Zirconia based electrolytes doping

Oxides exhibiting only high ion conductivity are mainly fluorite-related structures based on zirconia or ceria. Zirconia-based electrolytes are currently used in solid oxide fuel cells (SOFCs). The MIEC oxides are more attractive for separative membrane applications, and these oxides mainly belong to the following types fluorite-related oxides doped to improve their electron conduction, - ... 

The most commonly used electrolyte materials in SOFCs are based on zirconia and ceria doped with a suitable cation, normally a rare earth (see Chapter 9). The properties that make these two materials attractive for use in fuel cells are discussed in Section 4.4.4, and it is sufficient to note that the most important feature is that they are good oxygen ion conductors. We will focus here on some recent investigations of these materials, with emphasis placed on their methods of preparation. 

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. 

The maximum value of conductivity for ceria-based electrolytes is attained at a certain concentration of the dopant oxides, in a matmer similar to the zirconia electrolytes. As shown in fig. 7 (Takahashi and Iwahara 1966), the conductivity of Ce02 doped with... 

Typical electrolyte materials for SOFCs are oxides with low valence element substitutions, sometimes named acceptor dopants [13, 95] which create oxygen vacancies through charge compensation. For SOFC applications, there are various materials that have been explored as electrolyte, yttria-doped zirconia (YSZ) and gadolinium-doped ceria (GDC) are the most common materials used for the oxideconducting electrolyte. Above 800 °C, YSZ becomes a conductor of oxygen ions (02-) zirconia-based SOFC operates between 800 and 1100 °C. The ionic conductivity of YSZ is 0.02 S m at 800 °C and 0.1 S cm at 1000 °C. A thin electrolyte (25-50 (im) ensures that the contribution of electrolyte to the ohmic loss in the SOFC is kept to a minimum. 

Doped ceria has been suggested as an alternative electrolyte for low temperature SOFCs [6, 31, 32]. Reviews on the electrical conductivity and conduction mechanism in ceria-based electrolytes have been presented by Mogensen et al. [33] and Steele [34], Ceria possesses the same fluorite structure as the stabilised zirconia. Mobile oxygen vacancies are introduced by substituting Ce " with trivalent rare earth ions as shown in Eq. (1). The conductivity of doped ceria systems depends on the kind of dopant and its concentration. A typical dopant concentration dependence of the electrical conductivity in the (Ce02)i -x(Sm203)x system as reported by Yahiro etal. [3 5] is shown in Figure 4.9. 

During the past decades, many oxide formulations have been extensively examined in the search for candidate SOFC electrolyte materials. Zirconia-based compositions are still the best electrolytes at present owing to their good stability under reducing atmospheres, low electronic conductivity, and acceptable oxide ion conductivity above 800°C. The recent trend of SOFC development is to operate at lower temperatures. The lowest operation temperature limit of the cell, for thin YSZ electrolytes, is estimated to be about 700°C from YSZ conductivity and mechanical property data. Scandia-doped zirconia, which shows a higher conductivity than that of YSZ, could be preferred at temperatures below 700°C, if the cost of scandia was acceptable. 

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