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Polymer Degradation and Stability 97 (2012) 1010e1016

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Polymer Degradation and Stability

journal homepage: www.elsevier .com/locate/polydegstab

Application of TGA techniques to analyze the compositional and structuraldegradation of PEMFC MEAs

Hye-Jin Lee a,b, Min Kyung Cho a,b, Yoo Yeon Jo a, Kug-Seung Lee a, Hyung-Juhn Kim a, EunAe Cho a,Soo-Kil Kim c, Dirk Henkensmeier a, Tae-Hoon Lim a, Jong Hyun Jang a,*

a Fuel Cell Research Center, Korea Institute of Science and Technology (KIST), Hawolgok-dong, Seoul 136-791, Republic of Koreab School of Chemical and Biological Engineering, Seoul National University (SNU), Seoul 151-744, Republic of Koreac School of Integrative Engineering, Chung-Ang University, Seoul 156-756, Republic of Korea

a r t i c l e i n f o

Article history:Received 27 December 2011Received in revised form23 February 2012Accepted 9 March 2012Available online 19 March 2012

Keywords:Polymer electrolyte membrane fuel cells(PEMFC)Membrane Electrode assembly (MEA)DurabilityThermogravimetric analysis (TGA)

* Corresponding author. Tel.: þ82 958 5287; fax: þE-mail addresses: jhjang@kist.re.kr, jonghyun.jang

0141-3910/$ e see front matter � 2012 Elsevier Ltd.doi:10.1016/j.polymdegradstab.2012.03.016

a b s t r a c t

Thermogravimetric analysis (TGA) has been proposed as a new post-analysis tool of membrane electrodeassembly (MEA) for polymer electrolyte membrane fuel (PEMFC). Analysis of catalyst layer (CL) sampleswith various Nafion ionomer contents quantitatively confirmed that the Nafion ionomer decomposedfirst at around 320 �C and the support carbon at around 410 �C. For the degradation analysis of MEAs, theamount variation of components, including platinum (Pt) and attached gas diffusion layer (GDL), and thedegree of Pt agglomeration could be evaluated from weight changes and DTG peak shifts (the platinumactivation effect), respectively. For an MEA degraded by start-up/shut-down cycling, Pt agglomerationand significant GDL attachment, as well as invariant Pt amount and slight weight decrease of Nafion andsupporting carbon, could be analyzed by a single TGA measurement. Similar degradation analysis wasalso possible for stack-operated MEAs. This TGA analysis technique is expected to be effectively utilizedas a preliminary diagnosis tool in a routine characterization of degraded PEMFC MEAs.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Proton exchange membrane fuel cells (PEMFCs) have beenconsidered to be feasible power sources to replace conventionalfossil fuel-based technologies. Due to their advantages, such as highefficiency, low operating temperature, and low emission of green-house gases, PEMFCs can be used in various applications, such asautomobiles, stationary power generation, and portable electronicdevices. However, for commercialization, further development ofPEMFCs is required to increase their durability and reduce thematerial cost [1,2]. The durability targets for 2015 set by the U. S.Department of Energy (DOE) are 5000 h and 40,000 h for auto-motive systems and stationary applications, respectively [3]. Thelong-term durability of PEMFC single cell and stacks has beenevaluated under various operating conditions, including constantcurrent and power cycling [4e6], while the start-up/shut-downdurability has also been studied for vehicle applications [7e12]. Inaddition, the effects of contamination [13e15] and freeze/thawcycling [10,16,17] on PEMFC durability have been reported byvarious groups.

82 958 5199.@gmail.com (J.H. Jang).

All rights reserved.

During operation, the degradation of MEAs and stacks is moni-tored by various in-situ techniques. In general, electrochemicaltechniques, such as electrochemical impedance spectroscopy (EIS)[4e7,9,11,17e19], cyclic voltammetry (CV) [6,7,9,11e15,17,18,20e25],linear sweep voltammetry (LSV) [7,9,11,16,18,26], and galvanostaticanalysis [27] arewidely utilized for in-situ analysis. For example, thestructural changes in the catalyst layers (CLs) and physical degra-dation of the proton exchange membranes (PEMs) can be detectedby measuring the platinum (Pt) active surface area (by CV) andhydrogen crossover current (by LSV), respectively. In addition,physicochemical analyses, such as fluoride-ion emission rate (FER)[26,28e30] and gas chromatography (GC) [11], have been utilized tomonitor PEMFC degradation during cell operation.

After single-cell/stack operation, post-analyses are required toelucidate the detailed degradation mechanism of each componentin theMEAs and stacks. To study the PEMdegradation, the chemicaldecomposition can be identified by ion exchange capacity (IEC)[29e31] and infrared spectroscopy (IR) analysis [8,29,30], and thephysical deformation can be confirmed by scanning electronmicroscopy (SEM) [8,28,29]. In the case of CLs, the Pt particlegrowth can be determined by transmission electron microscopy(TEM) [6,8,9,12,24,25,32,33] and X-ray diffraction (XRD) [8,9,20,33],and the porous structure and thickness changes can be analyzed bySEM [6,8e10,14,18,33,34] and mercury porosimetry [16]. Also, the

Fig. 1. (a) TGA and (b) DTG curves of a decal MEA sample (Sample A) under airatmosphere (heating rate: 10 �C/min). The sample size was 1 cm2.

H.-J. Lee et al. / Polymer Degradation and Stability 97 (2012) 1010e1016 1011

Pt migration from the CL to the PEM has been detected by TEM andenergy dispersive spectrometer (EDS) analyses [9,25]. To developPEMFC MEAs and stacks, both in-situ analyses and post-analyseswill be required to fully understand the degradation behavior andenhance the long-term durability. For example, the combination ofin-situ analyses (polarization, CV, and CO2 analysis) and post-analyses (SEM, X-ray photoelectron spectroscopy (XPS) andinductively coupled plasma (ICP)) has demonstrated that corrosionof the cathode carbon and growth of Pt particles are primarilyresponsible for the substantial degradation that occurs duringstart-up/shut-down cycles [8,11].

In this study, thermogravimetric analysis (TGA) was used asa novel tool for post-analysis of PEMFC MEAs, and its usefulnesswas confirmed by analyzing MEA samples that had been degradedby start-up/shut-down cycling and a stack operation. Previously,TGA has been primarily used to evaluate the thermal stability of thepolymer membranes [30,35e37] and the carbon support [38]. Forexample, Baturina et al. demonstrated the occurrence of low-temperature carbon oxidation of a supported Pt catalyst and CLs,which was correlated to the thermal stability of the support carbonduring the operation of the PEMFC [38]. As a post-analysis tool, TGAis expected to provide the following information: (i) qualitativecomponent analysis utilizing different thermal stabilities, (ii)compositional analysis from the weight loss at different tempera-ture ranges, and (iii) characterization of microstructural changesfrom variations in the TGA curves and the peak temperatures in thedifferentiated curves. Because the sampling and analysis isstraightforward and less expensive than other techniques, TGAwillhave advantages for routine characterization, especially in industry.For example, TGA could be utilized to analyze various parts of MEAsin operated stacks, and further characterization using morecomplicated and expensive procedures, including electron probemicroanalysis (EPMA) and XRD, could subsequently be conductedonly on selected samples. In this way, the efficiency of the overalldiagnosis procedure will be enhanced, improving the cost- andtime-effectiveness in research and development.

2. Experimental

For TGA analysis, a Nafion ionomer sample was prepared bydrying a Nafion dispersion (Dupont Inc., D2021) at 60 �C ina vacuum oven. Additionally, a 1-cm2 Nafion membrane (DupontInc., NRE-212) was used as received. The catalyst/ionomer sampleswere prepared by mixing a carbon-supported platinum catalyst(Tanaka Kikinzoku Kogyo K.K., Pt 45.5 wt.%), a Nafion dispersion,isopropyl alcohol (Baker Analyzed HPLC Reagent), and triplydeionized water for 2 h and drying the mixtures in an oven toremove the solvents. The volumetric ratios of the Pt/C catalyst andthe Nafion ionomer were controlled to be 10:90 (N10), 30:70 (N30),and 50:50 (N50).

A homemade MEA sample was prepared using the decal-transfer method. First, the CLs with an ionomer content of 30 wt.%were coated on decal substrates (Kapton film, 51 mm) with a doctorblade and were then dried at room temperature under vacuum for24 h. Next, the CL-coated substrates were cut into 25-cm2 piecesand were placed on both sides of a Nafion membrane (Dupont Inc.,NRE-212) to form a substrate/CL/PEM/CL/substrate complex. Afterthe complex was hot-pressed at 140 �C for 4 min, the substrates onboth sides were removed to provide a decal MEA (Sample A).

For the degradation analysis by TGA, two MEA samples wereprepared from commercial MEAs: fresh one (Sample B1) anda degraded MEA by start-up/shut-down cycling (Sample B2). Theexperimental details and electrochemical characteristics werepreviously reported by Jo et al. [7]. As another example of degra-dation analysis, two samples were taken from different parts of

single stack-operated MEA (Sample C1 and Sample C2) and char-acterized by various techniques, including TGA. With an active areaof 10.24 cm2, the electrochemical characterizations were per-formed at a cell temperature of 65 �C. After the samples wereactivated for 20 h, polarization curves were measured with fullyhumidified hydrogen (anodes) and air (cathodes). The stoichio-metric ratios of hydrogen and air were 1.5 and 2.0, respectively. TheEIS was measured at a DC potential of 0.85 V over a frequency rangeof 10 kHze50 mHz (Zhaner, IM6). After electrochemical analysis,the GDLs were removed, and the remaining MEA samples were cutto a sample size of 1 cm2 for TGA. The cross-sectional microstruc-tures were analyzed with FESEM (Hitachi S-4100) and EPMA (JXA-8500F). XRD was performed using a Cu Ka (l ¼ 1.54056 �A) sourceover a 2q angular region of 15�e85� at a scan rate of 2�/min (RigakuATX-G thin-film diffractometer).

TGAwas performed on each sample under an air atmosphere ata temperature range of room temperature to 1000 �Cwith a heatingrate of 10 �C/min (Universal V4.2E TA Instruments, 2050 TGA).

3. Results and discussion

3.1. TGA analysis of a decal MEA sample

To represent a typical PEMFC MEA, a homemade MEA wasprepared using the decal-transfer method, and a sample of thedecal MEA (1 cm2) was analyzed by TGA under an air atmosphere(Fig. 1a). As the temperature increased to 200 �C, the sampleweightdecreased slightly with water evaporation; the weight of the dried

Fig. 2. (a) TGA and (b) DTG curves of Nafion membrane and Nafion ionomer samplesunder air atmosphere (heating rate: 10 �C/min).

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decal MEA sample was 11.36 mg at 200 �C. The MEA sample weightdecreased again starting at a temperature of approximately 300 �Cbecause of the thermal decomposition of its components. Attemperatures above 500 �C, the sampleweight was almost constant(0.53 mg at 600 �C). In the differential thermal gravimetry (DTG)curve, two peaks were identified at temperatures of 363 �C and432 �C, indicating that the weight loss by thermal decompositionoccurred in two steps (Fig. 1b). The weight losses corresponding tothe lower and higher temperature steps were determined to be2.43 mg and 8.40 mg, respectively, using the inflection point(373 �C) as the boundary temperature.

The decal MEA sample includes a Pt/C catalyst, a Nafion ion-omer, and a Nafion membrane. Assuming that the carbon supports,Nafion ionomer, and Nafion membrane were completely decom-posed at temperatures below 600 �C, the Pt weight could bedetermined to be 0.53 mg from the TGA data. At higher tempera-tures than 600 �C, small weight increase was observed probablydue to gradual Pt oxidation under air atmosphere.

Then, using the nominal Pt ratio in the Pt/C catalyst (45.5 wt.%)and the Nafion ionomer content in the CL (30 wt.%), the weights ofthe carbon support and Nafion ionomer could be calculated to be0.63 and 0.50 mg, respectively. By subtracting the total CL weight(1.66 mg) from the dried MEA weight, the PEM weight was esti-mated to be 9.70 mg, which was in reasonable agreement withthe product information for Nafion N212 (10 mg/cm2). Becausethe PEM weight (9.70 mg) is larger than either of the weightdecreases in the first and second thermal decompositions, itappears that the PEM was partially decomposed in the first step,and the remaining part was decomposed in the second, higher-temperature step.

The reproducibility of the TGA analysis was checked byanalyzing three samples from a single MEA. For the weightdecrease between 200 and 600 �C, the relative error was 2.0%. Inthe case of remaining weight at 600 �C, which corresponds to the Ptweight, the relative error was rather higher as 7.3%, due to the verysmall sample amount. Depending on the availability, the uncer-tainty would be decreased by controlling measurement conditions,including larger MEA samples.

3.2. TGA analysis of MEA components and DTG peak assignment

To assign the DTG peaks of the MEA samples, the TGA charac-teristics of the PEM and CL components were examined. First,a Nafion membrane sample (PEM) and Nafion ionomer wereanalyzed. The weight decrease of this PEM occurred in two steps,with DTG peak positions at 345 �C and 435 �C (Fig. 2). It has beenreported that the sulfonic acid groups decompose first at lowertemperatures (290e400 �C), and the side chains and main chainsdecompose at higher temperatures (400e560 �C) [39]. In the caseof Nafion ionomer, thermal decomposition temperature wasslightly lower, probably due to the different molecular weight [40].

Second, the Pt/C catalyst and Nafion ionomer mixture samples(CL) were analyzed as a function of the ionomer content. Thesamples were designated according to their ionomer content asN10, N30 and N50, where the number represents the Nafion ion-omer content relative to the total weight of the sample. As shown inFig. 3a, theweight loss by thermal decomposition increased and theresidue content decreased with an increase in the Nafion ionomercontent (and a decrease in the Pt content). In each sample, theweight loss occurred in two temperature regions, approximately320 �C and 410 �C. The DTG curves show that in samples witha higher ionomer content, the peak area in the lower temperatureregion (w1) was greater, but the peak area in the higher tempera-ture region (w2) was smaller (Fig. 3b), suggesting that the Nafionionomer decomposed at a lower temperature than did the carbon

supports in the Pt/C. This observation is consistent with theBaturina et al.’s report, where the evolved gases in TGA wereanalyzed by mass spectroscopy (TGAeMS) [38]. The mass spec-troscopy (MS) or Fourier transform infrared spectroscopy (FTIR), inconjunction with TGA, can be effectively utilized to study decom-position mechanisms and detect minor components. However, asquantitative analysis is not reliable with MS and FTIR, the simpleTGA analysis would be sufficient for most cases in routine MEAdegradation study.

For quantitative analysis, the DTG curves were fitted withmultiple Gaussian peaks, and the areas of the two DTG peaks weredetermined for each sample. When the 320 �C (w1) peak area wasplotted as a function of the ionomer content (Fig. 4a), a linearrelationship was observed. The slope in the linear regression was1.03 (r2 ¼ 0.988), indicating that the Nafion ionomers, when mixedwith Pt/C catalysts, are completely decomposed in the lowertemperature step at approximately 320 �C. This behavior demon-strates that the thermal decomposition of the Nafion ionomer wasgreatly enhanced by the Pt/C catalysts [36,38]. In the absence of Pt,the Nafion ionomer sample decomposed at 343 �C and 430 �C, ina similar manner as Nafion membranes (Fig. 2b) [35,39].

In Fig. 4b, the 410 �C (higher temperature) (w2) peak area andthe residue content at 600 �C (w3) were plotted as a function of thePt/C content. The peak area increased linearly with the Pt/C content,with a slope of 0.432 (r2 ¼ 0.920), confirming that the second DTGpeak is related to the thermal decomposition of the carbonsupports in the Pt/C catalyst. Also, the content of the residue (w3),which is mainly composed of Pt, was proportional to the Pt/C

Fig. 4. (a) Peak area of the lower temperature DTG peak as a function of the Nafionionomer content and (b) peak area of the higher temperature DTG peak and residueweight as a function of the Pt/C content.

Fig. 3. (a) TGA and (b) DTG curves of Pt/C catalyst and Nafion ionomer samples underair atmosphere (heating rate: 10 �C/min). The Nafion ionomer contents were varied at10, 30, and 50 wt.%.

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content, with a slope of 0.557 (r2 ¼ 0.998). The w3 slope of 0.557 ishigher than that expected based on the nominal Pt content of45.5 wt.% (product specification), demonstrating that the thermallystable components comprise 55.7 wt.% of the Pt/C catalyst. Thisexperimental result suggests a higher Pt content than specified orthe presence of a thermally stable impurity in the supportingcarbon.

In summary, these results demonstrated that (i) Nafion ion-omers and a minor portion of the Nafion membranes, includingsulfonic acid groups, are decomposed at the lower temperature(approximately 320 �C), (ii) the carbon supports and a majorportion of the Nafion membranes are decomposed at the highertemperature (approximately 410 �C), and (iii) the Pt and otherthermally stable parts remained as the residue content. With thisinformation, the TGA data of the decal MEA sample shown in Fig. 1were analyzed as follows. From the w3 (0.53 mg) in the TGA dataand the ash content of the Pt/C catalyst (55.7 wt.%), the weight ofthe catalyst was calculated to be 0.95 mg. The Nafion ionomerweight was 0.41 mg, with a CL ionomer content of 30 wt.%, and theNafion membrane weight was 10.00 mg. Therefore, at the lowertemperature (363 �C) step, the ionomer (0.41 mg) and part of theNafion membrane (2.02 mg) were thermally decomposed, anda weight decrease was observed. At the second step (432 �C), theremaining part of the Nafion membrane (7.98 mg) and the sup-porting carbon (0.42 mg) were burned, leaving Pt and thermallystable impurities.

3.3. TGA analysis of an MEA after start-up/shut-down cycling

The TGA analysiswas applied as an ex-situ technique to the start-up/shut-down durability test. It was reported by Jo et al. that thesingle-cell performance gradually decreased from 983 mA/cm2 to167 mA/cm2 (at 0.6 V) by 1200 unprotected start-up/shut-downcycles [7]. Various in-situ analyses were carried out during the celloperation, and significant degradation in the cathode catalysts wasdetected with the CV technique, with a decrease in the electro-chemical active surface area (EAS) from61.65m2/g to 4.92m2/g. ThePEM degradation was not severe, based on the LSV and EIS results.

TGA analysis was performed on the fresh MEA (Sample B1) andoperatedMEA (Sample B2) samples, as shown in Fig. 5. First, the ashcontent of Sample B2 (0.89 mg at 700 �C) was found to be similar tothat of Sample B1, demonstrating that the Pt loading did notsignificantly decrease during start-up/shut-down cycling, eventhough the EAS and cell performances decreased severely. Next, theattached GDL was detected for Sample B2 from theweight decreaseat temperatures higher than 600 �C, which appears as a small peakaround 630 �C in DTG plot. When interfacial stability is enhancedby degradation, GDLs cannot be completely removed from the CLduring TGA sample preparation, as observed by EPMA images. Also,it was experimentally confirmed that typical GDL samples werethermally decomposed in the temperature range of 600e700 �Cunder air atmosphere. The amount of attached GDL in Sample B2was estimated to be 0.15 mg, assuming a linear baseline at>600 �C.

Fig. 6. (a) Polarization curves and (b) Nyquist plots of impedance data at a DC potentialof 0.85 V for segmented MEAs for Sample C1 (closed circles) and Sample C2 (opencircles) of a stack-operated MEA (cell temperature: 65 �C). For the polarizationexperiments, hydrogen and air were supplied to the anodes and cathodes, respectively.

Fig. 5. (a) TGA and (b) DTG curves of a fresh commercial MEA sample (Sample B1, solidline) and a commercial MEA sample that was degraded by unprotected start-up/shut-down cycling (Sample B2, dashed line) under air atmosphere (heating rate: 10 �C/min).

H.-J. Lee et al. / Polymer Degradation and Stability 97 (2012) 1010e10161014

Third, the total weight of the thermally decomposable MEAcomponents (Nafion ionomer, Nafion membrane, and carbonsupport) was determined to be slightly larger in Sample B1(4.84 mg) than in Sample B2 (4.74 mg) from the weight decreasethat occurred between 200 �C and 600 �C. In the DTG data, the peakarea of the lower temperature region (320 �C) decreased by 0.31mg(corresponding to the Nafion ionomer and the less stable part of thePEM), while the peak area of the higher temperature region(approximately 410 �C) increased by 0.20 mg (corresponding to thecarbon support and the more stable part of PEM). It appears thatduring start-up/shut-down operation, the Pt activation effect onthe PEM thermal decomposition was weakened by Pt agglomera-tion, resulting in an increase in the DTG peak in the highertemperature region. Therefore, it was not possible to determinewhich component was responsible for the weight decrease of0.1 mg. In a typical PEMFC MEA, the 0.10 mg decrease wouldcorrespond to a 10% weight loss of the carbon support or the Nafionionomer in the CL. The third DTG peaks at 497 �C (Sample B1) and556 �C (Sample B2) appear to be related to the reinforcingcomponent in the PEM of commercial MEAs, and the peak area didnot change significantly (Sample B1: 0.74 mg; Sample B2: 0.75 mg).

Fourth, after unprotected operation, the TGA curveswere shiftedto higher temperatures. In the DTG curves, the first peak for theNafion ionomer and a minor portion of the PEM appeared ata similar position (320 �C) in Samples B1 and B2. However, thesecond peak, corresponding to the thermal decomposition of thecarbon support and PEM backbone, was shifted to a highertemperature (B1: 405 �C, and B2: 428 �C) and became broader. The

third set of peaks, which appear to be related to the additionalpolymer component in the PEM, also shifted from 497 �C to 556 �C.These peak shifts can be explained by the decreased activationeffect of platinum on thermal decomposition. When the carbonoxidation is enhanced by platinum and other metal catalysts withthe oxygen-transfer mechanism [41e43], the temperature decreasewill be dependent on the catalyst amount and available surfacearea. For example, it was reported that the ignition temperature ofcarbon was decreased from 430 �C to 390 �C when the Pt amountwas increased from 20 wt.% to 40 wt.% [38]. As the Pt amount wassimilar, it can be reasonably concluded from the temperature shiftsthat the Pt particles significantly agglomerated during start-up/shut-down cycling, which is consistent with the EAS decrease andTEM analysis results.

3.4. TGA analysis of a stack-operated MEA

One MEA was selected from an operated PEMFC stack, and itsdegradation distribution was analyzed by various in-situ and ex-situ techniques, including TGA. Two parts were cut from theselected MEA, and the local cell performances were evaluated. TheOCV values were similar between two samples (Sample C1: 0.914 Vand Sample C2: 0.923 V), but the decrease in cell voltage withincreasing current density was much larger for Sample C2, indi-cating severe local degradation at the region of Sample C2 (Fig. 6a).

Fig. 8. (a) TGA and (b) DTG curves of Sample C1 (solid line) and Sample C2 (dashedline) of a stack-operated MEA under air atmosphere (heating rate: 10 �C/min).

H.-J. Lee et al. / Polymer Degradation and Stability 97 (2012) 1010e1016 1015

The current densities at 0.6 V were 842 (Sample C1) and 115 mA/cm2 (Sample C2). With EIS, Sample C2 showed larger ohmic resis-tance (Sample C1: 0.05 U cm2 and Sample C2: 0.11 U cm2) andcharge transfer resistance (Sample C1: 3.58 U cm2 and Sample C2:14.13 U cm2) values compared to Sample C1, confirming nonuni-form MEA degradation during stack operation (Fig. 6b). CV analysisshowed that the EAS value of Sample C2 (6.10 m2/g) was muchsmaller than that of Sample C1 (17.88 m2/g), probably due to moresevere degradation in the cathodes, caused by mechanisms such asplatinum loss, platinum agglomeration, and Nafion ionomerdegradation. From the Pt (220) peak in the XRD patterns, the Ptcrystalline sizes were determined to be 7.0 nm (Sample C1) and9.3 nm (Sample C2), indicating more Pt agglomeration at the regionof Sample C2 (initial Pt size: 2.9 nm).

The cross-sectional SEM images demonstrated that the cathodelayer of Sample C2 was thinner and denser than that of Sample C1(Fig. 7). Severe degradation during stack operation could inducedensification of the cathode layers, possibly with a loss of cathodecomponents (Pt, supporting carbon, and Nafion ionomer).

As an additional ex-situ technique, TGA analysis was performedon Samples C1 and C2. Typical TGA characteristics were observed,as shown in Fig. 8, and the data were analyzed as follows. First, thePt weight was found to be similar between Samples C1 (0.94 mg)and C2 (0.99 mg) from the weight loss at 800 �C, even though thedegradation was more severe for Sample C2. Second, the weight ofthe attached GDL in Sample C2, which was observed in a SEMimage, was determined to be 0.88 mg. Third, the combined weightof the supporting carbon, the Nafion ionomer, and the PEM wasdetermined to be 5.14 mg (Sample C1) and 5.42 mg (Sample C2)from the difference between the dried sample weight at 200 �C(Sample C1: 6.08 mg; Sample C2: 7.30 mg) and the weight of Pt andthe GDL at 600 �C. This analysis result suggests that the weight losswas larger for Sample C1, but its influence on the local cellperformance was not critical.

Fourth, in the DTG data, the peak positions of Sample C2 were athigher temperature than those of Sample C1 because the Pt acti-vation effect was weaker for Sample C2. Because the Pt content wassimilar between Samples C1 and C2, as confirmed by the TGAtechniques, it can be concluded that the Pt agglomeration occurredmore severely at the region around Sample C2. In other words, the

Fig. 7. Cross-sectional SEM images of (a) Sample C1 and (b) Sample C2. The polymerelectrolyte membranes, cathode catalyst layers, and gas diffusion layers are indicatedas PEM, CCL, and GDL, respectively.

Pt particle growth could be evaluated from the peak shift in theDTG data.

As the Pt agglomeration and composition variation can beanalyzed in a single measurement, the TGA techniquewill be usefulfor a routine analysis of a large collection of degraded MEAs. Afterpreliminary diagnosis by TGA, detailed analyses for selectedsamples will be carried out focusing on the detected problem. Forexample, particle size can be quantitatively determined byconventional XRD and TEM techniques, while active Pt area can beelectrochemically evaluated by CV technique.

4. Conclusions

By TGA analysis of PEMFC MEA components, it was concludedthat (i) Nafion ionomers and a minor portion of the Nafionmembranes decomposed at the lower temperature (approximately320 �C), (ii) the carbon supports and a major portion of the Nafionmembranes decomposed at the higher temperature (approxi-mately 410 �C), and (iii) Pt and other thermally stable partsremained as the residue content. Based on this, degraded MEAscould be analyzed by the TGA technique to determine the variationinMEA components and to detect GDLs attached to CLs. In addition,Pt agglomeration during MEA degradation could be evaluated fromthe shifts of TGA curves (or DTG peaks) to higher temperatureswithdecreased platinum activation effect. As the compositional andstructural variations can be conveniently analyzed in a singlemeasurement, the TGA technique is expected to be very useful as anex-situ tool for preliminary evaluation of degraded PEMFC MEAs.

H.-J. Lee et al. / Polymer Degradation and Stability 97 (2012) 1010e10161016

Acknowledgments

This work was supported by the Joint Research Project fundedby the Korea Research Council of Fundamental Science and Tech-nology (KRCF), Republic of Korea, as a part of the “development andmechanism study of high performance and durable components forhigh-temperature PEMFCs.” This work was also supported bya grant (M2009010025) from the Fundamental R&D Program forCore Technology of Materials funded by the Korea GovernmentMinistry of Knowledge Economy.

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