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Evaluation of double perovskite Sr2FeTiO6−δ as potential cathode or anode materials for intermediate-temperature solid oxide fuel cells
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Evaluation of double perovskite Sr2FeTiO6 δ as potential cathode or anode materials for intermediate-temperature solid oxide fuel cells Weidong Li, Ye Cheng, Qingjun Zhoun, Tong Wei, Zepeng Li, Huiyu Yan, Zheng Wang, Xue Han College of Science, Civil Aviation University of China, Tianjin 300300, PR China Received 18 March 2015; received in revised form 30 May 2015; accepted 16 June 2015
Abstract A “cobalt-free” double perovskite electrode material with stoichiometric composition Sr2FeTiO6 δ (SFT) has been developed for intermediatetemperature solid oxide fuel cells (IT-SOFCs) using Ce0.8Sm0.2O1.9 (SDC) electrolyte. The reactivity tests show that the SFT electrode is chemically compatible with the SDC electrolyte at 1100 1C for 10 h. The electrical conductivity of SFT sample reaches 2.83–2.33 S cm 1 the temperature range of 600–800 1C. The thermal expansion coefficient of the SFT sample is 16.8 10 6 K 1 the temperature range of 30– 1000 1C in air, which is lower than that of most of the reported Co-based electrodes. Using NiO–SDC as anode, SFT as cathode, and SDC as supported electrolyte, the single cell exhibits maximum power density of 441 mW cm 2 at 800 1C. The area specific resistances of the SFT electrode on SDC electrolyte calcined at 1000 1C were found to be 0.051, 0.094 and 0.204 Ω cm2 at 800, 750 and 700 1C, respectively. Maximum power density of 335 mW cm 2 has been achieved for a symmetrical single cell with the configuration of SFT/SDC/SFT at 800 1C with wet H2 as fuel and ambient air as oxidant. These results indicate that SFT is a very promising electrode candidate for IT-SOFCs with SDC electrolyte. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Solid oxide fuel cells; Double perovskite; Cathode; Symmetrical cell
1. Introduction Solid oxide fuel cells (SOFCs) are electrochemical energy conversion devices which directly convert chemical energy to electricity with very high energy conversion efficiency [1,2]. Much effort in recent year has been devoted to develop intermediate temperature solid oxide fuel cells (IT-SOFCs) operating at 600–800 1C because of they possess the merits of both high and low temperature fuel cells, such as low environment impact, good fuel flexibility, high energy conversion efficiency and low system cost [3–5]. However, a major challenge for reducing operation temperature of SOFCs is the quick increase in cathode polarization resistance, such as La1 xSrxMnO3, cannot retain high electrochemical activity with decreasing temperature [6,7]. n
Corresponding author. Fax: þ86 22 24092514. E-mail addresses: [email protected], [email protected] (Q. Zhou).
During the past, Cobalt containing oxides of different composition, such as Ln1 xSrxCo1 yFeyO3 δ (Ln=rare-earth) [8–12], Ba1 xSrxCo1 yFeyO3 δ [5,13–16], LnBaCo2O5 þ δ (Ln=rare-earth) [17–21], and RBa(Co, M)4O7 (R=Y, Ca, In and M=Zn, Ga, Al) [22–27], have been extensively exploited as potential cathode materials for IT-SOFCs, owing to their excellent electrocatalytic activity for the oxygen reduction reaction (ORR). However, most of these materials usually show poor chemical stability, insufficient chemical compatibility with the electrolyte, high thermal expansion coefficient, high cost of cobalt element and easy evaporation. Therefore, it is significant to develop new cathode materials with high stability and sufficient electrocatalytic activity for IT-SOFCs which would be a long-term challenge. Recently, many “cobalt-free” cathode materials are reported, which show high compatibility with other cell components and good long term stability as well as favorable electrochemical performance [28–34].
http://dx.doi.org/10.1016/j.ceramint.2015.06.074 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: W. Li, et al., Evaluation of double perovskite Sr2FeTiO6 δ as potential cathode or anode materials for intermediate-temperature solid oxide fuel cells, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.074
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It is well known that Iron ions have higher chemical stability than cobalt ions because of the less flexible redox behavior of iron. Moreover, the cost of the iron is lower than that of cobalt, and it is beneficial for industrial application. Thus, Fe-based composite oxides as promising cathodes for SOFCs have attracted much attention in recent years, such as Ba0.5Sr0.5Zn0.2Fe0.8O3 δ [32], SrFe0.9Nb0.1O3 δ [33] and La0.6Sr0.4Fe0.8Cu0.2O3 δ [34]. Recently, several studies report on the structural, dielectric and magnetic properties of complex Sr2FeTiO6 δ double perovskite [35–37]. It has been demonstrated that Sr2FeTiO6 δ can be a very promising cathode material for SOFCs with YSZ electrolyte [38,39]. In this work, synthesis and the electrode performance of SFT on SDC electrolyte have been systematically studied. We first investigated the performance of SFT as a new cobalt-free cathode material for ITSOFCs. In addition, Considering Fe and Ti ions are stable cations which show very good stability of the perovskite structure against reduction [40,41]. Therefore, we also present preliminary results that SFT is used as both the anode and cathode materials. 2. Experimental sections 2.1. Material synthesis and cell fabrication Sr2FeTiO6 δ (SFT) was synthesized by the traditional solidstate reaction method. Stoichiometric amounts of commercial powders SrCO3 (99%), Fe2O3 (99.99%) and TiO2 (99.5%) were mixed by milling with ethanol for 5 h. The obtained precursors were dry pressed into pellets and then separately calcined at 1050, and 1150 1C for 12 h in air with one intermediate grinding. For the measurement of electrical conductivity and thermal expansion coefficient (TEC), the powders were pressed into pellets and sintered in air at 1200 1C for 5 h. Ce0.8Sm0.2O1.9 (SDC), and NiO powders were synthesized by a glycine-nitrate process (GNP). Stoichiometric amounts of Ce(NO3)3 6H2O (Z 99%) and Sm2O3 (99.9%) were used as the metal–ion sources. Sm2O3 was dissolved in nitric acid to form nitrates. According to the cation stoichiometry of the precursors, these nitrates were dissolved in deionized water with proper amount of glycine. The solutions were heated under continuous stirring until they changed to viscous gel and followed by combustion to form fine powders which were calcined at 600 1C for 2 h to get the final products. Dense SDC pellets were prepared by uniaxial mold pressing of SDC powders at 220 MPa and then sintered at 1400 1C for 10 h in air. The NiO powders for anode were also synthesized by the GNP method. Symmetrical cells of SFT/SDC/SFT for impedance studies were prepared by screen-printing cathode ink onto both sides of the SDC electrolyte pellets. After drying, the samples were sintered at 1000 1C for 2 h. Electrolyte-supported single fuel cells, NiO SDC/SDC/SFT and SFT/SDC/SFT were fabricated using 300-mm-thick SDC pellets. For single cells of NiO SDC/SDC/SFT studied in this work, the NiO–SDC (in a weight ratio of 65:35) ink was screen-printed on one side of
the SDC pellet and fired at 1250 1C for 4 h. SFT ink was then screen-printed on the other side of the SDC pellet and fired at 1000 1C for 2 h. For the symmetrical single cells SFT/SDC/ SFT, the SFT slurry was screen-printed onto both sides of the SDC electrolyte pellets and calcined at 1100 1C for 3 h in air. Single cells were sealed on an alumina tube by silver paste to test the cell performance. 2.2. Material characterization and cell test The phase purity and chemical compatibility of the prepared SFT powders were examined at room temperature by X-ray diffraction (XRD) (Rigaku-D-Max Ra system). Electrical conductivity as a function of temperature was determined using the van der Pauw method from 300 1C to 850 1C in air. TEC measurement was performed using a Netzsch DIL 402C dilatometer, which operated in a temperature range from 30 to 1000 1C with an air purge flow rate of 60 ml min 1 and heating rate was 5 1C min 1. Thermogravimetric analysis (TGA) was performed with a NETZSCH STA 449F3 simultaneous thermogravimetric analyzer, from 30 to 1000 1C with a heating rate of 10 1C min 1 in air with a flowing rate of 50 ml min 1. Scanning electron microscopy (SEM) micrographs were obtained with SS-550 scanning electron microscope (Shimadzu). For symmetrical cells, impedance spectra were recorded under OCV using an electrochemical impedance spectrum analyzer (Zaher Im6ex) in a frequency range of 0.1 Hz to 1 MHz with a signal amplitude of 10 mV. For the single cell tests, humidified H2 was used as the fuel at a flow rate of 50 mL min 1, whereas ambient air was supplied to cathode as the oxidant. 3. Result and discussion 3.1. Crystal structure and chemical compatibility The XRD pattern of the SFT sample shown in Fig. 1(a) indicates that a single-phase double-perovskite structure is formed. The patterns could be indexed on the basis of a cubic space group Pm-3m (no. 221). The lattice parameter and the
Fig. 1. Room temperature XRD patterns of (a) SFT sample sintered at 1150 1C for 12 h in air, (b) SFT–SDC powders calcined at 1000 1C for 10 h, and (c) SFT–SDC powders calcined at 1100 1C for 10 h.
Please cite this article as: W. Li, et al., Evaluation of double perovskite Sr2FeTiO6 δ as potential cathode or anode materials for intermediate-temperature solid oxide fuel cells, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.074
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cell volume of SFT are calculated to be a¼ 3.902549 Å and V ¼ 59.43537 Å3, respectively. Fig. 1(b) and (c) shows the XRD patterns of SFT and SDC mixture (1:1weight ratio) after annealing at 1000 and 1100 1C for 10 h in air, respectively. As shown in Fig. 1(b) and (c), no additional diffraction peaks are observed. This suggests that SFT oxide is chemically compatible with SDC electrolyte under such circumstances. It has been reported that SFT reacts with YSZ electrolyte after heating above 800 1C [38]. This means that the SFT is a more suitable electrode for SOFC based on SDC electrolyte.
3.2. Electrical conductivity The temperature dependence of the electrical conductivity of SFT in air is presented in Fig. 2. As shown in Fig. 2, the electrical conductivity increases with temperature, suggesting a semiconducting behavior, and a typical metallic behavior above 600 1C. The inflections on the electrical conductivity curves at 600 1C are attributed to the release of lattice oxygen and the thermal reduction of Fe4 þ to Fe3 þ and Fe3 þ to Fe2 þ on heating. The electrical conductivity values of the SFT sample are 2.83–2.33 S cm 1 in the temperature range of 600–800 1C and reaches a maximum value of 2.83 S cm 1 around 600 1C, which is much higher than that of Ba2CoMo0.5Nb0.5O6 δ cathode [42]. It agrees well with the previously reported in the literature [38]. Comparing with some typical cathode materials, the electrical conductivity of SFT is relative low. The relatively low electrical conductivity of SFT should influence the current collection and the charge-transfer processes for the whole electrode. However, with excellent cell design and well-coated current collector, the current can be expected to pass through the porous electrode with well-distributed. In addition, compared with the electronic conductivity, the surface exchange kinetics and bulk diffusion properties should be more important for SOFC electrode. In fact, several cathode compositions, for example, BaxSr1 xCo0.8Fe0.2O3 δ, BaNb0.05Fe0.95O3 δ, BaCo0.7Fe0.2Nb0.1O3 δ, and BaBi0.05Co0.8Nb0.15O3 δ, all from the perovskite family with relatively low conductivity, have since been reported to possess excellent electrochemical performances [43–46].
Fig. 2. Electrical conductivity of SFT in air as a function of temperature.
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3.3. TGA and thermal expansion behavior Fig. 3 shows the temperature dependences weight change for SFT powder in air. It can be clearly identified that the weight loss continues with the increasing of temperature. The TGA curves can be divided into two stages. First, from room temperature to around 360 1C is associated to the desorptions of physically adsorbed water and other gasses. Second, from 360 1C to 1000 1C the weight is reduced permanently, which is primarly attributed to the loss of oxygen from the lattice and the formation of oxygen vacancies followed by the reduction of iron ions, as described in Eq. (1). 2FeFe þ O o -2FeFe þ Vo þ O2 ðgÞ
ð1Þ
Fig. 4 shows the thermal expansion curves of the SFT sample in a temperature range of 30–1000 1C in air. As shown in Fig.4, the thermal expansion behavior of SFT sample shows two linear dependences in the temperature range of 30–350 1C and 350– 1000 1C, respectively. An obvious inflection point at around 350 1C is likely due to the thermally induced reduction of Fe ions from higher to lower valences and the loss of lattice oxygen as demonstrated by TGA results. The equilibrium given in Eq. (1) shift to the right with increasing temperature. In addition, the chemical expansion occurs at the higher temperature can contribute to the spin state transition of Fe ions in the lattice with increasing temperature [47]. The average TEC of the SFT
Fig. 3. TGA curve of the SFT powder as a function of temperature.
Fig. 4. Thermal expansion curves of SFT sample in the temperature range of 30–1000 1C in air.
Please cite this article as: W. Li, et al., Evaluation of double perovskite Sr2FeTiO6 δ as potential cathode or anode materials for intermediate-temperature solid oxide fuel cells, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.074
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sample is 16.8 10 6 K 1 between 30 and 1000 1C, which is lower than most cobalt-containing materials, e.g., BaxSr1 xCo0.8Fe0.2O3 δ [43], SrCo0.9Ta0.1O3 δ [48], SrCo1 yTiyO3 δ [49] and BaBi0.05Co0.8Nb0.15O3 δ [46], and is also lower than those of the cobalt-free electrodes like Ba0.95La0.05FeO3 δ [50], SrFe1 xTixO3 δ [51], and Ba0.5Sr0.5Fe0.9Nb0.1O3 δ [52]. Associated dates are summarized in Table 1. 3.4. AC impedance study The electrochemical properties of SFT as cathode for oxygen reduction reaction (ORR) have been assessed by applying symmetric cells with SDC as the electrolyte. Fig. 5 shows the Nyquist plots of SFT/SDC/SFT symmetric cell at different temperatures and SFT cathode on SDC electrolyte calcined at 1000 1C for 2 h. For comparison, the ohmic resistance was set as zero in the Nyquist plots. The difference between the low-frequency and the high-frequency intercepts on the real axis corresponds to the area specific resistance (ASR). As can be seen, the ASR value reduces significantly with the increase in operating temperature from 1.406 Ω cm2 at 600 1C to 0.521, 0.204, 0.094 and 0.051 Ω cm2 at 650, 700, 750 and 800 1C, respectively. The ASR values of the SFT cathodes are much lower than that of the reported double perovskite-type cathode materials. For example, the ASR
values are 0.076 Ω cm2 at 800 1C for Sr2Fe1.5Mo0.5O6 cathode on the LSGM electrolyte [53], and 0.74 Ω cm2 at 750 1C for Sr2Fe0.1Co0.9NbO6 cathode on CGO electrolyte [54]. Thus, SFT is expected to have higher catalytic activity for the reduction of molecular oxygen at intermediate temperatures. The excellent electrode performance of SFT can be attributed to the high oxygen surface exchange coefficient and ionic conductivity [39,55,56]. From the slope of the Arrhenius plot ln Rp versus 1000/T in Fig. 6, the activation energy of the SFT cathode can be determined to be 130.43 7 2.76 k J mol 1. 3.5. Performance of single-cell An SDC-based electrolyte-supported SOFC was fabricated in the configuration of NiO–SDC/SDC/SFT. The cathode was sintered at 1000 1C for 2 h in air. Fig. 7 shows the cell voltage and the power density as a function of the current density for single-cell operating at intermediate temperatures with humidified pure H2 as fuel and ambient air as oxidant. As shown in Fig. 7, the experimental maximum power densities are 74, 136, 227, 337 and 441 mW cm 2 at 600, 650, 700, 750 and 800 1C, respectively. The performance of the cell with the SFT cathode is good compared with previously reported other cobalt-free cathode measured under the similar conditions. For example, the power density of SrFe0.9Nb0.1O3 δ cathode based on 300 mm SDC electrolyte material, Pr2Ni0.6Cu0.4O4
Table 1 The average TECs of SFT and other electrode materials. Materials
TEC (x10 6 K 1)
Temperature range (1C)
Reference
BaxSr1 xCo0.8Fe0.2O3 δ SrCo0.9Ta0.1O3 δ SrCo1 yTiyO3 δ BaBi0.05Co0.8Nb0.15O3 δ SFT
19.95–20.44 21.4 21.2–23.0 20.7 16.8
50–1000 30–1000 30–1000 30–1000 30–1000
[43] [48] [49] [46] This work [50] [51] [52]
Ba0.95La0.05FeO3 δ 24.5 22.9–26.5 SrFe1 xTixO3 δ Ba0.5Sr0.5Fe0.9Nb0.1O3 δ 19.2 a RT: room temperature.
a
RT–1000 30–1000 a RT–1000
Fig. 6. Arrhenius plot of the ASR values for SFT cathode.
Fig. 5. Typical impedance spectra of SFT cathode with SDC electrolyte calcined at 1000 1C for 2 h.
Fig. 7. Electrochemical performance data of the NiO–SDC/SDC/SFT singlecell using pure H2 as fuel and ambient air as oxidant in the temperature range of 600–800 1C.
Please cite this article as: W. Li, et al., Evaluation of double perovskite Sr2FeTiO6 δ as potential cathode or anode materials for intermediate-temperature solid oxide fuel cells, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.074
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cathode using 30 mm SDC as the electrolyte material, and La2NiO4 cathode based on 300 mm SDC electrolyte material is only 407 mW cm 2, 308 mW cm 2, and 170 mW cm 2, at 800 1C, respectively [33,57,58]. Whereas, the power densities of SFT electrode are still a little lower than those of Co-base cathodes such as NdBa1 xCaxCo2O5 þ δ [59]. The cell performance would be further enhanced through optimizing the SFT electrode microstructures and using an SDC film as the electrolyte. The short-term stability test of the SFT cathode under fuel cell working conditions was further performed. Fig. 8 shows the cell maximum power density and the current density as a function of operation time at 750 1C. It can be seen that the cell power density and current density are very stable during the 40 h short-term testing, indicating that SFT material can be a very promising cathode candidate for the practical application in IT-SOFCs. 3.6. Microstructure of the single cell The cross-sectional microstructures of NiO-SDC/SDC/SFT cell after fuel cell test was analyzed by SEM, as shown in Fig. 9. From the SEM images, the SDC electrolyte is fairly dense without obvious pores. As can be seen from Fig. 9(a), the thickness of SDC electrolyte is 300 mm, where the thickness of the NiO–SDC anode and the SFT cathode is 21 mm and 33 mm, respectively. We can also find from Fig. 9(b) that the porous SFT has a good bonding and continuous contact with the dense SDC electrolyte at the interface.
Fig. 9. SEM images of (a) cross-section micrograph of the single cell NiO– SDC/SDC/SFT, and (b) cross-section micrograph of the half-cell SFT/SDC, after electrochemical tests.
3.7. Symmetrical cell performance Recently, there is considerable interest in developing symmetrical solid oxide fuel cells using simultaneously the same electrode material as anode and cathode. The symmetrical SOFCs would bring about additional advantages such as simplified fabrication procedures and minimized compatibility problems between components. To date, many materials have been evaluated as potential electrodes for symmetrical SOFCs, such as La0.75Sr0.25Cr0.5Mn0.5O3 δ [60], Sr2Fe1.5Mo0.5O6 δ [40], La0.8Sr0.2Sc0.8Mn0.2O3 δ [61], and La0.4Sr0.6Co0.2Fe0.7Nb0.1O3 δ [62]. In this study, the cell performance of SFT as symmetrical electrode was further investigated.
Fig. 8. The stability test of a single cell with a SFT cathode at 750 1C.
Fig. 10. (a) XRD patterns of SFT sintered at 1150 1C in air; (b) SFT heat treated in pure H2 at 800 1C for 10 h.
In order to examine the phase stability of SFT in the reducing atmosphere, SFT powders were heat-treated in humidified pure H2 at 800 1C for 10 h with the XRD patterns shown in Fig. 10(b). After treatment in wet H2 at 800 1C for 10 h, the element Fe and some unknown phases can be separated from the bulk material of SFT. This implies the structure of SFT in reducing atmosphere is not stable. Under the condition of hydrogen atmosphere at high temperature, the reaction that SFT is reduced to Fe is intrinsically thermodynamically favored in the forward direction.
Please cite this article as: W. Li, et al., Evaluation of double perovskite Sr2FeTiO6 δ as potential cathode or anode materials for intermediate-temperature solid oxide fuel cells, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.074
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By using the SFT as symmetrical anode/cathode electrodes, symmetrical SOFCs have been fabricated. The electrochemical performance has been investigated. Considering the preparation conditions, SFT/SDC/SFT symmetrical cells were finally prepared by screen-printing on the SDC electrolytes with SFT ink, and then co-fired at 1100 1C for 3 h. Fig. 11 shows the electrochemical impedance spectra of the SFT/SDC/SFT symmetrical half cells under open circuit conditions in air after sintering at 1100 1C for 3 h. The ASR decreased with the increase in operating temperature from 0.484 Ω cm2 at 650 1C to 0.207, 0.104 and 0.062 Ω cm2 at 700, 750 and 800 1C, respectively, which is similar to the values that at 1000 1C for 2 h (Section 3.4). Better adhesion between the SFT and SDC can be obtained at a calcining temperature of 1100 1C compared with 1000 1C. Therefore, the ASR of the cathode calcined at 1100 1C is similar to the values that at 1000 1C. The performance of the single cell was tested to prove that SFT can work as both cathode and anode simultaneously in a symmetrical fuel cells. Fig.12 shows the performance of an SDC electrolyte supported symmetrical fuel cell with the configuration of SFT/SDC/SFT tested under humidified pure H2. The maximum power densities are 335 and 221 mW cm 2 in H2 at 800 and 750 1C, respectively, which are comparable with that of most of the reported symmetrical electrodes [60,63–65]. The good cell performance in H2 indicates that
Fig. 11. Typical impedance spectra of SFT cathode with SDC electrolyte calcined at 1100 1C for 3 h.
Fig. 13. Impedance spectra of single cells SFT/SDC/SFT in humidified pure H2.
SFT should be a promising symmetrical electrode for ITSOFCs. The cell impedance spectra measured under open circuit conditions with humidified pure H2 as fuels are shown in Fig. 13. The ASR of the symmetrical cell is 0.510, 0.223, 0.116 and 0.075 Ω cm2 at 650 1C, 700 1C, 750 1C and 800 1C, respectively, which is better than that of the La0.7Sr0.3Fe0.7Ga0.3O3 δ as symmetrical electrode [66]. 4. Conclusions In summary, we demonstrate a novel cobalt-free double perovskite Sr2FeTiO6 δ (SFT) electrode synthesized by solidstate reaction method. SFT material had good chemical compatibility with SDC electrolyte below 1100 1C. SFT exhibits a semi-conduction to metallic conduction transition at around 600 1C with a maximum conductivity value of 2.83 S cm 1. A low TEC of 16.8 10 6 K 1 was demonstrated. Low area specific resistances (ASR) were obtained for the SFT electrode in the intermediate temperature range, suggesting that its high electrocatalytic activity for oxygen reduction. The single cells with the configuration of NiO– SDC/SDC/SFT show peak power density of 441 mW cm 2 at 800 1C using humidified H2 as the fuel and ambient air as the oxidant. In addition, SFT has also been evaluated as electrode for symmetrical solid oxide fuel cell. The peak power density of a single cell using SFT as symmetrical electrode and SDC as electrolyte reached 335 mW cm 2 at 800 1C using humidified H2 as the fuel and ambient air as the oxidant. Although further research is necessary to evaluate degradation with longterm performance in the single cell with SFT electrodes, preliminary indications that the double-perovskite SFT oxide appears to be a favorable IT-SOFCs electrode material. Acknowledgments
Fig. 12. Electrochemical performance data of the single cell with the SFT as symmetrical electrode in humidified pure H2 as fuel and ambient air as oxidant in the temperature range of 650–800 1C.
This work was supported by the National Undergraduate Training Programs for Innovation and Entrepreneurship (201410059033), the Fundamental Research Funds for the Central Universities (3122015L004), the National Natural Science Foundation of China (51002183, 11304380), Tianjin Reasearch Program of Application Foundation and Advanced Technology (14JCQNJC03700).
Please cite this article as: W. Li, et al., Evaluation of double perovskite Sr2FeTiO6 δ as potential cathode or anode materials for intermediate-temperature solid oxide fuel cells, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.074
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Evaluation of double perovskite Sr2FeTiO6 δ as potential cathode or anode materials for intermediate-temperature solid oxide fuel cells Weidong Li, Ye Cheng, Qingjun Zhoun, Tong Wei, Zepeng Li, Huiyu Yan, Zheng Wang, Xue Han College of Science, Civil Aviation University of China, Tianjin 300300, PR China Received 18 March 2015; received in revised form 30 May 2015; accepted 16 June 2015
Abstract A “cobalt-free” double perovskite electrode material with stoichiometric composition Sr2FeTiO6 δ (SFT) has been developed for intermediatetemperature solid oxide fuel cells (IT-SOFCs) using Ce0.8Sm0.2O1.9 (SDC) electrolyte. The reactivity tests show that the SFT electrode is chemically compatible with the SDC electrolyte at 1100 1C for 10 h. The electrical conductivity of SFT sample reaches 2.83–2.33 S cm 1 the temperature range of 600–800 1C. The thermal expansion coefficient of the SFT sample is 16.8 10 6 K 1 the temperature range of 30– 1000 1C in air, which is lower than that of most of the reported Co-based electrodes. Using NiO–SDC as anode, SFT as cathode, and SDC as supported electrolyte, the single cell exhibits maximum power density of 441 mW cm 2 at 800 1C. The area specific resistances of the SFT electrode on SDC electrolyte calcined at 1000 1C were found to be 0.051, 0.094 and 0.204 Ω cm2 at 800, 750 and 700 1C, respectively. Maximum power density of 335 mW cm 2 has been achieved for a symmetrical single cell with the configuration of SFT/SDC/SFT at 800 1C with wet H2 as fuel and ambient air as oxidant. These results indicate that SFT is a very promising electrode candidate for IT-SOFCs with SDC electrolyte. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Solid oxide fuel cells; Double perovskite; Cathode; Symmetrical cell
1. Introduction Solid oxide fuel cells (SOFCs) are electrochemical energy conversion devices which directly convert chemical energy to electricity with very high energy conversion efficiency [1,2]. Much effort in recent year has been devoted to develop intermediate temperature solid oxide fuel cells (IT-SOFCs) operating at 600–800 1C because of they possess the merits of both high and low temperature fuel cells, such as low environment impact, good fuel flexibility, high energy conversion efficiency and low system cost [3–5]. However, a major challenge for reducing operation temperature of SOFCs is the quick increase in cathode polarization resistance, such as La1 xSrxMnO3, cannot retain high electrochemical activity with decreasing temperature [6,7]. n
Corresponding author. Fax: þ86 22 24092514. E-mail addresses: [email protected], [email protected] (Q. Zhou).
During the past, Cobalt containing oxides of different composition, such as Ln1 xSrxCo1 yFeyO3 δ (Ln=rare-earth) [8–12], Ba1 xSrxCo1 yFeyO3 δ [5,13–16], LnBaCo2O5 þ δ (Ln=rare-earth) [17–21], and RBa(Co, M)4O7 (R=Y, Ca, In and M=Zn, Ga, Al) [22–27], have been extensively exploited as potential cathode materials for IT-SOFCs, owing to their excellent electrocatalytic activity for the oxygen reduction reaction (ORR). However, most of these materials usually show poor chemical stability, insufficient chemical compatibility with the electrolyte, high thermal expansion coefficient, high cost of cobalt element and easy evaporation. Therefore, it is significant to develop new cathode materials with high stability and sufficient electrocatalytic activity for IT-SOFCs which would be a long-term challenge. Recently, many “cobalt-free” cathode materials are reported, which show high compatibility with other cell components and good long term stability as well as favorable electrochemical performance [28–34].
http://dx.doi.org/10.1016/j.ceramint.2015.06.074 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: W. Li, et al., Evaluation of double perovskite Sr2FeTiO6 δ as potential cathode or anode materials for intermediate-temperature solid oxide fuel cells, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.074
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It is well known that Iron ions have higher chemical stability than cobalt ions because of the less flexible redox behavior of iron. Moreover, the cost of the iron is lower than that of cobalt, and it is beneficial for industrial application. Thus, Fe-based composite oxides as promising cathodes for SOFCs have attracted much attention in recent years, such as Ba0.5Sr0.5Zn0.2Fe0.8O3 δ [32], SrFe0.9Nb0.1O3 δ [33] and La0.6Sr0.4Fe0.8Cu0.2O3 δ [34]. Recently, several studies report on the structural, dielectric and magnetic properties of complex Sr2FeTiO6 δ double perovskite [35–37]. It has been demonstrated that Sr2FeTiO6 δ can be a very promising cathode material for SOFCs with YSZ electrolyte [38,39]. In this work, synthesis and the electrode performance of SFT on SDC electrolyte have been systematically studied. We first investigated the performance of SFT as a new cobalt-free cathode material for ITSOFCs. In addition, Considering Fe and Ti ions are stable cations which show very good stability of the perovskite structure against reduction [40,41]. Therefore, we also present preliminary results that SFT is used as both the anode and cathode materials. 2. Experimental sections 2.1. Material synthesis and cell fabrication Sr2FeTiO6 δ (SFT) was synthesized by the traditional solidstate reaction method. Stoichiometric amounts of commercial powders SrCO3 (99%), Fe2O3 (99.99%) and TiO2 (99.5%) were mixed by milling with ethanol for 5 h. The obtained precursors were dry pressed into pellets and then separately calcined at 1050, and 1150 1C for 12 h in air with one intermediate grinding. For the measurement of electrical conductivity and thermal expansion coefficient (TEC), the powders were pressed into pellets and sintered in air at 1200 1C for 5 h. Ce0.8Sm0.2O1.9 (SDC), and NiO powders were synthesized by a glycine-nitrate process (GNP). Stoichiometric amounts of Ce(NO3)3 6H2O (Z 99%) and Sm2O3 (99.9%) were used as the metal–ion sources. Sm2O3 was dissolved in nitric acid to form nitrates. According to the cation stoichiometry of the precursors, these nitrates were dissolved in deionized water with proper amount of glycine. The solutions were heated under continuous stirring until they changed to viscous gel and followed by combustion to form fine powders which were calcined at 600 1C for 2 h to get the final products. Dense SDC pellets were prepared by uniaxial mold pressing of SDC powders at 220 MPa and then sintered at 1400 1C for 10 h in air. The NiO powders for anode were also synthesized by the GNP method. Symmetrical cells of SFT/SDC/SFT for impedance studies were prepared by screen-printing cathode ink onto both sides of the SDC electrolyte pellets. After drying, the samples were sintered at 1000 1C for 2 h. Electrolyte-supported single fuel cells, NiO SDC/SDC/SFT and SFT/SDC/SFT were fabricated using 300-mm-thick SDC pellets. For single cells of NiO SDC/SDC/SFT studied in this work, the NiO–SDC (in a weight ratio of 65:35) ink was screen-printed on one side of
the SDC pellet and fired at 1250 1C for 4 h. SFT ink was then screen-printed on the other side of the SDC pellet and fired at 1000 1C for 2 h. For the symmetrical single cells SFT/SDC/ SFT, the SFT slurry was screen-printed onto both sides of the SDC electrolyte pellets and calcined at 1100 1C for 3 h in air. Single cells were sealed on an alumina tube by silver paste to test the cell performance. 2.2. Material characterization and cell test The phase purity and chemical compatibility of the prepared SFT powders were examined at room temperature by X-ray diffraction (XRD) (Rigaku-D-Max Ra system). Electrical conductivity as a function of temperature was determined using the van der Pauw method from 300 1C to 850 1C in air. TEC measurement was performed using a Netzsch DIL 402C dilatometer, which operated in a temperature range from 30 to 1000 1C with an air purge flow rate of 60 ml min 1 and heating rate was 5 1C min 1. Thermogravimetric analysis (TGA) was performed with a NETZSCH STA 449F3 simultaneous thermogravimetric analyzer, from 30 to 1000 1C with a heating rate of 10 1C min 1 in air with a flowing rate of 50 ml min 1. Scanning electron microscopy (SEM) micrographs were obtained with SS-550 scanning electron microscope (Shimadzu). For symmetrical cells, impedance spectra were recorded under OCV using an electrochemical impedance spectrum analyzer (Zaher Im6ex) in a frequency range of 0.1 Hz to 1 MHz with a signal amplitude of 10 mV. For the single cell tests, humidified H2 was used as the fuel at a flow rate of 50 mL min 1, whereas ambient air was supplied to cathode as the oxidant. 3. Result and discussion 3.1. Crystal structure and chemical compatibility The XRD pattern of the SFT sample shown in Fig. 1(a) indicates that a single-phase double-perovskite structure is formed. The patterns could be indexed on the basis of a cubic space group Pm-3m (no. 221). The lattice parameter and the
Fig. 1. Room temperature XRD patterns of (a) SFT sample sintered at 1150 1C for 12 h in air, (b) SFT–SDC powders calcined at 1000 1C for 10 h, and (c) SFT–SDC powders calcined at 1100 1C for 10 h.
Please cite this article as: W. Li, et al., Evaluation of double perovskite Sr2FeTiO6 δ as potential cathode or anode materials for intermediate-temperature solid oxide fuel cells, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.074
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cell volume of SFT are calculated to be a¼ 3.902549 Å and V ¼ 59.43537 Å3, respectively. Fig. 1(b) and (c) shows the XRD patterns of SFT and SDC mixture (1:1weight ratio) after annealing at 1000 and 1100 1C for 10 h in air, respectively. As shown in Fig. 1(b) and (c), no additional diffraction peaks are observed. This suggests that SFT oxide is chemically compatible with SDC electrolyte under such circumstances. It has been reported that SFT reacts with YSZ electrolyte after heating above 800 1C [38]. This means that the SFT is a more suitable electrode for SOFC based on SDC electrolyte.
3.2. Electrical conductivity The temperature dependence of the electrical conductivity of SFT in air is presented in Fig. 2. As shown in Fig. 2, the electrical conductivity increases with temperature, suggesting a semiconducting behavior, and a typical metallic behavior above 600 1C. The inflections on the electrical conductivity curves at 600 1C are attributed to the release of lattice oxygen and the thermal reduction of Fe4 þ to Fe3 þ and Fe3 þ to Fe2 þ on heating. The electrical conductivity values of the SFT sample are 2.83–2.33 S cm 1 in the temperature range of 600–800 1C and reaches a maximum value of 2.83 S cm 1 around 600 1C, which is much higher than that of Ba2CoMo0.5Nb0.5O6 δ cathode [42]. It agrees well with the previously reported in the literature [38]. Comparing with some typical cathode materials, the electrical conductivity of SFT is relative low. The relatively low electrical conductivity of SFT should influence the current collection and the charge-transfer processes for the whole electrode. However, with excellent cell design and well-coated current collector, the current can be expected to pass through the porous electrode with well-distributed. In addition, compared with the electronic conductivity, the surface exchange kinetics and bulk diffusion properties should be more important for SOFC electrode. In fact, several cathode compositions, for example, BaxSr1 xCo0.8Fe0.2O3 δ, BaNb0.05Fe0.95O3 δ, BaCo0.7Fe0.2Nb0.1O3 δ, and BaBi0.05Co0.8Nb0.15O3 δ, all from the perovskite family with relatively low conductivity, have since been reported to possess excellent electrochemical performances [43–46].
Fig. 2. Electrical conductivity of SFT in air as a function of temperature.
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3.3. TGA and thermal expansion behavior Fig. 3 shows the temperature dependences weight change for SFT powder in air. It can be clearly identified that the weight loss continues with the increasing of temperature. The TGA curves can be divided into two stages. First, from room temperature to around 360 1C is associated to the desorptions of physically adsorbed water and other gasses. Second, from 360 1C to 1000 1C the weight is reduced permanently, which is primarly attributed to the loss of oxygen from the lattice and the formation of oxygen vacancies followed by the reduction of iron ions, as described in Eq. (1). 2FeFe þ O o -2FeFe þ Vo þ O2 ðgÞ
ð1Þ
Fig. 4 shows the thermal expansion curves of the SFT sample in a temperature range of 30–1000 1C in air. As shown in Fig.4, the thermal expansion behavior of SFT sample shows two linear dependences in the temperature range of 30–350 1C and 350– 1000 1C, respectively. An obvious inflection point at around 350 1C is likely due to the thermally induced reduction of Fe ions from higher to lower valences and the loss of lattice oxygen as demonstrated by TGA results. The equilibrium given in Eq. (1) shift to the right with increasing temperature. In addition, the chemical expansion occurs at the higher temperature can contribute to the spin state transition of Fe ions in the lattice with increasing temperature [47]. The average TEC of the SFT
Fig. 3. TGA curve of the SFT powder as a function of temperature.
Fig. 4. Thermal expansion curves of SFT sample in the temperature range of 30–1000 1C in air.
Please cite this article as: W. Li, et al., Evaluation of double perovskite Sr2FeTiO6 δ as potential cathode or anode materials for intermediate-temperature solid oxide fuel cells, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.074
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sample is 16.8 10 6 K 1 between 30 and 1000 1C, which is lower than most cobalt-containing materials, e.g., BaxSr1 xCo0.8Fe0.2O3 δ [43], SrCo0.9Ta0.1O3 δ [48], SrCo1 yTiyO3 δ [49] and BaBi0.05Co0.8Nb0.15O3 δ [46], and is also lower than those of the cobalt-free electrodes like Ba0.95La0.05FeO3 δ [50], SrFe1 xTixO3 δ [51], and Ba0.5Sr0.5Fe0.9Nb0.1O3 δ [52]. Associated dates are summarized in Table 1. 3.4. AC impedance study The electrochemical properties of SFT as cathode for oxygen reduction reaction (ORR) have been assessed by applying symmetric cells with SDC as the electrolyte. Fig. 5 shows the Nyquist plots of SFT/SDC/SFT symmetric cell at different temperatures and SFT cathode on SDC electrolyte calcined at 1000 1C for 2 h. For comparison, the ohmic resistance was set as zero in the Nyquist plots. The difference between the low-frequency and the high-frequency intercepts on the real axis corresponds to the area specific resistance (ASR). As can be seen, the ASR value reduces significantly with the increase in operating temperature from 1.406 Ω cm2 at 600 1C to 0.521, 0.204, 0.094 and 0.051 Ω cm2 at 650, 700, 750 and 800 1C, respectively. The ASR values of the SFT cathodes are much lower than that of the reported double perovskite-type cathode materials. For example, the ASR
values are 0.076 Ω cm2 at 800 1C for Sr2Fe1.5Mo0.5O6 cathode on the LSGM electrolyte [53], and 0.74 Ω cm2 at 750 1C for Sr2Fe0.1Co0.9NbO6 cathode on CGO electrolyte [54]. Thus, SFT is expected to have higher catalytic activity for the reduction of molecular oxygen at intermediate temperatures. The excellent electrode performance of SFT can be attributed to the high oxygen surface exchange coefficient and ionic conductivity [39,55,56]. From the slope of the Arrhenius plot ln Rp versus 1000/T in Fig. 6, the activation energy of the SFT cathode can be determined to be 130.43 7 2.76 k J mol 1. 3.5. Performance of single-cell An SDC-based electrolyte-supported SOFC was fabricated in the configuration of NiO–SDC/SDC/SFT. The cathode was sintered at 1000 1C for 2 h in air. Fig. 7 shows the cell voltage and the power density as a function of the current density for single-cell operating at intermediate temperatures with humidified pure H2 as fuel and ambient air as oxidant. As shown in Fig. 7, the experimental maximum power densities are 74, 136, 227, 337 and 441 mW cm 2 at 600, 650, 700, 750 and 800 1C, respectively. The performance of the cell with the SFT cathode is good compared with previously reported other cobalt-free cathode measured under the similar conditions. For example, the power density of SrFe0.9Nb0.1O3 δ cathode based on 300 mm SDC electrolyte material, Pr2Ni0.6Cu0.4O4
Table 1 The average TECs of SFT and other electrode materials. Materials
TEC (x10 6 K 1)
Temperature range (1C)
Reference
BaxSr1 xCo0.8Fe0.2O3 δ SrCo0.9Ta0.1O3 δ SrCo1 yTiyO3 δ BaBi0.05Co0.8Nb0.15O3 δ SFT
19.95–20.44 21.4 21.2–23.0 20.7 16.8
50–1000 30–1000 30–1000 30–1000 30–1000
[43] [48] [49] [46] This work [50] [51] [52]
Ba0.95La0.05FeO3 δ 24.5 22.9–26.5 SrFe1 xTixO3 δ Ba0.5Sr0.5Fe0.9Nb0.1O3 δ 19.2 a RT: room temperature.
a
RT–1000 30–1000 a RT–1000
Fig. 6. Arrhenius plot of the ASR values for SFT cathode.
Fig. 5. Typical impedance spectra of SFT cathode with SDC electrolyte calcined at 1000 1C for 2 h.
Fig. 7. Electrochemical performance data of the NiO–SDC/SDC/SFT singlecell using pure H2 as fuel and ambient air as oxidant in the temperature range of 600–800 1C.
Please cite this article as: W. Li, et al., Evaluation of double perovskite Sr2FeTiO6 δ as potential cathode or anode materials for intermediate-temperature solid oxide fuel cells, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.074
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cathode using 30 mm SDC as the electrolyte material, and La2NiO4 cathode based on 300 mm SDC electrolyte material is only 407 mW cm 2, 308 mW cm 2, and 170 mW cm 2, at 800 1C, respectively [33,57,58]. Whereas, the power densities of SFT electrode are still a little lower than those of Co-base cathodes such as NdBa1 xCaxCo2O5 þ δ [59]. The cell performance would be further enhanced through optimizing the SFT electrode microstructures and using an SDC film as the electrolyte. The short-term stability test of the SFT cathode under fuel cell working conditions was further performed. Fig. 8 shows the cell maximum power density and the current density as a function of operation time at 750 1C. It can be seen that the cell power density and current density are very stable during the 40 h short-term testing, indicating that SFT material can be a very promising cathode candidate for the practical application in IT-SOFCs. 3.6. Microstructure of the single cell The cross-sectional microstructures of NiO-SDC/SDC/SFT cell after fuel cell test was analyzed by SEM, as shown in Fig. 9. From the SEM images, the SDC electrolyte is fairly dense without obvious pores. As can be seen from Fig. 9(a), the thickness of SDC electrolyte is 300 mm, where the thickness of the NiO–SDC anode and the SFT cathode is 21 mm and 33 mm, respectively. We can also find from Fig. 9(b) that the porous SFT has a good bonding and continuous contact with the dense SDC electrolyte at the interface.
Fig. 9. SEM images of (a) cross-section micrograph of the single cell NiO– SDC/SDC/SFT, and (b) cross-section micrograph of the half-cell SFT/SDC, after electrochemical tests.
3.7. Symmetrical cell performance Recently, there is considerable interest in developing symmetrical solid oxide fuel cells using simultaneously the same electrode material as anode and cathode. The symmetrical SOFCs would bring about additional advantages such as simplified fabrication procedures and minimized compatibility problems between components. To date, many materials have been evaluated as potential electrodes for symmetrical SOFCs, such as La0.75Sr0.25Cr0.5Mn0.5O3 δ [60], Sr2Fe1.5Mo0.5O6 δ [40], La0.8Sr0.2Sc0.8Mn0.2O3 δ [61], and La0.4Sr0.6Co0.2Fe0.7Nb0.1O3 δ [62]. In this study, the cell performance of SFT as symmetrical electrode was further investigated.
Fig. 8. The stability test of a single cell with a SFT cathode at 750 1C.
Fig. 10. (a) XRD patterns of SFT sintered at 1150 1C in air; (b) SFT heat treated in pure H2 at 800 1C for 10 h.
In order to examine the phase stability of SFT in the reducing atmosphere, SFT powders were heat-treated in humidified pure H2 at 800 1C for 10 h with the XRD patterns shown in Fig. 10(b). After treatment in wet H2 at 800 1C for 10 h, the element Fe and some unknown phases can be separated from the bulk material of SFT. This implies the structure of SFT in reducing atmosphere is not stable. Under the condition of hydrogen atmosphere at high temperature, the reaction that SFT is reduced to Fe is intrinsically thermodynamically favored in the forward direction.
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By using the SFT as symmetrical anode/cathode electrodes, symmetrical SOFCs have been fabricated. The electrochemical performance has been investigated. Considering the preparation conditions, SFT/SDC/SFT symmetrical cells were finally prepared by screen-printing on the SDC electrolytes with SFT ink, and then co-fired at 1100 1C for 3 h. Fig. 11 shows the electrochemical impedance spectra of the SFT/SDC/SFT symmetrical half cells under open circuit conditions in air after sintering at 1100 1C for 3 h. The ASR decreased with the increase in operating temperature from 0.484 Ω cm2 at 650 1C to 0.207, 0.104 and 0.062 Ω cm2 at 700, 750 and 800 1C, respectively, which is similar to the values that at 1000 1C for 2 h (Section 3.4). Better adhesion between the SFT and SDC can be obtained at a calcining temperature of 1100 1C compared with 1000 1C. Therefore, the ASR of the cathode calcined at 1100 1C is similar to the values that at 1000 1C. The performance of the single cell was tested to prove that SFT can work as both cathode and anode simultaneously in a symmetrical fuel cells. Fig.12 shows the performance of an SDC electrolyte supported symmetrical fuel cell with the configuration of SFT/SDC/SFT tested under humidified pure H2. The maximum power densities are 335 and 221 mW cm 2 in H2 at 800 and 750 1C, respectively, which are comparable with that of most of the reported symmetrical electrodes [60,63–65]. The good cell performance in H2 indicates that
Fig. 11. Typical impedance spectra of SFT cathode with SDC electrolyte calcined at 1100 1C for 3 h.
Fig. 13. Impedance spectra of single cells SFT/SDC/SFT in humidified pure H2.
SFT should be a promising symmetrical electrode for ITSOFCs. The cell impedance spectra measured under open circuit conditions with humidified pure H2 as fuels are shown in Fig. 13. The ASR of the symmetrical cell is 0.510, 0.223, 0.116 and 0.075 Ω cm2 at 650 1C, 700 1C, 750 1C and 800 1C, respectively, which is better than that of the La0.7Sr0.3Fe0.7Ga0.3O3 δ as symmetrical electrode [66]. 4. Conclusions In summary, we demonstrate a novel cobalt-free double perovskite Sr2FeTiO6 δ (SFT) electrode synthesized by solidstate reaction method. SFT material had good chemical compatibility with SDC electrolyte below 1100 1C. SFT exhibits a semi-conduction to metallic conduction transition at around 600 1C with a maximum conductivity value of 2.83 S cm 1. A low TEC of 16.8 10 6 K 1 was demonstrated. Low area specific resistances (ASR) were obtained for the SFT electrode in the intermediate temperature range, suggesting that its high electrocatalytic activity for oxygen reduction. The single cells with the configuration of NiO– SDC/SDC/SFT show peak power density of 441 mW cm 2 at 800 1C using humidified H2 as the fuel and ambient air as the oxidant. In addition, SFT has also been evaluated as electrode for symmetrical solid oxide fuel cell. The peak power density of a single cell using SFT as symmetrical electrode and SDC as electrolyte reached 335 mW cm 2 at 800 1C using humidified H2 as the fuel and ambient air as the oxidant. Although further research is necessary to evaluate degradation with longterm performance in the single cell with SFT electrodes, preliminary indications that the double-perovskite SFT oxide appears to be a favorable IT-SOFCs electrode material. Acknowledgments
Fig. 12. Electrochemical performance data of the single cell with the SFT as symmetrical electrode in humidified pure H2 as fuel and ambient air as oxidant in the temperature range of 650–800 1C.
This work was supported by the National Undergraduate Training Programs for Innovation and Entrepreneurship (201410059033), the Fundamental Research Funds for the Central Universities (3122015L004), the National Natural Science Foundation of China (51002183, 11304380), Tianjin Reasearch Program of Application Foundation and Advanced Technology (14JCQNJC03700).
Please cite this article as: W. Li, et al., Evaluation of double perovskite Sr2FeTiO6 δ as potential cathode or anode materials for intermediate-temperature solid oxide fuel cells, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.074
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Please cite this article as: W. Li, et al., Evaluation of double perovskite Sr2FeTiO6 δ as potential cathode or anode materials for intermediate-temperature solid oxide fuel cells, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.074