A novel guanidinium grafted poly(aryl ether sulfone) for high-performance hydroxide exchange membranes

Article abstract of DOI:10.1039/c0cc01834a

A novel poly(aryl ether sulfone) ionomer containing hexaalkylguanidinium groups was synthesized, and membranes formed from this polymer displayed large ionic clusters, high hydroxide conductivity, and excellent solubility in low boiling point water-soluble solvents such as ethanol and methanol.

Full text of DOI:10.1039/c0cc01834a

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A novel guanidinium grafted poly(aryl ether sulfone) for
high-performance hydroxide exchange membranesw
Qiang Zhang,ab Shenghai Lib and Suobo Zhang*b
Received 11th June 2010, Accepted 7th September 2010
DOI: 10.1039/c0cc01834a
A novel poly(aryl ether sulfone) ionomer containing hexa-
alkylguanidinium groups was synthesized, and membranes
formed from this polymer displayed large ionic clusters, high
hydroxide conductivity, and excellent solubility in low boiling
point water-soluble solvents such as ethanol and methanol.
polystyrene and are not very stable in alkaline environments.
For the design of novel HEMs, aromatic ionomers are
considered promising alternative materials for fuel cell
applications due to their high thermal stability, excellent
mechanical strength, and superior chemical resistance in acidic
and alkaline conditions. To date, most HEMs have been
constructed mainly from poly(aryl ether sulfone),9 poly-
A transition to renewable and low-carbon forms of energy is
being widely debated as a means of securing a sustainable
future for mankind.1 Fuel cells have the potential to provide
clean and efficient energy sources for stationary, traction, and
portable applications.2 Of the several different types of fuel
cells under development today, polymer electrolyte fuel cells
(PEFCs) have been recognized as a potential future power
source for zero-emission vehicles.3 Polymer electrolyte
membranes are one of the main components of PEFCs. Two
types of solid polymer electrolyte have been studied in fuel cell
research: proton exchange polymer membranes (PEMs) and
hydroxide exchange polymer membranes (HEMs). Over the
past decade, vast resources have been devoted to developing
PEMs.4 Although PEMs exhibit excellent chemical, mechan-
ical, and thermal stability as well as high ionic conductivity,
several disadvantages, such as slow anode kinetics, parasitic
methanol crossover, and the requirement for expensive platinum-
based catalysts, have obstructed the commercialization of
proton exchange membrane fuel cells (PEMFCs).5 To overcome
these hurdles, HEMs were developed. Hydroxide exchange
membrane fuel cells (HEMFCs) have numerous advantages
over PEMFCs in terms of both cathode kinetics and ohmic
polarization. The inherently faster kinetics of the oxygen
reduction reaction in an alkaline fuel cell allow the use of
non-noble and low-cost metal electrocatalysts, such as silver
and nickel. Fuel oxidation is more facile in alkaline media than
in acidic media.6 In addition, the direction of hydroxide ion
conduction opposes that of methanol crossover, thereby
mitigating or possibly eliminating fuel crossover.7
(phenylene),10
poly(2,6-dimethyl-1,4-phenylene
oxide),11
radiation-grafted PVDF, ETFT, and FEP.7b,12 Their
development, however, has been hampered by the poor solubility
and low hydroxide conductivity of the HEMs. One of the most
desirable properties of an ionomer for use in a catalyst layer is
excellent solubility in low boiling point water-soluble solvents,
such as ethanol, because these solvents are easy and safe to
handle, and they are removed during electrode preparation.9a
Because most of the polymers used as HEMs were not soluble
in ionomer form, their ability to form an efficient three-phase-
boundary structure in the catalyst layer has been questionable,
thereby limiting performance.13 The most commonly used
material for hydroxide exchange is a quaternary ammonium
hydroxide containing-polymer that exhibits poor hydroxide
conductivity, partly due to incomplete ionization of quaternary
ammonium hydroxide.10–12
Hexaalkylguanidinium salts are used as ionic liquids due to
their intrinsically useful properties, such as negligible vapor
pressures, elevated thermal and chemical stabilities under
thermal, basic, acidic, nucleophilic and oxidative conditions,
and large electrochemical windows.14 Furthermore, the pKa
of pentaalkylguanidine (pKa
= 13.8) is higher than
that of trimethylamine (pKa = 10.8), indicating that hexa-
alkylguanidinium hydroxide is a stronger base than quaternary
ammonium hydroxide.15 Due to their strongly basic character,
guanidines are considered superbases.16 Their high alkalinity
yields a high concentration of mobile hydroxide ions. More
recently, we have prepared novel HEMs that contain hexa-
alkylguanidinium functionalities exhibiting high hydroxide
conductivity and outstanding alkaline stability.17 They were
prepared by chloromethylation of the polysulfone, followed by
reaction with 1,1,2,3,3-pentamethylguanidine to form hexa-
alkylguanidinium groups. However, the chloromethylation
reaction is usually restricted due to the use of a toxic reagent,
chloromethyl methyl ether18 and the lack of precise control
over the degree and location of functionalization.6,19
Recently, Varcoe and co-workers reported the first example
of metal cation-free alkaline membrane electrode assemblies
for use in all-solid-state alkaline fuel cells with long-term
performance stability.8 The success of these assemblies has
inspired increasing interest in HEMs. Commercially available
anion exchange membranes are typically based on cross-linked
a Graduate School of Chinese Academy of Sciences,
Beijing 100049, China
In the present study, hexaalkylguanidinium hydroxide was
attached to poly(aryl ether sulfone), abbreviated PES-G-OH.
The synthesis of this polymer is illustrated in Scheme 1. We
developed a novel side chain type monomer (PPH-MPDA),
3,3-bis(4-hydroxyphenyl)-2-(3-(methylamino)propyl)isoindolin-
1-one, which contained an aliphatic diamine group. The
aromatic poly(aryl ether sulfone) with pendant diamine
b Key Laboratory of Advanced Ecomaterials,
Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, China.
E-mail: sbzhang@ciac.jl.cn; Fax: +86-431-85685653;
Tel: +86-431-85262118
w Electronic supplementary information (ESI) available: Experimental
procedures. See DOI: 10.1039/c0cc01834a
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Scheme 1 Synthesis of poly(aryl ether sulfone) containing hexa-
alkylguanidinium groups (PES-G-OH).
groups (PES-A) was obtained by reacting PPH-MPDA with
4,40-dichlorodiphenyl sulfone in the presence of K2CO3 as a
base and catalyst in NMP. Using typical polymerization
conditions, the aprotic dipolar reaction system could be
dehydrated using toluene until a final polymerization temperature
of 160–165 1C was reached. Careful control of the poly-
merization temperature (160–165 1C) was required to avoid
cross-linking. The hexaalkylguanidinium hydroxide-containing
polymer (PES-G-OH) was prepared by the reaction of Vilsmeier
salt and the diamine groups in PES-A, followed by hydroxide
ion exchange. The structure of the polymer was analyzed by
1H and 13C NMR analyses to examine the change in molecular
structure during the synthetic process (see ESIw). The peaks at
2.10 and 2.16 ppm were assigned to the chemical shifts of
protons on, respectively, the methyl or methylene groups
attached to the nitrogen atom in PES-A. After the reaction,
these peaks disappeared and new peaks emerged between
2.65 and 2.90 ppm that were assigned to the proton of the
hexaalkylguanidinium hydroxide. The 13C NMR spectra
also verified the structure of PES-G-OH. In addition to the
emergence of new peaks between 46.97 and 35.63 ppm, the
peak at 162.46 ppm, which corresponded to the carbon
atom at the centre of the hexaalkylguanidinium group, was
observed. 1H and 13C NMR revealed that the reaction proceeded
quantitatively.
Fig. 1 TEM image of Pd2+-stained membrane: PES-G-OH, IEC =
1.39 meq/g.
uptake (35.8% for PES-G-OH) and swelling ratio (15.2% for
PES-G-OH).
Transmission electron microscopy (TEM) analyses were
performed in the palladium form (Fig. 1). In the TEM image,
the ionic domains appear as a dark area due to their affinity
2ꢀ
with PdCl4 anion, and the white areas represent the hydro-
phobic regions of the polymer nanostructure.9b As can be seen
in Fig. 1, ionic clusters of PES-G-OH, recognized as dark
areas (ca. 40–50 nm in diameter) in the TEM images, could be
clearly observed. The average size of the ionic regions in the
PES-G-OH membrane was markedly larger than that in the
membranes containing quaternary ammonium groups, reported
by Zhuang’s group,9b which displayed ionic clusters tens of
nanometres in size. The excellent phase-separated morphology
of the PES-G-OH membrane was attributed to two factors: bulky
volume and strong hydrophilicity of the hexaalkylguanidinium
groups.
Perhaps the most important property of membranes
employed in HEMFC applications is the hydroxide conductivity.
Fig.
2 shows the dependence of membrane hydroxide
conductivity on temperature over the range 15–60 1C. For
comparison, the conductivity of Nafion 117 was also measured
under the same conditions. By way of comparison, Coates’
Recently, Hickner et al. reported a process in which the
polymer converted to its ionic form in solution and formed
a membrane, and they demonstrated that the membrane
promoted ionic aggregation and, thus, higher conductivity.20
In such a process, solubility is a key property of ionomers used
to form HEMs. The ionomer described here exhibited
outstanding solubility in polar aprotic solvents, such as
DMF, DMAc, DMSO, and NMP. Membranes were prepared
by dissolving the ionomers in NMP followed by casting on a
levelled Teflon sheet. Above all, the PES-G-OH ionomer
exhibited excellent solubility in low boiling point water-soluble
solvents (Table S1), such as ethanol and methanol, which was
similar to the solubility of quaternary phosphonium-containing
polymers reported by Yan and co-workers.9a The PES-G-OH
ionomer was insoluble in pure water, even at 100 1C, suggesting
that it could be used in the catalyst layer without incurring loss
due to dissolution.
The water uptake, swelling ratio, and ion exchange
capacity (IEC) of the membrane are reported in Table S2.
At 60 1C, PES-G-OH membrane exhibited a moderate water
Fig. 2 Temperature dependence of the hydroxide ion conductivity of
the membranes.
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HEMs (IEC = 1.40 meq/g) exhibited 0.028 S cmꢀ1 at 50 1C.21
We recently reported tetraalkylammonium-functionalized
HEM (IEC = 1.47 meq/g) conductivities of 0.018 S cmꢀ1 at
60 1C.9d The hydroxide conductivities of the PES-G-OH
membrane (IEC = 1.39 meq/g) were 0.037 S cmꢀ1 at 50 1C,
and 0.42 S cmꢀ1 at 60 1C, respectively. The PES-G-OH
membrane showed a hydroxide conductivity that was much
higher than those of the tetraalkylammonium-functionalized
HEMs mentioned above. This phenomenon was attributed to
the strong basicity of the hexaalkylguanidinium hydroxide
which provided a high concentration of mobile hydroxide
ions.
Notes and references
1 D. A. J. Rand and R. M. Dell, Hydrogen Energy Challenges
and Prospects, RSC, Cambridge, UK, 2008.
¨
2 M. Unlu, J. F. Zhou and P. A. Kohl, J. Phys. Chem. C, 2009, 113,
¨
11416–11423.
3 R. Bashyam and P. Zelenay, Nature, 2006, 443, 63–66.
4 (a) K. Miyatake, T. Shimura, T. Mikami and M. Watanabe, Chem.
Commun., 2009, 6403–6405; (b) S. Seesukphronrarak and A. Ohira,
Chem. Commun., 2009, 4744–4746.
5 (a) X. Kong, K. Wadhwa, J. G. Verkade and K. Schmidt-Rohr,
Macromolecules, 2009, 42, 1659–1664; (b) Y. Wan, B. Peppley, K.
A. M. Creber and V. T. Bui, J. Power Sources, 2010, 354, 23–31.
6 G. Wang, Y. Weng, D. Chu, R. Chen and D. Xie, J. Membr. Sci.,
2009, 332, 63–68.
7 (a) N. J. Robertson, H. A. Kostalik IV, T. J. Clark, P. F. Mutolo,
H. D. Abruna and G. W. Coates, J. Am. Chem. Soc., 2010, 132,
3400–3404; (b) J. R. Varcoe, R. C. T. Slade, E. L. H. Yee,
S. D. Poynton, D. J. Driscoll and D. C. Apperley, Chem. Mater.,
2007, 19, 2686–2693.
The thermal stability of the PES-G-OH membrane (TGA
curve) is reported in Fig S3. The PES-G-OH polymer showed
a 5% weight loss at 327 1C, which was 60 1C higher than that
of membranes containing quaternary ammonium hydroxide
groups (360 1C).9d As expected, the PES-G-OH membrane
exhibited a higher stability than the tetraalkylammonium-
functionalized membrane. In addition to excellent thermal
stability, PES-G-OH exhibited outstanding alkaline stability.
PES-G-OH membrane maintained its ionic conductivity even
after immersion in 10 M NaOH solution at 25 1C and 2 M
NaOH solution at 80 1C for 24 h (see Table S3 in the
Supporting Information). By contrast, the commercially
available AMI-7001S, MA-3475 and AMB-SS anion exchange
membranes soaked in 1 M KOH underwent severe color
change, which was observed in as little as 5 min.22 This
phenomenon has been attributed to deterioration of the
chemical structure.22 No change in color for PES-G-OH
membrane was observed during any soaking period, suggesting
negligible chemical deterioration. And the PES-G-OH
membrane maintained its ionic conductivity after immersion
in either DI water or 1 M NaOH for one week at room
temperature, indicating good alkaline stability.
8 J. R. Varcoe, R. C. T. Slade and E. L. H. Yee, Chem. Commun.,
2006, 1428–1429.
9 (a) S. Gu, R. Cai, T. Luo, Z. Chen, M. Sun, Y. Liu, G. He and
Y. Yan, Angew. Chem., Int. Ed., 2009, 48, 6499–6502; (b) J. Pan,
S. Lu, Y. Li, A. Huang, L. Zhuang and J. Lu, Adv. Funct. Mater.,
2010, 20, 312–319; (c) S. Lu, J. Pan, A. Huang, L. Zhuang and
J. Lu, Proc. Natl. Acad. Sci. U. S. A., 2008, 105, 20611–20614;
(d) J. Wang, Z. Zhao, F. Gong, S. Li and S. Zhang, Macro-
molecules, 2009, 42, 8711–8717.
10 (a) M. R. Hibbs, C. H. Fujimoto and C. J. Cornelius,
Macromolecules, 2009, 42, 8316–8321; (b) E. E. Switzer,
T. S. Olson, A. K. Datye, P. Atanassov, M. R. Hibbs, C. Fujimoto
and C. J. Cornelius, Electrochim. Acta, 2010, 55, 3404–3408.
11 (a) Y. Wu, C. Wu, J. R. Varcoe, S. D. Poynton, T. Xu and Y. Fu,
J. Power Sources, 2010, 195, 3069–3076; (b) L. Wu and T. Xu,
J. Membr. Sci., 2008, 322, 286–292; (c) Y. Wu, C. Wu, T. Xu,
X. Lin and Y. Fu, J. Membr. Sci., 2009, 338, 51–60.
12 (a) R. C. T. Slade and J. R. Varcoe, Solid State Ionics, 2005, 176,
585–597; (b) T. N. Danks, R. C. T. Slade and J. R. Varcoe,
J. Mater. Chem., 2002, 12, 3371–3373.
13 (a) G. Wang, Y. Weng, D. Chu, D. Xie and R. Chen, J. Membr.
Sci., 2009, 326, 4–8; (b) M. R. Hibbs, M. A. Hickner, T. M. Alam,
S. K. McIntyre, C. H. Fujimoto and C. J. Cornelius, Chem. Mater.,
2008, 20, 2566–2573; (c) J. S. Park, S. H. Park, S. D. Yim,
Y. G. Yoon, W. Y. Lee and C. S. Kim, J. Power Sources, 2008,
178, 620–626.
14 N. M. M. Mateus, L. C. Branco, N. M. T. Lourenc
C. A. M. Afonso, Green Chem., 2003, 5, 347–352.
15 R. S. Blackburn, A. Harvey, L. L. Kettle, J. D. Payne and
S. J. Russell, Langmuir, 2006, 22, 5636–5644.
16 T. Isobe, K. Fukuda and T. Ishikawa, J. Org. Chem., 2000, 65,
7770–7773.
17 J. Wang, S. Li and S. Zhang, Macromolecules, 2010, 43,
3890–3896.
18 (a) A. Blair and N. Kazerouni, Cancer, Causes Control, 1997, 8,
473–490; (b) S. Laskin, M. Kusschner, R. T. Drew, V. P. Capiello
and N. Nelson, Arch. Environ. Health, 1971, 23, 135–142.
19 M. Tanaka, M. Koike, K. Miyatake and M. Watanabe, Macro-
molecules, 2010, 43, 2657–2659.
20 J. Yan and M. A. Hickner, Macromolecules, 2010, 43, 2349–2356.
21 T. J. Clark, N. J. Robertson, H. A. Kostalik IV, E. B. Lobkovsky,
P. F. Mutolo, H. D. Abruna and G. W. Coates, J. Am. Chem. Soc.,
2009, 131, 12888–12889.
In summary, a novel side chain-type poly(aryl ether sulfone)
containing hexaalkylguanidinium groups was prepared via
typical nucleophilic substitution polymerization followed by
the reaction of the diamine groups with Vilsmeier salt. PES-G-
OH displayed excellent solubility in some low boiling point
water-soluble solvents, methanol and ethanol, and outstanding
stability. Enhanced phase separation and high hydroxide
conductivity were observed, which constituted improvements
over conventional quaternary ammonium hydroxide-containing
polymers.
¸ o and
This research was financially supported by the National
Basic Research Program of China (No. 2009CB623401), the
National Science Foundation of China (No. 50973106,
21074133 and 50825302), the Scientific and Technological
Planning Projects of the Ji Lin Province (No. 20080117), and
the Development of Scientific and Technological Project of the
Jilin Province (No. 20080620).
22 J. A. Vega, C. Chartier and W. E. Mustain, J. Power Sources, 2010,
195, 7176–7180.
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