Accelerated Life-time Tests including Different Load Cycling Protocols for High Temperature Polymer Electrolyte Membrane Fuel Cells

Electrochimica Acta 148 (2014) 15–25

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Accelerated Life-time Tests including Different Load Cycling Protocols for High Temperature Polymer Electrolyte Membrane Fuel Cells Yukwon Jeon a , So me Juon b , Hojung Hwang b , Jeongho Park b , Yong-gun Shul a,b, * a b

Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, South Korea Graduate program in New Energy and Battery Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, South Korea

A R T I C L E I N F O

A B S T R A C T

Article history: Received 22 July 2014 Received in revised form 15 September 2014 Accepted 8 October 2014 Available online 13 October 2014

The purpose of this work is to identify new accelerated life-time (ALT) test protocols for polymer electrolyte membrane fuel cell (PEMFC), which function at conditions of high temperature and low humidity (under 120
C 40%RH). A design of accelerated life protocols was studied to observe degradation phenomena under different load step cycling conditions, compared with the constant voltage test at 0.6 V. The effects of changes in frequency and potential range are explored, and different degradation rates are revealed. The test protocol of constant voltage test, load step cycling and with different potential range show total performance decay rates of 5.3 mAcm2h1, 8.4 mAcm2h1 and 10.2 mAcm2h1, respectively, while the highest total decay rate of load step cycling with frequency change is 17.0 mAcm2h1 including a rapid decrease of current density after 450 cycles under 120
C 40%RH. In respect to the operation of 500 cycles (35 h), material degradation failure mechanisms are investigated according to the electro- and physico-chemical characteristics of the MEA. The performances assessed by the life-time evaluation methods are strongly related to the increase in the membrane resistance and the release of the sulfate and fluoride ions, dominated more at a higher cycling frequency. Furthermore, in the contrast to the starting MEA, Pt aggregations of the catalysts and a decline in the electrochemical surface area (ECSA) are clearly observed at the end of the testing especially with a wider sweeping range, corresponded to the decrease in the electrochemical properties. ã 2014 Elsevier Ltd. All rights reserved.

Keywords: High temperature polymer electrolyte membrane fuel cell accelerated life-time test load cycling

1. Introduction Polymer electrolyte membrane fuel cells (PEMFCs) are promising candidates for transportation, stationary and portable electric power generations due to their simplicity, high energy density and easy recharging [1–3]. Nowadays, one of the challenges in the PEMFC is the operation under medium or high temperature (above 120
C) [4,5]. Increasing the temperature up to 120
C would improve the catalyst behavior, as it enhances the reaction kinetics, alleviates flooding by liquid water at the cathode electrode, and increases the tolerance to CO contained in the hydrogen from the reforming process or chloride, which may come from the water. Furthermore, high temperature operation will reduce the size of the thermal subsystem, and could potentially be co-generated using hot water ambient heat in the stationary system. This would simplify the architectural design and result in an overall increase of system efficiency, thereby advancing the commercialization of the

* Corresponding author. Tel.: +82-2-2123-2758, fax: +82-2-312-6401. E-mail address: [email protected] (Y.-g. Shul). http://dx.doi.org/10.1016/j.electacta.2014.10.025 0013-4686/ ã 2014 Elsevier Ltd. All rights reserved.

fuel cell system [6–9]. For these reasons, the department of Energy (DOE) funds multiple projects intended to modify the chemical structure of the high temperature membrane to enable high conductivity and durability for a future plan with a target of up to 120
C and relative humidity under 40% [10,11]. Therefore, the minimum operation condition of 120
C and relative humidity 40% was selected to design a tests for high temperature fuel cells. Among commercialization issues, such as performance, cost, reliability and long-term performance, for high temperature fuel cell, long-term durability is one of the most critical problems. Recently, DOE announced also the long-term durability target as >500 h for automobile applications, and as more than 20,000 h for stationary applications in order to represent the full range of external environment conditions [12–14]. Most existing studies on PEM fuel cells focus on the durability of degradation mechanism using different failure modes due to the additional demands. Recently, accelerated life-time test (ALT) spotlighted in the field of the degradation test because life-time tests under normal operation conditions are impractical [15–17]. ALT is a more preferred method, since it significantly reduces the experiment time and subsequent post-mortem analysis, while still providing

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critical information to expedite failures or degradation mechanisms by loading a stress severer than that of the normal operation conditions. There are various acceleration factors when examining the serious causes from different running atmosphere, which include (1) operation conditions such as temperature (-30120
C), gas flow, water management and humidity; and (2) operation models such as mechanical vibration, open circuit voltage (OCV) operation, load/potential (current/voltage/power) cycling, on-off cycling and freeze/thaw cycling [18–21]. This kinds of durability test methods open up discussions regarding pinholes, gas crossover, mechanical stability, polymer decomposition at the membrane and carbon support corrosion, Pt dissolution, agglomeration at the electrode chases in morphology, chemical degradation, and other degradation mechanisms via different electro- and physicochemical characterization of the fuel cell compounds [22–26]. In general, these degradations are exacerbated at elevated temperatures [4–9]. However, the lack of understanding of the degradation mechanism as well as the difficulty of performing made the condition of the operating at high temperature and low humidity requiring more effort. In this study, alternative PSFA membranes and ionomer with the tradename of AquivionTM from Solvay-Solexis was used as a reference since Nafion1 membrane has undeniable drawbacks including deterioration of water retention, proton conductivity and cell performance at high temperature and low humidified conditions [27,28]. Among these membranes, a short side chain perfluorosulfonic ionomer (SSC-PFSA, AquivionTM) shows high glass transition temperature of 127
C (long side chain ionomer: Nafion1 of 67
C at the same equivalent weights) in dry form [29]. Furthermore, larger crystallinity is observed in short side chain ionomers than in long side chain ionomers, resulting in more

reliable properties at a high temperature than Nafion1. It is proven in the reference that the AquivionTM membranes appeared to perform significantly better under high temperatures with lower ohmic loss resistance, hydrogen cross-over, and a better electro catalytic activity, which can be explained by the better properties at high temperature [29–31]. Furthermore, SSC-PFSA polymer have a good balance between transport properties and stability. The shorter side-pendent chains and the absence of the ether group of the tertiary carbon also gives better chemical and mechanical properties, making them more suitable for working at harsh conditions in fuel cell systems [32]. The other reasons for the better properties of the SSC-PFSA were not only due to the good chemical properties but also due to the effective water sorption polymer structure, especially at an elevated temperature, which was characterized by the motion of water within proton conducting membranes [33]. However, there are no systematic works regarding functional durability under the conditions of high temperature and low humidity using both SSC-PFSA membrane and ionomer. The purpose of this work is to provide valuable information regarding the PEMFC durability and membrane electrode assemblies (MEAs) degradation under 120
C 40%RH using new ALT test protocols operating at harsher conditions than the normal constant voltage test. For the high temperature operation, all MEAs of the single cells are based on AquivionTM to focus on only to the degradation by the different protocols. ALT protocols include load step cycling with different frequencies and voltage range to evaluate the degradation patterns of the MEAs. The performance drops during the ALT protocols were monitored while IV curve, electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were periodically

Fig. 1. Accelerated life-time test protocols in one cycle for single MEAs consisted of Aquivion E87-05 S under 120
C 40%RH: A. mode 1, B. mode 2, C. mode 3, D. mode 4.

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analyzed for 500 cycles. Post-test characterizations were conducted to observe the degradation of the electrode, membrane and interfaces between them. 2. Experimental 2.1. Preparation of membrane electrode assembly (MEA) AquivionTM E87–05S membrane (Solvay solexis, 50 mm thickness), AquivionTM ionomer solution (15 wt.% solution in lower aliphatic alcohol/H2O mix, EW = 870, Solvay Solexis) and Carbon supported Pt catalyst (Johnson Matthey, 40 wt.% Pt on carbon black) were used for the MEA preparation. Slurry containing 0.3 g of carbon supported Pt, 1.2 g de-ionized water, 0.8 g of AquivionTM ionomer solution and 3.6 g of isopropyl alcohol (Aldrich) were mixed and used for the catalyst ink. The catalyst slurries were mechanically stirred and ultrasonicated to allow homogeneous mixing of ionomer and Pt particles. Each step was repeated five times alternately. The catalyst inks were then sprayed onto the AquivionTM membrane using an airbrush gun with a catalyst loading of 0.4 mgcm2 at both sides, which have an active surface area of 1 cm2. Then, the catalyst-coated membranes were dried at 100
C for 1 h to remove residual solvent. The single fuel cell was prepared with the assembly of catalyst-coated membrane, gas diffusion media (SGL 10BC) and Teflon gasket, without hotpressing. 2.2. Measurement of single cell performance Several electrochemical experiments were carried out with a single PEMFC station (Fuel cell technology, USA). Pure hydrogen

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and oxygen were fed into the cell as a fuel and oxidation gas, respectively. The flow rates of H2 and O2 were 0.2 Lmin1 and 0.3 Lmin1, respectively, with a stoichiometry of 1/1.5 (H2/O2). The reactive feeding gases of hydrogen and oxygen were preheated to the same temperature as that of the water humidifier. Electrochemical experiments were performed under 120
C 40%RH without giving a back-pressure. Before the durability tests, activation step at constant current of 3 Acm2 for 1 hour and IV curve plotting was carried out three times. 2.3. Test Protocols (Constant voltage and Accelerated life-time tests) Various test protocols were conducted to investigate the influence of changing the conditions of the durability test, as shown in Fig. 1. For all ALT modes, the step potential was 0.05 V for each step with the total time for one cycle being 4.25 min. Mode 1 was a constant voltage test to be used as reference, and was performed at fixed operation condition of 0.6 V. Mode 2 was an accelerated life-time test (ALT) with a potential cycling at the range from 0.6 V to 1 V. In Mode 3, the range of the voltage sweep was 0.3 V to 1 V, whereas Mode 4 had the frequency with a voltage sweep of two times at a range of 0.6 V to 1 V for one cycle. Operation of 500 cycles, which is around 35 h, was performed for every ALT protocols, and IV curve, CV, LSV and EIS were measured every 100 cycles. To investigate reproducibility, we have tested each protocol at least 2 times and all the plots of the electrochemical tests includes error bars. The characterization data were analyzed by the materials from the second experiments of each mode. A KFM2030 (KIKUSUI) was used as electric load to determine polarization curves (current density vs. voltage), and PGSTAT-30 (Autolab) was used for an in situ EIS and CV. The EIS was

Fig. 2. Evaluation of current density profile versus operation cycles under test protocols for single MEAs consisted of Aquivion E87-05 S under 120
C 40%RH: A. mode 1, B. mode 2, C. mode 3, D. mode 4.

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performed at 0.6 V to examine the membrane/electrode resistances at an amplitude of 10 mV and a frequency range of 100 MHz– 10 kHz. To analyze the catalyst layer, the potential of the CV was scanned in the range of 0 to 1.35 V with a scan rate of 50 mVs1, whereas the LSV experiment was performed to determine a hydrogen crossover current with a sweep rate of 4 mVs1 and potential range of 0.0 to 0.5 V [19]. 2.4. Post-test characterization Five different measurements were attempted for the characterization of the physical properties. The morphological changes of the membrane were examined by a field emission scanning electron microscope (FE-SEM, JSM-6701F, JEOL) operated at 15 kV after sputtering the samples with platinum at 10 mA for 100 s. Ion exchange capacity (IEC) of the membranes was determined by acid-base titration. Dried polymer was stirred in 1 M of sodium chloride (NaCl, >99.5%, sigma-Aldrich) solution for 24 hours in order to release the H+ ions. Then, the mixture was titrated with 0.1 N of sodium hydroxide solution by Phenolphthalein (0.1%C20H14O4 solution, DUKSAN) which was used as a pH indicator. IEC of the sample was calculated by measuring the consumed amount of sodium hydroxide solution. To investigate the membrane and ionomer degradation, the fluoride and sulfate contents of the drain water from the anode and cathode were analyzed by using the Ion Chromatograph (Dionex ICS-2000) with a conductivity detector. AS19 Analytical column was applied to separate the fluoride ions from other anions and NaOH was used as an eluent. The morphological characteristics of the catalysts before and after the test modes were observed using a transmission electron microscope (HR-TEM, JEOL JEM-2100F). X-ray diffraction analysis (XRD) was recorded by Rigaku Miniflex AD11605 with X-ray diffractometer using Cu-Ka radiation (l = 1.5405 nm) in order to study the particle size of the electrocatalysts. The tube current was 30 mA with tube voltage of 30 kV. The 2-theta angular regions between 25
to 85
were explored at a scan rate of 5
min1. 3. Result and discussion 3.1. Electrochemical performance under different protocols As a key in this study, the long-term tests of the AquivionTM– based MEA have been performed at different protocols under high temperature and low humidity of 120
C 40%RH. Fig. 2A–D presents the evaluation monitoring versus the operating cycles under the protocols for modes 1, 2, 3 and 4, respectively. We noticed that the current density significantly decreased with time at all designed ALT protocols due to the severe operation conditions of high temperature with low humidity and different

stresses for 500 cycles, which indicates that the 2, 3 and 4 test modes can be considered as usable accelerated life-time tests. The biggest difference between constant voltage mode and potential sweeping mode is the declining trend of the current density at 0.6 V. The current densities, including load steps, for mode 2, 3 and 4 deteriorated rapidly for the first 100–200 cycles, and waned gradually after 200 cycles, while a gradual decrease for constant voltage mode was observed. It can be explained that a high impact of the voltage sweep accelerated the degradation at the first few cycles with it being most serious at mode 4. Interestingly, enormous performance decay after the test mode 4 was observed at the point of 450 cycles (32 h). This abrupt change in current density indicates the failure of fuel cell components due to the numbers of load cycles at 120
C 40%RH, likely membrane/catalyst and its interface degradation that will be discussed in further detail. Fig. 3A shows the evaluation of the polarization curves before initial and after the durability tests. The initial current densities of the single cells were all around 620 mAcm2 at 0.6 V. When the accelerated tests were conducted via each mode for 500 cycles, the single cell performances after the test mode 1, 2, 3 and 4 were 438 mAcm2, 325 mAcm2, 259 mAcm2 and 23 mAcm2, respectively at 0.6 V. The total decay rate can be plotted as linear line of the current density versus time, and the values are listed in Table 1. The test protocol of mode 1, 2, and 3 showed total performance decay rates of 5.3 mAcm2h1, 8.4 mAcm2h1 and 10.2 mAcm2h1, respectively, while the highest total decay rate of mode 4 was 17.0 mAcm2h1 including a rapid decrease of current density after 450 cycles under 120
C 40%RH. This performance drop can also be analyzed from data of the current density at 0.6 V and the maximum power densities, which were periodically measured after every 100 cycles for 500 cycles to investigate the performance-declining trend for each durability modes as shown in Fig. 3B–C. Fig. 3B–D shows the degradation tendency of the current density, power density and OCV for each 100 cycles including an error range from testing each protocols at least 2 times. As previously mentioned, the initial performance drop is revealed during the first 100–200 cycles particularly for the ALT protocols with much higher decay rate of that sections in Table 1. It can be seen that degradation behavior of the current density (Fig. 3B) and maximum power density (Fig. 3C) was different during each 100 cycles due to the change in the integrated power generation in each mode. However, compared with the different stresses of the wider voltage range and higher frequency, the trend in the current density drop until 400 cycles was similar to the maximum power density decreases that indicate both strains have a similar impact to the MEA for each different test cycling mode. After 400 cycles, however, the current density after the higher frequency ALT

Table 1 Performance monitoring of the developed durability protocols for single MEAs consisted of Aquivion E87-05 S for 35 h under 120
C 40%RH. Test

Mode 1 Mode 2 Mode 3 Mode 4

Hydrogen crossover current mAcm2

Initial Current density at 0.6 V mAcm2

Final Current density at 0.6 V mAcm2

Total Decay rate mAcm2h

Decay rates after each 100 cycles mAcm2h

0

624

438

5.31

0– 100 7.12

0

623

375

7.08

0

620

259

8

621

23

100–200

200–300

300–400

400–500

9.47

4.40

4.82

0.57

21.07

5.76

2.46

3.97

9.01

10.20

33.69

9.43

2.45

2.84

2.80

17.02

29.08

18.44

2.41

1.28

34.75

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Fig. 3. Polarization (I-V) curves for single MEAs consisted of Aquivion E87-05 S before and after each cycles of the test protocols under 120
C 40%RH.

suddenly decreased and the value of the performance decay rate was high, which shows an agreement with the decrease observed after 450 cycles as shown in Fig. 2D. As a result from Fig. 3 and Table 1, the speed of performance decay is much faster at high potential sweep frequency than at the constant voltage of 0.6 V, and even higher than that of the ALT with wider range of the voltage sweep, which means the protocol of mode 4 has a huge influence to the MEA degradation. This behavior is also related to the OCV drop after 500 cycles (35 h) under 120
C 40%RH as shown in Fig. 3D. The OCVs decreased from 0.94 V to 0.93, 0.93 and 0.91 V in mode 1, 2 and 3, respectively, while the OCV after test mode 4 was impressively decreased to 0.89 V. The relatively large OCV drop after mode 4 with huge impacts by the higher frequency may be due to the increased rate of H2O2, OH, and OOH generation, which accelerates membrane degradation including material loss from thinning of the membrane [27–30]. This possibility is related to the crossover of the reactants, which can be promoted at low humidity and high temperature [4]. To quantify the hydrogen crossover through the membranes, linear sweep voltammograms (LSV) experiments were performed to investigate hydrogen crossover current for all the MEAs after the durability test modes (Table 1). Hydrogen that crosses over to the cathode is oxidized, and the measured current corresponds to the oxidation of the hydrogen molecules [19,20]. In general, PEMs deliver a small crossover of hydrogen gas from the anode to the cathode, however if hydrogen crossover rate increases dramatically, macroscopic pinholes might form in the membrane [4]. Interestingly, the MEA after the test mode 4 shows a hydrogen

crossover current of 8 mAcm2 under 120
C 40%RH, whereas almost no hydrogen crossover was revealed after the other durability protocols, which is in an accordance with the OCV drops. It is worth noting that the declining tendency of the membrane properties was highly linked to the frequency amount of the stresses. The proton transport and charge-transfer resistance of electrode/membrane were also characterized using electro-chemical impedance spectroscopy (EIS) analysis to study the mechanism for the rapid decay of the performance. Fig. 4A shows the impedance spectra of the MEA fabricated with A8705S membrane and ionomer after each durability testing modes. The Nyquist plots had semi circles with ohmic resistance as the membrane resistance at the x-intercept point in a high frequency, and charge transfer resistance at the x-intercept point in low frequency, which was generally taken as the interface resistance of the electrode and membrane [17–19]. The ohmic resistance of the fresh MEA was 0.07 Vcm2, and a slight increase to 0.12 Vcm2 was observed after applying a constant voltage of 0.6 V. On the contrary, relatively high rises were observed after the ALT protocols with values of 0.22 Vcm2, 0.26 Vcm2 and 0.39 Vcm2 for mode 2, 3 and 4, respectively. From resistances for each cycle in Fig. 4B, we see a gradual increase of the membrane resistance during the first 100 cycles particularly for mode 3 and 4, whereas the membrane resistance increases after 300 cycles for the constant voltage mode. This may be consistent with the loss in the materials of the membranes resulting in the decrease of the membrane conductivity along with degradation in the performances. Another reason for

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Fig. 4. Electrochemical Impedance spectra for single MEAs consisted of Aquivion E87-05 S before and after each cycles of the test protocols under 120
C 40%RH.

the sharp increase after mode 4 in the ohmic resistance could also be associated with the formation of the hydrogen cross over that lead to the failure of the membrane in the localized region. Another behavior is envisaged from the increase of the polarization resistances, which generally confirms the cathode reaction kinetics in the MEA. With increasing stress such as wider range of the voltage sweep and more frequent voltage sweeping, the diameter of the loop in Fig. 4A significantly increased and moved to the positive direction, reflecting an increase in the charge transfer resistance within the interface of the membrane and catalyst layer. As shown in Fig. 4C for each cycles, the main difference of the resistances drop for EIS plots between mode 3 with wider sweep range and the other modes was much more distinctive from the first 100 cycles to the 400 cycles, which means a higher MEA degradation at the electrode and interfaces. However, the rapid drop after 500 cycles of the ALT results with the higher frequency is correlated to the numerous performance drop and polarization resistance increase that means the membrane degradation also affect the whole MEA structure. Fig. 5 shows the ECSAs (electrochemical active surface area) of the cathode after the durability tests from the cyclic voltammetry (CV) studies of Pt/C catalysts. The reduced ECSAs were calculated from the hydrogen adsorption charge on the smooth Pt surface by integration of CV profile after the double layer capacitance correction. We applied the following equation to obtain the ECSA (m2/g) of the Pt/C electrocatalyst on the cathode side of the MEAs [3,36]:

ECSA ¼

100  Q H GL

(1)

where, G is the electrical charge related with monolayer adsorption of the hydrogen on the Pt surface (generally 21 mC/ cm2), L is the Pt mass in the cathode (mg), and QH is the electrooxidation charge of the adsorbed hydrogen on the Pt surface integrated from the hydrogen adsorption peak. A significant

Fig. 5. Electrochemical active surface area (ECSA) for single MEAs consisted of Aquivion E87-05 S after each cycles of the test protocols under 120
C 40%RH.

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decrease in the ECSAs of the MEAs after the ALTs was observed in test modes 1 through 4 from the initial values of 20.49 m2g1 to 13.74 m2g1, 11.09 m2g1, 6.25 m2g1 and 2.78 m2g1, respectively. We can predict that a changes in the electrode micro structure due to the hydrophobic domain of the ionomer came into contact with catalyst surface rather than the hydrophilic groups, which reducing the triple boundary phase at high temperature and low hydration of the ionomer [11]. Even though an especially huge drop of 86.4% in the ECSA after the durability test mode 4 due to the influence of the hydrogen cross over after 32 h by the membrane degradation, the ALT with a wider potential range (mode 3) shows a higher dropping rate during the whole 500 cycles. Carbon corrosion could be an important reason of the ECSA drop. It may occur if the cathode is held at wide potential range with relatively high oxidation potentials due to the generation of oxygen atoms at the catalyst. At high temperatures, these oxygen atoms may react with the carbon substrate and/or water to generate the production of CO and CO2 [4]. Moreover, an irreversible deterioration of the cathode carbon corrosion is sometimes occurred during repeated load cycling especially with a wide potential range is presumably,

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which may generate potentials that favor peroxide formation closely related to the Pt dissolution to the catalyst aggregation. These two possibilities reduce the triple boundary phase with a loss in carbon and low hydration of the ionomer along with a decrease in the catalytic reaction [26,34,35]. This demonstrates that the ALT reduces the ECSA even more at wider range of voltage sweep (mode 3). Thus, electrode degradation is revealed to be another dominant reason for the drops in performance and durability for ALT protocols at high temperature and low humidity. All these electrochemical results may be due to the fact that running at severe operation conditions influences the inner structures of the MEA such as morphology of the membrane, catalyst and interface of the membrane/catalyst layer. The evidences for each degradation trends will be discussed in more detail. 3.2. Membrane and ionomer degradation To have a better understanding of the degradation of the polymer electrolyte and ionomer, FE-SEM, IEC, and IC analysis of

Fig. 6. FE-SEM images of cross sections and surface morphologies of the electrolyte membranes after the test protocols under 120
C 40%RH: A. fresh MEA, B. after mode 1, C. after mode 2, D. after mode 3, E. after mode 4.

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each MEA were carried out. In Fig. 6, the cross-sections for the thicknesses of each membrane are shown to compare the morphological change due to the direct impact to the membrane from different degradation protocols. In this experiment, the constant voltage of 0.6 V, which is relatively not that high of a voltage was used as reference, and shows minimal degradation in the membrane thickness with a value of 50.1 mm as reported in Table 2. However, the MEAs after the ALTs show small amount of thinning from bar thinkness of 53.0 mm, to 48.7 mm (mode 2), 48.1 mm (mode 3) and 46.0 mm (mode 4), respectively. The thinning of the membranes confirms a loss in the mass by the degradation of the polymer membrane which could be a clue for the large performance drop during the load cycling tests. According to this, a loss in the IEC (Ion Exchange Capacity) for all tested MEAs, especially for the designed ALTs, can be naturally revealed as listed in Table 2. These results are closely related to the ionic conductivity of the electrolyte membrane along with the OCV drops trend confirmed from the previous data. This result is also an agreement with the magnification image of the SEM (Fig. 6) in which pinholes were observed thoroughly in the membrane cross section surface. The wide-ranging morphological relaxation happens above the glass transition temperature (Tg) of a polymer, which may have a contrary effect on properties of the membrane. From this reason, it has been previously studied from our group that the A8705S membrane shows better stability than the Nafion 212 membrane due to its higher Tg but still shows mechanical and chemical degradation at the high temperature [32]. These observations of pinholes and thinning of the membrane is generally by the loss of mechanical attributes when dehydrated, resulting in shrinking, and cracking at a high temperature and low humidity [4–6]. This accelerates gas crossover with large consequences to the decrease in the cell performances. Because of the gas crossover, H2 and O2 may combine exothermically on Pt catalyst causing local hot spots that lead to larger pinholes like in the membrane after the durability test mode 4, where a destructive cycle can be initiated with increased crossover by membrane degradation. More detail information of the membrane as well as the ionomer chemical degradation can be investigated through the fluoride and sulfate release rates from the IC (Ionic Chromatography) results in Fig. 7. The fluoride and sulfate release rate at the cathode was higher than at the anode for all MEAs after the test since the ionic groups of the sulfonic and fluoride ions are firstly decomposed and transported across the damaged membrane with water from anode to cathode during the electro-osmotic process, especially, at the fuel cell operation at high temperature atmosphere [6]. The suggested cause to the formation of the impurities in the drain water is the decomposition of membrane and ionomer due to radical attacks. Both ions at the end group sites are susceptible to the radical attack by HOO and HO, which deteriorate the membrane particularly during the ALT tests and even faster for mode 4 [5]. This result could give an evidence that the amount of the high potential sweeping environment during

Fig. 7. Change in A. sulfate and B. fluoride release rate after the test protocols under 120
C 40%RH. Drained water from the anode and cathode of the single MEAs consisted of Aquivion E87-05 S was analyzed by IC: A. after mode 1, B. after mode 2, C after mode 3, D. after mode 4.

load steps at ALTs also cause a degradation of the side chain breakdown which can be correlated with the IEC decrease (Table 2). Furthermore, in the operation condition of high temperature and low humidity, the above action could be improved, and the thinning of the membranes is closely related. However, from the conclusion of the impact of the ALT and ALT with wider potential range is more to the electrode degradation, another possibility of the ions release is the ionomer decomposition from Pt catalyst mainly at the cathode side. Thus, this characterization results seem to be an explanation for not only an electrolyte membrane degradation but also a significant degradation of the electrode at 120
C 40%RH.

Table 2 Post-characterization of the membranes and catalysts for single MEAs consisted of Aquivion E87–05 S before and after the test protocols under 120
C 40%RH through SEM, IEC (Ion Exchange Capacity) of the membrane, TEM and XRD analysis. Test

Initial Mode 1 Mode 2 Mode 3 Mode 4

SEM

IEC

Thickness (mm)

(meqg

53.0 50.7 48.7 48.1 46.0

1.420 1.103 0.892 0.886 0.718

1

)

TEM

XRD

Mean Pt particle size (nm)

Peak position at (111) (2-theta)

Mean Pt particle size (nm)

2.33 2.54 3.53 3.97 3.58

39.72 39.93 40.54 40.58 40.51

2.43 2.77 3.63 3.97 3.66

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3.3. Catalyst degradation in the electrode

Fig. 8. XRD patterns of the Pt/C cathode catalysts after the test protocols under 120
C 40%RH: A. fresh MEA, B. after mode 1, C. after mode 2, D. after mode 3, E. after mode 4.

It has been recognized that catalyst layer operating at above 100
C is a problem due to the chemical and morphological instability by a carbon corrosion and Pt agglomeration [2–5]. Corrosion of the carbon support in the cathode may occur at relatively high oxidation potentials from the generation of oxygen atoms at the catalyst [26,34,35]. At the high temperature, oxygen atoms will react with the carbon and produce CO/CO2 which destroy and reduce the carbon materials as confirmed from the reduction on the XRD intensity as shown in Fig. 8. This carbon oxidation rate is higher at the high electrode potential that is the reason of the ECSA loss after ALT with a wider range and higher frequency (Fig. 5). In particular, more time of being in the wide potential range give a huge influence in the catalyst degradation not only by the carbon corrosion but also in the Pt agglomeration which is discussed below. For the electrochemical reaction of the catalyst layers, the change in the catalyst morphology gives evidence of the electrochemical reaction properties. The changes in Pt/C

Fig. 9. HR-TEM micrographs of the Pt/C cathode catalysts after the test protocols under 120
C 40%RH: A. fresh MEA, B. after mode 1, C. after mode 2, D. after mode 3, E. after mode 4.

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morphology in the electrode after each test modes for 35 h were compared using HR-TEM as shown in Fig. 9, and the average particle size of platinum is listed in Table 2. All the catalyst samples were collected from the cathode side after the durability tests. For the initial MEA, Pt particles were well-dispersed on the carbon support with an average particle size of 2.33 nm. After the durability test at constant voltage condition (mode 1), it was difficult to find any changes in the particle sizes (2.54 nm), whereas a slight decrease in Pt dispersion was observed. On the other hand, the sizes of the particles in the catalyst remaining after the three ALT tests were found to have significantly increased overall to the sizes of 3.53 (mode 2), 3.97 (mode 3) and 3.58 (mode 4) nm, respectively, as along with a lower dispersion of the Pt on the carbon support. The results of increase in the Pt particle sizes were also observed at the mean particle sizes calculated from powder x-ray diffraction patterns, as listed in Table 2. In all cases, XRD patterns, in Fig. 8, for the Pt/C before and after durability tests show the diffraction peaks corresponding to (111), (200), (220) and (311) planes that are characteristic of a face-centered cubic (fcc) structure of Pt (JCPDS-87e0646) [27]. The mean particles of the catalysts were calculated from the (111) diffraction peak following Scherrer's equation using shape constant (K) of 0.9. It is important to note that the overlapping of two Pt and one graphite diffraction peaks in the region between 35
and 50
makes deconvolution of the peaks rather inaccurate, and thus only an estimation of the mean particle size can be drawn from this method. The mean particle sizes from the XRD tended to be similar with that of HRTEM. There was no big change in the size of Pt particles after test mode 1 compared to the initial size of 2.43 nm. However, all the catalyst particles were enlarged after the ALT experiments to around 3.6 to 4.0 nm. The observation of Pt ripening and aggregation are probably due to the crystallite migration mechanism, which can be explained by Pt dissolution and re-precipitation phenomena (Ostwald ripening) accelerated by the operation at elevated temperature [2,12,20]. It can be discussed that the constant voltage testing was less affected by the Pt catalyst than the potential sweeping durability test. The faster sintering rate during repeated load cycling is presumably due to the cycling potential sweep especially at the wider voltage range, which may create potentials that favor peroxide formation closely related to the Pt dissolution. The potential cycling could eliminate the continuous passivation of Pt where less PtO and PtOO leads to more Pt dissolution and then generate the agglomeration of the Pt particles [8]. Furthermore, the ORR operation condition is too corrosive at the wider potential range to keep the original geometric characteristic of the nanosized Pt, and this could result in the loss of active surface area along with their reaction kinetics [25]. Another interesting phenomenon observed in XRD spectrum is that Pt crystallite shows not only a particle size increase but also a shift of Pt (111) peak, especially after the ALT durability tests. This slight change could be explained by the existence of an internal stress and, accordingly, the transformation into Pt crystallinity with a shrunk lattice. This observation is bound to cause decrease in the catalytic activity. 4. Conclusion We demonstrated different durability testing modes for high temperature PEMFC performances by using the SSC-PFSA polymer at the membrane and catalyst layer. Accelerated life-time (ALT) protocols using voltage sweeping were designed to modulate the high temperature PEM fuel cell operation, and to understand the degradation mechanism under 120
C 40%RH. A moderate degradation rate of the current density was detected for about 500 cycles (35 h), which was followed by a much more rapid decrease of

performance at high temperature operations. Different ranges and frequencies of the voltage sweep were compared with a mode of a constant voltage of 0.6 V as a reference to inspect the importance of the application of ALT protocols. Both modified ALTs were more efficient to estimate the life-time at short time due to the faster performance drop than the normal ALT. Electrochemical and morphological/structural changes at the catalyst and membrane were investigated to verify the cause of the performance degradation. While the MEA degradation originated from the composites failure after the ALT modes under 120
C 40% RH, the relatively severer degradation of the membrane after the ALT with more frequency was one reason for the loss in the material and hydrogen cross over. Serious impacts from the ALT modes, significantly after ALT mode with wider potential range, also show damages on the catalyst layer including the increase of the Pt particle size and dispersion which cause a decrease in the ECSA for chemical reaction. Based on all the experiment and characterization results, we could conclude that the developed ALT failure modes of the MEA likely originated from the composited failure that might be due to the stresses to both the membrane and catalyst layer imposed by the load cycle with voltage sweep under 120
C 40%RH. By changing the condition of the load cycling, the ALT with more frequency shows a relatively higher influence to the membrane degradation whereas the ALT with the wide potential range gives relatively more impact the electrode degradation, along with morphology and chemical change. These defective structure of the MEA affected the electrochemical properties and finally caused a performance drop. Therefore, this work has presented a degradation mechanism that can be explained by specific ALT durability testing modes for the high temperature PEM fuel cell operation. It was clear that these changes were the key point for the performance degradation at the developed ALT protocols. From these results, the newly developed new protocols, including number of diagnostics, are expected to give valuable information for the evaluation of the high temperature PEM fuel cell. Acknowledgement This work was supported by the New & Renewable Energy Technology Development Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (No. 2009T100200046) and by the National Research Foundation of the Ministry of Science, ICT, Future Planning (NRF2012M1A2A2671711), Republic of Korea. References [1] B. Steele, A. Heinzel, Materials for fuel-cell technologies, Nature 414 (2001) 345. [2] R. Borup, J. Meyers, B. Pivovar, Y.S. Kim, R. Mukundan, N. Garland, D. Myers, M. Wilson, F. Garzon, D. Wood, P. Zelenay, K. More, K. Stroh, T. Zawodzinski, J. Boncella, J.E. McGrath, M. Inaba, K. Miyatake, M. Hori, K. Ota, Z. Ogumi, S. Miyata, A. Nishikata, Z. Siroma, Y. Uchimoto, K. Yasuda, K. Kimijima, N. Iwashita, Scientific aspects of polymer electrolyte fuel cell durability and degradation, Chem. Rev. 107 (2007) 3904. [3] J.K. Koh, Y. Jeon, Y.I. Cho, J.H. Kim, Y.-G. Shul, A facile preparation method of surface patterned polymer electrolyte membranes for fuel cell applications, J. Mater. Chem. A 2 (2014) 8652. [4] J. Zhang, Z. Xie, J. Zhang, Y. Tang, C. Song, T. Navessin, Z. Shi, D. Song, H. Wang, D. P. Wilkinson, et al., High temperature PEM fuel cells, J. Power Sources 160 (2006) 872. [5] C. Yang, P. Costamagna, S. Srinivasan, J. Benziger and A. B. Bocarsly Approaches and technical challenges to high temperature operation of proton exchange membrane fuel cells, J. Power Sources 103 (2001) 1. [6] O. Savadogo, Emerging membranes for electrochemical systems: Part II. High temperature composite membranes for polymer electrolyte fuel cells (PEFC) applications, J. Power Sources 127 (2004) 135. [7] A.M. Jordan, C.B. Brian, Sulfonated Polybenzimidazoles for High Temperature PEM Fuel Cells, Macromolecules 43 (2010) 6706.

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