Recent advances in conjugated polymer energy storage

Recent Advances in Conjugated Polymer Energy StorageJared F.Mike,Jodie L.LutkenhausArtie McFerrin Department of Chemical Engineering,Texas A&M University,College Station,Texas77843 Correspondence to:J.L.Lutkenhaus(E-mail:jodie.lutkenhaus@)Received17December2012;revised9January2013;accepted11January2013;published online6February2013 DOI:10.1002/polb.23256ABSTRACT:This review covers recent advances in conjugated polymers and their application in energy storage.Conjugated polymers are promising cost-effective,lightweight,and flexi-ble electrode materials.The operating principles of conju-gated polymers are presented within the framework of their potential for energy storage.Special focus is given to polyaniline electrodes.Recent advances are reviewed includ-ing new methods of synthesis,nanostructuring,and assem-bly.Also,covered are applications that take full advantage of the mechanical properties of conjugated polymers and future applications of these novel materials.V C2013Wiley Periodicals,Inc.J.Polym.Sci.,Part B:Polym.Phys.2013,51, 468–480KEYWORDS:batteries;conducting polymers;conjugated poly-mers;energy;energy storage;flexible battery;flexible superca-pacitor;polyaniline;polymer battery;pseudocapacitor; supercapacitorsINTRODUCTION Storage of energy through electrochemical means(batteries,supercapacitors)is critical to providing portable power for technical applications such as laptops, smartphones,electric vehicles,military field electronics, pacemakers,glucose sensors,and many other devices.‘‘Plas-tic power,’’or energy and power sources consisting entirely (or mostly)of polymeric materials,offers several exciting opportunities for portable power.For example,a polymeric power source is potentially flexible,lightweight,moldable or castable,and multifunctional.This review will focus on the growing potential of conjugated polymers as electrodes for plastic power and recent advances occurring within the last 5years of the field.A particular focus will be placed on non-aqueous systems,which can achieve higher potentials. Conjugated polymers offer not only a means to store energy but also a means to do so independent of form factor.The mechanical properties of conjugated polymers are similar to those of a plastic,allowing them to be bent,flexed,or even stretched.1–8Without the need for a traditional prismatic or cylindrical cell,conjugated polymeric electrodes could be incorporated seamlessly into textiles,data tattoos,electronic paper,structural panels,and so forth.8–14Additionally,it is advantageous to directly combine plastic power with poly-mer solar cells or light emitting diodes in a streamlined manufacturing process to create low-cost fully flexible devices.10,15,16Aside from their mechanical versatility,conjugated polymers have several advantages such as synthetic tailorability,cost, and processability that make them ideal candidates for elec-trochemical energy storage.Their electrochemical and physi-cal properties may be tuned through organic synthetic tech-niques.1,17,18Furthermore,the raw materials and methods to produce these polymers are often economical.New synthetic approaches have enhanced processability such that conju-gated polymers can be processed via screen printing,doctor blading,or inkjet printing.19–21Other advantages include electrical conductivity and high coulombic efficiency with some polymers able to be cycled hundreds to thousands of times with little degradation.18,22,29In light of this,conju-gated polymers are ideal materials for hybrid composites in that they work synergistically with inorganic compounds that have high capacity but lack the conductivity and cyclability necessary for commercial application.23–26The combination of synthetic tuning,processing options,and me-chanical flexibility results in plastic power sources that can be designed to suit the device rather than vice versa.Most research in conjugated polymer electrodes for energy storage has focused on three polymers—polyaniline(PANI), polypyrrole,and polythiophene(PT)—and derivatives thereof,Figure1.18,27,28The focus has lain on these particu-lar polymers because of the presence of extensive back-ground research,the commercial availability of both mono-mers and polymers,and their ease of synthesis.The structures in Figure1are only a representative sample, whereas Nov a k’s1997review presents a more exhaustive list of structural variety.Of particular note is PANI,which was once a cathode in a PANI|lithium battery commercially available from Bridgestone-Seiko in the late1980s.29Table1 presents a comparison between common battery materials, LiCoO2and LiFePO4,30,31and the conjugated polymers pre-sented in Figure1.V C2013Wiley Periodicals,Inc.468JOURNAL OF POLYMER SCIENCE,PART B:POLYMER PHYSICS2013,51,468–480FEATURE ARTICLE JOURNAL OF POLYMERSCIENCEAlthough there are clear advantages,there are several issues that must be addressed before the full potential of conju-gated polymers can be realized.These include oxidative sta-bility,diffusion limitations of ions of dopants,low energy and power density,and low capacity.Several recent advances in the field have made significant headway in addressing these issues,and will be discussed in this review.OPERATING PRINCIPLESTo understand the operation of conjugated polymers within the framework of a battery electrode,we must first consider the fundamental properties of conjugated polymers them-selves.The conductivity and charge storage capabilities of conjugated polymers depend on electronic properties not possessed by common aliphatic polymers.The origin of interesting electronic and optical properties of conjugated polymers comes from the overlap of adjacent p -orbitals.The combination of molecular orbitals along the backbone of a conjugated polymer gives rise to a band structure analogous to that seen in inorganic semiconductors.37,38The effective conjugation length is the point at which no additional mono-mer units added to the chain affect the electronic propertiesof the polymer.39–41For organic semiconductors,the bottom of the conduction band is referred to as the lowest unoccu-pied molecular orbital (LUMO),while the top of the valence band is referred to as the highest occupied molecular orbital (HOMO).The values of the HOMO and LUMO energy levels determine properties such as oxidative stability,optical absorption wavelengths,and whether a polymer is more likely to be n-type or p-type.26,42Polymers can be classified as being either n-type or p-type based on whether they are easily reduced or oxidized,respec-tively.Electron rich structures that have higher HOMO levels make good p-type materials due to their ability to stabilize a positive charge,while electron poor structures are able to sta-bilize negative charges in low-lying LUMO levels,making them good n-type materials.43–45Generally speaking,n-doped poly-mers act as anodes,while p-doped polymers function well in cathodes.18There has been much work on p-type materials for cathodes,but there has been little focus on the synthesis of new n-type materials.46–48For fully flexible polymer batteries to be realized,both electrodes should be plastic,and so there is a need to research new n-type electrode materials as well.Conjugated polymers can be doped through either chemical or electrochemical means.49–51Figure 2illustrates this process for PT ,which can be p-doped or n-doped.Doping creates radi-cal cations or anions,also referred to as positive or negative polarons,respectively,which are delocalized along the poly-mer backbone and are a proposed mechanism for conductiv-ity.49,52,53Charge delocalization is coupled to structural reor-ganization to a quinoidal form and alters fundamental molecular vibrations.52The appearance of new subgap energy states and formation of quinoidal structure can be observed as changes in the UV/vis and IR/Raman spectra of poly-mers.49,52–56Conjugated polymers in their native state have very low conductivities,on the order of 10À4–10À9S/cm,but can attain very high conductivities when doped.38,57Poly(3,4-ethylenedioxythiophene)(PEDOT),for example,has exhibited conductivities as high as 1500S/cm in certain cases.58Jodie L.Lutkenhaus is an Assistant Professor of Chemical Engineering at Texas A&M University.Research specialties include polymeric electrodes for energy storage,polyelec-trolytes,ultrathin films and nanostructured polymers,as well as quartz crystal microbalance with dissipation.She received her B.S.in Chemical Engineering from The University of Texas at Austin in 2002and her Ph.D.in Chemical Engineering from the Massachusetts Institute of Technology in 2007.She received the NSF CAREER Award and ACS PRF-DNI in2011.Jared F.Mike received his B.S.from Youngstown State University in 2005and Ph.D.from Iowa State University in 2011with Professor Malika Jeffries-EL.Since 2011,he has been a postdoctoral researcher in Dr.Jodie Lutkenhaus’s laboratory at Texas A&M University.He is currently developing new conjugated polymers and studying their electrochemical performance as energy storage materials.He is also studying the effects of functionalization on the electrochemical properties of conjugatedpolymers.FIGURE 1Examples of common conjugated polymers found in energy storage research.JOURNAL OF POLYMERSCIENCEFEATUREARTICLEJOURNAL OF POLYMER SCIENCE,PART B:POLYMER PHYSICS 2013,51,468–480469To maintain charge balance,redox reactions associated with polymer doping are concurrent with the movement of ions into and out of the polymer matrix.59,60Although oxidation generally results in anion movement within the film,studies have shown that under certain conditions,cation movement occurs as well.61–63It could be inferred that analogous processes can also occur for anions during polymer reduc-tion.As polymer films grow thicker,they tend to respond more slowly due to ion transport limitations,giving rise to lower capacities as well as energy and power den-sities.6,18,64,65Polymer nanostructures such as nanowires,nanotubes,or nanoparticles can help to alleviate problems associated with ion diffusion by increasing the surface area and porosity.66–68TABLE 1Performance Characteristics of Conjugated Polymers as Compared to Current Inorganic Li-Ion Battery MaterialsCathode Operating Voltage (V vs.Li/Li þ)MW of repeat unit Theoretical Capacity a (mAh/g)Experimental Capacity (mAh/g)Energy Density (mWh/g)PowerDensity (mW/g)LiCoO 230$3.897.8727413530–144100–1,000LiFePO 431$3.2157.7617010990–10023–70Polyaniline 233–491.11294100–147300100Polypyrrole 1,323–465.074128260$10Polythiophene 33,34 3.1–482.12326829389PEDOT 35,36$2.7–4.2140.1619130–701–435–2,500aBased on a doping level of 1electron per monomerunit.FIGURE 2Illustration of PT electronic and chemical structural changes during a)oxidation (p-doping)and b)reduction (n-doping).A radical cation (positive polaron)is formed from oxidation,while a radical anion (negative polaron)is formed during reduction.High states of charging can lead to the formation of positive and negative bipolarons.Cations (C þ)and anions (A À)counterbal-ance charges in the polymer chain.FEATURE ARTICLEJOURNAL OF POLYMERSCIENCE470JOURNAL OF POLYMER SCIENCE,PART B:POLYMER PHYSICS 2013,51,468–480Besides ion diffusion,oxidative stability also affects the elec-trode’s performance.The number of monomer units over which a charge is delocalized determines the specific capacity of the electrode.For most conjugated polymers,this amounts to one unit of charge for every two to three mono-mer units,18,60although higher levels of charging are possi-ble.A higher specific capacity is realized with higher levels of oxidation or reduction.Unfortunately,high levels of charg-ing can lead to destabilization of the polymer matrix through unwanted interchain interactions or breakdown of polymer chains.69,70Additionally,highly charged species are much more likely to react with common electrolytes and/or oxygen in the air.69,71,72Low capacities can be mitigated to a limited extent through choice of dopant ion,electrolyte,or polymer structure.69,73–78There is evidence that strong interactions between the highly charged or doped conjugated polymer and a complementary polyion can serve to stabilize highly oxidized conjugated polymers.79Deficiencies associated with oxidative stability over many charge cycles have seen improvements with select composite materials such as gra-phene or carbon nanotubes.1,80–83SYNTHESISSynthesis of conjugated polymers for electrodes may be accomplished through electrochemical or chemical means.In general,oxidation is used to provide the driving force for polymer synthesis,whether it is through the addition of a chemical oxidant(e.g.,ammonium persulfate)or through the application of an oxidizing electrochemical potential to the surface of a submerged electrode.60As this type of synthesis is performed in an oxidizing environment,the resulting poly-mers are formed in an already doped state.PANI,for exam-ple,is synthesized in the emeraldine salt state under oxida-tive conditions.Reductive polymerization is not unheard of, but is rarely employed primarily due to limited applicabil-ity.84–87This is possibly one reason p-type polymers are more commonly studied than n-type polymers.Conjugated polymers are most often synthesized in a solution of mono-mer and electrolyte,but there are examples of solid-state or vapor deposition polymerizations as well.58,88,89Metal-cata-lyzed cross-couplings are another method for producing con-jugated polymers.90–94Unlike oxidative synthesis,polymers synthesized in this way are formed in a charge-neutral state. Unfortunately,these systems can sometimes require the presence of specific functional groups and sensitive reaction conditions not conducive to producing large quantities of material suitable for electrodes.However,promising advan-ces in direct arylation polymerizations are working toward eliminating the need for generating monomers with specific functional group requirements.95Metal-catalyzed polycon-densations are also a potential method for synthesizing new n-type or specially functionalized materials.90,96Several recent synthetic approaches have focused upon increasing the solubility or dispersibility of conjugated poly-mers to enhance their processability.Side chains are essen-tial for solubilizing conjugated polymers used in semicon-ducting applications,and similar concepts are applicable for electrochemical energy storage.97–100Side chain functionali-zation has been used to facilitate intimate mixing in compo-sites.32,101–103Conjugated polymer:polyelectrolyte complexes have also come a long way to improve polymer process-ing.104–106For example,poly(2-acrylamido-2-methyl-1-pro-panesulfonic acid)has been shown to both solubilize and stabilize PANI in complexes.79,107–109PANIPANI|lithium batteries are one of few examples of polymer-based energy storage to have been commercialized.Research interest in PANI continues to grow due to its oxidative stabil-ity,relatively high theoretical capacity,simple synthesis,and electrochemical reversibility.30,110PANI is an interesting case with its many redox states,111–113(Fig.3).Only emeraldine salt is conductive,although the conductivity of pernigraniline salt is still not entirely clear,and more recent reports suggest that it is.114–117PANI electrodes make use of the reversible reactions between fully reduced leucoe-meraldine base and partially oxidized emeraldine salt.At higher potentials,cycling stability can break down as emeral-dine salt irreversibly oxidizes to pernigraniline base23 because the reaction pathway to pernigraniline salt is not favored.There have been efforts to stabilize the pernigrani-line salt,79,116,117which allows for the doubling of PANI’s capacity and increasing its energy density.Although there are many advantages,problems associated with the utilization of PANI stem from insolubility,diffusion issues,cycling,and stability.Insolubility and intractability of PANI are often considered to be the main issues hindering its usage in energy storage.118These problems had tradition-ally been addressed through dispersing the polymer in solu-tions using a series of steps intended to remove aggregates and ensure a stable suspension.119More recently,template synthesis using polyanions and the synthesis of PANI nano-structures,most notably nanofibers,has enabled easy solu-tion suspension for PANI.67,118,120Cycling stability has been addressed by combining PANI with carbon materials such as graphene or carbon nanotubes.Carbonaceous materials store charge through non-faradaic reactions and possess high cy-cling stability.They help to maintain conductivity in a com-posite when PANI has been reduced to a non-conductive state.Therefore,composites of PANI and carbon materials are promising candidates for energy storage.1,46,80,83Among the many varieties of synthesizable nanostructures for conjugated polymers,there is particular interest in one-dimesional nanostructures such as nanotubes,nanorods,or nanofibers.121,122PANI nanofibers(PANI-NF)are especially attractive since they can be made economically and quickly in solution without the use of templates,additives,or compli-cated synthetic techniques.122Kaner and coworkers have developed synthetic methods for variable diameter PANI-NF that involve either rapidmixing of oxidant and monomer solu-tions or interfacial polymerization in phase-separated solvent systems(Fig.4).67,123–125The size of the nanofibers can be controlled by choice of reactant medium and acid.77,108,109JOURNAL OF POLYMERSCIENCE FEATUREARTICLE JOURNAL OF POLYMER SCIENCE,PART B:POLYMER PHYSICS2013,51,468–480471As expected,PANI-NFs show improved electrochemical per-formance over conventional solution-synthesized PANI (Table 2).126–128Perhaps the most attractive feature of PANI-NFs,however,is their ability overcome intractability issues because they are easily suspended in aqueous solution and remain so for extended periods of time.67As previously stated,nanostructured carbon materials such as graphene,carbon nanotubes,or even carbon black are de-sirable for their high conductivity and stability,147–150yet can suffer from lower capacity versus redox active materials due to the non-Faradaic nature of charge storage.80,151,152PANI is an ideal material for polymer/carbon composites because of its cyclability,electrochemical activity,and so forth;several examples are highlighted in Table 2.Carbonnanotubes or graphene/PANI composite electrodes are usu-ally formed by pressing into pellets,vacuum-assisted filtra-tion,or by polymerization on carbon nanotube surfaces or films,for example.130,131,133–135,139–144,146–158Although these electrodes can have good electrochemical characteristics,there are drawbacks to their fabrication such as requiring additional pre-or post-treatment of samples or special ex-perimental conditions to form a workingcathode.133,137,138,145,158The processability of PANI-NFs allows for the facile solution synthesis of composite electrodes.Hammond’s group has demonstrated the successful layer-by-layer assembly of PAN-I:multiwalled carbon nanotube films (Fig.5).159The result-ing film was highly porous,and was able to overcomeionFIGURE 4Transmission electron microscope (TEM)images of PANI-NF formed through (a)interfacial and (b)rapid mixing poly-merization (Reproduced from Refs.124and 125,with permission VC 2002American Chemical Society and V C 2004John Wiley &Sons,Inc.)FIGURE 3Illustration of the redox states of PANI and the chemical reactions associated with their interconversion.FEATURE ARTICLEJOURNAL OF POLYMERSCIENCE472JOURNAL OF POLYMER SCIENCE,PART B:POLYMER PHYSICS 2013,51,468–480T A B L E 2S e l e c t e d P o l y a n i l i n e C o m p o s i t e s w i t h G r a p h e n e o r S i n g l e -W a l l e d o r M u l t i -W a l l e d C a r b o n N a n o t u b e sC a t h o d e S p e c i f i c C a p a c i t a n c e (F /g )C h a r g i n g R a t e aE l e c t r o l y t eE n e r g y D e n s i t y (m W h /g )P o w e r D e n s i t y (m W /g )bN o t e sP A N I 12733c1M C a m p h o r s u l f o n i c a c i dB u l k P A N I s y n t h e s i z e d b y c h e m i c a l m e t h o dP A N I -N F 127122c1M C a m p h o r s u l f o n i c a c i dC h e m i c a l s y n t h e s i s o f P A N I n a n o f i b e r s u s i n g n a n o f i b e r s s e e d sP A N I -N F 129554d1.0A /g1M H 2S O 4I n t e r f a c i a l p o l y m e r i z a t i o nP A N I /P o r o u s C a r b o n1,270–2,200c66.7-.67A /g1M H 2S O 4173–30047,000–70E l e c t r o d e p o s i t i o n o f P A N I o n t o h i e r a r c h i c a l l y p o r o u s c a r b o n m o n o l i t hP A N I /P o r o u s C a r b o n768–900c5–0.5A /g1M H 2S O 4E l e c t r o d e p o s i t i o n o f P A N I o n t o m e s o p o r o u s c a r b o n m o n o l i t hP A N I /S W N T 130485c0.01A /c m 21M H 2S O 4120–2282,250–520E l e c t r o d e p o s i t i o n o f P A N I o n t o S W N T /b i n d e r f i l m o n s t a i n l e s s s t e e l s h e e tP A N I /S W N T 131236d10A /g1M L i C l O 41:1:1E C /D E C /D M C13162500E l e c t r o d e p o s i t e d P A N I o n f r e e -s t a n d i n g S W N T f i l mP A N I -N F /M W N T132417d1m A /c m21M L i P F 63:7E C :D M C$100–500300,000–300D i p p e d l a y e r -b y -l a y e r s o l u t i o n a s s e m b l y o n I T OP A N I /M W N T133360–424d1.0–0.2A /g1M H 2S O 46–10C h e m i c a l s y n t h e s i s o f P A N I o n ‘‘B u c k y P a p e r ’’m a d e b y f i l t r a t i o n o f M W N T sP A N I /M W N T 134650,c 360d$0.2A /g1M H 2S O 4C h e m i c a l s y n t h e s i s o n t o M W N T s i n s o l u t i o n ,p r e s s e d i n t o p e l l e tP A N I /C N T 135978c118A /g1M H 2S O 4E l e c t r o d e p o s i t e d P A N I o n v e r t i c a l l y o r i e n t e d c a r b o n n a n o t u b e a r r a yP A N I /M W N T 136332d1.0A /g S o l i d s t a t e H 2S O 4ÀP V A g e l7.12,189C h e m i c a l s y n t h e s i s o n t o ‘‘B u c k y P a p e r ’’P A N I /G r a p h e n e 137170(F /c m 3)c100A /c m 31M N a 2S O 4D i p p e d l a y e r -b y -l a y e r s o l u t i o n a s s e m b l y o n I T OP A N I /G r a p h e n e 138116–375c3.0–0.5A /g1M H 2S O 430.341,000D i p p e d l a y e r -b y -l a y e r s o l u t i o n a s s e m b l y o n I T OP A N I /G r a p h e n e 1391046c1m V /sQ 6M K O H39–14570,000–522C h e m i c a l s y n t h e s i s i n a s u s p e n s i o n o f g r a p h e n e o x i d e ,m i x e d w i t h b i n d e r s a n d c o a t e d o n t o N i f o a mP A N I -N F /G r a p h e n e 140531c0.2A /g1M H 2S O 4C h e m i c a l s y n t h e s i s i n a s u s p e n s i o n o f g r a p h e n e o x i d e ,m i x e d w i t h b i n d e r s a n d p r e s s e d o n t o S S s h e e tP A N I -N F /G r a p h e n e 141197–210d3.0–0.3A /g1M H 2S O 4M i x e d s u s p e n s i o n o f P A N I a n d g r a p h e n e f i l t e r e d t o p r o d u c e a f i l mP A N I -N F /G r a p h e n e 142321–526c1.0–0.2A /g2M H 2S O 4C h e m i c a l s y n t h e s i s i n a s u r f a c t a n t -s t a b i l i z e d s u s p e n s i o n o f g r a p h e n e o x i d e ,d r o p c a s t o n t o g l a s s y c a r b o n e l e c t r o d eP A N I N a n o w i r e s /G r a p h e n e 143227–555d2.0–0.2A /g1M H 2S O 4C h e m i c a l s y n t h e s i s o f P A N I n a n o w i r e s o n g r a p h e n e ,m i x e d w i t h b i n d e r ,p r e s s e d o n t o g o l d g r i dP A N I -g -G r a p h e n e 144250c1M H 2S O 4C h e m i c a l s y n t h e s i s o f P A N I g r a f t e d f r o m g r a p h e n e s h e e t s ,d r o p c a s t o n t o g l a s s y c a r b o n e l e c t r o d eP A N I N a n o r o d /G r a p h e n e 145974d2.5A /gH 3P O 4-P V A g e lC h e m i c a l s y n t h e s i s o n t o g r a p h e n e p a t t e r n e d b y m i c r o m o l d i n g i n c a p i l l a r i e sP A N I /G r a p h e n e /C N T 146341–569c10–0.1A /g1M H C l C h e m i c a l s y n t h e s i s o f P A N I o n C N T s ,s u s p e n d e d w i t h g r a p h e n e ,p r e s s e d o n t o c o n d u c t i v e t a p e o n P ta ,bF o r v a l u e s g i v e n a s a r a n g e ,t h e h i g h v a l u e f o r t h e c a p a c i t a n c e o r e n e r g y c o r r e s p o n d s t o t h e l o w v a l u e f o r c h a r g i n g r a t e s o r p o w e r .cT h r e e -e l e c t r o d e m e a s u r e m e n t .d T w o -e l e c t r o d e m e a s u r e m e n t .JOURNAL OF POLYMERSCIENCEFEATUREARTICLEJOURNAL OF POLYMER SCIENCE,PART B:POLYMER PHYSICS 2013,51,468–480473diffusion limitations at high rates of discharge.Several groups have also produced similar PANI/graphene films using layer-by-layer deposition methods.137,138,159Layer-by-layer films were formed by alternately immersing a surface-treated ITO-coated glass substrate into separate solutions of positively charged PANI-NFs and negatively charged func-tionalized carbon nanotubes or graphene.132,137,138,160,161Electrostatic interactions and entropic contributions enabled film formation at the surface.162This technique is advanta-geous in that it allows for the solution fabrication of intimate composites without the need for binders.In addition,layer-by-layer film formation is versatile and can be performed on many different surfaces such as textiles,walls,wood,glass,metal,and so forth.163,164With the advent of spray layer-by-layer assembly,165composites such as these could be rapidly manufactured on an industrial scale onto many substrates or even applied as paint.9,163–166PANI is also a promising material for composites with high capacity inorganic materials that lack ionic and electronic conductivity.23,167,168Our group has recently combined PANIand vanadium pentoxide in a layer-by-layer electrode (Fig.6).23V 2O 5is a high capacity metal oxide that shows promise for battery applications but suffers from poor elec-tronic conductivity and slow lithium ion diffusion.169,170By combining it with PANI,a composite can be formed that func-tions better than either material alone.23,171,172Motivated by the work of the Oliveira group,171,173we recently identified assembly conditions that form high capacity,energy dense cathodes.23,174By judiciously selecting the pH of assembly,the cycle thickness was dramatically increased,resulting in a shortened processing time.Overall,investigations of V 2O 5/PANI composites have demonstrated promise because the two components work together synergistically to improve coulombic efficiency and charge transport.23,171,172,175–177Our work has shown that the capacity is optimized at a par-ticular thickness,where thicker films are subject to ion diffu-sion limitations.23To address this issue,we are replacing conventionally synthesized PANI with PANI-NF to introduce a porous electrode morphology.174Other metal oxides and inorganics have also been successfully combined with PANI to produce synergistic composites.24,168,178–189FIGURE 5Illustration of the layer-by-layer assembly of PANI/carbon nanotube films (Reproduced from Ref.133,with permission VC 2011American ChemicalSociety.)FIGURE 6Illustration describing the steps of the layer-by-layer assembly process as applied to PANI/vanadium pentoxide compo-sites on ITO slides (Reproduced from Ref.21,with permission VC 2011American Chemical Society.)FEATURE ARTICLEJOURNAL OF POLYMERSCIENCE474JOURNAL OF POLYMER SCIENCE,PART B:POLYMER PHYSICS 2013,51,468–480。