发表英文文的期刊财经DOC

论文中英文摘 要

作者姓名:张浩

论文题目:电化学电容器用碳纳米管阵列及其复合电极的制备与性能

作者简介:张浩,男,1981年10月出生,2003年7月毕业于国防科技大学航天与材料工程学院,获应用化学专业工学学士学位,同年9月师从于防化研究院杨裕生院士/研究员,从事碳纳米管阵列及其复合材料制备与性能研究,于2016年6月获博士学位后,继续留在防化研究院杨裕生院士领导的"军用化学电源研究发展中心"从事教学科研工作.博士论文成果以系列论文形式集中发表在相关研究领域的权威刊物上.截至2016年,在国内外学术期刊上发表与博士论文相关学术论文27篇(18篇为第一作者),其中SCI论文21篇(有1篇IF>,10.0,共12篇IF>,3.0),已累计被引50余次(SCI检索),作为主要研究人员获军队科技进步二等奖一项,获2件国内专利授权,应邀为Springer出版社撰写一章名为《Supercapacitorsbasedon3DNanoarchitectures》的英文书籍文稿,并应邀成为JACS,ACSNano,Carbon,Electrochem.Commun.,Electrochim.Acta,Mater.Res.Bull.和Mater.Phys.Chem.等国内外期刊的论文审稿人.

中文摘 要

电化学电容器因具有高比功率和长寿命等突出优点在军用,民用两方面高速发展.电极材料是提高电化学电容器性能的关键,碳纳米管阵列(CNTA)具有优良的电导率和发达的中,大孔结构,有望获得优良的电容性能.高质量CNTA的生长机理与可控制备仍是全世界研究的热点,在集流体上直接生长CNTA难度更大,另外关于CNTA电容性能的报道较少,对CNTA电极的储能特点,性能优势认识不足.本论文以研究高质量CNTA的制备和高性能CNTA基电极为目标,设计并制备出几种高容量,高功率特性的CNTA及其复合电极,系统研究了这些电极的"原料—制备—结构—性能"间的关联,阐明了它们的储能特点.在高性能CNTA及其复合电极的纳米结构形成机理,可控制备和储能机制方面取得了一些成果.

(1)制备高性能CNTA电极的先决条件是制备出高质量的CNTA.本论文用电子束蒸发的方法在硅基底上镀上Al2O3缓冲层和Fe催化剂,对催化剂膜进行预处理,再应用CVD法制备出了高达1.3mm的CNTA.

研究表明,预处理条件对Fe/Al2O3催化剂膜的形貌,活性和随后的CNTA生长都有显着影响,只有形状规则,尺寸和分布密度合适的纳米化催化剂颗粒才具有高的催化活性,50SCCM(毫升/分)氢气流量下,600~800oC升温过程中预处理8min是较合适的条件,能得到由直径约30nm,分布均匀的高活性催化剂颗粒.基于系统的AFM,SEM,EDX研究分析,提出了预处理条件对催化剂膜形貌和CNTA生长的影响机制:高温条件下氢气可以显着改变催化剂/缓冲层/气氛的表面能,进而改变缓冲层和催化剂的形貌,氢气能促使氧化铝缓冲层形成一定起伏,而预处理形成的Fe纳米颗粒形成于氧化铝起伏层的顶部,进而催化生长CNT.

CNTA生长过程中,乙烯流量,氢气流量以及生长时间均会影响CNTA的高度,乙烯流量和氢气流量还会影响到催化剂上碳的"吸附—分解—扩散—析出"过程,进而对制得的碳纳米管的直径,管壁数和石墨层结构产生显着影响:乙烯流量越高,氢气流量越低,制得的CNT直径就越大,管壁越厚.因此,可通过控制生长条件制备具有特定性质的CNTA,如含有单壁或双壁碳纳米管的CNTA.CNTA的石墨层结构与其高度存在一定联系:CNTA越高石墨层结构越好.

(2)在硅基底上生长CNTA的工艺最为成熟,但硅导电性差,在其上生长的CNTA难以直接用做电极板.本论文设计出一种简便,有效的"切割-粘贴"(cut-paste)法:将硅基底上生长的CNTA切割下来,粘贴到导电集流体上制成CNTA电极,制得的CNTA电极能保持原始CNTA的微观结构.

CNTA电极在有机,无机电解液体系中分别获得25和15F/g比容量,以及良好的倍率性能,在无机体系中容量较小,这一反常现象的原因是CNTA具有强的疏水性.由于具有较大的孔径和通孔结构,CNTA电极在离子液体中的比容量和倍率性能与其在有机电解液中相当,这同其它多孔电容炭不同.

无序碳纳米管(ECNT)电极与CNTA电极微观结构的差异使得两种电极性质相差较大:CNTA电极具有更为发达的中孔结构和电子导电网络,从而获得高于ECNT电极的离子电导率和电子电导率,以及较低的等效串联电阻,因此具有较好的倍率性能.

(3)将CNTA直接生长在导电集流体上制备电极(直接法),能改善CNTA与集流体的接触,使电极获得更小的接触电阻和更优良的电化学性能.应用固相浮动催化法,在镀氧化铝缓冲层的不锈钢,钽和玻态炭基底上直接制备出85~90μm高的CNTA,应用负载催化法,在镀氧化铝缓冲层的不锈钢,钽和铜基底上直接制备高出达400μm的CNTA,研究表明只要基底的表面结构在高温还原性气氛下保持稳定,便能用直接法在其上制备出高质量的CNTA.这加深了对导电性基底上生长CNTA的认识,拓宽了可用于生长CNTA的基底的范畴.

直接法制备CNTA电极的比容量30F/g,略高于间接法制备的CNTA电极,倍率性能则要远高出后者,原因是CNTA与集流体接触更好,ESR更小,与负载催化法相比,固相浮动催化法制备的CNTA电极ESR值更小,功率特性更好.

CNTA基有机电解液体系电容器可获得高达3.5V的工作电位,较长的循环寿命,基于CNTA质量的比功率和比能量性能分别为928kW/kg和19Wh/kg.通过分析实验结果,参考双电层储能的相关理论,认为CNTA电容器有望获得1.39Wh/kg的比容量和62.5kW/kg的比功率.

(4)为进一步提高CNTA的比容量,提出了以新型双电层电容电极—碳纳米管阵列(CNTA)电极为基材,均匀沉积高比容量准电容电极材料的新思路.利用CNTA电极高比表面,规则大介孔及优异的三维导电网络结构,制备了具有独特纳米结构和优异储能特性的准电容/CNTA复合电极材料,包括具有纳米"管套管"结构的聚苯胺/CNTA复合材料和具有"蒲公英花状"纳米结构的氧化锰/CNTA复合材料(见图1),解决了目前存在的双电层电容电极材料比容量低,准电容电极材料倍率特性差的问题.

图1将准电容材料沉积到CNTA电极之上制备CNTA复合电极示意图

采用电沉积方法在CNTA基体中每根CNT表面可控地沉积7nm厚的聚苯胺(PANI)层,制备出具有纳米管阵列结构的PANI/CNTA复合电极材料,该材料具有1030F/g的比容量,294A/g的大电流密度下比容量仍有789F/g,5000次循环容量衰减5.5%,倍率性能和寿命均高于PANI/ECNT复合材料的性能.研究并提出该复合电极的储能机制:纳米尺寸的PANI层均匀地附着在CNTA上,首先,CNT之间的中,大孔和PANI活性层中的微孔构成了分级式的孔结构,有利于电解液离子的快速迁移,离子电导率较高,其次,在充放电的过程中,离子在电极纳米尺寸PANI层内的扩散长度(L)很短,保证了电极材料的高利用率,第三,由于CNT具有优异的导电性且直接与集流体(Ta箔)连接,使CNTA/Ta基体具有良好的导电性,为电荷传输提供了电子"高速公路",电极电子电导率较高,第四,作为基体的CNT具有优异的机械性能.

PANI/CNTA做锂电池正极材料,具有99mAh/g的比容量,20,60和260C的高倍率下,仍能获得57,54和37mAh/g的比容量,高于文献报道的PANI/ECNT复合材料的倍率性能.

(5)氧化锰(MO)做电化学电容器电极材料具有寿命长和密度高的优点.本论文以CNTA为基体,采用电沉积法制备出具有蒲公英花状纳米结构的MO微粒与CNTA的复合电极材料,复合材料中氧化锰的质量和体积比容量分别为199F/g和307F/cm3,在77A/g的大电流密度下仍能保持101F/g的比容量,倍率性能高于目前氧化锰及氧化锰基复合材料,20000次循环充放电后容量仅衰减3%.在该类复合材料中,纳米尺寸的氧化锰颗粒规则地分布在孔结构规则的CNTA上,且倾向位于在CNT间的接触点上.该类复合电极表现出优异电容性质与其这种独特的结构有关:首先,纳米管之间的中,大孔和氧化锰纳米花中的微孔构成了分级式的孔结构,有利于电解液离子的快速迁移,保证了在短时间内有充足的离子与活性物质作用,其次,在充放电过程中,氧化锰"花瓣"的纳米尺寸减少了离子在电极内的扩散长度(L),提高了质子交换的速率,保证了电极材料的高利用率,第三,由于CNT具有优异的导电性且直接与Ta箔连接,每个氧化锰纳米花均由至少两根高性能"导线"(CNT)与集流体相连,提高了电极的电子电导率,第四,作为基体的CNT具有优异的机械性能,可以减少由于机械因素引起的循环衰减.此外,与粉末状电极材料不同,MO/CNTA复合电极在电极制备过程中不需要任何的粘结剂和导电剂,制备工艺简单,且容易对电极进行后期整体加工,具有较好的研究前景.

研究表明,可通过使用添加剂和热处理进一步提高复合材料的性能:醋酸根能促使电沉积的氧化锰形成纳米薄片状结构,具有较高的比表面积,质量和体积比容量,分别为322m2/g,302F/g和453F/cm3,钴的掺入改善了电极材料的质子传导性,提高复合电极的电导率,降低ESR值,热处理使得复合电极中氧化锰晶化,倍率性能提高.

MO/CNTA复合电极在锂电池中具有较高的比容量(246mAh/g)和较好的倍率性能(50C下51mAh/g),高于文献报道的氧化锰基电极材料的结果.

两类CNTA基纳米复合材料的纳米结构形成机制和性能均在本学位论文中进行了深入的探讨,为其它相关纳米电极材料的设计,制备奠定了理论与实践基础,也开辟了合成同时具有高比表面,分级式孔结构以及优异导电性复合材料的新思路.这些先进功能材料将在各种化学电源,催化,化学传感器,微电子等方面有较广阔的应用前景.关 键 词:电化学电容器,碳纳米管阵列,聚苯胺,氧化锰,复合材料

PreparationandPerformanceofCarbonNanotubeArrayandCarbonNanotubeArray-basedCompositeElectrodeorElectrochemicalCapacitors

ZhangHao

ABSTRACT

Electrochemicalcapacitorsheattractedgreatinterestinbothmilitaryandpopularapplicationsbecausetheyhehighspecificpowerandlongcyclelife.Electrodematerialsarethekeyproblemofimprovingtheperformanceofelectrochemicalcapacitors.Carbonnanotubearray(CNTA)canobtaingoodcapacitiveperformancebecauseitpresentssuperiorelectronicconductivityanddevelopedmeso/macroporousstructure.ThefabricationofCNTAhasattractedgreatinterestintheworld,however,itisstillhardtogrowCNTAdirectlyoncurrentcollectors.Moreover,therearefewstudiesonthecapacitiveperformanceofCNTAandthesestudieallshortoftheunderstandingofCNTAelectrode'senergystoragecharacteristicsandadvantages.Inordertosolvetheseproblemsandmeetthedemandofelectrochemicalcapacitororhigh-performanceelectrodematerials,thisdissertationstudiedthepreparationandenergystoragecharacteristicsofCNTAelectrode,andthendesignedandsynthesizedseveralhighperformanceCNTA-basedpositematerials.Therelationshipsbetweenprecursor,fabrication,structure,andperformanceofthesematerialswerestudiedsystematically,theenergystoragecharacteristicsofthesenovelmaterialswereillustrated.Themainresultsareaollows:

(1)ThepreconditionoffabricationofCNTAelectrodeisthepreparationofCNTA.AcatalystfilmofFe(1.2nm)/Al2O3(30nm)wasdepositedbyelectron-beamevaporationontotheSisubstrate,heattreatedinH2,andthenusedtogrowa1.3-mm-longCNTAwithnicecrystallinitybychemicalvapordeposition(CVD).Ourresultsconfirmthatthemorphologyandactivityofcatalystfilmsareverysensitivetothepretreatmentconditions.Onlyuniformlydispersedanddenselypackedcatalystparticles,whichcanbepreparedbysimplyannealingthecatalystfilmat800°Cfor8minin50sccmH2,hehighactivity.BasedontheresultsofAFM,SEM,andEDX,wesuggestthatthecatalystparticlesareapttoformatthetopoftheAl2Oarticlesafterpretreatment,whichisduetothefactthatahydrogenatmospheregenerallyleadstoamuchbetterwettingbehiorandthenanincreaseofthecontactanglebetweencatalystparticleandbufferlayer.

EthyleneandhydrogenflowratesandgrowthtimeinfluencethelengthofCNTA,furthermore,ethyleneandhydrogenflowratescaninfluencethediameter,wallnumber,andgraphitesheetcrystallinityofnanotubes:largerethyleneflowrateorallerhydrogenflowrateleadstonanotubeswithlargerdiameterandmorewallnumber,andthenCNTAcontainingsingle-anddouble-walledCNTscanbesynthesizedbycontrollingthegrowthconditions.Inaddition,itioundthatlongerCNTAtendstopresentbettercrystallinityofgraphitesheets.Themechanioftheinfluenceofethyleneandhydrogenflowrates,growthtemperature,andtimeonthepropertiesofCNTAhasbeenproposed.ThegrowthprocessofCNTAhasbeenpresented.

(2)GrowthofCNTAonsiliconsubstrateiature,however,siliconisasemiconductorandcannotbeusedasacurrentcollector.CNTAelectrodewaabricatedbyasimpleandpowerful"cut-paste"method,whichdidnotdamagethemicrostructureoftheCNTA.CNTAelectrodepresents25and15F/ginanicandaqueouselectrolyte.ThehydrophobicpropertyofCNTleadstoanabnormalphenomenonthatCNTApresentslowerspecificcapacitanceinaqueouselectrolytethanthatinanicelectrolyte.Unliketraditionalporouscarbonmaterials,thecapacitiveandrateperformanceofCNTAinionicliquidelectrolyteareparabletothatinanicelectrolyte,thereasonisthatCNTApresentlargeporesandopenporestructure.Comparedtoentangledcarbonnanotube(ECNT)electrode,CNTApresentmoredevelopedmesoporousstructureandelectronicconductivepaths,andthenhashigherionicandelectronicconductivity,andlowerequivalentseriesresistance(ESR),thushasbetterrateperformance.

(3)GrowthofCNTAoncurrentcollectordirectlycanimprovethecontactsbetweenCNTAandsubstrate,ensuretheelectrodeobtaininglowercontactresistanceandthenbettercapacitiveperformance.85~90-μm-longCNTAsweregrownonstainlesssteel,Ta,andglassycarbonsubstratesbypyrolysisofironphthalocyanineandethylene.Upto420-μm-longCNTAsweregrownonstainlesssteel,Ta,andCusubstratesbythermalCVD.ExperimentalresultsindicatethatCNTAcanbegrownonanysubstratebyAl2O3bufferlayer-assistedCVDaslongasthissubstrateisstableinH2athightemperatures,thisunderstandingclarifythepreconditionofgrowingCNTAonconductivesubstrate,extendingtherangeofsubstratesthatcanbeusedtosynthesizeCNTA.

ThespecificcapacitanceofCNTAgrownoncurrentcollectorsis30F/g.TheseCNTAshemuchhigherratecapabilitythantheCNTAspastedoncurrentcollectors,whichisattributedtothattheformerelectrodeshelowerESR.CNTA-basedsupercapacitorwithanicelectrolytepresentshighworkinoltage(3.5V),longcyclelife,andgoodratecapability.Thepowerandenergydensitiesare928kW/kgand19Wh/kg,respectively(basedonthemassofCNTA).WeclaimthatCNTA-basedelectrochemicalcapacitorayobtainspecificenergyandpowerdensitiesof1.39Wh/kgand62.5kW/kg,respectively.

(4)InordertoenhancethecapacitanceofCNTA,pseudocapacitivematerials,suchasconductingpolymersandmetaloxides,aredopedintotheporesofhighlyconductiveandstableCNTAs(seeFig.1).ConsideringthatCNTAhasregularporestructure,highSSA,homogeneousproperty(binder-free),andsuperiorconductivity,depositingpseudo-capacitivematerialsonCNTAelectrodesisapromisingmethodforthefabricationofnovelpositeelectrodeswithsuperiorcapacitiveproperties.

Fig.1.Aschematicdiagramillustratinghowpseudo-capacitivematerialaybedepositedonCNTAelectrodeornovelCNTA-basedpositeelectrodes.

Polyaniline(PANI)/CNTApositeelectrodeweresynthesizedbyelectrodepositingofanilinenanolayeroneveryCNTsinCNTA.CNTAsupportwithgoodconductivity,largeporesize,regularporestructure,andgoodstabilityimprovebothpowerandcycleperformanceofpositeelectrode.Nanometer-scale(7nm)PANIlayeronCNTAexhibitsgoodelectrochemicalaccessibilityforelectrolyteionsandreducethedistancethationustbetransportedduringcharge-dischargeprocess,whichiundamentalforelectrodematerialsofsupercapacitorsshowinghighspecificcapacitanceandgoodratecapability.PANI/CNTApositeelectrodeischaracterizedbyhighspecificcapacitance(1030F/g),superiorratecapability(95%capacityretentionat118A/g),andhighstability(5.5%capacitylossafter5,000cycles),therateandcycleperformancesaremuchbetterthanthatofPANI/ECNTposites.TheenergystoragecharacteristicsofthePANI/CNTApositeareinvestigatedandstatedaollows.Nanometer-sizedPANIlayercoatseveryCNTinCNTAuniformly.Thisgeometryhasseveraladvantages.First,thePANInanolayerisconnecteddirectlywiththecurrentcollector(Tafoil)byelectron"superhighways"(CNTs),thusthissuperiorconductingworkalloworefficientchargetransportandenhancestheelectronicconductivityofthepositesignificantly.Second,thehighSSAandthenanometersize,whichreducesthediffusionlengthofionsinPANIphaseduringchargedischargeprocess,ensureahighutilizationofelectrodematerials,andthenahighspecificcapacity.Third,hierarchicallyporousstructureenhancestheionicconductivityofthePANI/CNTApositegreatly.Thephysicochemicalpropertiesoftheelectrolyteinmacroporesaresimilartothoseofthebulkelectrolytewiththelowestresistance.IonbufferingreservoirscanbeformedinmacroporesbetweennanotubestominimizethediffusiondistancestotheinteriorsurfacesofPANIphase.Fourth,theuseofCNTswithexceptionalmechanicalpropertiesasasupportandthegeometryofthenanometersizePNAIlayercanreleasethecycledegradationproblemscausedbymechanicalproblemsorvolumechangesandcanoverenanoparticleaggregation.Inaddition,asPANIphaseisconnectedtotheconductingframework,theneedforbindersorconductingadditives,whichaddextracontactresistanceorweight,iseliminated.Thus,thePANI/CNTApositeelectrodepresentsthebestelectrochemicalcapacitiveperformance.

WhenusedasaLi-ionbatterycathode,PANI/CNTAelectrodecandeliver99,57,54,and37mAh/gatdischargeratesof0.4,20,60,and260C.

(5)Manganeseoxide(MO)hashighdensityandlongcyclelifeinelectrochemicalcapacitors.TheelectrodepositionofMOonCNTAsubstrateleadstoacontrolledpreparationofmanganeseoxidenanoflower/CNTApositeelectrodeswithhierarchicalporousstructure,largesurfacearea,andsuperiorconductivity,everydandelionflower-likeMOnanoparticlelocatesatthejunctionsbetweenCNTsandthispositeelectrodeisbinder-free.Themanganeseoxideinthepositeelectrodepresentsexcellentratecapability(50.8%capacityretentionat77A/g),highcapacitance(199F/gand307F/cm3),andlongcyclelife(3%capacitylossafter20,000charge/dischargecycles),therateperformanceisbetterthananyMOorMO-basedpositematerialsreported.Theenergystoragecharacteristicsofthemanganeseoxide/CNTApositearediscussed.Manganeseoxidenanoflowersaregrowndirectlyonnanostructuredcurrentcollector(CNTA).Thisgeometryhasseveraladvantages.First,eachmanganeseoxidenanoflowerisconnecteddirectlywiththecurrentcollector(Tafoil)bytwoormoreelectron"superhighways"(CNTs),thus,thissuperiorconductingworkalloworefficientchargetransportandenhancestheelectronicconductivityofpositesignificantly.Second,thehighSSAandthenanometersize,whichreducesthediffusionlengthofionswithinmanganeseoxidephaseduringthecharge/dischargeprocess,ensureahighutilizationofelectrodematerials,andthenahighspecificcapacitance.Third,ahierarchicallyporousstructureenhancestheionicconductivityofthepositegreatly.Thephysicochemicalpropertiesoftheelectrolyteinmacroporesaresimilartothoseofthebulkelectrolytewiththelowestresistance.Ion-bufferingreservoirscanbeformedinmacroporestominimizethediffusiondistancestointeriorsurfacesofmanganeseoxide.Fourth,theuseofCNTswithexceptionalmechanicalpropertiesasasupportandthegeometryofthemanganeseoxidenanoflowercanreleasethecycledegradationproblemscausedbymechanicalproblemsorvolumechangesandcanoverenanoparticleaggregation.Inaddition,aseverymanganeseoxideparticleisconnectedtotheconductingframework,theneedforbindersorconductingadditives,whichaddextracontactresistanceorweight,iseliminated.Thus,themanganeseoxide/CNTApositeelectrodepresentsthebestelectrochemicalcapacitiveperformance.

TheperformanceofMO/CNTAcanbeenhancedbyadditivesandheattreatment:aceticcanmakeMOdepositormingnanosheetstructureandobtaininghighspecificsurfaceareaandcapacitanceof322m2/g,302F/g,and453F/cm3,CocanenhancetheconductivityandthenthepowerperformanceofMO,heattreatmentcanenhancetherateperformanceofMO.

WhenusedasaLi-ionbatterieselectrode,MO/CNTAelectrodedelivers246and51mAh/gatdischargeratesof0.4and50C,respectively.Theseresultsarebetterthantheperformancesofmanganeseoxide-basedmaterialsreportedpreviously.

ThemechanisofthenanostructureformingandtheenergystorageofthetwokindsofCNTA-basedmaterialshebeeninvestigated.OurworkrevealsthatCNTAisanidealsubstratefordepositingtransitionmetaloxidestosynthesizenovelnanomaterialsandopensupanovelrouteforthedirectsynthesisofadvancedfunctionalmaterialswithhierarchicalporousstructureandsuperiorconductivity.Thesematerialscanfindapplicationsinnotonlyenergystoragebutalsosensors,catalysis,andmicroelectronics.Keywords:Electrochemicalcapacitors,Carbonnanotubearray,Polyaniline,

Manganeseoxide,Compositematerials