Processing of yttrium-doped barium zirconate for high proton conductivity Peter Babilo Materials Science, California Institute of Technology, Pasadena, California 91125 Tetsuya Uda Materials Science, California Institute of Technology, Pasadena, California 91125; and Materials Science and Engineering, Kyoto University, Kyoto, Japan 606-8501 Sossina M. Haile a) 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, BaZr 0.8 Y 0.2 O 3- 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 (BaCeO 3 ), strontium cerate (SrCeO 3 ), and barium zirconate (BaZrO 3 ), have been widely studied in recent years as proton conducting electrolytes for a variety of electro- chemical 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 den- sities of 570 mW/cm 2 at 430 °C and 780 mW/cm 2 at 510 °C under air/hydrogen conditions. To circumvent the detrimental reaction of barium cerate with CO 2 , which would otherwise result in the formation of BaCO 3 and CeO 2 , the authors protected the thin electrolyte by de- positing it onto a dense layer of palladium foil, a well- known 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 CO 2 -containing atmospheres. 3 Thus, fuel cells based on this electrolyte would not require elaborate so- lutions for ensuring cell longevity. However, the conduc- tivity of doped barium zirconate as reported from 12 independent groups (Table I 4–15 ) 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 any real device and raising fundamental questions re- garding 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 than 1.2 × 10 -4 S/cm at 600 °C. The understanding of the properties of barium zirconate was revised substantially in 1999. In that year Kreuer 2 re- ported the conductivity of yttrium-doped barium zir- conate (BYZ) to be ∼5 × 10 -5 S/cm at just 140 °C, and this was quickly confirmed by Bohn and Schober in 2000. 7 In a later paper, Kreuer noted the poor reproduc- ibility of conductivity measurements of barium zir- conate 11 and argued that this is connected to the diffi- culty in fabricating polycrystalline compacts of the ma- terial. In the present study, we explore the sample preparation steps that are responsible for this poor repro- ducibility and report a methodology for consistently ob- taining high-density, high-conductivity barium zirconate with transport properties comparable to the best values reported in the literature. II. EXPERIMENTAL PROCEDURE The specific composition selected in this study was BaZr 0.8 Y 0.2 O 3-x (BYZ20). To obtain reproducible sintering behavior and electrical properties, a wide range a) Address all correspondence to this author. e-mail: smhaile@caltech.edu DOI: 10.1557/JMR.2007.0163 J. Mater. Res., Vol. 22, No. 5, May 2007 © 2007 Materials Research Society 1322