Journal of Membrane Science 362 (2010) 29–37
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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 ,∗
a
Institute of Applied Chemistry,National Chiao-Tung University,Hsin-Chu,Taiwan
b
Material and Chemical Rearch Laboratories,Industrial Technology Rearch Institute,Chutung,Taiwan c
玉米饼干
Graduate 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 28November 2009
Received in revid form 31May 2010Accepted 2June 2010
Available online 9 June 2010Keywords:Fuel cells
Polymer-electrolyte membrane Sulfonated polymer
Crosslinked membranes Proton conductor
a b s t r a c t
The prent 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 the cro
sslinked 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 compod 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-1membrane possd the highest lectivity for methanol fuel cells (3.38×105S cm −3s),approximately five times that of Nafion 117,thus implying its potential for practical u in high-energy-density devices.侍奉的意思
© 2010 Elvier B.V. All rights rerved.
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 gas [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 (SP
OP)[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/ba 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.:+886353131512.
E-mail address:u.edu.tw (F.-C.Chang).
lent crosslinked SPEEK [24,25].Nevertheless,the crosslinked polymer-electrolyte membranes usually display significant loss in proton conductivity due to low water uptake values caud by the crosslinking structure.Conquently,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 u in anion-exchange membranes [28,29],relatively few studies have exploited the role of zwitterionic crosslinked membranes in PEMs [30].The prent 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 tho of other sulfonated polymers.2.Experimental 2.1.Materials
4-Styrenesulfonic acid sodium salt hydrate (NaSS),4-vinylpyridine (4VP),poly(4-vinylpyridine)(P4VP),potassium disulfite (K 2S 2O 5)and 1,10-dibromodecane were purchad from
0376-7388/$–e front matter © 2010 Elvier B.V. All rights rerved.doi:10.sci.2010.06.004
30Y.-S.Ye et al./Journal of Membrane Science 362 (2010) 29–37
Sigma–Aldrich.All other chemicals were of reagent grade,obtained from Sigma–Aldrich and ud as received.
2.2.Synthesis of poly(styrene sulfonic acid-co-vinylpyridine)(NaSS-4VP)
The NaSS-4VP copolymer was synthesized according to Scheme 1through a free radical polymerization in deionized water at 70◦C under a nitrogen atmosphere for 8h.The chon reaction conditions were:[M ]total =4.5M,the initiator of [K 2S 2O 5]=1wt%,and monomer feed ratios of ([NaSS]:[4VP]=3:2mol%).The result-ing solution was precipitated into acetone and the NaSS-4VP polymer was filtered off,washed three times with acetone,and then dried under vacuum at 80◦C for 12h.The copolymer struc-ture and compositions were determining 1H NMR (using integral area of chemical shifts of monomer functional groups for quantita-tive analysis).The mole ratio of NaSS to 4VP was determined to be 1.58:1(25◦C,d 6-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 N pyridine),1040( asym S O),1010( sym S O)cm −1;Intrinsic viscosity (IV,in DMSO at 30◦C):2.83dl 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-3and SP-4was 1:1,4:3,2:1and 4:1)were dissolved in order to give ri to a 10wt%solution in DMSO at room temperature which was then stirred for 2h.The resulting solution was cast onto a glass plate and heated at 60◦C for 48h 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 2M HCl solution for 48h 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 500MHz NMR spectrometer.FTIR spectra were obtained with a Nicolet Avatar 320FTIR spectrometer;32scans were collected at a spectral resolution of 1cm −1(25◦C,d 6-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 e
quilibrated in 1.0M NaCl solution for 24h to exchange the protons with sodium ions.Sub-quently,the membrane was removed and rind with deionized water.The rin water was then collected and combined with the NaCl solution,which was titrated with 0.01mol 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 (W dry ,g).IEC =
C NaOH V NaOH
W dry惊蛰的意思
(1)
The completely dry crosslinked SS-4VP membranes were immerd in deionized water at room temperature for 24h 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 mass (W wet ).Subquently,the
T a b l e 1C h a r a c t e r i s t i c s o f t h e c r o s s l i n k e d S S -4V P m e m b r a n e s .
快乐旅游S a m p l e
P o l y m e r c o n t e n t (w t %)
C r o s s l i n k e r c o n t e n t (w t %)C r o s s l i n k i n g f r a c t i o n (m o l %)a
I o n -e x c h a n g e c a p a c i t y (m e q u i v g −1)
W a t e r u p t a k e (%)
M e t h a n o l u p t a k e (%)
P r o t o n c o n d u c t i v i t y (S c m −1)d M e t h a n o l p e r m e a b i l i t y ×10−6
(c m 2s −1)
S e l e c t i v i t y ×105
(S c m −3s )
C a l c u l a t e d I E C t h b
T i t r a t i o n I E C t i t c S P -174.625.490.32.522.4153.124.50.0710.213.38S P -279.520.572.82.84
路书2.6562.526.10.0820.392.10S P -385.514.548.13.423.3386.328.90.1101.750.63S P -492.27.823.94.063.83142.430.20.1436.460.22N a fio n 117
––––1.0235.662.10.0931.310.71
a
M o l a r r a t i o o f c r o s s l i n k e d p y r i d i n e u n i t s t o t o t a l p y r i d i n e u n i t s .b
I E C c a l c u l a t e d f r o m D S .c
I E C m e a s u r e d w i t h t i t r a t i o n .d
M e a s u r e d a t 30◦C a n d 90%R H .
Y.-S.Ye et al./Journal of Membrane Science362 (2010) 29–37
31
打屁板
行为动词Scheme1.The network structure of the crosslinked SS-4VP membranes. membranes were dried at1
20◦C for24h before their dry weights
(W dry)were measured.The water uptake(WU%)was calculated
according to the following equation:
WU(%)=W wet−W dry
W dry
×100%(2)
想念的反义词2.4.
3.Mechanical and thermal properties
The mechanical properties of the wet membranes were mea-sured by Instron-1211at the test speed of2mm/min,the size of the specie is60mm×10mm.For each testing,three measurements at least were recorded and average value was calculated.A DuPont Q100thermogravimetric analyzer(TGA)was utilized to investigate the thermal stability of the membranes;the samples(∼10mg)
were preheated to150◦C for15min to remove resdiual water before measured,then heated from ambient temperature to850◦C under
a nitrogen atmosphere at a heating rate of20◦C/min.
2.4.4.Oxidative and hydrolytic stability
Oxidative stability of the membranes was tested by immers-ing thefilms into Fenton’s reagent(30wt%H2O2containing 30ppm ferrous ammonium sulfate)at80◦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 expod to Fenton’s reagent(30wt%H2O2containing30ppm ferrous ammo-nium sulfate)at30◦C.The resultiing membranes were immerd in deionized water at room temperature for12h 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 at100◦C.The proton conductivity of the membranes was measured at30◦C and90%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(PGSTAT30),and the experiments involved scanning the ac frequency from100kHz to 10Hz at a voltage amplitude of10mV.The membrane(1cm 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 least30min before any measurement.Repeated measurements were taken at that given temperature with10min interval until no more change in conductivity was obrved.The proton conduc-tivity of the membrane was calculated from the obrved sample resistance from the relationship:
=
L
(3) where is the proton conductivity(in S cm−1),L is the distance between the electrodes ud to measure the potential(L=1cm).R is the impedance of the membrane(in ohm),which was measured at the frequency that produced the minimum imaginary respon, and A is the membrane ction area(in cm2)
The activation energy(E a,kJ mol−1),which is the minimum energy required for proton transport,was obtained for each mem-brane from the gradients of Arrhenius plots bad on the following equation:
=
−E a
RT(4) Here, is the proton conductivity(in S cm−1),R is the universal gas constant(8.314J 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 elwhere[22–25].Water desorption measurements were carried out on a TGA Q100to deter-mine the weight change of the sample over time at60◦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 electro
n microscope(TEM)operated at120kV.To stain the hydrophilic domains,the membrane was converted into its Pb2+form by immersing in1N Pb(Ac)2[lead(II) acetate]solution overnight and then rinsing with water.The mem-brane was dried under vacuum at80◦C for12h and then the sample
32Y.-S.Ye et al./Journal of Membrane Science
362 (2010) 29–37
Fig.1.(A)FTIR,(B)1H NMR and (C)13C NMR spectra of the NaSS-4VP copolymer.(D)A 13C NMR spectrum of SP-2(25◦C,d 6-DMSO).
was ctioned into 50-nm slices using an ultramicrotome.The slices were picked up with 200-mesh copper grids for TEM obrvation.
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 1555cm −1was attributed to the pyridine group,and the peaks at 1040and 1010cm −1were assigned to the vibration of sul-fonic acid groups.In Fig.1(B),the peaks at 6.6ppm corresponded to meta protons [marked as ‘c’in the molecular formula in Fig.1(B)]on the phenyl ring in NaSS-4VP,the peaks at 7.5and 8.3ppm were assigned to the protons adjacent to the sulfonated group (‘a’)and pyridine nitrogen atom (‘b’),respectively,and the peak at 1.5ppm was believ
ed to reprent the protons of methane and methylene groups (‘d’).In addition,the 13C 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 in Fig.1(C)]and 126.1ppm (‘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 process,model reactions of 1-bromobutane with pyridine (Scheme 2)in DMSO-d 6at 60◦C were investigated by
means of 1H NMR spectroscopy and the results are prented in Fig.2.As displayed in the 1H NMR spectrum,the formation of pyridinium salts was revealed by the shift of the signals from the protons of pyridine (7.37,7.75and 8.62)and alkyl group (1.78and 3.46)to 8.34,8.81and 9.54,and 1.96and 4.91,
respectively.The results confirmed that the 1-bromidebutane was almost completely reacted with pyridine via nucleophilic substitution after 400min.In the nucleophilic substitution reaction,the formation of pyridinium salts would occur if a haloalkyl group’s monomer was employed.
In the prent 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 each 1H NMR spectrum.One can obrve that the nucleophilic substitution reaction occurred during the solvent evaporation pro-cess.The degree of crosslinking incread relatively slowly upon increasing the content of the haloalkyl crosslinker as a result of the crosslinking reaction being inhibited by the network
structure.
Scheme 2.The reaction of 1-bromobutane with pyridine (model compound).
Y.-S.Ye et al./Journal of Membrane Science 362 (2010) 29–37
33
Fig.2.Model reactions of 1-bromobutane with pyridine in d 6-DMSO at 60◦C:the evolution of the 1H 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 in Scheme 2
)
.
Fig.3.FTIR spectra of NaSS-4VP,SP-1and SP-4.
Further evidence of a crosslinked SS-4VP structure was obtained from 13C NMR analysis.The 13C NMR spectrum of SP-2[Fig.1(D)]indicates the prence of the alkane group of carbons adjacent to the pyridinium salt at 62.2ppm [marked as ‘b’in the molecular formula in Fig.1(D)].The 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.3shows the spectra of pure NaSS-4VP,SP-1and SP-4membranes,indicating that the intensity of the peak at 1639cm −1incread 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 1261cm −1and its intensity incread with the reduction of the crosslinking density.The absorption band at 1261cm −1was assigned to the absorption arising from the asymmetric stretch-ing of the –SO 3−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 –SO 3−anions and pyridinium cations can attach to one another in the crosslinked SS-4VP to form ion pairs,which also give ri 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 in Fig.4.As shown in Fig.4,the thermal degradation of the crosslinked SS-4VP copolymer incread upon increasing the crosslinker content.The degradation temperatures of crosslinked SS-4VP membranes were all higher than 300◦C,suggesting that the prence 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 disclod by the DSC measurements,no glass tran-sition temperature (T g )was obrved for any of the crosslinked SS-4VP membranes in the temperature range from 30to 300◦C.The abnce 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 ud in PEFCs.
It is esntial for PEMs to posss adequate mechanical integrity to withstand fabrication of the membrane electrode asmbly.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 in Table 2.In the wet state,the sample showed superior mechanical properties,as oppod to Nafion 117,with tensile stress values in the range of 29.6–51.6MPa,elongation at break values of 2.2–6.3%and values of Young’s mod-ulus of 1.3–2.4GPa.The mechanical properties of the crosslinked SS-4VP membranes incread upon increasing the crosslinking ,the prence of the crosslinking structure enhanced the strength of the membranes.In addition,it is assumed that the acid–ba interaction restrict the molecular motion of the polymer chains resulting in stronger membranes.
The data prented 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 ud as func-tional PEM materials.
