Masayuki Dokiya

Thermal expansion of Gd-doped ceria and reduced ceria

Abstract The thermal expansion coefficients of CeO 2 and Gd-doped ceria were measured in the range from 100 to 873 K and those of reduced ceria were measured from 100 to 323 K. The thermal expansion coefficient of Ce 1− x Gd x O 2− x /2 became large with increasing the dopant content, x . The thermal expansion coefficient of reduced ceria became large with increasing the vacancy content of oxygen. These increases of the thermal expansion coefficients result from weakening binding energy due to the increase of oxygen vacancy. The thermal expansion coefficients of CeO 2 and Gd-doped ceria were calculated theoretically and they were in good agreement with experimental ones except for the higher temperature range.

SOFC system and technology

Compact size solid oxide fuel cells (SOFCs), which will be operated at reduced temperature, are becoming a frontier of R and D. These compact size SOFCs will fit well with intermittent loads, of which share in energy system is increasing today, whereas the “conventional SOFCs” will be effectively operated with stationary mode. For such compact size SOFCs, throttle down operation following intermittent loads will be profitable because low current density gives higher efficiency. SOFCs are not suitable for quick start up. It was estimated that the hot standby mode would be more acceptable than cold start mode from the viewpoint of heat loss. The merit of internal reforming will also be lost for the reduced operation temperature. In order to overcome this problem, an electrochemical oxidation of deposited carbon was tested and a new direct internal reforming concept using this carbon deposition was proposed. In order to develop cheap compact size, reduced temperature SOFCs, the feasibility of anode supported SOFCs was investigated on Zr(Sc)O2, Zr(Y)O2, Ce(rare earth)O2, (La,Sr)(Ga,Mg)O2 electrolytes, examining applicability of wet co-fire process and electrode activity. The feasibility was confirmed with zirconia, not yet with ceria due to its fragility, and pessimistic with (La,Sr)(Ga,Mg)O2 due to Ni diffusion during co-firing.

Electrical and Ionic Conductivity of Gd‐Doped Ceria

Electrical conductivity of Ce 1-y Gd y O 2-y/2-x (y = 0.1 1, 0.2), as a function of temperature and oxygen partial pressure, is measured with a complex impedance method. The increase of electrical conductivity with reducing oxygen partial pressure can be described well by a model that assumes constant mobility of both oxygen vacancies and electrons, based on an ideal-solution model of non-stoichiometry of Gd-doped ceria. Ionic conductivity is calculated, and its activation energy is discussed. Electronic conductivity is discussed also.

Electronic conductivity, Seebeck coefficient, defect and electronic structure of nonstoichiometric La1-xSrxMnO3

Abstract In order to elucidate the relationship between the electrical properties and composition ( d and x ) of La 1− x Sr x MnO 3+ d , precise measurements were made on the conductivity, σ , and Seebeck coefficient, Q , for the oxide with 0≤ x ≤0.7 as a function of T and P (O 2 ) up to 1273 K. Analysis was made for the high-temperature paramagnetic state using the nonstoichiometry data and defect and electronic structure models reported by the present authors. It was shown that σ and Q in the oxygen excess La 1− x Sr x MnO 3+ d ( d >0) are fixed to the value at those of the stoichiometric oxygen content, d =0. In the oxygen deficient La 1− x Sr x MnO 3+ d , they are essentially determined by the mean Mn valence and temperature. The predominant electrical conduction was found to take place by the electron hopping on the e g ↑ level of Mn. In La 1− x Sr x MnO 3+ d ( d ≤0) under the condition of z = x +2 d ≤1/3, σ is given by: σ=(2.8×10 6 /T){2 (−z 2 −z+6)(6−18z)/(17−z) 2 +(−z 2 −z+6) (z 2 +18z+5)/(17−z) 2 } exp {(−Ea/(kT)} where the activation energy Ea=−0.59(3+z)+2.00 eV . For z ≥1/3, it is given by: σ=(2.8×10 6 /T) z(1−z) exp {(−Ea/(kT)} where Ea=−0.036(3+z)+0.16 eV . Q is also described essentially by this model. However, the effect of minority carrier conduction is clearly found in Q in addition to the major conduction on e g ↑ level. The major carrier conduction is p-type and the minor carrier is n-type for z ≤0.5 and vice versa for z ≥0.5.

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.

Structural consideration on the ionic conductivity of perovskite-type oxides

Abstract Ionic conductivity of perovskite-type oxides is discussed in terms of structurally related parameters. Electrical conductivity data of perovskite-type oxides, reported as oxygen ion conductors, were collected from literatures and the correlation between the electrical conductivity of these compounds and the structurally related parameters, such as tolerance factor, specific free volume and oxygen deficiency was examined. The tolerance factor and the specific free volume were both a function of ionic radius and the tolerance factor decreased with increasing the specific free volume. The optimum tolerance factor was found to be around 0.96 due to the balance between the specific free volume and the tolerance factor in order to obtain the maximum electrical conductivity for A III B III O 3 -type perovskites. The optimum oxygen deficiency to obtain the maximum electrical conductivity was around 0.2. The effect of ionic size of dopant ions in A and B site on electrical conductivity was also discussed.

Performance of a La0.6Sr0.4Co0.8Fe0.2O3–Ce0.8Gd0.2O1.9–Ag cathode for ceria electrolyte SOFCs

Abstract Performance of an La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3 –CGO–Ag cathode (where CGO is Ce 0.8 Gd 0.2 O 1.9 ) on a Ce 0.8 Sm 0.2 O 1.9 electrolyte was studied by the three-probe AC impedance method at the steady state of various DC bias currents. The electrode consisted of a porous layer of La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3 –CGO (30 wt.%) and a layer of Ag particles coated on the surface. The Ag coating was used to improve the oxygen exchange reaction activity. As a result, an interface conductivity greater than 1 S/cm 2 was obtained at 600 °C with a low activation energy. The dependence of the AC impedance on the frequency was analyzed in order to clarify the reaction steps in the oxygen reduction mechanism.

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}.