Abstract— A facile metal nanoparticle synthesis (process time < 5

min) produced a highly active Pd nanoparticle catalyst for oxygen

reduction reaction. An electrode was dipped into separate solutions of

reducing agent and Pd ions to deposit amorphous Pd nanoparticles

whose catalytic activity was superior to that of commercial Pt/C

catalysts.

Keywords—Oxygen Reduction Reaction, Electrocatalysts

I. INTRODUCTION

T-based electrocatalysts are the most widely used catalysts

for oxygen reduction reaction (ORR) at the cathode of a

fuel cell. However, because these catalysts are expensive,

researchers have explored alternatives to use

lower-platinum-loaded or non-platinum-based electrocatalysts

for ORR [1-10]. Various nanostructured composites and alloys

have been proposed and demonstrated based on guidelines and

models from thermodynamic principles and quantum chemical

calculations. 9, 10 However, most of the synthetic processes

require long reaction times (on the order of hours or days), high

temperatures, vacuum conditions and complicated handling

procedures or complex chemical compositions, resulting in less

practical usage of such catalysts. If a facile and quick process

for synthesizing electrocatalysts could be found, we can easily

screen out varieties of nanostructured composites and alloys

designed from theoretical models for maximum catalytic

activity. Furthermore, such a synthetic process could be

automated for rapid research and development in combinatorial

chemistry. Here we propose a facile and quick metal

nanoparticle synthesis that directly deposits Pd nanoparticles

onto a solid-state electrode. Two different solutions of

chemicals (reducing agent and metal ion) were prepared but not

mixed. A solid-state electrode was sequentially dipped into

these solutions for a defined time on the order of seconds. By

this method, we demonstrated a stepwise metal-deposition

process to synthesize Pd nanoparticles for use as

non-platinum-based electrocatalysts for ORR. We performed

multiple cycles to actively tune and to optimize electrocatalytic

activity. The obtained Pd nanoparticles exhibited ORR activity

superior to that of a commercial Pt-loaded carbon (Pt/C)

catalyst.

Tan Chun Long Desmond, Lim Ji Zheng Shaune, Poon Kee Chun and

Hirotaka Sato*, are with School of Mechanical and Aerospace Engineering

(MAE), Nanyang Technological University (NTU), Singapore 639798. Fax: +65-6792-4062; Tel: +65-6790-5010; E-mail: hirosato@ntu.edu.sg

II. EXPERIMENT

The entire procedure for synthesizing the Pd nanoparticle

electrocatalysts was conducted under ambient conditions

without the need for heating, vacuum conditions or a power

supply apparatus. A 200 mM reducing agent solution [sodium

hypophosphite (NaH2PO2) or hydrazine (N2H4)] and a 2 mM

palladium chloride solution were separately prepared and

stocked. A glassy carbon (GC) disk electrode (3 mm in

diameter Bioanalytical Systems, Inc.) was first dipped into the

reducing agent solution for 10 s. After the electrode was air

gunned to remove excess solution that was adhered to it, it was

then dipped into the palladium chloride solution for another 10

s, thereby reducing the palladium ions to palladium metal

nanoparticles by utilizing only the quantity of reducing agent

adsorbed on the electrode (Pd electroless deposition). This

sequence of steps was repeated to tune and to optimize the

electrocatalytic activity, as described later. In conventional

nanoparticle catalyst preparation, the nanoparticles are

synthesized and stocked in a solvent, some of which are

adsorbed onto an electrode surface for use as a catalyst. To

disperse the nanoparticles well in the solvent, surfactants or

capping agents are usually introduced. These additives adhere

to the particles and can block electron transfer and mass

transport, thereby impairing catalytic activity [11-13]. Unlike

the conventional route to catalyst preparation, the presented

synthesis leads to the deposition of Pd nanoparticles directly

onto the electrode surface, making such additives unnecessary.

Further, it leads to the production of less chemical waste

because the entire metal consumed is actually deposited onto

the electrode surface.

III. RESULTS AND DISCUSSION

In the conventional electroless-deposition process, because

the reducing agent and metal ions coexist in a single solution, it

is usually necessary to moderately stabilize the metal ions by

adding complex agents to prolong the solution shelf life [14-16].

Stabilization inhibits metal deposition in the solution, until a

substrate whose surface is catalysed for the oxidation of the

reducing agent is introduced into the solution. In fact, as no

complex agent was introduced, Pd deposition took place

immediately after mixing the reducing agent and metal ion

solutions (Fig. S1, Supplementary Information). Because the

reducing agent and metal ions are not mixed in our stepwise

electroless-deposition method, the prepared solutions have a

long shelf life without the necessity of interventions.

Facile Synthesis of Highly Active Nanoparticle

Electrocatalysts for Oxygen Reduction Reaction

by Stepwise Pd Electroless Deposition

Tan Chun Long Desmond, Lim Ji Zheng Shaune, Poon Kee Chun, and Hirotaka Sato*

P

International Conference on Emerging Trends in Engineering and Technology (ICETET'2013) Dec. 7-8, 2013 Patong Beach, Phuket (Thailand)

http://dx.doi.org/10.15242/IIE.E1213045 71

The commercial Pt/C catalyst (20% Pt on Vulcan XC72

supplied from Sigma-Aldrich) electrode was prepared

according to the procedure described in the reference [17].

First, 100 mg of Pt/C powder was dispersed in 100 mL ethanol,

and it was immediately ultrasonicated before use. Next, using a

microsyringe, 7.5 µL of the Pt/C dispersion was cast onto the

GC electrode.

Figure 1A displays the ORR polarization curves measured in

an oxygen-saturated 0.1 M KOH solution of the Pd

nanoparticles synthesized by NaH2PO2. The Pd nanoparticles

were prepared by varying the number of deposition cycles and

dipping times. The Pd catalyst on the GC electrode prepared in

a single deposition cycle with a total dipping time of 20 s (10 s

for the NaH2PO2 solution and another 10 s for the Pd ion

solution) clearly exhibits electrocatalytic activity for ORR as

compared with the bare GC electrode. Moreover, with an

additional deposition cycle, for an additional 20 s of synthesis

time, the Pd catalyst exhibited a higher activity than the single

cycle. The overpotential was reduced by ~30 mV: the

polarization curve was positively shifted by ~30 mV. We

repeated the deposition cycle yet again and found the tendency

that as more deposition cycles were conducted, the polarization

curve shifted more positively. When the total synthesis time

was 120 s (20 s × 6 cycles), the given Pd nanoparticles

exhibited electrocatalytic activity superior to that of the

commercial Pt/C catalyst: the half-wave potential of 20 s × 6

cycles was −122 mV, whereas that of the Pt/C catalyst was

−138 mV. Among our prepared Pd nanoparticles, a total

synthesis time of 240 s (20 s × 12 cycles) exhibited the highest

catalytic activity at a half-wave potential of −108 mV.

Figure 1B displays the ORR polarization curves for Pd

nanoparticles synthesized by N2H4 under exactly the same

conditions as in the NaH2PO2 process. Similar to the NaH2PO2

process, more deposition cycles produced more positively

shifted polarization curves. However, the N2H4 process clearly

exhibited a lesser catalytic activity than the NaH2PO2 process.

The maximum catalytic activity in the N2H4 process was

obtained for a synthesis time of 180 s (20 s × 9 cycles) but with

a half-wave potential of −140 mV (slightly lower than that of

the Pt/C catalyst, −138 mV).

Figure 2 shows the microstructures (TEM, electron

diffraction) and compositions (EDS) of the Pd nanoparticles

synthesized by the NaH2PO2 and N2H4 processes. Pure Pd

nanoparticles were synthesized by the N2H4 process (Figure

2B), and the TEM images show ~50-nm-scaled clusters that

appeared as coagulated particles of a few nanometres each. The

Pd nanoparticles are crystalline since the electron-diffraction

Curr

en

t d

ensity / u

Acm

-2

Potential /mV vs. Ag/AgCl

-800

-700

-600

-500

-400

-300

-200

-100

0

-400 -350 -300 -250 -200 -150 -100 -50 0 50

GCPt/C20 sec x 1 cycle20 sec x 2 cycle20 sec x 3 cycle20 sec x 6 cycle20 sec x 9 cycle20 sec x 12 cycle

Fig. 1 Linear-scan voltammograms of Pd nanoparticles

synthesized by the stepwise electroless deposition process using

(A) NaH2PO2, (B) N2H4 as reducing agent, commercial

Pt-loaded, carbon (Pt/C) and bare glassy carbon (GC) electrodes

in an oxygen-saturated 0.1 M KOH solution at a scanning rate of

10 mV/s. The current is normalized by the geometrical surface

area of the corresponding electrode for current density.

Potential /mV vs. Ag/AgCl

Curr

en

t d

ensity / u

Acm

-2

-800

-700

-600

-500

-400

-300

-200

-100

0

-400 -350 -300 -250 -200 -150 -100 -50 0 50

GC

Pt/C

20 sec x 1 cycle

20 sec x 3 cycle

20 sec x 6 cycle

20 sec x 9 cycle

20 sec x 12 cycle

A

B

Fig. 2 Transmission electron microscopy images (TEM, 200 kV,

JEM-2010 from JEOL), electron-diffraction patterns and

energy-dispersive X-ray spectroscopy (EDS) of Pd nanoparticles

synthesized by the stepwise electroless deposition process using

(A) NaH2PO2 and (B) N2H4 as reducing agent. The EDS of (A)

gives Pd : P = 89 : 11 in at%, while that of (B) indicates that the

particles are pure palladium.

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

1.5 2.0 2.5 3.0 3.5

1.5 2.0 2.5 3.0 3.5

100 nm

100 nm

Pd

Pd

P

Intensity / a.u. A

B

Energy / keV

Energy / keV

International Conference on Emerging Trends in Engineering and Technology (ICETET'2013) Dec. 7-8, 2013 Patong Beach, Phuket (Thailand)

http://dx.doi.org/10.15242/IIE.E1213045 72

pattern shows clear Debye–Scherrer rings. In contrast to the

N2H4 process, the NaH2PO2 process (Figure 2A) formed less

coagulated nanoparticles with diameters of a few nanometres.

Pd particles obtained by simply mixing the NaH2PO2 and Pd

ion solutions were dispersed first but coagulated after hours

while the particles obtained by N2H4 coagulated soon after

mixing. Probably because nanoparticles were directly

deposited on the substrate surface in the stepwise synthesis

process, the Pd nanoparticles remained dispersed well and

possessed high catalytic active areas.

The halo electron-diffraction pattern in Figure 2A indicates

that the Pd nanoparticles synthesized by the NaH2PO2 process

are amorphous, and the EDS elemental analysis indicates that

they contain 11 at% phosphorous. The XRD profile of the Pd

nanoparticles also shows that the NaH2PO2 process forms

amorphous particles while the N2H4 process forms typical Pd

crystals. Phosphorous incorporated into Pd as a hypophosphite

ion decomposed to discharge electrons for Pd deposition, which

consequently disordered the Pd crystalline lattice and resulted

in the amorphous phase, as encountered in conventional

electroless deposition using NaH2PO2. Such amorphous-phase

of Pd particles would give high electrocatalytic activity (Figure

1A) as other amorphous particles and thin films exhibit

electrocatalytic activities.

In conclusion, we developed and demonstrated a synthesis

process for Pd nanoparticles whose electrocatalytic activity for

ORR can be tuned and optimized by repeating the dipping

process and introducing different reducing agents. The

optimized Pd nanoparticles were found to be superior to the

commercial Pt catalyst (Figure 1). This synthesis process has

the following benefits:

(1) Handling is remarkably facile (sequential dipping into

two solutions of simple chemical compositions), and reaction

time is short (20 s per deposition cycle).

(2) Catalytic activity is tunable by simply repeating the

deposition cycle.

(3) No vacuum, high temperature or an external power

source is needed.

(4) Less chemical waste is produced.

In addition, by introducing other metal ion solutions and

their mixtures into the process, other pure metals, alloys or

composite nanoparticles can be synthesized. Owing to these

benefits, nanoparticles can be widely designed and tailored

from theoretical models, not only for ORR electrocatalysis but

also for various purposes, and the synthetic process can be

automated towards combinatorial chemistry and

high-throughput screening.

ACKNOWLEDGMENT

This study was financially supported by the Nanyang

Assistant Professorship (NAP, M4080740). The authors

appreciate Ms. Koh Joo Luang, Ms. Heng Chee Hoon, Ms.

Yong Mei Yoke and Mr. Leong Kwok Phui at

Material/Biological & Chemical Laboratories at MAE, NTU,

for their continuous support and effort to set up and maintain an

excellent experimental environment.

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International Conference on Emerging Trends in Engineering and Technology (ICETET'2013) Dec. 7-8, 2013 Patong Beach, Phuket (Thailand)

http://dx.doi.org/10.15242/IIE.E1213045 73