Ceria Based Electrolytes for SOFC

 

S.P Kochhar¹* and A.P Singh²

¹Department of Applied Sciences and Humanities, Ferozepur College of Engineering and Technology, Ferozshah, 152004 India

²Punjab Technical University, Jalandhar-Kapurthala Highway Kapurthala-104041, India

*Corresponding Author E-mail: rosysbscet@yahoo.co.in

ABSTRACT:

Ceria based electrolytes are reviewed in terms of their structure and conductivity. The electrical conductivity depends on the various factors. This paper gives an extract of available data on the physical and electrochemical properties of pure and doped ceria.

 

 

 

1. INTRODUCTION:

Cerium based materials have been widely investigated in the last two decades for their applications. One problem arising from ceria-based electrolytes for SOFCs is that conventional sintering to full density needs temperatures exceeding 1300°C. As consequence, high grain growth rates and large grains were observed resulting in poor mechanical stability of the ceramic component. This characteristic is one of the major reasons which hinders the use of CeO₂ solid solution in SOFC applications despite its superior electrochemical performance

 

Although ceria based electrolytes are slightly reduced at low oxygen partial pressures and develop increasingly electronic conductivity, it has been shown that CGO based SOFC can be operated at temperatures as low as 700°C with high power output and high efficiency. Therefore, CeO₂ solid solutions are attractive electrolytes in SOFCs ^1. We have tried to review the data in terms of its applications in fuel cells mainly.

 

2. PHYSICAL PROPERTIES OF CeO₂ (Cerium (IV) oxide):

Cerium (IV) oxide, also known as ceric oxide, ceria, cerium oxide or cerium dioxide, is an oxide of the metal cerium. Pure CeO₂ is pale yellow in colour due to Ce (IV)–O(-II) charge transfer. The color of the ceria is also sensitive to the presence of other lanthanides. A presence of ~0.02% of Pr results in a buff color attributable to Ce (IV)-Pr (III) transitions.

 

With higher values of Pr (~2%) the oxide becomes a potential red coloured pigment². Grossly non-stoichiometric ceria samples are reported to be blue, related to Ce (IV)-Ce (III) transitions. In addition, as the oxide is usually produced by the calcination of a precursor salt, the observed color depends on the extent of that calcination.

 

Cerium also forms cerium (III) oxide, Ce₂O₃, but CeO₂ is the most stable phase at room temperature and under atmospheric conditions. In air or oxygen-containing environments, Ce (III) gets converted to tetravalent and Ce (IV) oxide is formed. The ceric ion is a powerful oxidizing agent and can be reduced by, oxalic acid, halogen acids, ferrous salts, and hydrogen peroxide but when it is associated with the strongly coordinating ligand, oxygen, is completely stabilized. Ceria exhibits oxygen deficiency of x in CeO_2-δ as large as 0 - 0.5.The removal of oxygen creates electrons as well as ionized oxygen vacancies, V^∙∙_o in the lattice. These materials are thus ionic and n-type semiconductors.

 

3.  DEFECT CHEMISTRY OF CERIA:

Ceria stores oxygen under oxygen excess conditions and in reducing environments, ceria based systems lose oxygen and develop electronic conductivity. If these materials are used in SOFCs they have to be protected on the anode side. Quasi-free electrons are introduced into fluorite lattice in such reducing atmosphere as a consequence the doped CeO₂ exhibits high oxide ionic conductivity in oxidizing atmosphere.

 

4.  FLUORITE STRUCTURE:

The fluorite structures are the most common and classical oxygen ion conducting oxide materials. The general formula of a fluorite oxide is AO₂, where A is a large tetravalent cation, the crystal structure consists of a simple cubic oxygen lattice with alternate body centers occupied by eight coordinated cations. The materials that readily exist in the fluorite crystal structure are uranium dioxide (UO₂), thorium dioxide (ThO₂) and ceria (CeO₂).

 

Once the molecular oxygen has been converted to oxygen ions it must migrate through the electrolyte to the fuel side of the cell and for such migration to occur, the electrolyte must possess a high ionic conductivity and no electronic conductivity. It must be fully dense to prevent short circuiting of reacting gases through it. Moreover, it should also be as thin as possible to minimize resistive losses in the cell. As with the other materials, it must be chemically, thermally, and structurally stable across a wide temperature range. In oxygen ion conductors, current flow occurs by the movement of oxide ions through the crystal lattice.

 

CeO₂ belongs to the family of fluorites. CeO₂ crystallizes in cubic face-centered crystal symmetry, with the Ce atoms in the corner of the cube and in the center of each of its faces, cerium coordinate 8 oxygen anion at the corners of a cube while oxygen is tetrahedrally surrounded by 4 cerium cations and the oxygen atoms occupy tetrahedral holes the lattice (Figure 1), this structure is open and it shows large tolerance for high levels of atomic disorder, which may be introduced either by doping, reduction or oxidation.

 

Figure 1: Fluorite structure of Ceria

 

5.  DOPED CERIA:

Doping of the fluorite oxides is usually achieved by substitution of the host cation with either a rare earth or an alkaline earth metal ion. The ionic conductivity of doped ceria is greater than that of stabilized zirconia for comparable doping conditions. This is a result of the larger ionic radius of Ce⁴⁺ (0.97 A⁰) than Zr⁴⁺ (0.84 A⁰), which results in a more open structure through which oxygen ions can easily migrate. The maximum ionic conductivity occurs at 10- 20 mol% substitutional dissolution for most of the dopants. For ceria based electrolytes, the highest oxygen ion conductivity is observed when the aliovalent doping cation radius is closest to the ionic radius of the host cation. For this reason, gadolinium doped ceria (CGO) is the extensively studied ceria based electrolyte.

 

5.1 Solubility of metal oxides in Ceria:

The ceria fluorite structure is also very tolerant to dissolution of lower valent metal oxides. CeO₂ is known to have a high solid solubility for many dopants (Table 1). The solubility of different metal oxides in ceria oxide varies a lot. Ca and Sr are reported to have a solubility of 9%, Y of more than 48.6% (1400°C). The rare earth oxides are soluble to more than 2-9 mol %. However, the solubility limit at low temperature might be lower as evidenced in the case of La (45% at 1600°C).

 

 

Table 1. Ionic radius and solubility of MO_x in CeO₂   for 8-fold coordination,  b. for 6-fold coordination ¹

 

 

6. IONIC CONDUCTIVITIES:

Ceria oxide doped with elements such as Ca, Sr, Ba, Y, Zr, Nb, La, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu have been investigated in the cited literature^3,4,5,6. They all show relatively high conductivities at intermediate temperatures compared to 8YSZ. The ionic conductivity in case of ceria electrolytes have been discussed in many ways such as dopants , microstructure, grain boundary, local structure, multiple doping, existence of micro domain, grain size, composition impurity (processing conditions). In this work it has been tried to review all these factors related to conductivity. The conductivity of doped ceria depends on the kind of dopant and its concentration. High bulk density pellets are necessary for ionic conductivity measurements, thermal expansion, and thermal conductivity studies. To have dense materials with high ionic conductivity, the study of sintering and microstructure of electrolytes are most important. It may be noted that sintering and densification behavior of ceria-based electrolytes is strongly dependent on the characteristics of raw powders.

 

6.1. Dopants:

The basic principle for the choice of a dopant, advocated by many researchers, is the ability of the dopant to minimize the internal strain of the lattice. For pure ceria, whose ionic conductivity is not particularly high because of low concentration of oxygen vacancies. The addition of dopants to the fluorite oxides results in creation of oxygen vacancies which are responsible for the ionic conductivity The ionic conductivity of ceria has been extensively investigated with respect to different dopants of Ca⁺², Sm⁺², Gd⁺³ ,Y⁺³, La⁺³, and dopant concentration. The conductivity depends on dopants Gd₂O₃, Y₂O₃, Sm₂O₃ and CaO give the highest maximum conductivities^3,5,6 but it is now well understood that ionic conductivities are obtained in lattices when ionic radius of dopant and host cation matches (Kilner,1983) suggested that for doped fluorite structures the radius of a dopant should match the radius of the host cation as close as possible. Kim in  1989 was able to calculate a critical ionic radius (r_c) for a dopant which would not change the volume of the host lattice and for ceria with trivalent dopants (r_c was 1.038Å).The highest conductivities are obtained in an undistorted lattice which results in highest conc. of oxide vacancies , however more work is needed to understand the correlation .

 

The highest ionic conductivities were reported for a doping level of 25 mol% Gd⁺³ (GdO_1.5)⁷ leading to 5% O₂ vacancies and 10 mol% GdO_1.5 ⁸. Sm⁺³ doped ceria due to small association enthalpy between dopant cation and oxygen vacancy in the fluorite lattice. Van Gool stated that in order to have high oxide ion conductivity the oxide ions on the various sites should have very similar energy. Wang et al showed that for ceria both oxide ion conductivity and activation energy are strong functions of dopant type as well as concentration.

 

6.2. Microstructure:

Many Researchers have revealed the relation of electrical conductivity with grain size, particularly specific grain boundary conductivity. In order to correlate the electrical properties with sample microstructure Brick Layer Model has been proposed⁹.

 

In this model (Figure 2) material is assumed to consist of an array of cubic shaped grains of equal size with edges having edge length, D, surrounded by a grain boundary of a specific thickness, σ_gb of length dg separated by flat grain boundaries of thickness δ_gb where δ_gb<< d_g .The current flow is assumed to be one-dimensional, and the curvature of the current paths at the corners of the grains is neglected. Depending on the relative magnitudes of the conductivities σ_gr and σ_gb, the current flows either through grains and across grain-boundaries or along grain-boundaries Using this model one can describe two different situations depending on the relative magnitudes of the grain interior conductivity (σ_gi) and grain boundary conductivity (σ_gb) .

 

Figure 2: (a) Sketch of the brick-layer-model, BLM, indicating grain diameter, D, grain-boundary thickness, d_gb, sample thickness, l, cross-sectional area, S and electrodes, E.

(b) current crossing grain and grain boundary perpendicular.

(c) current flows along parallel grain-boundaries

 

 

(i)   When σ_gi >>σ_gb then current will mainly flow through grains and across grain boundaries. In a two arc spectrum one can interpret the highest frequency arc representing the bulk response followed by the grain-boundary signal at lower/lowest frequency. It is important to note that for impedance spectra which exhibit two clearly separated arcs, one can immediately deduce that σ_gb < σ_gr since only a difference in conductivities of grain and grain-boundary can give a difference in characteristic frequencies.

 

(ii)  When σ_gb>>σ_gi the current path will be along grain boundaries. A ceramic sample will then show only one arc in the impedance plot and little information will be gained about the microstructure.

 

(iii) The electrical properties C_gi and C_gb and the micro structural property dg can be measured by impedance spectroscopy and from SEM micrographs (Christie et al., 1996) applied the BLM on Ce_0.8Gd_0.2O_2-x with micro

(iv) Meter and sub-micrometer sized grains. They found the grain-boundary conductivity being strongly dependent on the microstructure. Using conductivity data of grain-boundaries as a function of grain-size for Ce_0.8Gd_0.2O_1.9 they found that for grain sizes exceeding about 1 mm correlates well with the BLM. For smaller grain sizes in the range of 0.4 to 0.8 mm they found an abnormally high conductivity which they did not attribute to the presence of a grain-boundary phase. A further explanation has not been given by them. Therefore sintering and grain growth of the solid electrolyte are strictly controlled for the elevation of quality of SOFC. (Inaba et al, 1998) has reported the sintering behavior of undoped and Gd-doped ceria.

 

6.3. Grain boundary conductivity:

(Kilner, 1983) reported grain boundary resistance occurred due to an amorphous glassy phase caused by impurities, micro porosity in the boundaries or due to segregation of dopant ions. Scanlon et al studied segregation effect of the surface composition of ceramic Ce-Gd oxide. Christie et al, 1996 showed using electron microscopy that there was no foreign phase present in the grain boundaries. In spite of this fact the grain boundary having width in nanometer had a lower specific ionic conductivity than the bulk material in the temperature range of 250-500°C. (Chiang et al, 1997) was agree with these results based on the study of nanocrystalline CGO20 that grains having small size might affect the bulk conductivity, as dopant is extracted to the grain boundaries to an extent that the grains become practically undoped. The bulk conductivity is not affected so it is possible to fabricate the dense samples with low contribution from the grain boundaries to the total resistivity as shown recently by (Van herle et al., 1996) for 20 mol% YO

 

At high temperature, the contribution from σ_gb to the total conductivity is expected to be very small because the activation energy associated with oxygen ion migration within ceria lattice is much lower that for the σ_gb.. So role of the grain boundary conductivity measurements are influenced by sin_1.5. tering characteristics and the processing conditions. Density and amount of porosity of the pellet may change the measurements of conductivity. Densification is better with the increased amount of Gd doping because of the larger mobility of oxygen although grain growth is lower (H. Inaba, 1998). Results showed that the grain boundary conductivity is only weakly affected by the sintering additives like TMO (2% Co). Earlier Lewis et al showed CoCGO had enhanced specific grain boundary conductivity compared to CGO. Kleinlogel and Gauckler reported that σ_gb of CoCGO-20 was greater for the samples sintered at short dwell timer and with higher Co content.

 

6.4.  Existence of micro domains:

The presence of microdomains significantly affects the conductivity of doped ceria as a result of aging.(Mori et al, 2003) have shown that characteristics of microdomains significantly affect the conductivity of electrolytes and conductivity can further be optimized by controlling the no. and size of microdomains through adjusting doping level.

 

According to (Zhang et al, 2004) the engineering of microdomains would provide a new approach for achieving an enhancement in the ionic conductivity of solid oxide electrolyte. The relationship between the conductivity and dopant ionic size as well as the composition (x = 0.2) having maximum total conductivity could be explained by critical dopant concentration at which the microdomains start to form. When dopant content is high, conductivity decreases due to formation of microdomains but little information is available regarding critical dopant concentration above which the formation of microdomains is the major factor responsible for the conductivity decrease. Microdomains lead to decrease in conductivity as they prevent oxygen vacancies from passing through the lattice. Gd doped ceramics exhibit a significant decrease in conductivity at x ³ 0.2 and rapid increase in DH_a.

 

Dopant ionic size is important factor which determines the formation of microdomains and formation of microdomains is related to the solubility of the dopant in ceria so larger mismatch of ionic sizes between dopant and ceria leads to formation of microdomains comparing ionic sizes. Formation of microdomains follow the order La⁺³ > Gd⁺³ > Y³⁺.Ionic sizes of most rare earth oxides range from 1.02Å Y³⁺ to 1.16Å (La³⁺). Their critical concentration of the doped ceria could be at x = ~0.2 so maximum conductivity occurs at x ~0.2 for most of the ceria ceramics doped with rare earth oxide.

 

6.5. Total Ionic conductivity:

Earlier works were carried out using classical four point conductivity measurements on the sintered samples. This gives the total conductivity with little possibility of resolving it into bulk and grain boundary conductivities but with the help of ac impedance analysis by using two electrodes only the complex impedance Z of a specimen is measured at a large range of frequencies (0.003- 1000000 Hz).

 

Kevane et al. reported in 1963 that calcia (up. to 10 wt%) dissolved in ceria introduced extrinsic ionic conduction. Since then many works have been carried out on ceria doped with CaO and SrO, there is some scatter in the reported values of the conductivity as well as in the activation energy. The level is 0.1–0.2 S cm^-1 at 1000°C and the activation energies in the range of 0.6 to 1.0 eV were reported.

 

Kudo and Obayashi, 1976 examined the electrical properties of CeO doped with LnO (Ln=Y, La, to Yb). The data showed that the behavior for all of the lanthanide oxides was very similar to that of CaO and SrO doped ceria when compared on a vacancy concentration basis.

 

Yahiro et al. 1988 examined the total ionic conductivity of the series (CeO)_0.8 (LnO_1.5)_0.2 (Ln= La, Nd, Sm, Eu, Gd, Y, Ho, Tm, Yb) at 800°C.(CeO) _0.8 (SmO_1.5 )_0.2 was found to have the highest conductivity whereas that of (CeO)_0.8 (GdO_1.5)_0.2 was almost 50% lower. The authors found that the ionic conductivity increased with increasing ionic radius, from Yb to Sm, but decreased at r.>0.109 nm.

 

According to Steele, 2001 the highest ionic conductivity of the doped ceria is exhibited by the Ce_0.8 Gd_0.2O_1.9 in contradiction with the above findings. A plausible explanation for the differences in the value of ionic conductivities is due to the different methods of fabrication of the samples.

 

7.  USE OF SINTERING AIDS IN CERIA ELECTROLYTES:

Many attempts have been made to increase the sinterability of doped ceria powders by using fine powder by using small amount of sintering aids eg. Transition metal oxides (Co, Cu, Ni, Mn, Fe). These are being added to increase sintering rates. Recently Kleinlogel, 1999 reported that in CoO doped CGO, doping lowers the required sintering temperature for fully dense material from 1250º to 900ºC with average grain size @ 120nm. This was further confirmed by Lewis et al. By choosing the appropriate cobalt oxide concentration and sintering time/temperature we can get the dense ceramic electrolytes.

 

8.      CONCLUDING REMARKS:

Inspite of some drawbacks of ceria it holds a lot of potential to be an attractive material for SOFC so this review provides an overall insight of ceria based electrolytes which would be helpful to improve our knowledge about ceria and their applications in SOFC.

 

9. REFERENCES:

1.       Kleinlogel Christoph Martin Thesis (1999) “Cathode supported thin electolytes and nanosized ceria solid solutions for solid oxide fuel cells.”

2.       Kilbourn B.T., (1992) Cerium, A Guide To Its Role in Chemical Technology, Molycorp, NY.

3.       Kim D.J. (1989) J. Am. Ceram. Soc. 72 1415.

4.       Kilner J., Solid State Ionics (1983) 8, 201.

5.       Arai H., Kunisaki T., Shimizu Y.and Seiyama T.(1986) Solid State lonics 20, 241.

6.       Zhen Y.S., Milne S.J., Brook J.R., (1988) Sci. Ceram. 14, 1025.

7.       Ralph J.M., Kilner J.A., (1996)  B. Thorstenson (Ed),Second Eur. SOFC Forum, l2, 773

8.       Steele B.C.H, (2000) Solid State Ionics 129, 95.

9.       Beekmans N.M. and Heyne L.(1976). Electrochimica. Acta, 21, 303.

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13.     Van herle J., Horita T., Kawada T., Sakai N., Yokokawa H., Dokiya M., (1996) J. Eur. Ceram. Soc. 16, 961.

14.     Lewis G.L., et al., Solid State Ionics, (2002), 152-153, 567-573.

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16.     Zhang T.S, Ma. J, Kong L.B, Chan S.H and Kilner J.A (2004) Solid State Ionics 170, 209-217.

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19.     Yahiro H., Eguchi Y., Eguchi K., Arai H., (1988) J. Appl. Electro chem. 18 , 527.

20.     Steele B.C.H., Heinzel A., (2001) Nature, 414, 345.

 

 

Received on 16.09.2010        Modified on 15.10.2010

Accepted on 30.10.2010        © AJRC All right reserved