Degradation Studies and Sr Diffusion Behaviour in Anode Supported Cell after 3,000 h SOFC Short Stack Testing

Abstract

Solid Oxide Fuel Cells (SOFCs) are promising candidates for future energy conversion devices that transform chemical energy of fuel into electricity [1]. One of the main challenges in improving the performance and cost-effectiveness of SOFC is the control of the degradation processes, such as Sr diffusion and chemical interactions which can contribute to the overall reduction of the cell performance [2-4]. Several authors have evaluated the importance of cation diffusivities for surface segregation of Sr and thus for a major degradation mechanism of SOFC cathodes [5, 6].

In the present work, a 3000 hour degradation test was carried out at 780 ºC in a short stack under real time operation conditions. After the test, a cell from this stack was disassembled and samples from nine different areas were analyzed, looking for evidences of degradation phenomena taking place. A pristine sample was also analysed as reference.

The stack consist on coated ferritic stainless steel, anode supported cells and seal. The cells are supportend on a Ni-YSZ cermet anode with yttria-stabilized zirconia (YSZ) as electrolyte and a lanthanum strontium cobalt ferrite (LSCF) oxide as cathode material. A gadolinia-doped ceria (GDC) is used as barrier layer between cathode and electrolyte to prevent the formation of poorly conducting secondary phases, such as SrZrO₃ or La₂Zr₂O₇[7].

The aim of this work is to study the Sr diffusion, the effectiveness of GDC barrier layer and the evolution of LSCF cathode during operation. The deterioration of the performance measured in the cell is correlated with degradation mechanisms observed in post mortem experiments carried out in pristine and aged cells. The evolution of the Sr and other species in the cathode is examined by X-ray diffraction (XRD), confocal laser Raman spectroscopy, electron probe micro analyzer (EPMA-WDS), scanning electron microscopy equipped with an Energy-dispersive X-ray analyzer.

Besides, the concentrations of pollutants in cells were obtained by inductively coupled plasma optical emission spectrometry (ICP-OES) after dissolution in acid. A local microstructural and phase distribution study is carried out by means of transmission electron microscopy (TEM) and a high resolution scanning electron microscope coupled with electron energy-loss spectroscopy (EELS).

The results throw light on the evolution of the cathode/barrier layer/electrolyte system of SOFC cells working under real conditions after long operating time (Figure 1).

Figure 1. SEM images of the cross section and EPMA elemental distribution maps of the LSCF/GDC/YSZ in pristine and aged cells.

References

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[7] A. Arregui, L. M. Rodriguez-Martinez, S. Modena, M. Bertoldi, J. van Herle, V. M. Sglavo, Electrochimia Acta, 58 (2011) 312-321.

Acknowledgements

The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) Fuel Cells and Hydrogen Joint Undertaking (FCH-JU-2013-1) under grant agreement No 621207.