Sr Migration and Gd-Ce-Zr Interdiffusion in Solid Oxide Cells Under Different Fabrication and Testing Conditions

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This work aims to understand how solid oxide cell (SOC) fabrication methods and SOC operating conditions affect Sr migration into the yttria stabilized zirconia (YSZ) electrolyte through the rare-earth doped ceria barrier layer. Ni-YSZ electrode-supported SOCs and coupons were fabricated using (La0.6Sr0.4)0.95Co0.2Fe0.8O3-δ-Gd0.1Ce0.9O2-δ (LSCF-GDC) oxygen electrode and GDC or samaria-doped ceria (SDC) barrier layer. Barrier layers were either screen printed and sintered to YSZ electrolyte layer at 1260 °C for 2h, or deposited using thin film deposition techniques such as PLD, electron beam and sputtering. LSCF-GDC electrode layer was sintered onto the GDC barrier at different temperatures from 950 to 1100°C for 2h to determine the effect of firing temperature and barrier layer thickness on Sr migration. Several SOCs were tested at 750°C in a wide range of operating conditions, both in SOFC and SOEC mode, for up to 12,000 hours. The GDC/YSZ interfaces in coupons and SOCs were examined using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). Sr-Zr rich phase formation at GDC/YSZ interface was observed in coupons with LSCF-GDC sintered at 1100°C, but not in the other coupons sintered below 1100°C, unless the barrier layer was too thin and porous. Furthermore, no Sr migration into YSZ was observed in SOECs with varying testing time from 0 h to12,000 h. In addition, Gd and Ce diffusion into YSZ occurred when GDC was fired at 1260°C, but not in cells with GDC deposited using thin film techniques without additional high temperature firing. No significant difference was noted in the GDC-YSZ interdiffusion layer growth after SOC testing for 1,000-12,000 h. Aberration-corrected scanning transmission electron microscopy (STEM) was involved to identify the composition and crystal structure of the Sr-Zr rich phase, which was demonstrated to be SrZrO3 phase. After the long-term SOC tests, no SrZrO3 phase or Sr migration through GDC grain boundaries was observed in cells with 5-7 microns thick porous GDC barrier. This presentation will demonstrate how optimization of fabrication conditions, thermal treatment and fabrication techniques, could mitigate the formation of the insulating SrZrO3 phase.