Highly flexible all-solid-state supercapacitors based on carbon nanotube/polypyrrole composite films and fibers

Abstract

Flexible all-solid-state film-shaped supercapacitors in both film and fiber configurations have been fabricated by depositing polypyrrole (PPy) onto freestanding carbon nanotube (CNT) films and also by directly twisting the composite films into thin fibers. These CNT/PPy films and fibers maintain a porous structure in which PPy is coated on individual CNTs as well as their interconnections. The specific capacitance of a solid CNT/PPy composite film supercapacitor reaches 139.2 F g⁻¹ (27.8 mF cm⁻², 10 mV s⁻¹) and 331.4 F g⁻¹ (5 mV s⁻¹) in a solid fiber supercapacitor when directly twisting the CNT/PPy composite film into a fiber, which is attributed to the uniform PPy coating throughout the CNT film and both inside and outside of the fiber. These devices show high stability during 10 000 charge and discharge cycles, and good flexibility under bending and twisting. Our CNT/PPy film and fiber-shaped supercapacitors have many potential applications in flexible electronics and energy storage textiles.

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Introduction

Portable and wearable electronics have been a trend in recent years. Energy storage devices (batteries, traditional capacitors and supercapacitors), which are the key parts that hinder the electronics from being smart, are required to have properties such as being lighter, thinner and flexible.^1–3 Supercapacitors, also named electrochemical capacitors, are a new type of energy storage device between traditional capacitors and batteries.^4,5 They have a much higher energy density than traditional capacitors. Supercapacitors can provide fast charging–discharging, a long-cycle life, a wide working temperature range, and they are environment-friendly. In comparison with supercapacitors using liquid electrolytes, all-solid-state and flexible supercapacitors have excellent mechanical properties, such as being stretchable, twistable and compressible,^6,7 and their electrochemical performance can be maintained even under significant deformation. Apart from that, they eliminate problems associated with liquid electrolytes (e.g. leakage), and thus are lightweight and much safer.

For all-solid-state and flexible supercapacitors, the key part is to develop high-performance electrode materials. Carbon nanotubes (CNTs), with high electronic and thermal conductivity, excellent mechanical properties, good corrosion resistance, high temperature stability, and a large surface area,^8–10 have been used as electrodes for all-solid-state and flexible supercapacitors. However, the charge storage ability of pure carbon nanotubes is limited. To improve the performance of the CNT-based supercapacitors, active materials such as MnO₂,^11–14 graphene nanosheets,^15–18 NiCo₂O₄,¹⁹ and conductive polymers^20,21 have been coated onto the carbon materials as electrodes to achieve a high capacity. Currently, great attention has been given to composites of carbon nanotubes and conducting polymers. Polypyrrole (PPy), with a low cost, good environmental stability and high capacitance, has been considered as an ideal candidate.^22–24 However, PPy shows a weak flexibility and poor cycling stability, therefore coupling PPy with flexible and stable CNTs can obtain nanocomposite materials with a high flexibility and charge storage ability.

Fiber-shaped energy storage devices have the potential to assemble functional energy textiles, and represent an emerging research area. Twist-spun CNT fibers are strong and highly conductive, and can serve as ideal templates for fabricating composite fiber electrodes. To date, most of these electrodes are made by depositing PPy onto CNT fibers.^25,26 However, PPy molecules are usually deposited onto CNTs near the fiber surface and cannot reach the inner part of the densely twisted fiber, resulting in only a moderate increase of the capacitance. Therefore, finding a method to fabricate flexible CNT/PPy nanocomposite supercapacitors with uniform PPy loading throughout the bulk electrode can improve the capacitance of supercapacitors greatly.

In this paper, we designed a novel all-solid-state and highly flexible CNT/PPy supercapacitor with polyvinyl alcohol (PVA)/H₃PO₄ as the gel electrolyte, and CNT/PPy films as the flexible electrodes. The CNT/PPy films were further spun into CNT/PPy fibers to make a fiber-shaped supercapacitor. Owing to the synergistic effects between CNTs and PPy, the supercapacitor based on two parallel CNT/PPy films achieved a specific capacitance of 139.2 F g⁻¹, and that based on two twisted CNT/PPy fibers reached 331.4 F g⁻¹. Furthermore, their electrochemical performance can be maintained under different deformations such as bending and twisting.

Results and discussion

As is illustrated in Fig. 1a, a layer of PPy was coated onto the CNT to form the CNT/PPy coaxial structure. At the same time, the interconnected parts of the CNTs were welded by PPy, which further increases the mechanical strength of the CNT film. The CNT film was synthesized by a floating catalyst chemical vapor deposition (CVD) method,²⁷ the as-prepared CNT film was immersed in 30% H₂O₂ for three hours to remove the amorphous carbon and then in 15% HCl for two hours to remove the remaining catalyst. Electrochemical deposition of PPy was performed in a three-electrode system using a platinum wire as the counter electrode, an Ag/AgCl electrode as the reference electrode and the CNT film as the working electrode. The electrolyte was an aqueous solution containing 0.02 M KCl, 0.001 M HCl, and 0.05 M pyrrole. An image of a large-area free-standing and homogeneous purified CNT film is shown in Fig. 1b, which can be tailored into any shape for electrodepositing PPy to match the device requirement. As is shown in Fig. 1c, the CNT film maintained good transparency after a drying and cutting process. Furthermore, it was flexible and could be curved at a high degree (Fig. 1d). A scanning electron microscopy (SEM) image revealed the morphology of the CNTs in the film, as is shown in Fig. 1f, in which the film was made up of a random arrangement of CNTs with diameters of 3–25 nm. The porous structure provided a large space for the PPy modification in order to achieve a high loading flexible electrode for the supercapacitor. A flexible CNT/PPy film with a PPy deposition time of 200 seconds is shown in Fig. 1e. From the SEM image of the CNT/PPy composite film (Fig. 1g), the porous structure of the film was maintained, although the pore size decreased slightly due to the increased CNT diameter after PPy coating. These pores facilitated the infiltration of the electrolyte and the interconnected CNT/PPy network enhanced the transfer of the electrons, enabling an excellent capacitance performance of the supercapacitors based on the CNT/PPy composite films. While the embedded CNT network improved charge transport, a uniform PPy coating on the CNT network could enhance the electrode flexibility (and also mechanical strength), as demonstrated in previous work.²⁸ The resulting PPy/CNT composite film is flexible and can be twisted by tweezers (as is shown in Fig. 1e).

Fig. 1 (a) Illustration of the process of the CNT/PPy composite films. Optical images of the as-produced film (b) suspended in water, (c) after drying and cutting, and (d) in a bending state. (e) Optical image of the dry CNT/PPy composite film in a bending state. (f) SEM of the as-produced CNT film. (g) SEM of the CNT/PPy composite film.

The high magnification SEM image of the CNT/PPy composite film shown in Fig. 2a revealed that PPy was coated onto the individual CNTs and their bundles, while the coated nanotubes were intertwined together into a nanoporous three-dimensional bundle network. The PPy layer covered along the entire CNT length evenly, and the rough surface exhibited a larger specific surface area. As a result, all of the CNTs were coated by PPy and the tube diameters increased from 10–20 nm to 80–90 nm (Fig. 2b). More details of the morphology of the CNT/PPy composite film were obtained by transmission electron microscopy (TEM) (Fig. 2c). The nanoporous reticulate structure of the CNT/PPy composite film revealed by the TEM image is appropriate for the exchanging and transport of charges.

Fig. 2 Characterization of the CNT/PPy composite film. (a) SEM of the CNT/PPy composite film (deposition time of 200 s). (b) Higher magnification SEM image of the CNT/PPy composite film, clearly showing that the CNTs have been wrapped by PPy layers. (c) TEM image of the CNT/PPy composite film. (d) Raman spectra of the as-produced CNT film and CNT/PPy composite film.

Raman spectroscopy analysis was employed to further study the structural change after PPy coating (Fig. 2d). Raman spectra were recorded for the pure CNT film and CNT/PPy composite film with a deposition time of 200 s. The CNT film displayed two prominent peaks at 1590 and 1340 cm⁻¹ (ESI Fig. S1†) corresponding to the G and D bands, respectively. For the CNT/PPy composite film, the characteristic bands corresponding to PPy (1417, 1248, 1046, 985 and 930 cm⁻¹) appeared, indicating the successful coating of PPy on the CNT film. The ring deformation modes at 930 and 985 cm⁻¹ reflected different oxidation steps of the pyrrole unit; the peak at 1046 cm⁻¹ qualitatively concatenated with the electrical conductivity of the PPy. The peak at 1248 cm⁻¹ was assigned to C–H and N–H bending vibrations, and the peak at 1417 cm⁻¹ was assigned to ring stretching vibrations of the C–C bonds.

The CNT film is flexible and tends to aggregate when immersed in a solution, hence it cannot be used to deposit PPy directly. To solve this problem, we spread out the film onto a mould (Fig. S2†), then, the electrolyte can infiltrate the CNT film without inducing its aggregation. The performances of the CNT or CNT/PPy electrodes were first evaluated by cyclic voltammograms (CV) in a three-electrode system, and the mass specific capacitance was calculated by integrating over the full CV curves to determine the average value. As is shown in Fig. 3a, after depositing PPy for 200 s, an obvious deviation from the typical rectangular shape of the original CNT film appears, indicating that the charge storage mechanism switches from an electrochemical double-layer (EDL) mode for the pure CNT film to an oxidation–reduction mode for PPy. The integrated area of the CNT/PPy composite increased significantly which indicated that the deposition of the PPy layer can improve the specific capacitance.

Fig. 3 Capacitive properties of the CNT/PPy film-shaped supercapacitors. (a) CV curves of the CNT and CNT/PPy films (active area of 1 cm²). Except for (a), which was measured in a 3-electrode configuration, all of the CV curves and corresponding specific capacitances in (b–f) were measured and calculated as solid-state 2-electrode devices. (b), (c) CV curves and galvanostatic charge/discharge curves of the film-shaped supercapacitor. (d) Cycling performance of the film-shaped supercapacitor at 2 A g⁻¹ for 10 000 cycles. (e) Photographs of the film-shaped supercapacitor (2.5 cm²) when straight, bending at 120°, bending into an S shape and in a twisted state. (f) CV curves of the supercapacitor in different measurement conditions corresponding to (e) at a 100 mV s⁻¹ scan rate.

The specific capacitance of the CNT film electrode (1 cm²) increased to about 3.85 F g⁻¹ (0.46 mF cm⁻²) at 50 mV s⁻¹, and the specific capacitance of the CNT/PPy composite film electrode (1 cm²) was about 152.08 F g⁻¹ (18.25 mF cm⁻²) at 50 mV s⁻¹. Nyquist plots of the CNT and CNT/PPy films were obtained in a frequency range from 0.001 Hz to 100 KHz (Fig. S3a†). From the electrochemical impedance spectroscopy (EIS) curves of the CNT film and the CNT/PPy composite film, the small semicircles in a higher frequency range indicate a very small charge transfer resistance in the electrode systems. The CNT/PPy composite film electrode showed an approximately vertical EIS curve tail in a lower frequency range which indicates efficient ion diffusion in the electrodes and a nearly ideal capacitive behavior. The CV curves of the CNT/PPy composite film electrode with different scan rates (5 to 200 mV s⁻¹) are shown in Fig. S3b,† and the specific capacitance reached 494.04 F g⁻¹ (59.29 mF cm⁻²) at 5 mV s⁻¹ which was much higher than that of a previous RGO/MWCNT paper (193 F g⁻¹),²⁹ and that of a MWCNT/PPy film (204 F g⁻¹, 276.3 F g⁻¹).^8,30

To fabricate a highly flexible energy storage device, two pieces of as-synthesized CNT/PPy film electrodes were assembled together with a poly(vinyl alcohol) (PVA)/H₃PO₄ electrolyte separated with a piece of paper as the separator to form a sandwiched structure. In this flexible energy storage device, the CNT/PPy films work as two electrodes with PVA/H₃PO₄ as the electrolyte and the paper as a separator (as illustrated in Fig. S4†). The PVA/H₃PO₄ electrolyte has a high viscosity and is environmentally stable under ambient conditions. The PVA/H₃PO₄ electrolyte can infiltrate into the electrode materials due to the nanoporous reticulate structure of the CNT/PPy composite film, ensuring a well wetted electrolytic environment for the electrodes. A two-electrode configuration was used in the electrochemical measurement for evaluating the electrochemical performance of the all-solid-state supercapacitor based on the CNT/PPy composite film. The electrochemical measurements of the film-shaped all-solid-state supercapacitor were conducted at room temperature. Fig. 3b shows the CV curves of the as-fabricated device (with an active electrode area of 1 cm²) at a scan rate of 10 to 200 mV s⁻¹ within a potential window of 0–0.9 V and the specific capacitance of the supercapacitor was 139.2 F g⁻¹ (27.8 mF cm⁻²) at 5 mV s⁻¹. In the literature, there are a number of studies on CNT–pseudo polymer composite film supercapacitors. For example, the capacitance of an all-solid-state SC device assembled by two thin SWCNT films using PVA/H₃PO₄ was ∼36 F g⁻¹.³¹ All-solid-state SC devices were assembled by using two pieces of CNT/PPy, CNT/MnO₂ or CNT–WO₃ electrodes and PVA/H₂SO₄ as the gel electrolyte with total device volumetric capacitances of 4.9 F cm⁻³, 5.1 F cm⁻³, and 2.6 F cm⁻³.^28,32,33 Since our films are very thin (0.62 μm), the calculation of the electrode volume is not reliable.

A galvanostatic charge–discharge (GCD) test was performed in a fixed voltage range of 0–0.9 V with different current densities from 2 to 10 A g⁻¹ (Fig. 3c). The GCD curves showed almost equal charging and discharging times. With an increase of current density, an incomplete discharge occurs among the as-formed electric double layers, resulting in an appreciable IR drop (associated with internal resistance) at the beginning of the discharge process. A good electrical conductivity of the supercapacitor is a key factor for a good electrochemical performance and enhances the power density. The long-term cycle stability was measured by the GCD at a current density of 2 A g⁻¹ for 10 000 cycles and the result is shown in Fig. 3d. The capacitance of the supercapacitor decreased gradually during the whole process besides several slight fluctuations in some cycles. The capacitance of the supercapacitor was retained at around 65.6% of its initial value even after 10 000 cycles. After 8000 charge/discharge cycles, a slight increase in C/C₀ appears (Fig. 3d), which might be due to an improved electrolyte penetration into the porous CNT/PPy network, creating more ion diffusion paths and available active surfaces among the electrode. Furthermore, the good symmetry of both the charge and discharge profiles at a 2 A g⁻¹ charge–discharge current density for the 5000th to 5005th cycles of the device further indicated an excellent capacitive behavior (inset of Fig. 3d). The electrochemical performance of the device resulted from the introduction of the PPy layer which provided a good conductive path for electron and ion transfer and further enhanced the electrochemical kinetics and capacitance.

In practical applications, flexible supercapacitors need to work under a deformation state, which requires a stable performance of the supercapacitors under different deformations. Thus, an investigation on how the mechanical deformation affects the electrochemical properties of the film-shaped supercapacitor based on the CNT/PPy composite film becomes extremely important for the device reliability evaluation. The as-prepared CNT/PPy composite films were cut into strips and then fabricated into a film-shaped supercapacitor. Fig. 3e shows photographs of a flexible film device (area of 2.5 cm²) in the conditions of its original state, bending to a 120° angle, bending to an “S” shape, and twisting, respectively. No cracks in the device were observed upon bending and twisting, indicating an excellent flexibility of the film-shaped supercapacitor. To further verify the high stability of the film-shaped supercapacitor based on the CNT/PPy composite films, CV curves of the as-fabricated supercapacitor were compared before and after bending and twisting. As is shown in Fig. 3f, the CV curves were recorded in a freestanding device in its original state, bending to a 120° angle, bending to an “S” shape and twisting, at the same scanning rate of 100 mV s⁻¹ in a voltage window of 0.9 V. The CNT/PPy supercapacitor showed a high flexibility and stability under the bending and twisting test, as it was depicted that the areas of the curves under bending to a 120° angle, bending to an “S” shape and twisting were almost identical to that measured under normal straight conditions. The specific capacitances of the supercapacitor in different shapes were also similar to each other, and were 15.4 F g⁻¹ (2.86 mF cm⁻²) for the original state, 14.0 F g⁻¹ (2.6 mF cm⁻²) for bending at a 120° angle, 14.4 F g⁻¹ (2.68 mF cm⁻²) for the “S” shape, and about 14.0 F g⁻¹ (2.6 mF cm⁻²) for the twisting state. The CV curves almost overlap with each other, showing a good stability of the supercapacitor under different deformations.

Furthermore, the as-prepared CNT/PPy composite film can be twisted into a fiber. The CNT/PPy film with an increased film thickness (0.62 μm) over the original CNT film (0.2 μm) was cut into a regular strip with a size of 5 mm × 70 mm and spun into a uniform fiber (d = 138 μm as is shown in Fig. 4a). The CNT/PPy fiber displayed directional spirals on its surface which contributed a larger surface for transporting electrolyte ions. In addition, from the magnified SEM image of the surface, the nanoporous reticulate structure was still maintained after the twisting process. PPy was coated on each individual CNT in our CNT/PPy composite fibers fabricated by directly twisting a CNT/PPy composite film compared to those fibers.^11,34–38 Here, it seems that by simply twisting a composite film into a fiber, one can improve the specific capacitance of the supercapacitor. There might be two reasons for this. First, we have cut the film into narrow strips and then twisted each strip into a thin fiber, thus the mass of the CNT/PPy fiber is actually much less than that of the CNT/PPy film, which causes an increased mass specific capacitance. Second, twisting a film into a dense fiber improves the packing of the CNTs inside, and the electrical conductivity of the electrodes, which is favorable for fast charge transport. However, negative effects, such as the lower interfacial area between the two cylindrical fibers compared with that between the two flat films, may also play a role. Indeed, our method offers a potential approach toward making high-performance fiber-based energy systems. We first make a uniform composite film in which PPy is coated onto the surface of individual CNTs, bundles, and their junctions, with a controlled thickness. Thus, the PPy distribution is uniform throughout the twisted fiber, ensuring a maximum performance as supercapacitor electrodes.

Fig. 4 Capacitive properties of the CNT/PPy fiber-shaped supercapacitor. (a) CNT/PPy fiber fabricated by twisting the CNT/PPy film. (b) CV and (c) galvanostatic charge/discharge curves of the fiber-shaped supercapacitor. (d) Photographs of the fiber-shaped supercapacitor when straight, bending into an S shape and in a twisted state. (e) CV curves of the supercapacitor in different measurement conditions corresponding to (d) at a 200 mV s⁻¹ scan rate.

Two individual CNT/PPy composite fibers were used as symmetric electrodes coated by a thin layer of the PVA/H₃PO₄ gel electrolyte, and then twisted into a double-helical structure (as illustrated in Fig. S5†). We measured the electrochemical performance of the two-electrode flexible fiber-shaped supercapacitors. Fig. 4b shows CV curves of the fiber-shaped supercapacitor which exhibited a nearly rectangular shape at different scan rates in the range of 10 to 200 mV s⁻¹, showing a good electrochemical behavior. The fiber-shaped supercapacitor showed the highest specific capacitance of 331.4 F g⁻¹ at 5 mV s⁻¹, which was higher than those in previous reports (56 F g⁻¹ and 63.6 F g⁻¹).^4,34 The galvanostatic charge/discharge curves at different current densities (2–10 A g⁻¹) exhibited a symmetric triangle-like shape at all charge/discharge current densities (Fig. 4c), which was consistent with the rectangular-like shape in CV curves indicating a highly reversible electrochemical behavior.

Flexibility tests of the fiber-shaped supercapacitor have been performed by bending the fiber to an “S” shape and into a twisted state, Fig. 4d shows the images of the device in various conditions of an original state, under bending to an “S” shape, and under twisting. To further verify the high stability of the fiber-shaped supercapacitor, CV curves of the fiber-shaped supercapacitor were compared before and after bending and twisting. From the CV curves in different static deformation states at a scan rate of 200 mV s⁻¹ (Fig. 4e), the fiber-shaped supercapacitor showed negligibles change in CV characteristics in both the bending and twisting shapes. The CV curves overlapped with each other in the conditions of bending to an “S” shape and a twisted state, showing good stability under deformation conditions. The specific capacitances of the supercapacitor were 52.74 F g⁻¹ (original state), 50.08 F g⁻¹ (“S” shape), and 51.12 F g⁻¹ (twisted state).

Conclusions

In conclusion, an effective approach was designed to fabricate all-solid-state film-shaped and fiber-shaped supercapacitors with good performance, including high capacitances and excellent flexibility. The film-shaped supercapacitor (2.5 cm²) showed a high stability in the cyclic tests for 10 000 cycles and good flexibility under bending and twisting. In addition, the flexible CNT/PPy composite film could be twisted into a fiber. A fiber-shaped supercapacitor fabricated by twisting two composite fibers had a specific capacitance of 331.4 F g⁻¹ (5 mV s⁻¹), and the device could be deformed into various deformations. Our CNT/PPy film/fiber-shaped supercapacitors have a wide range of potential applications in portable, wearable, and body-integrated flexible electronics and energy storage devices.

Experimental

Preparation of the CNT films

The CNT films were directly produced by using the chemical vapor deposition (CVD) technique.²⁷ In order to remove the amorphous carbon and Fe particles, the as-prepared CNT films were purified by treatment in 30% H₂O₂ for three hours and then in 15% HCl for two hours. Then the CNT films were washed with deionized water, and dried in a vacuum at 45 °C.

Preparation of the film and fiber-shaped electrodes

Electrochemical deposition of PPy was performed in a three-electrode system using a platinum wire as the counter electrode, an Ag/AgCl electrode as the reference electrode and a CNT film as the working electrode. The electrolyte was an aqueous solution containing 0.02 M KCl, 0.001 M HCl, and 0.05 M pyrrole. PPy was in situ electrodeposited on the CNT film at a constant potential of 0.7 V versus the Ag/AgCl electrode for different periods. Then the CNT/PPy composite films were washed with deionized water and dried. Some CNT/PPy composite films were used as the film-shaped electrodes, and some films were cut into strips to spin into straight fibers and then used as the fiber-shaped electrodes.

Fabrication of the film and fiber-shaped supercapacitors

To fabricate the symmetric film-shaped supercapacitors, the PVA/H₃PO₄ gel electrolyte was prepared by adding 3 g of PVA and 3 g of H₃PO₄ into 30 mL of deionized water and was magnetically stirred at 90 °C for several hours until the solution became clear. Two CNT/PPy composite film electrodes were immersed into the as-prepared PVA/H₃PO₄ electrolyte for 1 hour. The electrodes were assembled together to fabricate the film-shaped supercapacitor. In addition, to fabricate the fiber-shaped supercapacitor, the CNT/PPy film was cut into regular strips with a size of 5 mm × 70 mm and spun into a uniform fiber. Then the as-prepared fibers were used as electrodes, which were placed in a twisted state and coated by the PVA/H₃PO₄ gel electrolyte.

Characterization

The morphology of the samples was obtained by SEM (JEOL JSM-6700F) and TEM (JEOL JEM-2100). Raman spectra were recorded with a spectrophotometer (Renishaw-in Via Reflex). Electrochemical measurements were carried out at room temperature using an electrochemical workstation (CorrTest CS2350). Cyclic voltammetry and galvanostatic charge–discharge tests were performed in a voltage window of 0–0.9 V at different scan rates and current densities, respectively. The electrochemical impedance spectroscopy measurements were performed at a frequency range from 100 kHz to 0.01 Hz. The mechanical flexibility test was carried out by manual control.

When tested in a three-electrode system, for a single electrode, its specific capacitance, C_s (F g⁻¹), can be calculated from the CV curves: , where s is the scan rate, V is the potential window, m is the mass of a single electrode and I is the current.

When a symmetric supercapacitor is charged, a voltage will build up across the two electrodes. The capacitance (C, F) of the device is: . For an ideal symmetric supercapacitor, the specific capacitance, C_s (F g⁻¹) for the active material can be derived from the capacitance of the device: , where m is the total mass of the active material (CNT/PPy).³⁹

Acknowledgements

The authors greatly acknowledge the financial support from the National Natural Science Foundation under the grants of NSFC 51502267 and the China Postdoctoral Science Foundation funded project (2015M582200).

References

1. G. Xu, B. Ding, P. Nie, L. Shen, J. Wang and X. Zhang, Porous Nitrogen-Doped Carbon Nanotubes Derived from Tubular Polypyrrole for Energy-Storage Applications, Chem.–Eur. J., 2013, 19(37), 12306–12312.
2. W. Tian, X. Mao, P. Brown, G. C. Rutledge and T. A. Hatton, Electrochemically Nanostructured Polyvinylferrocene/Polypyrrole Composites with Synergy for Energy Storage, Adv. Funct. Mater., 2015, 25(30), 4803–4813.
3. H. Sun, Y. Jiang, L. Qiu, X. You, J. Yang, X. Fu, P. Chen, G. Guan, Z. Yang, X. Sun and H. Peng, Energy Harvesting and Storage Devices Fused into Various Patterns, J. Mater. Chem. A, 2015, 3(29), 14977–14984.
4. R. Q. Xu, F. M. Guo, X. Cui, L. Zhang, K. L. Wang and J. Q. Wei, High Performance Carbon Nanotube Based Fiber-shaped Supercapacitors Using Redox Additives of Polypyrrole and Hydroquinone, J. Mater. Chem. A, 2015, 3(44), 22353–22360.
5. X. Y. Zhang, L. Q. Han, S. Sun, C. Y. Wang and M. M. Chen, MnO₂/C Composite Electrodes Free of Conductive Enhancer for Supercapacitors, J. Alloys Compd., 2015, 653, 539–545.
6. X. Pu, L. Li, M. Liu, C. Jiang, C. Du, Z. Zhao, W. Hu and Z. L. Wang, Wearable Self-Charging Power Textile Based on Flexible Yarn Supercapacitors and Fabric Nanogenerators, Adv. Mater., 2015, 28(1), 98–105.
7. C. J. Raj, B. C. Kim, W. J. Cho, W. G. Lee, S. D. Jung, Y. H. Kim, S. Y. Park and K. H. Yu, Highly Flexible and Planar Supercapacitors Using Graphite Flakes/Polypyrrole in Polymer Lapping Film, ACS Appl. Mater. Interfaces, 2015, 7(24), 13405–13414.
8. T. I. W. Schnoor, G. Smith, D. Eder, K. K. K. Koziol, G. Tim Burstein, A. H. Windle and K. Schulte, The Production of Aligned MWCNT/Polypyrrole Composite Films, Carbon, 2013, 60, 229–235.
9. M. Hughes, G. Z. Chen, M. S. P. Shaffer, D. J. Fray and A. H. Windle, Electrochemical Capacitance of a Nanoporous Composite of Carbon Nanotubes and Polypyrrole, Chem. Mater., 2002, 14(4), 1610–1613.
10. Y. Tsarfati, V. Strauss, S. Kuhri, E. Krieg, H. Weissman, E. Shimoni, J. Baram, D. M. Guldi and B. Rybtchinski, Dispersing Perylene Diimide/SWCNT Composites: Structural Insights at the Molecular Level and Fabricating Advanced Materials, J. Am. Chem. Soc., 2015, 137(23), 7429–7440.
11. J. Tao, N. Liu, W. Ma, L. Ding, L. Li, J. Su and Y. Gao, Solid-State High Performance Flexible Supercapacitors Based on Polypyrrole–MnO₂–Carbon Fiber Composite Structure, Sci. Rep., 2013, 3, 2286.
12. T. Gu and B. Wei, Fast and Stable Redox Reaction of MnO₂/CNT Composite Electrodes for Dynamically Stretchable Pseudocapacitors, Nanoscale, 2015, 7(27), 11626–11632.
13. T. G. Yun, B. I. Hwang, D. Kim, S. Hyun and S. M. Han, Polypyrrole–MnO₂-Coated Textile-Based Flexible-Stretchable Supercapacitor with High Electrochemical and Mechanical Reliability, ACS Appl. Mater. Interfaces, 2015, 7(17), 9228–9234.
14. Y. Liu, K. Shi and I. Zhitomirsky, Azopolymer Triggered Electrophoretic Deposition of MnO₂–Carbon Nanotube Composites and Polypyrrole Coated Carbon Nanotubes for Supercapacitors, J. Mater. Chem. A, 2015, 3(32), 16486–16494.
15. R. Sun, H. Chen, Q. Li, Q. Song and X. Zhang, Spontaneous Assembly of Strong and Conductive Graphene/Polypyrrole Composite Aerogels for Energy Storage, Nanoscale, 2014, 6(21), 12912–12920.
16. H. Kashni, L. Chen, Y. Ito, J. Han, A. Hirata and M. Chen, Bicontinuous Nanotubular Graphene–Polypyrrole Composite for High Performance Flexible Supercapacitors, Nano Energy, 2016, 19, 391–400.
17. L. Liu, X. M. Bian, J. Tang, H. Hu, Z. L. Hou and W. L. Song, Exceptional Electrical and Thermal Transport Properties in Tunable all-Graphene Papers, RSC Adv., 2015, 5(92), 75239–75247.
18. Y. Zhao, J. Liu, Y. Hu, H. Cheng, C. Hu, C. Jiang, L. Jiang, A. Cao and L. Qu, Highly Compression-Tolerant Supercapacitor Based on Polypyrrole-Mediated Graphene Foam Electrodes, Adv. Mater., 2013, 25(4), 591–595.
19. S. Xu, D. Yang, F. Zhang, J. Liu, A. Gou and F. Hou, Fabrication of NiCo₂O₄ and Carbon Nanotube Nanocomposite Films as a High-Performance Flexible Electrode of Supercapacitors, RSC Adv., 2015, 5(90), 74032–74039.
20. B. Liang, Z. Qin, J. Zhao, Y. Zhang, Z. Zhou and Y. Lu, Controlled Synthesis, Core–Shell Structures and Electrochemical Properties of Polyaniline/Polypyrrole Composite Nanofibers, J. Mater. Chem. A, 2014, 2(7), 2129–2135.
21. Z. Niu, P. Luan, Q. Shao, H. Dong, J. Li, J. Chen, D. Zhao, L. Cai, W. Zhou, X. Chen and S. Xie, A “Skeleton/Skin” Strategy for Preparing Ultrathin Free-Standing Single-Walled Carbon Nanotube/Polyaniline Films for High Performance Supercapacitor Electrodes, Energy Environ. Sci., 2012, 5(9), 8726–8733.
22. Y. Shi, L. Pan, B. Liu, Y. Wang, Y. Cui, Z. Bao and G. Yu, Nanostructured Conductive Polypyrrole Hydrogels as High-Performance, Flexible Supercapacitor Electrodes, J. Mater. Chem. A, 2014, 2(17), 6086–6091.
23. C. E. Zhao, J. Wu, S. Kjelleberg, J. S. C. Loo and Q. Zhang, Employing a Flexible and Low-Cost Polypyrrole Nanotube Membrane as an Anode to Enhance Current Generation in Microbial Fuel Cells, Small, 2015, 11(28), 3440–3443.
24. Y. Huang, J. Tao, W. Meng, M. Zhu, Y. Huang, Y. Fu, Y. Gao and C. Zhi, Super-High Rate Stretchable Polypyrrole-Based Supercapacitors with Excellent Cycling Stability, Nano Energy, 2015, 11, 518–525.
25. S. Ryu, P. Lee, J. B. Chou, R. Xu, R. Zhao, A. J. Hart and S. G. Kim, Fabrication of Extremely Elastic Wearable Strain Sensor Using Aligned Carbon Nanotube Fibers for Monitoring Human Motion, ACS Nano, 2015, 9(6), 5929–5936.
26. X. Cai, R. V. Hansen, L. Zhang, B. Li, C. K. Poh, S. H. Lim, L. Chen, J. Yang, L. Lai, J. Lin and Z. Shen, Binary Metal Sulfide and Polypyrrole on Vertically-Aligned Carbon Nanotubes Array/Carbon Fiber Paper as High-Performance Electrodes, J. Mater. Chem. A, 2015, 3(44), 22043–22052.
27. Z. Li, Y. Jia, J. Q. Wei, K. L. Wang, Q. K. Shu, X. C. Gui, H. W. Zhu, A. Y. Cao and D. H. Wu, Large Area, Highly Transparent Carbon Nanotube Spiderwebs for Energy Harvesting, J. Mater. Chem., 2010, 20(34), 7236–7240.
28. Y. Chen, L. H. Du, P. H. Yang, P. Sun, X. Yu and W. J. Mai, Significantly Enhanced Robustness and Electrochemical Performance of Flexible Carbon Nanotube-Based Supercapacitors by Electrodepositing Polypyrrole, J. Power Sources, 2015, 287, 68–74.
29. C. Yang, J. Shen, C. Wang, H. Fei, H. Bao and G. Wang, All-Solid-State Asymmetric Supercapacitor Based on Reduced Graphene Oxide/Carbon Nanotube and Carbon Fiber Paper/Polypyrrole Electrodes, J. Mater. Chem. A, 2014, 2(5), 1458–1464.
30. T. Qian, X. Zhou, C. F. Yu, S. S. Wu and J. Shen, Highly Dispersed Carbon Nanotube/Polypyrrole Core/Shell Composites with Improved Electrochemical Capacitive Performance, J. Mater. Chem. A, 2013, 1(48), 15230–15234.
31. M. Kaempgen, C. K. Chan, J. Ma, Y. Cui and G. Gruner, Printable Thin Film Supercapacitors Using Single-Walled Carbon Nanotubes, Nano Lett., 2009, 9, 1872–1876.
32. L. H. Du, P. H. Yang, X. Yu, P. Y. Liu, J. H. Song and W. J. Mai, Flexible Supercapacitors Based on Carbon Nanotube/MnO₂ Nanotube Hybrid Porous Films for Wearable Electronic Devices, J. Mater. Chem. A, 2014, 2, 17561–17567.
33. P. Sun, Z. W. Deng, P. H. Yang, X. Yu, Y. L. Chen, Z. M. Liang, H. Meng, W. G. Xie, S. Z. Tan and W. J. Mai, Freestanding CNT–WO₃ Hybrid Electrodes for Flexible Asymmetric Supercapacitors, J. Mater. Chem. A, 2015, 3, 12076–12080.
34. Y. Y. Shang, C. H. Wang, X. D. He, J. J. Li, Q. Y. Peng, E. Z. Shi, R. G. Wang, S. Y. Du, A. Y. Cao and Y. B. Li, Self-Stretchable, Helical Carbon Nanotube Yarn Supercapacitors with Stable Performance under Extreme Deformation Conditions, Nano Energy, 2015, 12, 401–409.
35. J. Ren, L. Li, C. Chen, X. L. Chen, Z. B. Cai, L. B. Qiu, Y. G. Wang, X. R. Zhu and H. S. Peng, Batteries: Twisting Carbon Nanotube Fibers for Both Wire-Shaped Micro-Supercapacitor and Micro-Battery, Adv. Mater., 2013, 25(8), 1224.
36. C. Choi, J. A. Lee, A. Y. Choi, Y. T. Kim, X. Lepró, M. D. Lima, R. H. Bauqhman and S. J. Kim, Flexible Supercapacitor Made of Carbon Nanotube Yarn with Internal Pores, Adv. Mater., 2014, 26(13), 2059–2065.
37. Z. B. Cai, L. Li, J. Ren, L. B. Qiu, H. J. Li and H. S. Peng, Flexible, Weavable and Efficient Microsupercapacitor Wires Based on Polyaniline Composite Fibers Incorporated with Aligned Carbon Nanotubes, J. Mater. Chem. A, 2013, 1(2), 258–261.
38. K. Wang, Q. H. Meng, Y. J. Zhang, Z. X. Wei and M. H. Miao, High-Performance Two-Ply Yarn Supercapacitors Based on Carbon Nanotubes and Polyaniline Nanowire Arrays, Adv. Mater., 2013, 25(10), 1494–1498.
39. P. H. Yang and W. J. Mai, Flexible Solid-State Electrochemical Supercapacitors, Nano Energy, 2014, 8, 274–290.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra07297f

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