ww.sciencedirect.com

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 5 7 6 0e1 5 7 6 5

Available online at w

ScienceDirect

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

Effects of membranes thickness on performance ofDMFCs under freeze-thaw cycles

Yumi Oh a,b, Sang-Kyung Kim a,c,*, Dong-Hyun Peck a, Doo-Hwan Jung a,c,Yonggun Shul b

a Fuel Cell Research Center, Korea Institute of Energy Research, 152 Gajang-ro, Yuseong-gu, Daejeon 305-343,

Republic of Koreab Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seoul 120-749,

Republic of Koreac Department of Advanced Energy Technology, University of Science and Technology, 217 Gajeong-ro, Yuseong-gu,

Daejeon 305-350, Republic of Korea

a r t i c l e i n f o

Article history:

Received 4 January 2014

Received in revised form

16 April 2014

Accepted 1 June 2014

Available online 15 August 2014

Keywords:

DMFC

Membrane electrode assembly

(MEA)

Freeze-thaw cycles

Performance degradation

Membrane thickness

* Corresponding author. Fuel Cell ResearchRepublic of Korea. Tel.: þ82 42 860 3366; fax

E-mail address: ksk@kier.re.kr (S.-K. Kimhttp://dx.doi.org/10.1016/j.ijhydene.2014.06.00360-3199/Copyright © 2014, Hydrogen Energ

a b s t r a c t

The effect of membrane thickness on the performance degradation of DMFCs was inves-

tigated during freeze-thaw cycling across temperatures ranging from �32 �C to 60 �C. Three

cells with Nafion membranes of varying thickness were prepared: Nafion 112, Nafion 115

and Nafion 117. Performance degradation was evaluated by comparing the changes to

electrode polarization, electrochemical impedance and cyclic voltammetry over a range of

freeze-thaw cycles. It was determined that freeze-thaw cycling affected the performance of

the three membrane electrode assemblies (MEA). A cell with a Nafion 112 membrane

showed a more significant increase in cathode overpotential than cells with either a Nafion

115 or Nafion 117 membrane. The charge transfer resistance of the cell with a thin

membrane was more affected by freeze-thaw cycles than the cells with thicker mem-

branes. All three cells showed a significant decrease in ECSA after freeze-thaw cycles.

Freeze-thaw cycles damaged the triple phase boundary region and decreased the ECSA of

cells with thinner membranes, which cause rapid performance degradation. The use of

thick membranes in MEAs was determined to be the effective method for reducing per-

formance degradation during freeze-thaw cycling.

Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

Introduction

Portable DMFC which has been highlighted for military uses

such as laptops, power sources, airplanes and etc. has to

withstand freezing temperatures [1e4]. A rapid degradation in

Center, Korea Institute o: þ82 42 860 3739.).31y Publications, LLC. Publ

performance has been reported for DMFCs that were sub-

jected to freeze-thaw cycles [5e7], yet there have been limited

investigations on the operation of DMFCs at subzero temper-

atures. One reason that has been reported for the rapid per-

formance degradation of DMFCs using liquid fuel in the anode

is remained water in the cell. When it stored at subzero

f Energy Research, 152 Gajang-ro, Yuseong-gu, Daejeon 305-343,

ished by Elsevier Ltd. All rights reserved.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 5 7 6 0e1 5 7 6 5 15761

temperature, the expansion of ice has been shown to raise

micromechanical damages, such as pinholes, cracks in the

catalyst layer and distortions in the membrane and re-

inforcements [8e13].

DMFCs consist of many components, including catalysts,

membranes, gas diffusion layers (GDLs), gaskets and bipolar

plates. Among them, the membrane degradation of MEAs is

one of important factors during freeze-thaw cycling of DMFCs.

It is reported that the dramatic ECSA loss on the cathode side

may have been attributable to the crossover problem due to

abrupt breakage of the membrane [14]. Methanol crossover,

which leads not only in a lower fuel efficiency, but also a

decrease in the overall cell voltage due to the mixed potential

on the cathode, is also one of the most challenging issues for

DMFCs [15]. During an operation of DMFC,methanol crossover

from the anode through the membrane to the cathode occurs

because of molecular diffusion and the electro-osmotic drag

[16]. The effects of methanol crossover were observed under

various operating conditions and membrane types with

different membrane permeability and thickness [17,18]. The

membrane thickness is a very important factor regarding the

methanol crossover problem. Thinner membranes have a

higher rate of methanol crossover but have smaller re-

sistances, raising the cell performance [19,20]. Fast degrada-

tion in performance of the cell with thinner membranes is

clear by a rapid increase in anode fuel crossover showing

membrane thinning and pinhole or crack formations [21].

Much effort has been made in order to understand the

correlation between freeze-thaw cycles and the performance

degradation in the DMFC. A comprehensive understanding

about the effect of freeze-thaw cycles and gas purging on

micro part changes of the DMFC, was researched in our pre-

vious work [22].The details of the performance degradation

were analyzed by comparing the change of polarization of

each electrode and the impedance. It was found that prior to

freezing the cell, nitrogen gas purging and air purging of the

electrodes were effective to retard the performance degrada-

tion in the DMFCs. An effective way to solve this problem is to

use gas to purge the water from the PEMFCs [12,23,24] and

DMFCs [22].

In the present paper, the effect of freeze-thaw cycles on the

performance degradation of DMFCs with Nafion membranes

of varying thickness was investigated. To understand the

specific reasons of performance degradation caused by freeze-

thaw cycling, the performance and electrode polarization of

fuel cells were measured for three MEAs with different

membrane types during cycle progression. The impedance of

the MEA was also analyzed after each cycle. Cyclic voltam-

metrymeasurements were also conducted and the changes to

areas of electrochemical activity were compared.

Experimental

MEAswere prepared according to the descriptions given in the

first part of this study [22]. A Hispec 12,100 with a Pt:Ru:C

composition of 48:24:28 and a Hispec 13,100 with a Pt:C

composition of 72:28 were used for the anode and cathode

catalysts, respectively. The anode and cathode catalysts were

both purchased from Johnson Matthey (UK). A 10% Nafion

dispersion solution (Dupont Fluoroproducts, USA), 1-propanol

and 2-propanol were used to make the catalyst slurry. The

slurry was brushed repeatedly onto 9 cm2 of the GDL on the

anode and the cathode until the Pt loading was 2 mg/cm2.

Three types of the MEAs were taken into account for DMFC

performance tests to understand the influence of varying

Nafion membrane thickness. To fabricate MEAs for the

DMFCs, an anode, Nafion membrane (Nafion 112 (50 mm),

Nafion 115 (120 mm) or Nafion 117 (175 mm)) and cathode were

pressed at 150 �C and 5MPa for 1min. TheMEAswere installed

into carbon plated with a flow field and supporting plates.

The cells were stored at �32 �C for 12 h (freezing) and then

1 M methanol was fed to heat up the cell at 60 �C for 30 min

(thawing). Then, a fuel cell performance test was performed

by supplying a 3 cc/min of 1 M methanol to the anode and a

130 cc/min stream of air to the cathode at 60 �C.After that, electrochemical impedance spectroscopy (EIS)

of the cells were analyzed by electrochemical impedance

spectroscopy (EIS) using the IM6ex spectroscope (ZAHNER

MESSTECHNNIK). The frequency range was from 0.1 Hz to

100 kHz, the amplitude was 10 mV, and the voltage was set to

0.4 V using a potentiostat. Impedance spectra were obtained

after the cells were stabilized for 5 min with supplies of 1 M

methanol to the anode and air to the cathode.

The electrochemically active surface areas (ECSA) of cath-

ode were analyzed by cyclic voltammetry (CV) using a bipo-

tentiostat AFCBP1 (Pine Instrument Company, Grove City,

Pennsylvania). The scan rate was 10 mV/s and the voltage

rangewas from 0 V to 0.6 V. The anodewas fedwith 10ml/min

of hydrogen gas and the cathode was supplied to 1 ml/min of

distilledwater during the voltage cycling. ECSAwas calculated

with the fifth cycle of CV test. These methods were repeated

during 20 freeze-thaw cycles.

Results and discussion

Performance test of DMFC with Nafion 112

As shown in Fig. 1, the effect of freeze-thaw cycling on the cell

with a Nafion 112 membrane were investigated between

�32 �C and 60 �C for 20 cycles. Before freezing the maximum

power density of the cell was 133.12 mW/cm2 as indicated in

Fig. 1-(a), however it was reduced to 40.81 mW/cm2with a

reduction of 69% after 20 freeze-thaw cycles,. Both the anode

polarization and the cathode polarization were increased,

however the change in the cathode polarization was higher

than that of the anode polarization, as indicated in Fig. 1-(b).

That implied that the increase in cathode polarization had a

greater impact on performance loss to lead the decrease of full

cell polarization during freeze-thaw cycles.

Fig. 2 shows the changes in electrode over potentials as a

function of the freeze-thaw cycle number at 100 mA/cm2.

After 20 freeze-thaw cycles, the increase in anode over-

potential was 8% and 29% increase in the cathode over-

potential was shown. The anode overpotential did not

significantly change throughout the increased freeze-thaw

cycles, whereas the change of the cathode overpotential

increased dramatically. The cathode polarization was

importantly decreased compared with the anode as shown in

Fig. 2 e The maximum power density and electrode

overpotentials as a function of the freeze-thaw cycle

number for cells with Nafion 112: - maximum power

density, anode overpotential at 100 mA/cm2 and

cathode overpotential at 100 mA/cm2.

Fig. 1 e Performance degradation of a DMFC with a Nafion

112 membrane by freeze-thaw cycling: (a) polarization and

power density versus current density as a function of

freeze-thaw cycle numbers and (b) polarization curves of

the full cell, anode and cathode.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 5 7 6 0e1 5 7 6 515762

Fig. 1-(b), and it indicated that the reduction in cathode per-

formance has been primarily attributed to the cell perfor-

mance loss.

Nyquist plots were introduced to analyze the reason of the

performance degradation using electrochemical character-

ization method. Contact resistances of flow field plates,

membrane, GDL and catalyst layer are included in ohmic

resistance [25]. Charge transfer resistance includes the acti-

vation and mass transport. Fig. 3 shows the variation of

Nyquist plot obtained before freezing and after 7, 11, 16 and

20 cycles. The impedance in the cell dramatically increased

as cycle number increased and the ohmic resistance and the

charge transfer resistances increased by 12% and 131%,

respectively after 20 freeze-thaw cycles. Obviously the in-

crease in the charge transfer resistance was the main cause

for performance loss during freeze-thaw cycling [17,18]. The

increase in the charge transfer resistance indicated that

serious mass transport problems that are related to the

reduced triple phase boundary occurred due to the formation

of some pinholes and cracks in the catalyst layer during ice

formation [22]. It indicates gas pores to the catalyst layer and

the electrolyte were interrupted by methanol crossover. This

can also be inferred that methanol crossover to the cathode

side could have poisoned the catalyst layer to result in the

decrease of the catalytic activity. Therefore, the increase in

cathode charge transfer resistance was the dominant factor

in performance degradation with repetitive freeze-thaw

cycles.

The effect of membrane thickness during freeze-thaw cycles

Three cells with different Nafion membranes were compared

with investigate the effect of membrane thickness on cell

performance. One cell had a Nafion 112 membrane, whereas

the other cells had either Nafion 115 or Nafion 117 membrane.

The three cells showed a gradual degradation in performance

correlating to the progression of freeze-thaw cycles, as shown

in Fig. 4(a). After 20 freeze-thaw cycles, the maximum power

Fig. 3 e Changes in impedance for cells with Nafion 112

during freeze-thaw cycles.

Fig. 4 e (a) Changes in maximum power density and (b)

changes in current density at maximum power density of

the cells with Nafion 112, Nafion 115 and Nafion 117 during

freeze-thaw cycling: - Nafion 112, Nafion 115 and

Nafion 117.

Fig. 5 e Changes in anode and cathode overpotentials of

cells with Nafion 112, Nafion 115 and Nafion 117 at

100 mA/cm2 during freeze-thaw cycling: - anode

overpotential of the cell with Nafion 112, anode

overpotential of the cell with Nafion 115, anode

overpotential of the cell with Nafion 117, , cathode

overpotential of the cell with Nafion 112, cathode

overpotential of the cell with Nafion 115 and cathode

overpotential of the cell with Nafion 117.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 5 7 6 0e1 5 7 6 5 15763

density decreased by 36% and 28% when the membrane was

Nafion 115 and Nafion 117, respectively. The cell with Nafion

112 had a 69% reduction in maximum power density after 20

freeze-thaw cycles. The thinmembrane, Nafion 112, wasmore

affected by freeze-thaw cycling than Nafion 115 and Nafion

117. In Fig. 4(b), the current density at the maximum power

density in the three cells also showed decreaseswith the same

pattern as themaximumpower density during 20 freeze-thaw

cycles. Due to damagedmembrane layer which resulted more

crossover, a higher content of water andmethanol was able to

be present at the cathode in the thinner membrane. The

presence of high amounts of water in the membrane of the

MEA before freezing could presumably allow more pinholes

and cracks on the membrane layer which are produced from

ice formation and a more volume change. Therefore, the cell

with thinner membrane showed greater performance degra-

dation in the cell during freeze-thaw cycling.

The increase in anode and cathode overpotential for the

cells at 100 mA/cm2 during freeze-thaw cycles is presented in

Fig. 5. Because there was a large increase in cathode over-

potential, no significant difference in anode overpotential was

observed for the three cells. The increase in cathode over-

potential of the cell with Nafion 112 was larger than the cells

with Nafion 115 and Nafion 117. Therefore, the performance

degradation of the cells was primarily caused by increases in

cathode overpotential and especially the cathode side of the

cell with thinner membrane had lower cathode performance

as a result of high rates of methanol crossover in the more

damaged membranes.

Changes of ohmic resistance and charge transfer resis-

tance in the three cells with Nafion 112, Nafion 115 and Nafion

117 during freeze-thaw cycles between �32 �C and 60 �C were

plotted in Fig. 6. After 20 cycles, the ohmic resistance of the

cell with Nafion 112 increased by 12%, whereas the ohmic

resistance increases of the cells with Nafion 115 and Nafion

117were 15% and 16%, respectively. A slight increase in ohmic

resistance caused by freeze-thaw cycling was observed.

However, all three cells had a much greater increase in charge

transfer resistance than ohmic resistance. After 20 freeze-

thaw cycles, the increase of the charge transfer resistance in

the cell with Nafion 112 was 95%, whereas the cells with

Nafion 115 and Nafion 117 only increased by 37% and 30%,

respectively. The increase in charge transfer resistance is

attributed to the reduced triple phase boundary, which is

associated with reduced cathode catalyst activity resulted

from methanol crossover during freeze-thaw cycling. It was

obvious that the cell that had a thinner membrane was more

vulnerable and showed the dramatic increases in the charge

transfer resistance. Therefore, using a thickerMEAmembrane

effectively reduced the increase in charge transfer resistance

Fig. 6 e Changes in ohmic and charge transfer resistances

of cells with Nafion 112, Nafion 115 and Nafion 117 during

freeze-thaw cycling: - ohmic resistance of the cell with

Nafion 112, ohmic resistance of the cell with Nafion 115,

ohmic resistance of the cell with Nafion 117,, charge

transfer resistance of the cell with Nafion 112, charge

transfer resistance of the cell with Nafion 115 and charge

transfer resistance of the cell with Nafion 117.

Fig. 7 e Changes in cyclic voltammetry graph of cells with

Nafion 112, Nafion 115 and Nafion 117 after 0, 3, 10, 20

freeze-thaw cycles: (a) Nafion 112, (b) Nafion 115 and (c)

Nafion 117.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 5 7 6 0e1 5 7 6 515764

that was caused by freeze-thaw cycling and can prevent the

increase of performance loss caused by repetitive freeze-thaw

cycles.

Cyclic voltammetry (CV) was analyzed on the cathode side

of the cells with Nafion 112, Nafion 115 and Nafion 117 during

freeze-thaw cycles at �32 �C (Fig. 7). The ECSA changes of the

three cells were calculated during freeze-thaw cycling, as

shown in Table 1. The three cells showed the same

decreasing trend in ECSAs after 20 freeze-thaw cycles. The

loss of ECSA indicates that either a reduction in catalyst ac-

tivity or changes to the microstructure of the catalyst layer

occurred after freeze-thaw cycling. Although the cell with

Nafion 112 showed the greatest reduction in performance, no

significant decrease in ECSA was observed. This means that

drastic performance degradation of the cell with Nafion 112

did not only result from damaged triple phase boundaries

which means microstructural changes such as micro cavity

and a detachment of ionomer from catalyst particles but also

from the decreased oxidation reduction reactions (ORR) on

the cathode which are originated from methanol crossover

during freeze-thaw cycling. The performance degradation of

the cell with Nafion 112, a thin membrane was also most

affected by these problems and the other factors such as the

increase in the cathode overpotential and the increase in the

ohmic and charge transfer resistance. However, the cells with

Nafion 115 and Nafion 117 showed a significant decrease in

cathode ECSA after 20 freeze-thaw cycles. Therefore, the

rapid performance degradation of cells with thinner mem-

branes is more related to the severe destruction of triple

phase boundary and the deteriorated catalytic activity from

repetitive freeze-thaw cycles than the cells with the thicker

membranes.

Conclusions

The effect of the DMFCswithmembranes of varying thickness

was evaluated during freeze-thaw cycling. At �32 �C, The

maximumpower density of cells with Nafion 112 decreased by

Table 1 e Changes to cathode ECSA during freeze-thawcycles for Nafionmembranes (m2/g) of varying thickness.

Cycle number Nafion 112 Nafion 115 Nafion 117

Before freezing 23.3 25.6 24.3

3 cycles 22.1 22.9 25.1

10 cycles 21.4 18.7 26.4

20 cycles 19.5 15.0 15.0

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 5 7 6 0e1 5 7 6 5 15765

69%, whereas cells with Nafion 115 and Nafion 117 decreased

by 36% and 28%, respectively. The performance loss was

mainly caused by an increase in cathode overpotential and

the cathode overpotential of the cell with thinner membrane

was more affected by repetitive freeze-thaw cycles due to

severe methanol crossover. From the impedance data, the

increase in charge transfer resistance was more significant

than the increase in ohmic resistance, which indicates that

the reduction in performance was mostly attributed to the

electrodes, especially the cathode, rather than themembrane.

Compared with the cells with the thicker membranes, the

charge transfer resistance of the cell with a thin membrane

was also more affected by freeze-thaw cycling due to the

formation of micro cavity and derachments of ionomer from

catalyst between membrane and catalyst and the decreased

catalytic activity. As a result, using thickmembranes for MEAs

is the goodmethod to reduce performance degradation during

freeze-thaw cycling.

Acknowledgments

This work was supported by the Next Generation Military

Battery Research Center Program of the Defense Acquisition

Program Administration and the Agency for Defense

Development.

r e f e r e n c e s

[1] Samsung SDI. Samsung SDI develops military portableDMFC. Fuel Cells Bull 2009;6:6e7.

[2] Kamarudin SK, Daud WRW, Ho SL, Hasran UA. Overview onthe challenges and developments of micro-direct methanolfuel cells (DMFC). J Power Sources 2007;163:743e54.

[3] Kamarudin SK, Achmad F, Daud WRW. Overview on theapplication of direct methanol fuel cell (DMFC) for portableelectronic devices. Int J Hydrogen Energy 2009;34:6902e16.

[4] Kamaruddin MZF, Kamarudin SK, Daud WRW, Masdar MS.Renew Sustain Energy Rev 2013;24:557e65.

[5] Luo Maji, Huang Chengyong, Liu Wei, Luo Zhiping, Pan Mu.Degradation behaviors of polymer electrolyte membrane fuelcell under freeze/thaw cycles. Int J of Hydrogen Energy2010;35:2989e93.

[6] Zhang Shengsheng, Yu Hongmei, Zhu Hong, Hou Junbo,Yi Baolian, Ming Pingwen. Effects of freeze/thaw cycles andgas purging method on polymer electrolyte membrane fuelcells. Chin J Chem Eng 2006;14:802e5.

[7] Gavello G, Zeng J, Francia C, Icardi UA, Graizzaro A,Specchia S. Experimental studies on Nafion® 112 single PEM-FCs exposed to freezing conditions. Int J Hydrogen Energy2011;36:8070e81.

[8] Kim S, Mench MM. Physical degradation of membraneelectrode assemblies undergoing freeze/thaw cycling:micro-structure effects. J Power Sources2007;174:206e20.

[9] Oszcipok M, Zedda M, Riemann D, Geckeler D. Lowtemperature operation and influence parameters on the coldstart ability of portable PEMFCs. J Power Sources2006;154:404e11.

[10] Oszcipok M, Riemann D, Kronenwett U, Kreideweis M,Zedda M. Statocs analysis of operational influences on thecold start behavior of PEM fuel cells. J Power Sources2005;145:407e15.

[11] Guo Q, Qi Z. Effect of freeze-thaw cycles on the propertiesand performance of membrane- electrode assemblies. JPower Sources 2006;160:1269e74.

[12] Hou J, Yu H, Zhang S, Sun S, Wang H, Yi B, et al. Analysis ofPEMFC freeze degradation at �20 �C after gas purging. JPower Sources 2006;162:513e20.

[13] Wang H, Hou J, Yu H, Sun S. Effects of reverse voltage andsubzero startup on the membrane electrode assembly of aPEMFC. J Power Sources 2007;165:287e92.

[14] Cha Hou-Chin, Chen Charn-Ying, Wang Rui-Xiang,Chang Chun-Lung. Performance test and degradationanalysis of direct methanol fuel cell membrane electrodeassembly during freeze/thaw cycles. J Power Sources2011;196:2650e60.

[15] Liu JG, Sun GG, Zhao FL, Wang GX, Zhao G, Chen LK, et al.Study of sintered stainless steel fiber felt as gas diffusionbacking in air-breathing DMFC. J Power Sources2004;133:175e80.

[16] Jiang Rongzhong, Chu Deryn. Comparative studies ofmethanol crossover and cell performance for a directmethanol fuel cell. J Electrochem Soc 2004;151:69e76.

[17] Ren X, Wilson MS, Gottesfeld S. High performance directmethanol polymer electrolyte fuel cells. J Electrochem Soc1996;143:12.

[18] Surapudi S, Narayn SR, Vamos E, Frank H, Halpert G,LaConti A, et al. Advances in direct oxidation methanol fuelcells. J Power Sources 1994;47:377e85.

[19] Liu JG, Zhao TS, Liang ZX, Chen R. Effect of membranethickness on the performance and efficiency of passivedirect methanol fuel cells. J Power Sources2006;153:61e7.

[20] Hwan Jung Doo, Hyeong Lee Chang, Soo Kim Chang, RyulShin Dong. Performance of a direct methanol polymerelectrolyte fuel cell. J Power Sources 1998;71:169e73.

[21] Yuan Xia-Zi, Zhang Shengshen, Wu Haijiang, Sun Jian Colin,Hiesgen Renate, Friedrich K Adreas, et al. Degradation of apolymer exchange membrane fuel cell stack with Nafion®

membranes of different thicknesses: Part I. In situ diagnosis.J Power Sources 2010;195:7594e9.

[22] Oh Yumi, Kim Sang-Kyung, Lim Seong yop, Jung Doo-Hwan,Peck Dong-Hyun, Shul Yonggun. The effects of freeze-thawcycling and gas purging on performance degradation indirect methanol fuel cells. Int J Hydrogen Energy2012;37:17268e74.

[23] Lee Sang-Yeop, Kim Sang-Uk, Kim Hyoung-Juhn, Jang JongHyun, Oh In-Hwan, Cho EunAe. Water removalcharacteristics of proton exchange membrane fuel cellsusing a dry gas purging method. J Power Sources2008;180:784e90.

[24] Ali Mokmeli, Saeed Asghari. An investigation into the effectof anode purging on the fuel cell performance. Int J HydrogenEnergy 2010;35:9276e82.

[25] Lee Yongtaek, Kim Bosung, Kim Yongchan, Li Xiaguo. Effectsof a microporous layer on the performance degradation ofproton exchange membrane fuel cells through repetitivefreezing. J Power Sources 2011;196:1940e7.