High oxide ion and proton conductivity in a disordered hexagonal perovskite

Abstract

Oxide ion and proton conductors, which exhibit high conductivity at intermediate temperature, are necessary to improve the performance of ceramic fuel cells. The crystal structure plays a pivotal role in defining the ionic conduction properties and the discovery of new materials is a challenging research focus. Here we show that the undoped hexagonal perovskite Ba7Nb4MoO20 supports pure ionic conduction with high proton and oxide ion conductivity at 510 °C (the bulk conductivity is 4.0 mS cm-1) and hence is an exceptional candidate for application as a dual-ion solid electrolyte in a ceramic fuel cell which will combine the advantages of both oxide ion and proton conducting electrolytes. Ba7Nb4MoO20 also showcases excellent chemical and electrical stability. Hexagonal perovskites form an important new family of materials for obtaining novel ionic conductors with potential applications in a range of energy-related technologies.

Relationship between the Crystal Structure and Electrical Properties of Oxide Ion Conducting Ba3W1.2Nb0.8O8.6

“…The results demonstrate that above 300 C the crystal structure rearranges, resulting in an increase in the percentage of (W,Nb)O4 tetrahedra within the average structure. The same structural rearrangement has been reported for Ba3MoNbO8.5 22 and results in an enhancement of the bulk ionic conductivity. 22 Ba3W1.2Nb0.8O8.6 is ~ 13.0% and increases to ~ 41.8% at 500 °C and ~ 45.5% at 600 °C.…”

“…The same structural rearrangement has been reported for Ba3MoNbO8.5 22 and results in an enhancement of the bulk ionic conductivity. 22 Ba3W1.2Nb0.8O8.6 is ~ 13.0% and increases to ~ 41.8% at 500 °C and ~ 45.5% at 600 °C. In contrast there are ~ 51.3% tetrahedra within the average structure of Ba3MoNbO8.5 at 300…”

“…C which increases to ~ 64.5% at 500 °C and ~ 64.9% at 600 °C 22 . This shows that the rate of increase of the (W,Nb)O4 tetrahedra upon heating from 300 C to 600 C is faster in Ba3W1.2Nb0.8O8.6 than in Ba3MoNbO8.5 enhancing the bulk conductivity at 600 C (Figure 1 Figure S5).…”

“…The thermal evolution of the bond lengths and angles are comparable to those previously described for Ba3MoNbO8.5. 22 The Ba3M'1+xM''1-xO8.5+x/2 (M' = W, Mo; M'' = Nb) phases all exhibit a second order Jahn-Teller distortion (SOJT) 32 (Table S2).…”

“…21 It has been suggested that the same structural rearrangement previously reported for Ba3MoNbO8.5 occurs in these compounds, preserving the high-temperature ionic conductivity. 21,22 In the work reported here, we examine further the relationship between the ratio of (W/Nb)O4 tetrahedra to (W/Nb)O6 octahedra, temperature and conductivity. The W/Nb ratio was altered so as to adjust the oxygen stoichiometry according to Nb 5+ -> W 6+ + ½ O 2-resulting in the synthesis of Ba3W1.2Nb0.8O8.6.…”