Method to Measure Area Specific Resistance and Chromium Migration Simultaneously from Solid Oxide Fuel Cell Interconnect Materials

Chromium evaporation is identified as a major degradation mechanism in solid oxide fuel cell (SOFC) stacks. The major chromium source is the commonly used stainless steel interconnects, thus raising a need for protective coatings on the interconnect steel. Ex situ characterization methods of protective coatings involve chromium evaporation measurements, area specific resistance (ASR) measurements and long-term exposure tests. To replicate stack conditions, commonly used ASR measurement setups should be further developed. This work presents an improved characterization method for steels and coatings and aims to be an extension to state-of-the-art characterization methods. The studied steel samples, bare or coated, are placed adjacent to palladium foils with a screenprinted lanthanum-strontium-cobalt (LSC) layer and the resistivity over the pair is measured. The method offers similar contact materials, chromium migration mechanisms, electrical contacts and chemical interactions, as seen in stacks. Further, it enables post-test chromium migration analysis with electron microscopy. Demonstration of the method validated that protective coatings hindered both oxidation and chromium migration from the substrate steels. The presented method could aid in accelerating protective coating development.

Analysis of High Temperature Degradation of Alloys in Solid Oxide Fuel Cell

Manuel Bianco

Solid oxide fuel cells (SOFCs) are approaching commercialization to improve power production efficiency. Currently, cost and lifetime reliability limit their spread in the market. The SOFC is ceramic-based, but refractory metal alloys are in fact the majority materials present in SOFC systems. The lifetime behaviour of SOFC stacks, and of the metallic components in particular, remain in need of further investigation. The degradation processes behind the possible failure of selected metal interconnect (MIC) and balance of plant (BoP) alloys were investigated. Samples were extracted from stacks operated for periods up to 18000 hours and analysed with scanning electron microscopy coupled with X-ray energy dispersion spectroscopy (SEM/EDS). Scanning transmission electron microscopy (STEM) coupled with EDS and electron diffraction was applied to identify corrosion products. Electrochemical impedance spectroscopy (EIS) analysis completed the characterization of the stacks. Degradation processes of steam generator BoP alloy were investigated with SEM/EDS as well. Various protective coating solutions to limit interconnect and related cathode degradation were tested as small coupon samples. Area specific resistance (ASR) monitoring and evaluation of Cr evaporation blocking verified the effectiveness of these solutions. A special set-up able to test 8 small MIC samples simultaneously in different gas atmospheres was built and used to correlate operating variables to material degradation through a design of experiment (DoE) approach. The results indicate that in commercial ferritic stainless steel, corrosion kinetics are fast in the first few thousand hours of operation and then tend to stabilize. The main influencing parameter is the initial density of the protective coating, independently of the steel and coating compositions. However, even porous coatings demonstrated to be viable for periods at least up to 2 years. The strong chemical interaction with the cell materials is their main drawback, but they present a competitive advantage in manufacturing cost. The steam generator alloy used in the BoP suffered stress corrosion cracking induced by the strong thermal gradient between hot and cold gas and the aggressive hot outlet gas atmosphere. Aluminizing treatment of the alloy seems to solve the issue.

EPFL2019

Evaluation of Protective Coatings for SOFC Interconnects

Michele Bianco, Jan Van Herle

Chromium poisoning is a widely recognized degradation process in solid oxide fuel cells (SOFC). Stainless steel interconnect plates, in direct contact with the cathode, have been identified as the major chromium source, raising a need for electrically conducting protective coatings for the interconnects. This work evaluates four different manganese-cobalt protective coatings manufactured on thin steel foils, made by three commercial companies and a research centre. Area specific resistance, coating microstructure, and chromium retention are compared. Measurements were done in a humid atmosphere over 1000 hours at 700 °C. An innovative measurement setup was used, in which the coated steel samples are stacked adjacent to thin palladium foils with a screen-printed lanthanum strontium cobalt layer, replicating an SOFC cathode. As a conclusion, TeerCoating Ltd’s and Turbocoating S.p.A’s coatings performed similar to the Sandvik Material Technology’s ceriumcobalt reference coating, and could be employed as such in SOFC applications.

2015

Analysis of High Temperature Degradation of Alloys in Solid Oxide Fuel Cell

Manuel Bianco

EPFL2019