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Polymer Degradation and Stability 93 (2008) 214e224www.elsevier.com/locate/polydegstab

Comparison of two accelerated Nafion� 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 Nafion� 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 additionof Nafion�, and an exchange method where Nafion� in the Fe2þ form is exposed to hydrogen peroxide. Accelerated aging experiments wereconducted over 3e5 days. Weight loss, fluoride ion release, ion exchange capacity, intrinsic viscosity, morphological characteristics, and dimen-sional changes were measured. FTIR spectra, mechanical properties and membrane barrier properties were also investigated.� 2007 Published by Elsevier Ltd.

Keywords: PEM fuel cell; Nafion�; Degradation; Fenton’s test

1. Introduction

Polymer electrolyte membrane fuel cells (PEMFCs) fueledwith hydrogen are an emerging alternative energy technologywith possible applications ranging from cell phones to homesand vehicles. These devices generate power by electrochemi-cally reacting hydrogen and oxygen in the presence of a cata-lyst. On the anode, hydrogen separates into protons andelectrons. The protons pass through the ion conducting poly-mer electrolyte membrane (PEM), which makes up part ofthe membrane electrode assembly (MEA), to the cathodewhere they recombine with the electrons and oxygen, typicallyfrom air, to produce electricity, water and heat. Along with therole of conducting protons, the PEM must also act as a gas bar-rier between the anode and cathode and provide mechanicalstrength to the MEA. A schematic of the different parts ofa fuel cell is shown in Fig. 1.

Though there are many parts of a fuel cell system that caninfluence reliability and durability, degradation of the mem-brane electrode assembly, and in particular the polymer

* Corresponding author. Tel.: þ1 519 888 4567x33301.

E-mail addresses: s2kundu@uwaterloo.ca (S. Kundu), lsimon@uwaterloo.

ca (L.C. Simon), mfowler@uwaterloo.ca (M.W. Fowler).

0141-3910/$ - see front matter � 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 theoverall system. The most common fuel cell electrolyte used to-day is Nafion�, a perfluorinated sulfonic acid polymer whosechemical structure is shown in Fig. 1. Nafion� consists ofa hydrophobic Teflon-like backbone, which provides mechan-ical stability, and side chains ending in hydrophilic sulfonicacid groups that allow proton conduction when the membranematerial is hydrated.

There are many causes and degradation modes related tofailure of polymer electrolyte membranes [1e7]. There hasrecently been considerable research on the chemical degrada-tion of the polymer electrolyte during normal fuel cell opera-tion. It has been proposed that carboxylic end groups left overfrom the Nafion� manufacturing process may be susceptibleto attack by radical species generated during fuel cell reactions[8]. The proposed mechanism is as follows:

Step 1: ReCF2COOHþOH�/ ReCF2� þ CO2þH2O

Step 2: ReCF2� þOH�/ ReCF2OH / ReCOFþHF

Step 3: ReCOFþH2O / ReCOOHþHF

The radical species, such as hydroxyl radicals, are thoughtto be formed by the decay of hydrogen peroxide which is anintermediate of the electrochemical oxygen reduction

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

215S. Kundu et al. / Polymer Degradation and Stability 93 (2008) 214e224

reaction; additionally it has been proposed that oxygen maypermeate through the polymer electrolyte membrane and re-act at the anode to also produce peroxide species [1,8e11].The presence of hydroxyl radicals has been confirmed by us-ing spin trapping techniques which can detect the species insitu [9]. There has also been evidence suggesting that perox-ide may not be the sole degrading species in a fuel cell [12].

Membrane degradation, by hydroxyl radical attack, hasbeen studied by ex situ accelerated methods. Accelerated test-ing is important to perform on these materials which can lastfor thousands of hours under regular operating conditions. Theoverall purpose of accelerated testing is to achieve the samedegradation effects in a shorter amount of time. Ex situ accel-erated chemical degradation experimentation of fuel cell ion-omers most commonly employs Fenton’s testing. Fenton’sreagents are made by combining hydrogen peroxide withFe2þ ions in order to produce radicals as follows:

Fenton’s reaction : H2O2 þ Fe2þ / Fe3þ þ OH� þ OH�

There have been two main methods employed to study thedegradation of Nafion� membranes by Fenton’s type reaction.The first method exposes the membrane to a solution of perox-ide and metal ions (solution method) while the second methodexchanges the metal ions with the acid sites of the polymerbefore exposure to peroxide (exchange method).

Healy et al. [11] compared degradation products from sam-ples of Nafion� degraded with a Fenton’s solution containing4e16 ppm Fe2þ and 29% H2O2 with degradation productsfrom an in situ experiment. Using F19 NMR they found thatmembranes degraded in a Fenton’s reagent released chemical

compounds that shared many chemical signals as thosereleased by membranes degraded during fuel cell testing.Not only did they identify fluoride ions in the Fenton’s solu-tion water but also identified a fluorinated species with similarcharacteristics as the side chains of Nafion�. It was suggestedthat as the fluorinated backbone of Nafion� degraded it wouldrelease the side chain components.

Wayne [13] studied the effect of Fenton’s testing withNafion� 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 �Cwith the longest immersion time being approximately 45 minand the shortest being 10 min. No significant changes in weightwere observed, although there were significant changes to themorphology. After degradation samples contained bubbleswhich appeared to split the membrane and samples becamemore opaque. The ion exchange capacity and the conductivityof the membrane were not observed to have changed. Finally,the membrane shrank slightly in one planar direction and elon-gated slightly in the opposite direction and the thickness in-creased from the production of bubbles.

Recently Tang and coworkers [14] performed degradationstudies with 30% peroxide and a combination of Fe, Cr, andNi to simulate ionic contaminants from stainless steel fuelcell components. Testing was conducted up to 96 h between80 �C and 90 �C with solution replenishment every 30 min.They found that fluoride containing polymer fragmentsentered into the degradation solution but no significant newpeaks in the FTIR spectra of the degraded membranes. Theyfurther found that the membrane morphology changed signif-icantly, developing bubbles with time.

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

Inaba and Kinumoto [10,15] performed similar work witha Fenton’s reagent consisting of 30% H2O2. Instead of addingiron and peroxide together and exposing Nafion� to the mix-ture they first exchanged iron and other metallic cations intosamples of Nafion� and then added peroxide. They also mea-sured fluoride release and sulfate release with ion chromatog-raphy from which they estimated that almost 70% of the initialnumber of CeF bonds were broken and almost 35% of SO3

�

groups had degraded. Furthermore, a small amount of S]Obonds was identified in the polymer film after degradationusing FTIR. A decrease in mechanical strength of the mem-brane and a loss of membrane thickness were also reported.LaConti and coworkers [1] described how the rate of loss offluoride ions from Nafion� was proportional to the logarithmof peroxide concentration used in the Fenton’s solution.

Peroxide in the absence of iron particles has also been shownto degrade samples of Nafion�. Qiao et al. [16] observed thatexposure of Nafion� to 30% H2O2 over a period of 30 days de-creased its conductivity of the membrane and water uptakecharacteristics. Furthermore, FTIR analysis of the membranesshowed the development of SeOeS bonds whose formationwas attributed to the oxidation of sulfonic acid groups.

To date there has not been a comparison of the differences be-tween the solution and exchange methods of accelerated degra-dation. This is important when determining which method bestreflects in situ degradation and therefore is the better accelerateddegradation method. This paper reports on the ex situ degrada-tion of Nafion� 112 membrane using a Fenton’s reagent with16 ppm Fe2þ, as well as Nafion� in the exchanged Fe2þ

form, over multiple days and on the investigation of chemical,morphological and mechanical changes in the membrane. Thegoal is to examine any differences between the two methodsof degradation as well as develop a set of features for eachdegradation method which can be compared against membranesdegraded in situ.

2. Experimental

2.1. Materials

The main material of study was Nafion� 112 provided byIon Power Corporation. A 30% peroxide solution was obtainedfrom Caledon Laboratory Chemicals and analytical gradeFeCl2$4H2O was obtained from Fisher Scientific. Catalystcoated membranes used in in situ experiments were suppliedby Ion Power Corporation and used Nafion� 112 as the elec-trolyte material.

Samples of Nafion� 112 were cut from a larger sheet andhad dimensions of approximately 7.5� 2.5 cm. Directionalityin the membrane can be determined by swelling characteris-tics. The machine direction will refer to the direction of thegreatest swelling and the cross-direction will refer to the direc-tion 90� from the machine direction. For further simplicity, thewidth will describe the machine direction and the length thecross-direction. All samples from a set were cut in the samedirection. Prior to degradation the samples were soaked in1 M H2SO4 at 72 �C for a minimum of 3 h.

2.2. Fenton’s test

Two different styles of Fenton’s test were employed in thisstudy. For the primary test, which will be referred to as thesolution method, a solution of Fe2þ and H2O2 was made towhich a sample of Nafion� was added. This method is similarto 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 Nafion�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 withFeCl2$4H2O to produce a 16 mg/L solution of Fe2þ ions.This solution (40 mL) was placed in a closed vial containingthe sample of Nafion�. A Teflon prong (of negligible size)was used to ensure that the sample of Nafion� did not floatinto the head space of the vial. Gas pressure was releasedfrom the vessel through a needle piercing the septum in thelid. The Nafion� and solution were kept in a 72 �C ovenand the solution of peroxide and iron was replaced by fresh so-lution changed approximately every 12 h. Chemical analysisof this solution was therefore also done every 12 h. The solu-tions 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 Nafion� wereplaced in a saturated solution of FeCl2$4H2O for 24 h. Thesamples were then rinsed and placed into vials to which40 mL of peroxide was added. The vials were kept ina 72 �C oven and replaced with fresh peroxide every 24 h.Samples were removed periodically, rinsed and saved for laterstudy. Saved samples were rinsed and conditioned in 1 MH2SO4 prior to further testing to remove any residual ironand return the membrane to the Hþ form.

2.3. Gravimetry

Samples of Nafion� were weighed by first drying the sam-ples for 24 h at 72 �C in open glass vials. The vials were cap-ped in the oven and then samples were transported to a MetlerAE 160 balance with 0.001 g accuracy. Samples were thenremoved from the vial and weighed.

2.4. Fluoride ion release

An Orion 611 Digital pH/millivolt meter was used witha fluoride ion selective electrode to measure the concentrationof fluoride ions in sample water. Calibration curves rangedfrom 0.001 M to 1 M. Both the concentration of fluorideions and the volume of sample water remaining after conduct-ing the degradation experiment were used to determine thetotal number of moles of fluoride released.

2.5. Ion exchange capacity (IEC)

Ion exchange capacity measurements were done by condi-tioning samples in 1 M H2SO4 at 72 �C for a minimum of 3 h,

217S. Kundu et al. / Polymer Degradation and Stability 93 (2008) 214e224

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

2.6. Viscometry

Samples of fresh and aged Nafion� were dissolved intosolutions of 50/50 wt% ethanol/water at 250 �C prior toviscosity measurements similar to the procedure used inRef. [17]. This dissolution procedure used here is usual forNafion� membranes and is not expected to cause significantdegradation. Prior to dissolution the aged Nafion� sampleswere conditioned in 1 M H2SO4 at 72 �C, rinsed, and driedto remove any iron and other ions from the membrane. Viscos-ity was measured in a constant temperature bath at 25 �C usinga Cannon Ubbelohde glass viscometer. Relative viscosity ofthe solutions was calculated using Eq. (1):

hr ¼t� t0

t0

1

Cð1Þ

where t is the elution time of the polymer/solvent sample andt0 is the elution time of the solvent. C is the concentration ofthe polymer in g/dl. Concentration was determined by mea-surement of the mass of dry membrane dissolved into a mea-sured mass of solvent of given density.

2.7. Fourier transform infrared (FTIR)

Samples of fresh and degraded membranes were analyzedwith FTIR. Measurements were conducted on a Bruker Tensor27 FTIR using an MVP 2 Series ATR accessory made by Her-rick Scientific Products Inc. The spectrum was collected after32 scans with a resolution of 4 cm�1. Background subtractionwas used and measurements were conducted at roomtemperature.

2.8. Scanning electron microscopy (SEM)

SEM analysis was carried out using an LEO SEM with fieldemission Gemini Column. X-ray compositional analysis wasdone using an electron dispersive spectroscopy (EDS) manu-factured by EDAX. The detection limit of the EDAX systemis atoms with atomic weights equal to or larger than carbon.Samples of Nafion� or catalyst coated membrane (CCM)were cut into squares of approximately 0.5� 0.5 cm2 andfixed to an aluminum stub with double sided conductivetape. Cross-sections were made by freeze fracture from a stripof sample submerged in liquid nitrogen. Once frozen, the sam-ple was broken in half while still submerged. Samples werealso sputter coated with gold to improve conductivity.

2.9. Dimensional measurements

The dimensions of the Nafion� films were measured usingdigital calipers at room temperature. Swollen measurements

were done with the Nafion� samples in liquid water. Dry sam-ples of degraded Nafion� tended to be wrinkled, and in orderto obtain measurements of dimensions the samples weregently pressed down with a glass slide.

2.10. Tensile testing

A Rheometrics Mini Mat 2000 tensile tester was used togenerate stressestrain data of materials. Samples of Nafion�were cut to a width of 0.6 cm and placed in the tension testingfixtures with a gap width of 5 mm. The load cell range was0e200 N. The strain rate was 2 mm/min at 25 �C. TheYoung’s modulus was estimated using the linear rise in thestressestrain curve corresponding to elastic behavior.

2.11. Dynamic mechanical thermal analysis (DMTA)

A Rheometrics DMTA V dynamic mechanical thermal an-alyzer was used to determine transition temperatures of theNafion� films. Experiments were done using a rectangulartension fixture with a gap width of 2.5 mm. The sampleswere cut from the degraded samples to a width of approxi-mately 0.6 mm as measured with a digital caliper. Experi-ments used a frequency of 1 Hz and 0.1 N initial staticforce. The temperature ramp rate was 2 �C/min at a constantstrain of 0.05%. Samples were tested from 40 �C to 140 �C.

2.12. Nitrogen crossover

Nitrogen gas crossover measurements, the amount of nitro-gen gas flow across the membrane with a given pressure differ-ential (regardless of transport mechanism), were performed byclamping Nafion� samples between two 1/4 inch face sealedfittings at room temperature. One fitting was pressurized withnitrogen gas to 5 psi and the other fitting was attached to a bub-ble column at atmospheric pressure. Crossover was measuredby timing the progression of bubbles through the graduatedcolumn. If no significant movement of the bubble (>0.1 mL)was seen after 10 min, then the samples were said to haveno crossover.

3. Results

Samples of Nafion� were degraded via two accelerateddegradation methods. In the solution method, samples wereexposed to a mixture of peroxide and Fe2þ ions; in theexchange method, samples of Nafion� in the Fe2þ formwere exposed to a peroxide solution.

3.1. Chemical changes in membrane structure

An initial sign of polymer degradation is weight loss over thetesting period. With both degradation methods the Nafion�membranes lost significant weight with time (Fig. 2). Theweight loss increased following and after 80 h of exposureexceeded 20% of the original weight in both cases. This was par-ticularly interesting since different amounts of iron are present

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

0 20 40 60 80 100 120 140

Time (h)

Wei

ght

Los

s

Degraded - Solution Method

Degraded - Exchange Method

Fig. 2. Percent weight loss of Nafion� 112 samples degraded by the solution

method and the exchange method.

0

20

40

60

80

100

120

0 20 60 40 80 100 120 140

Time (h)

Ave

rage

Cum

ulat

ive

Flu

orid

e L

oss

(µm

ol)

Degraded - Solution MethodDegraded - Exchange Method

Fig. 3. Cumulative fluoride loss from membrane samples degraded by the

solution and exchange methods.

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

and solution renewal intervals were different between the twomethods. One explanation is that with the solution methodiron, peroxide, and polymer must be in proximity to each otherfor degradation to occur. Radicals produced within the bulk so-lution may terminate by an alternate mechanism such as reactionwith another radical or with polymer fragments. On the otherhand with the exchange method iron is present within the poly-mer membrane and so a smaller amount of iron could potentiallycause the same amount of degradation in the polymer with thismethod. The important item to note is that weight loss with bothmethods produced similar results over the same period of timewith the procedures used.

Control samples, summarized in Table 1, established thatthe observed accelerated degradation was due to the combina-tion of iron and peroxide. Nafion� samples were exposed towater (Control 1) and a water/iron solution (Control 3). Boththese solutions showed little weight loss over the testingtime of 120 h though the sample in the water/iron solutionlost almost 5% of its original weight. A Nafion� samplewas exposed to 30% peroxide/water solution (Control 2)(without any Fe2þ) and showed no significant weight loss after80 h. Finally, Teflon films (Control 4) were also exposed toa 16 ppm Fenton’s solution to establish that the Teflon prongswere stable under the test conditions. The results showed thatthere was no significant change in weight of the Teflon films.

Fluoride ion release was measured with every solutionchange over the course of the experiments and the cumulativefluoride loss is shown in Fig. 3. Fluoride release is considered

Table 1

Comparison of weight loss of Nafion� control samples, Teflon, and degraded

Nafion�

Control 1 Control 2 Control 3 Control 4

Nafion� in

water

(120 h)

Nafion�þ 30%

HOOH

(96 h)

Nafion�þH2Oþ Fe2þ

(120 h)

TeflonþFenton’s

(96 h)

% Weight loss 2.82 1.18 5.39 �0.13

Cumulative fluoride

release (mmol)

Negligible 0.87 Negligible 0.06

to be an indicator of membrane degradation at the molecularlevel. It would be expected that for similar weight losses a similaramount of fluoride would be released. However as can be seen inFig. 3 the solution method of degradation lost more fluoride ionscumulatively than the exchange method of degradation. Onepossible explanation is that small polymer fragments make uppart of the mass leaving the polymer films and entering intothe surrounding solution. In the case of the solution methodthese polymer fragments can continue degrading since ironions and peroxide exist in the surrounding solution at high con-centrations. This is not the case with the exchange method sinceall the iron is bound in the polymer. The samples degraded by thesolution method for 120 h had a cumulative fluoride ion mea-surement of 100 mmol or 1.5% of the available fluorine in thepolymer structure. Samples degraded by the solution methodmeasured 34 mmol of fluoride ions cumulatively or 0.5% ofthe available fluorine. As discussed, it is expected that more fluo-rine atoms were still bonded in chain fragments leaving themembrane. Fluoride release from each of the control sampleswas negligible (Table 1).

Ion exchange capacity (IEC) measurements and chemicalanalysis using electron dispersive spectroscopy (EDS) wereperformed on the degraded membranes in order to determineif the ratio of backbone to side chain groups had been changedupon degradation. The results showed no significant change inthe ion exchange capacity or in the atomic ratio of oxygen tofluorine (O/F ratio) over the exposure time for both the solu-tion and the exchange methods. The value for the IEC was1.1 mmol/gdry-membrane for the fresh membrane and the O/Fatomic ratio was 0.18. Similar behavior was observed for sul-fur/oxygen ratio and sulfur/fluorine ratio. Other report in theliterature [10,15] based on ion chromatography data suggestedthat the backbone components degrade to a larger extent thanthe sulfonic acid groups, thus indicating that IEC should in-crease with degradation. This difference is probably due tothe fact that if sulfur containing chain segments may not be de-tected by ion chromatography. The results presented here indi-cate that the polymer remaining in the membrane had a similaraverage chemical composition to the original fresh polymer.

80090010001100120013001400150016001700

Wavenumber (cm-1)

Nor

mal

ized

Abs

orba

nce

A Fresh Nafion™ 112B Degraded - Solution Method 120 h

A

B

S-O

C-FC-F

C

C Degraded - Exchange Method 84h

Fig. 5. FTIR spectra for fresh Nafion� membrane samples, a sample degraded

via the solution method, and a sample degraded via the exchange method.

219S. Kundu et al. / Polymer Degradation and Stability 93 (2008) 214e224

Although the ratio of oxygen atoms in the side chains tofluorine atoms in the backbone did not change, degradationcould have removed backbone and side chain segments inthe same proportion. Viscosity measurements were carriedout to determine if the average molecular weight of the poly-mer membranes were changing. Samples of fresh Nafion�112, degraded sample from the exchange method experiment(84 h of degradation) and from the solution method (96 h ofdegradation) were dissolved in 50/50 by weight mixtures ofethanol and water. These samples were chosen because of theirhigh degree of degradation and similar weight loss. All sam-ples were in the hydrogen form and reduced viscosity was cal-culated by Eq. (1). Plots of reduced viscosity against theconcentration of Nafion� 112 are shown in Fig. 4. The plotsshow an almost exponential increase in reduced viscosity witha reduction in polymer concentration. This is a typical behav-ior of polyelectrolytes [18] and has been shown to be presentin sulfonated electrolytes, including Nafion�, by several au-thors [19,20]. The curves for fresh samples lie above thosefor samples degraded by the solution method (96 h). Thereare two possible explanations for this difference between freshand aged samples. One possibility is that the structure of therepeating units making up the polymer chains has changedand the other possibility is that the polymer molecular weighthas decreased, leading to changes in the size of aggregateswhen in solution. It has already been shown by IEC resultsthat this structure, in so far as the ratio of side chain compo-nents to backbone, has not changed. On the other hand, a mod-erate decrease in average molecular weight has been shown toshift the viscosity curves down by Cohen and coworkers [21].A change in molecular weight is also consistent with the sug-gested mechanisms of Nafion� degradation found in the liter-ature [8]. Interestingly, the reduced viscosity curves ofsamples degraded by the solution method and the exchangemethod are very similar, indicating that their average molecu-lar weight is also similar even though the weight loss of thespecific samples in question was different (Fig. 2).

0

1

2

3

4

5

6

7

8

0 0.05 0.1 0.15 0.2 0.25

Nafion™ Concentration (g/dl)

Red

uced

Vis

cosi

ty (

dl/g

)

Fresh Nafion™ 112

Degraded - Solution Method (96h)

Degraded - Exchange Method (84h)

Fig. 4. Viscosity data for dissolved samples of fresh Nafion� 112 and samples

degraded by the solution and exchange methods. Samples were dissolved in

50/50 ethanol/water mixtures at 250 �C.

Evidence of chemical changes in the membranes was inves-tigated by FTIR. Typical curves of the fresh and degradedspectra are shown in Fig. 5. The main peaks of interest areat 1147 cm�1 and 1204 cm�1 which are attributed to CeFvibration [15,22] and at 1054 cm�1 which is due to SeOstretching [16,23]. The peak at 1147 cm�1 was used to nor-malize the curves for comparison purposes. The FTIR resultsof degraded membranes and control samples reinforce IECand EDAX findings which showed no significant change inthe relative numbers of side chains to backbone segments indegraded samples as compared to fresh Nafion�. FTIR anal-ysis of the membranes degraded in this experiment by bothmethods as well as the control samples could not detect anynew peaks which could be correlated with exposure time de-spite the high level of degradation. This was also observedby Tang and coworkers [14].

The chemical characterization of Nafion� 112 membranesdegraded by a Fenton’s solution indicates that the only signif-icant change to membranes degraded by OH radicals in thisway was the average molecular weight. Changes to chemicalstructure, such as changes to the ratio of side chain groupsto backbone groups as well as changes in the chemical bondspresent in the membranes, were not detected here. Neverthe-less, exposure to the Fenton’s reagent severely degraded themembranes as seen by the loss in weight and fluoride loss data.

3.2. Changes in morphology and dimensionalcharacteristics

Despite the weight loss and IEC trends being similarbetween the two degradation methods, scanning electronmicroscopy (SEM) analysis of the Nafion� samples showedsignificant differences in the morphology of the samples. Inthe case of samples degraded by the solution method, thesurface was roughened by the presence of defects. Uponcloser examination of the membrane the defects appear tobe bubbles formed within the membrane (Fig. 6a and c) aswell as tears and bumps on the membrane surface. The bub-bles were in the order of hundreds of microns in diameter

Fig. 6. Comparison of surface morphologies between Nafion� 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.

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

while the bumps and tears were less than 10 mm. Overall, thenumber of defects increased with the time of exposure todegradation for both methods. The progression of degrada-tion for the solution method is shown in Fig. 7c,e,g. Themembrane splits into two halves after 24 h of exposure andafter long time samples degraded by the solution method de-veloped large pits within the two halves of the polymermembrane (Fig. 8). With exception of the pit which occursafter long exposure times, close examination of theseparate halves of the membrane shows that their morphol-ogy resembles that of the fresh membrane with no extradistinguishing features. The samples themselves were trans-lucent in appearance.

Samples degraded with the exchange method show fewerlarge bubbles on the surface as compared to the solutionmethod even after 84 h of exposure. The surfaces have smallerdefects and fewer holes penetrating the membrane as shown inFig. 6b and d. Overall, membranes degraded by this methodwere far smoother to the touch than the solution method sam-ples due to smaller number of large bubbles and tears. Cross-sections of samples degraded by the exchange method showa markedly different morphology than those degraded by thesolution method. The progression of degradation of samplesdegraded by the exchange method is shown in Fig. 7b,d,f.The surface of the membrane has the appearance of foamwith the centre of the membrane essentially remaining asone continuous phase. This foam appearance was not observedwith solution method samples even after long time. Controlsamples showed no change in morphology with time com-pared with new samples. The appearance of the membraneswas 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 inhow reactive species diffuse into the membrane as a resultof the differences in the experimental solutions.

A reduction in thickness is a common observation inNafion� samples degraded in fuel cells and is usually attrib-uted to chemical degradation [11]. A dimensional analysis ofthe membranes was performed to determine the effect of deg-radation by Fenton’s reagent on the sample’s shape and size.Fresh samples measured with calipers and SEM had thick-nesses between 0.04 mm and 0.06 mm. The SEM was usedto measure membrane thicknesses on aged samples sincecaliper measurements were affected by the rough surface fea-tures. Thickness measurements of solution method samplesexcluded the empty space created by bubbles and focused onthe combined thickness of the two halves of the membranes.For the exchange method, the degraded membranes had welldefined edges for measurement encompassing both the highlydegraded ‘‘foamed’’ edges and the less degraded centre. Suchresults showed that neither method of degradation produceda significant change in membrane thickness.

Nafion� samples preferentially swell in one planar direc-tion versus the other. Here, the sample width will be the direc-tion characterized by a high degree of swelling and the samplelength will be the direction characterized by less swelling.These dimensions changed significantly with degradation.With exposure using both methods the swollen width in-creased by 10e15% while at the same time the swollen lengthdecreased by 3e5% as shown in Fig. 9 after long exposuretimes. Changes in swelling in aged samples were muchmore significant than changes observed in control samples

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

solution method for 24 h, 48 h, and 120 h, respectively, 500� magnification.

221S. Kundu et al. / Polymer Degradation and Stability 93 (2008) 214e224

though Nafion� exposed to hydrogen peroxide did have anincrease in swollen width of almost 5% as shown in Table 2.Dry dimensions followed a similar trend. Additionally, agedsamples tended to swell more than fresh samples.

As the membrane degrades the total molecular weight ofthe polymer chains decreased as shown by viscosity measure-ments. This facilitates the unrestricted movement of thechains. The swelling behavior already shows that when givengreater mobility, through hydration, the entangled polymer

chains will preferentially relax in one direction. Thus, as thechains degrade the membrane will tend to gain length in onedirection over another.

3.3. Effect of degradation on mechanical propertiesand barrier properties

Degradation imparts many physical changes on membranemorphology and dimensions, hence it is expected that

Fig. 8. Nafion� membrane split by a bubble after degradation by the solution

method for 108 h.

Table 2

Comparison of dimensional changes in control samples

Sample Control 1 Control 2 Control 3 Control 4

Nafion� in

water

(120 h)

Nafion�þ30% HOOH

(96 h)

Nafion�þH2Oþ Fe2þ

(120 h)

TeflonþFenton’s

(96 h)

% Change in width 2.78 4.60 2.61 �2.50

% Change in length �1.11 �0.32 �2.10 �0.96

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

mechanical and barrier properties will also be affected. Themechanical properties were investigated by tensile stressestrain curves and by temperature scanning in dynamic me-chanical measurements. The barrier properties were studiedby measuring the crossover flow.

The Young’s modulus for both degradation methods wasobserved to decrease by 10e20% in relation to fresh samples(Fig. 10). This is consistent with a reduction in average molec-ular weight shown by the viscosity measurements. The shorterchains resulting from OH radical degradation also result in lesschain entanglements. This increases the mobility of the chainswhen stressed and therefore a lower Young’s modulus.

Transition temperatures were measured using the loss tan-gent (tan d) in the dynamic mechanical thermal analysis(DMTA). Typical curves for fresh and degraded Nafion� sam-ples are shown in Fig. 11. A loss tangent peak occurred be-tween 120 �C and 130 �C in all cases which is typical for

-20-15-10-505

101520

0 20 40 60 80 100 120 140

Time (h)

Cha

nge

Swollen WidthSwollen Length

-20-15-10-505

101520

Cha

nge

Swollen WidthSwollen Length

a

b

Fig. 9. Changes in length and width for samples degraded by the solution

method (a) and the exchange method (b).

this material [24] and is known as the a-transition. This tran-sition is associated with increased mobility of the ionic do-main, which tends to agglomerate into clusters within thepolymer [25]. The storage modulus (E0) of degraded sampleswas lower than fresh samples (Fig. 11) followed similar trendsas the Young’s modulus. Again, this is expected of polymerswith decreased molecular weights.

Fig. 11 also shows that the loss tangent curves of degradedsamples are also lower than the fresh samples. Furthermore,degraded samples showed an increase in a-transition tempera-ture with samples degraded by the solution method showingthe largest increase (Fig. 12).

Within a fuel cell, Nafion� electrolyte membranes are notonly responsible for transporting protons across the membranebut are also responsible for acting as a gas barrier between thetwo sides. Failure of the membrane as a gas barrier can resultin lower and unstable voltages or complete failure of the fuelcell. The gas barrier properties of the membranes degraded inthis work were evaluated as a function of time and are shownin Fig. 13. Fresh samples exhibited nitrogen flow less than0.0002 mL s�1. Samples degraded with the solution methodhad nitrogen crossover rates as high as 0.27 mL s�1. The sam-ples degraded by the exchange method all had negligiblecrossover rates. The reason for this extends from the morphol-ogy of the degraded samples, those degraded by the solutionmethod had many holes penetrating to the centre of the mem-brane facilitating crossover. In contrast, the membranesdegraded by the exchange method did not have these featuresand therefore could continue to act as a gas barrier.

60

70

80

90

100

110

120

130

140

150

160

0 20 40 60 80 100 120 140

Time (h)

Mod

ulus

(M

Pa)

Degraded - Solution Method

Degraded - Exchange

Fig. 10. Young’s modulus results for samples degraded by the solution and

exchange methods.

0

20

40

60

80

100

120

50 60 70 80 90 100 110 120 130 140 150

Temperature (oC)

E' (

MP

a)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Tan (

)

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

Fig. 11. Storage modulus (E0) and loss tangent (tan d) of fresh Nafion�, sam-

ple degraded by the solution method for 120 h, and sample degraded by the

exchange method for 84 h.

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0 20 40 60 80 100 120 140

Time (h)

Nit

roge

n C

ross

over

Flo

w (

ml s

-1)

Fig. 13. Crossover results for membranes degraded by the solution method.

Samples degraded by the exchange method showed negligible crossover.

223S. Kundu et al. / Polymer Degradation and Stability 93 (2008) 214e224

4. Conclusions

This work has presented a comparison of Nafion� mem-branes degraded by two ex situ accelerated degradationmethods promoting degradation by OH radical attack usinga Fenton’s type test. The two methods of Fenton’s testingthat were compared were a solution method which comprisesintroducing Nafion� membrane to a solution of peroxide andiron and an exchange method where a sample of Nafion� inthe Fe2þ form was exposed to a peroxide solution. It wasfound that the two degradation methods significantly degradedthe membrane over days of exposure time with samples loos-ing over 20% of their original weight over the testing time.The weight loss was accompanied with fluoride release andno change in the ratio of side chains to backbone. Viscositymeasurements of dissolved membranes suggest that the aver-age molecular weight decreased with degradation. Further-more, FTIR analysis also indicated that both methods ofdegradation did not produce any new chemical groups onthe membranes.

121

122

123

124

125

126

127

128

0 20 40 60 80 100 120

Time (h)

-tra

nsit

ion

tem

pera

ture

(o C

)

Solution MethodExchange Method

140

Fig. 12. a-Transition temperatures for membranes degraded by the solution

and exchange methods.

Despite there being no significant chemical differencesbetween the two degradation methods, differences in mor-phology of the degraded samples were observed. Analysisof the morphology showed that membranes degraded by thesolution method had many holes, tears, and large bubbleson the surface and cross-sections revealed that the bubblesoriginated at the centre of the membrane, splitting it intotwo. Membranes degraded by the exchange method did notsplit into two but instead areas close to the surface appeared‘foamy’. The thickness of the membranes did not change ap-preciably in both cases though with degradation, the swollenwidth increased while the membrane shrank slightly inlength.

Finally, mechanical and gas barrier properties were exam-ined for the degraded membranes. In both degradationmethods the modulus decreased with exposure time whichwas attributed to the reduction in molecular weight of thepolymer chains. This was also observed in dynamic mechani-cal results of the storage modulus. The peak height of the losstangent curve in dynamic mechanical experiments was lowerwith increased degradation. This may indicate that ionic clus-ters in the membrane, made up of sulfonic acid groups, shrankas the polymer was degraded. The transition temperature how-ever increased with exposure to the Fenton’s reagent whichmay indicate that the smaller ionic clusters were also morestable.

Acknowledgements

The authors would like to acknowledge Ion Power Inc. forsupporting this study and the Natural Sciences and Engineer-ing Research Council (NSERC) of Canada for financialsupport.

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