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Journal of Membrane Science 376 (2011) 225–232

Contents lists available at ScienceDirect

Journal of Membrane Science

journa l homepage: www.e lsev ier .com/ locate /memsci

lkali doped polyvinyl alcohol/multi-walled carbon nano-tube electrolyte forirect methanol alkaline fuel cell

en-Han Pana, S. Jessie Luea,c,∗, Chia-Ming Changb,c, Ying-Ling Liub,c,∗∗

Department of Chemical and Materials Engineering, Chang Gung University, Kwei-shan, Taoyuan 333, TaiwanDepartment of Chemical Engineering, Chung Yuan University, Chungli, Taoyuan 320, TaiwanR&D Center for Membrane Technology, Chung Yuan University, Chungli, Taoyuan 320, Taiwan

r t i c l e i n f o

rticle history:eceived 31 January 2011eceived in revised form 13 April 2011ccepted 15 April 2011vailable online 22 April 2011

a b s t r a c t

A novel route to functionalize polyvinyl alcohol (PVA) onto multi-walled carbon nano-tube (MWCNT)is reported in this work. FTIR, XPS, Raman spectroscopy, and TGA data confirmed PVA grafting ontothe MWCNT. The grafted PVA content was estimated to be 25% in the PVA–functionalized MWCNT. Anano-composite consisting of PVA and 0.05% functionalized CNT was successfully prepared using a solu-tion casting method. Water solubility and diffusivity were enhanced in the CNT-containing membranes.

eywords:irect methanol alkaline fuel cell (DMAFC)ell performanceethanol permeability

onductivityano-composite

The ionic conductivity of the potassium hydroxide (KOH)-doped PVA/CNT membrane was improved byadding the functionalized CNT, which might be ascribed to the ionic channels provided by the CNT. Themethanol permeability was suppressed in the CNT-containing sample. The alkali-doped electrolytes wereapplied in direct methanol alkaline fuel cells. An open-circuit potential and a peak power density of 0.86 Vand 39 mW cm−2 were obtained using a 2 M methanol fuel in 6 M KOH at 60 ◦C with the PVA/CNT/KOHelectrolyte, significantly higher than those without CNT incorporation.

. Introduction

Fuel cells have attracted much attraction as an alternative powerupply in many applications. Direct methanol fuel cells (DMFC) areuitable for portable electronic devices due to many advantages,ncluding high energy density, compact design, and room tempera-ure start-up. Most DMFCs use a proton-exchange membrane, suchs Nafion® from Dupont [1,2], as the membrane electrolyte. Thiserfluorosulfonic acid membrane suffers from methanol cross-over3–5], resulting in detrimental effects: cathode catalyst poison-ng and reduced cell voltage due to the mixed over-potential ofhe unwanted methanol oxidation due to methanol transfer fromnode to cathode. Researchers have recently paid more attentiono developing hydroxide conducting polymer electrolytes for direct

ethanol alkaline fuel cells (DMAFCs) [6–10].

In a DMAFC operation, hydroxide ions are generated on the

athode and consumed on the anode [11]. The hydroxide ions areransported from the cathode to anode and oppose the methanol

∗ Corresponding author at: Department of Chemical and Materials Engineering,hang Gung University, 259 Wen-hwa First Road, Kwei-shan, Taoyuan 333, Taiwan.el.: +886 3 2118800x5489; fax: +886 3 2118700.∗∗ Corresponding author at: Department of Chemical Engineering, Chung Yuanniversity, Chungli, Taoyuan 320, Taiwan. Tel.: +886 3 2654130;

ax: +886 3 2654199.E-mail addresses: jessie@mail.cgu.edu.tw (S.J. Lue), ylliu@cycu.edu.tw (Y.-L. Liu).

376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.memsci.2011.04.026

© 2011 Elsevier B.V. All rights reserved.

diffusion direction. The methanol oxidation rate is more favorablein an alkaline solution than in an acidic one [11–13]. Less expensivecathode catalysts are available to reduce the cell cost from usingprecious metals [14–16]. These advantages are the main moti-vation in the development of hydroxide-conducting membranes.Although one might be concerned that carbonate salt might haveformed on the anode of a DMAFC and this weak acid might havereduced the alkalinity of the methanol/KOH solution, we found thatthe K2CO3 formation during 100-h of continuous operation in analkaline direct methanol fuel cell with recycling anode feed did notaffect the cell performance because the amount of produced car-bonate was negligible and the potassium salt was soluble in theaqueous solution [9].

Although anion-exchange membranes have been adopted inDMAFC application, the cell performance is not as high as thosewith alkali-doped electrolytes [6–10]. Polyvinyl alcohol (PVA) isoften used as a base material for alkali doping, owing to its inex-pensiveness, hydrophilicity, and good film forming property. Theabundant hydroxyl groups provide good compatibility and uptakesubstantial amounts of alkali aqueous solution. Many modificationmethods, including cross-linking [17–19], copolymerization [20],and the addition of inorganic fillers [9,21–23] are used to fabricatePVA with enhanced mechanical strength. In addition, we found the

incorporation of nano-fumed silica particles into the PVA matrixenhanced the stability in water due to the physical cross-linkagemechanism, which suppressed the polymer crystal unfolding andthe membrane dissolution in water [24]. The ionic conductivities of

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26 W.-H. Pan et al. / Journal of Me

he alkali-doped PVA/filler composites were enhanced as comparedith the alkali-doped PVA electrolyte [9,10] due to the enlarged

ree volume size present in the polymer matrix caused by theano-filler [25]. This enlarged free volume size was big enough

or water molecules and hydroxide ion to permeate but hinderedarger methanol transport, thus reducing the methanol cross-overate [9,10].

In the last few years, carbon nano-tubes (CNT) have receiveduch attention due to their excellent mechanical, electrical,

hermal, and magnetic properties [26]. Some researchers founddvantages of polymer and CNT composites. Thomassin et al. [27]eported beneficial effects by dispersing 1–2% multiwalled car-on nano-tubes (MWCNT) into Nafion membranes. The Young’sodulus of the composite membranes was increased while theethanol permeability was decreased without adverse impact on

he ionic conductivity. Joo et al. [28] found that the mechani-al properties were improved by adding functionalized CNT intoulfonated poly(arylene sulfone) (sPAS) matrix. The DMFC perfor-ance using the CNT-containing sPAS was significantly improved

ompared with the neat sPAS film, owing to the former higheronic conductivity and decreased methanol permeability. Liu et al.29] demonstrated that blending Nafion–functionalized MWCNTould improve the proton-exchange hydrogen fuel cell power den-ity by 50% compared with pristine Nafion. Recently Liu et al. [30]eported that the incorporation of Nafion– and polybenzimida-ole (PBI)–functionalized MWCNT into PBI matrix also enhancedhe hydrogen fuel cell performance. The peak power density wasncreased by 13–32% as compared with the electrolyte without theunctionalized CNT addition [30].

In the present research, we investigate the DMAFC performancesing potassium hydroxide (KOH)-doped PVA/MWCNT electrolyte.he CNT surface modification and the PVA/CNT composite prepa-ation are reported. The enhanced cell voltage and power densityn fuel cells using the PVA/CNT/KOH electrolyte are correlated tohe membrane characteristics.

. Experimental

.1. Materials

Multi-walled carbon nano-tubes (MWCNT), with average diam-ters of 10–50 nm and lengths of 1–25 �m were received fromhe Carbon Nano-tube Co., Ltd., Incheon, Korea. The purity of theeceived MWCNT is 93%. MWCNT was washed with dimethyl-ulfoxide prior to use. Polyvinyl alcohol (PVA, average moleculareight of 89,000–98,000, more than 99% hydrolyzed), and potas-

ium hydroxide (KOH) were obtained from Sigma–Aldrich. Methyllcohol (HPLC grade, 99.9%) was from Acros Organics, Geel,elgium. Gas diffusion electrodes with 5 mg cm−2 Pt–Ru alloy (1:1)lack for the anode and 5 mg cm−2 Pt black for the cathode wereurchased from E-tek, Somerset, NJ, USA. Pure water with resistiv-

ty of 18 M� cm was produced using a Millipore water purifier (Elix/Milli-Q Gradient system, Millipore Corp., Bedford, MA).

.2. Preparation of PVA–functionalized MWCNTs

PVA–functionalized MWCNT was prepared using an ozone-ediated method as reported previously [31,32]. One gram of PVAas dissolved in 30 mL water at 70 ◦C, placed in a 100 mL one-ecked flask with a stirrer. The PVA solution was purged withzone gas for 15 min. The ozone gas flow rate was 6 L min−1 and the

zone concentration was 28 g m−3. The solution was then purgedith argon gas for 15 min to remove the free peroxide groups. Halfgram of CNT was added in the PVA solution quickly. The mix-

ure was stirred at 80 ◦C for 3 h to graft the PVA polymer onto the

e Science 376 (2011) 225–232

MWCNT. After the reaction was completed, the functionalized CNTand the solution were separated using a centrifuge and washedwith hot water until the solution was clear. The PVA–functionalizedCNT was collected using filtration and then dried over night. TheMWCNT and PVA reaction scheme is shown in Fig. 1.

2.3. PVA/CNT composite preparation

The PVA–functionalized CNT (0.0075 mg) was suspended in10 mL of deionized (DI) water under ultrasonication at room tem-perature. Fifteen grams of PVA were dissolved in 50 mL of DI waterat room temperature to form a polymer solution. The CNT sus-pension was mixed into the PVA solution. Another 75 mL of DIwater was used to rinse the beaker wall and remove the remainingCNT. The rinse solution was then combined with the CNT and PVAsolution. The solution (total of 135 mL) was heated to 90 ◦C underultrasonication and stirring (250 rpm) for 6 h to obtain a homoge-neous solution. The solution was cast on a glass with an applicationknife (model 3580, Elcometer Instrument Ltd., Edge Lane, England).A uniform thickness film was obtained after being dried in vacuumat 60 ◦C for 6 h. The resulting PVA composites consisted of 0.05%(weight basis) of PVA–functionalized CNT, unless stated otherwise.The thickness of the dried composite membrane was 130–180 �m.The PVA and PVA/CNT composites were immersed in KOH solutionof various concentrations for at least 24 h. These KOH-doped PVAand PVA composite were referred as PVA/KOH and PVA/CNT/KOH.

2.4. Characterization

Fourier transform infrared (FTIR) spectra of the functional-ized CNT, PVA and PVA/CNT composite membrane were obtainedthrough the attenuated total reflectance method using an FTIR(Perkin Elmer Spectrum One, Perkin Elmer Corp., Norwalk, CT,USA) equipped with a multiple internal reflectance apparatus and aZnSe prism as an internal reflection element. Raman spectra of thepristine and PVA–functionalized CNT were obtained using a Ren-ishaw InVia Raman spectrometer (3D Nanometer Scale Raman PLMicrospectrometer, Tokyo Instruments, Inc., Tokyo, Japan) employ-ing a He–Ne laser of 1 mW radiating on the sample operatingat 632.8 nm. Thermo-gravimetric analysis (TGA) was performedwith an instrument from the Thermal Analysis Incorporation (TA-TGA Q-500, TA Instrument, New Castle, DE, USA) under a nitrogenatmosphere at a heating rate of 10 ◦C min−1. X-ray photoelectronspectroscopy (XPS) analysis was conducted with a VG MicrotechMT-500 ESCA (Thermo Fisher Scientific Inc, Walthan, MA, USA)using an MgK� line as the radiation source. High-resolution trans-mission electron microscopy (HRTEM) was conducted with aJEOL JEM-2010 HR-TEM (JEOL Ltd, Tokyo, Japan). The membranemicro-structure was observed using field emission scanning elec-tron microscope (FESEM, model S-4800, Hitachi High-TechnologiesCorp., Tokyo, Japan) after being freeze-fractured in liquid nitro-gen and then sputtered with Pt. An X-ray diffraction (XRD, modelD5005D, Siemens AG, Munich, Germany) measurement was per-formed on PVA and PVA/CNT composite membranes to examinetheir crystallinity characteristics. The X-ray radiation was gener-ated using Cu K� (wavelength 1.54056 A) from an anode operatingat 40 kV and 40 mA. The scanning rate was 0.5◦ s−1 with a 0.02◦

resolution. The XRD was recorded over the angles 15–50◦.

2.5. Water uptake and diffusivity

The membrane water uptake was determined by measuring thedifference between the dry weight (Wo, in g) and total weight (Wtt,

W.-H. Pan et al. / Journal of Membrane Science 376 (2011) 225–232 227

of P

ic

M

rfit

l

wem

2

wArmsDt3bftTwPdtf

F

warVatt

C

wr

t

F

Fig. 1. Reaction mechanism

n g) after immersion in 25 ◦C DI water. The solvent uptake (M) wasalculated from the following equation:

= Wtt − Wo

Wo(1)

The water diffusion coefficient was calculated on the transientegime using the limiting slope method [33]. The diffusion coef-cient was fitted for a relative uptake data set as a function ofime:

n(

1 − Mt

M∞

)− ln

(8

�2

)= �2Dt

ı2(2)

here Mt and M∞ are the solvent uptake (in g g−1) at time t and atquilibrium, respectively, D is the diffusion coefficient and ı is theembrane thickness.

.6. Methanol permeability measurement

Methanol permeability measurements for 1 M methanol at 30 ◦Cere carried out on a PVA membrane and PVA/CNT composite.side-by-side diffusion cell consisting of two-compartment glass

eservoir (source and receiving reservoirs) was used to test the per-eability. The source reservoir was filled with a methanol aqueous

olution of volume VA and the receiving reservoir was filled withI water of volume VB. These two reservoirs were separated by

he membrane under test. The permeation cell was maintained at0 ◦C. The methanol concentration transported through the mem-ranes was determined by sampling a small amount of the solutionrom the receiving compartment at time intervals for concentra-ion measurement using a gas chromatograph (HP 4890A, Agilentechnologies Co. Ltd., St. Louis, MO, USA). The analytical columnas a fused silica capillary column (30 m × 0.32 mm, Supel-QTM

LOT, Supelco, St. Louis, MO, USA). The methanol permeability wasetermined from the methanol concentrations at various elapsedimes using the equation described in our previous paper [34]. Theollowing two equations are for the methanol mass conservation:

lux = VB

A

dCB

dt= −VA

A

dCA

dt(3)

here A is the effective membrane area, t is the time elapsed,nd CA and CB are the methanol concentrations in the source andeceiving reservoirs, respectively. In this experiment, VA equaledB for calculation simplicity. Taking mass balance on methanol byssuming insignificant volume changes in both reservoirs duringhe permeation experiment and negligible methanol sorbed insidehe membrane yields Eq. (4).

A,o = CA + CB (4)

here CA,o is the initial methanol concentration placed in the sourceeservoir.

In addition, Fick’s law holds true for a methanol diffusion process

hrough the membrane:

lux = −DdCm

dx= D

CmA − Cm

B

L= D

K(CA − CB)L

(5)

VA grafting onto MWCNT.

where D is methanol diffusion coefficient, Cm is methanolconcentration inside the membrane, x is the distance along trans-membrane direction, L the membrane thickness, K the partitionconstant relating methanol concentration inside membrane to thatin aqueous solution, and subscripts A and B represent the interfaceat the donor and receiving reservoirs, respectively.

Combining Eqs. (3)–(5) by eliminating CA and integrating bothsides of Eq. (3) result in the following equation:

lnCA,o

CA,o − 2CB= 2ADK

LV(t − to) (6)

where permeability P equals DK and can be obtained as a slope byplotting ln(CA,o/(CA,o − 2CB)) vs. 2At/LV.

2.7. Ionic conductivity measurements

The PVA and PVA/CNT membranes were immersed in a 2 or 6 MKOH solution for at least 24 h before conductivity measurement.A potentiostat (Autolab, PGSTAT-30, Eco Chemie B.V., Utrecht,Netherlands) was used to measure the alternative current (AC)impedance of the KOH-doped electrolytes. The electrolyte wassandwiched between two stainless steel electrodes, each with asurface area of 1.33 cm2, in a spring-loaded glass holder [19]. Thisapparatus was maintained in a chamber with controlled relativehumidity and temperature. The relative humidity of the chamberwas kept at about 99% and the testing temperature ranged from30 ◦C to 60 ◦C. The sample was scanned from 100 kHz to 100 Hz withoscillating amplitude of 5 mV. The bulk resistance was determinedfrom the Nyquist plot of the KOH-doped electrolytes according tothe procedure described in the literature [9,35]. The conductivity(�, in S cm−1) of the KOH-doped electrolyte was calculated usingthe following equation:

� = l

RbA(7)

where l is the thickness of the electrolyte (cm), Rb is the bulk resis-tance (�), and A is the contact area of the stainless steel electrodes(cm2) [9].

2.8. Cell performance measurements

The PVA/KOH and PVA/CNT/KOH membranes were sandwichedbetween anode and cathode gas diffusion electrodes to obtainmembrane electrode assemblies (MEA). The gas diffusion elec-trodes (combining catalysts on diffusion layers) were commercialproducts and purchased from E-tek. The cathode catalyst was highprecious (HP) Pt black on Vulcan XC-72R carbon nano-particles.The anode catalyst was HP Pt:Ru alloy (1:1) on Vulcan XC-72R.The gas diffusion layer was a hydrophobicity-controlled microp-orous layer on carbon cloth. The catalyst loading was 5 mg cm−2

for the anode and cathode. The effective area of the MEA was

5 cm2. The current density (I) and potential (V) of the DMAFC wererecorded on an electrical load (PLZ164WA electrochemical sys-tem, Kikusui Electronics Corporation, Tokyo, Japan) at a scan rateof 5 mA s−1. The power density was calculated as the product of

228 W.-H. Pan et al. / Journal of Membrane Science 376 (2011) 225–232

csd

3

3

aMsMgwMpotadMXo(Ma2batsb

fs(altttMPDM

tionalized MWCNT. The pristine MWCNT did not show obviousweight loss, whereas the weight loss of the PVA–functionalizedMWCNT was 20 wt% at 600 ◦C, which could be attributed to thethermal degradation of PVA chains wrapped on MWCNT. As the

Fig. 2. FTIR spectrum of PVA and PVA–functionalized MWCNT.

ell voltage and current density. The power density–current den-ity (P–I) curves were plotted to determine the maximum powerensity (Pmax).

. Results and discussion

.1. Characterization of PVA–functionalized MWCNT

PVA was chemically incorporated into MWCNT bundles usingn ozone-mediated process. The obtained PVA–functionalizedWCNT was characterized for the functional groups present. Fig. 2

hows the FTIR spectra of the PVA and the PVA–functionalizedWCNT. PVA exhibited major absorption peaks of C–O and –OH

roups at about 1150 and 3300 cm−1, respectively. These peaksere also observed in the FTIR spectrum of PVA–functionalizedWCNT. This demonstrates that the PVA–functionalized MWCNT

ossesses PVA chains and the PVA was successfully graftednto the MWCNT. The absorption peak at about 1605 cm−1 inhe PVA–functionalized MWCNT spectrum was attributed to thebsorption of C O groups, which were generated from the oxi-ation reaction in the ozone treatment. The PVA–functionalizedWCNT chemical structure was also further characterized withPS (Fig. 3). There was a significant oxygen signal and high intensityn the PVA–functionalized MWCNT in the wide-scan XPS spectrumFig. 3(a)). The C1s core-level spectrum of the PVA–functionalized

WCNT could be de-convoluted into 4 peaks of C–H, O C–C, C–O,nd C O species at binding energy of 285.0, 285.5, 286.5, and90.0 eV, respectively (Fig. 3(b)). The C–H, O C–C, C–O, and C Oonds originated from the grafted PVA. The C O groups couldrise from the un-hydrolyzed repeating units of PVA chains andhe oxidation reaction during the ozone treatment. These resultsupport the successful incorporation of PVA chains into MWCNTundles.

The changes in the MWCNT chemical structures due to PVAunctionalization were monitored using Raman spectroscopy. Ashown in Fig. 4, the pristine MWCNT shows a tangential bandG band) at about 1572 cm−1 and a disorder band (D Band) atround 1324 cm−1. The PVA–functionalized MWCNT exhibits simi-ar absorption peaks compared to pristine MWCNT, demonstratinghe presence of MWCNT structures. In the PVA functionalizationreatment, ozone-treatment on the PVA chains generated radicalshat reacted with the MWCNT. Some sp2-hybrid carbons in the

3

WCNT were converted into sp -hybrid in the reaction. As a result,VA–functionalized of MWCNT demonstrated an increase in the- to G-band intensity ratios (ID/IG) from 1.15 (for the pristineWCNT) to 4.41 (for the functionalized MWCNT). This phenom-

Fig. 3. XPS characterization of PVA–functionalized MWCNT: (a) wide scan spectrumand (b) C1s core-level spectrum.

ena has been reported in the polyurethane functionalized MWCNT[36].

Fig. 5 shows TGA thermograms of the PVA, MWCNT and func-

Fig. 4. Raman spectra of pristine MWCNT (blue line) and PVA–functionalizedMWCNT (red line). (For interpretation of the references to color in this figure legend,the reader is referred to the web version of this article.)

W.-H. Pan et al. / Journal of Membrane Science 376 (2011) 225–232 229

Fig. 5. TGA analysis in nitrogen of PVA (green line), pristine MWCNT (blue line) andPi

pscttlttrbPtMmfM

Table 1Water uptakes, diffusion coefficients, and methanol permeabilities in PVA andPVA/CNT composites at 25 ◦C.

Composite Water uptake(g g−1)

Water diffusioncoefficient(10−7 cm2 s−1)

Methanolpermeability(10−7 cm2 s−1)

the sorption history data using Eq. (2) and the water diffu-

VA–functionalized MWCNT (red line). (For interpretation of the references to colorn this figure legend, the reader is referred to the web version of this article.)

ure PVA showed a residue of about 20 wt% in the TGA analy-is, the PVA weight fraction of the PVA–functionalized MWCNTould be calculated to be about 25 wt%. It is noteworthy thathe weight loss of the functionalized MWCNT occurred at higheremperatures than those for the pristine PVA. The chemicalinkages between the PVA and the MWCNT could enhance thehermal stability of the grafted PVA chains. Moreover, ozoniza-ion and the subsequent addition reaction of PVA chains mightesult in a cross-linked structure and increase the thermal sta-ility of the PVA chains. Fig. 6 shows the HRTEM micrographs ofVA–functionalized MWCNT. Compared with the pristine MWCNT,he amorphous polymer layers covering the outer bundles of the

WCNT were observed on the functionalized MWCNT. The poly-

er layer thickness was about 3 nm. The TGA and HRTEM results

urther confirm the successful preparation of PVA–functionalizedWCNT.

Fig. 6. HR-TEM images of pristine MWCNT (upper) and PVA–function

PVA 2.81 2.14 3.57PVA/CNT 2.94 3.09 2.99

3.2. Characterization of the PVA/CNT composite

FE-SEM micrographs of the PVA and PVA/CNT composites areshown in Fig. 7. Both the PVA and the PVA/CNT surfaces weresmooth and little difference was found on their surface morphol-ogy. From the cross-sectional views of the PVA/CNT composites theCNT agglomerate was observed occasionally (Fig. 7), which mightbe due to van der Waals force between the CNT bundles.

The X-ray diffraction measurement was examined to study thecrystalline of the PVA and PVA/CNT membranes. Fig. 8 shows theirdiffraction patterns at 2� between 15 and 50◦ patterns. It is clearthat the pure PVA shows a remarkable peak for an orthorhombiclattice centered at a 2� of 19.9◦ [24], indicating its semi-crystallinenature. The d-spacing was calculated to be 4.45 A. Both samplesdemonstrated similar crystalline diffraction patterns but the peakintensity of the PVA/CNT membranes was slightly lower than thatfor the PVA film. It implies that the PVA/CNT crystallinity wasreduced compared with that of the PVA.

3.3. Water sorption and diffusion

Fig. 9 shows the typical sorption uptake history for PVA andPVA/CNT membranes. The water solubility in the PVA/CNT com-posites was slightly higher than that in the PVA film (2.94 g g−1

vs. 2.81 g g−1). The water diffusion coefficients were fitted from

sion coefficient was improved with increasing the CNT content(3.09 × 10−7 cm2 s−1 for PVA/CNT and 2.14 × 10−7 cm2 s−1 for PVA,Table 1). There are two possible reasons for this diffusivity increase.

alized MWCNT (lower) at 40 k, 100 k, and 300 k magnifications.

230 W.-H. Pan et al. / Journal of Membrane Science 376 (2011) 225–232

rface a

Tafcd

3

mmtw[aa[

Fig. 7. Field emission scanning electron micrographs of su

he polymer crystallinity may be reduced at the CNT presence [25]nd more amorphous region in the PVA favored the water dif-usion. Moreover, the embedded CNT had abundant grafted PVAhains, which may serve as a special hydrophilic channel for wateriffusion.

.4. Methanol permeability

To reduce methanol crossover, methanol permeabilityust be overcome. It was found that the methanol per-eability of the PVA/CNT composite was slightly reduced

o 2.99 × 10−7 cm2 s−1 as compared with the pristine PVA,hich had a methanol permeability of 3.57 × 10−7 cm2 s−1

10]. This reduction in methanol permeability may be

scribed to the lower compatibility of CNT and methanolnd the hindered pathway under the presence of CNT28].

Fig. 8. X-ray diffraction patterns of PVA and PVA/CNT membranes.

nd cross-sectional views on PVA and PVA/CNT composite.

3.5. Ionic conductivity of KOH-doped PVA and composite

The resistance data of the KOH-doped PVA and PVA/CNT com-posite at various temperatures were measured using the ACimpedance analyzer. Fig. 10 shows the Nyquist plot for PVA andPVA/CNT electrolytes with 2 M KOH at 30 ◦C. From the x-interceptsof the curves in Fig. 10, it was clear that PVA/CNT exhibited a lowerresistance than PVA, which might be caused by ionic channelsprovided by the incorporated functionalized CNT [37]. The func-tionalized MWCNT bundles were covered by a layer of PVA (Fig. 6)and full of hydroxyl groups on the CNT surface. These hydroxylgroups not only absorbed water molecules (as shown in the wateruptake in Table 1), but also took up KOH species. The hydroxide ionscould transfer through these hydroxyl groups and the conductivitywas enhanced in the PVA/CNT/KOH electrolyte. Table 2 summarizes

the conductivity data for PVA and PVA/CNT electrolyte doped with2 M and 6 M KOH at 30–60 ◦C. At an elevated temperature the con-ductivity increased due to higher ionic mobility. The PVA/CNT/KOHhad higher conductivity than PVA/KOH. The 6 M alkali concentra-

Fig. 9. Water sorption history for PVA and PVA/CNT composite membranes.

W.-H. Pan et al. / Journal of Membrane Science 376 (2011) 225–232 231

Fig. 10. Nyquist plots of PVA and PVA/CNT electrolytes doped with 2 M KOH at30 ◦C. The small insert shows the high frequency range data, which are extracted toobtained resistance and conductivity values.

Table 2Ionic conductivity (in 10−2 S cm−1) of PVA and PVA/CNT doped with 2 M and 6 MKOH.

Alkali concentration Membrane Temperature (◦C)

30 40 50 60

2 M KOH PVA 5.25 7.02 8.43 10.262 M KOH PVA/CNT 6.97 7.91 8.61 10.24

ts[

3

P6palm

Famw

Fig. 12. DMAFC voltage (left axis) and power density (right axis) as a function of cur-rent density at 30 and 60 ◦C using PVA/CNT/KOH electrolyte (anode: 2 M methanolin 6 M KOH with a flow rate of 5 mL min−1, cathode: humidified oxygen with a flow

6 M KOH PVA 5.64 7.28 9.01 10.886 M KOH PVA/CNT 7.13 8.38 9.97 11.76

ion also enhanced the electrolyte conductivity than 2 M. This isimilar to the results for PVA/fumed silica KOH-doped composites9].

.6. DMAFC performance

Fig. 11 shows the polarization curves for the DMAFC using theVA/KOH and PVA/CNT/KOH electrolytes with 2 M methanol inM KOH at 30 ◦C and 60 ◦C. The PVA/CNT/KOH electrolyte out-erformed the PVA/KOH electrolyte at both temperatures. The

ctivation over-potential drop was significantly improved in theow current density region for the CNT-containing sample. In the

edium current density region, the ohmic over-potential was more

ig. 11. Effect of CNT addition in PVA on DMAFC performance: voltage (left axis)nd power density (right axis) as a function of current density at 30 ◦C (anode: 2 Methanol in 6 M KOH with a flow rate of 5 mL min−1 , cathode: humidified oxygenith a flow rate of 100 mL min−1).

rate of 100 mL min−1).

severe in the cell using PVA/KOH electrolyte. This can be corre-lated to the lower conductivity in the PVA/KOH than that for thePVA/CNT/KOH (Table 2).

The higher temperature resulted in a higher cell voltage at thesame current density level. This is caused by the higher catalyticreaction kinetics at both electrodes and the higher conductivity.Fig. 12 shows the power density values of DMAFC using PVA/KOHand PVA/CNT/KOH electrolytes at 30 and 60 ◦C. The peak powerdensity reached 39 mW cm−2 with PVA/CNT/KOH electrolyte at60 ◦C. This value is significantly higher than most literature data. Wereported on a similar peak power density employing PVA/FS/KOHas the electrolyte. However, that FS load was 20%, much higher thanthe CNT content of 0.05% used in this study. The addition of a verysmall amount of the functionalized CNT can significantly improvethe ionic conductivity and the fuel cell performance.

4. Conclusion

A novel route to functionalize PVA onto the multiwall CNTis reported in this work. FTIR, XPS, Raman spectroscopy, andTGA data confirmed the polymer grafting onto the MWCNT.The grafted PVA content was estimated to be 25% in thePVA–functionalized MWCNT. A PVA composite consisting of 0.05%functionalized MWCNT was successfully prepared using a solution-casting method. Water solubility and diffusivity were enhanced inthe CNT-containing membranes as compared with the pristine PVA.The ionic conductivity of the KOH-doped membrane was improvedby adding the functionalized CNT, which might be ascribed to theionic channels provided by the CNT. The methanol permeabilitywas suppressed in the CNT-containing sample. The alkali-dopedelectrolytes were applied in direct methanol alkaline fuel cells.An open-circuit potential and a peak power density of 0.86 V and39 mW cm−2 were obtained using a 2 M methanol fuel in 6 M KOH at60 ◦C with the PVA/CNT/KOH electrolyte, significantly higher thanthose without CNT incorporation.

Acknowledgements

We thank the National Science Council of Taiwan for its finan-

cial support (NSC 98-2221-E-182-034). Valuable inputs from thereferees are greatly acknowledged.

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