Elsevier

Solid State Ionics

Volume 138, Issues 1–2, 1 December 2000, Pages 91-98
Solid State Ionics

Protonic conduction in Zr-substituted BaCeO3

https://doi.org/10.1016/S0167-2738(00)00777-3Get rights and content

Abstract

Zr-substituted BaCeO3 (BaCe0.9−xZrxY0.1O3−α) was synthesized and its electrical conduction behavior and chemical stability were investigated. Single phases were confirmed over the whole range of x values (0.0≤x≤0.9). These oxides exhibited pure protonic conduction in hydrogen-containing atmosphere while they showed protonic, oxide ionic and electronic mixed conduction under high oxygen partial pressure at elevated temperatures. The chemical stability against CO2 increased with an increase in zirconium content, although the protonic conductivity decreased with increasing x.

Introduction

It is well-known that some perovskite-type oxides based on BaCeO3, SrCeO3 and CaZrO3 exhibit good protonic conduction under hydrogen-containing atmosphere at elevated temperatures [1], [2], [3]. High temperature-type proton conducting solids have a great potential for applications, e.g. electrolytes for fuel cell, hydrogen separation, hydrogen sensor, etc. [4]. Properties needed for the electrolyte change depend on the way of use. Only high chemical and mechanical stability is required for sensor electrolytes. Although CaZrO3-based protonic conductors exhibit low conductivity, the chemical and mechanical strengths are very good. Therefore, a hydrogen sensor for molten metals, using it as a solid electrolyte, has been developed and now it is widely used in process control in the metal melting industry [5], [6]. On the other hand, fuel cell and hydrogen pumps need very high conductivity. BaCeO3-based protonic conductors show the highest conductivity among this class of conductors. However, since BaCeO3 is not stable in CO2-containing atmosphere below 800°C [7], [8], [9], [10], [11], it is not suitable for practical use. Recently, in order to solve this problem, many investigators have paid attention to the chemical stability of protonic conductors [12], [13], [14], [15], [16], [17]. In general, cerate-based protonic conductors have a high conductivity but rather poor chemical stability. In contrast, zirconate-based protonic conductors are stable but have rather low conductivity. These results suggest that solid solutions between cerate and zirconate may have both high protonic conductivity and good chemical stability. From this point of view, Wienströer and Ryu investigated the stability and conductivity of Zr-substituted, Gd- or Nd-doped barium cerates [14], [16]. Matzke and Cappadonia reported conductivity of Zr-substituted, Yb-doped strontium cerates [13]. However, these reports did not describe how to change ionic conduction by substituting Zr partially for Ce, and did not confirm the protonic conduction under hydrogen-containing atmosphere directly, which should be main interest in these materials.

In this paper, in order to clarify protonic conduction properties in Zr-substituted, Y-doped barium cerate, we investigated the conduction properties in this oxide using various electrochemical methods at elevated temperatures. In addition, chemical stability of the Zr-substituted specimens against CO2 at elevated temperature were also investigated.

Section snippets

Experimental

The specimen oxides of BaCe0.9−xZrxY0.1O3−α (0.0≤x≤0.9) were synthesized by a solid-state reaction method using BaCO3, CeO2, ZrO2 and Y2O3 (99.9% purity) as starting materials. The powders of the required proportion were mixed in a ball mill with ethanol for 10 h. The dried powders were calcined at 1400°C for 10 h in air. The calcined powders were ground in a ball mill with benzene for 20 h and then passed through a sieve with 200 mesh. The powder was pressed into cylindrical pellets by

Powder X-ray diffraction patterns

Fig. 1 shows the XRD patterns of BaCe0.9−xZrxY0.1O3−α after sintering. All specimens showed single phase. Symmetry of lattice was orthorhombic at 0≤x≤0.2 and cubic at 0.3≤x≤0.9. The lattice parameters and unit cell volume determined from XRD analysis are shown in Table 1. Increasing Zr concentration leads to a decrease of the lattice parameters. This decrease of parameters reflects the substitution of small Zr4+ for Ce4+-site. The ionic radii of six coordinated Ce4+ and Zr4+ are 0.87 and 0.72

Conclusion

Zr-substituted BaCeO3 (BaCe0.9−xZrxY0.1O3−α) shows single phases over the whole range of 0.0≤x≤0.9. These oxides exhibited pure protonic conduction under hydrogen atmosphere at high temperature while they showed protonic-, oxide ionic- and electronic-mixed conduction under high oxygen partial pressure. The chemical stability against CO2 increased with an increase in zirconium content, although the protonic conductivity decreased with increasing x. The electronic conductivities of these oxides

References (18)

  • H Iwahara et al.

    Solid State Ionics

    (1981)
  • T Yajima et al.

    Solid State Ionics

    (1991)
  • H Iwahara

    Solid State Ionics

    (1995)
  • T Yajima et al.

    Solid State Ionics

    (1995)
  • N Fukatsu et al.

    Solid State Ionics

    (1998)
  • M.J Scholten et al.

    Solid State Ionics

    (1993)
  • N Bonanos et al.

    Solid State Ionics

    (1995)
  • T Yajima et al.

    Solid State Ionics

    (1992)
  • T Matzke et al.

    Solid State Ionics

    (1996)
There are more references available in the full text version of this article.

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