Processing of yttrium-doped barium zirconate for high proton conductivity
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Tetsuya Uda Materials Science, California Institute of Technology, Pasadena, California 91125; and Materials Science and Engineering, Kyoto University, Kyoto, Japan 606-8501
Sossina M. Hailea) Materials Science, California Institute of Technology, Pasadena, California 91125 (Received 4 May 2006; accepted 25 January 2007)
The factors governing the transport properties of yttrium-doped barium zirconate (BYZ) have been explored, with the aim of attaining reproducible proton conductivity in well-densified samples. It was found that a small initial particle size (50–100 nm) and high-temperature sintering (1600 °C) in the presence of excess barium were essential. By this procedure, BaZr0.8Y0.2O3−␦ with 93% to 99% theoretical density and total (bulk plus grain boundary) conductivity of 7.9 × 10−3 S/cm at 600 °C [as measured by alternating current (ac) impedance spectroscopy under humidified nitrogen] could be reliably prepared. Samples sintered in the absence of excess barium displayed yttria-like precipitates and a bulk conductivity that was reduced by more than 2 orders of magnitude.
I. INTRODUCTION
Doped perovskites, such as barium cerate (BaCeO3), strontium cerate (SrCeO 3 ), and barium zirconate (BaZrO3), have been widely studied in recent years as proton conducting electrolytes for a variety of electrochemical devices including fuel cells. Among the fuel cell studies, the most impressive results are arguably those of Iguchi et al.1 Using yttrium-doped barium cerate as an electrolyte, these authors demonstrated power densities of 570 mW/cm2 at 430 °C and 780 mW/cm2 at 510 °C under air/hydrogen conditions. To circumvent the detrimental reaction of barium cerate with CO2, which would otherwise result in the formation of BaCO3 and CeO2, the authors protected the thin electrolyte by depositing it onto a dense layer of palladium foil, a wellknown hydrogen separation material. In contrast to barium cerate, barium zirconate, which exhibits proton conduction by a similar mechanism,2 is known to be stable in CO2-containing atmospheres.3 Thus, fuel cells based on this electrolyte would not require elaborate solutions for ensuring cell longevity. However, the conductivity of doped barium zirconate as reported from 12 independent groups (Table I4–15) varies widely, from a low of ∼1 × 10−6 to a high of 1 × 10−2 S/cm at 600 °C, introducing major challenges for its implementation in a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0163 1322 J. Mater. Res., Vol. 22, No. 5, May 2007 http://journals.cambridge.org Downloaded: 18 Jul 2014
any real device and raising fundamental questions regarding the proton transport mechanism. The earliest studies of doped barium zirconate, which appeared in the early 1990s, suggested that this material exhibits poor proton conductivity compared with doped barium cerate. Although their reported values differ by 2 orders of magnitude, Iwahara et al.,4 Manthiram et al.,5 and Slade et al.6 all agreed that the conductivity of the zirconate is no more th
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