enhanced anhydrous proton conductivity of polymer electrolyte membrane enabled by facile ionic...
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Enhanced anhydrous proton conductivity ofpolymer electrolyte membrane enabled byfacile Ionic liquid-based hoping pathways
Haoqin Zhang, Wenjia Wu, Jingtao Wang, TaoZhang, Benbing Shi, Jindun Liu, Shaokui Cao
PII: S0376-7388(14)00869-2DOI: http://dx.doi.org/10.1016/j.memsci.2014.11.033Reference: MEMSCI13321
To appear in: Journal of Membrane Science
Received date: 21 August 2014Revised date: 10 November 2014Accepted date: 18 November 2014
Cite this article as: Haoqin Zhang, Wenjia Wu, Jingtao Wang, Tao Zhang,Benbing Shi, Jindun Liu, Shaokui Cao, Enhanced anhydrous proton conductiv-ity of polymer electrolyte membrane enabled by facile Ionic liquid-basedhoping pathways, Journal of Membrane Science, http://dx.doi.org/10.1016/j.mem-sci.2014.11.033
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1
Enhanced anhydrous proton conductivity of polymer electrolyte membrane
enabled by facile ionic liquid-based hoping pathways
Haoqin Zhang,a Wenjia Wu,
a Jingtao Wang,
a,b,* Tao Zhang,
a Benbing Shi,
a Jindun Liu,
a Shaokui
Caob
aSchool of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou 450001, P. R.
China
bSchool of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450001, P. R.
China
*To whom correspondence should be addressed.
E-mails: [email protected]
Abstract:
Herein, a series of composite membranes based on sulfonated poly(ether ether
ketone) (SPEEK) and imidazole-type ionic liquid (ImIL) are prepared through
IL-swollen method as anhydrous electrolytes for fuel cell. The IL loading amount is
accurately controlled by preparation conditions (e.g., ultrasonic power, treatment
temperature, and treatment time). The influence of IL on physicochemical properties
of composite membrane is systematically investigated. The IL is enriched into the
ionic clusters of SPEEK matrix driven by electrostatic attractions, thereby broadening
them to form inter-connected channels. IL provides anhydrous hoping sites and
low-energy-barrier paths of imidazole-sulfonic acid pairs to composite membrane.
Through the channels, these sites form facile pathways and significantly enhance the
anhydrous conductivity of composite membrane. Particularly, the composite
membrane containing 43% IL achieves a 52 times higher conductivity (9.3 mS cm–1
)
than that of the control membrane (0.179 mS cm–1
) at 140 oC. Increasing IL loading
amount will further elevate the anhydrous conductivity. The dynamic IL release and
2
the concomitant conductivity of composite membrane are investigated. Moreover,
another team of composite membranes are prepared by solution casting method for
exploring the influence of preparation method on the microstructure, IL retention
ability, and conductivity of IL-incorporated membrane.
Keywords: imidazole-type ionic liquid; sulfonated poly(ether ether ketone);
composite membrane; anhydrous hoping site; proton conductivity; transfer
mechanism
3
1. Introduction
Polymer electrolyte membrane fuel cell (PEMFC) operated under elevated
temperatures and anhydrous conditions has attracted intensive attentions due to the
advantages of high tolerance of catalyst to CO, improved electrode kinetics, and
simplified heat/water management [1-6]. Currently, perfluorosulfonic acid membranes
(e.g., Nafion) are widely utilized in PEMFC because of their excellent
physicochemical properties and high proton conductivities (~0.1 S cm-1
) under
hydrated conditions [7]. However, these membranes suffer from serious conductivity
decline (<0.001 S cm-1
) and hence a 95% loss of fuel cell performance, caused by the
water evaporation under the desirable operation conditions [8]. The development of
alternative PEM, that possesses adequate anhydrous proton conductivity, is in great
demand for fuel cell technologies at present.
Theoretically, there are two mechanisms for proton transfer through a PEM: vehicle
mechanism (proton diffusing by forming a complex such as H3O+, H5O2
+, or H9O4
+)
and Grotthuss mechanism (proton hoping from one carrier site to another one through
hydrogen bonds) [9-11]. However, protons can only be transported via Grotthuss
mechanism under elevated temperatures and anhydrous conditions, as the water loss
makes the vehicle mechanism hardly occur. For Grotthuss mechanism, the proton
transfer relies on short-distance (< 0.275 nm) proton hoping between two carrier sites
[12]. It is indispensable to provide sufficient and efficient hoping sites in a membrane
to create facile transfer pathways for achieving high anhydrous conductivity.
To this end, tremendous efforts have been devoted to replacing water with
non-aqueous proton solvents (e.g., heterocycle [13-18] and ionic liquid [19-28]) or
4
creating facile hoping sites (e.g., acid-base pairs [29-33]), which allow protons
migrate in a water-free manner. Heterocycles, for example imidazole, is found to be
the closest proton solvent to water as testified by their similar pKa values (14.5 for
imidazole and 15.7 for water) [13]. As an amphoteric molecule, imidazole has two
nitrogen atoms: one atom accepts a proton while the other atom donates a proton. In
such a way, protons could be transported within one imidazole molecule through
reorientation or between two imidazole molecules through hydrogen bonds [14].
These features endow imidazole-filled membrane with considerable proton
conduction ability. Yamada et al. developed a composite membrane by incorporating
imidazole into poly (vinylphosphonic acid) membrane, and noted an anhydrous
conductivity up to 7 mS cm-1
at 150 oC when incorporating 80% imidazole, over ten
times higher than that of pristine membrane [16]. However, the heterocycle was
usually in free form and would leach from the membrane [18]. This leaching
increased the hoping distance for protons and meanwhile poisoned Pt catalyst in
cathode, leading to a decline of proton conductivity and fuel cell performance. Similar
to heterocycle, ionic liquid (IL) is an ideal proton solvent, which contains Brønsted
acid (proton donor) and base (proton acceptor). These donor and acceptor enable
protons to jump directly from the donor to acceptor [19, 20]. Additionally, the low
vapor pressure and good fluidity confer these hoping sites high mobility at elevated
temperatures, donating superior proton conduction ability [21, 22]. It was found that
IL-based composite membranes could acquire the anhydrous conductivity as high as
10 mS cm-1
at the temperature above 120 oC [23]. For these membranes, sufficient IL
5
loading amount was crucial to fast proton transfer by forming continuous pathways.
Generally, IL is incorporated into membrane through the following methods: solution
casting method [19, 20, 24-26, 34] and IL-swollen method [23, 27]. The former is
related to the IL incorporation by being dispersed into the membrane casting solution;
the latter is related to the IL incorporation by immersing the casted membrane in IL.
For the casting membrane, IL is random distribution within polymer matrix and
unable to form continuous pathway. By comparison, the IL in swollen membrane
concentrated in the ionic clusters of Nafion/SPEEK membrane. In such a way, IL
constructs continuous pathway for proton migration. The capillary forces and
interfacial interactions (electrostatic attractions) jointly enhance the IL retention
ability of composite membrane [28]. Despite these advantages, the investigations for
IL-swollen membrane, especially the control of IL loading amount and the conduction
mechanism of IL, are seldom conducted at present. As another promising anhydrous
carrier site, acid-base pairs had attracted increasing attention due to its distinct
transfer mechanism. Within the pair, proton donor (acid group) and acceptor (base
group) were closely jointed and protons could jump directly from donor to acceptor
[29, 30]. The Coulomb force within the pairs can promote the
protonation/deprotonation and the subsequent proton jumping, thereby reducing
enthalpy change [11]. In such a way, acid-base pairs work as low-energy-barrier path
for proton transfer. Yamada et al. incorporated monododecyl phosphate into
2-undecylimidazole to prepare composite membrane, and the generation of acid-base
pairs allowed the anhydrous conductivity reach 1.0 mS/cm at 150 oC, two orders of
6
magnitude higher than that of pure monododecyl phosphate or 2-undecylimidazole
[31]. However, the development of acid-base composite membrane with ordered and
controlled acid-base pairs remains a challenge [32, 33].
In this study, a series of composite membranes were prepared by incorporating
imidazole-type ionic liquid (ImIL) into acid matrix using IL-swollen method. SPEEK
was chosen as acidic matrix owing to its adequate structural stability and abundant
-SO3H aggregated ionic clusters [35]. The ImIL may play the following multiple roles:
(i) the coexistence of heterocycle and IL provided sufficient proton hoping sites; (ii)
the generated acid-base pairs along the surface of SPEEK clusters constructed facile
transfer pathways, imparting an enhanced proton transfer via Grotthuss mechanism.
The ImIL loading amount was controlled by the treatment conditions, including
ultrasonic power, treatment temperature, and treatment time. For the purpose of
comparison, another team of composite membranes containing the same ImIL loading
amount were prepared by solution casting method. The microstructure and
physicochemical properties of the composite membrane were investigated
systematically. The anhydrous proton conduction property and transfer mechanism of
composite membrane were evaluated in detail.
2. Experiment
2.1. Materials and chemicals
Poly(ether ether ketone) (Victrex®
PEEK, grade 381G) was supplied by Nanjing
Yuanbang Engineering Plastics Co., Ltd. Dimethylformamide (DMF) and sulfuric
acid were supplied from Kewei Chemistry Co., Ltd. 1-Butyl-3-methylimidazolium
tetrafluoroborate ([BMIm]+BF4
-, ImIL) was purchased from Lanzhou Institute of
7
Chemical Physics, Chinese Academy of Sciences. De-ionized water was used in all
experiments.
2.2. Synthesis of SPEEK and SPEEK control membrane
SPEEK was obtained via the post-sulfonation of PEEK: PEEK pellets (20.0 g) were
added into sulfuric acid solution (98 wt.%, 147 mL) at 25 oC. The reaction mixture
was stirred vigorously for a certain time at 45 oC, cooled to room temperature, and
then added into excessive water under continuous agitation. The precipitated SPEEK
was washed by water until pH value reached 7.0, and then it was dried first at 25 oC
for 48 h and then at 60 oC for 24 h under vacuum. Sulfonation degree (DS) of SPEEK
was tuned by controlling the reaction time, and for kinds of SPEEK with the DSs of
43%, 54%, 62% and 75% were synthesized, designated as SP-43, SP-54, SP-62, and
SP-75, respectively. The obtained SPEEK was utilized to prepare SPEEK control
membrane through solution casting method: SPEEK (0.7 g) was dispersed into DMF
(10 mL) at 25 oC and then stirred vigorously for 12 h. The resultant solution was cast
onto a glass plate and dried first at 60 ºC for 12 h, then at 80 ºC for 8 h.
2.3. Preparation of the composite membrane
The SPEEK control membrane (4.0 ×4.0 cm) was used for the preparation of
composite membrane. As illustrated in Scheme 1, the SPEEK control membrane was
immersed into ionic liquid at a given temperature and/or ultrasonic power. After being
immersed in IL for a certain time, the membrane was taken out and the surface IL was
removed, then it was dried in vacuum oven at 60 oC until a constant weight was
obtained. The IL loading amount within the resulting composite membrane was
8
determined by the following equation: IL loading amount (%) = %100×−
d
dw
W
WW,
where Wd and Ww were the weight of membrane before and after being incorporated
by IL. It should be noted that the thickness of SPEEK control membrane was around
78 µm, and it increased to the range of 85-92 µm after the incorporation of IL.
Scheme 1
2.4. Characterization
Fourier transform infrared (FTIR) spectra of the membrane was recorded on a
Nicolet MAGNA-IR 560 instrument with the resolution of 4 cm-1
at room temperature.
Cross-section of the membrane was observed on a scanning electron microscope
(SEM, JSM7500F) after freeze-fractured in liquid nitrogen and followed by sputtering
with gold. The chain packing of membrane was probed by small-angle X-ray
scattering (SAXS) and wide X-ray diffractometry (WXRD) using a Bruker D8
Advance ECO in the range of 0.1-4o and 5-60
o, respectively. Thermogravimetric
analysis (TGA) was conducted by TGA-50 SHIMADZU from 30 to 800 oC at 10
oC
min-1
under nitrogen atmosphere. Mechanical property of the membrane sample (1.0
cm×4.0 cm) was tested using an Instron Mechanical tester (Testometric 350 AX) at
the elongation rate of 2 mm min-1
at room temperature.
2.5. IL loss and IL retention
The IL leaching behaviors of composite membrane were determined in terms of IL
loss and IL retention. The composite membrane was dried in vacuum oven at 60 oC
until a constant weight (Wb), and then it was immersed in 40 ml water at room
temperature for a certain time. The membrane was dried in vacuum oven at 60 oC
until a constant weight (Wa). The IL loss was calculated using the formula: IL loss (%)
9
= %100×−
W
WW ab, W was the initial weight of IL in composite membrane. The IL
retention was calculated by equation: IL retention (%) = %100×−
d
da
W
WW, where Wd
was the weight of SPEEK in composite membrane. The IL loss and IL retention under
low humidity were tested through a similar way by keeping the membrane in a
climate box under a certain temperature and humidity.
2.6. Proton Conductivity Measurement
Proton conductivity of the membrane was measured in a conductivity cell by AC
impedance spectroscopy method. The membrane resistance was probed by a
frequency response analyzer (FRA, Compactstat, IVIUM Tech.) with oscillating
voltage of 20 mV over a frequency range of 105-1 Hz. The membrane sample was
dried in vacuum oven at 60 oC until it reached a constant weight. The anhydrous
conductivity was tested using dry air and the system was allowed to equilibrate at the
desired temperature for a few hours until a constant resistance (R, Ω) was obtained.
The conductivity (σ, Scm-1
) of the sample was obtained from σ=l/AR, where l (cm)
and A (cm2) were the membrane thickness and area, respectively.
3. Results and Discussion
3.1. Microstructure of the membrane
The chemical structure of membrane was qualitatively analyzed by FTIR spectra.
Fig. 1 revealed that all the membranes displayed three characteristic peaks at around
1225, 1083 and 1027 cm-1
, which were assigned to the symmetric and asymmetric
stretching vibrations of O=S=O in -SO3H groups, confirming the successful
sulfonation of PEEK backbone. Compared with SPEEK control membrane, IL
incorporation gave rise to two new peaks for composite membrane at 1580 and 1060
10
cm-1
, corresponding to imidazole cation (C-N stretching) and BF4- anion, respectively.
Meanwhile, the intensities of characteristic peaks for -SO3H in composite membrane
became weak, probably due to the formation of interactions between -SO3H of
SPEEK and imidazole of IL [36]. In addition, the peak that associated with -SO3-
(1109 cm-1
) could not be discriminated in composite membrane, indicating that the
acid (sulfonic acid) and base (imidazole) groups did not react to form imidazolium
salt, which should assemble into ordered acid-base pairs.
Fig. 1
SPEEK, composed of a hydrophilic side chain (terminated with sulfonic acid
groups) and a hydrophobic aromatic backbone, possessed distinct nanophase
separation. The side chain aggregated into ionic cluster, whereas the aromatic
backbone formed hydrophobic phase for mechanical strength [37]. The morphologies
of the as-prepared membranes were probed by SEM and SAXS. It could be seen in
Fig. 2 that the overall morphology of all the membranes was uniform without obvious
cracks or defects. For SPEEK control membrane, no macroscopic channel was
observed in Fig. 2a. In comparison, the incorporation of IL gave distinct continuous
channels to the composite membrane, and increasing the IL loading amount would
elevate the channel size and meanwhile increase the connectivity of these channels
(Fig. 2b-e). Such phenomena indicated that IL might be mainly stored in the ionic
clusters driven by the electrostatic interactions from -SO3H groups, and the
aggregation of IL broadened the clusters to form inter-connected channels.
Fig. 2
SAXS as a commonly-utilized technique is also available to probe ionic
11
clusters/channels, which are often caused by the nanophase separation between ionic
side chains and hydrophobic main chains. To be specific, the location and intensity of
SAXS peak can be employed to evaluate the size and number of the ionic cluster
/channel, respectively [38-41]. Accordingly, SAXS was utilized to further probe the
structure of the membrane and the results were shown in Fig. 3a. It could be found
that typical scattering peaks at the q values ranging of 0.50-0.56 nm-1
were observed
for all the membranes, inferring the existence of nanophase separation and ionic
cluster [41]. Compared with SPEEK control membrane (0.56 nm-1
), IL incorporation
increased the channel size of composite membranes as testified by the reduction of q
values to 0.53, 0.53, 0.52, and 0.50 nm-1
when incorporating 16%, 25%, 32%, and
43% IL, respectively. The Bragg spacing (d) was related to q according to the
equation: d = 2π/q, and the d was referred to the “center to center distance” between
two clusters and indicated the size of ionic clusters [42]. Accordingly, the reduction of
q implied the increase of cluster size. Such phenomena were reasonably attributed to
the enrichment of IL within the clusters, which were in agreement with the results by
SEM. Meanwhile, the incorporation of IL also enhanced the peak intensity of the
composite membranes, indicating the increased number of ionic clusters. Collectively,
the increased cluster size and cluster number corporately contributed to the increase of
connectivity of ionic cluster channels, and the IL-enriched continuous channels would
donate facile pathways for proton hoping through the carrier sites (i.e. imidazole,
ionic liquid, and acid-base pairs). To further investigate the influence of IL on the
morphology of SPEEK matrix, the membranes were subjected to WXRD. Fig.3b
12
revealed that all the membranes exhibited a broad crystalline band at 2θ=12-25°,
corresponding to the ordered stacking of hydrophobic backbone [43]. Compared with
SPEEK control membrane, the intensity decline of this band in composite membrane
should be duo to the plasticizing effect of IL on hydrophobic domains of SPEEK. The
plasticizing effect would weaken the interactions among backbones and thus destroy
the ordered stacking, making the composite membrane more flexible.
Fig. 3
3.2. Thermal and mechanical properties of the membrane
PEM should possess adequate thermal and mechanical stabilities to achieve the
ability for practical application in fuel cell. Thermal stability of SPEEK control
membrane and composite membrane were probed by TGA and the results were shown
in Fig. 4a. For SPEEK control membrane, three-step weight loss was observed: the
first stage was the water evaporation from membrane (30-150 oC); the second stage
was the pyrolysis of sulfonic acid group of SPEEK (270-400 oC); and the third stage
was the decomposition of SPEEK backbone (450-800 oC). By comparison, the
incorporation of IL altered the decomposition behavior of composite membrane. In
the first stage, composite membrane showed lower weight loss than that of SPEEK
control membrane. This finding should be attributed to the presence of IL, which
generated electrostatic interactions with -SO3H groups in the form of acid-base pairs,
thus suppressing the water absorption of SPEEK. In the second stage, the degradation
of IL and -SO3H allowed the composite membrane display higher weight loss than
SPEEK control membrane (15%) [44]. For example, the weight losses of SP-62-IL-16,
13
SP-62-IL-25, SP-62-IL-32, and SP-62-IL-43 during this stage were 27%, 33%, 41%,
and 49%, respectively. In the third stage, the weight loss for composite membrane was
ascribed to the degradation of SPEEK backbone, similar to that of SPEEK control
membrane. The char yield at 800 oC of composite membrane was lower than that of
control membrane as a result of the decomposition of filled IL. TGA results revealed
that all the membranes were thermally stable up to 250 oC, adequate for fuel cell.
Mechanical property of the membrane in term of tensile strength was shown in Fig.
4b. The continuous hydrophobic phase (aromatic backbones) endowed SPEEK
control membrane with good mechanical property, achieving the tensile strength of
35.5 MPa. The presence of IL destroyed the ordered stacking of SPEEK backbones
via plasticizing effect, resulting in a reduction of tensile strength. The strengths of
SP-62-IL-16, SP-62-IL-25, SP-62-IL-32, and SP-62-IL-43 were reduced to 28.6, 24.3,
22.1, and 19.8 MPa, respectively. In spite of the strength reduction, the composite
membranes possessed acceptable tensile strength when compared with that of recast
Nafion 115 (17.2 MPa).
Fig. 4
3.3. The effect of preparation conditions on IL loading amount
Since IL will donate additional proton-hoping sites, IL loading amount strongly
affects the proton conductivity of IL-based membranes. In this study, the IL loading
amount in composite membrane was tuned by preparation conditions, including
ultrasonic power, treatment temperature, and treatment time. As one common
strengthening process, ultrasonic treatment could accelerate the mass transfer by
14
reducing the resistance of boundary layer. The influence of ultrasonic power on IL
loading amount was illustrated in Fig. 5a. It could be found that the IL loading amount
was 3.5% under the ultrasonic power of 50 W for 8 h. Increasing ultrasonic power
would lower the resistance of boundary layer, thereby elevating IL loading amount.
For example, the IL loading amount of composite membrane elevated from 3.5% to
8.8% as the power increased from 50 to 200 W. With the increase of ultrasonic time,
the IL loading amount increased at first and then kept almost constant after treating 8
h. Such phenomenon should be due to the high molecule weight and viscosity of IL,
which made IL hardly diffuse deeply into ionic cluster. Accordingly, the IL loading
amount in composite membrane was limited (<9%) through ultrasonic treatment. Fig.
5b showed the temperature-dependent IL loading amount, which revealed that all the
membranes exhibited a continuous rise of IL loading as the temperature elevated from
25 oC to 85
oC. For instance, the loading amounts of SP-43, SP-54, SP-62, and SP-75
at 25 oC were 1.3 2.1, 3.0, and 3.8%, respectively; while they significantly increased
to 27.5, 52.6, 70.8, and 77.1% when the temperature reached 65 oC. Such phenomena
were ascribed to the temperature increase, which (i) would endow IL with decreased
viscosity and increased molecular kinetic energy and (ii) would afford the SPEEK
matrix enlarged ionic cluster, thus corporately promoting the diffusion of IL deeply
into ionic cluster. A sharp increase of IL loading amount was observed when the
temperature increased from 55 to 65 oC. This should be due to the obvious swelling of
SPEEK-based membranes, which efficiently enlarged the ionic cluster. Similar
behavior was observed in water swelling test of SPEEK-based membranes [45]. Fig.
15
5b also revealed that the DS of SPEEK strongly influenced IL loading amount. High
DS would afford the membrane large and more ionic clusters, which reduced the
diffusion resistance and meanwhile provided more space for IL storage. Besides, the
high -SO3H concentration would facilitate the IL transfer into ionic cluster through
electrostatic attractions. For example, the IL loading amounts of SP-43, SP-54, SP-62,
and SP-75 at 75 oC were 38.2, 64.9, 88.5, and 96.5%, respectively. Considering the
efficient swelling, the time-dependent IL loading amount was conducted under the
temperature of 65 oC. Fig. 5c suggested that with the increase of treatment time, the
IL loading amount increased at the first 6 h and then reached a constant value for all
the membranes. This finding indicated that the uptake of IL mainly occurred within
the beginning 6 h, and then the membrane reached a dynamic equilibrium for IL
incorporation.
Fig. 5
3.4. Proton conductivity
High anhydrous proton conductivity is essential for the performances of
elevated-temperature fuel cell, including operational fuel cell voltage and current
output. Considering the structural/mechanical stabilities, the IL loading amount was
controlled below 43% in the conductivity measurement. Anhydrous conductivities of
the as-prepared membranes under 40-140 °C were tested and the results were shown
in Fig. 6a. It was found that SPEEK control membrane exhibited a relatively low
proton conductivity of 0.17 mS cm–1
at 120 °C, three orders of magnitude lower than
that under hydrous conditions [46]. This finding corroborated that water played
pivotal role in proton transfer of –SO3H group by generating solvated specie to
16
dissociate H+ from the acid group, through which the resulting acid radical ion (–SO3
–)
could serve as proton carrier. Accordingly, the –SO3H groups in SPEEK control
membrane might lose their conduction ability in the absence of water. In comparison,
the incorporation of IL gave significant enhancement in proton transfer ability to
composite membrane, yielding a 20 times increase of conductivity (3.6 mS cm–1
)
when embedding only 16% IL. Increasing IL loading amount could further enhance
the transfer ability of composite membrane. For instance, the conductivities were
gradually enhanced to 5.4, 6.6, and 7.7 mS cm–1
as the IL loading amounts increased
to 25%, 32%, and 43%, respectively. Such observations inferred the superior
promotion of imidazole-typed ionic liquid on the proton conduction behavior of acidic
polymer membrane. The presence of ImIL within ionic channels would (i) provide
abundant anhydrous hoping sites (i.e., imidazole and ionic liquid, Scheme 2B and C)
for proton jumping and (ii) form acid-base (sulfonic acid-imidazole, Scheme 2A)
pairs along the channel surface, which created low-energy-barrier anhydrous paths.
Through the inter-connected channels, these hoping sites created continuous pathways
for efficiently transporting protons. In such a way, the conduction ability of -SO3H
was activated in the form of acid-base pairs. With the increase of IL loading amount,
the size and connectivity of transfer channels were increased, yielding a gradual
enhancement in anhydrous conductivity. For another, Fig. 6a also revealed that all the
membranes displayed a continuous increase of anhydrous conductivity with the
testing temperature. For example, the conductivities of SP-62-IL-32 increased from
0.93 to 7.4 mS cm–1
when the temperature elevated from 40 to 140 °C. The facilitated
17
mobility of proton carriers should be the main factor for conductivity increase. It
should be noted that SP-62-IL-43 achieved the maximum anhydrous conductivity of
9.3 mS cm–1
at 140 °C among the tested membranes, which was about 52 times of
that of SPEEK control membrane (0.18 mS cm–1
) under identical conditions.
Fig. 6
Scheme 2
To further investigate the functions of IL on proton conduction, activation energy
(Ea) for proton transfer through the membrane was calculated using Arrhenius
equation from temperature-dependent conductivity curves (Fig. 6b). Close to the
results in literature, SPEEK control membrane attained a Grotthuss-type Ea of 26.9 KJ
mol-1
[47, 48]. In comparison, incorporating IL lowered the Ea values of composite
membranes, allowing the Ea values reduce to 26.1, 25.2, 22.8, and 21.6 KJ mol-1
when
embedding 16%, 25%, 32%, and 43% IL, respectively. The increase of hoping sites
and channel size as well as the formation of acid-base pairs should contribute to this
barrier reduction. In summary, the reduced Ea together with the enhanced proton
conductivity further verified the distinct conduction ability of imidazole-type ionic
liquid within acidic polymer membrane, which constructed facile pathways for
Grotthuss-type proton transfer through IL-swollen method.
3.5. IL retention ability of the membrane
Generally, the IL leaching is a serious issue for IL-incorporated composite
membrane, because it will lead to the decline of proton conductivity. During the
18
operation of PEMFC under elevated temperatures and anhydrous conditions, water
molecules will be generated at cathode in the form of steam, part of which may cause
IL leaching from PEM. In order to investigate the IL retention ability of composite
membrane, IL loss and IL retention were tested under an extreme condition
(immersing the membrane into water). Fig. 7a depicted the IL loss curves of
composite membrane as a function of time when being immersed in de-ionized water.
It was found that all the composite membranes displayed a fast IL release at the
beginning 50 min, owing to the leaching of IL in free form. Like free water, the
free-form IL had weak interactions with membrane matrix and mainly located in the
center of ionic channels. After this stage, the IL content in composite membrane kept
almost constant with the testing time, and the retained IL was reasonable in bound
form. Like bound water, the bound-form IL had strong attractions with -SO3H groups
along channel surface. For composite membrane, high IL loading amount was related
to large IL-enriched channels, affording more free-form IL. Accordingly, SP-62-IL-43
displayed a higher IL loss (79%) during the first stage than SP-62-IL-16 (72%). Fig.
7b showed the time-dependent IL retention within composite membrane. Similar to
the results in Fig. 7a, the IL retention decreased obviously at the beginning 50 min
and then reached a constant value. Although SP-62-IL-43 suffered from a fast IL loss,
the increase of ionic channels (see SEM image) would provide more space for
interacting with IL. Accordingly, the retained IL within SP-62-IL-43 at equilibrium
(9.0%) was higher than that within SP-62-IL-16 (4.6%). The IL leaching would
reduce the amount of proton sites and thus decrease the proton conduction ability of
19
composite membrane. Fig. 7c showed the proton conductivity of composite
membrane during the IL release. Similarly to the behavior of IL retention, the drastic
IL release led to obvious reduction of proton conductivity within the beginning 50
min. For instance, the conductivities of SP-62-IL-16 and SP-62-IL-43 decreased from
5.1 to 1.8 mS cm–1
and from 9.3 to 2.9 mS cm–1
during this stage, respectively. The
high IL retention endowed SP-62-IL-43 with higher proton transfer ability than that of
SP-62-IL-16. After equilibrium, the retained IL was mainly in bound form, which
transported protons through acid-base pairs in a low-barrier manner. Consequently,
the constant value of conductivity of composite membrane (above 1.8 mS cm–1
) after
leaching was still much higher than that of SPEEK control membrane (0.18 mS cm–1
)
at 140 oC. Under low humidity, the IL leaching result of composite membrane is more
useful for its practical application in fuel cell. SP-62-IL-43, which contained
relatively high IL loading amount among all the composite membranes, was chosen as
representative for investigating the IL retention ability under 80 oC and 10% RH over
days. Fig. 7d revealed a similar IL loss trend to that when the membrane was
immersed in water: the IL loss increased at first and then reached a constant value.
Compared with the data in Fig. 7a, it could be found that the IL leaching from
composite membrane under low humidity was obviously reduced due to the lack of
water. For example, the constant IL loss of SP-62-IL-43 under 10% RH was only
8.1% after testing 168 h, about 1/10 of that when being immersed in water after
testing 150 min. As a result, the retained IL in SP-62-IL-43 under 10% RH (39.5%)
was much higher than that when being immersed in water (9.0%). Collectively, these
20
data implied that the as-prepared membranes had acceptable IL retention ability,
especially under low humidity.
Fig. 7
3.6 Comparison of the composite membranes prepared by IL-swollen and
solution casting methods
In this study, IL was incorporated into the ionic cluster of SPEEK membrane by
IL-swollen method. SEM images revealed that IL was mainly concentrated within
ionic clusters, affording composite membrane continuous IL channels. In this manner,
efficient transfer pathways were constructed for anhydrous proton jumping. This
approach might be different from the well-developed solution casting method,
through which the IL was randomly distributed within the membrane, failing to form
continuous pathways. In order to confirm this hypothesis, a series of composite
membranes with the same IL loading amount were prepared through solution casting
method: a certain amount of IL and SPEEK were dispersed into DMF under ultrasonic
treatment and stirring, and then the resultant solution was casted onto a glass plate and
dried. The microstructure, proton conductivity, and IL retention ability were
investigated.
The microstructure of composite membrane prepared by solution casting method
was probed by SEM (Fig. 8), which revealed that the overall morphologies of all the
membranes were uniform without obvious cracks or pinholes. Contrast to the swollen
membranes, no IL-aggregated channel was observed in the cross-section of whole
membrane. This phenomenon indicated that IL was uniformly distributed within the
21
SPEEK matrix on the molecular scale driven by the strong electrostatic interactions
between SPEEK and IL. These interactions would interfere with the nanophase
separation of SPEEK chains and thus suppressed the formation of bi-continuous
phases (i.e., hydrophilic cluster and hydrophobic domain).
Fig. 8
This structural feature could also be verified by the results of tensile strength. Fig. 9
revealed that IL incorporation markedly reduced the tensile strength of casting
membrane. The tensile strengths of SP-62-IL-16, SP-62-IL-25, SP-62-IL-32, and
SP-62-IL-43 were 27.2, 22.7, 19.5, and 17.7 MPa, respectively. The values of casting
membranes were all lower than those of the swollen membranes under identical
conditions. For instance, the tensile strengths of SP-62-IL-25 prepared by IL-swollen
method and solution casting method were 24.3 and 22.7 MPa, respectively.
Considering the existence of numerous voids (IL-aggregated channels) in the swollen
membrane, the low tensile strength of the casting membrane was probably ascribed to
the lack of continuous hydrophobic phase.
Fig. 9.
Anhydrous conductivity of the casting membrane at temperatures ranging from 40
to 140 °C was shown in Fig. 10a. Similarly, the incorporation of IL gave additional
hoping sites in the form of acid-base pairs, resulting in an enhancement of proton
conductivity of composite membrane. For instance, the anhydrous conductivities of
SP-62-IL-16, SP-62-IL-25, SP-62-IL-32, and SP-62-IL-43 at 120 °C were 2.3, 3.6,
4.9, and 6.2 mS cm-1
, respectively. For another, the anhydrous conductivities of all the
membranes increased with the testing temperature. Fig. 10b depicted the
22
temperature-dependent conductivities of the composite membranes prepared by
IL-swollen method and solution casting method containing the same IL loading
amount. It could be clearly seen that the swollen membrane possessed higher proton
transfer ability than that of the casting membrane at all temperatures. For instance, the
conductivities of SP-62-IL-16 at 120°C were 3.6 and 2.3 mS cm-1
for swollen
membrane and casting membrane, respectively, and these values of SP-62-IL-43 were
7.7 and 6.2 mS cm-1
. The random distribution of IL within the casting membrane
should be the main factor for its low conductivity, which made the membrane lack of
continuous conduction pathways and thus had high transfer resistance.
Fig. 10
The IL retention ability of casting membranes in terms of IL loss and IL retention
was tested and the results were shown in Fig.11a-b. It was found that the casting
membrane displayed similar trend to the swollen membrane: a fast IL release at the
beginning 50 min and then reaching a constant value. Compared with the swollen
membrane, the casting membrane attained higher IL retention. For example, the
retained IL loading amounts at equilibrium of the casting membranes were 5.0% and
10.8% for SP-62-IL-16 and SP-62-IL-43, respectively, higher than the 4.6% and 9.0%
of the swollen membranes. Such phenomena were probably due to the random
distribution of IL in the casting membrane, which allowed more IL to interact with
SPEEK chains and thus formed more bound-form IL. Fig 11c showed that the
conductivity of casting membrane continuously decreased during the IL release. By
comparison, it was surprising to found that the swollen membranes attained higher
23
proton conductivities than those of the casting ones under identical conditions. For
instance, the constant values of conductivity of the casting membranes were 0.9 and
1.8 mS cm–1
for SP-62-IL-16 and SP-62-IL-43, respectively, while those of the
swollen ones were 1.4 and 2.2 mS cm–1
. Considering the low IL retention of swollen
membranes, the high conductivity should be attributed to the inter-connected channels,
which worked as continuous conduction pathways. Furthermore, the IL retention
ability and the corresponding conductivity of the casting membrane under 80 oC and
10% RH were also performed and the results were shown in Fig. 11d-e. Despite the
similar IL release trend, the IL retention under this condition was higher than that
being immersed in water. The high IL retention then conferred the composite
membrane high proton conduction ability. The constant conductivity of the casting
SP-62-IL-43 under 80 oC and 10% RH (after testing for 168 h) was 3.5 mS cm
–1,
while this value for the membrane being immersed in water was 1.8 mS cm–1
(after
testing for 150 min). Compared with the casting membrane, the swollen membrane
displayed higher anhydrous conductivity under 80 oC and 10% RH, resulting from the
continuous conduction pathways. Collectively, these results implied that the swollen
membrane might be superior to the casting membrane in proton conduction property.
Fig. 11
4. Conclusions
In summary, a facile approach for highly conducting PEM under anhydrous
conditions was developed by incorporating imidazole-type ionic liquid into acidic
polymer membrane through IL-swollen method. During the preparation, IL loading
24
amount was accurately controlled by tuning the preparation conditions, including
ultrasonic power, treatment temperature, and treatment time. Besides, this approach
could allow IL aggregate into the ionic clusters of SPEEK matrix, forming
inter-connected conducting channels, and increasing IL loading amount could elevate
the size and connectivity of channels. Conductivity measurements revealed that the
presence of IL gave significant enhancement in anhydrous conductivity of SPEEK
membrane through the following multiple roles: the imidazole and ionic liquid
provided abundant anhydrous hoping sites to the membrane; the imidazole formed
acid-base pairs with sulfonic acid groups, which worked as low-energy-barrier
pathways along channel surface. The continuous pathways ensured the efficient
proton transfer through these hoping sites. Particularly, a 51 times increase of
conductivity (9.3 mS cm–1
) at 140 oC was acquired when incorporating 43% IL. In
addition, the electrostatic interactions conferred the composite membrane with
acceptable IL retention ability under low humidity. Compared with swollen membrane,
casting membrane exhibited random IL distribution and failed to form continuous
transfer pathway, thus showing lower anhydrous conductivity. Considering the facile
preparation method, tunable IL loading amount, and continuous transfer pathway, the
present study may offer a promising strategy for design of proton conducting
materials with high proton conductivity, especially under anhydrous and elevated
temperature conditions.
Acknowledgements
25
We gratefully acknowledge financial supports from National Natural Science
Foundation of China (21206151 and 21276244) and China Postdoctoral Science
Foundation (2012M521409).
Nomenclature
wt weight fraction (%)
DS sulfonated degree of PEEK (%)
Wd weight of the SPEEK control membrane (g)
Ww weight of the composite membrane (g)
Wb weight of the composite membrane before leaching test (g)
Wa weight of composite membrane after leaching test (g)
W initial weight of IL in composite membrane (g)
σ proton conductivity (S cm-1
)
l membrane thickness (cm)
A membrane surface area (cm2)
R membrane resistance (Ω)
d the Bragg spacing (nm)
q scattering peaks (°)
2θ crystalline band (°)
Ea activation energy for proton transfer (KJ mol-1
)
References
26
[1] J.-T Wang, R.F. Savinell, J.S. Wainright, M.H. Litt, H. Yu, A H2/O2 fuel cell using
acid doping polybenzimidazole as polymer electrolyte, Electrochim. Acta 41 (1996)
193-197.
[2] S.M. Haile, D.A. Boysen, C.R.I. Chisholm, R.B. Merle, Solid acids as fuel cell
electrolytes, Nature 410 (2001) 910-913.
[3] C. Shen, S.L. Hsu, Synthesis of novel cross-linked polybenzimidazole membranes
for high temperature proton exchange membrane fuel cells, J. Membr. Sci. 443 (2013)
138-143.
[4] Y. Zhu, W.H. Zhu, B.J. Tatarchuk, Performance comparison between high
temperature and traditional proton exchange membrane fuel cell stacks using
electrochemical impedance spectroscopy, J. Power Sources 256 (2014) 250-257.
[5] S. Yuan, X. Guo, D. Aili, C. Pan, Q. Li, J. Fang, Poly(imide benzimidazole)s for
high temperature polymer electrolyte membrane fuel cells, J. Membr. Sci. 454 (2014)
351-358.
[6] A. Ignaszak, C. Song, W. Zhu, J. Zhang, A. Bauer, R. Baker, V. Neburchilov, S. Ye,
S. Campbell, Titanium carbide and its core-shelled derivative TiC@TiO2 as catalyst
supports for proton exchange membrane fuel cells, Electrochim. Acta 69 (2012)
397-405.
[7] K.A. Mauritz, R.B. Moore, State of understanding of Nafion, Chem. Rev. 104
(2004) 4535-4585.
27
[8] J.S. Lee, T. Nohira, R. Hagiwara, Novel composite electrolyte membranes
consisting of fluorohydrogenate ionic liquid and polymers for the unhumidified
intermediate temperature fuel cell, J. Power Sources 171 (2007) 535-539.
[9] K.D. Kreuer, Proton Conductivity: Materials and Applications, Chem. Mater. 8
(1996) 610-641.
[10] T.J. Peckham, S. Holdcroft, Structure-Morphology-Property Relationships of
Non-Perfluorinated Proton-Conducting Membranes, Adv. Mater. 22 (2010)
4667-4690.
[11] J. Wang, X. Yue, Z. Zhang, Z. Yang, Y. Li, H. Zhang, X. Yang, H. Wu, Z. Jiang,
Enhancement of Proton Conduction at Low Humidity by Incorporating Imidazole
Microcapsules into Polymer Electrolyte Membranes, Adv. Funct. Mater. 22 (2012)
4539-4546.
[12] I. Presiado, J. Lal, E. Mamontov, A.I. Kolesnikov, D. Huppert, Fast Proton
Hopping Detection in Ice Ih by Quasi-Elastic Neutron Scattering, J. Phys. Chem. C
115 (2011) 10245-10251.
[13] R. Subbaraman, H. Ghassemi, T. Zawodzinski Jr, Triazole and triazole
derivatives as proton transport facilitators in polymer electrolyte membrane fuel cells,
Solid State Ionics 180 (2009) 1143-1150.
[14] W. Münch, K.D. Kreuer, W. Silvestri, J. Maier, G. Seifert, The diffusion
mechanism of an excess proton in imidazole molecule chains: first results of an ab
initio molecular dynamics study, Solid State Ionics 145 (2001) 437-443.
[15] M. Geormezi, C.L. Chochos, N. Gourdoupi, S.G. Neophytides, J.K. Kallitsis,
28
High performance polymer electrolytes based on main and side chain pyridine
aromatic polyethers for high and medium temperature proton exchange membrane
fuel cells, J. Power Sources 196 (2011) 9382-9390.
[16] M. Yamada, I. Honma, Anhydrous proton conducting polymer electrolytes based
on poly(vinylphosphonic acid)-heterocycle composite material, Polymer 46 (2005)
2986-2992.
[17] S. Wang, C. Zhao, W. Ma, G. Zhang, Z. Liu, J. Ni, M. Li, Preparation and
properties of epoxy-cross-linked porous polybenzimidazole for high temperature
proton exchange membrane fuel cells, J. Membr. Sci. 411-412 (2012) 54-63.
[18] C. Yang, P. Costamagna, S. Srinivasan, J. Benziger, A.B. Bocarsly, Approaches
and technical challenges to high temperature operation of proton exchange membrane
fuel cells, J. Power Sources 103 (2001) 1-9.
[19] J.T. Wang, S.L. Hsu, Enhanced high-temperature polymer electrolyte membrane
for fuel cells based on polybenzimidazole and ionic liquids, Electrochim. Acta 56
(2011) 2842-2846.
[20] A. Fernicola, S. Panero, B. Scrosati, Proton-conducting membranes based on
protic ionic liquids, J. Power Sources 178 (2008) 591-595.
[21] T. Welton, Room-Temperature Ionic Liquids. Solvents for Synthesis and
Catalysis, Chem. Rev. 99 (1999) 2071-2083.
[22] S. Yi, F. Zhang, W. Li, C. Huang, H. Zhang, M. Pan, Anhydrous
elevated-temperature polymer electrolyte membranes based on ionic liquids, J.
Membr. Sci. 366 (2011) 349-355.
29
[23] M.K. Mistry, S. Subianto, N.R. Choudhury, N.K. Dutta, Interfacial Interactions in
Aprotic Ionic Liquid Based Protonic Membrane and Its Correlation with High
Temperature Conductivity and Thermal Properties, Langmuir 25(16) (2009)
9240-9251.
[24] J. Xiang, R. Chen, F. Wu, L. Li, S. Chen, Q. Zou, Physicochemical properties of
new amide-based protic ionic liquids and their use as materials for anhydrous proton
conductors, Electrochim. Acta 56 (2011) 7503-7509.
[25] H. Ye, J. Huang, J.J. Xu, N.K.A.C. Kodiweera, J.R.P. Jayakody, S.G. Greenbaum,
New membranes based on ionic liquids for PEM fuel cells at elevated temperatures, J.
Power Sources 178 (2008) 651-660.
[26] H. Zhang, W. Feng, Z. Zhou, J. Nie, Composite electrolytes of lithium
salt/polymeric ionic liquid with bis(fluorosulfonyl)imide, Solid State Ionics 256 (2014)
61-67.
[27] V.D. Noto, E. Negro, J. Sanchez, C. Iojoiu, Structure-relaxation interplay of a
new nanostructured membrane based on tetraethylammonium
trifluoromethanesulfonate ionic liquid and neutralized Nafion 117 for
high-temperature fuel cells, J. Am. Chem. Soc. 132 (2010) 2183-2195.
[28] B. Lin, B. Qiu, L. Qiu, Z. Si, F. Chu, X. Chen, F. Yan,
Imidazolium-functionalized SiO2 nanoparticle doped proton conducting membranes
for anhydrous proton exchange membrane applications, Fuel Cells 13 (2013) 72-78.
[29] G. Liu, H. Zhang, J. Hu, Y. Zhai, D. Xu, Z. Shao, Studies of performance
degradation of a high temperature PEMFC based on H3PO4-doped PBI, J. Power
30
Sources 162 (2006) 547-552.
[30] H. Zhang, T. Zhang, J. Wang, F. Pei, Y. He, J. Liu, Enhanced proton conductivity
of sulfonated poly(ether ether ketone) membrane embedded by dopamine-modified
nanotubes for proton exchange membrane fuel cell, Fuel Cells 13 (2013) 1155-1165.
[31] M. Yamada, I. Honma, Anhydrous protonic conductivity of a self-assembled
acid-base composite material, J. Phys. Chem. B 108 (2004) 5522-5526.
[32] G.R. Goward, M.F.H. Schuster, D. Sebastiani, I. Schnell, H.W. Spiess,
High-resolution solid-state NMR studies of imidazole-based proton conductors:
structure motifs and chemical exchange from 1H NMR, J. Phys. Chem. B 106 (2002)
9322-9334.
[33] B.S. Hickman, M. Mascal, J.J. Titman, I.G. Wood, Protonic conduction in
imidazole: A solid-state 15
N NMR study, J. Am. Chem. Soc. 121 (1999) 11486-11490.
[34] P. Uchytil, J. Schauer, R. Petrychkovych, K. Setnickova, S.Y. Suen, Ionic liquid
membranes for carbon dioxide-methane separation, J. Membr. Sci. 383 (2011)
262-271.
[35] J. Wang, Z. Yue, J. Economy, Preparation of proton-conducting composite
membranes from sulfonated poly(ether ether ketone) and polyacrylonitrile, J. Membr.
Sci. 291 (2007) 210-219.
[36] P.R. Jothi, S. Dharmalingam, An efficient proton conducting electrolyte
membrane for high temperature fuel cell in aqueous-free medium, J. Membr. Sci. 450
(2014) 389-396.
31
[37] K. Schmidt-Rohr, Q. Chen, Parallel cylindrical water nanochannels in Nafion
fuel-cell membranes, Nat. Mater. 7 (2008) 75-83.
[38] Z. Jiang, X. Zhao, Y. Fu, A. Manthiram, Composite membranes based on
sulfonated poly(ether ether ketone) and SDBS-adsorbed graphene oxide for direct
methanol fuel cells, J. Mater. Chem. 22 (2012) 24862-24869.
[39] M.D. Bennett, D.J. Leo, G.L. Wilkes, F.L. Beyer, T.W. Pechar, A model of charge
transport and electromechanical transduction in ionic liquid-swollen Nafion
membranes, Polymer 47 (2006) 6782-6796.
[40] H. Mendil-Jakani, I.Z. Lopez, P.M. Legrand, V.H. Mareau, L. Gonon, A new
interpretation of SAXS peaks in sulfonated poly(ether ether ketone) (sPEEK)
membranes for fuel cells, Phys. Chem. Chem. Phys. 16 (2014) 11228-11235.
[41] A. Carbone, R. Pedicini, G. Portale, A. Longo, L. D’Ilario, E. Passalacqua,
Sulphonated poly(ether ether ketone) membranes for fuel cell application: Thermal
and structural characterization, J. Power Sources 163 (2006) 18-26.
[42] S.S. Sekhon, J. Park, E. Cho, Y. Yoon, C. Kim, W. Lee, Morphology studies of
high temperature proton conducting membranes containing hydrophilic/hydrophobic
ionic liquids, Macromolecules 42 (2009) 2054-2062.
[43] C.A. Kawaguti, K. Dahmouche, A.S. Gomes, Nanostructure and properties of
proton-conducting sulfonated poly(ether ether ketone) (SPEEK) and zirconia–SPEEK
hybrid membranes for direct alcohol fuel cells: effect of the nature of swelling solvent
and incorporation of heteropolyacid, Polym. Int. 61 (2012) 82–92.
[44] S.S. Sekhon, J. Park, J. Baek, S. Yim, T. Yang, C. Kim, Small-angle X-ray
32
scattering study of water free fuel cell membranes containing ionic liquids, Chem.
Mater. 22 (2010) 803-812.
[45] H. Zhang, C. Ma, J. Wang, X. Wang, H. Bai, J. Liu, Enhancement of proton
conductivity of polymer electrolyte membrane enabled by sulfonated nanotubes, Int. J.
Hydrogen Energ. 39 (2014) 974–986.
[46] A.N. Mondal, B.P. Tripathi, V.K. Shahi, Highly stable aprotic ionic-liquid doped
anhydrous proton-conducting polymer electrolyte membrane for high-temperature
applications, J. Mater. Chem. 21 (2011) 4117-4124.
[47] B.P. Tripathi, V.K. Shahi, Surface redox polymerized SPEEK–MO2–PANI (M=
Si, Zr and Ti) composite polyelectrolyte membranes impervious to methanol, Colloids
Surf. A 340 (2009) 10-19.
[48] Q. Che, B. Sun, R. He, Preparation and characterization of new anhydrous,
conducting membranes based on composites of ionic liquid trifluoroacetic
propylamine and polymers of sulfonated poly (ether ether) ketone or
polyvinylidenefluoride, Electrochim. Acta 53 (2008) 4428-4434.
Figure Captions
Scheme 1. Synthesis process of the composite membrane through IL-swollen method.
Scheme 2. Probably proton transfer mechanisms in composite membrane: (A) proton
hoping through acid-base pairs, (B) proton hoping through imidazole molecules,
and (C) proton hoping through ionic liquid.
Fig. 1. FTIR curves of SPEEK control membrane and composite membranes.
Fig. 2. SEM images of the cross-section of membranes: (a) SP-62, (b) SP-62-IL-16, (c)
33
SP-62-IL-25, (d) SP-62-IL-32, and (e) SP-62-IL-43.
Fig. 3. (a) SAXS and (b) WXRD curves of SPEEK control membrane and composite
membranes.
Fig. 4. (a) TGA curves and (b) tensile strength of SPEEK control membrane and
composite membranes.
Fig. 5. The effect of preparation conditions on IL loading amount in composite
membrane: (a) ultrasonic power, (b) treatment temperature, and (c) treatment time.
Fig. 6. (a) Temperature dependence and (b) Arrhenius plots of anhydrous conductivity
of SPEEK control membrane and composite membranes.
Fig. 7. (a) IL loss and (b) IL retention of the composite membranes (being immersed
in water) as a function of time. (c) Time dependence of anhydrous conductivity of
the composite membranes after being immersed in water (conductivity was
measured at 140 °C). (d) IL loss and IL retention of SP-62-IL-43 under 80 oC and
10% RH.
Fig. 8. SEM images of the cross-section of casting membranes: (a) SP-62-IL-16, (b)
SP-62-IL-25, (c) SP-62-IL-32, and (d) SP-62-IL-43.
Fig. 9. The tensile strength of SPEEK control membrane and casting membranes.
Fig. 10. (a) Temperature-dependent conductivity of the casting membranes under
anhydrous conditions; and (b) the comparison of temperature-dependent
conductivity of swollen membrane and casting membrane under anhydrous
conditions.
34
Fig. 11. (a) IL loss and (b) IL retention of the casting membranes (being immersed in
water) as a function of time. (c) The comparison of time-dependent conductivity
of the swollen membrane and casting membrane after being immersed in water
(testing at 140 °C). (d) IL loss and IL retention of the casting membrane
(SP-62-IL-43) under 80 oC and 10% RH. (e) The comparison of time-dependent
conductivity of the swollen membrane and casting membrane under 80 oC and
10% RH.
Graphical Abstract
Highlights
ImIL was incorporated into SPEEK to prepare composite membrane via
IL-swollen method.
ImIL loading amount was accurately controlled by the preparation conditions.
35
ImIL was enriched in ionic clusters to form facile and continuous transfer
channels.
ImIL gave significant enhancement in anhydrous conductivity to composite
membrane.
Comparison of preparation (IL-swollen or solution casting) method was
conducted.
1800 1600 1400 1200 1000 800
SP-62-IL-16
Wave number (cm-1)
Tra
ns
mit
tan
ce
(a
.u.)
1225 1083 1060
SP-62
SP-62-IL-25
SP-62-IL-32
SP-62-IL-43
15801027
Fig. 1. FTIR curves of SPEEK control membrane and composite membranes.
36
Fig. 2. SEM images of the cross-section of membranes: (a) SP-62, (b) SP-62-IL-16, (c)
SP-62-IL-25, (d) SP-62-IL-32, and (e) SP-62-IL-43.
0.2 0.4 0.6 0.8 1.0 1.2 1.4
Re
lati
ve
in
ten
sit
y (
a.u
.)
q (nm-1
)
SP-62
SP-62-IL-16
SP-62-IL-25
SP-62-IL-32
SP-62-IL-43
0.50 nm-1
0.52 nm-1
0.53 nm-1
0.56 nm-1
0.53 nm-1
10 20 30 40 50 60
2θ (degree)
R
ela
tiv
e i
nte
nsit
y (
a.u
.)
SP-62
SP-62-IL-16
SP-62-IL-25
SP-62-IL-32
SP-62-IL-43
(a) (b)
Fig. 3. (a) SAXS and (b) WXRD curves of SPEEK control membrane and composite
membranes.
37
100 200 300 400 500 600 700
20
40
60
80
100
We
igh
t (%
)
Temperature (oC)
SP-62
SP-62-IL-16
SP-62-IL-25
SP-62-IL-32
SP-62-IL-43
0
10
20
30
40
Nafion 115
SP-62
SP-62-IL-16
SP-62-IL-25SP-62-IL-32
Ten
sile
str
en
gth
(M
Pa)
Membranes
SP-62-IL-43
(a) (b)
Fig. 4. (a) TGA curves and (b) tensile strength of SPEEK control membrane and
composite membranes.
0 2 4 6 8 10 12 14 16
0
2
4
6
8
10
IL lo
ad
ing
(%
)
Time (h)
50W
100W
200WTemperature: 40
oC
20 30 40 50 60 70 80 90
0
20
40
60
80
100
SP-43
SP-54
SP-62
SP-75
IL l
oa
din
g (
%)
Temperature (oC)
Time: 6 h
(a) (b)
0 2 4 6 8 10 12
0
10
20
30
40
50
60
70
80
90
IL l
oa
din
g (
%)
Time (h)
SP-43
SP-54
SP-62
SP-75
Temperature: 65 oC
(c)
Fig. 5. The effect of preparation conditions on IL loading amount in composite
membrane: (a) ultrasonic power, (b) treatment temperature, and (c) treatment time.
38
40 60 80 100 120 140
0
2
4
6
8
10
pro
ton
co
nd
ucti
vit
y (
mS
cm
-1)
Temperature (°C)
SP-62
SP-62-IL-16
SP-62-IL-25
SP-62-IL-32
SP-62-IL-43
2.4 2.6 2.8 3.0 3.2
-12
-11
-10
-9
-8
-7
-6
-5
-4
SP-62 Ea= 26.9 KJ mol-1
SP-62-IL-16 Ea= 26.1 KJ mol-1
SP-62-IL-25 Ea= 25.2 KJ mol-1
SP-62-IL-32 Ea= 22.8 KJ mol-1
SP-62-IL-43 Ea= 21.6 KJ mol-1
Pro
ton
co
nd
uc
tiv
ity
ln
σ (
S c
m-1)
1000/T (K-1)
(a) (b)
Fig. 6. (a) Temperature dependence and (b) Arrhenius plots of anhydrous conductivity
of SPEEK control membrane and composite membranes.
0 20 40 60 80 100 120 140
0
10
20
30
40
50
60
70
80
IL l
os
s (
%)
Immersing time (min)
SP-62-IL-16
SP-62-IL-43
0 20 40 60 80 100 120 140
10
20
30
40
IL r
ete
nti
on
(%
)
Immersing time (min)
SP-62-IL-16
SP-62-IL-43
(a) (b)
0 20 40 60 80 100 120 140
0
2
4
6
8
10
Pro
ton
co
nd
uc
tiv
ity
(m
S c
m-1)
Immersing time (min)
SP-62
SP-62-IL-16
SP-62-IL-43
0 20 40 60 80 100 120 140 160
0
5
10
30
35
40
IL l
oss
an
d r
ete
nti
on
(%
)
Time (h)
IL loss
IL retention
(c) (d)
39
Fig. 7. (a) IL loss and (b) IL retention of the composite membranes (being immersed
in water) as a function of time. (c) Time dependence of anhydrous conductivity of the
composite membranes after being immersed in water (conductivity was measured at
140 °C). (d) IL loss and IL retention of SP-62-IL-43 under 80 oC and 10% RH.
Fig. 8. SEM images of the cross-section of casting membranes: (a) SP-62-IL-16, (b)
SP-62-IL-25, (c) SP-62-IL-32, and (d) SP-62-IL-43.
40
0
10
20
30
40
SP-62
Ten
sile
str
en
gth
(M
Pa
)
Membranes
SP-62-IL-43SP-62-IL-32
SP-62-IL-25
SP-62-IL-16
Fig. 9. The tensile strength of SPEEK control membrane and casting membranes.
40 60 80 100 120 140
0
2
4
6
8
Pro
ton
co
nd
ucti
vit
y (
mS
cm
-1)
Temperature (°C)
SP-62-IL-16
SP-62-IL-25
SP-62-IL-32
SP-62-IL-43
40 60 80 100 120 140
0
2
4
6
8
10
Pro
ton
co
nd
ucti
vit
y (
mS
cm
-1)
Temperature (oC)
SP-62-IL-16 (casting membrane)
SP-62-IL-16 (swollen membrane)
SP-62-IL-43 (casting membrane)
SP-62-IL-43 (swollen membrane)
(a) (b)
Fig. 10. (a) Temperature-dependent conductivity of the casting membranes under
anhydrous conditions; and (b) the comparison of temperature-dependent conductivity
of swollen membrane and casting membrane under anhydrous conditions.
41
0 20 40 60 80 100 120 140
0
10
20
30
40
50
60
70
80
IL l
os
s (
%)
Immersing time (min)
SP-62-IL-16
SP-62-IL-43
0 20 40 60 80 100 120 140
10
20
30
40
IL r
ete
nti
on
(%
)
Immersing time (min)
SP-62-IL-16
SP-62-IL-43
(a) (b)
0 20 40 60 80 100 120 140
2
4
6
8
Pro
ton
co
nd
ucti
vit
y (
mS
cm
-1)
Immersing time (min)
SP-62-IL-16 (casting membrane)
SP-62-IL-16 (swollen membrane)
SP-62-IL-43 (casting membrane)
SP-62-IL-43 (swollen membrane)
0 20 40 60 80 100 120 140 160
0
5
10
30
35
40
IL l
os
s a
nd
re
ten
tio
n
(%)
Time (h)
IL loss
IL retention
(c) (d)
0 20 40 60 80 100 120 140 160
3.5
4.0
4.5
5.0
Pro
ton
co
nd
ucti
vit
y (
mS
cm
-1)
Time (h)
SP-62-IL-43 (casting membrane)
SP-62-IL-43 (swollen membrane)
(e)
Fig. 11. (a) IL loss and (b) IL retention of the casting membranes (being immersed in
water) as a function of time. (c) The comparison of time-dependent conductivity of
the swollen membrane and casting membrane after being immersed in water (testing
at 140 °C). (d) IL loss and IL retention of the casting membrane (SP-62-IL-43) under
80 oC and 10% RH. (e) The comparison of time-dependent conductivity of the
swollen membrane and casting membrane under 80 oC and 10% RH.