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Preparation, chemical stability and electrochemical properties of LSCF–CBO composite cathodes
Materials Chemistry and Physics 88 (2004) 160–166
Preparation, chemical stability and electrochemical properties of LSCF–CBO composite cathodes Hui Zhao∗ , Lihua Huo, Liping Sun, Lijun Yu, Shan Gao, Jinggui Zhao College of Chemistry and Chemical Technology, Heilongjiang University, Harbin 150080, PR China Received 5 January 2004; received in revised form 29 June 2004; accepted 12 July 2004
Abstract La0.6 Sr0.4 Co0.2 Fe0.8 O3 –Ce0.7 Bi0.3 O2 (LSCF–CBO) composite electrode has been prepared by polymer–gel method. SEM results showed that the composite electrode formed good contact with Ce0.9 Gd0.1 O2−␦ (CGO) electrolyte after sintering at 900 ◦ C for 2 h. AC impedance spectroscopy and dc polarization measurements were used to study the composite electrode performances in different atmospheres. The (pO2 )1/2 relationship of Rp (polarization resistance) with oxygen partial pressure has been observed, which indicated that the oxygen adsorption/desorption process was the reaction rate limiting step. The polarization resistance decreased with increasing CBO contents. The optimum value of 35 vol.% CBO in LSCF resulted in 0.16 cm2 area specific resistivity (ASR) at 706 ◦ C in air, which indicated that the LSCF–CBO composite electrode was a promising cathode material in ITSOFC. © 2004 Elsevier B.V. All rights reserved. Keywords: Ceramics; Electrochemical techniques; Electrochemical properties
1. Introduction There is increasing interest in the development of intermediate temperature solid oxide fuel cells (ITSOFCs). Reduced operation temperature can reduce problems with sealing and thermal degradation, allows the use of lower-cost interconnect materials [1]. Although improvement has been made recently, reducing the operation temperature to 500–800 ◦ C without a significant decrease of the power density remains to be the challenge of this technology [2,3]. One of the major problems encountered is the large cathode overpotential caused by the reduced operation temperatures. LSCF has been found to be a possible candidate to solve this problem [4–6]. Unlike LSM, which is a poor ionic conductor, LSCF is a mixed electronic-ionic conductor with appreciable ionic conductivity. In this material, the exchange of oxygen molecules occurs at the whole electrode surface, and the oxygen ions can diffuse through the bulk of the mixed ∗ Corresponding author. Tel.: +86-4518-6607791; fax: +86-45186-682541. E-mail address: [email protected] (H. Zhao).
0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.07.002
conductor. LSCF has shown improved cathode properties, due to the rapid surface exchange kinetics, high oxygen vacancy concentration, and the extended triple phase boundary (TPB) from the electrolyte/electrode interface to the bulk of the electrode [4,7–9]. Studies have shown that the activation energy for oxygen self-diffusion in LSCF was high (about 186 kJ mol−1 )[10], thus the oxygen ionic conductivity will be small at low temperature. In order to enhance the ionic conductivity, a composite electrode with a highly ionic conducting phase has been explored [11]. It was found that the LSCF–CGO composite material exhibits greatly improved cathode performance. Dusastre and Kilner has reported that a four times lower area specific resistivity was found in the composites [11]. Although LSCF–CGO composite cathodes have exhibited much improved catalysis properties, research into alternative materials is needed. Recently, bismuth doped perovskite-type materials have been investigated as possible cathodes in ITSOFC [12]. In the previous study, bismuth doped ceria has shown high oxygen ionic conductivity, which may be used as electrolyte in ITSOFC [13–16]. Furthermore from practical point of view, CBO is cheaper than CGO. It is interesting to study the LSCF–CBO composite material
H. Zhao et al. / Materials Chemistry and Physics 88 (2004) 160–166
and its possible usage in ITSOFC. Here we report the preliminary results of LSCF–CBO composite electrode in the intermediate temperature range.
2. Experimental Ce0.9 Gd0.1 O2−␦ (CGO) powders (supplied by Rhodia) were pressed uniaxially at 350 MPa to form green pellet and then sintered at 1350 ◦ C for 8 h. The density of the obtained pellet, determined by Archimedes’ method, was over 95% of the theoretical value. Both sides of the pellet were roughed with 240# SiC grit paper and then ultrasonically cleaned. The La0.6 Sr0.4 Co0.2 Fe0.8 O3 (LSCF) powders were prepared according to ref. [17]. The Ce0.7 Bi0.3 O2 powders were prepared according to reference [13]. The electrode slurry was made with the desired amount of LSCF and CBO together with ethylene glycol and ball milled overnight. The compositions of the slurry were varied from 10 to 50 wt.% of CBO. The slurry was painted on one side of the CGO pellet to form an electrode area of 0.4 cm2 . Platinum paste was painted on the other side of the pellet in symmetric configuration. A Pt wire was used as reference electrode and put on the same side as the working electrode. The cell was first heated up to 500 ◦ C to eliminate the organic binders, and then to 900 ◦ C for 2 h. The structure and phase stability of the composite materials were characterized by X-ray powder diffraction on a Siemens D5005 diffractometer with a scan rate of 0.1◦ (2θ) min−1 . SEM was used to examine the morphology and microstructures of the sintered composite electrodes. The impedance spectra were recorded over the frequency range 1 MHz to 0.01 Hz using autolab PGstat30. The measurements were performed at equilibrium potential
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as a function of temperature (500–700 ◦ C) and oxygen partial pressure (10−3 –0.21 atm in an Ar/O2 mixture atmosphere). The obtained data was analyzed with Z-view software. The dc polarization experiments were performed by the chronoampermetry method [18], which involves a potential step followed by recording the current density as a function of time. The cathode overpotential was calculated according to the following Eq. ηWE = UWR − iRel where ηWE represents the cathode overpotential, UWR is the applied voltage between working electrode and reference electrode, i is the current flowing through the test cell and Rel is the resistance of the electrolyte obtained from the impedance spectrum.
3. Results and discussions 3.1. Chemical stability and sintering properties of the LSCF–CBO composites XRD experiments were first performed to check the phase stability of CBO with LSCF under different experimental conditions. The results were shown in Fig. 1. It was observed that CBO and LSCF powders remained their structures unchanged when sintered at 1000 ◦ C in air, or heated at 700 ◦ C under 0.6 V cathodic potential in argon-oxygen mixed atmospheres (with oxygen partial pressure equals to 0.001 atm). Two additional small diffraction peaks appeared at about 27 and 30◦ , respectively. This result indicated that a minor third phase has formed when the LSCF–CBO composite material was heat treated at 1000 ◦ C. At present, it is difficult to
Fig. 1. XRD results of LSCF–CBO composite materials sintered at different experimental conditions. Asterisks indicate the formation of third phase.
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Fig. 2. SEM results of the sintered composite electrodes: (a) 800 ◦ C; (b) 900 ◦ C; (c) 1000 ◦ C; (d) interface between LSCF–CBO composite electrode and CGO electrolyte.
identify the third phase, due to the limited data information. The sintering properties of LSCF–CBO composite materials on CGO electrolyte were further studied. Fig. 2 shows the SEM results of the composite electrode sintering at different temperatures for two hours. Below 800 ◦ C, LSCF and CGO show bad contact with each other. Sintering at 900 ◦ C results in a structure with moderate porosity and well-necked particles. When the composite electrode is sintered at 1000 ◦ C for 2 h, however, the over-sintering phenomenon is observed. The particle size is about 1 m, and the thickness of the composite electrode is about 10 m. 3.2. Electrochemical properties of the composite materials Fig. 3 shows typical Nyquist plots of the test cells measured at different temperatures and oxygen partial pressures, respectively. In reduced atmospheres, two arcs could be observed. The arc located at the low frequency side (arc 2) dominates the spectrum for all the measurement parameters and all samples. The arc at high frequency side (arc 1) can be well separated at higher temperatures. The corresponding relaxation frequencies of the two arcs (measured at 706 ◦ C) are around 1 kHz and 10 Hz, respectively. In air, however, the two arcs tend to merge into one depressed arc (see Fig. 3b, inset). In order to separate the contributions of the two arcs, an equivalent circuit with two distributed elements, similar to that reported in ref. [18] and [19], has been used to fit the
data. The fitting results are shown in Fig. 3. In the fitting model, the inductance at very high frequency is attributed to the artifacts of the device and platinum wires. Rel represents the intercept of the impedance spectrum at high frequency with the real axis. The distributed element represents a constant phase element (CPE) in parallel with a resistance (R). Each CPE has a CPE-T, which is related to the relaxation capacitance; and a CPE-P, which reflects the displacement of the center of the arc from the real axis. In this model, R1 represents the magnitude of arc 1, and R2 is that of arc 2, respectively. It is found that Rel decreases with the increase of measurement temperature, but remains unchanged with the change of oxygen partial pressure. We compared the conductivity calculated from Rel with the reported conductivity values of CGO electrolyte (Fig. 4). Obviously Rel is in good agreement with the reported values in the whole range of temperature. The activation energy obtained is 0.8 eV, which is a typical value for the oxygen ionic conduction in CGO electrolyte [11]. Therefore, we attributed Rel to the oxygen ionic conduction in CGO electrolyte. From Fig. 3, we can also see that both R1 and R2 decrease with increasing measurement temperatures and oxygen partial pressures. The roughly estimated capacitance values (obtained from the apex frequency of the two arcs, according to the equation = 1/RC) are about 1 × 10−4 and 1 × 10−2 F, respectively. The values are consistent with electrochemical reaction mechanism that occurs on electrode [20]. The CPE-P values for the two arcs were found to be 0.5 and 0.7, respectively. The values were
H. Zhao et al. / Materials Chemistry and Physics 88 (2004) 160–166
Fig. 3. Typical Nyquist plot and fitting model of the composite electrode with 35 vol.% CBO on CGO pellet measured under different oxygen partial pressures at (a) 571 ◦ C and (b) 706 ◦ C, respectively.
Fig. 4. Comparison results of Rel with the reported conductivity values of CGO electrolyte.
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similar to that of the reported LSM–YSZ composite electrode [19]. In order to investigate the effect of sintering temperature on the LSCF–CBO cathode properties, a number of different sintering conditions were studied. Fig. 5 gave the Nyquist plot of the composite cathodes sintered at different temperatures for 2 h and then measured at 706 ◦ C in air. From the impedance spectrum, we observed that the total polarization resistance was relatively large when the electrode was sintered at 800 ◦ C. Although the major contribution still came from the second arc, the amplitude of the first arc increased dramatically. This is due to the low sintering temperatures which results in poor contact between the LSCF and CBO particles and the electrode/electrolyte interface (Fig. 2a). The similar phenomena have been observed before in the investigation of LSM–YSZ and LSM–CBO composite cathode [21,22]. When the sintering temperature was 900 ◦ C, the total polarization resistance reduced. Considering the low melting point of Bi2 O3 doped CBO powders, a good contact has been formed between CBO and LSCF particles at this temperature, which in turn will reduce the grain boundary resistance. When the sintering temperature is increased to 1000 ◦ C, however, the polarization resistance increases again, due to the oversintering of LSCF–CBO composite electrode. In this case, the nano-size CBO powder may melt and cover the surface of LSCF powders at 1000 ◦ C, and the TPB in LSCF–CBO composite cathode decreased. Similar effects have been observed before in the reported LSM–YSZ and LSM–CGO composite materials [21,23]. The effects of CBO contents on the polarization resistance were further studied. Fig. 6 showed the total polarization resistance, Rp (given by the difference between the real-axis intercepts of the impedance spectrum) versus measurement temperatures for different composite cathodes. It was observed that the total polarization resistance first decreased with the increase of CBO content, and reached a minimum at about 35 vol.% CBO in LSCF powders. For high volume
Fig. 5. Nyquist plot of the composite cathodes sintering at different temperatures for 2 h and then measured at 706 ◦ C in air.
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Fig. 6. Arrhenius plot of the total polarization resistance for different composite cathode materials.
fractions, the polarization resistance increases rapidly. This result is roughly in agreement with the estimated trends obtained from the effective medium percolation theory (EMPT). According to EMPT model, 30% porosity (estimated from the SEM micrograph, as shown in Fig. 2b) will lead to 40% optimum volume fraction of CBO in the LSCF–CBO composite electrode. The discrepancy between the estimated value (40 vol.%) and the experiment results (35 vol.%) indicated that the electrode performance is not uniquely determined by the mixed ionic and electronic conductivity, but also by the inherent catalytic properties and the gas transport properties of the composite electrode. The similar conclusions have been obtained before from the study of LSCF–CGO composite electrode [11]. The corresponding activation energy is about 1.4 eV, similar to that of the pure LSCF electrode [11,24]. The activation energy remained almost unchanged when the CBO content was less than 35 vol.%, and then decreased with increasing CBO contents. Therefore, in the experiments below, only the optimum composite electrodes (with 35 vol.% CBO) were further studied.
Fig. 7. Variation of R1 and R2 with the temperatures measured at different oxygen partial pressures, respectively.
volved in the reactions [25–28]. According to (Fig. 8), we observed that n value was around 0.25 for R1 and 0.40 for R2 , respectively. In a previous study, the (pO2 )1/2 relationship has been observed in LSC, SSC and Pt materials. It
3.3. Oxygen partial pressure dependence of the composite electrode Oxygen partial pressure experiments have been performed to study the reaction mechanism that occurred on the composite electrode. Figs. 7 and 8 show the variations of R1 and R2 with temperatures and oxygen partial pressures, respectively. It was observed that both R1 and R2 decreased with increasing oxygen partial pressures and temperatures. The activation energies of R1 and R2 were 1.2 and 1.55 eV, respectively. The activation energies did not change with the oxygen partial pressure (Fig. 7). The change of R1 and R2 with the oxygen partial pressures (Fig. 8) could be described by the following expression: Rp = Rp o (pO2 )n . The value of n could give useful information about the type of species in-
Fig. 8. Variation of (a) R1 and (b) R2 with the oxygen partial pressures measured at different temperatures, respectively.
H. Zhao et al. / Materials Chemistry and Physics 88 (2004) 160–166
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Table 1 Comparison of the area specific resistance obtained from the EIS measurement in air and from the inverse of the linear expression between 0 and −20 mV Temperature (◦ C)
η/ia ( cm2 )
Rp b ( cm2 )
571 628 673 706
2.141 0.592 0.277 0.162
2.136 0.612 0.284 0.16
a The polarization resistance was calculated from the linear expression i = i0 ZFη/RT. b The polarization resistance was obtained from the EIS spectrum.
to optimize the microstructure and to improve the catalytic properties of the electrode. Fig. 9. The overpotential–current density curves measured at different temperatures in air.
was related to the oxygen adsorption/desorption process on electrode [29–33]. The (pO2 )1/4 relationship has been observed for LSC and Pt materials. It was considered as the contribution from the charge transfer process [32,33]. In our study, the composition and microstructure were different from the reported LSC, SSC and Pt electrode, but they exhibited the similar behaviors on oxygen partial pressures. So we proposed that arc 1 was caused by the charge transfer process on the electrode, whereas arc 2 was due to the oxygen adsorption/desorption process on the electrode. We could also observe that the polarization resistance corresponding to arc 2 was relatively large at low oxygen partial pressure, and it decreased more rapidly than that of arc 1. This trend indicated that the oxygen adsorption/desorption process was the rate limiting step under lower oxygen partial pressure, whereas at higher oxygen partial pressure, the charge transfer process became more important.
4. Conclusions • The composite electrode forms good contact with CGO electrolyte after sintering at 900 ◦ C for 2 h. • The oxygen adsorption/desorption process is the reaction rate limiting step under lower oxygen partial pressure, whereas the charge transfer process become more important at higher oxygen partial pressure. • The lowest specific resistivity obtained at 706 ◦ C in air is about 0.16 cm2 .
Acknowledgements The project was supported by Natural Science Foundation (ZJG03-4) and oversea foundation (1054HQ003) of Heilongjiang Province.
References 3.4. dc measurment Fig. 9 showed the dc polarization curves measured at different temperatures in air. It was observed that the current density increases with increasing temperatures. At low overpotential (less than 20 mV), we can expect a linear expression[5], i = i0 ZFη/RT, where i is the current density, i0 the exchange current density, η the overpotential, and F, R have their normal meanings, respectively. From the inverse of the derivative of i against η, we can obtain the area specific resistance. The value was comparable to the area specific resistance deduced from the impedance spectrum (Table 1). The area specific resistance obtained at 706 ◦ C in air was about 0.16 cm2 , comparable to the LSCF–CGO material [11]. Our results again proved that the polarization resistance can be reduced dramatically by forming a composite cathode with high electronic and ionic conducting materials, and that LSCF–CBO composite materials can be considered as possible cathodes for ITSOFC. Further work will be done
[1] N.Q. Minh, J. Am. Ceram. Soc. 76 (1993) 563. [2] S. Linderoth, in: A.J. Mc Evoy (Ed.), Proceedings of the Fourth European Solid Oxide Fuel Cell Forum, U. Bossel, Oberrohrdorf, Switzerland (publ.), 2000, p. 19. [3] E. Ivers-Tiffee, A. Weber, D. Herbstritt, J. Eur. Ceram. Soc. 21 (2001) 1805. [4] S.B. Adler, J.A. Lane, B.C.H. Steele, J. Electrochem. Soc. 143 (1996) 3554. [5] B.C.H. Steele, Solid State Ion. 75 (1995) 157. [6] B.C.H. Steele, Solid State Ion. 86–88 (1996) 1223. [7] C.C. Chen, M.M. Nasralla, H.U. Anderson, in: S.C. Singhal, H. Iwahara (Eds.), Proceedings of the Third International Symposium on Solid Oxide Fuel Cells, The Electrochemical Society Proceedings Series, Pennington, NY, 1999, p. 252. [8] L.-W. Tai, M.M. Nasrallah, H.U. Anderson, in: S.C. Singhal, H. Iwahara (Eds.), Proceedings of the Third International Symposium of the Solid Oxide Fuel Cells, The Electrochemical Society Proceedings Series, Pennington, NY, 1999, p. 241. [9] C.C. Chen, M.M. Nasralla, H.U. Anderson, M.A. Alim, J. Electrochem. Soc. 142 (1995) 491. [10] S.J. Benson, R.J. Chater, J.A. Kilner, in: T.A. Ramanarayanan (Ed.), Proceedings of the Third International Symposium on Ionic and
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H. Zhao et al. / Materials Chemistry and Physics 88 (2004) 160–166 Mixed Conducting Ceramics, Electrochemical Society Proceedings, Pennington, 1998, p. 596. V. Dusastre, J.A. Kilner, Solid State Ion. 126 (1999) 163. H. Zhao, M. Pijolat, J. Mater. Chem. 12 (2002) 3787. H. Zhao, S. Feng, W. Xu, Mater. Res. Bull. 35 (2000) 2379. H. Zhao, S. Feng, Chem. Mater. 11 (1999) 958. G. Li, L. Li, S. Feng, M. Wang, L. Zhang, X. Yao, Adv. Mater. 11 (1999) 146. G. Li, Y. Mao, L. Li, S. Feng, M. Wang, X. Yao, Chem. Mater. 11 (1999) 1259. S.R. Wang, T. Kato, S. Nagata, T. Honda, T. Kaneko, N. Iwashita, M. Dokiya, Solid State Ion. 146 (2002) 203. A. Espuirol, N. Brandon, N. Bonanos, J. Kilner, M. Mogensen, B.C.H. Steele, in: A.J. Mc Evoy (Ed.), Proceedings of the Fifth European Solid Oxide Fuel Cell Forum, U. Bossel, Oberrohrdorf, Switzerland (publ.), 2002, p. 225. N.T. Hart, N.P. Brandon, M.J. Day, J.E. Shemilt, J. Mater. Sci. 36 (2001) 1077. J.T.S. Irvine, D.C. Sinclair, A.R. West, Adv. Mater. 2 (1990) 132. S.W. Matin, Solid State Ion. 51 (1992) 19. H. Zhao, L.H. Huo, S. Gao, J. Power Sources 125 (2004) 149.
[23] E.P. Murray, M.J. Sever, S.A. Barnett, Solid State Ion. 148 (2002) 27. [24] B.C.H. Steele, Solid State Ion. 94 (1997) 239. [25] Y. Takeda, R. Kanno, M. Noda, Y. Tomida, O. Yamamoto, J. Electrochem. Soc. 134 (1987) 2656. [26] E. Siebert, A. Hammouche, M. Kleitz, Electrochim. Acta 40 (1995) 1741. [27] J. Mizusaki, K. Amano, S. Yamauchi, K. Fueki, Solid State Ion. 22 (1987) 313. [28] F. Mauvy, J.M. Bassat, E. Boehm, J.P. Manaud, P. Dordor, J.C. Grenier, Solid State Ion. 158 (2003) 17. [29] A. Endo, S. Wada, C.J. Wen, H. Komiyama, K. Yamada, J. Electrochem. Soc. 145 (1998) L35. [30] M. Koyama, C.J. Wen, T. Masuyama, J. Otomo, H. Fukunaga, K. Yamada, K. Eguchi, H. Takahashi, J. Electrochem. Soc. 148 (2001) 795. [31] Y.L. Wang, C.L. Chen, S.Y. Chen, C.W. Chu, A.J. Jacobson, J. Electrochem. Soc. 147 (2000) 4001. [32] H. Fukunaga, M. Koyama, N. Takahashi, C. Wen, K. Yamada, Solid State Ion. 132 (2000) 279. [33] H.X. Hu, M.L. Liu, J. Electrochem. Soc. 144 (1997) 3561.
Preparation, chemical stability and electrochemical properties of LSCF–CBO composite cathodes Hui Zhao∗ , Lihua Huo, Liping Sun, Lijun Yu, Shan Gao, Jinggui Zhao College of Chemistry and Chemical Technology, Heilongjiang University, Harbin 150080, PR China Received 5 January 2004; received in revised form 29 June 2004; accepted 12 July 2004
Abstract La0.6 Sr0.4 Co0.2 Fe0.8 O3 –Ce0.7 Bi0.3 O2 (LSCF–CBO) composite electrode has been prepared by polymer–gel method. SEM results showed that the composite electrode formed good contact with Ce0.9 Gd0.1 O2−␦ (CGO) electrolyte after sintering at 900 ◦ C for 2 h. AC impedance spectroscopy and dc polarization measurements were used to study the composite electrode performances in different atmospheres. The (pO2 )1/2 relationship of Rp (polarization resistance) with oxygen partial pressure has been observed, which indicated that the oxygen adsorption/desorption process was the reaction rate limiting step. The polarization resistance decreased with increasing CBO contents. The optimum value of 35 vol.% CBO in LSCF resulted in 0.16 cm2 area specific resistivity (ASR) at 706 ◦ C in air, which indicated that the LSCF–CBO composite electrode was a promising cathode material in ITSOFC. © 2004 Elsevier B.V. All rights reserved. Keywords: Ceramics; Electrochemical techniques; Electrochemical properties
1. Introduction There is increasing interest in the development of intermediate temperature solid oxide fuel cells (ITSOFCs). Reduced operation temperature can reduce problems with sealing and thermal degradation, allows the use of lower-cost interconnect materials [1]. Although improvement has been made recently, reducing the operation temperature to 500–800 ◦ C without a significant decrease of the power density remains to be the challenge of this technology [2,3]. One of the major problems encountered is the large cathode overpotential caused by the reduced operation temperatures. LSCF has been found to be a possible candidate to solve this problem [4–6]. Unlike LSM, which is a poor ionic conductor, LSCF is a mixed electronic-ionic conductor with appreciable ionic conductivity. In this material, the exchange of oxygen molecules occurs at the whole electrode surface, and the oxygen ions can diffuse through the bulk of the mixed ∗ Corresponding author. Tel.: +86-4518-6607791; fax: +86-45186-682541. E-mail address: [email protected] (H. Zhao).
0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.07.002
conductor. LSCF has shown improved cathode properties, due to the rapid surface exchange kinetics, high oxygen vacancy concentration, and the extended triple phase boundary (TPB) from the electrolyte/electrode interface to the bulk of the electrode [4,7–9]. Studies have shown that the activation energy for oxygen self-diffusion in LSCF was high (about 186 kJ mol−1 )[10], thus the oxygen ionic conductivity will be small at low temperature. In order to enhance the ionic conductivity, a composite electrode with a highly ionic conducting phase has been explored [11]. It was found that the LSCF–CGO composite material exhibits greatly improved cathode performance. Dusastre and Kilner has reported that a four times lower area specific resistivity was found in the composites [11]. Although LSCF–CGO composite cathodes have exhibited much improved catalysis properties, research into alternative materials is needed. Recently, bismuth doped perovskite-type materials have been investigated as possible cathodes in ITSOFC [12]. In the previous study, bismuth doped ceria has shown high oxygen ionic conductivity, which may be used as electrolyte in ITSOFC [13–16]. Furthermore from practical point of view, CBO is cheaper than CGO. It is interesting to study the LSCF–CBO composite material
H. Zhao et al. / Materials Chemistry and Physics 88 (2004) 160–166
and its possible usage in ITSOFC. Here we report the preliminary results of LSCF–CBO composite electrode in the intermediate temperature range.
2. Experimental Ce0.9 Gd0.1 O2−␦ (CGO) powders (supplied by Rhodia) were pressed uniaxially at 350 MPa to form green pellet and then sintered at 1350 ◦ C for 8 h. The density of the obtained pellet, determined by Archimedes’ method, was over 95% of the theoretical value. Both sides of the pellet were roughed with 240# SiC grit paper and then ultrasonically cleaned. The La0.6 Sr0.4 Co0.2 Fe0.8 O3 (LSCF) powders were prepared according to ref. [17]. The Ce0.7 Bi0.3 O2 powders were prepared according to reference [13]. The electrode slurry was made with the desired amount of LSCF and CBO together with ethylene glycol and ball milled overnight. The compositions of the slurry were varied from 10 to 50 wt.% of CBO. The slurry was painted on one side of the CGO pellet to form an electrode area of 0.4 cm2 . Platinum paste was painted on the other side of the pellet in symmetric configuration. A Pt wire was used as reference electrode and put on the same side as the working electrode. The cell was first heated up to 500 ◦ C to eliminate the organic binders, and then to 900 ◦ C for 2 h. The structure and phase stability of the composite materials were characterized by X-ray powder diffraction on a Siemens D5005 diffractometer with a scan rate of 0.1◦ (2θ) min−1 . SEM was used to examine the morphology and microstructures of the sintered composite electrodes. The impedance spectra were recorded over the frequency range 1 MHz to 0.01 Hz using autolab PGstat30. The measurements were performed at equilibrium potential
161
as a function of temperature (500–700 ◦ C) and oxygen partial pressure (10−3 –0.21 atm in an Ar/O2 mixture atmosphere). The obtained data was analyzed with Z-view software. The dc polarization experiments were performed by the chronoampermetry method [18], which involves a potential step followed by recording the current density as a function of time. The cathode overpotential was calculated according to the following Eq. ηWE = UWR − iRel where ηWE represents the cathode overpotential, UWR is the applied voltage between working electrode and reference electrode, i is the current flowing through the test cell and Rel is the resistance of the electrolyte obtained from the impedance spectrum.
3. Results and discussions 3.1. Chemical stability and sintering properties of the LSCF–CBO composites XRD experiments were first performed to check the phase stability of CBO with LSCF under different experimental conditions. The results were shown in Fig. 1. It was observed that CBO and LSCF powders remained their structures unchanged when sintered at 1000 ◦ C in air, or heated at 700 ◦ C under 0.6 V cathodic potential in argon-oxygen mixed atmospheres (with oxygen partial pressure equals to 0.001 atm). Two additional small diffraction peaks appeared at about 27 and 30◦ , respectively. This result indicated that a minor third phase has formed when the LSCF–CBO composite material was heat treated at 1000 ◦ C. At present, it is difficult to
Fig. 1. XRD results of LSCF–CBO composite materials sintered at different experimental conditions. Asterisks indicate the formation of third phase.
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Fig. 2. SEM results of the sintered composite electrodes: (a) 800 ◦ C; (b) 900 ◦ C; (c) 1000 ◦ C; (d) interface between LSCF–CBO composite electrode and CGO electrolyte.
identify the third phase, due to the limited data information. The sintering properties of LSCF–CBO composite materials on CGO electrolyte were further studied. Fig. 2 shows the SEM results of the composite electrode sintering at different temperatures for two hours. Below 800 ◦ C, LSCF and CGO show bad contact with each other. Sintering at 900 ◦ C results in a structure with moderate porosity and well-necked particles. When the composite electrode is sintered at 1000 ◦ C for 2 h, however, the over-sintering phenomenon is observed. The particle size is about 1 m, and the thickness of the composite electrode is about 10 m. 3.2. Electrochemical properties of the composite materials Fig. 3 shows typical Nyquist plots of the test cells measured at different temperatures and oxygen partial pressures, respectively. In reduced atmospheres, two arcs could be observed. The arc located at the low frequency side (arc 2) dominates the spectrum for all the measurement parameters and all samples. The arc at high frequency side (arc 1) can be well separated at higher temperatures. The corresponding relaxation frequencies of the two arcs (measured at 706 ◦ C) are around 1 kHz and 10 Hz, respectively. In air, however, the two arcs tend to merge into one depressed arc (see Fig. 3b, inset). In order to separate the contributions of the two arcs, an equivalent circuit with two distributed elements, similar to that reported in ref. [18] and [19], has been used to fit the
data. The fitting results are shown in Fig. 3. In the fitting model, the inductance at very high frequency is attributed to the artifacts of the device and platinum wires. Rel represents the intercept of the impedance spectrum at high frequency with the real axis. The distributed element represents a constant phase element (CPE) in parallel with a resistance (R). Each CPE has a CPE-T, which is related to the relaxation capacitance; and a CPE-P, which reflects the displacement of the center of the arc from the real axis. In this model, R1 represents the magnitude of arc 1, and R2 is that of arc 2, respectively. It is found that Rel decreases with the increase of measurement temperature, but remains unchanged with the change of oxygen partial pressure. We compared the conductivity calculated from Rel with the reported conductivity values of CGO electrolyte (Fig. 4). Obviously Rel is in good agreement with the reported values in the whole range of temperature. The activation energy obtained is 0.8 eV, which is a typical value for the oxygen ionic conduction in CGO electrolyte [11]. Therefore, we attributed Rel to the oxygen ionic conduction in CGO electrolyte. From Fig. 3, we can also see that both R1 and R2 decrease with increasing measurement temperatures and oxygen partial pressures. The roughly estimated capacitance values (obtained from the apex frequency of the two arcs, according to the equation = 1/RC) are about 1 × 10−4 and 1 × 10−2 F, respectively. The values are consistent with electrochemical reaction mechanism that occurs on electrode [20]. The CPE-P values for the two arcs were found to be 0.5 and 0.7, respectively. The values were
H. Zhao et al. / Materials Chemistry and Physics 88 (2004) 160–166
Fig. 3. Typical Nyquist plot and fitting model of the composite electrode with 35 vol.% CBO on CGO pellet measured under different oxygen partial pressures at (a) 571 ◦ C and (b) 706 ◦ C, respectively.
Fig. 4. Comparison results of Rel with the reported conductivity values of CGO electrolyte.
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similar to that of the reported LSM–YSZ composite electrode [19]. In order to investigate the effect of sintering temperature on the LSCF–CBO cathode properties, a number of different sintering conditions were studied. Fig. 5 gave the Nyquist plot of the composite cathodes sintered at different temperatures for 2 h and then measured at 706 ◦ C in air. From the impedance spectrum, we observed that the total polarization resistance was relatively large when the electrode was sintered at 800 ◦ C. Although the major contribution still came from the second arc, the amplitude of the first arc increased dramatically. This is due to the low sintering temperatures which results in poor contact between the LSCF and CBO particles and the electrode/electrolyte interface (Fig. 2a). The similar phenomena have been observed before in the investigation of LSM–YSZ and LSM–CBO composite cathode [21,22]. When the sintering temperature was 900 ◦ C, the total polarization resistance reduced. Considering the low melting point of Bi2 O3 doped CBO powders, a good contact has been formed between CBO and LSCF particles at this temperature, which in turn will reduce the grain boundary resistance. When the sintering temperature is increased to 1000 ◦ C, however, the polarization resistance increases again, due to the oversintering of LSCF–CBO composite electrode. In this case, the nano-size CBO powder may melt and cover the surface of LSCF powders at 1000 ◦ C, and the TPB in LSCF–CBO composite cathode decreased. Similar effects have been observed before in the reported LSM–YSZ and LSM–CGO composite materials [21,23]. The effects of CBO contents on the polarization resistance were further studied. Fig. 6 showed the total polarization resistance, Rp (given by the difference between the real-axis intercepts of the impedance spectrum) versus measurement temperatures for different composite cathodes. It was observed that the total polarization resistance first decreased with the increase of CBO content, and reached a minimum at about 35 vol.% CBO in LSCF powders. For high volume
Fig. 5. Nyquist plot of the composite cathodes sintering at different temperatures for 2 h and then measured at 706 ◦ C in air.
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Fig. 6. Arrhenius plot of the total polarization resistance for different composite cathode materials.
fractions, the polarization resistance increases rapidly. This result is roughly in agreement with the estimated trends obtained from the effective medium percolation theory (EMPT). According to EMPT model, 30% porosity (estimated from the SEM micrograph, as shown in Fig. 2b) will lead to 40% optimum volume fraction of CBO in the LSCF–CBO composite electrode. The discrepancy between the estimated value (40 vol.%) and the experiment results (35 vol.%) indicated that the electrode performance is not uniquely determined by the mixed ionic and electronic conductivity, but also by the inherent catalytic properties and the gas transport properties of the composite electrode. The similar conclusions have been obtained before from the study of LSCF–CGO composite electrode [11]. The corresponding activation energy is about 1.4 eV, similar to that of the pure LSCF electrode [11,24]. The activation energy remained almost unchanged when the CBO content was less than 35 vol.%, and then decreased with increasing CBO contents. Therefore, in the experiments below, only the optimum composite electrodes (with 35 vol.% CBO) were further studied.
Fig. 7. Variation of R1 and R2 with the temperatures measured at different oxygen partial pressures, respectively.
volved in the reactions [25–28]. According to (Fig. 8), we observed that n value was around 0.25 for R1 and 0.40 for R2 , respectively. In a previous study, the (pO2 )1/2 relationship has been observed in LSC, SSC and Pt materials. It
3.3. Oxygen partial pressure dependence of the composite electrode Oxygen partial pressure experiments have been performed to study the reaction mechanism that occurred on the composite electrode. Figs. 7 and 8 show the variations of R1 and R2 with temperatures and oxygen partial pressures, respectively. It was observed that both R1 and R2 decreased with increasing oxygen partial pressures and temperatures. The activation energies of R1 and R2 were 1.2 and 1.55 eV, respectively. The activation energies did not change with the oxygen partial pressure (Fig. 7). The change of R1 and R2 with the oxygen partial pressures (Fig. 8) could be described by the following expression: Rp = Rp o (pO2 )n . The value of n could give useful information about the type of species in-
Fig. 8. Variation of (a) R1 and (b) R2 with the oxygen partial pressures measured at different temperatures, respectively.
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Table 1 Comparison of the area specific resistance obtained from the EIS measurement in air and from the inverse of the linear expression between 0 and −20 mV Temperature (◦ C)
η/ia ( cm2 )
Rp b ( cm2 )
571 628 673 706
2.141 0.592 0.277 0.162
2.136 0.612 0.284 0.16
a The polarization resistance was calculated from the linear expression i = i0 ZFη/RT. b The polarization resistance was obtained from the EIS spectrum.
to optimize the microstructure and to improve the catalytic properties of the electrode. Fig. 9. The overpotential–current density curves measured at different temperatures in air.
was related to the oxygen adsorption/desorption process on electrode [29–33]. The (pO2 )1/4 relationship has been observed for LSC and Pt materials. It was considered as the contribution from the charge transfer process [32,33]. In our study, the composition and microstructure were different from the reported LSC, SSC and Pt electrode, but they exhibited the similar behaviors on oxygen partial pressures. So we proposed that arc 1 was caused by the charge transfer process on the electrode, whereas arc 2 was due to the oxygen adsorption/desorption process on the electrode. We could also observe that the polarization resistance corresponding to arc 2 was relatively large at low oxygen partial pressure, and it decreased more rapidly than that of arc 1. This trend indicated that the oxygen adsorption/desorption process was the rate limiting step under lower oxygen partial pressure, whereas at higher oxygen partial pressure, the charge transfer process became more important.
4. Conclusions • The composite electrode forms good contact with CGO electrolyte after sintering at 900 ◦ C for 2 h. • The oxygen adsorption/desorption process is the reaction rate limiting step under lower oxygen partial pressure, whereas the charge transfer process become more important at higher oxygen partial pressure. • The lowest specific resistivity obtained at 706 ◦ C in air is about 0.16 cm2 .
Acknowledgements The project was supported by Natural Science Foundation (ZJG03-4) and oversea foundation (1054HQ003) of Heilongjiang Province.
References 3.4. dc measurment Fig. 9 showed the dc polarization curves measured at different temperatures in air. It was observed that the current density increases with increasing temperatures. At low overpotential (less than 20 mV), we can expect a linear expression[5], i = i0 ZFη/RT, where i is the current density, i0 the exchange current density, η the overpotential, and F, R have their normal meanings, respectively. From the inverse of the derivative of i against η, we can obtain the area specific resistance. The value was comparable to the area specific resistance deduced from the impedance spectrum (Table 1). The area specific resistance obtained at 706 ◦ C in air was about 0.16 cm2 , comparable to the LSCF–CGO material [11]. Our results again proved that the polarization resistance can be reduced dramatically by forming a composite cathode with high electronic and ionic conducting materials, and that LSCF–CBO composite materials can be considered as possible cathodes for ITSOFC. Further work will be done
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