Electro catalytic performance of pt-supported poly (o-phenylenediamine) microrods for methanol oxidation reaction

Electro-catalytic performance of Pt-supported poly(o-phenylenediamine) microrods for methanoloxidation 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

� 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 � Pt supported poly (o-phenylenediamine) �Nanostructured materials � Electro-catalyst

T. Maiyalagan (&)

School of Chemical and Biomedical Engineering, Nanyang Technological University, 50 Nanyang

Drive, Singapore 639798, Singapore

e-mail: [email protected]

C. Mahendiran

Department of Chemistry, Anna University of Technology Tirunelveli, Nagercoil 629004, India

K. Chaitanya � F. Nawaz Khan

Chemistry Division, School of Advanced Sciences, VIT University, Vellore 632014, India

e-mail: [email protected]

R. Tyagi

Department of Chemistry, Deshbandhu College, Delhi University, Delhi 110007, India

123

Res Chem Intermed (2012) 38:383–391

DOI 10.1007/s11164-011-0354-3

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 HAuCl4and 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

384 T. Maiyalagan et al.

123

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

Electro-catalytic performance of Pt 385

123

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

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

Fig. 1 Optical micrographs of poly (o-phenylenediamine) microrods


123

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

20 30 40 50 60 70 80

Pt (

111)

Pt (

220)P

t (20

0)

C (0

02)

(b)

(a)

Inte

nsit

y (a

.u)

2θ (degrees)

(a) 20% Pt/C(b) 20% Pt/PoPD

(a) (b)

Fig. 2 a X-ray diffraction patterns and b EDX patterns of 20% Pt Pt/PoPd electro-catalysts prepared bysodium borohydride reduction

-200 0 200 400 600 800 1000 1200

-1.5

-1.0

-0.5

0.0

0.5

1.0

(b)

(a)

Potential (V) vs Ag/AgCl

Cu

rren

t d

ensi

ty (

mA

/cm

2 Pt)

Fig. 3 Cyclic voltammograms of a 20% Pt/PoPD and b 20% Pt/C in nitrogen-saturated 0.5 M H2SO4 ata scan rate of 50 mV/s


123

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

Table 1 Electrochemical properties of PoPD microrods-supported Pt and carbon-supported Pt

Electrode Electrochemical

surface area (m2 g-1)

Current density

(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

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2-0.5

0.0

0.5

1.0

1.5

2.0

Potential (V) vs Ag/AgCl

Cu

rren

t d

ensi

ty (

mA

/cm

2)

(b)

(a)

Fig. 4 Cyclic voltammograms of a 20% Pt/PoPD and b 20% Pt/C electrode in 0.5 M H2SO4/1 MCH3OH at 50 mV/s


123

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.

0 500 1000 1500 2000 2500 3000 35000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

(b)(a)

Time (seconds)

Cu

rren

t den

sity

(m

A/c

m2 )

Fig. 5 Chronoamperometric curves for the catalysts a 20% Pt/PoPD and b 20% Pt/C recorded at the0.7 V versus Ag/AgCl for 3,600 s in nitrogen-saturated 0.5 M H2SO4 and 1 M methanol at 25 �C


123

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