In situ electrochemical observation of anisotropic lattice contraction of La0.6Sr0.4FeO3-δ electrodes during pulsed laser deposition.

La0.6Sr0.4FeO3-δ (LSF) electrodes were grown on different electrolyte substrates by pulsed laser deposition (PLD) and their oxygen exchange reaction (OER) resistance was tracked in real-time by in situ PLD impedance spectroscopy (i-PLD) inside the PLD chamber. This enables measurements on pristine surfaces free from any contaminations and the direct observation of thickness dependent properties. As substrates, yttria-stabilized zirconia single crystals (YSZ) were used for polycrystalline LSF growth and La0.95Sr0.05Ga0.95Mg0.05O3-δ (LSGM) single crystals or YSZ single crystals with a 5 nm buffer-layer of Gd0.2Ce0.8O2-δ for epitaxial LSF film growth. While polycrystalline LSF electrodes show a constant OER resistance in a broad thickness range, epitaxially grown LSF electrodes exhibit a continuous and strong increase of the OER resistance with film thickness until ≈60 nm. In addition, the activation energy of the OER resistance increases by 0.23 eV compared to polycrystalline LSF. High resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) measurements reveal an increasing contraction of the out-of-plane lattice parameter in the epitaxial LSF electrodes over electrode thickness. Defect thermodynamic simulations suggest that the decrease of the LSF unit cell volume is accompanied by a lowering of the oxygen vacancy concentration, explaining both the resistive increase and the increased activation energy.

Real-time impedance monitoring of oxygen reduction during surface modification of thin film cathodes.

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.

Voltage and partial pressure dependent defect chemistry in (La,Sr)FeO3-δ thin films investigated by chemical capacitance measurements.

La0.6Sr0.4FeO3-δ (LSF) thin films of different thickness were prepared by pulsed laser deposition on yttria stabilized zirconia (YSZ) and characterized by using three electrode impedance spectroscopy. Electrochemical film capacitance was analyzed in relation to oxygen partial pressure (0.25 mbar to 1 bar), DC polarization (0 m to -600 m) and temperature (500 to 650 °C). For most measurement parameters, the chemical bulk capacitance dominates the overall capacitive properties and the corresponding defect chemical state depends solely on the oxygen chemical potential inside the film, independent of atmospheric oxygen pressure and DC polarization. Thus, defect chemical properties (defect concentrations and defect formation enthalpies) could be deduced from such measurements. Comparison with LSF defect chemical bulk data from the literature showed good agreement for vacancy formation energies but suggested larger electronic defect concentrations in the films. From thickness-dependent measurements at lower oxygen chemical potentials, an additional capacitive contribution could be identified and attributed to the LSF|YSZ interface. Deviations from simple chemical capacitance models at high pressures are most probably due to defect interactions.

A Novel Application for Low Frequency Electrochemical Impedance Spectroscopy as an Online Process Monitoring Tool for Viable Cell Concentrations.

New approaches in process monitoring during industrial fermentations are not only limited to classical pH, dO2 and offgas analysis, but use different in situ and online sensors based on different physical principles to determine biomass, product quality, lysis and far more. One of the very important approaches is the in situ accessibility of viable cell concentration (VCC). This knowledge provides increased efficiency in monitoring and controlling strategies during cultivations. Electrochemical impedance spectroscopy-EIS-is used to monitor biomass in a fermentation of E. coli BL21(DE3), producing a recombinant protein using a fed batch-based approach. Increases in the double layer capacitance (Cdl), determined at frequencies below 1 kHz, are proportional to the increase of biomass in the batch and fed batch phase, monitored in offline and online modes for different cultivations. A good correlation of Cdl with cell density is found and in order to get an appropriate verification of this method, different state-of-the-art biomass measurements are performed and compared. Since measurements in this frequency range are largely determined by the double layer region between the electrode and media, rather minor interferences with process parameters (aeration, stirring) are to be expected. It is shown that impedance spectroscopy at low frequencies is a powerful tool for cultivation monitoring.

Synthesis, Crystal Chemistry, and Electrochemical Properties of Li(7-2x)La3Zr(2-x)Mo(x)O12 (x = 0.1-0.4): Stabilization of the Cubic Garnet Polymorph via Substitution of Zr(4+) by Mo(6+).

Cubic Li7La3Zr2O12 (LLZO) garnets are exceptionally well suited to be used as solid electrolytes or protecting layers in "Beyond Li-ion Battery" concepts. Unfortunately, cubic LLZO is not stable at room temperature (RT) and has to be stabilized by supervalent dopants. In this study we demonstrate a new possibility to stabilize the cubic phase at RT via substitution of Zr(4+) by Mo(6+). A Mo(6+) content of 0.25 per formula unit (pfu) stabilizes the cubic LLZO phase, and the solubility limit is about 0.3 Mo(6+) pfu. Based on the results of neutron powder diffraction and Raman spectroscopy, Mo(6+) is located at the octahedrally coordinated 16a site of the cubic garnet structure (space group Ia-3d). Since Mo(6+) has a smaller ionic radius compared to Zr(4+) the lattice parameter a0 decreases almost linearly as a function of the Mo(6+) content. The highest bulk Li-ion conductivity is found for the 0.25 pfu composition, with a typical RT value of 3.4 × 10(-4) S cm(-1). An additional significant resistive contribution originating from the sample interior (most probably from grain boundaries) could be identified in impedance spectra. The latter strongly depends on the prehistory and increases significantly after annealing at 700 °C in ambient air. Cyclic voltammetry experiments on cells containing Mo(6+) substituted LLZO indicate that the material is stable up to 6 V.

Fast oxygen exchange and diffusion kinetics of grain boundaries in Sr-doped LaMnO3 thin films.

In this study, the contribution of grain boundaries to the oxygen reduction and diffusion kinetics of La0.8Sr0.2MnO3 (LSM) thin films is investigated. Polycrystalline LSM thin films with columnar grains of different grain sizes as well as epitaxial thin films were prepared by pulsed laser deposition. (18)O tracer exchange experiments were performed at temperatures from 570 °C to 810 °C and subsequently analyzed by secondary ion mass spectrometry (SIMS). The isotope concentration depth profiles of polycrystalline films clearly indicate contributions from diffusion and surface exchange in grains as well as in grain boundaries. Measured depth profiles were analyzed by finite element modeling and revealed the diffusion coefficients D and oxygen exchange coefficients k of both the grain bulk and grain boundaries. Values obtained for grain boundaries (Dgb and kgb) are almost three orders of magnitude higher than those of the grains (Dg and kg). Hence, grain boundaries may not only facilitate fast oxygen diffusion but also fast oxygen exchange kinetics. Variation of the A-site stoichiometry ((La0.8Sr0.2)0.95MnO3) did not lead to large changes of the kinetic parameters. Properties found for epitaxial layers without grain boundaries (Db and kb) are close to those of the grains in polycrystalline layers.

Mapping electrochemically driven gas exchange of mixed conducting SrTi0.7Fe0.3O3 - δ and Ce0.8Gd0.2O1.9 thin films by 18O tracer incorporation under reducing atmosphere.

Thermally and electrochemically driven 18O tracer exchange experiments in H2/H218O atmosphere were performed on SrTi0.7Fe0.3O3 - Î´ and Ce0.8Gd0.2O2 - Î´ thin films on single crystalline YSZ substrates. Noble metal current collectors were deposited on both films and electrochemically polarized during the exchange experiment. The resulting tracer distribution was analyzed by spatially resolved secondary ion mass spectrometry. Increased tracer fraction near the current collectors was found under cathodic polarization and decreased tracer fraction under anodic polarization. High cathodic bias leads to enhanced n-type electronic conductivity, which increases the extent of the electrochemically active zone.

Enhancing electrochemical water-splitting kinetics by polarization-driven formation of near-surface iron(0): an in†situ XPS study on perovskite-type electrodes.

In the search for optimized cathode materials for high-temperature electrolysis, mixed conducting oxides are highly promising candidates. This study deals with fundamentally novel insights into the relation between surface chemistry and electrocatalytic activity of lanthanum ferrite based electrolysis cathodes. For this means, near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) and impedance spectroscopy experiments were performed simultaneously on electrochemically polarized La0.6 Sr0.4 FeO3-δ (LSF) thin film electrodes. Under cathodic polarization the formation of Fe(0) on the LSF surface could be observed, which was accompanied by a strong improvement of the electrochemical water splitting activity of the electrodes. This correlation suggests a fundamentally different water splitting mechanism in presence of the metallic iron species and may open novel paths in the search for electrodes with increased water splitting activity.

Cation diffusion in La(0.6)Sr(0.4)CoO(3-δ) below 800 °C and its relevance for Sr segregation.

Cation diffusion was investigated in La0.6Sr0.4CoO3-δ (LSC) thin films on (100) yttria stabilized zirconia in the temperature range 625-800 °C. Isotopic ((86)Sr) and elemental tracers (Fe, Sm) were used to establish diffusion profiles of the cations in bi- and multi-layered thin films. The profiles were analyzed by time of flight-secondary ion mass spectrometry (ToF-SIMS). Grain and grain boundary diffusion coefficients of the cations were determined for LSC thin films with columnar grains - diffusion along grain boundaries is shown to be about three orders of magnitude faster than in grains. This could be verified for thin films with different grain size. A- and B-site cations showed very similar temperature dependencies with activation energies of ∼3.5 eV for bulk and ∼4.1 eV for grain boundary diffusion. The importance of cation diffusivities for surface segregation of Sr and thus for a major degradation mechanism of LSC cathodes in solid oxide fuel cells is discussed.

Electrochemical properties of La0.6Sr0.4CoO3 - δ thin films investigated by complementary impedance spectroscopy and isotope exchange depth profiling.

The oxygen exchange and diffusion properties of La0.6Sr0.4CoO3 - Î´ thin films on yttria stabilized zirconia were analyzed by impedance spectroscopy and 18O tracer experiments. The investigations were performed on the same thin film samples and at the same temperature (400 °C) in order to get complementary information by the two methods. Electrochemical impedance spectroscopy can reveal resistive and capacitive contributions of such systems, but an exact interpretation of the spectra of complex oxide electrodes is often difficult from impedance data alone. It is shown that additional isotope exchange depth profiling can significantly help interpreting impedance spectra by giving reliable information on the individual contribution and exact location of resistances (surface, electrode bulk, interface). The measurements also allowed quantitative comparison of electrode polarization resistances obtained by different methods.

A novel ToF-SIMS operation mode for sub 100 nm lateral resolution: Application and performance.

A novel operation mode for time of flight-secondary ion mass spectrometry (ToF-SIMS) is described for a TOF.SIMS 5 instrument with a Bi-ion gun. It features sub 100 nm lateral resolution, adjustable primary ion currents and the possibility to measure with high lateral resolution as well as high mass resolution. The adjustment and performance of the novel operation mode are described and compared to established ToF-SIMS operation modes. Several examples of application featuring novel scientific results show the capabilities of the operation mode in terms of lateral resolution, accuracy of isotope analysis of oxygen, and combination of high lateral and mass resolution. The relationship between high lateral resolution and operation of SIMS in static mode is discussed.

A High Temperature Harvestorer Based on a Photovoltaic Cell and an Oxygen Ion Battery.

Hybrid devices for combined energy harvesting and storage, i.e., harvestorers, are attractive solutions for powering small autonomous devices (e.g., "smart appliances", Internet of things nodes), which are ever more prominent as the digitalization and technologization of our society progresses. A concept for a high temperature (HT) harvestorer is presented, and the operational characteristics of a prototype device are discussed. It is based on photovoltaic (PV) energy harvesting and HT electrochemical energy storage. The HT-PV cells employ SrTiO3/La0.9Sr0.1CrO3-δ heterojunctions for energy harvesting and produce photovoltages up to 1 V and photocurrents of several mA cm-2 upon UV illumination at 350 °C. Electrochemical energy storage is realized by oxygen ion battery (OIB), a device based on mixed ionic and electronic conducting oxide thin film electrodes and an yttria stabilized zirconia electrolyte. The OIB exhibits capacities of up to 11 mC cm-2 (3 µA h cm-2) at 0.6 V (350 °C). A prototype harvestorer device was fabricated by integrating an HT-PV and an OIB cell into one device. This harvestorer was operated over several cycles consisting of harvesting and storing energy under illumination, followed by retrieval of the stored energy without illumination. Up to 3.5 mJ cm-2 (1 µW h cm-2) was stored with energy efficiencies up to 67%. Approaches for further optimization are discussed.

Engineering surface dipoles on mixed conducting oxides with ultra-thin oxide decoration layers.

Improving materials for energy conversion and storage devices is deeply connected with an optimization of their surfaces and surface modification is a promising strategy on the way to enhance modern energy technologies. This study shows that surface modification with ultra-thin oxide layers allows for a systematic tailoring of the surface dipole and the work function of mixed ionic and electronic conducting oxides, and it introduces the ionic potential of surface cations as a readily accessible descriptor for these effects. The combination of X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) illustrates that basic oxides with a lower ionic potential than the host material induce a positive surface charge and reduce the work function of the host material and vice versa. As a proof of concept that this strategy is widely applicable to tailor surface properties, we examined the effect of ultra-thin decoration layers on the oxygen exchange kinetics of pristine mixed conducting oxide thin films in very clean conditions by means of in-situ impedance spectroscopy during pulsed laser deposition (i-PLD). The study shows that basic decorations with a reduced surface work function lead to a substantial acceleration of the oxygen exchange on the surfaces of diverse materials.

Structural Characterization of La0.6Sr0.4CoO3-δ Thin Films Grown on (100)-, (110)-, and (111)-Oriented La0.95Sr0.05Ga0.95Mg0.05O3-δ.

In this study, a detailed structural characterization of epitaxial La0.6Sr0.4CoO3-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 La0.95Sr0.05Ga0.95Mg0.05O3-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.

The relevance of interfaces for oxide ion transport in yttria stabilized zirconia (YSZ) thin films.

Thin YSZ films were prepared on magnesia, sapphire and strontium titanate (STO) single crystals using pulsed laser deposition and, for comparison, by a sol-gel method on STO. The bulk and interfacial mass and charge transport properties of these films were investigated by complementary impedance spectroscopy and tracer diffusion measurements. In this context, a novel two-step tracer diffusion experiment is introduced. For YSZ films on sapphire and magnesia, grain bulk conductivities similar to those of polycrystalline samples were measured in most cases. Strongly blocking grain boundaries could be identified by impedance measurements. The films on sapphire and magnesia also exhibited good agreement between effective transport properties of impedance and tracer measurements. YSZ layers on strontium titanate single crystals, on the other hand, showed a strongly increased effective conductivity in impedance studies. However, in tracer diffusion experiments this could be unambiguously attributed to conduction in the substrate while the diffusion coefficient of YSZ on STO was comparable to that of YSZ films on other substrates. Moreover, the tracer diffusion experiments did not indicate any significant increase of oxide ion mobility on a free YSZ surface compared to a Pt|YSZ interface.

Reversible Ultrathin PtOx Formation at the Buried Pt/YSZ(111) Interface Studied In Situ under Electrochemical Polarization.

Three different platinum oxides are observed by in situ X-ray diffraction during electrochemical potential cycles of platinum thin film model electrodes on yttria-stabilized zirconia (YSZ) at a temperature of 702 K in air. Scanning electron microscopy and atomic force microscopy performed before and after the in situ electrochemical X-ray experiments indicate that approximately 20% of the platinum electrode has locally delaminated from the substrate by forming pyramidlike blisters. The oxides and their locations are identified as (1) an ultrathin PtOx at the buried Pt/YSZ interface, which forms reversibly upon anodic polarization; (2) polycrystalline ß-PtO2, which forms irreversibly upon anodic polarization on the inside of the blisters; and (3) an ultrathin α-PtO2 at the Pt/air interface, which forms by thermal oxidation and which does not depend on the electrochemical polarization. Thermodynamic and kinetic aspects are discussed to explain the coexistence of multiple phases at the same electrochemical conditions.

Crystal-Orientation-Dependent Oxygen Exchange Kinetics on Mixed Conducting Thin-Film Surfaces Investigated by In Situ Studies.

The oxygen exchange kinetics and the surface chemistry of epitaxially grown, dense La0.6Sr0.4CoO3-δ (LSC) thin films in three different orientations, (001), (110), and (111), were investigated by means of in situ impedance spectroscopy during pulsed laser deposition (i-PLD) and near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS). i-PLD measurements showed that pristine LSC surfaces exhibit very fast surface exchange kinetics but revealed no significant differences between the specific orientations. However, as soon as the surfaces were in contact with acidic, gaseous impurities, such as S-containing compounds in nominally pure measurement atmospheres, NAP-XPS measurements revealed that the (001) orientation is substantially more susceptible to the formation of sulfate adsorbates and a concomitant performance decrease. This result is further substantiated by a stronger increase of the work function on (001)-oriented LSC surfaces upon sulfate adsorbate formation and by a faster performance degradation of these surfaces in ex situ measurement setups. This phenomenon has potentially gone unnoticed in the discussion of the interplay between the crystal orientation and the oxygen exchange kinetics and might have far-reaching implications for real solid oxide cell electrodes, where porous materials exhibit a wide variety of differently oriented and reconstructed surfaces.

Improving and degrading the oxygen exchange kinetics of La0.6Sr0.4CoO3-δ by Sr decoration.

Minimizing the overpotential at the air electrode of solid oxide fuel cells (SOFC) is one of the key challenges regarding a broad applicability of this technology. Next to novel materials and geometry optimization, surface modification is a promising and flexible method to alter the oxygen exchange kinetics at SOFC cathode surfaces. Despite extensive research, the mechanism behind the effect of surface decorations is still under debate. Moreover, for Sr decoration, previous studies yielded conflicting results, reporting either a beneficial or a detrimental impact on the oxygen exchange kinetics. In this contribution, in situ impedance spectroscopy during pulsed laser deposition was used to investigate the effect of Sr containing decorations under different deposition conditions. Depending on deposition temperature and interactions with the gas phase, opposing effects of Sr decoration were found. In combination with near-ambient pressure X-ray photoelectron spectroscopy and non-ambient X-ray diffractometry, it was possible to trace this phenomenon back to different chemical environments of the surface Sr. At high temperatures, Sr is deposited as SrO, which can have a beneficial effect on the oxygen exchange kinetics. At low temperatures, SrCO3 adsorbates are formed from trace amounts of CO2 in the measurement atmosphere, causing a decrease of the oxygen exchange rate. These results are in excellent agreement with the concept of surface acidity as a descriptor for the effect of surface decorations, providing further insight into the oxygen exchange kinetics on SOFC cathode surfaces and its degradation. In addition, this study shows that Sr segregation itself initially does not lead to performance degradation but that segregated SrO readily reacts with acidic compounds, reducing the catalytic capability of mixed conducting oxides.

Electronic and ionic effects of sulphur and other acidic adsorbates on the surface of an SOFC cathode material.

The effects of sulphur adsorbates and other typical solid oxide fuel cell (SOFC) poisons on the electronic and ionic properties of an SrO-terminated (La,Sr)CoO3 (LSC) surface and on its oxygen exchange kinetics have been investigated experimentally with near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), low energy ion scattering (LEIS) and impedance spectroscopy as well as computationally with density functional theory (DFT). The experiment shows that trace amounts of sulphur in measurement atmospheres form SO2- 4 adsorbates and strongly deactivate a pristine LSC surface. They induce a work function increase, indicating a changing surface potential and a surface dipole. DFT calculations reveal that the main participants in these charge transfer processes are not sub-surface transition metals, but surface oxygen atoms. The study further shows that sulphate adsorbates strongly affect oxygen vacancy formation energies in the LSC (sub-)surface, thus affecting defect concentrations and oxygen transport properties. To generalize these results, the investigation was extended to other acidic oxides which are technologically relevant as SOFC cathode poisons, such as CO2 and CrO3. The results unveil a clear correlation of work function changes and redistributed charge with the Smith acidity of the adsorbed oxide and clarify fundamental mechanistic details of atomic surface modifications. The impact of acidic adsorbates on various aspects of the oxygen exchange reaction rate is discussed in detail.