Page 106 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

Chapter 4

Electrolytes

Tatsumi Ishihara, Nigel M. Sammes and Osamu Yamamoto

4.1 Introduction
The electrolyte for solid oxide fuel cells (SOFCs) must be stable in both reducing
and oxidising environments, and must have sufficiently high ionic with low
electronic conductivity at the cell operating temperature. In addition, the
material must be able to be formed into a thin, strong film with no gas leaks. Until
now, stabilised zirconia, especially yttria-stabilised zirconia, possessing the
fluorite structure, has been the most favoured electrolyte for SOFCs. Other fluorite
structured oxide ion conductors, such as doped ceria, have also been proposed as
the electrolyte materials for SOFCs, especially for reduced temperature operation
(600-800°C). More recently, a number of other materials, including perovskites,
brownmillerites and hexagonal structured oxides, have also been found to
possess good ionic conductivity. This chapter first describes fundamental and
practical aspects of fluorite structured electrolytes, and then it proceeds to
discuss the structure and properties of perovskites and other ion conductors.

4.2 Fluorite-Structured Electrolytes

Oxide ion conductivity was first observed in Zr02 containing 15 wt% Y203
(yttria-stabilised zirconia or YSZ) by Nernst [l] in the 1890s. In 193 7, Baur and
Preis [21 constructed the first solid oxide fuel cell using this electrolyte. Since that
time, many oxide systems have been examined as potential electrolytes for
SOFCs. An excellent review of solid oxide electrolytes was presented by Etsell and
Flengas in 1970 [3], while more recent conductivity data are summarised by
Minh and Taltahashi [4]. Figure 4.1 shows the temperature dependence of the
ionic conductivity for several oxides, indicating that YSZ is by no means the best
oxide ion conductor.
Bismuth oxide compositions [ 51 show the highest conductivity and several
other formulatioils are also superior to YSZ, particularly at temperatures below