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Journal of Membrane Science 367 (2011) 296–305

Contents lists available at ScienceDirect

Journal of Membrane Science

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

ulfonated poly(2,6-dimethyl-1,4-phenylene oxide) (SPPO) electrolyteembranes reinforced by electrospun nanofiber porous substrates for fuel cells

ung-Hyun Yuna, Jung-Je Wooa, Seok-Jun Seoa, Liang Wub, Dan Wub,ongwen Xub, Seung-Hyeon Moona,∗

School of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), 261 Cheomdan-gwagiro, Buk-gu, Gwangju 500-712, Republic of KoreaSchool of Chemistry and Materials Science, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China

r t i c l e i n f o

rticle history:eceived 26 May 2010eceived in revised form 3 November 2010ccepted 5 November 2010vailable online 12 November 2010

eywords:

a b s t r a c t

This study reports on a novel non-fluorinated composite polymer electrolyte membrane reinforced by anelectrospun nanofiber porous substrate (NFPS) having a symmetrically pore-filled structure. Sulfonatedpoly(2,6-dimethyl-1,4-phenylene oxide) (SPPO) was mechanically reinforced by electrospun and cross-linked bromomethylated poly(2,6-dimetyl-1,4-phenylene oxide) (cBPPO), and the effect of pore-filledSPPO with different ion exchange capacities was investigated. After physical and chemical treatments ofthe electrospun BPPO nanofiber mat (NFM), proton conducting membranes were successfully fabricated

uel cellPPO electrolyte membranerocessable electrospun nanofiber porousubstrateroton conductanceater management

without additional surface conducting layers. In particular, the affinity between hydrophilic conductingSPPO and hydrophobic reinforcing BPPO was considerably improved as both have the same backbonestructure. As a result, SPPO based membranes with a high water swelling of 90% were significantly sup-pressed to 20% due to the mechanical support of cBPPO NFPS. Moreover, the proton conductivities of theprepared membranes were improved from 0.03 to 0.08 S/cm by the increasing IEC, and greatly enhancedperformances were confirmed in a H2/O2 fuel cell. Finally, advantages for water management using the

s wer

prepared thin membrane

. Introduction

Polymer electrolyte fuel cells (PEFCs) are commonly consideredo be one of the most likely potential alternative power sourcesue to their convenient operating temperature, high energy con-ersion efficiency, and ease of integration into hybrid systems [1,2].n addition, byproducts from PEFCs such as water and heat areelatively environmentally friendly compared to the products ofypical combustion engines. As such, recent efforts have focused onhe commercialization of PEFCs in a range of applications, includ-ng use in portable devices, residences, stationary applications, andehicles [3].

Polymer electrolyte membranes (PEMs) are a core component inembrane–electrode assemblies (MEAs). Perfluorinated sulfonic

cid membranes such as Nafion®, Flemion®, Aciplex®, and Dow®

eries have been widely used due to their high proton conductiv-

ty and good thermal and chemical durabilities [4]. Despite theirdvantages, Nafion membranes also have a number of drawbacks,uch as excessive fuel crossover, high cost, and poor conductiv-ty at a high temperature under low humidity conditions [5]. To

∗ Corresponding author. Tel.: +82 62 715 2435; fax: +82 62 715 2434.E-mail address: shmoon@gist.ac.kr (S.-H. Moon).

376-7388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.memsci.2010.11.017

e discussed in terms of their corresponding polarization behaviors.© 2010 Elsevier B.V. All rights reserved.

overcome these problems, efforts have been made to developalternative fuel cell membranes [4,6]; among the candidates,non-fluorinated polymers such as polystyrene and its derivatives[7–9], sulfonated poly(arylene ether sulfone)s [10], sulfonatedpoly(arylene ether ketone)s [11], sulfonated polyimides [12], andsulfonated organic–inorganic hybrids [8] are notably being inves-tigated in attempts to render better oxidative stability and lowerfuel crossover via the introduction of a cross-linking structure [13].Hydrocarbon-based membranes without cross-linkage, however,are generally too swollen to employ in MEAs under sufficient ion-exchange capacity (IEC) conditions, and excessive water swellingdecreases proton conductivity and oxidative stability, and increasesfuel cross-over.

Attractive approaches for improving the membrane proper-ties (e.g., suppressing water swelling under high IEC) include: theapplication of a reinforcing method such as blending a mechani-cally stable framework into hydrocarbon based polymers [14,15];grafting sulfonated hydrocarbon based polymers onto support-ing materials using radiation or an electron beam [16,17]; and

monomer sorption onto porous or nonporous substrates [8,9]. Inaddition, reinforcing the membrane with a pore-filling structureconsisting of a supporting porous substrate and a conducting poly-mer has drawn attention for direct methanol fuel cells (DMFCs)[18–27] and PEFCs [28–32]. In pore-filling membranes, properties

brane Science 367 (2011) 296–305 297

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S.-H. Yun et al. / Journal of Mem

re affected by the substrate, as reported by Yamaguchi et al. [22]. Inrder to supply the growing demand for porous substrates, electro-pun poly(vinylidene fluoride) nanofibers were recently employed,ith Nafion impregnation reported by Choi et al. [33] for DMFC

pplication.Electrospinning deposition (ESD) is a straightforward method

or producing fine polymeric fibers which may then be used asembrane support for fuel cells and secondary batteries [34].

he electropun fibers can supply membranes with a high surface-o-volume ratio and porosity compared to conventional porouslms prepared via phase inversion or stretching methods [34,35].espite their advantages, however, electrospun non-fluorinatedydrocarbon composite polymers have not been well utilized assubstrate due to interfacial resistance between the supporting

nd conducting polymers.In this study, we present non-fluorinated composite reinforced

embranes having a pore-filling structure by employing chem-cally and thermally stable non-fluorinated polymers such asulfonated poly(phenylene oxide) (SPPO) [48] and bromomethy-ated poly(phenylene oxide) (BPPO) [36–39]. This study mayrovide additional benefits for PEFCs: (a) good affinity of supportingnd conducting polymers with minimized interfacial resistance; (b)asy fabrication of thin membranes; and (c) application of a pro-essable electrospun nanofiber porous substrate (NFPS) made fromn engineering plastic. Accordingly, it is expected that the newomposite membranes possess a low water swelling ratio and aigh proton conductivity through their three-dimensionally inter-onnected proton pathway.

. Experimental

.1. Materials

BPPO was kindly supplied by the University of Science andechnology of China and was purified based on its solubility in-methyl-2-pyrrolidone (NMP) obtained from Junsei Chemicalo. (Tokyo, Japan) prior to use. Poly(2,6-dimethyl-1,4-phenylenexide) (PPO) and chlorosulfonic acid (CSA, 99%) were purchasedrom Aldrich (Milwaukee, WI, USA), and chloroform (CHF, HPLCrade), methanol (MeOH), and N,N-dimethylformamide (DMF)rom Fisher Scientific Ltd. (Korea) were used as received.

.2. Synthesis of a conducting polymer, SPPO

SPPO was synthesized from 20 g of PPO; the PPO was sulfonatedsing chlorosulfonic acid in a 200 ml chloroform system [38–40].

n order to examine the effect of SPPO on the membrane proper-ies, a range of SPPO samples were synthesized depending on themount of chlorosulfonic acid at room temperature in the air. Theynthesized SPPO in sodium form (SPPONa) was dissolved in DMF,nd cast on a glass plate to form a film. The SPPONa cast films werehen treated by immersing them into 1 mol/l HCl solution for 5 hn order to exchange sodium into hydrogen form (SPPOH) prior tonalysis.

.3. Fabrication of electrospun and cross-linked BPPO NFPS

The purified BPPO was dissolved in NMP (0.40 g/ml) for over 24 ht 40 ◦C in order to obtain a homogeneous solution. The solutionas then electrospun at 16 kV using a horizontal electrospinning

eposition (ESD) system based on a 10 cm tip-to-collector distanceTCD), as described in Fig. 1. The injection rate was 0.30 ml/h, and aml disposal polyethylene syringe (Kovax Syringe®, Korea Vaccineo. Ltd., Korea) with a 23 gauge (inner diameter was 0.318 mm)etal needle was used as the working electrode. The electrospun

Fig. 1. Schematic illustration of a horizontal electrospinning deposition (ESD)system. The counter electrode rotated at 150 rpm, and horizontally oscillated at150 rpm to and from 15 cm.

BPPO nanofiber mat (NFM) was subsequently collected on the alu-minum foil covering the cylindrical counter electrode (rotated at150 rpm and horizontally oscillated at 150 rpm) at room tempera-ture in air. The NFM was then treated via hot-pressing at 50 ◦C under2000 psi for 30 min, to prepare a physically processable nanofiberporous substrate (NFPS). Finally, the cross-linked BPPO (cBPPO)NFPS was obtained via a post cross-linking reaction with ammoniasolution (28 wt.%, Junsei, Japan) for further SPPO impregnation.

2.4. Fabrication of membranes

The 35 wt.% SPPO solution in DMF was prepared at 40 ◦C for24 h to obtain homogenous solutions. Each of the SPPO solutionswas cast on a glass plate at a thickness of 150 �m by using aglass bar. Subsequently, the electrospun cBPPO NFPS impregnatedwith the cast solution and then sandwiched using another glassplate to obtain a plane surface. The glass plates were clipped andit was stepwisely dried at 40 ◦C for 5 h and 60 ◦C for 12 h undervacuum conditions. The excessive SPPO solution was extract outduring the low temperature drying conditions, and then the sand-wiched glass plates were subsequently removed. The membranewas finally dried at 40 ◦C for 12 h to remove residual solvent. Thefabricated membranes and Nafion®112 were then analyzed afterbeing treated with a 1 mol/l HCl solution for 5 h.

2.5. Structural confirmations

Fig. 2 presents the structural concept of the SPPO membrane interms of morphologies and chemical structures. The morphologiesof the electrospun NFPS and pore-filled membranes were exam-ined by scanning electron microscope (SEM, Hitachi S-4700, Japan).The distribution plot for the diameters of the electrospun fiberswas obtained after measuring 400 unit fiber diameters with animage tracer (IT Plus 4.0, Sometech Inc., Korea), where the meandiameter was obtained by Gaussian curve fitting. The chemicalstructures of PPO, BPPO, cBPPO, and SPPO were confirmed usinga Fourier transform infrared spectrometer (FTIR 460 plus, Jasco,Japan) and MiracleTM accessory (PIKE tech. Inc., USA), which wasdesigned to obtain a single reflection horizontal attenuated totalreflectance (HATR) of an Si window. Each sample was measured30 times at a scan resolution of 8 cm−1 in a wave number range of550–4000 cm−1.

2.6. Membrane properties

The proton conductivity of the membrane was measuredusing a four-probe electrochemical impedance cell [41] at roomtemperature. Impedance analyses were carried out using a poten-tiostat/galvanostat (Autolab PGSTAT 30, Eco Chemie, Netherland),operated via a galvanostatic method under a 0.1 mA AC amplitude

298 S.-H. Yun et al. / Journal of Membrane Science 367 (2011) 296–305

ed by

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Fig. 2. Schematic structure of the SPPO membrane reinforc

ver a frequency range of 1 MHz to 50 Hz. The membranes werenalyzed at the same time to minimize the differences in tem-erature and humidity during the measurement. The membraneesistance was determined at a zero phase angle from the Nyquistehavior. Finally, the proton conductivity k (S/cm) was calculatedsing:

= L

RWd(1)

here R is the experimentally determined membrane resistance,is the distance between the reference-sensing electrodes, and Wnd d are the width and thickness of the membrane, respectively.he proton conductance c (S/cm2) was calculated by dividing theroton conductivity by the membrane thickness.

The water uptake (WU) was calculated based on the weight ofhe dried (Wdry) and wet (Wwet) membranes as follows:

U(%) = Wwet − Wdry

Wdry× 100 (2)

The ion-exchange capacity was measured according to thehenolphthalein endpoint detection via the titration of an NaOHqueous solution. Before titration, the membrane in acid form wasoaked in a 0.5 mol/l NaCl aqueous solution for 12 h, and the equiv-lent of the protons released from the membrane in the solutionas detected by titrating a 0.01 mol/l NaOH (CNaOH) aqueous solu-

ion in the presence of phenolphthalein. Based on the titrated NaOHolume (VNaOH), the IEC (mequiv./g) was calculated using:

EC = CNaOHVNaOH

Wdry(3)

The swelling ratio in water (SW), a useful parameter for theumerical evaluation of dimensional stability, was then calculatedccording to:

W = Awet − Adry

Adry× 100 (4)

here Awet and Adry are the surface area of the membrane in driednd swollen states, respectively.

.7. Single cell test

The membrane performances were evaluated for a single cell5 cm2 active area, CNL, USA) in comparison with a Nafion®112

embrane (DuPont, USA) at 60 ◦C using 100% humidified pureydrogen and oxygen gases, in which the flow rates of hydro-en and oxygen gases were controlled at 110 ml/min using a massow controller. In order to fabricate electrodes, the catalyst pow-er (36 wt.% of Pt, Tanaka, Japan) and Nafion® ionomer solution

the electrospun cBPPO nanofiber porous substrate (NFPS).

(Aldrich, USA) were dispersed in isopropyl alcohol (IPA, 99.8%,Merck, Germany). The dispersion mixture was then ultrasonicatedfor 30 min and subsequently sprayed on a gas diffusion layer (GDL,10BC, SGL Carbon Group, Germany). The catalyst loading was con-tinuously calculated by weighing the electrode before and afterapplying the catalyst dispersion, and 0.40 mg/cm2 of Pt was loaded.The MEA was fabricated by sandwiching the membrane betweentwo electrodes without hot-pressing; the polarization curves wereobtained after MEA activation by operating continuous polarizationuntil a stable polarization was achieved.

3. Results and discussion

3.1. Synthesis of SPPO and cBPPO

The two reactions of cross-linking electrospun BPPO NFPS andthe sulfonation of PPO were confirmed by the transmittance FT-IR spectra, as shown in Fig. 3. The bromine group at the methylbranch of BPPO was detached from the polymer for the cross-linking process. The spectra of BPPO NFPS and cBPPO NFPS werethen compared to the spectrum of the PPO cast film (Fig. 3(a)and (b), respectively). The peak at 2920 cm−1 was assigned to thearomatic group, and the peak at 2850 cm−1 corresponds to thatof alkanes (–CH3). Those of the phenyl groups were observed at1604 and 1469 cm−1, and the characteristics of the symmetricaland asymmetrical stretching vibrations for C–O were observed at1303 and 1189 cm−1, respectively. Since the two polymers havethe same backbone structure, the above peaks were observed forboth BPPO NFPS and cBPPO NFPS. For the bromine group of BPPO,a new peak after bromination of PPO appeared at 562 cm−1 withinthe range suggested in a report [41], as presented in the magni-fied view in Fig. 3(b). From the covalently cross-linked structure,the vibration peaks of N–H in –CH2–NH–CH2– were expected at1480–1580, 1100–1160 and 3120–3550 cm−1; however, the FT-IRspectrum of the original BPPO showed other strong peaks in thesespectral regions, thereby obscuring the N–H vibration peaks [37].On the other hand, the peak for the bromine group at 562 cm−1

disappeared in the spectrum of cBPPO, as the bromine ions wereremoved after the cross-linking reaction. Therefore, it was con-firmed that the cross-linking reaction by ammonia took place atroom temperature.

The reaction for PPO sulfonation was also confirmed by FT-IR

spectra, which were compared to the PPO spectrum in Fig. 3(c).Similarly the peaks assigned to the aromatic, alkanes, phenyl, andthe C–O of the oxide chain appeared at the same wavenumbers dueto the fact that they all have the same backbone structure. However,introduction of a sulfonic acid group (–SO3

−H+) brought new peaks

S.-H. Yun et al. / Journal of Membrane Science 367 (2011) 296–305 299

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ig. 3. Confirmation of bromination, cross-linking, and sulfonation processes by FThe low wavenumber, and (c) SPPO film.

t 1396, 1070, and 675 cm−1, which were subsequently assigned tohe sulfonic acid group; another sulfonic acid band was observedt 1200 cm−1 in the aromatic ether band (at 1189 cm−1 for C–O)40,47]. The broad peak at around 3480 cm−1 was assigned to theulfonic acid group that appears in the non-fluorinated conductingolymer SPPO.

.2. Properties of the synthesized SPPO

Various SPPO membranes were prepared by varying the amountf CSA from 6.0 to 7.5 ml with 20 g of PPO to investigate the effectf pore-filled SPPO for different IECs, denoted as S6.0, S6.5, S7.0,

nd S7.5, respectively. Table 1 shows the properties of the syn-hesized SPPO base membranes. The thicknesses of the cast SPPO

embranes were relatively uniform in a dry state; in a wet state,owever, they gradually increased with increasing amounts of CSAue to the increasing IEC. The IEC of SPPO increased with increases

ectra: for (a) PPO, BPPO cast film, and cBPPO NFPS, with magnified spectra for (b)

in CSA, from 2.12 mequiv./g for S6.0 to 2.62 mequiv./g for S7.5.Also, the water uptake increased from 55.8% for S6.0 to 260.1%for S7.5, implying that the degree of sulfonation was successfullyincreased by increasing the amount of CSA from 6.0 to 7.5 ml. Withthe increasing IEC, the proton conductivity was also enhanced from0.09 to 0.15 S/cm for S6.0 and S7.0, respectively. The increasedproton conductivity with IEC implies that the water uptake val-ues were reasonable in enhancing the proton transfer through thefully hydrated membrane. For S7.5 however, the proton conduc-tivity significantly decreased despite the enhanced IEC, becauseof excessive water uptake of over 260%, which prohibited protonconduction by water flooding along the proton-conducting chan-

nel. Thus, S7.5 was excluded in the following study on the effectsof IEC because it did not display a similar trend with increasingIEC.

Each of the S6.0, S6.5, and S7.0 had high IEC and high pro-ton conductivity, though their water uptake values were too

300 S.-H. Yun et al. / Journal of Membrane Science 367 (2011) 296–305

Table 1Preparation of highly sulfonated PPO (SPPO) membranes having various ion exchange capacities.

SPPO Sulfonation history Cast SPPO membrane

CSA (ml) PPO (g) CHF (ml) ddrya (�m) dwet

b (�m) IEC (mequiv./g) WU (wt.%) k (S/cm)

S6.0 6.0 20 200 114 131 2.12 55.84 0.09S6.5 6.5 20 200 116 146 2.43 94.62 0.13S7.0 7.0 20 200 114 152 2.57 129.60 0.15S7.5 7.5 20 200 118 173 2.62 260.10 0.10

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a Thickness of the membrane in a dry state.b Thickness of the membrane in a wet state.

igh to be employed in MEAs. A membrane requiring a highater uptake generally accompanies poor dimensional stability,hich causes mechanical breakage in MEAs during hydration

nd dehydration conditions and high fuel crossover during itswollen state, thereby further decreasing the fuel cell perfor-ance. The SPPO membranes synthesized in this study require

igh water uptake to transfer protons. Also, excessive water in theEA fabric could cause water flooding, and an excessively highater requirement is not a merit in terms of water management

43,44]. In other words, excessive water uptake and swelling causenterfacial failure between the membrane and electrode surfaces,nd water deficiency at elevated temperatures or low humidityonditions decreases proton conduction through membranes. Tovercome these problems, a mechanical reinforcing method wasmployed by using physically and chemically modified electrospunFPS.

.3. Properties of electrospun cBPPO NFPS

Polymeric nanofibers as a substrate should be physically sus-ainable during handling. Also, solvent resistance during theore-filling process is a crucial problem for membrane fabrication.

n this regard, the physical and chemical feasibilities of electrospunPPO fibers were investigated to determine appropriate physi-al handling and chemical processes for membrane fabrication, ashown in Fig. 4. The purified BPPO was successfully electrospunnder the given conditions, though the electrospun BPPO NFM didot maintain its shape during handling (Fig. 4(a)). In contrast, elec-rospun BPPO NFPS was obtained after the modification of NFM,nd was successfully used as a porous substrate while maintainingts shape (Fig. 4(b)).

Since electrospun BPPO NFPS can be cross-linked by ammo-ia to prevent melting in DMF during SPPO impregnation, solventbsorption behaviors were investigated as shown in Fig. 4(c).he BPPO NFPS is soluble in NMP, whereas the cBPPO NFPS wasot soluble due to the cross-linked structure of cBPPO. From thegure, the BPPO NFPS was observed to be partially soluble inMF causing it to shrivel. In contrast, the cBPPO NFPS was not

oluble in DMF, but rather absorbed it completely. This resultmplies that the synthesized SPPO soluble in DMF, can be read-ly impregnated in cBPPO NFPS due to the good affinity of cBPPO

ith the solvent. Thus, the electrospun cBPPO NFPS was physi-ally and chemically processable for the use in SPPO membraneabrication.

The diameter of NFPS fibers affect their physical properties,ncluding mechanical strength, average pore size, and specificurface area, and is therefore an important parameter affecting

embrane properties. Fig. 5 shows the diameter distribution of

he electrospun fibers for the cBPPO NFPS. The diameters are seeno be normally distributed, such that a Gaussian distribution curveas assumed, and that the mean diameter of the fibers was around

55 nm.

3.4. Fabrication of reinforced SPPO membranes using cBPPO NFPS

As previously illustrated in Fig. 2, the membranes were fabri-cated by impregnating cBPPO NFPS with the cast SPPO solution.Note that the structural concept of a pore-filling membrane hasbeen clearly presented in a previous report [22], in which prac-tical pore-filling structures in the presence of surface conductinglayers have been an experimentally crucial point. Current meth-ods for employing electrospun fibrous substrate in a pore-fillingmembrane require a surface conducting layer, since surfaces areuneven due to complex fiber networks, whereas a phase inversedsubstrate could be more even [45]. A previous study on elec-trospun fibrous substrates reported that surface layers could beadditionally formed due to excessive impregnation [33]. The addi-tional layers may result in better interfacial contact with electrodes,however the influence of surface layers on the interfacial contactand MEA durability have yet to be demonstrated. On the otherhand, the surface layers could be more important for membrane’ssymmetry rather than for interfacial contact, since the membranecould be physically symmetric or asymmetric based on the addi-tionally formed surface layers. That leads different distribution ofhydrophilic polymers in-thickness direction. In our previous work,it was confirmed that an asymmetric membrane is beneficial towater management by controlling water diffusion in asymmetricfuel cells [46]. The focus of this study is on a pore-filled membranewith symmetric layers to demonstrate effects of reinforcement byemploying electrospun non-fluorinated NFPS. Hence, a symmet-rically pore-filled structure is desirable with an ignorable surfacelayers.

Fig. 6 shows the SEM images of the cross-sections and surfacesfor NFPS and the resultant SPPO membrane reinforced by NFPS.From the cross-sectional morphologies of Fig. 6(a) and (b), show-ing that the three-dimensionally interconnected network along thepores of the NFPS provides continuous proton conducting chan-nels with no pores left unfilled by SPPO. In addition, the thicknessof the electrospun NFPS was 30 �m when 1 g of BPPO was usedunder the given electrospinning conditions, the same as for theNFPS of the SPPO impregnated membranes. The fiber frames onthe surfaces were quite distinguishable (Fig. 6(c) and (d)), imply-ing that the SPPO conducting layers on the surfaces were slightlycoated at ignorable thickness. Therefore, it was confirmed that asymmetric structure without additional conducting layers couldbe successfully formed.

In a previous study, it was reported that porous and nonporousrod-type fibers were formed depending on solvents [42]. In order toexamine the type of the electrospun BPPO fiber, the morphology ofa unit BPPO fiber was examined. As presented in Fig. 6(e), the elec-trospun BPPO fiber was found to be a rod-type fiber without pores

on the surface and inside a unit fiber. With the expected symmet-ric structure, the hydrophilic SPPO was pore-filled completely. Thisresult implies that DMF is a good solvent for impregnating SPPOinto NFPS, as was previously reported. For this reason, there wereno interfacial defect between the hydrophobic cBPPO NFPS and

S.-H. Yun et al. / Journal of Membrane Science 367 (2011) 296–305 301

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ig. 4. Photograph showing the physical and chemical processability of electrospunorous substrate (NFPS) for physical processes, and subsequently cross-linked by amMP and DMF.

ydrophilic conducting SPPO, as shown in Fig. 6(f). It was also foundhat the affinity of the both polymers could be greatly enhanced ashey have the same backbone structure as well as DMF absorptionehavior.

.5. Properties of reinforced SPPO membranes based on cBPPOFPS

Since highly sulfonated base membranes such as S6.0, S6.5, and7.0 tend to expand due to the water swelling effect, they were

ig. 5. The scatter plot (�) of the frequency in electrospun BPPO nanofiber diameter; drmages, and distribution curve of the nanofiber (—-) obtained by Gaussian curve fitting.

: (a) the electrospun BPPO nanofiber mat (NFM) was modified into (b) a nanofiberfor chemical processes, such that (c) cBPPO NFPS was not dissolved while absorbing

reinforced by NFPS, denoted as S6.0NFPS, S6.5NFPS, and S7.0NFPS,respectively. Fig. 7 presents the water swelling ratio before andafter reinforcement with respect to the water uptake. The swellingratio of the base SPPO membranes were significantly suppressedby the mechanical support of NFPS, being reduced from 41 to 12%

for S6.0NFPS, 71 to 16% for S6.5NFPS, and 89 to 21% for S7.0NFPS.Accordingly, the water uptake was greatly reduced from 56 to30% for S6.0NFPS, 95 to 34% for S6.5NFPS, and 130 to 67% forS7.0NFPS. These results imply that the SPPO membranes were effi-ciently reinforced by NFPS, and that the electrospun cBPPO NFPS

awn based on 400 fiber sample diameters randomly measured from surface SEM

302 S.-H. Yun et al. / Journal of Membrane Science 367 (2011) 296–305

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Fu

ig. 6. SEM images of the prepared cBPPO NFPS for (a) the cross-section, (c) the suhe surface, and (f) the cross-sectional interfaces.

as suitable for use as a supporting matrix in a pore-filling mem-rane.

Proton conductivities of the fabricated membranes were subse-uently examined at room temperature under 100% RH conditions.s shown in Fig. 8(a), the impregnation of the SPPO while increas-

ng IEC from S6.0 to S7.0 led to a corresponding increase in the

roton conductivity from 0.03 to 0.08 S/cm. Also, S7.0NFPS reached.08 S/cm, which is comparable to 0.07 S/cm for Nafion®112; sim-

larly, the IEC for S6.0NFPS was 1.08 mequiv./g, almost the same asor Nafion®112 (Fig. 8(b)). On the other hand, the water uptake for

ig. 7. Effect of cBPPO NFPS as a mechanical support: swelling ratio and waterptake before (©, �, and �) and after (�, �, and �) reinforcement.

and (e) a unit fiber; the SPPO impregnated membrane for (b) the cross-section, (d)

S6.0NFPS was higher than that for Nafion®112, even though the IECand swelling ratio, were almost the same. Since the IEC of S6.0NFPSwas calculated based on the weight of the dried membrane, theweight for the nonconductive NFPS was included in Wdry of Eq.(3). Therefore, it is expected that the absolute density of the ion-exchangeable group for the unit volume of the conducting pathwaymight be higher than for Nafion®112. Moreover, the high density ofthe ion-exchangeable group in the conducting pathway (or waterchannel) is expected to enhance the fast and efficient proton con-duction via the structural diffusion of protons [47] during a fuel celloperation.

3.6. Single cell performance

The thickness of the membranes was around 30 �m, which wasmuch thinner than Nafion®112 in a swollen state. Once a thin mem-brane is formed, the proton conductance value becomes greaterthan that of thick membranes. Then, due to the high conductancevalue, a lower ohmic resistance is expected in the polarization curveduring fuel cell operation.

Fig. 9 shows the resultant polarization behaviors and powerdensity curves for the fabricated membranes and Nafion®112 dur-ing single cell operation, in which the synthesized membranesexhibit higher performance compared to Nafion®112. At a constantvoltage of 0.6 V, the power densities were 0.534, 0.588, 0.702, and0.816 W/cm2 for S6.0NFPS, Nafion®112, S6.5NFPS, and S7.0NFPS,

respectively. Around 140% of the enhanced power density was elec-trochemically converted via the S7.0NFPS membrane comparedto Nafion®112. The non-fluorinated composite SPPO membranesreinforced by NFPS were found to be a promising alternative poly-mer electrolyte membrane for fuel cell application.

S.-H. Yun et al. / Journal of Membrane Science 367 (2011) 296–305 303

Fig. 8. Proton conductivities and ion-exchange capacities (IEC) of Nafion®112 andtmp

ivpwiticlrmt

mlTipipichdwtfdc

Fig. 9. Single cell performances of the membranes in (a) and (b), and proton con-ductances in (c). The thicknesses of the membranes are 60, 28, 24, and 30 �m forNafion®112, S6.0NFPS, S6.5NFPS, and S7.0NFPS, respectively. Polarization curves

he synthesized S6.0NFPS, S6.5NFPS, and S7.0NFPS. The proton conductivities wereeasured via the membrane resistances obtained from Nyquist plots using the four-

robe electrochemical impedance cell.

In terms of their polarization behaviors, there were no signif-cant differences in the activation loss regions (AlRs), because aoltage drop in the AlR is mainly governed by thermodynamicarameters of catalytic activation and all the tests performed hereere carried out using the same electrode (0.40 mg Pt/cm2). Then,

n order to evaluate the membrane performance, the liner slope ofhe ohmic resistance region (OrR) is generally compared becauset exhibits the overall ohmic resistance associated with the protononductivity of a membrane. In particular, S6.0NFPS had a notablyow proton conductivity of 0.03 S/cm, while displaying a compa-able performance with the Nafion®112. The reason is that theembrane conductance values of both membranes were almost

he same (around 11 S/cm2), as presented in Fig. 9(c).On the other hand, there have been few reports discussing the

ass transfer region (MtR) in a polarization curve, partially due toow electrical efficiency at a low voltage in practical applications.he MtR, however, gives additional information to the OrR, sincet is dominantly associated with water behaviors in an MEA. In theolarization curves for Nafion®112 and S6.0NFPS, the ohmic behav-

or was nearly the same, though S6.0NFPS showed relatively highererformance in the high current region. The higher current release

ndicates a higher oxygen reduction reaction (ORR) in the cathode,onsequently producing a higher amount of water. In general, aigh water production rate in the cathode causes a rapid voltagerop due to mass transfer limitation dominantly due to not onlyater flooding at cathode but also anodic drying [46], in addition

o unavoidable ohmic resistance. Here, we operated the cells withully humidified hydrogen and oxygen so that the rapid voltagerop of Nafion®112 would be caused by mass transfer limitation atathode. Based on this assumption, the reason for the higher perfor-

were obtained in an H2/O2 fuel cell having a 5 cm2 effective area at 60 ◦C under 100%RH conditions. The gas flow rate was 110 ml/min and a 0.40 mg/cm2 Pt catalyst wasapplied.

mance in the high current region (MtR) for S6.0NFPS was partiallydue to the efficient back diffusion of water from the cathode toanode against the electro-osmosis drag (EOD) from the anode to

cathode. Also, it is expected that ultrathin membranes are favor-able in terms of water back diffusion to avoid cathodic flooding andanodic drying problems, wherein the membranes possess high IECwith sufficient water uptake, high proton conductance, and good

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imensional stability, as is found in thin membranes reinforced bylectrospun NFPS.

. Conclusions

We have developed a non-fluorinated composite polymer elec-rolyte membrane reinforced by electrospun NFPS. The electrospunFPS was further modified into processable NFPS by hot-pressingnd detaching techniques. Here, highly sulfonated SPPO base mem-ranes that were previously too swollen to apply in MEAs wereuccessfully reinforced by NFPS without requiring an additionalonducting layer on the membrane surface. Moreover, the affinityetween the hydrophilic conducting SPPO and hydrophobic sup-orting cBPPO NFPS was excellent, due in particular to the sameolymer backbone. As a result, the SPPO membranes reinforcedy the cBPPO NFPS showed greatly enhanced the fuel cell perfor-ance, based on the increased proton conductivity along with the

ncreased IEC of the SPPO. With the enhanced fuel cell performance,urther advantages for water management using these membranesre expected in comparison with Nafion®112.

cknowledgments

This work was supported by the New & Renewable Energy&D Program (No. 20093020030020-11-1-000) of the Korea Insti-ute of Energy Technology Evaluation and Planning (KETEP) grantunded by the Korea government Ministry of Knowledge Economy.he BPPO support by Prof. Tongwen Xu in USTC is also gratefullycknowledged.

eferences

[1] C. Bossi, A.D. Corno, M. Scagliotti, C. Valli, Characterisation of a 3 kW PEFC powersystem coupled with a metal hydride H2 storage, J. Power Sources 171 (2007)122–129.

[2] H.-S. Chu, F. Tsau, Y.-Y. Yan, K.-L. Hsueh, F.-L. Chen, The development of a smallPEMFC combined heat and power system, J. Power Sources 176 (2008) 499–514.

[3] S.J.C. Cleghorn, D.K. Mayfield, D.A. Moore, J.C. Moore, G. Rusch, T.W. Sherman,N.T. Sisofo, U. Beuscher, A polymer electrolyte fuel cell life test: 3 years ofcontinuous operation, J. Power Sources 158 (2006) 446–454.

[4] R. Souzy, B. Ameduri, Functional fluoropolymers for fuel cell membranes, Prog.Polym. Sci. 30 (2005) 644–687.

[5] B.-J. Chang, D.J. Kim, J.H. Kim, S.-B. Lee, H.J. Joo, Sulfonated poly(fluorene-co-sulfone)ether membranes containing perfluorocyclobutane groups for fuel cellapplications, J. Membr. Sci. 325 (2008) 989–996.

[6] M. Rikukawa, K. Sanui, Proton-conducting polymer electrolyte membranesbased on hydrocarbon polymers, Prog. Polym. Sci. 25 (2000) 1463–1502.

[7] T.A. Sherazi, S. Ahmad, M.A. Kashmiri, D.S. Kim, M.D. Guiver, Radiation-inducedgrafting of styrene onto ultra-high molecular weight polyethylene powder forpolymer electrolyte fuel cell application II. Sulfonation and characterization, J.Membr. Sci. 333 (2009) 59–67.

[8] R.-Q. Fu, J.-J. Woo, S.-J. Seo, J.-S. Lee, S.-H. Moon, Covalent organic/inorganichybrid proton-conductive membrane with semi-interpenetrating polymer net-work: preparation and characterizations, J. Power Sources 179 (2008) 458–466.

[9] R.-Q. Fu, J.-J. Woo, S.-J. Seo, J.-S. Lee, S.-H. Moon, Sulfonatedpolystyrene/polyvinyl chloride composite membranes for PEMFC applications,J. Membr. Sci. 309 (2008) 156–164.

10] S. Feng, Y. Shang, X. Xie, Y. Wang, J. Xu, Synthesis and characterization ofcrosslinked sulfonated poly(arylene ether sulfone) membranes for DMFC appli-cation, J. Membr. Sci. 335 (2009) 13–20.

11] J. Jaafar, A.F. Ismail, T. Matsuura, Preparation and barrier properties ofSPEEK/Cloisite 15A®/TAP nanocomposite membrane for DMFC application, J.Membr. Sci. 345 (2009) 119–127.

12] Q. Li, J.O. Jensen, R.F. Savinell, N.J. Bjerrum, High temperature proton exchangemembranes based on polybenzimidazoles for fuel cells, Prog. Polym. Sci. 34(2009) 449–477.

13] R. Borup, J. Meyers, B. Pivovar, et al., Scientific aspects of polymer electrolytefuel cell durability and degradation, Chem. Rev. 107 (2007) 3904–3951.

14] Y. Fu, A. Manthiram, M.D. Guiver, Blend membranes based on sulfonated

poly(ether ether ketone) and polysulfone bearing benzimidazole side groupsfor proton exchange membrane fuel cells, Electrochem. Commun. 8 (2006)1386–1390.

15] A. Mokrini, M.A. Huneault, Z. Shi, Z. Xie, S. Holdcroft, Non-fluorinated proton-exchange membranes based on melt extruded SEBS/HDPE blends, J. Membr.Sci. 325 (2008) 749–757.

[

[

e Science 367 (2011) 296–305

16] T. Yamaki, M. Asano, Y. Maekawa, Y. Morita, T. Suwa, J. Chen, N. Tsubokawa, K.Kobayashi, H. Kubota, M. Yoshida, Radiation grafting of styrene into crosslinkedPTEE films and subsequent sulfonation for fuel cell applications, Radiat. Phys.Chem. 67 (2003) 403–407.

17] B.N. Kim, D.H. Lee, D.H. Han, Characteristics of fuel cell membranes preparedby EB radiation grafting onto FEP with styrene derivatives, styrene and 2-methylstyrene, J. Electrochem. Soc. 155 (7) (2008) B680–B685.

18] T. Yamaguchi, H. Zhou, S. Nakazawa, N. Hara, An extremely low methanolcrossover and highly durable aromatic pore-filling electrolyte membrane fordirect methanol fuel cells, Adv. Mater. 19 (2007) 592–596.

19] T. Yamaguchi, F. Miyata, S.-I. Nakao, Pore-filling type polymer electrolytemembranes for a direct methanol fuel cell, J. Membr. Sci. 214 (2003) 283–292.

20] T. Yamaguchi, M. Ibe, B.N. Nair, S.-i. Nakao, A pore-filling electrolyte membrane-electrode integrated system for a direct methanol fuel cell application, J.Electrochem. Soc. 149 (11) (2002) A1448–A1453.

21] A. Yamauchi, T. Ito, T. Yamaguchi, Low methanol crossover and high perfor-mance of DMFCs achieved with a pore-filling polymer electrolyte membrane,J. Power Sources 174 (2007) 170–175.

22] T. Yamaguchi, F. Miyata, S.-i. Nakao, Pore filling membranes with a pore-filling structure for a direct methanol fuel cell, Adv. Mater. 15 (14) (2003)1198–1201.

23] T. Yamaguchi, H. Kuroki, F. Miyata, DMFC performances using a pore-fillingpolymerelectrolyte membrane for portable usages, Electrochem. Commun. 7(2005) 730–734.

24] J.H. Shim, I.G. Koo, W.M. Lee, Nafion-impregnated polyethylene–terephthalatefilm used as the electrolyte for direct methanol fuel cells, Electrochim. Acta 50(2005) 2385–2391.

25] J.-P. Shin, B.-J. Chang, J.-H. Kim, S.-B. Lee, D.H. Suh, Sulfonated polystyrene/PTFEcomposite membranes, J. Membr. Sci. 251 (2005) 247–254.

26] M.M. Nasef, N.A. Zubir, A.F. Ismail, M. Khayet, K.Z.M. Dahlan, H. Saidi, R. Rohani,T.I.S. Ngah, N.A. Sulaiman, PSSA pore-filled PVDF membranes by simultaneouselectron beam irradiation: preparation and transport characteristics of protonsand methanol, J. Membr. Sci. 268 (2006) 96–108.

27] M.M. Nasef, N.A. Zubir, A.F. Ismail, K.Z.M. Dahlan, H. Saidi, M. Khayet, Prepara-tion of radiochemically pore-filled polymer electrolyte membranes for directmethanol fuel cells, J. Power Sources 156 (2006) 200–210.

28] K.M. Nouel, P.S. Fedkiw, Nafion® based composite polymer electrolyte mem-branes, Electrochim. Acta 43 (1997) 2381–2387.

29] J. Shim, H.Y. Ha, S.-A. Hong, I.-H. Oh, Characteristics of the Nafion ionomer-impregnated composite membrane for polymer electrolyte fuel cells, J. PowerSources 109 (2002) 412–417.

30] F. Liu, B. Yi, D. Xing, J. Yu, H. Zhang, Nafion/PTFE composite membranes for fuelcell applications, J. Membr. Sci. 212 (2003) 213–223.

31] T.L. Yu, H.-L. Lin, K.-S. Shen, L.-N. Huang, Y.-C. Chang, G.-B. Jung, J.C. Huang,Nafion/PTFE composite membranes for fuel cell applications, J. Polym. Res. 11(2004) 217–224.

32] H.-L. Lin, T.L. Yu, K.-S. Shen, L.-N. Huang, Effect of Triton-X on the prepa-ration of Nafion/PTFE composite membranes, J. Membr. Sci. 237 (2004)1–7.

33] S.W. Choi, Y.-Z. Fu, Y.R. Ahn, S.M. Jo, A. Manthiram, Nafion-impregnated electro-spun polyvinylidene fluoride composite membranes for direct methanol fuelcells, J. Power Sources 180 (2008) 167–171.

34] G. Cheruvally, J.-K. Kim, J.-W. Choi, J.-H. Ahn, Y.-J. Shin, J. Manuel, P. Ragha-van, K.-W. Kim, H.-J. Ahn, D.S. Choi, C.E. Song, Electrospun polymer membraneactivated with room temperature ionic liquid: novel polymer electrolytes forlithium batteries, J. Power Sources 172 (2007) 863–869.

35] X. Wang, K. Zhang, Y. Yang, L. Wang, Z. Zhou, M. Zhu, B.S. Hsiao, B.Chu, Development of hydrophilic barrier layer on nanofibrous substrate ascomposite membrane via a facile route, J. Membr. Sci. 356 (2010) 110–116.

36] H. Yu, T.W. Xu, Fundamental studies of homogeneous cation exchange mem-branes from poly(2,6-dimethyl-1,4-phenylene oxide): membranes prepared bysimultaneous aryl-sulfonation and aryl-bromination, J. Appl. Polym. Sci. 100(2006) 2238–2243.

37] Y. Li, T.W. Xu, M. Gong, Fundamental studies of a new series of anionexchange membranes: membranes prepared from bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) and pyridine, J. Membr. Sci. 279 (2006)200–208.

38] B. Kruczek, T. Matsuura, Development and characterization of homogeneousmembranes de from high molecular weight sulfonated polyphenylene oxide, J.Membr. Sci. 146 (1998) 263–275.

39] S. Yang, C. Gong, R. Guan, H. Zou, H. Dai, Sulfonated poly(phenylene oxide)membranes as promising materials for new proton exchange membranes,Polym. Adv. Technol. 17 (2006) 360–365.

40] R.Y.M. Huang, J.J. Kim, Synthesis and transport properties of thin film compositemembranes. I. Synthesis of poly(phenylene oxide) polymer and its sulfonation,J. Appl. Polym. Sci. 29 (1984) 4017–4027.

41] T.W. Xu, J.-J. Woo, S.-J. Seo, S.-H. Moon, In situ polymerization: a novel routefor thermally stable proton-conductive membranes, J. Membr. Sci. 325 (2008)

209–216.

42] M. Bognitzki, W. Czado, T. Frese, A. Schaper, M. Hellwig, M. Steinhart, A. Greiner,J.H. Wendorff, Nanostructured fibers via electrospinning, Adv. Mater. 13 (1)(2001) 70–72.

43] K. Tüber, D. Pócza, C. Hebling, Visualization of water buildup in the cathode ofa transparent PEM fuel cell, J. Power Sources 124 (2003) 403–414.

bran

[

[

[

[Hydrogen bonding: a channel for protons to transfer through acid–base pairs,

S.-H. Yun et al. / Journal of Mem

44] P. Berg, K. Promislow, J.St. Pierre, J. Stumper, B. Wetton, Water management inPEM fuel cells, J. Electrochem. Soc. 151 (3) (2004) A341–A353.

45] A. Magistris, P. Mustarelli, F. Parazzoli, E. Quartarone, P. Piaggio, A. Bottino,Structure porosity and conductivity of PVdF films for polymer electrolytes, J.Power Sources 97-98 (2001) 657–660.

46] J.-J. Woo, S.-J. Seo, S.-H. Yun, T.W. Xu, J. Lee, S.-H. Moon, Asymmetric poly-mer electrolyte membranes for water management of fuel cells, Electrochem.Commun. 12 (2010) 148–151.

[

e Science 367 (2011) 296–305 305

47] L. Wu, C. Huang, J.-J. Woo, D. Wu, S.-H. Yun, S.-J. Seo, T.W. Xu, S.-H. Moon,

J. Phys. Chem. B 113 (36) (2009) 12265–12270.48] D. Wu, L. Wu, J.-J. Woo, S.-H. Yun, S.-J. Seo, T.W. Xu, S.-H. Moon, A simple

heat treatment to prepare covalently crosslinked membranes from sulfonatedpoly(2,6-dimethyl-1,4-phenylene oxide) for application in fuel cells, J. Membr.Sci. 348 (2010) 167–173.