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The present paper describes the electrochemical results and the post exposure microstructural characterization by means of SEM/EDS of a four layer SOFC stack after 34,000 h operation under load at 700 °C, showing low voltage degradation rate less than 0.3% per 1,000 h. Emphasis was put on the behavior of the ferritic interconnect steel and its interaction with glass sealants as well as contacting and coating materials. The interconnect steel had formed thin protective chromia scales. Gas distributing steel foils of 200 µm showed in some locations breakaway type oxidation. It was related to local overheating caused by locally occurring combustion of a defective cell. The interaction of the steel with the glass sealant showed good adhesion. Interdiffusion at the joint between nickel contact and steel interconnect at the anode side resulted in minor oxide formation on the Ni wires and in austenite formation in the steel without formation of σ‐FeCr formation at the steel/austenite interface. The steel showed excellent compatibility with the Manganese‐Cobalt‐Ferrite (MCF) chromium retention coatings. Chromium enriched phases were found near the interface between MCF coating and perovskite contact layer. Excess glass sealing material interacted with the contact layer without showing obvious detrimental effects.

The objective of this paper is to illustrate a variety of studies carried out to improve the quality of some particular glass-ceramic joining materials based on measured properties such as gas-tightness and mechanical resistance and demonstrate the feasibility of using the proposed materials for solid oxide fuel cells (SOFC) and solid oxide electrolysis cells (SOEC) applications. First, the sealing conditions have been optimized for the two selected compositions in the system MgO-BaO/SrO-B 2 O 3 -SiO 2 . Once the joining materials have been optimized, the gas-tightness has been measured as a function of the glass-ceramic crystallization degree, its thermal cycling behavior and the influence of a reducing atmosphere on this property. The electrical resistance at high temperature has also been studied. Subsequently, the chemical compatibility of the joints steel/glass-ceramic has been evaluated by means of the analysis of the cross-sections using SEM and EDX. Furthermore, the mechanical and chemical stability of the joints has also been studied as a function of the crystallization degree, the resistance vs. thermal cycling and the influence of a reducing atmosphere. Finally, the mechanical resistance of the joints regarding flexural loading has been characterized employing a 4-point bending method both at room temperature and at relevant high temperatures varying the seal thickness. Overall, the results verify that the developed and tested materials are promising for long term stable SOFC and SOEC applications in advanced stack designs aiding prolonged lifetime under thermal-cyclic conditions.

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