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

Abstract

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.

Factors Governing Oxygen Reduction in Solid Oxide Fuel Cell Cathodes

“…Although these materials are primarily electronic conductors ( t n,p ≈ 1), the very large concentrations of oxygen vacancies generated in these materials (as shown in Figure ) has been found to lead to rapid bulk oxygen ionic transport as well as increased rates of exchange of oxygen with the gas. The rates of these processes have been measured by a variety of techniques including electronic blocking methods, − 18 O 2 tracer techniques based on postmortem SIMS analysis, − relaxation methods involving the measurement of mass, conductivity, or other properties following a sudden change in P O 2 , ,− or direct measurement of permeation through a dense membrane. − Although we do not review this literature in detail, some of the highlights relevant to the present discussion are included.…”

Oxygen Exchange Kinetics over Sr- and Co-Doped LaFeO3

“…Fitting the transient responses with analytical solutions to Fick's second law is a common method to determine kinetic and transport properties for these types of materials. [19][20][21][22][23][24][25][26][27][28][29][30][31] These analyses are complemented by additional data from techniques such as isotopic analysis, in situ XRD, and Mo ¨ssbauer spectroscopy.…”

“…These properties, namely the chemical diffusion coefficient (neutral mass transfer under chemical potential gradients) and the oxygen exchange constant, are determined by subjecting mixed conductors to step changes in P O 2 . Fitting the transient responses with analytical solutions to Fick’s second law is a common method to determine kinetic and transport properties for these types of materials. – These analyses are complemented by additional data from techniques such as isotopic analysis, in situ XRD, and Mössbauer spectroscopy.…”

Comprehensive review of methane conversion in solid oxide fuel cells: Prospects for efficient electricity generation from natural gas

“…In particular, LSM and LSCF compositions have been among the most studied, and currently been employed in state-of-the-art SOFC cathode designs. Fundamental studies on perovskite-based MIEC cathode materials and their transport and kinetic properties have been reported elsewhere [257][258][259][260][261][289][290][291][292][293][294][295][296][297][298], and will not be reviewed here. Among the challenges, however, Sr-segregation to the surface has been a persistent problem that results in the degradation of cell performance [299][300][301].…”