Preparation of poly(vinyl alcohol)/montmorillonite/poly(styrene sulfonic acid) composite membranes...

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Current Applied Physics 11 (2011) S229eS237

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Current Applied Physics

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Preparation of poly(vinyl alcohol)/montmorillonite/poly(styrene sulfonic acid)composite membranes for hydrogeneoxygen polymer electrolyte fuel cells

Chun-Chen Yang*, Shwu-Jer Chiu, Shih-Chen KuoDepartment of Chemical Engineering, Ming Chi University of Technology, 84 Gungjuan Rd., Taipei Hsien 243, Taiwan, ROC

a r t i c l e i n f o

Article history:Received 23 September 2010Received in revised form3 November 2010Accepted 15 November 2010Available online 23 November 2010

Keywords:Poly(vinyl alcohol) (PVA)Montmorillonite (MMT)Poly(styrene sulfonic acid) (PSSA)Proton conductingPolymer electrolyte membrane fuel cell(PEMFC)

* Corresponding author. Tel.: þ88629089899; fax:E-mail address: ccyang@mail.mcut.edu.tw (C.-C. Y

1567-1739/$ e see front matter � 2010 Elsevier B.V.doi:10.1016/j.cap.2010.11.043

a b s t r a c t

The good performance poly(vinyl alcohol)/montmorillonite/poly(styrene sulfonic acid) (PVA/MMT/PSSA)composite membrane is prepared by a solution casting method. The characteristic properties of thecomposite membranes are investigated by thermal gravimetric analysis (TGA), differential scanningcalorimetry (DSC), dynamic mechanical analysis (DMA), X-ray diffraction (XRD), scanning electronmicroscopy (SEM), micro-Raman spectroscopy and AC impedance method. The ionic conductivities of thePVA/MMT/PSSA composite membranes in water at ambient temperature are of the order of 10�3 S cm�1,i.e., at 2.07 � 10�3e6.69 � 10�3 S cm�1. The ionic conductivity of the novel composite membrane isgreatly enhanced due to two proton ionic sources used, i.e., the modified MMT fillers and PSSA polymer.Under atmospheric pressure, the peak power densities of the polymer electrolyte membrane fuel cell(PEMFC) at 25 and 50 �C are 65.23 and 90.70 mW cm�2, respectively. The results indicate that the PEMFCcomprised of the PVA/MMT/PSSA composite membrane has a good electrochemical performance. Thiscomposite membrane is a potential candidate for the future PEMFC application.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

Polymer electrolyte membrane fuel cells (PEMFCs) [1e10] areconsidered as the most promising alternative green power sourcesfor the future EV transportation use, because they have high energyand power densities, good efficiency, low pollutant emission, andhigh utilization. During the last decades, progress has been achievedfor the PEMFC in materials, system design, manufacturing, andapplication. At present, aNafionmembrane iswidely used in PEMFC,due to its excellent chemical, mechanical stability, and high protonconductivity. However, the high cost of the Nafion membrane andthe lack of suitable hydrogen storagemedia are twomajor problemsfor PEMFC applications. Developing a cheap and high performancepolymer electrolyte membrane that facilitates the transport ofproton at different operating conditions can overcome the firstmajor problem on the commercializing fuel cell devices. Somearomatic hydrocarbon polymers based PEMFCs [2e7] are preparedto replace using the Nafion membrane. These polymer membranes,unfortunately, exhibit poor proton conductivity in a low humidityenvironment and restrict their further applications in PEMFCs.

Many efforts have been made by researchers to modify thepolymer electrolyte membrane. Inorganic-organic composite

þ886 29041914.ang).

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membrane is one of the effective methods, which has been studiedto enhance the water retention and to decrease the fuel crossover.Silica, zirconia, and titania are three major inorganic fillers. Wata-nabe et al. [4] first reported their work for the addition of hydro-scopic oxides into perfluorosulfonic acid membranes. Tang et al. [5]studied the Nafion/silica polymer membrane for fuel cell atelevated temperature. Chen et al. [6] investigated the Nafion/Zeolite nanocomposite polymer membrane for a direct methanolfuel cell application. The addition of nonconductive inorganic fillersinto the membrane normally results in a decrease of the protonconductivity. In order to avoid this phenomenon, some modifiedpolymer membranes with functionalized silica have been proposed[7e10] and several of them report about sulfonic acid functional-ized Nafion/silica composite membranes. Pereira et al. [7] exam-ined the mesostructured hybrid Nafion/silica composite membranefor high temperature fuel cell applications.

Nicolic et al. [9] prepared a cross-linkedpoly(vinyl alcohol) (PVA)polymer membrane for a hydrogen fuel cell by a gamma irradiationtechnique. Recently, Yang et al. [11,12] synthesized a cross-linkedPVA/MMT composite polymer membrane and applied themembrane to an acidic DMFC. However, a challenge for the PVA-based polymer membrane is that they show a poor protonconductivity and the acidic electrolyte may leak out from the poly-mer membranes. The main reason for this problem is that the PVApolymer itself does not contain any negatively charged ions or thenegative organic functional groups, such as carboxylic (eCOOH) or

C.-C. Yang et al. / Current Applied Physics 11 (2011) S229eS237S230

sulfonic acid (eSO3H) groups. Some negatively charged ionic groupsmust be grafted or blended into the PVA polymer host before usingPVA polymer membranes in a H2eO2 fuel cell. From this point ofview, the poly(styrene sulfonic acid) (PSSA) was chosen as a protondonor, because it can achieve reasonable proton conductivity for thePVA/PSSA blend composite polymer membrane. In addition, Rhimet al. [13] synthesized a PVA/sulfosuccinic acid (SSA) proton-con-ducting polymer membrane: the SSA containing both eSO3H andeCOOHgroups served as the cross-linking agent and a proton donorwith varying from 5 to 30wt.%.

Previous authors [14,15] have studied a sulfonated poly(etherether ketone)/poly(vinyl alcohol) (SPEEK/PVA blend), calleddouble-layer membrane, for a fuel cell. A sub-layer of PVA polymerclung to the anode and provided a good barrier for the alcoholcrossover. On the other hand, the SPEEK sub-layer maintained themechanical stability and a low swelling ratio. Kumar et al. [16]reported an interesting result about the poly(vinyl alcohol)/paratoluene sulfonic acid (PVA/PTSA) polymer membranes. The authorsintroduced a suitable amount of proton charge carriers (namely,eSO3H group), this hydrophilic PTSA greatly enhanced the ionicconductivity of PVA/PTSA polymer membrane. Moreover, Sahuet al. [17] investigated the effects of poly(styrene sulfonic acid)content on a PVA/PSSA blend polymer membrane and its applica-tion in a hydrogeneoxygen PEMFC. They revealed that a maximumproton conductivity of the PVA/PSSA blend membrane wasappeared when the weight percentage of PSSA appeared at 35wt.%.With the optimized PVA/PSSA polymer membrane, the PEMFCachieved a peak power density of 210 mW cm�2 at 500 mA cm�2 at75 �C, compared to a peak power density of only 38 mW cm�2 at80 mA cm�2 for the PEMFC with a pristine PVA membrane.

In this work, the constant amount of 10wt.% PSSA, i.e.,PVA:PSSA ¼ 1:0.10, was added in the PVA matrix. As we know,montmorillonite (MMT) is a well-known silicate material withlayered structure. Therefore, the addition of these hydrophilic MMTceramic fillers into the polymer matrix not only reduces the crystal-linity of the PVA polymer but also increases the amorphous phases ofPVA polymermatrix, resulting in an increase of its ionic conductivity[18,19]. A variety of ceramic fillers have been used in PVAmembrane,such as TiO2 (PVA/TiO2) [20], SiO2 (PEG/SiO2 and PVA/SiO2) [21], andhydroxyapatite (PVA/HAP) [22].

A suitable amount of the MMT fillers was first blended with thePSSA polymer in a weight ratio of 1:1 under continuous stirringcondition for 24h. These PSSA-blendedMMTfillersweredesignatedas the modified MMT fillers in this work. We attempted to dispersethe modified MMT fillers into the PVA and PSSA polymer matrix asthe solid plasticizers and ionic solid conductors,whichwere capableof enhancing the ionic conductivity. TGA and DSC experimentsanalyzed the thermal stability properties of the compositemembrane. DMA and SEM were applied to study the mechanicalproperties and surface morphology of the composite polymermembranes, respectively. XRDwas used to examine the crystallinityof PVA/MMT/PSSA solid polymer electrolytes (SPEs) and micro-Raman spectroscopy was used to investigate the chemical compo-sition information within each composite membrane. The ionicconductivity of proton-conducting composite polymer electrolytewas measured by AC impedance spectroscopy. The characteristicproperties of PVA/MMT/PSSA composite membranes with differentamounts (5,10,15, and20wt.%) of themodifiedMMTfillers (functionas the second ionic source) and 10wt.% PSSA polymer (act as themajor ionic source) were examined and discussed in detail.

Furthermore, a PEMFC with the air cathode, the hydrogenanode, and the PVA/MMT/PSSA composite polymer membrane wasassembled and compared the performance at operating tempera-tures of 25 and 50 �C, respectively. The electrochemical character-istics of the PEMFC employing the PVA/MMT/PSSA composite

membranes were investigated by the linear polarization, poten-tiostatic and galvanostatic methods; especially for the peak powerdensity of the PEMFC.

2. Experimental

2.1. Preparation of the PVA/MMT/PSSA composite membrane

PVA (Aldrich), PSSA (Aldrich, 18wt.% solution), nano-sized MMTfillers (Aldrich) and H2SO4 (Merck) were used as received withoutfurther purification. Degree of polymerization and saponification ofPVA were 1700 and 98e99%, respectively. The PVA/MMT/PSSAcomposite membrane was prepared by a solution casting method.The inert ceramic MMTclays were first blended with PSSA polymersolution in a weight ratio of 1:1 at room temperature for 24 h andfollowed by drying at 100 �C overnight, the obtained solid mixturepowders were further ground to fine particles. The ionic MMTfillers were further dried at 120 �C for 12 h before use. Theimpregnation pre-treatment process can transform the MMT froman inert filler to an ionic conducting filler (the MMT impregnatingwith eSO3H groups). Therefore, the proton conductivity of thecompositemembranes with thesemodifiedMMT fillers was greatlyimproved. Several different percentages (0e20wt.%) of the modi-fied MMT fillers were added slowly into the viscous solution understirring condition.

The obtained solution became homogeneous and viscous bystirring continuously at 85 �C for 3 h. The contents of the modifiedMMT fillers in the PVA and PSSA polymers were well controlled.About 5wt.% glutaraldehyde (GA, 25wt.% content in distilled water,Merck) was added into the viscous mixture polymer solution tocarry out the cross-linking reaction with PVA. The resulting viscousblend polymer solution was coated onto a glass plate. It was foundthat the viscous mixture polymer solution became gel when thecontent of the PSSA polymer (it also acted as a catalyst for the cross-linking reaction) was more than 10wt.%. The maximum content ofthe PSSA polymer, therefore, was controlled at 10wt.% in order toavoid the rapid formation high viscosity gel.

The thickness of the wet composite membrane is between 0.030and 0.050 cm. The sample of glass platewith viscous PVA/MMT/PSSAcomposite polymer was weighed again and then the excess waterwasallowed to evaporate slowlyat 60 �C ina relative humidityof 30%.The glass platewith the composite polymermembranewasweighedagain after water solvent was completely evaporated. The composi-tion of PVA/MMT/PSSA composite membrane was determined fromthe mass balance. The thickness of the dried composite membranewascontrolledbetween0.020and0.030 cm.Thedetailedpreparationmethods of the composite membranes based on PVA by a solutioncasting method have been reported in literature [11,12].

2.2. Crystal structure, surface morphology, thermal and mechanicalproperties

The surface morphology of the PVA/MMT/PSSA compositemembranes was investigated using a Hitachi S-2600H scanningelectron microscopy (SEM). Differential scanning calorimetry (DSC)(a Perkin Elmer Pyris 7 DSC system) and TGA (aMettler Toledo TGA/SDT 851e system) were used to examine the thermal properties ofthe PVA/MMT/PSSA composite membrane. DSC measurementswere carried out in a dry N2 atmosphere from 25 to 250 �C witha heating rate of 10 �C min�1. TGA measurements were carried outby heating from 25 to 600 �C under a N2 atmosphere at a heatingrate of 10 �C min�1 with a sample about 5e10 mg. Derivativethermogravimetry (DTG) curve is the first order differential of TGAcurve with respect to temperature. DMA were conducted using anRSA-III Instrument DMA-Thermal analyzer (TA) at a frequency of

Fig. 1. (a) TGA and (b) DTG thermographs for the PVA/PSSA/xwt.%MMT compositemembranes.

C.-C. Yang et al. / Current Applied Physics 11 (2011) S229eS237 S231

1 Hz and oscillation amplitude of 0.15 mm. DMA measurementswere carried out by heating from 25 to 150 �C under an air atmo-sphere at a heating rate of 5 �C min�1. The crystal structures of thePVA/MMT/PSSA composite membranes were examined usinga Philips X’Pert XRD with Cu Ka radiation of wavelength (l) of1.54056 Å for 2q angles between 10 and 80�.

2.3. Ionic conductivity measurements

The ionic conductivity of the PVA/MMT/PSSA compositemembrane was measured by an AC impedance method. The PVA/MMT/PSSA composite membranes were first immersed in a 2 MH2SO4 solution for at least 24 h, and then washed with D.I. waterseveral times before test. The composite membranes were clampedbetween stainless steel (SS304), ion-blocking electrodes, each ofsurface area 1.32 cm2, in a spring-loaded glass holder. In order tomeasure correct temperature, a thermocouple was put closely to thecomposite polymer membrane for temperature measurement. Eachsamplewas kept at the experimental temperature at least for 30minto reach equilibrium before measurement. AC impedance measure-ments were carried out using an Autolab PGSTAT-30 equipment (EcoChemie B.V., Netherlands). The AC spectra in the range of 100 kHz to10 Hz at an excitation signal of 5 mV were recorded. AC impedancespectra of the composite polymer membrane were recorded ata temperature range between 30 and 70 �C. The experimentaltemperatures were maintained constant within the variation of�0.5 �C by a convection oven. Each PVA/MMT/PSSA compositemembrane was examined at least three times.

2.4. Micro-Raman analyses

The micro-Raman spectroscopy analysis was carried out usinga Renishaw confocal microscopy Raman spectroscopy system witha microscope equipped with a 50� objective and a charge coupleddevice (CCD) detector. Raman excitation source was provided bya 632.8 nmHeeNe laser beam,which had the beampower of 17mWand was focused on the sample with a spot size of about 1 mm indiameter.

2.5. Preparation of the anode and the cathode

The catalyst slurry ink for the anode was prepared by using Pt/Ccatalyst (20wt.% of Pt), 15wt.% Nafion binder solution (Aldrich), anda suitable amount of distilled water and isopropyl alcohol (IPA). Theresulting Pt/C inks were first ultrasonicated for 2 h. The Pt/C inkswere loaded onto the carbon paper (GDL 10BB, SIGRACET, Germany)by an air spray method to achieve a loading of 1 mg cm�2. Theelectrode area was 5 cm2. The as-prepared anode was dried ina vacuum oven at 110 �C for 2 h. The air cathodewas prepared usingsimilar procedure as the anode, but the cathode with the Pt black(Alfa) inks of 1.0 mg cm�2.

2.6. Electrochemical property measurements

The PVA/MMT/PSSA composite membrane was sandwichedbetween the sheets of the anode and cathode and then pressed at25 �C under 100 kgf cm�2 for 5 min to obtain a membrane electrodeassembly (MEA). The electrode area of the MEA was about 5 cm2.The electrochemical measurements were carried out in a two-electrode system. The current densityevoltage (IeV) and the powerdensity (P.D.) curves for the PEMFC comprised of the PVA/MMT/PSSA composite membranes were recorded, respectively. All of theelectrochemical measurements were performed on an AutolabPGSTAT-30 electrochemical systemwith GPES 4.8 package software(Eco Chemie, Netherland). The electrochemical performances of the

PEMFC with H2 gas feed at a rate of 46 cm3 s�1 and O2 gas feed ata rate of 100 cm3 s�1 were examined in ambient pressure and at 25and 50 �C, respectively.

3. Results and discussion

3.1. Thermal and mechanical properties

Fig. 1(a) shows TGA thermographs for pure PSSA film, the PVA/5wt.%GA film, and the PVA/10wt.%PSSA/5e20wt.% MMT/5wt.%GAcomposite membranes, respectively. The TGA curve of pure PSSAfilm also shows three major weight loss regions, which appear asthree major peaks in the DTG curves, as shown in Fig. 1(b). The TGA

low

E

ndo up/ m

W

(1) PVA film

(2) PVA/5wt.% GA

(3) PVA/10wt.% PSSA/10wt.%M MT/5wt.% GA

(4) PVA/10wt.% PSSA/20wt.%M MT/5wt.% GA

(3)

(4)

Tm1

Tm2

C.-C. Yang et al. / Current Applied Physics 11 (2011) S229eS237S232

curve of PVA/5wt.%GA film (without any MMT fillers) shows threemajor weight loss regions, which appear as three correspondingmajor peaks in the DTG curves. The TGA curve of the PVA/10wt.%PSSA/xwt.%MMT polymer membranes shows a similar result asPVA/5wt.%GA film.

Thefirst regionat a temperatureof 50e90 �C (Tp,1¼72 �C) isdue tothe evaporation of weakly physical and strongly chemical boundedH2O; the weight loss of the membrane is approximately 7w8wt.%.The second transition region at around 300e400 �C (Tp,2¼ 334 �C) isdue to thedegradationofeSO3

� group (or calleddesulfonation)on thePSSA polymer and the totalweight loss corresponding to this phase isabout 27e28wt.%. The peak of the third transition at around400e450 �C (Tp,3 ¼ 421 �C) is as a result of the cleavage of the side-chain of the PSSA polymer, the total weight loss corresponding to thisphase is about 27e28wt.%. The totalweight loss is about 58wt.%whenthe temperature reaches at 600 �C.

For cross-linked PVA/5wt.%GA film, the first region at a temper-ature of 90e150 �C (Tp,1 ¼ 113 �C) is because of the evaporation ofphysical and strongly chemical bounded H2O and the weight loss ofthe membrane is approximately 4w5wt.%. The second transitionregion at around 250e400 �C (Tp,2¼ 340 �C) is due to the cleavage ofthe side-chain of PVA polymer and the total weight loss corre-sponding to this phase is about 78e79wt.%. In addition, it is alsofound that there is a shoulder peak at 288 �C (PVAdegradationpeak)at this thermal oxidation stage. The third transition region at around400e470 �C (Tp,3¼ 421 �C) is caused by the cleavage of the backboneof PVApolymer,which reflects the totalweight loss is approximately94e95wt.% at 600 �C.

For the as-synthesized PVA/MMT/PSSA composite membranes,the weight loss due to the evaporation of physical and stronglychemical bounded H2O at a temperature of 80e140 �C is very small,onlyabout 1w2wt.%. Thefirst transition region at around150e220 �C(Tp,1 ¼ 183 �C) is due to the degradation of eSO3

� group on the PSSApolymer, the total weight loss in this phase is about 29e30wt.%. Thesecond transition region at around 370e500 �C (Tp,2 ¼ 433 �C) isowning to the cleavage of the side-chain of PVApolymer and the totalweight loss is approximately 78e79wt.%. The peak of the thirdtransition at around 500e540 �C (Tp,3 ¼ 520 �C) is also caused by thecleavage of the backbone of PVA polymer, the total weight loss isabout 81e82wt.% at 600 �C.

The results for weight loss of the PVA/MMT/PSSA compositemembranes are summarized and listed in Table 1. Accordingly, thedegradation peaks of the cross-linked PVA/MMT/PSSA compositemembrane samples are less intense and shift towards highertemperature. It can be concluded that the thermal stability isimproved probably due to the effect of the addition of modifiedMMT fillers and the chemical cross-linked reaction between PVAand glutaraldehyde.

The DSC measurements were carried out by a hea-tingecoolingeheating cycle, called the HeCeH procedure. Thepurpose of the first heating cycle was to remove any thermalhistory of the PVA composite polymer membrane. The DSC (2nd

Table 1TGA results for weight loss of PVA/PSSA/MMT composite membranes with differentamounts of modified MMT fillers.

Loss(%) Temperatures

100 �C 200 �C 300 �C 400 �C 500 �C 600 �CTypes

Pure PSSA 13.0 16.5 18.8 40.1 54.3 57.5PVA/10%GA 1.29 23.97 36.13 44.51 82.66 85.00PVA//10%PSSA/5wt.%MMT/5%GA 1.18 19.71 34.73 44.12 78.42 81.03PVA/10%PSSA/10wt.%MMT/5%GA 1.43 17.74 33.94 44.46 76.71 79.88PVA/10%PSSA/20wt.%MMT/5%GA 1.48 18.82 32.61 41.63 67.98 72.29

heating) curves for the pure PVA film, the PVA/GA film, and thePVA/MMT/PSSA composite polymer membranes with various MMTcompositions (0e20wt.%) are shown in Fig. 2. An endothermic peakwas appeared at 225 �C, which corresponded to the meltingtemperature (Tm) of the pure PVA film. It had been reported that theTm of the pure PVA polymer with 98e99% degree of hydrolysis wasat 226 �C [11,12]. Moreover, an endothermic peak was presented at187 �C, which corresponded to the Tm of the PVA/GA film. It wasfound that the melting temperature, Tm, of the cross-linked PVA/GAfilm shifted toward lower temperature when the PVA film wascross-linked with GA.

In addition, two melting temperatures, Tm2 ¼ 187 �C andTm1 ¼ 174 �C, for the cross-linked PVA/10wt.%MMT/10wt.%PSSAcomposite membrane were found. In contrast, the two meltingtemperatures, Tm2 and Tm1, for the cross-linked PVA/20%MMT/10%PSSA composite membrane were at 178 �C and 167 �C, respectively.Both melting temperatures were shifted toward lower temperatureregion when the amount of added modified MMT fillers was addedincreasingly.

The above result indicates that the degree of crystallinity of thePVA/MMT/PSSA membrane gradually decreased when the amountof the modified MMT fillers increased. It was also found that there-crystallization peak temperature (Tc) was shifted to a lowertemperature direction and the Tc peak became broader from theDSC cooling curve (not shown here). This also indicated a changebetween a semi-crystalline phase and an amorphous phase.

The storage moduli (E0) vs. temperature curves of pure PVA film,the PVA/10wt.%PSSA film, and the PVA/0e20wt.%MMT/10wt.%PSSAcomposite membranes were measured (figure not shown here). Thestorage modulus of pure PVA film (E0¼6.60� 108 Pa) was lower thanthose of PVA/xwt.%MMT/10wt.%PSSA composite membranes (E0¼1.16w1.87�109Pa) at 30 �C. Itwas found that the storagemodulus ofPVA/MMT/PSSA composite membranes was slightly increased withincreasing the loading of the MMT fillers (up to 20wt.%). The storagemodulus started to decrease when the content of the modifiedMMTfillerswasover10wt.%, as listed inTable 2. As amatter of fact, the poorphysicochemical properties, the mechanical properties of the PVA/MMT/PSSA composite membrane were obtained when the quantity

Temperature/ o

C

50 75 100 125 150 175 200 225 250

Heat F

(1)

(2)

Tm

Fig. 2. DSC thermographs of pure PVA, the PVA/GA and the PVA/MMT/PSSA compositemembranes.

Table 2DMA results (storage modulus, tand vs. T) of the cross-linked PVA/PSSA/MMT composite membranes with different quantities of the modified MMT fillers.

E’/Pa PVA film PVA/10%PSSA PVA/10%PSSA/5wt.%MMT/5wt.%GA PVA/10%PSSA/10wt.%MMT/5wt.%GA PVA/10%PSSA/20wt.%MMT/5wt.%GA

Temp.

30 �C 6.60 � 108 2.23 � 109 1.87 � 109 1.72 � 109 1.16 � 109

60 �C 1.22 � 108 1.33 � 108 8.78 � 107 9.12 � 107 1.13 � 108

100 �C 1.17 � 108 1.57 � 108 8.02 � 107 7.99 � 107 1.08 � 108

150 �C 8.38 � 107 1.02 � 108 4.70 � 107 6.34 � 107 1.00 � 108

tan(d), oC 27.01 54.83 52.18 52.33 54.47

C.-C. Yang et al. / Current Applied Physics 11 (2011) S229eS237 S233

of the modified MMT fillers was 25wt.% (data not shown here).Actually, the amountofmodifiedMMTfillers added in the PVA/MMT/PSSA composite membrane needs to be well controlled. It can beconcluded that themaximum content of the modifiedMMT fillers inthe PVA/MMT/PSSA composite membrane was limited to 20wt.%.Interestingly, it was also found that the mechanical properties wereimprovedwhen the thermal treatmentwas carried out at 120 �C. Thismay be due to the annealing effect on the cross-linked PVA-basedcomposite membrane. In general, the annealing treatment willenhance the degree of cross-linking between the eOH group of thePVA polymer and the eCHO group of GA.

Fig. 3 shows the tan(d) vs. temperature curves of the PVA/MMT/PSSA composite membranes. The glass transition temperatures (Tg)canbe takenatapeak (tan(d)1) of the tan(d) curve. Theglass transitiontemperatures of pure PVA film (Tg, PVA) and all cross-linked PVA/x%MMT/10wt.%PSSA composite membranes are listed in Table 2. Theresults indicated that the glass transition temperatures of pure PVAfilm (considered as a Tg, PVA) and the PVA/10wt.%PSSA membraneswere at 27.01 and 54.83 �C, respectively. Apparently, it was observedthat there is only one tan(d) peak for the PVA/xwt.%MMT/10wt.%PSSAcomposite membranes. The tan(d) peaks for the compositemembranes, they were taken as the Tg, PVA, were located between 52and 54 �C. Interestingly, these tan(d) peaks were much broaden forthe PVA/xwt.%MMT/10wt.%PSSA composite membranes, ascompared with those of pure PVA film and the PVA/PSSA blendmembrane. The broader and lowering intensities for tan(d) peaks for

Temperature/ o

C

20 40 60 80 100 120 140 160

ta

n(

)/ a

.u

.

-0

(1). Pure PVA

(2). PVA/10wt.%PSSA/5wt.GA

(3). PVA/10wt.%PSSA/5wt.%MMT/5wt.%GA

(4). PVA/10wt.%PSSA/10wt.%MMT/5wt.%GA

(5). PVA/10wt.%PSSA/20wt.%MMT/5wt.%GA

(1)

(2)

(3)

(4)

(5)

Tg, PVA

Fig. 3. DMA curves for the pure PVA and the PVA/10wt.%PSSA/xwt.%MMT compositemembranes (the tan(d) vs. T curves).

the PVA/xwt.%MMT/10wt.%PSSA composite membranes are strongevidences for decreasing of the crystallinity of the compositemembranes.

3.2. XRD and SEM analyses

X-ray diffraction measurement was performed to examine thecrystallinity of the PVA/MMT/PSSA composite membrane. Fig. 4(a)shows the XRD pattern for the as-received MMT fillers and themodified MMT fillers containing PSSA polymer, respectively. It wasobserved that there were two XRD characteristic peaks for theseMMT materials at 2q angles of 1.75 and 3.30�. It was found that thediffraction peak positions are only slightly changed; however, thepeak intensity is greatly reduced and becomes broad. Fig. 4(b)illustrates the diffraction pattern of the PVA/MMT/PSSA composite

Fig. 4. XRD patterns for (a) the as-received MMT fillers and the modified MMT fillers;(b) the pure PVA film, the PVA/GA film, and the PVA/MMT/GA, PVA/10wt.%PSSA/20wt.%MMT/GA composite membranes.

C.-C. Yang et al. / Current Applied Physics 11 (2011) S229eS237S234

membranes that were prepared by adding different amounts of themodified MMT fillers. The PVA polymer is well-known exhibitinga semi-crystalline structure with a large crystalline peak at a 2qangle of 19e20� and a small amorphous peak at 39e40�. The peakat 19.5� is corresponded to the (101) plane for the pure PVA film.The peak intensity at 2q ¼ 19� became weaker for the PVA/5e20wt.%MMT composite polymer membranes, which could beseen clearly in Fig. 4(b). But, the intensities of the (101) peak forPVA/MMT/PSSA compositemembranes were greatly reduced whenthe amount of added modified MMT fillers was increased. Undersuch circumstances, it was clear that the amorphous domain in thePVA/MMT/PSSA composite polymer membranes was markedlyaugmented (i.e. the degree of crystallinity decreased).

Table 3 demonstrates the values of relative crystallinity (%) forthe PVA/MMT/PSSA composite polymer membranes with differentcompositions of MMT fillers. The relative crystallinity values of thePVA/MMT/PSSA composite membrane decreased from 100 to46.03%. Note that the domain of amorphous phases increases withan increase in the contents of the modified MMT fillers. There isa significantmotion of the polymer chain in the amorphous phase orsome defects existing at an interface between the polymer chainsand the MMT fillers. Therefore, the PVA/MMT/PSSA compositemembranes show good ionic transport property (data shown latersection). This is due to the more amorphous phase and flexibility oflocal PVA chain segmentalmotion in the PVA/MMT/PSSA compositemembranes.

The top and cross-sectional views of SEM photographs for thePVA/20wt.%MMT/10wt.%PSSA composite membranes can be foundin Fig. 5(a), (b), and (c), respectively. Furthermore, it was found thatthese hydrophilic MMT fillers are dispersed well into the PVApolymer matrix. The dimension for the modified MMT fillersembedded in PVA matrix is about 0.5 mm � 1w2 mmwith long thinoval shape, MMT fillers as shown clearly in Fig. 5(c). As we can see,these modified MMT fillers are completely mixed with PVA poly-mer. As a whole, the compatibility of the PVA polymer and themodified MMT fillers shows a uniform surface texture when thecontent of the modified MMT fillers is not over 20wt.%.

In short, both hydrophilic PVA polymer and the modified MMTfillers were homogeneous and fully compatible without any phaseseparation occurring while a suitable amount of the modified MMTfillers was added. It is well accepted that the suitable amount of themodified MMT fillers (also as the ionic conductors containingeSO3H groups) [18,19] can greatly increase the ionic conductivity ofthe composite membrane.

Fig. 5. SEM photographs for PVA/10wt.%PSSA/20wt.%MMT composite membranes:(a) top view; (b) and (c) cross-sectional views.

3.3. Micro-Raman analyses

3.3.1. Micro-Raman spectroscopy is a powerful tool to characterizethe PVA/MMT/PSSA composite membrane

Fig. 6(a) shows the micro-Raman spectra of pure PVA and PVA/xwt.%MMT/10wt.%PSSA composite membranes, respectively. Thespectra showed some strong characteristic scattering peaks for PVA

Table 3XRD results of PVA/10wt.%PSSA/xwt.%MMT composite membranes.

Types XRD peaks position

Position/deg. Intensity/a.u. Relative intensity/%Membranes

Pure PVA film 19.53� 18600 100.000wt.%MMT 19.41� 11100 59.685wt.%MMT 19.47� 11271 60.6010wt.%MMT 19.41� 8947 48.1020wt.%MMT 19.49� 8562 46.03

polymer at 1440, 1258, 1146, 919, and 860 cm�1 [11,12,22]. A verystrong peak for the PVA polymer at 1440 cm�1 was observed due tothe CeH bending and OeH bending. In addition, two additionalvibrational peaks for PVA polymer at 919 and 860 cm�1 were due tothe CeC stretching and several weak scattering peaks at 1258, 1146,1093, and 1066 cm�1 due to the CeC stretching and CeO stretchingwere shown in Fig. 6(a). For the PSSA, three characteristic scatteringpeaks for the PSSA polymer were at 1599, 1125, and 1041 cm�1, asseen in Fig. 6(a). More specifically, the peaks of 1599 cm�1 were dueto the C]C stretching and the peaks of 1125 and 1041 cm�1 were dueto the eSO3

� stretching.

Fig. 6. (a) micro-Raman spectra for the pure PVA film, and the PVA/10wt.%PSSA/20wt.%MMT composite membranes; (b) micro-Raman spectra for the PVA/10wt.%PSSA/Xwt.%MMT composite membranes.

Z '/ ohm

0 10 20 30 40 50 60 70

-Z

''/ oh

m

0

20

40

60

80

100

120

140

(1) 30o

C

(2) 40o

C

(3) 50o

C

(4) 60o

C

(5) 70o

C

(1)

(2)

(3)

(4)

(5)

Fig. 7. Nyquist plot for the PVA/20wt.%MMT/10wt.%PSSA composite membrane.

1000

1100

1200

100

120

(1). I-V at 25o

C

(2). I-V at 50o

C

(3). P.D. at 25o

C

(4). P.D. at 50o

C

(4)

C.-C. Yang et al. / Current Applied Physics 11 (2011) S229eS237 S235

The most important outcome with respect to the micro-Ramananalysis, it can be clearly seen that the intensities of these SO3-vibrational peaks (at 1041 and 1126 cm�1) for the PVA/MMT/PSSAcomposite membrane were slightly varied, as shown in Fig. 6(b). Avibrational peak of the eSO3

� at 1041 cm�1 is an indicator for thePSSA polymer. In other words, it indicated that the ionic conduc-tivity is increased; it is due to the existing negative ionic groups(-SO3

�) in the PVA/MMT/PSSA composite membrane. The additionof the PSSA polymer and the modified MMT (both containing the-SO3

� ionic sources) results in enhancing the proton conductivity.

Current density/ mA cm-2

0 25 50 75 100 125 150 175 200 225 250

Cell voltage/ m

V

300

400

500

600

700

800

900

P.D

./ m

W cm

-2

0

20

40

60

80

(1)(2)

(3)

Fig. 8. The IeV and P.D. curves for the PEMFC composed of the PVA/10wt.%PSSA/20wt.%MMT composite membrane with H2 and O2 gas fuels at 25 and 50 �C and atatmospheric pressure.

3.4. Ionic conductivity measurements

The typical AC impedance spectra of the PVA/MMT/PSSAcomposite membrane, synthesized by blending PVA and PSSA poly-mers with 20wt.% modified MMT fillers, could be obtained atdifferent temperatures, as shown in Fig. 7 (Nyquist plot). The ACspectra are typically non-vertical spikes for stainless steel (SS)blocking electrodes, i.e., anSSrPVA/MMT/PSSASPErSS cell. Analysis ofthe spectra yields information about the properties of the PVA/MMT/PSSA composite membrane, such as bulk resistance, Rb. The bulkresistance associated with the membrane conductivity was deter-mined from the high-frequency intercept of the impedancewith realaxis. Taking into account the thickness of the composite membranes,the ionic conductivity (s) was calculated from the Rb value, accordingto the equation: s ¼ L=Rb,A, where s is the proton conductivity ofthe compositemembrane (S cm�1), L is the thickness (cm) of the PVA/MMT/PSSA composite membrane, A is the cross-sectional area of the

blocking electrode (cm2), and Rb is the bulk resistance (ohm) ofa proton-conducting composite polymer membrane.

Typically, the Rb values of the PVA/20wt.%MMT/10wt.%PSSAcomposite membranes were on the order of 4e16 U and are highlydependent on the contents of themodifiedMMT fillers [18,19]. Notethat these composite polymer membranes were immersed in D.I.water for 24 h before measurement.

Table 4 shows the ionic conductivities of PVA/0e20wt.%MMT/10wt.%PSSA composite membranes at different temperatures. Aslisted in Table 4, the ionic conductivity value of the PVA/10wt.%PSSA composite membrane (without any MMT fillers) in water is2.07 � 10�3 S cm�1 at 30 �C. In contrast, the ionic conductivityvalues for PVA/xwt.%MMT/10wt.%PSSA composite membraneswith 5, 10, 15 and 20wt.% MMT fillers are 3.11 � 10�3, 3.62 � 10�3,

Table 4The ionic conductivities (S cm�1) of the PVA/10wt.%PSSA/xwt.%MMT compositemembranes.

s(S cm�1) 0%MMT 5%MMT 10%MMT 15%MMT 20%MMT

Temp.

30 �C 2.07 � 10�3 3.11 � 10�3 3.62 � 10�3 3.69 � 10�3 6.69 � 10�3

40 �C 2.47 � 10�3 3.82 � 10�3 4.64 � 10�3 4.88 � 10�3 7.72 � 10�3

50 �C 3.36 � 10�3 4.55 � 10�3 5.17 � 10�3 5.82 � 10�3 9.01 � 10�3

60 �C 4.27 � 10�3 5.19 � 10�3 5.78 � 10�3 6.97 � 10�3 10.4 � 10�3

70 �C 4.71 � 10�3 5.84 � 10�3 6.33 � 10�3 8.19 � 10�3 12.0 � 10�3

Table 5Electrochemical performances of the PEMFC comprised of the PVA/10wt.%PSSA/20wt.%MMT composite membrane with H2 and O2 feeds at 25 and 50 �C and atambient pressure.

Temperature 25 �C 50 �C

Parameters

Eocp/V 0.930 0.941Peak P.D./mW cm�2 90.70 65.23ip,max/mA cm�2 221.23 159.10Ep,max/V 0.410 0.410

C.-C. Yang et al. / Current Applied Physics 11 (2011) S229eS237S236

3.69 � 10�3 and 6.69 � 10�3 S cm�1 at 30 �C, respectively. It wasfound that the PVA/20wt.%MMT/10wt.%PSSA composite membranehas the highest ionic conductivity, s ¼ 6.69 � 10�3 S cm�1, atambient temperature.

By contrast, Sahu et al. [17] showed an ionic conductivity of1.30 � 10�3 S cm�1 for the PVA/PSSA blend membrane in fullyhumidified condition at 30 �C. Moreover, they also showed that theionic conductivity of a pristine PVA membrane was only1.0�10�5 S cm�1. According to the above results, it is seen clearly thatthe ionic conductivity of the PVA/MMT/PSSA membrane decreaseswhen the content of the modifiedMMT fillers is higher than 20wt.%.Furthermore, Huang et al. [23] also studied the proton-conductingmembranebasedonPVAandpoly (vinyl pyrrolidone) (PVP)withSSA.Their experimental results showed the proton conductivity of theorder of 10�3 S cm�1 for the PVA/PVP/SSA composite membranes.

According to the above results, it was observed that the ionicconductivity of the PVA/xwt.%MMT/10wt.%PSSA composite elec-trolytes in water was of the order of 10�3 S cm�1 at ambienttemperature. The temperature dependence of the ionic conduc-tivity is of the Arrhenius type: s ¼ so expð�Ea

RTÞ, where so is a pre-exponential factor, Ea is the activation energy, and T is thetemperature in Kelvins. From the log10 (s) vs. 1/T plots, the acti-vation energy (Ea) can be obtained for the PVA/20wt.%MMT/10wt.%PSSA composite electrolytes, which is highly dependent on theloading of modified MMT fillers [11,12]. The Ea values of the PVA/10wt.%PSSA/xwt.%MMT composite membrane are approximately8e12 kJ mol�1, which are lower than the general Nafion 117membrane having Ea value of 15e20 kJ mol�1.

The proton transport follows two mechanisms: one is the Grot-thus mechanism, which can be explained as a proton jump from onesolvent molecule to the next through hydrogen bonds; the other isthe vehicle mechanism, in which the proton diffuses together withsolvent molecules by forming a complex (i.e., H3Oþ) and subse-quently diffusing intact. Both the Grotthus and vehicle mechanismsmay be responsible for the composite polymer membrane protontransfer.

3.5. Electrochemical performance measurements

Fig. 8 shows the potentialecurrent density (IeV) and the powerdensity-current density curves for the PEMFC with H2 fuel and O2oxidant at 25and50 �C, respectively. At 25 �C, thepeakpowerdensityof 65.23 mW cm�2 was achieved for the PEMFC using PVA/20wt.%MMT/10wt.%PSSA composite membrane at Ep,max ¼ 0.410 V witha peak current density (ip,max) of 159.1 mA cm�2, as displayed clearlyin Table 5. At 50 �C, the peak power density of 90.70 mW cm�2 wasachieved for the PEMFC with PVA/20wt.%MMT/10wt.%PSSAcompositemembrane at Ep,max¼ 0.410Vwith a peak current density(ip,max) of 221.23 mA cm�2.

Only few studies based on the PVA-based composite polymermembrane for the PEMFC could be found in the literature. Bycomparison, the peak power density of 210 mW cm�2 for thePEMFC with gaseous H2 and O2 feed at 75 �C (w100%RH) andatmospheric pressure is achieved with the PVA/PSSA (25wt.%)

blend membrane [17]. However, the peak power density of38 mW cm�2 for the PEMFC is obtained with pure PVA membraneunder identical operating condition. By comparison, the differenceperformance of the peak power density in PEMFC is due to differentcompositions of the proton-conducting composite membrane andthe operating conditions. The performance characteristics of thePEMFC with different PVA/PSSA composite membranes aresummarized in detail and shown in Table 5.

The above results clearly manifest that the PEMFC comprised ofthe PVA/MMT/PSSA composite membrane showed comparableelectrochemical performance under ambient conditions. The meritis clear that the PVA/MMT/PSSA composite membrane is a cheapnon-perfluorosulfonated polymer membrane, as compared withthe perfluorosulfonated Nafion 117 membrane; which is anexpensive polymer membrane.

4. Conclusions

Theproton-conducting compositemembranes basedon thePVA,PSSA polymers, and the modified MMT fillers were prepared bya solution castingmethod. The ionic conductivities of the compositemembranes were of the order of 10�3 S cm�1 in water at ambienttemperature. Before the addition of MMT fillers, the MMT fillerswere pre-blended with the PSSA polymer in a weight ratio of 1:1.These modifiedMMT fillers were well dispersed into the mixture ofPVA and PSSA polymer matrix. The modified MMT filler containingPSSA polymerwas used as a second ionic source, which is capable ofenhancing the ionic conductivity, thermal properties, and dimen-sional stability on the PVA/MMT/PSSA composite membrane. Thepolymer electrolyte membrane fuel cell (PEMFC) comprised of theacidic PVA/MMT/PSSA composite membrane was assembled andexamined. The performance of the PEMFCusingH2 andO2 feedswasexamined at ambient temperature and atmospheric pressure.Moreover, it was found that the peak power densities at ambientpressure are 65.23 and 90.70mWcm�2 at 25 and 50 �C, respectively.In conclusion, the PVA/MMT/PSSAcompositemembranes are a goodcandidate for the future PEMFC applications.

Acknowledgements

Financial support from the National Science Council, Taiwan(Project No: NSC-96-2221-E131-009-MY2) is gratefullyacknowledged.

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