Hydrogen permeation and chemical stability of In-doped SrCe 0.95Tm0.05O3-δ membranes

Article abstract of DOI:10.1016/j.jallcom.2014.07.050

A series of SrCe_0.95-xIn _xTm_0.05O_3-δ (x = 0.00, 0.05, 0.10, 0.15, 0.20) samples were prepared by the sol-gel technique using ethylenediaminetetraacetic acid and citric acid as the complexing agents. The X-ray diffraction (XRD) measurements were carried out to study the formation of SrCe_0.75In_0.20Tm_0.05O_{3 -δ} oxide. The results indicate that In can dissolve in the orthorhombic lattice of strontium cerate to form a solid solution at 1300 °C. The effects of In doping on the sinterability, chemical stability and hydrogen permeation flux of SrCe_0.95Tm_0.05O _3-δ oxides were investigated by a variety of characterization methods. According to the results of XRD and scanning electron microscopy (SEM), increasing In content in the oxides causes lattice constriction and promotes grain growth during sintering process at 1300°C. The relative density and morphology variation of the sintered membranes reveals that In doping improves the sintering activity of SrCe_0.95Tm _0.05O_3-δ membrane. The stability test shows that the stability of strontium cerate against CO ₂ increases with the increasing In content. Hydrogen permeation through the SrCe_0.95-xIn_xTm _0.05O_3-δ membranes was carried out between 700 and 900°C using 40% H₂/He mixture as feed gas and Ar as sweep gas, respectively. The H₂ permeation flux decreases with the increase of the In content. Activation energies of SrCe _0.95Tm_0.05O_3-δ, SrCe_0.85In_0.10Tm_0.05O_{3 -δ} and SrCe_0.75In_0.20Tm _0.05O_3-δ are 36.61, 52.25 and 73.06 kJ/mol, respectively.

Full text of DOI:10.1016/j.jallcom.2014.07.050

Journal of Alloys and Compounds 616 (2014) 142–147
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Hydrogen permeation and chemical stability of In-doped
SrCe0.95Tm0.05O3ꢀd membranes
a
b
Wenhui Yuan a, , Chichi Xiao , Li Li
⇑
a School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, PR China
b School of Environmental Science and Engineering, South China University of Technology, Guangzhou 510006, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of SrCe0.95ꢀxInxTm0.05O3ꢀd (x = 0.00, 0.05, 0.10, 0.15, 0.20) samples were prepared by the sol–gel
technique using ethylenediaminetetraacetic acid and citric acid as the complexing agents. The X-ray dif-
fraction (XRD) measurements were carried out to study the formation of SrCe0.75In0.20Tm0.05O3ꢀd oxide.
The results indicate that In can dissolve in the orthorhombic lattice of strontium cerate to form a solid
solution at 1300 °C. The effects of In doping on the sinterability, chemical stability and hydrogen perme-
ation flux of SrCe0.95Tm0.05O3ꢀd oxides were investigated by a variety of characterization methods.
According to the results of XRD and scanning electron microscopy (SEM), increasing In content in the
oxides causes lattice constriction and promotes grain growth during sintering process at 1300 °C. The
relative density and morphology variation of the sintered membranes reveals that In doping improves
the sintering activity of SrCe0.95Tm0.05O3ꢀd membrane. The stability test shows that the stability of stron-
tium cerate against CO2 increases with the increasing In content. Hydrogen permeation through the
SrCe0.95ꢀxInxTm0.05O3ꢀd membranes was carried out between 700 and 900 °C using 40% H2/He mixture
as feed gas and Ar as sweep gas, respectively. The H2 permeation flux decreases with the increase of
the In content. Activation energies of SrCe0.95Tm0.05O3ꢀd, SrCe0.85In0.10Tm0.05O3ꢀd and SrCe0.75In0.20Tm0.05
O3ꢀd are 36.61, 52.25 and 73.06 kJ/mol, respectively.
Received 11 May 2014
Received in revised form 6 July 2014
Accepted 7 July 2014
Available online 14 July 2014
Keywords:
Perovskites
Strontium cerate
Chemical stability
Hydrogen permeation
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
as a hydrogen separation membrane in the atmosphere containing
O2 [9]. While the SrZrO3 and BaZrO3 oxides are hard to be applied
With the increasing shortage of petroleum resources, more and
more attention has been paid to the exploitation and the require-
ment of the renewable resources. Hydrogen is widely considered
as a potential substitute for fossil fuels because it is a clean,
efficient and renewable energy. Owing to many advantages such
as thermal stability, good mechanical strength and high hydrogen
selectivity, dense mixed protonic and electronic conducting
ceramic membranes, mostly based on the SrCeO3, BaCeO3, SrZrO3
and BaZrO3 perovskite oxides in a general formula of ABO3, have
attracted considerable attention over the past few decades for their
potential application for separating H2 at high temperature from
coal gasified gas streams [1–8].
for hydrogen separation due to their low protonic and electronic
conductivity. According to previous reports by Knight [10] and
Bonanos [11], the distorted orthorhombic structure of SrCeO3
effectively inhibits oxygen ion conduction, thereby enabling high
transference number for proton conduction. More recently, Zhang
and Zhao [12] found that partial substitution of Sr for Ba increased
the structural distortion of BaCe0.9Nd0.1O3ꢀd oxides and decreased
the oxygen ion conductivity. Therefore, among the perovskite
oxides researched, SrCeO3 based oxides are the most promising
materials for hydrogen separation.
The poor stability of SrCeO3 based materials at elevated temper-
atures in the atmosphere of CO2, however, limits their application
as hydrogen gas separation membranes. Extensive studies have
been focused on the development and optimization of the perfor-
mance of doped cerates in order to enhance their stabilities with-
out remarkably sacrificing their hydrogen permeation flux. One
effective approach to improve the chemical stability of SrCeO3
based oxides is to form a solid solution with SrZrO3 [13,14],
because the SrZrO3 based materials show excellent stability in
CO2 atmosphere. However, the SrZrO3 based materials have much
Among the oxides, the BaCeO3 based oxides exhibit the highest
conductivity, but they show oxygen ion conductivity comparable
to their proton conductivity, which makes it difficult to be used
⇑
Corresponding author. Present Address: School of Chemistry and Chemical
Engineering, South China University of Technology, Wushan, Tianhe, Guangzhou
510640, PR China. Tel./fax: +86 20 8711 1887.
E-mail address: cewhyuan@scut.edu.cn (W. Yuan).
http://dx.doi.org/10.1016/j.jallcom.2014.07.050
0925-8388/Ó 2014 Elsevier B.V. All rights reserved.

W. Yuan et al. / Journal of Alloys and Compounds 616 (2014) 142–147
143
analyzed with HP 5890 GC equipped with a Carboxen 1000 column. Only these
membranes displaying gas tight at room temperature were chosen for hydrogen
permeation.
higher sintering temperatures and lower protonic and electronic
conductivity [15], which are unfavorable for the fabrication of
dense membrane and for the separation of hydrogen gas. For
example, Kniep and Lin [14] reported that the hydrogen perme-
ation flux of SrCe0.95Tm0.05O3 membrane is about 5 times higher
than that of SrCe0.75Zr0.20Tm0.05O3 membrane under the same
conditions.
3. Results and discussions
3.1. Formation of the perovskite structure
Usually, the pervoskite oxides ABO3 with the B site doping of
metallic elements with high electronegativity show good chemical
stability [16,17]. Since the Indium (In) has much higher electroneg-
ativity than Cerium (Ce), the partial replacement of Ce with In has
showed a good stability against CO2 for BaCeO3 oxides [18,19]. So
the In doping is expected to increase the CO2 resistance of SrCeO3
oxides. However, the electrical conductivity of In-doped BaCeO3 is
rather low [20–22]. In general, the high and comparable electronic
and protonic conductions are always required for ceramic mem-
branes to be functional in non-galvanic hydrogen permeation.
However, hydrogen permeation properties are still limited by elec-
tron transport, even in the doped oxide system [5]. So, an effective
approach to improve the H2 permeability of SrCeO3 based ceramic
membranes is to improve their electronic conductivity. While the
electronic conductivity is inversely related to the ionization poten-
tial of the doping elements [3]. Due to the low third ionization
energy of Tm, high hydrogen permeation flux has been reported
for Tm-doped SrCeO3 membrane [3]. In order to achieve high
hydrogen permeation flux and good chemical stability, the ‘‘co-
doping’’ strategy [23] is applied on doped SrCeO3 with In and Tm.
In this work, SrCe0.95ꢀxInxTm0.05O3ꢀd (SCITm, 0 6 x 6 0.2) solid
solutions were prepared by sol–gel method. The chemical stability
and hydrogen permeation flux of SCITm oxides were systemati-
cally investigated by a variety of characterization methods.
XRD patterns of SrCe0.75In0.20Tm0.05O3ꢀ (SCITm20) intermedi-
a
ates sintered under different temperatures for 10 h are shown in
Fig. 1. By comparing the XRD spectra with the standard XRD
pattern, it can be seen that the powder is a mixture of metal oxide
(CeO2), carbonate salt (SrCO3) and a small amount of SrIn2O4 when
the calcination temperature is about 700 °C. The contents of metal
oxide and carbonate salt phases decrease gradually while the
content of Sr2CeO4 phase increases with increasing temperature.
So it is concluded that the metal oxide and the carbonate salt react
to form Sr2CeO4. As the temperature increased to 1000 °C, the pow-
der crystal phase is mainly composed of perovskite, but the small
amount of SrIn2O4 phase still exists in the sample, which means
that In is not melting into the perovskite host to form a solid solu-
tion. The SrIn2O4 phase is finally disappeared when the calcination
temperature reaches up to 1300 °C, which is evidence for the
miscibility of SrCeO3 and SrIn2O4. Therefore, a pure perovskite
structure of SCITm20 oxide can be obtained when the calcination
temperature is 1300 °C.
3.2. Membrane characteristics
Fig. 2 shows the XRD patterns of SCITm membranes sintered at
1300 °C for 10 h. As can be seen from Fig. 2(a), all specimens
showed a single perovskite phase for all of the characteristic peaks,
indicating that In dissolves in the orthorhombic lattice of stron-
tium cerate to form a solid solution at 1300 °C. Moreover, introduc-
ing In into the lattice changes the lattice parameters of the host to
some degree. As shown in Fig. 2(b), the peak sharpness decreases
and the peak position slightly shifts to higher angles with the
increase of In content, indicating that the substitution of Ce with
In induces smaller lattice parameters. This was confirmed by the
Rietveld analysis for the samples performed using TOPAS program
and the results are listed in Table 1. As shown in Table 1, the
decrease of the lattice parameters and lattice volume is caused
2. Experimental
2.1. Sample synthesis and fabrication
The SrCe0.95ꢀxInxTm0.05O3ꢀd (0 6 x 6 0.2) perovskite powder was synthesized by
a sol–gel method using ethylenediaminetetraacetic acid (EDTA) and citric acid (CA)
as chelating agents [24]. Sr(NO3)2, Ce(NO3)3ꢁ6H2O, In(NO3)3ꢁ4.5H2O, Tm(NO3)3ꢁ6H2O,
all in analytical grades, were used as the raw materials for metal ion sources. The
molar ratio of total metal ions to EDTA to CA in the solution was 1:1:1.5. The pH
of the solution was adjusted to about 8 by ammonia to prevent selective precipita-
tion. The mixed solution was then heated to 90 °C and stirred for 5 h to allow the
polymerization reactions to take place. Then, the remaining water was heated in
an evaporating dish on an electric furnace until self-igniting, yielding grey and
porous powders. The powders were calcined at 1000 °C for 10 h to remove organic
components. The calcined powders were uniaxially pressed into disks under the
pressure of 20 MPa for 10 min. The resulting disks were sintered in air at 1300 °C
1300 oC
for 10 h with a ramp rate of 2 °C minꢀ1
.
o
1100 C
2.2. Sample characterization
o
1000 C
900 oC
The crystalline structure of powders and membranes was measured by an X-ray
diffractometer (XRD, Germany, D8 Advance) with Cu radiation source
Ka
(k = 1.5418 Å). A step-scan mode was used to collect 2h data from 10–80°. The
X-ray tube voltage and current were set at 40 kV and 40 mA, respectively. The data
were analyzed by the Rietveld method using the TOPAS program. Microstructures
of the sintered membranes were observed by scanning electron microscopy (SEM,
JEOL JSM-6510). Gold sputter coating was performed on the samples under vacuum
before the measurements. The relative density of sintered membranes was mea-
sured based on Archimedes principle. The chemical stability of powder samples
was analyzed using a thermal analyzer (Germany, Netzsch STA 449C) under CO2
(30 mL minꢀ1) and N2 (20 mL minꢀ1) atmospheres at 700 °C for 4 h.
800 oC
700 oC
o
600 C
SrIn2O4
Sr2CeO4
SrCO3
CeO2
2.3. Hydrogen permeation measurement
10
20
30
40
50
60
70
80
2θ (o)
The hydrogen permeation flux through the SCITm membranes was measured
using a home-made high-temperature permeation cell that was described else-
where [25]. H2–He mixtures were introduced to the feed side, while Ar was used
as the sweeping gas on the permeate side. The downstream chamber effluent was
Fig. 1. XRD patterns of SCITm20 intermediates through calcinations of the
precursor at different temperatures for 10 h.

144
W. Yuan et al. / Journal of Alloys and Compounds 616 (2014) 142–147
method and the results are listed in Table 1. The results indicated
(b)
(a)
that partial substitution of In for Ce showed a substantial effect of
increasing relative density; even a 5 mol% substitution of In
markedly improved the relative density of SCTm membrane. The
relative density and morphology variation of the sintered
membranes reveals that In doping can improve the sintering
activity of SCTm membrane. According to the results of Fig. 1,
the SrIn2O4 phase is not miscible with perovskite phase until the
temperature reaches up to 1300 °C. So the improved sintering
activity of In-doped membranes might be mainly caused by the
SrIn2O4 phase, which serves as a sintering aid during sintering
process [27].
x=0.20
x=0.15
x=0.10
x=0.05
x=0.00
20
30
40
50
(o)
60
70
29
30
(o)
2θ
2θ
3.3. Chemical stability
Fig. 2. (a) XRD patterns of SrCe0.95ꢀxInxTm0.05O3ꢀd membranes calcined at 1300 °C
for 10 h and (b) magnification of 2h range of 28.5–30.5°.
It is well known that the main drawback of using SrCeO3 based
material as a membrane for hydrogen separation is its microstruc-
ture deterioration in the presence of CO2 at elevated temperature.
In this paper, to investigate the chemical stability of the oxides,
thermogravimetric measurements were used to record the weight
change of each sample under the atmosphere containing CO2 (60%
CO2 + 40% N2). As shown in Fig. 4, the weight increments of SCTm,
SCITm10 and SCITm20 samples are about 4.8%, 3.5%, 1.5%, respec-
tively. The increase in weight means that the SCITm membranes
react with CO2 in such a terrible circumstance [28]. The reaction
between SrCe0.95ꢀxInxTm0.05O3ꢀd and CO2 is considered as follow-
ing [29]:
by increasing In content. Such a result is expected due to the
smaller ionic radius of In3+ (6-coordination with oxygen, 0.80 Å)
in comparison with that of Ce4+ (6-coordination with oxygen,
0.87 Å). Similar trends were also observed when Zr was doped into
SrCe0.95Tm0.05O3ꢀd [14,25] and Tb was doped into SrCeO3 [26].
The morphology of the membranes was determined with the
aid of scanning electron microscopy and the corresponding images
are shown in Fig. 3. As can be seen from this figure, the doping of
In exerted a significant effect on the sintering process of the
membranes. With the increasing In content, smoother membrane
surface and larger grain can be obtained. For example, the average
grain sizes of SCTm, SCITm10 and SCITm20 are approximately 1.5,
SrCe0:95ꢀxInxTm0:05O3ꢀd þCO2 ! SrCO3 þCe0:95ꢀxInxTm0:05O2ꢀd ð1Þ
13 and 25 lm, respectively. The relative density of the membranes
was also calculated with experimental lattice parameters and
the corresponding chemical formulas via the typical Archimedes
So it is concluded that the chemical stability of SCITm oxides
against CO2 gradually increases with the increasing In content from
the increase of weight percentage.
Table 1
Lattice parameters, unit cell volume and relative density of the SCITm membranes sintered at 1300 °C for 10 h.
Composition
Lattice parameters unit cell (Å)
V (Å3)
Relative density
a
b
c
SCTm
8.5826
8.5765
8.5716
8.5524
8.5389
6.0015
5.9979
5.9937
5.9941
6.0000
6.1419
6.1288
6.1285
6.1276
6.1057
316.3596
315.2737
314.8603
314.1255
312.8215
0.862
0.967
0.930
0.952
0.986
SCITm05
SCITm10
SCITm15
SCITm20
Fig. 3. SEM micrographs of SrCe0.95ꢀxInxTm0.05O3ꢀd membranes sintered at 1300 °C for 10 h. (a) x = 0.00; (b) x = 0.05; (c) x = 0.10; (d) x = 0.15 and (e) x = 0.20.

W. Yuan et al. / Journal of Alloys and Compounds 616 (2014) 142–147
145
700
105
104
103
102
101
100
SrCO3
CeO2
600
(a)
500
(b)
400
300
(c)
200
100
0
0
50
100
150
200
250
10
20
30
40
50
60
70
80
(o)
Time (min)
2θ
Fig. 4. TG curves of SrCe0.95ꢀxInxTm0.05O3ꢀd powders exposed to 60% CO2 and 40%
N2 atmospheres at 700 °C for 4 h. (a) x = 0.00; (b) x = 0.10 and (c) x = 0.20.
Fig. 6. XRD patterns of SCTm and SCITm20 membrane surfaces exposed to 10% CO2,
40% H2 and 50% He mixture gas at 900 °C.
Long term stability of membranes was investigated by measur-
ing the H2 permeation through the membranes with a thickness of
1.2 mm as a function of time under 10% CO2, 40% H2 and 50% He
atmosphere at 900 °C, as shown in Fig. 5. Without In, hydrogen flux
of SCTm sample decreased quickly after the membrane exposed to
mixture gas containing 10% CO2 at 900 °C; it continued to decrease
with the time of exposure and decreased 46.6% for 10 h. Moreover,
the hydrogen flux of SCITm20 membrane decreased moderately in
the first 2 h after the membrane being exposed to the mixture gas
and approached to a constant with further exposure. The decrease
of H2 flux for SCITm20 membrane is about 19.8%, which is much
lower than that of SCTm.
After permeation experiments for these two membranes
exposed to the mixture gas containing 10% CO2, the upstream
membrane surfaces of both membranes were examined by XRD.
Fig. 6 shows the XRD patterns of membranes exposed to the mix-
ture gas. Both of the membranes react with CO2 to form SrCO3 and
CeO2. But the XRD peak intensity for CeO2 and SrCO3 phases with
respect to that of the perovskite phase for SCITm20 membrane is
much lower than that for the SCTm membrane. For example, the
intensity ratio of the CeO2 peak at about 28.5° to that of the perov-
skite peak at about 29.4° is about 1.09 for SCTm and only about
0.28 for the SCITm20 membrane. It is indicated that In doping hin-
ders the formation of CeO2 and SrCO3 and improves chemical sta-
bility of the strontium cerate oxides.
The above evidences demonstrate that partially replacement of
Ce by In indeed increases the chemical stability of strontium cerate
against CO2. This is due to the fact that In doping can reduce the
basicity of the SCTm oxide because of the relatively high electro-
negativity of In3+ (1.49) compared with that of Ce4+ (1.08), which
makes the oxide less easily react with acidic gases and stabilizes
the oxides against carbonation [16].
3.4. Hydrogen permeation
The hydrogen permeation fluxes through the SCITm mem-
branes as a function of H2 concentration were measured at
900 °C, as shown in Fig. 7. The H2 permeation flux increased with
the increase of upstream H2 concentration since the net driving
force for H2 permeation and the concentration of protons and elec-
trons on the feed side were enhanced by a higher H2 concentration.
According to the figure, the hydrogen fluxes of SCITm membranes
decrease with increasing In content. For instance, the H2 perme-
ation fluxes of SCTm, SCITm10 and SCITm20 at 900 °C under 50%
H2–He/Ar gradient are 0.054, 0.042, 0.033 mL cmꢀ2 minꢀ1
respectively.
,
To explore the underlying reasons for the decreased hydrogen
permeation of the In-doped samples, hydrogen fluxes for SCITm
membranes were measured under the condition of 40% H2–He/Ar
0.06
SCTm
SCITm10
SCITm20
0.050
SCTm
SCITm20
0.05
0.04
0.03
0.02
0.045
0.040
0.035
0.030
0.025
0.020
20
25
30
35
40
45
50
0
5
10
15
20
25
30
35
H2 concentration in the feed (%)
Time (min)
Fig. 7. Hydrogen fluxes of SrCe0.95ꢀxInxTm0.05O3ꢀd (x = 0.00, 0.10, 0.20) membranes
as function of H2 concentration (900 °C, upstream: H2 + He = 100 mL/min,
downstream: Ar = 30 mL/min).
Fig. 5. Time dependence of hydrogen flux through SCTm and SCITm20 membranes
at 900 °C (upstream, 10% CO2, 40% H2 and 50% N2; downstream, 100% Ar).
a

146
W. Yuan et al. / Journal of Alloys and Compounds 616 (2014) 142–147
Table 1, the lattice volume decreased with In content for the In-
-3.0
-3.5
-4.0
-4.5
-5.0
-5.5
doped samples, which means the proton mobility decreased with
increase of In content. On the other hand, the doping of In is
expected to reduce the basicity of the ceramic oxide because of
the relatively high electronegativity of In. While, the basicity of
oxide is beneficial to the formation of the protonic defects, as men-
tioned by Kreuer [31]. Therefore, the In-doping is disadvantageous
for the formation of protonic charge carriers, which result in the
increase of activation energy [32]. In general, the decrease in
hydrogen permeation flux for the In-doped samples could be
mainly induced by the decrease of protonic conductivity, but fur-
ther investigations are necessary for confirmation.
SCTm
SCITm10
SCITm20
4. Conclusion
0.84
0.88
0.92
0.96
1.00
1.04
1000/T (K-1)
The SrCe0.95ꢀxInxTm0.05O3ꢀd (0 6 x 6 0.2) perovskite oxides have
been synthesized by EDTA/CA complexing method. The XRD anal-
ysis results indicate that all samples are single-phase in the range
of x = 0.00–0.20 and the In doping leads to the decrease of lattice
parameters. The relative density and morphology variation of the
sintered membranes reveals that In doping can improve the sinter-
ing activity of SrCe0.95Tm0.05O3ꢀd membrane. Improved sintering
activity of In-doped membranes might be mainly caused by the
SrIn2O4 phase, which serves as a sintering aid during sintering
process. Moreover, the membrane stability test shows that In dop-
ing improved the chemical stability against CO2 and the chemical
stability increased with the increase of In content in the present
experimental condition. Finally, according to the hydrogen perme-
ation measurements, the In doping results in the decrease of the
hydrogen permeation flux in the temperature interval of 700–
900 °C. The decrease of the hydrogen permeation flux for the In-
doped membrane could be a result of the loss of protonic
conductivity.
Fig. 8. H2 permeation flux of SrCe0.95ꢀxInxTm0.05O3ꢀd (x = 0.00, 0.10, 0.20) mem-
branes versus 1000/T.
gradient at the temperature from 700 to 900 °C. Arrhenius plots of
H2 flux for In-doped SCTm membranes are shown in Fig. 8. Almost
linear relationships were observed between the logarithm of the
flux and 1000/T, indicating no change in the electronic and pro-
tonic conduction mechanism within the temperature range inves-
tigated. The activation energy, Ea (kJ molꢀ1) and pre-exponential
factor, A (mL cmꢀ2 minꢀ1) can be calculated by Arrhenius equation:
J ¼ AexpðꢀEa=RTÞ
ð2Þ
where J (mL cmꢀ2 minꢀ1) is the H2 permeation flux.
Table 2 compares the activation energies of SCITm membranes
and a few other doped SrCeO3 ceramic membranes reported in the
literature. The activation energies of SCTm, SCITm10 and SCITm20
are 36.61, 52.25, 73.06 kJ/mol, respectively. The activation energy
of SCTm membrane is in accordance with the result of Kniep and
Lin [14], which is 36.5 kJ/mol under 10% H2/He conditions. As
Cheng et al. [30] reported, H2 permeation for SCTm membrane in
the temperature range of 600–700 °C is limited by electronic
conduction with activation energy of 116 kJ/mol, while in the tem-
perature range of 600–700 °C the membrane is a mixed protonic-
electronic conductivity due to the low activation energy of 20 kJ/
mol.
In this case, the activation energy increased with the increase of
In content. The increase of the activation energy for the In-doped
membrane could be mainly induced by the loss of protonic con-
ductivity. On one hand, Kreuer [16] suggested that the proton
mobility increased with increasing lattice volume. As shown in
Acknowledgment
The authors gratefully acknowledge the financial support of
National Natural Science Foundation of China (No. 20976057)
and research fund of The Guangdong Provincial Engineering
Research Center of Green Fine Chemicals, China.
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Table 2
Comparison of activation energy for doped SrCeO3.
Sample
Operation
temperature
T (°C)
Atmosphere Activation Ref.
energy Ea
(kJ molꢀ1
)
SrCe0.95Tm0.05O3ꢀd
SrCe0.95Y0.05O3ꢀd
SrCe0.95Eu0.05O3ꢀd
700–950
600–700
700–950
Fa: 10%H2/He 20
[30]
S: 20%O2/Ar
F: H2/He
116
132
[33]
S: Ar
700–850
750–900
700–900
F: H2/Ar
80
[34]
S: He
SrCe0.95Tm0.05O3ꢀd
SrCe0.75Zr0.20Tm0.05O3ꢀd
SrCe0.95Tm0.05O3ꢀd
SrCe0.85In0.10Tm0.05O3ꢀd
SrCe0.75In0.20Tm0.05O3ꢀd
F: 10%H2/He
S: 20%O2/Ar
F: 40%H2/He
36.5
78.1
36.61
52.25
73.06
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