Performance of Small Stack for Intermediate Temperature-Operating Solid Oxide Fuel Cells Using Stainless Steel Interconnects

Recently we have started a project to develop 1 kW-class SOFC system for Residential Power Generation (RPG) application supported by Korean Government. For a 1 kW-class SOFC stack that can be operated at intermediate temperatures, we started to develop anode-supported, planar type SOFC to have advantages for commercialization of SOFCs considering mass production and using cost-effective interconnects such as ferritic stainless steels. Anode-supported single cells with thin electrolyte layer of YSZ were fabricated and their small stacks were built and evaluated. The size of anode-supported single cells after final sintering was about 5 × 5 cm², and the thickness of electrolyte and the cathode layer were about 20 μm and 30 μm, respectively. I-V and AC impedance characteristics of these single cells and small stacks were evaluated at intermediate temperature (650 ∼ 750°C) by using hydrogen gas as a fuel. We have already carried out long-term performance test for YSZ thin electrolyte single cell for above 26,000 h (3 years) at 750°C, applying 0.76 V with power density of 200 mW/cm². Using these YSZ thin electrolyte 5 × 5 cm² cells and Inconel interconnect plates coated by silver paste, the 15-cell and 60-cell short stack were prepared. The initial stack (15 cell) voltage at 150 mW/cm² was 12.5 V in hydrogen as fuel of 120 sccm/cell at 750°C and decreased to about 10.9 V at 500 h of operation time. It was then stabilized and kept until 4,000 h with a degradation rate of 10 mV/(1000 h, 1 cell). AC impedance of this small stack and microstructure of cell components were analyzed during and after the operation. Furthermore thin electrolyte cells and ferritic stainless steel interconnects were built into a 4-cell stack and the small stack was operated at 650°C for cost-effective planar SOFC RPG system. I-V and AC impedance characteristics of the small stack were evaluated at 650°C by using hydrogen as a fuel.