In Situ Impedance Analysis of Oxygen Exchange on Growing La0.6Sr0.4CoO3-δ Thin Films.

The further development of solid oxide fuel and electrolysis cells (SOFC/SOEC) strongly relies on research activities dealing with electrode materials. Recent studies showed that under operating conditions many perovskite-type oxide electrodes are prone to changes of their surface composition, leading to severe changes of their electrochemical performance. This results in a large scatter of data in literature and complicates comparison of materials. Moreover, little information is available on the potentially excellent properties of surfaces immediately after preparation, that is, before any degradation by exposure to other gas compositions or temperature changes. Here, we introduce in situ impedance spectroscopy during pulsed laser deposition (IPLD) as a new method for electrochemical analysis of mixed ionic and electronic conducting (MIEC) thin films during growth. First, this approach can truly reveal the properties of as-prepared MIEC electrode materials, since it avoids any alterations of their surface between preparation and investigation. Second, the measurements during growth give information on the thickness dependence of film properties. This technique is applied to La0.6Sr0.4CoO3-δ (LSC), one of the most promising SOFC/SOEC oxygen electrode material. From the earliest stages of LSC film deposition on yttria-stabilized zirconia (YSZ) to a fully grown thin film of 100 nm thickness, data are gained on the oxygen exchange kinetics and the defect chemistry of LSC. A remarkable reproducibility is found in repeated film growth experiments, not only for the bulk related chemical capacitance but also for the surface related polarization resistance (±10%). Polarization resistances of as-prepared LSC films are extraordinarily low (2.0 Ω cm2 in 40 µbar O2 at 600 °C). LSC films on YSZ and on La0.95Sr0.05Ga0.95Mg0.05O3-δ (LSGM) single crystals exhibit significantly different electrochemical properties, possibly associated with the tensile strain of LSC on LSGM.

How To Get Mechanistic Information from Partial Pressure-Dependent Current-Voltage Measurements of Oxygen Exchange on Mixed Conducting Electrodes.

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

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.

The Chemical Evolution of the La0.6Sr0.4CoO3-δ Surface Under SOFC Operating Conditions and Its Implications for Electrochemical Oxygen Exchange Activity.

Owing to its extraordinary high activity for catalysing the oxygen exchange reaction, strontium doped LaCoO3 (LSC) is one of the most promising materials for solid oxide fuel cell (SOFC) cathodes. However, under SOFC operating conditions this material suffers from performance degradation. This loss of electrochemical activity has been extensively studied in the past and an accumulation of strontium at the LSC surface has been shown to be responsible for most of the degradation effects. The present study sheds further light onto LSC surface changes also occurring under SOFC operating conditions. In-situ near ambient pressure X-ray photoelectron spectroscopy measurements were conducted at temperatures between 400 and 790 °C. Simultaneously, electrochemical impedance measurements were performed to characterise the catalytic activity of the LSC electrode surface for O2 reduction. This combination allowed a correlation of the loss in electro-catalytic activity with the appearance of an additional La-containing Sr-oxide species at the LSC surface. This additional Sr-oxide species preferentially covers electrochemically active Co sites at the surface, and thus very effectively decreases the oxygen exchange performance of LSC. Formation of precipitates, in contrast, was found to play a less important role for the electrochemical degradation of LSC.

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.

The Chemical Capacitance as a Fingerprint of Defect Chemistry in Mixed Conducting Oxides.

The oxygen stoichiometry of mixed conducting oxides depends on the oxygen chemical potential and thus on the oxygen partial pressure in the gas phase. Also voltages may change the local oxygen stoichiometry and the amount to which such changes take place is quantified by the chemical capacitance of the sample. Impedance spectroscopy can be used to probe this chemical capacitance. Impedance measurements on different oxides ((La,Sr)FeO3-δ = LSF, Sr(Ti,Fe)O3-δ = STF, and Pb(Zr,Ti)O3 = PZT) are presented, and demonstrate how the chemical capacitance may affect impedance spectra in different types of electrochemical cells. A quantitative analysis of the spectra is based on generalized equivalent circuits developed for mixed conducting oxides by J. Jamnik and J. Maier. It is discussed how defect chemical information can be deduced from the chemical capacitance.

Operando X-ray Investigation of Electrode/Electrolyte Interfaces in Model Solid Oxide Fuel Cells.

We employed operando anomalous surface X-ray diffraction to investigate the buried interface between the cathode and the electrolyte of a model solid oxide fuel cell with atomic resolution. The cell was studied under different oxygen pressures at elevated temperatures and polarizations by external potential control. Making use of anomalous X-ray diffraction effects at the Y and Zr K-edges allowed us to resolve the interfacial structure and chemical composition of a (100)-oriented, 9.5 mol % yttria-stabilized zirconia (YSZ) single crystal electrolyte below a La0.6Sr0.4CoO3-δ (LSC) electrode. We observe yttrium segregation toward the YSZ/LSC electrolyte/electrode interface under reducing conditions. Under oxidizing conditions, the interface becomes Y depleted. The yttrium segregation is corroborated by an enhanced outward relaxation of the YSZ interfacial metal ion layer. At the same time, an increase in point defect concentration in the electrolyte at the interface was observed, as evidenced by reduced YSZ crystallographic site occupancies for the cations as well as the oxygen ions. Such changes in composition are expected to strongly influence the oxygen ion transport through this interface which plays an important role for the performance of solid oxide fuel cells. The structure of the interface is compared to the bare YSZ(100) surface structure near the microelectrode under identical conditions and to the structure of the YSZ(100) surface prepared under ultrahigh vacuum conditions.

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.