Sol-gel processing of barium cerate-based electrolyte films on porous substrates

Barium cerate-based materials exhibit high ionic conductivities at intermediate temperatures and thus offer great potential for solid oxide fuel cell (SOFC) applications. In particular, {dollar}rm BaCesb{lcub}0.8{rcub}Gdsb{lcub}0.2{rcub}0sb3{dollar}(BCG) exhibits the highest ionic conductivities amongst the barium cerate-based materials studied. The performance of SOFCs can be further enhanced by fabricating these electrolytes in a thin-film form to reduce the resistive losses. In this work, two types of sol-gel processes, the Pechini and the colloidal process, are studied to fabricate dense BCG electrolyte films on porous substrates. Systematic studies are performed in each process to determine the critical factors affecting the film microstructure and to gain a fundamental understanding of the process. In the Pechini process, the nature of complexation agent and the molar ratio of complexation agent to metal cations (C-ratio) critically affect the film microstructure. Crack-free and dense BCG films are successfully prepared by modifying the chelating agent in the Pechini process and by optimizing the C-ratio and the viscosity of solutions. Studies in the colloidal approach indicate that the nature of dispersant and the precursor particle morphology are the critical factors that influence the microstructure of derived films. A "multiple coating" technique is developed to prepare dense BCG films on dense and porous substrates. The influence of Ni as a "sintering aid" is also observed. Further, SOFCs based on BCG electrolyte films are successfully constructed and their electrochemical properties are evaluated under H{dollar}sb2{dollar}-air fuel cell conditions. Synthesis of nano-engineered barium cerate-based electrolyte powders using modified Pechini process and alkoxide process is also studied. The single component oxide powders prepared using the alkoxide process contain aggregates of submicron size; each of the aggregate may further consist of nanometer sized primary particles. This type of powder morphology is highly desirable from the point of view of sintering and densification of colloidal films. Accordingly, results suggest that the use of these nano-engineered powders of controlled morphology aids in lowering the processing temperatures of BCG electrolyte films.