Electrocatalytic enhancement of methanol oxidation by adding CeO2 nanoparticle on porous electrode

Available online at www.sciencedirect.com: ScienceDirectRARE EARITHSELSEVIER .JOURNAL OF RARE EARTHS, VoL 30, No.1, Jan. 2012, P.29www.re-journal.com/en/Electrocatalytic enhancement of methanol oxidation by adding CeO2nanoparticle on porous electrodeFENG Xiaojuan (冯晓娟), SHI Yanlong (石彦龙), zHOU Hujuan (周慧娟)(College of Chemistry and Chemical Engineering, Hexi Universiy, Zhangye 734000, China)Received 7 April 2011; revised 25 August 2011Abstract: The polyanilin/polysulfone (PAN/PSF) composite flms were prepared by electropolymerization, and then CeOr-Pt particles werecodeposited into this composite film to obtain the CeO-Pl-modifed polyanilie/polysulfone (CeO2:-PV/PANPSF) electrodes. Their morphol-ogy and chemical component were characterized by feld emission scanning electron microscopy (FESEM) and energy dispersive X-rayspectroscopy (EDS), respectively. The results sbowed that the composite film had bi-layer structure with asymmetrical pores, and platinumand cerium oxide particles were homogencously dispersed in the modified flm electrodes. The cyclic voltammetry (CV) and electrocbemicalimpedance spectroscopy (EIS) techniques were applied to investigate the electocalytic activity of the Pt-CeO2/PAN/PSF electrodes. It wasindicated that appropriate amount of CeO2 could enhance the catalytic activity of Pt for methanol electro oxidation. Chronoamperometry (-t)measurements revealed that the Pt-CeO/PAN/PSF electrode was relatively endurable for intermediate production. In addition, different mix-ing amounts of Pt and CeO2 nanoparticles were also investigated in detail.Keywords: porous electrode; electrocatalytic activity; methanol oxidation; rare earthsIn the last decade, direct methanol fuel cells (DMFCs)advantage of being coated easily by other materials. Theyhave attracted extensive interest from scientists and engi-own high accessible surface area, good stability and low re-neers around the world-y because the fuel offers many ad-sistancel". Consequently, dispersing metal particles intovantages, such as higher energy density and lower weight.PAN flms is an attractive method to prepare electrocata-However, there are several barriers to their application, in-lysts' 8.19. Generally, these conductive polymers were di-cluding the poisoning of Pt electrocatalyst by adsorbed COrectly deposited onto the surface of the conductive substratespecies, as well as substantial power losses due to the rela-as working electrode by electrochemical polymerization, buttively low activity of the methanol oxidation catalysts4.the brittleness, unsteadiness and desquamation of active de-Therefore, Pt-based alloys or Pt/metal oxide catalysts arposited layer constitute major obstacles to its extensive ap-considered as a best choice for anodic and cathodic fuel cell.plication. Thus, preparing supporting materials with largeNowadays, many different metal mixed oxides such asporosity is necessary for Pt-CeO2 nanoparticles dispersion.CeO2, NiO, MoO, TiO2, V2Os, and La2O3 have been em-In the present investigation, a study was carried out mix-ployed to increase electrocatalytic activity for methanol oxi-ing Pt catalyst with various amounts of CeO2 nanoparticle asdation!5-1o. Particularly, CeO2 has received considerable at-anode materials. The porous PAN/PSF composite films weretention because cerium oxide is a fluorite oxide whoseprepared with some modifications. The platinum and CcO2cations can switch between +3 and +4 oxidation states and itparticles were deposited in this composite flm by CV depo~has the ability to act as an oxygen buffer because CeO2 cansition. It was found out that the oxidation currents dependedrelease oxygen reversibly. Researchers have described thaton amount of CeO2 content, and in this case, 7 wt%CeO2 exhibits high activity during the oxidation of CO atCeO2-PVPAN/PSF electrode had dramatically high current.acidic medial". In addition, the importance of the support-The results showed that the Pt catalyzing activity was im-ing material structure for the activity of the catalyst has beenproved while there was nearly no side efect from the badconcernedl2,1s!. Carbon and carbon nanotubes (CNTs), ap-electron conductivity of CeO2. It is expected that this com-peared to be promising supporting materials for platinumposite film can be used as a promising anode materials forand platinum-based oxides catalysts. However, the pristinedirect methanol fuel cells (DMFCs).surface of these materials is relatively inert and difficult tosupport particles homogeneouslyl4 l6, which often results in1 Experimentalthe agglomeration and drop of nanoparticles. Compared with中国煤化工other substrates, conducting polymer of polyaniline has theFoundation item: Project supported by the National Natural Science Foundation of China (20475077)MHCNMHGCorresponding author: FENG Xiaojuan (E mail: fengriajuan820@yaboo.cn; Tel: +86 936-8282066)DOI: 10.1016/S1002. 0721(10)60633-3.FENG Xiaojuan et al, Electrocatalytic enhancement of methanol oxidation by adding CeO2 nanoparicle on porous electrode31to obtain loading without agglomeration. Secondly, its holestrue oxidation signify catalytic effect and CcO2 acts as asupply an unblocked path for transporting reaction compo-promoter for methanol electro-oxidation.nents while its cross-link frameworks fumish the electro-The mechanism of methanol oxidation on CeO2-PVPAN/chemical reaction with large surface. So the composite filmPSF electrode can be speculated as follows: methanol is ini-is an excellent substrate for depositing electrocatalysts.tially adsorbed on the Pt, while simultaneously losing itsThe EDS data are shown in Fig. 2. The obvious signal ofmethanol proton to a basic oxide ion. The methoxy speciesplatinum and CeO2 indicates that two kinds of particles haveformed get oxidatively decomposed to CO speciesl(1):been deposited on the composite film. The peaks ofC, N andPt(CHzOH)axs- +Pt-(CO)as+4H*+4e~(1)S elements are due to polyaniline and polysulfone in theWith the increase of polar potential, water moleculecomposite film.gradually decomposed OH species which were adsorbed on2.2 Cyelic voltammetry (CV) for methanol electrooxi-CeO2, producing an species CeO2-(OH)as:CeO2+H2O-→+CeO2-(OH)ads+H*+e~(2)dationCeO2-(OH)ads species can transform CO-like poisoningThe cyclic voltammogram curves of PVPAN/PSF elec-species (COas) on Pt to CO2, releasing the active sites on Pttrodes with different amounts of CeO2 for methanol elec-for further electrochemical reaction.trooxidation are shown in Fig. 3, respectively. From the fig-Pt-COs+CeO2-(OH)ads- +Pt+CeO2+CO2+H+e~3)ure, we can see that methanol oxidation begins at approxi-It is reasonable to expect that the incorporation of the oxy-mately 500 mV and reaches its maximum peak current atgen storage material into the electrode surface might lead toabout 650 mV. It is notable that the peak current of metha-an increase in the local oxygen concentration, thereby in-nol electro-oxidation on CeO2-PVPAN/PSF is considerablycreasing the methanol electrooxidation activity.higher than that on PVPAN/PSF, the oxidation current in-creases with increase in the CeO2 content, and23 Current-time curves for methanol electrooxidation7%CeOx~PVPAN/ PSF electrode has the highest peak current,To evaluate the endurance of the composite electrodes, thebut when the CeO2 content arrives at 10% the current de-electrolysis was performed at constant potential of0.65 V increases suddenly. It is shown that appropriate amount is very1.0 mol/L CH;OH with 0.5 mol/L H2SO4 solution for 1 h.important for methanol eleectro-oxidation. Although CeO2Fig.4 shows typical current-time responses on 7%CeO2-PVdoes not have good electron conductivity, it can promote COPAN/PSF (1) and PVPAN/PSF (2) electrodes. As shown insignificantly. This result infers that Pt is responsible for thethe figure, both of them present a current decay in cur-800frent-time measurements, which was also observed for tradi-tional Pt atributing to the formation of some Pt oxide or ad-PiMa600sorbed intermediates such as co in methanol electrooxida-tion reaction!. However, the most interesting feature de-serving our attention is that the current decay on the4007%CeO2-PtPAN/PSF electrode is less than that on200CcMa1PVPAN/PSF electrode. This is supposed that the Pt-CeO2modified composite film electrode is relatively endurable forCI Kaintermediate production.CeLa1CuKa1 PtLal0.1234..56789102.4 Electrochemical impedance spectroscopy (EIS) formethanol electrooxidationFig. 2 EDS image of 7%CeO2-PVPAN/PSF electrodeElectrochemical impedance spectroscopy analysis is aJ1) PUPANPSF1.4十0.8 +(2) 59%CeO,PVPANPSF :+3) 79%CeO,-P/PANPSFJ(4) 10%CcO,;PtPAN/PSF1.00.6 15) 12%CeO),PVPANPSE0.8-自0.4+4)白0.6-,(.4-0.2-.2-1)_.0-(20.02+中国煤化工2500 30000 35000..2).40.6.8.0E1V (vs. SCE)TYHCNMHGFig. 3 Cyclic voltammetry of methanol electrooxidation on dfferentFig 4 Chroroaunperolneune curves oI r-ueO2/PAN/PSF (1) andPVPAN/PSF (2) electrodes in 1 mol/L CH2OH+0.5 molLcontent ofCeO2 inl molL CH;OH+0.5 mol/L H2SO4 solutionH2SO4 solution at 0.65 v.32JOURNAL OF RARE EARTHS, VoL 30, No, 1, Jar. 20121200300800.2V0.45 V70010.7V200。500g1; 300h 4000叶00)-1000501001502002503003500100200300400500-800 -600 - 400- 200210Z'/Z1QFig 5 Impedance specta of 7%CeOrPVPANPSF ectrode at dfferenet potentialsvery powerful tool to characterize the electrochemical proc-cating a change in the mechanism or at least a change in thess. To better evaluate the performance of the CeO2, the im-predominance of certain processes. Because Tafel slope ispedance plots of 7%CeO2rP/PAN/PSF electrode in 1 mol/Lsmall in the low potential region, the unit reaction involvingCHzOH+0.5 mol/L H2SO4 solution were carried out at dif-the first electron transfer is rate-determining step. So it canferent potentials. Fig 5 shows the impedance of methanolbe concluded that a splitting of the first C H bond ofelectrooxidation at potentials of 0.20, 0.45 and 0.7 V, reCH3OH molecule with the first electron transfer is rate-de-spectively. The EIS results indicate that the methanol elec-termining step of methanol electrooxidation at the low po-trooxidation on the 7%CeO2-P/PAN/PSF electrodes at vari-tentials. However, with an increase of potential, the Tafelous potentials shows different impedance behaviors. Theslope becomes big at high potentials, which indicates thattransition from resistive behavior to pseudo-inductive berate-determining step is changed. Reaction between ad-havior is more clearly evident in the corresponding Nyquistsorbed CO on Pt sites and the formation of local oxygen onplots (from 0.2 to 0.45 V). At 0.2 V, an arc is evident in theCeO2 sites were hypothesized to be rate-determining step.complex- plane plots, which indicates the presence of a resis-This result is consistent with the above EIS results.tive component. The resistive component may be due to thereaction resistant of the methanol dehydrogenation reaction3 Conclusionsand/or the oxidation process of intermediate producs!29. At0.45 V, pseudo-inductive behavior is observed, which hasThe CeO2-Pt/ PAN/PSF composite film electrode withbeen atributed to COds oxidation241. At high potential ofbi-layer asymmetrical porous structure was successfully0.70 V, the electrode maybe produce surface passivation,prepared by the electropolymerization method. The analysiswhich makes the real component of the impedance becomeof electrochemical method indicated that CeO2-Pt modifiednegative. The passivating layer protects the Pt surface fromcomposite film electrodes had good activity and high endur-contacting methanol, and so the higher potentials lead to lowance for methanol oxidation in comparison with Pt modifiedactivity of 7 wt.%CeOr-PVPAN/PSF electrode.composite film electrodes. By adding the CeOr nanoparticleson the composite film, the methanol electro-oxidation cur-2.5 Tafel curvesrent could be promoted greatly. The reason for the increasedTafel plot of the 7%CeO2-PVPAN/PSF electrode is shownactivity was analyzed and ascribed to the promotion of COin Fig. 6. 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