Chemical analysis of spray pyrolysis gadolinia-doped ceria electrolyte thin films for solid oxide fuel cells

Current solid oxide fuel cell research aims for the reduction of operating temperatures while maintaining power output to reduce the cost of operation. A promising strategy for achieving this goal is to replace common microcrystalline yttria-stabilized zirconia (YSZ) electrolytes of 10-200 mu m thickness with nanocrystalline gadolinia-doped ceria electrolytes (CGO) of 100-500 nm thickness deposited by spray pyrolysis. While decreasing the electrolyte thickness, we expect ohmic losses of the fuel cell to decrease linearly and can realize lower operation temperatures at equal efficiency. In this study, the chemical homogeneity of as-deposited and annealed Ce0.8Gd0.2O1.9-x thin films deposited by spray pyrolysis at 350 degrees C and annealed at 1000 degrees C were investigated. The chemical composition of the gadolinia-doped ceria films was studied by X-ray photoelectron spectroscopy and Ar+ sputtering as a function of film depth. After the topmost layer was removed by Ar+ sputtering, the thin films showed a surprisingly homogeneous dopant concentration of 23.4 +/- 0.6 at % gadolinia in ceria, independent of the film depth. However, spray-pyrolysis-related residues of the precursors (i.e., chlorine from the precursor salt, carbon from the pyrolysis solvents, and water) could be found at unexpected depths in the film and even after annealing at temperatures as high as 1000 degrees C. The pyrolytic decomposition of the spray pyrolysis thin films is not completely finished after deposition. Changes in the chemical composition may be present during solid oxide fuel cell operation of CGO electrolytes at 600-1000 degrees C.