Fabrication and characterization of micro-tubular solid oxide fuel cells with yttria stabilized zirconia and Hafnia-Erbia co-doped bismuth oxide bilayer electrolyte

Abstract

Current solid oxide fuel cell (SOFC) technology relies on YSZ electrolyte material due to its high stability under SOFC operation conditions. However, the ionic conductivity of YSZ decays as the operation temperature decreases to intermediate or low-temperature range.Commercialization of SOFC systems is hindered by the high manufacturing cost, which is directly related to the use of special and expensive system components because of the high operating temperatures of SOFCs. It is possible to use low-cost materials in system production if the operating temperature of SOFCs is reduced below 600 degrees C.In this study, we investigated the possibility of using 8% mol Yttria stabilized Zirconia (YSZ) and Hafnia-Erbia Co-doped Bismuth Oxide (HEB, ((Er2O3)(0.20)(Bi2O3)(0.80))(0.99)-(HfO2)(0.01)) bilayer electrolyte configuration to lower the operating temperatures of SOFC to 600 degrees C and below. The cell performances at low-temperature region (T < 600 degrees C) are aimed to be increased with the additional HEB layer by the increased oxygen reduction kinetics on the air electrolyte side.Using a combination of thermoplastic extrusion and dip coating processes, four distinct cell types were created. Cells with monolayer and bilayer electrolyte were generated for each cathode type using two different cathode types, Lanthanum Strontium Manganate (LSM) and LSM-HEB. On the cell performance and operating temperatures, the impacts of electrolyte configuration and cathode type were examined. It was also explored whether the HEB material would be more effective as a second electrolyte or as a composite cathode if it was added to the LSM. Under various cell operating circumstances (different oxygen partial pressure values), several cell parameters such as the ohmic resistance, exchange current density, and anodic and cathodic saturation current densities were derived by fitting the experimentally recorded current density-voltage curves into the polarization model. The polarization resistances were calculated using cell parameters and four different cells were compared with each other. The performances measurements of both cell configurations showed that the peak power density of the bi-layered electrolyte configuration improved by a factor of 1.45 and reached 0.94 W/cm(2) at 600 degrees C, while it was only 0.51 W/cm(2) for the monolayered YSZ electrolyte configuration. Additionally, the effect of HEB addition to LSM cathode resulted in a performance increase of %84 compared to the cell with LSM cathode only. The presence of HEB either as a second electrolyte or ionic conductive phase in the composite cathode is lowered activation polarization of the cells both bi- or monolayered electrolyte cells and resulted in increased performance.