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Oxide ionic conductivity and microstructures of Sm- or La-doped CeO2-based systems
Solid State Ionics 154 – 155 (2002) 461 – 466 www.elsevier.com/locate/ssi
Oxide ionic conductivity and microstructures of Sm- or La-doped CeO2-based systems Toshiyuki Mori a,*, John Drennan b, Jong-Heun Lee c, Ji-Guang Li a, Takayasu Ikegami a a
Sintered Body Group, Advanced Materials Laboratory, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki, 405-0044, Japan b Centre for Microscopy and Microanalysis, The University of Queensland, Saint Lucia, Brisbane, Qld 4072, Australia c School of Materials Science and Engineering, Seoul National University, Seoul, 151-742, South Korea Accepted 18 March 2002
Abstract The concept of crystallographic index termed the effective index is suggested and applied to the design of ceria (CeO2)based electrolytes to maximize oxide ionic conductivity. The suggested index considers the fluorite structure, and combines the expected oxygen vacancy level with the ionic radius mismatch between host and dopant cations. Using this approach, oxide ionic conductivity of Sm- or La-doped CeO2-based system has been optimized and tested under operating conditions of a solid oxide fuel cell. In the observation of microstructure in atomic scale, both Sm-doped CeO2 and La-doped CeO2 electrolytes had large micro-domains over 10 nm in the lattice. On the other hand, Sm or La and alkaline earth co-doped CeO2-based electrolytes with high effective index had small micro-domains around 1 – 3 nm in the microstructure. The large micro-domain would prevent oxide ion from passing through the lattice. Therefore, it is concluded that the improvement of ionic conductivity is reflected in changes of microstructure in atomic scale. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Ionic conduction solids (66.30.D); Microstructure crystals (61.70); Mixed conductivity (72.60); Selected area electron diffraction pattern (61.14.L); Crystalline solids (61.66)
1. Introduction Doped ceria (CeO2) systems which maintain fluorite-related structure have a long history of investigation as potential materials for oxide ion electrolytes. While the materials have found application as oxygen sensor, [1,2] electrochemical cells such as membrane reactor [3] and interlayer structures in certain designs of electrolyte/anode interfaces [4,5] *
Corresponding author. Tel.: +81-298-51-7449x2247; fax: +81-298-52-7449. E-mail address: [email protected] (T. Mori).
in fuel cells, the most attractive application of doped CeO2 would be as the solid electrolyte in SOFCs. The higher ionic conductivity of the material will reduce the operating temperature of SOFCs. CeO2 doped with the oxides of di- or trivalent metals possess higher oxide ionic conductivity than any reported yttria-stabilized zirconia electrolyte. [6,7] At high oxygen partial pressures, these CeO2-based oxides show high oxide ionic conductivity. At low oxygen partial pressures with associated anodic conditions, however, the Ce4 + ion can be partially reduced to Ce3 +. Quasi-free electrons are introduced into fluorite lattice in such reducing atmospheres. As
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a consequence the doped CeO2 exhibits high oxide ionic conductivity in oxidizing atmosphere, whereas electronic conductivity is developed under anodic conditions in the fuel cell. The stability in both oxidizing and reducing atmosphere is an essential requirement for the doped CeO2 systems. In order to overcome this problem and improve the conductivity, a crystallographic strategy for development of doped CeO2 has been proposed. In addition, the true microstructures of prepared specimens with compositions designed on the basis of the above approach should provide atomic scale information which may reveal information on the conduction mechanism. Recently, the electrolytic properties of samarium[8,9] and gadolinium [10,11]-doped CeO2 electrolytes have been examined from the viewpoint of application to SOFCs. These materials have the feature that the association enthalpy between dopant cation and oxygen vacancy in the fluorite lattice is small. [12,13] On the other hand, the enthalpy of La-doped CeO2 would be comparable to the value of Gd-doped CeO2 system. Therefore, La-doped CeO2 would be another candidate of high-quality electrolytes, although previous reports on La-doped CeO2 system did not show high ionic conductivity. In the present study, for improvement of ionic conductivity, we reintroduce the concept of crystallographic index termed the effective index [14,15] and apply the approach to Sm- and La-doped systems in order to maximize ionic conductivity. Moreover, the true microstructures were investigated using transmission electron microscopy (TEM). We examined the relationship between electrolytic properties and true microstructures. In a general theory of oxide ionic conduction, a motion of ions is described by an activated jump process. The diffusion coefficient in this process is given by D ¼ D0 expðDG=kT Þ ¼ cð1 NcÞZk2 m0 expðDS=kÞexpðE=kT Þ
ð1Þ
where k is jump distance, T is the absolute temperature, k is Boltzmann’s constant, m0 indicates the attempt frequency and E means activation energy. Since the ion mobility is defined by l = qD/kT (i.e. Nernst – Einstein relationship) and Nc is the fraction of
occupied sites, C corresponds to the density of ion sites in the sublattice, the ionic conductivity becomes r ¼ NcCql ¼ cðCq2 =kT ÞNcð1 NcÞZk2 m0 expðDS=kÞexpðE=kT Þ ¼ ðr0 =T ÞexpðE=kT Þ
ð2Þ
In high-quality electrolytes, Nc must approach 1/2. This corresponds to a situation where nearly all of the ions on a given sublattice are mobile. Moreover, the crystal structure should be arranged for easy motion of oxide ions from one equivalent site to the neighbor site. In other words, the suggested approach for fast oxide ionic conduction should satisfy simultaneously two criteria which include the condition for Nc c 1/2 and the condition for having the ideal configuration in a fluorite lattice. In this paper, we reintroduce the concept of the effective index. [14,15] The effective index for a fast oxide ionic conduction in doped CeO2 electrolytes was defined using ionic radii and the amount of oxygen vacancies that are produced by the dopant substituting on idealized crystallographic sites. According to Pauling’s first rule, the crystallographic coordination number of a cation is decided by the radius ratio between cation and anion. The coordination number of the cation in the case of oxides dominate the crystal structure. The crystal structure of CeO2-based oxide is related to the fluorite type lattice. The coordination number of the cation in the idealized fluorite structure is eight. For stable formation of the fluorite structure, the ionic radius ratio between the cation (rc) and anion (ra) ranges between 0.732 and 1 in the case of eightfold coordination. Up until now, this ratio (rc/ra) was not considered as a controlling factor in the ionic conductivity because it seemed to have no direct relationship to the defect structure of fluorite. However, this ratio can be effectively utilized by considering it in conjunction with the oxygen vacancy levels. Kilner et al. [12] indicated that association enthalpy between dopant cation and oxygen vacancy was minimized when the ionic radius mismatch between dopant cation and oxygen vacancy is least. Accordingly, it is inferred that the oxide ionic conductivity in fluorite could be enhanced by decrease of this ionic radius mismatch. However, this ionic radius ratio lacked the viewpoint of crystallography. Therefore, the high-
T. Mori et al. / Solid State Ionics 154 – 155 (2002) 461–466
quality electrolyte could not be proposed using this ionic radius ratio. In this study, the effective index was defined in Eq. (3) as an original index instead of (rc/ra) or (rd/rh). Effective index ¼ ðavg: rc =eff : ro Þ*ðrd =rh Þ
ð3Þ
where avg. rc, rd and rh are the average ionic radius of cation, the average ionic radius of dopant and ionic radius of the host cation (Ce4 + ), respectively. The eff. ro is effective oxygen ionic radius, which can be given by eff : ro ¼ 1:4 fð2 dÞ=2g
ð4Þ
where d is the level of expected oxygen vacancy, and ˚ ) of oxygen in 1.4 is the commonly used ionic radius (A oxides. The first term (avg. rc/eff. ro) in Eq. (3) becomes 1 when the cation is coordinated by eight anions in nondistorted fluorite structure. The second term (rd/rh) in Eq. (3) approaches 1 when association enthalpy between dopant cation and anion vacancy is minimized in the case of fluorite structure [12,13]. Accordingly, it is assumed that the doped CeO 2 reaches the ideal fluorite-related structure for high oxide ionic condition when the effective index goes toward 1. We concluded that the first term and second term in Eq. (3) would correspond to the condition for having an ideal configuration in fluorite and the condition for Nc c 1/2 in the fluorite lattice, respectively. Therefore, the authors assume that the doped CeO2 reaches the ideal fluorite structure for high ionic conduction when the effective index approaches 1. In this study, we think that nondistorted fluorite is one of the candidates of superior quality solid electrolyte. The suggested index would indicate the composition of nondistorted CeO2-based oxide. In addition, lattice distortion would change the microstructure at atom level in solid electrolyte. Accordingly, we conclude that the combination between the preparation of specimens on the basis of the hypothesis (i.e. effective index) and a careful observation of microstructure in prepared specimens is important for the creation of superior quality solid electrolyte. In order to confirm the validity of the suggested index, Sm x Ce 1 x O 2 d (x = 0.2, 0.21, 0.25), (Sm0.5Ca0.5) xCe1 x O2 d (x = 0.175, 0.20, 0.225, 0.25), (Sm 0 . 7 5 Sr 0 . 2 Ba 0 . 0 5 ) 0 . 1 7 5 Ce 0 . 8 2 5 O 1 . 8 9 1 , La x Ce 1 x O 2 d (x = 0.125, 0.15, 0.175, 0.20),
463
( L a 0 . 8 S r 0 . 2 ) x C e 1 x O 2 d (x = 0. 0 5, 0 .1 ) an d (La 1 x Sr 0.2 Ba x ) 0.175 Ce 0.825 O 2 d (x = 0.03, 0.05) were characterized as the low, intermediate and high index, respectively. The microstructure of the sintered bodies was investigated to elucidate the conduction mechanism using TEM. The electrical conductivity and operation properties of SOFC of specimens were also evaluated.
2. Experimental The starting materials used were commercial nitrate powders. Each powder was dissolved in distilled water, and the solutions were mixed in order to prepare the aforementioned fixed compositions. NH3+(NH4)2CO3 solution in a 1:1 ratio was dropped into the mixed solution for precipitation. The powder was dried after filtration and rinsing. The dried powder was calcined at 800 –1000 jC for 1 h in air. The green bodies were made using isostatic press. The sintering temperature was 1450 –1550 jC and sintering time was 4 h in each case. The crystal phases were identified using X-ray diffraction. The elements in the sintered bodies were analyzed using inductively coupled plasma (ICP) technique. The electrical conductivity of the sintered specimens was measured by DC-four point, DC-three point and AC-three point measurements. Power density of the specimens as electrolytes in planar SOFC was measured using O2 + H2 ( + H2O, 40 jC) cell. Analytical TEM was used to characterize these specimens.
3. Results and discussion Fig. 1 shows the relationship between effective index and electrical conductivity. The closed symbols indicate specimens with the same expected oxygen vacancy level. As this figure indicates, the electrical conductivity increased with increasing effective index, even if oxygen vacancy level was the same (i.e. specimens 2, 8 and 15). From this, it was concluded that the crystal structure of specimens 2, 8 and 15 are optimally modified to the preferable structure (i.e. small distorted structure) in the high index region. Moreover, the activation energy of specimens decreased from 88 kJ/mol in Sm0.2Ce0.8O1.9 to 67 kJ/
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Fig. 1. Relationship between effective index and conductivity. (1) Sm0.2Ce0.8O1.90, (2) Sm0.21Ce0.79O1.891, (3) Sm0.25Ce0.75O1.875, (4) (Sm0.5Ca0.5)0.175Ce0.825O1.87, (5) (Sm0.5Ca0.5)0.2Ce0.8O1.85, (6) (Sm0.5Ca0.5)0.225Ce0.775O1.84, (7) (Sm0.5Ca0.5)0.25Ce0.75O1.81, (8) (Sm0.75Sr0.2Ba0.05)0.175Ce0.825O1.891, (9) La0.125Ce0.875O1.94, (10) La0.15Ce0.85O1.925, (11) La0.175Ce0.825O1.91, (12) (La0.8Sr0.1)0.175 Ce0.825O1.90, (13) (La0.8Sr0.2)0.175Ce0.825O1.89, (14) (La0.77Sr0.2 Ba0.03)0.175Ce0.825O1.892, (15) (La0.75Sr0.2Ba0.05)0.175Ce0.825O1.891.
mol in (La0.75Sr 0.2Ba0.05) 0.175Ce0.825O 1.89. These results indicate that the suggested index is a useful tool for improvement of electrical conductivity. On the other hand, Table 1 summarizes the analyzed composition data in four representative examples. The chemical analysis showed that the sample composition was controlled well with only a very small deviation. It is concluded that the suggested index of all specimens can be calculated using the starting compositions of the specimens. Figs. 2 and 3 display high-resolution images and selected area electron diffraction patterns recorded from Sm0.2Ce0.8O1.9 and (La0.75Sr0.2Ba0.05)0.175Ce0.825 O1.89 sintered bodies, respectively. The big diffuse scatter clearly appeared in the background of electron diffraction patterns, although extra spots cannot be observed in it. This diffuse scatter indicates that these
Table 1 Chemical composition of specimens Sample
Chemical composition
Sm0.25Ce0.75O1.88 (Sm0.5Ca0.5)0.225Ce0.775 O1.84 La0.175Ce0.825O1.91(2) (La0.75Sr0.2Ba0.05)0.175 Ce0.825O1.891
Sm0.242Ce0.758O1.88 (Sm0.496Ca0.468)0.225 Ce0.783O1.84 La0.178Ce0.822O1.91(1) (La0.751Sr0.198Ba0.051)0.177 Ce0.823O1.890
Fig. 2. High-resolution image (h110iF) and selected area electron diffraction pattern (h110iF) recorded from Sm0.2Ce0.8O1.90 sintered body, dashed line area means micro-domain with ordered structure.
specimens have micro-domains with ordered structure and the interface between micro-domain and fluorite lattice is coherent in a grain. If the interface has a misfit, the extra spots will be seen in the diffraction pattern. Moreover, EDS analysis indicated that the grain boundaries in these specimens were very clean. The highresolution images indicate that the micro-domain has irregular shapes. While Sm0.2Ce0.8O1.9 sintered body has a relatively large micro-domain in the grain, the micro-domain size in (La 0 . 7 5 Sr 0 . 2 Ba 0 . 0 5 ) 0 . 1 7 5 Ce0.825O1.89 sintered body was much smaller. A large strain in the lattice of doped CeO2 would be created by doping lower valent cations on the Ce site. In order to minimize the distortion in the lattice, micro-domains form spontaneously in the lattice. Accordingly, it is concluded that the micro-domain size is minimized in the nondistorted fluorite structure. On the other hand, suggested index indicates the research direction to prepare a promising material (i.e. nondistorted fluorite structure) for high-quality electrolyte. Therefore, the
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gested index was higher than that of Sm0.2Ce0.8O1.9 with the low index. Accordingly, it is concluded that (La0.75Sr0.2Ba0.05) 0.175Ce0.825O1.89 would be less reducible than Sm0.2Ce0.8O1.9 in a humidified hydrogen atmosphere. Because of the high ionic conductivity and good reduction resistance, the maximum power density of (La0.75Sr0.2Ba0.05)0.175Ce0.825O1.89 reached 0.82 W/cm2 at 1000 jC. This value was over two times larger than that of Sm0.2Ce0.8O1.9. The power density of (La0.75Sr0.2Ba0.05)0.175Ce0.825O1.89 was 0.31 W/cm2 at 700 jC, which is sufficient for low-temperature application if we consider that the thickness of electrolyte was 0.5 mm. In this study, electrodes were not optimized in single-cell tests. Therefore, it is expected that the power density of
Fig. 3. High-resolution image (h110iF) and selected area electron diffraction pattern (h110iF) recorded from (La0.75Sr0.2Ba0.05)0.175 Ce0.825O1.891 sintered body, dashed line area means microdomain with ordered structure.
specimen with high index values such as (La0.75Sr0.2Ba0.05)0.175Ce0.825O1.89 sintered body has small microdomains and shows high conductivity. The authors expect that the specimen with interface between nanosize domain and fluorite lattice will show excellent electrolytic properties. Fig. 4a and b plots the potential and power density as a function of current density at cell temperature of 700, 800, 900 and 1000 jC using Sm0.2Ce0.8O1.9 and (La0.75Sr0.2Ba0.05)0.175Ce0.825O1.89 as the electrolytes, respectively. It is well known that CeO2 electrolytes exhibit n-type electronic conduction in the operating atmosphere of a fuel cell. The decrement of terminal potential corresponds to the n-type electronic conduction of reduced CeO2-based oxides. The terminal potential decreased with increasing operating temperature in both cases. However, the terminal potential of (La0.75Sr0.2Ba0.05)0.175Ce0.825O1.89 with high sug-
Fig. 4. Discharge performance of (a) Sm0.2Ce0.8O1.90 and (b) (La0.75Sr0.2Ba0.05)0.175Ce0.825O1.891 electrolyte. Small open symbols indicate the potential as a function of current density: (5) 1000, (D) 900, (o) 800, (q) 700 jC. Large closed symbols indicate the power density as a function of current density: (n) 1000, (E) 900, (.) 800, (z) 700 jC. Electrolyte thickness: 0.5 mm. Cathode: La0.9Sr0.1MnO3, anode: Ni/YSZ cermet (Ni/YSZ = 55:45 in volume ratio).
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(La0.75Sr0.2Ba0.05)0.175Ce0.825O1.89 could be further improved using the electrodes such as La1 xSrCoO3 cathode and Ni/CeO2 cermet anode.
4. Summary The effective index, as criterion for fast oxide ionic conduction in doped CeO2, was suggested using ionic radii and expected oxygen vacancy level in order to improve electrolytic properties. The relationship between microstructure and electrolytic properties was examined. The results obtained are summarized as follows; 1. Ionic conductivity increased with increasing suggested index. The activation energy of the specimens with high index value is lower than that of the specimens with low index value. Moreover, maximum power density of (La0.75Sr0.2Ba0.05)0.175Ce0.825O1.89 with high suggested index was higher than that of Sm0.2Ce0.8O1.9 with the low index. Accordingly, it is concluded that suggested index shows one of the future trends for development of high-quality electrolytes. 2. Selected area electron diffraction patterns recorded from sintered samples indicated the existence of micro-domains with ordered structure in doped CeO2 electrolytes. The interface between the microdomain and the fluorite lattice was coherent in a grain. The size of the micro-domain in Sm0.2Ce0.8O1.9 was much larger than that in (La 0.75 Sr 0.2 Ba 0.05 ) 0.175 Ce0.825O1.89. The control of domain size would be an important factor for development of improved CeO2 electrolytes. 3. The formation of micro-domains would be attributable to the lattice distortion in doped CeO2.
Since the suggested index indicates the composition of nondistorted CeO2-based oxide, the size of microdomains in (La0.75Sr0.2Ba0.05)0.175Ce0.825O1.89 with high suggested index would be minimized. 4. The size of micro-domain in (La 0.75Sr 0.2Ba0.05)0.175Ce0.825O1.89 would be controlled using advanced sol – gel processing. We also expect that the control of domain size in the lattice and the clarification of interface structure between microdomain and fluorite lattice will improve the electrolytic properties in (La0.75Sr0.2Ba0.05)0.175Ce0.825O1.89.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
H. Arai, J. Bull. Ceram. Soc. Jpn. 27 (2) (1992) 100. N. Yamazoe, N. Miura, MRS Bull. 24 (6) (1999) 37. K. Takehira, Catal. Catal. 39 (5) (1997) 330. K. Huang, R. Tichy, J.B. Goodenough, C. Milliken, J. Am. Ceram. Soc. 81 (10) (1998) 2581. T. Tsai, S.A. Barnett, J. Electrochem. Soc. 145 (5) (1998) 1696. H. Yahiro, Y. Baba, K. Eguchi, H. Arai, J. Electrochem. Soc. 135 (8) (1988) 2077. B.C.H. Steele, Solid State Ionics 129 (2000) 95. M. Go¨dickemeier, L.J. Gauckler, J. Electrochem. Soc. 145 (2) (1998) 414. C. Milliken, S. Guruswamy, A. Khandkar, J. Electrochem. Soc. 146 (3) (1999) 872. T. Kudo, H. Obayashi, J. Electrochem. Soc. 123 (3) (1976) 415. R. Doshi, V.L. Richards, J.D. Carter, X. Wang, M. Krumpelt, J. Electrochem. Soc. 146 (4) (1999) 1273. J.A. Kilner, C.D. Waters, Solid State Ionics 6 (1982) 253. V. Butler, C.R.A. Catlow, B.E.F. Fender, J.H. Harding, Solid State Ionics 8 (1983) 109. T. Mori, T. Ikegami, H. Yamamura, J. Electrochem. Soc. 146 (12) (1999) 4380. T. Mori, J.-H. Lee, J.-G. Li, T. Ikegami, G. Auchterlonie, J. Drennan, Solid State Ionics 138 (2001) 277.
Oxide ionic conductivity and microstructures of Sm- or La-doped CeO2-based systems Toshiyuki Mori a,*, John Drennan b, Jong-Heun Lee c, Ji-Guang Li a, Takayasu Ikegami a a
Sintered Body Group, Advanced Materials Laboratory, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki, 405-0044, Japan b Centre for Microscopy and Microanalysis, The University of Queensland, Saint Lucia, Brisbane, Qld 4072, Australia c School of Materials Science and Engineering, Seoul National University, Seoul, 151-742, South Korea Accepted 18 March 2002
Abstract The concept of crystallographic index termed the effective index is suggested and applied to the design of ceria (CeO2)based electrolytes to maximize oxide ionic conductivity. The suggested index considers the fluorite structure, and combines the expected oxygen vacancy level with the ionic radius mismatch between host and dopant cations. Using this approach, oxide ionic conductivity of Sm- or La-doped CeO2-based system has been optimized and tested under operating conditions of a solid oxide fuel cell. In the observation of microstructure in atomic scale, both Sm-doped CeO2 and La-doped CeO2 electrolytes had large micro-domains over 10 nm in the lattice. On the other hand, Sm or La and alkaline earth co-doped CeO2-based electrolytes with high effective index had small micro-domains around 1 – 3 nm in the microstructure. The large micro-domain would prevent oxide ion from passing through the lattice. Therefore, it is concluded that the improvement of ionic conductivity is reflected in changes of microstructure in atomic scale. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Ionic conduction solids (66.30.D); Microstructure crystals (61.70); Mixed conductivity (72.60); Selected area electron diffraction pattern (61.14.L); Crystalline solids (61.66)
1. Introduction Doped ceria (CeO2) systems which maintain fluorite-related structure have a long history of investigation as potential materials for oxide ion electrolytes. While the materials have found application as oxygen sensor, [1,2] electrochemical cells such as membrane reactor [3] and interlayer structures in certain designs of electrolyte/anode interfaces [4,5] *
Corresponding author. Tel.: +81-298-51-7449x2247; fax: +81-298-52-7449. E-mail address: [email protected] (T. Mori).
in fuel cells, the most attractive application of doped CeO2 would be as the solid electrolyte in SOFCs. The higher ionic conductivity of the material will reduce the operating temperature of SOFCs. CeO2 doped with the oxides of di- or trivalent metals possess higher oxide ionic conductivity than any reported yttria-stabilized zirconia electrolyte. [6,7] At high oxygen partial pressures, these CeO2-based oxides show high oxide ionic conductivity. At low oxygen partial pressures with associated anodic conditions, however, the Ce4 + ion can be partially reduced to Ce3 +. Quasi-free electrons are introduced into fluorite lattice in such reducing atmospheres. As
0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 4 8 3 - 6
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T. Mori et al. / Solid State Ionics 154 – 155 (2002) 461–466
a consequence the doped CeO2 exhibits high oxide ionic conductivity in oxidizing atmosphere, whereas electronic conductivity is developed under anodic conditions in the fuel cell. The stability in both oxidizing and reducing atmosphere is an essential requirement for the doped CeO2 systems. In order to overcome this problem and improve the conductivity, a crystallographic strategy for development of doped CeO2 has been proposed. In addition, the true microstructures of prepared specimens with compositions designed on the basis of the above approach should provide atomic scale information which may reveal information on the conduction mechanism. Recently, the electrolytic properties of samarium[8,9] and gadolinium [10,11]-doped CeO2 electrolytes have been examined from the viewpoint of application to SOFCs. These materials have the feature that the association enthalpy between dopant cation and oxygen vacancy in the fluorite lattice is small. [12,13] On the other hand, the enthalpy of La-doped CeO2 would be comparable to the value of Gd-doped CeO2 system. Therefore, La-doped CeO2 would be another candidate of high-quality electrolytes, although previous reports on La-doped CeO2 system did not show high ionic conductivity. In the present study, for improvement of ionic conductivity, we reintroduce the concept of crystallographic index termed the effective index [14,15] and apply the approach to Sm- and La-doped systems in order to maximize ionic conductivity. Moreover, the true microstructures were investigated using transmission electron microscopy (TEM). We examined the relationship between electrolytic properties and true microstructures. In a general theory of oxide ionic conduction, a motion of ions is described by an activated jump process. The diffusion coefficient in this process is given by D ¼ D0 expðDG=kT Þ ¼ cð1 NcÞZk2 m0 expðDS=kÞexpðE=kT Þ
ð1Þ
where k is jump distance, T is the absolute temperature, k is Boltzmann’s constant, m0 indicates the attempt frequency and E means activation energy. Since the ion mobility is defined by l = qD/kT (i.e. Nernst – Einstein relationship) and Nc is the fraction of
occupied sites, C corresponds to the density of ion sites in the sublattice, the ionic conductivity becomes r ¼ NcCql ¼ cðCq2 =kT ÞNcð1 NcÞZk2 m0 expðDS=kÞexpðE=kT Þ ¼ ðr0 =T ÞexpðE=kT Þ
ð2Þ
In high-quality electrolytes, Nc must approach 1/2. This corresponds to a situation where nearly all of the ions on a given sublattice are mobile. Moreover, the crystal structure should be arranged for easy motion of oxide ions from one equivalent site to the neighbor site. In other words, the suggested approach for fast oxide ionic conduction should satisfy simultaneously two criteria which include the condition for Nc c 1/2 and the condition for having the ideal configuration in a fluorite lattice. In this paper, we reintroduce the concept of the effective index. [14,15] The effective index for a fast oxide ionic conduction in doped CeO2 electrolytes was defined using ionic radii and the amount of oxygen vacancies that are produced by the dopant substituting on idealized crystallographic sites. According to Pauling’s first rule, the crystallographic coordination number of a cation is decided by the radius ratio between cation and anion. The coordination number of the cation in the case of oxides dominate the crystal structure. The crystal structure of CeO2-based oxide is related to the fluorite type lattice. The coordination number of the cation in the idealized fluorite structure is eight. For stable formation of the fluorite structure, the ionic radius ratio between the cation (rc) and anion (ra) ranges between 0.732 and 1 in the case of eightfold coordination. Up until now, this ratio (rc/ra) was not considered as a controlling factor in the ionic conductivity because it seemed to have no direct relationship to the defect structure of fluorite. However, this ratio can be effectively utilized by considering it in conjunction with the oxygen vacancy levels. Kilner et al. [12] indicated that association enthalpy between dopant cation and oxygen vacancy was minimized when the ionic radius mismatch between dopant cation and oxygen vacancy is least. Accordingly, it is inferred that the oxide ionic conductivity in fluorite could be enhanced by decrease of this ionic radius mismatch. However, this ionic radius ratio lacked the viewpoint of crystallography. Therefore, the high-
T. Mori et al. / Solid State Ionics 154 – 155 (2002) 461–466
quality electrolyte could not be proposed using this ionic radius ratio. In this study, the effective index was defined in Eq. (3) as an original index instead of (rc/ra) or (rd/rh). Effective index ¼ ðavg: rc =eff : ro Þ*ðrd =rh Þ
ð3Þ
where avg. rc, rd and rh are the average ionic radius of cation, the average ionic radius of dopant and ionic radius of the host cation (Ce4 + ), respectively. The eff. ro is effective oxygen ionic radius, which can be given by eff : ro ¼ 1:4 fð2 dÞ=2g
ð4Þ
where d is the level of expected oxygen vacancy, and ˚ ) of oxygen in 1.4 is the commonly used ionic radius (A oxides. The first term (avg. rc/eff. ro) in Eq. (3) becomes 1 when the cation is coordinated by eight anions in nondistorted fluorite structure. The second term (rd/rh) in Eq. (3) approaches 1 when association enthalpy between dopant cation and anion vacancy is minimized in the case of fluorite structure [12,13]. Accordingly, it is assumed that the doped CeO 2 reaches the ideal fluorite-related structure for high oxide ionic condition when the effective index goes toward 1. We concluded that the first term and second term in Eq. (3) would correspond to the condition for having an ideal configuration in fluorite and the condition for Nc c 1/2 in the fluorite lattice, respectively. Therefore, the authors assume that the doped CeO2 reaches the ideal fluorite structure for high ionic conduction when the effective index approaches 1. In this study, we think that nondistorted fluorite is one of the candidates of superior quality solid electrolyte. The suggested index would indicate the composition of nondistorted CeO2-based oxide. In addition, lattice distortion would change the microstructure at atom level in solid electrolyte. Accordingly, we conclude that the combination between the preparation of specimens on the basis of the hypothesis (i.e. effective index) and a careful observation of microstructure in prepared specimens is important for the creation of superior quality solid electrolyte. In order to confirm the validity of the suggested index, Sm x Ce 1 x O 2 d (x = 0.2, 0.21, 0.25), (Sm0.5Ca0.5) xCe1 x O2 d (x = 0.175, 0.20, 0.225, 0.25), (Sm 0 . 7 5 Sr 0 . 2 Ba 0 . 0 5 ) 0 . 1 7 5 Ce 0 . 8 2 5 O 1 . 8 9 1 , La x Ce 1 x O 2 d (x = 0.125, 0.15, 0.175, 0.20),
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( L a 0 . 8 S r 0 . 2 ) x C e 1 x O 2 d (x = 0. 0 5, 0 .1 ) an d (La 1 x Sr 0.2 Ba x ) 0.175 Ce 0.825 O 2 d (x = 0.03, 0.05) were characterized as the low, intermediate and high index, respectively. The microstructure of the sintered bodies was investigated to elucidate the conduction mechanism using TEM. The electrical conductivity and operation properties of SOFC of specimens were also evaluated.
2. Experimental The starting materials used were commercial nitrate powders. Each powder was dissolved in distilled water, and the solutions were mixed in order to prepare the aforementioned fixed compositions. NH3+(NH4)2CO3 solution in a 1:1 ratio was dropped into the mixed solution for precipitation. The powder was dried after filtration and rinsing. The dried powder was calcined at 800 –1000 jC for 1 h in air. The green bodies were made using isostatic press. The sintering temperature was 1450 –1550 jC and sintering time was 4 h in each case. The crystal phases were identified using X-ray diffraction. The elements in the sintered bodies were analyzed using inductively coupled plasma (ICP) technique. The electrical conductivity of the sintered specimens was measured by DC-four point, DC-three point and AC-three point measurements. Power density of the specimens as electrolytes in planar SOFC was measured using O2 + H2 ( + H2O, 40 jC) cell. Analytical TEM was used to characterize these specimens.
3. Results and discussion Fig. 1 shows the relationship between effective index and electrical conductivity. The closed symbols indicate specimens with the same expected oxygen vacancy level. As this figure indicates, the electrical conductivity increased with increasing effective index, even if oxygen vacancy level was the same (i.e. specimens 2, 8 and 15). From this, it was concluded that the crystal structure of specimens 2, 8 and 15 are optimally modified to the preferable structure (i.e. small distorted structure) in the high index region. Moreover, the activation energy of specimens decreased from 88 kJ/mol in Sm0.2Ce0.8O1.9 to 67 kJ/
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Fig. 1. Relationship between effective index and conductivity. (1) Sm0.2Ce0.8O1.90, (2) Sm0.21Ce0.79O1.891, (3) Sm0.25Ce0.75O1.875, (4) (Sm0.5Ca0.5)0.175Ce0.825O1.87, (5) (Sm0.5Ca0.5)0.2Ce0.8O1.85, (6) (Sm0.5Ca0.5)0.225Ce0.775O1.84, (7) (Sm0.5Ca0.5)0.25Ce0.75O1.81, (8) (Sm0.75Sr0.2Ba0.05)0.175Ce0.825O1.891, (9) La0.125Ce0.875O1.94, (10) La0.15Ce0.85O1.925, (11) La0.175Ce0.825O1.91, (12) (La0.8Sr0.1)0.175 Ce0.825O1.90, (13) (La0.8Sr0.2)0.175Ce0.825O1.89, (14) (La0.77Sr0.2 Ba0.03)0.175Ce0.825O1.892, (15) (La0.75Sr0.2Ba0.05)0.175Ce0.825O1.891.
mol in (La0.75Sr 0.2Ba0.05) 0.175Ce0.825O 1.89. These results indicate that the suggested index is a useful tool for improvement of electrical conductivity. On the other hand, Table 1 summarizes the analyzed composition data in four representative examples. The chemical analysis showed that the sample composition was controlled well with only a very small deviation. It is concluded that the suggested index of all specimens can be calculated using the starting compositions of the specimens. Figs. 2 and 3 display high-resolution images and selected area electron diffraction patterns recorded from Sm0.2Ce0.8O1.9 and (La0.75Sr0.2Ba0.05)0.175Ce0.825 O1.89 sintered bodies, respectively. The big diffuse scatter clearly appeared in the background of electron diffraction patterns, although extra spots cannot be observed in it. This diffuse scatter indicates that these
Table 1 Chemical composition of specimens Sample
Chemical composition
Sm0.25Ce0.75O1.88 (Sm0.5Ca0.5)0.225Ce0.775 O1.84 La0.175Ce0.825O1.91(2) (La0.75Sr0.2Ba0.05)0.175 Ce0.825O1.891
Sm0.242Ce0.758O1.88 (Sm0.496Ca0.468)0.225 Ce0.783O1.84 La0.178Ce0.822O1.91(1) (La0.751Sr0.198Ba0.051)0.177 Ce0.823O1.890
Fig. 2. High-resolution image (h110iF) and selected area electron diffraction pattern (h110iF) recorded from Sm0.2Ce0.8O1.90 sintered body, dashed line area means micro-domain with ordered structure.
specimens have micro-domains with ordered structure and the interface between micro-domain and fluorite lattice is coherent in a grain. If the interface has a misfit, the extra spots will be seen in the diffraction pattern. Moreover, EDS analysis indicated that the grain boundaries in these specimens were very clean. The highresolution images indicate that the micro-domain has irregular shapes. While Sm0.2Ce0.8O1.9 sintered body has a relatively large micro-domain in the grain, the micro-domain size in (La 0 . 7 5 Sr 0 . 2 Ba 0 . 0 5 ) 0 . 1 7 5 Ce0.825O1.89 sintered body was much smaller. A large strain in the lattice of doped CeO2 would be created by doping lower valent cations on the Ce site. In order to minimize the distortion in the lattice, micro-domains form spontaneously in the lattice. Accordingly, it is concluded that the micro-domain size is minimized in the nondistorted fluorite structure. On the other hand, suggested index indicates the research direction to prepare a promising material (i.e. nondistorted fluorite structure) for high-quality electrolyte. Therefore, the
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gested index was higher than that of Sm0.2Ce0.8O1.9 with the low index. Accordingly, it is concluded that (La0.75Sr0.2Ba0.05) 0.175Ce0.825O1.89 would be less reducible than Sm0.2Ce0.8O1.9 in a humidified hydrogen atmosphere. Because of the high ionic conductivity and good reduction resistance, the maximum power density of (La0.75Sr0.2Ba0.05)0.175Ce0.825O1.89 reached 0.82 W/cm2 at 1000 jC. This value was over two times larger than that of Sm0.2Ce0.8O1.9. The power density of (La0.75Sr0.2Ba0.05)0.175Ce0.825O1.89 was 0.31 W/cm2 at 700 jC, which is sufficient for low-temperature application if we consider that the thickness of electrolyte was 0.5 mm. In this study, electrodes were not optimized in single-cell tests. Therefore, it is expected that the power density of
Fig. 3. High-resolution image (h110iF) and selected area electron diffraction pattern (h110iF) recorded from (La0.75Sr0.2Ba0.05)0.175 Ce0.825O1.891 sintered body, dashed line area means microdomain with ordered structure.
specimen with high index values such as (La0.75Sr0.2Ba0.05)0.175Ce0.825O1.89 sintered body has small microdomains and shows high conductivity. The authors expect that the specimen with interface between nanosize domain and fluorite lattice will show excellent electrolytic properties. Fig. 4a and b plots the potential and power density as a function of current density at cell temperature of 700, 800, 900 and 1000 jC using Sm0.2Ce0.8O1.9 and (La0.75Sr0.2Ba0.05)0.175Ce0.825O1.89 as the electrolytes, respectively. It is well known that CeO2 electrolytes exhibit n-type electronic conduction in the operating atmosphere of a fuel cell. The decrement of terminal potential corresponds to the n-type electronic conduction of reduced CeO2-based oxides. The terminal potential decreased with increasing operating temperature in both cases. However, the terminal potential of (La0.75Sr0.2Ba0.05)0.175Ce0.825O1.89 with high sug-
Fig. 4. Discharge performance of (a) Sm0.2Ce0.8O1.90 and (b) (La0.75Sr0.2Ba0.05)0.175Ce0.825O1.891 electrolyte. Small open symbols indicate the potential as a function of current density: (5) 1000, (D) 900, (o) 800, (q) 700 jC. Large closed symbols indicate the power density as a function of current density: (n) 1000, (E) 900, (.) 800, (z) 700 jC. Electrolyte thickness: 0.5 mm. Cathode: La0.9Sr0.1MnO3, anode: Ni/YSZ cermet (Ni/YSZ = 55:45 in volume ratio).
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(La0.75Sr0.2Ba0.05)0.175Ce0.825O1.89 could be further improved using the electrodes such as La1 xSrCoO3 cathode and Ni/CeO2 cermet anode.
4. Summary The effective index, as criterion for fast oxide ionic conduction in doped CeO2, was suggested using ionic radii and expected oxygen vacancy level in order to improve electrolytic properties. The relationship between microstructure and electrolytic properties was examined. The results obtained are summarized as follows; 1. Ionic conductivity increased with increasing suggested index. The activation energy of the specimens with high index value is lower than that of the specimens with low index value. Moreover, maximum power density of (La0.75Sr0.2Ba0.05)0.175Ce0.825O1.89 with high suggested index was higher than that of Sm0.2Ce0.8O1.9 with the low index. Accordingly, it is concluded that suggested index shows one of the future trends for development of high-quality electrolytes. 2. Selected area electron diffraction patterns recorded from sintered samples indicated the existence of micro-domains with ordered structure in doped CeO2 electrolytes. The interface between the microdomain and the fluorite lattice was coherent in a grain. The size of the micro-domain in Sm0.2Ce0.8O1.9 was much larger than that in (La 0.75 Sr 0.2 Ba 0.05 ) 0.175 Ce0.825O1.89. The control of domain size would be an important factor for development of improved CeO2 electrolytes. 3. The formation of micro-domains would be attributable to the lattice distortion in doped CeO2.
Since the suggested index indicates the composition of nondistorted CeO2-based oxide, the size of microdomains in (La0.75Sr0.2Ba0.05)0.175Ce0.825O1.89 with high suggested index would be minimized. 4. The size of micro-domain in (La 0.75Sr 0.2Ba0.05)0.175Ce0.825O1.89 would be controlled using advanced sol – gel processing. We also expect that the control of domain size in the lattice and the clarification of interface structure between microdomain and fluorite lattice will improve the electrolytic properties in (La0.75Sr0.2Ba0.05)0.175Ce0.825O1.89.
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