Stability of metal oxides against Li/Na carbonates in composite electrolytes

Solid oxide-alkaline carbonate (Li 2 CO 3 : Na 2 CO 3 , 1 : 1 molar ratio) composite electrolytes were prepared using different solid oxide matrices (TiO 2 , HfO 2 , Yb 2 O 3 , Y 2 O 3 , Dy 2 O 3 , Gd 2 O 3 and La 2 O 3 ), to cover a wide range of ceramic chemical characteristics. The chemical and microstructural stability of these oxides with the mixed carbonates were studied by powder X-ray diffraction, scanning electron microscopy, infrared and laser Raman spectroscopic techniques after reacting them at 690 °C for 1 h in air. The electrical performance of selected composites was evaluated using impedance spectroscopy, in air. Amongst the oxides hereby tested, TiO 2 is found to be the most unstable in contact with the molten carbonates whereas Yb 2 O 3 is quite stable. The corresponding composites have ionic conductivities (3.3 × 10 −1 S cm −1 at 580 °C, in air) close to those reported for state-of-the-art ceria-based composite electrolytes. A draft equivalent circuit model underlines the transport in the carbonate phase and across the carbonate/oxide interfaces as the dominant contributions to the total conductivity of these composites. Yb 2 O 3 + Li 2 CO 3 : Na 2 CO 3 composites show chemical stability at operating temperatures in the order of 690 °C, standing as a potential candidate for intermediate temperature applications. Possible equivalent circuits corresponding to alternative pathways in composite electrolytes with alkaline carbonates and Yb 2 O 3 , a promising ceramic matrix.