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Abstract
AbstractThe 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 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.
Highlights
The successful market introduction and public acceptance of the high temperature solid oxide cells (SOC) technology require high performance, long-term stability and low costs of the corresponding stacks
A 30-cell stack with ESCs was operated for 3,370 h in Solid Oxide Electrolyzer (SOEC) and for 2,500 h in reversible SOFC/SOEC cycling mode at 70% steam conversion and 70% fuel utilization, respectively
During 3,370 h of SOEC operation at 820 °C and 70% SC, the stack remained gas-tight and the electrolysis voltage increased by +0.5% kh–1 (i.e., +12 mW cm2 kh–1 per RU), which is low compared to the degradation of other stacks found in literature
Summary
The successful market introduction and public acceptance of the high temperature solid oxide cells (SOC) technology require high performance, long-term stability and low costs of the corresponding stacks. The electrolysis voltage V (I) under operation at an electrical current I is the sum of the OCV and the resulting overvoltages DVtotal, which are generated by the resistances of the cell or the stack repeat unit. EIS spectra were recorded near OCV conditions, in order to analyze and qualify the degradation mechanisms without having electrical current induced temperature effects involved These spectra do not focus on the quantification of the degradation values of the long-term operations, but on the analysis of the differences between SOEC and reversible SOFC/SOEC operation. A nonlinear progression of the J–V-curve with increasing current density can be observed This behavior is typical for stacks with high gas tightness, which are operated with dry fuel gas in SOFC mode. At current densities higher than 50 mA cm–2 the J–V-curve proceeds
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