A new method for improving the ion conductivity of anion exchange membranes by using TiO2 nanoparticles coated with ionic liquid

Article abstract of DOI:10.1039/c6ra21355c

Recently a new method for increasing the ion conductivity of anion exchange membranes (AEM) was developed based on the novel materials ionic liquids (ILs). We mixed the ILs into the membrane directly instead of immobilizing onto the polymer backbone as in the traditional way. Nano-TiO₂ was introduced to stabilize the ILs in the membrane. The ILs were immobilized by the nano-TiO₂, acting as the "active sites" in the membrane, to enhance the mobility of the hydroxyl groups so as to increase the ion conductivity. Both pure ILs composite membranes and ILs-TiO₂ composite membranes were synthesized, and their properties were compared. ¹H nuclear magnetic resonance spectroscopy and Fourier transform infrared spectroscopy were used to analyze the structures of the composite membranes. The mechanical properties, thermal stabilities, ion conductivities, water uptakes, swelling ratios, and ion exchange capacities of the membranes were investigated. The interaction between the TiO₂ and ionic liquids was confirmed by X-ray diffraction. The stability of the ILs in the membrane was measured comprehensively. All these results show that this novel method is effective and promising for AEM applications.

Full text of DOI:10.1039/c6ra21355c

RSC Advances
View Article Online
View Journal | View Issue
PAPER
A new method for improving the ion conductivity
of anion exchange membranes by using TiO2
nanoparticles coated with ionic liquid
Cite this: RSC Adv., 2016, 6, 96768
Yuhao Chu, Yuenan Chen, Nanjun Chen, Fanghui Wang and Hong Zhu*
Recently a new method for increasing the ion conductivity of anion exchange membranes (AEM) was
developed based on the novel materials ionic liquids (ILs). We mixed the ILs into the membrane directly
instead of immobilizing onto the polymer backbone as in the traditional way. Nano-TiO
to stabilize the ILs in the membrane. The ILs were immobilized by the nano-TiO , acting as the “active
sites” in the membrane, to enhance the mobility of the hydroxyl groups so as to increase the ion
2
was introduced
2
2
conductivity. Both pure ILs composite membranes and ILs–TiO composite membranes were
1
synthesized, and their properties were compared. H nuclear magnetic resonance spectroscopy and
Fourier transform infrared spectroscopy were used to analyze the structures of the composite
membranes. The mechanical properties, thermal stabilities, ion conductivities, water uptakes, swelling
ratios, and ion exchange capacities of the membranes were investigated. The interaction between the
Received 25th August 2016
Accepted 5th October 2016
2
TiO and ionic liquids was confirmed by X-ray diffraction. The stability of the ILs in the membrane was
DOI: 10.1039/c6ra21355c
measured comprehensively. All these results show that this novel method is effective and promising for
www.rsc.org/advances
AEM applications.
which the traditional quaternary ammonium groups were
replaced by the metal-cation complex to improve the ion
Introduction
17
Recently, fuel cells have become a popular clean energy tech- conductivity. This work provided effectively method for devel-
1
–5
nology because of their effectiveness in energy conversion.
oping the novel cation, however, the cost of developing novel
Proton exchange membrane fuel cells (PEMFCs) have been cation is relatively high and the synthesis route is complex.
widely used in many elds like portable, stationary power and Many studies are focused on modifying the microstructures of
6
even electromobile. However, the high costs of the materials in the membranes, and hydrophilic/hydrophobic microphase
PEMFCs and complex water management have limited the separation is investigated so as to display well-dened ion
7,8
applications of PEMFCs. In the past few years, anion exchange conductive channels. Zhuang et al. have shown that
membrane fuel cells (AEMFCs) have been developed to over- hydrophilic/hydrophobic microphase separation can be ob-
come the disadvantages of PEMFCs such as the cost and tained by changing the length of the carbon chains graed with
9
18
stability of the catalyst. Instead of the platinum catalysts used conductive groups, introducing alkyl chains into the polymer
19
in PEMFCs, the catalysts used in AEMFCs can be non-noble backbone, and varying the distance between the conductive
metal catalysts, which can vastly reduce the cost and improve groups and alkyl chains. These three methods can build well-
20
10,11
the stability of the noble catalysts.
However, the low ion organized ion conductive channels in the membranes and
conductivity and poor dimensional stability of the anion greatly improve the ion conductivity. Another way to obtain
exchange membranes (AEMs) have hindered the development microphase separation is the use of block copolymers. Many
1
2,13
of AEMFCs.
become more and more urgent.
Ion conductivity has proved to be the most important issue properties of the membrane effectively. Multiple copolymers
Thus, the development of efficient AEMs has research groups have demonstrated that microphase separation
through the formation of diblock copolymers can improve the
14–16
in the development of AEMs.
made in recent years to improve the ion conductivity of AEMs. copolymers,
Zha et al. have developed a novel metal-cation-based AEM in phenolphthalein-based multiblock copolymers. AEMs based
on these copolymers achieved well properties.
Many developments have been have been done following this method, such as PPO diblock
2
1
2
2
aromatic
block
copolymers,
and
23
Herein, we developed a relatively simple method focus on
improving the ion conductivity of AEMs without changing the
microstructure of the membranes. Dissociated ionic liquids
State Key Laboratory of Chemical Resource Engineering, Institute of Modern Catalysis,
Department of Organic Chemistry, School of Science, Beijing University of Chemical
Technology, Beijing, 100029, P. R. China. E-mail: zhuho128@126.com; Tel:
were rst introduced into the membranes in AEMs. In our
+
86-10-64444919
96768 | RSC Adv., 2016, 6, 96768–96777
This journal is © The Royal Society of Chemistry 2016

View Article Online
Paper
RSC Advances
design, the ionic liquids act as the “active sites” in the Preparation of composites of ionic liquid and nano-TiO2
membrane and can accelerate the movement of the hydroxyl
groups. When the hydroxyl groups attempt to pass through the
Nano-TiO (0.8 g, 0.01 mol) was rst diluted in 5 ml of 50%
ethanol/water solution, and then 1-methy-3-methylimidazolium
2
membrane, the electrostatic attraction between the ILs and the
ionic liquid (2.24 g, 0.01 mol) was added into the solution to
hydroxyl groups can improve the mobility of the hydroxyl
form a uniform system. The solution was evaporated at room
groups so as to form the ion conductive channel in the
temperature under magnetic stirring for 30 min. The product
membrane, thus leading to the improvement in ion conduc-
tivity. In avoiding the run off the ILs under high relative
ꢀ
was then dried in the oven at 40 C for 24 h. Subsequently, the
ꢀ
product was maintained under vacuum for 5 days at 40 C to
humidity, we introduced nano-TiO
2
to stabilize the ILs in the
remove any residual solvent. The resultant powders were used
for powder X-ray diffraction and TEM measurements.
membrane. The interaction between the nano-TiO and ILs has
2
24–26
been conrmed in the literature.
We dispersed the nano-
TiO2 into the polymer chains sufficiently by the ultrasound
method, so the ILs can be “immobilized” in the membrane
Preparation of triethylamine-graed membrane (BPPO–TEA)
owing to the interaction between the ILs and nano-TiO
dispersed in the membrane offer ion conductive sites which oxide) (BPPO) with a degree of bromination of 50% was dis-
help form the conductive channels.
solved in 10 ml of dimethyl formamide (DMF). Excess triethyl-
2
. The ILs 0.25 g of bromomethylated poly(2,6-dimethyl-1,4-phenylene
In this work, we chose the common polymer backbone PPO amine was added into the solution to complete the
and the conventional conductive group triethylamine so that quaternization. Then the solution was cast on a at glass plate
ꢀ
the ability of ILs to improve the ion conductivity of AEMs can be and dried for 24 h in an oven at 120 C to form the BPPO–TEA
investigated comprehensively. The PPO was rstly brominated membrane.
into the bromine form and then substituted by the triethyl-
amine aer the Menshutkin reaction. Subsequently, different
contents of ILs were mixed into the membrane to prepare
a series of triethylamine–ionic liquid composite AEMs. A certain
Preparation of triethylamine–ionic liquid composite
membrane (BPPO–TEA–IL)
0.25 g of bromomethylated poly(2,6-dimethyl-1,4-phenylene
2
amount of nano-TiO was added into the membranes to form
oxide) (BPPO) with a degree of bromination of 50% was dis-
solved in 10 ml of DMF. Then triethylamine was added into the
solution to complete the quaternization. The reaction was
maintained at room temperature for 4–5 h, and then different
amounts (5%, 10%, 15%, 20%, and 25% weight of the polymer)
of the ionic liquid were dispersed into the polymer solution to
form solutions with different concentrations of ionic liquid.
Subsequently, the solutions were cast on at glass plates and
dried in an oven for 24 h to form BPPO–TEA–IL composite
membranes. The membranes were soaked in 1 M sodium
hydroxide aqueous solution for 48 h to nish the anion
exchange. Finally, the obtained composite membranes were
washed several times with deionized water and immersed in
deionized water before testing.
a series of triethylamine–nano TiO –ionic liquid composite
2
AEMs. The interactions between nano-TiO and ILs in the free
2
states and in the membranes were measured by XRD respec-
tively. The structure of the AEMs was studied by FTIR, NMR, and
SEM. The AEM properties ion exchange capacity (IEC), water
uptake, swelling ratio, hydroxide ion conductivity, mechanical
and thermal properties, and chemical stability were analyzed.
Experimental
Materials
Nano-TiO , N-methylimidazolium and methyl iodide were
2
purchased from Aladdin Chemical (Shanghai, China) and used
as received. PPO was purchased from Nanya Chemical
Company (Nanjing, China). All the solutions were stored over 4 Preparation of triethylamine–nano TiO
2
–ionic liquid
A˚ molecular sieves. All other chemicals were purchased from
composite membranes (BPPO–TEA–1TiO –IL)
2
the Beijing Chemical Reagent Store (China) and used without
further purication. Deionized water was used throughout this
work.
0
2
.0025 g of nano-TiO (1% of the weight of the polymer) was
dispersed into 10 ml of DMF. The solution was kept in an
ultrasonic bath for 12 h to make sure that the nano-TiO was
2
dispersed adequately. Then 0.25 g of polymer was dissolved into
the solution for 12 h to ensure that the polymer structure was
Preparation of 1-methy-3-methylimidazolium ionic liquid
uniformly lled with the nano-TiO
nization was achieved according to the above method, and
N-Methylimidazolium (1.5 g, 0.018 mol) and iodomethane (3.12 different amounts of the ionic liquid were added. The system
g, 0.021 mol) were mixed in a ask under electromagnetic was maintained at room temperature for 8 h to form nano-TiO
2
. Subsequently, the quater-
(
MeIL)
2
–
stirring. The mixture was kept at room temperature for two ionic liquid–polymer composite solutions with different
hours. The product was washed by ethyl acetate for three times concentrations of ionic uids. The solutions were cast on at
and evaporated under decompression to remove the residual glass plates and dried in an oven for 24 h to form the BPPO–
ꢀ
solvent. The melting point was measured to be 75 C (literature TEA–1TiO –IL composite membranes. The membranes were
2
ꢀ
data 79 C). The structure of the product was conrmed by soaked in 1 M sodium hydroxide aqueous solution before taken
NMR.
from the alkali solution. And then, the membranes were washed
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 96768–96777 | 96769

View Article Online
RSC Advances
Paper
by deionized water for several times and immersed in deionized parameters aer the membrane has been soaked in deionized
water before testing.
water for 24 h.
Swelling ratio (%) ¼ (L
w
ꢁ L
d
)/L
d
ꢂ 100%
(3)
Measurements
1
1
H NMR spectroscopy. The H NMR spectra of the ILs and where L
w
is the dimension of the wet membrane and L
d
is the
composite membranes were obtained by an AV400 NMR spec- dimension of the dry membrane.
trometer respectively. The TMS was added as the internal
The WU and SR measurements were made at room temper-
reference for chemical shi in our NMR measurement. The ature and 60 C.
ꢀ
ꢀ
temperature of analysis is between 20 to 25 C and concentra-
Ion conductivity. The ion conductivity of the composite
tion of samples is 50 mg sample in 0.5 ml deuterium reagent. membranes were measured by the standard two-electrode AC
Deuterated dimethyl sulfoxide was used as the solvent.
impedance method in deionized water on a Zahner Eennium
X-ray diffraction. The ionic liquid–nano TiO2 composite electrochemical workstation and the AC frequency ranging
particles and composite membranes were analyzed by X-ray from 1 Hz to 100 kHz. Before testing, the membranes were
diffraction on a Rigaku Miniex 300 powder diffractometer immersed in a 2 M NaOH solution for 48 h to nish the anion
connected to a goniometer (30 kV, 10 mA). The particles were exchange and then stored in deionized water for 24 h to remove
collected in the step scan mode with a step of 0.01 and counting the residual hydroxyl ions in the surface of the membranes. The
ꢀ
ꢀ
time of 0.5 s in the angular range of 15 to 40 with q–2q ion conductivity was calculated as follows:
conguration, and the membranes were measured in the
angular range of 10 to 70 .
Transmission electron microscopy (TEM). The composite
2
material containing ionic liquid and nano-TiO was analyzed by
TEM at 200 kV. The sample was prepared by the dispersion of
the solution on the carbon-coated copper grid.
Scanning electron microscopy (SEM). Samples of the BPPO–
TEA–IL and BPPO–TEA–1TiO –IL composite membranes were
2
ꢀ
ꢀ
s ¼ d/(RA)
(4)
where d is the thickness of the membrane, A is the area of the
electric eld covered by the membrane, and R is the membrane
resistance. The thickness of the membrane d is measured by the
spiral-micrometer, and the area of electric eld A is the exact
data 0.5 cm . The membrane resistance R was measured
through the AC impedance method.
Mechanical properties. A universal material testing machine
was used to study the mechanical properties of the composite
membranes at a crosshead speed of 10 mm min at room
temperature under 60% relative humidity (RH).
Chemical stability. The chemical stability of the BPPO–TEA–
IL and BPPO–TEA–1TiO –IL composite membranes were
2
2
analyzed by SEM. The surfaces of the membranes and the
distributions of the nanoparticles on the surfaces were
observed.
ꢁ1
Thermogravimetric analysis (TGA). TGA was performed on
a TG209C thermogravimetric analyzer in a nitrogen atmo-
ꢀ
sphere. The temperature range was 30–800 C and the heating
ꢀ
ꢁ1
rate was 15 C min
.
investigated to study the stability of the ionic liquid in the
membranes and the degradation of the cation. The membranes
were immersed in a 6 M NaOH solution for 1000 h at room
temperature. Before test, the membranes were stored in
deionized water for 24 h to remove the residual hydroxyl ions in
the surface of the membranes.
Ionic exchange capacity (IEC). The IECs of the composite
membranes were measured by the back titration method. The
membrane was rst soaked in 25 ml of 0.1 M HCl solution at
room temperature for 24 h, and then the solution was back
titrated by 0.1 M NaOH to pH ¼ 7. The membrane was
immersed in deionized water for 24 h to remove the residual
ꢀ
NaOH and then dried in a vacuum at 60 C for 24 h to achieve
a certain weight. The IEC was calculated as follows:
Results and discussion
Synthesis of methyl ILs and nano-TiO composite materials
IEC ¼ (VHCl ꢂ C ꢁ V
ꢂ C )/M
(1)
1
NaOH
2
2
The synthesis procedure for the methyl IL (MeIL) is given in
Scheme 1. The structure of MeIL is conrmed by the NMR
spectrum shown in Fig. 1. The peaks at 9.0 and 7.6 are assigned
to the hydrogens on the imidazole ring. The single peak around
1 2
where C and C are the concentrations and VHCl and VNaOH are
the volumes of the HCl and NaOH solutions, respectively, and M
is the weight of the membrane.
Water uptake (WU) and swelling ratio (SR). The WU of
a composite membrane was calculated by comparing the
weights of the membrane before and aer immersion in
deionized water:
4.0 is assigned to the hydrogens on the two methyl groups. The
peak between 3.0 and 3.5 belongs to the residual water, and the
peak at 2.5 is the solvent peak.
WU (%) ¼ (m
w
ꢁ m
d
)/m
d
ꢂ 100%
(2)
where WU is the water uptake, mw is the weight of the wet
membrane, and m is the weight of the dry membrane.
d
The SR of the membrane was measured in three directions
(
length, width, and thickness) by analyzing the variation of the Scheme 1 Synthesis route of methyl ionic liquid (MeIL).
96770 | RSC Adv., 2016, 6, 96768–96777
This journal is © The Royal Society of Chemistry 2016

View Article Online
Paper
RSC Advances
1
Fig. 1 H NMR spectra of methyl ionic liquid.
Fig. 3 TEM micrographs of composite particle.
The nano-TiO
to the self-assembly method. XRD was used to investigate the
interactions between the TiO nanoparticles and ILs. The
patterns for pure TiO nanoparticles, pure MeIL, and mixtures
2
composite materials were prepared according
2
ꢁ1
of store water of the membranes. The peaks at 2850 cm is
2
associated with the C–H bond and the peaks between 1500
of TiO and MeIL are shown in Fig. 2. The results show that the
mixture retains most of the crystalline structures of both
component materials, while the new peaks and peak displace-
ments indicate interactions between the TiO nanoparticles and
2
ILs at the atomic level. The TEM was analysed to investigate the
size of composite materials. The result is shown in Fig. 3. The
2
ꢁ
1
ꢁ1
cm to 1600 cm are ascribed to the characteristic peaks on
ꢁ
1
the benzene. There hardly any peaks appear at 620 cm , the
successful displace of C–Br is thus conrmed. The NMR test
results are shown in Fig. 5. The peaks at 9.0, 7.6, and 3.9 indi-
cate the successful introduction of the methyl ILs. The peaks
around 7.0 and 6.5 are assigned to the hydrogens on the
size of pure nano-TiO
and the size of composite materials is calculated to be between
0–70 nm. The larger size of the composite materials due to the
presence of IL layers and aggregation of nano-TiO2.
2
bought from Aladdin Chemical is 25 nm
benzene rings. The hydrogens on the groups –CH
CH CH can be observed between 1.2 and 3.5. The peak
around 3.0 is assigned to the residual water.
XRD was used to study the interactions between MeIL and
nano-TiO in the membranes. The patterns are shown in Fig. 6.
2
– and
–
2
3
5
2
Synthesis of triethylamine–nano TiO
membranes
2
–ionic liquid composite
The patterns for BPPO–TEA and BPPO–TEA–IL are similar in
general, showing that the IL is mixed into the polymer directly
without any interactions with the polymer chains. Aer the
The composite membranes were prepared according to the
route shown in Scheme 2. The route is composed of graing,
dispersing, and self-assembly. The FTIR spectra are shown in
doping of the nano-TiO
characteristic peaks of TiO
microstructure of the polymer has changed. Therefore, the
2
, some peaks have disappeared, and the
2
are observed, indicating that the
ꢁ
1
Fig. 4. The peaks around 3500 cm are associated with the
residual water in the membrane, which indicates the well ability
2
interactions between the TiO , IL, and polymer chains in the
membrane is conrmed.
Membrane morphology
Scanning electron microscopy (SEM) was used to analyze the
structure of the membranes. BPPO–TEA–IL and BPPO–TEA–
2
1TiO –15IL were observed severally. The micrographs are
shown in Fig. 7. The BPPO–TEA–IL membrane is homogeneous
in general, but some impurity particles can be observed. Aer
the addition of the nano TiO , the self-assembly interaction
2
between the MeILs and the surface of the nano TiO
the dispersion of the nano TiO –MeIL composite particles
because the nano TiO particles are well dispersed in the
2
enhances
2
2
membrane. No obvious agglomeration of the composite parti-
cles can be observed from the images. These results imply that
the prepared composite membranes have proper structure.
The EDX-mapping test was selected to investigate the
elements and their dispersion in the BPPO–TEA–1TiO –15IL
2
membrane. As shown in Fig. 8, the elements C, N, O, Br, Ti, and
2
Fig. 2 XRD patterns of TiO , methyl-ionic liquid, and composite
particle.
I are observed in the membrane, conrming the successfully
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 96768–96777 | 96771

View Article Online
RSC Advances
Paper
2
Scheme 2 Synthesis route of BPPO–TEA–TiO –IL composite membrane.
Fig. 4 FTIR spectra of the composite membranes.
Fig. 6 XRD patterns of BPPO–TEA, BPPO–TEA–15IL, and BPPO–
TEA–1TiO –15IL membranes.
2
Ionic conductivity
The ion conductivities of the BPPO–TEA–ILs composite
membranes were measured under 100% relative humidity (RH).
A membrane with only TEA was also synthesized and measured.
The results are shown in Fig. 10. The results indicate that the
introduction of even a small amount of IL can improve the ion
conductivity of the membrane. The ion conductivity of the
membrane increases with increasing IL content. Considering
that the polymer structure becomes loosened at high tempera-
tures, the IL molecules can run off from the membranes. As
a result, the ion conductivity of the membrane increases more
slowly at high IL contents and high temperatures, as shown in
Fig. 10.
1
Fig. 5 H NMR spectra of the composite membrane BPPO–TEA–
1
2
TiO –15IL.
2
introduction of nano TiO and MeIL. Fig. 9 shows the disper-
sion of the elements. The blue points stand for the element Ti,
the green points represent the element I, and the red points
stand for the element N. All the elements are uniformly
distributed in the membrane. The elements N and I aggregate
around the element Ti, indicating the self-assembly structure
TiO
conductivities of the membranes with TiO
detailedly to compare with those of the corresponding
membranes without TiO . The results were listed in Fig. 11. The
2
was added to stabilize the IL in the membrane. The ion
2
were measured
2
signicant increase in ion conductivity with the addition of
2
between the MeIL and the surface of the nano TiO .
96772 | RSC Adv., 2016, 6, 96768–96777
This journal is © The Royal Society of Chemistry 2016

View Article Online
Paper
RSC Advances
Fig. 9 (a) Dispersion of N in membrane BPPO–TEA–1TiO
(
(
2
–15MeIL,
b) dispersion of Ti in membrane, (c) dispersion of I in membrane, and
d) dispersion of all three elements.
Fig. 7 SEM micrographs of (A) BPPO–TEA–IL and (B) BPPO–TEA–
TiO –IL membranes.
2
Fig. 10 Ion conductivity versus temperature of BPPO–TEA–IL
membranes.
2
Fig. 8 EDX spectra of BPPO–TEA–1TiO –15MeIL membrane.
TiO indicates that the IL is effectively stabilized by the nano-
2
TiO
2
dispersed in the polymer. Like those of the corresponding
, the ion conductivities of the TiO –IL
membranes without TiO
2
2
composite membranes also increase with increasing IL content.
At an IL content of 30 wt%, the ion conductivity can reach 51.6
ꢁ
1
ꢀ
mS cm at 80 C, which is a great improvement over that of the
BPPO–TEA membrane. These results indicate that the addition
of nano-TiO can reduce the wastage of IL because the IL is
2
stored in the membrane. The self-assembly between the nano-
TiO and ILs can stabilize the ILs in the membrane so that more
2
ILs can play their roles in improving the ion conductivities of
the membranes.
The Arrhenius behaviors of the corresponding membranes Fig. 11 Ion conductivities of the BPPO–TEA–1TiO
2
–IL membranes.
without TiO and nano-TiO –ILs composite membranes were
2
2
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 96768–96777 | 96773

View Article Online
RSC Advances
Paper
investigated respectively. The transport activation energy E
a
of Table 1 WU and SR of composite membranes
an AEM is calculated as follows:
Water
Swelling
Ion
uptake
ratio
(%)
conductivity
E
a
¼ ꢁb ꢂ R
(5)
ꢁ1
(%)
(mS cm
)
where b is the slope of the approximately linear ln s–1000/T, Membrane
30 ꢀC 60 ꢀC 30 ꢀC 60 ꢀC 30 ꢀC 60 ꢀC
a
and R is the gas constant. The E values of the corresponding
ꢁ1
BPPO–TEA
18.67 28.37 9.32
18.95 27.36 9.14
19.41 29.29 9.37
19.56 28.76 9.35
8.95 14.37 4.14
9.31 14.29 4.37
9.56 14.57 4.35
12.67
12.89
12.36 13.27 21.37
12.96 14.96 24.01
7.89 21.64 30
7.83 8.16
8.12 14.27
membranes without TiO range from 11.47 to 14.81 kJ mol
which are close to the literature values of common AEMs.
,
2
BPPO–TEA–15IL
BPPO–TEA–20IL
BPPO–TEA–30IL
2
7–29
a 2
The E values of the nano-TiO –ILs composite membranes,
ꢁ1
which range from 7.36 to 9.47 kJ mol , are much smaller BPPO–TEA–1TiO –15IL
2
indicating the ion conductive channels in these membranes are BPPO–TEA–1TiO
less sensitive to the temperature and more stable in the solu-
2
–20IL
–30IL
7.33 24.5
7.96 31.25 39.02
37.02
BPPO–TEA–1TiO
2
tion. The introduction of nano-TiO
2
stabilizes the ILs and the
interaction between nano-TiO , IL and polymer backbones
2
make the polymer chains combine closer so that the ion
conductive channels created by the active sites of IL are
enhanced. The results are shown in Fig. 12.
water molecules at the higher temperature. When come to the
total results, we found an interesting phenomenon. The SRs of
the membranes vary within a small range with increasing of IL
content in the composite membrane. An explanation is given as
follows: the swelling ratio is mostly decided by the structure of
the polymer backbone and the content of ion conductive
groups, and the added ILs just act as the “transporters” in the
ion conduction process without changing the content of
conductive group trimethylamine or breaking the structure of
the polymer backbones. Thus, the addition of the ILs has no
obvious inuence on the SRs of the membranes because the
content of conductive groups and the structure of the polymer
backbones, the decisive factors for the SR, are the same each
30
IEC, water uptake, and swelling ratio of membranes
The water uptake (WU) and swelling ratio (SR) are important
properties of anion exchange membranes. These properties are
closely connected to the mechanical properties of AEMs. A low
swelling ratio oen leads to ne mechanical properties. The
WUs and SRs of the membranes were measured at room
ꢀ
temperature and 60 C. The results are shown in Table 1. The
ꢀ
WUs and SRs are higher at 60 C than at room temperature
because the loose structure of the polymer will absorb more
membrane. The ion conductivity of the BPPO–TEA–1TiO –30IL
2
membrane is 30% higher than that of the BPPO–TEA–1TiO2–
1
0IL membrane, but the SR is about the same for both
membranes. These results indicate that the addition of nano-
TiO and ILs creates some efficient ion conductive channels in
2
the membrane for increasing the ion conductivity without
breaking the original compact structure of the BPPO–TEA–IL
membrane. The properties of BPPO–TEA and BPPO–TEA–IL
membranes are also measured under the same condition. The
WUs and SRs of the two types membranes are higher than
BPPO–TEA–TiO –IL composite membranes, which indicates the
2
positive effect on mechanical properties of membranes aer the
addition of inorganic nano material.
2 2
The IECs of the BPPO–TEA–1TiO –15IL, BPPO–TEA–1TiO –
2
0IL and BPPO–TEA–1TiO –30IL membranes were measured by
2
back titration. The IEC values of the three membranes are close
because the ion conductive groups in the membranes are equal
in content. In order to investigate the role in increasing the ion
conductivity of the ILs in the membrane in depth, the ion
density (a) of the membranes and the ion mobility (m) of the
hydroxyl were calculated by the following equations:
a ¼ mdry ꢂ IEC/Vwet
m ¼ s/Fa
(6)
(7)
where mdry is the weight of the dry membrane, Vwet is the
volume of the wet membrane, s is the ion conductivity of the
membrane and F is the faraday constant. The results were listed
2
Fig. 12 Arrhenius plots of BPPO–TEA–IL and BPPO–TEA–1TiO –IL
membranes.
96774 | RSC Adv., 2016, 6, 96768–96777
This journal is © The Royal Society of Chemistry 2016

View Article Online
Paper
RSC Advances
Table 2 IEC, ion density and ion mobility of composite membranes
TEA membrane, BPPO–TEA–IL composite membrane, and
BPPO–TEA–1TiO –IL composite membrane were measured
respectively. The IL content in the membrane BPPO–TEA–
2
IEC
Ion density Ion mobility
ꢁ
1
ꢁ1
ꢁ4
2
ꢁ1 ꢁ1
Membrane
(mmol g ) (mol L
)
(ꢂ10 cm s
V )
1
TiO –10IL is 10%.
2
BPPO–TEA–1TiO
BPPO–TEA–1TiO
BPPO–TEA–1TiO
2
2
2
–15IL 1.47
–20IL 1.62
–30IL 1.76
2.16
2.06
2.14
1.04
1.23
1.51
The results are shown in Fig. 13. The curves show that each
membrane shows a three-step decomposition. The rst step
ꢀ
ꢀ
between 50 C and 120 C is assigned to the decomposition of
the bonded water and the residual solvent, which indicates the
appropriate water storage ability of the membrane. The second
ꢀ
ꢀ
decomposition between 200 C and 390 C is assigned to the
degradation of the quaternary ammonium groups and IL. The
in Table 2. The ion density of the three membranes are mostly
the same, which indicates the equal amount of ion conductive
groups and the same polymer backbone. However, the ion
mobility of the hydroxyl groups in the three membranes
increases with increasing IL content in the membranes. The
trend conrms that the ILs dispersed in the membrane really
contribute to the ion conductivity and improve the ion mobility
without participating in the ion conductivity, the fundamental
reason that the addition of ILs can improve the ion conductivity.
ꢀ
third stage aer 420 C is ascribed to the degradation of the
2
polymer backbone. Aer the addition of the nano-TiO , the
curve becomes more at, which conrms an improvement in
the thermal stability of the membrane. The test results show
that the composite membranes have thermal properties suit-
able for fuel cell applications.
Mechanical properties
The mechanical properties of the membranes are also very
important in the practical applications of AEMs. The mechan-
Thermal stability
Thermogravimetric analysis (TGA) was used to investigate the ical properties of the BPPO–TEA membrane, the two ratios of
thermal stability of the composite membranes. The pure BPPO– the BPPO–TEA–IL membranes, and the three ratios BPPO–TEA–
2
1TiO –IL composite membranes were measured at room
temperature (RT) and 60% RH, and the results are shown in
Table 3. The pure BPPO–TEA membrane has a tensile strength
of 28.412 MPa and an elongation at break of 4.04%. Aer the
addition of the IL, the tensile strength decreases to 25.885 MPa.
The tensile strength decreases with increasing IL content
because the introduction of the ILs loosens the structure of the
polymer chains in the membranes and reduces the crystallinity
of the membranes. The mechanical properties are higher in the
BPPO–TEA–1TiO
nucleating effects of the inorganic ller nano-TiO
interactions between the polymer chains. The tensile strengths
of the BPPO–TEA–1TiO –IL composite membranes are in the
2
–IL composite membranes because the
2
enhance the
2
range from 20.52 to 30.48 MPa, the elongation at break is in the
range from 4.54% to 6.15%, and the tensile modulus is in the
range from 646.92 to 921.13 MPa. The three properties decrease
with increasing IL content in the membranes, probably because
more IL in the membranes lead to more water absorption of the
membranes to inuence the mechanical properties of the
Fig. 13 TGA curves of three types of membranes.
Table 3 Mechanical properties of three types of membranes
Tensile strength
(MPa)
Tensile modulus
(MPa)
Elongation at
break (%)
Sample
BPPO–TEA
28.412
25.885
22.438
30.483
22.588
20.524
35.9
889.10
696.59
765.71
921.13
772.20
646.92
800
4.04
4.11
4.09
6.15
5.34
4.54
7.2
BPPO–TEA–15IL
BPPO–TEA–20IL
BPPO–TEA–1TiO
BPPO–TEA–1TiO
BPPO–TEA–1TiO
2
2
2
–15IL
–20IL
–30IL
PPO-g-Q72 (ref. 31)
32
sQPBI-X
20.4
14.3
570
621.8
12.5
89.8
PSAN90-[DAMIm][OH]20 (ref. 33)
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 96768–96777 | 96775

View Article Online
RSC Advances
Paper
observably improve the ion conductivities of the membranes
and the introduction of the nano-TiO can greatly stabilize the
2
MeIL in the membranes. The theoretical calculations showed
that the improvement of the ion conductivities of the
membranes is due to the improvement of ion mobility. The ion
2
conductivities of BPPO–TEA–1TiO –IL composite membranes
ꢁ
1
range from 21.64 to 31.25 mS cm at room temperature and
ꢁ
1
ꢀ
3
8.6 to 51.6 mS cm at 80 C. However, the water uptakes and
swelling ratios of the composite membranes are relatively low.
The thermal stabilities and mechanical properties of the
composite membranes are sufficiently high for AEM applica-
tions. These results showed that the new method for improving
the ion conductivity is promising for AEM applications.
Acknowledgements
Fig. 14 Variations of conductivity with time of three types of
The authors gratefully acknowledge the nancial support from
the National High Technology Research and Development
Program of China (no. 2011AA11A273), the National Natural
Science Foundation of China (no. 21176022, 21176023), the
International S&T Cooperation Program of China (no.
membranes.
membranes and the excess IL will reduce the nucleating effect
of nano TiO . In summary, the mechanical properties of the
2
BPPO–TEA–1TiO –IL membranes are proper compared with
2013DFA51860).
2
those of other membranes reported before.
Notes and references
Alkaline stability
1
2
3
4
5
J. Fan, H. Zhu, R. Li, N. Chen and K. Han, J. Mater. Chem. A,
014, 2, 9376–9385.
C. Xue, J. Zou, Z. Sun, F. Wang, K. Han and H. Zhu, Int. J.
Hydrogen Energy, 2014, 39, 7931–7939.
K. Han, J. Shen, C. Hayner, H. Ye, M. Kung and H. Kung, J.
Power Sources, 2014, 251, 331–337.
A. H. N. Rao, R. L. Thankamony, H.-J. Kim, S. Nam and
T.-H. Kim, Polymer, 2013, 54, 111–119.
The long-term chemical stability of the composite membranes
can be divided into two parts: the degradation of the cation and
wastage rate of the ILs. The pure BPPO–TEA membrane, BPPO–
2
2
TEA–ILs composite membrane and BPPO–TEA–1TiO –IL
composite membrane were immersed into a 4 M NaOH solution
at room temperature, and the ion conductivities of these
membranes were measured several times. Aer 280 h, the
membranes remained tough and the dimensions of the
membranes were proper, indicating the good stability of the
polymer backbones. The variations of ion conductivity with
time for various membranes are shown in Fig. 14. The degra-
dation rate of the ion conductivity of the pure BPPO–TEA
membrane is 78.4% because of the Hofmann elimination of the
quaternary ammonium salt under alkaline condition. Mean-
while, the degradation rates of the ion conductivities of the
BPPO–TEA–IL composite membranes are as high as 74.3%,
G. Merle, M. Wessling and K. Nijmeijer, J. Membr. Sci., 2011,
377, 1–35.
6
7
K. D. Kreuer, Chem. Mater., 2014, 26, 361–380.
H. W. Zhang and P. K. Shen, Chem. Rev., 2012, 112, 2780–
2832.
8
9
G. Couture, A. Alaaeddine, F. Boschet and B. Ameduri, Prog.
Polym. Sci., 2011, 36, 1521–1557.
A. Lai, L. Wang, X. Lin, Y. Zhuo, Q. Zhang, A. Zhu and Q. Liu,
J. Membr. Sci., 2015, 481, 9–18.
80.2% and 84.5%, indicating that the ILs run off from the
1
1
1
1
1
0 N. Li, J. Yong, A. H. Michel and C. Wang, J. Am. Chem. Soc.,
membranes during the test time. However, the run off of the ILs
in the membranes were effectively controlled aer the addition
2013, 135, 10124–10133.
1 Y. Zhuo, A. Lai, Q. Zhang, A. Zhu, M. Ye and Q. Liu, J. Membr.
Sci., 2015, 491, 138–148.
2 G. Nie, X. Li, J. Tao, W. Wu and S. Liao, J. Membr. Sci., 2015,
of the nano-TiO
ties of BPPO–TEA–1TiO
2
. The degradation rates of the ion conductivi-
–IL composite membranes were
2
reduced to 30.3%, 31.8% and 35.1%, conrming the role of the
474, 187–195.
nano TiO in stabilizing the ILs in the membranes.
2
3 Y. Zhao, H. Yu, F. Xie, Y. Liu, Z. Shao and B. Yi, J. Power
Sources, 2014, 269, 1–6.
4 S. Yun, J. Parrondo and V. Ramani, Derivatized Cardo, J.
Mater. Chem. A, 2014, 2, 6605–6615.
Conclusions
A new method was developed to improve the ion conductivity of 15 D. Y. Chen and M. A. Hickner, ACS Appl. Mater. Interfaces,
AEMs by introducing IL and nano-TiO into the membranes. 2012, 4, 5775–5781.
to prevent waste. The ion 16 N. W. Li and M. D. Guiver, Macromolecules, 2014, 47, 2175–
conductivity tests showed that the introduction of the MeIL can 2198.
2
MeIL was immobilized by nano-TiO
2
96776 | RSC Adv., 2016, 6, 96768–96777
This journal is © The Royal Society of Chemistry 2016

View Article Online
Paper
RSC Advances
1
1
1
2
2
2
7 Y. Zha, L. Melanie, M. Disabb, D. Zachary, A. H. Michael and 25 H. Chang, S. Chang, T. Hung, J. Jiang, J. Kuo and S. H. A. Lin,
N. Gregory, J. Am. Chem. Soc., 2012, 134, 4493–4496. J. Phys. Chem. C, 2011, 115, 23778–23783.
8 J. Pan, Y. Li, J. Han, G. Li, L. Tan, C. Chen, J. Lu and 26 T. Alammar, H. Noei, Y. Wang and A.-V. Mudring, Mild Yet,
L. Zhuang, Energy Environ. Sci., 2013, 6, 2912–2915. Nanoscale, 2013, 5, 8045–8055.
9 J. Pan, C. Chen, L. Zhuang and J. Lu, Acc. Chem. Res., 2012, 27 X. Lin, Y. Liu, D. P. Simon, L. Ai, R. V. John, L. Wu, Y. Li,
4
5, 473–481.
X. Liang, Q. Li and T. Xu, J. Power Sources, 2013, 233, 259–
268.
28 B. Qiu, B. Lin, Z. Si, L. Oiu, F. Chu, J. Zhao and F. Yan, J.
Power Sources, 2012, 217, 329–335.
0 J. Pan, C. Chen, Y. Li, L. Wang, L. Tan, G. Li, X. Tang, L. Xiao,
J. Lu and L. Zhuang, Energy Environ. Sci., 2014, 7, 354–360.
1 Y. Yamg and M. K. Daniel, Macromolecules, 2015, 48, 4471–
4
480.
29 X. Li, J. Tao, G. Nie, L. Wang, L. Li and S. Liao, RSC Adv.,
2014, 4, 41398–41410.
2 Y. Naoki, O. Hideaki, M. Junpei, N. Eriko, A. Koichiro,
W. Masahiro and M. Kenji, ACS Appl. Mater. Interfaces, 30 F. Song, Y. Fu, Y. Gao, J. Li, J. Qiao, X. Zhou and Y. Liu,
014, 6, 17044–17052. Electrochim. Acta, 2015, 177, 137–144.
3 A. Lai, L. Wang, C. Lin, Y. Zhuo, Q. Zhang, A. Zhu and Q. Liu, 31 J. Ran, L. Wu, Q. Ge, Y. Chen and T. Xu, J. Membr. Sci., 2014,
ACS Appl. Mater. Interfaces, 2015, 7, 8284–8292. 470, 229–236.
4 M. G. Izabelle, P. F. Clarissa, R. B. Caroline, Z. T. Aniele, 32 J. Li-cheng, L.-c. Steve, L. Bi-yun and H. Yi-lun, J. Membr. Sci.,
A. P. Marcos, A. V. Marcos, M. Giovanna, C. R. Lucas and 2014, 460, 160–170.
C. R. Danieli, ACS Appl. Mater. Interfaces, 2014, 6, 11536– 33 B. Lin, F. Chu, Y. Ren, B. Jia, N. Yuan, H. Shang, T. Feng,
1543. Y. Zhu and J. Ding, J. Power Sources, 2014, 266, 186–192.
2
2
2
1
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 96768–96777 | 96777