Tatsuya Kawada

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

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.

Oxygen reduction sites and diffusion paths at La0.9Sr0.1MnO3−x/yttria-stabilized zirconia interface for different cathodic overvoltages by secondary-ion mass spectrometry

Oxygen reduction active sites were investigated at the interface of O2/La0.9Sr0.1MnO3−x (LSM)/yttria-stabilized zirconia (YSZ) for three different overvoltages of cathodic polarization (η=−0.336 V, η=−0.185 V, and η=−0.090 V versus reference electrode). Isotopic oxygen (16O/18O) exchange under cathodic polarization and secondary ion mass spectrometry (SIMS) technique were examined to visualize the oxygen incorporation/reduction active sites. The LSM mesh pattern electrode was prepared to define the area and length of contact between LSM mesh and YSZ. Under cathodic polarization of η=−0.336 V, oxide ions can diffuse through the LSM to take part in the electrode reaction. From the SIMS images of YSZ surface, the active sites for oxygen reduction/incorporation are distributed in spots at the position where the LSM mesh was attached. Especially, O2/LSM/YSZ three phase boundary (TPB) is the main active site for oxygen incorporation/reduction, although some spots of high 18O concentration were observed at the center of the LSM mesh part. Under the lower polarization (η=−0.185 V, and η=−0.090 V), oxide ions did not diffuse through the LSM mesh. SIMS imaging analysis of YSZ surface showed high 18O concentration spots around the center of the LSM mesh part. The distribution of ionic currents at the surface of YSZ affects the distribution of active sites for oxygen incorporation.

Oxygen tracer diffusion coefficient of (La, Sr)MnO3 ± δ

Abstract To understand the mechanism of oxygen diffusion, the oxygen tracer diffusion coefficient, D o ∗ , of strontium-doped lanthanum manganites, La1 − xSrxMnO3 ± δ (x = 0.05, 0.10, 0.15 and 0.20) was measured as a function of composition, temperature and oxygen partial pressure. The measured D o ∗ was as low as 10−12 to 10−11 cm2 s−1 at 1000 °C, from which the ionic conductivity of 10−7 to 10−6 S/cm was estimated. Because of such a poor ionic conductivity, the possibility for the bulk diffusion of oxide ions to contribute to the cathode reactions in solid oxide fuel cells is considered to be sparse. The negative dependence of D o ∗ on the oxygen partial pressure suggests the vacancy mechanism for oxygen diffusion.

Reaction between solid oxide fuel cell materials

Abstract In fabrication processes of solid oxide fuel cells, high-temperature heat treatments cannot be avoided. It will give rise to mutual reaction and interdiffusion of the cell component materials: yttria-stabilized zirconia (YSZ, electrolyte), (La, Sr)MnO 3 (cathode), Ni-YSZ cermet (anode) and (La, Ca)CrO 3 (separator). Reactivity of LaMnO 3 and YSZ was estimated by thermodynamic calculations, and it was found that the nonstoichiometry at La site in LaMnO 3 plays an important role on the reaction. Diffusion of Mn into YSZ leads to increase of La activity at the interface and promotes the reaction. Electrical conductivity of YSZ decreases when Mn dissolves in the cubic phase of YSZ. Oxidation state of the dissolving Mn varies with partial pressure of oxygen and affects the electrical properties of YSZ. Migration of Ca from (La, Ca)CrO 3 separator to other cell components is one of the largest problems in the co-firing cell fabrication process because it prohibits the sintering of the separator.

Imaging of oxygen transport at SOFC cathode/electrolyte interfaces by a novel technique

Oxygen transport was examined around the O2/cathode/yttria-stabilized zirconia (YSZ) interfaces in solid oxide fuel cells (SOFCs). To visualize the kinetics of oxygen transport, isotope oxygen exchange ( 16 O/ 18 O exchange) and secondary ion mass spectrometry analysis (SIMS) were adopted for La0.85Sr0.15MnO3-mesh and gold (Au)-mesh cathodes. The mesh cathode surfaces promoted oxygen adsorption and surface oxygen exchange. Oxygen bulk diffusion in the mesh cathode was observed only at the La0.85Sr0.15MnO3-mesh. The active sites for oxygen incorporation were compared at the YSZ surfaces after removing the mesh cathodes. The La0.85Sr0.15MnO3-mesh/YSZ interface can be the active site for oxygen incorporation as well as the triple phase boundary (TPB). On the other hand, the Au/YSZ interface showed low 18 O concentration, and the active sites were limited to the TPB lines. Most active sites for oxygen incorporation were the O 2/cathode/YSZ interface (TPB) for La0.85Sr0.15MnO3-mesh and Au-mesh from the line analysis of the SIMS images. The expansion of the oxygen incorporation active site around the TPB was discussed on the basis of SIMS imaging analysis and electrochemical analysis. # 2002 Elsevier Science B.V. All rights reserved.