fuel cell cathode materials and influence of phase boundaries on oxygen exchange activity(2016) Stämmler, Sebastian; Maier, Joachim (Prof. Dr. rer. nat.)The present work deals with the investigation of dense pulsed laser deposited (PLD) La-Sr-Co-O thin films which contain one or more perovskite related phases with catalytic activity for the oxygen reduction reaction (ORR). For the performance of solid oxide fuel cells (SOFC), a high ORR activity of the cathode material is decisive, in particular when attempting to lower the operation temperature. There are literature reports that enhanced ORR activities can be reached at hetero-interfaces of two different electronically and ionically conducting cathode materials, as demonstrated for (La,Sr)CoO3-d perovskite phase / (La,Sr)2CoO4+d Ruddlesden-Popper phase ("TPB effect", Kawada and Sase et al. 2006 and Crumlin et al. 2010). So far, no systematic experiments were performed to quantify the strength of the TPB effect (i.e. contribution of TPBs to ORR activity per TPB length). The present work therefore concentrated on the fundamental investigation of the TPB effect, a proper chemical and morphological characterization of the self-assembled La-Sr-Co-O composite cathode films containing perovskite and Ruddlesden-Popper phase and the quantification of the strength of the TPB effect.Item Open Access New ternary and quaternary metal oxides of ruthenium and osmium(2006) Mogare, Kailash M.; Jansen, Martin (Prof. Dr. Dr. h. c.)Ternary and quaternary oxides of platinum metals are of particular interest because of their intriguing physical, especially magnetic, properties. A very few alkali metal ruthenates and osmates have been reported so far with ruthenium and osmium exhibiting high oxidation states. The scope of the present thesis is to synthesize and characterize new ternary and quaternary alkali metal ruthenates and alkali metal osmates with an acquisition of high oxidation states by the transition metals. To achieve this, high oxygen pressures at elevated temperatures have been applied by using steel autoclaves as reaction vessels. The synthesized alkali metal ruthenates and osmates are investigated qualitatively and quantatively and further characterized using techniques such as powder and single crystal X-ray diffraction, powder neutron diffraction, infra-red/Raman spectroscopy, thermal analysis and measurements of conductivity and magnetic properties. In the case of alkali metal oxo-ruthenates, new hexavalent ruthenates of the compositions A2RuO4 and A3B(RuO4)2 have been explored. Within this family, the compounds display a trend of increasing magnetic interactions with decrease in the size of alkali metals. These interactions are independent from the crystal structure type and can be correlated to the shortest Ru-Ru distances among the Ru6+ ions. Neutron diffraction studies are carried out on Na2RuO4, the ternary oxo-ruthenate displaying the highest magnetic transition temperature in this family, and the low temperature magnetic structure has been deduced from the same. A number of new ternary and quaternary alkali metal oxo-osmates containing heptavalent and octavalent osmium have been explored. Some more high valent osmium containing oxides, which have been predicted and sketchily studied earlier by spectroscopic methods, have now been characterized with their crystal structures and physical properties. Some of the new compounds explored in this work belong to the family of double or triple perovskites (A2BMO6 and A3BM2O9, respectively). Only a few double and triple perovskite oxides with alkali metal cations as the A and B cations are known till today. The new compounds represent the first osmium perovskites containing alkali metal cations at the A site. Sr2CrOsO6, a cubic double perovskite, is re-investigated and found to exhibit the highest Curie temperature known so far in the family of double perovskites. The compound belongs to the increasing list of double perovskites with the task of discovering high ferromagnetic ordering temperature for potential technological applications.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.