Proton trapping in yttrium-doped barium zirconate

Journal name:
Nature Materials
Volume:
12,
Pages:
647–651
Year published:
DOI:
doi:10.1038/nmat3638
Received
Accepted
Published online

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.

At a glance

Figures

  1. Proton trapping in BYZ20.
    Figure 1: Proton trapping in BYZ20.

    a, Long-range proton diffusivity exhibits downward curvature, as expressed in equation (3) (red line) with parameters given in Table 1. Red circles and triangles denote, respectively, the proton diffusivity and the ambipolar diffusivity between proton and deuterium determined by a.c. impedance spectroscopy (ACIS). The trap-free proton diffusivity calculated from the proton–dopant association (trapping) model (blue line) matches that determined from a neutron spin-echo (NSE) experiment (blue open circles11). b, Apparent activation energies for proton diffusion and constant activation energy for association-free proton diffusion. c, Trapped and trap-free proton concentrations computed from the proton–dopant association model, equation (2) and the experimentally measured total proton concentration (see Supplementary Information for full description of analysis methodology).

  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 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.

    Red and blue circles correspond to the proton concentration in the two distinct environments associated with higher and lower frequency resonances, respectively, after correcting the data for the Boltzmann factor. The former generally follows the concentration of trapped protons computed in the proton–dopant association (trapping) model (red line), whereas the latter follows the concentration of trap-free protons (blue line).

  3. Proton motion in yttrium-doped barium zirconate in the presence of trapping effects.
    Figure 3: Proton motion in yttrium-doped barium zirconate in the presence of trapping effects.

    a, Trapped, trap-free and apparent proton jump frequencies as a function of temperature. Blue circles represent values obtained by NMR (derived from Supplementary Fig. S11b) with an Arrhenius fit (dotted blue line). The red circles and triangles represent proton jump frequencies calculated, respectively, from the proton diffusivity and from the ambipolar diffusivity between protons and deuterium ions (Supplementary Methods)23. The solid red and blue lines represent the calculated jump frequencies of apparent protons and trap-free protons in the trapping model, respectively. b, Schematic representation of proton transport steps and their energetics.

  4. Electrolyte thickness and temperature required for an area-specific resistance of 0.15 Ω cm2, an accepted target value for efficient fuel cells.
    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.

    Red and blue lines represent values calculated with respective association energies of Eas of −20 kJ mol−1, predicted in atomistic simulation8, and −29 kJ mol−1, obtained in this work.

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Author information

Affiliations

  1. Japan Science and Technology Agency, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan

    • Yoshihiro Yamazaki
  2. Materials Science Department, California Institute of Technology, 1200 East California Boulevard, Pasadena, California 91125, USA

    • Yoshihiro Yamazaki,
    • Yuji Okuyama,
    • Juan C. Lucio-Vega &
    • Sossina M. Haile
  3. Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK

    • Frédéric Blanc &
    • Clare P. Grey
  4. Department of Chemistry, Stony Brook University, Stony Brook, New York 11794-3400, USA

    • Lucienne Buannic &
    • Clare P. Grey

Contributions

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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The authors declare no competing financial interests.

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