Y. L. Yang

Impedance Studies of Oxygen Exchange on Dense Thin Film Electrodes of La0.5Sr0.5CoO3 − δ

Solid-state electrochemical cells with dense oriented thin film electrodes of La 0.5 Sr 0.5 CoO 3-δ (LSCO) were prepared on (100) surfaces of single-crystal disks of yttria-stabilized zirconia (YSZ) by the pulsed laser deposition technique. Oxygen exchange at the electrodes was studied with alternating current impedance spectroscopy under various temperature and oxygen partial pressure conditions. Three distinctive features were observed in the impedance spectra from high to low frequency corresponding to contributions from the ionic conduction of the YSZ electrolyte, ionic transfer at the LSCO/YSZ interface, and the oxygen exchange on the LSCO electrode surface. An equivalent circuit model of the electrode process is used to fit the impedance data. The time constant for the oxygen surface exchange was derived from the impedance simulation. The surface chemical exchange coefficients, K chem , were calculated from the time constants as a function of temperature and pO 2 . k chem is 7 X 10 -4 cm/s at T = 700°C and pO 2 = I atm. The activation energy at pO 2 = 1 atm is 1.1 eV. The interfacial conductivity data were also derived from the impedance simulations as a function of temperature and pO 2 . The activation energy for the interfacial transport at pO 2 = 1 atm is 1.6 eV.

Oxygen permeation in dense SrCo0.8Fe0.2O3-δ membranes: Surface exchange kinetics versus bulk diffusion

Abstract Oxygen permeation measurements were made on dense discs of SrCo 0.8 Fe 0.2 O 3 − δ as a function of pressure gradient, 0.01 to 1 atm, temperature, 775 to 870 °C and disc thickness, 1 to 2.6 mm. Two different measuring configurations were used: with the pressure gradient applied parallel to the disc axis and with the pressure gradient applied between the edge and the two flat surfaces of the disc. This enabled us to take into account leakage effects due to the edge. A model is proposed for the dependence of the surface exchange current on the chemical potential drop at the gas-solid interface. Analysis of the pressure gradient and membrane thickness dependence of the permeation shows that the permeation is controlled by both bulk diffusion of oxide ions as well as exchange kinetics between gas-phase oxygen molecules and oxide ions. Values for the surface exchange rate coefficient k i 0 and the ambipolar diffusion coefficient D a are reported.

Oxygen Surface Exchange in Mixed Ionic Electronic Conductors: Application to La0.5Sr0.5Fe0.8Ga0.2 O 3 − δ

We propose an oxygen surface exchange model in which the effect of vacancies at the gas-mixed ionic electronic conductor interface are included and apply the model to isotope exchange, oxygen permeability, and electrical conductivity relaxation. We deduce relationships between the surface-exchange coefficients associated with these phenomena and extend the treatment of the conductivity relaxation to large changes in oxygen partial pressure, where the commonly used assumption of first order reaction rate breaks down We apply the model to interpret the permeation and electrical conductivity relaxation measurements in La 0.5 Sr 0.5 Fe 0.8 Ga 0.2 O 3-δ . Transport in the material is almost completely surface limited, and data were interpreted in terms of a single surface-exchange coefficient.

Electrical conductivity relaxation studies of an epitaxial La0.5Sr0.5CoO3−δ thin film

Abstract The oxygen surface exchange coefficient kchem of a La0.5Sr0.5CoO3−δ (LSCO) thin film has been determined from electrical conductivity relaxation measurements. The LSCO thin films were deposited on LaAlO3 (LAO) single crystal substrates by pulsed laser deposition (PLD). The electrical conductivity relaxation behavior of the film was measured at high temperature on switching the oxygen partial pressure between 0.01, 0.05, 0.10, 0.30, 0.50 and 1.00 atm. The kchem values were obtained by fitting the conductivity relaxation curves using a surface-limited kinetics model. The results show that kchem increases with temperature and with the oxygen partial pressure after the switch, but is not sensitive to the initial partial pressure. After prolonged heating at 900 °C, kchem increased substantially. The increase is associated with a change in the thin film surface morphology on prolonged heating.

The effect of the magnitude of the oxygen partial pressure change in electrical conductivity relaxation measurements: oxygen transport kinetics in La0.5Sr0.5CoO3−δ

Abstract The kinetics of oxygen transport in La 0.5 Sr 0.5 CoO 3− δ were investigated using the electrical conductivity relaxation technique in the temperature range from 600°C to 700°C and with oxygen partial pressures from 0.01 to 1.0 atm. The effects of oxygen partial pressure on the chemical diffusivity of oxygen, D chem , and surface exchange coefficient, k chem , were investigated by systematically changing the initial ( p 1 ) and the final ( p 2 ) oxygen partial pressures. Results showed that for oxygen partial pressure switches with p 1 / p 2 >20, a linear model for the exchange kinetics is no longer valid. The dependence of the exchange kinetics on the final oxygen partial pressure was determined.

Determination of oxygen permeation kinetics in a ceramic membrane based on the composition SrFeCo0.5O3.25−δ

The oxygen permeation through an oxide ceramic membrane with bulk composition SrFeCo0.5O3.25−δ has been measured as a function of both temperature and the oxygen pressure gradient across the membrane. The form of the pressure gradient dependence of the permeation indicates that the oxygen transport in the membrane is dependent primarily on the bulk diffusion rate. Although the permeation experiments were carried out at temperatures within, or very close to, the range where SrFeCo0.5O3.25−δ is stable as a pure single phase in air, the membrane was found to consist of SrFe1.5−xCoxO3.25−δ (x=∼0.42) together with fractions of Sr(Co1−xFex)O2.5−δ perovskite and Co–Fe oxide. Phase stability experiments indicate these phases formed as a stable assemblage when the membrane was densified at high temperature (1090°C) during final processing. The assemblage persists at the lower temperatures of the permeation experiments most likely because of slow re-equilibration kinetics.

Combustion synthesis and characterization of Sr and Ga doped LaFeO3

Abstract A homogeneous perovskite oxide La 0.5 Sr 0.5 Ga 0.2 Fe 0.8 O 3− δ (LSGFO) has been synthesized by self-propagating high-temperature synthesis (SHS). The homogeneity and the particle size of the combustion product may be increased by decreasing the cooling rate of the sample, either by increasing the sample diameter or by controlling the post-combustion temperature. The particle morphology depends on the gaseous and molten compounds formed as the combustion front passes through the sample. The perovskite oxide maintained its cubic structure at all temperatures in air. However, a decomposition of LSGFO occurred at 860°C under a simulated syngas environment (22% CH 4 +57% H 2 +21% CO 2 , oxygen partial pressure of about 10 −17 atm). The maximum electrical conductivity of a disc prepared from the SHS powder was 142 S/cm at 580°C under oxygen pressure of 1 atm. The LSGFO may be suitable for use as a membrane in syngas production since its thermal expansion in air and reducing environment are rather close at high temperature.

Oxygen surface exchange in mixed ionic electronic conductor membranes

Abstract We discuss the different oxygen surface exchange coefficients and the net surface exchange flux i=ciki0[exp(nμg/RT)−exp(2nμ/RT)] at gas–MIEC oxide interfaces, where ki=ki0(p/p0)n is the surface exchange coefficient, derived from isotope exchange experiments, p is the oxygen pressure, μg and μ are the chemical potentials of the gas and of the ion–electron hole pairs at the interface and ci is the oxygen ion concentration. The model is applied to oxygen permeation in planar and tubular membranes.

Oxygen permeation in SrCo0.8Fe0.2O3-δ membranes with porous electrodes

Abstract Measurements of oxygen flux, in the temperature range, 770–870 °C on planar membranes of SrCo 0.8 Fe 0.2 O 3−δ , consisting of a dense discs ~ 0.1 cm thick modified by coating with porous layers on both sides, yielded an enhancement in the oxygen flux, η ≈ 2, compared to the uncoated membrane. When only one side of the disc was coated with the porous layer on the high pressure side of the membrane then η ≈ 1.2, while when the modified side was on the low pressure side then η ≈ 1.4. The observed values of η were explained by a model of diffusion-reaction transport in mixed ionic electronic conductor membranes with porous electrodes, extended to the case of finite thickness of the dense layer. From a fit of the model to the experimental data, values of the surface exchange coefficient, k iO , and ambipolar oxygen ion-electron hole pair diffusion coefficient, D a , were deduced. At 870 °C, k iO = 2.5 × 10 −4 cm/s and D a = 5 × 10 −6 cm 2 /s.

Oxygen permeation, electrical conductivity and stability of the perovskite oxide La0.2Sr0.8Cu0.4Co0.6O3-x

Abstract The cubic perovskite La 0.2 Sr 0.8 Cu 0.4 Co 0.6 O 3− x has high electronic and ionic conductivity at high temperatures. The electronic conductivity initially increases with increasing temperature but then decreases due to loss of oxygen atoms from the structure. The oxygen permeability measured on disc membranes is comparable to values reported for other cobalt-containing perovskite oxides with high strontium content. The permeation flux decreases with time due to a transformation of the disordered cubic perovskite to an ordered structure related to Brownmillerite. This ordered structure is stable below 975°C in an oxygen partial pressure pO 2 =0.01 atm. Membranes that have been maintained at high temperature in an oxygen partial pressure gradient for >400 h show decomposition to a multi-phase assemblage that includes non-perovskite strontium cuprates. This decomposition may also contribute to the flux decline.