Enhancement of the interfacial polarization resistance of La0.6Sr0.4Co0.2Fe0.8O3-δ cathode by microwave-assisted combustion method
Introduction
Solid oxide fuel cells (SOFCs) have been considered as the most promising system in generating electrical energy for commercial applications because of their high efficiency, fuel flexibility, and absence of noble catalytic metals [1]. SOFCs operate at very high temperatures of 800–1000 °C. The development of SOFCs at operating temperatures ranging from 400 °C to 800 °C has been extensively investigated [2], [3]. New materials, including nano-perovskite-type materials, Ba1−xSrxCo0.8Fe0.2O3-δ [4], layered perovskite-related structures, Lan+1NinO3n+1 [5], and double-perovskite GdBaCoO5+δ [6], have been proposed as cathode components for SOFCs. These materials demonstrated potent performance at low temperatures. Among these materials, La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) is the most suitable candidate as a cathode material for intermediate temperature (IT-SOFC). LSCF is a perovskite-type material with high electronic and oxygen ion conductivity and shows good chemical stability with ceria-based electrolytes at operating temperatures [7]. The typical composition of a LSCF cathode material exhibits a low cathode polarization resistance of 0.036 Ω cm2 and a maximum power density of 744.6 mW/cm2 at 750 °C in an anode-supported single cell configuration using NiO-YSZ as an anode, YSZ as an electrolyte, and GDC as an interdiffusion barrier layer [8].
The performances of the mixed ionic–electronic conducting perovskite cathodes depend on chemical composition, powder properties, and processing temperature [9]. Powder properties, including microstructure or particle size, surface area, and phase purity, are influenced by synthesis or preparation methods [10], [11]. LSCF powders prepared through various methods and calcined at different temperatures present diverse morphological characteristics. The sensitivity and sintering temperature of preparation methods can also influence the performance of LSCF cathodes [12], [13]. Thus, an appropriate synthesis method should be adopted to achieve maximum efficiency. Numerous preparation techniques and approaches have been used and developed to improve the performance of LSCF cathodes. For example, LSCF materials have been synthesized through solid-state reaction [14], [15], spray pyrolysis [16], [17], [18], citrate–gel process [19], glycine–nitrate process (GNP) [20], [21], combined ethylenediaminetetraacetic acid (EDTA)–citrate process [22], [23], [24] and co-precipitation [25]. Each method generally produces powders with different properties, particularly the particle size, particle-size distribution, surface area, purity, and particle agglomeration degree.
The solution combustion technique is a simple synthesis method, that is, a self-sustaining combustion process to instantly produce ultrafine ceramic powders with improved shape, size, and properties, at low calcination temperature. Different ignition techniques have been developed to initiate combustion synthesis [26]. For example, microwave energy is successfully used as an initiating technique because of its enhanced uniform reaction medium, accelerated reaction rates and reduced reaction time [27], [28], [29]. Poli et al. [30] proposed the use of microwave energy to address process intensification by producing a pure NiAl intermetallic phase through combustion synthesis. Microwave ignition in combustion synthesis is different from the conventional heating tools, such as hotplate, oven, or muffle furnace; the microwave is used to transfer the required amount of electromagnetic energy and to convert this energy into heat through the polarization and movement of reactants [31]. Compared with conventional heating, microwave heating demonstrates a narrow distribution of the required energy to the reactive species even at a molecular level [32]. Therefore, heat is generated internally in reactants through microwave rather than ignition from external sources. Moreover, a thermal gradient can be prevented by the internal heating properties during microwave processing; thus, microwave heating provides a uniform environment for chemical reactions with very high heating rates [33].
The use of fuels and complexing agents in the solution combustion synthesis helps produce ultrafine, nanosized, homogeneous, high-porosity, and highly reactive ceramic oxide powders [23]. In the solution combustion synthesis, glycine, urea, citric acid, oxalyl hydrazine, and sucrose are commonly used as fuels to produce perovskite powder. GNP involves glycine and metal nitrates as fuel and oxidizers, respectively. However, the use of microwave energy has been rarely investigated to produce SOFC components. Ces´ario et al. [34] and Sun et al. [35] were the first to report the combustion synthesis of La0.6Sr0.4Co0.2Fe0.8O3-δ perovskite-type cathode and Ce0.8Sm0.2O1.9 (SDC) electrolyte by using urea and glycine as fuels, respectively, and by heating the reactants with microwave energy. We employed a microwave-assisted combustion method based on these advantages, using glycine as fuel to synthesize LSCF powders.
This study aimed to investigate the effects of combustion techniques, namely, microwave and conventional processes, on the phase formation, microstructure, surface area, and electrochemical properties of LSCF through GNP. The microstructure of the sintered symmetrical pellets was examined through field emission scanning electron microscopy (FESEM). Area-specific resistance (ASR) was also analyzed through electrochemical impedance spectroscopy (EIS) from 400 °C to 800 °C in air.
Section snippets
Synthesis of LSCF nanopowder
LSCF powder was prepared via glycine–nitrate combustion technique. Analytical-grade La(NO3)3·6H2O (99%), Sr(NO3)3 (99%), Co(NO3)2·6H2O (99%), and Fe(NO3)3·9H2O (99%) were used as metal salts. Glycine was used as fuel to initiate the combustion reaction. Metal salts were purchased from Merck Chemicals, Germany, whereas glycine was purchased from Friendemann Schmidt Chemicals, Germany. All reagents were used as received without further purification.
The metal nitrate precursor solution was
Thermogravimetry analysis (TGA)
TGA was performed to examine the decomposition behavior of LSCF powders produced through microwave and conventional heating. The TGA curves of M-LSCF and H-LSCF were measured as a function of temperature (30–1000 °C), as shown in Fig. 1(a) and 1(b), respectively. The weight loss stages of the TGA curve indicated that the uncalcined LSCF powder gradually underwent thermal decomposition as the temperature increased, and most changes in the TGA plot of the samples occurred at approximately 60–850
Conclusion
Pure single perovskite-phase LSCF powders were successfully prepared through a microwave-assisted GNP. The XRD patterns confirmed the formation of pure single perovskite phase at a low calcination temperature (800 °C for 5 h). The cathode prepared through microwave heating contained a fine porous structure with nanoparticles. The symmetrical cell based on LSCF powder prepared through microwave heating exhibited lower interfacial resistance at all operating temperatures than the cell based on the
Acknowledgement
This work was supported by the Universiti Kebangsaan Malaysia, Malaysia and the Ministry of Science, Technology, and Innovation Malaysia via the research sponsorship of GUP-2016-045 and 03-01-02-SF1079. The authors would like to extend their gratitude to the Center for Research and Instrumentation Management for support and to UKM for excellent testing equipment.
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