Identification of carbon deposition and its removal in solid oxide fuel cells by applying a non-conventional diagnostic tool

Operating solid oxide fuel cells (SOFC) in a specific industry-relevant environment can result in undesired performance alterations and subsequent morphological changes. Conventional characterization tools, such as polarization curve or electrochemical impedance spectroscopy, can be employed to gain a rough overview of the cell performance. Nevertheless, if these conventional tools fail and seemingly invisible degradation mechanisms occur inside the cell and system, such degradation cannot be identified. Eventually, these mechanisms can cause the system to collapse abruptly and irreversibly. In this study, novel online-monitoring tools based on total harmonic distortion analysis (THD) were employed to identify the most relevant degradation mechanism when using carbon-containing fuels, i.e. carbon deposition. The examination was performed on SOFCs of industrial size, methane was used as a fuel, and the cells were operated under a constant current load. The operating conditions were designed as required for application in auxiliary power units (APUs). The results reveal that the preliminary stage of degradation can be most quickly identified using the frequencies: 1 Hz, 10 Hz, 100 Hz, 2 kHz, 2.5 kHz, 4 kHz and 8 kHz. If the lower frequencies are removed and only the frequencies equal to or higher than 2 kHz are applied, almost the same information is obtained, and the measuring time can be significantly reduced. In addition, this study enabled us to identify a highly efficient strategy that can be used to remove carbon and to regain the initial cell performance. Carbon removal during the regeneration process was also successfully detected by applying the THD methodology.