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. “Plastic 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 5 years 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.1–8 Without 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.8–14 Additionally, it is advantageous to directly combine plastic power with polymer solar cells or light emitting diodes in a streamlined manufacturing process to create low-cost fully flexible devices.10, 15, 16
Aside from their mechanical versatility, conjugated polymers have several advantages such as synthetic tailorability, cost, and processability that make them ideal candidates for electrochemical energy storage. Their electrochemical and physical properties may be tuned through organic synthetic techniques.1, 17, 18 Furthermore, the raw materials and methods to produce these polymers are often economical. New synthetic approaches have enhanced processability such that conjugated polymers can be processed via screen printing, doctor blading, or inkjet printing.19–21 Other 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, 29 In light of this, conjugated 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.23–26 The combination of synthetic tuning, processing options, and mechanical 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, Figure 1.18, 27, 28 The focus has lain on these particular polymers because of the presence of extensive background research, the commercial availability of both monomers and polymers, and their ease of synthesis. The structures in Figure 1 are only a representative sample, whereas Novák's 1997 review 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 late 1980s.29 Table 1 presents a comparison between common battery materials, LiCoO2 and LiFePO4,30, 31 and the conjugated polymers presented in Figure 1.
Table 1. Performance Characteristics of Conjugated Polymers as Compared to Current Inorganic Li-Ion Battery Materials
Although there are clear advantages, there are several issues that must be addressed before the full potential of conjugated polymers can be realized. These include oxidative stability, 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.
To understand the operation of conjugated polymers within the framework of a battery electrode, we must first consider the fundamental properties of conjugated polymers themselves. 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 π-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, 38 The effective conjugation length is the point at which no additional monomer units added to the chain affect the electronic properties of the polymer.39–41 For organic semiconductors, the bottom of the conduction band is referred to as the lowest unoccupied 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, 42
Polymers can be classified as being either n-type or p-type based on whether they are easily reduced or oxidized, respectively. 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 stabilize negative charges in low-lying LUMO levels, making them good n-type materials.43–45 Generally speaking, n-doped polymers act as anodes, while p-doped polymers function well in cathodes.18 There has been much work on p-type materials for cathodes, but there has been little focus on the synthesis of new n-type materials.46–48 For 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.49–51 Figure 2 illustrates this process for PT, which can be p-doped or n-doped. Doping creates radical cations or anions, also referred to as positive or negative polarons, respectively, which are delocalized along the polymer backbone and are a proposed mechanism for conductivity.49, 52, 53 Charge delocalization is coupled to structural reorganization to a quinoidal form and alters fundamental molecular vibrations.52 The 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 polymers.49, 52–56 Conjugated polymers in their native state have very low conductivities, on the order of 10−4–10−9 S/cm, but can attain very high conductivities when doped.38, 57 Poly(3,4-ethylenedioxythiophene) (PEDOT), for example, has exhibited conductivities as high as 1500 S/cm in certain cases.58
To maintain charge balance, redox reactions associated with polymer doping are concurrent with the movement of ions into and out of the polymer matrix.59, 60 Although oxidation generally results in anion movement within the film, studies have shown that under certain conditions, cation movement occurs as well.61–63 It could be inferred that analogous processes can also occur for anions during polymer reduction. 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 densities.6, 18, 64, 65 Polymer nanostructures such as nanowires, nanotubes, or nanoparticles can help to alleviate problems associated with ion diffusion by increasing the surface area and porosity.66–68
Besides ion diffusion, oxidative stability also affects the electrode'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 monomer units,18, 60 although higher levels of charging are possible. A higher specific capacity is realized with higher levels of oxidation or reduction. Unfortunately, high levels of charging can lead to destabilization of the polymer matrix through unwanted interchain interactions or breakdown of polymer chains.69, 70 Additionally, highly charged species are much more likely to react with common electrolytes and/or oxygen in the air.69, 71, 72 Low capacities can be mitigated to a limited extent through choice of dopant ion, electrolyte, or polymer structure.69, 73–78 There 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.79 Deficiencies associated with oxidative stability over many charge cycles have seen improvements with select composite materials such as graphene or carbon nanotubes.1, 80–83
Synthesis 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.60 As this type of synthesis is performed in an oxidizing environment, the resulting polymers are formed in an already doped state. PANI, for example, is synthesized in the emeraldine salt state under oxidative conditions. Reductive polymerization is not unheard of, but is rarely employed primarily due to limited applicability.84–87 This 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 monomer and electrolyte, but there are examples of solid-state or vapor deposition polymerizations as well.58, 88, 89 Metal-catalyzed cross-couplings are another method for producing conjugated polymers.90–94 Unlike 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 advances in direct arylation polymerizations are working toward eliminating the need for generating monomers with specific functional group requirements.95 Metal-catalyzed polycondensations are also a potential method for synthesizing new n-type or specially functionalized materials.90, 96
Several recent synthetic approaches have focused upon increasing the solubility or dispersibility of conjugated polymers to enhance their processability. Side chains are essential for solubilizing conjugated polymers used in semiconducting applications, and similar concepts are applicable for electrochemical energy storage.97–100 Side chain functionalization has been used to facilitate intimate mixing in composites.32, 101–103 Conjugated polymer:polyelectrolyte complexes have also come a long way to improve polymer processing.104–106 For example, poly(2-acrylamido-2-methyl-1-propanesulfonic acid) has been shown to both solubilize and stabilize PANI in complexes.79, 107–109
PANI|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 stability, relatively high theoretical capacity, simple synthesis, and electrochemical reversibility.30, 110 PANI 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.114–117 PANI electrodes make use of the reversible reactions between fully reduced leucoemeraldine base and partially oxidized emeraldine salt. At higher potentials, cycling stability can break down as emeraldine salt irreversibly oxidizes to pernigraniline base23 because the reaction pathway to pernigraniline salt is not favored. There have been efforts to stabilize the pernigraniline salt,79, 116, 117 which 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.118 These problems had traditionally been addressed through dispersing the polymer in solutions using a series of steps intended to remove aggregates and ensure a stable suspension.119 More recently, template synthesis using polyanions and the synthesis of PANI nanostructures, most notably nanofibers, has enabled easy solution suspension for PANI.67, 118, 120 Cycling 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 cycling stability. They help to maintain conductivity in a composite 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, 83
Among the many varieties of synthesizable nanostructures for conjugated polymers, there is particular interest in one-dimesional nanostructures such as nanotubes, nanorods, or nanofibers.121, 122 PANI nanofibers (PANI-NF) are especially attractive since they can be made economically and quickly in solution without the use of templates, additives, or complicated synthetic techniques.122 Kaner and coworkers have developed synthetic methods for variable diameter PANI-NF that involve either rapidmixing of oxidant and monomer solutions or interfacial polymerization in phase-separated solvent systems (Fig. 4).67, 123–125 The size of the nanofibers can be controlled by choice of reactant medium and acid.77, 108, 109 As expected, PANI-NFs show improved electrochemical performance over conventional solution- synthesized PANI (Table 2).126–128 Perhaps 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.67
Table 2. Selected Polyaniline Composites with Graphene or Single-Walled or Multi-Walled Carbon Nanotubes
As previously stated, nanostructured carbon materials such as graphene, carbon nanotubes, or even carbon black are desirable for their high conductivity and stability,147–150 yet can suffer from lower capacity versus redox active materials due to the non-Faradaic nature of charge storage.80, 151, 152 PANI is an ideal material for polymer/carbon composites because of its cyclability, electrochemical activity, and so forth; several examples are highlighted in Table 2. Carbon nanotubes or graphene/PANI composite electrodes are usually formed by pressing into pellets, vacuum-assisted filtration, or by polymerization on carbon nanotube surfaces or films, for example.130, 131, 133–135, 139–144, 146–158 Although 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 experimental conditions to form a working cathode.133, 137, 138, 145, 158
The 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 PANI:multiwalled carbon nanotube films (Fig. 5).159 The resulting film was highly porous, and was able to overcome ion diffusion limitations at high rates of discharge. Several groups have also produced similar PANI/graphene films using layer-by-layer deposition methods.137, 138, 159 Layer-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 functionalized carbon nanotubes or graphene.132, 137, 138, 160, 161 Electrostatic interactions and entropic contributions enabled film formation at the surface.162 This technique is advantageous 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, 164 With the advent of spray layer-by-layer assembly,165 composites such as these could be rapidly manufactured on an industrial scale onto many substrates or even applied as paint.9, 163–166
PANI is also a promising material for composites with high capacity inorganic materials that lack ionic and electronic conductivity.23, 167, 168 Our group has recently combined PANI and vanadium pentoxide in a layer-by-layer electrode (Fig. 6).23 V2O5 is a high capacity metal oxide that shows promise for battery applications but suffers from poor electronic conductivity and slow lithium ion diffusion.169, 170 By combining it with PANI, a composite can be formed that functions better than either material alone.23, 171, 172 Motivated by the work of the Oliveira group,171, 173 we recently identified assembly conditions that form high capacity, energy dense cathodes.23, 174 By judiciously selecting the pH of assembly, the cycle thickness was dramatically increased, resulting in a shortened processing time. Overall, investigations of V2O5/PANI composites have demonstrated promise because the two components work together synergistically to improve coulombic efficiency and charge transport.23, 171, 172, 175–177 Our work has shown that the capacity is optimized at a particular thickness, where thicker films are subject to ion diffusion limitations.23 To address this issue, we are replacing conventionally synthesized PANI with PANI-NF to introduce a porous electrode morphology.174 Other metal oxides and inorganics have also been successfully combined with PANI to produce synergistic composites.24, 168, 178–189
OTHER CONJUGATED POLYMERS
Many of the strategies mentioned above for PANI have been applied to other conjugated polymers—primarily polythiophenes and polypyrroles with various levels of success. This section will cover recent developments in polymers other than PANI.
There are methods to increase charge capacity without drastically compromising polymer properties by including redox active components in the polymer structure. The Goodenough group reported improvements in both capacity and long-term cyclability of polypyrrole by covalently anchoring ferrocene groups to the polymer.75 Similar improvements were observed for a ferrocene functionalized triphenylamine polymer and a 2,2,6,6-tetramethylpiperidine-N-oxide functionalized PT.190, 191 Zhou et al. recently reported capacity enhancement of 300% in polypyrroles doped with Fe(CN)64− anions.76 A lesser enhancement was also observed for PEDOT and PT. It was postulated that since the Fermi level of Fe(CN)64− was close to that of the conjugated polymers used in the study, the anion served as a mediator for charge transfer between the polymer and electrolyte.76 In addition, the films were shown to exchange lithium cations during charge cycling instead of anions. Lithium is more mobile, and its small size prevents large structural changes during charge cycling.76 The same group also reported capacity enhancements when using redox active diphenylamine-4-sulfonate as anionic dopant.78 Similarly, Kim et al. showed improvements in capacitance on a polypyrrole/cellulose composite electrode when using an anthraquinone dopant ion.192
Promoting structural order through morphological control is important for polymeric electrodes.122 Regioregularity in poly(3-alkylthiophenes) has been linked to enhanced capacitance, conductivity, and stability.97, 193 Another good example of this lies in the comparison between poly(3-hexylthiophene) nanofibers versus bulk films. The nanostructuring promotes increased molecular order that allows for improved charge transport within the polymer matrix.194, 195
There are very few reports on n-type conjugated polymers for energy storage. A major issue involves the lack of reliable synthetic methods to produce large quantities of n-type polymer. There are, however, new ways to make them using metal-catalyzed cross-couplings,90, 96 although these developments are very recent as of the time of this publication and have not been widely applied to electrode materials as of yet. There is one example, however, of an n-type polyfluorene stabilizing silicon nanoparticles in a composite anode.25 The increased cycle life is attributed to the polymer's reversible lithium doping and the formation of an intimate composite with silicon.25 There are also examples of using side-chain functionalization to stabilize polythiophenes to n-doping.12, 69, 196 The Reynolds group has circumvented the poor polymerizability of n-type monomers by sandwiching an electron poor n-type heterocycle between two electron rich heterocycles to form a donor–acceptor polymer.47, 197 These n-dopable polymers were examined in symmetric supercapacitors using non-aqueous electrolyte. It was found these devices could attain operating voltages as high as 2.5 V and energy densities of ∼13 Wh/kg although they experienced a loss of electrochemical activity after 200 cycles.47
Diffusion limitations lead to low response times at high current densities, leading to low power efficiencies. So-called “self-doped” conjugated polymers possessing ionic side chains can have very fast response times and improved processability relative to their parent polymers, but can suffer from low conductivities.198, 199 This can be alleviated to an extent with nanostructuring.198 Recently, self-doped PANI has proven to be a useful energy storage material.199–204 One group has reported a specific capacitance of 408 F/g at a rate of 1 A/g for self-doped PANI on functionalized carbon cloth.201 Ghenaatian et al. attained a high capacity of 215 mAh/g for a Zn/self-doped PANI battery. Balsara and coworkers205, 206 have developed an interesting solution to overcoming diffusion limitations. Block copolymers of conducting PT and poly(ethylene oxide) were used to form films that had both large ionic and electronic conductivities. In bulk films, it was shown that polymer blocks segregated into solid state microdomains of ionically conductive PEO and electronically conductive PT (Fig. 7). The integrated nature of the resulting material was successfully employed in a solid-state battery.99
Because form factors are less of a concern for conjugated polymer electrodes, there have been efforts directed to take advantage of the flexibility of various substrates. Wallace's group has produced stretchable polypyrrole supercapacitor electrodes on a prestretched, gold-coated elastic substrate.3 Conductive papers made from composites of polymer with graphene or carbon nanotubes are promising flexible electrodes.131, 133, 157, 207 Zhao et al.208 report a compressible electrode made of a composite of “graphene foam” and polypyrrole. Cellulose is another popular flexible substrate due to its industrial importance, high surface area, and economic sources such as bacteria or wood.1, 5, 192, 209–213 Milczarek and Inganäs64 have reported a cathode composed of polypyrrole and lignin, which, unlike cellulose, has electrochemically active components as part of its complex structure. Fabric substrates have also drawn interest due to the potential for wearable electronics.7, 8, 11, 12, 14, 203, 214 A polyester/cellulose fabric cathode was demonstrated in a lithium battery by simply soaking the fabric in PEDOT:poly(styrene sulfonate) polymer ink.8 Yue et al. has coated nylon/lycra fabric with polypyrrole to form a stretchable electrode.7 All of these examples serve to demonstrate the promise of flexible plastic power into unconventional form factors.
This review has outlined some of the most successful methods for achieving high capacity, energy and power dense, stable polymer-based electrodes as of yet. Although major hurdles have been overcome in the way of nanostructuring and polymer synthesis, much of the most promising work is still in the beginning stages of research. Block copolymers and polymer functionalization have proven to be useful in overcoming diffusion limitations and increasing polymer response times and capacities. Complexation with a polyelectrolyte of complementary charge shows promise in stabilizing highly charged conjugated polymer species. PANI nanostructures enable their utilization in economical solution processing techniques. Layer-by-layer techniques are excellent ways to form intimate composites between various materials and conjugated polymers. With the advent of spray technology and water-processable conjugated polymers, it could be envisioned that electrodes could be manufactured on a variety of surfaces, enabling their direct incorporation with high-end electronics such as light emitting diodes or solar cells, where intimate integration with a power source would prove especially useful. A growing body of research is exploring the utility of many different substrates to take full advantage of the mechanical properties of conjugated polymers. Considering the new developments and current directions highlighted in this review, conjugated polymers will undoubtedly shape the future of energy storage.
A portion of this work was supported in part by the Welch Foundation (Grant No. A-1766).
Jodie L. Lutkenhaus is an Assistant Professor of Chemical Engineering at Texas A&M University. Research specialties include polymeric electrodes for energy storage, polyelectrolytes, 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 2002 and her Ph.D. in Chemical Engineering from the Massachusetts Institute of Technology in 2007. She received the NSF CAREER Award and ACS PRF-DNI in 2011.
Jared F. Mike received his B.S. from Youngstown State University in 2005 and Ph.D. from Iowa State University in 2011 with 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 conjugated polymers.