Ceria electrolytes ionic conductivity

It is considered that the bulk area specific resistance i o must be lower than l o = k/<7 = 0.15 Qcm, where L is the electrolyte thickness and a is its total conductivity, predominantly ionic [39]. At present, fabrication technology allows the preparation of reliable supported structures with film thicknesses in the range 10-15 pm consequently, the electrolyte ionic conductivity must be higher than 10 Scm. As shown in Figure 12.9, a few electrolytes (ceria-based oxides, stabihzed zirconias, and doped gallates) exceed this minimum ionic conductivity above 500 °C. 

Owing to the activation energy, at intermediate temperatures the electrolytic domain increases and at e.g., 500°C the electronic conductivity plays only a minor role. In view of the ionic conductivity being still high enough at these temperatures, ceria appears to be an appropriate electrolyte for intermediate temperature fuel cells. Further information on the properties of ceria and its use in SOFCs can be found in Ref.106,120... 

Whereas the ionic conductivity is always much lower than the electronic conductivity in pure reduced ceria, the situation is quite different in ceria doped with oxides of two- or three-valent metals due to the introduction of oxide ion vacancies, cf eqs. 15,2 and 15.3. A high vacancy concentration will shift eq. 15.1 to the left. This means that the ionic domain extended down to 10 atm or even lower in the temperature range of 600 - 1000 0. The electronic conductivity in air may be very low, and the doped cerias are under these conditions excellent electrolytes. The conductivity mechanism is the hopping of oxide ions to vacant sites, and the ionic conductivity, a may be expressed as... 

Although the emphasis here will, by necessity, be placed on more recent data, several key reviews of transport in nanocrystalline ionic materials have been presented, the details of which will be outlined first. An international workshop on interfacially controlled functional materials was conducted in 2000, the proceedings of which were published in the journal Solid State Ionics (Volume 131), focusing on the topic of atomic transport. In this issue, Maier [29] considered point defect thermodynamics and particle size, and Tuller [239] critically reviewed the available transport data for three oxides, namely cubic zirconia, ceria, and titania. Subsequently, in 2003, Heitjans and Indris [210] reviewed the diffusion and ionic conductivity data in nanoionics, and included some useful tabulations of data. A review of nanocrystalline ceria and zirconia electrolytes was recently published [240], as have extensive reviews of the mechanical behavior (hardness and plasticity) of both metals and ceramics [13, 234]. 

Figure 9.4 Total conductivity of stabilized zirconia (a) and doped ceria (b) solid electrolytes [34—40], compared to the oxygen ionic conductivity of pyrochlore-type Cd2Zr2O7 5 [41], (Cd,Ca)2Ti2O7 5 [42], (Cd,Ca)2Sn2O7 s [43], and 241707 a [44].

To reduce the resistance of the YSZ electrolyte, electrode-supported in particular anode-supported planar cells with thin film electrolyte of 5-20 pm are often adopted. To operate a SOFC at lower temperatures with high power output, an alternative electrolyte material with high ionic conductivity at low temperatures (500-650°C) should be used. Rare earth oxide-doped ceria (RDC, R is usually Y2O3, Gd203, and Sm203) is a material of choice due to its superior ionic conductivity, especially in low temperature range of 500-650°C [47]. 

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