Doped ceria ionic conductivity

In the search of high-performance SOFC anode, doped ceria have been evaluated as possible anode materials [9,10]. Comparing Ni-samaria-doped ceria (SDC) with Ni-YSZ, the Ni-SDC anode exhibits higher open-circuit voltages and a lower degree of polarization with either methanol as the fuel, as shown in Fig. 5, or methane as the fuel, as shown in Fig. 6. It was found that the depolarization ability of the anode is associated with the catalytic activity, the electrical conductivity, and the oxygen ionic conductivity of the anode materials [9]. It was also found that the anodic polarization and electro-catalytic activity strongly depend on the Ni content in the anode, and the optimum result for the Ni-SDC anode is achieved with 60... 

FIGURE 1.14 Ionic conductivity of doped ceria at 1073 K against the radius of dopant cation, rc shown in the horizontal axis is for the critical radius of divalent or trivalent cation, respectively. (Data from Inaba, H. and Tagawa, H., Solid State Ionics, 83, 1, 1996.)... 

It has been noted that the conductivity and activation energy can be correlated with the ionic radius of the dopant ions, with a minimum in activation energy occurring for those dopants whose radius most closely matches that of Ce4+. Kilner et al. [83] suggested that it would be more appropriate to evaluate the relative ion mismatch of dopant and host by comparing the cubic lattice parameter of the relevant rare-earth oxide. Kim [84] extended this approach by a systematic analysis of the effect of dopant ionic radius upon the relevant host lattice and gave the following empirical relation between the lattice constant of doped-ceria solid solutions and the ionic radius of the dopants. 

Different from the critical radius, Mori et al. [89] proposed a concept of effective crystallographic index to maximize the oxide ionic conductivity of doped ceria. The index, I, is defined as... 

TABLE 1.3 Ionic Conductivity Data for Selected Gadolinia-Doped Ceria (GDC)... 

Pure and doped ceria are mixed ionic and electronic conductors under reducing atmospheres, due to considerable p-type electronic conductivity under reducing conditions. The electronic conduction is due to the reduction of Cc4 to Ce3+ under... 

Ni-cermet type anodes have been improved by substituting the YSZ by ceria, gadolinium-doped ceria (GDC) and samarium-doped ceria (SDC). Ceria seems to increase the catalytic activity of the cermet for hydrogen oxidation, while SDC and GDC improve the ionic conductivity of the anode. Ni-ceria cermets are considered the main candidate for low-temperature SOFC [74],... 

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. 

One of the exceptions was the discovery of high ionic conductivity in appropriately doped FaGa03.128 129 As in the other oxide ion conductors, its ionic conductivity depends on both the dopant level as well as on the nature of the dopant. A major difference to ceria and zirconia is the presence of two cations that can be substituted the detailed defect chemistry of such solid solutions is far from being fully understood. Co-doping of Sr on A sites and Mg on B-sites leads to an ionic conductivity of ca. 0.12—0.17 S cm 1 at 800°C,130-133 which is similar to doped ceria but considerably exceeds the value of YSZ (ca. 0.03 S cm 1 at 800°C80 81). The activation energy also varies with composition and can be as low as ca. 0.6 eV.130 131 At about 600-700°C, the... 

Related systems It should be noted that specific properties for applications could be enhanced by using solid solutions, doped materials, and composites, instead of pure ceria. For example, ceria-zirconia solid solution is a well known ceria based material for enhanced OSC and high ionic conductivity for solid state fuel cell components. It is also used in the three way catalysts for automobile waste gas cleaning, because of the improved thermal stability, surface area, and reducibility. The synthesis, structure, and properties of ceria-zirconia have been actively studied for a long time. Di Monte and Kaspar et al. presented feature articles on the nanostructured ceria—zirconia-mixed oxides. The studies on phase, structures, as well as the microstructures are discussed and reviewed (Di Monte et al., 2004). 

S Atomistic simulation assisted synthesis and investigations The classical atomistic simulation techniques based on the pair potentials are suitable for the simulations of ceria nanoparticles even with a real sized model. Molecular d)mamics studies with several thousands of ions and up to hundreds of nanoseconds in a time scale have been carried out to interpret the diffusion, and crystal growth behaviors for pure and doped-ceria nanoparticles. Traditionally, the technique has been used to explore the oxygen ionic conductivity in ionic conductors such as ceria and zirconia (Maicaneanu et al., 2001 Sayle et al., 2006). 

Zhou, X.D., Scarfino, B., and Anderson, H.U., Electrical conductivity and stability of Gd-doped ceria/Y-doped zirconia ceramics and thin films. Solid State Ionics, 2004, 175 19-22. 

The conductivities of some ceria-containing compounds obtained from the literature have been compiled and shown as solid lines in Fig. 2.12. The main dopants for ceria belong to the alkaline earth or rare-earth metal series and the majority of doped samples exhibit conductivity values which fall into a rather limited band (gray band in Fig. 2.12), which points to a similar behaviour for all doped ceria samples. Exceptions are pure ceria and ceria doped with redox elements like Pr and Tb which give rise to electronic contribution to conductivity. An important requirement is that a homogeneous solid solution forms between the two oxides, which maintains the fluorite structure since the presence of a second phase or phase inhomogeneity due to insufficient solubility can affect ionic conductivity. The very low values of conductivity found for BaO and MgO-doped ceria were in fact attributed to the low solubility of these oxides into the lattice of When the... 

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