Evaluation of Variously Doped BaCeO 3 Ceramics as Protonic Conductors
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EVALUATION OF VARIOUSLY DOPED BaCeO, AS PROTONIC CONDUCTORS
CERAMICS
J.F. LIU AND A.S. NOWICK Henry Krumb School of Mines, Columbia University, New York,
NY 10027
ABSTRACT
Nd-doped BaCeO, is reported to be an excellent high-temperature protonic conductor, while the Gd-doped material is not. In order to better understand the role of different dopants, we have studied BaCeO, doped with 5 mol% of each Nd3+, Eu3 +, Gd3 +, Yb3 + and Ca2+. In each case samples were treated in an appropriate gas atmosphere at 9000C followed by measurement from 25 to 2000C in a "frozen-in" state. Protonic conduction was deduced from the isotope effect, i.e. comparing conductivity after treatment in D20-saturated gas with that for H20, as well as by the change between wet and dry treatments. In addition, we measured H20 uptake by observing the weight change between wet and dry treatments, and also carried out some galvanic cell measurements. It was concluded that, of the above dopants, only Nd and Ca give rise to appreciable protonic conduction. In the Nd case, the low activation energy of 0.54 eV is probably the proton hopping energy. On the other hand, Eu, Gd and Yb-doped materials appear to show the dominance of oxygenion conduction in the low temperature range and mixed oxygen-ion and electron-hole conductivity at elevated temperatures. The most likely reason for these differences seems to be centered on the ionic radii of the dopants. INTRODUCTION
The work of Iwahara and his group [1-5] has shown that acceptor doped provskite-structured oxides, notably SrCeO, and BaCeO,, can become good high-temperature protonic conductors after exposure to water vapor. They showed that Yb3 +-doped SrCeO, and Nd'+-doped BaCeO, ceramics were particularly suitable, and applied these materials to high-temperature fuel cells and sensors. Of all the materials studied, Nd3 +-doped BaCeO, gave the highest protonic conductivity, yet Gd'+-doped BaCeO, did not seem to be a protonic conductor [6]. A weakness of the measurements of Iwahara et al. is that they were all made at relatively high temperatures and at a single frequency. Thus, clear determination of the true bulk (lattice) conductivity, and the associated activation energy, was not possible. The present work is a study of the effects of various acceptor dopants on the bulk conductivity of BaCeO3 ceramics. All dopants used (Nd3+, Eu3+, Gd'+, Yb'+ and Ca2+) substitute for Ce'+ and are therefore acceptors [7), which are presumably compensated by oxygen-ion vacancies (Vs'). Then, treatment in water vapor allows for interstitial protons to enter the lattice through the reaction: H20 +
- 0 Ox0 + 2Wi
(1)
In this way protons replace oxygen vacancies as charge compensators for the acceptor dopants. In each case, samples were treated in an appropriate gas atmosphere at 9000C, while measurements were made in the range from 25 to 200 0 C so that, during the measurements, the material was in a frozen-in state with respect to any reactions with ambient gases. This method has been used previously in our labora
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