Self-repairable glass seals for solid oxide fuel cells
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Seals are required for a functioning solid oxide fuel cell (SOFC). These seals must function at high temperatures of 600–900 °C and in oxidizing and reducing environments of the fuels and air. Among the different type of seals, the metal–ceramic seals require significant attention, research, and development because the brittle nature of ceramics and glasses leads to fracture and loss of seal integrity and functionality. A novel concept of self-healing/self-repairable glass seals is proposed, developed, and used for making metal–glass–ceramic seals for application in SOFC for enhancing reliability and life. Glasses and glass–ceramics displaying self-healing behavior are investigated and used to fabricate seals. The performance of these seals under long-term exposure at higher temperatures coupled with thermal cycling is characterized. Self-repairability of these glass seals is also demonstrated by leak tests along with the long-term performance. An approach for studying the kinetics of crack healing in glasses and glass–ceramics responsible for self-repair is briefly described. I. INTRODUCTION
Solid oxide fuel cell (SOFC) is a solid-state energy conversion device that produces electricity by electrochemically combining fuel and oxidant across an ionic conducting oxide membrane at high temperatures.1 The planar SOFC configuration is superior to other configurations in terms of efficiency and power density but requires hermetic seals to prevent fuel–oxidant mixing and to provide electrical insulation to stacks.2,3 Seals required for a planar SOFC can be classified as metal–metal, ceramic– ceramic, and metal–ceramic seals (Fig. 1). Among these seals, metal–ceramic seals are particularly challenging because of their severe functional requirements as well as difficulty in selection of materials with conflicting requirements and associated processing. Glasses are most widely used to make metal–ceramic seals because these can be modified to have very close match of thermal expansion with other fuel cell components, and glass seals show good hermeticity along with good thermal and environmental stability.4–8 Although glasses are good option to seal SOFC components, they suffer from their inherent brittleness and can develop cracks in the seal area during thermal cycling or thermal shock. This can cause leakage of fuel from the seal leading to degradation in cell performance and system efficiency. Fortunately, there are ways to minimize this problem by using some innovative approaches developed by our research group such as layered composite seals9 and self-healing glass seals.10,11 Recently, our group has advanced a novel concept of active selfrepairable seals for SOFCs,10,12 which requires a fundamental understanding of the ability of glasses to self-heal/repair a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.188 J. Mater. Res., Vol. 27, No. 15, Aug 14, 2012
cracks at the SOFC operating temperatures. Therefore, the primary focus of this paper is a study of the crack-healing/selfrepair
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