Anion exchange membranes containing no β-hydrogen atoms on ammonium groups: Synthesis, properties, and alkaline stability

Article abstract of DOI:10.1039/d0ra09308d

Novel anion conductive polymer membranes have been designed and synthesized to investigate whether the absence of β-hydrogen atoms of ammonium groups affects the membranes' properties and chemical stability. The hydrophilic monomer, 2,2-bis(4-chlorobenzyl)-2-phenyl-ethylamine (3), was obtained via a two-step reaction with an overall yield of 98% under mild reaction conditions. Ni(0)-promoted copolymerization of 3 with 2,2-bis(4-chlorophenyl)hexafluoropropane (1) afforded high molecular weight copolymers (Mn = 12.8-19.6 kDa, Mw = 82.1-224.6 kDa). After quaternization with iodomethane, QBAF-BS polymers formed bendable, robust membranes from solution casting. The ion exchange capacity (IEC) of the membranes ranged from 1.50 to 2.44 mequiv. g-1. The membranes exhibited high hydroxide ion conductivity in water (up to 191 mS cm-1 at 80 °C for IEC = 2.25 mequiv. g-1), suggesting that the newly designed hydrophilic structure was effective in improving the ion conductivity. Based on small-angle X-ray scattering (SAXS) analyses and transmission electron microscopy (TEM) images, all membranes featured nano-phase separated morphology with a large dependence on the copolymer composition. The strain properties were improved on increasing the content of the hydrophilic component up to IEC = 2.25 mequiv. g-1, above which the strain became smaller due to the larger water absorption. The membranes were not stable under harsh alkaline conditions (in 8 M KOH at 80 °C) gradually losing the hydroxide ion conductivity. Compared to our previous AEMs which contained typical aliphatic ammonium groups, the lack of β-hydrogen atoms did not practically improve the alkaline stability of AEMs possibly due to the main chain degradation but contributed to higher ion conductivity. This journal is

Full text of DOI:10.1039/d0ra09308d

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Anion exchange membranes containing no b-
hydrogen atoms on ammonium groups: synthesis,
properties, and alkaline stability†
bcd
Cite this: RSC Adv., 2021, 11, 1030
Daniel Koronkaa and Kenji Miyatake
*
Novel anion conductive polymer membranes have been designed and synthesized to investigate whether
the absence of b-hydrogen atoms of ammonium groups affects the membranes' properties and chemical
stability. The hydrophilic monomer, 2,2-bis(4-chlorobenzyl)-2-phenyl-ethylamine (3), was obtained via
a two-step reaction with an overall yield of 98% under mild reaction conditions. Ni(0)-promoted
copolymerization of
3 with 2,2-bis(4-chlorophenyl)hexafluoropropane (1) afforded high molecular
weight copolymers (Mn ¼ 12.8–19.6 kDa, Mw ¼ 82.1–224.6 kDa). After quaternization with iodomethane,
QBAF-BS polymers formed bendable, robust membranes from solution casting. The ion exchange
capacity (IEC) of the membranes ranged from 1.50 to 2.44 mequiv. gꢀ1. The membranes exhibited high
hydroxide ion conductivity in water (up to 191 mS cmꢀ1 at 80 ꢁC for IEC ¼ 2.25 mequiv. gꢀ1), suggesting
that the newly designed hydrophilic structure was effective in improving the ion conductivity. Based on
small-angle X-ray scattering (SAXS) analyses and transmission electron microscopy (TEM) images, all
membranes featured nano-phase separated morphology with a large dependence on the copolymer
composition. The strain properties were improved on increasing the content of the hydrophilic
component up to IEC ¼ 2.25 mequiv. gꢀ1, above which the strain became smaller due to the larger
water absorption. The membranes were not stable under harsh alkaline conditions (in 8 M KOH at 80 ꢁC)
gradually losing the hydroxide ion conductivity. Compared to our previous AEMs which contained typical
aliphatic ammonium groups, the lack of b-hydrogen atoms did not practically improve the alkaline
stability of AEMs possibly due to the main chain degradation but contributed to higher ion conductivity.
Received 2nd November 2020
Accepted 16th December 2020
DOI: 10.1039/d0ra09308d
rsc.li/rsc-advances
therefore, inexpensive components can be used in the cells such
as catalysts, bipolar plates, and even membranes.1–4 Currently,
1. Introduction
technical issues associated with AEMFCs are the membranes
that are prone to hydroxide ion-induced degradation under the
basic conditions. One of the dominant degradation pathways is
the Hofmann elimination in which hydroxide ions attack
hydrogen atoms at b-position to the quaternary nitrogen,
resulting in the formation of an alkene and a neutral tertiary
amine.5
Polymer electrolyte fuel cells (PEFCs) have been regarded as
a viable solution for green, efficient and sustainable energy
conversion devices that could replace old, environmentally
harmful technologies using fossil fuels. The most prominent
and widely researched type of PEFC is the so-called proton
exchange membrane fuel cell (PEMFC). They are, however,
rather costly which hinders the widespread dissemination of
PEMFCs. In recent years, anion exchange membrane fuel cells
(AEMFCs) have emerged as an alternative option, which are
potentially advantageous over PEMFCs. In contrast to PEMFCs,
the operating conditions for AEMFCs are not acidic but basic,
Several synthetic methods have been developed and inves-
tigated to overcome this issue including but not limited to
comb-shaped polymers,6 pendant ammonium head groups
strategy,7 polymer blends,8 cross-linking,9 graing,10 and
ammonium groups modications.11 One of the promising
approaches to improve the alkaline stability of AEMs is to
design structures that are devoid of or have a minimal number
aInterdisciplinary Graduate School of Medicine and Engineering, University of
Yamanashi, 4 Takeda, Kofu 400-8510, Japan
bFuel Cell Nanomaterials Center, University of Yamanashi, 4 Takeda, Kofu 400-8510,
of b-hydrogen atoms to suppress Hofmann degradation.12–15
A
Japan. E-mail: miyatake@yamanashi.ac.jp
good example is poly(aryl imidazoliums) (PBIs) containing
quaternary imidazolium groups in the main chain that showed
outstanding stability in 10 M KOH at 100 ꢁC for as long as 168 h
while having 10 mS cmꢀ1 of Clꢀ conductivity at 25 ꢁC in
water.16,17 For examples beyond other PBIs,18–20 great alkaline
stability was reported for blend membrane of poly(vinyl alcohol)
cClean Energy Research Center, University of Yamanashi, 4 Takeda, Kofu 400-8510,
Japan
dDepartment of Applied Chemistry, Waseda University, Tokyo 169-8555, Japan. E-mail:
kmiyatake@aoni.waseda.jp
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra09308d
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(PVA) and poly(diallyldimethylammonium chloride) (PDDA) to received. Dimethylsulfoxide-d6 with 0.03% TMS (DMSO-d6,
survive in 8 M KOH up to 360 h with a hydroxide ion conduc- Acros Organics) and chloroform-d1 (CDCl3, Acros Organics)
tivity of 25 mS cmꢀ1 in water at 25 ꢁC.21,22 Benzyl halides in the were used as received.
polymer backbone or side chains when reacted with trimethyl-
amine (TMA) or hexamethylenetetramine (HMTA) formed
structures that contained no or less b-hydrogen atoms to the
quaternary nitrogen.23,24 In these cases, however, a different
2.2. Synthesis of monomers
2,2-Bis(4-chlorophenyl)-hexauoropropane (1). 2,2-Bis(4-
degradation mode became dominant where the hydroxide ion aminophenyl)-hexauoropropane (1.0 g, 3.0 mmol) was mixed
attacked at the a-carbon leading to deamination. In most cases, with conc. HCl (10 mL) in a 100 mL round-bottom ask while
however, synthesis of polymers with no b-hydrogen atoms cooled with an ice-water bath. To this suspension, a solution of
contained complex processes and therefore, a potential indus- sodium nitrite (0.52 g, 7.5 mmol) in water (10 mL) was added
trial scale-up would be costly.
dropwise over a period of 30 min. Aer the addition, the mixture
Previously, we developed partially uorinated anion was allowed to stir at 0–5 ꢁC for 30 min. To the reaction mixture
conductive polymers that exhibited high hydroxide ion a pre-cooled solution of copper(I) chloride (1.512 g, 15.3 mmol)
conductivity and mechanical strength. Copolymer membranes in conc. HCl (10 mL) was added dropwise to maintain the
(QPAF-4) containing peruoroalkylene and pendant trimethy- reaction temperature below 5 ꢁC. Then, the mixture was stirred
lalkylammonium groups exhibited reasonable hydroxide ion at r.t. for 2 h. Aer the reaction, the mixture was extracted with
conductivity up to 76 mS cmꢀ1 at 80 ꢁC in water. QPAF-4 three portions of DCM (20 mL). The organic layers were
ꢁ
membrane was stable in 1 M KOH at 80 C for 1000 h without collected, washed with brine and dried over sodium sulfate. The
losing conductivity, however, unstable under severer conditions mixture was evaporated under reduced pressure to obtain crude
in 8 M KOH.25 More recently, hexauoroisopropylidene groups product as an orange viscous liquid. The crude product was
were investigated as hydrophobic components to repress water puried via ash chromatography (eluent: hexane) to afford
swelling and consequently improve hydroxide ion conductivity pure 1 as a white solid (0.783 g, 65%). 1H NMR (500 MHz,
(up to 134 mS cmꢀ1 at 80 ꢁC) compared to that of QPAF-4.26 CDCl3): d 7.30 (d, J ¼ 9.2 Hz, 4H), 7.36 (d, J ¼ 8.1 Hz, 4H).
Since these polymers shared the same pendant trimethylalky-
lammonium groups, alkaline (in)stability was similar.
2,2-Bis(4-chlorobenzyl)-2-phenylacetonitrile (2). Anhydrous
THF (40 mL) was added to a three-neck round-bottom ask
In the present paper, we designed a novel hydrophilic under nitrogen atmosphere while cooled with an ice-water bath.
structure with no b-hydrogen atoms at the ammonium groups Sodium hydride (2.0 g, 50 mmol) followed by phenyl acetonitrile
for our partially uorinated polymers. Unlike most of the (1.17 g, 10 mmol) was added with vigorous stirring. To the
previous polymers, precursor monomer and polymers, and the mixture, 4-chlorobenzyl bromide (5.13 g, 25 mmol) was added.
resulting quaternized copolymers could be synthesized in facile The mixture was stirred at r.t. under nitrogen atmosphere for
processes. Synthesis, structure, and chemical and physical 24 h. Then, the mixture was cooled again with an ice-water bath,
properties of the copolymer membranes are reported to reveal and water (20 mL) was added dropwise to quench the reaction.
the effect of the hydrophilic component.
The mixture was extracted with three portions of diethyl ether
(20 mL). The organic layers were collected, washed with brine
and dried over sodium sulfate. The mixture was evaporated
under reduced pressure to obtain crude product as an orange
2. Experimental section
2.1. Materials
viscous liquid. The crude product was triturated thoroughly
1
2,2-Bis(4-aminophenyl)hexauoropropane (>98.0%), 4-chlor- with hexane to afford pure 2 as a white solid (1.81 g, 98%). H
obenzyl bromide (>95.0%), phenylacetonitrile (>98.0%), sodium NMR (500 MHz, CDCl3): d 3.27 (s, 4H), 7.13 (t, J ¼ 7.5 Hz, 4H),
hydride (60% dispersion in paraffin liquid), tetrahydrofuran 7.8 (d, J ¼ 8.0 Hz, 4H).
anhydrous (stabilized with 2,6-di-tert-butyl-4-methylphenol,
2,2-Bis(4-chlorobenzyl)-2-phenyl-ethylamine (3). A three-
>99.5%), diethyl ether anhydrous (stabilized with 2,6-di-tert- neck round-bottom ask equipped with a magnetic stirring
butyl-4-methylphenol, >99.5%), cesium carbonate (>98.0%), bar, a nitrogen inlet and outlet, and a reux condenser was
iodomethane (>98.5%), and 2,20-bipyridyl (bpy, >99%) were charged with anhydrous diethyl ether (30 mL). While cooled
purchased from Tokyo Chemical Industry Co., Ltd, and used as with an ice-water bath, LAH (1.05 g, 27.6 mmol) followed by
received. Hydrochloric acid (35.0–37.0%), potassium hydroxide a solution of 2 (2.0 g, 5.43 mmol) in anhydrous diethyl ether (20
(>86%), sodium chloride (99%), sodium carbonate (>99.8%), mL) was added. The mixture was heated to reux (40 ꢁC) for 2 h.
sodium thiosulfate (>99.0%), sodium nitrate (>99%), sodium The mixture was cooled again with an ice-water bath, and the
hydrogen carbonate (>99.5%), sodium nitrite (>98.5%), silver reaction was quenched by the dropwise addition of water (1.0
nitrate aq. (0.01 M), dimethyl sulfoxide (DMSO, >99%), bis(1,5- mL), 20 wt% sodium sulfate aqueous solution (1.0 mL), and
cyclooctadiene)nickel(0) (Ni(COD)2, 95%), N,N-dimethylaceta- water (3.0 mL). Aer stirring for 30 min to deactivate any
mide (>99%), hexane (>96%), dichloromethane (DCM, >99.5%), unreacted LAH, anhydrous MgSO4 was added to the mixture
silica gel N60 (spherical, neutral, 100–210 mm), sodium sulfate and allowed to stir for another 30 min. The resulting white
(>98.5%), magnesium sulfate, anhydrous (>95.0%), diethyl precipitate was removed by ltration through a celite plug. The
ether (>99.0%) and lithium aluminum hydride (LAH, >92.0%) removal of diethyl ether solvent under reduced pressure affor-
were purchased from Kanto Chemical Co., Inc., and used as ded pure 3 as a colorless oil (2.0 g, 99%), which solidied over
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extended storage below 10 ꢁC to form a white crystalline alkaline stability were carried out as described in our previous
powder. 1H NMR (500 MHz, CDCl3): d 0.94 (s, 2H), 2.80 (s, 2H), work.19
3.08 (s, 4H), 7.07 (t, J ¼ 8.0 Hz, 4H), 7.19 (d, J ¼ 8.0 Hz, 4H), 7.21–
7.33 (m, 5H).
3. Results and discussion
3.1. Monomer syntheses
2.3. Polymerization
The hydrophobic monomer (1) was synthesized from 2,2-bis(4-
A three-neck round-bottom ask equipped with a magnetic
aminophenyl)hexauoropropane
via
Sandmeyer-reaction
stirring bar, a nitrogen inlet and outlet, and a reux condenser
was charged with 1 (336.5 mg, 0.90 mmol), 3 (311 mg, 0.83
mmol), bpy (680 mg, 4.36 mmol), and dry DMAc (6 mL) under
nitrogen atmosphere. The mixture was heated to 80 ꢁC with
stirring, and Ni(COD)2 (1.200 g, 4.36 mmol) was added. The
polymerization was carried out at 80 ꢁC for 5 h. Aer cooled to
r.t., the mixture was poured into a large excess of 1/1 (by
volume) mixture of methanol and conc. hydrochloric acid. The
resulting precipitate was washed with a mixture of methanol/
hydrochloric acid, 1.0 M K2CO3 aq. twice, and ultra-pure water
(18 MU). Aer drying in vacuum at 60 ꢁC overnight, BAF-BSx
(where x refers to the titrated IEC) polymer was obtained as
a white brous solid (519.8 mg, 93%).
(Scheme S1†), of which chemical structure was conrmed by
1H, 13C, and 19F NMR spectra (Fig. S1–S3†). The hydrophilic
monomer (3) was synthesized via a two-step reaction as shown
in Scheme 1. The successful bis-4-chlorobenzylation of the
phenylacetonitrile was suggested by the disappearance of the
methylene proton peak (at 4.4 ppm, Fig. S4†) of the starting
phenylacetonitrile in the 1H NMR spectrum (Fig. S5†). In
addition, methylene peaks of 4-chlorobenzyl groups were
observed at 3.3 ppm (Fig. S5†), which were at higher magnetic
eld than those of 4-benzylbromide at 3.8 ppm (Fig. S6†). In the
13C NMR spectrum, peaks were well-assigned to the structure
(Fig. S7†). The second step was the reduction of the nitrile
groups to primary amine groups using LAH. The reduction
reaction proceeded well to provide the target compound
without producing any by-products. The structure of the
product was also conrmed by 1H and 13C NMR spectra (Fig. S5
and S7†). The new hydrophilic monomer (3) was obtained in
>98% overall yield.
2.4. Quaternization and membrane casting
A typical procedure is as follows. BAF-BS (target IEC ¼ 1.50
mequiv. gꢀ1 for the resulting quaternized polymer) (450 mg,
0.65 mmol of the primary amino groups) was dissolved in
DMSO (10 mL) in a round-bottom ask. To the solution, cesium
carbonate (480 mg, 1.47 mmol) and iodomethane (3.14 g, 22.1
mmol) were added separately. The mixture was stirred under
3.2. Polymerizations
Polymerization reactions. Ni(0)-promoted polycondensation
nitrogen atmosphere at 40 ꢁC for 36 h, and then added dropwise of the monomers 1 and 3 was carried out under the conditions
into a mixture of water (200 mL), sodium chloride (50 g), similar to those for our previously reported (Scheme 2).27
sodium carbonate (5 g), and sodium thiosulfate (1.58 g) to Comonomer feed ratios were controlled to obtain the nal
precipitate quaternized QBAF-BS as a white solid in Clꢀ ion quaternized copolymers QBAF-BS with targeted ion exchange
form (addition of sodium carbonate and sodium thiosulfate was capacities (IEC) ranging from 1.50 to 2.50 mequiv. gꢀ1. In all
effective to avoid the formation of I2). The ꢁproduct was ltered, cases, the copolymers were obtained as white brous products
washed with ultra-pure water, dried at 60 C overnight to yield in reasonably high yield, typically between 80–93% (Table 1).
a white solid (500 mg, 99%). To obtain the membranes, solution The precursor BAF-BS copolymers were readily soluble in
casting method was used with DMSO as solvent (5 mL). The organic solvents such as chloroform, DMSO or DMAc. The
solution was cast onto a at glass surface at 55 ꢁC overnight. The molecular weights estimated from GPC analyses were Mn
obtained colorless, bendable membranes were washed with 12.8–19.6 kDa, Mw ¼ 82.1–224.6 kDa with relatively high poly-
¼
water to remove any residual DMSO solvent.
dispersity (PDI ¼ 5.3–12.2). Similarly high PDIs were obtained
for our previous copolymers (BAF-QAF and QPAF-4) having
tertiary amine groups under similar polymerization conditions,
meaning less controllable Ni(0)-promoted reaction.25,26 In Fig. 1
the 1H NMR spectrum of BAF-BS2.00 copolymer as a typical
example is shown, in which all peaks were well-assigned to the
supposed structure suggesting successful copolymerization
reaction. The copolymer compositions calculated from the peak
2.5. Measurements
Mechanical analyses (dynamic mechanical analysis, stress
versus strain tests) were carried out for the membranes in Clꢀ
forms to avoid the effect of atmospheric CO2. For alkaline
stability tests and hydroxide ion conductivity measurements,
membranes in OHꢀ ion forms were used. Ion exchange was
carried out by immersing the membranes in 1.0 M KOH solu-
tion for 24 h at 40 ꢁC. Aer the ion exchange reaction, the
membranes were washed with ultra-pure water for 24 h at 40 ꢁC.
2ꢀ
For TEM images, the membranes were ion-exchanged to PtCl4
ion forms. The properties measurements such as dynamic
mechanical analysis (DMA), stress versus strain (SS) curves,
hydroxide ion conductivity, water uptake, transmission electron
microscopy (TEM), small-angle X-ray scattering (SAXS), and (3).
Scheme 1 Synthesis of 2,2-bis(4-chlorobenzyl)-2-phenyl-ethylamine
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Fig. 1 1H NMR spectra of QBAF-BS in DMSO-d6 (top) and BAF-BS in
CDCl3 (bottom).
by the disappearance of the peak of the primary amine groups at
ca. 1.0 ppm and the appearance of the prominent peak of
methyl protons on the quaternary ammonium groups at
2.8 ppm. The polymer backbone remained intact based on the
fact that no shi was observed in the 19F NMR peaks (Fig. S8†).
Ion exchange capacity (IEC) values calculated from the peak
integrals in the 1H NMR spectra of QBAF-BS copolymers are
summarized in Table 1. The titrated IEC (via Mohr-titration)
and NMR-estimated IEC values were in fair agreement.
Casting from DMSO solution provided transparent, colorless
and bendable membranes from all quaternized copolymers
(Fig. S9†). Membrane exibility seemed similar to that of our
group's previously developed BAF-QAF26 regardless of its
composition (the chemical structure of BAF-QAF polymer is
shown in Fig. S10†).
3.3. Morphology
Scheme 2 Synthesis of QBAF-BS copolymers.
Cross-sectional TEM images of QBAF-BS membranes with
different IEC values, stained with tetrachloroplatinate ions are
shown in Fig. 2. The membranes exhibited phase-separated
1
integrals in the H NMR spectra were in good accordance with morphology with the dark areas ascribed to hydrophilic and
the feed comonomer composition within acceptable errors.
bright areas ascribed to hydrophobic domains, respectively.
Methylation or quaternization reaction of the precursor BAF- Both hydrophilic and hydrophobic domains were nearly
BS copolymers was then carried out with large excess of iodo- spherical in shape, where the hydrophilic domains were similar
methane in DMSO solution in the presence of cesium carbonate in size, around 1.1–1.3 nm in diameter regardless of their IEC
to obtain QBAF-BS copolymers. The resulting products were (1.3 ꢂ 0.32 nm for 1.50 mequiv. gꢀ1, 1.1 ꢂ 0.21 nm for 2.00
white, brous solids, and fully soluble in DMSO and DMAc, and mequiv. gꢀ1 and 1.2 ꢂ 0.24 nm for 2.44 mequiv. gꢀ1). The
even soluble in methanol when the counter ions were chloride average size of the hydrophobic clusters slightly increased as
ions, but insoluble in less polar solvents such as chloroform. 1H increasing IEC (1.22 ꢂ 0.23 nm for 1.50 mequiv. gꢀ1, 1.49 ꢂ
NMR spectrum of QBAF-BS suggested a complete quaterniza- 0.20 nm for 2.00 mequiv. gꢀ1 and 1.90 ꢂ 0.31 nm for 2.44
tion reaction by comparison with that of the precursor BAF-BS mequiv. gꢀ1). It is noted that the interface of the hydrophilic
in Fig. 1. The quantitative methylation reaction was suggested and hydrophobic domains was more distinct and sharper in
Table 1 Copolymer composition, target and obtained IECs, molecular weight and yield of QBAF-BS copolymers
Composition (1 : 3)
Feeda
Molecular weightc (kDa)
IEC (mequiv. gꢀ1
)
No.
Obtainedb
Mn
Mw
Mw/Mn
NMRb
Titration
Yield (%)
QBAF-BS1.50
QBAF-BS1.75
QBAF-BS2.00
QBAF-BS2.25
QBAF-BS2.50
1.07 : 1.00
0.76 : 1.00
0.51 : 1.00
0.34 : 1.00
0.19 : 1.00
1.17 : 1.00
0.74 : 1.00
0.51 : 1.00
0.35 : 1.00
0.22 : 1.00
12.8
18.1
16.4
18.4
19.6
84.5
6.6
8.6
11.1
12.2
5.3
1.39
1.52
1.99
2.24
2.32
1.50
1.63
2.02
2.25
2.44
93
88
82
87
80
155.6
181.9
224.6
104.0
a
The molar ratio of the comonomers in the polymerization. b Calculated from the peak integrals in the 1H NMR spectra. c Measured for BAF-BS
copolymers.
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Fig. 3 SAXS patterns of QBAF-BS membranes with IEC ¼ 1.50 mequiv.
g
ꢀ1, IEC ¼ 2.00 mequiv. gꢀ1, and IEC ¼ 2.44 mequiv. gꢀ1 in Clꢀ ion
forms at 30%, 50%, 70% and 90% relative humidity.
the TEM results, d-spacing slightly increased as increasing the
IEC. Furthermore, the smaller peak supported the above-
mentioned idea of less ordered (or more randomized)
morphology for the higher IEC membranes.
3.4. Water uptake and ion conductivity
Fig. 4 shows water uptake (WU) and l-value (number of absorbed
water molecules per ammonium group) of QBAF-BS (and BAF-
QAF for reference) membranes in OHꢀ ion forms at 30 ꢁC as
a function of the IEC. WU increased with increasing IEC to 86 wt%
for QBAF-BS2.25, and jumped to 260 wt% for the highest IEC
membrane QBAF-BS2.44. The l-value remained nearly constant
(ca. 20) up to IEC ¼ 2.25 mequiv. gꢀ1, and also jumped to 59 at the
highest IEC membrane. The large increase in l suggests that the
water molecules were located in the hydrophobic domains as well
Fig. 2 TEM images of QBAF-BS membranes with IEC ¼ 1.50 mequiv.
g
ꢀ1 (top), IEC ¼ 2.00 mequiv. gꢀ1 (middle), and IEC ¼ 2.44 mequiv. gꢀ1
(bottom), all stained with tetrachloroplatinate ions.
contrast for lower IEC membranes. The result suggests that
ionic groups were more incorporated into the hydrophobic
domains in the higher IEC membranes probably because of the
considerably larger composition of the hydrophilic components
causing larger apparent size of the hydrophobic clusters.
To investigate the morphology under humidied conditions,
ꢁ
small-angle X-ray scattering (SAXS) was measured at 40 C and
30–90% RH (relative humidity) for the QBAF-BS membranes in
Clꢀ ion forms. Fig. 3 shows the scattering intensity as a function
of the scattering vector (q) at different RH.
A distinct peak was observed at q ¼ 0.66 nmꢀ1 (or d-spacing
¼ 9.5 nm) for the membrane with IEC ¼ 1.50 mequiv. gꢀ1. As
the humidity increased, the peak slightly developed suggesting
that the peak was associated with the hydrophilic clusters. A
similar but much smaller peak was observed for higher IEC
membranes with only minor dependence on the humidity. The
peaks were roughly at q ¼ 0.73 nmꢀ1 (or d-spacing ¼ 8.6 nm)
and q ¼ 0.57 nmꢀ1 (or d-spacing ¼ 11.0 nm) for IEC ¼ 2.00
mequiv. gꢀ1 and IEC ¼ 2.44 mequiv. gꢀ1, respectively. Similar to
Fig. 4 Water uptake (at r.t.) and l-values of QBAF-BS and QBAF-BS
membranes as a function of ion exchange capacity.
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as in the hydrophilic domains in the highest IEC membrane. This
is probably not contradictory to the above-mentioned TEM images
and SAXS data, where higher IEC membranes contained less
ordered morphology. For comparison, BAF-QAF membranes
sharing the same hydrophobic component showed water uptake
increase from 40 wt% to 98 wt% and l increase from 14 to 23 for
1.30 and 2.40 mequiv. gꢀ1, respectively.26 A large jump of the water
uptake and l in BAF-QAF membrane at IEC ¼ 2.40 mequiv. gꢀ1
,
which was not observed for BAF-QAF membranes, might be
ascribed to the ammonium groups attached closer to the polymer
main chain. QBAF-BS membranes absorbed the same amount of
water at 80 ꢁC (Fig. S11†), suggesting good dimensional stability
in hot water.
The hydroxide ion conductivity of QBAF-BS membranes was
ꢁ
measured in water at 30 C and plotted as a function of IEC in
Fig. 5. Unlike BAF-QAF membranes whose conductivity
increased monotonously as increasing the IEC, QBAF-BS
Fig. 6 Temperature dependence of the hydroxide ion conductivity of
QBAF-BS membranes.
membranes exhibited
a maximum conductivity of 102.3
mS cmꢀ1 at IEC ¼ 2.25 mequiv. gꢀ1. The conductivity decreased
to 83.2 mS cmꢀ1 for the highest IEC membrane (2.44 mequiv.
gꢀ1). The excess water absorption at the highest IEC caused
signicant swelling of the membrane, resulting in the decrease
in practical (volumetric) IEC and thus, the decrease in the
conductivity. Similar behavior was observed with our previous
membranes with large water absorbability.25,28 Compared to
other quaternized polymer membranes devoid of b-hydrogen
atoms (conductivity smaller than 100 mS cmꢀ1 at 25 ꢁC),22,23
QBAF-BS membranes showed higher hydroxide ion conductivity
3.5. Mechanical properties
Dynamic mechanical analyses (DMA) of the QBAF-BS
membranes were carried out in Clꢀ ion forms. In the
humidity dependence at 80 ꢁC (Fig. 7a), storage (E0) and loss (E00)
moduli decreased gradually as increasing the humidity. No
glass transition behaviour was observed for the three
membranes in the tan d (¼ E0/E00) curves. Similarly, in the
temperature dependence at 60% RH, the viscoelastic properties
showed only minor changes without detectable peaks (Fig. 7b).
The results are similar to those of the BAF-QAF membranes,26
indicating that the viscoelastic properties and their
temperature/humidity dependences were mostly related with
the hydrophobic components.
(102.3 mS cmꢀ1 at 30 C) for its relatively low IEC values.
ꢁ
Fig. 6 shows the temperature dependence of the hydroxide
ion conductivity of the QBAF-BS membranes. All membranes
followed Arrhenius-type temperature dependence from 30 to
80 ꢁC with low apparent activation energies ranging between
10.2–11.7 kJ molꢀ1 suggesting a similar ion conduction mech-
anism for this series of membranes regardless of the IEC value.
The Ea values were also similar to those of the BAF-QAF
membranes (11–13 kJ molꢀ1), where migration of hydrated
hydroxide ions via vehicular mechanism was suggested.26
Overall, QBAF-BS2.25 showed the best-balꢁanced water uptake
ꢁ
and conductivity properties within the 30 C to 80 C temper-
ature range.
Fig. 7 (a) Humidity (at 80 ꢁC), and (b) temperature (at 60% RH)
Fig.
5 Hydroxide ion conductivity of QBAF-BS and BAF-QF dependence of DMA curves of QBAF-BS membranes and BAF-QAF
membranes (in water, at 30 ꢁC) as a function of ion exchange capacity. membrane for reference.
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The stress versus strain curves of the QBAF-BS membranes
were also measured in Clꢀ ion forms at 80 ꢁC and 60% RH
(Fig. 8). The results are summarized in Fig. 9. Young's modulus
slightly decreased with increasing IEC up to 2.25 mequiv. gꢀ1
and dropped at 2.44 mequiv. gꢀ1. The maximum stress at break
showed similar IEC dependence. The IEC dependence of those
properties is related with an increase in the absorbed water
which acted as a plasticizer decreasing the membrane stiff-
ness.29 In contrast, the maximum strain increased with
increasing the IEC up to 2.25 mequiv. gꢀ1, opposite IEC
dependence from those of the Young's modulus and maximum
stress. Although the membranes were bendable, the obtained
maximum strains were relatively small with the highest for
QBAF-BS2.25 (39%). Increase of the maximum strain with IEC
supported the idea that the absorbed water increased the elas-
ticity of the membranes. However, QBAF-BS2.44 showed a drop
in the maximum strain presumably due to its much larger water
absorbability (Fig. 4) and lower molecular weight (Mn ¼ 19.6
kDa, Mw ¼ 104 kDa) than those of QBAF-BS2.25 (Mn ¼ 18.4 kDa,
Fig. 9 Mechanical properties (Young's modulus, elongation at break
and maximum stress) of QBAF-BS membranes.
the major degradation modes that are unique to this novel
chemical structure. Furthermore, under the same conditions,
BAF-QAF membrane showed major degradation.26 Thus, severe
conditions served as
a logical reference conditions for
comparing QBAF-BS with BAF-QAF. Fig. 10 shows the remaining
conductivity (in water at 40 ꢁC) as a function of the testing time.
Hydroxide ion conductivity of low IEC membranes (IEC ¼ 1.50
and 1.63 mequiv. gꢀ1) was lost aer 300 h, with remaining
conductivities of 4 mS cmꢀ1 (6% of the initial conductivity) for
IEC 1.50 mequiv. gꢀ1 and 1 mS cmꢀ1 (1% of the initial
conductivity) for IEC 1.63 mequiv. gꢀ1, respectively. The
membrane with medium IEC (2.00 mequiv. gꢀ1) also showed
continuous degradation up to 150 h, at which the test was
terminated with a remaining conductivity of 22 mS cmꢀ1 (22%
Mw
¼
224.6 kDa) (Table 1). Compared with BAF-QAF
membranes, QBAF-BS membranes exhibited similar values
and IEC dependence of the Young's modulus (BAF-QAF ¼ 5.2–
8.9 GPa). QBAF-BS showed larger strain with a maximum of
38.8% for IEC 2.25 mequiv. gꢀ1 membrane (Fig. 8), compared to
13% for BAF-QAF (IEC 1.50 mequiv. gꢀ1) membrane, which
could be attributed to the hydrophilic component. The larger
molecular weights (Mw
¼
84.5–224.3 kDa) of QBAF-BS
membranes than those (Mw ¼ 77–115 kDa) of BAF-QAF
membranes were also accountable. It is concluded that the
viscoelastic properties were dominated by the hydrophobic
components while the strain properties were more related with
the hydrophilic components, absorbed water, and molecular
weight.
conductivity retention). The higher IEC (¼ 2.25 mequiv. gꢀ1
)
membrane showed minor loss of the conductivity up to 150 h,
however, the conductivity dropped aer this time period to 19
mS cmꢀ1 (85% loss of its initial conductivity) at 300 h. The
highest IEC membrane (¼ 2.44 mequiv. gꢀ1) showed severe
swelling under the testing conditions causing mechanical
failure that prohibited conductivity measurements. According
to the estimated IEC values from the remaining conductivity
values in Fig. 10, membranes with IEC < 2.00 mequiv. gꢀ1
featured degradation with similar rate of the conductivity loss. A
likely explanation for this result is in the increased activation
energy of the hydroxide ion attack with higher l-values at higher
IECs.30
3.6. Alkaline stability
The ability to withstand harsh alkaline conditions is a crucial
property of AEM membranes for alkaline fuel cell applications.
The accelerated alkaline stability test using highly concentrated
alkaline solution (8 M KOH) at elevated temperature (80 ꢁC)
used in this study was chosen not to reveal the expected life-
span of this membrane in fuel cell conditions but to reveal
Despite the lack of hydrogen atoms at b-position to the
quaternary nitrogen atoms, thereby eliminating the chance for
Hofmann degradation, QBAF-BS membranes showed major
ꢁ
loss of the hydroxide ion conductivity in 8 M KOH at 80 C. To
investigate the degradation mechanism of QBAF-BS
Fig. 8 Stress versus strain curves of QBAF-BS membranes at 80 ꢁC Fig. 10 Time course of hydroxide ion conductivity (at 40 ꢁC) of QBAF-
and 60% RH.
BS membranes in 8 M KOH at 80 ꢁC.
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membranes, 1H, 19F NMR and FTIR spectra were measured and
compared with those of the pristine membranes. In Fig. 11, 1H
NMR spectra of the QBAF-BS1.50 membrane is shown (see
Fig. S12 and S13† for other membranes), where the prominent
peak of the methyl groups attached to the quaternary trimethyl
ammonium groups at 2.82–2.83 ppm disappeared aer the
stability test. A faint peak appeared at 5.3 ppm for all
membranes (Fig. S14†). Furthermore, more prominent peaks
appeared at 0.8, 1.1, 1.85 and around 2.0 ppm. 19F NMR spectra
were nearly identical to those of the pristine samples (Fig. S15–
S17†). In the FTIR spectra (Fig. 12, S18, and S19†), the peak at
ca. 1245 cmꢀ1 assignable to the stretching vibration of C–N+
bonds became smaller and new peaks assignable to the
stretching vibration of CH3–N bonds appeared at 2767 cmꢀ1
and 2816 cmꢀ1 in the post-test samples. Based on the NMR and
IR spectral data, suggested degradation mechanisms are
summarized in Fig. 13. The disappearance of the quaternary
ammonium peaks in the 1H NMR spectra together with the
appearance of a prominent peak at 1.95–2.10 ppm assignable to
methyl protons of tertiary amines suggests the demethylation of
the trimethylammonium groups as a major degradation mode,
leading to loss of IEC and conductivity (Fig. 13(i)). The changes
in the FTIR spectra also support this idea. The appearance of
Fig. 12 FTIR spectra of the pristine (black, 0 h) and post-test (red, 300
h) QBAF-BS1.50 membrane.
1
the proton signals at 1.85 ppm in the H NMR spectrum could
be the result of nucleophilic attack of the hydroxide ions on the
methylene groups connecting with the quaternary ammonium
groups to form alcohols (Fig. 13(ii)). In addition, the peaks at
5.3 ppm, and 0.8 and 1.1 ppm are indicative of phenolic protons
and aryl cyclopropane groups, respectively, formed possibly via
the mechanism shown in Fig. 13(iii). Loss of membrane exi-
bility observed for QBAF-BS would be a typical consequence of
main chain degradation and a decrease in molecular weight
(GPC measurement was unavailable for the quaternized poly-
mers due to the strong interactions with the GPC columns). The
post-test membranes were not strong enough for mechanical
properties measurements. Since the 19F NMR spectra showed
no changes, the hydrophobic component is believed to have
remained intact through the alkaline stability test. Overall, the
novel structure of QBAF-BS was effective in eliminating
Hofmann-type degradation, which was the major degradation
pathway for BAF-QAF polymers.26 However, the loss in ion
conductivity and mechanical strength was consequence of the
demethylation as major and nucleophilic main chain as minor
Fig. 13 Possible hydroxide ion-induced degradation mechanism of
QBAF-BS membranes and suggested chain cleavage mechanism
based on the post-test NMR and FTIR spectra.
degradation modes, respectively. In order to evaluate the fuel
cell performance and long-term stability of the membranes, we
tried preparation of catalyst coated membrane and fuel cell
evaluation several times, only ended up in mechanical failure of
the membranes. Higher molecular weight and/or smaller poly-
dispersity would be needed for mechanically durable
membranes. Since QBAF-BS is highly hydroxide ion conductive
and soluble in methanol, fuel cell test will be conducted both as
membrane and electrode ionomer (binder).
4. Conclusions
A series of novel anion conducting copolymers (QBAF-BS) con-
taining hexauoroisopropylidene and quaternary trimethyl-
amine groups with no b-hydrogen atoms have been synthesized
and characterized in detail. The copolymers showed excellent
membrane forming capability resulting in freestanding, stiff
Fig. 11 1H NMR spectrum of the pristine (black, 0 h) and post-test (red,
300 h) QBAF-BS1.50 membrane. The red asterisks (*) denote the new
peaks that appeared after the alkaline stability test.
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1038 | RSC Adv., 2021, 11, 1030–1038
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