Electrochemical performance and long-term durability of a 200 W-class solid oxide regenerative fuel cell stack

The electrochemical performance and durability of a 200 W-class solid oxide regenerative fuel cell (SORFC) stack are investigated for cyclic mode-changing and long-term operation. Three unit cells (10 cm × 10 cm), each based on a Ni - yttria-stabilized zirconia (YSZ) fuel electrode, a scandia-stabilized zirconia (ScSZ) electrolyte and a Sr-doped LaCoO₃ (LSC) - gadolinia-doped ceria (GDC) air electrode, are used for the stack development. Delamination of the air electrode is suppressed by using a mixed ionic- and electronic-conducting air electrode with no oxygen excess non-stoichiometry, and gas leakage is minimized by using novel glass-ceramic composite sealants. Excellent electrochemical performance is achieved in a single cell level by minimizing the ohmic and electrode polarizations, and the three-cell stack is successfully configured without major performance loss associated with electrical contacts, gas supply or sealing. Stable operation is confirmed at a thermal neutral voltage for 1000 h in the solid oxide electrolysis cell (SOEC) mode, and the periodic change of the operation mode between the solid oxide fuel cell (SOFC) and SOEC is found to accelerate the performance degradation. The effect of cyclic mode-changing on the stability of the SORFC stacks is discussed in detail based on the observations from the post-mortem microstructural analysis.