Katsuhiko Yamaji

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.

Effect of SO2 Concentration on Degradation of Sm0.5Sr0.5CoO3 Cathode

A trace amount of SO2 in air is not a negligible factor on degradation of SOFC cathode. SO2 acts like Cr vapors which cause Cr-poisoning, and easily reacts with the strontium component in lanthanum-cobaltite based SOFC cathodes. Effects of SO2 concentration on degradation of an Sm0.5Sr0.5CoO3 (SSC) cathode were investigated in order to evaluate degradation of SSC cathode in an accelerated mode. Three different concentrations of SO2 (100 ppm, 1 ppm, and 5 ppb) were supplied to the cathode with air during cell tests at T = 1073 K Degradation of SSC cathode was accelerated with increasing SO2 concentration. In the thin SO2 conditions (1 ppm and 5 ppb SO2), SO2 in air was captured on the surface of the SSC cathode. SO2 reacted with the strontium component in the SSC cathode to form SrSO4. In the thick SO2 condition (100 ppm SO2), SO2 reacted with the strontium and samarium components. Too excess SO2 concentration in air was not suitable to examine cathode degradation in an accelerated mode because other degradation modes were appeared. One ppm - SO2 in air was applicable to accelerate the degradation of SSC cathode.

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.

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.

Chemical stability of the La0.9Sr0.1Ga0.8Mg0.2O2.85 electrolyte in a reducing atmosphere

Abstract Chemical Stability of La 0.9 Sr 0.1 Ga 0.8 Mg 0.2 O 2.85 as an SOFC electrolyte was investigated in humidified hydrogen at 1273 K. After annealing at 1273 K, significant changes were observed in the surface morphology of the electrolyte, and La(OH) 3 , LaSrGaO 4 and some unknown phases were formed, which can be interpreted as a result of the vaporization of Ga components such as Ga 2 O. After annealing at 1073 K, changes in the morphology were small, but a slight amount of Ga depletion was also observed. The existence of Pt enhances the Ga depletion from the electrolyte in the reducing atmosphere probably because of the formation of Pt–Ga alloy.

Evaluation of Fe-Cr Alloys as Interconnects for Reduced Operation Temperature SOFCs

The stability of Fe-Cr alloys was investigated in H 2 -H 2 O atmosphere at 1073 K for solid oxide fuel cell (SOFC) interconnects. Oxide scale layers formed at the surface of Fe-Cr alloys changed the surface morphology of the alloy. The phases identified by X-ray diffraction were spinel oxides and Cr 2 O 3 at the surface. The diffusion profiles of several major and minor elements were measured by secondary ion mass spectrometry in its depth profile mode. The distribution of cations was in the following order from the free surface: Mn-, Fe-, La-rich layer → Cr-rich layer → Si-rich layer → Al 2 O 3 inner oxides and alloy. The growth rate of the scale layer was determined from the thickness of the scale with a parabolic growth relationship: x 2 (μm 2 )=3.6 X 10 -6 (μm 2 s -1 )×t(s).

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.

Stability of Fe-Cr alloy interconnects under CH4-H2O atmosphere for SOFCs

The chemical stability of Fe-Cr alloys (ZMG232 and SUS430) was examined under humidified CH 4 gases at 1073 K to simulate the real anode atmosphere in SOFC operation. Surface microstructure change and oxide scale layer formation were observed on the oxidized Fe-Cr alloy surfaces. The main reaction products were Mn-Cr-(Fe) spinels for both alloys. Secondary ion mass spectrometry (SIMS) was applied to measure the elemental distribution of minor and major elements around the oxide scale/alloy interface. A high concentration of Mn on the oxide scale surface suggested the fast diffusion of Mn in the oxide scale to form the spinels. Annealing in CH 4 -H 2 O made the oxide scale thicker with duration time on the alloy surface. The parabolic growth rates (k p ) of oxide scale layer were evaluated from the thickness of oxide scales by secondary ion mass spectrometry (SIMS) depth profiles, which were calculated to the following: k p = 6.25 x 10 -6 μm 2 /s for SUS430 and kp = 4.42 x 10 -6 μm 2 /s for ZMG232. The electrical conductivity of oxidized alloys showed the semi-conductor temperature dependence for both alloys. The electrical conductivity of oxidized ZMG232 alloy was higher than that of oxidized SUS430.

Application of Electrophoretic Deposition Technique to Solid Oxide Fuel Cells

The technological feasibility of applying electrophoretic deposition to solid oxide fuel cells was investigated by making small tubular cathode substrates and cathode/electrolyte/anode multilayers. A small tube made of La{sub x}MnO{sub 3{+-}d}(LM), (La{sub 0.7}Sr{sub 0.3}){sub 0.9}MnO{sub 3{+-}d}, or (La{sub 0.85}Sr{sub 0.15}){sub 0.95}MnO{sub 3{+-}d} was obtained by depositing lanthanum manganite on graphite from an iodine added acetone bath or an isopropanol bath. The graphite was oxidized and removed during sintering, resulting in a tubular LM with one end closed. A multilayer consisting of porous LM/dense YSZ/porous NiO-YSZ was obtained by deposition of yttria-stabilized zirconia on presintered LM and subsequent codeposition of NiCO{sub 3} powder and YSZ powder on the YSZ-deposited LM. Finally, this LM(presintered)/YSZ(as-deposited)/NiCO{sub 3}-YSZ(as deposited) layer was cofired.