The alloys Cr5Fe1Y 2 O 3 and the ferritic steel Crofer22APU are typical alloys used as solid oxide fuel cell ͑SOFC͒ interconnect materials. Alloy Cr5Fe1Y 2 O 3 is an oxide dispersion strengthened ͑ODS͒ 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 ͑LSM͒ and La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3 ͑LSCF͒ 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.

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.

Electrochemical behavior of the ͕La 0.65 Sr 0.3 MnO 3 ͑LSM͒/porous functional layer͑FL͒/8 mol % Y 2 O 3 -stabilized ZrO 2 ͑8YSZ͒ electrolyte͖ 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 m 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/8YSZ/gas͖ 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/gas͖ 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.Strontium-doped lanthanum manganite ͑LSM͒ is used as cathode material in solid oxide fuel cells ͑SOFCs͒. It was shown earlier 1 that mixing of LSM and Y 2 O 3 -stabilized ZrO 2 ͑YSZ͒ allows the enhancement of the electrochemical performance of the cathode. Different measures for optimization of the cathode performance ͑varia-tion of LSM/YSZ ratio, electrode thickness, microstructure, and production methods͒ were introduced. 2-12 Recently, noticeable progress has been achieved in this area. The symmetric or anodesupported single cells with the double-layered porous cathode made of an A-site deficient LSM layer and a composite ͑LSM-8YSZ͒ functional layer show good electrical behavior and low aging rate at 800°C without a chromium source. 12-14 However, the degradation of such cells is significant in the presence of chromium containing alloys 13,14 which are generally considered as interconnects 15,16 in planar SOFC stacks.The chromium poisoning of LSM cathodes with different levels of Sr-doping and A-site deficiency was investigated by several research groups using different materials as a chromium source ͑ODS alloy, Crofer22APU, SUS430, Inconel 600͒. 14,17-25 The loss of cathodic activity was demonstrated in all cases. However, discrepancies in the degradation of the electrochemical performance and in the cathode microstructure were reported. Different degradation mechanisms were therefore proposed. For instance, the total polarization resistance of the LSM cathode in combination with 3 mol % Y 2 O 3 doped zirconia electrolyte without a composite layer strongly increased over 4-16 h at temperatures of 900-1000°C with air flow under current load and was accompanied by the formation of Cr, Mn spinel crystals. [17][18][19][20] The aut...

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The thermochemical and structural stability of complex perovskites A1-x A′ x B1-y-z B′ y B′′ z O3±δ (A, A′ = La, Sr and B, B′, B′′ = Ni, Mn, Fe, Co) was explored in H2−Ar atmosphere (5% H2−95% Ar) in a wide temperature range by thermogravimetric analysis, differential thermal analysis, XRD, and HRTEM. All perovskites showed good thermochemical stability in a temperature range of 25−300 °C. Reduction of the perovskites occurs at temperatures higher than 300 °C and can be interpreted as a multistep process. At the initial stage of exposure to H2−Ar, a small weight gain was observed. This might indicate direct sorption of hydrogen into the lattice, forming hydride−oxide phases. On the other hand, the oxide lattice could reduce to form water, and then, the evolved water is reincorporated into the lattice to give a small weight gain. This is followed by dramatic weight loss. Water was found to be the main gaseous product formed during reduction. Complex perovskites, depending upon composition, rapidly lose up to 6−12 mol % of the lattice oxygen, which is accompanied by phase or structural transformations in the solid. Further mechanism and kinetics of reduction strongly depend on temperature. The rate of reduction at intermediate temperatures (500−700 °C) becomes slow, probably due to a local stabilization of La(OH)3 in extremely humidified hydrogen-containing atmosphere. The complete reduction of perovskites can occur at 800 °C. On long-term annealing, the perovskite containing three transition elements and Sr on the B and A sublattices, respectively, showed better thermochemical stability in hydrogen-containing atmosphere. The results suggest that the presence of structural defects and their mobility in the oxygen sublattice are important factors determining the thermochemical stability of perovskites.