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Nanostructured conducting polymer hydrogels for energy storage applications Ye Shi,† Lele Peng† and Guihua Yu* Conducting polymer hydrogels are emerging as a promising class of polymeric materials for various technological applications, especially for energy storage devices due to their unique combination of advantageous features of conventional polymers and organic conductors. To overcome the drawbacks of conventional synthesis, new synthetic routes in which acid molecules are adopted as both crosslinkers and dopants have been developed for conducting polymer hydrogels with unique 3D hierarchical porous nanostructures, resulting in high electrical conductivity, large surface area, structural tunability and hier-
Received 23rd May 2015, Accepted 25th June 2015 DOI: 10.1039/c5nr03403e www.rsc.org/nanoscale
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archical porosity for rapid mass/charge transport. The newly developed conducting polymer hydrogels exhibit high performance when applied as active electrode materials for electrochemical capacitors or as functional binder materials for high-energy lithium-ion batteries. This feature article summarizes the synthesis of conducting polymer hydrogels, presents their applications in energy storage, and discusses further opportunities and challenges.
Introduction
Nanostructured materials have become critically important in a wide range of applications from renewable energy, electronics, and photonics to medical and life science, because of their unusual physical/chemical properties due to the confined dimensions of such materials.1–4 They are promising to
Materials Science and Engineering Program and Department of Mechanical Engineering, The University of Texas at Austin, Austin, TX 78712, USA. E-mail: [email protected] † These authors contributed equally.
Ye Shi
Ye Shi is a Materials Science graduate student at the University of Texas at Austin. He received his B.S. and M.S. degrees in Polymer Science and Engineering at Zhejiang University. In fall 2013, he began graduate studies with Prof. Guihua Yu at University of Texas at Austin, focusing on rational synthesis, modification and tunable properties of conductive polymer nanomaterials and their applications in energy storage and conversion devices and other functional devices/systems.
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address critical challenges faced by many energy storage technologies owing to their advantageous features, such as larger electrochemically active surface area, shortened pathways for charge/mass transport, and better accommodation of the strain within electrodes. Among various nanostructured materials, hydrogels with highly cross-linked networks of polymer chains are of particular interest because they present three-dimensional (3D) structures at both microscale and nanometer levels. Different from inorganic nanomaterials, hydrogels exhibit mechanical properties ranging from soft and weak to hard and tough, owing to their unique composition with large quantity of liquid dispersed within a solid.5 Due to
Lele Peng
Lele Peng received his B.S. degree in Materials Science and Engineering from the University of Science and Technology of China (USTC) in 2012. He is currently pursuing his PhD in Materials Science and Engineering at the University of Texas at Austin under the supervision of Prof. Guihua Yu. His current interests include the synthesis and characterization of 2D nanomaterials and their hybrids for energy storage and conversion, especially for lithium ion batteries and high performance supercapacitors.
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its crosslinking structure and swelling nature,6 a hydrogel usually possesses a highly porous microstructure, high surface area, good compatibility with bio- or other hydrophilic molecules, and tunable mechanical properties, thus becoming an ideal material for diverse applications in energy, electronics, and medical/life science.7,8 Conductive polymers usually possess highly π-conjugated polymeric chains. Typical conductive polymers include polyacetylene (PA), polyaniline (PANI), polypyrrole (PPy), polythiophene (PTh), poly( phenylenevinylene) (PPV), etc. (Fig. 1b). The conductivity of conductive polymers could be tuned in a wide range from 10−10 up to 104 S cm−1 through chemical or electro-
Fig. 1 (a) Representative applications of conducting polymer hydrogels (CPHs). (b) The chemical structures of typical conductive polymers.
Dr Guihua Yu is an Assistant Professor of Materials Science and Engineering at the University of Texas at Austin. He received his B.S. degree with the highest honor in chemistry from the University of Science and Technology of China, earned PhD in chemistry at Harvard University in 2009, followed by postdoc at Stanford University. His research has been focused on rational synthesis and selfDr Guihua Yu assembly of functional organic nanostructures and two-dimensional nanostructured solids for advanced energy technologies, and fundamental understanding of their structure–property–performance relationship.
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chemical redox reactions which is called “doping”,9 endowing the conductive polymers the ability to act as insulators, semiconductors or conductors by changing the dopants and/or the level of doping.10–13 In addition, doping is a reversible process which makes the backbone of conductive polymers positively ( p-doping) or negatively charged (n-doping), enabling a number of device applications such as energy storage and conversion devices, sensors, and actuators.14 Conductive polymers can also be rationally designed and fabricated to nanostructures, thus providing new exciting features including tunable conductivities, flexibility, and mixed conductive mechanism15 which lowers the interfacial impedance between electrodes and electrolytes. As a special class of conductive polymer materials, conducting polymer hydrogels (CPHs) synergize the advantages of hydrogels and organic conductors.16 CPHs with 3D network structures provide an electrically conductive yet mechanically robust framework:7 the continuous conductive backbones can promote charge transportation,17 the porous structure facilitates the diffusion of ions and molecules, and the swelling nature offers additional effective interface between molecular chains and solution phases. Compared with their bulk form, CPHs also exhibit other unique properties such as improved mechanical properties due to effective accommodation of the strain caused by electrochemical reaction, lightweight and good processability.18,19 As a result, CPHs have been explored for wide-ranging applications such as energy conversion and storage, sensors, actuators, medical and bio-devices, and superhydrophobic coatings (Fig. 1).20–23 In this feature article, the synthetic approaches and fabrication strategies for CPHs are presented, followed by a detailed discussion on their energy storage applications, including supercapacitors and Li-ion batteries, as nanostructured CPHs provide large effective electrochemical interface with electrolyte ions,24 which could promote highly efficient ion and electron transport and electrochemical processes.25 This article will also comment on the current limitations and perspectives for further developing energy storage applications of CPHs.
2. Synthesis and processing of CPHs The polymerization of conductive polymers typically involves an electrochemical process of oxidation, a chemical process of coupling and eliminating protons.10 In a typical polymerization process, first, a radical cation is generated by the oxidation of one monomer and then couples with another radical cation to form a dimer after the loss of two protons. The oxidation step could be induced by various methods such as chemical oxidation, electrochemical process, and photochemical initiation. The dimer could be further oxidized and coupled with radical cations to form oligomers. Following the sequence of oxidation, coupling, and deprotonation, this propagation continues until the polymer is finally obtained.26
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Conventional synthetic methods for CPHs
Traditional synthetic routes include polymerization of monomers in an existing non-conductive hydrogel matrix which serves as template, copolymerization of conductive polymers with non-conductive polymers, and crosslinking conductive polymers such as poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) by multivalent metal ions (Fe3+ or Mg2+).16 The hydrogel-template method is most commonly used (Fig. 2a). Typically, a hydrogel template matrix is first prepared and de-swollen, and then re-swollen in the solution of monomers for conductive polymer synthesis which is in situ polymerized by electrochemical polymerization or by adding chemical oxidants. PANI or PPy–PNIPAM [ poly(N-isopropylacrylamide)],27 PPy–pHEMA [ poly(2-hydroxyethyl methacrylate)],28 PEDOT–alginate,29 PEDOT–PAA ( polyacrylic acid),30 PEDOT–PAMPS [poly(2-acrylamido-2-methyl-1-propanesulfonic acid)]31 and PPy–PAAM ( polyacrylamide)32 have been successfully synthesized via this strategy. To increase the surface area, a micro- or nanostructured template would be further applied.33 For example, poly(lactide-co-glycolide) (PLGA) nanofibers were adopted as templates to electrochemically deposit conductive polymers, thus forming crosslinked CPHs (Fig. 2b). However, the limited scalability of template-guided synthesis inhibits its further application. Another method to fabricate a conducting polymer hydrogel is the copolymerization of monomers for conductive polymers with other monomers for nonconductive polymers. Non-conductive polymers can act as a component of main chains of copolymers or as a crosslinking part to crosslink conductive counterparts. In this route, both non-conductive hydrogels and conductive polymer precursors
Fig. 2 (a) Conventional synthetic methods for CPHs, in which monomers of conductive polymers are polymerized onto non-conductive hydrogel matrix templates. Reproduced with permission from ref. 16. Copyright 2013 Royal Society of Chemistry. (b) SEM images of poly(lactide-co-glycolide) (PLGA) nanoscale fibers used as the template (top panel) and PEDOT hydrogels synthesized based on the PLGA template (bottom panel). Reproduced with permission from ref. 33. Copyright 2006 WILEY-VCH Verlag GmbH & Co.
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are placed together and polymerized either simultaneously or in a two-step process by chemical oxidation or electrochemical polymerization. Through this method, a PPy–pHEMA hybrid hydrogel is synthesized and its physical and chemical properties could be tuned by further adding other acrylate, methacrylate and acrylamide monomers.34 CPHs synthesized by the conventional method consist of both conductive and nonconductive components, and may lead to deteriorated electrical properties over time. Moreover, excessive metal ions adopted in the polymerization may reduce the compatibility of hydrogels with other molecules. In terms of potential scalability, the conventional synthetic methods discussed above have significant limitations to overcome. 2.2
Newly developed methods for CPH synthesis
A new synthetic route for conductive hydrogels which exhibits facile processability, excellent electronic properties, high electrochemical activity and high biocompatibility (Fig. 3a and b) has recently been reported.7 Phytic acid is adopted as the gelator and dopant in the synthetic process and conductive hydrogel networks could be directly formed. PANI and PPy hydrogels have been successfully synthesized via this method. The gelation mechanism of the CPH could be explained by the ability of each phytic acid molecule to interact with more than one conductive polymer chain. The gelation is typically completed within several minutes and can be induced in a wide range of molar ratios of monomer to phytic acid, and with diverse oxidative initiators such as ammonium persulphate (APS) and hydrogen peroxide. Moreover, the reaction could be conducted under biphasic conditions.35 The SEM images (Fig. 3c) show that the dehydrated PANI hydrogels are constructed with coral-like dendritic nanofibers with uniform diameters of 60–100 nm while the fully swollen PANI hydrogels are demonstrated by an AFM image (Fig. 3d) to consist of nanofibers of about 200–300 nm in diameter, showing their high level of hydration. In addition, the phytic acid renders CPHs a significant hydrophilicity owing to an excess of phosphorus groups (Fig. 3e). The swelling nature and hydrophilic properties of CPHs may provide additional interface areas between polymer chains and solution phases to support more active reaction sites and anchoring sites for active molecules or particles. The framework of the as-prepared CPHs is free of insulating polymers, thus providing an ideal 3D interconnected path for electron transport.36,37 The obtained PANI hydrogel reaches a conductivity of 0.11 S cm−1 at room temperature, which is the highest reported value for conducting polymer hydrogels (typically in the range of 10−4– 10−2 S cm−1 reported in the literature). A highly porous 3D nanostructure with hierarchical porosity (the first level is the 100 nm scale pores between the branched nanofibers and the second level is the bigger micron sized pores marked by the white arrows in Fig. 3c) can be achieved on a large-scale and with long-term stability in this synthesis. Such 3D porous structures offer large open channels on the microscale and mesoscale pores which facilitate the diffusion of ions and
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Fig. 3 (a) Schematic illustration of the 3D hierarchical microstructure of the gelated PANI hydrogel where phytic acid plays the role as a dopant and a crosslinker. (b) A photograph of the PANI hydrogel. (c) A SEM image showing the interconnected network of dendritic PANI nanofibers. (d) AFM image of the PANI hydrogel. (e) Contact angle of phytic acid doped PANI. (f ) Nitrogen adsorption–desorption isotherm of the dehydrated PANI hydrogel. Reproduced with permission from ref. 7. Copyright 2012 Proceedings of the National Academy of Sciences of the United States of America.
molecules and also provide a large specific surface area which is measured to be ∼42 m2 g−1 and could be tuned from 40 to 100 m2 g−1 (Fig. 3f ). Moreover, the synthetic route needs no external ingredients such as surfactants or templates, thus greatly enhancing its processability and universality. Adopting phytic acid as a crosslinker and a dopant, another synthetic approach based on multiphase interfacial reactions could be applied for CPHs (Fig. 4a).35 In a typical synthesis, an aqueous solution of an oxidative reagent and another solution containing a mixture of the monomer, organic solvent and phytic acid are prepared and then mixed together. The monomers would polymerize at the interface, resulting in hollowsphere microstructures (Fig. 4b and c) which are further interconnected by the crosslinker. In this method, the microstructure of the obtained CPHs could be simply tuned by adjusting the ratio of monomer to crosslinker, as well as different organic solvents, which finally leads to tunable mechanical and electrochemical properties of CPHs. For example, by forming an interconnected hollow-sphere microstructure, inherently stiff and brittle PPy based CPHs with a rigid conjugated-ring backbone can exhibit a tunable effective elastic modulus that are capable of withstanding large effective strains (Fig. 4d). Pan et al. synthesized PPy-based CPHs which consist of interconnected hollow spheres with polydisperse diameters ranging from hundreds of nanometres to several microns.35 The hollow-sphere structure brings the PPy foam structural elasticity, enabling an ultrasensitive pressure sensor. The diameters, size dispersions and shell thicknesses can be adjusted by using different organic solvents, including 1-butanol, t-butanol, s-butanol and t-amyl alcohol. The micro-
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Fig. 4 (a) Schematic of the structural elasticity of the hollow-spherestructured PPy. (b) TEM image of PPy showing its interconnected hollow-sphere structure, scale bar: 1 μm. (c) SEM images of the PPy hydrogel with a pyrrole : phytic acid ratio of 5 : 1; (d) six consecutive compression tests on the PPy thin film. Reproduced with permission from ref. 35. Copyright 2013 Nature Publishing Group.
structure could be also tuned by changing the ratio of monomer to phytic acid. Yu et al. found that the PPy hydrogel turns from the hollow-sphere dominant structure to the particle-dominant one when the concentration of the crosslinker is significantly lowered. The PPy hydrogels resulting from
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3. Energy storage applications of CPHs
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3.1
Fig. 5 Micropatterned PANI hydrogel arrays fabricated by (a) ink-jet printing and (b) spray coating. (c) Photograph of a microelectrode array based on PEDOT on a flexible gel substrate. Reproduced with permission from ref. 39. Copyright 2010 American Chemical Society.
different Py : phytic acid ratios also show different electrochemical properties.38 Owing to their facile synthesis and tunable mechanical properties, the CPHs show high scalability and processability that they could be processed by scalable techniques such as ink-jet printing or spray coating, to fabricate the desired micropatterns for large arrays of electrochemical devices (Fig. 5).39 Ink-jet printing of the polymer is often hampered by the limited solubility and high viscosity of polymer solutions. This difficulty is overcome by separately depositing two distinct solutions which contain an oxidative initiator and phytic acid, and monomers for conductive polymer synthesis respectively onto the substrate. The patterned PANI hydrogel is formed when two solutions are able to interact and the morphology of the printed hydrogel is found to be the same as that of the bulk synthesized hydrogel. The conductive hydrogel can also be processed by spray coating to form micropatterns of millimetre size. Similar to ink-jet printing, the two solutions are alternatively deposited multiple times through poly(dimethylsiloxane) (PDMS) soft stencil masks onto a range of substrates. Through these approaches we can fabricate large arrays of patterned CPHs, leading to highly conductive, functional microelectrodes potentially useful for supercapacitors, lithium batteries, biosensors, chemical sensors, and other bioelectrodes. These newly developed synthetic methods for nanostructured CPHs offer the ability to rationally design and control the morphology and microstructure of conductive polymers, as well as tuning their mechanical and electrochemical properties. With these synthetically tunable chemical/physical characteristics, the newly developed CPHs possess the possibility to meet requirements in various applications ranging from functioning as active electrode materials to serving as other functional components (e.g. as functional binder materials), thus becoming important candidates for energy storage devices, which is detailed in the following sections.
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CPHs as active electrodes for electrochemical capacitors
As designed to take advantage of the near-surface charge storage mechanism, electrochemical capacitors (ECs), often called ‘supercapacitors’, show high power density and can store and release the energy within the time frame of a few seconds,40 thus being projected as future power sources for high-power applications such as electric vehicles, utility loadlevelling, as well as back-up power for computer memory, lasers, and pulsed-light generators. Conductive polymers, such as PANI, PPy, and their derivatives, have been widely investigated for EC applications as pseudocapacitive materials, owing to their fast and reversible redox reactions at the electrode surface.41 These π-conjugated polymers have shown high gravimetric and volumetric pseudocapacitance in various non-aqueous electrolytes with operating voltages of ∼3–4 V. Their synergized properties from both traditional polymers and organic conductors can potentially meet the requirements for flexible, lightweight, and environmentally friendly devices. However, when used as bulk materials, conductive polymers suffer from a limited cycling stability due to structural change which leads to the fast decay of their electrochemical performance. Since EC systems involve ionic and electronic transport processes at the electrode surface and the electrode/electrolyte interfaces, a highperformance EC electrode requires high electrical conductivity, high ion-accessible surface and interface area, fast ionic transport and good electrochemical stability. Nanostructuring conductive polymers is found to be an effective approach to improve the performance of conductive polymers in EC systems. CPHs are emerging as a promising class of materials for EC applications because their 3D hierarchical nanostructure can provide an excellent interface for electrolyte ions to access the electroactive surface of CPHs and an intrinsically conducting and robust framework that promotes the charge/mass transport, thus improving the electrochemical usage of active materials. The 3D nanostructured PANI hydrogels have been demonstrated as high-performance EC electrodes recently.7 The PANI hydrogel electrode shows remarkably small charge transfer resistance, suggesting favorable ion transport within the 3D continuous framework (Fig. 6a). The excellent electrochemical activity of this hydrogel based electrodes was demonstrated by galvanostatic charge–discharge measurements in which the active PANI hydrogel materials yielded a specific capacitance of ∼480 F g−1 at a current density of 0.2 A g−1. In addition, the PANI hydrogel based electrodes exhibited excellent rate performance with only ∼7% capacitance loss when the current density was increased by a factor of 10 (∼450 F g−1 at 0.5 A g−1 decreased to ∼420 F g−1 at 5 A g−1), indicating an exceptional rate capability for high power performance (Fig. 6b and c). The high rate performance can be attributed to the facile electronic
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Fig. 6 (a) TEM image showing the nanostructured network of the dehydrated PANI hydrogel. The white arrows denote the micron sized pores in the PANI hydrogel. (b) Galvanostatic discharge profiles at various current densities (0.5–5 A g−1) of PANI hydrogel based electrodes. (c) Summary plot of specific capacitance values vs. current density for PANI hydrogel-based electrodes. (Inset) Cycling curve at a high current rate of 5 A g−1. Reproduced with permission from ref. 7. Copyright 2012 Proceedings of the National Academy of Sciences of the United States of America. (d) CV curves of the PPy hydrogel based supercapacitor under different bending conditions at a scan rate of 100 mV s−1. (e) Specific capacitance of the full cell versus current densities. (f ) Cycling performance over 3000 charge–discharge cycles under a bending curvature of 3 mm. Reproduced with permission from ref. 38. Copyright 2014 Royal Society of Chemistry.
and ionic transport stemming from the hierarchically conductive network. Moreover, compared to bulk films of electroactive polymers that suffer from limited cyclability due to the swelling and shrinking of polymer chains during charging and discharging processes, PANI hydrogel-based electrodes showed good cycling stability, which is another key requirement in the operation of supercapacitors. The hydrogel electrodes retained as high as ∼91% and ∼83% of initial capacitance over 5000 cycles and 10 000 cycles respectively at a high current density of 5 A g−1, which can be attributed to the highly porous interconnected nanostructures in the conducting polymer hydrogels that can accommodate the swelling and shrinking of the polymer network during cycling processes. In addition, the PPy hydrogel is also demonstrated to be a promising material for EC application. Recently, Yu et al. successfully synthesized the 3D nanostructured conductive PPy hydrogels with structure-derived elasticity by an organic/ aqueous biphasic interfacial synthesis.38 This strategy yielded PPy hydrogels with tunable pore size by controlling the ratio of crosslinker ( phytic acid) to pyrrole monomers. Similar to PANI hydrogel based electrodes, the electrodes fabricated by PPy hydrogels show high electrochemical performance with a specific capacitance of 380 F g−1, excellent rate capability, and areal capacitance as high as 6.4 F cm−2 at a mass loading of 20 mg cm−2 (Fig. 6e and f ). More importantly, the unique 3D porous nanostructure composed of interconnected polymer nanospheres endowed the hydrogel based electrodes exceptional mechanical properties, making it possible to fabricate a highly flexible energy storage device. An all solid-state supercapacitor was fabricated by assembling two pieces of dry PPy
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hydrogel electrodes sandwiched within a PVA–H2SO4 gel-like electrolyte. The electrochemical tests of solid-state supercapacitors under different bending conditions showed that the encircled areas (i.e. storage capacitance) in the closed CV curves remain almost the same as the curvature of the supercapacitor increases (Fig. 6d) and the capacitance change is still negligible under extensive bending. This remarkable flexibility can be explained by the fact that the deformation of the PPy backbone during bending could be accommodated by the porous space enclosed in the PPy network and the PPy hydrogel can strongly adhere to the current collector. The 3D nanostructured CPHs possess a highly porous microstructure, a large surface area, excellent compatibility with other hydrophilic molecules (e.g. electrolyte solution), and tunable mechanical properties originated from the crosslinking structure and swelling nature, thus becoming a promising material for ECs. However, side reactions accompanied with a large surface area can’t be eliminated completely and the dopants may negatively affect the chemical stability, hence the performance of hydrogel based electrodes. Thus new approaches involving surface chemical modification or molecular-level modification which passivates the surface to avoid undesirable reactions need to be developed. In addition, combining CPHs with other functional materials such as carbon based materials could take advantage of the synergistic effects in which CPHs provide superior pseudocapacitance, while the other components act as a framework that helps sustain from large strains in the charging/discharging cycling process, thus becoming another potential pathway to further improve the performance of EC electrodes based on CPHs.
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3.2 CPHs as functional binders in high-energy Li-ion batteries As an important type of secondary battery, Li-ion batteries have dominated the market for consumer electronics as energy storage devices due to their high storage capacity, high efficiency, lightweight, and portability.42 Pure conductive polymers had been explored as electrode materials since 1980s and a comprehensive review has been conducted by Haas et al.15 Conductive polymers may serve as both anodic and cathodic materials, however, they are commonly used as a cathode in Li-ion batteries. Conductive polymers show several attractive features such as good processability, low cost, tunable molecular modification, and lightweight when applied as electrodes. However, poor stability during cycling and low conductivity at reduced states significantly inhibit their further promotion in Li-ion batteries. In addition to functioning as active energy storage electrodes, conductive polymers can serve as binder materials in Li-ion batteries. An efficient binder system is essential to maintain the mechanical and electrical integrity of electrodes in energy devices.43 In a traditional system, polymer binders such as polyvinylidene fluoride (PVDF) adhere the active electrode particles and other additives together to hold the mechanical integrity while conductive additives ensure the conductivity of the entire electrode. However, this classic binder system can’t work well for next-generation ultrahigh-capacity electrodes which typically involve large volume/structural change during the lithiation/ delithiation process and would delaminate the polymer layer coated on the surface of active materials.44,45 And conductive additives tend to aggregate during volume expansion, resulting in the destruction of electronic connections. Therefore, an ideal
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binder system for ultrahigh-capacity electrodes should have the following characteristics: (1) inherent electronic conductivity in a battery operation environment, (2) mechanical adhesion and ductility with tolerance of large volume change, and (3) electrolyte uptake to warrant high ionic conductivity.46 Conductive polymers have been studied and applied as the binder in high-capacity electrodes due to their dual functionalities as a mechanical binder and a conductive additive. To enhance the performance of conductive polymers, Liu et al.46 developed a systematic way combining chemical synthesis, quantum calculation, spectroscopic, and mechanical testing tools to tailor the conductive polymer from the molecular level. Based on the designed electronic and mechanical properties, the tailored polymer binder achieves both high conductivity for electron conduction and mechanical integrity. Apart from molecular tailoring of conductive polymers, controlling the micro/nanostructure and electrochemical interface between electrodes and electrolytes is another efficient way to enhance their performance when acting as electrode binders, especially for the CPHs with 3D nanostructures due to their unique combination of properties of large surface area, short conduction path for electrons, high porosity for ionic diffusion and good compatibility with active materials. Recently, Yu et al. adopted in situ polymerization of PANI hydrogels, to form a bi-functional conformal coating that binds to the Si surface and also serves as a continuous 3D pathway for electron conduction.47 The synthesis could be conducted by chemically mixing two solutions, with one containing the initiator and the other containing the monomer and the crosslinker ( phytic acid) which can potentially bind with the SiO2 on the Si particle surfaces via hydrogen bonding and electrostatic interaction (Fig. 7a). Moreover the doping by the
Fig. 7 (a) Schematic of porous Si nanoparticle/conducting polymer hydrogel composite electrodes. Each Si nanoparticle is encapsulated within a conductive polymer surface coating and is further connected to the highly porous hydrogel framework. (b) Electrochemical cycling of the in situ polymerized Si particle–PANI composite electrodes at a charge/discharge current of 1.0 A g−1. Reproduced with permission from ref. 47. Copyright 2013 Nature Publishing Group. (c) Schematic of flexible electrodes using the composite of active nanoparticles, CNT and PEDOT:PSS. (d) Photograph of a flexible TiO2–PEDOT:PSS–CNT film. (e) Cycling stability of a thick Si–PEDOT:PSS–CNT electrode at 0.1C rate. Reproduced with permission from ref. 49. Copyright 2014 WILEY-VCH Verlag GmbH & Co.
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phytic acid makes PANI positively charged, thus generating electrostatic interactions with the negatively charged surface oxide, which is also thought to contribute to the improved cycle lifetime. There are several key design features for these unique Si– CPH hybrid electrode systems. First, the porous hydrogel matrix has a large empty space to permit the large volume expansion of the Si particles during the lithiation process. Second, the conductive and continuous 3D framework of the CPH, as well as the conformal conductive coating surrounding Si particles, helps provide a good electrical connection to the particles. Third, when compared to other 3D nanostructures such as metals and carbon which act simply as conductive additives, the conducting hydrogels enable multiple functions including coating, conductive binder and surface modification. Therefore the 3D nanostructured CPH reduces the weight fraction of the binder and the conductive filler of a battery and improves the SEI interface between the electrode and electrolyte. The capacity of a Si nanoparticle–PANI composite electrode varies from 2500 mA h g−1 to 1100 mA h g−1 when the charge/discharge rate changes from 0.3 to 3.0 A g−1. Most impressively, the Si–PANI hybrid electrode exhibits superior cycling performance (Fig. 7b). At a high current density of 6.0 A g−1, ∼91% of electrode capacity was still retained after 5000 cycles. Yu et al. further extended the conductive polymer based electrode to a ternary system which chemically integrates PPy, Si nanoparticles and carbon nanotubes (CNTs).48 The 3D ternary electrode based on Si/PPy/CNT was achieved by mixing the Si nanoparticles and CNTs with the pre-obtained hydrogel precursor that was synthesized by adding pyrrole and phytic acid. The hierarchically designed 3D nanostructured ternary electrode exhibited a remarkable reversible capacity of ∼1600 mA h g−1 and an average Coulombic efficiency of ∼99.5% over 1000 cycles. Moreover, the capacity retention sustained over 85% after 1000 deep charge/discharge cycles. The incorporation of CNT additives greatly enhanced the electron transport/conduction capability of the acid-doped conductive PPy framework by improving both the physical connections and electrical contact of Si nanoparticles with the external 3D conductive framework, thereby maximizing the effective electrochemical utilization of the active materials and ensuring a reversible lithium insertion/extraction process even at high current rates. This promising material design and the concept of the scalable synthesis method are expected to be useful for other alloy-type anodes such as germanium, tin, and tin oxides. A similar strategy was also applied by Bao et al. recently.49 To address the problem that most electrochemically active conductive polymers suffer from mechanical brittleness, low capacity (
FEATURE ARTICLE
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Nanostructured conducting polymer hydrogels for energy storage applications Ye Shi,† Lele Peng† and Guihua Yu* Conducting polymer hydrogels are emerging as a promising class of polymeric materials for various technological applications, especially for energy storage devices due to their unique combination of advantageous features of conventional polymers and organic conductors. To overcome the drawbacks of conventional synthesis, new synthetic routes in which acid molecules are adopted as both crosslinkers and dopants have been developed for conducting polymer hydrogels with unique 3D hierarchical porous nanostructures, resulting in high electrical conductivity, large surface area, structural tunability and hier-
Received 23rd May 2015, Accepted 25th June 2015 DOI: 10.1039/c5nr03403e www.rsc.org/nanoscale
1.
archical porosity for rapid mass/charge transport. The newly developed conducting polymer hydrogels exhibit high performance when applied as active electrode materials for electrochemical capacitors or as functional binder materials for high-energy lithium-ion batteries. This feature article summarizes the synthesis of conducting polymer hydrogels, presents their applications in energy storage, and discusses further opportunities and challenges.
Introduction
Nanostructured materials have become critically important in a wide range of applications from renewable energy, electronics, and photonics to medical and life science, because of their unusual physical/chemical properties due to the confined dimensions of such materials.1–4 They are promising to
Materials Science and Engineering Program and Department of Mechanical Engineering, The University of Texas at Austin, Austin, TX 78712, USA. E-mail: [email protected] † These authors contributed equally.
Ye Shi
Ye Shi is a Materials Science graduate student at the University of Texas at Austin. He received his B.S. and M.S. degrees in Polymer Science and Engineering at Zhejiang University. In fall 2013, he began graduate studies with Prof. Guihua Yu at University of Texas at Austin, focusing on rational synthesis, modification and tunable properties of conductive polymer nanomaterials and their applications in energy storage and conversion devices and other functional devices/systems.
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address critical challenges faced by many energy storage technologies owing to their advantageous features, such as larger electrochemically active surface area, shortened pathways for charge/mass transport, and better accommodation of the strain within electrodes. Among various nanostructured materials, hydrogels with highly cross-linked networks of polymer chains are of particular interest because they present three-dimensional (3D) structures at both microscale and nanometer levels. Different from inorganic nanomaterials, hydrogels exhibit mechanical properties ranging from soft and weak to hard and tough, owing to their unique composition with large quantity of liquid dispersed within a solid.5 Due to
Lele Peng
Lele Peng received his B.S. degree in Materials Science and Engineering from the University of Science and Technology of China (USTC) in 2012. He is currently pursuing his PhD in Materials Science and Engineering at the University of Texas at Austin under the supervision of Prof. Guihua Yu. His current interests include the synthesis and characterization of 2D nanomaterials and their hybrids for energy storage and conversion, especially for lithium ion batteries and high performance supercapacitors.
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its crosslinking structure and swelling nature,6 a hydrogel usually possesses a highly porous microstructure, high surface area, good compatibility with bio- or other hydrophilic molecules, and tunable mechanical properties, thus becoming an ideal material for diverse applications in energy, electronics, and medical/life science.7,8 Conductive polymers usually possess highly π-conjugated polymeric chains. Typical conductive polymers include polyacetylene (PA), polyaniline (PANI), polypyrrole (PPy), polythiophene (PTh), poly( phenylenevinylene) (PPV), etc. (Fig. 1b). The conductivity of conductive polymers could be tuned in a wide range from 10−10 up to 104 S cm−1 through chemical or electro-
Fig. 1 (a) Representative applications of conducting polymer hydrogels (CPHs). (b) The chemical structures of typical conductive polymers.
Dr Guihua Yu is an Assistant Professor of Materials Science and Engineering at the University of Texas at Austin. He received his B.S. degree with the highest honor in chemistry from the University of Science and Technology of China, earned PhD in chemistry at Harvard University in 2009, followed by postdoc at Stanford University. His research has been focused on rational synthesis and selfDr Guihua Yu assembly of functional organic nanostructures and two-dimensional nanostructured solids for advanced energy technologies, and fundamental understanding of their structure–property–performance relationship.
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chemical redox reactions which is called “doping”,9 endowing the conductive polymers the ability to act as insulators, semiconductors or conductors by changing the dopants and/or the level of doping.10–13 In addition, doping is a reversible process which makes the backbone of conductive polymers positively ( p-doping) or negatively charged (n-doping), enabling a number of device applications such as energy storage and conversion devices, sensors, and actuators.14 Conductive polymers can also be rationally designed and fabricated to nanostructures, thus providing new exciting features including tunable conductivities, flexibility, and mixed conductive mechanism15 which lowers the interfacial impedance between electrodes and electrolytes. As a special class of conductive polymer materials, conducting polymer hydrogels (CPHs) synergize the advantages of hydrogels and organic conductors.16 CPHs with 3D network structures provide an electrically conductive yet mechanically robust framework:7 the continuous conductive backbones can promote charge transportation,17 the porous structure facilitates the diffusion of ions and molecules, and the swelling nature offers additional effective interface between molecular chains and solution phases. Compared with their bulk form, CPHs also exhibit other unique properties such as improved mechanical properties due to effective accommodation of the strain caused by electrochemical reaction, lightweight and good processability.18,19 As a result, CPHs have been explored for wide-ranging applications such as energy conversion and storage, sensors, actuators, medical and bio-devices, and superhydrophobic coatings (Fig. 1).20–23 In this feature article, the synthetic approaches and fabrication strategies for CPHs are presented, followed by a detailed discussion on their energy storage applications, including supercapacitors and Li-ion batteries, as nanostructured CPHs provide large effective electrochemical interface with electrolyte ions,24 which could promote highly efficient ion and electron transport and electrochemical processes.25 This article will also comment on the current limitations and perspectives for further developing energy storage applications of CPHs.
2. Synthesis and processing of CPHs The polymerization of conductive polymers typically involves an electrochemical process of oxidation, a chemical process of coupling and eliminating protons.10 In a typical polymerization process, first, a radical cation is generated by the oxidation of one monomer and then couples with another radical cation to form a dimer after the loss of two protons. The oxidation step could be induced by various methods such as chemical oxidation, electrochemical process, and photochemical initiation. The dimer could be further oxidized and coupled with radical cations to form oligomers. Following the sequence of oxidation, coupling, and deprotonation, this propagation continues until the polymer is finally obtained.26
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Conventional synthetic methods for CPHs
Traditional synthetic routes include polymerization of monomers in an existing non-conductive hydrogel matrix which serves as template, copolymerization of conductive polymers with non-conductive polymers, and crosslinking conductive polymers such as poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) by multivalent metal ions (Fe3+ or Mg2+).16 The hydrogel-template method is most commonly used (Fig. 2a). Typically, a hydrogel template matrix is first prepared and de-swollen, and then re-swollen in the solution of monomers for conductive polymer synthesis which is in situ polymerized by electrochemical polymerization or by adding chemical oxidants. PANI or PPy–PNIPAM [ poly(N-isopropylacrylamide)],27 PPy–pHEMA [ poly(2-hydroxyethyl methacrylate)],28 PEDOT–alginate,29 PEDOT–PAA ( polyacrylic acid),30 PEDOT–PAMPS [poly(2-acrylamido-2-methyl-1-propanesulfonic acid)]31 and PPy–PAAM ( polyacrylamide)32 have been successfully synthesized via this strategy. To increase the surface area, a micro- or nanostructured template would be further applied.33 For example, poly(lactide-co-glycolide) (PLGA) nanofibers were adopted as templates to electrochemically deposit conductive polymers, thus forming crosslinked CPHs (Fig. 2b). However, the limited scalability of template-guided synthesis inhibits its further application. Another method to fabricate a conducting polymer hydrogel is the copolymerization of monomers for conductive polymers with other monomers for nonconductive polymers. Non-conductive polymers can act as a component of main chains of copolymers or as a crosslinking part to crosslink conductive counterparts. In this route, both non-conductive hydrogels and conductive polymer precursors
Fig. 2 (a) Conventional synthetic methods for CPHs, in which monomers of conductive polymers are polymerized onto non-conductive hydrogel matrix templates. Reproduced with permission from ref. 16. Copyright 2013 Royal Society of Chemistry. (b) SEM images of poly(lactide-co-glycolide) (PLGA) nanoscale fibers used as the template (top panel) and PEDOT hydrogels synthesized based on the PLGA template (bottom panel). Reproduced with permission from ref. 33. Copyright 2006 WILEY-VCH Verlag GmbH & Co.
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are placed together and polymerized either simultaneously or in a two-step process by chemical oxidation or electrochemical polymerization. Through this method, a PPy–pHEMA hybrid hydrogel is synthesized and its physical and chemical properties could be tuned by further adding other acrylate, methacrylate and acrylamide monomers.34 CPHs synthesized by the conventional method consist of both conductive and nonconductive components, and may lead to deteriorated electrical properties over time. Moreover, excessive metal ions adopted in the polymerization may reduce the compatibility of hydrogels with other molecules. In terms of potential scalability, the conventional synthetic methods discussed above have significant limitations to overcome. 2.2
Newly developed methods for CPH synthesis
A new synthetic route for conductive hydrogels which exhibits facile processability, excellent electronic properties, high electrochemical activity and high biocompatibility (Fig. 3a and b) has recently been reported.7 Phytic acid is adopted as the gelator and dopant in the synthetic process and conductive hydrogel networks could be directly formed. PANI and PPy hydrogels have been successfully synthesized via this method. The gelation mechanism of the CPH could be explained by the ability of each phytic acid molecule to interact with more than one conductive polymer chain. The gelation is typically completed within several minutes and can be induced in a wide range of molar ratios of monomer to phytic acid, and with diverse oxidative initiators such as ammonium persulphate (APS) and hydrogen peroxide. Moreover, the reaction could be conducted under biphasic conditions.35 The SEM images (Fig. 3c) show that the dehydrated PANI hydrogels are constructed with coral-like dendritic nanofibers with uniform diameters of 60–100 nm while the fully swollen PANI hydrogels are demonstrated by an AFM image (Fig. 3d) to consist of nanofibers of about 200–300 nm in diameter, showing their high level of hydration. In addition, the phytic acid renders CPHs a significant hydrophilicity owing to an excess of phosphorus groups (Fig. 3e). The swelling nature and hydrophilic properties of CPHs may provide additional interface areas between polymer chains and solution phases to support more active reaction sites and anchoring sites for active molecules or particles. The framework of the as-prepared CPHs is free of insulating polymers, thus providing an ideal 3D interconnected path for electron transport.36,37 The obtained PANI hydrogel reaches a conductivity of 0.11 S cm−1 at room temperature, which is the highest reported value for conducting polymer hydrogels (typically in the range of 10−4– 10−2 S cm−1 reported in the literature). A highly porous 3D nanostructure with hierarchical porosity (the first level is the 100 nm scale pores between the branched nanofibers and the second level is the bigger micron sized pores marked by the white arrows in Fig. 3c) can be achieved on a large-scale and with long-term stability in this synthesis. Such 3D porous structures offer large open channels on the microscale and mesoscale pores which facilitate the diffusion of ions and
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Fig. 3 (a) Schematic illustration of the 3D hierarchical microstructure of the gelated PANI hydrogel where phytic acid plays the role as a dopant and a crosslinker. (b) A photograph of the PANI hydrogel. (c) A SEM image showing the interconnected network of dendritic PANI nanofibers. (d) AFM image of the PANI hydrogel. (e) Contact angle of phytic acid doped PANI. (f ) Nitrogen adsorption–desorption isotherm of the dehydrated PANI hydrogel. Reproduced with permission from ref. 7. Copyright 2012 Proceedings of the National Academy of Sciences of the United States of America.
molecules and also provide a large specific surface area which is measured to be ∼42 m2 g−1 and could be tuned from 40 to 100 m2 g−1 (Fig. 3f ). Moreover, the synthetic route needs no external ingredients such as surfactants or templates, thus greatly enhancing its processability and universality. Adopting phytic acid as a crosslinker and a dopant, another synthetic approach based on multiphase interfacial reactions could be applied for CPHs (Fig. 4a).35 In a typical synthesis, an aqueous solution of an oxidative reagent and another solution containing a mixture of the monomer, organic solvent and phytic acid are prepared and then mixed together. The monomers would polymerize at the interface, resulting in hollowsphere microstructures (Fig. 4b and c) which are further interconnected by the crosslinker. In this method, the microstructure of the obtained CPHs could be simply tuned by adjusting the ratio of monomer to crosslinker, as well as different organic solvents, which finally leads to tunable mechanical and electrochemical properties of CPHs. For example, by forming an interconnected hollow-sphere microstructure, inherently stiff and brittle PPy based CPHs with a rigid conjugated-ring backbone can exhibit a tunable effective elastic modulus that are capable of withstanding large effective strains (Fig. 4d). Pan et al. synthesized PPy-based CPHs which consist of interconnected hollow spheres with polydisperse diameters ranging from hundreds of nanometres to several microns.35 The hollow-sphere structure brings the PPy foam structural elasticity, enabling an ultrasensitive pressure sensor. The diameters, size dispersions and shell thicknesses can be adjusted by using different organic solvents, including 1-butanol, t-butanol, s-butanol and t-amyl alcohol. The micro-
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Fig. 4 (a) Schematic of the structural elasticity of the hollow-spherestructured PPy. (b) TEM image of PPy showing its interconnected hollow-sphere structure, scale bar: 1 μm. (c) SEM images of the PPy hydrogel with a pyrrole : phytic acid ratio of 5 : 1; (d) six consecutive compression tests on the PPy thin film. Reproduced with permission from ref. 35. Copyright 2013 Nature Publishing Group.
structure could be also tuned by changing the ratio of monomer to phytic acid. Yu et al. found that the PPy hydrogel turns from the hollow-sphere dominant structure to the particle-dominant one when the concentration of the crosslinker is significantly lowered. The PPy hydrogels resulting from
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3. Energy storage applications of CPHs
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3.1
Fig. 5 Micropatterned PANI hydrogel arrays fabricated by (a) ink-jet printing and (b) spray coating. (c) Photograph of a microelectrode array based on PEDOT on a flexible gel substrate. Reproduced with permission from ref. 39. Copyright 2010 American Chemical Society.
different Py : phytic acid ratios also show different electrochemical properties.38 Owing to their facile synthesis and tunable mechanical properties, the CPHs show high scalability and processability that they could be processed by scalable techniques such as ink-jet printing or spray coating, to fabricate the desired micropatterns for large arrays of electrochemical devices (Fig. 5).39 Ink-jet printing of the polymer is often hampered by the limited solubility and high viscosity of polymer solutions. This difficulty is overcome by separately depositing two distinct solutions which contain an oxidative initiator and phytic acid, and monomers for conductive polymer synthesis respectively onto the substrate. The patterned PANI hydrogel is formed when two solutions are able to interact and the morphology of the printed hydrogel is found to be the same as that of the bulk synthesized hydrogel. The conductive hydrogel can also be processed by spray coating to form micropatterns of millimetre size. Similar to ink-jet printing, the two solutions are alternatively deposited multiple times through poly(dimethylsiloxane) (PDMS) soft stencil masks onto a range of substrates. Through these approaches we can fabricate large arrays of patterned CPHs, leading to highly conductive, functional microelectrodes potentially useful for supercapacitors, lithium batteries, biosensors, chemical sensors, and other bioelectrodes. These newly developed synthetic methods for nanostructured CPHs offer the ability to rationally design and control the morphology and microstructure of conductive polymers, as well as tuning their mechanical and electrochemical properties. With these synthetically tunable chemical/physical characteristics, the newly developed CPHs possess the possibility to meet requirements in various applications ranging from functioning as active electrode materials to serving as other functional components (e.g. as functional binder materials), thus becoming important candidates for energy storage devices, which is detailed in the following sections.
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CPHs as active electrodes for electrochemical capacitors
As designed to take advantage of the near-surface charge storage mechanism, electrochemical capacitors (ECs), often called ‘supercapacitors’, show high power density and can store and release the energy within the time frame of a few seconds,40 thus being projected as future power sources for high-power applications such as electric vehicles, utility loadlevelling, as well as back-up power for computer memory, lasers, and pulsed-light generators. Conductive polymers, such as PANI, PPy, and their derivatives, have been widely investigated for EC applications as pseudocapacitive materials, owing to their fast and reversible redox reactions at the electrode surface.41 These π-conjugated polymers have shown high gravimetric and volumetric pseudocapacitance in various non-aqueous electrolytes with operating voltages of ∼3–4 V. Their synergized properties from both traditional polymers and organic conductors can potentially meet the requirements for flexible, lightweight, and environmentally friendly devices. However, when used as bulk materials, conductive polymers suffer from a limited cycling stability due to structural change which leads to the fast decay of their electrochemical performance. Since EC systems involve ionic and electronic transport processes at the electrode surface and the electrode/electrolyte interfaces, a highperformance EC electrode requires high electrical conductivity, high ion-accessible surface and interface area, fast ionic transport and good electrochemical stability. Nanostructuring conductive polymers is found to be an effective approach to improve the performance of conductive polymers in EC systems. CPHs are emerging as a promising class of materials for EC applications because their 3D hierarchical nanostructure can provide an excellent interface for electrolyte ions to access the electroactive surface of CPHs and an intrinsically conducting and robust framework that promotes the charge/mass transport, thus improving the electrochemical usage of active materials. The 3D nanostructured PANI hydrogels have been demonstrated as high-performance EC electrodes recently.7 The PANI hydrogel electrode shows remarkably small charge transfer resistance, suggesting favorable ion transport within the 3D continuous framework (Fig. 6a). The excellent electrochemical activity of this hydrogel based electrodes was demonstrated by galvanostatic charge–discharge measurements in which the active PANI hydrogel materials yielded a specific capacitance of ∼480 F g−1 at a current density of 0.2 A g−1. In addition, the PANI hydrogel based electrodes exhibited excellent rate performance with only ∼7% capacitance loss when the current density was increased by a factor of 10 (∼450 F g−1 at 0.5 A g−1 decreased to ∼420 F g−1 at 5 A g−1), indicating an exceptional rate capability for high power performance (Fig. 6b and c). The high rate performance can be attributed to the facile electronic
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Fig. 6 (a) TEM image showing the nanostructured network of the dehydrated PANI hydrogel. The white arrows denote the micron sized pores in the PANI hydrogel. (b) Galvanostatic discharge profiles at various current densities (0.5–5 A g−1) of PANI hydrogel based electrodes. (c) Summary plot of specific capacitance values vs. current density for PANI hydrogel-based electrodes. (Inset) Cycling curve at a high current rate of 5 A g−1. Reproduced with permission from ref. 7. Copyright 2012 Proceedings of the National Academy of Sciences of the United States of America. (d) CV curves of the PPy hydrogel based supercapacitor under different bending conditions at a scan rate of 100 mV s−1. (e) Specific capacitance of the full cell versus current densities. (f ) Cycling performance over 3000 charge–discharge cycles under a bending curvature of 3 mm. Reproduced with permission from ref. 38. Copyright 2014 Royal Society of Chemistry.
and ionic transport stemming from the hierarchically conductive network. Moreover, compared to bulk films of electroactive polymers that suffer from limited cyclability due to the swelling and shrinking of polymer chains during charging and discharging processes, PANI hydrogel-based electrodes showed good cycling stability, which is another key requirement in the operation of supercapacitors. The hydrogel electrodes retained as high as ∼91% and ∼83% of initial capacitance over 5000 cycles and 10 000 cycles respectively at a high current density of 5 A g−1, which can be attributed to the highly porous interconnected nanostructures in the conducting polymer hydrogels that can accommodate the swelling and shrinking of the polymer network during cycling processes. In addition, the PPy hydrogel is also demonstrated to be a promising material for EC application. Recently, Yu et al. successfully synthesized the 3D nanostructured conductive PPy hydrogels with structure-derived elasticity by an organic/ aqueous biphasic interfacial synthesis.38 This strategy yielded PPy hydrogels with tunable pore size by controlling the ratio of crosslinker ( phytic acid) to pyrrole monomers. Similar to PANI hydrogel based electrodes, the electrodes fabricated by PPy hydrogels show high electrochemical performance with a specific capacitance of 380 F g−1, excellent rate capability, and areal capacitance as high as 6.4 F cm−2 at a mass loading of 20 mg cm−2 (Fig. 6e and f ). More importantly, the unique 3D porous nanostructure composed of interconnected polymer nanospheres endowed the hydrogel based electrodes exceptional mechanical properties, making it possible to fabricate a highly flexible energy storage device. An all solid-state supercapacitor was fabricated by assembling two pieces of dry PPy
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hydrogel electrodes sandwiched within a PVA–H2SO4 gel-like electrolyte. The electrochemical tests of solid-state supercapacitors under different bending conditions showed that the encircled areas (i.e. storage capacitance) in the closed CV curves remain almost the same as the curvature of the supercapacitor increases (Fig. 6d) and the capacitance change is still negligible under extensive bending. This remarkable flexibility can be explained by the fact that the deformation of the PPy backbone during bending could be accommodated by the porous space enclosed in the PPy network and the PPy hydrogel can strongly adhere to the current collector. The 3D nanostructured CPHs possess a highly porous microstructure, a large surface area, excellent compatibility with other hydrophilic molecules (e.g. electrolyte solution), and tunable mechanical properties originated from the crosslinking structure and swelling nature, thus becoming a promising material for ECs. However, side reactions accompanied with a large surface area can’t be eliminated completely and the dopants may negatively affect the chemical stability, hence the performance of hydrogel based electrodes. Thus new approaches involving surface chemical modification or molecular-level modification which passivates the surface to avoid undesirable reactions need to be developed. In addition, combining CPHs with other functional materials such as carbon based materials could take advantage of the synergistic effects in which CPHs provide superior pseudocapacitance, while the other components act as a framework that helps sustain from large strains in the charging/discharging cycling process, thus becoming another potential pathway to further improve the performance of EC electrodes based on CPHs.
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3.2 CPHs as functional binders in high-energy Li-ion batteries As an important type of secondary battery, Li-ion batteries have dominated the market for consumer electronics as energy storage devices due to their high storage capacity, high efficiency, lightweight, and portability.42 Pure conductive polymers had been explored as electrode materials since 1980s and a comprehensive review has been conducted by Haas et al.15 Conductive polymers may serve as both anodic and cathodic materials, however, they are commonly used as a cathode in Li-ion batteries. Conductive polymers show several attractive features such as good processability, low cost, tunable molecular modification, and lightweight when applied as electrodes. However, poor stability during cycling and low conductivity at reduced states significantly inhibit their further promotion in Li-ion batteries. In addition to functioning as active energy storage electrodes, conductive polymers can serve as binder materials in Li-ion batteries. An efficient binder system is essential to maintain the mechanical and electrical integrity of electrodes in energy devices.43 In a traditional system, polymer binders such as polyvinylidene fluoride (PVDF) adhere the active electrode particles and other additives together to hold the mechanical integrity while conductive additives ensure the conductivity of the entire electrode. However, this classic binder system can’t work well for next-generation ultrahigh-capacity electrodes which typically involve large volume/structural change during the lithiation/ delithiation process and would delaminate the polymer layer coated on the surface of active materials.44,45 And conductive additives tend to aggregate during volume expansion, resulting in the destruction of electronic connections. Therefore, an ideal
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binder system for ultrahigh-capacity electrodes should have the following characteristics: (1) inherent electronic conductivity in a battery operation environment, (2) mechanical adhesion and ductility with tolerance of large volume change, and (3) electrolyte uptake to warrant high ionic conductivity.46 Conductive polymers have been studied and applied as the binder in high-capacity electrodes due to their dual functionalities as a mechanical binder and a conductive additive. To enhance the performance of conductive polymers, Liu et al.46 developed a systematic way combining chemical synthesis, quantum calculation, spectroscopic, and mechanical testing tools to tailor the conductive polymer from the molecular level. Based on the designed electronic and mechanical properties, the tailored polymer binder achieves both high conductivity for electron conduction and mechanical integrity. Apart from molecular tailoring of conductive polymers, controlling the micro/nanostructure and electrochemical interface between electrodes and electrolytes is another efficient way to enhance their performance when acting as electrode binders, especially for the CPHs with 3D nanostructures due to their unique combination of properties of large surface area, short conduction path for electrons, high porosity for ionic diffusion and good compatibility with active materials. Recently, Yu et al. adopted in situ polymerization of PANI hydrogels, to form a bi-functional conformal coating that binds to the Si surface and also serves as a continuous 3D pathway for electron conduction.47 The synthesis could be conducted by chemically mixing two solutions, with one containing the initiator and the other containing the monomer and the crosslinker ( phytic acid) which can potentially bind with the SiO2 on the Si particle surfaces via hydrogen bonding and electrostatic interaction (Fig. 7a). Moreover the doping by the
Fig. 7 (a) Schematic of porous Si nanoparticle/conducting polymer hydrogel composite electrodes. Each Si nanoparticle is encapsulated within a conductive polymer surface coating and is further connected to the highly porous hydrogel framework. (b) Electrochemical cycling of the in situ polymerized Si particle–PANI composite electrodes at a charge/discharge current of 1.0 A g−1. Reproduced with permission from ref. 47. Copyright 2013 Nature Publishing Group. (c) Schematic of flexible electrodes using the composite of active nanoparticles, CNT and PEDOT:PSS. (d) Photograph of a flexible TiO2–PEDOT:PSS–CNT film. (e) Cycling stability of a thick Si–PEDOT:PSS–CNT electrode at 0.1C rate. Reproduced with permission from ref. 49. Copyright 2014 WILEY-VCH Verlag GmbH & Co.
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phytic acid makes PANI positively charged, thus generating electrostatic interactions with the negatively charged surface oxide, which is also thought to contribute to the improved cycle lifetime. There are several key design features for these unique Si– CPH hybrid electrode systems. First, the porous hydrogel matrix has a large empty space to permit the large volume expansion of the Si particles during the lithiation process. Second, the conductive and continuous 3D framework of the CPH, as well as the conformal conductive coating surrounding Si particles, helps provide a good electrical connection to the particles. Third, when compared to other 3D nanostructures such as metals and carbon which act simply as conductive additives, the conducting hydrogels enable multiple functions including coating, conductive binder and surface modification. Therefore the 3D nanostructured CPH reduces the weight fraction of the binder and the conductive filler of a battery and improves the SEI interface between the electrode and electrolyte. The capacity of a Si nanoparticle–PANI composite electrode varies from 2500 mA h g−1 to 1100 mA h g−1 when the charge/discharge rate changes from 0.3 to 3.0 A g−1. Most impressively, the Si–PANI hybrid electrode exhibits superior cycling performance (Fig. 7b). At a high current density of 6.0 A g−1, ∼91% of electrode capacity was still retained after 5000 cycles. Yu et al. further extended the conductive polymer based electrode to a ternary system which chemically integrates PPy, Si nanoparticles and carbon nanotubes (CNTs).48 The 3D ternary electrode based on Si/PPy/CNT was achieved by mixing the Si nanoparticles and CNTs with the pre-obtained hydrogel precursor that was synthesized by adding pyrrole and phytic acid. The hierarchically designed 3D nanostructured ternary electrode exhibited a remarkable reversible capacity of ∼1600 mA h g−1 and an average Coulombic efficiency of ∼99.5% over 1000 cycles. Moreover, the capacity retention sustained over 85% after 1000 deep charge/discharge cycles. The incorporation of CNT additives greatly enhanced the electron transport/conduction capability of the acid-doped conductive PPy framework by improving both the physical connections and electrical contact of Si nanoparticles with the external 3D conductive framework, thereby maximizing the effective electrochemical utilization of the active materials and ensuring a reversible lithium insertion/extraction process even at high current rates. This promising material design and the concept of the scalable synthesis method are expected to be useful for other alloy-type anodes such as germanium, tin, and tin oxides. A similar strategy was also applied by Bao et al. recently.49 To address the problem that most electrochemically active conductive polymers suffer from mechanical brittleness, low capacity (
