Electrolyte fluorite-type materials

Yttria-stabilized zirconia f[Zrlj YJ02, /2) is known in the literature as YSZ and has a fluorite-type structure [67] (see Figure 2.16). This material has a high oxygen ion conductivity and is, therefore, applied as a high-temperature electrolyte material, for example, in high-temperature fuel cells [68,73],... 

Let us briefly consider in this context two important classes of materials. The first are the oxides of fluorite type such as Zr02 or Ce02. They can accommodate high concentrations of lower valent cations. Y203 doping of Zr0210 leads to the very important YSZ electrolyte. 

Complex FCC oxides of the fluorite type represent oxygen-conduction solid electrolytes (SOE s). They comprise a typical class of materials for the manufacture of sensors of oxygen activity in complex gas mixtures, oxygen pumps, electrolyzers and high-temperature fuel elements. These materials are based on doped oxides of cerium and thorium, zirconium and hafnium, and bismuth oxide. Materials based on zirconium oxide, for example, yttrium stabilized zirconia (YSZ) are the most known and studied among them. This fact is explained both by their processibility and a wide spectrum of practical applications and by the possibility to conduct studies on single crystals, which have the commercial name "fianites" and are used in jewelry. 

Solid electrolytes. These correspond to soHd materials in which the ionic mobility is insured by various intrinsic and extrinsic defects and are called solid ion conductors. Common examples are ion-conducting solids with rock salt or halite-type solids with a Bl structure (e.g., a-AgI), oxygen-conducting solids with a fluorite-type Cl structure (A"02), for instance CaF and yttria-stabilized zirconia (YSZ, ZrO with 8 mol.% Y O,), a pyro-chlore structure (A BjO ), perovskite-type oxides (A"B" 03), La Mo O, or solids with the spinel-type structure such as beta-aluminas (NaAl 0 ) for which the ionic conduction is ensured by Na mobility. 

Yttria-stabilized Z1O2, discovered by Nemst [39], is still one of the state-of-the-art SOFC electrolyte materials which was used to demonstrate the first SOFC (and the first solid electrolyte fuel cell) in 1937 at ETH-Ziirich [40]. Electronic defect concentrations are negligibly low [41]. As can be observed in Fig. 6.1, the ionic conductivity of fluorite-type oxides stabilized with hypovalent elements exhibits a maximum at a certain dopant concentration above which defect interactions occur [42-45]. As shown in Fig. 6.2, it is known that the peak value of the ionic... 

A SOFC was proposed by Baur and Preis as far back as 1937 based upon an electrolyte of stabilised zirconia with metallic electrodes. Since then stabilised zirconia has been the electrolyte that has received most attention by fuel cell developers. Most zirconia electrolytes are based upon either yttria or scandia stabilisation of the tetragonal poly-morph, commonly referred to as YSZ and ScSZ, respectively, although a number of alternative dopants have been investigated (Tables 2.1 and 2.2). Conventionally the substitution level is between 3mol% and 8 mol% for the yttria-based materials and at 10-12 mol% for the scan-dia-based materials. The choice of the dopant level is dictated by a compromise between mechanical robustness and overall conductivity, as summarised in Table 2.1. Substitution of zirconia results in the stabilisation of either the tetragonal or cubic polymorphs adopting the fluorite type structure as shown in Figure 2.2. This substitution... 

An alternative approach to reduce the operating temperature is to use new electrolyte materials such as scandium doped zirconia (SeSZ), and rare earth doped ceria (RDC) which have the fluorite type stmeture, or lanthanum gallate based oxides such as (La,Sr)(Ga,Mg)03 (LSGM) with perovskite stmeture, all of which have higher ionic conductivities than YSZ. An 1 kW class SOFC stack with (La,Sr)(Ga,Mg,Co)03 electrolyte was demonstrated by the collaboration of Mitsubishi Materials Corp. and The Kansai Electric Power Co., Inc. in 2001,... 

In 1994, new solid electrolytes with high conductivity at low temperatures were found in the form of doped lanthanum gallates [142]. Since the selfdiffusion coefficients of cations are apparently fundamentally larger in the perovskite-type oxides than in fluorite oxides, lanthanum gallate electrolyte and associated electrode materials tend to react too easily at the temperatures of fabrication and operation of cells. 

---