Solid State Ionics 201 (2011) 21–26

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Solid State Ionics

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PVA nano composite membrane for DMFC application

Jatindranath Maiti, Nitul Kakati, Seok Hee Lee, Seung Hyun Jee, Young Soo Yoon ⁎Energy and Sensor Laboratory, School of Materials Science and Engineering, Yonsei University, 134 Shinchon Dong, Seoul 120-749, South Korea

⁎ Corresponding author. Tel.: +82 2 2123 2847; fax:E-mail address: yoonys@yonsei.ac.kr (Y.S. Yoon).

0167-2738/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.ssi.2011.07.013

a b s t r a c t

a r t i c l e i n f o

Article history:Received 22 July 2010Received in revised form 15 July 2011Accepted 18 July 2011Available online 13 September 2011

Keywords:Polyvinyl alcohol (PVA)Sulfonated MWCNTFluorinated MMTPolymer nano composite membraneDMFC

A new PEM composite membrane comprising of polyvinyl alcohol (PVA), sulfonic acid functionalized CNT andfluorinated MMT has been fabricated. Composite polymer membrane has been prepared by simple solutioncasting method. Composite properties have been evaluated by using thermal gravimetric analysis (TGA),scanning electron microscopy (SEM), and FTIR techniques. The proton conductivity, methanol crossover andwater uptake properties of newly fabricated membrane have been studied. The polymer membrane showsgood thermal properties. The water content is in the range of 35–45%. Especially, it has been found that thefluorinated MMT used in this study plays a decisive role in water uptake and acts as a hydrophobic surface forcontrolling the swelling. The proton conductivities and the methanol permeabilities of all the membranes arein the range of 10−3 to 10−2 S/cm and 2.08×10−6 cm2/s at room temperature, respectively.

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1. Introduction

Fuel cell technology is a benign process and has the potential tobecome future generation green energy for portable electronics andvehicle propulsion [1–3]. Now a day, research on the improvement ofpolymermembrane for directmethanol fuel cell application is oneof thechallenging assignments to resolve the balance between protonconductivity andmethanol crossover [4–6]. The large-scale commercialutilization of Nafion® in DMFC causes some issues such as low powerdensity which is due to methanol crossover and dehydration at hightemperature [4,7]. Low cost membrane is also required for DMFCcommercial application without sacrificing their properties. Reductionof membrane cost could be achieved by using non-fluorinated polymerelectrolytes with a cheaper polymer. From commercial point of view,PVA is a possible candidate to be used as amembrane for DMFC becauseof its low cost, good chemical stability, film-forming ability, and highhydrophilicity and availability of cross-linking sites to create a stablemembrane with goodmechanical properties and selective permeabilityto water [8–11]. Furthermore, the PVA polymer used is biodegradable,nonhazardous, and environmentally benign [12]. PVA based compositemembranes if optimized may serve as a potential alternative proton-conducting membrane for direct methanol fuel cell applications. Wateruptake is also important in determining the ultimate performance ofproton exchangemembranematerials. In essentially all currentpolymerbased membrane, water is required to facilitate proton conductivity.However, absorbed water also affects the mechanical properties of themembrane by acting as a plasticizer, lowering the Tg andmodulus of the

membrane [13]. Careful control of water uptake is critical for reducingadverse effects of swelling anddegradationof themechanical propertiesof the membrane in humid environments, as well as inducing stressesbetween themembraneand theelectrodes. Both conductivity andwateruptake rely heavily on the concentration of ion conducting units (mostcommonly sulfonic acid) in the polymer membrane. Varying the ioncontent of the membrane can control both its water uptake andconductivity. While it is desirable to maximize the proton conductivityof the membrane by increasing its ion content (decreasing equivalentweight), other physical properties must be considered. Too many ionicgroupswill cause themembrane to swell excessively withwater, whichcompromisesmechanical integrity and durability. Polymer swelling canbe reduced by crosslinking with suitable crosslinking agent, whereasproton conductivity can be increased by the formation of hybridcomposites by the incorporation of proton conductors such as sulfonicacids [14]. In the past fewdecades, nano clay (MMT) [15–17] and carbonnanotubes (CNTs) [18] were studied intensively as typical nano fillers toincorporate into polymer matrices. The dispersion of nano clay inpolymer can result in a reduction of moisture absorption, thermalstability, barrier properties, and flammability as well as significantenhancements of modulus, strength, and hence overall performance ofnanocomposite [16]. Furthermore, clay is inexpensive relative totraditional reinforcing materials and environmentally friendly. Thesulfonic acid functionalized carbonnano tube basedpolymer compositeshas been revealed a remarkable improvement of proton conductivityand also capable of increasing the mechanical stability along with adecrease in methanol permeability [18].

Herein we present a chemical strategy to increase the sulfonic acidcontent and better channel like network for proton transport of PVAmembrane by incorporating sulfonic acid functionalized multi walledcarbon nanotubes. The membranes with highly hydrophobic

22 J. Maiti et al. / Solid State Ionics 201 (2011) 21–26

fluorinated surface exhibited improved proton conductivity andreduced methanol permeability at a relatively low water uptake[19]. The hydrophobic blocks can serve as matrix for mechanicalstrength and limited swelling. We have integrated both hydrophobicsurface and acidic group by using two different nano filler. Thestrategy of using both sulfonated CNT and fluorinated MMT to enrichproton conductivity, self humidification, thermal and barrier proper-ties of the membrane might be helpful in alleviating many significantdifficulties associated with fuel cell.

In this work, composite membrane composed of polyvinyl alcohol(PVA), sulfonic acid functionalized CNT and fluorinated MMT havebeen prepared by simple solution casting method. This is a newapproach to add two different functionalized nano filler into samematrix for studying the water uptake, proton conductivity, methanolpermeability and thermal stability of composite membrane.

2. Experimental

2.1. Materials

Polyvinyl alcohol (PVA) (molecular weight of 31,000–50,000),sulfosuccinic acid (SSA) (70 wt% in water solution), Hexafluoropho-sphoric acid (65 wt.% solution in water), Montmorillonite K10, all thechemicals were purchased from Aldrich company. Multi walled CNTwas purchased from EM Power, South Korea.

2.2. Sulfonation of MWCNT

First, purification of MWCNT (1 g) was carried out by refluxing theCNT with 200 ml of 60% HNO3 at 120 °C for 4 h to remove the metalparticles. The mixture was diluted, centrifuged and washed withexcess DI water. The purified product was dried at 70 °C in vacuumoven for overnight.

Second, the sulfonation of MWCNT was executed in presence of(NH4)2SO4 [20]. 0.25 g of ammonium sulfate dissolved in 5 ml DIwater was mixed with 0.25 g MWCNT. After that, the mixture waswell agitated; it was heated at 235 °C for as long as 30 min. It isbelieved that at 235 °C, (NH4)2SO4 decomposes to generate SO3, andthe formed SO3 reacts with carbon via its surface hydrogen atoms tohave −SO3H groups linked onto it [21].

NH4ð Þ2SO4→2NH3 + H2O + SO3 ð1Þ

Carbon−H + SO3→Carbon−SO3H: ð2Þ

2.3. Fluorination of MMT

MMT (1.2 g) was dispersed in 100 ml of deionized water using anultrasonic bath. The suspension was stirred with 1 N H2SO4 (20 ml) atroom temperature for 2 h in order to increase its surface activity and toremove impurities. The mixture was washed with excess deionizedwater and dried in vacuum oven at 70 °C for 12 h. 2 ml hexafluoropho-sphoric acid (60 wt.% solution in water) and MMT (1 g) in 5 ml waterwere mixed and the mixture was stirred using a Teflon beaker at roomtemperature for 24 h [22]. The product was washed in a mixture ofisopropyl alcohol andwater (1:1) for preventing agglomeration ofMMTparticle resulting inhigher surface areaproduct. Finally thematerialwasdried in vacuum oven at 70 °C for 12 h.

2.4. Membrane preparation

10 wt.% of PVA in water was stirred continuously at 90 °C for 6 huntil the solution mixture reached a homogeneous solution. Then thePVA solutions were mixed with sulfonated MWCNT (1wt.%),fluorinated MMT (1 wt.%) and cross linking agent sulfosuccinic acid

(SSA) (10 wt.%) and the mixture was stirred at room temperature for24 h. After that, the homogeneous solutions were poured onto aplastic petridish. The cast polymer solutions were allowed to dry in airat room temperature for 24 h. The fully driedmembranes were peeledoff away from the petridish, and then heated in an oven at 120 °C for1 h to make cross linking reaction. The membranes were stored in DIwater before use. The membrane thickness was in the range of 100–150 μm.

2.5. FTIR

Fourier transform infrared (FT-IR) spectroscopic measurementswere performed using a JASCO FT-IR 300E device.

2.6. Thermogravimetric analysis

The membrane thermal stability was evaluated by using athermogravimetric analyzer TA Q 50 system TGA. The samples werescanned at a heating rate of 10 °C/min under flow of nitrogen.

2.7. Surface morphology and chemical composition characterization

A scanning electron microscope with an energy dispersive X-rayspectroscopy system (FESEM JSM-6700F, JEOL coupled with INCAenergy dispersive X-ray spectroscopy) was used to evaluate themembrane microstructure and chemical composition.

2.8. Water uptake

The water uptake of the membranes was determined bymeasuring the change in the weight before and after the hydration.Pre-dried membranes were immersed in deionized water for 24 h,and then surface attached water onto the membrane was removedwith filter paper. After that, the wettedmembraneweight (Wwet) wasdetermined as quickly as possible. The weight of dry membrane(Wdry) was determined after completely drying it in vacuum at 60 °Cfor 24 h. The water uptake (%) value of themembranes was calculatedby using the following equation

Water uptake %ð Þ = Wwet−Wdry

WdryX 100:

2.9. Proton conductivity measurements

Proton conductivity measurements were carried out at ambienttemperature after equilibrating themembrane in de-ionizedwater for1 day. The proton conductivity cell was composed of two 5 mmdiameter platinum electrodes. The membrane sample was sand-wiched between the platinum electrodes. Proton conductivity of themembranes was measured by an impedance spectroscopy using aSolartron 1260 gain phase analyzer, interfaced to a Solartron 1480multistat. The measurement was carried out in a potentiostatic modein the frequency range of 0.1 Hz to 10 MHz with 5 mV of oscillatingvoltage. The laboratory made four probe conductivity cell was used.The conductivity cell was placed in the head-space of a temperaturecontrolled sealed vessel which was maintained at 100% relativehumidity. Proton conductivity (σ) of the samples was calculated fromimpedance data using the following equation [23]:

σ =L

RWD

where R is the membrane resistance derived from the impedancevalue at zero phase angle, L is the distance between two potential

23J. Maiti et al. / Solid State Ionics 201 (2011) 21–26

sensing platinum electrodes, W and D are the width and thickness ofthe membrane respectively.

2.10. Methanol crossover

Methanol permeability measurement was carried out with ahome-made permeation measuring cell that had two compartments.Compartment Awas filled with 150 ml 20% (v/v) methanol solution inde-ionized water, and compartment B was filled with 150 ml de-ionized water. The membrane was mounted between the twocompartments, and the diameter of the diffusion area was 3.0 cm.The solutions in both compartments were magnetically stirred. Themethanol concentration in compartment B was monitored using arefractive index detector (RI BT600, Younglin Instrument Co., Korea)through a 1-mm diameter silicon tube with a 1.0 ml min−1 constantflow driven by a Master flex pump. The output signal was convertedby a data module (Autochro, Younglin Instrument Co., Korea) andrecorded by a personal computer. Methanol permeability (P) wasobtained by means of the following relationship [24]:

CB ðtÞ ¼A PV B L

CA ðt� toÞ

where CA is the initial methanol concentration in compartment A;CB(t) the methanol concentration in compartment B at diffusiontime t; VB the volume of de-ionized water in compartment B; L thethickness of the membrane; and A is the effective permeating area.

3. Results and discussion

3.1. FTIR

Themembranewas prepared by solution castingmethod as shownin scheme 1. FTIR studies have been carried out on samples containingPVA and sulfonated MWCNT, and composite membrane (Fig. 1). In

Scheme 1. Membra

pure PVA, we observe a doublet peak around 1000–1300 cm−1 and abroad region around 3000–3500 cm−1. They are characteristic of PVAand have been assigned to C–O stretching and O–H stretching,respectively. In sulfonated MWCNT, two peaks at 1030 cm−1 andaround 700 cm−1 have been assigned to S_O and S–O symmetricstretching, respectively. Upon blending sulfonated MWCNT and SSAwith PVA, the S_O and S–O stretching characteristics of membranegrow up while the C–O stretching characteristic of PVA decreases.More direct evidence of the percentage of sulfur and fluorine comesfrom EDX studies, as will be discussed later.

3.2. Thermal properties

The thermal stability of the PVA nano composite membrane wasevaluated through the TGA experiments. The thermogravimetricanalysis (TGA) results for the PVA and composite membranes areshown in Fig. 2. Pure PVA sample exhibited two thermal decompo-sition stages. The first occurred at 285 °C and the second at 400–450 °C. These two stages reflect the breakage of the side and mainbackbone polymer chains, respectively [25]. At the end of the analysis,at 800 °C, the PVA had 5% residue. The TGA curve of the PVAmembrane showed three consecutive weight losses arising from theprocesses of thermal solvation, thermal desulfonation, and thermooxidation and degradation of the polymermatrix. The first weight lossof about 5 wt.% at 100 °C is closely associatedwith the loss of absorbedwater molecules. Most of these absorbed water molecules aresupposed to be in a bound state, rather than in the free molecularstate [26]. The second weight loss of about 35 wt.% at around 150–380 °C corresponds to the loss of sulfonic acid group by thedesulfonation and a breakage of some portion of polymer chains aswell as breakage of the ester bonds. In the third weight loss of about40 wt.% at temperatures N400 °C is due to the decomposition of themain chains of the PVA [27]. The decomposition temperature of PVAwas 285 °C. That temperature increased to 410 °C with chemicalcrosslinking and inorganic nano filler addition. Many researchers have

ne preparation.

Fig. 1. FTIR spectra of PVA, sulfonated MWCNT and membrane.

Fig. 3. (a) SEM photograph of the PVA/S-MWCNT/F-MMT/SSA (membrane 1) and(b) PVA/S-MWCNT/SSA (membrane 2).

24 J. Maiti et al. / Solid State Ionics 201 (2011) 21–26

pointed out that incorporation of nanoparticles into a polymer matrixenhanced the membranes thermal stability for PVA. Mbhele et al. [28]claimed that the thermal degradation occurs from free radicalformations at weak bonds and/or chain ends, followed by theirtransfer to adjacent chains via interchain reaction. The presence of thenanofiller restricted themobility of the polymer chains, prohibited thefree radical transfer and, therefore, suppressed the thermal degrada-tion. It could therefore be concluded that the thermal stability wasimproved due to the additive effect of theMMT and CNT fillers and thechemical crosslinking reaction between the OH group on the PVA andthe COOH group on the SSA.

3.3. SEM and EDX

The morphology and composition of composite membraneshave been analyzed by a SEM microscope. SEM photographs for thePVA/S-MWCNT/F-MMT/SSA (membrane 1) and PVA/S-MWCNT/SSA(membrane 2) composite polymer membrane are shown in Fig. 3 (a)and (b), respectively. The basic difference between these membranesis that membrane 1 contains F-MMT and membrane 2 does notcontain F-MMT except this all other compositions are same for both.No relevant morphological features have been noticed in thesemembranes although some difference on the surface morphologyfor membrane 1 has been detected. Many different sizes of aggregatesor chunks that are randomly distributed on the top surface have been

0 100 200 300 400 500 600 700 800 900

0

20

40

60

80

100

Wei

gh

t (%

)

Temperature (oC)

PVA polymer PVA nano composite

Fig. 2. TGA plot of PVA and nano composite membrane.

observed for membrane 1 (PVA/S-MWCNT/F-MMT/SSA). The com-patibility of membrane 2 (without F- MMT) is still uniform andhomogenous. This indicates that the nano F-MMT was not properlydispersed within the PVA polymer matrix, as shown in Fig. 3.However, it is clearly seen that cracks have been generated on thetop of the surface in the both type (with or without MMT) ofmembrane. The degree of sulfonation and fluorination has beencalculated by EDX measurement as shown in Fig. 4, 0.16 wt.% of thesulfur is attached to the MWCNT by this method (Fig. 4 a). Indeed theamount of sulfonation is low and can be systematically varied bychanging the concentration of ammonium sulfate. The degree offluorination on the MMT is 0.62 wt.% (Fig. 4b).

3.4. Water uptake

The water uptake of the PVA nano composite membranes is37 wt.%. While the water uptake of pure PVA membrane (24.4 wt%) iscomparable to that of Nafion®117 membrane, the PVA nanocomposite membrane exhibited a remarkably higher water uptakeowing to the high hydrophilic nature of cross linking agent and thedisruption of the highly ordered arrangement of pristine PVA chainindividually [29]. We have prepared PVA nano composite membranewithout fluorinatedMMT to study the positive effect of the addition ofhydrophilic SSA as well as sulfonated CNT and the negative effect ofhydrophobic fluorinated MMT function on the water uptake. Thewater uptake of PVA nano composite without fluorinated MMT is

Fig. 4. (a) EDX analysis of S-MWCNT, (b) EDX analysis of F-MMT.

Fig. 5. Proton conductivity of membrane 1 (PVA/S-MWCNT/F-MMT/SSA) andmembrane 2 (PVA/S-MWCNT/SSA) in the temperature range from 25 to 80 °C under100% RH conditions.

25J. Maiti et al. / Solid State Ionics 201 (2011) 21–26

45 wt.% which is higher than that of the fluorinated MMT basedmembrane. The fluorinated MMT decreases the water content byincreasing the hydrophobic surface property. This result shows thatwater uptake can be controlled by using hydrophobic–hydrophilicinteraction to reach a balance.

3.5. Proton conductivity and methanol permeability

The proton conductivity of the membrane is a key property thatdirectly affects operational fuel cell voltage. The proton conductivitymeasurements of the membrane were run at RH 100% at roomtemperature in the longitudinal direction by AC impedance spectros-copy. The proton conductivities of the hybridmembranesmeasured ata temperature range between 25 and 80 °C. The proton conductivityvalue of membrane 1 (PVA/S-MWCNT/F-MMT/SSA) and membrane 2(PVA/S-MWCNT/SSA) is 0.006 S/cm and 0.004 S/cm at 30 °C, respec-tively. We have added same amount of sulfonic acid source (1 wt.%sulfonated MWCNT and 10 wt.% crosslinking agent with respect toPVA) in both type of membrane. Both type of membrane show moreor less same order of proton conductivity. Proton conductivityincreases with increasing temperature in both cases (Fig. 5). It wasreported that proton conductivity of PVA based membrane increasedwith increasing the content of crosslinking agent (SSA) [30,31]. Inour case, observed proton conductivity is low compared to othermembranes (reported proton conductivity in the order of 10−2 S/cm)[31,32] due to low level of sulfonic group in MWCNT as well as

crosslinking agent present in the membrane. Water uptake plays acritical role in proton conduction because it is the major carrier ofprotons. However, excess swelling in water reduces the membrane'smechanical strength. Typically, many polymer electrolyte membranesswell or even become soluble in water when the sulfonation level

26 J. Maiti et al. / Solid State Ionics 201 (2011) 21–26

increases in order to obtain high proton conductivity [30]. In our case,crosslinked as well as hydrophobic–hydrophilic interaction bothapproach are applied to balance water uptake and proton conductivity.

The methanol permeability of the membranes 1 and 2 has beencalculated as 2.08×10−6 cm2/s and 4.13×10−6 cm2/s, respectively. Itis known that methanol permeates through hydrophilic ionicchannels; especially free water molecules and that proton aretransported by hopping between ionic sites due to hydrogen bondingbetween bound water molecules as well as through ionic channels.Therefore, it is expected that the methanol permeability should bedecreased due to the MMT particles acting as materials for blockingthe methanol transport and/or for reducing free water. Howevermethanol permeability of our membrane is high compared to othertype of PVA membranes (methanol permeability in the range of10−8–10−7 cm2/s) [31,32]. This is possibly due to the crackgeneration on the surface of the membrane. MMT does not act as ablocking material for methanol transport in our study owing to theiragglomerate structure.

For comparison purposes, methanol permeability and protonconductivity of Nafion 115 membranes were also measured using thesame apparatus and testing conditions. The values of methanolpermeability and proton conductivity obtained were 1.78×10−6 cm2/sand0.112 S/cmat30 °C, respectively. In relation to this study,mostof themembranes prepared herein have similar methanol permeabilitycompared to Nafion 115 membrane. Proton conductivity values are,however, lower than that of theNafion115. Further attempts have yet tobe made to improve the proton conductivity as well as methanolpermeability of the PVA nanocomposite membrane. This might beachieved by optimizing the degree of sulfonation and the degree ofcrosslinking of the membrane.

4. Conclusions

In the present work, crosslinked PVA nanocomposite membranecontaining sulfonic acid and fluorine group has been prepared bysimple solution techniques and also evaluated as a potential polymerelectrolyte membrane in direct fuel cell application. Especially,sulfonic acid functionalized MWCNT and fluorine functionalizedMMT have been effectively introduced into the PVA matrix withsulfosuccinic acid as a crosslinked agent. The proton conductivity ofthe membrane is in the range of 0.004 to 0.01 S/cm. The low protonconductivity is due to low level of sulfonic acid group present in themembrane. The methanol permeability of the membrane is2.08×10−6 cm2/s. It has been found that the water uptake of themembrane can be controlled by using fluorine functionalized MMT.Our future attempt is to obtain a uniform, improved property and to

demonstrate the practical applicability of using this PVA compositemembrane without cracks and nano-sized chunks or aggregate inDMFC.

Acknowledgments

This research was supported by the Pioneer Research CenterProgram through the National Research Foundation of Korea fundedby the Ministry of Education, Science and Technology (2011-0001676).

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