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

Abstract

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 (SrCo 0.5 Fe 0.5 O 3Ϫ␦ ) 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 yttria-stabilized zirconia 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.Significant attention has been focused recently on the oxygen conduction properties of certain nonstoichiometric oxides as cathodes for solid oxide fuel cells 1,2 and as membranes for highly selective oxygen separation. 3 A variant of the latter application is related to membrane reactors for syngas production. 4 In such applications, reliable information is required about the nonstoichiometry of the material, kinetics of the surface exchange reaction for oxygen, as well as bulk diffusivity of oxygen, as a function of temperature and partial pressure of oxygen. Various techniques available for measuring these properties, mostly based on the relaxation of oxygen concentration in the material after a step change at a surface, have been widely discussed. [5][6][7][8][9][10][11] We have investigated the construction and methodology in relation to a solid-state electrochemical cell, first designed and applied by Belzner et al. 5 for simultaneous measurement of the chemical diffusion coefficient and the thermodynamic enhancement factor of oxygen in manganites. 6 The methodology had to be improved for studying materials with high diffusion coefficients such as SrCo x Fe 1Ϫx O 3Ϫ␦ because of rate limitation imposed by the surface exchange reaction of oxygen. 7,8 The difficulties involved technical problems in cell construction due to reactivity of the sampleelectrolyte interface and theoretical problems related to data analysis. In particular, the difficulty in extracting the surface exchange coefficient due to interference from the impedances of cell components other than the sample has been discussed. 12 In this connection, electrochemical impedance spectroscopy ͑E...

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