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 s ealed 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 potentia l step at short and long times. It is shown that, while the formation of in terfacial phases by reaction between the sample and the electrolyte may aff ect the surface exchange coefficient, the chemical diffusion coefficient da ta determined by the present approach are independent of such interfacial p henomena. The consistency of data obtained from several specimens with vary ing 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 cel l. The new cell allows monitoring of the permeation rate by electrochemical means. The steady-state permeation data obtained by the permeation cell ar e consistent with the chemical-diffusion and surface-exchange coefficients measured by the blocked diffusion cell as long as the assumptions of the re lated theoretical models are satisfied. This is a further validation of the diffusion model and the related methodology developed here for obtaining t he necessary data for characterizing oxygen exchange and transport in such materials. (C) 1999 The Electrochemical Society. S0013-4651(98)06-048-0. Al l rights reserved.