Shuangyan Wang

Rapid oxygen ion diffusion and surface exchange kinetics in PrBaCo2O5+x with a perovskite related structure and ordered A cations

As part of an investigation of new cathode materials for intermediate temperature solid oxide fuel cells, we have investigated particular perovskite oxides with ordered A cations which, in turn, localize the oxygen vacancies into layers. The oxygen exchange kinetics of polycrystalline samples of the oxygen-deficient double perovskite PrBaCo2O5+x (PBCO) have been determined by electrical conductivity relaxation (ECR) and by oxygen-isotope exchange and depth profiling (IEDP). The ECR and IEDP measurements reveal that PBCO has high electronic conductivity and rapid oxygen ion diffusion and surface exchange kinetics. Both techniques demonstrate that the oxygen kinetics in this structure type are significantly faster than in corresponding disordered perovskites.

Oxygen exchange kinetics of epitaxial PrBaCo2O5+δ thin films

The oxygen exchange kinetics of thin films of the oxygen-deficient double perovskite PrBaCo2O5+δ (PBCO) have been determined by electrical conductivity relaxation (ECR) and by oxygen-isotope exchange and depth profiling (IEDP). Microstructural studies indicate that the PBCO films, prepared by pulsed laser deposition, have excellent single-crystal quality and epitaxial nature. The ECR and IEDP measurements reveal that the PBCO films have high electronic conductivity and rapid surface exchange kinetics, although the ECR data indicate the presence of two distinct kinetic pathways. The rapid surface kinetics compared with those of other perovskites suggest the application of PBCO as a cathode material in intermediate-temperature solid oxide fuel cells.

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.

Oxygen Transport Kinetics in SrFeO3 − δ , La0.5Sr0.5FeO3 − δ , and La0.2Sr0.8Cr0.2Fe0.8 O 3 − δ Measured by Electrical Conductivity Relaxation

The chemical diffusion and surface exchange coefficients were measured for SrFeO 3 - δ , La 0 . 5 Sr 0 . 5 FeO 3 - δ , and La 0 . 2 Sr 0 . 8 Cr 0 . 2 Fe 0 . 8 O 3 - δ at 790 ≤ T ≤ 1000°C and 0.01 ≤ pO 2 ≤ 1 atm by electrical conductivity relaxation. Values of the chemical diffusion (D c h e m ) and surface exchange (k c h e m ) coefficients were obtained by monitoring the variation of electrical conductivity after an abrupt change in oxygen partial pressure. The vacancy diffusion coefficient (D v ), oxygen ion self-diffusion coefficient (Do), and surface exchange coefficients at thermal equilibrium (k e x ) were calculated using the measured thermodynamic factors and were compared with literature data for other compositions.

An electrical conductivity relaxation study of La0.6Sr0.4Fe0.8Co0.2O3−δ

Abstract Electrical conductivity relaxation (ECR) experiments were carried out on La 0.6 Sr 0.4 Fe 0.8 Co 0.2 O 3− δ (LSFCO) in the temperature range of 840 to 910 °C and in the oxygen partial pressure range of 0.01 to 0.05 atm. The effects of annealing at 900 °C on the oxygen transport properties were investigated for individual samples. The values of D O 2− =D O / Γ O and k ex = k chem / Γ O agree well with the result from isotope-exchange depth profile (IEDP) experiments.

Impedance studies of dense polycrystalline thin films of La2NiO4+δ

The cathodic kinetics of 3000 A thick La2NiO4+δ (LNO) films were studied using AC impedance spectroscopy at 400–620 °C and in 0.01–1.0 atm O2. The dense polycrystalline LNO films were deposited on both sides of yttrium stabilized zirconia (YSZ) (100) single crystal substrates by pulsed laser deposition (PLD). Three distinct features were observed in their AC impedance spectra. The high frequency (HF) feature is assigned to the oxygen ion conduction of the YSZ electrolyte, the medium frequency (MF) feature to the interfacial resistance between the film and the electrolyte, and the low frequency (LF) feature to the oxygen reduction reaction at the film surface. For the LNO film, we conclude that the electrode reaction is limited by the surface exchange reaction. The surface exchange coefficient, ki, is calculated from the LF resistance. The values of ki agree well with those obtained previously for a ceramic sample.