Towards a wastewater energy recovery system: The utilization of humidified ammonia by a solid oxide fuel cell stack

This study presents the results of investigations on performance and durability of an ammonia-supplied MK352 solid oxide fuel cell stack with electrolyte supported cells and chromium based interconnects. The performance evaluation revealed no significant differences between ammonia and equivalent hydrogen/nitrogen gases as fuel, which was a result of the excellent ammonia conversion rates up to 99.99%. When using high ammonia flow rates, temperature measurements inside the stack revealed a temperature drop due to the endothermic ammonia decomposition of up to 18.8 K, which proceeded preferentially at the fuel inlet region. An 1000 h durability test with humidified ammonia in 80% fuel utilization condition was performed, which resulted in a stack performance degradation rate of about 1.1%/1000 h. Tests with hydrogen/nitrogen fueled reference stacks revealed similar degradation rates during the initial 1000 h. Post-mortem analyses by scanning electron microscopy and energy-dispersive X-ray spectroscopy revealed no significant micro-structural deterioration of the functional layers of the anode, but nitriding effects on the nickel contact meshes and chromium nitrides were found in the material structure of the interconnects. Also, an oxide layer was found between interconnect and contact meshes at the anode, which appears to be the main cause of the performance degradation.