A soluble and conductive polyfluorene ionomer with pendant imidazolium groups for alkaline fuel cell applications

Article abstract of DOI:10.1021/ma202159d

Solvent processable polyfluorene ionomers with pendant imidazolium groups were synthesized and characterized. The synthesized polymeric membranes are transparent, flexible, and mechanically strong and exhibit hydroxide ion conductivity above 10^-2

Full text of DOI:10.1021/ma202159d

Article
pubs.acs.org/Macromolecules
A Soluble and Conductive Polyfluorene Ionomer with Pendant
Imidazolium Groups for Alkaline Fuel Cell Applications
Bencai Lin, Lihua Qiu, Bo Qiu, Yu Peng, and Feng Yan*
Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Polymer Science and Engineering,
College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, P. R. China
S
* Supporting Information
ABSTRACT: Solvent processable polyfluorene ionomers with
pendant imidazolium groups were synthesized and charac-
terized. The synthesized polymeric membranes are trans-
parent, flexible, and mechanically strong and exhibit hydroxide
ion conductivity above 10−2 S/cm at room temperature. The
membranes are soluble in polar aprotic solvents such as
DMSO and DMF, while insoluble in water and aqueous methanol. The solubility properties of polyfluorene ionomers enable the
use for not only alkaline anion-exchange membrane but also ionomer electrode material. 1H NMR and hydroxide ion
conductivity measurements demonstrated an excellent chemical stability of the synthesized polyfluorene ionomers in high-pH
solution at elevated temperatures. The results of the study suggest a feasible approach for the synthesis and practical applications
of alkaline anion-exchange membranes (AEMs).
acidic conditions, and it is desirable to develop new type of
polymeric anion-exchange membranes (AEMs).
INTRODUCTION
■
Fuel cells have been recognized as one of the most promising
power generation technologies that could provide clean and
efficient energy for stationary, transportation, and portable
electronics.1−6 Among the several types of fuel cells, proton-
exchange membrane fuel cells (PEMFCs), which operate under
acidic conditions, are attracting a great deal of interest due to
their high power density, high energy-conversion efficiencies,
low starting temperature, and easy handling.3 Proton-exchange
membranes (PEMs), which act as a barrier between the fuel
and oxidant streams and simultaneously transport protons, have
been considered as one of the key components of a PEMFC.1,2
The most commonly used PEMs, represented by Nafion
perfluorosulfonic acid membranes, have high proton con-
ductivities, good mechanical properties, and excellent chemical
stability. However, application of PEMFCs is limited by their
exclusively dependence of platinum catalysts because of the
limited platinum resource in nature. Therefore, there is growing
interest in developing anion-exchange membranes for alkaline
anion-exchange membrane fuel cells (AEMFCs), in which the
charge carriers are hydroxide ions (or other anions) instead of
protons.
Compared with traditional PEMFCs, one main advantage of
AEMFCs is their potential to use non-precious-metal-based
electrocatalysts such as silver, cobalt, or nickel due to the low
overpotentials associated with electrochemical reactions at high
pH.5,6 Furthermore, AEMFCs also offer the fuel flexibility,
reduced fuel (such as methanol) crossover, and enhanced
reaction kinetics for both oxygen reduction and fuel
oxidation.7−9 All these advantages make the AEMFC
technology financially and technically doable. However,
application of Nafion membranes in PEMFCs is limited to
A variety of polymeric AEMs containing quaternary
ammonium groups, such as polysulfone,10−12 radiation-grafted
poly(tetrafluoroethene-co-hexafluoropropylene) (FEP), poly-
(vinylidene fluoride) (PVDF), poly(ethylene-co-tetrafluoro-
ethylene) (ETFE),13,14 poly(2,6-dimethyl-1,4-phenylene
oxide) (PPO),15 poly(ether imide) (PEI),16 and poly(vinyl
alcohol) (PVA),17,18a have been synthesized and extensively
studied. These membranes were typically prepared by attach-
ment of chloromethyl groups to polymer backbones and
followed by quaternization to form ammonium salts. However,
chloromethyl ether is carcinogenic and harmful to human, and
the precise control over the degree and location of
functionalization is usually difficult. Therefore, it is desirable
to prepare the AEMs in a relatively simple synthetic route,
especially without the use of chloromethyl ether.18
Imidazolium-based ionic liquids (ILs) have been recently
applied for the preparation of anhydrous PEMs because of their
unique physicochemical properties, such as good thermal
stabilities, low volatility, and high ion conductivity.19,20 The
excellent ion-exchange capabilities of ILs makes them very
useful in ion-exchange applications.21 Recently, AEMs based on
the alkaline imidazolium-type ILs have been studied by several
groups.22,23 Compared with membranes based on quaternary
ammonium salts, imidazolium group functionalized AMEs
show comparable ionic conductivity, however, better chemical
stability in high-pH solution.23 Yan et al. recently synthesized
imidazolium-type IL-based AEMs via in situ cross-linking of 1-
Received: September 23, 2011
Revised: November 2, 2011
Published: November 30, 2011
© 2011 American Chemical Society
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temperature, the solvent and unreacted 1,2-dimethylimidazole were
removed under vacuum. The residue was washed with ethyl acetate
three times prior to being dried under vacuum at 80 °C for 24 h to
vinyl-3-methylimidazolium iodide and followed by anion-
exchange with hydroxide ions.22a This approach provided facile
synthetic access to membranes with high conductivity and
mechanical properties and low swelling degree. However, the
cross-linked AMEs are insoluble in all solvents and could not be
used in the catalyst layer to build an efficient three-phase
boundary to improve the utilization of the catalyst particles.
Therefore, developing of solvent processable AEMs is in great
demand.24
Herein we present a facile synthetic strategy for the synthesis
of solvent-soluble AEMs based on alkaline imidazolium-type
IL-functionalized polyfluorene ionomers. The membranes are
mechanically strong and soluble in polar aprotic solvents such
as DMSO and DMF while insoluble in water and aqueous
methanol. The solvent processable polyfluorene ionomers show
high hydroxide conductivity and could be applied as both
AEMs and electrode materials.
1
produce the final yellow solid (2.45 g, 95.3%). H NMR (400 MHz,
DMSO) δ: 9.58 (d, 2H), 7.80−7.82 (d, 2H), 7.64 (d, 2H), 7.58−7.54
(br, 6H), 7.51(br, 2H), 7.48(br, 2H), 6.87−6.89 (d, 4H), 3.94−3.90
(t, 4H), 3.66 (s, 6H), 2.42 (s, 6H), 2.07 (br, 4H), 1.45 (br, 4H), 1.02
(br, 4H), 0.54 (br, 4H).
Synthesis of Polyfluorene Ionomer Containing Pendant
Alkaline Imidazolium Ionic Liquids. A typical synthesis procedure
was as follows: A mixture containing 4.34 g of 2,7-di(4′-phenol)-9,9-
bis(6′-(1,2-dimethylimidazole))fluorene bromine (5.0 mmol), 1.44 g
of 4,4′-dichlorodiphenyl sulfone (5.0 mmol), and 3.58 g of Cs2CO3
(11.0 mmol) were added into a 100 mL three-neck flask equipped with
a magnetic stirrer, a Deane-Stark trap, and a nitrogen inlet. Then, 25
mL of dried DMSO and 13 mL of toluene were added into the
reaction flask under a nitrogen atmosphere. The reaction bath was
heated to 130 °C to dehydrate the system for 4 h. After toluene and
water was distilled off, the temperature of reaction was raised gradually
to 140 °C and then stirred for 8 h. The reaction was cooled to room
temperature, and 20 mL of DMSO was added to dilute the highly
viscous solution, and the solution was then poured into deionized
water. The precipitated polymers were washed with deionized water
and ethanol several times prior to being dried under vacuum at 80 °C
for 24 h to produce the final product.
EXPERIMENTAL SECTION
■
Materials. 1,2-Dimethylimidazole, 1,6-dibromohexane, 2,7-dibro-
mofluorene, 4-carboxybenzeneboronic acid, 4,4′-dichlorodiphenyl
sulfone, [1,1′-bis(diphenylphosphino)ferrocene]dichloropalladium(II)
(PdCl2(dppf)), chloroform, dichloromethane, N,N-dimethylforma-
mide (DMF), dimethyl sulfoxide (DMSO), N,N-dimethylacetamide
(DMAc), N-methylpyrrolidone (NMP), tetrahydrofuran, toluene,
acetonitrile, cesium carbonate, ethyl ether, ethyl acetate, potassium
hydroxide, tetrabutylammonium bromide (TBAB), sodium hydroxide,
and hydrochloric acid were used as purchased. Distilled deionized
water was used for all experiments.
Membrane Preparation and Evaluation. The membranes were
prepared by casting a DMF solution of synthesized polyfluorene
ionomers (2 wt %) onto a leveled Teflon sheet. To avoid air bubbles,
the membranes were slowly dried at 80 °C for 12 h and then dried in
vacuo at 100 °C for 24 h, forming thin films with the thickness of
about 25 μm. The resultant membranes were immersed in a N2-
saturated 1 M KOH (or Na2CO3) solution at 60 °C for 48 h to
Synthesis of 2,7-Dibromo-9,9-bis(6′-bromohexyl)fluorene
(Compound 1). 2,7-Dibromo-9,9-bis(6′-bromohexyl)fluorene was
synthesized as documented in previous literature.25 Briefly, a mixture
of 2,7-dibromofluorene (5.0 g, 15 mmol), 1,6-dibromohexane (30
mL), TBAB (0.1 g), and sodium hydroxide (30 mL, 50 wt %) aqueous
solution was stirred at 60 °C for 8 h under nitrogen. After diluting the
reaction mixture with dichloromethane, the organic layer was washed
with water and brine. The separated organic layer was dried over
magnesium sulfate, and dichloromethane was evaporated. The
unreacted 1,6-dibromohexane was distilled in vacuum, and 2,7-
dibromo-9,9-bis(6′-bromohexyl)fluorene (7.6 g, 75.8%) was obtained
as a white crystal by chromatography with petroleum ether as the
eluent. 1H NMR (CDCl3) δ (ppm): 7.53−7.50 (d, 2H) 8.0 Hz),
7.46−7.44 (d, 2H), 7.42 (s, 2H), 3.30−3.27 (t, 4H), 1.93−1.89 (t,
4H), 1.68−1.55 (m, 4H), 1.20−1.17 (m, 4H), 1.09−1.06 (m, 4H),
0.57(m, 4H).
convert the membrane from Br− to OH− or CO3 form.22 This
2−
process was repeated three times to ensure a complete conversion
displacement. Then the converted membranes were immersed in a N2-
saturated deionized water for 24 h and washed with deionized water
until the pH of residual water was neutral.
Characterization. 1H NMR spectra were recorded on a Varian 400
MHz spectrometer. Fourier transform infrared (FT-IR) spectra were
recorded on a Varian CP-3800 spectrometer in the range 4000−400
cm−1. Thermal analysis was carried out on Universal Analysis 2000
thermogravimetric analyzer (TGA). Samples were heated from 40 to
500 °C at a heating rate of 10 °C/min under a nitrogen flow. The
tensile properties of membranes were measured by using an Instron
3365 at 25 °C at a crosshead speed of 5 mm/min. Scanning electron
microscopy (SEM) images were taken with a Philips XL 30 FEG
microscope with an accelerating voltage of 10 kV. Energy dispersive X-
ray spectroscopy (EDX) measurements were performed with the
spectrometer attached on the Hitachi S-4700 FESEM.
Synthesis of 2,7-Di(4′-phenol)-9,9-bis(6′-bromohexanyl)-
fluorene (Compound 2). 2,7-Di(4′-phenol)-9,9-bis(6′-
bromohexanyl)fluorene was synthesized as follows:26 A mixture
containing 2,7-dibromo-9,9-bis(6′-bromohexyl)fluorene (5.0 g, 7.7
mmol), 4-carboxybenzeneboronic acid (1.2 g, 8.5 mmol) in THF
(40 mL), and 2.0 M aqueous K2CO3 solution (30 mL) was charged
with argon for 30 min. Then 160 mg of PdCl2(dppf) was added under
an argon atmosphere, and the mixture was then stirred at 70 °C for 24
h. The crude product was extracted with 50 mL of CHCl3 three times.
The combined organic phase was washed with brine and dried over
anhydrous MgSO4, and the solvent was removed under vacuum. The
residue was purified by silica gel column chromatography using ethyl
acetate/petroleum ether 1/4 (v/v) as eluent to obtain white solids
Hydroxide Ion Conductivity. The resistance value of the
membranes was measured over the frequency range from 1 Hz to 1
MHz by four-point probe alternating current (ac) impedance
spectroscopy using an electrode system connected with an electro-
chemical workstation (CHI660C). All the samples were fully hydrated
in N2 saturated deionized water for at least 24 h prior to the
conductivity measurement. Conductivity measurements under fully
hydrated conditions were carried out in a chamber filled with a N2-
saturated deionized water to maintain the relative humidity at 100%
during the experiments. All the samples were equilibrated for at least
30 min at a given temperature. Repeated measurements were taken
with 10 min interval until no more change in conductivity was
observed. The ionic conductivity σ (S/cm) of a given membrane can
be calculated from
1
(3.46 g, 66.2%). H NMR (400 MHz, CDCl3) δ: 7.74−7.73 (d, 2H),
7.58 (d, 2H), 7.56 (d, 2H), 7.54−7.52 (s, 2H), 7.49 (d, 2H), 6.96−
6.94 (d, 4H), 4.88 (s, 4H), 3.26 (t, 4H), 3.28−3.24 (br, 4H), 2.04−
2.00 (br, 4H), 1.68−1.61 (br, 6H), 1.21−1.16 (d, 4H), 1.10−1.07 (br,
4H), 0.70 (br, 4H).
Synthesis of 2,7-Di(4′-phenol)-9,9-bis(6′-(1,2-
dimethylimidazole))fluorene Bromine (Compound 3). A sol-
ution of 2,7-di(4′-phenol)-9,9-bis(6′-bromohexanyl)fluorene (2.0 g,
2.96 mmol) and 1,2-dimethylimidazole (1 mL, 12.5 mmol) in 20 mL
of CH3CN was stirred at 70 °C for 24 h. Upon cooling down to room
where l is the distance (cm) between two stainless steel
electrodes, A is the cross-sectional area (cm2) of the membrane,
obtained from the membrane thickness multiplied by its width,
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a
Scheme 1. Synthetic Procedure and Photograph of Polyfluorene Ionomer Membrane
a
Reagents and conditions: (a) 50 wt % KOH, TBAB, 60 °C, 8 h; (b) 4-carboxybenzeneboronic acid, PdCl2(dppf), 1 mol %, 70 °C, 24 h, (c) 1,2-
dimethylimidazole, 70 °C, 24 h.
1
Figure 1. H NMR spectra of synthesized polyfluorene ionomer with pendant ionic liquids.
of 0.01 M HCl standard solution for 24 h. The solutions were then
titrated with a standardized NaOH solution using phenolphthalein as
an indicator. The IEC value was calculated using the expression
and R is the membrane resistance value from the ac impedance
data (Ω).
Water Uptake and Swelling Ratio. The membrane samples were
soaked in the N2-saturated deionized water at room temperature for 24
h. The hydrated polymer membranes were taken out, and the excess
water on the surface was removed by wiping with a tissue paper and
weighed immediately (Ww). Then the wet membrane was dried under
vacuum at 80 °C until a constant dry weight was obtained (Wd). The
water uptake W was calculated with the equation
where V0,NaOH and Vx,NaOH are the volume of the NaOH
consumed in the titration without and with membranes,
respectively, CNaOH is the mole concentration of the NaOH,
which are titrated by the standard oxalic acid solution, and mdry
is the mass of the dry membranes. Three replicates were
conducted for each sample.
where Wd and Ww are the mass of the dry and water-swollen
samples, respectively.
The swelling ratio was characterized by linear expansion ratio
(LER), which was determined by the difference between wet and dry
dimensions of a membrane sample (3 cm in length and 1 cm in
width). The calculation was based on the equation
Membrane Stability in Alkaline Solution. The alkaline stability of
the membranes was examined by immersing the membrane samples in
N2-saturated 1 M KOH solution at 60 °C. The degradation of polymer
1
membranes was evaluated by measuring the changes of H NMR, FT-
IR, and hydroxide conductivity.
RESULTS AND DISCUSSION
■
Herein we present a facile synthetic strategy for the synthesis of
solvent-soluble AEMs based on alkaline imidazolium-type ionic
liquids (ILs)-functionalized polyfluorene ionomers. Scheme 1
shows the synthetic procedure for polyfluorene ionomers. We
developed a novel fluorene derivative (compound 3) which
contains saturated alkyl side chains and imidazolium groups.
where Xwet and Xdry are the lengths of wet and dry membranes,
respectively.
Ion-Exchange Capacity (IEC). Ion-exchange capacities (IEC) were
determined by a back-titration. The AEMs were immersed in 100 mL
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Figure 2. FT-IR spectra of polyfluorene ionomers in (A) Br− and (B) OH− forms and (C) in OH− form immersing in 1 M KOH for 400 h.
Figure 3. Energy dispersive X-ray (EDX) spectra for polyfluorene ionomer membranes in (A) Br− and (B) OH− forms with silicon as the substrate.
Figure 4. SEM images of polyfluorene ionomer membrane in OH− form: surface (A) and cross-sectional view (B).
The purity and chemical structure of the synthesized
compounds 1, 2, and 3 were confirmed by 1H NMR
measurements (see Supporting Information). The polyfluorene
ionomer with pendant imidazolium groups was obtained from
compound 3 and 4,4′-dichlorodiphenyl sulfone by nucleophilic
substitution polycondensation (Scheme 1). By employing
monomers with the ionic liquid moiety already present, AEM
synthesis is greatly simplified because postpolymerization
modifications could be avoided. The chemical structure of the
resultant polyfluorene ionomers was characterized by 1H NMR.
Figure 1 exhibits the expected chemical shifts and intensities for
a pure polyfluorene ionomer containing pendant alkaline
imidazolium ionic liquids (inserted chemical structure and
chemical shift assignments). Chemical shifts of the fluorene
units are observed at 7.69 and 7.80 ppm. The chemical shifts at
2.44, 3.67 ppm and the inserted spectrum which shows several
minor chemical shifts at 7.51 ppm are attributed to the attached
imidazolium groups.
The chemical structure of the resultant polyfluorene
ionomers was further investigated by means of FT-IR spectrum.
Figure 2 shows the FT-IR spectra of the polyfluorene ionomer
membranes in Br− and OH− forms produced by solution
casting. Membranes in both two forms show the absorption
bands of methylene (CH2) at 2856−2935 cm−1. The peaks at
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1465−1637 cm−1 confirm the existence of polyphenylene.
Absorption peaks at around 1586 and 744 cm−1 arise from
vibrational mode of imidazolium cations. The peaks at 1317
and 1151 cm−1 confirm the existence of sulfone (SO2), and the
peak at 1245 cm−1 is the characteristic peak of aryl ethers. The
results clearly confirm the successful copolymerization of
compound 3 and 4,4′-dichlorodiphenyl sulfone. Compared
with Figure 2A, a stronger absorption peak at around 3400
cm−1 in Figure 2B,C is attributed to the stretching vibration of
O−H groups, indicating the successful anion change of Br− to
OH−. Furthermore, no obvious changes between parts B and C
of Figure 3 are observed, indicating that the copolymer is stable
in alkaline solutions. Moreover, the results of energy dispersive
X-ray (EDX) spectra (Figure 3) show that no bromine could be
detected in OH− form, which again confirms the anion-
exchange of the polyfluorene ionomers.
stabilities. Both the membranes show a slight weight loss (less
than 4 wt %) below 200 °C, probably corresponding to the
evaporation of absorbed water or solvents (such as DMF). The
weight loss region at temperatures above 300 °C may due to
the degradation of the copolymer backbone. The thermal
stability of the AEMs produced in this work is better than that
of reported quaternary ammonium polysulfone10a and quater-
nary guanidinium poly(arylene ether sulfone)s containing
hydroxide groups.11a These results confirm that this type of
alkaline IL-based copolymer membranes indeed confers a high
thermal stability, far beyond the range of interest for application
in AEMFCs.
The tensile strength of the membrane in OH− form is 50.41
MPa, with the Young’s modulus of 1610.32 MPa, and values of
elongation at break of 12.63% (Table 1). Compared with
Membranes were prepared by dissolving the polyfluorene
polymers in DMF followed by casting on a leveled Teflon
sheet. The membranes were dried under vacuum to remove
residual solvent. The resultant membranes are transparent and
flexible and can be easily cut into any desired sizes and shapes
(Scheme 1). The morphology of the membranes in OH− form
was investigated by scanning electron microscopy (SEM). SEM
images of the surface and cross section of the membranes
showed that the produced membranes are uniform, compact,
and without any visible pores (Figure 4). In addition, the
synthesized alkaline polyfluorene ionomers exhibited out-
standing solubility in polar aprotic solvents, such as DMF,
DMAc, DMSO, and NMP, while insoluble in water and
aqueous methanol. These solubility properties of polyfluorene
ionomers enable the use for not only alkaline anion-exchange
membrane but also ionomer electrode material.
Table 1. Mechanical Properties of Polyfluorene Ionomer,
Nafion-117, and PSQNOH-50
tensile strength
(MPa)
tensile modulus
(MPa)
elongation at
break (%)
membrane
polyfluorene
ionomer
50.41
1610.32
12.63
Nafion-117
PSQNOH-5011d
21.11
51.2
6.60
370.62
28.4
1520
Nafion-117, AEMs in this study showed a higher tensile
strength and tensile modulus while a lower elongation at break
because of the rigid ring-structured backbone. The mechanical
properties of AEMs in the present work are equivalent to
PSQNOH-50 which has similar polymeric backbone.11d These
results indicate that the AEMs produced in this work are tough
and ductile enough for potential use as AEM materials.
The thermal stability of AEMs is always a concern for
AEMFCs because some AEMs based on quaternary ammonium
salts are usually unstable in KOH solution at elevated
temperature.10a Aromatic polymers are generally considered
as the preferred candidates for high-temperature fuel cell
applications due to their excellent thermal stability. Figure 5
Table 2 shows the values of ion-exchange capacity (IEC),
water uptake, swelling degree, and conductivity of the produced
polyfluorene ionomer membranes. The IEC values are
generally considered responsible for ion transfer and thus are
an indirect and reliable approximation of the ion conductivity.
The IEC value of produced membrane is calculated to be 0.98
mmol/g, which is close to the theoretical value of polyfluorene
(1.04 mmol/g). Swelling behavior of the membrane is an
essential factor influencing the mechanical properties and the
morphologic stability of membranes. Generally, the water
uptake and swelling degree increase with the IEC values.
However, the membranes have less water uptake and swelling
degree compared to the cross-linked membrane with the same
IEC value,22a probably due to the presence of the aromatic
backbone in the membrane. It should be noted that the water
swelling degree of the membranes is even lower than that of
Nafion-117 under the same experimental conditions. Such a
good dimensional stability of membranes suggests a feasible
approach for practical applications in fuel cells.
The hydroxide conductivity of the membranes is of particular
important and plays a significant role in fuel cell performance.
The conductivity of AEMs is significantly influenced by the IEC
values because it links to the density of ionizable functional
group in the membranes. As the IEC values increasing, which
implying the improved hydrophilic properties of the membrane,
the membrane becomes more hydrophilic and absorbs more
water, which facilitates ion transport. Here, the conductivity of
fully hydrated AEMs was measured by the four-probe method.
The choice of the four-probe instead of two-probe method for
the conductivity measurements is because the effect of contact
Figure 5. TGA curves of polyfluorene ionomer membranes under
nitrogen flow. Heating rate: 10 °C/min.
shows the typical thermogravimetric analyzer (TGA) curves of
produced copolymer membranes in Br− and OH− forms which
were recorded under a nitrogen flow from 40 to 500 °C at a
heating rate of 10 °C/min to assess their short-term thermal
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Table 2. Ion-Exchange Capacity (IEC), Water Uptake, Swelling Degree, and Conductivity of AEMs
b
b
IEC (mequiv g−1
)
water uptake (%)
swelling degree (%)
conductivity (×10−2 S cm−1
)
a
membrane
theor
expt
0.98
30 °C
60 °C
30 °C
60 °C
30 °C
60 °C
polyfluorene ionomer
Nafion-117
PSQNOH-5011d
1.04
0.91
17.26
19.48
24.76
27.11
19
5.56
6.09
10.81
12.87
12
2.35
5.91
4.28
9.55
3.20
1.90
1.85
a
b
Calculated from monomer ratio. After immersing in water at 60 °C for 24 h, average of two trials.
resistance on the four-probe ionic conductivity is much lower
than that on two-probe method, especially in high humidity.27
For comparison, the conductivity of AEMs was also measured
by two-probe method, and the results are shown in Figure S4
(see Supporting Information). Figure 6 shows the reproducible
of produced AEMs was investigated by immersing the
membrane samples in 1 M KOH solution at 60 °C. The
changes of ionic conductivity and the chemical structure of the
tested membranes were characterized. Figure 7 shows 1H NMR
1
Figure 7. H NMR spectra of polyfluorene ionomer membranes in
Br− form and OH− form and after immersing in 1 M KOH solution at
60 °C for 400 h.
Figure 6. Conductivity Arrhenius plots of polyfluorene ionomer
membranes in OH− and CO32− forms and Nafion-117 as a function of
temperature.
spectra of the membranes after the alkaline stability test. It
should be noted that no new chemical shifts of polymers were
observed even after 400 h testing period in 1 M KOH at 60 °C,
indicating an excellent alkaline stability of the membranes in
high-pH alkaline solution. These results further support the FT-
IR spectra results, as shown in Figure 2.
plot of ionic conductivity of polyfluorene ionomer membranes
in both OH− and CO3 forms as a function of temperature.
2−
For comparison, the conductivity of Nafion-117 was also
measured under the same conditions. The ionic conductivity of
the membranes gradually increases with the increase of
temperature. For example, the ionic conductivity of polyfluor-
ene ionomer membranes in OH− form increased from 2.35 ×
10−2 to 4.28 × 10−2 S/cm, if the temperature rose from 30 to
60 °C (Figure 6). This is mainly due to faster migration of ions
and higher diffusivity with the increasing of temperature.
Simultaneously, the polymeric chain will be more flexible and
more water will be adsorbed by the membrane at a higher
temperature, leading to a more swollen membrane structure
and wider ion transferring channel, resulting in more propitious
ion transfer.11,22a,c The conductivity of polyfluorene ionomer
membranes in OH− form was about 2-fold higher than that in
The alkaline stability of the polyfluorene ionomer mem-
branes was further proved by measuring the hydroxide
conductivity changes after certain testing time. The membranes
maintain their conductivity even after immersion in 1 M KOH
solution at 60 °C for 400 h, and the IEC value is 0.92 mequiv/
g, which is close to the initial value (0.98 mequiv/g). All these
results indicate excellent long-term alkaline stability of AEMs in
high-pH environment (Figure 8). AEMs functionalized with
quaternary ammonium salts usually show high room temper-
ature conductivity of hydroxide. However, hydroxide is an
aggressive nucleophile and could degrade the quaternary
ammonium cation sites on the polymeric AEMs because of
nucleophilic substitution and (or) Hofmann elimination
reaction, especially at high pH and elevated temperature.10
Although the Hofmann elimination reaction could be avoided
by synthesizing β-hydrogen-absent quaternary ammoniums,
quaternary ammonium-functionalized polymers are still suffer-
ing from the nucleophilic substitution reaction. The excellent
alkaline stability of alkaline ionic liquid-functionalized poly-
fluorene ionomers is probably due to the resonance effect of
the conjugated imidazole rings, which reduce the positive
charge density of the cation and weaken the interaction with
2−
CO3 form probably because the infinite diffusion coefficient
of OH− anions is about 3.7 times higher than that of CO3
2−
anions.11b The results suggest other factors such as cation−
anion interaction or insufficient equilibrium time may also
impact the anion mobility.11b The hydroxide conductivities of
the synthesized polyfluorene ionomer membranes is compara-
ble to the ionic conductivity of tetraalkylammonium-function-
alized AEMs with the similar IEC values.11a
Chemical stability of the AEMs is still a challenge for the
practical application of fuel cells, especially in high-pH
environment of alkaline fuel cells. Here, the alkaline stability
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Article
Higher Education (20103201110003), Program for Scientific
Innovation Research of College Graduate in Jiangsu Province
(CXZZ11-0103), and a Project Funded by the Priority
Academic Program Development of Jiangsu Higher Education
Institutions. We thank Prof. Q. Li and Dr. F. Meng for
mechanical properties measurements.
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ASSOCIATED CONTENT
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* Supporting Information
1H NMR spectra of synthesized compounds and the
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free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: fyan@suda.edu.cn.
■
ACKNOWLEDGMENTS
■
This work was supported by Natural Science Foundation of
China (Nos. 20874071, 20974072, and 21174102), The
Natural Science Foundation of Jiangsu Province
(BK2011274), Research Fund for the Doctoral Program of
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