T. Kawada

Determination of Oxygen Vacancy Concentration in a Thin Film of La0.6Sr0.4CoO3 − δ by an Electrochemical Method

Equivalent circuit analysis was undertaken on a mixed conductor electrode/solid oxide electrolyte system. In a limited case where surface reaction is the predominant rate-controlling process, the equivalent circuit was simplified to parallel connection of a resistor (surface reaction resistance) and a capacitor (chemical capacitance due to oxygen nonstoichiometry). Equilibrium oxygen vacancy concentration was correlated with the chemical capacitance. The model was applied to a dense film of La 0.6 Sr 0.4 CoO 3-δ deposited on a sintered plate of Ce 0.9 Gd 0.1 O 1.95 by a laser ablation method. Frequency response of the electrochemical impedance was measured under dc bias at 873-1073 K in O 2 -Ar gas mixtures. The observed capacitance was extremely large, e.g., around 0.1 to I F cm -2 for the 1.5 μm thick film. The oxygen vacancy concentration in the film was calculated from the capacitance and compared with the literature data measured by thermogravimetry. The film was found to show smaller oxygen nonstoichiometry. The enthalpy for oxygen vacancy formation in the film was about 40 kJ mol -1 larger than the bulk.

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.

Oxygen isotope exchange with a dense La0.6Sr0.4CoO3−δ electrode on a Ce0.9Ca0.1O1.9 electrolyte

Abstract Oxygen isotope exchange experiments were carried out with a dense La0.6Sr0.4CoO3−δ film (0.5 μm thick) deposited on a Ce0.9Ca0.1O1.9 substrate by a laser ablation method. The isotope exchange profile was measured from the surface into the electrolyte by a secondary ion mass spectrometer (SIMS). The oxygen diffusion through the La0.6Sr0.4CoO3−δ film was fast enough not to make any observable gradient in oxygen isotope concentration inside the film. The surface isotope exchange rate, k*, was calculated from the diffusion profile into the electrolyte layer. The electrochemical impedance, σE, was compared with k*. The oxygen partial pressure dependence of those two parameters were quite similar. The absolute value of k* was larger than expected from σE by a factor of 2 or higher.

Mass transport properties of Ce0.9Gd0.1O2−δ at the surface and in the bulk

Abstract The isothermal thermogravimetry was performed on the system of Ce 0.9 Gd 0.1 O 2− δ (CGO10) to determine the oxygen nonstoichiometry, δ , in the temperature range between 1073 and 1173 K and the oxygen partial pressure range of 10 −22 –1 bar. In addition, chemical diffusion coefficients, D chem , and surface reaction rate constants, k , were obtained in CO–CO 2 and H 2 –H 2 O atmospheres through the analysis of a weight relaxation process. The values of obtained chemical diffusion coefficients were consistent with those estimated from partial conductivity and oxygen nonstoichiometry through the P O 2 regions from predominant ionic conduction to predominant electronic conduction. Surface reaction rate constants are not directly related with oxygen partial pressure but depend on the gas species and the composition. The isotope exchange depth profiling with secondary ion mass spectrometry (SIMS) was done to examine the relationship between k and surface exchange coefficients, k s . It was shown that the hydrogen partial pressure dependence of k s converted from k was the same, although the absolute value was about 1 over 30 of those from SIMS analysis.

Nonstoichiometry of Ce1−XYXO2−0.5X−δ (X=0.1, 0.2) ☆

Abstract Nonstoichiometry of the fluorite-type oxide solid solutions Ce 1− X Y X O 2−0.5 X − δ ( X =0.1, 0.2) was measured as a function of temperature ( T =973–1373 K) and oxygen partial pressure ( P (O 2 )=10 −2 –10 −24 atm) by means of thermogravimetry. The result shows that the nonstoichiometry of Ce 1− X Y X O 2−0.5 X − δ ( X =0.1, 0.2) is not explained by a simple point defect model, therefore, defect association models are suggested. From the calculation, it is found that (Ce Ce ′V O Ce Ce ′) x is the major defect association not only in Ce 1− X Y X O 2−0.5 X − δ ( X =0.1, 0.2), but also in pure CeO 2 for nonstoichiometries less than 0.10. It is also found that the defect association (Ce Ce ′V O Ce Ce ′) x is dominating at lower temperature and smaller X composition.

Structure and polarization characteristics of solid oxide fuel cell anodes

Abstract Several nickel-YSZ (yttria stabilized zirconia) cermet electrodes were prepared by slurry coating method and their dc and ac polarization behaviors were investigated. Characteristics of the cermet electrodes were found to be affected by preparation conditions such as (1) nickel content, (2) pre-calcination temperature and (3) baking temperature. These variations of polarization behaviors were analyzed using a simple equivalent circuit model. Impedances of ionic path through YSZ particles and of electronic path through nickel particles were found to be dominant factors to determine the electrode characteristics.

A novel technique for imaging electrochemical reaction sites on a solid oxide electrolyte

Oxygen isotope was used to investigate the active electrochemical reaction site on a solid oxide electrolyte. The isotope exchange reaction was performed under current flow, and the distribution of the incorporated isotope was analyzed by a secondary ion mass spectrometer. The results were compared with calculations using a simple model. The lateral resolution of the present method was estimated to be around 1 μm. The quenching process and the imaging resolution should be improved to investigate further details.

Hydrogen permeability in (CeO2)0.9(CaO)0.1 at high temperatures

The atomic hydrogen (1/2H2) permeability, JH, in (CeO2)0.9(GdO1.5)0.1 (fluorite-type) was measured at 1800–800 K. Two tubular specimens (SP(L), SP(S)) of different lengths sintered at 1970 K in air were used to eliminate the permeation through supporting materials (Pt rings, alumina tube and alumina disk). The JH was 2.81×10−6–1.50×10−8/mol h−1 cm−1, when it was assumed that PH2=7.34×103 Pa and PH2O=2.28×103 Pa outside of both specimens and that the Ar flow rates to the inside of SP(L) and SP(S) were 20.9 and 9.4–16.0 cm3 min−1, respectively. LogJH decreased with decreasing temperature and was proportional to the inverse temperature at 1800–1100 and 1100–800 K, and the activation energies were 1.12±0.01 and 0.18±0.01 eV, respectively. (CeO2)0.9(GdO1.5)0.1 would be an electron–proton mixed conductor under H2–H2O atmosphere at high temperatures. The protonic conductivity was roughly calculated from the JH and from the hydrogen partial pressures inside and outside the specimens, its values being 1.4×10−4–7.2×10−7/S cm−1 and 1.2×10−6/S cm−1 at 1070 K.

Hydrogen permeability of YSZ single crystals at high temperatures

Abstract The atomic hydrogen (H) permeability, J H , of 10YSZ single crystal (fluorite-type) was measured at 1800–1050 K using the Nigara-method . Two tubular specimens (φ 1.30×0.93 cm; length: 3.35 cm (Specimen(L)) and 1.00 cm (Specimen(S))), annealed at 1570 K for 6 h in air, were used to eliminate the permeations of supporting materials (Pt rings, alumina tube, and alumina disk). Measurement was carried out after the specimen had been kept at 1800 K for 20 h. The J H was 3.97×10 −7 –2.79×10 −9 /mol h −1 cm −1 , when it was assumed that P H 2 =7.36×10 3 Pa and P H 2 O =2.30×10 3 Pa outside of both specimens and that the Ar flow rates to the inside of Specimen(L) and Specimen(S) were 21.5 and 16.9–19.9 cm 3 min −1 , respectively. Log J H decreased with the decrease of temperature and was proportional to the inverse temperature. The activation energies were 0.61±0.09, 1.60±0.05, and 0.79±0.02 eV at 1800–1680, 1680–1330, and 1330–1050 K, respectively. The protonic conductivity, which was roughly calculated from the J H and the hydrogen partial pressures inside and outside of the specimens, was 2.4×10 −6 –1.5×10 −8 /S cm −1 at 1800–1050 K. The J H of other 10YSZ single crystal specimens, which were not annealed and which were measured after being kept at 1800 K for 1 h, was lower than that of the above specimens, especially at high temperatures, and the activation energies were 1.27±0.03, 0.15±0.03, and 1.49±0.09 eV at 1800–1425, 1425–1150, and 1150–1050 K, respectively. The 10YSZ single crystal would be an electron–proton mixed conductor under H 2 –H 2 O atmosphere at high temperatures. Since the J H of 12YSZ single crystal specimens, which were annealed at 1870 K for 24 h in air and were measured after being kept at 1800 K for 18 h, was less than 1×10 −9 /mol h −1 cm −1 at 1800 K, it was difficult to determine the relation between its hydrogen permeability and temperature.

The atomic hydrogen permeability in (CeO2)0.85(CaO)0.15 at high temperatures

Abstract The atomic hydrogen permeability, J H , in fluorite-type (CeO 2 ) 0.85 (CaO) 0.15 was measured to confirm the influence of CaO concentration in CeO 2 at 1050–1800 K. The decrease of J H with reducing temperature and the proportionality of log J H to inverse temperature are suggested. The estimated activation energies are 1.0 5 ±0.01, 1.3 5 ±0.02 and 0.2 2 ±0.04 eV at 1800–1400, 1400–1250 and 1250–1050 K, respectively. The J H of (CeO 2 ) 1− X (CaO) X decreases with increasing X and proportional to X 5/2 at 1800–1300 K under H 2 –H 2 O atmosphere. H 2 is decomposed to 2H + and 2e − on the surface of the specimen and they permeate it. They hop between oxygen ions and cerium ions, respectively. It would be difficult for H + to cross over Vo, which was produced by CaO, then J H decreased with increasing X . (CeO 2 ) 1− X (CaO) X would be the electron–proton mixed conductor under H 2 –H 2 O atmosphere at high temperatures.