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Journal of Membrane Science 303 (2007) 258–266

Sulfonated poly(arylene ether sulfone)–silica nanocompositemembrane for direct methanol fuel cell (DMFC)

Chang Hyun Lee a, Kyung A. Min a, Ho Bum Park a,b, Young Taik Hong c,Byung Ok Jung d, Young Moo Lee a,∗

a School of Chemical Engineering, Hanyang University, Seoul 133-791, South Koreab Department of Chemical Engineering, University of Texas at Austin, Burnet Road 10100, Austin, TX 78758, United Statesc Advanced Materials Division, Korea Research Institute of Chemical Technology, Yusung, Daejeon 305-600, South Korea

d School of Applied Chemistry and Chemical Engineering, Seoul National University of Technology, South Korea

Received 8 June 2007; received in revised form 10 July 2007; accepted 16 July 2007Available online 22 July 2007

bstract

Inorganic nanoparticles in nanocomposite membranes significantly affect the characteristics of those membranes, such as proton and methanolransport behavior, membrane durability, and electrochemical single cell result. Therefore, the inorganic nanoparticles should be deliberatelyhosen to fabricate composite membranes with desirable properties for DMFC. In this study, sulfonated poly(arylene ether sulfone) (SPAES) andydrophilic fumed silica (SiO2) were used as a polymer matrix and an inorganic nanoparticle, respectively. The SiO2 nanoparticles have variousurface areas (150, 200, 300, and 380 m2 g−1) and average particle sizes (7, 12, and 14 nm). The SiO2 nanoparticles are evenly dispersed in thePAES matrix by aid of a non-ionic surfactant (Pluronics® L64). Interestingly, SiO2 particles with a high surface area and small particle size showed

he best results: high proton conductivity, long membrane life time under oxidative conditions, good dimensional stability, outstanding single cell

erformance, and reduced methanol crossover. Moreover, SiO2 content plays an important role in membrane microstructures and membraneroperties such as proton conductivity and methanol barrier behavior. An excessive SiO2 content caused a large aggregation of SiO2 particles,eading to the deterioration of mechanical properties in nanocomposite membranes. In the present study, optimal SiO2 content for maximizing theuel cell performance of current nanocomposite membranes was ca. 2 wt.%.

2007 Elsevier B.V. All rights reserved.

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eywords: Organic–inorganic nanocomposite; Sulfonated poly(arylene ether s

. Introduction

Organic–inorganic nanocomposites have attracted muchnterest as polymer electrolyte membranes (PEMs) for fuel cells,ince inorganic nanoparticles in a polymer matrix might improveechanical strength [1,2], proton conductivity [1,3,4], fuel bar-

ier properties [1,5], and membrane durability [1]. For thisurpose, inorganic nanoparticles should be distributed homoge-eously in the polymer phase, which is strongly related to higheliability of the membrane performances in the nanocompos-

tes [1,7]. Usually, a well-known sol–gel process is used to formwell-distributed inorganic microstructure within the polymeratrix. However, membrane morphology and physicochemical

∗ Corresponding author. Tel.: +82 2 2220 0525; fax: +82 2 2291 5982.E-mail address: ymlee@hanyang.ac.kr (Y.M. Lee).

t[csSbu

376-7388/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.memsci.2007.07.026

); Silica nanoparticle; Direct methanol fuel cell; Non-ionic surfactant

roperties based on the fabrication method can be easily affectedy a variety of factors such as pH, temperature and pressure inhe reaction medium, solvent and alkoxide precursors [7–10],hich make reproducible membrane formation difficult.Meanwhile, direct mixing of nanoparticles may be much pre-

erred owing to convenient incorporation of nanoparticles, ifhe nanoparticles are well dispersed within polymer matrix. Inur previous studies [1,6], amphiphilic surfactants were used asispersants to help improved dispersion of SiO2 nanoparticlesithin hydrophilic sulfonated polymers. The surfactants affect

he formation of microstructures such as crosslinked structure1] and interpenetrating polymer network [6], depending on thehemical structures of polymer matrices as well as those of the

urfactants themselves. Among SiO2 nanoparticles, hydrophiliciO2 was more effective than hydrophobic SiO2 with respect tooth proton conductivity and methanol barrier property, partic-larly for direct methanol fuel cells (DMFCs) [1,6].

C.H. Lee et al. / Journal of Membrane Science 303 (2007) 258–266 259

Table 1Nomenclature of SPAES–SiO2 nanocomposites containing SiO2 particles with various physical properties

Sample Type of SiO2 Surface area ofSiO2 (m2 g−1)

Average particlesize (nm)

SiO2 content (%) SiOH conc.a (mmol g−1)

Pristine SPAES – – – 0 –CSPAES–SiO2-200-1 Aerosil 200 200 12 1 0.75SPAES–SiO2-150-1 Aerosil 150 150 14 1 0.63SPAES–SiO2-200-1 Aerosil 200 200 12 1 0.75SPAES–SiO2-300-1 Aerosil 300 300 7 1 1.25SPAES–SiO2-380-1 Aerosil 380 380 7 1 1.48SPAES–SiO2-380-0.5 0.5SPAES–SiO2-380-1 1SPAES–SiO2-380-2 Aerosil 380 380 7 2 1.48SS

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PAES–SiO2-380-3PAES–SiO2-380-5

a SiOH concentration on the Aerosil® surface determined using the lithium a

In the present study, Pluronic® L64 and hydrophilic SiO2ere selected as a dispersant and an inorganic nanoparticle,

espectively. Also, we chose poly(arylene ether sulfone) as aolymer matrix. The main goal of this study is to systematicallynvestigate the effects of the surface area and average particleize of hydrophilic SiO2 particles on the properties of polymeranocomposite membranes. Moreover, the relationship betweeniO2 content and membrane performances were correlated. In

his research, electrochemical single cell performance of theresent system is compared with that using Nafion 117 underhe same DMFC operation conditions.

. Experimental

.1. Materials

The chemicals 4,4′-dichlorodiphenylsulfone (DCDPS) and,4′-dihydroxybiphenyl (BP) used for synthesizing poly(arylenether sulfone) were purchased from Tokyo Kasei Co. (Tokyo,apan) and used after recrystallization with ethanol and a dryingrocess under vacuum at 120 ◦C for 1 day. DCDPS was con-erted to 3,3′-disulfonated DCDPS (SDCDPS, yield = 91.4%)hrough direct sulfonation using SO3 (28%, Aldrich, WI, USA)11,12]. N-Methylpyrrolidinone (NMP) and dimethylacetamideDMAc) were purchased from Aldrich Chemical Co. (WI, USA)nd used as a solvent for polymer synthesis and a casting solvent,espectively. Toluene and potassium carbonate (K2CO3) wereurchased from Aldrich Chemical Co. and used as received. Aommercial surfactant, Pluronic® L64 (PEO13-PPO30-PEO13,ASF, Ludwigshafen, Germany) was used as a dispersant to dis-

ribute nanoparticles homogeneously. Nanosized SiO2, Aerosil®

50, 200, 300, and 380 in Table 1 were purchased from Degussahemical Co. (Dusseldorf, Germany) and dried at 80 ◦C and–5 mmHg for 2 days before use.

.2. Fabrication of SPAES–SiO2 nanocomposite

Prior to fabrication of the organic–inorganic nanocom-osite, pristine SPAES (inherent viscosity measured usingstwald viscometer at 30 ◦C = 2.02 dl g−1 and ion exchange

dNoa

35

um hydride method [16,18].

apacity (IEC) measured by a conventional titration methodD2187) = 1.62 mequiv. g−1) was synthesized through polycon-ensation of sodium salt-form SDCDPS (4 mmol, 3.93 g),CDPS (6 mmol, 3.45 g), and BP (10 mmol, 3.72 g) [12]. Afterrying in a vacuum oven at 120 ◦C, SPAES powder was re-issolved in DMAc (1 wt.%) and filtered using 1 �m PTFElter to remove impurities in the solution. Then, a mixture ofwt.% Aerosil® 200 and 3 wt.% L64 in DMAc was added

o the SPAES solution, and mechanically stirred for 1 day at0 ◦C to obtain a yellow SPAES–SiO2 nanocomposite. OtherPAES–SiO2 nanocomposites were also fabricated via the samerocedure using SiO2 particles having various surface proper-ies and average particle sizes. CSPAES–SiO2-200-1 containingnly 1 wt.% Aerosil 200 without L64 was also fabricated ascontrol membrane. Moreover, SPAES–SiO2 nanocompositesased on Aerosil® 380 were prepared by varying SiO2 con-ents (0.5, 1, 2, 3, and 5 wt.% SiO2 per SPAES weight). Moreample information in detail is summarized in Table 1. Here,ll membranes were carefully prepared with a nominal thick-ess of 50 �m to minimize their thickness effect on membraneerformances.

.3. Membrane formation and acidification

SPAES–SiO2 nanocomposite membranes in sodium salt-orm were prepared using a solution-casting method. The castembranes were dried at 60 ◦C for 8 h and heated at 80 ◦C for

4 h, 100 ◦C for 6 h, and 120 ◦C for 8 h in a vacuum oven. Then,hese membranes were peeled off from glass plates in deion-zed water and, then, immersed in a boiling 1 M H2SO4 solutionor 1 h. Finally, the acidified membranes were treated in boilingater to remove excessive free H2SO4 molecules in the acidifiedembranes.

.4. Evaluation of SPAES–SiO2 nanocomposite membranes

The particle size of SiO2 (nm) in DMAc was measured using

ynamic light scattering (angle = 90◦) on a Malvern Zetasizerano ZS (Malvern Instruments, Malvern, UK) over a pH rangef 4.5–7.5 at 30 ◦C. The concentrations of all samples were fixedt 0.5% (w/v). A field emission scanning electron microscopy

2 bran

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StdttiSasponding z-average diameter of SiO2 agglomerates decreased.However, there was little variation in the diameter of SiO2agglomerates derived from SiO2 nanoparticles of a given indi-vidual particle size (7 nm).

60 C.H. Lee et al. / Journal of Mem

FE-SEM, JEOL Model JSF 6340F, Tokyo, Japan) and a trans-ission electron microscopy (TEM, FEI company, Hillsboro,R, USA) were used to check the distribution of SiO2 nanopar-

icles in a SPAES matrix. For TEM analysis using a Tecnai G2un-type apparatus (running voltage = 200 kV), samples wereltra-sectioned in liquid nitrogen using a microtome equippedith a diamond knife and placed on a 200-mesh copper grid.Water uptake (%) of nanocomposite membranes was mea-

ured by their weight difference after soaking in deionized watert 30 ◦C for 1 day. The dimensional stability (%) of membranesas evaluated by comparing their volumetric change before and

fter equilibrium water uptake. Prior to the measurement, eachample was dried in a vacuum oven at 120 ◦C for 1 day andmmersed in water at 30 ◦C for 1 day. The state of water in theembranes was characterized by measuring the endothermic

eak derived from the fusion enthalpy of water in the tempera-ure range of −50 to 5 ◦C using differential scanning calorimetryDSC, DSC 2010 thermal analyzer, TA instrument, New Cas-le, DE, USA) [13]. The d-spacing value (A) was obtained from

wide angle X-ray diffraction (WAXD) pattern in the rangef 5◦ ≤ 2θ ≤ 50◦ (Rigaku Denki Model RAD-C diffractometer,okyo, Japan). The average d-spacing value can be an effectivearometer to estimate average distance between polymer chainsr changes in microstructure of membrane.

The proton conductivity (σ, S cm−1) of each sample (dimen-ion: 1 cm × 4 cm) was obtained using the equation σ = l/(R × S)fter measuring the ohmic resistance (R, �) with the four-pointrobe alternating current (ac) impedance spectroscopic method14]. Here, l (cm) and S (cm2) are the distance between refer-nce electrodes and the cross-sectional area of each membrane,espectively. The electrode system was set up in the thermallyontrolled water bath and connected with an impedance/gainhase analyzer (Solartron 1260) and an electrochemical inter-ace (Solartron 1287, Farnborough Hampshire, ONR, UK). Theethanol permeability (PMeOH, cm3 cm cm−2 s−1) was deter-ined by measuring the concentration gradient through theembrane sample between two-chamber diffusion cells filledith 10 M (∼34%) methanol solution and deionized water in the

emperature range from 30 to 90 ◦C. The methanol concentrationas detected using gas chromatography (Shimadtzu, GC-14B,okyo, Japan) equipped with a thermal conductivity detectorTCD) [15].

The physical strength of membrane samples was measuredsing an Instron mechanical testing machine (INSTRON-1708,oston, MA, USA) using ASTM D882. The resistance to hydro-en peroxide radical was examined in Fenton’s reagents at0 ◦C under harsh (30 ppm ferrous ammonium sulfate in 30 wt.%2O2) and mild (2 ppm Ferrous sulfate in 3 wt.% H2O2) condi-

ions. Each measurement was carried out repeatedly at least fiveimes to ensure good reproducibility.

.5. Fabrication of membrane-electrode assemblies (MEAs)

MEAs based on nanocomposite membranes were fabricatedsing the catalyst-coated membrane (CCM) method [1,15]. Forhe anode, 4.0 g of Nafion ionomer solution (5 wt.%, EW = 1100)nd 2.0 g of Pt–Ru black (Johnson Matthey Fuel Cell, PA, USA)

FcS

e Science 303 (2007) 258–266

ere mixed in 40 g of water–isopropyl alcohol. After sonicatingor 1 h, a well-dispersed catalyst slurry was carefully sprayednto the membranes at 30 ◦C to avoid thermal sintering of cata-yst nanoparticles. Subsequently, the slurry containing Pt blacks a cathode catalyst was sprayed on the other side of the mem-ranes. The catalyst loading of each electrode was 3 mg cm−2.

. Results and discussion

.1. Pluronic® L64 as a dispersant

The hydrophilic properties of SiO2 are due to the silanolroup (–Si–OH) on the surface of SiO2 nanoparticles [16].n incorporation of amphiphilic Pluronics® at a greater-

han-critical micelle concentration (CMC) into the suspensionedium (i.e. m-cresol) containing SiO2 nanoparticles caused the

hysical interaction between –Si–OH groups on the SiO2 (core)urface and poly(ethylene oxide) (PEO) moieties in Pluronics®

shell) via hydrogen bonding, in turn resulting in the core–shellormation.

Fig. 1 shows the z-average diameters of SiO2 particles andiO2-L64 core-shell measured in DMAc using photon correla-

ion spectroscopy [17]. Here, the z-average diameter is a meaniameter based on the intensity of scattered light. In each case,he z-average diameter of the SiO2 particles is much largerhan the reported individual particle size. This means that annteraction between vicinal and bridged silanol groups on theiO2 surface significantly contributed to the formation of SiO2gglomerates. When small SiO2 particles were used, the corre-

ig. 1. Z-average diameters of core–shells composed of hydrophilic SiO2 parti-les and L64 (©) in comparison with those of only SiO2 particles (�) in DMAc.ee particle size of samples in Table 1.

C.H. Lee et al. / Journal of Membrane Science 303 (2007) 258–266 261

F PAESa

pctSwchaDrs

cwitIoSoTta

Scd

apdeaetadaSts

3m

ig. 2. FE-SEM surface image of (a) CSPAES–SiO2-200-1 without L64 and (b) Snd (d) SPAES–SiO2-300-1 with a magnification of 1000.

The addition of L64 in SiO2, however, changes the surfaceroperties of SiO2. The z-average diameter of the SiO2-L64ore–shell increases with decreasing SiO2 particle size. In par-icular, this trend seemed to depend more on the surface area ofiO2 particles than on their size. This result can be explainedith the concentration of –Si–OH groups of each SiO2 parti-

le in Table 1. That is, SiO2 particles with large surface areaave high –Si–OH content per unit mass, which improved theffinity with PEO moieties of L64 at measured CMC of L64 inMAc greater than 3.8 g/l. Therefore, a large amount of L64 sur-

ounded the SiO2 surface and, consequently, created a core–shelltructure with a large diameter.

In the presence of L64, SiO2 nanoparticles are added with theore–shell structure into the SPAES solution and are dispersedithin the SPAES matrix after drying. Fig. 2 shows FE-SEM

mages of the SPAES–SiO2 composite membranes containinghe same content (1 wt.%) of SiO2 with various particle sizes.n CSPAES–SiO2-200-1, the aggregation of SiO2 particles isbservable with the naked eye. On the other hand, the state ofiO2 dispersion is not easily seen in the FE-SEM surface images

f the SPAES–SiO2 nanocomposite membranes containing L64.he existence of SiO2 (bright dots) can be confirmed only in

he energy-dispersive spectrometer (EDS) images in Fig. 2(c)nd (d). The number of SiO2 particle per unit area is less in

mt

–SiO2-200-1 with L64, and Si-mapping EDS images of (c) SPAES–SiO2-200-1

PAES–SiO2-200 than in SPAES–SiO2-300 with the same SiO2ontent and density (50 g L−1 [16]), because the possibility ofetecting relatively large SiO2 particles is decreased.

The TEM image in Fig. 3 clearly displays the remark-ble property of L64 as a dispersant in the present sulfonatedoly(arylene ether) system. Considering both that the electronensity of SiO2 (∼0.3 electron A−3 [19]) is higher than any otherlements in the composites and that the sizes of dark spotsre similar to those of incorporated SiO2 particles, it can bexplained that the dark spots are due to individual SiO2 nanopar-icles rather than to SiO2 agglomerates. This means L64 aidedll types of SiO2 particles used in this study in forming theark SiO2 cluster phase across the entire SPAES matrix withoutny aggregation of their particles. Furthermore, the hydrophiliciO2 particles are mainly located near hydrophilic domains in

he nanocomposite membranes [1,6] because of the hydrophilicurface properties of the SiO2-L64 core–shell.

.2. PEM performances of SPAES–SiO2 nanocompositeembranes

The existence of water molecules in sulfonated polymerembranes considerably affected proton and methanol transport

hrough narrow hydrophilic channels surrounded by negatively

262 C.H. Lee et al. / Journal of Membrane Science 303 (2007) 258–266

(b) SP

catmsmta–p

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F

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Fig. 3. TEM images of (a) SPAES–SiO2-200-1,

harged fixed ions, such as sulfonic acid (–SO3H) [20]. Usu-lly, a high degree of sulfonation of polymer membranes leadso high water uptake, poor methanol barrier property, and weak

echanical properties. Meanwhile, the proton conductivity ofulfonated polymer membranes at low IEC values is too low toake them available in the fuel cells. From this point of view,

he fabrication of nanocomposite membranes can be a desirablepproach to limiting severe water uptake – even at high IECand to maintain a proper water sorption level for reasonable

roton conduction.Fig. 4 shows the water sorption behavior of the SPAES–SiO2

anocomposites, illustrating a method for enhancing both protononductivity and methanol barrier property. SiO2 nanoparticlesell-dispersed around hydrophilic domains of SPAES might

educe free volume elements where absorbed water moleculesight remain and decrease water uptake. The surface proper-

ies of SiO2 nanoparticles contributed to the reduction of waterptake in nanocomposites. The isoelectric point (IEP) of SiO2

s 2 [21], at which –Si–OH groups on SiO2 surface exist inhe neutral state. In an acidic medium such as PEM (pH 0–1)22] with pH below the IEP of SiO2, –Si–OH groups are con-erted into the aquo state Si–(OH2)+ [23]. The SiO2 particles

ig. 4. Water sorption behavior of SPAES–SiO2 nanocomposite membranes.

bpctimST

mSca9Stoorgpm

AES–SiO2-300-1, and (c) SPAES–SiO2-380-1.

re able to act as an additional proton conductor and strengthenecondary hydrogen bonding with –SO3

− groups in SPAES.he strong interaction between Si–(OH2)+ and/or –SO3

− groupsauses the reduction of d-spacing values of polymer, meaninghat an average interchain distance of SPAES became narrown the composite membranes (see Table 2). The same resultsere obtained in sulfonated polyimide–silica nanocomposites

1]. The resulting polymer morphologies induced low waterptake and, thereby, contributed to high-dimensional stabilityn the nanocomposite membranes. Simultaneously, –Si–(OH2)+

roups interacted with water molecules and increased boundater content responsible for proton conduction. Notice that

hese effects are conspicuously observed in the nanocompos-tes containing SiO2 particles with high surface area. In additiono hydrophilic SiO2 nanoparticles, hydrophilic PEO moieties in64 also contributed to an increase of bound water content. How-ver, the effect of L64 was not considered in this study, because64 content was the same in all the nanocomposite membranes.

Fig. 5 shows a relationship between molecular transportehavior and the surface area of SiO2 in SPAES–SiO2 nanocom-osite membranes. Both proton and methanol moleculesan transport more easily through membranes at elevatedemperatures. Unlike proton conduction, which exhibits a sim-lar activation energy (∼9 kJ mol−1), temperature-dependentethanol permeation increases with high activation energy inPAES–SiO2 nanocomposites rather than in pristine SPAES (seeable 2).

Prior to the influence of SiO2 surface area on PEM perfor-ances, SiO2 dispersion effect was investigated. The aggregatediO2 particles in CSPAES–SiO2-200-1 widen the average inter-hain distance of the SPAES chains, as shown in Table 2,nd endow the membrane with a methanol permeability (e.g..38 × 10−7 cm3 cm cm−2 s−1 at 30 ◦C) higher than that inPAES (e.g. 6.64 × 10−7 cm3 cm cm−2 s−1 at 30 ◦C) in all the

emperature range. Meanwhile, the incorporation of SiO2 with-ut L64 might contribute to improving the proton conductivityf SPAES up to a level similar to that of SPAES–SiO2-200-1 fab-

icated with L64. This is because the positively charged –Si–OHroups in the acidic medium also endow the SiO2 particles withroton conduction capability and, hence, the nanocompositeembrane with improved proton conductivity.

C.H. Lee et al. / Journal of Membrane Science 303 (2007) 258–266 263

Table 2Physicochemical properties of SPAES–SiO2 nanocomposite membranes

Sample d-Spacing value (A) Dimensional changea (%) Activation energy for protonconductionb (kJ mol−1)

Activation energy for methanolpermeationc (kJ mol−1)

Pristine SPAES 5.04 58.6 9.0 30.2CSPAES–SiO2-200-1 5.13 64.2 9.0 43.7SPAES–SiO2-150-1 4.86 50.5 9.4 31.9SPAES–SiO2-200-1 4.50 31.8 9.0 33.7SPAES–SiO2-300-1 4.34 21.5 9.2 39.7SPAES–SiO2-380-1 3.68 18.7 9.0 40.9SPAES–SiO2-380-0.5 3.92 21.2 9.2 39.0SPAES–SiO2-380-1 3.68 18.7 9.0 40.9SPAES–SiO2-380-2 4.25 18.9 9.2 41.2SPAES–SiO2-380-3 4.50 24.9 9.3 44.7SPAES–SiO2-380-5 4.74 36.4 9.3 48.6

a Dimensional change = (Vs − Vd)/Vd × 100, Here, Vs and Vd are volumes of membrane samples (5 cm × 5 cm) after swelling for 1 day in liquid water at 30 ◦Ca ◦

of 30–ge of

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nd drying for 1 day in vacuum oven at 110 C, respectively.b Obtained from proton conductivity data measured in the temperature rangec Obtained from methanol permeability data measured in the temperature ran

In contrast, well-dispersed SiO2 nanoparticles act as selec-ive barriers to bulky methanol–water complexes and reduce

ethanol permeability. The SiO2 nanoparticles act as additionalroton conductors and improve bound water content, result-ng in promoted proton transport through the nanocomposite

embranes. As stated above, these effects of SiO2 nanopar-icles on transport phenomena are evidently observed in theanocomposite membranes containing SiO2 particles with highurface area. In summary, choosing hydrophilic SiO2 nanopar-icles with small particle size and high surface area is preferredor composite-type PEMs with excellent proton conductivity andethanol barrier property.

.3. Optimum SiO2 content in SPAES–SiO2 nanocomposite

embranes

In addition to the surface properties of SiO2 nanoparticles,iO2 content affects membrane properties [10]. Very low SiO2

r–ST

ig. 5. Molecular transport behavior of SPAES–SiO2 nanocomposite membranes: (PAES–SiO2-150-1 (�), SPAES–SiO2-200-1 (�), SPAES–SiO2-300-1 (�), and SPA

90 ◦C.30–90 ◦C.

ontent is not enough to enhance PEM performances, whilexcessive SiO2 content makes composite membranes brittle,eading to deterioration of their mechanical strength. Accord-ngly, optimization of SiO2 content is necessary to fabricateomposite-type PEMs with superior properties. Based on theverall PEM performances described above, Aerosil 380 withhe largest surface area and the smallest average particle sizeas chosen as the inorganic nanoparticle for this process.Fig. 6 shows the relationship between water sorption behavior

nd PEM performances with respect to SiO2 content. Differ-ng from the general expectation that an increasing quantityf hydrophilic SiO2 leads to a consecutive increase of waterptake in the nanocomposites, a peculiar water uptake behav-or was observed at SiO2 content below 1 wt.% in Fig. 6(a). In

egion I, the strong interaction between the positively chargedSi–OH groups on the SiO2 surface and –SO3H groups inPAES decreases the average interchain distance of SPAES (seeable 2), leading to reduced water uptake. The well-dispersed

a) methanol permeability and (b) proton conductivity of pristine SPAES (�),ES–SiO2-380-1 (�).

264 C.H. Lee et al. / Journal of Membrane Science 303 (2007) 258–266

roton

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Sfe3

3

acsosbh[he

TM

S

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Fig. 6. Effect of SiO2 content on (a) methanol permeability and (b) p

iO2 nanoparticles play an important role in decreasing waterptake via reduction of free volume within the SPAES matrix.s a result, the methanol permeability in the SPAES–SiO2anocomposite decreases at SiO2 content below 1 wt.%, show-ng its high dependence on water uptake.

However, in region II, SiO2 nanoparticles begin to aggre-ate over 1 wt.%, which physically hinders interaction amongPAES chains. Consequently, their average interchain distance

ncreases, resulting in high water swelling and thereby lowethanol barrier properties. On the other hand, proton conduc-

ivity of SPAES–SiO2 nanocomposites successively increasesith SiO2 content, where this trend is consistent with that of

heir bound water content, as shown in Fig. 6(b).The mechanical properties in Table 3 list physical parameters

mportant in investigating optimal SiO2 content. L64 is effec-ive in forming a SiO2 cluster phase within the SPAES matrixt low SiO2 content (below 1 wt.%). The SiO2 nanoparticlesct as reinforcing agents [24,25], improving the tensile strengthf SPAES–SiO2 nanocomposite membranes and increasing the

longation at break. By contrast, aggregated SiO2 particles inSPAES–SiO2-200-1 weaken both its tensile strength and, to

ome extent, elongation in comparison with SPAES–SiO2-200-having the same SiO2 content. Moreover, adding excessive

sHip

able 3echanical properties and radical durabilities of SPAES–SiO2 nanocomposite memb

ample Tensile strength (MPa) Elongation (%

ristine SPAES 57.6 17.6SPAES–SiO2-200-1 48.3 12.3PAES–SiO2-150-1 58.4 17.3PAES–SiO2-200-1 60.0 17.5PAES–SiO2-300-1 64.6 17.8PAES–SiO2-380-1 65.4 20.6PAES–SiO2-380-0.5 64.3 19.3PAES–SiO2-380-1 65.4 20.6PAES–SiO2-380-2 62.8 17.8PAES–SiO2-380-3 52.9 15.4PAES–SiO2-380-5 24.0 9.9

a Measured at 30 ◦C under the harsh condition (30 ppm ferrous ammonium sulfateb Measured at 30 ◦C under the mild condition (2 ppm ferrous sulfate in 3 wt.% H2O

conductivity in SPAES–SiO2 nanocomposites based on Aerosil 380.

iO2 content reduces the availability of L64 and results in theormation of large SiO2 agglomerates. In particular, the wors-ning of mechanical properties is visible at SiO2 content overwt.%.

.4. Chemical stability and electrochemical performances

The feasibility of SPAES–SiO2 nanocomposite membraness practical PEMs was evaluated by measuring their chemi-al stability and single cell performances. Here, the chemicaltability is one of the important factors evaluating the lifetimef PEMs under harsh fuel cell conditions. Even perfluorinated-ulfonic acid membranes including Nafion® with a stable C–Fackbone suffer from the membrane degradation caused byydrogen peroxide (H2O2) as a by-product of catalytic reaction26–28]. The chemical degradation occurs especially notably inydrocarbon-based polymers including sulfonated poly(arylenether)s [13,15,29–31].

Table 3 gives examples for improvement of the oxidative

tability of SPAES via the incorporation of SiO2 particles.ydrophilic SiO2 particles seem to be somewhat effective in

ncreasing the tolerance of a radical attack independent of SiO2article dispersion. This trend could be clearly observed in

ranes

) Oxidative stability

τ1 (h)a τ2 (h)a τ1 (h)b τ2 (h)b

16 25 235 33016 27 240 33516 28 240 34017 29 245 34518 31 250 35519 33 265 36518 30 247 35319 33 265 36518 31 261 35816 27 255 34515 25 245 340

in 30 wt.% H2O2).

2).

C.H. Lee et al. / Journal of Membran

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(

ig. 7. Electrochemical single cell performances measured at 90 ◦C to investi-ate effects of (a) surface properties of SiO2 and (b) SiO2 content.

he SPAES–SiO2 nanocomposite containing L64. In particu-ar, SiO2 particles of small size and high surface area helphe composite membranes to enhance the oxidative radicaltability irrespective of measurement conditions. Furthermore,he highest tolerance to free radical attack is observed inPAES–SiO2-380-1 with 1 wt.% SiO2 content. The addition oferosil 380 above 3 wt.% induces irregular aggregation of SiO2articles within the SPAES matrix and weakens the chemicaltability of the corresponding composite membranes.

Fig. 7 reports the electrochemical single cell performancesf MEAs based on SPAES–SiO2 nanocomposite membraneseasured at 90 ◦C under feed flow rates of 1 cm3 min−1 ofM MeOH and 150 cm3 min−1 of air at atmospheric pressure.afion 117 and pristine SPAES are used as standard samples.afion 117 exhibits much lower single cell results than all

he SPAES membranes because of its relatively low protononductivity (1.11 × 10−1 S cm−1) and high methanol perme-bility (1.35 × 10−5 cm3 cm cm−2 s−1) at the given temperature90 ◦C). The effect of SiO2 on electrochemical performancess insignificant in SPAES–SiO2-150-1 containing SiO2 witharge particle size (Fig. 7(a)). Note, however, that hydrophiliciO2 with smaller particle size and higher surface area is moreffective in preventing methanol transport and enhancing pro-on transport through the nanocomposite membranes than anyther SiO2 particles. Eventually, SPAES–SiO2-380-1 showed anutstanding electrochemical result, superior to pristine SPAES

ecause of improved proton conductivity and methanol barrierroperty. Despite a small amount (1 wt.%) of Aerosil 380, theaximum power density and current density at 0.4 V were 25%

nd 35% larger than those of pristine SPAES, respectively.

e Science 303 (2007) 258–266 265

The single cell performances of PEM are significantly depen-ent upon SiO2 content in the membrane (see Fig. 7(b)).erformance increases as up to 2 wt.% SiO2 content and dras-

ically decreases above 3 wt.% SiO2 content. This trend isonsistent with that of selectivity (Φ = σPMeOH

−1), a charac-eristic factor for evaluation of both proton conductivity and

ethanol permeability. The Φ value may be used as an indi-ect factor for prediction of the electrochemical single cellerformance prior to MEA fabrication. SPAES–SiO2-380-21.91 × 106 S s cm−3) with the highest proton conductiv-ty exhibited a higher Φ value than SPAES–SiO2-380-11.85 × 106 S s cm−3) with the lowest methanol permeabilityt the same operation temperature. Accordingly, SPAES–SiO2-80-2 shows excellent DMFC performances (maximum powerensity = 114.4 mW cm−2 at 380 mA cm−2 and current densityt 0.4 V = 234.8 mA cm−2), indicating that 2 wt.% is optimaliO2 content in this system.

At present, we are focusing on a long-term DMFC operationased on SPAES–SiO2-380-2, MEA fabrication using catalyticinders derived from non-perfluorinated ionomers, and devel-pment of nanocomposite membranes with improved oxidativetability. The related work will be reported as compared with theesults from SPAES–SiO2 nanocomposite membranes.

. Conclusions

The following conclusions can be drawn from this study:

1) SPAES–SiO2 nanocomposite membranes containing SiO2nanoparticles with various surface properties and contentswere successfully fabricated using L64 as a dispersant.L64 at above CMC in the suspension medium contain-ing SiO2 nanoparticles led to the formation of a SiO2-L64core–shell. The z-average diameter of the core–shell wasproportional to the surface area of SiO2 rather than to SiO2particle size. After the incorporation of SiO2 nanoparticleswith core–shell structure, well-dispersed SiO2 cluster phasewas formed near hydrophilic domains of the nanocompos-ite membranes without any SiO2 aggregation. Note thatthe direct mixing of inorganic nanoparticles together withdispersants was useful in the fabrication of highly reliableorganic–inorganic nanocomposite membranes.

2) The surface properties of SiO2 nanoparticles affected notonly the molecular transport behavior but also the durabilityof membranes. In particular, the hydrophilic SiO2 with smallparticle size and high surface area was effective in enhanc-ing proton conductivity and lowering methanol permeabilityin SPAES–SiO2 nanocomposite membranes via easy con-trol of water sorption behavior. The SiO2 nanoparticles alsocontributed to the enhancement of resistance to free radi-cal attack in Fenton’s solution regardless of measurementconditions.

3) Overall PEM performances in SPAES–SiO2 nanocomposite

membranes were influenced by SiO2 content. The pro-ton conductivity continuously increased with SiO2 content,whereas the methanol barrier property increased up to1 wt.% SiO2, but rapidly decreased above 3 wt.%. The trends

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66 C.H. Lee et al. / Journal of Mem

in mechanical properties and chemical stability were verysimilar to those of the methanol barrier property. It meansSiO2 particles can act to some extent as reinforcing agentsand chemical stabilizers.

4) All SPAES–SiO2 nanocomposite membranes exhibitedoutstanding electrochemical single cell performances com-pared with that of Nafion 117 because of their high protonconductivity and low methanol permeability. Consideringthe electrochemical results determined using measurementsof molecular transport behavior and membrane stability,incorporating Aerosil 380 with the smallest particle sizeand the highest surface area at 2 wt.% SiO2 content was themost desirable in the fabrication of the current SPAES–SiO2nanocomposite as potential PEMs for DMFC.

cknowledgements

The authors would like to thank Research Park/LG Chemtd., and the Ministry of Commerce, Industry and Energy, for

unding this research in the framework of the Korean gov-rnment/industry joint project for the development of 50 Wirect methanol fuel cell systems. This study was supported byNational RD & D Organization for Hydrogen & Fuel Cell.”.H. Lee and K.A. Min are very grateful to the BK21 Project

or fellowships.

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