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Yttria-stabilized zirconia (YSZ) has become an indispensable solid electrolyte material in modern solid oxide fuel and electrolysis cells (SOFCs/SOECs) as well as oxygen sensors. The oxygen ionic conductivity of YSZ is among the highest known so far. For energy efficiency optimization of SOFCs and lowering the high-temperature degradation of electrodes, the oxygen ionic conductivity needs to be further enhanced. This would allow for a reduction in application temperature. Despite extensive regular point defect-doping strategies, this key issue remains unsolved. Here, we investigate the role of mechanically induced dislocations (line-defects) on electrical conductivity and oxygen transport in bulk YSZ. An advanced mechanical deformation approach is employed to generate distinctly aligned dislocation-rich and -deficient regions. The in-depth electrical characterization of these regions exhibited highly conducting effects of dislocation-induced strain inside the bulk material. Furthermore, targeted oxygen tracer diffusion experiments prove enriched oxygen incorporation within the dislocation bundles. Therefore, the potential of mechanically induced dislocations is elucidated as a design element to tune the bulk ionic transport in YSZ.

It is shown that the defect chemistry of acceptor-doped SrTiO3is understood in great detail, and that this permits the accurate description, not only of the equilibrium electronic and ionic conductivities, but also of non-equilibrium situations generated by electrical or chemical driving forces, and even of the quenched state. Recent investigations on internal and external boundaries provide astonishingly precise insights into the pertinent structure–property relations.

Fast Li-ion conducting garnets have shown excellent performance as chemically stable solid state Li electrolytes even at room temperature. However, because of phase formation and Li loss during preparation, reliably obtaining high Li-ion conductivities remains challenging. In this work, we show that an additional defect chemical species needs to be considered, namely, oxygen vacancies. We prove the existence of oxygen vacancies in all six investigated sample types: Ta-, Al-, and Ga-stabilized cubic Li7La3Zr2O12(LLZO) polycrystals and Ta-stabilized LLZO single crystals. Isotope exchange three-dimensional analysis was used to characterize surface oxygen exchange (k*) and bulk oxygen diffusion (D*) enabled by the oxygen vacancies present in the LLZO variants. Remarkably high k* values of 10–11–10–8cm s–1and D* values of 10–15–10–11cm2s–1were found at 350 °C in air. In a further data analysis, the differences between the compositions are investigated, the concentration of oxygen vacancies is estimated, and the possible effects on the cation defect chemistry and phase formation of LLZO are discussed.

The oxygen incorporation and evolution reaction on mixed conducting electrodes of solid oxide fuel or electrolysis cells involves gas molecules as well as ionic and electronic point defects in the electrode. The defect concentrations depend on the gas phase and can be modified by the overpotential. These interrelationships make a mechanistic analysis of partial pressure-dependent current–voltage experiments challenging. In this contribution it is described how to exploit this complex situation to unravel the kinetic roles of surface adsorbates and electrode point defects. Essential is a counterbalancing of oxygen partial pressure and dc electrode polarization such that the point defect concentrations in the electrode remain constant despite varying the oxygen partial pressure. It is exemplarily shown for La0.6Sr0.4FeO3−δ(LSF) thin film electrodes on yttria-stabilized zirconia how mechanistically relevant reaction orders can be obtained from current–voltage curves, measured in a three-electrode setup. This analysis strongly suggests electron holes as the limiting defect species for the oxygen evolution on LSF and reveals the dependence of the oxygen incorporation rate on the oxygen vacancy concentration. A virtual independence of the reaction rate from the oxygen partial pressure was empirically found for moderate oxygen pressures. This effect, however, arises from a counterbalancing of defect and adsorbate concentration changes.

Revealing whether dislocations accelerate oxygen ion transport is important for providing abilities in tuning the ionic conductivity of ceramic materials. In this study, we report how dislocations affect oxygen ion diffusion in Sr-doped LaMnO3(LSM), a model perovskite oxide that serves in energy conversion technologies. LSM epitaxial thin films with thicknesses ranging from 10 nm to more than 100 nm were prepared by pulsed laser deposition on single-crystal LaAlO3and SrTiO3substrates. The lattice mismatch between the film and substrates induces compressive or tensile in-plane strain in the LSM layers. This lattice strain is partially reduced by dislocations, especially in the LSM films on LaAlO3. Oxygen isotope exchange measured by secondary ion mass spectrometry revealed the existence of at least two very different diffusion coefficients in the LSM films on LaAlO3. The diffusion profiles can be quantitatively explained by the existence of fast oxygen ion diffusion along threading dislocations that is faster by up to 3 orders of magnitude compared to that in LSM bulk.

Any substantial move of energy sources from fossil fuels to renewable resources requires large scale storage of excess energy, for example, via power to fuel processes. In this respect electrochemical reduction of CO2may become very important, since it offers a method of sustainable CO production, which is a crucial prerequisite for synthesis of sustainable fuels. Carbon dioxide reduction in solid oxide electrolysis cells (SOECs) is particularly promising owing to the high operating temperature, which leads to both improved thermodynamics and fast kinetics. Additionally, compared to purely chemical CO formation on oxide catalysts, SOECs have the outstanding advantage that the catalytically active oxygen vacancies are continuously formed at the counter electrode, and move to the working electrode where they reactivate the oxide surface without the need of a preceding chemical (e.g., by H2) or thermal reduction step. In the present work, the surface chemistry of (La,Sr)FeO3−δand (La,Sr)CrO3−δbased perovskite-type electrodes was studied during electrochemical CO2reduction by means of near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) at SOEC operating temperatures. These measurements revealed the formation of a carbonate intermediate, which develops on the oxide surface only upon cathodic polarization (i.e., under sufficiently reducing conditions). The amount of this adsorbate increases with increasing oxygen vacancy concentration of the electrode material, thus suggesting vacant oxygen lattice sites as the predominant adsorption sites for carbon dioxide. The correlation of carbonate coverage and cathodic polarization indicates that an electron transfer is required to form the carbonate and thus to activate CO2on the oxide surface. The results also suggest that acceptor doped oxides with high electron concentration and high oxygen vacancy concentration may be particularly suited for CO2reduction. In contrast to water splitting, the CO2electrolysis reaction was not significantly affected by metallic particles, which were exsolved from the perovskite electrodes upon cathodic polarization. Carbon formation on the electrode surface was only observed under very strong cathodic conditions, and the carbon could be easily removed by retracting the applied voltage without damaging the electrode, which is particularly promising from an application point of view.

Improvement of solid oxide fuel cells strongly relies on the development of cathode materials with high catalytic activity for the oxygen reduction reaction. Excellent activity was found for perovskite-type oxides such as La1−xSrxCoO3−δ(LSC), but performance degradation, probably caused by surface composition changes, hinders exploitation of the full potential of LSC. This study reveals that the potentially very high activity of the LSC surface can be traced back to few very active sites. Already tiny amounts of SrO, for example, 4% of a monolayer, deposited on an LSC surface, lead to severe deactivation. Co, on the other hand, causes (re-)activation, suggesting that active sites are strongly related to Co being present at the surface. These insights could be gained by a novel method to measure changes of the electrochemical performance of thin film electrodes in situ, while modifying their surface: impedance spectroscopy measurements during deposition of well-defined fractions of monolayers of Sr-, Co- and La-oxides by single laser pulses in a pulsed laser deposition chamber.

Perovskite-type La0.57Li0.29TiO3(LLTO) is a promising solid electrolyte material with high Li-ion conductivity. However, its experimental electrochemical stability window is not precisely known, and thus the compatibility with potential electrode materials is partly unclear. In this contribution, we present results from electrochemical and analytical experiments to elucidate the stability of LLTO when being polarized with Li-ion-blocking Pt electrodes. Above 2.5 V, a darkened color front starts moving from the cathode to the anode, leading to electrolyte degradation. While first-principles calculations predict the appearance of new phases as decomposition products, we find zones with modified defect chemical properties originating from the anode and cathode. The darkened zone forming at the cathode contains Ti3+polarons with high mobility, which leads to a mixed ion-electron conductivity, already for a very small Li excess concentration. Next to the anode a spatially very confined, weakly conductive Li depletion zone forms. The spatially confined but substantial Li depletion near the anode could be quantified by analytical laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). In contrast to first-principles calculations, no new phases were found near the anode, according to synchrotron-based grazing incidence XRD measurements.

Lithium lanthanum titanate Li0.29+δLa0.57TiO3(LLTO) is a promising material in Li ion battery application, due to its ambient stability and high ionic conductivity. When it is subjected to a high Li chemical potential, additional Li ions intercalate into vacant A sites, which is balanced by the reduction of Ti4+ions to Ti3+. At this point, LLTO becomes a mixed ion and electron conductor, which means that it undergoes a transition from an electrolyte to a high rate capable electrode material in the potential range below ca 1.7 V vs Li metal. However, the exact voltage of the transition from electrolyte to the electrode, as well as the electronic conductivity of reduced LLTO were still unknown. Here, we investigate the thermodynamics of lithium insertion as well as ion and electron conductivity of reduced LLTO by employing a galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS). We can show that LLTO gradually changes from electrolyte material to a mixed conductor, with an ion transference number that depends on the Li chemical potential. Lastly, we present a defect chemical model that fits excellently to the U(δ) curves and the conductivity data.

The changes of the surface morphology and the surface chemistry of LSC thin films grown on different substrates were tracked for 100 hours under SOFC operation conditions. Atomic force microscopy was used to monitor the formation of particles at the LSC surface. Depending on the thin film structure (polycrystalline vs. epitaxial), different particle formation dynamics were observed. Electron microscopy was employed to investigate the chemistry of the segregated particles and revealed that the particles were Sr- and S-rich. Secondary ion mass spectrometry and X-ray photoelectron spectroscopy measurements were performed on degraded LSC thin films, which also found significant amounts of sulfur on the LSC surface, despite no deliberate addition of sulfur compounds, as well as A-site cation enrichment. Impedance spectroscopy was used to track the polarization resistance of LSC grown on YSZ over the same degradation period and a strong increase inthe polarization resistance and in its activation energy was revealed (1.09 to 1.73 eV). The experimental results indicate that sulfur adsorption on LSC surfaces is omnipresent in the investigated conditions and even trace amounts of sulfur compounds present in nominally pure measurement gases account for particle formation and multiple degradation effects under operating conditions.

The oxygen exchange activity of mixed conducting oxide surfaces has been widely investigated, but a detailed understanding of the corresponding reaction mechanisms and the rate-limiting steps is largely still missing. Combined in situ investigation of electrochemically polarized model electrode surfaces under realistic temperature and pressure conditions by near-ambient pressure (NAP) XPS and impedance spectroscopy enables very surface-sensitive chemical analysis and may detect species that are involved in the rate-limiting step. In the present study, acceptor-doped perovskite-type La0.6Sr0.4CoO3-δ(LSC), La0.6Sr0.4FeO3-δ(LSF), and SrTi0.7Fe0.3O3-δ(STF) thin film model electrodes were investigated under well-defined electrochemical polarization as cathodes in oxidizing (O2) and as anodes in reducing (H2/H2O) atmospheres. In oxidizing atmosphere all materials exhibit additional surface species of strontium and oxygen. The polaron-type electronic conduction mechanism of LSF and STF and the metal-like mechanism of LSC are reflected by distinct differences in the valence band spectra. Switching between oxidizing and reducing atmosphere as well as electrochemical polarization cause reversible shifts in the measured binding energy. This can be correlated to a Fermi level shift due to variations in the chemical potential of oxygen. Changes of oxidation states were detected on Fe, which appears as FeIIIin oxidizing atmosphere and as mixed FeII/IIIin H2/H2O. Cathodic polarization in reducing atmosphere leads to the reversible formation of a catalytically active Fe0phase.

The ionic conductivity of a halide perovskite can be boosted by light. This may be detrimental for perovskite solar cells, but also holds promise for solid-state photoelectrochemical devices.

Comparative (electro)catalytic, structural, and spectroscopic studies in hydrogen electro-oxidation, the (inverse) water-gas shift reaction, and methane conversion on two representative mixed ionic–electronic conducting perovskite-type materials La0.6Sr0.4FeO3−δ(LSF) and SrTi0.7Fe0.3O3−δ(STF) were performed with the aim of eventually correlating (electro)catalytic activity and associated structural changes and to highlight intrinsic reactivity characteristics as a function of the reduction state. Starting from a strongly prereduced (vacancy-rich) initial state, only (inverse) water-gas shift activity has been observed on both materials beyond ca. 450 °C but no catalytic methane reforming or methane decomposition reactivity up to 600 °C. In contrast, when starting from the fully oxidized state, total methane oxidation to CO2was observed on both materials. The catalytic performance of both perovskite-type oxides is thus strongly dependent on the degree/depth of reduction, on the associated reactivity of the remaining lattice oxygen, and on the reduction-induced oxygen vacancies. The latter are clearly more reactive toward water on LSF, and this higher reactivity is linked to the superior electrocatalytic performance of LSF in hydrogen oxidation. Combined electron microscopy, X-ray diffraction, and Raman measurements in turn also revealed altered surface and bulk structures and reactivities.

Even though solid oxide fuel/electrolysis cells (SOFC/SOEC) are already commercially available, the effect of electrochemical polarization on the electrochemical properties and overpotentials of individual electrodes is largely unexplored. This is partly due to difficulties in separating anode and cathode impedance features and overpotentials of operating fuel cells. For this, we present a novel three-electrode geometry to measure single-electrode impedance spectra and overpotentials in solid oxide cells. With this new design, we characterise polarised porous La0.6Sr0.4FeO3−δ(LSF) electrodes by simultaneous impedance spectroscopy and ambient pressure XPS measurements. With physically justified equivalent circuit models, we can show how the overpotential-dependent changes in the impedance and XPS spectra are related to oxygen vacancy and electronic point defect concentrations, which deterimine the electrochemical properties. The results are overall in very good agreement with the key findings of several previous studies on the bulk defect chemistry and surface chemistry of LSF. They show for example the exsolution of Fe0particles during cathodic polarisation in H2+ H2O atmosphere that decrease the polarization resistance by roughly one order of magnitude.

Abstract

The influence of lattice strain on the oxygen exchange kinetics and diffusion in oxides was investigated on (100) epitaxial La1–xSrxCoO3−δ(LSC) thin films grown by pulsed laser deposition. Planar tensile and compressively strained LSC films were obtained on single-crystalline SrTiO3and LaAlO3. 18O isotope exchange depth profiling with ToF-SIMS was employed to simultaneously measure the tracer surface exchange coefficient k* and the tracer diffusion coefficient D* in the temperature range 280–475 °C. In accordance with recent theoretical findings, much faster surface exchange (∼4 times) and diffusion (∼10 times) were observed for the tensile strained films compared to the compressively strained films in the entire temperature range. The same strain effecttensile strain leading to higher k* and D*was found for different LSC compositions (x= 0.2 and x= 0.4) and for surface-etched films. The temperature dependence of k* and D* is discussed with respect to the contributions of strain states, formation enthalpy of oxygen vacancies, and vacancy mobility at different temperatures. Our findings point toward the control of oxygen surface exchange and diffusion kinetics by means of lattice strain in existing mixed conducting oxides for energy conversion applications.

La0.6Sr0.4CoO3-δ(LSC) thin film electrodes of about 80 nm thickness were prepared via a sol-gel route on yttria-stabilized zirconia (YSZ) and gadolinium doped ceria (GDC) solid electrolytes. Impedance measurements on microelectrodes fabricated from these films revealed very low polarization resistances for electrochemical oxygen exchange at intermediate temperatures (500–600 ºC). Prerequisite for the very fast oxygen reduction kinetics was a limitation of the preparation (annealing) temperature to 600 ºC. Already after ca.15 minutes annealing of the sol-gel prepared layer at this temperature, electrodes were electrochemically highly active despite low crystallinity (almost XRD amorphous). LSC electrodes on GDC showed much higher polarization resistances. However, this originated from an additional interfacial resistance rather than from kinetically slow LSC surfaces.

The crystal structure of K2Al2O3F2,prepared at 800 °C by ceramic methods, was determined from conventional laboratory X‐ray powder diffraction data. The compound crystallizes in the monoclinic space group C2/m(No. 12) with a= 11.21675(8), b= 8.16351(6), c= 6.12301(5) Å β= 88.8108(6) ° and Z= 4. Unit cell and space group suggestions were found by using the TOPAS program. Starting positional parameters for potassium and aluminum atoms were obtained from 548 reflections by the methods implemented in the FOX program. Oxygen and fluorine atom positions were determined by structural considerations. The structure was refined using the fundamental parameter approach of TOPAS, which gave the final residuals (%) RBragg= 1.6 and Rwp= 4.6. The crystal structure consists of individual layers of oxygen connected AlO3F tetrahedral groups with the fluorine atoms separating the layers. Empirical energy calculations agree with the crystal structure refinements and suggest no or little oxygen–fluorine exchange. Between and perpendicular to the sheets, the potassium ion can move without steric hindrance. Conductivity measurements from room temperature to 300 °C show decent potassium ion conductivity.

La0.6Sr0.4CoO3–δ(LSC) thin‐film electrodes are prepared on yttria‐stabilized zirconia (YSZ) substrates by pulsed laser deposition at different deposition temperatures. The decrease of the film crystallinity, occurring when the deposition temperature is lowered, is accompanied by a strong increase of the electrochemical oxygen exchange rate of LSC. For more or less X‐ray diffraction (XRD)‐amorphous electrodes deposited between ca. 340 and 510 °C polarization resistances as low as 0.1 Ω cm2can be obtained at 600 °C. Such films also exhibit the best stability of the polarization resistance while electrodes deposited at higher temperatures show a strong and fast degradation of the electrochemical kinetics (thermal deactivation). Possible reasons for this behavior and consequences with respect to the preparation of high‐performance solid oxide fuel cell (SOFC) cathodes are discussed.

A way is suggested for determining the effective three phase boundary width of electrodes on solid electrolytes from the difference between the electrolyte resistance of the primary and the secondary current distribution at a circular microelectrode. It is shown by means of numerical finite element calculations how the potential distribution in the solid electrolyte depends on the frequency and how the corresponding variation of the electrolyte resistance can lead to an additional semicircle in the complex impedance plane. An equivalent circuit is introduced for analyzing such a situation and thus for determining the additional ohmic resistance due to current constriction close to the three phase boundary region of the electrode. This additional resistance is quantitatively related to the effective three phase boundary width and thus can serve as a tool to estimate the size of the electrochemically active region of electrode reactions such as oxygen reduction or hydrogen oxidation on oxide ion conducting solid electrolytes.

Even though the electrochemical oxygen reduction reaction at mixed conducting oxide electrodes is highly important for several applications of solid electrolytes a thorough discussion of the kinetics of electron and ion transfer steps at the corresponding electrode surface is not available yet. A straightforward application of current–voltage I–V relations derived for charge transfer reactions at electrodeelectrolyte interfaces turns out to be inappropriate. In this contribution, a model is presented that relates concentrations of electrochemically active species and electrostatic potential steps at the electrodegas interface of mixed conducting electrodes to the applied overpotential, and thus I–Vformulas for the corresponding electron and ion transfer reactions are obtained. Depending on the specific parameters surprising effects are found such as an additional factor of two in the exponents of the I–Vrelation, irrelevance of the symmetry factor or limiting currents even if charge transfer is rate determining.

In this paper we present a set of simple analytical solutions for the parallel and the serial contributions of space charge zones to the overall conduction in bicrystals and (brick layered) ceramics for a variety of situations. The different situations are characterized by signs of the space charge potential, by different charge numbers of the carriers and by the presence or absence of acceptor or donor dopants. We can give solutions for arbitrary charge numbers as long as we restrict to sufficiently high space charge potentials and can presuppose equilibrium for the mobile carriers. A recent analysis of impedance measurements of nanocrystalline ceria based on a space charge model is briefly mentioned as a specific example, as it reflects the occurrence of highly n-conducting accumulation layers in addition to strongly blocking oxygen vacancy depletion layers.

In this paper we present a set of simple analytical solutions for the parallel and the serial contributions of space charge zones to the overall conduction in bicrystals and brick layered ceramics for a variety of situations. The different situations are characterized by signs of the space charge potential, by different charge numbers of the carriers and by the presence or absence of acceptor or donor dopants. We can give solutions for arbitrary charge numbers as long as we restrict to sufficiently high space charge potentials and can presuppose equilibrium for the mobile carriers. A recent analysis of impedance measurements of nanocrystalline ceria based on a space charge model is briefly mentioned as a specific example, as it reflects the occurrence of highly n-conducting accumulation layers in addition to strongly blocking oxygen vacancy depletion layers.

An impedance technique is presented using microelectrodes to quantitatively investigate bulk and grain boundary properties of polycrystals with highly conductive grain boundaries. The connections between the experimentally available resistance data and the local grain boundary properties are achieved by finite element simulations. Experiments on polycrystalline AgCl are presented which demonstrate the applicability of the method and permit the determination of the ionic conductivity enhancement at the grain boundaries. Under the assumption that an enhanced Ag vacancy concentration in the space charge layer is the origin of the increased conductivity, the space charge potential is determined to be 300 mV.

Lateral inhomogeneities of grain boundaries caused by partially wetting grain-boundary phases or pores induced in the grain boundary give rise to current-constriction resistances close to grain-to-grain contacts. The present work presents a quantitative three-dimensional analysis of the resulting grain-boundary impedance. Numerical finite-element calculations of the impedance spectra of polycrystalline samples characterized by typical imperfect contact patterns are performed and analyzed. The validity of existing models is checked, and an interpolation formula for the grain-boundary resistance is given, using the fraction of contacted area and the number of contact spots per grain as parameters. The magnitude of the effects that can be expected in realistic ceramics is emphasized, as well as the question of the extent to which further conclusions on the grain-boundary mechanism can be drawn from the temperature, partial-pressure, bias, and grain-size dependence of the impedance spectra.

Von besonderer Aktualität in der Kinetik sind Untersuchungen in überkritisch fluider Phase. Die hochauflösende Spektroskopie liefert immer mehr grundlegende Daten für das chemische Verhalten der Moleküle. Membranforschung profitiert von wachsenden Kenntnissen über komplexe (teilgeordnete) Flüssigkeiten. Verschiedene Anwendungen, wie Brennstoffzellen, forcieren die Forschung zum Massen‐ und Ladungstransport in Festkörpern.

The polarization resistance of La0.6Sr0.4FeO3-?(LSF) thin film electrodes was investigated by electrochemical impedance spectroscopy. Different states were considered, ranging from pristine films right after deposition to films degraded under various conditions. Impedance measurements performed inside the pulsed laser deposition chamber (in-situ) revealed the existence of a very facile, low resistive state of as-deposited thin films with up to two orders of magnitude lower resistances compared to conventional ex-situ measurements and literature data. Ex-situ and in-situ measurements show different dependencies of polarization resistances, suggesting a different oxygen exchange reaction mechanism on the as-deposited films. The time dependence of the polarization resistance indicates that two degradation mechanisms with different timescales are responsible for the much higher resistances found in ex-situ measurements. Degradation mechanisms are discussed and conclusions with respect to the rate determining step of oxygen exchange on different surfaces are drawn.

Electrochemical reactions at solid|gas interfaces of mixed ionic electronic conductors (MIEC), such as oxygen reduction or evolution, differ substantially from usual electrochemical reactions in aqueous solutions. Overpotentials do not directly translate to electrostatic surface potentials but act mainly by changing the concentration of point defects in the MIEC. This has severe consequences for the mechanistic interpretation of current voltage curves of MIEC electrodes. In this contribution it is shown how overpotential dependent defect concentrations affect the current-voltage curves of oxygen reduction and oxygen evolution at MIEC surfaces. Exemplarily, quantitative current-voltage curves are deduced from the known defect chemical data set (Brouwer diagram) of La0.6Sr0.4FeO3 − δ(LSF). Various curve shapes result, from Tafel-like exponential relations to essentially voltage independent limiting currents. Tafel slopes have a very different meaning compared to charge transfer limited reactions at metal electrode interfaces. It is shown how mechanistic information can be obtained from the difference of anodic and cathodic Tafel slopes or by comparing exchange current densities and ac resistances. Moreover, partial pressure dependences of anodic and cathodic currents are deduced, showing that exponents of power laws often do not indicate whether atomic or molecular oxygen species are involved in the rate limiting step.