Nanoparticles of Pt/HxMoO3 electrodeposited in poly(3,4-ethylenedioxythiophene)- poly(styrene sulfonic acid) as the electrocatalyst for methanol oxidation

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Journal of Power Sources 185 (2008) 807–814

Contents lists available atScienceDirect

Journal of Power Sources

j o u r n a l h o m e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / j p o w s o u r

Nanoparticles of Pt/H

x

MoO

3

electrodeposited in poly

(3,4-ethylenedioxythiophene)-poly(styrene sulfonic acid)

as the electrocatalyst for methanol oxidation

Chung-Wen Kuo

a

_{, Chinnaiah Sivakumar}

b

_{, Ten-Chin Wen}

a,∗ a_{Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan}

b_{Electrodics & Electrocatalysis Division, Central Electrochemical Research Institute, Karaikudi 630006, Tamil Nadu, India}

a r t i c l e i n f o

Article history:

Received 23 May 2008

Received in revised form 11 July 2008 Accepted 15 July 2008

Available online 30 July 2008

Keywords: Pt/HxMoO3 PEDOT-PSS XPS SEM Methanol oxidation

a b s t r a c t

Nanoparticles of platinum and hydrous molybdenum oxide (Pt/HxMoO3) were successfully

elec-trodeposited onto poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonic acid) (PEDOT-PSS) ﬁlm by chronocoulometry (0.2 C). Various loadings of Pt/HxMoO3particles onto the PEDOT-PSS electrode were

achieved using the co-deposition technique. The existence of Pt/HxMoO3 particles was determined

through characterization by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) anal-ysis. XPS results revealed that deposited Pt and molybdenum were metallic Pt and HxMoO3, respectively.

XRD analysis showed a decrease of Pt crystalline facets for the incorporation of HxMoO3into

PEDOT-PSS-Pt, indicating a strong interaction between Pt and HxMoO3. Scanning electron microscopy (SEM) results

revealed a uniform dispersion of Pt/HxMoO3particles, with the particle size of 70–90 nm, in the

PEDOT-PSS matrix. The cyclic voltammetry study and chronopotentiometry measurements demonstrated that the PEDOT-PSS-Pt/HxMoO3electrode had superior electrocatalytic activity of methanol oxidation with less

CO poisoning. The presence of amorphous HxMoO3particles on the Pt surface minimized CO poisoning of

methanol oxidation.

1. Introduction

Direct methanol fuel cells (DMFCs) are considered a promising solution to future energy problems because of their high-energy conversion efﬁciency, low pollutant emission, low operating tem-perature, and the simplicity of handling and processing liquid fuel [1,2]. The development of DMFCs has the problem of slow methanol oxidation reaction kinetics at the Pt anode catalyst. This is mainly due to the self-poisoning of Pt surfaces caused by the adsorp-tion of reacadsorp-tion intermediates CO and CO-like species[3]. A few researchers have attempted to reduce the amount of adsorbed CO on Pt surfaces by employing added metal catalysts such as Ru, Sn, W, Mo, and Os to promote CO oxidation[4–6]. Molybdenum can improve the activity of platinum toward the oxidation of methanol when it is added into platinum as an alloy element or a compound [7,8]. The mechanism of methanol oxidation on Pt in the pres-ence of molybdenum oxide species has been reported in literature [9–12]. The Mo oxide species in Pt promote the oxidative removal of adsorbed CO intermediates. The enhancement of surface

cat-∗ Corresponding author. Tel.: +886 6 2385487; fax: +886 6 2344496.

E-mail address:[email protected](T.-C. Wen).

alytic activity by Pt-Mo oxide interaction and the large surface area achieved with low loading levels improves DMFCs.

Conducting polymer matrix is a solid support alternative to carbon that can be used as a DMFC electrode catalyst because it provides uniform distribution of metal nanoparticles into the polymer matrix in a 3D manner. Pt particles dispersed in the poly-mer matrix (PEDOT-PSS) exhibit interesting catalytic properties [13–16]because of the large number of catalytic sites. In the past few years, several articles have been published on the electro-catalytic properties of platinum alloys such as Pt-Ru incorporated into conducting polymeric matrix[17,18]. It has been shown that a modified electrode with polymer has better catalytic performance than a modified electrode without polymer toward the oxidation of methanol. The high surface area of conducting polymers has made them popular as supporting materials in the development of new electrocatalysts[19,20]. Because of the relatively high electric conductivity of conducting polymers, it is possible to shuttle the electrons through polymer chains between the electrodes and dis-persed metal particles where the electrocatalytic reaction occurs. Thus, an efficient electrocatalyst can be achieved in the composite materials.

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808 C.-W. Kuo et al. / Journal of Power Sources 185 (2008) 807–814 HxMoO3particles. PEDOT is a high surface area conducting

poly-mer with good environmental stability, high electrical conductivity, very good ﬁlm forming properties, and a high degree of doping com-pared to PANI and other conducting polymers[21]. It can be used as an electrode catalyst support for Pt particles for the oxidation of small organic molecules such as methanol. Ghosh and Inganas[21] reported that PEDOT-PSS is a non-stoichiometric polyelectrolyte complex of PEDOT and PSS, with an excess of the latter component. PEDOT-PSS is commercially available as stable aqueous dispersion. The network structure and SO3−groups of PEDOT-PSS[22]permit

increased uptake of Pt4+_ions_[13]_{and facilitate the uniform}

dis-tribution of Pt particles in a polymer matrix. The polymer matrix is used as a protective layer to prevent nanoparticles aggregation after their formation. Since HxMoO3 can promote the oxidative

removal of adsorbed CO intermediates on Pt surfaces[23,24], dif-ferent methods of loading Pt/HxMoO3particles entrapped in the

PEDOT-PSS matrix are examined to ﬁnd the best electrocatalytic activity and stability toward methanol oxidation.

2. Experimental

A PEDOT-PSS (Alfa, 1.34 wt%) matrix electrode was prepared by spin coating 100␮l (2 drops) of dispersion onto indium-tin oxide (ITO) substrate at 2000 rpm for 1 min. After the completion of the spin coating process, films were dried at 150◦C for 3 min. A thin film of PEDOT-PSS formed over a cleaned indium-tin oxide elec-trode (1.0 cm× 1.0 cm). Before each experiment, ITO coated glass was cleaned in an ultrasonic bath using detergent, double distilled water, and isopropanol, then dried with a dry nitrogen flow fol-lowed by UV-O3treatments for 30 min.

Pt, HxMoO3, and Pt/HxMoO3 particles were incorporated

into the PEDOT-PSS ﬁlm by electrochemical deposition/co-deposition from 0.01 M HCl + 0.1 M KCl solution containing 5 mM H2PtCl6·6H2O (PEDOT-PSS-Pt), 1 mM Na2MoO4·2H2O

(PEDOT-PSS-HxMoO3), 5 mM H2PtCl6·6H2O, and 1 mM Na2MoO4·2H2O

(PEDOT-PSS-Pt/HxMoO3) at a constant potential of −0.2 V (vs.

Ag/AgCl). For comparison purposes, Pt, HxMoO3, and Pt/HxMoO3

particles were also deposited on a bare ITO electrode (Pt, E-HxMoO3, and Pt/HxMoO3) with a process similar to the deposition

of Pt, HxMoO3, and Pt/HxMoO3particles onto PEDOT-PSS,

respec-tively. Constant deposition charge of 0.2 C was maintained for all deposition processes, except that PEDOT-PSS-HxMoO3 and

E-HxMoO3 electrodes were kept at 0.04 C. After deposition, the

electrodes were rinsed with double distilled water for 5 min and then dried at 150◦C for 3 min.

Electrochemical characterizations of PSS-Pt, PEDOT-PSS-HxMoO3, and PEDOT-PSS-Pt/HxMoO3 composite electrodes

were carried out using a PGSTAT20 electrochemical analyzer, AUTOLAB Electrochemical Instrument (The Netherlands). All exper-iments were performed in a three-component cell. An Ag/AgCl electrode (in 3 M KCl), Pt wire, and ITO coated glass plate (1 cm2

area) were used as reference, counter, and working electrodes, respectively. A Luggin capillary, whose tip was set at a distance of 1–2 mm from the surface of the working electrode, was used to minimize errors due to the iR drop in the electrolytes. The cat-alytic activities of E-HxMoO3, E-Pt/HxMoO3, PEDOT-PSS-Pt, and

PEDOT-PSS-Pt/HxMoO3 composite electrodes were examined in

0.1 M CH3OH + 0.5 M H2SO4solution at the potential range of−0.2

to 1.0 V with a scanning rate of 50 mV s−1.

X-ray photoelectron spectroscopy (XPS) was performed using ESCA 210 spectrometers. XPS spectra employed Mg K␣ (h = 1253.6 eV) irradiation as the photon source, with a primary tension of 12 kV. The pressure during the scans was approximately 10−10mbar. Morphological and crystalline behavior changes among electrodes fabricated with indium-tin oxide as the substrate, and

E-Pt/HxMoO3, PEDOT-PSS-Pt, and PEDOT-PSS-Pt/HxMoO3 ﬁlms

were compared using scanning electron microscopy (SEM) (Philips X1-40 FEG.), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD) analysis, respectively. X-ray diffraction spectra for the as-prepared electrodes were obtained by exposing the samples to a Sieman D5000 X-ray source with Cu K␣ (1.542 Å) as a target in diffraction angles (2) ranging from 10◦ to 90◦ with a scanning rate of 4◦min−1. An Impedance Spectrum Analyzer, IM6 (ZAHNER, Germany), with Thales software was employed to measure and analyze the ac impedance spectra of electrodes obtained at various element resistances. The potential amplitude of ac was kept at 10 mV and a frequency range of 50 mHz to 100 kHz was used.

3. Results and discussion

Conducting PEDOT-PSS film used as a catalyst support was formed in ITO glass by the spin coating process. The PEDOT-PSS film thickness (202 nm) and electrochemical stability were mea-sured by ellipsometry and the cyclic voltammetry (CV) technique as described elsewhere [13]. To investigate the electrocatalytic activity of Pt/HxMoO3nanoparticles loaded PEDOT-PSS film, two

different approaches were taken: (i) electrocodeposition of various amounts Pt/HxMoO3particles onto PEDOT-PSS and (ii)

electrode-position of various amounts of HxMoO3particles onto constant Pt

(0.1 mg cm−2) particles incorporated PEDOT-PSS ﬁlm. The detailed procedure of the deposition of Pt/HxMoO3particles in the

PEDOT-PSS ﬁlm was described in Section 2. The cyclic voltammetry technique was chosen to characterize the Pt and Pt/HxMoO3

par-ticles embedded in PEDOT-PSS ﬁlm in 0.5 M H2SO4and compared

with bare E-Pt, E-Pt/HxMoO3electrodes.

3.1. Deposition of Pt and co-deposition of Pt/HxMoO3particles on

the PEDOT-PSS electrode

Pt and Pt/HxMoO3 nanoparticles were electrodeposited on

PEDOT-PSS ﬁlm using the chronocoulometry technique. The detailed procedure of the deposition of Pt and Pt/HxMoO3

par-ticles is described in Section 2. Fig. 1 shows the typical cyclic voltammograms of Pt and Pt/HxMoO3 particles loaded

PEDOT-PSS ﬁlm in the potential range of−0.2 and 1.0 V vs. Ag/AgCl at

Fig. 1. Cyclic voltammograms of (a) PEDOT-PSS-Pt/HxMoO3prepared using the

co-deposition technique, (b) PEDOT-PSS-Pt and (c) PEDOT-PSS-Pt/HxMoO3obtained

by the deposition of HxMoO3on PEDOT-PSS-Pt electrodes in 0.5 M H2SO4with a

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C.-W. Kuo et al. / Journal of Power Sources 185 (2008) 807–814 809

Fig. 2. (a) S 2p XPS core-level spectra of (I) PEDOT-PSS, (II) PEDOT-PSS-Pt, and (III) PEDOT-PSS-Pt/HxMoO3. (b) Mo 3d XPS core-level spectra of (I) E-HxMoO3, (II)

PEDOT-PSS-HxMoO3, (III) E-Pt/HxMoO3, and (IV) PEDOT-PSS-Pt/HxMoO3. (c) Supplementary deconvolution of Mo 3d core level spectra of PEDOT-PSS-Pt/HxMoO3. (d) Pt 4f XPS core-level

spectra of (I) E-Pt, (II) PEDOT-PSS-Pt, (III) E-Pt/HxMoO3, and (IV) PEDOT-PSS-Pt/HxMoO3.

a scanning rate of 50 mV s−1 in 0.5 M H2SO4. A clear skin

tex-ture of hydrogen adsorption–desorption of Pt nanoparticles with no sharp peaks was observed[20]in the potential range of−0.2 and 0.1 V, which indicates that the Pt particles were dispersed uni-formly in the PEDOT-PSS 3D-random matrix (curve a). Because of the PEDOT-PSS matrix, the electrodeposited Pt and Pt/HxMoO3

nanoparticles have a high surface area (1.978 mC cm−2for PEDOT-PSS-Pt and 3.220 mC cm−2 for PEDOT-PSS-Pt/HxMoO3) compared

to Pt and Pt/HxMoO3particles deposited on the bare ITO electrode

(ﬁgure not shown). An additional new redox peak was observed at 0.254 V on the anode side and a broad peak at 0.282 V on the cathode side for Pt/HxMoO3 particle loaded PEDOT-PSS ﬁlm

(curve a). This indicates that the particles of HxMoO3 were

co-deposited with Pt particles and that a trioxide of Mo exists in the redox transition between +4 and +6 states with non-stoichiometric mixed valences (HxMoO3/HyMoO3; 0 < y < x < 2), which is similar

to the hydrogen molybdenum bronze obtained by the reduction of molybdates in acid solutions [9,24,25]. A close examination of the hydrogen adsorption–desorption region of CV of PEDOT-PSS-Pt and PEDOT-PEDOT-PSS-Pt/HxMoO3 electrodes produced almost

the same peak current values, indicating that the co-deposited

HxMoO3particles do not affect the real surface area of Pt particles

in the PEDOT-PSS-Pt/HxMoO3 matrix. During co-deposition

pro-cess, HxMoO3particles are covered on the surface of Pt particles

and hence aggregation of Pt particles is avoided in PEDOT cata-lyst support. But electrodeposition of Pt particles onto PEDOT-PSS matrix results Pt particles agglomeration. HxMoO3 particles may

be located nearby or adjacent to Pt (1 1 1) surface of Pt particles. We observed high electro-active surface area and electrocatalytic activity of PEDOT-PSS-Pt/HxMoO3 catalyst toward methanol

oxi-dation compared to the catalyst prepared by electrodeposition of Pt particles on PEDOT-PSS matrix. Hence HxMoO3 particles

can be assisted to incorporate Pt particles in PEDOT-PSS matrix with uniform distribution and avoided particles aggregation to certain extent. However, a reduced electro-active surface of Pt particles was noticed on the Pt/MoOx/C catalyst prepared by the

impregnation of Pt particles on MoOx/C dispersed in ethanol

solu-tion [24] compared to the PEDOT-PSS-Pt/HxMoO3 catalyst. To

obtain the maximum loading of Pt/HxMoO3particles onto

PEDOT-PSS matrix, co-deposition (deposition charge = 0.2 C) of various amounts of HxMoO3was performed between 0.5 mM of [Na2MoO4]

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814 C.-W. Kuo et al. / Journal of Power Sources 185 (2008) 807–814 was absorbed on the surface of platinum particles during the

methanol oxidation process and HxMoO3 appears in the higher

oxidation state, which can oxidize CO molecules adsorbed on the surface of platinum particles. To interpret the impedance results, an equivalent circuit was used to ﬁt the EIS data inFig. 7(inset). In this Rs(RctCPE) circuit, Rs represents ohmic resistance of the solution, while RctCPE represents the parallel combination of the charge-transfer resistance (Rct) and the constant phase element (CPE). The parallel combination (Rct and CPE) leads to a depressed semicircle in the corresponding Nyquist impedance plot.

According to experiment data based on the equivalent cir-cuit, it is clear that the lowest charge-transfer resistance (Rct = 438 ) can be observed for PEDOT-PSS-Pt/HxMoO3 than

PEDOT-PSS-Pt (Rct = 1439 ) due to the presence of HxMoO3

in the PEDOT-PSS matrix. This can be explained by HxMoO3

oxidizing the CO on the platinum surface to form CO2. The

charge-transfer resistance of PEDOT-PSS-Pt/HxMoO3is lower than

that of Pt-HxMoO3 (Rct = 637 ). This means that the

embed-ding of Pt/HxMoO3 nanoparticles inside the PEDOT-PSS matrix

may lead to a faster charge transfer in the parallel PEDOT-PSS ﬁlm/solution and Pt/HxMoO3/solution interface than in the

Pt/HxMoO3(microparticles)/solution. The PEDOT-PSS matrix may

also hinder the formation of strongly absorbed poisonous species.

4. Conclusion

A high electro-active surface area (3.220 mC cm−2) of Pt/HxMoO3 particles was successfully electrodeposited into

the PEDOT-PSS matrix for methanol oxidation. The existence of HxMoO3 particles in PEDOT-PSS-Pt/HxMoO3 was veriﬁed using

cyclic voltammetry. The 3d5/2and 3d3/2 core level spectra of Mo

further support the existence of HxMoO3. XPS of S elements

ana-lyzed that PEDOT-PSS-Pt indicates a strong interaction between platinum and sulfonic groups of PEDOT-PSS. XRD results fur-ther conﬁrmed that the crystalline lattice of platinum might be inﬂuenced by the existence of amorphous HxMoO3in composite

systems. The composite PEDOT-PSS-Pt/HxMoO3 based electrode

proved to be a promising catalyst for methanol oxidation. The PEDOT-PSS-Pt/HxMoO3 electrode exhibits a high current density

(2.220 mA cm−2) and a low onset potential toward methanol oxidation. The PEDOT-PSS matrix provides an environment for dispersing Pt particles with less aggregation, which is evident from SEM results. HxMoO3 favors the transformation of CO to

carbon dioxide on the platinum surface. A clean Pt surface then becomes available for the oxidation of methanol. The enhanced electrocatalytic activity of Pt in PEDOT-PSS might allow a decrease in the use of Pt content in DMFCs applications.

Acknowledgement

The ﬁnancial support of this work by the National Sci-ence Council of Taiwan under grants NSC-95-221-E-006-325,

2811-E-006-021, ET-7-006-006-ET, and NSC-95-2211-E-006-409-MY3 is gratefully acknowledged.

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