Determination of Oxygen Nonstoichiometry and Diffusivily in Mixed Conducting Oxides by Oxygen Coulometric Titration I. Chemical Diffusion in La085r02CoO3_5

Oxygen coulometric titration has been applied to measure chemical diffusion in La0 45r0 2CoO36 between 700 and 1000°C. The transient current response to a potentiostatic step has been transformed from the time domain to the frequency domain. The equivalent circuit used to fit the resulting impedance data contains the element that describes the finite-length diffusion of oxygen into the sample specimen. Other elements included are the gas-phase capacitance and the sum of the gas-phase diffusion resistance and that associated with the limited surface exchange kinetics of the sample. The chemical diffusion coefficient of perovskite La0 85s02CoO3_3 has been determined as a function of temperature and oxygen partial pressure. Its value can be represented by D (cm°/s) = 5.91 X exp [(—135 kJ/mol)/RT], and turns out to be practically independent of oxygen partial pressure in the range 102 — 0.209 bat Introduction Mixed ionic and electronic conducting perovskites La,.,,SrCoO3_6 receive wide attention to date because of their potential applications as gas separation membranes, oxidation catalysts, and as electrodes in solid oxide fuel cells and oxygen sensors. High ionic conductivity in these materials at elevated temperatures results from large oxygen vacancy concentration in conjunction with a high vacancy diffusivity. Although the ionic transference number remains below a value of 0.01, the ionic conductivity may be one to two orders of magnitude higher than that of stabilized zirconia. An obvious consideration to the design of the aforementioned applications is to know how these parameters depend on environmental parameters like oxygen partial pressure and temperature. Changes in the oxygen vacancy concentration as a function of temperature and oxygen partial pressure are commonly measured using thermogravimetry or coulometric titration. An advantage of the latter technique is that electrical currents and voltages can be measured with high accuracy. An additional advantage is that coulometric titration permits direct control of the oxygen stoichiometry of the sample specimen, which allows easy determination of thermodynamic quantities such as the partial energy and entropy associated with oxygen incorporation into the oxide."2 Chemical diffusion coefficients can be measured simultaneously by monitoring the rate at which the electrochemical cell proceeds to equilibrium after the cell is imposed to a sudden change in either the voltage or the current. Thus obtained chemical diffusion coefficients can be used to calculate the corresponding vacancy diffusion coefficients when combined with equilibrium data of the oxygen stoichiometry as a function of oxygen partial pressure. The value of the vacancy diffusion coefficient can also be obtained from data of the ionic conductivity. However, the direct measurement of ionic conductivity in predominant electronically conducting oxides is often complicated due to short-circuiting paths for oxygen transport, such as diffusion of oxygen species along the oxide surface or via the gas phase through rapid exchange. This leads to overestimates of the ionic conductivity.3 Another problem is the possibility of an interfacial reaction between the oxide and the blocking electrode material or the glass used for sealing to suppress parasitic contributions to oxygen transport. Also, the ionic conductivity can be determined from steady-state oxygen permeation experiments, assuming negligible influence of the oxygen exchange kinetics at the gas solid interface.4 It may be noted that such an assumption is not required when using coulometric titration. Since the time dependence of bulk diffusion differs from that of the surface exchange kinetics, these can be distinguished by analysis of the experimental data in the frequency domain. In this paper, the oxygen nonstoichiometry and diffusivity of perovskite La0 4Sr0 2CoO3.3 are measured by oxygen coulometric titration. The chemical diffusion coefficient is determined from analysis of the transient current response to potentiostatic steps. Oxygen nonstoichiometry data which includes determination of the partial energy and entropy associated with oxygen incorporation are presented in Part II of this paper.5 Theory Chemical diffusion.—When an oxygen chemical potential gradient is present in the bulk of a mixed oxygen ion and electron conducting oxide, oxygen ions diffuse in the direction toward low oxygen chemical potential. The oxygen flux J0 is related to the gradient in oxygen chemical potential p,, as given by the Wagner equation 6.7