Browsing by Author "Maier, Joachim (Prof. Dr.)"

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Open Access
Ab initio study of point defects in the bulk and on surfaces of an SrTiO3 crystal
(2009) Alexandrov, Vitaly; Maier, Joachim (Prof. Dr.)
The main goal of the present thesis was the theoretical study of substitutional iron impurities and oxygen vacancies present in different charge states in the bulk and on the SrTiO$_{3}$ (001) surface by means of first-principles simulations. Our first step was to examine basic properties of perfect perovskite crystals. We found for SrTiO$_{3}$ and SrFeO$_{3}$ parent compounds that the best agreement with experimental data on lattice constant, bulk modulus, cohesive energy and optical band gap is provided by the hybrid Hartree-Fock and the density functional theory (DFT) approach in the linear combination of atomic orbitals approximation with the so-called B3PW functional which was therefore chosen as a main tool in the study. In order to study SrFe$_{x}$Ti$_{1-x}$O$_{3}$ solid solutions, we carried out a series of calculations for various iron contents (50, 6.25, 3.70, 3.125, and 1.85 at.\%) in doped SrTiO$_{3}$. We found that the Jahn-Teller (JT) distortion around an Fe$^{4+}$ ion is the largest for the most dilute solid solution, becomes less pronounced for 50\% iron doping, and disappears in a pure SrFeO$_{3}$. This tendency in changing the magnitude of the JT distortion agrees well with the EXAFS experimenatal data extrapolated to the dilute defect limit. Also, the electronic structure calculations indicate that SrFe$_{x}$Ti$_{1-x}$O$_{3}$ containing more than 50\% iron is metallic, and that its conductivity is caused by a strong mixing of O $2p$ and Fe $3d$ ($e_{g}$) states in the pre-Fermi energy region. We found that the iron impurity insertion energy of 1.79 eV (1.85\% iron content) is very close to that for 6.25\% iron, and this energy decreases down to 1.59 eV for 50\% and lastly to 1.57 eV for a pure SrFeO$_{3}$. Calculations of iron impurities on the SrTiO$_{3}$ (001) surfaces revealed that the Fe$^{4+}$ ion has the propensity to segregate from the bulk to both SrO and TiO$_{2}$ facets, with the segregation energies of 0.32 and 0.48 eV, respectively. The Mulliken population analysis indicates that the presence of Fe$^{4+}$ ion in the subsurface plane significantly reduces the charge of the nearest oxygen atom at the topmost SrO surface (in a comparison to the substituted Ti$^{4+}$), thus severely diminishing its basic properties. We also examined the neutral and positively single-charged oxygen vacancies ($F$ and $F^{+}$ centers), both in the bulk and on the SrTiO$_{3}$ (001) surfaces. It was found that the neutral vacancy has an even higher tendency than Fe$^{4+}$ ions to surface segregation, being $\thicksim$1.0 eV for SrO and 1.4 eV for TiO$_{2}$ surfaces. The defect energy level becomes much more shallow when going from the bulk (0.77 eV below the bottom of the conduction band at the $\Gamma$-point of the BZ) to SrO (0.27 eV) and TiO$_{2}$ surfaces (very shallow level, almost degenerate with the CB within an accuracy of the method). Our simulations of the bulk charged $F^{+}$ center show that it has a deeper energy level (1.20 eV) than the neutral defect revealing also a lower vacancy formation energy. The charged nature of the center results in a more pronounced relaxation around the defect with a repulsive interaction with neighboring titanium atoms. This relaxation becomes even stronger at the surface. Thus, a common feature of both types of vacancies is the more shallow energy levels on the surfaces compared to the bulk, particularly, for the TiO$_{2}$ facet, while the effect is less pronounced for the $F^{+}$ center. We simulated additionally the diffusion of oxygen species by means of the density functional theory combined with the nudged elastic band (NEB) method. We have shown that the calculated activation barrier for diffusion of oxygen vacancy along the TiO$_{2}$ surface is almost by a factor of three smaller than in the bulk (0.14 vs. 0.38 eV). Adsorption energy of oxygen atom atop Ti ion for the TiO$_{2}$ facet is as large as 2.13 eV being considerably higher than that atop Sr ion on SrO facet (0.57 eV). Moreover, the creation of surface oxygen vacancy nearby the O atom adsorbed atop Ti ion leads to a significant increase in the O$_{ads}$-Ti binding energy. Because of such a strong adsorbate-adsorbent bonding, penetration of the adsorbed O atom into the surface layer could occur predominantly when the very mobile surface oxygen vacancy meets adsorbed oxygen atom. Simulation of a drop of the adsorbed O atom into the oxygen vacancy nearby reveals a distinguishable but extremely small activation barrier, $\thicksim$0.01 eV. Thus, we predict almost no-barrier soaking of the adsorbed O atom into the surface layer, fast surface diffusion of the oxygen vacancies and a much slower diffusion in the bulk. Considerable part of our study was devoted to the examination of oxygen vacancies (neutral and charged) as important ionic charge carriers under confinement conditions. This aspect is of paramount importance in nanoionic systems in which the boundary zones overlap and not only the density but in addition the nano-size spacing at interfaces becomes a key factor.
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.
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.
Open Access
Ba1-xSrxCoyFe1-yO3-delta SOFC cathode materials : bulk properties, kinetics and mechanism of oxygen reduction
(2009) Wang, Lei; Maier, Joachim (Prof. Dr.)
This work is mainly concerned with the mixed conducting perovskite solid solution materials family Ba1-xSrxCoyFe1-yO3-delta (BSCF) which is discussed as solid oxide fuel cell (SOFC) cathode material. The aim is to get an improved understanding of the complex oxygen reduction reaction on such oxides in general, and in particular for the application as catalytically active cathode in SOFC. As a SOFC cathode candidate, the stability of BSCFs with regard to the application was first studied on powders synthesized from the metal nitrates by the glycine nitrate process. With respect to the stability towards electrolyte materials, at 750 °C (Ba0.5Sr0.5)1.04Co0.8Fe0.2O3-delta (BSCF5080) already reacts with 8 mol % Y2O3-ZrO2 (YSZ). This undesired reactivity is more pronounced than that of La0.6Sr0.4Co0.8Fe0.2O3-delta and YSZ. At 850 °C, BSCF5080 even starts to react with Ce0.9Ge0.1O2-delta (CGO). The increased reactivity of BSCF5080 may mainly come from the higher A-/B- cation size mismatch and therefore BSCFs with lower Ba content or Ba-deficiency may show higher stability with the electrolytes. Concerning the stability of BSCFs under CO2-containing atmosphere, a high Ba content is detrimental. Even Ba-deficiency can not effectively inhibit the formation of carbonates. As a result, Ba-containing perovskite oxide cathodes require highly purified oxidizing atmosphere which is not very practical. Some of the BSCF compositions with cubic perovskite structure from the synthesis slowly transform to noncubic phases at intermediate temperatures. For the investigated compositions, no obvious relationship between the cubic perovskite stability at intermediate temperatures and the Goldschmidt tolerance factor could be established. However, BSCF with lower Co content shows higher long-term phase stability, therefore Co/Fe ratio may play an important role. Co changes its oxidation state more easily and the ionic radius change upon the change of the oxidation state facilitates the formation of face-sharing octahedra with shorter B-O bond length and the resulting noncubic perovskites. The oxygen reduction kinetics and mechanism was studied on geometrically well defined microelectrodes. 100 nm thick dense thin films of the respective compositions were deposited by pulsed laser deposition (PLD) on (100)-oriented YSZ single crystals. X-ray diffraction (XRD) shows the films maintain high phase purity and exhibit a highly textured structure which depends on the film thickness, the exact cation composition and the substrate. Scanning electron microscopy (SEM) measurements confirm that the thin films prepared under the present deposition conditions are dense films with columnar growth. The thin films were subsequently patterned into circular microelectrodes with 20 to 100 µm diameter by photolithography. Silver paste and foil were attached to the back side of the sample as counter electrode. Impedance spectra were recorded on these microelectrodes and the temperature, oxygen partial pressure and dc bias dependence was studied. It is found that the oxygen reduction on these BSCF microelectrodes proceeds through the "bulk path", i. e., incorporation of oxygen into the electrode on the whole electrode surface, and subsequent oxygen ions diffusion through the electrode bulk. The rate of oxygen incorporation is limited by the oxygen surface reaction rate. Based on the equivalent circuit developed from previous works, the surface resistance Rs corresponding to the oxygen surface incorporation reaction can be quantitatively compared for different cation compositions in BSCF. The temperature dependence of Rs is similar for the six BSCF compositions studied and the activation energies are in the range of 1.3 to 1.8 eV. In the oxygen partial pressure P(O2) dependence, all compositions have an exponent between -0.5 and -1 in the logRs-logP(O2) plot indicating the reaction order of the oxygen molecules is 1. Both cathodic and anodic dc bias decrease Rs. Applied cathodic bias values up to 400 mV do not introduce obvious irreversible changes on a less degraded ("fresh") sample while a cathodic bias of 300 mV already obviously decreases Rs and introduces irreversible changes on a more degraded ("aged") sample. Due to the difficulty in directly detecting the coverage and nature of the intermediate oxygen species on the surface of the cathodes under SOFC operation conditions, correlations between the oxygen incorporation reaction rate kq calculated from Rs and various bulk materials properties were studied and used to supply information on the detailed oxygen reduction mechanism. There is no straightforward correlation between kq and the lattice constant, the electronic conductivity, the Goldschmidt tolerance factor or the oxidation enthalpy. However, kq increases nonlinearly with increasing oxygen vacancy concentration cVö which indicates that the oxygen vacancies are involved in the rate-determining step of the oxygen incorporation reaction. The nonlinear increase of kq with increasing cVö was assumed to be caused by the increased oxygen vacancy mobility. To confirm this assumption, acquiring reliable oxygen vacancy diffusion coefficient data is necessary. Due to the relatively low density of sintered pellets and the complexity in the experiments, oxygen vacancy diffusion coefficients from conductivity relaxation experiments for BSCF5080 were not accurate enough. However, reliable oxygen vacancy diffusion coefficients were obtained on 250 nm dense BSCF thin films deposited by PLD on (100)-oriented MgO single crystals. A gas-tight gold cover layer was deposited on top of the BSCF film by evaporation. In order to enable oxygen isotope exchange between BSCF and the atmosphere, a cut about 30 µm wide with relatively sharp edges was created through the gold and BSCF layers with an automatic dicing saw. Isotope exchange was carried out at different temperatures and later on the 18O concentration profile in the quenched samples was analyzed by time-of-flight secondary ion mass spectrometry (TOF-SIMS). The oxygen vacancy diffusion coefficients DVö calculated from the extracted oxygen tracer diffusion coefficient D* for BSCF5080 are significantly higher than that of (La,Sr)(Mn,Fe,Co)O3-delta perovskites. The activation energy Ea of DVö for BSCF5080 is only 0.47 (+/- 0.04) eV which is much lower than that of Ba0.5Sr0.5FeO3-delta (BSF, 1.1 (+/- 0.1) eV), SrFeO3-delta (SF, 0.9 (+/- 0.1) eV) and (La,Sr)(Mn,Fe,Co)O3-delta (~ 0.9 eV). The high DVö and low Ea is thought to be due to low cation charge (A2+, fraction of Co2+) and high polarizability (Ba2+, Co2+). With DVö calculated from D*, the correlation between kq and DVö is confirmed. While the increase of kq from (La,Sr)(Mn)O3+/-delta (LSM) to (La,Sr)(Co,Fe)O3-delta (LSCF) is mainly due to the increase of the oxygen vacancy concentration, the increase of kq from LSCF/SF to Ba0.5Sr0.5Co0.8Fe0.2O3-delta mainly comes from the accelerated (surface) oxygen vacancy diffusion. Based on the experimental observations and the conclusions from DFT calculations on LaMnO3 slabs, the rate-determining step of the oxygen incorporation reaction on BSCF is proposed to be the diffusion of an oxygen vacancy towards an adsorbed O2- resulting in an O2(2-) adsorbed at the vacancy position, the dissociation of which is fast. There are intrinsic problems of BSCF when it is applied as SOFC cathode, such as the undesired reactivity with the electrolyte, the carbonate formation under CO2-containing atmosphere and the long-term phase transformation. It is almost impossible to solve them only by varying the cation composition within this material family. Therefore BSCF may not be applicable as a SOFC cathode. Nevertheless, based on the proposed oxygen incorporation mechanism for BSCF, the combination of high oxygen vacancy concentration and mobility is an adequate criterion for selecting other SOFC cathode candidates. In BSCF, the high oxygen vacancy concentration is due to the fact that the 4+ oxidation state of B-site cations is not so stable, and therefore the 2+ oxidation state at A-site is mainly compensated by the formation of oxygen vacancies. To avoid the problems introduced by Ba and maintain the average oxidation state of A-site to be 2+, perovskites with a combination of 1+ alkali metals and 3+ rare earth metals at A-site were studied. The solubility of Ag in (La,Ag)Co0.4Fe0.6O3-delta perovskites is rather limited. For K-containing (La,K)Co0.4Fe0.6O3-delta and (Nd,K)Co0.4Fe0.6O3-delta perovskites, it is difficult to get phase pure powders. Rs from impedance spectroscopy studies of the material with a nominal composition La0.5Co0.4Fe0.6O3-delta and a predominant perovskite phase is one order of magnitude higher than LSCF and this is most probably due to the phase impurities. Based on the mechanism suggested in this thesis, fast oxygen exchange materials should have a high oxygen vacancy mobility. This motivates to investigate perovskites with highly polarizable A-cations. In this respect, Bi3+ instead of Ba2+ would be an interesting candidate, which also might help to reduce the carbonate formation problem.
Open Access
Beiträge zur Sensorik redox-aktiver Gase
(2002) Kamp, Bernhard; Maier, Joachim (Prof. Dr.)
## Chemische Diffusion von Sauerstoff in Zinndioxid ## Resistive Taguchi-Sensoren basieren häufig auf dem n-Halbleiter Zinndioxid. Als wichtigste Ursache für Drift dieser Sensoren werden Änderungen der Sauerstoffstöchiometrie des SnO2 angesehen. Diese gehen auf die Sauerstoffausbaureaktion zurück: Oo = Vo°° + 2e' + 1/2 O2 (Oo : reguläres Oxidion, Vo°° : Sauerstoffleerstelle, e' : Leitungselektronen). Der Transport dieser Defekte erfolgt durch chemische Diffusion, welche damit von großer Relevanz für das Drift-Verhalten der Taguchi-Sensoren ist. In dieser Arbeit sollten die Abhängigkeit des chemischen Diffusionskoeffizienten D von der Temperatur, dem Sauerstoff-Partialdruck p(O2), dem Dotiergehalt des Kristalls und der kristallographischen Orientierung untersucht werden. Es wurden an Einkristallen Leitfähigkeits-Relaxationsexperimente durchgeführt. Die beoachtete Sprungantwort war mit drei Relaxationprozessen wesentlich komplizierter als nach dem gängigen Defektmodell erwartet. Zur Identifikation der einzelnen Prozesse wurde die Abhängigkeit der Leitfähigkeitsänderung (bzw. ihrer Abklingzeit) von der p(O2)-Änderung und Probendicke betrachtet. So wurde die anfängliche Änderung einem Oberflächeneffekt und der zweite Vorgang der chemischen Diffusion von Sauerstoff zugeordnet. Die Gesamtänderung der Leitfähigkeit durch den zweiten und dritten Relaxationsvorgang war deutlich größer als nach einfachen defektchemischen Modellen ( p(O2)^(-1/4)-Gesetz ) erwartet. Zusätzlich wurden ESR-Relaxationsexperimente durchgeführt, bei denen nach einem p(O2)-Sprung die Änderung des ESR-Signals eines redox-aktiven Dotierions (Eisen(III) bzw. Mangan(IV) ) verfolgt wurde. Es konnte bestätigt werden, daß der o.g. zweite Vorgang der chemischen Diffusion entspricht. Auch hier wurde ein zusätzlicher langsamerer Vorgang gefunden. Da die ESR-Methode ein über das gesamte Probenvolumen gemitteltes Signal liefert, muß es sich dabei um eine Komplizierung der Defektchemie des Zinndioxids und nicht nur um einen Oberflächeneffekt handeln. Als Erklärung wurde vorgeschlagen, daß durch das Schottky-Gleichgewicht gebildete 4-fach ionisierte Zinnleerstellen in 3-fach ionisierte Zinnleerstellen übergehen und so die Konzentration an Leitungselektronen beeinflussen. Defektchemische Modelle auf Basis dieser Annahme zeigen, daß dies die o.g. Erhöhung der p(O2)-Abhängigkeit der Leitfähigkeit erklären kann. Es wurde keine signifikante Abhängigkeit des chemischen Diffusionskoeffizienten von Dotiergehalt oder Kristallorientierung gefunden. Die Temperaturabhängigkeit des chemischen Diffusionskoeffizienten ließ sich mit der Beziehung D = exp(-3.9±1.8) cm^2 s^-1 exp(-(1.1±0.2)eV / kT ) beschreiben. Interne Redoxreaktionen wie z.B. das Gleichgewicht zwischen Eisen(III) und Eisen(II), können die chemische Diffusion des Sauerstoffs in Oxiden verlangsamen (Trappingeffekt). Mit einem Modell wurde unter Berücksichtigung des Trappings die Abhängigkeit des chemischen Diffusionskoeffizienten von der Elektronenkonzentration berechnet. Dabei wurde das "einfache" Modell der Defektchemie des SnO2 verwendet, in welches nur der Sauerstoff-Ausbau und Trapping an Dotierionen eingingen. Mit einer kombinierten ESR- und Leitfähigkeits-Methode wurden zuvor die benötigten Gleichgewichtskonstanten der relevanten redox-Gleichgewichte bestimmt: Standardwerte der freien Enthalpien (Abstand der Störstellen zum Leitungsband): Eisen(III)/Eisen(II): 0.35eV, Mangan(III)/Mangan(IV): 0.76eV (800°C). Die berechnete deutliche Abhängigkeit des chemischen Diffusionskoeffizienten von der Leitungselektronenkonzentration stand in Widerspruch zu experimentellen Ergebnissen. Auch dies zeigt, daß das "einfache" defektchemische Modell nicht ausreichend ist. Die Abweichungen können wiederum durch die o.g. redox-Aktivität der Zinnleerstellen erklärt werden. Aus der Unabhängigkeit der gemessenen chemischen Diffusionskoeffizienten von der Konzentration an Leitungselektronen und dem Gehalt an redox-aktiven Verunreinigungen läßt sich schließen, daß die in dieser Arbeit ermittelten chemischen Diffusionskoeffizienten die Untergrenze der bei anderen Dotiergehalten möglichen Werte darstellen. Mit der Beziehung t = L^2 / 2D lassen sich daher für Taguchi-Sensoren Mindestwerte für charakteristische Zeiten t von Driftprozessen angeben (L=charakteristische Länge, z.B. der Radius der SnO2-Körner im Sensor). Man muß dafür annehmen, daß die chemische Diffusion des Sauerstoffs geschwindigkeitsbestimmend für den Driftvorgang ist. Jedoch sind die so gefundenen charakteristischen Zeiten wesentlich geringer als in der Literatur angegebene Werte für reale Driftvorgänge an Taguchi-Sensoren. Dies läßt sich dadurch erklären, daß nicht die chemische Diffusion geschwindigkeitsbestimmend für den Sauerstoffaustausch des Sensor-Materials ist, sondern die Oberflächenreaktion, durch die Sauerstoff zwischen der Oberfläche des Materials und der Gasphase ausgetauscht wird. ## Protonenleitende Perowskite als Elektrolyte für amperometrische Stickoxidsensoren ## Im zweiten Teil der Arbeit wurde die Eignung perowskitischer Protonenleiter als Elektrolyt amperometrischer Gassensoren untersucht. Zunächst wurde ein Barium-Calcium-Niobat Ba(Ca0.39Nb0.61)O2.91 (BCN) aufgrund seiner relativ hohen chemischen und mechanischen Stabilität unter verschiedenen Perowskiten ausgewählt. Leitfähigkeitsmessungen ergaben, daß die protonische Leitfähigkeit durch die Korngrenzen stark vermindert wurde (z.B. bei 250°C um einen Faktor 10). Die elektronische Leitfähigkeit wurde jedoch kaum verändert. Die Korngrenzen steigern damit die elektronische Überführungszahl des Materials. Es wurden Polarisationsmessungen in verschiedenen Atmosphären (befeuchtete Argon-Atmosphäre, sowie O2, CO-, NO-, NO2-haltige Atmosphären) durchgeführt. Mit Platin- und Gold-Elektroden auf BCN ließ sich NO2 selektiv reduzieren. Bis ca. 350°C ließ sich eine hohe Selektivität erzielen, die jedoch bei höheren Temperaturen zurückging. In Gegenwart von NO wurde ähnlich wie bei NO2 eine Reduktionsreaktion beobachtet. Als wahrscheinliche Ursache für die Stickoxidselektivität wurde die oberflächliche Zersetzung des BCN unter Bildung von Nitraten angeführt. Es konnte keine detektierbare Umsetzung von CO festgestellt werden. Die starke Querempfindlichkeit gegenüber H2 ist an Platinelektroden ausgeprägter als an Goldelektroden. Problematisch war die Neigung des BCN zur Versprödung und seine Zersetzung zum Erdalkali-Nitrat. Letztere Reaktion schreitet jedoch nach der Bildung einer sehr dünnen Reaktionsschicht (ca. 5-10 Elementarzellen) nur noch sehr langsam voran. Abschließend wird der Einsatz perowskitischer Protonenleiter in Kombination mit einer durch Bildung von Erdalkali-Nitraten erzielten Selektivität der Stickoxidreduktion als ein aussichtsreicher Weg zu einem selektiven und im Aufbau vergleichsweise einfachen amperometrischen Sensor für Stickoxide bewertet. Zur technischen Umsetzung dieses Ziels sind jedoch Elektrolytmaterialen mit verbesserten Eigenschaften gegenüber BCN notwendig. Wichtig sind dabei vor allem eine höhere chemische und mechanischen Stabilität sowie geringere elektronische Leitfähigkeiten.
Open Access
Boundary effects on the electrical conductivity of cerium oxide thin films
(2013) Göbel, Marcus; Maier, Joachim (Prof. Dr.)
Boundary effects in mixed and ionic conductors and in particular space charge layer (SCL) effects are a major field of research in solid state ionics since they are known to strongly affect the electrical transport properties of the materials. In particular, in this study the boundary effects of cerium oxide were investigated which due to its high ionic conductivity is a material of large relevance for a wide range of applications such as solid oxide fuel cells (SOFCs), oxygen membranes and catalysis. Here the thin film geometry was applied which offers the advantage of a usually well defined microstructure allowing to access effects at both the film-substrate interface (FSI) and the grain boundaries (GBs). The ceria thin films were grown with pulsed laser deposition (PLD) on various substrates. Thin films were prepared of (1) different doping contents (nominally pure, acceptor doped and donor doped), (2) different microstructures (epitaxial and nanocrystalline) and (3) different thicknesses (between 20 and 450 nm). Their microstructure was characterized with XRD, SEM, TEM and electron diffraction. The defect chemistry and conductivity properties of the samples were investigated with impedance spectroscopy. Additionally, a software to numerically compute the SCL profiles and conductivity effects in ceria was developed. The numerical approach allowed for the determination of all relevant SCL profile characteristics and for the correlation of them with both the material properties and the resulting conductivity effects. Notably, the numerical approach was observed to yield precise results without the use of further assumptions also for asymmetric and mixed cases in contrast to the well known analytical solutions. This allowed for a test of the assumptions made in the analytical solutions which resulted in the development of improved analytical relationships. Remarkably, the improved analytical approach which is not restricted to ceria but generally applicable was observed to yield very reliable outcomes even for complex situations. In most cases the boundary effects were found to dominate the conductivity of the investigated ceria thin films. Effects at both the film-substrate interface and at the grain boundaries were observed. As expected in the framework of the SCL theory no significant FSI effect was detected in strongly acceptor doped films. In epitaxial, nominally pure ceria films grown on Al2O3 <0001> the conductivity was observed to be reduced at the FSI in agreement with the SCL theory. In nanocrystalline, acceptor doped samples grown on SiO2 <0001> a significant decrease of the conductivity at the GBs was observed in accordance with a SCL potential of 0.32 ± 0.05 V. For this set of samples a thickness dependence of the grain size was detected resulting in a considerable change of the conductivity with film thickness. Therefore, it could be demonstrated that a thickness dependent conductivity in polycrystalline samples can not always be assigned to FSI effects; a finding of relevance for a number of similar studies on interface effects. Nanocrystalline, acceptor doped films grown on Al2O3 <1-102> and MgO <100> substrates were found to exhibit a less significant decrease of the ionic conductivity and, hence, a smaller SCL potential of 0.19± 0.05 V. This is most likely the result of the smaller lattice mismatch between substrate and CeO2 film resulting in less pronounced GB core charges. Thin films prepared at room temperature, characterized by a nanocrystalline microstructure with very small grains, were measured to exhibit very pronounced SCL effects (decrease of the ionic conductivity by 3 orders of magnitude). Most likely here the Mott-Schottky assumption is fulfilled much better resulting in a particularly strong depletion of the oxygen vacancies. Remarkably, for these samples the pO2 of the electrolytic domain boundary, was found to be shifted by 29 orders of magnitude compared with what is expected from an extrapolation of the literature bulk data. Two superimposing effects were found to be the origin of this drastic shift: In addition to (1) the pronounced SCL effects, (2) already in the bulk the electronic conductivity was measured to be strongly increased. Both effects also affected the activation energies. Remarkably, in this case the electronic activation energy fell even below the ionic value resulting in an increase of the electronic conductivity contribution for decreasing temperatures in marked contrast to the properties typically observed in ceria. Notably, in donor doped ceria the electronic conductivity was observed to be decreased at the GBs. The recorded conductivity data indicates that either a negative SCL potential or a change of the electron mobility at the GBs is the cause of this effect. The investigation could also confirm the presence of oxygen interstitial defects in donor doped cerium oxide which was suggested in earlier studies. Also the oxygen insertion enthalpy was measured.
Open Access
Charge carrier defect chemistry of nanoscopic SrTiO3
(2012) Lupetin, Piero; Maier, Joachim (Prof. Dr.)
The study of ionic and electronic conduction properties of nanosized objects has revealed, in the last years, a variety of fascinating effects as the conduction properties of nanocrystalline materials are dominated if not fully controlled by the grain boundaries. The basis for the understanding of such effects is provided by the field of nanoionics, which allows the elucidation of defect chemistry not only for well separated boundary zones but also in the more exciting mesoscopic range where the distance of the interfaces (grain size) is on the order or below the characteristic decay length of a semi-infinite interface. In the present study, strontium titanate (SrTiO3) has been taken as a model system to investigate these aspects. Notably, SrTiO3 is an excellent example for electroceramic oxides in general and for the family of perovskites in particular, thanks to its pronounced stability and its well studied defect chemistry at the macroscale. Furthermore, it exhibits a great technological relevance for several different applications such as anode for solid oxides fuel cells, varistors as well as substrate for high temperature superconductors. In the field of solid state ionics its importance is due to the fact that it is a mixed ionic and electronic conductor, with characteristic variations in the typical window of experimental conditions. In this contribution the electrical properties of SrTiO3 are investigated at the nanoscale, when no unperturbed bulk is present and the overall electrical properties are clearly dominated by the grain boundaries. Acceptor (iron) as well as donor (niobium) doping has been used to adjust the properties of the material (conductivity, space charge potential) even in the mesoscopic regime. In order to investigate size effects on the conduction properties the preparation of nanostructured SrTiO3 with a grain size smaller than 100 nm comes to the fore. This implies the optimization of the synthesis procedure at low temperature as well as of the sintering process. For the synthesis procedure three different methods are considered, namely co-precipitation, combustion and solvothermal and the densification was carried out using the high pressure field assisted sintering, also known as spark plasma sintering. Once the preparation of the nanostructured material is achieved, the attention is focused on the characterization of the electrical properties. In this context, the oxygen non-stoichiometry is considered as a key element, since it plays a crucial role in determining whether SrTiO3 is a p-type, n-type or ionic conductor. Therefore, the conduction properties have been investigated over a broad range of oxygen partial pressures and temperatures by means of impedance spectroscopy. In the case of undoped SrTiO3 (characterized by intrinsic acceptor impurities), the stoichiometry variation of the mesoscopic situation, in which the space charge zones overlap, reveals several exciting size-induced phenomena such as: increase of n-type conductivity by several orders of magnitude, an equally great depression of p-type conductivity and an even stronger drop of the oxygen vacancy conductivity when compared to the macroscopic situation. This generates a giant shift of the conductivity minimum by several orders of magnitude in terms of partial pressure. The results can be explained in the light of space charge effects occurring as a consequence of a positive charge excess in the grain boundary core. Huge size effects are observed also in intentionally acceptor doped nanocrystalline SrTiO3 and the difference with respect to the nominally pure case can be explained by the higher doping level. Another aspect, considered in this study concerns the possibility of tuning the grain boundary properties. This point is particularly relevant in mesoscopic materials, in which the grain boundaries control the overall charge transport. This goal is achieved by adding the dopant at the grain boundaries in order to modify only locally the stoichiometry. In particular, in SrTiO3 it is observed that the addition of acceptors, namely iron, at the grain boundaries yields to a core-shell situation within the grain in which the highly conductive shell short-circuits the bulk and determines the overall conduction properties of the material. Particularly intriguing are the studies on donor (niobium) doped SrTiO3, which is a well known n-type conductor at relatively high temperatures in the high oxygen partial pressure range. Surprisingly, the nanocrystalline material showed p-type conductivity in oxidizing conditions at 550°C and a blocking effect of the grain boundaries with respect of the electron transport when the material switches to the n-type regime. The whole set of results make nanocrystalline SrTiO3 a formidable master example of defect chemistry in the nanocrystalline regime and demonstrate the enormous power of size as degree of freedom in modern materials research.
Open Access
Charge carrier formation, mobility and microstructure of sulfonated polyelectrolytes for electrochemical applications
(2015) Wohlfarth, Andreas; Maier, Joachim (Prof. Dr.)
Polyelectrolytes are materials consisting of a polymer backbone with covalently attached positively or negatively charge groups including their counterions. Sulfonated polyelectrolytes are a specific class, which is especially interesting for electrochemical application as they can be used to separate the electrodes and mediate the electrochemical reactions taking place at anode and cathode by conducting a specific ion; this ion may be H+ in the case of PEM-fuel cells or Li+ and Na+ in various battery systems. The key challenges for the development of these electrolytes is the combination of good mechanical properties and high ion transport as well as high electrochemical stability. High ionic conductivity of polyelectrolytes depends on the presence of small polar solvents to ensure efficient dissociation and mobility of the counterions. These processes cannot be understood by merely considering electrostatics (i.e. Deby-Hückel approach) such as in Manning counterion condensation theory. Specific interactions between solvent-ion-polymer and the molecular conformations have to be taken into account as well. This is one of the results of the present work using sulfonated polyelectrolytes, different cations and solvents as model systems with a combined approach of experimental techniques and simulations. The dissociation behavior of sulfonated polysulfones was investigated by a combined electrophoretic (E) NMR, pulsed magnetic field gradient (PFG) NMR and conductivity approach. Since the results from the NMR experiments, especially from E-NMR which is by far no standard measurement, are crucial for the key conclusions drawn in this thesis, some critical issues of this technique are studied and discussed in detail. E-NMR is essentially a PFG-NMR experiment with an applied electric field; the applied voltage can reach up to 300 V. Therefore, it was necessary to determine a measurement window in which no decomposition or other interfering effects appeared. In addition, polymers gererally exhibit some polydispersity with the low molecular weight fraction showing a higher diffusion coefficients and drift velocities, which had to be taken into account. By concentrating ionic groups on the polymer, specific polyelectrolyte effects show up. Dissociation is no longer complete, the interaction between ionic charges and the solvent is heavily modified and correlations of ionic motion start to appear. According to a MD-simulation, this very much depends on the polymer conformation and position of the ionic groups as well as the chemical nature of the solvent. Once the density of ionic groups (-SO3H) of polysulfones reaches a point where their average separation is of the order of the Bjerrum length of water, the degree of counterion condensation is shown to depend on details of the molecular structure and the accessible conformations of the polymer chain. In this regime, well-defined ionic aggregates occur, i.e. triple-ions form. The conformational details depend on the degrees of freedom and specific interactions between ions and solvent. When it comes to ion conducting membranes, increasing the ion exchange capacity (decreasing the average separation of ionic groups) is a common measure to increase ionic conductivity. However, the results on dissociation and conductivity of synthesized polysulfones containing octasulfonated units (currently the material with highest known IEC) clearly reveal the limit of this approach. The short separation of ionic charges in such systems at high concentrations additionally leads to electrostatic interactions between neighboring polymer strands. This is the driving force for a nanoscale ordering in polyelectrolyte membranes. Different kinds of solvents, ions and ion exchange capacities directly affect the microstructure formation. Finally, the effects of acid-base interactions between sulfonic acid-based polyelectrolytes and weakly basic modified polymers were investigated as blending of both is a way to form stable membranes for electrochemical applications. Here, the membrane formation process and the resulting properties, in particular proton conductivity, microstructure and mechanical strength have been studied. The developed polymer blends are the first example in which an improvement of mechanical properties not goes along with a significant decrease of proton conductivity. Key to success was to use a hydrophilic polymer with a high IEC and a hydrophobic polymer with a low number of basic groups. In summary, this thesis provides insides into the charge carrier formation process, the transport and microstructure of sulfonated polyelectrolytes by identifying the relevant molecular interactions. Together with the superior mechanical properties of the developed blend membranes, this work significantly contributes to solve the key challenges for electrolytes in electrochemical application.
Open Access
Defect chemistry and photo-ionic effects in bromide and iodide perovskites
(2023) Wang, Ya-Ru; Maier, Joachim (Prof. Dr.)
Bromide and iodide perovskites, especially their mixtures, hold great potential for opto-electronic application due to their optical absorption properties in the visible range. While it is established that these materials are mixed ionic-electronic conductors, their ionic transport properties both in the dark and under light are poorly understood. The present work deals with the defect chemistry and photo-ionic effects in halide perovskites, including iodide and bromide perovskites with special focus on the photo induced phase separation (photo de-mixing) in mixed bromide - iodide perovskites. The first part covers the defect chemical study of bromide perovskites, including 3D MAPbBr3 and 2D Dion-Jacobson (PDMA)PbBr4. The results reveal that both 3D MAPbBr3 and 2D (PDMA)PbBr4 are mixed ionic-electronic conductors. 2D (PDMA)PbBr4 show three orders of magnitude lower ionic conductivity compared with 3D MAPbBr3. This implies that dimensionality reduction is an effective strategy for reducing ion migration in these systems. From a bromine partial pressure dependent study, it is concluded that MAPbBr3 and 2D(PDMA)PbBr4 are both P-type conductor and that the surface reaction is the limiting process for the incorporation and exporation of the Br2 gas. A non-monotonic dependence of the electronic conductivity on bromine partial pressure is detected for both 2D and 3D bromide perovskites. It can be attributed to the reversible formation and dissociation of AuBrx on the gold electrode and perovskites interface. The second part covers the investigation of the thermodynamic properties of the 2D mixed halide perovskites under light. It has been shown that light can be used as a knob for inducing photo de-mixing from single phase 2D mixed halide perovskites to to I-rich and Br-rich phases. In the dark, the photo de-mixed phases re-mix with complete reversibility of both their optical and structural properties, demonstrating the full miscibility of mixed bromide-iodide perosvkites in the dark. The temperature-dependence of absorption spectra for the photo de-mixed phases gave clear evidence for a miscibility gap under light, from which photo de-mixed phases’ compositions are extracted. The photo-miscibility-gap is mapped and confirmed by various methods. The shape of the photo-miscibility-gap shows limited variation in the 0.01 - 0.1 sun illumination intensity range. The non-encapsulation of surface, however, demonstrated a widening of the photo-miscibility gap. The third part covers the kinetic analysis and mechanistic investigation of photo de-mixing in 2D mixed halide perovskites. Simultaneous monitoring of the electrical conductivity and optical absorption allows for a local probe of electronic and ionic charge carriers, and the composition evolution. Furthermore, time dependent phase distribution is investigated with the aid of top view SEM, showing that I-rich nanodomains forming along the grain boundaries at early times after light exposure with further formation of such domains also within the grain at longer times. Local elementary distribution is probed with TEM. From the temperature dependent de-mixing half-time, an activation energy for photo de-mixing of 0.39 eV is obtained. Finally, together with DFT calculation on defect formation energy of the mixture of different defect type, a mechanistic description for photo de-mixing, both from molecular and microscopic level is proposed. The last part deal with the phase stability study of mixed halide perovskites in other dimensionalities, including 3D and nanocrystal based thin films. 3D mixed halide perovskites show that similar to 2D mixed halide perovskites, photo de-mixing occur in two stage. Different from the full reversibility of 2D, photo degradation of 3D perovskites into PbI2 in the dark over long time scales is observed. The nanocrystalline mixed halide perovskite (BA-MAPb(I0.5Br0.5)3) thin films show no de-mixing in contrast to 2D and 3D under same measurement conditions. Nanocrystals mixtures show superior phase stability under the same illumination condition, with neither degradation nor de-mixing. This thesis contributes to the understanding of the defect chemistry and ion transport properties of bromide and iodide perovskites, with specific focus on photo de-mixing in mixed halide perovskites. These findings will aid compositional engineering related to halide mixtures to enable optimization of optoelectronic devices as well as the development of other emerging systems exploiting photo-ionic effects.
Open Access
Defect chemistry in alkali peroxides and superoxides
(2014) Gerbig, Oliver; Maier, Joachim (Prof. Dr.)
Während Bildung und Transport von Punktdefekten in Oxiden an vielen Beispielen untersucht wurde, ist die Defektchemie der Metall-Peroxide und Superoxide praktisch unbekannt. Diese Thematik ist sowohl von grundlegendem Interesse (was sind die relevanten Ladungsträger, ergeben sich durch die Peroxid- und Superoxidionen Unterschiede zu Oxiden etc.) als auch von technologischer Bedeutung für neuartige elektrochemische Energiespeicher. Bei der elektrischen Entladung von Alkalimetall-Sauerstoff-Batterien mit nicht-wässrigen Elektrolyten reagieren Sauerstoff und Alkalimetall-Ionen unter Aufnahme der Elektronen aus dem elektrischen Stromkreis zu Alkalimetallperoxiden oder -superoxiden. Im Vergleich zu herkömmlichen Alkalimetall-Ionen-Batterien, bei denen die Alkalimetall-Ionen von einem Wirtsmaterial aus üblicherweise schweren Übergangsmetalloxiden- oder phosphaten aufgenommen werden (durch Interkalation oder Phasenumwandlung), könnten deutlich höhere spezifische Energiedichten erreicht werden, die insbesondere für Elektroantriebe von Kraftfahrzeugen erforderlich sind. Da die Alkalimetallperoxide und -superoxide bei jedem Entlade- und Ladezyklus neu gebildet und wieder zersetzt werden, kommt der Transport- und Reaktionskinetik dieser Materialien eine erhebliche Bedeutung für die Leistungsfähigkeit der Batterien zu. In der vorliegenden Arbeit werden die elektrischen Transporteigenschaften, die Sauerstoffaustauschkinetik sowie die Defektchemie der Peroxide von Lithium, Natrium und Kalium sowie der Superoxide von Kalium, Rubidium und Cäsium untersucht. Zur elektrochemischen Charakterisierung wurden Presslinge hergestellt und ionisch blockierende Elektroden (Titan, Gold, Platin) durch Kathodenzerstäubung aufgebracht. Die elektrische Leitfähigkeit der Materialien wurde mittels Impedanzspektroskopie und Stöchiometriepolarisation mit Gleichstrom in ihre ionische und elektronischen Anteile zerlegt. Aus dem transienten Verhalten der Stöchiometriepolarisation ließ sich außerdem der chemische Diffusionkoeffizient (ambipolare Diffusion von ionischen und elektronischen Ladungsträgern) gewinnen. Die Sauerstoffpartialdruckabhängigkeit der elektronischen Leitfähigkeit gibt Aufschluss über die elektronischen Majoritätsträger. Für die Alkalimetall-Peroxide ergibt sich eine zunehmende elektronische Leitfähigkeit mit zunehmendem Sauerstoffpartialdruck gemäß einer p-Leitung. Mit Hilfe der Elektronenspinresonanzspektroskopie können Superoxidionen als Defektspezies in den Peroxiden identifiziert werden. Dieser Defekt lässt sich als ein an einem regulären Peroxid-Platz lokalisiertes Defektelektron (Loch) veranschaulichen. Der wahrscheinlichste elektronische Leitungsmechanismus ist daher der Sprung eines solchen Defektelektrons zu einem benachbarten Peroxid-Ion. Das genau umgekehrte Bild ergibt sich für die Alkalimetall-Superoxide. Hier zeigt die zunehmende elektronische Leitfähigkeit mit abnehmendem Sauerstoffpartialdruck auf einen n-Leitfähigkeit an. Die Überschusselektronen sind am wahrscheinlichsten an Superoxid-Ionen gebunden und bilden Peroxidionen als Defektspezies im Material. Die ionischen Majoritätsladungsträger lassen sich in Lithiumperoxid durch die Verwendung von Elektroden aus Lithium-Aluminium | Lithiumiodid (reversibel für Kationen, polarisierend für Anionen) dem Lithiumionen-Teilgitter zu ordnen. Für den Transport der Kationen ergibt sich aus der Erhöhung der ionischen Leitfähigkeit mit zunehmendem Gehalt an Donoratomen ein Leerstellenmechanismus. Die relativ hohen Aktivierungsenergien für ionischen und elektronischen Transport erklären sich durch Berücksichtigung von Assoziaten aus Lithiumionenleerstellen und Donoren sowie Lithiumionenleerstellen und Löchern. Auf der Grundlage der Ergebnisse wird ein defektchemisches Modell vorgeschlagen und diskutiert. Die Abhängigkeiten der Defektkonzentrationen vom Sauerstoffpartialdruck bzw. von der Alkalimetallaktivität werden abgeleitet und als sogenannte Kröger-Vink-Diagramme graphisch dargestellt. Der Sauerstoffaustausch der Alkaliperoxide und superoxide wurde durch Sauerstoff-Isotopenaustausch mit anschließender Analyse der Gasphasenzusammensetzung mittels Massenspektrometer untersucht. In Lithiumperoxid, Natriumperoxid sowie Kaliumsuperoxid erfolgt der Sauerstoffaustausch maßgeblich unter Brechung der kovalenten Sauerstoffbindung. Dabei wird die Austauschkinetik mit zunehmendem Kationenradius deutlich schneller. Hingegen ermöglicht die hohe Polarisierbarkeit der Kationen im Cäsiumsuperoxid sogar den Einbau und den Volumentransport von Sauerstoff unter Beibehaltung der kovalenten Bindung. Aus der Sauerstoffisotopenrelaxation lassen sich der Diffusionskoeffizient des Superoxids sowie eine Untergrenze für die Sauerstoffaustauschreaktionsrate bestimmen. Während die Diffusion im Vergleich zu Sauerstoffelektrolyten wie Yttrium-dotiertem Zirkonoxid langsam verläuft, ist die Oberflächenreaktion in Cäsiumsuperoxid verglichen mit hochdotierten Kathodenmaterialien für Hochtemperatur-Brennstoffzellen sehr schnell. Zusammenfassend lässt sich festhalten, dass in der vorliegenden Arbeit die Defektchemie für eine Reihe von Metall-Peroxiden und Superoxiden systematisch und im Detail untersucht wurde. Hierbei wurden auch Transportgrößen wie die elektronische und ionische Leitfähigkeit, Diffusionskoeffizienten und Oberflächenreaktionsraten bestimmt sowie Transportmechanismen aufgeklärt. Diese Ergebnisse sind einerseits von grundlegendem Interesse aber auch hochgradig relevant für das Verständnis und die Entwicklung von neuartigen elektrochemischen Energiespeichern.
Open Access
Defect chemistry of bulk and thin film lithium chalcogenides
(2019) Lorger, Simon; Maier, Joachim (Prof. Dr.)
Open Access
Defect chemistry of mixed conducting perovskites : interplay of protonic defects, oxygen vacancies and electron holes
(2022) Raimondi, Giulia; Maier, Joachim (Prof. Dr.)
Open Access
Effects of ionic and electronic charge carriers in nanostructured TiO2 on lithium storage
(2012) Shin, Ji-Yong; Maier, Joachim (Prof. Dr.)
As far as efficient energy storage device in electric vehicles or smart grids are concerned lithium batteries came to the fore due to their high energy and power densities. However, it is still challenging to develop reasonably high rate and energy density batteries because of the limitation in intrinsic electrochemical properties of electrode materials. For electrode materials in Li-batteries, excellent performance can be only achieved by short diffusion length (L) and high chemical diffusivity (D) of Li. In recent years, therefore many efforts to control the time dependence have been conducted in the field of Li-batteries. As operation of Li-batteries involves both ionic and electronic charge carriers, fundamental understanding of the role of each charge carrier is of great importance. Using titanium dioxide (TiO2) as a model material, the main aim of this study therefore is to primarily understand effects of size and morphology and defect chemistry on the overall lithium transport and storage properties and furthermore to propose a strategy to design high performance electrode materials from the standpoint of defect chemistry. In the first part, effects of size and morphology of titania (variation of transport length of mainly ionic charge carriers (i.e. Li+)) on the Li storage are studied. Hierarchical nanoporous TiO2 particles with a large interfacial proportion were successfully prepared by hydrolysis and its electrochemical performance investigated. Due to the significantly shortened transport length of Li+ ion by nano-structuring, the materials showed excellent Li storage performance (showing both enhanced bulk and interfacial Li storage contributions) compared to nonporous materials. Especially, the nature of interfacial storage behavior is systematically discussed and its great importance for Li-batteries emphasized. The second part of the thesis mainly focuses on effects of charge carriers concentration (variation of electronic charge carrier concentration) on the Li storage properties. To increase electronic charge carrier concentrations in nanostructured TiO2, two different strategies are applied: (1) formation of frozen-in native defects (oxygen-deficient TiO2-d nanoparticles) and (2) homogeneous n-type doping by extrinsic defects (Nb5+-substituted mesoporous TiO2 nanoparticles). The results are discussed on the basis of multiple experimental techniques as well as a theoretical defect chemical analysis. These different strategies not only allow to develop relevant adjusting screws for optimizing the performance, they also allow a deeper understanding of ionic and electronic charge carriers in the context of Li-batteries.
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.
Open Access
Eigenschaften und Anwendungen von Netzwerken aus Kohlenstoff-Nanoröhren
(2006) Kaempgen, Martti; Maier, Joachim (Prof. Dr.)
Die vorliegende Arbeit über dünne CNT-Netzwerke befasst sich sowohl mit den grundlegenden elektronischen und optischen Eigenschaften , als auch mit neuen Möglichkeiten der Anwendungen. Grundsätzlich lassen sich die optischen Eigenschaften von CNT-Netzwerken als geeignete Mittelung über alle spektroskopisch erfassten Moleküle verstehen. Die Charakterisierung mittels der Absorptionsspektroskopie an freistehenden CNT-Netzwerken erlaubt einen maximalern Wellenlängenbereich (UV/VIS/NIR/-MIR) ohne limitierende Substrate. Aussagen über den Dotierungsgrad (FIR-Bereich) und zur chemischen Funktionalisierung (IR) sind genauso möglich, wie die Beobachtung optischer Übergänge zwischen den van-Hove-Singularitäten (VIS/NIR) und des Absorptionsmaximums der Plasmonen (UV). Die Ramanstreuung hingegen wurde als bereits bewährtes Werkzeug zur Bestimmung des mittleren Durchmessers der CNTs und zum Nachweis vorhandener Defekte eingesetzt. Dabei zeigte sich, dass eine maximale Leitfähigkeit des CNT-Netzwerkes bei einer Defekt-Konzentration von etwa 2% erreicht wird, was dem Dotierungsgrad entspricht und dementsprechend auch durch p-Dotierung erklärt werden kann. Weiterhin konnte demonstriert werden, dass die Transparenz der CNT-Netzwerke ein sehr geeignetes Mittel zur Beschreibung der Dichte darstellt und sich durch ein modiflziertes Beer'sche Gesetzes auch quantitativ ausdrücken lässt. Diese erstmalig aufgezeigte Möglichkeit machte auch die Kombination mit der Perkolationstheorie möglich, was zu einer neuen Beziehung führte, mit der sich die Abhängigkeit der Leitfähigkeit von der Transparenz durch einfach zu messende Parameter sehr gut beschreiben lässt. Die Überprüfung verschiedener vorgeschlagener Modelle auf den temperaturabhängigen elektrischen Transport ergab, dass diese Eigenschaften am besten über fluktuierende Ladungsschwankungen an den Tunnelkontakten zwischen 1-dimensionalen metallisch leitenden Bereichen (1D-FAT.Modell) beschreiben lässt. Die Schwächen aller untersuchten Modelle liegt darin, dass die halbleitenden CNTs nicht berücksichtigt werden. Bei die Anwendungen stellen einfache dünne CNT-Netzwerke bereits transparente, flexible und leitfähige Schichten dar, die in Konkurrenz mit herkömmlichem Zinn-dotierten Indiumoxid (ITO) stehen könnten. Zwar konnte die absolute Leitfähigkeit des ITO bislang nicht erreicht werden, allerdings reicht der erzielte Oberflächenwiderstand (1kOhm bei 90% Transparenz) bereits für viele Anwendungen aus, sodass zumindest teilweise auf das teure ITO verzichtet werden könnte. Erstmalig konnten auch transparente und flexible Transistoren auf Basis dünner CNT-Netzwerke realisiert werden. Das gelang durch Stapeln von CNT-Netzwerken unterschiedlicher Dichte, die sich auch elektronisch unterschiedlich verhalten (halbleitend bzw. metallisch leitend) und welche durch eine dünne isolierende Schicht getrennt sind. Die Mobilität des dadurch enstehenden transparenten und flexiblen Transistors ist deutlich höher als die von organischen Dünnschicht-Transistoren. Anwendungstechnisch erscheint das vor allem für das wachsende Gebiet der sog. Plastik-Elektronik interessant zu sein. Strukturierte CNT-Netzwerke weisen aufgrund ihrer geringen Dimensionen aber auch besondere Diffusionsverhältnisse in elektrochemischen Anwendungen auf (Ultramikroeletroden). Das konnte erstmalig an freistehenden Bündeln aus CNTs demonstriert werden können, die sich durch isolierende Beschichtungen prinzipiell noch weiter strukturieren lassen. Die hohe Transparenz dünner CNT-Netzwerke kann aber auch dazu genutzt werden, auf den CNTs aufgebrachte Materialien optisch zu charakterisieren. Dazu wurde im Rahmen dieser Arbeit erstmalig die elektrochemische Abscheidung eines Polymers (Polyanilin) auf einem dünnen CNT-Netzwerk demonstriert. Die anschließenden pH-abhängigen UV/VIS-Spektren stimmten sehr gut mit bereits bekannten Spektren des Polyanilins überein. Anderseits diente das CNT-Netzwerk auch zur elektrischen Kontaktierung des Polymers, sodass die Sensoreigenschaften des Polymers ausgenutzt werden konnten (pH-Sensor). Es zeigte sich, dass der potentiometrische pH-Response dieses CNT/Polyanilin-Sensor mit dem eines herkömmlichen pH-Meters aus brüchigem Glas durchaus vergleichbar ist. Die statistische Verteilung von Metallclustern erscheint für die Katalyse im allgemeinen und für Niedertemperatur-Brennstoffzellen im besonderen interessant. Bisher konnten CNTs aber lediglich als mengenmäßig geringes Additiv auf der Oberfläche kommerzieller Elektroden aus amorphem Kohlenstoff (etwa als Trägermaterial für den Katalysator) eingesetzt werden. Die morphologischen Eigenschaften der CNTs erlauben aber die Konstruktion einer kompletten neuartigen Gasdiffusionselektrode, die ausschließlich aus CNTs besteht, was im Rahmen dieser Arbeit erstmalig demonstriert werden konnte. Das stellt insbesondere für mobile Applikationen, in denen Gewicht und Volumen eine größere Bedeutung zukommen, eine interessante Alternative dar.
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.
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.
Open Access
First-principles calculations of LaMnO3 surface reactivity
(2008) Mastrikov, Yuri; Maier, Joachim (Prof. Dr.)
The main aim of this Thesis is to model elementary processes at the Solid Oxide Fuel Cell (SOFC) cathode on the atomic level. As a model cathode material we use LaMnO3. For this purpose we have chosen the Generalized Gradient Approximation (GGA) method within the Density Functional Theory (DFT) as implemented into the VASP computer code.. One of the main reasons to choose this code is its powerful structure optimisation algorithm. As it is demonstrated in the Thesis, despite certain flaws of the DFT method, it gives very reasonable structural and energetic parameters for such strongly correlated materials as LaMnO3. We calculated the atomic and electronic structure of the perfect LaMnO3 and of its bare surfaces; we modelled adsorbed oxygen on the MnO2-terminated surface, as well as O vacancies in the bulk and on the (001) surface. Along with static properties, we also calculated adsorbed O and O vacancy migration energies, LaMnO3 cohesive energy and surface formation energies. The electronic density distribution was analysed by means of the electron difference maps and the effective atomic charges calculated by means of the topological (Bader) analysis. Special attention was paid to the energetics and charge redistribution upon adsorption of O atoms and O2 molecules on the LaMnO3 (001) surface.
Open Access
First-principles thermodynamic study of oxygen vacancies in ABO3-type perovskites
(2017) Arrigoni, Marco; Maier, Joachim (Prof. Dr.)
Open Access
Grain boundary characterization of electroceramics : acceptor-doped BaZrO3, an intermediate temperature proton conductor
(2011) Shirpour, Mona; Maier, Joachim (Prof. Dr.)
Acceptor doped BaZrO3 exhibit a high bulk proton conductivity of 2.10-2 S/cm to 5.10-5 S/cm in the temperature range of 500°C to 100°C. Nevertheless, highly resistive grain boundaries and notoriously small grain sizes seriously hamper the application of this material as an electrolyte in intermediate-temperature solid oxide fuel cells. Up to now, none of the usual methods (such as powder synthesis via chemical methods, reactive sintering, and sintering aids) could considerably improve the specific GB conductivity of this material. Investigation on sintering properties showed that acceptor-doped BaZrO3 exhibits a limited grain growth resulting in grain sizes 10 times smaller than undoped BaZrO3. Larger grains, by decreasing the GB number density, can improve the total conductivity of BaZrO3. Nevertheless, due to the limited range for the expected grain size increase, a modification of specific GB conductivity seems much more effective. This work is focused on characterization of grain boundaries to get a better understanding of the blocking effect in this perovskite structure proton conductor. It was attempted to check the most frequently reported origins for the blocking character of GBs in ion conductors: (i) Presence of continuous secondary/amorphous phase at GB (ii) Distorted crystallographic structure of grain boundary region which can result in a low water solubility and/or lower proton mobility in GB region (iii) Inhomogeneous distribution of dopant cations in GB region (iv) Depletion of protonic charge carriers in space charge layers which form to compensate an excess charge in the GB core Based on TEM images showing clean GB, secondary phases at the GB could be ruled out as origin for the blocking character. The similar change of bulk and GB conductivity upon switching from dry to wet atmosphere indicated that hydration behaviors and proton mobility in the GB region do not significantly differ from bulk. Due to high vapor pressure of barium oxide, the conventionally sintered samples typically suffer a certain barium loss due to the long-term high-temperature sintering step. The effect of barium loss is more pronounced for the GB conductivity than for the bulk. Although some extra barium can be added in the starting composition to compensate the loss, composition control is not easy using the conventional sintering method. This situation could be improved by “Spark Plasma Sintering” which allows one to minimize Ba deficiency due to much shorter sintering times. The sintered ceramic achieves a higher density and offers the possibility of applying an additional post-heat treatment. EDXS-TEM studies on GBs with different heat treatments (resulting in strongly different conductivities) showed that Y as well as Sc dopants segregate to the GB region, and that GBs with a higher amount of segregation exhibit a lower resistivity. Y+3 and Sc+3 cations were chosen due to their different ionic radius; the large Y+3 cation exhibits a large size mismatch in Zr+4 site in contrast to Sc+3 with a negligible size mismatch. The comparison revealed that for those two dopants segregation is mainly driven by electrostatic forces due to a positively charged core, which is responsible for depletion of protons, holes and oxygen vacancies in the adjacent space charge zones. The segregated dopants increase the GB conductivity partly by directly compensating the positive charge in the core, and partly by accumulation in the space charge zone decreasing the thickness of the depletion layer. Further evidence for a positive core charge comes from measurements on strongly reduced n-conducting Y-doped BaZrO3, where the blocking GB character disappears. This finding is a clear evidence for a positively charged core and consequently related depletion/accumulation layers in adjacent space charge zones. The expected non-linear current-voltage character of the proposed space charge zones was confirmed studying the electrical behavior of GBs under DC-bias. For this purpose, a large grain sample, allowing to apply a reasonable DC bias over each GB, was prepared in an Infrared Image Furnace. The observed voltage-dependent GB resistances and capacitances are consistent with a Schottky-type barrier at the interface. Two models, with and without interface states, are discussed to explain voltage dependence of the GB capacitance. Based on these findings, as an alternative core charge compensation, the Ba-site was doped with Cs+ cations. has a high tendency to segregate to the GB region due to its size mismatch (rCs+ = 1.88 Å, rBa+2 = 1.61 Å) and electrostatic interaction with positive core. The conductivity measurements showed that only 1 at.% of Cs can increase GB conductivity of Y-doped BaZrO3 by more than 2-3 orders of magnitude. In summary, from the systematic structural, chemical and electrical characterization a positive GB core charge resulting in space charge depletion of protons could be identified as the major causes for the blocking effect of GBs in acceptor-doped BaZrO3. This investigation has been successfully used to increase conductivity of GB by adjusting heat treatment or by applying adequate GB decoration. These results may be extended to other electroceramic materials for which GB properties are important.