Electrolytic Activity of Carbon-supportedPt-Au Nano Particles For

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Electrochimica Acta 52 (2007) 5599–5605

Electrocatalytic activity of carbon-supported Pt–Aunanoparticles for methanol electro-oxidation

In-Su Park, Kug-Seung Lee, Dae-Sik Jung, Hee-Young Park, Yung-Eun Sung ∗,1

School of Chemical & Biological Engineering and Research Center for Energy Conversion & Storage,

Seoul National University, Seoul 151-744, South Korea

Received 7 September 2006; received in revised form 14 December 2006; accepted 15 December 2006

Available online 21 January 2007

Abstract

Pt-modified Au nanoparticles on carbon support were prepared and analyzed as electrocatalysts for methanol electro-oxidation. In this paper,

a novel chemical strategy is described for the preparation and characterization of carbon-supported and Pt-modified Au nanoparticles, which

were prepared by using a successive reduction process. After preparing Au colloid nanoparticles (∼3.5 nm diameter), Au nanoparticles were

supported spontaneously on the surface of carbon black in the aqueous solution. Then a nanoscaled Pt layer was deposited on the surface of

carbon-supported Au nanoparticles by the chemical reduction. The structural information and electrocatalytic activities of the Pt-modified Au

nanoparticles were confirmed by transmission electron microscopy (TEM), X-ray diffractometry (XRD) and cyclic voltammetry (CV). The results

indicate that carbon-supported Au nanoparticles were modified with the reduced Pt atoms selectively. The Pt-modified Au nanoparticles showed the

higher electrocatalytic activity for methanol electro-oxidation reaction than the commercial one (Johnson–Matthey). The increased electrocatalytic

activity might be attributed to the effective surface structure of Pt-modified Au nanoparticles, which have a high utilization of Pt for surface reaction

of methanol electro-oxidation.

© 2007 Elsevier Ltd. All rights reserved.

Keywords: Direct methanol fuel cell; Electrocatalyst; Successive reduction process; Pt-modified Au nanoparticles; Methanol electro-oxidation

1. Introduction

Direct methanol fuel cells (DMFCs) have been considered

the ideal fuel cell system for fuel cell-based mobile power sup-

ply systems. Indeed, there are certain advantages to be found in

the use of methanol as fuel, not the least of which is its high

energy density and the fact that it stays in liquid state at room

temperature [1,2]. However, there are some unsolved techni-

cal problems involved in the commercialization of DMFCs: the

high loading of noble metal electrocatalysts, the slow kinetics

of electrode reaction and the crossover of methanol through themembrane, etc. The most serious problem is the high loading of

Pt and its alloy electrocatalysts. Much effort has been devoted

to increasing electrocatalytic activity and reducing the loading

of noble metal catalysts [3–11].

∗ Corresponding author. Tel.: +82 2 880 1889; fax: +82 2 888 1604.

E-mail address: [email protected] (Y.-E. Sung).1 ISE member.

Since electrocatalytic reactions are strongly dependent on

the surface structure of metal catalysts, the atom-leveled design

of the surface structure plays a significant role in a high cat-

alytic activity and the utilization of electrocatalysts. Therefore,

surface-modified electrocatalysts have attracted much attention

due to their unique structure and new electronic and electro-

catalytic properties [12–15]. Various methods can be employed

in preparing the surface-modified nanoparticles, such as under-

potential deposition (UPD)[16–23], thermal treatment approach

[24], spontaneous formation [25–28], and successive reduction

process [29,30].Recently, the nanoparticles with uniform Pt-group overlay-

ers (down to 1–2 monolayers) were prepared with the UPD

approach and applied for the oxygen reduction [19–22] and

methanol oxidation [23]. Although the UPD redox replace-

ment technique offers many interesting results, the chemical

approachesare important for the practical application of surface-

modified electrocatalysts. The low-priced and high catalytic

activated electrocatalysts can be prepared if highly dispersed

nanoparticles are used as a substrate for depositing the elec-

0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.electacta.2006.12.068

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5600 I.-S. Park et al. / Electrochimica Acta 52 (2007) 5599–5605

trocatalytic active materials with the chemical methods. Zhao

and Xu reported the preparation of Pt-modified Au nanoparti-

cles by using an aqueous hydrogen reduction method [30]. The

utilization of prepared Pt was nearly 100%, which is remarkable

result. However, the Au nanoparticles (∼10 nm diameter) were

too big to obtain a high specific surface area for depositing the

active elements. Therefore, the highly dispersed metallic sub-

strate is a prerequisite for obtaining a high catalytic activity of

surface-modified electrocatalysts.

In the present experiment, the carbon-supported ∼3.5 nm

Au nanoparticles were adapted as the substrate and the suc-

cessive reduction process was used for depositing Pt on the

surface of Au nanoparticles. The merits of gold nanoparticle

as a metal substrate for catalyst design is its ability to prepare

routinely mono-dispersed colloid nanoparticles having a wide

diameter range andgood stabilityin theacid electrolytes[30,31].

The prepared electrocatalysts were extended for electrocatalytic

applications. The central point of this study is the application of

carbon-supported and highly dispersed Au nanoparticles for the

preparation of surface-modified electrocatalysts.

2. Experimental

2.1. Preparation of carbon-supported and Pt-modified Au

nanoparticles

All aqueous solutions were made with deionized (DI) water,

which was further purified with a Milli-Q system (Millipore

water, 18.2 M cm). The following materials were obtained

from Aldrich: HAuCl4·3H2O, H2PtCl6· xH2O, sodium citrate

tribasic dihydrate (Na3C6H5O7·2H2O), NaBH4 and l-ascorbic

acid (C6H8O6). All chemicals were of analytical grade and wereused as received.

The carbon-supported 30mass% Au nanoparticles (30AuC)

were synthesized as follows (the gold nanoparticles used in

this study were prepared as described in detail elsewhere [22]):

first, Au nanoparticles of ca. 3.48 nm diameter were prepared by

adding HAuCl4·3H2O (0.0579 g) to 800 ml of H2O with vigor-

ous stirring, followed by the addition of aqueous sodium citrate

(0.1 g) 1 min later. After an additional minute, NaBH4 (0.03 g)

and sodium citrate (0.1 g) dissolved solution was added. The

solution was stirred for 30 min, and then adequate carbon black

(Vulcan XC-72R, 0.0672 g) was added. During the stirring for

48 h, gold colloid particles were supported spontaneously on

the surface of the carbon black particle. After supporting Aunanoparticles on the surface of the carbon black particles, the

red color of the solvent changed to a transparent color. The

mass percent of Au in the 30AuC was optimized experimen-

tally by trial and error. Generally, the optimum mass percent

of spontaneously supported metal nanoparticles is dependent

on the specific surface area of support due to the finite size

of particle and interparticle repulsion [22,32]. The cleaning

process of the solution was conducted by precipitation and

decantation.

The preparing procedures of Pt-modified Au nanoparticles

on carbon support were as follows: first, an adequate amount

of Pt precursor solution was added to 400 ml of the as-prepared

30AuC-dispersed solutions. Next, an adequate amount of ascor-

bic acid solution was added [29]. After stirring for 20 h, the

resultant solution was precipitated, washed and dried in the vac-

uum oven at 350 K. The prepared electrocatalysts were coded

as Au–Pt[ x], x denoting the atomic Pt/Au ratio within the

nanoparticles. The characteristics of Au-Pt[ x] electrocatalysts

were also compared with the 40 mass% Pt/C (Johnson–Matthey,

PtC[JM]).

2.2. Catalyst characterization

Samples for TEM were prepared by placing a drop of solu-

tion onto a carbon-coated copper grid and were examined

using a JEOL 2010 transmission electron microscopy oper-

ated at 200 kV. Analysis of X-ray diffraction was performed

with Rigaku D/MAX 2500 operated with a Cu K source

(λ= 1.541 A) at 40kV and 100mA.

2.3. Electrochemical measurement

Cyclic voltammogram was obtained in a conventional three-

electrode electrochemical cell using glassy carbon electrode

(6 mm diameter) as the working electrode, platinum wire as

the counter electrode and saturated calomel electrode (SCE)

as the reference electrode. Electrochemical measurements were

all recorded and reported versus normal hydrogen electrode

(NHE). The glassy carbon electrode was polished with 1, 0.3,

0.05m-Al2O3 slurry and washed ultrasonically with Milli-

pore water before use. The catalyst inks were prepared by

mixing carbon-supported catalysts, Millipore water, a 5 wt.%

Nafion® solution (Aldrich Chem. Co) as the binding mate-

rial and 2-propanol (JUNSEI). The 200 l of Millipore water,572l of Nafion® solution and 8 ml of 2-propanol per 0.1 g

of catalysts were mixed and then stirred until ink had formed

homogeneously. The catalyst ink was dropped on the glassy

carbon electrode with a micropipette, and the carbon electrode

was then dried in a vacuum oven. Electrochemical experiments

were performed with an AUTOLAB (Eco Chemie). Solutions

of 0.5M H2SO4 and 1M CH3OH/0.5 M H2SO4 were purged

with nitrogen gas prior to measuring. In order to identify

the surface composition and activities of the carbon-supported

electrocatalysts, voltammetry was conducted in the potential

between 0.05–1.5V versus NHE at a scan rate of 20 mV/s.

Prior to the measurement of carbon-supported electrocatalysts,the cyclic voltammetry was conducted with a scan rate of

50 mV/s as the cleaning step of electrode in the solution of 0.5 M

H2SO4.

3. Results and discussion

3.1. TEM analysis

The Pt-modified Au nanoparticles on carbon support were

characterized by HRTEM [33]. The surface-averaged disper-

sion, D (ratio of atoms that are on the surface to the total

number of atoms in the particle), was calculated according to



I.-S. Park et al. / Electrochimica Acta 52 (2007) 5599–5605 5601

the following equation:

D = 6v

s

inid

2i

inid 3i

(1)

where ni is the frequency of the occurrence of particles of the

size d i; v the volume per metal atom in the bulk; s is the average

area occupied by a metal atom on the surface. The prefactor 6is valid for spherical particles. For determining the particle size

distribution from low-magnification images, all particles were

regarded as spherical. Fig.1(a) shows an image of the highly dis-

persed Au nanoparticles on the carbon black. The dispersion and

mean particle diameter of carbon-supported Au nanoparticles

were 0.2936 and 3.48 nm, respectively. Fig. 1(b) and (c) shows

the TEM images of Au–Pt[0.5] and Au–Pt[1.0], respectively. In

the Pt-modified Au nanoparticles on carbon support, the high

dispersion and similar surface particle coverage as 30AuC were

sustained. Particle size distributions were compared in Fig. 1(d).

The mean particle diameters were increased and the widths of

distributions were narrowed according to the increased amount

of Pt. The decreased widths of distributions indicate that no new

additional nucleation took place on the surface of the carbon

black particles during the formation of Pt-modified Au nanopar-

ticles, and the growth rates of the nanoparticles were more rapid

in the small particles than in the large ones. The increased size

of the resultant nanoparticles with the increased amount of Pt

is in agreement with the increase expected from the following

equation:

DAu–Pt[x] = DAu

1 +

V m(Pt)[Pt]

V m(Au)[Au]

1/3

(2)

Fig. 1. TEM images and particle size distributions of Pt-modified Au nanoparticles on carbon support: (a) TEM image of carbon-supported Au nanoparticles, (b)

TEM image of Au–Pt[0.5] nanoparticles, (c) TEM image of Au–Pt[1.0] nanoparticles, and (d) comparison of particle size distributions.




Fig. 2. Comparison of experimental and calculateddiameters in the Pt-modified

Au nanoparticles.

where V m is the mole volume; [ ] the overall concentration of thetwo metals involved; DAu–Pt[ x] the diameter of the Pt-modified

Au particles; DAu is the diameter of the carbon-supported Au

particles [29]. Fig. 2 shows the relation between the experi-

mental and calculated diameters, and the mean particle sizes

Fig. 3. X-ray diffraction profiles of Pt-modified Au nanoparticles.

are given in parentheses. The experimental diameters of thePt-modified Au nanoparticles were corresponded to the diame-

ter which was obtained from the calculation, although an error

range was detected. In this study, the experimental diameters of

30AuC, Au–Pt[0.5] and Au–Pt[1.0] are 3.48, 3.80 and 4.55 nm,

Fig. 4. (a) Cyclic voltammograms in 0.5 M H2SO4, (b) mass-specific Au surface area (m2Au/gAu) and exposed Au site percentages (%), and (c) mass-specific Pt

areas (m2Pt/gPt) and (m

2Pt/gtotalmetal). Current densities vs. potential plots were measured in 0.5 M H2SO4 at room temperature.




respectively. The particle size of Au–Pt[1.0] suggests that the

thickness of the deposited Pt layer is 0.535 nm and corresponds

to about 1.5 atomic layer for the Pt atom (that is, d Pt =0.36nm)

[30,34]. These results indicate that the reduced Pt atoms were

deposited selectively on the surface of carbon-supported Au

nanoparticles.

3.2. XRD analysis

For obtaining the crystallographic information, an XRD mea-

surement was conducted. The wide-range XRD profiles showed

the main peaks of crystalline Au nanoparticles. The main peaks

of Au nanoparticles occurred at 38.18◦ (111), 44.39◦ (200),

64.58◦ (2 2 0) and 77.55◦ (3 1 1). This result indicates that the

Au nanoparticles present stably in the Pt-modified Au nanopar-

ticles. The short-range XRD profiles are shown in Fig. 3. As

shown in Fig. 3, a change in the main peaks are noticeable from

the Au–Pt[0.75] onward. Thatis, another peakappeared between

(1 1 1) main peaks of the Au and Pt nanoparticles. This might be

due to the modified structure of Au nanoparticles with Pt. Thefurther analysis has to be performed for the exact identification

of Pt phase in the Pt-modified Au nanoparticles.

3.3. CV characteristics

The surface structures of bimetallic nanoparticles can take

various and complex shapes [35]. Analyzing the surface compo-

sitionis very importantsincethe Au nanoparticleswere modified

with the Pt atoms, and the catalytic reactions are sensitive to the

surface structure. Cyclic voltammetry was performed with solu-

tions of 0.5 M H2SO4 for obtaining the surface composition and

characteristics of surface-modified nanoparticles, and the results

are represented in Fig. 4. Fig. 4(a) shows the cyclic voltammo-

gram, and the y-axis represents the mass-specific current density

( A / gAu). The reduction of Au oxide (1.2 V) and Pt oxide (0.7 V)

areshown in Fig.4(a). A modification of thesurface composition

occurred in conjunction with the increased Pt. That is, the reduc-

tion current of Au oxide decreased while the reduction current

of Pt oxide increased as the amount of Pt increased. This result

suggests that the reduced Pt atoms were deposited on the sur-

face of the Au nanoparticles selectively. From the charge of the

oxide reduction peak in the negative-going scan, the real surface

area of nanoparticles was determined as follows: 400 C cm−2

for Au and 440C cm−2 for Pt [36,37]. Friedrich et al. insisted

that the determination of the real surface area from oxide reduc-

Fig. 5. Electrocatalytic activity of Pt-modified Au nanoparticles on carbon support for methanol electro-oxidation: (a) current densities ( A / gtotalmetal) vs. potential

plots, (b) current densities ( A / gPt) vs. potential plots, and (c) mass-specific current densities at 0.6 V vs. NHE. Current densities vs. potential plots were measured in

0.5M H2SO4 +1 M CH3OH at room temperature.




tion is more practical than from hydrogen desorption because

of the higher charges involved [36]. The exposed Au site per-

centages Γ Au (%) were calculated according to the following

equation:

Γ Au (%) =AAu/gAu

A0Au/gAu

× 100 (3)

where AAu / gAu and A0Au/gAu are mass-specific Au surface areas

in the Au–Pt[ x] and 30AuC, respectively. For the calculation

of mass-specific active areas, Pt and Au masses were estimated

from the used amount of electrocatalysts on the basis of assump-

tion. Mass-specific Au surface areas are shown in Fig. 4(b) and

exposed Au site percentages are given in parentheses. As noted

in Fig. 4(b), the surface area of 30AuC was 22.7 m2 /g, and

the exposed Au site percentage of Au–Pt[1.0] was 7.40. This

result indicates that the Au nanoparticles were coated by the

reduced Pt atoms partially, and a Pt skin-like surface structure

was present in the Au–Pt[1.0] electrocatalyst. In addition, the

mass-specific active areas were calculated from the reductionarea of Pt oxide, as shown in Fig. 4(c). The filled squares and

empty circles representthe mass-specificPt surface area in terms

of Pt andtotalmetal weights, respectively. Them2Pt/gPt increased

as the amount of Pt decreased. The m2Pt/gPt of Au–Pt[0.25] was

207.9 m2 /g and this active area corresponded to 88.4% of Pt uti-

lization [30]. In the m2Pt/gtotalmetal, the Au–Pt[0.75] showed the

highest value in the prepared electrocatalysts. In other words,

the Au–Pt[0.75] had the most efficient surface structure for the

surface electrocatalytic reaction.

3.4. Methanol electro-oxidation

The catalytic activity of Pt-modified Au nanoparticles for

methanol electro-oxidation reaction was measured by obtain-

ing a voltammogram in the solution of 1 M CH3OH/0.5 M

H2SO4. As shown in Fig. 5(a) and (b), the reported current here

was normalized to the amount of total metal and Pt, respec-

tively. For the calculation of mass specific current densities,

Pt and Au masses were estimated from the used amount of

electrocatalysts on the basis of assumption. Fig. 5(c) showed

the mass-normalized currents, which were obtained at 0.6 V

versus NHE. The empty circle represents the current density

in terms of total metal content. The measured mass-specific

activity of PtC[JM] was 17.87 A/g [38]. The Pt-modified Au

nanoparticles showed as much catalytic activity as the com-mercial one, while Au–Pt[0.75] showed the highest relative

activity at 0.6 V. This demonstrates that similar catalytic activ-

ity was obtained in the Pt-modified Au nanoparticles by using

a smaller amount of Pt. This trend corresponds to that of

m2Pt/gtotalmetal. The filled squares represent the current den-

sity in terms of the Pt amount and show that the catalytic

activities increased as the loading amount of Pt decreased.

Most notably, the Au–Pt[0.25] showed about a 250% increase

in catalytic activity compared to that of the commercial one.

This distinction might be due to the effective surface struc-

ture of Pt-modified Au nanoparticles for the electrocatalytic

reaction.

4. Conclusions

The Pt-modified Au nanoparticles on carbon support were

prepared by a successive chemical reduction process. As indi-

cated by theTEM andXRD analysis,the successively reduced Pt

atoms were deposited on the surface of Au nanoparticles selec-

tively. From the CV analysis, the reduction current of Au oxide

decreased while the reduction current of Pt oxide increased

according to increased Pt. In other words, the surface struc-

ture of Au nanoparticles was modified by successive reductions

of Pt element. The Pt-modified Au nanoparticles on carbon

support represented the similar catalytic activity with the com-

mercial pure Pt electrocatalysts by using a smaller amount of

Pt. This increased catalytic activity might be attributed to the

high utilization of Pt atoms for the electrocatalytic reaction. By

using carbon-supported nanoparticle substrate and the succes-

sive chemical reduction method, a high performance electrode

with a small amount of Pt could be prepared and readily applied

to other electrocatalytic reactions, as well as to the methanol

electro-oxidation reaction.

Acknowledgements

This work was supported by the Ministry of Commerce,

Industry and Energy, the KOSEF through the Research Cen-

ter for Energy Conversion & Storage and the Korea Research

Foundation (Grant #KRF-2004-005-D00064).

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Documents

Electrolytic Activity of Carbon-supportedPt-Au Nano Particles For