Analysis of electrochemical degradation phenomena of a 60-cell SOC stack operated in reversible SOFC/SOEC cycling mode

The paper presents the performance and degradation of a 1 kW SOC stack with 60 cells, which has been operated for 4200 h mainly in reversible SOFC/SOEC cycling mode. The stack was manufactured by the industrial project partner E&KOA (Daejeon, Korea) within the frame of the Korean-German project “Solid Oxide Reversible Fuel Cell / Electrolysis Stack” (SORFES). This project focused on the operation and improvement of a commercial scale 1 kW SOC stack in reversible SOFC/SOEC cycling mode in order to enhance the hydrogen productivity and its utilization. The reversible SOFC/SOEC cycles aimed to cover stack operation with intermittent renewable electricity supply (e.g. of photovoltaics and/or wind) based on the scenario of future expansions of gas pipelines, H2 storage capacities and electrical power grids within Europe. At the beginning of operation high performances and electrical efficiencies of 1068 W and 57 % in SOFC and 1950 W and 116 % in SOEC were achieved at 80 % fuel utilization. The stack revealed good homogeneity along the height with uniform electrical contacts of the cells in the stack. During reversible SOFC/SOEC cycling a rather stable stack behavior with degradation rates of −1.7 %/kh in SOFC mode and + 2.3 %/kh in SOEC mode was observed. In both modes the repeat units in the middle of the stack showed higher degradation compared to the bottom and top, which can be explained by thermo-mechanical stresses due to temperature gradients along the stack height during reversible SOFC/SOEC cycling. The degradation was dominated by the increase of the ohmic- and gas concentration resistances, whereas the degradation of the electrochemical activity of the electrodes played a minor role. It is very likely that the geometrical contact area between the oxygen electrode and the interconnector plate might have degraded. Therefore, further activities concentrate on the optimization of the reversible switching procedure in terms of temperature changes and gradients and on the development of contact elements with higher tolerance towards thermo-mechanical stresses.