Highly active PtRu catalysts supported on carbon nanotubes prepared by modified impregnation method for methanol electro-oxidation

Article abstract of DOI:10.1016/j.electacta.2008.05.003

A simple and rapid synthesis method (denoted as modified impregnation method, MI) for PtRu/CNTs (MI) and PtRu/C (MI) was presented. PtRu/CNTs (MI) and PtRu/C (MI) catalysts were characterized by transmission electron microscopy (TEM) and X-ray diffractometry. It was shown that Pt-Ru particles with small average size (2.7 nm) were uniformly dispersed on carbon supports (carbon nanotubes and carbon black) and displayed the characteristic diffraction peaks of Pt face-centered cubic structure. Cyclic voltammetry and chronoamperometry showed that the Pt-Ru/CNTs (MI) catalyst exhibited better methanol oxidation activities than Pt-Ru/C (MI) catalyst and commercial Pt-Ru/C (E-TEK) catalyst. The single cells with Pt-Ru/CNTs (MI) catalyst exhibited a power density of 61 mW/cm², about 27% higher than those single cells with commercial Pt-Ru/C (E-TEK) catalyst.

Full text of DOI:10.1016/j.electacta.2008.05.003

Electrochimica Acta 53 (2008) 7807–7811
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Highly active PtRu catalysts supported on carbon nanotubes prepared by
modified impregnation method for methanol electro-oxidation
∗
Zhiming Cui, Changpeng Liu, Jianhui Liao, Wei Xing
Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, Jilin, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A simple and rapid synthesis method (denoted as modified impregnation method, MI) for PtRu/CNTs (MI)
and PtRu/C (MI) was presented. PtRu/CNTs (MI) and PtRu/C (MI) catalysts were characterized by transmis-
sion electron microscopy (TEM) and X-ray diffractometry. It was shown that Pt-Ru particles with small
average size (2.7 nm) were uniformly dispersed on carbon supports (carbon nanotubes and carbon black)
and displayed the characteristic diffraction peaks of Pt face-centered cubic structure. Cyclic voltammetry
and chronoamperometry showed that the Pt-Ru/CNTs (MI) catalyst exhibited better methanol oxidation
activities than Pt-Ru/C (MI) catalyst and commercial Pt-Ru/C (E-TEK) catalyst. The single cells with Pt-
Ru/CNTs (MI) catalyst exhibited a power density of 61 mW/cm , about 27% higher than those single cells
with commercial Pt-Ru/C (E-TEK) catalyst.
© 2008 Elsevier Ltd. All rights reserved.
Received 26 January 2008
Received in revised form 25 April 2008
Accepted 5 May 2008
Available online 9 May 2008
Keywords:
Direct methanol fuel cell
PtRu catalyst
Methanol electro-oxidation
Impregnation method
Carbon nanotubes
2
1. Introduction
Preparation methods have an important influence on the parti-
cle size and size distribution of PtRu catalysts; therefore, numerous
Direct methanol fuel cells (DMFCs) are commonly considered
as candidates for future energy-generating devices, but there are
two key problems inhibiting the commercialization of DMFCs: the
poor kinetics of methanol oxidation reactions in the anode, and
methanol crossover from anode to cathode causing poisoning of
the cathode [1–3]. On the issue of improving the kinetics of the
methanol oxidation reaction, Pt- and PtRu-based nanoparticles are
of major interest as anode catalysts for DMFCs. Up to now, PtRu cat-
alysts have still been the most active binary catalysts. The oxidation
of CH OH to CO at “low” potentials takes place via a bifunctional
mechanism posited by Watanabe and Motoo [4,5]. It is assumed
that Ru provides an oxygenated surface species by dissociating
water at the Ru sites at lower potentials against pure sites, leading to
the accelerated CO2 formation and a decrease in the CO poisoning,
thus improving the CO tolerance. Several groups have investigated
the origin of the Ru enhancement by using various types of model
bimetallic Pt/Ru electrodes, including Pt/Ru alloy [6–10] and Ru-
decorated Pt electrodes [11–15]. The Urbana-Champaign group
led by Wieckowski reported the combined electrochemistry NMR
studies have been directed toward the synthesis [18–27]. To achieve
fine dispersion and high utilization, PtRu catalysts are usually sup-
ported on some high-surface-area materials such as carbon black
and CNTs. Among the synthetic methods reported in the liter-
ature for PtRu catalysts, the most popular are colloidal method
[18–23] and impregnation method [24–29]. The colloidal method
often uses surfactants or polymer as the stabilizers which not
only protect the nanocatalysts but also create a space for nano-
size particles to grow [23]. But how to remove the stabilizer after
reduction becomes a major problem; in particular, repeated filter-
ing and washing procedures are time-consuming and easy to cause
the loss of noble metals [30,31]. Impregnation method includes
an impregnation step, followed by a reduction step. During the
impregnation step, the precursors of Pt and Ru are mixed with
high-surface-area catalyst supports in aqueous solution to form a
homogeneous mixture. In comparison to colloidal method, impreg-
nation method is relatively simple, but Pt-Ru catalysts prepared by
this method do not show good control of particle size and distribu-
tion [31,32].
3
2
(
EC NMR)/electrochemistry approach, which provides new insights
In this work, we presented a simple and rapid route (denoted
as modified impregnation method) to prepare PtRu/CNTs (MI) and
PtRu/C (MI). The PtRu catalysts prepared in this work were charac-
terized by transmission electron microscopy and X-ray diffraction.
The methanol electro-oxidation activities of the PtRu catalysts were
also discussed and compared with commercial PtRu catalysts.
into the promotion of CO tolerance in Pt/Ru catalysts [16,17].
∗ Corresponding author. Tel.: +86 431 5262223; fax: +86 431 5685653.
E-mail address: xingwei@ciac.jl.cn (W. Xing).
0
013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2008.05.003

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Z. Cui et al. / Electrochimica Acta 53 (2008) 7807–7811
Fig. 1. TEM images of PtRu nanoparticles supported on multiwalled CNTs (a) and Vulcan XC-72 carbon (b).
2. Experimental
2.3. Physicochemical characterization of catalysts
2.1. Materials
Transmission electron microscopy (TEM) analysis was carried
out with a JEOL2010 microscope operating at 200 kV with nomi-
nal resolution. Samples were first ultrasonicated in alcohol for 1 h
and then deposited on 3 mm Cu grids. The composition and con-
tent of Pt and Ru in the Pt-Ru/CNTs and Pt-Ru/C catalysts were
evaluated with energy-dispersive X-ray (EDX) analysis in a scan-
ning electron microscope (JEOL JAX-840). X-ray diffraction spectra
The metal sources were an aqueous solution of H PtCl6 and
2
RuCl3 freshly prepared in order to avoid the formation of complex
2
+
[
RuO(H O) ] , which is difficult to be reduced [33]. Multiwalled
2 4
2
CNTs with ≥95% in purity, 40–300 m /g in specific surface area,
1
0–20 nm in diameter and 5–15 ␮m in length, were purchased from
Shenzhen Nanotech Port Co., Ltd., China. The as-purchased CNTs
were surface oxidized by H SO -HNO mixture (8.0 M for each acid)
were measured with a Philips PW1700 diffractometer using Cu K␣
(ꢁ = 1.5405 A˚ ) radiation source operating at 40 kV and 30 mA.
2
4
3
for 4 h in an ultrasonic bath [34]. Vulcan XC-72 carbon black was
purchased from Cabot Corporation. Deionized water (18.23 Mꢀ)
was produced by a Milli-Q ultrapure system from Millipore Ltd.,
USA.
2
.4. Electrochemical measurements
Electrochemical measurements were carried out with an EG &
G mode 273 potentiostat/galvanostat and a conventional three-
electrode test cell. Glassy carbon (GC) was polished with 0.05 ␮m
alumina suspension before each experiment and served as an
underlying substrate of the working electrode. The catalyst ink was
prepared by ultrasonically dispersing the mixture of 5 mg catalysts,
2
.2. Preparation of PtRu nanoparticles on the MWCNTs and on
the Vulcan XC-72 Carbon
Pt-Ru/CNTs (20 wt% Pt + 10 wt% Ru) and Pt-Ru/C (20 wt%
Pt + 10 wt% Ru) were prepared by a modified impregnation method
and the preparation procedure is briefly described below using
CNTs as example. The surface-oxidized CNTs were suspended in
1
mL ethanol, and 50 ␮L 5 wt.% Nafion solution. 10 ␮L catalyst ink
was pipetted and spread on the glassy carbon disk. Then the elec-
◦
trode was dried at 80 C for 1 h. A Pt foil and an Ag/AgCl were used
as the counter and reference electrodes, respectively. All potentials
in this report were quoted against Ag/AgCl. All electrolyte solutions
were deaerated by high-purity nitrogen for 15 min prior to any mea-
deionized water and treated in an ultrasonic bath. Then H PtCl6
and RuCl3 solution were added dropwise under mechanically
2
stirred conditions for 2 h. Then 5% (v/v) NH ·H O solution was
3
2
surement. The electrolyte solution was 1 M CH OH in 0.5 M H SO .
added to adjust PH of the mixture solution to above 12, there-
3
2
4
2−
3+
The cyclic voltammetries of methanol were measured between
0.2 and 0.8 V vs. Ag/AgCl at 20 mV/s. For chronoamperometry, the
fore PtCl6
and Ru were precipitated due to the formation
−
of (NH ) PtCl and Ru(OH)3 and the precipitate was adsorbed
4
2
6
electrode potential was fixed at 0.5 V vs. Ag/AgCl.
on CNTs. With dropwise addition of NaBH4 solution, (NH ) PtCl
6
4
2
and Ru(OH)3 were reduced to Pt and Ru nanoparticles. Sub-
sequently, the suspension was filtered and washed with hot
2.5. Performance tests for single cells
−
deionized water until no Cl was detected. Finally, the resulting
◦
catalysts were air-dried at 100 C in a drying oven for 24 h and
The performance of a single DMFC with the anodic Pt-Ru/CNT
(MI) catalyst was measured and compared with that with the
anodic Pt-Ru/C (MI) and the Pt-Ru/C (E-TEK) catalyst (20 wt%
Pt + 10 wt% Ru). Pt/C catalyst (20 wt.% Pt) was used as the cathodic
then kept in desiccator. The PtRu catalysts supported on CNTs
and carbon black were noted Pt-Ru/CNTs (MI) and Pt-Ru/C (MI),
respectively.

Z. Cui et al. / Electrochimica Acta 53 (2008) 7807–7811
7809
Fig. 2. Particle size distributions for PtRu nanoparticles supported on multiwalled CNTs (a) and Vulcan XC-72 carbon (b).
catalyst. The catalyst loading was 4 mg cm−2 for both the anode and
◦
◦
The diffraction peak at about 24 (carbon black) and 25.5 (CNTs)
[37,38] observed is attributed to the hexagonal graphite structure
(0 0 2), which can reflect the graphite degree of a carbon material.
CNTs have higher diffraction peak (0 0 2) than carbon black, indi-
cating that CNTs have higher graphite degree and better electrical
conductivity than carbon black.
the cathode and the effective electrode area of the single cell was
2
9
cm . The fabrication of the membrane electrode assembly (MEA)
is the same as that in the literature [35]. The performance of the
single cell was measured with a Fuel Cell Test System (Arbin Co.).
−
.0 M CH OH solution with a flow rate of 20 mL min and oxygen
3
1
2
−1
with a flow rate of 0.5 L min at 0.05 MPa were used. The single
◦
cell was operated at 80 C.
3.2. Electrochemical analysis
3. Results and discussion
The Pt-Ru/CNTs and Pt-Ru/C catalysts were characterized at
room temperature by cyclic voltammetry (CV) in electrolytes of
0.5 M H SO4 and 1 M CH OH at a scan rate of 20 mV/s shown in
3.1. TEM, EDX and XRD analyses
2
3
Fig. 5. For comparison, a commercially available catalyst (20 wt%
Part a and b of Fig. 1 show typical TEM images for the Pt-Ru/CNTs
and Pt-Ru/C. It is shown that Pt-Ru particles with small average size
were uniformly dispersed on carbon supports (carbon nanotubes
and carbon black). Histograms of metal particle diameters were
obtained by measuring the size of 200 particles in random regions
as shown in Fig. 2. The average diameters for metal nanoparticles
are 2.7 nm for Pt-Ru/CNTs (MI) and 2.9 nm for Pt-Ru/C (MI). These
observations indicate that PtRu catalysts with uniform distribution
and small average size can be synthesized by this modified impreg-
nation method. It can be explained as follows: it is known to all that
PtRu nanoparticles suspended in solution are easily aggregated.
Therefore, before reduction step, PtCl62− and Ru in the solution
3+
were firstly precipitated in the form of (NH ) PtCl and Ru(OH)3
4
2
6
and then the precipitates were adsorbed on high-surface-area car-
bon supports. After the precipitates were reduced, the resulted PtRu
nanoparticle were anchored to high-surface-area carbon supports
and thus inhibited aggregating.
Fig. 3 shows the EDX results for Pt-Ru/CNTs and Pt-Ru/C samples.
The weight contents of Pt-Ru in Pt-Ru/CNTs catalysts and in Pt-Ru/C
catalysts are 28.94% and 28.46%, respectively. The Pt/Ru atomic ratio
is 1.05 for Pt-Ru/CNTs, and 1.08 for Pt-Ru/C, close to that in the
precursor solution.
Typical XRD patterns of the Pt-Ru/CNTs and Pt-Ru/C catalysts
are shown in Fig. 4, from which the crystalline lattice fringes of
the bimetallic alloy nanoparticles were confirmed. The XRD pat-
terns displayed the (1 1 1), (2 0 0), (2 2 0), and (3 1 1) reflections,
confirming that the catalysts exhibit the fcc structures of Pt. The
Pt(1 1 1) diffraction peaks for these Pt-Ru catalysts are shifted to
higher positions than that for Pt/C, which can be interpreted as evi-
◦
◦
dence of alloying. Diffraction peaks near 38 and 44 in 2Â from
Ru are not observed, possibly because Ru has entered the Pt lattice
and formed the Pt-Ru alloy or Ru exists as the amorphous form [36].
Fig. 3. EDX spectra of PtRu/CNTs (a) and PtRu/C catalysts (b).

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Z. Cui et al. / Electrochimica Acta 53 (2008) 7807–7811
Fig. 4. XRD patterns of the catalysts prepared by modified impregnation method.
Fig. 6. Current vs. time plots measured by the CA method in1 M CH3OH + 0.5 M
H2SO4 solutions at 0.5 V (vs. Ag/AgCl) at room temperature.
Pt + 10 wt % Ru supported on carbon, E-TEK, Inc.) was also studied
under the same experimental conditions. It can be observed from
Fig. 5 that the onsets of methanol oxidation peaks all begin at about
Cyclic voltammetry and chronoamperometry showed that the
Pt-Ru/CNTs (MI) catalyst exhibited better methanol oxidation activ-
ities than Pt-Ru/C (MI) catalyst and commercial Pt-Ru/C (E-TEK)
catalyst. It can be explained that Pt-Ru/CNTs (MI) prepared by mod-
ified impregnation method have uniform distribution and small
average size of PtRu particles and CNTs have higher graphite degree
and thus better electrical conductivity than carbon black.
0.2 V (vs. Ag/AgCl), and the peak potentials of methanol oxidation
are similar, at about 0.53 V (vs. Ag/AgCl). However, the peak current
2
2
densities are different, 34 mA/cm for Pt-Ru/CNTs (MI), 27 mA/cm
for Pt-Ru/C (MI), 24 mA/cm2 for Pt-Ru/C (E-TEK) catalyst. Pt-Ru
catalysts prepared in this work exhibit better methanol oxida-
tion activities than commercial Pt-Ru/C (E-TEK) catalyst, which is
attributed to uniform distribution and small average size of PtRu
nanoparticles. In addition, the Pt-Ru/CNTs (MI) shows superior
activity of methanol electro-oxidation to Pt-Ru/C (E-TEK) and Pt-
Ru/C (MI), which is associated closely with two aspects: the novel
modified impregnation method, and surface structures and elec-
tronic properties of CNTs [39,40].
Chronoamperometry (CA) curves for the three Pt-Ru catalysts
are shown in Fig. 6. These curves reflect the activity and stability of
the three Pt-Ru catalysts to catalyze methanol oxidation. Obviously,
the decay in the methanol oxidation current with time was differ-
ent. The Pt-Ru/CNTs shows the highest initial current density and
limiting current density, which indicates the best catalytic activity
and stability.
3.3. Direct methanol fuel cell performance test
Fig. 7 shows the current density–voltage curves of the single
DMFC with different anodic catalysts. Among the three tested Pt-
Ru catalysts, the Pt-Ru/CNTs shows the highest performance, e.g.,
2
when the current density is 200 mA/cm , the voltage of the cell with
the anodic Pt-Ru/CNTs is 0.30 V, cf. 0.26 V for Pt-Ru/C and 0.23 V for
E-TEK catalyst. It is shown that the single cells with Pt-Ru/CNTs
(
MI) have higher open circuit voltage (OCV = 0.72 V) and maxi-
2
mal power density (Pmax = 61 mW/cm ) than those of Pt-Ru/C (MI,
OCV = 0.68 V, Pmax = 52 mW/cm ) and Pt-Ru/C (E-TEK, OCV = 0.65 V,
Pmax = 48 mW/cm ). This result is consistent with that of CVs and
2
2
the reasons have been discussed in detail in Section 3.2.
Fig. 5. Cyclic voltammograms recorded at 20 mV/s in 1 M CH3OH + 0.5 M H2SO4
solutions at room temperature.
◦
Fig. 7. Single cell performance using different PtRu catalysts at the anode at 80 C.

Z. Cui et al. / Electrochimica Acta 53 (2008) 7807–7811
7811
4
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