Polyaniline Porous Network for

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Highly Stable CNT/PANI Porous Network for Multifunctional Applications Wenqi Zhao, Yibin Li, Shiting Wu, Dezhi Wang, Xu Zhao, Fan Xu, Mingchu Zou, Hui Zhang, Xiaodong He, and Anyuan Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11984 • Publication Date (Web): 24 Nov 2016 Downloaded from http://pubs.acs.org on November 27, 2016

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ACS Applied Materials & Interfaces

Highly Stable CNT/PANI Porous Network for Multifunctional Applications Wenqi Zhao,1,2 Yibin Li,1* Shiting Wu,2 Dezhi Wang,3 Xu Zhao,1 Fan Xu,1 Mingchu Zou,2 Hui Zhang,2 Xiaodong He,1 Anyuan Cao2* 1

Centre for Composite Materials and Structures, Harbin Institute of Technology, Harbin 150080, P. R. China

2

Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, P.

R. China 3

Institute of Petrochemistry, HLJ Academy of Sciences, Harbin 150040, P. R. China

*Corresponding authors: Email: [email protected], [email protected]

Abstract

Three-dimensional carbon nanotube (CNT) networks with high porosity and electrical conductivity have many potential applications in energy and environmental areas, but the network structure is not very stable due to weak inter-CNT interactions. Here, we coat a thin polyaniline (PANI) layer on as-synthesized CNT sponge to obtain a mechanically and electrically stable network, and enable multifunctional applications. The resulting CNT/PANI network serves as stable strain sensors, highly compressible supercapacitor electrode with enhanced volume-normalized capacitance (632 F/cm3), and reinforced nanocomposites with the PANI as intermediate layer between the CNT fillers and polymeric matrix. Our results provide a simple and controllable method for achieving high-stability porous networks composed of CNTs, graphene, or other nanostructures. Keywords:

carbon

nanotube,

polyaniline,

stable

network,

nanocomposite 1

ACS Paragon Plus Environment

multifunctional

application,


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Introduction

Carbon nanotube (CNT) and graphene aerogels are recently emerged three-dimensional (3D) porous materials with potential applications in various areas such as sensors, composites, energy and environmental cleanup.1-6 Compared to traditional aerogels based on silica or ceramics, these carbon nanomaterial-based aerogels possess not only high mechanical integrity and flexibility (thus, they are also called foams or sponges), but also excellent electrical conductivity. As a result, exciting applications such as flexible conductive energy conversion or storage electrodes, and functional nanocomposites, have been explored extensively.7-10 To this end, a robust and stable 3D CNT or graphene network is essential for achieving reversible behavior and reliable performance. As most of aerogels consist of CNT skeletons or graphene sheets stacked onto each other by self-assembly, the interaction between randomly overlapped struts where only van der Waals force is present, is typically very weak. Upon deformation, many events such as rotation and kink formation of individual skeletons, as well as the slippage between them, could occur and consequently lead to plastic deformation of the aerogel structure and unstable electrical behavior. Although covalent bonding between graphene oxide sheets can be introduced during fabrication,11 the improvement in mechanical robustness is very limited.

Introducing a foreign coating onto the internal structural units has proven to be an effective way to reinforce the 3D porous aerogels. As demonstrated in others and our work, amorphous carbon, oxides and selected polymers have been deposited into CNT sponges and graphene based aerogels to make hybrid networks.12-14,16,17 Those hybrid sponges show improved elasticity and recovery during repeated compression, and also serve as bulk porous conductive electrodes for supercapacitors and 2


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Li-ion batteries.10,15 As revealed by the above studies, the amorphous carbon or pseudo-polymer layer is coated uniformly on the CNT struts and inter-CNT joints, resulting in a fixed 3D network that can prevent slippage between CNTs during deformation. While applications as supercapacitor or battery electrode has been demonstrated with moderate performance, the reinforced CNT network with high stability remains not fully utilized. In particular, introducing a pseudo-polymer such as polypyrrole (PPy) or polyaniline brings multifunctionality into the aerogels and enables more applications.

Here, we show that a CNT/PANI sponge consisting of a controlled-thickness PANI coating on the CNT network serves as a multifunctional 3D porous material for strain sensors, compressible supercapacitor electrodes, and reinforced conductive nanocomposites. The PANI coating is introduced into the CNT sponge through a simple electro-deposition method, creating a highly stable 3D network under large-strain compression with improved mechanical robustness and electrical conductivity. Furthermore, the porous CNT/PANI network can be compressed severely to reach a volume-normalized capacitance of 632 F/cm3. Finally, we also demonstrate a CNT-reinforced epoxy composite containing an intermediate PANI layer at the filler-matrix interface.

Results and discussion

The fabrication of 3D CNT/PANI network and its three potential applications, including compressive strain sensors, compressible supercapacitor electrodes, and reinforced nanocomposites, are illustrated in Figure 1a. Starting from a bulk CNT sponge as template, we infiltrate aniline monomers into the porous space within the sponge, and induce polymerization around each individual CNT by π-π inter-linkage through an electro-deposition process. After that, the CNT 3



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sponge has been converted into a CNT/PANI network, and the strut diameters have increased from originally 20-40 nm to currently 50-100 nm according to scanning electron microscopy (SEM) characterization (Fig. 1b, Supporting Information Fig. S1). A conformal PANI layer is coated onto the CNTs and inter-CNT junctions, where three overlapped multi-walled nanotubes are welded together by the PANI coating and the core-shell structure can be clearly observed in the broken cross-section (Fig. 1c). At this PANI thickness (∼40 nm), there is still much space between adjacent struts and the sponge remains highly porous, which facilitates further applications as compressible sensors and electrodes. Transmission electron microscopy (TEM) images also reveal the presence of internal multi-walled nanotubes and outer PANI layer, as well as welded cross-junctions of CNTs in contact (Fig. 1d). For this core-shell configuration, the thickness of the PANI layer can be tuned in the range from several nanometers to about 35 nm by varying the deposition period. Accordingly, the bulk sponge density increases from 10 to 48 mg/cm3. In addition, the chemical composition of the PANI shell has been characterized by Raman spectrum, which shows several distinct peaks centered at 1174 cm-1 (in-plane C-H bending), 1346 cm-1 (C-N'+ stretching), 1404 cm-1 (C-C stretching), 1468 cm-1 (C=N stretching) and 1565 cm-1 (C=C stretching) (Fig. S2a), indicating successful polymerization during the electro-deposition process.35 Thermogravimetric analysis (TGA) reveals gradual weight reduction up to about 600 °C before the combustion of CNTs, which is slightly delayed compared with the original CNT sponge (Fig.S2b). The PANI coating has also confirmed by X-ray diffraction (XRD). Compared to the original CNT sponge, we observe pronounced peaks centered at 15.2°, 20.1° and 25.8°, which are characteristic of PANI in its doped emeraldine salt form.36

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We have investigated the electrical response in the CNT and CNT/PANI sponges under compression, as this is a critical prerequisite for bulk porous materials to serve as flexible (or compressible) sensors and electrodes. First, the PANI coating increases the bulk conductivity from 81 S/m (CNT sponge) to 186 S/m in the CNT/PANI network. We think there are mainly two reasons for the improved conductivity: a) the deposited PANI layer can weld the cross-junction between CNTs (thus, reducing contact resistance between CNTs); b) in addition to CNT networks, the continuous and uniform PANI layer also can work as conductive pathways. Second, the stability of electrical response is greatly improved after PANI coating. It has been demonstrated that porous CNT networks show resistance change upon compressive loading and unloading, a result of densification and releasing of internal CNTs.18 Here, although the resistance (R) still changes in each compression cycle, both the base resistance (R0) and the magnitude of resistance change (∆R,

∆R=R0-R) show considerable fluctuation over 17 tested cycles (Fig. 2a). This unstable behavior is attributed to the unstable network structure, in which CNTs change orientation and form bundles, as studied before.12 In contrast, the resistance change in the CNT/PANI sponge is highly reversible over cyclic compression, and the values of R0 and ∆R remain constant over many cycles (Fig. 2b). At this compressive strain (ε =20%), the resistance change is relatively small, resulting in a modest ∆R (∼3%). When the maximum strain increased to 50%, ∆R also rises up to 22% due to further densification while the CNT/PANI network maintains the same stable behavior as for small strains over 500 cycles (Fig. 2c). Underlying the stable electrical performance is the enhanced mechanical properties including the compressive strength and elastic recovery after 1000 compression cycles to different strains (ε =10% to 50%) (Fig. 2d and Fig. S3). SEM images of samples after cyclic tests reveal well maintenance of the 3D CNT/PANI network due to the strengthening by the PANI coating, 5



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as compared with the pure CNT sponge (Fig. S4). Such conductive CNT/PANI network with high structural stability when subjected to large-degree cyclic deformation, is very attractive for application as stress/strain sensors, although currently the sensitivity (∆R per strain) is relatively low and needs further improvement.

The 3D conductive CNT network covered by a pseudo-polymer layer is an ideal electrode material for supercapacitors. Both PANI and PPy are pseudo-capacitive materials and can be introduced to CNT sponge as supercapacitor electrodes. Compared with PPy, PANI possesses better capacitive properties such as higher specific capacitance according to our previous and others work.19,37 However, for porous aerogels to serve as supercapacitor electrodes, one of the major concerns is their low volumetric capacitance due to the high porosity, limiting cell assembly and practical applications. 20 Here, considering the stable CNT/PANI network, we manually compressed it into a thin membrane with a thickness reduction by nearly 60-fold (from initially 3 mm down to 55 µm) (illustrated in Fig. 3a). SEM characterization shows that the 3D network has been collapsed, but the interconnection between PANI-fixed CNTs is retained (Fig. 3b, 3c). This process has removed more than 98% of porous space among the sponge, yet with the same active material mass. Correspondingly, the bulk density increases from 0.028 g/cm3 (before compression) to 1.195 g/cm3 (after). Cyclic Voltammogram (CV) curve of the densified membrane (at a scan rate of 5 mV/s, measured in a three-probe station) is indeed close to the sponge before densification (Fig. 3d). It shows that the network structure is maintained after collapsing, with only slight loss of internal surface area. This result also indicates that improving the volumetric capacitance of the porous CNT/PANI network by severe compression is possible.

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Based on this, we have assembled symmetric supercapacitor cells by clamping two identical CNT/PANI sponges from two sides with a filter paper in the middle. About a 33 nm-thick PANI coating is selected here, by comparing samples with different PANI thicknesses (Fig. S6). Then, we carried out systematic tests on the assembled cells consisting of densified CNT sponge or CNT/PANI membrane electrodes, including their and galvanostatic charge-discharge measurements at a serious of current densities (1 to 10 A/g) (Fig. 3e) and CV characteristics across a voltage window of 1.0 V at scan rates of 5 to 200 mV/s, electrochemical impedance spectroscopy (EIS), and capacitance retention over 2000 cycles (Fig. S5). The coulombic efficiency can reach 95.2% and 93.5% at large current densities of 10 A/g and 5 A/g but decrease to 86.6% (2 A/g) and 55.6% (1 A/g) due to the limited electrochemical reversibility. The mass and volume-specific capacitance values are obtained from the charge-discharge curves. Specifically, the gravimetric capacitance of the CNT/PANI cell is 528 F/g at a current density of 1 A/g, and decreases to 455 F/g at 10 A/g. This is mainly due to the synergistic effect of the pseudo-reactions from the PANI coating (as disclosed in low scan rate CVs) and the embedded conductive network, as generally observed in polymer-carbon hybrid structures. Simultaneously, the volumetric capacitance (CV) reaches 632 and 544 F/cm3 at 1 and 10 A/g, respectively (Fig. 3f). In recent years, improving CV of 3D porous carbon-based aerogels has been an increasingly important issue, and there are already several studies on graphene (rather than CNTs) aerogels by mainly two approaches: 1) densification of the porous electrode materials and 2) introduction of pseudocapacitive materials.21-27 Y. Zhu’s group fabricated a 3D graphene architecture using polyurethane (PU) sponge as template, which obtained a relatively low CV of 149 F/cm3 because of the presence of the many macropores among the sponge.28 X. Duan’s group compressed a densified graphene framework (with an increased density of 0.71 g/cm3) with a CV of 212 F/cm3.29 7



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Pure graphene aerogels can not reach high CV even after densification. Q. Yang’s group fabricated graphene/PANI aerogels through vacuum drying, leading to volume shrinkage and dense packing of the monolith. They obtained a high CV of 800 F/cm3 at 0.1 A/g, but at larger current densities the CV values are about 600 F/cm3 (1 A/g) and 540 F/cm3 (10 A/g). 30 These results are similar to our compressed CNT/PANI sponges (632 F/cm3 and 544 F/cm3, respectively). Energy density and power density are also two key parameters to judge the performance of supercapacitors. According to formulae (3)~(7), high specific capacitance, large voltage window and small equivalent series resistance (RESR) can give rise to high energy density and power density. Our CNT/PANI sponge shows a small voltage drop at the beginning of the discharge curve, corresponding to a low RESR (0.037Ω) (Fig. 3e). The maximum energy density of the compact sponge is 17.77 Wh/L (14.87 Wh/kg) at a power density of 6.38 kW/L (5.34 kW/kg). The maximum power density is 31.56 kW/L (26.41 kW/kg) at a energy density of 15.31 Wh/L (12.81 Wh/kg) (inset of Fig.3f). Because our electrochemical tests were conducted in an aqueous system with a relative small voltage window (compared to organic system), the obtained energy and power densities are located at a moderate level (among other carbon materials and conducting polymers).

Our CNT/PANI sponges are not only candidates for supercapacitors, but also suitable to fabricate reinforced nanocomposites due to their robust scaffolds and highly open cell structures, as demonstrated in previous CNT sponge and graphene aerogel based epoxy resin composites.

4,8,31,32

First, we prepared the bulk composites through infiltrating epoxy resin into the porous CNT/PANI network directly (Fig. 4a). Here, compared with previous binary nanocomposites, the difference in our composites is the presence of an intermediate PANI layer, which may promote the interaction

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between the epoxy matrix and CNT fillers. To study the function of the PANI intermediate layer, we have conducted tensile tests on pure epoxy, epoxy/CNT sponge and epoxy/CNT/PANI sponge (Fig. 4b and S8). Compared with pure epoxy, the tensile stress of epoxy/CNT sponge decreases from in average 128.3 to 100.5 MPa, possibly due to the non-thorough infiltration of viscous epoxy into the CNT sponge during fabrication, resulting in some defects (e.g. pores) in the composite. These defects can be eliminated by better control of the epoxy infiltration process. However, the tensile stress of the epoxy/CNT/PANI composites reach 141.8 MPa, about 40% increase than that of the epoxy/CNT composites, and also higher than pure epoxy. To analyze the mechanical enhancement in our ternary composites, we have characterized the fractured surface of those samples after tensile testing. First, as shown in SEM images, both the CNTs and CNT/PANI nanotubes are uniformly distributed in the epoxy matrix as individual skeletons (Fig. S7). Second, we find that in the cross section there are exposed CNTs and CNT/PANI segments which are extracted from the epoxy matrix during fracture, as well as hollow holes in which those skeletons have been removed (Fig. 4c and 4d). Close view of these holes reveals difference between the two samples. In the epoxy/CNT/PANI composite, some hollow PANI nanotubes can be observed, indicating that the CNT cores are extracted from the PANI shells and the latter are retained within the epoxy matrix. It reveals that there are two kinds of failure modes involved in the epoxy/CNT/PANI composites: 1) fracture between the epoxy and PANI shells and 2) fracture between CNT cores and PANI shells, which is different from the epoxy/CNT composites where only one fracture mode (between epoxy and CNTs) occurs. The second failure mode indicates that the PANI intermediate layer can enhance the interaction between CNTs and epoxy, which is distinct from traditional binary composites. Such filler-matrix interfacial enhancement is critical for improving the mechanical properties, as demonstrated in our 9



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epoxy/CNT/PANI composites. We have also tested the electrical conductivity of CNT and CNT/PANI sponges, epoxy/CNT and epoxy/CNT/PANI composites (Fig. 4e). The electrical conductivity of the epoxy/CNT composites (46 S/m) is only 56% of the original CNT sponge (81 S/m), while the conductivity of the epoxy/CNT/PANI composites (133 S/m) is 71% of CNT/PANI sponge (186 S/m). Such a relatively higher electrical conductivity in our epoxy/CNT/PANI composites is attributed to the introduction of the semiconducting PANI coating and the fixing of the conductive CNT network. This is useful in developing highly conductive functional nanocomposites for various applications.

Conclusion

We have deposited a uniform PANI coating on the 3D CNT network, and obtained highly a stable structure with enhanced mechanical and electrical properties. The resulting CNT/PANI sponge shows stable resistance change during large-strain cycles, and also can be compressed to work as supercapacitor electrodes with large volumetric capacitance. Furthermore, the PANI coating serves as an intermediate layer between the CNT fillers and epoxy matrix, and consequently improves the mechanical properties. Our multifunctional CNT/PANI sponges have potential applications in many areas such as strain sensors, supercapacitors and nanocomposites.

Experimental Section

1. Fabrication of CNT sponges and CNT/PANI core–shell sponges

CNT sponges were synthesized by chemical vapor deposition (CVD) using ferrocene and 1,2-dichlorobenzene as the catalyst and carbon precursors according to the previous work of our group.33 An as-grown bulk CNT sponge 10


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block was directly used as the working electrode in the potential range from -0.2 to 0.8 V at sweep rate of 40 mVs-1 for 100~500 cycles in 1 M H2SO4 and 0.05 M aniline aqueous solution34. The electrodeposition of PANI was performed using a three-electrode electrochemical workstation (Parstat 4000 Instruments, American). A Pt wire and a saturated calomel electrode (SCE) were employed as the counter and reference electrodes, respectively. Subsequent to electrodeposition, the electrode was washed with distilled water several times, and then freeze-dried to maintain the porous structure.

2. Fabrication of CNT sponge or CNT/PANI sponge reinforced composites

An epoxy resin solution was prepared by mixing an epoxy base agent and curing agent (Wessex Resins, PRO-SET 125 Resin/226PF Hardener) in the ratio of 10:3 by weight. Bulk CNT sponge and CNT/PANI sponge were immersed into the epoxy solution for infiltration. The solution was put in a vacuum oven for 10 min to allow an epoxy flow into the porous structure and removing gas bubbles. Then, the sponges were picked out of the solution and epoxy resin was cured at 50°C for 16 hours and 80°C for 3 hours to form solid nanocomposites.

3. Material characterization and mechanical measurements

Microstructure and morphology of samples including CNT sponges before and after compression, CNT/PANI sponges before and after testing, were characterized using SEM (Hit-achi S4800) and TEM (FEI G2 T20, 200 kV). Raman spectra were recorded with a micro-Raman spectrometer (Renishaw in Via plus). The structures of the prepared sponges were analyzed by TGA was conducted on a TGA Q600 analyzer from 20 to 800 °C under air at a heating rate of 20 °C min-1. Mechanical tests were carried out by a single-column static instrument (Instron 5843) equipped with 10 N load cell (compression tests) and 2000 N load cell (tensile tests). Specimens of 3 mm×2.5 mm ×25 mm were used for tensile tests.

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4. Electrochemical measurements

Firstly, press the hybrid CNT/PANI sponge to a film along height direction and dried under 60°C in air rather than freeze-dried to fabricate the compact CNT/PANI core–shell sponges. Electrochemical characterization of supercapacitors including cyclic voltammetry (CV), galvanostatic charge/discharge and electrochemical impedance spectroscopy (EIS) were carried out in 1 M H2SO4 electrolyte with the same equipment in a three-electrode cell as in the electrodeposition as well as a two-electrode cell. EIS measurements were carried out in the frequency range from 100 kHz to 0.01 Hz at open circuit potential with an ac perturbation of 5 mV. The specific gravimetric and volumetric capacitances of the sponge electrodes were calculated from discharging curves according to eqs 1 and 2, respectively.

CS (F / g) =

2I∆t m∆V

(1)

CV (F / cm3 ) = ρ × CS

(2)

where I is the response current (A), m is the mass of a single sponge electrode (g), ∆V is the potential range during the discharge process (V), ρ (mg/cm3) is the density of the electrode material, and ∆t is the discharging time (s).

The specific energy density and power density of the supercapacitor cell were estimated by using the following formula and normalizing to the mass of the two carbon electrodes.

E(Wh / kg) =

CS ∆V 2 8× 3.6

(3)

EV (Wh / L) = ρ × E

(4)

∆V 2 P(W / kg) = 4RESR m

(5)

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PV (W / L) = ρ × P

(6)

The effective series resistance was estimated using the voltage drop at the beginning of the discharge, Vdrop, at certain constant current I, according to

RESR =

Vdrop 2I

(7)

Supporting Information

The characterization of the CNT/PANI sponges, including Raman, TGA, XRD. Mechanical behavior and mechanism study on elastic recovery of CNT/PANI sponges. Electrochemical performance of symmetric cells. Capacitive properties comparison between the CNT sponge and CNT/PANI sponges with different PANI thickness by the three-electrode configuration test. SEM images and tensile stress-strain curves of epoxy/CNT and epoxy/CNT/PANI composites. This information is available free of charge via the Internet.

Acknowledgement

This work was financially supported by the National Nature Science Foundation of China (NO. 51325202). Y.L. and X.H., acknowledge the Natural Science Foundation in China (NSFC 11272109) and the Ph. D. Programs Foundation of Ministry of Education of China (20122302110063).

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Figure 1. Structural characterization of the CNT/PANI sponges. (a) Photographs and structural models of as-prepared CNT sponge before and after electrodeposition of PANI, and the models of strain sensors, supercapacitors and nanocomposites for CNT/PANI sponges. (b) SEM images of CNT/PANI sponge. (c) Close-up view of (b) showing the coaxial structure of PANI layer uniformly coated on CNTs. (d) TEM images of the core-shell structures in CNT/PANI sponge. (e) Variation of PANI thickness and the bulk density. Inset shows model of polymer-wrapped CNTs.

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Figure 2. Electromechanical properties of original and polymer-strengthened CNT sponges. (a) Change in resistance of the CNT sponge over 17 selected cycles at the compressed (ε=20%) and recovered state. (b) Change in resistance of the CNT/PANI sponge over 36 selected cycles showing reproducible and stable values at the compressed (ε=20%) and recovered state. Inset: Relative resistance change (∆R/R0) over 8 cycles among the cyclic test of (b). (c) Selected ∆R/R0 curves in cycling stability test of CNT/PANI sponge under repeated applied strain of 50% (d) Selected stress-strain curves of CNT/PANI sponge for 1000 cycles at a large strain (ε=50%) showing remarkable elasticity.

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Figure 3. Electrochemical performance of symmetric cells. (a) models of CNT/PANI sponge before and after densification. (b) SEM image of CNT/PANI sponge. Inset, photograph of (b). (c) SEM image of the compact CNT/PANI sponge. Inset, cross-section images of (c). (d) CV curves of the CNT/PANI and compact CNT/PANI sponges at 5 mV/s. (e) Galvanostatic charge/discharge curves of the compact CNT/PANI supercapacitor. (f) Calculated gravimetric and volumetric capacitances of the corresponding cell. Inset, Ragone plots of gravimetric energy density versus gravimetric power density and volumetric energy density versus volumetric power density for the compact CNT/PANI supercapacitor.

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Figure 4. epoxy/CNT/PANI nanocomposites. (a) Illustration of the fabrication process and the model of microstructure for epoxy/CNT/PANI sponge after tensile testing. (b) Comparison of stress and strain for pure epoxy, epoxy/CNT and epoxy/CNT/PANI composites obtained by tensile testing. All the data are average value of three specimens. (c) SEM image of the fractured surface of the epoxy/CNT composite after tensile testing and (d) SEM image of the epoxy/CNT/PANI composite after tensile testing. (e) Comparison of electrical conductivities of CNT sponge, CNT/PANI sponge, epoxy/CNT sponge and epoxy/CNT/PANI sponge. Inset shows models of composite nanotubes.

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Polyaniline Porous Network for

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