Synthesis and characterization of a novel crosslinkable side-chain sulfonated poly(arylene ether sulfone) copolymer proton exchange membranes

Article abstract of DOI:10.1039/d0ra02987d

A series of novel crosslinkable side-chain sulfonated poly(arylene ether sulfone) copolymers (S-SPAES(x/y)) was prepared from 4,4′-biphenol, 4,4′-difluorodiphenyl sulfone, and a new difluoro aromatic monomer 1-(2,6-difluorophenyl)-2-(3,5-dimethoxyphenyl)-1,2-ethanedione (DFDMED) via co-polycondensation, demethylation, and further nucleophilic substitution of 1,4-butane sultone. Meanwhile, quinoxaline-based crosslinked copolymers (CS-SPAES(x/y)) were obtained via cyclo-condensation between S-SPAES(x/y) and 3,3′-diaminobenzidine. Both the crosslinkable and crosslinked copolymer membranes exhibit good mechanical properties and high anisotropic membrane swelling. Crosslinkable S-SPAES(1/2) with an ion exchange capacity (IEC) of 2.01 mequiv. g-1 displays a relatively high proton conductivity of 180 mS cm-1 and acceptable single-cell performance, which is attributed to its good microphase separation resulting from the side-chain sulfonated copolymer structures. Compared with S-SPAES(1/1) (IEC of 1.68 mequiv. g-1), crosslinked CS-SPAES(1/2) with a comparable IEC exhibits a larger conductivity of 157 mS cm-1, and significantly higher oxidative stability and lower membrane swelling, suggesting a distinct performance improvement due to the quinoxaline-based crosslinking. This journal is

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Synthesis and characterization of a novel
crosslinkable side-chain sulfonated poly(arylene
ether sulfone) copolymer proton exchange
membranes†
Cite this: RSC Adv., 2020, 10, 24772
*
*
Shouping Wang, Fugang He, Qiang Weng, Diao Yuan, Pei Chen, Xinbing Chen
and Zhongwei An
A series of novel crosslinkable side-chain sulfonated poly(arylene ether sulfone) copolymers (S-SPAES(x/y))
was prepared from 4,40-biphenol, 4,40-difluorodiphenyl sulfone, and a new difluoro aromatic monomer 1-
(2,6-difluorophenyl)-2-(3,5-dimethoxyphenyl)-1,2-ethanedione (DFDMED) via co-polycondensation,
demethylation, and further nucleophilic substitution of 1,4-butane sultone. Meanwhile, quinoxaline-
based crosslinked copolymers (CS-SPAES(x/y)) were obtained via cyclo-condensation between S-
SPAES(x/y) and 3,30-diaminobenzidine. Both the crosslinkable and crosslinked copolymer membranes
exhibit good mechanical properties and high anisotropic membrane swelling. Crosslinkable S-SPAES(1/2)
with an ion exchange capacity (IEC) of 2.01 mequiv. gꢀ1 displays a relatively high proton conductivity of
180 mS cmꢀ1 and acceptable single-cell performance, which is attributed to its good microphase
separation resulting from the side-chain sulfonated copolymer structures. Compared with S-SPAES(1/1)
(IEC of 1.68 mequiv. gꢀ1), crosslinked CS-SPAES(1/2) with a comparable IEC exhibits a larger conductivity
of 157 mS cmꢀ1, and significantly higher oxidative stability and lower membrane swelling, suggesting
a distinct performance improvement due to the quinoxaline-based crosslinking.
Received 2nd April 2020
Accepted 28th May 2020
DOI: 10.1039/d0ra02987d
rsc.li/rsc-advances
achieved from copolymerization between sulfonated and non-
sulfonated monomers.8,22,26–28
1. Introduction
Among the reported sulfonated aromatic copolymers for fuel
cell applications, poly(arylene ether) based ionomers are the
most studied,10–12,14,15,22–25,27,28 which is attributed to their good
stability and easy fabrication. However, the sulfonated
poly(arylene ether sulfone) (PAES) with high ion exchange
capacity (>2.0 mequiv. gꢀ1) generally exhibits large swelling,
which leads to undesirable qualities in the fuel cells. Therefore,
many efforts have been focused on developing novel sulfonated
PAES structures with acceptable membrane swelling and good
proton conductivity, in which the crosslinking method and
side-chain strategies are commonly utilized to suppress
swelling and improve the conductivity.11,18,22,27,28
In recent years, several kinds of crosslinking methods have
been reported to develop novel crosslinked PEMs with high
performance, including ionically-crosslinked acid/base blend
membranes,29 covalently-crosslinked membranes via esteri-
cation reaction,30 thermally-activated covalent crosslinked
membranes,31–33 covalently-crosslinked membranes via ther-
mally or photochemically-activated radical crosslinking reac-
tion,34–36 and quinoxaline-based covalent ionically-crosslinked
membranes.27,28,37,38 Crosslinking is considered to be effective
to balance the membrane properties but there are few reported
crosslinkable monomers or copolymers for PEMs.
Polymer electrolyte membrane (PEM) fuel cells have been widely
studied for years and focused on the achievement of promising
clean energy sources for transportation, and stationary and
portable power applications due to their low polluting capa-
bility and good power density,1–3 wherein PEM, as one of the key
components for achieving ion conduction and obstructing fuel
crossover, has attracted widespread interest. Among the re-
ported PEMs, peruorosulfonate ionomers exhibit large proton
conductivity and good stability4–6 but the high cost and large
fuel crossover encumber their further applications in fuel cells.7
Up to now, many kinds of sulfonated aromatic ionomers with
good performance have been extensively developed to act as
PEMs.8–25 In general, the ionomers are prepared via a post-
sulfonation strategy, in which sulfonic acid groups are directly
introduced into the polymer structures bearing the reactive
sites,10–13,15,17,21,25 as well as through a pre-sulfonation method
Key Laboratory of Applied Surface and Colloid Chemistry (MOE), Shaanxi Key
Laboratory for Advanced Energy Devices, Shaanxi Engineering Laboratory for
Advanced Energy Technology, School of Materials Science and Engineering, Shaanxi
Normal University, Xi'an 710119, PR China. E-mail: chenxinbing@snnu.edu.cn;
chenpei@snnu.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra02987d
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Crosslinkable groups in the monomer contribute to the 62.50 mmol), 210 mL of absolute diethyl ether, and 60 mL of
ꢁ
formation of a covalently-crosslinked structure and to achieve anhydrous toluene with stirring at ꢀ10 C for about 3 h. Aer
reduced sulfonic acid consumption for ionic or covalent cross- the reaction, 3,5-dimethoxybenzyl magnesium bromide was
linking, such as propenyl groups for thermally- or obtained. To the above solution, 2,6-diuorobenzonitrile
photochemically-activated radical crosslinking34 and benzil (6.0189 g, 43.27 mmol) in 50 mL of absolute diethyl ether was
groups for quinoxaline-based crosslinking.27 Here, a new added slowly with stirring and reuxing under nitrogen atmo-
diuoro aromatic monomer 1-(2,6-diuorophenyl)-2-(3,5- sphere. Aer the addition, the solution was stirred under reux;
dimethoxyphenyl)-1,2-ethanedione (DFDMED) with cross- meanwhile, the reaction was monitored via thin layer chroma-
linkable ethanedione and ionizable dimethoxy groups was tography. Aer about 6 h, the reaction was quenched using
designed and synthesized; meanwhile, DFDMED-based cross- 80 mL of 0.1 M HCl, keeping the solution below 25 ꢁC. Then, the
linkable side-chain poly(arylene ether sulfone) copolymers separated organic layer was hydrolyzed in 200 mL of 1 M HCl
bearing pendant butanesulfonic acid were prepared from 4,40- under reux for 6 h. The system was cooled to room tempera-
biphenol, 4,40-diuorodiphenyl sulfone, and DFDMED. ture, separated, and extracted with diethyl ether, and washed
Furthermore, quinoxaline-based crosslinked copolymers were with water to neutral pH. The combined organic phase was
obtained via cyclo-condensation between the crosslinkable dried over magnesium sulphate. Aer the removal of the solvent
copolymers and 3,30-diaminobenzidine. The properties of the and unreacted 2,6-diuorobenzonitrile in vacuo, the residue was
crosslinkable and crosslinked copolymer membranes, such as puried via column chromatography on silica gel using petro-
proton conductivity, membrane swelling, water uptake, meth- leum ether/dichloromethane as the eluent to give the objective
anol permeability, oxidative stability, and fuel cell performance, compound DFDME with purity above 98% for GC measurement
have been investigated and discussed.
and 46% yield.
Characterization data of DFDME: 1H-NMR (300 MHz,
D3CCOCD3, TMS): d (ppm) 3.74 (s, 6H), 4.13 (s, 2H), 6.34 (s, 1H),
6.37 (s, 2H), 6.90 (t, 2H), 7.35 (m, 1H). IR (KBr, pellet, cmꢀ1):
3060, 3030, 2947, 2860, 1625, 1586, 1565, 1466, 1234, 1137,
1050, 810, 773. EI-MS m/z (rel. int.): 292.17 (M+, 30), 264.17 (37),
151.16 (18), 141.03 (100), 113.08 (10), 91.06 (6).
2. Experimental
2.1. Materials and reagents
4,40-Diuorodiphenyl sulfone (DFDPS) and 3,5-dimethox-
ybenzyl bromide was obtained from Beijing Zhongsheng Hua-
teng Technology Co. Ltd. and Zhejiang Shouerfu Chemical Co.
Ltd, respectively. 4,40-Biphenol (BP) and 3,30-diaminobenzidine
(DAB) were acquired from Shanghai Aladdin Reagent Co. Ltd.
Other solvents and reagents were purchased from Sinopharm
Chemical Reagent.
2.2.2. Preparation
of
1-(2,6-diuorophenyl)-2-(3,5-
dimethoxy phenyl)-1,2-ethanedione (DFDMED). To a 250 mL
round-bottom ask equipped with a magnetic stirrer and
condenser, DFDME (5.6118 g, 19.20 mmol), CuBr2 (4.2883 g,
19.20 mmol), 30 mL of dimethylsulphoxide (DMSO), and 30 mL
of ethyl acetate were added. The solution was stirred under
reux overnight, then cooled to room temperature. The reaction
mixture was diluted with water and extracted with chloroform
three times; then, the combined organic layer was dried over
magnesium sulphate. Aer the removal of the solvent in vacuo,
the residue was puried via column chromatography on silica
gel using petroleum ether/tetrahydrofuran as the eluent and
then recrystallized from a mixture solution of petroleum ether
and dichloromethane to give the objective compound DFDMED
with purity above 99% for GC measurement and 84% yield.
Characterization data of DFDMED: 1H-NMR (300 MHz,
DMSO-d6, TMS): d (ppm) 3.83 (s, 6H), 6.94 (s, 1H), 7.12 (s, 2H),
7.34 (t, 2H), 7.78–7.88 (m, 1H). IR (KBr, pellet, cmꢀ1): 3097,
2918, 2859, 1695, 1647, 1596, 1546, 1514, 1460, 1368, 1235,
1135, 1045, 855, 808, 671. EI-MS m/z (rel. int.): 306.22 (M+, 6),
165.13 (100), 137.12 (23), 122.08 (17), 107.11 (10).
2.2. Preparation of the diuoro aromatic monomer
DFDMED
Two-step reactions including the nucleophilic addition of 2,6-
diuorobenzonitrile and oxidation of the corresponding 1,2-
disubstituted ethanone were utilized to prepare the diuoro
aromatic monomer DFDMED, which contains a crosslinkable
ethanedione and two ionizable dimethoxy groups, as shown in
Scheme 1. The synthetic procedure is described below.
2.2.1. Preparation
of
1-(2,6-diuorophenyl)-2-(3,5-
dimethoxy phenyl)-ethanone (DFDME). Under nitrogen
protection, 3,5-dimethoxybenzyl bromide (9.9993 g, 43.27
mmol) was added into the mixture of magnesium (1.5193 g,
2.3. Preparation of DFDMED-based side-chain sulfonated
copolymers (S-SPAES(x/y))
A
series of side-chain sulfonated copolymers BP-DFDPS/
DFDMED(x/y) (coded as S-SPAES(x/y)), where x/y refers to the
molar ratio of DFDPS to DFDMED, was obtained via one-pot
high temperature copolymerization, as well as demethylation
and further nucleophilic substitution, as shown in Scheme 2.
Scheme 1 Synthesis of the difluoro aromatic monomer DFDMED.
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potassium carbonate (0.95 g, 6.87 mmol) were added; then, the
ꢁ
reaction was carried out at 100 C for 24 h. Aer the reaction,
the resulting solution was poured into ethyl acetate to obtain
a precipitate. The precipitate was washed to neutral pH and
then immersed in ethanol at 40 ꢁC for 24 h. Aer washing, the
precipitate was ltered and dried to give the side-chain
sulfonated copolymer S-SPAES(1/1).
2.4. Fabrication of proton exchange membranes
The crosslinkable copolymer membrane was fabricated via
a casting method, whose procedure is described as follows. The
obtained copolymer was rstly dissolved in DMSO to obtain
a 5 wt% solution and then the solution was ltered. The
resulting ltrate was cast onto a glass plate at 80 ꢁC, which was
ꢁ
dried at 100 C for 12 h. The as-cast salt form membrane was
immersed in 1 M HCl solution at 50 ꢁC for 48 h to achieve
proton exchange. Then, the membrane was soaked in water at
40 ꢁC for hours and thoroughly washed with deionized water
until the rinsed water became neutral. Aer drying in vacuum at
ꢁ
100 C for 12 h, the proton exchange membrane with a thick-
Scheme
2 Synthesis of the crosslinkable side-chain sulfonated
ness of 40–70 mm was obtained.
copolymers S-SPAES(x/y).
The crosslinked copolymers BP-DFDPS/DFDMED/DAB(x/y/z)
(coded as CS-SPAES(x/y)) were prepared via the cyclo-
condensation of DAB and the benzil group in the DFDMED
unit, where the quinoxaline cross-linkages were built in the
copolymer structures, as shown in Scheme 3. In the synthetic
procedures, the molar content of DAB was set to 30 mol% based
on DFDMED for all the crosslinked membranes; in other words,
z ¼ 0.3y. The crosslinked copolymer membranes could be
prepared via the following procedure. The copolymer S-
SPAES(x/y) in salt form was changed to the proton form via
soaking in 1 M HCl solution at 50 ꢁC for 48 h. The obtained
proton form S-SPAES(x/y) was ltered and washed three times
with deionized water, and then immersed in trimethylamine for
72 h to give the ammonium salt form copolymer. Then, it was
dissolved in m-cresol to obtain a 4 wt% solution and the ob-
tained solution was ltered. The resulting ltrate was added
into a 50 mL three-necked ask equipped with a magnetic
stirrer, a condenser, and a nitrogen inlet. Aer a given amount
Herein, the synthetic procedure of S-SPAES(1/1) is chosen as an
example and described below.
2.3.1. Preparation of DFDMED-based side-chain PAES
copolymer (S-PAES(1/1)-OCH3). To a 50 mL three-necked ask
equipped with
a magnetic stirrer, a Dean-Stark trap,
a condenser, and a nitrogen inlet, 4,40-biphenol (0.7262 g, 3.90
mmol), DFDPS (0.4958 g, 1.95 mmol), DFDMED (0.5972 g, 1.95
mmol), anhydrous potassium carbonate (0.5929 g, 4.29 mmol),
12.0 mL of DMSO, and 15.0 mL of toluene were added. Aer
stirring at 130 ꢁC for 4 h, the reaction mixture was slowly heated
to 150 ꢁC to remove the azeotrope of toluene and the produced
water. Then, the solution was stirred at 150 ꢁC for about 3 h till
a highly viscous solution was obtained. The resulting solution
was poured into water to obtain a ber-like precipitate. Then,
the copolymer was thoroughly washed with water and meth-
anol, and dried at 120 ꢁC in vacuum.
2.3.2. Preparation of demethylated side-chain copolymer
(S-PAES(1/1)-OH). To a 500 mL three-necked ask equipped
with a magnetic stirrer, a condenser, and a nitrogen inlet, 1.69 g
of S-PAES(1/1)-OCH3 and 150 mL of chloroform were added.
Aer the copolymer was dissolved completely under nitrogen
protection, 4.9 mL of boron tribromide in 50 mL of dichloro-
methane was slowly added at ꢀ30 ꢁC and then, the reaction
ꢁ
carried out at ꢀ30 C for 5 h. Aer the reaction, the resulting
solution was poured into ice water to obtain a white precipitate.
The precipitate was ltered and washed to neutral pH, and then
dried to give the demethylated copolymer S-PAES(1/1)-OH.
2.3.3. Preparation of the side-chain sulfonated copolymer
(S-SPAES(1/1)). To a 250 mL three-necked ask equipped with
a magnetic stirrer, a condenser, and a nitrogen inlet, 1.42 g of S-
PAES(1/1)-OH and 100 mL of DMSO were added. Aer the
copolymer was dissolved completely under nitrogen protection,
1,4-butanesulfonic acid lactone (0.92 g, 6.76 mmol) and
Scheme 3 Synthesis of the crosslinked side-chain sulfonated copol-
ymers CS-SPAES(x/y).
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of DAB was added in the ltrate, the solution was stirred at where ts and ws represent the thickness and width of the swollen
120 ꢁC for 3 h and then 140 ꢁC for 4 h (the gelation took place if membrane, respectively, while d refers to the membrane width
the reaction was continued for a longer time). The resulting between the two electrodes.
solution was cast onto a glass plate at 80 ꢁC, which was dried at
The methanol permeability (PM) value was acquired using
ꢁ
ꢁ
100 C for 12 h and then cured at 180 C in vacuum for 5 h to 32 wt% methanol feed solution from a liquid permeation
promote crosslinking. The as-cast crosslinked ammonium salt apparatus composed of a water-swollen membrane sample, and
form membrane was immersed in 1 M HCl solution at 50 ꢁC for feed and permeate parts,27,28 where gas chromatography (Shi-
48 h to achieve proton exchange. Then, the crosslinked madzu GC2014C) was utilized to monitor the methanol
membrane was post-treated as mentioned above.
concentrations of the feed (Cf) and the permeate (Cp). The PM
was calculated from the following equation:
2.5. Characterization
PM ¼ CpVpL/(ACft)
The structures of the monomer and copolymers were charac-
terized through spectral methods, including infrared spectros-
copy (IR) on a Bruker Equinox 55 spectrometer with the
absorption spectra recorded in the range from 4000 to
400 cmꢀ1, proton nuclear magnetic resonance (1H-NMR) spec-
troscopy on a Bruker AV 300 instrument with tetramethyl silane
(TMS) as the internal standard, and mass spectroscopy on a GC/
EI-MS Thermo DSQ II instrument with m/z 50–650.
The molecular weights of the copolymers were measured via
a Waters-Breeze gel permeation chromatography (GPC) system,
while the mechanical property was studied using an Instron
3342 universal testing machine. Thermal stability was studied
using thermogravimetric analysis (TGA) with a TA 600SDT
where L, A, Vp, and t refer to the membrane thickness, effective
area, permeate volume, and time, respectively.
The microphase separation morphologies of the membranes
were determined on a transmission electron microscope (TEM,
Tecnai G2 F20 instrument). Firstly, the copolymers in the
proton forms were changed to the Ag+ forms via immersion in
AgNO3 solutions at 40 ꢁC for 72 h. Aer washing with deionized
water, the copolymers with Ag+ forms in DMSO solutions were
then cast onto copper grids for TEM measurement. The sample
ꢁ
is needed to be dried in vacuum at 80 C before the test. The
surfaces of the membranes were characterized via a Bruker Icon
Dimension atomic force microscope (AFM) and a Hitachi SU-
8020 ultra-high resolution scanning electron microscope (SEM).
instrumenꢀt1 in helium, in which the heating rate was
ꢁ
10 C min
.
3. Results and discussion
2.6. Measurements
3.1. Synthesis and characterization of DFDMED
Through-plane (Dtc) and in-plane (Dlc) dimensional changes were
studied to represent the degree of membrane swelling, while
water uptake (WU) studies reveal the membrane absorbability.
Both of them can be obtained via the difference in weight and
dimensions (thickness, diameter) for the circular membranes
before and aer soaking in water. WU, Dtc, and Dlc were
calculated via the following equations:
In the synthetic procedures, the preparation of 3,5-dimethox-
ybenzyl magnesium bromide is needed to be performed at
ꢀ10 ꢁC in a mixture solution of diethyl ether and toluene, where
toluene is utilized to increase the solubility of 3,5-dimethox-
ybenzyl bromide, while low temperature prevents self-coupling.
The chemical structure of DFDMED was characterized via
1H-NMR, IR, and GC/EI-MS. From the 1H-NMR spectrum
(Fig. 1), ve kinds of protons with the quantity ratio of
1 : 2:2 : 1:6 were conrmed, wherein a big peak at 3.83 ppm
refers to the dimethoxy protons and a peak at 6.94 ppm
WU ¼ [(W ꢀ Wd)/Wd] ꢂ 100%
Dtc ¼ [(t ꢀ td)/td] ꢂ 100%
Dlc ¼ [(l ꢀ ld)/ld] ꢂ 100%
where Wd, td, and ld represent the weight, thickness, and
diameter of the dry membrane and are obtained aer drying in
vacuum, while W, t, and l refer to the ones for the wet
membrane and are acquired by weighing the wet membrane
without water on the surface aer soaking at
a given
temperature.
Proton conductivity (s) was measured in deionized water at
a given temperature using a Hioki 3532-80 electrochemical
impedance spectroscopy instrument with frequency range from
100 Hz to 100 KHz, where a rectangular membrane and two
pairs of platinum plate electrodes were utilized to build
a sandwich test xture system. The s can be calculated from the
following equation:
s ¼ d/(tswsR)
Fig. 1 1H-NMR spectrum of DFDMED in DMSO-d6.
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Table 1 Molecular weights and reduced viscosities of the poly(arylene ether sulfone) copolymers
Mn (104 g molꢀ1
)
Mwa (104 g molꢀ1
)
Mw/Mn
hb (dL gꢀ1
)
a
Copolymer
S-PAES(1/1)-OCH3
S-PAES(2/3)-OCH3
S-PAES(1/2)-OCH3
4.57
4.28
4.00
8.01
6.86
6.43
1.75
1.60
1.61
0.82
0.75
0.71
a
Measured at 30 ꢁC using THF as the solvent and polystyrene as tge standard. b 0.5 g dLꢀ1 in CHCl3 at 30 ꢁC.
represents the phenyl protons between two methoxy groups. molecular weight distributions (Mw/Mn) of the copolymers are
This is in good accordance with the structure of DFDMED. From 1.32–1.58, indicating typical polycondensation. Further post-
the IR data, the absorption bands at 1695 and 1647 cmꢀ1 are sulfonation was carried out via demethylation and nucleophilic
assigned as the stretching vibrations of the C]O bond, while substitution with 1,4-butane sultone to prepare the crosslinkable
the peaks at 1596 and 1235 cmꢀ1 are ascribed to the vibrations side-chain sulfonated copolymers S-SPAES(x/y) (Scheme 2).
of the phenyl ring and the C–F bond, respectively. The strong
The structures of the copolymers S-SPAES(x/y) were
peaks at 1460 and 1368 cmꢀ1 are attributed to the vibration of conrmed via IR and 1H-NMR spectroscopy. From the IR
the C–H bond in the methoxy group. Meanwhile, the absorption spectrum of S-SPAES(1/1) in Fig. 2, the symmetric and asym-
bands at 855 and 671 cmꢀ1 are assigned as the C–H out-of-plane metric stretching vibrations of C]O in the ethanedione unit
bending vibrations for 1,3,5- and 1,2,3-substituted benzenoid appear at 1665 cmꢀ1, while the absorption peaks at 1040, 1065,
structures, respectively. From the GC/EI-MS data, the peak at m/ and 1105 cmꢀ1 are assigned to the vibration of O]S]O in the
z 306.22 with the relative intensity of 6% is assigned as the sulfonic acid group. In addition, the peaks at 1600, 1590, and
molecular ion peak, which is consistent with the calculated 1495 cmꢀ1 are attributed to the vibrations of the phenyl skel-
molecular weight of 306.26. In addition, the peak at m/z 165.13 eton. Fig. 3(a) and (b) demonstrate the 1H-NMR spectra of S-
with 100% relative intensity is ascribed to the elimination of the PAES(1/1)-OCH3 and S-PAES(1/1)-OH, respectively, while
diuorobenzoyl radical from the molecular ion species. These Fig. 3(c) reveals the 1H-NMR spectrum of S-SPAES(1/1). The
results prove that DFDMED has the structure proposed in characteristic signal at 3.71 ppm in Fig. 3(a) is ascribed to the
Scheme 1.
3.2. Characterization of the copolymers
As shown in Table 1, the prepared crosslinkable copolymers
bearing two pendant methoxyl groups (S-PAES(x/y)-OCH3) display
highly reduced viscosities of 0.71–0.82 dL gꢀ1, where the molar
ratio of DFDPS to DFDMED is set as 1/1, 2/3, and 1/2 for x/y,
respectively. Meanwhile, the copolymers have high molecular
weights with Mw and Mn of 6.43–8.01 ꢂ 104 g molꢀ1 and 4.00–
4.57 ꢂ 104 g molꢀ1 (Table S1 and Fig. S1 in the ESI†), respectively,
which indicates good random co-polycondensation. The
Fig. 3 1H NMR spectra of S-PAES(1/1)-OCH3, S-PAES(1/1)-OH, and S-
Fig. 2 IR spectra of S-SPAES(1/1) and CS-SPAES(1/1).
24776 | RSC Adv., 2020, 10, 24772–24783
SPAES(1/1).
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Table 3 Mechanical properties of the crosslinkable and crosslinked
aliphatic protons of the methoxy group in DFDMED, while the
signal at 7.96 ppm is identied as the aromatic proton in
DFDPS, suggesting good random co-polycondensation. Aer
demethylation, S-PAES(1/1)-OH displays much lower charac-
teristic methyl protons at 3.71 ppm in Fig. 3(b); meanwhile,
a new signal at 9.65 ppm is identied as the hydroxyl proton. It
is noted that the extent of demethylation is about 80% via
quantication from the 1H-NMR spectrum. For S-SPAES(1/1),
the aliphatic protons at 3.90, 2.50, and 1.70 ppm (Fig. 3(c))
indicate incorporation of pendant butanesulfonic acid groups
into the side-chains of the copolymer. The other 1H-NMR
spectra of the copolymers are shown in Fig. S2 and S3 in the
ESI.†
membranesa
Code
YM (GPa)
MS (MPa)
EB (%)
S-SPAES(1/1)
S-SPAES(2/3)
S-SPAES(1/2)
CS-SPAES(1/1)
CS-SPAES(2/3)
CS-SPAES(1/2)
0.93
0.97
0.92
1.09
1.13
1.00
40.95
42.86
48.63
44.61
49.26
55.63
7.21
10.98
7.13
6.32
7.48
6.62
a
YM: young's modulus; MS: maximum stress; EB: elongation at break.
The morphologies of the membranes are shown in Fig. 4 for
S-SPAES(1/1) and CS-SPAES(1/1) in the Ag+ forms, wherein the
bright and dark domains in the TEM images represent the
hydrophilic and hydrophobic phases, respectively. The
membranes display obvious microphase separations, which is
attributed to their side-chain sulfonated copolymer structures.
From Fig. 4(a) and (b), CS-SPAES(1/1) shows smaller average
hydrophilic domains than S-SPAES(1/1), which may be due to
the restraining aggregation of the sulfonic acid groups resulting
from the crosslinking. Meanwhile, the AFM images of the
membranes are shown in Fig. 5(a)–(f), wherein the dark and
bright domains are identied as the hydrophobic and hydro-
philic phases, respectively. It was found that both S-SPAES(x/y)
and CS-SPAES(x/y) exhibit good microphase separations, which
is consistent with the TEM results. From Fig. 5, it is noted that
the hydrophilic domain size of the uncrosslinked membrane is
3–20, 4–22, and 6–28 nm for S-SPAES(1/1), S-SPAES(2/3), and S-
SPAES(1/2), respectively. The sizes of the hydrophilic domains
show a slight enlargement with the increase in the sulfonated
units. For the crosslinked membranes, the hydrophilic domain
size is 1–8, 1–11, and 2–16 nm for CS-SPAES(1/1), CS-SPAES(2/3),
and CS-SPAES(1/2), respectively. Compared to the uncros-
slinked membranes, the crosslinked ones exhibit much smaller
hydrophilic domains, which is attributed to their crosslinked
copolymer structures. In addition, the SEM images in Fig. 6 and
S4 in the ESI† reveal that the obtained crosslinked and
uncrosslinked membranes have homogeneous morphologies
and dense membrane surfaces.
3.3. Characterization of the crosslinked copolymers
Fig. 2 shows the IR spectrum of the crosslinked sulfonated
copolymer CS-SPAES(1/1). Compared to the crosslinkable S-
SPAES(1/1), CS-SPAES(1/1) displays characteristic absorption
bands of the quinoxaline ring at 1424 cmꢀ1 (shoulder peak) and
the characteristic stretching vibration of the C–N bond at
1220 cmꢀ1, which indicates the formation of the quinoxaline
cross-linkage. Meanwhile, the crosslinked sulfonated copoly-
mers CS-SPAES(x/y) are insoluble in common aprotic solvents,
while the crosslinkable ones have good solubility in aprotic
solvents (Table 2). The solubility results further prove the
formation of crosslinking aer cyclo-condensation. In addition,
according to the feed ratio of DAB to DFDMED (0.3/1), the
maximum crosslinking degree is up to 60%.
3.4. Characterization of the copolymer membranes
The copolymer membranes are obtained from the solution
casting method, wherein the crosslinked membranes are
fabricated using cyclo-condensation reaction solutions.27,28
The membranes have good mechanical properties (Table 3),
where the Young's modulus (YM), maximum stress (MS), and
elongation at break (EB) are 0.92–0.97 GPa, 41–49 MPa, and
7.1–11.0% for the crosslinkable membranes S-SPAES(x/y),
while they are 1.00–1.13 GPa, 45–56 MPa, and 6.3–7.5% for the
crosslinked CS-SPAES(x/y), respectively. The results reveal that
CS-SPAES(x/y) has slightly better mechanical properties than
S-SPAES(x/y), which is ascribed to the crosslinking structure in
the membranes.
Table 2 Solubilities of the crosslinkable and crosslinked membranesa
Code
CHCl3
DMSO
NMP
DMAc
DMF
S-SPAES(1/1)
S-SPAES(2/3)
S-SPAES(1/2)
CS-SPAES(1/1)
CS-SPAES(2/3)
CS-SPAES(1/2)
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
++
++
++
ꢀ
++
++
++
ꢀ
++
++
++
ꢀ
++
++
++
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
a
DMSO: dimethyl sulfoxide; NMP: 1-methyl-2-pyrrolidone; DMAc: N,N-
dimethyl-acetamide; DMF: N,N-dimethylformamide; ++: soluble at
room temperature; ꢀ: insoluble.
Fig. 4 TEM images of the membranes in Ag+ forms. (a) S-SPAES(1/1),
(b) CS-SPAES(1/1).
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Fig. 5 AFM images of the membranes. (a) CS-SPAES(1/1), (b) CS-SPAES(2/3), (c) CS-SPAES(1/2), (d) S-SPAES(1/1), (e) S-SPAES(2/3), and (f) S-
SPAES(1/2).
WU represents the water absorbed in the membrane, which
can ensure the dissociation and transportation of the proton, as
well as the construction of an interconnected ion channel. In
general, a large WU contributes to enhanced proton conduc-
tivity; however, this will easily lead to excessive swelling and
thus cause dimensional mismatch in fuel cell applications.
From Table 4 and Fig. 7, it is observed that WU increases
steadily on promoting IEC from 1.40 to 2.01 mequiv. gꢀ1, and it
is signicantly enhanced with increasing temperature from 25
to 80 ꢁC, respectively. It is noted that CS-SPAES(1/2) exhibits
much lower WU (24.9%) than the crosslinkable S-SPAES(1/2)
(46.2%) at 80 ꢁC, which is attributed to its smaller IEC and
3.5. IEC, WU, dimensional change, and membrane stability
The basic properties of crosslinkable and crosslinked side-chain
sulfonated copolymer membranes are listed in Table 4. The IEC
values are calculated from a titration method, where 0.05 M
NaOH solution was chosen to titrate the released proton in the
soaking solution (15 wt% NaCl) containing the sample
membrane and phenolphthalein was utilized as the indicator.
From Table 4, it can be seen that the membranes have IECs of
1.68–2.01 mequiv. gꢀ1 for S-SPAES(x/y) and 1.40–1.72 mequiv. gꢀ1
for CS-SPAES(x/y), respectively, which are comparable to those of
the reported membranes.10–12,15,24,25,27–30
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Fig. 7 Temperature dependence of WU for the crosslinkable and
crosslinked membranes.
(S1, l of 27) and the corresponding crosslinked membrane R2
(CS1-1, l of 20) with comparable IECs.28
Fig. 6 SEM micrographs of the surfaces for the membranes. (a) CS-
The through-plane (Dtc) and in-plane (Dlc) membrane swelling
are shown in Table 4 and Fig. 8. Similar to WU, the dimensional
changes increase steadily on promoting the IEC from 1.40 to
2.01 mequiv. gꢀ1 and they are signicantly enhanced with
increasing temperature from 25 to 80 ꢁC. From Table 4, it can be
seen that the membranes exhibit the anisotropic membrane
swelling ratio (Dtc/Dlc) of 1.08–1.23, which suggests that the
membranes display slightly larger swelling in the thickness
than in the plane. This is different from the isotropic
membrane swelling of the main-chain SPAES reference
membranes R1 and R2.28 The difference in the membrane
swelling is attributed to their side-chain sulfonated copolymer
structures. The crosslinked membranes exhibit Dtc of 9.3–
12.6% and Dlc of 8.6–11.6% at 80 ꢁC, which are signicantly
lower than that of the crosslinkable membranes (Dtc of 14.1–
16.9% and Dlc of 11.6–14.2%), suggesting the effective inhibi-
tion in the polymer chain relaxation resulting from the cross-
linking structures. In addition, the side-chain S-SPAES(1/2) and
CS-SPAES(1/2) exhibit much smaller dimensional changes in
both the through-plane and in-plane directions than R1 (Dtc of
31% and Dlc of 29%) and R2 (Dtc of 18% and Dlc of 17%) in spite
of their comparable IECs,28 which is in accordance with the
above WU.
SPAES(2/3) and (b) S-SPAES(2/3).
crosslinking structures. In addition, CS-SPAES(1/2) has lower
WU than S-SPAES(1/1) (29.5%) in spite of its slightly higher IEC,
which reveals that the crosslinking restrains the relaxation of
the copolymer chains in water and thus leads to a decrease in
WU.
Another objective parameter related to the membrane
absorbability, namely, the number of absorbed water molecules
per ionic group (l), is utilized to analyze the WU of the
membranes with different IECs,12,15,25,27,28,37,38 which is calcu-
lated via equation, l ¼ (10 ꢂ WU)/(18 ꢂ IEC). The membranes
exhibit l values of 8–13 at 80 ꢁC (Table 4), which is comparable
to the reported main-chain SPAESs (IECs of 1.20–2.74 mequiv.
gꢀ1 and l of 9–48)27,28,37,38 and the side-chain SPAESs (IECs of
0.88–1.70 mequiv. gꢀ1 and l of 8–21)12,15,25 but lower than that of
Naon 117 (l of 19).10 The crosslinked membranes CS-SPAES(x/
y) show lower l (8) than the corresponding crosslinkable S-
SPAES(x/y) (10–13), which is in accordance with the above WUs.
In addition, the side-chain crosslinkable (S-SPAES(1/2)) and
crosslinked membranes (CS-SPAES(1/2)) display much lower l
values than the reported main-chain reference membrane R1
Table 4 Basic properties of the crosslinkable and crosslinked membranes
Size
WU (%)
25 ꢁ
l
changea (%)
Code
Membrane
IEC (mequiv. gꢀ1
)
C
80 ꢁ
C
25 ꢁ
C
80 ꢁC
Dtc
Dlc
Dt/laa
S-SPAES(1/1)
S-SPAES(2/3)
S-SPAES(1/2)
CS-SPAES(1/1)
CS-SPAES(2/3)
CS-SPAES(1/2)
BP-DFDPS/DFDMED(1/1)
BP-DFDPS/DFDMED(2/3)
BP-DFDPS/DFDMED(1/2)
BP-DFDPS/DFDMED/DAB(1/1/0.3)
BP-DFDPS/DFDMED/DAB(2/3/0.9)
BP-DFDPS/DFDMED/DAB(1/2/0.6)
1.68
1.88
2.01
1.40
1.67
1.72
19.8
26.9
33.9
10.1
13.4
15.3
29.5
38.3
46.2
20.6
22.7
24.9
7
8
9
4
4
5
10
11
13
8
8
8
14.1
15.7
16.9
9.3
11.2
12.6
11.6
12.8
14.2
8.6
9.8
10.4
1.22
1.23
1.19
1.08
1.14
1.21
a
At 80 ꢁC.
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Fig. 8 Temperature dependence of the in-plane dimensional changes
for the crosslinkable and crosslinked membranes.
Fig. 9 TGA curves of the crosslinkable and crosslinked membranes.
Membrane durability under hydroxy and/or hydroperoxy
radical attacks was evaluated commonly from the oxidative
stability in Fenton's reagent (3 wt% H2O2 + 2 ppm Fe2SO4) at
decomposition temperatures above 200 ꢁC indicate high
membrane stability for fuel cell application under common
operation temperature. In addition, CS-SPAES(x/y) shows
slightly higher thermal stability than S-SPAES(x/y) because of
their quinoxaline-based crosslinking, which is in good agree-
ment with the literature.27,28,37,38
ꢁ
80 C and the elapsed time (s), for which the membrane sheet
keeps its shape in the Fenton's reagent with periodical shaking
until it breaks into pieces, is chosen to represent the oxidative
stability. From Table 5, the crosslinkable membranes S-
SPAES(x/y) show comparable oxidative stability (s of 100–143
min) to R1 (s of 125 min), whereas the crosslinked membranes
CS-SPAES(x/y) exhibit much higher oxidative stability (s of 301–
351 min) than R2 (s of 260 min),28 which reveals that the side-
chain crosslinked copolymer structures are more suitable for
the preparation of durable membranes than the main-chain
ones. This is attributed to the good microphase separation
structure in the present membranes, which leads to low WU and
membrane swelling, thus restricting the radical damage
domains and enhancing the membrane durability. In addition,
CS-SPAES(1/2) shows much higher oxidative stability (s of 301
min) than the corresponding crosslinkable S-SPAES(1/2) (s of
100 min) and S-SPAES(1/1) (s of 143 min) with comparable IEC,
which is due to covalent crosslinking, as well as low WU and
dimensional changes. This is in agreement with the literature.22
The TGA curves in Fig. 9 reveal the thermal properties of the
membranes. Both S-SPAES(x/y) and CS-SPAES(x/y) exhibit three-
step degradation, wherein the sulfonic acid groups, pendant
alkoxy chain, and copolymer matrix decompose at 200–300 ꢁC,
300–400 ꢁC, and above 400 ꢁC, respectively. High
3.6. Conductivity and methanol permeability
Generally, proton conductivity (s) varies with the temperature
and humidity as well as IEC and WU of the membranes. Table 5
lists the s values in water and the apparent activation energy
(DEa) of the conductivity, while Fig. 10 shows the temperature
dependence of s for the obtained membranes. From Table 5
and Fig. 10, it can be seen that s enhances gradually with
increasing temperature from 25 to 80 ꢁC, and increases steadily
on promoting the IEC from 1.40 to 2.01 mequiv. gꢀ1, respec-
tively, which is similar to WU and the dimensional changes. The
s values are 144–180 mS cmꢀ1 (80 ꢁC) for S-SPAES(x/y) with IECs
of 1.68–2.01 mequiv. gꢀ1 and 102–157 mS cmꢀ1 for CS-SPAES(x/
y) with IECs of 1.40–1.72 mequiv. gꢀ1, respectively, where the
larger s values for the crosslinkable membranes are ascribed to
their higher IECs and larger hydrophilic domains (Fig. 4 and 5)
than those of the crosslinked ones. In addition, the apparent
activation energy (DEa) of s is evaluated for the membranes in
ꢁ
the temperature range of 25–80 C. The membranes have DEa
Table 5 The s, DEa, s, PM, and 4 values of the crosslinkable and crosslinked membranes
sa (mS cmꢀ1
)
Code
25 ꢁ
C
80 ꢁ
C
DEa (kJ molꢀ1
)
sb time (min)
PM (10ꢀ7 cm2 sꢀ1
c
)
4c (104 S cmꢀ3 sꢀ1
)
S-SPAES(1/1)
S-SPAES(2/3)
S-SPAES(1/2)
CS-SPAES(1/1)
CS-SPAES(2/3)
CS-SPAES(1/2)
40
57
67
28
39
46
144
163
180
102
145
157
20.2
16.5
15.7
20.5
18.3
18.1
143
123
100
351
329
301
2.72
4.37
7.12
1.02
1.31
1.64
14.7
13.0
9.4
27.5
29.8
28.0
a
In water. b s: oxidative stability; time: the elapsed time aer which the membranes became broken. c At 32 wt% methanol solution and 25 ꢁC.
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membranes have much higher s but signicantly lower WU than
the reference main-chain ones except R1 with comparable IECs.28
These results suggest that the side-chain sulfonated copolymer
structure probably contributes to the achievement of a better
balance between the WU and s via the introduction of DFDMED-
based hydrophilic units into the membranes. In addition, CS-
SPAES(1/2) exhibits larger or comparable s value (157 mS cmꢀ1
)
but much lower WU (24.9%) than the reported crosslinked
membranes with comparable IECs (R2, R6, and R7),10,27,28
whereas its s value and WU are signicantly lower than those of
R8 in spite of their comparable IECs.14
PM refers to the level of methanol crossover for PEM, which is
commonly utilized to evaluate fuel crossover in the polymer
electrolyte fuel cells to some extent. Meanwhile, another param-
eter (4) calculated from the ratio of s to PM is generally utilized to
assess the membrane performance.25,27,28,37,38 From Tables 5 and
6, it is observed that the present side-chain sulfonated copolymer
Fig. 10 Temperature dependence of s for the crosslinkable and
crosslinked membranes.
membranes show much smaller PM (1.02–7.12 ꢂ 10ꢀ7 cm2 sꢀ1
)
values of 15.7–20.5 kJ molꢀ1, which is comparable to the
ꢀ1
but higher 4 values (9.4–29.8 ꢂ 104 S cmꢀ3
s
) than the repre-
reference membranes (8–24 kJ molꢀ1) with IECs of 0.88–2.74
sentative Naon 112 (PM of 24 ꢂ 10ꢀ7 cm2 sꢀ1 and 4 of 4.2 ꢂ 104
S cmꢀ3 sꢀ1) and Naon 117 (PM of 16 ꢂ 10ꢀ7 cm2 sꢀ1 and 4 of 3.7
mequiv. gꢀ1
.
It is noted that the side-chain S-
10,24,25,27,28,37
SPAES(1/1) and CS-SPAES(1/2) exhibit higher s values (144 and
157 mS cmꢀ1) than the main-chain reference membrane R3
(Table 6, s of 120 mS cmꢀ1) and the crosslinked R2 (s of 141
mS cmꢀ1) with comparable IECs; in spite of this, the s of
S-SPAES(1/2) is lower than that of R1.28 This suggests that the
side-chain sulfonated copolymer structures are probably bene-
cial for building ion transport channels than the main-chain
ones. In addition, the s of CS-SPAES(1/2) is higher than that
of S-SPAES(1/1) with comparable IEC due to its slightly higher
ion concentration resulting from lower dimensional changes.
The comparisons of IEC, WU, and s values between the
membranes and some reported ones are summarized in Table 6.
The crosslinkable S-SPAES(1/2) displays signicantly larger s and
lower WU than the side-chain poly(arylene ether) membrane R5
ꢂ
104 cmꢀ3 sꢀ1).40,41 The S-SPAES(1/2) has lower PM
S
(7.12 ꢂ 10ꢀ7 cm2 sꢀ1) but higher 4 (9.4 ꢂ 104 S cmꢀ3 sꢀ1) than R1
(PM of 11.6 ꢂ 10ꢀ7 cm2 sꢀ1 and 4 of 8.4 ꢂ 104 S cmꢀ3 sꢀ1) and R4
(PM of 19.7 ꢂ 10ꢀ7 cm2 sꢀ1 and 4 of 7.7 ꢂ 104 S cmꢀ3 sꢀ1);28,39 this
is ascribed to its low membrane swelling. However, the cross-
linked membrane CS-SPAES(1/2) exhibits much lower PM
(1.64 ꢂ 10ꢀ7 cm2 sꢀ1) but signicantly larger 4 (28.0 ꢂ 104 S
cmꢀ3 sꢀ1) than the crosslinked R2 (PM of 4.9 ꢂ 10ꢀ7 cm2 sꢀ1 and
4 of 9.2 ꢂ 104 S cmꢀ3 sꢀ1) and R6 (PM of 5.5 ꢂ 10ꢀ7 cm2 sꢀ1 and 4
of 7.1 ꢂ 104 S cmꢀ3 sꢀ1),27,28 which is attributed to its low
membrane swelling resulting from the side-chain sulfonated
copolymer structures.
with comparable IEC;11 meanwhile, it shows reduced WU by 3.7. The single-cell performance
about 78% and decreased s by only about 43% than R4.39
The single-cell PEMFC performance of S-SPAES(1/2) was evalu-
Furthermore, the present side-chain sulfonated copolymer
ated at 60 ꢁC at 100% RH via a Ningbo Beit fuel cell test system,
Table 6 Comparison of IEC, WU, s, PM, and 4 among the PEMs
sb (mS cmꢀ1
)
PM
c
IEC
WUaa
(%)
(conc./temp.)
10ꢀ4
4
Code
Membrane
(mequiv. gꢀ1
)
25 ꢁ
C
80 ꢁ
C
(10ꢀ7 cm2 sꢀ1
)
(S cmꢀ3 sꢀ1
)
Ref.
S-SPAES(1/2)
CS-SPAES(1/2)
2.01
1.72
1.96
1.76
1.65
2.75
2.01
1.65
1.74
1.75
0.91
0.91
46.2
24.9
99
63
53
208
70 (90 ꢁC)
42
45.5
43
67
46
98
45
43
151
180
157
197
141
120
317
154
111
152
385
7.12 (32/25)
1.64 (32/25)
11.6 (32/25)
4.9 (32/25)
4.8 (32/25)
19.7 (3.2/25)
9.4
28.0
8.4
9.2
9.0
This
This
28
28
28
39
11
27
10
R1
R2
R3
R4
R5
R6
R7
R8
BP-SDFDPS/DFB (1/1)
BP-SDFDPS/DFB/DAB (1/1/0.2)
BP-SDFDPS/DFB (2/3)
SQMPAEK-2.5
SPAEK(X9.1Y8.8)
BP-CBFBS/DFB/DAB (1/1/0.2)
sPh-PESK-13
7.7
58 (30 ꢁC)
39
77
5.5 (32/25)
7.1
HBSPE/SGO 0.75
8.4 (32/25)
24 (10/30)
16 (8.0/30)
14
40
41
Naon 112
Naon 117
(100)
57
4.2
3.7
29
125
a
b
c
Water uptake at 80 ꢁC. In water at 25 (or 30) and 80 ꢁC. At the conditions of methanol concentration (wt%) and temperature (ꢁC) given in
parentheses.
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Research Funds for the Central Universities (GK201801001 and
GK201703030).
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Fig. 11 Polarization (open symbols) and power density (solid symbols)
curves of the single cell based on S-SPAES(1/2) membranes at 60 ꢁC in
100% RH condition.
as shown in Fig. 11. The maximum power density of the S-
SPAES(1/2)-based fuel cell was 314.5 mW cmꢀ2, which was
comparable to R5 with same the IEC (324 mW cmꢀ2).11 The S-
SPAES(1/2)-based membrane-electrolyte assembly (MEA) had
an acceptable single-cell PEMFC performance, which is
ascribed to its reasonably large s, low PM, and excellent
dimensional stability. Furthermore, it is known that the opti-
mization of MEA fabrication and the regulation of the fuel cell
operating conditions tend to improve the performance of the
MEA; the corresponding research is ongoing.
4. Conclusions
Herein, a new crosslinkable diuoro aromatic monomer
DFDMED is developed and a series of DFDMED-based cross-
linkable sulfonated copolymers was prepared via post-
sulfonation. Furthermore, the crosslinked copolymers were
obtained through quinoxaline-based crosslinking between S-
SPAES(x/y) and DAB. The crosslinkable and crosslinked
membranes generally have a higher s than the main-chain ones
because of their side-chain sulfonated copolymer structures,
which are benecial for building ion transport channels. S-
SPAES(1/2) with an IEC of 2.01 mequiv. gꢀ1 exhibits good
performance, such as a high s of 180 mS cmꢀ1, a low PM of 7.12
ꢂ 10ꢀ7 cm2 sꢀ1, a large 4 of 9.4 ꢂ 104 S cmꢀ3 sꢀ1, and
a maximum power density of 314.5 mW cmꢀ2, indicating its
possible application prospect in PEMFCs.
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