Exploratory Study of Novel Materials Used for Intermediate Temperature Solid Oxide Fuel Cell (IT-SOFC) Electrolytes

Sammanfattning: Currently, yttria-stabilized zirconia (YSZ) is still the dominant electrolyte material in commercial SOFC applications. But it has severe drawbacks due to its high operating temperatures. In present work, two electrolyte materials: Hf0.69Y0.31O2-δ (YSH) used as oxygen ion conductor and BaZr0.5Ce0.3Ln0.2O3-δ (BZCLn532, Ln=Y, Sm, Gd, Dy) used as protonic conductors were studied at intermediate temperatures (IT, 500-700 ℃). The work is focused on the following parts:1)      A pure and well-crystallized YSH powder was successfully synthesized by using a modified solid state reaction method. The obtained YSH is in a fluorite cubic structure with a lattice parameter 5.140674 Å from the Rietveld refinement analysis. A YSH ceramic material with a relative density of 97.5% is obtained by a conventional sintering at a temperature of 1650 ℃. The oxygen ion conductivity of the YSH ceramic is 3.65×10-5 S cm-1 at a temperature of 700 ℃, which is too low for oxygen ion conductor applications. In contrast, there is an obvious enhancement of the protonic conductivity, when the testing temperature is higher than 600 ℃. The conductivity that tested in a moist atmosphere at a temperature of 700 ℃ is 5.19×10-5 S cm-1, which is 1.4 times higher than the oxygen ion conductivity.2)      BaZr0.5Ce0.3Ln0.2O3-δ (BZCLn532, Ln=Y, Sm, Gd, Dy) based electrolytes were successfully synthesized by using a cost-effective solid state reactive sintering (SSRS) method with 1 wt.% NiO as a sintering aid. Based on the obtained conductivities of BZCLn532 compounds measured in a dry air atmosphere and a moist air atmosphere, BaZr0.5Ce0.3Y0.2O3-δ (BZCY532) and BaZr0.5Ce0.3Dy0.2O3-δ (BZCD532) compounds are demonstrated to be good candidates for both oxygen ion conductor and proton conductor materials for solid oxide fuel cells operating at intermediate temperatures.In summary, proton conducting ceramic materials represent one type of promising materials for future IT-SOFCs electrolyte applications.