Role of gas-phase composition on the performance of ceria-based composite electrolytes

• Ceria-based composites interact significantly with the surrounding gas phase.
• Electrode impedance decreases from CO2 to O2 and to H2.
• Formation of hydrogenated species explains enhanced conductivity in H2.
• Differences in bulk conductivity are governed by the mixed carbonates phase.

Ceria-based composites (ranging from 30 to 70 vol%) including a mixture of Li and Na carbonates (1:2 M ratio, respectively) were sequentially exposed to pure CO2, O2 and a mixture of 10 vol% H2 diluted in N2, at temperatures ranging from 300 to 580 °C, to study the role of composite/gas interaction on electrical performance. Impedance spectroscopy measurements performed under these working conditions were complemented by microstructural and structural characterization. Changes in conductivity and electrode impedance when changing the atmosphere from CO2 to O2 and to H2 confirmed changes in concentration and/or in mobility of charge carriers and electroactive species. Novel species identified either by X-ray diffraction (hydrated carbonates, only present at the surface of the samples), or by infrared spectroscopy (hydrogen carbonate ions), might be related to the observed conductivity enhancement under H2.