Glass-based Sealants for Application in Planar Solid Oxide Fuel Cell Stack

One of the major challenges for the commercialization of planar anode-supported solid oxide fuel cell (SOFC) is the development of suitable high temperature sealant. The sealants must maintain gas-tightness between the anode and the cathode compartments to avoid intermixing of fuel and oxidant at high operating temperature (700 degrees-800 degrees C), be an electrical insulator and chemically non-reactive with the stack components. Due to the high commercial importance of such sealants, very few sealing compositions are available in the open literature and have to be developed by the individual SOFC developer. In this context the present article gives an overview on such sealant development with particular emphasis on glass-ceramics based sealants. Other than glass-based sealants, very recent approaches such as compressive and self healing seals also show some potential as SOFC sealants. The review of literature is followed by description of the progress of work related to,development of glass based sealant at our Institute for application in the developed planar anode-supported SOFC stack. Under this developmental activity, a number of glass systems having SiO2 and B2O3 as glass formers and BaO, MgO and CaO as the alkaline earth metal oxide content have been investigated and thoroughly characterized. Based on the findings, some of these glasses have been used for making seals between different types of SOFC components, e.g. ferritic stainless steel (Crofer22APU) (metallic interconnect) and 8 mol% yttria-stabilized zirconia (YSZ) (electrolyte) under various combinations, viz. metal-metal, metal-ceramic and ceramic-ceramic. A novel concept of "bi-layered sealing" consisting of two different glass compositions having graded CTE values has been implemented for the first time during actual SOFC stack operation. The performance of the developed sealants has been evaluated in several working SOFC short stacks (up to 6-cell) for a maximum period of 20 days. It is observed that the open circuit voltage (OCV) of these stacks is very near to its theoretical value and remains same during the stack testing period and thus ensured the potentiality of the developed sealant for SOFC application.