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

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 SrCo0.5Fe0.5O3−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.