Synthesis and electrochemical study of Pt-based nanoporous materials

Article abstract of DOI:10.1016/j.electacta.2008.02.028

In the present work, a variety of Pt-based bimetallic nanostructured materials including nanoporous Pt, Pt-Ru, Pt-Ir, Pt-Pd and Pt-Pb networks have been directly grown on titanium substrates via a facile hydrothermal method. The as-fabricated electrodes were characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction and electrochemical methods. The active surface areas of these nanoporous Pt-based alloy catalysts are increased by over 68 (Pt-Pd), 69 (Pt-Ru) and 113 (Pt-Ir) fold compared to a polycrystalline Pt electrode. All these synthesized nanoporous electrodes exhibit superb electrocatalytic performance towards electrochemical oxidation of methanol and formic acid. Among the five nanoporous Pt-based electrodes, the Pt-Ir shows the highest peak current density at +0.50 V, with 68 times of enhancement compared to the polycrystalline Pt for methanol oxidation, and with 86 times of enhancement in formic acid oxidation; whereas the catalytic activity of the nanoporous Pt-Pb electrode outperforms the other materials in formic acid oxidation at the low potential regions, delivering an enhanced current density by 280-fold compared to the polycrystalline Pt at +0.15 V. The new approach described in this study is suitable for synthesizing a wide range of bi-metallic and tri-metallic nanoporous materials, desirable for electrochemical sensor design and potential application in fuel cells.

Full text of DOI:10.1016/j.electacta.2008.02.028

Electrochimica Acta 53 (2008) 6944–6952
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Synthesis and electrochemical study of Pt-based nanoporous materials
Jingpeng Wanga,b, Peter Holt-Hindlea, Duncan MacDonalda, Dan F. Thomasb, Aicheng Chena,∗,1
a Department of Chemistry, Lakehead University, Thunder Bay, Ontario P7B 5E1, Canada
b Department of Chemistry, University of Guelph, Guelph, Ontario N1G 2W1, Canada
a r t i c l e i n f o
a b s t r a c t
Article history:
In the present work, a variety of Pt-based bimetallic nanostructured materials including nanoporous Pt,
Pt–Ru, Pt–Ir, Pt–Pd and Pt–Pb networks have been directly grown on titanium substrates via a facile
hydrothermal method. The as-fabricated electrodes were characterized by scanning electron microscopy,
energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction and electro-
chemical methods. The active surface areas of these nanoporous Pt-based alloy catalysts are increased
by over 68 (Pt–Pd), 69 (Pt–Ru) and 113 (Pt–Ir) fold compared to a polycrystalline Pt electrode. All these
synthesized nanoporous electrodes exhibit superb electrocatalytic performance towards electrochemical
oxidation of methanol and formic acid. Among the five nanoporous Pt-based electrodes, the Pt–Ir shows
the highest peak current density at +0.50 V, with 68 times of enhancement compared to the polycrys-
talline Pt for methanol oxidation, and with 86 times of enhancement in formic acid oxidation; whereas
the catalytic activity of the nanoporous Pt–Pb electrode outperforms the other materials in formic acid
oxidation at the low potential regions, delivering an enhanced current density by 280-fold compared to
the polycrystalline Pt at +0.15 V. The new approach described in this study is suitable for synthesizing
a wide range of bi-metallic and tri-metallic nanoporous materials, desirable for electrochemical sensor
design and potential application in fuel cells.
Received 16 November 2007
Received in revised form 8 February 2008
Accepted 9 February 2008
Available online 16 February 2008
Keywords:
Fuel cells
Platinum-based catalysts
Electrooxidation
Nanoporous networks
Hydrothermal method
© 2008 Elsevier Ltd. All rights reserved.
1. Introduction
for the oxidation of SOMs [8,9]. However, the performance of a pure
Pt catalyst suffers considerable deterioration due to the formation
Platinum-based nanomaterials with high surface areas have
been receiving increased attention due to their unique properties
and a number of impressive applications in catalysis, fuel cells, and
chemical sensors [1–4]. Driven by the increasing need for cleaner
and more efficient engines in transportation applications, and by
the longer lifetimes and higher energy densities achievable in com-
pact power sources, polymer electrolyte membrane (PEM) fuel cells
are being developed as the most attractive alternatives to com-
bustion engines [5–7]. Of the common fuels generally considered,
hydrogen is mechanistically the easiest to be oxidized and no CO2 is
produced. However, the commercialization of the hydrogen fuel cell
still has considerable challenges related to production, purification,
distribution and storage. There has been growing interest in study-
ing the direct electrocatalytic oxidation of small organic molecules
(SOMs), such as formic acid and methanol for potential use as fuels.
The electrochemical oxidation of fuels requires the use of a
catalyst to achieve the current densities required for practical appli-
cations; Pt-based catalysts are some of the most efficient materials
of strongly adsorbed poison species. In particular, Pt is an excellent
catalyst for the dehydrogenation of SOMs but, on the other hand,
it is extremely susceptible to poisoning by CO, thiols and halides.
These remain strongly adsorbed on the Pt surface and block the
active sites from further catalysis, resulting in a dramatic decrease
in efficiency and overall performance. Efforts to minimize this poi-
soning have been centered around the addition of co-catalysts such
as Ru, Sn, Au, W, Mo, Os, Re, Rh, Pd, Pb, Bi and Ir to Pt to promote
CO oxidation [9–18]. In particular, the incorporation of ruthenium
into the Pt catalyst has yielded the best results, making the PtRu
catalyst the industry and research standard [8–11,14]. Two mech-
anisms have been proposed to address the enhancement of Pt
catalytic properties toward methanol oxidation by Ru. One is the
bi-functional mechanism: Ru facilitates the generation of oxygen-
like species on the catalyst surface which oxidize the poison species
[19–21]. The other is the electronic ligand-effect mechanism: the
modification of the electronic properties of Pt via a Pt–Ru orbital
overlap [22,23].
In recent years, fabrication of Pt-based nanomaterials has been
carried out via a variety of preparative strategies, such as the
impregnation of metal precursors followed by chemical or physical
reduction [24–32], electrodeposition [33–37], electroless deposi-
tion [38–40] and sol–gel derived process [41]. Given this extensive
∗
Corresponding author. Tel.: +1 807 3438318; fax: +1 807 3467775.
E-mail address: aicheng.chen@lakeheadu.ca (A. Chen).
ISE member.
1
0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2008.02.028

J. Wang et al. / Electrochimica Acta 53 (2008) 6944–6952
6945
exploration into the synthesis of Pt-based electrocatalysts, it stands
out that the electrocatalytic performance of the Pt-based cata-
lysts is highly dependent on not only the nature of the metal
elements, but also on a variety of surface conditions of the syn-
thesized materials (i.e. surface composition, morphology, impurity
contamination). Hence, an ideal synthesis method of Pt-based
electrocatalysts should be facile, cost-effective, controllable, repro-
ducible and free of surface contaminants.
networks. Then the autoclave was sealed and heated at 180 ◦C for
10 h. After cooling to room temperature, the coated Ti plate was
annealed in a tube furnace at 250 ◦C under argon for 2 h; and then
rinsed with pure water.
2.3. Instruments and electrochemical experiments
The surface morphology and composition of the synthesized
samples were characterized using scanning electron microscopy
(SEM) (JEOL JSM 5900LV) equipped with an energy dispersive
X-ray spectrometer (EDS) (Oxford Links ISIS). Surface elemental
compositions based on quantitative EDS analysis are reported in
average values of readings taken at five different spots on each
sample surface. The X-ray diffraction (XRD) patterns of the as-
prepared samples were recorded using a Philips PW 1050-3710
Diffractometer with Cu K␣ radiation. XRD patterns were compared
and assigned according to the database of the International Cen-
tre for Diffraction Data (ICDD). The surface composition of the
nanoporous Pt–Ir sample was further analyzed by X-ray photo-
electron spectroscopy (XPS) (Omicron EA-125 energy analyzer and
a multi-channel detector), using a monochromatic Mg K␣ X-ray
source (hꢁ = 1253.6 eV). The X-ray source operated at an output
of 220 W. All binding energies (BE) reported were corrected using
the Cl 2p3/2 peak at 199 eV as an internal standard. Atomic sen-
sitivity factors were employed for calculating the surface metallic
compositions from the integrated peak areas.
Cyclic voltammetry (CV), chronoamperometry (CA) and elec-
trochemical impedance spectroscopy (EIS) were carried out using
the three-electrode cell system described in our previous studies
[47,48]. In all cases a platinum coil was used as the counter elec-
trode and was flame annealed before each experiment. The working
electrode was the Ti-supported nanoporous Pt, Pt–Ru, Pt–Ir, Pt–Pb
and Pt–Pd networks. For comparison, a smooth polycrystalline Pt
wire with surface area 1.0 cm2 was also used as a working elec-
trode. A saturated calomel electrode (SCE) was used as the reference
and was connected to the investigated electrolyte through a salt
bridge. All potentials reported in this paper are referred to the SCE
reference electrode. Data acquisition and analysis were performed
with a Solartron 1287 Potentiostat and a Solartron 1252B Frequency
Response Analyzer using the software CorrWare and ZPlot. Unless
otherwise specified, the geometric surface area of each electrode
was used to calculate the current density. The amplitude of modu-
lation potential for the electrochemical impedance measurements
was 10 mV and the frequency was varied from 40 kHz to 25 mHz. All
solutions were de-aerated with ultra-pure argon (99.999%) before
measurements and argon was passed over the top of the solution
during the experiments. Carbon monoxide (PRAXAIR, 99.9%) was
used for the CO-stripping study. Before scanning the applied poten-
tial, the working electrode was held at −0.1 V for 360 s while a
stream of pure CO was bubbled into the solution during the first
180 s. Then the solution was purged with pure Ar for the remaining
3-min holding period. Immediately following the electrode pre-
treatment, two cycles of CV were recorded for each electrode. All
measurements were conducted at room temperature (22 2 ◦C).
Recently, we have directly grown three-dimensional (3D)
nanoporous Pt networks on titanium substrates through
a
hydrothermal-assisted seed growth method and our study has
shown that the active surface area of the formed nanoporous Pt
network electrode is much larger than that of polycrystalline Pt
[42]. Through a hydrothermal assisted precipitating process, the 3D
nanoporous Pt networks were further decorated with Ru, produc-
ing well-defined 3D nanoporous Pt–Ru bimetallic networks. The Ru
decoration significantly enhances the electrocatalytic activity of the
nanoporous Pt networks not only by lowering the onset potential,
but also by greatly increasing the current density for methanol oxi-
dation [43]. In addition, we further developed this hydrothermal
method and fabricated 3D nanoporous networks of Pt–Ru [44] and
Pt–Ir [45,46].
In the present work, we report on the direct growth of
nanoporous Pt and Pt–M networks (where M = Ru, Ir, Pb or Pd, with
the atomic ratio of Pt:M close to 50:50) on Ti substrates using the
novel one-step hydrothermal method and the systematic study of
the electrocatalytic performance of the synthesized nanoporous Pt,
Pt–Ru, Pt–Ir, Pt–Pb, Pt–Pd catalysts using the electrochemical oxi-
dation of CO, methanol and formic acid as a probe. To the best
of our knowledge, this is the first time to systemically compare
the physical and electrochemical properties of Pt, Pt–Pb, Pt–Pd,
Pt–Ru, and Pt–Ir nanomaterials fabricated by one universal syn-
thesis method. The major advantages of the synthesis method we
introduced here are that it is facile, reproducible and universal. Our
study has shown that the innovative approach described in this
study is suitable for synthesizing a wide range of bi-metallic and/or
tri-metallic nanoporous materials with large active surface areas
and high electrochemical performance.
2. Experimental
2.1. Materials
Titanium plates (99.2%) with
a
thickness of 0.5 mm
were purchased from Alfa Aesar and cut into small pieces
(1.25 cm × 0.80 cm). Methanol (99.9%), H2PtCl6·6H2O, Pb(NO3)2,
RuCl3, IrCl3, PdCl2, formaldehyde solution (37 wt.% in water),
sulfuric acid (99.999%) and formic acid (96.0%) were used as
received from Aldrich. Nanopure® water (18.2 Mꢀ cm) was used
to prepare all solutions. All other chemicals were analytical grade
and were used as received from commercial sources.
2.2. Synthesis of nanoporous Pt-based networks
The hydrothermal method used to fabricate the nanoporous
Pt-based network electrodes is similar to that in our previous
report [44]. Briefly, a Ti plate was washed in acetone followed by
Nanopure® water, and then etched in an 18 wt.% HCl solution at
85 ◦C for 10 min. The etched Ti substrate was transferred into a
teflon lined autoclave containing 10 ml of an aqueous mixture of
inorganic metal precursors and an excess reducing agent. The pre-
cursors used in this study were H2PtCl6, Pb(NO3)2, RuCl3, IrCl3 and
PdCl2. Formaldehyde was used as the reducing agent. For compari-
son, the initial concentration of the H2PtCl6 precursor was kept the
same (5.4 mM) in fabricating the nanoporous Pt and different Pt–M
3. Results and discussion
3.1. Surface characterization
The surface morphology and particle size of the as-synthesized
Pt-based materials were examined by SEM at a magnification of
10,000. Fig. 1 shows five SEM images of the as-synthesized Pt (a)
and the bimetallic Pt–M electrocatalysts, where M = Pb (b), Pd (c),
Ru (d) and Ir (e). It is obvious that the Ti substrates are well cov-
ered by the catalyst networks and their surfaces consist of irregular

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J. Wang et al. / Electrochimica Acta 53 (2008) 6944–6952
Fig. 1. SEM images recorded at a magnification of 10,000 for the as-synthesized Pt-containing catalysts: (a) nanoporous Pt; (b) Pt–Pb (50%); (c) Pt–Pd (50%); (d) Pt–Ru (50%);
and (e) Pt–Ir (50%). (f) The corresponding EDS spectra of the samples as shown in (a–e).
pores ranging from tens of nanometers to several micrometers in
diameter. The nanoporous Pt networks (Fig. 1a) are composed of
Pt nanoparticles with several tens of nanometers in diameter; the
Pt–Pb (Fig. 1b) and Pt–Pd (Fig. 1c) networks both have relatively
uniform particle sizes of around 80–100 nm, whereas the Pt–Ru
(Fig. 1d) and Pt–Ir (Fig. 1e) networks have relative larger particle
size, ranging from tens of nanometers to hundreds of nanometers.
All these SEM images reveal that the synthesized Pt-based materials
possess particle-based nanoporous network morphology.
Our previous study has shown that the Pt-based bimetallic cat-
alysts can be prepared with various bimetallic compositions by
adjusting the ratio of the precursors [44–46]. It is well known that
among most Pt-containing intermetallic phases, the one-to-one
phase containing an equimolar ratio of Pt and the co-catalyst metal
always exhibits the highest electrocatalytic activity [18,49]. Thus,
in the present work, electrodes with only the equimolar ratio of
Pt and each co-catalyst were fabricated and systematically stud-
ied. Fig. 1f presents the corresponding EDS spectra of these five
Pt-based nanoporous electrodes. Quantitative EDS elemental anal-
ysis reveals that the surface compositions of the four bimetallic
Pt-based nanoporous samples are all at a normalized atomic ratio
of Pt:M close to 50:50 with a deviation less than 5%. Another impor-
tant aspect in the EDS spectra is that there is no discernible carbon
or oxygen signal, indicating that the synthesized nanoporous net-
works are free of surface organic impurities.
As seen in Fig. 1f, the Ir peak strongly overlaps with the Pt sig-
nal; we employed XPS to further analyze the composition of the
nanoporous Pt–Ir sample. Fig. 2a shows the XP spectra in the Pt 4f
region of the Pt–Ir (solid line) and nanoporous Pt samples (dashed
line); and Fig. 2b presents the XP spectrum in the Ir 4f region of
the synthesized Pt–Ir sample. The two 4f binding states of Pt in
the nanoporous Pt sample are identified as 4f7/2 and 4f5/2 at 71.0
and 74.3 eV, which are consistent with previous studies [51–54].
Whereas the 4f7/2 and 4f5/2 doublets of Pt in the nanoporous Pt–Ir
sample are found at 71.4 and 74.7 eV, respectively. As shown in
Fig. 2b, the core level 4f7/2 and 4f5/2 doublets of Ir in the Pt–Ir sam-
ple appear at 60.6 and 63.6 eV, respectively; whereas the typical
values of Ir metal 4f doublets are 60.9 and 63.9 eV [51,52]. Hence, a
positive shift of 0.4 eV in binding energy occurs for the Pt 4f7/2 in
the Pt–Ir sample, and a negative shift of 0.3 eV in binding energy
occurs for the Ir 4f7/2. As the shift of binding energy of core-level
orbital is closely correlated with a change of electron density, these
results indicate an intra-atomic charge transfer from the Pt atom
to the Ir atom. Similar Pt–Ir electronic interaction has also been

J. Wang et al. / Electrochimica Acta 53 (2008) 6944–6952
6947
Fig. 3. XRD patterns recorded on the as-labeled Ti substrates and nanoporous Pt-
containing networks.
2ꢂ values for pure Pt and pure Pd/Ir/Ru; and there are no distinct
(1 1 1), (2 0 0) and (2 2 0) reflections characteristic of pure Pd, Ir,
Ru in each corresponding spectrum. All the evidence indicates
that the Pt phase and the added co-catalyst Pd, Ru or Ir phase are
partially or fully alloyed, rather than a simply physical mixture of
the particles of the two metals [41,54–56]. As for the XRD patterns
of the synthesized nanoporous Pt–Pb networks, all peaks, except
those from Ti substrate, can be indexed to the PtPb hexagonal NiAs
crystal structure. As there are no other distinct crystalline phases
present in significant amount, the Pt and Pb components are
considered fully alloyed [31,57,58]. The average crystallite size of
each Pt-containing samples was further calculated using Scherrer
formula [59], revealing that the nanoporous Pt, Pt–Pb, Pt–Pd, Pt–Ir,
Pt–Ru networks possess average crystallite sizes of 24, 35, 18, 6 and
4 nm, respectively. Comparison of the XRD-derived grain sizes and
the SEM-estimated particle sizes (Fig. 1) indicates that the synthe-
sized Pt-containing nanocrystals are somewhat agglomerated, as is
expected since no surfactants are used in their fabrication process.
Fig. 2. XP spectra of the Pt 4f (a) and Ir 4f (b) regions for the nanoporous Pt (red
dashed line) and Pt–Ir (solid black line) networks. (For interpretation of the refer-
ences to color in this figure legend, the reader is referred to the web version of the
article.)
reported by other groups [16,54]. The surface atomic Pt/Ir ratio was
determined based on the quantitative integration of Pt 4f and Ir
4f peak areas with consideration of the Pt and Ir atomic sensitiv-
ity factors [50]. The calculated Pt/Ir atomic ratio of the synthesized
nanoporous Pt–Ir networks is 45:55, which is consistent with the
EDS analysis.
Phase and structure characterization on the synthesized Pt-
based nanoporous electrodes were achieved by XRD analysis.
Fig. 3 shows the XRD patterns of a bare Ti substrate and all the
five synthesized Pt-based nanoporous materials. All the diffraction
patterns of the Ti substrate precisely matched the patterns of a
hexagonal Ti crystal in the ICDD database (PDF No. 44-1294). These
Ti diffraction patterns, with much less intensities, are also seen in
the XRD spectra of the remaining five Pt-containing samples. The
diffraction patterns of the nanoporous Pt, Pt–Pd, Pt–Ir and Pt–Ru
all display the (1 1 1), (2 0 0) and (2 2 0) reflections, characteristic of
the face centered cubic (fcc) crystal structure. However, the (1 1 1),
(2 0 0) and (2 2 0) peaks of the Pt–Pd, Pt–Ir and Pt–Ru were found to
have discernible positive shifts in 2ꢂ values compared to the corre-
sponding nanoporous Pt peaks. Such an increase in 2ꢂ values of the
(1 1 1), (2 0 0) and (2 2 0) peaks corresponds to decreased d-spacing
values and decreased lattice constants due to the incorporations
of Pd, Ir and Ru, respectively. This is confirmed by quantitative cal-
culations on lattice constant (a) based on the diffraction angles of
the (2 2 0) reflection peaks. Our results show that the synthesized
Pt–Pd, Pt–Ir, and Pt–Ru materials have a values of 0.389, 0.388, and
0.388 nm, respectively, all of which are smaller than the a value
of nanoporous Pt 0.392 nm. More importantly, the peak lines of
Pt–Pd, Pt–Ir and Pt–Ru’s patterns appear exactly between those
3.2. Electrochemical oxidation of surface adsorbed CO
CO has been identified as an intermediate as well as the major
poison species for Pt catalysts in the process of oxidation of most
SOMs (e.g. formic acid, methanol). At low electrode potentials, CO
remains strongly adsorbed, especially at the bridging and threefold
hollow sites of Pt surfaces [18,47]. Therefore, one of the most impor-
tant merits of an effective catalyst is its efficient removal of surface
adsorbed CO. To compare the performance of the as-synthesized
nanoporous electrodes towards CO poisoning, CO stripping exper-
iments were carried out in a 0.1 M H2SO4 solution and the results
are presented in Fig. 4. The most obvious common characteristics
in Fig. 4a–c are: first, the hydrogen adsorption peaks of these elec-
trodes are fully suppressed in the first positive-going scan due to
pre-adsorbed CO blocking the active surface sites; second, a broad
shoulder-like wave and a subsequent sharp current peak result-
ing from CO oxidation appear in the potential range from 0.0 to
0.6 V; third, the evidence (i) that the hydrogen desorption peaks
appear in the first negative-going scan and (ii) that the second cyclic
voltammogram resembles the CV in a CO free solution indicates
the complete oxidation of adsorbed CO during the first forward
scan. In particular, the nanoporous Pt–Ru electrode outperforms
the other three electrodes (nanoporous Pt, Pt–Ir and Pt–Pd) in terms

6948
J. Wang et al. / Electrochimica Acta 53 (2008) 6944–6952
are more than 100 mV more positive than those of Pt–Ru. This can
be explained by the bi-functional mechanism [19–21]: the Ru sites
at the Pt–Ru boundaries facilitate the generation of oxygenated
species thus oxidizing the adsorbed CO on Pt sites at a lower anode
potential. It is also noteworthy that the nanoporous Pt–Ir electrode
generated the highest peak current density upon CO oxidation.
As for the nanoporous Pt–Pb electrode, the onset potential of
CO oxidation is difficult to discern due to the simultaneous onset of
surface oxidation. The peak current density of the Pt–Pb electrode
for CO oxidation is the lowest, but the peak potential is similar to
that of the nanoporous Pt, except for its broader shape. In contrast
to all the other Pt-based electrodes, the nanoporous Pt–Pb elec-
trode (Fig. 4d) possesses an interesting feature, that is the hydrogen
adsorption/desorption is still fully suppressed in the second cycle
of CV, despite there being no more adsorbed CO present on the
electrode surface. That Pt–Pb surface heavily suppresses the hydro-
gen adsorption/desorption and that it behaves less actively towards
extrinsically adsorbed CO than the other Pt-containing electrodes
should be considered as the consequence of the Pt–Pb’s unique alloy
structure. In the hexagonal NiAs type crystal structure, the surface
Pt–Pt distance has been found to be dilated due to the incorporation
of Pb atoms, which effectively prevents CO from binding at bridge
or threefold hollow sites of Pt atoms [40,58].
Surface adsorbed CO monolayer oxidation has also been widely
used as an effective method to evaluate the active surface area
of Pt-based electrocatalysts [60]. Calculated from the peak areas
associated with CO oxidation in Fig. 4a–c, the consumed charges
upon CO stripping on nanoporous Pt, Pt–Pd, Pt–Ru and Pt–Ir sur-
faces are 20.4, 28.8, 29.3 and 47.8 mC, respectively. Assuming a
charge to surface area value of 420 ␮C/cm2 used for CO stripping
on a polycrystalline Pt surface [37,60], the electrodes’ active sur-
face areas were calculated to be 48.6 (nanoporous Pt), 68.6 (Pt–Pd),
69.8 (Pt–Ru) and 113.8 (Pt–Ir) cm2, respectively. Due to the infea-
sibility of this method for the Pt–Pb surface, we are unable to
quantitatively determine the active surface area of the fabricated
Pt–Pb electrode at this point. However, after comparison of Pt–Pb
with the other nanoporous Pt-containing electrodes in terms of
the XRD-derived crystallite size, SEM-estimated particle size, and
the observed porous morphology, the active surface area of the
Pt–Pb network should be comparable to that of the nanoporous Pt
and Pt–Pd networks. The control test of CO stripping shows that
the smooth polycrystalline Pt electrode used in this study pos-
sesses an active surface area of 1.0 cm2. These results indicate that,
despite of the same geometric surface area for each electrode, the
active surface areas of these 3D nanoporous Pt-containing elec-
trodes are much larger than that of the smooth polycrystalline Pt
electrode—the largest one (Pt–Ir) being over 113 times larger than
that of the polycrystalline Pt.
3.3. Electrochemical oxidation of methanol
The electrocatalytic activity of the as-synthesized nanoporous
Pt-based electrodes towards methanol oxidation was first studied
by cyclic voltammetry. Fig. 5a presents the positive-going por-
tions of the CV curves in a 0.1 M H2SO4 + 0.1 M CH3OH solution
at a potential scan rate of 20 mV/s. It is apparent that all the five
nanoporous Pt-containing electrodes have significantly enhanced
current densities compared to the bulk polycrystalline Pt. In par-
ticular, among these six Pt-containing electrodes, the peak current
density, appearing at different potentials, increases in the order of
bulk polycrystalline Pt (0.28 mA/cm2 at +0.51 V) < nanoporous Pt
(15.1 mA/cm2 at +0.60 V) < Pt–Ru (17.5 mA/cm2 at +0.50 V) < Pt–Pd
(19.8 mA/cm2 at +0.53 V) < Pt–Pb (48.3 mA/cm2 at +0.69 V) < Pt–Ir
(65.1 mA/cm2 at +0.50 V). In terms of the onset potential of
methanol oxidation, Pt–Ru outperforms the others at a potential
Fig. 4. Cyclic voltammograms of CO stripping on the nanoporous Pt-containing elec-
trodes: (a) Pt–Ru (blue) and nanoporous Pt (black); (b) Pt–Ir; (c) Pt–Pd; (d) Pt–Pb.
Two cycles of the CV were recorded for each electrode in a 0.1 M H2SO4 at a poten-
tial scan rate of 20 mV/s: the first sweep cycle is shown in solid line; and the second
sweep cycle is drawn in dashed line. (For interpretation of the references to color in
this figure legend, the reader is referred to the web version of the article.)
of its very low onset and peak potential of CO oxidation, which are
at about −0.03 and 0.30 V, respectively; while the nanoporous Pt,
Pt–Ir and Pt–Pd electrodes have similar onset potentials at about
0.08 V and peak potentials centered around 0.41 V, both of which

J. Wang et al. / Electrochimica Acta 53 (2008) 6944–6952
6949
Fig. 5. (a) Positive-going portions of cyclic voltammograms of the nanoporous Pt-
containing electrodes in a solution of 0.1 M CH3OH + 0.1 M H2SO4 at a potential scan
rate of 20 mV/s: Pt–Ir (red); Pt–Ru (blue); Pt–Pd (pink); Pt–Pb (green); nanoporous Pt
(black) and bulk polycrystalline Pt (brown). Positive-going portions of cyclic voltam-
mograms shown in (a) are re-plotted in (b) for five selected electrodes with current
densities shown as per active surface area. (For interpretation of the references to
color in this figure legend, the reader is referred to the web version of the article.)
of −0.05 V, while the rest of the nanoporous electrodes have a sim-
ilar onset potential at around +0.10 V. In addition, it is interesting to
note that the CV curves presented in Fig. 5a have their own charac-
teristics. For instance, nanoporous Pt and Pt–Pd have shoulder-like
waves at 100 mV negative to the large peak; Pt–Ru and Pt–Pb exhibit
a single yet very broad peak during the methanol oxidation; Pt–Ir,
unlike the other electrodes, has a relatively sharp and large peak.
Although these as-synthesized nanoporous electrodes all have
the same geometric area, their active surface areas are quite dif-
ferent as seen from the previous discussion. Hence the methanol
oxidation curves of the five selected Pt-containing electrodes were
re-plotted in Fig. 5b, with current densities shown per their active
surface areas. The nanoporous Pt, Pt–Ru and Pt–Pd electrodes
have approximately similar peak current densities to the poly-
crystalline Pt, indicating the much higher current densities of the
those nanoporous electrodes seen in Fig. 5a are mainly due to their
much larger active surface areas. The nanoporous Pt–Ir once again
surpasses the other electrodes with respect to the peak current den-
sity, showing that the incorporation of Ir into Pt greatly enhances
the electrode’s intrinsic electrocatalytic activity towards methanol
oxidation.
We further measured the steady-state current of the
nanoporous Pt-based electrodes at different electrode potentials.
The chronoamperometric plots recorded in a 0.1 M CH3OH + 0.1 M
H2SO4 solution are presented in Fig. 6. The electrode potential was
first held at −0.10 V for 60 s, then stepped up to +0.30 V (Fig. 6a),
+0.50 V (Fig. 6b) and +0.80 V (Fig. 6c) for 500 s. The steady-state
current densities are summarized in Table 1. It is clearly shown
that the nanoporous Pt-based electrodes have much higher current
densities than the bulk polycrystalline Pt under all the three inves-
Fig. 6. Chronoamperometry of the nanoporous Pt-containing electrodes in a solu-
tion of 0.1 M CH3OH + 0.1 M H2SO4. The electrode potentials was first held at −0.10 V
for 60 s, then stepped up to +0.30 V (a), +0.50 V (b) and +0.80 V (c), respectively. In all
cases, line a, b, c, d, e and f represent the CA curves of nanoporous Pt (black), Pt–Pb
(green), Pt–Pd (pink), Pt–Ru (blue), Pt–Ir (red) and bulk polycrystalline Pt (brown),
respectively. (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of the article.)
tigated electrode potentials. Suggested by the positive-going CV
curves of each electrode shown in Fig. 5a, 0.30 V is where the onset
of methanol oxidation occurs on the electrodes. At this relatively
low anode potential, the nanoporous Pt–Ru electrode exhibits
the highest current density. This is attributed to the Pt–Ru “bi-
functional” catalytic activity [19–21]. The current density of Pt–Ir
Table 1
Steady-state current densities (mA/cm2) at three applied potential values for
methanol oxidation at the nanoporous Pt-based electrodes in 0.1 M CH3OH + 0.1 M
H2SO4
+ 0.30 V vs. SCE
+0.50 V vs. SCE
+0.80 V vs. SCE
Pt
0.1
2.6
7.2
4.8
0.9
1.7
0.4
13.2
15.7
27.4
24.5
15.6
0.1
1.1
1.1
1.6
3.9
0.8
NP–Pt
Pt–Ru
Pt–Ir
Pt–Pb
Pt–Pd

6950
J. Wang et al. / Electrochimica Acta 53 (2008) 6944–6952
Fig. 8. Positive-going portions of the cyclic voltammograms of the nanoporous Pt-
containing electrodes in a solution of 0.1 M HCOOH + 0.1 M H2SO4 at a potential scan
rate of 20 mV/s: Pt–Ir (red); Pt–Ru (blue); Pt–Pd (pink); Pt–Pb (green); nanoporous
Pt (black) and bulk polycrystalline Pt (brown). (For interpretation of the references
to color in this figure legend, the reader is referred to the web version of the article.)
Fig. 7. Nyquist plots from electrochemical impedance spectroscopy at the potential
of +0.3 V in a solution of 0.1 M CH3OH + 0.1 M H2SO4, recorded for each Pt-containing
electrode: (a) nanoporous Pt (black); (b) Pt–Pb (green); (c) Pt–Pd (pink); (d) Pt–Ru
(blue); (e) Pt–Ir (red); (f) bulk polycrystalline Pt (brown). The amplitude of mod-
ulation potential is 10 mV. The frequency changes from 40 kHz to 25 mHz. (For
interpretation of the references to color in this figure legend, the reader is referred
to the web version of the article.)
rials exhibit the strongest catalytic activity towards the oxidation
of CO and methanol in terms of peak current density. This is con-
sistent with recent findings, which show positive effects brought
on by the addition of Ir to a Pt catalyst on the electrochemical
oxidation of methanol, carbon monoxide and hydrogen oxidation
[16,28,41,45,47,63]. It has been suggested that the incorporated Ir,
similar to the role of Ru in Pt–Ru systems, likely stabilizes the
adsorbed surface OH group, thus assisting in the oxidation of CO
or other adsorbed intermediates [28,54]. In the case of Pt–Pb, on
the one hand, Pb cannot provide adsorbed oxygenated species for
CO and methanol oxidation [64]; on the other hand, as discussed
earlier, the Pt–Pb surface is poisoned by the COads to a lesser extent
than a pure Pt surface, a process which is promoted by the “third
body effect” [40,65] wherein the addition of Pb to Pt surfaces dilates
the Pt–Pt distance and reduces the number of adsorption sites for
CO due to geometrical hindrance. Therefore, the nanoporous Pt–Pb
electrode exhibits a compromised performance towards methanol
oxidation: behaving inertly at lower potential regions; and produc-
ing remarkable high current densities in the high potential regions.
ranks the second, indicating that the incorporation of Ir to the Pt
surface also significantly promotes methanol oxidation at the low
electrode potential. The nanoporous Pt–Pd and Pt–Pb electrodes
both exhibit much lower current densities than the nanoporous
Pt–Ru electrode. This may be due to a lower performance in
the dehydrogenation of methanol at this low potential. Under
0.50 V (Fig. 6b), the steady-state current density decreases in the
order of nanoporous Pt–Ir > Pt–Pb > Pt–Pd ≈ Pt–Ru > nanoporous
Pt ꢀ polycrystalline Pt, indicating that under this near-peak
potential, the catalytic activities of all the bimetallic materials are
significantly enhanced compared to pure Pt materials. Especially
in the case of Pt–Ir, its steady-state current density reached a value
over 68 times larger than that of the polycrystalline Pt. When these
electrodes were tested under the potential of 0.80 V, their current
densities all dramatically dropped due to the oxide formation
under this high electrode potential.
3.4. Electrochemical oxidation of formic acid
Fig. 7 compares the Nyquist plots of all the Pt-containing elec-
trodes at 0.30 V in a solution of 0.1 M CH3OH + 0.1 M H2SO4. The
inset is the Nyquist plot of the polycrystalline Pt as both its imag-
inary and real components of impedance are extremely high. The
frequency was changed from 40 kHz to 25 mHz. It can be seen from
Fig. 7 that the electrochemical impedance increases as follows:
Pt–Ru < Pt–Ir < nanoporous Pt < Pt–Pd < Pt–Pb ꢁ polycrystalline Pt.
This order exactly matches the results of chronoamperometry
shown in Fig. 6a, suggesting that the significantly enhanced cat-
alytic activity of Pt–Ru and Pt–Ir electrodes at low anode potential
may be associated with their much lower charge transfer resistance
compared to the rest of the nanoporous electrodes [44,45].
We further investigated the electrocatalytic activity of the
synthesized nanoporous Pt-based electrodes towards formic
acid oxidation. Despite a lower theoretical energy density than
methanol, formic acid has its own advantages as a fuel for
fuel cells, such as ease in transportation and storage, low fuel
crossover through the membrane and high theoretical open cir-
cuit potential [46]. Fig. 8 presents forward-scanning CV curves of
the nanoporous Pt-containing electrodes and the polycrystalline
Pt electrode recorded in a 0.1 M H2SO4 + 0.1 M HCOOH solution.
All the nanoporous Pt-based electrodes exhibit much higher cur-
rent than the polycrystalline Pt electrode. Two anodic waves
From a mechanistic point of view, our electrochemical results on
the oxidation of CO and methanol at the nanoporous Pt–Ru elec-
trode are well supported by the “bi-functional” mechanism [43,44].
Nevertheless, it is important to point out that the exact bimetallic
composition of the Pt–Ru system also plays a vital role in deter-
mining the best catalytic effect. For example, Pt–Ru alloys with an
equimolar ratio of Pt and Ru have been found to yield the best
performance for CO and HCOOH oxidation [49,61]. In the case of
methanol, it has been reported that Pt–Ru alloys with 10–30% Ru
are more suitable for the dissociative chemisorption and oxidation
of methanol fuel [61,62]. In addition, the nanoporous Pt–Ir mate-
Table 2
Steady-state current densities (mA/cm2) at three applied potential values for formic
acid oxidation at the nanoporous Pt-based electrodes in 0.1 M HCOOH + 0.1 M H2SO4
+ 0.15 V vs. SCE
+0.30 V vs. SCE
+0.50 V vs. SCE
Pt
0.1
4.6
5.9
9.8
28.3
14.1
0.7
11.1
22.8
24.6
29.1
33.6
0.4
14.6
28.3
34.4
25.2
19.6
NP–Pt
Pt–Ru
Pt–Ir
Pt–Pb
Pt–Pd

J. Wang et al. / Electrochimica Acta 53 (2008) 6944–6952
6951
are observed in each curve and the peak potentials are strongly
reaction Eq. (2) produces adsorbed carbon monoxide ( COads
)
which acts as a poison intermediate and requires a higher elec-
trode potential to be further oxidized into CO2. In the case of
the polycrystalline Pt electrode, the electrochemical oxidation of
formic acid occurs mainly through the dehydration pathway. This
is why the current is very small at the low electrode potentials. In
contrast, a significantly enhanced current is seen in the CV curve
of the nanoporous Pt–Pb electrode at the low electrode potential
region and is attributed to the direct oxidation of formic acid, indi-
cating that the electrochemical oxidation of formic acid on the
nanoporous Pt–Pb electrode is mainly through the dehydrogena-
tion path. The electrochemical oxidation of formic acid on the other
four nanoporous electrodes takes place through both the dehydro-
genation and the dehydration paths. All these results show that by
changing the composition of the Pt-based nanoporous electrode,
one can easily tune the mechanisms of the electrochemical oxida-
tion of formic acid.
dependant on the nature of each co-catalyst. The onset poten-
tial of formic acid oxidation increases in the following order:
Pt–Pb < Pt–Pd < Pt–Ir ≈ Pt–Ru ≈ nanoporous Pt < polycrystalline Pt.
The mechanism of formic acid oxidation on platinum electrodes has
been well studied, involving the so-called dual pathways which are
dehydrogenation and dehydration, represented by the following
equations [7,29,39,66–71]:
HCOOH → CO2 + 2H+ + 2e− dehydrogenation path
(1)
HCOOH → COads + H2O → CO2 + 2H+ + 2e−
dehydration path
(2)
The preferred reaction for the complete oxidation of formic acid
is the dehydrogenation reaction Eq. (1), in which CO2 is directly
formed through the formation of adsorbed active intermediates
Chronoamperometry was employed to further measure the
steady-state current densities of the nanoporous Pt-containing
electrodes at different electrode potentials in formic acid oxidation.
Due to the high electrocatalytic activities of the Pt–Pb and Pt–Pd
electrodes in the low potential region indicated by Fig. 8, chronoam-
perometric plots were compared at +0.15 V (Fig. 9a), +0.30 V (Fig. 9b)
and +0.50 V (Fig. 9c). The steady-state current densities are summa-
rized in Table 2. It can be clearly seen that the nanoporous Pt–Pb
electrode possesses the highest steady-state current density—over
280 times of that of polycrystalline Pt. Pt–Pd electrode, ranking only
second to Pb at this potential, surpassing the remaining four elec-
trodes in current density. In the case of +0.30 V, Pt–Pd and Pt–Pb
exchanged their ranks in current density, while still outperform-
ing the remaining 4 electrodes. As suggested by other researchers
[39,58,72], this can be explained by the preferred dehydrogenation
pathway adopted by Pt–Pb and Pt–Pd at low potential regions. As
the applied potential was raised to +0.50 V, Pt–Ir and Pt–Ru took
over the first and second places in steady-state current density.
At this intermediate potential, Pt–Ir and Pt–Ru electrodes showed
a much higher activity than pure Pt due to the rapid enhance-
ment in CO2 production rate by the effective removal of COads
formed in the dehydration reaction, which is referred to a bi-
functional mechanism analogous to methanol oxidation [46,70]. In
all cases, the Pt–Pb electrode is able to retain very large current
densities (25–29 mA/cm2) from low to intermediate potentials; the
Pt–Pd electrode can only offer its largest current density at +0.30 V;
whereas the Pt–Ru, Pt–Ir and nanoporous Pt electrodes are capable
of delivering much larger current densities at +0.50 V than those at
lower potentials. In the case of Pt–Ir, its current density at +0.50 V
is 86 times of that of polycrystalline Pt electrode.
(
COOHads). In contrast, formic acid oxidation via a dehydration
4. Conclusions
In summary, novel 3D nanoporous bimetallic Pt–Ru, Pt–Ir,
Pt–Pb and Pt–Pd networks have been fabricated by a one-step
facile hydrothermal method that allows controllable variation
of the bimetallic compositions. For ease of comparison, these
nanoporous Pt-based electrodes were prepared at an equimolar
ratio of Pt to the co-catalyst. XRD results indicate that the Pt
atoms are alloyed with the co-catalyst components, resulting in
a hexagonal crystal structure for Pt–Pb, and fcc crystal structures
with contracted lattice constants for Pt–Pd, Pt–Ru and Pt–Ir mate-
rials. XRD-derived crystallite sizes, together with particle sizes
observed in SEM images indicate particle agglomeration during the
hydrothermal reduction process. The nanoporous Pt–Ir surface was
further characterized by XPS, results of which showed a positive
shift of 0.4 eV in binding energy for the Pt 4f7/2 and a negative
shift of 0.3 eV in binding energy for the Ir 4f7/2, indicating a unique
Fig. 9. Chronoamperometry of the nanoporous Pt-containing electrodes in a solu-
tion of 0.1 M HCOOH + 0.1 M H2SO4. The electrode potentials were first held at
−0.10 V for 60 s, then stepped up to +0.15 V (a), +0.30 V (b) and +0.50 V (c), respec-
tively. In all cases, line a, b, c, d, e and f represent the CA curves of nanoporous Pt
(black), Pt–Pb (green), Pt–Pd (pink), Pt–Ru (blue), Pt–Ir (red) and bulk polycrystalline
Pt (brown), respectively. (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of the article.)

6952
J. Wang et al. / Electrochimica Acta 53 (2008) 6944–6952
electronic interaction between Pt and Ir atoms. The electrocatalytic
performance of these nanoporous Pt-based electrodes was inves-
tigated towards methanol and formic acid oxidation. It is difficult
to quantitatively compare our electrochemical results with those
yet published as our electrodes are Ti-supported catalysts, which
were formed onsite in one single step without extra additives,
whereas a large fraction of fuel cell catalysts reported in most elec-
trochemistry journals were prepared with the addition of Nafion,
polymer binder and carbon black, and applied onto glassy carbon
substrate; in light of providing a standard point, we specifically
included a bulk polycrystalline Pt electrode as a comparable target.
The adsorbed CO monolayer stripping experiment reveals that the
active surface areas of the nanoporous Pt-containing electrodes
are much larger than that of the polycrystalline Pt electrode. It also
demonstrated enhanced catalytic activities towards CO oxidation
on the nanoporous Pt–Ru electrode. Moreover, our electrochemical
results show that for methanol oxidation, the Pt–Ru electrode
outperforms the other electrodes in having a low onset poten-
tial of −0.05 V, and the Pt–Ir electrode delivers a substantially
enhanced current density at +0.50 V with 68 times of enhancement
compared to polycrystalline Pt, whereas the nanoporous Pt–Pb
electrode exhibits low activity at the low potential regions and
is very active in delivering a remarkable high current density
at the higher potential regions; for formic acid oxidation, the
nanoporous Pt-containing electrodes distinguished themselves by
favoring different reaction pathways, by virtue of which the Pt–Pb
electrode at +0.15 V, and the Pt–Ir electrode at +0.50 V produced
current densities over 280 times and 86 times larger than that of
polycrystalline Pt. The approach described in this study is suitable
for synthesizing a wide range of bi-metallic and tri-metallic
nanoporous materials with large active surface areas and high
electrocatalytic activity, desirable for novel electrochemical sensor
design and new electrocatalyst development.
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