Werner Sitte

Stability of the SOFC Cathode Material ( Ba , Sr ) ( Co , Fe ) O3 − δ in CO2-Containing Atmospheres

The stability of the solid oxide fuel cell (SOFC) cathode material Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ in CO 2 -containing atmospheres (4 X 10 -4 ≤ pCO 2 /bar ≤ 5 × 10 -2 ) is investigated by precision thermogravimetry (TG) and mass spectrometry (MS) as a function of temperature (20 ≤ T/°C ≤ 950). The desorption of O 2 and CO 2 from samples with different pretreatments is compared. Oxygen exchange at 300 < T/°C < 700 is significantly impaired by CO 2 -containing atmospheres. At 600°C the kinetics of carbonate formation in CO 2 -rich atmosphere is described by a linear-parabolic rate law. A pronounced dependence of the rate constant on the CO 2 content is suggested. Temperature cycles at 0 s pC0 2 /bar ≤ 5 X 10 -2 and pO 2 = 0.2 bar, which show effects due to oxygen exchange of the perovskite and due to CO 2 , are analyzed based on reference experiments in a CO 2 -free atmosphere, and under consideration of the TG-MS results. The decomposition temperature of the carbonate in contact with an atmosphere of pCO 2 = 5 X 10 -2 bar amounts to 807°C. Complementary, the impact of carbonate formation on the oxygen exchange kinetics is investigated by conductivity relaxation measurements. A severe degradation of the surface oxygen exchange coefficient is observed after 3-10 days of exposure to ambient air. Regeneration and activation of the oxygen exchange kinetics is demonstrated after treatment in a CO 2 -free atmosphere at 825°C.

Nonstoichiometry and transport properties of strontium-substituted lanthanum cobaltites

Abstract In La1−xSrxCoO3−δ (LSC), the oxygen nonstoichiometry and transport properties change with variations in Sr content, temperature and oxygen partial pressure. The purpose of this study was to investigate and correlate these parameters. The extent of oxygen deficiency was investigated by oxygen exchange measurements over the range from 300 to 900 °C and 1.9×10−2⩾pO2⩾1.0×10−4 bar. The oxygen deficiency was found to increase with decreasing oxygen partial pressure and/or increasing temperature. The chemical diffusion coefficient and the surface oxygen exchange coefficient were determined simultaneously by fitting transient re-equilibration curves to the respective diffusion equation at T=725 °C. The electronic conductivity was studied as a function of temperature and oxygen partial pressure at 1.9×10−2≤pO2≤1.0×10−4 bar. Furthermore, these data were correlated with those of the nonstoichiometry to yield conductivity values for constant oxygen content. In the case of La0.6Sr0.4CoO3−δ (LSC04), the temperature dependence of the isostoichiometric conductivities yields activation energies of 0.02≤Ea≤0.05 eV.

Fast grain boundary diffusion and rate-limiting surface exchange reactions in polycrystalline materials

Analytical solutions to the diffusion equations for fast grain boundarydiffusion in polycrystalline solids of finite thickness bounded by two parallel surface planes are presented. The grain boundarydiffusion coefficient may be higher by several orders of magnitude than the bulk (volume) diffusion coefficient. The microstructure of the polycrystalline sample has been modeled by means of an isolated grain boundary as well as an array of parallel grain boundaries. The surface concentration has been assumed to vary continuously during the diffusion process, as the surfaceexchange reactions of the diffusing species are considered to be fairly slow and equilibrium between the surface and the constant diffusion source (gas phase) is not attained. Two different cases for surfacediffusion at the interfacepolycrystalline sample/diffusion source are taken into account, viz. negligible and fast surfacediffusion. The pertinent analytical solutions allow the calculation of two-dimensional diffusion profiles in thin films and average concentration profiles for semi-infinite diffusion systems. In addition, the time dependence of the total amount of diffusing species exchanged between the diffusion source and the solid sample has been calculated. The theoretical results are interpreted in terms of Harrison’s classification of diffusion regimes. The parallel grain boundary model enables the calculation of diffusion profiles for both type-A and type-B kinetics. In the case of type-A kinetics expressions for the effective diffusion coefficient as well as the effective surface exchange coefficient are proposed, which depend on the corresponding bulk values, the microstructure of the specimen, and the respective surface condition.

Oxygen tracer diffusion in donor doped barium titanate

Oxygen exchange with the ambient atmosphere and oxygen diffusion are assumed to play a decisive role in the re-oxidation process of positive temperature coefficient (PTC) resistors based on donor doped barium titanate. 18O tracer experiments with subsequent time-of-flight secondary ion mass spectrometry (ToF-SIMS) measurements were thus carried out to investigate the oxygen diffusion properties of donor doped barium titanate. Fast grain boundary diffusion was found at temperatures between 750 °C and 900 °C. Moreover, evidence is given for a position dependent diffusion coefficient close to the surface. The secondary phase developing during the production process is shown to be Ti-rich and hardly any oxygen tracer exchange with this secondary phase could be observed. This suggests that grain boundary diffusion does not take place via such secondary phases. Rather, evidence of diffusion along an oxygen vacancy enriched space charge region is found.

Microstructure and ionic conductivity of strontium-substituted lanthanum cobaltites

Abstract La 0.4 Sr 0.6 CoO 3− δ powders were synthesized by the glycine nitrate process. X-ray powder diffraction and selected area electron diffraction (SAED) in the transmission electron microscope (TEM) were used to determine the basic crystal structure of the perovskite samples. Additionally, energy-filtering transmission electron microscopy (EFTEM) was used to evaluate the homogeneity of the samples at the nanometer level. The ionic conductivity σ i of La 0.4 Sr 0.6 CoO 3− δ was obtained from galvanostatic polarization experiments as a function of oxygen non-stoichiometry for 3− δ values between 2.70 and 2.81 (corresponding to p O 2 values between 10 −4 and 10 −1 bar) at 775 and 825 °C. It could be observed that σ i shows a maximum which shifts towards larger values of the oxygen non-stoichiometry with increasing temperature. The maximum value was observed at 3− δ =2.79 and 2.77 for 775 and 825 °C, respectively. This behavior has been reported to be indicative of the formation of vacancy-ordered structures which are expected to lower the mobility of oxygen vacancies. The TEM investigations revealed a superstructure within microdomains which were crystallographically oriented perpendicular to each other and were found to be around 100 nm in size.

Oxygen Exchange Kinetics of the IT-SOFC Cathode Material Nd2NiO4+δ and Comparison with La0.6Sr0.4CoO3-δ

For the promising intermediate temperature solid oxide fuel cell (IT-SOFC) cathode material Nd 2 NiO 4+δ , chemical surface exchange coefficients k chem and chemical diffusion coefficients D chem of oxygen have been determined by conductivity relaxation measurements and compared with results for La 0.6 Sr 0.4 CoO 3-δ between 575 and 725°C. At 725°C and an oxygen partial pressure of 0.1 bar, k chem and D chem of Nd 2 NiO 4+δ amount to 1×10 -3 cm s -1 and 2 x 10 -6 cm 2 s -1 , respectively, which are higher than those for La 0.6 Sr 0.4 Co0 3-δ . However, due to high activation energies, a strong decrease of both kinetic parameters is observed for Nd 2 NiO 4+δ upon temperature reduction. Activation energies of k chem and D chem are lower for La 0.6 Sr 0.4 CoO 3-δ , leading to faster oxygen exchange compared to Nd 2 NiO 4+δ at 600°C. Electronic conductivities of Nd 2 NiO 4+δ amount to 100-125 S cm -1 while those of La 0.6 Sr 0.4 CoO 3-δ are between 1600 and 2200 S cm -1 in the investigated temperature and oxygen partial pressure range. Ionic conductivities and surface exchange resistances, which were calculated from the kinetic parameters, further show superior oxygen transport properties of La 0..6 Sr 0.4 CoO 3-δ compared to Nd 2 NiO 4+δ in pure O 2 -Ar atmospheres.

Long-Term Oxygen Exchange Kinetics of Nd2NiO4 + δ in H2O - and CO2-Containing Atmospheres

The degradation of the solid oxide fuel cell cathode material Nd 2 NiO 4+δ with respect to the oxygen exchange kinetics in H 2 O- and CO 2 -containing atmospheres has been investigated at 700°C. The material exhibits excellent stability in dry Ar + 1% O 2 atmosphere over 1000 h. No appreciable degradation has been observed after exposure to a CO 2 -rich atmosphere, but a decrease in the chemical surface exchange coefficient of oxygen by 1 order of magnitude has been found after 1000 h under moist conditions (30% relative humidity). Post-test X-ray photoelectron spectroscopy depth analysis showed significant changes in the Nd:Ni cation ratio within the topmost 100 nm of the H 2 O-degraded sample. It is concluded that the degradation mechanism involves hydroxides as intermediate species even if the predominant species in the degraded surface is NdNiO 3 .

Electrochemical polarization measurements on mixed conducting oxides

Abstract A solid-state galvanostatic polarization method is presented for the simultaneous determination of the chemical diffusion coefficient D and the ionic conductivity σ i of mixed conducting oxides as a function of nonstoichiometry. The method is especially designed for oxides which react with the solid electrolyte. With the symmetrical electrochemical cell used, the sample is located between two ionic electrodes separated by thin porous platinum interlayers to avoid any side reactions. Polarization measurements are applied to homogeneous samples of La 0.4 Sr 0.6 CoO 3− δ in the p O 2 range between 10 −4 and 10 −1 bar (3− δ =2.68–2.81) at 825 °C. D decreases with increasing oxygen content. σ i shows a maximum at 3− δ =2.77 and decreases with decreasing oxygen content indicating the possible formation of vacancy ordered structures. Values for D from the polarization measurements at T =825 °C and from oxygen exchange measurements at the same temperature are in good agreement with each other.

Electrical properties of grain boundaries in interfacially controlled functional ceramics

The mass and charge transport properties as well as structure and composition of grain boundaries in interfacially controlled electroceramic materials are reviewed. Recent advances in the field of donor doped (n-conducting) barium titanate are emphasized, comprising experimental methods such as the characterization of the structure and composition of grain boundaries, the experimental determination of the electrical properties as a function of temperature, oxygen partial pressure and dc bias, the investigation of diffusion along grain boundaries involving conductivity relaxation experiments and 18O - tracer exchange measurements. In addition, phenomenological modeling of transport properties of grain boundaries in donor doped BaTiO3 are outlined in detail, encompassing finite element simulations of diffusion processes as well as modified Schottky barrier models which are suitable for the calculation of the electrical properties as a function of temperature and dc bias.

Thermodynamic modeling of the Cr – S system

Abstract All known phases of the chromium – sulfur binary system have been taken into consideration within a thorough thermodynamic analysis of phase equilibria and thermodynamic data at 1 bar total pressure over the entire composition range between 25 °C and temperatures above the liquidus. Gibbs energy modeling for ten phases has been performed. A limited set of model quantities were optimized by which the experimental data can be reproduced simultaneously within experimental limits of error. New interpretations of complex phase relations are obtained. The extended modified quasichemical model is applied for the liquid chromium – sulfur phase. The Gibbs energy of high-temperature chromium pyrrhotite solution is described by a two-sublattice model within the framework of the compound energy formalism. The sulfur solubility of solid chromium is modeled using a substitutional approach. The Gibbs energies of six stoichiometric compounds are also modeled. The first internally consistent description of all thermodynamically stable phases known in the literature for the chromium – sulfur system is presented.