SDC/Na₂CO₃ nanocomposite: New freeze drying based synthesis and application as electrolyte in low-temperature solid oxide fuel cells

A key issue to develop low-temperature solid oxide fuel cells (LTSOFCs) is to develop new electrolyte materials with enhanced ionic conductivity. Recently, SDC/Na₂CO₃ nanocomposite, as a proton and oxide co-ion conductor, has been developed as promising electrolyte candidates for LTSOFCs, where Na₂CO₃ as the secondary phase performs several crucial functions. However, it's difficult to control the homogeneity of Na ₂CO₃ phase in the composite by the current methods for composite fabrication. In this study, we report a new freeze drying technique to fabricate SDC/Na₂CO₃ nanocomposites with different content of Na₂CO₃. Structural and morphological study confirmed that the homogeneity of both SDC and Na₂CO₃ phases in the nanocomposite is well controlled by the freeze drying technique. The effect of Na₂CO₃ content on proton and oxygen ion conductivities of SDC-carbonate samples were investigated by the four-probe d.c. measurement. Proton conductivity transformation around 350 °C has been observed for all the SDC/Na₂CO₃ nanocomposites due to the glass transition of amorphous Na₂CO₃ phase, and the proton conductivity is dependent on Na₂CO₃ content. While oxygen ion conductivity deceases with the increasing of Na₂CO₃ volume fraction in the nanocomposite. Finally, SOFCs were fabricated using SDC/Na₂CO₃ nanocomposite samples and tested for electrochemical performances. The excellent performance of SOFCs using SDC/Na₂CO₃ nanocomposite electrolyte verifies that nanocomposite approach is an effective way to fabricate electrolyte with enhanced ionic conductivity for LTSOFCs.