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Synthesis and physicochemical characterization of nanocrystalline cobalt doped lanthanum strontium ferrite
Solid State Sciences 13 (2011) 1022e1030
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Synthesis and physicochemical characterization of nanocrystalline cobalt doped lanthanum strontium ferrite Chaubey Nityanand a, *, Wani Bina Nalin b, Bharadwaj Shyamala Rajkumar b, Chattopadhyaya Mahesh Chandra a a b
Department of Chemistry, University of Allahabad, Allahabad 211002, India Chemistry Division, Bhabha Atomic Research Center, Trombay,Mumbai 400085, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 May 2010 Received in revised form 18 January 2011 Accepted 29 January 2011 Available online 4 March 2011
Nanosized crystallites of La0.6Sr0.4Fe0.8Co0.2O3d (LSCF), a promising cathode material for Intermediate Temperature-Solid Oxide Fuel Cells (IT-SOFCs) has been synthesized by alternative methods like ceramic route, polymerisable complex process and gel-combustion method and calcined at different temperatures. X-ray diffraction studies were used for the determination of phase purity, crystal structure and average crystallite size of the samples. Microstructure of LSCF samples was studied by SEM. Temperature-programmed reduction studies were done for evaluating the redox behavior of the samples prepared by alternative methods. The electrical conductivity measurements of sintered samples were carried out at elevated temperatures using four-probe method. The electrical conductivity of sample synthesized by gel-combustion method was more as compared to sample prepared by polymerisable complex process and ceramic route. The electrical conductivity decreased at high temperature due to loss of oxygen and the formation of oxygen vacancies. Thermo-dilatometry was used to study the linear thermal expansion behavior of the samples which shows that there is an observable increase in thermal expansion at high temperatures. Ó 2011 Elsevier Masson SAS. All rights reserved.
Keywords: Perovskite oxides Cathode materials Crystal structure Electrical conductivity Thermal expansion Redox behavior
1. Introduction Solid Oxide Fuel Cell (SOFC) is a solid-state electrochemical device that converts the chemical energy of a fuel directly into electrical energy [1e4]. At present, the main targets in the development of solid oxide fuel cells to achieve commercial viability are high durability, high power density and low production costs. This can partially be obtained by operating SOFCs at lower temperatures that can accelerate the commercialization of this energy conversion technology and hence has been the prime focus of recent research and development activities. However, lowering the operating temperature not only increases ohmic losses at the electrolyte but also polarization loss at the anode and the cathode. To keep up with the performance of traditional SOFCs that operate between 900 and 1000 C, new materials with improved performance have to be used. To enhance the oxygen ion conductivity of the electrolyte at the reduced temperature La1x SrxGa1yMgyOz, gadolinia doped ceria etc. can be used to replace the yttria stabilized zirconia [5].
* Corresponding author. Tel./fax: þ91 532 2466534. E-mail address: [email protected] (C. Nityanand). 1293-2558/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2011.01.026
The polarization losses occur mainly due to the electrode overpotentials, especially at the cathode [6]. In order to prevent significant deterioration at lower temperature, cathode materials should retain their electrochemical activity and oxygen ion conductivity so as to secure sufficient activity for electrocatalytic reaction, as well as maintain a high surface exchange rate of oxygen at the gas/cathode interface [7]. Thus, selecting a high performance cathode material which is compatible with a suitable electrolyte is the major challenge faced in the development of medium temperature SOFC. The transport of oxide ion through the cathode is advantageous in enhancing the possible reaction pathways and increases the active area into the cathode volume. Sr doped lanthanum manganite is used as a cathode material in SOFCs at the operating temperature of 1000 C but at lower temperature it has high electrode resistance. The mixed ionic and electronic conductivity of the cathode can be made better by mixing an ion conducting material such as YSZ with an electronically conducting material La1xSrxMnO3d [8,9]. Another way to increase the mixed conductivity of the cathode is to substitute the conventional materials with perovskite oxides having high value of mixed conductivity [8e10]. Use of mixed ionic electronic conductor at the cathode is expected to lower the cathode polarization. Thus in the past decades, there has been growing interest in perovskite-type
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oxides with high oxygen-ionic and electronic conductivity such as La1xSrxCoO3d [11e13]. But the thermal expansion coefficient (TEC) value of La1xSrxCoO3d is relatively high and it reacts with the electrolyte YSZ above 900 C to form La2Zr2O7 and SrZrO3 leading to degradation of the cell [14]. However substitution of Co with Fe decreases the thermal expansion [15]. It was also seen that doping with Sr increases the ionic conductivity very much at the same time doping with Fe decreases the ionic conductivity to small extent [16]. Cathode materials with higher performance at lower temperature such as La0.6Sr0.4Fe0.8Co0.2O3d (LSCF) can substitute La1ySryMnO3x (LSM) the performance of which decreases rapidly when the operating temperature is below 800 C. It has been found that electrical properties of the components of SOFCs can be improved to a great extent by decreasing the size of particles to nanoscale range. In the present study an attempt has been made to synthesize nanocrystalline LSCF samples by alternative routes like polymerisable complex process, gel-combustion method and ceramic route and a comparative study of their microstructure, thermal expansion behavior and electrical conductivity was carried out. 2. Experimental 2.1. Synthesis LSCF was prepared by three different routes as given below. 2.1.1. Polymerisable complex process Stoichiometric amounts of La(NO3)3$6H2O, Sr(NO3)2, Co (No3)2$6H2O and Fe(NO3)3$9H2O were dissolved in distilled water to get a concentration of 0.03M. Concentration of citric acid solution was fixed according to the molar ratios of citric acid to total metal ions which was varied between 3 and 5. Citric acid solution was mixed with 10 mL ethylene glycol solution and was added to the metal ions solution with stirring vigorously to get a homogeneous yellow solution. The resultant solution was heated with continuous stirring at 80 C for 12 h to remove the excess solvent and advance the polymerization between citric acid and ethylene glycol. After heating for 12 h the solution became highly viscous. The color of the solution changed from yellow to orange and at last a glassy resin was formed. The resin formed was dried in hot air oven at 100 C and then calcined at higher temperatures. 2.1.2. Gel-combustion process Stoichiometric amounts of metal nitrates were dissolved in distilled water to produce transparent mixed metal-nitrate solution. Glycine (NH2CH2COOH), which is able to bind the metal ions and acts as a fuel in combustion reaction was then added to the mixed metal-nitrate solution. The molar ratio of glycine to oxidant was set to 1. The transparent aqueous solution containing metal nitrates and glycine was heated on a hot plate under stirring. It was converted to a viscous gel due to thermal dehydration and ignited to a flame at about 200 C, resulting in fine powder with evolution of large amount of gases. The large volume of gases generated during such type of auto-ignition process quickly cools the powder formed which leads to nucleation of crystallites without further growth. 2.1.3. Ceramic route Stoichiometric amounts of high purity La2O3, SrCO3, Co3O4 and Fe2O3 were used as starting materials. La2O3 was preheated at 900 C to avoid the formation of hydroxide. Precursors were ground in an agate mortar and pestle, and after grinding the mixture was pelletized, fired at 900 C for 7 days. The pellet was again cracked, reground and finally sintered at 950 C for 10 days to get the singlephase product.
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2.2. Characterization FTIR spectra of powders prepared by different processes were recorded at room temperature using a spectrometer (FTLA, 2000, Make: ABB Ltd.) with a resolution of 4 cm1. Thermal analysis was done on TG/DTA Equipment (SETRAM model 92-16.18) in air from room temperature to 1300 C. Powder X-ray diffraction data for the samples were collected on a Philips X-ray Diffractometer (PW1710) with Ni filtered Cu-K-alpha1 radiation and using silicon as an external standard. The XRD patterns were recorded in a continuous scan mode with a step width of 0.02 and at a scanning rate of 1 2q min1. The indexing of XRD patterns were done by POWD (version 2.2) program. For particle size distribution, Small Angle X-Ray Scattering (SAXS) experiments were performed on Rigaku RINTPC2000 in the region of small angle scattering 0.02e5 . Scanning Electron Microscope (JEOL JXA-8100) was used to see the grain size and consistency of the microstructure of calcined powders and ceramic specimens. SEM was operated with an acceleration voltage of 20 kV using a Tungsten anode. The samples were given 5 nm of carbon coating before taking micrographs. Thermal expansion measurements were done on SETSYS Evolution TMA 1600 (Setaram Instrumentation, France). For evaluating redox behavior as a function of sample composition, TPR profiles were recorded with a TPDRO-1100 analyser (Thermoquest, Italy) using H2 (5%) þ Ar gas mixture as a reduction medium and O2 (5%) þ He gas mixture as oxidation medium with a heating rate of 6 C/min. 20 mg of the sample was taken for TPR/TPO study. Samples were pretreated by heating in inert atmosphere at 600 C for 1 h prior to recording the TPR run. The DC electrical conductivity measurements were carried out using four-probe method with sintered bars. Pt paste (MaTeck Gmbh, Germany) was painted on the square cross-section edges of the sample and along one of the rectangular edges (separated by distance L) to form current and voltage electrodes, respectively. The sample was heat treated at 1073 K for 4 h to ensure good bonding between the electrodes and the sample. Two Pt wires attached to thin Pt foils were made to act as current contacts, and two voltage contacts were made with Pt wires connected to the voltage electrodes using Pt paste. The voltage between the two inner electrodes and the current through the sample were recorded at each temperature. The electrical conductivity s was calculated by the equation:
s ¼ LI=VA Where A is the electrode area, V is voltage between the inner electrodes separated by distance L and I is the current through the sample.
3. Results and discussion 3.1. Synthesis To prevent the precipitation occurring over the whole concentration process and to maintain the homogeneity of the metal ions in the resin on a molecular scale, the molar ratio of citric acid to metal ions should be high enough. So we have investigated minimum molar ratio of chelating ligand to total metal ions in the precursor of LSCF. When the molar ratio of citric acid to metal ions were in the range of 3e5, no visible precipitation was observed during the polymerization process. The pH value of the precursor solution was less than 2. The drying rate was kept to be moderate and drying temperature was kept below 80 C for producing a transparent intermediate resin by polymerisable complex process. The organic precursors are widely used in the synthesis of mixed oxide powders [17e19]. In the present work polyesterification
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between hydrocarboxylic acids such as citric acid, and polyhydroxy alcohols such as ethylene glycol has been used to produce phase pure cobalt substituted lanthanum strontium ferrite powders. In the ester reaction, carboxyl end of citric acid and hydroxyl end of ethylene glycol react and a water molecule is released. The acid acts as a chelating agent that stabilizes the cations dissolved in the solution. The cations are stabilized due to columbic attraction forces between the carboxylic or hydroxyl groups of the carrier materials and metal cations. Due to this stabilization, metal cations are distributed homogeneously in the solution and are stabilized in the pre-ceramic precursor after solvent removal. After calcinations and organic burn-out, an amorphous powder is obtained. At higher temperatures, crystallization of the desired phases takes place. Due to the homogeneity in molecular level, lower diffusion distances for the cations are required to obtain the desired crystal phase, hence phase pure products are obtained at lower temperature as compared to the ceramic route [20]. Glycine is one of the cheapest amino acids and contains minimum amount of carbon contents with respect to other fuels used in the gel-combustion method. The glycine molecule contains a carboxylic acid group at one end and an amine group at the other end both of which participate in the complexation of metal ions. This “zwitterionic” character allows effective complexation with metal cations of varying ionic size. Thus, glycine first forms complexes with metal cations, which increases their solubility and prevents selective precipitation as water is evaporated and later it serves as fuel for the combustion reaction being oxidized by the nitrate ions. 3.2. FTIR The FTIR spectra of the glycine along with LSCF samples prepared are shown in Figs. 1 and 2 to see the completion of the combustion reaction. The comparison of the spectra in Fig. 2 shows that glycine has been destroyed completely during the combustion process, since no characteristics peaks of glycine was found in the sample
Fig. 2. FTIR spectra of LSCF samples prepared by gel-combustion process. (A) calcined at 150 C( B) calcined at 800 C, (C) Glycine.
prepared by combustion method. A broad absorption band around 3450 cm1 appeared in the IR spectra of the samples prepared by both the methods which are characteristics of absorbed water or hydroxyl group (OeH stretching) in alcohol. It also shows absorption bands in the vicinities of 1725 and 1190 cm1, which can be attributed to monodentate ligand of metal with carbonyl groups (COO) of citric acid in the case of samples prepared by polymerisable complex process. In the case of samples prepared by polymerisable complex process a band w925 cm1 is due to symmetric vibrations of CeOeC bonds and trace amount of nitrate ions was indicated by the band around 1400 cm1. All these bands disappears from FTIR spectrum of the sample heated at higher temperatures of 800e850 C, indicating that the impurities have been removed completely. The small bands appearing between 800 and 500 cm1 are mainly due to metal oxide bond [21]. 3.3. TG-DTA The various stages of removal of adsorbed moisture, organics, decarboxylation reactions, combustion, and crystallization can be understood from these thermal analysis results. Fig. 3 shows simultaneous TG/DTA curve of LSCF precursors prepared by gelcombustion method. Weight loss of about 7% occurred between 30 and 200 C which could be attributed to the loss of absorbed moisture in the sample. This weight loss is accompanied by a broad endothermic peak around 160 C. Again a weight loss of 16% occurred between 200 and 900 C which is likely due to decomposition of precursors to oxides, loss of carbon residue by oxidation and also from decomposition of residual nitrates. In the DTA plot an exothermic peak is present at 310 C, followed by a broad endothermic peak which can be attributed to oxidation of carbon reside and decomposition of precursors to oxides to form the perovskite oxide. Similarly decomposition path of LSCF precursors prepared by polymerisable complex process is also delineated. The formation of perovskite oxide starts at 400 C and completes at 850 C, which is also supported by XRD results. 3.4. XRD
Fig. 1. FTIR spectra of LSCF samples prepared by polymerisable complex process at molar ratio of citric acid to metal ions ¼ 4, calcined (A) at 850 C (B) at 150 C; at molar ratio of citric acid to metal ions ¼ 5 and calcined (C) at 850 C (D) at 150 C.
The synthesized powders were calcined in air at higher temperatures to investigate the evolution of crystalline phases. As shown in Fig. 4. LSCF samples prepared by polymerisable complex process give the rhombohedral phase after heat treatment at 850 C for 5h while the LSCF sample prepared by gel-combustion process
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Fig. 3. TG/DTA pattern of LSCF precursor prepared by gel-combustion method.
gives the rhombohedral phase at 800 C for 4h. In the ceramic route, perovskite phase was formed after heating at 900 C for 7 days along with traces of impurity phases of La2O3 (PCPDF no. 741144). In this method phase pure perovskite was formed only after heating at 950 C for 10 days as shown in Fig. 5. These observations can be explained as follows: The solid state or ceramic method involves the mechanical mixing of the solid constituents (oxides/carbonates), repeated grinding and annealing at elevated temperatures over a long duration. The solid-state reactions are diffusion controlled reactions and follow a parabolic law given as
dx=dt ¼ kx1 where ‘x’ is the degree of reaction (thickness of the product layer), t is the time and k is a constant. Since the reactants are high melting refractory solids it is only at high temperatures that the ions have sufficient thermal energy to enable them to jump off their normal lattice sites and diffuse through the crystals. Initially the reaction is faster and thereafter the rate becomes slow as the reactants have to diffuse through the product layer that is formed in order to come in
Fig. 5. XRD pattern showing the formation of LSCF sample by ceramic route (P perovskite phase,# impurity phase of La203)
contact with one another. The reactants are ground and mixed thoroughly as the subsequent reaction rate depends to a large extent on the particle size of reactants, the degree of homogenization achieved on mixing and the intimacy of contact between the grains as well as the effect of temperature. Thus the rate-limiting step is diffusion of ions through the product layer. In gel-combustion and polymerisable complex process, there is homogenization of ions at molecular level and hence the reaction rates are faster and one gets phase pure composition at relatively lower temperature. In ceramic method of preparation, we observed unreacted La2O3 as impurity phase. Grinding the partially reacted powder and reheating brings fresh surfaces in contact and the unreached La2O3 gets into the lattice of partially formed sample. Thus, the lines due to La2O3 disappear on repeated grinding and reheating. The average crystallite size of samples prepared by gelcombustion method and polymerisable complex process was evaluated from x-ray line broadening analysis using the Scherrer equation, which is between 15 and 17 nm. The average crystallite size of LSCF sample prepared by ceramic route is 22 nm. In this work we get rhombohedral perovskite La0.6Sr0.4Fe0.8Co0.2O3ed with space group R3c synthesized by different routes. Fig. 6 shows the XRD pattern of LSCF samples prepared by different routes after heating at 1400 C in air. All the samples have rhombohedral structure and average crystallite size between 27 and 31 nm. The lattice parameters of La0.6Sr0.4Fe0.8Co0.2O3ed sample prepared by different routes and after giving heat treatment at different temperatures are given in Tables 1 and 2. The lattice parameters have been given in hexagonal cell dimensions. From the Fig. 6 we see that the small impurity phase of SrLaCoO4 (PCPDF no.832412) was present in sample prepared by gel-combustion method and heated at 1400 C. 3.5. Particle size distribution
Fig. 4. XRD pattern of LSCF samples: A to C are prepared by polymerisable complex process at molar ratio of citric acid to metal ions 3, 4 and 5 respectively and calcined at 850 C for 5 h, D is prepared by gel-combustion process and calcined at 800 C for 4 h and E is prepared by ceramic route.
The particle size distribution is used to determine the agglomeration of particles in the samples. The data measured by Small Angle X-ray Scattering were examined for determination of size and size distributions of nanocrystalline LSCF samples obtained after heat treatment. From Fig. 7, it is clear that the agglomeration is between 2 nm and 29 nm for LSCF sample prepared by
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C. Nityanand et al. / Solid State Sciences 13 (2011) 1022e1030 Table 2 Cell parameters and cell volumes of La0.6Sr0.4Fe0.8Co0.2O3d prepared by different routes and heated at 1400 C. Method
Structure
a0(Ǻ) a
c0(Ǻ) a
v0(Ǻ)3 b
Polymerisable complex process Gel-combustion method Ceramic route
Rhombohedral Rhombohedral Rhombohedral
5.4674 5.4955 5.4944
13.4436 13.4112 13.4035
348.03 350.77 350.41
a b
Fig. 6. XRD pattern of LSCF synthesized by different route and calcined at 1400 C in air, (# impurity phase of SrLaCoO4).
polymerisable complex process and between 3 nm and 40 nm for the same prepared by gel-combustion process. For ceramic route the agglomeration is between 6 and 43 nm. The particles were found to be spherical in shape. From the Fig. 7c we can see that the sample prepared by ceramic route contains hard agglomerates in comparison to the samples prepared by other routes which in turn affect the sintered density and microstructure that can be seen from bulk density and SEM results. The samples prepared by polymerisable complex process and gel-combustion method have narrow particle size distribution in comparison to ceramic route as can be seen from Fig. 7a and b. 3.6. Thermal expansion results The linear thermal expansion curves for LSCF sample prepared at 1400 C by different methods are shown in the Fig. 8. All the thermal expansion curves are non-linear with inflection occurring w650e800 C. The thermal expansion curves were fitted by two straight lines. There is a change in the slope in the expansion behavior in higher temperature range that is the expansion takes place at higher rate in the high temperature range. The value of average linear thermal expansion coefficient (a) of LSCF samples prepared by different routes is given in the Table 3 in the different temperature ranges. The average TEC value in high temperature range is nearly double than that in low temperature range in all the samples. The change in slope observed in high temperature range in the thermal expansion behavior is considered due to loss of lattice oxygen and the formation of oxygen vacancies [22,23].
0.0002. 0.1.
Simultaneously thermal reduction of cations in B site occurs from high valence state of Co4þ and Fe4þ to lower trivalent states Co3þ and Fe3þ to maintain the electrical neutrality. The valence changes are followed by an increase of the ionic radius particularly for the reduction of Co4þ (0.067 nm) to Co3þ (0.075 nm). Due to these reductions, a decrease in the BeO bond occurs according to Pauling’s second rule, and in this way the size of BO6 octahedra increases which enhance the lattice expansion [22]. The thermal expansion coefficient of LSCF sample in the temperature range 100e700 C is 14.97 106 K1 and 15.38 106 K1 for ceramic route and polymerisable complex process respectively and for gel-combustion process it is 14.31 106 K1 in the temperature range 100e650 C. 3.7. TPR/TPO TPR/TPO studies of LSCF samples prepared by different methods were done to observe the redox behavior of LSCF, which is used as a cathode material for solid oxide fuel cells. This method is based on the temperature-programmed reduction and oxidation of materials and is used for the release and uptake of oxygen from lattice of cathode materials which is responsible for change of stoichiometry. We may mention that Fe and Co are only reducible species in LSCF sample. Substitution of La on the A site by Sr at constant stoichiometry contributes to a change of valence for some of the iron atoms from Fe3þ to Fe4þ due to the aliovalence of Sr2þ vs La3þ. Since Fe4þ reduces easily to Fe3þ which in turn increases the movement of oxygen ions and oxygen vacancies are formed upon heating the material in He atmosphere and result in a peak in the TPR results. The oxygen vacancies in the perovskite material are filled when it is heated in an oxygen atmosphere which gives peak in the TPO results. The peak temperature is denoted as Tmax. From Fig. 9 we see
Table 1 Cell parameters and cell volumes of La0.6Sr0.4Fe0.8Co0.2O3d prepared by different routes. a0(Ǻ) a
c0(Ǻ) a
v0(Ǻ)3 b
Polymerisable complex process (850 C) Citric acid to metal ion 3 Rhombohedral Citric acid to metal ion 4 Rhombohedral Citric acid to metal ion 5 Rhombohedral
5.4957 5.5035 5.4949
13.4296 13.4273 13.5215
351.30 352.21 353.56
Gel-combustion method (800 C)
Rhombohedral
5.5001
13.5453
354.87
Ceramic route (950 C)
Rhombohedral
5.4921
13.4112
350.32
Method
Structure
a b
0.0002. 0.1.
Fig. 7. Particle size distribution of LSCF samples calcined at 850 C for 5 h (from SAXS measurements), (a) by polymerisable complex process, (b) by gel-combustion method and (c) by ceramic route.
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Fig. 8. Thermal expansion behavior of LSCF sample prepared by different methods.
that in ceramic route reduction of Fe starts at w200 C and is completed at w475 C with Tmax at 382 C, while in the case of sample prepared by gel-combustion method reduction of Fe starts at relatively lower temperature w150 C and completes at w780 C with Tmax at w615 C. The reduction peak of Fe is sharp with low intensity (area) in the case of sample prepared by ceramic route whereas the sample prepared by gel-combustion method has broad reduction peak with relatively high intensity (area) of the signal. From the Fig. 9 we see that amount of H2 consumed (or the amount of oxygen released) is more in the case of LSCF sample prepared by gel-combustion method. The increased reducibility is considered due to high oxygen ion mobility [24], thus we can conclude that the high reducibility of LSCF sample prepared by gel-combustion method is related with high concentration of oxygen vacancies through which the migration of oxygen ion takes place [25]. This result will be supported by electrical conductivity result in the Section 3.9. Thus we see that the reduction profile of LSCF sample depends on particle morphology and crystallite size [26]. We also see that in the gel-combustion method reduction starts at relatively low temperature and amount of hydrogen consumed is also higher which indicates that sample crystallite size obtained in gelcombustion method has an important role in the reduction behavior of sample. Reduction of Fe and Co takes place at different temperatures. In the gel-combustion method reduction of Co takes place between w785 Cew1070 C with Tmax at 923 C, while in the case of sample prepared by ceramic route reduction of Co takes place in two steps with Tmax at 528 C and 713 C.Fig. 10 shows that
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Fig. 9. TPR profile for LSCF samples in flowing H2 (5%) at 6 C min1.
in the case of sample prepared by gel-combustion method simul taneous oxidation of Co and Fe starts at w100 C and is completed at w700 C with Tmax at 516 C. 3.8. Sintering behavior of LSCF powders prepared by different routes For density measurements the samples were pelletized (10 mm diameter) at the pressure of 4000 kg cm2 and dense samples were obtained by sintering the pellets at 1400 C for 6 h in air. Several of the bulk properties are related to the sintered density such as electrical conductivity and thermal expansion behavior. Dense compositions show much higher total conductivity and lower activation energy. The bulk densities of all these samples were calculated by Archimedes method. Since 1673 K is the fabrication temperature of SOFCs, it is necessary to study the property of material sintered at this temperature. Theoretical density of the samples was calculated by using the expression
dth ¼ zM=0:6023V
Table 3 Average TEC value in different temperature range for La0.6Sr0.4Fe0.8Co0.2O3d prepared by different routes. Synthesis route
Temperature range ( C)
Average TEC (1 106 K1)
Ceramic route
100e700 100e1000 700e1000
14.97 19.19 27.70
Polymerisable complex process
100e700 100e1000 700e1000
15.38 19.76 28.54
Gel-combustion Method
100e650 100e1000 800e1300
14.31 18.98 27.01 Fig. 10. TPO/TPR profiles for LSCF sample prepared by gel-combustion method.
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where M (in atomic mass units) is the mass of one formula unit, z is the number of such chemical units in one unit cell of the crystal and V (in Å3) is the volume of the crystalline unit cell as determined by XRD. The relative density was calculated by
2Co3þ 4Co2þ þ Co4þ
(1)
with an equilibrium constant as:
Relative density ¼ ðMeasured sintered density=Theoretical densityÞ 100
The highest density (97.12% of theoretical density) was observed for the powders obtained by gel-combustion method due to its superior powder properties. It is believed that soft agglomeration and high surface area of this powder results in better packing at the green stage of the pellet which helps sintering in an articulated manner in comparison to other powders. The powder obtained by polymerisable complex process gives the sintered density (94% of theoretical density) lower than the powder obtained by gelcombustion method. The powder obtained by ceramic route gives lowest sintered density (69.70% of theoretical density). This is due to hard agglomeration in the samples prepared by ceramic route which is also supported by the particle size distribution results. Due to hard agglomeration density after sintering decreases which is due to increasing attraction between particles with decreasing particle size due to the van der Waals interaction [27]. 3.9. Electrical conductivity measurements The perovskite LSCF is a mixed ionic and electronic conductor [7] due to presence of holes and oxygen vacancies. The ionic conductivity is about two orders of magnitude lower than the electronic conductivity, so it is assumed that measured value of conductivity is mainly due to electronic conductivity. According to classification of solid-state electrochemistry, SOFC cathodes lie in intercalation electrodes category. The presence of host particles and guest particles results in the conductivity. The guest particles inhabit sites within a lattice provided by host particles. The guest particles move between sites in the host lattice and the concentration of guests can be changed by adding or removing it from the host lattice [28]. Electrical conductivity measurements of sintered samples of LSCF were carried out using four-probe method. Change in conductivity behavior for the LSCF samples prepared by ceramic route, gel-combustion method and polymerisable complex process is shown in Fig. 11. From the figure we see that conductivity of all the samples increases with increasing temperature. It can be seen that the plots are almost linear at low temperatures. These results suggest that the conductivity mechanism is by thermally activated hopping of p-type small polarons between the localized states. The electron passes through the triple and tetravalent state of Co and Fe and results the electronic conductivity. Here the electrical conductivity is given by the following Arrhenius equation.
ih i.h i2 h Co3þ ¼ exp½ DGD =kT KD ¼ Co2þ Co4þ
(2)
In which expression, ΔGD is the disproportionation energy. At low temperatures, the Co ions are mainly in low-spin CoIII state (S ¼ 0). At higher temperatures, high-spin paramagnetic Co3þ(S ¼ 2) state becomes more predominant. Both CoIII and Co3þ can coexist over a considerable temperature range. As the temperature increases further, charge disproportionation occurs as stated above. Due to hoping through B2þ and B3þ site, the n-type small polarons also took part in the electrical conduction of LSCF. The charge disproportionation of Co increased both n- and p-type carriers, furthermore enhanced the electrical conductivity. We also see that at high temperature the plots show a negative deviation from the linearity which shows that the small polaron conduction is not dominating mechanism at higher temperatures. At high temperature, oxygen loss increases and consequently there is decrease in the concentration and mobility of electronic carriers [15,31]. It is supposed that the high oxygen vacancy density present at higher temperature behaves as dispersion centers, or immobilizes the electrons and thus reduces carrier mobility [32]. The thermal expansion results support the lattice oxygen loss in LSCF at high temperature which is responsible for reduction of electrical conductivity at high temperatures. From the Fig. 11 we see that LSCF sample prepared by ceramic route and sintered at 1400 C for 10 h has higher conductivity value than the sample sintered at 1200 C for 10 h. We also see that LSCF sample prepared by gel-combustion method has marginally higher conductivity value than the sample prepared by polymerisable complex process. The sample prepared by ceramic route has lowest conductivity value. This is due to the fact that sample prepared by ceramic route has much larger
s ¼ A=T expð Ea =kTÞ where A is material constant containing the carrier concentration term, Ea is the activation energy for hopping conduction, k is the Boltzmann’s constant, T is the absolute temperature. According to this equation the pre-exponential term will decrease with increase in temperature, but the exponential term will increase with temperature. The mechanism of charge disproportionation has been generally accepted in explaining electrical and magnetic properties of LaCoO3 based compounds [29,30]. It involves the transfer of d electrons 4 , e2 ) to the adjacent low-spin CoIII (t 6 ) from the high-spin Co3þ (t2g g 2g followed by forming equal number of n- and p-type carriers according to the reaction:
Fig. 11. Log (s T) vs 1000/T plot for LSCF prepared by different routes. (a) ceramic route sintered at 1200 C (b) ceramic route sintered at 1400 C(c) polymerisable complex process sintered at 1400 C (d) gel-combustion method sintered at 1400 C.
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Fig. 12. SEM micrograph of LSCF powder prepared by gel-combustion method and calcined at 900 C.
particles than that prepared by gel-combustion method and polymerisable complex process which is also supported by particle size distribution and SEM results. The higher electrical conductivity of LSCF sample prepared by gel-combustion method is due to higher bulk density (97.12% of theoretical density) of the sample. So it is clear that the method of preparation affects the conduction behavior of the sample. Lai et al. [33], have studied the composition La0.6Sr0.4Co0.8Fe0.2O3d, as compared to the composition studied by us viz., La0.6Sr0.4Co0.2Fe0.8O3d. One can expect that the cobalt-rich composition will have higher conductivity as LaCoO3 has much higher conductivity compared to LaFeO3 [34]. Consequently, the conductivity values from this paper cannot be compared to our results. The method of preparation adopted by Shao et al. [35] (Citric AcideEDTA) is different from the ones adopted in our work (solid state, gel-combustion and polymerisable complex methods). The highest conductivity observed by them is 284 S cm1 at 700 C for the samples sintered at 1200 C. The highest conductivity observed by us is quite low i.e. 130 S cm1 at 600 C for the sample prepared
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Fig. 14. SEM micrograph of LSCF sample prepared by ceramic route and sintered at 1400 C.
by gel-combustion method and sintered at 1400 C. SEM micrographs of our samples show sufficient porosity which is advantageous for SOFC applications of this material as cathode. The low conductivity could be due to increased porosity of the sample. 3.10. SEM results The SEM micrographs of LSCF samples, prepared by different methods and after giving heat treatments are shown in Figs. 12e14. The SEM micrograph of samples show that compound synthesized by different routes have different micro-structural characteristics. SEM micrograph shows that powders form loose agglomerates. The particles are bonded together by weak van der Waals forces with no significant local sintering among the particles. The particles have spherical shape. The sample prepared by different routes has different grain sizes after sintering. The sample prepared by solidstate route has much larger particles with a size of w10 mm. On the other side sample prepared by gel-combustion method gives smaller particles in the range of 800e900 nm. The micro-structural characteristics of samples determine the conduction behavior of samples due to which the sample prepared by ceramic route has the lowest conductivity while the gel-combustion method gives highest conductivity. The conductivity of sample prepared by polymerisable complex process is slightly lower than the sample prepared by gel-combustion method. SEM micrograph shows that after sintering at 1400 C the samples have necessary porous structure which is essential for a cathode material in SOFCs.
4. Conclusion
Fig. 13. SEM micrograph of LSCF sample prepared by gel-combustion method and sintered at 1400 C.
Nanocrystalline LSCF samples for application in IT-SOFCs as a cathode material have been synthesized by different processes. The synthesized samples have to be calcined at 800 C for 4 h in the case of gel-combustion process, at 850 C for 5 h in the case of polymerisable complex process and at 950 C for 10 days in ceramic route to yield phase pure products. The average crystallite size is between 15 and 17 nm for the sample prepared by gel-combustion method and polymerisable complex process as determined by Xray line broadening of diffraction patterns. The average crystallite size of LSCF sample prepared by ceramic route is 22 nm. All the
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samples have rhombohedral structure with space group R3c. As a consequence of particle size reduction a large area of interface between the electrolyte and electrodes will be obtained, which is beneficial in the fabrication of IT-SOFCs. Thermal expansion results shows that the average TEC value, for LSCF samples in high temperature range is nearly double than that in low temperature range. TPO/TPR studies shows that method of preparation affects the redox behavior of the samples. LSCF samples prepared by gelcombustion method and polymerisable process have higher conductivity value than the sample prepared by ceramic route. Acknowledgments Authors gratefully acknowledges Board of Research in Nuclear Sciences, Department of Atomic Energy, Government of India, India, (No.2007/37/17/BRNS) for providing financial support during this research work and Nanotechnology Application Center, University of Allahabad, Allahabad, for providing characterization facility for SEM and SAXS. References [1] [2] [3] [4] [5] [6] [7] [8]
N.Q. Minh, J. Am. Ceram Soc. 76 (3) (1993) 563. R.M. Ormerod, Chem. Soc. Rev. 32 (1) (2003) 17. A. Lashtabeg, S.J. Skinner, J. Mater. Chem. 16 (2006) 3161. L.A. Chick, L.R. Pederson, G.D. Maupin, J.L. Bates, L.E. Thomas, G.J. Exarhos, Mater. Lett. 10 (1990) 6. M. Kurumada, H. Hara, F. Munakata, E. Iguchi, Solid State Ionics 176 (2005) 245. M. Sahibzada, B.C.H. Steele, K. Zheng, R.A. Rudkin, I.S. Metcalfe, Catal. Today 38 (1997) 459. A. Mai, V.A.C. Haanappel, S. Uhlenvruck, F. Tietz, D. Stover, Solid State Ionics 176 (2005) 1341. J.M. Ralph, J.T. Vaughey, M. Krumpelt, Electrochem. Soc. Proc. Volume 16 (2001) 466.
[9] J.W. Kim, A.V. Virkar, K.-Z. Fung, K. Mehta, S.C. Singhal, J. Electrochem. Soc. 146 (1999) 69. [10] V.V. Kharton, A.P. Viskup, D.M. Bochkov, E.N. Naumovich, O.P. Reut, Solid State Ionics 110 (1998) 61. [11] J. Mizusaki, J. Tabuchi, T. Matsuura, S. Yamauchi, K. Fueki, J. Electrochem. Soc. 136 (1989) 2082. [12] Y. Teraoka, H.M. Zhang, K. Okamoto, N. Yamazoe, Mater. Res. Bull. 23 (1998) 51. [13] O. Yamamoto, Y. Takeda, R. Kanno, M. Noda, Solid State Ionics 22 (1987) 241. [14] H.Y. Tu, Y. Takeda, N. Imanishi, O. Yamamoto, Solid State Ionics 117 (1999) 277. [15] L.W. Tai, M.M. Nasrallah, H.U. Anderson, D.M. Sparlin, S.R. Sehlin, Solid State Ionics 76 (1995) 259. [16] D. Waller, J.A. Lane, et al., in: B. Thorstensen, U. Bossel (Eds.), Second European Solid State Forum, Proceedings, Switzerland, May 6e10 (1996), p. 737. [17] GCh Kostogloudis, Ch Ftikos, A.A. Khanlou, A. Naoumidis, D. Stover, Solid State Ionics 134 (2000) 127. [18] F. Riza, Ch Ftikos, F. Tietz, W. Fischer, J. Eur. Ceram Soc. 21 (2001) 1769. [19] M.P. Pechini, U.S. Patent No. 3, 330, 697, July (1967). [20] M.A. Gulgun, O.O. Popoola, W.M. Kriven, J. Am. Ceram Soc. 77 (2) (1994) 531. [21] H.-Y. Tian, W.-G. Luo, X.-H. Pu, P.-S. Qiu, X.-Y. He, A.-I. Ding, Thermochim. Acta. 360 (1) (2000) 57. [22] GVh Kostogloudis, P. Fertis, Ch Fitkos, J. Eur. Ceram Soc. 18 (1998) 2209. [23] V.V. Kharton, A.A. Yaremchenko, M.V. Patrakeev, E.N. Naumovich, F.M.B. Marques, J. Eur. Ceram Soc. 23 (2003) 1417. [24] H. Ullmann, N. Trofimenko, Solid State Ionics 119 (1999) 1e8. [25] M.A. Pena, J.L. Fierro, Chem. Rev. 101 (2001) 1981e2107. [26] M.R. Pai, B.N. Wani, B. Sreedhar, S. Singh, N.M. Gupta, J. Mol. Catal. A 246 (2006) 128. [27] H. Ferkel, R.J. Hellming, Nanostruct. Mater. 11 (1999) 617. [28] P.G. Bruce (Ed.), Solid State Electrochemistry, Cambridge University Press, Scotland, 1995, p. 163. [29] R.R. Heikes, R.C. Miller, R. Mazelsky, Physica 30 (1964) 1600. [30] P.M. Raccah, J.B. Goodenough, J. Appl. Phys. 19 (1968) 1209. [31] L.W. Tai, M.M. Nasrallah, H.U. Anderson, D.M. Sparlin, S.R. Sehlin, Solid State Ionics 76 (1995) 273. [32] J.B. MacChesney, R.C. Sherwood, J.F. Potter, J. Chem. Phys. 43 (1965) 1907. [33] B.K. Lai, A.C. Johnson, H. Xiong, S. Ramanathan, J. Power Sources 186 (2009) 115. [34] R.V. Wandekar, Ph.D. Thesis, Mumbai University, 2007. [35] J. Shao, Y. Tao, J. Wang, C. Xu, W.G. Wang, J. Alloys Compounds 484 (2009) 263.
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Synthesis and physicochemical characterization of nanocrystalline cobalt doped lanthanum strontium ferrite Chaubey Nityanand a, *, Wani Bina Nalin b, Bharadwaj Shyamala Rajkumar b, Chattopadhyaya Mahesh Chandra a a b
Department of Chemistry, University of Allahabad, Allahabad 211002, India Chemistry Division, Bhabha Atomic Research Center, Trombay,Mumbai 400085, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 May 2010 Received in revised form 18 January 2011 Accepted 29 January 2011 Available online 4 March 2011
Nanosized crystallites of La0.6Sr0.4Fe0.8Co0.2O3d (LSCF), a promising cathode material for Intermediate Temperature-Solid Oxide Fuel Cells (IT-SOFCs) has been synthesized by alternative methods like ceramic route, polymerisable complex process and gel-combustion method and calcined at different temperatures. X-ray diffraction studies were used for the determination of phase purity, crystal structure and average crystallite size of the samples. Microstructure of LSCF samples was studied by SEM. Temperature-programmed reduction studies were done for evaluating the redox behavior of the samples prepared by alternative methods. The electrical conductivity measurements of sintered samples were carried out at elevated temperatures using four-probe method. The electrical conductivity of sample synthesized by gel-combustion method was more as compared to sample prepared by polymerisable complex process and ceramic route. The electrical conductivity decreased at high temperature due to loss of oxygen and the formation of oxygen vacancies. Thermo-dilatometry was used to study the linear thermal expansion behavior of the samples which shows that there is an observable increase in thermal expansion at high temperatures. Ó 2011 Elsevier Masson SAS. All rights reserved.
Keywords: Perovskite oxides Cathode materials Crystal structure Electrical conductivity Thermal expansion Redox behavior
1. Introduction Solid Oxide Fuel Cell (SOFC) is a solid-state electrochemical device that converts the chemical energy of a fuel directly into electrical energy [1e4]. At present, the main targets in the development of solid oxide fuel cells to achieve commercial viability are high durability, high power density and low production costs. This can partially be obtained by operating SOFCs at lower temperatures that can accelerate the commercialization of this energy conversion technology and hence has been the prime focus of recent research and development activities. However, lowering the operating temperature not only increases ohmic losses at the electrolyte but also polarization loss at the anode and the cathode. To keep up with the performance of traditional SOFCs that operate between 900 and 1000 C, new materials with improved performance have to be used. To enhance the oxygen ion conductivity of the electrolyte at the reduced temperature La1x SrxGa1yMgyOz, gadolinia doped ceria etc. can be used to replace the yttria stabilized zirconia [5].
* Corresponding author. Tel./fax: þ91 532 2466534. E-mail address: [email protected] (C. Nityanand). 1293-2558/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2011.01.026
The polarization losses occur mainly due to the electrode overpotentials, especially at the cathode [6]. In order to prevent significant deterioration at lower temperature, cathode materials should retain their electrochemical activity and oxygen ion conductivity so as to secure sufficient activity for electrocatalytic reaction, as well as maintain a high surface exchange rate of oxygen at the gas/cathode interface [7]. Thus, selecting a high performance cathode material which is compatible with a suitable electrolyte is the major challenge faced in the development of medium temperature SOFC. The transport of oxide ion through the cathode is advantageous in enhancing the possible reaction pathways and increases the active area into the cathode volume. Sr doped lanthanum manganite is used as a cathode material in SOFCs at the operating temperature of 1000 C but at lower temperature it has high electrode resistance. The mixed ionic and electronic conductivity of the cathode can be made better by mixing an ion conducting material such as YSZ with an electronically conducting material La1xSrxMnO3d [8,9]. Another way to increase the mixed conductivity of the cathode is to substitute the conventional materials with perovskite oxides having high value of mixed conductivity [8e10]. Use of mixed ionic electronic conductor at the cathode is expected to lower the cathode polarization. Thus in the past decades, there has been growing interest in perovskite-type
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oxides with high oxygen-ionic and electronic conductivity such as La1xSrxCoO3d [11e13]. But the thermal expansion coefficient (TEC) value of La1xSrxCoO3d is relatively high and it reacts with the electrolyte YSZ above 900 C to form La2Zr2O7 and SrZrO3 leading to degradation of the cell [14]. However substitution of Co with Fe decreases the thermal expansion [15]. It was also seen that doping with Sr increases the ionic conductivity very much at the same time doping with Fe decreases the ionic conductivity to small extent [16]. Cathode materials with higher performance at lower temperature such as La0.6Sr0.4Fe0.8Co0.2O3d (LSCF) can substitute La1ySryMnO3x (LSM) the performance of which decreases rapidly when the operating temperature is below 800 C. It has been found that electrical properties of the components of SOFCs can be improved to a great extent by decreasing the size of particles to nanoscale range. In the present study an attempt has been made to synthesize nanocrystalline LSCF samples by alternative routes like polymerisable complex process, gel-combustion method and ceramic route and a comparative study of their microstructure, thermal expansion behavior and electrical conductivity was carried out. 2. Experimental 2.1. Synthesis LSCF was prepared by three different routes as given below. 2.1.1. Polymerisable complex process Stoichiometric amounts of La(NO3)3$6H2O, Sr(NO3)2, Co (No3)2$6H2O and Fe(NO3)3$9H2O were dissolved in distilled water to get a concentration of 0.03M. Concentration of citric acid solution was fixed according to the molar ratios of citric acid to total metal ions which was varied between 3 and 5. Citric acid solution was mixed with 10 mL ethylene glycol solution and was added to the metal ions solution with stirring vigorously to get a homogeneous yellow solution. The resultant solution was heated with continuous stirring at 80 C for 12 h to remove the excess solvent and advance the polymerization between citric acid and ethylene glycol. After heating for 12 h the solution became highly viscous. The color of the solution changed from yellow to orange and at last a glassy resin was formed. The resin formed was dried in hot air oven at 100 C and then calcined at higher temperatures. 2.1.2. Gel-combustion process Stoichiometric amounts of metal nitrates were dissolved in distilled water to produce transparent mixed metal-nitrate solution. Glycine (NH2CH2COOH), which is able to bind the metal ions and acts as a fuel in combustion reaction was then added to the mixed metal-nitrate solution. The molar ratio of glycine to oxidant was set to 1. The transparent aqueous solution containing metal nitrates and glycine was heated on a hot plate under stirring. It was converted to a viscous gel due to thermal dehydration and ignited to a flame at about 200 C, resulting in fine powder with evolution of large amount of gases. The large volume of gases generated during such type of auto-ignition process quickly cools the powder formed which leads to nucleation of crystallites without further growth. 2.1.3. Ceramic route Stoichiometric amounts of high purity La2O3, SrCO3, Co3O4 and Fe2O3 were used as starting materials. La2O3 was preheated at 900 C to avoid the formation of hydroxide. Precursors were ground in an agate mortar and pestle, and after grinding the mixture was pelletized, fired at 900 C for 7 days. The pellet was again cracked, reground and finally sintered at 950 C for 10 days to get the singlephase product.
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2.2. Characterization FTIR spectra of powders prepared by different processes were recorded at room temperature using a spectrometer (FTLA, 2000, Make: ABB Ltd.) with a resolution of 4 cm1. Thermal analysis was done on TG/DTA Equipment (SETRAM model 92-16.18) in air from room temperature to 1300 C. Powder X-ray diffraction data for the samples were collected on a Philips X-ray Diffractometer (PW1710) with Ni filtered Cu-K-alpha1 radiation and using silicon as an external standard. The XRD patterns were recorded in a continuous scan mode with a step width of 0.02 and at a scanning rate of 1 2q min1. The indexing of XRD patterns were done by POWD (version 2.2) program. For particle size distribution, Small Angle X-Ray Scattering (SAXS) experiments were performed on Rigaku RINTPC2000 in the region of small angle scattering 0.02e5 . Scanning Electron Microscope (JEOL JXA-8100) was used to see the grain size and consistency of the microstructure of calcined powders and ceramic specimens. SEM was operated with an acceleration voltage of 20 kV using a Tungsten anode. The samples were given 5 nm of carbon coating before taking micrographs. Thermal expansion measurements were done on SETSYS Evolution TMA 1600 (Setaram Instrumentation, France). For evaluating redox behavior as a function of sample composition, TPR profiles were recorded with a TPDRO-1100 analyser (Thermoquest, Italy) using H2 (5%) þ Ar gas mixture as a reduction medium and O2 (5%) þ He gas mixture as oxidation medium with a heating rate of 6 C/min. 20 mg of the sample was taken for TPR/TPO study. Samples were pretreated by heating in inert atmosphere at 600 C for 1 h prior to recording the TPR run. The DC electrical conductivity measurements were carried out using four-probe method with sintered bars. Pt paste (MaTeck Gmbh, Germany) was painted on the square cross-section edges of the sample and along one of the rectangular edges (separated by distance L) to form current and voltage electrodes, respectively. The sample was heat treated at 1073 K for 4 h to ensure good bonding between the electrodes and the sample. Two Pt wires attached to thin Pt foils were made to act as current contacts, and two voltage contacts were made with Pt wires connected to the voltage electrodes using Pt paste. The voltage between the two inner electrodes and the current through the sample were recorded at each temperature. The electrical conductivity s was calculated by the equation:
s ¼ LI=VA Where A is the electrode area, V is voltage between the inner electrodes separated by distance L and I is the current through the sample.
3. Results and discussion 3.1. Synthesis To prevent the precipitation occurring over the whole concentration process and to maintain the homogeneity of the metal ions in the resin on a molecular scale, the molar ratio of citric acid to metal ions should be high enough. So we have investigated minimum molar ratio of chelating ligand to total metal ions in the precursor of LSCF. When the molar ratio of citric acid to metal ions were in the range of 3e5, no visible precipitation was observed during the polymerization process. The pH value of the precursor solution was less than 2. The drying rate was kept to be moderate and drying temperature was kept below 80 C for producing a transparent intermediate resin by polymerisable complex process. The organic precursors are widely used in the synthesis of mixed oxide powders [17e19]. In the present work polyesterification
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between hydrocarboxylic acids such as citric acid, and polyhydroxy alcohols such as ethylene glycol has been used to produce phase pure cobalt substituted lanthanum strontium ferrite powders. In the ester reaction, carboxyl end of citric acid and hydroxyl end of ethylene glycol react and a water molecule is released. The acid acts as a chelating agent that stabilizes the cations dissolved in the solution. The cations are stabilized due to columbic attraction forces between the carboxylic or hydroxyl groups of the carrier materials and metal cations. Due to this stabilization, metal cations are distributed homogeneously in the solution and are stabilized in the pre-ceramic precursor after solvent removal. After calcinations and organic burn-out, an amorphous powder is obtained. At higher temperatures, crystallization of the desired phases takes place. Due to the homogeneity in molecular level, lower diffusion distances for the cations are required to obtain the desired crystal phase, hence phase pure products are obtained at lower temperature as compared to the ceramic route [20]. Glycine is one of the cheapest amino acids and contains minimum amount of carbon contents with respect to other fuels used in the gel-combustion method. The glycine molecule contains a carboxylic acid group at one end and an amine group at the other end both of which participate in the complexation of metal ions. This “zwitterionic” character allows effective complexation with metal cations of varying ionic size. Thus, glycine first forms complexes with metal cations, which increases their solubility and prevents selective precipitation as water is evaporated and later it serves as fuel for the combustion reaction being oxidized by the nitrate ions. 3.2. FTIR The FTIR spectra of the glycine along with LSCF samples prepared are shown in Figs. 1 and 2 to see the completion of the combustion reaction. The comparison of the spectra in Fig. 2 shows that glycine has been destroyed completely during the combustion process, since no characteristics peaks of glycine was found in the sample
Fig. 2. FTIR spectra of LSCF samples prepared by gel-combustion process. (A) calcined at 150 C( B) calcined at 800 C, (C) Glycine.
prepared by combustion method. A broad absorption band around 3450 cm1 appeared in the IR spectra of the samples prepared by both the methods which are characteristics of absorbed water or hydroxyl group (OeH stretching) in alcohol. It also shows absorption bands in the vicinities of 1725 and 1190 cm1, which can be attributed to monodentate ligand of metal with carbonyl groups (COO) of citric acid in the case of samples prepared by polymerisable complex process. In the case of samples prepared by polymerisable complex process a band w925 cm1 is due to symmetric vibrations of CeOeC bonds and trace amount of nitrate ions was indicated by the band around 1400 cm1. All these bands disappears from FTIR spectrum of the sample heated at higher temperatures of 800e850 C, indicating that the impurities have been removed completely. The small bands appearing between 800 and 500 cm1 are mainly due to metal oxide bond [21]. 3.3. TG-DTA The various stages of removal of adsorbed moisture, organics, decarboxylation reactions, combustion, and crystallization can be understood from these thermal analysis results. Fig. 3 shows simultaneous TG/DTA curve of LSCF precursors prepared by gelcombustion method. Weight loss of about 7% occurred between 30 and 200 C which could be attributed to the loss of absorbed moisture in the sample. This weight loss is accompanied by a broad endothermic peak around 160 C. Again a weight loss of 16% occurred between 200 and 900 C which is likely due to decomposition of precursors to oxides, loss of carbon residue by oxidation and also from decomposition of residual nitrates. In the DTA plot an exothermic peak is present at 310 C, followed by a broad endothermic peak which can be attributed to oxidation of carbon reside and decomposition of precursors to oxides to form the perovskite oxide. Similarly decomposition path of LSCF precursors prepared by polymerisable complex process is also delineated. The formation of perovskite oxide starts at 400 C and completes at 850 C, which is also supported by XRD results. 3.4. XRD
Fig. 1. FTIR spectra of LSCF samples prepared by polymerisable complex process at molar ratio of citric acid to metal ions ¼ 4, calcined (A) at 850 C (B) at 150 C; at molar ratio of citric acid to metal ions ¼ 5 and calcined (C) at 850 C (D) at 150 C.
The synthesized powders were calcined in air at higher temperatures to investigate the evolution of crystalline phases. As shown in Fig. 4. LSCF samples prepared by polymerisable complex process give the rhombohedral phase after heat treatment at 850 C for 5h while the LSCF sample prepared by gel-combustion process
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Fig. 3. TG/DTA pattern of LSCF precursor prepared by gel-combustion method.
gives the rhombohedral phase at 800 C for 4h. In the ceramic route, perovskite phase was formed after heating at 900 C for 7 days along with traces of impurity phases of La2O3 (PCPDF no. 741144). In this method phase pure perovskite was formed only after heating at 950 C for 10 days as shown in Fig. 5. These observations can be explained as follows: The solid state or ceramic method involves the mechanical mixing of the solid constituents (oxides/carbonates), repeated grinding and annealing at elevated temperatures over a long duration. The solid-state reactions are diffusion controlled reactions and follow a parabolic law given as
dx=dt ¼ kx1 where ‘x’ is the degree of reaction (thickness of the product layer), t is the time and k is a constant. Since the reactants are high melting refractory solids it is only at high temperatures that the ions have sufficient thermal energy to enable them to jump off their normal lattice sites and diffuse through the crystals. Initially the reaction is faster and thereafter the rate becomes slow as the reactants have to diffuse through the product layer that is formed in order to come in
Fig. 5. XRD pattern showing the formation of LSCF sample by ceramic route (P perovskite phase,# impurity phase of La203)
contact with one another. The reactants are ground and mixed thoroughly as the subsequent reaction rate depends to a large extent on the particle size of reactants, the degree of homogenization achieved on mixing and the intimacy of contact between the grains as well as the effect of temperature. Thus the rate-limiting step is diffusion of ions through the product layer. In gel-combustion and polymerisable complex process, there is homogenization of ions at molecular level and hence the reaction rates are faster and one gets phase pure composition at relatively lower temperature. In ceramic method of preparation, we observed unreacted La2O3 as impurity phase. Grinding the partially reacted powder and reheating brings fresh surfaces in contact and the unreached La2O3 gets into the lattice of partially formed sample. Thus, the lines due to La2O3 disappear on repeated grinding and reheating. The average crystallite size of samples prepared by gelcombustion method and polymerisable complex process was evaluated from x-ray line broadening analysis using the Scherrer equation, which is between 15 and 17 nm. The average crystallite size of LSCF sample prepared by ceramic route is 22 nm. In this work we get rhombohedral perovskite La0.6Sr0.4Fe0.8Co0.2O3ed with space group R3c synthesized by different routes. Fig. 6 shows the XRD pattern of LSCF samples prepared by different routes after heating at 1400 C in air. All the samples have rhombohedral structure and average crystallite size between 27 and 31 nm. The lattice parameters of La0.6Sr0.4Fe0.8Co0.2O3ed sample prepared by different routes and after giving heat treatment at different temperatures are given in Tables 1 and 2. The lattice parameters have been given in hexagonal cell dimensions. From the Fig. 6 we see that the small impurity phase of SrLaCoO4 (PCPDF no.832412) was present in sample prepared by gel-combustion method and heated at 1400 C. 3.5. Particle size distribution
Fig. 4. XRD pattern of LSCF samples: A to C are prepared by polymerisable complex process at molar ratio of citric acid to metal ions 3, 4 and 5 respectively and calcined at 850 C for 5 h, D is prepared by gel-combustion process and calcined at 800 C for 4 h and E is prepared by ceramic route.
The particle size distribution is used to determine the agglomeration of particles in the samples. The data measured by Small Angle X-ray Scattering were examined for determination of size and size distributions of nanocrystalline LSCF samples obtained after heat treatment. From Fig. 7, it is clear that the agglomeration is between 2 nm and 29 nm for LSCF sample prepared by
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C. Nityanand et al. / Solid State Sciences 13 (2011) 1022e1030 Table 2 Cell parameters and cell volumes of La0.6Sr0.4Fe0.8Co0.2O3d prepared by different routes and heated at 1400 C. Method
Structure
a0(Ǻ) a
c0(Ǻ) a
v0(Ǻ)3 b
Polymerisable complex process Gel-combustion method Ceramic route
Rhombohedral Rhombohedral Rhombohedral
5.4674 5.4955 5.4944
13.4436 13.4112 13.4035
348.03 350.77 350.41
a b
Fig. 6. XRD pattern of LSCF synthesized by different route and calcined at 1400 C in air, (# impurity phase of SrLaCoO4).
polymerisable complex process and between 3 nm and 40 nm for the same prepared by gel-combustion process. For ceramic route the agglomeration is between 6 and 43 nm. The particles were found to be spherical in shape. From the Fig. 7c we can see that the sample prepared by ceramic route contains hard agglomerates in comparison to the samples prepared by other routes which in turn affect the sintered density and microstructure that can be seen from bulk density and SEM results. The samples prepared by polymerisable complex process and gel-combustion method have narrow particle size distribution in comparison to ceramic route as can be seen from Fig. 7a and b. 3.6. Thermal expansion results The linear thermal expansion curves for LSCF sample prepared at 1400 C by different methods are shown in the Fig. 8. All the thermal expansion curves are non-linear with inflection occurring w650e800 C. The thermal expansion curves were fitted by two straight lines. There is a change in the slope in the expansion behavior in higher temperature range that is the expansion takes place at higher rate in the high temperature range. The value of average linear thermal expansion coefficient (a) of LSCF samples prepared by different routes is given in the Table 3 in the different temperature ranges. The average TEC value in high temperature range is nearly double than that in low temperature range in all the samples. The change in slope observed in high temperature range in the thermal expansion behavior is considered due to loss of lattice oxygen and the formation of oxygen vacancies [22,23].
0.0002. 0.1.
Simultaneously thermal reduction of cations in B site occurs from high valence state of Co4þ and Fe4þ to lower trivalent states Co3þ and Fe3þ to maintain the electrical neutrality. The valence changes are followed by an increase of the ionic radius particularly for the reduction of Co4þ (0.067 nm) to Co3þ (0.075 nm). Due to these reductions, a decrease in the BeO bond occurs according to Pauling’s second rule, and in this way the size of BO6 octahedra increases which enhance the lattice expansion [22]. The thermal expansion coefficient of LSCF sample in the temperature range 100e700 C is 14.97 106 K1 and 15.38 106 K1 for ceramic route and polymerisable complex process respectively and for gel-combustion process it is 14.31 106 K1 in the temperature range 100e650 C. 3.7. TPR/TPO TPR/TPO studies of LSCF samples prepared by different methods were done to observe the redox behavior of LSCF, which is used as a cathode material for solid oxide fuel cells. This method is based on the temperature-programmed reduction and oxidation of materials and is used for the release and uptake of oxygen from lattice of cathode materials which is responsible for change of stoichiometry. We may mention that Fe and Co are only reducible species in LSCF sample. Substitution of La on the A site by Sr at constant stoichiometry contributes to a change of valence for some of the iron atoms from Fe3þ to Fe4þ due to the aliovalence of Sr2þ vs La3þ. Since Fe4þ reduces easily to Fe3þ which in turn increases the movement of oxygen ions and oxygen vacancies are formed upon heating the material in He atmosphere and result in a peak in the TPR results. The oxygen vacancies in the perovskite material are filled when it is heated in an oxygen atmosphere which gives peak in the TPO results. The peak temperature is denoted as Tmax. From Fig. 9 we see
Table 1 Cell parameters and cell volumes of La0.6Sr0.4Fe0.8Co0.2O3d prepared by different routes. a0(Ǻ) a
c0(Ǻ) a
v0(Ǻ)3 b
Polymerisable complex process (850 C) Citric acid to metal ion 3 Rhombohedral Citric acid to metal ion 4 Rhombohedral Citric acid to metal ion 5 Rhombohedral
5.4957 5.5035 5.4949
13.4296 13.4273 13.5215
351.30 352.21 353.56
Gel-combustion method (800 C)
Rhombohedral
5.5001
13.5453
354.87
Ceramic route (950 C)
Rhombohedral
5.4921
13.4112
350.32
Method
Structure
a b
0.0002. 0.1.
Fig. 7. Particle size distribution of LSCF samples calcined at 850 C for 5 h (from SAXS measurements), (a) by polymerisable complex process, (b) by gel-combustion method and (c) by ceramic route.
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Fig. 8. Thermal expansion behavior of LSCF sample prepared by different methods.
that in ceramic route reduction of Fe starts at w200 C and is completed at w475 C with Tmax at 382 C, while in the case of sample prepared by gel-combustion method reduction of Fe starts at relatively lower temperature w150 C and completes at w780 C with Tmax at w615 C. The reduction peak of Fe is sharp with low intensity (area) in the case of sample prepared by ceramic route whereas the sample prepared by gel-combustion method has broad reduction peak with relatively high intensity (area) of the signal. From the Fig. 9 we see that amount of H2 consumed (or the amount of oxygen released) is more in the case of LSCF sample prepared by gel-combustion method. The increased reducibility is considered due to high oxygen ion mobility [24], thus we can conclude that the high reducibility of LSCF sample prepared by gel-combustion method is related with high concentration of oxygen vacancies through which the migration of oxygen ion takes place [25]. This result will be supported by electrical conductivity result in the Section 3.9. Thus we see that the reduction profile of LSCF sample depends on particle morphology and crystallite size [26]. We also see that in the gel-combustion method reduction starts at relatively low temperature and amount of hydrogen consumed is also higher which indicates that sample crystallite size obtained in gelcombustion method has an important role in the reduction behavior of sample. Reduction of Fe and Co takes place at different temperatures. In the gel-combustion method reduction of Co takes place between w785 Cew1070 C with Tmax at 923 C, while in the case of sample prepared by ceramic route reduction of Co takes place in two steps with Tmax at 528 C and 713 C.Fig. 10 shows that
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Fig. 9. TPR profile for LSCF samples in flowing H2 (5%) at 6 C min1.
in the case of sample prepared by gel-combustion method simul taneous oxidation of Co and Fe starts at w100 C and is completed at w700 C with Tmax at 516 C. 3.8. Sintering behavior of LSCF powders prepared by different routes For density measurements the samples were pelletized (10 mm diameter) at the pressure of 4000 kg cm2 and dense samples were obtained by sintering the pellets at 1400 C for 6 h in air. Several of the bulk properties are related to the sintered density such as electrical conductivity and thermal expansion behavior. Dense compositions show much higher total conductivity and lower activation energy. The bulk densities of all these samples were calculated by Archimedes method. Since 1673 K is the fabrication temperature of SOFCs, it is necessary to study the property of material sintered at this temperature. Theoretical density of the samples was calculated by using the expression
dth ¼ zM=0:6023V
Table 3 Average TEC value in different temperature range for La0.6Sr0.4Fe0.8Co0.2O3d prepared by different routes. Synthesis route
Temperature range ( C)
Average TEC (1 106 K1)
Ceramic route
100e700 100e1000 700e1000
14.97 19.19 27.70
Polymerisable complex process
100e700 100e1000 700e1000
15.38 19.76 28.54
Gel-combustion Method
100e650 100e1000 800e1300
14.31 18.98 27.01 Fig. 10. TPO/TPR profiles for LSCF sample prepared by gel-combustion method.
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where M (in atomic mass units) is the mass of one formula unit, z is the number of such chemical units in one unit cell of the crystal and V (in Å3) is the volume of the crystalline unit cell as determined by XRD. The relative density was calculated by
2Co3þ 4Co2þ þ Co4þ
(1)
with an equilibrium constant as:
Relative density ¼ ðMeasured sintered density=Theoretical densityÞ 100
The highest density (97.12% of theoretical density) was observed for the powders obtained by gel-combustion method due to its superior powder properties. It is believed that soft agglomeration and high surface area of this powder results in better packing at the green stage of the pellet which helps sintering in an articulated manner in comparison to other powders. The powder obtained by polymerisable complex process gives the sintered density (94% of theoretical density) lower than the powder obtained by gelcombustion method. The powder obtained by ceramic route gives lowest sintered density (69.70% of theoretical density). This is due to hard agglomeration in the samples prepared by ceramic route which is also supported by the particle size distribution results. Due to hard agglomeration density after sintering decreases which is due to increasing attraction between particles with decreasing particle size due to the van der Waals interaction [27]. 3.9. Electrical conductivity measurements The perovskite LSCF is a mixed ionic and electronic conductor [7] due to presence of holes and oxygen vacancies. The ionic conductivity is about two orders of magnitude lower than the electronic conductivity, so it is assumed that measured value of conductivity is mainly due to electronic conductivity. According to classification of solid-state electrochemistry, SOFC cathodes lie in intercalation electrodes category. The presence of host particles and guest particles results in the conductivity. The guest particles inhabit sites within a lattice provided by host particles. The guest particles move between sites in the host lattice and the concentration of guests can be changed by adding or removing it from the host lattice [28]. Electrical conductivity measurements of sintered samples of LSCF were carried out using four-probe method. Change in conductivity behavior for the LSCF samples prepared by ceramic route, gel-combustion method and polymerisable complex process is shown in Fig. 11. From the figure we see that conductivity of all the samples increases with increasing temperature. It can be seen that the plots are almost linear at low temperatures. These results suggest that the conductivity mechanism is by thermally activated hopping of p-type small polarons between the localized states. The electron passes through the triple and tetravalent state of Co and Fe and results the electronic conductivity. Here the electrical conductivity is given by the following Arrhenius equation.
ih i.h i2 h Co3þ ¼ exp½ DGD =kT KD ¼ Co2þ Co4þ
(2)
In which expression, ΔGD is the disproportionation energy. At low temperatures, the Co ions are mainly in low-spin CoIII state (S ¼ 0). At higher temperatures, high-spin paramagnetic Co3þ(S ¼ 2) state becomes more predominant. Both CoIII and Co3þ can coexist over a considerable temperature range. As the temperature increases further, charge disproportionation occurs as stated above. Due to hoping through B2þ and B3þ site, the n-type small polarons also took part in the electrical conduction of LSCF. The charge disproportionation of Co increased both n- and p-type carriers, furthermore enhanced the electrical conductivity. We also see that at high temperature the plots show a negative deviation from the linearity which shows that the small polaron conduction is not dominating mechanism at higher temperatures. At high temperature, oxygen loss increases and consequently there is decrease in the concentration and mobility of electronic carriers [15,31]. It is supposed that the high oxygen vacancy density present at higher temperature behaves as dispersion centers, or immobilizes the electrons and thus reduces carrier mobility [32]. The thermal expansion results support the lattice oxygen loss in LSCF at high temperature which is responsible for reduction of electrical conductivity at high temperatures. From the Fig. 11 we see that LSCF sample prepared by ceramic route and sintered at 1400 C for 10 h has higher conductivity value than the sample sintered at 1200 C for 10 h. We also see that LSCF sample prepared by gel-combustion method has marginally higher conductivity value than the sample prepared by polymerisable complex process. The sample prepared by ceramic route has lowest conductivity value. This is due to the fact that sample prepared by ceramic route has much larger
s ¼ A=T expð Ea =kTÞ where A is material constant containing the carrier concentration term, Ea is the activation energy for hopping conduction, k is the Boltzmann’s constant, T is the absolute temperature. According to this equation the pre-exponential term will decrease with increase in temperature, but the exponential term will increase with temperature. The mechanism of charge disproportionation has been generally accepted in explaining electrical and magnetic properties of LaCoO3 based compounds [29,30]. It involves the transfer of d electrons 4 , e2 ) to the adjacent low-spin CoIII (t 6 ) from the high-spin Co3þ (t2g g 2g followed by forming equal number of n- and p-type carriers according to the reaction:
Fig. 11. Log (s T) vs 1000/T plot for LSCF prepared by different routes. (a) ceramic route sintered at 1200 C (b) ceramic route sintered at 1400 C(c) polymerisable complex process sintered at 1400 C (d) gel-combustion method sintered at 1400 C.
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Fig. 12. SEM micrograph of LSCF powder prepared by gel-combustion method and calcined at 900 C.
particles than that prepared by gel-combustion method and polymerisable complex process which is also supported by particle size distribution and SEM results. The higher electrical conductivity of LSCF sample prepared by gel-combustion method is due to higher bulk density (97.12% of theoretical density) of the sample. So it is clear that the method of preparation affects the conduction behavior of the sample. Lai et al. [33], have studied the composition La0.6Sr0.4Co0.8Fe0.2O3d, as compared to the composition studied by us viz., La0.6Sr0.4Co0.2Fe0.8O3d. One can expect that the cobalt-rich composition will have higher conductivity as LaCoO3 has much higher conductivity compared to LaFeO3 [34]. Consequently, the conductivity values from this paper cannot be compared to our results. The method of preparation adopted by Shao et al. [35] (Citric AcideEDTA) is different from the ones adopted in our work (solid state, gel-combustion and polymerisable complex methods). The highest conductivity observed by them is 284 S cm1 at 700 C for the samples sintered at 1200 C. The highest conductivity observed by us is quite low i.e. 130 S cm1 at 600 C for the sample prepared
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Fig. 14. SEM micrograph of LSCF sample prepared by ceramic route and sintered at 1400 C.
by gel-combustion method and sintered at 1400 C. SEM micrographs of our samples show sufficient porosity which is advantageous for SOFC applications of this material as cathode. The low conductivity could be due to increased porosity of the sample. 3.10. SEM results The SEM micrographs of LSCF samples, prepared by different methods and after giving heat treatments are shown in Figs. 12e14. The SEM micrograph of samples show that compound synthesized by different routes have different micro-structural characteristics. SEM micrograph shows that powders form loose agglomerates. The particles are bonded together by weak van der Waals forces with no significant local sintering among the particles. The particles have spherical shape. The sample prepared by different routes has different grain sizes after sintering. The sample prepared by solidstate route has much larger particles with a size of w10 mm. On the other side sample prepared by gel-combustion method gives smaller particles in the range of 800e900 nm. The micro-structural characteristics of samples determine the conduction behavior of samples due to which the sample prepared by ceramic route has the lowest conductivity while the gel-combustion method gives highest conductivity. The conductivity of sample prepared by polymerisable complex process is slightly lower than the sample prepared by gel-combustion method. SEM micrograph shows that after sintering at 1400 C the samples have necessary porous structure which is essential for a cathode material in SOFCs.
4. Conclusion
Fig. 13. SEM micrograph of LSCF sample prepared by gel-combustion method and sintered at 1400 C.
Nanocrystalline LSCF samples for application in IT-SOFCs as a cathode material have been synthesized by different processes. The synthesized samples have to be calcined at 800 C for 4 h in the case of gel-combustion process, at 850 C for 5 h in the case of polymerisable complex process and at 950 C for 10 days in ceramic route to yield phase pure products. The average crystallite size is between 15 and 17 nm for the sample prepared by gel-combustion method and polymerisable complex process as determined by Xray line broadening of diffraction patterns. The average crystallite size of LSCF sample prepared by ceramic route is 22 nm. All the
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samples have rhombohedral structure with space group R3c. As a consequence of particle size reduction a large area of interface between the electrolyte and electrodes will be obtained, which is beneficial in the fabrication of IT-SOFCs. Thermal expansion results shows that the average TEC value, for LSCF samples in high temperature range is nearly double than that in low temperature range. TPO/TPR studies shows that method of preparation affects the redox behavior of the samples. LSCF samples prepared by gelcombustion method and polymerisable process have higher conductivity value than the sample prepared by ceramic route. Acknowledgments Authors gratefully acknowledges Board of Research in Nuclear Sciences, Department of Atomic Energy, Government of India, India, (No.2007/37/17/BRNS) for providing financial support during this research work and Nanotechnology Application Center, University of Allahabad, Allahabad, for providing characterization facility for SEM and SAXS. References [1] [2] [3] [4] [5] [6] [7] [8]
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