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Subsolidus phase equilibria in the NiO–CeO2 and La2O3–CeO2–Fe2O3 systems
Materials Research Bulletin, Vol. 33, No. 8, pp. 1175–1183, 1998 Copyright © 1998 Elsevier Science Ltd Printed in the USA. All rights reserved 0025-5408/98 $19.00 1 .00
PII S0025-5408(98)00103-2
SUBSOLIDUS PHASE EQUILIBRIA IN THE NiO–CeO2 AND La2O3–CeO2–Fe2O3 SYSTEMS
M. Hrovat*, J. Holc, S. Bernik, and D. Makovec Joz˘ef Stefan Institute, Jamova 39, 1001 Ljubljana, Slovenia (Refereed) (Received October 31, 1997; Accepted December 19, 1997)
ABSTRACT Subsolidus equilibria in air in the NiO–CeO2 and La2O3–CeO2–Fe2O3 systems were studied. In the NiO–CeO2 and Fe2O3–CeO2 systems, no binary compound and no solid solubility were detected. La2O3 was soluble in CeO2, forming a cubic fluorite solid solution up to La0.5Ce0.5O1.75, whereas no solid solubility of CeO2 in La2O3 was detected. In the La2O3–CeO2–Fe2O3 system, no ternary compound was found. The tie line is between LaFeO3 and CeO2. © 1998 Elsevier Science Ltd
KEYWORDS: A. oxides, C. electron microscopy, C. X-ray diffraction, D. phase equilibria INTRODUCTION A fuel cell is a device for direct conversion of chemical energy into electrical energy. Basically it consists of a cathode, an anode, and an electrolyte. Oxidant is fed to the cathode, and reducent (fuel) to the anode. The electrolyte, through which the ion current flows, prevents the mixing of oxidant and fuel. High-temperature fuel cells with a solid oxide electrolyte (SOFC) work at the present time at temperatures around 1000°C. The advantage of SOFCs for production of electrical energy is their high efficiency of 50 – 60%; some yield estimates (calculated and not based on experimental data from “real” SOFC prototypes) are as high as 70 – 80%. Also, nitrous oxides are not produced and the amount of CO2 released per kWh, due to their high efficiency, is about 50% less than for power sources based on
*To whom correspondence should be addressed. 1175
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combustion, making the SOFC an “environmentally friendly” method of power generation [1–3]. Due to the high operating temperatures of today’s SOFCs, the choice of materials is limited mainly to ceramics. The solid electrolyte in SOFC cells, which must be dense without open porosity, be stable under oxidizing or reducing conditions, and have high ionic and low electronic conductivity, is usually yttria-stabilized cubic zirconia (YSZ). The cathode (air electrode) is based on semiconducting oxides, and the anode (fuel electrode) is customarily a mixture of metallic nickel and yttria stabilized zirconia. Single fuel cells are joined in series by the interconnect. The interconnect material must withstand both oxidizing and reducing atmospheres and have high electronic and low (preferably none) ionic conductivity. Lanthanum chromite– based perovskites are mostly used. Extensive and comprehensive reviews of materials for SOFCs are presented in refs. 4 and 5. The next generation of SOFCs will have lower operating temperatures of about or below 800°C. One of the reasons for lowering the temperature is to replace ceramic interconnect materials by metallic high-temperature alloys, because it is easier and therefore less expensive to fabricate interconnect components from a metal than from a brittle ceramic. However, the conductivity of YSZ decreases by nearly an order of magnitude when the temperature is decreased from 1000 to 800°C. Some other ceramic material would be needed for the solid electrolyte. In the literature, doped CeO2 is mentioned as the most suitable “candidate.” The logarithm of the conductivity of YSZ and Ce0.8Gd0.2O22x vs. reciprocal temperature is shown in Figure 1 [6]. Gadolinia-doped ceria has a higher specific conductivity over the whole temperature range. The activation energies of ionic conductivity of gadolinia-doped ceria and YSZ are about 0.75 and 1 eV, respectively [4,7]. For an SOFC operating at 1000°C, cathode materials are mostly based on (La12xSrx)MnO3. It is a relatively good electronic conductor (resistivity 0.1 ohmzcm at 1000°C), but its ionic conductivity is low [8,9]. At lower operating temperatures, polarization losses of manganites are too high for the efficient operation of an SOFC. Data in the literature [5,10 –13] indicate that cobalt-doped ferrite with the nominal composition (La12xSrx)(Fe0.8Co0.2)O32z would be the material of “choice” due to its high electronic and ionic conductivity. As anode materials for a ceria-based solid electrolyte, some authors [5,14] have described the use of a mixture of nickel and ceria, instead of nickel and YSZ, for better material compatibility between the anode and ceria-based electrolyte. Anodes are usually prepared by sintering a mixture of nickel oxide and ceria in air (as in the case of nickel oxide and YSZ) and then reducing NiO to metallic Ni. The aim of this work was to investigate subsolidus phase equilibria (in air) in the CeO2–NiO and La2O3–CeO2–Fe2O3 systems. Results would indicate possible interactions between a ceria-based electrolyte and a LaFeO3-based cathode, or nickel oxide and ceria, during sintering of an anode material. Data in the literature indicate that there is no stable binary compound in the CeO2–La2O3 system [5,12]. The binary compound LaFeO3 with a melting point at about 1890°C exists in the La2O3–Fe2O3 system. Another compound, LaFe12O19, is stable only in the narrow temperature range between 1380 and 1420°C [15]. The lowest melting temperature in this system is at 1430°C. Between ceria and iron oxide, two compounds are reported, FeCe2O4 [16] and FeCeO3 [17]. However, they are not part of the CeO2–Fe2O3 system, because the valency of the cerium ion is Ce31 for both compounds, while the valency of the iron ion is Fe21 in FeCe2O4 and Fe31 in FeCeO3. FeCe2O4 has been prepared by firing a mixture of Fe2O3 and CeO2 at
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FIG. 1 The logarithm of conductivity of YSZ and Ce0.8Gd0.2O22x vs. reciprocal temperature.
temperatures around 1350°C in a reducing CO/CO2 atmosphere, and FeCeO3 by firing a stoichiometric mixture of CeO2, Fe2O3, and metallic Fe in a sealed evacuated quartz tube at 800 – 850°C. Pound [18] reported that the solid solubility of NiO in CeO2 is about 10%, while Ranslov et al. [19] did not find any solid solubility between CeO2 and NiO. EXPERIMENTAL For experimental work, La(OH)3 (Ventron, 99.9%), Fe2O3 (Alfa, 99.9%), and CeO2 (KochLight, 99.9%) were used. The samples were mixed in isopropyl alcohol, pressed into pellets, prefired at 1000°C, and fired up to five times at 120°C, with intermediate grinding. The firing temperature of 1200°C was used because CeO2 is stoichiometric; i.e., Ce ions are in the 41 state up to this temperature in air [20]. For firing, pellets were placed on platinum foils. The compositions of the relevant samples in the La2O3–Fe2O3–CeO2 system are shown in Figure 2. Samples with the compositions represented in the La2O3-rich part of the diagram were stored, after being fired, in petroleum oil. Fired materials were characterized by X-ray powder diffraction analysis with a Philips PW 1710 X-ray diffractometer using Cu Ka radiation. X-ray spectra were measured from 2u 5 20° to 2u 5 70° in steps of 0.02°. Silicon was used as the internal standard. A Jeol JXA-840A scanning electron microscope equipped with a Tracor-Northern energy-dispersive X-ray (EDX) analyzer was used for overall microstructural and compositional analysis. Samples prepared for scanning electron microscopy (SEM) were mounted in epoxy in cross-sectional orientation and then polished using standard metallographic techniques. Prior to SEM analysis, the samples were coated with carbon to provide electrical conductivity and avoid charging effects. The sample with the nominal composition Ce0.5La0.5O1,75 was milled in an agate mortar after repeated firing, and the powder was deposited on a copper grid–supported transparent
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FIG. 2 The proposed subsolidus ternary phase diagram of the La2O3–CeO2–Fe2O3 system at 1200°C in air. The tie line is between LaFeO3 and CeO2(ss). carbon foil. Electron diffraction patterns were taken on transparent particles of the powder sample, using a transmission electron microscope (Jeol 2000 FX) operating at 200 keV. RESULTS AND DISCUSSION Results of the X-ray powder diffraction analysis of relevant samples fired in air at 1200°C are summarized in Table 1. The numbers of samples in the La2O3–CeO2–Fe2O3 system correspond to the numbers marked in the phase diagram in Figure 2. Nominal compositions of samples and phases identified after firing are given. TABLE 1 Results of X-ray Diffraction Analysis of Some Compositions in the NiO–CeO2 and La2O3–CeO2–Fe2O3 Systems, Fired in Air at 1200°C Sample
Nominal composition
Identified phases
1 2 3 4 5 6 7 8 9 10 11 12
CeO2 1 NiO Fe2O3 1 4CeO2 Fe2O3 1 2CeO2 2Fe2O3 1 4CeO2 1 La2O3 5Fe2O3 1 6CeO2 1 2La2O3 Fe2O3 1 2CeO2 1 La2O3 3CeO2 1 La2O3 2CeO2 1 La2O3 CeO2 1 La2O3 Fe2O3 1 4CeO2 1 2La2O3 2CeO2 1 Fe2O3 1 3La2O3 CeO2 1 Fe2O3 1 2La2O3
CeO2, NiO Fe2O3, CeO2 Fe2O3, CeO2 LaFeO3, CeO2, Fe2O3 LaFeO3, CeO2, Fe2O3 LaFeO3, CeO2 CeO2(ss)a CeO2(ss) CeO2(ss), La2O3, La(OH)3b LaFeO3, CeO2(ss) LaFeO3, CeO2(ss), La2O3, La(OH)3 LaFeO3, CeO2(ss)
a
Solid solution of La2O3 in CeO2. The presence of La(OH)3 is due to the partial reaction of La2O3 with air moisture.
b
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FIG. 3 The microstructure of the Fe2O3–CeO2 sample fired at 1200°C. Darker grains are Fe2O3 and lighter grains are CeO2. X-ray powder diffraction analysis indicated that no binary compound formed between NiO and CeO2, fired at 1200°C in air. This was confirmed by EDX microanalysis. Solid solubility of either NiO in CeO2 or CeO2 in NiO was not detected by either X-ray diffraction data or EDX analysis, which is in agreement with the results reported in ref. 19. The microstructure of the Fe2O3–CeO2 sample fired at 1200°C in air is shown in Figure 3. The material is a mixture of dark Fe2O3 and lighter CeO2 grains. EDX analysis did not indicate any solid solubility or any binary compound; this was confirmed by X-ray diffraction
TABLE 2 Calculated Cell Parameters of Materials with Compositions x/2La2O3 1 (1 2 x)CeO2 fired at 1200°C CeO2(ss) (cubic) x
a (nm)
0 0.2 0.25 0.4 0.5 0.55a 0.66 0.7 0.8 0.9 1
0.5407(1) 0.5481(5) 0.5515(4) 0.5549(3) 0.5593(1) 0.5592(2) 0.5603(5) 0.5589(4) 0.5587(3) 0.5587(5)
La2O3 (hexagonal) a (nm)
c (nm)
0.3941(5) 0.3944(4) 0.3941(4) 0.3945(5) 0.3934(6)
0.613(1) 0.613(1) 0.613(1) 0.613(1) 0.613(1)
Parameters of the La2O3 cell for the material with x 5 0.55 could not be calculated from X-ray data with sufficient precision, due to the low intensity of La2O3 peaks.
a
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FIG. 4 Calculated cell parameters of materials with compositions x/2 La2O3 1 (1 2 x) CeO2, fired at 1200°C, as a function of x. The lattice dimension of CeO2 cubic fluorite solid solution monotonously increases with increasing lanthanum content up to x 5 0.5. analysis (Table 1). FeCe2O4 [16] and FeCeO3 [17] compounds with the cerium ion in 31 state were not obtained under the described experimental conditions. As seen in Table 1, in the CeO2–La2O3 system there are two regions, the region of solid solution of La2O3 in CeO2 and the two-phase region of CeO2(ss) and La2O3. In Table 2, the
FIG. 5 Electron diffraction pattern of the Ce0.5La0.5O1.75 compound in the ,110. zone axis. Superstructural spots correspond to the doubling of the fluorite cubic cell.
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FIG. 6 The microstructure of material with the nominal composition 2CeO2 1 Fe2O3 1 La2O3, fired at 1200°C. The material is a mixture of darker LaFeO3 and lighter CeO2 phase. calculated cell parameters of materials in the system CeO2–1/2La2O3 fired at 1200°C are given. The number in brackets indicates the accuracy of the last significant digit. Cell parameters as a function of x are graphically presented in Figure 4. The lattice dimension of CeO2 cubic fluorite solid solution monotonically increases with increasing lanthanum content up to x 5 0.5, due to the larger ionic radius of La31 (0.115 nm) compared with Ce41 (0.101 nm). For samples with x . 0.5, the cell parameters of the fluorite solid solution do not change further. Cell parameters a and c of the La2O3 hexagonal cell remained constant in the region between x 5 0.5 and x 5 1, indicating that there is no solid solubility of CeO2 in La2O3, at least that can be detected by X-ray powder diffraction. A sample with composition Ce0.5La0.5O1.75 was analyzed by transmission electron microscopy (TEM). Figure 5 shows the electron diffraction pattern taken on a crystalline particle of this sample. Only two types of spots are visible: strong spots corresponding to the fluorite structure in the ,110. zone axis and weaker superstructural spots. These superstructural spots correspond to the doubling of the fluorite cubic cell. Thus, the electron diffraction pattern matches the cubic cell with a cell parameter of about 1.1 nm. The present
FIG. 7 The microstructure of material with nominal composition CeO2 1 Fe2O3 1 2La2O3, fired at 1200°C. The material is a mixture of darker LaFeO3 and lighter CeO2(ss) phase.
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structure is isostructural with the compound Ce0.5Nd0.5O1.75, reported by Dixon et al. [21]. The compound La2Ce2O5 (La0.5Ce0.5O1.75) with an ordered fluorite structure is the end member of CeO2 solid solutions; i.e., LaxCe12xO22x/2 for x 5 0.5. The subsolidus phase equilibria in the La2O3–CeO2–Fe2O3 system at 1200°C are shown in Figure 6. No ternary compound was found. The tie line is between the LaFeO3 and CeO2 range of solid solutions. The microstructures of materials with the nominal compositions 2CeO2 1 Fe2O3 1 La2O3 and CeO2 1 Fe2O3 1 2La2O3, fired at 1200°C, are shown in Figures 6 and 7, respectively. Both materials consist of a two-phase mixture of darker LaFeO3 and lighter CeO2 grains (Fig. 6) or CeO2(ss) grains (Fig. 7). No solid solution of CeO2 in LaFeO3 was found. The results obtained indicate that LaFeO3-based cathode material does not react with CeO2-based solid electrolyte in an SOFC. CONCLUSIONS Subsolidus equilibria in the NiO–CeO2 and La2O3–CeO2–Fe2O3 systems were studied. The aim was to investigate possible interactions between a ceria-based solid electrolyte in new generation SOFCs and LaFeO3-based cathode or nickel oxide and ceria during sintering of anode material. All samples were fired in air at 1200°C. In the NiO–CeO2 system, no binary compound was found. Solid solubilities of either NiO in CeO2 or CeO2 in NiO were not detected. In the CeO2–Fe2O3 system, no binary compound was found. The compounds FeCe2O4 and FeCeO3 with the cerium ion in 31 state, reported in the literature, could not be synthesized under our experimental conditions. In the CeO2–La2O3 system, a solid solution La12xCexO22x/2 existed. The unit-cell dimensions of CeO2 cubic fluorite solid solution monotonically increased with increasing lanthanum content up to x 5 0.5. The compound La2Ce2O7 (La0.5Ce0.5O1.75) with an ordered fluorite structure, which is isostructural with the compound Ce0.5Nd0.5O1.75 reported in the literature, was the end member of CeO2 solid solution. No solid solubility of CeO2 in La2O3 was detected. In the La2O3–CeO2–Fe2O3 system no ternary compound was found. The tie line was between LaFeO3 and CeO2. There was no solid solubility of CeO2 in LaFeO3. The results obtained in both systems (i.e., NiO–CeO2 and La2O3–CeO2–Fe2O3), indicate that a ceria-based solid electrolyte in SOFC would not react with either NiO on the anode side or LaFeO3-based materials on the cathode side. ACKNOWLEDGMENT The authors thank Ms. Medeja Gec for the preparation of samples for microstructural analysis. The financial support of the Ministry of Science and Technology of Slovenia is gratefully acknowledged. REFERENCES 1. M. Mogensen and N. Christiansen, Europhys. News 24, 7 (1993). 2. H. Tagawa, in Proceedings of the Third International Symposium. on Solid Oxide Fuel Cells, ed. S.C. Singhal and H. Iwahara, pp. 6 –15, The Electrochemical Society, Pennington, NJ, USA (1993). 3. S. Kartha and P. Grimes, Phys. Today 47 (11), 54 (1994).
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4. N.Q. Minh, J. Am. Ceram. Soc. 76 (3), 563 (1993). 5. B.C.H. Steele, in First European Solid Oxide Fuel Cell Forum Proceedings, ed. U. Bossel, pp. 375–397, Eur. SOFC Forum Secretariat, Baden, Switzerland (1994). 6. A.M. Azad, S. Larose, and S.A. Akbar, J. Mater. Sci. 29, 4135 (1994). 7. M. Godickemeier, K. Sasaki, and L.J. Gauckler, in Proceedings of the Fourth International Symposium on Solid Oxide Fuel Cells SOFC-IV, ed. M. Dokiya, O. Yamamoto, T. Tagawa, and S.C. Singhal, pp. 1072–1081, The Electrochemical Society, Pennington, NJ, USA (1995). 8. A. Hammouche, E. Siebert, and A. Hammou, Mater. Res. Bull. 24, 367 (1989). 9. J.H. Kuo, H.U. Anderson, and D.M. Sparlin, J. Solid State Chem. 87, 55 (1990). 10. K. Nisancioglu and T.M. Gur, Proceedings of the Third International Symposium on Solid Oxide Fuel Cells, ed. S.C. Singhal and H. Iwahara, pp. 267–275, The Electrochemical Society, Pennington, NJ, USA (1993). 11. L.O. Jerdal and R. Tunold, Proceedings of the Fourth International Symposium on Solid Oxide Fuel Cells SOFC-IV, ed. M. Dokiya, O. Yamamoto, T. Tagawa, and S.C. Singhal, pp. 544 –553, The Electrochemical Society, Pennington, NJ, USA (1995). 12. C.C. Chen, M.M. Nasrallah, H.U. Anderson, and M.A. Alim, J. Electrochem. Soc. 142 (2), 491 (1995). 13. D. Waller, J.A. Lane, J.A. Kilner, R.J. Chater, P.S. Manning, and B.C.H. Steele, Second European Solid Oxide Fuel Cell Forum Proceedings, ed. B. Thorstensen, pp. 737–750, Eur. SOFC Forum, Oberrohrdorf, Switzerland (1996). 14. K. Eguchi, M. Kayano, Y. Kunisa, and H. Arai, Proceedings of the Fourth International Symposium on Solid Oxide Fuel Cells SOFC-IV, ed. M. Dokiya, O. Yamamoto, T. Tagawa, and S.C. Singhal, pp. 676 – 685, The Electrochemical Society, Pennington, NJ, USA (1995). 15. V.L. Moruzzi and M.W. Shafer, J. Am. Ceram. Soc. 43, 367 (1960). 16. B.F. Belov, A.V. Goroh, V.P. Demchuk, N.A. Dorschenko, and Z.A. Somojlenko, Inorg. Mater. 19 (2), 231 (1983). 17. M. Robbins, G.K. Wertheim, A. Menth, and R.C. Sherwood, J. Phys. Chem. Solids 30 (7), 1823 (1969). 18. B.G. Pound, Solid State Ionics 52, 183 (1992). 19. J. Ranslov, F.W. Poulsen, and M. Mogensen, Solid State Ionics 61, 277 (1993). 20. D. Makovec, Z. Samardz˘ija, and D. Kolar, Proceedings Third Euroceramics, ed. P. Duran and J.F. Fernandez, Vol. 1, pp. 961–966 (1996). 21. S. Dixon, J. Marr, E.E. Lachowski, J.A. Gard, and F.P. Glasser, Mater. Res. Bull. 15, 1811 (1980).
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SUBSOLIDUS PHASE EQUILIBRIA IN THE NiO–CeO2 AND La2O3–CeO2–Fe2O3 SYSTEMS
M. Hrovat*, J. Holc, S. Bernik, and D. Makovec Joz˘ef Stefan Institute, Jamova 39, 1001 Ljubljana, Slovenia (Refereed) (Received October 31, 1997; Accepted December 19, 1997)
ABSTRACT Subsolidus equilibria in air in the NiO–CeO2 and La2O3–CeO2–Fe2O3 systems were studied. In the NiO–CeO2 and Fe2O3–CeO2 systems, no binary compound and no solid solubility were detected. La2O3 was soluble in CeO2, forming a cubic fluorite solid solution up to La0.5Ce0.5O1.75, whereas no solid solubility of CeO2 in La2O3 was detected. In the La2O3–CeO2–Fe2O3 system, no ternary compound was found. The tie line is between LaFeO3 and CeO2. © 1998 Elsevier Science Ltd
KEYWORDS: A. oxides, C. electron microscopy, C. X-ray diffraction, D. phase equilibria INTRODUCTION A fuel cell is a device for direct conversion of chemical energy into electrical energy. Basically it consists of a cathode, an anode, and an electrolyte. Oxidant is fed to the cathode, and reducent (fuel) to the anode. The electrolyte, through which the ion current flows, prevents the mixing of oxidant and fuel. High-temperature fuel cells with a solid oxide electrolyte (SOFC) work at the present time at temperatures around 1000°C. The advantage of SOFCs for production of electrical energy is their high efficiency of 50 – 60%; some yield estimates (calculated and not based on experimental data from “real” SOFC prototypes) are as high as 70 – 80%. Also, nitrous oxides are not produced and the amount of CO2 released per kWh, due to their high efficiency, is about 50% less than for power sources based on
*To whom correspondence should be addressed. 1175
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combustion, making the SOFC an “environmentally friendly” method of power generation [1–3]. Due to the high operating temperatures of today’s SOFCs, the choice of materials is limited mainly to ceramics. The solid electrolyte in SOFC cells, which must be dense without open porosity, be stable under oxidizing or reducing conditions, and have high ionic and low electronic conductivity, is usually yttria-stabilized cubic zirconia (YSZ). The cathode (air electrode) is based on semiconducting oxides, and the anode (fuel electrode) is customarily a mixture of metallic nickel and yttria stabilized zirconia. Single fuel cells are joined in series by the interconnect. The interconnect material must withstand both oxidizing and reducing atmospheres and have high electronic and low (preferably none) ionic conductivity. Lanthanum chromite– based perovskites are mostly used. Extensive and comprehensive reviews of materials for SOFCs are presented in refs. 4 and 5. The next generation of SOFCs will have lower operating temperatures of about or below 800°C. One of the reasons for lowering the temperature is to replace ceramic interconnect materials by metallic high-temperature alloys, because it is easier and therefore less expensive to fabricate interconnect components from a metal than from a brittle ceramic. However, the conductivity of YSZ decreases by nearly an order of magnitude when the temperature is decreased from 1000 to 800°C. Some other ceramic material would be needed for the solid electrolyte. In the literature, doped CeO2 is mentioned as the most suitable “candidate.” The logarithm of the conductivity of YSZ and Ce0.8Gd0.2O22x vs. reciprocal temperature is shown in Figure 1 [6]. Gadolinia-doped ceria has a higher specific conductivity over the whole temperature range. The activation energies of ionic conductivity of gadolinia-doped ceria and YSZ are about 0.75 and 1 eV, respectively [4,7]. For an SOFC operating at 1000°C, cathode materials are mostly based on (La12xSrx)MnO3. It is a relatively good electronic conductor (resistivity 0.1 ohmzcm at 1000°C), but its ionic conductivity is low [8,9]. At lower operating temperatures, polarization losses of manganites are too high for the efficient operation of an SOFC. Data in the literature [5,10 –13] indicate that cobalt-doped ferrite with the nominal composition (La12xSrx)(Fe0.8Co0.2)O32z would be the material of “choice” due to its high electronic and ionic conductivity. As anode materials for a ceria-based solid electrolyte, some authors [5,14] have described the use of a mixture of nickel and ceria, instead of nickel and YSZ, for better material compatibility between the anode and ceria-based electrolyte. Anodes are usually prepared by sintering a mixture of nickel oxide and ceria in air (as in the case of nickel oxide and YSZ) and then reducing NiO to metallic Ni. The aim of this work was to investigate subsolidus phase equilibria (in air) in the CeO2–NiO and La2O3–CeO2–Fe2O3 systems. Results would indicate possible interactions between a ceria-based electrolyte and a LaFeO3-based cathode, or nickel oxide and ceria, during sintering of an anode material. Data in the literature indicate that there is no stable binary compound in the CeO2–La2O3 system [5,12]. The binary compound LaFeO3 with a melting point at about 1890°C exists in the La2O3–Fe2O3 system. Another compound, LaFe12O19, is stable only in the narrow temperature range between 1380 and 1420°C [15]. The lowest melting temperature in this system is at 1430°C. Between ceria and iron oxide, two compounds are reported, FeCe2O4 [16] and FeCeO3 [17]. However, they are not part of the CeO2–Fe2O3 system, because the valency of the cerium ion is Ce31 for both compounds, while the valency of the iron ion is Fe21 in FeCe2O4 and Fe31 in FeCeO3. FeCe2O4 has been prepared by firing a mixture of Fe2O3 and CeO2 at
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FIG. 1 The logarithm of conductivity of YSZ and Ce0.8Gd0.2O22x vs. reciprocal temperature.
temperatures around 1350°C in a reducing CO/CO2 atmosphere, and FeCeO3 by firing a stoichiometric mixture of CeO2, Fe2O3, and metallic Fe in a sealed evacuated quartz tube at 800 – 850°C. Pound [18] reported that the solid solubility of NiO in CeO2 is about 10%, while Ranslov et al. [19] did not find any solid solubility between CeO2 and NiO. EXPERIMENTAL For experimental work, La(OH)3 (Ventron, 99.9%), Fe2O3 (Alfa, 99.9%), and CeO2 (KochLight, 99.9%) were used. The samples were mixed in isopropyl alcohol, pressed into pellets, prefired at 1000°C, and fired up to five times at 120°C, with intermediate grinding. The firing temperature of 1200°C was used because CeO2 is stoichiometric; i.e., Ce ions are in the 41 state up to this temperature in air [20]. For firing, pellets were placed on platinum foils. The compositions of the relevant samples in the La2O3–Fe2O3–CeO2 system are shown in Figure 2. Samples with the compositions represented in the La2O3-rich part of the diagram were stored, after being fired, in petroleum oil. Fired materials were characterized by X-ray powder diffraction analysis with a Philips PW 1710 X-ray diffractometer using Cu Ka radiation. X-ray spectra were measured from 2u 5 20° to 2u 5 70° in steps of 0.02°. Silicon was used as the internal standard. A Jeol JXA-840A scanning electron microscope equipped with a Tracor-Northern energy-dispersive X-ray (EDX) analyzer was used for overall microstructural and compositional analysis. Samples prepared for scanning electron microscopy (SEM) were mounted in epoxy in cross-sectional orientation and then polished using standard metallographic techniques. Prior to SEM analysis, the samples were coated with carbon to provide electrical conductivity and avoid charging effects. The sample with the nominal composition Ce0.5La0.5O1,75 was milled in an agate mortar after repeated firing, and the powder was deposited on a copper grid–supported transparent
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FIG. 2 The proposed subsolidus ternary phase diagram of the La2O3–CeO2–Fe2O3 system at 1200°C in air. The tie line is between LaFeO3 and CeO2(ss). carbon foil. Electron diffraction patterns were taken on transparent particles of the powder sample, using a transmission electron microscope (Jeol 2000 FX) operating at 200 keV. RESULTS AND DISCUSSION Results of the X-ray powder diffraction analysis of relevant samples fired in air at 1200°C are summarized in Table 1. The numbers of samples in the La2O3–CeO2–Fe2O3 system correspond to the numbers marked in the phase diagram in Figure 2. Nominal compositions of samples and phases identified after firing are given. TABLE 1 Results of X-ray Diffraction Analysis of Some Compositions in the NiO–CeO2 and La2O3–CeO2–Fe2O3 Systems, Fired in Air at 1200°C Sample
Nominal composition
Identified phases
1 2 3 4 5 6 7 8 9 10 11 12
CeO2 1 NiO Fe2O3 1 4CeO2 Fe2O3 1 2CeO2 2Fe2O3 1 4CeO2 1 La2O3 5Fe2O3 1 6CeO2 1 2La2O3 Fe2O3 1 2CeO2 1 La2O3 3CeO2 1 La2O3 2CeO2 1 La2O3 CeO2 1 La2O3 Fe2O3 1 4CeO2 1 2La2O3 2CeO2 1 Fe2O3 1 3La2O3 CeO2 1 Fe2O3 1 2La2O3
CeO2, NiO Fe2O3, CeO2 Fe2O3, CeO2 LaFeO3, CeO2, Fe2O3 LaFeO3, CeO2, Fe2O3 LaFeO3, CeO2 CeO2(ss)a CeO2(ss) CeO2(ss), La2O3, La(OH)3b LaFeO3, CeO2(ss) LaFeO3, CeO2(ss), La2O3, La(OH)3 LaFeO3, CeO2(ss)
a
Solid solution of La2O3 in CeO2. The presence of La(OH)3 is due to the partial reaction of La2O3 with air moisture.
b
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FIG. 3 The microstructure of the Fe2O3–CeO2 sample fired at 1200°C. Darker grains are Fe2O3 and lighter grains are CeO2. X-ray powder diffraction analysis indicated that no binary compound formed between NiO and CeO2, fired at 1200°C in air. This was confirmed by EDX microanalysis. Solid solubility of either NiO in CeO2 or CeO2 in NiO was not detected by either X-ray diffraction data or EDX analysis, which is in agreement with the results reported in ref. 19. The microstructure of the Fe2O3–CeO2 sample fired at 1200°C in air is shown in Figure 3. The material is a mixture of dark Fe2O3 and lighter CeO2 grains. EDX analysis did not indicate any solid solubility or any binary compound; this was confirmed by X-ray diffraction
TABLE 2 Calculated Cell Parameters of Materials with Compositions x/2La2O3 1 (1 2 x)CeO2 fired at 1200°C CeO2(ss) (cubic) x
a (nm)
0 0.2 0.25 0.4 0.5 0.55a 0.66 0.7 0.8 0.9 1
0.5407(1) 0.5481(5) 0.5515(4) 0.5549(3) 0.5593(1) 0.5592(2) 0.5603(5) 0.5589(4) 0.5587(3) 0.5587(5)
La2O3 (hexagonal) a (nm)
c (nm)
0.3941(5) 0.3944(4) 0.3941(4) 0.3945(5) 0.3934(6)
0.613(1) 0.613(1) 0.613(1) 0.613(1) 0.613(1)
Parameters of the La2O3 cell for the material with x 5 0.55 could not be calculated from X-ray data with sufficient precision, due to the low intensity of La2O3 peaks.
a
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FIG. 4 Calculated cell parameters of materials with compositions x/2 La2O3 1 (1 2 x) CeO2, fired at 1200°C, as a function of x. The lattice dimension of CeO2 cubic fluorite solid solution monotonously increases with increasing lanthanum content up to x 5 0.5. analysis (Table 1). FeCe2O4 [16] and FeCeO3 [17] compounds with the cerium ion in 31 state were not obtained under the described experimental conditions. As seen in Table 1, in the CeO2–La2O3 system there are two regions, the region of solid solution of La2O3 in CeO2 and the two-phase region of CeO2(ss) and La2O3. In Table 2, the
FIG. 5 Electron diffraction pattern of the Ce0.5La0.5O1.75 compound in the ,110. zone axis. Superstructural spots correspond to the doubling of the fluorite cubic cell.
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FIG. 6 The microstructure of material with the nominal composition 2CeO2 1 Fe2O3 1 La2O3, fired at 1200°C. The material is a mixture of darker LaFeO3 and lighter CeO2 phase. calculated cell parameters of materials in the system CeO2–1/2La2O3 fired at 1200°C are given. The number in brackets indicates the accuracy of the last significant digit. Cell parameters as a function of x are graphically presented in Figure 4. The lattice dimension of CeO2 cubic fluorite solid solution monotonically increases with increasing lanthanum content up to x 5 0.5, due to the larger ionic radius of La31 (0.115 nm) compared with Ce41 (0.101 nm). For samples with x . 0.5, the cell parameters of the fluorite solid solution do not change further. Cell parameters a and c of the La2O3 hexagonal cell remained constant in the region between x 5 0.5 and x 5 1, indicating that there is no solid solubility of CeO2 in La2O3, at least that can be detected by X-ray powder diffraction. A sample with composition Ce0.5La0.5O1.75 was analyzed by transmission electron microscopy (TEM). Figure 5 shows the electron diffraction pattern taken on a crystalline particle of this sample. Only two types of spots are visible: strong spots corresponding to the fluorite structure in the ,110. zone axis and weaker superstructural spots. These superstructural spots correspond to the doubling of the fluorite cubic cell. Thus, the electron diffraction pattern matches the cubic cell with a cell parameter of about 1.1 nm. The present
FIG. 7 The microstructure of material with nominal composition CeO2 1 Fe2O3 1 2La2O3, fired at 1200°C. The material is a mixture of darker LaFeO3 and lighter CeO2(ss) phase.
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structure is isostructural with the compound Ce0.5Nd0.5O1.75, reported by Dixon et al. [21]. The compound La2Ce2O5 (La0.5Ce0.5O1.75) with an ordered fluorite structure is the end member of CeO2 solid solutions; i.e., LaxCe12xO22x/2 for x 5 0.5. The subsolidus phase equilibria in the La2O3–CeO2–Fe2O3 system at 1200°C are shown in Figure 6. No ternary compound was found. The tie line is between the LaFeO3 and CeO2 range of solid solutions. The microstructures of materials with the nominal compositions 2CeO2 1 Fe2O3 1 La2O3 and CeO2 1 Fe2O3 1 2La2O3, fired at 1200°C, are shown in Figures 6 and 7, respectively. Both materials consist of a two-phase mixture of darker LaFeO3 and lighter CeO2 grains (Fig. 6) or CeO2(ss) grains (Fig. 7). No solid solution of CeO2 in LaFeO3 was found. The results obtained indicate that LaFeO3-based cathode material does not react with CeO2-based solid electrolyte in an SOFC. CONCLUSIONS Subsolidus equilibria in the NiO–CeO2 and La2O3–CeO2–Fe2O3 systems were studied. The aim was to investigate possible interactions between a ceria-based solid electrolyte in new generation SOFCs and LaFeO3-based cathode or nickel oxide and ceria during sintering of anode material. All samples were fired in air at 1200°C. In the NiO–CeO2 system, no binary compound was found. Solid solubilities of either NiO in CeO2 or CeO2 in NiO were not detected. In the CeO2–Fe2O3 system, no binary compound was found. The compounds FeCe2O4 and FeCeO3 with the cerium ion in 31 state, reported in the literature, could not be synthesized under our experimental conditions. In the CeO2–La2O3 system, a solid solution La12xCexO22x/2 existed. The unit-cell dimensions of CeO2 cubic fluorite solid solution monotonically increased with increasing lanthanum content up to x 5 0.5. The compound La2Ce2O7 (La0.5Ce0.5O1.75) with an ordered fluorite structure, which is isostructural with the compound Ce0.5Nd0.5O1.75 reported in the literature, was the end member of CeO2 solid solution. No solid solubility of CeO2 in La2O3 was detected. In the La2O3–CeO2–Fe2O3 system no ternary compound was found. The tie line was between LaFeO3 and CeO2. There was no solid solubility of CeO2 in LaFeO3. The results obtained in both systems (i.e., NiO–CeO2 and La2O3–CeO2–Fe2O3), indicate that a ceria-based solid electrolyte in SOFC would not react with either NiO on the anode side or LaFeO3-based materials on the cathode side. ACKNOWLEDGMENT The authors thank Ms. Medeja Gec for the preparation of samples for microstructural analysis. The financial support of the Ministry of Science and Technology of Slovenia is gratefully acknowledged. REFERENCES 1. M. Mogensen and N. Christiansen, Europhys. News 24, 7 (1993). 2. H. Tagawa, in Proceedings of the Third International Symposium. on Solid Oxide Fuel Cells, ed. S.C. Singhal and H. Iwahara, pp. 6 –15, The Electrochemical Society, Pennington, NJ, USA (1993). 3. S. Kartha and P. Grimes, Phys. Today 47 (11), 54 (1994).
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4. N.Q. Minh, J. Am. Ceram. Soc. 76 (3), 563 (1993). 5. B.C.H. Steele, in First European Solid Oxide Fuel Cell Forum Proceedings, ed. U. Bossel, pp. 375–397, Eur. SOFC Forum Secretariat, Baden, Switzerland (1994). 6. A.M. Azad, S. Larose, and S.A. Akbar, J. Mater. Sci. 29, 4135 (1994). 7. M. Godickemeier, K. Sasaki, and L.J. Gauckler, in Proceedings of the Fourth International Symposium on Solid Oxide Fuel Cells SOFC-IV, ed. M. Dokiya, O. Yamamoto, T. Tagawa, and S.C. Singhal, pp. 1072–1081, The Electrochemical Society, Pennington, NJ, USA (1995). 8. A. Hammouche, E. Siebert, and A. Hammou, Mater. Res. Bull. 24, 367 (1989). 9. J.H. Kuo, H.U. Anderson, and D.M. Sparlin, J. Solid State Chem. 87, 55 (1990). 10. K. Nisancioglu and T.M. Gur, Proceedings of the Third International Symposium on Solid Oxide Fuel Cells, ed. S.C. Singhal and H. Iwahara, pp. 267–275, The Electrochemical Society, Pennington, NJ, USA (1993). 11. L.O. Jerdal and R. Tunold, Proceedings of the Fourth International Symposium on Solid Oxide Fuel Cells SOFC-IV, ed. M. Dokiya, O. Yamamoto, T. Tagawa, and S.C. Singhal, pp. 544 –553, The Electrochemical Society, Pennington, NJ, USA (1995). 12. C.C. Chen, M.M. Nasrallah, H.U. Anderson, and M.A. Alim, J. Electrochem. Soc. 142 (2), 491 (1995). 13. D. Waller, J.A. Lane, J.A. Kilner, R.J. Chater, P.S. Manning, and B.C.H. Steele, Second European Solid Oxide Fuel Cell Forum Proceedings, ed. B. Thorstensen, pp. 737–750, Eur. SOFC Forum, Oberrohrdorf, Switzerland (1996). 14. K. Eguchi, M. Kayano, Y. Kunisa, and H. Arai, Proceedings of the Fourth International Symposium on Solid Oxide Fuel Cells SOFC-IV, ed. M. Dokiya, O. Yamamoto, T. Tagawa, and S.C. Singhal, pp. 676 – 685, The Electrochemical Society, Pennington, NJ, USA (1995). 15. V.L. Moruzzi and M.W. Shafer, J. Am. Ceram. Soc. 43, 367 (1960). 16. B.F. Belov, A.V. Goroh, V.P. Demchuk, N.A. Dorschenko, and Z.A. Somojlenko, Inorg. Mater. 19 (2), 231 (1983). 17. M. Robbins, G.K. Wertheim, A. Menth, and R.C. Sherwood, J. Phys. Chem. Solids 30 (7), 1823 (1969). 18. B.G. Pound, Solid State Ionics 52, 183 (1992). 19. J. Ranslov, F.W. Poulsen, and M. Mogensen, Solid State Ionics 61, 277 (1993). 20. D. Makovec, Z. Samardz˘ija, and D. Kolar, Proceedings Third Euroceramics, ed. P. Duran and J.F. Fernandez, Vol. 1, pp. 961–966 (1996). 21. S. Dixon, J. Marr, E.E. Lachowski, J.A. Gard, and F.P. Glasser, Mater. Res. Bull. 15, 1811 (1980).