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Perfluorosulfonic acid-functionalized Pt/carbon nanotubecatalysts with enhanced stability and performance for usein proton exchange membrane fuel cells

Daping He, Shichun Mu *, Mu Pan

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology,

Wuhan 430070, China

A R T I C L E I N F O

Article history:

Received 20 May 2010

Accepted 24 August 2010

Available online 31 August 2010

A B S T R A C T

To further improve lifetime and performance of carbon nanotube (CNT) supported Pt cata-

lysts for proton exchange membrane fuel cells, perfluorosulfonic acid (PFSA) was intro-

duced by a simple colloid route to functionalize Pt (PFSA-Pt/CNT) catalysts. Here the

PFSA is available as a binder to tightly anchor Pt nano-particles onto the CNT surfaces,

and as a proton conductor to increase the triple phase boundary zone of the catalysts.

The prepared Pt nano-particles ranging from 2 to 5 nm in diameter are uniformly deposited

on CNTs. A high catalytic activity of this novel composite catalyst was observed by both

cyclic voltammetry and oxygen reduction reaction (ORR) measurements. The loss rate of

the electrochemically active area of the PFSA-Pt/CNT catalyst decreases by a factor of

two in comparison with that of the plain Pt/CNT catalyst. Meanwhile, a lower loss rate

for the new catalyst was also observed by electrochemically-accelerated durability testing

for the ORR activity. These results indicate that the stability of the new catalyst is signifi-

cantly improved over that of the plain Pt/CNT catalyst by introduction of PFSA.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Proton exchange membrane fuel cells (PEMFCs) are a promis-

ing power technology for applications in portable, stationary

power sources and for automotive engines [1–3]. However, a

significant challenge hindering large-scale implementation

of PEMFCs is the performance loss during extended operation

and repeated cycling of fuel cells. Investigations have re-

vealed that the degradation of catalysts is one of the most

important reasons contributing to this performance loss

[4,5]. Currently, Pt nano-particles dispersed on high surface

area carbon (Pt/C) is the preferred catalyst in PEMFC system.

The corrosion of the carbon support leads to agglomeration

of Pt nano-particles under extremely harsh working environ-

ments, which is another important reason for catalyst deteri-

oration [4–6]. Moreover, some Pt particles get entrapped in

deep cracks in the carbon support and this causes the ab-

sence of the important triple phase boundary reaction zone

which is essential for PEMFC operation [6,7].

To solve these issues, some research groups introduced

CNTs as more effective supports of Pt catalysts. CNTs do

not have deep cracks and they possess many beneficial prop-

erties such as high electrical conductivity, high chemical sta-

bility, as well as extremely high mechanical strength and

modulus [8–10]. Several research papers reported that CNT-

supported Pt catalysts increase the catalytic activity [11–13]

and also show high electrochemical stability [14,15] compared

to conventional Pt/C catalysts. However, CNTs can also be

gradually oxidized for example in the presence of defects or

impurities at nanotube surfaces [16–18]. In addition, the inert

surface of CNTs leads to a weaker metal-support interaction

as well as a low dispersion of metal particles on CNT surfaces

0008-6223/$ - see front matter � 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.carbon.2010.08.045

* Corresponding author: Fax: +86 27 87879468.E-mail address: msc@whut.edu.cn (S. Mu).

C A R B O N 4 9 ( 2 0 1 1 ) 8 2 – 8 8

avai lab le at www.sc iencedi rec t .com

journal homepage: www.elsev ier .com/ locate /carbon

Author's personal copy

[19,20]. Therefore, producing well-dispersed metal particles

on CNT surfaces and further improving their electrochemical

stability remains an important challenge. Currently, an en-

hanced electrochemical stability for Pt/CNT catalyst can be

achieved for example by nitrogen doping [21,22]. Also, silica

layers were employed to prepare improved carbon nano-

tube-supported Pt metal particles [23,24]. However, more sim-

ple solutions are needed and the fundamental mechanism for

stabilizing PEMFC catalysts needs further investigation.

The perfluorosulfonic acid (PFSA) polymer, such as Nafion

as a proton conductor, can enhance the catalytic activity of Pt

catalysts by increasing the triple phase boundary reaction

zone. Addition of PFSA into the catalytic layer can be em-

ployed to improve the dispersion of Pt nano-particles on sup-

ports and to increase the ionic (protonic) conductivity of the

catalytic layer [25,26]. In previous work, we applied Nafion

to titanium diboride supported Pt catalysts, which improved

the dispersion of Pt nano-particles on inert surfaces of ceram-

ics [27]. In addition, Tian et al. [28] and our previous studies

[29] showed that SO3� end groups of Nafion act as a stabilizer

and facilitate the reaction for ORR at Pt nano-particles. They

also enhance the metal-support interaction. Thus, the PFSA

is important to improve the lifetime and performance of

CNT-supported Pt catalysts. Here, we demonstrated that Pt

nano-particles can be produced well-dispersed on the CNT

surfaces and the stability of the resulting composite catalyst

was greatly improved with additional PFSA.

2. Experimental

2.1. Preparation of PFSA-Pt/CNT and Pt/CNT catalysts

A volume of 20 ml H2PtCl6ÆH2O solution (1.5 mg ml�1), 100 ll

5 wt.% PFSA Nafion (Dupont Co.) and 200 ml ethylene glycol

were mixed and refluxed at 130 �C with the pH value of the

solution adjusted to 8–10 by adding NaOH solution. When

the color of the solution changed from yellowish to dark,

the PFSA-stabilized colloidal Pt nano-particles were formed.

Next, previously suspended single-walled carbon nanotubes

(SWCNTs) in ethylene glycol were added to the Pt colloid with

further agitation for 11 h. After filtration, washing, and drying

at 100 �C in a vacuum oven, PFSA functioned 20 wt.% Pt

loaded CNT catalysts were obtained. For comparison, the Pt/

CNT catalyst as a benchmark was prepared using the same

method except the addition of PFSA.

2.2. Characterization of the catalysts

Morphology studies of the prepared catalysts were performed

by JEOL 2010 transmission electron microscopy (TEM). The

prepared catalysts were dispersed ultrasonically in ethanol.

Energy dispersive X-ray analysis (EDX) was used to determine

the metal contents in the samples. An aliquot of this solution

was deposited on a grid and dried at room temperature. X-ray

diffraction (XRD) analysis was performed using a Rigaku X-

ray diffractometer with CuKa radiation source. The XRD scans

were investigated at 2 min�1 from 20� to 90�. The average

crystallite size (D) of Pt nano-particles can be obtained

according to the Scherrer equation [30] using the Pt (220) peak.

D ¼ kkb

cos h ð1Þ

here k = 0.89; k (the X-ray wave length for Cu radiation) =

0.1541 nm; b is the half height peak width; h is the Bragg angle

corresponding to the peak maximum; D, denotes the average

crystallite size. Infrared spectra of the prepared PFSA-Pt/CNT

catalyst were carried out to detect the presence of PFSA in Pt/

CNT catalysts by Fourier transform infrared spectroscopy

(Nicolet MAGNA-IR 560, FTIR, USA). The reference PFSA was

prepared by evaporating 5.0 wt.% PFSA liquid. The electro-

chemical performance of the catalysts was characterized with

a conventional triple electrode system. A saturated calomel

electrode (SCE) was used as reference, and a platinum wire

was used as counter electrode. The catalyst powders were

mixed with deionized water and 5 wt.% Nafion solution, and

then coated onto a mirror-polished glassy carbon disk elec-

trode (f = 3 mm) as the working electrode. The cyclic voltam-

mograms (CVs) were obtained in aqueous 0.5 M H2SO4 at the

scan rate of 50 mV s�1 in a potential window of 0–1.2 V versus

Fig. 1 – (A) Transmission electron microscope (TEM) images, (B) energy dispersive X-ray (EDX) patterns.

C A R B O N 4 9 ( 2 0 1 1 ) 8 2 – 8 8 83

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reversible hydrogen electrode (RHE) at room temperature.

H2SO4 solution was saturated with pure argon in order to ex-

pel oxygen from the solution. The specific electrochemical ac-

tive area (ECA) was calculated based on the following relation

[31,32].

ECA ¼ QH

m� qH

ð2Þ

here QH is the charge for Hupd adsorption, m is the loading

amount of metal, and qH (210 lC cm�2) is the charge required

for monolayer adsorption of hydrogen on Pt surfaces. Electro-

chemically-accelerated durability testing (ADT) was em-

ployed to evaluate the long-term performance of catalysts.

ADT is an inexpensive and time-effective method for screen-

ing catalysts in terms of stability and performance [33]. Using

the same experimental system for CV testing and for ADT,

data were obtained by scanning the applied potential be-

tween 0.6 and 1.20 V versus RHE. In addition, the ORR perfor-

mance of the as-prepared catalysts was evaluated by rotating

disk electrode (RDE) techniques in 0.5 M H2SO4 electrolyte

with a sweep rate of 10 mV s�1 and a speed of 1600 rpm at

room temperature.

3. Results and discussion

In Fig. 1A, we show the high uniformity of prepared PFSA-sta-

bilized Pt nano-colloids, with an average size of 3 nm for Pt

nano-particles. EDX studies of individual metal particles

(Fig. 1B) reveal the presence of elements C, F and S on the Pt

nano-colloid derived from the PFSA molecular layers on the

outer surface of the metal particles.

Fig. 2 shows that crystal Pt nano-particles (as the high-res-

olution TEM images shown in the insets) are well dispersed

on the wall of CNTs in the PFSA-Pt/CNT catalysts with a par-

ticle size ranging from 2 to 5 nm (Fig. 2A). In contrast, a poor

dispersion of Pt nano-particles on the wall of CNTs (Fig. 2B) is

observed in the Pt/CNT catalysts, indicating a non-uniform

distribution of Pt nano-paticles on the support surface with-

out the PFSA stabilizer.

XRD patterns of both PFSA-Pt/CNTand Pt/CNT catalysts are

show in Fig. 3A. Peaks at 2h = 39.7�, 46.46�, 67.7� and 81.42�are

assigned to the (1 1 1), (2 0 0), (2 2 0), and (3 1 1) planes of Pt,

respectively, displaying a typical diffraction pattern of fcc lat-

tice for Pt. The calculated average crystallite size of Pt for

PFSA-Pt/CNT and Pt/CNT was 2.9 and 3.2 nm, respectively.

FTIR spectra of the PFSA and PFSA-Pt/CNT catalysts are gi-

ven in the Fig. 3B. For the PFSA-Pt/CNT catalysts, the strong

characteristic band at 1150 cm�1 and 1230 cm�1 can be as-

signed to CF2 stretching vibration corresponding to the bands

at 1154 cm�1 and 1234 cm�1 in PFSA. The other relatively

bands, 1059 cm�1 is associated with �SO3 stretching vibra-

tion; 978 cm�1 is associated with the C–O–C symmetric

stretching vibration, related to the band at 1060 cm�1 and

980 cm�1 in PFSA. These results also verify the presence of

PFSA in the as-prepared Pt/CNT catalysts, which is consistent

with the EDX results.

Fig. 4 shows CV curves of both catalysts recorded at room

temperature. The ECA is calculated from measuring the

charge collected in the hydrogen adsorption/desorption re-

gion after double-layer correction and assuming a value of

210 lC cm�2 for the adsorption in a hydrogen monolayer

[31,32]. Significantly, the PFSA-Pt/CNT provides a higher ECA

(72.46 m2 g�1) than that of Pt/CNT (52 m2 g�1). The ECA in-

creases 40% compared to that of Pt/CNT.

Polarization curves for the ORR on these catalysts are

shown in Fig. 5. The diffusion-limiting currents were obtained

in the potential region below 0.6 V, whereas a mixed kinetic-

diffusion control region occurs between 0.7 and 0.9 V. The ki-

netic current was calculated from the ORR polarization curve

according to the Koutecky-Levich equation [34] (Eq. (3)).

1i¼ 1

ikþ 1

idð3Þ

here i denotes the experimentally measured current, id is the

diffusion-limiting current, and ik is the kinetic current. Then,

Fig. 2 – (A) TEM images of the PFSA-Pt/CNT, the inset is an

HRTEM image of PFSA-Pt/CNT. (B) Pt/CNT catalysts, the inset

is an HRTEM image of Pt/CNT.

84 C A R B O N 4 9 ( 2 0 1 1 ) 8 2 – 8 8

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the kinetic current was calculated based on the following

equation.

ik ¼i� idid � i

ð4Þ

The calculated results are shown in Fig. 5. The ORR activ-

ities were calculated at 0.9 V versus RHE in 0.5 M H2SO4 solu-

tion. The Pt mass activity of PFSA-Pt/CNT (6.97 mA mg�1) is

1.30 times of that of Pt/CNT (5.33 mA mg�1), indicating that

the novel catalyst has a higher ORR activity compared to Pt/

CNT. The improved catalytic activity can be ascribed to the

increased triple phase boundary on the PFSA stabilized Pt

nano-particle surfaces as well as to the well-dispersed Pt

nano-particles on the CNT support, moreover, the presence

of SO3� end groups in the poly-electrolyte chain structures

facilitates the reaction species transfer process for ORR [27].

Fig. 6A and B show CVs for both catalysts before and after

ADT. The hydrogen adsorption peak related to ECA in CVs de-

creases with increasing numbers of potential cycles. Fig. 6C

shows the ECA loss of Pt versus the electrochemical oxidation

cycle number. The ECA loss for both catalysts decreases with

potential cycling under the same electrochemically acceler-

ated testing conditions.

However, it should be noted that the ECA loss of the PFSA-

Pt/CNT is always far lower than that of Pt/CNT in the range of

3 to 6000 cycles. Significantly, the ECA loss becomes more

sluggish during 3000 to 6000 cycles. The ECA loss rate is only

4.86 · 10�3 m2 g�1 cycle�1, which is about two times lower

than that of the Pt/CNT (8.55 · 10�3 m2 g�1 cycle�1). This indi-

cates the lifetime of PFSA-Pt/CNT is about two times longer

than that of Pt/CNT under the same electrochemical acceler-

ation test.

Fig. 6D shows the ORR curves for both catalysts after 1000

cycles. It can be seen that the Pt/CNT catalyst has remarkable

110 mV degradation in half-wave potential after the ADTwhile

the corresponding shift for the PFSA-Pt/CNT catalyst is only

80 mV. The result shows that the PFSA-Pt/CNT catalyst has a

lower loss rate in ORR activity than Pt/CNT catalyst. These

changes of ORR activities are consistent with the ECA loss as

mentioned above.

The degradation mechanism of Pt/CNT could be explained

as shown in Fig. 7A. Three mechanisms are generally

Fig. 3 – (A) XRD patterns of PFSA-Pt/CNT and Pt/CNT

catalysts; (B) FTIR spectra of the prepared PFSA-Pt/CNT and

the reference PFSA.

Fig. 4 – CV curves and ECA of PFSA-Pt/CNT and Pt/CNT

catalysts.

Fig. 5 – Current–potential curves for ORR in O2-satuated

0.5 mol l�1 H2SO4 and comparisons of ORR mass activities of

catalysts.

C A R B O N 4 9 ( 2 0 1 1 ) 8 2 – 8 8 85

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accepted to contribute toward the loss of ECA for carbon sup-

ported Pt nano-particles. First, electrochemical corrosion oc-

curs on Pt metal particles (Position a). The Pt corrosion

mechanism [35] at the anode is

Pt ! P4þt þ 4e� ð5Þ

Whereas at the cathode it is

PtO2 þ 4Hþ þ 2e� ! P2þt þ 2H2O ð6Þ

Fig. 6 – CV curves of PFSA-Pt/CNT (A) and Pt/CNT (B) by ADT, changes of ECA (C) and ORR activity (D) of catalysts with the

increased potential cycles in 0.5 mol l�1 H2SO4 at room temperature.

Fig. 7 – Schematic of ECA loss for Pt nanoparticles on carbon support (A) and for PFSA-stabilized Pt on carbon support (B).

86 C A R B O N 4 9 ( 2 0 1 1 ) 8 2 – 8 8

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Second, unsupported Pt metal particles are formed due to

the corrosion of the carbon support (Position b). The corrosion

reaction [36] is given in Eq. (7).

Cþ 2H2O! CO2 þ 4Hþ þ 4e� ð7Þ

The presence of Pt promotes this corrosion reaction (Posi-

tion c) [6]. The detached Pt particles are readily agglomerated

on the surface of supports or ionomers [37,38]. Third, Pt metal

particles or their aggregates grow through Ostwald ripening,

that is, Pt atoms with Pt particles are transported onto other

particles to form larger particles (Position d) [23,39,40]. Corre-

spondingly, the degradation mechanism of PFSA-Pt/CNT

could be well explained by Fig. 7B, where the Pt nano-particles

are stabilized with PFSA, the electrochemical corrosion of Pt

could be suppressed with the protection of PFSA (Position

a1) compared to that without PFSA protection (Position a in

Fig. 7A). Although the carbon support can be degraded, the

triple phase boundary always exits on the Pt particle surfaces,

and this slows down the loss of ECA (Position b1). In addition,

the ability of PFSA molecules to adhere to the Pt surfaces like

tentacles could anchor Pt particles on the support surfaces

although some detachment of Pt particles does take place (Po-

sition c1). Furthermore, the PFSA could effectively prevent Pt

nano-particles from aggregation on support surfaces because

of a strong steric hindrance effect of PFSA with SO3� groups

(Position d1). In the first 3000 electrochemical oxidation cy-

cles, all of these factors may be at work to some extent during

the decrease of Pt ECA for PFSA-Pt/CNT catalysts, which lead

to a lower ECA loss rate than that observed for Pt/CNT cata-

lyst. During 3000–6000 potential cycles, these factors (espe-

cially for b1, c1 and d1 positions) may play a positive role

and sharply reduce the ECA loss, which lead to a stabilization.

Also, it is important to consider support factors. Compared

to carbon black, the CNT has high chemical stability and un-

ique mechanical property [10], which could effectively de-

crease the support degradation.

4. Conclusions

A novel CNT-supported Pt catalyst has been successfully

developed to improve the lifetime and performance of Pt/

CNT catalysts. It is unique for this catalyst that the PFSA (Naf-

ion) offers (i) an extended triple phase boundary on Pt sur-

faces as a proton conductor, which enhances electro-

catalytic activity, and (ii) a strong metal-support interaction

as both a binder and a stabilizer for Pt nano-particles, which

improves the catalyst lifetime. As a result, Pt nano-particles

can be well dispersed on inert surfaces of CNTs. The novel

catalyst shows a high electro-catalytic activity and an excel-

lent electrochemical stability. This presents a simple and

effective approach to produce highly stable Pt catalysts which

have a considerable potential for application in PEMFCs and

other devices.

Acknowledgments

This work was supported by The National Natural Science

Foundation of China (NSFC) (50972112, 50632050), The New

Century Excellent Talent Program of Ministry of Education

of China (MOE) (No. NCET - 07 - 0652) and the Fundamental

Research Funds for the Central Universities.

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