Technology Overview
One of the critical issues in designing and fabricating a high-performance, planar, solid oxide fuel cell (SOFC) stack is the development of the appropriate materials and techniques for hermetically sealing the metal and ceramic components. Researchers at Pacific Northwest National Laboratory (PNNL) have developed a method for sealing SOFC components that has shown ultra-low leak rates when applied to hybrid compressive mica seals.
The National Energy Technology Laboratory selected PNNL as the national laboratory to support its SOFC program, in part to develop technologies for the Solid State Energy Conversion Alliance (SECA). Basic to the needs was finding a way to stop leakages in SOFCs.
Researchers at PNNL developed a method for overcoming the chemical, mechanical, thermal, and electrical limitations of glass and air brazed seals, which commonly fracture during repeated thermal cycling, forming electrical shorting, or performance degradation under SOFC operating conditions.
The high-performance mica-based compressive seals have overcome these limitations by using a "mica sandwich," where a metal (such as silver) or a glass layer is put on either side of the mica. When heated up to 800-930°, the metal layer conforms to the irregularities in the surfaces, forming a seal of very low leakage.
Advantages
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Better control over chemical reaction between H2 and O2, which translates into higher fuel cell efficiency
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Use comprehensive—SOFC parts are not physically bonded, which allows the reuse of some components
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Environmentally friendly—mica is a natural mineral containing no hazardous elements
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Method allows materials to resist build-up of residual stresses—mica actually removes potential stresses
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Materials are commercially available and low cost, allowing for easier implementation of the method
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Design allows for wider range of materials selection
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Excellent thermal cycle stability—materials have been tested over 1000 thermal cycles in dual environments
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Excellent durability— materials have been tested over 28,366 hours at 800°C in dual environments.