RARE METALSVol. 24, No. 1, Mar 2005, p.33Platinum-polyaniline-modified carbon fiber electrode forthe electrooxidation of methanolWU Kezhong'2), MENG Xu), WANG Xindong), and LI Jingling')1) Mtallurgical and Ecological Engineering School, University of Science and Technology Beiing, Bejing 100083, China2) Department ofChemisty, Hebei Normal University, Shjjazhuang 050016, China(Received 2004-04-05)Abstract: Platinum was electrodeposited onto a polyaniline-modified carbon fiber electrode by the cyclic voltammetricmethod in sulfuric acid, which may enable an increase in the level of platinum utilization curently achieved in electrocata-lytic systems. This electrode preparation consists of a two-step procedure: first electropolymerization of aniline onto carbonfiber and then electrodeposition of platinum. The catalytic activity of the platinum-polyaniline-modified carbon fiber elec-trode (Pt/PAni/C) was compared with that of a bare carbon fiber electrode (Pt/C) by the oxidation of methanol. The maxi-mum oxidation current of methanol on PVPAni/C is 50.7 mA.cmi 2, which is 6.7 times higher than 7.6 mA:.cm 2 on the PUC.Scanning electron microscopy was used to investigate the dispersion of the platinum particles of about 0.4 um.Key words: physical chemistry; electrodeposition; polyaniline; catalytic oxidation; platinum utilization[This project was financially supported by the National Natural Science Foundation of China (Nos. 50074003 and50274010) and the National High- Tech Research and Development Program of China (863 Program)]1Introductionchromic effects, well-behaved electrochemistry, andmoderately high conductivity in the doped form [3]It is well known that the highly dispersed plati-have made it a candidate of great technologicalnum catalyst has been widely employed as the anodepromise to modify electrodes.for the electrooxidation of methanol in the directIn the present work, platinum particles were elec-methanol fuel cell (DMFC) [1]. To make the mosttrodeposited onto a bare carbon fiber electrode (Pt/C)efficient use of a catalytic metal, it is desirable toand a polyaniline-modified carbon fiber electrodeobtain the highest degree of metal dispersion. Re-(Pt/PAni/C) by the cyclic votammetric method. Thecently, the deposition of Pt particles onto conductingcatalytic activity of both electrodes was studied bypolymer-modified electrodes has gained wide inter-he electrooxidation of methanol in sulfiuric acidest for catalytic purposes [2], because the electro-electrolyte. Scanning electron microscopy (SEM)polymerization of some organic monomers are gen-was employed to give information about the mor-erally homogeneous, strongly adherent to the elec-phology and nature of the deposited electrocatalysttrode surface, and chemically stable. The electro-particles.polymerization obtained enlarges the effective sur-face area by diminishing the size of Pt particles.2 ExperimentalPolyaniline (PAni) occupies the most importantplace among the organic conducting polymers. ItsAllwere performed中國煤化工unique properties such as ease of preparation inwith a2ton Applied Re-aqueous medium, good stability in air, simplicity insearchTYHCNMHGby using adoping，improved electronic properties, electro-two-compartment glass cell with a three-electrodeCorresponding author: WANG XindongE-mail: echem@ustb.edu.cn34RARE METALS, Vol. 24, No.1, Mar 2005configuration at 25°C. The working electrode wascreased slightly under repeated potential scanning,carbon fibers, which were extra-high-strength poly-the oxidation current at about -0.12 V grew almostacrylonitrile-based fibers manufactured in tows ofcontinuously under this condition. This characteristic500 and 1000 flaments. The diameter of a singlewave indicates that the hydrogen adsorption reactionfilament was 5 um. During the manufacture of theoccurs at the surface of the Pt deposited on the car-carbon fibers, the filaments were dip-coated with anbon fiber eletrode. The large overpotential (-0.15 Vepoxy resin, the so-called sizing step, in order to im-to 0.25 V) observed in the first potential scan shouldprove their handling properties. The geometric areabe due to the formation of Pt nuclei on the bare car-of the electrode was 0.5 cm2. A platinum disc wasbon fiber surface. After the formation of these nuclei,used as the counter electrode and a saturated calomelthe electrodeposition of Pt occurs on the Pt surfaceelectrode (SCE) as the reference electrode. All po-at more positive potentials. The behavior shown intentials were presented on the SCE scale. The sub-figure 1 seems to be a typical example of nucleationstrate-working electrode was washed with twice dis-nd growth of the electrodeposition. Note that hy-tilled water. The surface morphology of the elec-drogen is evolved rapidly at potentials more negativetrodes was observed by JSM-6301F model SEMhan -0.21 V. Furthermore, a poorly defined wave(LEO-1450, UK).was observed at -0.15 V as a shoulder of the hydro-All chemicals were purchased from Beijinggen evolution reaction, which should be due to aChemical Reagent Company. Aniline was purifiedfurther reduction step of PtCl2- 。Similar resultsby ditillation with zinc dust and stored under nitro-have been reported by Itaya [4].gen gas. Other reagents were of analytical grade and0.005used as received. The solutions were prepared withtwice distilled water.0.0002(3 Results and discussion5 -0.0053.1 Cyclic voltammetry of the electrodeposition ofP-0.010Platinum particles were electrodeposited onto- -0.015both the bare carbon fiber and the polyaniline-0.250.50 0.75modifed carbon fiber in 5 mmol/L H2PtCl6+ 0.5E/V vs. SCEmol/L H2SO4 solution by cyclic voltammetry (CV)Figure 1 Cyclic voltammograms recorded at a barebetween -0.2 and 0.55 V at 50 mV/s for 20 cycles.carbon fiber electrode.Figure 1 shows typical CV curves of the electro-deposition of Pt obtained on the bare carbon fiberThe PtPAni/C electrode was prepared in twoelectrode (recorded the lst, 2nd, 5th, 11th, 18th, andsteps: first electropolymerization of aniline and then20th cyclic). In the first negative scan from 0.55 V,electrodeposition of platinum. The electropolymeri-the current due to the reduction of PtCl2- com-zation of 0.2 mol/L freshly distilled aniline, dis-menced at about 0.3 V. However, larger reductionsolved in 0.5 mol/L H2SO4, was carried out using thecurrents were observed in the reversible scan. Thiscyclic voltammetric technique at 50 mVs -1 betweenenhancement was observed clearly in the second-0.2 and 0.85 V for 10 cycles. The film thicknesspotential scan, where the current commenced at(0.31 μum) can be calculated by the height of the firstabout 0.4 V. Note that the onset potential of0.4 V isanodic peak in the redox process of polyaniline, asin fair accord with the equilibrium potential of thereporte中國煤化工experiment forreduction reaction of Pt+. Although the curent dueplatinulied out using a!Y;CNMH(to the reduction reaction of Pt+ described above in-polyaninc-lulou calvuIl IIUCI electrode， asWu K.Z. et al, Platinum- polyaniline-modified carbon fiber electrode for the electrooxidation of methanol 35shown in figure 2 (the lst, 2nd, 5th, 6th, 18th, andtheir size and distribution. Figure 3 shows the SEM20th cyclic). The behavior is very similar to that ob-micrographs of platinum catalyst particles that havetained at the bare carbon fiber electrode. However,been deposited on the bare carbon fiber and thethe reduction current corresponding to the depositionpolyaniline-modified carbon fiber by cyclic potentialof Pt is reduced by about one-fourth of the current(-0.2 to 0.55 V). It is very different that the platinumobserved at the bare carbon fiber electrode. This ef-particles are distributed on both electrodes, suggest-fect has been explained by Donnan rejection becauseing that the formation of nuclei and the growth ofpolyaniline is a conducting film [4] and has conju-platinum particles are influenced markedly by thegated double bonds, which promotes delocalizationelctrode surface. There are less active sites of theof electrons. In addition, it is noteworthy that thereformation of nuclei on the surface of the bare carbonare two reduction peaks (0.24 and -0.07 V) and twofiber electrode, so the growth of platinum particlesoxidation peaks (0.47 and 0.18 V) in the first scan,accordingly is limited. It is seen from figure 3(a) thatthis may be associated with redox of polyanilinethe diameters of platinum particles are from 50 nmbetween -0.2 and 0.55 V in acidic media [6]. How-to 1 um, and the diameters increase with increasingever, the peaks (0.47 and 0.24 V) of redox of poly-platinum loading on the surface of the bare carbonaniline are inhibited from the second scan because offiber electrode. Nevertheless, the platinum particlesthe deposition of Pt. Then, the formation of Pt nucleiare distributed in an almost homogeneous manner aton the polyaniline-modified carbon fiber surface wasthe surface of the polyaniline-modified carbon fiberobserved by the large overpotential (0.25-0.1 V) inelectrode in approximately spherical-like shape withthe second potential scan.diameters of about 0.4 um. Polyaniline as the con-ducting polymer can augment the active sites of the0.010formation of Pt nuclei on the surface of carbon fibers.0.005This result shows more homogeneous and small size0.000(a之-0.005-0.010-0.015-0.25 0.000.250.50 0.75E1V vs. SCEFigure 2 Cycdlic voltammograms recorded continu-ously at a polyaniline-modified carbon fiber electrode.(b)On the basis of the result shown in figures 1 and 2,a typical procedure of the potential limits was em-ployed for the electrodeposition as follows: the po-tential limits were 0.55 and- -0.2 V for successivepotential cycles, respectively. Under these limitedconditions, it is reasonably expected that nucleationoccurs in the initiatory potential cycle and Pt parti-cles grow in the successive repeated scans.3.2 SEM characterizationFigure中國煤化工surfaces of theSEM is used to provide morphological informa-barMYHCN M H Gd the platinum-tion of the dispersed platinum particles, especiallypolyaniline-modifed carbon fiber electrode (b).30RARE METALS, Vol. 24, No.1, Mar 2005microparticles on the Pt/PAni/C than that on thelytic activity for methanol oxidation is.Pt/C under the same condition.3.3 Cyclic voltammograms of the electrocatalytic4 Conclusionsoxidation of methanolThe platinum-polyaniline-modified carbon fiberA strong dependence of the platinum catalytic ac-electrode enlarges the effective surface area by di-tivity on the catalyst particle size and its distributionminishing the size of Pt particles. The electrocata-within the matrix of conducting polymers is markedlytic activity of the PtPani/C electrode for methanolfor the oxidation reaction of small molecules [7].oxidation is connected to the platinum and real sur-The electrocatalytic activity of the P/C andface area. Also, the relationship between distributionPtPAni/C electrodes is studied for the oxidation ofand electrocatalytic activity can be established frommethanol. The electrocatalytic oxidation of methanolhe oxidation current of methanol. The maximumwas examined in 0.5 mol/L CH3OH + 1 mol/Loxidation current of methanol on PtPAni/C is 50.7H2SO4 solution by a cyclic voltammetry technique atmA.cm-，which is 6.7 times higher than 7.650 mV:s- between -0.2 and 0.85 V. The cyclicmA.cm~- on the Pt/C. Scanning electron microscopyvoltammograms of the electrocatalytic oxidation ofwas used to investigate the dispersion of the plati-methanol with the two different electrocatalytic elec-num particles of about 0.4 um.trodes are shown in figure 4. It is seen that twoelectrodes exhibit electrocatalytic activity for theReferencesmethanol oxidation, while the anodic peak currentheight of methanol oxidation, which appears in the1]Thompson S.D, Jordan L.R, and Forsyth M, Plati-positive going sweeps, changes greatly as a functionnum electrodeposition for polymer electrolyte mem-of the platinum particle size and distribution of thebrane fuel cell, Electrochim. Acta, 2001, 46: 1657.electrodeposition. The homogeneous and higher2] Mikhaylova A.A, Molodkina E.B, Khazova O.A.,and Bagotzky V.S, Electrocatalytic and adsorptiondispersion (i.e. high surface area of active sites) ofproperties of platinum microparticles electrodepos-the platinum particles will be of benefit to the ad-ited into polyaniline films, J. Electroanal. Chem,sorption and oxidation reaction of methanol, corre-2001, 509: 119.sponding to higher oxidation current densities in3] Rajendran V., Gopalan A., Vasudevan T., Chen W. C,figure 4, and the curent peak height of methanoland Wen T C., Growth behavior of polyaniline filmsoxidation (at 0.64 V) also increases from 7.6 to 50.7deposited by pulse potentiostatic method [], MatermA.cm- 2 (figure 4). In other words, the greater theChem. Phys, 2000,65:320platinum distribution is, the higher the electrocata-4] Itaya K, Takahash H, and Uchida I, Electrodepodi-tion of Pt ultramicroparticles in nafion films on0.075p一Pt/PAni/Cglassy carbon electrodes, J. Electroanal. Chem, 1986,-.. Pt/C208: 373.5] Stilwell D.E. and Park Su-Moon, Electrochemistry信0.025of conductive polymers, J. Electrochem. Soc, 1998,135 (10): 2491.0.0006] Zotti G, Cattarin S, and Comisso N, Electrodeposi--0.025tion of polythiophene, polyprrole and polyaniline bythe cyclic potential sweep method, J. Electroanal.-0.0520.50.00.51.0Chem, 1987, 235: 259.7] Frelink T. Visscher W., and Van Veen J.A.R., Parti-E/V vs. SCEcle中國煤化工platinum catalystsFigure 4 Cyclic voltammograms of the electrocata-Y片CNM H G°>l, J Electroanal.lytic oxidation of methanol on Pt/C and PtPAni/C elec-o山. vw.trodes.