Carbon Nanotubes Supported Pt-Ru-Ni as Methanol Electro-Oxidation Catalyst for Direct Methanol Fuel

Available online at www.sciencedirect.comScienceDirect|P|Joumal oNatural Gas CemnstryJournal of Natural Gas Chemistry 16(2007)162- 166SCIENCE PRESSArticle .Carbon Nanotubes Supported Pt. Ru-Ni as MethanolElectro-Oxidation Catalyst for Direct Methanol Fuel CellsFei Ye'，Shengzhou Chen2，Xinfa Dong'，Weiming Lin1,21. School of Chemical and Energy Engineering, South China University of Technology, Guangzhou 510640, Guangdong, China;2. School of Chemistry and Cherical Engineering, Guangzhou University, Guangzhou 510006, Guangdong, China[ Manuscript received November 27, 2006; revised February 27, 2007 ]Abstract: Carbon nanotubes (CNTs) supported Pt- Ru and Pt-Ru-Ni catalysts were prepared by chem-ical reduction of metal precursors with sodium borohydride at room temperature. The crystallographicproperties and composition of the catalysts were characterized by X-ray diffraction (XRD) and energydispersive X-ray (EDX) analysis, and the catalytic activity and stability for methanol electro-oxidationwere measured by electrochemical impedance spectroscopy (EIS), linear sweep voltammetries (LSV), andchronoamperometry (CA). The results show that the catalysts exhibit face centered cubic (fcc) structure.The particle size of Pt _Ru-Ni/CNTs catalyst is about 4.8 nm. The catalytic activity and stability of thePt-Ru-Ni/CNTs catalyst are higher than those of Pt-Ru/CNTs catalyst.Key words: carbon nanotubes; Pt-Ru-Ni/CNTs; methanol electro oxidation; direct methanol fuel cells1. Introductionreleased at the anode [5].CH3OH + H2O→CO2 + 6H+ +6e- (Ea = 0.046 V)(1)Methanol is a fundamental product of C1 chem-At the cathode, the protons migrating throughistry industry. The production processes of methanolthe electrolyte membrane, together with the electronsare simple, with rich recourses, such as natural gas,passing through the outer circuit combine with oxy-coal, heavy oil, and so forth [1]. Therefore methanolgen to form water.has many applications in the chemical industry. In re-cent years, direct methanol fuel cells (DMFCs) have3/202 + 6H+ +6e~→3H2O(Ec= 1.23 V) (2)attracted an increasing attention because of theirThe overall reaction in a DMFC is the catalyticfavorable advantages, such as simple system struc-conversion of methanol with oxygen to carbon dioxideture, compatibility with current petroleum distribu-and water; with a maximum thermodynamic voltagetion network, high energy density, as well as low tem-of 1.18V at 25 °C.perature operation [2- 4]. It is suggested that amongthe various types of fuel cells, DMFCs show the mostCH3OH+ 3/2O2→CO2 + 2H2O(Eell= 1.18 V)promising prospect for portable applications (laptops,(3)PDAs，mobile phones, etc.). In operation, DMFCs中国煤化工fort has been madeoxidize methanol with water to form carbon dioxide,to th0HCNMH Gof DMFCs, severalprotons, and electrons. The carbon dioxide is thenproblennotoudaur心w icouIvCu in terms of efficiencyCorresponding author. Tel: 020-87113023; E mail: wmlin@gzhu.edu.cnThe project is supported by the National Natural Science Foundation of China (20576023), the Science and TechnologyProject of Guangzhou City (2005 J1-C0361) and the Key Project of Education Bureau of Guangzhou City (2052).Journal of Natural Gas Chemistry Vol. 16 No. 22007163and power density. One of the problems is the low1:1) and Pt-Ru-Ni (with an atomic ratio of 6:3:1) cata-activity of methanol electro- oxidation in the anode.lysts were prepared by chemical reduction of H2PtCl6,It is well known that Pt is the most active metalRuCl3，and NiCl2 precursors with sodium borohy-for methanol electro- oxidation; however, Pt is easydride at room temperature. The metal loading of theto be poisoned by CO-like species produced duringtwo catalysts was 20% in weight. Appropriate amountthe methanol electro-oxidation and thus loses contin-of CNTs, metal precursors, and ultra-pure water wereuous high catalytic activity [5]. Therefore, Pt-basedultrasonically mixed for 30 min and then mechanicallybimetal, ternary, or quaternary catalysts have beenstirred for 2 h. Excess quantities of 0.2 M sodiumused to improve the CO-tolerance characteristics ofborohydride solution were added drop- by-drop to thePt through the bifunctional or electronic effect [3].mixtures and then the bath was stirred for 3 h for theRecently, Pt-Ru-Ni alloy or Vulcan XC-72 supportedcomplete reduction of the metals. Finally, the mix-Pt-Ru-Ni catalysts have been reported to show highertures were filtered, washed, and dried in an oven atactivity and stability in comparison to state-of-the-art80°C for 2 h.Pt-Ru catalysts [6- 10].Since the discovery of carbon nanotubes (CNTs)2.3. Characterizations of catalystsin 1991 [11], they have been widely used in manyfields. In the fuel cell areas, they could be of inter-X-ray diffraction (XRD) powder patterns of the .est as catalyst supports [12,13]. CNTs show highlyCNTs and catalysts were obtained on a XD-3 X-rayelectrochemically accessible surface area and offerdiffractometer (Beijing Purkinje General Instrumenta remarkable electronic conductivity compared withCo., Ltd., China) using a Cu-Ka source operating atthe commonly used Vulcan carbon black. Previ-36 kV and 20 mA. The scanning range and rate areous studies have showed that CNTs supported cata-10°- 90° and 8° /min, respectively. Chemical compo-lysts exhibit better performance of methanol electro-sition analyses of the catalysts were carried out onoxidation as compared to conventional carbon blackan energy dispersive X-ray (EDX) analyzer (Oxford(XC-72) supported catalysts [14- - 17]. However, lttleINCA300) attached to a scanning electron microscoperesearch has been devoted to CNTs supported ternary(LEO 1530 VP, Germany).catalysts [18]. This study presents the methanolelectro- oxidation results on CNTs supported Pt-Ru-2.4. Electrochemical measurementsNi catalyst.The electrochemical measurements were per-formed in a solution of 0.5 M H2SO4 and 1 M CH3OH2. Experimentalat room temperature, using a conventional three-2.1. Oxidative pretreatment of CNTselectrode cell and a Solartron SI 1287 electrochemi-cal interface and SI 1260 impedance/ gain-phase ana-Well- aligned multi-walled CNTs with puritylyzer. A Pt mesh and a saturated calomel electrodehigher than 95% were purchased from Shenzhen Nan-(SCE，-0.241 V vs. NHE) were used as counter-otech Port Co., Ltd., China. The main range of di-electrode and reference electrode, respectively. Pt-ameter, length, and surface area of the CNTs wasRu/CNTs or Pt-Ru-Ni/CNTs modified glassy car-10- 20 nm, 5-15 purmn, and 40- 300 m2 /g, respectively.bon (GC, Johnson Matthey) electrode was used asIn order to increase the concentration of grafting sitesthe working electrode. The GC electrode was pol-on the walls of CNTs, an oxidative pretreatment wasished by 1.0, 0.3, and 0.05 μum alumina (CHI Inc.,performed by refluxing with a mixture of concen-USA), respectively, and then washed in ethanol andtrated sulfuric and nitric acids (1:1 v/v, 98% and 70%,ultra- pure water ultrasonically. 3 mg catalyst andrespectively) at 90 °C for 5 h. Following this, the1 ml solution (20% isopropanol+ 73.75% H20+6.25%CNTs were filtered and washed using ultra-pure wa-Nafion (5 wt%, Fluka)) were mixed ultrasonically forter (18.23 MN) until the pH of the filtrate became 730 min.10 μ slurry was pipetted onto the surface ofand consequently dried in an oven at 110 °C for 5 h.thethe solvent evapora-tion中国煤化工btained. The appar2.2. Preparation of catalystsent sTYHC N M H Grode was 0.196 cm2,and the specific loading of the catalyst was aboutCNTs supported Pt-Ru (with an atomic ratio of30 1ugmetal/cm2. The impedance spectra were reg-164Fei Ye et al./ Journal of Natural Gas Chemistry Vol. 16 No.22007istered at frequencies from 100 KHz to 0.05 Hz withand Pt-Ru-Ni/CNTs are 5.6 and 4.8 nm, respectively.an amplitude of 10 mV at a potential of 0.4 V vs.No peaks associated with either Ru or Ni metal or ox-SCE, and ZPlot and ZView software were used toide species are observed, implying that Ru or Ni maymeasure and analyze the impedance data. Linearenter Pt fcc-center to form Pt Ru or Pt-Ru-Ni alloys,sweep voltammetries (LSV) were plotted within a po-or partially present as oxide species but not clearly betential range from -0.241 to 0.50 V vs. SCE with a .discerned by X-ray diffraction.scanning rate of 5 mV/s, and the chronoamperometryEDX measurement was used to analyze the com-(CA) profiles were obtained at a potential of 0.40 Vposition of the catalysts.EDX spectra of Pt-Ru-vs. SCE with a polarization time of 30 min.Ni/CNTs and Pt-Ru/CNTs are shown in Figure 2,and the calculated chemical composition of the two3. Results and discussioncatalysts are listed in Table 1. It is evident that Pt,Ru, and Ni are present on the CNTs support, indi-3.1. Characterizations of catalystscating that the H2PtCl6, RuCl3, and NiCl2 precur-sors can be reduced to their respective metal phasesFigure 1 shows the XRD patterns of CNTs sup-by sodium borohydride at room temperature. Aport and Pt-Ru-Ni/CNTs and Pt-Ru/CNTs cata-cording to the composition values in Table 1, the ra-lysts. The first peak located at about 25.5° irtio of Pt:Ru for Pt-Ru/CNTs catalyst is 48.87:51.13,the three samples is associated with the CNTs sup-and the ratio of Pt:Ru:Ni for Pt-Ru-Ni/CNTs cata-port. Four characteristic peaks corresponding t(lyst is 58.03:34.88:7.09, which are quite close to the(111), (200), (220), and (311) planes of the fcc crys-theoretical values. Wang and co-workers [9] preparedtalline Pt are observed in the catalysts' patterns, andPt-Ru/C and Pt-Ru-Ni/C by the same method, andthe corresponding peak angles a lttle shift to higherthe ratio of Pt:Ru (theoretic value 1:1) for Pt-Ru/C20 values of 39.760, 46.240, 67.45", and 81.28 for purecatalyst was 56.3:43.7, and the ratio of Pt:Ru:Ni (the-Pt fcc, indicating that the alloy catalysts have single-oretic value 6:3:1) for Pt-Ru-Ni/C was 63.2:30.1:6.7.phase disordered structures, and the lattice constantsdecrease because of Ru or Ni substitution in Pt fcc-center [9]. The average particle size may be roughly(acalculated from Pt (220) FWHM according to Debye-Scherrer Equation [6,9,19]:会0.9XCuKa(4)B20 cos θmaxPPtwhere, L is the average particle size, λCuKa is theRuNINAPrPtX-ray wave-length (1.5406 A), B20 is the full widthat half maximum, and 0max is the angle at peak max-imum. The average particle sizes for Pt-Ru/CNTsP(11)P+(200)C(002)Pu(311) .101214Energy (keV)Figure 2. EDX spectra of Pt-Ru-Ni/CNTs (a) andPt-Ru/CNTs (b) catalystsTable 1. The atomic compositions of the catalysts3070中国煤化工sition (at.%)determined by EDX20/(°)CNMHG- Pt RuNiFigure 1. XRD patterns of CNTs support (1), andPt-Ru/CNTs5050一48.87 51.13Pt-Ru/CNTs (2) and Pt-Ru-Ni/CNTsPt-Ru-Ni/CNTs 60 30 1058.03 34.88 7.09(3) catalystsJournal of Natural Gas Chemistry Vol. 16 No. 2 20071653.2. Electrochemical measurements of the cat-the Pt-Ru-Ni/CNTs catalyst shows a higher currentalystsdensity at all potential than Pt-Ru/CNTs catalyst,indicating superior catalytic activity, which may beElctrochemical impedance spectroscopy (EIS)attributed to the promoting efct of Ni in Pt-Ru-was used to determine the charge transfer resistanceNi/CNTs catalyst for methanol electro-oxidation.(Rct) during the methanol oxidation process on thePt-Ru/CNTs and Pt Ru-Ni/CNTs catalysts. Fig-0.004ure 3 shows the Nyquist plots obtained in a solution of0.0030.5 M H2SO4 and 1 M CH3OH at a potential of 0.40 Vvs. SCE. It is interesting that for Pt-Ru-Ni/CNTs0.002catalyst, an inductive loop appears at low frequencies，which could be attributed to the kinetics of the COad“(2)oxidation [20- 22]. Using the ZView software andthe equivalent circuits [22], the impedance data werefitted, and the values of various circuit elements wereobtained. The Rct values are 231.3 and 135.3 S forPt- Ru/CNTs and Pt-Ru-Ni/CNTs, respectively. A-0.2 -0.1 00.1.2 0.3 0.0.5significant decrease of Rct values for Pt-Ru-Ni/CNTsE1(V vs. SCE)indicates a smaller reaction resistance and higher cat-Figure 4. Linear sweep voltammetries of methanolalytic activity for Pt-Ru-Ni/CNTs electrode.electro-oxidation on Pt-Ru-Ni/CNTs (1)and Pt-Ru/CNTs (2) catalystsTest conditions: solution 0.5 M H2SO4 and 1 M CH3OH, scan(1)rate 5 mV/s, room temperature-80The chronoamperometryCA) profiles of.-60methanol electro- oxidation on Pt- Ru/CNTs and PRu-Ni/CNTs catalysts at a potential of0.4 V vs. SCEg -40are shown in Figure 5. At a potential of 0.4 V vsSCE, methanol can be continuously electro-oxidizedon the surfaces of Pt- based catalysts. However, theaccurmulation of reaction intermediates would ap-pear if the removal reaction cannot be in line with20that of methanol electro-oxidation. Therefore, the50015200decrease in electro-oxidation current density will oC-210Figure 3. Nyquist plots for Pt-Ru/CNTs (1) andcur, and the decrease with less extent is indicativePt-Ru-Ni/CNTs (2) catalystsof better CO-resistance. For the two catalysts, theTest conditions: solution 0.5 M H2SO4 and 1 M CHzOH, po-onset current densities, as well as, the decrease intential 0.40 V Vvs. SCE, room temperatureelectro-oxidation current density are different. TheThe catalytic activities of Pt-Ru/CNTs and Pt-onset current densities at Pt-Ru/CNTs and Pt-Ru-Ru-Ni/CNTs were also analyzed by linear sweepNi/CNTs electrodes are about 0.00030 A/cm2 andvoltammetries (LSV) with scanning from -0.241 to0.00045 A/cm2, respectively. And Pt-Ru-Ni/CNTs0.50 V vs. SCE and a scan rate of 5 mV/s (Fig-have higher current densities throughout the testure 4). Theoretically, methanol electro- oxidation maytime, that is, the current density at Pt-Ru-Ni/CNTs .proceed at 0.04 V v8. NHE, but the potential foelectrode at 1800 s is almost twice larger than that atmethanol electro oxidation on Pt-based catalysts isPt-Ru/CNTs electrode. These results further demon-much larger than the theoretical value due to the poi-strate the improved catalytic activity and stabilityson of Pt sites by the reaction intermediates, suchof Pt-Ru-Ni/CNTs catalyst in comparison to Pt-as formaldehyde, formalic acid, carbon monoxide,Ru中国煤化工:and so on. According to Figure 4, the onset po-and stability oftential of methanol electro oxidation on the Pt-Ru-Pt-RuMYHC N M H Gectro oxidation canNi/CNTs catalyst is approximately similar to thatbe explained by the electronic effect and the pres-of Pt-Ru/CNTs, that is, 0.08 V vs. SCE. However，ence of Ni(OH)2 and NiOOH species on the Pt-Ru-166Fei Ye et al./ Journal of Natural Gas Chemistry Vol. 16 No.2 2007Ni/CNTs catalyst [6,9]. The electronic transfer fromvorable properties of Ni hydroxide species on the Pt-Ni to Pt may contribute to the decrease of Pt-CORu-Ni/CNTs catalyst. Therefore, Pt-Ru/CNTs cat-binding energy and the releasing of more Pt sites foralyst can be used as an efficient anode catalyst forcontinuous methanol electro- oxidation. The Ni hy-direct methanol fuel cells.droxide layer that is present on the Pt- Ru-Ni/CNTscatalyst surface may have favorable properties, suchReferencesas proton and electronic conductivities, anticorrosionunder methanol electro-oxidation conditions, and the[1] TijmP J A, Waller F J, Brown D M. Appl Catal A:General, 2001, 221(1-2): 275facilitating oxidation of the poisoning CO-like species[2] Wasmus S, Kuver A. 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