La(Sr)Ga(Fe)O3 perovskite oxide as a new mixed ionic-electronic conductor for oxygen permeating membrane

Article abstract of DOI:10.1149/1.1524174

Fe-doped La_1-xSr_xGaO₃ exhibits a high conductivity (～1 S/cm) and a high oxygen permeation rate. In particular, the highest conductivity and the oxygen permeating rate were attained at La_0.7Sr_0.3Ga_0.6Fe_0.4O₃ (LSGF). Although a surface catalyst is required, oxygen permeation rate from air to Ar was as high as 2.5 cm³ std/min cm² at 1273 K and 0.3 mm membrane thickness. Oxygen permeation rate from air to Ar increased in the following order, La_0.6Sr_0.4CoO₃ > La_0.9Sr_0.1CoO₃ = Sm_0.5Sr_0.5CoO₃ ? Sm_0.6Sr_0.4CoO₃, as the surface catalyst. Since the p₀₂ gradient becomes larger, the oxygen permeation rate drastically increased by changing from air-Ar to CH₄-air condition. The products were only CO and H₂, having a molar ratio (H₂/CO ratio) of almost two. Electronic hole conduction was only observed in LSGF polarization measurement and the oxide ion conductivity estimated is as high as s = 0.6 S cm^-1 at 1073 K. At high p_o2, the main defect in LSGF is O_i″ and Fe_Fe and at intermediate p_o2, concentration of Fe_Fe is balanced with that of O_i″. The estimated transport number of oxide ion was ca. 0.6, which is in a good agreement with that estimated by the electromotive force in H₂-O₂ gas concentration cells.

Full text of DOI:10.1149/1.1524174

Journal of The Electrochemical Society, 150 ͑1͒ E17-E23 ͑2003͒
E17
0013-4651/2002/150͑1͒/E17/7/$7.00 © The Electrochemical Society, Inc.
La„Sr…Ga„Fe…O3 Perovskite Oxide as a New Mixed
Ionic-Electronic Conductor for Oxygen Permeating Membrane
,z
*
Tatsumi Ishihara, Yuko Tsuruta, Yu Chunying, Toshitsune Todaka,
Hiroyasu Nishiguchi, and Yusaku Takita
Department of Applied Chemistry, Faculty of Engineering, Oita University, Dannoharu 700,
Oita 870-1192, Japan
Fe-doped La1ϪxSrxGaO3 exhibits a high conductivity ͑ϳ1 S/cm͒ and a high oxygen permeation rate. In particular, the highest
conductivity and the oxygen permeating rate were attained at La0.7Sr0.3Ga0.6Fe0.4O3 ͑LSGF͒. Although a surface catalyst is
required, oxygen permeation rate from air to Ar was as high as 2.5 cm3 std/min cm2 at 1273 K and 0.3 mm membrane thickness.
Ͼ
Oxygen permeation rate from air to Ar increased in the following order, La0.6Sr0.4CoO3
La0.9Sr0.1CoO3 ϭ Sm0.5Sr0.5CoO3
ӷ Sm0.6Sr0.4CoO3 , as the surface catalyst. Since the pO gradient becomes larger, the oxygen permeation rate drastically in-
2
creased by changing from air-Ar to CH4-air condition. The products were only CO and H2 , having a molar ratio (H2 /CO ratio͒
of almost two. Electronic hole conduction was only observed in LSGF polarization measurement and• the oxide ion conductivity
estimated is as high•as s ϭ 0.6 S cmϪ1 at 1073 K. At high pO , the main defect in LSGF is OiЉ and FeFe , and at intermediate pO
,
2
2
concentration of FeFe is balanced with that of OiЉ . The estimated transport number of oxide ion was ca. 0.6, which is in a good
agreement with that estimated by the electromotive force in H2-O2 gas concentration cells.
© 2002 The Electrochemical Society. ͓DOI: 10.1149/1.1524174͔ All rights reserved.
Manuscript submitted January 22, 2002; revised manuscript received July 3, 2002. Available electronicallyNovember 21, 2002.
Methane (CH4) is the major component in natural gas which is
an abundant natural resource. Therefore, conversion of CH4 into
useful compounds is an important subject at present. From the view-
point of the useful utilization of CH4 , the partial oxidation of CH4 is
attractive, because the reaction gives a synthesis gas at CO:H2
ϭ 1:2, which is suitable for the synthesis of methanol or hydrocar-
bons. Pure oxygen gas is an essential reactant for this reaction and
as far as the cost is concerned, separation of air into O2 and N2 by a
simple method should be considered. Separation of air into O2 and
N2 by a mixed electronic and oxide ionic conducting ceramic mem-
brane is an ideal method for obtaining pure oxygen because of its
simple structure and low energy consumption.1 However, separation
the oxygen permeation membrane for CH4 partial oxidation. In this
study, therefore, mixed electronic and oxide ionic conductivity in
Fe-doped La(Sr)GaO3 was investigated in detail. Furthermore,
oxygen-permeating properties through La(Sr)Ga(Fe)O3 ͑LSGF͒
membrane under CH4 partial oxidation were studied.
Experimental
Sample preparation.—Fe-doped La(Sr)GaO3 was prepared by a
conventional solid-state reaction using La2O3 ͑99.99%, Wako Pure
Chemicals͒, SrCO3 ͑reagent grate, Wako Pure Chemicals͒, Ga2O3
͑99.99%, Kishida Chemicals͒, and Fe2O3 ͑99.5%, Wako Pure
Chemicals͒. Metal oxides of the desired compositions were milled in
an Al2O3 mortar and pestle and the mixture subsequently calcined at
1273 K for 6 h. The powder after calcination was mixed again and
pressed into a disk ͑20 mm diam͒ followed by isostatic pressing at
2700 kg/cm2 for 20 min. The obtained disks were sintered at 1773 K
for 6 h. Finally, disks were ground to 0.5 mm thickness on a dia-
mond wheel. It is also noted that change in the composition during
preparation is not observed by inductively coupled plasma ͑ICP͒
measurement. In order to improve the surface activity for oxygen
dissociation, each disk was painted on both surfaces with
La(Sr)CoO3 ͑denoted as LSC͒ or Sm(Sr)CoO3 ͑SSC͒ slurry at 10
mm diam with a brush. La0.6Sr0.4CoO3 was mainly used for this
purpose. LSC or SSC was prepared by calcination of the mixture of
reagent-grade La(NO3)3 , Sm2O3 , Sr(NO3)2 , and (CH3COO)2Co
͑Kishida Chemicals͒ at 1273 K for 6 h.
of oxygen with mixed conducting membrane requires pO gradient.
2
Since pO in CH4 stream is smaller than 10Ϫ20 atm, application of
2
oxygen separation with mixed conducting ceramic membrane for
CH4 partial oxidation is an ideal application of the mixed conduct-
ing ceramic membranes.
It is well known that La(Sr)Fe(Co)O3 ͑LSFC͒ perovskite oxides
exhibit superior mixed conductivity and, consequently, a high oxy-
gen permeation rate is attained on these perovskite oxides.2 In par-
ticular, it is reported that SrCoO3 doped with Ca and Fe for Sr and
Co sites, respectively, exhibits an extremely high oxygen permeation
rate.3 However, LSFC is easily reduced in CH4 atmosphere and it is
reported that failure of the membrane due to reduction sometimes
occurs when LSFC is used as the oxygen-permeating membrane for
CH4 partial oxidation.4 It is reported that SrFeCo0.5O3 is stable
against reduction and it can be used as the oxygen-permeating mem-
brane for the partial oxidation of methane.5,6 However, synthesis of
this oxide is rather difficult, and further improvement in the oxygen-
permeating rate is required for the application of the mixed
electronic-oxide ionic conductor to the oxygen generator for CH4
partial oxidation.
Oxygen permeation and CH4 partial oxidation measure-
ments.—An oxygen gas concentration cell of air-Ar, schematically
shown in Fig. 1, was used for the simple oxygen permeation mea-
surement. Permeating oxygen from air to Ar was analyzed by using
a gas chromatograph. Molten Pyrex glass was used for the gas seal.
Since no N2 in Ar sweep gas was detected by the analysis with gas
chromatographs, it was confirmed that the physical gas leakage
through the sample and gas seal was negligible.
The same gas concentration cell as shown in Fig. 1 was also used
for CH4 partial oxidation. In the case of CH4 partial oxidation, Ni
and LSC were used as the catalysts for the CH4 and oxygen side,
respectively, at 10 mm and 40 ␮m diam and thickness. Ni catalyst
was obtained by applying the commercial NiO ͑reagent grade, Wako
Pure Chemical͒ and before reaction, NiO was reduced to Ni by
flowing H2 at 1273 K for 1 h. The outlet gas from the membrane
In our previous study, it was found that Ni- or Fe-doped
La(Sr)GaO3 was a mixed electronic hole and oxide ionic conductor
and the material was stable over a wide range of oxygen partial
pressures (pO ).7,8 In particular, it was found that Fe-doped
2
La(Sr)GaO3 exhibited high electrical conductivity and an oxygen-
permeating rate.9 Therefore, Fe-doped La(Sr)GaO3 is expected as
* Electrochemical Society Active Member.
z E-mail: isihara@cc.oita-u.ac.jp
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E18
Journal of The Electrochemical Society, 150 ͑1͒ E17-E23 ͑2003͒
Figure 3. Arrhenius plots of the electrical conductivity of
La1ϪxSrxGa0.6Fe0.4O3 in pO ϭ 10Ϫ5 atm.
2
An Al2O3 dense plate was connected to an Fe-doped LaGaO3 plate
͑1 mm in thickness͒ with a molten Pyrex glass ring and Pt paste ͑ca.
10 mm diam͒ attached to the specimens. The reference electrode
was attached close to the pumping electrode and the setup was put
into air flow stream. A constant potential was applied using a poten-
tiostat ͑Hokuto Denko, HFBA-501͒ and current was measured with
a digital multimeter ͑Advantest, R6451͒. Commercial oxygen was
always used as the reference gas. The electrode area for oxygen
pumping is much larger than that of the side area of the disks, and
the oxygen flux incorporated from the side of the disks does not
significantly influence the estimated electronic conductivity.
Figure 1. Schematic view of the gas concentration cell used for oxygen
permeation as well as CH4 partial oxidation.
reactor was analyzed by the gas chromatographs with a thermal
conductive detector ͑TCD͒ with a molecular sieve 5A column.
Electrochemical measurements.—Electrical conductivity was
measured with a generally used dc four-probe method in the con-
ventional gas flow cell which is not shown. Partial pressure of oxy-
gen was controlled by using N2-O2 , CO-CO2 , and H2-H2O gas
mixture and monitored with an oxygen sensor using Ca0.15Zr0.85O2
electrolyte. The transport number of oxide ion was estimated by the
ratio of the measured electromotive force ͑emf͒ in the H2-O2 cell to
that estimated by the Nernst equation. The setup for the H2-O2 cell
is quite similar to that shown in Fig. 1. The Wagner polarization
method ͑ion-blocking method͒ was used for the separation of elec-
tronic conductivity from oxide-ion conductivity. The experimental
setup for the polarization method is schematically shown in Fig. 2.
Results and Discussion
Electrical conducting property of LSGF.—Figure 3 shows
Arrhenius
plots
of
the
electrical
conductivity
of
La1ϪxSrxGa0.6Fe0.4O3 at pO ϭ 10Ϫ5 atm. It is seen that all speci-
2
mens exhibit a high total conductivity in the range 1-10 S/cm. The
total conductivity increased with increasing amount of Sr and at-
tained a maximum at x ϭ 0.3 at temperature lower than 973 K.
Conductivity of all specimens exhibited metal-like temperature de-
pendence at temperatures higher than 973 K, namely, conductivity
decreased despite increasing temperature. The generally accepted
explanation for this involves the loss of oxygen from the sample at
high temperature at the expense of electron holes. This is also re-
lated to the thermal reduction of Fe. The conductivity reached a
maximum at x ϭ 0.2 at temperatures Ͼ973 K. In any case, it can be
said that La1ϪxSrxGa0.6Fe0.4O3 exhibits a high total conductivity.
Figure
4
shows
the
electrical
conductivity
of
La1ϪxSrxGa0.6Fe0.4O3 as a function of oxygen partial pressure. In the
pO range higher than pO ϭ 10Ϫ5 atm, the electrical conductivity
2
2
increased with increasing oxygen partial pressure and pO depen-
2
dence of electrical conductivity is almost pϪO1/4 . Consequently, it is
2
clear that the main electronic charge carrier in La1ϪxSrxGa0.6Fe0.4O3
is the electronic hole, which could be assigned to the formation
of Fe4ϩ
,
namely, FeG•a
.
The electronic conductivity of
La1ϪxSrxGa0.6Fe0.4O3 was almost independent of the oxygen partial
pressure in a range of pO ϭ 10Ϫ15 to 10Ϫ5 atm. In our previous
Figure 2. Schematic view of the experimental setup for the polarization
method.
2
study on La0.8Sr0.2Ga0.8Mg0.2ϪxFexO3 ͑LSGMF͒,10 the valence num-
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Journal of The Electrochemical Society, 150 ͑1͒ E17-E23 ͑2003͒
E19
Figure 5. Temperature dependence of the transport number of oxide ions in
La1ϪxSrxGa0.6Fe0.4O3 .
Figure 4. Electrical conductivity of La1ϪxSrxGa0.6Fe0.4O3 as a function of
the oxygen partial pressure.
doped LaGaO3 exhibits simultaneously high oxide ionic and elec-
tronic hole conduction. The largest permeation rate of oxygen is
theoretically achieved at 0.5 of the transport number of oxide ion
when the total conductivity is the same as shown in Eq. 1. There-
fore, Fe-doped LaGaO3 perovskite-type oxide is suitable as a mixed
conductor for the oxygen-permeating membrane. Since the highest
electrical conductivity was obtained, LSGF is highly attractive as
the mixed electronic-oxide ionic conducting material. Contribution
of the electronic charge carrier was further studied by an ion-
blocking method.
Electronic conductivity can be separated from that of oxide ion
with the Wagner’s polarization ͑ion-blocking͒ method. In this study,
contribution of electronic conductivity to the total conductivity was
estimated using the polarization method. Figure 6 shows Ie vs. V
curves for LSGF. In the usual case, ‘‘S’’-shaped curves are observed
in these polarization measurements on samples having electronic ͑n͒
and hole ͑p͒ conduction.12,13 However, electronic current Ie in-
creased monotonically with increasing applied potential in the case
of LSGF. This suggests that p-type conductivity was always domi-
nant in LSGF and no n-type conductivity was observed at applied
ber of Fe in LSGMF was measured with electron spin resonance
͑ESR͒ and it was seen that the ESR signal assigned to Fe3ϩ in-
creased by reducing the sample. Therefore, ESR data suggests that
Fe3ϩ is the dominant species and Fe4ϩ forms in the oxidizing atmo-
¨
sphere, expressed as the following equation by Kroger-Vink notation
FexGa ϩ 1/2 O2 ϩ VO¨
ϭ FeG•a ϩ OxO ϩ h •
Formation of Fe4ϩ
͒
͓1͔
͑
At low pO atmosphere, it is considered that Fe2ϩ is formed as
2
the following equation
2FexGa ϩ OxO ϭ 2FeGЈa ϩ 1/2 O2 ϩ VO¨
Formation of Fe2ϩ
͒
͑
͓2͔
Therefore, in the intermediate pO range, these two equations should
2
be balanced and as following
3FexGa ϭ FeG•a ϩ h • ϩ 2FeGЈa
͓3͔
Since no gaseous oxygen is included in this equation, the electrical
conductivity is independent of pO , that is shown in Fig. 4. There-
2
fore, amount of Fe4ϩ and hole should be balanced with that of Fe2ϩ
as shown in the following equation
Fe •
ϩ
p • ϭ 2 FeЈ
͔
Ga
͓4͔
͓
͔
͓
͔
͓
Ga
in which Fe •
,
p • , and FeЈ represent the amount of Fe4ϩ
,
͓
͔
͓
͔
͓
͔
Ga
Ga
electronic hole, and Fe2ϩ, respectively. Although the number of the
iron compounds consisting of Fe4ϩ is very limited, it is reported that
Fe4ϩ is formed in perovskite-type oxide, SrFeO3 .11 However, con-
sidering the stable valence number, trivalent iron is the dominant
species both in O2 and H2 atmosphere.
The temperature dependence of the transport number of oxide
ion in La1ϪxSrxGa0.6Fe0.4O3 , which was estimated by emf in the
H2-O2 cell, is shown in Fig. 5. Obviously, emf in H2-O2 cells using
La1ϪxSrxGa0.6Fe0.4O3 were almost half those estimated by the
Nernst equation, and, furthermore, it changed by changing the pO
Figure 6. Ie vs. V curves obtained with the ion-blocking technique for LSGF
2
difference across the membrane. Therefore, it can be said that Fe-
at 1173 K.
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E20
Journal of The Electrochemical Society, 150 ͑1͒ E17-E23 ͑2003͒
Figure 7. pO dependence of hole and oxide-ion conductivity in LSGF at
2
1172 K.
Figure 8. Temperature dependence of the permeation rate of oxygen
through LSGF and LSGM membranes ͑0.5 mm thick͒.
La0.9Sr0.1Ga0.8Mg0.2O3 was short-circuited by using external lead wire.
potentials less than 0.5 V. Electronic hole conductivity can be esti-
mated by the slope of Ie vs. V curves. Also, the applied potential
across the sample corresponds to the Nernst emf values across the
sample. Since pure oxygen was used for the reference gas, we can
estimate the oxygen partial pressure p ͑pumping͔͒ in the pump-
͓
O
2
in LSGF is higher than that of LSGM, since the rate-limiting step
for the oxygen permeation through LaGaO3 membrane is the bulk
diffusion step. This is because the oxygen permeation rate
monotonically increased with decreasing the thickness of membrane
in both cases. The oxygen permeation rate of LSGF is 2.3 times
higher than that of LSGM. This improvement in oxygen permeation
rate is also in good agreement with the improved oxide-ion conduc-
tivity shown in Fig. 3. In any case, considering the oxygen-
permeating rate of 2.1 cm3 std/cm2 min at 1173 K reported for
La0.6Ba0.4Co0.8Fe0.2O3 ,2 it can be said that LSGF exhibits large oxy-
gen permeation rate which can be attributed to its fast oxide-ion
conductivity.
Figure 9 shows the effects of the thickness of LSGF membrane
on the oxygen permeation rate. The oxygen permeation rate through
the mixed electronic-oxide ionic conducting membrane is theoreti-
cally expressed by the following equation when the bulk diffusion
step is the rate-limiting step14
ing room from the applied potential and pO (Ref.) ϭ 1 atm by
2
using the Nernst equation
pO pumping͒ ϭ exp Ϫ4VF/RT͒
͓5͔
͑
͑
2
Here, V indicates applied potential and F, R, and T are Faraday
constant, gas constant, and temperature, respectively.
p-Type conductivity was estimated based on the polarization
measurement and is shown in Fig. 7 as a function of oxygen partial
pressure. By subtracting the p-type conductivity from the total con-
ductivity, the oxide-ion conductivity was also estimated, which is
also shown in Fig. 7. Since oxide-ion conductivity is independent of
the oxygen partial pressure, two points at high and low pO were
2
calculated in Fig. 7. It is clear that p-type conductivity in LSGF
gradually decreased with decreasing oxygen partial pressure and it
became slightly steep at pO smaller than 10Ϫ5 atm. The depen-
2
dence of p-type conductivity on pO is ca. 1/50 at pO
2
2
ϭ 0.21 ϫ 10Ϫ5 atm. Since the pO dependence of total conductiv-
2
ity in Fig. 3 for the corresponding range is ca. 1/30, the pO depen-
2
dence of the estimated p-type conductivity is in reasonable agree-
me•nt. These results also suggest that the main defects in LSGF are
FeGa and electronic hole, and the amount of FeG•a is balanced with
that of V.O. . In addition, the estimated transport number of oxide ion
by the polarization method was 0.59, which is also in good agree-
ment with that by emf in the H2-O2 gas concentration cell, 0.63. It
is also noted that the oxide-ion conductivity in LSGF is extremely
high ͑ca. 0.6 S/cm͒, which is 2.7 times higher than that of oxide-ion
conductivity in La0.8Sr0.2Ga0.8Mg0.2O3 ͑LSGM͒.
Oxygen-permeating property in LSGF.—Figure 8 shows the tem-
perature dependence of the permeation rate of oxygen through
LSGF membrane. In this figure, the oxygen permeation rate through
an LSGM membrane at short-circuited condition is also shown as a
reference. It is clear that a high oxygen permeation rate was
achieved on LSGF membrane at all temperatures. The oxygen per-
meation rate was attained to be 1.8 cm3 std/cm2 min at 1273 K,
which is almost three times higher than that of LSGM membrane at
the short-circuited condition. The higher oxygen permeation rate of
LSGF than that of LSGM suggested that the oxide-ion conductivity
Figure 9. Effects of the thickness of LSGF membrane on the oxygen per-
meation rate. (La0.6Sr0.4CoO3 was used for the surface catalyst.͒
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Journal of The Electrochemical Society, 150 ͑1͒ E17-E23 ͑2003͒
E21
rate at 1173 K attained a value of 2.5 cm3 std/cm2 min when the
thickness of the LSFC membrane was 0.3 mm. Therefore, the rate-
limiting step for oxygen permeation through LSGF membrane
seems to be the bulk diffusion process for the thickness range stud-
ied in this paper. Consequently, it is expected that further increase in
oxygen permeation rate is obtained by decreasing membrane thick-
ness. However, with decreasing temperature, dependence of oxygen
permeation rate on membrane thickness became insignificant. This
suggests that the surface reaction rate became slower and was the
rate-determining steps with decreasing temperature. Therefore, in
order to improve the oxygen permeation rate at 873 K, improvement
in catalytic activity is required.
Effects of the surface catalysts on the oxygen permeation rate in
LSGF membrane were further studied and the results are shown in
Fig. 10. The oxygen permeation rate was strongly dependent on the
surface catalyst. Since almost no oxygen permeation was observed
through LSGF membrane when no catalyst was applied as shown in
Fig. 10, surface catalyst is essential for obtaining the high oxygen
permeation rate. This suggests that the surface activity of LSGF
to oxygen dissociation reaction is low; however, oxide ion diffusiv-
ity in bulk is high. Consequently, the oxygen permeation through
LSGF membrane is strongly influenced by surface catalyst, and it
was seen that the oxygen permeation from air to Ar at 1273 K
Figure 10. Effects of the surface catalysts on the oxygen permeation rate
from air to Ar through LSGF membrane ͑0.5 mm͒.
Ͼ
increased in the following order. La0.6Sr0.4CoO3
La0.8Sr0.2CoO3
Լ Sm0.5Sr0.5CoO3 ӷ Sm0.6Sr0.4CoO3 as the surface catalyst. It was
reported that Sm0.5Sr0.5CoO3 exhibits the highest activity to the ca-
thodic reaction of solid oxide fuel cells ͑SOFCs͒ using
LaGaO3-based electrolyte at intermediate temperature.15,16 There-
fore, it is considered that SmCoO3 is highly active for the oxygen
dissociation reaction. However, La0.6Sr0.4CoO3 , which is not so
highly active to the cathodic reaction of SOFC, exhibited the highest
activity for oxygen permeation. Therefore, it seems likely that the
surface reaction steps contained in the oxygen permeation are dif-
ferent from those in the cathode of the fuel cell. In any case, it was
seen that La0.6Sr0.4CoO3 is the preferable surface catalyst for the
LSGF oxygen-permeating membrane among the examined oxides.
RT␴ ␴
ph
p1
e
i
JO
ϭ
ln
͓6͔
16F2 ␴ ϩ ␴ ͒t
2
͑
e
i
where JO is oxygen permeation rate, t the thickness of membrane, T
2
temperature, R gas constant, ph and p1 are oxygen partial pressures,
F the Faraday constant, and ␴ and ␴ are the electronic and oxide
e
i
ionic conductivity, respectively. Based on this equation, JO is in-
2
versely proportional to the thickness of membrane. As shown in Fig.
9, the oxygen permeation rate monotonically increased with de-
creasing thickness of the membrane, and the oxygen permeating
Figure 11. Temperature dependence of oxygen permeation rate, CH4 conversion, and yield of CO and H2 . (La0.6Sr0.4CoO3 and Ni were used for air and CH4
side catalysts, respectively.͒
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Journal of The Electrochemical Society, 150 ͑1͒ E17-E23 ͑2003͒
Figure 12. Oxygen permeation rate and the product yield in CH4 partial oxidation as a function of membrane thickness at 1273 K.
The oxygen-permeating property of LSGF under CH4 partial
oxidation.—Application of LSGF to the oxygen-permeating mem-
brane for CH4 partial oxidation was further studied. Figure 11 shows
the temperature dependence of oxygen permeation rate, CH4 con-
version, and yield of CO and H2 . It is clear that the oxygen perme-
ation rate increased drastically by changing Ar to CH4 in the perme-
ation side. This increase in oxygen permeation is predicted from Eq.
the thickness of the LSGF membrane or increasing the pO gradient
across the membrane.
2
Figure 12 shows the oxygen permeation rate and the product
yield in CH4 partial oxidation as a function of membrane thickness
at 1273 K. It is clear that the oxygen permeation rate monotonically
increased with decreasing the thickness of the LSGF membrane.
These results also suggested that the rate-limiting step for the CH4
partial oxidation is a bulk diffusion step even for 0.3 mm thick
membrane. The permeation rate from air to CH4 was 12 cm3 std/min
cm2 at 1273 K as shown in Fig. 12. In accordance with the increase
in the amount of permeating oxygen, CH4 conversion as well as
yield of CO and H2 also increased monotonically. Since the amount
of carbon was almost balanced before and after the reaction, no
carbon seems to form on Ni catalyst. Absence of carbon formation
on Ni catalyst was also confirmed by visual examination after partial
oxidation reaction. H2 /CO ratio was always close to two. Conse-
quently, it seems that no carbon was deposited during CH4 partial
oxidation in the LSGF membrane reactor and the CH4 partial oxi-
dation into CO/H2 gas was the dominant reaction. Therefore, the
CH4 partial oxidation process combined with the oxygen separation
membrane using mixed oxide ion conductor is highly attractive for
the conversion of CH4 into liquid fuels, because the size of the
reactor is small and the process is also simple.
Figure 13 shows the X-ray diffraction ͑XRD͒ patterns before and
after CH4 partial oxidation for 12 h. It was seen that no significant
difference was observed on the XRD patterns of the LSGF mem-
brane before and after the CH4 partial oxidation reaction. Therefore,
the LSGF membrane has enough stability against reduction and oxi-
dation. SEM observation also confirmed that there is no crack ob-
served on the surface of the membrane. Absence of crack formation
during CH4 partial oxidation is also confirmed by no N2 leakage
observed in the product stream. Consequently, it can be concluded
that LSGF has good chemical stability, which is enough for the
oxygen-permeating membrane for the CH4 partial oxidation.
6 because the pO gradient becomes steeper. The permeation rate
2
attained a value of ca. 8 cm3 std/min cm2 at 1273 K when the
thickness of the electrolyte was 0.5 mm. CH4 conversion of 40%
and yield of CO and H2 at the same value to CH4 conversion were
also obtained at 1273 K. Because the yield of CO and H2 were
almost the same as that of CH4 conversion and CO2 formation was
negligibly small, it is obvious that the CH4 partial oxidation domi-
nantly proceeds in the membrane reactor using LSGF as the oxygen-
permeating membrane.
Effects of oxygen partial pressure in oxidant on the permeation
rate through LSGF membrane under the CH4 partial oxidation were
further studied and the results are compared in Fig. 11. It is reason-
able that oxygen permeation rate further increased by changing from
air to oxygen at all temperatures examined. This suggests that the
rate-limiting step of the oxygen permeation under the CH4 partial
oxidation is still a bulk diffusion process. The amount of oxygen
permeation was as high as 12 and 6 cm3 std/min cm2 at 1273 and
1073 K, respectively, when the thickness of the membrane is 0.5
mm. CH4 conversion was also improved by changing air to oxygen.
However, formation of CO2 was observed and it became significant
with decreasing reaction temperature when oxygen was used as oxi-
dant. This suggests that the surface activity of the Ni catalyst used
for the partial oxidation of CH4 was not high enough, and the
amount of permeating oxygen became excess when pure oxygen
was used as a source of oxygen. Therefore, further increase in the
yield of CO and H2 can be achieved ͑if the activity of Ni can be
improved by improving dispersion͒. It is also noted that the pressur-
izing air also gives the same positive effects on the oxygen perme-
ation rate. Even at 873 K, the rate-determining step for oxygen
permeation seems to be the bulk diffusion, and so it is expected that
the oxygen permeation rate can be further improved by decreasing
Conclusion
Although mixed oxide-ion conductivity in LaGaO3-based oxide
has not been studied extensively, it was found that Fe-doped,
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Journal of The Electrochemical Society, 150 ͑1͒ E17-E23 ͑2003͒
Acknowledgments
E23
The authors acknowledge financial support from a Grant-in-Aid
for Science Promotion from the Ministry of Education, Culture,
Sports, Science, and Technology of Japan ͑no. 11102006 and
13355032͒. Financial support from NEDO is also appreciated.
Oita University assisted in meeting the publication costs of this article.
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