Oxygen tracer diffusion and surface exchange kinetics in Ba0.5Sr0.5Co0.8Fe0.2O3 - δ

The oxygen tracer diffusion coefficient, Db∗, and the oxygen tracer surface exchange coefficient, k, were measured in Ba0.5Sr0.5Co0.8Fe0.2O3 - δ (BSCF5582) over the temperature range of 310-800 °C and the oxygen partial pressure range of 1.3 × 10- 3-0.21 bar. Several measurement techniques were used: isotope exchange followed by depth profiling (IEDP) within individual single grains or line scanning (IELS) along the sample cross-section and gas-phase analysis (GPA). Surface exchange kinetics was initially found to be slow and presumably inhibited by the formation of a passivating layer on the sample surface. High temperature pre-anneals (900-950 °C) changed the nature of this layer and enhanced surface exchange. Fast bulk oxygen diffusion and surface exchange kinetics were observed after high temperature pre-anneals within the temperature range studied. The activation energies for 18O tracer diffusion and surface exchange at 0.21 bar were 0.72 ± 0.05 and 1.10 ± 0.15 eV, respectively. The tracer diffusion coefficient showed weak dependence upon oxygen partial pressure, whereas the surface exchange coefficient exhibited strong oxygen partial pressure dependence. The microstructure of the samples (the porosity and grain size) had a profound effect on the measured tracer diffusion coefficient. © 2014 Elsevier B.V

Determination of Kinetic Parameters and Identification of the Rate-Determining Steps in the Oxygen Exchange Process for LaNi_0.6Fe_0.4O_{3−<i>δ</i>}

The mixed ionic and electronic oxide LaNi0.6Fe0.4O3−δ (LNF) is a promising ceramic cathode material for solid oxide fuel cells. Since the reaction rate of oxygen interaction with the cathode material is extremely important, the present work considers the oxygen exchange mechanism between O2 and LNF oxide. The kinetic dependence of the oxygen/oxide interaction has been determined by two isotopic methods using 18O-labelled oxygen. The application of the isotope exchange with the gas phase equilibrium (IE-GPE) and the pulsed isotope exchange (PIE) has provided information over a wide range of temperatures (350–800 °C) and oxygen pressures (10–200 mbar), as each method has different applicability limits. Applying mathematical models to treat the kinetic relationships, the oxygen exchange rate (rH, atom × cm−2 × s−1) and the diffusion coefficient (D, cm2/s) were calculated. The values of rH and D depend on both temperature and oxygen pressure. The activation energy of the surface exchange rate is 0.73 ± 0.05 eV for the PIE method at 200 mbar, and 0.48 ± 0.02 eV for the IE-GPE method at 10–20 mbar; for the diffusion coefficient, the activation energy equals 0.62 ± 0.01 eV at 10–20 mbar for the IE-GPE method. Differences in the mechanism of oxygen exchange and diffusion on dense and powder samples are observed due to the different microstructure and surface morphology of the samples. The influence of oxygen pressure on the ratio of contributions of different exchange types to the total oxygen exchange rate is demonstrated. For the first time, the rate-determining step in the oxygen exchange process for LNF material has been identified. This paper discusses the reasons for the difference in the mechanisms of oxygen exchange and diffusion

Oxygen surface exchange kinetics of Ba0.5Sr0.5Co0.8Fe0.2O3−δ

The mechanism of oxygen exchange between the gas phase and Ba0.5Sr0.5Co0.8Fe0.2O3−δoxide was evaluated by considering the inhomogeneity of the oxide surface. The applicability of existing models for the analysis of the oxygen exchange mechanism was considered. A new model with a dissociation step was suggested. The rate-determining steps of the oxygen exchange process were revealed under different experimental conditions. The change in the rate-determining step occurred at 600-650 °C. The probable cause was considered taking into account the parameter of nonequivalency of adsorption centers. A relationship between the oxygen isotope redistribution rates and the rates of the elementary steps in a “gas phase-solid oxide” system was revealed

Oxygen surface exchange and diffusion in Pr 1.75 Sr 0.25 Ni 0.75 Co 0.25 O 4±: δ

Oxygen surface exchange and diffusion in Pr 1.75 Sr 0.25 Ni 0.75 Co 0.25 O 4±δ have been investigated using two methods: pulsed isotope exchange (PIE) and oxygen isotope exchange with gas phase equilibration (IE GPE). Oxygen surface exchange kinetics is considered in the framework of two-step models including two consecutive stages: dissociative adsorption of oxygen and incorporation of oxygen adatoms into the crystal lattice. The rates of oxygen heterogeneous exchange (r H ) as well as the rates of dissociative adsorption (r a ) and oxygen incorporation (r i ) have been calculated. The applicability of the two-step model is discussed based on the concept of a novel two-step mechanism with distributed rates of dissociative adsorption and incorporation of oxygen. It is shown that the two-step model can be applicable for the description of oxygen exchange kinetics in Pr 1.75 Sr 0.25 Ni 0.75 Co 0.25 O 4±δ only at temperatures below 750 °C. Above this temperature, only the statistical model with distributed rates can be used. At low temperatures (<750 °C), the oxygen incorporation rate is found to be smaller than the rate of oxygen dissociative adsorption. Thus, under these experimental conditions the stage of oxygen incorporation is considered to be rate-determining. When increasing the temperature, the difference between r a and r i decreases and the stages become competing. The oxygen isotope exchange kinetic profiles obtained using the IE GPE method are found to be complicated and include a surface exchange stage as well as at least two diffusion relaxation processes. The reasons for the existence of these two processes are discussed