Air Side Contamination in SOFC Stack Testing
Solid Oxide Fuel Cells (SOFCs) are likely candidates for electricity production with high efficiency and promise reasonable fuel tolerance. While fuel impurities are known anode poisons, less attention is brought to air-purity, especially to air-side contaminants from exogenous sources, which are studied here. This work aims to quantify pollutants and identify their sources during SOFC testing in stack configuration. Post-analyses of a long-term test have shown that performance degradation was mainly due to cathode pollutants originated upstream of the cell, therefore their source identification is crucial. The compressed air system, feeding the airflow to the cathode, was investigated by filtering and subsequent chemical analysis of the filters. Hot-air-sampling was redone in situ at the cathode air entry during a new test run to assess the contaminant concentrations in air in SOFC test conditions. In addition, the behavior of SOFC proximal system components, i.e. alloy oxidation, was characterized separately. The study focused on chromium contamination from Cr-containing high temperature alloys used in Balance-of-Plant (BoP) components, i.e. heat-exchangers, tubing and flanges in direct contact with the airflow at high temperature. Concentrations of volatile Cr-species in air under SOFC testing conditions were compared to Cr-accumulation on the tested cell as well as to Cr-evaporation rates from BoP alloys which were individually characterized regarding oxidation behavior. Evaporated Cr quantities were found to saturate the air with Cr-vapors at the cathode air-inlet, as confirmed by the in-situ measurement of volatile species in the hot airflow. Moreover, these amounts are well correlated to accumulated Cr in the cell after long term testing. Si-containing sub-oxide scale layers of heat-resistant BoP alloys were identified as silicon contamination source besides sub-micron dust from environmental air. The analyses of laboratory air quality revealed sulfur in sufficient quantity to act as a cathode pollutant. The results of this study suggest guidelines to reduce air side contamination of SOFC stacks from exogenous sources.