Cathode contacting in SOFC stacks

In SOFC stacks the electrical contact between the ceramic cathode layer and the metallic interconnector (IC) has to be adjusted carefully to minimize the contact resistance and slow down degradation mechanisms. In JÜLICH, ICs made of Crofer22APU or Crofer22H are coated with a MnCo1.9Fe0.1O2 spinel (MCF) applied by atmospheric plasma spraying (APS), which prevents the diffusion of volatile Cr species from the steel into the cell. Materials for the cathode contact layer have to be sufficiently good electronic conductors, stable in oxidizing atmosphere, chemically stable and compatible with the adjacent layers, and show a thermal expansion behavior similar to MCF and the cathode material, i. e. La0.58Sr0.4Co0.2Fe0.8O3-x (LSCF).Perovskites of La and Co, doped with Fe, Mn, or Cu on the B-site have been applied by wet powder spraying (WPS) onto the pre-coated IC. These compounds offer a good electrical conductivity and an appropriate thermal expansion but, depending on the composition, they may form several phases instead of a stable single phase. [1] Alternatively, LSCF can also be used for cathode contacting and first tests show promising results with better stack performances than the previously mentioned compounds. Although the thermal expansion coefficient of LSCF is quite large compared to those of MCF and the IC material, the ability for thermal cycles is given. Furthermore, no problems arising from chemical reactions or interdiffusion are expected when the contact layer is made of the same material as the cathode.As an alternative to WPS, the contact layer is screen printed onto the cell. At present, the full cathode is being covered although the contact layer is required only between the IC ribs and the cell. Applying the contact layer as stripes could reduce the amount of material needed and at the same time improve the gas supply from the interconnect channels to the active sites for reduction in the cathode since the gas transport may be the performance limiting factor at the cathode side. [2]A different possibility for contacting the cathode is to use metal foam sheets that are very easy to handle. Two metal foams (CoNi and CuMn, provided by Alantum, Rep. of Korea) have been examined regarding their electrical and mechanical properties at high temperatures to evaluate the suitability for SOFC stacks.References:[1] Montero, Fischer, Tietz, Stöver, Cassir and Villarreal, Solid State Ionics 180, 731 (2009)[2] Geisler, Kromp, Weber and Ivers-Tiffée, Journal of The Electrochemical Society 161, F778 (2014