Degradation analysis of a 60-cell SOC stack during reversible SOFC/SOFC cycling

Abstract

Although in recent years the degradation rates of SOFC and SOEC stacks in single mode long-term operation have been steadily decreased, the understanding of degradation mechanisms during reversible SOFC/SOEC operation remains an important and challenging issue. The paper focusses on the performance and degradation analysis of a 60-cell SOC stack which has been operated during 2000 h in reversible SOFC/SOEC cycling mode. The stack with fuel electrode supported cells was manufactured by the industrial project partner E&KOA (Daejeon, Korea) based on the proprietary technology which was developed within the Korean-German project “Solid Oxide Reversible Fuel Cell / Electrolysis Stack” (SORFES). This project aims on the development and improvement of the core component technology for a 1 kW reversible SOC stack in order to enhance the hydrogen productivity and its utilization. After the initial performance test the stack has been operated at 750°C with 40 reversible SOFC/SOEC cycles. Each cycle consisted of 24 h operation in SOFC mode at 80% fuel utilization and 24 h in SOEC mode at 80% steam conversion. This test procedure aims to cover stack operation with intermittent renewable electricity supply (e.g. of photovoltaics and/or wind). The stack was electrochemically characterized by jV-curves and electrochemical impedance spectroscopy (EIS). In order to analyze the temperature changes and gradients two thermocouples were integrated inside the stack in the air manifold inlet and outlet. The paper presents the initial performance and homogeneity along the repeat units (RUs) of the stack in SOFC and SOEC at the beginning of operation. The degradation, especially of the OCV, the power density and the area specific resistance (ASR) of the different RUs are discussed. Moreover, the contribution of the individual resistances, specifically of the ohmic-, the electrode polarization- and the gas concentration resistances, of the RUs are presented. The influence of temperature gradients and thermo-mechanical stresses during reversible exothermic (SOFC) and endothermic (SOEC) cycling are outlined and discussed.