A facile synthesis method produced highly active Pd nanoparticle catalysts for oxygen reduction reaction (ORR) in under 5 minutes. An electrode was dipped in separate solutions of reducing agent and Pd ions to deposit amorphous Pd nanoparticles. Repeatedly dipping the electrode increased catalytic activity, with the highest activity achieved after 12 cycles of 20 seconds each. The Pd nanoparticles produced using sodium hypophosphite as the reducing agent showed superior ORR activity compared to commercial Pt/C catalysts.

1. 
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

2. 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
Currentdensity/uAcm
-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
GC
Pt/C
20 sec x 1 cycle
20 sec x 2 cycle
20 sec x 3 cycle
20 sec x 6 cycle
20 sec x 9 cycle
20 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
Currentdensity/uAcm
-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
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

3. 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