Towards a practical tool for online monitoring of solid oxide fuel cell operation: An experimental study and application of advanced data analysis approaches

Reliability and durability are key criteria for solid oxide fuel cell (SOFC) systems in order to gain a market acceptance and establish a positive reputation. To ensure SOFCs reach their reliability and durability targets, feedback regarding the stack degradation sources is required in order to balance the system operation for acceptable power supply; while maintaining longevity. Indeed, a compromise on one or both accounts may be required for many systems that will enter the field for the first time, since there are so many unknowns for the SOFC technologies. Systems including SOFC stacks are being designed and tested for small stationary power generators and large-scale plants, as well as for automotive auxiliary power and range extension systems. This ensures that appropriate contermeasures can be taken, thereby considerably extending lifetime of SOFC systems. An advanced approach, the distribution of relaxation time analysis of electrochemical impedance spectra is used to identify excitation frequencies altered by SOFC regulation or degradation effects. The technique used and results contained in this paper provide key information, which can be used in the development of a low cost and practical online monitoring hardware tailored specifically for SOFCs. The impacts of diverse operating parameters on an industrial sized SOFC cell performance are identified. Moreover, specific time constants for a practically relevant example stack degradation mode (carbon deposition) are determined. In a further step, the key frequencies for carbon deposition may be monitored online using the tool. A key finding of this work is that the processes can be successfully isolated via EIS excitation without using a reference electrode. This is of great importance for using the methods developed to isolate the key feedback frequencies in a practical experimental setup. Furthermore, this paper demonstrates why avoiding, and if necessary, the removal of carbon deposits, on the SOFC anode, are essential to understand and enact for reliable and durable SOFCs systems to break into and hold a market position.