Long-term conductivity stability of acceptor-doped Na0.54Bi0.46TiO3−δ

Abstract

Sodium bismuth titanate (NBT) based compounds have drawn significant attention because of high oxygen-ion conductivity which makes them a promising electrolyte candidate for solid oxide fuel cells. The present work investigates the long-term conductivity stability of Na0.54Bi0.46TiO3−δ doped with acceptor cations such as Mg2+ and Ga3+ in 5% H2-95% N2 environment at 600 °C for 100 h. In-situ conductivity measurement performed using impedance spectroscopy indicates a significant loss in the conductivity in the first 20 h of ageing. Following which, the conductivity appears to be stable for the next 20 h. However, a sudden gain in conductivity is seen after 40 h of ageing in reducing conditions. XRD analysis indicates the formation of Bi2O3 based compound which on further ageing decomposes to metallic Bi. SEM analysis reveals the change in grain morphology during the course of ageing with the microstructure of 100 h aged sample is shown to contain several ruptured grains. XPS results indicate the partial reduction of Ti4+ to Ti3+ which possibly leads to high n-type electronic conductivity in aged samples. The conductivity loss is attributed to the decomposition of the perovskite phase while the sudden conductivity increase after 40 h of ageing is explained through n-type electronic conduction inside the NBT lattice.