Gd0.2Ce0.8O2 Diffusion Barrier Layer between (La0.58Sr0.4)(Co0.2Fe0.8)O3-δ Cathode and Y0.16Zr0.84O2 Electrolyte for Solid Oxide Fuel Cells: Effect of Barrier Layer Sintering Temperature on Microstructure

Combining high-performance (La0.58Sr0.4)(Co0.2Fe0.8)O3-delta (LSCF) cathodes with Y-doped ZrO2(YDZ) electrolytes in solid oxide fuel cells leads to the formation of SrZrO3 (SZO) as secondary phase with exceedingly low oxygen ion conductivity and poor catalytic capability. A promising prevention strategy is the insertion of Gd-doped CeO2 (GDC) as a reaction barrier. In this work, screen-printed GDC layers were sintered on YDZ substrates at temperatures varying from 1100 to 1400 degrees C. Subsequently, screen-printed LSCF was sintered on top at 1080 degrees C. The goal of this work was to understand microstructure formation during the two subsequent sintering processes by the analysis of nanometer-scale elemental distributions, crystal structures, and grain sizes of this extended cathode/electrolyte interface as a function of the GDC sintering temperature. Various representative regions on all samples were analyzed by transmission electron microscopy combined with energy dispersive X-ray spectroscopy with high spatial resolution. For the lowest GDC sintering temperature an almost continuous ion-blocking SZO layer is formed during subsequent LSCF sintering. Although GDC densification does not occur, SZO formation is increasingly suppressed if higher GDC sintering temperatures are applied. This is attributed to a dense interdiffusion layer forming at the GDC/YDZ interface, which increases in thickness with GDC sintering temperature and contains a Zr-depleted region adjacent to the porous GDC layer. The dense and Zr-poor sublayer prevents the reaction of Sr species transported via gas phase during LSCF sintering with the Zr-rich YDZ substrate. The microstructural features have a pronounced influence on the electrical performance. Electrochemical impedance spectroscopy measurements at open circuit voltage conditions reveal differences in the polarization resistance of up to 3 orders of magnitude.