Electrochemical Properties of Ce0.5Sr0.5 (Co0.8Fe0.2)1−xZrxO3−δ Cathode Materials for Low Intermediate Temperature Solid Oxide Fuel Cells (LIT-SOFCs) Synthesized by Sol-Gel Method
Ce0.5Sr0.5 (Co0.8Fe0.2)1−x ZrxO3−δ (CSCFZ) powders were synthesized by the sol-gel method and characterized to study structural and electrochemical properties. X-ray diffractometer (XRD) patterns of all samples give nanosized particles of a high-degree crystalline cathode having a cubic-type perovskite structure of space group Pm-3m with the existence of oxygen vacancies in the lattices. The results have the perovskite phase with average crystallite sizes of 26.57 nm, 18.14 nm, 18.13 nm, and 18.12 nm with porosities of 9.93%, 9.87%, 9.50%, and 9.08% for x = 0, 0.1, 0.15, and 0.2, respectively. Scanning electron microscope (SEM) micrographs showed the presence of pores on the microstructure. Average grain sizes of prepared samples found from SEM images were in the range of 105.30–183.02 nm. The partial substitution of zirconium at the B-site shows more stable materials than the host without decreasing the porosity that much. The results of electronic conductivity analyzed by the four-probe dc technique show that the conductivity of synthesized materials increases with the increment of both dopant concentration and temperature by the decrement of area specific resistances. The electrical conductivity of CSCFZ steadily increased with the increment of temperature which reached 42.76 Scm−1 at around 450°C.
Using fossil fuels as energy sources cause environmental issues by affecting the atmosphere. These environmental issues affect human life and which leads to the innovation of alternative energy sources such as fuel cells. The usage of a safe, desirable, and clean energy carrier instead of fossil fuels has a substantial impact on diminishing air pollution . Among fuel cells, SOFCs are the most attractive due to their fuel flexibility and high energy conversion efficiency on high operating temperatures [2, 3]. However, high operation temperature puts strict requirements on SOFCs component materials which lead to rapid degradation of the cells . Research on the preparation and characterization of nanostructured materials is well known as a significant field in recent science and technology [4, 5]. These nanometer-sized inorganic materials can represent an extensive range of optical and electrical properties . The science and engineering of nanometer-sized materials is currently employed for the development of many scientific, industrial, ecological, and technological fields . Biology, medicine, chemistry, pharmacy, agriculture, food industry, and materials science are the main fields that have been benefited from the great technological progress developed in nanoscience .
SOFCs convert the chemical energy of fuels straightforwardly into electrical energy with high efficiencies and minimal emissions. The three components of the fuel cell are an electrolyte layer in contact with a porous anode and cathode on either side . To decrease the cost of SOFCs, there is significant interest in lowering the operating temperature of SOFCs . SOFCs are based on O2- conducting electrolytes and give a chance to use higher hydrocarbons directly without a reforming step . Porosity plays a great role in the overall cell performance. Perovskite-type cathode materials have been widely studied as cathode materials for SOFCs due to the possibility of both A-site and B-site doping . Their structure offers wide flexibility to improve the properties of materials, such as catalytic activity, chemical stability, and thermal property .
Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF5080) has reached high oxygen permeability and was predictable to be structurally stable based on the evaluation of the Goldschmidt tolerance factor and is the main component in many following studies . Although BSCF shows excellent electrochemical performance, it has a high thermal expansion coefficient (TEC) with the value of 20 × 10−6K−1 between 50°C and 1000°C which is problematic for cells using this cathode material due to a mismatched TEC with those of other cell components . Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) at 700°C was developed by using two sintering approaches. The reactive sintered body of hexagonal-phase BSCF obtained by the one-step process showed higher electrical conductivity of 6.35 Scm−1 at 500°C than the two-step sintered BSCF of electrical conductivity 4.15 Scm−1 .
Darb, in 2008, synthesized and characterized Ba0.5Sr0.5Co0.2Fe0.8O3−δ and Ba0.5Sr0.5Co0.8Fe0.2O3−δ, among which the second composition has shown to be promising regarding physical properties . Composite cathodes containing Ba0.5Sr0.5Co0.8Fe0.2O3−δ and La0.6Sr0.4Co0.8Fe0.2O3−δ have lower ASR than BSCF and LSCF cathodes . The values of ASR are 0.46 and 2.77 Ω·cm2 at 700°C and 600°C, respectively, for the composite cathode Ba0.5Sr0.5Co0.8Fe0.2O3−δ and La0.6Sr0.4Co0.8Fe0.2O3−δ in 1 : 1 weight ratio. Compared to other cathode materials, the electronic conductivity of LSCF is very high with a maximum value of 215.58 Scm−1 at 823 K, whereas BSCF has a much lower electronic conductivity 14.87 Scm−1 at 773 K . The TEC of BSCF is larger than the TEC of LSCF [17, 18]. Comparing with LSCF, BSCF is a better cathode material because it has the lowest surface resistance.
According to Yusof and Rahman, BSCF has been reported as a good and effective cathode material due to high conductivity, excellent oxygen transport, catalytic activity, and a potential candidate for a low-temperature solid oxide fuel cell (LT-SOFC) . In their study, the nanocomposite cathode BSCF-samarium-doped ceria (SDC) was developed for LT-SOFC, and the behavior of the nanoceramic powders was examined . The results obtained by their study confirm that selecting a suitable calcination temperature is important in eliminating secondary phases in the BSCF-SDC composite cathode and enhancing its properties. These secondary phases come from the reaction of the cathode and electrolyte because of a mismatch in the thermal expansion coefficient.
Perovskite Sr1−xCexCoO3−δ (0.05 ≤ x ≤ 0.15) has been prepared by a sol-gel technique and studied for intermediate temperature solid oxide fuel cell cathodes . As SOFC cathodes, Sr1−xCexCoO3−δ materials have sufficiently high electronic conductivities and excellent chemical compatibility with SDC electrolyte . At 1100 K, the oxygen atoms in Sr0.95Ce0.05CoO3−δ show large and highly anisotropic displacement factors, suggesting significant ionic mobility . SOFCs have faced many challenges in commercializing them. Though most of the problems associated with performance and material issues have been resolved to a certain extent, commercial applications of SOFCs still suffer from high costs and high-temperature requirements . Research studies regarding SOFCs are focused on the range 500–700°C; the slow cathode reaction in this range is the source of many energy losses .
Perovskite cathodes, LSCF and BSCF, have good electronic conductivity but their TECs are not matched with the electrolyte . Replacing barium or lanthanum on A-site with cerium improves the problem by lowering the TEC of the cathode, but its electronic conductivity is not good because of the formation of unreacted secondary CeO2 peak for a concentration of cerium >15% . However, more concentration of cerium (50%) leads to giving good migration of charge carriers by creating more oxygen vacancies. However, if there is any unreacted Ce due to excess doping, it will be balanced by doping other elements in B-site.
The addition of high valence elements on B-site improves the cubic structure of the base material, which in turn increases electronic conductivity of the cathode . Although there were many cathodes used (such as LSM, LSC, LSCF, and BSCF), the thermo-mechanical suitability between electrolyte and cathode can be improved by mixing cathode material with a new element or a small impurity such as zirconium and titanium . Such composite cathode will have better electronic conductivity, and the overpotential of the cathode can be reduced by adding zirconium or titanium .
In this work, we prepared perovskite Ce0.5Sr0.5 (Co0.8Fe0.2)1−x ZrxO3−δ (CSCFZ) oxide by a sol-gel technique. The crystal structure of Ce0.5Sr0.5 (Co0.8Fe0.2)1−x ZrxO3−δ and the crystalline nature of the materials were investigated by X-ray diffraction (XRD). Its electrochemical performance as a cathode for SOFCs was examined in cathodic symmetrical cells. The relationship between the structure and properties of Ce0.5Sr0.5 (Co0.8Fe0.2)1−x ZrxO3−δ as a cathode for SOFCs has been correlated, and the microscopic reason for the observed good performance has been revealed.
2.1. Material Preparation Method
Cathode materials of Ce0.5Sr0.5 (Co0.8Fe0.2)1−xZrxO3−δ (CSCFZ) for x = 0, 0.1, 0.15, and 0.2 were synthesized by the sol-gel method. The raw materials and ingredients used are powders of the following AR-grade Sigma-Aldrich and Otto, India chemicals, as shown in Table 1.
Five grams of CSCFZ materials were chosen as target products. The individual nitrates of precursor powders were mixed by weighing properly with a digital microbalance and dissolved in distilled water. A stoichiometric amount of citric acid (1 : 1 ratio with a mole of nitrates), which plays the role of gelation agent, was dissolved in distilled water and mixed with the solution of metal nitrates by continuously stirring with a magnetic stirrer without heat. Then, the solution is stirred on a magnetic stirrer with a hot plate. To avoid the appearance of hydroxides and base salts at higher pH values, it is important checking the pH value is ∼7. The pH of the solution was adjusted by adding ammonia solution drop by drop using a saker and seeing the color of the pH paper by dropping a small amount of the solution by saker on it. After adjusting the pH of the solution, it was slowly heated for 6 hrs at 80°C over a temperature-controlled magnetic stirrer with a hot plate until the solution becomes a viscous solid called gel. During the reaction, the mixture is stirred continuously at high speed to avoid any agglomeration and for preparing the homogenous solution. After this, continue heating with reduced stirring speed for few minutes. Then, the gel is dropped from the beaker to the ceramic bowl and heated in a hot air oven for 7 hrs at 300°C until the gel becomes black ash. The presintered powders are grinded for 1 hr continuously with an agate mortar. The pre-sintering temperature was taken for 3 hrs at 800°C and final sintering was taken for 5 hrs at 950°C. The pellets were made by using a hydraulic press with stainless steel die by applying a load of 12 ton (having 9 mm diameter and 2 mm thickness). In the end, the prepared pellets were sintered at 950°C for 5 hrs in the furnace by putting it in a closed alumina crucible to get the final composition and the desired pure phase Perovskite before characterization and measurement.
X-ray diffraction had been carried out to study the crystalline nature and phase purity of all four samples. The data was taken by Xpert Philips XRD using Cu-Kα radiation with a wavelength of λ = 0.1542 nm at a scanning rate of 20/min with a Cu anode of 40 kV and 30 mA. Scanning was taken for 2θ values ranging from 20°C to 80°C. Scanning electron microscopy (SEM) is applied to investigate the grain morphology of prepared samples. A high-resolution SEM (ZEISS) was used to see the morphology of synthesized samples and to relate them with the physical behaviors of the particles used as a cathode material. In the study, the SEM analysis was carried out by using the sintered pellets of the samples. Electrical conductivity for all synthesized samples was measured by the four-probe (contacts) dc method.
2.3. Fabrication and Testing of Cells
Two grams of the powder was taken with 0.04 gram methylcellulose (2% of the powder) and grinded for 30 minutes continuously. SDC-based electrolytes were fabricated by the sol-gel technique and pelletized by a hydraulic press with stainless steel die by applying a load of 12 ton (having 9 mm diameter and 2 mm thickness) and sintered at 1200°C for 5 hrs. Then, CSCFZ layers with submicron size were first coated on the surfaces of the electrolyte support using a screen-printing technique. Slurries of the cathode were fabricated using a mixture of α-terpineol and methylcellulose. Finally, the slurries of cathode materials were coated on top of the SDC layer and fired at 900°C for 5 hrs to study electrical properties. To verify the electrochemical performances of the cathodes, screen printed circular SDC with active areas of 0.741, 0.752, 0.743, and 0.7435 cm2 per unit length for concentrations of x = 0, 0.1, 0.15, and 0.2 were constructed. The configuration of the cell was cathode and solid SDC electrolyte. For the electrochemical tests, Ag was pasted on the surface of the cathode as a current collection.
3. Results and Discussion
3.1. XRD Analysis
Crystallographic characterization of Ce0.5Sr0.5 (Co0.8Fe0.2)1−x ZrxO3−δ (x = 0, 0.1, 0.15, 0.2) was carried out at room temperature. Figure 1 shows the XRD patterns of the as-synthesized Ce0.5Sr0.5 (Co0.8Fe0.2)1−x ZrxO3−δ (x = 0, 0.1, 0.15, 0.2) at room temperature, which shows pure, well-crystallized perovskite phases, indexed in a cubic unit cell for both materials. The crystallinity nature of the materials increased as the doping of zirconium increases. The impurity peak (I) that came from unreacted CeO2 minimized from the figures for high concentration of Zr and the sharpness of peaks increased. XRD reveals high-degree crystalline cathodes with cubic-type structure having the space group of Pm-3m. Ten symmetrical diffraction peaks were observed in the XRD spectra (111) for 2θ = 28.47, (200) for 2θ = 33.26, (211) for 2θ = 41.35, (220) for 2θ = 47.63, (221) for 2θ = 49.99, (311) for 2θ = 56.50, (222) for 2θ = 59.28, (321) for 2θ = 66.17, (400) for 2θ = 68.48, and (331) for 2θ = 76.81. This XRD pattern matches with standard single-phase cubic perovskite structure having the space group Pm-3m (JCPDS File No.: 109462) .
From XRD peaks, lattice parameters were calculated using UnitCellWin software. The mean crystallite size was calculated from the peaks broadening by using the Debey Scherrer equation from XRD data :
The density of the samples increased as partial substitution of lighter elements with heavier elements increases due to the concentration of dopant Zr on the B-site. As a result, the porosity of samples decreases by a small amount due to dopant. The materials are porous than earlier findings which are around 5% . Even though porosity is one of the key criteria for SOFC cathode materials, the stability of material also plays a great role in the performances of SOFCs. The stability depends on the crystallinity nature of a material, and it was observed that the crystallinity nature of the samples increased as the concentration of dopant on the B-site increases. The variation of parameters is more likely a compensatory response of the crystal structure to accommodate dopants of different ionic sizes as compared to those of host ions which causes the shrinkage of a crystal lattice that leads to a decrease in the values of lattice constants.
3.2. SEM Analysis
As it is clearly seen from Figure 2, the crystal consists of a periodic arrangement of the unit cell into a lattice and exhibited complete densification with a presence of pores on the surface. The average grain sizes in each of the synthesized series get smaller when the dopant concentration increases as also evident from XRD data. Further reduction in average grain sizes of particles for doped samples in Figure 2 is owing to the incorporation of the dopant in perovskite cathodes, and it is in the range of 105.3025–183.025 nm. The average grain sizes of all the samples were calculated by the linear intercept technique.
3.3. Electrochemical Analysis
Electrical conductivities are the sum of electronic and oxygen ionic conductivity, owing to the co-presence of electron-hole pairs and oxygen vacancies. It is well known that the ionic conductivity is much lower than electronic conductivities in perovskite oxides. Therefore, it can be assumed reasonably that the measured conductivity values refer to the electronic conductivities alone .
The electrical conductivity of CSCF for x = 0, steadily increased with temperature and then reached a level of approximately 15.62 Scm−1 at 450°C, while that of CSCFZ continuously increased with temperature up to 42.76 Scm−1 at the same temperature. The electrical conductivities are in a good range as shown in earlier findings , and it is much better than findings obtained by Yamaguchi et al. at 500°C  within a temperature range of interest (250–450°C). Increment of temperature leads to high electronic conductivity of the cathode by decreasing the interface resistance, and this in turn leads to high electrochemical performance. Thus, the cathodic resistance of the materials would be lower.
The temperature dependence of electronic conductivity of all the prepared samples was presented in Figure 3 in the form of ln (σT) versus . The value of activation energies was calculated from the slope of the graphs and have a value of 0.83, 0.81, 0.77, and 0.72 eV for x = 0, 0.1, 0.15, and 0.2, respectively, which is less than the values of earlier findings of 1.15 eV [26, 30].
An effective measure of the electrochemical performance of CSCFZ cathodes is area specific resistance (ASR) also called polarization resistances, which can be obtained from four-probe measurements on a symmetrical cell at various temperatures. The decrease in performance was followed by an increase in the ASR of the cell. Figure 4 shows that ASR values of CeZr0, CeZr1, CeZr2, and CeZr3 (x = 0, 0.1, 0.15, and 0.2) cathodes were measured on temperature range of 250–450°C in air. ASR of CSCFZ cathodes is decreased with the rising of temperature and have values ranging from 0.098–0.1295 Ω·cm2 to 250–450°C temperature range for concentration x = 0.2. From this point of view, the concentration x = 0.2 CSCFZ material can be considered as better cathodic candidates. The obtained ASR results are much lower than previous literature reported on the cells 1.74 Ω·cm2 at 600°C [31–33], showing that this cathode can be used in the LIT-SOFCs and are in the range of 0.07–0.3 Ω·cm2 : where s is DC conductivity, t is the thickness of the sample, R is the resistance of the sample, and A is the area of the sample .
As it is clearly seen from Table 3, electronic conductivity is increased with the increment of both doping concentration and temperature. As it was seen from X-ray diffraction patterns, the presence of a secondary phase (CeO2) leads to low electrical conductivity because the appearance of this additional phase (peak) leads to an increase in the overall resistance of the button cell. Therefore, CSCFZ at x = 0 concentration exhibits the poorest performance among all the cathode materials prepared and characterized.
Ce0.5Sr0.5 (Co0.8Fe0.2)1−xZrxO3−δ (CSCFZ) powders for x = 0 and (0.1 ≤ x ≤ 0.2) in a step sizes increment of 0.05 had been synthesized by the sol-gel method and characterized to explore their properties to use as a cathode material for low intermediate temperature solid oxide fuel cells (LIT-SOFCs). The crystal structure, microstructure, thermal properties, and electronic conductivity of the synthesized samples have been determined by X-ray diffraction (XRD), scanning electron microscopy (SEM), and the four-probe dc technique. The XRD result gives that all samples are single phases with a cubic perovskite-type structure with an average crystallite size of 26.57 nm, 18.14 nm, 18.13 nm, and 18.12 nm for x = 0, 0.1, 0.15, and 0.2, respectively. From the results obtained by XRD data, the lattice parameters of the prepared samples were 5.42293 Å, 5.40527 Å, 5.40213 Å, and 5.40200 Å. The samples were porous with porosities of 9.93%, 9.87%, 9.50%, and 9.08%. Results of SEM micrograph show that the crystals consist of a periodic arrangement of the unit cell into a lattice and exhibit complete densification with a presence of pores in the surface with the average grain sizes ranging from 105–183 nm. The obtained results from the four-probe show ASR is in the range of 0.07–0.3Ω·cm2 and decreases with the increment of temperature and dopant concentration which leads to the increase of electronic conductivity of the prepared cathode materials in the range of 9.51–42.76 Scm−1 for a temperature range of (250–450°C). The increase of zirconium concentration has a positive effect on electronic conductivity by minimizing/removing the appearance of the unwanted secondary phase (CeO2).
The data used to support the findings of this study are included within the article and are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
The authors would like to express their gratitude to the department of physics and chemistry of Osmania University and central university, Hyderabad, India, for allowing to use their laboratory facilities.
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