Proton trapping in yttrium-doped barium zirconate

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Abstract

The environmental benefits of fuel cells have been increasingly appreciated in recent years. Among candidate electrolytes for solid-oxide fuel cells, yttrium-doped barium zirconate has garnered attention because of its high proton conductivity, particularly in the intermediate-temperature region targeted for cost-effective solid-oxide fuel cell operation, and its excellent chemical stability. However, fundamental questions surrounding the defect chemistry and macroscopic proton transport mechanism of this material remain, especially in regard to the possible role of proton trapping. Here we show, through a combined thermogravimetric and a.c. impedance study, that macroscopic proton transport in yttrium-doped barium zirconate is limited by proton–dopant association (proton trapping). Protons must overcome the association energy, 29 kJ mol−1, as well as the general activation energy, 16 kJ mol−1, to achieve long-range transport. Proton nuclear magnetic resonance studies show the presence of two types of proton environment above room temperature, reflecting differences in proton–dopant configurations. This insight motivates efforts to identify suitable alternative dopants with reduced association energies as a route to higher conductivities.

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Figure 1: Proton trapping in BYZ20.
Figure 2: Comparison of trapped and trap-free proton concentrations obtained from high-temperature solid-state 1H magic-angle-spinning NMR measurements and from the proton–dopant association model.
Figure 3: Proton motion in yttrium-doped barium zirconate in the presence of trapping effects.
Figure 4: Electrolyte thickness and temperature required for an area-specific resistance of 0.15 Ω cm2, an accepted target value for efficient fuel cells27.

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Acknowledgements

This work was supported by the Japan Science Technology Agency, PRESTO and the Gordon and Betty Moore Foundation. Y.O. thanks the JSPS Institutional Program for Young Researcher Overseas Visits. F.B., L.B. and C.P.G. acknowledge financial support from the NSF under grant DMR0804737; F.B. also thanks the EU Marie Curie actions for an International Incoming fellowship 2011–2013 (grant no. 275212) and Clare Hall, University of Cambridge, UK for a Research fellowship. We thank L. Sperrin and B. Y. Zhu for fruitful discussions.

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Y.Y. designed the experiments, Y.Y. and J.C.L-V. synthesized the samples, Y.O. and Y.Y. performed and analysed the electrochemical and thermogravimetric measurements, Y.Y. and S.M.H. derived the diffusion equations for proton trapping, L.B and Y.Y. performed the NMR measurements, F.B., Y.Y., L.B. and C.P.G. analysed the NMR results, Y.Y., F.B., Y.O., L.B., C.P.G. and S.M.H. discussed the results, and Y.Y., F.B., C.P.G. and S.M.H. co-wrote the manuscript.

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Correspondence to Yoshihiro Yamazaki.

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The authors declare no competing financial interests.

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Yamazaki, Y., Blanc, F., Okuyama, Y. et al. Proton trapping in yttrium-doped barium zirconate. Nature Mater 12, 647–651 (2013). https://doi.org/10.1038/nmat3638

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