Kinetics of CO2 reduction in KOH/methanol electrolyte on Pb electrode

Journal of Harbin Institute of Technology (Ner Series), Vol. 14, No. 2, 2007Kinetics of CO2 reduction in KOH/ methanol electrolyte on Pb electrodeSONG Shuang, HE Zhi-qiao, YE Jie xu, CHEN Jian-meng宋爽，何志桥，叶 杰旭,陈建孟(ollege of Biogial and Enionmentel Enginering, Zhejiag Univernity of Technology , Hanghou 310032, China)nol electrolyte at diferent temperatures and pressures. A graphite electrode was employed as the counter elec-trode, and an Ag|AgCl (sat. KC1) eletrode was used as the reference eletrode. The Tafel plots of the prod-ucts by the electrochemical reduction of CO2 showed that the formation process of HC00H difered from that ofCo and the reduction of CO2 was not limited by the difusion of CO2 in the investigated potential range. Kinetieanalysis indicated that the reaction orders were 0. 573 for electrochemical reduction of CO2 to CO and 0. 671 forCO2 to HC0OH in the cathodic direction.Key words: CO2 ; electrochemical reduction; Pb electrode; kineticsCLC number: TQ150. 1Document code: AArticle ID: 1005-9113(2007 )02 0240-04As natural oil reserves become depleted, attentiondirecet electrolysis would provide both oxygen to supportwill increasingly focus on alternative energy sources.life and carbon monoxide, which can be used as substituteThe energy and raw materials for this are currently defor hydrogen fuel4l. The specific electrochemical reac-rived from the bumning of coal, with the accompanyingtions and chemical reactions are as follows:release of CO2 as a by-product “，CO2 causes thethe anodicgreenhouse effect and consequent disruption of the nat-40H~→2H20 +O2 +4e.ural environment. Therefore ，conversion and utilizationthe cathodieof CO2 is an important subject in moderm chemistry. As2H20 + 2CO2 +4e- +40H~ +2C0.CO2 is one of the most thermodynamically stable carbon the total reactioncompounds, a highly energetic reductant or an extermnal2CO2-→2C0 +02.source of energy is required to convert it into other car-Large amounts of investigation have been carriedbon compounds. Electrochemical reduction of CO2 isout on the electrochemical reduction of CO2 in water,believed to be one of the most efective methods for themethanol and some organic solvents with different elec-removal and conversion of CO2 because of its conven-trodes to the compounds such as methane, ethanol ,ient operation，simple reductive productions and over-HCOOH and so on (65-10]. However, there are few re-coming the high redox potential of CO2/CO2 2- 3 .ports about the study on the kinetics for the electro-Moreover, the byproduct of respiration is CO2. Onchemical reduction of CO2. In the present work, weearth we exhale CO2 back into the atmosphere and it dif-.studied the partial curent density for CO and HC0OHfuses into the extremely large volume of air that makesformation and the kinetics of the electrochemical reduc-up the atmosphere. This is very important because if thetion of CO2 to CO and HCOOH on Pb electrode inCO2 concentration in the air is very high, greater than0.1 mol/L KOH/ methanol electrolyte at diferent tem-1% for long-term exposure and greater than 3% forperatures and pressures.short-term exposure , deterioration in performance , head-ache and maybe even death can result. In manned1 Experimentalspacecraft an astronaut is given pure oxygen and hebreathes out a mixture of oxygen and CO2. If the mixtureElctrochemical reduction of CO2 was performed inof CO2 and oxygen were ejected, astronauts would havean H-type cell. A Pb (99. 99%) foil was used as theto cary about 6 times as mush oxygen for the same dura-working electrode and a graphite electrode was em-tion. Thus a system that removes CO2 from the air isployed as the counter electrode. The cathode compart-necessary in a manned spacecraft. Spltting CO2 by usingment was separated from the anode compartment withReceived 2005 -08 -29.中国煤化工Sponsored by the National Nalural Science Foundation of China ( Grant No. 504080ence Funatin od Chin(Grunt No. M203034 and Y505036).MYHCNMHG●240●Joumal of Harbin Institute of Technology (New Serie), Vol 14, No.2, 2007a membrane. The cathode potential was measured withCO2 to the electrode in the KOH methanol was 8-respect to an Ag|AgCl (sat. KCl) electrode, connect-chieved in the operating CO2 pressure region.ed with the catholyte through a salt bridge. The Pb foilused as the electrode required polishing each surfacewith successively finer grades of alumina powder down1.6to 0.05 μm, followed by the removal of grease with ac-etone. Finally , the Pb electrode was chemically condi-tioned by immersion in 18 mol/L sulfuric acid for 20 sand was rinsed with pure water and the electrolyte.A discontinuous electroreduction procedure wasused. According to current potential curves, afer CO2gas was bubbled into the catholyte for 30 min, theCO2-saturated solution was reduced at a cathodic polar-ization in the range of -1.5 to -2.1 V vs AgIAgCl0.80 2.4 2.8(sat. KCl) at different temperatures and - 0.6 to0.-0.9 V vs Ag|AgCl (sat. KCl) at different pres-gjsures, respectively. Moreover, a total of 30 coulombs▲258K, O273K, 1 1298 Kof charge passed through the cell.Catholyte and anolyte, 0. 1 mol/L KOH methanolCaseous products obtained during the electrore-Fig. 1 Tafel plots of co formation by eletrochemicalduction were collected in a gas collector and analyzedreduction of CO2 on a Pb electrode inby Nicolet Aratar 370 DTGS FT-IR Analyzer. Productsmethanol at different temperaturessoluble in the catholyte were analyzed using ion chro-matography.2 Results and Discussion1.42.1 Partial Current Density for CO and HCOOHFormation1.2 fIn order to discuss the infuence of the potentialon the partial current densities ( PCD) for CO andHCOOH formation in KOH/ methanol electrolyte, the1.0Tafel plots of the products by CO2 electrochemical reduction on the Pb electrode at different operating tem-01.41822 2.6 3.0 3.4peratures were evaluated. The results are ilustrated inlgjFigs. 1 and 2. The log current density for the formationof HC00H and CO increased linearly. The Tafel slope▲258 K, O273K,I 1298 KCatholyte and anolyte, 0. 1 molVL KOH methanolof HCOOH formation differed from that of CO formationFig.2 Tafel plots of HCOOH formation by electrochemicalregardless of temperature, although both HCOOH andCO were products of the two-electron reduction of CO2.The results indicated that the formation process ofHCOOH differed from that of CO and the productions2.2 Electrode Kineticsof them were not limited by the difusion of CO2 in thisA steady-state curent- voltage relationship is as-potential range. Therefore , adequate transport of CO2sumed of the formto the electrode in K0H/methanol catholyte might bej = jo[exp(aFmn/RT) -exp( -BFmn/RT)]. (1)achieved at different operating temperatures.where η is the overpotential, j is the current density, joTafel plots for CO and HCOOH formation on theis the exchange current density, n is the number of e-Pb electrode at different operating pressures are shownlectrons involved in the reaction, a is the cathodicin Figs.3 and 4, respectively. At every CO2 pressure,transfer coefficient and β is the anodie transfer coeffi-the log curent densities of CO and HCOOH formationcient(a +β = 1). In our experiments, the electro-kept linearly increasingly， and the Tafel slope ofchemical reaction process was in the high overpotentialHCOOH formnation differed from that of CO formationregion (j>j), Eq. (1) is simplified to .regardless of CO2 pressure. The results indicated thatn=中国煤化工303RT/anP)ljHC0OH and Co forned independently of each other,(2)and it could be found that a sufficiently high supply ofHY HCNMHG●241●Journal of Harbin Institute of Technol6gy Necu. Seria)hn0lL.(4, No. 2, 2007.0)Tab.2 The parameters of kinetics for CO and HCOOHformation at different pressureslic0HCOOH,p/MPa- aj%/(A.m-2) a in/(A.m-2)fo.0.0.08630. 1090.112 41. 4000. 09344. 259.0.105 66. 0690.08859.0200.104 86.0990.081.73. 1317 104.592.2 tThe variation of the logarithm of the exchange cur-.71.9、2.1、2.32:rent density versus 1/T is plotted in Fig.5 and Fig. 6shows the exchange curent density in the Tafel regiona a function of the CO2 pressure. These figures00.2 MPa. x0.4.MPa.10.8 MPa, ▲1.2MPp .showed clearly that the least squares correlation whichCatholyte aad anolytpl 0. i mqi/L KOH/methamo'resulted from these analyses wasFig.3 Tafel plots of CQ formation by electrochemicaljo.co = 105P0204 exp(-7127.8/T)，(3)reduction of CO2 on a Pb electrode inmethart at different pressuresjo.HCOOH = 10*pang.x0exp( -6577.6/T). (4)wherejn was in A/cm2 for T from 258 to 298 K, CO2pressures from 0.2 to 1.2 MPa.0.40.2十d.国-0.20.3-0.4-0.6。-0.8.|0.21.0-1.2202.2、 2.4hgj-1.8-1 10.2M年10MPo0.8 Mrr9x12MP8-20 -0.00340.00360.0038Calholyle and anolyte, 0.1 mal/L KOH merthanolFig.4 Tafel plots of HCOOHformformation by electrochemicalVT/K+reduction of CO2 on a Pb gelectrgde in methanol at■HCOOH,▲co .diferent pressuresFig.5 Relationship between the exchange currentdensity and the temperatureAccording t:thia'equation, , the hkintie paramelersofa and jo fon CO,and HCQOHformation at differentoperating temperatures and for which at different oper-2.0 tating pressures were calculateidiroitrithe slope andin-tercept:iof the Tafel plotslof η on' lgj as shdwn inFigs. 1 -4, respectively. The detailed data re tabula-主1.8tedin Tabs.1 artd 2. From Tabs.1 and 2 , ;the reiactioni imeversible tolsome extent..Tabrhrs) The parameters: 鴨kinetics for co and HCOOH.w formation.然diterent temperatures."沙k”山=HCQOHadiav (A.m-)叫1ai’ jo1(A.m~2)gp2980.078。‘2. 231“0.146".~ 0.41027361906..5) 0.273)750E.0.045中国煤化工(5)2580. 1210. 0540. 1810.014CNMHnge curent densityYHGol CO2●242●.Jourmal of Harbin Instiute of Technology (New Series)，Vol. 14, No. 2, 2007Without consideration of the Nemnst effect, the re-tion.action orders can be estimated from the following equa-References:tionm= (algjo/alg Pco2)r.em"(5)[1] StevensC B, Rada T, Raguse B. Energy storge by theelectrochemical reduction of CO2, to Co at a porous Auwhere Pcon is CO2 pressure and φa is the equilibriumflm. Journal of Electroanalytical Chenistry, 2002, 526 (1potential.-2): 125-133. .Accounting for the Nermnst effect, the reaction or-2] Sullivan B P. Electrochemical and Electrocatalytic Reduc-ders for the cathodic partial curents n, Z is .tion of CO2. Amslerdam: Elsevier, 1993.Z =m+a.(6)[3] Xu YJ, ChenF M, Zhu M Y. The progress in the reduc-From Eqs. (3), (4), (5) and (6) and consid-tion technology of CO2. Chemical Industry and Engineeringering the average values of a for CO and HCOOH for-Progress, 1995 (3): 22-27.mation acording to Tab.2, the calculated reaction or-[4] Breedlove B K, Ferrence G M，Washington J, et al. Aphotoeletochemical approach to spliting CO2 for a man-ders for CO and HCO0H formation were 0. 573 andned mission to mars. Materials and Design, 2001, 220.671, respectively. Consequently, it was found that(7): 577 - 584.the reaction of HCOOH formation was more active than[5] Mizuno T, Kawamo M, Kaneco s, et al. Electrochemicalthat of CO formation.reduction of CO2 at Ti and hydrogen - storing Ti electrodesin KOH - methanol. Electrochimica Acta, 1998, 43 (8):3 Conclusion899 -907.[6] KanecoS, liba K, Ohta K, et al. Electrochemical reduc-The electrochemical reduction of CO2 on a Pb etion of CO2 on Ag in KOH + methanol at low temperature.Elctrochimica Acta, 1998, 44 (4): 573 - 578.lectrode in 0. 1 mol/L KOH/ methanol electrolyte was[7] Kaneco S, liba K, Ohta K, et al. Electrochemical reduc-studied. The Tafel plots of CO and HCOOH by thetion of CO2 on Au in KOH + methanol at low temperature.electrochemical reduction of CO2 indicated that the for-Joumal of Electroanalytical Chemistry, 1998, 441 (1 -mation process of HCOOH differed from that of CO and2): 215 -220.the reduction of CO2 was not limited by the diffusion of8] KanecoS, Hiei N H, Xing Y, et al. Electrochemical con-CO2 in the polential range. The correlations that resul-version of carbon dioxide to methane in aqueous NaHCO3ted from analysis of the logarithm of the exchange cursolution at less than 273 K. Electrochemical Acta, 2002,48 (1): 51 -55.rent density versus 1/T and the log exchange curent[9] Todoroki M, Hara K, Kudo A, et al. Electrochemical re-density on the log CO2 pressure in the operating condi-duction of high pressure at Pb, Hg and In electrodes in antion wereaqucous KHCO， solution. Jourmal of Electroanalyticaljo.co = 10°.2poQ.4exp( - 7127. 8/T) ,Chemistry, 1995, 394 (1 -2): 199 -203.jo.Hcoom = 10*- Pco2: sexp( - 6577. 6/T).[10]Schrebler R, Cury P, Herrena F,et al. Study of the elec-trochemical redurtion of CO2 on electrodeposited rhenium eThe reaction orders in the cathodic direction forlectrodes in methanol media. Journal of ElectroanalyticalCO and HCOOH formation were 0.573 and 0.671 con-Chemistry, 2001, 516(1-2): 23 -30.sidering the Nernst ffct. The experimental results[11]Zha Q X. Introduction to Electrode Process Kinetics. Beishowed that the reaction of HCOOH formation was morejing: Science Press, 2002.active than that of CO formation in the operating condi-中国煤化工MYHCNMHG●243●