Electrochemical impedance spectroscopy of SOFC and SOEC stacks

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

Electrochemical impedance spectroscopy for solid oxide fuel cell (SOFC) and electrolysis (SOEC) stacks

High temperature solid oxide fuel cells (SOFC) and solid oxide electrolysis cells (SOEC) have a high potential as future power generation systems with high efficiency, low noise and low emissions. This covers the wide field of, e.g. stationary SOFC μ-CHP (combined heat and power), mobile SOFC APU (auxiliary power unit), SOEC power-to-gas and combined SOFC/SOEC power-to-gas-to-power systems. However, the successful development of the solid oxide cell (SOC) stacks requires high quality reliable and reproducible test results, which enables 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 [1, 2]. The presentation focuses on the improvement of the understanding and of the electrochemical behavior of SOC stack repeat units both in fuel cell (SOFC) and electrolysis (SOEC) mode. The stacks have been investigated in different projects by electrochemical impedance spectra, current voltage curves (jV-curves) and gas analysis. In the first part the quality of the results is presented in terms of reproducibility and repeatability among different testing partners and among different test methods. The corresponding results have been obtained within the European funded project “SOCTESQA” (Solid oxide cell and stack testing and quality assurance) [3]. Moreover, the most relevant parameters influencing the reliability of the EIS spectra and jV-curves are outlined. In the next part the electrochemical results of the SOC stacks operated at different characteristic conditions are presented and discussed. This includes stacks operated at high current density and at high fuel utilization and stacks with high contact resistance [4]. In order to understand the electrochemical behavior of these three cases the different resistances of the repeat units have been determined with an equivalent circuit. Both experimental and modeling results are presented and discussed. The results were obtained in the German funded project “ZeuS 3”. In the last part of the presentation degradation issues of the SOC stacks are addressed. Aspects for determining reliable and reproducible degradation rates are discussed. Moreover, results of stacks with and without Cr-evaporation protection layer from the German funded project “Smart” are presented. The stacks have been operated in SOFC mode galvanostatically at 300 mA/cm2 for 10.000 h. The increases of the different resistances in the stack repeat units which have been determined by EIS spectra during operation are outlined. Additionally, the most relevant degradation mechanisms are discussed

Long-Term Behavior of a Solid Oxide Electrolyzer (SOEC) Stack

The successful market introduction of the solid oxide fuel/electrolysis cell technology for power-to-gas applications requires the reduction of the degradation rates and the better understanding of the degradation mechanisms of the stacks. Therefore, the paper reports and compares the long-term behavior of a solid oxide cell stack in electrolysis and reversible fuel cell/electrolysis operation. The 30-cell stack with electrolyte supported cells was supplied by Sunfire GmbH (Dresden/Germany) in the German funded RSOC Project. The stack was operated for 3370 h in electrolysis and afterwards for 2500 h in reversible fuel cell/electrolysis mode, each at 70 % gas conversion. In the beginning of the test, the stack showed high gas tightness, good performances and high efficiencies in both SOEC and SOFC operations. During 3370 h of SOEC operation a low degradation of +0.5 %/1000 h was measured. During 2500 h of reversible fuel cell/electrolysis cycling, the gas tightness of the stack slightly decreased, which led to a temperature increase, and higher degradation rates were observed. The increase of the ohmic resistance contributed mostly to the degradation. Optimized operating conditions for reversible cycling and increasing the purity of the supplied water are foreseen in order to minimize stack degradation in reversible operation

Long term behaviour of solid oxide electrolyser (SOEC) stacks

The paper reports and compares the long term operation behavior of a solid oxide electrolysis (SOEC) stack in electrolysis and reversible SOFC/SOEC operation. The 30-cell stack with electrolyte supported cells (ESC) was supplied by Sunfire GmbH (Dresden/Germany) as part of the German funded project “Reversible Solid Oxide Cell" (RSOC). The stack was operated for 3370 h in SOEC and for 2000 h in reversible SOFC/SOEC operation mode at 70 % steam conversion and 70 % fuel utilization, respectively. After the initial performance test, current-voltage (jV) characteristics and electrochemical impedance spectra (EIS) were recorded regularly. The degradation of the electrochemical properties, e.g. open circuit voltage (OCV), electrical stack power, electrical stack efficiency and the resistances of the repeat units (RU), during SOEC and reversible SOFC/SOEC long term operations are compared to each other and discussed

Electrochemical impedance spectroscopy of SOFC and SOEC stacks

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