High-performance oxygen electrodes for solid oxide fuel cells (SOFC) require a highly efficient material selection for the oxygen reduction reaction that takes places near the cathode/electrolyte interface. La1-xSrxCo1-yFeyO3-δ (LSCF) delivers fast oxygen-exchange kinetics and is therefore utilized as cathode material for high performing state-of-the-art anode-supported fuel cells. The main disadvantage of LSCF is its instability against Zr-based electrolytes. A diffusion barrier interlayer consisting of Gd doped cerium oxide (GDC) is therefore inserted to prevent the formation of an insulating SrZrO3 phase. This study presents new results on the importance of the GDC manufacturing conditions. A complex interaction of GDC with LSCF and YSZ during fabrication is found with a tremendous influence on the performance of the cathode/electrolyte interface of SOFCs.
Combining high-performance (La 0.58 Sr 0.4 )(Co 0.2 Fe 0.8 )O 3-δ (LSCF) cathodes with Y-doped ZrO 2 (YDZ) electrolytes in solid oxide fuel cells leads to the formation of SrZrO 3 (SZO) as secondary phase with exceedingly low oxygen ion conductivity and poor catalytic capability. A promising prevention strategy is the insertion of Gd-doped CeO 2 (GDC) as a reaction barrier. In this work, screen-printed GDC layers were sintered on YDZ substrates at temperatures varying from 1100 to 1400 °C. Subsequently, screen-printed LSCF was sintered on top at 1080 °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.
Interdiffusion phenomena and secondary phase formation at the interface La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) / Gd 0.2 Ce 0.8 O 2-δ (GDC) / Y 0.16 Zr 0.84 O 2-δ (YSZ) are correlated to linear and non-linear losses in symmetrical and full SOFC cells. FIB/SEM (focussed ion beam / scanning electron microscopy) tomography is applied for determining the local distribution of the primary phases LSCF, GDC, and YSZ and elemental analysis via STEM/EDXS (scanning transmission electron microscopy / energy dispersive X-ray spectroscopy) provides information on the secondary phase SrZrO 3 (SZO) and the interdiffusion between GDC and YSZ (ID). This reveals the effect of GDC co-sintering temperature (varied from 1100 • C to 1400 • C), alongside the sintering of LSCF at 1080 • C, on these multi-layered microstructures. Electrochemical impedance spectra on symmetrical cells show that the polarization resistance (ASR cat) of the cathode/electrolyte interface is pronouncedly affected by three orders of magnitude, changing the overall power density of anode supported SOFC single cells at 0.8V by as much as a factor of 20. In conclusion, the chemical composition of the ID has a tremendous impact on the electrochemical efficiency of the investigated LSCF/GDC/YSZ interface, and processing parameters of anode supported cells with LSCF cathode have to be carefully chosen for individual SOFC cell concepts.
The functionality of oxygen electrodes for solid oxide fuel cells (SOFC) is mainly determined by intrinsic material properties and fabrication controlled microstructure parameters for an enhanced oxygen reduction reaction at the solid/gas interface. La1-xSrx Co1-yFeyO3-δ (LSCF) delivers high oxygen-exchange kinetics and is widely used as a cathode material for high performing anode-supported fuel cells. LSCF is however chemically instable against Zr-based electrolytes (f.e. Y stabilized ZrO2 - YSZ) and is therefore separated by a 5-8 µm thick diffusion barrier interlayer, consisting of rare earth doped Cerium oxide (f.e. doped with Gd - GDC). This study shows that variations in GDC manufacturing conditions tremendously influence the cathode performance. The LSCF-GDC-YSZ layer sequence reveals a complex interacting nature during fabrication that needs to be understood for further optimization and reproducible performance of the oxygen/cathode /electrolyte interface in SOFCs.
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