Oxygen Exchange and Diffusion Coefficients of Strontium‐Doped Lanthanum Ferrites by Electrical Conductivity Relaxation

Abstract

Electrical conductivity relaxation experiments were performed on thin specimens of Lai_rSrFe0a_, (x =0.1, 0.4) at oxygen partial pressures Po, = 10 -1 bar in the temperature range 923 to 1223 K. The transient response of the electrical conductivity after a sudden change of the ambient oxygen partial pressure was analyzed in the frequency domain. The latter procedure allowed diffusion-limited and surface exchange-limited kinetics of re-equilibration to be distinguished. The response of specimens with thiçknesses of 350 to 460 p.m indicated diffusion-controlled kinetics at Po,> 0.03 bar. The chemical diffusion coefficients, D, were found invariant with oxygen pressure. At 1073 K the absolute values were D = 6.5 x 10-6 cm2 -i for x = 0.1 and D = 1.1 X 10 cm2 s1 for x = 0.4, with activation energies of about 80 kJ/mol. The equilibration process was governed by surface exchange at Po, < 0.01 bar. The surface exchange coefficient, k0, was proportional to pa,, where n = 0.65 to 0.85. This pressure dependency was interpreted in terms of a slow surface process involving an oxygen molecule and a surface oxygen vacancy, and causes the observed sharp transition from diffusion-to exchange-controlled kinetics. The activation energy of k0 was estimated at 110 to 135 kJ/mol.

In-Operando Determination of SOFC Cathode Oxygen Surface Exchange Coefficients for Enhanced Oxygen Reduction Reaction Kinetics

“…Previous studies reported that values of k ex for LSC are 1-2 orders of magnitude higher than k ex for LSF. [27][28][29][30] However, these reported experimental results were measured using depth profiling or conductivity relaxation methods, which lack the surface sensitivity that the surface exchange technique that is reported here can provide. The surface exchange process measured with these methods is measured either under the diffusion-limited regime of the exchange process, or under the mixed diffusion-surface regime, due to the use of sample thickness.…”

The Significance of Gas-Phase Mass Transport in Assessment ofkchemandDchem

“…Surface exchange and bulk diffusion coefficients obtained from ECR measurements have been shown to exhibit a discrepancy between steps performed from high to low (reduction) and low to high (oxidation) oxygen partial pressures in a number of reports [3,4,5,6,7]. The apparent difference in oxidation and reduction values is in general more pronounced at decreasing p O 2 .…”

“…By performing a step change in oxygen partial pressure, which will lead to a change in the oxygen non-stoichiometry, and monitoring the electronic conductivity response, the change in non-stoichiometry over time and thus the oxygen transport kinetics can be deduced. The validity of this technique, especially for materials exhibiting high oxygen exchange rates and/or at low partial pressures of oxygen, has however been questioned [3,4]. In this study we will investigate the validity of ECR in terms of transport coefficient assessment both from an experimental and a theoretical modelling approach.…”

Comparison of Electrochemical Properties of La0.6Sr0.4FeO3-δThin Film Electrodes: Oxidizing vs. Reducing Conditions

“…19,20 Several studies were performed on the defect chemistry and transport properties of LSF [21][22][23][24][25][26][27][28][29][30][31] [2] Fe 4+ states can be interpreted as electron holes (h • ) and those determine the electronic conductivity at high oxygen partial pressure as they are the majority mobile charge carrier. [20][21][22] Towards lower oxygen partial pressures (and/or higher temperatures) the equilibrium of Eq.…”