New proton conducting membranes with high retention of protic ionic liquids

Article abstract of DOI:10.1039/c0jm02878a

New proton conducting membranes, exhibiting a high proton conductivity (10^-2 S cm^-1, without humidification at 160°C), based on the ionic liquid (IL) 1-butyl-3-methylimidazolium tetrafluoroborate (BMIm-BF₄) were investigated. The incorporation of a similar initial IL loading was found to reduce the proton conductivity loss of a sulfonated polydiacetylene based membrane, i.e. SPDA70-IL70 by 19.2% and IL-based Nafion by 64.6%; additionally, the SPDA70-IL70 was able to maintain a proton conductivity of more than 10^-3 S cm^-1 after centrifugation. The introduction of both a cross-linked structure and a high degree of sulfonation into the polymer matrix, resulted in significant improvements to the membranes' thermal properties, the dispersion of ionic domains, and the retention of IL - together with a reduction of proton conductivity loss. The membranes, which have a high degree of sulfonation with thermally cross-linkable diacetylene in the polymer's main chain, avoid the drawback of poor miscibility commonly associated with sulfonated polymers and cross-linking agents. When the composite membrane was used in a H₂/O₂ fuel cell without humidification at 100°C, a power density > 250 mA cm^-2 was achieved together with a maximum current denisty of 122 mW cm^-2.

Full text of DOI:10.1039/c0jm02878a

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PAPER
New proton conducting membranes with high retention of protic ionic liquids†
Yun Sheng Ye,a Ming Yao Cheng,b Ji Yong Tseng,a Gao-Wei Liang,a John Rick,a Yao Jheng Huang,c
c
Feng Chih Chang and Bing Joe Hwang
a
*
Received 31st August 2010, Accepted 11th November 2010
DOI: 10.1039/c0jm02878a
New proton conducting membranes, exhibiting a high proton conductivity (10ꢀ2 S cmꢀ1, without
humidification at 160 ꢁC), based on the ionic liquid (IL) 1-butyl-3-methylimidazolium
tetrafluoroborate (BMIm-BF4) were investigated. The incorporation of a similar initial IL loading was
found to reduce the proton conductivity loss of a sulfonated polydiacetylene based membrane, i.e.
SPDA70-IL70 by 19.2% and IL-based Nafion by 64.6%; additionally, the SPDA70-IL70 was able to
maintain a proton conductivity of more than 10ꢀ3 S cmꢀ1 after centrifugation. The introduction of both
a cross-linked structure and a high degree of sulfonation into the polymer matrix, resulted in significant
improvements to the membranes’ thermal properties, the dispersion of ionic domains, and the retention
of IL—together with a reduction of proton conductivity loss. The membranes, which have a high
degree of sulfonation with thermally cross-linkable diacetylene in the polymer’s main chain, avoid the
drawback of poor miscibility commonly associated with sulfonated polymers and cross-linking agents.
When the composite membrane was used in a H2/O2 fuel cell without humidification at 100 ꢁC, a power
density > 250 mA cmꢀ2 was achieved together with a maximum current denisty of 122 mW cmꢀ2
.
less poisoning at the anode, and easy heat and water manage-
ments of the stacks.9–13 For water-swelled polymeric systems,
however, the conductivity is strongly humidity dependent, so
they cannot be used above 100 ꢁC due to the loss/evaporation of
water, which results in a significant loss of conductivity.14 In
contrast to which ILs are suitable for high temperature appli-
cations, because they can act as proton transferring carriers in
anhydrous conditions, while exhibiting: excellent chemical and
thermal stabilities, non-flammability and non-volatility. There-
fore, many studies have been carried out to develop IL-based
membranes with high proton conductivities that can operate at
Introduction
Ionic liquids (ILs) are composed of organic cations (e.g., imi-
dazolium, pyridinium, or tetraalkylammonium) and anions
(halide, tetrafluoroborate, hexafluorophosphate, triflate, ami-
dotriflate, etc); they have highly variable aqueous solubilities
(depending upon the anion), ranging from highly soluble to
immiscible. ILs are attracting significant attention, in many fields
of chemistry and industry, because of their unique physico-
chemical properties, such as their high ionic conductivity, high
polarity, high density, high heat capacity, and high thermal and
chemical stability.1–5 These characteristic features of ILs offer
suitable properties for polymer electrolyte membranes (PEMs)
and have been extensively studied to replace liquid electrolytes,
which are toxic, flammable and able to leach out.6–8
higher temperatures (100–200 C) in anhydrous conditions.15–29
ꢁ
The development of IL-based PEMs mainly focuses on mixtures
of well-known polymers with IL using various strategies, such
as: the in situ polymerization of vinyl monomers in ILs,18–20
doping polymers with ILs,21–23 and solution casting IL-polymer
solutions.24–29
For polymer electrolyte membrane fuel cells (PEMFCs), the
PEM is one of the key issues because it acts as an electrolyte to
transport protons from the anode to the cathode. One of the
main challenges in the development of PEMFCs is to develop
membrane materials with high operating temperatures (above
Although the IL-based membranes allow PEMFCs to operate
at higher temperatures in anhydrous conditions while avoiding
water dependent conductivity, they have a major drawback, i.e.
the progressive release of ILs leading to a long-term decline in the
performance of the fuel cells.30,31 To solve the leaching problem,
approaches incorporating inorganic fillers32,33 into the polymer
membranes and the formation of cross-linkable structures
appear to be efficient approaches towards retarding the degree of
methanol diffusion and water uptake, while enhancing the poly-
mer’s mechanical properties and dimensional stability.34–38 More
recently, Yan and Lu et al.20 reported protic IL-based hybrid
proton conducting membranes, wherein the addition of silica
ꢁ
100 C), to achieve higher reaction kinetics at both electrodes,
aDepartment of Chemical Engineering, National Taiwan University of
Science and Technology, Taipei, Taiwan. E-mail: bjh@mail.ntust.edu.tw
bGraduate Institute of Engineering, National Taiwan University of Science
and Technology, Taipei, Taiwan
cInstitute of Applied Chemistry, National Chiao-Tung University, Hsin-
Chu, Taiwan
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c0jm02878a
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fillers into the composite membranes was shown to both enhance
the conductivity and prevent the release of the IL component
from the composite membranes. To the best of our knowledge,
the incorporation of cross-linking structures in the PEMs will
enhance the retention of IL in the membrane; however, the
presence of the cross-linking structure appears to lower proton
conductivity. Thus, the development of more efficient
membranes with both improved proton conductivities and
enhanced retention of IL in the membrane remains an important
challenge.
was attached a water condenser and an inlet and outlet for
oxygen. The reaction mixture was warmed to 50 ꢁC while oxygen
was bubbled through. After 1 h, different ratios of the monomers
PBPB and PBPBS (15 wt% solid content) in 10 mL of DMSO
were added to the catalyst mixture to generate different polymers
(Table 1). After 24 h, 8 mL of DMSO was added to the viscous
solution, and the polymer was precipitated in 400 mL of meth-
anol containing 2% HCl. The gray fibrous polymer was filtered
and immersed in 50 mL of methanol containing 5% HCl with
stirring over night. The solid was filtered and washed with
ꢁ
In the present study, we describe the synthesis and character-
ization of IL-based cross-linked membranes. The thermal
cross-linkable diacetylene-containing polymer, sulfonated poly-
diacetylene (SPDA), was synthesized and used as the polymer
matrix because the diacetylene groups have the ability to cross-
link under irradiation or with heat treatment without the
evolution of volatile byproducts.39,40 In addition, such main
chain type cross-linked polymers avoid the drawback of poor
miscibility often associated with common sulfonated polymer
and cross-linking agents. Polymers with various degrees of
sulfonation (DS) of SPDA were synthesized via oxidative
coupling polymerization and were blended with 1-butyl-3-
methylimidazolium tetrafluoroborate (BMIm-BF4) to form IL-
based membranes (SPDA-IL) after a thermal cross-linking
process. The influences of the DS and the network structure on
the properties of the resulting composite membrane were
systematically investigated with respect to: the IL uptake, the
leaching of IL by centrifugation, morphology, ionic conductivity
and thermal stability. In addition, the influence of IL retention
properties on proton conductivity, based on SPDA and recast
Nafion containing IL, has also been compared in this study.
methanol–water several times and dried under vacuum at 60 C
(yield 73%) (Scheme 1).
1H NMR (d6-DMSO): d 1.58 (g), 4.91 (a), 6.86 (e), 6.95 (c),
7.00 (d), 7.13 (f), 7.32 ppm (b) (Fig. 1).; 13C NMR: d 31.33 (19),
41.99 (18), 56.37–58.35 (3, 10, 13), 70.76 (1, 11), 77.04 (2, 12),
114.74 (15), 116.23–117.63 (5, 6, 8), 128.30 (16), 138.38 (7),
144.08 (17), 148.96 (9), 151.61 (4), 155.33 ppm (14) (Fig. 2).
Membrane preparation
SPDA and the BMIm-BF4 (cf. Table 1) were dissolved in DMSO
at room temperature, to give a 15 wt% solution which was stirred
overnight. The resulting solution was cast onto a glass plate and
heated at 60 ꢁC for 24 h to slowly evaporate the solvent and then
heated at 180 ꢁC for 4 h. The resulting membranes were blotted
with kimwipes to remove surplus IL. The Nafion-IL membrane
was prepared by a solution casting method described elsewhere.26
Monomer and polymer characterization
1H NMR and 13C NMR spectra were recorded at 25 ꢁC using an
INOVA 500 MHz NMR spectrometer. Solid state 13C NMR
spectra were recorded on a Buuker-400 MHz instrument using
tetramethylsilane as an internal reference. FTIR spectra were
obtained with a Nicolet Avatar 320 FTIR spectrometer; 32 scans
were collected at a spectral resolution of 1 cmꢀ1. The ion
exchange capacities (IECs) were determined by titration as in our
previous study.38,39 Wide-angle X-ray diffraction (WAXD)
spectra were recorded for powdered samples using a Rigaku D/
max-2500 type X-ray diffraction instrument. SAXS experiments
were performed using the BL23A SWAXS instrument at the
National Synchrotron Radiation Research Center (NSRRC),
Experimental section
Materials
Nafion (5%) dissolved in a solvent comprising water and meth-
anol, Bisphenol A ($99%), 2,5-dihydroxybenzenesulfonate,
tetra-n-butylammonium bromide ($98.0%), sodium tetra-
fluoroborate ($98.0%), N,N,N0,N0-tetramethylethylenediamine
(TMEDA) ($99%), copper-iodine (CuI) ($99.5%), pyridine
($99%), hydrochloride acid (36.5–38.0%), dimethylformamide
(DMF) ($99%), acetonitrile (HPLC grade), acetone ($99.9%),
ethanol (EtOH) ($99.5%) and dimethyl sulfoxide (DMSO)
($99.9%) were purchased from Sigma-Aldrich and used as
received. 1-Bromobutane ($99%), 1-methylimidazole ($99%),
propargyl bromide (80 wt. % in toluene) and potassium
carbonate ($99.0%) were purchased from Acros and used as
ꢀ
Taiwan, using a 10 keV (wavelength l ¼ 1.24 A) beam with a 0.5
nm diameter. The scattering wavevector transfer q ¼ 4plꢀ1sinq is
defined by l and the scattering angle 2q of X-rays. Samples for
SAXS (thickness ¼ 150ꢂ200 mm) were sealed between two thin
Kapton windows (80 mm thickness each) and measured at an
ambient temperature ꢂ26 ꢁC. The proton conductivity of the
membrane was determined with an ac electrochemical impedance
analyzer (PGSTAT 30), and the experiments involved scanning
the ac frequency from 100 kHz to 10 Hz at a voltage amplitude of
10 mV. A DuPont Q100 thermo-gravimetric analyzer (TGA) was
utilized to investigate the thermal stability of the membranes; the
samples (ꢂ10 mg) were heated from ambient temperature to 850
received.
4,40-(Propane-2,2-diyl)bis[(prop-2-ynyloxy)benzene]
(PBPB), potassium 2,5-bis(prop-2-ynyloxy)benzenesulfonate
(PBPBS) and the ionic liquid [1-butyl-3-methylimidazolium tet-
rafluoroborate (BMIm-BF4)] were synthesized according to
a modified procedure described elsewhere.41,42 (See the ESI†).
Sulfonated polydiacetylene (SPDA)
ꢁC under a nitrogen atmosphere at a heating rate of 20 ꢁC minꢀ1
.
To a dried three-neck, round-bottomed flask were added: CuI
(11.5 mg, 0.11 mmol), N,N,N0,N0-tetramethylethylenediamine
(TMEDA) (14 mg, 0.12 mmol), pyridine (0.185 g, 2.35 mmol),
and 5 mL of anhydrous dimethyl sulfoxide (DMSO). To the flask
The stability of the membranes was tested by measuring their
ꢁ
proton conductivities at 160 C, after thermal treatment at the
same temperature for a designated time. The centrifugal test was
carried out using a centrifugal process as detailed in our previous
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Table 1 Characterization of sulfonated polydiacetylene membrane and composite membrane
Theoretical content of PBPBS
(mol%)a
Measured content of PBPBS
(mol%)b
Theoretical IEC
(meq gꢀ1
Measured IEC
Reaction temp.
(ꢁC)d
b
c
Polymer
)
(meq gꢀ1
)
SPDA50
SPDA60
SPDA70
PDA
50
60
70
0
49.5
58.8
68.0
0
1.68
2.04
2.41
0
1.57
1.99
2.32
0
200.9
187.9
175.9
209.4
a
Determined from the initial feed ratio of PBPBS. b Calculated from 1H NMR spectra. c IEC measured with titration. d Reaction exotherm temperature
obtained from DSC at a heating rate of 20 ꢁC.
Scheme 1 Schematic representation of the preparation of the cross-
linked SPDA-IL membrane.
study.43 The IL-based membrane was poured into centrifugal
filters (MICORSEP 30 K OMEGA, PALL). The centrifugal
process was performed at 20000 rcf (relative centrifugal force)
and 25 ꢁC for several cycles until no weight loss was observed in
the membrane. The proton conductivity of these membranes was
measured at 160 ꢁC. The electrode system, electrochemical
Fig. 1 (a) 1H-NMR (d6-DMSO, 25 ꢁC) spectrum of PBPB, PBPBS and
SPDA50. (b) 1H-NMR (d6-DMSO, 25 ꢁC) spectrum of SPDA50-70.
procedure, fabrication of membrane electrode assembly (MEA),
and the single-cell test procedure are described in the ESI†.
Results and discussion
FTIR spectrum is in agreement with the structure of the mono-
mer PBPBS as indicated by the presence of bands at 3286 and
1181 cmꢀ1, corresponding to propargyl and sulfonic acid struc-
tures, respectively.
Monomer and polymer synthesis
Monomers PBPB and PBPBS were prepared with high yields
from propargyl bromide. The 1H-NMR (d6-DMSO, 25 ꢁC)
spectra of the monomers are shown in Fig. 1a, wherein the
presence of propargyl protons, at 2.54 and 3.53 ppm respectively,
for the monomers PBPB and PBPBS is apparent. The corre-
sponding 13C NMR (d6-DMSO, 25 ꢁC) shows resonances at
78.28 and 79.76 ppm, indicating the propargyl structure. The
The synthesis of the three sulfonated polydiacetylenes (SPDA)
SPDA50, 60 and 70, was carried out using the oxidative coupling
method developed by Hay (Scheme 1).44 SPDA50-70 were
obtained using three different PBPB to PBPBS ratios. The degree
of sulfonation (DS) in the final copolymers was determined by 1H
NMR (d6-DMSO, 25 ꢁC) from the integration of aromatic
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Fig. 2 13C-NMR spectrum of non-crosslinked and crosslinked SPDA50.
groups of the monomers PBPB [marked as ‘f’ and ‘e’ in the
molecular formula in Fig. 1b] and PBPBS, [marked as ‘b’, ‘c’, and
‘d’ in the molecular formula in Fig. 1b]. Table 1 shows that the
PBPB to PBPBS ratios in the final polymers were found to be
very close to those in the feed ratios, allowing them to be easily
adjusted and controlled. Moreover, a similar result was also
observed for the measured ion exchange capacity (IEC) values
for SPDA membranes, which were close to the theoretical values.
The 1H NMR and 13C NMR spectra of the polymers were
consistent with their assigned structures. No residual ethynyl
protons (–C^CH), due to unreacted acetylene groups (–C^C–),
Fig. 3 (a) DSC curves of non-crosslinked membranes (PDA and
SPDA50–70). (b) DSC curves of uncross-linked SPDA50 after thermal
treatments (at 180 ꢁC) with designated time (30–240 min).
1
were observed for the polymers by H NMR (Fig. 1a and 1b).
The 13C NMR spectra also showed that the terminal ethynyl
carbons were transformed into diacetylenic carbons after poly-
merization. For example, the two diacetylenic carbons at 70.59
and 77.04 ppm in the spectrum of SPDA50 (Fig. 2) replace the
two ethynyl carbons at 78.28 and 79.76 ppm in the spectra of the
monomers PBPB and PBPBS respectively. These results indicate
that the different DS of SPDA were successfully synthesized by
oxidative coupling.
gradually and disappeared completely after 240 min. These
results indicate the complete cross-polymerization of the diac-
ꢁ
etylene in the SPDA membrane after 240 min at 180 C.
The cross-linking of the membrane after the incorporation of
IL into the polymer matrix was examined by FTIR. Fig. 4 shows
the FTIR spectra of the non-crosslinked SPDA50, crosslinked
SPDA50, SPDA50-IL composite membrane, and pure BMIm-
BF4. SPDA50 was selected as a typical example to analyze the
cross-linking behavior of IL-based membranes. The spectrum of
pure BMIm-BF4 shows various characteristic peaks associated
with the IL structure. The first two bands at 3166 and 2943 cmꢀ1
Cross-linking behavior of polymer with ionic liquid
The cross-linking of SPDA50–70 and PDA (polydiacetylene) was
examined by DSC as shown in Fig. 3a. Pure PDA exhibits
a single sharp peak at 209.9 ꢁC, which is attributed to the cross-
polymerization of diacetylene. It has been reported that the
cross-polymerization of diacetylene often takes place in this
temperature range for aliphatic diacetylenes.45–47 Fig. 3a shows
that the SPDA membranes (SPDA50–70) display an exotherm
that began arounꢁd 130.8 ꢁC, peaked above 175.9 ꢁC, and
extended to 318.4 C. The nature of such a dramatic difference
may be due to structural rearrangements facilitated by the higher
DS of aromatic diacetylenes during cross-linked polymerization.
In addition, for SPDA membranes, the exotherm peak temper-
ature decreased with an increase in the DS of the membrane.
These observations simply reflected the increase in the local
concentration of sulfonated segments with reactive diacetylene
groups facilitating the ease of thermal activation.48,49 The
SPDA50 was taken as an example to study the curing behavior
by DSC after thermal treatment (at 180 ꢁC) with designed times
(30–240 min). As shown in Fig. 3b, the exotherm decreased
Fig.
4 FTIR spectrum of non-crosslinked SPDA50, crosslinked
SPDA50, SPDA50-IL composite membrane, and pure Bmin-BF4.
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represent the –CH groups of the imidazole ring of the Bmin-BF4,
where the signal for the C]C double bonds of the imidazole ring
is around 1572 cmꢀ1. The bands at 1121 and 754 cmꢀ1 correspond
to strength vibrations of the B–F bonds in BF4ꢀ, relating to the
structure of the anion.50,51 The spectrum of SPDA50 before
curing was characterized by the symmetric stretching vibration of
the C–O–C groups at 1223 cmꢀ1, the _C–C stretching vibration
band at 912–970 cmꢀ1, and the symmetric stretching vibration of
the sulfonic acid groups around 1186 cmꢀ1. After cross-linking,
the intensity of the _C–C stretching vibration band at 912–970
cmꢀ1 decreased, while a new peak appeared at 1733 cmꢀ1 (due to
carbonyl groups). These results suggest that the –CH2O– linkage
is involved in the thermal-curing process to a certain extent and is
responsible for the formation of the new carbonyl in the cross-
linking process. These results are similar to the previous results
for thermal cross-linked polydiacetylene,46 indicating that
a Claisen rearrangement reaction has taken place during the
thermal cross-linking process (Scheme 1).52,53 The 13C NMR
spectrum of SPDA50 (Fig. 2) also confirmed extensive reactions
involving the diacetylene and methyleneoxy groups, as the peaks
at 70.59 and 77.04 ppm (alkyne carbons) and at 56.37–58.27 ppm
(–CH2O–) almost completely disappeared. Similar results also
showed in other SPDA-IL membranes which indicate that the
diacetylene groups were completely reacted, incorporating IL
into the polymer matrix. Fig. 5 shows a photograph of the
resulting SPDA-IL membrane (SPDA70-IL70 with the thickness
of 160 mm). These membranes are flexible and can be cut into any
desired sizes and shapes, whereas the pure form polymeric matrix
(SPDA) (without ILs) is stiff and brittle.
Ionic liquid uptake and centrifugal test
The ionic liquid used in the present study was 1-butyl-3-meth-
ylimidazolium tetrafluoroborate (BMIm-BF4). This IL has been
reported to a have dynamic viscosity of 2.33 P at 30 ꢁC and
a conductivity of 0.86 mS cmꢀ1 at 60 ꢁC.54 The loss after
centrifugation and thermal decomposition temperatures of
SPDA membranes with different ratios of IL loading are shown
in Table 2. The amount of absorbed IL into the membranes
impregnated after the cross-linking procedure is less than the
theoretical content of IL. In comparison with the Nafion-IL
membrane, the difference between the theoretical and the
measured content of IL in SPDA is similar. The lower IL uptake
for the measured value can be mainly attributed to the presence
of cross-linking in the polymer chains that restricts IL swelling in
Fig. 5 A photograph of the SPDA-IL membrane (SPDA70-IL70).
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the membrane. Generally, the absorption of IL in the membrane
is dependent on the availability and accessibility of ionic groups.
As the membrane content of sulfonic acid groups increases, more
become accessible and available for ion exchange with the cation
of the IL. Therefore, the content of IL in the SPDA membrane is
expected to increase with increasing DS. A decrease in the
membrane weight was observed after the leaching test. Choud-
hury et al.22 demonstrated that the IL molecules in the membrane
can be classified into two parts: (a) where the IL is physically
residing within the membrane and is not bound through inter-
action, and (b) where the IL interacts with the membrane
through electrostatic interactions. In this study, the centrifuge
was used to probe the IL holding levels by physical separation of
the solid (polymer) and the liquid phase (IL). The centrifugal test
was carried out to determine the amount of IL that may be lost
from the membrane. This test does not reflect real operating
conditions for the membranes, but it helps to evaluate the long-
term stability of these composite membranes. The percentage
losses (%) and residual weights (wt %) of IL are shown in Table 2.
All samples were subjected to a rotation speed of 20 000 rpm in
the centrifugal test. Fig. 6 shows the amount of IL lost from the
sample SPDA70-IL70 as a function of rpm. It was evident that
a centrifuge rotation speed of 20 000 rpm could easily differen-
tiate between strongly interacting IL and weakly interacting IL.
The amount of IL lost from the SPDA membranes (all with DS ¼
50), was compared to the amount of IL initially loaded. The
results indicated that a fraction of the IL was physically residing
within the membrane and not bound through interactions, and
was hence lost during centrifugation. The retaining capability
within the polymer for the IL was improved by increasing the DS
of SPDA membrane, indicating that to a significant degree ILs
interact with the membrane through electrostatic interactions
(Scheme 2). To clarify the effects of cross-linking on the retention
of the IL, SPDA membranes (SPDA50-IL70) with various
degrees of cross-linking (49.5–100%) were investigated (ESI†). It
is not surprising that the IL loss after the centrifugation test
decreased as the degree of membrane cross-linking increased, due
to the presence of more cross-linked structure able to prevent the
release of IL. The retention of IL, from SPDA50-IL70, was
compared to that of SPDA70-IL70 and Nafion-IL50, since
Scheme
2
Graphical representation of
a
crosslinked SPDA-IL
membrane.
similar IL loadings are found in these membranes. Higher
amounts of IL were lost during centrifugation from the Nafion-
IL50 membrane (22.0%) than the SPDA70-IL70 membrane
(10.5%), indicating a higher number of sulfonic acid groups
(IEC ¼ 2.32 meq gꢀ1) to be present in the SPDA70-IL70
membrane. In addition, the network structure throughout the
membrane allows for the entrapment of more IL in the
membrane and prevents the release of IL component from
the composite membranes.
Morphological changes in the presence of ionic liquid
In order to understand and explain the ionic conductivity results
obtained from the cross-linked SPDA containing hydrophilic IL
(BMIm-BF4), the morphology of the membranes was studied by
X-ray diffraction (XRD) and small-angle X-ray scattering
(SAXS). In general, the cluster network model 55,56 and the core
shell model57,58 are the two main models most widely used to
explain the morphologies and ionic conductivities of water
swollen Nafion membranes. These models feature a spherical
ionic cluster phase within a continuous matrix phase. On the
basis of quantitative analysis of published SAXS data, it has been
shown that the angular position of the ionomer peak in the
SAXS profile represents the average spacing between centers of
ionic clusters. Recently, the morphology of proton conductivity
membranes containing ILs has been investigated by small-angle
X-ray scattering (SAXS).17,26,59 Swelling of the membrane ionic
domains was expected to cause changes in the ionomer’s
morphology, especially in the ionomer peak of the SAXS profile.
SAXS scattering profiles, plotted as an intensity vs. scattering
vector q on a log–log plot, at various IL swelling levels and DS of
the cross-linked SPDA are shown in Fig. 7a. The ‘‘ionomer peak’’
is presented in SPDA50-IL50 membrane at q ¼ 0.077 Aꢀ1 (d ¼
ꢀ1
ꢀ
ꢀ
81.6 A) and q ¼ 0.107 A (d ¼ 58.7 A). The presence of a weak
ionomer peak at q ¼ 0.107 Aꢀ1 (d ¼ 58.7 A) may result from the
ꢀ
restriction of the ionic domains by the cross-linked structure,
leading to the small size of ionic domains containing IL. The
membrane used in the present study was a cross-linked polymer
Fig. 6 Amount of IL lost from sample SPDA70-IL70 as a function of
rpm.
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compressed in the membrane due to the distribution of IL along
the polymer chains, or that the polymer chains were restricted by
IL domains to produce the different formation of cross-linked
structure.
Ionic conductivity and single-cell test of composite membranes
The ionic conductivities (s) of SPDA-IL and Nafion-IL
membranes in anhydrous conditions were plotted as a function
of temperature, see Fig. 8a. The corresponding conductivity
increased exponentially with temperature from the order of 10ꢀ4
S cmꢀ1 up to 10ꢀ2 S cmꢀ1 at 160 ꢁC. In general, the proton
conductivity is strongly dependent on its nanostructure and
water content. The decrease in the water content of the
membrane results in low conductivity, due to not all of the acid
sites being dissociated and a low level of interaction among water
molecules, via hydrogen bonding. The conductivity of the IL-
based membranes shows a similar trend to the water-based
membrane. It may be due to the presence of IL resulting in an
enhancement in the dissociation of protons from the sulfonated
polymer causing an increase in proton transport mobility. The
SPDA with a higher DS shows better ionic conductivity in the
same range of temperatures (80–160 ꢁC), which may be due to the
larger number of sulfonic acid groups. In addition, the SPDA
Fig. 7 (a) SAXS spectra of SPDA50-IL50, SPDA50-IL60, SPDA50-
IL70, SPDA60-IL70 and SPDA70-IL70. (b) XRD spectra of SPDA50,
SPDA50-IL10, SPDA50-IL50, SPDA50-IL70 and SPDA70-IL70.
and such cross-linked structures generally form small ionic
domains.34–38 The ionomer peak systematically shifted to a lower
q as the IL content increased, i.e., 0.077 Aꢀ1 and 0.107 Aꢀ1 at 50
wt % to 0.072 Aꢀ1 and 0.103 Aꢀ1 at 70 wt % BMIm-BF4,
respectively. This increase in inter-cluster spacing may corre-
spond to an increase in cluster size through plasticization with
IL. An increase in d spacing is generally observed for many
ionomers in the literature.17,26,59 In case of the membrane con-
taining the same IL loading, the ionomer peak slightly decreases
with an increase in the DS of the SPDA matrix (Table 2), which is
related to the presence of the higher content of sulfonic acid
groups in the membrane. Moreover, the ionomer peak at q ¼
0.103 Aꢀ1 (d ¼ 61.0 A) disappeared with higher DS (DS ¼ 70).
ꢀ
Although the clustering of IL occurs due to aggregation in the
hydrophobic cross-linked structure, as more sulfonic acid groups
in the membrane can favor the accessibility of hydrophilic IL to
ion exchange with sulfonic acid groups of the membrane.
Therefore, a higher degree of DS in the polymer matrix allows
more IL to ion exchange with sulfonic acid groups and results in
homogeneous distribution of IL in the membrane matrix.
The morphological change in the presence of IL was also
checked by XRD, as shown in Fig. 7b. For a pure SPDA
membraneꢁ, the boarder amorphous peak appeared at a value of
ꢀ
2q ¼ 30.06 (d ¼ 2.97 A). Upon incorporating 10 wt% of BMIm-
BF4, a new boarder shoulder appeared at 2q ¼ 21.04ꢁ (d ¼ 4.22
Fig. 8 Ionic conductivities as a function of temperature of SPDA-IL and
Nafion-IL membranes in anhydrous conditions. (b) Ionic conductivity of
the IL-uptake membranes before and after centrifugation.
ꢀ
A), an increased trend was observed with an increase in the IL
loading in the membrane, suggesting that the polymer matrix was
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with higher DS may provide better ion-conducting paths for
proton conduction in the composite membrane. These results are
consistent with previous results for membranes with different
DS.60–62
Fig. 8b shows the ionic conductivities of the IL-uptake
membranes before and after the centrifugal process. The Nafion-
IL50 membrane shows a higher conductivity (1.7 mS cmꢀ1
)
compared to the SPDA-IL membranes at 160 ꢁC, which is related
to a longer flexible spacer (free volume) that facilitates the
transportation of protons.63,64 After centrifugation, the conduc-
tivities of all membranes decreased by more than the as-prepared
ones (made without any centrifugation test), indicating a strong
dependence on IL uptake in the membrane. In the case of
a water-swelled electrolyte membrane, there are two main
mechanisms for proton transport, namely proton hopping
(Grotthuss mechanism) and diffusion (vehicular mechanism).64
The two mechanisms may be adopted in this work as well. Before
centrifugation, protons transport may occur by both hopping
and diffusion in the membrane. However, after centrifugation,
proton transport via diffusion is limited, since the amount of IL
in the bulk phase that can facilitate proton diffusion is reduced. It
is similar to proton transport under low humidity conditions for
water-based electrolyte membranes. It is simply the case that
after centrifugation, the SPDA membrane with a higher DS
exhibits better proton transport, implying the presence of more
sulfonic acid groups as well as a higher amount of IL in the
membrane. In addition, the trend of variation in the proton
conductivity is consistent with our supposition, i.e. that the
conductivity reduction is less for membranes with higher DS.
For Nafion-IL composite membranes, a 64.7% reduction in its
conductivity was observed after centrifugation (from 1.70 mS
cmꢀ1 to 0.63 mS cmꢀ1). In contrast, the conductivity of the
SPDA70-IL70 membrane decreased from 1.04 mS cmꢀ1 to 0.84
mS cmꢀ1, i.e. only a 19.2% in reduction. Although the Nafion-
Fig. 9 TGA curves of pure IL (BMIm-BF4), SPDA50, Nafion-IL50 and
SPDA50-IL membranes.
for weight losses of 5% and 10% are listed in Table 2. The TGA
curve of pure IL exhibits excellent thermal stability up to 400.5
ꢁC, followed immediately by one-step decomposition. For cross-
linked SPDAꢁ50 membranes, it shows a continuous weight loss
ꢁ
between 225 C and 520 C, which is attributed to the decom-
position of sulfonic acid groups and aliphatic segments of
propargyl. However, the onset temperatures for 5 and 10 wt%
weight losses are above 310 ꢁC and 340 respectively, for all
SPDA-IL membranes. It is evident that the decomposition by
desulfonation of sulfonic acid groups has been suppressed and
shifted to higher temperatures for all of the SPDA-IL
membranes. It has been reported that the thermal stability of
sulfonated polymers in the sodium form is much higher in
comparison with that in acid form.55,56 This observation confirms
that the decomposition of sulfonic acid groups and aliphatic
segments of propargyl is inhibited by the presence of IL since the
protons originating from the sulfonic acid groups are partially
replaced by the bulky cation of the IL, resulting in the increase of
the thermal stability of the membrane.
based composite membrane possesses
a relatively higher
conductivity than the sulfonated polymer, its proton conduc-
tivity significantly decreases after centrifugation, indicating the
benefits of both a higher DS and the presence of a network
structure in the membrane.
Membranes employed in PEFCs operating in the range
100ꢂ200 ꢁC, require a high degree of stability for long-term
ꢁ
The H2/O2 fuel cell was operated at 100 C using a SPDA50-
IL70 and SPDA70-IL70 composite membrane without humidi-
fication (ESI†). The maximum power density was 98 mW cmꢀ2
for the SPDA50-IL70 membrane and 122 for the SPDA70-IL70,
which is approximately a 24% increase. This may be due to the
higher DS of the SPDA70-IL70 increasing both the membrane
IL retention and the proton conductivity. However, the perfor-
mance is still much lower than that of conventional PEMFCs
with hydrated ion exchange membranes. It is worth noting that
the SPDA membrane was operated without humidification of the
supplied gases (H2 and O2). Hence, this approach allows this type
of IL-based cross-linked membrane to have potential applica-
tions in high-temperature polymer electrolyte membrane fuel
cells.
Thermal stability of composite membranes
The thermal stabilities of pure IL (BMIm-BF4), SPDA50,
Nafion-IL50 and SPDA50-IL membranes were investigated by
TGA as shown in Fig. 9. The corresponding onset temperatures
Fig. 10 Time-dependant proton conductivities of the SPDA-IL
membranes under high temperature treatment (160 ꢁC).
2730 | J. Mater. Chem., 2011, 21, 2723–2732
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A new type of sulfonated polymer, containing diacetylene groups
in the main chain, was successfully synthesized via an oxidative
coupling approach. FTIR, NMR and DSC were used to confirm
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linked membrane potential applications in high-temperature
polymer electrolyte membrane fuel cells.
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Acknowledgements
We thank financial support from the National Science Council
(under contract number NSC-99-2120-M-011-001), facilities
from the National Taiwan University of Science and Tech-
nology, and the National Synchrotron Radiation Research
Center (NSRRC) is gratefully acknowledged.
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