Journal of Energy Chemistry - DICPpemfc.dicp.ac.cn/201911.pdf · 2019. 12. 11. · 96 L. Qu et al. / Journal of Energy Chemistry 35 (2019) 95–103 The degradation of MEAs during

Journal of Energy Chemistry 35 (2019) 95–103

Contents lists available at ScienceDirect

Journal of Energy Chemistry

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

Effect of electrode Pt-loading and cathode flow-field plate type on the

degradation of PEMFC

Lijuan Qu a , b , Zhiqiang Wang a , Xiaoqian Guo a , Wei Song a , Feng Xie a , Liang He a , b , Zhigang Shao a , ∗, Baolian Yi a

a Fuel Cell System and Engineering Group, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China b University of Chinese Academy of Sciences, Beijing 10 0 049, China

a r t i c l e i n f o

Article history:

Received 27 July 2018

Revised 5 September 2018

Accepted 11 September 2018

Available online 19 September 2018

Keywords:

Proton exchange membrane fuel cell

Electrode platinum loading

Current-variation cycle

Traditional solid plate

Water transport plate

a b s t r a c t

The electrode Pt-loading has an effect on the number of active sites and the thickness of catalyst layer,

which has huge influence on the mass transfer and water management during dynamic process in PEM-

FCs. In this study, membrane electrode assemblies with different Pt-loadings were prepared, and PEMFCs

were assembled using those membrane electrode assemblies with traditional solid plate and water trans-

port plate as cathode flow-field plates, respectively. The performance and electrochemical surface area of

cells were characterized to evaluate the membrane electrode assemblies degradation after rapid current-

variation cycles. Scanning electron microscope and transmission electron microscope were used to in-

vestigate the decay of catalyst layers and Pt/C catalyst. With the increase of Pt-loading, the performance

degradation of membrane electrode assemblies will be mitigated. But higher Pt-loading means thicker

catalyst layer, which leads to a longer pathway of mass transfer, and it may result in carbon material

corrosion in membrane electrode assemblies. The decay of Pt/C catalyst in cathode is mainly caused by

the corrosion of carbon support, and the degradation of anode Pt/C catalyst is a consequence of migra-

tion and aggregation of Pt particles. And using water transport plate is beneficial to alleviating the age of

cathode Pt/C catalyst.

© 2018 Published by Elsevier B.V. and Science Press on behalf of Science Press and Dalian Institute of

Chemical Physics, Chinese Academy of Sciences

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. Introduction

Proton exchange membrane fuel cell (PEMFC) is considered as

promising energy power [1] that possesses less pollution [2,3] ,

igh efficiency [4–6] , and outstanding start-up rate [3,7–9] . Lately,

remendous progresses have been made for the commercial pop-

larization of PEMFC, particularly in the auto industry [10,11] .

owever, the expensive membrane electrode assemblies (MEAs)

12] and the suboptimal durability [13] have been the primary

roblems which impede the widespread application of PEMFC.

The rate of cathode oxygen reduction reaction (ORR) is sluggish,

hich is an important challenge for the development of PEMFC.

ntil now, Pt-based electrocatalysts are the best choices in prac-

ical terms [14] . However, since platinum is rare metal, its high

rice accounts for an important portion of the expensive mate-

ial cost of MEA [12] . Decreasing Pt-loading of MEA without cell

erformance loss is the aim of many researches on ORR electro-

atalyst. Although there are great efforts of researches on Pt-free

∗ Corresponding author. E-mail address: [email protected] (Z. Shao).

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ttps://doi.org/10.1016/j.jechem.2018.09.004

095-4956/© 2018 Published by Elsevier B.V. and Science Press on behalf of Science Press

RR electrocatalyst, owing to their poor performance and unsat-

sfying stability, it is very difficult for Pt-free electrocatalyst to be

sed in practical application. Therefore, most ORR electrocatalysts

sed today are based on Pt, which are dispersed on the carbon

lack support in the form of nanoparticles. With state-of-the-art

t/C catalysts, it is equally important to achieve platinum-loading

eduction as well as enhanced catalyst activity and MEA durability

15] . Thus it is necessary to understand the distinct degradation of

EAs with different Pt-loadings.

Comparing with steady-state, the dynamic operation is much

ore noticeable [16] . When it comes to automotive applications,

EMFC will undergo many drastic current changes, and its volt-

ge will oscillate. Since fuel is pure hydrogen and the easy na-

ure of hydrogen oxidation reaction, the anode potential is ap-

roximate value of reversible hydrogen potential, which indicates

hat cathode experiences potential oscillation when the cell volt-

ge changes [17] . When voltage changes, the corrosion of carbon

aterial can influence the long-term durability of PEMFCs [17] . Be-

ides, platinum is highly stable with both low and high cell voltage

n PEMFC. However, when cathode potential changes rapidly, plat-

num will dissolve quickly [18,19] . Thus for a long time, the cell

erformance will seriously degrade.

and Dalian Institute of Chemical Physics, Chinese Academy of Sciences

https://doi.org/10.1016/j.jechem.2018.09.004http://www.ScienceDirect.comhttp://www.elsevier.com/locate/jechemmailto:[email protected]://doi.org/10.1016/j.jechem.2018.09.004

96 L. Qu et al. / Journal of Energy Chemistry 35 (2019) 95–103

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The degradation of MEAs during dynamic processes is highly

impacted by water management. Water management is always

deemed to be a significant factor for optimal performance and

durability of PEMFC [20–22] . It is one of the major technical chal-

lenges to achieve proper proton exchange membrane hydration

without electrode flooding in PEMFCs. One way to improve the

water management is controlling the relative humidity of the re-

actants [23] . The other promising way to improve water manage-

ment is introducing water transport plates (WTPs), which is put

forward by the United Technologies Corporation (UTC) [24,25] . The

characteristics of WTPs have been detailed described in references

[24–26] .

Until now, there is no related report about the effect of elec-

trode Pt-loading on the degradation of MEAs in the PEMFC with

WTP as flow-field plates. Therefore, in this paper, MEAs with dif-

ferent Pt-loadings were prepared. Besides, to compare the effect

of cathode flow-field plates on MEAs degradation, traditional solid

plate (SP) and WTP were used as cathode flow-field plate respec-

tively, and different PEMFCs were assembled with different MEAs.

Following, the degradation degree of PEMFCs performance was

compared after current-variation cycles with measurements of po-

larization curves and cyclic voltammetry (CV) curves. In addition,

scanning electron microscope (SEM) and transmission electron mi-

croscope (TEM) were used to study the microstructure degradation

of MEAs.

2. Experimental

2.1. Preparation of MEAs

MEAs, with an active area of 5 cm 2 (2 cm ∗2.5 cm), were as-sembled with catalyst-coated proton exchange membrane (CCM)

and gas diffusion layer (GDL). The CCM and GDL were home-

made. Catalyst ink, which consisted of commercial catalyst pow-

der (70 wt% Pt/C, Johnson Matthey Corporation), Nafion solution

(5 wt%, DuPont Corp.) and iso-propyl alcohol, was prepared. Then

homogeneous catalyst ink was sprayed on the one side of pro-

ton exchange membrane (Nafion®212, DuPont) to prepare CCM.

Each MEA included two CCMs, with the side without catalyst

being sticked together. Four types of MEAs were prepared with

different Pt-loadings CCMs. The Pt-loading of CCM on both an-

ode and cathode was the same, and one side Pt-loadings were

0.1 mg Pt cm −2 , 0.2 mg Pt cm −2 , 0.3 mg Pt cm −2 , 0.4 mg Pt cm −2 , respec-

tively. Our home-made GDL was carbon paper (Toray, TGP-H-060)

as the substrate, with polytetrafluoroethylene (PTFE, 25 wt%) and

carbon black impregnating it. The MEAs were prepared by plac-

ing the GDLs on the anode and cathode side of CCMs, and subse-

quently by hot pressing at 140 °C and 0.2 MPa abs for 2 min.

2.2. Fuel cell design

A special single cell was designed [24] . For the WTPFC, the

cathode flow-field plate was WTP. And for the SPFC, the cath-

ode flow-field plate was SP. The anode flow-field plates were SPs.

Circulating water flowed through the hollowed channels between

flow-field plate and current collector plate, as well as saturated the

WTP. All the flow-field plates were made of graphite with thick-

ness of 1.3 mm, and parallel gas flow-field was machined with fol-

lowing dimensions: rib width of 0.8 mm, channel width of 0.8 mm,

and channel depth of 0.8 mm.

2.3. Fuel cell test system

The PEMFC test station was home-made. The test station could

control the operating parameters (such as relative humidity of re-

actants, cell temperature, gases flow rate and backpressure) during

ests. KFM 2030 (Kikusui, Japan) was used as the electric load in

he testing processes, and it could record data automatically. The

ell temperature was kept at 65 °C for every experiment. Both an-de and cathode gases were humidified by bubbling gas through

istilled water tanks held at an assigned temperature. Before gases

ere fed into, they were first humidified by passing through their

orresponding humidifiers.

.4. Degradation experiment conditions

Degraded MEAs were achieved by carrying out rapid current-

ariation cycles experiments with fuel cells by employing elec-

ric load. For a single current-variation cycle, the current density

volved as the following process: maintaining at 0 mA cm −2 for 2 s,hanging from 0 mA cm −2 to 600 mA cm −2 taking 1 s, maintainingt 600 mA cm −2 for 2 s, changing from 600 mA cm −2 to 0 mA cm −2

aking 1 s ( Fig. 1 ). It took about 6 s for each degradation cycle, and

here were 80,0 0 0 current-variation cycles in all for every MEA be-

ore degradation test stopped. Thus, the degradation procedure of

ach MEA lasted about 133.3 h. In the course of last 30,0 0 0 degra-

ation cycles, the anode was fed with saturated humidified hydro-

en (99.9%) at the flow rate of 80 mL min −1 and saturated humidi-ed air served as oxidant at the flow rate of 120 mL min −1 . At theeginning and the end of degradation cycles, cell performance was

ecorded respectively, with measurements of polarization curves

nd CV curves.

.5. Polarization curves

To study the performance degradation, polarization curve

as obtained. For every measurement, the measuring condi-

ions were maintained at the same. The flow rate of hydrogen

0.15 MPa abs ) was 100 mL min −1 and the air (0.15 MPa abs ) flow rate

as 800 mL min −1 . The pressure of circulating water was main-ained at 0.11 MPa abs . Both H 2 and air were saturated humidi-

ed. Before polarization curve test, cells were fully activated to a

teady-state.

.6. Cyclic voltammetry curves

In order to calculate electrochemical surface area (ECSA) of

EAs, CV measurements were carried out. For every measure-

ent, the test conditions were identical, cell cathode was fed with

aturated humidified nitrogen (0.15 MPa abs ) and its flow rate was

20 mL min −1 . Saturated humidified hydrogen (0.15 MPa abs ) wasupplied to anode with the flow rate of 80 mL min −1 . Before CVest, nitrogen and hydrogen purged cell until the cell open circuit

oltage was 0.1 V or below. The cathode potential scanned from

.08 V to 1.2 V (versus the reference electrode) with the scanning

ate of 50 mV s −1 at 65 °C. The CV curves were measured withHI-630C (CH Instruments, Inc.) with cathode serving as the work

lectrode, and anode acting as reference electrode (dynamic hydro-

en electrode, DHE) and counter electrode. The ECSA was calcu-

ated from integrated hydrogen desorption area of CV curves with

.21 mC cm Pt −2 as the conversion factor.

In order to compare the performance and electrochemical char-

cters of MEAs, electrochemical characterizations of all MEAs were

onducted in an identical PEMFC with SPs as flow-field plates.

.7. Transmission electron microscopy

In order to investigate the Pt/C catalyst degradation of differ-

nt MEAs, TEM images were obtained with employing JEOL JEM-

0 0 0EX transmission electron microscope, operating at 120 kV. Fol-

owing the sizes of Pt particles were analyzed. The size distribution

f Pt particles was obtained by calculating 300 particles size on

ach TEM image.

L. Qu et al. / Journal of Energy Chemistry 35 (2019) 95–103 97

Fig. 1. Schematic drawing of current density evolution of one cycle.

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.8. Scanning electron microscopy

The cross-section of the fresh and degraded MEAs was prepared

y riving MEAs in liquid nitrogen. The morphology of the cross

ection of MEAs was obtained with JEOL JSM-6360LV scanning

lectron microscope. Thus, the changes of microstructure from the

ristine and degraded MEAs were observed.

. Results and discussion

.1. Electrochemical characterization of MEAs

The polarization curves of MEAs before and after 80,0 0 0

urrent-variation cycles for various Pt-loading MEAs are shown in

ig. 2 . And the corresponding voltage loss percent at 10 0 0 mA cm −2

s shown in Fig. 4 (b). It is obvious that the performance loss is

ess when the Pt-loading increases. And the performance decline

f MEAs degraded in WTPFC is obviously lower than that de-

raded in SPFC. The cell voltage decline percent at 10 0 0 mA cm −2

f MEAs degraded in SPFC is 42.27%, 16.13%, 10.46%, corresponding

ig. 2. Comparison of polarization behavior of MEAs before and after current-variation cy

d) 0.4 mg cm −2 . (The Pt-loadings in all figures in this paper are one side electrode platin

t-loading of 0.2 mg cm −2 , 0.3 mg cm −2 , 0.4 mg cm −2 . After degra-ation cycles, the voltage at 10 0 0 mA cm −2 is −0.466 V when thet-loading is 0.1 mg cm −2 ( Fig. 4 (a)). However, when it comes toEAs degraded in WTPFC, the voltage decrease at 10 0 0 mA cm −2

s 16.36% (0.1 mg cm −2 ), 9.09% (0.2 mg cm −2 ), 7.84% (0.3 mg cm −2 ),.22% (0.4 mg cm −2 ) ( Fig. 4 (b)). Because the repeated rapid current-ariation operation can lead to the irreversible oxidation, dissolu-

ion, migration and aggregation of the cathode Pt in entire cathode

ayer [27] , which will cause the decay of MEAs, the performance

eclines.

A high-performance electrode in PEMFCs should have continu-

us electrolyte pathways to access the Pt surface throughout the

L. Carbon support corrosion will change the porous structure of

atalyst and catalyst/ionomer interfaces, which will lead to the de-

rease of proton conductivity in CL [28] . Thus the ohmic resistance

f degraded MEAs increases, such as the result in Fig. 2 (a). In ad-

ition, altering the porous structure in MEAs will lead to increas-

ng mass transfer resistance of reactants, which also results in de-

reased performance.

The CV curves of different MEAs before and after degrada-

ion cycles are presented in Fig. 3 . And corresponding ECSA de-

line percent is shown in Fig. 4 (d). The ECSA decline percent of

EAs degraded in SPFC is 72.53%, 59.90%, 47.73% and 60.58% with

lectrode Pt-loading of 0.1 mg cm −2 , 0.2 mg cm −2 , 0.3 mg cm −2 and.4 mg cm −2 , respectively. And the ECSA loss of MEAs degraded

n WTPFC is 48.61% (0.1 mg cm −2 ), 46.96% (0.2 mg cm −2 ), 37.27%0.3 mg cm −2 ), 50.37% (0.4 mg cm −2 ). During the current-variationycles, cells have undergone open circuit state, which means the

igh potential of cathode. It is reported that the Pt can be oxi-

ized to Pt–O at high potential, and the Pt–O can be chemically

issolved in solution, which will cause the Pt loss or precipitation

y reduction [29] . The ECSA should depend on two elements, the

ne is Pt particles number attached on carbon support, and the

ther is the Pt particles size [30] . The dissolved Pt ions may diffuse

nto the PEM and be chemically reduced by hydrogen crossover

rom anode, which can lead to the decrease of Pt particles num-

er on carbon support. Cooperating with the Ostwald ripening and

cles for various Pt-loading MEAs: (a) 0.1 mg cm −2 ; (b) 0.2 mg cm −2 ; (c) 0.3 mg cm −2 ; um loading.)

98 L. Qu et al. / Journal of Energy Chemistry 35 (2019) 95–103

Fig. 3. Comparison of CV curves characterization of MEAs before and after current-variation cycles for various Pt-loadings MEAs: (a) 0.1 mg cm −2 ; (b) 0.2 mg cm −2 ; (c) 0.3 mg cm −2 ; (d) 0.4 mg cm −2 .

Fig. 4. (a) MEAs performance and (b) their degradation percent at 10 0 0 mA cm −2 ; (c) normalized ECSA and (d) ECSA decline percent of MEAs before and after 80,0 0 0 current-variation cycles with different Pt-loadings.

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aggregation of cathode Pt nanoparticles, the cell ECSA declines.

ECSA loss will make a significant influence on the drop of ORR ki-

netics, and MEAs performance declines.

It is noted that the ECSA loss percent of MEAs with Pt-loading

of 0.4 mg cm −2 is higher than that of 0.2 mg cm −2 , 0.3 mg cm −2 ,and it is even higher than that of 0.1 mg cm −2 with MEAs degradedin WTPFCs. It might be owing to that the CL of MEAs with Pt-

loading of 0.4 mg cm −2 is too thick. Thicker CL can impede themass transfer in MEAs. During dynamic processes, the obstructed

ass transfer may lead to local starvation of reactants, which can

oost the corrosion of carbon materials. Electrochemical corrosion

f catalyst carbon-support will lead to the electrical isolation of Pt

articles because they are apart from the carbon-supports. These

t particles will tend to aggregate and grow up, which might be

he dominating factors that lead to the ECSA loss of Pt/C catalyst.

owever, although the ECSA degradation percent of MEAs with Pt-

oading of 0.4 mg cm −2 is the biggest, its performance loss is min-mum. That can be attributed to the greater basic amount of Pt/C

L. Qu et al. / Journal of Energy Chemistry 35 (2019) 95–103 99

Fig. 5. Voltage evolution of SPFC (a) and WTPFC (b) with Pt-loading of 0.1 mg cm −2 .

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atalyst for Pt-loading of 0.4 mg cm −2 . Despite the largest ratio lossf ECSA, the quantity of residual healthy Pt/C after 80,0 0 0 current-

ariation cycles is yet high, therefore the MEAs performance is still

igh and the performance loss is least.

It is obvious that the loss of performance and ECSA of MEAs

egraded in WTPFC are less than MEAs degraded in SPFC. With

he MEAs after current-variation cycles in SPFC, the performance

t 10 0 0 mA cm −2 declines about 0.257 V, 0.101 V and 0.064 V cor-esponding Pt-loading of 0.2 mg cm −2 , 0.3 mg cm −2 , 0.4 mg cm −2 .

ith regard to degraded MEA of Pt-loading 0.1 mg cm −2 , the volt-ge at 10 0 0 mA cm −2 is even lower than 0 V. However, when itomes to the MEAs degraded in WTPFC, performance decline is

bout 93 mV, 55 mV, 49 mV and 20 mV, respectively. Which is

uch smaller than that of MEAs degraded in SPFC, implying bet-

er durability of MEAs in WTPFC. This is owing to the ability of

TP improving water management [24,25] . Because of the wa-

er drainage function of WTP, excessive water is transported from

athode flow-field channels to circulating water chamber; if there

s insufficient of water in MEAs, WTP transports water from circu-

atory water chambers to MEAs [31] .

Fig. 5 shows the voltage evolution of SPFC ( Fig. 5 (a)) and

TPFC ( Fig. 5 (b)) with Pt-loading of 0.1 mg cm −2 . It is obvious that

Fig. 6. (a) TEM image and (b) Pt particles size histogram

PFC voltage at 600 mA cm −2 fluctuates severely at about 50,0 0 0egradation cycles, but voltage of WTPFC is more stable. And

bout 60,0 0 0 cycles, SPFC voltage at 600 mA cm −2 is lower than V occasionally, which is influenced by inadequate water manage-

ent. When it is at the end of degradation cycles, the voltage (at

00 mA cm −2 ) of SPFC is invariably under 0 V, which means thathe MEA is damaged seriously. With regard to WTPFC, voltage is

ore stable, and at later period of degradation cycles, performance

f WTPFC is much higher than that of SPFC. Therefore, the MEAs

egradation caused by water management can be mitigated with

TP as cathode flow-field plate.

.2. Physical characterization of MEAs

In order to monitor the degradation of Pt/C catalyst, TEM im-

ges of catalyst before and after current-variation cycles with dif-

erent MEAs were obtained. Fig. 6 shows the TEM image of Pt/C

atalysts and the distribution of Pt particles size before current-

ariation cycles. The average Pt particle size of original Pt/C cata-

yst is about 3.22 nm.

When it comes to MEAs after 80,0 0 0 current-variation cycles,

he Pt particles in cathode of all MEAs grow larger, and agglomer-

te can be clearly observed ( Fig. 7 ). It is obvious that after current-

ariation cycles, the size distributions of cathode Pt particles are

uch wider and there are long tails in the large particle size part.

t is account for that under fuel cell running conditions, oxygen is

dsorbed, split and converted to water on catalyst active sites, and

ome fundamental reaction processes are accompanied by struc-

ural changes of Pt catalyst. Repeated rapid potential cycling can

ead to the mixed state of Pt catalyst with various structural con-

itions, which may cause the degradation of cathode Pt catalyst

32] . In addition, there is report that rapid potential variation can

ead to Pt oxidation and dissolution [33,34] , and for Ostwald ripen-

ng, small particles shrink and the other big particles grow [35] . All

hese factors lead to the growing up and aggregation of Pt parti-

les. The long tails in the large particle size part may derive from

he micrometer-scale platinum dissolution-diffusion-precipitation

echanism [34] .

After current-variation cycles, with regard to the MEAs de-

raded in SPFC, the average sizes of Pt particles in cathode are

.30 nm, 6.73 nm, 7.08 nm and 7.79 nm, respectively, corresponding

he Pt-loadings of 0.1 mg cm −2 , 0.2 mg cm −2 , 0.3 mg cm −2 and.4 mg cm −2 ( Fig. 7 (a), (c), (e) and (g)). It is obvious that theverage Pt particle size of cathode catalysts becomes larger as

he cathode Pt-loading increases. This is a result of more serious

orrosion of carbon support with higher cathode Pt-loading, which

eans a thicker CL. Thicker CL results in longer distance of mass

ransfer, which may cause uneven reactants distribution. Uneven

uel distribution in a PEMFC causes “no fuel” regions, which

ill be occupied by oxygen permeating through the membrane,

esulting in a potential jump of the cathode to meet the demand

f current, thus carbon corrosion is accelerated [28] . Because Pt

of Pt/C catalyst before current-variation cycles.

100 L. Qu et al. / Journal of Energy Chemistry 35 (2019) 95–103

Fig. 7. Comparison of TEM images of cathode catalysts after current-variation cycles for various Pt-loading MEAs: (a) and (b) 0.1 mg cm −2 ; (c) and (d) 0.2 mg cm −2 ; (e) and (f) 0.3 mg cm −2 ; (g) and (h) 0.4 mg cm −2 . (a), (c), (e), and (g) degraded in SPFC; (b), (d), (f), and (h) degraded in WTPFC.

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particles are anchored on carbon support, carbon corrosion can

cause Pt particles detaching from it. Pt particles dissolve, diffuse

and redeposit onto larger particles, thus the size of Pt particles

increase [17] . So, a direct effect of thicker CL is harmful for mass

transfer in MEAs, which can aggravate the carbon material decay.

The corrosion of carbon support can cause the detachment of Pt

particles from it and aggregation of Pt particles [30] . In the case

of oxidant starvation on cathode, hydrogen pump will occur and

hydrogen is generated in cathode. The presence of hydrogen on

athode could chemically generate heat on the platinum particles

36] . The solubility of Pt increases with temperature [17] , so local

xidant starvation can accelerate the dissolution of Pt particles.

pecifically, the density of Pt particles on carbon support after

ynamic cycles decreases obviously in comparison with the initial

t/C catalyst. Additionally, there is obviously bare carbon support,

nd the distribution of Pt particles on carbon support is uneven,

hich can be contributed to that Pt particles could detach from

arbon support under the potential cycling condition [37] . When it

L. Qu et al. / Journal of Energy Chemistry 35 (2019) 95–103 101

Fig. 8. Comparison of TEM images of SPFC and WTPFC anode catalysts after current-change cycling for various Pt-loading MEAs: (a) and (b) 0.1 mg cm −2 ; (c) and (d) 0.2 mg cm −2 ; (e) and (f) 0.3 mg cm −2 ; (g) and (h) 0.4 mg cm −2 . (a), (c), (e) and (g) degraded in SPFC; (b), (d), (f) and ( h) degraded in WTPFC.

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omes to the MEAs degraded in WTPFC, after current-variation cy-

les, the average Pt particle sizes in cathode are 5.84 nm, 5.88 nm,

.04 nm and 6.01 nm, respectively, corresponding to the Pt-loadings

f 0.1 mg cm −2 , 0.2 mg cm −2 , 0.3 mg cm −2 and 0.4 mg cm −2 ( Fig. (b), (d), (f) and (h)), which are also much larger than that of the

nitial Pt/C catalyst. Besides, there is aggregation of Pt for all MEAs.

omparing with MEAs degraded in SPFC, after current-variation

ycles, the average Pt particle size of MEAs degraded in WTPFC is

ess. In addition, the dispersion of Pt particles on carbon support

s better than that of MEAs degraded in SPFC. It implies that

sing WTP is beneficial to alleviating the degradation of cathode

atalysts during the dynamic process. Moreover, the average Pt

article sizes of MEAs degraded in WTPFC after current-variation

ycles are approximate, and it might be that WTP can mitigate the

nfluence of CL thickness on mass transfer effectively because of

ts ability to improve water management.

Fig. 8 shows the comparison of TEM images of anode catalysts

fter current-variation cycles for various Pt-loading MEAs. After

102 L. Qu et al. / Journal of Energy Chemistry 35 (2019) 95–103

Fig. 9. Comparison of SEM images of cathode catalyst layer before (a) and (d) and after (b), (c), (e) and (f) current-variation cycles for various Pt-loading MEAs: (a), (b) and

(c) 0.1 mg cm −2 ; (d), (e) and (f) 0.2 mg cm −2 . (b) and (e) degraded in SPFC; (c) and (f) degraded in WTPFC.

Fig. 10. Comparison of SEM images of anode catalyst layer before (a) and (d) and after (b), (c), (e), and (f) current-variation cycles for various Pt-loading MEAs: (a), (b), and

(c) 0.1 mg cm −2 ; (d), (e), and (f) 0.2 mg cm −2 . (b) and (e) degraded in SPFC; (c) and (f) degraded in WTPFC.

L. Qu et al. / Journal of Energy Chemistry 35 (2019) 95–103 103

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ynamic cycles, the average Pt particle sizes are approximate with

ll MEAs. Comparing with initial Pt/C catalyst, the Pt particles grow

p a little ( < 0.4 nm). However, it cannot be ignored that the aggre-

ation of anode Pt/C catalyst is more serious than that of cathode

t/C catalyst. But the distribution of anode Pt particles on carbon

upport is more even than that of cathode Pt particles. It can be

ttributed to that the cathode has suffered high potential during

urrent-variation cycles, and the Pt oxide layers forming and re-

oval repeatedly, which leads to the quicker decay of cathode Pt/C

atalyst [38] .

Because the difference of performance degradation between

EAs degraded in SPFC and WTPFC increases as the electrode Pt-

oading decreases, MEAs with lower Pt-loading (0.1 mg cm −2 and.2 mg cm −2 ) were chosen to compare the difference of CL decay.

Fig. 9 shows the SEM images of cathode CL. It can be seen

hat the thickness of cathode catalyst layer from fresh MEAs

ith Pt-loading 0.1 mg cm −2 is 0.648 μm ( Fig. 9 (a)). However, af-er 80,0 0 0 current-variation cycles, the thickness of cathode CL for

EA degraded in SPFC is 0.5 μm ( Fig. 9 (b)) and that for MEA de-raded in WTPFC is 0.592 μm ( Fig. 9 (c)). For the case of Pt-loading.2 mg cm −2 , the thickness of cathode CL changes from 0.592 μmfresh) ( Fig. 9 (d)) to 0.889 μm (MEA degraded in SPFC) ( Fig. 9 (e))nd 1.246 μm (MEA degraded in WTPFC) ( Fig. 9 (f)). The thicknessf cathode CL of MEAs degraded in WTPFC is higher than that of

EAs degraded in SPFC, and it can be contributed to the severer

athode carbon support corrosion of MEAs degraded in SPFC. Se-

ious carbon support corrosion can bring about running off of Pt

articles and attenuation of CLs. The above state further demon-

trates that taking advantage of WTP as cathode flow-field plate is

ood for the durability of MEAs during the current-variation cycles.

Fig. 10 presents the SEM images of anode CL. It is obviously that

he CL thickness of degraded MEAs is approximately the same with

resh MEAs when the Pt-loading is uniform, which also proves that

he degradation of anode catalyst is a consequence of migration

nd aggregation of Pt particles, rather than carbon support corro-

ion.

. Conclusions

The performance and ECSA of MEAs were characterized to eval-

ate the MEAs degradation after 80,0 0 0 rapid current-variation cy-

les. With the increase of Pt-loading, MEA performance degrada-

ion will be mitigated. But when the Pt-loading is 0.4 mg cm −2 ,he degraded percent of ECSA is largest, which may result from

he weaker mass transfer in thicker cathode CL, and blocked mass

ransfer can lead to degradation of carbon materials. Besides, the

oss of performance and ECSA of MEAs degraded in SPFC is higher

han that of MEAs degraded in WTPFC.

SEM and TEM images confirm that the WTP as cathode flow-

eld plate can mitigate the degradation of Pt/C catalyst caused by

ass transfer in CL, because of the ability of WTP to improve the

ater management of PEMFC. Moreover, it is concluded that the

athode Pt/C catalyst decay is mainly caused by the corrosion of

arbon support, and the degradation of anode Pt/C catalyst is a

onsequence of migration and aggregation of Pt particles.

cknowledgments

This work was financially supported by the National Key

esearch and Development Program of China (Grant no.

016YFB0101208), NSFC-Liaoning Joint Funding (Grant no.

1508202) and the National Natural Science Foundations of

hina (Grant no. 61433013 and 91434131 )

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Effect of electrode Pt-loading and cathode flow-field plate type on the degradation of PEMFC1 Introduction2 Experimental2.1 Preparation of MEAs2.2 Fuel cell design2.3 Fuel cell test system2.4 Degradation experiment conditions2.5 Polarization curves2.6 Cyclic voltammetry curves2.7 Transmission electron microscopy2.8 Scanning electron microscopy

3 Results and discussion3.1 Electrochemical characterization of MEAs3.2 Physical characterization of MEAs

4 Conclusions Acknowledgments References