(Invited) Challenges with Ceria Diffusion Barrier Layer

Abstract

Sr-doped perovskite has been the most commonly used oxygen electrode material for solid oxide cells due to their high conductivity and catalytic activity for oxygen reduction and oxygen evolution reactions. Direct contact of the electrode material with the doped zirconia electrolyte leads to formation of resistive Sr- and La-zirconate phases during cell fabrication and long-term operation leads to performance loss. A doped ceria barrier layer is applied between the electrolyte and the oxygen electrode to avoid the reactions that form the resistive phase. The ceria layer is applied using a variety of processes and screen printing is the preferred method as the oxygen electrode also uses the same fabrication option. The barrier layer is sintered at high temperature to target a thin, dense layer. The barrier layer sintering temperature needs to balance good resulting density without any reaction with the electrolyte. The ceria-zirconia solid solution is more resistive than the end members. Thus, it is accepted to limit the sintering temperature which results in a porous barrier layer.OxEon stacks tested for several thousand hours were post-test analyzed at the Pacific Northwest National Laboratory using an array of electron microscopes, and Sr migration to the zirconia interface through pores and discontinuity in the barrier layer was confirmed. The use of Sr-free electrode composition between the barrier layer and the Sr-containing electrode layer did not prevent Sr migration to the electrolyte interface when discontinuities are present in the barrier layer.In addition, the dopant in ceria, samaria in this case, also migrates into zirconia during long-term operation while as fabricated cells do not show migration of samaria.Investigations are ongoing to improve the density of the barrier layer without deleterious interdiffusion during fabrication.