Long-Term Behavior of a SOC Stack in Reversible Fuel Cell/Electrolysis Operation
Authors: Lang Michael , Braig Michael , Alqubati Nageeb , Szabo Patric 
Published: July 2021

The German funded project “Reversible Solid Oxide Cell" (RSOC) aims to demonstrate and to optimize a reversible SOC system made by Sunfire GmbH (Dresden/Germany) in the industrial power-to-gas (P2G) plant of Audi AG in Werlte (Germany). The produced H2 may be either used for industrial processes and/or for fueling stations or be converted by methanation into synthetic fuels. It can also be used to generate electricity, which will be fed into the grid especially during low renewable energy supply. For the development of the RSOC system, a fundamental understanding of the corresponding stack behavior during long-term operation under system relevant reversible SOFC/SOEC cycling conditions is obligatory. Moreover, the understanding of the degradation and the degradation mechanisms of the SOC stacks during long-term operation for several thousand hours, especially in reversible SOFC/SOEC mode, remain one of the most important and challenging issues. Therefore, this paper presents the long-term behavior and the degradation results of a 30-cell stack with electrolyte supported cells (ESCs) operated for 9200 h in reversible SOFC/SOEC mode. The stack was supplied by Sunfire GmbH and is identical to the stacks used in the Audi RSOC system in Werlte. The long-term stack test can be separated into three different operating phases, which are described in the following.
In the initial performance test during the first 400 h, the stack showed a high gas tightness as well as a very good and homogeneous performance. In SOFC, a stack power of 560 W (146 mWcm-2) and an electrical efficiency of 45% were achieved at 70% H2 utilization. In SOEC, a stack power of -1.86 kW (-486 mWcm-2) was required in order to split 70% of the supplied steam into H2. At the given operating conditions 10.2 SLPM of H2 was produced with a high electrical stack efficiency of 99%. In the following 8600 h of long-term operation altogether 326 reversible SOFC/SOEC cycles were performed. The SOFC/SOEC cycles consisted of 9 h of SOEC operation at 820°C and 70% SC (-383 mAcm-2), 9 h of SOFC operation at 750°C and 70% FU (183 mAcm-2) and of two operating mode switching phases each with a duration of 3 h. During reversible operation, very low degradation rates of -4.4 mVkh-1 (SOFC) and +1.9 mVkh-1 (SOEC) per repeat unit (RU) were observed. The relative degradation values were -0.53 %kh-1 in SOFC and +0.15 %kh-1 in SOEC. These degradation rates are the lowest degradation values of reversibly cycled stacks ever reached up to now. However, the degradation of the RUs was not homogeneous. The RUs located at the endplates showed the highest degradation rates and the degradation rates tended to become lower with increasing stack height. Electrochemical impedance spectra (EIS) have shown, that the increase of the ohmic resistance contributed mostly to the degradation rates, whereas the electrode polarization resistances and the gas concentration resistance remained almost constant during reversible SOFC/SOEC operation. The increase in resistances has led to an increase of the stack temperature of +3 Kkh-1 in SOFC and +5 Kkh-1 in SOEC. This resulted in enhanced temperature gradients inside the stack during operation. The resulting increase in thermo-mechanical stresses have led to the deterioration of the gas tightness of three RUs in the middle of the stack. Due to the progressive degradation and the further temperature increase in the third operation phase after 9000 h, the stack operation was stopped after 9200 h and 337 of reversible cycles.
In order to identify the degradation mechanisms, the future activities focus on the disassembling and the post-mortem analysis of the stack components by scanning electron microscopy (SEM) and energy dispersive microanalysis (EDX). Moreover, the results gained from this stack test will be used for the optimization of the operation of the RSOC system at Audi AG. In this context, the adjustment of the stack cooling with air may significantly reduce the thermo-mechanical stresses in the stacks, thus leading to minimization of the system degradation during reversible SOFC/SOEC long-term operation. The outcome and the results of the RSOC project will foster the further market introduction of the RSOC technology for P2G applications.