Pt-Ru Catalysts Prepared by a Modified Polyol Process for Direct Methanol Fuel Cells

2012年10月贵金属Oct. 2012第33卷增刊1Precious MetalsVol. 33, No. SPt-Ru Catalysts Prepared by a Modified PolyolProcess for Direct Methanol Fuel CellsZHANG Junmin, ZHU Fangfang, ZHANG Kunhua, LIU Weiping, GUAN Weiming*(State Key Laboratory of Advanced Technologiecs for Comprehensive Utilization ofPlatinum Metals, Kunming Institute ofPreciousMetals, Kunming 650106, China. *E- mail:gwm@ipm.com.cn)Abstract: Supported PtRu/C catalysts used in direct methanol fuel cells (DMFCs) were prepared by a newmodified polyol method. Transmission electron microscopy (TEM), X-ray diffaction (XRD) and cyclicvoltammograms (CVs) were carried out to characterize the morphology, composition and the electrochemicalproperties of the PtRu/C catalyst. The results revealed that the PtRu nanoparticles with small average particle size(≈2.5 nm), and highly dispersed on the carbon support. The PtRw/C catalyst exhibited high catalytic activity andanti- poisoned performance than that of the JM PtRu/C. It is imply that the modifed polyol method is efficient forPtRu/C catalyst preparation.Key words: PtRu/C catalysts; modified polyol method; direct methanol fuel cells(DMFCs); electrochemicalperformanceCLC number: 0643.36Document Code: AArticle ID: 1004-0676(2012)S1-0222-05At present, the diret methanol fuel cells(DMFCs) is being widely investigated and considered as a possiblepower source for electric vehicles and other portable applications. However, reformate gas of methanol or anyother carbon-containing fuel is always contaminated with a small amount of some ppm of CO. The platinumcatalyst is rapidly poisoned due to strong CO adsorption on the platinum surface, leading to a significant decreaseof the power output. On the other hand, in order to effectively utilize the catalysts, their small particles are usuallydispersed to onto the conductive carbon supports. In this process, the conventional methods, such asimpregnation'", colloidal method2 1 and ion exchange*, are not adequate ftom the viewpoint of offering highmetal surface area or good dispersion of metal particles. To avoid the above disadvantages, an alternativeanti-poisoned PtRu/C catalyst prepared by a modified polyol method has been investigated.In this study, we report a simple procedure for the preparation of PtRu/C catalysts. This method bears bothadvantages of the impregnation, co-precipitation and ion exchange. The preparation of process can be simple andthe catalyst grain-size distribution is easy-control. Therefore, the PtRw/C catalyst prepared by this procedure withsmall average particle size highly dispersed on the carbon support and represented high catalytic activity.中国煤化工Received date: 2012-03-21MHCNMHG..No. S1ZHANG Junmin et al: Pt-Ru Catalysts Prepared by a Modified Polyol Process for Direct Methanol Fuel Cells2231 Experimental1.1 Catalyst synthesisThe metal precursors Pt(acac)2 and Ru(acac)3 were dissolved totally in diphenyl ether with heating to 100"C,The oleic acid, oleamide and carbon black were put in as surfactants and supporter respectively. Then the reducerwas added in the solution and the temperature was increased up to 250C and kept constant for 30 min so that themetals were reduced adequately. After cooling, the mixture was filtered and the filter cake was dried in a vacuumoven at 85"C for 2 h. The catalysts were marked as IPM-PtRu/C..2 Physical characterization techniquesThree techniques were employed to physically characterize the catalysts: X -ray diffraction (XRD), thetransmission electronic microscopy (TEM) with analychusis of energy dispersive X-rays (EDX). The instrumentswere a Rigaku D/max-RC using the Cu Ka radiation and v =2%/min. The transmission electronic microscopy(TEM) study was done using a Tecnai G2 20 (Finland) microscope operating at 200 kV, coupled to an energydispersive X-ray analysis (EDX) spectrometer. The samples for the TEM analysis were prepared by ultrasonicallydispersing the catalyst powders in ethanol. A drop of the suspension was applied onto a carbon coated copper gridand was dried in air.1.3 Elctrochemical performance measurementsThe cyclic voltammetry spectra were recorded using a potentiostat/galvanostat model 273 electrochemicalstation. Five milligram of Pt-based catalyst was suspended in 1 mL of ethanol and 50 μL of Nafion⑧solution (5wt% DuPont) to prepare catalyst ink. Then 10 μL ink was transferred with an injector to clean glassy carbon diskelectrode (with area of 0.07069 cm' ) as the electrode. A mixture solution containing 0.5 mol/L H2SO4 and 0.5mol/L CHzOH was used as electrolyte. During the experiment the temperature was kept constant at 25"C. Thescan rate was 20 mV/s, and a saturated calomel electrode (SCE) was used as reference electrode, and a platinumwire was used as counter electrode. The data were recorded after three cyclic curves overlapped each other.2 Results and discussion2.1 XRDCurve (a) in Fig. 1 shows the XRD patterns of the 20wt% PtC catalyst prepared by the modified polyolmethod. The characteristic diffaction peaks of the fcc Pt demonstrate that a successful reduction of Pt precursorto metallic form has been achieved.XRD results of the IPM-PtRu/C (2:1) was shown in curve(b) of Fig.1. Compared with curve (a), the lack of peak of thew111Ru and its oxide could indicate the alloying of Pt andRu(characteristic diffraction peaks of Ru is shown at gP(20})38.49(100)，44.0*(101),58.3*(102) and69.49(110)respectively.). Moreover, it can be observed that the人人diffraction peaks of the IPM-PtRu/C slightly shift to higherBragg angles compared with Pt/C, which indicates thedecrease of the lattice constant with the presence of Ru. Suchevidence accounts for the presence of a IPM-PtRu alloy in thecatalyst, where the platinum atoms (atomic radius=1.38A) on中国煤化工。ctrocatalyststhe lattice points of the face centered cubic lattice are replacedMHC N M H GptRu/C(2:1)by the smaller ruthenium atoms (atomic radius=1 .325A).224Precious MetalsVol. 33Furthermore, the line broadening of (11) and (220) at 2θ of nearly 39°, 69° respectively also can be attribute tothe replacement of the platinum atoms on the lattice points by ruthenium atoms.2.2 TEMFig.2 shows TEM images of the IPM-PtRu/C nanoparticles and the corresponding particle size distributionhistogram. As it can be seen, the metal nanoparticles are homogeneously dispersed on the surface of the carbonsupport, which could be associated to the presence of an excess of stabilizers. The particle size distribution with2.54 nm average particle size is uniform in the range of 1.8 ? ~ 3.4 nm and no agglomerates of metal particles wereobserved.bFig.2 TEM micrographs of the PtRu/C prepared by modified polyol method(a TEM micrographs, b Particle size distribution histogram)The composition of the PtoRu40C electrocatalysts was determined by EDX analysis. The actual composition(Pt3Ru36) was found to be very close to the nominal value, indicating a good incorporation of Ru into the catalyst.2.3 Electrochemical performanceFig.3(a) shows the cyclic voltammogram studies of two catalysts, which were carried out in 0.5 mol/LCH2OH and 0.5 mol/L H2SO4 solution. As it can be seen. the take- off voltage of IPM-PtRu/C and JM-PtRu/C formethanol electrochemical oxidation were 0.18 V (vs. SCE), 0.37 V(vs. SCE) respectively. When forward scanvoltage was raised to 0.5 V (vs. SCE) , the current density of IPM-PtRu/C and JM-PtRu/C for methanolelectrochemical oxidation were 98.9 mA/mg, 351.2 mA/mg respectively. Besides, the maximum current densityvalues of IPM-PtRu/C and JM-PtRu/C for methanol electrochemical oxidation were 439.6 mA/mg, 287.5 mA/mgrespectively. All the results revealed that the IPM-PtRu/C catalyst with low take-off voltage and high currentdensity possess high electro-catalytic activity for methanol electrochemical oxidation than that of the JM-PtRu/Ccatalyst.Besides, Fig.3(b) shows the chronoamperometry curve studies of two catalysts. As it can be seen, currentdensities of two catalysts were all depressing with time before reach steady state. However, current densitity ofJM-PtRw/C display sharp depress than that of IPM-PtRu/C, that is to say, current densitity of IPM-PtRu/C wasalways higher than that of JM-PtRu/C, after 400 s, the current density value of PM-PtRu/C was still 69.6 mA/mg.Therefore, the IPM-PtRu/C catalyst bear higher electro-catalytic stabili中国煤化工mical oxidationthan the JM-PtRu/C catalyst."YHCNMHG.No. S1ZHANG Junmin et al: Pt-Ru Catalysts Prepared by a Modified Polyol Process for Direct Methanol Fuel Cells225IPM-PtRu/C catalyst which possesses high electro-catalytic activity and stability displayed excellentelectrochemical performance. This is due to poisoning of catalytic surface by CO species coming fromdissociative adsorption of methanol at platinum. The modification of platinum catalysts with ruthenium partiallyprevents the poisoning by CO species. Concretely speaking, the process methanol electrochemical oxidation onPVC electrode includes two steps: Firstly, dissociative methanol was adsorbed by the exposed Pt sites, COads asthe main intermediate product was generated. Secondly, COads was oxidated into CO2. Reaction formula is asfollows:CH3OH- +COads+4H*+4e^COd+4H*+H2O→>CO2+6H*+2eIntense absorption of COads intermediate product on the exposed Pt sites prevented methanol oxidation, PtVCcatalyst was poisoned consequently.The modification of platinum catalysts with ruthenium not only parially prevents the poisoning by COspecies, but also reduces the take-off voltage for methanol electrochemical oxidation. This can be explained byBifunctional Mechanism' 6o. When the platinum catalysts was modified by ruthenium, the reaction processmethanol electrochemical oxidation on PtRu/C electrode was as follows:CHjOH- +COHads+3H*+3e^COHas+3Pt-→>COHds+3H*+3e^Ru+H2O-→Ru-OHds+H*+e-Pt-COHas+Ru-OHas-→CO2+3Pt+Ru+2H*+2eAt first, dissociative methanol which was dissociated into carbonaceous intermediate products under lowelectrode potential was absorbed by the exposed Pt sites, and then the Pts-COHads absorbing products were gotten.At the same time, H2O was dissociated by ruthenium into oxygenic item that was absorbed by the exposed Rusites under low electrode potential. At last, oxidation-reduction reaction occurred between Ptz-COHads andRu-OHads. Above all, adquent Ru-OHads are essential for keeping Ptz-COHads oxidation to preventing poisoning ofcatalytic surface by carbonaceous intermediate species. Therefore, the modification of platinum catalysts withruthenium contributes a lot to a superior electrochemical activity.Compared with JM-PtRu/C catalys, IPM-PtRu/C catalyst displayed excellent electrochemical performance.The small particle size and homogeneous disperse of JM-PtRu/C catalyst contribute a lot.JM 20%-PIRuC--- JM 20%PIRu/C- IPM 20%-PIRuCa500 t--- IPM 20% PtRu/C50040000300200 t1010000002040808101.2100 200300 400 500 600PotentialNV V SCETimes/Sec.Fig. 3 Electrochemical performance of PtRu/C and JM 20%PtRu中国煤化工.5 mol/LH2SO4 solutionDYHCNMHG(a Cyclic voltammogram of two catalysts, b Chronoamperometry curve of two catalysts)226Precious MetalsVol. 33Even though there is agreement about influence of reaction mechanism on the kinetics of the ORR, Mucheffort in this area is still needed to understand the way in which all parameters, such as particle shape, size, andsize distribution combined affect the catalytic properties of PtM/C cathode materials.ConclusionsA new modified polyol method is adopted for synthesis of the supported IPM-PtRw/C catalysts, the alloyingparticle of Pt and Ru was prepared by this method, and the particle sizes of IPM-PtRu/C catalysts with 2.5 nmaverage particle sizes are in the range of 1.8~ 3.8 nm. IPM-PtRu/C catalysts exhibit high electro-catalytic activityand stability than JM-Pt/C, The electrochemical performance of the IPM-PtRu/C catalysts ilustrate that the newmodifed polyol method is a promising way for catalysts preparation.References:[1] Pozio A, De Francesco M. Comparison of high surface Pt/C catalysts by cyclie voltammetry[J]. J Power Sources, 2002, 105:13-19.2] Petrow H G Allen R G Finelyarticulated clloidal platinum compound and sol for producing the same, and method ofpreparation: US. Patent, 4044193([P]. 1977-09-22.3] Prabhuram I, Wang X C, Hui L, et al. Synthesis and characterization of surfactant- stabilized Pt/C nanocatalysts for fuelellapplications[J]. J Phys Chem B, 2003, 107: 11057-11064.4] Yasuda K, Nishimura Y. The deposition of ultrafine platinum particles on carbon black by surface ion exchange- increase inloading amount[J] Materials Chemistry and Physics , 2003, 82: 921-928.5] Xiong L. Kannan A M. Manthiram A. Pt-M(M=Fe. Co. 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