Oxygen Exchange and Transport in (La0.6Sr0.4)0.98FeO3-d – Ce0.9Gd0.1O1.95 Dual-Phase Composites

Simona Ovtar,Peter Vang Hendriksen,Kion Norrman,Martin Søgaard

doi:10.1149/2.1031803jes

Simona Ovtar, Peter Vang Hendriksen + Show 2 more

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https://doi.org/10.1149/2.1031803jes

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Abstract

The chemical diffusion coefficient and the effective surface exchange coefficient (kex) of dual-phase (La0.6Sr0.4)0.98FeO3-d (LSF) − Ce0.9Gd0.1O1.95 (CGO) composites containing between 30 and 70 vol.% of CGO were determined by electrical conductivity relaxation (ECR) at high oxygen partial pressures (10−3 < pO2 < 1 atm) and at temperatures between 600°C and 900°C. The surface impurity segregation was detected by TOF-SIMS analysis. A large enhancement of kex was observed with increasing CGO fraction in the composite. kex was increased from 3.51 × 10−5 cm/s for a pure LSF to 1.86 × 10−4 cm/s for a 70 vol.% of CGO in the composite at 750°C for a pO2 change from 0.2 to 1.0 atm. The experiments demonstrate that the kex is enhanced due to a synergistic effect between the two phases, and suggest a direct involvement of CGO phase in the oxygen surface exchange reaction. Possible mechanisms that could account for the synergy are the oxygen exchange process occurs also on the CGO surface, for example a spillover of absorbed oxygen ions from the LSF surface to the CGO surface or/and scavenging of impurities from one phase to another, thereby improving the oxygen exchange properties of the cleaned phase.

Highlights

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A large focus has been on the specific composition La0.6Sr0.4Co0.2Fe0.8O3-d 4,13–15 (LSCF), which is especially promising for solid oxide fuel cells (SOFC)/solid oxide electrolysis cells (SOEC) applications due to a good compromise between oxygen transport properties and thermomechanical properties
The ionic conductivity increases with decreasing pO2 and the electronic conductivity is above a pO2 of 10−12 atm predominantly p-type and below 10−12 atm n-type.[8]

Summary

Introduction

General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. Users may download and print one copy of any publication from the public portal for the purpose of private study or research. Perovskites within the (La,Sr)(Fe,Co)O3 class of materials can potentially find application as catalysts, electrode materials in solid oxide fuel cells (SOFC) or solid oxide electrolysis cells (SOEC), in gas sensors and in oxygen transport membranes.[1,2,3,4] Both end members of the class; the iron free one (strontium doped lanthanum cobaltate)[5,6] as well as the cobalt free one (strontium doped lanthanum ferrite)[7,8,9,10,11,12] have been intensively studied. The oxygen surface exchange coefficient (kO) and the oxygen diffusion coefficient (DO) are commonly quantified using oxygen isotope diffusion profiles obtained from secondary ions mass spectrometry (SIMS),[16] and from electrical conductivity relaxation (ECR)[11] experiments (where one measures chemical diffusion coefficient (Dchem) and effective surface exchange coefficient (kex), and more recently from Pulsed Isotope Exchange (PIE), using powder samples.[17]

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