Manufacturing and characterization of the porous supported
Ba0.5Sr0.5Co0.8Fe0.2O3-δ oxygen transport membranes
*
Wei-Xin Kao (高維欣), Tai-Nan Lin (林泰男), Ming-Wei Liao (廖明威),
Yu-Ming Chen (陳佑明), Chun-Yen Yeh (葉俊彥), Hong-Yi Kuo (郭弘毅)
Nuclear Fuels and Materials Division, Institute of Nuclear Energy Research
核能研究所，核子燃料與材料組
Abstract
The Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) powder is synthesized by glycine-nitrate combustion (GNC) process and the
oxygen transport membrane (OTM) using BSCF is prepared by tape casting method. The porous supported OTM is
composed of the porous supported substrate and the dense membrane with a thickness of 120μm. The oxygen
permeation flux is measured in an air and argon gradient and the oxygen permeability is determined based on the
operating temperature and the porosity of porous supported substrate. The oxygen permeation flux is increased with
the increasing operating temperature as well as the inlet gas flow rates. The results indicate that the oxygen
permeation flux is strongly influenced by the increasing of the inlet gas flow rates and operating temperatures. The
oxygen permeation flux of the OTM with porous supported substrate porosity of 26 % is higher than that of 18 % with
a fixed device thickness. This result indicates that the higher porosity of porous substrate can provide larger surface
exchange area for improving the oxygen catalytic rate. The maximum oxygen permeation flux with substrate porosity
of 26 % is 1.9 ml min-1 cm-2 at 900°C with air flow rate of 100 ml min-1 in feed side and argon flow rate of 200 ml
min-1 in permeated side, respectively.
Keywords: oxygen transport membranes, oxygen permeation flux, porous supported layers, Ba0.5Sr0.5Co0.8Fe0.2O3-δ
(BSCF) powder
1. Introduction
An important strategy for reducing CO2 emission
is carbon capture and storage (CCS). The emission of
CO2 from fossil power plant can be reduced by oxyfuel
combustion [1-2]. In an oxyfuel power plant, the fossil
fuel is combusted by pure oxygen and formed primarily
CO2 and H2O in the outlet gas [2-3]. The CO2 can be
captured easier and cheaper than when air is used in the
combustion process. The required oxygen can be
provided by oxygen transport membranes (OTMs) [1,
3-4]. In principle, the OTMs allow oxygen ion diffusion
through vacancies in the crystal lattice of the OTMs
when the oxygen partial pressure is high enough in the
feed side to provide the driving force for oxygen ion
transportation across the OTMs to the permeate side
[1-2]. Therefore, the OTMs can separate oxygen at
purity of 100 % from air.
The mixed ionic-electronic conducting (MIEC)
oxides are multifunctional materials that widely used as
cathode material of solid-oxide fuel cells (SOFCs) and
oxygen permeation membranes [1,4-5]. Among the
various MIEC materials, the most promising materials
are perovskites with the formula ABO3-δ.The material
Ba1-xSrxCoyFe1-yO3-δ is known to have high oxygen
permeation flux and phase stability under long-term test
at high temperatures [4, 6]. According to prior reports,
the BSCF membrane with 1~1.5mm thickness exhibited
a oxygen permeation flux of 1.35-4.0 ml min-1 cm-2 at
900~925ºC [6-8]. The oxygen permeation flux base on
dense membrane diffusion can be described by the
Wagner equation [3, 9] :
𝑗𝑜2 = −
ln 𝑃"𝑂2
𝑅𝑇
𝜎𝑒𝑙 𝜎𝑖𝑜𝑛
∫
𝑑 ln 𝑃𝑂2
2
16𝐹 𝐿 ln 𝑃′ 𝑂 𝜎𝑒𝑙 + 𝜎𝑖𝑜𝑛
2
where 𝑗𝑜2 is the oxygen permeation flux, R is the
gas constant, L is the dense membrane thickness, F is
the Faraday constant, σel is the material electronic
conductivity, σion is the material ionic conductivity,
𝑃′ 𝑂2 and 𝑃"𝑂2 are the high and low oxygen partial
pressure, respectively.
According to the Wagner equation, the oxygen
permeation flux can be increased by decreasing the
membrane thickness. Thus, to develop OTMs with high
oxygen permeation flux, it is essential to reduce the
thickness of OTMs for increase oxygen permeation flux.
The oxygen permeation flux through BSCF membrane
is mainly controlled by oxygen diffusion rate and
oxygen gas-solid surface exchange rate. The porous
supported membrane is needed for the mechanical
stability, when the thickness reduces very low 3) and the
porous supported layer can increase the surface area
exposed the gas phase [9-10].
In this study, the Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF)
powder is synthesized by glycine-nitrate combustion
(GNC) process and investigated using X-ray diffracted
(XRD) to ensure phase purity. According to prior
reports, the BSCF OTM was mainly adopting die-press
to prepare the disc-shaped green membrane, followed
by sintering to obtain the OTM device. In our study, the
BSCF green membrane made by tape casting process in
fabricating the green slip with and without designated
amount of pore former, forming the dense and porous
membrane structure, followed by lamination of selected
layers of the slips and sintering to obtain the BSCF
OTM device. The detailed fabricating procedures of
tape casting processes were described in our previous
studies [11]. The microstructure of the BSCF OTM is
characterized by field emission scanning electron
microscopy (SEM). The operational variables of the
oxygen permeation flux, such as operation temperature,
and feed gas flow rates have been investigated and
reported in this study. The novelty of this study is to
control the casting recipe as well as the porosity of the
porous support, thus forming OTM device with larger
size, and exhibits reliable oxygen permeation property.
measured by the nitrogen in the permeated gas while
using air as feed gas.
2. Experimental
2.1 BSCF powder and membrane preparation
The Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) powders were
synthesized using the glycine-nitrate process. In this
case, stoichiometric amounts of Ba(NO3)2 (SHOWA,
99 %), Sr(NO3)2 (Merck, 99%), Co(NO3)2·6H2O
(SHOWA, 98 %), and Fe(NO3)3·9H2O (Merck, 99 %)
were used as the starting raw materials. Metal nitrates
were dissolved in distilled water and then the glycine
was added to the solution. The mixture was heated on a
hot plate, evaporated to a viscous gel, and ignited with
a flame, resulting in a BSCF ash of a dark-gray color.
The phase identification of BSCF was performed by
X-ray diffraction (XRD, Bruker, D8 advance) using
CuKλ radiation in the range of 2= 20o - 80o with a
scanning speed of 2o (2 min-1. The oxide slurry was
prepared by the mixing of BSCF powders, the
dispersant, binder, plasticizer as well. BSCF green
substrate was tape-casted with individual layer
thickness of ~ 130 µm. Layers of green tapes were cut
and assembled with certain area and membrane
thickness. The green membrane was then subjected to a
hot-press process via a laminator for several times and
further calcined at 1100 °C for 4 hours to form the
BSCF oxygen transportation membrane. The slurries
for the porous supported layer containing 6 % and 10 %
graphite powder (Alfa Aesar) as pore former in relation
to the BSCF powder content were prepare leading to
porosity of the porous supported layers of 18 % and 26
%, respectively, whereas the slurry for the dense
membrane did not contain any pore former. The
porosity of porous substrates was determined by
Archimedes method. Both the BSCF dense membrane
and porous supported layer were prepared by tape
casting process (ECS, Model CS-8). Then, the BSCF
dense green tape was printed onto the BSCF porous
green tape and then sintered at 1100 °C for 4 hours.
Field emission scanning electron microscopy (FE-SEM,
Hitachi, S-4800) and XRD were used to evaluate the
microstructure and crystal structure.
2.2 Oxygen permeation flux measurement
The OTM sample was set in a lab-scale
measurement station for the oxygen permeation
property investigations. Fig. 1 illustrates the testing
setup for oxygen permeation measurement. The
membrane was cut to a circle with diameter of 2 cm
seated on the alumina tube, followed by sealing at 820
°C by ceramic paste. The measurement was conducted
under various temperature and air/argon flow rates. The
oxygen permeation flux was thus investigated by
analyzing the oxygen volume ratio of argon in the
outlet port via online gas chromatography (GC,
INFICON, Micro GC 3000) and the leakage was
Fig.1 Schematic illustration for the oxygen permeation
testing setup
3. Results and Discussion
3.1Microstructure and phase anal ysis
Fig. 2 shows the XRD results for the BSCF
powders. The as-synthesized BSCF powders reveal
poor crystallinity. The as-synthesized powders show
the weak crystallinity of cubic perovskite structure, and
there were many miscellaneous peaks in the XRD
pattern. The powders were followed by calcination in
air for 4 hours at 600~1100 °C. The crystallinity
improves with calcination temperature and the
completed crystal structure of single cubic perovskite
phase is observed after calcination above 950°C. These
diffraction peaks have been identified by previous
work.11) The cubic perovskite structure of single-phase
BSCF was obtained via calcination above 1000 °C. Fig.
3 shows the cross-sectional SEM image for the BSCF
membrane after 1100 °C calcination. The thickness of
the membrane is about 480 µm and the dense and
pore-free microstructure can be observed, ensuring the
oxygen ion to effectively transport across the
membrane. Fig. 4 shows the cross-sectional SEM
image for the BSCF membrane with porous supported
layer. The dense and pore-free microstructure can be
observed, ensuring the oxygen ion to effectively
transport across the membrane. It clearly shows that the
BSCF dense membrane with a thickness of
approximately 120 µm and exhibits the well interfacial
contacts with the porous supported layer. The porous
supported substrates have the porous microstructures to
assure the good diffusion for air and hence provide
large surface area to enhance the oxygen surface
exchange kinetics. The density of BSCF dense layer is
estimated to be ~5.35 g cm-3. The porosity of porous
substrates containing 6 % and 10 % graphite as pore
former are estimated to be ~ 18 % and ~ 26 %,
respectively.
3.2 Oxygen permeation flux measurements
The oxygen permeation test result at various
temperatures is shown in Fig. 5 with fixed argon flow
rates of 100 mlmin-1. The oxygen permeation flux
increases with temperature and it reaches 1.05
mlmin-1cm-2 at 900 °C. In Fig. 5, the variation of
oxygen permeation flux with different inlet air flow rate
at 900 °C. The flux increases with the elevated air flow
rate which suggests increased oxygen partial pressure
in the inlet port. The oxygen permeation test result at
various temperatures is shown in Fig. 6 with fixed
air/argon flow rates of 100 mlmin-1. The oxygen
permeation flux increases with temperature and it
reaches 1.05 mlmin-1cm-2 at 900 °C.
Fig. 2 XRD patterns for BSCF powders calcined at
various temperatures.
Fig. 3 Cross-sectional SEM image for
membrane after 1100 °C calcination
BSCF
Fig. 5 Oxygen permeation flux for BSCF membrane
with different air flow rates at 900 °C
Fig. 6 Oxygen permeation flux for the BSCF
membrane at different testing temperature
Fig. 4 SEM micrographs of the cross section of the
OTMs with porous supported layer of (A) 18 %
porosity and (B) 26 % porosity.
Fig. 7 shows oxygen permeation flux as a function
of air flow rate on the feed side at different operating
temperatures. The sweep gas flow rate of argon at the
permeate side is fixed at 100 ml min-1. In the
temperature range from 700 to 900 °C, it is found that
the oxygen permeation flux is increased with increasing
air flow rate as well as the operating temperature. When
the operating temperature is at 700 °C, the oxygen
permeation flux increased slightly as air flow rate
increase. However, the permeation flux values became
more sensitive to the air flow rate at higher operating
temperatures. The oxygen permeation flux increased
sharply with the increasing air flow rates at 800 and
900 °C up to air flow rates of 130 ml min-1, then the
oxygen permeation flux increase tends to maintain
certain stability with a further raise of the air flow rates.
Fig. 7 Effect of air flow rates on the oxygen
permeation flux at different operating
temperatures with argon flow rate of 100 ml
min-1. (The porosity of porous supported layers
is 26 %)
Fig. 8 shows the oxygen permeation flux as a
function of argon flow rate on the permeated side at
different operating temperatures. The air flow rate at
the feed side is fixed at 100 ml min-1. From Fig. 7 and
Fig. 8, it shows that the oxygen permeation flux
increases with operation temperature as expected, due
to the fact that the elevated temperature can enhance
the surface exchange kinetics and contribute to a
significance effect on oxygen permeability for oxygen
permeation. From Fig. 8, the oxygen permeation flux
increases from 1.64 to 1.90 ml min-1 cm-2 while the
sweep gas (argon) flow rate increases from 100 to 200
ml min-1 at 900 °C. It shows that the increase in the
sweep gas flow rate is positive effect for the oxygen
permeation flux. This indicates that the increase of
sweep gas reduces the oxygen partial pressure in the
permeated side, hence providing a greater driving force
for the permeated oxygen throughout the membrane to
be carried away for the GC measurement. From the
above results, the high sweep gas and air feed flow
rates are needed to increase driving force between the
permeate side and feed side for enhance the for oxygen
permeation. These results are in agreement with
previous reports by other researcher [11-13]. The
oxygen permeation flux is reached 1.90 ml min-1 cm-2
at the 900 °C with air flow rates of 100 ml min-1 and
argon flow rates of 200 ml min-1, respectively.
Fig. 9 shows the correspondence of the oxygen
permeation flux as a function of air flow rate on the
feed side at different porous supported substrate
porosity. The effect of the air flow rates and operation
temperature on the oxygen permeation flux is similar to
that of above mentioned results. The oxygen
permeation fluxes are 1.12 and 1.64 ml min-1 cm-2 with
the porosity of porous supported layer are 18% and 26
% with air flow rate of 100 ml min-1 and Ar flow rate of
100 ml min-1 at 900 °C, respectively. It shows that the
oxygen permeation flux increase with the increasing
porosity of porous supported layer. This result indicates
that the oxygen permeation flux increases with the
increasing porosity, so as to improve the oxygen
surface exchange kinetics in the same manner.
Fig. 8 Effect of argon flow rates on the oxygen
permeation flux at different operating
temperatures with air flow rate of 100 ml min-1.
(The porosity of porous supported layers is 26
%)
Fig. 9 Effect of air flow rates on the oxygen permeation
flux with porous supported layer of different
porosity at 900 °C. (Flow rate of argon = 100 ml
min-1)
Fig. 10 shows oxygen permeation flux as a function
of air flow rate on the feed side with different thickness
of the porous support layer. The oxygen permeation
flux of the porous support layer thickness with 580 μm
and 360 μm with 18% of porosity are 1.13 and 1.52 ml
min-1 cm-2 with air flow rate of 100 ml min-1 and Ar
flow rate of 100 ml min-1 at 900oC. It shows that the
oxygen permeation flux increase with the decreasing
thickness of porous supported layer. This result
indicates that the bulk diffusion from porous supported
layer increase with the decreasing thickness of porous
supported layer.
Fig. 10 The oxygen permeation flux of the porous
support layers with 580 μm and 360 μm with
18% of porosity. (Flow rate of argon = 100
ml min-1)
The summary of the oxygen permeation flux with
different thickness of dense membrane layer is shown
in Table 1. It is shown that the oxygen permeation flux
increase from 1.05 to 1.34 ml min-1cm-2 with the
decreasing thickness of dense membrane layer from
480 to 120μm. This result indicates that the increasing
thickness of dense membrane layer caused the bulk
diffusion resistance increases and the oxygen
permeation flux decrease.
By introducing the support concept in the membrane
fabrication, the effect of the thickness and porosity of
the porous supported substrate on oxygen permeation
flux is also shown in Table 1. The porous substrate
provides certain mechanical strength as well as the air
permeation path for the gas transporting through the
porous skeleton to reach the thin dense membrane. It is
shown that the OTMs with the 360μm porous supported
substrate thickness exhibits higher oxygen permeation
flux than that with 580μm porous supported layer
thickness at 900 °C while air flow rate of 100 ml min-1
and argon flow rate of 100 ml min-1. As can be seen,
the OTMs with the 26 % porous supported substrate
porosity exhibits higher oxygen permeation flux than
that with 18% porosity at the same operation conditions
and the same thickness. It indicates the porous
supported layer provides enhanced surface exchange
kinetics and this result in agreement with the
above-mentioned discussion. Then, the oxygen
permeation is mainly dominated by bulk diffusion with
the porous supported substrate porosity of 18 %.
For comparison, OTMs with dense structure and
with porous supported substrate are evaluated. At 900
°C, the oxygen permeation fluxes for OTMs with dense
structure (thickness: 120μm) and with porous supported
layer (thickness of dense membrane / porous supported
layer: 120 / 580μm, porosity of porous supported layer:
26%) are 1.34 and 1.64 mlmin-1cm-2, respectively. The
total thickness of OTM with porous supported layer
with 26% porosity of porous supported layer is 700μm
(thickness of dense membrane /porous supported layer:
120 / 580μm). That thickness is higher than the OTM
with dense structure (thickness: 120μm), but the
oxygen permeation flux is higher than the OTM with
dense structure. It indicates that made a porous
supported layer on dense OTM should be enhanced
surface exchange kinetics for oxygen catalytic and
improved the oxygen permeation flux of the OTM.
Table 1. Summary of the oxygen permeation flux with different thickness and porosity of oxygen transport
membranes at 900 oC (air and argon flow rate is 100 ml min-1, respectively)
Thickness of dense membrane /porous
Oxygen permeation flux
Porosity of porous supported
supported layer (µm)
(ml min-1cm-2)
layer (%)
480
1.05
-420
1.18
-120
1.34
-120 / 580
1.13
18
120 / 360
1.52
18
120 / 580
1.64
26
4. Conclusions
BSCF powders are synthesized successfully via GNC
in the article and proved to be a potential candidate for
the application of oxygen transport membrane by tape
casting technique. The single-phase cubic perovskite
structure can be obtained by calcination at a
temperature above 1000 °C. By tape casting, BSCF
oxygen transportation membrane can be fabricated for
evaluation of oxygen permeation behavior. According
to the SEM results, the dense OTM and porous
supported OTM with the dense membrane and the
thickness of the dense membrane are approximately
480μm and 120μm, respectively. The oxygen
permeation flux increases with the increasing sweep gas
and air flow rates. This result indicates that the increase
sweep gas and air flow rates can increase the oxygen
partial pressure gradient between the permeate side and
the feed side to provide higher driving force for oxygen
permeation. The oxygen permeation fluxes of the
OTMs with dense structure are 1.05, 1.18, and 1.34 ml
min-1 cm-2 with the thickness are 480, 420, and 120μm,
respectively. The oxygen permeation flux of the larger
porosity of porous supported substrate is higher than
that with lower porosity at the same operation
conditions. The oxygen permeation fluxes are 1.12 and
1.64 ml min-1 cm-2 with the porosity of porous
supported layer are 18 % and 26 % with air flow rate of
100 ml min-1 and argon flow rate of 100 ml min-1 at 900
°C, respectively. For the effective argon sweep gas
effect, the maximum oxygen permeation flux with
substrate porosity of 26 % is 1.90 ml min-1 cm-2 at 900
°C with air flow rate of 100 ml min-1 in feed side and
argon flow rate of 200 ml min-1 in permeated side. This
result shows that the porous structure of the porous
supported substrate can provide the large effective
surface area to enhance oxygen surface exchange
kinetics for increased oxygen permeation flux [8,14].
On the other hand, with certain combination of device
fabrication setup, the thickness of the porous substrate
provides certain mechanical strength as well as the air
permeation path for the gas transporting through the
porous skeleton to reach the thin dense membrane.
The oxygen permeation fluxes for OTMs with dense
structure (thickness: 120μm) and with porous supported
layer (thickness of dense membrane / porous supported
layer: 120 / 580μm, porosity of porous supported layer:
26%) are 1.34 and 1.64 mlmin-1cm-2, respectively.
Those different OTMs own the same thickness of dense
membrane, and the OTM with porous supported layer
with higher oxygen permeation flux than that without
porous supported layer. As mentioned previously, the
reduced membrane thickness is an effective approach to
elevate the oxygen permeation flux. However, the
porous supported layer porosity is an important issue in
the development of thin dense membrane with high
oxygen permeation flux.
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製備鋇鍶鈷鐵氧化物氧傳輸膜與其特性分析
*高維欣 林泰男 廖明威 陳佑明 葉俊彥 郭弘毅
行政院原子能委員會核能研究所 核子燃料與材料組
摘
要
鋇鍶鈷鐵氧化物(Ba0.5Sr0.5Co0.8Fe0.2O3-BSCF)) 粉體由甘胺酸-硝酸鹽法(GNC)製備並利用刮刀成型法製備
BSCF 氧傳輸膜，此多孔支撐型 BSCF 氧傳輸膜由多孔性基板與 120 µm 緻密層所組成。其氧滲透通量在不同
操作溫度、支撐層孔隙度下被量測，氧滲透通量隨溫度與進氣速率增加而增加，這結果顯示氧滲透通量會隨
溫度與進氣速率增加而有強烈影響。具 26%孔隙度之多孔性基板的多孔支撐型 BSCF 氧傳輸膜，其氧滲透通
量比具 18%孔隙度之多孔性基板的多孔支撐型 BSCF 氧傳輸膜來的大。這結果顯示，高孔隙度之多孔性基板
可提供較大的交換面積去改善對氧的催化速率。其具 26%孔隙度之多孔性基板的多孔支撐型 BSCF 氧傳輸膜
之最大氧滲透通量在 900 °C 為 1.9 mlmin-1cm-2 (空氣與氬氣進料流率分別為 100 ml min-1 及 200 ml min-1)。
關鍵詞：氧傳輸膜、氧滲透通量、多孔支撐層、鋇鍶鈷鐵氧化物粉體