Electrochemical Studies on Na-β”-Alumina + Yttria-Stabilized Zirconia (YSZ) Composite Mixed Na+-Ion-O2−-Ion Conductors

Abstract

Composite Na-β”-alumina + yttria-stabilized zirconia (YSZ) samples were fabricated by vapor phase conversion of sintered α-alumina + YSZ two-phase samples. In the converted samples Na-β”-alumina and YSZ were both contiguous. The samples thus are mixed Na+-ion-O2−-ion conductors. Discs were tested separately under an oxygen chemical potential differential (H2-H2O-N2 gas mixture on one side and air on the other – fuel cell mode) and under a sodium chemical potential differential (Na-β-alumina + Na-β”-alumina on one side and α-alumina + Na-β-alumina on the other side). In both cases finite open circuit voltages were measured. In testing in fuel cell mode, the initially observed open circuit voltage was lower than expected for a single phase YSZ membrane, which was attributed to the transient flow of Na+ ions. After prolonged testing, the surface exposed to hydrogen containing gas mixture degraded forming a thin layer of α-alumina. The ionic conductivity of the samples was measured using both AC and DC methods.