Sr and Co Vapor-Phase Transport from LSCF Cathodes

Abstract

Strontium transport contributes to degradation mechanisms in SOFCs, especially strontium zirconate formation at the interface of the cathode barrier layer with the YSZ electrolyte. High temperatures required for cathode sintering lead to Sr transport during cell construction. SEM analysis of SOFCs with LSCF cathodes shows Sr at the YSZ/GDC barrier layer interface, but no Sr throughout the GDC barrier layer itself, as would be expected for solid-phase transport. Vapor-phase transport was identified as a possible mechanism that would lead to these observations. A series of tests were performed to differentiate between surface diffusion and vapor-phase diffusion of species from LSCF cathode material at typical sintering temperatures (up to 1100°C). A GDC substrate was printed with LSCF and separated by an air gap from a YSZ target substrate. First a simple spacer ring was used to create the air gap, and strontium transport was observed via SEM-EDS (energy dispersive spectroscopy). Then, various geometries with long surface paths were employed to reduce the possibility of surface transport. The target substrates were analyzed via x-ray photoelectron spectroscopy (XPS) with high sensitivity for surface species. An open air gap did not show evidence of Sr or Co transport. A refined, enclosed air gap with long surface path length did show such evidence for Sr and Co, indicating an enclosed area to maintain significant vapor pressure was necessary for transport. An SOFC with LSCF in direct contact with a barrier layer would also provide such an environment for vapor-phase transport to occur. Furthermore, spatially resolved analysis showed a relatively uniform distribution of Sr and Co across the target substrate. This supports the vapor-phase mechanism as Sr and Co transported via surface diffusion would have been fixed preferentially to the outer edge of the target substrate. These results have implications for adjusting sintering conditions and barrier layer requirements to prevent the formation of undesired strontium zirconate. This work was performed for the US DOE Office of Fossil Energy’s Solid Oxide Fuel Cell program under the Cathode Task.