Carbon nanotubes-Nafion composites as Pt-Ru catalyst support for methanol electro-oxidation in acid

Available online at www.sciencedirect.comJOURINALOFScienceDirectNATURAL GASSNGCJ CHEMISTRYEL SEVIERJoumal of Natural Gas Chemistry 18(2009)199-204www.elsevier.com/locate/jngcCarbon nanotubes-Nafion composites as PtRu catalyst support formethanol electro-oxidation in acid mediaShengzhou Chen'*, Fei Yel2，Weiming Lin',2I. School of Chemistry and Chemical Engineering, Guangzhou Universit, Guangzhou 510006, Guangdong, China;2. School of Chemical and Energy Engineering, South China University of Technology% Guangzhou 510640, Guangdong. China.(eceived February 23, 2009; Revised May 4, 2009; Available online June 22, 2009 ]AbstractCarbon nanotubes-Nafion (CNTs-Nafion) composites were prepared by impregnated CNTs with Nafion in ethanol solution and characterizedby FT-IR. Pt-Ru catalysts supported on CNTs-Nafion composites were synthesized by microwave- asisted polyol process. The physical andelectrochemical properties of the catalysts were characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), Costipping voltammetry, cyclic voltammery (CV) and chronoamperomety (CA). The results showed that the Nafion incorporation in CNTS-Nafion composites did not significantly aler the oxygen-containing groups on the CNTs surface. The Pr-Ru catalyst supported on CNTs-Nafioncomposites with 2 wt% Nafion showed good dispersion and the best Co oxidation and methanol electro oxidation activities.Key wordscarbon nanotubes-Nafion composites; Pt-Ru catalysts; methanol electro oxidation1. Introductionbe not only accessible to the reactants, but also electricallyconnected to the current collectors, and ionically connectedto the electrolyte membrane [5]. A typical DMFCs electrodeDirect methanol fuel cells (DMFCs) have received a lot ofis prepared by painting or spraying catalyst inks that containattention as the promising power sources due to high energya matrix of electrolyte phase (usually Nafion ionomer) anddensity, simple and easily handled structure, easy transporta-carbon-supported catalyst on either the gas diffusion layer ortion and storage of cheap liquid fuel as well as low tempera-the electrolyte membrane [1]. Usually, the utilization of Ptture operation [1,2]. They can be widely used in electric mo-is only about 20% [6] and the dispersion of Nafion ionomertors or portable devices, such as laptops, MP4, mobile phones,in the catalyst layer is not uniform [7]. Since, the sulfonicand so forth [3]. In the anode of a DMFC, methanol is oxi-acid micelles in Nafion ionomer have an approximate diam-dized with water to form carbon dioxide, electrons and pro-eter of 4- 200 nm [8], the Pt particles located in carbon poretons, which transport through the proton exchange membranewith the diameter less than 4 nm are ionically inaccessible.to combine with oxygen and electron in the cathode to formOn the other hand, the aggregation of Nafion on the Pt par-water, so the current is produced in the outer circuit when theticle surface will form a barrier to the reactants, loweringelectrons move from the anode to the cathode [4]. However,the electrochemically active sites of the catalyst. Therefore,one of the most serious barriers for the widespread commer-these electrodes show poor electrolyte contact or poor reac-cialization of DMFCs is the low electro-oxidation activity oftion diffusion. To overcome these disadvantages, one mustmethanol in the anode, and so large amount of precious metalincrease the catalyst loading, causing to high cost of the elec-Pt or Pr-containing alloy is needed. Three transport processestrode materials.involve in the anodic electro-oxidation of methanol, i.e, elec-To solve these problems, the electrolyte is first mixed withtron transport, proton transport (H+), chemical species trans-carbon support to form carbon-Nafion composites before theport (methanol, CO, CO2, etc.) and thus an efficient electrodepreparation of the catalyst. Pt or P1-Ru were electrodepositedshould maximize the three-phase boundary region. Gener-on the Nafion-bonded carbon [9} or cryogel carbon-Nafionally it is recognized that the effective electrocatalysts shouldsuppor中国煤化Ilization due to the de-●Coresponding author. Te: +686039366507; Pax: +6020-39366507; E.mal:|Y片This work was supported by National Natural Science Foundation of China NO.C N M H Grtholoy Dprtmear ofGuangdong Province (NO.2008B010800036; NO.20088010800037)CoprightO2009, Dalian Insitue of Chemical Physics, Chinese Academy of Sciences. All rights reseved.doi:10.1016/S1003-9953(08)60108-5200Shengzhou Chen et al./ Joural of. Naural Gas Chemisty Vol. I8No.2 2009position of Pt particles taking place in the three phase reactionwith 50 ml ethylene glycol in a two-necked flask. Aboutzone. Recently, Park et al. [11] found that the incorporation70 mg CNTs-Nafion composites were added to the mix-of ionomer in the carbon support enhances the contact areature, and its pH value was adjusted to 10 by adding 0.5 Mbetween the catalyst particles and reduces the total amount ofNaOH in ethylene glycol. The solution was ultrasonicatedionomer required in the anode catalyst layer, which facilitatesfor 30 min and placed in an experimental microwave oventhe mass transport of reactants and products. Scibioh et al. (Nanjing Jiequan Microwave Equipment Co, Ltd, 700 W) and[12] reported that the ionomer-coated carbon supports can ex-heated for 3 min. The solution was then cooled, filtered andtend the three-phase boundary and so enhance the utilizationwashed with ethanol and ultra-pure water for 6 times. Finally,of the catalyst particles. In addition, Nafion is also a nega-the Pt-Ru/CNTs catalysts were dried in a vacuum oven attively charged polymer, and it can act as a stabilizer during80°C forSh. .the preparation of Pt-Ru/C catalyst to improve the methanolelectro-oxidation performance [I3].2.3. Characterization of suppor and catalystsCarbon nanotubes (CNTs) show highly electrochemicallyaccessible surface area and offer a remarkable electronic con-Fourier transfom infraredspectroscopy of theductivity. They have been used as catalyst supports forCNTS and CNTs-Nafion composites was obtained on afuel cells [14,15]. Previous studies have showed that CNTssupported catalysts exhibit better performance of methanolThermo Nicolet 380 spectrometer in 400- 4000cm-1 range.electro-oxidation as compared to conventional carbon blackThe samples were mixed with KBr and well ground before be-(XC-72) supported catalysts [16- 19]. In the present study,ing pressed into wafers. X-ray diffraction (XRD) patterns ofCNTs were impregnated with Nafion in ethanol solution tothe catalysts were obtained on an XD-3 X-ray diffractometerform CNTs-Nafion composites, which were used as the sup-(Beijing Purkinje General Instrument Co, Ltd) using a Cu-K。port of Pt-Ru catalysts for methanol electro-oxidation. TIsource operating at 36 kV and 20 mA. The scanning range andeffect of Nafion content on the oxygen-containing groups inrate are 10°~90° and 2%/min, respectively. The morphologyCNTs and the performance of the methanol electro-oxidationanalysis of Pt-Ru/CNTs-0 and Pt-Ru/CNTs-2 catalysts wasconducted by a transmission electron microscope (TEM) ofwas investigated.JEOL, JEM-100CXII operating at 200 kV.2. Experimental2.4. Electrochemical measurement of the catalysts2.1. Preparation of CNTs-Naf ion composites .The electrochemical measurements were performed atMulti- walled CNTs with purity higher than 95% wereroom temperature using a conventional three-electrode cellpurchased from Chengdu Organic Chemicals Co. Ltd. Theand a Solartron SI 1287 electrochemical interface. A Pt meshmain range of diameter of the CNTS was 8~15 nm with theand a saturated calomel electrode (SCE, -0.241 V vs. NHE)length of 50 μm. The surface area was 233 m2/g. An oxida-were used as a counter electrode and reference electrode, re-tive pretreatment of the CNTs was performed by ultrasonic inspectively. Pt-Ru/CNTs modified glassy carbon (GC, John-a mixture of concentrated sulfuric acid (98%) and nitric acidson Matthey) electrode was used as a working electrode. TheGC electrode was polished by 1.0, 0.3 and 0.05 um alumina(70%) with the volume ratio of 1: 1 at room temperature for(CHI Inc.), respectively, and then washed in ethanol and in2 h. Afterwards, the CNTs were filtered and washed by ultra-ultra-pure water in succession by ultrasonic. Catalyst of 2 mgpure water (18.23 M2) until the pH of the filtrate became 7.and 1 ml solution of 20% isopropanol +73.75% H2O +6.25%They were then dried in a vacuum oven at 90°C for 5 h.CNTs-Nafion composites were prepared by dispersingNafion (5 wt%) were mixed ultrasonically for 30 min. SluryCNTs in an ethanol solution of Nafion (5 wt%, Fluka). Theof 6 ul was pipetted onto the surface of the polished GC elec-solution was strred for 24 h, and then heated in a water bath attrode (4 mm in diameter). After the solvent evaporation, the80 °C, and finally dried in a vacuum oven at 80 °C for4 h. Theworking electrode was obtained. The apparent surface area ofNafion content in the CNTS was varied as 0, 1, 2 and 5 Wt%the GC electrode was 0.126 cm2, and the specific loading ofand the CNTs-Nafion composites samples were designated asthe catalyst was about 28.6 μgmeta/cm2. For the CO strip-ping voltammetry, carbon monoxide (99.9% pure) was ab-CNTs-0, CNTs-1, CNTS-2 and CNTs-5, respectively.sorbed onto the PL-Ru/CNTs catalysts by bubbling it in thesupporting electrolyte of 0.5 M H2SO4 solution for 20 min,2.2. Preparation of catalystswhile the potential was held at 0.091V vs. SCE. Then a; nurged for 30 min to remove the Co dis-The Pt-Ru/CNTSs-0, Pt-Ru/CNTs-I, Pt-Ru/CNTSs-2 andsolve中国煤化工re potential was then cy-P-Ru/CNTs-5 catalysts were synthesized by microwave-cled; SCE for three cycles atassisted polyol process. The Pt-Ru content in each sample10 nMHC NM H Gvwere ltedinasowas 30 wt% with a Pt: Ru atomic ratio of 1:1. 2.67 mllution of 0.5 M H2SO4+1 M CH3OH within a potential rangeof 0.038 M H2PtCl6 in ethylene glycol solution and 2.78 mlbetween -0.241 and 0.759V vs. SCE with a scanning rateof 0.036 M RuCl3 in ethylene glycol solution were mixedof 20 mV/s, then chronoamperometry profiles were plotted inJoumal of Natural Gas Chemistry Vol. 18 No. 2200920167.450 and 81.28° for pure Pt fcc, indicating that the Pt-RuNafion composites supported Pt-Ru catalysts and commercial alloy catalysts have single- phase- disordered structures, andPt-Ru/C-JM catalyst from Johnson & Mathey Co.the lattice constants decrease because of Ru substitution inPt fcc-center [22]. Peaks associated with either Ru metal or3. Results and discussionoxide species are not detected, implying that Ru may enter Ptfcc-center to form Pt-Ru alloys, or parially present as oxidespecies but not clearly discerned by X -ray diffraction. The av-To increase the concentration of metal anchoring siteserage particle size could be roughly calculated from Pt (220)on the walls of CNTS, the CNTs are often functional-FWHM according to Debye-Scherrer equation [23]:ized by an oxidative pretreatment in concentrated HNO3and/or H2SO4 [20,21], so it is of great significance to un-L = 0.9XCuKa/B2oCos0max(1)derstand whether the oxygen-containing groups on the CNTs-where L is the average particle size,入CuKa is the X-ray wave-Nafion composites are atered or covered by the Nafion in- length (1.5406 A), B2o is the full width at half maximum, andcorporation. FT-IR spectra of the CNTs and CNTs-Nafion0max is the angle at peak maximum. The results are listedcomposites are shown in Figure 1. The bands located atin Table 1. The average particle sizes for Pt-Ru/CNTs-0, Pt-about 3400, 1740, 1650, 1570 and 1100cm-1 can be at- Ru/CNTs-1, Pt-Ru/CNTs-2, Pt-Ru/CNTs-5 are 2.7, 2.6, 2.5tributed to the vibration of O-H, C=0, O-H, C-C andand 2.6 nm, respectively. It can be seen that the incorporationC-O bonds, respectively. It is clear that the CNTs ultrason-of Nafion in the CNTs support almost has no effect on theicated in concentrated HNO3 and H2SO4 can form oxygen- metal particle sizes of the Pt-Ru catalysts. This may be due tocontaining groups such as carboxyl, carbonyl and hydroxylthe homogeneous and fast microwave heating of the ethyleneon the surface, and the surface oxygen-containing groups areglycol solution with high dielectric constant and the dielectricnot significantly altered or covered with the incorporation of loss [24,25].Nafion in CNTs-Nafion composites.00 r98 k96 tet(4旨92 t(2)88 F86 t10 203040)0201(° )4000 3500 3000 2500 2000 1500 1000 500Wavenumber (cm ')Figure 2. X-ray dffrcion pttens ofPr Ru/CNTs-0(1), Pr-Ru/CNTs-.1 (2),Figure 1. FT-IR secta of CNTs-0 (), CNTs-1 (2) CNTs-2 (3) andPT-Ru/CNTS-2 (3) and Pt-Ru/CNTs-5 (4) catalystsCNTs-5(4)Microwave- assisted polyol process can reduce the tem-Figure 2 displays the X-ray diffraction (XRD) patternsperature gradients in the reaction medium, so the metalof (1) Pt-Rw/CNTs-0, (2) Pr-Ru/CNTs-1, (3) Pr-Ru/CNTs-2,nanoparticles can be nucleated and grown up in a more(4) Pt-Ru/CNTs-5. The first peak located at a 20 value of uniform reducing environment. Transmission electron mi-about 25.59 is associated with the carbon nanotubes support.croscopy (TEM) imaging was used to examine the effectFour characteristic peaks corresponding to 111), (200), (220)of Nafion incorporation on the morphology of the catalysts.and (31 1) planes of the face-centered cubic crystalline of PtFigure 3 shows the TEM images of Pt-Ru/CNTs-0 and Pt-can be observed, and the corresponding 20 values are shifted Ru/CNTs-2 catalysts. It can be seen from Figure 3(a) thatto higher as compared to the 20 values of 39.769, 46.249,PL-Ru particles exhibit poor formnation on the CNTs withoutTable 1. Average particle size, co srpping and cydlic voltammetry results of P-RwCNTs catalystsAverage particle.Co sripping voltammetry M中国煤化ImmetryCatalystsize (nm)4onset potentialpeak potentials curent density (Acm2)P-RuCNTS-02.0.1480.339ETYHCNMHG0.0015Pr-RwCNTs-10.1450.3200.4870.0022PL-RuCNTs-22:0.1380.2910.4720.0025Pt-Ru/CNTs-520.1410.3020.4860.0012* Values determined by XRD analysis202Shengzhou Chen et al Jourmal of Naural Gas Chemisty Vol. I8No.2 2009Pt-Ru/CNTs-0 catalyst, suggesting that the catalysts have bet-20082ter oxidation kinetics for the CO oxidation reaction. The PL-Ru/CNTs-2 catalyst with 2 wt% Nafion incorporation showedthe best CO oxidation activity, with the lowest peak potentialof0.291 V vs. SCE.0.00120010- P-Rw/CNTs-0. Pr-RwCNTs-1Shmen0008 E. PI-RuCNT-2 .CNTs-50.00060.00000004 F0.00.6E(V v以. SCE)ShmFigure 4. CO sipping voltammogans for PrRuICNTs catalysThe cyclic voltammograms of methanol electro-oxidationFigure 3. TEM images of Pr-Ru/CNTs-0 (细) and P-Ru/CNTs-2 (b) catalystsfor various Pt-_Ru/CNTs catalysts are shown in Figure 5,and the peak potentials as well as the peak curtent den-Nafion incorporation, and some small particles display seri-sities are also summarized in Table 1. Methanol electro-ous aggregation. When the CNTs were incorporated withoxidation on Pt-Ru catalyst has been studied for many years.2 Wt% Nafion, the morphology of Pt-Ru/CNTs-2 catalyst isIt is generally accepted that at low potential, methanol andimproved, and the Pt-Ru particles are uniformly dispersed onintermediates, such as formaldehyde, formic acid, CO andthe outer walls of CNTs, as shown in Figure 3(b). The par-so forth, would adsorb on the catalyst, and the reactionticle size distribution is about 2- 4 nm, which is just in thewould continue only when Ru dissociatively adsorbs H2Orange of the ideal particle size of Pt-Ru catalyst for methanolto form Ru-OHads [27,28]. An oxidative peak would occurelectro-oxidation [26].during the positive scanning because at high potential theIt is generally suggested that the methanol eletrocatalyticOH strongly adsorbs on thecatalyst surface,blocking theoxidation on Pt-Ru catalyst can be divided into two steps,methanol electro-oxidation. Another oxidative peak wouldi.e, methanol dehydrogenation and CO-like intermediates re-appear during the negative scanning due to re-oxidation ofmoval [27]:methanol and intermediates after the reduction of stronglyPt+ CHzOH一+ Pt-COads +4H+ +4e~adsorbed OH. The low peak potential and high peak cur-rent density in the positive scanning will be an indicationRu+ H20-→Ru~ OHads+H++e~3)of better methanol electro-oxidation activities. It can be seenPt-COads + Ru_OHads- + Pt+ Ru+CO2+H++e~ (4)from Figure 5 and Table 1 that the activities of the Pt-Ru/CNTs catalyst are related to the Nafion content in theThe CO-like intermediates, if not removed as soon asCNTs-Nafion composites. In accordance with the resultspossible, might act as poisons to the catalytic sites and willof CO stripping voltammetry, the best methanol electro-block the methanol adsorption and dehydrogenation, there-oxidation activity is achieved on the Pt-Ru catalyst sup-fore a good catalyst for methanol electro-oxidation shouldported on CNTs-Nafion composites with 2 wt% Nafion,also catalyze the oxidation of Co [4]. CO stipping voltam-with the lowest peak potential of 0.472V vs. SCE and themetry can give preliminary information about the electrocat-highest peak current density of 0.0025 A/cm2. The stabil-alytic performance of the Pt-Ru catalyst. Figure 4 shows theity of the Pt-Ru/CNTs and commercial Pt-Ru/C-JM catalystsCO stripping voltammograms of Pt-Ru catalysts supported onwere examined by chronoamperometry. Figure 6 shows theCNTS and CNTs-Nafion composites. Table 1 summarizes， nrofiles of methanol electro-oxidationthe onset potentials and peak potentials for the CO oxida-for中国煤化工Pr-Ru/C-JM catalysts.tion reaction. As indicated in Table I, the incorporation ofTheCYHactivity and staility,Nafion has litle effect on the onset potentials of CO oxida-with.CN MHGxAvem2 a he end oition over the Pt-Ru/CNTs catalysts. However, the peak poten-900 s, and the decreasing order of current densities for othertials of CO oxidation over Pt-Ru catalysts supported on CNTS-catalysts is Pr-Ru/CNTS- 1(0.00049 A/cm2) > PI-Ru/CNTs-0Nafion composites shifted to lower values as compared with(0.00041 Acm2) > Pt-Rw/C-JM (0.00040 A/cm2) > Pt-Ru/Joumal of Natural Gas Chemistry Vol. 18 No.22009203to the active sites [12]. Therefore, there is an optimum amount0.0025of Nafion in CNTs-Nafion composites. Our present study- P-Ru/CNTs-00.0020. Pr-RwCNTSsIshows that the optimum content of Nafion is 2 wt%.- PI-Rw/CNTs-20.0015Pr-Rw/CNTs-54. Conclusions0.0010Carbon nanotubes-Nafion (CNTs-Nafion) composites0.0005prepared by impregnated CNTs with Nafion in ethanol solu-0.0000tion, were used as the support of Pl-Ru catalyst for methanolelectro-oxidation. The FT-IR and XRD results showed that the-.0005055incorporation of Nafion in CNTs-Nafion composites does not-0.0010significantly alter the oxygen-containing groups on the CNTs0.20..2.6surface, and almost has no effect on the metal particle sizesE(V v9. SCE)of the Pt-Ru/CNTs synthesized by microwave assisted polyolFigure 5. Cyclic voltammograms of mcthanol ectroxidation for Ptprocess. The Pt-Ru catalyst supported on CNTS-Nafion com-RuCNTs catalystsposites with 2 wt% Nafion showed the best actities of COoxidation and methanol electro-oxidation, which might be dueto improved dispersion of Pt-Ru particles as well as the ex-0.0008tension of the three-phase boundary region between reactants,catalyst and Nafion proton conducting ionomer. Therefore,0.0007CNTs-Nafion composites can be used as an efficient supportof the methanol electro-oxidation catalyst for direct methanol0.0006 .fuel cell applications.(3)References(1)0.0004[1] Wasmus s, Kuver A. 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J[23] Radmilovic V, Gasteiger H A, Ross P N. J Catal, 1995, 154(1):Power Sources, 2006, 155(2): 959828] Watanabe M, Motoo s. J Elecroanal Chem, 1975, 60(1); 275DALIAN INSTITUTE OF CHEMICAL PHYSICS,CHIN∈SE ACADEMY OF SCIENCESLab Director and Division Directors,Dalian National Laboratory for Clean EnergyDalian National Laboratory for Clean Energy (DNL), based mainly in Dalian Institute of Chemical Physics (DICP), ChineseAcademy of Sciences (CAS), is looking for outstanding candidates for DNL director and directors for its nine research divi-sions: optimized utilization of fossil energy, low carbon catalysis and engineering, energy saving & energy environment,fuel cell & energy storage, hydrogen energy, biomass energy, solar energy, maritime renewable energy, basic & strategicstudies on energy, and service center for energy researches. More details about DICP can be found at htp://www.dicp.ac.cn/.Successful candidates for these positions should have a Ph. 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A generous start-up research fund for eachsuccessful candidate will also be provided,Candidates, who are interested in these positions, should send a complete CV and publication list to Dr. Hua' an Zhang, De-partment of Personnel (86- 411-84379556, talents@dicp.ac.cn), Mr. Zhiyuan Mao (maozy @ dicp.ac.cn), Prof. Can Li (canli@dicp.ac.cn) and Prof. Tao Zhang (taozhang@dicp.ac.cn).Dalian Institute of Chemical Physics,Chinese Academy of Sciences中国煤化工MYHCNMHG