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Oxide ion transport in Pb( Zr x Ti1- x) O3 ( PZT) was investigated using 18 O tracer diffusion and time-of-flight secondary ion mass spectrometry analysis. Particularly diffusion in the near-surface regions (a few 100 nm in depth) is discussed and strong evidence for a space charge layer near the surface is provided. In this layer, an enrichment of oxygen vacancies is found at temperatures up to 600°C, resulting in box-shaped tracer diffusion profiles. At 650°C, however, the profile shape is modified and reveals a depletion of oxygen vacancies close to the surface. Accordingly, a change in the polarity of the surface charge takes place between 600°C and 650°C. The tracer diffusion profiles are very sensitive to surface modification, caused by etching or annealing at higher temperature, and also depend on the sintering procedure. Moreover, application of an external field during the tracer experiments affected the tracer diffusion at the cathode, while leaving the profile at the anode unperturbed. Quantitative analysis of the profiles was performed by means of numerical calculations based on Gouy-Chapman or Mott-Schottky space charge models. Estimates of the oxygen vacancy concentration and ionic transference number of donor-doped PZT are provided. [ABSTRACT FROM AUTHOR]

Pulsed laser deposited La0.6Sr0.4CoO3-δ (LSC) thin film electrodes on yttria stabilized zirconia (YSZ) single crystals were investigated by impedance spectroscopy, time of flight secondary ion mass spectrometry (ToF-SIMS) and inductively coupled plasma optical emission spectrometry (ICP-OES). Effects caused by different film deposition temperatures, thermal annealing and chemical etching were studied. Correlations between changes in electrode polarization resistance of oxygen reduction and surface composition were found. At high deposition temperatures and after thermal annealing an inhomogeneous cation distribution was detected in the surface-near region, most manifest in a significant Sr enrichment at the surface. An activating effect of chemical etching of LSC is described, which can lower the polarization resistance by orders of magnitude. Chemistry behind this activation and thermal degradation was analyzed by ToF-SIMS and ICP-OES measurements of in-situ etched LSC films. The latter allow quantitative depth resolved compositional analysis with nominally sub nm resolution. High resolution scanning electron microscopy images illustrate the accompanying changes in surface morphology. All measurements suggest that stoichiometric LSC surfaces intrinsically exhibit very high activity towards oxygen reduction. [ABSTRACT FROM AUTHOR]

In this study the electrochemically active region of oxygen incorporation into yttria stabilized zirconia (YSZ) was visualized by means of 18O tracer incorporation experiments on dense Pt thin film microelectrodes combined with ToF-SIMS analysis. The localization and the shape of the incorporation zone were found to strongly depend on the polarization of the electrode. In case of lower overpotentials the active zone next to the three phase boundary (TPB) was frame-shaped and located beneath the Pt electrode. Increases in polarization led to an extension of the incorporation zone along the free YSZ surface. Owing to the low temperature of 300–330 °C a profile-broadening caused by diffusion in YSZ could be minimized and quantitatively separated from the measured profiles. The TPB-width (i.e.the decay length of electrochemical activity) was determined to be approximately 1.0–1.3 μm at these temperatures. [ABSTRACT FROM AUTHOR]

Abstract: The electrochemical oxygen exchange reaction on yttria stabilized zirconia (YSZ) was investigated by means of Pt microelectrodes. The circular-shaped electrodes were prepared by sputter deposition of platinum onto a heated YSZ substrate and photolithographic micro-structuring of the Pt film. Electrochemical characterization was carried out by impedance spectroscopy at temperatures ranging from 700 to 900°C. Analysis of the results obtained on differently sized microelectrodes showed that the main part of the polarization resistance of O2 reduction scales with the inverse length of the three phase boundary (TPB). Consequently, Pt(O2)/YSZ represents a so-called surface path system with the TPB-zone as the active region for oxygen incorporation and the rate-limiting step being located close to this TPB. At 700°C a TPB-related reciprocal electrode resistance (electrode “conductivity”) of 6.6±0.9 10−7 Ω−1 cm−1 with an activation energy of 1.36±0.11eV was determined. An additional, much smaller contribution to the polarization resistance scales with the inverse electrode area and is interpreted in terms of a limited oxygen reservoir at the Pt/YSZ interface. Several implications of temperature gradients resulting in microelectrode measurements (e.g. thermovoltage) are discussed. [Copyright &y& Elsevier]

The influence of SrO–WO3 additive on the piezoelectric properties and electric conductivity of lead zirconate–titanate (PZT) near the morphotropic phase boundary was investigated. Starting from the composition 0.995 Pb(Zr0.525Ti0.475)O3 + 0.015 SrO + 0.005 WO3 (corresponding to 0.5 mol% of Sr3WO6) the fraction of WO3 was increased until Sr:W = 1:1. These samples were compared to a set of samples of PZT with WO3 addition but without SrO. Sr3WO6 has pseudocubic K2NaAlF6 structure with part of the strontium being octahedrally coordinated. In a solid solution with PZT this opens the possibility of realizing Sr on a B-site adjacent to a B-site occupied by W. Piezoelectric properties were measured on sintered disc samples under high field conditions. Impedance spectroscopy was used to investigate the conductivity and dielectric properties of the ceramics at various temperatures. The combined addition of SrO and WO3 increases the piezoelectric strain compared to samples of PZT with WO3 addition only. The results in polarization and conductivity indicate that the donor effect of WO3 is counteracted by the addition of SrO. [ABSTRACT FROM AUTHOR]

In the context of the search for fundamental size effects regarding electrical conduction, we analyze the transition from semi-infinite or isolated to overlapped depletion layers in appreciably dense nanocrystalline SrTiO3 ceramics occurring if the grain size becomes smaller than ∼ 100 nm. At these small grain sizes, the bulk impedance signal that is seen clearly for microcrystalline samples disappears, leaving only one impedance response, which can be attributed to the space-charge zones. Space-charge potentials and Mott—Schottky lengths are calculated and shown to be consistent with the assumption of the mesoscopic condition. In accordance with the increased homogeneity of the mesoscopic situation in an 80 nm sample, and unlike for microcrystalline samples, the impedance response is characterized by almost an ideal capacitance that no longer originates from space-charge polarization. These conclusions are corroborated by detailed numerical calculations using linear irreversible thermodynamics. [ABSTRACT FROM AUTHOR]

Abstract: Geometrically well-defined, dense thin film microelectrodes of the mixed conducting solid oxide fuel cell cathode material La0.6Sr0.4Co0.8Fe0.2O3−δ have been prepared by pulsed laser deposition and standard photolithographic techniques on yttria-stabilised zirconia substrates. The electrochemical properties of these model electrodes were investigated by impedance spectroscopy as a function of temperature and dc bias. It is shown that an equivalent circuit derived rigorously in Ref. [J. Jamnik and J. Maier, Phys. Chem. Chem. Phys. 3, 1668–1678 (2001).] for a cell with a mixed conducting electrode provides an appropriate description for this experimental system, enabling a correct interpretation of the measured impedance data. Under zero or small dc bias, the electrochemical resistance is dominated by the oxygen exchange reaction at the surface of the electrode, with minor contributions from the electrode/electrolyte interface and the ohmic resistance of the electrolyte. The main capacitive process is associated with oxygen stoichiometry changes in the bulk of the electrode (chemical capacitance), while an additional electrode/electrolyte interfacial capacitance is also present. The temperature and dc bias dependencies of these processes are discussed in terms of defect chemistry. [Copyright &y& Elsevier]

Conductivity profiles of nitrogen-graded zirconia from nitridation of 2 mol% yttria-doped tetragonal zirconia polycrystals (2Y-TZP) were determined using microcontact impedance spectroscopy. Local conductivity variations were interpreted in terms of nitrogen concentration and the morphological distribution of the tetragonal precipitates in the two-phase region of nitrogen partially stabilized zirconia (N-PSZ). Nitrogen incorporation in the stabilized zirconia was found to strongly decrease the local conductivity below 400°C, even though the oxygen vacancy concentration increased nominally. This behavior is ascribed to the strong vacancy–vacancy interaction leading to vacancy ordering. The strong interaction is indicated by the formation of ordered-vacancy phases in nitrogen-doped zirconia. Percolation of elongated tetragonal precipitates of lesser nitrogen concentration and thus of higher conductivity explains the conductivity anisotropy observed by microcontact measurements as well as the large effective dielectric constants. [ABSTRACT FROM AUTHOR]

For large-grained manganese zinc (MnZn) ferrite ceramics, grain misorientation determined by electron backscatter diffractions and grain-boundary resistance measured using microcontact impedance spectroscopy have been correlated. The degree of oxidation of grain boundaries and, hence, the barrier height depends on the overall grain-boundary network as well as on the individual boundary structure; therefore, a statistical analysis has been performed based on several hundreds of local measurements. When the boundaries are divided into low- and high- resistance groups, statistically significant differences in rotation axis and angle distributions are found. The misorientation distribution of the low-resistance boundary group is suggested to reflect the low-energy configurations of boundary planes in MnZn ferrites. [ABSTRACT FROM AUTHOR]

Microcontact impedance spectroscopy in conjunction with conventional impedance techniques has been performed to elucidate microstructural and compositional inhomogeneities in polycrystalline Mn-Zn ferrites. Microcontact studies reveal the occurrence of oxygen stoichiometry gradients in these typically prepared ferrite ceramics together with a strong spatial resistance variation from boundary to boundary. This leads to a reinterpretation of the conventional impedance spectra of ferrites. [ABSTRACT FROM AUTHOR]

The atomistic structure and electrical properties of a symmetrical 5.4° [001] tilt grain boundary in Fe-doped SrTiO[sub 3] have been investigated, respectively, by means of various transmission electron microscopy (TEM) techniques and impedance spectroscopy. In weak-beam dark-field images, the grain boundary is revealed to consist of a periodic array of dislocations; high-resolution TEM images show that the dislocation cores are separated by regions of strained lattice. The impedance response of the bicrystal has been measured in the frequency range 20 Hz ≤ f ≤ 10[sup 6] Hz as a function of temperature and oxygen partial pressure. The transport of charge across the array of dislocations that form the grain boundary is strongly hindered. Analysis of the impedance data in terms of a double-Schottky-barrier model yields a spacecharge potential that exhibits a weak dependence on temperature and oxygen partial pressure and is ∼0.55 V in the investigated regime. [ABSTRACT FROM AUTHOR]

Hebb–Wagner polarization technique was applied to Fe-doped SrTiO3 single and bicrystals: as far as a single crystal is concerned, the polarization is complicated by additional internal redox reactions, and it has to be appropriately generalized; as far as a bicrystal is concerned, it was demonstrated how the electronic and ionic partial conductivities of a grain boundary can be separated with this technique. The partial conductivities and the charge carrier concentrations thus obtained clearly demonstrate that the electronic and the ionic charge carriers are depleted in the space charge region, more specifically that the depletion of the ionic ones is more pronounced, being in agreement with the double charge of oxygen vacancies. [Copyright &y& Elsevier]

Mixed conducting oxides are frequently studied as electrode materials for solid oxide fuel or electrolysis cells. However, in principle their ability to change stoichiometry depending on the oxygen chemical potential also enables charge storage typical for battery electrodes: Formally neutral oxygen can be incorporated by annihilating oxygen vacancies and creating electron holes, similar to the incorporation of lithium in lithium ion battery electrodes. Electrodes with different reducibility should thus allow operation of an oxide ion battery. This is exemplified for perovskite‐type La0.6Sr0.4FeO3−δ (LSF), La0.5Sr0.5Cr0.2Mn0.8O3−δ (LSCrMn), and La0.9Sr0.1CrO3−δ (LSCr) electrodes with blocked oxygen surface exchange. Cells with thin film electrodes are prepared by pulsed laser deposition on yttria stabilized zirconia electrolytes. Charge/discharge cycles and impedance measurements are performed on half cells between 350 and 500 °C and related to the materials defect chemistry. Electrode capacities of up to 350 mA h cm−3 and <0.25 % capacity loss over 19 cycles are measured at potentials between 0.2 and −1.1 V versus O2 at 1 bar. Full cells with LSF cathodes and LSCrMn anodes are successfully operated at 350 to 400 °C with electrode related energy densities up to 70 mW h cm−3, coulomb efficiencies >99 % and good cycling performance. This novel type of rechargeable battery may open further application fields such as elevated temperatures (>200 °C), where common cation‐based cells are not applicable so far. [ABSTRACT FROM AUTHOR]

The original article can be found online at https://doi.org/10.1038/s41467-020-18563-w. Correction to: I Nature Communications i , https://doi.org/10.1038/s41467-020-18563-w, published online 23 September 2020. [Extracted from the article]

Exsolution of metal nanoparticles from perovskite-type oxides is a very promising approach to obtain catalysts with superior properties. One particularly interesting property of exsolution catalysts is the possibility of electrochemical switching between different activity states. In this work, synchrotron-based in-situ X-ray diffraction experiments on electrochemically polarized La0.6Sr0.4FeO3-δ thin film electrodes are performed, in order to simultaneously obtain insights into the phase composition and the catalytic activity of the electrode surface. This shows that reversible electrochemical switching between a high and low activity state is accompanied by a phase change of exsolved particles between metallic α--Fe and Fe-oxides. Reintegration of iron into the perovskite lattice is thus not required for obtaining a switchable catalyst, making this process especially interesting for intermediate temperature applications. These measurements also reveal how metallic particles on La0.6Sr0.4FeO3-δ electrodes affect the H2 oxidation and H2O splitting mechanism and why the particle size plays a minor role. Exsolution, nanoparticle precipitation from oxide materials during reduction, offers a promising approach to heterogeneous catalysis. Here, authors examine the switching behavior between high and low activity and correlate the electro-catalytic activity to exsolved phases from complex oxides. [ABSTRACT FROM AUTHOR]

Detailed insight into electrochemical reaction mechanisms and rate limiting steps is crucial for targeted optimization of solid oxide fuel cell (SOFC) electrodes, especially for new materials and processing techniques, such as Ni/Gd-doped ceria (GDC) cermet anodes in metal-supported cells. Here, we present a comprehensive model that describes the impedance of porous cermet electrodes according to a transmission line circuit. We exemplify the validity of the model on electrolyte-supported symmetrical model cells with two equal Ni/Ce0.9Gd0.1O1.95-δ anodes. These anodes exhibit a remarkably low polarization resistance of less than 0.1 Ωcm2 at 750 °C and OCV, and metal-supported cells with equally prepared anodes achieve excellent power density of >2 W/cm2 at 700 °C. With the transmission line impedance model, it is possible to separate and quantify the individual contributions to the polarization resistance, such as oxygen ion transport across the YSZ-GDC interface, ionic conductivity within the porous anode, oxygen exchange at the GDC surface and gas phase diffusion. Furthermore, we show that the fitted parameters consistently scale with variation of electrode geometry, temperature and atmosphere. Since the fitted parameters are representative for materials properties, we can also relate our results to model studies on the ion conductivity, oxygen stoichiometry and surface catalytic properties of Gd-doped ceria and obtain very good quantitative agreement. With this detailed insight into reaction mechanisms, we can explain the excellent performance of the anode as a combination of materials properties of GDC and the unusual microstructure that is a consequence of the reductive sintering procedure, which is required for anodes in metal-supported cells. [ABSTRACT FROM AUTHOR]

The impact of UV irradiation on Fe‐doped SrTiO3 (Fe:STO) single crystals is investigated at elevated temperatures. Illumination leads to incorporation of oxygen into the single crystals and thus to a decreasing oxygen vacancy concentration and oxidation of Fe3+ to Fe4+. The Fe4+ ions cause a color change from transparent/brownish to black. This photochromic blackening due to stoichiometry changes at elevated temperatures is irreversible at room temperature, but annealing at high temperatures, for example at 700 °C, can restore the original stoichiometry and color. Absorbance changes due to UV irradiation are monitored by ex situ and in situ UV–vis spectroscopy experiments and changes in electrical properties are measured by van der Pauw measurements and in‐plane electrochemical impedance spectroscopy. After 1140 min of illumination at 440 °C, for example, electrical measurements reveal a conductivity increase by more than a factor of 5 due to the enhanced hole concentration in blackened Fe:STO. In addition, UV illumination increases the oxygen chemical potential up to a calculated p(O2) of more than 109 Pa in Fe:STO. Hence, UV light can be used to tune the color, but also electrical properties of Fe:STO by directly impacting the bulk defect concentrations. [ABSTRACT FROM AUTHOR]

In this study, a detailed structural characterization of epitaxial La 0.6 Sr 0.4 CoO 3 − δ (LSC) films grown in (100), (110), and (111) orientations was conducted. LSC is a model air electrode material in solid oxide fuel and electrolysis cells and understanding the correlation of bulk structure and catalytic activity is essential for the design of future electrode materials. Thin films were grown on single crystals of the perovskite material La 0.95 Sr 0.05 Ga 0.95 Mg 0.05 O 3 − δ cut in three different directions. This enabled an examination of structural details at the atomic scale for a realistic material combination in solid oxide cells. The investigation involved the application of atomic force microscopy, X-ray diffraction, and high-resolution transmission electron microscopy to explore the distinct properties of these thin films. Interestingly, ordering phenomena in both cationic as well as anionic sublattices were found, despite the fact that the thin films were never at higher temperatures than 600 °C. Cationic ordering was found in spherical precipitates, whereas the ordering of oxygen vacancies led to the partial transition to brownmillerite in all three orientations. Our results indicate a very high oxygen vacancy concentration in all three thin films. Lattice strains in-plane and out-of-plane was measured, and its implications for the structural modifications are discussed. [ABSTRACT FROM AUTHOR]

Photo-active heterojunctions were prepared by growing thin La 0.9 Sr 0.1 CrO 3-δ films on SrTiO 3 (STO) single crystals substrates via pulsed laser deposition. The photo-electrochemical characteristics of these heterojunctions under UV illumination were investigated by DC measurements and electrochemical impedance spectroscopy (EIS). Under UV illumination, the photo-current produced by these junctions increases over time by almost an order of magnitude to 1 to 2 mA cm-2 at 350 °C. Two effects governing this increase can be distinguished: A first process, independent of the surrounding atmosphere, is related to a stoichiometry polarization of the STO single crystal upon current flow. A second process is highly sensitive to contaminants in the surrounding atmosphere and is strongly accelerated in pure, synthetic atmospheres. We suggest that it is related to photo-oxidation of the STO single crystal. EIS revealed that both processes result in a decrease of both the electronic transport resistance through the STO single crystal and, to a lesser degree, the space charge resistance of the photo-active junction. Operando EIS under illumination and with DC bias was used to record the cells' photo-power characteristics and identify individual loss processes. Owing to the illumination induced strong increase of the STO single crystal conductivity, the photo-currents of such enhanced cells are limited by the space charge resistance. • The photo-current of SrTiO 3 |La 0.9 Sr 0.1 CrO 3 hetero-junctions continually increases under UV illumination. • Two mechanisms govern this effect: One is atmosphere independent and one strongly depends on atmosphere. • The atmosphere independent mechanism is caused by stoichiometry polarization in the SrTiO 3 crystal. • The atmosphere dependent mechanisms is related to photo-oxidation of the SrTiO 3 crystal. [ABSTRACT FROM AUTHOR]

Tailoring the electrical conductivity of functional ceramics by introducing dislocations is a comparatively recent research focus, and its merits were demonstrated through mechanical means. Especially bulk deformation at high temperatures is suggested to be a promising method to introduce a high dislocation density. So far, however, controlling dislocation generation and their annihilation remains difficult. Although deforming ceramics generate dislocations on multiple length scales, dislocation annihilation at the same time appears to be the bottleneck to use the full potential of dislocations‐tailoring the electrical conductivity. Here, we demonstrate the control over these aspects using a micromechanical approach on yttria‐stabilized zirconia ‐ YSZ. Targeted indentation well below the dislocation annihilation temperature resulted in extremely dense dislocation networks, visualized by chemical etching and electron channeling contrast imaging. Microcontact‐impedance measurements helped evaluate the electrical response of operating individual slip systems. A significant conductivity enhancement is revealed in dislocation‐rich regions compared to pristine ones in fully stabilized YSZ. This enhancement is mainly attributed to oxygen ionic conductivity. Thus, the possibility of increasing the conductivity is illustrated and provides a prospect to transfer the merits of dislocation‐tuned electrical conductivity to solid oxygen electrolytes. [ABSTRACT FROM AUTHOR]

Diffusion of transition metals across the cathode–electrolyte interface is identified as a key challenge for the practical realization of solid‐state batteries. This is related to the formation of highly resistive interphases impeding the charge transport across the materials. Herein, the hypothesis that formation of interphases is associated with the incorporation of Co into the Li7La3Zr2O12 lattice representing the starting point of a cascade of degradation processes is investigated. It is shown that Co incorporates into the garnet structure preferably four‐fold coordinated as Co2+ or Co3+ depending on oxygen fugacity. The solubility limit of Co is determined to be around 0.16 per formula unit, whereby concentrations beyond this limit causes a cubic‐to‐tetragonal phase transition. Moreover, the temperature‐dependent Co diffusion coefficient is determined, for example, D700 °C = 9.46 × 10−14 cm2 s−1 and an activation energy Ea = 1.65 eV, suggesting that detrimental cross diffusion will take place at any relevant process condition. Additionally, the optimal protective Al2O3 coating thickness for relevant temperatures is studied, which allows to create a process diagram to mitigate any degradation with a minimum compromise on electrochemical performance. This study provides a tool to optimize processing conditions toward developing high energy density solid‐state batteries. [ABSTRACT FROM AUTHOR]

Abstract: Composites of ion and electron conducting phases allow for interfacial storage of a neutral component via space charge effects. The present contribution considers the rate of transport‐controlled incorporation and excorporation in such artificial mixed conductors. The upper limit of the relaxation time is determined by interfacial job‐sharing diffusion which is analyzed in greater detail. In general cases bulk phases assist by migration processes (dual phase transport). For more complex morphological situations, the considerations are complemented by numerical calculations. Thus the treatment also offers a pertinent approach with respect to the kinetics of solid state supercapacitors. Selected experimental results are included in the discussion. [ABSTRACT FROM AUTHOR]

All-oxide model-composite electrodes were established consisting of a thin, micropatterned, electronically conducting oxide, which acts as a current collector, and a thin film of gadolinia-doped ceria, which is an electrochemically highly active mixed conductor under reducing atmospheres. The choice of the current collecting oxides was based on their electronic conductivity assessed by measurements of thin films using the van der Pauw method. Lanthanum and niobium doped strontium titanate as well as alumina doped zinc oxide, were investigated this way in a humid hydrogen atmosphere. Promising materials were incorporated as a current collector into model-composite microelectrodes and tested for their stability and efficiency in electrochemically activating the microelectrode. Alumina doped zinc oxide, while being an excellent electron conductor, showed severe stability problems at temperatures above 600 °C. However, a microelectrode with a current collector of niobium doped strontium titanate (Sr 0·9 Ti 0.8 Nb 0·2 O 3 ) performed comparable to an electrode with a Pt current collector and, additionally, showed an improved tolerance to sulphur poisoning in a humid hydrogen atmosphere with 10 ppm of hydrogen sulphide. [ABSTRACT FROM AUTHOR]