Ionic Transport Properties in Nanocrystalline Ce0.8A0.2O2-δ (with A = Eu, Gd, Dy, and Ho) Materials
The ionic transport properties of nanocrystalline 20 mol% Eu, Gd, Dy, and Ho doped cerias, with average grain size of around 14 nm were studied by correlating electrical, dielectric properties, and various dynamic parameters. Gd-doped nanocrystalline ceria shows higher value of conductivity (i.e., 1.8 × 10−4 S cm−1 at 550°C) and a lower value of association energy of oxygen vacancies with trivalent dopants Gd3+ (i.e., 0.1 eV), compared to others. Mainly the lattice parameters and dielectric constants (ε∞) are found to control the association energy of oxygen vacancies in these nanomaterials, which in turn resulted in the presence of grain and grain boundary conductivity in Gd- and Eu-doped cerias and only significant grain interior conductivity in Dy- and Ho-doped cerias.
KeywordsNanostructures Electrical transport Dielectric relaxations Association energy
The microcrystalline rare-earth-doped ceria-based materials are very good oxide ionic conductors at intermediate temperatures, and these materials have been considered as alternative of the yttria-stabilized zirconia as the electrolyte of the solid oxide fuel cell (SOFC) [1, 2]. Major problem associated with the ceria-based electrolytes is the reduction of Ce4+ to Ce3+ under reducing condition at temperatures above 700°C, resulting high electronic conductivity, which is detrimental to the function of electrolyte in the SOFCs . However, it has been observed that the reduction for ceria-based materials can be neglected at temperatures below 700°C . In order to use the ceria-based materials as electrolyte below 700°C, it is highly desirable to increase the ionic conductivity at lower temperatures. In the case of polycrystalline ceria-based electrolytes, the impurities such as Si and Ca segregate at grain boundaries and form thin blocking layers within the grain boundary network , which affect the grain boundary conductivity. It has been considered that, by reducing the grain size of the materials, total amount of impurities can be spread over a large interfacial area and the effect of impurities can be reduced. Hence, the grain boundary conductivity as well as overall ionic conductivity of the materials can be increased. It has been also speculated that the grain boundary may provide faster diffusion pathway for ionic defects resulting in enhanced ionic conductivity in the finely grained materials . It is therefore expected that, by reducing the grain size of the ceria-based materials into nano scale, conductivity of the materials can be increased, and the materials can be used at lower temperatures to avoid the reduction of Ce4+ to Ce3+, which leads to the electronic conductivity of the materials, so that it would be helpful in reducing the operating temperature of SOFC. Since the reduction of operating temperature of SOFC (which is expected to be the alternative energy source for future generation) has been a subject of interest world wide, recently a considerable interest has been increased on the development of nanostructured ceria-based electrolyte materials. Various synthesis and processing methods have been used to prepare the pure ceria or doped ceria nanocrystalline materials with desired properties. These include gel casting process , chemically processed method and IGC method , carbonate co-precipitation method  etc. Apart from nanoparticles, the doped or undoped cerias in the form of nanotubes, nanorods, and nanowires are also being considered for useful application in fuel cells. The morphology controlled synthesis of various nanorods, nanotubes, and nanowires of ceria-based materials have been proposed by various authors elsewhere [10, 11, 12, 13]. However, since the physical and chemical properties of these nanomaterials are synthesis process and grain size dependent, these properties are currently under debate and not yet clearly understood.
In this work, we have discussed the electrical and dielectric properties and their correlations in the nanostructured 20 mol % of Eu-, Gd-, Dy-, and Ho-doped ceria-based materials for in-depth understanding of ionic transport process in nano scale. A comparison study of various dynamic parameters has been carried out to have a clear idea of the fundamental reason behind the variation in association energy of oxygen vacancies in these nanostructured materials with dopants of different ionic radii and its effect on ionic conductivity of these nanostructured materials.
The nanocrystalline materials of 20 mol% Eu-, Gd-, Dy-, and Ho-doped cerium oxide (Ce0.8A0.2O2-δ,A = Eu, Gd, Dy, and Ho) were prepared by citrate auto ignition method using CeO2, Eu2O3, Gd2O3, Dy2O3, and Ho2O3 as starting materials, as described in our previous work on Ho-doped ceria . The precursors obtained for different materials were calcined at 800°C for 5 h to get single phase nanocrystalline materials. The phase purity and crystal structures of calcined powders were studied by X-ray diffraction and high-resolution TEM. The ionic transference numbers of the nanocrystalline material in pellet form were calculated in air atmosphere using the Wagner’s dc polarization technique as described by Baral and Sankaranarayanan , and these values are found to be around 85 at 500°C. The ac electrical and dielectric properties of materials (in pellet form) were studied using electrochemical impedance spectroscopy. The impedance measurements were carried out using two probe method in air, varying the temperature from 250 to 550°C and the frequency from 1 to 10 MHz. The silver paste was also used on both sides of pellets (sintered at 800 for 2 h each) as electrodes in the impedance measurement as well as dc polarization technique. The densities of the pellets of different materials used for both the experiments were between 93 and 95% of the theoretical density. Since the calcination temperature of powder materials and sintering temperature of the pellets were same, the average grain size in the sintered pellets are found to remain same and the nanocrystallinity in the pellets is maintained in the sintered pellets, as it was observed in our previous result .
Results and Discussion
Values of conductivity and activation energies in different materials
σgi(S cm−1) (at 550°C)
σgb(S cm−1) (at 550°C)
σ (S cm−1) (550°C)
5.8 × 10−4
7.3 × 10−4
1.8 × 10−4
3.45 × 10−4
4.4 × 10−4
1.39 × 10−4
1.36 × 10−4
1.4 × 10−4
The values of dielectric constants (ε∞) of the materials at different temperatures
Generally, the activation energy for the bound motion is considered as the migration energy of free vacancy (Vo▪▪) in the long range motion . The total activation energy (Eσ) being the sum of the association energy (Easso) and migration energy (Em), the association energies in EuDC0.2 is found to be 0.14 eV (Easso(Eu) = 0.91–0.77 eV). Similarly, in case of DyDC0.2, it is 0.24 eV (since Easso(Dy) = 1.16–0.92 eV), and in the case of HDC0.2 material, the value of association energy is found to be 0.24 eV . It is observed that in case of EuDC0.2 and DyDC0.2 materials, the migration energy (Em) of oxygen ions in the long-range motion calculated from the dielectric relaxation peaks, match well with the activation energies (EH) for hopping of charge carriers, as in case of HDC0.2 . Hence, these values also support that in these nanomaterials, the migration of oxygen ions takes place through hopping, which was predicted earlier with respect to the behaviour of frequency spectra of conductivity. However, in the case of GDC0.2, the M″ spectra do not show any peak above 300°C, except at few lower temperatures. So it was difficult to find out the migration energy of oxygen vacancies from modulus spectra. Since the value of hopping energy is same as the migration energy of oxygen vacancies, as observed in the case of Eu-, Dy- and Ho-doped materials, the migration energy of the oxygen vacancies in the material GDC0.2 can be taken as 0.97 eV. Hence, the association energy in the GDC0.2 must be 0.1 eV (i.e., Easso(Gd) = 1.07 − 0.97 eV).
(where r is the distance between the oxygen vacancy and its associated Ho3+, and εr is the dielectric constant) , must be higher. This could be the reason for the high value of association energy (i.e., 24 eV) between the oxygen vacancy and its associated Ho3+, in the nanocrystalline HDC0.2 material.
Similarly, based on values of lattice constants, the inter ionic spacing (r) in the case of material DyDC0.2 can be expected to be smaller compared to that of EuDC0.2 and GDC0.2, even though it must be larger than the value of “r” in HDC0.2. The values of dielectric constant ε∞ of the material DyDC0.2 are also found to be mush lower than that of others, at all the temperatures. This must be the reason for high value of the columbic interaction energy between the oxygen vacancy (Vo▪▪) and the trivalent dopant Dy3+, which results in a high value of association energy (0.24 eV) in the nanocrystalline DyDC0.2 material, as in case of HDC0.2. Since in the case materials HDC0.2 and DyDC0.2, the oxygen vacancies are strongly associated with trivalent dopant ions, it is expected that the local motion of oxygen vacancies around the dopants in side the grains mainly contributes to the conductivity of the materials, and hence this leads to the presence of only the grain interior conduction as shown in complex impedance plots of DyDC0.2 and HDC0.2 materials (Fig. 3c, 3d).
However, in the case of GDC0.2 and EyDC0.2, the values of dielectric constants are comparatively higher and since the lattice constants are comparatively larger, the inter ionic distances must be larger compared to that of DyDC0.2 and HDC0.2. This could be the reason of lower values of association energies (i.e., 0.1 and 0.14 eV, respectively) in the materials GDC0.2 and EuDC0.2 compared to that of DyDC0.2 and HDC0.2. Even, comparing the materials GDC0.2 and EuDC0.2, the values of dielectric constant of GDC0.2 is higher than that of EuDC0.2 as shown in Table 2 and of course the value of “r” can expected to be larger in GDC0.2 than that in the case of EuDC0.2. This clearly indicates that the columbic interaction energy must be lower in GDC0.2. Therefore, association energy in GDC0.2 (i.e., 0.1 eV) is lower compared to that in case of EuDC0.2 (i.e., 0.14 eV).
The values of association energy in the nanocrystalline GDC0.2 and EuDC0.2 materials being much smaller (than that of DyDC0.2 and HDC0.2), the oxygen vacancies must be free at higher temperatures. Generally, at higher temperatures, the concentration of free oxygen vacancy is expected to be more at grain boundary regions, which results in the significant grain boundary conduction in both the GDC0.2 and EuDC0.2 materials, as shown in their complex impedance plots (Fig. 3a, 3b). However, the migration energy (Em) of free oxygen vacancies being comparatively smaller (i.e., 0.77 eV) in EuDC0.2 than that of others, there must be an easy migration of free oxygen vacancies in the grain boundary regions at higher temperatures. This resulted in a bend in the Arrhenius plot of conductivity (shown in Fig. 4) of EuDC0.2, with a lower value of total activation energy (i.e. 0.91 eV) above 470°C.
Comparing with our previous results on 15 mol% Gd-doped ceria , the present GDC0.2 material has larger lattice constant and hence inter ionic separation between them must be larger compared to that in GDC0.15. So this should have resulted in lower association energy in GDC0.2. But much higher value of dielectric constant (ε∞) of GDC0.15 compared to that of GDC0.2 resulted in a lower value of the association energy in GDC0.15 (i.e., 0.07 eV). So, from these results it can be predicted that the values of lattice constants and dielectric constants play major roles in controlling the association energy in these nanostructured rare-earth-doped cerias.
The electrical and dielectric properties of nanocrystalline 20 mol % Gd-, Eu-, Dy-, and Ho-doped ceria have been thoroughly studied. Various dynamic parameters such as lattice constants, conductivity, hopping energy, dielectric constants, migration energies, association energy are obtained in the intermediate temperature range and compared. All the materials show good range of ionic conductivity for the potential application, in intermediate temperature region. Gd-and Eu-doped ceria exhibited the presence of both grain interior and grain boundary conductivity, whereas in the case of Dy- and Ho-doped materials, the grain interior conductivity is only observed. GDC0.2 shows high value conductivity and low value of association energy compared to others as observed in microcrystalline materials. Oxygen ions follow the hopping mechanism during conduction in all the nanostructured materials. Comparing with present and reported results, it is observed that the values of lattice constants and dielectric constants mainly control the association energy of oxygen vacancies, which in turn decides the presence of both grain and grain boundary conductivities or only the grain interior conductivity in these nanostructured rare-earth-doped cerias.
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