DMFC中甲醇渗透的电化学研究

第12卷第2期电化学Vol.12 No.22006年5月ELECTROCHEMISTRYMay 2006Aricle ID:1006-3471 (2006 )02-0148-06An Electrochemical Investigation ofMethanol Crossover in DMFCs .YOU Meng-di' ,CHENG Xuan*2 , LIU Lian' ,ZHANG Lu'(1. Department of Chemistry ,2. Department of Materials Science and Engineering,Slate Key Laboratory for Physical Chemistry of Solid Sufaces ,Xiamen University, Xiamen 361005 , Fujian ,China)Abstract: Methanol crossover and its eflet on the open-circuit volage (OCV) in DMFCs were studied usingcyclic voltammetry and chronoamperometry under stationary condition and at ambient temperature . An H-shapecell was constructed and a simulative DMFC test was carried out to investigate methanol crossover through Na-fion@ 117 from anode to cathode. The results revealed that the amount of methanol in the cathode side is depend-ent upon the time of penetration. As the concentration of methanol increased , the hydrogen adsorption-desorptionon the surface of electrode was suppressed and a shoulder peak appeared during the forward sweep for methanoloxidation. The simulative DMFC test also showed that the methanol crossover caused a sudden decline in the0CV.Key words : Methanol crossover, Methanol oxidation, Cyclic voltanmetry , ChronoamperometyCLC Number: TM 911.4Document Code: A1 Introductionanol oxygen fuel cell is 1.18 V at 25 C. This valueResearch and development activities on directis comparable to that for a hydrogen oxygen fuel cell,methanol fuel cells ( DMFCs) have gained importancewhich is 1. 23 vl4. However, in practice, DMFCsin recent years because of their potential applicationshave a much lower open circuit voltage (0CV). Oneas stationary and portable power sources'l. Methanolof the major reasons is that methanol can crossis an atractive fuel because its energy density is muchthrough the proton exchange membrane (PEM) , suchhigher than that of hydrogen, and it is an inexpensiveas Nafion, to reach the cathode side via physicalliquid that is easy to handle, store and transport(2].diffusion ( by a concentration gradient) and electro-DMFCs provide the most versatile options for cleanosmotic drag ( by protons). Such crossover not onlyand eficient power production31. It can be a usefulresults in a waste of fuel, but also causes an interalchemical short to the fuel cells and lowers the cellpower source over a wide spectrum of energy require-performance. Most of the methanol crossing over willments, from national defense to civil use and manyother fields. As a fuel cell, DMFCs are the mostbe electrochemically oxidized at the cathode. Such anoxidation reaction lowers the cathode potential and al-promising candidates for portable power applications.A thermodynamic reversible potential for a meth-so consumes some cathode reactant5-7].The two most commonly used methods of deter-Received date :2005-11-22, * Corresponding author ,TEL;(86592)2187701 ,E-mail :xcheng@ xmu. edu. cnSupported by the Key Projeet, the Nationai Natural Science Foundation of China (20433060)●150.电化学2006年situ , methanol permeability was obtained. In chrono-Peak C)，methanol dissociation and adsorption on theamperometric measurements, a potential of0.85 V aelectrode surface also suppressed the adsorption-de-gainst SCE reference electrode was applied and thesorption of hydrogen and decreased their peak currentsteady -state currents were determined. A simulativedensities.DMFC test was carried out using the H-shape cell to140study methanol crossover on OCV. The carbon sup-120Ccro/mol.L-Bported RuPt and carbon supported Pt were, respec-0.1IItively, used as the anode and cathode catalysts.目800.25Adding required volume of 1 mol . L-1 CH3 0H in600.750.5 mol●L-' H2SO, to anode side of the cell and an40....... 1equal volume of 0.5 mol ●L-' H2 SO, to cathode”20Iside. Air was supplied to the cathode side by a com-pressor at ambient pressure. Cyclic voltammery was-20-0.20.00.20.40.60.810used to analyze the solution in the cathode side ex-situr4--十一8(21after the cell operated for different periods of time. Aseries of standard methanol solutions were also ob-tained using cyclic voltammetry in a home-made half三20cell for a comparison. All the electrochemical experi-ments were performed using AUTOLAB PGSTAT30-0.20.00.20.40.60.8 1.0electrochemical workstation and at ambient tempera-EV(vs.SCE)ture.a)0.803Results and DiscussionFigure 1a shows typical CV curves for standard百50+0.75methanol solutions in 0.5 mol●L~H2SO4. The re-400.70示gions of hydrogen adsorption-desorption(I) and meth-一*- peak currentanol oxidation(II) are enlarged for more detailed in-一●- peak position0.65formation. The curent densities and potentials oPeak A are given in Fig. 1b. In general, the peak.0.0 0.20.4 0.60.81.00.60current density (I,) and peak potential ( E。) foCrn,ox/mol ●L1methanol oxidation ( Peak A) during the forwardsweep were found to rise with an increase in methanolFig.1 Cyclic voltammograms for standard methanol solu-concentration as evident in Fig. 1b. This could be at-tions in 0.5 mol●L-' HSO, solution with the en-tributed to the increased coverage with methanol aslarged hydrogen regions(a) and the detailed infor-the concentration increases，which might decrease themation for Peak A (b)amount of adsorbed oxygen containing speciesscan rate: 50 mV●g-1(OH) on the surface of the electrode. The amountof increased OH formation at a more positive poten-Cyclic voltammograms obtained from the cathodetial results in a faster rate of methanol oxidation andside of H-shape cell after different periods of time at aaccordingly increases the current density of peakroom temperature and under a stationary condition are“A"[14). On the other hand, as the concentration in-provided in Fig. 2. The cathode initially containedcreases，the peak related to methanol oxidation0.5 mol●L-' H2SO, solution, while the anode con-showed a shoulder at a lower polential (indicated as tained 0.5 mol ●I-1 H,SO, and 1 mol . L CH,OH第2期游梦迪等:甲醇渗透的电化学研究●151●solutions. After each time interval , the solution of the0CV for 10 hours. The CV curve from the standardcathode side was analyzed in-situ using cyclic voltam-solutionof 1 mol●L-' CH,OH (Fig. 1a), the CVmetry. It can be seen that the peak current density forcurve obtained from the cathode side of H-cell undermethanol oxidation ( Peak A') during the forwardstationary for 10h (Fig. 2a) are compared with thesweep increased with time of penetration increasing,CV curve from the cathode side of H-cell operated atindicating the increased amount of methanol crossed0CV for 10h in Fig.5. It is obvious that the peak offrom the anode side to the cathode side. Comparedmethanol oxidation appeared , suggesting the presencethe CV curves in Fig, 2 with the standard CV curvesof methanol crossed from the anode to the cathode.in Fig. 1a, the shapes of methanol oxidation peaksHowever, as compared with those observed from th(A and A')and shoulder peaks (C and C'), as wellstandard methanol solution, the position, shape andas the hydrogen adsorption-desorption behaviors remagnitude of Peak A and B were significantly differ-mained unchanged, while the shape of Peak B' ob-ent. Nevertheless， for the same permeation timeserved during the reverse scan was more well defined(10h) in 1 mol●L-' CH3 0H, methanol crossedthan that of Peak B.through Nafion⑧117 from anode to cathode under sta-The chronoamperomograms obtained at the anodetionary condition was more severe than that operatedand cathode sides of H cell after different permeationat 0CV condition.time are presented in Fig. 3. The steady-state currentdensities were then evaluated and plotted against the60cathodeII)hpermeation time in Fig.3c. It is evident that the...3hsteady-state current density for methanol oxidation in-creased with permeation time in the cathode side but20.....8hdecreased in the anode side. After 48 hours, the cur-rent densities for methanol oxidation were found not tobe equal showing no equilibrium of methanol in both-0.2 0.00.20.40.60.8 1.0sides. The oxidation current density for the cathodeside is only4.26 mA● cm-' while that for the anodeA' II5040个side is7.02 mA●cm - 2. The results were signifcant-ly diferent from that reported by Ramyal1s] due to dif-0。20百ferent pretreated membranes and different experimen-10-tal systems. It should be pointed that there was no g~-6itation in the present work.-0.20000.2040.608100E/V(vs.SCE)Using the H-shape cell, a simulative DMFC testwas carried out to study the effect of methanol cross-Fig.2 Cyclic voltammograms obtained at the cathodeover on OCV. The 0CV as a function of operatingside of H-cell after diferent time intervals withtime is shown in Fig. 4. It was observed that the 0CVthe enlarged hydrogen regionsgradually increased at the beginning, then declinedrapidly from 0.42V to 0.11V ( indicated by an ar-4 Conclusionsrow), and finally stabilized at about 0.1V after oneMethanol crossover through Nafion⑧ membranesand half an hour. The decrease in 0CV might be(117) was investigated directly by cyclic voltammetrycaused by the crossover of methanol from the anode toand chronoamperometry. The amount of methanol incathode. To venify this point, the solution in the cath-the cathode side was dependent on the time of pene-ode side was analyzed ex-situ using cyclic voltamme-tration. The peak current density and potential forty after the simulative DMFC cell was operated atmethanol oxidation during the forward sweep in●152.电化学2006年30nanodecathade8.0一目20}... 33h20}....33h7.515}.1548h.0.5方ande0102030402030 4001020304050t/s因)b)Fig.3 Chronoamperomnograms obtained at the anode side (a) and cathode side (b) of H-cell afer dfferent timeintervals variation of the steady-state current densities with permeation time( c)0.5gradually at the beginning then declined rapidly fromH-cell at 0CV0.4Anode: lmol.L' CH,0H+0.Smal.L H,so,0.42 V to0. 11 V, and finally stabilized at aboutCathode: bbled vit ir in 0.5mol.L' HSO,0.1 V after one and half an hour..3)0.24References :[1] Scott K,Taama W. Performance of a direct methanol fu-0.1-el cll[J]. J. Appl Elctrochem. 1998, 28 :289.[2] McNicol B D, Rand D AJ, Williams K R. Direct meth ,0246810thanol-air fuel cells for road transportation[J]. J. PowerSources, 1999, 83:15 ~31.Fig.4 Variation of 0CV with operating time during a[3] Ren Xiaoming, Zelenay Piotr, Thomas Sharon,et al.simulative DMFC testRecent advances in direct methanol fuel cells at AlamosNational Labortory[J]. J Power Sourees, 2000, 86:111 ~ 116.140[4] Qi Z C,Kaufman A. Open circuit volage and methanolcrossover in DMFCs[J].J. Power Sources, 2002, 110:tandard nolutiotatinuy coe 10k177 ~ 185.80oCv oerted.h[5] Ticoli v, Crreta N, Bartoloxzi M. A comparative in60-vestigation of proton and methanol transport in fluorina-40ted ionomeric membranes[J]. J Electrochem. Soc. ，20 t2000, 147 :1286 ~ 1290.0[6] Satoru Hikita, Kinitaka Yasuo Nakajima. 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Methanol[12] Kiver A ,Poje-Kamloh K. Comparalive study of meth-permeability studies on sulphonated polyphenylene 0x-anol crossover across electropolymerized and commer-ide membrane for direct methanol fuel cel[J].J. Newcial proton exchange membrane electrolytes for the acidMaterils for Electrochemical Systems. 2001, 4:115 ~direet methanol fuel cell[J]. Electrochimica Acta,DMFC中甲醇渗透的电化学研究游梦迪',程璇"，刘连'，张 璐'(1.厦门大学化学系; 2.厦门大学材料科学与工程系,固体表面物理化学国家重点实验室,福建厦门361005)摘要:设计并建立甲醇渗 透测试体系和模拟直接甲醇燃料电池( DMFC)运行体系,分别考察静态条件下Hcell中甲醇的渗透和运行条件下甲醇渗透对0CV的影响循环伏安和计时电流法测试表明:随着渗透时间的延长,阴极侧的甲醇浓度增加;甲醇浓度增加,氧化峰电流增大,峰电位正移,氢在电极表面的吸脱附受到抑制,同时甲醇的正向氧化电流曲线出现肩峰模拟DMFC实验测试结果表明:OCV先逐渐上升,接着发生突降,大约1.5 b后趋于稳定.关键词:甲醇渗透; 甲醇氧化;循环伏安;计时电流