The successful market introduction of the solid oxide fuel/ electrolysis cell technology for power-togas 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 3,370 h in electrolysis and afterwards for 2,500 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 3,370 h of SOEC operation a low degradation of +0.5%/ 1,000 h was measured. During 2,500 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.

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.

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) in the frame 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.

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