Degradation of a Solid Oxide Cell (SOC) stack during reversible fuel cell / electrolysis operation

Kurzfassung

Solid oxide fuel cells / electrolysis cells (SOFCs/SOECs) are predestined for the integration into power-to-gas systems due to their high efficiency, reversibility between SOFC and SOEC operation and fuel flexibility. Reliable long-term stability of SOC stacks under industrial operating conditions is a key factor for the commercial market introduction of the technology. Within the framework of the project Reversible Solid Oxide Cell (RSOC), which is a coorporation with Audi AG, two RSOC modules are integrated into an existing power-to-gas plant in Werlte. At DLR lab-scale tests with similar stacks are performed to better understand the behavior of the system. This thesis presents the long-term behavior of a 30-cell stack with electrolyte supported cells with 127.8 cm² active area under reversible SOFC/SOEC operation for 6500 h. The stack was supplied by Sunfire GmbH. The cells in the stack consist of a Ni-GDC fuel electrode, a 3YSZ electrolyte and a LSCF-GDC air electrode. During the test program 226 reversible SOFC/SOEC cycles were performed. At the end of this thesis, the stack is still running for additional 1500 h. One cycle lasts 24 h and consists of 9 h under current load in SOFC and SOEC each. Between both operating modes the operation conditions are switched for 3 h. In SOFC the stack is operated at 750 °C with a gas composition of 40 % H2 + 60 % N2 on the fuel electrode and air on the air electrode. In SOEC the operating temperature is set to 820 °C and the gas flow on the fuel electrode consists of 80 % H2O + 10 % H2 + 10 % N2, while the air electrode is supplied with air. The current density is set to 183 mA/cm² in SOFC and 383 mA/cm² in SOEC, both corresponding to 70 % gas conversion. Approximately every 2000 h the reversible SOFC/SOEC operation is interrupted in order to characterize the stack and its repeat units (RUs) with current-voltage-curves (jV-curves) and electrochemical impedance spectroscopy (EIS). With the obtained values the degradation of the stack and the RUs regarding voltage, resistances and temperature is determined.