Surface Segregation and Phase Formation in Thin Films of Sofc Cathode Materials

Davis, Jacob; Yu, Yang; Cetin, Deniz; Ludwig, Karl; Pal, Uday B.; Gopalan, Srikanth; Basu, Soumendra N.

doi:10.1002/9781118888735.ch31

Cited by 3 publications

(4 citation statements)

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“…Strontium segregation to the surface of LSCF-6428 during annealing at elevated temperatures in the form of strontium oxide (SrO) and strontium carbonate (SrCO 3 ) has been reported previously [6,7]. Oh et al reported that the formation of Sr-rich precipitates was enhanced with increasing temperature and oxygen partial pressure [6].…”

Section: Introductionmentioning

confidence: 91%

“…Oh et al reported that the formation of Sr-rich precipitates was enhanced with increasing temperature and oxygen partial pressure [6]. Davis et al reported SrCO 3 formation on LSCF-6428 thin films after annealing at 800°C in air, using hard x-ray photoelectron spectroscopy (HAXPES) [7]. The formation of SrCO 3 was attributed to the presence of CO 2 in the atmosphere.…”

Section: Introductionmentioning

confidence: 99%

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Effect of atmospheric CO2 on surface segregation and phase formation in La0.6Sr0.4Co0.2Fe0.8O3− thin films

Luo

Cetin

et al. 2014

Applied Surface Science

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The effects of atmospheric CO 2 on surface segregation and phase formation in La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF-6428) were investigated. (001)-oriented LSCF-6428 thin films were deposited on lattice matched (110)-oriented NdGaO 3 (NGO) substrates by Pulsed Laser Deposition (PLD). Using the synchrotron technique of Total Reflection X-ray Fluorescence (TXRF), it was found that the kinetics of Sr surface segregation was enhanced when annealing at 800°C in a high CO 2 partial pressure, as compared to a similar anneal in a CO 2-free atmosphere, with the oxygen partial pressure being constant in both cases. HArd X-ray PhotoElectron Spectroscopy (HAXPES) measurements showed that the contribution of the surface carbonate to surface oxide phases increased significantly for the sample annealed in the high-CO 2 atmosphere. Atomic Force Microscopy (AFM) studies showed enhanced surface phase formation during the high-CO 2 partial pressure anneal. Density functional theory (DFT) calculations provide a thermodynamic basis for the enhanced kinetics of surface segregation in the presence of atmospheric CO 2 .

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Section: Introductionmentioning

confidence: 91%

Section: Introductionmentioning

confidence: 99%

Effect of atmospheric CO2 on surface segregation and phase formation in La0.6Sr0.4Co0.2Fe0.8O3− thin films

Luo

Cetin

et al. 2014

Applied Surface Science

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“…It has been reported previously that strontium tends to segregate to the surface in forms of strontium oxide (SrO) and strontium carbonate (SrCO 3 ) during annealing at SOFC working temperatures (2,5). The electrochemical performance of the device would be degraded not only because of the electrically insulating nature of SrO and SrCO 3 , but also because the formation of these surface secondary phases reduces the effective LSCF surface area for oxygen reduction reaction (ORR) process.…”

Section: Introductionmentioning

confidence: 99%

Electronic Structure Measurements of Heteroepitaxial Solid Oxide Fuel Cell Cathode Thin Films

Yu¹,

Ludwig²,

Pal³

et al. 2014

ECS Trans.

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Synchrotron based total reflection x-ray fluorescence (TXRF) and hard x-ray photoelectron spectroscopy (HAXPES) technique have been utilized to measure the compositional changes and evolution of surface chemistry of strontium doped lanthanum cobalt iron oxide (LSCF) annealed at 800 °C, as a function of strontium dopant concentration and gas composition. TXRF is a useful way to depth-profile the surface composition of material at elevated temperatures in real time. The capability of HAXPES to overcome surface contamination issues helps better understand the segregation phenomena. This study demonstrates that the formation of Sr carbonate results in enhanced kinetics of Sr surface segregation in LSCF thin films with presence of CO2 when annealing at 800 °C and lowering the Sr dopant concentration results in a decrease Sr surface segregation.

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“…6 Strontium segregation has also been observed in the widely used cathode material La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 (LSCF). [7][8][9][10][11][12][13][14] Oh et al showed that precipitates form on grain surfaces upon annealing after 50 hours. 11 Segregation has been found for other ABO 3 perovskites including La 0.8 Sr 0.2 CoO 3±δ .…”

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confidence: 99%

Surface Segregation in Lanthanum Strontium Manganite Thin Films and Its Potential Effect on the Oxygen Reduction Reaction

Davis

Ludwig

Smith

et al. 2017

J. Electrochem. Soc.

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A limiting factor for solid oxide fuel cell (SOFC) performance is the oxygen reduction reaction (ORR) that occurs at the cathode surface. This study examines changes in the surface composition of (La 0.8 Sr 0.2 ) 0.95 MnO 3 (LSM-20), a well-known SOFC cathode material. Heteroepitaxial thin films of LSM-20 were grown on lattice matched single crystal LaAlO 3 and NdGaO 3 substrates. Total Reflection X-ray Fluorescence (TXRF) measurements showed surface manganese enrichment on annealing at 800 • C in air. X-ray absorption near edge spectroscopy (XANES) showed that this leads to a redistribution of manganese 3+, 4+, and 2+ valence states at the surface. This was confirmed by defect modeling, which showed that surface Mn segregation leads to an increase in the surface concentration of oxygen vacancies and the Mn +4 species, both of which are expected to be beneficial for ORR. Solid oxide fuel cells (SOFCs) are chemical to electrical energy conversion devices that have several advantages, including potential efficiency over 70%, fuel flexibility, low emissions, and low degradation over long time scales.1 Commonly, SOFCs are operated at high temperatures (∼ 800• C). Incorporation of oxygen into the cell at the cathode surface, known as the oxygen reduction reaction (ORR) is identified as a rate-limiting step in an SOFC.2 The overall ORR can be expressed as:Equation 1 actually consists of several sub-steps including oxygen adsorption on the surface of the cathode, electron transfer to form oxygen ions, and incorporation of oxygen ions into oxygen vacancies, among others. For an electronic conducting cathode material like strontium-doped lanthanum manganite (LSM), the charge transfer occurs at the triple phase boundary (TPB), 2 requiring the adsorbed oxygen to be transported to the TPBs.Recent research examines how changes in cation dopant concentration impacts surface phase formation.3,4 Cations in these materials segregate to grain surfaces and create phases that can both enhance (as small, nanoscale adsorption sites) oxygen incorporation and block (large insulating phases) incorporation sites.Total reflection X-ray fluorescence (TXRF) studies on LSM films with 30 at% Sr A-site doping have suggested that strontium segregates to the surface and form strontium enriched nanoparticles.5 Yildiz et al. examined Mn K-edge XANES of LSM and saw no dependence on polarization at high temperature. Yildiz and coworkers also looked at La L III -edge XANES for LSM and LCM and suggested that there is an increase in the number of electronic La 5d-band vacancies in the near surface region of these materials. 15,20,21 In contrast, Shao-Horn and co-workers have shown that a low concentration of surface phases may in fact be beneficial to overall cell performance by promoting surface exchange.22 Understanding how the surface chemistry changes during initial heating may reveal a way to exploit the self-decorating nature of these materials. Thus, understanding surface segregation/phase formation at the cathode surface and their effects on t...

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Surface Segregation and Phase Formation in Thin Films of Sofc Cathode Materials

Cited by 3 publications

References 11 publications

Effect of atmospheric CO2 on surface segregation and phase formation in La0.6Sr0.4Co0.2Fe0.8O3− thin films

Effect of atmospheric CO2 on surface segregation and phase formation in La0.6Sr0.4Co0.2Fe0.8O3− thin films

Electronic Structure Measurements of Heteroepitaxial Solid Oxide Fuel Cell Cathode Thin Films

Surface Segregation in Lanthanum Strontium Manganite Thin Films and Its Potential Effect on the Oxygen Reduction Reaction

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