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Pd particle size effects on oxygen electrochemicalreduction

Weijiang Zhou a,*, Miao Li a, Ovi Lian Ding a, Siew Hwa Chan a,b,*,Lan Zhang a, Yanhong Xue a

aEnergy Research Institute @ NTU, Nanyang Technological University, 50 Nanyang Drive,

637553 Singapore, Singaporeb School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue,

639798 Singapore, Singapore

a r t i c l e i n f o

Article history:

Received 27 June 2013

Received in revised form

27 January 2014

Accepted 29 January 2014

Available online 22 February 2014

Keywords:

Electrocatalysis

Oxygen reduction reaction

Palladium

Proton exchange membrane fuel

cells

Particle size effect

* Corresponding authors. Energy Research InSingapore. Tel.: þ65 6790 5591; fax: þ65 6790

E-mail addresses: wjzhou@ntu.edu.sg (W0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2014.01.1

a b s t r a c t

A series of carbon-supported Pd nanoparticles catalysts with average sizes ranging from 2.7

to 8.7 nm was synthesized by an aqueous phase reduction method in the presence of

citrate and evaluated for the oxygen reduction reaction (ORR). It was found that the average

Pd particle size can be tuned and controlled by the citrate to Pd precursor ratio and the

effect of citrate concentration on Pd particle sizes was carefully examined. The catalysts

were also characterized by transmission electron microscopy, X-ray diffraction and X-ray

photoelectron spectroscopies, cyclic voltammetry and rotating disc electrode paleography.

Measurements of the ORR activities using a rotating disk electrode show a monotonic in-

crease in specific mass activity with increasing Pd dispersion. On the other hand, the

specific surface activity displayed a volcano curve with the maximum value between 5.0

and 6.0 nm. The particle size effect could be attributed to a combination of several size-

dependent effects, i.e. changes in the distribution of low index planes on the surface, the

relative abundance of low coordination sites, and the Pd electronic states.

Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

Introduction

There has been significant progress over the last two decades

on improving the performance of proton exchangemembrane

fuel cells (PEMFCs) and reducing the Pt catalyst loading in the

fuel cells. However, the Pt loading in the cathode remains high

because of the sluggish kinetic towards oxygen reduction re-

action (ORR). The issue related to poor ORR is one of the key

factors affecting the performance of the PEMFC system. In

view of the high cost of Pt and the scarcity of this precious

stitute @ NTU, Nanyang4862.

. Zhou), mshchan@ntu.ed2014, Hydrogen Energy P97

metal, the decrease of Pt loading in the cathode is a R&D

imperative, and often one of the most extensively studied

subjects in PEMFC research community [1,2]. The issue has

become more profound in direct methanol fuel cells (DMFCs)

wheremethanol crossover brings about selectivity issue in the

catalysis at cathode. Pt is active to both ORR and the direct

electro-oxidation of the diffused fuel molecules. The estab-

lishment of a mixed potential at the cathode significantly

decreases the fuel cell performance and the efficiency of

cathodic Pt utilization. The development of active and cost-

effective catalysts for cathode with excellent methanol (or

Technological University, 50 Nanyang Drive, 637553 Singapore,

u.sg (S.H. Chan).ublications, LLC. Published by Elsevier Ltd. All rights reserved.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 6 4 3 3e6 4 4 26434

other crossovered fuel) tolerance for DMFCs is always one of

the key challenges [3].

As a noble metal, palladium (Pd) and Pd-containing mate-

rials have been investigated as an electrocatalyst for ORR in

the cathode and for formic acid oxidation reaction in the

anode. Pd is the second best catalyst in the noble metal family

after Pt for many chemical reactions. Furthermore, Pd is less

expensive and more abundant than Pt, which could lead to a

reduction in fuel cell cost if Pd can substitute for Pt on a large

scale. Monolayer Pd or Pd nanoparticles were reported to be

more active than multilayer, single crystal or bulk Pd, indi-

cating that the structures and the preparation processes of the

Pd catalyst have significant impact on the catalysis [4,5]. This

may not be too surprising since finely dispersed Pd nano-

particles had previously shown enhanced catalytic activity in

formic acid oxidation [6,7]. Pd is also more selective towards

ORR in the presence of crossovered methanol in DMFCs. In

recent years, it was demonstrated that Pd nanoparticles are

more active than Pt nanoparticles in alkaline environment

towards ORR [5,15]. Moreover, Pd has been explored as an

electrocatalyst for high-temperature PEMFCs. In high-

temperature PEMFCs, both anode and cathode reactions are

fast enough that Pd-based catalysts can be employed to

replace the more costly platinum. These are highly desirable

attributes of Pd or Pd alloy for an ORR catalyst in PEMFCs and

alkaline fuel cells (AFCs) [5,6,8e15]. Hence, it is significant to

explore the application of nanosized Pd catalysts in PEMFCs

[8,16]. Besides, the value of Pd as a catalytic metal has also

been ascertained in the industry for heterogeneous catalytic

reactions such as selective hydrogenation [17] and dehydro-

genation reactions, methanation [18], CO oxidation [19] and

NO reduction [20]. However, Pd catalysts are often deposited

on a suitable support with low metal loading (e.g. �5 wt%),

which is too low for the electrocatalysis. It is desirable to

develop highly dispersed nanosized Pd catalyst with a high

metal loading to increase the number of catalytic active sites

for “difficult” reactions such as ORR at low temperatures (not

more than 200 �C). Many preparation methods have been

adopted to produce highly dispersed Pdmetal on carriers such

as metal oxides or carbonaceous materials. Chemical reduc-

tion in aqueous reactions is always a favorable route because

of its simplicity and low operational cost. For example,

different alcohol solvents were employed in the impregnation

method to control the size of Pd nanoparticles supported on

carbon material [16]. Several reducing agents have been tried

to prepare carbon-supported Pd catalysts at different pH

conditions [8]. Polyol synthesis method, in which ethylene

glycol exhibits at least two functions of reducing and

dispersing effects, is one of the most popular methods for

synthesizing large amount of Pt-based nanocatalysts and Pd

nanocatalysts after some modification. Essentially, it is

necessary to develop a low cost method based on aqueous

solution for synthesizing Pd-based electrocatalysts with

tunable particle sizes. The literature reports unveiled the

importance of the procedure in preparing the catalysts, indi-

cating that there is a large room for further improving the

properties of Pd catalysts such as the particle size and

homogeneity.

The ORR is a multi-step reaction which is affected by the

surface morphology and electronic states of the catalyst [21].

Since Pd is a potential Pt substitute for ORR, it is important to

explore if one could manipulate the Pd particle size to achieve

the desired catalytic properties. In this study, we investigated

the particle size effect on ORR with several carbon-supported

nanosized palladium catalysts (10 wt%) in the size range of

2.7e8.7 nm [22,23].

Experimental

Preparation method

All chemicals, PdCl2, trisodium citrate dihydrate, sodium

borohydride (NaBH4), perchloric acid (HClO4), and HCl were

purchased from SigmaeAldrich and used as received. Carbon

XC-72R (SBET ¼ 240 m2 g�1) was purchased from Cabot and

refluxed in 5 M HNO3 solution for 5 h before use. In a typical

process for synthesizing 150 mg sample Pd/C-8, Pd/C electro-

catalysts with 10 wt% Pd were prepared as follows: A diluted

trisodium citrate solution (e.g. 30 mL of 0.0375 M aqueous

solution) was added dropwise to stirred carbon slurry

(5 mg mL�1 water) in water. The mixture was stirred for

1 h before a stoichiometric amount (14.1 mL) of 0.01 M PdCl2solution (PdCl2 in 0.1 M HCl) was introduced dropwise under

stirring. The mixture was then stirred for 2 h after the addi-

tion. An excess of freshly prepared ice-cold NaBH4 solution

was then added dropwise with stirring. The reduction was

performed to completion in an ice-water bath and stirred

overnight to deposit completely Pd nanoparticles onto carbon

surface. The solid Pd/C product recovered after successive

centrifugal separation, washing with copious DI water and

drying in vacuum at 70 �C overnight was denoted as Pd/C-N,

where N is the molar ratio of citrate to Pd used in the prepa-

ration. The effect of molar ratio of citrate to Pd was examined

on the final Pd particle sizes.

Physicochemical characterization

X-ray diffraction (XRD) characterization was carried out on a

Bruker GADDS diffractometer using a CuKa source

(l ¼ 1.54056 �A) operating at 40 kV and 40 mA. Transmission

electron microscopy (TEM) was performed on a JEOL JEM 2010

microscope operating at 200 kV accelerating voltage. X-ray

photoelectron spectroscopic (XPS) analysis of the sampleswas

carried out on a VG scientific ESCALAB MKII spectrometer and

the narrow scan Pd3d XPS spectra were de-convoluted by

XPSPEAK (version 4.1) after correction by adventitious carbon

(graphite C1s level at 284.5 eV) [25].

Electrochemical characterization

Electrochemical measurements were carried out on an Auto-

lab potentiostat/galvanostat (with GPES software v4.9) using a

conventional 3-electrode cell and 0.1 M HClO4 supporting

electrolyte. The working electrode was a thin layer of Nafion-

impregnated catalyst cast on a glassy carbon disc (5 mm

diameter, 0.196 cm2) held in a Teflon cylinder. The catalyst

layer was prepared from a Pd/C ink produced by ultrasonically

dispersing 5.0 mg of catalyst in 1.0 mL ethanol and 50 mL

Nafion alcoholic solution (5 wt%, Aldrich) [24]. In the RDE

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 en e r g y 3 9 ( 2 0 1 4 ) 6 4 3 3e6 4 4 2 6435

study, 20 mL of the catalyst ink was dispersed onto the surface

of a glassy carbon RDE (rotating disk electrode) with a micro-

pipette followed by drying in oven at 75 �C for 15 min. The Pd

loading on the glassy carbon electrode was 48.5 mg cm�2 and

kept constant for all samples. A Pt gauze and an AgjAgClelectrode were used as the counter electrode and the refer-

ence electrode, respectively. The working electrode potential

was controlledwith respect to the AgjAgCl reference electrode

by the Autolab. All potentials have been converted to the RHE

scale in this paper. The electrolyte solution was de-aerated

with high purity of Ar prior to the electrochemical pro-

cedures. CO-stripping voltammetry began with bubbling CO

into the electrolyte solution at 0.04 V for 15 min. The electro-

lyte was then degassed by bubbling Ar for 30 min to expel the

unabsorbed CO. The electrode potential was scanned from

0.04 V to 1.2 V. Thereafter the scan direction was reversed to

end the scan at 0.04 V. For RDE measurements the electrolyte

was saturated with high purity of oxygen by bubbling. All

polarization measurements were performed at a scan rate of

5 mV s�1 in the cathodic direction under different rotational

speeds. Catalyst activities were expressed in both gravimetric

and surface area units.

Results and discussion

The XRD spectra for various as-prepared Pd/C catalysts are

shown in Fig. 1. For most of the catalysts especially those

synthesizedwith low citrate/Pd ratios, the characteristic (111),

(200), (220), (311) peaks of face-centered cubic (fcc) Pd diffrac-

tions (JCPDS, Card No. 05-0681) could be found at 2q values of

about 40�, 47�, 68� and 82�, respectively. Therewere no obvious

diffraction peaks that could be assigned to palladium oxides.

The size of the Pd particles was calculated by the Scherrer

equation using the full width at half maximum (FWHM) of the

(220) diffraction in the 2q range of 60e75� [26] where the only

contribution from carbon support was a linear background

and a more accurate determination of FWHM was allowed.

The Pd lattice parameter and interatomic distance were also

Fig. 1 e XRD patterns of Pd/C samples synthesized with

different citrate/Pd molar ratio.

calculated based on the (220) diffraction peak. The summary

of results in Table 1 shows that there are orderly increases in

Pd lattice parameter and interatomic distance compared to

the bulk values (0.389 nm) with the decrease in particle size,

even though the deviations from the bulk values were rela-

tively small [27e29]. The variations in interatomic distance

brought about by changes in particle size would affect the

density of Pd sites on the surface, and consequently the

catalyst activity in ORR. Besides palladium crystal structure

changes, the interaction between Pd and the carbon support,

the adsorption of the possible citrate residuals on metal sur-

face could also contribute to the size-dependent dilation of

lattice parameter and interatomic distance [27,30,31]. Such

residuals could be indicated to some extent by cyclic vol-

tammetry (CV) in HClO4 electrolyte if they existed on the

catalyst surface. A very weak oxidation peak was barely

visible in the first anodic scan and disappeared in subsequent

scans, which could be related to the possible citrate residuals.

This study has demonstrated that most of the citrate-related

residuals have been removed during the separation and

washing process. It was observed that the (220) diffraction

peaks have become increasingly indiscernible with the in-

crease in Pd dispersion and finally they are suitable for use to

calculate the particle size and lattice parameter. The average

particle size of the metallic phase, in this case, was based on

TEM data by counting more than 150 particles in several

randomly selected regions (Fig. 2). There is a good agreement

in the average particle sizes obtained from TEM and XRD

study, although they are not exactly identical (Table 1). It was

found that a high citrate/Pd ratio was favorable to the syn-

thesis of small Pd particles with a narrow size distribution.

Specifically, Pd particles (sample Pd/C-8) could be achieved as

small as 2.7 at a citrate/Pd ratio of 8. On the other hand, the

average particle sizewas as large as 8.7 nmwhen the ratiowas

decreased to one. Lowering the citrate/Pd ratio also resulted in

a broader size distribution, besides the increase in particle

size. Some agglomerates of Pd nanoparticles were also found

in samples of large average particle size. Fig. 2 also shows a

linear relationship between the average Pd particle size and

the molar ratio of citrate/Pd which is useful for the prepara-

tion of the specific Pd particle size within this size range.

Apparently, the citrate ions had stabilized the Pd nano-

particles against agglomeration into larger particles. It is hy-

pothesized that citrate ions were first anchored to the carbon

support through the carbonyl groups on the carbon surface

formed during the support treatment by nitric acid. Then, the

carboxyl group of citrate partly coordinated with the surface

Pd nanoparticles deposited on the carbon surface. From the

measured particle sizes, specific surface areaswere calculated

by assuming spherical geometry of the particles. The calcu-

lated values are denoted as STEM in Table 1.

The surface states of the Pd nanoparticles were inferred

from their core level Pd3d XPS spectra. The high resolution XPS

spectra of the Pd 3d level are characterized by the spin-orbit

split components corresponding to the Pd 3d3/2 and Pd 3d5/2

doublet (Fig. 3). The binding energies of the measured Pd 3d5/2

peaks for all samples are slightly higher than the bulk values

(i.e. 335.1 eV for bulk Pd 3d5/2 spectra). The shift could be

caused by particle size effects and/or interactions between the

Pd nanoparticles and the carbon support. The Pd 3d spectra

Table 1 e The summary of catalyst characterization (TEM, XRD and CO-stripping tests).

Pd/C-1 Pd/C-2 Pd/C-4 Pd/C-5 Pd/C-6 Pd/C-8

Particle size (nm) from TEM 8.67 7.14 5.83 5.05 3.75 2.72

Particle size (nm) from XRD 8.21 6.91 5.67 4.98 3.42 e

Palladium interatomic distance (�A) 3.9042 3.9052 3.9083 3.9095 3.9108 e

Palladium lattice parameters (�A) 2.7607 2.7614 2.7636 2.7644 2.7654 e

Specific surface area (m2/g Pd, STEM)a 57.6 69.9 85.6 98.8 133.1 183.5

ECSAH (m2/g Pd)b 16.8 21.4 47.7 56.7 72.4 79.5

ECSACO (m2/g Pd)b 41.6 47.6 64.9 69.7 92.9 132.5

ECSAO (m2/g Pd)c 43.7 45.2 57.8 69.3 93.7 149.0

ECSAH/STEM (%) 29.2 30.6 55.7 57.4 54.4 43.3

ECSACO/STEM (%) 72.2 68.1 75.8 70.5 69.8 72.2

ECSAO/STEM (%) 75.5 64.7 67.5 70.1 70.4 81.2

a Calculated from the empirical formula: S ¼ 6000/(rd), where r is the density of Pd, 12.02 g/cm3, and d is the average particle size (nm, TEM

results).b The calculation methods are shown in the references [34,35].c The calculation method is cited from reference [36].

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 6 4 3 3e6 4 4 26436

was de-convoluted into two or three doublet as shown in

Fig. 3, and only Pd 3d5/2 sub-peaks of every sample are dis-

cussed here. The peak of the most intense doublet located

between 335.4 eV and 335.8 eV is assigned to be the signature

of the metallic Pd, while the second component peak with

binding energy ranging from 336.3 to 336.8 eV is assigned to be

the surface PdO, which could not be detected by XRD. For the

two samples with very small average Pd particle size (2.7 nm

and 3.8 nm), there is another small peak at the binding energy

of 338.0 eV which could be associated to the PdOx surface

phase. Besides, the XPS analysis shows a general, albeit slight

increases in the Pd3d binding energy (and the corresponding

de-convoluted peaks) with decreasing particle size. More

specifically, the binding energy for Pd 3d5/2 of surface metallic

Pd increases from 335.4 eV to 335.8 eV when the average

particle size decreases from 8.7 nm to 2.7 nm. This orderly

shift in binding energy is mainly caused by the different par-

ticle sizes of the Pd since the use of same carrier (XC-72 car-

bon) has removed much of the direct effects related to the

carbon support. The direction of the shift indicates a state of

electron deficiency in small Pd nanoparticles, which was

initially found to exist in Pd nanoparticles smaller than 2 nm

[32] but recent reports showed that itmay also appear in larger

nanoparticles [6,33]. Fig. 3 also shows that the oxidized Pd

surface contents are higher in smaller Pd nanoparticles. For

instance, the total surface oxide content (PdO and PdOx) is 50%

for the Pd/C-8 sample (2.7 nm) but is less than 20% for the Pd/

C-1 sample (8.7 nm). Since XRD had not detected the presence

of a bulk oxide phase, the oxides detected by XPS were mainly

surface bound. The increase in the number of step and kink

sites in smaller Pd nanoparticles could also bring about the

strong affinity for oxygen and consequently the increase in

surface Pd oxides.

Cyclic voltammetry in the absence of oxygen could be used

to infer proton adsorption/desorption behavior, water disso-

ciation, and metal surface oxidization and reduction on the

catalyst surface. The H-adsorption/desorption region or the

stripping peak of pre-adsorbed CO can also be used to esti-

mate the catalyst electrochemical surface area (ECSA), which

is more relevant to ORR and other electrochemical reactions

than the gross surface areas calculated from TEM or XRD

particle size data [34,35]. Fig. 4 shows the voltammograms of

various Pd/C samples in 0.1 M HClO4 aqueous electrolyte. The

amplified H-desorption region in inset B shows more clearly

the changes in the voltammetric features with Pd particle size

in the range of 8.7 to 2.7 nm. There are notable differences

between the H-adsorption/desorption regions of nanosized Pd

and nanosized Pt [36,37]. The H-desorption region of Pd can be

de-convoluted into several peaks (simulated with Origin

software), as shown in inset C of Fig. 4, which could be

attributed to proton adsorption on and desorption from

different Pd low index planes, namely the (111), (100), (110)

planes [38,39]. It is well known that the distribution of various

Pd crystallographic planes is particle size dependent. In this

study, it is found that component (a), which appears to be

more positive in potential (w0.2 V), increases its percentage

presence in ECSAH with the decrease in particle size. Hence

this component could be associated with proton desorption

from the Pd (100) planes, where hydrogen adsorbed most

strongly. The earliest proton desorption occurs at around

0.095 V (component (c)), which could be associated with the Pd

(110) planes and are considered to be the most active one

[40,41]. It is also found in this figure that the ratio of compo-

nent (c) in ECSA is almost invariant with particle size.

Component (b), where proton desorbs at around 0.14 V has a

diminishing presence in ECSAH with the decrease in particle

size. It is difficult to affirmatively attribute this component to

proton desorption from Pd (111) plane but it must contain an

element of it to some extent. Therefore, it can be seen here

that the ratio of (100) planes to (111) planes is increased with

the decrease of the average Pd particle size. This finding is

consistent with the results of Greund et al. [16]. The different

H-adsorption/desorption behavior herein has confirmed the

modification of the Pd surface morphology related to particle

size changes. The different surface morphology is expected to

affect the ORR process, and different behaviors in surface

oxide formation (Inset D of Fig. 4) may be used as an indica-

tion. The onset of the formation of surface oxide, denoted as

eOHads, occurs at a more negative potential for the smaller Pd

nanoparticles. The oxide peaks are also more intense indi-

cating more extensive oxidation of the surface of small Pd

nanoparticles due to the abundance of sites that have strong

Fig. 2 e The typical TEM images and histogram of various Pd/C samples. (A) Pd/C-8; (B) Pd/C-6; (C) Pd/C-5; (D) Pd/C-4; (E) Pd/C-

2; (F) Pd/C-1, and (G) the relationship between the average palladium particle size (TEM) and the molar ratio of citrate to

palladium chloride.

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 en e r g y 3 9 ( 2 0 1 4 ) 6 4 3 3e6 4 4 2 6437

affinity for adsorbed oxygen. The reduction of oxide in the

reverse scan is obviously different for nanoparticles with

different average particle sizes. It can be seen from inset E that

there are two reduction peaks for the sample of larger Pd

nanoparticles (such as Pd/C-1 and Pd/C-2) and only one peak

for the sample of smaller Pd nanoparticles occurring at more

negative potentials, and it is believed that the later is attrib-

uted to the narrow and uniformnanoparticle size distribution.

The double reduction peaks must be related to the wider

particle size distribution of those samples of larger average

Fig. 3 e X-ray photoelectron spectra of Pd3d of various Pd/C samples. (a) Pd/C-1; (b) Pd/C-2; (c) Pd/C-4; (d) Pd/C-5; (e) Pd/C-6; (f)

Pd/C-8.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 6 4 3 3e6 4 4 26438

nanoparticles (TEM inset results), as found from the inset E

that one of the peaks occurs almost at the same potentials of

the samples of smaller nanoparticles. And the other reduction

peak taking place at more positive potential is related to the

bigger nanoparticles. There is also a detectable negative shift

in corresponding reduction peaks with decreasing particle

size. This is indication that the reduction of surface oxides is

easier on the larger Pd particles, and becomes difficulty on the

smaller particles. The double reduction peaks and the poten-

tial difference are more easily distinguished with lower CV

scan rate. The trend concurs well with the relative ease of

surface oxide formation in the forward anodic scan. The

different reducibility of oxides on large particles is mademore

discernible in sample Pd/C-1 and Pd/C-2 because of the

broader size distributions there. The specific ECSA of various

Pd/C samples was estimated from the composite H-adsorp-

tion/desorption region and oxide reduction peaks based on

the assumed charge values of 210 mC cm�2 and 405 mC cm�2 for

the hydrogen UPD coulometry and the surface PdOmonolayer

reduction, and denoted as ECSAH and ECSAO, respectively

(Table 1) [34,36].

The ECSA can also be estimated from anodic stripping

voltammetry based on the charge associated with the oxida-

tive stripping of a monolayer of pre-adsorbed CO and denoted

as ECSACO. FromTable 1, it can be seen that the data of ECSACO

and ECSAO are very homologous except the data for the

sample Pd/C-8. This divergence could be resulted from the

smaller nanoparticle size of this sample which has stronger

oxygen affinity and more surface oxidized Pd species

including even some surface PdO2-like species. Table 1 also

shows that there is a positive correlation between ECSACO and

the surface areas calculated from TEM measurements (STEM)

(ECSACO is about 70% of STEM for all samples). The corre-

spondence between ECSAH and STEM is not so good. This could

be due to lots of factors such as different adhesive strength of

proton and CO, and some dissolution of adsorbed hydrogen

into Pd bulk [42]. Since ECSA represents the specific surface

area accessible to electrochemically active molecules, the

ECSACO data were used as the basis for some of the calcula-

tions shown later.

The polarization curves of ORR on various Pd/C samples

were measured by RDE in oxygen-saturated 0.1 M HClO4

electrolyte at 298 K at different rotation rates to access the

kinetic data (see Supplementary Information Fig. S1). Based on

our present ORR experiments, no obvious effects from the

possible citrate residues were observed. After careful washing

with copious DI water, it is believed that the possible residues

of citrate and the possible effect are negligible. The numbers

of electrons exchanged in ORR were calculated from the

widely accepted KouteckyeLevich first order equation

[43e45]. Different from the previous reports that ORR is a 4-

electron reaction on Pd catalysts [11,46,47], the numbers of

electrons transferred, n, which were calculated based on the

present experiment results are less than 4 and are different for

different samples. The computed results presented in Fig. 5

shows a parabolic relationship between n and the Pd particle

Fig. 4 e The CV results of various Pd/C samples in 0.1 M

HClO4 electrolyte (A). The inset B is the partial enlarged

detail of the H-desorption region of A in the potential

region from 0 V to 0.3 V. The inset C is the demonstration

of the H-desorption region difference of sample Pd/C-8 and

Pd/C-2.

Fig. 5 e The relationship between the numbers of electron

transferred and average palladium particle size and

electrochemical surface area.

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 en e r g y 3 9 ( 2 0 1 4 ) 6 4 3 3e6 4 4 2 6439

size. While n increases with average particle sizes decrease, it

approaches, but is not exactly 4 even for the smallest Pd

particles tested here. The value n however regresses linearly

with ECSACO and increases monotonically with the later. This

indicates that most of oxygen is reduced via the 4-electron

pathway on the Pd catalysts, and only small amount of oxy-

gen is reduced to H2O2 as the byproduct by the 2-electron

pathway. The study confirmed that the average particle size

of Pd catalysts has a significant effect on the reaction path-

ways. Generally, as shown in Table 2, the 4-electron pathway

is more prevalent among the smaller Pd nanoparticles. This is

understandable as the OeO bond cleavage is facilitated due to

the stronger affinity of small Pd particles for the oxygen

molecules shown previously. Table 2 shows that the specific

mass activity of Pd ðmAmg�1Pd Þ is also benefited from the

decrease in average particle size and hence a high Pd disper-

sion is recommended for improved metal utilization.

Although the current activity is still inferior to that of Pt under

methanol-free acidic conditions [48], it can be seen from the

study that the supported nanoparticles Pd shows a very

promising potential towards ORR.

The Tafel slopes measured from the high current density

region are in the range of 113e123 mV decade�1 for all Pd/C

samples, and are very close to the theoretical value (�118 [49]).

Different from the number of electrons transferred, there

appears to be no dependence of the Tafel slopes on particle

size. Hence to a first approximation, the rate-determining step

(RDS) of ORR on all Pd/C samples is likely to be the same. The

exchange current densities were also calculated and shown in

Table 2, which gradually increases with the improvement of

Pd dispersion. More notably the exchange current densities

measured here are at least 10 times higher than those of

sputtered Pd which again confirms that the preparation con-

ditions and the nano-structures of Pd catalysts determine

their activity [3,4,50]. The kinetic current was determined via

the KouteckyeLevich equation and normalized by the Pd

ECSACO to provide some indications of specific activities, or

average turnover numbers. Both the raw data before correc-

tion for mass transfer effects at 0.85 V and the data after

correction display a volcano plot relationshipwith the average

Pd size with maxima occurring in the particle size range

5.0e6.0 nm (Fig. 6). Although experimental conditions are not

exactly similar and different supporting electrolytes were

used, it is interesting to find the optimal Pd particle sizes from

different group are very similar, namely in the range of

5e6 nm [51,52].

Based on the above characterization by XRD and TEM, it

can be clearly demonstrated that the well-dispersed, nano-

sized Pd crystallites were successfully synthesized on the

carbon support. The Pd particle size distribution can be

significantly narrowed down by using a high molar ratio of

citrate/Pd in the synthesis, and the desired average particle

size can be achieved and controlled easily in aqueous solution

by changing citrate/Pd ratio within the range of ration

Table 2 e The summary of ORR results of different catalysts.

Sample Pd/C-1 Pd/C-2 Pd/C-4 Pd/C-5 Pd/C-6 Pd/C-8

Particle size (nm) from TEM 8.67 7.14 5.83 5.05 3.75 2.72

Number of electron transferred 3.40 3.48 3.52 3.65 3.78 3.96

Tafel slope (mV decade�1) 113.4 115.1 114.6 115.4 117.5 122.9

Specific mass activities (mA cm�2Pd ) at 0.85 V 2.27 3.63 6.00 7.59 8.02 8.73

Exchange current density (10�6 mA cm�2Pd ) 1.74 2.19 3.41 4.29 5.04 6.39

Kinetic current density (mA cm�2Pd ) at 0.85 V Raw data 4.68 8.73 9.24 10.89 8.7 6.59

Mass-trans free data 5.37 7.67 9.36 8.71 8.77 6.71

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 6 4 3 3e6 4 4 26440

reported herein. Compared to other synthesis such as polysol

method, the present method is cheaper and easier to operate

with tunable Pd nanoparticle sizes and narrow particle size

distribution. Therefore, the current method of preparation is

suitable and efficient for producing supported Pd nano-

particles even at a high metal loading of 10 wt%. Pd oxides

were detected by XPS while not by XRD as the formation of

oxides is mainly confined to the nanoparticle surfaces. Cyclic

voltammetry showed that the surface Pd oxides are electro-

chemically reducible and most of citrate residuals can be

removed bywashingwith copious DI water. The voltammetric

features in the H-adsorption/desorption region and XRD sug-

gest the dependence of electrochemical property on nano-

particle sizes. XPS measurements also detected minor

electronic effects and changes in oxygen affinity among the

smaller Pd nanoparticles. The trend to higher binding energy

with reduced particle size reflects an increase in the density of

state near the Fermi level; resulting in stronger interaction

between the Pd surface atoms and adsorbed oxygen mole-

cules. Corroborating evidence from cyclic voltammetry came

mainly from the surface oxide formation potential region

where the smaller Pd nanoparticles were found to dissociate

Fig. 6 e The relationship between the average palladium

particle size (nm) and the specific ECSA activity ðmA cmL2Pd Þ

at 0.85 V.

water at more negative potentials, the negative shift in the

surface oxide reduction peaks, and more surface PdO species

which brings about a larger ECSAO data. The stronger affinity

of smaller Pd particles for oxygenated species can be a double-

edged sword for ORR [53]. The strong affinity of small Pd

nanoparticles for oxygen can facilitate the dissociative

chemisorption of oxygen molecules into OHads, which is the

main intermediate in the 4-electron pathway of oxygen

reduction. On the other hand, a stronger affinity for oxygen

can also bring about an adverse effect on the removal of

eOHads as water, inhibiting the regeneration of active surface

sites for the next cycle of oxygen adsorption and reaction;

thus leading to an overall reduction in ORR efficiency.

The monotonic decrease in specific mass activity with the

increase in Pd particle size mirrors the trend found for Pt

catalysts, which promotes the endeavors to find more effec-

tive procedures to decrease the noble metal particle sizes [54].

This is to be expected since the fraction of surface atoms (and

hence available surface sites to some extent) per particle de-

creasesmonotonically with the increase of particle size. There

are however other effects. The specific current normalized by

ECSA can differentiate better these effects. The produced

classic volcano plot between specific area activity and Pd

particle size confirms that ORR is a structure-sensitive reac-

tion where different arrangements of the surface atoms leads

to different intrinsic catalytic activities, and different distri-

butions of low index planes on the particle surface with size

can result in significant nonlinearity in the observed trend.

On the surface of small Pd particles, the concentration of

contiguous Pd surface sites of high coordination numbers is

lower than that on the large particles. On the other hand,

there are more abundant low coordination sites (such as

flaws, kinks, steps and edges) which are strong binders for

oxygen and oxygenated species. Consequently, adsorption of

molecular oxygen ismuch stronger in smaller particles, which

is beneficial to OeO bond cleavage, favoring the 4-electron

pathway in ORR on Pd catalysts. If this were the only consid-

eration, there would be a monotonic inverse relationship be-

tween activity and Pd particle size. Excessive affinity for

oxygen, on the other hand, can interfere with the facile

removal of reaction intermediates such as eOH, inhibiting the

regeneration of Pd surface sites for the next catalytic cycle.

The decreased specific area activities on very small particles,

the negative shift in the potential for reduction of surface

palladium oxides, are experimental evidence for the decrease

in available active sites for ORR at a given potential. The

electronic state of Pd particles can also contribute to oxygen

reduction. The stronger interaction is favorable to the oxygen

molecule adsorption, OeO bond cleavage but is adversarial to

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 en e r g y 3 9 ( 2 0 1 4 ) 6 4 3 3e6 4 4 2 6441

O�ads reactionwith Hþ and site regeneration.With the decrease

of the particle size, the proliferation of low coordination sites

and electronic effects lead to unduly strong interaction be-

tween oxygenated species and surface Pd atoms resulting in

the deactivation of these surface sites. These two effects may

work well or against dependent on the particle size, resulting

in the experimentally observed volcano behavior.

Conclusions

Carbon-supported Pd nanoparticles can be prepared at the

high metal loading of 10 wt% by a simple aqueous solution

chemistry approach without any expensive and unwieldy

surfactants. The Pd particle size and narrow particle size

distribution are tunable and controllable by playing with the

molar ratio of citrate to Pd in the synthesis. The variation in Pd

particle size brings about not only changes in surface

morphology, but also the electronic states of Pd. Although the

specificmass activity in ORR increasesmonotonically with the

decrease in Pd particle size and the increase in ECSA, the

specific surface area activity transverses a maximum value as

the particle size decreases from 8.7 nm to 2.7 nm. The strong

particle size effects can be attributed to a combination of

several effects: the ORR activities of different low index

planes, types of surface sites and electronic effects in catal-

ysis. In different particle size ranges, some of these effects

may work well or against, but all of them are related to the

interaction between adsorbed oxygen or oxygenated species

with surface Pd atoms. The most favorable Pd particle size

towards ORR at room temperature is in the range of

5.0e6.0 nmwhich apparently has the best balance of all of the

above effects.

Appendix A. Supplementary data

Supplementary data related to this article can be found at

http://dx.doi.org/10.1016/j.ijhydene.2014.01.197.

r e f e r e n c e s

[1] Brouzgou A, Song SQ, Tsiakaras P. Low and non-platinumelectrocatalysts for PEMFCs: current status, challenges andprospects. Appl Catal B Environ 2012;127:371e88.

[2] Markovi�c NM, Schmidt TJ, Stamenkovi�c V, Ross PN. Oxygenreduction reaction on Pt and Pt bimetallic surfaces: aselective review. Fuel Cells 2001;1:105e9.

[3] Song S, Wang Y, Tsiakaras P, Shen P. Direct alcohol fuel cells:a novel non-platinum and alcohol inert ORR electrocatalyst.Appl Catal B Environ 2008;78:381e7.

[4] Pan W, Zhang X, Ma H, Zhang J. Electrochemical synthesis,voltammetric behavior, and electrocatalytic activity of Pdnanoparticles. J Phys Chem C 2008;112:2456e61.

[5] Alexeyeva N, Sarapuu A, Tammeveski K, Vidal-lglesias F,Solla-Gullon J, Feliu J. Electroreduction of oxygen on Vulcancarbon supported Pd nanoparticles and Pd-M nanoalloys inacid and alkaline solutions. Electrochim Acta2011;56:6702e8.

[6] Ha S, Larsen R, Masel RI. Performance characterization of Pd/C nanocatalyst for direct formic acid fuel cells. J PowerSources 2005;144:28e34.

[7] Zhou WP, Lewera A, Larsen R, Masel RI, Bagus PS,Wieckowski A. Size effects in electronic and catalyticproperties of unsupported palladium nanoparticles inelectrooxidation of formic acid. J Phys Chem B2006;110:13393e8.

[8] Alvarez GF, Mamlouk M, Kumar S, Scott K. Preparation andcharacterization of carbon-supported palladiumnanoparticles for oxygen reduction in low temperature PEMfuel cells. J Appl Electrochem 2011;41:925e37.

[9] Jukk K, Alexeyeva N, Sarapuu A, Ritslaid P, Kozlova J,Sammelselg V, et al. Electroreduction of oxygen on sputter-deposited Pd nanolayers on multi-walled carbon nanotubes.Int J Hydrogen Energy 2013;38:3614e20.

[10] Pires FI, Villullas HM. Pd-based catalysts: influence of thesecond metal on their stability and oxygen reductionactivity. Int J Hydrogen Energy 2012;37:17052e9.

[11] Salvador-Pascual JJ, Citalan-Cigarroa S, Solorza-Feria O.Kinetics of oxygen reduction reaction on nanosized Pdelectrocatalyst in acid media. J Power Sources2007;172:229e34.

[12] Moreira J, del Angel P, Ocampo AL, Sebatian P, Montoya J,Castellanos R. Synthesis, characterization and application ofa Pd/Vulcan and Pd/C catalyst in a PEM fuel cell. Int JHydrogen Energy 2004;29:915e20.

[13] Shim JH, Kim J, Lee C, Lee Y. Porous Pd layer-coated Aunanoparticles supported on carbon: synthesis andelectrocatalytic activity for oxygen reduction in acid media.Chem Mater 2011;23:4694e700.

[14] Brouzgou A, Podias A, Tsiakaras P. PEMFCs and AEMFCsdirectly fed with ethanol: a current status comparativereview. J Appl Electrochem 2013;43:119e36.

[15] Nagle LC, Burke LD. Anomalous electrochemical behaviour ofpalladium in base. J Solid State Electrochem 2010;14:1465e79.

[16] Huang YJ, Liao JH, Liu CP, Lu TH, Xing W. The size-controlledsynthesis of Pd/C catalysts by different solvents for formicacid electrooxidation. Nanotechnology 2009;20:105604e8.

[17] Silverstr-Albero J, Rupprechter G, Freund HJ. Atmosphericpressure studies of selective 1,3-butadiene hydrogenation onwell-defined Pd/Al2O3/NiAl(110) model catalysts: effect of Pdparticle size. J Catal 2004;240:58e65.

[18] Vannice MA, Garten RL. Supported palladium catalysts formethanation. Ind Eng Chem Prod Res Dev 1979;18:186e91.

[19] Oh S, Hoflund GB. Chemical state study of palladium powderand ceria-supported palladium during low-temperature COoxidation. J Phys Chem A 2006;110:7609e13.

[20] Hepburn JS, Stenger HG. No reduction by Al2O3-supportedrhodium, palladium, and platinum. 2. Effects of SO2

poisoning. Energy Fuels 1988;2:289e92.[21] Greeley J, Rossmeisl J, Hellman A, Nørskov JK. Theoretical

trends in particle size effect for the oxygen reductionreaction. Z Phys Chem 2007;221:1209e20.

[22] Wang RF, Liao SJ, Fu ZY, Ji S. Platinum free ternaryelectrocatalysts prepared via organic colloidal method foroxygen reduction. Electrochem Comm 2008;10:523e6.

[23] Zhou WJ, Lee JY. Particle size effects in Pd-catalyzedelectrooxidation of formic acid. J Phys Chem C2008;112:3789e93.

[24] Tamizhmani G, Dodelet JP, Guay D. Crystallite size effects ofcarbon-supported platinum on oxygen reduction in liquidacids. J Electrochem Soc 1996;143:18e23.

[25] Rodrıguez-Nieto FJ, Morante-Catacora TY, Cabrera CR.Sequential and simultaneous electrodeposition of Pt-Ruelectrocatalysts on a HOPG substrate and the electro-oxidation of methanol in aqueous sulfuric acid. J ElectroanalChem 2004;571:15e26.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 6 4 3 3e6 4 4 26442

[26] Radmilovic V, Gasteiger HA, Ross PN. Structure andchemical-composition of a supported Pt-Ru electrocatalytsfor methanol oxidation. J Catal 1995;154:98e106.

[27] Sun Y, Frenkel AI, Isseroff R, Shonbrun C, Forman M, Shin K,et al. Characterization of palladium nanoparticles by usingX-ray reflectivity, EXAFS, and electron microscopy. Langmuir2006;22:807e16.

[28] Teranishi T, Miyake M. Size control of palladiumnanoparticles and their crystal structures. Chem Mater1998;10:594e600.

[29] Heinemann K, Poppa H. In situ TEM evidence of latticeexpansion of very small supported palladium particles. SurfSci 1985;156:265e74.

[30] Tsubaki N, Fujimoto K. Promotional SMSI effect on supportedpalladium catalysts for methanol synthesis. Top Catal2003;22:325e35.

[31] Tribolet P, Kiwi-Minsker L. Palladium on carbon nanofibersgrown on metallic filters as novel structured catalyst. CatalToday 2005;105:337e43.

[32] Wilson OM, Knecht MR, Garcia-Martinez JC, Crooks RM.Effect of Pd nanoparticle size on the catalytic hydrogenationof allyl alcohol. J Am Chem Soc 2006;128:4510e1.

[33] Ramos AL, Alves PD, Aranda D, Schmal M. Characterizationof carbon supported palladium catalysts: inference ofelectronic and particle size effects using reaction probes.Appl Catal A 2007;277:71e81.

[34] Schmidt T, Gasteiger H, Stab GD, Urban PM, Kolb DM,Behm RJ. Characterization of high-surface areaelectrocatalysts using a rotating disk electrode configuration.J Electrochem Soc 1998;145:2354e8.

[35] Maillard F, Schreier S, Hanzlik M, Savinova E, Weinkauf S,Stimming U. Influence of particle agglomeration on thecatalytic activity of carbon-supported Pt nanoparticles inCO monolayer oxidation. Phys Chem Chem Phys 2005;7:385e93.

[36] Chierchie T, Mayer C, Lorenz WJ. Structural-changes ofsurface oxide layers on palladium. J Electroanal Chem1982;135:211e20.

[37] Giordano N, Passalacqua E, Pino L, Arico A, Antonucci V,Vivaldi M, et al. Analysis of platinum particle-size andoxygen reduction in phosphoric-acid. Electrochim Acta1991;36:1979e84.

[38] Min M, Cho J, Cho K, Kim H. Particle size and alloying effectsof Pt-based alloy catalysts for fuel cell applications.Electrochim Acta 2000;45:4211e7.

[39] Hamann CH, Hamnett A, Vielstich W. Electrochemistry.Weinheim: Wiley-VCH; 1998.

[40] Wang ZL. Transmission electron microscopy of shape-controlled nanocrystals and their assemblies. J Phys Chem B2000;104:1153e75.

[41] Hoshi N, Kida K, Nakamura M, Nakada M, Osada K.Structural effects of electrochemical oxidation of formic acidon single crystal electrodes of palladium. J Phys Chem B2006;110:12480e4.

[42] Lee K, Savadog O, Ishihara A, Mitsuchima S, Kaiya N, Ota K.Methanol-tolerant oxygen reduction electrocatalysts basedon Pd-3D Transition metal alloys for direct methanol fuelcells. J Electrochem Soc 2006;153:A20e4.

[43] Bard AJ, Faulkner LR. Electrochemical methods. New York:John Wiley & Sons, Inc.; 2001.

[44] Hsueh K, Gonzalez E, Srinivasan S. Electrolyte effects onoxygen reduction kinetics at platinum e a rotating-ring diskelectrode analysis. Electrochim Acta 1983;28:691e7.

[45] Zheng J, Wang X, Fu R, Yang D, Li P, Lv H, et al.Microstructure effect of carbon nanofibers on Pt/CNFselectrocatalyst for oxygen reduction. Int J Hydrogen Energy2012;37:4639e47.

[46] Burbe LD, Casey JK. An examination of the electrochemical-behavior of palladium electrodes in acid. J Electrochem Soc1993;140:1284e91.

[47] Raghuveer V, Manthiram A, Bard AJ. Pd-Co-Moelectrocatalyst for the oxygen reduction reaction in protonexchange membrane fuel cells. J Phys Chem B2005;109:22909e12.

[48] Gasteiger HA, Kocha SS, Sompalli B, Wagner FT. Activitybenchmarks and requirements for Pt, Pt-alloy, and non-Ptoxygen reduction catalysts for PEMFCs. Appl Catal B Environ2005;56:9e35.

[49] Murthi VS, Urian RC, Mukerjee S. Oxygen reduction kineticsin low and medium temperature acid environment:correlation of water activation and surface properties insupported Pt and Pt alloy electrocatalysts. J Phys Chem B2004;108:11011e23.

[50] Savadogo O, Lee K, Oishi K, Mitsushima S, Kamiya N, Ota K.New palladium alloys catalyst for the oxygen reductionreaction in an acid medium. Electrochem Commun2004;6:105e9.

[51] Jiang L, Hsu A, Chu D, Chen R. Size-dependent activity ofpalladium nanoparticles for oxygen electroreduction inalkaline solutions. J Electrochem Soc 2009;156:B643e9.

[52] Perez J, Gonzalez ER, Ticianelli EA. Oxygen electrocatalysison thin porous coating rotating platinum electrodes.Electrochim Acta 1998;44:1329e39.

[53] Lima F, Zhang J, Shao MH, Sasaki K, Vukmirovic M,Ticianelli E, et al. Catalytic activity-d-band center correlationfor the O2 reduction reaction on platinum in alkalinesolutions. J Phys Chem C 2007;111:404e10.

[54] Watanabe M, Sei H, Stonehart P. The influence of platinumcrystallite size on the electroreduction of oxygen. JElectroanal Chem 1989;261:375e87.