In the following study, the effect of strontium deposition at the interface between GDC compatibility layer and electrolyte was investigated in fuel cell (SOFC) and electrolysis mode (SOEC). The cells were composed by a nickel-yttria stabilized zirconia (Ni-YSZ) support electrode on the feed side, a YSZ electrolyte, a gadolinium doped ceria (GDC) compatibility layer and lanthanum strontium cobalt ferrite (LSCF)-GDC oxygen electrode. The interfaces between the YSZ electrolyte, the GDC compatibility layer and the oxygen electrode in pristine and aged samples extracted from segmented cells and short stacks operated in SOFC or SOEC mode for up to 10,000 h were imaged by 3-D focused ion beam-scanning electron microscopy (FIB-SEM) serial sectioning. Further, during the imaging of a sample aged for 10,000 h in SOEC mode, the acceleration voltage was changed automatically every 10 sections from 1.7 kV for the acquisition of the secondary electron/secondary ion (SESI) and energy-selective backscattered (EsB) signals to 10 kV for elemental mapping by energy-dispersive X-ray spectroscopy (EDX). Features with a distinct grayscale value, morphology and spatial distribution are detected in the EsB data at the interface between the YSZ electrolyte and GDC compatibility layer. 2-D and 3-D EDX analyses show that these inclusions contain strontium, zirconium and oxygen. Transmission electron microscopy (TEM)-EDX with selected area electron diffraction (SAED) performed on regions of interest confirms that the detected features correspond to a SrZrO₃ secondary phase. The 3-D FIB-SEM data was then segmented including the SrZrO₃ secondary phase for quantitative analyses of its morphology and spatial distribution. The evolution upon SOFC and SOEC operation was examined by measuring the volume fractions, 3-D phase and neck size distributions and interfacial surface areas. The analysis revealed that the secondary phase is already present in the pristine samples and is located at the interface between the GDC compatibility layer and YSZ electrolyte. The SrZrO₃ inclusions do not form a connected phase. Most of their interfacial surface area is with the pore phase, which is twice larger than that with the GDC phase. The 2-D and 3-D EDX measurements do not reveal a clear continuous Sr-gradient across the GDC compatibility layer. Therefore, the secondary phase may have formed by gas-phase transport or surface diffusion of Sr, mostly during the cell sintering. SOFC operation for 4,700 h causes only subtle changes. In contrast, an almost continuous layer of SrZrO₃ forms after 10,000 h in SOEC mode, leading apparently to the mechanical weakening of the interface. Delamination just below this secondary phase layer is observed over large areas at several locations, in both 2-D and 3-D analyses. The segmented 3-D volumes were meshed and finite-element (FE) transports were performed to quantify the detrimental effect on the effective conductivity. The degradation was estimated by comparing the conductivity with and without the presence of the SrZrO₃ secondary phase. The operation in SOFC mode caused a limited decrease of the conductivity, while the effects on the performance are more significant after SOEC operation.

Keywords: SOFC, SOEC, Sr-migration, 3-D characterization