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

O xide-ion conductors are an exciting class of materials. 1There 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. [2][3]3][4][5][6] In the past decade, the pace of research on oxide-ion conductors has been rapid.A wide range of materials, e.g., ZrO 2 or CeO 2 -based fluorites, 7-10 LaGaO 3based perovskites, [11][12][13] La 2 Mo 2 O 9 -based LAMOX family, 14 lanthanum silicate-based apatites, 15 stabilized δ-Bi 2 O 3 16-20 and the Bi 4 V 2 O 11 -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-Bi 2 O 3 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, pO 2 -T) at the fuel electrode. 22econd, many stabilized δ-Bi 2 O 3 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. [18][19]][20][21][22][23] Wachsman and co-workers have demonstrated the first obstacle can be overcome by the creation of bilayer electrolytes, based on Gd-stabilized CeO 2 (GDC) and stabilized δ-Bi 2 O 3 . 24,25In this arrangement, the highly conducting δ-Bi 2 O 3 layer is protected from the low pO 2 at the fuel electrode by the more robust GDC (also an oxide-ion conductor) thus raising the pO 2 experienced by the δ-Bi 2 O 3 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-Bi 2 O 3 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. 18,23,[26][27]][28] In 2014, we reported a new family of oxide-ion conductors based on the ferroelectric perovskite sodium bismuth titanate (Na 0.5 Bi 0.5 TiO 3 , NBT). 29,30In particular, high levels of oxideion conduction were found in bismuth-deficient NBT, i.e., Na 0.50 Bi 0.49 TiO 2.985 , NB 0.49 T. The high ionic conductivity originates from oxygen vacancies generated because of Bi deficiency, as well as high anion mobility as a result of highly