INFLUENCE OF CHEMICAL AGENT IN SYNTHESIZING STRONTIUM- DOPED LANTHANUM COBALTITE POWDER
Adbullah Abdul Samat1, Mahendra Rao Somalu1, Andanastuti Muchtar1, 2 and Nafisah Osman3
1Fuel Cell Institute, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia
2Department of Mechanical Engineering, Faculty of Built and Environment, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia
3Faculty of Applied Sciences, Universiti Teknologi MARA, 02600 Arau, Perlis, Malaysia
Corresponding author: [email protected]
ABSTRACT
A simple low temperature synthesis route has been performed in preparing single perovskite phase of La0.6Sr0.4CoO3-δ (LSCO) for cathode application in intermediate temperature proton conducting solid oxide fuel cell (SOFC). A sol-gel method assisted with a combined chelating agent namely citric acid and ethylenediaminetetraacetic acid (EDTA) was used in this study. Ethylene glycol (EG) and activated carbon (AC) were employed as a combined surfactant/dispersion agent. The synthesized powders were characterized by XRD, SEM/EDS and PSA for phase formation, microstructure and particle size analysis, respectively. The results of the analysis have been presented in this paper accordingly.
Keywords: LSCO; cathode; modified sol-gel; combined surfactant/dispersion agent;
INTRODUCTION
Solid oxide fuel cells (SOFCs) have attracted huge attention worldwide due to the demand for clean, secure and renewable energy. It offers few remarkable advantages including high energy conversion efficiency, low/zero pollutant and fuel flexibility.
Unfortunately, its high cost limits commercial use for the high operating temperature.
Thus, the reduction of working temperature from high to intermediate temperature of SOFCs becomes the urgent demand for broad commercialization. Intermediate temperature SOFCs (IT-SOFC) especially proton conducting SOFCs (H+-SOFC) attract much interest for application at intermediate temperature. It has some advantages as compared with oxygen conducting SOFC (O2--SOFC) such as low activation energy, high energy efficiency and avoid fuel dilution [1]–[3].
At the intermediate temperature, two technical barriers which need to overcome in order for IT-SOFCs performance to be comparable to that in high temperature are ohmic resistance of electrolyte and relatively low catalytic activity of electrodes, especially cathode [4], [5]. The development of proper cathode materials for intermediate temperature H+-SOFC in order to improve materials compatibility and interfacial polarization resistance still remains a challenge. Many cobalt-containing perovskite- type mixed ionic and electronic conductors (MIECs) have been extensively studied as the cathode materials. One of them is strontium-doped lanthanum cobaltite, La0.6Sr0.4CoO3-δ (LSCO). It has shown good catalytic activity and low polarization resistance at intermediate temperature [6], [7]. However, the performance on the cathode materials depends on the way how they are being produced. Different method of synthesis will result in different microstructure properties of cathode, which then determine its performance such as gas diffusion and electrocatalytically reaction [8], [9].
Wet chemical methods (WCMs) are more favorable for cathode preparation since smaller particle size, better purity and higher surface areas can be obtained. These good properties then leading to a better cathode performance [8], [10]. One of the WCMs is sol-gel technique. In this technique, employed chemical agents such as surfactant or dispersion agent will affect the particle size of the produced powder. In our previous work [11], we have employed ethylene glycol (EG) as surfactant and activated carbon (AC) as dispersion agent, separately. The average particle size obtained was 751.2 nm and 1046.3 nm for EG and AC, respectively. Since our intention is to produce nano size cathode particles, we did some modification on this sol-gel technique by combining EG and AC as a combined surfactant/dispersion agent. The effect of the combination of the surfactant and dispersion agent compared to the single surfactant or dispersion agent on particle size is presented in our present work. The results obtained in this present study are expected to be the important findings in achieving better cathode properties for better performance of intermediate temperature H+-SOFC.
EXPERIMENTAL
Powder Synthesis
La0.6Sr0.4CoO3-δ (LSCO) powder was prepared via a modified sol-gel method. Citric acid and ethylenediaminetetraacetic acid (EDTA) was used as a combined chelating agent; ethylene glycol (EG) and activated carbon (AC) were used as a combined surfactant/dispersion agent. Analytical grade of metal nitrate salts of lanthanum, La(NO3)3.6H2O, strontium, Sr(NO3)2 and cobalt, Co(NO3)2.6H2O were used as precursor materials. A mixture of these precursor materials was firstly dissolved in 100 mL deionized water. Then, calculated amounts of citric acid and EDTA were slowly added into the solution mixture, accordingly. After that, pH of the solution was adjusted
% 100 phase
Perovskite
m
I I I
p p
rate of 5 ºC min-1 for 5 hours. The produced powders were labelled as EG/AC700, EG/AC800, EG/AC900 and EG/AC1000, respectively.
Powder Characterization
X-ray Diffractometer (XRD Bruker D8 Advance) using a monochromatic Cu-Kα radiation source (λ = 0.1540558 nm) with Ni-filtered was used to confirm the phase formation of LSCO perovskite powder. The XRD was operated at 40 kV and 40 mA for the 2 theta (θ) ranging from 20º to 80º. The percentage of perovskite phase present in the calcined powders was calculated by using the following equation:
(1)
The Ip refers to the maximum intensity of the perovskite phase and Im is the maximum intensity of the impurity phases. Morphology of the powder was examined by a Merlin Compact Zeiss Field Emission Scanning Electron Microscope (FESEM) equipped with Energy Dispersive X-ray Spectrometer (EDS). The imaging was performed in Secondary Electron (SEI) mode using an accelerating voltage of 15 kV at magnification of 50000 times. Particle size distribution analysis of the powders was done using Nano- ZS Particle Size Analyzer (PSA) manufactured by Malvern using deionized water as dispersant.
RESULTS AND DISCUSSION
The XRD patterns of LSCO powders after calcined at different calcination temperatures are shown in Figure 1. The strongest peaks of all the XRD patterns were matched with JCPDS file no. 48-0121 with cubic (Pm-3m) structure. As the calcination temperature increases, the peaks intensity in the XRD patterns was enhanced and become dominant phase. A single perovskite phase of LSCO was obtained at calcination temperatures of 900 °C and 1000 °C using EG and AC as a combined surfactant/dispersion agent. The calcined powders at 700 °C and 800 °C showed weak crystallinity of the LSCO perovskite phase since some impurities phases which are cobalt oxide (CoO), strontium cobalt oxide (SrCoOx) and strontium oxide (SrO) were detected in their XRD patterns.
A summary of phase formation analysis was presented in Table 1. The results obtained in the present work are in line with the results obtained from our previous work [11]
which used EG and AC as a separate surfactant and dispersion agent, respectively. A single perovskite phase of LSCO was also obtained at calcination temperature of 900 °C as shown in Figure 2. The powders were labelled as EG900 and AC900, respectively.
Figure 1: XRD patterns of the LSCO powders prepared using combined surfactant/dispersion agent after calcined at different temperatures
Figure 2: XRD patterns of a single perovskite phase of LSCO obtained using combined and separate surfactant and/or dispersion agent at 900 °C
Table 1: A summary of phase formation analysis of LSCO powders after calcined at different temperatures
Calcination Temperature (°C)
Crystalline Phase Percentage of Perovskite Phase (%)
700 LSCO, CoO, SrO, SrCoOx 67.41
800 LSCO, CoO, SrO, SrCoOx 85.33
900 LSCO 100.00
1000 LSCO 100.00
Figure 3(a) shows the SEM image of the single perovskite phase of LSCO powder after calcined at 900 °C (EG/AC900). Figure 3(b) and 3(c) shows the SEM images of the single perovskite phase of EG900 and AC900, respectively. It can be seen that the powder consist of homogeneous and almost identical particles. The particles formed well-bonded porous network and well connected with each other. Figure 4 shows EDS spectrum of the EG/AC900 powder. The figure shows the presence of lanthanum (La), strontium (Sr), cobalt (Co), oxygen (O) and carbon (C). The elemental composition distribution analysis of the powder is presented in the Table 2. It can be seen that the calculated mol fraction obtained from EDS measurement was close to the nominal fraction. The high residual carbon content might be due to the incomplete removal of carbon from activated carbon used.
Figure 3: SEM images of (a) EG/AC900 powder; (b) EG900 and (c) AC900 at magnification of 50Kx
Figure 4: EDS spectrum of EG/AC900 powder
Table 2: EDS data of weight percentage and mol fraction for EG/AC900 powder
Element wt% mol Calculated mol fraction Nominal mol fraction
La 34.56 0.25 0.61 0.60
Sr 11.64 0.13 0.32 0.40
Co 24.32 0.41 1.00 1.00
1.93 2.00
O 15.34 0.96
C 14.14 1.18
Table 3 shows the comparison of average particle size diameter of EG/AC900 powder with EG900 and AC900 powders. It is clearly seen that the particle size is significantly decreased when the combined surfactant/dispersion agent was employed in producing the powder. It has smaller particle size compared those which were prepared using single surfactant and dispersion agent. The discrepancy between the results might be due to the different properties of the surfactant and dispersion agent employed as they have different molecular structure and different relative bond strength towards metal cations during synthesizing process. Additionally, the reaction mechanism of both
Table 3: Average particle size diameter of the produced LSCO powders
Sample Average particle size diameter (nm)
EG/AC900 508.1
EG900 751.2
AC900 1046.3
CONCLUSIONS
LSCO powder was successfully prepared using a modified sol-gel method by employing ethylene glycol (EG) and activated carbon (AC) as a combined surfactant/dispersion agent. A single perovskite phase of the LSCO was obtained at calcination temperatures of 900 °C and 1000 °C. The produced powder (EG/AC900) consists of homogenous and well-distribution of particles. The employment of the combined surfactant/dispersion agent significantly reduce the particle size as the produced powder has smaller particle size as compared to those which have been prepared using single surfactant and dispersion agent, respectively. The outcomes from this study are expected to embark a significant knowledge in optimizing the microstructure of sample in order to produce better cathode properties for better performance of intermediate temperature H+-SOFC.
ACKNOWLEDGEMENT
The authors would like to thank the Ministry of Education (MOE) of Malaysia for the Fundamental Research Grant Scheme (FRGS/2/2013/TK06/UKM/02/9 and FRGS/1/2014/SG06/UiTM/02/2) and Universiti Kebangsaan Malaysia (UKM) via research sponsorship of DLP-2014-004. The first author is thankful to MOE and Universiti Malaysia Perlis (UniMAP) for the SLAB/SLAI scholarship. Facilities support from Centre for Research and Instrumentation Management (CRIM) of UKM and Universiti Teknologi MARA (UiTM) is gratefully acknowledged.
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