Oxygen transport through dense La0.6Sr0.4Fe0.8Co0.2O3-d perovskite-type permeation membranes

In this study, we examine the parameters that govern the overall oxygen flux through a permeation membrane. The chemical diffusion (D) and the surface exchange (k) coefficients for oxygen in La0.6Sr0.4Fe0.8Co0.2O3-d were determined as a function of temperature using a specially designed electrochemical cell combined with impedance spectroscopy. Typically, D=1.7 10-5 cm2/s and k=3.6 10-4 cm/s in air at 900°C. These values were compared with literature 18O/16O isotope exchange data. Oxygen permeation measurements were also performed on the same material in an air/Ar gradient, in the temperature range of 800 to 1000°C. At 900°C, the oxygen flux across a 1.53 mm thick membrane was 8.0 10-8 mol/(cm2s). The measured fluxes were compared with fluxes calculated on the basis of the D and k values using expressions derived from Fick’s law. Comparison showed that the flux is controlled by both bulk diffusion and surface exchange, even for such thick membranes, and that the apparent k? varies significantly from one experiment to another.

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Stefan Diethelm, Jan Van Herle
2004
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https://infoscience.epfl.ch/record/53457?ln=fr

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Oxygen transport and nonstoichiometry in SrFeO3-delta

Alexandre Closset, Stefan Diethelm, Jan Van Herle

Chemical diffusion (D) and surface exchange (k) coefficients for SrFeO3-delta were measured using an electrochemical cell combined with electrochemical impedance spectroscopy (EIS) and potential step technique (PS) in the temperature range of 850-915°C. A value of ~ 4x10-5 cm2/s and a k value of ~ 8x10-5 cm/s were obtained at 900°C. Slow scan (0.5-3 microV/s) cyclic voltametry (CV) was performed in the same temperature range, using the same electrochemical cell to obtain oxygen nonstoichiometry data. The oxygen nonstoichiometry (delta) at 900°C in air was determined as 0.4. A plateau corresponding to delta = 0.5 was observed below an oxygen partial pressure (pO2) of 10-6 atm. These results were shown to be consistent with the literature data. Nonstoichiometry data were further analysed using the existing defect models, and the limits of the independent point defect approximation and the necessity of considering interactions between point defects and clusters were established. Keywords: Strontium Ferrate/Ferrite; Oxygen transport; Oxygen nonstoichiometry.

2000

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Determination of Chemical Diffusion and Surface Exchange Coefficients of Oxygen by Electrochemical Impedance Spectroscopy

Alexandre Closset, Stefan Diethelm, Jan Van Herle

A rigorous mathematical model is developed for the complex impedance of a solid-state electrochemical cell, which is commonly used for the measurement of oxygen transport, oxygen exchange kinetics and thermodynamic properties of nonstoichiometric mixed conducting oxides. The model leads to a simple equivalent circuit for the cell with unambiguous definition of the physical significance of its components. A method is proposed for the analysis of experimental data. The methodology thus developed is validated by comparing the experimental data measured for a well-studied perovskite (SrCo0.5Fe0.503-delta) with the results obtained from the completely equivalent potential-step technique. In addition, various electrochemical properties of the other cell components, such as Pt electrodes and YSZ electrolyte, also obtainable from measurements, show good agreement with the available literature data. The cell design, which significantly minimizes the gas space in contact with the sample, has a clear advantage over similar relaxation cells in terms of reducing the dominating effect of the gas phase capacitance in numerical data analysis. A possible disadvantage, however, is the large impedance of the oxygen pump at low oxygen partial pressures, which may in a similar manner obstruct deconvolution of the sample properties from the measured data.

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Simultaneous determination of chemical diffusion and surface exchange coefficients of oxygen by the potential step technique

Alexandre Closset, Stefan Diethelm, Jan Van Herle

Oxygen diffusion is treated in a dense electronically conducting cobaltate pellet blocked ionically on one surface, electronically on the other, and sealed on its cylindrical periphery. A procedure is developed for extracting the chemical diffusion and surface exchange coefficients for oxygen by use of the asymptotic equations derived for the current response to a potential step at short and long times. It is shown that, while the formation of interfacial phases by reaction between the sample and the electrolyte may affect the surface exchange coefficient, the chemical diffusion coefficient data determined by the present approach are independent of such interfacial phenomena. The consistency of data obtained from several specimens with varying thickness and manner of interfacing with the electrolyte validates the diffusion model and the method used for data analysis. An oxygen permeation cell is also developed in this work as a modification of the diffusion cell. The new cell allows monitoring of the permeation rate by electrochemical means. The steady-state permeation data obtained by the permeation cell are consistent with the chemical-diffusion and surface- exchange coefficients measured by the blocked diffusion cell as long as the assumptions of the related theoretical models are satisfied. This is a further validation of the diffusion model and the related methodology developed here for obtaining the necessary data for characterizing oxygen exchange and transport in such materials.

1999

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