High-Performance Cathode/Electrolyte Interfaces for SOFC

Introduction

La_1-xSr_xCo_1-yFe_yO_3-δ (LSCF) cathodes are best suited for high power densities of anode-supported cells even at low operating temperatures (1). This requires the development of a rather sophisticated processing route, because a poorly conducting SrZrO₃(SZO) phase may develop at the cathode/electrolyte interface (2). The Gd-doped Ceria (GDC) interlayer, which blocks the SZO reaction, densifies only at rather high sintering temperatures. This leads to GDC/YSZ interdiffusion with a lower ionic conductivity (3).

This study presents a high-resolution analysis of the temperature-dependent reaction processes between LSCF-cathodes, GDC-interlayers and Zr-based electrolytes.

Experimental

Symmetrical LSCF-GDC-YSZ cells (YSZ - Y₂O₃ stabilized ZrO₂) were prepared, for which the sintering temperature of the screen printed GDC interlayer (T_sinter,GDC) was systematically varied between 1100…1400 °C. Porous LSCF was applied by screen printing subsequently and sintered at 1080 °C for 3h. A comprehensive scanning transmission electron microscope (STEM) analysis was performed, including energy dispersive x-ray spectroscopy mappings (EDXS) for a high spatial resolution of the elemental distribution along the interfaces. The corresponding cell performance was tested by means of current-voltage curves (Fig. 3) and electrochemical impedance spectroscopy measurements (EIS) with symmetrical cells and anode-supported cells (ASC) provided by Forschungszentrum Jülich (JÜLICH).

Results and Discussion

The symmetrical cell tests reveal a complex interaction between LSCF and GDC, depending on sintering temperature and time. Besides Sr, also Co and Fe evaporate and play a yet underestimated role as sintering aid during densification. In the presence of gaseous Co-species cation diffusivity is significantly enhanced (4) and leads to Gd demixing in the Gd_0.2Ce_0.8O_2-δ (GDC) interlayer (Fig. 1). A side effect is the formation of GdFeO₃ and CoO grains inside the GDC layer. Furthermore, Gd³⁺ is likely to diffuse into the adjacent layers YSZ and LSCF (Fig. 1). This seems to be crucial, as already small amounts of Gd in LSCF lead to a drastic performance decrease. We assume that the smaller Gd³⁺ cation tilts the rhombohedral LSCF to the orthorhombic structure (5). At the GDC/YSZ-interface, a gradually increase in SZO phase is observed from high (Fig. 2) to low GDC sintering temperatures (Fig. 1). Understanding the structural and electrical interplay of these phases is essential. We will correlate our findings from STEM, EDXS, C/V (see Fig. 3) and EIS measurements and propose guidelines for processing high-performance anode-supported cells at a high reliability level.

Conclusions

The complex interplay between LSCF cathode and GDC interlayer during fabrication is discussed. LSCF constituents Co and Fe play a yet underestimated role as sintering aid for screen printed GDC layers during LSCF sintering. The consequence is pronounced cation diffusion along the cathode/electrolyte interface which affects the electrochemical performance drastically. In summary, optimum conditions for GDC fabrication will be presented that have practical relevance in every large-scale SOFC fabrication routine.

References

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