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

Abstract

Solid oxide–alkaline carbonate (Li₂CO₃ : Na₂CO₃, 1 : 1 molar ratio) composite electrolytes were prepared using different solid oxide matrices (TiO₂, HfO₂, Yb₂O₃, Y₂O₃, Dy₂O₃, Gd₂O₃ and La₂O₃), 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, TiO₂ is found to be the most unstable in contact with the molten carbonates whereas Yb₂O₃ 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. Yb₂O₃ + Li₂CO₃ : Na₂CO₃ composites show chemical stability at operating temperatures in the order of 690 °C, standing as a potential candidate for intermediate temperature applications.