Impedance spectroscopy study of Au electrodes on Gd-doped CeO2 (GDC) – Molten Li2CO3+Na2CO3 (LNC) composite electrolytes

Abstract

We herein report an impedance spectroscopy study of Au electrodes on Gd-doped CeO2 (GDC) – molten Li2CO3+Na2CO3 (LNC) composite electrolytes in O2 and O2+CO2 atmospheres. Complementary measurements of Au on GDC alone are provided for supporting insight. We find that the adsorption of CO2 on GDC in O2+CO2 atmospheres effectively blocks oxygen adsorption and severely slows oxygen reduction kinetics. The conductivity of the composite is dominated by the GDC phase in the solid-solid temperature region, while the LNC phase dominates above its melting point, and no further enhancement e.g. by interfacial effects are found. The incorporation of LNC melt into GDC results in a significant reduction in the polarisation resistance of Au electrodes in O2 atmospheres, as the melt mediates the reaction by a peroxide mechanism. In O2+CO2 atmospheres, however, the polarisation resistance of Au electrodes on GDC-LNC membranes is significantly higher, higher even than that on GDC. This we assign again to the blocking adsorption of CO2 (or carbonate) on the surfaces of ceria and the sluggish transport and reactions now mediated by carbonate-carried oxide species (CO42−) instead of peroxide species.