Ceria-Based Oxide Ion Conductors

In Table 4.2, the conductivity data for doped ceria are summarised. Ce02-Gd203 and Ce02-Sm203 show an ionic conductivity as high as 5 x 10 S/cm at 500°C, corresponding to 0.2 Q cm ohmic loss for an electrolyte of 10 pm thickness. These compositions are attractive for low temperature SOFCs and have been extensively examined. 

Ceria-based oxide ion conductors are reported to have purely ionic conductivity at high oxygen partial pressures. At lower oxygen partial pressures, as prevalent on the anode side of an SOFC, these materials become partially reduced. This leads to electronic conductivity in a large volume fraction of the electrolyte extending from the anode side. When a cell is constructed with such 

5 Fabrication of Zr02 and Ce02-Based Electrolyte Films 

Under the operating conditions of the EVD process, the oxide exhibits both oxide ion and electronic conductivity. Thus, the oxide ion flux during the oxide growth is balanced by an electron flux, thereby preserving the electroneutrality of the oxide. The growth rate of the oxide is commonly described by the classical 

Conventional screen printing techniques have also been used to prepare thin electrolyte films in electrode-supported designs [45]. Cells with electrolyte thicknesses between 3 and 30 pm have been fabricated. The total cell resistance with a 4 pm thick electrolyte was 0.105 cm at 700°C which corresponds to a 10 S/cm conductivity value of YSZ at 700°C. In Table 4.3, the preparation methods for SOFC electrolytes used by several organisations are summarised. 

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A key factor in the possible applications of oxide ion conductors is that, for use as an electrolyte, their electronic transport number should be as low as possible. While the stabilised zirconias have an oxide ion transport number of unity in a wide range of atmospheres and oxygen partial pressures, the BijOj-based materials are easily reduced at low oxygen partial pressures. This leads to the generation of electrons, from the reaction 20 Oj + 4e, and hence to a significant electronic transport number. Thus, although BijOj-based materials are the best oxide ion conductors, they cannot be used as the solid electrolyte in, for example, fuel cell or sensor applications. Similar, but less marked, effects occur with ceria-based materials, due to the tendency of Ce ions to become reduced to Ce +. 

The activation energy for oxide ion conduction in the various zirconia-, thoria- and ceria-based materials is usually at least 0.8 eV. A significant fraction of this is due to the association of oxide vacancies and aliovalent dopants (ion trapping effects). Calculations have shown that the association enthalpy can be reduced and hence the conductivity optimised, when the ionic radius of the aliovalent substituting ion matches that of the host ion. A good example of this effect is seen in Gd-doped ceria in which Gd is the optimum size to substitute for Ce these materials are amongst the best oxide ion conductors. Fig. 2.11. 

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. 

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