Journal of Membrane Science 334 (2009) 9–15
Contents lists available at ScienceDirect
Journal of Membrane Science
journal homepage: www.elsevier.com/locate/memsci
Amphoteric ion exchange membrane synthesized by radiation-induced graft
copolymerization of styrene and dimethylaminoethyl methacrylate into
PVDF film for vanadium redox flow battery applications
Jingyi Qiu a , Junzhi Zhang b , Jinhua Chen c , Jing Peng a , Ling Xu a , Maolin Zhai a,∗ ,
Jiuqiang Li a , Genshuan Wei a
a
Beijing National Laboratory for Molecular Science (BNLMS), Department of Applied Chemistry, College of Chemistry and Molecular Engineering,
Peking University, Beijing 100871, PR China
b
State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing 100084, PR China
c
Conducting Polymer Materials Group, Environment and Industrial Materials Research Division, Quantum Beam Science Directorate,
Japan Atomic Energy Agency (JAEA), 1233 Watanuk-machi, Takasaki, Gunma 370-1292, Japan
a r t i c l e
i n f o
Article history:
Received 23 December 2008
Received in revised form 11 February 2009
Accepted 12 February 2009
Available online 24 February 2009
Keywords:
Radiation grafting
Styrene
Dimethylaminoethyl methacrylate
Amphoteric ion exchange membrane
Vanadium redox flow battery
a b s t r a c t
Poly(vinylidene difluoride) (PVDF) film was grafted with styrene (St) and dimethylaminoethyl methacrylate (DMAEMA) using ␥-irradiation techniques. Through subsequent sulfonation and protonation
processes, a new kind of amphoteric ion exchange membrane (AIEM) was synthesized. The grafting
yield (GY) increased with absorbed dose and leveled off at about 60 kGy. The composition of poly(Stco-DMAEMA) grafts was correlated to the ratio of St to DMAEMA monomer in the feed. Micro-FTIR and
XPS analyses testified that the grafting and sulfonation of St unit in poly(St-co-DMAEMA) grafts had
been carried out as designed. Further characterizations showed that the properties of the AIEM strongly
depended on the composition and GY of the film, i.e. higher content of DMAEMA brought lower permeability of vanadium ions and conductivity, while higher GY led to higher water uptake, ion exchange capacity
(IEC) and conductivity. Finally, an AIEM with a GY of 26.1% was assembled and tested in the vanadium
redox flow battery (VRFB) system. It was found that the VRFB assembled with the AIEM maintained an
open circuit voltage (OCV) higher than 1.2 V after placed for 68 h, which was much longer than that with
the Nafion117 membrane. Therefore, this work provides a novel method to develop potential substitute
of Nafion membranes to be applied in VRFB system.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
Since the vanadium redox flow battery (VRFB) was proposed by
Skyllas-Kazacos in 1985 [1,2], it has attracted more and more attention as a promising energy storage system. Ion exchange membrane
(IEM) is the key component of the battery and should possess the
following properties to obtain a high energy efficiency and long
cycle life: high ionic conductivity; low permeability of vanadium
ions; good chemical stability and low price. Unfortunately, up to
now the commercial membranes cannot satisfy all of the above
requirements. For instance, some of the membranes show poor
stability in VRFB electrolyte solution due to the oxidation degradation induced by VO2 + [3], while some of the membranes with good
stability suffer from the crossover of vanadium ions, which causes
serious self-discharge of the battery [4]. Consequently, research on
∗ Corresponding author. Tel.: +86 10 62753794; fax: +86 10 62753794.
E-mail addresses: [email protected] (J.Y. Qiu), [email protected] (M. Zhai).
0376-7388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.memsci.2009.02.009
exploiting IEMs with improved properties is of great importance to
the large-scale application of VRFB.
Radiation grafting technique has been widely used as a alternative routes to prepare IEMs for electrochemical applications
[5–9]. This technique has exhibited obvious advantages over traditional methods that the composition and resulting properties of
the membrane can be easily controlled by applying appropriate
grafting conditions. Various hydrocarbon and fluorocarbon films
have been used as base films to prepare IEM using this technique.
Comparing with the hydrocarbon polymers, the fluorocarbon polymers were much more favored due to their outstanding thermal
and chemical stability, which makes them particularly suitable for
preparing highly stable IEMs. PVDF [10], poly(tetrafluoroethylene)
(PTFE) [11], poly(ethylene-co-tetrafluoroethylene) (ETFE) [5],
poly(tetrafluoroethylene-co-hexafluoro-propylene) (FEP) [8], etc.
have been grafted with different monomers to prepare a variety
of IEMs, which have exhibited satisfying chemical stability in fuel
cell or VRFB. Nevertheless, the application of these membranes
in VRFB is still limited due to the respective shortcomings. For
10
J.Y. Qiu et al. / Journal of Membrane Science 334 (2009) 9–15
instance, cation exchange membranes usually exhibit high conductivity but suffer from high permeability of vanadium ions [4], while
anion exchange membranes effectively suppress the vanadium ions
crossover but are simultaneity restricted by lower conductivity
[12,13]. Accordingly, a novel IEM that combines high conductivity
and low vanadium ions crossover is prerequisite for the industrialization of VRFB.
The amphoteric ion exchange membrane (AIEM) with both
cation and anion exchange capabilities was first proposed by Sollner
in 1932 [14]. Due to the controllability of the ion exchange property
by adjusting the composition or the external conditions, the AIEM is
expected to have potential applications in many fields [15–17]. Several kinds of IEMs prepared by introducing anion exchange groups
to the cation exchange membrane like Nafion117 have been applied
in VRFB [18–21]. It has been found that the crossover of vanadium
ions through these membranes has been suppressed as the anion
exchange groups incorporated, and meanwhile these membranes
maintain a rather high conductivity. Besides, the water transport
through the membrane has been also reduced due to the decrease
in vanadium ions permeation.
In this work, a novel AIEM is to be prepared via a handy radiation
grafting approach. The parameters of the preparation procedure, as
well as the characteristics of the resulting AIEM are to be investigated. The loading test of AIEM in the VRFB system is to be performed to evaluate the prospect for its application in VRFB systems.
2. Experimental
2.1. Materials
PVDF film with a thickness of 20 ␮m was supplied by Kureha
Company (Japan) and used as the base film. St and DMAEMA (Acros)
with a purity of more than 99% was used without further purification. Chlorosulfonic acid was purchased from Beijing Yili Fine
Chemical Co. Ltd. VOSO4 ·3H2 O (analytical reagent) was supplied
by Shanghai Luyuan Fine Chemical Plant. Nafion117 membrane (Du
Pont) was immersed into deionized water for 24 h prior to use.
2.2. Preparation of the AIEM
The preparation route of the AIEM is shown in Fig. 1. A piece of
PVDF film was washed with acetone and dried to constant weight.
Then the film with known weight was immersed into monomer
solution of acetone. After bubbling with nitrogen for about 15 min
and sealed, the sample was subjected to ␥-ray irradiation from
a 60 Co source (Peking University). After irradiated with certain
absorbed dose, the film was placed in a Soxhlet’s extraction device
to remove the residue monomers and homopolymers by acetone
for 24 h. Then the film was dried to constant weight in vacuum and
weighed. GY was calculated as follows:
GY (%) =
Wg − W0
× 100
W0
(1)
where Wg and W0 are weights of the film after and before grafting,
respectively.
To study the effect of the monomer ratio on the membrane properties, three monomer solutions (total concentration of 2 M) with
St/DMAEMA mole ratios of 3:1, 1:1 and 1:3 were used.
The grafted film was sulfonated in a 0.2 M chlorosulfonic acid
solution of 1,2-dichloromethane at room temperature for 8 h, and
then hydrolyzed with distilled water at 60 ◦ C overnight. After that,
the film was protonated in 1 M HCl for 6 h, and then kept in distilled
water at room temperature prior to further characterization.
2.3. Micro-FTIR and elemental analysis of the grafted film
Micro-FTIR analysis was performed on a Nicolet (Magna-IR 750)
spectrometer. The spectra were measured in absorbance mode in a
wave number range of 4000–600 cm−1 .
To determine the content of St and DMAEMA, the grafted film
was subjected to elemental analysis using an Elementar Analysensysteme GmbH (model: vario EL). The mole ratio of DMAEMA to
St (nDMAEMA /nSt ) in poly(St-co-DMAEMA) grafts was determined
according to the following equation:
104(100 + GY )PN
nDMAEMA
=
nSt
14GY − (100 + GY )PN 157
(2)
where PN is the content of nitrogen; 14, 104 and 157 are molar mass
of nitrogen, St and DMAEMA, respectively.
The X-ray photoelectron spectroscopy (XPS) analysis was performed with an AXIS-Ultra instrument from Kratos Analytical using
monochromatic Al K␣ radiation (225 W, 15 mA, 15 kV) and lowenergy electron flooding for charge compensation. To compensate
for surface charges effects, binding energies were calibrated using C
Fig. 1. Preparation route of the AIEM.
J.Y. Qiu et al. / Journal of Membrane Science 334 (2009) 9–15
1s hydrocarbon peak at a binding energy (BE) of 284.8 eV. The data
were converted into VAMAS file format and imported into CASA
XPS software package for manipulation and curve fitting.
2.4. IEC, ion conductivity and water uptake of the AIEM
For anion exchange capacity measurement, a piece of dried
AIEM with certain weight was immersed into 0.05 M HCl solution
overnight and stirred occasionally. The anion exchange capacity
was determined by titrating the above solution with 0.05 M NaOH
solution.
In contrast, to determine the cation exchange capacity, a piece of
dried AIEM with certain weight was immersed into 0.05 M NaOH
solution for 24 h. Then the solution was titrated with 0.05 M HCl
solution.
The proton conductivity of the AIEM was obtained by impedance
spectroscopy measurement using a CHI660 electrochemical work
station with an AC perturbation of 10 mV. The AIEM was hydrated
for 24 h before determination and clamped between two Pt electrodes for recording of the impedance spectroscopy. The proton
conductivity () was calculated as
L
=
RS
(3)
where R is the real impedance taken at zero imaginary impedance
in the impedance spectroscopy, L and S are the thickness and area
of the AIEM between the electrodes, respectively.
To evaluate the water uptake of AIEM, the sample with certain weight was immersed into deionized water. After equilibrating
for 24 h, the membrane was taken out and the water adhering
to the surface was quickly wiped using filter paper, and then the
membrane was weighed again. The water uptake was calculated
according to the following equation:
Water uptake (%) =
Ww − Wd
× 100
Wd
(4)
where Ww and Wd are the weights of the AIEM in the wet and dry
state, respectively.
2.5. Permeability of vanadium ions through the AIEM
The permeability of vanadium ions through the AIEM was investigated using the equipment as shown in Fig. 2. 1.5 M V(IV) solution
was prepared by dissolving VOSO4 ·3H2 O in 2 M H2 SO4 solution. As
shown in Fig. 2, the left reservoir was filled with vanadium ion solution and the right one was filled with a 1.5 M MgSO4 in 2 M H2 SO4
solution. MgSO4 was used to balance the ionic strength and reduce
the osmotic pressure. The area of the membrane exposed to the
solution was 1.77 cm2 and the volume of solutions in both sides
Fig. 2. Illustration of equipment for permeability of vanadium ions test.
11
was 25 mL. The MgSO4 solution was taken for inductively coupled
plasma atomic emission spectrometry (ICP-AES) (Leeman, Profile)
analysis at a regular time and the concentration of vanadium ions
in the solution was determined. The permeability of vanadium ions
through the Nafion117 membrane was also tested using the same
method for comparison.
2.6. VRFB performance of the AIEM
The VRFB was fabricated by sandwiching the membrane
between two pieces of graphite carbon electrodes. 1.5 M V(II)/V(III)
and V(IV)/V(V) in 2.5 M H2 SO4 solutions were used as the electrolytes in the positive and negative half cells, respectively. The
membrane area was 5 cm2 and the volume of the electrolytes solution in each half cell was 40 mL. The battery was first charged to
1.6 V at a current density of 40 mA cm−2 . The OCV was measured
then at room temperature.
3. Results and discussion
3.1. Synthesis of the AIEM
Fig. 3 shows the effect of dose on the GY of grafted PVDF film.
It can be seen that GY increases with dose and levels off after
about 60 kGy. At a dose of 20 kGy, grafted PVDF film with a GY of
about 30% can be prepared. Moreover, it can be seen from Table 1
that the initial monomer ratio has less effect on the total GY but
strongly affect the nDMAEMA /nSt in poly(St-co-DMAEMA) grafts. Similar phenomenon has also been observed in radiation-induced graft
copolymerization of St and DMAEMA into PTFE film [22]. One reason is that St and DMAEMA tend to react with each other to form
a kind of alternate copolymer according to their reactivity ratios
calculated using Fineman-Ross method, r1 = 0.510 and r2 = 0.716
[23]. Another one is that the radical yield of St is higher than
that of DMAEMA in the same condition [22,23]. Therefore, when
DMAEMA/St increased from 1:3 to 3:1 in the feed, nDMAEMA /nSt only
increased from ca. 0.4 to 1.4.
The grafted film is further sulfonated and protonated to synthesize a new kind of AIEM (Fig. 1). The composition of the AIEM
can be easily modulated by controlling the initial monomer ratio,
which is advantageous to the application in VRFB. Because the AIEM
with higher content of DMAEMA may exhibit lower permeability of vanadium ions due to the Donnan exclusion effect between
–R3 NH+ groups of protonated DMAEMA unit and vanadium ions,
while higher content of St will lead to higher conductivity after
subsequent sulfonation.
Fig. 3. Effect of dose on GY of grafted PVDF films.
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J.Y. Qiu et al. / Journal of Membrane Science 334 (2009) 9–15
Table 1
Elemental analysis and conductivity of the AIEM with different compositions.
DMAEMA/St in the feed
Dose (kGy)
GY (%)
Weight percent (%)
nDMAEMA /nSt in the grafted film
Conductivity (S/cm)
PC
PH
PN
1:3
10
15
20
11.9
17.2
25.9
42.73
43.85
46.19
3.76
3.93
4.43
0.36
0.49
0.71
0.42
0.40
0.42
0.027
0.038
0.052
1:1
10
15
20
11.6
18.1
26.1
41.87
44.39
46.54
3.83
4.21
4.41
0.51
0.74
1.05
0.81
0.78
0.88
0.024
0.033
0.046
3:1
10
15
20
12.1
17.9
24.7
42.80
44.51
46.37
3.14
4.31
4.61
0.66
0.92
1.22
1.45
1.41
1.49
0.019
0.029
0.037
Fig. 4 shows the FTIR spectra of PVDF, grafted PVDF and the
AIEM (denoted as A–C, respectively). Comparing with original PVDF
film, the grafted film shows the absorption bands at 1600 cm−1 ,
1490 cm−1 and 1450 cm−1 which are due to the C C in-plane
stretch vibration of the benzene ring, while the absorption bands
at 760 cm−1 and 702 cm−1 are due to the aromatic C–H deformation of the mono-substituted benzene ring [24–26]; the grafting of
DMAEMA can be testified by the presence of absorption band at
1720 cm−1 which is assigned to the stretch vibration of carbonyl
group (C O) in the ester bond (C(O)O) in DMAEMA [27]. For the
AIEM, the sulfonation of the St can be confirmed by the presence
of the sharp absorption bands at 1030 cm−1 and 1010 cm−1 , which
can be assigned to the absorption of the –SO3 H group in the AIEM.
Moreover, it can be seen that the absorption bands at 760 cm−1 and
702 cm−1 disappear, which also indicates that the benzene ring in
St has been completely sulfonated.
The grafted PVDF and the AIEM were also subjected for XPS analysis. Fig. 5(A) and (B) demonstrate the C 1s core-level spectra of the
grafted PVDF and the AIEM, respectively. Both of the spectra can
be curve-fitted with four peak components at BEs of 284.9, 286.2,
288.9 and 291.0 eV, which can be attributed to CH2 , C–O, C(O)O and
CF2 species, respectively [28,29]. It can be found that no new peak
component is observed after sulfonation. From the N 1s core-level
spectra (Fig. 5C), the N 1s spectra of the grafted film can be curvefitted with one peak component at BE of 399.5 eV, attributing to the
–N(CH3 )2 group of DMAEMA unit in poly(St-co-DMAEMA) grafts. As
for the AIEM (Fig. 5D), the N 1s core-level spectra can be curve-fitted
with two peak components with BEs of 399.8 and 402.3 eV, owing
to –N(CH3 )2 and –NH+ (CH3 )2 groups, respectively, which certifies
that most of the DMAEMA unit in poly(St-co-DMAEMA) grafts has
been protonated during the hydrolysis process [28]. Consequently,
it can be concluded from the FTIR and XPS analysis that both of St
Fig. 4. FTIR spectra of PVDF film (A); grafted PVDF (B, GY = 26.1%) and the AIEM (C,
GY = 26.1%).
and DMAEMA have been introduced to the base film and the AIEM
has been prepared as designed.
3.2. Effect of the membrane composition on the conductivity and
permeability of vanadium ions
To evaluate the effect of membrane composition on the properties of the AIEM, the conductivity and permeability of vanadium
ions of the AIEM with different composition was tested. It can be
seen from Table 1 that conductivity of the AIEM increases with the
content of St. The AIEM with nDMAEMA /nSt of 0.42 and the GY of
25.9% gives a rather high conductivity, i.e. 0.052 S/cm. This can be
attributed to the fact that the conducting of proton is mainly carried
out by the sulfonic groups of the AIEM.
The crossover of vanadium ions through the IEM has been a
serious problem to the VRFB as it causes serious self-discharge
and results in low efficiency. In our previous work [13], a kind of
anion exchange membrane has been prepared by radiation grafting
of DMAEMA. Comparing with Nafion117 membrane, it was found
that the permeability of vanadium ions through the anion exchange
membrane with a GY of 40% has been reduced to only 1/20–1/40
of that through Nafion117 membrane, which is due to the Donnan
exclusion effect between the cations in the membrane and the vanadium ions. Mohammadi and Skyllas-Kazacos [19] have prepared
several kinds of IEMs by composing cation exchange membranes
with anion exchange resin to obtain a better performance. However,
the membrane stability is not satisfying ascribed to the preparation procedure [19,30]. Recently, the IEMs prepared by introducing
cation charged groups onto the Nafion117 membrane have also
been proposed [18,20,21]. It was found that the permeability of
vanadium ions through the membrane is evidently reduced because
of the Donnan exclusion effect, meanwhile rather high conductivity
was still maintained. These works are beneficial to the membrane
design for VRFB in despite of the slightly decrement of conductivity after modification. Nevertheless, the high cost of the Nafion117
membrane will still be a problem making against to the commercialization of VRFB.
The permeability (P) of vanadium ions through the prepared
AIEM was measured here. The change of the vanadium concentration in MgSO4 side with time is shown in Fig. 6. It can be seen that
the concentration of vanadium ions in MgSO4 side for Nafion117
membrane increased much faster than that for the AIEM. As for the
AIEM with different composition, it can be found that the concentration of vanadium ions decreases with increasing the content of
DMAEMA, which can be explained that high content of DMAEMA
leads to more significant Donnan exclusion effect and therefore
lower permeability of vanadium ions. The concentration of vanadium ions in the MgSO4 side as a function of time can be defined
according to the following equation:
V
dct
P
= S (c0 − ct )
L
dt
(5)
J.Y. Qiu et al. / Journal of Membrane Science 334 (2009) 9–15
13
Fig. 5. XPS analysis of the grafted PVDF and AIEM (GY = 26.1%).
where V is the volume of the solution in both sides; S is the area
of the membrane exposed to the solution; P is the permeability of
vanadium ions; L is the thickness of the membrane; c0 is the initial
concentration of the vanadium solution in the left reservoir; ct is the
vanadium concentration in the MgSO4 side at time (t). Assumption
is made that P is independent on the concentration.
According to Eq. (5) and Fig. 6, the permeability (P) through the
AIEM and Nafion117 membrane has been calculated and listed in
Table 2. It can be found that the permeability of V(IV) through the
AIEM is only about 1/10 of that through the Nafion117 membrane.
Moreover, as nDMAEMA /nSt increases from 0.42 to 1.45, P decreases
by about 25%.
Accordingly, the AIEM has shown low permeability of vanadium
ions as well as a rather high conductivity. The results were contributed to the fact that multivalent vanadium cations are difficult
to be adsorbed onto the cationic charged AIEM compared to hydrogen proton because of Donnan exclusion effect [31]. Moreover, it
can be concluded from the above results that the composition is
of great importance to the membrane conductivity and permeability of vanadium ions. On the other hand, by controlling the
grafting conditions, such as the initial monomer ratio or GY, the
AIEM with required properties can be prepared through a convenient approach. To further investigate the effect of GY on membrane
properties, the AIEM prepared in the solution with monomer ratio
of 1:1 was characterized in the following.
3.3. Effect of GY on IEC, ionic conductivity and water uptake
The effect of GY on the cation and anion exchange capacity of
the AIEM is shown in Fig. 7. It can be seen that both of the cation
and anion exchange capacities increase with GY of the AIEM. Moreover, the cation exchange capacity is higher than the anion exchange
capacity at all GY, which accords with the results obtained in the elemental analysis and further indicated that the sulfonation process
has been completely carried out. For the AIEM with a GY of 26.1%, the
AIEM has a cation exchange capacity of 0.83 mmol g−1 and an anion
exchange capacity of 0.71 mmol g−1 . The IEC of Nafion117 membrane is reported as 0.98 mmol g−1 , which is a little higher than the
cation exchange capacity of the AIEM with a GY of 26.1%.
The relationship between the conductivity and GY of the AIEM
is shown in Fig. 8. Obviously, the conductivity of the AIEM increases
with the GY, which is attributed to the fact that more ion exchange
groups are introduced into the membrane at high GY. At a GY of
26.1%, the AIEM possesses a conductivity of 0.046 S cm−1 , which is
similar to that of Nafion117 membrane (0.05 S cm−1 ). In our previous work [13], a kind of anion exchange membrane has been
Table 2
Permeability of vanadium ions through the AIEM with different composition and
Nafion117 membrane.
Fig. 6. Permeation behavior of vanadium ions through the AIEM with different composition and Nafion117 membrane.
Membrane
Thickness (␮m)
nDMAEMA /nSt
P (×10−7 cm2 /min)
Nafion117
178
25
–
0.42
7.93
0.91
25
25
0.88
1.49
0.73
0.67
AIEM
14
J.Y. Qiu et al. / Journal of Membrane Science 334 (2009) 9–15
Fig. 7. Effect of GY on IEC of the AIEM.
Fig. 9. OCV of the VRFB assembled with the AIEM.
prepared using radiation grafting technique and it was found that
the conductivity of the anion exchange membrane is lower than
that of the AIEM at the same GY. The increase in conductivity is
attributed to the sulfonic groups introduced by sulfonation of St
unit in poly(St-co-DMAEMA) grafts, which is favored to the proton
conducting.
Water uptake is an important property of the IEM as it relates to
the conductivity and dimensional stability of the membrane. The
water uptake of the AIEM in deionized water is also shown in Fig. 8.
As expected, high GY leads to high water uptake, indicating the
increase of hydrophilicity within the hydrophobic PVDF membrane,
which is due to the incorporation of more ion exchange groups as
a consequence of the increase in GY. As shown in Fig. 8, at ca. 26.1%
GY, the water uptake of the AIEM is about 36%, which is slightly
higher than that of the Nafion117 membrane (30%).
ity test, i.e. much lower permeability of vanadium ions through the
AIEM than that through the Nafion117 membrane. Accordingly, due
to much lower permeability of vanadium ions through the AIEM, the
VRFB assembled with the AIEM is expected to have lower degree of
self-discharge and higher energy efficiency, which will be investigated in further experiments.
3.4. OCV of VRFB with the AIEM
The OCV result of VRFB with the AIEM is shown in Fig. 9. It has
been found that OCV of the VRFB assembled with the Nafion117
membrane decreased rapidly after staying for about 14 h [13,18].
In contrast, the VRFB assembled with the AIEM with a 26.1% GY
exhibits a much better performance: maintaining OCV above 1.2 V
for about 68 h, which is much longer than that obtained with the
Nafion117 membrane. The OCV performance indicates that the selfdischarge of the VRFB with Nafion117 is more serious than that of
the VRFB with the AIEM, which is corresponding to the permeabil-
4. Conclusions
A novel AIEM aiming for VRFB applications has been prepared
through a radiation grafting approach. Further characterization
shows that the AIEM with required properties can be conveniently
prepared by choosing proper grafting conditions like the initial
monomer ratio and the absorbed dose. The obtained AIEM has
exhibited the advantages over both anion and cation exchange
membranes in VRFB: much lower permeability of vanadium ions
than cation exchange membranes due to the Donnan exclusion
effect; higher conductivity comparing to the exchange membranes.
We hope that this work provides a convenient method for preparing
new kinds of IEMs for VRFB and other applications.
Acknowledgements
The Scientific Research Foundation for the Returned Overseas
Chinese Scholars of States Education Ministry is acknowledged for
supporting this research.
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Fig. 8. Effect of GY on conductivity and water uptake of the AIEM.
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