High oxide ion and proton conductivity in a disordered hexagonal perovskite


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 that 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.

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Fig. 1: Oxide ion conductivity of Ba7Nb4MoO20.
Fig. 2: Proton conductivity in Ba7Nb4MoO20 under humidified conditions (pH2O 0.021 atm).
Fig. 3: Comparison of the bulk conductivity of Ba7Nb4MoO20 with other leading ionic conductors.
Fig. 4: The crystal structure of Ba7Nb4MoO20 and thermal rearrangement of the oxygen fractional occupancies.
Fig. 5: Hydration of Ba7Nb4MoO20.

Data availability

The data that support the findings of this study are available from the authors on reasonable request.


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This research was supported by the Leverhulme trust and EPSRC (MISE). We also acknowledge STFC-GB for provision of beamtime at the Institut Laue Langevin.

Author information

A.C.M. and S.F. designed the study. S.F. performed the synthesis and electrical characterization of Ba7Nb4MoO20 and analysed the data along with K.S.M. and E.J.W. The transport and pO2 measurements were performed at the University of St. Andrews by S.F., P.A.C. and C.S. with guidance from J.T.S.I. The structural characterization was performed by S.F. and K.S.M. with guidance from A.C.M, E.J.W., J.M.S.S. and C.R. S.F. performed the thermogravimetric measurements and analysed the results. A.C.M. and S.F. wrote the manuscript with E.J.W. A.C.M. directed the project.

Correspondence to Sacha Fop or Abbie C. Mclaughlin.

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Supplementary Figs. 1–31, Tables 1–3, discussion and references.

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Fop, S., McCombie, K.S., Wildman, E.J. et al. High oxide ion and proton conductivity in a disordered hexagonal perovskite. Nat. Mater. (2020). https://doi.org/10.1038/s41563-020-0629-4

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