Alexandre Closset

Simultaneous Determination of Chemical Diffusion and Surface Exchange Coefficients of Oxygen by the Potential Step Technique

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.

Study of oxygen exchange and transport in mixed conducting cobaltates by electrochemical impedance spectroscopy

Abstract Oxygen diffusion is treated in a dense electronically conducting perovskite pellet blocked ionically on one surface, electronically on the other, and sealed on the cylindrical surface. Oxygen exchange at the electronically blocked surface is assigned first order reaction kinetics. An equivalent circuit model is suggested for the cell impedance by the Laplace transform of Fick’s second law. The methodology thus developed for this technique is applied to determine the chemical diffusion and surface exchange coefficients of SrCo 0.5 Fe 0.5 O 3− δ interfaced with air in a solid state electrochemical cell employing a YSZ electrolyte as an oxygen pump. These parameters measured by EIS are compared with the values obtained by the potential step (PS) method on the same electrochemical system. The two approaches are equivalent for characterizing diffusion of oxygen vacancies, but EIS is more appropriate for direct measurement of the exchange coefficient. Calculation of this parameter from the PS data may not be straightforward. The two techniques are complementary for complete characterization of oxygen exchange and transport in mixed conducting oxides.

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

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.

Study of oxygen exchange and transport in mixed conducting electroceramics

Abstract Oxygen diffusion is treated in a dense electronically conducting perovskite pellet blocked ionically on one surface, electronically on the other, and sealed on the periphery. Oxygen exchange at the electronically blocked surface is assigned first order reaction kinetics. An equivalent circuit model is suggested for the cell impedance by the Laplace transform of Fick’s second law. Asymptotic expressions are employed to extract the slope of the coulometric titration curve and the chemical diffusion coefficient from electrochemical-impedance-spectroscopy (EIS) data. NLLS fit with the theoretical model is performed to evaluate the surface oxygen exchange coefficient at the interface of the electrochemical cell. The methodology is applied to determine the chemical diffusion and surface exchange coefficients of oxygen in SrCo 0·5 Fe 0·5 O 3−δ , interfaced with a YSZ electrolyte. The experimental results are used to link the processes corresponding to the diffusion of oxygen vacancies to the ionic conductivity of the material. The data is applicable to solid oxide fuel cell cathodes, oxygen permeation membranes and related systems.