Effects of Trace Copper on Shrinkage and Low-Temperature Co-Firing of Solid Oxide Fuel Cells

Abstract

Solid oxide fuel cells (SOFCs) are an attractive power generation due to their high energy efficiency and possibility of waste heat recovery with operating at high temperature. SOFCs were fabricated by commonly three steps with pre-sintering of anode, dip-coating electrolyte and screen printing cathode materials cause of different shrinkages. It is hard to commercialize with these high manufacturing costs of multi-step process. SOFCs have been focused on low-cost materials and process for commercializing with high efficiency and performance. 8 mol % yttria-stabilized zirconia (8 YSZ) has been considered as one of the most conventional material for SOFC electrolyte. The 8 YSZ electrolytes doped with Cu contents which used as a sintering aid leads to densify the electrolyte and increase the shrinkage and performance. So we attempted to fabricate with one step co-firing process with 8 YSZ electrolytes at low temperature as 1250 oC. First of all, we found addition of trace Cu is effective to improve the sinterability of the YSZ electrolytes. The YSZ electrolyte with 100ppm CuO shows better sinterability and ionic conductivity than the pure YSZ electrolyte. The YSZ electrolyte with 100ppm CuO was densified below 1300 oC and the increased oxygen vacancy concentration leads to have higher ionic conductivity. Furthermore, the performances of the single-layer YSZ cells with 100ppm CuO were about 1.5 times higher(0.51 W cm2) than the cell with the pure YSZ electrolyte (0.36 W cm2) at 800 oC. The GDC/YSZ (100 ppm CuO) bi-layer cells also showed higher cell performance at 700 oC which shows similar tendency to the single-layer YSZ cell tests. These show that addition of 100 ppm CuO into the YSZ electrolyte is greatly effective for improvement of SOFC performance with energy saving during sintering process. According to these results of enhancing the sinterability and ionic conductivity, co-firing process was developed by addition of 5000 ppm Cu to the YSZ electrolytes and anodes at 1250 oC which is seriously lower than conventional multi-step fabrications with the sintering temperature above 1500 oC. The sinterability of the YSZ electrolyte was significantly improved 1250 oC by the 5000 ppm Cu. The co-fired cells with one- and two-step exhibited reasonable OCV value of 1.016 V and 1.011 V, respectively. Maximum power density of two-step cell with 0.71 W cm-2 at 850 oC which was similar value compared to conventional cell but the one-step cell has low maximum power density with 0.14 W cm-2. Above 1200 oC as sintering temperatures, the LSM/YSZ cathode produces insulating phases at the interfaces and low porosity makes cathode polarization. As a result, the cathode should be improved to be stable at 1250 oC, or the co-firing temperatures should be reduced below 1200 oC.