High Oxide-Ion Conduction in Rb-Containing Oxides

Abstract

Oxide-ion conductors are essential for clean energy applications, such as solid oxide fuel cells (SOFCs). Since the Rb⁺ ion is the second largest available cation, some Rb-containing oxides are expected to have low activation energy for oxide-ion conductivity and high oxide-ion conductivity at low temperatures. However, Rb-containing oxide-ion conductors are very rare. Herein, we report the high oxide-ion conductivity of the palmierite-type Rb₅BiMo₄O₁₆ (e.g., 2.3 mS cm^–1 at 560 °C), which was discovered by a combined technique of computer screening through bond-valence-based energy calculations of 475 compositions of Rb-containing oxides, synthesis, and characterization of the structural and transport properties. Rb₅BiMo₄O₁₆ exhibits a high oxide-ion conductivity of 0.14 mS cm^–1 at 300 °C, which is 29 times higher than that of yttria-stabilized zirconia (YSZ) at 300 °C and comparable to the leading oxide-ion conductors with tetrahedral moieties. The high ionic conductivity below 480 °C is mainly due to the low activation energy for ionic conductivity, which can be attributed to the large free volume of Rb₅BiMo₄O₁₆. The extremely large anisotropic thermal motion of oxygen atoms and the rotational motion of MoO₄ tetrahedra are also responsible for the high conductivity. Rb₅BiMo₄O₁₆ is stable at high temperatures under a CO₂ flow, under a wet air flow, and under a wet 5% H₂ in a N₂ flow, and at about 21 °C in water. Rb₅RMo₄O₁₆ materials (R: La, Pr, Nd, Sm, Gd, Tb, and Er) also exhibit significant conductivity. The discovery of Rb-containing oxides with high conductivity and high stability would open a new avenue for oxide-ion conductors.