Studies of ammonia-fueled SOFC stacks: Insights into possible failure causes

Abstract

Ammonia (NH 3 ), as a carbon-free hydrogen carrier, is considered a promising alternative fuel. However, it is a highly reactive substance and induces corrosion of some materials. Ammonia decomposes into H 2 and N 2 at temperatures above 500 °C, making it suitable as fuel for solid oxide fuel cells (SOFCs). Yet, the direct utilization of ammonia in SOFC stacks (so-called DA-SOFC) may lead to the deterioration of stack components. The article presents an experimental, detailed insight into the performance, durability, and degradation of 10-cell and 3-cell stacks. The stack’s electrochemical characterization demonstrated a high-power density in short-term operation (up to 230 mW cm −2 for 24 A at 680 °C and a voltage for each cell higher than 0.8 V). However, the failure of stacks running on open-circuit or galvanostatic conditions was observed after a few hours of operation. This study is focused on finding the reasons and mechanisms of these accidents. For this purpose, the post-mortem stacks components were examined using scanning electron microscopy/energy-dispersive X-ray spectroscopy, X-ray diffraction, GDOES, and a profilometer. Detailed insight into the degradation process led to novel conclusions about the stack components’ nitriding process, which was correlated with steel deformation and short-cut. The microstructure of interconnects contained nitrogen at a concentration of up to 8 % wt, as indicated by EDS measurements, and confirmed by GDOES tests. SEM images revealed fractures and the segregation of components in the interconnect. The presence of chromium nitrides was disclosed by XRD studies. The combination of temperature stresses and chemical deformation of the Crofer 22 APU interconnects, exceeding cell thickness, leads to a short-circuit and failure of all measured stacks.

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