Cyclic degradation analysis of reversible Solid Oxide Cell short-stack

The deployment of renewable energy systems has been increasing steadily in recent years. The intermittency associated with the renewable electricity supply can be addressed through the usage of energy storage and release systems. In this framework, reversible solid oxide cells (rSOC) integrated in power-to-gas-to-power systems can convert excess electricity into multiple useful fuels (solid oxide electrolysis (SOE) mode) and restitute it when its renewable production is too low (solid oxide fuel cell (SOFC) mode). To make rSOC competitive and durable, low degradation for the targeted lifetime must be achieved. In BALANCE project (H2020 GA no:731224), with the purpose of assessing the degradation of rSOC in both modes, a cyclic durability test as shown in the graphical abstract was conducted on a 5-cell short stack (100 cm2 active area each) designed by CEA, using H2- electrode supported Ni-YSZ cells manufactured by DTU. The composition of the inlet gases was H2/N2:50/50 in SOFC mode and H2/H2O:20/80 in SOE mode. Steady-state initial performance revealed an improvement for all the repeatable units (RUs) in SOFC mode. In SOE mode, only RU 4 and 5 showed enhanced performance while RU1-3 degraded. Afterwards, J-V characteristic curves showed loss of performances in both modes all along the experiment. A distribution of relaxation time analysis performed on the electrochemical impedance spectra measured regularly throughout the test revealed that the oxide ion transport through the H2-electrode and the O2-electrode is responsible for the observed degradation, over the 1400 h test duration.