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...

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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.

As part of two different stack tests with four-plane short stacks and their intensive post-test characterization, two varying diffusionrelated degradation mechanisms were investigated. The first was a short-term test (~1250h) with two different chromium evaporation protection layers on the air-side metallic interconnect and frame and the second was a long-term endurance test (~ 35,000h). For the first stack, two planes were coated with a manganese oxide layer applied by wet powder spraying (WPS), while the other two planes were coated with a manganese-cobalt-iron spinel layer by atmospheric plasma spraying (APS). The voltage loss in the planes with a WPS-coated interconnect was markedly higher than in those coated by means of APS. Finally, it was shown that the microstructure of the layers plays a key role in minimizing Cr evaporation. In this stack, gas-phase diffusion prevails over degradation. In the long-term stack, severe degradation due to solid-state manganese diffusion was observed. This paper draws an interaction hypothesis.

The thermal expansion of La0.5Sr0.5Co0.25Fe0.75O3 (LSCF55) is investigated both by first principles phonon calculations combined with the quasi‐harmonic approximation (QHA) and by experimental approaches. Within the framework of the QHA, the volumetric thermal expansion coefficient of rhombohedral LSCF55 is calculated as αV,GGA = 50.34 × 10−6 K−1. For comparison, the lattice expansion and the volume expansion of LSCF55 grain are measured by in situ high‐temperature X‐ray diffractometer (HT‐XRD). An anisotropic thermal expansion of rhombohedral LSCF55 with αa,hex = 10.89 × 10−6 K−1 and αc,hex = 21.18 × 10−6 K−1 is obtained. The volumetric thermal expansion coefficient is measured as αV,HT‐XRD = 43.17 × 10−6 K−1. In addition, the effectively isotropic expansion coefficients of a polycrystalline LSCF55 bar specimen are measured using a vertical high‐performance thermo‐mechanical analyzer and yield αl,bar specimen = 17.37 × 10−6 K−1 and αV,bar specimen = 52.11 × 10−6 K−1.