Nature and Functionality of Oxygen/Cathode/Electrolyte-Interfaces in SOFCs

Modern solid oxide fuel cells (SOFC) use a high-performance 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 (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 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.

The present study is a combined microstructural and electrochemical characterization of LSCF-GDC-YSZ interfaces after different sintering temperatures, and gives novel insights on the functionality of GDC interlayers for enhanced cell performance.

Symmetric-cell samples were fabricated by screen-printing Gd_0.2Ce_0.8O_2-d layers on both sides of YSZ substrates and sintered at 1150, 1200, 1250, 1300, 1350 and 1400°C, followed by a porous LSCF layer on top, sintered at 1080°C. Thereby, the interface characteristics between gas phase/cathode/interlayer(s)/electrolyte were substantially modified. All of them were studied by scanning electron microscopy (SEM) using focused ion beam (FIB) for cutting. This delivered an greyscale information of the interface characteristics, which then was combined with energy dispersive x-ray (EDXS) mapping from scanning transmission electron microscopy (STEM). Detailed results will be presented in [3]. The area specific resistance of the cathodic polarization process (ASR_cat) was determined by impedance spectroscopy in the range from 600°C to 800°C, which gave the measure of functionality.

Evidently, the nature of the GDC barrier layer depends on sintering temperature (Fig. 1), and controls the area specific resistance of the cathodic polarization ASR_cat (Fig.2). Figure 1 shows four SEM/FIB greyscale images of LSCF – GDC –YSZ interfaces before and after LSCF sintering at 1080°C (GDC sintered at 1100 °C (left) and 1400°C (right)). The interdiffusion region was analyzed by HAADF-STEM imaging, and EDXS mapping classified elemental distributions of Zr, Y, Gd, Ce, Co, and Sr at the YSZ–GDC interface (see [3]). The blue colored area visualizes the GDC/YSZ interdiffusion layer, its thickness increases with GDC sintering temperature. The red colored area visualizes the existence of a SrZrO3 phase as blocking layer, its volumetric portion decreases with GDC sintering temperature. Microstructure evolution after GDC sintering (Fig.1a) and 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 SrZrO₃ 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 three orders of magnitude. The interface changes from a layer sequence LSCF/(Co-doped)GDC/SZO/GDC-YSZ/YSZ at 1100°C gradually to LSCF/(Co-doped)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, 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   “effective GDC manufacture conditions” 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.

[3]         V. Wilde, H. Störmer, J. Szász, E. Ivers-Tiffée, D. Gerthsen, 227th ECS Proceedings (2015) to be published

Image Captions

Figure 1 Segmented SEM picture of a FIB cut at the LSCF cathode/GDC interlayer/YSZ electrolyte interface: a) after sintering GDC at1100 °C (left) and at 1400 °C (right). b) after sintering LSCF at 1080°C. Blue shaded areas indicate GDC/YSZ interdiffusion, red areas the SrZrO3 phase.

Figure 2 Temperature dependence of the cathodic polarization resistance (ASRcat) for LSCF-GDC cells in laboratory air with different GDC layers sintered at 1150°C, 1200°C, 1250°C, 1300°C, 1350°C and 1400°C, respectively.