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H-cell中甲醇的渗透和运行条件下甲醇渗透对OCV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 h后趋于稳定.Methanol crossover and its effect on the open-circuit voltage(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 thatthe amountof 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 theOCV.作者联系地址：厦门大学化学系,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,厦门大学化学系,厦门大学化学系 福建厦门361005Author's Address: 1.Department ofChemistry,2.Department ofMaterials Science and Engineering,State Key Laboratory for Physical Chemistry ofSolid Surfaces,Xiamen University,Xiamen361005,Fujian,Chin

CHENG Xuan

LIULian

YOUMeng-di

ZHANG Lu

刘连

张璐

游梦迪

程璇

Xiamen University Institutional Repository

第 12卷 　第 2期2006年 5月电化学ELECTROCHEM ISTRYVol. 12　No. 2May 2006A rticle ID: 100623471 (2006) 0220148206Received date: 2005211222, 3 Corresponding author, TEL: (862592) 2187701, E2mail : xcheng@ xmu. edu. cnSupported by the Key Project, the National Natural Science Foundation of China (20433060)An Electrochem ical Investigation ofM ethanol Crossover in DM FCsYOU Meng2di1 , CHENG Xuan3 2 , L IU L ian1 , ZHANG Lu1(1. D epartm en t of Chem istry, 2. D epa rtm en t of M ateria ls Science and Engineering,S ta te Key L abora tory for Physica l Chem istry of Solid Surfaces,X iam en U niversity, X iam en 361005, Fu jian, China)Ab s trac t:Methanol crossover and its effect on the open2circuit voltage (OCV ) in DMFCs were studied usingcyclic voltammetry and chronoamperometry under stationary condition and at ambient temperature . An H2shapecell was constructed and a simulative DMFC test was carried out to investigate methanol crossover through Na2fionµ 117 from anode to cathode. The results revealed that the amount of methanol in the cathode side is depend2ent upon the time of penetration. A s the concentration of methanol increased, the hydrogen adsorp tion2desorp tionon the surface of electrode was supp ressed 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 theOCV.Key wo rds: Methanol crossover, Methanol oxidation, Cyclic voltammetry, ChronoamperometryCLC Number: 　TM 911. 4　　　　　　　　Document Code: 　A1　 In troductionResearch and development activities on directmethanol fuel cells (DMFCs) have gained importancein recent years because of their potential app licationsas stationary and portable power sources[ 1 ] . Methanolis an attractive fuel because its energy density ismuchhigher than that of hydrogen, and it is an inexpensiveliquid that is easy to handle, store and transport[ 2 ].DMFCs p rovide the most versatile op tions for cleanand efficient power p roduction[ 3 ] . It can be a usefulpower source over a wide spectrum of energy require2ments, from national defense to civil use and manyother fields. A s a fuel cell, DMFCs are the mostp rom ising candidates for portable power app lications.A thermodynam ic reversible potential for a meth2anol oxygen fuel cell is 1. 18 V at 25 ℃. This valueis comparable to that for a hydrogen oxygen fuel cell,which is 1. 23 V[ 4 ]. However, in p ractice, DMFCshave a much lower open circuit voltage (OCV ). Oneof the major reasons is that methanol can crossthrough the p roton exchange membrane ( PEM ) , suchas Nafionµ, to reach the cathode side via physicaldiffusion ( by a concentration gradient) and electro2osmotic drag ( by p rotons). Such crossover not onlyresults in a waste of fuel, but also causes an internalchem ical short to the fuel cells and lowers the cellperformance. Most of the methanol crossing over willbe electrochem ically oxidized at the cathode. Such anoxidation reaction lowers the cathode potential and al2so consumes some cathode reactant[ 527 ] .The two most commonly used methods of deter2m ining methanol crossover to date are monitoring theCO2 flux from the cathode effluent gas and electro2chem ical techniques. The former is mainly using anop tical IR CO2[ 8 ] sensor or gas chromatography [ 9 ] .Measurement by p recip itation as BaCO3 has also beenreported [ 10 ] . The method of CO2 measurement is con2venient for studying effects on methanol crossover ofDMFC operating conditions, particularly cell currentdensity. In this method, it is assumed that theCH3 OH crossed to cathode is comp letely oxidized toCO2 , which is unlikely in p ractical operation. Fur2thermore, the crossover of CO2 from the anode to thecathode is ignored. Thus, the measurement ofCH3 OH crossover by monitoring CO2 at the cathode islikely to be inaccurate and requires lengthy and care2ful calibration when used.Electrochem ical techniques are also widely usedto study methanol crossover. A voltammetric methodhas been developed in Los A lamos National Laborato2ries[ 7 ] . During the measurement, nitrogen was intro2duced into the cathode side and a positive voltage wasapp lied using a power supp ly. The reaction occurringat the cathode is the oxidation of methanol that crossesthrough the membrane. W hen the app lied voltage ishigh enough to quickly oxidize all the methanol diffu2sing to the cathode side, a lim iting current is a2chieved. This lim iting current app roximately rep re2sents the rate of methanol crossover at an open cir2cuit. By recording the potential ( E ) of a PtRu /Celectrode using potentiometric method during CH3 OHcrossover, it was found that the slope ( dE / d t) of Eversus t ( time ) curve is p roportional to methanolcrossover rate[ 11 ]. From the time required to reach theequilibrium concentration of CH3 OH on both sides ofthe PEM , the methanol crossover rate can be calculat2ed. A new and convenient app roach was establishedto estimate methanol permeability through such simp leelectrochem ical techniques as cyclic voltammetry andchronoamperometry[ 12 ]. Experiments were carried outin a two2compartment cell with the membrane separa2ting the compartments. From the slope of permeabilitycurve at various intervals, the methanol permeabilityhas been calculated. The abovementioned methodsare more convenient and faster than conventional CO2analysis method, but can only p rovide information onmethanol crossover at an open circuit, which is differ2ent from conventional CO2 analysis method.In addition, differential electrochem ical massspectrometry (DEMS) was first developed and intro2duced by Wolter and Heitbaum in early 1980’s [ 12 ] .A methanol sensor based on the amperometric methodand direct measuring the methanol concentrationmethod was also used to study methanol crossover[ 13 ].In this work, the methanol crossover through Na2fionµ117 was investigated using cyclic voltammetryand chronoamperometry. An H2shape cell was con2structed and a simulative DMFC test was carried outto investigate methanol crossover on OCV at ambienttemperature.2　Exper im en ta lPretreated Nafionµmembranes ( 117 ) werestored in ultra2pure water before used. The p reviousthermal history of the membranes was found to affectthe ability of the membrane to take up water. Hence,membranes for all the studies were subjected to thesame treatment and a fresh membrane was used foreach study.Electrochem ical measurements were carried outin an H2shape cell with the membrane separating thetwo compartments as anode and cathode sides. Cyclicvoltammetry and chronoamperometry were used tostudy the methanol permeability of the membrane in aconventional three2electrode cell. A carbon supportedp latinum on a glassy carbon ( Pt/C /GC ) and asmooth p latinum electrode were used as the workingand counter electrodes. The base electrolyte was 0. 5mol·L - 1 H2 SO4. Saturated calomel electrode ( SCE)was used as a reference electrode throughout the ex2periments. The initial voltage and sweep rate were -0. 241V ( vs. SCE) and 50 mV·s- 1 , respectively,throughout the CV tests. Permeability was studied byintroducing required volume of 1 mol·L - 1 CH3OH in0. 5 mol·L - 1 H2 SO4 to anode side of the cell and anequal volume of 0. 5 mol·L - 1 H2 SO4 to cathodeside. By analyzing the solution of the cathode side in2·941·第 2期 　　 游梦迪等 :甲醇渗透的电化学研究situ, methanol permeability was obtained. In chrono2amperometric measurements, a potential of 0. 85 V a2gainst SCE reference electrode was app lied and thesteady2state currents were determ ined. A simulativeDMFC test was carried out using the H2shape cell tostudy methanol crossover on OCV. The carbon sup2ported RuPt and carbon supported Pt were, respec2tively, used as the anode and cathode catalysts.Adding required volume of 1 mol·L - 1 CH3 OH in0. 5 mol·L - 1 H2 SO4 to anode side of the cell and anequal volume of 0. 5 mol·L - 1 H2 SO4 to cathodeside. A ir was supp lied to the cathode side by a com2p ressor at ambient p ressure. Cyclic voltammetry wasused to analyze the solution in the cathode side ex2situafter the cell operated for different periods of time. Aseries of standard methanol solutions were also ob2tained using cyclic voltammetry in a home2made halfcell for a comparison. A ll the electrochem ical experi2ments were performed using AUTOLAB PGSTAT30electrochem ical workstation and at ambient tempera2ture.3　Results and D iscussion　　Figure 1a shows typ ical CV curves for standardmethanol solutions in 0. 5 mol·L - 1 H2 SO4. The re2gions of hydrogen adsorp tion2desorp tion ( I) and meth2anol oxidation ( II) are enlarged for more detailed in2formation. The current densities and potentials ofPeak A are given in Fig. 1b. In general, the peakcurrent density ( Ip ) and peak potential ( Ep ) formethanol oxidation ( Peak A ) during the forwardsweep were found to rise with an increase in methanolconcentration as evident in Fig. 1b. This could be at2tributed to the increased coverage with methanol asthe concentration increases, which m ight decrease theamount of adsorbed oxygen containing species(OHads ) on the surface of the electrode. The amountof increased OHads formation at a more positive poten2tial results in a faster rate of methanol oxidation andaccordingly increases the current density of peak“A”[ 14 ] . On the other hand, as the concentration in2creases, the peak related to methanol oxidationshowed a shoulder at a lower potential ( indicated asPeak C) , methanol dissociation and adsorp tion on theelectrode surface also supp ressed the adsorp tion2de2sorp tion of hydrogen and decreased their peak currentdensities.Fig. 1　Cyclic voltammogram s for standard methanol solutions in0. 5 mol·L - 1 H2 SO4 solution with the enlarged hydro2gen regions( a) and the detailed information for Peak A( b)scan rate: 50 mV·s- 1Cyclic voltammogram s obtained from the cathodeside of H2shape cell after different periods of time at aroom temperature and under a stationary condition arep rovided in Fig. 2. The cathode initially contained0. 5 mol·L - 1 H2 SO4 solution, while the anode con2tained 0. 5 mol·L - 1 H2 SO4 and 1 mol·L- 1CH3 OH·051· 　　　　　　　　　　　　电 　化 　学 2006年solutions. After each time interval, the solution of thecathode side was analyzed in2situ using cyclic voltam2metry. It can be seen that the peak current density formethanol oxidation ( Peak A′) during the forwardsweep increased with time of penetration increasing,indicating the increased amount of methanol crossedfrom the anode side to the cathode side. Comparedthe CV curves in Fig. 2 with the standard CV curvesin Fig. 1a, the shapes of methanol oxidation peaks(A and A′) and shoulder peaks (C and C′) , as wellas the hydrogen adsorp tion2desorp tion behaviors re2mained unchanged, while the shape of Peak B ’ ob2served during the reverse scan was more well2definedthan that of Peak B.The chronoamperomogram s obtained at the anodeand cathode sides of H2cell after different permeationtime are p resented in Fig. 3. The steady2state currentdensities were then evaluated and p lotted against thepermeation time in Fig. 3c. It is evident that thesteady2state current density for methanol oxidation in2creased with permeation time in the cathode side butdecreased in the anode side. After 48 hours, the cur2rent densities for methanol oxidation were found not tobe equal showing no equilibrium of methanol in bothsides. The oxidation current density for the cathodeside is only 4. 26 mA·cm - 2 while that for the anodeside is 7. 02 mA·cm - 2. The results were significant2ly different from that reported by Ramya[ 15 ]due to dif2ferent p retreated membranes and different experimen2tal system s. It should be pointed that there was no ag2itation in the p resent work.U sing the H2shape cell, a simulative DMFC testwas carried out to study the effect of methanol cross2over on OCV. The OCV as a function of operatingtime is shown in Fig. 4. It was observed that the OCVgradually increased at the beginning, then declinedrap idly from 0. 42V to 0. 11V ( indicated by an ar2row) , and finally stabilized at about 0. 1V after oneand half an hour. The decrease in OCV m ight becaused by the crossover of methanol from the anode tocathode. To verify this point, the solution in the cath2ode side was analyzed ex2situ using cyclic voltamme2try after the simulative DMFC cell was operated atOCV for 10 hours. The CV curve from the standardsolution of 1 mol·L - 1 CH3 OH ( Fig. 1a) , the CVcurve obtained from the cathode side of H2cell understationary for 10h ( Fig. 2a) are compared with theCV curve from the cathode side of H2cell operated atOCV for 10h in Fig. 5. It is obvious that the peak ofmethanol oxidation appeared, suggesting the p resenceof methanol crossed from the anode to the cathode.However, as compared with those observed from thestandard methanol solution, the position, shape andmagnitude of Peak A and B were significantly differ2ent. Nevertheless, for the same permeation time(10 h) in 1 mol·L - 1 CH3 OH, methanol crossedthrough Nafionµ117 from anode to cathode under sta2tionary condition was more severe than that operatedat OCV condition.Fig. 2　Cyclic voltammogram s obtained at the cathode sideof H2cell after different time intervals with the en2larged hydrogen regions4　Conclusion sMethanol crossover through Nafionµ membranes(117) was investigated directly by cyclic voltammetryand chronoamperometry. The amount of methanol inthe cathode side was dependent on the time of pene2tration. The peak current density and potential formethanol oxidation during the forward sweep in2·151·第 2期 　　 游梦迪等 :甲醇渗透的电化学研究Fig. 3　Chronoamperomogram s obtained at the anode side ( a) and cathode side ( b) of H2cell after different time intervalsvariation of the steady2state current densities with permeation time ( c)Fig. 4　Variation of OCV with operating time during a sim2ulative DMFC testFig. 5　A comparison of voltammogram s obtained at thecathode side of H2cell under stationary conditionfor 10 h and after the cell was operated at OCVfor 10 hcreased, while the desorp tion and adsorp tion of hy2drogen on the electrode surface were supp ressed withan increase in methanol concentration. During thesimulative DMFC test, it was found that the OCV rosegradually at the beginning then declined rap idly from0. 42 V to 0. 11 V, and finally stabilized at about0. 1 V after one and half an hour.R efe rence s:[ 1 ]　Scott K, Taama W. Performance of a direct methanol fu2el cell[ J ]. J. App l Electrochem. 1998, 28: 289.[ 2 ] McN icol B D, Rand D A J, W illiam s K R. D irect meth2anol2air fuel cells for road transportation [ J ]. J. PowerSources, 1999, 83: 15～31.[ 3 ] 　Ren Xiaom ing, Zelenay Piotr, Thomas Sharon, et al.Recent advances in direct methanol fuel cells at A lamosNational Laboratory[ J ]. J. Power Sources, 2000, 86:111～116.[ 4 ]　Q i Z G, Kaufman A. Open circuit voltage and methanolcrossover in DMFCs[ J ]. J. Power Sources, 2002, 110:177～185.[ 5 ]　Tricoli V, Carretta N, Bartolozzi M. A comparative in2vestigation of p roton and methanol transport in fluorina2ted ionomeric membranes [ J ]. J. Electrochem. Soc. ,2000, 147: 1286～1290.[ 6 ]　Satoru H ikita, Kim itaka Yasuo Nakajima. Measurementof methanol crossover in direct methanol fuel cell [ J ].JSAE Review, 2001, 22: 151～156.[ 7 ]　Ren Xiaom ing, Sp ringer Thomas E, Zawodzinski Thom2as A, et al. Methanol transport through Nafion mem2branes electro2osmotic drag effects on potential stepmeasurements[ J ]. J. Electrochem Soc. , 2000, 147:466～474.[ 8 ]　Dohle H, D ivisek J, Mergel J , et al. Recent develop2ment of the measurement of the methanol permeation ina direct methanol fuel cell [ J ]. J. Power Sources,2002, 105: 274～282.[ 9 ]　Bogdan Gurau, Eugene Smotkin S. Methanol crossoverin direct methanol fuel cells: a link between power and·251· 　　　　　　　　　　　　电 　化 　学 2006年energy density[ J ]. J. Power Sources, 2002, 112: 339～352.[ 10 ] 　J iang Rongzhong, Chu Deryn. Comparative studies ofmethanol crossover and cell performance for a DMFC[ J ]. J. Electrochem. Soc. , 2004, 151: A69～A76.[ 11 ] 　Nookala Munichandraiah, Kimberly McGrath, SuryaPrakash G K, et al. A potentiometric method of moni2toring methanol crossover through polymer electrolytemembranes of directmethanol fuel cells [ J ]. J. PowerSources, 2003, 117: 98～101.[ 12 ]　Küver A, Potje2Kam loth K. Comparative study of meth2anol crossover across electropolymerized and commer2cial p roton exchange membrane electrolytes for the aciddirect methanol fuel cell [ J ]. Electrochim ica Acta,1998, 43: 2527～2535.[ 13 ]　Narayanan S R, Valdez T I, Chun W. Design and oper2ation of an electrochem icalmethanol concentration sen2sor for direct methanol fuel cell system s[ J ]. Electro2chem ical and Solid2State Letters, 2000, 3: 117～120.[ 14 ]　W en Gangyao (文纲要 ) , Zhang Ying (张颖 ) , YangZhenglong (杨正龙 ) , et al. Investigation of methanolanode electrooxidation catalysts[ J ]. Electrochem istry,1998, 1: 73～78.[ 15 ]　Ramya K, V ishnup riya B, Dhathathreyan K S. Methanolpermeability studies on sulphonated polyphenylene ox2ide membrane for direct methanol fuel cell[ J ]. J. NewMaterials for Electrochem ical System s. 2001, 4: 115～120.DM FC中甲醇渗透的电化学研究游梦迪 1 , 程　璇 3 2 , 刘　连 1 , 张　璐 1(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H2cell中甲醇的渗透和运行条件下甲醇渗透对 OCV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 h后趋于稳定.关键词 : 　甲醇渗透 ; 甲醇氧化 ; 循环伏安 ; 计时电流·351·第 2期 　　 游梦迪等 :甲醇渗透的电化学研究

An Electrochemical Investigation of Methanol Crossover in DMFCs

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An Electrochemical Investigation of Methanol Crossover in DMFCs

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