1. Res Chem Intermed (2012) 38:383–391
DOI 10.1007/s11164-011-0354-3
Electro-catalytic performance of Pt-supported poly
(o-phenylenediamine) microrods for methanol
oxidation reaction
T. Maiyalagan • C. Mahendiran • K. Chaitanya •
Richa Tyagi • F. Nawaz Khan
Received: 16 May 2011 / Accepted: 22 July 2011 / Published online: 4 August 2011
Ó Springer Science+Business Media B.V. 2011
Abstract Poly (o-phenylenediamine) (PoPD) microrods were obtained by inter-
facial polymerization using ferric chloride as oxidant and without any template or
functional dopant. Pt/PoPD nanocatalysts were prepared by the reduction of chlo-
roplatinic acid with sodium borohydride, and the composite catalysts formed were
characterized by X-ray diffraction and electrochemical methods. The nanocom-
posite of Pt/PoPD microrods has been explored for their electro-catalytic perfor-
mance towards oxidation of methanol. The electro-catalytic activity of Pt/PoPD was
found to be much higher (current density 1.96 mA/cm2 at 0.70 V) in comparison to
Pt/Vulcan electrodes (the current density values of 1.56 mA/cm2 at 0.71 V) which
may be attributed to the microrod morphology of PoPD that facilitate the effective
dispersion of Pt particles and easier access of methanol towards the catalytic sites.
Keywords Methanol oxidation Á Pt supported poly (o-phenylenediamine) Á
Nanostructured materials Á Electro-catalyst
T. Maiyalagan (&)
School of Chemical and Biomedical Engineering, Nanyang Technological University, 50 Nanyang
Drive, Singapore 639798, Singapore
e-mail: maiyalagan@gmail.com
C. Mahendiran
Department of Chemistry, Anna University of Technology Tirunelveli, Nagercoil 629004, India
K. Chaitanya Á F. Nawaz Khan
Chemistry Division, School of Advanced Sciences, VIT University, Vellore 632014, India
e-mail: nawaz_f@yahoo.co.in
R. Tyagi
Department of Chemistry, Deshbandhu College, Delhi University, Delhi 110007, India
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2. 384 T. Maiyalagan et al.
Introduction
Though fuel cells are considered as potent energy conversion devices, they have not
been evolved as an economically viable, socially acceptable, easily manipulative
tool for energy conversion [1–8]. A fuel cell essentially consists of three
components: the two electrodes and an electrolyte. Pt and Pt-based noble metals
have been employed as electro-catalysts in both the anode and cathode, and
nowadays there are attempts to reduce the amount of noble metal loading in the
electrodes by dispersing the noble metals on suitable electronically conducting
supports with low metal loadings. To decrease the platinum loading as well as to
improve the oxidation rate and electrode stability, considerable efforts have been
applied to study the electrode materials for the direct electrochemical oxidation of
methanol [9–17]. Carbon is the most common catalyst support material that
conducts only electrons in the electro-oxidation reaction. An alternative is to
develop a catalyst support that conducts both protons and electrons efficiently [18–
20] by employing conducting polymers possessing both protonic and electronic
conductivity. Recently, conducting polymer matrices are being employed as catalyst
support materials for the oxidation, and metal nanoparticles dispersed on conducting
polymer support provide access to a large number of catalytic sites with the
possibilities of the recovery of spent catalyst.
Catalytic particles dispersed on conducting polymers, mainly polypyrrole (PPY)
and polyaniline (PANI), have received considerable attention as the electrode
material for methanol oxidation [21–23]. The conducting and electroactive
polymers, such as poly(o-phenylenediamine) (PoPD), have a greater potential in
various fields of technology due to their interesting properties, different from those
of the usual conducting polymers, like PANI or PPY, which make them promising
for applications in electro- and bioelectro-chemical sensors. One of these properties
relates to an unusual dependence of the electric conductivity on the redox state of
the PoPD polymer. Different from PANI or PPY, PoPD shows high conductivity in
its reduced state, whereas the oxidized state is insulating. This determines the
electrochemical properties of PoPD, since many electrode redox processes have
been shown to take place within a relatively narrow potential window, correspond-
ing to the reduced (conducting) form of this polymer. Within this potential window,
electro-catalytic oxidation of some species takes place, making it possible to use
PoPD for electro-catalytic applications, such as the electro-oxidation of coenzyme
NADH, electro-oxidation of methanol [24–27] and oxygen reduction [28–30].
PoPD is usually obtained through electrochemical polymerization. In this
polymerization method, the PoPD obtained usually exhibits an irregular morphol-
ogy. 1D nano-structured PoPD is obtained by mixing aqueous solutions of HAuCl4
and OPD without any surfactants or templates [30]. However, the materials obtained
were not pure. Hence in this article, we describe an efficient method for the
preparation of PoPD microrods with different lengths using ferric chloride as
oxidant, which takes advantage of the easy removal of FeCl2 by simple washing
with water; the PoPDs so obtained were of high purity [31].
In the present investigation, the composite material based on Pt-supported PoPD
microrods has been compared with that of the Pt/C electrodes for electro-oxidation
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3. Electro-catalytic performance of Pt 385
of methanol. These materials were characterized using X-ray diffraction (XRD) and
cyclic voltammetry (CV). The Pt-supported PoPD microrods electrode exhibited
better catalytic activity and stability compared to the 20 wt% Pt supported on the
Vulcan carbon electrode.
Experimental
Materials
The present study was carried out in aqueous solutions. Purified water obtained by
passing distilled water through a milli Q (Millipore) water purification system was
used. The reagents o-Phenylenediamine (oPD), ferric chloride (FeCl3), chloroplatinic
acid (H2PtCl6), ruthenium chloride (RuCl3), sodium borohydride (NaBH4), sodium
hydroxide (NaOH) were purchased from Aldrich (India) and used as received.
Methanol and sulphuric acid were obtained from Fischer Chemicals (India). Nafion 5
wt% solution was obtained from Dupont (USA) and was used as received.
Synthesis of POPD microrods
The PoPD microrods were obtained by chemical oxidation of oPD using ferric
chloride as an oxidant. In a typical procedure, different contents of oPD monomer
were dissolved in 30 mL distilled water at room temperature. Then, 10 mL aqueous
solutions of ferric chloride (the molar ratio of ferric chloride to oPD is 1:1) were
added to the above mixtures under vigorous stirring at room temperature for 5 h.
The resulting precipitates were washed with water twice and filtered. Finally, the
products were dried in vacuum at 50 °C for 24 h.
Synthesis of Pt supported PoPD microrods
Amounts of 0.1 gm of PoPD and 2 mL of 50 mM H2PtCl6 were mixed with 100 mL
of distilled water. Then, 2 mL NaBH4 solution (50 mg mL-1) was added drop-wise
to the above solution with vigorous stirring at room temperature. Stirring was
continued overnight before the solid phase was recovered by filtration and then
washed copiously with water. The recovered solid was dried overnight under
vacuum at room temperature.
An aqueous solution of H2PtCl6 was prepared, and the pH of the solution was
adjusted to 6.8 by adding NaOH. A freshly prepared aqueous solution of NaBH4
was added to this solution under stirring at room temperature. After the addition of
NaBH4, the color of the solution changed to brown, indicating the formation of
nanoparticles. The solution was stirred overnight.
Characterization
The morphologies of the microrods were observed by an optical microscope
(Olympus BX-100).The phases and lattice parameters of the catalyst were
123

4. 386 T. Maiyalagan et al.
characterized by X-ray diffraction (XRD) patterns employing a Shimadzu XD-D1
˚
diffractometer using Cu Ka radiation (k = 1.5418 A) operating at 40 kV and
48 mA. XRD samples were obtained by depositing composite-supported nanopar-
ticles on a glass slide and drying the latter in a vacuum overnight.
Electrochemical measurements
All electrochemical measurements were performed in a conventional three-
electrode cell at room temperature. A Pt wire was used as a counter electrode.
All electrochemical potentials in the present study are given versus an Ag/AgCl
(saturated KCl) reference electrode. Glassy Carbon (GC) (electrode
area = 0.196 cm2) was polished to a mirror finish with 0.05-lm alumina
suspensions before each experiment which served as an underlying substrate of
the working electrode. In order to prepare the composite electrode, the catalysts
were dispersed ultrasonically in water at a concentration of 1.0 mg mL-1 and a
20-lL aliquot was transferred on to a polished glassy carbon substrate. After the
evaporation of the water, the resulting thin catalyst film was covered with 5 wt%
Nafion solution. Then, the electrode was dried at 353 K and used as the working
electrode. A solution of 1.0 M CH3OH in 0.5 M H2SO4 was used to study the
methanol oxidation activity.
Results and discussion
The PoPD microrods were obtained by interfacial polymerization and Fig. 1 shows
the optical micrograph of uniform PoPD microrods of several micrometers in length
and with diameters of approximately 1–3 lm, which suggested that they were
fabricated successfully by the interfacial polymerization method. The XRD pattern
of the catalyst (Pt/PoPD) is shown in Fig. 2. The peaks consistent with face-
centered cubic (fcc) as expected for Pt were clearly observed for the Pt/PoPD
catalyst. The XRD patterns were used to estimate the average particle size using the
Scherrer equation:
˚
dðAÞ ¼ kk=b cosh
where k is a coefficient (0.9), k the wavelength of X-ray used (1.54056 A°), b the
full-width half maximum and h is the angle at position of peak maximum. The mean
particle size obtained from the XRD patterns were 4.48 nm for Pt/PoPD and 2.8 nm
for Pt/C. To further identify the composition of these Pt/PoPD materials, the EDX
analysis (Fig. 2b) was used to analyze the content of Pt in the Pt/PoPD composite
which revealed a satisfactorily result close to the theoretical value (i.e., 20%)
calculated based on the fact that Pt precursors being reduced completely and the
entire reduced Pt particles are incorporated into the Pt/PoPD composite.
The cyclic voltammograms of Pt nanoparticles-supported PopD microrods
exhibiting hydrogen adsorption–desorption peaks corresponding to platinum
oxidation–reduction in 0.5 M H2SO4 at a scan rate of 50 mV/s are shown in
123

5. Electro-catalytic performance of Pt 387
Fig. 1 Optical micrographs of poly (o-phenylenediamine) microrods
Fig. 3. The appropriate cyclic voltammogram pattern of Pt nanoparticles-supported
PopD microrods was attained within five cycles in the background electrolyte. The
electrochemical surface areas (ECSA) calculated using the hydrogen desorption
peak and used to evaluate the surface area of the Pt-based catalysts are given in
Table 1. The surface area of Pt-supported PopD microrods was found to be higher
than the surface area of Pt/C [14]. The electrochemical surface area of various
polymer electrodes area is reported and the electrochemical surface area of Pt/C
electrode consistent with the previously reported electrodes is given in Table 1.
The electro-catalytic activity of the particulate composite electrodes for methanol
oxidation was studied by cyclic voltammetry. Figure 4 shows the cyclic voltam-
mograms (CVs) of the electrodes in 1.0 M MeOH ? 0.5 M H2SO4 at 25 °C with a
123

6. 388 T. Maiyalagan et al.
(a) (b)
Pt (111)
(a) 20% Pt/C
(b) 20% Pt/PoPD
C (002)
Pt (200)
Intensity (a.u)
(a)
Pt (220)
(b)
20 30 40 50 60 70 80
2θ (degrees)
Fig. 2 a X-ray diffraction patterns and b EDX patterns of 20% Pt Pt/PoPd electro-catalysts prepared by
sodium borohydride reduction
1.0 (a)
(mA/cm Pt )
2
0.5 (b)
0.0
Current density
-0.5
-1.0
-1.5
-200 0 200 400 600 800 1000 1200
Potential (V) vs Ag/AgCl
Fig. 3 Cyclic voltammograms of a 20% Pt/PoPD and b 20% Pt/C in nitrogen-saturated 0.5 M H2SO4 at
a scan rate of 50 mV/s
scan rate of 50 mV s-1. The specific activity of the catalysts normalized to Pt
surface area is shown in Table 1. The oxidation current of Pt/PoPD electrode
(If = 1.96 mA/cm2) was also higher than that of Pt/C (1.56 mA/cm2). During the
reverse scan, the oxidation peak at 0.50 V was obtained with a peak current of
Ib = 1.45 mA/cm2. This peak is attributed to the release of adsorbed CO or CO-like
species, which can be generated via incomplete oxidation of methanol in the
forward scan [33]. These carbonaceous species are mostly in the form of linearly
bonded Pt=C=O, which are oxidized in the reaction of the backward scan peak [33].
Thus, the ratio of the forward anodic peak current density (If) to the reverse
anodic peak current density (Ib), i.e., If/Ib, suggests a tolerance to carbonaceous
species accumulation on catalysts during methanol electro-oxidation. The low If/Ib
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7. Electro-catalytic performance of Pt 389
Table 1 Electrochemical properties of PoPD microrods-supported Pt and carbon-supported Pt
Electrode Electrochemical Current density
surface area (m2 g-1) (mA/cm2)
Pt/C 78.4 1.56
Pt/C [38] 84 1.39
Pt/PoPD microrods 96 1.96
Pt/poly(N-acetylaniline) nanorods [39] – 2.60
Pt/poly(o-anisidine) nanofiber [40] – 3.56
Pt/polypyrrole [41] 117 14.1
Pt/CNx [41] 114 7
2.0 (a)
Current density (mA/cm )
2
1.5 (b)
1.0
0.5
0.0
-0.5
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
Potential (V) vs Ag/AgCl
Fig. 4 Cyclic voltammograms of a 20% Pt/PoPD and b 20% Pt/C electrode in 0.5 M H2SO4/1 M
CH3OH at 50 mV/s
indicates poor oxidation of methanol to carbon dioxide during the forward anodic
scan and excessive accumulation of carbonaceous residues on the catalyst surface.
On the other hand, the high If/Ib indicates excellent oxidation of methanol during the
reverse anodic scan and less accumulation of residues on the catalyst. The reported
If/Ib value for the commercial E-TEK catalyst is *1 [34, 35]. The ratio observed for
Pt–Ru after vigorous heat treatment is *1.30 [36]. In the present study, the ratio is
observed to be 1.34 for Pt/PoPD catalysts, larger than the ratio of 1.1 [37] which
was calculated for the Pt/C (E-TEK) sample. Therefore, the higher tolerance of the
catalysts to incompletely oxidized species is another important reason for the higher
efficiency of Pt/PoPD catalysts.
The stability of Pt/PoPD and Pt/C was tested by chronoamperometric curves for
methanol oxidation as shown in Fig. 5.The current density has been normalized to
Pt surface area for the evaluation of the catalytic activity of the electrodes. In the
curves of all composite catalysts there was a sharp initial current drop, followed by a
123

8. 390 T. Maiyalagan et al.
1.6
1.4
Current density (mA/cm )
2
1.2
1.0
0.8
0.6
0.4
(a)
(b)
0.2
0.0
0 500 1000 1500 2000 2500 3000 3500
Time (seconds)
Fig. 5 Chronoamperometric curves for the catalysts a 20% Pt/PoPD and b 20% Pt/C recorded at the
0.7 V versus Ag/AgCl for 3,600 s in nitrogen-saturated 0.5 M H2SO4 and 1 M methanol at 25 °C
slow decay. A possible reason for the slow decay of current density is the poisoning
effect by COads intermediates [32]. As observed from Fig. 5, methanol oxidation on
Pt/PoPD gives a higher oxidation current than that on Pt/C during the process. This
indicated that the direct oxidation of methanol on Pt/PoPD was enhanced.
Therefore, Pt/PoPD had better poisoning-tolerance ability than Pt/C. At the same
time, the Pt/PoPD maintained the highest current density compared to Pt/C
electrodes. This was mainly due to the more facilitative methanol oxidation on Pt/
PoPD, which was in agreement with the aforementioned CV results. The more facile
diffusion of both liquid fuel and products of the microrods-supported catalyst
structure interpenetrated with the electrolyte network. Therefore, the utilization
efficiency of catalysts becomes higher. The microrods arrays may have a great
potential in the application of direct methanol fuel cells.
Conclusions
Synthesis and characterization of conducting PoPD microrods-incorporated Pt
nanoparticles are reported here. Good catalytic activity was observed for the electro-
oxidation of methanol at the metal–polymer microrods composite electrode. The Pt-
loaded PoPD microrods not only increase the electronic-ionic contact but also
provide an easier electronic pathway between the electrode and the electrolyte,
which increases the reactant accessibility to the catalytic sites. The electro-catalytic
activity of the microrods-based electrode was compared with those of 20 wt% Pt
supported on the Vulcan carbon electrode, using cyclic voltammetry. The Pt-loaded
PoPD microrods electrode exhibited better catalytic activity and stability than the
20 wt% Pt supported on the Vulcan carbon electrode.
123

9. Electro-catalytic performance of Pt 391
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