Surface chemistry of La0.6Sr0.4CoO3−δ thin films and its impact on the oxygen surface exchange resistance

Rupp, Ghislain M.; Téllez, Helena; Druce, John; Limbeck, Andreas; Ishihara, Takeshi; Kilner, John A.; Fleig, Jürgen

doi:10.1039/c5ta05279c

Cited by 117 publications

(157 citation statements)

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“…This suggests that the as-prepared LSCF particles are terminated by a single SrO rocksalt layer. A similar result was obtained by the same method on as-prepared LSC thin films 16,24 and by low energy ion scattering measurements on as-prepared LSCF thin films, 25 despite different thermal history. The aged LSCF electrode surfaces showed a strongly enhanced amount of Sr (from 1.30 ± 0.27 to 3.90 ± 0.92 nmol/cm 2 ), see Fig.…”

Section: Resultssupporting

confidence: 83%

Mechanisms of Performance Degradation of (La,Sr)(Co,Fe)O_3-δSolid Oxide Fuel Cell Cathodes

Wang

Yakal-Kremski

Yeh

et al. 2016

J. Electrochem. Soc.

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131

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Symmetric cells with porous La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) electrodes on Gd 0.1 Ce 0.9 O 1.95 (GDC) electrolytes were aged at 800 • C for 800 hours in ambient air. Electrochemical impedance spectroscopy (EIS) measurements performed periodically at 700 • C showed a continuous increase of the polarization resistance from 0.15 to 0.34 · cm 2 . Three-dimensional (3D) tomographic analysis using focused ion beam-scanning electron microscopy (FIB-SEM) showed negligible changes due to the ageing, suggesting that the observed resistance increase was not caused by electrode morphological evolution. However, an increased amount, by a factor of 3, of a water-soluble Sr rich surface phase on the aged LSCF electrode was detected by an etching procedure coupled with inductively coupled plasma-optical emission spectrometry (ICP-OES). The electrochemical analysis in combination with the microstructural parameters determined by FIB-SEM was used to examine the effect of Sr segregation on the rate of oxygen surface exchange, based on the Adler-Lane-Steele (ALS) model. 1-6However, a number of MIEC materials including LSCF exhibit Sr surface segregation, which has been proposed to hinder the oxygen surface exchange process. [7][8][9][10][11] This is believed to be an important issue for the stability of advanced SOFCs utilizing MIEC cathodes. Still, most of the Sr segregation observations have been made on thin film samples, where it is straightforward to measure surface composition using techniques such as X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and secondary ion mass spectrometry (SIMS).12-15 Recently Rupp et al. 16 reported a novel technique utilizing chemical etching with on-line inductively coupled plasma optical emission spectrometry (ICP-OES) detection to successfully quantify Sr-rich surface phases on dense La 0.6 Sr 0.4 CoO 3-δ (LSC) thin films. There is only one report of a measurement of a practical porous LSCF electrode that showed Sr segregation, measured via XPS, along with electrochemical performance degradation. 11The distinction between thin film and bulk electrode samples is potentially important. Thin films are often characterized by columnar growth with very high grain boundary densities, they may exhibit significant stress, and the thermal history is usually very different from that of porous electrodes (preparation mostly below 800• C). Although the measured properties of well-prepared polycrystalline thin-film electrodes normally agree reasonably well with values from bulk materials, 17 in some cases such as epitaxial thin film electrodes, properties may deviate significantly.18 Also, SOFC stacks with LSCF cathodes have been shown to provide reasonably stable long-term performance, 19 an observation that appears to be at odds with the Sr segregation and related degradation observed for thin-film LSCF.In this work, we investigated the degradation mechanisms, especially Sr surface segregation, of porous LSCF cathodes. LSCF symmetric-electrode cells with Gd 0.1 Ce 0.9 ...

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Section: Resultssupporting

confidence: 83%

Mechanisms of Performance Degradation of (La,Sr)(Co,Fe)O_3-δSolid Oxide Fuel Cell Cathodes

Wang

Yakal-Kremski

Yeh

et al. 2016

J. Electrochem. Soc.

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131

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“…6,10,17,19,22,34 The degradation on LSC at elevated temperatures renders itself in two apparent ways. These are the formation of Sr-rich precipitates at the surface, as in the present study, and the formation of a thin SrO termination layer at the top surface shown by low energy ion scattering and by ICP mass spectroscopy.…”

Section: Discussion On Implicationsmentioning

confidence: 99%

Effect of crystal orientation on the segregation of aliovalent dopants at the surface of La_0.6Sr_0.4CoO₃

2018

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The effect of crystal orientation on aliovalent dopant segregation at the surface of La 0.6 Sr 0.4 CoO 3 as a model perovskite oxide was investigated. La 0.6 Sr 0.4 CoO 3 pellets were produced and then annealed in air at 800 C for 2-5 hours to drive the cation segregation at the surface. To quantify the chemical state of the pellets' surface, including cation composition and secondary phase precipitates rich in Sr, the pellets were characterized by X-ray photoelectron spectroscopy and Auger electron spectroscopy. To systematically assess the correlation between crystal orientation and dopant cation segregation, selected regions at the surface of the annealed pellets were analyzed by scanning electron microscopy and electron backscatter diffraction. Investigation of more than 300 grains revealed that the area coverage of the Sr-rich precipitates on grains having a {001} orientation was more than 3 times higher compared to that on the {101} and {111} orientations. On the other hand, the number density of precipitates was very similar on each grain orientation. This grain orientation dependent behavior indicates that the exposed facet plays an important role in dopant cation segregation, especially in the growth of secondary phases, likely by altering the surface energy and charged defect concentration. The present study provides insights into the importance of atomic arrangements in determining the surface stability and cation segregation tendency on perovskite oxides.

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“…The preferred cathode materials for SOFC are mainly mixed ionic and electronic conducting (MIEC) perovskites like (La,Sr)(Co,Fe)O 3−δ (LSCF) and (La,Sr)CoO 3−δ (LSC). These materials deliver a high electrochemical performance, but due to the segregation of the Sr from the A-site of the perovskite lattice to the surface (Finsterbusch et al, 2012;Rupp et al, 2015), they are prone to reacting with volatile Cr species (Simner et al, 2006). According to the nucleation theory (Jiang and Zhen, 2008), the degradation mechanism is believed to be driven by a kinetically fast chemical reaction between the segregated Sr species and the volatile Cr species.…”

Section: Introductionmentioning

confidence: 99%

Accelerated Testing of Chromium Poisoning of Sr-Containing Mixed Conducting Solid Oxide Cell Air Electrodes

et al. 2018

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A straightforward method for accelerated testing of Cr poisoning phenomena on mixed ionic and electronic conducting (MIEC) Solid Oxide Cell (SOC) air electrode materials was developed. Cr 2 O 3 -powder is mixed with an organic matrix and screen printed onto anode-supported button cells with (La,Sr)(Co,Fe)O 3−δ (LSCF) cathode. Then, a thermal treatment was conducted to achieve the formation of a well-defined amount of SrCrO 4 on the cathode surface within only a few hours. For the proof of concept, single cell measurements were done to investigate the influence of this new kind of Cr deposition. As reference, a cell poisoned via gas phase diffusion and a cell without any Cr contamination were characterized in the same manner. According to the impedance data, the polarization resistance for both cells, which were contaminated with Cr species, increased. The expected relationship between deposited amount of Cr and increase of polarization resistance was found. ICP-OES analysis proves the high reproducibility of the method as the Cr content is only a function of the Cr paste composition and almost independent of external poisoning conditions. Due to its scalability and reproducibility, this method is proposed to be a new tool for screening of cathode materials and the investigation of different stages of Cr poisoning prior to more sophisticated and time consuming investigations like stack tests.

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Surface chemistry of La_0.6Sr_0.4CoO_3−δ thin films and its impact on the oxygen surface exchange resistance

Abstract: Correlation between the surface chemistry of LSC thin films analyzed by LEIS and ICP-MS and the oxygen exchange kinetics.

Cited by 117 publications

References 50 publications

Mechanisms of Performance Degradation of (La,Sr)(Co,Fe)O_3-δSolid Oxide Fuel Cell Cathodes

Mechanisms of Performance Degradation of (La,Sr)(Co,Fe)O_3-δSolid Oxide Fuel Cell Cathodes

Effect of crystal orientation on the segregation of aliovalent dopants at the surface of La_0.6Sr_0.4CoO₃

Accelerated Testing of Chromium Poisoning of Sr-Containing Mixed Conducting Solid Oxide Cell Air Electrodes

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Surface chemistry of La0.6Sr0.4CoO3−δ thin films and its impact on the oxygen surface exchange resistance

Abstract: Correlation between the surface chemistry of LSC thin films analyzed by LEIS and ICP-MS and the oxygen exchange kinetics.

Cited by 117 publications

References 50 publications

Mechanisms of Performance Degradation of (La,Sr)(Co,Fe)O3-δSolid Oxide Fuel Cell Cathodes

Mechanisms of Performance Degradation of (La,Sr)(Co,Fe)O3-δSolid Oxide Fuel Cell Cathodes

Effect of crystal orientation on the segregation of aliovalent dopants at the surface of La0.6Sr0.4CoO3

Accelerated Testing of Chromium Poisoning of Sr-Containing Mixed Conducting Solid Oxide Cell Air Electrodes

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Surface chemistry of La_0.6Sr_0.4CoO_3−δ thin films and its impact on the oxygen surface exchange resistance

Mechanisms of Performance Degradation of (La,Sr)(Co,Fe)O_3-δSolid Oxide Fuel Cell Cathodes

Mechanisms of Performance Degradation of (La,Sr)(Co,Fe)O_3-δSolid Oxide Fuel Cell Cathodes

Effect of crystal orientation on the segregation of aliovalent dopants at the surface of La_0.6Sr_0.4CoO₃