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

9 Pages·2019·4.391 MB·English

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

Journal of Energy Chemistry 35 (2019) 95–103
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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 a b s t r a c t
Article history: The electrode Pt-loading has an effect on the number of active sites and the thickness of catalyst layer,
Received 27 July 2018 which has huge influence on the mass transfer and water management during dynamic process in PEM-
Revised 5 September 2018 FCs. In this study, membrane electrode assemblies with different Pt-loadings were prepared, and PEMFCs
Accepted 11 September 2018
were assembled using those membrane electrode assemblies with traditional solid plate and water trans-
Available online 19 September 2018
port plate as cathode flow-field plates, respectively. The performance and electrochemical surface area of
Keywords: cells were characterized to evaluate the membrane electrode assemblies degradation after rapid current-
Proton exchange membrane fuel cell variation cycles. Scanning electron microscope and transmission electron microscope were used to in-
Electrode platinum loading vestigate the decay of catalyst layers and Pt/C catalyst. With the increase of Pt-loading, the performance
Current-variation cycle degradation of membrane electrode assemblies will be mitigated. But higher Pt-loading means thicker
Traditional solid plate catalyst layer, which leads to a longer pathway of mass transfer, and it may result in carbon material
Water transport plate 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
1. Introduction ORR electrocatalyst, owing to their poor performance and unsat-
isfying stability, it is very difficult for Pt-free electrocatalyst to be
Proton exchange membrane fuel cell (PEMFC) is considered as used in practical application. Therefore, most ORR electrocatalysts
a promising energy power [1] that possesses less pollution [2,3] , used today are based on Pt, which are dispersed on the carbon
high efficiency [4–6] , and outstanding start-up rate [3,7–9] . Lately, black support in the form of nanoparticles. With state-of-the-art
tremendous progresses have been made for the commercial pop- Pt/C catalysts, it is equally important to achieve platinum-loading
ularization of PEMFC, particularly in the auto industry [10,11] . reduction as well as enhanced catalyst activity and MEA durability
However, the expensive membrane electrode assemblies (MEAs) [15] . Thus it is necessary to understand the distinct degradation of
[12] and the suboptimal durability [13] have been the primary MEAs with different Pt-loadings.
problems which impede the widespread application of PEMFC. Comparing with steady-state, the dynamic operation is much
The rate of cathode oxygen reduction reaction (ORR) is sluggish, more noticeable [16] . When it comes to automotive applications,
which is an important challenge for the development of PEMFC. PEMFC will undergo many drastic current changes, and its volt-
Until now, Pt-based electrocatalysts are the best choices in prac- age will oscillate. Since fuel is pure hydrogen and the easy na-
tical terms [14] . However, since platinum is rare metal, its high ture of hydrogen oxidation reaction, the anode potential is ap-
price accounts for an important portion of the expensive mate- proximate value of reversible hydrogen potential, which indicates
rial cost of MEA [12] . Decreasing Pt-loading of MEA without cell that cathode experiences potential oscillation when the cell volt-
performance loss is the aim of many researches on ORR electro- age changes [17] . When voltage changes, the corrosion of carbon
catalyst. Although there are great efforts of researches on Pt-free material can influence the long-term durability of PEMFCs [17] . Be-
sides, platinum is highly stable with both low and high cell voltage
in PEMFC. However, when cathode potential changes rapidly, plat-
inum will dissolve quickly [18,19] . Thus for a long time, the cell
∗ Corresponding author.
E-mail address: [email protected] (Z. Shao). performance will seriously degrade.
https://doi.org/10.1016/j.jechem.2018.09.004
2095-4956/©2018PublishedbyElsevierB.V.andSciencePressonbehalfofSciencePressandDalianInstituteofChemicalPhysics,ChineseAcademyofSciences
96 L. Qu et al. / Journal of Energy Chemistry 35 (2019) 95–103
The degradation of MEAs during dynamic processes is highly tests. KFM 2030 (Kikusui, Japan) was used as the electric load in
impacted by water management. Water management is always the testing processes, and it could record data automatically. The
deemed to be a significant factor for optimal performance and cell temperature was kept at 65 °C for every experiment. Both an-
durability of PEMFC [20–22] . It is one of the major technical chal- ode and cathode gases were humidified by bubbling gas through
lenges to achieve proper proton exchange membrane hydration distilled water tanks held at an assigned temperature. Before gases
without electrode flooding in PEMFCs. One way to improve the were fed into, they were first humidified by passing through their
water management is controlling the relative humidity of the re- corresponding humidifiers.
actants [23] . The other promising way to improve water manage-
ment is introducing water transport plates (WTPs), which is put 2.4. Degradation experiment conditions
forward by the United Technologies Corporation (UTC) [24,25] . The
characteristics of WTPs have been detailed described in references Degraded MEAs were achieved by carrying out rapid current-
[24–26] . variation cycles experiments with fuel cells by employing elec-
Until now, there is no related report about the effect of elec- tric load. For a single current-variation cycle, the current density
trode Pt-loading on the degradation of MEAs in the PEMFC with evolved as the following process: maintaining at 0 mA cm −2 for 2 s,
WTP as flow-field plates. Therefore, in this paper, MEAs with dif- changing from 0 mA cm −2 to 600 mA cm −2 taking 1 s, maintaining
ferent Pt-loadings were prepared. Besides, to compare the effect at 600 mA cm −2 for 2 s, changing from 600 mA cm −2 to 0 mA cm −2
of cathode flow-field plates on MEAs degradation, traditional solid taking 1 s ( Fig. 1 ). It took about 6 s for each degradation cycle, and
plate (SP) and WTP were used as cathode flow-field plate respec- there were 80,0 0 0 current-variation cycles in all for every MEA be-
tively, and different PEMFCs were assembled with different MEAs. fore degradation test stopped. Thus, the degradation procedure of
Following, the degradation degree of PEMFCs performance was each MEA lasted about 133.3 h. In the course of last 30,0 0 0 degra-
compared after current-variation cycles with measurements of po- dation cycles, the anode was fed with saturated humidified hydro-
larization curves and cyclic voltammetry (CV) curves. In addition, gen (99.9%) at the flow rate of 80 mL min −1 and saturated humidi-
scanning electron microscope (SEM) and transmission electron mi- fied air served as oxidant at the flow rate of 120 mL min −1 . At the
croscope (TEM) were used to study the microstructure degradation beginning and the end of degradation cycles, cell performance was
of MEAs. recorded respectively, with measurements of polarization curves
and CV curves.
2. Experimental
2.5. Polarization curves
2.1. Preparation of MEAs
To study the performance degradation, polarization curve
MEAs, with an active area of 5 cm 2 (2 cm ∗2.5 cm), were as- was obtained. For every measurement, the measuring condi-
tions were maintained at the same. The flow rate of hydrogen
saenmd bgleads wdiiftfhu sicoanta llyasyte-rc o(aGteDdL )p. rTohtoen CeCxMch aanngde GmDeLm bwrearnee h(CoCmMe-) (w0a.1s5 8M0P0a m absL ) m winas − 11 . 0T0h me Lp mreinss −u1r ea nodf tchier cauilra t(i0n.1g5 wMaPtae arb sw ) aflso wm arainte-
made. Catalyst ink, which consisted of commercial catalyst pow-
der (70 wt% Pt/C, Johnson Matthey Corporation), Nafion solution tained at 0.11 MPa abs . Both H 2 and air were saturated humidi-
fied. Before polarization curve test, cells were fully activated to a
(5 wt%, DuPont Corp.) and iso-propyl alcohol, was prepared. Then
steady-state.
homogeneous catalyst ink was sprayed on the one side of pro-
ton exchange membrane (Nafion®212, DuPont) to prepare CCM. 2.6. Cyclic voltammetry curves
Each MEA included two CCMs, with the side without catalyst
being sticked together. Four types of MEAs were prepared with In order to calculate electrochemical surface area (ECSA) of
different Pt-loadings CCMs. The Pt-loading of CCM on both an- MEAs, CV measurements were carried out. For every measure-
ode and cathode was the same, and one side Pt-loadings were ment, the test conditions were identical, cell cathode was fed with
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- saturated humidified nitrogen (0.15 MPa abs ) and its flow rate was
tively. Our home-made GDL was carbon paper (Toray, TGP-H-060) 120 mL min −1 . Saturated humidified hydrogen (0.15 MPa abs ) was
as the substrate, with polytetrafluoroethylene (PTFE, 25 wt%) and supplied to anode with the flow rate of 80 mL min −1 . Before CV
carbon black impregnating it. The MEAs were prepared by plac- test, nitrogen and hydrogen purged cell until the cell open circuit
ing the GDLs on the anode and cathode side of CCMs, and subse- voltage was 0.1 V or below. The cathode potential scanned from
quently by hot pressing at 140 °C and 0.2 MPa abs for 2 min. 0.08 V to 1.2 V (versus the reference electrode) with the scanning
rate of 50 mV s −1 at 65 °C. The CV curves were measured with
2.2. Fuel cell design CHI-630C (CH Instruments, Inc.) with cathode serving as the work
electrode, and anode acting as reference electrode (dynamic hydro-
A special single cell was designed [24] . For the WTPFC, the gen electrode, DHE) and counter electrode. The ECSA was calcu-
cathode flow-field plate was WTP. And for the SPFC, the cath- lated from integrated hydrogen desorption area of CV curves with
ode flow-field plate was SP. The anode flow-field plates were SPs. 0.21 mC cm Pt −2 as the conversion factor.
Circulating water flowed through the hollowed channels between In order to compare the performance and electrochemical char-
flow-field plate and current collector plate, as well as saturated the acters of MEAs, electrochemical characterizations of all MEAs were
WTP. All the flow-field plates were made of graphite with thick- conducted in an identical PEMFC with SPs as flow-field plates.
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, 2.7. Transmission electron microscopy
and channel depth of 0.8 mm.
In order to investigate the Pt/C catalyst degradation of differ-
2.3. Fuel cell test system ent MEAs, TEM images were obtained with employing JEOL JEM-
20 0 0EX transmission electron microscope, operating at 120 kV. Fol-
The PEMFC test station was home-made. The test station could lowing the sizes of Pt particles were analyzed. The size distribution
control the operating parameters (such as relative humidity of re- of Pt particles was obtained by calculating 300 particles size on
actants, cell temperature, gases flow rate and backpressure) during each TEM image.
L. Qu et al. / Journal of Energy Chemistry 35 (2019) 95–103 97
Pt-loading of 0.2 mg cm −2 , 0.3 mg cm −2 , 0.4 mg cm −2 . After degra-
dation cycles, the voltage at 10 0 0 mA cm −2 is −0.466 V when the
Pt-loading is 0.1 mg cm −2 ( Fig. 4 (a)). However, when it comes to
MEAs degraded in WTPFC, the voltage decrease at 10 0 0 mA cm −2
is 16.36% (0.1 mg cm −2 ), 9.09% (0.2 mg cm −2 ), 7.84% (0.3 mg cm −2 ),
3.22% (0.4 mg cm −2 ) ( Fig. 4 (b)). Because the repeated rapid current-
variation operation can lead to the irreversible oxidation, dissolu-
tion, migration and aggregation of the cathode Pt in entire cathode
layer [27] , which will cause the decay of MEAs, the performance
declines.
A high-performance electrode in PEMFCs should have continu-
ous electrolyte pathways to access the Pt surface throughout the
CL. Carbon support corrosion will change the porous structure of
catalyst and catalyst/ionomer interfaces, which will lead to the de-
crease of proton conductivity in CL [28] . Thus the ohmic resistance
Fig. 1. Schematic drawing of current density evolution of one cycle. of degraded MEAs increases, such as the result in Fig. 2 (a). In ad-
dition, altering the porous structure in MEAs will lead to increas-
ing mass transfer resistance of reactants, which also results in de-
2.8. Scanning electron microscopy
creased performance.
The CV curves of different MEAs before and after degrada-
The cross-section of the fresh and degraded MEAs was prepared
tion cycles are presented in Fig. 3 . And corresponding ECSA de-
by riving MEAs in liquid nitrogen. The morphology of the cross
cline percent is shown in Fig. 4 (d). The ECSA decline percent of
section of MEAs was obtained with JEOL JSM-6360LV scanning
MEAs degraded in SPFC is 72.53%, 59.90%, 47.73% and 60.58% with
electron microscope. Thus, the changes of microstructure from the
electrode Pt-loading of 0.1 mg cm −2 , 0.2 mg cm −2 , 0.3 mg cm −2 and
pristine and degraded MEAs were observed.
0.4 mg cm −2 , respectively. And the ECSA loss of MEAs degraded
in WTPFC is 48.61% (0.1 mg cm −2 ), 46.96% (0.2 mg cm −2 ), 37.27%
3. Results and discussion (0.3 mg cm −2 ), 50.37% (0.4 mg cm −2 ). During the current-variation
cycles, cells have undergone open circuit state, which means the
3.1. Electrochemical characterization of MEAs
high potential of cathode. It is reported that the Pt can be oxi-
dized to Pt–O at high potential, and the Pt–O can be chemically
The polarization curves of MEAs before and after 80,0 0 0
dissolved in solution, which will cause the Pt loss or precipitation
current-variation cycles for various Pt-loading MEAs are shown in
Fig. 2 . And the corresponding voltage loss percent at 10 0 0 mA cm −2 by reduction [29] . The ECSA should depend on two elements, the
one is Pt particles number attached on carbon support, and the
is shown in Fig. 4 (b). It is obvious that the performance loss is
other is the Pt particles size [30] . The dissolved Pt ions may diffuse
less when the Pt-loading increases. And the performance decline
into the PEM and be chemically reduced by hydrogen crossover
of MEAs degraded in WTPFC is obviously lower than that de-
graded in SPFC. The cell voltage decline percent at 10 0 0 mA cm −2 from anode, which can lead to the decrease of Pt particles num-
ber on carbon support. Cooperating with the Ostwald ripening and
of MEAs degraded in SPFC is 42.27%, 16.13%, 10.46%, corresponding
Fig. 2. Comparison of polarization behavior of MEAs before and after current-variation cycles for various Pt-loading 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 . (The Pt-loadings in all figures in this paper are one side electrode platinum 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.
aggregation of cathode Pt nanoparticles, the cell ECSA declines. mass transfer may lead to local starvation of reactants, which can
ECSA loss will make a significant influence on the drop of ORR ki- boost the corrosion of carbon materials. Electrochemical corrosion
netics, and MEAs performance declines. of catalyst carbon-support will lead to the electrical isolation of Pt
It is noted that the ECSA loss percent of MEAs with Pt-loading particles because they are apart from the carbon-supports. These
of 0.4 mg cm −2 is higher than that of 0.2 mg cm −2 , 0.3 mg cm −2 , Pt particles will tend to aggregate and grow up, which might be
and it is even higher than that of 0.1 mg cm −2 with MEAs degraded the dominating factors that lead to the ECSA loss of Pt/C catalyst.
in WTPFCs. It might be owing to that the CL of MEAs with Pt- However, although the ECSA degradation percent of MEAs with Pt-
loading of 0.4 mg cm −2 is too thick. Thicker CL can impede the loading of 0.4 mg cm −2 is the biggest, its performance loss is min-
mass transfer in MEAs. During dynamic processes, the obstructed imum. 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
SPFC voltage at 600 mA cm −2 fluctuates severely at about 50,0 0 0
degradation cycles, but voltage of WTPFC is more stable. And
about 60,0 0 0 cycles, SPFC voltage at 600 mA cm −2 is lower than
0 V occasionally, which is influenced by inadequate water manage-
ment. When it is at the end of degradation cycles, the voltage (at
600 mA cm −2 ) of SPFC is invariably under 0 V, which means that
the MEA is damaged seriously. With regard to WTPFC, voltage is
more stable, and at later period of degradation cycles, performance
of WTPFC is much higher than that of SPFC. Therefore, the MEAs
degradation caused by water management can be mitigated with
WTP as cathode flow-field plate.
3.2. Physical characterization of MEAs
In order to monitor the degradation of Pt/C catalyst, TEM im-
ages of catalyst before and after current-variation cycles with dif-
ferent MEAs were obtained. Fig. 6 shows the TEM image of Pt/C
catalysts and the distribution of Pt particles size before current-
variation cycles. The average Pt particle size of original Pt/C cata-
lyst is about 3.22 nm.
When it comes to MEAs after 80,0 0 0 current-variation cycles,
the Pt particles in cathode of all MEAs grow larger, and agglomer-
ate can be clearly observed ( Fig. 7 ). It is obvious that after current-
variation cycles, the size distributions of cathode Pt particles are
much wider and there are long tails in the large particle size part.
It is account for that under fuel cell running conditions, oxygen is
Fig. 5. Voltage evolution of SPFC (a) and WTPFC (b) with Pt-loading of 0.1 mg cm −2 . adsorbed, split and converted to water on catalyst active sites, and
some fundamental reaction processes are accompanied by struc-
tural changes of Pt catalyst. Repeated rapid potential cycling can
lead to the mixed state of Pt catalyst with various structural con-
catalyst for Pt-loading of 0.4 mg cm −2 . Despite the largest ratio loss ditions, which may cause the degradation of cathode Pt catalyst
of ECSA, the quantity of residual healthy Pt/C after 80,0 0 0 current- [32] . In addition, there is report that rapid potential variation can
variation cycles is yet high, therefore the MEAs performance is still lead to Pt oxidation and dissolution [33,34] , and for Ostwald ripen-
high and the performance loss is least. ing, small particles shrink and the other big particles grow [35] . All
It is obvious that the loss of performance and ECSA of MEAs these factors lead to the growing up and aggregation of Pt parti-
degraded in WTPFC are less than MEAs degraded in SPFC. With cles. The long tails in the large particle size part may derive from
the MEAs after current-variation cycles in SPFC, the performance the micrometer-scale platinum dissolution-diffusion-precipitation
at 10 0 0 mA cm −2 declines about 0.257 V, 0.101 V and 0.064 V cor- mechanism [34] .
responding Pt-loading of 0.2 mg cm −2 , 0.3 mg cm −2 , 0.4 mg cm −2 . After current-variation cycles, with regard to the MEAs de-
With regard to degraded MEA of Pt-loading 0.1 mg cm −2 , the volt- graded in SPFC, the average sizes of Pt particles in cathode are
age at 10 0 0 mA cm −2 is even lower than 0 V. However, when it 6.30 nm, 6.73 nm, 7.08 nm and 7.79 nm, respectively, corresponding
comes to the MEAs degraded in WTPFC, performance decline is the Pt-loadings of 0.1 mg cm −2 , 0.2 mg cm −2 , 0.3 mg cm −2 and
about 93 mV, 55 mV, 49 mV and 20 mV, respectively. Which is 0.4 mg cm −2 ( Fig. 7 (a), (c), (e) and (g)). It is obvious that the
much smaller than that of MEAs degraded in SPFC, implying bet- average Pt particle size of cathode catalysts becomes larger as
ter durability of MEAs in WTPFC. This is owing to the ability of the cathode Pt-loading increases. This is a result of more serious
WTP improving water management [24,25] . Because of the wa- corrosion of carbon support with higher cathode Pt-loading, which
ter drainage function of WTP, excessive water is transported from means a thicker CL. Thicker CL results in longer distance of mass
cathode flow-field channels to circulating water chamber; if there transfer, which may cause uneven reactants distribution. Uneven
is insufficient of water in MEAs, WTP transports water from circu- fuel distribution in a PEMFC causes “no fuel” regions, which
latory water chambers to MEAs [31] . will be occupied by oxygen permeating through the membrane,
Fig. 5 shows the voltage evolution of SPFC ( Fig. 5 (a)) and resulting in a potential jump of the cathode to meet the demand
WTPFC ( Fig. 5 (b)) with Pt-loading of 0.1 mg cm −2 . It is obvious that of current, thus carbon corrosion is accelerated [28] . Because Pt
Fig. 6. (a) TEM image and (b) Pt particles size histogram 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.
particles are anchored on carbon support, carbon corrosion can cathode could chemically generate heat on the platinum particles
cause Pt particles detaching from it. Pt particles dissolve, diffuse [36] . The solubility of Pt increases with temperature [17] , so local
and redeposit onto larger particles, thus the size of Pt particles oxidant starvation can accelerate the dissolution of Pt particles.
increase [17] . So, a direct effect of thicker CL is harmful for mass Specifically, the density of Pt particles on carbon support after
transfer in MEAs, which can aggravate the carbon material decay. dynamic cycles decreases obviously in comparison with the initial
The corrosion of carbon support can cause the detachment of Pt Pt/C catalyst. Additionally, there is obviously bare carbon support,
particles from it and aggregation of Pt particles [30] . In the case and the distribution of Pt particles on carbon support is uneven,
of oxidant starvation on cathode, hydrogen pump will occur and which can be contributed to that Pt particles could detach from
hydrogen is generated in cathode. The presence of hydrogen on carbon 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.
comes to the MEAs degraded in WTPFC, after current-variation cy- is better than that of MEAs degraded in SPFC. It implies that
cles, the average Pt particle sizes in cathode are 5.84 nm, 5.88 nm, using WTP is beneficial to alleviating the degradation of cathode
6.04 nm and 6.01 nm, respectively, corresponding to the Pt-loadings catalysts during the dynamic process. Moreover, the average Pt
of 0.1 mg cm −2 , 0.2 mg cm −2 , 0.3 mg cm −2 and 0.4 mg cm −2 ( Fig. particle sizes of MEAs degraded in WTPFC after current-variation
7 (b), (d), (f) and (h)), which are also much larger than that of the cycles are approximate, and it might be that WTP can mitigate the
initial Pt/C catalyst. Besides, there is aggregation of Pt for all MEAs. influence of CL thickness on mass transfer effectively because of
Comparing with MEAs degraded in SPFC, after current-variation its ability to improve water management.
cycles, the average Pt particle size of MEAs degraded in WTPFC is Fig. 8 shows the comparison of TEM images of anode catalysts
less. In addition, the dispersion of Pt particles on carbon support after 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
dynamic cycles, the average Pt particle sizes are approximate with 2016YFB0101208), NSFC-Liaoning Joint Funding (Grant no.
all MEAs. Comparing with initial Pt/C catalyst, the Pt particles grow U1508202) and the National Natural Science Foundations of
up a little ( < 0.4 nm). However, it cannot be ignored that the aggre- China (Grant no. 61433013 and 91434131 )
gation of anode Pt/C catalyst is more serious than that of cathode
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