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versión On-line ISSN 0717-9707
J. Chil. Chem. Soc. v.54 n.4 Concepción dic. 2009
J. Chil. Chem. Soc., 54, Nº 4 (2009), págs. 445-447.
PREPARATION OF CE0.8GD0.2O1.9 SOLID ELECTROLYTE BY THE SOL-COMBUSTION METHOD
JIHAI CHENGa,b*, WEITAO BAOa, DECHUN ZHUa, CHANGAN TIANa, QIYI YINa, MING DINGb
aDepartment of Chemistry and Materials Engineering, Hefei University, Hefei, Anhui, 230022, China. e-mail: firstname.lastname@example.org
bKey Lab of Powder and Energy Sources Materials, Hefei University, Hefei, Anhui, 230022, China.
Ce0.8Gd0.2O1.9 (GDC) powders were prepared by a novel sol-combustion method using citric acid as the chelating reagent. The effects of different sintering temperatures were investigated on the evolution of crystalline phase, particle size, and lattice parameters of the resulted GDC powders. The results have shown that ultrafne GDC powders of around 40nm in diameter were successfully prepared by the sol-combustion technique. The powders showed a high sinterability, and a relative of 95.2% of the theoretical density was obtained at a sintering temperature of 1250oC. Electrochemistry measurements showed that the GDC ceramics had relatively high oxygen ionic conductivity in low-temperature range.
Key words: SOFC; Doped CeO2; Electrical conductivity; Sol-combustion method; Electrolyte.
Solid oxide fuel cell (SOFC) is a new chemical energy source with many advantages, such as transformation of chemical energy to electrical energy with high effciency and low pollution to environment 1-3. Traditional solid oxide fuel cells based on yttria-stabilized zirconia (YSZ) can maintain suffciently high ionic conductivity. However, the high operating temperature (above 1000oC) results in both higher costs and system degradation 4,5. Intermediate-temperature solid oxide fuel cells (IT-SOFCs) with an operating temperature of 500oC-600oC have been investigated by a number of researchers previously 6,7.
Ceria-based oxides have been suggested as one of the most promising electrolyte materials for IT-SOFCs. These materials show higher oxygen ionic conductivity at relatively lower temperature, compared with YSZ materials. Therefore, many studies have been carried out on ceria-based oxides doped with rare-earth elements, and many progresses have been made with regard to their properties as electrolytes for SOFC 8,13.
Doped ceria oxide powders, such as Ce1-xSmxO2-y, Ce1-xGdxO2-y, Ce1-xYxO2-y, etc. (y=0.5x), have been prepared by the traditional solid-state reaction, in which oxide precursors were milled and calcined at a high temperature. The latter may cause the degradation of the electric and mechanical properties of the sintered bodies due to the impurity contamination 14. Therefore, many soft-chemistry routes have been used to produce precursors at a lower temperature, as it is the case of fuorite precursors, to improve the preparation of ceria-based fuorite powders for IT-SOFCs application 15,18.
In this study, Ce0.8Gd0.2O1.9 powders were synthesized by a sol-combustion method. Their sinterability and electrical conductivity were examined.
Powder samples with the general formula of Ce0.8Gd0.2O1.9 (GDC) were synthesized by the sol-combustion method, as shown in Fig.1.
Analytical pure cerium nitrate (Ce(NO3)3·6H2O, China National Medicines Corporation Ltd., China, containing CeO2 39 wt.%), gadolinia (China National Medicines Corporation Ltd., China, 99.99wt.%), were used as starting materials. Cerium nitrate was dissolved in deionized water and the stoichiometric amount of gadolinia was dissolved in nitrate solution, then they were mixed together with citric acid solution (the mole ratio of cation to citric acid was 1/1.5). The pH value of the system was adjusted to 7-8 with ammonia solution under continuous stirring at 40oC and a homogeneous sol was formed. The sol was then heated at 120oC for about 1h; a white alveolate precursor was obtained. The precursor was then calcined at different temperatures (650oC, 750oC, and 850oC) to get the fnal composition powders. The obtained powders were pressed into pellets under a pressure of about 200MPa, and then sintered at different temperatures (1000-1300ºC) in air for 4h.
X-ray diffraction (XRD) analysis was performed with a rotating diffractometer (model: Rigaku-D/Max-γB, Rigaku, Japan) with the CuKα line (λ=0.15406nm), and the diffractograms were scanned in 2θ from 10° to 70° at a rate of 6°/min. The crystal size of the calcined powders was estimated using the Scherrer formula.
Ionic conductivity of the sintered GDC materials was measured on the sintered pellet samples. Silver paste was used as electrodes painted on both sides of each pellet. The measurement of AC impedance was performed in air with an electrochemical workstation CHI660B at a frequency range of 0.1Hz to 100 kHz. The measurement curves were conducted in the temperature range from 500oC to 800oC with an interval of 50oC.
RESULTS AND DISCUSSION
Fig.2 shows the X-ray diffraction patterns of Ce0.8Gd0.2O1.9 powders calcined at 650oC, 750oC, and 850oC. All diffraction peaks match well with ICDD PDF# 43-1002; no additional diffraction peaks were found, indicating that the substitution of Gd3+ ions into the Ce4+ sites took place.
It can be seen that all samples were single phase with cubic fuorite structure; all the diffraction peaks can be attributed to 111, 200, 220, 311, 222, 400, 331 and 420 refections of the single phase cubic structure of fuorite CeO2, and there were no other peaks of other phases. In the sol-gel process, no other cations were added and the original cations did not lose, so the result material was pure production with a general formula Ce0.8Gd0.2O1.9. This suggests that GDC powders with single phase can be successfully synthesized by the sol-combustion method by calcinations of the gel above 650oC, whereas ceria-based electrolytes prepared by conventional solid state reaction require calcinations temperatures around 1000oC. (at 650oC, 750oC, and 850oC ).
The mean grain size of Ce0.8Gd0.2O1.9 powders was calculated to be about 40nm based on Scherrer formula.
After heating at 650oC for 4h, the lattice parameter of the Ce08Gd02O19 crystallized in a cubic fuorite structure is a=0.54317nm, which is slightly larger than that of the CeO2 (a=0.5411nm). When Ce4+ ions of CeO2 were partially substituted by Gd3+ ions, Ce0 8Gd0 O was formed. Though the ionic radius of Gd3+ (0.0938nm) is smaller than that of Ce4+ (0.1034nm), the substitution does not distort the structure of CeO2 signifcantly, as shown by the similarity in the crystal lattice parameters, which are the same within the experimental error.
Since the electrolyte must be gastight, it is said that an acceptable relative density of impervious ceramic electrolyte should be higher than 94% 2. Relative density of the sintered pellets was measured using the Archimedes method. Fig.3 shows the relative density of GDC ceramics sintered at different temperatures. The GDC powders showed a high sinterability; density of sintered GDC ceramics increases as sintering temperature increases, and a maximum relative density value (95.2%) of the theoretical one was obtained at sintering temperature of 1250oC, which satisfed the requirement for SOFC electrolytes operating at intermediate temperature range.
The ionic conductivity of sintered GDC samples was measured by AC impedance technique. The impedance spectra data were ftted with suitable equivalent circuit of the zsimpwin software to distinguish the bulk resistance from the grain-boundary resistance, and the conductivities were calculated. For a common ceramic electrolyte sample, the contributions to conductivity can be ascribed to the different conduction processes occurring in the bulk (a high-frequency semicircle originates in bulk conduction and dielectric processes), grain boundaries (an intermediate-frequency semicircle provides information on the grain-boundary and/or impurity contribution to ionic conduction), and electrode/electrolyte interfaces (a low-frequency semicircle or arc is generally due to ion and electron transfers at the sample surface contacting the electrode). The total resistance of electrolyte can be expressed as:
where L is the sample thickness and S is the electrode area on the sample surface. These parameters are compiled in Table1. With the increase of the operating temperature, the total resistance decreases, and the electrical conductivity datum, σ, increases. The conductivity of GDC sintered at 1250oC for 4h in air reached 7.64×10-2S·cm-1 under the testing temperature of 800oC.
The conductivity data were analyzed further using the Arrhenius equation.
where E is activation energy of electrical conduction, k is the Boltzmann constant, T is the absolute temperature and σ0 is a pre-exponential factor, being constant in a certain temperature range. Fig.4 shows the Arrhenius plots of the electrical conductivity for the sample sintered at 1250ºC at the temperature range from 500oC to 800oC. The activation energy of the total conductivity for the sample sintered at 1250oC was estimated to be 0.87eV.
Fig.5 shows the Arrhenius plots of the electrical conductivity for the GDC samples treated after different sintering temperatures at various testing temperatures. It can be seen from Fig.5 that the electrical conductivities of the GDC samples for a given testing temperature increased as sintering temperature increased up to 1250oC, while the total electrical conductivities of the samples sintered at 1300oC and 1250oC are almost the same. This phenomenon means that the pellets were dense when sintered up to1250oC. However, when sintered beyond 1250ºC, grain size of GDC samples grew rapidly to form coarse and big grains, and at the same time, more and larger pores appeared. Thus, the density of the pellets began to increase much more slowly. So the electrical conductivity of the sample sintered at 1300oC was similar to or not much higher than that of the sample sintered at 1250oC. The reason may be that the lattice oxygen loss of this system at high temperatures caused the decrease of electrical conductivity due to the reduction of charge carrier concentration 19.
Nanosized Ce0.8Gd0.2O1.9(GDC) powders were prepared by a new sol-combustion method. When calcined over 650oC, the sample presents a single phase with cubic fuorite structure. At sintering temperatures higher than1250oC, the sintered pellet samples become denser much more slowly than below that temperature. The sample sintered at 1250oC showed relatively high electrical conductivity and low activation energy as electrolytes in the intermediate-temperature SOFC. The conductivity of GDC sintered at 1250oC and tested at 800oC can reach 7.64×10-2S·cm-1; the activation energy of the total conductivity for the sample was 0.87eV.
This work was supported by the Scientifc Research & Development Foundation of Hefei University (No. 08KY014ZR).
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(Received: April 29, 2009 - Accepted: October 22, 2009).