Analysis of Electrochemical Degradation Phenomena of SOC Stacks Operated in Reversible SOFC/SOEC Cycling Mode

Abstract

In recent years the degradation rates of high temperature stacks with solid oxide cells (SOC) during steady-state long-term operation in fuel cell (SOFC) and electrolysis (SOEC) mode have been steadily decreased. In contrast, the quantification and understanding of degradation mechanisms of SOC stacks during reversible SOFC/SOEC cycling operation still remains a challenging issue. Therefore, the present paper focusses on the detailed analysis and discussion of degradation phenomena of two SOC stacks during galvanostatic steady-state SOFC and reversible SOFC/SOEC cycling operation. The stacks with fuel electrode supported cells of Elcogen (Estonia) were fabricated by the industrial project partner E&KOA (Daejeon, Korea) within the KoreanGerman project “Solid Oxide Reversible Fuel Cell/Electrolysis Stack” (SORFES). The first 10-cell stack was tested at DLR during 1400 h and the results were used to improve the second 6-cell stack, which was operated at E&KOA during 2800 h. For electrochemical characterization jV-curves and electrochemical impedance spectroscopy were measured. The results between galvanostatic steady-state SOFC operation and reversible SOFC/SOEC cycling are compared. The degradation of the open circuit voltages, the performances and the resistances of the individual repeat units are presented and discussed. Moreover, possible degradation mechanisms are outlined.