Designing Hierarchically Nanostructured Conductive Polymer Gels for

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Perspective

Designing Hierarchically Nanostructured Conductive Polymer Gels for Electrochemical Energy Storage and Conversion Ye Shi, and Guihua Yu Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b04879 • Publication Date (Web): 18 Feb 2016 Downloaded from http://pubs.acs.org on February 20, 2016

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Chemistry of Materials

Designing Hierarchically Nanostructured Conductive Polymer Gels for Electrochemical Energy Storage and Conversion Ye Shi,† and Guihua Yu*,† †Materials Science and Engineering Program and Department of Mechanical Engineering, The University of Texas at Austin, Texas 78712, United States

ABSTRACT: Nanostructured conductive polymers have been widely researched for various applications such as energy storage and conversion, chemical/biological sensors, biomedical devices. Recently, novel synthetic methods which adopt doping molecules as crosslinker have been developed to prepare conductive polymer gels (CPGs) with crosslinked network and 3D hierarchically porous nanostructures. The CPGs, as well as their derived carbon frameworks, exhibit high electrical conductivity, large surface area, structural tunability and hierarchical porosity for rapid mass/charge transport, which contribute to their high performance when applied for energy storage and conversion devices. This Perspective highlights the key features of CPGs and their derived carbon frameworks, discuss their possibilities in terms of rational synthesis and energy-related applications, and prospects future directions for their technological development.

1. Introduction Since the invention of conductive polyacetylene in 1970s, conductive polymers have received considerable research interests from both academia and industry owing to their ability to offer tunable electrical conductivity while maintaining properties associated with conventional polymers, such as ease of synthesis and flexibility in processing.1-3 Till now, a variety of conductive polymers have been developed. The most typical conductive polymers include polyacetylene (PA), polyaniline (PANI), polypyrrole (PPy), polythiophene (PTh), poly(phenylenevinylene) (PPV), etc. The intrinsic conductivity of conductive polymer results from their πconjugated chains and “doping” process which involves chemical or electrochemical redox reactions and protonation.4 The conductivity of conductive polymers is associated with the molecular structures of materials,5 the level of doping, as well as the ordering of molecular packing, thus offering a wide range of tunability.6-9 Recently, rapid advancement in nanoscale science and technology promoted the development of nanostructured conductive polymers.10-12 Because of the unusual physical/chemical properties associated with confined dimensions of nano-scale structure13-15 and conjugated polymeric chains, nanostructured conductive polymers have been exploited for a range of applications from energy conversion and storage, electronics, biological and chemical sensors to medical science.9, 16-21 Among diverse nanostructures, conductive polymer gels (CPGs) with threedimensional (3D) hierarchical structures constructed by

highly cross-linked networks of polymer chains are of particular interest since they possess hierarchically porous microstructure, high surface area (40-100 m2 g-1), exceptional compatibility with bio- or other hydrophilic molecules, and tunable chemical/physical properties (Figure 1).22-26 CPGs represent a promising class of polymeric materials for energy storage and conversion devices because they could provide an electrically conductive yet monolithic framework to promote transport of electrons,24, 27 offer micro- and meso-scale pores within the polymeric matrix to facilitate the diffusion of ions and molecules, as well as extending effective interface between molecular chains and electrolyte for redox reactions.28 The porous structure of CPGs can also help accommodate the strain caused by volume change during electrochemical reaction and offer the possibility to fabricate light-weight and flexible devices.12, 29 CPGs have recently been derived to carbon framework materials via thermal treatment and activation.30-32 The resulted carbon material has been demonstrated to be promising for energy storage and conversion owing to its high electrocatalytic activity. The derived carbon framework maintains the hierarchical nanostructure of CPGs, thus constructing a 3D carbon network for electron transport, providing pores for ion diffusion and large quantity of active sites for electrocatalytic reactions. In addition, the carbon framework could be doped by multi-elements such as N and P from the CPG precursor which could further improve the activity.

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by further adding other acrylate, methacrylate and acrylamide monomers.39 CPGs synthesized by conventional method consist of both conductive and nonconductive components, which may lead to deteriorated electrical properties over time. In terms of potential scalability, conventional synthetic methods discussed above have significant limitations to overcome since the scalability of template-guided synthesis is limited. 2.2 Synthetic methods for dopant molecules crosslinked CPGs

Figure 1. Schematic illustration of nanostructured CPGs and chemical structures of typical conductive polymers.

In this Perspective, we will introduce the dopant molecules crosslinked CPGs and their derived carbon framework synthesis, present the applications of CPGs for energy storage and conversion devices including lithium ion batteries, supercapacitors and electrocatalysts, discuss the possibilities and limitations of CPGs in terms of synthesis and applications, and explore future directions for the development of CPG.

2. Synthesis of CPGs and their derived carbon frameworks 2.1 Conventional synthetic methods for CPGs CPGs have been synthesized by polymerization of monomers in existing non-conductive hydrogel matrix, copolymerization of conductive polymer with nonconductive polymers, or crosslinking conductive polymers by multivalent metal ions (Fe3+ or Mg2+).33 In the first method, the non-conductive hydrogel serves as template and the monomer of conductive polymers is introduced during the re-swollen process of hydrogel template. Then the conductive part is polymerized electrochemically or by adding chemical oxidants. PPy-pHEMA [poly(2hydroxyethyl methacrylate)],34 PEDOT-alginate,35 36 PEDOT-PAA (polyacrylic acid), PEDOT-PAMPS [poly(2acrylamido-2-methyl-1-propanesulfonic acid)]37 and PPyPAAM (polyacrylamide)38 have been successfully synthesized via this strategy. CPGs could also be synthesized by copolymerization of monomers of conductive polymer and non-conductive polymers. Non-conductive polymers can act as component of main chains of copolymer or the crosslinking part to crosslink conductive counterparts. In this route, both non-conductive hydrogel and conductive polymer precursors are placed together and polymerized either simultaneously or in a two-step process by chemical oxidation or electrochemical polymerization. Through this method, PPy-pHEMA hybrid hydrogel is synthesized and its physical and chemical properties could be tuned

Like most hydrogels using crosslinkers to interconnect the polymer chains and build up a polymeric network, CPGs have recently been synthesized using a templatefree method in which dopant molecules with multifunctional groups were adopted as crosslinker (Figure 2A).24 Two typical crosslinkers are phytic acid and copper phthalocyanine-3,4',4’’,4’’’-tetrasulfonic acid tetrasodium salt (CuPcTs). Each of these molecules could interact with more than one conductive polymer chain by protonating the nitrogen groups, thus a mesh-like hydrogel network would be formed through this crosslinking effect. In a typical synthesis, two precursor solutions containing monomers, dopant molecules and oxidative initiator, respectively, are mixed together and the gelation is typically completed within several minutes (Figure 2B). It should be noted that the ratios of conductive polymer monomer to dopant molecules could be tuned in a wide range and diverse initiators such as ammonium cerous sulphate (APS) and hydrogen peroxide could be adopted. The morphology and structure of CPGs could be tuned via using different initiators. The surface area of CPGs could be tuned by adopting different polymer materials and adjusting the synthetic conditions such as monomer/crosslinker ratio, solvents, reaction time. The synthetic route also shows good processibility and universality since no external ingredients such as surfactants or templates are needed. PANI based CPGs were successfully synthesized by using phytic acid as the crosslinker and dopant. SEM images show the 3D porous foam morphology of the PANI hydrogel (Figure 2C). The foam-like nanostructures are constructed with coral-like dendritic nanofibers with uniform diameters of 60–100 nm. Two levels of pores could be found in the nanostructured PANI hydrogel. The first level was the pores between the branched nanofibers (the average gap size), and the second one was the micron size pores. Recently, PANI 3D networks were also synthesized using salicylic acid as the dopant via a slow chemical oxidation and self-assembly polymerization. The dopant enabled synthesis could be combined with other synthetic approaches for crosslinked gels with novel microstructures and structure-derived mechanical properties, as revealed by the successful demonstration of interfacial synthesis (Figure 2D and 2E).40 In such synthesis, the water in the solution with conductive polymer monomers and phytic acid is replaced by organic solvent and


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the polymerization would occur at the interface between two phases, resulting in interconnected hollow-sphere microstructure (Figure 2F). The microstructure parameters such as the diameters of micro spheres, size dispersions and shell thickness could be adjusted by using different organic solvents, as well as changing the ratio of polymer monomers to phytic acid, leading to tunable mechanical and chemical properties of obtained CPGs.41 Figure 2E shows a PPy CPG foam synthesized by the interfacial synthesis which exhibits greatly enhanced elasticity to withstand 100 g weight.

Figure 2. (A) Schematic of the 3D hierarchical microstructure of the gelated PANI hydrogel using phytic acid as dopant and crosslinker. (B) A photograph of the PANI hydrogel. (C) A SEM image showing the interconnected network of dendritic PANI nanofibers. Reprinted with permission from ref. 24. Copyright 2012 Proceedings of the National Academy of Sciences of the United States of America. (D) Schematic of interfacial synthesis of the hollow-sphere-structured PPy. Reprinted with permission from ref. 40. Copyright 2014 Nature Publishing Group. (E) A photograph shows that a spongy PPy gel sample placed under a balance weight of 100 g. (F) TEM image of PPy showing its interconnected hollow-sphere structure, scale bar: 1 μm. Reprinted with permission from ref. 41. Copyright 2014 Royal Society of Chemistry.

The molecular and geometric structures of dopant molecules also play an important role in determining microstructures, as well as electrochemical and mechanical properties of CPGs. Recently, PPy gel with interconnected fiber structure was synthesized using CuTcPs as dopant and crosslinker (Figure 3A). Controlled experiments and fundamental studies reveal that the formation of this nanostructure could be explained by supramolecular selfassembly mechanism in which electrostatic interaction and hydrogen bonding between the functional dopant molecules and polymer chains help align the polymer chains to form 1D nanostructure which further evolve to mesh-like network.42 Control PPy samples were also synthesized by using indigo carmine and indigo carmine dehydrate as the control dopants which have molecular structures similar to a half and a quarter of CuPcTs molecule, respectively (Figure 3B and 3C). The binaryfunctional indigo carmine led to PPy hydrogel consisted of ‘necklace-like’ 1D nanostructure with particles aligned

to form fibers while single functional isatin-5-sulfonic acid sodium salt led to a granular nanostructure with morphology similar to that of the PPy synthesized without dopants. The result demonstrates the self-sorting and steric effect of multi-functional dopant molecules on the nanostructures of PPy hydrogels. It also indicates the potential to tune the properties of CPGs by controlling the molecular structures of dopants. The electrical and electrochemical properties of CPGs are closely related to their microstructures and the dopants used in synthesis. The resulting CPGs are highly conductive since the framework is free of insulating polymers and the crosslinkers act as acid dopants as well. In addition, the crosslinking network builds up an ideal 3D interconnected path for electron transport.43, 44 The obtained PANI hydrogel reaches a record conductivity of 0.11 S cm-1 at room temperature by using phytic acid as the crosslinker while PPy hydrogel achieves even higher conductivity of 7.8 S cm-1 using CuPcTs as the dopant since CuPcTs is a good organic semiconductor (Figure 3D). The CPGs are also chemically and electrochemically active owing to the formation of hierarchically porous nanostructure with long-term stability. The 3D hierarchically porous nanostructures offer large open channels by containing both micro- and meso-scale pores, thus facilitating the diffusion of ions and molecules. The swelling nature of this polymeric network may provide additional interface between polymer chains and solution phase to support more active reaction sites and anchoring sites for active molecules, thus boosting the possibility of CPGs to be applied in energy storage and conversion devices.33, 45

Figure 3. (A-C) SEM images of different nanostructured PPy hydrogels with various dopants: (A) CuPcTs as dopant, (B) indigo carmine as dopant, and (C) isatin-5-sulfonic acid sodium salt dehydrate as dopant. Scale bar: 1 μm. (D) Conductivities of PPy CPGs synthesized by different dopants. Reprinted with permission from ref. 42. Copyright 2015 American Chemical Society.

In terms of potential scalability and processability, the newly developed CPGs could be processed by scalable techniques such as ink-jet printing or spray coating, making it possible to fabricate desired micropatterns for large arrays of devices.46 The precursor solutions could be separately deposited onto the substrate and gels will form in the area that both solutions overlapped, thus avoiding the



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block of inkjet nozzle by gelation and high viscosity of polymer solutions. The morphology of printed CPGs is found to be the same as that of bulk synthesized hydrogel. The synthetic parameters that are tunable in the synthesis of CPGs include but are not restricted to the solvents, the ratio of monomer to crosslinker, and the initiators. New crosslinkers with multi-functional groups which can react and gelate with conductive polymer chains are under exploiting. The properties of crosslinkers such as the molecular weight, steric effect, and rigidity of molecules play important roles in determining the properties of resulted CPGs. In the future research, the seeking of new crosslinkers will be an important and interesting direction and co-crosslinkers can be also added to diversify the networking structure of CPGs. 2.3 Synthesis of CPG derived carbon frameworks The dopant-enabled CPGs have recently been derived to carbon frameworks by thermal annealing process (Figure 4A). The CPGs with conjugated polymeric framework is an ideal precursor for carbon framework synthesis since they could be readily converted into porous carbon without using any sacrificial template. The dopant molecules play an important role in determining the final structure and properties of resulted carbon framework. The crosslinking molecules such as phytic acid with high degradation temperature could prevent pore collapsing during carbonization and in-situ formed organophosphates during carbonization of phytic acid could help retain lower molecular weight species, thus reaching a high carbon yield. In addition, elements such as N and P provided by PANI and phytic acid could act as dopants in the resulting carbon networks, making it possible to tune the chemical potential of graphitic carbon networks.30 The synthesis of highly porous graphitic carbon frameworks from CPGs usually involves three steps. The hydrogel is firstly converted into an aerogel by freeze drying, thus maintaining its hierarchically porous structure. Then a subsequent pyrolysis is applied for the conductive polymer aerogel leading to the formation of graphitic carbon networks. At last, a chemical activation process by mixing the graphitic carbon with KOH followed by a heat treatment could be used to further increase the porosity and surface area. It should be noted that the surface area and pore structure of the hydrogel-derived graphitic carbon could be tuned by adjusting the annealing temperature and activation conditions. Typical SEM and TEM images of obtained carbon framework are shown in Figure 4B and 4C.30

Figure 4. (A) Illustration of transformation of phytic acid cross-linked PANI (left) into doped graphene-like carbon sheets (right). Reprinted with permission from ref. 30. Copyright 2015 American Chemical Society. (B, C) SEM (B) and TEM (C) images of PANI derived carbon framework. Inset in B: Digital images of PANI aerogel before (left) and after (right) pyrolysis at 1,000 °C. Reprinted with permission from ref. 31. Copyright 2015 Nature Publishing Group. (D) Schematic illustration of shape fixing via salt recrystallization method. Reprinted with permission from ref. 32. Copyright 2015 American Chemical Society.

To better maintain the 3D network structure of CPGs, a strategy called “shape fixing via salt recrystallization” method was developed (Figure 4D).32 The 3D PANI network fabricated via a self-assembly process was mixed with supersaturated NaCl solution in a beaker. The water was then evaporated so that the NaCl recrystallized around the PANI on the bottom of the beaker. This cycle of the addition of NaCl solution, water evaporation, and NaCl recrystallization was repeated until the whole PANI was fully buried so that it appeared to be tightly sealed inside the NaCl crystal. Thus, the original shape of the 3D PANI network would be fixed by NaCl crystal. At last, the NaCl crystal-sealed PANI was subsequently thermal treated to form carbon framework.

3. Applications of CPGs in energy storage and conversion devices 3.1 CPGs as active electrodes for electrochemical capacitors Electrochemical capacitors (ECs), often called “supercapacitors”, have become a key member among electrochemical energy storage systems that utilize the near-


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surface charge storage. Supercapacitors can store and release energy within time frame of a few seconds, as well as showing exceptional cycle life, thus useful in a variety of applications ranging from consumer electronics, hybrid vehicles, to large industrial scale power and energy management.47, 48 Conductive polymers, such as PANI, PPy and their derivatives have been widely studied for supercapacitors as pseudocapacitive materials which take advantage of fast and reversible electron-exchange reactions at or near the electrode surface.49-53 These conjugated polymers show high gravimetric and volumetric pseudocapacitance in various non-aqueous electrolytes with operating voltage of ~3 V. With mechanical features similar to conventional polymers, conductive polymer based supercapacitors can be fabricated to flexible and lightweight devices. However, bulk conductive polymers suffer from poor cycle life and fast decayed capacitance in high rate cycling process, possibly due to high volume change during charge/discharge processes and decrease of conductivity caused by the change of doping states. To improve the performance of conductive polymers for EC applications, efforts need to be directed towards the modification of surface states and improvement of electrical conductivity, ionic transport rate and electrochemical stability of conductive polymers. The unique structure of CPGs establish a continuous network to promote the transport of electrons, provide short diffusion path for electrolyte ions to access the electroactive surface of polymeric matrix, and construct a hierarchically porous architecture with intrinsic elasticity to accommodate the volume change and facilitate the diffusion of electrolyte ions (Figure 5A). The excellent performance of CPGs for supercapacitor was proved by the electrochemical tests of PANI hydrogel in 1 M H2SO4 electrolyte.24 The PANI hydrogel exhibited a specific capacitance of ~480 F g-1 at a current density of 0.2 A g-1 and excellent rate performance with only ~7% capacitance loss when the current density was increased by a factor of 10. The exceptional rate capability for high power performance could be attributed to the enhanced electronic and ionic transport brought by the hierarchically porous structure. The porous structure which could accommodate the volume change caused by swelling and shrinking of polymer matrix also contributed to good cycling stability of PANI hydrogel that it could retain 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 (Figure 5B). The dopant molecules for crosslinking could also enhance the conductivity of the CPGs and contribute to improved electrochemical performance. The conductivities of CuPcTs doped PPy and pristine PPy were measured to be 7.8 and 0.07 S·cm−1. Owing to the higher conductivity, 1D morphology, and more porous structure, the specific capacitance of the CuPcTs doped PPy was calculated to be as high as ∼400 F·g−1 at 0.2 A·g−1, whereas that of pristine PPy was only 232 F· g−1 (Figure 5C and 5D).42

Figure 5. (A) TEM image showing the nanostructured network of PANI hydrogel. The white arrows denote the micron size pores in PANI hydrogel. (B) Summary plot of specific capacitance vs. current density for PANI hydrogel-based -1 electrodes. (Inset) Cycling curve at high current rate of 5 A g . Reprinted with permission from ref. 24. Copyright 2012 Proceedings of the National Academy of Sciences of the United States of America. (C) SEM image of different nanostructured PPy hydrogels with CuPcTs as dopant. (D) Specific capacitance vs. current density for CuPcTs−PPy and pristine PPy. Reprinted with permission from ref. 42. Copyright 2015 American Chemical Society. (E) CV curves of the PPy hydrogel based supercapacitor under diﬀerent bending conditions -1 at a scan rate of 100 mV s . (F) Speciﬁc capacitance of the full cell versus current densities. Reprinted with permission from ref. 41. Copyright 2014 Royal Society of Chemistry.

Combining the mechanical property of conventional polymers and electrical property of organic conductors, CPGs are emerging as a promising material for the fabrication of flexible supercapacitors. The possibility to tune the mechanical properties arising from synthetic routes such as interfacial synthesis further promoted flexible supercapacitor devices based on CPGs.54, 55 Recently, Yu et al.41 prepared PPy hydrogels with structure-derived elasticity by an organic/aqueous biphasic interfacial synthesis. The PPy hydrogel based electrodes not only showed high rate cycling performance, but also demonstrated successful application for highly flexible devices. An all solid-state supercapacitor was fabricated by assembling two pieces of dry PPy gel electrodes sandwiched within a PVA–H2SO4 gel-like electrolyte. The electrochemical tests of this solid-state supercapacitor at 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 and the capacitance change is still negligible even at a highly



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bending state (Figure 5E). This remarkable flexibility could be explained by the fact that deformation of the PPy backbone during bending could be largely accommodated by the porous space and the PPy hydrogel can strongly adhere to current collector. PPy CPG showed 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 (Figure 5F). The performance of CPGs based supercapacitors could be further improved. Accompanied with large surface area, undesired side reactions will potentially rise and the stability of dopants may be deteriorated during these reactions. To minimize these issues, the surface of CPGs could be chemically modified to passivate the surface and the molecular structure of polymeric chains should be tailored with functional side chains or copolymer sequence to adjust the chemical potential of CPGs, thus improving their stability during electrochemical reactions. With the porous structure and tunable mechanical properties, CPGs can act as an ideal matrix to hybridize with other organic or inorganic materials to take advantage of the synergistic effects in which CPGs provide pseudocapacitance, while the other components act as a framework that helps sustain from large strains in charging/discharging cycling process. For examples, interpenetrating networks could be constructed by introducing another gel network with desirable mechanical properties into CPG matrix and carbon based materials such like CNTs can be added to reinforce the CPG skeleton. With excellent compatibility with other functional molecules or particles, CPGs can also serve as the host for other electrochemically active materials, such as transition metal oxides particles to further enhance the energy density of supercapacitor devices. With these optimized material systems, the scaled-up fabrication of micro-patterned and highly flexible devices on various substrates will be promising for practical applications.

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3.2 CPGs as functional binder in high energy Li-ion batteries Lithium ion battery is a class of rechargeable batteries in which lithium ions move from the negative electrode to the positive electrode during discharging and back during charging. Compared to supercapacitor, lithium ion battery shows limitations on cycle life and charge/discharge rate, but possesses high storage capacity, high efficiency, relatively light weight and portability, thus having dominated the market of consumer electronics as energy sources.56-58 Conductive polymers could serve as both anodic and cathodic electrode materials in lithium ion battery through reversible interactions between lithium ions and polymer chains.59, 60 They have been studied for battery application since 1980s and a comprehensive review has been conducted by Haas et al.61 Although conductive polymers show several advantages such as good processability, low cost, and light weight when applied as electrodes, key issues such as poor stability and low conductivity at reduced states have not been solved for conductive polymers, thus inhibiting their further applications. Conductive polymers are more commonly used as modification materials for energy storage. The coating of conductive polymers could enhance the performance of other cathodes and anodes battery materials by improving their low intrinsic electronic conductivity, building up effective paths for electronic transport and Li ion diffusion, and enhancing electrochemical activity. However, modification by conductive polymer may also induce undesirable side reaction and require more complicated processing procedures. Some conductive polymers (such as PPy) are oxygen sensitive under discharged state and CPGs could be over-oxidated by excess charge. In Li-ion batteries, Li reacting with the electrolyte yields basic, nucleophilic species which in turn would react with the radical cations present in oxidized, conjugated polymers such as PAni and PPy. These undesirable side reactions may lead to the decrease of conductivity of CPGs and harm the performance of devices.62, 63

Figure 6. (A) Schematic of porous Si nanoparticle/conducting polymer gel composite electrodes. (B) Electrochemical cycling of -1 the in-situ polymerized Si particle-PANI composite electrodes at a charge/discharge current of 1.0 A g . Reprinted with permission from ref. 69. Copyright 2013 Nature Publishing Group. (C) Schematic of the formation of 3D Si/PPy/CNT ternary electrode. Reprinted with permission from ref. 70. Copyright 2013 American Chemical Society. (D) Schematic of flexible electrodes using


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composite of active nanoparticles, CNT and PEDOT:PSS. (E) Cycling stability of a thick Si-PEDOT:PSS-CNT electrode at 0.1C rate. Reprinted with permission from ref. 71. Copyright 2014 Wiley-VCH Verlag.

Another important role that conductive polymers may play is to serve as binder materials in lithium ion battery. Unlike conventional binder system in which polymeric materials and conductive carbon additives are needed to hold mechanical integrity and high conductivity of electrodes,64, 65 conductive polymers could play dual functions owing to their synergetic features of conventional polymers and organic conductors, thus avoiding the potential aggregation of conductive additives and detaching of polymer layer during cycling.66, 67 To improve the performance of conductive polymers for binder materials, several strategies have been developed. Liu et al. proposed a systematic way combining chemical synthesis, quantum calculation, spectroscopic, and mechanical testing tools to design unconventional conductive polymer.68 Based on the designed electronic and mechanical properties, the tailored polymer binder achieves both high conductivity for electron conduction and mechanical integrity. Another way is the adoption of conductive polymers with controlled micro/nano structure and modified electrochemical interface between electrode and electrolyte. CPGs have emerged as a suitable candidate due to their combination of properties such as large surface area, tunable mechanical property, high electrical conductivity, high porosity for ionic diffusion and good compatibility with active materials. Recently, a high-performance lithium ion battery anode consisting of Si nanoparticles and PANI gel nanoframework was developed (Figure 6A).69 PANI gel was insitu polymerized to form a bi-functional conformal coating which bond to surface of Si particles via hydrogen bonding and served as a continuous 3D pathway for electronic conduction. The capacity of this Si nanoparticlePANI composite electrode varies from 2,500 mAh g-1 to 1,100 mAh g-1 at charge/discharge rate from 0.3 to 3.0 A g-1 and retains ~91% after 5,000 cycles at a high current density of 6.0 A g-1 (Figure 6B). The superior performance could be explained by the following mechanism. First, the porous hydrogel matrix accommodates the large volume expansion of the Si particles during lithiation. Second, the conducting gels play multiple functions including coating, conductive binder and surface modification, thus reducing the weight fraction of the binder and enabling a deformable and stable SEI on the Si surface. Impedance measurements and SEM images of electrodes after cycling confirmed that a uniform and thin SEI formed on the electrode. Third, the electrostatic interaction between positively charged PANI due to doping of phytic acid and negatively charged surface oxide could also contribute to the improved cycling performance. The CPGs based binder system in lithium ion battery has been further extended to ternary system. Yu et al. showed the hierarchically designed 3D nanostructured ternary PPy-Si-CNT electrode (Figure 6C),70 exhibited a remarkable reversible capacity of ∼1600 mAh g-1 and capacity retention of over 85% after 1000 cycles. The incorporation of CNTs greatly enhanced 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. Similar strategy was applied by Bao et al.71 in which PEDOT:PSS hydrogel was in-situ synthesized in the solution containing super long CNTs and electrochemically active nanoparticles (Figure 6D). In the hybrid hydrogel system, CNT creates an ideal scaffold with high mechanical robustness and long-range conductivity. For two kinds of electrochemically active materials, TiO2 based electrodes achieved a capacity of 76 mAh g–1, while Si nanoparticle based electrodes reached a high areal capacity of 2.2 mAh cm–2 (Figure 6E). The gravimetric energy density and power density of lithium ion battery could be potentially enhanced since the loading of binder could be as low as 10 wt% of the electrode material and the porous structure facilitates the diffusion of electrolyte. In addition, the solution synthesis and electrode fabrication process are scalable and compatible with existing slurry coating technique in battery manufacturing industry, thus opening the door for CPGs based binder materials into the practical industry manufacturing. However, some issues still remain to be solved. First, the mechanical strength of CPGs is relatively poor due to their intrinsically rigid backbones. Second, the volumetric capacity of CPGs based electrodes is lower than that of traditional electrodes because the highly porous structure would lead to inferior packing. Third, selfdischarge may become serious due to undesirable reactions caused by increased surface area. At last, the conductivity of CPGs, especially at reduced states still needs to be enhanced for high rate applications. New strategies need to be developed in future research. The mass loading for Si-CPGs anode was relatively low, in the range of 0.3-0.5 mg/cm2, which is restricted by the mechanical strength of CPGs. Further progress is needed in improving the mass loading that requires design and synthesis of CPGs with better mechanical properties and some recent gel works have demonstrated that the mass loading could be enhanced to around 2 mg/cm2. Controlling microstructure and tailoring at the molecular level should be combined to enhance the mechanical and electrical properties of CPGs. The backbones of CPGs could be modified by side chains or co-polymerized with other polymer sequences and new crosslinkers/dopants need to be exploited. The molecular modification provides a powerful tool to tune the properties of CPGs while maintaining the interesting microstructures. Chemical modification should be applied on the surface of CPGs to passivate surface to avoid undesirable reactions, as well as improving the electrical and chemical contacts between polymer matrix and active materials. For example, the positively charged conductive polymers could be modified by negatively charged polyelectrolyte to enhance the interactions between polymer chains and metal particles. Beyond the modification at the molecular level, hybrid polymers



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could be prepared to counteract the drawbacks of CPGs. Hydrogels such as PVA, PEG with interconnected chains structure could help enhance the mechanical strength and elasticity, as well as providing abundant –OH groups, thus becoming potential candidates to develop interpenetrating double-network system with CPGs. 3.3 CPGs derived carbon framework for energy storage CPGs have been investigated as a platform to develop new material systems, such as porous graphitic carbons

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for energy storage devices. Porous graphitic carbons, such as 3D porous graphene network are an important class of carbon materials for wide ranging applications from energy storage, catalysts, to sorbents, owing to their high intrinsic electronic conductivity and large surface area.72, 73 Compared to traditional porous carbon materials such as activated carbons which require extensive purification to eliminate large amount of impurities and usage of templates to control pore size and pore connection, porous graphitic carbon frameworks show additional advantages of large pore volume and low cost.

Figure 7. (A) Supercapacitor fabricated by carbon framework on FET film, and specific capacitance vs. current density of supercapacitor electrodes made from different porous carbon framework. Reprinted with permission from ref. 30. Copyright 2015 American Chemical Society. (B) Schematic of the basic configuration of a primary Zn–air battery and their specific capacities normalized to the mass of the consumed Zn. Reprinted with permission from ref. 31. Copyright 2015 Nature Publishing Group. (C) Charge/discharge voltage profiles at a C/5 current rate and long-term cycling stability for carbon framework/polysulfide and KB/polysulfide electrode after equilibrium, respectively. The discharging curve starts with plateaus at 2.4 and 2.05 V, while the charging curve displayed overlapped plateaus starting from 2.4 V. Reprinted with permission from ref. 30. Copyright 2015 American Chemical Society.

CPGs can be used as an ideal precursor for the synthesis of 3D porous graphitic carbon networks. Compared to conventional methods such as random stacking of individual graphene sheets which suffers from severe aggregation and usage of graphene oxides as building blocks which requires strong oxidative and reductive chemicals, adoption of CPGs as precursor show advantages that their intrinsic 3D cross-linked and hierarchically porous architecture gives rise to large surface area and pore volume without deteriorating the pore connectivity and their facile synthesis is favorable for large-scale production. The as-synthesized highly porous graphitic carbon could reach surface area as high as 4073 m2 g-1, large pore volume of 2.26 cm-3, hierarchical pore architecture, high electrical conductivity and electrochemical activity, and good stability.29 Based on these unprecedented properties, energy storage devices with high performance have been developed by adopting the porous graphitic carbon as electrode materials. By directly spray coating carbon suspension based ink onto polyethylene terephthalate (PET)

sheets, flexible polyimide films, or silicon wafers, micropatterned supercapacitors could be prepared (Figure 7A). The relatively small carbon particle size offers scalability and high flexibility for processing and the interconnected fibers provide good mechanical flexibility. The devices show high performance in both aqueous and organic electrolyte. The highly porous graphitic carbon showed a capacitance of 225 and 162 F g-1 at a current density of 0.5 and 50 A g-1, respectively, in 0.5 M H2SO4. The N dopant may have contributed to the capacitance due to pseudocapacitive effect. In addition, high mass loading and highly stable cycling performance could be attained that the porous graphitic carbon electrodes can retain ~83% of the initial capacitance as mass loading increased from 1 to 11 mg cm-2 and reach capacitance retention of 95% after 10000 cycles at 5 A g-1.30 The porous graphitic carbon has also been applied for lithium-sulfur batteries (Figure 7C).30 The ultrahigh surface area and polar doping atoms provide more active sites for lithium sulfide deposition and the interconnect-


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ed framework could maintain conductive pathways, thus providing the porous graphitic carbon based battery with high cycling stability and high mass lading. The areal capacity reached as high as 4.2 mAh cm−2 and the electrodes can retain∼80% of initial capacity at C/5. The high specific capacity attained at high mass loading can be attributed to the effective hierarchically porous conductive architecture and the doping atoms of N and O for the strong LixS interaction that controls the formation of lithium sulfide species and maintains high active material utilization. In addition to lithium ion battery, Zn-air battery using porous graphitic carbon as air cathode has been developed by Dai et al31 (Figure 7B). Both primary and rechargeable Zn–air batteries using carbon framework as the electrocatalyst were developed. The open-circuit potential of the two-electrode primary Zn–air battery is as high as ∼1.48 V, suggesting a good catalytic performance of bifunctional electrocatalyst. The carbon framework could facilitate an efficient diffusion of oxygen gas and electrolyte to the active sites. The battery showed a current density of ~70 mA cm-2 and a peak power density of ~55 mW cm-2, as well as good rechargeability. 3.4 CPGs and their derived carbon framework for energy conversion The oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are crucial reactions for electrochemical power generators such as fuel cells, lithium-air battery, and generation of oxygen.54, 74-77 Conventional catalysts for ORR are Pt based materials and catalysts for OER usually base on transition metal oxides.31 These electrochemical catalysts are expensive and metal-dependent. Carbon materials doped with nitrogen have been demonstrated to be an efficient, low-cost, metal-free alternative to Pt for ORR. Co-doping N-doped carbon nanomaterials with a second heteroatom, such as B, S or P, can modulate the electronic properties and surface polarities to further increase ORR activity. However, no truly metal-free bifunctional ORR and OER catalyst has been reported for conventional carbon materials and the efficiency of carbon based catalysts needs to be further improved.

Figure 8. (A) Schematic of active sites on edges and in pores. (B) Steady-state plots of ORR polarization. Reprinted with permission from ref. 32. Copyright 2015 American Chemical

Society. (C) RRDE measurements of ORR at an N and P codoped carbon framework electrode with different catalyst loadings. (D) The N and P co-doped carbon framework shows the electrocatalytic activities towards both ORR and OER. Reprinted with permission from ref. 31. Copyright 2015 Nature Publishing Group.

Recently carbon frameworks derived from dopant crosslinked CPGs are found to be a promising candidate for both ORR and OER. Other than traditional porous carbon materials synthesized by template method which usually fail in increasing the number of active reaction sites because the templates hinder contact between the nitrogen-containing precursors and metals, CPGs derived carbon frameworks can provide a large number of active sites since no templates are needed during the synthesis. In addition, they are porous enough to accommodate the linkage of various species-transport channels to their active sites: voids for O2 and water, a polymer chain for protons, and a carbon framework for electrons. The doping of N or other elements could be also tuned by changing the crosslinking molecules and polymer precursors. Wei et al.32 used the self-assembled PANI with a 3D network structure to synthesize carbon framework by fixing and fully sealing PANI precursor inside NaCl via recrystallization of NaCl solution. The original 3D structure of the PANI is well-preserved, and various pores, from micro- to meso- to macro-size, are formed in large quantities in the high temperature pyrolysis. These pores resulted in a high density of ORR active sites located along the efficient mass-transport pathway, leading to a high utilization of active sites (Figure 8A). The resulted carbon framework exhibited high conductivity, ORR activity and good stability in acidic electrolytes (Figure 8B). Proton exchange membrane fuel cell was fabricated using the carbon framework as the cathode catalyst and it produced a peak power of 600 mW cm−2, indicating the 3D PANI derived carbon material is among the best nonprecious metal catalysts for the ORR. N and P co-doped carbon framework synthesized by carbonization of phytic acid crosslinked PANI hydrogel is further demonstrated to show bifunctional catalytic activities towards ORR and OER (Figure 8C and 8D).31 Mechanism studies revealed that the N and P co-doping generates synergistic effects to improve the electrocatalytic activities towards both OER and ORR since the overpotential was reduced. The minimum overpotentials of N,P co-doped graphene for ORR and OER are 0.44 V and 0.39 V respectively, lower than those of the best catalysts identified theoretically (∼0.45 V for ORR on Pt and ∼0.42 V for OER on RuO2). CPGs also show great potential in energy conversion applications. Gel type materials have been widely used as quasi solid state electrolyte in dye-sensitized solar cells (DSSC) to replace liquid electrolyte, thus avoiding leakage and volatilization of liquid. In addition, gel electrolytes show high ionic conductivity, good interfacial filling properties, and good long-term stability, owing to their ability to absorb and maintain large amount of water. Conductive polymers have been regarded as promising



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candidates for development of gel electrolyte in DSSC due to low cost, high conductivity, high specific capacitances, good stabilities, and catalytic activity for I−/I3− reaction. Wu et al.78 synthesized poly(acrylic acid)-gpoly(ethylene glycol)/polyaniline (PAA-g-PEG/PANI) gel and used it as quasi solid state electrolyte for DSSC. The hybrid CPG possesses a porous network structure which endows it the ability to absorb large quantity of liquid. PANI could enhance the conductivity of gel electrolyte and a maximum value of 11.50 mS/cm was reached. As second chain of PANI is formed, the whole system becomes an interpenetrated network structure and an electrical conductive network is formed with PANI chains. Benefited from the excellent conductivity of PAA-gPEG/PANI, the light-to-electric energy conversion efficiency of DSSC was improved significantly reaching 6.38%. PPy was also used for fabrication of gel electrolyte in DSSC. Li et al.79 developed poly(hydroxyethyl methacrylate/glycerol) poly(HEMA/GR) gel with a 3D framework and integrated it with PPy. The PPy based gel electrolyte showed a significantly enhanced ionic conductivity, transport kinetics and electrocatalytic activity for the I−/I3− redox couple. The quasi-solid state DSSCs with this gel electrolyte yielded a reasonable conversion efficiency of 6.63%. Although the conductive gels adopted in these works are conventional hybrid gels, we could expect the application of dopant molecules crosslinked CPGs with improved mechanical properties since they show high conductivity, good catalytic activity and porous structure for ion transport.

4. Outlook and opportunities With hierarchically porous structure and high surface area, CPGs can act as the matrix for the introduction of second gel network, thus enabling the development of hybrid gel systems with interpenetrated network structure. Hybrid gel systems could bring additional new features, such as stimuli-responsive property, self-healing property and enhanced bio-compatibility, thus opening the door to new functional energy devices such as selfhealing and/or responsive energy devices.80, 81 Though practical devices based on hybrid hydrogels have not been realized, some potentially useful materials have been synthesized and reported. For example, a self-healing hybrid gel was recently developed by incorporating a supramolecule into conductive PPy gel matrix.78 The hybrid gel combines the high conductivity of PPy gel and selfhealing property of supramolecular gel, and exhibits enhanced mechanical strength and excellent elasticity due to its unique binary network structure, which provide the possibility for the fabrication of highly stretchable and flexible energy storage and conversion devices. The selfhealing behavior is efficient and could be observed at room temperature without any external stimuli, owing to dynamic assembly or disassembly of supramolecules and association or dissociation of metal-ligand bonds (Figure 9). This conductive, room-temperature self-healing gel material takes unique advantage of supramolecular chem-

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istry and polymer nanoscience, and shows its potential applications in various fields such as self-healing electronics, artificial skins, soft robotics, biomimetic prostheses and energy storage. Yu et al.82 also developed a hybrid hydrogel based on PNIPAM and CPGs with a unique combination of high electrical conductivity, high thermoresponsive sensitivity and good mechanical properties. By proper modification and device design, these hybrid CPGs could play an increasingly important role in various fields such as energy storage, biomedical application, and chemical sensors.

Figure 9. (A) SEM image of PPy/supramolecule hybrid gel and illustration of self-healing mechanism. (B) Self-healing behavior of PPy/supramolecule hybrid gel. Reprinted with permission from ref. 81. Copyright 2015 American Chemical Society.

Given the flexibility of chemical modification and their processability, CPGs could be further extended to wider applications. Figure 10 shows the perspective applications of CPGs and their derived carbon framework materials. In portable electronics, the shape of the energy source is a particular limiting factor for the creation of practical and aesthetic devices. The adoption of flexible and stretchable power devices would be a promising solution. Inspired by structure-derived mechanical properties and high scalability and processability, CPGs have been regarded as potential candidates for the fabrication of highly flexible and stretchable energy storage devices. To realize practical devices based on CPGs, two strategies would be future researching directions in this field. The first is to improve the mechanical strength and elasticity of CPGs by molecular modification or compositing with other materials. Although flexible supercapacitor based on PPy hydrogel prepared by interfacial synthesis has been realized, most CPGs suffer from mechanical brittleness and rigidity and may not bear large strain deformation, and large shape deformation. The introduction of polymer segments with high toughness or elasticity could potentially improve the strength and elasticity of CPGs and hybrid materials consisting of CPGs and a second reinforcing component could also modify the mechanical properties of CPGs. The second strategy is to develop printable flexible energy storage devices based on CPGs. Printing technologies which deposit electrode on flexible


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substrates are promising for the fabrication of flexible devices because of their large-scale, good flexibility, and low-lost features.55 Because CPGs have the ability to be processed by ink-jet printing or spray coating, future efforts regarding printable energy-storage devices based on CPGs that can provide large-scale, cheap production processes and high flexibility is needed.

based on CPGs with new exciting functionalities need to be further explored. The carbon framework derived from CPGs could be modified by chemical doping, surface modification and hybridizing with other functional materials.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

Notes The authors declare no financial competing interest.

Biographies Ye Shi is a Materials Science and Engineering graduate student at the University of Texas at Austin. He received BS and M.S. degrees in Polymer Science and Engineering at Zhejiang University. In fall 2013, he began graduate studies with Prof. Guihua Yu at the University of Texas at Austin, focusing on the synthesis and modification of conductive polymer nanomaterials and their applications in energy storage and functional devices. Figure 10. Perspective applications of CPGs and their derived carbon framework materials.

For the CPGs derived carbon frameworks, the structures and resulting properties such as specific surface area, pore size and size distribution, and chemical status of carbon atoms could be controlled by using different CPG precursors and CPGs with tuned microstructures. New dopant molecules could also introduce new doping elements into carbon frameworks to tune their electrochemical properties. The obtained carbon framework materials can act as catalyst alone or combined with metal particles such as Pt and Pd particles to catalyze chemical reactions owing to its 3D structure and high electrocatalytic activity.83 The hierarchically nanostructured carbon framework can also applied as 3D current collector to improve the performance of energy storage and conversion devices such as lithium ion battery and fuel cell. Owing to its high conductivity, CPGs derived carbon framework can also be fabricated to flexible and transparent electrodes and used for flexible and portable electronic devices. For future development, simulation/modelling studies, and state-of-the-art microscopic and spectroscopic techniques should be adopted to acquire deeper understanding of the fundamental knowledge about CPGs, such as relationship between nano/micro-structure and mechanical properties, effects of polymeric chains assembling states and surface chemistry on charge transport, electrochemical dynamics at polymer and biomolecule/electrolyte/inorganic materials interfaces. Based on these studies, advanced techniques in organic synthesis and nanofabrication technologies could be used to delicate control over the electrical, electrochemical, mechanical and surface properties of CPGs. Moreover, composites

Dr. Guihua Yu is an Assistant Professor of Materials Science and Engineering at the University of Texas at Austin. He received his BS degree with the highest honor in chemistry from University of Science and Technology of China, earned his Ph.D. in chemistry at Harvard University in 2009, followed by postdoc training at Stanford University. His research has been focused on rational synthesis and selfassembly of functional organic nanostructures and twodimensional nanostructured solids for advanced energy technologies, and fundamental understanding of the structure–property–performance relationship of these new synthetic nanoscale materials.

ACKNOWLEDGMENT We are grateful for the financial support from National Science Foundation award NSF-CMMI-1537894, 3M Nontenured Faculty Award, and the Welch Foundation grant F-1861.

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Designing Hierarchically Nanostructured Conductive Polymer Gels for

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