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WITHDRAWN: Synthesis and characterization of cobalt-free SrFe0.8Ti0.2O3-δ cathode powders synthesized through combustion method for solid oxide fuel cells
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Synthesis and characterization of cobalt-free SrFe0.8Ti0.2O3-d cathode powders synthesized through combustion method for solid oxide fuel cells Nurul Akidah Baharuddin a, Andanastuti Muchtar a,b,*, Mahendra Rao Somalu a, Noor Shieela Kalib a, Nor Fatina Raduwan a a
Fuel Cell Institute, Universiti Kebangsaan Malaysia, 43600, UKM Bangi, Selangor, Malaysia Centre for Materials Engineering and Smart Manufacturing (MERCU), Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia
b
article info
abstract
Article history:
Cobalt-free SrFe0.8Ti0.2O3-d cathode were synthesized through combustion method. Results
Received 25 May 2017
of the thermogravimetric and Fourier transform infrared analyses suggested that a
Received in revised form
perovskite oxide started to form at temperatures above 1100 C. X-ray diffraction and
11 September 2017
Rietveld refinement analyses confirmed that the single-phase cubic structure (Pm-3m) of
Accepted 15 January 2018
SrFe0.8Ti0.2O3-d cathode was produced after calcination at 1300 C. The average sizes of the
Available online xxx
particles were 1.0827 and 1.4438 mm as revealed by field emission scanning electron microscopy and dynamic light scattering analysis, respectively. In addition, energy dispersive
Keywords:
X-ray analysis coupled with mapping revealed the homogenous distribution of elements in
Solid oxide fuel cell
the cathode. The thermal expansion coefficient of the SrFe0.8Ti0.2O3-d cathode is
Cobalt-free cathode
16.20 106 K1. For the electrochemical behavior, the area specific resistance of cathode
Perovskite
(0.60e13.57 U cm2) was obtained at 600e800 C and the activation energy is 121.77 kJ mol1.
Particle size
This work confirmed the potential of SrFe0.8Ti0.2O3-d cathode in intermediate temperature
Area specific resistance
solid oxide fuel cell. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Solid oxide fuel cells (SOFCs) are a type of an energy converter device with a high operating temperature of approximately 1000 C [1]. Reduction of the operation temperature to the intermediate range (600e800 C) is important in minimizing material cost and in avoiding cell degradation [2e4]. However, this approach reduces the oxygen reduction reaction rate in
the cathode component due to the existing material limitation (lanthanum strontium manganite); thus, new materials have been proposed for intermediate-temperature SOFC cathodes [5]. Perovskite cathode was introduced to fill this research gap. Perovskite oxide is suitable as intermediate-temperature SOFC due to its mixed ionic-electronic conducting property; through this property, the oxygen reduction reaction not only occurs at triple phase boundary (the specific point where cathode, electrolyte, and gas meet) but also throughout the
* Corresponding author. Fuel Cell Institute, Universiti Kebangsaan Malaysia, 43600, UKM Bangi, Selangor, Malaysia. E-mail address: [email protected] (A. Muchtar). https://doi.org/10.1016/j.ijhydene.2018.01.210 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Baharuddin NA, et al., Synthesis and characterization of cobalt-free SrFe0.8Ti0.2O3-d cathode powders synthesized through combustion method for solid oxide fuel cells, International Journal of Hydrogen Energy (2018), https://doi.org/ 10.1016/j.ijhydene.2018.01.210
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bulk cathode [6]. Introduction of perovskite cathode reduces the area specific resistance (ASR), which is a desired outcome in developing novel cathode materials for intermediatetemperature SOFCs. A number of perovskite cathode operated at this temperature range have been reported. Perovskite cathode materials, such as La0.6Sr0.4Co0.2Fe0.8O3-d (LSCF), NdBa1-xCo2O5þd, and Pr1-xBaCo2O5þd, have been reported; all of these cathodes contain cobalt [7e9]. Cobalt-containing cathodes was believed to exhibit high electro-catalytic activity. Despite this advantage, cobalt-containing cathodes exhibit high thermal expansion coefficient (TEC) due to the presence of cobalt compared with the commonly used electrolytes, such as Sm0.2Ce0.8O1.9 (SDC). SDC electrolyte is known for its high ionic conductivity under intermediate-temperature SOFC environment; the TEC value of SDC is ~12.5 106 K1 [10,11], whereas the average TEC of cobalt-containing intermediate-temperature SOFCs is 20e22 106 K1 [12,13]. One way to reduce the TEC of cobalt-containing cathode is to introduce an electrolyte material into the cathode, which is then called composite cathode (e.g., LSCF-SDC) [14]. Besides that, development of cobalt-free cathode was considered to minimize the TEC difference between cathode and electrolyte components. A large TEC difference between both components causes delamination of the cathode/electrolyte layer, thereby accelerating cell degradation. Cobalt-free cathodes (e.g., Ba0.5Sr0.5Fe0.8Cu0.1Ti0.1O3-d and Ba1-xLaxFeO3-d) have been developed to address the TEC mismatch issue [15,16]. These cathodes exhibit reduced TEC with an ASR lower than 1.0 U cm2. This ASR value remains higher than that of cobalt-containing cathodes (e.g., LSCF with an ASR of 0.062 U cm2 at 744 C) [17]. Thus, various materials have been explored to reduce the ASR of cobalt-free cathodes. SrFeO3-d materials were first introduced into oxygen gas sensor application [18]. These materials were subsequently introduced in SOFC field and applied in cobalt-free cathodes because of their excellent performance under oxygen reduction environment. SrFeO3-d-based cathodes, such as SrFe1-xNbxO3-d and SrFe0.95Ti0.05O3-d display ASR values lower than 1.0 Ucm2. ASR is strongly related to the purity of cobalt-free cathode materials, in which the presence of secondary phase induces the formation of an additional insulation layer. This layer acts as insulator, which reduces the electronic conductivity of a cathode [19]. Thus, pure cathode materials must be synthesized. In this work, cobalt-free SrFe0.8Ti0.2O3-d cathode was produced through combustion. This study aims to produce fine and pure cathode powders through wet processing technique rather than through the traditional solid-state method. The purity, particle size, and homogeneity of elements in the SrFe0.8Ti0.2O3-d cobalt-free cathode powder was specifically discussed. In addition, the electrochemical capability of this pure SrFe0.8Ti0.2O3-d cobalt-free cathode was determined by calculating the ASR.
Material and methods Material In this work, SrFe0.8Ti0.2O3-d precursor powders were produced through combustion. Strontium nitrate (Sr(NO3)2), iron nitrate (Fe(NO3)3$9H2O), titanium butoxide (Ti(C4H9O)4), and glycine
(NH2CH2COOH) were purchased from SigmaeAldrich. Ti(C4H9O)4 was hydrolyzed to form titanium hydroxide (TiO(OH)2), which was subjected to nitration to produce TiO(NO3)2. The precursor nitrate solution was added with NH2CH2COOH and mixed on a hotplate for 45 h before increasing the temperature to 150 C. After 90 min, the temperature was further increased to 350 C to initiate combustion and consequently produce SrFe0.8Ti0.2O3-d precursor powder.
Thermal decomposition, structural and thermal expansion analyses Weight loss and decomposition behavior of the precursor powders were studied using thermogravimetric analysis (TGA) (Pyris Diamond TG/DTG analyzer) and Fourier transform infrared (FTIR) spectroscopy (PerkinElmer). The structural and physical properties of the powders calcined at 1300 C for 5 h were characterized using X-ray diffraction (XRD) (D8-Advance; Bruker, Germany), field emission electron microscopy (FESEM) (JSM-6701F; JEOL, USA), and dynamic light scattering (DLS) (Zetasizer Nano ZS; Malvern Instrument, UK). For average particle size measurement using FESEM micrograph, at least 100 particles were considered. Measurement was conducted using Microsoft Visio (version 2010). Elemental analysis was performed through energy-dispersive X-ray analysis (EDX) coupled with FESEM. The SrFe0.8Ti0.2O3-d cathode powders were subsequently pressed into a cylindrical bulk (diameter 13 mm) and then sintered at 1300 C prior to TEC analysis using a dilatometer (L75 PT Horizontal; LINSEIS, USA). The average TEC value for SrFe0.8Ti0.2O3-d cathode was calculated based on the thermal expansion curve result.
Fabrication of symmetrical cell and analysis of electricalelectrochemical performance Cathode ink was also prepared. Pure SrFe0.8Ti0.2O3-d cathode powders were mixed with acetone and dispersant (Hypermer KD15) by using a high-energy ball milling machine (FRITSCH PULVERISETTE 6, Germany) at a rotational speed of 250 rpm for 2 h before drying overnight. The cathode powders (26 vol %) were subsequently milled using a triple-roll mill (EGM-65, ELE, China) with addition of terpineol (solvent) and ethyl cellulose (binder) to form cathode inks. Prior to half and symmetrical cell production for electrical conductivity and electrochemical impedance spectroscopy (EIS) analysis, Sm0.2Ce0.8O1.9 (SDC) (Sigma-Aldrich) powders were pressed into coin-shaped structure under a pressure of 52 MPa to form an electrolyte substrate. The SDC electrolyte was sintered at 1400 C for 6 h. The as-prepared cathode inks were then screen printed 10 times on one side (half-cell) and both sides (symmetrical cell) of the electrolyte surface with an active area, A, of 1 cm2 prior to sintering at 1300 C for 2 h. The half-cell was first subjected to the electrical conductivity analysis using the van der Pauw method at 600 Ce800 C under an air flow rate of 200 ml/min. Next, the symmetrical cell was used for EIS analysis (potentiostat PGSTAT302 N, Metrohm Autolab, Netherlands) coupled with NOVA software (version 1.10) conducted at a voltage amplitude of 10 mV under the same environment and operation temperature with the electrical conductivity analysis.
Please cite this article in press as: Baharuddin NA, et al., Synthesis and characterization of cobalt-free SrFe0.8Ti0.2O3-d cathode powders synthesized through combustion method for solid oxide fuel cells, International Journal of Hydrogen Energy (2018), https://doi.org/ 10.1016/j.ijhydene.2018.01.210
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Results and discussion Thermal decomposition analysis of precursor powders The thermal decomposition analysis of the SrFe0.8Ti0.2O3d precursor powders indicated two stages of weight loss. Fig. 1 shows the TGA and derivative thermogravimetric (DTG) curves. At a temperature below 600 C, the weight loss recorded was due to the elimination of water and nitrate molecules [20]. The DTG curve shows that a significant weight loss occurred at 800 Ce1000 C. The weight loss observed in this region was due to the elimination of carbon residue from the powders [21]. The weight loss stopped nearly at 1080 C, and the TGA curve started to flatten above this temperature. Thus, calcination of SrFe0.8Ti0.2O3-d precursor powders should be conducted at temperatures higher than 1080 C. To prove this theory, we calcined the precursor powders at two temperatures (700 C and 1300 C). The precursor and calcined SrFe0.8Ti0.2O3-d were subjected to FTIR analysis to observe the formation of pure SrFe0.8Ti0.2O3d powders (Fig. 2). The presence of cathode material produced from perovskite oxide is confirmed by the formation of downward peak at a wavenumber of ~600 cm1 [22]. The FTIR spectra of the three powder samples peaked at the above-mentioned wavenumber. Moreover, additional peaks were observed in the precursor powders and in powders calcined at 700 C. At a wavenumber of ~900 cm1, the peaks were attributed to the carbonate functional group of the remaining carbon. This peak was not observed in powders calcined at 1300 C because of the elimination of carbon residue at ~900 C. The low intensity peaks at 1400-1600 cm1 belonged to the O-H functional group (bending mode) [23]. Based on the FTIR analysis, perovskite oxide formed after calcination at 1300 C. However, the exact phase of this perovskite oxide remains unknown and can only be determined using XRD analysis.
Phase purity analysis of SrFe0.8Ti0.2O3-d powders XRD analysis was conducted to confirm the phase of perovskite oxide. Fig. 3 shows the XRD pattern of cobalt-free
Fig. 1 e TGA-DTG curves of precursor SrFe0.8Ti0.2O3-d powders.
Fig. 2 e FTIR spectra of precursor and calcined SrFe0.8Ti0.2O3-d powders.
cathode powders after calcination at 1300 C. Pure SrFe0.8Ti0.2O3-d cathode powders were successfully produced through combustion. No secondary phases (undesired peaks) were observed. The synthesized SrFe0.8Ti0.2O3-d cathode powders exhibit a cubic phase structure (group Pm-3m) and an average crystallite size of 62.22 nm (pattern #PDF 01-0706858). The lattice parameter, a, for SrFe0.8Ti0.2O3-d cathode is 3.8793 A (0.38793 nm), whereas the powders density, r, is 5.40 g cm3. Comparison of the a of SrFe0.8Ti0.2O3-d cathode with that of SrFeO3-d (0.3860 nm) [24] showed that doping 0.20 mol ratio of Ti into the structure increased the value of a or in other words caused the lattice to expand. The expansion of lattice is due to the substitution of Ti4þ into Fe4þ, leading to the changes in Fe4þ/Fe3þ fraction. The Fe3þ fraction increased, and given that Fe3þ is larger than Fe4þ, the lattice expands accordingly [25]. A study reported the same phase structure for pure cobaltfree cathode produced using SrFe0.9Ti0.1O3-d through solegel method [26]. Similar to other wet processes (e.g., sol‒gel
Fig. 3 e XRD pattern of SrFe0.8Ti0.2O3-d powders.
Please cite this article in press as: Baharuddin NA, et al., Synthesis and characterization of cobalt-free SrFe0.8Ti0.2O3-d cathode powders synthesized through combustion method for solid oxide fuel cells, International Journal of Hydrogen Energy (2018), https://doi.org/ 10.1016/j.ijhydene.2018.01.210
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method), combustion produces pure cobalt-free cathode materials with small crystallite size. A small crystallite results in fine cathode particles. A particle can be produced from many crystallites; thus, the particle size is expected to be larger than the crystallite size [27]. To determine the average particle size of cobalt-free SrFe0.8Ti0.2O3-d cathode, we measured the particles size through FESEM and DLS analyses. The result is discussed in the following section.
Physical characterization of SrFe0.8Ti0.2O3-d powders The average particle size of SrFe0.8Ti0.2O3-d powders was measured through FESEM (Fig. 4a) and DLS analyses (Fig. 4b); the average crystallite size recorded (Zave) are 1.0827 and 1.4438 mm, respectively. The Zave values expressed in micrometer were recorded in this work due to particle agglomeration during sample preparation [28]. In this study, both FESEM and DLS measured the agglomerate size, and the smallest particle size recorded is 200e300 nm. Additionally, Fig. 4a indicates the minimum particle size obtained through FESEM. The average particle size recorded is smaller than that of other cobalt-free cathode powders synthesized using solid-state method (e.g., Sr0.9K0.1FeO3-d) [29]. The homogeneity of element distribution in cobalt-free cathode SrFe0.8Ti0.2O3-d powders was also studied. Fig. 5 shows the EDX mapping result for SrFe0.8Ti0.2O3-d powders. All elements (Sr, Fe, and Ti) were evenly distributed in the same spot due to sufficient mixing during preparation of the precursor solution before combustion [30]. Based on the EDX mapping analysis, the weight percentage of each element is similar to the calculated value, and well-distributed elements were found in SrFe0.8Ti0.2O3-d cathode powders produced using combustion. A pure, fine, and well-distributed element in cobalt-free cathode SrFe0.8Ti0.2O3-d powders can enhance
Fig. 5 e EDX spectrum and elemental mapping results for SrFe0.8Ti0.2O3-d powders.
the electrical/electrochemical performance of the SOFC component. The relationship among these parameters should be studied in future works.
Thermal expansion coefficient of cobalt-free SrFe0.8Ti0.2O3-d cathode The as-synthesized cobalt-free SrFe0.8Ti0.2O3-d cathode was expected to achieve lower TEC value compared with the existing cobalt-containing cathode. Fig. 6 shows the thermal expansion curve of SrFe0.8Ti0.2O3-d cathode in air at 30 Ce1000 C. The average TEC value of 16.20 106 K1 was
Fig. 4 e FESEM micrograph of SrFe0.8Ti0.2O3-d powders (a) and average particle size distribution measured through (b) FESEM and (c) DLS analyses. Please cite this article in press as: Baharuddin NA, et al., Synthesis and characterization of cobalt-free SrFe0.8Ti0.2O3-d cathode powders synthesized through combustion method for solid oxide fuel cells, International Journal of Hydrogen Energy (2018), https://doi.org/ 10.1016/j.ijhydene.2018.01.210
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obtained in this work. Further change in relative length with increasing temperature is due to the expansion of lattice with respect to reduction of cation valence (Fe4þ to Fe3þ) [31]. The reported TEC values for cobalt-free cathode is lower than 19 106 K1 [31,32]. Additionally, the TEC for cobalt-free SrFe0.8Ti0.2O3-d is not only lower than that of cobaltcontaining cathode as mentioned above but also better compared with other SrFeO3-d-based cathodes, such as SrFe0.95Ti0.05O3-d (TEC ¼ 25.9 106 K1) synthesized through solid-state reaction method [33]. Lower TEC value obtained in this work is believed due to be due to the high purity of cobaltfree cathode itself.
Electrical conductivity of cobalt-free SrFe0.8Ti0.2O3-d cathode As mentioned, cobalt-free SrFe0.8Ti0.2O3-d cathode is a perovskite-structured material with mixed ionic-electronic conducting property. Thus, knowing its electrical conductivity is significant in predicting the cathode performance. Fig. 7 shows the electrical conductivity of cobalt-free SrFe0.8Ti0.2O3-d cathode under the operation temperature of 600e800 C in flowing air. The conductivity increased as the operation temperature increased, which confirmed the semi-conductor behavior of cobalt-free SrFe0.8Ti0.2O3-d cathode. The conductivity was expected to reduce at elevated temperatures as the cathode behavior changed into metallic type due to the lattice oxygen loss and the reduction of the Fe ions [34]. The increasing oxygen loss produced more vacancies which acted as electron trapper, resulting in the reduction of carrier mobility and conductivity [35]. For the host material of SrFeO3-d, the transition temperature was found at above 400 C. As for doped SrFeO3-d cathodes like SrFe0.8Ti0.2O3-d, the transition state varied depending on the dopant species and concentration [36]. In this work, the conductivity of cobalt-free SrFe0.8Ti0.2O3-d cathode did not show any change in behavior even at the maximum operation temperature (800 C), suggesting that the introduction of Ti as dopant in B-site stabilized the lattice and increased the transition temperature margin. The maximum conductivity value of 28.02 Scm1 was achieved at 800 C. This value is considered low compared to cobalt-containing
Fig. 7 e Electrical conductivity for cobalt-free SrFe0.8Ti0.2O3-d cathode at 600e800 C.
cathodes such as LSCF and BSCF reported elsewhere with conductivity of 30e400 Scm1 in the intermediate temperature range (600e800 C) of SOFC [37]. This difference is predictable due to the good conductivity property of cobalt itself besides its excellent electro-catalytic characteristic [3]. Nevertheless, the conductivity of cobalt-free SrFe0.8Ti0.2O3d cathode synthesized in this study is better compared to the same material produced using the solid-state method [38]. In a previous work, SrFe0.8Ti0.2O3-d exhibited its highest conductivity of ~5 Scm1 at temperature around 600 C. At the same operation temperature, the electrical conductivity of 6.81 Scm1 was achieved for the cathode produced in this work due to a higher cathode purity, synthesized using the combustion method rather than the solid-state technique. In addition, the same material reported by a previous work underwent a transition of conductivity behavior at ~500 C, which is near the transition temperature of the undoped material mentioned earlier. Therefore, producing the cathode material by the solid-state method does not guarantee doping success as the margin of the transition temperature does not expand due to its unstabilized cubic structure. The maximum conductivity (28.02 Scm1) yield in this work is far from the theoretical suggested value of 100 Scm1. However, this finding is not an ending in the measurement of cathode performance. Previous work by Zhou et al. (2015) proved that the cobalt-free (Nd0.9La0.1)2(Ni0.74Cu0.21Al0.05)O41 still d cathode with electrical conductivity below 60 S cm managed to achieved desired electrochemical performance with ASR below the targeted value of 1 U cm2 [39]. The ASR value determined the cathode potential in a real working environment. Thus, the electrochemical performance test was conducted on the cobalt-free SrFe0.8Ti0.2O3-d cathode.
Electrochemical performance of cobalt-free SrFe0.8Ti0.2O3-d cathode
Fig. 6 e Thermal expansion curve of SrFe0.8Ti0.2O3-d cathode at 30e1000 C in air.
EIS analysis of the cobalt-free SrFe0.8Ti0.2O3-d cathode was performed at 600e800 C to obtain the ASR values of the cathode film. An ASR value of lower than 1 U cm2 was aimed
Please cite this article in press as: Baharuddin NA, et al., Synthesis and characterization of cobalt-free SrFe0.8Ti0.2O3-d cathode powders synthesized through combustion method for solid oxide fuel cells, International Journal of Hydrogen Energy (2018), https://doi.org/ 10.1016/j.ijhydene.2018.01.210
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where the active area of cathode, A, is 1 cm2. The ASR of cobalt-free SrFe0.8Ti0.2O3-d cathode are 0.60, 1.07, 2.33, 5.01, and 13.57 U cm2 at 800 C, 750 C, 700 C, 650 C, and 600 C, respectively. The ASR values obtained for cobalt-free SrFe0.8Ti0.2O3-d cathode is slightly lower compared with some previous results. Grilo et al. (2016) reported that ASR for the cobalt-free Pr0.5Sr0.5Fe0.8Cu0.2O3 cathode at an operation temperature of 800 C is 0.62 U cm2 [42], whereas other researchers reported an ASR value of 1.1 U cm2 for the newly developed cobaltcontaining GdCoO3 cathode [43]. The ASR value for common
cobalt-containing cathodes, such as La0.6Sr0.4Co0.2Fe0.8O3-d, for intermediate-temperature SOFC is below 0.10 U cm2 [44], which is still far lower than the cobalt-free SrFe0.8Ti0.2O3-d cathode. However, this ASR value can be improved by introducing a pore former to increase the oxygen diffusion rate at the cathode and by increasing the cathode solid contents at the ink preparation stage so that the active particle that participates in oxygen reduction reaction is increased [45e47]. A comparison of the ASR value of SrFe0.8Ti0.2O3-d cathode with another Ti-doped SrFeO3-d cathode (Ti ¼ 0.05, 0.10, 0.15) shows that the previously reported work has lower ASR value of 0.034e0.053 U cm2 at 800 C [48]. However, this lower ASR value is believed to be due to the introduction of silver paste as current collector. Compared with other studies, this work tested the symmetrical cell without any additional conductive layer, allowing the measurement of the real resistance of cathode. The ln (ASR) in the function of temperature (1000/T) graph was plotted based on the ASR values calculated above (Fig. 9). The slope of the linear fitting was used to calculate the activation energy, Ea, of the cathode [49]. The Ea value for the cobalt-free SrFe0.8Ti0.2O3-d cathode is 121.77 kJ mol1, which is lower compared with Ea for SrFeO3-d (173.70 kJ mol1) [50] due to the stabilization of the cubic phase with the introduction of titanium as dopant [51]. To observe the effect of the dopant amount on the electrochemical performance of the cathode, the cobalt-free SrFe0.8Ti0.2O3-d produced in this study was compared with SrFe0.9Ti0.1O3-d cathode synthesized in a previous work by Yang et al. (2015) [26]. The Ea calculated from ASR values reported by Yang et al. is 110.49 kJ mol1. Minimum dopant was suggested as increasing the titanium led to the expansion of lattice parameter. The introduction of Ti4þ with a large ionic size into the Fe lattice, which comprised Fe4þ and Fe3þ, caused the lattice to expand [52]. The Big lattice reduced the oxygen transport pathway, which resulted in a higher Ea value. Even though the Ea for SrFe0.8Ti0.2O3-d cathode is higher compared to the SrFe0.9Ti0.1O3-d, its activation energy is still low compared with that of other cobalt-free cathodes, such as BaFe0.95Sn0.05O3-d, SrNb0.1Fe0.9O3-d, and Nd0.5Sr0.5Fe0.8Cu0.2O3-dSm0.2Ce0.8O1.9, whose Ea values are 124, 133, and 125.3e133.4 kJ mol1, respectively [53e55]. The ASR and Ea
Fig. 8 e EIS Nyquist plots for cobalt-free SrFe0.8Ti0.2O3-d cathode at 600e800 C.
Fig. 9 e Arrhenius plot for cobalt-free SrFe0.8Ti0.2O3-d cathode at 600e800 C.
for the newly developed cathode materials. Through EIS analysis, a Nyquist plot consisting of impedance values was generated. Nyquist plot for SOFC cathode generally operates in this intermediate-temperature region consisting of arcs that belongs to three frequency ranges: high, medium, and low. At a high frequency range of ~106 Hz, the arc that appeared was attributed to the resistance of the electrolyte grain boundary. This high-frequency arc is unavailable in EIS analysis of SrFe0.8Ti0.2O3-d cathode as shown in Fig. 8. This phenomenon is due to the very low resistance of the electrolyte SDC in the selected operation temperature range. This high frequency arc is expected to appear only when the temperature is reduced to that of the lower-temperature SOFC region, that is, below 600 C. The arc in Fig. 8 belongs to medium-low frequency regions, in which the equivalent circuit were yields through the Nyquist curve fitting. Based on the equivalent circuit in Fig. 8 (inset), the R0 attributed to the ohmic resistance will not be include in calculating the cathode's ASR. The ASR value for cobalt-free SrFe0.8Ti0.2O3-d cathode was calculated using R1 and R2. R1 comprises the resistance at the interfacial between cathode-electrolyte surface, whereas R2 contributed from resistance of bulk cathode in the oxygen adsorptiondesorption process [40]. These interfacial and bulk resistance values must be summed up to determine the total polarization resistance of the cathode component. ASR of the cathode is measured in Eq. (1) [41] as follows: ASR ¼ (R1 þ R2) A / 2
(1)
Please cite this article in press as: Baharuddin NA, et al., Synthesis and characterization of cobalt-free SrFe0.8Ti0.2O3-d cathode powders synthesized through combustion method for solid oxide fuel cells, International Journal of Hydrogen Energy (2018), https://doi.org/ 10.1016/j.ijhydene.2018.01.210
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values demonstrate that the cobalt-free SrFe0.8Ti0.2O3-d cathode can be potentially used in intermediate-temperature SOFC.
Conclusion In this research, pure cobalt-free SrFe0.8Ti0.2O3-d cathode powder was synthesized through combustion. SrFe0.8Ti0.2O3-d powders possess a cubic Pm-3m structure and an average crystallite size of 62.22 nm. FESEM and DLS analyses revealed that the average particle sizes were 1.0827 and 1.4438 mm, respectively. These values are smaller than those of other cobalt-free SrFeO3-dbased cathode (Sr0.9K0.1FeO3-d) produced using solid-state method. Moreover, well-distributed elements were observed in the as-synthesized SrFe0.8Ti0.2O3-d powders. The cobalt-free SrFe0.8Ti0.2O3-d cathode was confirmed to have a low TEC value of 16.20 106 K1. The lowest ASR value of 0.60 U cm2 at 800 C was obtained for cobalt-free SrFe0.8Ti0.2O3-d. Reducing the ASR value to lower than 0.1 U cm2 should be the focus of future works. Improving oxygen diffusion through adjustment of the cathode porosity is proposed.
Acknowledgements This work was supported by the Universiti Kebangsaan Malaysia (UKM) (Grant No.: GUP-2016-045) and the Ministry of Higher Education (Grant No.: FRGS/1/2015/TK10/UKM/01/2). The authors would also like to acknowledge the Centre for Research and Instrumentation Management in UKM for allowing us to use their excellent testing equipment.
references
[1] Sun C, Hui R, Roller J. Cathode materials for solid oxide fuel cells: a review. J Solid State Electrochem 2010;14:1125e44. [2] Hua C-H, Chou C-C. Preparation of nanoscale composite LSCF/GDCS cathode Materials by microwave sintering for intermediate-temperature SOFC Applications. Ceram Int 2015;41:1e5. O, [3] Rolle A, Mohamed HAA, Huo D, Capoen E, Mentre Vannier RN, et al. Ca3Co4O9þd, a growing potential SOFC cathode material: impact of the layer composition and thickness on the electrochemical properties. Solid State Ionics 2016;294:21e30. [4] Anwar M, Mas A, Abdalla AM, Somalu MR, Muchtar A. Effect of sintering temperature on the microstructure and ionic. Process Appl Ceram 2017:67e74. [5] Cheng Y, Zhou Q, Li W, Wei T, Li Z, An D, et al. Ba0.9Sr0.1Co0.9In0.1O3-d perovskite as cathode material for ITSOFC. J Alloys Compd 2015;641:234e7. [6] Baharuddin NA, Muchtar A, Somalu MR. Short review on cobalt-free cathodes for solid oxide fuel cells. Int J Hydrogen Energy 2017;42:9149e55. [7] Shong W-J, Liu C-K, Lu C-W, Wu S-H, Lee R-Y. Characteristics of La0.6Sr0.4Co0.2Fe0.8O3eCu2O mixture as a contact material in SOFC stacks. Int J Hydrogen Energy 2017;42:1170e80. [8] Sun J, Liu X, Han F, Zhu L, Bi H, Wang H, et al. NdBa1xCo2O5þd as cathode materials for IT-SOFC. Solid State Ionics 2016;288:54e60.
7
[9] Zhang L, Yao G, Song Z, Niu B, Long W, Zhang L, et al. Effects of Pr-deficiency on thermal expansion and electrochemical properties in Pr1-xBaCo2O5þd cathodes for IT-SOFCs. Electrochim Acta 2016;212:522e34. [10] Yin S, Li M, Zeng Y, Li C, Chen X, Ye Z. Study of Sm0.2Ce0.8O1.9 (SDC) electrolyte prepared by a simple modified solid-state method. J Rare Earths 2014;32:767e71. [11] Zhao L, Huang X, Zhu R, Lu Z, Sun W, Zhang Y, et al. Optimization on technical parameters for fabrication of SDC film by screen-printing used as electrolyte in IT-SOFC. J Phys Chem Solids 2008;69:2019e24. [12] Ding X, Gao X, Shen J, Wang J, Zhu W, Wang X, et al. Cobaltfree Sr0.7Y0.3CuO2þd as a cathode for intermediatetemperature solid oxide fuel cell. Int J Hydrogen Energy 2014;39:1030e8. [13] Xu Q, Huang DP, Zhang F, Chen W, Chen M, Liu HX. Structure, electrical conducting and thermal expansion properties of La0.6Sr0.4Co0.8Fe0.2O3-d-Ce0.8Sm0.2O2-d composite cathodes. J Alloys Compd 2008;454:460e5. [14] Harris J, Metcalfe C, Marr M, Kuhn J, Kesler O. Fabrication and characterization of solid oxide fuel cell cathodes made from nano-structured LSCF-SDC composite feedstock. J Power Sources 2013;239:234e43. MO, Shao Z. Cobalt-free [15] Yang G, Shen J, Chen Y, Tade Ba0.5Sr0.5Fe0.8Cu0.1Ti0.1O3d as a bi-functional electrode material for solid oxide fuel cells. J Power Sources 2015;298:184e92. [16] Ding X, Gao X, Zhu W, Wang J, Jiang J. Electrode redox properties of Ba1xLaxFeO3d as cobalt free cathode materials for intermediate-temperature SOFCs. Int J Hydrogen Energy 2014;39:12092e100. [17] Fan ESC, Kuhn J, Kesler O. Suspension plasma spraying of La0.6Sr0.4Co0.2Fe0.8O3d cathodes: influence of carbon black pore former on performance and degradation. J Power Sources 2016;316:72e84. [18] Lap C, Huang H, Chia W, Liu H, Huang Y, Siu M, et al. Sensors and Actuators B : chemical Effect of annealing temperature on the crystallization and oxygen sensing property of strontium titanate ferrite sol e gel thin films. Sensors Actuators B Chem 2013;187:20e6. € dl N, Bucher E, Gspan C, Egger A, Ganser C, Teichert C, [19] Schro et al. Phase decomposition in the chromium- and siliconpoisoned IT-SOFC cathode materials La0.6Sr0.4CoO3-d and La2NiO4þd. Solid State Ionics 2016;288:14e21. [20] Agthe M. Rare earth oxide nanopowder (RE ¼ Nd, Eu, Gd, Ho, Y, Yb) by combustion synthesis, sulfation and calcination role of the initial structure. 2011. [21] Shahgaldi S, Yaakob Z, Khadem DJ, Ahmadrezaei M, Daud WRW. Synthesis and characterization of cobalt-free Ba0.5Sr0.5Fe0.8Cu0.2O3-d perovskite oxide cathode nanofibers. J Alloys Compd 2011;509:9005e9. [22] Naveenkumar A, Kuruva P, Shivakumara C, Srilakshmi C. Mixture of fuels approach for the synthesis of SrFeO3d nanocatalyst and its impact on the catalytic reduction of nitrobenzene. Inorg Chem 2014;53:12178e85. [23] Ghosh S, Dasgupta S. Synthesis, characterization and properties of nanocrystalline perovskite cathode materials. Mater Sci 2010;28:427e38. [24] Ghaffari M, Huang H, Tan OK, Shannon M. Band gap measurement of SrFeO3-d by ultraviolet photoelectron spectroscopy and photovoltage method. Crystengcomm 2012;14:7487e92. [25] Waerenborgh JC, Rojas DP, Shaula AL, Mather GC, Patrakeev MV, Kharton VV, et al. Phase formation and iron oxidation states in SrFe(Al)O3-d perovskites. Mater Lett 2005;59:1644e8. [26] Yang G, Su C, Chen Y, Dong F, Tade MO, Shao Z. Cobalt-free SrFe0.9Ti0.1O3d as a high-performance electrode material for
Please cite this article in press as: Baharuddin NA, et al., Synthesis and characterization of cobalt-free SrFe0.8Ti0.2O3-d cathode powders synthesized through combustion method for solid oxide fuel cells, International Journal of Hydrogen Energy (2018), https://doi.org/ 10.1016/j.ijhydene.2018.01.210
8
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e8
oxygen reduction reaction on doped ceria electrolyte with favorable CO2 tolerance. J Eur Ceram Soc 2015;35:2531e9. Hajarpour S, Gheisari K, Honarbakhsh Raouf A. Characterization of nanocrystalline Mg0.6Zn0.4Fe2O4 soft ferrites synthesized by glycine-nitrate combustion process. J Magn Magn Mater 2013;329:165e9. Bahar Basim G, Khalili M. Particle size analysis on wide size distribution powders; Effect of sampling and characterization technique. Adv Powder Technol 2015;26:200e7. Hou S, Alonso JA, Goodenough JB. Co-free, iron perovskites as cathode materials for intermediate-temperature solid oxide fuel cells. J Power Sources 2010;195:280e4. Mirjalili F, Chuah L, Mohamad H. Effect of stirring time on synthesis of ultra fine a -Al2O3 powder by a simple sol-gel method. J Ceram Process Res 2011;12:738e41. Lu J, Yin YM, Li J, Xu L, Ma ZF. A cobalt-free electrode material La0.5Sr0.5Fe0.8Cu0.2O3-d for symmetrical solid oxide fuel cells. Electrochem Commun 2015;61:18e22. Ling Y, Zhao L, Lin B, Dong Y, Zhang X, Meng G, et al. Investigation of cobalt-free cathode material Sm0.5Sr0.5Fe0.8Cu0.2O3d for intermediate temperature solid oxide fuel cell. Int J Hydrogen Energy 2010;35:6905e10. Yu X, Fan J, Xue L. Performance optimization of SrFe0.95Ti0.05O3d cathode for intermediate temperature SOFC. Ceram Int 2014;40:13627e34. Wei B, Lu¨ Z, Huang X, Liu M, Li N, Su W. Synthesis, electrical and electrochemical properties of Ba0.5Sr0.5Zn0.2Fe0.8O3d perovskite oxide for IT-SOFC cathode. J Power Sources 2008;176:1e8. Shuyan L, Lu¨ Z, Huang X, Su W. Thermal, electrical, and electrochemical properties of Nd-doped Ba0.5Sr0.5Co0.8Fe0.2O3d as a cathode material for SOFC. Solid State Ionics 2008;178:1853e8. Kozhevnikov VL, Leonidov IA, Patrakeev MV, Mitberg EB, Poeppelmeier KR. Electrical properties of the ferrite SrFeOy at high temperatures. J Solid State Chem 2001;158:320e6. Rembelski D, Viricelle JP, Combemale L, Rieu M. Characterization and comparison of different cathode materials for SC- SOFC : LSM, BSCF, SSC, and LSCF. Fuel Cells 2012;12:256e64. Zhang Z, Chen D, Dong F, Xu X, Hao Y, Shao Z. Understanding the doping effect toward the design of CO2tolerant perovskite membranes with enhanced oxygen permeability. J Memb Sci 2016;519:11e21. Zhou Q, Gao Y, Wang F, An D, Li Y, Zou Y, et al. Novel cobaltfree cathode material (Nd0.9La0.1)2(Ni0.74Cu0.21Al0.05)O4þd for intermediate-temperature solid oxide fuel cells. Ceram Int 2015;41:639e43. Gao L, Zhu M, Li Q, Sun L, Zhao H, Grenier J. Electrode properties of Cu-doped Bi0.5Sr0.5FeO3-d cobalt-free perovskite as cathode for intermediate-temperature solid oxide fuel cells. J Alloys Compd 2017;700:29e36. Molin S, Lewandowska-Iwaniak W, Kusz B, Gazda M, Jasinski P. Structural and electrical properties of Sr(Ti, Fe)O3-d materials for SOFC cathodes. J Electroceramics 2012;28:80e7.
[42] Grilo DF, Macedo DA, Ces R, Moura CG, Paul D, Nascimento RM. Cobalt-free perovskite Pr0.5Sr0.5Fe1-xCuxO3d (PSFC) as a cathode material for intermediate temperature solid oxide fuel cells. Mater Chem Phys 2016;180:256e62. [43] Lenka RK, Mahata T, Patro PK, Tyagi a K, Sinha PK. Synthesis and characterization of GdCoO3 as a potential SOFC cathode material. J Alloys Compd 2012;537:100e5. [44] Muhammed Ali SA, Anwar M, Somalu MR, Muchtar A. Enhancement of the interfacial polarization resistance of La0.6Sr0.4Co0.2Fe0.8O3-d cathode by microwave-assisted combustion method. Ceram Int 2016;43:4647e54. [45] Prestat M, Morandi A, Heel A, Holzer L, Holtappels P, Graule TJ. Effect of graphite pore former on oxygen electrodes prepared with La0.6Sr0.4CoO3-d. Electrochem Commun 2010;12:292e5. [46] de Larramendi IR, Ortiz-Vitoriano N, Dzul-Bautista IB, Rojo T. Designing Perovskite Oxides for Solid Oxide Fuel Cells. Perovskite Mater. - Synth. Characterisation, Prop. Appl. 2016. p. 589e617. [47] Somalu MR, Muchtar A, Daud WRW, Brandon NP. Screenprinting inks for the fabrication of solid oxide fuel cell films: a review. Renew Sustain Energy Rev 2017;75:426e39. [48] Yu X, Long W, Jin F, He T. Cobalt-free perovskite cathode materials SrFe1xTixO3d and performance optimization for intermediate-temperature solid oxide fuel cells. Electrochim Acta 2014;123:426e34. [49] Fan L, Zhang H, Chen M, Wang C, Wang H, Singh M, et al. Electrochemical study of lithiated transition metal oxide composite as symmetrical electrode for low temperature ceramic fuel cells. Int J Hydrogen Energy 2013;38: 11398e405. [50] Wang L. BSCF SOFC Cathode Materials: bulk properties, kinetics and mechanism of oxygen reduction. 2009. ndez-Ropero AJ, Porras-Va zquez JM, Cabeza A, [51] Ferna pez D, Losilla ER. High valence Slater PR, Marrero-Lo transition metal doped strontium ferrites for electrode materials in symmetrical SOFCs. J Power Sources 2014;249:405e13. [52] Sendilkumar A, Babu PD, Manivel Raja M, Reddy VR, Gupta A, € ssbauer investigation of Srinath S. Structural and Mo nanocrystalline SrFe1-xTixO3-d. J Am Ceram Soc 2013;96:2973e8. [53] Dong F, Ni M, He W, Chen Y, Yang G, Chen D. An efficient electrocatalyst as cathode material for solid oxide fuel: BaFe0.95Sn0.05O3-d. J Power Sources 2016;326:459e65. [54] Jiang S, Zhou W, Niu Y, Zhu Z, Shao Z. Phase transition of a cobalt-free perovskite as a high-performance cathode for intermediate-temperature solid oxide fuel cells. ChemSusChem 2012;5:2023e31. [55] Yin J, Zhang C, Yin Y, Shi H, Lin Y, Lu J, et al. Anodesupported single-chamber solid oxide fuel cell based on cobalt-free composite cathode of at intermediate temperatures. J Power Sources 2015;286:217e23.
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Synthesis and characterization of cobalt-free SrFe0.8Ti0.2O3-d cathode powders synthesized through combustion method for solid oxide fuel cells Nurul Akidah Baharuddin a, Andanastuti Muchtar a,b,*, Mahendra Rao Somalu a, Noor Shieela Kalib a, Nor Fatina Raduwan a a
Fuel Cell Institute, Universiti Kebangsaan Malaysia, 43600, UKM Bangi, Selangor, Malaysia Centre for Materials Engineering and Smart Manufacturing (MERCU), Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia
b
article info
abstract
Article history:
Cobalt-free SrFe0.8Ti0.2O3-d cathode were synthesized through combustion method. Results
Received 25 May 2017
of the thermogravimetric and Fourier transform infrared analyses suggested that a
Received in revised form
perovskite oxide started to form at temperatures above 1100 C. X-ray diffraction and
11 September 2017
Rietveld refinement analyses confirmed that the single-phase cubic structure (Pm-3m) of
Accepted 15 January 2018
SrFe0.8Ti0.2O3-d cathode was produced after calcination at 1300 C. The average sizes of the
Available online xxx
particles were 1.0827 and 1.4438 mm as revealed by field emission scanning electron microscopy and dynamic light scattering analysis, respectively. In addition, energy dispersive
Keywords:
X-ray analysis coupled with mapping revealed the homogenous distribution of elements in
Solid oxide fuel cell
the cathode. The thermal expansion coefficient of the SrFe0.8Ti0.2O3-d cathode is
Cobalt-free cathode
16.20 106 K1. For the electrochemical behavior, the area specific resistance of cathode
Perovskite
(0.60e13.57 U cm2) was obtained at 600e800 C and the activation energy is 121.77 kJ mol1.
Particle size
This work confirmed the potential of SrFe0.8Ti0.2O3-d cathode in intermediate temperature
Area specific resistance
solid oxide fuel cell. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Solid oxide fuel cells (SOFCs) are a type of an energy converter device with a high operating temperature of approximately 1000 C [1]. Reduction of the operation temperature to the intermediate range (600e800 C) is important in minimizing material cost and in avoiding cell degradation [2e4]. However, this approach reduces the oxygen reduction reaction rate in
the cathode component due to the existing material limitation (lanthanum strontium manganite); thus, new materials have been proposed for intermediate-temperature SOFC cathodes [5]. Perovskite cathode was introduced to fill this research gap. Perovskite oxide is suitable as intermediate-temperature SOFC due to its mixed ionic-electronic conducting property; through this property, the oxygen reduction reaction not only occurs at triple phase boundary (the specific point where cathode, electrolyte, and gas meet) but also throughout the
* Corresponding author. Fuel Cell Institute, Universiti Kebangsaan Malaysia, 43600, UKM Bangi, Selangor, Malaysia. E-mail address: [email protected] (A. Muchtar). https://doi.org/10.1016/j.ijhydene.2018.01.210 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Baharuddin NA, et al., Synthesis and characterization of cobalt-free SrFe0.8Ti0.2O3-d cathode powders synthesized through combustion method for solid oxide fuel cells, International Journal of Hydrogen Energy (2018), https://doi.org/ 10.1016/j.ijhydene.2018.01.210
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bulk cathode [6]. Introduction of perovskite cathode reduces the area specific resistance (ASR), which is a desired outcome in developing novel cathode materials for intermediatetemperature SOFCs. A number of perovskite cathode operated at this temperature range have been reported. Perovskite cathode materials, such as La0.6Sr0.4Co0.2Fe0.8O3-d (LSCF), NdBa1-xCo2O5þd, and Pr1-xBaCo2O5þd, have been reported; all of these cathodes contain cobalt [7e9]. Cobalt-containing cathodes was believed to exhibit high electro-catalytic activity. Despite this advantage, cobalt-containing cathodes exhibit high thermal expansion coefficient (TEC) due to the presence of cobalt compared with the commonly used electrolytes, such as Sm0.2Ce0.8O1.9 (SDC). SDC electrolyte is known for its high ionic conductivity under intermediate-temperature SOFC environment; the TEC value of SDC is ~12.5 106 K1 [10,11], whereas the average TEC of cobalt-containing intermediate-temperature SOFCs is 20e22 106 K1 [12,13]. One way to reduce the TEC of cobalt-containing cathode is to introduce an electrolyte material into the cathode, which is then called composite cathode (e.g., LSCF-SDC) [14]. Besides that, development of cobalt-free cathode was considered to minimize the TEC difference between cathode and electrolyte components. A large TEC difference between both components causes delamination of the cathode/electrolyte layer, thereby accelerating cell degradation. Cobalt-free cathodes (e.g., Ba0.5Sr0.5Fe0.8Cu0.1Ti0.1O3-d and Ba1-xLaxFeO3-d) have been developed to address the TEC mismatch issue [15,16]. These cathodes exhibit reduced TEC with an ASR lower than 1.0 U cm2. This ASR value remains higher than that of cobalt-containing cathodes (e.g., LSCF with an ASR of 0.062 U cm2 at 744 C) [17]. Thus, various materials have been explored to reduce the ASR of cobalt-free cathodes. SrFeO3-d materials were first introduced into oxygen gas sensor application [18]. These materials were subsequently introduced in SOFC field and applied in cobalt-free cathodes because of their excellent performance under oxygen reduction environment. SrFeO3-d-based cathodes, such as SrFe1-xNbxO3-d and SrFe0.95Ti0.05O3-d display ASR values lower than 1.0 Ucm2. ASR is strongly related to the purity of cobalt-free cathode materials, in which the presence of secondary phase induces the formation of an additional insulation layer. This layer acts as insulator, which reduces the electronic conductivity of a cathode [19]. Thus, pure cathode materials must be synthesized. In this work, cobalt-free SrFe0.8Ti0.2O3-d cathode was produced through combustion. This study aims to produce fine and pure cathode powders through wet processing technique rather than through the traditional solid-state method. The purity, particle size, and homogeneity of elements in the SrFe0.8Ti0.2O3-d cobalt-free cathode powder was specifically discussed. In addition, the electrochemical capability of this pure SrFe0.8Ti0.2O3-d cobalt-free cathode was determined by calculating the ASR.
Material and methods Material In this work, SrFe0.8Ti0.2O3-d precursor powders were produced through combustion. Strontium nitrate (Sr(NO3)2), iron nitrate (Fe(NO3)3$9H2O), titanium butoxide (Ti(C4H9O)4), and glycine
(NH2CH2COOH) were purchased from SigmaeAldrich. Ti(C4H9O)4 was hydrolyzed to form titanium hydroxide (TiO(OH)2), which was subjected to nitration to produce TiO(NO3)2. The precursor nitrate solution was added with NH2CH2COOH and mixed on a hotplate for 45 h before increasing the temperature to 150 C. After 90 min, the temperature was further increased to 350 C to initiate combustion and consequently produce SrFe0.8Ti0.2O3-d precursor powder.
Thermal decomposition, structural and thermal expansion analyses Weight loss and decomposition behavior of the precursor powders were studied using thermogravimetric analysis (TGA) (Pyris Diamond TG/DTG analyzer) and Fourier transform infrared (FTIR) spectroscopy (PerkinElmer). The structural and physical properties of the powders calcined at 1300 C for 5 h were characterized using X-ray diffraction (XRD) (D8-Advance; Bruker, Germany), field emission electron microscopy (FESEM) (JSM-6701F; JEOL, USA), and dynamic light scattering (DLS) (Zetasizer Nano ZS; Malvern Instrument, UK). For average particle size measurement using FESEM micrograph, at least 100 particles were considered. Measurement was conducted using Microsoft Visio (version 2010). Elemental analysis was performed through energy-dispersive X-ray analysis (EDX) coupled with FESEM. The SrFe0.8Ti0.2O3-d cathode powders were subsequently pressed into a cylindrical bulk (diameter 13 mm) and then sintered at 1300 C prior to TEC analysis using a dilatometer (L75 PT Horizontal; LINSEIS, USA). The average TEC value for SrFe0.8Ti0.2O3-d cathode was calculated based on the thermal expansion curve result.
Fabrication of symmetrical cell and analysis of electricalelectrochemical performance Cathode ink was also prepared. Pure SrFe0.8Ti0.2O3-d cathode powders were mixed with acetone and dispersant (Hypermer KD15) by using a high-energy ball milling machine (FRITSCH PULVERISETTE 6, Germany) at a rotational speed of 250 rpm for 2 h before drying overnight. The cathode powders (26 vol %) were subsequently milled using a triple-roll mill (EGM-65, ELE, China) with addition of terpineol (solvent) and ethyl cellulose (binder) to form cathode inks. Prior to half and symmetrical cell production for electrical conductivity and electrochemical impedance spectroscopy (EIS) analysis, Sm0.2Ce0.8O1.9 (SDC) (Sigma-Aldrich) powders were pressed into coin-shaped structure under a pressure of 52 MPa to form an electrolyte substrate. The SDC electrolyte was sintered at 1400 C for 6 h. The as-prepared cathode inks were then screen printed 10 times on one side (half-cell) and both sides (symmetrical cell) of the electrolyte surface with an active area, A, of 1 cm2 prior to sintering at 1300 C for 2 h. The half-cell was first subjected to the electrical conductivity analysis using the van der Pauw method at 600 Ce800 C under an air flow rate of 200 ml/min. Next, the symmetrical cell was used for EIS analysis (potentiostat PGSTAT302 N, Metrohm Autolab, Netherlands) coupled with NOVA software (version 1.10) conducted at a voltage amplitude of 10 mV under the same environment and operation temperature with the electrical conductivity analysis.
Please cite this article in press as: Baharuddin NA, et al., Synthesis and characterization of cobalt-free SrFe0.8Ti0.2O3-d cathode powders synthesized through combustion method for solid oxide fuel cells, International Journal of Hydrogen Energy (2018), https://doi.org/ 10.1016/j.ijhydene.2018.01.210
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Results and discussion Thermal decomposition analysis of precursor powders The thermal decomposition analysis of the SrFe0.8Ti0.2O3d precursor powders indicated two stages of weight loss. Fig. 1 shows the TGA and derivative thermogravimetric (DTG) curves. At a temperature below 600 C, the weight loss recorded was due to the elimination of water and nitrate molecules [20]. The DTG curve shows that a significant weight loss occurred at 800 Ce1000 C. The weight loss observed in this region was due to the elimination of carbon residue from the powders [21]. The weight loss stopped nearly at 1080 C, and the TGA curve started to flatten above this temperature. Thus, calcination of SrFe0.8Ti0.2O3-d precursor powders should be conducted at temperatures higher than 1080 C. To prove this theory, we calcined the precursor powders at two temperatures (700 C and 1300 C). The precursor and calcined SrFe0.8Ti0.2O3-d were subjected to FTIR analysis to observe the formation of pure SrFe0.8Ti0.2O3d powders (Fig. 2). The presence of cathode material produced from perovskite oxide is confirmed by the formation of downward peak at a wavenumber of ~600 cm1 [22]. The FTIR spectra of the three powder samples peaked at the above-mentioned wavenumber. Moreover, additional peaks were observed in the precursor powders and in powders calcined at 700 C. At a wavenumber of ~900 cm1, the peaks were attributed to the carbonate functional group of the remaining carbon. This peak was not observed in powders calcined at 1300 C because of the elimination of carbon residue at ~900 C. The low intensity peaks at 1400-1600 cm1 belonged to the O-H functional group (bending mode) [23]. Based on the FTIR analysis, perovskite oxide formed after calcination at 1300 C. However, the exact phase of this perovskite oxide remains unknown and can only be determined using XRD analysis.
Phase purity analysis of SrFe0.8Ti0.2O3-d powders XRD analysis was conducted to confirm the phase of perovskite oxide. Fig. 3 shows the XRD pattern of cobalt-free
Fig. 1 e TGA-DTG curves of precursor SrFe0.8Ti0.2O3-d powders.
Fig. 2 e FTIR spectra of precursor and calcined SrFe0.8Ti0.2O3-d powders.
cathode powders after calcination at 1300 C. Pure SrFe0.8Ti0.2O3-d cathode powders were successfully produced through combustion. No secondary phases (undesired peaks) were observed. The synthesized SrFe0.8Ti0.2O3-d cathode powders exhibit a cubic phase structure (group Pm-3m) and an average crystallite size of 62.22 nm (pattern #PDF 01-0706858). The lattice parameter, a, for SrFe0.8Ti0.2O3-d cathode is 3.8793 A (0.38793 nm), whereas the powders density, r, is 5.40 g cm3. Comparison of the a of SrFe0.8Ti0.2O3-d cathode with that of SrFeO3-d (0.3860 nm) [24] showed that doping 0.20 mol ratio of Ti into the structure increased the value of a or in other words caused the lattice to expand. The expansion of lattice is due to the substitution of Ti4þ into Fe4þ, leading to the changes in Fe4þ/Fe3þ fraction. The Fe3þ fraction increased, and given that Fe3þ is larger than Fe4þ, the lattice expands accordingly [25]. A study reported the same phase structure for pure cobaltfree cathode produced using SrFe0.9Ti0.1O3-d through solegel method [26]. Similar to other wet processes (e.g., sol‒gel
Fig. 3 e XRD pattern of SrFe0.8Ti0.2O3-d powders.
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method), combustion produces pure cobalt-free cathode materials with small crystallite size. A small crystallite results in fine cathode particles. A particle can be produced from many crystallites; thus, the particle size is expected to be larger than the crystallite size [27]. To determine the average particle size of cobalt-free SrFe0.8Ti0.2O3-d cathode, we measured the particles size through FESEM and DLS analyses. The result is discussed in the following section.
Physical characterization of SrFe0.8Ti0.2O3-d powders The average particle size of SrFe0.8Ti0.2O3-d powders was measured through FESEM (Fig. 4a) and DLS analyses (Fig. 4b); the average crystallite size recorded (Zave) are 1.0827 and 1.4438 mm, respectively. The Zave values expressed in micrometer were recorded in this work due to particle agglomeration during sample preparation [28]. In this study, both FESEM and DLS measured the agglomerate size, and the smallest particle size recorded is 200e300 nm. Additionally, Fig. 4a indicates the minimum particle size obtained through FESEM. The average particle size recorded is smaller than that of other cobalt-free cathode powders synthesized using solid-state method (e.g., Sr0.9K0.1FeO3-d) [29]. The homogeneity of element distribution in cobalt-free cathode SrFe0.8Ti0.2O3-d powders was also studied. Fig. 5 shows the EDX mapping result for SrFe0.8Ti0.2O3-d powders. All elements (Sr, Fe, and Ti) were evenly distributed in the same spot due to sufficient mixing during preparation of the precursor solution before combustion [30]. Based on the EDX mapping analysis, the weight percentage of each element is similar to the calculated value, and well-distributed elements were found in SrFe0.8Ti0.2O3-d cathode powders produced using combustion. A pure, fine, and well-distributed element in cobalt-free cathode SrFe0.8Ti0.2O3-d powders can enhance
Fig. 5 e EDX spectrum and elemental mapping results for SrFe0.8Ti0.2O3-d powders.
the electrical/electrochemical performance of the SOFC component. The relationship among these parameters should be studied in future works.
Thermal expansion coefficient of cobalt-free SrFe0.8Ti0.2O3-d cathode The as-synthesized cobalt-free SrFe0.8Ti0.2O3-d cathode was expected to achieve lower TEC value compared with the existing cobalt-containing cathode. Fig. 6 shows the thermal expansion curve of SrFe0.8Ti0.2O3-d cathode in air at 30 Ce1000 C. The average TEC value of 16.20 106 K1 was
Fig. 4 e FESEM micrograph of SrFe0.8Ti0.2O3-d powders (a) and average particle size distribution measured through (b) FESEM and (c) DLS analyses. Please cite this article in press as: Baharuddin NA, et al., Synthesis and characterization of cobalt-free SrFe0.8Ti0.2O3-d cathode powders synthesized through combustion method for solid oxide fuel cells, International Journal of Hydrogen Energy (2018), https://doi.org/ 10.1016/j.ijhydene.2018.01.210
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obtained in this work. Further change in relative length with increasing temperature is due to the expansion of lattice with respect to reduction of cation valence (Fe4þ to Fe3þ) [31]. The reported TEC values for cobalt-free cathode is lower than 19 106 K1 [31,32]. Additionally, the TEC for cobalt-free SrFe0.8Ti0.2O3-d is not only lower than that of cobaltcontaining cathode as mentioned above but also better compared with other SrFeO3-d-based cathodes, such as SrFe0.95Ti0.05O3-d (TEC ¼ 25.9 106 K1) synthesized through solid-state reaction method [33]. Lower TEC value obtained in this work is believed due to be due to the high purity of cobaltfree cathode itself.
Electrical conductivity of cobalt-free SrFe0.8Ti0.2O3-d cathode As mentioned, cobalt-free SrFe0.8Ti0.2O3-d cathode is a perovskite-structured material with mixed ionic-electronic conducting property. Thus, knowing its electrical conductivity is significant in predicting the cathode performance. Fig. 7 shows the electrical conductivity of cobalt-free SrFe0.8Ti0.2O3-d cathode under the operation temperature of 600e800 C in flowing air. The conductivity increased as the operation temperature increased, which confirmed the semi-conductor behavior of cobalt-free SrFe0.8Ti0.2O3-d cathode. The conductivity was expected to reduce at elevated temperatures as the cathode behavior changed into metallic type due to the lattice oxygen loss and the reduction of the Fe ions [34]. The increasing oxygen loss produced more vacancies which acted as electron trapper, resulting in the reduction of carrier mobility and conductivity [35]. For the host material of SrFeO3-d, the transition temperature was found at above 400 C. As for doped SrFeO3-d cathodes like SrFe0.8Ti0.2O3-d, the transition state varied depending on the dopant species and concentration [36]. In this work, the conductivity of cobalt-free SrFe0.8Ti0.2O3-d cathode did not show any change in behavior even at the maximum operation temperature (800 C), suggesting that the introduction of Ti as dopant in B-site stabilized the lattice and increased the transition temperature margin. The maximum conductivity value of 28.02 Scm1 was achieved at 800 C. This value is considered low compared to cobalt-containing
Fig. 7 e Electrical conductivity for cobalt-free SrFe0.8Ti0.2O3-d cathode at 600e800 C.
cathodes such as LSCF and BSCF reported elsewhere with conductivity of 30e400 Scm1 in the intermediate temperature range (600e800 C) of SOFC [37]. This difference is predictable due to the good conductivity property of cobalt itself besides its excellent electro-catalytic characteristic [3]. Nevertheless, the conductivity of cobalt-free SrFe0.8Ti0.2O3d cathode synthesized in this study is better compared to the same material produced using the solid-state method [38]. In a previous work, SrFe0.8Ti0.2O3-d exhibited its highest conductivity of ~5 Scm1 at temperature around 600 C. At the same operation temperature, the electrical conductivity of 6.81 Scm1 was achieved for the cathode produced in this work due to a higher cathode purity, synthesized using the combustion method rather than the solid-state technique. In addition, the same material reported by a previous work underwent a transition of conductivity behavior at ~500 C, which is near the transition temperature of the undoped material mentioned earlier. Therefore, producing the cathode material by the solid-state method does not guarantee doping success as the margin of the transition temperature does not expand due to its unstabilized cubic structure. The maximum conductivity (28.02 Scm1) yield in this work is far from the theoretical suggested value of 100 Scm1. However, this finding is not an ending in the measurement of cathode performance. Previous work by Zhou et al. (2015) proved that the cobalt-free (Nd0.9La0.1)2(Ni0.74Cu0.21Al0.05)O41 still d cathode with electrical conductivity below 60 S cm managed to achieved desired electrochemical performance with ASR below the targeted value of 1 U cm2 [39]. The ASR value determined the cathode potential in a real working environment. Thus, the electrochemical performance test was conducted on the cobalt-free SrFe0.8Ti0.2O3-d cathode.
Electrochemical performance of cobalt-free SrFe0.8Ti0.2O3-d cathode
Fig. 6 e Thermal expansion curve of SrFe0.8Ti0.2O3-d cathode at 30e1000 C in air.
EIS analysis of the cobalt-free SrFe0.8Ti0.2O3-d cathode was performed at 600e800 C to obtain the ASR values of the cathode film. An ASR value of lower than 1 U cm2 was aimed
Please cite this article in press as: Baharuddin NA, et al., Synthesis and characterization of cobalt-free SrFe0.8Ti0.2O3-d cathode powders synthesized through combustion method for solid oxide fuel cells, International Journal of Hydrogen Energy (2018), https://doi.org/ 10.1016/j.ijhydene.2018.01.210
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where the active area of cathode, A, is 1 cm2. The ASR of cobalt-free SrFe0.8Ti0.2O3-d cathode are 0.60, 1.07, 2.33, 5.01, and 13.57 U cm2 at 800 C, 750 C, 700 C, 650 C, and 600 C, respectively. The ASR values obtained for cobalt-free SrFe0.8Ti0.2O3-d cathode is slightly lower compared with some previous results. Grilo et al. (2016) reported that ASR for the cobalt-free Pr0.5Sr0.5Fe0.8Cu0.2O3 cathode at an operation temperature of 800 C is 0.62 U cm2 [42], whereas other researchers reported an ASR value of 1.1 U cm2 for the newly developed cobaltcontaining GdCoO3 cathode [43]. The ASR value for common
cobalt-containing cathodes, such as La0.6Sr0.4Co0.2Fe0.8O3-d, for intermediate-temperature SOFC is below 0.10 U cm2 [44], which is still far lower than the cobalt-free SrFe0.8Ti0.2O3-d cathode. However, this ASR value can be improved by introducing a pore former to increase the oxygen diffusion rate at the cathode and by increasing the cathode solid contents at the ink preparation stage so that the active particle that participates in oxygen reduction reaction is increased [45e47]. A comparison of the ASR value of SrFe0.8Ti0.2O3-d cathode with another Ti-doped SrFeO3-d cathode (Ti ¼ 0.05, 0.10, 0.15) shows that the previously reported work has lower ASR value of 0.034e0.053 U cm2 at 800 C [48]. However, this lower ASR value is believed to be due to the introduction of silver paste as current collector. Compared with other studies, this work tested the symmetrical cell without any additional conductive layer, allowing the measurement of the real resistance of cathode. The ln (ASR) in the function of temperature (1000/T) graph was plotted based on the ASR values calculated above (Fig. 9). The slope of the linear fitting was used to calculate the activation energy, Ea, of the cathode [49]. The Ea value for the cobalt-free SrFe0.8Ti0.2O3-d cathode is 121.77 kJ mol1, which is lower compared with Ea for SrFeO3-d (173.70 kJ mol1) [50] due to the stabilization of the cubic phase with the introduction of titanium as dopant [51]. To observe the effect of the dopant amount on the electrochemical performance of the cathode, the cobalt-free SrFe0.8Ti0.2O3-d produced in this study was compared with SrFe0.9Ti0.1O3-d cathode synthesized in a previous work by Yang et al. (2015) [26]. The Ea calculated from ASR values reported by Yang et al. is 110.49 kJ mol1. Minimum dopant was suggested as increasing the titanium led to the expansion of lattice parameter. The introduction of Ti4þ with a large ionic size into the Fe lattice, which comprised Fe4þ and Fe3þ, caused the lattice to expand [52]. The Big lattice reduced the oxygen transport pathway, which resulted in a higher Ea value. Even though the Ea for SrFe0.8Ti0.2O3-d cathode is higher compared to the SrFe0.9Ti0.1O3-d, its activation energy is still low compared with that of other cobalt-free cathodes, such as BaFe0.95Sn0.05O3-d, SrNb0.1Fe0.9O3-d, and Nd0.5Sr0.5Fe0.8Cu0.2O3-dSm0.2Ce0.8O1.9, whose Ea values are 124, 133, and 125.3e133.4 kJ mol1, respectively [53e55]. The ASR and Ea
Fig. 8 e EIS Nyquist plots for cobalt-free SrFe0.8Ti0.2O3-d cathode at 600e800 C.
Fig. 9 e Arrhenius plot for cobalt-free SrFe0.8Ti0.2O3-d cathode at 600e800 C.
for the newly developed cathode materials. Through EIS analysis, a Nyquist plot consisting of impedance values was generated. Nyquist plot for SOFC cathode generally operates in this intermediate-temperature region consisting of arcs that belongs to three frequency ranges: high, medium, and low. At a high frequency range of ~106 Hz, the arc that appeared was attributed to the resistance of the electrolyte grain boundary. This high-frequency arc is unavailable in EIS analysis of SrFe0.8Ti0.2O3-d cathode as shown in Fig. 8. This phenomenon is due to the very low resistance of the electrolyte SDC in the selected operation temperature range. This high frequency arc is expected to appear only when the temperature is reduced to that of the lower-temperature SOFC region, that is, below 600 C. The arc in Fig. 8 belongs to medium-low frequency regions, in which the equivalent circuit were yields through the Nyquist curve fitting. Based on the equivalent circuit in Fig. 8 (inset), the R0 attributed to the ohmic resistance will not be include in calculating the cathode's ASR. The ASR value for cobalt-free SrFe0.8Ti0.2O3-d cathode was calculated using R1 and R2. R1 comprises the resistance at the interfacial between cathode-electrolyte surface, whereas R2 contributed from resistance of bulk cathode in the oxygen adsorptiondesorption process [40]. These interfacial and bulk resistance values must be summed up to determine the total polarization resistance of the cathode component. ASR of the cathode is measured in Eq. (1) [41] as follows: ASR ¼ (R1 þ R2) A / 2
(1)
Please cite this article in press as: Baharuddin NA, et al., Synthesis and characterization of cobalt-free SrFe0.8Ti0.2O3-d cathode powders synthesized through combustion method for solid oxide fuel cells, International Journal of Hydrogen Energy (2018), https://doi.org/ 10.1016/j.ijhydene.2018.01.210
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values demonstrate that the cobalt-free SrFe0.8Ti0.2O3-d cathode can be potentially used in intermediate-temperature SOFC.
Conclusion In this research, pure cobalt-free SrFe0.8Ti0.2O3-d cathode powder was synthesized through combustion. SrFe0.8Ti0.2O3-d powders possess a cubic Pm-3m structure and an average crystallite size of 62.22 nm. FESEM and DLS analyses revealed that the average particle sizes were 1.0827 and 1.4438 mm, respectively. These values are smaller than those of other cobalt-free SrFeO3-dbased cathode (Sr0.9K0.1FeO3-d) produced using solid-state method. Moreover, well-distributed elements were observed in the as-synthesized SrFe0.8Ti0.2O3-d powders. The cobalt-free SrFe0.8Ti0.2O3-d cathode was confirmed to have a low TEC value of 16.20 106 K1. The lowest ASR value of 0.60 U cm2 at 800 C was obtained for cobalt-free SrFe0.8Ti0.2O3-d. Reducing the ASR value to lower than 0.1 U cm2 should be the focus of future works. Improving oxygen diffusion through adjustment of the cathode porosity is proposed.
Acknowledgements This work was supported by the Universiti Kebangsaan Malaysia (UKM) (Grant No.: GUP-2016-045) and the Ministry of Higher Education (Grant No.: FRGS/1/2015/TK10/UKM/01/2). The authors would also like to acknowledge the Centre for Research and Instrumentation Management in UKM for allowing us to use their excellent testing equipment.
references
[1] Sun C, Hui R, Roller J. Cathode materials for solid oxide fuel cells: a review. J Solid State Electrochem 2010;14:1125e44. [2] Hua C-H, Chou C-C. Preparation of nanoscale composite LSCF/GDCS cathode Materials by microwave sintering for intermediate-temperature SOFC Applications. Ceram Int 2015;41:1e5. O, [3] Rolle A, Mohamed HAA, Huo D, Capoen E, Mentre Vannier RN, et al. Ca3Co4O9þd, a growing potential SOFC cathode material: impact of the layer composition and thickness on the electrochemical properties. Solid State Ionics 2016;294:21e30. [4] Anwar M, Mas A, Abdalla AM, Somalu MR, Muchtar A. Effect of sintering temperature on the microstructure and ionic. Process Appl Ceram 2017:67e74. [5] Cheng Y, Zhou Q, Li W, Wei T, Li Z, An D, et al. Ba0.9Sr0.1Co0.9In0.1O3-d perovskite as cathode material for ITSOFC. J Alloys Compd 2015;641:234e7. [6] Baharuddin NA, Muchtar A, Somalu MR. Short review on cobalt-free cathodes for solid oxide fuel cells. Int J Hydrogen Energy 2017;42:9149e55. [7] Shong W-J, Liu C-K, Lu C-W, Wu S-H, Lee R-Y. Characteristics of La0.6Sr0.4Co0.2Fe0.8O3eCu2O mixture as a contact material in SOFC stacks. Int J Hydrogen Energy 2017;42:1170e80. [8] Sun J, Liu X, Han F, Zhu L, Bi H, Wang H, et al. NdBa1xCo2O5þd as cathode materials for IT-SOFC. Solid State Ionics 2016;288:54e60.
7
[9] Zhang L, Yao G, Song Z, Niu B, Long W, Zhang L, et al. Effects of Pr-deficiency on thermal expansion and electrochemical properties in Pr1-xBaCo2O5þd cathodes for IT-SOFCs. Electrochim Acta 2016;212:522e34. [10] Yin S, Li M, Zeng Y, Li C, Chen X, Ye Z. Study of Sm0.2Ce0.8O1.9 (SDC) electrolyte prepared by a simple modified solid-state method. J Rare Earths 2014;32:767e71. [11] Zhao L, Huang X, Zhu R, Lu Z, Sun W, Zhang Y, et al. Optimization on technical parameters for fabrication of SDC film by screen-printing used as electrolyte in IT-SOFC. J Phys Chem Solids 2008;69:2019e24. [12] Ding X, Gao X, Shen J, Wang J, Zhu W, Wang X, et al. Cobaltfree Sr0.7Y0.3CuO2þd as a cathode for intermediatetemperature solid oxide fuel cell. Int J Hydrogen Energy 2014;39:1030e8. [13] Xu Q, Huang DP, Zhang F, Chen W, Chen M, Liu HX. Structure, electrical conducting and thermal expansion properties of La0.6Sr0.4Co0.8Fe0.2O3-d-Ce0.8Sm0.2O2-d composite cathodes. J Alloys Compd 2008;454:460e5. [14] Harris J, Metcalfe C, Marr M, Kuhn J, Kesler O. Fabrication and characterization of solid oxide fuel cell cathodes made from nano-structured LSCF-SDC composite feedstock. J Power Sources 2013;239:234e43. MO, Shao Z. Cobalt-free [15] Yang G, Shen J, Chen Y, Tade Ba0.5Sr0.5Fe0.8Cu0.1Ti0.1O3d as a bi-functional electrode material for solid oxide fuel cells. J Power Sources 2015;298:184e92. [16] Ding X, Gao X, Zhu W, Wang J, Jiang J. Electrode redox properties of Ba1xLaxFeO3d as cobalt free cathode materials for intermediate-temperature SOFCs. Int J Hydrogen Energy 2014;39:12092e100. [17] Fan ESC, Kuhn J, Kesler O. Suspension plasma spraying of La0.6Sr0.4Co0.2Fe0.8O3d cathodes: influence of carbon black pore former on performance and degradation. J Power Sources 2016;316:72e84. [18] Lap C, Huang H, Chia W, Liu H, Huang Y, Siu M, et al. Sensors and Actuators B : chemical Effect of annealing temperature on the crystallization and oxygen sensing property of strontium titanate ferrite sol e gel thin films. Sensors Actuators B Chem 2013;187:20e6. € dl N, Bucher E, Gspan C, Egger A, Ganser C, Teichert C, [19] Schro et al. Phase decomposition in the chromium- and siliconpoisoned IT-SOFC cathode materials La0.6Sr0.4CoO3-d and La2NiO4þd. Solid State Ionics 2016;288:14e21. [20] Agthe M. Rare earth oxide nanopowder (RE ¼ Nd, Eu, Gd, Ho, Y, Yb) by combustion synthesis, sulfation and calcination role of the initial structure. 2011. [21] Shahgaldi S, Yaakob Z, Khadem DJ, Ahmadrezaei M, Daud WRW. Synthesis and characterization of cobalt-free Ba0.5Sr0.5Fe0.8Cu0.2O3-d perovskite oxide cathode nanofibers. J Alloys Compd 2011;509:9005e9. [22] Naveenkumar A, Kuruva P, Shivakumara C, Srilakshmi C. Mixture of fuels approach for the synthesis of SrFeO3d nanocatalyst and its impact on the catalytic reduction of nitrobenzene. Inorg Chem 2014;53:12178e85. [23] Ghosh S, Dasgupta S. Synthesis, characterization and properties of nanocrystalline perovskite cathode materials. Mater Sci 2010;28:427e38. [24] Ghaffari M, Huang H, Tan OK, Shannon M. Band gap measurement of SrFeO3-d by ultraviolet photoelectron spectroscopy and photovoltage method. Crystengcomm 2012;14:7487e92. [25] Waerenborgh JC, Rojas DP, Shaula AL, Mather GC, Patrakeev MV, Kharton VV, et al. Phase formation and iron oxidation states in SrFe(Al)O3-d perovskites. Mater Lett 2005;59:1644e8. [26] Yang G, Su C, Chen Y, Dong F, Tade MO, Shao Z. Cobalt-free SrFe0.9Ti0.1O3d as a high-performance electrode material for
Please cite this article in press as: Baharuddin NA, et al., Synthesis and characterization of cobalt-free SrFe0.8Ti0.2O3-d cathode powders synthesized through combustion method for solid oxide fuel cells, International Journal of Hydrogen Energy (2018), https://doi.org/ 10.1016/j.ijhydene.2018.01.210
8
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e8
oxygen reduction reaction on doped ceria electrolyte with favorable CO2 tolerance. J Eur Ceram Soc 2015;35:2531e9. Hajarpour S, Gheisari K, Honarbakhsh Raouf A. Characterization of nanocrystalline Mg0.6Zn0.4Fe2O4 soft ferrites synthesized by glycine-nitrate combustion process. J Magn Magn Mater 2013;329:165e9. Bahar Basim G, Khalili M. Particle size analysis on wide size distribution powders; Effect of sampling and characterization technique. Adv Powder Technol 2015;26:200e7. Hou S, Alonso JA, Goodenough JB. Co-free, iron perovskites as cathode materials for intermediate-temperature solid oxide fuel cells. J Power Sources 2010;195:280e4. Mirjalili F, Chuah L, Mohamad H. Effect of stirring time on synthesis of ultra fine a -Al2O3 powder by a simple sol-gel method. J Ceram Process Res 2011;12:738e41. Lu J, Yin YM, Li J, Xu L, Ma ZF. A cobalt-free electrode material La0.5Sr0.5Fe0.8Cu0.2O3-d for symmetrical solid oxide fuel cells. Electrochem Commun 2015;61:18e22. Ling Y, Zhao L, Lin B, Dong Y, Zhang X, Meng G, et al. Investigation of cobalt-free cathode material Sm0.5Sr0.5Fe0.8Cu0.2O3d for intermediate temperature solid oxide fuel cell. Int J Hydrogen Energy 2010;35:6905e10. Yu X, Fan J, Xue L. Performance optimization of SrFe0.95Ti0.05O3d cathode for intermediate temperature SOFC. Ceram Int 2014;40:13627e34. Wei B, Lu¨ Z, Huang X, Liu M, Li N, Su W. Synthesis, electrical and electrochemical properties of Ba0.5Sr0.5Zn0.2Fe0.8O3d perovskite oxide for IT-SOFC cathode. J Power Sources 2008;176:1e8. Shuyan L, Lu¨ Z, Huang X, Su W. Thermal, electrical, and electrochemical properties of Nd-doped Ba0.5Sr0.5Co0.8Fe0.2O3d as a cathode material for SOFC. Solid State Ionics 2008;178:1853e8. Kozhevnikov VL, Leonidov IA, Patrakeev MV, Mitberg EB, Poeppelmeier KR. Electrical properties of the ferrite SrFeOy at high temperatures. J Solid State Chem 2001;158:320e6. Rembelski D, Viricelle JP, Combemale L, Rieu M. Characterization and comparison of different cathode materials for SC- SOFC : LSM, BSCF, SSC, and LSCF. Fuel Cells 2012;12:256e64. Zhang Z, Chen D, Dong F, Xu X, Hao Y, Shao Z. Understanding the doping effect toward the design of CO2tolerant perovskite membranes with enhanced oxygen permeability. J Memb Sci 2016;519:11e21. Zhou Q, Gao Y, Wang F, An D, Li Y, Zou Y, et al. Novel cobaltfree cathode material (Nd0.9La0.1)2(Ni0.74Cu0.21Al0.05)O4þd for intermediate-temperature solid oxide fuel cells. Ceram Int 2015;41:639e43. Gao L, Zhu M, Li Q, Sun L, Zhao H, Grenier J. Electrode properties of Cu-doped Bi0.5Sr0.5FeO3-d cobalt-free perovskite as cathode for intermediate-temperature solid oxide fuel cells. J Alloys Compd 2017;700:29e36. Molin S, Lewandowska-Iwaniak W, Kusz B, Gazda M, Jasinski P. Structural and electrical properties of Sr(Ti, Fe)O3-d materials for SOFC cathodes. J Electroceramics 2012;28:80e7.
[42] Grilo DF, Macedo DA, Ces R, Moura CG, Paul D, Nascimento RM. Cobalt-free perovskite Pr0.5Sr0.5Fe1-xCuxO3d (PSFC) as a cathode material for intermediate temperature solid oxide fuel cells. Mater Chem Phys 2016;180:256e62. [43] Lenka RK, Mahata T, Patro PK, Tyagi a K, Sinha PK. Synthesis and characterization of GdCoO3 as a potential SOFC cathode material. J Alloys Compd 2012;537:100e5. [44] Muhammed Ali SA, Anwar M, Somalu MR, Muchtar A. Enhancement of the interfacial polarization resistance of La0.6Sr0.4Co0.2Fe0.8O3-d cathode by microwave-assisted combustion method. Ceram Int 2016;43:4647e54. [45] Prestat M, Morandi A, Heel A, Holzer L, Holtappels P, Graule TJ. Effect of graphite pore former on oxygen electrodes prepared with La0.6Sr0.4CoO3-d. Electrochem Commun 2010;12:292e5. [46] de Larramendi IR, Ortiz-Vitoriano N, Dzul-Bautista IB, Rojo T. Designing Perovskite Oxides for Solid Oxide Fuel Cells. Perovskite Mater. - Synth. Characterisation, Prop. Appl. 2016. p. 589e617. [47] Somalu MR, Muchtar A, Daud WRW, Brandon NP. Screenprinting inks for the fabrication of solid oxide fuel cell films: a review. Renew Sustain Energy Rev 2017;75:426e39. [48] Yu X, Long W, Jin F, He T. Cobalt-free perovskite cathode materials SrFe1xTixO3d and performance optimization for intermediate-temperature solid oxide fuel cells. Electrochim Acta 2014;123:426e34. [49] Fan L, Zhang H, Chen M, Wang C, Wang H, Singh M, et al. Electrochemical study of lithiated transition metal oxide composite as symmetrical electrode for low temperature ceramic fuel cells. Int J Hydrogen Energy 2013;38: 11398e405. [50] Wang L. BSCF SOFC Cathode Materials: bulk properties, kinetics and mechanism of oxygen reduction. 2009. ndez-Ropero AJ, Porras-Va zquez JM, Cabeza A, [51] Ferna pez D, Losilla ER. High valence Slater PR, Marrero-Lo transition metal doped strontium ferrites for electrode materials in symmetrical SOFCs. J Power Sources 2014;249:405e13. [52] Sendilkumar A, Babu PD, Manivel Raja M, Reddy VR, Gupta A, € ssbauer investigation of Srinath S. Structural and Mo nanocrystalline SrFe1-xTixO3-d. J Am Ceram Soc 2013;96:2973e8. [53] Dong F, Ni M, He W, Chen Y, Yang G, Chen D. An efficient electrocatalyst as cathode material for solid oxide fuel: BaFe0.95Sn0.05O3-d. J Power Sources 2016;326:459e65. [54] Jiang S, Zhou W, Niu Y, Zhu Z, Shao Z. Phase transition of a cobalt-free perovskite as a high-performance cathode for intermediate-temperature solid oxide fuel cells. ChemSusChem 2012;5:2023e31. [55] Yin J, Zhang C, Yin Y, Shi H, Lin Y, Lu J, et al. Anodesupported single-chamber solid oxide fuel cell based on cobalt-free composite cathode of at intermediate temperatures. J Power Sources 2015;286:217e23.
Please cite this article in press as: Baharuddin NA, et al., Synthesis and characterization of cobalt-free SrFe0.8Ti0.2O3-d cathode powders synthesized through combustion method for solid oxide fuel cells, International Journal of Hydrogen Energy (2018), https://doi.org/ 10.1016/j.ijhydene.2018.01.210