Insight into surface segregation and chromium deposition on La0.6Sr0.4Co0.2Fe0.8O3−δ cathodes of solid oxide fuel cells

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In high temperature solid oxide fuel cells (SOFCs), electrode/electrolyte interfaces play a critical role in the electrocatalytic activity and durability of the cells. In this study, thermally and electrochemically induced electrode/electrolyte interfaces were investigated on pre-sintered and in situ assembled (La 0.8 Sr 0.2 ) 0.90 MnO 3 (LSM) and La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) electrodes on Y 2 O 3 -ZrO 2 (YSZ) and Gd 0.2 Ce 0.8 O 2 (GDC) electrolytes, using atomic force microscopy (AFM) and electrochemical impedance spectroscopy (EIS). The results indicate that thermally induced interface is characterized by convex contact rings with depth of 100-400 nm and diameter in agreement with the particle size of pre-sintered LSM and LSCF electrodes, while the electrochemically induced interfaces under cathodic polarization conditions on in situ assembled electrodes are characterized by particle-shaped contact marks or clusters (50-100 nm in diameter). The number and distribution of contact clusters depend on the cathodic current density as well as the electrode and electrolyte materials. The contact clusters on the in situ assembled LSCF/GDC interface are substantially smaller than that on the in situ assembled LSM/GDC interface likely due to the high mixed ionic and electronic conductivities of LSCF materials. The results show that the electrochemically induced interface is most likely resulting from the incorporation of oxygen species and cation interdiffusion under cathodic polarization conditions. However, the electrocatalytic activity of electrochemically induced electrode/electrolyte interfaces is comparable to the thermally induced interfaces for the O 2 reduction reaction under SOFC operation conditions. © The Author Solid oxide fuel cell (SOFC) is one of the most efficient technologies for the conversion of chemical energy of fuels such as hydrogen and natural gas directly into electrical power. It employs a solid oxide electrolyte such as yttria-stabilized zirconia (YSZ) that serves as an ionic conductor in the temperature range between 600• C to 1000• C. The electrolyte separates the cathode from the anode and conducts oxygen ions from the cathode to the anode where they react electrochemically with the fuel. Electrons are then released to an external circuit, which provides a useful source of electrical power. The electrochemical reactions such as O 2 reduction at the cathode and H 2 oxidation at the anodes occur at the electrode/electrolyte interface regions.1-3 Thus, in SOFCs, electrode/electrolyte interfaces play a vital role in the mechanism and kinetics of the electrochemical reactions and in the fundamental understanding of the relationship between the electrochemical processes and microstructure.4-10 The cell performance and durability also depend strongly on the microstructural change at the interface under SOFC operation conditions. 11,12Lanthanum strontium manganite (LSM) is one of the most common cathode materials for SOFCs because of high electrical conductivity, good thermal and chem...

Section: Resultsmentioning

confidence: 99%

Thermally and Electrochemically Induced Electrode/Electrolyte Interfaces in Solid Oxide Fuel Cells: An AFM and EIS Study

Jiang

2015

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“…This is the most common location for the deposition of Cr species when using a mixed ionicelectronic conducting (MIEC) cathode such as LSCF. 25,27,35 Investigating the cathode in greater detail revealed some deterioration in approximately the last 10 μm facing the GDC barrier layer. Compared to the upper bulk of the cathode, this area appears slightly fractured.…”

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confidence: 99%

“…For LSCF cathodes, the degradation product is often found under the channels of the flow field. 9,24,25 The most widespread theory describing the reaction of gaseous Cr with Sr-containing cathodes such as LSCF is the nucleation theory. 9 It is based on a chemical reaction of the Cr species in the gas phase with segregated SrO on the cathode surface, which is kinetically fast and results in the formation of Sr-Cr-(O)-nuclei (1).…”

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confidence: 99%

Insight into the Reaction Mechanism of (La_0.58Sr_0.40)(Co_0.20Fe_0.80)O_3-δCathode with Volatile Chromium Species at High Current Density in a Solid Oxide Fuel Cell Stack

Beez

Yin

Menzler

et al. 2017

Anode-supported solid oxide fuel cells with different Cr protection layers on the metallic interconnect were operated in a short stack at 700 • C for 1240 h. The current density was raised sequentially from 0.5 A cm −2 during the first 240 h of operation to 0.75 A cm −2 for a further 1000 h. After operation, the (La,Sr)(Co,Fe)O 3-δ (LSCF) cathode layers were analyzed with respect to Cr interaction by both wet chemical and microstructural methods. For cells equipped with interconnects coated with a dense APS protection layer, the amount of Cr on the cathode was in the range of a few μg. For cells with a porous WPS coating on the interconnect, the amount of Cr was in the range of 110-160 μg cm −2 and Cr-containing phases were detected by SEM analysis both on top of the cathode layer and also at the LSCF/GDC interface, which has rarely been observed before. In addition, a deterioration of the cathode microstructure near the LSCF/GDC interface was observed. With respect to the high current density during operation, a theory was developed which explains both the Cr deposition at the LSCF/GDC interface and also the deterioration of the cathode. Climate change, limitation of resources, technical and political obstacles associated with nuclear power and changes in the global energy economy are only a few reasons why the world needs alternative concepts for its future energy supply. One key technology offering decentralized energy supply is solid oxide fuel cells (SOFC).1 Their high efficiency; fuel flexibility and scalability allows them to be used as decentralized power plants or, on a smaller scale, in auxiliary power units or range extenders in mobile applications. 2-4During the development and optimization of SOFC stacks and systems, attention was not only focused on performance but also on the application of cost-efficient materials. One major improvement was the establishment of metallic interconnects, offering high electronic and thermal conductivity and mechanical stability while also decreasing the price per repeating unit compared to full ceramic stack designs.5 However, the preferentially used Cr-containing steels lead to pronounced performance degradation due to the evaporation of Cr compounds, which then react with the cathode. Under the oxidizing conditions on the cathode side, hexavalent Cr species such as CrO 3 and CrO 2 (OH) 2 evaporate from the oxide scale of the interconnect or balance-of-plant components and react with the LSCF cathode material to form a Sr-and Cr-containing oxide scale thereby decreasing cell performance. 3,6,7 This effect is known to be influenced by parameters such as Cr partial pressure, air humidity and temperature. [8][9][10] To avoid this Cr-related degradation, different strategies have been developed to either reduce the Cr partial pressure to an acceptable level for the desired operating time or to increase the Cr tolerance of the cathode material itself. The most promising technique for avoiding Cr poisoning is to coat the interconnect steel with a preferentially dense pro...

“…13 In the case of LSCF electrodes, deposition of Cr species preferentially take place on the surface of the LSCF electrode, resulted from the interaction between the segregated SrO and gaseous Cr species. 14,15 Both the humidity in the air stream and operation temperature have a significant effect on the Cr deposition. 16 Cr deposition decreases significantly with the decrease of temperature, most likely due to the reduced Sr segregation as well as the decrease of the partial pressure of gaseous Cr species at reduced temperatures.…”

mentioning

confidence: 99%

Co-Deposition and Poisoning of Chromium and Sulfur Contaminants on La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δCathodes of Solid Oxide Fuel Cells

Wang

O’Donnell

et al. 2015

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The presence of both chromium and sulfur (Cr/S) contaminants on the microstructure and electrocatalytic activity properties of La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) electrodes of solid oxide fuel cells (SOFCs) is studied, using Confocal laser Raman spectroscopy, XRD, scanning electron microscopy, X-ray photoelectron spectroscopy (XPS) and electrical conductivity relaxation (ECR) methods. LSCF dense bar samples were heat treated in the presence of Cr 2 O 3 and 20 ppm SO 2 and in the temperature range of 600-900 • C. The deposition and reaction products between LSCF and Cr/S depend on the temperature: SrCrO 4 only forms on LSCF samples at 900 • C and 800 • C, while formation of SrSO 4 phase occurs at all temperatures studied. The results indicate that sulfur shows a higher activity with LSCF, as compared to gaseous Cr species. Segregated SrO is more likely to react with gaseous Cr species at higher temperatures, however, reaction with SO 2 is more pronounced at lower temperatures, forming SrSO 4 . ECR results indicate that co-deposition of Cr and sulfur significantly deteriorates the surface exchange and diffusion processes for the O 2 reduction reaction on LSCF electrodes. The durability of a solid oxide fuel cell (SOFC) is critically related to the degradation behavior of its cathodes such as La 0.8 Sr 0.2 MnO 3 (LSM) and La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) in the presence of impurity species such as chromium from the chromia-forming interconnect, silica, boron and volatile alkaline elements from the glass seals sealant and sulfur from air.1-8 Among them, gaseous chromium species vaporized from the chromium oxide scale of chromia-forming metallic interconnect are probably the most investigated contaminants affecting the performance of SOFCs' cathodes. The mechanism and process of the deposition and poisoning of chromium species at the cathodes of SOFCs have been extensively investigated, including LSM, 9-11 LSCF 12 and Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ (BSCF). 13 In the case of LSCF electrodes, deposition of Cr species preferentially take place on the surface of the LSCF electrode, resulted from the interaction between the segregated SrO and gaseous Cr species.14,15 Both the humidity in the air stream and operation temperature have a significant effect on the Cr deposition. 16 Cr deposition decreases significantly with the decrease of temperature, most likely due to the reduced Sr segregation as well as the decrease of the partial pressure of gaseous Cr species at reduced temperatures. 17The sulfur in the form of SO 2 or H 2 S in the air stream is another important contaminant affecting the performance stability of SOFC cathodes. SO 2 content in air can be in the range of 10-340 μg m −3 (3.5 × 10 −3 -0.12 ppm) in cities. 18 Despite the low concentration of SO 2 , sulfur in the air stream can accumulate at the cathodes of SOFC cells, degrading the performance.5 Wang et al. 19 studied the polarization performance behavior of LSCF in the presence of 0.1 ppm SO 2 and observed two-stages of performance degrad...