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2 electrolyte materials and compatible cathode materials
Solid State Ionics 201 (2011) 81–86
Contents lists available at SciVerse ScienceDirect
Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
Synthesis and characterization of apatite-type La9.67Si6-xAlxO26.5-x/2 electrolyte materials and compatible cathode materials J. Zhou, X.F. Ye, J.L. Li, S.R. Wang ⁎, T.L. Wen CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences (SICCAS), 1295 Dingxi Road, Shanghai 200050, PR China
a r t i c l e
i n f o
Article history: Received 22 January 2011 Received in revised form 5 May 2011 Accepted 20 July 2011 Available online 9 September 2011 Keywords: Solid oxide fuel cell (SOFC) Apatite-type electrolyte Cathode Ionic conductivity
a b s t r a c t Apatite silicates have recently been reported as promising electrolyte materials for intermediate temperature solid oxide fuel cells (IT-SOFCs). In this work, a series of apatite-type compounds La9.67Si6-xAlxO26.5-x/2 (LSAO) with x = 0–2 are synthesized by the sol–gel process at calcining temperature of 800–900 °C. Thermal expansion coefficient, relative density and electrical conductivity of these samples with different Al doped contents are investigated. A symmetrical cell, which is composed of La9.67Si5AlO26 electrolyte and (La0.74Bi0.10Sr0.16)MnO3+δ (LBSM) cathode, is fabricated and electrochemically characterized. LBSM cathode shows a good electrochemical performance, which proves LBSM to be a promising candidate cathode for LSAO-based electrolyte. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Solid oxide fuel cells (SOFCs) are regarded as promising electrochemical devices for their environmental friendship, fuel flexibility, and high energy-conversion efficiency. However, the traditional SOFCs operate at high temperature, which limits the selection of materials and influences the long-term stability of SOFCs. In order to promote the application of SOFCs, the recent researches and development of SOFCs have been focused on lowering the operating temperature, which can be achieved by using higher oxide ion-conducting electrolyte materials or by making thinner electrolyte membranes. One candidate for the electrolyte materials with high conductivity at lower temperature is the apatite-type Lanthanum silicate. Since the first report on apatite-type lanthanum silicate published by Nakayama [1], many researchers [2–6] have indicated that apatitetype lanthanum silicates show higher oxide ion conductivity than those of conventional oxide ion conductors at intermediate temperature. Moreover, no significant variation of conductivity is observed with the change of the oxygen partial pressure and their electronic conductivity is negligible over a wide range of P(O2)[7,8]. In order to make the electrolyte dense enough to separate the air and fuel compartments, apatite-type Lanthanum silicates should be sintered at very high temperatures (N1600 °C) or expensive sintering technology should be used [10–12]. So they are somewhat difficult to be used as
⁎ Corresponding author. Tel.: + 86 21 52411520; fax: + 86 21 52413903. E-mail addresses: [email protected] (J. Zhou), [email protected] (S.R. Wang). 0167-2738/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2011.07.014
electrolyte of SOFCs [3,4,6,7,9]. Recently, aluminum doped lanthanum silicates with Al doping onto Si site have attracted much attention due to their better properties [7,13–16]. However, the influences of different Al doping amount on La9.67Si6O26.5 have seldom been systematically studied. In order to complete the lanthanum silicates system, a series work was carried out. It is still a challenge to find compatible electrode materials for electrolyte materials of apatite-type Lanthanum silicates, although many efforts have been done [21–23]. The cathodic symmetrical cells with La9Sr1Si6O26.5 electrolyte were tested by Bonhomme et al. and Tsipis et al., but the cells did not exhibit good enough performances. As reported in the reference [24], the perovskite structure (La0.74Bi0.10Sr0.16)MnO3+δ (LBSM) cathode exhibited good performance at intermediate temperature. Meanwhile the LBSM cathode has a good sintering property due to sol–gel synthesis process. The TEC of LBSM is also similar to traditional electrolyte materials. Moreover, the TEC of LBSM is similar to that of apatite-type Lanthanum silicates. Consequently, LBSM is assumed to be a good candidate cathode for this kind of electrolyte if a good electrochemical performance could be exhibited. In the present work, a series of apatite-type compounds La9.67Si6-xAlxO26.5-x/2 (x = 0–2) powders are synthesized by the sol– gel process, which is considered as a good method for preparation of ultra-fine powders [6,17] compared to the solid state reaction technology [3,4,8,9,11,18–20]. Then these obtained materials were characterized by phase composition, sintered density and electrical conductivity. We also fabricated and measured the cathodic symmetrical cell with (La0.74Bi0.10Sr0.16)MnO3+δ as the cathode and La9.67Si5AlO26 as the electrolyte. The effects of interface between the La9.67Si5AlO26 electrolyte and cathode materials on the electrochemical reaction were studied.
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2. Experimental
showed by scanning electron microscopy (SEM, JXA-8100, JEOL Co. Ltd., Japan).
2.1. Synthesis of La9.67Si6-xAlxO26.5-x/2 powders and their characterization 3. Results and discussion The series of La9.67Si6-xAlxO26.5-x/2 powders were synthesized by the sol–gel process. Tetraethyl orthosilicate (TEOS, N97.1%) was dissolved in a mixture of ethanol, deionized water and nitric acid, and stirred to get a transparent solution. Lanthanum oxide (99.99%, calcined at 1200 °C for 2 h) was dissolved in nitric acid, and then citric acid and ethylene glycol were added into it. After hydrolyzing for 1 h, the TEOS solution and Al(NO3)3·9H2O (99%) solution were added to the nitric Lanthanum solution. The mixed solution reacted at room temperature for 3–4 h, and then was put in 60 °C drying oven until a light yellow sol was formed. The sol was moved to 120–150 °C drying oven and dried to form a gel. The gel was ball milled for 2 h, calcined at 800–900 °C for 4 h and then ball milled for 1.5 h with zirconia balls again. Then pure apatite-type powders can be obtained. The flow chart of the experimental procedure is shown in Fig. 1. X-ray-diffraction (XRD) analysis (D/max 2550 V) was performed with Cu Kα source to determine the phases. The particle distributions of powders were measured by laser diffraction granulometry (Mastersizer 2000, MALVERN, England). The as-synthesized apatite powders were uniaxially pressed into pellets and bars under 200 MPa and sintered at 1450 °C–1500 °C for 4 h in air, burying in zirconia powder, which can make the pellets homogeneous sintering and prevent the reaction of pellets with Aluminum oxide substrate. The thermal expansion coefficients (TECs) of these samples were measured by means of a NETZSCH DIL 402PC dilatometer within the temperature range from 20 °C to 1300 °C at heating rate of 5 K min − 1. And the conductivity of those bars was measured by the four-probe method using an Electrochemical Workstation (IM6e, ZAHNER).
2.2. Cell fabrication and measurement The sintered LSAO pellets mentioned above were used as the electrolyte of the symmetric cell for the electrochemical impedance analysis, and their thickness was about 2.48 mm. The cathode materials used in this work was (La0.74Bi0.10Sr0.16)MnO3−δ, which was provided by co-workers from our group [24]. The cathode powder was ground with terpineol based slurry in anagate mortar to prepare a paste, which was then screen-printed onto the both sides of the LSAO electrolytes and sintered at 900 °C for 3 h to form the symmetric cells. Then those cells were tested using the impedance analyzer IM6e over a frequency range of 10 mHz–1 MHz with an excitation potential of 20 mV. The microstructure and the morphology of the symmetric cell structure were
Fig. 1. Flow-chart of synthesis for La9.67Si6-xAlxO26.5-x/2 powder.
3.1. Characterization of the electrolyte materials 3.1.1. Phase structure and microstructure of La9.67Si6-xAlxO26.5-x/2 powders Based on the TG-DTA data of La9.67Si5AlO26 gel, these gel powders were calcined between 800 °C and 900 °C. The XRD diffraction patterns of these powders are presented in Fig. 2. We can see from Fig. 2 that a single oxy-apatite phase is obtained without any other phases, no matter how much the doping content of Al is. It is different from previous report that a secondary phase of LaAlO3 was observed after annealing [25]. The particle sizes of La9.67Si5.75Al0.25O26.375 and La9.67Si5AlO26 powders measured by laser diffraction granulometry and the secondary electron microscopic image of the La9.67Si5AlO26 powder are shown in Fig. 3. It is clear that the crystallite size of the powder lays around 100 nm and big particles are aggregates of small crystallite. Those powder were ball milled twice, the distribution of powder depends on the distribution of zirconia balls. The two maxima were due to the two different diameter styles zirconia balls. And this distribution of secondary aggregates could provide high packing density, which are good for sintering. 3.1.2. Sintering properties of the apatite materials Considering the cost and simplicity of sintering technology, the traditional sintering technology was chosen. Those La9.67Si6-xAlxO26.5-x/2 powders were pressed into pellets and then sintered at 1450 °C–1500 °C for 4 h. As previously reported, the relative density of La9.67Si6-xAlxO26.5-x/2 pellets is too low by the sol–gel synthesis process; however as an electrolyte for SOFCs, La9.67Si6-xAlxO26.5-x/2 pellets need higher relative density to meet the demand. In the same preparation condition, the obtained relative density of samples with different doping amount (x=0, 0.25, 0.5, 0.75, 1, 1.25, 1.5, 2) were 91.7%, 75.5%, 70.3%, 93.0%, 97.5%, 93.2%, 83.1%,and 66.1%, respectively, which were calculated depending on theoretical and measured densities (Fig. 4). The series of apatite-type powders were synthesized by the same process excepting different Al dopant amount. Therefore, it can be concluded that the Al dopant amount has obvious effects on the promotion of densification. We inferred that the doping element Al make the structure have slight change. Table 2shows the cell parameters of all samples being calculated on the pattern of XRD diffraction, which proved the change of structure. And we need following experimental data to infer what change of structure. 3.1.3. Characterization of the electrolyte materials The thermal expansion coefficient (TEC) of La9.67Si6-xAlxO26.5-x/2 with different Al doping amount is compared with cathode materials
Fig. 2. XRD pattern of the La9.67Si6-xAlxO26.5-x/2 samples obtained at 800 °C–900 °C for 6 h.
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Fig. 3. Particle size distribution of the La9.67Si5.75Al0.25O26.375 (a) and the La9.67Si5AlO26 (b, c) powder.
[26,27] and YSZ in Table 1. TEC values of La9.67Si6-xAlxO26.5-x/2 vary between 9.36 and 9.88 × 10 − 6 K − 1 with different doping content, and they are similar to that of the traditional YSZ electrolyte. As a result, many electrode materials can meet the TEC matching with these La9.67Si6-xAlxO26.5-x/2 electrolyte materials, if only a good contact of them can be obtained without interface reaction. The electrical conductivities of the LSAO samples were measured in air by a conventional four-probe method. The total conductivities of La9.67Si6-xAlxO26.5-x/2 materials are shown in Fig. 5, and the dependence of electrical conductivity on Al content is shown in Fig. 6. We can see from the figure that La9.67Si6O26.5 shows the highest electrical conductivity, which is in agreement with the report of Shaula's [7].
They indicated that the maximum ionic conductivity was found for the phase containing 26.5 oxygen atoms per unit formula. It has been known that oxide-ion is the charge carrier in the La9.67Si6-xAlxO26.5-x/2 materials [28,29]. It could be assumed that Al doping makes the interstitial oxide ions decreased and isolated [SiO4] 4− tetrahedron more distorted. While adding a little Al dopant, the decreasing in interstitial oxide ions plays a dominant role. At this occasion, the electrical conductivity decreases with the increase Al dopant content. Of course, one possible reason of conductivity decrease may also be the different relative density of different samples, as mentioned above. However, in this case, the activation energy should not change much, because the relative density of different samples should have no impact on the activation energy. As is shown in Fig. 7, the
Table 1 The thermal expansion coefficient of the electrolyte and electrode materials in air.
Fig. 4. The relative density against Al dopant concentration.
Composition
structure
Temperature range, °C
Thermal expansion coefficient, α*106 K− 1
La9.67Si6O26.5 La9.67Si5.75Al0.25O26.375 La9.67Si5.5Al0.5O26.25 La9.67Si5.25Al0.75O26.125 La9.67Si5AlO26 La9.67Si4.75Al1.25O25.875 La9.67Si4.5Al1.5O25.75 La9.67Si4Al2O25.5 La0.6Sr0.4Co0.2Fe0.8O3+δ (La0.8Sr0.2)0.98MnO3+δ (La0.74Bi0.10Sr0.16)MnO3−δ ZrO2–8 mol%Y2O3
Apatite Apatite Apatite Apatite Apatite Apatite Apatite Apatite Perovskite Perovskite Perovskite Fluorite
25–850 25–850 25–850 25–850 25–850 25–850 25–850 25–850 100–600 27–1000 25–800 30–900
9.88 9.76 9.48 9.36 9.73 9.52 9.79 9.54 15.3(27) b11.2(28) 11.6(24) 10.5
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Table 2 The cell parameters of La9.67Si6-xAlxO26.5-x/2 samples. Composition
a
c
Volume
La9.67Si6O26.5 La9.67Si5.75Al0.25O26.375 La9.67Si5.5Al0.5O26.25 La9.67Si5.25Al0.75O26.125 La9.67Si5AlO26 La9.67Si4.75Al1.25O25.875 La9.67Si4.5Al1.5O25.75 La9.67Si4Al2O25.5
9.704 9.710 9.723 9.712 9.708 9.713 9.714 9.740
7.178 7.186 7.188 7.194 7.208 7.203 7.214 7.214
585.4 586.8 588.5 587.7 588.2 588.5 589.6 592.7
activation energy of the La9.67Si6-xAlxO26.5-x/2 materials decrease with Al doping content, and the lowest activation energy is found for the dopant amount between 1/5 and 1.25/4.75. This result implies some structure changing, for example the distortion of isolated [SiO4] 4− tetrahedron, which makes the carrier to move more easily. Therefore, while adding a little Al dopant, the decreasing in interstitial oxide ions plays a dominant role. The electrical conductivity decreases with the increase Al dopant content in this case. While adding more Al dopant, the structure distorting plays a dominant role, so the electrical conductivity increases with the Al dopant content. And when Al/Si ratio is more than 1/5, the lack of carriers became serious, because the apatite-type phases contained 26 oxygen atoms per unit formula. When the Al/Si was more than 1/5, the infection point of activation energy occurs in Fig. 7 for Al/Si ratio between 1/5 and 1.25/4.75, and it can be speculated that conductive mechanism has changed, for example, the carrier may change from the interstitial oxide ions to the vacancy of oxide ions. As a result, the electrical conductivity decreased again. La9.67Si6O26.5 and La9.67Si5AlO26 have acceptable conductivity for application as an electrolyte in SOFC, and it is noted that the activation energy is lower than that of traditional materials. Therefore, this kind of electrolyte materials can be used at lower operating temperature.
Fig. 6. The electrical conductivity versus Al content.
3.2.2. Cathode electrochemical performance Based on these results mentioned above, the symmetrical cell LBSM/LSAO/LBSM was prepared, and then measured by impedance spectroscopy (Fig. 9), for which the electrolyte was La9.67Si5AlO26. As seen in Fig. 9, the total polarization resistance of two sides electrolyte/electrode is 2.77, 1.625 and 1.27 Ω cm 2 at 750 °C, 850 °C and 900 °C, respectively. Hence the cathode polarization resistance is 1.385, 0.813, 0.635 Ω cm 2 at 750 °C, 850 °C and 900 °C, respectively. It indicates that the interface polarization has been improved compared to the best result reported in the literature, in which the polarization resistance of La2Ni0.8Cu0.2O4+δ cathode was 3.2 Ω cm 2 at 700 °C and
3.2. Cathode material for LSAO electrolyte 3.2.1. Chemical compatibility As mentioned before [24], the TEC of LBSM is close to LSM, which ensures the TEC compatibility of the LBSM cathode with the LSAO material. It is then valuable to investigate the electrochemical properties of the LBSM cathode on LSAO electrolyte. After the mixture of 50 wt.% LBSM and 50 wt.% LSAO being calcined at 1200 °C for 10 h, the XRD diffraction was performed and the pattern is presented in Fig. 8. It indicates that no third phase exists in the detectable range, proving a good chemical compatibility between the cathode and the electrolyte.
Fig. 7. The activation energy versus Al content.
Fig. 5. The conductivity of La9.67Si6-xAlxO26.5-x/2 samples.
Fig. 8. XRD pattern of LSAO + LBSM composite materials sintered at 1200 °C for 10 h.
J. Zhou et al. / Solid State Ionics 201 (2011) 81–86
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LBSM/LSAO. From the SEM image, no other layer or third phase at the interface is observed. Considering the results in Fig. 8, it can be concluded that the structures of apatite and perovskite remain unchanged although a few element has interdiffused. Fig. 9 shows that the result of polarization resistance is not influenced by the slight element diffusion. These results indicate the potential utilization of LBSM as the cathode material on LSAO electrolyte. 4. Conclusion
Fig. 9. AC impedance spectra of LBSM cathode sintered at 900 °C for 3 h in air.
1.5 Ω cm 2 at 800 °C without PrOx surface modification [23]. Fig. 10 is a typical fracture cross-section SEM image of symmetrical LBSM/LSAO/LBSM cell after operation. Both a porous cathode and a dense electrolyte were obtained, and the two layers have good contact without obvious segregation seen at the interface. The average grain particle size of LBSM cathode is around 500 nm, which might be why the cathode has good sintering property and good electrochemical performance. Fig. 11 shows the results of Electron probe micro-analyzer (EPMA) examination on the symmetrical cell, and it can be concluded that a few Mn and Si elements interdiffused nearby the interface of
The series of La9.67Si6-xAlxO26.5-x/2 (x = 0–2) powders with apatitetype single phase were synthesized by the sol–gel process. The obtained powders show good sintering property, which can be made into dense electrolyte membranes by traditional sintering technology. Al doping contents have a significant effect on the electrolyte properties, such as thermal expansion coefficient, relative density, electrical conductivity and activation energy. The series of electrolyte materials exhibit moderate TECs. La9.67 Si 6 O 26.5 (x = 0) among the series exhibits the highest electrical conductivity. However, compromising the relative density and electrical conductivity, La9.67Si5AlO26 was used as the electrolyte material of the symmetrical cells for investigation. The LBSM cathode exhibits a good performance on LSAO electrolyte, and the cathode polarization resistance was 1.385, 0.813, 0.635 Ω cm2 at 750 °C, 850 °C and 900 °C, respectively. Although a few elements interdiffuse at the interface of the electrolyte and the cathode, it does not influence the cathode performance as we can see. Therefore LBSM may be a promising candidate cathode for LSAO-based electrolyte in SOFC. Acknowledgements The authors are grateful for the financial support from the Science and Technology Commission of Shanghai Municipality No. 09DZ1206600. References
Fig. 10. Typical fracture cross-section SEM image of symmetrical LBSM/LSAO/LBSM cell.
Fig. 11. Electron probe micro-analyzer cross-section images of the LBSM cathode on the LSAO electrolyte.
[1] S. Nakayama, M. Sakamoto, J. Eur. Ceram. Soc. 18 (1998) 1413. [2] L. Leon-Reina, E.R. Losilla, M. Martinez-Lara, S. Bruque, M.A.G. Aranda, J. Mater. Chem. 14 (2004) 1142. [3] J.E.H. Sansom, D. Richings, P.R. Slater, Solid State Ion. 139 (2001) 205. [4] A. Najib, J.E.H. Sansom, J.R. Tolchard, P.R. Slater, M.S. Islam, Dalton Trans. (2004) 3106. [5] J.E.H. Sansom, P.R. Slater, Solid State Ion. 167 (2004) 23. [6] S.W. Tao, J.T.S. Irvine, Mater. Res. Bull. 36 (2001) 1245. [7] A.L. Shaula, V.V. Kharton, F.M.B. Marques, J. Solid State Chem. 178 (2005) 2050. [8] S. Beaudet-Savignat, A. Vincent, S. Lambert, F. Gervais, J. Mater. Chem. 17 (2007) 2078. [9] A. Mineshige, T. Nakao, M. Kobune, T. Yazawa, H. Yoshioka, Solid State Ion. 179 (2008) 1009. [10] A. Chesnaud, C. Bogicevic, F. Karolak, C. Estournes, G. Dezanneau, Chem. Commun. (2007) 1550. [11] W. Gao, H.L. Liao, C. Coddet, J. Power Sources 179 (2008) 739. [12] C.Y. Ma, P. Briois, J. Bohlmark, F. Lapostolle, A. Billard, Ionics 14 (2008) 471. [13] H. Zhang, F. Li, J. Jin, Q. Wang, Y. Sun, 16th International Conference on Solid State Ionics, ELSEVIER SCIENCE BV, 2007, p. 1024. [14] T. Kharlamova, S. Pavlova, V. Sadykov, M. Chaikina, T. Krieger, O. Lapina, D. Khabibulin, A. Ishchenko, V. Zaikovskii, C. Argirusis, J. Frade, Symposium on Chemistry and Processes for the Design of Metal Oxide Nanoparticles held at the 2007 EMRS Fall Meeting, WILEY-BLACKWELL, Warsaw, POLAND, 2007, p. 939. [15] G. Lucazeau, N. Sergent, T. Pagnier, A. Shaula, V. Kharton, F.M.B. Marques, J. Raman Spectrosc. 38 (2007) 21. [16] A. Mineshige, T. Nakao, Y. Ohnishi, M. Kobune, T. Yazawa, H. Yoshioka, Electrochemistry 77 (2009) 146. [17] S. Celerier, C. Laberty-Robert, F. Ansart, C. Calmet, P. Stevens, 9th Electroceramics Congress, ELSEVIER SCI LTD, Cherbourg, FRANCE, 2005, p. 2665. [18] A. Brisse, A.L. Sauvet, C. Barthet, S. Georges, J. Fouletier, Solid State Ion. 178 (2007) 1337. [19] Y. Masubuchi, M. Higuchi, T. Takeda, S. Kikkawa, Solid State Ion. 177 (2006) 263. [20] A. Vincent, S.B. Savignat, F. Gervais, 9th Conference and Exhibition of the EuropeanCeramic-Society, ELSEVIER SCI LTD, Portoroz, SLOVENIA, 2005, p. 1187. [21] A. Brisse, A.L. Sauvet, C. Barthet, S. Beaudet-Savignat, A. Fouletier, Fuel Cells 6 (2006) 59. [22] S.B. Savignat, M. Chiron, C. Barthet, 9th Conference and Exhibition of the European-Ceramic-Society, ELSEVIER SCI LTD, Portoroz, SLOVENIA, 2005, p. 673. [23] E.V. Tsipis, V.V. Kharton, J.R. Frade, Electrochim. Acta 52 (2007) 4428.
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[24] J.L. Li, S.R. Wang, Z.R. Wang, R.Z. Liu, T.L. Wen, Z.Y. Wen, J. Power Sources 179 (2008) 474. [25] T. Kharlamova, S. Pavlova, V. Sadykov, O. Lapina, D. Khabibulin, T. Krieger, V. Zaikovskii, A. Ishchenko, A. Salanov, V. Muzykantov, N. Mezentseva, M. Chaikina, N. Uvarov, J. Frade, C. Argirusis, 16th International Conference on Solid State Ionics, ELSEVIER SCIENCE BV, Shanghai, PEOPLES R CHINA, 2007, p. 1018.
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Contents lists available at SciVerse ScienceDirect
Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
Synthesis and characterization of apatite-type La9.67Si6-xAlxO26.5-x/2 electrolyte materials and compatible cathode materials J. Zhou, X.F. Ye, J.L. Li, S.R. Wang ⁎, T.L. Wen CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences (SICCAS), 1295 Dingxi Road, Shanghai 200050, PR China
a r t i c l e
i n f o
Article history: Received 22 January 2011 Received in revised form 5 May 2011 Accepted 20 July 2011 Available online 9 September 2011 Keywords: Solid oxide fuel cell (SOFC) Apatite-type electrolyte Cathode Ionic conductivity
a b s t r a c t Apatite silicates have recently been reported as promising electrolyte materials for intermediate temperature solid oxide fuel cells (IT-SOFCs). In this work, a series of apatite-type compounds La9.67Si6-xAlxO26.5-x/2 (LSAO) with x = 0–2 are synthesized by the sol–gel process at calcining temperature of 800–900 °C. Thermal expansion coefficient, relative density and electrical conductivity of these samples with different Al doped contents are investigated. A symmetrical cell, which is composed of La9.67Si5AlO26 electrolyte and (La0.74Bi0.10Sr0.16)MnO3+δ (LBSM) cathode, is fabricated and electrochemically characterized. LBSM cathode shows a good electrochemical performance, which proves LBSM to be a promising candidate cathode for LSAO-based electrolyte. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Solid oxide fuel cells (SOFCs) are regarded as promising electrochemical devices for their environmental friendship, fuel flexibility, and high energy-conversion efficiency. However, the traditional SOFCs operate at high temperature, which limits the selection of materials and influences the long-term stability of SOFCs. In order to promote the application of SOFCs, the recent researches and development of SOFCs have been focused on lowering the operating temperature, which can be achieved by using higher oxide ion-conducting electrolyte materials or by making thinner electrolyte membranes. One candidate for the electrolyte materials with high conductivity at lower temperature is the apatite-type Lanthanum silicate. Since the first report on apatite-type lanthanum silicate published by Nakayama [1], many researchers [2–6] have indicated that apatitetype lanthanum silicates show higher oxide ion conductivity than those of conventional oxide ion conductors at intermediate temperature. Moreover, no significant variation of conductivity is observed with the change of the oxygen partial pressure and their electronic conductivity is negligible over a wide range of P(O2)[7,8]. In order to make the electrolyte dense enough to separate the air and fuel compartments, apatite-type Lanthanum silicates should be sintered at very high temperatures (N1600 °C) or expensive sintering technology should be used [10–12]. So they are somewhat difficult to be used as
⁎ Corresponding author. Tel.: + 86 21 52411520; fax: + 86 21 52413903. E-mail addresses: [email protected] (J. Zhou), [email protected] (S.R. Wang). 0167-2738/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2011.07.014
electrolyte of SOFCs [3,4,6,7,9]. Recently, aluminum doped lanthanum silicates with Al doping onto Si site have attracted much attention due to their better properties [7,13–16]. However, the influences of different Al doping amount on La9.67Si6O26.5 have seldom been systematically studied. In order to complete the lanthanum silicates system, a series work was carried out. It is still a challenge to find compatible electrode materials for electrolyte materials of apatite-type Lanthanum silicates, although many efforts have been done [21–23]. The cathodic symmetrical cells with La9Sr1Si6O26.5 electrolyte were tested by Bonhomme et al. and Tsipis et al., but the cells did not exhibit good enough performances. As reported in the reference [24], the perovskite structure (La0.74Bi0.10Sr0.16)MnO3+δ (LBSM) cathode exhibited good performance at intermediate temperature. Meanwhile the LBSM cathode has a good sintering property due to sol–gel synthesis process. The TEC of LBSM is also similar to traditional electrolyte materials. Moreover, the TEC of LBSM is similar to that of apatite-type Lanthanum silicates. Consequently, LBSM is assumed to be a good candidate cathode for this kind of electrolyte if a good electrochemical performance could be exhibited. In the present work, a series of apatite-type compounds La9.67Si6-xAlxO26.5-x/2 (x = 0–2) powders are synthesized by the sol– gel process, which is considered as a good method for preparation of ultra-fine powders [6,17] compared to the solid state reaction technology [3,4,8,9,11,18–20]. Then these obtained materials were characterized by phase composition, sintered density and electrical conductivity. We also fabricated and measured the cathodic symmetrical cell with (La0.74Bi0.10Sr0.16)MnO3+δ as the cathode and La9.67Si5AlO26 as the electrolyte. The effects of interface between the La9.67Si5AlO26 electrolyte and cathode materials on the electrochemical reaction were studied.
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2. Experimental
showed by scanning electron microscopy (SEM, JXA-8100, JEOL Co. Ltd., Japan).
2.1. Synthesis of La9.67Si6-xAlxO26.5-x/2 powders and their characterization 3. Results and discussion The series of La9.67Si6-xAlxO26.5-x/2 powders were synthesized by the sol–gel process. Tetraethyl orthosilicate (TEOS, N97.1%) was dissolved in a mixture of ethanol, deionized water and nitric acid, and stirred to get a transparent solution. Lanthanum oxide (99.99%, calcined at 1200 °C for 2 h) was dissolved in nitric acid, and then citric acid and ethylene glycol were added into it. After hydrolyzing for 1 h, the TEOS solution and Al(NO3)3·9H2O (99%) solution were added to the nitric Lanthanum solution. The mixed solution reacted at room temperature for 3–4 h, and then was put in 60 °C drying oven until a light yellow sol was formed. The sol was moved to 120–150 °C drying oven and dried to form a gel. The gel was ball milled for 2 h, calcined at 800–900 °C for 4 h and then ball milled for 1.5 h with zirconia balls again. Then pure apatite-type powders can be obtained. The flow chart of the experimental procedure is shown in Fig. 1. X-ray-diffraction (XRD) analysis (D/max 2550 V) was performed with Cu Kα source to determine the phases. The particle distributions of powders were measured by laser diffraction granulometry (Mastersizer 2000, MALVERN, England). The as-synthesized apatite powders were uniaxially pressed into pellets and bars under 200 MPa and sintered at 1450 °C–1500 °C for 4 h in air, burying in zirconia powder, which can make the pellets homogeneous sintering and prevent the reaction of pellets with Aluminum oxide substrate. The thermal expansion coefficients (TECs) of these samples were measured by means of a NETZSCH DIL 402PC dilatometer within the temperature range from 20 °C to 1300 °C at heating rate of 5 K min − 1. And the conductivity of those bars was measured by the four-probe method using an Electrochemical Workstation (IM6e, ZAHNER).
2.2. Cell fabrication and measurement The sintered LSAO pellets mentioned above were used as the electrolyte of the symmetric cell for the electrochemical impedance analysis, and their thickness was about 2.48 mm. The cathode materials used in this work was (La0.74Bi0.10Sr0.16)MnO3−δ, which was provided by co-workers from our group [24]. The cathode powder was ground with terpineol based slurry in anagate mortar to prepare a paste, which was then screen-printed onto the both sides of the LSAO electrolytes and sintered at 900 °C for 3 h to form the symmetric cells. Then those cells were tested using the impedance analyzer IM6e over a frequency range of 10 mHz–1 MHz with an excitation potential of 20 mV. The microstructure and the morphology of the symmetric cell structure were
Fig. 1. Flow-chart of synthesis for La9.67Si6-xAlxO26.5-x/2 powder.
3.1. Characterization of the electrolyte materials 3.1.1. Phase structure and microstructure of La9.67Si6-xAlxO26.5-x/2 powders Based on the TG-DTA data of La9.67Si5AlO26 gel, these gel powders were calcined between 800 °C and 900 °C. The XRD diffraction patterns of these powders are presented in Fig. 2. We can see from Fig. 2 that a single oxy-apatite phase is obtained without any other phases, no matter how much the doping content of Al is. It is different from previous report that a secondary phase of LaAlO3 was observed after annealing [25]. The particle sizes of La9.67Si5.75Al0.25O26.375 and La9.67Si5AlO26 powders measured by laser diffraction granulometry and the secondary electron microscopic image of the La9.67Si5AlO26 powder are shown in Fig. 3. It is clear that the crystallite size of the powder lays around 100 nm and big particles are aggregates of small crystallite. Those powder were ball milled twice, the distribution of powder depends on the distribution of zirconia balls. The two maxima were due to the two different diameter styles zirconia balls. And this distribution of secondary aggregates could provide high packing density, which are good for sintering. 3.1.2. Sintering properties of the apatite materials Considering the cost and simplicity of sintering technology, the traditional sintering technology was chosen. Those La9.67Si6-xAlxO26.5-x/2 powders were pressed into pellets and then sintered at 1450 °C–1500 °C for 4 h. As previously reported, the relative density of La9.67Si6-xAlxO26.5-x/2 pellets is too low by the sol–gel synthesis process; however as an electrolyte for SOFCs, La9.67Si6-xAlxO26.5-x/2 pellets need higher relative density to meet the demand. In the same preparation condition, the obtained relative density of samples with different doping amount (x=0, 0.25, 0.5, 0.75, 1, 1.25, 1.5, 2) were 91.7%, 75.5%, 70.3%, 93.0%, 97.5%, 93.2%, 83.1%,and 66.1%, respectively, which were calculated depending on theoretical and measured densities (Fig. 4). The series of apatite-type powders were synthesized by the same process excepting different Al dopant amount. Therefore, it can be concluded that the Al dopant amount has obvious effects on the promotion of densification. We inferred that the doping element Al make the structure have slight change. Table 2shows the cell parameters of all samples being calculated on the pattern of XRD diffraction, which proved the change of structure. And we need following experimental data to infer what change of structure. 3.1.3. Characterization of the electrolyte materials The thermal expansion coefficient (TEC) of La9.67Si6-xAlxO26.5-x/2 with different Al doping amount is compared with cathode materials
Fig. 2. XRD pattern of the La9.67Si6-xAlxO26.5-x/2 samples obtained at 800 °C–900 °C for 6 h.
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Fig. 3. Particle size distribution of the La9.67Si5.75Al0.25O26.375 (a) and the La9.67Si5AlO26 (b, c) powder.
[26,27] and YSZ in Table 1. TEC values of La9.67Si6-xAlxO26.5-x/2 vary between 9.36 and 9.88 × 10 − 6 K − 1 with different doping content, and they are similar to that of the traditional YSZ electrolyte. As a result, many electrode materials can meet the TEC matching with these La9.67Si6-xAlxO26.5-x/2 electrolyte materials, if only a good contact of them can be obtained without interface reaction. The electrical conductivities of the LSAO samples were measured in air by a conventional four-probe method. The total conductivities of La9.67Si6-xAlxO26.5-x/2 materials are shown in Fig. 5, and the dependence of electrical conductivity on Al content is shown in Fig. 6. We can see from the figure that La9.67Si6O26.5 shows the highest electrical conductivity, which is in agreement with the report of Shaula's [7].
They indicated that the maximum ionic conductivity was found for the phase containing 26.5 oxygen atoms per unit formula. It has been known that oxide-ion is the charge carrier in the La9.67Si6-xAlxO26.5-x/2 materials [28,29]. It could be assumed that Al doping makes the interstitial oxide ions decreased and isolated [SiO4] 4− tetrahedron more distorted. While adding a little Al dopant, the decreasing in interstitial oxide ions plays a dominant role. At this occasion, the electrical conductivity decreases with the increase Al dopant content. Of course, one possible reason of conductivity decrease may also be the different relative density of different samples, as mentioned above. However, in this case, the activation energy should not change much, because the relative density of different samples should have no impact on the activation energy. As is shown in Fig. 7, the
Table 1 The thermal expansion coefficient of the electrolyte and electrode materials in air.
Fig. 4. The relative density against Al dopant concentration.
Composition
structure
Temperature range, °C
Thermal expansion coefficient, α*106 K− 1
La9.67Si6O26.5 La9.67Si5.75Al0.25O26.375 La9.67Si5.5Al0.5O26.25 La9.67Si5.25Al0.75O26.125 La9.67Si5AlO26 La9.67Si4.75Al1.25O25.875 La9.67Si4.5Al1.5O25.75 La9.67Si4Al2O25.5 La0.6Sr0.4Co0.2Fe0.8O3+δ (La0.8Sr0.2)0.98MnO3+δ (La0.74Bi0.10Sr0.16)MnO3−δ ZrO2–8 mol%Y2O3
Apatite Apatite Apatite Apatite Apatite Apatite Apatite Apatite Perovskite Perovskite Perovskite Fluorite
25–850 25–850 25–850 25–850 25–850 25–850 25–850 25–850 100–600 27–1000 25–800 30–900
9.88 9.76 9.48 9.36 9.73 9.52 9.79 9.54 15.3(27) b11.2(28) 11.6(24) 10.5
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Table 2 The cell parameters of La9.67Si6-xAlxO26.5-x/2 samples. Composition
a
c
Volume
La9.67Si6O26.5 La9.67Si5.75Al0.25O26.375 La9.67Si5.5Al0.5O26.25 La9.67Si5.25Al0.75O26.125 La9.67Si5AlO26 La9.67Si4.75Al1.25O25.875 La9.67Si4.5Al1.5O25.75 La9.67Si4Al2O25.5
9.704 9.710 9.723 9.712 9.708 9.713 9.714 9.740
7.178 7.186 7.188 7.194 7.208 7.203 7.214 7.214
585.4 586.8 588.5 587.7 588.2 588.5 589.6 592.7
activation energy of the La9.67Si6-xAlxO26.5-x/2 materials decrease with Al doping content, and the lowest activation energy is found for the dopant amount between 1/5 and 1.25/4.75. This result implies some structure changing, for example the distortion of isolated [SiO4] 4− tetrahedron, which makes the carrier to move more easily. Therefore, while adding a little Al dopant, the decreasing in interstitial oxide ions plays a dominant role. The electrical conductivity decreases with the increase Al dopant content in this case. While adding more Al dopant, the structure distorting plays a dominant role, so the electrical conductivity increases with the Al dopant content. And when Al/Si ratio is more than 1/5, the lack of carriers became serious, because the apatite-type phases contained 26 oxygen atoms per unit formula. When the Al/Si was more than 1/5, the infection point of activation energy occurs in Fig. 7 for Al/Si ratio between 1/5 and 1.25/4.75, and it can be speculated that conductive mechanism has changed, for example, the carrier may change from the interstitial oxide ions to the vacancy of oxide ions. As a result, the electrical conductivity decreased again. La9.67Si6O26.5 and La9.67Si5AlO26 have acceptable conductivity for application as an electrolyte in SOFC, and it is noted that the activation energy is lower than that of traditional materials. Therefore, this kind of electrolyte materials can be used at lower operating temperature.
Fig. 6. The electrical conductivity versus Al content.
3.2.2. Cathode electrochemical performance Based on these results mentioned above, the symmetrical cell LBSM/LSAO/LBSM was prepared, and then measured by impedance spectroscopy (Fig. 9), for which the electrolyte was La9.67Si5AlO26. As seen in Fig. 9, the total polarization resistance of two sides electrolyte/electrode is 2.77, 1.625 and 1.27 Ω cm 2 at 750 °C, 850 °C and 900 °C, respectively. Hence the cathode polarization resistance is 1.385, 0.813, 0.635 Ω cm 2 at 750 °C, 850 °C and 900 °C, respectively. It indicates that the interface polarization has been improved compared to the best result reported in the literature, in which the polarization resistance of La2Ni0.8Cu0.2O4+δ cathode was 3.2 Ω cm 2 at 700 °C and
3.2. Cathode material for LSAO electrolyte 3.2.1. Chemical compatibility As mentioned before [24], the TEC of LBSM is close to LSM, which ensures the TEC compatibility of the LBSM cathode with the LSAO material. It is then valuable to investigate the electrochemical properties of the LBSM cathode on LSAO electrolyte. After the mixture of 50 wt.% LBSM and 50 wt.% LSAO being calcined at 1200 °C for 10 h, the XRD diffraction was performed and the pattern is presented in Fig. 8. It indicates that no third phase exists in the detectable range, proving a good chemical compatibility between the cathode and the electrolyte.
Fig. 7. The activation energy versus Al content.
Fig. 5. The conductivity of La9.67Si6-xAlxO26.5-x/2 samples.
Fig. 8. XRD pattern of LSAO + LBSM composite materials sintered at 1200 °C for 10 h.
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LBSM/LSAO. From the SEM image, no other layer or third phase at the interface is observed. Considering the results in Fig. 8, it can be concluded that the structures of apatite and perovskite remain unchanged although a few element has interdiffused. Fig. 9 shows that the result of polarization resistance is not influenced by the slight element diffusion. These results indicate the potential utilization of LBSM as the cathode material on LSAO electrolyte. 4. Conclusion
Fig. 9. AC impedance spectra of LBSM cathode sintered at 900 °C for 3 h in air.
1.5 Ω cm 2 at 800 °C without PrOx surface modification [23]. Fig. 10 is a typical fracture cross-section SEM image of symmetrical LBSM/LSAO/LBSM cell after operation. Both a porous cathode and a dense electrolyte were obtained, and the two layers have good contact without obvious segregation seen at the interface. The average grain particle size of LBSM cathode is around 500 nm, which might be why the cathode has good sintering property and good electrochemical performance. Fig. 11 shows the results of Electron probe micro-analyzer (EPMA) examination on the symmetrical cell, and it can be concluded that a few Mn and Si elements interdiffused nearby the interface of
The series of La9.67Si6-xAlxO26.5-x/2 (x = 0–2) powders with apatitetype single phase were synthesized by the sol–gel process. The obtained powders show good sintering property, which can be made into dense electrolyte membranes by traditional sintering technology. Al doping contents have a significant effect on the electrolyte properties, such as thermal expansion coefficient, relative density, electrical conductivity and activation energy. The series of electrolyte materials exhibit moderate TECs. La9.67 Si 6 O 26.5 (x = 0) among the series exhibits the highest electrical conductivity. However, compromising the relative density and electrical conductivity, La9.67Si5AlO26 was used as the electrolyte material of the symmetrical cells for investigation. The LBSM cathode exhibits a good performance on LSAO electrolyte, and the cathode polarization resistance was 1.385, 0.813, 0.635 Ω cm2 at 750 °C, 850 °C and 900 °C, respectively. Although a few elements interdiffuse at the interface of the electrolyte and the cathode, it does not influence the cathode performance as we can see. Therefore LBSM may be a promising candidate cathode for LSAO-based electrolyte in SOFC. Acknowledgements The authors are grateful for the financial support from the Science and Technology Commission of Shanghai Municipality No. 09DZ1206600. References
Fig. 10. Typical fracture cross-section SEM image of symmetrical LBSM/LSAO/LBSM cell.
Fig. 11. Electron probe micro-analyzer cross-section images of the LBSM cathode on the LSAO electrolyte.
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