Oxygen exchange kinetics of La0.4Sr0.6FeO3?? by simultaneous application of conductivity relaxation and carrier gas coulometry

Preis, Wolfgang

doi:10.1016/j.ssi.2003.12.045

Cited by 71 publications

(38 citation statements)

References 11 publications

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“…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 .…”

Section: Motivation and Initial Resultsmentioning

confidence: 99%

The Significance of Gas-Phase Mass Transport in Assessment ofk_chemandD_chem

Lohne

Søgaard

Wiik

2013

J. Electrochem. Soc.

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In this work, the validity of electrical conductivity relaxation (ECR) as a method for the assessment of chemical surface exchange, k chem , and bulk diffusion, D chem , coefficients is investigated with respect to mass transport limitations in the gas phase. A model encompassing both the oxygen surface exchange, mass transport in the bulk sample and the gas phase was set up and solved under different conditions using finite element software. It is found that the transport of oxygen in the gas phase is insufficient at low oxygen partial pressures, causing a concentration boundary layer at the sample surface to develop. This significantly decreases the driving force for oxygen exchange. The effect of mass transport limitations on the computed apparent transport coefficients is shown to be pronounced and surface exchange coefficients are shown to deviate as much as one order of magnitude from the set values. When mass transport limitations are pronounced, a discrepancy between oxidation and reduction values of the apparent k chem and D chem is evident and modelled apparent activation energies for k chem are shown to decrease significantly. The validity of the apparent transport coefficients can be improved by changing the experimental parameters, however the surface exchange coefficient is extremely sensitive to insufficient transport of oxygen in the gas phase and improvements are in general marginal. A criteria for the validity of D chem is introduced while no such measure could be introduced for k chem . The effect of experimental parameters and material properties on mass transport limitations are presented and general recommendations concerning the assessment of k chem and D chem are given.

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Section: Motivation and Initial Resultsmentioning

confidence: 99%

The Significance of Gas-Phase Mass Transport in Assessment ofk_chemandD_chem

Lohne

Søgaard

Wiik

2013

J. Electrochem. Soc.

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“…Humidification of the test gases was realized by passing the gas streams through gas washing bottles which were filled with deionized water and placed into a thermostat. To achieve relative humidities (r.h.) of 30% and 60%, the thermostat was set to 6°C and 17°C, from the conductivity relaxation data using non-linear regression of the corresponding solutions of the diffusion equation [27,28]. The effects of chromium poisoning were simulated by placing a chromium pellet upstream close to the specimen thus exposing the sample to chromia-containing gas flows.…”

Section: Electrical Conductivity Relaxation Measurementsmentioning

confidence: 99%

Long-term stability of the IT-SOFC cathode materials La0.6Sr0.4CoO3−δ and La2NiO4+δ against combined chromium and silicon poisoning

et al. 2015

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Long-term degradation effects of combined Cr-and Si-poisoning on the promising IT-SOFC cathode materials La 0.6 Sr 0.4 CoO 3-δ and La 2 NiO 4+δ were investigated at 700°C in dry and humid atmospheres for subsequent periods of 1000 hours using dcconductivity relaxation measurements. Degradation-induced changes in chemical composition and morphology of the contaminated sample surfaces were studied by atomic force microscopy, X-ray photoelectron spectroscopy and scanning electron microscopy with energy and wavelength dispersive X-ray analysis. Upon exposure to humid, Cr-and Si-containing gas flows both materials exhibit a strong decrease of the chemical surface exchange coefficient of oxygen by a factor 110 and 40 for La 0.6 Sr 0.4 CoO 3-δ and La 2 NiO 4+δ , respectively, which can be attributed to the formation of Cr-containing crystallites on the degraded sample surfaces. Post-test analyses confirm large amounts of Cr accompanied by a Sr-enrichment within the first 600 nm of the surface of La 0.6 Sr 0.4 CoO 3-δ , indicating the decomposition of the perovskite phase by SrCrO 4 -formation. For La 2 NiO 4+δ the penetration depth of chromium is significantly less and Cr-traces up to a depth of up to 140 nm were determined by depth profiling. For both compounds silicon was found to spread in small patches across the entire sample surface as determined by elemental mapping analysis.

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“…[156] or La 0. 4 Sr 0.6 FeO 3Àd [157] ). The different character of the electronic structure of the "electron-rich" Sr(Fe x Ti 1Àx )O 3Àd materials drastically affects oxygen incorporation kinetics.…”

Section: High Iron Contentmentioning

confidence: 99%

How Is Oxygen Incorporated into Oxides? A Comprehensive Kinetic Study of a Simple Solid‐State Reaction with SrTiO₃ as a Model Material

2008

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The kinetics of stoichiometry change of an oxide--a prototype of a simple solid-state reaction and a process of substantial technological relevance--is studied and analyzed in great detail. Oxygen incorporation into strontium titanate was chosen as a model process. The complete reaction can be phenomenologically and mechanistically understood beginning with the surface reaction and ending with the transport in the perovskite. Key elements are a detailed knowledge of the defect chemistry of the perovskite as well as the application of a variety of experimental and theoretical tools, many of them evolving from this study. The importance of the reaction and transport steps for (electro)chemical applications is emphasized.

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Oxygen exchange kinetics of La0.4Sr0.6FeO3?? by simultaneous application of conductivity relaxation and carrier gas coulometry

Cited by 71 publications

References 11 publications

The Significance of Gas-Phase Mass Transport in Assessment ofk_chemandD_chem

The Significance of Gas-Phase Mass Transport in Assessment ofk_chemandD_chem

Long-term stability of the IT-SOFC cathode materials La0.6Sr0.4CoO3−δ and La2NiO4+δ against combined chromium and silicon poisoning

How Is Oxygen Incorporated into Oxides? A Comprehensive Kinetic Study of a Simple Solid‐State Reaction with SrTiO₃ as a Model Material

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Oxygen exchange kinetics of La0.4Sr0.6FeO3?? by simultaneous application of conductivity relaxation and carrier gas coulometry

Cited by 71 publications

References 11 publications

The Significance of Gas-Phase Mass Transport in Assessment ofkchemandDchem

The Significance of Gas-Phase Mass Transport in Assessment ofkchemandDchem

Long-term stability of the IT-SOFC cathode materials La0.6Sr0.4CoO3−δ and La2NiO4+δ against combined chromium and silicon poisoning

How Is Oxygen Incorporated into Oxides? A Comprehensive Kinetic Study of a Simple Solid‐State Reaction with SrTiO3 as a Model Material

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The Significance of Gas-Phase Mass Transport in Assessment ofk_chemandD_chem

The Significance of Gas-Phase Mass Transport in Assessment ofk_chemandD_chem

How Is Oxygen Incorporated into Oxides? A Comprehensive Kinetic Study of a Simple Solid‐State Reaction with SrTiO₃ as a Model Material