Comparison of two accelerated Nafion™ degradation experiments

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Polymer Degradation and Stability 93 (2008) 214e224 www.elsevier.com/locate/polydegstab

Comparison of two accelerated NafionÔ degradation experiments Sumit Kundu, Leonardo C. Simon*, Michael W. Fowler Department of Chemical Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, Canada N2L 3G1 Received 7 August 2007; received in revised form 4 October 2007; accepted 7 October 2007 Available online 12 October 2007

Abstract This work describes a systematic investigation of the degradation of NafionÔ 112 membranes using a Fenton’s accelerated aging experiment. Two variations of the experiment were compared: a solution method where iron ions and peroxide exist together in solution prior to the addition of NafionÔ, and an exchange method where NafionÔ in the Fe2þ form is exposed to hydrogen peroxide. Accelerated aging experiments were conducted over 3e5 days. Weight loss, fluoride ion release, ion exchange capacity, intrinsic viscosity, morphological characteristics, and dimensional changes were measured. FTIR spectra, mechanical properties and membrane barrier properties were also investigated. Ó 2007 Published by Elsevier Ltd. Keywords: PEM fuel cell; NafionÔ; Degradation; Fenton’s test

1. Introduction Polymer electrolyte membrane fuel cells (PEMFCs) fueled with hydrogen are an emerging alternative energy technology with possible applications ranging from cell phones to homes and vehicles. These devices generate power by electrochemically reacting hydrogen and oxygen in the presence of a catalyst. On the anode, hydrogen separates into protons and electrons. The protons pass through the ion conducting polymer electrolyte membrane (PEM), which makes up part of the membrane electrode assembly (MEA), to the cathode where they recombine with the electrons and oxygen, typically from air, to produce electricity, water and heat. Along with the role of conducting protons, the PEM must also act as a gas barrier between the anode and cathode and provide mechanical strength to the MEA. A schematic of the different parts of a fuel cell is shown in Fig. 1. Though there are many parts of a fuel cell system that can influence reliability and durability, degradation of the membrane electrode assembly, and in particular the polymer

* Corresponding author. Tel.: þ1 519 888 4567x33301. E-mail addresses: [email protected] (S. Kundu), lsimon@uwaterloo. ca (L.C. Simon), [email protected] (M.W. Fowler). 0141-3910/$ - see front matter Ó 2007 Published by Elsevier Ltd. doi:10.1016/j.polymdegradstab.2007.10.001

electrolyte membrane, is the most important. Furthermore, failure of the membrane can lead to safety issues with the overall system. The most common fuel cell electrolyte used today is NafionÔ, a perfluorinated sulfonic acid polymer whose chemical structure is shown in Fig. 1. NafionÔ consists of a hydrophobic Teflon-like backbone, which provides mechanical stability, and side chains ending in hydrophilic sulfonic acid groups that allow proton conduction when the membrane material is hydrated. There are many causes and degradation modes related to failure of polymer electrolyte membranes [1e7]. There has recently been considerable research on the chemical degradation of the polymer electrolyte during normal fuel cell operation. It has been proposed that carboxylic end groups left over from the NafionÔ manufacturing process may be susceptible to attack by radical species generated during fuel cell reactions [8]. The proposed mechanism is as follows: Step 1: ReCF2COOH þ OH / ReCF2 þ CO2 þ H2O Step 2: ReCF2 þ OH / ReCF2OH / ReCOF þ HF Step 3: ReCOF þ H2O / ReCOOH þ HF The radical species, such as hydroxyl radicals, are thought to be formed by the decay of hydrogen peroxide which is an intermediate of the electrochemical oxygen reduction

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Fig. 1. Schematic of a polymer electrolyte membrane fuel cell.

reaction; additionally it has been proposed that oxygen may permeate through the polymer electrolyte membrane and react at the anode to also produce peroxide species [1,8e11]. The presence of hydroxyl radicals has been confirmed by using spin trapping techniques which can detect the species in situ [9]. There has also been evidence suggesting that peroxide may not be the sole degrading species in a fuel cell [12]. Membrane degradation, by hydroxyl radical attack, has been studied by ex situ accelerated methods. Accelerated testing is important to perform on these materials which can last for thousands of hours under regular operating conditions. The overall purpose of accelerated testing is to achieve the same degradation effects in a shorter amount of time. Ex situ accelerated chemical degradation experimentation of fuel cell ionomers most commonly employs Fenton’s testing. Fenton’s reagents are made by combining hydrogen peroxide with Fe2þ ions in order to produce radicals as follows: Fenton’s reaction : H2 O2 þ Fe2þ / Fe3þ þ OH þ OH There have been two main methods employed to study the degradation of NafionÔ membranes by Fenton’s type reaction. The first method exposes the membrane to a solution of peroxide and metal ions (solution method) while the second method exchanges the metal ions with the acid sites of the polymer before exposure to peroxide (exchange method). Healy et al. [11] compared degradation products from samples of NafionÔ degraded with a Fenton’s solution containing 4e16 ppm Fe2þ and 29% H2O2 with degradation products from an in situ experiment. Using F19 NMR they found that membranes degraded in a Fenton’s reagent released chemical

compounds that shared many chemical signals as those released by membranes degraded during fuel cell testing. Not only did they identify fluoride ions in the Fenton’s solution water but also identified a fluorinated species with similar characteristics as the side chains of NafionÔ. It was suggested that as the fluorinated backbone of NafionÔ degraded it would release the side chain components. Wayne [13] studied the effect of Fenton’s testing with NafionÔ 117 in peroxide solutions ranging from 12% to 24%, iron concentrations of 2.8 mM (112 ppm) to 5.0 mM (280 ppm), at temperatures ranging between 20  C and 70  C with the longest immersion time being approximately 45 min and the shortest being 10 min. No significant changes in weight were observed, although there were significant changes to the morphology. After degradation samples contained bubbles which appeared to split the membrane and samples became more opaque. The ion exchange capacity and the conductivity of the membrane were not observed to have changed. Finally, the membrane shrank slightly in one planar direction and elongated slightly in the opposite direction and the thickness increased from the production of bubbles. Recently Tang and coworkers [14] performed degradation studies with 30% peroxide and a combination of Fe, Cr, and Ni to simulate ionic contaminants from stainless steel fuel cell components. Testing was conducted up to 96 h between 80  C and 90  C with solution replenishment every 30 min. They found that fluoride containing polymer fragments entered into the degradation solution but no significant new peaks in the FTIR spectra of the degraded membranes. They further found that the membrane morphology changed significantly, developing bubbles with time.

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Inaba and Kinumoto [10,15] performed similar work with a Fenton’s reagent consisting of 30% H2O2. Instead of adding iron and peroxide together and exposing NafionÔ to the mixture they first exchanged iron and other metallic cations into samples of NafionÔ and then added peroxide. They also measured fluoride release and sulfate release with ion chromatography from which they estimated that almost 70% of the initial number of CeF bonds were broken and almost 35% of SO 3 groups had degraded. Furthermore, a small amount of S]O bonds was identified in the polymer film after degradation using FTIR. A decrease in mechanical strength of the membrane and a loss of membrane thickness were also reported. LaConti and coworkers [1] described how the rate of loss of fluoride ions from NafionÔ was proportional to the logarithm of peroxide concentration used in the Fenton’s solution. Peroxide in the absence of iron particles has also been shown to degrade samples of NafionÔ. Qiao et al. [16] observed that exposure of NafionÔ to 30% H2O2 over a period of 30 days decreased its conductivity of the membrane and water uptake characteristics. Furthermore, FTIR analysis of the membranes showed the development of SeOeS bonds whose formation was attributed to the oxidation of sulfonic acid groups. To date there has not been a comparison of the differences between the solution and exchange methods of accelerated degradation. This is important when determining which method best reflects in situ degradation and therefore is the better accelerated degradation method. This paper reports on the ex situ degradation of NafionÔ 112 membrane using a Fenton’s reagent with 16 ppm Fe2þ, as well as NafionÔ in the exchanged Fe2þ form, over multiple days and on the investigation of chemical, morphological and mechanical changes in the membrane. The goal is to examine any differences between the two methods of degradation as well as develop a set of features for each degradation method which can be compared against membranes degraded in situ.

2.2. Fenton’s test Two different styles of Fenton’s test were employed in this study. For the primary test, which will be referred to as the solution method, a solution of Fe2þ and H2O2 was made to which a sample of NafionÔ was added. This method is similar to that reported by Healy et al. [11]. In the alternative method, similar to the method reported by Inaba and Kinumoto [10,15], Fe2þ ions were exchanged into a sample of NafionÔ which was then submerged in H2O2 and allowed to degrade. The second method will be referred to as the exchange method. The solution method used 30% H2O2 mixed with FeCl2$4H2O to produce a 16 mg/L solution of Fe2þ ions. This solution (40 mL) was placed in a closed vial containing the sample of NafionÔ. A Teflon prong (of negligible size) was used to ensure that the sample of NafionÔ did not float into the head space of the vial. Gas pressure was released from the vessel through a needle piercing the septum in the lid. The NafionÔ and solution were kept in a 72  C oven and the solution of peroxide and iron was replaced by fresh solution changed approximately every 12 h. Chemical analysis of this solution was therefore also done every 12 h. The solutions were stored in polyethylene bottles for fluoride analysis. Every 24 h three samples were removed from the study, rinsed, and saved for later study. In the exchange method the samples of NafionÔ were placed in a saturated solution of FeCl2$4H2O for 24 h. The samples were then rinsed and placed into vials to which 40 mL of peroxide was added. The vials were kept in a 72  C oven and replaced with fresh peroxide every 24 h. Samples were removed periodically, rinsed and saved for later study. Saved samples were rinsed and conditioned in 1 M H2SO4 prior to further testing to remove any residual iron and return the membrane to the Hþ form. 2.3. Gravimetry

2. Experimental 2.1. Materials The main material of study was NafionÔ 112 provided by Ion Power Corporation. A 30% peroxide solution was obtained from Caledon Laboratory Chemicals and analytical grade FeCl2$4H2O was obtained from Fisher Scientific. Catalyst coated membranes used in in situ experiments were supplied by Ion Power Corporation and used NafionÔ 112 as the electrolyte material. Samples of NafionÔ 112 were cut from a larger sheet and had dimensions of approximately 7.5  2.5 cm. Directionality in the membrane can be determined by swelling characteristics. The machine direction will refer to the direction of the greatest swelling and the cross-direction will refer to the direction 90 from the machine direction. For further simplicity, the width will describe the machine direction and the length the cross-direction. All samples from a set were cut in the same direction. Prior to degradation the samples were soaked in 1 M H2SO4 at 72  C for a minimum of 3 h.

Samples of NafionÔ were weighed by first drying the samples for 24 h at 72  C in open glass vials. The vials were capped in the oven and then samples were transported to a Metler AE 160 balance with 0.001 g accuracy. Samples were then removed from the vial and weighed. 2.4. Fluoride ion release An Orion 611 Digital pH/millivolt meter was used with a fluoride ion selective electrode to measure the concentration of fluoride ions in sample water. Calibration curves ranged from 0.001 M to 1 M. Both the concentration of fluoride ions and the volume of sample water remaining after conducting the degradation experiment were used to determine the total number of moles of fluoride released. 2.5. Ion exchange capacity (IEC) Ion exchange capacity measurements were done by conditioning samples in 1 M H2SO4 at 72  C for a minimum of 3 h,

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rinsing the samples and then placing them in a solution of 1 M NaCl at 72  C for 24 h. The sample was then removed and the remaining liquid was titrated with 0.01 M NaOH using an ORION 420A digital pH meter.

were done with the NafionÔ samples in liquid water. Dry samples of degraded NafionÔ tended to be wrinkled, and in order to obtain measurements of dimensions the samples were gently pressed down with a glass slide.

2.6. Viscometry

2.10. Tensile testing

Samples of fresh and aged NafionÔ were dissolved into solutions of 50/50 wt% ethanol/water at 250  C prior to viscosity measurements similar to the procedure used in Ref. [17]. This dissolution procedure used here is usual for NafionÔ membranes and is not expected to cause significant degradation. Prior to dissolution the aged NafionÔ samples were conditioned in 1 M H2SO4 at 72  C, rinsed, and dried to remove any iron and other ions from the membrane. Viscosity was measured in a constant temperature bath at 25  C using a Cannon Ubbelohde glass viscometer. Relative viscosity of the solutions was calculated using Eq. (1):

A Rheometrics Mini Mat 2000 tensile tester was used to generate stressestrain data of materials. Samples of NafionÔ were cut to a width of 0.6 cm and placed in the tension testing fixtures with a gap width of 5 mm. The load cell range was 0e200 N. The strain rate was 2 mm/min at 25  C. The Young’s modulus was estimated using the linear rise in the stressestrain curve corresponding to elastic behavior.

hr ¼

t  t0 1 t0 C

ð1Þ

where t is the elution time of the polymer/solvent sample and t0 is the elution time of the solvent. C is the concentration of the polymer in g/dl. Concentration was determined by measurement of the mass of dry membrane dissolved into a measured mass of solvent of given density.

2.11. Dynamic mechanical thermal analysis (DMTA) A Rheometrics DMTA V dynamic mechanical thermal analyzer was used to determine transition temperatures of the NafionÔ films. Experiments were done using a rectangular tension fixture with a gap width of 2.5 mm. The samples were cut from the degraded samples to a width of approximately 0.6 mm as measured with a digital caliper. Experiments used a frequency of 1 Hz and 0.1 N initial static force. The temperature ramp rate was 2  C/min at a constant strain of 0.05%. Samples were tested from 40  C to 140  C. 2.12. Nitrogen crossover

2.7. Fourier transform infrared (FTIR) Samples of fresh and degraded membranes were analyzed with FTIR. Measurements were conducted on a Bruker Tensor 27 FTIR using an MVP 2 Series ATR accessory made by Herrick Scientific Products Inc. The spectrum was collected after 32 scans with a resolution of 4 cm1. Background subtraction was used and measurements were conducted at room temperature. 2.8. Scanning electron microscopy (SEM) SEM analysis was carried out using an LEO SEM with field emission Gemini Column. X-ray compositional analysis was done using an electron dispersive spectroscopy (EDS) manufactured by EDAX. The detection limit of the EDAX system is atoms with atomic weights equal to or larger than carbon. Samples of NafionÔ or catalyst coated membrane (CCM) were cut into squares of approximately 0.5  0.5 cm2 and fixed to an aluminum stub with double sided conductive tape. Cross-sections were made by freeze fracture from a strip of sample submerged in liquid nitrogen. Once frozen, the sample was broken in half while still submerged. Samples were also sputter coated with gold to improve conductivity. 2.9. Dimensional measurements The dimensions of the NafionÔ films were measured using digital calipers at room temperature. Swollen measurements

Nitrogen gas crossover measurements, the amount of nitrogen gas flow across the membrane with a given pressure differential (regardless of transport mechanism), were performed by clamping NafionÔ samples between two 1/4 inch face sealed fittings at room temperature. One fitting was pressurized with nitrogen gas to 5 psi and the other fitting was attached to a bubble column at atmospheric pressure. Crossover was measured by timing the progression of bubbles through the graduated column. If no significant movement of the bubble (>0.1 mL) was seen after 10 min, then the samples were said to have no crossover. 3. Results Samples of NafionÔ were degraded via two accelerated degradation methods. In the solution method, samples were exposed to a mixture of peroxide and Fe2þ ions; in the exchange method, samples of NafionÔ in the Fe2þ form were exposed to a peroxide solution. 3.1. Chemical changes in membrane structure An initial sign of polymer degradation is weight loss over the testing period. With both degradation methods the NafionÔ membranes lost significant weight with time (Fig. 2). The weight loss increased following and after 80 h of exposure exceeded 20% of the original weight in both cases. This was particularly interesting since different amounts of iron are present

40.00 Degraded - Solution Method 35.00

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Weight Loss

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Time (h)

and solution renewal intervals were different between the two methods. One explanation is that with the solution method iron, peroxide, and polymer must be in proximity to each other for degradation to occur. Radicals produced within the bulk solution may terminate by an alternate mechanism such as reaction with another radical or with polymer fragments. On the other hand with the exchange method iron is present within the polymer membrane and so a smaller amount of iron could potentially cause the same amount of degradation in the polymer with this method. The important item to note is that weight loss with both methods produced similar results over the same period of time with the procedures used. Control samples, summarized in Table 1, established that the observed accelerated degradation was due to the combination of iron and peroxide. NafionÔ samples were exposed to water (Control 1) and a water/iron solution (Control 3). Both these solutions showed little weight loss over the testing time of 120 h though the sample in the water/iron solution lost almost 5% of its original weight. A NafionÔ sample was exposed to 30% peroxide/water solution (Control 2) (without any Fe2þ) and showed no significant weight loss after 80 h. Finally, Teflon films (Control 4) were also exposed to a 16 ppm Fenton’s solution to establish that the Teflon prongs were stable under the test conditions. The results showed that there was no significant change in weight of the Teflon films. Fluoride ion release was measured with every solution change over the course of the experiments and the cumulative fluoride loss is shown in Fig. 3. Fluoride release is considered Table 1 Comparison of weight loss of NafionÔ control samples, Teflon, and degraded NafionÔ

% Weight loss 2.82 Cumulative fluoride Negligible release (mmol)

120 Degraded - Solution Method Degraded - Exchange Method 100 80 60 40 20 0

0

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60

40

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120

140

Time (h)

Fig. 2. Percent weight loss of NafionÔ 112 samples degraded by the solution method and the exchange method.

Control 1 NafionÔ in water (120 h)

Average Cumulative Fluoride Loss (µmol)

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Control 2 NafionÔ þ 30% HOOH (96 h)

Control 3 NafionÔ þ H2O þ Fe2þ (120 h)

Control 4 Teflon þ Fenton’s (96 h)

1.18 0.87

5.39 Negligible

0.13 0.06

Fig. 3. Cumulative fluoride loss from membrane samples degraded by the solution and exchange methods.

to be an indicator of membrane degradation at the molecular level. It would be expected that for similar weight losses a similar amount of fluoride would be released. However as can be seen in Fig. 3 the solution method of degradation lost more fluoride ions cumulatively than the exchange method of degradation. One possible explanation is that small polymer fragments make up part of the mass leaving the polymer films and entering into the surrounding solution. In the case of the solution method these polymer fragments can continue degrading since iron ions and peroxide exist in the surrounding solution at high concentrations. This is not the case with the exchange method since all the iron is bound in the polymer. The samples degraded by the solution method for 120 h had a cumulative fluoride ion measurement of 100 mmol or 1.5% of the available fluorine in the polymer structure. Samples degraded by the solution method measured 34 mmol of fluoride ions cumulatively or 0.5% of the available fluorine. As discussed, it is expected that more fluorine atoms were still bonded in chain fragments leaving the membrane. Fluoride release from each of the control samples was negligible (Table 1). Ion exchange capacity (IEC) measurements and chemical analysis using electron dispersive spectroscopy (EDS) were performed on the degraded membranes in order to determine if the ratio of backbone to side chain groups had been changed upon degradation. The results showed no significant change in the ion exchange capacity or in the atomic ratio of oxygen to fluorine (O/F ratio) over the exposure time for both the solution and the exchange methods. The value for the IEC was 1.1 mmol/gdry-membrane for the fresh membrane and the O/F atomic ratio was 0.18. Similar behavior was observed for sulfur/oxygen ratio and sulfur/fluorine ratio. Other report in the literature [10,15] based on ion chromatography data suggested that the backbone components degrade to a larger extent than the sulfonic acid groups, thus indicating that IEC should increase with degradation. This difference is probably due to the fact that if sulfur containing chain segments may not be detected by ion chromatography. The results presented here indicate that the polymer remaining in the membrane had a similar average chemical composition to the original fresh polymer.

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8 Fresh Nafion™ 112

Reduced Viscosity (dl/g)

7

Degraded - Solution Method (96h) Degraded - Exchange Method (84h)

6

S-O A

B

C

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Wavenumber (cm-1) Fig. 5. FTIR spectra for fresh NafionÔ membrane samples, a sample degraded via the solution method, and a sample degraded via the exchange method.

Evidence of chemical changes in the membranes was investigated by FTIR. Typical curves of the fresh and degraded spectra are shown in Fig. 5. The main peaks of interest are at 1147 cm1 and 1204 cm1 which are attributed to CeF vibration [15,22] and at 1054 cm1 which is due to SeO stretching [16,23]. The peak at 1147 cm1 was used to normalize the curves for comparison purposes. The FTIR results of degraded membranes and control samples reinforce IEC and EDAX findings which showed no significant change in the relative numbers of side chains to backbone segments in degraded samples as compared to fresh NafionÔ. FTIR analysis of the membranes degraded in this experiment by both methods as well as the control samples could not detect any new peaks which could be correlated with exposure time despite the high level of degradation. This was also observed by Tang and coworkers [14]. The chemical characterization of NafionÔ 112 membranes degraded by a Fenton’s solution indicates that the only significant change to membranes degraded by OH radicals in this way was the average molecular weight. Changes to chemical structure, such as changes to the ratio of side chain groups to backbone groups as well as changes in the chemical bonds present in the membranes, were not detected here. Nevertheless, exposure to the Fenton’s reagent severely degraded the membranes as seen by the loss in weight and fluoride loss data. 3.2. Changes in morphology and dimensional characteristics

5 4 3 2 1 0

A Fresh Nafion™ 112 C-F B Degraded - Solution Method 120 h C-F C Degraded - Exchange Method 84h

Normalized Absorbance

Although the ratio of oxygen atoms in the side chains to fluorine atoms in the backbone did not change, degradation could have removed backbone and side chain segments in the same proportion. Viscosity measurements were carried out to determine if the average molecular weight of the polymer membranes were changing. Samples of fresh NafionÔ 112, degraded sample from the exchange method experiment (84 h of degradation) and from the solution method (96 h of degradation) were dissolved in 50/50 by weight mixtures of ethanol and water. These samples were chosen because of their high degree of degradation and similar weight loss. All samples were in the hydrogen form and reduced viscosity was calculated by Eq. (1). Plots of reduced viscosity against the concentration of NafionÔ 112 are shown in Fig. 4. The plots show an almost exponential increase in reduced viscosity with a reduction in polymer concentration. This is a typical behavior of polyelectrolytes [18] and has been shown to be present in sulfonated electrolytes, including NafionÔ, by several authors [19,20]. The curves for fresh samples lie above those for samples degraded by the solution method (96 h). There are two possible explanations for this difference between fresh and aged samples. One possibility is that the structure of the repeating units making up the polymer chains has changed and the other possibility is that the polymer molecular weight has decreased, leading to changes in the size of aggregates when in solution. It has already been shown by IEC results that this structure, in so far as the ratio of side chain components to backbone, has not changed. On the other hand, a moderate decrease in average molecular weight has been shown to shift the viscosity curves down by Cohen and coworkers [21]. A change in molecular weight is also consistent with the suggested mechanisms of NafionÔ degradation found in the literature [8]. Interestingly, the reduced viscosity curves of samples degraded by the solution method and the exchange method are very similar, indicating that their average molecular weight is also similar even though the weight loss of the specific samples in question was different (Fig. 2).

219

0

0.05

0.1

0.15

0.2

0.25

Nafion™ Concentration (g/dl) Fig. 4. Viscosity data for dissolved samples of fresh NafionÔ 112 and samples degraded by the solution and exchange methods. Samples were dissolved in 50/50 ethanol/water mixtures at 250  C.

Despite the weight loss and IEC trends being similar between the two degradation methods, scanning electron microscopy (SEM) analysis of the NafionÔ samples showed significant differences in the morphology of the samples. In the case of samples degraded by the solution method, the surface was roughened by the presence of defects. Upon closer examination of the membrane the defects appear to be bubbles formed within the membrane (Fig. 6a and c) as well as tears and bumps on the membrane surface. The bubbles were in the order of hundreds of microns in diameter

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Fig. 6. Comparison of surface morphologies between NafionÔ degraded by the solution method and the exchange method. (a) Solution method 100 magnification, (b) exchange method 100 magnification, (c) solution method 500 magnification, and (d) exchange method 500 magnification.

while the bumps and tears were less than 10 mm. Overall, the number of defects increased with the time of exposure to degradation for both methods. The progression of degradation for the solution method is shown in Fig. 7c,e,g. The membrane splits into two halves after 24 h of exposure and after long time samples degraded by the solution method developed large pits within the two halves of the polymer membrane (Fig. 8). With exception of the pit which occurs after long exposure times, close examination of the separate halves of the membrane shows that their morphology resembles that of the fresh membrane with no extra distinguishing features. The samples themselves were translucent in appearance. Samples degraded with the exchange method show fewer large bubbles on the surface as compared to the solution method even after 84 h of exposure. The surfaces have smaller defects and fewer holes penetrating the membrane as shown in Fig. 6b and d. Overall, membranes degraded by this method were far smoother to the touch than the solution method samples due to smaller number of large bubbles and tears. Crosssections of samples degraded by the exchange method show a markedly different morphology than those degraded by the solution method. The progression of degradation of samples degraded by the exchange method is shown in Fig. 7b,d,f. The surface of the membrane has the appearance of foam with the centre of the membrane essentially remaining as one continuous phase. This foam appearance was not observed with solution method samples even after long time. Control samples showed no change in morphology with time compared with new samples. The appearance of the membranes was also different from the solution method, the membrane

changed from being transparent to being opaque and white. The difference in morphology is attributed to differences in how reactive species diffuse into the membrane as a result of the differences in the experimental solutions. A reduction in thickness is a common observation in NafionÔ samples degraded in fuel cells and is usually attributed to chemical degradation [11]. A dimensional analysis of the membranes was performed to determine the effect of degradation by Fenton’s reagent on the sample’s shape and size. Fresh samples measured with calipers and SEM had thicknesses between 0.04 mm and 0.06 mm. The SEM was used to measure membrane thicknesses on aged samples since caliper measurements were affected by the rough surface features. Thickness measurements of solution method samples excluded the empty space created by bubbles and focused on the combined thickness of the two halves of the membranes. For the exchange method, the degraded membranes had well defined edges for measurement encompassing both the highly degraded ‘‘foamed’’ edges and the less degraded centre. Such results showed that neither method of degradation produced a significant change in membrane thickness. NafionÔ samples preferentially swell in one planar direction versus the other. Here, the sample width will be the direction characterized by a high degree of swelling and the sample length will be the direction characterized by less swelling. These dimensions changed significantly with degradation. With exposure using both methods the swollen width increased by 10e15% while at the same time the swollen length decreased by 3e5% as shown in Fig. 9 after long exposure times. Changes in swelling in aged samples were much more significant than changes observed in control samples

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Fig. 7. Comparison of SEM cross-sections of NafionÔ samples degraded by the solution method and by the exchange method. (a) Fresh NafionÔ samples, 2000 magnification. (b, d, f) NafionÔ degraded by the exchange method for 24 h, 60 h, and 84 h, respectively, 2000 magnification. (c, e, g) NafionÔ degraded by the solution method for 24 h, 48 h, and 120 h, respectively, 500 magnification.

though NafionÔ exposed to hydrogen peroxide did have an increase in swollen width of almost 5% as shown in Table 2. Dry dimensions followed a similar trend. Additionally, aged samples tended to swell more than fresh samples. As the membrane degrades the total molecular weight of the polymer chains decreased as shown by viscosity measurements. This facilitates the unrestricted movement of the chains. The swelling behavior already shows that when given greater mobility, through hydration, the entangled polymer

chains will preferentially relax in one direction. Thus, as the chains degrade the membrane will tend to gain length in one direction over another. 3.3. Effect of degradation on mechanical properties and barrier properties Degradation imparts many physical changes on membrane morphology and dimensions, hence it is expected that

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Table 2 Comparison of dimensional changes in control samples

Fig. 8. NafionÔ membrane split by a bubble after degradation by the solution method for 108 h.

20 15 10 5 0 -5 -10 -15 -20 20 15 10 5 0 -5 -10 -15 -20

Control 1 NafionÔ in water (120 h)

Control 2 NafionÔ þ 30% HOOH (96 h)

Control 3 NafionÔ þ H2O þ Fe2þ (120 h)

Control 4 Teflon þ Fenton’s (96 h)

% Change in width % Change in length

2.78 1.11

4.60 0.32

2.61 2.10

2.50 0.96

this material [24] and is known as the a-transition. This transition is associated with increased mobility of the ionic domain, which tends to agglomerate into clusters within the polymer [25]. The storage modulus (E0 ) of degraded samples was lower than fresh samples (Fig. 11) followed similar trends as the Young’s modulus. Again, this is expected of polymers with decreased molecular weights. Fig. 11 also shows that the loss tangent curves of degraded samples are also lower than the fresh samples. Furthermore, degraded samples showed an increase in a-transition temperature with samples degraded by the solution method showing the largest increase (Fig. 12). Within a fuel cell, NafionÔ electrolyte membranes are not only responsible for transporting protons across the membrane but are also responsible for acting as a gas barrier between the two sides. Failure of the membrane as a gas barrier can result in lower and unstable voltages or complete failure of the fuel cell. The gas barrier properties of the membranes degraded in this work were evaluated as a function of time and are shown in Fig. 13. Fresh samples exhibited nitrogen flow less than 0.0002 mL s1. Samples degraded with the solution method had nitrogen crossover rates as high as 0.27 mL s1. The samples degraded by the exchange method all had negligible crossover rates. The reason for this extends from the morphology of the degraded samples, those degraded by the solution method had many holes penetrating to the centre of the membrane facilitating crossover. In contrast, the membranes degraded by the exchange method did not have these features and therefore could continue to act as a gas barrier.

160

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150

Degraded - Exchange

140 Swollen Width Swollen Length

b

Modulus (MPa)

Change

Change

mechanical and barrier properties will also be affected. The mechanical properties were investigated by tensile stresse strain curves and by temperature scanning in dynamic mechanical measurements. The barrier properties were studied by measuring the crossover flow. The Young’s modulus for both degradation methods was observed to decrease by 10e20% in relation to fresh samples (Fig. 10). This is consistent with a reduction in average molecular weight shown by the viscosity measurements. The shorter chains resulting from OH radical degradation also result in less chain entanglements. This increases the mobility of the chains when stressed and therefore a lower Young’s modulus. Transition temperatures were measured using the loss tangent (tan d) in the dynamic mechanical thermal analysis (DMTA). Typical curves for fresh and degraded NafionÔ samples are shown in Fig. 11. A loss tangent peak occurred between 120  C and 130  C in all cases which is typical for

Sample

130 120 110 100 90 80

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Time (h) Fig. 9. Changes in length and width for samples degraded by the solution method (a) and the exchange method (b).

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Time (h) Fig. 10. Young’s modulus results for samples degraded by the solution and exchange methods.

S. Kundu et al. / Polymer Degradation and Stability 93 (2008) 214e224 0.30

Fresh Nafion™ 112 Degraded - Exchange Method, 84h. Degraded - Solution Method, 120h

100

0.6 0.5 0.4

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Tan ( )

E' (MPa)

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Fig. 11. Storage modulus (E0 ) and loss tangent (tan d) of fresh NafionÔ, sample degraded by the solution method for 120 h, and sample degraded by the exchange method for 84 h.

4. Conclusions This work has presented a comparison of NafionÔ membranes degraded by two ex situ accelerated degradation methods promoting degradation by OH radical attack using a Fenton’s type test. The two methods of Fenton’s testing that were compared were a solution method which comprises introducing NafionÔ membrane to a solution of peroxide and iron and an exchange method where a sample of NafionÔ in the Fe2þ form was exposed to a peroxide solution. It was found that the two degradation methods significantly degraded the membrane over days of exposure time with samples loosing over 20% of their original weight over the testing time. The weight loss was accompanied with fluoride release and no change in the ratio of side chains to backbone. Viscosity measurements of dissolved membranes suggest that the average molecular weight decreased with degradation. Furthermore, FTIR analysis also indicated that both methods of degradation did not produce any new chemical groups on the membranes. 128

-transition temperature (oC)

Nitrogen Crossover Flow (ml s-1)

0.7

120

223

127

Fig. 13. Crossover results for membranes degraded by the solution method. Samples degraded by the exchange method showed negligible crossover.

Despite there being no significant chemical differences between the two degradation methods, differences in morphology of the degraded samples were observed. Analysis of the morphology showed that membranes degraded by the solution method had many holes, tears, and large bubbles on the surface and cross-sections revealed that the bubbles originated at the centre of the membrane, splitting it into two. Membranes degraded by the exchange method did not split into two but instead areas close to the surface appeared ‘foamy’. The thickness of the membranes did not change appreciably in both cases though with degradation, the swollen width increased while the membrane shrank slightly in length. Finally, mechanical and gas barrier properties were examined for the degraded membranes. In both degradation methods the modulus decreased with exposure time which was attributed to the reduction in molecular weight of the polymer chains. This was also observed in dynamic mechanical results of the storage modulus. The peak height of the loss tangent curve in dynamic mechanical experiments was lower with increased degradation. This may indicate that ionic clusters in the membrane, made up of sulfonic acid groups, shrank as the polymer was degraded. The transition temperature however increased with exposure to the Fenton’s reagent which may indicate that the smaller ionic clusters were also more stable.

126

Acknowledgements

125

The authors would like to acknowledge Ion Power Inc. for supporting this study and the Natural Sciences and Engineering Research Council (NSERC) of Canada for financial support.

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References

Time (h) Fig. 12. a-Transition temperatures for membranes degraded by the solution and exchange methods.

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