Effect of enhanced oxygen reduction activity on oxygen permeation of La0.6Sr0.4Co0.2Fe0.8O3−δ membrane decorated by K2NiF4-type oxide

Accepted Manuscript Effect of enhanced oxygen reduction activity on oxygen permeation of La0.6Sr0.4Co0.2Fe0.8O3-δ membrane decorated by K2NiF4-type oxide Ning Han, Shuguang Zhang, Xiuxia Meng, Naitao Yang, Bo Meng, Xiaoyao Tan, Shaomin Liu PII:

S0925-8388(15)31064-1

DOI:

10.1016/j.jallcom.2015.09.086

Reference:

JALCOM 35361

To appear in:

Journal of Alloys and Compounds

Received Date: 21 July 2015 Revised Date:

10 September 2015

Accepted Date: 11 September 2015

Please cite this article as: N. Han, S. Zhang, X. Meng, N. Yang, B. Meng, X. Tan, S. Liu, Effect of enhanced oxygen reduction activity on oxygen permeation of La0.6Sr0.4Co0.2Fe0.8O3-δ membrane decorated by K2NiF4-type oxide, Journal of Alloys and Compounds (2015), doi: 10.1016/ j.jallcom.2015.09.086. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT

Effect of enhanced oxygen reduction activity on oxygen permeation of La0.6Sr0.4Co0.2Fe0.8O3-δ membrane decorated by K2NiF4-type oxide

§

Department of Chemical Engineering, Curtin University, Perth WA 6845, Australia

Corresponding author. E-mail: [email protected](S. Zhang), [email protected](X. Tan).

SC

*

RI PT

Ning Han,† Shuguang Zhang,*,† Xiuxia Meng,† Naitao Yang,† Bo Meng,† Xiaoyao Tan,*, ‡ Shaomin Liu§ † School of Chemical Engineering, Shandong University of Technology, Zibo 255049, China ‡ Department of Chemical Engineering, Tianjin Polytechnic University, Tianjin 300387, China

Abstract: Asymmetric dense La0.6Sr0.4Co0.2Fe0.8O3-δ hollow fiber membranes were prepared by the

M AN U

joint phase inversion and sintering processes. Surface treatment by hydrochloric acid-etching and decoration of dispersed porous (La0.5Sr0.5)2CoO4+δ thin layer, was applied to improve the surface exchange reaction kinetics of oxygen reduction. Oxygen permeation performance of the unmodified and modified membranes was investigated under different operating conditions. Experimental results indicate that both surface treatment strategies effectively accelerate the oxygen permeation rate of the

TE D

resultant membranes by the improved surface exchange kinetics; in particular, the oxygen fluxes were enlarged from 0.036-1.021 ml·min-1·cm-2 of the bare membrane to 0.201-1.311 ml·min-1·cm-2 of the decorated membrane by (La0.5Sr0.5)2CoO4+δ with a minimum improvement percentage of 28%

EP

when operation temperature varied from 700 to 1000 °C with helium flow rate of 150 ml·min-1. Furthermore, better flux enhancement of the surface-modified membrane was observed at lower

AC C

temperature regimes than at higher temperatures indicates at lower temperatures the surface exchange reaction kinetics play a larger role in controlling the overall oxygen transport through the membrane; in other words, the relative limiting effect of the ionic bulk diffusion becomes more noticeable at higher temperatures. In addition to the significant oxygen flux improvement, the surface decorated membrane also displays high permeation stability under the investigated operating conditions.

Key words: hollow fiber membrane, perovskite, K2NiF4-type oxide, oxygen permeability, dispersed surface decoration

ACCEPTED MANUSCRIPT

1. Introduction Oxygen is one of the most important chemicals in this contemporary society as its application

RI PT

has been reached almost every sector in our economy from chemical industries, metal manufacturing, waste treatment, medical/health care, space/military industries, and the huge newly emerging markets of clean energy industry for carbon capture and storage [1,2]. Unfortunately, current industrial tonnage oxygen production is still achieved by cryogenic distillation process, a

SC

100-year-old expensive technique. Thus, if a new cost-effective technique is available to replace the conventional technology, it will be a great advancement to our modern economy and environmental

M AN U

conservation.

Dense ceramic membranes made from mixed oxygen ionic and electronic conductors (MIEC) have attracted significant interest due to the advantages over the conventional processes, including high selectivity (nearly 100% for oxygen) and energy- and cost-efficiency [3-7]. Although small pilot scale facilities associated with this ceramic membrane technology has been set up by Air Product and

TE D

Chemicals, large-scale applications has not been realized due to the commercial requirements for the oxygen flux rates, the material stability and the high temperature sealing techniques to build up the membrane units under high temperature operating conditions. Among the MIEC membranes studied,

EP

ABO3-type perovskite oxides are the focus of research due to their high oxygen permeation flux rates [8-10]. For instance, La0.6Sr0.4Co0.2Fe0.8O3-δ, one of the most typical perovskite materials presenting

AC C

intermediate oxygen flux but relatively high stability has been widely investigated as oxygen separating membrane and SOFC cathode catalyst for many fundamental studies. Consequently, in this work, La0.6Sr0.4Co0.2Fe0.8O3-δ has been selected as the demonstration membrane material to show the effective strategies of surface modification to improve the oxygen flux. The oxygen transport rate through a dense membrane from a certain material is jointly determined by the driving force of oxygen concentration gradient across the membrane and the overall resistances of bulk diffusion and surface exchange reactions [11,12]; thus with a fixed oxygen concentration gradient, the oxygen permeability could be improved by reducing membrane thickness or surface modification [13,14]. Owing to the asymmetric structure of perovskite hollow fiber

ACCEPTED MANUSCRIPT membranes employed in this work, the oxygen flux through such membrane may be more significantly limited by the surface exchange reactions occurring at the gas/solid interface. Under such circumstances, the oxygen flux can then be effectively improved by enhancing the oxygen surface exchange kinetics. This can be realized by two traditional schemes [14-17] as showing in

RI PT

Fig.1: (1) surface roughening by acid-etching or simply abrading with emery paper or coating the membrane surfaces with porous layers which can be of the same material to increase the effective surface area (Fig.1D), (2) coating the membrane surfaces by a second phase with superior oxygen

TE D

M AN U

SC

exchange activity to catalyze a faster oxygen exchange reaction (Fig.1B).

EP

Fig.1. Schematic of surface treatment processes: (A) untreated surface, hereinafter referred to as bare or original surface; (B) traditional surface modification; (C) dispersed-second-phase

AC C

particle modification; (D) surface roughening by acid-etching or abrading with emery paper; (E) surface roughening followed by dispersed-second-phase particle modification, hereinafter referred to as decoration. Note: the number and line thickness of blue arrows denote the magnitude of oxygen surface exchange reaction rate. Recently, Yang et al. carried out a series of electrochemical studies on oxygen reduction reaction (ORR) activity of surface-modified epitaxial perovskite thin films for SOFCs [18,19]. Their results demonstrate that an electrode decorated with a secondary phase can strongly affect its ORR activity [20,21]. However, such influence from decoration differs greatly with different material phase, thickness, and morphology of the surface secondary phase (Fig.1C). Due to the similar

ACCEPTED MANUSCRIPT requirements on oxygen reduction applied on the SOFC cathode surface and the MIEC membrane, the decoration strategy can be borrowed from SOFC fields to modify the conventional perovskite membranes with improved ORR kinetics for oxygen separation as long as the decoration process is fully optimized for best efficiency. In view of the above situation, acid etching followed by

RI PT

dispersed-second-phase particle decoration, a combinatorial method promises to improve the surface modification effect synergistically, was employed here, as shown in Fig.1E. The unmodified and acid-etched systems were also investigated for comparison purpose.

In this study, La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) hollow fiber membrane was chosen as the

SC

substrate membrane and (La0.5Sr0.5)2CoO4+δ, and a K2NiF4-type oxide was used as the surface decoration surface catalyst to improve the ORR in LSCF membrane for oxygen separation. To realize

M AN U

this purpose, the gas-tight La0.6Sr0.4Co0.2Fe0.8O3-δ hollow fiber membranes were firstly prepared through the phase inversion/sintering technique.22 Then a dispersed (discontinuous) porous thin layer of (La0.5Sr0.5)2CoO4+δ was deposited on the outer surface of the membranes. The effect of surface decoration on the oxygen permeation performance was explored theoretically and experimentally.

2. Experimental section

TE D

2.1 Synthesis of powders

The powders of La0.6Sr0.4Co0.2Fe0.8O3-δ (hereinafter referred to as LSCF) and (La0.5Sr0.5)2CoO4+δ (hereinafter referred to as LSC214) were prepared through the sol–gel method. As an example, the

EP

synthesis process of LSCF powder can be briefly described as below. Stoichiometric amount of La(NO3)3·6H2O, Sr(NO3)2, Co(NO3)3·6H2O and Fe(NO3)3·9H2O were dissolved in deionized water

AC C

to form an aqueous solution, and then citric acid and ethylene glycol were added with the mole ratio for citric acid: ethylene glycol: metal ions of 1.5:1.8:1. The transparent solution was evaporated at 80°C under continuous stirring to remove excess water until a viscous gel was obtained. The gel was then transferred to a container placed on a hot plate. Further heating was conducted till self-combustion triggered to form LSCF powder precursor. The as-synthesized powder was subsequently calcined at 800 °C for 4 h to remove the residual carbon and obtain the desired LSCF perovskite structure material. In order to facilitate the subsequent hollow fiber membrane preparation, the calcined LSCF powder was ground and crushed in ethanol by a planet-type ball-miller for 24 h, followed by sieving through a sifter of 200-mesh to exclude larger agglomerates. The K2NiF4-type

ACCEPTED MANUSCRIPT oxide LSC214 powder was prepared in a similar way.

2.2 Fabrication of LSCF hollow fiber membrane The LSCF hollow fiber membrane precursors were fabricated with the calcined and ball-milled LSCF powder by the phase inversion-sintering method [22]. In this work, the spinning mixture was

RI PT

composed of 66.7 wt% calcined LSCF powder, 6.60 wt% polyethersulfone (PESf) and 26.7 wt% 1-methyl-2-pyrrolidinone (NMP). At room temperature, the viscosity of the spinning mixture was measured as 15500 mPa·s at the shear rate of 3 rpm. A spinneret with the orifice diameter/inner diameter of 3.0/1.2 mm was used. Deionized water and tap water were used as the internal and

SC

external coagulants, respectively. After drying, the hollow fiber precursors were sintered at 1350 °C for 4 h to obtain the impermeable hollow fiber membranes. The gas-tightness of the hollow fibers

M AN U

was evaluated by nitrogen leakage experiment [23]. Diameters of the LSCF hollow fiber membranes prepared here are about 2.00 mm (outer) and 1.25 mm (inner).

2.3 Membrane surface etching and decoration

Gas tight hollow fiber membrane surface modification was carried out by chemical etching in

TE D

acid solution or deposited by additional catalyst. A typical preparation procedure for acid etching can be briefly described below. One hollow fiber (35 cm in length) with the bottom end sealed by Teflon was immersed lengthwise in hydrochloric acid solution (1 wt %) in a glass test tube (with an o.d. 1.2 cm, i.d. 1.0 cm and length 40cm). After immersion time for 1-2 seconds, the hollow fiber was

EP

immediately removed outside the acid solution and suspended in the atmosphere for 5 minutes. The reacted fiber was rinsed thoroughly with deionised water and dried at 150oC in an electric oven for

AC C

subsequent characterization. For membrane surface decoration by additional catalyst, a controlled dip-coating method was adopted to deposit the surface activation layer. The thickness of which can be modulated by changing the particle loading of coating slurry (Table 1) as well as immersion times.

Table 1.

Formula of the LSC214 slurry for surface decoration

Experimental parameters Compositions of the slurry

Values

ACCEPTED MANUSCRIPT 15 wt%

2 - butanone

26.7 wt%

ethanol

53.3 wt%

triethanolamine(TEA)

0.5 wt%

polyvinyl butyral(PVB)

2 wt%

polyethylene glycol(PEG)

0.5 wt%

dibutyl phthalate(DBP)

0.5 wt%

SC

high purity graphite

1.5%

Pulling speed

48 h

M AN U

Ball-milling time Dipping times

RI PT

(La0.5Sr0.5)2CoO4+δ powder

One 60 mm·min-1

2.4 Oxygen permeation measurement

TE D

Oxygen permeability of the bare original and the surface decorated hollow fiber membranes was evaluated by the permeation cell with the configuration schematically shown in Fig. 2. The LSCF hollow fiber in length of 30 cm was connected successively to the glass tube and

EP

silicone tube using high-temperature silicone sealant (Tonsan New Materials and Technology Co., Beijing). A larger quartz tube (18 mm in diameter and 400 mm in length) was used to accommodate

AC C

the hollow fiber membrane, connected on both sides with small-diameter quartz tubes and sealed with epoxy glue. The permeation test module was put in a tubular furnace with inside diameter of 22 mm and effective heating length of 50 mm. To avoid the sealing failure of the two joints with organic sealant were kept 50 mm away from the high temperature zone of the furnace. Air was fed into the shell side of the test module and helium was passing through the hollow fiber lumen side to collect the permeated oxygen [14,24]. The gas flow rates were measured by mass flow controllers (D08-8B/ZM, Shanxi Chuangwei, China). Gas composition was monitored online by a gas chromatograph (Agilent 6890N) fitted with a 5 Å molecular sieve column (3 mm in diameter and 3 m in length) and a TCD detector. Highly pure argon with the flow rate of 40 ml·min-1 was used as the

ACCEPTED MANUSCRIPT carrier gas. Oxygen permeation tests were carried out by varying the helium gas flow rate between

SC

RI PT

100-300 ml·min-1 under the operating temperature between 700-1000 °C.

M AN U

Fig.2. Schematic diagram of hollow fiber oxygen permeation test module The oxygen permeation flux is calculated by J O2 =

V ( xO2 − (21 / 78) x N 2 ) Am

Where V is the permeate gas flow rate (ml·min-1), xO2 and xN 2 are the oxygen and nitrogen

Am =

π ( Do − Di ) L ln( Do / Di )

TE D

concentrations in the permeate stream (%). Am (cm2), the effective membrane area, is calculated by , in which L , Do and Di are the effective length for oxygen permeation, outer

EP

diameter and inner diameter of the hollow fiber membrane, respectively [25]. All the values of oxygen permeation fluxes and other gas flow rates were calculated on the basis of standard

AC C

temperature and pressure (STP), and at least three measurements were conducted for each experimental condition. The leakage percentage of the oxygen through the area sealed at high temperatures was less than 0.5% for all experiments. To further investigate the influence of surface decoration on oxygen permeation performance of LSCF membrane, the improvement factor of oxygen flux for the acid-etched and LSC214-decorated LSCF hollow fiber membranes were calculated according to the formula as follows: Improvement Factor =

J O∗ 2 − J O2 J O2

Where J*O2 is the oxygen permeation flux through the modified LSCF membrane by

ACCEPTED MANUSCRIPT acid-etching or LSC214 decoration and JO2 is the oxygen permeation flux through the unmodified LSCF membrane at the same operating conditions of temperature and sweep gas flow rate.

2.5 Characterization techniques The crystal phases of the LSCF hollow fiber membrane and LSC214 powder were determined by

RI PT

X-ray diffraction method (XRD, BRUKER D8 ADVANCE) using a monochromatized Cu-Kα radiation source. Continuous scan mode was used to collect 2θ data from 20° to 80° with each step of 0.02° and 0.1 s. The X-ray tube voltage and current were set at 40 kV and 30 mA, respectively. All the XRD tests were taken under similar condition. Microstructures of the original and decorated

SC

hollow fiber membranes were investigated using a field scanning electron microscope (SEM, FEI

characterization.

3. Results and discussion 3.1 Materials characterization (110)

M AN U

SIRION 200). Gold sputter coating was performed on the samples under vacuum before the SEM

TE D

AC C 20

(211)

(111)

(101)

EP

Intensity(a.u.)

(200)

30

bare membrane 1350°C (220)

(310)

acid-etched membrane 1350°C

precursor powder 800°C

40

50 60 2theta(degree)

70

80

Fig.3. XRD patterns of the LSCF powder and hollow fiber membranes

X-ray diffraction (XRD) patterns of the sintered LSCF powder, original pure LSCF membrane and the acid-etched LSCF membrane are shown in Fig.3. The characteristic peaks of the LSCF powder calcined at 800 °C agree very well with the standard XRD patterns of rhombohedral perovskite phase (JCPDS:86-1665), while the pure LSCF membrane and the acid-etched LSCF membrane calcined at 1350 °C are confirmed to be cubic perovskite phase (JCPDS:75-0279) [26].

ACCEPTED MANUSCRIPT This difference on XRD patterns can be attributed to the influence of different high temperature treatment [27]. It also can be seen that, apart from the cubic perovskite phase, no other characteristic peaks can be detected in the XRD pattern of the blank LSCF membrane and the acid-etched membrane. This indicates that the acid-etching does not contaminate the membranes with other

RI PT

impurity phases [28]. In addition, the intensity of the corresponding characteristic peaks of the hollow fiber membranes is much larger than the powder, implying that the crystallinity of the LSCF

20

TE D

M AN U

Intensity(a.u.)

SC

sintered at 1350°C is much better than the powder calcined at 800 °C.

30

40

50

60

70

80

EP

2theta(degree)

Fig.4.

XRD pattern of the LSC214 powder (T=1000 °C)

AC C

It is well known that in K2NiF4-type A2BO4 composite oxides, the radii of A and B ions should fulfill the following requirement [29]: RA/RB=1.7-2.4

As the structure factors of La2CoO4 and (La0.5Sr0.5)2CoO4 satisfy this relationship, both of them can form the K2NiF4-type structure under appropriate conditions. In general, the K2NiF4-type composite oxide crystallizes into two phases [30,31], i.e. orthorhombic phase (space group: Fmmm) and tetragonal phase (space group: I4/mmm). Fig. 4 shows XRD pattern of the LSC214 powder sintered at 1000 °C, which appears to be of the orthorhombic phase (JCPDS:72-0937). The partial substitution of La3+ by Sr2+ in the La2CoO4 lattice

ACCEPTED MANUSCRIPT has been realized successfully.

3.2 Morphology of the hollow fiber membranes The SEM micrographs of the bare and surface-decorated LSCF hollow fiber membranes are shown in Fig. 5 and Fig. 6, respectively. It can be seen that the prepared membranes demonstrate an

RI PT

integrated asymmetric structure. The fully densified layer is located in the outer part of the membrane wall while the sponge-like porous layer is present in the inner region, which is formed due to the solvent diffusion within the membrane precursor occurring during the spinning process. The outer/inner diameter of the hollow fiber is around 2.00/1.25 mm. Fig.5(e) displays the

SC

micrograph of the outer surface of LSCF membrane. LSCF grain boundaries can be clearly seen with grain sizes ranging from 0.4 to 2 µm. Although some cavities emerge, all of them are isolated pores

M AN U

and no thorough porosities can be detected by the nitrogen leakage test under the pressure difference of 2 bar at room temperature.

In order to get a more porous outer surface, the hollow fiber was etched with diluted acidic solution under careful controlling to avoid the obvious membrane thickness reduction. A thin porous LSC214 layer was deposited on the outer surface of the acid-etched membrane. At the first glance, no

TE D

much change can be inspected after the acid-etching and surface decoration compared to the general structures of the hollow fibers (Fig. 6b). The results of acid-etching and surface modification are given in Fig.6 (c), (d) and (f). However, from the SEM images at higher magnification, the

EP

microstructure change can be clearly observed. The LSC214 phase with thickness of about 9 µm (marked by arrow in Fig. 6(d)) was deposited on the outer surface of the membrane in a

AC C

discontinuous manner as some areas of bare membrane domains with clear LSCF grains could be detected(Fig.6(f)). The LSC214 particles fused and dispersed uniformly on the outer surface of membrane were perfectly integrated with the LSCF membrane substrate. Such a well-integrated interface condition is favorable for the formation and extension of triple-phase boundary (TPB), at which the LSCF phase, LSC214 phase and air phase converge together. Recent research indicates this can greatly minimize the boundary resistance of oxygen ions immigration and enhance the surface exchange kinetics of oxygen [20,21]. Thus we hypothesize that the LSC214-decorated LSCF hollow fiber membrane may improve the oxygen permeation rate greatly owing to its high catalytic activity for surface reactions.

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Fig.5. SEM micrographs of the bare LSCF hollow fiber membrane. (a) cross section, (b) wall section, (c) dense layer, (d) inner surface, (e) outer surface of the hollow fiber. The inner surface micrographs of the bare and surface-decorated LSCF hollow fiber membranes are demonstrated in Fig. 5(d) and Fig. 6(e), respectively. As expected, that the acid-etching or LSC214-decorating made little influence on the microstructure of the inner surface as the sample was

ACCEPTED MANUSCRIPT protected to grantee the surface etching or decoration only occurring on external surface. Actually,

AC C

EP

TE D

M AN U

SC

RI PT

the inner surface in porous structure is beneficial to the surface oxygen exchange reaction.

Fig.6. SEM micrographs of the acid-etched and LSC214-decorated LSCF hollow fiber membranes. (a) cross section, (b) wall section, (c) & (d) acid-etched and LSC214-decorated

ACCEPTED MANUSCRIPT dense layer, (e) inner surface, (f) outer surface of the hollow fiber.

3.3 Oxygen permeation performance of the hollow fiber membranes Table 2 lists the oxygen fluxes reported in literature and from in the present work. The comparison shows that the bare LSCF membrane prepared in this work possesses nearly similar

RI PT

oxygen fluxes as previously reported. Experimental results of oxygen fluxes varying with the flow rates of sweep gas at different operating temperatures are shown in Fig.7. It can be seen that the oxygen flux rates of the acid-etched LSCF membranes is better than that of the bare LSCF

SC

membrane, and the LSC214-decoration on the outer surface of the acid-etched LSCF membranes further increased the oxygen fluxes greatly.

Comparison of oxygen permeation fluxes reported in literature and present data.

LSCF

50

LSCF

50

M AN U

Table 2.

950

0.56

[32]

LSCF

100

950

0.61

Present work

LSCF

100

950

0.64

[32]

LSCF

100

1000

1.15

Present work

HCl acid-etched

46

850

0.35

[28]

900

0.60

[32]

Helium flow rate Material

EP

Ba0.5Sr0.5Co0.9Nb0.1O3

AC C

100

-modified

Oxygen permeation flux

(°C)

(ml·min-1·cm-2)

950

0.50

[11]

TE D

(ml·min-1·cm-2)

Temperature

Reference

Ag-modified

52

900

0.62

[11]

LSC214-modified

100

900

0.78

Present work

Fig.7 shows that the operating temperature has a great effect on the oxygen permeation performance of all these tested LSCF membranes. With the flow rate of helium gas at a constant of 150 ml·min-1, for example, the oxygen fluxes though the acid-etched and LSC214-decorated LSCF membranes had been improved dramatically from 0.036 to 1.021, 0.067 to 1.131 and 0.201 to 1.311 ml·min-1·cm-2, respectively, when the operating temperature was raised from 700 to 1000 °C. Higher operating temperature improves the oxygen permeation by enhancing the bulk diffusion rate and the

ACCEPTED MANUSCRIPT surface-exchange rate. 700°C 750°C 800°C 850°C 900°C 950°C 1000°C

-2 -1

(a)

1.0 0.8 0.6

RI PT

Oxygen permeation flux, ml· min · cm

1.2

0.4 0.2

1.4

150 200 250 -1 flow rate of helium, ml· min

1.2 1.0 0.8 0.6 0.4 0.2 0.0

1.6

700°C 750°C 800°C 850°C 900°C 950°C 1000°C

300

700°C 750°C 800°C 850°C 900°C 950°C 1000°C

(c)

AC C

-1

150 200 250 -1 flow rate of helium, ml· min

EP

-2

100

Oxygen permeation flux, ml· min · cm

300

M AN U

(b)

TE D

-1

Oxygen permeation flux, ml· min · cm

-2

100

SC

0.0

1.4 1.2 1.0 0.8 0.6 0.4 0.2

100

150 200 250 -1 flow rate of helium, ml· min

300

Fig.7. The variation of oxygen permeation fluxes along with the increasing sweep rate of

ACCEPTED MANUSCRIPT helium gas and temperature. (a) bare LSCF membrane, (b) acid-etched LSCF membrane, (c) LSC214-decorated LSCF membrane. In addition, it is evident that the oxygen permeation fluxes are positively associated with the flow rates of helium gas. Higher sweep rate of helium can raise the oxygen permeation flux owing to

RI PT

the enlarged oxygen gradient by lowering the oxygen partial pressure of the permeate side and vice versa. At operating temperature of 1000 °C, for instance, the oxygen permeation fluxes through the bare, acid-etched and LSC214-decorated LSCF membranes are enhanced from 0.942 to 1.218, 1.018 to 1.319 and 1.145 to 1.633 ml·min-1·cm-2, respectively, when the flow rate of helium gas was

SC

adjusted from 100 to 300 ml·min-1. By comparison of the effects of the sweep gas flow rate and operating temperature, we can comment that the operating temperature parameter is much more

M AN U

important for the oxygen permeation through these LSCF membranes. Thus the driving forces for oxygen permeation through the mixed conducting ceramic membranes are associated not only with the oxygen partial pressure gradient but also with the temperature. To some extent, oxygen permeation is a temperature-determining process.

For the commercial consideration, 1 ml·min-1·cm-2 is regarded as the threshold value of oxygen

TE D

permeation flux given the membrane provides sufficient stability [33]. Fig.7 indicates that this threshold value could not be reached by using bare membrane even the operating temperature was raised to 950 °C, while it seems that this target can be achievable for the LSC214-decorated LSCF

EP

membrane to be operated at temperatures higher than 900 °C. Thus the surface-decoration makes LSCF membrane closer to commercial applications. By coating a porous LSC214 layer on the outer

AC C

surface of LSCF membrane, the oxygen permeation flux is improved greatly as the porous LSC214 layer provides not only high specific surface area facilitating the quick diffusion of oxygen molecules to the membrane surface, but also remarkably enhances the surface exchange reaction rate of oxygen ion (high catalytic activity). To further investigate the influence of surface decoration on oxygen permeation performance of LSCF membrane, the improvement factor of oxygen flux for the acid-etched and LSC214-decorated LSCF hollow fiber membranes were calculated and plotted against operating temperature and sweep gas rates in Fig.8.

ACCEPTED MANUSCRIPT

(a)

B

flow rate C of helium, ml· min D 100 E 150 F 200 250 300

1.0 0.8

-1

0.6 0.4 0.2 0.0 750

800

850

900

Temperature, °C

(b)

4 3 2

flow rate of helium, ml· min 100 150 200 250 300

TE D

1 0

1000

-1

M AN U

Enhancement factor

5

950

SC

700

RI PT

Enhancement factor

1.2

700

750

800 850 900 Temperature, °C

950

1000

AC C

EP

Fig.8. The enhancement factors of oxygen permeation fluxes for acid-etched and LSC214-decorated LSCF hollow fiber membranes against operating temperature. (a) acid-etched LSCF membrane, (b) LSC214-decorated LSCF membrane. (Air feed flow rate= 200 ml·min-1). It is observed that the LSC214-decorated LSCF membranes gave much bigger improvement factors in term of oxygen flux than the acid-etched membranes within the entire operating temperature range (700-1000 °C). For both kinds of membranes, it also can be easily identified that remarkable improvements of oxygen permeation fluxes appear at lower than higher operating temperature range (700-850 °C). At 750 °C and the sweep gas rate of 100 ml·min-1, for example, the oxygen permeation fluxes through the bare, acid-etched and LSC214-decorated LSCF membranes were 0.046, 0.084 and 0.249 ml·min-1·cm-2, respectively. Compared with bare LSCF membrane, the flux enhancement factors of acid-etched and LSC214-decorated LSCF membranes were 0.82 and 4.22,

ACCEPTED MANUSCRIPT respectively. It reveals the high catalytic activity of LSC214 material, which is consistent with the result of Yang Shao-Horn’s study on interfacial ORR kinetics at intermediate temperature (500-800°C) [18,20]. In addition, the flux improvement factors always decreases with increasing temperature and are

RI PT

less related with the sweep gas rates on the whole. For instance, the flux improvement factors of acid-etched and LSC214-decorated LSCF membranes were decreased from 1.15 to 0.08, 4.93 to 0.15, respectively, with the temperature rising from 700 to 1000 °C, respectively. It is well known that the oxygen permeation rate is usually determined by bulk diffusion process and the surface exchange

SC

reaction of oxygen. The phenomenon described above implying the rate-determining step of the oxygen permeation process varies with the change in operating temperature. At lower temperatures,

M AN U

the oxygen permeation process is mainly determined by the rate of surface exchange reaction. However, along with the increase of temperature, the rate-determining role of the bulk oxygen diffusion gradually predominates over that of the surface exchange reaction because the activation energy of bulk diffusion is much larger than that of surface exchange reaction. As a result, the enhancement factor of oxygen permeation flux by surface decoration is relatively reduced. Similar

TE D

phenomena had been observed previously [28,33].

3.4 Apparent activation energy of oxygen permeation process To further investigate the effect of surface treatment/decoration on oxygen permeation

EP

performance of LSCF membrane from kinetic points of view, Arrhenius activation energy theory was employed here [34-36]. Fig.9 demonstrates Arrhenius plots of the oxygen permeation flux through

AC C

all kinds of LSCF membranes as a function of operating temperature at a constant helium flow rate of 100 ml·min-1. Obviously, each Arrhenius curve consists of two straight lines: high temperature part and intermediate temperature part. The apparent activation energies of all curves were calculated and summarized in Fig. 9. For all three kinds of LSCF membranes studied here, it can be seen that the apparent activation energy of high temperature scope (850-1000 °C) is lower than that of intermediate temperature scope (700-850 °C). And within them, the sequences of apparent activation energies for both temperature scopes are exactly the same: (1) high temperature scope, LSC214-decorated (46.59 kJ·mol-1) < acid-etched (80.31 kJ·mol-1) < bare (107.28 kJ·mol-1); (2) intermediate temperature scope, LSC214-decorated (65.85 kJ·mol-1) < acid-etched (97.27 kJ·mol-1) <

ACCEPTED MANUSCRIPT bare (115.73 kJ·mol-1).

-14.0

apparent

=46.59kJ· mol

-14.5

-15.5

apparent

Eb,1 = -1 80.31kJ· mol

-16.0

Eb,2

2

-1

ln(JO ), mol· s ⋅cm

-2

-15.0

Ec,2

apparent

-17.0

Ea,1

apparent

=107.28kJ· mol

-17.5 -18.0 0.75

0.80

apparent

apparent

0.85

0.90

1000/T, K

-1

=97.27kJ· mol

=115.73kJ· mol

M AN U

Ea,2

-1

=65.85kJ· mol

-1

SC

-16.5

a b c

-1

RI PT

Ec,1

0.95

1.00

-1

1.05

-1

Fig.9. Arrhenius plots of the oxygen permeation flux through the LSCF hollow fiber membranes. (a) bare membrane; (b) acid-etched membrane; (c) LSC214-decorated membrane.

100 ml·min-1).

TE D

(Temperature varies from 700 °C to 1000 °C; Air flow rate: 200 ml·min-1; Helium flow rate:

Acid-etching provides more surface area (i.e. active sites) than the original LSCF membrane to

EP

improve the surface oxygen reduction reaction kinetics, resulting in the decrease of apparent activation energies. Besides the high specific surface area effect, the high catalytic activity of

AC C

LSC214-decoration significantly raises the surface exchange reaction rate of oxygen, therefore the apparent activation energies further reduced dramatically to nearly half of those of the original bare LSCF membrane. As mentioned in part 3.3, the mechanism of oxygen permeation through the hollow fiber membrane is very complicated. The oxygen transport resistance mainly comes from these two processes, surface exchange reactions and bulk diffusion, depending on the operating temperature. Detailed explanation and understanding of the mechanism behind this complex process is desperately needed now. Only in this way, new oxygen transport materials with better performance could be designed and prepared for applications, such as oxygen permeation membrane, SOFC cathode, oxygen sensor, and so on. For that reason, we will investigate the oxygen transport

ACCEPTED MANUSCRIPT mechanism in detail next by using first principle calculation method at the atomic level.

3.5 Stability test

RI PT

0.8

0.6

SC

0.4

0.2

0.0 0

20

40

M AN U

-1

Oxygen permeation flux, ml· min · cm

-2

1.0

60

80 100 120 140 160 180 200 Time, h

Fig.10. Oxygen permeation flux as function of operating time at 900 °C. (Air flow rate: 200

TE D

ml·min-1; Helium flow rate: 100 ml·min-1).

To be a good membrane material for oxygen permeation, it should have not only sufficient high oxygen permeation flux, but also good stability for long term operation under certain oxygen partial

EP

pressure. A long-term operation test on the LSC214-decorated LSCF hollow fiber membrane was conducted under the condition as follows: T= 900 °C, flow rate of sweep gas = 100 ml·min-1and air

AC C

flow rate = 200 ml·min-1. The variation of oxygen permeation flux along with the operation time is plotted in Fig.10. It can be observed that the oxygen permeation flux of LSC214-decorated LSCF membrane system keeps steady, varying from 0.78 to 0.81 ml·min-1·cm-2, in the entire operation time of more than 200 hours at 900 °C. This implies that the LSC214-decorated LSCF hollow fiber membrane is highly stable at least under the test condition.

4. Conclusions The powders of perovskite oxide LSCF and K2NiF4-type oxide LSC214 were successfully prepared by the sol-gel method. Asymmetric gas-tight LSCF hollow fiber membranes were

ACCEPTED MANUSCRIPT fabricated through the phase inversion-sintering process. Surface treatment was conducted to improve the surface exchange reaction kinetics of oxygen reduction. The outer surface of the LSCF membranes was corroded firstly by diluted hydrochloric acid to increase the roughness, and then it was decorated with dispersed porous thin layer of the K2NiF4-type composite oxide LSC214. It is

RI PT

found that surface treatment indeed contributes to the improvement of surface catalytic activity. Compared to the 0.036-1.021 ml·min-1·cm-2 of bare membrane, the oxygen permeation fluxes of the LSC214-decorated membrane were dramatically enhanced to be 0.201-1.311 ml·min-1·cm-2, when temperature rose from 700 to 1000 °C (helium flow rate: 150 ml·min-1). Results also indicate that the

SC

rate-determining step of the oxygen permeation process changes from the surface exchange reaction to bulk diffusion along with increasing temperature, resulting in the decrease of the flux

M AN U

enhancement factors. In addition, the surface decorated membrane also exhibited high oxygen permeation stability during the entire operating 200 hours at 900 °C without flux decay. In brief, as a high active catalyst at the intermediate temperature range, the K2NiF4-type composite oxide LSC214 is a promising candidate that can be applied to improve oxygen permeation for practical industrial

Acknowledgements

TE D

applications in the future.

The authors gratefully acknowledge the financial supports for this work provided by Shandong Provincial Natural Science Foundation (No. ZR2012BQ010)，the Scientific Research Foundation for

EP

the Returned Overseas Chinese Scholars (State Education Ministry of China) and the National

AC C

Natural Science Foundation of China (21176146).

References

[1] X. Zhu, H. Liu, Y. Cong, W. Yang. Novel dual-phase membranes for CO2 capture via an oxyfuel route. Chemical Communications, 48 (2012), pp. 251-253. [2] N. H. Othman, Z. Wu, K. Li. A micro-structured La0.6Sr0.4Co0.2Fe0.8O3−δ hollow fiber membrane reactor for oxidative coupling of methane. Journal of Membrane Science, 468 (2014), pp. 31-41. [3] Y. Liu, X. Zhu, M. Li, H. Liu, Y. Cong, W. Yang. Stabilization of Low-Temperature Degradation in Mixed Ionic and Electronic Conducting Perovskite Oxygen Permeation Membranes. Angewandte Chemie International Edition, 52 (2013), pp. 3232-3236.

ACCEPTED MANUSCRIPT [4] Y. Wei, W. Yang, J. Caro, H. Wang. Dense ceramic oxygen permeable membranes and catalytic membrane reactors. Chemical Engineering Journal, 220 (2013), pp. 185-203. [5] X. Zhu, M. Li, H. Liu, T. Zhang, Y. Cong, W. Yang. Design and experimental investigation of oxide ceramic dual-phase membranes. Journal of Membrane Science, 394-395 (2012), pp. 120-130. [6] R. Wang, B. Meng, X. Meng, X. Tan, J. Sunarso, L. Liu, S. Liu. Highly stable La0.6Sr0.4Co0.2Fe0.8O3−δ hollow fiber membrane for air separation swept by steam or steam mixture. Journal of Membrane Science, 479 (2015), pp. 232-239. [7] Z. Wang, Y. Kathiraser, T. Soh, S. Kawi. Ultra-high oxygen permeable BaBiCoNb hollow fiber membranes and

RI PT

their stability under pure CH4 atmosphere. Journal of Membrane Science, 465 (2014), pp. 151-158.

[8] D. Schlehuber, E. Wessel, L. Singheiser, T. Markus. Long-term operation of a La0.58Sr0.4Co0.2Fe0.8O3−δ-membrane for oxygen separation. Journal of Membrane Science, 351 (2010), pp. 16-20.

[9] H. Wang, C. Tablet, A. Feldhoff, J. Caro. A Cobalt-Free Oxygen-Permeable Membrane Based on the Perovskite-Type Oxide Ba0.5Sr0.5Zn0.2Fe0.8O3-δ. Advanced Materials, 17 (2005), pp. 1785-1788.

SC

[10] X. Dong, Z. Xu, X. Chang, C. Zhang, W. Jin. Chemical Expansion, Crystal Structural Stability, and Oxygen Permeability of SrCo0.4Fe0.6-xAlxO3-δ Oxides. Journal of the American Ceramic Society, 90 (2007), pp. 3923-3929. [11] X. Tan, Z. Wang, H. Liu, S. Liu. Enhancement of oxygen permeation through La0.6Sr0.4Co0.2Fe0.8O3−δ hollow fiber membranes by surface modifications. Journal of Membrane Science, 324 (2008), pp. 128-135.

M AN U

[12] J. Sunarso, S. Baumann, J. M. Serra, W. A. Meulenberg, S. Liu, Y. S. Lin, J. C. Diniz da Costa. Mixed ionic–electronic conducting (MIEC) ceramic-based membranes for oxygen separation. Journal of Membrane Science, 320 (2008), pp. 13-41.

[13] A. Leo, S. Smart, S. Liu, J. C. Diniz da Cost. High performance perovskite hollow fibers for oxygen separation. Journal of Membrane Science, 368 (2011), pp. 64-68.

[14] T. Kida, S. Ninomiya, K. Watanabe, N. Yamazoe, K. Shimanoe. High Oxygen Permeation in Ba0.95La0.05FeO3-δ Membranes with Surface Modification. Applied Materials & Interfaces, 2 (2010), pp. 2849-2853.

TE D

[15] T. H. Lee, Y. L. Yang, A. J. Jacobsona, B. Abeles, S. Milner. Oxygen permeation in SrCo0.8Fe0.2O3-δ membranes with porous electrodes. Solid State Ionics, 100 (1997), pp. 87-94. [16] A. Leo, S. Liu, J. C. Diniz da Costa, Z. Shao. Oxygen permeation through perovskite membranes and the improvement of oxygen flux by surface modification. Science and Technology of Advanced Materials, 7 (2006), pp. 819-825.

EP

[17] A. Thursfield, I. S. Metcalfe. Air separation using a catalytically modified mixed conducting ceramic hollow fiber membrane module. Journal of Membrane Science, 288 (2007), pp. 175-187. [18] E. Mutoro, E. J. Crumlin, H. Pöpke, B. Luerssen, M. Amati, M. K. Abyaneh, M. D. Biegalski, H. M. Christen, L.

AC C

Gregoratti, J. Janek, Y. Shao-Horn. Reversible Compositional Control of Oxide Surfaces by Electrochemical Potentials. The Journal of Physical Chemistry Letters, 3 (2012), pp. 40-44. [19] L. Wang, S. Imashuku, A. Grimaud, D. Lee, K. Mezghani, M. A. Habib, Y. Shao-Horn. Enhancing Oxygen Permeation of Electronically Short-Circuited Oxygen-Ion Conductors by Decorating with Mixed Ionic-Electronic Conducting Oxides. ECS Electrochemistry Letters, 2 (2013), pp. F77-F81. [20] J. W. Han, B. Yildiz. Mechanism for enhanced oxygen reduction kinetics at the (La,Sr)CoO3−δ/(La,Sr)2CoO4+δ hetero-interface. Energy & Environmental Science, 5 (2012), pp. 8598-8607. [21] E. Mutoro, E. J. Crumlin, M. D. Biegalski, H. M. Christen, Y. Shao-Horn. Enhanced oxygen reduction activity on surface-decorated perovskite thin films for solid oxide fuel cells. Energy & Environmental Science, 4 (2011), pp. 36893696. [22] X. Tan, Y. Liu, K. Li. Preparation of LSCF Ceramic Hollow-Fiber Membranes for Oxygen Production by a Phase-Inversion/Sintering Technique. Industrial & Engineering Chemistry Research, 44 (2005), pp. 61-66. [23] X. Tan, Y. Liu, K. Li. Mixed conducting ceramic hollow-fiber membranes for air separation. AIChE Journal, 51

ACCEPTED MANUSCRIPT (2005), pp. 1991-2000. [24] J. Hong, P. Kirchen, A. F. Ghoniem. Interactions between oxygen permeation and homogeneous-phase fuel conversion on the sweep side of an ion transport membrane. Journal of Membrane Science, 428 (2013), pp. 309-322. [25] X. Tan, Z. Wang, K. Li. Effects of Sintering on the Properties of La0.6Sr0.4Co0.2Fe0.8O3−δ Perovskite Hollow. Industrial & Engineering Chemistry Research, 49 (2010), pp. 2895-2901. [26] G. Thornton, B. C. Tofield, A. W. Hewat. A Neutron Diffraction Study of LaCoO3 in the Temperature Range 4.2
RI PT

[27] A. Aguadero, D. Pérez-Coll, J. A. Alonso, S. J. Skinner, J. Kilner. A New Family of Mo-Doped SrCoO3−δ Perovskites for Application in Reversible Solid State Electrochemical Cells. Chemistry of Materials, 24 (2012), pp. 2655-2663.

[28] Z. Wang, H. Liu, X. Tan, Y. Jin, S. Liu. Improvement of the oxygen permeation through perovskite hollow fiber membranes by surface acid-modification. Journal of Membrane Science, 345 (2009), pp. 65-73. 30 (1979), pp. 353-356.

SC

[29] D. Ganguli. Cationic Radius Ratio and Formation of K2NiF4-Type Compounds. Journal of Solid State Chemistry, [30] U. Lehmann, Hk. Müller-Buschbaum. Ein Beitrag zur Chemie der Oxocobaltate(II): La2CoO4, Sm2CoO4. Zeitschrift für anorganische und allgemeine Chemie, 470 (1980), pp. 59-63.

M AN U

[31] M. V. Kniga, I. I. Vygovskii, E. E. Klementoviah. Russ. J. Inorg. Chem.(Eng. Trans.), 24 (1979), pp. 652. [32] D. Han, J. Wu, Z. Yan, K. Zhang, J. Liu, S. Liu. La0.6Sr0.4Co0.2Fe0.8O3−δ hollow fiber membrane performance improvement by coating of Ba0.5Sr0.5Co0.9Nb0.1O3−δ porous layer. RSC Advances, 4 (2014), pp. 19999-20004. [33] B. C. H. Steele. Oxygen ion conductors and their technological applications. Materials Science and Engineering B, 13 (1992), pp. 79-87.

[34] J. M. Serra, J. Garcia-Fayos, S. Baumann, F. Schulze-Küppers, W. A. Meulenberg. Oxygen permeation through tape-cast asymmetric all-La0.6Sr0.4Co0.2Fe0.8O3−δ membranes. Journal of Membrane Science, 447 (2013), pp. 297-305.

TE D

[35] T. T. Norton, J. Ortiz-Landeros, Y. S. Lin. Stability of La–Sr–Co–Fe Oxide–Carbonate Dual-Phase Membranes for Carbon Dioxide Separation at High Temperatures. Industrial & Engineering Chemistry Research, 53 (2014), pp. 2432-2440.

[36] H. Huang, S. Cheng, J. Gao, C. Chen, J. Yi. Phase-inversion tape-casting preparation and significant performance enhancement of Ce0.9Gd0.1O1.95–La0.6Sr0.4Co0.2Fe0.8O3−δ dual-phase asymmetric membrane for oxygen separation. Materials

AC C

EP

Letters, 137 (2014), pp. 245-248.

ACCEPTED MANUSCRIPT Acid etching followed by dispersed-second-phase particle decoration was employed.

(2)

The K2NiF4-type composite oxide was chosen to modify the perovskite membranes.

(3)

The oxygen flux enhancement factors always decreases with increasing temperature.

(4)

LSC214-decorated LSCF membrane is highly stable under the test condition.

AC C

EP

TE D

M AN U

SC

RI PT

(1)