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 2006An Electrochemical Investigation a06-3471(2006)02-0148-06Article ID: 100Methanol crossover in DMFCsYOU Meng-di', CHENG Xuan", LIU Lian, ZHANG Lu(1. Department of Chemistry, 2. Department of Materials Science and EngineeringState Key Laboratory for Physical Chemistry of Solid SurfacesXiamen University, Xiamen 361005, Fujian, China)Abstract: 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 tonvestigate methanol crossover through Na-fion117from anode to cathode The results revealed that the amount of methanol in the cathode side is dependent 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 methanolidation The simulative dmfc test also showed that the methanol crossover caused a sudden decline in theKey words: Methanol crossover, Methanol oxidation, Cyclic voltammetry, ChronoamperometryCLC Number: TM 911.4Document Code:A1 Introductionanol oxygen fuel cell is 1. 18 V at 25 C. This valueResearch and development activities on direct is comparable to that for a hydrogen oxygen fuel cellwhich is 1.23 vl4 However, in practice, DMFCsmethanol fuel cells( DMFCs) have gained importancein recent years because of their potential applicationshave a much lower open circuit voltage(OCv).Oneas stationary and portable power sources. Methof the major reasons is that methanol can crossis an attractive fuel because its energy density is much through the proton exchange membrane(PEM),suchas Nafion to reach the cathode sidehigher than that of hydrogen, and it is an inexpensiveliquid that is easy to handle, store and transport 2diffusion( by a concentration gradient) and electroDMFCs provide the most versatile options for cleanosmotic drag( by protons ). Such crossover not onlyand efficient power production. It can be a useful results in a waste of fuel, but also causes an intemalchemical short to the fuel cells and lowers the cellpower source over a wide spectrum of energy requirements, from national defense to civil use and manyperformance. Most of the methanol crossing over willbe electrochemically oxidized at the cathode Such another fields. As a fuel cell. DMFCs are the mostoxidation reaction lowers the cathode potential and alpromising candidates for portable power applicationssible potential for a meth. so camost commonly used methods of deter中国煤化工Received date: 2005-11-22, Corresponding author, TEL: (86-592)218CNMHGupported by the Key project, the National Natural Science Foundation of China( 20433060第2期游梦迪等:甲醇渗透的电化学研究mining methanol crossover to date are monitoring the are more convenient and faster than conventional CO 2CO2 flux from the cathode effluent gas and electro- analysis method, but can only provide information onchemical techniques. The former is mainly using an methanol crossover at an open circuit, which is differoptical IR CO, 8 sensor or gas chromatography ( 9. ent from conventional CO, analysis methodMeasurement by precipitation as BaCO, has also beenIn addition differential electrochemical massreported( 10 The method of CO, measurement is con- spectrometry(DEMS)was first developed and intro-venient for studying effects on methanol crossover of duced by Wolter and Heitbaum in early 1980sDMFC operating conditions, particularly cell current A methanol sensor based on the amperometric methodIn this method, it is assumed that the and direct measuring the methanol concentrationCH,OH crossed to cathode is completely oxidized to method was also used to studyanol crossover (131CO2, which is unlikely in practical operation. Fur-In this work, the methanol crossover through Nathermore, the crossover of Co, from thee anodeto the fion@ 117 was investigated using cyclic voltammetrycathodered. Thus, the measurement of and chronoamperometry. An Hpe cell was con-CH, OH crossover by monitoring CO, at the cathode is structed and a simulative dmfc test was carried oulikely to be inaccurate and requires lengthy and care- to investigate methanol crossover on OCV at ambientful calibration when usedemperatureElectrochemical techniques are also widely used2 Experimentalto study methanol crossover. A voltammetric methodhas been developed in Los Alamos National Laborato-Pretreated Nafion@ membranes(117)wereDuring the measurement, nitrogen was introstored in ultra-pure water before used. The previousduced into the cathodethermal history of the membranes was found to affectapplied using a power supply. The reaction occurringthe ability of the membrane to take up water. Henceat the cathode is the oxidation of methanol that crossesmembranes for all the studies were subjected to thethrough the membrane. Whenapplied voltage is same treatment and a fresh membrane was used forhigh enough to quic kly oxidize all the methanol diffu- each studysing to the cathode side, a limiting current is a-Electrochemical measurements were carried outchieved. This limiting current approximately repre-in an H-shape cell with the membrane separating thesents the rate of methanol crossover at an open cir- two compartmentscuit. By recording the potential(E)of a PRw/c voltammetry and chronoamperometry were used toelectrode using potentiometric method during CH, OH study the methanol permeability of the membrane in acrossover, it was found that the slope( dE/dt)of E conventional three-electrode cell. A carbon supportedversus t( time)curve is proportional to methanol platinum on a glassy carbon( Pt/C/GC)and acrossover rate[. From the time required to reach the smooth platinum electrode were used as the workingequilibrium concentration of CH, OH on both sides of and counter electrodes. The base electrolyte was 0.the pem, the methanol crossover rate can be calculat. mol.L-H2S0. Saturated calomel electrode(scEed. A new and convenient approach was established was used as a reference electrode throughout the ex-to estimate methanol permeability through such simple periments. The initial voltage and sweep rate wereelectrochemical techniques as cyclic voltammetry and 0. 241V(vs. SCE)and 50 mV.", respectivelythibility was studied by中国煤化工in a two-compartment cell with the membrane separa- intromol·LCH2OHinting the compartments. From the slope of permeability 0CNMH Ge of the cell and arcurve at various intervals, the methanol permeability equal volume of 0. 5 mol . L H2 SO4 to cathodehas been calculated. The abovementioned methods side By analyzing the solution of the cathode side in50电化学06年methanol permeability was obtained. In chrono- Peak C)lol dissociation and adsorption on theometric measurements, a potential of 0.85 Velectrode surface also suppressed the adsorptionnst SCE reference electrode was applied and the sorption of hydrogen and decreased their peak currentsteady-state currents were determined. A simulative densitiesDMFC test was carried out using the H-shape cell tostudy methanol crossover on OCV. The carbon sup-mol·L-1rted RuPt and carbon supported Pt were, respecIItively, used as the anode and cathode catalysts.80……0.25Adding required volume of 1 mol. L-CH,OH in0.750.5 mol L"'H,SO, to anode side of the cell and an 40equal volume of 0.5 mol. L-H, SO 4 to cathodede. Air was supplied to the cathode side by a com-02000.2040.60.81.0used to analyze the solution in the cathode side ex-situ E 4uter the cell operated for different periods of time. Aseries of standard methanol solutions were also ob-tained using cyclic voltammetry in a home-made halfcell for a comparison. All the electrochemical experi-ments were performed using AUTOLAB PGSTAT30020.002040.60.81.0cal workstation and at ambient tempera-EV(vs SCE)3 Results and DiscussionFigure la shows typical CV curves for standard500.75methanol solutions in 0.5 mol.L-H.SO. The re-070gions of hydrogen adsorption-desorption (I)and meth一 *-peak currentanol oxidation(Il)are enlarged for more detailed in-300.65formation. The current densities and potentials ofPeak a are given in Fig. Ib. In general, the peak0.600.00.2040.60.81.0current density (I,)and peak potential (E. )forC/mol·L1methanol oxidation Peak A) during the forwardsweep were found to rise with an increase in methanolFig. 1 Cyclic voltammograms for standard methanol soluconcentration as evident in Fig. 1b. This could be attions in 0. 5 mol - L H,SO4 solution with the en-tributed to the increased coverage with methanol aslarged hydrogen regions( a)and the detailed inforthe concentration increases, which might decrease thefor Peak A(b)amount of adsorbed oxygen containing speciesscan rate:50mV·s(OH d)on the surface of the electrode The amountof increased OH d formation at a more positive potenCyclic voltammograms obtained from the cathodetial results in a faster rate of methanol oxidation andshape cell after different periods of time at aaccordingly increases the current density of peak roor中国煤化工 tionary condition are“A”141. On the other hand, as the concentrationCNMHG initially containedcreases,the peak related to methanol oxidation 0.5 mol. L-I H SO, solution while the anode con-d a shoulder at a lower potential( indicatedtained0.5mol·LH2SO4 and 1 mol·LCH3OH第2期游梦迪等:甲醇渗透的电化学研究151solutions. After each time interval, the solution of the ocv for 10 hours. The Cv curve from the standardcathode side was analyzed in-situ using cyclic volta- solution of 1 mol.L CH, OH( Fig. 1a), the CVmetry. It can be seen that the peak current density for curve obtained from the cathode side of H-cell undermethanol oxidation( Peak A' )during the forward stationary 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 crossed OCV for 10h in Fig. 5. It is obvious that the peak offrom the anode side to the cathode side. Compared methanol oxidation appeared, suggesting the presencethe Cv curves in Fig. 2 with the standard CV curves of methanol crossed from the anode to the cathodein Fig. la, the shapes of methanol oxidation peaks However, as compared with those observed from the(A and A')and shoulder peaks(C and C"), as well standard methanol solution, the position, shape andas the hydrogen adsorption-desorption behaviors re- magnitude of Peak a and B were significantly differmained unchanged, while the shape of Peak Bob- ent. Nevertheless, for the same permserved during the reverse scan was more well-defined (10 h) in 1 mol .L CH, OH, methanol crosthan that of peak bthrough Nafion 117 from anode to cathode under sta-The chronoamperomograms obtained at the anode tionary condition was more severe than that operateand cathode sides of H-cell after different permeation at OCV conditiontime are presented in Fig 3. The steady-state currentdensities were then evaluated and plotted against thepermeation time in Fig 3c. It is evident that the计tAsteady-state current density for methanol oxidation in-creased with permeation time in the cathode side but33hdecreased in the anode side. After 48 hours the cur-48hent densities for methanol oxidation were found not tobe equal showilibrium of methanol in both020.002040.6081.0sides. The oxidation current density for the cathodeside is only 4.26 mA. cm while that for the anodeIIside is 7. 02 mA. cm". The results were significant-405ly different from that reported by Ramyadue to dif-ferent pretreated membranes and different experimen-tal systems. It should be pointed that there was no ag-10itation in the present work020.00.2040.60.81Using the H-shape cell, a simulative DMFC testE/V(vs SCE)was carried out to study the effect of methanol cross- Fig. 2 Cyclic voltammograms obtained at the cathodeover on OCV. The OCV as a function of operatingside of H-cell after different time intervals withtime is shown in Fig 4. It was observed that the OCvgradually increased at the beginning, then declinedrapidly from 0.42V to 0. 11v(indicated by an ar- 4 Cconclusionsrow), and finally stabilized at about 0. 1V after oneMethanol crossover through Nafion@membranesand half an hour. The decrease in ocv might be中国煤化 Ty cycle voltammetrycaused by the crossover of methanol from the anode to andunt of methanol inCNMHGcathode. To verify this point, the solution in the cath- the CauwasLunt un the timeof peneode side was analyzed ex-situ using cyclic voltamme- tration. The peak current density and potential fortry after the simulative DMFC cell was operated at methanol oxidation during the forward sweep in-152电化学200625201515-r-anode≤6.5a eatable0⑩o-203040691020304050t/sFig 3 Chronoamperomograms obtained at the anode side(a)and cathode side(b)of H-cell after different timeintervals variation of the steady-state current densities with permeation time(c)gradually at the beginning then declined rapidly fromH-cell at oCAnode: Imol.L CH,OH+0.SmalL" H,So,0. 42 V to 0. 11 V, and finally stabilized at aboutCathode: babbled with air in 0. Smol.L H,so0.1 V after one and half an hour0References0.1[1] Scott K, Taama W. Performance of a direct methanol fuel cell[J]. J. Appl Electrochem. 1998, 28: 289[2]McNicol B D, Rand DAJ, williams K R. Direct meth-anol-air fuel cells for road transportation[ J].J.PowerSources,1999,83:15~31Fig 4 Variation of OCV with operating time during a[3]National Laboratory[J]. J. Power Sources, 2000,86111-116140[4]QiZ G, Kaufman A. 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Electrochemistmembranes of direct methanol fuel cells [J.J.Power1998,1:73~7Sources,2003,117:98-101[15] Ramya K, Vishnupriya B, Dhathathreyan KS Methanol[12] Kuver A, Potje-Kamloth K. Comparative study of meth-permeability studies on sulphonated polyphenylene ox-anol crossover across electropolymerized and commer-ide membrane for direct methanol fuel cell J].J.Newcial proton exchange membrane electrolytes for the acidMaterials for Electrochemical Systems. 2001, 4: 115direct methanol fuel cell [J]. Electrochimica ActaDMFC中甲醇渗透的电化学研究游梦迪',程璇”,刘连!,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Hel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1.5h后趋于稳定关键词:甲醇渗透;甲醇氧化;循环伏安;计时电流中国煤化工CNMHG