Research Progress on Applications of Polyaniline (PANI) for Electrochemical Energy Storage and Conversion

Abstract

Conducting polyaniline (PANI) with high conductivity, ease of synthesis, high flexibility, low cost, environmental friendliness and unique redox properties has been extensively applied in electrochemical energy storage and conversion technologies including supercapacitors, rechargeable batteries and fuel cells. Pure PANI exhibits inferior stability as supercapacitive electrode, and can not meet the ever-increasing demand for more stable molecular structure, higher power/energy density and more N-active sites. The combination of PANI and other active materials like carbon materials, metal compounds and other conducting polymers (CPs) can make up for these disadvantages as supercapacitive electrode. As for rechargeable batteries and fuel cells, recent research related to PANI mainly focus on PANI modified composite electrodes and supported composite electrocatalysts respectively. In various PANI based composite structures, PANI usually acts as a conductive layer and network, and the resultant PANI based composites with various unique structures have demonstrated superior electrochemical performance in supercapacitors, rechargeable batteries and fuel cells due to the synergistic effect. Additionally, PANI derived N-doped carbon materials also have been widely used as metal-free electrocatalysts for fuel cells, which is also involved in this review. In the end, we give a brief outline of future advances and research directions on PANI.

Full text

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Materials2020,13,548;doi:10.3390/ma13030548www.mdpi.com/journal/materials
Review
ResearchProgressonApplicationsofPolyaniline
(PANI)forElectrochemicalEnergyStorage
andConversion
ZhihuaLi*andLiangjunGong

MaterialsSciencesandEngineering,CentralSouthUniversity,Changsha41000,Hunan,China
*Correspondence:ligfz@mail.csu.edu.cn;Tel.:+138‐7312‐0818
Received:29November2019;Accepted:20January2020;Published:23January2020
Abstract:Conductingpolyaniline(PANI)withhighconductivity,easeofsynthesis,highflexibility,
lowcost,environmentalfriendlinessanduniqueredoxpropertieshasbeenextensivelyappliedin
electrochemicalenergystorageandconversiontechnologiesincludingsupercapacitors,
rechargeablebatteriesandfuelcells.PurePANIexhibitsinferiorstabilityassupercapacitive
electrode,andcannotmeettheever‐increasingdemandformorestablemolecularstructure,higher
power/energydensityandmoreN‐activesites.ThecombinationofPANIandotheractivematerials
likecarbonmaterials,metalcompoundsandotherconductingpolymers(CPs)canmakeupforthese
disadvantagesassupercapacitiveelectrode.Asforrechargeablebatteriesandfuelcells,recent
researchrelatedtoPANImainlyfocusonPANImodifiedcompositeelectrodesandsupported
compositeelectrocatalystsrespectively.InvariousPANIbasedcompositestructures,PANIusually
actsasaconductivelayerandnetwork,andtheresultantPANIbasedcompositeswithvarious
uniquestructureshavedemonstratedsuperiorelectrochemicalperformanceinsupercapacitors,
rechargeablebatteriesandfuelcellsduetothesynergisticeffect.Additionally,PANIderivedN‐
dopedcarbonmaterialsalsohavebeenwidelyusedasmetal‐freeelectrocatalystsforfuelcells,
whichisalsoinvolvedinthisreview.Intheend,wegiveabriefoutlineoffutureadvancesand
researchdirectionsonPANI.
Keywords:PANI;electrochemicalenergystorageandconversion;composites;supercapacitor;
rechargeablebattery;fuelcell

1.Introduction
Withtherapiddevelopmentofenergy,supplyingofenergycannotmeettheemergingdemand
[1]duetotheincreasingenergyconsumption,whichacceleratesenergyshortage,henceenergy
storageandconversionplayasignificantroleinovercomingthechallenge.Todate,differentkinds
ofenergystorageandconversiontechnologieshavebeendevelopedtodealwiththeenergycrisis.
Amongthem,threekindsofcrucialelectrochemicalenergystorageandconversiontechnologies
includingsupercapacitors,rechargeablebatteriesandfuelcells[1,2]takethedominance.Figure1
showstheRagoneplotofsupercapacitors,rechargeablebatteriesandfuelcells.

Materials2020,13,5482of45
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Figure1.Ragoneplotforvarioussupercapacitors,batteriesandfuelcells[20](reproducedwith
permissionfromElsevier).
Toalargeextent,theperformanceofelectrochemicalenergystorageandconversiondevicesare
determinedbytheelectrodematerials[2].Carbonspecies,metalcompoundsandconducting
polymers(CPs)arethemaintypesusedaselectrodematerials.Carbonspeciesthatusedaselectrode
materialstypicallyincludegraphene[4–7],carbonnanotubes[1],porousnano‐carbons[4,8,9]and
activatedcarbon[10]duetotheirfastchargingcapabilitiesandhighconductivity[1].However,their
lowloadingdensitywillresultinlowenergydensity,whichlargelylimitstheirapplicationsfor
energystorageandconversion.Metalcompoundsexhibitnaturalabundanceandmultielectronredox
capability,buttheyareassociatedwithlowconductivityandeaseofself‐aggregation.
Conductingpolymers(CPs)arederivedfromintrinsicallyconductingpolymers(ICPs)that
discoveredin1960[11].Itcausedmuchattractionfromresearchersbecauseofthepromising
propertiesandpotentialapplicationsofICPssincethediscovery.CPbaseddevicesexhibithigher
specificcapacitancethandouble‐layercapacitors,moreover,theyhavefasterkineticsthanmost
inorganicbatteries,whichcannarrowthegapbetweencarbonbasedcapacitorsandinorganic
batteries,indicatingthepromisingpotentialofCPsinelectrochemicalenergystorageandconversion

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[3].AmongseveralcommonCPslikepolyphenyl,polypyrrole,polythiophene,polyphenylacetylene
andpolyaniline(PANI),PANIgeneratesthemostattractionowingtoitseasiersynthesis,lowercost
monomer,highertheoreticalconductivity(3407Fg−1),widerrangeofworkingpotentialwindowand
betterstabilitycomparedwiththeotherCPs[11].Therefore,PANIhasbeenarisingsuperstarinthe
fieldofelectrochemicalenergystorageandconversion.
TheconductivityofPANIisderivedfromitsuniquemolecularstructure.In1987,AlanGMac
Diarmid[12]proposedaPANIstructuralmodelinwhichbenzenestructuralunitsandquinoid
structuralunitsco‐existed,thisstructuralmodelhasbeenwidelyrecognizedbythescientific
community.DiarmidbelievesthattheconductivityofPANIisobtainedviadopingandde‐doping
thePANImolecularchain,thatis,thePANImolecularchaincontainsaseriesofreducedstructural
unitsandoxidizedstructuralunits,anditsstructuralformulaisasfollows(Figure2):

Figure2.Molecularstructureofpolyaniline(PANI).
Where:yrepresentsthedegreeofreductionofPANI,andmayalsoindicatethedegreeofdoping
ofthemolecularchain.Wheny=1,itmeansthatPANIisinafullyreducedstate(benzene‐type
structuralunit),itiscalledleucoemeraldinebase(LEB);wheny=0,itmeansthatPANIisinafully
oxidizedstate(quinoid‐typestructuralunit),itiscalledpernigranilinebase(PB);whenyisbetween
0and1,indicatingadopedstateinwhichanoxidationstateandareducedstateco‐exist(thebenzene
structuralunitandthequinoidstructuralunitco‐exist).Wherein,wheny=0.5,thatis,thedopantis
alternatelydopedinthePANImolecularchain,atthistime,thePANIisinanintermediateoxidation
state,andtheconductivityofthePANIafterdopingisoptimal,theoptimalstateiscalledemeraldine
base(EB).Ingeneral,thePANIthatemployedaselectrodeisthemixtureofthethreestates,butthe
highportionoftheEBstateisgreatlydesirableinordertoobtaintheoptimalperformanceofPANI.
PANIcanbesynthesizedbychemicalorelectrochemicalpolymerizationofmonomeraniline
[13].VariousmorphologiesofPANIcanbeobtainedthroughdifferentsynthesismethods:chemical
polymerizationusuallyleadstonanotubes,nanofibers,nanospheres,nanorods,nanoflakesandeven
nanoflowers,theaccuratemorphologiesarestronglydependedonreactiveconditionsandthespecies
ofoxidants;whileelectrochemicalpolymerizationonlyleadstonanofibersandfilms.Differentfrom
thechemicalpolymerization,themorphologiesofPANImainlyrelyonthenaturesofthesubstrates.
PANIcancombinewithotheractivematerialstoformahybridsystem.PANIbasedcompositeshave
extraadvantagescomparedwithpurePANI,theextraadvantagesdependonthetypesofextraactive
materials,andthecompositesusuallyshowimprovedproperties.InvariousPANIbasedcomposite
structures,PANIusuallyactsasaconductivelayerandnetwork,andtheresultantPANIbased
compositeswithvariousuniquestructureshavedeliveredsuperiorelectrochemicalperformancedue
tothesynergisticeffect,whichwillbeintroducedinthefollowingsections.
PANIhasbeenwidelyusedinenergystorageandconversion,includingsupercapacitors,
rechargeablebatteriesandfuelcells.Whenusedforsupercapacitors,PANIistheactiveelectrode
materialthatactsasachargecarrierduringtheredoxreaction.However,purePANIoftensuffers
fromseveredegradationofcapacitanceandinefficientcapacitancecontributionduring
pseudocapacitiveprocess,whichcanbeattributedtotheinvolvementoftheswelling,shrinkageand
crackingofPANIwhiledoping/dedopingofPANI.Fortunately,PANIwithhighflexibilitycanwell
combinewithotheractivematerialslikecarbonmaterials,metalcompoundsandCPstoformPANI
basedcompositeswithenhancedsupercapacitiveperformance.Asforrechargeablebatteries,itisa
smartstrategytodesignPANImodifiedelectrodematerials.TheadditionofPANIcannotonlymake
upforthedisadvantageslikecyclinginstability,lowconductivityandstructuralinstabilitythatexist

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inconventionalinorganicelectrodes,butalsoexploittheadvantagestocontributesynergisticeffect.
Besides,thehybridstructuresalsoplayanimportantroleinelectrochemicalperformanceofthe
compositeelectrodes.Todatenow,Ptsupportedonporouscarbonmaterialsisstillthemostusedas
anelectrocatalystforfuelcells.However,highcostofPtseverelyhindersthecommercializationof
fuelcells.Toenlargeitsutilization,variousinexpensivemetalslikeFe,Mo,Mn,Niandtheir
compoundsarechosentofabricatenon‐noblemetalelectrocatalysts,andthealternativeshavebeen
demonstratedtoaddressexcellentelectrocatalyticperformanceforoxygenoxidationreaction(ORR),
hydrogenevolutionreaction(HER)andhydrogenoxidationreaction(HOR).Formethanoloxidation
reaction(MOR),hithertothemainstrategyistodesignPt–M(Co,Mo,Ni,Fe,MnandRu)alloys
catalystswithlowerPtcontent.PANIwithhighconductivity,tunablemorphologiesandhigh
flexibilityisanidealsupportasmetalelectrocatalysts.PANIsupportedmetalelectrocatalystsshow
goodelectrocatalyticactivity,furthermore,PANIcanbeusedassupportfornon‐noblemetal
electrocatalysts,whichcancutthecost.Apartfromhighconductivity,tunablemorphologiesand
highflexibility,PANIsupportedmetalelectrocatalystscaneffectivelysuppresstheagglomeration
throughimprovingthedispersionoftheactivecatalysts.Inaddition,PANIanditsderivativescan
beusedasthecarbonprecursortopreparemetal‐freenon‐nobleORRelectrocatalysts[14]with
enhancedelectrocatalyticactivityowingtoitshighNcontent.Nevertheless,therearefewerreports
oncatalystsforoxygenevolutionreaction(OER)relatedtoPANI,henceitmaybeanother
challengingdirectionforfuelcells.
SeveralreviewsonelectrochemicalapplicationofPANIhavebeenpublishedinrecentyears.
However,mostofthemonlyreportexclusiveaspectofapplicationinthefieldofelectrochemistry.
Forexample,Snooketal.[3]summarizedmajorconductingpolymers(CPs)includingPANI,PPy
andPThappliedinsupercapacitors,aswellastheircompositeswithCNTsandinorganicbattery.
Mengetal.[20]reviewedconductingpolymers(CPs)includingPANI,PPyandPThusedas
supercapacitorelectrodematerials,andpointedoutdevelopmentdirectionsofCP‐based
supercapacitorsinthefuture.Luoetal.[173]reportedapplicationsofPANIforLi‐ionbatteries,Li‐
sulfurbatteriesandsupercapacitors.Itisseenfromtheabovementionedthatmajorworkon
electrochemicalapplicationsofPANIarefocusedonneithersupercapacitorsnorbatteries,butPANI
hasbeendemonstratedtoshowgreatpotentialinvariousaspectsonelectrochemistry.Therefore,
herewemeanacomprehensivereviewisdesirabletofillinthegap.Inthisreview,thewiderangeof
applicationsofPANIforelectrochemicalenergystorageandconversiontechnologiesincluding
supercapacitors,rechargeablebatteriesandfuelcellsareaddressedindetail(asseeninFigure3),
including:(1)PANIbasedsupercapacitorelectrodes;(2)PANImodifiedrechargeablebatteries
electrodesincludinglithium‐ionbatteries,lithium‐sulfurbatteriesandsodium‐ionbatteriesand(3)
PANI‐basedsupportedmetalelectrocatalystsandPANI‐derivedcarbonbasedmetal‐free
electrocatalystsforfuelcells.Intheend,wealsodiscussthefutureadvancesandresearchdirections
onPANI.

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
Figure3.Thearchitectureofthisreview.
2.ApplicationsofPANIforSupercapacitors
Supercapacitors,namelyultracapacitorsorelectrochemicalcapacitors,anewenergystorage
devicebetweenconventionalcapacitorsandbatteries[15],areconsideredasthepromising
electrochemicalenergystorage/conversiontechnologyduetoitshighspecificpower,longcycle
lifespanandfastcharge/dischargerate[16].Supercapacitorsaregenerallysortedintotwocategories
[17,18]basedonastoragemechanism:anelectrostaticdouble‐layercapacitor(EDLC)and
pseudocapacitor.EDLCsmainlygenerate/storeenergyviaadsorbing/desorptiononthesurfaceofthe
electrodebyapureelectrostaticcharge.Pseudocapacitors,alsoknownasFaradayquasi‐capacitors,
mainlygeneratepseudo‐capacitancebyareversibleredoxreactiononthesurfaceandnearthesurface
ofpseudo‐capacitoractiveelectrodematerials(transitionmetaloxides(TMOs)andconducting
polymers(CPs)[19]),therebyrealizingenergystorageandconversion.Theschematicsofan
electrostaticdouble‐layercapacitor(EDLC)(a)andpseudocapacitorareillustratedinFigure4.

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
Figure4.Schematicsofanelectrostaticdouble‐layercapacitor(EDLC)(a)andpseudocapacitor;(b)
[20](reproducedwithpermissionfromElsevier).
Asweknow,theperformanceofsupercapacitorsstronglydependsonthepropertiesofthe
employedelectrodematerials.Therearethreecategoriesofmainelectrodematerialsthatareadopted
insupercapacitors:(1)carbonmaterials;(2)conductingpolymers(CPs)and(3)transitionmetal
oxides(TMOs)[19,21].Carbon‐basedmaterialsusedaselectrodematerialsofEDLCshavebeen
extensivelystudied,anditisdemonstratedthattheyexhibitEDLC‐typebehaviorwithhighpower
density,lowcostandtunableporosity,buttheysufferfromlowenergydensity[14,22–24].For
pseudocapacitors,transitionmetaloxides(TMOs)andconductingpolymers(CPs)aretwotypesof
electrodematerialsthatarewidelyemployed.TMOscandisplaymultipleoxidationstates[1,14]
underlowactivationenergy,buttheyregrettablysufferfromlowcapacitance,inflexibleand
instability[14].
CPs,anotherpromisingelectrodematerialacknowledgedashavinghighspecificcapacitance,
perfectflexibility,goodstabilityandeaseofsynthesis,showafascinatingprospectinsupercapacitive
(pseudocapacitive)electrodes.Asmentionedabove,PANIhasmoreadvantagesovertheotherCPs
likeeaseofsynthesis,lowcostmonomer,hightheoreticalconductivity(3407Fg−1),awiderangeof
workingpotentialwindowandgoodstability,thereforePANIisthemostexploredmaterialthatis
usedaspseudocapacitiveelectrodesamongCPs[25–28].Inordertoimproveitsproperties,extensive
effortshavebeenmade,whicharemainlyassociatedwithcombiningPANIwithothermaterials(like
carbon‐basedmaterials,TMOs).Inrecentyears,substantialworkhasbeendevotedintocarbon‐based
materialsandTMOs,howeverreportsonPANIarerelativelyrare.Asakindofnovelelectrode
materialwithpromisingproperties,itdeservesmoreresearchandreportsbecauseitprovidesnew
thoughtforobtainingsupercapacitorelectrodewithsuperiorperformance.Inthissection,wereview
theresearchprogressofpurePANIorPANIbasedcompositesaspseudocapacitiveelectrode
materials.
2.1.PurePANI
AsanoutstandingCPwithuniquecharacteristics,PANIwithmultipleredoxstateshasexcellent
pseudocapacitiveperformance,thereforemanyresearchershavetriedtoutilizePANIin
supercapacitorssinceitsdiscovery.Anearlierreportonitsapplicationinsupercapacitorsappeared
in2001,FlorenceFusalbaetal.[29]studiedelectrochemicalcharacterizationofPANI.Theyevaluated
stabilityofPANI‐PANIsupercapacitorviaconstantcurrentcycling,alossofabout60%oftheinitial
chargeafter1000cycleswasdeliveredintheend.Theyattributedittohighionicorelectronic

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resistanceduetothecompactPANIfilms,andviewedthatthemorphologyofPANIstrongly
impactedelectrochemicalpropertiesbyinfluencingiondiffusionintothePANImatrixduringredox
reaction.MoreandmoreresearchesjustdemonstratedthatPANImorphologyplaysacrucialrolein
itselectrochemicalproperties.ChenHongandhiscoworkers[30,31]investigatedsupercapacitive
performancesofPANIunderdifferentmorphologies(granular,flakeandnanofiber)ofPANIfilms
preparedbythepulsegalvanostaticmethod(PGM)andgalvanostaticmethod(GM).Thenanofibrous
PANIdisplayedbettercapacitiveperformance,whichattributedtoitslagerspecificsurfaceand
betterelectronicorionicconductivity.
Obviouslyitisimportanttodesignarationalnanostructureforthepurposeofenhancingthe
propertiesofPANIsupercapacitiveelectrodes.Infact,thenanostructureofPANIisstrongly
determinedbythesynthesismethod,thereforeitiscrucialtofindarationalsynthesismethod.Liuet
al.[32]preparedporousPANIviatheinsituaqueouspolymerizationmethod,andcomparedits
electrochemicalcapacitanceperformancewithnonporousPANI.TheporousPANIpossesssmaller
andmorepores,furthermoretheporespresentedmorerandomarrangementcomparedtothe
nonporousPANI(Figure5).Morefascinatingly,theporousPANIexhibitedhighspecificcapacitance
as837Fg−1underthecurrentdensityof10mAg−1,muchhigherthanthatofthenonporousones(519
Fg−1),whileitsexperimentalcapacitance(1570Fg−1)wasjustabout77%oftheoreticalvalue(2027F
g−1),indicatingthatonly77%ofPANImakesacontributiontothecapacitanceability.Sivakkmarand
hiscoworkers[33]fabricatedPANInanofiberswithinterfacialpolymerizationandinvestigatedits
propertieswhenusedasasupercapacitorelectrode.Thetestshowedthattheinitialspecific
capacitance(554Fg−1)decreasedrapidlywhilecycling,andthevaluedecreasedto57Fg−1after1000
cycles.Furthermoretheyfoundthatonlyafraction(31%)ofthetheoreticalvalueofPANInanofibers
wasutilized.

Figure5.SEMimages:(a)nonporousPANIand(b)porousPANIandTEMimages:(c)nonporous
PANIand(d)porousPANI[32](reproducedwithpermissionfromElsevier).
Insummary,purePANIusedassupercapacitorelectrodematerialshasbeeninvestigatedalot,
howeveritsunstablecyclingandinefficientcontributionsignificantlylimititspracticalapplications
forsupercapacitors.HenceitisurgenttofabricatevariousPANIbasedcompositesforthepurpose
ofenhancingtheelectrochemicalpropertiesofsupercapacitors.

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2.2.PANIBasedComposites
Asdiscussedabove,purePANImaybeunsatisfactorytobeappliedinthesupercapacitor
electrodebecauseofitsinstabilityandlimitedcapacitancecontribution.Togetoverthesechallenges,
researchersaretryingtocombinePANIwithothermaterialstopreparebetterelectrochemical
propertiesofsupercapacitorelectrodes.Fortunately,highflexibilityofPANImakesitavailablefor
PANItocombinewithotheractivematerialstoformPANIbasedcomposites[1],moresatisfyingly,
thesePANIbasedcompositesshowpromisingpropertieswhenusedassupercapacitorelectrodes.
ThePANIbasedcompositescanbesimplycategorizedintotwotypes:binaryandternary.Herewe
willreviewtherecentdevelopmentandprogressofbinaryandternaryPANIbasedcompositesin
detail.
2.2.1.PANIBasedBinaryComposites
ItisprovedthatPANIiscompatiblewithmanytypesofmaterialslikecarbonmaterials,metal
compoundsandCPs.Inthefollowingsection,wewilldiscusstheresearchprogressofvariousPANI
basedbinarycomposites.
PANI/CarbonBinaryComposites
Carbonmaterialsascommonsupercapacitorelectrodeshaveattractedmuchconcernfrom
researchers.Knownasacknowledged,carbonmaterialshavelotsofoutstandingadvantages,suchas
highconductivity,highspecificsurfacearea,goodstability,perfectelectricalpropertiesandsoon
[34–36].PANI/carbonbinarycompositesarepromisingtoachieveenhancedperformancedueto
excellentelectricalpropertiesandgreatstabilityofPANIandcarbonmaterials.Manyidealcarbon
materialsincludingporouscarbon,CNTs,grapheneandcarbonnanofibershavebeendemonstrated
toexhibitoutstandingpropertieswhencombinedwithPANI.Porouscarbonpossesshighspecific
surfacearea,goodstabilityandlargeporositythatPANIlacks,whichindicatesthatitisavailableto
preparePANI/porouscarboncompositeswithimprovedproperties.Chenetal.[34]preparedPANI
viaelectrochemicalpolymerization,andloadeditontheas‐preparedporouscarbonelectrodes.It
wasdisplayedthattheinitialspecificcapacitanceofthePANI‐basedcapacitor(PC)wasashighas
180Fg−1,thatis,almostdoubleasthebare‐carboncapacitor(BC,92Fg−1).After1000cycles,the
specificcapacitanceofPCdecreasedfrom180to163Fg−1,indicatingitsgoodcyclingstability.More
recently,Zhangetal.[35]adoptedafacileandeconomicalmethodtoobtainpolyaniline/cellulous‐
derivedhighlyporousactivatedcarbons(PANI/C‐ACs)composites.TheyfabricatedC‐ACsskeleton
viathe“selectivesurfacedissolution”(SSD)method[36],inwhichfilterpaperwasusedasthecarbon
precursor,thenPANInanorodswereuniformlygrownontotheas‐preparedC‐ACsskeleton.Figure
6illustratedtheprocedureofPANI/C‐ACssynthesis.Whileusingitassupercapacitorelectrode,it
exhibitedexcellentspecificcapacitance(765Fg−1at1A/g)andhighcyclingstability(capacitance
retentionas91%after5000cycles),whichwasmuchbetterthanpurePANI.Besidesactivatedcarbon
(AC),orderedmesoporouscarbon(OMC)isalsoatypeofporouscarbonwithhigherspecificsurface
area(1000–2000m2/g)thanordinaryporouscarbon.Asatypeofcarbonspecieswithuniqueelectrical
double‐layercapacitance,OMCisdesiredtocombinewithPANIwithexcellentfaradaiccapacitance,
thePANI/OMCcompositesarepromisingtolargelyimprovesupercapacitorperformances.Basedon
thefeasibility,somesignificantresearch[37–41]onPANI/OMCcompositeshasbeendone.Intheir
reports,PANI/OMCcompositesaresuccessfullysynthesizedbyinsitupolymerizationorchemical
polymerization,andtheirsuperiorityinsupercapacitorelectrodeshavebeenillustratedindetail.
HoweverdifferentnanostructuresofPANIusuallyexhibitdifferentelectrochemicalperformances,
nanofibers,nanorodsandnanowhiskersofPANIthatdepositedontothesurfaceofOMCshowed
differentspecificsurfaceareasandcyclingstabilitiesduetodifferentstructure‐activityrelationships.
HereinwetaketheworkofYanetal.asanexample.They[40]synthesizedPANInanowhiskers
(PANI‐NWs)/orderedmesoporouscarbon(CMK‐3)compositethroughchemicaloxidative
polymerization,andstudieditselectrochemicalperformanceswhileusedasasupercapacitor
electrode.CMK‐3,ahighlyorderedhexagonallymesoporouscarbon,ownsmoresuperior

Materials2020,13,5489of45
electrochemicalperformancesthanordinarycarbonmaterials.Asexpected,thePANI‐NWs/CMK‐3
showedexcellentcapacitanceretention(90.4%after1000cycles)andhighelectrochemicalcapacitance
attributingtouniqueverticalarraysofPANI‐NWsandorderedframeworkofCMK‐3.Asanother
porouscarbon,orderedmacroporouscarbonsareverysimilartoOMCinmanyaspects.Itcanalso
combinewithPANI,andexhibitsbetterelectricaldouble‐layercapacitancethanPANI/OMC
compositesowingtoitsmacroporestructuresthatdifferentfromOMC[20].Carbonsphere‐type
materialsarealsoakindofimportantporouscarbonmaterials.Likeotherporouscarbon,they
usuallypossesshighspecificsurfaceareaandporestructures,whiletheirporestructureisverysmall,
soitismoreconvenientforelectronstotransportfromtheelectrolytetosupercapacitorelectrode
surface,whichcancontributetohighconductivityandgoodEDLCperformancessignificantly,
whereasitisapopularstrategytocombineitwithPANIwithgoodFaradaiccapacitiveproperties.
Shenetal.[42]fabricatednano‐hollowcarbonspheres(nano‐HCS)/PANIcompositesviainsitu
chemicaloxidativepolymerization.Anelectrochemicaltestdisplayedthatthemaximumspecific
capacitancereached435Fg−1,andthecapacitanceretentionwasabout60%after2000cycles.They
declaredthatthecompositesarepromisingforsupercapacitorapplications.

Figure6.SchematicofPANI/C‐ACssynthesisprocedure[35](reproducedwithpermissionfrom
Elsevier).
Carbonnanotubes(CNTs)havebeenatypeofhotmaterialssincethediscoveryin1991.
Especiallyinenergystorage/conversion,itholdsmuchpromiseduetoitsoutstandingelectrical
properties,whileitscapacitancevalueisfairlylow(generally40–80Fg−1)attributingtoitssmall
specificsurfacearea[43],soitisurgenttoimproveitscapacitanceproperties.Severalreversible
oxidationstatesofPANIendowsitwiththefeasibilitythatenhancingthepropertiesofCNTsthrough
fabricatingPANI/CNTscomposites.Khomenkoetal.[43]firstlyemployedPANI/multi‐wallcarbon
nanotubes(MWCNTs)compositesinsupercapacitorelectrodes.Theyobtainedthecompositesvia
chemicaloxidativepolymerizationmethod,thePANIdepositedontothesurfaceoftheas‐prepared
MWCNTsduringthepolymerization.Thecompositesasapositiveelectrodeexhibitedaspecific
capacitanceof320Fg−1(almosteighttimesasthatofMWCNTs)andalossofabout8%ofinitial
capacitanceafter50cycles.RightafterKhomenko,Dengetal.[44]synthesizedCNTs/PANI
nanocompositeviathedepositionofPANIonthesurfaceofCNTs,whichisafacileandcheapmethod
astheyclaimed.Inthenanocomposite,CNTsworkedastheskeletonsoastoincreasethespecific
surfaceareaofdepositedPANI,whichservedastheskin.Theuniqueskeleton/skinstructureand

Materials2020,13,54810of45
excellentpseudocapacitanceofuniformlycoatedPANIlargelyenhancedthespecificcapacitanceof
thecompositewherethevaluereached183Fg−1,muchhigherthantheCNTsas47Fg−1.Yang’sgroup
[45]preparednitrogen‐containingCNTs/PANInanocompositewithtunablemorphologiesbyhigh
temperaturetreatment.WhenthecompositewasusedasthesupercapacitorelectrodeinKOH
solution,theyshowedhighspecificcapacitance(163Fg−1at700°Cand0.1Ag−1)andgoodcycling
stability.Morerecently,Wangandhiscoworkers[46]reportedanovelmethodtofabricateflexible
ultrathinall‐solid‐statesupercapacitorswithexcellentelectrochemicalperformances.They
synthesizedasinglewallcarbonnanotubes(SWCNTs)/PANIelectrodefilmandaPVA/H3PO4
electrolytethroughspray‐printingandspin‐coatingmethodsrespectively.TheSWCNT/PANI
electrodepresentedaconsiderableinitialspecificcapacitanceof355.5Fg−1whenthemassratioof
SWCNT:PANIis1:1.Moreover,itscapacitanceretentionreached87.2%ofitsinitialspecific
capacitanceafter5000cycles,whoseelectrochemicalpropertiesaresuperiortotheformer[43–45],the
authorheldtheviewthattheflexibleultrathinall‐solid‐statesupercapacitorispromisingtopavethe
wayforadvancedapplicationsofenergystorage.
Graphene,apopularlystudiedmaterialwithextraordinaryelectrical,mechanicalandthermal
properties,hasattractedextensiveconcernfromresearchers.Furthermore,graphenehasbeenthehot
materialsservedassupercapacitorelectrodesduetoitsexcellentconductivityandconsiderable
theoreticalsurfacearea(2630m2/g)[47–49].Additionally,graphenepossesseshighstructuralstability
thatPANIlacks,henceitisafairlyidealmaterialtocombinewithPANIforthepurposeofoptimizing
supercapacitorperformances.Wuetal.[47]preparedafree‐standing,flexiblechemicallyconverted
graphene(CCG)/polyanilinenanofibers(PANI‐NFs)compositefilmwithalayeredstructureinthe
stableaqueousdispersionsbyvacuumfiltrationofthemixeddispersions.Theas‐preparedcomposite
filmwaswithhighconductivityof550S/m,highspecificcapacitanceof210Fg−1at0.3Ag−1and
stablecyclingproperty(94%ismaintainedafter1000cycles),muchbetterthanthatofpurePANI‐
NFs,indicatingthattheadditionofgraphenedisplaysanoticeablepromotiononPANI.Zhangetal.
[48]obtainedagraphene/PANInanofibercompositebyinsitupolymerizationinthepresenceof
grapheneoxide(GO).Theyfoundthatcomposites’electrochemicalpropertieswereatdifferent
conditionswhenthecontentofGOchanged,moreover,thehighestspecificcapacitanceof480F/gat
0.1A/gwasachievedwhenGOcontent(massratio)was80%,and70%ofinitialcapacitancewas
retainedover1000cycles.Theyexplainedthattheindividualgraphenesheetiseasytoagglomerate
whileatservice,butitcanbeimprovedbytheadditionofGOthatishardtoagglomerate.Generally,
grapheneappliedinsupercapacitorsissynthesizedthroughtheHummer’smethodorthemodified
Hummer’smethodonaccountofitslowcostandhighyields[1],thenextractsthegrapheneoxide
(GO)andreducedGO(RGO),twoimportantgraphenederivatives,andPANI/GO(orRGO)
compositeshavebeenextensivelystudiedinrecentyears.Xuetal.[49]reportedafacilemethodto
fabricatetheGO/PANInanocompositeviainsitupolymerizationwiththeassistanceofsupercritical
carbondioxide(SCCO2).Figure2‐4demonstratesthesynthesisprocess.AswecanseeinFigure7,
polymerizationoccurredunderCO2atmosphere,whichcaneffectivelypromotethedispersionofthe
anilinemonomer.TunablemorphologyofthePANI/GOnanocompositecanbeachievedthrough
controllingtheconcentrationofanilineduringpolymerization.Whentheconcentrationwas0.1M,
thenanocompositeachievedahighspecificcapacitanceof425F/gat0.2A/gandstillretained83%of
initialcapacitanceover500cycles,superiortoindividualPANIatthesamecondition.Asthegroup
illustrated,thesynergisticeffectbetweenthenanosizedPANIandGOwithhighspecificsurfacearea
leadstotheexcellentelectrochemicalproperties.Wangandcoworkers[50]designedasoftchemical
routetopreparePANIdopedwithGOsheet.Theobtainednanocompositepossesshighconductivity
of10S/mat22°Candhighspecificcapacitanceof531F/gat0.2A/g,betterthanthatofpurePANI
(216F/g).However,PANI/GOcomposites’capacitanceisstillnotsohighduetotheintrinsicsurface
natureofGO[51],whichmightinfluencetheefficientelectrontransfer,that,inturn,couldhinderits
usageinsupercapacitors.ReducedGO(RGO)cansignificantlyincreasecapacitanceandconductivity
becauseofadecreaseofoxygen‐containingfunctionalgroupsandrecoveryoftheperfectgraphene
structure,anditisstronglydemonstratedbyLuo’swork[51].Intheirwork,RGOwascompounded

Materials2020,13,54811of45
withPANIbythefollowingstrategy:firstly,GOwasreducedbyglucoseandammonia;thenPANI
wasuniformlyinsitupolymerizedontotheas‐preparedRGOnanosheets.Thereductiondegreeof
GOwasmeasuredbythereductiontime.ConductivityandcapacitanceofRGOwasincreasedwith
anincreaseofreductiontimeduetodecreaseofOcontentonthesurfaceofGO.Particularly,the
optimumPANI/RGOsupercapacitiveperformanceswereachievedwherespecificcapacitancewasas
highas1045F/gandahighretentionof97%after1000cyclesoccurredatareductiontimeof1h.

Figure7.SchematicdemonstrationofthesynthesisprocessofPANI/GOnanocomposite.(reprinted
withpermissionfrompreviousliterature[49]©2012AmericanChemicalSociety).
Carbonnanofibersalsoattractsomeattentionfromresearchers.Morerecently,Meltem’sgroup
[52,53]reportedonthepreparationoffree‐standingflexiblePANI/carbonnanofiberelectrodesbythe
sol–gelandelectrospinningmethod.Comparedwithanindividualcarbonnanofiberelectrode,the
hybridelectrodewaswithhighspecificcapacitanceof234F/gandgreatcyclingstabilitywitha
capacitanceretentionof90%after1000cycles,alongwithhighenergydensityof32Wh/kgatapower
densityof500W/kgbenefitingfromexcellentpseudocapacitivepropertiesofPANIcoatingontothe
carbonnanofiber.
Inconclusion,itisapromisingstrategytopreparePANI/carbonbinarycompositestooptimize
theelectrochemicalperformances.Morphologyandstructureofthecompositeshaveakeyimpacton
theelectrochemicalpropertiesofPANIbasedelectrodes,henceitiscrucialtodesignarationalway
toachievethetargets.Ingeneral,themorphologyofnanofiber,nanowhiskerorfree‐standingflexible
3D‐structureworksbetterthanothermorphologiesandstructures.However,thelimited
improvementofspecificcapacitanceisstilltheproblemofcarbonmaterials[20],fortunately,metal
compounds(especiallymetaloxides)makeupfortheshortage,therefore,PANI/metalcompound
deservesextensiveinvestigationsinsupercapacitors.
PANI/MetalCompoundsBinaryComposites
Exceptforcarbonmaterials,metalcompoundsalsohavebeenpopularlystudiedduetotheir
largecapacitanceabilityandexcellentstability.However,theirconductivityisfairlylow,thusitis
necessarytoinvolveintheassistanceofCPs(especiallyPANI)withhighconductivity.Inrecent

Materials2020,13,54812of45
years,metaloxides,metalchlorides,metalsulfidesandmetalnitridesarecommonlyusedtocombine
withPANIassupercapacitorelectrodesforimprovingtheelectrochemicalperformance.
Metaloxides,especiallytransitionmetaloxides,recognizedasacandidateinthefieldofa
supercapacitorbenefitingfromtheirextraordinarystabilityandoutstandingelectronstorageability,
aremostusedamongmetalcompounds.Howevertheirinferiorconductivityandpoorresistanceto
acidgreatlyhindertheirapplication.Inordertoovercomethelimitation,manyresearchersfabricate
CPs/metaloxideshybridswithexcellentconductivity,andPANI/metaloxidesaremorecommonly
reported.InthefieldofPANI/metaloxideshybrids,MnO2ismostwidelystudiedbecauseofitslower
cost,availableinabundance,excellentelectricalandcapacitivebehaviors,alongwithenvironmental
friendliness[54–58].Liuetal.[59]reportedafacileone‐stepmethodtoelectrochemicallysynthesize
thePANI/MnO2compositeviapulseelectrodeposition.Wherein,MnO2particleswereuniformly
dispersedontothesurfaceofPANInanorods.Thepreparedcompositepossessesahighspecific
capacitanceof810F/gat0.5A/gandacapacitanceretentionof86.3%after1000cycles,muchhigher
thanpurePANI,reflectingthattheadditionofMnO2hassynergisticeffectsbetweentheinvolved
materials.NovelworkwasdonebyHuo’sgroup[60].Firstly,theypreparedPANI/MnO2nanofibers
throughinterfacialchemicalpolymerization.Duringthesynthesis,4‐amino‐thiophenol(4‐ATP)
actedasthestructure‐directingagentontheAusubstrate;thenthenanofibercompositewithasize
of30nmtransferredintothemicrospherebyself‐assembly.ThePANI/MnO2nanofibermicrosphere
electrodeobtainedpreferablespecificcapacitanceof765F/gat1mA/cm2in1MNa2SO4solution,and
highcyclingstability(acapacitancedecreaseofjust14.9%after400cycles),whichconfirmthatthe
hybridmightbeapromisingsupercapacitorelectrodematerial.Ranandcoworkers[61]fabricateda
nano‐PANI@MnO2hybridwithatubularshapeviaasurfaceinitiatedpolymerizationtechnique.The
compositeelectrodeshowedgoodelectrochemicalperformancesincludingaspecificcapacitanceof
386F/gin1MNaNO3electrolytewiththepotentialwindowrangefrom0to0.6V,excellentstability
(aretentionof79.5%after800cycles)aswellasperfectEDLCperformance.Comparedwith
individualPANIorMnO2,thehybrid’sperformancesignificantlyimproved,whichcanascribetoa
synergisticeffectbetweenthetwocompositions,Xieetal.[62]revealedthatthestructureofMnO2
coulddecidethepropertiesofPANI/MnO2compositeelectrodesintheirstudy.Fourcrystal
structuresofMnO2werefabricatedintheirwork,theyareα‐MnO2,β‐MnO2,γ‐MnO2andδ‐MnO2
respectively.Wherein,γ‐MnO2exhibitedthebesteffectthroughcomparingtheelectrochemical
performanceofthefourcrystallographicstructures,thusitwaselectedtosynthesisγ‐MnO2/PANI
nanocompositeelectrodethroughtheinsitupolymerizationmethod.Theγ‐MnO2/PANImodified
electrodeshowedenhancedelectrochemicalperformance(493F/gat0.5A/gand95%capacitance
retentionafter1000cycles)thanindividualγ‐MnO2orPANIwhilethemassratioofγ‐MnO2toPANI
was1:5.Theauthorfurtherexplainedthatthreecrucialfactorsleadtothat:highenergydensityand
excellentcyclingstabilityofγ‐MnO2,highconductivityofPANI,alongwithinterconnectedreticular
structurebetweenthetwocompositions.
BesidesMnO2,PANI/othermetaloxidescompositeelectrodes’propertiesarealsostudied
extensivelyandalotofprogresshasbeenmadeinrecentyears.Forinstance,Atesetal.[63]compared
electrochemicalperformanceofPANI/CuO,PPy/CuOandPEDOT/CuOnanocompositefilmsthat
electrochemicallyfabricatedonglassycarbonelectrode.ResultsdisplayedthatPANI/CuOhas
advantagesovertheotherswithahighestspecificcapacitanceof286.35F/gat20mV/s,whilethe
highestspecificcapacitancevalueofPPy/CuOandPEDOT/CuOwas20.78F/gand198.89F/gat5
mV/srespectively.ItcanbeobviouslyincludedthatPANI/CuOwasthemostidealcandidatefor
supercapacitorelectrodematerials.Giri’sgroup[64]successfullyobtainedRuO2/PANIcompositevia
insituoxidativepolymerizationandinvestigatedtheeffectonelectrochemicalperformancewhile
dopingwithRu(Ш).Astheelectrochemicalcharacterizationsshowed,aspecificcapacitanceof425
F/ghadbeenachievedafteradditionofRuO2,whilethepurePANIwas160F/g,meanwhile,the
composite’scyclingstabilityalsolargelyincreased,reflectingthatRuO2broughtaboutasynergistic
effect.Morerecently,Prasankumarandcoworkers[65]preparedthePANI/Fe3O4compositeviain
situpolymerizationinthepresenceofmicrowaveobtainedFe3O4nanoparticles.Theprepared

Materials2020,13,54813of45
compositeexhibitedhighspecificcapacitanceof572F/gat0.5A/gandpronouncedlong‐termcycling
stability(82%capacitanceretentionover5000cycles).Thegroupviewedthatitisofgreatpotential
tobeappliedasefficientsupercapacitorelectrodes.Wang’sgroup[75]successfullypreparedthe
SnO2@PANInanocompositeasbelow(schematicallyillustratedinFigure8):Tostart,SnOwas
obtainedbyultrasonicationinthepresenceofethanolamine(ETA).Thenfollowedbyoxidationto
SnO2byinsituoxidationandthepolymerizationofaniline.TheoptimumSnO2@PANIcomposite
(18.73wt%PANI)possessesaspecificcapacitanceashighas335.5F/gat0.1A/g,excellentrate
capability(108.8F/gat40A/g)andperfectcyclingstability(nocapacitancedecayafter1000cycles),
superiortopurePANI,whichbenefitsfromthesynergisticeffectwheretheenhancedstabilityis
derivedfromSnO2andtheenhancedcapacitanceisderivedfromPANI.

Figure8.SchematicdiagramofpreparationofSnOandSnO2@PANInanocomposite(reproduced
from[75]withpermissionfromTheRoyalSocietyofChemistry).
Apartfrommetaloxides,othermetalcompoundslikemetalchlorides,metalsulfidesandmetal
nitridesalsoareappropriatetobeexploitedaselectrodematerialsforsupercapacitor.Comparedwith
metaloxides,theyaremorestableinacidicelectrolyte,buttheirelectricalconductivityand
electrochemicalperformancearenotsowellasmetaloxides.Dhibaretal.[66]synthesized
PANI/CuCl2compositeswithvariousdopinglevelsofCuCl2(1,2,3and4wt%)throughinsitu
oxidativepolymerization,usingAPS (Ammonium persulphate)asanoxidantintheHClmedium.
ElectrochemicalmeasuresshowedthatPANI/CuCl2(2wt%)exhibitedthemaximumspecific
capacitanceof626F/gat10mV/s,meanwhilethecompositeelectrodewaswiththemaximumpower
density(8158.5W/kgat200mV/s)andmaximumenergydensity(222.57Wh/kgat10mV/s),which
indicatethatthecontentofCuCl2hasanimportanteffectoncomposites’electrochemical
performance.Morelately,Zhangetal.[67]developedatemplate‐assistedtechniquetosynthesize
MoS2/PANIhollowmicrospheres.PANIwasdepositedontothesurfaceofhollowMoS2
microspheres.TheformationprocessofMoS2/PANIhollowmicrospheresisvividlydisplayedin
Figure9.TheypointedoutthathollowMoS2microspherestructureprovidealargenumberofion
channelsandlargesurfacearea,whichisintheflavorofiontransportinanelectrolyte.Whenthe
massratioofMoS2:PANIwas1:2,theelectrodepossessmaximumspecificcapacitanceof364F/gat5
mV/sandcapacitanceretentionof84.3%after8000cyclesat10A/g.FromDhibarandZhang’swork,
wecanseethatthecontentofmetalcompoundhasacrucialeffectoncomposites’electrochemical
performance,anditisnotalinearrelationshipbetweenthecontentandelectrochemicalperformance,
thebestelectrochemicalperformancecorrespondstoacertainvalueofcontent.Xiaetal.[77]designed
aPANI/TiNcore–shellnanowirearrays(NWAs)structurebyafacileelectrodepositiontechnique.
ThePANI/TiNNWAselectrodewaswithaveryhighspecificcapacitanceof1064F/gat1A/ganda
stablecapacitanceretentionof95%after200cyclesbenefitingfromhighelectronicconductivityand
capacitystoragederivedfromthecore–shellNWAsstructure.

Materials2020,13,54814of45

Figure9.SchematicillustrationoftheformationofMoS2/PANIhollowmicrospheres[67](reproduced
withpermissionfromElsevier).
PANI/CPsBinaryComposites
PANIcancombinewithotherCPstoformco‐polymers.Duetotheintrinsicelectrical
conductivityinCPs,alongwiththeirexcellentpseudocapacitiveperformance,theseco‐polymers
usuallypossessenhancedsupercapacitiveperformancederivedfromasynergisticeffect.
Additionally,theco‐polymersaredesirablebecauseoftheirlow‐costsynthesis,highenergystorage
capacity,highyieldsandenvironmentalfriendliness[68–72].
Inrecentyears,moreandmoreinvestigationsonPANI/CPscompositesaredone,andmoreand
moresatisfactoryproperties(especiallyelectrochemicalproperties)onthemareexploredout,
obviouslytheymightholdmuchpromiseforservingforsupercapacitors.Zhangetal.[72]designed
novelPANI/PPydouble‐wallednanotubearrays(DNTAs;asschematicallyillustratedinFigure10).
ThefabricatedhybridDNTAswereusedastheworkingelectrodetostudyitselectrochemical
properties.ThestudiesshowedthatthePANI/PPyDNTAsexhibitedahighspecificcapacitanceof
693F/gat5mV/s,whichwasmuchhigherthanPPyDNTAs(250F/gatsamecondition),outstanding
ratecapabilityandexcellentlong‐termcyclingstability(7.6%capacitancelossafter1000cycles).They
highlightedthatbothofPANIandPPymadeacontributiontoimprovedelectrochemical
performance.Veryrecently,Yangetal.[73]co‐polymerizedPANIandPEDOTbyamolecularbridge
providedbyphyticacid.ThePANI/PEDOTco‐polymerhydrogelwaswitha3D‐networkstructure
ofPEDOTsheetswherePANIwasinlaid.Whileusedasasupercapacitorelectrode,itshowed
outstandingelectrochemicalperformanceandhighlyenhancedmechanicalpropertiesascribingtoa
synergisticeffectanduniquemolecularinteractionsbetweenPANIandPEDOT.

Figure10.SchematicdiagramofthesynthesisofPANI/PPydouble‐wallednanotubearrays(DNTAs;
reprintedwithpermissionfrompreviousliterature[72]©2014AmericanChemicalSociety).

Materials2020,13,54815of45
OtherPANIBasedBinaryComposites
Apartfromcarbon,metalcompoundsandCPs,somecompositesthatconsistofPANIandother
materialsalsohavebeenfoundtoshowimprovedpropertiesrecently,includingPANI/puremetal,
PANI/otherorganicmaterialsandsoon.Tangetal.[74]developedagreenstrategytoobtaina
PANI/AgcompositewhereAgnanoparticleswereuniformlydispersedontothesurfaceofPANIvia
reductionAgsubstratewiththeassistanceofvitaminC.Thecompositeachievedthemaximum
specificcapacitanceof553F/gat1A/g,whichwasmuchhigherthanpurePANI(316F/gat1A/g),
attributingtothesynergisticeffectbetweenPANIandAg.Itscyclingstability(about90%capacitance
retentionafter1000cycles)andratecapabilitywereingoodcondition.Moreover,itexhibitedahigh
electricalconductivityof215.8S/m,itisbecauseAgnanoparticlespromotechargetransferbetween
theactivecomponents,thusleadingtoenhancedspecificcapacitanceandconductivity.Chenetal.
[85]designedacabbage‐likePANI/hydroquinonecompositemicrospherethroughinsitu
polymerization.Electrochemicalinvestigationsdemonstratedthatthenanocompositeexhibitedgreat
electrochemicalproperties,thatis,ahighspecificcapacitanceof126F/gat5mV/sand85.1%
capacitanceretentionafter500cyclesat1A/g.Astheauthorviewed,PANIprovidedelectronic
conductivechannelsforthehydroquinoneandhydroquinonethatactasapseudocapacitance
component.
2.2.2.PANIBasedTernaryComposites
Asreviewedabove,PANIbasedbinarycompositeshavemadealotofcontributionto
supercapacitorelectrodes,however,theoptimumelectrochemicalpropertiessuchasconductivity,
specificcapacitanceandcyclingstabilitystillcannotbeachievedfully[20].Currently,inorderto
reachtheutmostelectrochemicalperformancelevels,moreandmoreattentionsarefocusedon
ternarycomposites.
PANI/metaloxides/carbonmaterialsternarycompositesaremostappliedinsupercapacitors
sinceCPs,metaloxidesandcarbonmaterialsarethemostoutstandingactivematerialsfor
supercapacitorelectrodes.SankarandSelvan[76]designedaternaryhybridsupercapacitorelectrode
wheretheobtainedMnFe2O4nanoparticlesweredispersedontheflexiblegraphenesheetsviathe
hydrothermalmethodandwerewrappedwithPANIviatheinsituchemicalpolymerizationmethod.
IntheternaryMnFe2O4/graphene/PANIhybridstructure,theMnFe2O4andPANIfunctionedasa
spacerthatavoidsreattachmentbetweentheflexiblegraphenesheets,andPANIandgraphene
functionedasaconductivenetworkthatpromotesiontransferfromanelectrolyteintoanelectrode.
Asexpected,thecompositeexhibitedsatisfactoryelectrochemicalperformance,includingahigh
specificcapacitanceof241F/gat0.5mA/cm2(7.5timeshigherthanMnFe2O4)andperfectcycling
stability(100%capacitanceretentionafter5000cycles).Aswecansee,theternarypossessesmore
pronouncedelectrochemicalperformance(especiallycyclingstability)thanthebinarycomposites,it
mainlyattributestothestrongersynergisticeffectbetweenthethreeconstituentsthanthebinary
ones.Fu’sgroup[78]developedafacileelectro‐polymerizationstrategytofabricatea3Dpolystyrene
microsphere‐reducedgrapheneoxide/MnO2/PANI(3DrGN‐MnO2‐PANI)coaxialarrayscomposite.
Duringthefabricationprocessofthe3DrGN‐MnO2‐PANIcomposite,thepolystyrene(PS)was
insertedbetweentherGNtemplates,whichenlargedthespecificsurfaceareaoftherGN;PANIand
MnO2dispersedontotherGNtemplateswitharraysandnanoflakestructurerespectively,which
shortenedtheiondiffusionpath,enlargedinterfacialareaandfastenselectricalpathways.As
expected,theternarycompositefilmelectrodeshowedahighspecificcapacitanceof1181F/gat1
A/gandgoodcyclingstabilitywith89.1%capacitanceretentionafter1000cyclesat20A/g.The
enhancedelectrochemicalperformanceprovesthatthe3DrGN‐MnO2‐PANIwouldplayasignificant
roleinenergystoragesystems.ItisveryrecentlythatJeyaranjanandcoworkers[79]reportedahighly
scalableternaryporoushierarchicalPANI/RGO/CeO2hybridmicrospherepreparedbyaspray
dryingmethod.Theobtainedternarymicrospherewaswithahighspecificcapacitanceof684F/g,
goodratecapabilityandexcellentlong‐termcyclingstabilitywith92%capacitanceretentionafter

Materials2020,13,54816of45
6000cycles.Theimprovedelectrochemicalpropertiesattributetothefunctionalandsynergisticeffect
betweenthethreeconstituents.
PANI/metaloxides/carbonmaterialsarenottheexclusivePANIbasedternarycompositesfor
supercapacitorelectrodes.Infact,theothercomponentsexceptPANIneedtomakeupforthe
shortagesofPANIforthepurposeoffulfillingthemaximumsynergisticeffectlevelaspossible.As
forPANI,thedisadvantagesincludingthepoorstability,notsohighspecificsurfacearea,relatively
lowelectronicconductivityandeaseofagglomerationareurgenttobeenhancedinthePANIbased
ternarysystems,thustheothertwocomponentsthatcanactasoptimizingthesedisadvantagesof
PANIaresuitabletoformPANIbasedternarysystems.Forexample,Kimetal.[80]synthesized
ternaryAg/MnO2/PANInanocompositefilmsforsupercapacitorelectrodesthroughanovelpulsed
potentialelectro‐depositionstrategy.Inthecomposite,Agprovidesahighelectronicconductivity
andfastiontransfer,furthermore,AgandMnO2shapedasuniformvermicularmorphologywhile
thepurePANIshapedasagglomeratedvermicular‐likestructure,bothofwhichimprovethe
electrochemicalperformanceofthecompositeelectrode.Asaresult,theternarycompositeexhibited
muchbetterelectrochemicalperformancethanpurePANIfilmwithahighcalculatedspecific
capacitanceof621F/gand800F/gfromCVandCDrespectively,aswellasgoodstability(83%
capacitanceretentionafter750cycles).Luetal.[81]preparedPANI@TiO2/Ti3C2Txternarycomposite
withahierarchicalstructureviaahydrothermaltreatmentwithadditionoftheinsitupolymerization
process.Asnewactivematerials,thelayeredTi3C2Txprovidedahighspecificsurfaceareaandanice
frameworkthatisbeneficialtoiontransfer,andPANInanoflakesandTiO2nanoparticlescanincrease
theactivematerials’surfacearea.Aselectrochemicalmeasuresshowed,thenanocompositea
remarkablestabilityof94%capacitanceretentionafter8000cyclesat1A/g.Theyexplainedthatthe
highspecificcapacitanceof188.3F/gat10mV/s,whileTiO2/Ti3C2Txwasabouthalfofthat,anda
hierarchicalarchitectureandthesynergisticeffectofcombiningPANInanoflakeswithTiO2/Ti3C2Tx
compositeresultedintheenhancedelectrochemicalperformance.Morerecently,Zhuandcoworkers
[82]developedaneffectivemethodtoconstructhierarchicalZnO@metal‐organicframework
(MOF)@PANIcore–shellnanorodarraysoncarboncloth(CC)forasupercapacitorelectrode.The
schematicconstructionofZnO@MOF@PANInanoarraysonCCisvividlyillustratedinFigure11.As
risingporousmaterials,MOFisassociatedwithconsiderablehighspecificsurfaceareaandfast
electronandiontransfer,andZnOisrelatedwithexcellentstability,hencetheZnO@MOF@PANI
mightbeagoodcombinationforasupercapacitorelectrode.Inthecompositearchitecture,ZnO
nanorodsactedasthecorethatsupportstheMOF@PANIshell.Resultsshowedthattheternary
compositepossessesahighspecificcapacitanceof340.7F/gat1.0A/g,goodratecapabilitywith84.3%
capacitanceretentionfrom1.0to10A/gandlong‐termcyclingstabilityof82.5%capacitanceretention
after5000cyclesat2.0A/g.Theenhancedelectrochemicalpropertiesbenefitedfromtheunique
hierarchicalcore–shellarchitectureandthesynergisticeffectofthethreecomponentsastheauthor
viewed.

Figure11.SchematicillustrationofsynthesisofZnO@MOF@PANInanoarraysoncarboncloth(CC)
[82](reproducedwithpermissionfromElsevier).

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Table1presentsthepreparationmethodandelectrochemicalperformanceofsometypicalPANI
basedsupercapacitorelectrodematerials.
Insummary,PANIbasedternarycompositesshowgreatpotentialinsupercapacitors.
Furthermore,theelectrochemicalperformanceisdeeplyinfluencedbythenatureofthecomponents
thatcombinedwithPANIandthestructureormorphologyofthecomposites,thusitisimportantto
lookformaterialswithasynergisticeffect,andtodesignasuitablenano‐structureormorphology,
thenresultinextensiveinvestigations.Obviously,studyonPANIbasedternaryelectrodematerials
hasbecomeahotdirectioninthefieldofenergystorageandconversion.
Table1.ThepreparationmethodandelectrochemicalperformanceofsometypicalPANIbased
supercapacitorelectrodematerials.
MaterialsPreparationMethodMaximumSpecific
CapacitanceCycleStability
PANI[33]interfacialpolymerization554Fg−1at10mAg−157Fg−1after
1000cycles
PANI/POROUSCARBON[34]electrochemical
polymerization180Fg−1at1Ag−1163Fg−1after
1000cycles
PANI/C‐ACS[35]selectivesurface
dissolution(SSD)method765Fg−1at1A/g91%after5000
cycles
PANI‐NWS/CMK‐3[40]chemicaloxidative
polymerization90.4%after1000
cycles
NANO‐HCS/PANI[42]insituchemicaloxidative
polymerization435Fg−1at1Ag−160%after2000
cycles
PANI/MWCNTS[43]chemicaloxidative
polymerization320Fg−1at10mAg−18%after50
cycles
CNTS/PANI[44]depositionofPANIonthe
surfaceofCNTs183Fg−1at10mAg−1
NITROGEN‐CONTAINING
CNTS/PANI[45]hightemperaturetreatment163Fg−1at700°C
and0.1Ag−1
SWCNTS/PANI[46]spray‐printingmethod355.5Fg−1at0.1A
g−1
87.2%after5000
cycles
CCG/PANI‐NFS[47]vacuumfiltrationthe
mixeddispersions210Fg−1at0.3Ag−194%after1000
cycles
GRAPHENE/PANI
NANOFIBER[48]insitupolymerization480F/gat0.1A/g70%after1000
cycles
GO/PANI[49]insitupolymerization425F/gat0.2A/g83%after500
cycles
GO/PANI[50]asoftchemicalroute531F/gat0.2A/g
RGO/PANI[51]insitupolymerization1045F/gat0.1A/g97%after1000
cycles
PANI/CARBONNANOFIBER
[52,53]
sol–gelandelectrospinning
method234F/gat0.1A/g90%after1000
cycles
PANI/MNO2[59]pulseelectrodeposition810F/gat0.5A/g86.3%after1000
cycles
PANI/MNO2NANOFIBER
MICROSPHERE[60]
interfacialchemical
polymerization765F/gat1mA/cm285.1%after400
cycles
NANO‐PANI@MNO2[61]surfaceinitiated
polymerization
386F/gwiththe
potentialwindow
rangefrom0to0.6V
79.5%after800
cycles
PANI/CUO[63]insitupolymerization286.35F/gat20mV/s
RUO2/PANI[64]insituoxidative
polymerization425F/gat1mA/cm2
PANI/FE3O4[65]insitupolymerization572F/gat0.5A/g82%over5000
cycles

Materials2020,13,54818of45
SNO2/PANI[75]insituoxidative
polymerization335.5F/gat0.1A/g
nocapacitance
decayafter1000
cycles
PANI/CUCL2[66]insituoxidative
polymerization626F/gat10mV/s
MOS2/PANI[67]template‐assistedtechnique364F/gat5mV/s84.3%after8000
cycles
PANI/TINNWAS[77]electrodepositiontechnique1064F/gat1A/g95%after200
cycles
PANI/PPYDNTAS[72]PPycoatedontothePANI693F/gat5mV/s92.4%over1000
cycles
PANI/AG[74]Agnanoparticlesdispersed
ontothesurfaceofPANI553F/gat1A/g90%after1000
cycles
MNFE2O4/GRAPHENE/PANI
[76]
insituchemical
polymerization
241F/gat0.5
mA/cm2
100%after5000
cycles
3DRGN‐MNO2‐PANI)[78]electro‐polymerization1181F/gat1A/g89.1%after1000
cycles
PANI/RGO/CEO2[79]spraydryingmethod684F/gat1A/g92%after6000
cycles
AG/MNO2/PANI[80]pulsedpotentialelectro‐
deposition
621F/gand800F/g
at1A/gfromCV
andCDrespectively
83%after750
cycles
PANI@TIO2/TI3C2TX[81]
hydrothermaltreatment
withadditionofinsitu
polymerizationprocess
188.3F/gat10mV/s94%after8000
cycles
3.ApplicationsofPANIforRechargeableBatteries
Asanotherimportantelectrochemicalenergystorageandconversiondevice,rechargeable
batteries,alsocalledsecondarybatteries,havebeenextensivelyusedowningtoitshighenergy
density,goodportability,lowcost,safeandexcellentstability.Justlikesupercapacitors,their
electrochemicalperformancesaregreatlyrelatedwithelectrodeproperties.Conventionalelectrode
materialsforrechargeablebatteriesaremainlymetalormetallicderivatives,buttheyusuallysuffer
frompoorstability,inferiorconductivity,lowratecapabilityandvoltagedecrease[83].PANI
providesnovelfeasibilityfordesigningelectrodesofrechargeablebatteriesduetoitshighflexibility,
highelectricalconductivityandlow‐costsynthesis.Moreover,lotsofresearchhasprovedthe
applicationsofPANIforimprovingtheelectrochemicalperformanceofrechargeablebatteries,thus
PANIiswidelyusedinthatfield.Inthischapter,wewillemphaticallydiscussthethreetypesof
rechargeablebatteriesthataremostappliedandstudied:lithium‐ionbatteries(LIBs),lithium‐sulfur
batteries(LSBs)andsodium‐ionbatteries(SIBs).Theelectrodematerials’designforLIBs,LSBsand
SIBsthatareassociatedwithPANIwillbealsoreviewedindetail.
3.1.Lithium‐IonBatteries(LIBs)
Amongrechargeablebatteries,LIBsarethemostpromisingsuperstarusedinportableelectronic
devicesduetotheirhighpower/energydensity,goodportability,excellentcyclingstabilityand
environmentalfriendliness.Asarisingsecondarybatteries,LIBsmainlyrelyonlithiumionsmoving
betweenthepositiveelectrode(cathode)andthenegativeelectrode(anode)tooperate.Whenitis
charging,Li+ionstransferfromthecathodetotheanodethroughtheelectrolyte,thenresultsinaLi‐
richstateinananode;whendischarging,itisthecontrary,thatis,thecathodeisinaLi‐richstate.
ThemechanismisillustratedinFigure12.

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
Figure12.SchematicillustrationoftheworkingmechanismofLIBs.
SincethecathodeandanodeprovidethespaceforLi+ionstransfer,manyresearchershavebeen
tryingtoseekelectrodematerialswithsuperiorelectrochemicalperformancetooptimizethe
characteristicsofLIBs.Inthenextsection,wewilldiscusstherecentresearchprogressonPANI
modifiedcathode/anodematerialsforLIBsindetail.
3.1.1.PANIModifiedCathodeMaterials
Asoneofthemostpromisingenergystoragedevices,LIBshavebeenextensivelyinvestigated
andrapidlydevelopedsincethe1970s[84].Ingeneral,mostcathodematerialsforLIBsaremadefrom
Li‐containingtransitionmetaloxides,furthermore,thehigherLicontentinthecompoundendows
thecathodewithbetterelectrochemicalperformance,whereasLi‐richTMOsarethepromising
cathodematerials.However,relativelylowconductivity,cyclinginstabilityandstructuralinstability
arechallengesthatremaintobetackled.Fortunately,PANIwithhighconductivity,goodstability
andexcellentflexibilitycanovercometheseproblems,soPANImodifiedcathodematerialsare
wanted.Uptodatenow,Li‐richcathodematerialshaveundergonevariousgenerationslikeLiMxOy
(M=Co,Ni,Mn,V),LiFePO4,Li(Ni1−xCox)O2,Li(Ni1−x−yMnxFey)O2andLi(Ni1−x−yCoxMy)O2(M=Fe,Al,
Mn).Amongthem,LiCoO2,LiFePO4,LiV3O8,Li(NixCoyMn1−x−y)O2andLi(NixMnyFe1−x−y)O2havebeen
commonlychosentopreparePANImodifiedcathodematerialsforLIBs.
AsthefirstgenerationofthepromisingcathodematerialsforLIBs,LiCoO2hasbeen
commerciallyappliedduetoitshighLicontentandstablecyclicbehavior.Infact,LiCoO2isdispersed
ontotheelectrodesurfacewithasolidpowder,itisnecessarytodopeconductivemediumintothe
electricalwiringofLiCoO2powder[86].PANIissuitabletoactastheconductivemediumduetoits
highelectricalconductivity.Karimaetal.[86,87]developedaPickeringemulsionroutetoproduce
PANI/LiCoO2nanocompositeswithwell‐orderedlayeredstructure.ConductiveandflexiblePANI
additivecanconnecttheLiCoO2powderswell,andshortenchargetransportpathways,thereby
realizingincreasedconductivityandcapability.Asaconsequence,allofthenanocompositeswith
variousmassratioofPANIshowedenhancedspecificcapabilitiescomparedwiththatofpristine
LiCoO2of136mAh/g.

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LiFePO4(LFP)isthenextgenerationofpromisingcathodematerialafterLiCoO2owningtoits
excellentcyclingstability,lowcost,securityandlargecapacity.However,blamingforits1Dchannels
forLiextraction[88],therearestilltwolimitationsincludingpoorelectronicconductivityandslow
Li+iondiffusionthathinderitsapplicationsforLIBs[89].Carboncoatingisausefulstrategyto
improvetheelectronicconductivityofLFP,butaC‐LFPcompositeiselectrochemicallyinactive,
fortunately,electrochemicallyactivePANIispromisingtoformaPANI/C‐LFPcompositeby
combiningwithC‐LFPorsubstituteC‐LFPforPANI‐LFPcomposite.InSu’swork[90],aPANI‐CSA
(camphorsulfonicacid)/C‐LFPcompositewaspreparedbycoatingC‐LFPwithPANI‐CASinm‐
cresolsolution.Thecompositecathodesdeliveredenhancedspecificdischargespecificcapacityand
ratecapability.Inparticular,10%PANI‐CSA/C‐LFPachievedaspecificcapacityvalueashighas
165.3mAh/g.TheyattributedittotheelectrochemicallyactivePANIadditives.Recently,Fagundes
andcoworkers[91]reportedaPANI/LFPcompositecathodematerialforLIBs.Thecompositewas
fabricatedviaanalternativesynthesisroute,whichwasbeneficialtoenhancedelectrochemical
propertiesandfastelectrontransferrateoftheLPF.Therefore,thecompositeshowedalower
oxidationandreducingpotential(ΔEp=0.20V)thanthatofLFP(ΔEp=0.41V)accordingtotheCV.
Gongetal.[92]adoptedaPANI/poly(ethyleneglycol)(PEG)co‐polymertosimultaneouslymodify
theelectronicconductivityandLiiondiffusionrateoftheLFP.Themodificationwasderivedfrom
thesynergisticeffectbetweenPANIandPEG:PANIservedasasuperiorconductivemedium,while
PEGservedasanexcellentsolventforlithiumsaltsandbecamethebestknownpolymerionic
conductor.Inreturn,thePANI/PEGco‐polymermodifiedC‐LFPbasedcathodeachievedahigh
specificcapacityof125.3mAh/gat5°C,aswellasexcellentcyclingstabilityof95.7%capacity
retentionafter100cyclesat0.1°C.
LiV3O8iswell‐knownasapromisingcathodematerialforLIBsduetoitslargecapacity,being
chemicallystableanditslowcost,butpureLiV3O8suffersfromashortcycliclifeandpoorrate
capability.CoatingPANIonLiV3O8isofgreatusetomakeupforthedisadvantagesofpureLiV3O8.
Guoetal.[93]chemicallysynthesizedaLiV3O8/PANInanocompositeviatheoxidative
polymerizationmethod.Inthenanocompositecathode,PANIcoatingactedasaconductivenetwork
structure,moreoverwell‐crystallizedregionsandamorphous‐likeregionscoexistinit,bothofwhich
boostelectrontransferandlithiumionchemicalcoefficients,thenresultinginbetterelectrochemical
propertiesthanpristineLiV3O8,whichincludedanenhancedcyclicstability(about95%capacity
retentionafter55cycles)andsuperiorratecapability.
Inrecentyears,thetrimetallicLi‐containingoxidescathodematerialslikeLi(NixCoyMn1−x−y)O2
andLi(NixMnyFe1−x−y)O2arouselotsofinterestbecauseoftheirwellratecapability,highspecific
capacity,lowcostandenvironmentalfriendliness.However,withtheincreaseofNicontent,they
usuallyundergotheevaporationlossduringtheLiioncalcinationandinferiorcyclicstabilitydueto
residualLi2CO3andLiOHimpuritiesderivedfromsidereactionsaftereverycycle[94].Likewise,
coatingPANIontomonometallicandbimetallicLi‐containingoxidestoformmodifiedcathode
materialscanovercometheseobstacles.Songetal.[95]obtainedPANImodifiedPANI‐coated
Li(Ni0.8Co0.1Mn0.1)O2cathodematerialforLIBsviaasolutionmethod(Figure13c)andevaluatedits
electrochemicalperformance.PANIcoatingcanremovetheresidualLicompoundslikeLi2CO3and
LiOH(Figure13a,b)effectivelyandoptimizetheinterfacialelectrochemicalreactions.Thus,thePANI
modifiedcathodeachievedahighinitialdischargespecificcapacityof193.8mAh/gwithawell
capacityretentionof96.25%after80cyclesat1°Candremarkableratecapabilityathighcurrent
density(1°C,2°Cand5°C).Karthikeyanandhiscoworkers[96]testedthehybrid
PANI/Li(Ni1/3Mn1/3Fe1/3)O2(0.2molPANI)cathode’selectrochemicalcharacteristicsinhalf‐cell
configuration.Asaconsequence,itmaintained86%ofinitialdischargecapacityafter40cycles,
particularly,itcanexhibitremarkablecyclicabilityatultra‐highcurrentdensitiesof5,30and40°C.
TheyattributedittothePANIadditive,thatis,PANIhighlyenhancedtheconductivenatureofthe
half‐cellsystemandenabledefficientinsertionandextractionoftheLiion.

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Figure13.ReactionsofprotonatedPANIwithresiduallithiumcompoundslikeLi2CO3(a)andLiOH
(b)and(c)schematicillustrationofpreparationofPANI‐coatedLi(Ni0.8Co0.1Mn0.1)O2cathodematerial.
InadditiontoPANImodifiedcathodematerials,otherPANIbasedcompositeslikePANI/Zn
[97],PANI/Cu/Li2MnSiO4[98]andPANI@RGO/PW12[99]havebeenrecentlyusedascathode
materialsforLIBsduetotheiruniqueelectrochemicalcharacteristics.Furthermore,theyachieved
enhancedelectrochemicalpropertiesincludingspecificcapacity,cyclicstabilityandratecapability,
hencetheyaresignificantlyhotdirectionsforcathodematerialsofLIBs.
Infact,PANIitselfcanalsobeemployedasacathodematerialforLIBs.Basedonhigh
conductivityandflexibilityprovidedbyPANI,togetherwithshortchargetransportpathways
providedbythenanostructure,thePANInanowirearraysascathodematerialforLIBsdeliveredan
initialdischargecapacityof159.83mAh/gandexceptionalcyclingstability(119.79mAh/gretained
after100cyclesat30mA/g)[134].
3.1.2.PANIModifiedAnodeMaterials
Todate,commonanodematerialsforLIBsincludesiliconbasedmaterials(SiandSiOx),metal
oxides(SnO2,TiO2,NiO,Fe2O3andFe3O4)andmetalsulfides(MoS2andSnS2).Nevertheless,theyare
usuallyrelatedtopoorconductivity,structuralinstabilityandeaseofself‐aggregation.Aslike
cathodematerials,theseobstaclescanbealsoovercomebythePANIcoating.
Silicon(Si)anodematerialforLIBshasbeenapromisingresearchfocusbecauseofitshighest
ever‐knowntheoreticalcapacityof4200mAh/g.However,Siisasemiconductoroflowconductivity,
additionally,itusuallyundergoesseverevolumeexpansion(>300%)duringintercalationand
extractionoftheLiion[100].Promisingly,thetwoproblemscanbegreatlysolvedbyPANIcoating.
Manyefforts[100–105]havebeendevotedtomodifytheSianodebyfabricatingaSi/PANIcore–shell
nanocompositeanode.IntheSi/PANIcore–shellstructure,SiandPANIactascoreandshell
respectively,andPANIistightlyanchoredtonano‐SibyacovalentbondbetweenPANIandSi.A
PANI‐encapsulatedshellprovidesalargespaceforvolumeexpansionandshrinkageofSicore
duringintercalationandextractionoftheLiion,whichpromotesthecontactofelectrodematerials,
thenbringsforthhigherreversiblecapacityandbettercyclingstability.Besides,thecoatingofthe
conductivePANIlayersignificantlyenablesafastLiionandelectronictransfer,thusresultingin
excellentconductivity.Inaddition,dopingPANI/Sicompositeswithotheractivematerialsisanother
smartstrategytomodifySianodes’electrochemicalproperties.Carbonmaterials(graphite[106],

Materials2020,13,54822of45
graphene[107–109],RGO[110]andCNTs[111])andmetaloxides(CeO2[112]andTiO2[113])are
conventionallychosentotheactivematerials.GraphitewithpowdersfacilitatesbetterdisperseofSi
nanoparticlesandelectriccontactofactivematerials,whicheffectivelyimproveselectrochemical
properties,sotheternaryPANI/Si/graphitecompositecanachieveahighinitialcapacityof1392
mAh/gandstablecapacityretentionof62.2%after95cycles[106].GrapheneandRGOsheetscanbe
tightlyattachedtothePANIlayerthroughintimateenhancementofπconjugationbetweenthem.
Thelayer–layerstructureofPANI/graphene(orRGO)sheetcanencapsulateSinanoparticlestoform
anewcore–shellarchitecture[107,109]orgluenano‐Sitoformsandwich‐likenanoarchitecture
[108,110]withhighelasticmodulusandhightensilestrength.Bothofthearchitecturesprovided
conductiveandprotective3Dnetworktoavoidstructuraldamagederivedfromvolumeexpansion
duringintercalationandextractionoftheLiion.Additionally,the3Dnetworkcanpromisingly
promoteelectronandLiiontransfer,andthenresultinenhancedconductivity.Aunique
CNTs/PANI/Sicompositewithcore–sheatharchitecturebasedonCNTsfoamexhibitsenhanced
capacityandstabilityinZhou’sstudy[111].The3DinterconnectedporousCNTsprovidesubstantial
conductivechannelsfortheLiionandelectrontransfer.Moreover,aPANIsheathofferedSiwith
enoughspacetoexpand,ensuringstructuralintegrityofelectrodeduringLiioninsertionand
extraction.Inreturn,anenhancedinitialspecificcapacityof1954mAh/gandstablecyclicability(727
mAh/gmaintainedafter100cycles)wereachieved.Therearesimilarsynergisticeffectsthatexistin
theSi@PANI@CeO2composite[112]andSi@PANI@TiO2compositewithadoublecore–shell[113]
structure:ontheonehand,PANIservedasaprotectingelectrodeagainststructuredamagederived
fromvolumeexpansionofSi,ontheotherhand,PANIandlithiatedTiO2orCeO2highlypromoted
electronictransport.Asanoutcome,modifiedanodesexhibitedenhancedconductivityandcycling
stabilitycomparedtopristineSiandPANI/Sicomposite.
Siliconoxide(SiOx)isatypeofrisinganodematerialforLIBsinrecentyears,butlikeSi,low
conductivityandlargevolumeexpansionduringlithiation/de‐lithiationseverelyhinderitspractical
applications.Theseobstaclescanbewelltackledbydesigningcore–shellsandwich‐likePANI‐
wrappedSiOx/CNTs[114]orfabricatingSiOx/PANI/Cu2Ocompositeanodes[115,116].Inthedual
core–shellsandwichstructurePANI/SiOx/CNTscomposite(asseeninFigure14)[114],theCNTs
enhancetheconductivityofSiOxandthePANIendureslargevolumeexpansionduringlithiation/de‐
lithiationofSiOx.OwingtothesynergisticeffectbetweenPANIandSiOx,ahighcharge/discharge
capacityof1156/1178mAh/gat0.2A/gand728/725mAh/gretainedafter60cyclesat8A/gwas
achieved.AhollowSiOxcoatedwithPANI/Cu2OnanocompositecanbepreparedbytheStober
method[115,116].Thenanostructureeffectivelyrelievedthevolumeexpansionduringlithiation/de‐
lithiation,moreover,PANIandCu2Odual‐coatingsignificantlyenhancedreversibilityand
conductivityofSiOxandpreventeditdroppingfromtheelectrodesurface.
SnO2hasbeenactivelyemployedasapromisinganodematerialforLIBsduetoitshigh
theoreticalspecificcapacityof790mAh/g,lowdischargepotential,lowcostandnaturalabundance.
However,therearestillthreeproblemsthathamperitscommercialization:(1)poorcyclingstability;
(2)lowelectronicandionicconductivityand(3)enormousvolumeexpansion(>200%)during
lithiation/de‐lithiationprocess.Intensiveeffortshavebeendevotedtotacklethesedisadvantagesby
synthesizingPANI/SnO2basedternarycomposites.Guoetal.[117]reportedanin‐situ
polymerizationsol–gelroutetoprepareSnO2‐Fe2O3@PANIcomposite.ThegrowthofSnO2‐Fe2O3
particleswasfirstlysuppressedbythePANIontheiroutersurfaceduringpolymerization,nextthe
fullcoatingofacarbonshellencapsulatedtheFe2O3particlesinthethermaltreatment,whichforms
auniqueSnO2‐Fe2O3@Cstructure,inwhichSnO2‐Fe2O3particlesweretightlycoatedwithPANIand
theouterPANIshelleffectivelyrestrictstheiragglomeration,resultinginenhancedstability.
Additionally,theintroductionofacarbonlayerachievesimprovedelectronicconductivity.Hence
theuniquestructureoftheSnO2‐Fe2O3@Cnanocompositesignificantlyimprovesitselectrochemical
properties,achievingthefullyreversiblereactionandalloyreactionofSnO2.Enhancedcapacity
retentionofover1000mAh/gat400mA/gafter380cyclesandexcellentrateperformanceof611
mAh/gat1600mA/gwerereported.Anovel3DternaryPANI/SnO2/RGOnanostructurewas

Materials2020,13,54823of45
successfullydesignedasananodeforLIBsviaaneasydip‐coatingofPANI@SnO2andgraphene
dispersiononCufoam(Figure3‐4c)inDing’s[118]study.Inthenanostructure,PANIactedasthe
conductivematrixaswellasthegluethatbindthehollowSnO2nanoparticlesonRGOsheetstightly
toavoidaggregationwhilecycling,whichgreatlyimprovedtherateperformance;thehollowSnO2
nanoparticlesactedasthebufferforenormousvolumechangesduringinsertion/extractionofLi,and
providedactivespotsforvitiation,whichresultedinenhancedcyclingstability;theassemblyof
PANI@SnO2,RGOandCufoamwithstrongcontactachievesultra‐fastelectrontransportbya3D
expressway,whicheffectivelyenhanceselectronicconductivityandrateperformance.Aspredicted,
thenanocompositeexhibitedexcellentrateperformanceof268mAh/gat1000mA/gandcycling
stability(749mAh/gofinitial772mAh/gwasretainedafter100cyclesat100mA/g),muchhigher
thanSnO2/RGO,PANI/SnO2andpureSnO2(Figure15a,b).Yietal.[119]synthesized3Dexpanded
graphite(EG)/PANI/SnO2compositebythesolvothermalmethodfollowedbyin‐situoxidative
polymerization.Thelong‐ordered3DEGlayerstructuregreatlyenduredvolumeexpansionofSnO2,
PANIcanreducetheelectriccontactbetweenelectrodematerials,thedoublesynergiesbroughtforth
enhancedelectrochemicalcharacteristicsincludingahighinitialreversiblecapacityof1021mAh/gat
0.1A/gand408mAh/gretainedafter100cycles.Recently,Wangandcoworkers[120]designeda
novel1DPANI@SnO2@MWCNTcompositeastheanodematerialforLIBs.TheMWCNTcanprovide
convenientelectrontransferchannelsandaneffectivebufferforhugevolumechangesofSnO2,
furthermore,thesynergyoftheconductivePANIandMWCNTskeletonsignificantlyenhancedthe
electronicandionicconductivityoftheternary1DPANI@SnO2@MWCNTcomposite.Inreturn,the
compositedeliveredahighreversiblecharge/dischargespecificcapacityof878/888mAh/gat0.2A/g
over100cyclesand524/527mAh/gat1.0A/gover150cycles.


Materials2020,13,54824of45
Figure14.(a)SchematicillustrationofpreparationofPANI/SiOx/CNTsand(b)digitalphotographics
ofPANI/SiOx/CNTs[114](reproducedwithpermissionfromElsevier).

Figure15.(a)RatecapabilitiesoftheSnO2@PANI/rGOnanocomposites,SnO2@PANI,SnO2/RGOand
SnO2respectively;(b)cyclicbehaviorsoftheSnO2@PANI/rGOcomposites,SnO2/rGO,SnO2@PANI
andSnO2atthecurrentdensityof100mA/gand(c)aschematicillustrationofthepreparationofthe
PANI/SnO2/RGOnanocomposite[118](reproducedwithpermissionfromElsevier).
ComparedtosiliconbasedmaterialsandSnO2,TiO2isasuperiorcandidateforLianode
materialsasaresultofitssmallvolumeexpansion(<4%),reliablesecurityandexcellentcycling
stability.Unfortunately,thenano‐TiO2tendstoagglomerateanddecomposewhilecycling,which
resultincapacityfadinganddecreasesinelectroactivesites.Asaconsequence,therateof
lithiation/delithiationslowsdown.DopingtheconductivePANI/RGO[121]orPANI/GO[122]phase
inTiO2isaneffectivewaytoresolveit.BothofPANI/RGOandPANI/GOcanbeconstructedintoa
sandwichstructurewithTiO2.IntheTiO2/PANI/RGOsandwichstructure,theTiO2nanoparticles
sandwichedbetweenPANIandRGOnanosheets,whichcouldeffectivelypreventtheagglomeration
ofTiO2nanoparticlesandenablefastinsertionandextractionoftheLiion.Asaresult,thecomposite
anodeshowedenhancedrateperformanceandcyclingstability(dischargecapacityof149.8mAh/g
accompanyingCoulombicefficiencyof99.19%at1000mA/gafter100cycles)comparedwithpure
TiO2[121].Differentfromtheformer,PANInanorodswereverticallygrownonbothsidesof
amorphousTiO2‐GOnanosheetstoformastableTiO2/PANI/GOsandwichstructure.TheGO
networkprovidesmanyconductivechannelsforelectrontransportandallowednanoscalePANIand
TiO2tosettlewellontotheGOnetworktoformastablesandwichstructure,whichledtoenhanced
electronicconductivityandstability.Anexcellentinitialdischargecapacityof1335mAh/gat50mA/g
and435mAh/gafter250cyclesat100mA/gwerereported[122].
NiOisknownasasemiconductoroflowconductivity,andthenanoscaleNiOparticlesareeasily
convertedtoinactiveLi2Othroughthedischargereaction:NiO+2Li=Ni+Li2O,leadingtopoor
electriccontactbetweenLi‐activenanoparticlesandsubstrate.Moreover,NiOnanoparticlestendto
agglomerateandforminactivebulkparticleswhilecycling.Inordertotackleit,Huangetal.[123]

Materials2020,13,54825of45
developedanickelfoam‐supportedNiO/PANIcompositeasananodematerialforLIBs.Itwaswith
enhancedconductivityandstabilitybygluingNiOflakestightlyonthePANIlayer,whichcan
preventNiOlosscausedbyasideeffect.Furthermore,weakerpolarizationensuresbetter
reversibilityandcyclingperformance.TheNiO/PANIelectroderetained520mAh/gafter50cycles
at1°C,higherthanpristineNiOelectrodeof440mAh/g.TheNiO/PANIcompositecore–shellarrays
[124]couldworkbetterthantheformer.Itnotonlyexhibitedweakerpolarization,butalsoshowed
excellentLiionstoragecapabilityandenhancedelectronicconductivitywerederivedfromthe
introductionofconductivePANInetworkthatcouldenablefastelectronandiontransferand
improvestructuralstability.Therefore,anenhancedspecificcapacityof780mAh/gat0.1A/g,
superiorratecapabilityandcyclingstabilitytobareNiOwereobserved(Figure16).

(a)(b)
Figure16.ElectrochemicalcomparisonofNiOnanoflakearraysandNiO/PANIcore/shellarrays.(a)
Ratecapabilityand(b)cyclingstabilityat0.1A/g[124](reproducedwithpermissionfromElsevier).
Asakindofmetaloxideofnaturalabundance,hightheoreticalcapacity(1007mAh/g)andlow
cost,Fe2O3hasbeenpopularlydevelopedasanodematerialsforLIBs.Maetal.[125]successfully
synthesizedPANI‐coatedhollowFe2O3nanoellipsoidsviaasolvothermaltechniquefollowedbya
post‐coatingprocess.Theporousandhollowstructurecaneffectivelyaccommodatethevolume
changeofFe2O3,whilePANIcoatingactedasaconductivemediumthatgreatlyimprovedthe
electronicandionicconductivityinadditiontobufferingtheexpansion/contractionoftheactive
materialduringelectrochemicalreactions.Consequently,thePANI/Fe2O3compositedelivered
chargecapacitiesof366,223.4and105.8mAh/gat0.5,1.0and2.0Crespectively,andmaintaineda
chargecapacityof412.1mAh/gafter150cyclesat0.2°C,indicatingenhancedratecapabilityaswell
asextendedcycliclifespan.
Fe3O4,namelymagnetite,isnewlydevelopedasthePANImodifiedanodematerialforLIBs.
SimilartoFe2O3,naturalabundance,hightheoreticalcapacity(926mAh/g)andlowcostareits
attractions,buthugevolumeexpansionandsevereparticleagglomerationwhilecyclingneedtobe
overcome.AuniqueFe3O4@PANIcompositewithyolk–shellmicro‐nanoarchitecturewasobtained
byWang’sgroup[126].Figure17vividlyillustratesitsformationprocess.Themicro/nanostructure
isfavoredforpreventingthebulkFe3O4fromaggregationwhilecycling,theporousyolksandvoid
spacescanshortentransportlengthforLiionsandelectrons,andalsoprovideextrasitesforion
storage,andthePANIlayercaneffectivelyimprovetheconductivity,resultinginenhanced
electrochemicalperformance.Asexpected,ahighreversiblecapacityof982mAh/gafter50cyclesat
100mA/gandanoutstandingratecapabilityof734.6mAh/gat1000mA/gwereachieved.Coating
layeredgrapheneontoPANInanofiber‐anchoredFe3O4isalsoasmartstrategy[127].Itwas
demonstratedthatthegraphene/Fe3O4/PANIshowedasuperiorreversiblespecificcapacityof1214
mAh/g,extraordinaryratecapability,lowvolumeexpansion,enhancedcyclingstabilityand99.6%

Materials2020,13,54826of45
coulombicefficiencyover250cycles,owingtothecollectiveeffectoflayeredgraphene,Fe3O4hollow
rods,aswellasthesuperiorconductivityofPANI.

Figure17.SchematicillustrationofformingofFe3O4@PANIyolk–shellmicro‐nanoarchitecture[126]
(reproducedwithpermissionfromElsevier).
MoS2possessesaunique2DlayeredstructurethatenablesfastLiinsertionandextraction.
However,MoS2stillundergoespoorrateperformanceandfastcapacitydecaybecauseofpoor
electricalconductivitybetweenS–Mo–Ssheetswhileusedasanelectrodematerial.Aneffective
improvementistohybridMoS2withconductiveadditiveslikeconductingPANI[128]thatcan
greatlyimprovetheconductivityandstability.The3DhierarchicalMoS2/PANInanoflowers(Figure
18)werepreparedbyasimplehydrothermalmethod[129].Suchhierarchicalarchitecturesprovided
sufficientvoidspacefortheLiiontodiffuseandensuredstructuralintegrityoftheelectrodematerial.
TheflexiblePANIchainsnotonlyimprovedelectricalconductivity,butalsomaintainedhierarchical
architecturesofthenanoflowersduringheattreatment.Enhancedelectrochemicalperformancewas
achievedduetothesynergyofMoS2andPANI,aswellastheunique3Dhierarchicalstructuresof
MoS2/PANInanoflowers.



Materials2020,13,54827of45
Figure18.SEMimagesofthe3DhierarchicalMoS2/PANInanoflowers(a,b)andMoS2/Cnanoflowers
(d,e).PhotographsoftwotypesofChineseroses(c,f).Schematicillustrationofsynthesisof
MoS2/PANIandMoS2/Cnanoflowers(g;reprintedwithpermissionfrompreviousliterature[129]©
2014AmericanChemicalSociety).
SnS2hasasimilarlayeredstructurelikeMoS2thatisbeneficialtoLiioninsertion.The2D
SnS2@PANInanoplateswithalamellarsandwichnanostructurecanprovideagoodconductive
networkbetweenneighboringnanoplates,shortenthepathforiontransportintheactivematerial
andalleviatetheexpansionandcontractionoftheelectrodematerialduringcharge–discharge
processes,resultinginenhancedelectrochemicalperformance.Asaconsequence,theSnS2@PANI
nanoplateelectrodedeliveredahighinitialreversiblecapacityof968.7mAh/g,excellentcycling
stability(75.4%capacityretentionafter80cycles)andanoutstandingratecapability(356.1mAh/gat
5A/g)[130].
Inrecentyears,PANImodifiedbinarymetalorbinarymetaloxidescompositeshavebeentried
toemployasanodematerialsforLIBs.SuchasPANI/Sn‐Cunanotubes[131],PANI/Co3O4‐CuO[132]
andPANI/Cu3Mo2O9[133],inthesearchitectures,thePANIlayereffectivelyrelievesthestress
associatedwithvolumechangesofthebinarycompoundsandimprovesconductivity.Furthermore,
uniquecompositestructureshelpalot.Forexample,3DporousPANIhydrogel/Sn‐Cunanotubes
structureprovidesnetworkforelectronandLiiontransport,resultinginimprovedelectrical
conductivity;thePANI/Co3O4‐CuOraspberrydesigncanresultinlotsofadvantageslikesuppressed
agglomeration,aneffectiveelectricalcontact,enhancedcyclingstability,aswellasalowercharge
transferresistancewhilecycling.Asaresultofthesynergisticeffect,enhancedelectricalperformance
includingreversiblecapacity,cyclinglifespanandratecapabilitywereachieved[132].
3.2.Lithium‐SulfurBatteries(LSBs)
Lithium‐sulfurbatteries(LSBs)arethenextgenerationofpromisingrechargeablebatterieswith
highspecificenergyof2600Wh/kgandhightheoreticalspecificcapacityof1675mAh/g.
ConventionalLSBschoosemetalLiastheanodematerialandsulfurasthecathodematerial,sulfur
canreactwithLitoformLi2S.However,theprocessofreductionreactionisverycomplicated,many
sidereactionsareinvolvedinthereduction,asaresult,manyresidualsideproductsfromS8tosoluble
lithiumpolysulfidesLi2SxwillbereducedontheLianodeinaparasiticreaction,resultinginthe
shuttlemechanismandlowcoulombicefficiency[135].Furthermore,theconductivityisverylowdue
totheinsulatingnatureofsulfur,andthesulfurcathodeusuallysuffersfromhugevolumechange
whilecycling,leadingtopoorcyclingstabilityandinferiorrateperformance.Considerableefforts
haverecentlybeenmadetomodifytheelectrochemicalperformanceofthesulfurcathodelikeadding
anabsorbingagenttoabsorbpolysulfides,designingLi‐protectingseparatorsandencapsulating
sulfurinconductingpolymermatrix.PANIcoatingissuitabletoencapsulatesulfurinthePANI
matrixowningtoitsflexibility,highconductivity,slightsolubilityinorganicelectrolyteandporous
architecture.
TheprocessofencapsulatingsulfurinPANImatrixtoformPANI/Scompositecanbecalledthe
vulcanizationreaction.Duringthereaction,partialSatomssubstituteHatomsonthearomaticrings
byreactingwiththeunsaturatedbondsinPANIchainsduringheattreatment,theninter/intra‐chain
disulfidebondsareformedonthesidechain.Yanetal.[136]designedanano‐poroussulfur/PANI
(SPANI)compositebyinsituchlorinatedsubstitutionandvulcanizationreactions.Inthemain
backbonechaininSPANI,sulfurwasefficientlyencapsulatedinthePANIchain,andtheobtained
SPANIchainprovidedelectronicconductivityandelectrostaticattractionforcetostabilize
polysulfideanionswhilecycling,moreover,thedisulfidesidechaincanactasthesecond
electrochemicalredoxcomponent,resultinginenhancedcapacitybehavior.Consequently,excellent
reversiblecapacityof750mAh/g,superiorcyclingstability(89.7%capacityretentionafter200cycles
at0.3°C)andhighrateperformancewereobserved.Duanandcoworkers[137]producedaPANI‐
coatedsulfurcompositecathodeforLSBsbythelayer‐by‐layerassemblymethod,followedbythe

Materials2020,13,54828of45
crosslinkingandheattreatmentprocess.TheouterPANIshellcanprotectthesulfur(S8)and
polysulfidesfromdissolutionintoLianode,whileallowingLitobepermeableduringthe
charge/dischargeprocess.Furthermore,theconductivePANIlayerprovidessubstantialconducting
channelsforelectrontransport.Thefinalproductshowedahighconductivityof0.23S/cm.
Structuralinstabilitywhilecyclingderivedfromsulfurseverelylowerthecyclingstability,
additionally,solublelithiumpolysulfideshavebeenapotentialhazardtoratecapability.Henceitis
crucialtodesignarationalstructuretosurpassthesedisadvantages.Maetal.[138]preparedahollow
PANIsphere/sulfurcompositeviaavaporphaseinfusiontechnique.Thesulfurwasdepositedonto
bothoftheinnerandouterofthePANIhollowsphere,thevoidspaceinthePANI/sulfur
nanoparticleseffectivelybufferedvolumeexpansionwhilecycling(Figure19).ThePANIshellalso
preventedthedissolutionandmigrationofpolysulfides,aswellasimprovedtheelectronicandionic
conductivity.Moreover,thechemicalbondbetweenPANIandSwasformedduringheattreatment,
whichinhibitedtheshuttleeffect.Aspredicted,excellentoverallelectrochemicalperformance
includinghighreversiblecapacityof602mAh/gafter1000cyclesat0.5°Candcoulombicefficiency
ashighas97%wererealized.Zhouetal.[139]obtainedPANI/Syolk–shellstructurenanocomposite
byinsitupolymerization,followedbytheheattreatmentprocess.Intheyolk–shellstructure,sulfur
astheyolkwasencapsulatedinPANIshell,whichhelpedtoimmobilizethepolysulfidesand
accommodatethevolumeexpansionofsulfur,asaresult,excellentperformanceincludingcycling
stabilityandhighcapacityretentionwereobtained.

Figure19.SchematicillustrationofthehollowspherePANI/Scompositeduringthecharge/discharge
process.(a)TheinitialPANI‐Scomposite,(b)thecycledPANI‐Scomposite,(c)thelithiatedPANI‐S
compositeand(d)theschematicillustrationofintegrityofthehollowPANI‐Scathodewithsevere
volumechangeduringcharge/dischargeprocess(reproducedfrom[138]withpermissionfromThe
RoyalSocietyofChemistry).
Encapsulatingsulfur/carboncompositesinPANIcoatingsisanothereffectiveway.Different
fromvulcanizedPANI/Scompositesastheformer,theinsitupolymerizationofPANIcoatingson
thesulfur/carboncompositescanbeaccomplishedwithoutheatreaction.Derivedfrompromising
advantagesofbothPANIandcarbonmaterials,theternaryS‐C@PANIhybridcathodesforLSBshold
promiseforsuperiorelectrochemicalperformanceduetoasuperiorsynergisticeffect.PANIcanbe
polymerizedinsituontothegraphene(G)andGOsheettoobtainthePANI‐GandPANI‐GO
membrane,thesulfurnanoparticlesaresandwichedbetweenthemembranestoformaternary
sandwichstructure.TheconductivePANI‐GandPANI‐GOnetworksnotonlybufferedahuge
volumeexpansionofthesulfur,butalsomitigatedthediffusionoflithiumpolysulfidestotheLi

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anodebyachemicalinteractionbetweentheiminegroup(–N=)ofthequinoidringandpolysulfides,
finallyresultedinexcellentelectrochemicalperformance[140,141].CNTsarewidelyemployedas
supportingmaterialsbecauseofhighconductivity,largespecificsurfaceareaandexcellent
mechanicalproperties.ConsideringtheadvantagesofPANIandCNTs,itisasmartstrategyto
developaternarycompositewheresulfursupportedbyMWCNTsandcoatedwithPANI,thatis,the
MWCNTs‐S@PANIasacathodematerialforLSBs.Theuniquesandwicharchitectureeffectively
avoideddissolutionanddiffusionofpolysulfides,andtheMWCNTsprovidedconductivenetwork,
flexiblePANIaccommodatedvolumechangewhilecycling,resultinginenhancedcyclingbehavior
andratecapability[142].Porouscarbonmaterialslikeactivatedcarbonandmesoporouscarboncan
provideanefficientconductivenetworkforS,whilePANIcoatingfurtherreducesthevolumetric
effectofSandfacilitateselectronicconduction,aswellaspreventslithiumpolysulfidesfrom
dissolvinginanelectrolyte,thusthecompositeelectrodesexhibitedenhancedelectrochemical
characteristics[143,144].Inaddition,acetyleneblackisalsoapromisingcarbonmaterialtoform
PANI@S‐acetyleneblackcompositeascathodeforLSBs[145].S‐acetyleneblackpowderwas
encapsulatedinPANIcoatingastheshell,whichaccommodatedvolumeexpansionwhilecycling.
Moreover,theefficientconductivenetworkprovidedbytheacetyleneblack,togetherwiththestrong
affinitytosulfurandpolysulfidesprovidedbyPANI,enabledtheuniformdispersionofthesulfur,
promotedthetransportationofionsandenhancedthecyclicperformanceoftheLSBs.
3.3.Sodium‐IonBatteries(SIBs)
SimilartoLIBs,sodium‐ionbatteries(SIBs)relyontheinsertionandextractionofNaionto
operate.ComparedtoLielement,Naelementisofhighernaturalabundanceandlowercost,hence
SIBshavegainedlotsofrecognitionasaneffectivealternativetoLIBs.Aswellknowntous,
electrochemicalperformanceofrechargeablebatteriesislargelydeterminedbyelectrochemical
propertiesofelectrodematerials.ThustheSIBsdesirablyrequireustobeequippedwithadvanced
electrodes.Justliketheothersecondarycells,PANIholdsmuchpromiseforSIBsduetoitsversatile
electrochemicalproperties.Similarly,inthenextsection,wewilloverviewtherecentdevelopment
onPANImodifiedcathode/anodematerialsforSIBsindetail.
3.3.1.PANIModifiedCathodeMaterials
Ironphosphate(FePO4)isanimportantcathodematerialinvestigatedforSIBs.However,poor
electronicconductivityandinferiorionicdiffusionseverelyhinderitscommercialization.Carbonized
PANInanorods(CPNRs)areofgreatpotentialforenhancingelectrochemicalpropertiesofFePO4
[146].TheCPNRs/FePO4compositewassynthesizedbyacarbonizationprocessofthePANI/FePO4
composite,whichwaspreparedthroughamicroemulsionmethod.IntheaspreparedCPNRs/FePO4
composite,CPNRsprovidedhigh‐speedpathwaysforelectrontransport,enablingfastcharge
transferduringtheprocesssodiation/desodiation,moreover,FePO4canloadonthesurfaceofCPNRs
byanoncovalentbond,decreasingtheresistanceofchargetransfer.Asthecathodematerial,ahigh
initialdischargespecificcapacityof140.2mAh/g,withthevaluebeingretainedat134.4mAh/gafter
120cycleswasachieved.
Na3V2(PO4)3(NVP)isapromisingcathodematerialforSIBswithasuperionicconductor
framework,largechannelsforchargetransferandexcellentthermalability.Nevertheless,bareNVP
ispoorofelectricalconductivity,whichgreatlyrestrictsitspracticalapplication.Chenetal.[147]
designeddoublecoatingNVP(NVP@C@HC)toimprovetheelectrochemicalproperties.Wherein,
theHCwasderivedfrompyrolyticPANI/NVPwastightlyanchoredtothesurfaceofCandPANI
coatingviachelatinginteractionsofcitricacidandvanadate,whicheffectivelysuppresses
agglomerationofNVPparticleswhilecyclingandenhancesconductivity.Furthermore,thedouble
carbonlayersbuffervolumechangeduringthecharge/dischargeprocess.Asexpected,thedouble
carbon‐coatedporousNVP@C@HCcompositedeliveredanexcellentratecapability(60.4mAh/gat
50°C)andalong‐termcyclability(capacityretentionof83.3%at40°Cafter3000cycles).

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Veryrecently,anovelNa(Ni1/3Mn1/3 Fe1/3)O2(NNMF)embeddedontheconductivePANI
backbonewasdemonstratedtoexhibitenhancedelectrochemicalperformancewhileusedasa
cathodeforSIBs[148].TheuniformdispersionofNNMFonPANIensuredbetterelectricalcontact
betweenelectrolyteandactivematerials,moreover,shortNaiontransferpathwaysandbuffered
mechanicalstressprovidedbytheporousPANInetworksignificantlyleadtoenhancedcapacity
behaviorandcyclability.
SimilartoLIBs,asaconductivepolymer,PANIitselfisalsoanidealcathodematerialforSIBs.
NanostructuredPANIlikenanofiberPANIpresentedconsiderablespecificsurfaceareaandelectrical
conductivity,moreover,PANIwithhighflexibilityeffectivelyimprovedstructuralstabilityand
cyclingstability[149,150].
3.3.2.PANIModifiedAnodeMaterials
SinceNaion(0.102nm)isbiggerthantheLiion(0.076nm),itismorechallengingtoseeka
suitablehostanodematerialforSIBs.Inordertocopewiththischallenge,modifiedPANIbased
materialswithadvancedelectrochemicalperformancehaverecentlybeenemployedasanodematerials
forSIBs.
SnO2anodeperformancehasbeendemonstratedtoachievelongcycliclifeandexcellentrate
performancebyformingacore–shellstructuredSnO2hollowsphere(SnO2‐HS)/PANIcomposite
electrode[151].TheuniquehollowstructureoftheSnO2coreandtheflexiblePANIbufferlayercan
alleviatevolumeexpansionofSnO2andaggregationofgeneratedSnparticlesduringcycling.
Therefore,ahighreversiblecapacityof213.5mAh/gover400cyclesat300mA/gwasdelivered.
TransitionmetalsulfideslikeCo3S4andMoS2havebeenusedtofabricatethePANImodified
anodeforSIBs.Co3S4@PANInanotubeswereformedviatheuniformcoatingonbothouterandinner
surfacesofCo3S4nanotubes,whichwereobtainedbyafacileself‐templatehydrothermalroutebased
ontheKirkendaleffect[152].TheconductivePANIlayerenableselectronandNaiontransportand
preventsCo3S4nanotubesfromstructuralcollapseorpulverizationwhilecycling.Finally,the
compositeachievedahighmaintainedcapacityof252.2mAh/gofafter100cyclesat200mA/g,much
higherthanthatofbareCo3S4nanotubes(58.2mAh/gretainedofinitialvalueof815.3mAh/gafter
100cyclesat200mA/g).Theinterlayerspacingof2DMoS2canbeenlargedbytheintroductionofthe
conductivePANIlayertoformaMoS2/PANIheterostructure[153].Theenlargedinterlayerspacing
remarkablyfacilitateslargeNaiondiffusion,aswellasimprovesconductivity.Furthermore,the
inter‐overlappedMoS2/PANInanosheetscanretainstablestructuralintegritywhilecyclingdueto
thestrongcoordinationabilitybetweenMoandnitrogenatoms.Asaconsequence,thehybridanode
exhibitedhighcapacity,ratecapabilityandlongcycliclife.
InadditiontodirectlyfunctioningasactiveanodematerialsforSIBs,carbonizedPANIisalso
extensivelyutilizedduetosuperiorNa‐storageperformance.Forexample,PANIwasalso
investigatedastheSIBanodeinaPANIcarbonized3Dporouscarbon‐coatedgraphenehybrid
system[154],whereSiO2andPANIlayerweresuccessivelydepositedonthesurfaceof3Dporous
graphene(3DPG;Figure20A).Theuniquearchitecturethatiscomposedof3DPGnetworksanda
porousPANI‐convertedcarboncoatingcanendowthehybridwithhighelectricalconductivity,rapid
ionintercalation,substantialactivesites,shortionicdiffusionpathwaysandhighstructuralstability
forefficientNa‐storage.Consequently,the3DPG@CcompositedisplayedremarkableNa‐storage
performance,initialdischargecapacityashighas824mAh/gat50mA/g,highreversiblecapacity,
enhancedcyclingstability(323mAh/g after1000cyclesat1000mA/g)comparedtopure3DPGand
carbon(Figure20B),alongwithexcellentratecapability(207mAh/gat10A/g).

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

Figure20.(A)Schematicillustrationofformationof3Dporousgraphene@Cand(B)long‐termcycling
behaviorofthe3Dporousgraphene@Ccompositeat1000mA/g[154](reproducedwithpermission
fromElsevier).
AsaN‐richCPs,SandNco‐dopedS/Ncarbonnanotubes(S/N‐CT)systemcanbeformedvia
carbonizationofanS‐containingPANIderivative[155].NincarbonizedPANIenablesefficientNa
adsorptionperformanceandhighelectricalconductivitytoenhanceNa‐storageperformance;the
introductionofSintoacarbonmatrixcanfurtherenlargeinterlayerspacing,offeractivesitesand
shorteniondiffusiondistancetoimproverateperformanceandcyclingstability.Inagreementwith
theprediction,theas‐preparedS/N‐CTanodeforSIBsdeliveredareversiblecapacityashighas340
mAh/gat0.1A/gandanexcellentcyclingstability(94%capacityretentionafter3000cyclesat5A/g).
Table2presentsthepreparationmethodandelectrochemicalperformanceofsometypicalPANI
modifiedrechargeablebatterieselectrodematerials.
Table2.ThepreparationmethodandelectrochemicalperformanceofsometypicalPANImodified
rechargeablebatterieselectrodematerials.
MaterialsPreparationMethodMaximum
SpecificCapacityCycleStability
PANI/LICOO2[86,87]Pickeringemulsionroute136mAh/g
PANI‐CSA/C‐LFP[90]coatingC‐LFPwithPANI‐
CASinm‐cresolsolution165.3mAh/g
PANI/PEG[92]125.3mAh/g95.7%after100
cycles
LIV3O8/PANI[93]oxidativepolymerization95%after55cycles
PANI/LI(NI0.8CO0.1MN0.1)O2
[95]solutionmethod193.8mAh/g96.25%after80
cycles
PANI/LI(NI1/3MN1/3FE1/3)O2[96] 86%after40cycles
PANI[134]chemicaloxidative
polymerization159.83mAh/g
119.79mAh/g
retainedafter100
cycles
PANI/SI/GRAPHITE[106] 1392mAh/g62.2%after95
cycles
CNTS/PANI/SI[111]1954mAh/g727mAh/gretained
after100cycles
PANI/SIOX/CNTS[114]1156mAh/g728mAh/gretained
after60cycles

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SIOX/PANI/CU2O[115,116] 1178mAh/g725mAh/gretained
after60cycles
PANI/SNO2/RGO[118]
dip‐coatingofPANI@SnO2
andgraphenedispersionon
Cufoam
772mAh/g749mAh/gretained
after100cycles
EG/PANI/SNO2[119]
solvothermalmethod
followedbyin‐situ
oxidativepolymerization
1021mAh/g408mAh/gretained
after100cycles
PANI@SNO2@MWCNT[120] 888mAh/g527mAh/gretained
after150cycles
TIO2/PANI/GO[122]1335mAh/g435mAh/gafter250
cycles
PANI/FE2O3[125]
solvothermaltechnique
followedbyapost‐coating
process
366mAh/gat2.0C
412.1mAh/g
retainedafter150
cyclesat0.2C
FE3O4@PANI[126]997mAh/g982mAh/gretained
after50cycles
GRAPHENE/FE3O4/PANI[127] 1214mAh/g86%after50cycles
SNS2@PANI[130]968.7mAh/g75.4%after80
cycles
NANO‐POROUS
SULFUR/PANI[136]
insituchlorinated
substitutionand
vulcanizationreactions
750mAh/g89.7%after200
cycles
CPNRS/FEPO4[146]microemulsionmethod140.2mAh/g
134.4mAh/g
retainedafter120
cycles
NVP@C@HC[147] 
83.3%after3000
cycles
S/N‐CT[155]340mAh/g94%after3000
cycles
4.ApplicationsofPANIforFuelCells
Asatypeofnovelenergystorageandconversiondevicesofgreen,sustainableandefficient
energystorage,fuelcellshavebeenactivelydevelopedduringtheseyears.Unlikesupercapacitors
andrechargeablebatteries,thefuelcellscanrealizethedirectconversionfromchemicalenergyto
electricalenergy.Commonfuelcellsincludingdirectmethanolfuelcell(DMFC),protonexchange
membranefuelcell(PEMFC),polymerelectrolytemembranefuelcell(PEMFC),alkalinefuelcell
(AFC),Znaircellandmicrobialfuelcell(MFC)haveattractedsubstantialresearchfromscientistsin
thelastdecade.CPsholdalotofpromiseforuseaselectrocatalystsoffuelcellsduetotheirhigh
conductivity,tunablemorphologiesandhighflexibility.TheseCPsupportedandCPderived
electrocatalystsusuallyshowhighcatalyticactivityandgoodstability.AmongCPs,PANIbased
electrocatalystshavebeendemonstratedtodelivertheexcellentcatalyticactivityinahydrogen
oxidationreaction(HOR),oxygenreductionreaction(ORR),hydrogenevolutionreaction(HER)and
methanoloxidationreaction(MOR).Inthissection,wewillgiveasummaryofrecentresearchof
PANIsupportedmetalelectrocatalystsandPANIderivedmetal‐freeelectrocatalysts.
4.1.PANI‐BasedSupportedMetalElectrocatalysts
PANIwithhighconductivity,varioustunablemorphologiesandhighflexibilitycaneffectively
enhancetheconductivity,catalyticactivityandstability,soithasbeenactivelyusedtosupportmost
metalelectrocatalysts.Platinum(Pt)isthemostusedmetalelectrocatalystsasaresultof
extraordinaryMOR,HERandORRcatalyticactivity[156,157].PANIwithananowiresnetwork
structurewasusedassupportforPtdispersioninordertoenlargetheapplicationofPt[158].The
PANInanowiresnetworkarchitecturewasbeneficialtodistributePtnanoparticlesandtocreate
conductivechannels,leadingtoformahybridnanocatalystwithexcellentelectrocatalyticactivityin

Materials2020,13,54833of45
MOR.InadditiontoindividualPANIsupportedPtcatalysts,PANIbasedcompositeslikePANI/C,
PANI/MWCNTsandPANI/SnO2havebeeninvestigatedasstableandMORcatalyticsystems[159–161].
Nevertheless,highcostofPtisthemainproblemthatlimitsitscommercialutilization,thusitis
desirabletolookfornon‐noblemetalsortheircompoundsasthealternativestoreplacePtcatalyst.
Yuanetal.[162]designedanefficientPANI/carbonblack(PANI/C)composite‐supportediron
phthalocyanine(FePc)asanORRelectrocatalystforFePcinanair‐cathodesingle‐chamberMFC.The
resultingPANI/FePc/CcompositecatalystexhibitedbettercatalyticactivitythanbarePt,PANI/Cand
FePc/C,reflectingPANIadditiveenhancedtheactivityofCinORR.Moreover,thepowerpercostof
thePANI/C/FePccatalystwas7.5timesgreaterthanthatofthebarePtcatalyst,whichindicatedthat
thePANI/FePc/CcompositecanbeapromisingalternativetoPtcatalystinMFC.Zhang’sgroup[163]
hasdemonstratedthataPANIsupportedMoS2electrocatalystachievedactiveHERcatalytic
performance.TheflexiblePANIeffectivelypreventsMoS2fromaggregation,ensuringtheuniform
andverticaldispersiononthePANIbrancheswithhighedgeexposureofMoS2nanosheets.Apart
fromMoandFe,variousnon‐noblemetalelectrocatalysts,suchasMn,Co,Niandtheircompounds
havebeenstudiedtosubstituteforPt.
Protonexchangemembranefuelcell(PEMFC)isalsoapromisingfuelcellofhighpowerdensity.
Pt‐basedelectrocatalystsarecommonelectrodematerialsforORRandHOR.However,themain
obstacleisstillthedearcostofPt.ManyPt‐freenon‐noblemetal‐basedcatalystshavebeendiscovered
asnovelcatalystswithhighORRactivity,butnon‐noblematerialsasHORcatalystsarerarely
reported.Recently,Guoandcoworkers[164]discoveredthattheFeNPs‐PANI/CNTcatalyst
synthesizedbycontrollableself‐assemblycouldbeanappropriateHORcatalystinthePEMFC.HOR
kineticscanbewrittenasH2→Had→H+,thewholekinetictransferefficiencyislargelydeterminedby
theintermediateHad.Promisingly,Fe‐HadreversibilitywouldmaketheFe‐basedcatalystmaintain
stableintheacidcondition.Furthermore,PANI/FeNPsinterfacecaneffectivelystrengthenmass
transferandrealizedtherecoveryofactivesitesinthepresenceofconductiveandflexiblePANI
support,drivingHORintermittentlyevenathighpotential.Asaresult,theHad
adsorption/desorptionprocesscanberapidlydrivenatlowpotential,resultingintheremarkable
catalyticactivity,powerdensityashighas161W/kgandhighdurability.Theworkhaspavedthe
wayfornon‐noblematerialsasHORcatalysts.
Asmentionedabove,PANIsupportedPtshowsgoodcatalyticbehaviorinMOR,thereforethe
PANI/Ptcatalystscouldhelpalotindirectmethanolfuelcell(DMFC).Forinstance,Gharibi’sstudy
[165]provedthatthePANI/C(vulcanXC‐72)supportedPtelectrocatalystexhibitedbettercatalytic
performancethantraditionalPt/CwithanafionelectrocatalystinDMFC.InconventionalDMFC,
carbonXC‐72actsassupportasPt,whilenafionactsasabinderandprotonconductor.However,the
agglomerationofcarbonparticlesandslowchargetransportseverelyhinderitspracticalapplication.
Gharibietal.substitutednafionforPANInanowires.ConductivePANInanowirestructureactedas
anefficientcarrierforelectronandprotontransport,enhancingelectricalconductivityandincreasing
methanoldiffusioncoefficient.Additionally,asaflexiblebinder,PANIwithuniquearchitecture
significantlysuppressedtheagglomerationofcarbonparticles.TheresultantPt/C‐PANI
electrocatalystshowedbetterelectrocatalyticperformancethanPt/Cwithnafionelectrocatalyst.
Meantime,highcostofPtmightbeadisadvantage,butitcouldbewellovercomebythebinarymetal
catalysts[159].
Anovelpoly(pyrrole‐coaniline)(PPCA)hollownanosphere(HN)supportedPdnanoflowers
(PdNFs)wasdesignedformethanolelectrooxidation[166].ThePdNFswereelectrodepositedfrom
anaqueoussolutionof0.01MPdCl2and0.5MH2SO4inafacileone‐stepmethod,whilethePPCA
HNwasobtainedbyinsituemulsionpolymerization.PdNFsonaPPCAHNcoatedglassycarbon
electrode(GCE)wasfinallyfabricatedviatheelectrochemicalmethod.ConductivePPCAHNco‐
polymercouldsurpassinglyimproveconductivitycomparedtoexclusiveconductivePANIorPPy,
aswellasenlargedspecificsurfacearea.TheresultingPdNFs/PPCAHN/GCEdemonstratedbetter
electrocatalyticactivitythanPdNFs/PANI,PdNFs/PPyandindividualPdNFs.

Materials2020,13,54834of45
PANI‐functionalizedcanbeaneffectivesupportasmetalelectrocatalystsinfuelcells,especially
inprotonexchangemembranefuelcell(PEMFC).Pt/CNTcatalystsshowenhancedelectrocatalytic
activityandstabilitycomparedtoPtelectrocatalystsforPEMFC.However,Ptnanoparticleswere
hardtodeposituniformlyanddirectlyontohighlygraphitizedCNTsurfacewithoutactivefunctional
groups,theintroductionofbridgingconductiveandstablePANIwascapableofenhancingthe
bindingstrengthbetweenPtandCNTbyπ–πbondingprovidedbyPANI(Figure21).Moreover,the
Pt–NbondingendowedPtnanoparticleswithhigherdispersion,whosesizedistributionrangedfrom
2to4nm,bringingforthenhancedelectrocatalyticactivityoftheresultantPt‐PANI/CNTcatalyst[167].

Figure21.MolecularinteractioninthepreparedPt‐PANI/CNTcatalyst(reprintedwithpermission
frompreviousliterature[167]©2011AmericanChemicalSociety).
AnewPt‐C@PANIcore–shellstructuredcatalystwasdevelopedforPEMFC.ThethinPANI
layerwasdirectlypolymerizedontothePt‐Csurface.TheuniqueThePANI‐decoratedcore–shell
architecturecouldinduceelectrondelocalizationbetweenthePtdorbitalsandthePANIπ‐
conjugatedligandaccompanyingwithelectrontransferfromPttoPANI,whichexplainedthe
enhancedcatalyticactivityanddurability.Furthermore,thePt‐C@PANI(30%)addressedthebest
catalyticactivityandsuperiordurabilitycomparedwiththenon‐PANI‐decoratedPt‐Ccatalyst,
indicatingthethicknessofPANIshellmighthaveaninfluenceoncatalyticproperties,inwhichthe
suitablethickness(5nm)ofthePANIshellgreatlyprotectedthecarboncorefromdirectexposureto
thecorrosivesurroundings[168].
4.2.PANI‐DerivedCarbonBasedMetal‐FreeElectrocatalysts
AsaN‐containingcarbonmaterial,thepropertiesofPANIareexpectedtogetimprovedwhile
dopingheteroatoms(N,B,S,OandP)tothePANIchains,andPANIhasbeenwidelyattemptedto
fabricateadvancedmetal‐freecatalystsupportbydopingheteroatomstoit.Heteroatom‐dopedPANI
derivedporouscarbon‐basedcatalystsaregenerallyfocusedonmodifyingORRactivitysinceORR
isinhugedemandforsustainableandnon‐nobleelement.PANI‐derivedN‐ andO‐doped
mesoporouscarbon(PDMC)asasustainableandnon‐nobleORRelectrocatalysthasbeen
demonstratedtodeliverextraordinaryelectrochemicalcatalyticactivitytowardORR[169].PDMC

Materials2020,13,54835of45
waspreparedbypolymerizingPANIinsituwithintheporesofSBA‐15mesoporoussilica,followed
bysubjectingPANI/SBA‐15tocarbonizationunderaninertatmosphere,andfinallyetchingawaythe
silicaframework(Figure22).Thefinalmetal‐freePDMCtowardORRshowedevenbetter
electrocatalyticactivitythanPt‐PANI/SBA‐15athighcurrentdensityandachievedpreferablefour
electronspathwaytowardORR,whichcouldbeascribedtothesynergisticactivitiesofNandO
speciesthatwereimplantedintoit.Itissuggestedthatthemetal‐freePDMCispromisingtochallenge
conventionalparadigmsthatPtbasedcatalysts.InspiredbySilva’swork,cheaperGOandgraphene
comparedwithSBA‐15wereusedtosynthesizePANI‐derivedcarbon‐basedPNCNandGNR/PANI
metal‐freeelectrocatalysts.OwingtohighspecificsurfaceareaandhighNcontentofGOand
graphene,aswellasrespectiveuniquestructures,idealcatalyticactivitiesandstabilitywere
deliveredforORR[170,171].
Inrecentyears,someresearchersdiscoveredthattheelectrocatalyticperformanceofthePANI
derivedmetal‐freecatalystscouldbefurtherenhancedbyco‐dopingoftransitionmetals.Inorderto
understandthemechanismonenhancedactivationrelatedtothetransitionmetaldopants,Penget
al.[172]studiedtheeffectoftheadditionofvarioustransitionmetals(Mn,Fe,Co,NiandCu)onthe
structureandperformanceofthedopedcarboncatalystsM‐PANI/C‐Mela,accompaniedwitha
metal‐freecatalystasareference.SEMshowedthatdopingwithFeandMnledtoagraphene‐like
structure,whiledopingwithCo,NiandCuledtoadisorderedornanosheetstructure.Catalysts
dopedwithtransitionmetalsexhibitedenhancedcatalyticperformancecomparedtothemetal‐free
reference,moreover,theirORRactivityfollowedtheorderofNi<Mn<Cu<Co<Fe(Figure23),
whichwasconsistentwiththeorderoftheiractiveNcontents.Assuggestedinthepaper,the
collectiveeffectofthethreeaspectsmayresultinthevariousperformanceenhancementofthe
transitionmetaldopants,thatis,theNcontent/activeNcontent,metalresidue,aswellasthesurface
areaandporestructureofthecatalyst.Nevertheless,theenhancedperformanceisnotdeterminedby
anysingerfactor.WecanfurthermeanthatthePANIwouldmakeanindispensablecontributiondue
totheextensiveactiveNsitesprovidedbyit.

Figure22.SchematicillustrationofsynthesisofthePDMC(reprintedwithpermissionfromprevious
literature[169]©2013AmericanChemicalSociety).

Materials2020,13,54836of45

Figure23.Half‐wavepotentialvs.RHEandcurrentdensityofM‐PANI/C‐Melacatalystsin0.1M
KOH(reprintedwithpermissionfrompreviousliterature[172]©2014AmericanChemicalSociety).
5.ConclusionsandOutlook
Inthisreview,wepresentedhereimportantresearchprogressontheapplicationsofPANIfor
electrochemicalenergystorageandconversion.PurePANIwithhighconductivity,easeofsynthesis,
highflexibility,lowcost,environmentalfriendlinessanduniqueredoxcharacteristicsisakindof
activeandeconomicalelectrodematerialforsupercapacitors(pseudocapacitors)andrechargeable
batteries(cathodematerial).Duetoaconsiderablespecificsurfacearea,advantageousporestructures
andhighNcontent,PANIderivedporouscarbonmaterialsarealsosuitabletoserveassupportfor
anelectrocatalystoffuelcells.However,whileusedasasupercapacitiveelectrode,poorcycling
stabilityandinefficientcapacitancecontributionaredelivered.Additionally,withtherapid
developmentofenergy,morestablemoleculararchitecture,higherpower/energydensityandmore
N‐activesitesaregreatlydesirable,whileindividualPANIcannotmeettheever‐increasingdemand.
Therefore,itisnecessarytocombinePANIwithotheractivematerialslikecarbonmaterials,metal
compoundsandotherCPs.InvariousPANIbasedcompositestructures,PANIgenerallyactsasa
conductivelayerandnetwork,andtheresultantPANIbasedcompositeswithvariousunique
structureshaveexhibitedsuperiorelectrochemicalperformanceinsupercapacitors,rechargeable
batteriesandfuelcellsduetothesynergisticeffect.However,therestillaresomedisadvantagesthat
remaintobeimproved:
 PANIishardtocommercializeintheelectrochemicalfieldduetoitsrelativelyhighcostandlow
practicabilitycomparedwithconventionalinorganicmaterials.
 PANIishardtomaintainastablestructurebecauseofthede‐dopingphenomenoncausedby
light,electricity,magnetism,thermal,etc.whenusedaselectrodematerialsforsupercapacitors
andrechargeablebatteries.
 Itishardtobalancetheelectrochemicalperformanceandmechanicalpropertieswhileapplying
PANIinelectrochemicalenergystoragetechnologiesincludingsupercapacitors,rechargeable
batteriesandfuelcells.
Aswecansee,thecomprehensivepropertiesofPANIneedyettobeenhanced,thereforefuture
researchshouldfocusondevelopmentofuniquenanostructuresofPANIwithhighersurfaceareas

Materials2020,13,54837of45
andconductivitiesforsuchapplications.Inaddition,thereareyetsomeresearchgapsthatshouldbe
filled:forsupercapacitorsandrechargeablebatteries,worksondesigningtailor‐madederivativesof
PANIandfunctionalizedPANIneedtobedeeplyexplored;asforfuelcells,reportsoncatalystsfor
oxygenevolutionreaction(OER)relatedtoPANIarelacking,itshouldbedeeplyexplored.
PANIisconsideredasoneoftheusefulelectronicandintrinsicCPsanditsapplicationsin
electrochemicalenergystorageandconversionfieldhavebeendepictedminutelyinthisreview.We
knowthatPANIhasmanyuniqueproperties,anditislikelytobeusefulinotherfields,soitis
suggestedthattheapplicationofPANIshouldbeextendedtootherfields,whichcangreatlyenlarge
itsrangeofuse.Forinstance,instabilityofnano‐fluidscausedbythegradualsedimentation(orscale
formation)ofnanoparticlessimultaneouslywithagglomeratingorclusteringofnanoparticlesinside
thebasefluid[174]couldbeimprovedbyPANIcoating.PANIwithexcellentflexibilitycan
effectivelyencapsulatethenanoparticles,avoidingtheagglomeratingorclusteringofnanoparticles
insidethebasefluid,asaresult,highlyefficientheattransferofnano‐fluidswouldbeachieved.Itis
suggestedherePANIcouldbeexploredintheapplicationofnano‐fluidsforheattransfer,thatis,it
deservesmoreresearchinthisfield,andtogoastepfurther,itdeservesmoreresearchinothernovel
fields.Byreadingthisreview,researcherscanunderstandthedevelopmentaltendencyofPANIin
thefieldofelectrochemicalenergystorageandconversion,moreover,theycanbeinspiredtodiscuss
andstudywhetherPANIhavepotentialapplicationsinotherfields.
Finally,manufacturingcostisalsoacrucialfactorforcommercialization,insummary,future
advanceswillrequirecontinuousexplorationsandendeavorsindesigninguniquenanostructuresof
PANIwithhighersurfaceareasandconductivities,extendingapplicationfieldsanddevelopingcost‐
effectivemanufacturingtechnologies.
AuthorContributions:Allauthorscontributedequallytothismanuscript.Z.L.andL.G.were

responsiblefor
thereviewconceptanddesign.L.G.wereresponsible

fordraftpreparation.Z.L.andL.G.wereresponsiblefor
thereviewandeditingofthemanuscript.Z.L.andL.G.wereresponsiblefortherevisionofthemanuscript.All
authorshavereadandagreedtothepublishedversionofthemanuscript.
Funding:ThisworkwassupportedbyagrantofCentralSouthUniversity.
ConflictsofInterest:Theauthorsdeclarenoconflictofinterest.
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