Bilayer Electrolytes for Low Temperature and Intermediate Temperature Solid Oxide Fuel Cells – A Review

Abstract

The commercialisation of solid oxide fuel cell (SOFC) technology would benefit dramatically by a reduction of the operating temperature to a lower range (500–650 °C). Unfortunately, the ionic conductivity of YSZ and electrode performance decrease significantly at low temperatures resulting in low power density SOFCs. Doped ceria materials have ionic conductivity orders of magnitude higher than YSZ and have been extensively explored as an alternative electrolyte material. However, doped ceria reduces under fuel conditions at the anode side resulting in internal leakage current. This work is primarily focused on reviewing the recent developments of the concept of a bilayer electrolyte SOFC where ceria is the main electrolyte and the second electrolyte serves to block the leakage current. A thorough survey of works in the literature reveals that bismuth oxide/ceria bilayer electrolyte SOFCs yield higher power density compared to zirconia/ceria bilayer electrolyte SOFCS mainly due to the much higher ionic conductivity of stabilised bismuth oxide compositions compared to YSZ. A proper ceria/bismuth oxide thickness ratio is of great importance and hence needs to be tuned carefully. In addition, bilayer electrolytes can serve other functions in SOFC structures such as a diffusion barrier layer between the electrolyte and adjacent electrodes or a fast oxide ion conductor to promote catalytic activity toward oxygen reduction.