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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 Department of Mechanical and Materials Engineering, 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 powders were synthesized through the combustion
Received 14 June 2018
method. Results of thermogravimetric and Fourier transform infrared analyses suggested
Received in revised form
that a perovskite oxide started to form at temperatures above 1100 C. X-ray diffraction and
17 October 2018
Rietveld refinement analyses confirmed that the single-phase cubic structure (Pm-3m) of
Accepted 15 November 2018
the SrFe0$8Ti0$2O3-d cathode was produced after calcination at 1300 C. The average sizes of
Available online xxx
the 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
Keywords:
dispersive X-ray analysis coupled with mapping revealed the homogeneous distribution of
Solid oxide fuel cell
elements in the cathode. The thermal expansion coefficient of the SrFe0$8Ti0$2O3-d cathode
Cobalt-free cathode
is 16.20 106 K1. For the electrochemical behavior, the area specific resistance of cathode
Perovskite
(0.60e13.57 U cm2) was obtained at 600e800
Particle size
121.77 kJ mol1. This work confirmed the potential of a SrFe0$8Ti0$2O3-d cathode in the in-
Area specific resistance
termediate temperature solid oxide fuel cell.
C, and the activation energy was
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Solid oxide fuel cells (SOFCs) are a type of energy converter device with a high efficiency and operating temperature of approximately 1000 C [1,2]. Reducing the operating temperature to the intermediate range (600e800 C) is important in minimizing material cost and avoiding cell degradation [3e5]. However, this approach reduces the performance of the main
components of SOFCs, such as the anode, electrolyte, and cathode. The oxygen reduction reaction rate in the cathode is due to existing material limitations (lanthanum strontium manganite). Thus, new materials have been proposed to improve the performance of intermediate-temperature SOFC cathodes [6,7]. Perovskite cathode was introduced to fill this research gap. Perovskite oxide is suitable as an intermediatetemperature SOFC due to its mixed ionic-electronic
* 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.2018.11.142 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article 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, https://doi.org/10.1016/ j.ijhydene.2018.11.142
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conducting property, wherein the oxygen reduction reaction occurs not only at the triple-phase boundary (the specific point where cathode, electrolyte, and gas meet) but also throughout the bulk cathode [8]. Introducing of perovskite cathode reduces the area specific resistance (ASR), which is the desired outcome in developing novel cathode materials for intermediate-temperature SOFCs. A number of perovskite cathodes operated at this temperature range have been reported. Perovskite cathode materials, such as La0$6Sr0$4Co0$2 Fe0$8O3-d (LSCF), NdBa1-xCo2O5þd, and Pr1-xBaCo2O5þd, have been reported; they all contain cobalt [9e11]. Cobaltcontaining cathodes were believed to exhibit high electrocatalytic activity. Despite this advantage, the presence of cobalt in these cathodes causes a higher thermal expansion coefficient (TEC) compared with that of the commonly used electrolytes, such as Sm0.2Ce0$8O1.9 (SDC). SDC electrolyte is known for its high ionic conductivity under an intermediatetemperature SOFC environment; the TEC value of SDC is ~12.5 106 K1 [12,13], whereas the average TEC of cobaltcontaining intermediate-temperature SOFCs is from 20 to 22 106 K1 [14,15]. One way to reduce the TEC of a cobaltcontaining cathode is to introduce an electrolyte material into the cathode, which is called a composite cathode (e.g., LSCF-SDC) [16]. Moreover, the 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 [17,18]. Despite the reduction in TEC to below 20.0 106 K1, the ASR value remains higher than that of cobalt-containing cathodes (e.g., LSCF with an ASR of 0.062 U cm2 at 744 C) [19]. Thus, various materials have been explored to reduce the ASR of cobalt-free cathodes. SrFeO3-d materials were first introduced into the oxygen gas sensor application [20]. These materials were subsequently introduced in the SOFC field and applied in cobalt-free cathodes because of their excellent performance in an oxygen reduction environment displaying ASR values lower than 1.0 U cm2 for example, SrFe0$9Nb0$1O3-d (0.138 U cm2) and SrFe0$95Ti0$05 O3-d (0.12 U cm2), both at 750 C [21,22]. Nb and Ti are excellent candidates for B-site dopant (as Fe substitutes) in the SrFeO3-dbased cathode; however, in terms of the ionic radius tolerance factor, Ti showed a better range of 0.986e1.009 according to the literature [23]. At an operating temperature of 800 C, the ASR value for SrFe0$95Ti0$05O3-d cathode reached 0.058 U cm2. Unfortunately, this value remains questionable because of the addition of silver paste as the current collector layer at the surface of the electrodes alongside the current built-in collecting layer, such as silver or platinum wire/mesh [21]. In this work, Ti-doped SrFeO3-d cobalt-free cathode was produced through the combustion method with a different dopant mole ratio (0.2) against to the literature (0.05). The purity, particle size, and homogeneity of the elements in the SrFe0$8Ti0$2O3-d cobalt-free cathode powder were specifically discussed. Moreover, the electrochemical capability of this pure SrFe0$8Ti0$2O3-d cobalt-free cathode was determined by
calculating the ASR without an additional current collecting layer at cathode surfaces.
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 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) powders (Sigma-Aldrich) were pressed into coin-shaped structure to form an electrolyte substrate at 52 MPa. The SDC electrolyte was sintered at 1400 C for 6 h.
Please cite this article 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, https://doi.org/10.1016/ j.ijhydene.2018.11.142
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Then, the as-prepared cathode inks were 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 subjected to electrical conductivity analysis using the van der Pauw technique at 600e800 C with an air flow rate of 200 ml/min, as shown in Fig. 1 (a). Then, the symmetrical cell was used for EIS analysis (potentiostat PGSTAT302 N, Metrohm Autolab, Netherlands) coupled with NOVA software (version 1.10). The analysis was conducted at 10 mV in the same environment and operating temperature as the electrical conductivity analysis. The collection of current is solely dependent on the platinum mesh located at the top and bottom of the sample holder as shown in Fig. 1 (b), without the addition of a current collector such as silver paste, on the cathode surfaces. After the cathode performance was evaluated, the microstructure of the cathode layer was characterized via field emission scanning electron microscopy (FESEM) analysis. FESEM (Zeiss SUPRA 55VP, Germany) was conducted on the surfaces and cross-sections of the SrFe0$8Ti0$2O3-d cathode film for microstructural analysis. The FESEM surface micrograph was further analyzed using ImageJ software (National Institutes of Health, USA) to determine the porosity of the SrFe0$8Ti0$2O3-d cathode film.
Results and discussion Thermal decomposition analysis of precursor powders The thermal decomposition analysis of the SrFe0$8Ti0$2O3-d precursor powders indicated two stages of weight loss. Fig. 2 shows the TG analysis (TGA) and derivative thermogravimetric (DTG) curves. The weight loss recorded at 600 C was due to the elimination of water and nitrate molecules [24]. Then, a slight increase in weight was recorded at about 500 C due to the buoyancy effect [25]. In TGA, as the temperature increased, the density of the atmosphere in the balance decreased and caused an apparent mass gain in the sample or
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Fig. 2 e TGA-DTG curves of precursor SrFe0·8Ti0·2O3-d powders.
crucible [25,26]. This phenomenon is usually recorded at the early stage of heating and is depended on parameters, such as sample/crucible volume and atmospheric density. These parameters are affected by the temperature; the effects may vary from one experiment to another [26]. The DTG curve shows that a considerable weight loss occurred at 800e1000 C. The weight loss observed in this region was due to the elimination of carbon residue from the powders [27]. The weight loss stopped at ~1080 C, and the TGA curve started to flatten above this temperature. Thus, the calcination of SrFe0$8Ti0$2O3-d precursor powders should be conducted at temperatures higher than 1080 C. To observe the elimination of the undesired phase, calcination was conducted at temperatures below and above 1080 C. The temperature range was studied in our previous work with different compositions of SrFe0$5Ti0$5O3-d [28]. In this work, the minimum and maximum tested calcination temperatures (700 and 1300 C) were utilized to observe the phase elimination in SrFe0$8Ti0$2O3-d powders.
Fig. 1 e Schematic of the (a) Van der Pauw technique for electrical conductivity measurement and, (b) EIS measurement. Please cite this article 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, https://doi.org/10.1016/ j.ijhydene.2018.11.142
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The precursor and calcined SrFe0$8Ti0$2O3-d were subjected to FTIR analysis to observe the formation of pure SrFe0$8Ti0$2O3-d powders (Fig. 3). The presence of cathode material produced from perovskite oxide is confirmed by the formation of downward peak at a wavenumber of ~600 cm1 [29]. 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 OeH functional group (bending mode) [30]. 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 X-ray diffraction (XRD) analysis was conducted to confirm the phase of perovskite oxide. Fig. 4 shows the XRD pattern of cobalt-free 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 same phase structure for SrFe0$8Ti0$2O3-d was reported by Cowin et al. (2015) which was produced using a solgel technique. The pure cubic phase SrFe0$8Ti0$2O3-d powders were produced only after triple stage calcination at 600 C (2 h), 1100 C (24 h) and 1300 C (12 h) [31]. The overall duration for the heat treatment process of 38 h is too long compared with only 5 h in this study. Asides from the sol-gel method, the polymerized complex method was also applied by Demizu et al. (2017) who reported pure SrFe0$8Ti0$2O3-d powders [32]. However, the tetragonal structure with the space group I4/mm
Fig. 4 e XRD pattern of SrFe0·8Ti0·2O3-d powders.
is generally unsuitable for cathode because the cubic phase is more stable. 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) [33] 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 [34,35]. A study reported the same phase structure for pure cobaltfree cathode produced using SrFe0$9Ti0$1O3-d through solegel method [36]. Similar to other wet processes (e.g., sol‒gel 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 [37]. 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
Fig. 3 e FTIR spectra of precursor and calcined SrFe0·8Ti0·2O3-d powders.
The average particle size of SrFe0$8Ti0$2O3-d powders was measured by using FESEM (Fig. 5a) and DLS analyses (Fig. 5b); the average particle size recorded (Zave) was 1.0827 and 1.4438 mm, respectively. Zave values expressed in micrometer were recorded due to particle agglomeration during sample preparation [38]. In this study, FESEM and DLS were used to measure the agglomerate sizes, and the smallest particle size recorded is 200e300 nm. Morever, Fig. 5a indicates that the minimum particle size was obtained through FESEM. The average particle size recorded is smaller than that of SrFe0$8Ti0$2O3-d powders produced by a polymerized complex method (2 mm) and other cobalt-free cathode powders synthesized using solid-state method (e.g., Sr0$9K0$1FeO3-d) [32,39]. Fig. 6 shows the EDX mapping result for SrFe0$8Ti0$2O3-d powders. The mapping indicates homogeneous elemental (Sr, Fe
Please cite this article 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, https://doi.org/10.1016/ j.ijhydene.2018.11.142
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Fig. 5 e FESEM micrograph of SrFe0·8Ti0·2O3-d powders (a) and average particle size distribution measured through (b) FESEM and (c) DLS analyses.
and Ti) distribution in cobalt-free cathode SrFe0$8Ti0$2O3-d powders. Moreover, the weight percentage of each element is confirmed to be similar to the initially calculated value.
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 that of the existing cobalt-containing cathode. Fig. 7 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 obtained in this work. The further change in the relative
Fig. 6 e EDX spectrum and elemental mapping results for SrFe0·8Ti0·2O3-d powders.
length with increasing temperature is due to the expansion of the lattice with respect to the reduction of cation valence (Fe4þ to Fe3þ) [40]. The reported TEC values for cobalt-free cathode is lower than 19 106 K1 [40e43]. In addition, the TEC values for cobalt-free SrFe0$8Ti0$2O3-d is not only lower than that of the cobalt-containing cathode, as mentioned above, but is also better than that of other SrFeO3-d-based cathodes, such as SrFe0$95Ti0$05O3-d (TEC ¼ 25.9 106 K1) synthesized through solid-state reaction method [21]. This difference in TEC values is not only limited to the cathode composition, but is also influenced by properties of the material including bond strength, lattice structure, crystal defects and impurities [44e46]. This finding will require further studies to allow a better understanding of TEC behavior on cobalt-free cathode materials.
Fig. 7 e Thermal expansion curve of SrFe0·8Ti0·2O3-d cathode at 30e1000 C in air.
Please cite this article 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, https://doi.org/10.1016/ j.ijhydene.2018.11.142
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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. 8 shows the electrical conductivity of cobalt-free SrFe0$8Ti0$2 O3-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 [47]. The increasing oxygen loss produced more vacancies which acted as electron trapper, resulting in the reduction of carrier mobility and conductivity [48]. 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 [49]. In this work, the conductivity of cobalt-free SrFe0$8Ti0$2O3-d cathode did not show any change in behavior even at the maximum operating temperature (800 C), thereby suggesting that the introduction of Ti as a 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 lower compared with that of cobalt-containing cathodes, such as LSCF and BSCF with a conductivity of 30e400 Scm1 in the intermediate temperature range (600e800 C) in SOFC [50]. This difference is expected because of the good conductivity of cobalt, aside from its excellent electro-catalytic characteristic [4]. In comparison, the electrical conductivity of SrFe1-xTixO3-d (x ¼ 0.00e0.15) cobalt-free cathode in this work is remarkably different from that the literature, which is 72 cm1 at 650 C. The bulk cathode tested in the literature work has high density reaching up to 94%, thus explaining the better electrical path given that the grains are well-connected [51].
Fig. 8 e Electrical conductivity for cobalt-free SrFe0·8Ti0·2 O3-d cathode at 600e800 C.
Nevertheless, the conductivity of cobalt-free SrFe0$8Ti0$2 O3-d cathode synthesized in this study is better compared to the same material produced using the solid-state method [52]. 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)O4-d cathode with electrical conductivity below 60 S cm1 still managed to achieved desired electrochemical performance with ASR below the targeted value of 1 U cm2 [53]. 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 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 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. 9. 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. 9 belongs to the medium-low frequency regions, in which the equivalent circuit was obtained through Nyquist curve fitting. On the basis of the equivalent circuit in Fig. 9 (inset), the R0 attributed to the ohmic resistance will not be included in calculating the ASR of the cathode. The ASR value for cobalt-free SrFe0$8Ti0$2O3-d cathode was calculated using R1 and R2. The values for R1 and R2, which are shown in Table 1. R1 comprises the resistance at the interfacial between the cathode-electrolyte surface, whereas R2 is contributed by from the resistance of bulk cathode in the oxygen adsorptiondesorption process [54]. These interfacial and bulk resistance values must be summed up to determine the total polarization
Please cite this article 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, https://doi.org/10.1016/ j.ijhydene.2018.11.142
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Fig. 9 e EIS Nyquist plots for cobalt-free SrFe0·8Ti0·2O3 d cathode at 600e800 C.
Table 1 e EIS fitting parameters for SrFe0.8Ti0.2O3d cathode. Operation temperature ( C) 800 750 700 650 600
R1 (U)
R2 (U)
RT ¼ R1þR2 (U)
ASR ¼ RT/2 (U cm2)
1.02 1.94 2.51 4.83 9.77
0.18 0.20 2.15 5.19 17.36
1.20 2.14 4.66 10.02 27.13
0.60 1.07 2.33 5.01 13.57
resistance of the cathode component. The ASR of the cathode is measured in Eq. (1) [55] as follows: ASR ¼ RT A=2
(1)
where RT ¼ R1 þ R2 and the active area of cathode, A, is 1 cm2. The ASR of cobalt-free SrFe0$8Ti0$2O3-d cathode is 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 are slightly lower compared than those of other previous results. Grilo et al. (2016) reported that the ASR for the cobalt-free Pr0.5Sr0$5Fe0$8Cu0$2O3 cathode at an operating temperature of 800 C is 0.62 U cm2 [56], whereas other researchers reported an ASR value of 1.1 U cm2 for the newly developed cobalt-containing GdCoO3 cathode [57]. The ASR
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value for common cobalt-containing cathodes, such as La0$6Sr0$4Co0$2Fe0$8O3-d, for intermediate-temperature SOFC is below 0.10 U cm2 [58], which is still far lower than that of 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 the oxygen reduction reaction is increased [59e61]. Microstructural analysis using FESEM on the cobalt-free SrFe0$8Ti0$2O3-d cathode film showed that the film contains pores (Fig. 10). The acceptable porosity range for SOFC electrode is between 20 and 40% [62,63]. The FESEM surface micrographs were then subjected to image analysis using ImageJ, and a surface porosity of 22.6 ± 0.5% was observed, thereby proving that the cobalt-free SrFe0$8Ti0$2O3-d cathode can still be improved by introducing a pore former. 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 obtained a lower ASR value of 0.034e0.053 U cm2 at 800 C [64]. However, this low ASR value is believed to be due to the introduction of silver paste as the current collector given that the additional current collector layer can overshadow the real cathode performance in an oxygen reduction reaction environment [65]. Unlike 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. 11). The slope of the linear fitting was used to calculate the activation energy, Ea, of the cathode [66]. The Ea value for the cobalt-free SrFe0$8Ti0$2O3-d cathode is 121.77 kJ mol1, which is lower than the Ea value for SrFeO3-d (173.70 kJ mol1) [67] due to the stabilization of the cubic phase with the introduction of titanium as dopant [23]. 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) [36]. The Ea calculated from ASR values reported by Yang et al. is 110. These materials were subsequently introduced in SOFC field and 9 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 [35]. The big lattice reduced
Fig. 10 e FESEM surface and cross-sectioned micrograph for SrFe0·8Ti0·2O3-d cathode film. Please cite this article 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, https://doi.org/10.1016/ j.ijhydene.2018.11.142
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Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2018.11.142.
references
Fig. 11 e Arrhenius plot for cobalt-free SrFe0·8Ti0·2O3-d cathode at 600e800 C.
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$2 O3-d-Sm0.2Ce0$8O1.9, whose Ea values are 124, 133, and 125.3e133.4 kJ mol1, respectively [68e70]. The ASR and Ea 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. SrFe0.8Ti0.2O3d 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-d-based 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). 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.
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Available online at www.sciencedirect.com
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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 Department of Mechanical and Materials Engineering, 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 powders were synthesized through the combustion
Received 14 June 2018
method. Results of thermogravimetric and Fourier transform infrared analyses suggested
Received in revised form
that a perovskite oxide started to form at temperatures above 1100 C. X-ray diffraction and
17 October 2018
Rietveld refinement analyses confirmed that the single-phase cubic structure (Pm-3m) of
Accepted 15 November 2018
the SrFe0$8Ti0$2O3-d cathode was produced after calcination at 1300 C. The average sizes of
Available online xxx
the 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
Keywords:
dispersive X-ray analysis coupled with mapping revealed the homogeneous distribution of
Solid oxide fuel cell
elements in the cathode. The thermal expansion coefficient of the SrFe0$8Ti0$2O3-d cathode
Cobalt-free cathode
is 16.20 106 K1. For the electrochemical behavior, the area specific resistance of cathode
Perovskite
(0.60e13.57 U cm2) was obtained at 600e800
Particle size
121.77 kJ mol1. This work confirmed the potential of a SrFe0$8Ti0$2O3-d cathode in the in-
Area specific resistance
termediate temperature solid oxide fuel cell.
C, and the activation energy was
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Solid oxide fuel cells (SOFCs) are a type of energy converter device with a high efficiency and operating temperature of approximately 1000 C [1,2]. Reducing the operating temperature to the intermediate range (600e800 C) is important in minimizing material cost and avoiding cell degradation [3e5]. However, this approach reduces the performance of the main
components of SOFCs, such as the anode, electrolyte, and cathode. The oxygen reduction reaction rate in the cathode is due to existing material limitations (lanthanum strontium manganite). Thus, new materials have been proposed to improve the performance of intermediate-temperature SOFC cathodes [6,7]. Perovskite cathode was introduced to fill this research gap. Perovskite oxide is suitable as an intermediatetemperature SOFC due to its mixed ionic-electronic
* 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.2018.11.142 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article 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, https://doi.org/10.1016/ j.ijhydene.2018.11.142
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conducting property, wherein the oxygen reduction reaction occurs not only at the triple-phase boundary (the specific point where cathode, electrolyte, and gas meet) but also throughout the bulk cathode [8]. Introducing of perovskite cathode reduces the area specific resistance (ASR), which is the desired outcome in developing novel cathode materials for intermediate-temperature SOFCs. A number of perovskite cathodes operated at this temperature range have been reported. Perovskite cathode materials, such as La0$6Sr0$4Co0$2 Fe0$8O3-d (LSCF), NdBa1-xCo2O5þd, and Pr1-xBaCo2O5þd, have been reported; they all contain cobalt [9e11]. Cobaltcontaining cathodes were believed to exhibit high electrocatalytic activity. Despite this advantage, the presence of cobalt in these cathodes causes a higher thermal expansion coefficient (TEC) compared with that of the commonly used electrolytes, such as Sm0.2Ce0$8O1.9 (SDC). SDC electrolyte is known for its high ionic conductivity under an intermediatetemperature SOFC environment; the TEC value of SDC is ~12.5 106 K1 [12,13], whereas the average TEC of cobaltcontaining intermediate-temperature SOFCs is from 20 to 22 106 K1 [14,15]. One way to reduce the TEC of a cobaltcontaining cathode is to introduce an electrolyte material into the cathode, which is called a composite cathode (e.g., LSCF-SDC) [16]. Moreover, the 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 [17,18]. Despite the reduction in TEC to below 20.0 106 K1, the ASR value remains higher than that of cobalt-containing cathodes (e.g., LSCF with an ASR of 0.062 U cm2 at 744 C) [19]. Thus, various materials have been explored to reduce the ASR of cobalt-free cathodes. SrFeO3-d materials were first introduced into the oxygen gas sensor application [20]. These materials were subsequently introduced in the SOFC field and applied in cobalt-free cathodes because of their excellent performance in an oxygen reduction environment displaying ASR values lower than 1.0 U cm2 for example, SrFe0$9Nb0$1O3-d (0.138 U cm2) and SrFe0$95Ti0$05 O3-d (0.12 U cm2), both at 750 C [21,22]. Nb and Ti are excellent candidates for B-site dopant (as Fe substitutes) in the SrFeO3-dbased cathode; however, in terms of the ionic radius tolerance factor, Ti showed a better range of 0.986e1.009 according to the literature [23]. At an operating temperature of 800 C, the ASR value for SrFe0$95Ti0$05O3-d cathode reached 0.058 U cm2. Unfortunately, this value remains questionable because of the addition of silver paste as the current collector layer at the surface of the electrodes alongside the current built-in collecting layer, such as silver or platinum wire/mesh [21]. In this work, Ti-doped SrFeO3-d cobalt-free cathode was produced through the combustion method with a different dopant mole ratio (0.2) against to the literature (0.05). The purity, particle size, and homogeneity of the elements in the SrFe0$8Ti0$2O3-d cobalt-free cathode powder were specifically discussed. Moreover, the electrochemical capability of this pure SrFe0$8Ti0$2O3-d cobalt-free cathode was determined by
calculating the ASR without an additional current collecting layer at cathode surfaces.
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 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) powders (Sigma-Aldrich) were pressed into coin-shaped structure to form an electrolyte substrate at 52 MPa. The SDC electrolyte was sintered at 1400 C for 6 h.
Please cite this article 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, https://doi.org/10.1016/ j.ijhydene.2018.11.142
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Then, the as-prepared cathode inks were 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 subjected to electrical conductivity analysis using the van der Pauw technique at 600e800 C with an air flow rate of 200 ml/min, as shown in Fig. 1 (a). Then, the symmetrical cell was used for EIS analysis (potentiostat PGSTAT302 N, Metrohm Autolab, Netherlands) coupled with NOVA software (version 1.10). The analysis was conducted at 10 mV in the same environment and operating temperature as the electrical conductivity analysis. The collection of current is solely dependent on the platinum mesh located at the top and bottom of the sample holder as shown in Fig. 1 (b), without the addition of a current collector such as silver paste, on the cathode surfaces. After the cathode performance was evaluated, the microstructure of the cathode layer was characterized via field emission scanning electron microscopy (FESEM) analysis. FESEM (Zeiss SUPRA 55VP, Germany) was conducted on the surfaces and cross-sections of the SrFe0$8Ti0$2O3-d cathode film for microstructural analysis. The FESEM surface micrograph was further analyzed using ImageJ software (National Institutes of Health, USA) to determine the porosity of the SrFe0$8Ti0$2O3-d cathode film.
Results and discussion Thermal decomposition analysis of precursor powders The thermal decomposition analysis of the SrFe0$8Ti0$2O3-d precursor powders indicated two stages of weight loss. Fig. 2 shows the TG analysis (TGA) and derivative thermogravimetric (DTG) curves. The weight loss recorded at 600 C was due to the elimination of water and nitrate molecules [24]. Then, a slight increase in weight was recorded at about 500 C due to the buoyancy effect [25]. In TGA, as the temperature increased, the density of the atmosphere in the balance decreased and caused an apparent mass gain in the sample or
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Fig. 2 e TGA-DTG curves of precursor SrFe0·8Ti0·2O3-d powders.
crucible [25,26]. This phenomenon is usually recorded at the early stage of heating and is depended on parameters, such as sample/crucible volume and atmospheric density. These parameters are affected by the temperature; the effects may vary from one experiment to another [26]. The DTG curve shows that a considerable weight loss occurred at 800e1000 C. The weight loss observed in this region was due to the elimination of carbon residue from the powders [27]. The weight loss stopped at ~1080 C, and the TGA curve started to flatten above this temperature. Thus, the calcination of SrFe0$8Ti0$2O3-d precursor powders should be conducted at temperatures higher than 1080 C. To observe the elimination of the undesired phase, calcination was conducted at temperatures below and above 1080 C. The temperature range was studied in our previous work with different compositions of SrFe0$5Ti0$5O3-d [28]. In this work, the minimum and maximum tested calcination temperatures (700 and 1300 C) were utilized to observe the phase elimination in SrFe0$8Ti0$2O3-d powders.
Fig. 1 e Schematic of the (a) Van der Pauw technique for electrical conductivity measurement and, (b) EIS measurement. Please cite this article 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, https://doi.org/10.1016/ j.ijhydene.2018.11.142
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The precursor and calcined SrFe0$8Ti0$2O3-d were subjected to FTIR analysis to observe the formation of pure SrFe0$8Ti0$2O3-d powders (Fig. 3). The presence of cathode material produced from perovskite oxide is confirmed by the formation of downward peak at a wavenumber of ~600 cm1 [29]. 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 OeH functional group (bending mode) [30]. 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 X-ray diffraction (XRD) analysis was conducted to confirm the phase of perovskite oxide. Fig. 4 shows the XRD pattern of cobalt-free 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 same phase structure for SrFe0$8Ti0$2O3-d was reported by Cowin et al. (2015) which was produced using a solgel technique. The pure cubic phase SrFe0$8Ti0$2O3-d powders were produced only after triple stage calcination at 600 C (2 h), 1100 C (24 h) and 1300 C (12 h) [31]. The overall duration for the heat treatment process of 38 h is too long compared with only 5 h in this study. Asides from the sol-gel method, the polymerized complex method was also applied by Demizu et al. (2017) who reported pure SrFe0$8Ti0$2O3-d powders [32]. However, the tetragonal structure with the space group I4/mm
Fig. 4 e XRD pattern of SrFe0·8Ti0·2O3-d powders.
is generally unsuitable for cathode because the cubic phase is more stable. 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) [33] 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 [34,35]. A study reported the same phase structure for pure cobaltfree cathode produced using SrFe0$9Ti0$1O3-d through solegel method [36]. Similar to other wet processes (e.g., sol‒gel 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 [37]. 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
Fig. 3 e FTIR spectra of precursor and calcined SrFe0·8Ti0·2O3-d powders.
The average particle size of SrFe0$8Ti0$2O3-d powders was measured by using FESEM (Fig. 5a) and DLS analyses (Fig. 5b); the average particle size recorded (Zave) was 1.0827 and 1.4438 mm, respectively. Zave values expressed in micrometer were recorded due to particle agglomeration during sample preparation [38]. In this study, FESEM and DLS were used to measure the agglomerate sizes, and the smallest particle size recorded is 200e300 nm. Morever, Fig. 5a indicates that the minimum particle size was obtained through FESEM. The average particle size recorded is smaller than that of SrFe0$8Ti0$2O3-d powders produced by a polymerized complex method (2 mm) and other cobalt-free cathode powders synthesized using solid-state method (e.g., Sr0$9K0$1FeO3-d) [32,39]. Fig. 6 shows the EDX mapping result for SrFe0$8Ti0$2O3-d powders. The mapping indicates homogeneous elemental (Sr, Fe
Please cite this article 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, https://doi.org/10.1016/ j.ijhydene.2018.11.142
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Fig. 5 e FESEM micrograph of SrFe0·8Ti0·2O3-d powders (a) and average particle size distribution measured through (b) FESEM and (c) DLS analyses.
and Ti) distribution in cobalt-free cathode SrFe0$8Ti0$2O3-d powders. Moreover, the weight percentage of each element is confirmed to be similar to the initially calculated value.
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 that of the existing cobalt-containing cathode. Fig. 7 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 obtained in this work. The further change in the relative
Fig. 6 e EDX spectrum and elemental mapping results for SrFe0·8Ti0·2O3-d powders.
length with increasing temperature is due to the expansion of the lattice with respect to the reduction of cation valence (Fe4þ to Fe3þ) [40]. The reported TEC values for cobalt-free cathode is lower than 19 106 K1 [40e43]. In addition, the TEC values for cobalt-free SrFe0$8Ti0$2O3-d is not only lower than that of the cobalt-containing cathode, as mentioned above, but is also better than that of other SrFeO3-d-based cathodes, such as SrFe0$95Ti0$05O3-d (TEC ¼ 25.9 106 K1) synthesized through solid-state reaction method [21]. This difference in TEC values is not only limited to the cathode composition, but is also influenced by properties of the material including bond strength, lattice structure, crystal defects and impurities [44e46]. This finding will require further studies to allow a better understanding of TEC behavior on cobalt-free cathode materials.
Fig. 7 e Thermal expansion curve of SrFe0·8Ti0·2O3-d cathode at 30e1000 C in air.
Please cite this article 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, https://doi.org/10.1016/ j.ijhydene.2018.11.142
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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. 8 shows the electrical conductivity of cobalt-free SrFe0$8Ti0$2 O3-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 [47]. The increasing oxygen loss produced more vacancies which acted as electron trapper, resulting in the reduction of carrier mobility and conductivity [48]. 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 [49]. In this work, the conductivity of cobalt-free SrFe0$8Ti0$2O3-d cathode did not show any change in behavior even at the maximum operating temperature (800 C), thereby suggesting that the introduction of Ti as a 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 lower compared with that of cobalt-containing cathodes, such as LSCF and BSCF with a conductivity of 30e400 Scm1 in the intermediate temperature range (600e800 C) in SOFC [50]. This difference is expected because of the good conductivity of cobalt, aside from its excellent electro-catalytic characteristic [4]. In comparison, the electrical conductivity of SrFe1-xTixO3-d (x ¼ 0.00e0.15) cobalt-free cathode in this work is remarkably different from that the literature, which is 72 cm1 at 650 C. The bulk cathode tested in the literature work has high density reaching up to 94%, thus explaining the better electrical path given that the grains are well-connected [51].
Fig. 8 e Electrical conductivity for cobalt-free SrFe0·8Ti0·2 O3-d cathode at 600e800 C.
Nevertheless, the conductivity of cobalt-free SrFe0$8Ti0$2 O3-d cathode synthesized in this study is better compared to the same material produced using the solid-state method [52]. 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)O4-d cathode with electrical conductivity below 60 S cm1 still managed to achieved desired electrochemical performance with ASR below the targeted value of 1 U cm2 [53]. 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 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 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. 9. 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. 9 belongs to the medium-low frequency regions, in which the equivalent circuit was obtained through Nyquist curve fitting. On the basis of the equivalent circuit in Fig. 9 (inset), the R0 attributed to the ohmic resistance will not be included in calculating the ASR of the cathode. The ASR value for cobalt-free SrFe0$8Ti0$2O3-d cathode was calculated using R1 and R2. The values for R1 and R2, which are shown in Table 1. R1 comprises the resistance at the interfacial between the cathode-electrolyte surface, whereas R2 is contributed by from the resistance of bulk cathode in the oxygen adsorptiondesorption process [54]. These interfacial and bulk resistance values must be summed up to determine the total polarization
Please cite this article 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, https://doi.org/10.1016/ j.ijhydene.2018.11.142
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Fig. 9 e EIS Nyquist plots for cobalt-free SrFe0·8Ti0·2O3 d cathode at 600e800 C.
Table 1 e EIS fitting parameters for SrFe0.8Ti0.2O3d cathode. Operation temperature ( C) 800 750 700 650 600
R1 (U)
R2 (U)
RT ¼ R1þR2 (U)
ASR ¼ RT/2 (U cm2)
1.02 1.94 2.51 4.83 9.77
0.18 0.20 2.15 5.19 17.36
1.20 2.14 4.66 10.02 27.13
0.60 1.07 2.33 5.01 13.57
resistance of the cathode component. The ASR of the cathode is measured in Eq. (1) [55] as follows: ASR ¼ RT A=2
(1)
where RT ¼ R1 þ R2 and the active area of cathode, A, is 1 cm2. The ASR of cobalt-free SrFe0$8Ti0$2O3-d cathode is 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 are slightly lower compared than those of other previous results. Grilo et al. (2016) reported that the ASR for the cobalt-free Pr0.5Sr0$5Fe0$8Cu0$2O3 cathode at an operating temperature of 800 C is 0.62 U cm2 [56], whereas other researchers reported an ASR value of 1.1 U cm2 for the newly developed cobalt-containing GdCoO3 cathode [57]. The ASR
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value for common cobalt-containing cathodes, such as La0$6Sr0$4Co0$2Fe0$8O3-d, for intermediate-temperature SOFC is below 0.10 U cm2 [58], which is still far lower than that of 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 the oxygen reduction reaction is increased [59e61]. Microstructural analysis using FESEM on the cobalt-free SrFe0$8Ti0$2O3-d cathode film showed that the film contains pores (Fig. 10). The acceptable porosity range for SOFC electrode is between 20 and 40% [62,63]. The FESEM surface micrographs were then subjected to image analysis using ImageJ, and a surface porosity of 22.6 ± 0.5% was observed, thereby proving that the cobalt-free SrFe0$8Ti0$2O3-d cathode can still be improved by introducing a pore former. 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 obtained a lower ASR value of 0.034e0.053 U cm2 at 800 C [64]. However, this low ASR value is believed to be due to the introduction of silver paste as the current collector given that the additional current collector layer can overshadow the real cathode performance in an oxygen reduction reaction environment [65]. Unlike 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. 11). The slope of the linear fitting was used to calculate the activation energy, Ea, of the cathode [66]. The Ea value for the cobalt-free SrFe0$8Ti0$2O3-d cathode is 121.77 kJ mol1, which is lower than the Ea value for SrFeO3-d (173.70 kJ mol1) [67] due to the stabilization of the cubic phase with the introduction of titanium as dopant [23]. 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) [36]. The Ea calculated from ASR values reported by Yang et al. is 110. These materials were subsequently introduced in SOFC field and 9 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 [35]. The big lattice reduced
Fig. 10 e FESEM surface and cross-sectioned micrograph for SrFe0·8Ti0·2O3-d cathode film. Please cite this article 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, https://doi.org/10.1016/ j.ijhydene.2018.11.142
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Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2018.11.142.
references
Fig. 11 e Arrhenius plot for cobalt-free SrFe0·8Ti0·2O3-d cathode at 600e800 C.
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$2 O3-d-Sm0.2Ce0$8O1.9, whose Ea values are 124, 133, and 125.3e133.4 kJ mol1, respectively [68e70]. The ASR and Ea 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. SrFe0.8Ti0.2O3d 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-d-based 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). 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.
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Please cite this article 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, https://doi.org/10.1016/ j.ijhydene.2018.11.142
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Please cite this article 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, https://doi.org/10.1016/ j.ijhydene.2018.11.142