Influence of Cathode Polarization Change on Chromium Deposited on Electrolyte Surface Near Cathode Reaction Sites of SOFC

It has been clarified that chromium deposition occurs significantly on the electrolyte surface near the cathode reaction sites in consequence of cathode polarization. The deposited chromium probably remains on the surface, not diffusing into the electrolyte. Therefore, focusing on the behavior of the deposited chromium, such as crystal growth or dissipation by cathode polarization change is suitable for clarification of the deposition mechanism in detail.

   In this study, we investigated influence of cathode polarization change on the deposited chromium on the electrolyte surface by using NiO/YSZ or NiO/GDC for the cathode material. We used NiO for the cathode material because of the following two reasons. 1) Effect of cathode polarization can easily be studied because the predetermined cathode polarization is generated at the cathode reaction sites with smaller current flow. 2) Lower reactivity of NiO with chromium vapor than conventional cathode materials (e.g. LSCF, LSM) can make the analysis simpler.

   Although ohmic resistances of these cathodes using NiO are higher than usual, they showed enough performance to control cathode polarization. Chromium poisoning tests were conducted at 700 ^oC for 100 hours keeping cathode polarization constant at 200 mV. At the end of the chromium poisoning tests, several different procedures to stop the cell tests were executed concerning the timing to remove the cathode polarization

   After the chromium poisoning tests at 200 mV, chromium deposition was observed on the surface of the electrolyte near the cathode/electrolyte interface as observed in LSM cathode. The deposited chromium segregated at the interface of NiO and YSZ for the NiO/YSZ cathode by decreasing cathode polarization, which suggests nucleation under cathode polarization at 200 mV and growth of chromium compounds after decreasing the cathode polarization. On the other hand, the deposited chromium decreased for the NiO/GDC cathode after decreasing cathode polarization.