Investigation on electrical conductivity of LSM/YSZ8, LSM/Ce0.84Y0.16O0.96 and LSM/Ce0.42Zr0.42Y0.16O0.96 composite cathodes of SOFCs

Abstract

In the present work, the effect of solid electrolyte oxygen ion conductor composition on the electrical conductivity of strontium-doped lanthanum manganite (LSM, La0.7Sr0.3MnO3)-based composite cathodes were investigated. Three types of solid electrolytes including; yttria stabilized zirconia (Zr0.84Y0.16O0.96, YSZ8), yttria doped cerium oxide (Ce0.84Y0.16O0.96, CY), and ytrria doped cerium/zirconium oxide (Ce0.42Zr0.42Y0.16O0.96, CZY) were used for preparing composite cathodes of solid oxide fuel cells (SOFCs). Three types of composite cathodes were also fabricated, i.e., LSM-x wt% YSZ (x = 10–80), LSM–y wt.% CY and LSM–z wt.% CZY (y = z = 30, 50 and 70) by solid mixing and dry pressing.All the samples were sintered at 1450 °C to produce dense electrolytes and composite cathode samples for electrical conductivity measurement. The effect of remained porosity on the electrical conductivity was studied on the cathode with the composition of LSM–50 wt.% YSZ. To fabricate samples including different amount of remained porosities, the LSM–50 wt.% YSZ samples were sintered in the range of 1100–1450 °C for 5 h. The X-ray diffraction analysis showed that a single phase solid solution is created in the system of CY and CZY and also it indicated that the three used solid electrolytes and LSM were chemically compatible and no reaction occurred between them during sintering process of the composite cathodes at 1450 °C. The SEM results exhibited that the second phases and porosities are uniformly distributed inside the composite cathodes and perfect interface is created between the electrolytes and LSM phases. Electrical conductivity of the three different used electrolytes, pure LSM and all the dense and porous composite cathodes, were investigated in the temperature range of 300–900 °C in air by means of four probe DC measurement method.Among the different tested dense composite cathodes, CY-based composite material offers the highest electrode conductivity in the temperature range of 300–900 °C. For the composite cathodes, it was shown that by decreasing the sintering temperature and increasing the amount of remained porosity, the conductivity decrease and for all the used conductivity measuring temperature, they were merged to each other, so that the effect of measuring temperature on electrical conductivity nearly disappeared.