Carbon Deposition and Deformation of Ni-YSZ Cermet in Hydrocarbon-Containing Gases

For the full-scale commercialization of SOFC systems, the durability and reliability of the cells and stacks should be improved. Carbon deposition and following deformation of the anode and is one of issues to be solved. For SOFC systems, hydrogen-rich gas, which is obtained through the steam reforming of hydrocarbons, is supplied to the anode. In practical SOFC systems, the steam/carbon molar ratio (S/C) is kept high enough to prevent carbon deposition thermodynamically. However, at the start-up/shut-down processes or failures in the reforming system, etc.,hydrocarbon fuel with low S/C may be introduced into the anode. In Ni-YSZ cermet, which is the most common anode material in SOFCs, Ni is highly active for the decomposition of the hydrocarbon gases and carbon formation. If the low-S/C hydrocarbon fuel was supplied to Ni-YSZ cermet, carbon would deposit on Ni. Such carbon deposition causes not only deterioration of conductivity and electrochemical activity but also significant volume expansion of Ni-YSZ cermet.

From the backgrounds mentioned above, in this study, we quantitatively evaluated the carbon deposition and expansion of Ni-YSZ cermet during exposure to 10%CH₄-N₂, 1%CH₄-N₂, 1%C₂H₆-N₂ or 1%C₃H₈-N₂with S/C = 0.1 or 1.0 in the temperature range between 973 and 1173 K. The dilatometry and thermogravimetry measurements as well as posterior analyses by SEM/EDX and XRD were performed for blocks of Ni-YSZ cermet.

When S/C was 0.1, significant carbon deposition and expansion were observed during the exposure to hydrocarbon gases. The expansion reached 1% within 5 hours for 1%CH₄-N₂ and 1 hour for the others. According to SEM/EDX analyses, the amount of deposited carbon was found inside of the specimen after the exposure, although its distribution differed among hydrocarbon gas species. Carbon segregated in the near surface of the specimens for 1%C₂H₆-N₂ and 1%C₃H₈-N₂, while distributed in the whole parts of the specimens for 1% and 10%CH₄-N₂. The temperature dependency of the expansion was also quite deferent among hydrocarbon gas species. The expansion ratio was smallest at 973 K for 10%CH₄-N₂ and 1%CH₄-N₂, while smallest at 1173 K for 1%CH₂H₆-N₂ and 1%C₃H₈-N₂. In the presentation, the expansion mechanism will be discussed based on the obtained results.