Mass spectrometry to quantify and compare the gas barrier properties of radiation grafted membranes and Nafion®

Journal of Membrane Science 472 (2014) 55–66

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Mass spectrometry to quantify and compare the gas barrier properties of radiation grafted membranes and Nafions Zhuoxiang Zhang a, Raphaël Chattot a,b,c, Lukas Bonorand a, Kaewta Jetsrisuparb a,d, Yves Buchmüller a, Alexander Wokaun a, Lorenz Gubler a,n a

Electrochemistry Laboratory, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland Univ. Grenoble Alpes, LEPMI, F-38000 Grenoble, France c CNRS, LEPMI, F-38000 Grenoble, France d Department of Chemical Engineering, Khon Kaen University, Khon Kaen 40002, Thailand b

art ic l e i nf o

a b s t r a c t

Article history: Received 28 February 2014 Received in revised form 8 August 2014 Accepted 11 August 2014 Available online 28 August 2014

This study describes a method using mass spectrometry (MS) to quantify the gas barrier properties of different classes of ionomer membranes and the base film used in radiation grafted membranes for polymer electrolyte fuel cells. The effects of relative humidity (RH), temperature and graft level are investigated for different types of radiation grafted membrane with dissimilar chemical compositions. The results show that the radiation grafted membranes and Nafions 212 share the same trend of increasing gas permeability with RH, while the ETFE base film does not show RH dependency. With addition of nitrile-containing comonomer units in the grafted chains, the permeability of reactant gases in the membranes can be remarkably reduced. On the basis of experimental results, a physical model is proposed to describe the effect of water contents on the gas transport in the radiation grafted membranes and to explain the effect of graft level on gas barrier properties and the difference of gas barrier properties at high and low hydration levels between Nafions membranes and radiation grafted membranes. Using helium as a tracer, the deterioration of the gas barrier properties of the radiation grafted membrane during an accelerated stress test at open circuit voltage (OCV) is recorded in situ. & 2014 Elsevier B.V. All rights reserved.

Keywords: Proton exchange membrane Radiation grafting Gas permeability Gas barrier properties Mechanism of gas transport

1. Introduction Using hydrogen and air (or pure oxygen) as reactants, polymer electrolyte fuel cells (PEFCs) convert the chemical energy stored in the fuel directly into electricity with high efficiency and only produce water, thereby eliminating all local emissions such as carbon dioxide. In this energy conversion device, the polymer electrolyte membrane is required to provide the following functionalities: (i) transporting protons from the anode to the cathode, (ii) electrically insulating the two electrodes of a cell, (iii) separating the reactant gases. In order to prevent the direct contact between reactant gases through a permeation process, proton exchange membranes are required to possess good gas barrier properties. It has been found that permeation of the reactant gases is related to several critical aspects of PEFCs, such as fuel efficiency, the open circuit voltage (OCV) of a cell and membrane degradation. The direct contact between H2 and O2 in the presence of the noble metal catalyst leads to a chemical reaction without

n Correspondence to: Paul Scherrer Institut, OVGA/19, CH-5232 Villigen PSI, Switzerland. Tel.: þ 41 56 310 2673; fax: þ 41 56 310 4416. E-mail address: [email protected] (L. Gubler).

http://dx.doi.org/10.1016/j.memsci.2014.08.020 0376-7388/& 2014 Elsevier B.V. All rights reserved.

generating electricity and thus lowers the energy conversion efficiency of the fuel. Furthermore, the loss of cell voltage under open circuit conditions can be attributed to hydrogen crossover and the resulting oxygen reduction reaction overpotential at the cathode [1]. More importantly, in terms of the lifetime of a polymer electrolyte membrane used in the PEFC, the crossover of the two reactant gases is considered an essential origin of reactive intermediates, such as hydroxyl radicals, which are responsible for the chemical attack on the polymer chains and subsequent chain scission [2–5]. Therefore, the gas permeability in a proton exchange membrane is a crucial criterion for the assessment for membranes applied in the PEFC. Proton exchange membranes prepared by the radiation grafting method have been developed with the prospect of low cost, while aiming at maintaining or improving the functionalities of perfluoroalkylsulfonic acid (PFSA) ionomer membranes, such as the Nafions series, which are still an important cost driver for the fuel cell stack for early market with low production volume [6]. The fundamentals, polymer design strategies and recent developments in the area of radiation grafted ion-conducting polymers for application in PEFCs have been reviewed recently [7]. Radiation grafting for preparing membranes is basically a modification process of a pre-formed base film using a potentially wide selection of monomers with unique

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properties. The process provides a wide range of possibilities to design the composition and architecture of the membranes through careful variation of the irradiation and grafting conditions, as well as selection of grafted monomers [8]. Typically, the polymers used as base film for radiation grafted fuel cell membranes have high chemical, mechanical and thermal stability, which include fluorinated polymers such as poly (tetrafluoroethylene-co-hexafluoropropylene) (FEP), poly(tetrafluoroethylene-coperfluoropropyl vinyl ether) (PFA), and poly(tetrafluoroethylene) (PTFE) or partially fluorinated polymers such as poly (vinylidene fluoride) (PVDF) and poly(ethylene-alt-tetrafluoroethylene) (ETFE) [9,10]. The base films are functionalized to provide ion exchange sites and thus ionic conductivity. This starts with irradiating base films to generate active centers throughout the bulk of film using ionizing radiation. The activated film is then brought into contact with a solution containing monomers amenable to radical-induced polymerization. Then, the grafted film is sulfonated to introduce sulfonic groups, and thus proton exchange sites. With an optimized synthesis process, the fuel cell performance provided by radiation grafted membranes can be similar to that of Nafions 212 [6]. During a fuel cell test under simulated automotive operating conditions with dynamic load protocol, several thousand hours of durability have been attained with optimized radiation grafted membranes in comparison with MEAs based on commercially available PFSA membranes. The lifetime of the radiation grafted membranes dramatically exceeded that of Nafions 212 and also compared favorably against that of stateof-the-art Nafions XL-100, which is mechanically and chemically stabilized [6]. Some fundamental studies have been performed in order to develop radiation grafted membranes with improved gas barrier properties. For instance, addition of cross-linking comonomers, such as divinylbenzene (DVB), into the grafting reaction mixture can lead to effectively reduced gas crossover rate through the membrane. A trade-off between gas barrier properties and proton conductivity, however, has to be established, since a cross-linked network can also significantly limit the transport of protons [11]. Another method that does not require sacrificing proton conductivity is to introduce nitrile-containing co-monomers, for example methacrylonitrile (MAN) or acrylonitrile (AN), which can improve the gas barrier properties of radiation grafted membranes and thus significantly increase the lifetime of the membrane [12]. Recently, a study focusing on chemical degradation of styrene based radiation grafted membrane as a function of relative humidity (RH) showed that the rate of membrane degradation decreases with decreasing RH and follows the trend of H2 crossover, suggesting that the chemical attack on this kind of ionomer is directly related to the number of reactive intermediates formed as a result of gas crossover [13]. Although the fuel cell performance and lifetime of radiation grafted membrane have been significantly improved, which makes this class of ionomers a promising alternative to PFSA materials for fuel cell application, there are few systematic studies dedicated to the gas barrier properties of radiation grafted membranes. In contrast, there has been a large number of reports discussing permeability of PFSA based ionomers using various methods. Ogumi et al. initiated an investigation on the permeation of hydrogen and oxygen through Nafions membranes using an electrochemical monitoring technique (EMT) [14]. The solubility and diffusion constant of both gases were measured in several types of media at 25 1C, which is, however, far lower than the normal operating temperature (80–90 1C) of PEFCs. Also using the EMT, Sethuraman et al. evaluated the gas transport properties of Nafions membranes in a Devanathan–Stachurski type diffusion cell [15]. An extensive comparison of solubility and diffusion coefficient of oxygen in the membranes obtained in this study to those reported in the literature showed good agreement. By replicating fuel cell conditions, another of their study correlated

oxygen permeation rates with the peroxide formation rates at the anode [16], which are generally linked to the formation of the radical intermediates in PEFCs. Broka and Ekdunge measured oxygen and hydrogen permeability through Nafions 117 membrane by means of gas chromatography (GC) at different levels of RH and temperature, finding that the permeability of both gases increased with RH and temperature, although the permeability of hydrogen was around two times higher than that of oxygen [17]. Using a precise gas flowmeter, Sakai et al. also observed the permeability of hydrogen to be about twice as high as that of oxygen in the Nafions membrane, and their results suggested that the electrocatalyst plated on the surface of the membrane did not hinder the transport of gas [18]. Another study from this group, which was focused on gas diffusion in dried and hydrated Nafions membranes, respectively, demonstrated that the water absorbed in the membrane significantly increased the diffusivity to values approaching those in water, although the solubility was slightly decreased when the membrane was hydrated, which suggested that gases diffuse through clusters and channels consisting of ionic aggregates in the hydrated state [19]. There are a number of studies where hydrogen crossover rates in different classes of ionomers were measured electrochemically in a fuel cell configuration [12,20,21]. Also based on a fuel cell setup, through utilizing an IrOx electrode at the anode and a Pt electrode at the cathode, the oxygen permeability can be determined from the oxygen reduction reaction current limited by the oxygen crossover rate [22]. Methods based on mass spectrometry have been developed recently as in situ technique to perform local gas permeation analysis for PFSA membranes in the fuel cell [23,24]. For the category of radiation grafted membranes, Kallio et al. prepared a series of polystyrenesulfonic acid (PSSA) based membranes with various base polymers, and the diffusion and solubility coefficients of hydrogen and oxygen at 20 1C and a fixed pressure were obtained chronoamperometrically in a microelectrode cell [25]. In this study, we present a method to measure both hydrogen and oxygen permeability in radiation grafted membranes by using mass spectrometry (MS). Compared to the widely used electrochemical method for diagnosis of membrane mechanical integrity [20,26], the MS based method offers flexibility that allows us to measure both of H2 and O2 permeability in the same fuel cell configuration. Moreover, by admixing an inert tracer gas, such as helium, to the reactant gas, this method can be employed for online monitoring of membrane degradation during fuel cell tests, which will be reported in this study. For non-conducting films, in addition, the MS based method provides a tool to quantify their gas barrier properties, which the electrochemical method cannot, and furthermore to investigate the effect of the base film on the permeability of the ultimately obtained grafted membranes. This study aims to quantify the gas barrier properties of different kinds of radiation grafted membranes, as well as the ETFE base film and Nafions 212 membrane for comparison. Furthermore, the effects of RH, co-monomers, temperature and graft level (GL) are investigated for different radiation grafted membranes. With the permeability information of both base film and grafted membranes, it allows us to further tailor the gas barrier properties of radiation grafted membranes for fuel cell application by tuning base films, graft level, monomer combinations, etc. A physical model is proposed to illustrate the gas permeation process through the radiation grafted membrane at different levels of hydration, which, together with the permeability data of Nafions 212 membrane and various radiation grafted membranes, can provide insights into the similarities and dissimilarities between PFSA membranes and PSSA based radiation grafted membranes. Last but not least, using the inert gas helium as a tracer, the loss in membrane mechanical integrity was monitored in situ during an accelerated stress test at OCV.

Z. Zhang et al. / Journal of Membrane Science 472 (2014) 55–66

2. Methodology and experimental details 2.1. Methodology The measurement of gas permeability in the membrane is performed by supplying the target gas (H2 or O2, respectively) together with helium (10 vol%) to the anode compartment of the fuel cell. The molar fraction of the permeant target gas and helium is measured in the exhaust stream at cathode compartment using a quadrupole mass spectrometer (Pfeiffer Prisma 200M1). Since we cannot measure the crossover rates of H2 and O2 at the same time, through comparing the permeant concentrations of helium from two individual measurements, we can examine if the permeant concentrations of the two kinds of target gas measured together with helium were obtained under identical humidification conditions. Basically, the cathode compartment is fed with pure nitrogen as a carrier gas during the measurement, and the dilution of the target gas and helium at the anode compartment by permeating nitrogen is neglected as the concentration of the target gas and helium is much higher at the anode than the measured nitrogen concentration (  1000 ppm). Fig. 1 illustrates the used methodology for obtaining the permeability of the target gas (TG) and helium. The target gas concentration is measured at the outlet of the cathode compartment at a given pressure difference between anode and cathode, such as 1.5 bara at the cathode compartment and 2.0 bara at the anode compartment. Note that the gas crossover in this configuration is merely driven by gas diffusion as a result of the difference in the partial pressure of the target gas between anode (feed side) and cathode (permeate side), since a pristine membrane without defects, which would cause convective gas transport, is employed.

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Firstly, by applying the ideal gas law and considering the calibration conditions of the mass flow controller (i.e. 0 1C, 1.01325 bar), the known fluxes (in ml min  1) are converted to a mole based unit (mol s  1). The total flux of all gases (Jt,c in mol s  1) at the exhaust of the cathode compartment can be obtained from the measured N2 concentration (X N2 ;c in mol%) at the cathode compartment and the known N2 flow rate (J N2 ;c in mol s  1), as shown in Eq. (1). Then, combining the total flux with the concentration of the permeant target gas (XTG,c in mol%) measured by the MS yields the flux of the permeant target gas (JTG,c in mol s  1), as shown in Eq. (2). Under different measuring conditions, the molar fraction of the target gas in the anode compartment (XTG,a in mol%) varies with RH and can be determined using Eq. (3), assuming that the water concentration at the cathode compartment (X H2 O;c ), which can also be measured by the MS, is equal to the water concentration at the anode compartment (X H2 O;a ). Note that the permeation of gas through the membrane is driven by the partial pressure gradient and thus related to the molar fraction of the target gas at the anode compartment. The gas crossover rate (jcrossover in mol s  1 cm  2) can be obtained by normalizing the flux of the permeant target gas to the molar fraction of the target gas at the anode compartment and the geometry of the membrane (cf. Eq. (4)). By systematically changing the pressure difference between anode and cathode, accomplished by maintaining a constant pressure (1.5 bara) at the cathode and varying the pressure at the anode (Fig. 1(a)), a linear regression analysis of the gas crossover rate vs. pressure difference (Δp) yields the permeance (mol s  1 cm  2 kPa  1) of the permeant target gas as the slope, as the example in Fig. 1(b) demonstrates. The permeance multiplied by the thickness of a membrane yields the permeability of the membrane (mol cm s  1 cm  2 kPa  1). Using the MS method, the water concentration in the cell can be measured and thus the values of RH in the cell can be determined precisely. J N2 ;c X N2 ;c

ð1Þ

J TG;c ¼ J t;c X TG;c

ð2Þ

J t;c ¼

X TG;a ¼ ð1  X H2 O;c Þ

jcrossover ¼

J TG;a J TG;a þ J He;a

J TG;c X TG;a A

ð3Þ

ð4Þ

2.2. Experimental setup for gas permeability measurement

Fig. 1. (a) Schematic of gas flow configuration in the cell and notation of various quantities involved in the permeability measurement; (b) an example of the permeability obtained by linear regression analysis of the measured crossover rates at a range of differential pressure values.

A schematic diagram of the experimental setup employed in this work is presented in Fig. 2. K-type thermocouples were inserted into the end plates of both the cathode and the anode to control the temperature of the cell. Typically, in the permeability measurement, the cell temperature was set to 80 1C. During the permeability measurement, nitrogen and a mixture of target gas and helium were supplied in a co-flow configuration to the compartments of cathode and the anode, respectively. At the cathode compartment, nitrogen was supplied with a flow rate of 50 ml min  1, while at the anode compartment, the mixture of target gas and helium was supplied with a total flow rate of 200 ml min  1, in which the target gas accounts for 90%. Using the back pressure regulator, the pressure at the cathode compartment was maintained at 1.5 bara, whereas the pressure at the anode compartment was set to different values as shown in Fig. 1(a). Gas humidification was carried out by using bubble type humidifiers connected to the gas source and the inlets of the cell. For the

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measurement under completely dry conditions, this connection was closed and the gases were directly fed to the cell via a bypass. The mass spectrometer was connected to the outlet of the cathode compartment to measure the molar fractions of the target gases, helium and water. Calibration for the mass spectrometer was conducted before starting a new individual measurement for a membrane. Prior to supplying the target gas and helium to the anode compartment, both compartments were fed with pure nitrogen humidified to a certain RH that was used in the following measurement of gas permeability, and the “background molar fraction” of each gas of interest was measured. During the following permeability measurement, the “background molar fraction” was subtracted to obtain the real molar fraction of the permeant molecules of interest. 2.3. Preparation of membranes and single fuel cell During a typical membrane synthesis procedure, ethylenetetrafluoroethylene copolymer (ETFE) films with thickness 25 μm (Tefzels 100LZ) purchased from Dupont were washed in ethanol and dried in vacuum at 60 1C. Then they were electron-beam irradiated with a particular dose according to the ultimately obtained membranes (cf. Table 1) at Leoni Studer AG (Däniken, Switzerland) and subsequently stored at  80 1C until they were grafted. The grafting solutions comprised 20 vol% monomer (ether

pure styrene for styrene grafted membrane or a mixture of αmethylstyrene (AMS) with co-monomers for co-grafted membranes, cf. Table 1), 70 vol% isopropanol and 10 vol% deionized water. A stainless steel reactor, in which 6 pre-irradiated ETFE films and grafting solution were loaded, was purged with N2 for 1 h to remove oxygen before starting the reaction. Following these steps, the reactor was placed in a water bath at a thermostatically controlled temperature of 60 1C for styrene based membranes and 55 1C for AMS based membranes. The reaction time for preparing each kind of membrane (cf. Table 1) follows from the polymerization kinetics of the particular grafting monomer(s) and the required graft level. The grafted films were then removed from the reactor, washed with toluene and acetone in sequence to remove homopolymer and remaining monomer, and dried under vacuum at 80 1C overnight. The mass based GL can be determined as a ratio of the weight increase after grafting with respect to the initial film weight [27]. To introduce ion exchange sites, the grafted films were sulfonated with chlorosulfonic acid in dichloromethane (2 vol% for styrene based membranes and 10 vol% for AMS based membranes, respectively) at room temperature for 5 h, followed by hydrolysis in deionised water at 80 1C. The synthetic routes for the preparation of the radiation grafted membranes used in this study are shown in Fig. 3. The obtained membrane was dried in the air before it was laminated with gas diffusion electrodes

Fig. 2. Schematic of the experimental setup used in this study of gas permeability with mass spectrometry.

Table 1 Ex situ properties and preparation details of the membranes used in this study. Conductivity and thickness were measured in water swollen state at room temperature. Composition

Graft level (wt-%)

Irradiation dose (kGy)

Grafting time (h)

Ion exchange capacity (IEC) (mmol g  1)

Water uptake (wt%)

Thickness (wet) (μm)

Through-plane conductivity (mS cm  1)

Grafted S (S 25) Grafted S (S 66) Grafted AMS–AN Grafted AMS–MAN Nafions 212

 25  66  49  48 n.a.

1.5 1.5 15 15 n.a.

1 8 24 12 n.a.

1.677 0.06 2.777 0.05 1.81 7 0.02 1.577 0.04 1.107 0.02

327 3 1077 5 637 2 407 1 347 2

407 1 567 1 507 2 507 2 567 1

64 7 10 1557 10 747 12 417 4 1077 5

Z. Zhang et al. / Journal of Membrane Science 472 (2014) 55–66

ETFE

activated ETFE

grafted grafted styrene/AMS AN/MAN

radiation grafted proton exchange membrane

active center molar fraction of styrene or AMS with respect to the total number of grafted monomers

Fig. 3. The synthetic routes for the preparation of PSSA (i.e. styrene or AMS) based radiation grafted membranes with (i.e. AMS/MAN and AMS/AN co-grafted membranes) or without nitrile-containing co-monomers (i.e. pure styrene grafted membrane) used in this study. For both nitrile-containing AMS based co-grafted membranes, x is around 0.5, while x is 1 for the pure styrene grafted membrane.

(GDEs) to form an MEA. More details regarding the synthesis of each kind of membrane can be found elsewhere [6,28,29]. The membranes were laminated together with GDEs (type ELE162, Johnson Matthey Fuel Cells) with a platinum loading of 0.4 mg/cm2 to form MEAs in a hot-press at defined temperature, load, and duration (110 1C/2.5 MPa/180 s). Subsequently, the MEAs were assembled into a graphite single cell, which comprised a straight flow field machined into graphite plates. The cell with an active area of 16 cm2 was employed for gas permeability measurement and OCV hold test in this work. For the cell assembly, a gasket (100 mm PTFE) and sub-gasket (25 mm PEN) were used. 2.4. OCV hold test and in situ characterization of membrane integrity

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previously [11]. The gas permeability of the nitrile-containing co-grafted membrane is measured and reported in this study (cf. Section 3.3). The permeability values of hydrogen and oxygen for the ETFE base film were also measured in order to quantify the inherent gas barrier properties of the base film and investigate its role in the ultimate gas permeability of the grafted and sulfonated membrane. In terms of PFSA materials, the gas permeability of Nafions 212 was evaluated in comparison with the radiation grafted membranes, because it is a benchmark membrane on the market and there have been data reported in the literature which can be used to validate the method we employed in the experiments. Also, with the comparison of permeability data between the PFSA membrane and the radiation grafted membranes, the similarities and dissimilarities in gas barrier properties between these two classes of ionomer can be examined and elucidated. 3.1. Permeability in Nafions 212, pure styrene grafted membrane and ETFE Hydrogen and oxygen permeability as a function of RH in Nafions 212 (N212), pure styrene grafted and sulfonated membrane with a graft level of 25% (S 25) and ETFE base film is presented in Fig. 4. The styrene based membrane shows lower permeability values for both hydrogen and oxygen than the ETFE base film under the completely dry condition (i.e. RH is near 0%). However, a gradual increase in gas permeability with increasing RH is seen in the styrene based membrane, in which the permeability of hydrogen and oxygen is around 1.5 and 3 times higher, respectively, than that in the ETFE base film at the highest RH (i.e. RH is near 100%). In contrast, there is no change in gas barrier properties for the ETFE base film as a function of RH. With increasing RH, the permeability of hydrogen and oxygen in Nafions 212 shows a more pronounced

During the OCV hold test, the MEAs with pure styrene grafted membrane (25% GL) were operated in the single cell at 80 1C and 1.5 bara backpressure on both sides. H2 and O2 were fed to the anode and cathode with a flow rate of 180 ml min  1 and 50 ml min  1, respectively. Helium as an inert tracer gas was added to the anode stream with a flow rate of 20 ml min  1. The membrane degradation was evaluated at two RH levels (50% and 100%), while maintaining the other operating conditions identical. The cell was tested at OCV for 24 h, during which cell voltage and crossover rate of helium were continuously measured.

3. Results The permeability of hydrogen and oxygen in a range of polymer electrolyte membranes as function of RH were measured in the single cell as described above, and the results are shown and discussed in this section. Typically, the measurement was conducted at 80 1C, except for the case where the effect of temperature on permeability is examined. For PSSA based radiation grafted membrane, the pure styrene grafted membrane is selected as the baseline for the radiation grafted membranes due to its most basic chemical composition. In this sense, the pure styrene grafted membrane is a model system for fundamental studies on radiation grafted membranes, such as gas transport mechanism, due to its simple chemistry, which does not involve the effects of cografted units in the grafted chains. However, this membrane displays a poor stability under fuel cell operating conditions [12,30]. Introduction of cross-linkers [27,31,32] and nitrilecontaining co-monomers [12,33] into the grafting system, which produces co-grafted membranes, can significantly improve the durability of styrene based radiation grafted membranes in the fuel cell. The effect of cross-linker (e.g. divinylbenzene) on gas crossover rates in radiation grafted membrane has been reported

Fig. 4. Gas permeability in the ETFE base film (25 μm), Nafions 212 (N212) and a styrene grafted membrane with graft level of 25% (S 25) measured at 80 1C.

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increase than in the styrene based radiation grafted membrane, although both of them share low permeability values at the extremely dry condition. At the highest RH, the permeability values of hydrogen and oxygen in Nafions 212 are more than 2 and 3 times higher, respectively, than in the styrene based radiation grafted membrane. The difference in gas barrier properties between PFSA membranes and radiation grafted membranes has been observed before using an electrochemical method to measure hydrogen crossover current density at 100% RH [12], with which the results from this study are in good agreement. It again shows that radiation grafted membranes can offer superior gas barrier properties compared to PFSA based materials. The permeability of oxygen in Nafions 212 is on the same order of magnitude as the results reported in the literature, which were obtained using an electrochemical method, for example, 1.8  10  13 mol cm s  1 cm  2 kPa  1 at  100% RH and 1.2  10  13 mol cm s  1 cm  2 kPa  1 at  50% RH, respectively [22]. 3.2. Effect of graft level on gas barrier properties of membrane For the proton exchange membranes prepared by radiation grafting, the proton conduction functionality is introduced with the grafted component and subsequent sulfonation. Therefore, the graft level (GL), which is defined as a ratio of the weight increase after grafting with respect to the initial weight of the base film, is a fundamental parameter for tuning those fuel cell relevant membrane properties, such as proton conductivity [8,9]. The effect of GL on membrane barrier properties, however, is still unknown. In our study, the effect of graft level on the gas barrier properties of radiation grafted membranes is evaluated by measuring the gas permeability in the two styrene grafted and sulfonated

membranes with identical composition but different graft level (25% and 66%, represented by S 25 and S 66, respectively) (Fig. 5). Note that the influence of graft level on gas barrier properties of membranes can only be discussed for membranes sharing identical chemical structure, because any change in chemical structure, for example addition of co-monomer, could lead to different gas barrier properties, which will be discussed later. Interestingly, in both cases of hydrogen and oxygen, the permeability in the membrane with higher graft level is lower than that in the membrane with lower graft level at lower RH; however, the membrane with higher GL shows a more pronounced increase in permeability when the hydration level of the membrane approaches saturation, leading to higher gas permeability for S 66 than for S 25 in the high RH region. This phenomenon will be discussed in detail in Section 4. In fact, the membrane with higher GL displayed a higher degradation rate than the membrane with lower GL during two identical accelerated stress tests at OCV and 100% RH, for which the higher crossover rates of reactant gases and concomitantly higher formation rate of reactive intermediates could be responsible [12,34]. Of course, in addition to the influence of crossover of the reactant gases, which is an essential requirement for the formation of membrane degrading species [2], membrane chemical degradation in the fuel cell is a complex phenomenon which is also associated with RH conditions [13,21,35–37], cathode Pt dissolution [38–40], catalyst types [40,41], current densities [41–43], operating temperature [44–46], partial pressure [47,48], ion contaminations [4,49], etc. Nevertheless, the degradation behavior of different membranes tested under given identical testing conditions should not be distinguished by the factors mentioned above. This means that, in our case, the different degradation rates of the two membranes with identical chemical composition but different GL ought to be merely the consequence of different GL and resulting gas barrier properties under given identical testing conditions. The results from Fig. 4 imply that the membrane with higher GL may show an improved chemical stability in the low RH region. 3.3. Gas barrier properties of nitrile-containing membranes

Fig. 5. Gas permeability in the pure styrene grafted membranes with a GL of 25% and 66%, respectively, at 80 1C.

The aim of the topic described in this section is to investigate the gas barrier properties of co-grafted membranes prepared by copolymerization of AMS with a nitrile-containing co-monomer into the ETFE matrix. The rationale to use AMS instead of pure styrene is to improve the durability of the membranes. Membranes grafted with styrene only show a very low lifetime under fuel cell operating conditions, which is a result of the poor oxidative stability of the PSSA grafts [12,30]. One of the main weaknesses is the presence of the weak α-hydrogen [5,50,51]. Strategies to mitigate degradation encompass various approaches. One of the options is to target the αhydrogen and replace it ;with another group, such as –CH3, as in the case of α-methylstyrene (AMS). AMS is a cheap and readily available monomer. However, it shows poor radical polymerization kinetics and has to be copolymerized with a suitable co-monomer to enable grafting [7]. Therefore, in our earlier work we reported the cografting of AMS with methacrylonitrile (MAN) [30,52]. The membranes obtained showed a significantly improved durability in the fuel cell compared to styrene only grafted membranes. The improvement in stability was thought to be caused by replacement of styrene with AMS. One may however object to this premature conclusion, since the effect of MAN has not been independently investigated. Hence, to investigate a potential co-monomer effect, MAN and acrylonitrile (AN) were co-grafted together with styrene, respectively [12]. The obtained styrene–MAN and styrene–AN co-grafted membranes both showed a substantially improved stability compared to styrene-only grafted membranes. As a reason for this, the lower H2

Z. Zhang et al. / Journal of Membrane Science 472 (2014) 55–66

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properties of AMS co-grafted membranes can be attributed to the nature of nitrile-containing co-grafted units, which can enhance the stiffness of chains and the chain-to-chain binding, leading to hindered gas transport [12,53]. The AMS based membranes also show increasing gas permeability towards high RH, which is consistent with the findings obtained with the pure styrene grafted membrane. 3.4. Effect of temperature on gas barrier properties of membrane The measured permeability of hydrogen and oxygen in the pure styrene grafted membrane at various temperatures is plotted in Fig. 7 against RH. In general, an increase in permeability of both hydrogen and oxygen with temperature can be observed at a given RH. This trend was also found in case of PFSA ionomers [15,22,24,54]. It is generally believed that the effect of temperature on gas permeability in polymers is related to thermally activated segmental motion of polymer chains. With increasing temperature, thermal motion grows, and the probability of larger voids increases, which results in an increase in the diffusion coefficient [55]. Additionally, an increased solubility coefficient of oxygen with temperature has been observed in the particular case of PFSA ionomer (e.g. Nafion 112) [15], yet, there is a discrepancy between different membranes in terms of the change of gas solubility coefficient with temperature. 3.5. OCV hold tests and in situ measurement of membrane mechanical integrity

Fig. 6. Gas permeability in the AMS based co-grafted membranes compared to the pure styrene grafted membrane at 80 1C. AMS–MAN and AMS–AN represent AMS/ MAN co-grafted membrane and AMS/AN co-grafted membrane, respectively, both of which were grafted into ETFE base film.

crossover current density in the co-grafted membranes was put forward. Here, in this study, we explore the influence of the nitrilecontaining co-monomers on the gas barrier properties of the AMS based co-grafted membranes. Fig. 6 shows the permeability of hydrogen and oxygen in the AMS based co-grafted membranes, which include two different nitrile-containing co-monomer units, MAN and AN, in comparison to the permeability in the pure styrene grafted membrane. Note that, compared to the chemical composition of the optimized radiation grafted membrane that showed superior durability [6], the AMS–MAN co-grafted membrane contains the main constituents, but does not include cross-linker in order to highlight the role of nitrile-containing units in gas barrier properties. Gubler et al. compared the stability of a pure styrene grafted membrane and an AMS–MAN co-grafted membrane, and found that the latter one exhibited a durability of  500 h, which was a significant increase in lifetime by a factor of 10 compared to the former one [30]. The intrinsic protection at the α-position of AMS due to replacing H with a methyl group can contribute to enhancing chemical stability of the grafted chains, since the αC–H bond is likely to be cleaved upon the attack on the styrene sulfonic acid units by reactive intermediates, leading to chemical degradation. However, in Fig. 6, the fact that the AMS–MAN co-grafted membrane has better gas barrier properties than the styrene grafted membrane over the entire RH range suggests that a reduced formation rate of the reactive intermediates as a result of the lower gas crossover can also improve the durability of the membrane. At most RH levels, the AMS–AN co-grafted membrane shows similar gas permeability as the AMS–MAN co-grafted membrane, implying that a similar stability in the fuel cell may be expected. The improved gas barrier

A typical approach to assess the chemical stability of polymer electrolytes in fuel cells is to operate the cell at open circuit voltage

Fig. 7. The effect of temperature on gas permeability in the pure styrene grafted membrane (GL 25%).

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are dramatically decomposed according to the data reported previously [13], although the gas crossover rate does not increase sharply, indicating formation of pinholes and hence the cell should be shut down immediately. This is because, as shown in Fig. 4, the ETFE backbone can still offer good gas barrier properties even though the membrane is severely aged. It suggests that, unlike PFSA membranes, chemical degradation can be decoupled from mechanical membrane failure for radiation grafted membranes. A slight decrease in the permeation rate of He is found in case of 50% RH, which is probably a result of the humidification capability of the bubblers gradually degrading with time owing to a gradual drop in the water level, and slightly reduced RH in the cell causes the change of gas permeability in the membrane.

4. Discussion Fig. 4 compares the permeability of the styrene based radiation grafted membrane and its original base film, as well as Nafions 212, as a function of RH. Furthermore, Fig. 5 shows different effects of graft level on the gas barrier properties of the radiation grafted membrane between high and low RH. The gas transport mechanism as a function of RH for PFSA materials, such as Nafions series, has been extensively studied [14,17,19,54]. However, the mechanism of gas transport phenomena between high and low RH for

ETFE base film crystalline phase

Fig. 8. Cell voltage loss and helium permeation increase for the pure styrene grafted membrane tested at OCV and 80 1C for 24 h.

(OCV) for extended periods of time [56,57]. In Fig. 8 the voltage decay of the cells comprising the pure styrene grafted membrane (25% GL) during two 24 h OCV hold tests at 80 1C and 1.5 bara is plotted. Two RH conditions, (i.e.,  100% and  50%, respectively) were used to evaluate the loss of membrane integrity during the accelerated stress tests with different humidification levels. In case of 100% RH, at the beginning of the test, the cell voltage rapidly decreases with the formation of oxides on the Pt surface; whereas no decrease in the OCV of the cell operated at 50% RH is observed during the first several hours, probably because relatively low water content in the MEA may change the surface properties of Pt, which could affect the formation oxides. The evolution of OCV shows that, under full humidification conditions, the OCV decay is more rapid and pronounced than at 50% RH. This is in good agreement with the observations gained from the tests at 2.5 bara that we reported previously [13]. The history of OCV decay in the two tests suggests that, on the one hand, chemical degradation of the tested membrane is reduced with decreased RH, which, on the other hand, can lead to a lower loss in membrane mechanical integrity. This argument can be verified by in situ monitoring the evolution of the gas permeation rate in the tested membranes using helium as a tracer (Fig. 8). Note that we cannot directly measure the crossover rate of neither hydrogen nor oxygen, because a fraction of the permeant gases react with each other before they are probed by the MS. A notable difference in the development of gas permeation rates against the testing time between high and low RH is observed. The permeation rate of He in the membrane tested at  100% RH increases by roughly 30% over the time as a result of severe chemical degradation. In contrast, without severe chemical degradation, the mechanical integrity of the membrane tested at  50% RH is maintained during the OCV hold test. Actually, after the OCV hold test, the grafted chains in the membrane tested at 100% RH

amorphous phase

dry state

fully hydrated state

grafted and unhydrated ionic domain (hydrophilic) grafted and hydrated ionic domain (hydrophilic) gas (H or O ) diffusion pathway

Fig. 9. Schematic of the physical model presented in this study to explain the gas permeation process in radiation grafted membrane and ETFE base film at different RH.

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PSSA based radiation grafted membranes, to the best of our knowledge, has not been illustrated yet. A physical model (Fig. 9) that describes the microstructure of the radiation grafted membrane should help elucidate the experimental results shown above. This physical model features the twophase morphological structure in the radiation grafted membrane, comprising hydrophobic and hydrophilic domains. This two-phase morphological structure in radiation grafted membrane has been recently studied and confirmed by Balog and co-workers using small-angle X-ray scattering (SAXS) technique, which indicated that the hydrophilic domains consist of ionic clusters, but the spatial distribution of ionic domains is not uniform in radiation grafted polymer electrolytes based on the semi-crystalline polymer matrix, such as ETFE [58]. In their most recent work, the nano-scale structure is modeled and the nano-scale tortuosity of the aqueous phase of radiation grafted membrane estimated by combining the techniques of small-angle neutron scattering (SANS) and clipping of correlated random waves (CRW) [59]. The results revealed that the tortuosity decreases with the water content in radiation grafted membranes. In the case of PFSA membranes, such as the Nafions series, a similar ionic cluster based model has been proposed many years ago and basically regarded the material as microporous [60–62]. The model has been generally used to discuss the experimental observations in the transport of proton, water and reactant gases [14,63–67]. Recently, in addition, on the basis of SAXS and SANS data, the microstructure of Nafions could be described by other, more refined models, such as elongated polymeric bundles [68,69], networks of water filled channels [70,71], parallel invertedmicelle cylinders [72] and flat water films [73]. In our study, the base film of the radiation grafted membranes is ETFE, which does not show varying gas permeability with change in RH due to its hydrophobic nature (Fig. 4). Since the crystalline phase is basically impenetrable to permeant molecules, the transport of hydrogen or oxygen molecules must occur in the free volume of the amorphous phase around the crystallites [55], as depicted in Fig. 9(a). The free volume that is at a subnanometer scale is deemed essential to the gas diffusion and permeation in polymers [74]. After the membrane preparation procedure, which includes irradiation, grafting and sulfonation into the ETFE base film (Fig. 3), the membrane is obtained with a hydrophobic–hydrophilic hybrid morphological structure. In this case, the crystallinity in the ETFE domains is lowered during the membrane synthesis [75]. However, the grafted and sulfonated chains fill the free volume in the amorphous phase of ETFE and hence are likely to reduce the number of voids for the transport of gas molecules. In addition, intra- and intermolecular interactions between the ionic groups can lead to intra- and inter-chain spacing in the amorphous region smaller than that in the ETFE base film [76]. In analogy, smaller free volume in dry Nafions membranes than in nonploar PTFE was observed. [74]. Therefore, gas transport primarily occurs in the amorphous hydrophobic phases in the dry radiation grafted membranes [77], which results in higher tortuosities for gas diffusion than their base film ETFE, as depicted in Fig. 9(b). This can account for the lower permeability of hydrogen and oxygen in the styrene based radiation grafted membrane at relatively dry conditions (e.g. 0% and 30% RH) in comparison with the permeability in the ETFE base film (Fig. 4). Likewise, the membrane with a higher GL and a concomitantly higher volume fraction of grafted ionic groups, such as S 66 shown in Fig. 5, has a higher degree of filling of grafted chains and stronger binding between polymer chains than the membrane with a lower GL, such as S 25 shown in Fig. 5. Consequently, the gas permeability of S 66 is lower than that of S 25 at relatively dry conditions (e.g. 0%, 30%, 50% RH in Fig. 5).

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With the increase of RH, the water content and hydration number of the radiation grafted membranes increase [78]. The promoted fluctuations in the free volume of the hydrophilic constituents have been found due to the plasticization effect of water [79,80]. This plasticization effect of water can also apply to ionomer materials, which results in improved gas transport in the hydrophilic phase [25,74]. Under relatively dry conditions, however, the non-uniform distribution of ionic domains in the radiation grafted membrane has a negative impact on the percolation and connectivity of the hydrophilic phase [58]. Thus, facilitation of gas transport due to hydration is not sufficient to exceed the impact of the interaction between the sulfonate groups on gas diffusion at low hydration level. Hence, in the low RH region (cf. Figs. 4 and 5), the ETFE base film is still more permeable than the partially hydrated radiation grafted membrane, and the membrane with a higher GL (e.g. S 66) is less permeable than the membrane with a lower GL (e.g. S 25). In contrast, with more homogeneous distribution of ionic domains [58], the gas transport in Nafions 212 is far more sensitive to the facilitation provided by the absorbed water, as shown in Fig. 4. Nevertheless, as the membrane becomes highly hydrated with increasing RH, the ionic domains swell and neighboring ionic domains coalesce to form larger hydrated ionic domains. Consequently, a hydrated network for gas transport can be formed in the radiation grafted membrane, as illustrated in Fig. 9(c), which substantially improves the movement of gas molecules through the membrane. For the radiation grafted membranes, accordingly, the membrane with a lower GL, which has lower water uptake in fully swollen state (Table 1), exhibits better gas barrier properties than the membrane with a higher GL in the high RH region (Fig. 5), which contrasts the gas barrier properties between the two membranes with different GL in the low RH region. Likewise, in this case, the radiation grafted membrane becomes more permeable than the ETFE base film (Fig. 4). It is worth noting that, at high hydration level, a continuous channel of liquid water may form across the membrane [66], which can boost the gas diffusivity approaching the value in pure water, as suggested in the case of Nafions membranes [19]. Nevertheless, this might not occur in our case, because in that case, the gas permeability in the membranes should sharply increase in the high RH region, which, however, was not observed in the experiments. This is probably a result of the non-uniform distribution of ionic domains in the radiation grafted membranes. In this study, the permeation measurements of H2 and O2 in polymer membranes were performed in the steady-state using the MS based method, because our aim was to examine their gas barrier properties in the fuel cell and the relevant influential factors. It is known that permeability is the product of diffusion coefficient and solubility coefficient [55]. By applying MS to the non-steady-state measurement protocols, such as differential or transient methods [81], the deconvolution of diffusion and solubility coefficients becomes in principle possible. Although this would be interesting for further investigations, it is beyond the scope of this study. However, it is still possible to qualitatively discuss the changes of diffusion and solubility coefficients for radiation grafted membranes with the available data in the literature, which can help us understand the way water content influences the permeability of radiation grafted membranes. As mentioned above, the water absorbed in the Nafions membrane significantly increases the diffusivity but reduces the solubility slightly when the membrane is hydrated [19]. Regarding radiation grafted membranes, such as ETFE based pure styrene grafted membranes, it has been found that, at a fixed RH, with increasing ion exchange capacity (IEC), the diffusion coefficient and permeability of oxygen increases, whereas the solubility of oxygen marginally decreases [82]. This phenomenon was attributed to

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increasing water content with increasing IEC, which indicates that the dependence of the diffusion coefficient and the solubility coefficient on water content is shared between PFSA and PSSA based membranes. Our results show that the increase of permeability in Nafions 212 membrane as a function of RH is much more pronounced than that in all radiation grafted membranes. This phenomenon, as mentioned above, could be partially because of the less homogeneous distribution of ionic domains over the radiation grafted membrane than Nafions. In addition, this observation may be explained on the basis of a different availability of free volume as a function of the water content in the membranes. It has been found that, for the radiation grafted membranes, the volume expansion of the membranes upon hydration is directly proportional to the amount of absorbed water, corresponding to zero excess volume of mixing [34]. This means that, although the plasticization effect of the absorbed water can enhance fluctuations in the free volume of the polymer, the total free volume in the radiation grafted membranes may not be remarkably increased. By contrast, as reported previously [34,83], Nafions membranes expand more than the volume of the absorbed water, which indicates a positive excess volume of mixing. It implies that the total free volume in Nafions membranes increases as a result of absorbing water. Consequently, upon hydration, the gas transport in Nafions membranes is enhanced in more pronounced manner than that in the radiation grafted membrane. In the dry state, however, similar permeability is observed in Nafions 212, the pure styrene grafted membrane (GL 25%) and EFTE (Fig. 3). This observation correlates well with the result that the gas diffusion coefficient and solubility coefficient in the potassium form of the Nafions membrane in the dry state were similar to those in PTFE as the backbone material of Nafions [19]. These results indicate that the gas permeability, for both PFSA membrane and PSSA based radiation grafted membrane in the dry state, is mainly governed by the hydrophobic backbone materials.

5. Conclusions A method that uses a mass spectrometer to quantify and compare the gas barrier properties of different classes of polymer electrolyte membrane in the fuel cell is developed. The permeability of hydrogen and oxygen in a range of radiation grafted membranes, ETFE base film and Nafions 212 is measured as a function of RH. The results show that the radiation grafted membranes and Nafions 212 share the same trend of increasing gas permeability with RH and temperature, while the ETFE base film does not show RH dependency. At a given RH, the radiation grafted membranes have consistently better gas barrier properties than Nafions 212. The effect of graft level on gas barrier properties has been examined for the pure styrene grafted membranes, showing that the membrane with higher graft level is less permeable than the membrane with lower graft level at low hydration level, whereas the membrane with higher graft level becomes more permeable when the membrane is highly hydrated. Nitrilecontaining co-grafted membranes based on AMS are investigated, showing similar gas barrier properties of the AMS–MAN and the AMS–AN co-grafted membranes. Both of these nitrile-containing membranes have lower gas permeability than the pure styrene grafted membrane. A physical model based on the hydrophilic–hydrophobic morphological structure is used to illustrate the permeation process of gases in the radiation grafted membranes between dry and hydrated state. Upon absorbing water, the gas transport in the membranes can be facilitated by increasing the fluctuations of

free volume in the hydrophilic domains, and this facilitation can be significantly enhanced with increasing water content in the membranes. This model explains well the effect of graft level on gas barrier properties and difference in sensitivity of gas permeability to changes in water content between PFSA membranes and radiation grafted membranes. The loss in mechanical integrity of the pure styrene grafted membrane during an accelerated stress test at OCV is recorded in situ over time by means of helium as a tracer. The results show that loss in membrane mechanical integrity is effetely limited under a drier testing condition, as a result of much less chemical degradation compared to the fully hydrated state. Although the membrane tested under full humidification suffered from severe chemical degradation and showed an increase in gas permeability, no mechanical failure was detected during the test, indicating that chemical degradation can be effectively decoupled from membrane mechanical failure in case of radiation grafted membranes.

Acknowledgment Z.Z. and Y.B. acknowledge financial support by the Swiss Federal Office of Energy (contract no. 102245) and the Swiss National Science Foundation (project no. 2000_132382), respectively. The authors would like to thank Thomas Gloor for assistance with the maintenance of the test bench and Jürg Thut for drawing the schematic of the experimental setup. Valuable discussions with Dr. Stefan Kreitmeier regarding the methodology and data processing are highly appreciated. References [1] S.A. Vilekar, R. Datta, The effect of hydrogen crossover on open-circuit voltage in polymer electrolyte membrane fuel cells, J. Power Sources 195 (2010) 2241–2247. [2] H. Liu, F.D. Coms, J. Zhang, H.A. Gasteiger, A.B. LaConti, Chemical degradation: correlations between electrolyzer and fuel cell findings, in: F.N. Büchi, M. Inaba, T.J. Schmidt (Eds.), Polymer Electrolyte Fuel Cell Durability, Springer Science þBusiness Media, New York, 2009, pp. 71–118. [3] J. Wu, X.Z. Yuan, J.J. Martin, H. Wang, J. Zhang, J. Shen, S. Wu, W. Merida, A review of PEM fuel cell durability: degradation mechanisms and mitigation strategies, J. Power Sources 184 (2008) 104–119. [4] V.O. Mittal, H.R. Kunz, J.M. Fenton, Membrane degradation mechanisms in PEMFCs, J. Electrochem. Soc. 154 (2007) B652–B656. [5] L. Gubler, S.M. Dockheer, W.H. Koppenol, Radical (HO, H and HOO) formation and ionomer degradation in polymer electrolyte fuel cells, J. Electrochem. Soc. 158 (2011) B755–B769. [6] L. Gubler, L. Bonorand, Radiation grafted membranes for fuel cells: strategies to compete with PFSA membranes, ECS Trans. 58 (2013) 149–162. [7] L. Gubler, Polymer design strategies for radiation-grafted fuel cell membranes, Adv. Energy Mater. 4 (2014) 1300827. [8] S. Alkan-Gürsel, L. Gubler, B. Gupta, G. Scherer, Radiation grafted membranes, Adv. Polym. Sci. 215 (2008) 157–217. [9] L. Gubler, G.G. Scherer, Radiation-grafted proton conducting membranes, in: W. Vielstich, H.A. Gasteiger, A. Lamm, H. Yokokawa (Eds.), Handbook of Fuel Cells, John Wiley & Sons Ltd, Chichester, UK, 2010. [10] J. Chen, M. Asano, Y. Maekawa, M. Yoshida, Suitability of some fluoropolymers used as base films for preparation of polymer electrolyte fuel cell membranes, J. Membr. Sci. 277 (2006) 249–257. [11] L. Gubler, H. Ben youcef, S. Alkan-Gürsel, A. Wokaun, G.G. Scherer, Cross-linker effect in ETFE-based radiation-grafted proton-conducting membranes: I. Properties and fuel cell performance characteristics, J. Electrochem. Soc. 155 (2008) B921–B928. [12] Z. Zhang, K. Jetsrisuparb, A. Wokaun, L. Gubler, Study of nitrile-containing proton exchange membranes prepared by radiation grafting: performance and degradation in the polymer electrolyte fuel cell, J. Power Sources 243 (2013) 306–316. [13] Z. Zhang, Y. Buchmüller, A. Wokaun, L. Gubler, Degradation study of radiation grafted membranes under low humidity conditions in polymer electrolyte fuel cells, ECS Electrochem. Lett. 2 (2013) F69–F72. [14] Z. Ogumi, T. Kuroe, Z.i. Takehara, Gas permeation in SPE method: II. Oxygen and hydrogen permeation through Nafion, J. Electrochem. Soc. 132 (1985) 2601–2605. [15] V.A. Sethuraman, S. Khan, J.S. Jur, A.T. Haug, J.W. Weidner, Measuring oxygen, carbon monoxide and hydrogen sulfide diffusion coefficient and solubility in Nafion membranes, Electrochim. Acta 54 (2009) 6850–6860.

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