High Ionic Conductivity with Low Degradation in A-Site Strontium-Doped Nonstoichiometric Sodium Bismuth Titanate Perovskite

O conductors are an exciting class of materials. There has been a continued drive for the development of oxide-ion conductors because of their potential applications in various important technological devices such as solid oxide fuel cells (SOFCs), oxygen separation membranes, oxygen sensors, and oxygen pumps. In the past decade, the pace of research on oxide-ion conductors has been rapid. A wide range of materials, e.g., ZrO2 or CeO2-based fluorites, 7−10 LaGaO3based perovskites, La2Mo2O9-based LAMOX family, 14 lanthanum silicate-based apatites, stabilized δ-Bi2O3 16−20 and the Bi4V2O11-based BIMEVOX family 21 have been reported, among which Bi-based materials exhibit the highest known oxygen-ion conductivity. Despite their attractive levels of oxide-ion conductivity at intermediate temperatures, i.e., 400−600 °C, it is challenging to implement stabilized δ-Bi2O3 materials as an electrolyte for intermediate temperature solid oxide fuel cells (ITSOFCs) for two reasons. First, they are prone to chemical reduction/ decomposition under the required operating conditions (partial oxygen pressure−temperature, pO2−T) at the fuel electrode. Second, many stabilized δ-Bi2O3 materials are known to suffer from degradation (or aging) of the oxide-ion conductivity in this temperature range due to a combination of phase transformations and anion ordering. The former generally occur at >600 °C, whereas the latter is often dominant at ∼500 °C but becomes less pronounced at lower temperatures due to the slower kinetics associated with anion ordering. Wachsman and co-workers have demonstrated the first obstacle can be overcome by the creation of bilayer electrolytes, based on Gd-stabilized CeO2 (GDC) and stabilized δ-Bi2O3. 24,25 In this arrangement, the highly conducting δ-Bi2O3 layer is protected from the low pO2 at the fuel electrode by the more robust GDC (also an oxide-ion conductor) thus raising the pO2 experienced by the δ-Bi2O3 sufficiently to avoid chemical reduction/decomposition and therefore taking advantage of its higher oxide-ion conductivity. The same group has studied degradation effects in δ-Bi2O3 over many years and has shown that appropriately sized rare earth dopants and codoping with other elements, e.g., Dy and W at various levels can be effective in suppressing the degradation of ionic conductivity at ∼500 °C. In 2014, we reported a new family of oxide-ion conductors based on the ferroelectric perovskite sodium bismuth titanate (Na0.5Bi0.5TiO3, NBT). 29,30 In particular, high levels of oxideion conduction were found in bismuth-deficient NBT, i.e., Na0.50Bi0. 49TiO2.985, NB0.49T. The high ionic conductivity originates from oxygen vacancies generated because of Bi deficiency, as well as high anion mobility as a result of highly polarizable Bi with its 6s lone pair electrons and weak Bi−O bonds, which provide pathways with low diffusion barriers. The predominant oxide-ion conduction in NB0.49T has been confirmed by O diffusion profiles and electromotive force (EMF) measurements, which show an ionic transport number ti > 0.9 at 600−700 °C. Enhancement of the bulk conductivity (σb) by more than half an order of magnitude was achieved by acceptor-doping Mg onto the Ti-site (B-site) of NB0.49T to create a higher level of oxygen vacancies. Unfortunately, further enhancement of σb by increasing the doping level of Mg 2+ has not been achieved. Experimentally, the solubility of Mg on the Ti-site is rather low, i.e., a Ti-rich secondary phase has been observed in 2 at% Mg-doped NB0.49T. 29 Theoretically, firstprinciples calculations have recently predicted acceptor dopants on the Ti-site to significantly increase the oxygen migration barriers by binding with oxygen vacancies, which suppresses the mobility of oxygen ions and is detrimental to optimizing the ionic conduction. The same study proposed that acceptor doping on the A-site of the perovskite cell may be a more effective method to suppress binding with oxygen vacancies and therefore result in optimization of the ionic conductivity in NBT-based materials. Here, we report a Sr-doped NB0.49T, Na0.50Bi0. 47Sr0.02TiO2. 975, in which Sr replaces Bi on the A-site of NBT with the creation of oxygen vacancies. This A-site acceptor doping mechanism enhances σb compared with NB0.49T perovskite but more importantly, Sr-doped NB0.49T exhibits low degradation of σb with time at intermediate temperatures, as well as reasonable stability under reducing atmosphere, which makes it a promising electrolyte material for ITSOFCs. Sr-doped NB0.49T ceramics were prepared by the solid-state reaction method. Appropriate amounts of each powder was ball milled and then calcined twice at 800 °C. Pellets were sintered at 1150 °C for 2 h. Sintered pellets were phase-pure by XRD, dense (>95% of the theoretical X-ray density) and with an average grain size of ∼5 μm as shown by SEM on thermally etched surfaces. EDX analysis on the polished surface (without thermal etch) shows the composition is close to its nominal value and thus confirms the suggested doping mechanism. Electrical conductivity was obtained from ac impedance spectroscopy. A typical complex impedance plane (Z*) plot of Sr-doped NB0.49T showed three arcs, from high to low frequency representing the response from the grains (bulk), grain boundaries and electrode effects, respectively. An