Electrocatalytic oxidation of methanol on carbon-nanotubes/graphite electrode modified with platinum

Available online at www.sciencedirect.comsCIENCE dDIRECToTransactions ofNonferrous MetalsSociety of ChinaScienceTrans. Nonferrous Met. Soc. China 17(2007) 214- -219Presswww.csu.edu.cn/ysxb/Electrocatalytic oxidation of methanol on carbon-nanotubes/ graphite electrodemodified with platinum and molybdenum oxide nanoparticlesGAN Yong ping(甘永平), HUANG Hui(黄辉), ZHANG Wen-kui(张文魁)College of Chemical Engineering and Materials Science, Zhejiang University of Technology,Hangzhou 310032, ChinaReceived 11 November 2005; accepted 28 September 2006Abstract: Electrochemical codeposition and electrocatalytic properties of platinum and molybdenum oxide nanoparticles (Pt-MoO,)on carbon-nanotubes/ graphite electrode for methanol oxidation were invetigated. The micrograph and elemental composition of theresulting Pt-MoO/CNTs/graphite electrode were characterized by scanning electron microscopy(SEM) and energy dispersive X-rayspectroscopy(EDS). The results show that the Pt-MoOx particles with the average size of about 50 nm are highly dispersed on theCNTs surface. The Pt-MoO/CNTs/graphite electrode delivers excellent electrocatalytic properties for methanol oxidation. Thehighest mass activity(Am) reaches 264.8 A/g at the loading mass of 159.3 ug/cm2. This may be atrbuted to the small particle size andhigh dispersion of Pt-MoOx catalysts deposited on the CNTs surface. The kinetic analysis from electrochemical impedancespectroscopy(EIS) reveals that the existed MoO、phase can improve the chemisorptive and catalytic properties for methanoloxidation.Key words: methanol oxidation; carbon nanotubes; molybdenum oxides; nanoparticles; fuel cellsRecently, noble metal catalysts intermixed with1 Introductioninorganic oxides are of considerable interest fcatalysis[8- 10]. The oxide is used to physically separateThere is an increasing interest in the development ofthe catalytic particles and decrease their agglomerationthe direct methanol fuel cell(DMFC) due to its possibleate. Meanwhile, the oxide is believed to modify theuse in electric vehicles[1-2]. The performance of DMFCelectronic nature of the Pt particles, thus afecting theirhas improved markedly in the past years. Despite manychemisorptive and catalytic properties. The improvementefforts devoted to the DMFC development, there stillof CO-tolerance or catalytic activity foremain problems to be overcome in terms of efficiencyoxidation on the Pt catalyst by the addition of certainand power density. One of the reasons is the relativelykinds of oxides such as WO3, MoO3 and TiO2 makesslow kinetics of methanol oxidation reaction at the anode. such system attractive for the application inPlatinum has high activity for methanol oxidation andDMFC[1-14]. On the other hand, to apply thesewas used as anode catalysts for many years[3- -4].catalyst systems in practical uses， the supportingHowever, Pt catalyst will be poisoned by the adsorbedmaterials with high surface area are necessary tocarboxyl species derived from methanol oxidation. Idisperse catalyst particle and reduce the Pt loading underorder to solve these problems, Pt-based binary andthe condition of keeping the high catalytic activity.ternary metallic catalysts have been investigatedRecently，carbon nanotubes(CNTs) are promisingextensively. The Pt-Ru binary catalyst is commonlysupporting materials because of their interestingacepted as the best catalyst for methanol oxidation[5- -7].However, there is a signifcant problem concerning theResearchers have shown much interest in producingrelatively sparsity of Ru. The fraction of Ru in Pt-groupCNTs-supported Pt catalyst for oxygen reduction,metal resources is less than 10%, which is much smallerhydrogen and methanol oxidation reactions[15- 19].than that of Pt. Therefore, it is important to develop a However, to中国煤化工，work aboutnew anode catalyst without Ru species.Ptoxides (especMHCNM HGited on CNTsCorresponding author:Tel/Fax: +86-571-88320394; E-mail: hhui@zjut.edu.cn.GAN Yong-ping, et al/Trans. Nonferrous Met. Soc. China 17(2007)215electro-catalyst for methanol oxidation.and scanning electron microscopy(SEM) (Hitachi S-4700)In this paper, molybdenum oxide (MoOx) wasequipped with energy dispersive X-ray spectroscopyexamined as a potential oxide phase to improve the(EDS) (Vantage EST, NORAN).electrocatalytic properties of Pt catalyst for methanolThe electrocatalytic performance of Pt-MoO/CNTs/oxidation. We employed a simple electrochemicalgraphite electrode for methanol oxidation was evaluatedcodeposition method to disperse the Pt-MoO、by CV in a solution of 1.0 mol/L CH;OH in 0.5 mol/Lnanoparticles on CNTs support. The advantage of thisH2SO4. Electrochemical impedance spectroscopy(EIS)method is the mixing of metal and oxide on awas measured using an EG&G PARC Model 5210microscopic level, simple procedure for deposition andlock-in amplifier interfaced to Model 273 potentiostat.easy control of the loadingmass. TheThe impedance data were collected in the AC frequencyPt-MoO/CNTs/ graphite electrode is expected to improverange form 100 kHz to 0.1 Hz with an excitation signalelectrocatalytic properties for methanol oxidation due toof 10 mV. All experiments were carried out at ambientthe metal-oxide interaction and high surface area of thetemperature. All the potentials were referred to the SCE.catalyst particles with the low loading level.3 Results and discussion2 Experimental3.1 Elctrochemical codeposition of Pt-MoOx nano-2.1 Preparation of CNTs/graphite electrodeparticles on CNTs/graphite electrodeThe CV curves of Pt-MoO, nanoparticles grown onThe graphite electrode was sequentially polishedwith 1.0, 0.3 and 0.05 μum alumina emery paper, until athe CNTs/graphite electrode are shown in Fig.1. Thmirror-like surface was obtained. Then it was placedinner to outer curves correspond to the first and third CVunder ultrasonic conditions in double distilled water anddone continuously at 20 mV/s. The peak currentsethanol. The CNTs used in this work were obtained fromincrease with the increase in the number of cycles, whichChengdu Institute of Organic Chemistry and the puritymeans that the particles are gradully grown on thewas more than 95%. Further purification wassurface of the CNTs/graphite electrode. In the potentialaccomplished by ultrasonically agitating the CNTs inrange from 0.7 V to -0.10 V, there are one reductionconcentrated nitric acid at 25 C for 24 h. 2.5 mg ofeak at 0.062 V (peak a) and two correspondingacid-treated CNTs was dispersed in 10 mL acetone withoxidation peaks located at 0.225 V (peak a'), and 0.617 Vthe aid of ultrasonic agitation to give 0.25 mg/L black(peak a"). These peaks may be ascribed to the redoxsuspension, and cast on the surface of graphite electrode.reactions of molybdate species. Additionally, a pair ofFinally, the solvent acetone was evaporated in air to formpeaks (b/b) at the potential of about- -0.20 V areattributed to the deposition of Pt particles and hydrogena CNTs/ graphite electrode.adsorption/desorption on the deposited Pt surface.2.2 Co-deposition and electrochemical studies of Pt-MoOx nanoparticles on CNTs/graphite electrode-10-Using a standard three-electrode cell, Pt-MoOxnanoparticles were electrodeposited on a CNTs/graphiteelectrode from 2.2 mmol/L H2PtCl6+0.2 mol/L Na2MoO4+2.2 mol/L H2SO4 solutions by cyclic voltammetry(CV)吉ξ 10一under the condition of scan potential between 1.05 V and-0.25 V (vs SCE) and scan rate of 20 mV/s. An EG&GM273 potentiostat was employed for the deposition and20个electrochemical studies of Pt-MoOx nanoparticles. ThCNTs/graphite with a definite area of 1.0 cm2 was used30七as the working electrode. A platinum foil served as the00.1.2counter electrode and a saturated calomel electrode(SCE)φ(vs SCE)/Vwas used as the reference electrode. The loading mass ofFig.1 Cyclic voltammograms of Pt-MoO, particles grown onPt-MoOx catalyst was determined from the mass gain ofCNTs/graphite electrodeCNTs/graphite electrode after deposition. Th中国煤化工micrograph and elemental composition of theThe SEM"NTs/graphiteCNMH GPt-MoO,/CNTs/graphite electrode were investigated byelectrode in Fil.MYH.。ui-uuensional webtransmission electro microscopy(TEM) (Philip 200 UT)structure. The Pt-MoO， nanoparticles are deposited.GAN Yong ping, et a/Trans. Nonferrous Met. Soc. China 17(2007)216uniformly on the surface of the CNTs. The averageparticle size is about 50 nm. Also, the TEM image ofPt-MoO/CNTs particles is inserted in Fig.2(a). The Ptparticles with size of about 20- -40 nm are dispersed inPt-MoO,/graphite electrode under the same depositioncondition is shown in Fig.2(b). The sizes of Pt-MoOxparticles deposited on the graphite electrode are muchlarger, with diameters typically in the range of 100 -200nm. The interesting three- dimension structure, smallparticle size and high dispersion of Pt-MoO/CNTs121620composite may result in large valuable suface area andEnergy/keVgood electrocatalytic properties for methanol oxidation.Fig.3 EDS pattern of Pt-MoO/CNTs/graphite electrode (Pt-MoO, loading 62.7 ug/cm2)@●Pt-4550 nm-1501一15.0kVSEM5-0L-0.40.40.1.o(vs SCE)/VFig.4 CV curves of CNTs/graphite 1 and Pt-MoO/CNTs/graphite 2 electrodes (Pt-MoOx loading 159.3 ug/cm2 ) at 20mV/s in 0.5 mol/L H2SO4+ 1.0 mol/L CH;OH solutionsand Pt-MoOJ/CNTs/graphite electrodes in 0.5 mol/LH2SO4+1.0 mol/L CH;OH solutions. From curve 1 inzJUT 15.0kV 13.5mm SEMFig.4, it can be seen that the background current of theFig.2 SEM images and TEM image (inserted) ofCNTs/graphite electrode, which is the nature of thePt-MoO/CNTs/graphite electrode (Pt-MoOx loading 62.7double-layer capacitance, is large due to the high surfaceμg/cm2) (a) and Pt- MoO, graphite electrode (Pt-MoOx loadingarea of CNTs. Additionally, a pair of broad redox peaks57.4 ug/cm2)(b)between 0.3 V and 0.5 V can be observed. This maycorrelate with the redox behavior of the carboxylic acidFig.3 shows the EDS pattern of Pt-MoO/CNTs/groups (such as一(COOH)ads and一(OH)ad)[20]. Nographite electrode. This indicates that Pt, Mo are thecurrent peak of methanol oxidation is observed,major elements plus C and O. The quantitative EDSindicating that the CNTs/graphite electrode has noanalysis further shows that the Pt/Mo molar ratio is closeobvious electrocatalytic activity for methanol oxidation.to 3:2. In addition, we also tried to characterize theFor the Pt-MoO/CNTs/graphite electrode, however, twoPt-MoOJ/ CNTs/graphite electrode by X-raycurrent peaks of methanol oxidation are observed indiffraction(XRD) analysis, however, the MoOx phasecurve 2 in Fig.4. The oxidation peak(Ep) obtained fromwas not identified probably due to the low crystallinity ofthe positive-going scan is due to methanol oxidation, asthe electrodeposited MoOx.shown in Fig.5， its peak current density(Jp) isproportional to中国煤化工n rate (v),3.2 Electrochemical properties of Pt-MoO,/CNTs/suggestingYHCNMHGanol at thegraphite electrodePt-MoOJ/CNTs/g1 apuue ciecuvue liay ue controlled byFig.4 shows the CV curves of the CNTs/graphitediffusion process..GAN Yong ping, et a/Trans. Nonferrous Met. Soc. China 17(2007)217350300Pt-M0OJ/CNTs/graphite的250-个70之200-P/CNTs/graphite150-二5040-是100Pt-MoOJ/graphite503(011520255100150200 250 300v12/(mV.s-1)I/2Loading mss/(ug.cm-2)Fig.5 Dependence of peak current density (p) obtained fromFig.6 Relationship between mass activity and loading mass forpositive-going CV scan on v12 (Pt-MoO, loading 159.3 ug/cm2)methanol oxidation (Scan rate ofCV is 20 mV/s)3.3 Comparison of PtMoO,/CNTs/graphite， Ptmethanoloxidation,electrochemicalimpedanceMoO,/graphite and Pt/CNTs/graphite electrodesspectroscopy(EIS) was carried out on the Pt-MoOJFor comparison, the mass activit(Am) of the CNTs/graphite electrode at the potentials of 0.55 V andcatalysts is defined by peak current density per unit of0.70 V, respectively. For comparison, the EIS of theloading mass and calculated by the following equation: .Pt/CNTs/graphite electrode is also presented in Fig.7.The EIS shape of both electrodes is similar, but the sizeAg='rx103(1of primary semicircle on the Pt-MoO/CNTs/graphitemelectrode is smaller than that of the Pt/CNTs/ graphitewhere Jp (mA/cm) is obtained by the peak currentelectrode. This implies that the Pt-MoOJ/CNTs/graphiteobtained from the forward CV scan divided by exposedelectrode provides fast reaction kinetics for methanolsurface area of the electrode, and ma (μg /cm2) is loadingoxidation. At the potential of 0.55 V, the impedance plotmassof the Pt-MoO、 catalysts for Pt-MoO/CNTs/of the Pt-MoO/CNTs/graphite electrode exhibits agraphite, Pt-MoOJ/ graphite electrode, or the Pt catalystsclockwise capacitive-inductive loop (Fig.7(a)). Thefor Pt/CNTs/graphite electrode. Fig.6 shows theinductive behavior in low frequency range reveals thatrelationship between Am and md for methanol oxidation.the COads coverage decreases with increasing potential,It is clear that the Am of the Pt-MoO/CNTs/graphiteand the decreasing COads leads to an increase of Faradaicelectrode is much higher than that of the Pt-MoO/current[21]. According to bifunctional mechanism ofgraphite electrode at both low and high ms. Themethanol oxidation, a reasonable explanation is that withmaximum of Am (264.8 A/g) is obtained at m=159.3increasing potential, the large amounts of OHads areμg/cm2. As shown in Fig.2, the particle size of theformed on MoOx sites and react with COads and decreasePt-MoOx catalyst on the CNTs/graphite electrode is aboutits coverage. The decreasing surface coverage of COads50 nm, which is much smaller than that on the graphitewill contribute to the adsorption of methanol on the Ptcatalyst and enhance the Faradaic current. The similar(about 100- 200 nm). The smaller the particle size is, theinductive behavior in EIS at about 0.55 V is alsobigger the specific surface area is, and so higher theobserved recently at the PtRu/C anode catalyst of acatalytic activity is. The results show that the particleDMFC[22- -23]. Additionally, it is worth mentioning thatsize of the Pt-MoO、 catalysts and nature of thehe inductive response in low frequency at thesupporting materials are very important factors on thePt/CNTs/graphite electrode is not obvious. This meanscatalytic activity. And also, the Pt-MoO/CNTs/graphitethat molybdenum oxide contributes to the formation ofexhibits higher Am compared with the Pt/CNTs/graphitechemisorbed hydroxyl, and may result in highelectrodes at high ms. This indicates that the codepositionelectrocatalytic activity.of Pt-MoOx oxide can improve the catalytic activity ofPtFor methanol oxidation at high potential (0.70 V),catalyst.the impedance plots of both electrodes are drasticallychanged. The中国煤化Ind the origin3.4 EIS of methanol oxidation on Pt-MoO,/CNTs/anti-clockwiseHCNMHC°thescondgraphite electrodequadrant with tic 1calHUIClll U1 uueIn order to further investigate mechanism ofbecoming negative (see Fig.7()), which is probably due.GAN Yong-ping, et al/Trans. Nonferrous Met. Soc. China 17(2007)2185Pt+CHzOH- - Pt(CO)u+4Pt- Haa(2)80- (a)φ=0.55VPt+MoO,+H2O-→(MoO,)- -OHad+Pt- -Had(3)-Pt-MoOJ/CNTs/graphite-P/CNTs/graphite5Pt- Had- →+ 5Pt+5H*+5e(4)是40Pt(CO)ad+(MoO,)- -OHd→CO2+H*+Pt+MoO.+e (5)4 Conclusions1) The Pt-MoO/CNTs/graphite electrode shows an-2excellent electrocatalytic activity for methanol oxidation.2405080 100 120This may be attributed to the small particle size and highZrea/92dispersion of Pt-MoO, catalysts on the suface of CNTs.2) The mixing of Pt and MoOx nanoparticles on a60F(b)φ=0.70V●一Pt-MoO/CNTs/graphitemicroscopy level provides a modification of the40-。一PUCNTs/graphitechemisorptive and surface catalytic properties for20methanol oxidation.3) The Pt-MoO/CNTs/ graphite electrode has good“1applications in DMFC because of the low loading massof Pt-MoOx catalysts and high catalytic activity for-20-40References-80-60-40-20 020 40 6([1] WITHAM C K, CHUN W, VALDEZ T I, NARAYANAN s R.Performance of direct methanol fuel cells with sputter-depositedFig.7 Nyquist plots of EIS for methanol oxidation onanode catalyst layers []. 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