In order to ensure the continuous operation of (i) solid oxide fuel cells (SOFC) as the main parts of auxiliary power units (APUs) or stationary power generators, and (ii) solid oxide electrolysis cells (SOEC) as highly-efficient systems for advanced fuel generation, it is of crucial importance to be able to determine specific cell processes and to identify diverse potential degradation mechanisms at the earliest possible stage. This ensures the electrochemical processes within reversible solid oxide cell (rSOC) systems to be optimized with the aim to achieve the maximum overall efficiency for energy and fuel generation. Moreover, if degradation is identified at early stage, appropriate countermeasures can be taken, thereby considerably extending the lifetime of the rSOC system under operation. In this study, industrial-scale rSOCs are analyzed by means of electrochemical impedance spectroscopy (EIS) as well as advanced electrochemical tools - (i) distribution of relaxation times (DRT) analysis, and (ii) total harmonic distortion (THD) analysis. The DRT approach is applied in order to isolate the processes involved in both operating modes and to deliver suggestions for the overall operation optimization. Eventually, a practical tool applicable for online-monitoring systems - total harmonic distortion analysis (THD) is applied to identify carbon deposition degradation mechanisms in a high temperature fuel cell system, which was induced by fueling SOFC with methane. This method enabled detection of specific frequencies for the failure mode mentioned, thus making a basis for fast development of cost-effective and practical online monitoring tool. This technique enables to in-operando control rSOC systems, to identify diverse degradation mechanisms at initial state and to prolong the lifetime of the technology used.
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