Gd3+ and Sm3+ co-doped ceria based electrolytes for intermediate temperature solid oxide fuel cells

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Gd

3þ

and Sm

3þ

co-doped ceria based electrolytes for

intermediate temperature solid oxide fuel cells

Feng-Yun Wang

a,b

, Songying Chen

a,c

, Sooﬁn Cheng

a,*

a_{Department of Chemistry, National Taiwan University, Taipei 106, Taiwan} b_{Chemistry Department, Foshan University, Foshan 528000, PR China} c_{Institute of Catalysis, Zhejiang University, Hangzhou 310028, PR China}

Received 18 May 2004; accepted 21 May 2004 Available online 15 June 2004

Abstract

Co-doped ceria of Ce1aGdaySmyO20:5a, wherein a¼ 0:15 or 0.2, 0 6 y 6 a, were prepared for intermediate temperature solid oxide fuel cells (ITSOFCs). Their structures and ionic conductivities were characterized by X-ray diffraction and AC impedance spectroscopy. All the electrolytes were found to be ceria based solid solutions of fluorite type structures. However, co-doping effect was observed more apparent for the electrolytes with a¼ 0:15 than for those with a ¼ 0:2. In comparison to the singly doped ceria, the co-doped ceria of Ce0:85Gd0:15ySmyO1:925, wherein 0:05 6 y 6 0:1, showed much higher ionic conductivities at 773–973 K. These co-doped ceria are more ideal electrolyte materials of ITSOFCs.

Keywords: Doped ceria electrolyte; Co-doping eﬀect; Conductivity; Solid oxide fuel cells

1. Introduction

Doped ceria have been considered as one of the most promising electrolyte materials for intermediate tem-perature solid oxide fuel cells (ITSOFCs) [1,2]. These materials demonstrate much higher ionic conductivity at relatively lower temperatures in comparison to that of yttrium-stabilized zirconia (YSZ). Among the various

dopants been studied, Gd3þ and Sm3þ singly doped

ceria (abbreviated as CGO and CSO) were reported to have the highest conductivity [1,2]. Besides, many studies [3–5] have been carried out on co-doped ceria. However, controversial results on co-doping eﬀect were reported. For example, Herle et al. [3] found that co-doped ceria with 3, 5, or 10 dopants showed signiﬁcant higher ionic conductivity (by 10–30%) than the best singly doped materials, whereas, Yoshida et al. [5,6]

found that a doubly doped ceria with La3þ_{and Y}3þ_did

not show any synergistic eﬀects on ionic conductivity.

Since CGO and CSO are believed the most conductive

electrolytes [1,2], Gd3þ _{and Sm}3þ _{co-doped ceria are}

probably good and even better electrolytes. However, there is still a lack of study reported on these materials.

In this work, Gd3þ _{and Sm}3þ _{co-doped ceria materials}

were prepared and characterized. The eﬀect of co-dop-ing on structure and conductivity was studied in

com-parison to singly doped ceria. High conductive

electrolytes were found.

2. Experimental

The starting materials were the nitrate salts of reagent grade (Acros) and used as purchased. The aqueous

so-lutions of each metal ion of Ce3þ_{, Gd}3þ_{, and Sm}3þ_were

prepared by dissolving the nitrate salt in distilled water and diluting them to desired concentrations. An aque-ous solution of citric acid (CA) and polyethylene glycol

(PEG) in a weight ratio of CA/PEG¼ 60 was also

pre-pared and was termed as CP solution. According to the

composition (Ce1aGdaySmyO2y, wherein a¼ 0:15 or

0.2, 0 6 y 6 a) of the electrolyte samples, diﬀerent *

Corresponding author. Tel.: 8017; fax: +886-2-2363-6359.

E-mail address:[email protected](S. Cheng).

Electrochemistry Communications 6 (2004) 743–746

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volumes of metal ion solutions were taken and mixed in a beaker. Then, CP solution was added until the molar number of citric acid was equal to the total number of the metal ions in the beaker. The mixed solution was evaporated under stirring at 353 K until the solution gelled. The gel was dried at 378 K, ground and calcined in air at 973 K for 4 h, and then ground again to form calcined powder. The powder was uniaxially pressed under 750 MPa into green pellets using a stainless steel die with 13 mm diameter. The green pellets were further sintered at 1773 K for 14 h to form dense pellets.

For structure analysis, the dense pellets were ground to powder again and were identiﬁed at room tempera-ture using a PANalytial X-ray diﬀractometer (XRD) (Cu Ka radiation, 45 kV, 40 mA). For conductivity measurement, Ag paste was brushed onto both sides of the dense pellet, and then sintered at 1073 K for 30 min to form Ag electrodes. Pt leads were attached to the electrodes using Ag paste and were sintered again at 1073 K for 30 min. Impedance was measured using the two probe method with a Autolab Impedance Analyzer over the frequency range of 0.01 Hz–1 MHz and with a voltage amplitude of 30 mV. The measurements were taken at constant temperatures within 473–973 K and in air (25 sccm).

3. Results and discussion 3.1. Crystal structures

For all the ceria based electrolyte samples of this work, it was found that the doped ceria pellet samples (sintered at 1773 K) showed the same XRD patterns as the powder samples ( just calcined at 973 K) of the same compositions, except that the XRD peaks were much sharper for the former than for the latter, and that the XRD patterns of the doped ceria samples are diﬀerent

from those of pure Gd2O3 and Sm2O3, but similar to

that of pure CeO2. These results indicate that the doped

ceria samples are all solid solutions of ﬂuorite type structures that were formed in the calcining process and crystallized better in the sintering process.

More accurate XRD measurement with quartz as inner standard showed that the 2h values of the doped ceria shift slightly toward lower angle in comparison to

that of pure CeO2. This is because the ionic radius

de-creases in the order of Sm3þ>Gd3þ>Ce4þ, the

substi-tution of Ce4þ with Sm3þ and Gd3þ in the lattice of

CeO2 would enlarge the crystal lattice. As shown in

Fig. 1, the lattice constant of Ce1yGdyO20:5x and

Ce1ySmyO20:5yincreased linearly with y, but the slope

is lower for the former (0.0813) than for the latter

(0.134) because Gd3þ is smaller than Sm3þ. For the

same reason, the lattice constant of Ce0:85Gd0:15y

SmyO1:925 and Ce0:8Gd0:2ySmyO1:9 both increased

linearly with y, and for the same y, the lattice constant

was smaller for Ce0:85Gd0:15ySmyO1:925 than for

Ce0:8Gd0:2ySmyO1:9. The results in Fig. 2 follows

Ve-gard’s rule [7], further suggesting that all the doped ceria samples of this work are ceria based solid solutions. 3.2. Conductivities

It was documented that the main contribution of the conductivity of ceria based compounds in air was oxide ionic conductivity (>99.5%) and that from electronic conductivity was negligible [2,8]. In this paper, the conductivity measured in air was treated as the oxide ionic conductivity only. In the following context, the word ‘‘conductivity’’ is used to represent the total con-ductivity of grain and grain boundary.

Fig. 2. shows the conductivities of Ce1xGdxO20:5x,

wherein 0:1 6 x 6 0:2 in air and at 473–973 K in the form

0.00 0.05 0.10 0.15 0.20 5.410 5.415 5.420 5.425 5.430 5.435 5.440

Lattice Constant (A)

y (molar fraction) Ce 1-ySmyO2-0.5y Ce_1-yGd_yO_2-0.5y Ce 0.85Gd0.15-ySmyO1.925 Ce_0.85Gd_0.15-ySm_yO_1.925

Fig. 1. Dependence of lattice constant on the composition (y) of dif-ferent doped ceria electrolytes (sintered at 1773 K for 14 h).

0.10 0.12 0.14 0.16 0.18 0.20 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Ln(Sigma.T/S.cm -1.K) Gd doping, x 673K 773K 873K 973K

Fig. 2. Eﬀect of Gd doping (x) on the conductivity of pellet samples with nominal composition of Ce1xGdxO20:5xin air and at diﬀerent

temperatures.

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of ln(r T) vs. x. It can be seen that the maximum

con-ductivity emerged at x¼ 0:15 when temperature P 773

K, but at x¼ 0:1 when temperature <773 K. At 773–973

K, the operating temperature of ITSOFC, the samples

with x¼ 0:15 and 0.2 showed higher conductivity than

that with x¼ 0:1. Therefore, in the following co-doping

eﬀect study, total dopant content was conﬁned to 0.15 and 0.2.

In order to investigate co-doping eﬀect and at the same time avoid the eﬀect of oxide ion vacancy con-centration, a set of samples with nominal composition of Ce0:85Gd0:15ySmyO1:925, wherein 0 6 y 6 0:15, were

prepared and studied. The conductivities of these sam-ples in air and at diﬀerent temperatures were shown in Fig. 3 in the form of ln(r T) vs. y. It can be seen that co-doped samples showed apparently higher conductivities than singly doped samples. This suggests that co-doping eﬀect exists. At 973 K, the conductivity of the co-doped

samples reached the maximum of 0.0475 S cm1, higher

than the best results in the literatures for CGO (0.0316

S cm1) [9] and CSO (0.041 S cm1) [10].

Similarly, another set of samples with nominal

com-position of Ce0:8Gd0:2ySmyO1:9, wherein 0 6 y 6 0:2,

were prepared and studied. The conductivities of these samples in air and at different temperatures were shown in Fig. 4 in the form of ln(r T) vs. y. It can be seen that the conductivities of the co-doped ceria were higher than those of the singly doped ceria when temperature 6 773 K, but between those of the singly doped ceria when temperature >773 K. This suggests that co-doping effect exists here only when temperature 6 773 K. Similarly, the changes of co-doping effect in different temperature

ranges were also observed on Ce0:9Sm0:03Y0:03Ox, and

ceria doped with La3þand Y3þ [3–6].

Co-doping eﬀect has been discussed in terms of con-ﬁgurational entropy [11]. Comparing with singly doped

ceria solid solution, co-doped ceria solid solution had larger conﬁgurational entropy, and therefore had higher ionic conductivity. However, this theory can not explain

the fact that Ce0:85Gd0:1Sm0:05O1:925 and Ce0:85Gd0:05

Sm0:1O1:925 has similar conﬁgurational entropy, but the

former showed higher conductivity then the latter (see

Fig. 3), and that Ce0:8Gd0:15Sm0:05O1:9 had higher

con-ﬁgurational entropy than Ce0:85Gd0:1Sm0:05O1:925, but

the former showed lower conductivity than the latter (see Figs. 3 and 4). It has been proved that the ionic conductivity of doped ceria is related to the lattice dis-tortion [1,2]. Therefore, the co-doping eﬀect must be related not only to conﬁgurational entropy but also to

the lattice distortion away from pure CeO2. Because

Ce0:85Gd0:1Sm0:05O1:925has a lattice constant more close

to pure CeO2 than Ce0:85Gd0:05Sm0:1O1:925 (as shown in

Fig. 1), the former has higher ionic conductivity than the

latter. For the same reason, Ce0:85Gd0:1Sm0:05O1:925has

higher ionic conductivity than Ce0:8Gd0:15Sm0:05O1:9.

4. Conclusions

Co-doped ceria with nominal composition of

Ce1aGdaySmyO20:5a, wherein a¼ 0:15 or 0.2,

0 6 y 6 a, were prepared and studied in comparison to singly doped ceria on the structure and conductivity. The crystal structures of all the samples were ﬂuorite type ceria based solid solutions. Co-doping eﬀect was

observed more apparent for the samples with a¼ 0:15

than for those with a¼ 0:2. In comparison to the

singly-doped ceria, the co-singly-doped ceria of Ce0:85Gd0:15y

-SmyO1:925, wherein 0:05 6 y 6 0:1, showed much higher

ionic conductivities at 773–973 K. These co-doped ceria are more ideal electrolyte materials of ITSOFCs. -0.02 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 -3 -2 -1 0 1 2 3 4 ln(sigma.T/S.cm -1.K)

Sm doping, y (molar fraction) 673K 773K 873K 973K

Fig. 3. Eﬀect of Sm doping (y) on the conductivity of pellet samples with nominal composition of Ce0:85Gd0:15ySmyO1:925 in air and at

diﬀerent temperatures. 0.00 0.05 0.10 0.15 0.20 -4 -3 -2 -1 0 1 2 3 Ln(Sigma.T/S.cm -1.K)

Sm doping, y (molar fraction) 673K

773K 873K 973K

Fig. 4. Eﬀect of Sm doping (y) on the conductivity of pellet samples with nominal composition of Ce0:8Gd0:2ySmyO1:9in air and at

dif-ferent temperatures.

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Acknowledgements

The ﬁnancial support from CTCI Foundation, Tai-wan is gratefully acknowledged. The authors also thank Prof. Ben-Zu Wan, Department of Chemical Engineer-ing, National Taiwan University, for providing equip-ments on AC impedance measurement.

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