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

Abstract

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...

A Detailed Post Mortem Analysis of Solid Oxide Electrolyzer Cells after Long-Term Stack Operation

“…As stated previously, chemical analysis showed that chromium can also be found in higher amounts at the air inlet than at the air outlet. This phenomenon has been shown before: strontium chromate is more likely to form at the air inlet than at the air outlet (25). Additionally, LSCF is obviously not stable over 18 000 hours in electrolysis mode and strontium diffuses into the electrolyte as well as to the top of the cell.…”

Thermodynamic perspective of Sr‐related degradation issues in SOFCs

“…We applied this diagram to explain Cr species deposition under real stack operation conditions. More detailed information can be found in the co‐work with A. Beez …”

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

“…The amount of Cr deposited on the cathode strongly depends on the Cr protection coating applied to the interconnect and the operating temperature (Beez et al, 2017). For stacks with less efficient Cr protection coatings (e.g., porous coatings applied by wet powder spraying), which were operated for up to 17.000 h, we measured a Cr content in the range of 100-200 µg cm −2 on the stack level (Menzler et al, 2011;Menzler and Batfalsky, 2012;Beez et al, 2017).…”

“…The amount of Cr deposited on the cathode strongly depends on the Cr protection coating applied to the interconnect and the operating temperature (Beez et al, 2017). For stacks with less efficient Cr protection coatings (e.g., porous coatings applied by wet powder spraying), which were operated for up to 17.000 h, we measured a Cr content in the range of 100-200 µg cm −2 on the stack level (Menzler et al, 2011;Menzler and Batfalsky, 2012;Beez et al, 2017). So far a non-linear correlation between the amount of Cr deposited on an LSCF cathode and the operation time of a stack was measured (Menzler and Batfalsky, 2012) and a Cr content in the range of 200 µg cm −2 is the highest value reached for JÜLICH ASC stacks so far.…”

“…The amount of Cr 2 O 3 powder in the paste was calculated considering the wet layer thickness of the screen used for the screen printing process. It was targeted to achieve a Cr deposition, which is comparable to average Cr contents found in LSCF cathodes after several thousand hours of stack operation with JÜLICH specific stack design (∼100-200 µg cm −2 ) (Menzler et al, 2011;Beez et al, 2017). The Cr content of the paste was adjusted to 6.2 wt.-% (equals 0.1 g Cr 2 O 3 per 1 g organic matrix) with the aim to achieve a Cr deposition of ∼100 µg cm −2 .…”