Electrochemical impedance spectroscopy of SOFC and SOEC stacks

Abstract

The successful development of the solid oxide fuel/electrolysis cell technology requires high quality, reliable and reproducible test results which enable the proper understanding of the corresponding electrochemical processes. Especially the different resistances, the overvoltages under electrical current and the degradation mechanisms in the stacks are not well understood. In this context, temperature and fuel gas composition gradients along the cell area and along the height of the stack play an important role under operation. Electrochemical impedance spectroscopy (EIS) is a very useful tool in order to close this knowledge gap. Therefore, the paper focuses on the improvement of the understanding of the electrochemical behavior of SOC stack repeat units with focus on electrochemical impedance spectroscopy. Both, the fuel cell (SOFC) and the electrolysis (SOEC) operation are addressed. The corresponding results have been obtained within the European funded project “SOCTESQA” (Solid oxide cell and stack testing and quality assurance). The relevant issues and parameters which influence the quality of the EIS spectra are outlined. Moreover, the results of the spectra are discussed in terms of reproducibility among different test methods. This includes the validation of the low frequency impedances with the area specific resistances of the current-voltage curves. Both operating modes (SOFC and SOEC) are compared to each other. Additionally, the high frequency impedances are validated with high current interrupt (HCI) measurements. The results obtained by fitting of the spectra with an equivalent circuit are compared with theoretical calculations.