CYO–BZCYO composites with enhanced proton conductivity: Candidate electrolytes for low-temperature solid oxide fuel cells

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 3 0 4 4 e1 3 0 5 2

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CYOeBZCYO composites with enhanced proton conductivity: Candidate electrolytes for low-temperature solid oxide fuel cells Jianbing Huang a,*, Li Zhang b, Cheng Wang c, Ping Zhang b a

State Key Laboratory of Multiphase Flow in Power Engineering, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, PR China b Faculty of Material Science and Chemical Engineering, China University of Geosciences, Wuhan 430074, PR China c Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, PR China

article info

abstract

Article history:

Novel composite oxide ion conductors were developed based on a fluorite-type Ce0.8Y0.2O1.9

Received 20 December 2011

(CYO) and a perovskite-type BaZr0.1Ce0.7Y0.2O2.9 (BZCYO) synthesized by the carbonate co-

Received in revised form

precipitation route at 700
C and the solegel process at 1000
C, respectively. When sintered

9 May 2012

at 1400

Accepted 10 May 2012

1.21  102 S cm1 at 600
C, respectively, in dry air and wet hydrogen. CYOeBZCYO

Available online 12 June 2012

composites sintered at 1400
C showed much lower conductivity than CYO and BZCYO in




C, CYO and BZCYO showed an ac conductivity of 1.60  102 S cm1 and

dry air, but they showed enhanced conductivity in wet hydrogen. The highest conductivKeywords:

ities of 3.27  102 S cm1 at 500
C and 9.40  103 S cm1 at 400
C were achieved in wet

Ce0.8Y0.2O1.9(CYO)

hydrogen by the composite containing 30wt.% BZCYO, which are 3e5 times higher than

BaZr0.1Ce0.7Y0.2O2.9(BZCYO)

those of CYO and BZCYO, making this composite material a promising candidate as an

Composite electrolyte

electrolyte for low-temperature solid oxide fuel cells (LT-SOFCs).

Proton conductivity

Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

Low-temperature solid oxide fuel

reserved.

cells (LT-SOFCs)

1.

Introduction

To accelerate the commercialization of solid oxide fuel cell (SOFC) technology, enormous efforts have been made to reduce the operating temperature from a traditional high temperature of around 1000
C down to intermediate (600e800
C) and low (400e600
C) temperature range [1e3]. This reduced-temperature operation will allow easier cell & stack construction, enable cheaper materials to be used, and improve long-term cell stability and stack durability. By reducing the operating temperature to 500
C or below, due to its capability to start up rapidly, SOFC has the potential as

power source for electric vehicle applications in combination with energy storage devices, e.g. rechargeable batteries or electrochemical supercapacitors. However, electrolyte resistance and electrode polarization increase greatly with the decreasing temperature, which reduce the performance of SOFC. Improving the electrolyte performance for low temperature operation is achieved by reducing the electrolyte thickness and using new electrolyte materials with high ion (O2/Hþ) conductivity at low temperatures [4e6]. Fluorite-type ceria (CeO2) based oxides have been extensively studied as electrolyte materials for low-temperature solid oxide fuel cell (LT-SOFC) operating at 500e600
C and

* Corresponding author. Tel.: þ86 29 82665591; fax: þ86 29 82669033. E-mail address: [email protected] (J. Huang). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.05.040

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 3 0 4 4 e1 3 0 5 2

made much progress [7e10]. It has been reported that some singly doped ceria, such as Ce1xGdxO2d (CGO), Ce1xSmxO2d (CSO) and Ce1xYxO2d (CYO) show an oxygenion conductivity as high as 102 S cm1 at 600
C [4], corresponding to that of conventional yttria-stablized zirconia (YSZ) at 800
C. While doped ceria exhibits mixed ionicelectronic conducting behavior in the anodic environment due to the reduction of Ce4þ to Ce3þ. This reduction causes internal electronic transport in the electrolyte and expansion of the lattice, resulting in significant decrease in cell voltage and mechanical strength. The electronic conduction can be suppressed by lowering the operating temperature, and doped ceria becomes a predominantly ionic conductor below 450
C even in the fuel atmosphere [11], but its ionic conductivity needs further improvement for LT-SOFC application. Oxide proton conductors are also being applied as electrolytes for LT-SOFC because of their low activation energy for proton conduction. Many perovskite-type oxides show high proton conductivity in reducing and humid atmospheres at low temperature [12]. Doped barium cerate (BaCeO3) such as BaCe0.8Y0.2O2.9 (BCYO) possesses a proton conductivity of 102 S cm1 at 600
C, and it is a pure ion conductor not suffering from the reduction at low oxygen partial pressure. Ito et al. [13] had demonstrated the highest fuel cell performances at 400e600
C based on a 0.7 mm thick BCYO electrolyte. However, BaCeO3 is not chemically stable in an atmosphere containing CO2 and H2O, which makes it inadequate as electrolyte for fuel cell application, especially for use with hydrocarbon or syngas fuels. The chemical stability can be improved by the introduction of Zr to partially replace Ce, but the proton conductivity is normally reduced due to its poor sinterability [14]. BaZr0.1Ce0.7Y0.2O3d (BZCYO) achieves high proton conductivity, as well as sufficient chemical and thermal stability over the wide range of LT-SOFC operating conditions, which is regarded as candidate electrolyte for LTSOFC [15]. Two-phase composite electrolyte based on doped ceria phase and doped barium cerate phase has been successfully prepared using ceramic composite technology by Zhu et al. [16] for the first time and excellent fuel cell performances has been demonstrated below 600
C based on an approximately 0.3e0.5 mm thick electrolyte. It is found that doped barium cerate phase can block the internal electron current caused by the reduction of doped ceria phase and improve the total ion conductivity in SOFC operation [17]. This composite effect has been proved by some doped ceria-based composites. For example, doped ceriaecarbonate composites show hybrid oxygen-ion and proton conduction with extremely enhanced ion conductivity in fuel cell environment accounting for interfacial proton conduction [18]. In contrast, doped ceriadoped barium cerate composites have better thermal stability than doped ceriaecarbonate composites. However, the study on such functional two-phase composites based on oxide ion conductors for SOFC electrolytes is very limited. To our knowledge, Ce0.8Y0.2O1.9 (CYO) and BaZr0.1Ce0.7Y0.2O2.9 (BZCYO) composites have never been studied. These composites are expected to possess mixed ion conduction, enhanced ion conductivity, suppressed electron conductivity and improved chemical stability.

13045

In this work, novel CYOeBZCYO composites were prepared using ceramic composite technology and their electrical conductivities under dry air and wet hydrogen (3% H2O) atmospheres at 400e700
C were investigated to evaluate their feasibility as LT-SOFC electrolytes. The phase stability of CYOeBZCYO composites was studied to determine the sintered temperature.

2.

Experimental

2.1.

Materials preparation

Ce0.8Y0.2O1.9 (CYO) powders were synthesized by the carbonate co-precipitation route. Stoichiometric amounts of Ce(NO3)3$6H2O (A.R.) and Y2O3 (A.R.) were dissolved in distilled water and dilute nitric acid, respectively, then the Y(NO3)3 solution was added to the Ce(NO3)3 solution to form 0.5 M metal nitrate solution. According to the molar ratio of metal ion to carbonate ion in 1:2, a certain amount of Na2CO3 was dissolved in distilled water to form 0.5 M Na2CO3 solution. Then the metal nitrate solution was dropwise added into Na2CO3 solution under vigorous stirring to form a white precipitate at room temperature. The precipitate was vacuum-filtered, washed for three times with hot deionized water and anhydrous ethanol, followed by drying at 80
C for 12 h to obtain CYO precursor. Finally, the precursor was lightly crushed in an agate mortar and calcined at the temperature between 600
C and 800
C in air for 2 h to obtain pure CYO powders. BaZr0.1Ce0.7Y0.2O2.9 (BZCYO) powders were synthesized by the solegel process. The starting materials were Ba(NO3)2 (A.R.), ZrO2 (A.R.), Ce(NO3)3$6 H2O (A.R.), Y2O3 (A.R.) and EDTA (ethylenediamietetraacetic acid, A.R.). ZrO2 and Y2O3 were dissolved in nitric acid to form 1 M solution, respectively. Stoichiometric amounts of Ba(NO3)2 and Ce(NO3)3$6H2O were dissolved in distilled water, then the Zr(NO3)4 and Y(NO3)3 solutions were added into the above solution to form metal precursor solution with the molar ratio of Ba2þ:Zr4þ:Ce3þ:Y3þ as 1:0.1:0.7:0.2. The molar ratio of EDTA to the total metal ion was set at 2:1. Ammonium hydroxide (NH3 33%) was added to promote the dissolution of EDTA in deionized water, and adjust the pH value to 9 when the metal precursor solution was added dropwise to the EDTA solution. A polymeric gel was obtained after evaporating the water at 80
C under continuous stirring. The gel was heat-treated at 250
C in air for 6h, and then the resultant was ground thoroughly and calcined at the temperature between 800
C and 1200
C in air for 4 h to obtain pure BZCYO powders. CYOeBZCYO composite powders containing 20wt.%, 30wt.%, 50wt.% and 70wt.% BZCYO phase were prepared by mixing pure CYO and BZCYO powders in anhydrous ethanol, and dispersing the mixture by ultrasonic oscillation, then grinding the suspensions thoroughly in an agate mortar, followed by drying in an infrared oven. All composite powders and pure CYO and BZCYO powders were uniaxially pressed into pellets with 13 mm in diameter and 1 mm in thickness under a pressure of 300 MPa. Then the pellets were sintered in air at the temperature from 1200 to 1450
C for 5 h.

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2.2.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 3 0 4 4 e1 3 0 5 2

Characterization measurements

The crystal structure and phase purity of calcined CYO and BZCYO powders were characterized using an X-ray diffractometer (XRD, D8 Advance, Bruker AXS Corp., German) with CuKa radiation (l ¼ 0.15406 nm 40 kV and 100 mA). The densities of sintered CYO and BZCYO electrolyte pellets were measured by the standard Archimedes method. The surface morphology of CYO and BZCYO electrolyte pellets sintered at different temperatures were characterized using a scanning electron microscope (SEM, JSM-4500, JEOL, Japan). The phase stability of sintered CYOeBZCYO composite samples was also studied by XRD. The electrical conductivity of CYOeBZCYO composite electrolytes as well as pure CYO and BZCYO electrolytes was measured in dry air and wet hydrogen (3% H2O), respectively, using the standard two-probe ac impedance spectroscopy (IM6E, Zahner, Germany). Silver paste was painted onto either side of the electrolyte pellets and fired at 600
C for 1 h to act as electrodes. Impedance data were taken from 700
C to 400
C (50
C at intervals) and a frequency range of 0.1 Hze1 MHz using an excitation voltage of 10 mV.

calculated using the formula b2 ¼ b2m  b2s , where bm is the measured FWHM and bs (taken as 0.1704) is the FWHM of a standard silicon sample. The calculated particle sizes of CYO powders calcined at 600, 700 and 800
C are 18, 30, and 57 nm, respectively. Here, CYO powder calcined at 700
C for 2 h was chosen as one constituent phase for CYOeBZCYO composites. Fig. 2 shows the XRD patterns of BZCYO powders calcined at 800, 1000 and 1200
C, respectively. As evidence from the patterns, BaCO3 phase and fluorite phase were detected in BZCYO powder after calcination at 800
C. As the calcination temperature increased up to 1000
C, XRD showed pure BaCeO3 with orthorhombic perovskite structure as a result of the solid solution formation with Ce, Zr and Y statistically distributed in the lattice. It indicates that solegel process is effective to lower the phase formation temperature of BZCYO, compared with conventional solid-state reaction method [19]. The crystallite sizes of BZCYO particles are also calculated from FWHM data for (002) crystal face using Scherrer formula. The calculated particle sizes of BZCYO powders calcined at 800, 1000 and 1200
C are 15, 20, and 23 nm, respectively. Here, BZCYO powder calcined at 1000
C for 4 h was selected as the second phase for CYOeBZCYO composites.

3.2.

3.

Results and discussion

3.1.

Calcination of CYO and BZCYO powders

The surface microstructures of CYO electrolytes prepared from CYO powder calcined at 700
C and sintered at 1200, 1300 and 1400
C for 5 h, respectively, were observed by SEM, as shown in Fig. 3aec. Seen from Fig. 3a, the sintering necks were born at 1200
C between the neighboring particles, and the grain sizes ranged from about 0.2 to 1 mm. In the meantime, many small pores were distributed in the grain boundaries. When the sintered temperature increased to 1300
C (Fig. 3b), the grain sizes grew up to about 0.5e1.5 mm, and the pore size and number decreased significantly. The grain sizes increased to about 1.5e3 mm at the sintered temperature of 1400
C, and the surface structure became rather dense with the formation of isolated close pores. In general, large grain sized material showed a high conductivity, due to a significant increase in

o

Intensity (a.u.)

800 C

(400)

(311) (222)

(200)

(220)

(111)

Fig. 1 shows the XRD patterns of CYO powders calcined at 600, 700, and 800
C, respectively. These patterns confirm the formation of the cubic fluorite structure for all three samples. The degree of XRD peak broadening decreased with the increasing calcination temperature from 600 to 800
C, which indicates the increase of the crystallite size. The crystallite sizes of the particles are calculated from FWHM data for (111) crystal face using Scherrer formula D ¼ 0.9l/bcosq, where D is the crystallite size in nm, l the radiation wavelength (0.15406 nm in present case, Cu target), q the diffraction angle, and b is the corrected line width at half peak intensity, b can be

Sintering of CYO and BZCYO electrolytes

o

700 C o

600 C 10

20

30

40

50

60

70

2 (degree)

Fig. 1 e XRD patterns of CYO powders after being calcined at 600, 700, and 800
C for 2 h.

Fig. 2 e XRD patterns of BZCYO powders after being calcined at 800, 1000 and 1200
C for 5 h.

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Fig. 3 e SEM micrographs of the surface views of the CYO pellets sintered at (a) 1200
C, (b) 1300
C and (c) 1400
C, and the BZCYO pellets sintered at (d) 1300
C, (e) 1400
C, and (f) 1450
C for 5 h.

grain boundary conductivity (sgb), but a small decrease in grain interior conductivity (sgi) [20]. Thus, CYO electrolyte should be sintered at high temperatures to promote grain growth and reduce the number of defects and pores along grain boundaries in order to minimize grain boundary resistance. Shown in Fig. 3def are the surface views of BZCYO electrolytes prepared from BZCYO powder calcined at 1000
C and sintered at 1300, 1400, and 1450
C for 5 h, respectively. It can be observed in the micrograph (Fig. 3d) that the sample sintered at 1300
C was not fully dense. The grain size is estimated to be about 0.3 mm showing uniform distribution in the sintered body. With the increasing of sintered temperature, the grains grew up gradually due to the diffusion and transfer of cations, and the connected open pores were compressed and deformed to isolated close pores. At the sintering temperature of 1450
C (Fig. 3f), the number of pores on the surface of the sample decreased evidently, and the contact between adjacent particles became much tighter resulting in an average grain size of about 1 mm. But the sample sintered at 1450
C was still not fully densified. The effect of sintered temperature on the density of the CYO and BZCYO specimens from the corresponding powders calcined at 700
C and 1000
C, respectively, is illustrated in Table 1. The relative densities of CYO and BZCYO electrolytes were calculated according to the theoretical densities derived from the XRD data referring to the corresponding crystal structures and the densities measured by Archimedes method. As the sintered temperature increased, all sintered samples became increasingly dense. It is noted that a relative density of 95.1% was obtained by the CYO sample sintered at 1400
C, which is about 200
C lower than that by conventional

solid-state reaction method [21]. Low sintered temperature of electrolyte facilitates the selection of electrode materials for SOFC application and the reduction of cell fabrication cost. However, the BZCYO sample had comparatively low relative density, below 90% at 1450
C, due to low rates of grain growth. The poor sinterablity of BZCYO electrolyte will inevitably affect grain boundary resistance and gas permeability, which restricts its application in SOFC.

3.3. Electrical conductivity of sintered CYO and BZCYO electrolytes Fig. 4 shows the Arrhenius plot of the electrical conductivity in dry air for CYO electrolyte pellets from the powder calcined at 700
C sintered at 1200, 1300 and 1400
C for 5 h, respectively. An almost linear relationship is found in the measured temperature range of 400e700
C for each sample. It can be seen that the electrical conductivity of CYO electrolyte increases with the measured temperature studied. This trend is consistent with the behavior of ion conductors. Therefore, CYO electrolyte pellets sintered at different temperatures are proposed to be oxygen-ion conductors in air via the migration

Table 1 e The relative densities of the CYO and BZCYO pellets sintered at different temperatures. Sample

Relative density (%)


CYO BZCYO

1200 C

1300
C

1400
C

1450
C

69.8

85.3 66.1

95.1 80.7

89.3

13048

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 3 0 4 4 e1 3 0 5 2

o

o

t ( C)

t ( C) 600

4

550

500

750 700 650

400

3.5

1200 C o 1300 C o 1400 C

500

450

400

o

Wet hydrogen (3% H2O)

1300 C o

0.873eV 0.916eV

0

550

1400 C

3.0

2 1

600

4.0

o

Dry air

3 -1 ln( T) (Scm K)

450

-1 ln( T) (Scm K)

750 700 650

0.943eV

o

1450 C

2.5 0.391eV

2.0

0.381eV

1.5 1.0 0.378eV

-1

0.5 0.0

-2

1.0

1.0

1.1

1.2

1.3 -1 1000/T (K )

1.4

Fig. 4 e Arrhenius plot of the electrical conductivity in dry air for CYO electrolyte pellets from the powder calcined at 700
C sintered at 1200, 1300 and 1400
C for 5 h, respectively.

of oxygen vacancies. These vacancies are produced from the defect reaction (1) using Kro¨gereVink notation as follows: CeO2

 Y2 O3 ƒƒ! 2Y0Ce þ 3O O þ VO

1.1

1.5

(1)

Fig. 4 also exhibits an increase in the oxygen-ion conductivity with the sintered temperature. The CYO sample sintered at 1400
C shows the highest oxygen-ion conductivity, 0.016 S cm1 at 600
C, which is much higher than that of the CYO pellet prepared by the oxalate co-precipitation route and sintered at 1500
C for 5 h [20]. The increase in electrical conductivity with sintered temperature from 1200 to 1400
C is attributed to the increase in the grain size (Fig. 3aec) resulting in the decrease of grain boundary resistance. The activation energies for oxygen-ion conduction in CYO electrolytes sintered at different temperatures are calculated from the Arrhenius equation s ¼ A/Texp(Ea/kT ). It is seen from Fig. 4 that the activation energy (Ea) for oxygen-ion conduction decreases from 0.943 eV for CYO sample sintered at 1200
C to 0.873 eV at 1400
C, indicating that sintering at 1400
C is beneficial for ion conduction at much lower temperature. Note that the CYO sample sintered at 1400
C shows comparable activation energy with the CYO samples prepared by citric acidenitrate combustion process [22], but the former has higher oxygen-ion conductivity than the latter. Fig. 5 shows the Arrhenius plot of the electrical conductivity in wet hydrogen (3% H2O) for BZCYO electrolytes from the powder calcined at 1000
C sintered at 1300, 1400 and 1450
C, respectively. An approximately linear relationship is also found in the measured temperature range for each BZCYO sample and the electrical conductivity increases with the measured temperature, indicating that the sintered BZCYO electrolyte pellets are pure proton conductors in wet hydrogen atmosphere. As to AⅡBⅣO3 perovskite-type oxides, oxygen vacancies are introduced into the perovskite structure

1.2

1.3 -1 1000/T (K )

1.4

1.5

Fig. 5 e Arrhenius plot of the electrical conductivity in wet hydrogen (3% H2O) for BZCYO electrolyte pellets from the powder calcined at 700
C sintered at 1300, 1400 and 1450
C for 5 h, respectively.

by aliovalent doping. The doping reaction for an acceptor Y on the B site of BaCeO3 in dry atmospheres during the synthesis process can be written as follows: CeO2

 0  Y2 O3 þ 2BaO ƒƒ! 2Ba Ba þ 2YCe þ 5OO þ VO

(2)

When the sample is sintered under oxygen-containing atmospheres the reaction (3) may occur 1   V O þ O2 ðgÞ5OO þ 2h 2

(3)

In dry hydrogen, the proton can be produced in the form of hydroxyl according to the reaction (4) 

 H2 þ 2O O þ 2h 52OHO

(4)

In wet hydrogen, the proton conduction may appear due to incorporation of water into the sample to form hydroxyl species according to the reactions (5) and (6) 1   H2 OðgÞ þ 2O O þ 2h 52OHO þ O2 ðgÞ 2

(5)

  H2 OðgÞ þ O O þ VO 52OHO

(6)

It is believed that protons can migrate by hopping from the OHO site to O O site nearby causing this material to exhibit the proton conductivity. From Fig. 5, it is seen that the proton conductivity of BZCYO samples increases with the sintered temperature from 1300
C to 1450
C. This agrees with the SEM analysis (Fig. 3def) that showed an increasing grain size with increasing sintered temperature. A larger grain size leads to a smaller grain boundary surface, resulting in higher electrical conductivity for the samples with increasing sintered temperature. At 600
C, the proton conductivity values were measured at 7.89  103 S cm1 for the sample sintered at 1300
C, increasing up to 1.21  102 S cm1 at 1400
C and 1.28  102 S cm1 at 1450
C. From the fitted Arrhenius curves

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3.4. Phase stability of CYOeBZCYO composite electrolytes

o

t ( C) 750 700 650

3.5. Electrical conductivity of CYOeBZCYO composite electrolytes Fig. 7 presents the Arrhenius plots of the electrical conductivity in dry air for CYOeBZCYO composite electrolytes containing 20wt.% BZCYO sintered at 1300, 1400 and 1450
C, respectively. It is obvious that all the fitted Arrhenius curves can be separated into two straight lines at about 500
C for

500

450

400

0.752eV

2 -1

0.751eV 0.672eV

1

0.718eV Dry air

0

o

1300 C o 1400 C o 1450 C

0.631eV

-2 1.0

1.1

1.2

1.3

1.4

1.5

-1

1000/T (K )

Fig. 7 e Arrhenius plots of the electrical conductivity in dry air for CYOeBZCYO composite electrolytes containing 20wt.% BZCYO sintered at 1300, 1400 and 1450
C, respectively.

each sample sintered from 1300
C to 1450
C, indicating that the conduction mechanism of the composite alters at this temperature. Ceria phase exhibits pure oxygen-ion conduction in dry air. For doped barium cerate in dry air, oxygen-ion conductivity is suggested to be dominant at low temperatures, while electron hole conductivity is suggested to be dominant at elevated temperatures [24]. Thereby, the composite acts as oxygen-ion conductor at lower temperatures and mixed oxygen-ion and electron hole conductor at higher temperatures in dry air. It can be seen in Fig. 7 that the composite electrolyte sintered at 1400
C showed the highest electrical conductivity compared with the samples sintered at 1300
C and 1450
C in the measured temperature range, which can be attributed to the comprehensive effect of grain sizes and impurities. On one hand, the grain sizes of both CYO phase and BZCYO phase will increase with increasing sintered temperature, leading to higher electrical conductivity. On the other hand, the increasing content of impurities including Ba2YZrO6 and Ba2ZrO4 may cause the deterioration of ion conduction especially in the interfacial regions. Fig. 8 shows the Arrhenius plots of the electrical conductivity in wet hydrogen (3% H2O) for CYOeBZCYO composite electrolytes containing 20wt.% BZCYO sintered at 1300, 1400 and 1450
C, respectively. It is evident that the conduction mechanism alters at about 600
C in wet hydrogen atmosphere. It is well known that electron conduction can be introduced into doped ceria at low oxygen partial pressure and high temperature according to the reaction (7). 1  0 O O 52e þ VO þ O2 2

Fig. 6 e XRD patterns of CYOeBZCYO composite electrolytes containing 20wt.% BZCYO sintered at 1300, 1400, and 1450
C for 5 h, respectively.

550

0.772eV 3

-1

Fig. 6 shows the XRD patterns of CYOeBZCYO composite electrolytes containing 20wt.% BZCYO sintered at 1300, 1400, and 1450
C for 5 h, respectively. The diffraction pattern of the composite sample sintered at 1300
C consisted of two phases: the pure fluorite phase from CYO and the pure perovskite phase from BZCYO. After sintered at 1400
C for 5 h, a minor of additional Ba2YZrO6 phase (JCPDS Card Number 47-0385) and Ba2ZrO4$2.15H2O phase (JCPDS Card Number 43-0653) is detected, indicating that phase segregation and interdiffusion arose in the two-phase system during high temperature sintering and hygroscopic absorption occurred when exposed in ambient atmosphere. When the sintered temperature increased to 1450
C, the proportion of Ba2YZrO6 cubic phase in the composite electrolyte increased significantly, in contrast, the proportions of CeO2 fluorite phase and BaCeO3 perovskite phase decreased.

600

4

ln( T) (Scm K)

shown in Fig. 5, the activation energies are 0.378 eV, 0.381 eV, and 0.391 eV, respectively when the sintered temperature of BZCYO electrolyte increases from 1300
C to 1450
C. The proton conductivity for BZCYO sample sintered at 1400
C in this study are much higher than that for BZCYO samples prepared by gel-casting and sintered at 1550
C [23] in the measured temperature range of 400e700
C, and the activation energy for the former is much lower than that for the latter, although the relative density of the former (80.7%) is lower than that for the latter (95%).

(7)

In wet hydrogen environment, ceria phase exhibits a mixed oxygen-ion, proton and electron conduction [25], and barium cerate phase exhibits pure proton conduction. At lower

13050

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 3 0 4 4 e1 3 0 5 2

o

o

t ( C) 750 700 650

600

550

t ( C) 500

450

400

750 700 650

6

4 3

0.439eV

5

0.462eV

-1

ln( T) (Scm K)

-1

ln( T) (Scm K)

0.853eV Wet hydrogen (3% H2O) o

1450 C

1

500

0.930eV

0

0.652eV

-1 0.607eV

-2

x =0wt.% x =20wt.% x =30wt.% x =50wt.% x =70wt.% x =100wt.%

-4

o

1300 C

-5 -1

1.1

1.2

(1-x)CYO-x BZCYO 0.416eV

-3

1400 C

1.0

400 Dry air

0.580eV

0.705eV

o

0

450

1

0.756eV

3

2

550

2

4

0.580eV

600

0.873eV 0.756eV

1.3

1.4

-6

1.5

1.0

-1

1.1

1.2

1000/T (K )

temperatures (below 600
C), the electron conduction of CYO phase is not significant and it can be blocked by the pure proton conductor BZCYO, thus CYOeBZCYO composite containing 20wt.% BZCYO shows hybrid oxygen-ion and proton conduction in this condition. The composite sample sintered at 1400
C has the lowest activation energy 0.756 eV below 600
C, which is higher than that of pure BZCYO in wet hydrogen atmosphere. At higher temperature (above 600
C), the electron conduction becomes innegligible, thus the composite is a mixed ion-electron conductor. Among the three composite samples, the sample sintered at 1400
C still exhibits the highest electrical conductivity in wet hydrogen. This confirms that the presence of impurities will deteriorate the electrical conductivity of the composite. Hence, the appropriate sintered temperature is set at 1400
C for CYOeBZCYO composite electrolytes. It is seem from Figs. 7 and 8 that the composite sample shows much higher electrical conductivities in wet hydrogen than in dry air at the measured temperature range of 400e700
C. In dry air, the conductivity is only 1.03  102 and 7.0  104 S cm1 at 600
C and 400
C, respectively; while it increases to 8.49  102 and 5.9  103 S cm1 in wet hydrogen at the same temperatures, respectively. Similar result has been reported by Sun et al. [26] for the BaCe0.8Sm0.2O3deCe0.8 Sm0.2O2d composite (weight ratio 1:1); however, this composite electrolyte shows much lower conductivity than the BaZr0.1Ce0.7Y0.2O3deCe0.8Y0.2O2d composite (weight ratio 1:4) in this study either in dry air or in wet hydrogen atmosphere. Fig. 9 shows the electrical conductivity measured in dry air as a function of temperature for CYOeBZCYO composite electrolytes with different compositions sintered at 1400
C for 5 h. It is clear from Fig. 9 that the activation energies decrease gradually with the increase of BZCYO content in the CYOeBZCYO system, but the conductivities of CYOeBZCYO

1.4

1.5

Fig. 9 e Electrical conductivity measured in dry air as a function of temperature for CYOeBZCYO composite electrolytes with different compositions sintered at 1400
C for 5 h.

composite electrolytes are much lower than those of pure CYO and BZCYO electrolytes at elevated temperatures. This may be caused by the increasing amount of impurities including Ba2YZrO6 and Ba2ZrO4 in the composites with increasing BZCYO content. For the composites containing 20wt.% and 30wt.% BZCYO, both CYO phase and BZCYO phase can conduct oxygen-ion via oxygen vacancies. Moreover, consecutive two-phase interfaces can be formed in the matrix of CYO phase which facilitates the interfacial conduction, despite of the presence of small amount of impurities. o

t ( C) 750 700 650

600

550

500

450

400

6 0.439eV

5

0.414eV

Wet hydrogen (3% H2O) (1-x)CYO-x BZCYO

4 3 0.895eV -1 ln( T) (Scm K)

Fig. 8 e Arrhenius plots of the electrical conductivity in wet hydrogen (3% H2O) for CYOeBZCYO composite electrolytes containing 20wt.% BZCYO sintered at 1300, 1400 and 1450
C, respectively.

1.3 -1

1000/T (K )

0.579eV 0.756eV

2 1 0.399eV 0

0.525eV 0.381eV 0.711eV x =0wt.% x =20wt.% x =30wt.% x =50wt.% x =70wt.% x =100wt.%

-1 -2 -3 -4 -5 -6 1.0

1.1

1.2

1.3

1.4

1.5

-1

1000/T (K )

Fig. 10 e Electrical conductivity measured in wet hydrogen (3% H2O) as a function of temperature for CYOeBZCYO composite electrolytes with different compositions sintered at 1400
C for 5 h.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 3 0 4 4 e1 3 0 5 2

Fig. 10 shows the electrical conductivity measured in wet hydrogen (3% H2O) as a function of temperature for CYOeBZCYO composite electrolytes with different compositions sintered at 1400
C for 5h. At low temperatures (<600
C), the activation energies decrease gradually with the increasing of BZCYO content, but at high temperatures (600
C), the difference in activation energy for the CYOeBZCYO composite electrolytes is not marked. Composition dependences of conductivity measured in wet hydrogen for the CYOeBZCYO system are also illustrated in Fig. 10. With increasing amount of BZCYO, the conductivity of the composite electrolytes firstly increases to a maximum value then decreases sharply especially when the BZCYO content exceeds 50wt.%. The minimum conductivity is obtained by the composite electrolyte containing 70wt.% BZCYO, and the maximum conductivities are achieved by the composite electrolyte containing 20wt.% BZCYO at 550e700
C and the composite electrolyte containing 30wt.% BZCYO at 400e500
C. Comparing the composite electrolyte containing 30wt.% BZCYO with pure BZCYO, the conductivity is greatly enhanced at low temperatures. The highest conductivities of 3.27  102 S cm1 and 9.40  103 S cm1 are achieved at 500
C and 400
C, respectively, which are about 3e5 times higher than those of pure CYO and BZCYO, implying the composite effect is functioned in the CYOeBZCYO system. The composite effect can be ascribed to the interfacial proton conduction via oxygen vacancy or/and electron hole. Further studies will be carried out to explore the detailed conduction mechanism in the CYOeBZCYO system.

4.

Conclusion

Novel composite electrolytes were developed based on a fluorite-type CYO synthesized by the carbonate co-precipitation route at 700
C and a perovskite-type BZCYO synthesized by the solegel process at 1000
C. The appropriate sintered temperature for the composite electrolyte should not exceed 1400
C to maintain phase stability and high conductivity. Compared with CYO and BZCYO, the composite electrolytes showed much lower conductivity in dry air, however, they showed greatly enhanced conductivity in wet hydrogen (3% H2O). The highest conductivities of 3.27  102 S cm1 and 9.40  103 S cm1 were achieved at 500
C and 400
C in wet hydrogen by the composite containing 30wt.% BZCYO, indicating that this highly proton-conductive composite is a promising candidate as electrolyte for LT-SOFCs.

Acknowledgments This work was financially supported by the National Natural Science Foundation (NSFC) of China (No. 50902083), the NSFC Fund for Creative Research Groups (No. 50821064), the National Basic Research Program of China (No. 2012CB215401) and the fundamental research funds for the central universities.

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