Preliminary data on subsolidus phase equilibria in the La2O3Al2O3Mn2O3 and La2O3Al2O3Fe2O3 systems

J O U R N A L OF M A T E R I A L S S C I E N C E L E T T E R S 14 (1995) 1684 1687

Preliminary data on subsolidus phase equilibria in the La203-Cr203-Y203 and La203-Cr203-Zr02 systems M. HROVAT, S. BERNIK, J. HOLC, D. KOLAR, B. DACAR

Jo2ef Stefan Institute, University of Ljubljana, Jamova 39, 61000 Ljubljana, Slovenia

A fuel cell is a device for direct conversion of chemical energy into electrical energy, consisting basically of electrolyte and electrodes. Oxidant is fed to the cathode and reducent (fuel) to the anode. The concept is more than 150 years old, and was first reported in 1839 by Sir William Grove [1]. For a description of fuel cell development see, for example, [2]. High temperature fuel cells with solid oxide electrolyte work at temperatures around 1000°C. The first fuel cell with a zirconia based solid electrolyte was constructed nearly 60 years ago (1937) by Baur and Preis [3]. The advantage of high temperature solid oxide fuel cells (SOFC) for production of electrical energy is their high efficiency of 50-60%: some estimates are even up to a yield of 70-80%. Also, nitrous oxides are not produced and the amount of CO2 released per kWh is, due to the high efficiency, around 50% less than for power sources based on combustion [4-8]. The fuel is hydrogen, an H2/CO mixture, or hydrocarbons because the high temperature of operation makes possible the internal (in situ) reforming of hydrocarbons with water vapour [9]. The cross-sections of two basic constructions of SOFC, tubular and planar, are shown schematically in Figs 1 and 2, respectively. Due to the high operating temperatures the choice of materials is limited mainly to ceramics. An extensive and comprehensive review of materials for SOFC was presented in [10]. The electrodes (cathode and anode ) must be porous to permit the diffusion of oxygen and fuel to the zirconia electrolyte, and must have high electrical conductivities. Electrode polarization losses are reduced if the electrode material is also an ionic conductor. SrO doped LaMnO 3 and a mixture of metallic nickel and stabilized zirconia are generally used for cathode and anode, respectively. The solid electrolyte, which must be dense without open porosity, stable under oxidizing or reducing conditions and have high ionic and low electa'onic conductivity, is Y203 (8%) stabilized cubic ZrO2 (YSZ). The interconnect, which must also withstand both oxidizing and reducing atmospheres, is based on doped LaCrO3. It must have high electronic and low ionic conductivity. Some of the desirable characteristics of SOFC materials are summarized briefly in Table I. Although LaMnO3 based perovskites do not react with Y203 [11], it is well known that they react with zirconia, thus degrading the desirable long-term (the 1684

aim is 50000 working hours or nearly 6 years) performance [12-16]. At the interface between the ZrO 2 solid electrolyte and LaMnO3 cathode material, La2Zr207 is slowly formed. The specific electrical

/te

zlrconla Figure 1 Cross-section of the tubular design of an SOFC. The porous cathode and its coating of dense solid electrolyte are deposited on the porous ZrO2 based carrier tube. Electrical contact with the anode of the next cell is obtained with nickel felt. Air flows through the carrier tube and the file1 flows between the tubes.

Interconnect Anode [ ~ Electrolyte Cathode

Figure 2 Cross-section of the planar design of an SOFC. The air and the fuel flow through channels in the interconnect. 0261-8028

© 1995 Chapman & Hall

T A B L E I Some desirable characteristics of ceramic materials for SOFC Component

Solid electrolyte Cathode Anode Interconnect

Resistance against: Oxidation

Reduction

Yes Yes Not necessary Yes

Yes Not necessary Yes Yes

resistivity of La2Zr207 is more than two orders of magnitude higher than that of YSZ [17] (1500 ~ c m and 10 ~2 cm at 1000 °C, respectively), which increases the cell losses due to increased internal resistivity and therefore decreases its yield. The interactions between the interconnect material, i.e. lanthanum chromites, and other SOFC components were studied less extensively than the above mentioned interactions between cathode materials and YSZ. LaCrO3 and LaMnO 3 form solid solution on the interface between the cathode and anode [18, 19]. On the contact between the metallic nickel based anode and the interconnect NiCr204 spinel is formed [20, 21]. As shown in Figs 1 and 2, the interconnect is in contact with YSZ solid electrolyte in the case of the tubular construction and with the Ni/YSZ anode in the case of the planar construction. However, at least to the author's knowledge, possible interactions between LaCrO3 and YSZ (i.e. with ZrO2 or Y203) are not described in the open literature. The aim of the work reported here was to investigate the subsolidus phase equilibria in the LazO3-Cr203-Y203 and LazO3-Cr203-ZrO 2 systems. The results could imply possible interactions that can occur between Y203 or ZrO2 from Y203 stabilized zirconia and the LaCrO 3 based interconnect. No binary compound exists in the ZrO2-Cr203 system [22]. In the La203-Y203 system investigated by Coutures and Foex [23] the perovskite compound LaYO 3 decomposes (in solid) at around 1500 °C. The lowest melting temperature in the system is at 2200 °C. The compound La3YO 6 tentatively reported previously by Cassedane and Forester [24] was not confirmed. The binary compound YCrO3 with congruent melting point around 2300 °C is present in the Y 2 0 3 - C r 2 0 3 system with eutectic temperatures at 2020 °C on the Y203 side and at 2070 °C on the Cr203 side [25]. YCrO 3 was also proposed as a possible material for the interconnect [10]. Phase equilibria in the La203-Cr203 system are similar to those in the Y 2 0 3 - C r 2 0 3 system with the melting point of LaCrO 3 perovskite at 2430 °C [26]. In the ZrO2-Cr203 system the cubic pyrochlore compound, or rather solid solution with a nominal composition La2Zr207, melts congruently around 2300 °C. There are also extended solid solutions between La203 and La2Zr207 on one side and ZrO2 and La2Zr207 on the other. The lowest melting temperature (eutectic at the La203 side) is over 1900°C [27]. It can be seen that in all of the mentioned binary systems the liquidus temperatures,

Ionic conductivity

Electronic conductivity

Yes Desirable Desirable No

No Yes Yes Yes

either eutectics or melting points of compounds, are rather high, around or over 2000 °C. For experimental work, La(OH)3 (Ventron, 99.9%), Z r Q (Ventron, 99.9%), Cr203 (Merck, +99%) and 5(203 (Alfa Products, 99.99%) were used. The samples were mixed in ethyl alcohol, pressed into pellets and fired with intermediate grinding. Binary compounds were synthesized by firing at 1200 °C. During firing pellets were placed on platinum foils. The compositions of the relevant samples are shown in Figs 3 and 4. The regions of solid solutions are not

Or203

YCrO3~

Y203

LaCr03

YkaO 3

ka203

Figure 3 The proposed ternary phase diagram (subsolidus) of the La203-Cr203 Y203 system. The tie lines are between LaCrO 3LaYO 3, LaCrO3-Y203, and LaCrO3-YCrO 3.

Figure 4 The microstructure of a polished sample composed of Y203 and pre-reacted LaCrO3, fired at 1400 °C. The material is a mixture of darker LaCrO3 and lighter Y203 grains.

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shown. The results were evaluated by X-ray powder analysis, scanning microscopy and energy dispersive X-ray microanalysis (EDS). The proposed ternary phase diagram (subsolidus) of the La203-er203-Y203 system is shown in Fig. 3. The tie lines are between LaCrO3-LaYO3, LaCrO3Y203, and LaCrO3-YCrO3. The microstructure of a polished sample composed of 57203 and pre-reacted LaCrO3, fired at 1400 °C, is presented in Fig. 4. The material is a mixture of darker (and larger) LaCrO3 and lighter Y203 grains. Standardless EDS microanalysis indicated very limited solid solution of around 2% La203 in Y203 and less than 2% of Y203 in LaCrO3. The proposed ternary phase diagram (subsolidus) of the La203-Cr203-ZrO 2 system is shown in Fig. 5. The tie lines are between LaCrO3-ZrO2 and LaCrO3La2Zr207. No solid solution of either ZrO2 in LaCrO3 or La203 and/or Cr203 in ZrO2 was detected by standardless EDS microanalysis. The microstructure of a polished sample composed of ZrO2 and pre-

Cr203

ZrO2

La2Zr207

La203

Figure 5 The proposed ternary phase diagram (subsolidns) of the LazO3-Cr203-ZrO2 system. The tie lines are between LaCrO3LazZr207 and LaCrO3-ZrO 2.

Figure 6 The microstructure o'f a polished sample composed of ZrO2 and pre-reacted LaCrO3, fired at 1400 °C. The material is a mixture of lighter LaCrO3 and darker ZrO2 grains.

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reacted LaCrO3, fired at 1400 °C, is presented in Fig. 6. The material is a mixture of lighter LaCrO3 and dark grey ZrO2 grains. The results indicate that neither ZrO2 nor Y203 from YSZ solid solution reacts with the LaCrO3based interconnect, forming any other compound which could possibly degrade long-term SOFC performance.

Acknowledgement The financial support of the Ministry of Science and Technology of Slovenia is gratefully acknowledged.

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22. R. S. ROTH, J. R. DENNIS and H. F. McMURDIE (editors), "Phase diagrams for ceramists", Vol. VI, 1987 supplement (American Ceramic Society, Westervilte, Ohio, 1987) Fig. 6334, p. 72. 23. J. COUTURES and M. FOEX, J. Solid St. Chem. 11 (1974) 294. 24. E. M. LEVIN, H. F. McMURDIE and M. K. RESER (editors), "Phase diagrams for ceramists", Vol. III, 1975 supplement (American Ceramic Society, Westerville, Ohio, 1975) Fig. 4419, p. 153 (after J, Cassedane and H. Forester, C. R. Acad. Sci. 253 (1961) 2954).

25. Idem, ibid. Fig. 4402, p. 146. 26. Idem, ibid. Fig. 4397, p. 144. 27. E. M. LEVIN, C. R. ROBINS and H. F. McMURDIE (editors), "Phase diagrm~as for ceramists", Vol. II, 1969 supplement (American Ceramic Society, Westervitle, Ohio, t969) Fig. 2374, p. 103.

R e c e i v e d 23 January a n d accepted 2 M a y 1995

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