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4D scanning transmission electron microscope (STEM) techniques have been increasingly featured among the electron microscopy characterization approaches, as they provide a perspective of improved information retrieval from samples overall. To make 4D STEM experiments as viable as conventional STEM image acquisition, the recording of diffraction patterns with a pixelated detector at fast frame rates, sufficient sensitivity to capture single electron hits, and high dynamic range is necessary. This paper addresses the recent development in hybrid-pixel detector technology that now allows 4D STEM experiments with a similar setup to conventional STEM imaging with pixel collection time under 10 µs. Application examples on virtual STEM detectors and crystal phase-orientation mapping are presented.

Focused-ion-beam/scanning-electron-microscopy (FIB-SEM) tomography has become increasingly important to investigate the three-dimensional (3D) distribution of different phases in inhomogeneous materials. However, quantitative analysis of 3D structures by FIB-SEM tomography requires adequate image segmentation that is typically based on thresholds in the intensity histograms of SEM images. Recently machine learning (ML) segmentation algorithms have emerged with the opportunity to tune the algorithm based on prior knowledge of SEM image contrast. In this work we have performed FIB-SEM tomography of 3D printed nanoporous polymer structures with a special focus on SEM image segmentation and pore size determination. By applying Monte Carlo (MC) simulations, SEM image contrast of the pore/polymer interface was analysed. The performance of several traditional segmentation algorithms on simulated SEM images was found to lead to erroneous results on pore sizes. Training the ML segmentation algorithm by the knowledge obtained from MC simulations yields more reliable segmentation results. Evaluated pore sizes and analysis of further 3D material properties show a strong dependence on the segmentation method, which emphasize the importance of the segmentation process in the 3D reconstruction of FIB-SEM data.

Understanding cell performance is essential for selecting cell components and the processing parameters for solid oxide fuel cells. The scale of relevant microstructural features in electrodes, electrolyte and supporting substrate covers several orders of magnitude. This contribution will demonstrate how advanced correlative multi-scale tomography can be used to identify those parameters: ranging from millimeter to nanometer scale. We employ optical microscopy, X-ray computed tomography (μ-CT), focused ion beam-scanning electron microscopy tomography and energy-dispersive X-ray spectroscopy–scanning transmission electron microscopy. Additional investigations by selected area electron diffraction allow a determination of the underlying crystal structures. An SOFC design based on the co-sintering of an inert substrate with various functional layers on top is used as a blueprint, allowing further methodological development. The effect of interdiffusion between phases and development of secondary phases on microstructure and chemical composition will be shown. Furthermore, porosity and tortuosity extracted individually from all porous layers will allow modeling of gas diffusion loss contributions within the co-fired cell structure. This exemplifies how correlative tomography helps to understand specific contributions to overall cell performance.

(Ba0.5Sr0.5)(Co0.8Fe0.2)O3−δ (BSCF) with mixed ionic and electronic conductivity is promising as oxygen separation membrane. However, its exceptional oxygen permeability is linked to the cubic perovskite phase, which destabilizes below 825 °C. This work is concerned with the stabilizing effect of partial B-site substitution in BSCF by Ti (10%) and Nb (5% and 10%). In addition, the effect of B-site substoichiometry on the formation of CoO precipitates as nucleation sites for other secondary phases is studied. The focus of this work is laid on the analysis of the phase constitution in bulk material between 600 and 800 °C. Scanning electron microscopy and (scanning) transmission electron microscopy with high spatial resolution in combination with energy-dispersive X-ray spectroscopy are applied which allow to detect even small volume fractions of secondary phases. It is confirmed by quantitative analysis that Ti and Nb substitution reduces the formation of secondary phases up to ~ 95%. There is a solubility limit of Nb in BSCF, which leads to the precipitation of Nb-rich precipitates already at 5% Nb substitution in contrast to Ti which is fully soluble even at 10% substitution. Despite of that, BSCF doped with 5% Nb in combination with 5% substoichiometric B-site occupation is the most promising material, because it shows the lowest overall volume fraction of secondary phases (~ 0.8 vol% at 700 °C) and an almost negligible degradation rate in additionally recorded long-term resistivity measurements.

An SOFC design based on co-sintering of functional layers on an inert substrate is investigated. Understanding the impact of cell fabrication parameters on cell performance as well as the identification of the key aging mechanisms are important issues. This contribution will demonstrate how advanced correlative multiscale tomography (such as X-ray computed tomography (μ-CT), focused ion beam – scanning electron microscopy (FIB-SEM) tomography and energy dispersive X-ray spectroscopy - scanning transmission electron microscopy (EDXS-STEM)) can be used to identify performance critical cell components and gas transport limitations. The resulting microstructure data can be linked to cell performance measured via electrochemical impedance spectroscopy (EIS). Impedance data from full cells are compared to cathode/anode measurements on symmetrical cell geometry to quantify the different contributions. This exemplifies how performance and microstructural data can help to understand specific contributions to overall cell performance and degradation.

Scanning electron microscopy (SEM) is an indispensable characterization technique for materials science. More recently, scanning electron microscopes can be equipped with scanning transmission electron microscopy (STEM) detectors, which considerably extend their capabilities. It is demonstrated in this work that the correlative application of SEM and STEM imaging techniques provides comprehensive sample information on nanomaterials. This is highlighted by the use of a modern scanning electron microscope, which is equipped with in-lens and in-column detectors, a double-tilt holder for electron transparent specimens and a CCD camera for the acquisition of on-axis diffraction patterns. Using multi-walled carbon nanotubes and Pt/Al2O3 powder samples we will show that a complete characterization can be achieved by combining STEM (mass-thickness and diffraction) contrast and SEM (topography and materials) contrast. This is not possible in a standard transmission electron microscope where topography information cannot be routinely obtained. We also exploit the large tilt angle range of the specimen holder to perform 180 degrees STEM tomography on multi-walled carbon nanotubes, which avoids the missing wedge artifacts.

Focused ion beam (FIB) ��� scanning electron microscopy (SEM) serial sectioning tomography has become an important tool for three���dimensional microstructure reconstruction of solid oxide fuel cells (SOFC) to obtain an understanding of fabrication���related effects and SOFC performance. By sequential FIB milling and SEM imaging a stack of cross���section images across all functional SOFC layers was generated covering a large volume of 3.5��10$^{4}$ ��m$^{3}$. One crucial step is image segmentation where regions with different image intensities are assigned to different material phases within the SOFC. To analyze all relevant SOFC materials, it was up to now mandatory to acquire several images by scanning the same region with different imaging parameters because sufficient material contrast could otherwise not be achieved. In this work we obtained high���contast SEM images from a single scan to reconstract all functional SOFC layers consisting of a Ni/Y$_{2}$O$_{3}$���doped ZrO$_{2}$ (YDZ) cermet anode, YDZ electrolyte and (La,Sr)MnO$_{3}$/YDZ cathode. This was possible by using different, simultaneous read���out detectors installed in a state���of���the���art scanning electron microscope. In addition, we used a deterministic approach for the optimization of imaging parameters by employing Monte Carlo simulations rather than trial���and���error tests. We also studied the effect of detection geometry, detecting angle range and detector type.

The outstanding oxygen permeability of the perovskite Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) and its applicability as a cathode in solid oxide fuel cells are remarkable, yet have been hindered by the formation of secondary phases at T < 840 °C and the subsequent degradation. The other main drawback to BSCF is related to the formation of carbonates in the presence of CO2. These degrade its excellent oxygen surface-exchange kinetics. In this work, 10% Y-doped Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF10Y) is electrochemically, microstructurally, and chemically characterized in O2- and CO2-containing atmospheres as porous cathodes in symmetrical cells with Gd-doped ceria as electrolyte. Experiments in oxygen/nitrogen gas mixtures (pO2 = 0.02–0.40 atm) at T = 600–900 °C showed high performance with a cathode specific resistance of 49.9 mΩ cm2 at 600 °C in air (pO2 = 0.21 atm), which is very comparable to 47.8 mΩ cm2 for undoped BSCF, but which deteriorates constantly under the same conditions. Moreover, adding significant amoun...

The effect of B-site cation substitution on the phase stability of Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) in O2- and CO2-containing atmospheres has been studied thoroughly by our group. In particular, a doping concentration of 10 mol-% yttrium (Y) was found to strongly reduce or suppress secondary phase formation extending the stability of the cubic phase regime. Characterization by electrochemical impedance spectroscopy (EIS) of both, pure and Y-doped BSCF electrodes, is performed in oxygen-nitrogen mixtures as well as in CO2-containing atmospheres using symmetrical SOFC cells with Ce0.9Gd0.1O2-δ (CGO) as electrolyte from 600 to 900 °C. Analysis of the EIS data in combination with the distribution of relaxation times (DRT) reveals an enhanced tolerance of the Y-doped BSCF cathodes towards the adsorption of CO2 molecules. The enhanced stability of the cubic phase and reduced poisoning effect on the oxygen reduction reaction in CO2- containing atmospheres by doping, is further confirmed by scanning electron microscopy (SEM) and analytical (scanning) transmission electron microscopy ((S)TEM) combined with energy dispersive X-ray spectroscopy (EDXS).

Here, amorphous silica nanoparticles (NPs), one of the most abundant nanomaterials, are used as an example to illustrate the utmost importance of surface coverage by functional groups which critically determines biocompatibility. Silica NPs are functionalized with increasing amounts of amino groups, and the number of surface exposed groups is quantified and characterized by detailed NMR and fluorescamine binding studies. Subsequent biocompatibility studies in the absence of serum demonstrate that, irrespective of surface modification, both plain and amine-modified silica NPs trigger cell death in RAW 264.7 macrophages. The in vitro results can be confirmed in vivo and are predictive for the inflammatory potential in murine lungs. In the presence of serum proteins, on the other hand, a replacement of only 10% of surface-active silanol groups by amines is sufficient to suppress cytotoxicity, emphasizing the relevance of exposure conditions. Mechanistic investigations identify a key role of lysosomal injury for cytotoxicity only in the presence, but not in the absence, of serum proteins. In conclusion, this work shows the critical need to rigorously characterize the surface coverage of NPs by their constituent functional groups, as well as the impact of serum, to reliably establish quantitative nanostructure activity relationships and develop safe nanomaterials.

In this study, a novel approach of silica-PVA hybrid aerogel synthesis is presented based on the simultaneous sol-gel reaction of the silica precursor tertramethoxysilane (TMOS) and thermally impacted non-solvent induced phase separation of polyvinyl alcohol (PVA). The method based on: (1) water from PVA solution is required for hydrolysis of TMOS; (2) methanol from TMOS solution, is a non-solvent for PVA. In the result of sol-gel reaction, phase separation and drying during extraction with supercritical CO2, the hybrid SiO2-PVA aerogels were obtained. Results indicated that, incorporation of the silica network into the PVA network caused an increase of the specific surface area up to 618 m2/g from 38.3–110 m2/g for pure PVA. The Young's modulus was associated with the bulk density. The obtained hybrid SiO2-PVA aerogels are a result of two phenomena – the chemical reaction of silica precursor (sol-gel process) and the physical transformation in PVA solution (phase separation).

The application of mixed ionic–electronic conducting (Ba0.5Sr0.5)(Co0.8Fe0.2)O3−δ (BSCF) as gas separation membrane is up to now hampered by secondary phase formation which impairs the excellent oxygen permeation properties of this material. In this work, we have studied the impact of grain size and A/B-cation ratio on secondary phase formation in BSCF and Y-doped (Ba0.5Sr0.5)(Co0.8Fe0.2)0.9Y0.1O3−δ (BSCF10Y) by electron microscopic techniques before and after long-term thermal exposure at an application-relevant temperature (~760 °C). A large content of secondary phases is found in samples with small grain sizes because grain boundaries provide nucleation sites for secondary phases. Higher sintering temperatures increase the grain sizes and substantially reduce the content of secondary phases. Variations of the A/B-cation ratio between (Ba0.5Sr0.5)0.95(Co0.8Fe0.2)O3−δ and (Ba0.5Sr0.5)1.05(Co0.8Fe0.2)O3−δ do not lead to a change of the composition of the cubic BSCF phase but changes the volume fraction of Co3O4 precipitates which are already formed during sintering. BSCF with an excess of A-site cations contains the smallest overall amount of secondary phases in undoped BSCF due to the minimization of Co3O4 precipitation during sintering and the reduction of nucleation sites for other secondary phases at application-relevant temperatures. Secondary phase formation in BSCF10Y can be almost completely suppressed due to the stabilization of the cubic BSCF phase by Y-doping and large grain sizes after high-temperature sintering.

The performance of electroceramic high temperature oxygen-transport membranes (OTM) is deter­mined by ionic and electronic transport processes. Oxygen transport kinetics is determined by the transport parameters, namely the surface exchange coefficient k δ and the bulk diffusion coefficient D δ. Mixed-conducting perovskite oxides, e.g., A1-xSrxCo1-yFeyO3- δ (A = Ba, La, Pr…) [1], are materials with excellent oxygen transport properties. Especially Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) shows an excellent OTM performance [2] in its cubic phase [3] at high temperatures (T > 850 °C). For an energy-efficient application it makes sense to use these oxygen separation membranes at lower temperatures (T< 700 °C). However, the occurrence of secondary phases in this temperature range [4] leads to a decreased oxygen transport through the BSCF membrane. Secondary phase formation can, though, be suppressed by suitable doping strategies [5,6]. OTM permeation occurs due to an oxygen partial pressure (pO2) gradient applied across the dense membrane. Reducing the thickness of the membrane increases the oxygen flux. However, below a certain thickness, the surface transfer of oxygen becomes rate-limiting; an improvement in oxygen flux can only be realized by surface activation, e.g., by a nanostructured layer. A promising candidate for such a layer is La0.6Sr0.4CoO3-δ (LSC) [7]. On the one hand, nanoporous LSC can enlarge the geo­metrical surface area and consequently increase the effective k δ value which is important especially at the low-pO2 side of the membrane because oxygen transport parameters strongly decrease with lower pO2 [8]. On the other hand, the occurrence of chemical “hetero-interfaces” as previously reported in LSC [9,10] may also improve oxygen surface exchange. The oxygen transport parameters (k δ, D δ) of uncoated and LSC-coated dense BSCF ceramic bulk samples are determined by electrical conductivity relaxation (ECR) measurements. Combining the results from these ECR measurements under varying partial pressures (pO2 = 10-2…1 bar) and transmission electron microscopy (TEM) analysis the influence of the nanostructured functional LSC layer on the dense BSCF ceramic membrane is investigated and discussed. Elemental distributions before and after aging obtained by analytical scanning TEM are presented to address cation interdiffusion between LSC and BSCF. References: [1] Y. Teraoka et al., Chem. Lett., 1743 (1985). [2] S. Baumann et al., J. Membrane Sci. 277, 198 (2011). [3] Z. Shao et al., J. Membrane Sci. 172, 177 (2000). [4] P. Müller et al., Chem. Mater. 25, 564 (2013). [5] P. Haworth et al., Sep. Purif. Technol. 81, 88 (2011). [6] M. Meffert, L.-S. Unger et al., manuscript in preparation. [7] J. Hayd et al., J. Power Sources 196, 7263 (2011). [8] C. Niedrig et al., Solid State Ionics 283, 30 (2015). [9] M. Sase et al., Solid State Ionics 178, 1843 (2008). [10] J. Hayd et al., J. Electrochem. Soc. 160, F351 (2013).

(Ba0.5Sr0.5)(Co0.8Fe0.2)O3−δ (BSCF) is a promising material with mixed ionic and electronic conductivity which is considered for oxygen separation membranes. Selective improvement of material properties, e.g. oxygen diffusivity or suppression of secondary phase formation, can be achieved by B-site doping. This study is concerned with the formation of Co-oxide precipitates in undoped BSCF at typical homogenization temperatures of 1,000°C, which act as undesirable nucleation sites for other secondary phases in the application-relevant temperature range. Y-doping successfully suppresses Co-oxide formation, whereas only minor improvements are achieved by Sc-doping. To understand the reason for the different behavior of Y and Sc, the lattice sites of dopant cations in BSCF were experimentally determined in this work. Energy-dispersive X-ray spectroscopy in a transmission electron microscope was applied to locate dopant sites exploiting the atom location by channeling enhanced microanalysis technique. It is shown that Sc exclusively occupies B-cation sites, whereas Y is detected on A- and B-cation sites in Y-doped BSCF, although solely B-site doping was intended. A model is presented for the suppression of Co-oxide formation in Y-doped BSCF based on Y double-site occupancy.

Transmission electron microscopy (TEM) with low-energy electrons has been recognized as an important addition to the family of electron microscopies as it may avoid knock-on damage and increase the contrast of weakly scattering objects. Scanning electron microscopes (SEMs) are well suited for low-energy electron microscopy with maximum electron energies of 30 keV, but they are mainly used for topography imaging of bulk samples. Implementation of a scanning transmission electron microscopy (STEM) detector and a charge-coupled-device camera for the acquisition of on-axis transmission electron diffraction (TED) patterns, in combination with recent resolution improvements, make SEMs highly interesting for structure analysis of some electron-transparent specimens which are traditionally investigated by TEM. A new aspect is correlative SEM, STEM, and TED imaging from the same specimen region in a SEM which leads to a wealth of information. Simultaneous image acquisition gives information on surface topography, inner structure including crystal defects and qualitative material contrast. Lattice-fringe resolution is obtained in bright-field STEM imaging. The benefits of correlative SEM/STEM/TED imaging in a SEM are exemplified by structure analyses from representative sample classes such as nanoparticulates and bulk materials.

Transport measurements on the two dimensional electron system in Al2O3 SrTiO3 heterostructures indicate significant noncrystalline anisotropic behavior below T = 30 K. Lattice dislocations in SrTiO3 and interfacial steps are suggested to be the main sources for electronic anisotropy. Anisotropic defect scattering likewise alters magnetoresistance at low temperature remarkably and influences spin-orbit coupling significantly by the Elliot Yafet mechanism of spin relaxation resulting in anisotropic weak localization. Applying a magnetic field parallel to the interface results in an additional field induced anisotropy of the conductance, which can be attributed to Rashba spin orbit interaction. Compared to LaAlO3 SrTiO3, Rashba coupling seems to be reduced indicating a weaker polarity in Al2O3 SrTiO3 heterostructures.

In this study, the influence of long-term annealing at intermediate temperatures on oxygen transport of Ba0.5Sr0.5Co0.8Fe0.2O3 d (BSCF) and 3 mol% Zr-doped BSCF (BSCF-Z3) ceramics with different grain sizes was studied by means of in situ electrical conductivity relaxation (ECR) measurements. Ceramics with different grain sizes in the range of 3–80 mm were obtained by varying the temperature and dwell time during sintering. For both compositions, the apparent values of the chemical diffusion coefficient Dchem and surface exchange coefficient kchem extracted from the data of ECR measurements are found to decrease with the time of annealing. The strongly correlated decreases in Dchem and kchem, being greater in magnitude at a lower grain size and temperature, are observed significantly more pronounced for BSCF than for BSCF-Z3. The results from microstructural analysis provide a clear rationale for the observations from ECR. High-resolution transmission electron microscopy (TEM) images recorded before and after annealing under pure oxygen at 700 C for 13 d show excessive formation of hexagonal and plate-like lamellar precipitates at the grain boundaries and in the interior of grains of BSCF ceramics during the annealing process, whilst secondary phase formation is restricted solely to the grain boundary regions in BSCF-Z3. The possible importance of grain boundaries in determining the oxygen surface exchange kinetics is emphasized.

The solid oxide fuel cell (SOFC) offers a great potential for stationary energy conversion applications because of its high combined heat and power efficiency. It is of great interest to understand performance relevant aspects of both material selection and manufacturing optimization. A design, which is made by co-sintering of inert substrate and functional layers, offers great possibilities for further methodological development. Findings can be transferred to different cell concepts. To understand and link fabrication parameters to the cell performance and identify aging mechanisms, sophisticated investigation of the 3D microstructure is necessary. This contribution will demonstrate how advanced correlative multiscale tomography can be used to identify microstructural parameters ranging from millimeter to nanometer scale. We combine optical microscopy (OM), X-ray computed tomography (μ-CT), focused ion beam – scanning electron microscopy (FIB-SEM) tomography and energy dispersive X-ray spectroscopy - scanning transmission electron microscopy (EDXS-STEM). The resulting microstructure data can be used to visualize performance critical cell areas such as inhomogeneities and secondary phases. In addition, pore and tortuosity distributions obtained from data of different length scales allow modeling of gas diffusion losses of all cell layers. This exemplifies how correlative tomography helps to understand specific contributions to overall cell performance. Figure 1

A fast method for determination of the Co-valence state by electron energy loss spectroscopy in a transmission electron microscope is presented. We suggest the distance between the Co-L3and Co-L2white-lines as a reliable property for the determination of Co-valence states between 2+ and 3+. The determination of the Co-L2,3white-line distance can be automated and is therefore well suited for the evaluation of large data sets that are collected for line scans and mappings. Data with a low signal-to-noise due to short acquisition times can be processed by applying principal component analysis. The new technique was applied to study the Co-valence state of Ba0.5Sr0.5Co0.8Fe0.2O3-d(BSCF), which is hampered by the superposition of the Ba-M4,5white-lines on the Co-L2,3white-lines. The Co-valence state of the cubic BSCF phase was determined to be 2.2+ (±0.2) after annealing for 100 h at 650°C, compared to an increased valence state of 2.8+ (±0.2) for the hexagonal phase. These results support models that correlate the instability of the cubic BSCF phase with an increased Co-valence state at temperatures below 840°C.

Ba0.5Sr0.5Co0.8Fe0.2O3–d (BSCF) compacts were prepared and annealed under application-relevant temperatures between 700 and 1000 °C for 100 h. The microstructure and chemical composition was investigated by electron microscopic techniques and energy dispersive X-ray spectroscopy (EDXS) to obtain a detailed insight into the formation of secondary phases which are involved in the degradation of the ionic conductivity of BSCF. Secondary phases are precipitated from the cubic BSCF phase at temperatures ≤900 °C. In addition to the well-known hexagonal phase, another secondary phase was identified which has the same crystal structure as Ban+1ConO3n+3(Co8O8) with n ≥ 2 which is denoted as a BCO-type phase. Regions with plate-like morphology are formed which contain thin lamellae of the cubic, hexagonal, and BCO-type phase. The negative impact of the secondary phases on the material performance is discussed.

The two-dimensional electron system (2DES) in $\mathrm{LaAl}{\mathrm{O}}_{3}/\mathrm{SrTi}{\mathrm{O}}_{3}$ (LAO/STO) heterostructures, showing superconductivity below 200 mK at ambient pressure, displays incipient localization under hydrostatic pressure $p$. Even small $p$ significantly affects the transport properties for $10\phantom{\rule{0.16em}{0ex}}\mathrm{K}lTl100\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ where impurity scattering dominates. The distinct $T$ dependence of impurity scattering results in a minimum of the sheet resistance ${R}_{\mathrm{s}}(T)$, which has been often attributed to Kondo scattering. We show that the strong suppression of the dielectric permittivity $\ensuremath{\varepsilon}(p)$ of STO enhances impurity scattering via the loss of screening and leads to incipient localization of charge carriers with increasing hydrostatic pressure. For $Tl10\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ negative logarithmic corrections to the conductance are attributed to electron-electron interaction. Weak localization and antilocalization, if present, are dominated by the pivotal role of impurity scattering by, e.g., oxygen vacancies, for the transport properties of the 2DES in LAO/STO heterostructures. This interpretation is favorited by exploiting the strong dependence of the dielectric permittivity of STO on pressure. Measurements of the magnetoresistance corroborate this interpretation.

High-temperature devices such as solid oxide fuel cells (SOFCs) and oxygen transport membranes (OTMs) require materials with high chemical, microstructural stability as well as high catalytic activity towards the oxygen reduction reaction. In its cubic phase the mixed ionic-electronic conductor Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) presents outstanding oxygen transport properties and exceedingly low cathode resistance [1]. However, the application in OTMs has been hampered by its degrading oxygen permeability in the relevant temperature range (600 – 800 °C). It was shown by our group [2], that the multivalent Co-cation prefers within this temperature range a higher valence which destabilizes the cubic structure favoring the hexagonal phase formation. Various doping strategies with transition metals like, e.g., Y have shown beneficial effects with respect to stabilizing the cubic BSCF phase and preservation of the oxygen conductivity [3,4]. In this direction the effect of B-site partial substitution with Y, Ti or Nb has been studied thoroughly by our group using scanning electron microscopy (SEM) and analytical (scanning) transmission electron microscopy ((S)TEM) combined with energy dispersive X-ray spectroscopy (EDXS). In addition, the oxygen reduction reaction of pure and doped BSCF electrodes is investigated in oxygen-nitrogen mixtures (pO2 = 0.02 to 0.4 atm) as well as in CO2-atmospheres (0.1…1 vol. % CO2) using symmetrical SOFC cells with Ce0.9Gd0.1O2-δ (CGO) as electrolyte from 600 to 800 °C. Characterization by electrochemical impedance spectroscopy (EIS) combined with the distribution of relaxation times (DRT) reveals an enhanced tolerance of doped BSCF cathodes, in particular Y-doped, towards the adsorption-desorption of CO2 molecules. Channeling enhanced microanalysis (ALCHEMI) investigations on Y-doped BSCF demonstrate the coexistence of Y ions on both cation lattice sites with up to 55 % of the Y-ions located on the unintended A-lattice site [5,6]. Overall this study shows that enhancing the phase stability and the CO2 tolerance of BSCF by a rational substitution of the B-site is plausible while the high performance and its excellent oxygen transport properties are kept. References: [1] Z. Shao, S.M. Haile. Nature, 431 (2004) 170-173. [2] P. Müller, M. Meffert, H. Störmer, D. Gerthsen. Microsc. Microanal., 19 (2013) 1595-1605. [3] P. Haworth, S. Smart, J. Glasscock, J.C. Diniz da Costa. Sep. Purif. Technol., 94 (2012) 16-22. [4] M. Meffert, L-S Unger, L. Grünewald, H. Störmer, S. F. Wagner, E. Ivers-Tiffée, D. Gerthsen. J. Mater. Sci., 52 (2017) 2705-2719. [5] J. C. H. Spence and J. Taftø. J. Microsc., 130 (1983) 147-154. [6] M. Meffert, H. Störmer, D. Gerthsen. Microsc. Microanal., 22 (2016) 113-121.

The mixed ionic-electronic conducting cathode (La, Sr)(Co, Fe)O3-δ (LSCF) is utilized worldwide for solid oxide fuel cells (SOFCs) operated at intermediate temperatures [1] because of its excellent oxygen reduction reaction rate. The challenge of using LSCF as cathode material is the complex nature of the interface to Zirconia-based electrolytes, which may counteract this advantage [2, 3]. The composite cathode (La, Sr)MnO3 (LSM) – Y2O3 doped ZrO2(YSZ) is an adequate alternative for higher temperatures only. LSM-YSZ offers fewer regions of oxygen reduction (triple phase boundaries) due to spatially separated ionically (YSZ) and electronically (LSM) conductive phases. If electrochemical and microstructural parameters of both types of electrodes are quantified by appropriate techniques, the performance can be modeled and compared for different operating conditions. Advanced imaging techniques such as focused ion beam/scanning electron microscopy (FIB/SEM) or X-ray tomography are promising techniques for microstructure quantification enabling 3D reconstructions of µm- and sub-µm-scaled multiphase electrodes [4, 5, 6]. Reconstructing the composite cathode, in particular, comprises the challenge of overcoming the weak material contrast between LSM and YSZ using scanning electron microscopy. This contribution will show how FIB/SEM tomography can help to identify different microstructural parameters such as porosity, particle size, surface area and tortuosity. We will compare LSCF and LSM-YSZ cathodes and enlighten new challenges in image reconstruction, parameter acquisition and parameter interpretation for the different material sets. [1] H. Yokokawa, et al., J. Power Sources, 182, p. 400 (2008) [2] J. Szasz, et al., ECS Trans., 66(2), p. 79 (2015) [3] Yokokawa, et al., Solid State Ionics, 262, p. 454 (2014) [4] J.R. Wilson, et al., Nature Materials 5(17), p. 541 (2006) [5] J. Joos, et al., Electrochim. Acta 82, p. 268 (2012) [6] Y. C. K. Chen-Wiegart, et al., J. Power Sources, 218, p. 348 (2012) Figure 1

The cubic perovskite-type (Ba0.5Sr0.5)(Co0.8Fe0.2)O3-δ (c-BSCF) is a material with mixed electronic-ionic conductivity and extraordinary high oxygen permeability which makes c-BSCF interesting for applications such as, e.g., oxygen separation membranes and cathodes in solid oxide fuel cells. However, applications are up to now hampered by the degradation of the oxygen permeability in the relevant temperature range between 700 and 900 oC. Particularly detrimental is the formation of the hexagonal (h-)BSCF phase below 840 oC [1]. Moreover, several other secondary phases were identified that are believed to also adversely affect oxygen permeation [2]. Secondary phase formation is connected with the multivalent B-site Co-cation which increases its valence state with rising temperature. Substitution of B-site Co-cations by monovalent transition metals, e.g. Y or Zr, is considered as a promising strategy to stabilize c-BSCF [3]. Another issue is related to the degradation of BSCF in CO2-containing atmospheres where the formation of carbonate precipitates on the surface [4] is accompanied by the reduction of the oxygen surface exchange coefficient [5]. In this work we will address the stabilization of c-BSCF by Y-doping and the effect of Y-doping on the degradation of the electrochemical properties in CO2-containing atmospheres. Bulk samples were prepared by the mixed oxide route with compositions of (Ba0.5Sr0.5)(Co0.8Fe0.2)1-xYxO3- δ (x = 0 and 0.1, BSCF and BSCF10Y) which were subjected to annealing for 240 h between 640 oC and 880 oC in air. In addition, symmetrical cell samples with porous BSCF and BSCY10Y layers were fabricated by screen-printing on both sides of Ce0.9Gd0.1O2-δ (CGO) substrates. The samples were in-situ sintered at 900 °C for 10 h and electrochemically characterized by electrical impedance spectroscopy (EIS) at 700 oC in synthetic air with CO2-concentrations up to 3 %. Distribution of relaxation time (DRT) curves and area specific resistances are derived from EIS data which allow to monitor degradation processes. We use scanning electron microscopy (SEM) and analytical (scanning) transmission electron microscopy ((S)TEM) combined with energy dispersive X-ray spectroscopy (EDXS) to study the structural and chemical material properties. Different phases in bulk samples are visualized by SEM imaging using the specimen preparation procedure outlined in [6]. Fig. 1 shows SEM images of bulk BSCF and BSCF10Y after annealing at temperatures between 640 oC and 880 oC for 240 h. Inserted schemes illustrate secondary phase formation. BSCF (Figs. 1a-c) contains a large volume fraction of secondary phases comprising Co3O4, Ban+1ConO3n+3(Co8O8) (n ≥ 2) (BCO) and h-BSCF depending on the annealing temperature. Secondary phase formation in BSCF10Y is completely suppressed at 880 oC. Only small volume fractions of h-BSCF are detected at grain boundaries after annealing at 760 and 640 oC. Fig. 2 presents DRT curves of BSCF/CGO/BSCF and BSCF10Y/CGO/BSCF10Y cells at T=700 °C before and after 1 % CO2 addition for 50 h. A signal at 100 Hz increases strongly in BSCF/CGO/BSCF (Fig. 2a) which is absent before CO2 addition. The same signal is present in BSCF10Y/CGO/BSDF10Y (Fig. 2c) but with a significantly lower amplitude. SEM images show a high density of small precipitates on the surface of BSCF (Fig. 2b) which are present in a much smaller concentration on BSCF10Y (Fig. 2d). The small precipitates are absent if annealing is performed without CO2. We note that the 100 Hz signal disappears after switching off CO2 indicating that the origin of the process at 100 Hz could be related to CO2-adsorption. Our study shows that Y-doping with a sufficiently high Y-concentration strongly reduces or even suppresses secondary phase formation in BSCF. Y-doping is also promising to reduce degradation of the electrochemical properties in CO2-containing atmospheres. STEM-EDXS analysis are currently carried out to analyze secondary phases after annealing in CO2-containing air. References [1] S. Švarcová et al., Solid State Ionics 178, 1787 (2008) [2] K. Efimov et al., Chem. Mater. 22, 5866 (2010) [3] S. Yakovlev et al., Appl. Phys. Lett. 96, 254101 (2010) [4] M. Arnold, J. Membr. Sci. 293, 44 (2007) [5] C. Niedrig, Electrochemical Performance and Stability of (Ba0.5Sr0.5)(Co0.8Fe0.2)O3-δ for Oxygen Transport Membranes, Dissertation, KIT Scientific Publishing, 2015 [6] P. Müller et al., Solid State Ionics 206, 57 (2012) Figure 1

Transmission electron microscopy (TEM) is a standard technique for microstructure studies with a resolution that meanwhile has reached values better than 0.1 nm. Essential is the capability of obtaining images and diffraction patterns from the same small specimen region which facilitates crystal structure determination of small precipitates. Particularly unique is the possibility to perform atomic-scale chemical analyses by energy dispersive X-ray spectroscopy and electron energy loss spectroscopy (EELS). Information on the electronic material properties like, e.g., valence states or bond configurations between neighbor atoms is obtained by the electron near edge structure of ionization edges in EELS spectra. In addition to these well-known capabilities, many more imaging modes and evaluation techniques are available which yield additional material properties and render electron microscopy an extremely powerful technique. We will illustrate the capabilities of analytical TEM for the example of (Ba0.5Sr0.5)(Co0.8Fe0.2)O3-δ (BSCF) which is a promising and highly complex material for oxygen separation membranes. BSCF suffers from degradation of the oxygen diffusivity at temperatures below 840 oC which is attributed to the decomposition of the cubic phase into several secondary phases [1,2]. Composition and crystal structure analyses of the different phases combined with Co-valence state determination sheds light on the origin of the instability of the cubic phase. Indispensable is the determination Co-valence state in the cubic and hexagonal BSCF phases which required the introduction of a new evaluation technique due to the overlap of the Co-L2,3ionization edges with ionization edges of other elements in BSCF [3]. Doping with Yttrium has been used to stabilize the cubic Sr1−x Y x Co1−y Y y O3−δ phase [4] which also appears to be a promising strategy for BSCF. Substitution of B-site Co- and Fe-cations in BSCF by Y3+-cations with a fixed valence state should lead to the stabilization of the Co-valence and, hence, the stabilization of the cubic BSCF phase. The volume fraction of secondary phases is indeed substantially reduced in Y-doped BSCF. Only the hexagonal BSCF phase is observed which is confined to grain boundaries (Fig. 1). In contrast, a large volume fraction of different secondary phases is found in undoped BSCF (Fig. 2). A closer inspection suggests, however, that the suppression of secondary phases in Y-doped samples could be partially related to the lack of nucleation sites rather than the stabilizing effect of Y-doping. To achieve an improved understanding of the effect of Y-doping, the occupation sites of Y-cations were analyzed by atom location by channeling enhanced microanalysis (ALCHEMI) as proposed by Spence and Taftø [5]. ALCHEMI analyses demonstrate that Y-cations occupy A- and B-sites. Despite substitution of B-site cations, 25 % to 40 % of the Y-cations occupy A-sites depending on the doping level. This indicates the complex effect of Y-doping in BSCF which will be discussed. References [1] P. Müller, H. Störmer, M. Meffert, L. Dieterle, C. Niedrig, S.F. Wagner, E. Ivers-Tiffée, D. Gerthsen, Chem. Mater. 25 (2013) 564. [2] C. Niedrig, S. Taufall, M. Burriel, W. Menesklou, S. F. Wagner, S. Baumann, E. Ivers-Tiffée, Solid State Ionics 197 (2011) 25. [3] P. Müller, M. Meffert, H. Störmer, D. Gerthsen, Microsc. Microanal. 19 (2013) 1595. [4] K. Zhang, R. Ran, L. Ge, Z. Shao, W. Jin, N. Xu, Journal of Alloys and Compounds 474 (2009) 477. [5] J. Taftø, J. Spence, Science 218 (1982) 49. Figure 1