Takanori Otake

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.

Protonic-Electronic Mixed Conduction and Hydrogen Permeation in BaCe0.9 − x Y 0.1Ru x O 3 − α

The protonic-electronic mixed conductors are of great interest for their potential applications particularly for the hydrogen separation that is essential for hydrogen production from hydrocarbons. This paper deals with the mixed conduction properties of BaCe 0 . 9 - x Y 0 . 1 Ru x O 3 - α (x = 0-0.1) in which Ru is partially substituted for Ce in the high-temperature proton conductor, BaCe 0 . 9 Y 0 . 1 O 3 - α . Appreciable hydrogen permeation through the Ru-doped materials was observed and is attributed to ambipolar diffusion. The mixed conducting mechanism is discussed in terms of the defect chemistry and electronic structures revealed by the electrochemical and spectroscopic measurements.

Electrochemical Behaviors of Mixed Conducting Oxide Anodes for Solid Oxide Fuel Cell

To clarify guidelines for a high-performance mixed conducting oxide anode, electrochemical behaviors of mixed conducting oxide anodes were studied on the oxides, La 0.9 Ca 0.1 Cro 0.8 Al 0.2 O 3 , La 0.9 Ca 0.1 Cr 0.2 Al 0.3 O 3 , Sr 0.9 La 0.1 TiO 3 , Sr 0.8 La 0.2 TiO 3 , SrTi 0.97 Nb 0.03 O 3 , Ce 0.9 Gd 0.1 O 1.95 , and Ce 0.992 Nb 0.008 O 2 . The hydrogen oxidation on the oxide anodes is studied by ac impedance and steady-state polarization measurements. In the ac impedance measurements, a slow relaxation process of the order of 10 -2 Hz was observed with the CeO 2 -based and La 0.9 Ca 0.1 Cr 0.2 Al 0.8 O 3 anodes. The corresponding pseudocapacitances are 10 4 -10 6 μF cm -2 . These pseudocapacitances are identified as the chemical capacitance due to the variation of the nonstoichiometric oxygen content of the electrode material. At the same electrode potential, the CeO 2 -based anodes showed a far higher steady-state current than the LaCrO 3 - and the SrTiO 3 -based anodes. The extension of the reaction zone beyond the three phase boundary is estimated from our experimental results. The reaction zone of the Ce 0.9 Gd 0.1 O 1.95 anode extends from the three-phase boundary to the electrode/gas interface. To estimate what determines the electrode performance of the oxide anodes, the effect of the material property and the electrode microstructure was studied. The effect of the material property is much larger than that of the electrode microstructure. High ionic conductivity and catalytic activity with a certain level of electronic conductivity is required for a high-performance oxide anode.

Determination of the Reaction Zone in Gadolinia-Doped Ceria Anode for Solid Oxide Fuel Cell

In order to elucidate the reaction zone in porous Ce 0.9 Gd 0.1 O 1.95-δ anodes on yttria-stabilized zirconia (YSZ), ac impedance and steady-state polarization measurements are carried out in H 2 -H 2 O-Ar gas mixtures with different electrode thicknesses, 7, 20, and 45 μm anodes. Steady-state polarization current becomes larger as the electrode becomes thicker, while the current per unit surface area shows similar value. Therefore, the current seems to be a function of the electrode surface area. In ac impedance measurements, an extremely large pseudo-capacitance is observed to be as large as 10 5 -10 6 μF cm -2 . The measured pseudo-capacitance is caused by the nonstoichiometric perturbation of the oxygen content in the anode material. Both steady-state polarization and ac impedance measurements suggest that the reaction zone extends from the interface of YSZ/porous Ce 0.9 Gd 0.1 O 1.95-δ anodes toward the outer gas phase. The distribution of electrochemically active zone is semiquantitatively estimated. The activity for the surface reaction gradually decreases with increasing the distance from the electrode/electrolyte interface, and the reduction rate of the activity becomes high as the electrode thickness increases.

Ce3+ concentration in ZrO2–CeO2–Y2O3 system studied by electronic Raman scattering

Ceria doped yttria stabilized zirconia (ZrO2–CeO2–Y2O3 system) shows electron–ion mixed conduction at high temperatures under a reduced atmosphere. In this system, the electronic conductivity is caused by electrons, which may diffuse by hopping conduction between Ce4+ and Ce3+. We applied the electronic Raman scattering technique on the system in order to measure the concentration of Ce3+ in samples. For the X=0.6 and 0.8 samples in [(ZrO2)1−X (CeO2)X]0.9(Y2O3)0.1, the oxygen partial pressure dependence of the electronic Raman intensities agrees well with that of the oxygen non-stoichiometry measured by thermogravimetry. From this result, it is confirmed that all of the introduced electrons at reduced condition are trapped at cerium centers in these materials.

Phase diagram calculations of ZrO2-Based ceramics with an emphasis on the reduction/oxidation equilibria of cerium ions in the ZrO2-YO1.5-CeO2-CeO1.5 system

Phase diagram calculations that were made previously for the ZrO2-MO m/2 (m = 2, 3, 4) systems and for the ZrO2-YO1.5-MO m/2 (M = transition metals) systems have been extended to the ZrO2-YO1.5-CeO2(-CeO1.5) system to make an attempt to explain (1) thermogravimetric (TG) results as a function of oxygen potential, (2) electronic conductivity as a function of oxygen potential, and (3) a miscibility gap observed in air. The interaction parameters for the CeO2-CeO1.5-YO1.5 system were obtained from the reported oxygen nonstoichiometry in CeO2−x and rate earth doped ceria, (Ce,RE)O2−δ . The interaction parameters for the ZrO2-CeO2 subsystem were obtained so as to reproduce the observed miscibility gap at 1273 K. Those thermodynamic properties can reproduce consistently the experimental behaviors of the electronic conductivity and the TG results in the (Zr1−x Ce x )0.8Y0.2O1.9 solid solutions; these indicate the enhancement of reduction of CeO2.

Oxygen transport properties in ZrO2–CeO2–Y2O3 by SIMS analyses

Abstract A mixed conductor, ZrO 2 –CeO 2 –Y 2 O 3 (Ce–YSZ), exhibits anomalous electrical properties depending on oxygen partial pressure and CeO 2 concentration. We measured surface reaction rates and oxygen tracer diffusion coefficients of YSZ with various CeO 2 concentrations by secondary ion mass analyses. The Ce concentration dependence of oxygen tracer diffusion coefficient in air corresponds with that of the total electrical conductivity, over which the ionic conductivity is dominant. The surface exchange coefficient, however, had a higher value in the region of 20∼60 mol% CeO 2 containing YSZ compared to other compositions. Ce–YSZ, which contains this region of CeO 2 concentration, exhibits high electronic conduction at reducing conditions. The ionic and electronic conductivities are considered to affect the oxygen tracer diffusion coefficient and the surface exchange coefficient, respectively.

Surface reaction and transport kinetics of hydrogen through palladium-based membranes under gas co-existing with hydrogen atmospheres

In order to understand how gas co-existing with hydrogen affects the hydrogen permeability of a silver 23wt%-palladium membrane, the surface reaction of hydrogen was evaluated from the continuity of the surface reaction rate and the bulk diffusion flux of hydrogen based on the results of hydrogen-permeation measurements. The interference effect of the co-existing species was quantified as a function of temperature, partial pressure and the components of the co-existing gas. In order to investigate the behaviour of the co-existing species on the membrane surface, in situ observation using the Polarisation Modulated Infrared Reflection Absorption Spectroscopy (PM-IRRAS) was carried out. An infrared absorption peak due to the adsorption of carbon monoxide on the membrane was observed under the atmosphere of 7.8% carbon monoxide – 2.3% water vapour – 89.9% hydrogen. From the dependence of the infrared absorption peak on temperature, co-existing carbon monoxide was found to adsorb onto the membrane surface irreversibly.