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Durability study of Pt–Pd/C as PEMFC cathode catalyst Zhi-Min Zhou a,b , Zhi-Gang Shao a, *, Xiao-Ping Qin a,b , Xu-Guang Chen a , Zi-Dong Wei c , Bao-Lian Yi a a Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Fuel Cell R&D Center, Dalian Liaoning 116023, China b Graduate School of Chinese Academy of Sciences, Beijing 100049, China c Chongqing University, Chongqing 400045, China article info Article history: Received 9 October 2009 Received in revised form 10 December 2009 Accepted 10 December 2009 Available online 31 December 2009 Keywords: PEMFC Pt–Pd/C Catalyst Durability Potential cycling Dynamic loading abstract In this paper, Pt–Pd/C and Pt/C catalysts were evaluated and compared. The catalysts were evaluated as oxygen reduction reaction (ORR) catalysts in half cell test under potential cycling, and cathode catalysts in single cell test under dynamic loading simulating the vehicle operation. Physical and electrochemical techniques were applied to investigate the structure, performance and durability of those catalysts. The electrochemical active surface area (ECA) loss, particle size distribution, polarization behavior and electrochem- istry impedance spectroscopy (EIS) suggested that the Pt–Pd/C showed a better durability than Pt/C. ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. 1. Introduction Proton exchange membrane fuel cells (PEMFCs) are consid- ered as promising alternative power sources for trans- portation due to the high energy efficiency and clean utilization. Within the last decades, much progress has been made for PEMFCs in materials, system design, manufacturing and application. Up to now, durability is thought to be one of the major problems hindering the commercialization of PEMFCs besides cost, hydrogen storage, etc. While the lifetime for stationary used PEMFCs has already exceeded 3 years, the lifetime for transportation used PEMFCs is still below 3000 h, which is nearly half of the 2010/2015 target lifetime 5000 h set by U.S. DOE [1]. It is known that PEMFCs will gradually degrade in performance due to many factors including operation, water-heat management, and materials deterioration, etc [2]. Membrane electrode assemble (MEA) is the most important component of PEMFCs. The deterioration of MEA directly leads to the degradation of PEMFCs. One of the most important causes for MEA deterioration is the degradation of catalyst in the cathode side due to the high voltage, low pH and oxidation environment [3]. For the most popular Pt/C catalyst, it is reported that the nano-sized platinum supported on carbon black will gradually grow larger during long-term operation, thus reduce the electrochemical active area (ECA) and resulted in an irreversible performance loss [4]. This degradation will be even severer when PEMFCs are operated under dynamic loadings. Borup investigated the degradation behavior of single cell under cycling dynamic loading and found that the growth extent of cathode catalyst * Corresponding author. Tel.: þ86 411 84379153. E-mail address: [email protected] (Z.-G. Shao). Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he international journal of hydrogen energy 35 (2010) 1719–1726 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.12.056

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Page 1: Durability study of Pt-Pd/C as PEMFC cathode catalystpemfc.dicp.ac.cn/__local/1/62/EB/0198B78881D9DB91D381494F0CE_673DBC7… · Durability study of Pt–Pd/C as PEMFC cathode catalyst

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 5 ( 2 0 1 0 ) 1 7 1 9 – 1 7 2 6

Avai lab le a t www.sc iencedi rec t .com

j ourna l homepage : www.e lsev ier . com/ loca te /he

Durability study of Pt–Pd/C as PEMFC cathode catalyst

Zhi-Min Zhou a,b, Zhi-Gang Shao a,*, Xiao-Ping Qin a,b, Xu-Guang Chen a,Zi-Dong Wei c, Bao-Lian Yi a

a Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Fuel Cell R&D Center, Dalian Liaoning 116023, Chinab Graduate School of Chinese Academy of Sciences, Beijing 100049, Chinac Chongqing University, Chongqing 400045, China

a r t i c l e i n f o

Article history:

Received 9 October 2009

Received in revised form

10 December 2009

Accepted 10 December 2009

Available online 31 December 2009

Keywords:

PEMFC

Pt–Pd/C

Catalyst

Durability

Potential cycling

Dynamic loading

* Corresponding author. Tel.: þ86 411 843791E-mail address: [email protected] (Z.-G

0360-3199/$ – see front matter ª 2009 Profesdoi:10.1016/j.ijhydene.2009.12.056

a b s t r a c t

In this paper, Pt–Pd/C and Pt/C catalysts were evaluated and compared. The catalysts were

evaluated as oxygen reduction reaction (ORR) catalysts in half cell test under potential

cycling, and cathode catalysts in single cell test under dynamic loading simulating the

vehicle operation. Physical and electrochemical techniques were applied to investigate the

structure, performance and durability of those catalysts. The electrochemical active

surface area (ECA) loss, particle size distribution, polarization behavior and electrochem-

istry impedance spectroscopy (EIS) suggested that the Pt–Pd/C showed a better durability

than Pt/C.

ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1. Introduction water-heat management, and materials deterioration, etc [2].

Proton exchange membrane fuel cells (PEMFCs) are consid-

ered as promising alternative power sources for trans-

portation due to the high energy efficiency and clean

utilization. Within the last decades, much progress has been

made for PEMFCs in materials, system design, manufacturing

and application. Up to now, durability is thought to be one of

the major problems hindering the commercialization of

PEMFCs besides cost, hydrogen storage, etc. While the lifetime

for stationary used PEMFCs has already exceeded 3 years, the

lifetime for transportation used PEMFCs is still below 3000 h,

which is nearly half of the 2010/2015 target lifetime 5000 h set

by U.S. DOE [1]. It is known that PEMFCs will gradually degrade

in performance due to many factors including operation,

53.. Shao).sor T. Nejat Veziroglu. Pu

Membrane electrode assemble (MEA) is the most important

component of PEMFCs. The deterioration of MEA directly leads

to the degradation of PEMFCs. One of the most important

causes for MEA deterioration is the degradation of catalyst in

the cathode side due to the high voltage, low pH and oxidation

environment [3]. For the most popular Pt/C catalyst, it is

reported that the nano-sized platinum supported on carbon

black will gradually grow larger during long-term operation,

thus reduce the electrochemical active area (ECA) and resulted

in an irreversible performance loss [4].

This degradation will be even severer when PEMFCs are

operated under dynamic loadings. Borup investigated the

degradation behavior of single cell under cycling dynamic

loading and found that the growth extent of cathode catalyst

blished by Elsevier Ltd. All rights reserved.

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particles was greater than that observed during steady-state

testing [5]. The half cell electrochemical potential cycling test

also revealed that higher upper potential and scan frequency

lead to a higher degradation rate [5]. Darling and Meyer

calculated the Pt dissolution under potential cycling and found

platinum will dissolve rapidly when transitioning from low to

high potentials [6]. The proposed degradation mechanisms of

Pt/C catalysts include coarsening of Pt particles, Pt dissolution,

carbon corrosion [7,8] and loss of contact of ionomer and Pt [9].

Much work has been tried to improve the performance and

durability of the catalyst. One approach is using alloy or

bimetallic catalysts [10]. However, if cheap transition metals

are used, they may dissolve and move into the polymer phase.

Zhang studied the Pt–Au/C catalyst and demonstrated that

this bimetallic catalyst exhibited fairly high durability under

half cell potential cycling test [11]. However, gold is still a very

expensive metal. In the past a few years, much attention has

been paid to Palladium and its alloys. Adzic and coauthors

made a series of study on the ORR activity of Pt-monolayer on

model Pd crystal surface substrate as well as Pd-monolayer on

model Pt crystal surface substrate through experimental and

computational methods [12–17]. They reported that when Pd

is added to Pt, a change in the electronic structure may lead to

a higher ORR activity in half cell evaluation. Li prepared Pt–Pd/

C catalyst using ethylene glycol (EG) reflux reduction method

and found that this kind of catalyst poses higher ORR activity

than Pt/C catalyst in half cell test [18]. Lim synthesized Pd–Pt

bimetallic nanodendrites consisting of a dense array of Pt

branches on a Pd core and found that the Pd–Pt nanodendrites

were two and half times more active on Pt mass for the ORR

than the state-of-the-art Pt/C catalyst [19]. Peng and Yang

using a sequential synthetic method prepared carbon sup-

ported Platinum-on-palladium bimetallic heterogeneous

nanostructure, in which 3-nm Pt particles grew on the

surfaces of 5-nm Pd nanoparticles. The electrochemical study

showed not only enhancement in electrocatalytic activity for

ORR but also improved stability in comparison to a commer-

cial platinum catalyst [20]. Based on the summaries above,

one can notice that Pt–Pd/C is a promising catalyst for PEMFCs.

In order to meet the practical applications, more investiga-

tions should be taken on its durability, especially under full

fuel cell test conditions under dynamic loading.

In this paper, the Pt–Pd/C and Pt/C catalyst was prepared by

an ethylene glycol reduction process. The designed metal

weight ratio is 50% which was preferred in practical PEMFC

application. In order to get a highly dispersion of fine metal

particles with such metal weight ratio, a high surface area

carbon black was used as the support. The Pt–Pd/C catalyst was

then compared with home-made Pt/C and commercial Pt/C

catalysts in terms of durability in both half cell and single cell

under dynamic loading simulating the vehicle operation.

2. Experimental

2.1. Catalyst preparation

The synthesis of the Pt–Pd/C catalyst was following the

ethylene glycol (EG) reflux reduction method described in Ref.

[18]. A high surface area carbon black was used as the support.

The carbon black bought from Dalian Sunrise Power Co., Ltd.

(P.R. China) has a high specific surface area of 770 m2 g�1.

Briefly, calculated amount of the carbon black was dispersed

in an EG solution by adding H2PtCl6 and PdCl2 under vigorous

stirring. The pH of the solution was then adjusted to 12 with

2 M NaOH. The mixture was kept at 130 �C for 3 h under the

protection of high purified nitrogen. After washing and drying

the Pt–Pd/C catalyst was obtained. The designed atom ratio

was Pt:Pd¼ 3:1. A Pt/C catalyst was prepared following the

same way. A commercial Pt/C catalyst from Tanaka Kikinzoku

Co., Ltd (Japan) was also used for comparison. For conve-

nience, the catalysts were denoted as Pt3Pd/S770, Pt/S770 and

TKK 46.6%, respectively.

2.2. Half cell test

The half cell test was conducted on a thin film rotating disk

electrode (TFRDE) in a three electrode electrochemical system.

It is a simple and fast way widely used to investigate the

activity and stability of fuel cell catalysts [21]. The TFRDE was

prepared as following: 5 mg catalyst was dispersed in

a mixture of 50 mL 5% Nafion solution and 4 ml ethanol. The

mixture was agitated by ultrasonic for 30 min to form an ink.

10 mL of this ink was then dropped on a glassy carbon rotating

disk electrode and dried to yield a thin film electrode. The

geometric area of the TFRDE is 0.1256 cm2 and the metal

loading is ca. 45 mg cm�2. During the durability test, the TFRDE

was subjected to a potential cycling test. Specifically, the

catalyst was repeatedly scanned from 0.6 V to 1.2 V versus

SHE at a rate of 50 mV s�1 in 0.5 M H2SO4 deaerated with high

purified N2. Cyclic voltammetry (CV) was recorded at the

beginning and every 100 cycles of the potential sweep by a CHI

600C potentistat from 0 to 1.2 V versus SHE at 50 mV s�1. The

ECA can be obtained by integrating the CV profile after sub-

tracting the double layer from the hydrogen adsorption peak

of each CV curve. The charge related to a monolayer H

adsorption to the catalyst crystal was supposed to be

210 mC cm�2.

The ORR curves were measured in an O2 saturated 0.5 M

H2SO4 from 1.0 V to 0.241 V versus SHE at 5 mV s�1 with

a rotating speed of 1600 rpm.

2.3. Single cell test

Based on the half cell test results, single cell test was con-

ducted to the Pt3Pd/S770 and TKK 46.6% catalysts on an Arbin

FCTS BT2000 model. The metal loading of all the Gas Diffusion

Electrode (GDE) was 0.5 mg cm�2. The MEA was manufactured

by hot pressing the GDE onto each side of a Nafion�-115

membrane. The single cell with an active area of 5 cm2 was

operated for 400 h under a cyclic dynamic loading. Within one

single cycle the current was varying in the following

sequence: 0 mA cm�2 for 60 s, 200 mA cm�2 for 60 s,

400 mA cm�2 for 180 s, 300 mA cm�2 for 60 s, 600 mA cm�2 for

60 s and 200 mA cm�2 for 60 s. The time of one cycle is 480 s, so

each single cell experienced 3000 cycles during the test. The

dynamic loading test was carried on an Arbin BT 2000 electric

load. A schematic presenting the controlled currents and

voltage reactions is given in Fig. 1. The cell temperature was

85 �C, which is between the typical 60–90 �C operation range

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Fig. 1 – Schematic of the dynamic loading in single cell

dynamic loading test.

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and high to accelerate the degradation rate. The reactant gas

was controlled as 35 ml min�1/800 ml min�1 for H2/air at

0.2 MPa. Polarization curves were recorded periodically under

the same operation parameters. After the I–V plot was recor-

ded, the in-situ electrochemical impedance spectroscopy (EIS)

was measured. The perturbation amplitude for the sine signal

was 20 mV over a frequency range from 10 kHz to 100 mHz.

CV was also periodically recorded to monitor the degradation

of ECA after purging the cathode with N2. The scan was from

0 to 1.0 V vs. SHE at 50 mV s�1. All the electrochemical char-

acterizations on single cell were carried on a potentiostat

Model 2273 A from EG&G Princeton Applied Research.

2.4. Physical characterization

X-ray diffraction (XRD) measurements were performed on

a PANalytical X’Pert Pro diffractometer with nickel-filtered

Cuka radiation at 40 kV and 40 mA. The lattice parameter A0

and the mean diameter of catalysts were calculated by Bragg

and Scherrer formula [22].

Transmission electron microscopy investigations of all

catalysts before and after the half cell test were carried on

a JEOL JEM-2011 electron microscope operating at 120 kV.

Fig. 2 – XRD patterns of Pt–Pd/C and Pt/C catalysts.

Particle size distribution diagrams were obtained by counting

all particles on each TEM picture.

Inductively coupled plasma-atomic emission spectroscopy

(ICP-AES) analyses were conducted for the element analysis

on Leeman Plasma–Spec–I equipment.

3. Results and discussion

3.1. Physical characterization

The powder XRD patterns and TEM images are shown in

Figs. 2 and 3, respectively. The lattice parameter (A0) and the

Fig. 3 – TEM images of fresh Pt–Pd/C and Pt/C catalysts: (a)

Pt3Pd/S770; (b) Pt/S770; (c) TKK 46.6.

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Table 1 – Physical characterization of catalysts.

Catalyst Peak pos. 220/(2q) Lattice parameterA0 (nm)

Mean particlesize d (nm)

Metal weightpercent (ICP-AES)

Atomic ratioPt/Pd (ICP-AES)

Pt3Pd/S770 67.88 0.390 2.97 48 3.3

Pt/S770 67.26 0.393 1.60 48 –

TKK 46.6% 67.23 0.394 2.51 45 –

Fig. 4 – Particle size histograms of fresh Pt–Pd/C and Pt/C

catalysts (a) Pt3Pd/S770; (b) Pt/S770; (c) TKK 46.6%

measured from TEM images in Fig. 3.

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mean particle size derived from (220) peak are summarized in

Table 1. All catalysts showed a face-centered cubic (FCC)

structure. The (220) peak of Pt3Pd/S770 is positively shifted

with a shortening of crystal parameter, indicating that Pd has

an obvious effect on the Pt crystal size. The particle size of

Pt3Pd/S770 increased by nearly 1.37 nm compared with

Pt/S770 due to the high precipitation tendency of Pd when

pH> 7. The atomic ratio of Pt/Pd is 3.3 close to the designed

value. The particle morphology of fresh catalysts is shown in

Fig. 3. Compared with Pt/S770, the metal particles for Pt3Pd/

S770 are larger and more agglomerates can be found, thus

resulted in a wider distribution of Pt3Pd/S770 than Pt/S770

presented in Fig. 4. The mean diameter of metal particles

measured by TEM is little larger than the value measured by

XRD, because particles smaller than 1 nm are hard to count in

TEM [23].

Fig. 5 – Electrochemical characterization of fresh Pt–Pd/C

and Pt/C catalysts: (a) CV curves; (b) ORR curves.

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Fig. 6 – Comparison of normalized ECA for various catalysts

during potential cycling.

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3.2. Electrochemistry test in half cell

The CV and ORR curves of as-prepared Pt3Pd/S770 and Pt/S770

catalysts are shown in Fig. 5. The hydrogen adsorption/

desorption peak area of the Pt3Pd/S770 declines as its particle

is larger. Another difference in Fig. 5(a) is the Pt–O reduction

peak of Pt3Pd/S770 shifts by 0.024 V compared to Pt/S770. It is

believed that a shift of the Pt–O reduction peak to higher

potential reflects a decrease of the adsorption strength for the

oxygen containing species on the surface, e.g. OHads [24,25].

Although Pt weight and ECA of Pt3Pd/S770 is less than Pt/S770,

its ORR curve is very close to the latter, which means Pt3Pd/

S770 poses a high activity comparable to pure Pt/S770.

However, the improvement of the ORR as Li[18] described was

not found, which may result from different carbon supports

and metal/carbon ratios.

Potential cycling is a widely used way to test the stability

and durability of fuel cell catalysts [24,26,27]. The potential

window chosen in this paper is 0.6–1.2 V vs. SHE, which is

a typical operation region of H2/O2 PEMFC. Fig. 6 shows the

normalized ECA variation of different catalyst during the half

cell potential cycling test. The ECA was normalized to the

initial ECA before potential cycling. As the results indicated,

all catalysts gradually decrease in ECA as the potential scan

proceeds. After the half cell test, nearly 55% of the initial ECA

of Pt3Pd/S770 has been lost, compared to 68% of Pt/S770 and

65% of TKK 46.6%. It can be clearly seen that Pt3Pd/S770 has

the lowest decreasing rate, while Pt/S770 has the highest one.

The ECA degradation is listed in Table 2.

Table 2 – Catalyst degradation in half cell.

Catalyst ECA loss Mean particle size by TEM (nm)

Before test After test

Pt3Pd/S770 55% 3.18 4.52

Pt/S770 68% 2.24 5.36

TKK 46.6% 65% 2.75 5.27

Fig. 7(a)–(c) presents the TEM images of all the catalysts after

the potential cycling test. The size distribution derived from

Fig. 7 is presented in Fig. 8. Particle sizes before and after the

potential cycling are also listed in Table 2. After the potential

cycling all particles grow larger and many agglomerates can be

clearly observed. However, the metal particle of Pt3Pd/S770

becomes the smallest one and the Pt/S770 becomes the largest

one. After the potential cycling, the size distribution of Pt/S770

becomes much wider with a tail toward to large particle size. It

is reported this shape of the particle size distribution may

ascribe to the micrometer-scale platinum dissolution–diffu-

sion–precipitation mechanism [23]. However, the same distri-

bution behavior is not found on Pt3Pd/S770, which suggests that

Fig. 7 – TEM images of Pt–Pd/C and Pt/C catalysts after

potential cycling: (a) Pt3Pd/S770; (b) Pt/S770; (c) TKK 46.6%.

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Fig. 8 – Particle size histograms of Pt–Pd/C and Pt/C

catalysts after potential cycling (a) Pt3Pd/S770; (b) Pt/S770;

(c) TKK 46.6% measured from TEM images in Fig. 7.

Fig. 9 – Comparison of polarization behavior after different

operation time for various catalysts: (a) Pt3Pd/S770; (b) TKK

46.6%.

Table 3 – Catalyst degradation in single cell.

Catalyst Voltage at 200 mA cm�2 (V) ECA loss

Before test After test

Pt3Pd/S770 0.758 0.749 37%

TKK 46.6% 0.748 0.703 58%

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the addition of Pd may change the degradation mechanism of

the catalyst. Mayrhofer [28] observed that Pt particles detached

from the carbon support under potential cycling. So ECA should

depend on two terms: one is the number of particles survived

on carbon support and the other is the size of those particles.

Actually, one could see that the particles on carbon are much

less in Fig. 8b than Fig. 8a. A further discussion will be con-

ducted later.

3.3. Durability test in single cell

Based on the results of half cell test, Pt3Pd/S770 and TKK 46.6%

were chosen to be evaluated under single cell condition,

which is more similar to the real operation conditions of

vehicles. Both cells experienced open circuit voltage, mediate

current density and high current density in each cycle which

was repeated 3000 times during the whole 400 h test. The

polarization curves of cells with Pt3Pd/S770 and TKK 46.6% are

shown in Fig. 9. As listed in Table 3, the initial performance of

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Fig. 11 – Comparison of normalized ECA for various

catalysts cathode under single cell operation.

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the two cells are close to each other, though the cell using

Pt3Pd/S770 are slightly better. The voltage at 200 mA cm�2 is

0.758 V for cell with Pt3Pd/S770 and 0.748 V for cell with TKK

46.6%. After 400 h operation, the performances of both cells

are degraded. The voltage at 200 mA cm�2 decreased to 0.749 V

for cell with Pt3Pd/S770 and 0.703 V for cell with TKK 46.6%.

The Nyquist plots at 200 mA cm�2 are shown in Fig. 10. All

plots present a single arc indicating the electrode process is

dominated by the interfacial kinetics of the ORR process [29]. It

is believed that the high-frequency intercept of the impedance

arc on the real axis Ro represents the total ohmic resistance of

the cell, while the diameter of the arc, Rct, is a measure of the

charge transfer resistance of the ORR. From Fig. 10, it can

be seen that the Ro of both cells did not change much during the

operation suggesting the100%RH humidification waskept well.

However, the Rct of both cells did increase obviously after the

test. The Rct of cell using TKK 46.6% increased much larger than

the Rct of cell using Pt3Pd/S770. This may be an explanation of

higher performance in the latter cell after the long time test.

The ECA change of single cell with running time is also

shown in Fig. 11 and Table 3. It is observed that the degrada-

tion phenomenon is similar to what found in half cell test. The

ECA declines faster in the cell using TKK 46.6% than in the cell

of Pt3Pd/S770. And the ECA loss may make a major contribu-

tion to the drop in ORR kinetics.

3.4. Proposed degradation mechanisms

The single cell test results along with the half cell test results

demonstrated that addition of Pd may improve the durability

of the Pt/C catalysts for PEMFC cathode. The direct evidences

are the growth of particle size and ECA loss. As pointed out in

the Introduction section, there are several mechanisms sug-

gested to explain this degradation. Shao-Horn and coauthors

discussed a few different mechanisms through experimental

and theoretical computation, and conclude that the sub-

monolayer dissolution of Pt nanoparticles may be the gov-

erning factor for the surface area loss [7]. In details, the Pt can

be oxidized to Pt–O between ca. 0.9 and 1.2 V. The oxidized Pt

then can be chemically dissolved into the solution, thus

causing a Pt loss or precipitation by reduction. A mathematic

Fig. 10 – Comparison of EIS for single cell using various

catalysts.

model of this mechanism built by Darling [6] also demon-

strated that the oxidation played an important role in the

dissolution of Pt. Although this mechanism needs more

investigations to be identified, it is acceptable to use it to

explain the durability enhancement of Pt–Pd/C catalyst in this

paper. Gasteiger studied the ORR activity at different Pt crystal

surface and supposed that the specific ORR activity can be

reflected by the position of Pt–O reduction peak [25]. When the

peak position is more positive, it means the catalyst binds

oxygen species more weakly and thus the specific ORR activity

is higher. As observed in Section 3.2, the position of Pt–O

reduction peak is shifted to more positive, which means the

Pt–Pd binds oxygen species more weakly than pure Pt. It is

reported that the addition of Pd may change the electronic

structure and d-band center of the catalyst surface, thus

weaken the adsorption of oxygen species [16]. So, here we

proposed a possible mechanism as following: the introduction

of Pd can alleviate the oxidation of catalyst, reduce its disso-

lution and finally improve the durability.

There are also other possible mechanisms. Holby and

coauthors studied the impact of particle size distribution on

related degradation rate by experiment and theoretic simu-

lation [30]. They found that larger particles are more stable

than smaller ones due to lower Gibbs–Thomson energy.

Adding Pd made the catalyst size larger than pure Pt but

without obvious deterioration of ORR activity. This may at the

same time enhance the stability of the catalyst. Carbon

corrosion also can cause coalescence of particles and

detachment of Pt particles from the support, just like what the

Fig. 8b of Pt/S770 reveals. Whether the addition of Pd could

alleviate the carbon corrosion is not clear yet. And the

degradation mechanisms still needs more work to be clarified.

4. Conclusions

The Pt–Pd/C catalyst was prepared and compared with home-

made and commercial Pt/C catalysts. Both half cell test and

single cell test demonstrated that this bimetal catalyst

showed a comparable performance and better durability than

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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 5 ( 2 0 1 0 ) 1 7 1 9 – 1 7 2 61726

conventional Pt/C catalyst. The possible reasons for the

durability enhancement are discussed. Firstly, Pd may modify

the electronic structure of catalyst surface and thus alleviate

the dissolution of it; secondly, increased particle size may also

improve the resistance to degradation; lastly, less detachment

from carbon support has been observed for Pt–Pd catalyst.

Further work is still needed to clarify the degradation

mechanisms.

Acknowledgements

This work was financially supported by the National High

Technology Research and Development Program of China (863

Program, No. 2007AA05Z137, 2008AA11A105) and the National

Natural Science Foundations of China (No. 20636060, 20936008).

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