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Degradation mechanism of electrocatalyst duringlong-term operation of PEMFC

Chul Goo Chung, Lim Kim, Yong Wook Sung, Jinwoo Lee, Jong Shik Chung*

Department of Chemical Engineering, POSTECH, San 31, Hyoja-Dong, Nam-Gu, Pohang 790-784, Republic of Korea

a r t i c l e i n f o

Article history:

Received 7 April 2009

Received in revised form

6 August 2009

Accepted 30 August 2009

Available online 26 September 2009

Keywords:

Polymer electrolyte membrane

fuel cell

Electrocatalyst

Degradation mechanism

Pt sintering

Pt dissolution

* Corresponding author. Tel.: þ82 54 279 226E-mail address: jsc@postech.ac.kr (J.S. Ch

0360-3199/$ – see front matter Crown Copyrigdoi:10.1016/j.ijhydene.2009.08.094

a b s t r a c t

Long-term operation of a polymer electrolyte membrane fuel cell (PEMFC) was carried out

in constant-current (CC) and open-circuit-voltage (OCV) modes. The main factors causing

electrocatalyst deactivation were found to be Pt sintering and dissolution. In Pt sintering,

growth in particle size occurred mostly during the initial stage of operation (40 h).

Pt dissolution occurred mostly at the cathode, rather than the anode, due to chemical

oxidation of Pt to PtO by residual oxygen present in the cathode layer, resulting in a gradual

decrease in cell performance during long-term operation. After the dissolution of PtO in

water, Pt2þ was formed, which migrated from the cathode to the membrane phase, and

was re-deposited as Pt crystal upon reduction by crossover hydrogen, as was confirmed by

transmission electron microscopy (TEM) after long-term operation. Under normal oper-

ating conditions, there exists a balance at the cathode between chemical oxidation by

oxygen and electrochemical reduction by input electrons. Therefore, Pt dissolution at the

cathode is accelerated by an imbalance of these reactions under OCV conditions or by

a high O2 concentration in the feed.

Crown Copyright ª 2009 Published by Elsevier Ltd on behalf of Professor T. Nejat Veziroglu.

All rights reserved.

1. Introduction decrease in cell performance is exhibited by PEMFCs as the

Fuel cells convert chemical energy directly into electrical

energy and are one of the most promising tools for providing

environmentally clean power. For mobile applications such as

portable electronics and vehicles, polymer electrolyte fuel

cells (PEMFCs) are considered more attractive than other types

of fuel cells because of their relatively low operating temper-

atures and high specific power densities [1]. However, the

successful commercialization of PEMFCs would require

a reduction in the amount of the expensive platinum catalyst

used and a way to prevent degradation of the electrodes

during long-term operation. To date, most research has been

heavily focused on the development of new, more effective

catalysts and the preparation of high-performance electrodes

[2–9]. An equally important consideration is that a steady

7; fax: þ82 54 279 8453.ung).ht ª 2009 Published by Els

operation time increases, due to degradation of the

membrane electrode assembly (MEA) [10,11]. Degradation of

the MEA may be caused by (1) loss of activity of the electro-

catalyst, (2) carbon corrosion and (3) degradation of the poly-

mer-electrolyte membrane, among which the first item is

known to be the most important factor.

Concerning the deactivation of the electrocatalyst, sintering

of Pt during cell operation via Ostwald ripening has been

investigated by many research groups [12–18]. For example, the

results by Wilson et al. [14] showed a w60% reduction in surface

area of the Pt catalyst after a long operation of 4000 h. The

dissolution of platinum during cell operation has attracted

much attention since the loss of electrode catalyst is predomi-

nantly caused by this phenomenon. It is well known that

cathodic Pt can be dissolved and migrate into the membrane

evier Ltd on behalf of Professor T. Nejat Veziroglu. 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 e n e r g y 3 4 ( 2 0 0 9 ) 8 9 7 4 – 8 9 8 1 8975

phase during PEMFC operation [8,9,11,16,17] and that Pt ions in

the membrane phase can be chemically reduced by crossover

hydrogen from the anode side. Wang et al. have shown that the

position of reduced platinum in the membrane phase is affected

by gas compositions at both the anode and cathode [15–17].

Until now, the two deactivating factors of sintering and Pt

dissolution have been investigated independently. However,

the two issues are not separable and must occur simulta-

neously and/or sequentially during the operation of an actual

cell. In the present study, we investigated how the degrada-

tion of the MEA is affected by each of the two factors (sintering

and dissolution) during long-term cell operation. A major

focus was on revealing the mechanism of Pt dissolution at the

cathode, which was found to be a major contributor to the

deactivation of MEA cells during long-term operation.

2. Experimental

2.1. Activation of MEA

The MEA employed in this experiment was a commercial grade

(GORE� PRIMEA�, 25 cm2 surface area); both the cathode and

anode were supplied with a Pt loading of 0.4 mg/cm2. The

thicknesses of the cathode, membrane and anode were 10–

15 mm, 18–25 mm and 10–15 mm, respectively. A carbon paper

from SGL Co. was used for the gas diffusion layer (GDL) in the

unit cell. Before operating the unit cell, the fresh MEA was

activated, since an appreciable enhancement in the cathode

reactivity can be achieved by a proper activation process

[19,20]. The activation was conducted by operating the fresh

cell at 70 �C at a constant-voltage of 0.4 V for 8–10 h with feeds

of hydrogen and air, each with its relative humidity maintained

at 100%. The molar feed rates of hydrogen and oxygen in air

were held at 50% or 100% excess of the stoichiometric amount

for operating the cell in constant-current mode at 80 mA/cm2.

These values correspond to stoichiometric ratios (SR) of 1.5 and

2.0 for hydrogen and oxygen in air, respectively.

2.2. Unit cell tests

After the activation was completed, operation of the unit cell

was carried out at a temperature of 70 �C, with inlet gases

having a relative humidity of 100%. The flow rates of H2 and O2

were kept at SR values of 1.5 and 2.0, respectively. Either

constant-current (CC) mode (80 mA/cm2) or open-circuit-

voltage (OCV) mode was adopted for long-term operation.

During cell operation, polarization curves were taken at

regular intervals to investigate the degradation trend in cell

performance. Caution was taken to take the polarization

curves only as needed since excessive measurement could

affect cell performance. After operating the unit cell for a finite

period, it was disassembled and subjected to physical char-

acterization of the electrodes and membrane.

2.3. MEA characterization

For characterization after a finite period of operation, the MEA

was removed from the unit cell housing, and then the carbon

paper was removed from each side of the MEA. For TEM

analysis, a small piece was excised from the MEA and

embedded in epoxy resin (Araldite 502 embedding kit, Luft’s

Formula, Polyscience, Inc). The epoxy-embedded MEA was

dried for 30 min and cured at 60 �C for more than 12 h, fol-

lowed by a sectioning process using an ultra-microtome with

a diamond knife (DIATOME). The sliced sample (50–100 nm

thickness) was positioned on a copper grid (150 mesh) for

transmission electron microscopy (TEM, PHILIPS - CM200)

analysis. Micro-scale morphology and concentration of Pt in

the cross-section of the MEA were observed by using energy

dispersive X-ray spectroscopy (EDS) in the TEM. The cross-

section of the MEA was analyzed by scanning electron

microscopy (FE–SEM, Hitachi S4300SE, Japan) and identified by

an energy dispersive X-ray spectrometer (EDX). An X-ray

diffraction (XRD) pattern was observed with an M18XHF (Mac

Science Co.) diffractometer employing CuKa radiation

(l¼ 1.5405 A) as the X-ray source. The diffraction pattern was

examined in the regions of 2q¼ 5–90�, with a scanning speed

of 4.0�/min and with the X-ray gun operating at 40 kV and

200 mA. The crystalline structure was identified by the

comparison of measured XRD patterns with standard JCPDS

files. All the XRD measurements were done with new MEA

samples that were reacted for finite periods. After the reaction

test was carried out, the MEA was taken out of the reactor, and

the catalyst in the anode (or cathode) side of the MEA was

scraped with a knife after soaked in ethanol. The collected

catalyst samples were dried overnight prior to obtaining XRD

spectra.

3. Results and discussion

3.1. Cell degradation during long-term operation

Fig. 1 shows the polarization curves obtained, after activation

of the fresh MEA, at various intervals of operation, from the

shortest span, 48 h, to the longest, 1784 h, in CC mode at

80 mA/cm2. A large jump in the cell performance was

observed upon the initial activation treatment of the MEA,

after which, however, there was a steady and gradual degra-

dation of the cell performance as operation time increased.

The effect of the operation period on the growth of Pt particle

sizes in the anode and cathode was determined by means of

the X-ray line broadening method, and the results are shown

in Table 1. Pt particles in the fresh MEA are very small, aver-

aging 13.5 A in size. After the activation of the MEA, the size of

Pt in the cathode slightly increased, to 15.6 A, whereas the Pt

size in the anode was unchanged. During the CC-mode oper-

ation, Pt in both the anode and cathode grew rapidly, reaching

35.0–35.9 A at 40 h, after which there was a steady but a slow

increase in Pt size in the cathode only. However, the size

increase was negligible, from 35.9 A to 37 A during the

remaining period of operation (from 40 to 1784 h). The anode

Pt was very stable during the long-term operation, and the size

was completely unchanged within the range of measurement

error. The same kind of operation was carried out in open-

circuit-voltage (OCV) mode, and the results are shown in Fig. 2.

The cell operating conditions (temperature, feed rates and

humidity) were kept exactly the same as those in the CC-mode

Fig. 2 – Polarization curves of operated cell in open-circuit-

voltage (OCV) mode.Fig. 1 – Polarization curves of operated cell in constant-

current mode (80 mA/cm2).

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 e n e r g y 3 4 ( 2 0 0 9 ) 8 9 7 4 – 8 9 8 18976

trial. The pattern of the cell degradation is similar to that of

the CC mode, seen in Fig. 1.

To compare the results in more detail, the data in Fig. 1 and

Fig. 2 were re-plotted to show curves of voltage vs. time at

various fixed current densities under OCV- (Fig. 3(a)) and CC-

mode (80 mA/cm2) operation (Fig. 3(b)). The average voltage

decrease rates were 50 mV/1,000 h in OCV mode and 25 mV/

1,000 h in CC mode (80 mA/cm2), respectively. In both OCV and

CC modes, the rates of voltage decrease were not changed by

varying the current density. These results establish that the

OCV mode operation showed a twofold faster degradation of

cell voltage than the CC mode. Under OCV conditions, the

applied current was too small to dissolve sufficient quantities

of cathode Pt, even though the cell voltage was sufficiently

high. It is thus clear that the cathode Pt was likely oxidized to

PtO (or PtO2) through adsorption of oxygen, which must be

followed by chemical dissolution in the acidic aqueous solu-

tion. The oxidation of Pt can be accelerated by the absence of

the cathode reduction current, which would prohibit the

chemical oxidation of Pt [21].

The data presented in Figs. 1 and 2 were re-plotted in

a third way to show curves of the cell performance vs. time at

Table 1 – Pt particle size by XRD at various operatingtimes in constant-current mode (80 mA/cm2).

Operating time Particle size [A]

Anode Cathode

Fresh MEA 13.52 13.52

Activated MEA 13.50 15.59

20 h 16.89 16.90

40 h 35.01 35.86

325 h 34.50 36.61

582 h 35.21 37.04

1,784 h 34.51 37.05

various fixed voltages under OCV (Fig. 4(a)) and CC (80 mA/

cm2) modes (Fig. 4(b)). The cell performance, on the y-axis,

represents power density (current density multiplied by

voltage), which was normalized by dividing the power density

at time t by that at time zero. Firstly, it is interesting to observe

that there was little change in the cell performance in the low-

voltage (high-current) regions (<0.6 V), whereas a noticeable

change was observed at higher voltages (0.6 V and 0.8 V). We

suggest that, at low-voltages, the reaction is limited by mass-

transfer of reactant due to a high reaction rate (high-current

density). Therefore, to observe the real degradation status of

an electrode, measurement should be done in high-voltage

regions, where the reaction is not diffusion-limited due to

a relatively low current density. From the results in Fig. 4, we

can classify the changing pattern of the cell performance into

three categories by operating period: (i) a dramatic decrease

during initial stage of 40 h; (ii) an increase in the middle stage,

from 40 h to 300 h; and (iii) a monotonic slow decrease during

the last stage, from 300 h to the final time. In the middle stage,

however, some differences were noticed between the OCV

and CC modes of operation. The OCV mode showed a slower

rate of increase for a longer period at the lower voltages of 0.4

and 0.5 V, whereas, in CC mode, the performance showed

a faster rate of increase for a much shorter period at the higher

voltages of 0.6–0.8 V.

Table 1 presents changes in the Pt particle size observed

during cell operation. Initial Pt particle size in these electrodes

is very small, 13.5 A on average. However, the size increased

rapidly to w35–36 A at the initial 40 h-stage of cell operation in

both the cathode and anode, after which no further increase

was seen in the anode, but a small increase was seen in the

cathode. The particle growth of Pt was not dependent on

whether Pt exists in the anode or cathode or whether the cell

was operated at OCV or CC mode. We therefore think that the

initial rapid increases seen in particle size were caused by

the sintering (agglomeration) of Pt particles, probably because

Fig. 3 – Comparison of voltage drop at each current density vs. operating time. (a) OCV mode (b) constant-current mode

(80 mA/cm2).

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 e n e r g y 3 4 ( 2 0 0 9 ) 8 9 7 4 – 8 9 8 1 8977

the initial Pt size was too small to remain fixed on the carbon

surface. This indicates that very small particles (below

w2 nm) might be useless for PEMFC electrodes unless a way

could be found to anchor such small Pt particles firmly onto

the carbon surface.

After Pt particles in both the anode and cathode were

increased to 35–36 A at 40 h of operation, the Pt size in the

anode was nearly constant, and that in the cathode exhibited

a slight increase to 37 A at 1784 h of operation. However, the

cell performance decreased steadily, as shown in Fig. 3, at a rate

of 25–50 mV per 1000 h, depending on operating method.

Fig. 5 shows the Pt distribution pattern and morphology in

a cross-section of MEA analyzed by SEM-EDX. Compared with

Fig. 4 – Comparison of cell performance at each voltage value v

(80 mA/cm2).

fresh MEA (Fig. 5a), MEA operated with air for 1784 h showed

some loss of Pt, especially at the cathode (Fig. 5b). To our

surprise, compared with the MEA operated with air for 1784 h,

the MEA operated with pure oxygen for the even shorter time

of 500 h showed more loss of Pt at the cathode (Fig. 5c). This

was also accompanied by a greater decrease in Pt-layer

thickness in the cathode, establishing that the oxygen

concentration has a strong effect for Pt loss and carbon

oxidation in the cathode.

Fig. 6 shows a TEM observation for the MEA operated with

air for 1784 h. A great deal of Pt has been moved away from the

cathode and deposited in the membrane phase. No Pt depo-

sition in the membrane phase near the anode was observed

s. operating time. (a) OCV mode (b) constant-current mode

Fig. 5 – SEM-EDX images of cross-section of MEA under each condition: (a) Fresh MEA; (b) after 1,784 h of operation in

constant-current mode (80 mA/cm2) (Fuel: H2/Air); (c) after 500 h of operation in constant-current mode (80 mA/cm2)

(Fuel: H2/O2).

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 e n e r g y 3 4 ( 2 0 0 9 ) 8 9 7 4 – 8 9 8 18978

(data not shown), indicating that Pt deposited in the

membrane phase originated from the dissolution of Pt from

the cathode [21].

3.2. Degradation mechanism of electrocatalyst

From the XRD results and polarization curves of the MEA in

long-term cell operation, it seems that cell performance

rapidly decreased during the initial stage of 40 h due to sin-

tering or agglomeration of Pt particles, resulting in a signifi-

cant loss of Pt surface area. During the middle stage, from 40 h

to 300 h, the activity is slightly increased, probably due to

improvements in the physical properties of the interior elec-

trode structure [19,20]. A monotonic and continuous slow

decrease during last stage is due to Pt loss by dissolution at the

cathode [22]. Pt dissolution was confirmed from results of

Fig. 6 – TEM images of cross-section of MEA after 1,784 h operation in constant-current mode (80 mA/cm2) (Fuel: H2/Air):

(a) between anode interface and membrane; (b) between cathode interface and membrane.

Fig. 7 – TEM images of cross-section of MEA after 300 h

operation in constant-current mode (80 mA/cm2) (Fuel: H2/

Air): (a) between anode interface and membrane;

(b) between cathode interface and membrane.

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 e n e r g y 3 4 ( 2 0 0 9 ) 8 9 7 4 – 8 9 8 1 8979

SEM-EDX and TEM analyses, as presented in Figs. 5 and 6.

Among the possible reactions at each electrode, the degra-

dation mechanisms of the electrocatalyst can be inferred from

the following reactions.

Pt2þ þH2 / Pt0þ 2Hþ (1)

Pt0þ 1⁄2 O2 / PtO (2)

PtOþ 2Hþ/ Pt2þþH2O (3)

Pt2þ þ 2e�/ Pt0 (4)

Pt0 / Pt2þþ 2e� (5)

At the anode, electrochemical oxidation of Pt (5) can be

balanced with the chemical reduction by hydrogen (1). Like-

wise, at the cathode, chemical oxidation with oxygen (2) and

subsequent dissolution of PtO in water (3) can be balanced

with electrochemical reduction (4). Therefore, Pt oxidation

reactions (transformation of Pt into Pt2þ) can be accelerated by

an imbalance of the reduction reactions. A higher amount of

Pt loss observed at the cathode than at the anode indicates

that the chemical effect (chemical oxidation with oxygen at

the cathode and chemical reduction with hydrogen at anode)

is dominant compared to the electrochemical effect. Thus, in

the case of the anode, the chemical reduction force of

hydrogen would prevent the electrochemical oxidation of Pt.

Conversely, in the case of the cathode, it is expected that PtO

produced by the reaction of platinum and oxygen would be

converted to Pt2þ by reaction with dissolved Hþ in the water

phase. Pt2þ is transferred to the membrane or the GDL phase,

following the movement of water during fuel cell operation,

and Pt2þ in the membrane phase could be re-deposited as Pt0

after reduction by crossover hydrogen. This is a plausible

Fig. 8 – TEM images of cross-section of MEA after operation

in constant-current mode (80 mA/cm2) with air for the first

300 h and with oxygen for an additional 200 h: (a) between

anode interface and membrane; (b) in membrane;

(c) between cathode interface and membrane.

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 e n e r g y 3 4 ( 2 0 0 9 ) 8 9 7 4 – 8 9 8 18980

explanation for the gradual disappearance of platinum parti-

cles in the cathode and the formation of new platinum

particles in the membrane, away from the cathode, during

long-term operation.

Especially in the case of the cathode, operation at low

current (high-voltage) or high O2 concentration in the feed

results in a high concentration of residual oxidation in the

cathode, which promotes the formation of PtO and its

subsequent dissolution to Pt2þ. Pt2þ ions in the cathode must

move toward the membrane or GDL, resulting in accelerated

deactivation of cell performance. The decreased cell perfor-

mance observed at low current is explained by the results

shown in Fig. 3. As mentioned in Section 3.1, the average rate

of voltage decrease in OCV mode (50 mV/1,000 h) was two

times higher than that in CC mode (80 mA/cm2), which clearly

supports the suggested mechanism of Pt dissolution. Figs. 7

and 8 show the effects of oxygen concentration in the feed on

the Pt dissolution during operation under constant-current

mode (80 mA/cm2) with air for the first 300 h and with oxygen

for an additional 200 h. In order to see the O2 concentration

effect, the stoichiometric ratio of oxygen to the cathode was

held constant at 2.0 for these two cases by reducing the flow

rate of the oxygen feed to 21/79, comparable to that of air.

Compared to the air feed, the pure oxygen feed (Fig. 8) showed

more Pt dissolution in the membrane phase, with a sharper Pt

band that moved farther toward the anode side. These results

again confirm that higher oxygen concentration at the

cathode accelerates the chemical oxidation of Pt by Eq. (3),

resulting in more rapid degradation of cell performance.

However, at normal operating conditions, PtO formed by

residual oxygen in the cathode and Pt2þ ion dissolved in the

acidic water solution (wpH 5.4) can be reduced back to Pt at

the cathode by electrons transferred from the anode under

high-current conditions, resulting in re-precipitation of plat-

inum on the cathode by Eq.(4): Pt2þþ 2e�/ Pt0. Therefore, it is

thought that there is a balance between chemical oxidation by

oxygen and electrochemical reduction by electrons generated

in high-current CC mode. Due to this balance, the relative

degradation of the cell proceeded more slowly in current-

generating mode. On the contrary, in OCV conditions, current

does not flow, and the rate of chemical oxidation is much

faster than that of electrochemical reduction of Pt2þ, which

caused rapid deactivation of the cells in the case of OCV

conditions or high O2 concentration.

4. Conclusions

This work presents observations of electrocatalyst degrada-

tion during long-term operation of a PEMFC. Based on the

results, the following conclusions can be drawn. Cell degra-

dation should be measured in the high-voltage region (0.6–

0.8 V), where the reaction is not mass-transfer limited. During

long-term operation, the initial rapid decrease in cell perfor-

mance at 40 h was mostly caused by Pt particle size growth,

after which the Pt particle size remained unchanged at about

3.5 nm. Then, the activity gradually increased until 300 h,

which was caused by an improvement in the physical prop-

erties of MEA in the presence of current during the reaction.

Thus, this activity increase was not observed in the OCV

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 e n e r g y 3 4 ( 2 0 0 9 ) 8 9 7 4 – 8 9 8 1 8981

mode. After passing 300 h, a monotonic decrease in the

activity was observed due to the Pt particle dissolution,

especially at the cathode. Pt dissolution inside the cathode is

generally caused by chemical oxidation by residual oxygen

present in the cathode. Therefore, the rate would be acceler-

ated in OCV mode (low current) or by keeping oxygen

concentration high in the feed. The Pt dissolution rate can be

somewhat retarded in CC mode, which reduces residual

oxygen at the cathode.

Acknowledgements

This work was supported by the New & Renewable Energy

R&D program (2004-N-FC12-P-01-0-000) under the Korea

Ministry of Commerce, Industry and Energy (MOCIE).

r e f e r e n c e s

[1] Ralph TR, Hogarth MP. Catalysis for low temperature fuelcells. Platinum Met-Rev 2002;46(1):3–14.

[2] Abaoud HA, Ghouse M, Lovell KV, Al-Motairy GN. A hybridtechnique for fabricating PEMFC’s low platinum loadingelectrodes. Int J Hydrogen Energy 2005;30:385–91.

[3] Curtin DE, Lousenberg RD, Henry TJ, Tangeman PC,Tisack ME. Advanced materials for improved PEMFCperformance and life. J. Power Sources 2004;131:41–8.

[4] Tailoka F, Fray DJ, Kumar RV. Application of Nafionelectrolytes for the detection of humidity in a corrosiveatmosphere. Solid State Ionics 2003;161:267–77.

[5] Smitha B, Sridhar S, Khan AA. Solid polymer electrolytemembranes for fuel cell applications. J Memb Sci 2005;259:10–26.

[6] Passalacqua E, Lufrano F, Squadrito G, Patti A, Giorgi L.Influence of the structure in low-Pt loading electrodes forpolymer electrolyte fuel cells. Electrochim Acta 1998;43(24):3665–73.

[7] Chun YG, Kim CS, Peck DH, Shin DR. Performance ofa polymer electrolyte membrane fuel cell with thin filmcatalyst electrodes. J Power Sources 1998;71:174–8.

[8] Wang M, Woob K, Lou T, Zhaia Y, Kim D. Defining catalystlayer ingredients in PEMFC by orthogonal test and C–Vmethod. Int J Hydrogen Energy 2005;30:381–4.

[9] Ambrosioa EP, Franciaa C, Manzolib M, Penazzia N,Spinellia P. Platinum catalyst supported on mesoporouscarbon for PEMFC. Int J Hydrogen Energy 2008;33:3140–5.

[10] Huang C, Tan K, Lin J, Tan KL. XRD and XPS analysis of thedegradation of the polymer electrolyte in H2–O2 fuel cell.Chem Phys Lett 2003;371:80–5.

[11] Knights SD, Colbow KM, St-Pierre J, Wilkinson DP. Agingmechanisms and lifetime of PEFC and DMFC. J Power Sources2004;127:127–34.

[12] Borup Rod L, Davey John R, Garzon Fernando H, Wood DavidL, Inbody Michael A. PEM fuel cell electrocatalyst durabilitymeasurements. J Power Sources 2006;163:76–81.

[13] Ferreira PJ, la O GJ, Shao-Horn Y, Morgan D, Makharia R,Kocha S, et al. Instability of Pt/C electrocatalysts in protonexchange membrane fuel cells. J Electrochem Soc 2005;152(11):A2256–71.

[14] Wilson MS, Garzon FH, Sickafus KE, Gottesfeld S. Surfacearea loss of supported platinum in polymer electrolyte fuelcells. J Electrochem Soc 1993;140(10):2872–7.

[15] Cleghorn SJC, Mayfield DK, Moore DA, Moore JC, Rusch G,Sherman TW, et al. A polymer electrolyte fuel cell life test: 3years ofcontinuousoperation. J Power Sources 2006;158:446–54.

[16] Teranishi K, Kawata K, Tsushima S, Hirai S. Degradationmechanism of PEMFC under open circuit operation.Electrochem Solid-State Lett 2006;9(10):A475–7.

[17] Min M, Cho J, Cho K, Kim H. Particle size and alloying effectsof Pt-based alloy catalysts for fuel cell applications.Electrochim Acta 2000;45:4211–7.

[18] Taniguchi A, Akita T, Yasudaa K, Miyazakia Y. Analysis ofdegradation in PEMFC caused by cell reversal during airstarvation. Int J Hydrogen Energy 2008;33:2323–9.

[19] Qi Z, Kaufman A. Enhancement of PEM fuel cell performanceby steaming or boiling the electrode. J Power Sources 2002;109:227–9.

[20] Qi Z, Kaufman A. Activation of low temperature PEM fuelcells. J Power Sources 2002;111:181–4.

[21] Kim L, Chung CG, Sung YW, Chung JS. Dissolution andmigration of platinum after long-term operation ofa polymer electrolyte fuel cell under various conditions. JPower Sources 2008;183:524–32.

[22] Schulze M, Schneider1 A, Gulzow E. Alteration of thedistribution of the platinum catalyst in membrane-electrodeassemblies during PEFC operation. J Power Sources 2004;127:213–21.