Stability of Glass Ceramic Sealants in Atmospheres with Increased Water Contents

Abstract

For Solid oxide fuel cells (SOFC), operating at temperatures above 800 °C, glass ceramic sealants are state of the art. The properties of the glass ceramics are adapted to the metallic and ceramic join partners, operation conditions, as also to the atmospheres: fuel gas and air. In SOFCs, glass ceramic sealants offer reliable, long term stable operation over several 10,000 hours of service. The demand to operate solid oxide cells (SOC) in fuel cell- as also in electrolysis cell mode increases the demands of the stability of the sealants against atmospheres. In the electrolysis mode, the SOC-stacks were impinged by gas with high water contents to produce H2. Water contents of up to 95 % and temperatures above 800 °C are hard conditions for all applied materials of SOC stacks. In addition, glass ceramic sealants have to electrical insulator under applied electric voltage. In the electrolysis mode, the applied voltage is even higher than in fuel cell mode. In conclusion, the electrolysis mode is much harsher as fuel cell mode and glass ceramic sealants have to be adapted to this. Glasses in the system BaO-CaO-Al2O3-B2O3-SiO2 with La2O3-additions have the potential to form glass ceramics with extraordinary high coefficient of thermal expansion (CTE)values, which are suitable for use as sealants compatible with Crofer 22 APU interconnect. Their characteristics have been generally described in various studies and are also in focus of our own work and have proven their suitability for use under SOFC operating conditions. However, achieving such high CTE values is especially challenging when these glasses must be designed for use in stacks operating with electrolyte supported cells because sealing and operating temperature are usually higher than in other stack designs. Their behavior under simulated electrolysis conditions together with a direct correlation of test by using these glasses as sealants in real stack designed to operate with electrolyte supported cells has not been investigated yet and presents the scope of this study.For this study, a series of glasses has been prepared as glass powders and characterized in terms of relevant thermophysical properties such as softening behavior, thermal expansion and formation of crystalline phases. La2O3-free sealing glasses established as standards for use in commercial stacks served as references and results of both groups of sealing glasses have been compared. The new La2O3-containing glasses showed CTE-values up 10.8 ppm K-1 at a glass transition temperature ranging between 640°C and 650°C for a partially crystallized microstructure. The softening and crystallization characteristics analyzed by hot stage microscopy and DSC/TGA allow sealing processes of stacks components in the temperature range between 900°C and 950°C. Helium-leakage rates of sealed sandwich type samples made of Crofer 22 APU showed values below 10-8 mbar l-1 s-1. From suitable glasses sandwich type sample sealings made of two Crofer 22 APU coupons have been prepared and exposed to a dual atmosphere in a special test rig under simulated electrolysis conditions at 850 °C with different electrical voltages applied across the sealants and a water content of 50 % in the steam atmosphere. In this out-of-stack environment, the resistance of the glass sealants can be recorded while being treated with high water atmosphere and electric voltage of up to 5 V at 850 °C. The development of resistance over 300 hours of aging show which glass composition fit to the conditions and which exhibit high degradation rates. Afterwards cross sections have been prepared and analyzed by SEM. Effects observed in the interfacial regions glass-Crofer 22 APU and glass-atmosphere have been correlated with the electrical measurements. A 30 layers stack was tested under typical SOEC conditions for approx. 3200 hours. Four different glass types were used and compared in terms of their degradation rate and the ASR value. The selection of the glasses was made according to their thermal expansion and softening behavior as well as their insulating properties.