Modern solid oxide fuel cells (SOFC) use a high-performance mixed ionic-electronic conducting (MIEC) La_1-xSr_xCo_1-yFe_yO_3-δ (LSCF) cathode. This perovskite, when directly applied to Y₂O₃ stabilized ZrO₂ (YSZ), forms a blocking layer of Strontiumzirconate (SrZrO₃ - SZO). This secondary phase reaction is prevented by inserting a dense Gd-doped Ceria (GDC) between LSCF and YSZ [1]. Mostly, the GDC interlayer is screen printed and the correct density is assured by subsequent sintering. This high temperature treatment however causes a low-ionic conductivity GDC-YSZ interdiffusion phase [2]. The correct balance between GDC density and GDC-YSZ interdiffusion is crucial for achieving highly functional GDC interlayers. Microstructure evolution after GDC sintering (Fig.1a) and after LSCF sintering (Fig.1b) reveals the dramatic influence of LSCF as sintering aid for porous, fine grained GDC layers. On the other hand, GDC sintered at 1400°C shows effective inhibition of SZO formation, but enhanced GDC-YSZ interdiffusion. The changing nature of the gas/cathode/electrolyte interface goes hand in hand with a drastic decrease in ASR_cat by over two orders of magnitude. The interface changes from a layer sequence LSCF/GDC/SZO/GDC-YSZ/YSZ at 1100°C gradually to LSCF/GDC/GDC-YSZ/YSZ at 1400°C. We will discuss the origin of this solid/gas interface in detail, and link the effects to electrochemical performance, i.e., contributions of the electrolyte phases (GDC, GDC-YSZ interdiffusion, YSZ) to the ohmic resistance, and contributions of LSCF, SZO and GDC to the cathodic polarization resistance (ASR_cat).

These findings confirm a complex heterogeneous solid/gas interface at a mixed conducting cathode/electrolyte side of a SOFC cell. Its drastic influence on electrochemical performance points to the necessity of understanding the nature of MIEC cathode/electrolyte interfaces for individual cell concepts.

References

[1]         H. Yokokawa, N. Sakai, T. Horita, K. Yamaji, M.E. Brito, H. Kishimoto, J. Alloys Compd. 452 (2008) 41.

[2]         A. Tsoga, A. Naoumidis, A. Gupta, D. Stöver, Mater. Sci. Forum 308-311 (1999) 794.