Performance degradation and microstructure changes in freeze–thaw cycling for PEMFC MEAs with various initial microstructures

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Performance degradation and microstructure changes in freezeethaw cycling for PEMFC MEAs with various initial microstructures Sang-Yeop Lee a, Hyoung-Juhn Kim a, EunAe Cho a, Kug-Seung Lee a, Tae-Hoon Lim a, In Chul Hwang b, Jong Hyun Jang a,* a b

Fuel Cell Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Korea Corporate Research and Development Division, Hyundai-Kia Motors, Gyeonggi-do 446-912, Republic of Korea

article info

abstract

Article history:

When the temperature of a fuel cell vehicle is repeatedly reduced to subzero temperatures,

Received 17 May 2010

volume changes by water/ice transformations and frost heave mechanism can cause

Received in revised form

microstructural changes in membraneeelectrode assemblies (MEA), and a resultant

11 August 2010

permanent decrease in the performance of fuel cell stacks. In this study, five MEAs man-

Accepted 18 August 2010

ufactured by different methods, were tested under repeated freezeethaw (FeT) cycles between
20  C and 10  C, and the variations in their electrochemical and microstructural characteristics were analyzed according to the initial microstructures. When the MEAs

Keywords:

were prepared by spraying catalyst inks on polymer membranes, no significant micro-

Freezeethaw cycle

structural changes were observed. In the case of two supplied MEAs, void formations at the

Ohmic loss

electrolyte/electrode interface or vertical cracks within the catalyst layers were observed

Microstructure

after 120 FeT cycles. Void formation seems to be responsible for performance degradation

Polymer electrolyte membrane

as a result of ohmic loss, but the effect of cracks in the catalyst layers was not confirmed. In

fuel cell (PEMFC)

120 FeT cycles, activation overpotentials and concentration overpotentials did not increase

Membraneeelectrode assembly (MEA)

significantly for any of the MEAs, even although gradual decreases in the electrochemically active surface area of the platinum catalysts and changes in the porous structure were observed. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Polymer electrolyte membrane fuel cells (PEMFCs) are expected to replace conventional combustion engines in vehicles and to be used as power sources for houses and small buildings. To be commercially viable, it is important that PEMFC systems perform reliably under practical operating conditions. The introduction of PEMFC systems with higher reliability and a prolonged lifetime will also reduce annual usage costs. When used as power sources for vehicles, PEMFC

systems operate under more severe conditions than in residential applications. For example, it has been reported that frequent startup/shutdown of PEMFC stacks causes significant degradation, therefore controlled startup/shutdown procedures should be used in fuel cell vehicles (FCVs) to extend their lifetime [1,2]. Another important technical issue is PEMFC operation at subzero temperatures. In FCVs, the normal operating temperature of the PEMFC stacks is above 60  C. However, when the FCVs are parked outside, the stack temperature can

* Corresponding author. Tel.: þ82 2 958 5287; fax: þ82 2 958 5199. E-mail addresses: [email protected], [email protected] (J.H. Jang). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.08.070

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fall to below 0  C in winter in cold regions. When stored at subzero temperatures, water retained in the catalyst layers (CLs) and the gas diffusion layers (GDLs) is transformed to ice, and the accompanying volume increase can cause structural damage to the membraneeelectrode assemblies (MEAs) and the GDLs [3e5]. With FeT cycling, the frost heave mechanism also can be responsible for the PEMFC degradation [6e8]. As a result, PEMFC systems can be permanently damaged by repeated freezeethaw (FeT) cycles, influencing the cold-start characteristics and cell performances [3e16]. For example, Oszcipol et al. reported that, after 10 cold startup, the power density decreased by about 50% with the significant Pt active area decrease and ohmic resistance increase [4]. In some studies, significant degradation was not found with three FeT cycles (
10e80  C) [17] and 385 FeT cycles (
40e80  C) [18]. The MEA degradation is expected to be reduced by controlling shutdown procedures and by designing MEA microstructures that are more tolerant to volume changes resulting from water/ice transformations. For example, it has been reported that the amount of water remaining in a system can be reduced by gas purging at shutdown, thereby enhancing performance stability in FeT cycles [9,11,19]. Under identical shutdown procedures, the amount of residual water and its distribution inside MEAs will vary depending on the MEA microstructures, such as pore volume of cathode catalyst layer (CCL), thickness of polymer electrolyte membranes (PEMs), and PEM/CCL interfacial structure. Therefore, the structural damage and the resultant performance degradation caused by repeated FeT cycles will differ depending on the method and conditions used in manufacture of the MEA. However, the freezeethaw characteristics of a variety of MEAs with different microstructures have not yet been fully examined experimentally. In this study, five MEAs manufactured by various methods with different conditions were tested under repeated FeT cycles. The cell performances and electrochemical characteristics were evaluated as a function of the FeT cycle number. In addition, microstructural changes in each MEA were analyzed and correlated with performance decay. Finally, the effect of initial MEA microstructures on the MEA degradation in FeT cycling was discussed.

2.

Experimental

2.1.

Membraneeelectrode assemblies (MEAs)

In this study, three laboratory-manufactured MEAs (A, A_P, and A_HP) and two MEAs supplied by Hyundai-Kia Motors (B and C) were evaluated under FeT cycles. The lab-manufactured MEAs were catalyst-coated membranes (CCM) prepared by spraying catalyst inks on both sides of Nafion membranes (NRE-212; DuPont). The catalyst inks were prepared by mixing a carbon-supported platinum catalyst (Pt 45.5 wt.%; Tanaka K. K.), isopropyl alcohol (Analytical Grade HPLC Reagent; Baker), and a Nafion ionomer dispersion in Hþ form (EW1100, 5 wt.%; DuPont). The platinum loading was controlled to be 0.4 mg/cm2 for both the anode and cathode layers, and the active electrode area was 25 cm2. The prepared CCMs were then evaluated without a further pressing step

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(MEA A), after pressing at room temperature (MEA A_P), or after hot-pressing at 140  C (MEA A_HP).

2.2.

Single-cell test with FeT cycling

The MEAs were assembled from gas diffusion media (Sigracet 10BC; SGL Carbon Inc.), Teflon gaskets, and graphite blocks. The single-cell unit was installed at a fuel cell test station (P-780; CNL) and the cell temperature was increased to 65  C. The evaluation procedure in this study consisted of three steps: (i) initial activation and analysis; (ii) FeT cycling (1 set ¼ 15 cycles); and (iii) re-activation and analysis (Fig. 1). The single-cell was initially activated at a current density of 1.2 A/cm2 at 65  C. Fully humidified hydrogen and air were supplied to the anodes and cathodes, respectively. The stoichiometric ratio was fixed at 1.5 for hydrogen and 2.0 for air. The single-cell performance was characterized by galvanostatic polarization experiments (ieV curves) with increasing current density. Electrochemical impedance spectroscopy (EIS) was conducted at a dc potential of 0.85 V with an ac frequency range of 10 kHze0.01 Hz and an amplitude of 5 mV (IM6; Zanher-Electrik). The cathode gas was then switched to nitrogen, and cyclic voltammetry (CV) was performed between 0.06 V and 1.2 V at a scan rate of 50 mV/s, and linear sweep voltammetry (LSV) was performed at a scan rate of 2 mV/s. In an FeT cycling step, the single cells were operated at 65  C for 30 min (current density 0.8 A/cm2) to generate water in the cathode layers; every gas valve was closed. The single-cell unit was then disconnected from the test station and placed in an environmental chamber. The temperature of the environmental chamber was gradually decreased to
20  C over a period of 2 h (cooling rate 0.71  C/min). After stabilizing at
20  C for 2 h, the chamber temperature was increased and decreased repeatedly between
20  C and 10  C to induce water/ice transformations at the PEM/CL interface and in the CLs (heating and cooling rates 0.25  C/min). The changes in cell temperature with chamber temperature are shown in Fig. 1b. After 15 FeT cycles between
20  C and 10  C (1 set), the chamber temperature was increased to 65  C and the singlecell unit was transferred to the test station. After re-activation at 0.8 A/cm2 for 30 min, electrochemical analysis was carried out to evaluate the effects of FeT cycles on cell performance. For each MEA, the FeT cycling steps and re-activation/analysis steps were repeated eight times, corresponding to 120 FeT cycles. Before and after the FeT cycling operation, the surface and cross-sectional microstructures were examined by scanning electron microscopy (SEM; XL-30 FEG ESEM; FEI Co., XL-30 FEG ESEM) with an acceleration voltage of 15.0 kV. The porous structure of the MEAs was examined by mercury porosimetry with an intrusion pressure between 0 and 30,000 psi (AutoPore IV 9500; Micromeritics).

3.

Results and discussion

3.1.

Initial microstructures

The initial microstructures of MEAs were characterized by cross-sectional SEM and mercury porosimetry. The PEM

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thickness of the MEA A, MEA A_P, and MEA A_HP was about 50 mm, which is larger than that of MEA B (ca. 17 mm) and MEA C (ca. 30 mm). The CL thickness was similar for MEA A series and MEA B (ca. 11 mm), while that of MEA C is about 7 mm. When the cross-sectional images of MEA A_HP were compared with those of MEA A, significant structural change by hot-pressing was not observed in SEM analysis, even though surface roughness was decreased. Fig. 2a shows the initial accumulated pore volumes, which were normalized by MEA area, instead of sample weight. It can be noticed that the total pore volumes are larger for MEA A series over MEA B and MEA C, and pore volumes increase mainly at two pore diameter ranges, around 0.05 mm and 50 mm. The smaller pores represent gas channels between the Pt/C catalyst and ionomer materials in the catalyst layers (submicron pores, diameter 0.01e1 mm); micrometer scale pore diameters should be interpreted as secondary pores, interfacial voids, and cracks (micron pores, diameter > 1 mm). The diameter of submicron pores was similarly around 0.05 mm for every MEA. The initial pore volumes of submicron pores were similar for MEA A (1.29 mL/cm2), MEA A_P (1.32 mL/cm2), MEA A_HP (1.14 mL/cm2), and MEA B (1.26 mL/cm2) (Fig. 2b). The initial

a

80 Activation

3.2.

The effects of FeT cycling on cell performance were examined by varying the cell temperature between
20  C and 10  C for 120 cycles (Fig. 1a). The cell temperature was controlled by placing the single-cell unit in an environmental chamber. The actual temperature variation of the single cell was slightly more gradual than that of the chamber temperature (Fig. 1b).

-2

Accumulated Vpore / L cmMEA

60

o

T/ C

40 20 0 -20 0

20

40

60

80

100

120

140

160

Cell performance variation with FeT cycling

a

Re-activation & Analysis

15 F-T cycles

& Analysis

submicron pore volume of MEA C (0.59 mL/cm2) was much lower compared to other MEAs (42e52%), while the CL thickness of MEA C was about 64% of other MEAs, indicating that the CL of MEA C is more thin and dense. In the case of micron pores, the pore diameter of MEA A_HP (ca. 20 mm) was much smaller compared to the MEA A, probably due to the porous structure changes by hot press step; and the micron pores of MEA C were more distributed. The micron pore volume of MEA B was much smaller (41e51%) compared to the MEA A series of that of the MEA A series. The micron pore volume of MEA C was slightly higher than that of MEA B, but the difference in CL thickness should be considered in discussing the porous structures.

3

2

1

0

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0.01

t/h

Pore Volume / L cmMEA

0

o

T/ C

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-10

-20

-30

0

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30

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b

Cell temperature Chamber temperature

20

0.1

100

P Pore di diameter t / m

-2

b

MEA A MEA A_P MEA A_HP MEA B MEA C

4

40

50

60

t/h Fig. 1 e (a) Experimental procedure for freezeethaw cycling tests, and (b) typical temperature variation profiles of single-cell unit and environmental chamber.

Micron pores Submicron pores

4

3

2

1

0

A

A_P

A_HP

B

C

Fig. 2 e Initial MEA porous structures measured by mercury porosimetry. (a) Accumulated pore volume plots, and (b) submicron pore volumes (0.01e1 mm) and micron pore volumes (> 1 mme).

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initial 30 cycles 60 cycles 90 cycles 120 cycles

0.6

-2

0.8

E/V

0.7

MEA A

1.0

E @ 1.0 A ccm / V

a

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0.4

0.5

0.4

MEA A MEA A_P MEA A_HP MEA B MEA C

0.3

0.0

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i / A cm

0.8

1.0

1.2

0

15

30

45

60

75

90

105

120

F-T cycle number

-2

Fig. 4 e Single-cell voltage variations with FeT cycling.

b

MEA B

1.0

initial 30 cycles 60 cycles 90 cycles 120 cycles

E/V

0.8

0.6

0.4

0.0

0.2

0.4

0.6

i / A cm

c

0.8

1.2

MEA C

1.0

initial 30 cycles 60 cycles 90 cycles 120 cycles

08 0.8

E/V

1.0

-2

overpotential and ohmic losses. The cell voltage of MEA A changed randomly, as can be seen from the ieV curves. However, after pressing (MEA A_P) or hot-pressing (MEA A_HP), the cell performance became more stable with FeT cycle number. In the case of MEA B, the cell voltage initially increased until 30 FeT cycles to deliver high performances between 30 and 75 cycles, and then suddenly decreased after 75 FeT cycles. After initial degradation (ca. 40 mV in 15 cycles), the cell voltage of MEA C slowly decreased in further FeT cycling (ca. 40 mV in 105 cycles). Each MEA will have a different PEM/CL interfacial stability, porous structure of the catalyst layers, and hydrophilicity, depending on the MEA production method and conditions. Even under identical operating conditions, therefore, the amount and distribution of water generated in each MEA will be different. In FeT cycling, the volume expansion caused by the phase transformation of water to ice, as well as the frost heave mechanism, will take place in each MEA in different ways, resulting in corresponding morphological variations. At the same time, the electrochemical characteristics of the MEAs, such as the oxygen reduction reaction (ORR) kinetics (activation overpotential),

0.6

0.4

MEA A MEA A_P MEA A_HP MEA B MEA C

0.15

0.6

i / A cm

0.8

1.0

1.2

-2

Fig. 3 e Polarization curves as a function of FeT cycle number for (a) MEA A, (b) MEA B, and (c) MEA C.

The cell performances and electrochemical characteristics were evaluated every 15 FeT cycles. Fig. 3 shows variations in ieV curves with FeT cycling. The cell voltages at a current density of 1 A/cm2 were plotted as a function of FeT cycle number in Fig. 4. At this current density, the concentration overpotential plays an important role, as do the activation

2

0.4

0.10

cm

0.2

Rohm /

0.0

0.05

0.00 0 00

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60

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105

120

F-T cycle number Fig. 5 e Ohmic resistance variations with FeT cycling.

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OCV / V

1.00

0.95 MEA A MEA A_P MEA A_HP MEA B MEA C

0.90

b

0

30

45

60

75

90

105

120

90

105

120

F-T cycle number 2.5 MEA A MEA A_P MEA A_HP MEA B MEA C

-2 2

2.0

iLSV @ 0.4 V / mA cm

15

1.5

1.0

0.5

0.0

0

15

30

45

60

75

F-T cycle number Fig. 6 e (a) Open circuit potential, and (b) hydrogen crossover currents in LSV as a function of FeT cycle number.

proton conduction properties (ohmic loss), and gas transportation (concentration overpotential), will be changed accordingly. Therefore, to understand the effects of FeT cycling on cell performance, individual variations in overpotential terms with FeT cycles should be analyzed and related to the morphological changes for each MEA.

3.3.

Effect of FeT cycles on ohmic losses

The ohmic resistance was measured from the high frequency intercept in ac impedance spectra and the values were plotted as a function of FeT cycle number (Fig. 5). The three laboratory-manufactured MEAs showed stable ohmic resistances with FeT cycling: 89  5 mU cm2 (MEA A), 88  7 mU cm2 (MEA A_P), and 98  3 mU cm2 (MEA A_HP). MEA C also maintained a stable ohmic resistance of 65  2 mU cm2 over 120 FeT cycles. In the case of MEA B, the ohmic resistance suddenly increased during the sixth set of FeT cycles (76e90 cycles). The average ohmic resistance was 64  4 mU cm2 up to 75 cycles and 122  6 mU cm2 from 90 cycles. This ohmic resistance increase of 58 mU cm2 corresponds to an additional ohmic loss of 58 mV at 1.0 A/cm2. The cell voltage in the stable

region up to 75 cycles was 0.631 V, but the cell voltage decreased to 0.575 V after the sixth set of FeT cycles (76e90 cycles). From the size of the cell voltage decrease (56 mV), it can be concluded that the performance decay of MEA B, which occurred at 76e90 cycles, was mainly due to an increase in ohmic resistance. Generally, ohmic resistance in PEMFC systems is composed of PEM resistance, contact resistance, and wire resistances. If the sudden increase in the ohmic resistance of MEA B is due to a PEM resistance increase, significant PEM degradation would be expected to occur between 76 and 90 FeT cycles. Although separate measurement of PEM resistance and contact resistance is difficult, physical degradation of PEMs can be detected from open circuit voltage (OCV) values (from polarization experiments) and hydrogen cross-over currents (LSV). Fig. 6a shows the OCV values as a function of FeT cycle number. No significant changes in OCVs were observed in any of the MEAs, indicating that physical damage, such as pin-hole formation and cracking, was not severe. The averaged OCV values for MEA A (0.993 V), MEA A_P (1.000 V), and MEA_HP (0.998 V) were higher than those for MEA B (0.966 V) and MEA C (0.967 V), due to the differences in gas cross-over rates with various PEM thicknesses. The gas cross-over currents at 0.4 V determined by LSV plotted with FeT cycle number are shown in Fig. 6b. No significant variation in gas cross-over current was observed for any of the MEAs, indicating that the PEM of MEA B was not physically damaged in FeT cycles 76e90. As can be inferred from the OCV values, the hydrogen cross-over currents at 0.4 V measured by LSV were higher for MEA B and MEA C, which had thinner membranes.

3.4.

Microstructure variation with FeT cycling

After FeT cycling, the cross-sectional and surface morphologies, and porous structures were characterized by SEM and mercury porosimetry, respectively. No significant changes were observed at the anode sides of the PEMs in any of the MEAs. For the three laboratory-manufactured MEAs (MEA A, MEA A_P, and MEA A_HP), no structural variations, such as void and crack formation at the PEM/cathode interface and cathode layers, were detected after FeT cycling (Figs. 7 and 8). It seems that the direct spraying of the catalyst layer onto the PEMs provided good PEM/CL interfacial properties, and structural degradation at the PEM/CCL interfaces and in the cathode layers could be prevented during FeT cycles. In the case of MEA B, which experienced a large increase in ohmic resistance, severe void formation was observed at the PEM/CCL interface after FeT cycling (Fig. 7d). In addition, crack formation was detected on the surface of cathode layers (Fig. 8d). Fig. 9 shows the effect of 120 FeT cycles on the porous structure of MEA B. During the FeT cycles, the total pore volume of MEA B increased from 2.16 mL/cm2 to 4.04 mL/cm2 (Fig. 9a), probably due to void formation at the PEM/cathode interface. In the pore size distribution plots (Fig. 9b), peaks can be identified centered at a pore diameter of 56 nm (initial) and 54 nm (after FeT cycling). For 120 FeT cycles, the submicron pore volume of MEA B decreased from 1.26 mL/cm2 to 1.01 mL/cm2 (diameter 0.01e1 mm), while the micron pore volume increased from 0.90 mL/cm2 to 3.04 mL/cm2 (diameter > 1 mm).

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Fig. 7 e Cross-sectional SEM images after 120 FeT cycles for (a) MEA A, (b) MEA A_P, (c) MEA A_HP, (d) MEA B and (e) MEA C. The cathode catalyst layers and polymer electrolytes membranes are indicated as CCL and PEM, respectively.

For MEA C, the local void formation was not observed in the cross-sectional SEM images, even though CCL delamination could be identified. It seems that the thin and dense catalyst layers of MEA C contributed to the PEM/CCL interfacial stability. Considering that the ohmic resistance was stably maintained, it seems that the PEM/CCL interfacial strength was weakened in FeT cycling and catalyst layers were delaminated during cell disassembly and sample preparation. The micron pore volume of MEA C did not change, while the submicron pore volume slightly increased from 0.59 mL/cm2 to 0.67 mL/cm2. On the surface of cathode layer, large numbers of cracks were observed after FeT cycling, as shown in Fig. 8e. EDX analysis confirmed that the cracks became deeper and that the PEM surface was exposed. From these experimental observations, it can be concluded that the MEA degradation by FeT cycling differently occurred according to the initial microstructures. For the MEA A, MEA A_P, and MEA A_HP, the structural stability with FeT cycling can be related to the high PEM/CCL interfacial strength with the CCM methods, and possibly large amount of micron pores. In contrast, under identical FeT cycling condition, the MEA B experienced severe void formation at PEM/CCL interface, and resultant cell performance decay with ohmic resistance increase. For MEA C, cracks were developed in the cathode layers, but the overall cell performance was not influenced.

The structural variation of MEA B and MEA C seems to be related to the volume expansion with water/ice transformation [5] and the frost heave mechanism [6e8], which will be investigated in further studies.

3.5. Activation overpotential and concentration overpotential The activation overpotential (hact) and the concentration overpotential (hconc), which are related to the ORR kinetics and to mass transportation, were calculated using Eq. (1) [20], where EOCV is the reversible cell voltage (theoretical OCV), A is the Tafel slope, i0 is the ORR exchange current density, in is the leakage current density, and Rohm is the ohmic resistance of the fuel cell system. Ohmic resistances (Rohm) from ac impedance measurements were used to plot iR-compensated cell voltages as a function of current density, and non-linear data fitting was performed, typically with data points at current densities up to 80 mA/cm2. From the fitted parameters, the activation overpotential was calculated as A log [(i þ in)/i0], with a reversible cell voltage (EOCV) at 65  C of 1.177 V [21,22]. The remaining voltage loss that was not associated with activation overpotential and iR loss was ascribed to concentration overpotential.

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Fig. 8 e Surface SEM images of cathode catalyst layers after 120 FeT cycles for (a) MEA A, (b) MEA A_P, (c) MEA A_HP, (d) MEA B and (e) MEA C.

EðiÞ ¼ EOCV
hact ðiÞ
iRohm
hconc ðiÞ   i þ in
iRohm
hconc ðiÞ ¼ EOCV
Alog i0

(1)

The activation overpotential did not increase significantly with FeT cycling for any of the MEAs (Fig. 10a). For each MEA, using cyclic voltammograms recorded under a nitrogen atmosphere in the cathodes, the electrochemically active surface areas (EAS) were calculated from the electrochemical proton desorption charge (Q) around 0.15 V

(Fig. 10b). In this study, EAS is the platinum/ionomer interfacial area normalized by the MEA active area (AMEA). When the platinum loading is not accurately known or can be varied during experiments, as in this study, the kinetic and structural parameters are better represented by areanormalized values. The EAS gradually decreased with FeT cycling in a similar manner for all the MEAs. The platinum/ionomer area losses after 120 FeT cycles were 24% (MEA A_HP and MEA B), 25% (MEA C), 29% (MEA A), and 32% (MEA A_P). It therefore seems that an EAS decrease of

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a

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6 initial after F-T cycles

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@ 1.0 A cm c /V

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Pore diameter / m Fig. 9 e (a) Accumulated pore volumes as a function of pore diameter, and (b) pore size distribution of MEA B before and after FeT cycling.

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F-T cycle number Fig. 10 e (a) Activation overpotential values, and (b) electrochemically active surface areas as a function of FeT cycle number.

up to 32% did not influence the catalytic activity of the catalyst layers (2) 0.4 MEA A MEA A_P MEA A_HP MEA B MEA C

0.3 -2 2

The concentration overpotential values at 1.0 A/cm2 are presented in Fig. 11. It can be seen that hconc for MEA A fluctuated throughout the FeT cycling test, resulting in cell voltage variations. However, they became stable with pressing (MEA A_P) or hot-pressing (MEA A_HP). It seems that the rough surface of the catalyst layer in MEA A resulted in a high level of water trapping between the catalyst layer and the gas diffusion layers. As the catalyst layer became smoother with post treatment, water trapping no longer occurred. In the case of MEA B, the hconc initially decreased until 30 FeT cycles to reach stable values in continued cycling (0.11  0.06 V). This hconc variation is considered to be responsible for the voltage enhancement of MEA B in initial FeT cycling (Fig. 4), probably as a result of improved gas transport with microstructure changes in initial FeT cycling. The hconc of MEA C with less porous catalyst layers was relatively higher and its variation with FeT cycling could be well correlated with the cell performance degradation (Fig. 4).

@ 1.0 A cm / V

Q 0:21 mC=cm2  AMEA

conc

EAS ¼

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F-T cycle number Fig. 11 e Concentration overpotential variations with FeT cycling.

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Conclusions

The electrochemical and microstructural variations of five MEAs were evaluated under FeT cycling between
20  C and 10  C. For laboratory-manufactured MEAs, prepared by direct spraying of catalyst inks onto membranes, microstructural changes were not observed in SEM images, and the ohmic resistance did not change in the FeT cycling. In MEA B, which was supplied, void formation at the electrolyte/electrode interface was observed after the FeT cycling test, which could be correlated with the ohmic resistance increase and resultant cell voltage decrease after 75 FeT cycles. In MEA C, which was also supplied, cracks developed in the thin and dense catalyst layers in 120 FeT cycles, but the cell performance was not significantly influenced. It can therefore be concluded that the FeT durability is related to the initial microstructures that are determined by the MEA preparation method and fabrication conditions. In repeated FeT cycling, cell performance was mainly influenced by void formation at the electrolyte/electrode interface, and the effects of porous structure variation and crack formation in the catalyst layers were not significant. Based on activation overpotential values, it was also confirmed that degradation of catalytic activity did not occur in 120 FeT cycles, even although the platinum/ionomer interface gradually decreased.

Acknowledgements This work was supported by the New and Renewable Energy R&D Program and the National RD&D Organization for Hydrogen and Fuel Cell under the Ministry of Knowledge Economy, Republic of Korea, as part of the “Development of 200 kW Class PEMFC System for Bus” (2005-N-F12-P-01). This work was also supported by the New & Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (No.2009T100200046).

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