Remarkably High Oxide Ion Conductivity at Low Temperature in an Ordered Fluorite‐Type Superstructure

Abstract

Oxide ion conductors are technologically important materials because of their potential applications in oxygen sensors and pumps, as dense membranes for oxygen permeation, catalysts, and as electrolytes for solid oxide fuel cells (SOFCs). [1][2][3][4] To be efficient in various applications, candidate materials should possess a conductivity of at least 10 À2 S cm À1 at deviceoperating temperatures; currently commercially used yttriastabilized zirconia (YSZ) reaches this target at 700 8C.[1]Given the drive towards lowering device-operating temperatures, there is a strong impetus and a great challenge for materials chemists to develop materials with enhanced ionic mobility and superior low-temperature oxide ion conductivity. [5,6] A better understanding of generic structural features and pathways which facilitate ionic mobility at lower temperature is a key step in reaching this goal.Here we report a remarkably high oxide ion conductivity at low temperatures (300-500 8C) in an ordered pseudo-cubic 3 3 3 [8,9] By contrast and unusually, our materials crystallize as stable ordered superstructures, and do not undergo phase transitions to lower symmetry and lower conductivity polymorphs. Our ab initio molecular dynamics (AIMD) simulations reveal the structural features and mechanisms which facilitate the high oxide ion mobility at low temperatures, and provide conceptual insight readily applicable to other materials and structure types.The high-temperature cubic fluorite-type bismuth oxide, d-Bi 2 O 3 , with intrinsic oxygen vacancies, shows the highest oxide ion conductivity measured in any material (around 1 Scm À1 at 750 8C); [10] however, it is only thermodynamically stable in the narrow range between 730 and 824 8C.[11] There has been considerable interest in stabilizing the highly conducting d-Bi 2 O 3 phase by isovalent or aliovalent cation substitution to preserve oxide ion conductivity at lower temperatures. For example, 20 % substitution of Er into Bi 2 O 3 results in oxide ion conductivity of 2 10 À2 S cm À1 at 500 8C and 0.4 S cm À1 at 700 8C.[12] Double cation substitution has yielded even higher conductivities at low temperatures (300-500 8C); the best examples include Dy-W, [13] Pr-V, [7] and the recently reported La-Re [8] À3 -10 À2 S cm À1 at 300-400 8C, approaching the Cu-doped layered Bi 2 VO 5.5 (BICUVOX), which itself has the disadvantage of two-dimensional, anisotropic conductivity. Although the relative chemical instability of Bi oxides under reducing conditions has so far hampered their applications in SOFCs, the use of bilayer electrolytes can overcome this issue. [14] In addition to high oxide ion conductivity, bismuthbased oxides show electrocatalytic activity and therefore also have great potential for applications in electrochemical oxygen separation. [15,16] A common structural feature in the best d-Bi 2 O 3 -based oxide ion conductors reported so far is that doping stabilizes simple cubic structures with a % 5.5 and space group Fm " 3m. [7][8]13] By comparison, doped d-Bi 2 O 3 material...

The mechanism of oxide ion conductivity in bismuth rhenium oxide, Bi28Re2O49

“…The key conclusion is that the ability of Re to support four-, five-and six-fold coordination environments is important in this material, in that it provides a mechanism for 'self-doping' of This work extends the range of oxide ion conductors in which the variable cation coordination environments and the O 2-exchange between the highly polarisable Bi-O sublattice and a polyhedral metal-O sublattice facilitate the oxide ion mobility. The most notable example of this type of behaviour is the remarkable low-temperature oxide ion conductivity in the Bi 1-x V x O 1.5+x materials (x = 0.087, 0.095; σ~3.9 ×10 -2 S/cm at 500 o C) [32] where an additional favourable structural feature is the pseudo-cubic ordered superstructure. conductivity based on these concepts and mechanisms.…”

Cooperative mechanisms of oxygen vacancy stabilization and migration in the isolated tetrahedral anion Scheelite structure

“…Apart from tetrahedra-based structures, the advantage of tetrahedral units on transporting oxide anions has also been demonstrated in the traditional fluorite-based superstructures, for example, Bi 1− x V x O 1.5+ x system containing isolated VO 4 tetrahedra diluted in the fluorite lattice, where V 5+ cations have variable coordination numbers (from 4 to 6) 16,17 . The x  = 0.087–0.095 composition range demonstrates high oxide ion conductivity (up to 0.04 S cm −1 at 500 °C) and adopts a pseudo-cubic 3 × 3 × 3 fluorite superstructure with VO 4 tetrahedra apart from each other by ~6.7 Å without interaction between them 16 .…”

“…The x  = 0.087–0.095 composition range demonstrates high oxide ion conductivity (up to 0.04 S cm −1 at 500 °C) and adopts a pseudo-cubic 3 × 3 × 3 fluorite superstructure with VO 4 tetrahedra apart from each other by ~6.7 Å without interaction between them 16 . In this superstructure, ab initio molecular dynamic (MD) simulations demonstrated that the isolated VO 4 tetrahedra participate to transport oxide anions, which migrate by hopping among the vacancies in Bi-O fluorite slabs and are transported by the diluted VO 4 tetrahedra through a process of continuous formation, rotation, and breaking up of highly coordinated VO 4+ n polyhedra 16 . Increase of the V 5+ content in Bi 1− x V x O 1.5+ x leads to closer VO 4 tetrahedra, which constrains their rotation and thus reduces the oxide ion mobility, as revealed in Bi 46 V 8 O 89 ( x  = 0.148) 17 .…”

Localization of Oxygen Interstitials in CeSrGa3O7+δ Melilite

“…1,2 Pure oxide ion conductors with negligible electronic conduction can be used as electrolytes in SOFCs, a promising technology for clean energy conversion with high efficiency and fuel flexibility. 3 The mixed oxide ionic and electronic conductors have potential applications as cathodes in SOFCs, dense membranes for oxygen permeation, and syn-gas production from natural gas.…”