Natsuko Sakai

Thermodynamic stabilities of perovskite oxides for electrodes and other electrochemical materials

Abstract We report the chemical thermodynamic approach of examining of stabilities of electrodes, interconnections and electrolytes. The stability of conductive complex oxides can be discussed in terms of (1) thelattice stability of particular structures as a function of composition, (2) the valence stability of transition metal ions against oxidative or reductive decompositions, (3) the abilities of the formation of lattice defects, and (4) the chemical stability against reactions with other components and with gaseous substances. Although these properties exhibit complicated variations from oxide to oxide, the fundamental energetics related to these stabilities were found to be relatively simple across the transition metal series. This simplicity will make it possible to gain an insight into the stability issues by using, as a good chemical scale, the ionic radii of transition metal ions. After a chemical thermodynamic representation of these materials, chemical reactions at their interfaces can be thermodynamically analyzed by chemical equilibria calculations and construction of chemical potential diagrams.

Active Sites Imaging for Oxygen Reduction at the La0.9Sr0.1MnO3 − x /Yttria‐Stabilized Zirconia Interface by Secondary‐Ion Mass Spectrometry

Active sites for oxygen reduction were investigated at the interface of O{sub 2}/La{sub 0.9}Sr{sub 0.1}MnO{sub 3{minus}x} (LSM)/yttria-stabilized zirconia (YSZ). Isotopic oxygen ({sup 16}O/{sup 18}O) exchange under cathodic polarization and secondary-ion mass spectrometry (SIMS) analysis were examined to visualize the oxygen reduction active sites. The LSM mesh pattern electrode was prepared to define the contact area of LSM/YSZ. Under cathodic polarization, oxygen can diffuse through the dense LSM via oxygen vacancy, which promotes the electrode reaction. By SIMS imaging technique, the active sites for oxygen reduction were clearly determined as spots around the O{sub 2}/LSM/YSZ three-phase boundary. The line analysis of the SIMS image enabled the authors to draw a contour map of the {sup 18}O concentration in the cross section of YSZ. The diffusion paths were clearly visualized in the contour map. The width of the active sites for oxygen reduction is estimated to be less than 1 {micro}m under the examined condition.

Characteristics of Slurry‐Coated Nickel Zirconia Cermet Anodes for Solid Oxide Fuel Cells

Polarization characteristics were studied in nickel and yttria stabilized zirconia cermet (Ni-YSZ cermet) electrodes in order to search for an adequate fabrication condition of a planar solid oxide fuel cell. Several electrodes were prepared by the slurry coating method, and dc and ac polarization behaviors were investigated at 1273 K in H 2 -H 2 O atmosphere. An experimental equation was derived to describe a typical dc polarization curve of a Ni-YSZ cermet electrode. Effects of fabrication temperature, nickel content, and material powder were investigated in terms of deviation from this equation. Anodic overvoltage with a carefully prepared electrode was decreased to about 0.12 V at 1 A/cm 2

Sinterability and electrical conductivity of calcium-doped lanthanum chromites

Calcium-doped lanthanum chromites, (La1−xCax) (Cr1−y Cay O3, have been synthesized to investigate effects of calcium doping on sinterability and electrical conductivity. X-ray diffractometric results have revealed that in addition to normal perovskites (La1−xCaxCrO3), chromium-deficient perovskites can exist as a single phase in the composition region 0.1 <x < 0.3, although the deficit of chromium is small. These chromium-deficient perovskites show a good sinterability even in air at 1873 K. Electrical conductivity of these perovskites has been measured as functions of temperature and oxygen potential. It has been found that electrical conductivity of the chromium-deficient perovskites increases almost linearly with total calcium content. The magnitudes of electrical conductivity are comparable to those of strontium-doped lanthanum chromites.

Chemical Thermodynamic Considerations in Sintering of LaCrO3 ‐ Based Perovskites

This paper reports on the thermodynamic properties of LaCrO{sub 3} and related compounds estimated and utilized to correlate sinterability with phase relations in the La-M-Cr-O (M = Mg, Ca, Sr) systems. Chemical equilibria calculations reveal that calculated vapor pressures of gaseous chromium oxides (especially CrO{sub 3}) can be well correlated with our previous experimental results on sinterability. The poor sinterability can be ascribed to the formation of a thin layer of Cr{sub 2}O{sub 3}(s) which is formed from incongruently vaporized CrO{sub 3}(g) at the interparticle neck during the initial stage of sintering in air. Improvement of sinterability of Cr{sub 2}O{sub 3}-based oxides can be achieved by removing this Cr{sub 2}O{sub 3} layer or by decreasing CrO{sub 3}(g) vapor pressure to prevent the formation of Cr{sub 2}O{sub 3} layer. These suggest that (La{sub 1{minus}x}Ca{sub x})CrO{sub 3} perovskites are the most suitable materials for high sinterability, since these materials have the low CrO{sub 3} vapor pressure without precipitation of La{sub 2}O{sub 3}.

Oxygen permeation modelling of perovskites

A point defect model was used to describe the oxygen nonstoichiometry of the perovskites La0.75Sr0.25CrO3, La0.9Sr0.1FeO3, La0.9Sr0.1CoO3 and La0.8Sr0.2MnO3 as a function of the oxygen partial pressure. Form the oxygen vacancy concentration predicte by the point defect model, the ionic conductivity was calculated assuming a vacancy diffusion mechanism. The ionic conductivity was combined with the Wagner model for the oxidation of metals to yield an analytical expression for the oxygen permeation current density as a function of the oxygen partial pressure gradient. A linear boundary condition was used to show the effect of a limiting oxygen exchange rate at the surface.

Thermodynamic analysis of reaction profiles between LaMO3 (M=Ni,Co,Mn) and ZrO2

This paper discusses chemical reactions of LaMO{sub 3}(M = Ni, Co, Mn) and ZrO{sub 2} which have been analyzed using chemical potential diagrams for the La-Zr-M-O systems under a condition of P(O{sub 2}) = 1 bar. A log a(M)/a(Zr) vs. loga(La)/a(Zr) plot is appropriate for representing diffusion paths, because the layered arrangement of the reaction zone corresponds well to the geometrical configuration of stability polygons of reactants (LaMO{sub 3} and ZrO{sub 2}) and products (La{sub 2}Zr{sub 2}O{sub 7}, MO, La{sub 2}ZrMO{sub 6}, etc.). a complicated arrangement including a ternary perovskite phase, La{sub 2}ZrNiO{sub 6}, observed in the La{sub 2}NiO{sub 4}/YSZ couple can be represented by a simple diffusion path having the same slope in the chemical potential diagram as in the LaCoO{sub 3}/YSZ couple. The effect of lanthanum nonstoichiometry for La{sub y}mnO{sub 3} has been discussed in relation to La{sub 2}Zr{sub 2}O{sub 7} formation and manganese dissolution into YSZ cubic phases.

Low temperature fabrication of (Y,Gd,Sm)-doped ceria electrolyte

A general procedure based on the oxalate coprecipitation route was developed to produce very sinterable ceria (CeO2) powder doped with Gd2O3, Sm2O3 or Y2O3. The method is simple, reliable and very reproducible. Without any milling step, powder of 20 mol% GdO1.5, SmO1.5 or YO1.5-CeO2 could be fired translucent dense (97% relative to theoretical) at 1300 °C (4 h) already, as pressed compacts. This compares to typical values of 95% relative density obtained after firing at 1500–1600 °C in the standard literature. Cast tapes from the intensively milled powders were fired equally dense at 1400 °C (2 h), among the best results yet reported for tape cast doped ceria. Owing to their high density, excellent ionic conductivity values were observed for the doped ceria electrolytes, on the order of 5–7 S m−1 at 750 °C in air.

Oxygen reduction mechanism at porous La1−xSrxCoO3−d cathodes/La0.8Sr0.2Ga0.8Mg0.2O2.8 electrolyte interface for solid oxide fuel cells

The oxygen reduction mechanism was investigated at the porous La1−xSrxCoO3−d cathode/La0.8Sr0.2Ga0.8Mg0.2O2.8 electrolyte interface (x=0.2, 0.3, 0.4). The polarization resistance, measured from the impedance spectra, was compared in the samples of La1−xSrxCoO3−d as functions of x, temperatures, and applied DC voltages. The polarization resistance decreased with an increase of x values in La1−xSrxCoO3−d and with the applied cathodic voltage. The polarization resistance of the higher Sr-concentration in La1−xSrxCoO3−d showed the lower dependence on cathodic overpotential. The values of the activation energy of the interface conductivity (inverse of the polarization resistance) were similar for all La1−xSrxCoO3−d samples (127–143 kJ mol−1) at zero applied voltage (E=0 V). However, under cathodic polarization, the activation energy decreased as the applied voltage became more negative, which indicates a change of the reaction mechanism under cathodic polarization. Under cathodic polarization, oxide ion diffusion in the bulk La1−xSrxCoO3−d can be one of the main factors determining the reaction rates.

Recent Developments in Solid Oxide Fuel Cell Materials

Recent developments in solid oxide fuel cell (SOFC) materials and stacks have been reviewed mainly from the viewpoint of the materials chemistry associated with stack development. Firstly the general features of SOFCs are described to clarify the materials problems which need to be solved and the technology that needs to be developed. The main, characteristic features of conventional SOFC materials are described with the emphasis put on the materials problems associated with the respective materials. Then the materials problems associated with stack development are described for tubular stacks and for planar stacks with oxide interconnect and with metal interconnect. Finally, new materials, design and processing are discussed in relation to new applications of SOFCs. The emphasis is placed on the fact that these are new challenges and therefore a more fundamental strategy for materials and stack development should be constructed on the basis of the materials science and technology developed mainly for conventional materials.