Alternative platinum electrocatalyst supporter with micro/nanostructured polyaniline for direct methanol fuel cell applications

Article abstract of DOI:10.1016/j.electacta.2011.04.026

In this study, a series of micro/nanostructured polyanilines were synthesized and their morphology-dependent electrochemical properties for acting as a catalyst supporter for direct methanol fuel cell (DMFC) applications were investigated. These micro/nanostructures include submicron spheres, hollow microspheres, nanotubes, and nanofibers. Among the four micro/nanostructures, polyaniline nanofibers (PANF) manifest their superiority in high electrochemical active surface. Accordingly, PANF is adopted as the catalyst supporter thereafter. To couple with the use of the alternative catalyst supporter, this study also investigates the effect of reductant type on morphology and electrocatalytic properties of the PANF-supported Pt particles through a chemical reduction reaction. TEM images indicate that formic acid as a reductant results in well-dispersed Pt particles on the PANF surface. On the other hand, aggregations of Pt are observable when NaBH₄ is selected as a reductant. Moreover, the methanol oxidation current density measured with the Pt/PANF electrode being prepared by using formic acid is double that by using NaBH₄. Compared with Pt/XC-72, the Pt/PANF electrode possesses higher electrocatalytic activity and exhibits double power density. Moreover, Pt/PANF is superior to Pt/XC-72 in the aspect of operation stability based on a continuous discharge for 5 h.

Full text of DOI:10.1016/j.electacta.2011.04.026

Electrochimica Acta 56 (2011) 5679–5685
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Alternative platinum electrocatalyst supporter with micro/nanostructured
polyaniline for direct methanol fuel cell applications
Y.F. Huang, C.W. Lin∗, C.S. Chang, M.J. Ho
Department of Chemical and Materials Engineering, National Yunlin University of Science and Technology, 123, Sec. 3, University Road, Douliou, Yunlin, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study, a series of micro/nanostructured polyanilines were synthesized and their morphology-
dependent electrochemical properties for acting as a catalyst supporter for direct methanol fuel cell
(DMFC) applications were investigated. These micro/nanostructures include submicron spheres, hollow
microspheres, nanotubes, and nanofibers. Among the four micro/nanostructures, polyaniline nanofibers
(PANF) manifest their superiority in high electrochemical active surface. Accordingly, PANF is adopted as
the catalyst supporter thereafter. To couple with the use of the alternative catalyst supporter, this study
also investigates the effect of reductant type on morphology and electrocatalytic properties of the PANF-
supported Pt particles through a chemical reduction reaction. TEM images indicate that formic acid as a
reductant results in well-dispersed Pt particles on the PANF surface. On the other hand, aggregations of Pt
are observable when NaBH4 is selected as a reductant. Moreover, the methanol oxidation current density
measured with the Pt/PANF electrode being prepared by using formic acid is double that by using NaBH4.
Compared with Pt/XC-72, the Pt/PANF electrode possesses higher electrocatalytic activity and exhibits
double power density. Moreover, Pt/PANF is superior to Pt/XC-72 in the aspect of operation stability
based on a continuous discharge for 5 h.
Received 2 October 2010
Received in revised form 6 April 2011
Accepted 7 April 2011
Available online 27 April 2011
Keywords:
Polyaniline nanofibers
Catalyst supporter
Methanol oxidation
Direct methanol fuel cells
© 2011 Elsevier Ltd. All rights reserved.
1. Introduction
particles, which is a major factor leading to the poison of Pt catalyst
[13].
Direct methanol fuel cells (DMFCs) are highly attractive power
sources for a variety of applications, due to their high-energy
efficiency, low emission, low noise, and environmentally friendly
nature [1,2]. Despite these advantages, however, some problems
still prevent the commercialization of DMFCs. These problems
include the fuel crossover from anode to cathode, the poison of plat-
inum catalyst, and the reliability and durability of the membrane
electrode assembly (MEA). The poison of the Pt catalyst presents
one of the greatest challenges for DMFCs, as it causes a signifi-
cant loss in their performance [3,4]. Researchers have attempted
to overcome the catalyst poison problem, including the modifi-
cation of Pt with different metals and the selection of a catalyst
supporter [5–9]. It has been shown that the use of polyaniline as
the catalyst supporter is a simple and useful way of reducing the
catalyst poison problem [10–12]. Polyaniline can form a specific
interaction with Pt and helps to reduce the absorbance of CO on Pt
Several recent studies reported that nanostructured polyaniline
has a large accessible surface area and a low electric resistance,
making it a particularly suitable catalyst supporter for promoting
the catalytic ability of Pt particles. For example, Chen et al. and
Guo et al. [14,15] reported that ultra-high density Pt particles can
be grown on the surface of polyaniline nanofiber (PANF), and the
Pt/PANF catalysts have higher methanol oxidation ability than the
Pt/XC-72 carbon and the commercial E-Tek catalysts. Researchers
have reported similar results using polyaniline nanotubes as sup-
porters. For example, Rajesh et al. [16,17] reported that the Pt
supported on the polyaniline nanotubes exhibited excellent cat-
alytic activity and stability in comparison with the Pt supported on
the XC-72 carbon with conventional polyaniline. Moreover, a recent
study [18] reported the use of hollow polyaniline microspheres as
catalyst supporters to enhance the electrocatalytic capability of the
deposited metal.
Although many studies have reported the use of PANI as
catalyst supporters, their shape-dependent electrochemical prop-
erties have not been systematically studied. The morphology of
micro/nanostructures plays an important role in their electrochem-
ical properties, such as electronic conductivity, effective surface
area, and interfacial charge transfer resistance [19]. Requirements
for a suitable catalyst supporting material include good electric
∗
Corresponding author at: Department of Chemical and Materials Engineering,
National Yunlin University of Science and Technology, 123 Section 3, University
Road, Douliou, Yunlin 640, Taiwan. Tel.: +886 5 534 2601x4613;
fax: +886 5 531 2071.
E-mail address: lincw@yuntech.edu.tw (C.W. Lin).
0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2011.04.026

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Y.F. Huang et al. / Electrochimica Acta 56 (2011) 5679–5685
conductivity, high specific surface area, and high stability in acid
media. On the other hand, although the previous studies reported
that using polyaniline nanostructures as supporters can signifi-
cantly enhance Pt catalytic properties [14–17,20,21], it is worth
noting that none of them investigated the single cell performance of
these Pt/PANI nanocatalysts. To our best knowledge, only one study
reports the test results of DMFC performance with PtRu/PANI and
PtRu/C catalysts [22,23]. However, that study draws an inconsistent
conclusion that a lower performance of PtRu/PANI is obtained when
compared with PtRu/C [23].
To clarify the advantage of using adequate nanostructured
polyaniline as catalyst supporter for DMFC applications, this study
aims to investigate the electrochemical properties of polyaniline
with different micro-/nano-structures (i.e. submicron spheres, hol-
low microspheres, nanofibers, and nanotubes) to determine a most
suitable shape of Pt supporter. Morphology and catalytic ability of
Pt particles deposited on the synthesized PANI supporter through
different reduction agents are also investigated. To demonstrate the
benefit of using a PANI nanostructure as a Pt supporter, we make
a series of comparative study on the single cell tests of DMFC in
terms of polarization curve and operation stability under the use of
identical Pt catalyst supported on the PANI nanostructure and the
carbon black XC-72, respectively. It is worthy to remind that the
reason why we deposit Pt rather than Pt–Ru on PANI as the elec-
trocatalyst for DMFC in the current study is to avoid complicating
the benefit of diminishing the Pt poison with PANI. After all, the use
of Pt–Ru as catalyst has manifested an effect upon lowering the Pt
poison for DMFC operations [5,6].
dried in an oven at 50 ◦C for 1 day. The morphologies of these
micro/nanostructures were executed with a field emission scan-
ning electron microscope (FE-SEM; JSM-7401F, JEOL, Japan) and
a transmission electron microscopy (TEM; JEM-1400, JEOL, Japan)
operating at 120 kV.
2.3. Deposition of Pt particles on the PANI and XC-72 carbon
black supporters
The catalyst supporters (PANI or XC-72 carbon black) were
firstly dispersed in 10.3 mL of a K2PtCl4 aqueous solution (4.6 mM)
in a 20 mL vial, followed by the addition of 2.5 mL formic acid (99%)
in the solution to prepare the PANI- or XC-72-supported Pt cata-
lysts. These mixtures were stirred at room temperature for 16 h
to reduce the Pt precursors completely. Then, the solution was
isolated using gravity filtration and washed several times with
deionized water. For comparison, a similar reaction was performed
by using the K2PtCl4 solution and 2.8 mL of 0.05 M NaBH4 as reduc-
tion agent. UV–vis spectrophotometer (Lambda 850, Perkin Elmer,
USA) was used to identify the completion of reaction of K2PtCl4
during the reduction process.
2.4. Electrochemical characterization of PANI, Pt/PANI, and
Pt/XC-72
The cyclic voltammetry curve of the resulting polyaniline
micro/nanostructures and the Pt catalysts supported on PANI and
XC-72 carbon black were conducted with a three-electrode setup.
The working electrode with a circular area of 1 cm2 was prepared as
follows. The PANI micro/nanostructures, Pt/PANI or Pt/XC-72 were,
respectively, mixed ultrasonically with isopropanol in a glass vial
with a weight ratio of 1:30. Then, a suitable amount of Nafion®
solution was added to the dispersion and mixed until a uniform
paste was obtained. The weight ratio of Pt plus catalyst supporter
(i.e. PANI micro/nanostructures, Pt/PANI or Pt/XC-72) to Nafion is
1:1.34. This ink was then spread on the surface of the gas diffu-
sion media (15% wet proof, HEPHAS energy, Taiwan) and dried
in an oven at 60 ◦C. Besides, the weights of electrode before and
after the coating of slurries were measured carefully to determine
the exact quantity of polyaniline or Pt/polyaniline in the elec-
trode and the measured weight was also used to normalize the
CV curves. Ag/AgCl and platinum wire were used as the reference
and counter electrode, respectively. The cyclic voltammetry curves
were obtained in a 1 M methanol and 0.5 M H2SO4 solution. The
scan range was from −0.2 to 1.0 V versus Ag/AgCl, and the scan rate
2. Experimental
2.1. Materials
Aniline monomers (ꢀ99.5%, Fluka) and ammonium persul-
fate (APS, Riedel-de Haën) were used to synthesize polyaniline
nanofibers, nanotubes, submicron spheres, and hollow micro-
spheres with the introduction of hydrochloric acid (37%,
MERCK-GR), Methanol (99.8%, Riedel-de Haën), 1.6-hexandiol
(99%, Aldrich), or 1-propanol (98%, J.T. Baker) in the reaction
solutions. Potassium tetrachloroplatinate (II) (K2PtCl4, 99.9%), the
precursor of platinum, was purchased from Acros Organics. The
reduction agents – formic acid (98%) and sodium borohydride
(NaBH4, 98%) – were purchased from Acros Organics. Vulcan XC-
72 carbon black was obtained from Cabot Company. Pt/C (50% Pt
on carbon black—Alfa Aesar, USA) was used as a cathode elec-
trocatalyst. All chemicals were used as received without further
purification.
was 50 mV s−1
.
2.2. Synthesis of polyaniline micro/nanostructures
2.5. Preparation and evaluation of direct methanol fuel cells
The reactions were implemented in a 20-mL vial. The prepara-
tion of nanofibers typically involved mixing an aqueous solution of
aniline in 1 M HCl acid (5 mL) and another oxidant (ammonium per-
oxydisulfate) solution in the same doping acid (5 mL). The solutions
were then poured rapidly into a glass vial and shaken vigorously
for 30 s, and then put in an ultrasonic water bath (5210, Bran-
son, USA) at 60 ◦C. The reaction was executed at a molar ratio of
[ANI]/[APS] equal to 0.8:1 and the concentration of aniline was
0.2 mol L−1. The nanotubes, hollow microspheres, and submicron
spheres were synthesized with the same procedure, using the
acid-free 1 M methanol, 1 M 1-propanol, and 2 M 1.6-hexandiol
acid-free solutions, respectively. The reaction temperature for nan-
otubes, submicron spheres, and hollow microspheres remained at
0 ◦C. The resulting PANI samples were isolated by gravity filtra-
tion and washed in de-ionized water. Finally, the products were
The anode was fabricated using gas diffusion media as a sup-
porter with a given metal loading of 3 mg cm−2. The Pt particles
were deposited on the PANI nanostructures and XC-72 carbon
black to be anode catalyst, respectively. The cathode was pre-
pared by using a carbon-supported Pt catalyst (50% Pt on carbon
black—Alfa Aesar, USA) with a metal loading of 3 mg cm−2. The
added Nafion® as binder in both anode and cathode is 10 wt%.
2 M methanol solutions were fed at a fixed rate of 3 mL min−1 and
the oxygen flow rate was fixed at 100 SCCM. The pressure at the
cathode side is one atmosphere. The fabrication procedures and
conditions of the MEA were set according to the previous litera-
ture, using the Nafion® 115 as the proton conducting membrane
[24]. The DMFC performance with an active area of 6.25 cm2 was
evaluated utilizing a fuel cell test station (Model 2200, MACCOR,
Inc., USA).

Y.F. Huang et al. / Electrochimica Acta 56 (2011) 5679–5685
5681
Fig. 1. SEM and TEM (corner) images of polyaniline micro/nanostructures: (a) nanofiber, (b) nanotubes, (c) submicron spheres, and (d) hollow microspheres. The reaction
time for nanofibers and nanotubes is 90 min; the reaction time for submicron spheres and hollow microspheres is 45 min. The scale bar of the TEM image is 500 nm for
nanofibers and 200 nm for the others, respectively.
3. Results and discussion
have reported that the fibrous polyaniline may have a higher
molecular weight than that of nanotubes and spheres [32–34].
Regarding this, we consider here that the difference in current
density of the polyaniline species may be due to their different
morphologies and/or molecular weights, which results in different
electrochemical surface area. Besides, as there is a charge transfer
energy barrier between the PANI micro/nanostructured supporter
and the electrode substrate (i.e. carbon cloth in this case), this
higher current density of nanofibers may be attributed to a close
surface contact between the nanofibers and the electrode substrate
as well as higher surface active area [19]. These characteristics
are favorable for a catalyst supporter to fabricate highly efficient
electronic devices. Accordingly, this study selected the PANF as
the catalyst supporter for DMFC applications hereafter. In order to
pursue an optimum electrode capability, this study investigated
the effects of the reduction agent type and the PANF amount on
the electrocatalytic properties of the Pt/PANF catalysts.
3.1. Synthesis and characterization of polyaniline
micro/nanostructures
Fig. 1(a) and (b) shows the SEM images coupled with TEM images
(placed at corner) of the PANI micro/nanostructures synthesized in
different polymerization conditions. As Fig. 1(a) shows, the poly-
merization of aniline monomers in 1 M HCl solution at 60 ◦C can
produce polyaniline nanofibers with a diameter of 60–80 nm. How-
ever, Fig. 1(b) shows that the polymerization of aniline in 1 M
methanol acid-free solutions at 0 ◦C led to the formation of nan-
otubes with an outer diameter of 200 nm and an inner diameter
of 0–50 nm. Fig. 1(c) and (d) shows that PANI submicron spheres
with an outer diameter of ∼400 nm and hollow microspheres with
an outer diameter of ∼1 ␮m and a thickness of 50 nm can be
obtained from 1.6-hexandiol and 1-propanol acid-free solutions,
respectively. These results agree well with our previous findings
regarding the different growth routes of aniline monomers in dif-
ferent polymerization environments and conditions [25–28]. The
different growth routes can therefore produce different morpholo-
gies of the polyaniline micro/nanostructures. However, the details
of state-of-the-art of morphology control of polyaniline nanostruc-
tures are beyond the scope of the current study and can be referred
to our previous publications [25–28].
Fig.
2
presents the CV curves of different PANI
micro/nanostructures in a 1 M methanol and 0.5 M H2SO4 aqueous
solution. All PANI samples exhibit the electrochemical activities
that resulted from a typical redox response. Redox peaks at around
0.25 V and 0.7 V serve for the conversion of leucoemeraldine to
emeraldine, and emeraldine to pernigraniline, respectively. The
peak at 0.65 V corresponds to the redox reaction of degradation
products (hydroquinone to quinone) [29,30]. As can be observed
in Fig. 2, the PANI nanofibers are superior to nanotubes, submicron
spheres, and microspheres in the aspect of current density. This
higher current density stands for the higher effective surface area
that is accessible to electrolytes [31]. Specifically, previous studies
Fig. 2. Cyclic voltammogram curves of polyaniline micro/nanostructures in 1 M
CH3OH and 0.5 M H2SO4 solutions.

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Fig. 3. TEM images of PANF (5 mg) supported Pt catalysts fabricated with different
reduction agents.
3.2. Effects of reductant type on the morphology and oxidation
ability of the resulting Pt/PANF catalysts
Fig. 3 shows TEM images of the PANF-supported Pt catalysts
using different reduction agents. Fig. 3(a) shows aggregated Pt
particles on the PANF obtained from using NaBH4 as the reduc-
tion agent. On the other hand, using formic acid as the reduction
agent can yield well-dispersed Pt particles on the PANF, as shown
in Fig. 3(b). Fig. 4 shows UV–vis spectra of the reaction solu-
tions obtained before and after the reduction process (the PANF
Fig. 5. UV–vis spectra of the reaction solutions obtained at different reaction stages
with the using of different reduction agents.
in the solutions were filtered out). A sharp peak at 253 nm was
observed from the pre-reaction solution and which can be related
2−
to the absorbance of PtCl4
ions [35]. However, after the reduc-
2−
tion of PtCl4 by using formic acid or NaBH4 as reduction agent,
the peak at 253 nm disappeared. These results indicate that both
formic acid and NaBH4 can completely reduce the Pt2+ ions into Pt
atom during the reaction process. Thus, the Pt weight ratio of the
obtained Pt/PANF in the solutions with an adding PANF amount
of 5 mg was calculated to be about 66.5% [15]. To further iden-
tify the composition of these Pt/polyaniline materials, this study
used EDX technique to analyze the content of Pt in the Pt/PANI
composite [36–38]. As Table 1 shows, the Pt weight percentages
in both the Pt/PANI composites using the different reductants are
quite similar and satisfactorily closed to the theoretical value (i.e.
66.5%) which is calculated based on the Pt precursors being reduced
completely and the entire reduced Pt particles are incorporated
into the Pt/PANI composite. Accordingly, it can be inferred that
both different reduction agents can reduce the same amount of
Pt on the PANF supporters, even the resulting morphologies of the
PANF-supported Pt particles are significantly different. The differ-
ent morphology may be attributed to the different reduction rate
of the Pt precursor with the use of different reductants. As Fig. 5
shows, NaBH4 leads to a higher reduction rate than formic acid. The
absorbance of the peak at ∼250 nm with the NaBH4 is only one-fifth
Fig. 4. UV–vis spectra of the reaction solutions obtained before and after the reduc-
tion process with addition of 5 mg PANF.

Y.F. Huang et al. / Electrochimica Acta 56 (2011) 5679–5685
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Table 1
The EDX analysis results of the Pt/PANF compositions using different reductants.
Type of reductant
Weight ratio (%)
Atomic ratio (%)
C
N
Cl
Pt
Total
C
N
Cl
Pt
Total
Formic acid
NaBH4
27.98
30.73
4.23
3.60
0.52
1.08
67.27
64.59
100.0
100.0
77.88
80.53
10.11
8.09
0.49
0.96
11.53
10.42
100.0
100.0
to that with the formic acid at the reaction time of 10 min. The
higher reduction rate from NaBH4 may lead to a higher concentra-
tion of Pt nucleate formed at the initial reaction stage and thus leads
to the aggregation of Pt particles during the reaction process [39].
In contrast, the lower reduction rate from formic acid may allow
the as-synthesized Pt particles to be deposited on the polyaniline
surface and thus prevent the aggregation of metal particles during
the reaction process [40,41]. We therefore infer that the relatively
homogenous morphology of the Pt particles with the use of formic
acid can be ascribed to the lower reducing rate.
amounts of PANF, all Pt/PANF catalysts show a similar methanol
oxidation current peak at 0.7 V in the forward scan. However, a
smallest current peak at 0.47 V during the reverse scan can be
obtained with the addition of 10 mg PANF in the reaction solution.
This peak corresponds to the removal of the incompletely oxidized
carbonaceous species formed in the forward scan. Moreover, after
oxidizing in the backward scan, these carbonaceous species gener-
ally take the form of linearly bonded Pt
C O, which is the primary
factor leading the poison of the Pt catalyst [42,43]. These results
indicate that the increase of PANF amount in the reaction solutions
On the other hand, it is worth noting that these different mor-
phologies of Pt particles can lead to different methanol oxidation
abilities of the Pt/PANF catalysts. Fig. 6 shows the CV curves of
Pt/PANF catalysts obtained from the use of different reduction
agents. As can be seen, formic acid resulted in Pt/PANF catalyst with
a significantly higher current peak corresponding to the oxidation
of methanol (0.7 V), which is about two times higher than that of the
Pt/PANF catalyst obtained by using NaBH4. The higher electrocat-
alytic surface (owing to the well-dispersed and smaller Pt particles
on the PANF surface) is responsible for the higher methanol oxi-
dation current peak, as it provides a higher accessible surface to
the electrolyte for the Pt particles [14]. In short, the above results
indicate that the use of formic acid as a reduction agent helps the
formation of uniformly distributed Pt particles on the PANF sur-
face, and it is the well dispersed morphology of Pt promoting the
methanol oxidation ability of Pt/PANF catalysts.
helps to prevent the formation of linearly bonded Pt C O on the Pt
particles and consequently enhances the CO tolerance of Pt catalyst.
These different CO tolerance-related behaviors may be due to
the different Pt loading densities on the PANF surfaces. Fig. 8
shows the TEM image of the Pt/PANF catalysts prepared with
adding different amounts of PANF to the reaction solutions. Fig. 8(a)
shows a homogeneous Pt particle deposited on the PANF with the
adding of 10 mg PANF to the reaction solution. However, Fig. 8(b)
shows that decreasing the PANF amount to 2.5 mg leads to an
ultra high density of Pt particles with the formation of a thicker
Pt particle layer on the PANF surface. It is worth noting that this
thicker Pt layer on the PANF surface may block the interaction
between the outside Pt particles and the PANF supporters. The
specific interaction formed between Pt and PANI helps prevent
the adsorption of CO on the Pt particle [44,45]. Accordingly, it is
reasonable to suggest that a suitable amount of Pt with a homo-
geneous distribution on the PANF surface enhances the catalyst
poisoning tolerance of the Pt/PANF catalyst, and can be achieved
simply by adjusting the PANF loading amount in the reaction solu-
tion.
3.3. Effects of PANF amount on the catalytic ability of Pt/PANF
In examination of the advantages of using PANF as a Pt sup-
porter, this study investigated the catalytic properties of Pt/PANF
fabricated with the addition of different PANF amounts in the
reaction solutions. Fig. 7 shows the CV curves of the Pt/PANF cata-
lysts fabricated with the addition of different PANF amounts in the
K2PtCl4 solutions, ranging from 2.5 to 10 mg, at the same Pt load-
ing amount in the electrodes. As can be observed, regardless of the
Along with the successful improvement of the catalytic ability
and poisoning tolerance of the PANF-supported Pt catalysts, this
study compares the associated catalytic properties of the PANF-
supported Pt with commercial XC-72 carbon black-supported Pt
to demonstrate the advantage of using PANF as Pt supporters for
DMFC applications.
Fig. 6. Cyclic voltammograms of PANF supported Pt catalysts fabricated with dif-
ferent reduction agents in 1 M CH3OH and 0.5 M H2SO4 solutions with addition of
5 mg PANF.
Fig. 7. Cyclic voltammograms of PANF supported Pt catalysts fabricated with the
addition of different PANF amounts in 1 M CH3OH and 0.5 M H2SO4 solutions.

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Y.F. Huang et al. / Electrochimica Acta 56 (2011) 5679–5685
Fig. 8. TEM images of (a) 10 mg and (b) 2.5 mg PANF supported Pt catalysts.
Fig. 10. Polarization curves of the DMFC using (a) XC-72 and (b) PANF supported Pt
catalysts as anode. These catalysts were produced with the addition of 10 mg PANF
or XC-72 in the reaction solution.
3.4. Comparison of electrochemical properties of Pt/PANF and
Pt/XC-72
Fig. 9 shows the CV curves of the commercial XC-72 carbon-
and PANF-supported Pt catalysts in 1 M methanol and 0.5 M H2SO4
solutions. The PANF-supported Pt catalysts show a slightly higher
oxidation current (0.7 V) than that of the XC-72-supported Pt cat-
alysts in the forward scan, indicating that the PANF-supported Pt
catalysts have better methanol oxidation ability. However, in the
reverse scan, the PANF-supported Pt catalyst exhibits a significantly
lower current peak (0.47 V). Consequently, the Pt/PANF shows a sig-
nificantly higher If/Ib ratio than that of Pt/XC-72. The If/Ib ratios for
the Pt/PANF and Pt/XC-72 catalysts are 1.45 and 0.87, respectively.
The higher If/Ib ratio indicates that the Pt/PANF catalyst has a better
ability to oxidize methanol to carbon dioxide [42,43]. Accordingly,
it is reasonable to infer that the use of PANF as a catalyst supporter
can enhance the Pt catalyst to possess better methanol oxidation
ability and higher poisoning tolerance than XC-72, and these are
significant characteristics related to the performance and operation
stability of DMFC.
Fig. 10 shows the polarization curves of the PANF- and XC-72-
supported Pt catalysts as anode in DMFC with the feeding of 2 M
methanol and oxygen at different temperatures. With the increase
of cell temperature from 30 to 60 ◦C, the Pt/XC-72 catalyst demon-
strates a consistent improvement in DMFC performance from 4 to
11 mW cm−2, as can be seen in Fig. 10(a). However, when using
Pt/PANF as the anode catalyst, polarization curves in Fig. 10(b) indi-
cate that an increase of cell temperature from 30 to 60 ◦C results
in an increase of power density from 8 to 22 mW cm−2. It should
be noted that the apparently low current densities measured for
both Pt/PANF and Pt/XC-72 are due to the use of Pt instead of Pt/Ru
alloy as catalyst in this preliminary study of DMFC tests. A compar-
ison between Fig. 10(a) and (b) indicates that the power densities
obtained from the Pt/PANF catalyst are all twice as high as that
of the Pt/XC-72 at each corresponding temperature. These results
manifest that the use of PANF as a catalyst supporter can efficiently
enhance the performance of DMFC.
On the other hand, note that the DMFCs may yield a significant
difference in performance stability if different anode catalysts are
used. Fig. 11 presents the DMFC performances of the Pt/XC-72 and
Pt/PANF catalysts obtained before and after the discharge at 0.3 V
and 50 ◦C for 5 h. As can be seen in Fig. 11(a), the cell with Pt/XC-72
catalyst experiences a serious irreversible decline of power density
Fig. 9. Cyclic voltammograms of PANF and XC-72 supported Pt catalysts in 1 M
methanol and 0.5 M H2SO4 solutions. The added amount of PANF and XC-72 is 10 mg.

Y.F. Huang et al. / Electrochimica Acta 56 (2011) 5679–5685
5685
soning tolerance. Measurements of DMFC test show that the use
of PANF as Pt supporter in anode can yield twofold higher power
density and better operation stability as compared to the use of
XC-72. This study demonstrates an alternative and efficient way to
enhance both methanol oxidation ability and CO poisoning toler-
ance of Pt through adopting PANF as a catalyst supporter coupled
with formic acid as a reduction agent.
Acknowledgement
The authors are grateful to the National Science Council of
Taiwan (ROC) for the financial support of this work under Grant
no. 99-2221-E-224-069-MY2.
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This study investigates the morphology-dependent electro-
chemical properties of polyaniline micro/nanostructures as catalyst
supporters in DMFC applications. Polyaniline nanofibers with
higher electrochemical surface area and lower resistance of charge
transfer to electrode manifest themselves to be highly suitable
candidate. Well dispersed Pt particles on the PANF surface can be
achieved by using formic acid as a reduction agent and can therefore
enhance the catalytic ability of Pt. The If/Ib ratio from the CV curve
for the Pt/PANF and Pt/XC-72 catalysts is 1.45 and 0.87, respec-
tively, indicating the use of PANF as a catalyst supporter renders
the Pt catalyst better methanol oxidation ability and higher poi-