Enhanced electrocatalytic performance due to anomalous compressive strain and superior electron retention properties of highly porous Pt nanoparticles

Article abstract of DOI:10.1016/j.jcat.2012.04.004

The shape and structure of electrocatalysts at the nanoscale level have a decisive effect on their activity and durability in low-temperature fuel cells. Herein, we report the discovery of unexpected structural phenomena in exotic nanostructures: the anomalous compressive strain and superior electron retention properties of highly porous Pt (HP-Pt) nanoparticles synthesized using a weakly interacting organic capping agent, tetradecyl trimethyl ammonium bromide. Even though the particle size of the HP-Pt nanoparticles was much larger than those of commercial electrocatalysts, bond length shortening occurred anomalously, and the downshifted d-band center eventually led to increased oxygen reduction reaction activity. This is because the HP-Pt nanoparticles had a highly porous urchin-like dendritic structure, interestingly in the single-crystalline phase, despite the large particle size. In addition, their electron retention properties were superior to those of commercial samples, which led to drastically enhanced stability against Pt dissolution at high potentials.

Full text of DOI:10.1016/j.jcat.2012.04.004

Journal of Catalysis 291 (2012) 69–78
Contents lists available at SciVerse ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Enhanced electrocatalytic performance due to anomalous compressive strain
and superior electron retention properties of highly porous Pt nanoparticles
Dae-Suk Kim a, Cheonghee Kim b, Jung-Kon Kim a, Jun-Hyuk Kim a, Ho-Hwan Chun c, Hyunjoo Lee b,
Yong-Tae Kim a,
⇑
a School of Mechanical Engineering, Pusan National University, Busan 609-735, Republic of Korea
b Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul 120-749, Republic of Korea
c Global Core Research Center for Ships and Offshore Plants (GCRC-SOP), Pusan National University, Busan 609-735, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
The shape and structure of electrocatalysts at the nanoscale level have a decisive effect on their activity
and durability in low-temperature fuel cells. Herein, we report the discovery of unexpected structural
phenomena in exotic nanostructures: the anomalous compressive strain and superior electron retention
properties of highly porous Pt (HP-Pt) nanoparticles synthesized using a weakly interacting organic cap-
ping agent, tetradecyl trimethyl ammonium bromide. Even though the particle size of the HP-Pt nanopar-
ticles was much larger than those of commercial electrocatalysts, bond length shortening occurred
anomalously, and the downshifted d-band center eventually led to increased oxygen reduction reaction
activity. This is because the HP-Pt nanoparticles had a highly porous urchin-like dendritic structure,
interestingly in the single-crystalline phase, despite the large particle size. In addition, their electron
retention properties were superior to those of commercial samples, which led to drastically enhanced
stability against Pt dissolution at high potentials.
Received 24 December 2011
Revised 31 March 2012
Accepted 7 April 2012
Available online 17 May 2012
Keywords:
Electrocatalysts
Extended X-ray absorption fine structure
High-resolution powder diffraction
Oxygen reduction reaction
Highly porous Pt nanoparticles
Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction
devoted toward enhancing the ORR activity and durability of
Pt-based cathode electrocatalysts for PEMFCs.
One of the most serious hurdles to be overcome in the commer-
cialization of proton exchange membrane fuel cells (PEMFCs) is the
considerable activation loss in the cathode because of the slow
kinetics of the oxygen reduction reaction (ORR), which in turn re-
sults in a large amount of Pt usage for cathode electrocatalysts and
thereby increases the production cost of such fuel cells [1,2].
Furthermore, the rather fast deterioration of the cathode electro-
catalysts, mainly due to both Pt dissolution and coalescence, makes
it difficult to meet the US Department of Energy targets for durabil-
ity: 5000 h for transportation and 40,000 h for stationary applica-
tions [3,4]. For these reasons, considerable effort has been
The most widely considered idea is alloy formation (with either
solid-solution or phase-segregated structures) with various transi-
tion metals. Representative alloys reported to date include a Pt3Co
solid solution [5], Pt3Ni [6], Pt modified with Au clusters [4], a PtCu
phase-segregated structure [7], and Pt-on-Pd nanodendrite sys-
tems [8]. Although these alloys have shown enhanced ORR activity
(due to the compressive strain effect caused by alloying with tran-
sition metals having small atomic radii and the ligand effect caused
by the overlapping of electronic structures), leaching of the transi-
tion metals, which is considered to be one of the most serious
problems in terms of durability, is inevitable in most alloy electro-
catalysts (except for some phase-segregated alloys). Another
challenge in improving the ORR activity and durability is control-
ling the shape of the electrocatalyst particles, which can exist as
spheres [9], cubes [10], tetrahedra [11], cages [12], wires [13],
and sheets [14]. In particular, dendritic nanoparticles have been
synthesized intensively [15] and evaluated for various electrocata-
lytic reactions, including oxygen reduction [8] and small-molecule
oxidation [16], because these nanoparticles have a highly porous
structure, which leads to a large surface area per unit mass.
Lim et al. reported that Pd–Pt bimetallic nanodendrites synthe-
sized by the seed-growth technique demonstrated an activity 2.5
times higher than that of commercial Pt/C [8]. They concluded that
Abbreviations: HP-Pt, highly porous Pt; TTAB, tetradecyl trimethylammonium
bromide; ORR, oxygen reduction reaction; PEMEC, proton exchange membrane fuel
cell; HRPD, high-resolution powder diffraction; EXAFS, extended X-ray absorption
fine structure; XANES, X-ray absorption near-edge structure; DOE, US Department
of Energy; DI, deionized; TEM, transmission electron microscopy; HRTEM, high-
resolution TEM; CCD, charge-coupled device; XRD, X-ray diffraction; PLS, Pohang
Light Source; XPS, X-ray photoelectron spectroscopy; FCC, face-centered cubic;
RHE, reversible hydrogen electrode; CV, cyclic voltammetry; TF-RDE, thin-film
rotating disk electrode; ECSA, electrochemical surface area; RDS, rate-determining
step; 1NN, first nearest neighbor; RDF, radial distribution function; DW, Debye–
Waller.
⇑
Corresponding author. Fax: +82 51 514 0685.
E-mail address: yongtae@pusan.ac.kr (Y.-T. Kim).
0021-9517/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jcat.2012.04.004

70
D.-S. Kim et al. / Journal of Catalysis 291 (2012) 69–78
the enhanced ORR activity was attributable to an increase in the
surface area caused by the dendritic shape and the facets like
Pt(111) preferred for the ORR. The degree of durability enhance-
ment was relatively less remarkable than the activity increase,
and the authors did not discuss in detail the mechanism of such
an improvement. Similarly, Peng and Yang demonstrated the en-
hanced ORR activity and durability of Pt-on-Pd dendritic nanopar-
ticles formed through a sequential synthetic method [15]. They
elucidated that the origin of the ORR activity enhancement was
the improved tolerance against OH poisoning, which was indicated
by the positive shift of the onset and peak potentials for hydroxyl
ion adsorption during the forward sweep in cyclic voltammetry
(CV). They also suggested that the durability enhancement could
be explained by the favored interfacial structures between the Pt
and Pd supports and the larger particle size of the Pt-on-Pd nano-
particles. However, in most papers on dendritic nanoparticles, little
discussion has been provided on the most essential origin of per-
formance enhancement, such as the compressive strain effect for
ORR activity [7] and the electron retention property identified by
the d-band vacancy change at high potentials for durability [4,17].
In this study, we report the discovery of unexpected structural
phenomena in exotic nanostructures, that is, the anomalous com-
pressive strain and superior electron retention properties of highly
porous Pt (HP-Pt) nanoparticles synthesized using a weakly inter-
acting organic capping agent, tetradecyl trimethyl ammonium bro-
mide (TTAB). To date, most studies on the formation of highly
porous or dendritic nanoparticles have employed surfactants or
polymers with strong interactions with metal surfaces, which were
difficult to remove and may have acted as surface poisons [18,19].
However, since TTAB can be removed readily from the nanoparticle
surfaces, it was expected that enhanced electrocatalytic perfor-
mance could be demonstrated. In order to elucidate the origin of
the enhanced ORR activity and durability for HP-Pt nanoparticles,
we performed a precise analysis of their fine structures and elec-
tronic energy states. From various synchrotron studies, it was
found that compressive strain was exerted on the lattice, influenc-
ing the d-band center position; the d-band vacancy, which affects
the oxidation potential, hardly changed at high potentials. For an
in-depth analysis of the ORR activity enhancement resulting from
the small compressive strain-induced change in the lattice con-
stant, we employed fine-structure analysis techniques like high-
resolution powder diffraction (HRPD) with Rietveld refinement
and extended X-ray absorption fine structure (EXAFS) measure-
ments with FEFF analysis using a synchrotron beam. Furthermore,
in order to understand the origin of the stability against Pt dissolu-
tion through examination of the d-band vacancy change, we inves-
tigated the change in the white line in the Pt L3 absorption edge in
detail by applying potential steps using the in situ X-ray absorption
near-edge structure (XANES) technique; we used a specially de-
signed homemade measurement cell for this analysis. In the study,
the electrocatalytic properties of HP-Pt nanoparticles are compared
with those of several commercial samples, such as Pt/C E-TEK, Pt/C
E-TEK (300) (samples of Pt/C E-TEK heat-treated at 300 °C in a
hydrogen atmosphere in order to maximize the durability), and
Pt/C Premetek.
(99 + %, Aldrich) solution 0.5 mL as the reducing agent was in-
jected, and the solution was maintained at 90 °C for 7hrs. The total
volume of the solution was maintained at 10 mL, and final concen-
trations were 2 mM for K2PtCl4, 50 mM for TTAB, and 6 mM for
L-ascorbic acid. The product was centrifuged at 3000 rpm for
30 min. The supernatant solution was separated and centrifuged
again at 12,000 rpm for 10 min. The precipitate was collected and
redispersed in 5 mL of deionized (DI) water by sonication. For the
preparation of carbon-supported 20 wt.% HP-Pt nanoparticles, car-
bon supports (Vulcan XC-72R) were dispersed in DI water by son-
ication at a frequency of 70 kHz for 10 min. After sonication, the
prepared HP-Pt nanoparticles, equivalent to 20% weight ratio of
carbon, were dispersed in the above solution by stirring for 5 h.
The precipitate was filtered and washed several times with DI
water and ethanol and dried overnight in a vacuum oven at
50 °C. Commercial Pt/C E-TEK, Pt/C E-TEK annealed at 300 °C for
1 h (called Pt/C E-TEK (300)), and Pt/C Premetek with a metal load-
ing of 20 wt.% were employed for comparison with the HP-Pt
nanoparticles.
2.2. Catalyst characterization
The morphologies of the electrocatalyst samples were investi-
gated using transmission electron microscopy (TEM, JEOL JEM-
2011). Specimens for TEM observation were prepared by placing
a drop of the particle-dispersed solution onto a copper grid. The
microscope system was operated at an accelerating voltage of
200 keV. Two hundred nanoparticles randomly selected from the
TEM images were used to measure the average size of each sample.
The indexes of the surface facets and lattice fringes of the Pt nano-
particles were determined from high-resolution transmission elec-
tron microscopy (HRTEM) and Fourier transform data. All the
images were recorded with a charge-coupled device (CCD) camera.
X-ray diffraction (XRD) experiments on the synthesized pow-
ders were carried out using the synchrotron radiation of the 8C2
HRPD beamline at Pohang Light Source (PLS). The incident X-rays
were monochromatized to a wavelength of 1.54950 Å by a dou-
ble-bounce Si(111) monochromator. The diffraction pattern was
scanned in the 2h range of 10–151°, with a counting time of 4 s
per 0.02°. Scherrer’s equation was used to estimate the particle size
from the XRD results. For this purpose, the (111) peak of the Pt
face-centered cubic (FCC) structure at 2h = 40° was selected. The
XRD patterns were analyzed by the Rietveld refinement program
Fullprof [20] to determine the lattice parameters of the synthesized
powders. From all the profiles in the Fullprof program, a pseudo-
Voigt function was selected as the profile function.
X-ray photoelectron spectroscopy (XPS) experiments were per-
formed at the PLS 8A1 undulator (U7) beamline equipped with a
variable-included angle plane-grating monochromator. The inci-
dent photon energy was 630 eV, and photoelectrons emitted from
the surface of the Pt/C electrocatalysts, which were adhered to on
the carbon tape, were collected, and their energies were analyzed
by an electron analyzer (Physical Electronics: Model PHI 3057 with
an Omega lens and a 16-channel detector). Although Ar-ion bom-
bardment cleans the sample surface, it also produces some nega-
tive effects, such as variation of the surface topography and
preferential sputtering; hence, we did not clean the sample surface
by Ar-ion bombardment. The binding energies of the photoemis-
sion spectra were calibrated using the peak position of the Au 4f
line at 84.0 eV from a standard Au sample located next to the sam-
ple of interest. The XPS binding energies were analyzed using the
XPS spectral data processing software XPS International to process
the distribution of the Pt species and the relative intensities of the
synthesized powder. Details of fitting analysis are provided in
Fig. S2 of Supporting information.
2. Experimental section
2.1. Catalyst synthesis
In a typical synthesis, aqueous solutions of 20 mM K2PtCl4 (98%,
Aldrich) solution 1 mL, 400 mM tetradecyltrimethylammonium
bromide (TTAB; 99%, Aldrich) solution 1.25 mL and 7.25 mL of DI
water were mixed in a 20-mL vial at room temperature. The mix-
ture was heated at 90 °C for about 5 min until the solution became
clear. When the solution became clear, 120 mM L-ascorbic acid

D.-S. Kim et al. / Journal of Catalysis 291 (2012) 69–78
71
Pt L3 edge (11,564 eV) X-ray absorption spectroscopy (XAS)
3. Results and discussion
spectra were recorded at the BL7C1 beam line of PLS with a ring cur-
rent of 120–170 mA at 2.5 GeV. A Si(111) double-crystal mono-
chromator was employed to monochromatize the X-ray photon
energy. The data were collected in the transmission mode with
He and N2 gas-filled ionization chambers as detectors. Higher-order
harmonic contamination was eliminated by using detuning to re-
duce the incident X-ray intensity by ca. 40%. Energy calibration
was completed using standard Pt foil. Every experiment on the Pt/
C electrocatalysts was conducted using a homemade measurement
cell with an aluminum holder for the XAS powder experiments. The
in situ XANES data were recorded using a potentiostat (Biologic
VSP) via chronoamperometry measurements with potential steps
in N2-saturated 0.1 M HClO4 solutions. All the spectra were
recorded at room temperature. The spectra were processed using
the program IFEFFIT [21] (IFEFFIT version 1.2.11, Copyright 2008,
Matthew Newville, University of Chicago, http://cars9.uchica-
go.edu/ifeffit/) with background subtraction (AUTOBK) [22] and
normalization. The phase shift and back-scattering amplitude were
calculated theoretically using the FEFF 9.03 code [23]. Fourier trans-
form for the EXAFS spectra was performed in the range of approx-
imately 2.5–16 Åꢀ1 in k-space and 1–3 Å in the R window. From
these analyses, structural parameters such as the coordination
3.1. Basic morphology and structure of HP-Pt nanoparticles
The morphology and lattice structure of the HP-Pt nanoparticles
were identified from the bright-field and dark-field TEM images. As
shown in Figs. 1a–c, the commercial samples are spherical with a
size of about 3–4 nm. On the other hand, the HP-Pt nanoparticles
shown in Fig. 1d have urchin-like dendritic shapes and are highly
porous, with an average diameter of about 13 nm. Fourier transfor-
mation of the dark-field image obtained by TEM allows us to image
the reciprocal lattice points for the hkl planes and thereby enables
us to analyze the lattice structure and check the morphology
simultaneously. It is interesting to note that the reciprocal lattice
points, rather than the Ewald circle, are clearly shown in the Fou-
rier-transformed dark-field image, as seen in the inset of Fig. 1e.
This implies that the HP-Pt nanoparticles are not polycrystalline
secondary particles but primary particles with lattice structures
in the single-crystalline phase. Furthermore, the facets of the HP-
Pt nanoparticles are identified to consist of Pt(111) and (110)
rather than (100) or high-index planes, as observed in Fig. 1f.
This understanding is also supported by the HRPD patterns ob-
tained using a synchrotron beam followed by Rietveld refinement.
Fig. 2a shows the HRPD patterns and the reflection peaks, indexed
as (111), (200), (220), (311), (222), (331), and (420), corre-
sponding to face-centered cubic Pt with a lattice constant a =
3.930 Å (PDF standard cards, JCPDS 10-1311, space group
Fm3m[225]). The average particle size of the electrocatalysts was
calculated from the full-width at half-maximum of the Pt(111)
peak using Scherrer’s equation:
number (N), bond distance (R), Debye–Waller factor (
inner potential shift ( E0) were calculated.
Dr
2j ), and
D
2.3. Electrochemical measurements
All electrochemical measurements were performed in a three-
electrode electrochemical cell connected to a potentiostat (Biologic
VSP) at room temperature. A glassy carbon electrode (5 mm in
diameter) was used as the working electrode; it was polished with
diamond solution and Al2O3 slurries and then washed in DI water
with sonication before the experiments. HP-Pt/C powder (2 mg)
was dispersed well in DI water (2 mL) using an ultrasonicator,
kk
b cos h
d ¼
ð1Þ
where d is the average particle size (Å); k, the shape-sensitive coef-
ficient (0.9); k, the wavelength of the radiation used (1.5490 Å); b,
the full-width at half-maximum (in radians) of the peak; and h,
the angle at the position of the peak maximum (in radians). The
peak corresponding to Pt(111) at around 2h = 40° was used in the
calculation of the Lorentzian function. The calculated crystallite
sizes were 12.9 nm for HP-Pt nanoparticles, 3.3 nm for Pt/C E-TEK,
6.2 nm for Pt/C E-TEK (300), and 3.8 nm for Pt/C Premetek, which
are in good agreement (within the margin of error) with the values
evaluated from the TEM images (Table 1). In the Rietveld refine-
ment results, it is interesting to note that the lattice parameter of
the HP-Pt nanoparticles is the smallest among those of the four
kinds of electrocatalysts studied. In addition, as shown Fig. 2b, the
peak position corresponding to hkl (111) for HP-Pt/C was shifted
to a higher angle in comparison with that for the other commercial
samples. This indicated that a compressive lattice strain was ex-
erted on the lattice, which could lead to a change in the electronic
structure and thereby in the electrocatalytic activity [7].
and 20
l
L of the dispersion was dropped onto the glassy carbon
electrode with a micropipette to afford a Pt loading of 20
l
g/cm2
on the electrode. Uniform, thin HP-Pt/C films were prepared by
evaporating the water at room temperature. Nafion solution
(0.025 wt.%, 20 lL) was dropped onto a suspension-dried glassy
carbon electrode and dried in a vacuum oven at 70 °C for 30 min.
A Pt wire and a silver chloride electrode (Ag/AgCl sat. 3.5 M KCl)
were used as the counter and reference electrodes, respectively.
The Ag/AgCl reference electrode was calibrated on a daily basis
using a Pt disk electrode and H2-satuated 0.1 M HClO4 electrolyte
to ensure that its potential did not drift. All potentials were con-
verted to the reversible hydrogen electrode (RHE) scale. CV mea-
surements were performed in O2-free 0.1 M HClO4 electrolyte
(obtained by bubbling high-purity N2 gas through the electrolyte
for 1 h). The electrodes were cycled in the potential range between
0.05 and 1.15 V (versus RHE) at a scan rate of 20 mV/s after electro-
chemical cleaning with a quick scan (scan rate: 200 mV/s) for 50
cycles. After the CV measurements, the ORR was performed in
the same potential range in O2-saturated 0.1 M HClO4 at a scan rate
of 5 mV/s with rotation speeds of 100, 400, 900, 1600, and
2500 rpm. The durability tests were carried out at room tempera-
ture in O2-saturated 0.1 M HClO4 solutions with a cyclic potential
sweep between 0.6 and 1.1 V (versus RHE) at a sweep rate of
50 mV/s for 4000 cycles [8]. Then, CV measurements were per-
formed in N2-saturated 0.1 M HClO4 electrolyte in the potential
range between 0.05 and 1.15 V (versus RHE) at a scan rate of
20 mV/s. Pt/C E-TEK, Pt/C E-TEK (300), and Pt/C Premetek were
used for the same set of electrochemical measurements under
the same conditions. All the current densities were normalized to
the geometric area of the rotating disk electrode, and all electro-
chemical measurements were carried out at room temperature.
3.2. Investigation of ORR activity
In order to investigate the ORR activity of the HP-Pt nanoparti-
cles, we conducted thin-film rotating disk electrode (TF-RDE) mea-
surements [24]. Fig.
3 shows the results of electrochemical
measurements for the HP-Pt nanoparticles and several commercial
samples. The specific and mass activities of the samples were
determined by calculating jk, the mass-transport-corrected kinetic
current. For this calculation, the surface area employed was the
electrochemical surface area (ECSA) calculated from the hydrogen
adsorption/desorption region in CV, as follows [25]:
QH
L ꢁ qH
ECSA ¼
ð2Þ

72
D.-S. Kim et al. / Journal of Catalysis 291 (2012) 69–78
Fig. 1. TEM images of (a) Pt/C E-TEK (average size: 2.6 1.2 nm), (b) Pt/C E-TEK (300) (average size: 5.1 1.1 nm), (c) Pt/C Premetek (average size: 2.8 0.9 nm), (d) HP-Pt
nanoparticles/C, and (e and f) HRTEM image of HP-Pt nanoparticles (average size: 13.1 2.7 nm longest axis, 100% dendritic shapes) with the inset of Fourier transform
pattern for a single HP-Pt nanoparticle.
where L is the catalyst loading (l
g/cm2) on the working electrode;
1
j
1
1
1
1
¼
þ
¼
þ
ð3Þ
QH, the charge for the hydrogen adsorption (mC/cm2), calculated as
the mean value of the amounts of charge exchanged during the
electroadsorption and electrodesorption of H2 on the catalyst sites
after double-layer correction [24]; and qH, the charge required to
oxidize a monolayer of H2 on Pt (0.210 mC/cm2) [26].
jk jd jk
B
x1=2
in which
0:62nFACO D2O=3
2
2
B ¼
ð4Þ
g1=6
As seen in Table 2, both the specific and mass activities of the
HP-Pt nanoparticles were higher than those of the commercial
samples. At 0.9 V (versus RHE), the specific activity obtained for
where j is the experimentally obtained current; jk, the kinetic cur-
rent; jd, the diffusion-limited current; n, the number of electrons
transferred; F, the Faraday constant (F = 96485.3399 C/mol); A, the
geometric area of the electrode (A = 0.19625 cm2); CO2, the O2 con-
centration in the electrolyte (CO2 = 1.26 ꢁ 10ꢀ3 mol/L); DO2 the dif-
fusion coefficient of O2 in HClO4 solution (DO2 = 1.93 ꢁ 10ꢀ5 cm2/s);
the HP-Pt nanoparticles was 680.22
Pt/C E-TEK, Pt/C E-TEK (300), and Pt/C Premetek were 213.13,
84.08, and 223.35
A/cm2Pt, respectively. While the trend for the
l
A/cm2Pt, whereas those for
l
mass activity was almost same as that for the specific activity,
the degree of activity enhancement was less appreciable. It should
be noted that in the case of the Pt/C E-TEK (300) samples (heat-
treated at 300 °C in a hydrogen atmosphere), the limiting current
could not reach the theoretical value based on the Levich equation
at 1600 rpm. In fact, for Pt/C E-TEK (300) homogeneous thin-film
deposition of catalysts on the glassy carbon (working) electrode
could not be effected by the aqueous catalyst solution dropping
method; this was because unlike in the case of the other samples,
the surface of the carbon support after annealing was too inert for
the formation of a homogeneous catalyst solution in water. There-
fore, the geometric area of the glassy carbon electrode was incom-
pletely filled with the catalyst particles, so that the limiting current
could not reach the theoretical value calculated from the Levich
equation. Detailed results for the ORR activity evaluation at various
rotation speeds are provided in Fig. S1 of Supporting information.
As shown in Fig. 3c, the electrode current densities measured at
constant potentials were used to produce a Koutecky–Levich plot
[27], which represents the inverse current density (jꢀ1) as a func-
tion of the inverse square root of the rotation rate (xꢀ1/2) and indi-
cates the first-order kinetics with respect to molecular oxygen. The
Koutecky–Levich equation is written as
and
g, the viscosity of the electrolyte (
g
= 1.009 ꢁ 10ꢀ2 cm2/s) [28].
The slope of the lines (jꢀ1 versus xꢀ1/2) is 1/B. The calculated value
of B was 0.0915 mA sꢀ1/2 for the four Pt/C catalysts, and the number
of transferred electrons was calculated to be four for these four
kinds of Pt electrocatalysts.
The rate-determining step (RDS) in the ORR is the one-electron-
transfer reaction shown below in reaction (5), for which the theo-
retical Tafel slope is estimated to be ꢀ118 mV/decade on the basis
of the assumptions that the onset potential is independent of the
coverage of surface oxygen species and that the cathodic transfer
coefficient is 0.5 [29].
S þ O2 þ Hþ þ eꢀ ! S ꢀ O2H
ð5Þ
The Tafel slope under acidic conditions, in particular in an aqueous
HClO4 solution, in which the anion is hardly adsorbed on the elec-
trode surface, is generally divided into two regions: ꢀ59 mV/decade
at higher potentials (about 1.0–0.8 V versus RHE) and ꢀ118 mV/dec-
ade at potentials low enough for the formation of an oxide-free sur-
face. It is well known that such a deviation in the Tafel slope at high
potentials in aqueous HClO4 solution arises from the Temkin condi-
tions, where the potential is dependent on the surface coverage of

D.-S. Kim et al. / Journal of Catalysis 291 (2012) 69–78
73
among those of all the samples tested. Therefore, it is once again
proved that the HP-Pt nanoparticles show the fastest kinetics and
are therefore the best electrocatalysts for ORR among these four
samples.
(a)
observed
Rietveld refinement
3.3. Discussion of enhanced ORR activity
HP-Pt/C
It is well known that the catalytic activity is significantly af-
fected by the surface strain that results from changes in the size
or shape of catalyst particles or from alloy formation with a base
metal that has a different bulk Wigner–Seitz radius [32–34]. This
is because the adsorption strength of the adsorbate in the transi-
tion state, the key factor determining the catalytic activity, can
be tuned with the d-band structure of the catalyst particles, which
in turn varies markedly with surface strain. On the basis of the sur-
face strain effect, we found a clue to the origin of the enhanced ORR
activity of the HP-Pt nanoparticles in the EXAFS measurements. As
seen in Fig. 4a, the peak at around 2.6 Å, corresponding to the Pt–Pt
first nearest neighbor (1NN) of the radial distribution function
(RDF) for the HP-Pt nanoparticles, was clearly shifted to a lower r
value as compared to that of the commercial samples, indicating
that compressive strain was exerted on the lattice of these HP-Pt
nanoparticles. Indeed, the r value for the peak position in the EX-
AFS RDF is incorrect because of the phase shift for the absorber–
neighbor pair, and the exact bond length for the Pt–Pt 1NN should
be determined by fitting to the theoretical model using the widely
available FEFF code. The obtained bond length was 2.754 Å for the
HP-Pt nanoparticles, which was about 0.013 Å shorter than that for
the Pt/C E-TEK commercial sample. The detailed EXAFS parameters,
such as coordination number, energy shift, and Debye–Waller
(DW) factor, can be found in Table 3. In general, it is widely recog-
nized that a smaller particle has a shorter bond distance owing to
the increased surface tension [35,36]. Hence, there is a clear incon-
sistency between our data and this general phenomenon. We be-
lieve that this discrepancy is attributable to three factors: the
highly porous urchin-like morphology, the single-crystalline phase,
and the lower degree of surface oxidation for HP-Pt nanoparticles.
Even though the HP-Pt nanoparticle size is about 13 nm, the sur-
face-to-volume ratio is quite higher than that of a spherical particle
because of the highly porous urchin-like morphology. It has been
reported that a porous structure can lead to compressive strain
in the lattice and thereby result in shorter bond lengths [37]. In
addition, in spite of their somewhat complicated and porous
shapes, which can readily lead to high structural disorder and
can be formed from the agglomeration of several primary particles,
the HP-Pt nanoparticles are in the single-crystalline phase rather
than a polycrystalline phase composed of primary particles. In gen-
eral, the crystalline phase has both shorter bond lengths and lower
DW factors than the amorphous phase [38]. In addition, the DW
factor for smaller spherical nanoparticles with shorter bond
lengths is usually higher than that for larger particles [39,40]; nev-
ertheless, the DW factor for the HP-Pt nanoparticles is low, in spite
of their short bond lengths. This is because the HP-Pt nanoparticles
are in the well-ordered single-crystalline phase. Hence, we believe
that the bond length of the HP-Pt nanoparticles is anomalously
small in spite of their relatively large size because of the synergistic
Pt/C Premetek
Pt/C E-TEK (300)
Pt/C E-TEK
30 40 50 60 70 80 90 100 110 120 130
2θ
(b)
HP-Pt/C
Pt/C Premetek
Pt/C E-TEK (300)
Pt/C E-TEK
30
40
50
60
2θ
Fig. 2. (a) HRPD patterns for Pt peaks with Rietveld refinement. The reflection
peaks, indexed as (111), (200), (220), (311), (222), (331), and (420), correspond
to face-centered cubic Pt (PDF standard cards, JCPDS 10-1311, space group
Fm3m[225]). (b) Comparison of magnified peak of hkl (111).
OH produced by water dissociation [30]. Density functional calcula-
tions have shown that the ORR activation energy at the neighboring
sites of an adsorbate molecule increases because the identical direc-
tions of the induced dipole moments for the oxygen molecules and
the adsorbate (OH) species give rise to repulsive forces between
them [31]. Therefore, the potential varies with surface coverage in
this range, and the Tafel slope is not ꢀ118 mV/decade (the value ob-
tained for the one-electron-transfer reaction (RDS), wherein there is
no dependence of the potential on surface coverage) but ꢀ59 mV/
decade. The surface oxide is gradually reduced with the cathodic
scan, and eventually, the transition occurs at the point where the
surface OH coverage cannot have any effect on the shift of the poten-
tial. In other words, it occurs at the potential of the transition from
Temkin to Langmuirian conditions. Hence, a catalyst with a more po-
sitive transition potential can show faster ORR kinetics in aqueous
HClO4 solution. As shown in Fig. 3d, the transition potential for the
HP-Pt nanoparticles is 0.92 V (versus RHE), which is the highest
Table 1
Average particle size and lattice parameter of catalysts from HRPD data shown in Fig. 2.
Catalyst
Average particle
size (nm)
Lattice parameter
(Å)
Reliability factors
Pt/C E-TEK
3.3
6.2
3.8
3.91
3.92
3.93
3.91
Rp = 45.20%, Rwp = 40.10%, Rexp = 35.10%, S = 1.14
Rp = 27.80%, Rwp = 24.40%, Rexp = 15.67%, S = 1.56
Rp = 27.20%, Rwp = 25.70%, Rexp = 16.21%, S = 1.59
Rp = 24.60%, Rwp = 23.20%, Rexp = 14.88%, S = 1.56
Pt/C E-TEK (300)
Pt/C Premetek
HP-Pt/C
12.9

74
D.-S. Kim et al. / Journal of Catalysis 291 (2012) 69–78
(a)
(b)
0.6
0.4
0
-1
Pt/C E-TEK
0.2
Pt/C E-TEK (300)
Pt/C Premetek
HP-Pt/C
-2
-3
-4
-5
-6
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
j
j
Pt/C Premetek
HP-Pt/C
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.2
0.4
0.6
0.8
1.0
1.2
E / V (vs RHE)
E / V (vs RHE)
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
(c)
(d)
-59 mV/dec
1.05
1.00
0.95
0.90
0.85
0.80
0.75
-118 mV/dec
2 e-
j
4 e-
Pt/C E-TEK
Pt/C E-TEK
Pt/C E-TEK (300)
Pt/C Premetek
Pt/C E-TEK (300)
Pt/C Premetek
HP-Pt/C
Pt nanodendrites/C
0.0
0.1
0.2
0.3
-1
0
1
-2
ω
-1/2 / (rad/s)
-1/2
log (jk/mA cm )
Fig. 3. (a) CV curves for Pt/C E-TEK and HP-Pt/C nanoparticle catalysts in N2-saturated 0.1 M HClO4 solution with a scan rate of 20 mV/s. (b) ORR polarization curves for Pt/C E-
TEK, Pt/C E-TEK (300), Pt/C Premetek, and HP-Pt/C in O2-saturated 0.1 M HClO4 solution with sweep rate of 5 mV/s and rotation of 1600 rpm. (c) Koutecky–Levich plots for
ORR at 0.8 V (versus RHE). (d) Tafel plots for ORR (dashed lines indicate transition potentials from low Tafel slope to high Tafel slope).
Table 2
Half-wave potential (at 1600 rpm), ECSA and ORR activities at 0.9 V (versus RHE) for commercial Pt/C catalysts and HP-Pt/C.
Specific activity (l )
A/cm2Pt
Catalyst
Half-wave potential (V versus RHE)
LPt
(l
gPt/cm2)
ECSA (m2/gPt
)
Mass activity (lA/lgPt)
Pt/C E-TEK
0.85
0.79
0.85
0.88
20
20
20
20
65.07
36.37
51.08
27.89
144.28
34.63
119.52
204.05
213.13
84.08
223.35
680.22
Pt/C E-TEK (300)
Pt/C Premetek
HP-Pt/C
effect caused by the high surface-to-volume ratio, which in turn is
due to the exotic urchin-like morphology and the well-ordered sin-
gle-crystalline phase. Another possible reason for the anomalous
short bond length of the HP-Pt nanoparticles is the low degree of
surface oxidation resulting from the relatively larger particle size.
It has been reported that a high oxidation state of Pt has the effect
of increasing the Pt–Pt bond length [41]. Therefore, the high oxida-
tion state of the Pt surface compared to that of the HP-Pt nanopar-
ticles can result in a relatively longer Pt–Pt bond. A lower oxidation
state for the HP-Pt nanoparticles can also help in lowering the
bond length, although this effect is not so critical. The compressive
strain exerted on the lattice of these HP-Pt nanoparticles is conse-
quently thought to be the main origin of the enhanced ORR activ-
ity. The increased overlap of the d-band electrons between the
metal atoms, brought about by the compressive strain, results in
bandwidth broadening, and in order to keep the d-band occupancy
fixed, the energy of the d-band electron moves down from the Fer-
mi level. The downshifted d-band center position weakens the
adsorption of atomic oxygen, and the ORR activity is enhanced
by the decreased overpotential [32–34]. In summary, the origin
of the enhanced ORR activity of the HP-Pt nanoparticles is their un-
ique, highly porous and urchin-like dendritic structure, which can
give rise to compressive strain owing to the high surface-to-vol-
ume atomic ratio.
In addition to the compressive strain effect, the high proportion
of advantageous facets for the ORR, such as Pt(111), in the HP-Pt
nanoparticles can be considered another reason for the enhanced
ORR activity. It is interesting to note that while the mass activity
for the HP-Pt nanoparticles was only about 1.4 times higher than
those of the commercial samples, the specific activity was over
four times higher, as shown in Table 2. This result can be attributed
to the fact that the surface of the HP-Pt nanoparticles consists
mainly of facets that are advantageous for ORR, like Pt(111) or
Pt(110), even though the large particle size leads to a small ECSA.
As shown in Fig. 3a, the shape of the CV curve in the hydrogen
adsorption/desorption potential region is more rectangular for
the HP-Pt nanoparticles than for the commercial samples. This is
characteristic of the Pt(111) facet showing the best ORR activity
in an acid medium containing non-adsorbing anions [42]. The high
proportion of Pt(111) can be also identified from the HRTEM im-
age, as shown in Fig. 1f. Accordingly, while the increase in activity
per Pt mass unit was not so high, a massive increase appeared in

D.-S. Kim et al. / Journal of Catalysis 291 (2012) 69–78
75
hinder the ORR activity by acting as blocking sites with very strong
OH adsorption. Mayrhofer et al. reported that the ORR specific
activity decreases for small particles with high oxophilicity, be-
cause the active sites for oxygen adsorption or O–O splitting are
effectively blocked by the strong OH adsorption at these low-coor-
dinated sites [43]. As shown in Fig. 4b, the white line for the com-
mercial samples is much higher and the edge energy (E0) is shifted
to the higher side as compared to that for the HP-Pt nanoparticles.
Furthermore, there is a strong peak at around R = 1.6 Å in the RDF
(without consideration of the phase shift in the EXAFS model), cor-
responding to the Pt–O 1NN peak, for two types of commercial
samples (Pt/C E-TEK and Premetek), as seen in Fig. 4a. These results
imply that the commercial samples with sizes of 3–4 nm have a
high proportion of low-coordinated sites at which OH is prone to
be adsorbed by the dissociation of water in the air. However, after
the samples were annealed in a reductive atmosphere, the strong
peak corresponding to surface oxidation in the EXAFS RDF de-
creased in size because of the reduction of both the amount of sur-
face hydroxide and the number of unstable low-coordinated sites.
In the case of the HP-Pt nanoparticles, the number of low-coordi-
nated sites is small because of the lowest-intensity white line
and the lowest absorption energy (E0) in the XANES spectra and
the absence of the oxidation peak in the EXAFS RDF. Hence, in sum-
mary, the origins of the enhanced ORR activity for HP-Pt nanopar-
ticles are the compressive surface strain leading to a downshift of
the d-band center position, the high proportion of advantageous
facets like Pt(111), and the small proportion of low-coordinated
sites acting as ORR blocking sites.
(a)
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
(b)
Pt/C E-TEK
Pt/C E-TEK (300)
Pt/C Premetek
HP-Pt/C
3.4. Investigation of stability against Pt dissolution
Apart from electrocatalytic activity, the durability problem for
electrocatalysts is one of the most significant hurdles to be over-
come in the commercialization of fuel cells. It is widely recognized
that the degradation mechanisms of Pt-based cathode electrocata-
lysts can be categorized into four types: Pt dissolution, Pt agglom-
eration (sintering), carbon corrosion, and base-metal leaching for
alloy electrocatalysts [44]. Among these, the issues most closely re-
lated to this study are Pt dissolution and sintering, because the
electrocatalysts adopted for this investigation are Pt-only nanopar-
ticles on the same carbon black support (Vulcan XC-72). Notably,
many works have indicated that the growth of the catalyst particle
size after fuel cell operation is due to the Ostwald ripening caused
by dissolution and redeposition, rather than coarsening through
migration [45,46]. Because of this, we investigated in detail the sta-
bility of the nanoparticles against Pt dissolution by accelerated po-
tential cycling between 0.6 and 1.1 V (versus RHE). The stability
was determined by comparing the ECSA before and after cycling.
As seen in Fig. 5, for the HP-Pt nanoparticles, 27.3% of the ECSA
is lost because of Pt dissolution after 4000 cycles, while the losses
for E-TEK, E-TEK (300), and Premetek are 47.8%, 35.7%, and 53.1%,
respectively. This result indicates that the HP-Pt nanoparticles
have better durability as well as higher activity for the ORR than
do the commercial samples tested.
11540
11560
11580
Energy (eV)
11600
11620
Fig. 4. (a) Fourier transforms and (b) XANES data of the Pt L3edge for Pt/C E-TEK, Pt/
C E-TEK (300), Pt/C Premetek, and HP-Pt/C.
Table 3
Best-fit structural parameters for Pt–Pt1NNs obtained from analysis of the Pt L3edge
EXAFS spectra.
Catalyst
N
Rj (Å)
r2 (ꢁ10ꢀ6) (A2)
5.1
4.6
4.9
4.1
r-Factor (ꢁ10ꢀ2
1.89
1.67
1.95
1.75
)
Pt/C E-TEK
9.1
2.76 (3)
2.77 (3)
2.76 (3)
2.75 (4)
Pt/C E-TEK (300)
Pt/C Premetek
HP-Pt/C
10.5
9.2
10.8
the activity per Pt surface area unit. This is in good agreement with
a previous report, which stated that the specific activity for ORR in-
creases with the proportion of low-index planes and particle size
[43]. Furthermore, Markovic et al. showed that the order of ORR
activity of the three low-index facets was Pt(111) P (110) ꢂ
(100) in perchloric acid [42]. Therefore, we can conclude that the
high proportion of advantageous facets like Pt(111) in the HP-Pt
nanoparticles is another clear reason for their enhanced ORR
activity.
The small proportion of low-coordinated sites can be consid-
ered the third origin of the enhanced ORR activity of the HP-Pt
nanoparticles; this is because a small proportion of these sites
can result in decreased surface oxidation by OH adsorption as iden-
tified from the XANES and EXAFS results depicted in Fig. 4. It is
widely recognized that for small particle sizes, the proportion of
low-coordinated sites increases on the surface, and such sites can
3.5. Discussion of enhanced stability against Pt dissolution
It is well known that Pt dissolution is the most serious degrada-
tion factor at intermediate or high (ꢃ0.8 V versus RHE) potentials
for Pt/C electrocatalysts [47]. The following three reactions can
be considered as the origin of Pt dissolution [48]:
Pt ! Pt2þ þ 2eꢀ ðPt dissolutionÞ
ð6Þ
ð7Þ
ð8Þ
Pt þ H2O ! PtO þ 2Hþ þ 2eꢀ ðPt oxide formationÞ
PtO þ 2Hþ ! Pt2þ þ H2O ðPt oxide dissolutionÞ

76
D.-S. Kim et al. / Journal of Catalysis 291 (2012) 69–78
(a)
(b) 0.4
1.5
1.0
0.5
0.0
Pt/C E-TEK (300)
Pt/C E-TEK
0.3
0.2
0.1
0.0
-0.5
-1.0
-1.5
-2.0
-0.1
-0.2
-0.3
-0.4
-0.5
j
j
47.8% loss of ECSA
35.7% loss of ECSA
Initial state
After 4000 cycles
Initial state
After 4000 cycles
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
E / V (vs RHE)
E / V (vs RHE)
(c) 0.8
0.6
(d)0.3
Pt/C Premetek
HP-Pt/C
0.2
0.4
0.1
0.2
0.0
0.0
-0.1
-0.2
-0.3
-0.4
-0.5
-0.2
-0.4
-0.6
-0.8
-1.0
27.3% loss of ECSA
53.1% loss of ECSA
Initial state
after 4000 cycles
Initial state
After 4000 cycles
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
E / V (vs RHE)
E / V (vs RHE)
Fig. 5. CV Curves for (a) Pt/C E-TEK catalyst, (b) Pt/C E-TEK (300), (c) Pt/C Premetek, and (d) HP-Pt/C before and after 4000 cycles of accelerated durability tests.
(a)
(b)
HP-Pt/C
1.2
Pt/C E-TEK
1.2
0.8
0.4
0.0
0.8
0.4
0.0
0.47 V
0.77 V
0.97 V
1.07 V
0.47 V
0.77 V
0.97 V
1.07 V
11560
11580
11600
11560
11580
11600
Energy (eV)
Energy (eV)
(c) 0.25
(d)
0.20
0.15
0.10
0.05
0.00
Pt/C E-TEK
HP-Pt/C
0.4
0.6
0.8
1.0
1.2
E / V (vs. RHE)
Fig. 6. XANES spectra obtained with (a) Pt/C E-TEK and (b) HP-Pt/C nanoparticles at the Pt L3 edge at different potentials. (c) Comparison of the change of the absorption edge
peaks of the XANES spectra for Pt/C E-TEK and HP-Pt/C as a function of potential, obtained with electrocatalysts at different potentials in 0.1 M HClO4. (d) Homemade
measurement cell used for the in situ XANES experiments.

D.-S. Kim et al. / Journal of Catalysis 291 (2012) 69–78
77
require a much higher oxidation potential for dissolution, and
therefore, this is the decisive origin of their increased durability.
Such improved electron retention properties at high potentials
can be attributed to the surface conditions of the HP-Pt nanoparti-
cles. As shown in Fig. 7 and Table 4, the proportion of fully reduced
Pt is much higher in the HP-Pt nanoparticles, which have fewer
low-coordinated sites on the surface, than in the other samples.
This is in good agreement with the results found in the literature
[44]; the surfaces with a high proportion of low-coordinated sites
show a high oxidation state owing to an increase in the number of
initially adsorbed oxygen species. This phenomenon can readily be
identified in the Pt/C E-TEK (300) heat-treated sample, for which
the EXAFS RDF peak corresponding to the Pt–O 1NN disappeared
after heat treatment. Hence, the low proportion of low-coordinated
sites for HP-Pt nanoparticles can be considered to be the main ori-
gin of their enhanced electron retention properties and the elec-
tron-rich phase on their surface.
Pt4f7/2
Pt4f5/2
Pt/C E-TEK
Pt/C E-TEK (300)
Pt/C Premetek
HT-Pt/C
70
72
74
76
78
Binding Energy (eV)
The above conclusion can be further supported by the fact that
the shape change in the CV curve in the region of OH formation
(around 0.6 V) is more distinct in the commercial samples, which
have a higher proportion of low-coordinated sites such as step-
edge, kink, and corner sites because of their smaller particle size,
as indicated by the arrows in Fig. 5. This is because such low-coor-
dinated sites are more prone to be dissolved during potential cy-
cling than high-coordinated sites such as terrace sites. Hence, the
potential for OH formation is greatly shifted to the positive side.
In contrast, the current profile change of OH formation for the
HP-Pt nanoparticles is relatively smaller after cycling, demonstrat-
ing that the nanoparticle surface is originally composed of
high-coordinated sites and can therefore be more tolerant to
dissolution.
The final state effect may be another significant reason for the
enhanced electron retention characteristics observed at small pro-
portions of low-coordinated sites. It is widely recognized that it is
much more difficult for small particles to make up for the holes
generated by photoelectron emission because of their discrete
electronic structure; therefore, the binding energy is shifted to a
higher value in X-ray photoemission spectra [49]. In the case of
the HP-Pt nanoparticles, since the primary particle size is large,
the final state effect can be minimized because of the absence of
such a discrete electronic structure; consequently, a high propor-
tion of the electron-rich phase is obtained, as indicated by the Pt
4f core peak.
70
72
74
76
78
Binding Energy (eV)
Fig. 7. XPS spectra for Pt4f peak. The inset plot shows the XPS spectrum fitting of
the Pt4f photoemission from HP-Pt/C.
Dissolved Pt is redeposited onto a larger neighboring particle by
Ostwald ripening or precipitated on the proton exchange
membrane by the hydrogen molecules permeating from the anode.
Pt loss occurs through these processes, and thus the performance of
the fuel cell gradually deteriorates.
The highly improved stability against Pt dissolution in the case
of the HP-Pt nanoparticles can be explained from two viewpoints:
the electronic energy state of the surface and the unique morphol-
ogy with large primary particle size. First, strong evidence for the
electronic energy state was obtained from the XANES experiments
performed with potential steps applied using a specially designed
homemade cell (the details are shown in Fig. 6d). The white line
in the XANES is a strong peak due to the dipole-allowed transition
of the core electron to the continuum state. Hence, the white line
would increase remarkably for a catalyst that is prone to ioniza-
tion, or, in other words, is easily dissolved upon applying a poten-
tial. Fig. 6 shows the change in the white line at the Pt L3
absorption edge; this change is induced upon applying a potential
to cause electronic excitation from the Pt 2p3/2 core orbital formed
through spin–orbit coupling to the Pt d-band vacancy. It is interest-
ing to note that the degree of white-line increase at high potentials
for the HP-Pt nanoparticles is the lowest among that for all the
samples, implying that these nanoparticles are the most stable cat-
alysts against Pt dissolution. That is to say, the HP-Pt nanoparticles
4. Conclusions
The HP-Pt nanoparticles synthesized using a weakly interacting
organic capping agent, TTAB, showed enhanced ORR activity and
stability against Pt dissolution. The origins of their enhanced ORR
activity and stability were identified by performing EXAFS
measurements and in situ XANES studies. The high proportion of
surface atoms (due to the urchin-like morphology) in the single-
crystalline phase, despite the large primary particle size, led to
the generation of compressive strain through the high surface ten-
sion; therefore, the ORR activity was enhanced by the downshifted
d-band center position. The unique urchin-like morphology also
provided a high proportion of facets advantageous for the ORR,
such as Pt(111) and (110). In terms of durability, the stability
against Pt dissolution was markedly enhanced because of the im-
proved electron retention property and the decreased proportion
of low-coordinated sites, which played key roles in the upshift of
the oxidation potential in comparison with that of the spherically
shaped commercial samples. From the above discussion, we can
conclude that the highly porous urchin-like morphology is a prom-
ising structure for ORR electrocatalysts used in PEMFCs.
Table 4
Distribution of Pt 4f7/2 species and relative intensities.
Sample
Assigned chemical Binding energy Relative
state
(eV)
intensity (%)
Pt/C E-TEK
Pt(0)
Pt(II)
Pt(IV)
71.0
71.8
72.7
27.1
8.8
20.3
Pt/C E-TEK (300)
Pt/C Premetek
Pt(0)
Pt(II)
Pt(IV)
71.0
71.8
72.7
30.0
13.6
12.9
Pt(0)
Pt(II)
Pt(IV)
71.0
71.8
72.7
28.6
14.0
6.4
HP-Pt nanoparticles/C Pt(0)
71.0
71.8
72.7
39.4
5.7
8.1
Pt(II)
Pt(IV)

78
D.-S. Kim et al. / Journal of Catalysis 291 (2012) 69–78
[18] Y.J. Song, Y.B. Jiang, H.R. Wang, D.A. Pena, Y. Qiu, J.E. Miller, J.A. Shelnutt,
Acknowledgments
Nanotechnology 17 (2006) 1300.
[19] L. Wang, H.J. Wang, Y. Nemoto, Y. Yamauchi, Chem. Mater. 22 (2010) 2835.
[20] J. Rodríguez-Carvajal, Phys. B Condens. Matter. 192 (1993) 55.
[21] B. Ravel, M. Newville, J. Synchrotron Radiat. 12 (2005) 537.
This research was supported by a National Research Foundation
of Korea Grant funded by the Korean Government (2011-0003334,
2011-0027954, and 2011-0030658) and a Grant (M2009010025)
from the Fundamental R&D Program for Core Technology of Mate-
rials funded by the MKE, Republic of Korea.
[22] M. Newville, P. Lıvinßš, Y. Yacoby, J.J. Rehr, E.A. Stern, Phys. Rev. B47 (1993)
¯
14126.
[23] J.J. Rehr, R.C. Albers, Rev. Mod. Phys. 72 (2000) 621.
[24] T.J. Schmidt, H.A. Gasteiger, G.D. Stab, P.M. Urban, D.M. Kolb, R.J. Behm, J.
Electrochem. Soc. 145 (1998) 2354.
[25] M.De. Pozio, A. Francesco, F. Cemmi, L. Cardellini, J. Giorgi, Power Sources 105
(2002) 13.
[26] J. Perez, E.R. Gonzalez, E.A. Ticianelli, Electrochim. Acta 44 (1998) 1329.
[27] M.H. Shao, T. Huang, P. Liu, J. Zhang, K. Sasaki, M.B. Vukmirovic, R.R. Adzic,
Langmuir 22 (2006) 10409.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jcat.2012.04.004.
[28] J. Zhang, Y. Mo, M.B. Vukmirovic, R. Klie, K. Sasaki, R.R. Adzic, J. Phys. Chem. B
108 (2004) 10955.
[29] M. Gattrell, B. MacDougall, In Handbook of Fuel Cells, vol. 2, John Wiley & Sons
Ltd., England, 2003. p. 443.
References
[30] M.A. Damjanovic, J.O.M. Genshaw, J. Bockris, Chem. Phys. 45 (1966) 4057.
[31] J.J. Mortensen, B. Hammer, J.K. Nørskov, Phys. Rev. Lett. 80 (1998) 4333.
[32] M. Mavrikakis, B. Hammer, J.K. Nørskov, Phys. Rev. Lett. 81 (1998) 2819.
[33] J.X. Wang, H. Inada, L. Wu, Y. Zhu, Y. Choi, P. Liu, W.P. Zhou, R.R. Adzic, J. Am.
Chem. Soc. 131 (2009) 17298.
[34] Y. Xu, A.V. Ruban, M. Mavrikakis, J. Am. Chem. Soc. 126 (2004) 4717.
[35] I.E. Beck, V.V. Kriventsov, D.P. Ivanov, V.I. Zaikovsky, V.I. Bukhtiyarov, Nucl.
Instrum. Methods A 603 (2009) 108.
[36] Y.T. Kim, K. Ohshima, K. Higashimine, T. Uruga, M. Takata, H. Suematsu, T.
Mitani, Angew. Chem.– Int. Edit. 4 (5) (2006) 407.
[37] A. Witkowska, Di Cicco, E. Principi, Phys. Rev. B 76 (2007) 12.
[38] S.K. Sarkar, S. Kababya, S. Vega, H. Cohen, J.C. Woicik, A.I. Frenkel, G. Hodes,
Chem. Mater. 19 (2007) 879.
[39] B. Johannessen, P. Kluth, D.J. Cookson, G.J. Foran, M.C. Ridgway, Nucl. Instrum.
Methods B 246 (2006) 45.
[40] S.M. Kluth, B. Johannessen, P. Kluth, C.J. Glover, G.J. Foran, M.C. Ridgway, Nucl.
Instrum. Methods B 238 (2005) 264.
[41] Y. Nagai, T. Hirabayashi, K. Dohmae, N. Takagi, T. Minami, H. Shinjoh, S.
Matsumoto, J. Catal. 242 (2006) 103.
[42] N.M. Markovic, H.A. Gasteiger, P.N. Ross, J. Electrochem. Soc. 144 (1997) 1591.
[43] J.J. Mayrhofer, B.B. Blizanac, M. Arenz, V.R. Stamenkovic, P.N. Ross, N.M.
Markovic, J. Phys. Chem. B 109 (2005) 14433.
[1] H.A. Gasteiger, S.S. Kocha, B. Sompalli, F.T. Wagner, Appl. Catal. B – Environ. 56
(2005) 9.
[2] R.R. Adzic, J. Zhang, K. Sasaki, M.B. Vukmirovic, M. Shao, J.X. Wang, A.U. Nilekar,
M. Mavrikakis, J.A. Valerio, F. Uribe, Top. Catal. 46 (2007) 249.
[3] W. Shuangyin, J. San Ping, W. Timothy John, G. Jun, W. Xin, J. Phys. Chem. C 113
(2009) 18935.
[4] J. Zhang, K. Sasaki, E. Sutter, R.R. Adzic, Science 315 (2007) 220.
[5] V.R. Stamenkovic, B.S. Mun, M. Arenz, K.J.J. Mayrhofer, C.A. Lucas, G.F. Wang,
P.N. Ross, N.M. Markovic, Nat. Mater. 6 (2007) 241.
[6] V.R. Stamenkovic, B. Fowler, B.S. Mun, G.F. Wang, P.N. Ross, C.A. Lucas, N.M.
Markovic, Science 315 (2007) 493.
[7] P. Strasser, S. Koh, T. Anniyev, J. Greeley, K. More, C.F. Yu, Z.C. Liu, S. Kaya, D.
Nordlund, H. Ogasawara, M.F. Toney, A. Nilsson, Nat. Chem. 2 (2010) 454.
[8] B. Lim, M.J. Jiang, P.H.C. Camargo, E.C. Cho, J. Tao, X.M. Lu, Y.M. Zhu, Y.A. Xia,
Science 324 (2009) 1302.
[9] N.C. Bigall, T. Hartling, M. Klose, P. Simon, L.M. Eng, A. Eychmuller, Nano Lett. 8
(2008) 4588.
[10] T.S. Ahmadi, Z.L. Wang, A. Henglein, M.A. El-Sayed, Chem. Mater. 8 (1996)
1161.
[11] T.S. Ahmadi, Z.L. Wang, T.C. Green, A. Henglein, M.A. El-Sayed, Science 272
(1996) 1924.
[12] Y. Song, R.M. Garcia, R.M. Dorin, H. Wang, Y. Qiu, J.A. Shelnutt, Angew. Chem.
Int. Edit. 45 (2006) 8126.
[44] Y. Shao-Horn, W.C. Sheng, S. Chen, P.J. Ferreira, E.F. Holby, D. Morgan, Top.
Catal. 46 (2007) 285.
[13] J. Chen, T. Herricks, M. Geissler, Y. Xia, J. Am. Chem. Soc. 126 (2004) 10854.
[14] Y. Song, W.A. Steen, D. Pena, Y.-B. Jiang, C.J. Medforth, Q. Huo, J.L. Pincus, Y. Qiu,
D.Y. Sasaki, J.E. Miller, J.A. Shelnutt, Chem. Mater. 18 (2006) 2335.
[15] Z.M. Peng, H. Yang, J. Am. Chem. Soc. 131 (2009) 7542.
[16] Y.W. Lee, M. Kim, Y. Kim, S.W. Kang, J.H. Lee, S.W. Han, J. Phys. Chem. C 114
(2010) 7689.
[45] J. Aragane, T. Murahashi, T. Odaka, J. Electrochem. Soc. 135 (1988) 844.
[46] T. Akita, A. Taniguchi, J. Maekawa, Z. Siroma, K. Tanaka, M. Kohyama, K.
Yasuda, J. Power Sources 159 (2006) 461.
[47] P.J. Ferreira, G.J. la O’, Y. Shao-Horn, D. Morgan, R. Makharia, S. Kocha, H.A.
Gasteiger, J. Electrochem. Soc. 152 (2005) A2256.
[48] M. Pourbalx, M. Pourbalx, N A C E International, Texas, 1974, p. 644.
[49] G.K. Wertheim, S.B. DiCenzo, S.E. Youngquist, Phys. Rev. Lett. 51 (1983) 2310.
[17] K. Sasaki, J.L. Zhang, J. Wang, F. Uribe, R. Adzic, Res. Chem. Intermediat. 32
(2006) 543.