J. Mater. Sci. Technol., 212, 28(9), 828 832. Cobalt-free Composite Ba.5 Sr.5 Fe.9 Ni.1 O 3 δ Ce.8 Sm.2 O 2 δ as Cathode for Intermediate-Temperature Solid Oxide Fuel Cell Xiangfeng Chu 1), Feng Liu 1), Weichang Zhu 2), Yongping Dong 1), Mingfu Ye 1) and Wenqi Sun 1) 1) School of Chemistry and Chemical Engineering, Anhui University of Technology, Maanshan 2432, China 2) School of Materials Science and Engineering, Anhui University of Technology, Maanshan 2432, China [Manuscript received September 2, 211, in revised form January 4, 212] New cobalt-free composites consisting of Ba.5 Sr.5 Fe.9 Ni.1 O 3 δ (BSFN) and Ce.8 Sm.2 O 2 δ (SDC) were investigated as possible cathode materials for intermediate-temperature solid oxide fuel cell (IT-SOFC). BSFN, which was synthesized by auto ignition process, was chemically compatible with SDC up to 11 C as indicated by X-ray diffraction analysis. The electrical conductivity of BSFN reached the maximum value of 57 S cm 1 at 45 C. The thermal expansion coefficient (TEC) value of BSFN was 3.9 1 6 K 1, much higher than that of typical electrolytes. The electrochemical behavior of the composites was analyzed via electrochemical impedance spectroscopy with symmetrical cells BSFN-SDC/SDC/BSFN-SDC. The area specific interfacial polarization resistance (ASR) decreased with increasing SDC content of the composite. The area specific interfacial polarization resistance (ASR) at 7 C is only.49,.34 and.31 Ω cm 2 when 3, 4, and 5 wt% SDC was cooperated to BSFN, respectively. These results suggest that BSFN-SDC is a possible candidate for IT-SOFC cathode. KEY WORDS: Ba.5Sr.5Fe.9Ni.1O 3 δ ; Cathode; Intermediate-temperature solid oxide fuel cell (IT-SOFC); Solid oxide fuel cell 1. Introduction La 1 x Sr x MnO 3 δ perovskite (LSM) is regarded as one of the most promising cathode materials for solid oxide fuel cells (SOFC) due to its high thermal and chemical stability [1] to the typical electrolyte. However, the cathode polarization plays a critical role in the whole electrode reaction process, especially at low temperatures [2,3]. LSM is limited in the application of cathode for intermediate-temperature solid oxide fuel cell (IT-SOFC) because of its high polarization resistance [4]. In order to improve cathode performance, one effective strategy is to replace LSM with higher catalytic active materials at lower temperatures. For example, Wei et al. [5] examined Ba.5 Sr.5 Zn.2 Fe.8 O 3 δ (BSZF) as a cobalt-free Corresponding author. Prof., Ph.D.; Tel.: +86 555 2311551; Fax: +86 555 2311822; E-mail address: xfchu99@ahut.edu.cn (X.F. Chu). cathode for IT-SOFC and found that the symmetrical BSZF cathode on Ce.8 Sm.2 O 2 δ (SDC) electrolyte showed the area specific interfacial polarization resistance (ASR) of.48 Ω cm 2 at 65 C. Zhao et al. [6] prepared Ba.5 Sr.5 Fe.8 Cu.2 O 3 δ (BSFC) via auto ignition process and found that the ASR of BSFC was as low as.137 Ω cm 2 at 7 C. Ling et al. [7] reported that the ASR of a cobalt-free cubic perovskite oxide Sm.5 Sr.5 Fe.8 Cu.2 O 3 δ (SSFCu) was only.85 Ω cm 2 at 7 C. In this work, new cobalt-free composites BSFN-SDC were prepared and their electrochemical performance as the IT-SOFC cathode were investigated. 2. Experimental BSFN was synthesized by auto ignition process [8]. Stoichiometric amount of Ba(NO 3 ) 2, Sr(NO 3 ) 2, Fe(NO 3 ) 3 9H 2 O and Ni(NO 3 ) 2 6H 2 O were dissolved
X.F. Chu et al.: J. Mater. Sci. Technol., 212, 28(9), 828 832. 829 in distilled water to form an aqueous solution, and then citric acid, as complexation agent, was added with the mole ratio of citric acid/metal of 1.5:1. The solution was then heated till self-combustion occurred. The as-synthesized powders were subsequently calcined at 1 C for 3 h to obtain fine BSFN powders. SDC was synthesized by carbonate coprecipitation [9]. To assess the phase reaction between BSFN cathode and SDC electrolyte, the chemical compatibility of the BSFN cathode with the SDC electrolyte was investigated by sintering the mixed powders of BSFN and SDC in a weight ratio of 1:1 at 11 C for 3 h. Phase identification of the calcined powders was performed with X-ray diffraction (D/Max-gA, Japan). Electrical conductivity (σ) measurements were performed with a four-probe d.c. method on H.P. multimeter (Model 3441) from 4 to 8 C with increments of 5 C in air. Thermal expansion of the specimen was measured from 3 to 1 C using a dilatometer (SHI- MADZU5) at a heating rate of 1 C min 1 in air. Symmetrical electrochemical cell with the configuration of BSFN-SDC/SDC/BSFN-SDC applied for the impedance research was measured by electrochemical impedance spectroscopy (EIS) (IM6e, Zahner,.1 Hz 1 MHz) from 55 to 7 C with increments of 5 C. The powder SDC was uniaxially dry-pressed into pellet (Ø13 mm, 1 mm thick) at 2 MPa, and the pellet were sintered at 14 C for 5 h. Composite BSFN-SDC in various weight ratios were mixed thoroughly with a 6 wt% ethylcellulose terpineol binder to prepare the cathode slurry, respectively, which were painted on both faces of SDC electrolyte. Then the cells were fired at 1 C for 3 h in air. Afterwards, a silver paste was screen-printed onto both faces of the cathode, and then fired at 8 C for 2 h in air to form a symmetrical electrochemical cell. Hereafter, the composite BSFN-SDC in various weight ratios will be referred to as BSFN7-SDC3, BSFN6-SDC4 and BSFN5-SDC5 for convenience (the number is weight ratio (%)). A scanning electron microscopy (SEM, JEOL JSM-64) was used to observe the microstructure of BSFN-SDC composite cathode on the surface of SDC electrolyte and cross-section of BSFN-SDC/SDC after EIS testing. 3. Results and Discussion 3.1 XRD patterns of the BSFN-SDC, BSFN and SDC Fig. 1 presents XRD patterns of the BSFN powder after being calcined at 1 C for 3 h and the SDC powder. Sharp lines demonstrate a well-developed crystallization of BSFN and all the peaks can be well indexed as a cubic perovskite structure with the space group of Pm3m (211). Fig. 1 also shows the XRD spectra of mixed powders of BSFN and SDC Intensity / a.u. BSFN+SDC 11 o C BSFN 1 o C SDC 6 o C 2 3 4 5 6 7 8 2 / deg. Fig. 1 XRD patterns of the BSFN-SDC, BSFN and SDC Conductivity / S cm -1 6 5 4 3 2 1 35 4 45 5 55 6 65 7 75 8 85 Temp. / o C Fig. 2 Temperature dependence of the conductivity for BSFN sample measured in the range of 4 8 C in air after sintered at 11 C for 3 h. All the peaks can be attributed to either BSFN or SDC, indicating that BSFN is chemically compatible with SDC up to 11 C. 3.2 Temperature dependence of the conductivity for BSFN sample Fig. 2 shows the temperature dependence of the electrical conductivity of the BSFN sample in air. The total electrical conductivity was measured on a BSFN rectangular bar sintered at 125 C for 5 h in air. It is clear that the conductivity increases with increasing temperature and reaches the maximum value of 57 S cm 1 at about 45 C, then the conductivity begins to decrease when the temperature is high than 45 C because of the loss of the lattice oxygen at elevated temperature. The conductivity values of the BSFN sample in the temperature range of IT-SOFC (4 6 C) are 28 57 S cm 1. The conductivity of LSM was 3.5 S cm 1 at 1 C [1]. In fact, several perovskite electrode materials with low conductivity have been reported with good electrochemical performance. For example, the maximum conductivities of Ba.5 Sr.5 Co 1 y Fe y O 3 δ (BSCF) [11] were about 2
83 X.F. Chu et al.: J. Mater. Sci. Technol., 212, 28(9), 828 832. dl/l.3.25.2.15.1.5. 2 4 6 8 1 Temp. / o C Fig. 3 Thermal expansion of the sample of BSFN at a heating rate of 1 C min 1 in air 4 S cm 1, which is very close to that of BSFN. 3.3 Thermal expansion of the BSFN sample Thermal expansion curve in Fig. 3 shows the average thermal expansion coefficient (TEC) value of BSFN is 3.9 1 6 K 1 and gradually increases in the high temperature region. The TEC value of SDC was 12.8 1 6 K 1[12], indicating that there should be significant thermal expansion mismatch between BSFN cathode and SDC electrolyte. The mismatch of TEC between the electrolyte and cathode will result in delamination at the cathode/electrolyte interface, or cracking of the electrolyte because of the stress developed upon heating and cooling. However, the mismatch can be minimized with the addition of SDC to form composite cathode. 3.4 Electrochemical characterization The typical impedance spectra of the symmetrical cell BSFN-SDC/SDC/BSFN-SDC at various temperatures (at 55, 6, 65, 7 C) in air are shown in Fig. 4, which is fitted using equivalent circuit program by Zview. The electrolyte contribution has been subtracted from the overall impedance. The model used to fit the impedance data was based on the model shown in Fig. 5. Similar models have been used in the literature for comparable systems [13 16]. The polarization resistance (R p ) of a single electrode may be calculated as R p =1/2(R 1 + R 2 ), where R 1 and R 2 are the total electrode contributions. The ohmic resistances R s of the electrolyte and R p from the impedance fitting of samples are presented in Table 1. As shown in Fig. 6, the ASR decreases with increasing SDC content at the same operating temperature. As expected, the increase of the measuring temperature results in a significant reduction of the ASR. It is worthy to note that the ASR of the BSFN7- SDC3, BSFN6-SDC4 and BSFN5-SDC5 cathode are.49,.34 and.31 Ω cm 2 in air at 7 C, respectively. The ASR is typically used in the field of SOFC to quantify all resistances associated with the electrode [17]. Although the ASR of BSFN-SDC is higher than those of BSZF, BSFC and SSFCu, it is lower than that of LSM-SDC (.94 Ω cm 2 ) prepared by screen-printed method at 7 C [18]. This implies 3 25 2 15 1 5 (a) 55 o C 8 6 4 2 (b) 6 o C 3. 2.5 2. 1.5 1..5 5 1 15 2 25 3 (c) 65 o C...5 1. 1.5 2. 2.5 3. 2 4 6 8 1..8.6.4.2 (d) 7 o C...2.4.6.8 1. Fig. 4 Impedance spectra of the BSFN-SDC cathode in various weight ratios with SDC serving as the electrolyte measured at 55 7 C. (( ) BSFN7-SDC3; ( ) BSFN6-SDC4; ( ) BSFN5-SDC5)
X.F. Chu et al.: J. Mater. Sci. Technol., 212, 28(9), 828 832. 831 Fig. 5 Equivalent circuit of BSFN-SDC in air Table 1 Fitted equivalent circuit polarization resistances for electrode Cathode composition T / C R s/ω cm 2 Area polarization resistances (ASR)/Ω cm 2 R 1 R 2 R p 55 14.2 8.4 7.81 8.11 BSFN5-SDC5 6 7.81 3.19 2.26 2.73 65 4.74.79.8.79 7 3.9.23.39.31 55 12.29 1.72 1.3 1.38 BSFN6-SDC4 6 6.79 3.34 3.12 3.23 65 4.12.86.97.92 7 2.69.31.38.34 55 14.9 8.49 33.23 2.86 BSFN7-SDC3 6 11.51 4.49 4.3 4.26 65 6.76 1.23 1.34 1.28 7 4.21.41.57.49 ln((t/asr) / K -1 cm -2 ) 8.5 8. 7.5 7. 6.5 6. 5.5 5. 4.5 4. 3.5 BSFN5-SDC5 E a =1.6 ev BSFN6-SDC4 E a =1.66 ev BSFN7-SDC3 E a =1.8 ev 1. 1.5 1.1 1.15 1.2 1.25 (1/T) / K -1 Fig. 6 Arrhenius plot of polarization resistance for BSFN-SDC cathode in various weight ratios that the activity of BSFN-SDC cathode is higher than that of traditional LSM-SDC cathode. As shown in Fig. 6, the activation energy (E a ) of BSFN-SDC cathode decreases with the SDC content. This may result from expanding the reaction zone beyond three-phase boundaries with increasing SDC content. However, it is worthy to note that the electrical conductivity of BSFN-SDC cathode will be too low with over-high content of SDC in the cathode. This may result in the difficulty of collecting current in practice. 3.5 Microstructure of cathode/electrolyte The microstructure of BSFN-SDC/SDC is shown in Fig. 7. The cathodes are porous, indicating that Fig. 7 SEM images of BSFN-SDC composite cathode after EIS testing: (a) surface of SDC electrolyte, (b) crosssection of BSFN-SDC/SDC
832 X.F. Chu et al.: J. Mater. Sci. Technol., 212, 28(9), 828 832. there is a good absorbility to the O 2. The thickness of cathodes was determined from SEM micrograph. The average thickness of BSFN-SDC is about 2 µm. 4. Conclusion New cobalt-free composites BSFN-SDC as a possible cathode material have been investigated on their chemical compatibility, electrical conductivity and polarization resistances in various weight ratios. BSFN is chemically compatible with SDC at 11 C. The conductivity values of the BSFN sample in the temperature range of IT-SOFC (4 6 C) are 28 57 S cm 1. The polarization resistance decreases with increasing SDC content in cathode material. The polarization resistance of BSFN5-SDC5 is.31 Ω cm 2 in air at 7 C and is much lower than that of traditional LSM-SDC cathode material. Therefore, BSFN- SDC is a possible candidate for IT-SOFC cathode material. REFERENCES [1 ] N.Q. Minh: J. Am. Ceram. Soc., 1993, 76, 563. [2 ] T. Horita, K. Yamaji, N. Sakai, Y.P. Xiong, T. Kato, H. Yokokawa and T. Kawada: J. Power Sources, 22, 16, 224 [3 ] H. Yamaura, T. Ikuta, H. Yahiro and G. Okada: Solid State Ionics, 25, 176, 269. [4 ] E.P. Murray, T. Tsai and S.A. Barnett: Solid State Ionics, 1998, 11, 235. [5 ] B. Wei, Z. Lv, X.Q. Huang, M.L. Liu, N. Li and W.H. Su: J. Power Sources, 28, 176, 1. [6 ] L. Zhao, B.B. He, X.Z. Zhang, R.R. Peng, G.Y. Meng and X.Q. Liu: J. Power Sources, 21, 195, 1859. [7 ] Y.H. Ling, L. Zhao, B. Lin, Y.C. Dong, X.Z. Zhang, G.Y. Meng and X.Q. Liu: Int. J. Hydrog. Energy, 21, 35, 695. [8 ] W.Q. Sun, Z. Shi, S.M. Fang, L.T. Yan, Z.W. Zhu and W. Liu: Int. J. Hydrog. Energy, 21, 35, 7925. [9 ] H.B. Li, C.R. Xia, M.H. Zhu, Z.X. Zhou and G.Y. Meng: Acta Mater., 26, 54, 721 [1] C.T. Yang, W.J. Wei and A. Roosen: Mater. Chem. Phys., 23, 81, 134. [11] Z.H. Chen, R. Ran, W. Zhou, Z.P. Shao and S.M. Liu: Electrochim. Acta, 27, 52, 7343. [12] Q. Xu, D.P. Huang, F. Zhang, W. Chen, M. Chen and H.X. Liu: J. Alloy. Compd., 28, 454, 46. [13] Z.Y. Jiang, L. Zhang, K. Feng and C.R. Xia: J. Power Sources, 28, 185, 4. [14] H. Lv, Y.J. Wu, B. Huang, B.Y. Zhao and K.A. Hu: Solid State Ionics, 26, 177, 91. [15] M. Shah and S.A. Barnett: Solid State Ionics, 28, 179, 259. [16] L. Zhao, B.B. He, Y.H. ling, Z.Q. Xun, R.R. Peng, G.Y. Meng and X.Q. Liu: Int. J. Hydrog. Energy, 21, 35, 3769. [17] Z.P. Shao and S.M. Haile: Nature, 24, 431, 17. [18] X.Y. Xu, C.B. Cao, C.R. Xia and D.K. Peng: Ceram. Int., 29, 35, 2213.