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

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.

Sintering and characterization of W–Y and W–Y2O3 materials

Nadine Baluc, Zbigniew Oksiuta, Lyubomira Veleva

W-(0.3-2)Y2O3 and W-(0.3-2)Y materials (in wt.%) have been manufactured by conventional powder metallurgy methods including dry mechanical alloying of elemental powders in Ar atmosphere, cold pressing in air using a pressure in the range of 150-250 MPa and sintering at 1800 degrees C either in vacuum or in At atmosphere. The mechanical alloying time was optimized by means of X-ray diffractometry and scanning electron microscopy analyses of the particles. It was found that it should not exceed 15 h, in order to obtain small and homogeneous particle sizes and crystallite sizes and to avoid formation of WC impurities. The density of the sintered specimens was found to increase with Y2O3 and Y contents. The highest density of about 90% was obtained for the W-2Y material, while the highest microhardness of 1790 HV0,2 was measured for the W-2Y(2)O(3) material. Scanning electron microscopy observations revealed that sintering occurred, at least partially, from liquid phase, the amount of the corresponding dense regions increasing with the Y2O3 or Y content. (c) 2008 Published by Elsevier B.V.

Elsevier2009

Advanced analytical electron microscopy applied to Solid Oxide Cell materials and their degradation

Stéphane François Jean Poitel

Electron microscopy offers a wide range of techniques to study materials. Environmental Scanning Electron Microscopy (ESEM), in particular, allows the characterisation of samples at high-temperature using a laser heating stage, and under various gaseous atmospheres leading to direct observation of the sample surface morphology dynamics in conditions that are close to those in real operating devices (i.e. at high temperature in various gases). Solid Oxide Fuel Cells (SOFC) operating around 800°C in both oxidising and reducing gases, with proven co-generation efficiencies of 90%, offer very promising solutions in the ongoing energy transition. However, production costs and materials degradation in those demanding operating conditions remain significant barriers to their large-scale deployment. Studying SOFC materials' degradation through a combination of ESEM with other advanced microscopy techniques fulfilled the double aim of demonstrating the power of an ESEM and improving the understanding of SOFC materials' degradation during operation. Two materials from the SOFC stack were selected for degradation case studies at around 800°C in oxidising/reducing gases: coated ferritic steel interconnect and Ni-YSZ anode. The objectives were to explore the parameters' influence (temperature, heating ramp, pressure, gas exposure...), set the boundaries and define the guidelines of the ESEM technique while demonstrating its capabilities. The two different materials sets were selected for their relevance both for the SOFC field and for their different specific characteristics allowing a wide range of experiments to be explored. A specific method for the study of the oxidation of coated steel was developed combining ESEM, Focused Ion Beam (FIB) and Scanning Transmission Electron Microscopy (STEM). Post-test segmentation of the in-situ observed images demonstrated the possibility to obtain quantitative data. FIB sample preparation from the exact site of ESEM observation allowed to correlate the underlying microstructure with the in-situ surface observation. Applying this method, oxidation mechanisms of several coatings were characterised and specific behaviours observed and explained such as the appearance of oxide ridges and the behaviour of a cerium barrier layer in case of cobalt-cerium coated steel. The heating ramp speed and the oxidation temperature were demonstrated to be crucial parameters to obtain the desired microstructure. The nitriding pretreatment of interconnect was investigated too and shows promising results for improving the steel oxidation resistance. Another combination of techniques was developed for the study of the Ni-YSZ anode by complementing the ESEM observation with continuous Mass Spectrometer (MS) acquisition. The first reduction of the NiO-YsZ anode was analysed under gas flows of hydrogen and hydrocarbons. Redox cycling effect on the anode microstructure was investigated and the resulting observations were in line with models from the literature for the reduction and re-oxidation kinetics. Finally, the demonstration of an active solid oxide cell in the microscope operating at more than 700°C under mixed gas paves the way to further possible studies approaching the device's observation in real operating conditions. Limitation of the ESEM techniques are discussed and recommendations for further improvements and perspectives given.

EPFL2020

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

Michele Bianco, Jan Van Herle

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.

2019

Advanced analytical electron microscopy applied to Solid Oxide Cell materials and their degradation

Stéphane François Jean Poitel

EPFL2020