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The electrochemical properties of the composite electrodes of LaNi0.6Fe0.4O3−δ-Ce0.9Gd0.1O1.95 in various compositions were measured by electrochemical impedance spectroscopy as a function of temperature and p(O2). The enhancement of area-specific conductivity, σE, was found higher than that of the single-phase porous LaNi0.6Fe0.4O3−δ electrode. σE greatly depends on their volume ratio, where σE increases with the increase of the volume fraction of Ce0.9Gd0.1O1.95 phase up to 50%. Above 50% of the volume ratio, σE decreases which could be due to the loss of its surface area density and connectivity between LaNi0.6Fe0.4O3−δ particles. The enhancement degree of σE was evaluated using the transmission line model for the porous-mixed ionic and electronic electrodes. The calculated model is close to the measured σE, showing that the enhancement of σE is not only because of the enhancement of ionic conducting pathways but also possibly due to the decrease of the surface reaction resistivity upon contact with the Ce0.9Gd0.1O1.95.

Two-step thermochemical cycle for CO2 splitting has been studied using cerium oxide (CeO2-δ) as a redox material. Since oxygen release capacity of CeO2-δ increases by doping ZrO2, a lot of studies have been conducted on CeO2-ZrO2 solid solutions as state of the art redox materials for this cycle. In this study, further doping of Sc3+ into CeO2-ZrO2 solid solution was attempted with an expectation to increase amount of available oxygen vacancies which could improve redox kinetics. Scandium was chosen since the ionic radius is close to Zr4+and oxygen vacancy is introduced by the aliovalent doping. Oxygen nonstoichiometry of Ce0.8Zr0.15Sc0.05O2-δ was investigated by a coulometric titration method and chemical diffusion coefficient and surface reaction rate constant by an electrical conductivity relaxation method. Oxygen vacancy formation of Ce0.8Zr0.15Sc0.05O2-δ upon reduction was similar to that of Ce0.8Zr0.2O2-δ, while chemical diffusion coefficient in a high oxygen partial pressure region became larger by introducing Sc3+. Those were discussed in correlation with the reaction kinetics in the thermochemical cycle.

Relevant factors affecting an oxygen reduction reaction (ORR) in a heterostructure electrode comprising a Ce0.9Gd0.1O1.95 porous layer on LaNi0.6Fe0.4O3 − δ film electrode were evaluated using electrochemical impedance spectroscopy and isotope depth profile analyses. Despite the LaNi0.6Fe0.4O3 − δ film – porous Ce0.9Gd0.1O1.95 electrode having higher interfacial conductivity, σE, compared to a bare LaNi0.6Fe0.4O3 − δ film electrode, σE behavior as a function of p(O2) was similar to each measured temperature. This exhibits a good correlation with the isotopic oxygen depth profile analysis where the evaluated surface oxygen exchange coefficient, k*, showed enhancement compared to bare LaNi0.6Fe0.4O3 − δ film and exhibited the same behavior on p(O2). Based on the analyses, there are two plausible reasons for this enhancement: (a) the formation of triple phase boundaries at the junction of LaNi0.6Fe0.4O3 − δ film, porous Ce0.9Gd0.1O1.95, and gas phase, where the incorporation process during an ORR process takes place via the porous Ce0.9Gd0.1O1.95; and (b) a modification of the subsurface layer just below the film surface where the presence of diffused Ce and Gd appears to accelerate the ORR process.

In order to elucidate the electronic conduction mechanism and defect chemical model of LaNi 0.4 Fe 0.6 O 3 − δ at high temperatures, electrical conductivity ( σ ), Seebeck coefficient ( S ), and oxygen vacancy concentration ( δ ) of LaNi 0.4 Fe 0.6 O 3 − δ were measured as a function of oxygen partial pressure ( p (O 2 )) and temperature. Relatively large σ and small positive S values are observed, which indicates the contribution of ionic conduction is negligibly small to thermoelectric power and the major electronic carrier is a hole. From the analysis of σ and S , it is expected that the LaNi 0.4 Fe 0.6 O 3 − δ has small polaron hopping conduction mechanism where an electron is localized on the Fe. The slope of δ vs p (O 2 ) shows a minimum value near the stoichiometric composition and the δ increases as p (O 2 ) reduces and temperature increases. The relationship between δ vs. log p (O 2 ) is analyzed by a localized electron model and the behavior of the oxygen nonstoichiometry of LaNi 0.4 Fe 0.6 O 3 − δ can be explained.

The oxygen nonstoichiometry of the perovskite-type oxide, LaNi0.6Co0.4O3−δ, was measured at various temperatures and oxygen partial pressures (p(O2)) by high temperature gravimetry and coulometric titration. The oxygen nonstoichiometry appeared independent of p(O2) in the range of 10− 4 bar > p(O2), which was in good agreement with the tendency of lattice parameter determined by the high-temperature X-ray diffraction (HT-XRD). The oxygen nonstoichiometry behavior was analyzed using the defect equilibrium model assuming delocalized electron. In addition, the diffusion coefficient, D*, and the surface reaction rate constant, k*, were obtained by the isotope exchange and secondary ion mass spectrometry. The Dv estimated from the D* showed similarity with those reported for other perovskite oxides which supported the reliability of δ determined in this work.

The surface oxygen exchange coefficient ( k *) of LaNi 0.6 Fe 0.4 O 3−δ (LNF) coated with porous Ce 0.9 Gd 0.1 O 1.95 (GDC) was determined by the isotope exchange technique. The LNF coated with GDC was subjected to isotope exchange treatment where 16 O 2 was abruptly changed into 18 O 2 at a range temperature of 873–1073 K under 10 − 1 and 10 − 2 bar of the oxygen partial pressure ( p (O 2 )). The line-scanning mode of secondary ion mass spectrometry was used to determine the surface oxygen exchange of the LNF coated with GDC. The k * of LNF coated with GDC is enhanced compared to that of the bare LNF due to the formation of triple phase boundary between LNF, GDC, and gas phase of oxygen. The role of GDC is enhancing the incorporation process of the oxygen dissociated at the LNF surface. The adsorption and dissociation of oxygen takes place at LNF surface and incorporated into GDC porous.

In order to investigate the relevant factors affecting the transport properties of solid oxide fuel cell cathodes upon coating with Ce0.9Gd0.1O1.95 (GDC), the surface oxygen exchange coefficient, k*, and oxide ion diffusivity, D*, of La0.6Sr0.4CoO3-δ (LSC) and La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) were measured by a combination of isotope exchange technique and secondary ion mass spectrometry (SIMS). The k* of LSCF/GDC enhanced compared to a bare LSCF for all measured temperatures. In contrast, the k* of LSC/GDC only showed an enhancement compared to the bare LSC at relatively low temperatures (below 873 K) but not at higher temperatures. From these results, GDC has a less significant effect on the k* of LSC compared to LSCF. This suggests that the enhancement of k* is possibly induced only when the oxygen vacancy concentration, δ, of the cathode is lower than that of GDC, although another factor may affect the enhancement. The enhancement of k* is attributed to the formation of triple-phase boundary where a spill-over mechanism controlled the oxygen reduction reaction. The change of rate-determining step of LSCF due to the GDC coating suggests that the enhancement of the k* is not only with respect to the incorporation process, but it is possibly enhancing the reaction steps that involve δ as well.

Finite element method (FEM) simulations were carried out for modeling the electrical conductivity relaxation (ECR) process of a mixed ionic electronic conductor upon abrupt change of oxygen partial pressure. Oxygen diffusion pathway was simulated considering enhanced surface exchange rate on the current and voltage terminals formed on the specimen. The results suggested that the vacancy diffusion coefficient, DV, ECR, obtained from ECR method appears to be dependent on vacancy concentration, δ, even though the true DV is constant. The effects of the following parameters were investigated; degree of catalytic effect, surface reaction rate constant and sample dimension. Change in the sample dimension in a realistic range did not effectively suppress the modification of diffusion paths, and the change of apparent chemical diffusion coefficient, Dchem, app, was inevitable when surface reaction rate constant, kchem, and Dchem are in the same range. The results of the FEM simulation were confirmed by ECR experiments with catalytic active (Pt) and inactive (Au) electrodes.

In this study, the crystal structures and the thermal expansibilities of La 1 − x Sr x MnO 3 + δ (LSM) with oxygen stoichiometric composition, δ = 0, were studied by an atmosphere and temperature controlled X-Ray diffractometer (XRD). Pre-heat-treatment conditions for the samples and oxygen partial pressures for XRD measurements at high temperature were carefully selected to keep oxygen stoichiometric composition. The crystal structures of La 1 − x Sr x MnO 3 at room temperature were rhombohedral, tetragonal, and cubic symmetry for 0.2 ≤ x ≤ 0.45, 0.5 ≤ x ≤ 0.6, x = 0.7, respectively. The second order phase transition from rhombohedral to cubic symmetry was confirmed for x = 0.4 compound at 923–973 K. Meanwhile, the second order phase transition from tetragonal to cubic symmetry was confirmed for x = 0.5 compound at 623–673 K, x = 0.6 compound at 423–473 K, respectively. Furthermore, the pure thermal expansibilities of LSM without chemical expansion were discussed based on XRD analysis.

La0.6Sr0.4Co1 − yFeyO3 − δ (LSCF) mixed-conducting perovskite oxides, a SOFC cathode candidate material, exhibit oxygen nonstoichiometry under reducing atmosphere and elevated temperatures. Reduction of the B-site transition metals and formation of oxygen vacancies result in chemical expansion of the LSCF lattice. Knowledge of both thermal and chemical lattice expansion as a function of composition is an essential tool to tailor mismatches between different materials exposed to reducing and high temperature environments and avoid mechanical failure of the device. In this paper, thermal and chemical expansion coefficients were determined by measurement of the lattice parameters by HT-XRD as a function of pO2 (in the range of 10− 4 to 1 bar), temperature (773–1173 K) and B-site composition. The lattice parameters were correlated with the previously investigated oxygen nonstoichiometry of the LSCF series. While the thermal expansion coefficient increases almost linearly with increasing Co content, the chemical expansion coefficient appears to be independent of B-site composition.

The Young's modulus and the shear modulus of Ce0.9Gd0.1O2 − δ (GDC) were evaluated by using a resonance method in the temperature range from room temperature to 1273 K and in the p(O2) range from 10− 24 to 10− 1 bar. It was found that the elastic moduli of GDC under oxidizing conditions gradually decreased with increasing temperature in a constant atmosphere and was almost independent of p(O2) at a constant temperature. However, under reducing conditions, the elastic moduli apparently decreased as p(O2) decreased even at a constant temperature, and such a decrease became more considerable under lower p(O2) conditions. The variation in the elastic moduli of GDC with p(O2) seemed to reflect the variation in the oxygen nonstoichiometry δ. It was demonstrated that the elastic moduli are affected not only by the change in the interatomic distance due to the chemical and/or the thermal expansion but also by the variation in the oxygen nonsotichiometry and the average valence of cerium ions.

The oxygen nonstoichiometry of La0.6Sr0.4FeO3 − δ was measured at intermediate temperatures (773 to 1173 K) between 1 bar and the decomposition oxygen partial pressure by thermogravimetry and coulometric titration. The decomposition of the ABO3 perovskite phase was found to occur at low oxygen partial pressures (below 10− 20 bar). Using an atmosphere-controlled high-temperature XRD setup, the rhombohedral lattice parameters were obtained between 10− 4 and 1 bar at 773 to 1173 K. A phase transition from rhombohedral to cubic might be expected to occur at high temperatures and for δ near the plateau at δ = [Sr] / 2. The lattice expansion was separated into “pure” thermal and chemically induced expansion by combining the lattice parameters with the oxygen nonstoichiometry data. The linear thermal expansion was formulated with a “pure” thermal expansion coefficient of αth = 11.052 · 10− 6 K− 1 and a chemical expansion coefficient of αchem = 1.994 · 10− 2. The results were compared with previous data obtained for La0.6Sr0.4Co1 − yFeyO3 − δ with y = 0.2–0.8. La0.6Sr0.4FeO3 − δ was confirmed to show the highest thermo-chemical stability. While the chemical expansion of La0.6Sr0.4Co1 − yFeyO3 − δ seems little affected by the iron content, the thermal expansion coefficient was the lowest for La0.6Sr0.4FeO3 − δ.

Change in the lattice parameters of La0.6Sr0.4Co1 − yFeyO3 − δ (y = 0.2, 0.4, 0.6 and 0.8) was observed by in-situ XRD under the various temperature and oxygen partial pressure conditions for thermo-chemical expansivity study. By combination of the lattice parameters and oxygen non-stoichiometry, the linear thermal expansion rates were formulated in each compound. The obtained formulas are expressed as a linear function of temperature and oxygen deficiency with the standard deviations from measured data less than 1.2%. Based on the formulas, apparent thermal expansions were separated into thermally induced part and chemically induced part. Thermal expansion coefficient αth, which is proportional constant for temperature, is increased with increasing Co content in La0.6Sr0.4Co1 − yFeyO3 − δ while chemical expansion coefficient αchem, which is proportional constant for oxygen deficiency, shows very small dependence on the composition. Increase in apparent thermal expansion with increasing Co content in La0.6Sr0.4Co1 − yFeyO3 − δ was caused by the combination of αth and αchem.