Low-temperature solid-oxide fuel cells (SOFCs) have attracted much attention worldwide because of their potential long-term stability and economical competitiveness for many (including residential and automotive) applications. [1][2][3][4][5][6][7][8][9][10][11] To date, the best available electrolyte material seems to be doped ceria, which, potentially, meets most of the requirements for fuel-cell operation below 600°C. Several critical issues still remain, however, including electronic conductivity and uncertain mechanical integrity under fuel-cell operating conditions. The discovery of a new solid electrolyte for low-temperature SOFCs is a grand challenge for the SOFC community. Proton conductors are promising candidates as electrolytes for low-temperature SOFCs because of their low activation energy for proton conduction. Many perovskite-type oxides show high proton conductivity in a reducing atmosphere. [12,13] One of the major challenges for this type of proton conductor is a proper compromise between conductivity and chemical stability. For example, doped BaCeO 3 has sufficiently high ionic conductivity, but the chemical stability in an atmosphere containing CO 2 and H 2 O is inadequate for fuel-cell applications. [14,15] Because BaCeO 3 and BaZrO 3 easily form solid solutions, it is possible to replace a desired fraction of Ce in BaCeO 3 with Zr to form a solid solution that exhibits both adequate proton conductivity as well as sufficient chemical and thermal stability over a wide range of conditions relevant to fuelcell operation. [16][17][18] Here, we report a new composition,, in the barium-zirconiumcerium-yttrium (BZCY) family that, at temperatures below 550°C, displays the highest ionic conductivity of all known electrolyte materials for SOFC applications. The performance characteristics of a single cell based on BZCY7 are very promising. In order to investigate the chemical stability of BZCY7 and Ba(Ce 0.8 Y 0.2 )O 3 (BCY20) in a CO 2 -containing atmosphere, powder samples of both compositions were exposed to 2 % CO 2 (balanced with H 2 ) at 500°C for one week. The X-ray diffraction (XRD) patterns shown in Figure 1 suggest that BCY20 decomposed to BaCO 3 , CeO 2 , and Y 2 O 3 during the exposure whereas BZCY7 remained unchanged, implying that BZCY7 is stable at 500°C in an atmosphere containing 2 % CO 2 . Similarly, XRD examination of BZCY7 powder samples before and after exposure to H 2 containing 15 % H 2 O for one week, also shown in Figure 1, indicate that the structure of BZCY7 remained unchanged, implying that BZCY7 is stable at 500°C in an atmosphere containing 15 % water vapor. The partial substitution of Ce by Zr indeed increased the chemical stability of the material in an atmosphere containing CO 2 and H 2 O. Figure 2 shows the ionic conductivity of BZCY7 in a humid 4 % H 2 /Ar atmosphere, together with the conductivities of three conventional SOFC electrolyte materials: yttria-stabilized ZrO 2 (YSZ), La 0.8 Sr 0.2 Ga 0.8 Mg 0.2 O 3 (LSGM), and Ce 0.8 Gd 0.2 O 3 (GDC). The conductivity of...
