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

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

Understanding the Current-Voltage Behavior of High Temperature Solid Oxide Fuel Cell Stacks

High temperature solid oxide fuel cell (SOFC) stacks are highly efficient and environmentally friendly electrochemical systems, which convert the chemical energy of fuel gases with oxygen from air directly into electrical energy. During operation of SOFC stacks under system operating conditions pronounced temperature and fuel gas composition gradients along the cell area and along the height of the stack occur. Therefore, in contrast to SOFC cells, the electrochemical behavior of SOFC stacks is much more complex and has not sufficiently been studied. Specially, a shortcoming exists in terms of understanding the homogeneity, performance loss mechanisms, and various resistances and overvoltages within the stack repeat components. Therefore, this paper focuses on the improvement of the understanding and of the interpretation of different current-voltage curves of solid oxide fuel cell stack repeat units. Three different cases are discussed: repeat units with high power performance, with high cell contact resistance and with high fuel utilization. The stacks were investigated by current-voltage curves, electrochemical impedance spectroscopy and gas analysis. In order to understand the electrochemical behavior of these three cases both experimental and modeling results are presented, compared and discussed

Long term behaviour of solid oxide electrolyser (SOEC) stacks

The successful market introduction and public acceptance of the solid oxide fuel/electrolysis cell technology requires high performance, long-term stability and low costs of the corresponding cells and stacks. However, especially the degradation of SOEC stacks during long-term operation for several thousand hours in electrolysis and reversible SOFC/SOEC mode are not yet well understood. The paper presents the long-term behavior and the degradation results of a 30-cell stack with electrolyte supported cells operated in reversible SOFC/SOEC mode. The stack was supplied by Sunfire GmbH (Dresden/Germany) as part of the German funded project Reversible Solid Oxide Cell. The reversible cycles with duration of 24 h consist of 9 h SOFC operation at 70 % fuel utilization, 9 h SOEC operation at 70 % steam conversion and two switching phases, each with duration of 3 h. The stack is currently being operated for 6300 h under these reversible SOFC/SOEC cycling conditions. After the initial performance test, current-voltage curves and electrochemical impedance spectra were recorded every 2000 h. The electrochemical properties such as open circuit voltages, area specific resistances, power densities, fuel utilization, steam conversion rate and impedance values of the stack and of selected repeat units are presented. The homogeneity among the repeat units is discussed in context with the measured temperature gradients in the stack. The degradation and the change of the resistances, e.g. the ohmic resistances, the polarization resistances of the electrodes and the gas concentration resistances, during the reversible SOFC/SOEC long-term operation are presented and compared to each other. Furthermore, the degradation mechanisms of the different cell and stack components are discussed

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

Additional file 1: of Estimating the carbon fluxes of forests with an individual-based forest model

Simulation results for the three parameterizations. (DOCX 826 kb

Electrochemical Quality Assurance of Solid Oxide Electrolyser (SOEC) Stacks

High temperature solid oxide cells (SOC) are highly efficient and environmentally friendly electrochemical systems for the H2/H2O and/or CO/CO2 redox reactions. The cells can be operated reversely either in electrolysis (SOEC) or fuel cell (SOFC) mode which facilitates this technology for power-to-gas-to-power application in renewable energy storage systems. However, the successful market introduction and public acceptance of the SOEC technology require high quality, reliability and reproducibility of the corresponding cells and stacks. Therefore, in the European funded project "Solid oxide cell and stack testing, safety and quality assurance" (SOCTESQA) pre-normative test modules and programs for high temperature solid oxide cells and stacks have been developed. Different EU project partners have tested identical SOC stacks in several testing campaigns with the same test programs. The paper presents and compares the results of the stacks in SOEC operation which have been obtained by application of the three most important test modules, e.g. current-voltage characteristics, electrochemical impedance spectroscopy and operation at constant current. The results are analyzed and discussed in context to the test input parameters, e.g. gas temperatures and steam supply stability. Quality aspects like repeatability and reproducibility among the different partners and among different test methods are statistically evaluated and discussed.JRC.C.1-Energy Storag

Electrochemical Quality Assurance of Solid Oxide Electrolyser (SOEC) Stacks

High temperature solid oxide cells (SOC) are highly efficient and environmentally friendly electrochemical systems for the H2/H2O and/or CO/CO2 redox reactions. The cells can be operated reversely either in electrolysis (SOEC) or fuel cell (SOFC) mode which facilitates this technology for power-to-gas-to-power application in renewable energy storage systems. However, the successful market introduction and public acceptance of the SOEC technology require high quality, reliability and reproducibility of the corresponding cells and stacks. Therefore, in the European funded project “Solid oxide cell and stack testing, safety and quality assurance” (SOCTESQA) pre-normative test modules and programs for high temperature solid oxide cells and stacks have been developed. Different EU project partners have tested identical SOC stacks in several testing campaigns with the same test programs. The paper presents and compares the results of the stacks in SOEC operation which have been obtained by application of the three most important test modules, e.g. current-voltage characteristics, electrochemical impedance spectroscopy and operation at constant current. The results are analyzed and discussed in context to the test input parameters, e.g. gas temperatures and steam supply stability. Quality aspects like repeatability and reproducibility among the different partners and among different test methods are statistically evaluated and discussed