1. INTRODUCTION:

Solid oxide fuel cells SOFC gain significant interest in recent years due to its potential in generation of electrical energy via environmental friendly approach. SOFC is an electrochemical device that utilizes the chemical energy of a fuel and directly coverts it into electrical energy. SOFCs have several advantages such as a high efficiency, multi-fuel capability and co-generation over the other fuel cells and are commonly operated at high temperature (800-1000 °C).¹ However, the high temperature of SOFCs causes both physical and chemical degradation of the SOFCs cell materials.² Recently, there has been a considerable amount of research and development to reduce the operating temperature at an intermediate level ~600-800 °C.³ Therefore, IT–SOFC system requires not only new cathode and electrolyte materials but also sophisticated microstructure development of the cathode.

Lanthanum Strontium Manganite (LSM) is widely used as a cathode material in SOFCs because of its excellent mechanical, thermal and chemical stability at high temperature.⁴ In order to decrease the resistance as well as cell operating temperature, cathode materials which have higher conductivity than LSM in the intermediate temperature range should be used.⁵ LSCF-based materials are considered to be suitable candidates for intermediate temperature application.⁶

In the present study, we synthesized La_0.6Sr_0.4Co_0.3Fe_0.8O₃, (LSCF) materials with a high conductivity using gel-combustion method. We studied the characteristics of synthesized LSCF powder for microstructural and electrochemical performance by using SEM, XRD, FTIR, TGA-DTA, and impedance spectroscopy.

2. MATERIAL AND METHODS:

2.1. Chemicals and reagents:

All of the reagents were of analytical grade with the mass fraction purity of 0.99 and used as received without further purification. La(NO₃)₃.6H₂O and FeCl₃·6H₂O were purchased from Thomas Baker, Mumbai, India. Sr(NO₃)₂.6H₂O, Citric acid, and Ethylene glycol, NaOH and HNO₃ were purchased from E. Merck, Mumbai, India and Co(NO₃)₂.6H₂O were purchased from Alfa Aesar, Great Britain, UK.

2.2 Synthesis of LSCF:

In a typical synthesis of LSCF precursor, La(NO₃)₃.6H₂O (25.98 g), Sr(NO₃)₂.6H₂O (8.46 g), Co(NO₃)₂·6H₂O (5.82 g) , Fe(NO₃)₃·9H₂O (32.31 g) were first dissolved and mixed in a 100 mL of distilled water by magnetic stirring at room temperature. After complete dissolution of the metal nitrates fixed amount of citric acid, CA (19.22 g) as chelating agent and ethylene glycol, EG (6.21g) as esterification agent were added into the metal nitrate solution. The molar ratio of metal: citric acid: ethylene glycol was maintained at 1:1.5:0.5. The resultant solution was heated and stirred at 75 ^°C for 3hrs that result in formation of dark brown gel like matter due to the chemical formation of polymerization resin. Initially, the metal nitrate solution containing CA and EG underwent boiling and dehydrated, followed by decay which accompanied by the release of large amounts of gases. The mixture had taken on a more viscous appearance on continuing heating and lastly results in formation of complex polymerized resin. The combined complex precursor was heated at 80 ^°C for 10 h followed by calcinations at different temperature of 400 ^°C, 700 ^°C and 900 ^°C for 3 h to give black colored LSCF powder.

2.3 Characterization of synthesized LSCF:

The synthesized LSCF was characterized by BET surface area Micromeritics-2020, X-ray diffraction technique for the characterization of minerals. XRD was recorded on X'PERT-PRO diffractometer operated at 40 kV/30 mA, using CuK_α₁ radiation with a wavelength of 1.54 Å in the wide angle region from 20 to 80° on 2θ scale. FTIR (Perkin Elmer FT-IR spectrometer) was carried out to analyze functional groups, using potassium bromide (KBr) disk method. SEM studies were carried out using a scanning electron microscope (JEOL SEI) at electron acceleration voltages of 15 kV. The thermal stability was studied by thermo gravimetric analysis (TGA), under a dynamic air atmosphere at a heating rate of 10 min⁻¹using a Perkin Elmer TGA instrument. Impedance spectra were measured by means of Solartron machine, performed at a temperature of 400-900 °C with temperature intervals of 50 °C.

3. RESULTS AND DISCUSSION:

The specific surface areas were measured using BET analysis to obtain information about the granularity of the synthesized powder. As a result, the specific surface area of the as-synthesized powders was 47 m² g^-1, with this value decreasing to 30, 17, 15 and 13 m² g^-1 with calcination temperature of 80, 400, 700 and 900 °C, respectively. Thus, the LSCF powders with less grain growth and high specific surface area could be fabricated at the temperature of under 700 °C. However, the BET area of LSCF powders at 800 °C decreased to about 17 m² g^-1, indicating the suitable powder size for preparing slurry of cathode electrode.

Fig. 1. shows the XRD pattern of the as synthesized powder and the powder calcined at different temperature. The obtained peaks are in good agreement with rhombohedral perovskite structure of LSCF powder. It was further observed that the peak intensity and width increases simultaneously with the increase of calcined temperature from 80 °C to 900 °C. This also indicates about the enhancement of crystallinity with increase of temperature. The average crystallite size of sample was also evaluated from mathematical expression given by Scherrer. The equation given as:

D = 0.94 λ / β cos θ

The average crystallite size calculated for four different temperatures at 80 ⁰C, 400 ⁰C, 700 ⁰C and 900 ⁰C as 72, 63, 44 and 29 nm, respectively.

Fig. 1. XRD spectra of synthesized LSCF nanomaterials at different temperatures.

FTIR spectra of the citric acid along with LSCF sample prepared are displayed in Fig. 2. Abroad and intense absorption band appear at around 3450 cm^-1 indicated the presence of absorbed water or OH stretching group in alcohol. It can be clearly observed from the spectra that with increase of temperature from 80 °C to 900 °C the intensity of the OH peak decreases considerably which further indicates about the loss of absorbed water from the sample at higher temperature. The presence of peak at 1731 cm^-1 attributed to asymmetric C=O stretching while symmetric C=O appear at 1423 cm^-1.⁷ These peaks, however, seems to disappear with the increase in temperature. The small band appearing between 800 to 500 cm^-1 mainly due to the metal oxide interaction.⁸

Fig. 2.FTIR spectra of synthesized LSCF nanomaterials at different temperatures.

The SEM micrograph of the LSCF powder prepared at 400 and 900 °C depicted in Fig. 3. which can be observed that the powder form at 700 °C the powder form loose agglomerate which may be due to the van der Waal force that exist among the particles. The particles are mainly spherical shape with a nanomatric size. However, with the increase of calcination temperature upto 900 °C the weakly agglomerated particle starts producing dense agglomerated particles. The estimated average particle size of the LSCF powder calcined at 400 and 900 °C are 19 and 58 nm respectively. It has been also observed that changed in calcination temperature significantly influences specific surface area of LSCF nanopowder, as the calcination temperature increases the particle size of LSCF nanopowder also increases but the specific surface area reportedly decreases. The specific area of the LSCF powder investigated at 400 and 900 °C is 38.2 and 16.6 m²/gm respectively. The LSCF powder with high specific surface area and smaller particle size is highly desirable for cathode material which results in large triple phase boundary.⁹

Fig. 3. SEM images of synthesized LSCF nanomaterials.

The TG/DTA curve of LSCF precursors prepared by polymerisable complex process has been given in Fig. 4. The measure weight loss seems to occur in two steps. In first step weight loss of about 44% occur between 50 and 250 °C which could be corroborated to the loss of absorbed moisture in the sample. In second step weight loss of about 57% is observed upto 400 ⁰C which was also accompanied by the exothermic peak at 360 °C in the DTA curve which correspond to the decomposition of precursors to oxides to form the perovskite oxide. Fig. 4. exhibit about the formation of perovskite oxide starts at 400 °C and completes at 600 °C.

Fig. 4. TGA/DTA of synthesized LSCF nanomaterials at different temperatures.

Fig. 5. Impedance spectra of LSCF powders at two different temperatures.

The impedance spectra for the LSCF samples, sintered at 1000 °C for 4 hours were recorded at different temperature from 400 °C to 900 °C. The representative spectrum of LSCF prepared by gel combustion route at 400 °C and 900 °C is shown in Fig. 5. The each figure there exist two semi circles which correspond to bulk boundary and green boundary of as synthesized LSCF powder. The total conductivity of LSCF samples at different temperature of 400 °C and 900 °C were calculated to be 0.89 Scm^-1 and 0.94 S cm^-1, respectively. The calculated conductivity value of samples found to be much higher than reported value 0.065 and 0.034 S cm^-1 at 800 °C and 700 °C, respectively, by ¹⁰ for the sample LSGM. Arrhenius plot of conductivity of LSCF samples is given in Fig. 6. It reveals that ionic conductivity of the sample increases with increasing temperature. The temperature dependence of conductivity plots are according to Arrhenius law and conductivity can be represented as follow:

σ = (A/T) exp (-Ea/kT)

where A is the pre-exponential factor, T is the absolute temperature, k is the Boltzmann’s constant, and Ea is the activation energy.

Fig. 6. Arrhenius plot of [log (σT)] versus 1,000/T for LSCF sample synthesized by gel- combustion method

According to above equation the plot of [log (σT)] versus 1,000/T for the LSGM sample is presented in Fig.6.The activation energy of total conduction estimated from the Arrhenius plot of LSGM sample prepared by gel combustion method is ~0.68 eV and the value of pre exponential factor is 2.23 x 10⁶. It can be revealed from Fig. 6 that the Arrhenius plot show significant curvature at about 700 ^◦C and the activation energy at low temperature found to be greater than that at high temperature. In present case the plot comprises of two prominent linear regions. The initial three data point belongs to low temperature region and last three data points corresponds to high temperature region. The non-linear plots for Arrhenius model indicated substantial change in dominance of ionic conductivity from intergrain conductivity at low temperature to intragrain conductivity at high temperature. At high temperature the intergrain conductivity contributed marginally to the total conductivity than the intragrain conductivity while the intergrain conductivity is truly responsible for ionic conductivity at a low temperature.¹¹ At low and high temperature linearity of the plots change and thus the activation energy change with increasing temperature. The change in activation energy can be noticed at 700 °C. Activation energy for low temperature is 0.76 eV and for high temperature it is estimated to be 0.68 eV. The activation energy also provides insight about the migration energy responsible for the oxygen vacancies. At the high temperature range it has been found that all the oxygen vacancies are free and the activation energy reflects only the migration energy for the oxygen vacancies.¹² Moreover the increase in electrical conductivity with increasing temperature may be attributed to the small polar conduction.

4. CONCLUSIONS

The advanced LSCF cathode material was synthesized by the modified gel-combustion method. The investigation of microstructural detail suggests that surface area, crystallinity and crystallite size decreases with increasing temperature from 80 ⁰C – 900 ⁰C. Study on thermal analysis reveals that perovskite structure formation starts at 400 ⁰C and finally completes at 600 ⁰C. The conductivity measurement suggests that the as-synthesized LCSF cathode powder shows reasonably high electrical conductivity of 0.89 S cm^-1 obtained at intermediate temperature of 700 °C. Therefore the study suggests that the as-synthesized cathode material LCSF can perform suitably at intermediate temperature.

5. ACKNOWLEDGEMENTS

Raj Mani is thankful to UGC, New Delhi for the award of SRF under Rajiv Gandhi National Fellowship. R.K. Gautam and S. Banerjee are grateful to the UGC and CSIR for the award of SRF. The authors gratefully acknowledge, Department of Physics, University of Allahabad, Allahabad, for providing characterization facility for Electrochemical Impedance Spectra.

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Received on 01.05.2015 Modified on 16.05.2015

Asian J. Research Chem 8(6): June 2015; Page 389-393

DOI: 10.5958/0974-4150.2015.00062.0