Summary
This paper focuses on the long‐term tests for 1000 hours of different Membrane‐Electrode‐Interconnect Assemblies (MEIAs), consisting of a steel interconnect and an asymmetrical solid electrochemical cell based on the Ce0.8Sm0.2O2−δ electrolyte, the double‐layer working electrode with the La2NiO4+δ – Ce0.8Sm0.2O2−δ composite functional layer and the LaNi0.6Fe0.4O3−δ current collector layer, and the platinum reference electrode (O2, LaNi0.6Fe0.4O3−δ|La2NiO4+δ – Ce0.8Sm0.2O2−δ|Ce0.8Sm0.2O2−δ|Pt, O2). These tests were carried out on MEIAs with two types of Cr‐based stainless steels, Crofer 22 APU and 08X17T, with and without protective coatings at 850°C in air. The electrochemical performance of MEIAs was studied without polarization as well as under cathodic and anodic polarization (current density was 0.5 A cm−2) by means of electrochemical impedance spectroscopy (EIS) and the distribution of relaxation times method (DRT). It was found that the proposed protective coatings for the stainless steels inhibit chromium poisoning without polarization and under anodic polarization: for Crofer 22 APU the degradation of the polarization resistance after ~1100 hours of exposure was 0.35 ± 0.05 Ω cm2, and for 08X17T it was from 0.40 ± 0.05 Ω cm2. In the case of cathodic polarization, the degradation of the polarization resistance remained at a very high level, comparable to uncoated samples: for Crofer 22 APU and 08X17T with coatings, it was about 0.6 to 0.8 Ω cm2 after ~1100 hours. Post‐test analysis of the microstructure and chemical composition of the interconnect ‐ double‐layer electrode interface was carried out using scanning electron microscopy (SEM), wavelength dispersive X‐ray spectrometry (WDS), and respective image analysis of the obtained data. The influence of chromium poisoning on the degradation was considered for different stages of the electrode performance. It was demonstrated that chromium predominantly accumulates in the current collector layer in the case of anodic polarization and in the functional layer in the cases of open circuit and cathodic polarization. For the first time, a combined electrochemical‐chemical degradation mechanism of the La2NiO4+δ – Ce0.8Sm0.2O2−δ functional layer was discussed.
Highlights
Long‐term tests of Membrane‐Electrode‐Inteconnect Assembles (MEIAs) were done.
Chromium poisoning was detected on MEIAs with the interconnects without coating.
Cathodic polarization terminates the protective properties of the coatings.
Chromium poisoning decreases electrode activity to oxygen exchange and diffusivity.
A combined electrochemical‐chemical degradation mechanism of the functional layer was revealed.
: Two grades of chromium-free alloys were studied in order to apply them as interconnectors for solid oxide fuel cells. The surface modification methods were proposed for each alloy with the purpose of forming of oxide scales considering the required physicochemical properties. Investigations of the structure and properties of the obtained oxide scales were performed and the efficiency of the chosen surface modification methods was approved. The samples with the surface modification exhibited higher conductivity values in comparison with the nonmodified samples. A compatibility study of samples with surface modification and glass sealant of chosen composition was accomplished. The modified samples demonstrated good adhesion during testing and electrical resistance less than 40 mOhm/cm2 at 850 °C in air, which allowed us to recommend these alloys with respective modified oxide scales as interconnectors for SOFC.
Институт высокотемпературной электрохимии (ИВТЭ) УрО РАН, г. Екатеринбург Уральский федеральный университет имени первого Президента России Б.Н. Ельцина (УрФУ), г. Екатеринбург Институт металлургии (ИМЕТ) УрО РАН, г. Екатеринбург Белорусский государственный технологический университет (БГТУ), г. Минск, Беларусь
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