Composites Science and Technology 70 (2010) 181–185
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Composites Science and Technology
journal homepage: www.elsevier.com/locate/compscitech
Ceria (Sm3+, Nd3+)/carbonates composite electrolytes with high electrical
conductivity at low temperature
Wei Liu a, Yanyi Liu b, Bin Li a, Taylor D. Sparks c, Xi Wei a, Wei Pan a,*
a
State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
Department of Materials Science and Engineering, University of Washington, WA, USA
c
Material Department, College of Engineering, University of California, Santa Barbara, CA 930106-5050, USA
b
a r t i c l e
i n f o
Article history:
Received 31 March 2009
Received in revised form 11 August 2009
Accepted 14 October 2009
Available online 20 October 2009
Keywords:
A. Ceramic–matrix composites (CMCs)
B. Electrical properties
E. Sol–gel methods
a b s t r a c t
Composite electrolytes composed of Sm3+ and Nd3+ co-doped ceria (SNDC) and binary carbonates
(Li2CO3–Na2CO3) were investigated with respect to their microstructure, morphology and electrical conductivity. As a function of temperature, the electrical conductivity of the composite electrolytes was measured in air. The addition of (Li/Na)2CO3 to SNDC enhanced the high temperature conductivity. The
conductivity also rose sharply around the intenerating and melting point of carbonates. The best performance of 0.01 S/cm at 481 °C was achieved for the composite electrolyte containing 20 wt.% carbonates.
It is also estimated that the reason for the conductivity enhancement of SNDC is that the number of
oxygen transfer routes increases at the interface between SNDC and (Li/Na)2CO3.
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
As one of the clean and the most efficient energy conversion
devices, solid oxide fuel cells (SOFCs), have attracted much attention and developed rapidly in recent years [1–3]. Tremendous effort has been focused on lowering the operating temperature of
the SOFCs in order to attain significant benefits, such as reduced
contact resistance between electrodes and electrolyte, elevated
stability of the cell, decreased fabrication cost and increased potential for mobile applications [4–7]. Given the difficulty in maintaining good conductivity at low temperature in existing materials, an
alternate solution to the problem is to decrease the thickness of the
electrolyte or use a highly ionic conducting electrolyte [1].
Doped ceria (DCO) has been regarded as one of the most
promising electrolyte materials for intermediate-temperature (IT,
600–800 °C) or low-temperature (LT, 300–600 °C) SOFCs, as its
activation energy for oxygen diffusion is much lower than that of
yttria stabilized ZrO2 (YSZ) [8–12]. Among the DCO materials,
Gd3+ and Sm3+ singly doped ceria were found to have the highest
ionic conductivity [8,9,11–14]. However, as a consequence of the
reduction of Ce4+ to Ce3+, this group of electrolyte materials shows
a mixed ionic-electronic conducting behavior at high temperature
or in a reducing atmosphere, resulting in a significant decrease in
voltage, power output and efficiency of the cell [13,15]. One solution to this problem is to enhance the ionic conductivity through
the use of multi- or co-dopants [14–16]. Herle et al. [17] found that
* Corresponding author. Tel.: +86 10 6277 2858; fax: +86 10 6277 1160.
E-mail address: [email protected] (W. Pan).
0266-3538/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.compscitech.2009.10.006
co-doped ceria with multi-dopants had significantly higher ionic
conductivity in air than the best singly doped ceria. First principle
modeling and calculations done on the co-doped ceria by Andersson et al. [18] indicated that Nd3+/Sm3+ or Pr3+/Gd3+ co-doped ceria
which had an effective atomic number between 61 (Pm) and 62
(Sm) should possess the highest electrical conductivity. These
promising compositions predicted by Andersson et al. have been
authenticated by Omar et al. [19,20] and Liu et al. [10] experimentally. It is believed that co-doping could suppress the ordering of
oxygen vacancies and increase the pre-exponential factor in the
Arrhenius relationship [19–21], thereby could decrease the association enthalpy between the oxygen vacancies and the doping cations [22], lower the activation energy and enhance the ionic
conductivity. In addition, the increase of configuration entropy
[20–22], modification of elastic strain in the crystal lattice
[11,22] and change of the grain boundary composition [20,22]
may be the sources of the improvement of the conductivity.
Another method to overcome the problem caused by ceria
reduction is to use functional composite materials based on doped
ceria and various salts (carbonates, sulphates, halides or hydrates)
[4,5]. It was shown that the two-phase electrolytes, found to be coionic (O2/H+) conductors during fuel cell operation under the H2/
air atmosphere, displayed high ionic conductivity of 0.01–1 S/cm in
the IT/LT region [4]. Doped ceria/carbonate is one of the most typical composite solid electrolytes [6,23–29]. Wang et al. [30] fabricated the core–shell nanocomposite materials consisting of
Sm2O3 doped ceria core and amorphous Na2CO3 shell for the first
time, and found super-ionic conductivity of 0.1 S/cm (300 °C).
However, the conduction mechanism is still unclear.
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W. Liu et al. / Composites Science and Technology 70 (2010) 181–185
In our previous work, Liu et al. [10] have successfully obtained
Sm3+ and Nd3+ co-doped ceria powder through the combustion of a
citrate/nitrate gel, and found that the electrical conductivity of sintered Ce0.8Sm0.1Nd0.1O1.9 (SNDC) could reach 0.012 S/cm at 500 °C
and the activation energy was determined to be 0.63 eV. In this paper, based on our former work, SNDC/(Li/Na)2CO3 composite electrolytes were prepared in order to increase the ionic conductivity
further at lower temperatures by the citric-nitrate combustion
synthesis route and co-fired process. Additionally, the effects of
carbonate content on the phases, structure and electrical conductivity of SNDC/(Li/Na)2CO3 composite electrolytes have been
studied.
2. Experimental
Sm3+ and Nd3+ co-doped ceria (SNDC, Ce0.8Sm0.1Nd0.1O1.9) powders prepared by the citric-nitrate combustion process [10] were
mixed with Li2CO3 (98.0 wt.%, Guangdong Xilong Chemical Co.,
Ltd., China) and Na2CO3 (99.8 wt.%, Beijing Modern Eastern Fine
Chemicals Co., Ltd., China). The molar ratio of Li2CO3 and Na2CO3
is 2:1. The mixtures were milled thoroughly with alcohol medium,
and subsequently dried in the oven at 100 °C for 24 h. The dried
powders were then heat-treated at 680 °C in air for 40 min, after
which the mixtures were ground into fine powders (100–
300 nm), dry-pressed into pellets (20 mm in diameter and 1 mm
thick) uniaxially and fired again at 600 °C for 1 h. Densities of the
sintered disk-shaped pure SNDC was more than 95% and the
SNDC/(Li/Na)2CO3 pellets were 82–85% of theoretical density using
an automatic gas pycnometer (AccuPyc 1340, Micromeritics, USA).
Ag was pasted onto both sides of the samples to serve as the electrode, Ag wires were then attached with Ag adhesive paste to perform ionic conductivity measurements. Afterwards the samples
were baked at 550 °C for 30 min. The weight ratios of the composites (SNDC:(Li/Na)2CO3) electrolytes were 90:10, 80:20, 70:30 and
60:40 respectively.
The thermal behaviors of the composites were investigated by
thermogravimetry (TG)/differential scanning calorimetry (DSC)
(STA409PC, Netzsch, Selb, Germany) with a heating rate of 10 °C/
min at the temperature range of 25–700 °C in air. X-ray diffraction
(XRD) (D/MAX-RB X-ray diffractometer, Rigaku, Akishima-Shi, Japan) was carried out on the SNDC/(Li/Na)2CO3 composites for
phase identification, and morphology of the composites was examined using scanning electron microscopy (SEM) (JSM-6460LV, JEOL,
Japan). The ionic conductivity of the sintered pellets was investigated using a four-probe DC conductivity measurement with a current range of 20–500 mA.
Fig. 1. DSC curves for SNDC/(Li/Na)2CO3 composites with various carbonate
contents.
Fig. 2 shows the XRD patterns of the sintered specimens of the
SNDC/(Li/Na)2CO3 composites compared with pure SNDC and (Li/
Na)2CO3 (Li2CO3:Na2CO3 = 2:1, mol. ratio). All the peaks of SNDC
exhibit the single cubic fluorite CeO2 without any traces of
Sm2O3 or Nd2O3. The lack of Sm2O3 or Nd2O3 secondary phase in
SNDC corresponds with our previous work [10], which means that
the Sm3+ and Nd3+ ions could be fully doped into CeO2 crystal. The
XRD pattern also indicates that most of (Li/Na)2CO3 exists as an
amorphous phase in SNDC/(Li/Na)2CO3 composites, only small
amount of crystalline carbonates can be identified. It is estimated
from the XRD patterns that the SNDC/(Li/Na)2CO3 composites are
the unreacted mixtures of SNDC and (Li/Na)2CO3, which is also
identified by DSC tests since there are no unaccounted exothermic/endothermic peaks in the DSC curves from the synthesis and
firing process.
The fracture surface morphologies of the SNDC/(Li/Na)2CO3
composite pellets, sintered at 600 °C with three different carbonate
content of 10, 20 and 40 wt.%, were imaged by SEM. There are light
and gray regions distributed uniformly shown in Fig. 3. According
to the EDX results in Fig. 4, the higher contrast regions represent
the crystalline SNDC phase containing a larger percentage of heavy
cations than the gray regions representing the amorphous carbon-
3. Results and discussion
The DSC curves of the SNDC/(Li/Na)2CO3 powders with four different weigh ratios are shown in Fig. 1. Only one endothermic peak
around 500 °C in each curve is attributed to the bulk melting of (Li/
Na)2CO3, and it is about 10 °C lower than the melting point of the
pure (Li/Na)2CO3 phase. It also can be seen that the peaks shift towards higher temperature with increasing the binary carbonate
content. This phenomenon was also observed by other researchers
[5,31]. As is known, the melting point of solid phase decreases as
the matter size becomes smaller, thereby when the content of
(Li/Na)2CO3 powders milled with SNDC in liquid medium, the (Li/
Na)2CO3 powders can be easily crushed and well dispersed in the
SNDC powders, whereas when the content increases, the (Li/
Na)2CO3 powders will agglomerate and form larger particles, hence
the eutectic temperature increases near to the real melting point.
From Fig. 1 it also can be seen that the (Li/Na)2CO3 phase intenerates around 450 °C.
Fig. 2. XRD patterns of SNDC/(Li/Na)2CO3 sintered at 600 °C.
W. Liu et al. / Composites Science and Technology 70 (2010) 181–185
183
Fig. 4. SEM and spectrum images of the SNDC/(Li/Na)2CO3 composites cross-section
surface. (a) EDX points; (b) spectrum 1; (c) spectrum 2.
Fig. 3. SEM images of the SNDC/(Li/Na)2CO3 composites cross-section surface. (a)
(b)
SNDC:(Li/Na)2CO3 = 80:20;
(c)
SNDC:(Li/
SNDC:(Li/Na)2CO3 = 90:10;
Na)2CO3 = 60:40.
ates. It also can been seen from the SEM photos that SNDC is coated
by (Li/Na)2CO3. Additionally, there are no apparent larger pores
shown in the SEM photos, but it is difficult to conclude whether
sub-micrometer pores exist or not. It is estimated that the carbonates, forming the liquid phase around 500 °C, serve as a flux to enhance the densification of SNDC/(Li/Na)2CO3 composites at a low
sintering temperature. The microstructure shown in the photos
might provide ionic conduction with more fast paths at the interface between SNDC and (Li/Na)2CO3. This will be detailed later.
In this study, the electrical conductivity of ceria based electrolytes measured in air can be expressed as follows:
rðTÞ ¼
Ea
exp T
kB T
r0
ð1Þ
where T indicates the absolute temperature, kB the Boltzmann constant, Ea the activation energy, and r0 is a constant in a certain temperature range limited by the crystal unit cell [32]. At lower
intermediate temperatures, most of the oxygen vacancies are bound
at various traps, thus Ea of oxygen diffusion can be seen as a sum of
migration enthalpy (DHm) of oxygen ions and the association enthalpy (DHa) of the complex defect associates [20]. It can be seen
clearly from Eq. (1) that the ionic conductivity can increase with
increasing the value of r0 or decreasing that of Ea.
The conductivity r data for the SNDC/(Li/Na)2CO3 composites
can be calculated from resistance data measured at different temperatures with the standard relationship:
r¼
1L
RS
ð2Þ
where L is the thickness and S is the effective electrode area.
The typical plots of ln(rT) vs. 1000/T for SNDC/(Li/Na)2CO3 composite electrolytes with various carbonate contents are shown in
Fig. 5. It is observed that there is a leap of conductivity around
the carbonate transition temperature of SNDC/(Li/Na)2CO3 (around
the eutectic point of (Li/Na)2CO3) for all composite samples, which
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W. Liu et al. / Composites Science and Technology 70 (2010) 181–185
Fig. 5. Arrhenius plot for SNDC and SNDC/(Li/Na)2CO3 composite electrolytes with
various carbonate contents.
is totally different from the conductivity behavior of SNDC. Interestingly, the ln(rT) vs. 1000/T curves of the SNDC/(Li/Na)2CO3 composites are parallel to that of SNDC at both the higher and lower
temperature ranges. At higher temperature range (>550 °C), the
electrical conductivities of the SNDC/(Li/Na)2CO3 composites are
higher than that of SNDC, while lower at lower temperature range
(<450 °C). In this research the electrical conductivity of the SNDC/
(Li/Na)2CO3 composites can reach as high as 0.01 S/cm at 481 °C.
In Fig. 5, it can be seen from the curves of ln(rT) vs. 1000/T of
SNDC/(Li/Na)2CO3 composites that the electrical conductivity is
not directly proportional to the carbonate content. For example,
beyond the optimum ratio of 80:20, the higher carbonate content
reduces the electric conductivity at high temperature (as 60:40
shown in Fig. 5). From this observation, it is concluded that the
conductivity may not be dominated by Li+, Na+, and CO2
3 conduction ions through the carbonate phase. It therefore can be inferred
that the conductive mechanism of SNDC/(Li/Na)2CO3 composites
may be dominated by the oxygen ion transfer as in SNDC. Furthermore, the CO2
3 ion conduction contribution must be very little due
to the minor volume percentage of CO2 (0.03%) in air. Additionally,
cations such as Li+ and Na+ do not act as charge carriers in this
composite electrolyte, because they are blocked by a DC voltage
[33]. Therefore, the conduction measured is mainly supplied by
oxygen ions for SNDC/(Li/Na)2CO3 composites.
Since the curves in Fig. 5 are parallel at both higher and lower
temperature ranges, it seems that the activation energy Ea in pure
SNDC is the same as that in the SNDC/(Li/Na)2CO3 composite. We
propose that the increased ionic conductivity at high temperature
must then be explained by an increase in the value of r0 from Eq.
(1). This parameter is associated with the concentration of mobile
ions and the ion jump distance [27]. Therefore, high mobile ion
concentration and long jump distance at the interface between
SNDC and (Li/Na)2CO3 may contribute to the high conductivity at
the higher temperature range. Moreover, the so-called super-ionic
phase might exist at the interface between SNDC and carbonates,
forming the space charge zones, where the defect concentrations
are much higher than that in the bulk [4,25,27]. Wang et al. [30]
fabricated the SDC/Na2CO3 core–shell nanocomposite and they
also suggested that the enhancement of conductivity may be relevant to the interfacial conduction mechanism.
For the SNDC/(Li/Na)2CO3 composite, compared with the bulk,
the interface may supply oxygen ions with much more transfer
routes, especially at elevated temperature (>500 °C). In the transition temperature region (see Fig. 5), the carbonates will soften and
melt gradually, and the melted carbonates will percolate into the
interspaces in the SNDC/(Li/Na)2CO3 composite resulting in enhanced pore-free consistency. By filling interfacial interspaces there
will be more oxygen conduction routes at the interface, which results in the increase of the conductivity dramatically. Below the
transition temperature, the conductivity of the SNDC/(Li/Na)2CO3
composite decreases for two reasons. Firstly, the ionic conductivity
of SNDC/(Li/Na)2CO3 in the more ordered solid phase becomes lower
than that of SNDC, and in this case the dispersive (Li/Na)2CO3 phase
acts as the conductive barrier in the composites. Secondly, the volume change accompanying the liquid to solid transition necessitates
the creation of interfacial porosity removing conductivity paths.
The activation energy can be obtained by the slopes of the
ln(rT) vs. 1000/T curves. The activation energy of all the SNDC/
(Li/Na)2CO3 composites is close to that of SNDC (Ea = (1.117 ±
0.004) eV) at both higher and lower temperature ranges, with the
exception of the composite with the weight ratio of 60:40. This
suggests that the ionic conductive mechanism for SNDC/
(Li/Na)2CO3 is similar to that of SNDC, so in both materials the oxygen ions are the main charge carriers. Calculated from Eq. (1), the
activation energy for SNDC/(Li/Na)2CO3 (SNDC:(Li/Na)2CO3 =
80:20) in the transition region is (5.092 ± 0.098) eV which is much
higher at both higher and lower temperature ranges. The higher
activation energy may be a result of including the intenerating
and melting enthalpy of the carbonates.
The reduction of conductivity at high temperature and more
interestingly, the slight change in the activation energy, for the
high carbonates content (SNDC:(Li/Na)2CO3 = 60:40) composite is
not yet completely understood. However, it may be relevant to
the larger volume ratio of liquid (Li/Na)2CO3 which does not contribute to the ion conduction in the system and becomes deleterious for the measurement.
4. Conclusions
In this work, SNDC/(Li/Na)2CO3 composite electrolytes with
various carbonate contents were prepared. XRD results indicate
that no reaction occurred between the co-doped ceria and the carbonates. The ionic conductivity of the composite electrolytes was
measured in air. It was shown that around the transition temperature the conductivity changed rapidly, which may because of the
increased oxygen conduction routes, and the activation energy of
the SNDC/(Li/Na)2CO3 composite is much higher than that of
SNDC. At both higher and lower temperature ranges, the activation
energy of the SNDC/(Li/Na)2CO3 composite is close to that of SNDC,
Ea = (1.117 ± 0.004) eV, and it is estimated that SNDC/(Li/Na)2CO3
has the similar electrical conductive mechanism to that of SNDC.
It was found that the ionic conductivity of the SNDC/(Li/Na)2CO3
composites (SNDC:(Li/Na)2CO3 = 80:20) reached as high as 0.01 S/
cm at 481 °C.
Acknowledgments
This research was supported by TOYOTA MOTOR Corp.
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