Improvement of homogeneity and interfacial properties of radiation grafted membranes for fuel cells using diisopropenylbenzene crosslinker

Journal of Membrane Science 381 (2011) 102–109

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Improvement of homogeneity and interfacial properties of radiation grafted membranes for fuel cells using diisopropenylbenzene crosslinker H. Ben youcef, Lorenz Gubler ∗ , Annette Foelske-Schmitz, Günther G. Scherer Electrochemistry Laboratory, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland

a r t i c l e

i n f o

Article history: Received 20 December 2010 Received in revised form 8 July 2011 Accepted 12 July 2011 Available online 23 July 2011 Keywords: Crosslinking Diisopropenylbenzene Dimensional stability ETFE Proton exchange membrane Radiation grafting

a b s t r a c t A novel crosslinked radiation grafted poly(ethylene-alt-tetrafluoroethylene) (ETFE) based membrane has been obtained by use of the crosslinker 1,3-diisopropenylbenzene (DIPB). The investigated membranes are based on 25 ␮m thick ETFE film grafted with styrene/DIPB. Grafted films and proton-exchange membranes with varying DIPB concentration and fixed graft level (∼25%) were prepared. The influence of the crosslinker on various ex situ and in situ fuel cell relevant membrane properties was investigated in detail. The composition of the styrene/DIPB in the bulk and in the near-surface region was determined. Infrared spectroscopy analysis revealed homogeneous crosslinker distribution in the surface and bulk of the grafted films, whereas XPS showed a poorer grafts concentration at the surface in the highly crosslinked samples. A qualitative relationship between the surface hydrophilicity measured by the contact angle and the interfacial property of the membrane–electrode assembly (MEA) determined by electrochemical impedance spectroscopy during fuel cell operation was observed. The interfacial properties, dimensional stability and hydrogen barrier properties of the membranes improved substantially with increasing DIPB concentration. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The proton exchange membrane (PEM) is the heart of the low-temperature (80–100 ◦ C) fuel cell. Performance (i.e., conductivity), stability (chemical, thermal and mechanical), durability and reliability in fuel cell operation are essential requirements for the membrane [1–3]. Several alternatives to the state-of-theart and commercially available perfluorinated membranes, such as Membranes of the Nafioin(R) type, have been developed, directed towards the reduction of the material cost [4,5]. One alternative and low-cost method for the preparation of proton exchange membranes is based on the radiation-induced grafting technique [2,6]. Radiation grafting allows the functionalization of preformed and low-cost polymer films by means of radical polymerization reaction of selected vinyl monomers. The technique is versatile and offers, on the one hand, the possibility to use a variety of combinations between base films and monomers, and, on the other hand, to easily tune the desired membrane properties in a wide range of adjustable preparation parameters. An optimal combination of the base film properties, the radiation dose, the crosslinking, and the selected grafted monomers are the key parameters to develop low-cost and durable polymer electrolyte membranes for fuel cells [2,7].

∗ Corresponding author. Tel.: +41 56 3102673; fax: +41 56 3104416. E-mail address: [email protected] (L. Gubler). 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.07.021

Crosslinking is a method to increase the lifetime of radiation grafted membranes for fuel cells [2,6]. The degree of crosslinking is a crucial parameter to be optimized to improve the chemical, mechanical, and dimensional stability of radiation grafted membranes [6,8–11]. Various crosslinking strategies have been followed up to now to improve the oxidative stability of the membranes and to reduce the loss of grafted polymer chains [2]. Different crosslinkers (divinylbenzene (DVB), triallyl cyanurate (TAC), bis(vinyl phenyl)ethane (BVPE)) were used for the preparation of radiation grafted membranes in combination with styrene to improve their chemical stability and durability [12–15]. One of the major challenges during crosslinking is to control the membrane uniformity and the grafts homogeneity in-plane and through the film thickness. Divinylbenzene (DVB) was extensively used as crosslinking agent for styrene grafted into poly(ethylene-alt-tetrafluoroethylene) (ETFE) and poly(tetrafluoroetylene-co-hexafluoropropylene) (FEP) films at Paul Scherrer Institut (PSI) [2,6,8–10,16,17]. The used commercial DVB is of technical quality and a mixture of different isomers (∼80% of 1,3- and 1,4-divinylbenzene and ∼20% of 1,3- and 1,4ethylvinylbenzene) (Fig. 1). Thus, the difference in reactivity of the different isomers in presence of styrene increases the complexity of the grafting kinetics [9,18]. The incorporation of the crosslinker BVPE, reported by Lehtinen et al. and Yamaki et al., did not lead to an improvement in the gas barrier property of the grafted membranes, and no proof of the crosslinker incorporation (i.e., efficiency of crosslinking) by any spectroscopic technique has been given

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103

2. Experimental 2.1. Materials

a)

b)

e)

c)

d)

Fig. 1. Monomers of commercially available DVB ((∼80% of 1,3-divinylbenzene (a) and 1,4-divinylbenzene (b)) and ∼20% of 1,3-ethylvinylbenzene (c) and 1,4ethylvinylbenzene (d)) and of DIPB (1,3-diisopropenylbenzene (e)).

[13,15]. Due to its reactivity being similar to that of styrene, the BVPE crosslinker grafted with styrene was pointed out to be more homogeneously incorporated into the interior of irradiated PTFE base film compared to the case of DVB grafted with styrene [13]. The BVPE used in that study was a mixture of different isomers and allowed the preparation of more randomly crosslinked membranes with uniform and high crosslinking density as stated by the authors [13]. The increase of the DVB concentration in the grafting solution, more than 10% (v/v) in the total monomer volume, restricts the maximum reachable graft level even at longer grafting reaction time [6,9]. In addition to the complex morphology observed in crosslinked grafted membranes [11], the high reactivity of active DVB isomers as compared to the one of styrene (r(1,4-DVB) = 2.08, r(styrene) = 0.77; r(1,3-DVB) = 0.88, r(styrene) = 0.61) [19] induces a concentration gradient of the crosslinker through the thickness of the grafted film, with a substantially higher concentration at the surface [9]. To improve the through-plane homogeneity of crosslinked grafted membranes, a new crosslinker, 1,3-diisopropenylbenzene (DIPB) (Fig. 1), was used. DIPB cannot be grafted by itself into the irradiated film, as it is the case for DVB, and needs a suitable co-monomer. DIPB was selected because of its protected ␣-position and its use in combination with the ␣-methylstyrene/methacrylonitrile (AMS/MAN) system. However, the crosslinking efficiency (i.e., the extent of the crosslinker incorporation into the membrane) and effectiveness (i.e., the reactivity of the second double bond of the crosslinker) have not been proven so far [20]. This study aims to take advantage of the more favorable copolymerization kinetics of DIPB (r(1,3-DIPB) = 0.8, r(styrene) = 1.2 [19]) to prepare membranes with improved uniformity of crosslinker content through the thickness of the membrane. In addition, the higher purity (97% pure monomer) of DIPB in comparison with DVB (mixture of 4 isomers (Fig. 1)) simplifies the grafting system and thus the interpretation of results. The incorporation and effectiveness of the DIPB as crosslinker were investigated as a first objective. We report a detailed study on the styrene/DIPB grafting kinetics into irradiated ETFE base film. An FTIR-ATR analysis and DSC measurements of the grafted films were carried out and a comparison to results obtained with DVB/styrene grafted films is presented. Moreover, the effect of the DIPB/styrene molar ratio on the ex situ and in situ fuel cell relevant properties of styrene grafted ETFE based membranes are correlated.

In this study, the base polymer is an ETFE (Tefzel® 100LZ) film with a thickness of 25 ␮m and a density of ∼1.73 g cm−3 , purchased from DuPont (Circleville, USA). The monomer used during membrane preparation was styrene (purum grade; Fluka) and the crosslinker was 1,3-diisopropenylbenezene (DIPB) (97%; Aldrich). The solvent for the grafting reaction was a mixture of isopropanol (analytical grade; Fisher Scientific) and pure water obtained from a Seralpur® PRO 90 CN system (conductivity < 0.1 ␮S cm−1 ). Sulfonation was carried out using chlorosulfonic acid (Fluka) in dichloromethane (Fluka) as reagent. All monomers for the grafting reaction were used as received (without removal of the stabilizer/inhibitor). 2.2. Preparation of grafted films and membranes ETFE films were cut into 14 × 16 cm2 pieces and washed in ethanol, then dried at 60 ◦ C under vacuum. The dried ETFE films were electron beam irradiated under air at RT with a dose of 1.5 kGy at Leoni-Studer AG (Däniken, Switzerland) at conditions described elsewhere (accelerating voltage of 2.2 MV, dose rate 15.1 ± 1.1 kGy s−1 ) [6]. Subsequently, the films were stored at −80 ◦ C until used. Grafting reactions were carried out in two types of reactors. The glass reactor (60 mL) was used for studying the grafting kinetics, whereas the stainless steel reactor (600 mL) was used for the preparation of the samples to be assembled in single fuel cells [6]. All grafting reactions were performed under a nitrogen atmosphere, using a solution consisting of 20% (v/v) monomer (styrene/DIPB mixture), 70% (v/v) isopropanol and 10% water. The DIPB concentration in the initial grafting solution was varied from 0%, 5%, 10%, 15% to 20% (v/v) (volume of DIPB with respect to total monomer volume), which corresponds to a DIPB concentration change from 0%, 1%, 2%, 3% to 4% based on the total volume of grafting solution. The reactor was placed in a thermostatic bath at 60 ◦ C and the grafting reaction performed for different reaction times. The grafted films were extracted with toluene overnight and dried at 80 ◦ C under vacuum. The graft level (GL) was then calculated based on the weight of the film before (W0 ) and after grafting (Wg ) using the following equation: GL (%) =

Wg − W0 × 100 W0

The kinetics of the grafting reaction of styrene/DIPB into ETFE base film was performed in glass reactors (60 mL) for different DIPB concentrations (0%, 5%, 10%, 15%, 20%). Proton conducting membranes were obtained by sulfonation of the grafted films with chlorosulfonic acid in dichloromethane (2% (v/v)) solution at room temperature for 5 h, followed by hydrolysis/swelling in deionized water at 80 ◦ C for 8 h. 2.3. Ex situ characterization of grafted films and membranes The bulk composition of the grafted films was determined by Fourier transform infrared spectroscopy (FTIR), using a Perkin Elmer FTIR System 2000 spectrometer. The probing of the surface of the grafted films by the attenuated total reflectance (ATR) technique was carried out using a Golden GateTM system (Specac LTD., UK) with a 45◦ ZnSe crystal. The setup allows a penetration depth of 1–2 ␮m in the studied wavenumber range (800–1500 cm−1 ). The curve-fitting was performed using GRAMS/386 software (version 3.02) from Galactic Industries.

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A chemical analysis of the surface of the grafted films, with different crosslinker content, was performed via X-ray photoelectron spectroscopy (XPS) with an ESCALAB 220iXL (Thermo Scientific, USA, formerly V.G. Scientific) using Mg K␣ radiation for excitation (h = 1253.6 eV). The X-ray source was operated at reduced power of 100 W in order to minimize X-ray induced damage of the samples. XPS measurements were carried out in the constant analyzer energy (CAE) mode with a pass energy of 20 eV. Charge correction was performed according to the CF2 signal of ETFE, which was reported to occur at a binding energy of 291.3 eV [21]. The melting temperature and crystallinity of the membranes were determined by means of a Perkin Elmer DSC7. The DSC measurements were performed on membranes converted to salt form (KCl solution (0.5 M)) and dried under vacuum at 60 ◦ C. The conversion of the membranes into salt form reduces the content of absorbed water after drying and its negative effect on the obtained thermograms [22]. The heating curves were recorded in a temperature range of 30 → 300 ◦ C under nitrogen atmosphere at a constant heating rate of 20 ◦ C min−1 . The recrystallization behaviour of the grafted film was followed by first heating the sample to 300 ◦ C and holding it for 10 min at 300 ◦ C. Afterwards the thermograms were recoded in a temperature range of 300 → 30 ◦ C at the same operating conditions (cooling rate of 20 ◦ C min−1 ) as for the heating curves. The inherent crystallinity was calculated using the following equation: Inherent crystallinity =

Hf H0



1+

GL 100



The ion-exchange capacity, proton conductivity, water uptake and hydration number of ETFE based membranes were determined in fully swollen state at room temperature. The details of the measurement procedures of the membrane’s bulk properties can be found elsewhere [6,16]. The thickness of the grafted films and the membranes (in wet/dry state) were measured using a digital thickness gauge (MT12B Heidenhain, Germany). In-plane conductivity was determined by the 4-probe method using a BT-112 conductivity cell from Bekktech (Colorado, USA). Contact-angle measurements of membranes were performed using deionized water by means of a G2 contact-angle measurement system (Krüss GmbH, Germany). Prior to the measurement, the membranes were fixed on PVC plates and dried overnight at room temperature. All measurements were performed on membranes equilibrated in air at ambient conditions (room temperature and RH%) on the same day.

Fuel cell tests of the different ETFE based membranes were performed for ∼90 h, at a temperature of 80 ◦ C and at a constant current density of 500 mA cm−2 . The H2 /O2 stoichiometry was 1.5/1.5 for the whole current-density range and the tests were carried out at 100% RH in a 30 cm2 active area single cell [10]. The high frequency resistance (HFR) was continuously measured at 1 kHz by means of an AC milliohm meter model 3566 from Tsuruga (Osaka, Japan), and data points were recorded each 5 min. 3. Results and discussion 3.1. Influence of DIPB content on the kinetics of grafting and film composition The influence of DIPB/styrene molar ratio on the grafting kinetics of the ETFE irradiated films was investigated (Fig. 2). The initial grafting rate decreases gradually with the DIPB/styrene molar ratio in the initial grafting solution for all DIPB concentration (5–20% DIPB). For the uncrosslinked films (0% DIPB) and at an early phase of grafting (<4 h), the achieved graft levels (GL) were higher than those of the crosslinked samples. However, for longer reaction times (>8 h) the grafted films with DIPB concentration of 5% and 10% yield higher GLs than in case of the uncrosslinked films. Assuming that the grafting reaction proceeds according to the grafting front mechanism [23], the GL is governed by reactivity and diffusion of the monomer(s). Initially, the grafting takes place at the surface of irradiated films, thereafter the reaction becomes diffusion controlled. We note here that the DIPB by itself cannot be grafted into irradiated ETFE base films and styrene is required as a co-monomer. Indeed, no significant grafting was observed when tests were performed using pure DIPB and different irradiated films with low (1.5 up to 15 kGy) and high (50 up to 100 kGy) irradiation doses. The reactivity ratios reported for styrene and DIPB copolymerized by free radical polymerization (suspension) at 100 ◦ C are 1.2 and 0.8, respectively [19]. Hence, both terminal radicals of styrene and DIPB on the growing chain prefer to react with styrene, making the addition of DIPB less likely [19]. The influence of crosslinking on the grafting kinetics of styrene/DVB into ETFE films was investigated in detail and the same observations on the evolution of the GL were made [9]. However, the complexity of DVB is related mainly to the use of technical grade (DVB-tech) containing a mixture of 80% active crosslinking isomers and 20% of non-active isomers (Fig. 1). Based on the kinetics study, grafted films with fixed graft level of 25.7 ± 1.5% were prepared using different DIPB/styrene molar ratios in the initial grafting solution and with varying the reaction

2.4. In situ characterization of ETFE based membranes

0% 5%

10 % 15 %

20 %

120

Graft level (%)

Selected grafted films with a fixed graft level of ∼25% were sulfonated and the resulting membranes were characterized for their fuel cell relevant properties. The ETFE based membranes, with different DIPB concentration, were laminated with JM ELE162 electrodes (Johnson Matthey Fuel Cells, Swindon, UK) with a noble metal loading of 0.4 mg Pt cm−2 by hot-pressing at 110 ◦ C/5 MPa/180 s to form a membrane electrode assembly (MEA). The in situ characterization of the MEAs, namely by electrochemical impedance spectroscopy (EIS), polarization curves, and H2 crossover measurements were performed as described elsewhere [10]. The EIS spectra were recorded using a Zahner IM6 system (Zahner Messtechnik, Kronach, Germany) at a DC current density of 0.5 A cm−2 . From the EIS spectra represented in the Nyquist plot, the intercept at high frequency was interpreted as ohmic resistance, while the polarization resistance was defined as the diameter of the semi-circle like spectrum. The H2 crossover is measured in situ at 80 ◦ C after operating the cell in H2 /N2 for more than 1 h at a flow rate of 200 ml min−1 .

90

60

30

0 0

5

10

15

20

25

Reaction time (h) Fig. 2. Grafting kinetics of styrene/DIPB into ETFE based films for different DIPB concentrations. The DIPB concentration was varied from 0% to 20% (vol%) in the initial grafting solution.

Absorbance

a) 798

20 %

% DIPB 0%

800

780

760

Molar ratio (crosslinker/styrene) (grafted film) (%)

H. Ben youcef et al. / Journal of Membrane Science 381 (2011) 102–109

g-styrene/DIPB (Trans) g-styrene/DIPB (ATR) g-styrene/DVB (Trans) g-styrene/DVB (ATR)

25

b)

20 15 10 5 0 0

740

105

5

10

15

20

Molar ratio (crosslinker/styrene) (initial solution) (%)

Wavenumber (cm-1)

Fig. 3. (a) FTIR spectra of the ETFE grafted films (GL = 25%) at different DIPB concentration in the initial grafting solution. (b) Molar ratio of DIPB/styrene and DVB/styrene in the ETFE grafted films in the surface (ATR) and in the bulk (trans) versus the molar ratio of DIPB/styrene in the initial grafting solution. Values of DVB/styrene molar ratios are extracted from Ref. [9].

time from 1 h (0% DIPB) up to 14 h (20% DIPB). The grafted films were characterized by means of FTIR and FTIR-ATR to determine their composition. The addition of DIPB into the grafted films was confirmed by the increase of the peak intensity at around 800 cm−1 with the crosslinker content (Fig. 3a). Fitting the peaks at 800 cm−1 and 1493 cm−1 , assigned to the C–H disubstituted aromatic ring vibration of the DIPB and to the C–C mono-substituted aromatic stretch vibration of styrene, respectively, was exploited to quantify the DIPB/styrene composition in the near surface and in the bulk of the grafted films. The styrene content was determined using a calibration curve of the peak (1493 cm−1 ) of pure styrene grafted into ETFE base film at different graft levels (GL (%)), which is normalized by the characteristic peak of ETFE (1325 cm−1 ): Peak area (1493 cm−1 ) = P1 × GL (%) Peak area (1325 cm−1 ) where P1 = (4.67 ± 0.04) × 10−3 ; R2 = 0.99. Due to the fact that the DIPB alone cannot be grafted, the content of DIPB was deduced by calculating the difference between the GL measured by film weighing and the value obtained by FTIR. The composition of the grafted films was determined and the values of the DIPB/styrene molar ratio in the near-surface region and in the bulk are reported in comparison with the composition in the initial grafting solution (Fig. 3b). No significant difference was observed between the DIPB/styrene molar ratio up to around 3.5% in the initial grafting solution and that incorporated in the grafted film. However, a decrease in the effective incorporated DIPB concentration is observed starting from around 7.4% and the deviation is more pronounced for the higher concentrations (11.8% and 16.8%). In case of DVB/styrene, the same trend is observed and stronger deviation between the molar ratio in the feed and in the grafted film is found at higher crosslinker concentration (Fig. 3b). For DVB crosslinked membranes, we observed vibrations assigned to pendant double

bonds at 1630 cm−1 [6]. For DIPB crosslinked membranes, were did not observe any pendant double bonds over the entire range of DIPB concentrations, indicating a more effective crosslinking, i.e., making better use of the second double bond. It is well known that the reactivity of both double bonds in the DVB is different. When the first double bond reacts with a macro-radical, the reactivity of the second double bond is influenced, and as the chain grows its reactivity decreases [24]. Thus, the absence of the double bond signal highlights the crosslinker effectiveness (i.e., reactivity of the second double bond of the crosslinker) of the pendant isopropenyl group of the DIPB/styrene system as compared to the vinylic pendant group of the DVB/styrene monomer combination. It is observed that the composition in the near-surface and the entire bulk of the grafted films is comparable and the value increases with the DIPB content in the initial grafting solution (Fig. 3b). In the DVB/styrene system the molar ratio of the grafted films in both the surface (determined by ATR mode) and the bulk (determined by transmission mode) follows the same trend (Fig. 3b). However, the comparison of the molar ratio of DVB/styrene is found to be higher in the surface than in the bulk of the film. The obtained results show clearly the increased homogeneity of the DIPB/styrene distribution in comparison with the DVB/styrene crosslinked ETFE and FEP based films [9,18]. 3.2. Ex situ membrane characterization The ETFE based film is a semi-crystalline material and due to the membrane preparation (i.e., grafting, sulfonation, hydrolysis and swelling steps) the crystallinity and mechanical properties of the material are affected [22]. The inherent crystallinity of the crosslinked membranes with fixed graft level (GL ∼ 25%) and with a degree of sulfonation exceeding 96% was determined (Table 1). The calculation of the inherent crystallinity takes into account the dilution effect caused by the introduction of the amorphous grafted

Table 1 Melting temperature and crystallinity of ETFE-g-styrene-co-DIPB based membranes at different DIPB/styrene molar ratios. Membrane

GL (%)

Reaction time (h)

Molar ratioa of crosslinker to styrene (%)

Melting temperature (◦ C)

ETFE-g-styrene ETFE-g-styrene-co-DIPB ETFE-g-styrene-co-DIPB ETFE-g-styrene-co-DIPB ETFE-g-styrene-co-DIPB ETFE base film

26.7 26.3 27.2 24.5 23.8 0

1.00 1.45 2.83 5.78 14 0

0.0 3.5 6.7 9.0 11.5 –

275.6 275.8 275.2 276.0 274.4 270.0

a

Determined by FTIR.

± ± ± ± ± ±

1.0 0.5 0.5 0.4 0.0 0.6

Inherent crystallinity (%) 24.6 23.9 24.4 22.1 21.6 32.5

± ± ± ± ± ±

2.7 0.8 4.5 1.7 1.8 0.8

106

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3.5 % 0%

DI th e g sin

Nafion-212

ea

60

cr

6.7 %

90

In

Heat flow endo up

9.0 %

PB

-1

Conductivity (mS·cm )

11.5 %

co

nt

en

t

120

30

0 4

Cooling 200

220

8

12

16

Hydration number (n(H2O)/n(SO3H)) 240

260

Temperature (°C) Fig. 4. Crystallization thermograms of ETFE-g-styrene-co-DIPB at different DIPB/styrene molar ratio in the grafted films.

polymer [22]. The inherent crystallinity and the melting temperature of the membranes do not show any significant change with the increase of crosslinker content. However, the crystallinity of the ETFE base film (33%) is higher than that of the ETFE based membranes, which is mainly related to the disruption of crystallites in the latter due to water uptake and swelling [6,22]. Furthermore, the creation of entanglements and network and the hindrance of the polymer chain mobility may explain the observed increase of the melting temperature of membranes in comparison with the base film. The crystallization thermograms of ETFE based grafted films, with various crosslinker concentrations, were recorded at a constant cooling rate of 20 ◦ C min (Fig. 4). A single broad re-crystallization peak (exothermic) was observed for the uncrosslinked ETFE grafted film, whereas thermograms of the crosslinked samples show two peaks. The first peak maximum appearing at 252 ◦ C was observed for all grafted films including the 0% DIPB sample. A second peak, which is characteristic only of the crosslinked grafted films, appeared at a temperature below 252 ◦ C, and was even more shifted to lower temperature with the increase of the DIPB/styrene molar ratio. The alteration of the exothermic peak shape with the DIPB content is an indication of the inhibition of crystallization [2,6]. Due to the stability of radicals in the crystalline phase and due to diffusion limitation, it is assumed that the majority of grafting occurs in the amorphous phase and only surface grafting on the crystallites occurs [25]. It is therefore clear that two type of crystal domains are present in the crosslinked grafted films [8]. The first type crystallizes easily and is unlikely to be affected by the grafting/crosslinking of grafted chains (core of crystallites). The second crystallite type is the one most affected by the crosslinking (surface-near regions in the crystallites). Thus, the delay of the crystallization observed for the second peak in the crosslinked grafted films is attributed to the creation of more entanglements and a denser network, which reduces the mobility of the polymer chains and their ability to form crystalline domains. The same observation was made for the DVB crosslinked grafted films based on ETFE, where a multiplicity of crystallization peaks appeared in crosslinked grafted films and membranes [6]. The membrane ex situ fuel cell relevant properties, namely ion exchange capacity, water uptake, dimensional stability and proton conductivity of the ETFE based membrane, were determined as a function of the molar ratio of DIPB/styrene (Table 2). By increasing the molar ratio of DIPB/styrene from 0 to 11.5% in the membranes, the through-plane conductivity decreases from a value of

Fig. 5. Through-plane conductivity versus hydration number of ETFE-g-styrene-coDIPB based membrane with varying DIPB/styrene molar ratio.

107 mS cm−1 to 19 mS cm−1 , while the ion exchange capacity values only show a slight decrease (Table 2). Likewise, the in-plane conductivity values decrease from 82 mS cm−1 to 41 mS cm−1 . The obtained conductivity values (in-plane and through-plane) of the Nafion® 212 were comparable to the values measured for the uncrosslinked ETFE based membrane (Table 2). Likewise, the water uptake is reduced upon crosslinking, which can be attributed to the formation of a compact network structure of the grafted chains and the decrease in the volume available in the membrane to accommodate water. Thus, the observed decrease in conductivity is obvious, considering the decrease in swelling and hydration number, as a consequence of the reduced mobility of protons. Moreover, an approximately linear correlation between conductivity and hydration level is observed for all ETFE based membranes and Nafion® 212, showing the direct correlation between the proton mobility and the number of water molecules per sulfonic acid group (Fig. 5) [16]. Similar to the water uptake, the area and volume shrinking (wet → dry) decrease with crosslinking, i.e., the dimensional stability of the crosslinked membranes is improved and the expected mechanical stress generated by hydration/dehydration in the membrane-electrode assembly is reduced (Table 2). We note here that the Nafion® 212 showed the highest dimensional shrinking (wet → dry) in comparison with all grafted samples. The same observations were made for the DVB/styrene grafted ETFE based membranes [9]. Likewise, the anisotropy in conductivity, which is defined as the ratio of the in-plane over the through-plane conductivity, was studied. For Nafion® 212 a negligible anisotropy (conductivity ratio (in-plane/through-plane) ∼ 0.95) was observed for measurements obtained in fully swollen state. The ETFE based membranes with lower DIPB molar ratio (0% and 3.5%) showed the same small anisotropy value of ∼0.77, whereas this anisotropy vanished for the 6.7% DIPB based membrane (∼1.05). A high conductivity anisotropy value of ∼2.25 was measured for the 11.5% DIPB based membrane. The substantial change in the ratio of the in-plane to the through-plane conductivity is somewhat surprising, considering the largely uniform distribution of DIPB (Fig. 3b) and deserves more attention in the future. The conductivity is not only sensitive to the crosslinker distribution, but is governed to a large extent by the morphology of the bi-phasic nanostructure of the membrane and its probable anisotropy, which may change as a function of the crosslinking density. In general terms, the crosslinking hinders the mobility of the chains, thus the ability of some ionic sites to get connected with already formed ionic aggregates might be restricted [26]. Conductivity anisotropy was already observed for different membranes and correlation to the morphological change was investigated [27,28]. However, for prac-

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107

Table 2 Ion exchange capacity, water uptake and hydration number of ETFE-g-styrene-co-DIPB based membrane at different DIPB/styrene molar ratio. GL (%)

Molar ratioa (%)

Ion exchange capacity (mmol g−1 )

26.7 26.3 27.2 24.5 23.8 Nafion® 212

0.0 3.5 6.7 9.0 11.5 –

1.72 1.71 1.79 1.61 1.65 1.10

a b

± ± ± ± ± ±

0.05 0.02 0.01 0.08 0.03 0.02

Water uptake (% m) 44.4 31.9 24.9 24.4 20.2 38.9

± ± ± ± ± ±

Hydration number

2.4 1.9 4.0 0.3 4.4 1.3

14.3 10.4 7.7 8.5 6.8 21.5

± ± ± ± ± ±

1.0 0.5 1.2 0.5 1.4 0.5

In-plane conductivityb (mS cm−1 ) 81.7 67.0 57.2 51.1 41.2 80.2

± ± ± ± ± ±

6.4 1.0 4.5 4.1 2.6 2.4

Through-plane conductivityb (mS cm−1 ) 107.5 86.7 54.2 42.0 18.3 77.7

± ± ± ± ± ±

10.8 9.6 5.6 3.6 8.3 3.7

Volume shrinking wet → dry (%)

Area shrinking wet → dry (%)

37.5 29.9 26.7 21.7 18.9 47.7

26.6 21.8 20.2 16.8 16.8 32.1

DIPB/styrene, determined by FTIR. Measured in fully swollen state at room temperature.

tical application in the fuel cell, the through-plane conductivity is the most relevant parameter to improve. We note here that depending on the different membrane processing and treatments, e.g., stretching and hot-pressing, the observed anisotropy might be affected [27,28]. In order to investigate the surface wetting properties of ETFE based membranes, water contact-angle measurements were performed using static sessile drop method (Fig. 6). The contact-angle measurement performed on dried membranes showed no significant changes up to 6.7% DIPB concentration, while the values increase, starting from around 9% DIPB. This observed increase in surface hydrophobicity is not fully understood yet. However, it is suggested that the creation of a network induces a restriction on the grafted chains’ mobility, thus reducing the reorientation capability of the sulfonic groups towards the surface. It is assumed that the crosslinking to some extent restricts the surface re-arrangement, hindering the formation of more hydrophilic local structure [2,29,30]. In analogy to the contact-angle measurement results, XPS investigation on grafted films was directed towards quantification of the surface chemical structure and composition (Fig. 7). The recorded C 1s spectra of the different crosslinked grafted films show that the surface composition is strongly influenced by the extent of crosslinking. The uncrosslinked film shows a signal at ∼284.6 eV typically observed for hydrocarbons and suggesting the existence of polystyrene grafts on the surface. This signal is observed for the DIPB/styrene molar ratio up to 6.7% with a noticeable decrease in intensity. Starting from a ∼9% molar ratio of DIPB the well-defined signal at ∼284.6 eV disappears, proving the absence of hydrocarbons on the crosslinked sample surface. The surface concentration of the grafted com-

ponent is high in the uncrosslinked film, as similarly observed for poly(tetrafluroethylene-co-hexafluoropropylene) (FEP) grafted films and membranes using styrene/DVB [2,31]. As the content of crosslinker increases in the grafted membrane, the surface became poorer in polystyrene grafts [2,31]. We note here that the observed change in the surface composition of the grafted films and membranes are measured under ultra high vacuum conditions [2,31]. Even though the experimental conditions of the XPS measurement are different from the contact-angle measurement condition, performed on the dry membranes, the obtained results correlate quite well. The surface composition and interfacial behaviour of styrene/DIPB grafted ETFE based films and resulting membranes appear to be affected by the styrene/DIPB molar ratio. 3.3. Fuel cell performance To correlate the measured ex situ surface and bulk properties of the crosslinked grafted membranes to the in situ fuel cell relevant properties, MEAs were assembled as described in the experimental section and tested at a constant current density of 500 mA cm−2 . Measurement of single fuel cell performance (polarization curves), ac impedance spectra and H2 crossover were taken after 90 h of operation. The fuel cell performance of the different assembled MEAs was evaluated and polarization curves were recorded after steady state operation at 80 ◦ C. Interestingly, all the cells using different crosslinked membranes showed comparable fuel cell performances as observed in the polarization curves (Fig. 8a). The measured high frequency resistance (HFR) of the single cells showed an increase with the crosslinker content and values from 57 mOhm cm2 up to 93 mOhm cm2 were determined. The obtained values correlate quite well with the observed ex situ measured values of conductivity for the membranes in fully swollen state. The open circuit voltage (OCV) of all MEAs with different DIPB/styrene

C-Hn

CF2-CH2

90

CF2-CH2

80

I (a.u.)

Contact angle (deg)

100

70

0% 3.5%

Nafion-212

60 0

3

6

9

12

Molar ratio (DIPB/styrene) (%) in grafted film Fig. 6. Contact-angle measurement using water at RT of dry ETFE-g-styrene/DIPB membranes at different DIPB/styrene molar ratio compared with Nafion® 212 membrane. Measurement performed 30 s after the water drop is placed on the dry membrane surface.

6.7% 9.0% 11.5% ETFE base film 296

292

288

284

Binding energy (eV) Fig. 7. C 1s XP spectra for ETFE grafted films at different DIPB/styrene molar ratio: peak assignment as follows: 291.3 eV: CF2 –CH2 , 286.6 eV: CF2 –CH2 , 284.6 eV: C–Hn (hydrocarbons).

H. Ben youcef et al. / Journal of Membrane Science 381 (2011) 102–109

Cell voltage (V)

1.0

a)

0.8 0.6

11.5 % 9% 6.7 % 3.5 % 0% Nafion-212

0.4

b)

500 400 300 200

Nafion-212

100

0.10

Ohmic resistance 2 (mOhm·cm )

2

HF resistance (Ohm·cm )

0.2

Polarization resistance 2 (mOhm·cm )

108

0.08

0.06

80 Nafion-212 60

40

0.04 0

300

600

900

0

1200

3

6

9

12

Molar ratio (DIPB/styrene) in grafted film (%)

-2

Current density (mA·cm )

Fig. 8. (a) Polarization curves of MEAs based on ETFE-g-styrene/DIPB membranes with different DIPB/styrene molar ratio. Recorded at 80 ◦ C at fully humidification condition of both anode and cathode, H2 /O2 : 1.5/1.5 gas stoichiometry. (b) Ohmic resistance and polarization resistance of MEAs based on ETFE-g-styrene/DIPB membranes. Ac impedance spectra recorded at a dc current density of 500 mA cm−2 (frequency range: 0.1 Hz–50 kHz). MEAs are assembled using JM electrodes (0.4 mg Pt cm−2 ).

In order to evaluate the mechanical integrity of the membrane after 90 h of fuel cell operation, the in situ H2 crossover was measured by operating the cell in H2 /N2 mode and applying a cell voltage of 0.5 V (Fig. 9). It is believed that the gas crossover plays an important role in controlling the overall membrane degradation rate, either on the anode side or on the cathode side [32]. The H2 -crossover gradually decreases with the extent of crosslinking, in particular the crosslinked membranes with a DIPB concentration of more than 6.7% in the membrane showed a lower H2 permeability in comparison to a Nafion® 212 membrane. A correlation between the ex situ dimensional stability (wet → dry) and the membrane’s mechanical integrity measured in situ via H2 crossover was already established. It was stated that changes in the membrane’s hydration when assembled with electrodes in a single cell may cause a build-up of internal stress, leading to mechanical deterioration during fuel cell operation [33]. Furthermore, crosslinking with DVB was found to improve the ex situ chemical stability of the grafted membranes. So far, no correla-

1.0

-2

H2 crossover (mA·cm )

molar ratios is not affected by the composition of the membranes. This result is quite different from the previous results obtained for the DVB/styrene system but performed at that time with a different electrode, E-TEK (ELAT LT140EWSI), with a different loading (0.5 mg Pt cm−2 ) and different gas diffusion layer [10]. To understand the observed similar performances, even with the increase of ohmic resistance of the membranes, impedance measurements were performed. The derived ohmic resistance and polarization resistance values from the ac impedance spectra, recorded at a current density of 500 mA cm−2 , are presented in Fig. 8b. The initially measured ohmic resistance value of 50 mOhm cm−2 for the uncrosslinked membrane increases with the increase of the DIPB/styrene molar ratio, reaching a value of 87 mOhm cm−2 for the highly crosslinked membrane (11.5% DIPB). However, the polarization resistance does not show any significant change with the increase in DIPB/styrene molar ratio up to 9%, a marked decrease is only observed for the highly crosslinked membrane (11.5% DIPB) (Fig. 8b). The observed trend for the extracted ohmic resistance (impedance spectroscopy) correlates well with the ohmic resistance measured via the online HFR technique at 1 kHz, and is obviously associated with the decrease of membrane conductivity with increasing DIPB/styrene molar ratio (Table 1). The polarization resistance, which is assumed to be a measure of the quality of the membrane–electrode interface [14], shows a clear correlation with the contact-angle measurement. So far, the observed trend is different from the one observed for the styrene/DVB based ETFE membranes, however assembled with a different electrode material (E-TEK electrodes) [10]. Furthermore, the values reported in this work are higher by a factor of ∼2.3 than the values reported for the MEAs using E-TEK electrodes, expressed also in the poor adhesion behaviour between the JM electrode type and our grafted membranes. Using water, a contact-angle value of 127◦ was measured on the active surface of the JM electrode as received, implying hydrophobicity. Thus, the high polarization resistance, which induces a loss in proton transport at the MEA interface, may account for the comparable performance between the 0% and 11.5% DIPB crosslinked membrane.

0.8 Nafion-212 0.6

0.4 0

3

6

9

12

Molar ratio (DIPB/styrene) in grafted film (%) Fig. 9. Hydrogen crossover of ETFE-g-styrene/DIPB based MEAs for different DIPB/styrene molar ratio. Hydrogen crossover measured in H2 /N2 mode at 80 ◦ C.

H. Ben youcef et al. / Journal of Membrane Science 381 (2011) 102–109

tion between the in situ chemical stability, ex situ chemical stability and H2 crossover was investigated [6,9]. In the next step, ex situ chemical stability (treatment in a 3% H2 O2 solution at 60 ◦ C) of ETFE-g-styrene/DIPB membranes will be investigated in addition to their in situ durability fuel cell accelerated aging tests. 4. Conclusions Crosslinking styrene using DIPB was performed successfully for the preparation of radiation grafted membranes based on ETFE. Throughout the study, the effectiveness of the crosslinking using DIPB was proved using FTIR, FTIR/ATR, and DSC measurements. Homogeneous distribution of the DIPB/styrene molar ratio through the grafted films was obtained, mainly due to the lower reactivity of the DIPB in comparison with both DVB isomers. The increase of the crosslinker concentration in the initial grafting solution results in a different composition in the grafted films. The diffusion limitation of the monomer, due to creation of a network was observed only for higher DIPB/styrene molar ratios of more than 9% (mol/mol) in the grafted films. The increase of the DIPB/styrene molar ratio in the grafted films and resulting membranes induced changes in the ex situ fuel cell relevant properties. The conductivity and water uptake were reduced substantially as the crosslinker concentration increased, while the dimensional stability was improved. The ohmic resistance of the crosslinked membranes increased with the crosslinker content as expected, while the in situ interfacial properties of the MEAs were improved for the highly crosslinked sample (11.5%). So far, the relationship between DIPB content and the overall cell performance is not yet completely understood. However, we note here that recent investigation on improving the interface between the membrane and electrode was carried out and the impregnation of the membrane, using ethanol/Nafion ionomer (5%) solution, was pointed out as a necessary step to improve the adhesion between the ETFE based membrane and the electrodes. A clear correlation between the in situ interfacial properties of the MEAs and the surface composition (XPS) were obtained. Ex situ and in situ durability tests of these membranes will be investigated as next step and the potential of using DIPB as crosslinker for the grafted membranes will be compared with the use of DVB based membranes with the same composition. References [1] G.G. Scherer, Polymer membranes for fuel cells, Ber. Bunsenges. Phys. Chem. 94 (1990) 1008–1014. [2] S.A. Gürsel, L. Gubler, B. Gupta, G.G. Scherer, Radiation grafted membranes, Adv. Polym. Sci. 215 (2008) 157–217. [3] L. Gubler, G.G. Scherer, A proton-conducting polymer membrane as solid electrolyte–function and required properties, Adv. Polym. Sci. 215 (2008) 1–14. [4] B. Smitha, S. Sridhar, A.A. Khan, Solid polymer electrolyte membranes for fuel cells – a review, J. Membr. Sci. 259 (2005) 10–26. [5] R. Souzy, B. Ameduri, B. Boutevin, G. Gebel, P. Capron, Functional fluoropolymers for fuel cell membranes, Solid State Ionics 176 (2005) 2839–2848. [6] H. Ben youcef, Ph.D. Thesis ETH No. 18215, Swiss Federal Institute of Technology, Zürich, Switzerland, 2009, doi:10.3929/ethz-a-005773247. [7] T. Yamaki, Quantum-beam technology: a versatile tool for developing polymer electrolyte fuel-cell membranes, J. Power Sources 195 (2010) 5848–5855. [8] H. Ben youcef, S.A. Gürsel, A. Buisson, L. Gubler, A. Wokaun, G.G. Scherer, Influence of radiation-induced grafting process on mechanical properties of ETFE-based membranes for fuel cells, Fuel Cells 10 (2010) 401–410.

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