A

taPCsm©

K

1

seelHbtmtbriwfi

a

0d

Electrochimica Acta 52 (2007) 5873–5878

A new route for the electrodeposition of platinum–nickelalloy nanoparticles on multi-walled carbon nanotubes

Yue Zhao a, Yifeng E a, Louzhen Fan a,∗, Yongfu Qiu b, Shihe Yang b,∗∗a Department of Chemistry, Beijing Normal University, Beijing 100875, China

b Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China

Received 8 December 2006; received in revised form 2 February 2007; accepted 6 March 2007Available online 12 March 2007

bstract

An electrochemical method was developed to deposit platinum (Pt) and nickel (Ni) nanoparticles on multi-walled carbon nanotubes (MWCNTs)hrough a three-step process. X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDX) show the alloy formation for Pt and Ni withratio of 1:1. The presence of Pt(0), Ni(0), Ni(OH)2, NiOOH and slight NiO was deduced from XPS data. Electrocatalytic properties of the resultingtNi/MWCNT electrode for methanol oxidation reaction were investigated. Compared with Pt/MWCNT, an appreciably improved resistance to

O poisoning was observed for the PtNi/MWCNT electrode, which was interpreted by a mechanism based on the bifunctional catalysis. The

uccessful preparation of PtNi/MWCNT nanocomposites opens a new path for an efficient dispersion of the promising electrocatalysts in the directethanol fuel cells.2007 Elsevier Ltd. All rights reserved.

t; Met

ccbilarennNact

eywords: Electrodeposition; Platinum–nickel alloy; MWCNT; Electrocatalys

. Introduction

The direct methanol fuel cells (DMFCs) have received con-iderable attentions for applications in transportation, portablelectronics, and residential power sources, due to their highnergy density, relatively low operating temperatures, zero orow emission of pollutants, and minimal corrosion problems.owever, the commercial viability of DMFCs is still hinderedy several factors, including the low catalytic activity of elec-rodes both for the oxygen reduction reaction (ORR) and for the

ethanol oxidation reaction, the high costs of the Pt-based elec-rocatalysts, and the susceptibility of the catalysts to be poisonedy the CO-like intermediates formed in the methanol oxidationeaction [1–5]. The most common solution to these problemss to employ bi- or tri-metallic catalysts that combine platinumith other metals such as Ni, Co, etc., and disperse the metals

nely on proper supports [6].

Over the last two decades, various Pt-based bimetallic cat-lysts have been studied [7–10]. Among them, PtNi bimetallic

∗ Corresponding author. Fax: +86 10 58802075.∗∗ Corresponding author.

E-mail addresses: lzfan@bnu.edu.cn (L. Fan), chsyang@ust.hk (S. Yang).

bHcaaaPM

013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2007.03.020

hanol oxidation

atalyst has attracted more interests [11–17]. Ni alloying with Ptan improve the methanol oxidation by lowering the electronicinding energy in Pt. In addition, the presence of Ni oxidesn the catalyst provides an oxygen source for CO oxidation atower potentials. On the other hand, the PtNi alloy also presentsn improved activity for the oxygen reduction than Pt alone byeducing the cell voltage when used as a cathode. In order tonhance the catalytic activity of the PtNi alloy, two major issueseed to be considered concerning the extent of dispersion and theature of support such as multi-walled carbon nanotubes (MWC-Ts). As a new form of carbon, MWCNTs have been regarded

s a new support for metal catalysts due to their small size, highhemical/thermal/mechanical stabilities and large surface areao volume ratio [18–22].

At present, high surface area noble metal electrodes haveeen successfully prepared via several routes [17,23–25].owever, electrodeposition of PtNi alloy nanoparticles on

arbon nanotubes has not been achieved so far. Considering thedvantages of electrodeposition, such as high purity of deposits

nd simple procedure of deposition, herein we suggest a newpproach based on a so-called three-step process to deposittNi alloy nanoparticles on MWCNTs. The resulting PtNi/WCNT catalysts have been characterized by transmission

5 ica A

edsPv

2

2

fanIdrNpUSa(Dpw

2

i(ma

tpcae

2

uecewar

e(ser

ia

3

3

pcao1rtat((tatM

naand 4f5/2 lines appear at 70.91 eV and 74.13 eV, respectively,with the theoretical ratio of peak areas of 4:3. The peaks for Pt2+

and Pt4+ at 73.8 eV and 74.6 eV, respectively, were not found,indicating that Pt is present in the zero-valent metallic state in

874 Y. Zhao et al. / Electrochim

lectron microscope (TEM), X-ray diffraction (XRD), energyispersive X-ray spectroscopy (EDX) and X-ray photoelectronpectroscopy (XPS). The electrocatalytic properties of thetNi/MWCNT electrode have been investigated by cycleoltammetry (CV) method.

. Experimental

.1. Materials

Multi-walled carbon nanotubes (MWCNTs) were purchasedrom Shenzhen Nanotech. Port. Co., Ltd., Shenzhen, China,nd further purified prior to use by stirring in concentrateditric acid for 12 h. Toluene (C6H5CH3) (Park Co., Dublin,reland) was dried with sodium and refluxed for 6 h beforeistillation and then stored in the present of sodium. Othereagents are as follows: potassium sulfate (K2SO4) (A.R.,acalai Tesque, Inc., KYOTO, Japan); potassium tetrachloro-latinate (II) (Cl4K2Pt) (99.9%-Pt, International Laboratory,SA); nickel (II) chloride hexahydrate (NiCl2·6H2O) (98%-Ni,igma–Aldrich Laborchemikalien GmbH, Germany); sulfuriccid (H2SO4) (A.R., BeiHua, Inc., Beijing, China) and ethanolCH3CH2OH) (A.R., Chemical Reagent, Inc., Tianjin, China).eionized water (20 ± 1 ◦C, pH 7, ρ = 18.3 M�/cm) wasurified by passing through an EASY pure compact ultrapureater system (Barnstead Co., USA).

.2. Electrode preparation

The glassy carbon (GC) electrode (3 mm diameter) was pol-shed to a mirror finish with emery paper and alumina slurry1.0 �m, 0.3 �m), and ultrasonically cleaned in toluene for a fewinutes, then dried with a high-purity nitrogen stream immedi-

tely before use.One milligram purified MWCNTs was dispersed in 10 ml

oluene by ultrasonication for 30 min to give a 0.1 mg/ml sus-ension. Fifteen microlitres of the suspension was directlyast on GC electrode surface and evaporated in the solventt room temperature to prepare the MWCNTs film modifiedlectrode.

.3. Measurements

A conventional cell with a three-electrode configuration wassed throughout this work. The MWCNTs/GC electrode wasmployed as the working electrode, a platinum wire served as theounter electrode and a Ag/AgCl electrode was used as the refer-nce electrode. Electrochemical measurements were performedith a CHI705A (CH Instruments, Inc., USA) electrochemical

nalyzer and the potentials were measured and reported withespect to the Ag/AgCl electrode.

Morphologies of the as-synthesized PtNi/MWCNTs werexamined on a Hitachi 600 transmission electron microscope

TEM), which was operated at 300 kV. X-ray photoelectronpectroscopy data were obtained with an ESCALab220i-XLlectron spectrometer from VG Scientific using 300 W Al K�adiation. The base pressure was about 3 × 10−9 mbar. The bind-

cta 52 (2007) 5873–5878

ng energies were referenced to the C1s line at 284.8 eV fromdventitious carbon.

. Results and discussion

.1. Physical characterization and structural studies

A good dispersion of the catalyst on the carbon support is arecondition for attaining good electrocatalytic activity in a fuelell reaction. Fig. 1 shows the typical TEM image of MWCNTsfter electrochemical deposition. A homogeneous dispersionf nanoparticles largely spherical in shape and approximately0 nm in size can be observed. From the energy dispersive X-ay spectroscopy (EDX) of this sample (Fig. 2), it can be seenhat Pt and Ni are the major elements with an atomic ratio ofpproximately 1:1. XRD analyses demonstrate the characteris-ic peaks of the Pt fcc structure (see Fig. 3). For PtNi/MWCNTcurve a in Fig. 3), no peaks for fcc Ni are observed, but the 2θ of1 1 1) peak has angle shift from 39.84 of Pt/MWCNT (curve b)o 40.48. The angle shifts of the Pt peaks are the indication for thelloy formation between Pt and Ni [26], which is the evidencehat the PtNi alloy nanoparticles are successfully deposited on

WCNTs.To investigate the chemical nature of these PtNi alloy

anoparticles, XPS analyses were performed. Spectra of Pt 4fnd Ni 2p are shown in Fig. 4a and b, respectively. The Pt 4f7/2

Fig. 1. A typical TEM micrograph of the electrode after electrodeposition.

Y. Zhao et al. / Electrochimica Acta 52 (2007) 5873–5878 5875

tcitfiAa8r4oasNia

3

Mt

Fe

Fig. 2. EDX spectrum of PtNi/MWCNT.

he nanoparticles. In contrast, the Ni 2p3/2 spectrum shows aomplex structure with intense satellite signals of high bind-ng energy adjacent to the main peaks, which may be ascribedo a multielectron excitation (shake-up peaks) (Fig. 4b). Curvetting of the Ni 2p3/2 signals gives different nickel species.fter considering the shake-up peaks, the Ni 2p3/2 XPS peaks

t the binding energies of 852.6 eV, 854.0 eV, 855.8 eV, and57.1 eV are ascribed to Ni(0), NiO, Ni(OH)2, and NiOOH,espectively [26]. Furthermore, with reference to pure Pt, the Ptf XPS spectrum of PtNi on MWCNT experiences peak shiftsf −0.29 eV and −0.30 eV for Pt 4f7/2 (71.20 eV for pure Pt)nd Pt 4f5/2 (74.53 eV), respectively, indicating an electronictructural change of Pt when it is alloyed with Ni. Most likely,i(0) occupies the platinum lattice, and the metallic grains are

ntermixed with amorphous Ni oxides, such as NiO, Ni(OH)2nd NiOOH revealed in the XPS spectra.

.2. The formation of PtNi/MWCNT

The electrochemical deposition of PtNi nanoparticles onWCNTs was carried out by making use of an established

hree-step process [27–29] but with important improvement (as

Fig. 3. XRD analysis of PtNi/MWCNT (a) and Pt/MWCNT (b).

s(ecspKptctni

cvt(+fTt

ig. 4. X-ray photoelectron spectra of (a) Pt 4f and (b) Ni 2p of PtNi/MWCNTlectrode.

hown in Scheme 1): (1) generation of oxide functional groupsquinoid, carbonyl and carboxyl) at the defect sites located at thends and/or the sidewalls of the carbon nanotubes by potentialycling (200 mV s−1) from +1.8 V to −0.4 V in 0.5 M K2SO4olutions for 10 min; (2) oxidation of PtCl42− and Ni2+ to com-lexes of Pt(IV) and Ni(III) on the MWCNT surface from2PtCl4 + NiCl2 + 0.1 M K2SO4 (pH 4) aqueous solutions byotential-step method. The potential was increased from 0.3 Vo 1.3 V with pulse width of 0.001 s and this was carried out suc-essively until a steady pulse current reached; (3) conversion ofhe surface complexes on the MWCNTs to platinum and nickelanoparticles through potential cycling from +1.0 V to −0.26 Vn 0.1 M H2SO4 solutions to the steady state.

In order to investigate the oxidation of PtCl42− and Ni2+ toomplexes of Pt(IV) and Ni(III) on the MWCNT surface, cyclicoltammogram of MWCNT/GC (pre-treated in K2SO4 solu-ion) in 2 M K2PtCl4 + 1 M NiCl2 + 0.1 M K2SO4 was obtainedFig. 5). The lower limit of the potential scan was restricted to

0.3 V in order to prevent the electrodeposition of Pt(0) or Ni(0)rom the solution. Two irreversible anodic peaks are observed.he peak at 1.05 V corresponds to the oxidation of PtCl42−

o Pt(IV) complex. As reported, the oxygen atom from func-

5876 Y. Zhao et al. / Electrochimica Acta 52 (2007) 5873–5878

roces

twcwhpcTcPbr

eeaNcssot

FN

[affrtw

3

sNvPt−a

Scheme 1. Schematic illustration of the three-step p

ional groups such as quinoid, carbonyl and carboxyl, whichere generated on the surfaces of MWCNTs during electro-

hemical pre-treatment, serves as one of the two axial ligandshen the planar complex of Pt(II) is oxidized to form the octa-edral complex of Pt(IV) [27–29]. Another irreversible anodiceak at 0.92 V is relative to the oxidation of Ni2+ to Ni(III)omplex, which is in accord with a previous work [30–32].herefore, it is reasonable to believe that in our three-step pro-ess, the complexes of Pt(IV) and Ni(III) were the precursors oftNi nanoparticles on the MWCNTs, which were then obtainedy potential scanning in 0.1 M H2SO4 solution in the potentialange from +1.0 V to −0.26 V to steady state.

It has been reported that electrochemical pre-treatments oflectrode surfaces result in a pronounced enhancement for thelectrodeposition of Ni-containing species such as Ni(OH)2nd NiOOH [26,33,34]. Clearly, the electrodeposition of thei-containing species is necessarily accompanied by the pre-

ipitation of OH− ions generated at the electrode surface by in

itu electrochemical reduction of adsorbed oxygen-containingpecies. Our previous work has demonstrated the formation ofxide functional groups (carbonyl, hydroxyl and carboxyl) onhe surfaces of MWCNTs by electrochemical pre-treatments

ig. 5. Cyclic voltammograms of MWCNT/GC electrode in 2 M K2PtCl4 + 1 MiCl2 + 0.1 M K2SO4 (pH 4) solution (scan rate: 100 mV s−1).

icts

Ft

s for electrochemical synthesis of PtNi/MWCNTs.

29]. We believe that the production of Ni(OH)2 and NiOOHre somehow related to the electrochemical process for the oxideunctionalization of MWCNTs. As for NiO, it is known to beormed in very small amount by electrochemical treatment andelated to surface passivation of the electrode [26]. Probably,hese Ni-containing species are finely dispersed and inter-mixedith the PtNi nanoparticles.

.3. Activity for oxygen reduction and methanol oxidation

For comparison, Pt/MWCNT was also obtained under theame experimental conditions as PtNi/MWCNT except that noiCl2 was used for the second step. Fig. 6 shows the cyclicoltammograms of the resulting PtNi/MWCNT (curve a) andt/MWCNT (curve b) in 0.1 M H2SO4. It can be seen that

he reversible hydrogen adsorption/desorption peaks between0.25 V and 0.1 V and preoxidation/reduction between 0.4 V

nd 1.0 V of the Pt surface are clearly seen for Pt/MWCNT,

ndicating the presence of Pt, whereas for the PtNi/MWCNT, theurrent densities associated with the reversible hydrogen adsorp-ion region have decreased due to the site blocking effect [35],uggesting that the high dispersion of the PtNi alloy nanoparti-

ig. 6. Cyclic voltammograms of PtNi/MWCNT (a) and Pt/MWCNT (b) elec-rodes in 0.1 M H2SO4 solution (scan rate: 100 mV s−1).

Y. Zhao et al. / Electrochimica Acta 52 (2007) 5873–5878 5877

Ft

cncppasol

mtCepePwTPPw(cahIPtestb

P0Fiw

FiA

fftTs(cNiambioartNoC(

4

aaPPaeptathan Pt/MWCNT in methanol solution. The increased resis-

ig. 7. Cyclic voltammograms of PtNi/MWCNT (a) and Pt/MWCNT (b) elec-rodes in 2 M CH3OH + 0.1 M H2SO4 solution (scan rate: 100 mV s−1).

les with a disordered surface structure was obtained and thato surface enrichment of platinum was found. Significantly, itan be observed that the onset of the oxide formation and theeak potential of oxide reduction are shifted to more positiveositions by 20 mV and 70 mV, respectively, indicating that thelloying of Pt with Ni inhibits the chemisorption of OH on the Ptites at potentials above 0.6 V. This may have a beneficial effectn the oxygen adsorption at low overpotentials, and thus mayead to the enhancement of the ORR kinetics [21,36].

The catalytic activity of PtNi/MWCNT catalyst was alsoeasured in the oxidation of methanol, which is a useful applica-

ion in the direct methanol fuel cell (DMFC). Fig. 7 shows typicalVs of PtNi/MWCNT electrode (curve a) and Pt/MWCNTlectrode (curve b). Notably, the shapes of the CV curves andeak potentials are in line with other works typically using Ptlectrode, but the onset potential for methanol oxidation fortNi/MWCNT electrode occurs at an anodic potential 0.26 V,hich is about 50 mV lower than that for Pt/MWCNT (0.31 V).he current with the alloy electrode is also smaller than with thet electrode, which can be explained by the smaller amount oft in the alloy electrode. On the other hand, the ratio of the for-ard anodic peak current (If) to the reverse anodic peak current

Ib) can be used to describe the catalyst tolerance to carbona-eous species accumulation [37]. Ordinarily, If/Ib can be useds a performance index of a catalyst for the conversion and aigher If/Ib value implies better oxidation of methanol to CO2.n our experiments, the ratio was estimated to be about 2.0 for thetNi/MWCNT electrode, even higher than the value of 1.4 for

he Pt/MWCNT electrode. Such a high value for PtNi/MWCNTlectrode indicates that most of the intermediate carbonaceouspecies were oxidized to CO2 in the forward scan, suggestinghat the interaction of Pt and Ni leads to the less poisoning of Pty the CO-like intermediates formed during methanol activation.

The current density–time plots of PtNi/MWCNT andt/MWCNT electrodes in 2.0 M CH3OH + 0.1 M H2SO4 at.66 V (versus Ag/AgCl) for 3600 s were further measured.

ig. 8 shows such typical i–t plots with the current normal-

zed to the initial current (i/i0) to indicate the fractional decayith time. Pure Pt is easily poisoned by CO-like intermediates

ttF

ig. 8. Current–time plots of (a) PtNi/MWCNT and (b) Pt/MWCNT electrodesn 2 M CH3OH + 0.1 M H2SO4 solutions with a constant potential of 0.66 V vs.g/AgCl.

ormed during methanol activation. The current decay was rapidor Pt/MWCNT electrode (curve b), reaching 45% of the ini-ial current in 10 min and 30% of the initial value in an hour.his means that for Pt/MWCNT, 70% of the Pt surface activeites were lost after 1 h. In contrast, for PtNi/MWCNT electrodecurve a), 75% of initial current in 10 min and 68% of initialurrent after 1 h were retained, which suggests that pairing thei with Pt decreases the opportunity of Pt poisoning. Accord-

ng to the bifunctional catalysis, the mechanism is explaineds follows: from XPS data, the alloy sites consisted mainly ofetallic Pt and Ni(II). Pt(0) is the initial active site for the C–H

ond cleavage in methanol adsorbed on its surface, while Ni(II)s the neighboring site providing mobile oxygen species for thexidation of Pt-bonded CO to form CO2 [38]. The cooperativection between Pt and Ni (with different oxidation states) in theemoval of adsorbed CO intermediates releases the Pt sites forhe next round of action. Therefore, the interface between Pt andi is of utmost importance in the catalysis of methanol electro-xidation, which increases the resistance of the Pt catalysts toO poisoning (as shown in Fig. 7) and then extends its lifetime

as shown in Fig. 8).

. Conclusion

This work suggests a new route of electrodepositing PtNilloy nanoparticles on MWCNTs. On the basis of the char-cterizations by TEM, EDX, XRD and XPS, the as-preparedtNi nanoparticles on the surface of MWCNT consist of 1:1tNi alloy containing mainly Pt(0) and Ni(0), Ni(OH)2, NiOOHnd a minor amount of NiO. The PtNi/MWCNTs were used aslectrocatalysts for the oxidation of methanol and were com-ared with that of Pt/MWCNT. Cyclic voltammetry showedhat the PtNi/MWCNT has a beneficial effect on the oxygendsorption at low overpotentials, and has a higher If/Ib value

ance to CO poisoning of Pt is attributed to effectiveness ofhe neighboring Ni sites in removing the reaction intermediates.urther studies are warranted to understand the detailed deposi-

5 ica A

tti

A

SBN(

R

[[[[

[[

[[

[

[[

[

[[[[[

[[[

[[

[

[[

878 Y. Zhao et al. / Electrochim

ion mechanism, optimize the electrocatalytic efficiency, and taphe potential of this new catalyst dispersion/deposition methodn the direct methanol fuel cells.

cknowledgments

This work is financially supported by National Naturalcience Foundation of China (20473014), the Major Stateasic Research Development Programs (2004CB719903) andNSFC-RGC administrated by the UGC of Hong Kong

N HKUST604/04).

eferences

[1] A. Arico, S. Srinivasan, V. Antonucci, Fuel Cells 2 (2001) 133.[2] J.S. Ye, H.F. Cui, Y. Wen, W.D. Zhang, G.Q. Xu, F.S. Sheu, Microchim.

Acta 152 (2006) 267.[3] T.R. Ralph, M.P. Hogarth, Platinum Met. Rev. 46 (2002) 3.[4] S. Gottesfeld, I.D. Raistrick, S. Srinivasan, J. Electrochem. Soc. 134 (1987)

1455.[5] F. Freund, J. Lang, T. Lehman, K.A. Starz, Catal. Today 27 (1996) 279.[6] J.F. Drillet, A. Ee, J. Friedemann, B. Schnyder, V.M. Schmidt, Electrochim.

Acta 47 (2002) 1983.[7] H.F. Cui, J. Ye, X. Liu, W.D. Zhang, Nanotechnology 17 (2006) 2334.[8] V. Jalan, E.J. Taylor, J. Electrochem. Soc. 130 (1983) 2299.[9] G.T. Glass, G.L. Cahen, G.E. Stoner, E.J. Taylor, J. Electrochem. Soc. 134

(1987) 58.

10] B.C. Beard, P.N. Ross, J. Electrochem. Soc. 137 (1990) 3368.11] S. Mukerjee, S. Srinivasan, J. Electroanal. Chem. 357 (1993) 201.12] S. Mukerjee, S. Srinivasan, M.P. Soriaga, J. Phys. Chem. B 99 (1995) 4577.13] S. Mukerjee, S. Srinivasan, M.P. Soriaga, J. McBreen, J. Electrochem. Soc.

142 (1995) 1409.

[

[[[

cta 52 (2007) 5873–5878

14] M.K. Min, J. Cho, K. Cho, H. Kim, Electrochim. Acta 45 (2000) 4211.15] M.T. Paffet, G.J. Beery, S. Gottesfeld, J. Electrochem. Soc. 135 (1988)

1431.16] K.T. Kim, Y.G. Kim, J.S. Chung, J. Electrochem. Soc. 142 (1995) 1531.17] T. Toda, H. Igarashi, H. Uchida, M. Watanabe, J. Electrochem. Soc. 146

(1999) 3750.18] J. Li, A. Cassell, L. Delzeit, J. Han, M. Meyyappan, J. Phys. Chem. B 106

(2002) 9299.19] G. Tamizhmari, G.A. Capuano, J. Electrochem. Soc. 141 (1994) 968.20] U.A. Paulus, A. Wokaun, G.G. Scherer, T.J. Schmidt, V. Stamenkovic, N.M.

Markovic, P.N. Ross, J. Phys. Chem. B 106 (2002) 4181.21] V. Stamenkovic, T.J. Schmidt, P.N. Ross, N.M. Markovic, J. Phys. Chem.

B 106 (2002) 11970.22] E. Antoli ni, R.R. Passos, E.A. Ticianelli, Electrochim. Acta 48 (2002) 263.23] J. Shim, D.Y. Yoo, J.S. Lee, Electrochim. Acta 45 (2000) 1943.24] N.J. Alonso-Vante, Chem. Phys. 93 (1996) 702.25] Y. Xing, J. Phys. Chem. B 108 (2004) 19255.26] G. Casella, M.R. Guascito, M.G. Sannazzaro, J. Electroanal. Chem. 462

(1999) 202.27] D.J. Guo, H.L. Li, J. Electroanal. Chem. 573 (2004) 197.28] L. Xu, F.L. Li, S.J. Dong, J. Electroanal. Chem. 383 (1995) 133.29] Y. Zhao, L.Z. Fan, H.Z. Zhong, Y.F. Li, S.H. Yang, Adv. Funct. Mater.,

2007, in press.30] S. Bruckenstein, M.J. Shay, Anal. Chem. 188 (1985) 131.31] H. Angerstein-Kozlowska, B.E. Conway, A. Hamelin, L. Stoicoviciu, J.

Electroanal. Chem. 228 (1987) 429.32] H. Prakash, P. Natarajan, J. Photochem. Photobiol. A: Chem. 168 (2004)

81.33] B. Wu, H. Lei, C. Cha, Y. Chen, J. Electroanal. Chem. 377 (1994) 227.34] D. Rand, R. Woods, J. Electroanal. Chem. 31 (1971) 29.

35] J. Mathiyarasu, A.M. Remona, A. Mani, K.L. Phani, V.J. Yegnaraman,

Solid State Electrochem. 8 (2004) 968.36] A. Anderson, Electrochem. Acta 47 (2002) 3759.37] Y. Lin, X. Cui, C. Yen, C.M. Wai, J. Phys. Chem. B 109 (2005) 14410.38] F. Liu, J.Y. Lee, W. Zhou, J. Phys. Chem. B 108 (2004) 17959.