THE EFFECT OF POWERFUL OXYGEN EVOLUTION METAL-OXIDE ON THE ELECTROOXIDATION OF METHANOL

ACTA METALLURGICA SINICA (ENGLISH LET TERS)Vol. 14 No. 2 pp 103-108 April 2001THE EFFECT OF POWERFUL 0XYGEN EVOLUTION METALOXIDEON THE ELECTROOXIDATION OF METHANOLX.C. Chen, Y.X. Lis, J. Li, JJ. Chen and H.Z. LiutTFADepartment of Metallurgy Science and Engineering, Central South Liniversity, Changsha 10083, ( "hinaManuscript received 6 November 2000; in revised form 16 February 2001An improved thermal decompostion method 29 used to prepare modified 1n111 beseanode. Some researches have been carried out to learn about the orygen cr/lutonproperties of RuMn, RuCo, RuCe and RuEu etc. in H2SO4 solution and their +fuclon the elcroridation of rmethanol i added anto the calalytc layers of P1;T. rler-trodes. The result indicates thal the Pt/Ti ecrodes conlarnng RuMIn, Ru( ”mdRuEu elc. melal oride catalysts with the powerful evolution property erhithit日hiyherelectro-catalylic activily towards melhanol elecroridatun than Pt/Ti electrorde- 10桥out composile metal oride calalysts.KEY WORDS direct methanol fuel ell, mclal oride coatmg, oxygen erolulomactvily, eleclrooridation1. IntroductionThe direct methanol fucl cell (DMFC) is a pronising power source for electrir vehicles1-+i. .but its development is hampered by the large anodic owervoltagc. For methanol. six eler-trons must be exchanged for complete oxidation annd the oxidat ion kinetics is inherentlyslowed, so some intermediates are formed during methanol oxidation. In fuel cells. as theplatinum alone is not a sufficiently active methanol oxidation electrocatalyst. P1- lasedmultimetallic catalysts earn the attention for this application. Up to now Pt-Ru has beenbelieved the best bimetallic catalyst, because Ru atoms react with H2O to provide OHadsfor CO oxidation. Recent years, more complicated multimetallic oxides (e.g. P1-R-.Pt-Mo-Sn and Pt-Ru-W-Sn etc.) have been introduced to the DMFC to strengthe: theelectrocatalytic property of traditional anode material such as Pt or Pt-Ru catalystIn the process of electrooxidation H2SO4 solution on the molifed titaniun basi an-ode (DSA) for electro-deposition of Zn2+, we found the combination of Ru with soI1transitional metal oxides was able to greatly lower the overvoltagc of oxygen evolut ionprocessl6]. The electrooxidation process of the modifed titaniumi base anode in H2SO1solution contains the decomposition of H2O and the formation of |0]. Conseqnertly thecomposite metal oxide for tbe modified titanium base anode is also possible u inprovethe electrooxidation property of methanol if combined with Pt catalyst based 01 n prunptrcomposition.Several metbods, such as electro-deposition, thermal decomposition and chennical redluc-tion of platinum, have been reported for the preparation of platinum based electrorde'r““。PtM/C-Nafion anode is believed the best electrode for DMFC. but its preparation is C'Iplicated, and its active area is hard to deternine.Therefore it is not oveniern forPtM/C-Nafion anode to be adopted to investigate the electrocatalytic propertics of variedmetal oxides. In this paper thr modified titanium base anode is used to investigate tl术中国煤化工MYHCNMHGfect of oxygen evolution metal oxide on the electrooxidation of methanol. The preparationmethod for the modifed titanium base anode is very simple. The electrocatalytic prop-erties of the modified titanium base anode can also be conveniently modulated throughregulating the coating composition, with a good reproducibility. Furthermore it is easyto determine the active area of DSA and the Pt-loaded-weight. Of course the experimentresult can also be applied for the anode material of DMFC.2. Experimental Details2.1 Electrode preparationThe preparation of the titanium base anode coating is divided into two steps: pretreat-ment of Ti fakes and preparation of electrocatalytic coating. After sand blasted, titaniumfiakes were first degreased with solution made up of acetone and CCl4, etched in HCI(30%) at 609C for an hour, then washed by distilled water. The SnSb film was formed at3509cl10] and the the composite oxide surface coating was formed at 400C. After classified,the substrates with SnSb coating were dipped into their respective chloride coating solu-ion (precursor), dried in an air oven at 120C for 9 minutes and then treated by heatingin muffale furnace for 10 minutes. This work was repeated 5 times. The final anncalingtreatment in muffale furnace at 4009C lasted an hour. The morphologies of electrodes wereobserved by JSE 7200 scanning electron microscope. The samples were also analyzed byX ray diffraction.2.2 Electrochemical measurementFor long-term testing of the catalytic activity of the modified electrodes, a threeelectrode cell was used. The current density for methanol oxidation was recorded as thefunction of electrode potential. The electrode was firstly pre activated by cycling the elec-trode in 0.1mol/L H2SO4 for 15 minutes and then in 0.1mol/LH2SO4 + 1.0mol/LCH3OHfor 15 minutes. After pre activation, the electrode was dried and transferred to a cllcontaining methanol for the electrochemical measurement. The counter electrode was aplatinum foil. A saturated calomel electrode (SCE) was used as the reference electrode.The electrochemical experiment was carried out on a Model 173 potentialstat/ galvanostat.The working electrode was coated with PTFE flms to expose suitable geoimnetrie arcasbased on experiment requirement. The result was plotted on an X-Y recorder (YEWTYPE 3033). All the measurements were carried out at the temperature of 30土29C.3. Results and Discussion3.1 Electrocatalysis of methanol on the surface of electrodeA typical cyclic voltammogram of the Pt/Ti anode with a SnSb base layer in 1mol/LCH3OH + 0.1mol/L H2SO4 is shown in Fig.1. In the anodic scan, the electrochemicalionization of adsorbed H takes place between -0.4 and 0.0V, followed by the oxidationof methanol on the surface of electrode from 0.3 to 0.9V. Compared with Fig.1 (electrodein 0.1mol/L H2SO4 solution), two obvious oxidation peaks appear in Fig.1 (electrode in1mol/L CH3OH+0.1mol/L H2SO4). In Fig.1, nethanol begins its electrooxidation at about0.4V. Before the sweep potential reaches 0.55V. because of the poisoning of the elex:trodeby carbon monoxide that is generated during the oxidation, the oxidation of methanolby reaction with water to make carbor dioxide is restricted. With the increase of sweep .中国煤化工MYHCNMHG105potential, the adsorbed carbon monoxide is oxidized after the sweep potential reaches0.55V, and the maximum current density appears at about 0.68V. Variation of currentpeak in Fig.1 embodies the complexity of electrooxidation process of methanol on thesurface of Pt/Ti electrode. Fig.1 shows that the current peak appears at a lower potentialin reverse sweep than that in positive sweep because beforc reverse sweep potent ial reachesH adsorbed area, formation of carbon monoxide molecules is restricted. Thus met hanolcan be oxidized at a lower potential (the reverse current peak appears at 0.45V). and theminimum value of electrooxidation potential is 0.27V.3.2 Improvement of electrode preparationPlatinum-ruthenium is believed the best bimetallic oxides catalyst for electrooxidationof methanol, because adsorbed OHads oxidant is formed at a lower potential than thialon Pt sites. At suitable electrode potentials, water displacement occurs 011 Ru sitcs withformation of Ru-OH groups on the surface of the catalyst,Ru+ H20- + Ru-OH+H+ +eThe fnal step is the reaction of Ru-OH groups with neighboring methanolic residucs t(make carbon dioxidesRu-OH+Pt-C0- + Ru+Pt+CO2+H+ +e(21The electrocatalytic activity of electrode was lowed if the coating solution (precursoriwas made from H2PtCl6 mixed with RuCl3. XRD patterns indicated that some H2PtCl,molecules were not completely changed into Pt but existed as PtO2 because of the effectof RuO2 decomposed by RuCl3 reaction with O2. So an improved thermal decomposingmethod was used to make the electrode coating including RuO2 and Pt. The improvenentof electrocatalytic activity by Ru is also proved by Fig.2. In this new method, Ti electrodeswith SnSb base layers were firstly covered with the H2PtCg coating solution ther heat-treated by the oven and the muffale furnace, next RuC]3 solution was used. This procedurewas also repeated 5 times. So we can use this method to investigate the electrooxidationof methanol on the electrode with a metal oxide anode coating.n 140，0.1mol/L HSO↓120 一PtRn/Tis 30+ 1mol/L CH,OH1002 士P(/T; 80}60]-0.6-04-0.20002 040608 1.0 120.00.1020.3040.50.60.70.8Valtage,Valtage. VFig.1 The cyclic voltarmograms of modifed anode Fig.2 The electrocatalytic activity cormparisonin the solution of 0.1mol/L H2SO4 + 1mol/Lbetween Pl/Ti and PtRu/Ti made b; theCHgOH and in 0.1mol/L H2SO4 (Electrode:new method (Scan rate: 2mV/s).Pt-SnSb/Ti, scan rate: 10mV/s).中国煤化工MYHCNMHG1063.3 The ffeet of orygen evolution metal-oride on the electrooridation of methanolAs shown in Fig.3, the polarization curves of several Ru/Ti clectrodes doped with \In.Co, Eiu, P1 and Ce oxides separately are measured in 0.Imol/L H2SO4 solution. Sumeresearches have confirmedl4- -6| the enhancement of elctroratalytic activity if Ru oxide isadded into the anode coating, the over- voltage being lowered notably. Some mcasurementsare also tried to enhance the electrocatalytic activity of Ru/Ti electrode. Zhang etr.11- 13|attempted to use MnO2 or rare-earth oxides to enhance the oxygen cvolution activityof titanium anode and obtained a desirable result in the application of electro-dleposit ionprocess of zinc. F ig.3 indicates the effect of Mn, Co, Eu, P't and Ce on the oxygen evolutionactivity. Compared with PtRu/Ti electrode, modified electrode doped with Mn or Co oxideare all enhanced of their electrocatalytic activity but RuCe/Ti electrode can't exhibit anyenbancement. Oxygen evolution efirct of RuEu is improved just as the voltage below1.55V, but less than that of PtRu after the electrode potential is swept exceeding 1.55V.In Fig.4, the electrooxidation of methanol begins at 0.2V, and the peak current demnsityvalue (60muA/cm2) is obtained just as the electrode potential is swept at 0.7V. Fron Fig4.we can fnd that the Pt- WOr catalyst exhibits a higher catalytic activity towards met hanolelectrooxidation than platinized Ti electrode without additives, and the main.nn current.density reaches 80mA/cm2. Quite a lot of literatures have reportcd about the efcct of Won anodic electrocatalysis activity in DMFCI8.91. According to speculation, the promct ionalactivity of WO; is related to the W(VI)/W(IV) redox couple acting as a surface mediator .for the oxidation of surface methanolic residues.5U rRuMn40]- -- RuPt00-+ - RuCoRuEu10050-D0-5000.9 1.0 1.1 1.2 1.3 1.4 1.51.6 1.7 1.80.0 0.1 0.2 1.3 0.4 0.5 0.6 0.70.8Voltage. "Voltage. VFig.3 Effect of composite metal oxide on the po-lig4 Effert of composite neLal-oxide ore thelarization curves in 0.1mol/L I2SO4 s0-electro-oxidation of rmet hanol in 0. lmol;Llution (Scan rate: 2mV/s. temperature:1I2S0」+ Imol/I. (H;OH (Scal raler30°C).2mV/s, temperature: 30"(").Polarization curves for RuMn-Pt. RuCo-Pt and RuEu-Pt catalytic made by the ther-mal decomposition method are shown. in Fig.4. With the increase of sweep potential,their polarization curves embody the eiectrooxidation difference in the sare eleelrolyte.The RuMn- Pt composite oxides exhibit all the sarne electrocatalytic infucnce as RCo-Ptfirstly. but the current density of the later can kcep incrcasing until the maxinun Ccurrt ntis obtained at 0.75V. However the current density for methanol electrooxidation afectedl byRuCo-Pt composite oxides does not rise after the sweep potential is over 0.65V. RuF-P1中国煤化工MYHCNMHG107shows almost the sarne effect as W-Pt composite oxide for Inet hanol electrooxidation.In the process of clectrooxidation, methanol molecures are firstly adsorbed on Pt sir-faces, followed by dehydrogenation of adsorbed specics. The dehydrogenation is repeateduntil all the hydrogen has been stripped out and CO2 is formed as the final product. Allintermediates in the oxidation of methanol are oxidized into CO2 in the presence of (Hads. .Whereas the partially dehydrogenated fragments may act as poisons in the absencc ofOHads. The existence of powerful oxygen evolution meral oxides is beneficial to the ad-sorption of oxygen containing species. resulting in the significant enhancement of electrodeelctrocatalytic activity.3.4 Electrode charaterizationScanning electron microscopy (SEM) wasused to observe the morphologies of elec-trodes modified by multimetallic compos-ite oxides. Fig.5 shows the surface micro-graph of anode coating layer prepared fromRuCl3+ MnCl2 solution and H2PtCl6 solu-tion after polarization. Our previous worklI4lindicated that the coating before polariza-tion is fat and has no visible cracks, whileafter polarization it became rough and hadsome obvious local cracks due to the penetra-tion of the electrolyte along fine cracks andgrain boundery into the coating. Fig.5 indi-Fig.5 SEM image of anode coated with RuMn-Ptcates that the coating doped with RuMn-Ptafter polarization.is rough and has homogeneous fine cracks,which leads to a very high activc suface area. Furthermore, as the SnSb coating is usedas the base of composite oxides catalyst, the electron conductivity and the anti-passivityagainst the acid of these electrodes are inherently enhancedl10l. All these call contributcsubstantially to the enbancement of electrocatalytic activity. In the anodic scan of Fig.I.the electrochemical ionization of adsorbed H lakes placo between -0.4 and 0.0V. There-fore, intergration of the i/E curve results in the voltammetric charge, which can be lusedfor measuring the electrochemical active area of the titanium bascd anode. In addition, asreported in Ref.{15], the stripping of saturated CO adlayers can also be used as a mcasrefor the active surface. The voltammetric charges for some mnodified anode are obtained asfollows: Pt/Ti (0.0070C-cm -2), PtRuEu/Ti (0.0067C-cm-2), PtRuCo/Ti (0.0071C.cm- 2)and PtRuMn/Ti (0.0070C.cm -2), this result indicates that the active areas for four elec.trodes above are almost the same. In fact the active areas of titanium lase anodes withthe same geornetry area value are often different. Their perfrrnance evaluation shouldbe realized by normalizing the polarization curves. Although not being nornalized. thepolarization curves of the above four electrodes in this paper can also cmnbodv the lectrocatalytic activity diffcrence because of their almost the same active area.中国煤化工MYHCNMHG108 .4. ConclusionThe most significant sse in the development of useful low-cost higlh-ficiency mct hanolfuel cells for generating elcctric current is the poisoning of the platinurm anodc by rarbonmonoxide that is generated during the oxidation. ( arbon nmonoxide molecures for:edfrorn the early steps of methanol oxidation adsorb on and block polyerystalline plat inrumelectrode surfaces and are not oxidized away by reaction with wiater to make rarbon Ixideunless the anode potential increased to a certain value. It has been forund that l)y ， 'Jlingsome metal oxides to the svstcm such as Ru. W or Mo oxide the carbon monoxido canbe oxidized at a lower potential. Compared the anode polarization curves of Ru.\In/Ti.RuCo/Ti, RuEu/Ti and RuCe/Ti electrode with that of RuPt/Ti clectrode, RuN1n andRuCo composite oxides are found to exhibit a strengthen of oxygen evolution artivity.RuEu composite oxide catalyst only exhibits a slight enhancement below 1.55V. h itsoxygen evolution activity declines after the sweep potential exceeds 1.55V.Adding these composite oxides into the electrocatalytic coating layers of titaniun Hhaseanodes separately, RuMn and RuCo were found to greatly enhance the eletrooxidationactivity of methanol, all these may be ascribed to the promotion of oxidation of