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Journal of Membrane Science
j o u r n a l h o m e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / m e m s c i
Preparation and characterization of high-durability zwitterionic crosslinked
proton exchange membranes
Yun-Sheng Ye
a, Wen-Yi Chen
b, Yao-Jheng Huang
a, Ming-Yao Cheng
c, Ying-Chieh Yen
a,
Chih-Chia Cheng
a, Feng-Chih Chang
a,∗aInstitute of Applied Chemistry, National Chiao-Tung University, Hsin-Chu, Taiwan
bMaterial and Chemical Research Laboratories, Industrial Technology Research Institute, Chutung, Taiwan cGraduate Institute of Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan
a r t i c l e i n f o
Article history:
Received 28 November 2009 Received in revised form 31 May 2010 Accepted 2 June 2010
Available online 9 June 2010 Keywords: Fuel cells Polymer-electrolyte membrane Sulfonated polymer Crosslinked membranes Proton conductor
a b s t r a c t
The present paper describes the development of a novel proton exchange membrane comprising a poly (styrene sulfonic acid-co-4-vinylpyridine) copolymer that was crosslinked with a haloalkyl crosslinker through the formation of ionic bonding linkages. The nucleophilic substitution of these crosslinked mem-branes as well as a model reaction between pyridine and 1-bromobutane was confirmed by nuclear magnetic resonance (NMR). Tough and flexible membranes of a high mechanical strength were prepared. They demonstrated very elevated thermal, hydrolytic and oxidative stabilities as compared to other sul-fonated polymers. The crosslinked membrane composed of zwitterionic molecules with a crosslinking fraction of 90.3, denoted SP-1, exhibited a proton conductivity of ca. 7.1× 10−2S cm−1at 30◦C under 90% relative humidity; a value comparable to that of Nafion 117. Moreover, the crosslinked SP-1 membrane possessed the highest selectivity for methanol fuel cells (3.38× 105S cm−3s), approximately five times that of Nafion 117, thus implying its potential for practical use in high-energy-density devices.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Proton exchange membranes (PEMs) are key components in solid polymer electrolyte fuel cells (PEFCs) in which they provide an ionic pathway for proton transfer and prevent
mix-ing of the reactant gases [1–4]. A large number of PEMs
have been prepared from sulfonated aromatic polymers,
includ-ing sulfonated poly(aryl ether sulfone) (SPES) [5,6], sulfonated
polyphosphazene (SPOP) [7], poly(benzimidazole) (SPBI) [8,9],
sulfonated polyimide (SPI) [10–12], and sulfonated poly(ether
ether ketone) (SPEEK) [13,14]. In view of improving the
per-formance of PEMs, crosslinking appears to be an efficient and simple approach for reducing the degrees of methanol diffusion and water uptake while enhancing the mechanical proper-ties and dimensional stability. Numerous reports have been dedicated to the methods for crosslinking polymer-electrolyte membranes, and techniques include the ionic crosslinking of acid/base blend membranes (e.g., Nafion/polyaniline composites)
[15], the sulfonation of polysulfone/polybenzimidazole (SPSF/PBI)
[16]the UV-assisted photo-crosslinking of SPEEK[17–20], the
sul-fonation of poly(phthalazinone ether ketone) (SPPEK) [21], the
covalent crosslinked polyvinyl alcohol (PVA)[22,23], and the
cova-∗ Corresponding author. Tel.: +886 3 53131512. E-mail address:[email protected](F.-C. Chang).
lent crosslinked SPEEK [24,25]. Nevertheless, these crosslinked
polymer-electrolyte membranes usually display significant losses in proton conductivity due to low water uptake values caused by the crosslinking structure. Consequently, improving the chemical and mechanical stabilities of sulfonated polymer membranes with-out detrimentally affecting their proton conductivity and methanol crossover still remains an important challenge.
Although polymers derived from 4-vinylpyridine (4VP) have
been quaternized with alkyl halides[26,27]to form crosslinked
polymers for use in anion-exchange membranes[28,29], relatively
few studies have exploited the role of zwitterionic crosslinked
membranes in PEMs[30]. The present study describes the
synthe-sis of poly(styrene sulfonic acid-co-vinylpyridine) (NaSS-4VP) and its reaction with a crosslinker containing haloalkyl groups with the aim of forming zwitterionic crosslinked membranes exhibit-ing high oxidative and hydrolytic stabilities, adequate mechanical properties, a high proton conductivity, and a low methanol crossover relative to those of other sulfonated polymers.
2. Experimental
2.1. Materials
4-Styrenesulfonic acid sodium salt hydrate (NaSS),
4-vinylpyridine (4VP), poly(4-4-vinylpyridine) (P4VP), potassium
disulfite (K2S2O5) and 1,10-dibromodecane were purchased from
0376-7388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2010.06.004
tive analysis). The mole ratio of NaSS to 4VP was determined to be 1.58:1 (25◦C, d6-DMSO). 1H NMR (DMSO-d 6):ı = 0.65–2.39 (d), 6.05–7.15 (c), 7.15–7.89 (a), 7.89–8.81 (b) ppm; 13C NMR (DMSO-d 6): ı = 122.8–124.5 (f), 125.6–126.9 (b), 126.9–128.2 (c) 144.9–147.2 (a and d),
149.3–151.2 (e), 153.2–155.0 (g) ppm; IR: 1555 (C Npyridine),
1040 (asymS O), 1010 (symS O) cm−1; Intrinsic viscosity (IV, in
DMSO at 30◦C): 2.83 dl g−1.
2.3. Film casting and membrane acidification
Desired amounts of NaSS-4VP and the haloalkyl crosslinker
1,10-dibromodecane (cf.Table 1; molar ratio of pyridine groups
of NaSS-4VP to 1, 10-dibromobutane for SP-1, SP-2, SP-3 and SP-4 was 1:1, 4:3, 2:1 and 4:1) were dissolved in order to give rise to a 10 wt% solution in DMSO at room temperature which was then stirred for 2 h. The resulting solution was cast onto a glass plate
and heated at 60◦C for 48 h in order to complete the crosslinking
reaction. The dried membrane was soaked in methanol at room temperature to remove the residual solvent, and then peeled from the glass plate upon immersion in deionized water. The crosslinked NaSS-4VP membrane in acidic form (SS-4VP) was obtained after immersion in a 2 M HCl solution for 48 h and washing with deion-ized water until the pH reached 6–7.
2.4. Characterization of the membranes 2.4.1. Copolymer characterization
1H NMR spectra were recorded at 25◦C using an INOVA 500 MHz
NMR spectrometer. FTIR spectra were obtained with a Nicolet Avatar 320 FTIR spectrometer; 32 scans were collected at a spectral
resolution of 1 cm−1(25◦C, d6-DMSO).
2.4.2. Water uptake and ion-exchange capacity of the membranes The ion-exchange capacities (IECs) were determined by
titra-tion. A membrane in H+form was first equilibrated in 1.0 M NaCl
solution for 24 h to exchange the protons with sodium ions. Sub-sequently, the membrane was removed and rinsed with deionized water. The rinse water was then collected and combined with the
NaCl solution, which was titrated with 0.01 mol L−1NaOH using a
0.1% phenolphthalein solution in ethanol/water as the end-point of
exchangeable protons to the weight of the dry membrane (Wdry, g).
IEC=CNaOHVNaOH
Wdry
(1) The completely dry crosslinked SS-4VP membranes were immersed in deionized water at room temperature for 24 h and were then swiftly extracted, blotted with filter paper to remove any excess water from the membrane surfaces, and immediately
weighed to obtain their wet masses (Wwet). Subsequently, the Table
1 Characteristics of the crosslinked SS-4VP membranes. Sample Polymer content (wt%) Crosslinker content (wt%) Crosslinking fraction (mol%) a Ion-exchange capacity (mequiv g − 1) W ater uptake (%) Methanol uptake (%) Proton conductivity (S cm − 1) d Calculated IEC th b Titration IEC tit c SP-1 74.6 25.4 90.3 2.52 2.41 53.1 24.5 0.071 SP-2 79.5 20.5 72.8 2.84 2.65 62.5 26.1 0.082 SP-3 85.5 14.5 48.1 3.42 3.33 86.3 28.9 0.110 SP-4 92.2 7.8 23.9 4.06 3.83 142.4 30.2 0.143 Nafion 117 – – – – 1.02 35.6 62.1 0.093 a Molar ratio of crosslinked pyridine units to total pyridine units. b IEC calculated from DS. c IEC measured with titration. d Measured at 30 ◦C and 90% RH.
Scheme 1. The network structure of the crosslinked SS-4VP membranes.
membranes were dried at 120◦C for 24 h before their dry weights
(Wdry) were measured. The water uptake (WU %) was calculated
according to the following equation:
WU (%)= Wwet− Wdry
Wdry × 100%
(2)
2.4.3. Mechanical and thermal properties
The mechanical properties of the wet membranes were mea-sured by Instron-1211 at the test speed of 2 mm/min, the size of
the specie is 60 mm× 10 mm. For each testing, three measurements
at least were recorded and average value was calculated. A DuPont Q100 thermogravimetric analyzer (TGA) was utilized to investigate
the thermal stability of the membranes; the samples (∼10 mg)were
preheated to 150◦C for 15 min to remove resdiual water before
measured, then heated from ambient temperature to 850◦C under
a nitrogen atmosphere at a heating rate of 20◦C/min.
2.4.4. Oxidative and hydrolytic stability
Oxidative stability of the membranes was tested by
immers-ing the films into Fenton’s reagent (30 wt% H2O2 containing
30 ppm ferrous ammonium sulfate) at 80◦C. The oxidatve
stabil-ity was evaluated by recording the time when the membranes was dissolved completely. The proton conductivities of mem-branes plotted with respect to the time they were exposed to
Fenton’s reagent (30 wt% H2O2containing 30 ppm ferrous
ammo-nium sulfate) at 30◦C. The resultiing membranes were immersed
in deionized water at room temperature for 12 h and were then measured using an electrode system. Hydrolytic stability of the membranes was tested proton conductivities and weight loss for
membrane after soaking in water at 100◦C. The proton conductivity
of these membranes was measured at 30◦C and 90% RH. The
elec-trode system and electrochemical procedure was according Section 2.4.5.
2.4.5. Proton conductivity
The proton conductivity of the membrane was determined with an ac electrochemical impedance analyzer (PGSTAT 30), and the experiments involved scanning the ac frequency from 100 kHz to 10 Hz at a voltage amplitude of 10 mV. The membrane (1 cm in diameter) was sandwiched between two smooth stainless steel
disk electrodes in a cylindrical PTFE holder. The cell was placed in a thermal and humid controlled chamber for measurement. At a given temperature and humidity, the samples were equilibrated for at least 30 min before any measurement. Repeated measurements were taken at that given temperature with 10 min interval until no more change in conductivity was observed. The proton conduc-tivity of the membrane was calculated from the observed sample resistance from the relationship:
= RAL (3)
where is the proton conductivity (in S cm−1), L is the distance
between the electrodes used to measure the potential (L = 1 cm). R is the impedance of the membrane (in ohm), which was measured at the frequency that produced the minimum imaginary response,
and A is the membrane section area (in cm2)
The activation energy (Ea, kJ mol−1), which is the minimum
energy required for proton transport, was obtained for each mem-brane from the gradients of Arrhenius plots based on the following equation:
= −Ea
RT (4)
Here, is the proton conductivity (in S cm−1), R is the universal
gas constant (8.314 J mol−1K), and T is the absolute temperature
(K).
2.4.6. Methanol permeability and water desorption
The methanol diffusion coefficient of the membrane was mea-sured using a two-chamber liquid permeability cell. A detailed
description of this cell can be found elsewhere[22–25]. Water
desorption measurements were carried out on a TGA Q100 to
deter-mine the weight change of the sample over time at 60◦C. The water
diffusion coefficient was calculated according Ref[34].
2.4.7. Membrane morphology
The membrane morphologies were characterized using a JEOL JEM-1200CX-II transmission electron microscope (TEM) operated at 120 kV. To stain the hydrophilic domains, the membrane was
converted into its Pb2+form by immersing in 1 N Pb(Ac)
2[lead(II)
acetate] solution overnight and then rinsing with water. The
Fig. 1. (A) FTIR, (B)1H NMR and (C)13C NMR spectra of the NaSS-4VP copolymer. (D) A13C NMR spectrum of SP-2 (25◦C, d6-DMSO).
was sectioned into 50-nm slices using an ultramicrotome. The slices were picked up with 200-mesh copper grids for TEM observation.
3. Results and discussion
3.1. Characterization and crosslinking reaction
1H NMR,13C NMR (d
6-DMSO, 25◦C), and FTIR experiments were
performed in order to confirm the structure of the copolymer. Fig. 1(A) shows the FTIR spectrum of the copolymers. The sharp
peak at 1555 cm−1was attributed to the pyridine group, and the
peaks at 1040 and 1010 cm−1were assigned to the vibration of
sul-fonic acid groups. InFig. 1(B), the peaks at 6.6 ppm corresponded to
meta protons [marked as ‘c’ in the molecular formula inFig. 1(B)]
on the phenyl ring in NaSS-4VP, the peaks at 7.5 and 8.3 ppm were assigned to the protons adjacent to the sulfonated group (‘a’) and pyridine nitrogen atom (‘b’), respectively, and the peak at 1.5 ppm was believed to represent the protons of methane and methylene
groups (‘d’). In addition, the13C NMR spectrum of the NaSS-4VP
copolymer [Fig. 1(C)] illustrated the characteristic carbons
adja-cent to the sulfonated group and pyridine nitrogen atom at 150.2
[marked as ‘b’ in the molecular formula inFig. 1(C)] and 126.1 ppm
(‘e’), respectively.
The pyridine groups of the NaSS-4VP copolymer were able to interact with the 1-bromobutane thereby forming pyridinium
salts via nucleophilic substitution as previously reported[26–29]
To study the processes, model reactions of 1-bromobutane with
pyridine (Scheme 2) in DMSO-d6 at 60◦C were investigated by
means of1H NMR spectroscopy and the results are presented in
Fig. 2. As displayed in the1H NMR spectrum, the formation of
pyridinium salts was revealed by the shift of the signals from the protons of pyridine (7.37, 7.75 and 8.62) and alkyl group (1.78 and 3.46) to 8.34, 8.81 and 9.54, and 1.96 and 4.91, respectively. These results confirmed that the 1-bromidebutane was almost completely reacted with pyridine via nucleophilic substitution after 400 min. In the nucleophilic substitution reaction, the formation of pyridinium salts would occur if a haloalkyl group’s monomer was employed.
In the present study,1H NMR spectroscopy was also utilized to
analyze the degree of crosslinking between the pyridine groups of
NaSS-4VP and the haloalkyl groups.Table 1displays the relative
ratios of crosslinked pyridine units to the total number of pyridine groups determined from integration of the corresponding signals
in each1H NMR spectrum. One can observe that the nucleophilic
substitution reaction occurred during the solvent evaporation pro-cess. The degree of crosslinking increased relatively slowly upon increasing the content of the haloalkyl crosslinker as a result of the crosslinking reaction being inhibited by the network structure.
Fig. 2. Model reactions of 1-bromobutane with pyridine in d6-DMSO at 60◦C: the
evolution of the1H NMR spectra of the reaction mixture with time (the designated
signals belong to protons from the starting halide and the formed pyridinium salt, as indicated inScheme 2).
Fig. 3. FTIR spectra of NaSS-4VP, SP-1 and SP-4.
Further evidence of a crosslinked SS-4VP structure was obtained
from13C NMR analysis. The13C NMR spectrum of SP-2 [Fig. 1(D)]
indicates the presence of the alkane group of carbons adjacent to the pyridinium salt at 62.2 ppm [marked as ‘b’ in the molecular
formula inFig. 1(D)]. These results suggest that the crosslinked
SS-4VP structure was obtained after the solvent evaporation pro-cess.
In addition, the effect of interaction between the pyridinium cation and sulfonated anion on the crosslinked SS-4VP membrane
was also investigated by FTIR spectra. Fig. 3 shows the spectra
of pure NaSS-4VP, SP-1 and SP-4 membranes, indicating that the
intensity of the peak at 1639 cm−1increased after the crosslinking
reaction and acid treatment. According to previous studies[31,32],
this peak can be assigned to a ring vibration of the pyridinium cations, which were produced through proton transfer from the sulfonic acid groups and hydrochloric acid to the pyridine groups. In the spectrum of SS-4VP membranes, a new peak appeared
at 1261 cm−1 and its intensity increased with the reduction of
the crosslinking density. The absorption band at 1261 cm−1 was
assigned to the absorption arising from the asymmetric
stretch-ing of the –SO3− anions as previously described [33]. With the
reduction of crosslinking density, more pyridinium cations were
Fig. 4. TGA curves of P4VP and the crosslinked SS-4VP membranes.
produced through proton transfer from the sulfonic acid groups.
Therefore, we conclude that –SO3−anions and pyridinium cations
can attach to one another in the crosslinked SS-4VP to form ion pairs, which also give rise to the formation of ionic interaction of the polymer chains.
3.2. Thermal and mechanical properties
The thermal stabilities of crosslinked SS-4VP membranes were determined by TGA and DSC. The TGA curves for the crosslinked
SS-4VP are given inFig. 4. As shown inFig. 4, the thermal degradation
of the crosslinked SS-4VP copolymer increased upon increasing the crosslinker content. The degradation temperatures of crosslinked
SS-4VP membranes were all higher than 300◦C, suggesting that
the presence of the alkane crosslinking structure and the inter-action between acidic (sulfonic acid groups) and basic (basic nitrogen) units in the membrane improved the thermal stability of SS-4VP. As disclosed by the DSC measurements, no glass
tran-sition temperature (Tg) was observed for any of the crosslinked
SS-4VP membranes in the temperature range from 30 to 300◦C.
The absence of a glass transition temperature originated from the
nature of the ionomer, with its elevated ion concentration[35].
Such improved thermal properties are very desirable for electrolyte materials used in PEFCs.
It is essential for PEMs to possess adequate mechanical integrity to withstand fabrication of the membrane electrode assembly. When subjected to hot pressing, the electrodes could be easily peeled away from the MEA due to the deformation of the mem-brane. The experimental results on mechanical modulus, strength and elongation properties for the crosslinked SS-4VP membranes
at room temperature were summarized inTable 2. In the wet state,
the sample showed superior mechanical properties, as opposed to Nafion 117, with tensile stress values in the range of 29.6–51.6 MPa, elongation at break values of 2.2–6.3% and values of Young’s mod-ulus of 1.3–2.4 GPa. The mechanical properties of the crosslinked SS-4VP membranes increased upon increasing the crosslinking density, i.e., the presence of the crosslinking structure enhanced the strength of the membranes. In addition, it is assumed that the acid–base interaction restrict the molecular motion of the polymer chains resulting in stronger membranes.
These data presented in Table 2indicated that the addition
of alkane crosslinker enhanced the mechanical properties of the resulting membranes to some extent, respective of the ratio of alkane crosslinker. The membranes obtained good mechanical properties and were strong and tough enough to be used as func-tional PEM materials.
The IEC value for Nafion 117 was also measured and found to be
1.02 mequiv g−1—a result that agreed well with previously reported
data[36]. The reliability of the measurement method could thus
be validated. The IEC values of the crosslinked SS-4VP membranes
ranged from 2.41 to 3.83 mequiv g−1. The decrease in IEC values
with the increase in crosslinker content was caused by the decrease in the overall sulfonic acid content. Theoretical IEC values could be calculated based on the composition of the starting mixture and the following assumptions: (i) that no residual crosslinker was left in the membranes; (ii) that the weight variations due to the nucle-ophilic substitution reaction were negligible; (iii) that all of the sulfonic acid groups in SS-4VP contributed to the IEC; (iv) that all of the residual pyridinium salts contributed to the IEC.
Previous studies have indicated that the formation of the acid–base complexes within PEMs give rise to perturbations when measuring the IEC. This is the result of the exchange of the acid protons of the PEMs being more difficult as compared to for other
kinds of sulfonated polymers[37–40], which was also the case in
the present study. To eliminate this effect, the membrane was kept in the titration solution until the measurement was completed.
The WU of sulfonated polymers is known to have a pro-found effect on membrane conductivity and mechanical properties
[4]. However, excessively high WU levels can result in
mem-brane fragility and dimensional changes, which leads to the loss of mechanical properties. Basically, the amount of WU in the crosslinked sulfonated polymers is strongly dependent upon the amount of crosslinker and is also related to IEC values. The WU was determined from the ratio of the weight of the water absorbed by the membrane when immersed in water, with respect to the dry membrane weight. The WU of the crosslinked SS-4VP membranes
was evaluated at 30◦C and is presented inTable 1. As expected,
the crosslinking caused a significant suppression of the membrane swelling and the water uptake in the crosslinked membranes was thus lower, presumably because of the decrease in the overall sul-fonic acid content and the reduced free volume within the PEM. The water retention of membranes could provide indirect evidence of the variation in the WU with the increase in temperature and reduction in relative humidity (RH). Moreover, it is a very important parameter in view of their application in PEFCs.
The water desorption curves of crosslinked SS-4VP membranes
are shown inFig. 5. As can be seen, the water diffusion coefficients
of the crosslinked SS-4VP membranes demonstrate considerable decreases with an increasing crosslinker content. As the increase in crosslinking density, the water mobility decreased and hence the penetration of the water molecules through the membrane became difficult. In addition, the crosslinked materials were pyridinium
salts (Scheme 1), and as such provided additional sites for water
absorption giving rise to an improved ability for water retention. The lower the diffusion coefficient, the stronger the membranes
The type of crosslinker, the crosslinking density, and the microstructural change that occurs after crosslinking all have dra-matic effects on the water uptake, the state of water, and the
proton conductivity of crosslinked membranes[17–25]. The
pro-ton conductivities of the crosslinked SS-4VP membranes decreased with the crosslinker content and the associated decrease in water content. Nevertheless, although the SP-1 membrane exhibited the highest crosslinking density among all of the membranes, it still presented an adequate conductivity that was close to that of Nafion
117 (i.e., 7.1× 10−2S cm−1for SP-1 and 9.3× 10−2S cm−1for Nafion
117). Furthermore, the proton conductivity of the SP-1 membrane
surpassed that of Nafion 117 and reached a value of 0.14 S cm−1at
60◦C. The presence of both hydrogen bonds and ionic interactions
in the membrane (Section3.1) led to the formation of a random
pro-ton conductive pathway that facilitated propro-ton transfer[40–42]. In
addition, the introduction of the alkane and long crosslinker was believed to induce the formation of efficient hydrophilic channels within the polymer matrix, thereby enhancing the proton
conduc-tivity[43]. TEM micrographs of the SP-1 membrane (Fig. 6) also
indicate that the addition of crosslinker in the membrane results in a better distribution of the ionic clusters.
Fig. 7 presents Arrhenius plots of the proton conductivities measured at various temperatures. The proton conductivity of the crosslinked SS-4VP membrane was thermally stimulated since higher proton conductivities were expected at higher tempera-tures.
The activation energy (Ea) of these crosslinked SS-4VP
mem-branes (SP-1 and SP-2), obtained from the Arrhenius plots, lay
Fig. 5. The water desorption of the crosslinked SS-4VP membranes. The numbers in the boxes correspond to the water diffusion coefficients (×10−5cm−2s−1).
Fig. 6. TEM image of SP-1 membrane.
Fig. 7. Proton conductivities of the crosslinked SS-4VP membranes and Nafion 117, plotted as functions of the inverse temperature. The numbers in the boxes corre-spond to the activation energies (Ea, kJ mol−1).
within the range of 14–40 kJ mol−1. This was in decent
agree-ment with the results from the Grotthus-type mechanism[43].
The Ea-value of the SP-1 membrane (i.e., 18.0 kJ mol−1) was the
highest among all the investigated composites. This was a result of the higher amount of dense alkane network structure
retard-ing the evaporation of water at high temperatures[10], and thus
resulting in a less dramatic reduction of the proton conductivity. However, for SP-3 and SP-4, the plots of the conductivity vs. tem-perature cannot fit Arrhenius equation (non-linear relationship). This is probably because the membranes highly swelled under the conductivity test environment.
The proton conductivity of the crosslinked SS-4VP membranes
as a function of the relative humidity (RH) at 60◦C is presented
inFig. 8. For all crosslinked SS-4VP membranes, the proton con-ductivity decreased drastically as the RH decreased; a common phenomenon that has been observed for many other sulfonated polymer membranes. However, the relationship between the pro-ton conductivity and the RH indicates that the reduction of the proton conductivity was less dramatic when increasing the
crosslinker content at low RH and 60◦C, which also implied that
the dense alkane network, acid–base complex and additional pyri-dinium salts displayed a higher propensity for retaining water, thus limiting the dependence of the proton conductivity on hydration. On the other hand, the proton conductivity of the SP-1 was
main-tained at 6.4× 10−3S cm−1at 50% RH—a result comparable to that
Fig. 8. Conductivities of the crosslinked SS-4VP membranes plotted as functions of the relative humidity at 60◦C.
of Nafion 117. These findings were in good agreement with the water desorption data mentioned above. In addition, the proton conductivity was measured at low RH, and the proton transport therefore depended strongly on the distance between the hopping sites. However, since the crosslinked SS-4VP membranes com-prised a combination of acidic and basic sites and thus acted as a proton transfer medium, the site-to-site jumping distances were minimized and the proton transfer processes were facilitated under
low RH conditions[44–46].
Methanol permeability is an important membrane property in DMFC applications since the crossover of methanol from the anode to the cathode leads to a lower cell voltage and a decreased fuel
efficiency. As can be seen inTable 1, the methanol permeability of
the crosslinked SS-4VP membranes decreased upon increasing the crosslinking density. This was due to the presence of the crosslinker between the polymer chains preventing excessive water swelling while simultaneously retarding the degree of methanol crossover [17–25]. The methanol permeability of the SP-1 membrane was a mere 16% of that of Nafion 117 under equivalent conditions
(i.e., 2.10× 10−7cm2s−1for SP-1 and 1.31× 10−6cm2s−1for Nafion
117), despite that it absorbed more water. The underlying reason
was presumed to be the lower methanol uptake behavior (Table 1)
and the acid–base interaction acting as a methanol barrier[37].
For practical PEFC applications, PEMs need to exhibit high
proton conductivities (>10−2S cm−1) and low methanol
perme-abilities (<10−6cm2s−1). The ratio of the proton conductivity to
the methanol permeability, known as the selectivity (˚), is thus
an effective parameter for evaluating membrane performance in DMFCs. The selectivity of the SP-1 membrane was found to be ca. 5
times that of Nafion 117, as can be seen inTable 1, indicating that
the SP-1 membrane provided a superior performance as opposed to Nafion 117 with regard to being used in DMFCs.
3.5. Hydrolytic and oxidative stability
The oxidative stability of the crosslinked SS-4VP membranes
was determined using Fention’s reagent at both 25 and 80◦C
(Table 2). Fenton’s reagent is often employed to simulate the
oxidative reactions from radical species, such as OH• and HOO•,
during fuel cell operation. In the present case, the choice was made to utilize a higher concentration of Fenton’s reagent (i.e.,
30 wt% H2O2 and 30 ppm FeSO4) than what had been used in
previous studies (i.e., 3 wt% H2O2 and 2 ppm FeSO4) to measure
the oxidative stability[45–48]. Despite that the non-crosslinked
4VP copolymer was water-soluble, all of the crosslinked SS-4VP membranes were either completely insoluble or became
Fig. 9. Proton conductivities of the crosslinked SS-4VP membranes plotted as func-tions of the time they were exposed to Fenton’s reagent at 25◦C. (The conductivity
of SS-4VP membrane was measured at 30◦C and 90% RH.)
revealing that the oxidative stability of the crosslinked SS-4VP membranes was superior to that previously reported for
sul-fonated polymers[47–56]. The test carried out at 80◦C indicated
that the oxidative stability of the crosslinked SS-4VP membranes increased as the crosslinking density was raised due to the
crosslinking structure retarding the permeation of H2O2into the
membrane. The crosslinked SS-4VP membranes also possessed the potential of providing matrices with higher chemical
sta-bility due to the zwitterionic throughout the membrane [21].
The hydrolytic stability of a PEM is a crucial property for long-term fuel cell operation, and the testing of this characteristic for the crosslinked SS-4VP membranes was performed by immersing
them in deionized water at 100◦C and determining the time that
elapsed before the hydrated membranes began to lose their proton conductivity.
Table 3lists the proton conductivity and weight loss data of the SP-1 membrane before and after the aging tests. No changes in either shape or appearance were observed in the membrane up to after 6 days, thus demonstrating that there was no obvious hydrolysis during the treatment. Moreover, the SP-1 membrane retained more than 98.1% of its original weight after soaking in
water for 28 days at 100◦C, and the membranes maintained high
proton conductivities of 5.5× 10−2S cm−1(to be compared to the
initial proton conductivity of 7.1× 10−2S cm−1) thus revealing
their hydrolytic stability at high temperatures.Fig. 9 illustrates
the proton conductivities of the crosslinked SS-4VP membranes plotted with respect to the time they were exposed to Fenton’s
reagent (30 wt% H2O2containing 30 ppm ferrous ammonium
sul-fate at 30◦C). The SP-1 membrane maintained adequate proton
conductivities (>0.01 S cm−1) after 120 h (The conductivity of
SS-4VP membrane was measured at 30◦C and 90% RH). In contrast,
other sulfonated polymers[45–54]have been seen to decompose
after 24–60 h under similar testing conditions. Such oxidative and
permeability of the crosslinked SP-1 membranes despite their high IEC values. The proton conductivity was comparable or even superior to that of Nafion 117 at high temperatures and 90% RH. The crosslinked SP-1 membrane possessed the highest selectivity
(i.e., 3.38× 105S cm−3s); a result approximately five times that of
Nafion 117, thus implying its potential for practical applications in high-energy-density devices.
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