Development of Melilite‐Type Oxide Ion Conductors

Abstract

Lowering the operating temperature of solid oxide fuel cells (SOFCs) requires high performance oxide ion conductor electrolytes. Recently tetrahedra-based structures have been attracting considerable attention for oxide ion conductor development, among which the layered tetrahedral network melilite structure appears particularly interesting owing to its remarkable capability to accommodate and transport interstitial oxide ions, compared with isolated tetrahedral anion structures. Stabilization and migration mechanisms of interstitial oxide ions in melilites have been systematically investigated using local structural relaxation from both electrostatic Coulomb interaction and chemical bonding aspects based on atomic and electronic structures respectively using experimental and theoretical approaches. These reveal cationic size and chemical bonding effects on stabilization and migration mechanisms of interstitial oxide ions. Lately, full crystallization from glass, an innovative synthesis method, was employed to produce new metastable melilite oxide ion conductors which are inaccessible using classic solid state reaction owing to cationic size effect. Finally, the thermal and chemical stability at low temperature and the high oxide ion conductivity of the best melilite oxide ion conductors based on LaSrGa 3 O 7 are likely to provide real possibilities of applications of melilite-type electrolytes in SOFCs and other related devices.

Rare earth elements based oxide ion conductors

“…On the other hand, trivalent RE cations contribute toward the stabilization of the oxygen interstitials by forming ionic bonding with the excess oxide ions, 132 which requires large RE cations. More information on the development of melilite-type oxide ion conductors can be found in a review recently published by Zhou et al 133…”

Experimental and Theoretical Solid-State 29Si NMR Studies on Defect Structures in La9.33+x(SiO4)6O2+1.5x Apatite Oxide Ion Conductors

“…Understanding the mechanisms on the oxide anionic defect stabilization and oxide ion mobility is imperative in order to improve the oxide ion conductivity and design new oxide ion conductors with better performance. Recently, the materials based on tetrahedral units, for example, apatites, melilites, , and scheelites, , have attracted more and more attention on developing oxide ion conducting electrolytes owing to their high ionic conductivity. ,, The deformation and rotation flexibility of tetrahedral units facilitate the stabilization and transportation of oxygen interstitial or vacancy defects even within the low symmetric structures, ,, in contrast with the traditional fluorite and perovskite-type electrolytes. , …”

La2Ga3O7.5: A Metastable Ternary Melilite with a Super-Excess of Interstitial Oxide Ions Synthesized by Direct Crystallization of the Melt

“…In La 1+x AE 1−x Ga 3 O 7+x/2 (AE = Ca 2+ , Sr 2+ , or Ba 2+ ), interstitial oxide ions (O int ) are incorporated into the framework close to the centers of the pentagonal rings by tuning the La 3+ /AE 2+ ratio (x). 13 This creates GaO 5 trigonal bipyramidal units locally by coordination of O int to gallium ions within the tetrahedral layer, which is stabilized by a substantial bonding contribution from the two adjacent La 3+ /Sr 2+ cations. 14,15 At elevated temperatures, these interstitial oxide ions act as charge carriers, primarily by diffusion within the Ga−O framework layer.…”

“…Its structure (general formula: A 2 B 3 O 7 ) consists of a two-dimensional framework of corner-sharing 3- and 4-connected BO 4 tetrahedra forming distorted pentagonal channels perpendicular to the stacking axis, which alternates with a layer of A cations in an 8-fold coordination by oxide ions. In La 1+ x AE 1– x Ga 3 O 7+ x /2 ( AE = Ca 2+ , Sr 2+ , or Ba 2+ ), interstitial oxide ions (O int ) are incorporated into the framework close to the centers of the pentagonal rings by tuning the La 3+ / AE 2+ ratio ( x ) . This creates GaO 5 trigonal bipyramidal units locally by coordination of O int to gallium ions within the tetrahedral layer, which is stabilized by a substantial bonding contribution from the two adjacent La 3+ /Sr 2+ cations. , At elevated temperatures, these interstitial oxide ions act as charge carriers, primarily by diffusion within the Ga–O framework layer. , Consequently, the ionic conductivity (σ) depends directly on the concentration of interstitial oxide ( x ) in the framework.…”