Mass transport and electrochemical properties of sodium bismuth titanate based electrolyte materials
Oxide-ion conductors have drawn significant attention because of their crucial technical applications such as solid oxide fuel cells (SOFCs), oxygen separation membranes and sensors. Recently, sodium bismuth titanate (Na0.5Bi0.5TiO3, NBT) has been studied for its superior oxide-ion conductivity. In this thesis, the oxide-ion conductivity of acceptor-doped NBT is reported. A range of acceptor-type ions (K+, Li+, and Al3+) with varying doing levels are introduced into NBT, and their effects on
... their effects on material properties, including crystal structure, electrical conductivity, and oxygen transport properties, were investigated. Overall, low levels of acceptor dopants can be introduced on both cation sites of NBT through an ionic compensation mechanism that generates additional oxygen vacancies which improve oxide-ion conductivity to the levels of ~ 10-3 S cm-1 at 600 oC. Among the three acceptor dopants, a maximum enhancement of more than one order of magnitude was achieved by substituting Li on the A-site (7 x 10-3 S cm-1) or Al (3 x 10-3 S cm-1) on the B-site of NBT ceramics. The oxygen diffusion coefficients are in excellent agreement with the values derived from Electrochemical Impedance Spectroscopy data, confirming that the oxygen ions are the main charge carriers in the system. Finally, a degradation test was conducted under a variety of atmospheres and initial results show that Li- and Al-doped NBT materials were stable with a slight decay in conductivity. Compared with other well-known oxide-ion conductors, acceptor-doped NBT materials demonstrate superior ionic conductivity at temperatures above 600 oC and impressive durability under oxidizing atmospheres in the intermediate-temperature range. In short, the acceptor-doped NBT-based materials are excellent candidates for IT-SOFC applications.