Elena Konysheva

Chromium Poisoning of Perovskite Cathodes by the ODS Alloy Cr5Fe1Y2O3 and the High Chromium Ferritic Steel Crofer22APU

The alloys Cr5Fe1Y 2 O 3 and the ferritic steel Crofer22APU are typical alloys used as solid oxide fuel cell (SOFC) interconnect materials. Alloy Cr5Fe1Y 2 O 3 is an oxide dispersion strengthened (ODS) alloy developed by Plansee, Reutte, Austria, for use at high temperature. A typical material for medium-temperature SOFC, is the high chromium ferritic steel Crofer22APU supplied by Thyssen Krupp VDM, Germany. The two alloys form different oxide scales which affect chromium poisoning. Chromium vaporization as source term and electrochemical degradation of La 1-x Sr x MnO 3 (LSM) and La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3 (LSCF) describing the poisoning were studied for the two alloys. The dynamics of the chromium deposition in porous perovskite cathodes was studied by a dc method and impedance spectroscopy. Electrical degradation of the LSM cathode by alloy Cr5Fe1Y 2 O 3 was significantly higher than for Crofer22APU. The microstructure of the cells was studied after measurements by scanning and energy filtering transmission electron microscopy. Significant amounts of chromium were observed at the TPB in the functional layer of cells, with the LSM cathode giving insight into the degradation mechanism. Cells tested with the LSCF cathode clearly show Cr poisoning. Formation of large SrCrO 4 crystals was observed on the surface of the LSCF cathode.

Chromium Poisoning of the Porous Composite Cathode Effect of Cathode Thickness and Current Density

Electrochemical behavior of the (Lao 0.65 Sr 0.3 MnO 3 (LSM)/porous functional layer(FL)/8 mol % Y 2 O 3 -stabilized ZrO 2 (8YSZ) electrolyte} half cell in the presence of a Cr source was studied by dc method and impedance spectroscopy. The porous FL was made of a LSM and 8YSZ powder mixture. The degradation rate of the cell was found to depend strongly on the FL thickness. The increase in the thickness of the FL up to 13 μm leads to a significant reduction of the Cr poisoning of the cathode. The effect of high current density (up to 0.5 A/cm 2 ) on the long-term performance of half cells in the presence of chromium source was investigated. A few processes occur near the {LSM/SYSZ/gas} contact under current load in the presence of a chromium source: chemical transformations of this contact accompanied by the formation of chromium oxide under a relatively high current load, destruction of the electrolyte and penetration of chromium into the electrolyte along the grain boundaries. The chemical nature and rate of the transformations taking place near the {LSM/8YSZ/gas} contact under current load, most probably, determine the electrochemical performance of the cell for the initial operation period whereas the two other processes could contribute to the degradation during long-term performance.

On the Existence of A‐Site Deficiency in Perovskites and Its Relation to the Electrochemical Performance

The low range of A-site deficiency in perovskite structures with Ni cations was verified by neutron powder diffraction, transmission electron microscopy, and thermogravimetric analysis. A thermodynamic approach has been utilized, for the first time, to predict the extent of A-site deficiencies within the perovskite structure, introducing simple prediction criteria that could be adopted for designing advanced materials.

Crystal Structure, Oxygen Nonstoichiometry, and Conductivity of Mixed Ionic–Electronic Conducting Perovskite Composites with CeO2

Conductivity, structure, and oxygen nonstoichiometry of compositions in the following series (100 - x) La 0.6 Sr 0.4 CoO 3±δ ·xCeO 2 (LSCCe) and (100 ― x) La 0.8 Sr 0.2 MnO 3±δ ·xCeO 2 (LSMCe), where x = 0, 2, 5, and 10 mol %, are studied. CeO 2 has a low solubility limit in the rhombohedral structure. Compositions containing more than 2 mol % ceria are two-phase and consist of a perovskite constituent with rhombohedral structure and ceria with cubic structure. Small additions of CeO 2 influence the conductivity of the modified perovskites in a different way. The conductivity in the LSCCe series dramatically decreases with an addition of up to 10 mol % CeO 2 . However, the total conductivity in the LSMCe series only changes slightly. The conductivity of the perovskites as a function of oxygen partial pressure (0.21 to 1 × 10- 4 atm) is discussed. Oxygen nonstoichiometry (δ) depending on temperature and oxygen partial pressure varies in the LSCCe series in a wider range (0 ≤ δ ≤ 0.23) compared to that in the LSMCe series (0.06 ≤ δ < 0.14). In contrast to the LSMCe series, the compositions in the LSCCe series show a strong relationship between lattice parameters, oxygen nonstoichiometry, and conductivity. X-ray photoelectron spectroscopy indicates that distortion of Co-O bonds takes place, which could affect electronic conductivity of perovskites.

Red-ox behaviour in the La0.6Sr0.4CoO3±δ-CeO2 system

The compositions in the (100 − x)La0.6Sr0.4CoO3±δ-xCeO2 (LSCC) system with 5 ≤ x ≤ 76 are two-phase at room temperature. They consist of the modified perovskite with rhombohedral symmetry (Rc) and modified ceria with fluorite structure (Fmm). The cross-dissolution of La, Sr, Co and Ce cations between the initial La0.6Sr0.4CoO3±δ (LSC) and CeO2 takes place and results in the modification of the initial phases. This is particularly important for the modified ceria. The lattice parameter of the modified ceria increases due to the dissolution of La and Sr cations with larger ionic radii, thereby changing noticeably the oxygen sublattice in the fluorite structure. Above 300 °C LSCCx composites are three-phase due to the reversible change in the symmetry from rhombohedral (Rc) to cubic (Pmm) within the perovskite phase. Red-ox behaviour of the LSCC composites has been explored under air and argon atmospheres in terms of evolution of the chemical composition at the grains surface and phase interfaces, formation of oxygen vacancies and thermochemistry of this process. Reversible red-ox behaviour was observed in LSCCx with x = 8–37 most probably due to an observed high surface concentration of Co cations, that can be easily involved in the reduction/re-oxidation cycle. The increase in the surface concentration of Ce4+ cations together with the decrease in surface concentration of Co cations seems to result in the differences in the reduction and oxidation behaviour under air in LSCCx with x = 57–76. Formation of oxygen vacancies in LSC, LSCC02 and LSCCx with x = 5–76 in air was not accompanied by any distinct thermal events. This process becomes more endothermic with further increase in oxygen nonstoichiometry (δ) above certain values: δ > 0.08 in LSC, δ > 0.13 in LSCC02, and LSCC with x = 5–76. The LSCCx with x = 5–37 and with x = 57–76 show slightly different reduction behaviour under a(o2) = 7.4 × 10−5. In the composites with a relatively low CeO2 content, the extent of the reduction is proportional to the Co content in a composition, whereas the reduction of the LSCCx with x = 57–76 was more significant than expected. The changes in the enthalpy of oxygen vacancy formation and the kinetics of reduction have been discussed.

Fluctuation of surface composition and chemical states at the hetero-interface in composites comprised of a phase with perovskite structure and a phase related to the Ruddlesden–Popper family of compounds

Surfaces and interfaces in composite solids can possess particular compositions and properties, thereby governing the functions of materials. X-ray photoelectron spectroscopy (XPS) has been used for the first time to explore the cation rearrangement between the surface and bulk material of the crystallites in the two-phase composites formed in the La–Sr–Pr–Co–O system. The two-phase composites contain a perovskite phase (major fraction) and a layered perovskite-like phase (La,Sr,Pr)2CoO4±δ related to the Ruddlesden–Popper family of compounds. The difference between the surface and nominal composition was revealed for all the composites explored. The surface concentrations of Pr and La cations are lower compared to the nominal stoichiometry, suggesting their preferable dissolution into the volume of the crystallites. Both Pr3+ and Pr4+ cations coexist at the surface. The surface of all the composites is enriched in Sr cations that could exist as SrCO3, SrO2, and Sr(OH)2 individual surface phases as well as SrO on the surface of the perovskite phase. The second phase plays an important role in balancing the surface composition at the hetero-interface in the composites. The rise in the fraction of (La,Sr,Pr)2CoO4±δ in the composites changes the “surface Sr:bulk Sr” ratio, increasing the later contribution due to the stronger accommodation of Sr cations within its crystal lattice and decreasing the surface depletion of Co cations. In addition to Co3+ and Co4+ cations, a small fraction of Co2+ cations exists in the near-surface region of the composites. The surface of all the composites is strongly enriched in oxygen (by 11.8–13.3 at %). The O1s spectra of the composites contain contributions from the lattice oxygen related to the perovskite phase and layered (La,Sr,Pr)2CoO4±δ phase as well as different surface oxygen states. The effect of surface treatment on the evolution of the surface composition has been discussed.

Reduction of CeO2 in composites with transition metal complex oxides under hydrogen containing atmosphere and its correlation with catalytic activity

Reduction behavior of pure and doped CeO2, the multi-phase La0.6Sr0.4CoO3·xCeO2, La0.8Sr0.2MnO3·xCeO2, and La0.95Ni0.6Fe0.4O3·xCeO2 composites, was studied under hydrogen containing atmosphere to address issues related to the improvement of electrochemical and catalytic performance of electrodes in fuel cells. The enhanced reduction of cerium oxide was observed initially at 800°C in all composites in spite of the presence of highly reducible transition metal cations that could lead to the increase in surface concentration of oxygen vacancies and generation of the electron enriched surface. Due to continuous reduction of cerium oxide in La0.6Sr0.4CoO3·x-CeO2 and La0.8Sr0.2MnO3·xCeO2 (up to 10 h) composites the redox activity of the Ce4+/Ce3+ pair could be suppressed and additional measures are required for reversible spontaneous regeneration of Ce4+. After 3 h exposure to H2-Ar at 800°C the reduction of cerium oxides and perovskite phases in La0.95Ni0.6Fe0.4O3·xCeO2 composites was diminished. The extent of cerium oxide involvement in the reduction process varies with time, and depends on its initial deviation from oxygen stoichiometry (that results in the larger lattice parameter and the longer pathway for O2− transport through the fluorite lattice), chemical origin of transition metal cations in the perovskite, and phase diversity in multi-phase composites.

Effect of Minor Additions of CeO2 on Conductivity of Perovskites with Mixed Ionic-Electronic Conductivity

Conductivity, structure and thermochemical stability of compositions in the following series (100-x)La0.6Sr0.4CoO3•xCeO2 (LSCC), (100-x)La0.8Sr0.2MnO3•xCeO2 (LSMC) and (100-x)La0.95Ni0.6Fe0.4O3•xCeO2 (LNFC), where x= 0, 2 and 10 mol%, were studied. CeO2 has slightly higher solubility limit in La0.6Sr0.4CoO3 and La0.8Sr0.2MnO3 compared to La0.95Ni0.6Fe0.4O3, forming LSCC2 and LSMC2 single phase solid solutions. All compositions containing more than 2 mol% ceria were two phase and consisted of a perovskite constituent with rhombohedral structure and ceria with cubic structure. Small additions of CeO2 influence the conductivity of the modified perovskites in a different way. Conductivity in the LSCC series dramatically decreased. Adding up to 10 mol % CeO2 slightly changed the total conductivity in the LSMC series in spite of the presence of the secondary phase. The conductivity of the perovskites as a function of oxygen partial pressure (0.21-10-4 atm) is discussed.