Open access peer-reviewed chapter

Recent Progress on Electrochemical Capacitors Based on Carbon Nanotubes

Written By

Emilia Grądzka and Krzysztof Winkler

Submitted: April 25th, 2017 Reviewed: October 13th, 2017 Published: December 20th, 2017

DOI: 10.5772/intechopen.71687

From the Edited Volume

Carbon Nanotubes

Edited by Mohammed Muzibur Rahman and Abdullah Mohamed Asiri

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Abstract

This review is focused on the theoretical and practical aspects of electrochemical capacitors based on carbon nanotubes. In particular, recent improvements in the capacitance properties of the systems are discussed. In the first part, the charge storage mechanisms of the electrochemical capacitors are briefly described. The next part of the review is devoted to the capacitance properties of pristine single- and multi-walled carbon nanotubes. The major portion of the review is focused on the capacitance properties of modified carbon nanotubes. The electrochemical properties of nanotubes with boron, nitrogen, and other atoms incorporated into the carbon network structure as well as nanotubes modified with different functional groups are discussed. Special attention is paid to the composites of carbon nanotubes and conducting polymers, transition metal oxides, carbon nanostructures, and carbon gels. In all cases, the influences of different parameters such as porosity, structure of the electroactive layer, conductivity of the layer, nature of the heteroatoms, solvent and supporting electrolyte on the capacitance performance of hybrid materials are discussed. Finally, the capacitance properties of different systems containing carbon nanotubes are compared and summarized.

Keywords

  • carbon nanotubes
  • carbon nanoparticles
  • conducting polymers
  • composites
  • electrochemical capacitors
  • charge storage materials

1. Introduction

The last two decades can be considered a nanotechnology revolution. With each passing day, increasing attention is paid to the discovery of new nanoscale materials due to the miniaturization of devices in many areas. Nanomaterials are everywhere, from cosmetics and clothes to medicine and electronic devices. At the same time, the increased development of electronic devices requiring energy storage systems, such as batteries and supercapacitors, is starting to play a crucial role in everyday life. Supercapacitors, also called electrochemical capacitors, are energy saving units that can provide a huge amount of energy in a short time. Compared with batteries, electrochemical capacitors offer great advantages of high power capability, high rates of charge and discharge, high cycle life, flexible packaging and low weight [1, 2, 3, 4].

An interesting class of nanomaterial for storage devices is carbon nanotubes due to their large open surface that is completely exposed to electrolyte ions, good electrical conductivity, high surface area, mechanical strength, good corrosion resistance, chemical stability and low mass density [4, 5, 6, 7, 8]. However, so far, pristine CNTs have not met the commercial requirements for energy storage devices applications due to their poor dispersion in solvents, low electrochemical characteristics in the electrolytes, and low specific capacitance [7, 8]. The capacitance properties of carbon nanotubes may be improved by surface functionalization or by the formation of composites with redox-active systems such as conducting polymers or metal oxides. On the other hand, carbon nanotubes are excellent conducting supports to improve the properties of materials with poor conductivity, stability, and capacitance performance. Hence, carbon nanotubes-based nanocomposites or nanohybrids have recently been intensively developed because of their superior properties compared to the individual component alone. Much research on supercapacitors has aimed at increasing power and energy densities as well as lowering fabrication cost [8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18].

The objective of this review is to highlight theoretical and practical aspects of the capacitance properties of carbon nanotubes and their composites. In particular, recent progress in improving the capacitance properties of systems based on carbon nanotubes is discussed. In the first part, the charge storage mechanisms in electrochemical capacitors are briefly described. The next part of the review is devoted to the capacitance properties of pristine carbon nanotubes and carbon nanotubes modified by heteroatoms or functional groups. Special attention is paid to the composites of carbon nanotubes with conducting polymers, carbon nanostructures, transition metal oxides, and carbon gels.

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2. Principles and mechanism of charge storage

Electrochemical energy storage may be classified as encompassing either batteries or electrochemical capacitors (ECs), which are also known as supercapacitors, ultracapacitors, electrical double-layer capacitors, pseudocapacitors, gold capacitors or power coaches [19, 20, 21]. Supercapacitors fill the gap between batteries that exhibit high energy density and high power density conventional capacitors, covering several orders of magnitude both in energy and in power density. Batteries compared to supercapacitors store and deliver more energy with slower charge and discharge times. In the case of conventional capacitors, they have very high energy density in comparison to supercapacitors. Supercapacitors also exhibit low heating level, safety, long-term operation stability, low weight, and flexible packaging [4, 22, 23]. Due to these properties of semiconductors, they can be applied in many fields of electrotechnology. They can be used in electric and hybrid vehicles [3, 24, 25], high-energy pulsed lasers [26], mobile phones [26, 27, 28], laptops [27], and cameras [27, 28].

The charge-storage mechanism of electrochemical capacitors is controlled by two principal mechanisms: (i) a non-Faradaic electrostatic interaction resulting from ion adsorption at the electrode/electrolyte interface defined as electric-double layer capacitance (EDLC) (Figure 1a) and (ii) an electrochemical oxidation/reduction reaction in electroactive materials accompanied with Faradaic charge transfer named pseudocapacitance (Figure 1b) [4, 29, 30, 31, 32].

Figure 1.

Schematic diagram of supercapacitors: (a) EDLC and (b) pseudocapacitor (M represents the metal atom). Reproduced with permission from Ref. [32]. Copyright 2014 The Royal Society of Chemistry.

Typical supercapacitors consist of two electrodes separated by a dielectric material or electrolyte solution. In the case of EDLC, where the capacitance comes from the electrostatic charge accumulated at the electrode/electrolyte interface and strongly depends on the surface area of the electrode accessible to the electrolyte ions, mostly porous carbons are used as electrode materials due to their low price, facile synthesis, and sustainability. They are prepared by heat treatment and subsequent chemical activation with organic materials or, in the case of “carbide-derived carbons” (CDCs), by extracting metal atoms in metal carbides. More exotic materials, such as carbon nanotubes (CNTs) and graphene, are also being developed for supercapacitor applications. The operating voltage of supercapacitors depends on the solvent and electrolytes separating both electrodes. Tetraethylammonium tetrafluoroborate is often used as the supporting electrolyte in organic solvents. This salt offers a relatively high operating voltage of approximately 2.5 V and a high ionic conductivity of 20–60 mS cm−1. In aqueous solutions, a maximum potential of 1.2 V can be obtained. Moreover, room-temperature ionic liquids with an operating voltage as high as 4 V could also be used as electrolyte systems. However, supercapacitors based on ionic liquids have poor device power performance due to their low ionic conductivity, i.e., below 20 mS cm−1 [23, 30, 33].

The specific capacitance for EDLC is assumed to follow that of a parallel-plate capacitor:

C = ε r ε 0 dA E1

where εr is the electrolyte dielectric constant, ε0 is the permittivity of vacuum, A is the specific surface area of the electrode, and d is the effective thickness of the EDLC. Depending on the pore size of the electrode material, two models are proposed: an electric double cylinder capacitor (EDCC) model that describes mesoporous carbon electrodes (Figure 2a) and an electric wire-in-cylinder capacitor (EWCC) model corresponding to microporous carbon electrodes (Figure 2b). The equation describing the capacitance in these two models can be written as:

C = ε r ε 0 bln b b d A E2
C = ε r ε 0 bln b a 0 A E3

Figure 2.

Schematic diagrams (top views) of (a) a negatively charged mesopore with solvated cations approaching the pore wall to form an electric double-cylinder capacitor and (b) a negatively charged micropore of radius b with cations lining up along the pore axis to form an electric wire-in-cylinder capacitor. Reproduced with permission from Ref. [34]. Copyright 2008 Wiley-VCH Verlag.

where b is the pore radius, d is the distance of approaching ions to the surface of the carbon electrode, and a0 is the effective size of counterions. When the pores are large enough and the pore curvature is no longer significant, the EDCC model is reduced to the traditional planar EDLC model described by Eq. (1) [30, 34].

In contrast to EDLC, pseudocapacitance results from faradaic fast reversible redox reactions involving electrode material. During electrochemical reactions, the electrode material is reduced or oxidized and doped with counterions from the electrolyte solution. In this case, conducting polymers and metal oxides or hydroxides as electrode materials are used [30]. High-area carbon electrodes also exhibit a small pseudocapacitance component due to electrochemically active redox functionalities [22]. Pseudocapacitance is described by the following equation:

C = d Δ q d Δ V E4

where Δq is the charge related to the faradaic process in the potential range ΔV. The pseudocapacitance can reach higher values than the EDLC capacitance. However, redox capacitors exhibit lower power density due to poor electrical conductivity and lack of stability during the cycle.

The maximum energy stored in supercapacitors is given by

E = 1 2 CV 2 E5

where C is the capacitance and V is the operating voltage limited by the thermodynamic stability of the electrolyte solution. The total power of a supercapacitor is described by the following equation:

P = V 2 4 R E6

where R is the internal resistance of the capacitor coming from the various types of resistances associated with the intrinsic electronic properties of the electrode material and electrolyte solution, mass transfer resistance of the ions in the electrode material, and contact resistance between the current collector and the electrode [22, 23, 30, 31].

The electrode is one of the most important components for charge storage and plays a crucial role in determining the energy and power density of supercapacitors [29]. In EDLC, the high surface area resulting from the highly microporous structure of carbon electrode materials such as carbon aerogels, carbon black or carbon cloth is unfavorable for electrolyte wetting and rapid ionic motions, especially at high current loads [22]. The most frequently used activated carbon electrodes exhibit low electrolyte accessibility and poor electrical conductivity. These two effects are responsible for limited energy density and high internal resistance. For this reason, carbon nanotubes (CNTs) with nanoscale size, controllable size distribution, large surface area, high mesoporosity, electrolyte accessibility, and good electrical properties are very promising candidates for replacing carbon materials as the electrode materials in high-performance capacitors. Because of this fact, an extended part of this work will focus on supercapacitors based on carbon nanotubes [4, 29].

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3. Capacitance properties of single- and multi-walled carbon nanotubes

Carbon nanotubes exhibit large open surface area, excellent mechanical strength, chemical stability, low mass density, and relatively good electrical conductivity. All these properties make them a very good candidate for the electroactive material in charge storage devices. In these capacitors, the ions of electrolyte are adsorbed on the charged surface of carbon nanotubes, producing a Helmholtz layer. The capacitance properties of these systems depend on the number of graphene walls (single-, double-, or multi-wall), the nature of the electrode material, the composition of the electrolyte solution, and the structure of the carbon nanotube layer. The specific capacitance of pristine CNTs is relatively low and ranges from 2 to 45 F g−1 for SWCNTs [35, 36, 37, 38, 39, 40, 41, 42] and 3 to 80 F g−1 for MWCNTs [36, 37, 38, 42, 43, 44]. The capacitance properties of thin films of CNTs are collected in Table 1.

Carbon nanostructure Specific capacitance (F g−1) Experimental conditions Reference
SWCNTs 2 0.1 M Na2SO4 [39]
17 1 M NaCl [40]
18 1 M NaCl [35]
19 5 M KOH [35]
20 6 M KOH [41]
21 1 M NaCl [35]
24 7 M H2SO4 [35]
40 6 M KOH [38]
64 1 M H2SO4 [42]
MWCNTs 3 7 M KOH [43]
9 7 M KOH [43]
14 1 M H2SO4 [42]
14 6 M KOH [38]
17 4 M H2SO4 [43]
26 4 M H2SO4 [43]
30 1 M H2SO4 [44]
36 6 M KOH [38]
38 1 M Na2SO4 [44]
62 6 M KOH [38]
78 1 M H2SO4 [38]
80 6 M KOH [38]

Table 1.

Selected values of specific capacitances obtained for thin films of carbon nanotubes.

PC, propylene carbonate.

Such a large scattering of reported specific capacitance values is mainly related to the different procedures of CNT deposition on the conducting electrode surface and, therefore, different structures of CNTs films. Carbon nanotubes can be attached to electrodes by direct growth [45], manual manipulation [46], random spreading [47], deposition in a dc current electric field [48], or gas flow [49]. Specially designed substrates are used for film formation of vertically oriented CNTs [50, 51, 52, 53]. Two of the most common orientations of CNTs are displayed schematically in Figure 3. In general, films of vertically oriented carbon nanotubes exhibit much better electrical and capacitance properties in comparison to films formed from randomly oriented nanotubes [6, 50, 54, 55, 56, 57]. Figure 4 shows the structure of the SWCNT layer formed under CVD growth procedure. The highest specific capacitance of 52 F g−1 and excellent electrochemical stability were reported for such vertical structures of SWCNTs [55].

Figure 3.

Schematic representation of nanotubes orientation at the electrode surface.

Figure 4.

SEM image of the aligned carbon nanotubes formed under CVD procedure. Reproduced with permission from Ref. [6]. Copyright 2011 American Chemical Society.

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4. Covalent modification of carbon nanotubes and their capacitance properties

The covalent functionalization of carbon nanotubes provides modification of their physicochemical properties. Insertion of functional groups to the carbon network solves the problem of their poor dispersion capability. Stable dispersions of modified carbon nanotubes are particularly important in the formation of thin films of capacitors or capacitive devices. Moreover, tethering electron-donating or electron-withdrawing groups on CNT surfaces changes their electronic properties by the doping effect [22, 58, 59, 60, 61, 62]. Such groups exhibit electrochemical activity resulting in a pseudo-Faradaic capacitance effect.

Park and co-workers showed the relationship between the electrochemical activity of functional groups attached to the CNT surface and their capacitive characteristics [58]. They found that MWCNTs covalently functionalized by carboxylic, sulfonic, and amine groups (Figure 5) showed a two- to fourfold increase in capacitance over that of pristine MWCNTs due to pseudocapacitive charging-discharging arising from the presence of functional groups. Functionalized CNTs also form more stable dispersions in deionized water and polar organic solvents. Such dispersions can be used in the formation of mechanically stable and uniform capacitor films. Functionalized CNTs, however, exhibit a lower surface area due to the reduction in the average pore size as a consequence of the presence of surface functional groups [58].

Figure 5.

Schematic illustration of the covalent functionalization of CNTs. Reproduced with permission from Ref. [58]. Copyright 2013 The Royal Society of Chemistry.

Figure 6 shows the effect of the addition of oxygen redox-active molecules on the electrochemical performance of CNTs. Nanotubular materials can be treated (CNTs-T) chemically [62, 63, 64, 65, 66, 67, 68, 69, 70], electrochemically [59, 71, 72], photochemically [73, 74], and using plasma-induced techniques [75, 76]. The chemical modifications are usually performed in concentrated nitric acid or in a mixture of nitric and sulfuric acids. Cyclic voltammograms recorded for raw CNTs exhibit pseudorectangular cathodic and anodic profiles, which are the characteristics of ideal capacitors. In the case of CNTs-T, a pair of voltammetric peaks is observed. They are related to the redox reactions of functional groups on the CNTs-T surface:

E7
E8
E9

Figure 6.

Cyclic voltammograms of (a) raw CNTs and (b) CNTs-T electrodes in 1 M H2SO4. Reproduced with permission from Ref. [62]. Copyright 2014 Elsevier.

A higher current observed for CNTs-T suggests that their surface area is larger compared to that of raw CNTs. The surface of oxygen-containing functional groups also decreases surface resistivity and enhances surface wettability, offering more accessible sites for the physisorption of electrolyte ions and increasing the ionic conductivity at the electrode/electrolyte interface. In the procedure proposed by Wang and co-workers [62], the specific capacitance was increased from 28 F g−1 for raw CNTs to 85 F g−1 for CNTs-T in H2SO4 solution. Changing the electrolyte to H2SO4/hydroquinone mixture provides a drastic improvement of the specific capacitance to 508 and 3199 F g−1 for CNTs and CNTs-T, respectively [62].

The distribution and type of oxygen-containing functional groups depend on the type (Figure 7a) and concentration (Figure 7b) of the oxidizing agent [66, 67].

Figure 7.

(a) Influence of the oxidant on the distribution of oxygen-containing functional groups, measured on MWCNTs. (b) Influence of % w/w HNO3 on the level of oxidation, as well as the distribution of oxygen-containing functional groups. Reproduced with permission from Ref. [66]. Copyright 2011 Elsevier.

The oxidation of CNTs is a principle reaction for further functionalization of the carbon network. The combination of CNTs with graphene oxide in a lamellar graphene-CNT structure, r[GO/CNT], shown in Figure 8 causes an increase in the electrolyte-accessible surface area due to the intercalation of CNTs between the stacked GO sheets with associated large electrochemical active sites, thus improving conductivity through the formation of a 3D network aided by CNTs. Such a covalently linked CNTs-graphene system exhibits capacitance performance much superior than that of other carbon-based electrodes [77].

Figure 8.

Synthesis scheme depicting the reaction between a GO layer and a CNT and a view of the fabricated r[GO-CNT] film. Reproduced with permission from Ref. [77]. Copyright 2013 Wiley-VCH Verlag.

The high specific capacitance of CNTs could also be obtained with the incorporation of heteroatoms such as N [61, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87], B [86, 87, 88, 89], P [86, 90], and F [91]. The type of defect created by the heteroatom influences the kind of conduction generated ranging from n-type transport (N substitution doping) to p-type conduction (B substitution of boron in lattice) [92]. The existence of heteroatoms significantly enhances the specific capacitance values of carbon materials by the pseudocapacitance effect [83, 88]. Gueon and co-workers found that nitrogen-doped carbon nanotubes have a specific capacitance three times higher compared to undoped carbon nanomaterial [79]. Nitrogen-containing pentagons (Figure 9) can induce strong bending of the nanotubes and affect the alignment of the CNT lattice, resulting in the creation of donor states near the Fermi level and hence the specific capacitance [79, 80]. The effect of B-doping on the capacitance can be explained by modification of the space charge layer in carbon. The presence of boron leads to the enhancement of the space-charge layer capacitance through increasing the number of holes as a charge carrier or the DOS change at the Fermi level [88].

Figure 9.

Schematic representation of N doping in the carbon nanomaterial. Reproduced with permission from Ref. [92]. Copyright 2013 De Gruyter.

Thermal oxidation [88] or acid treatment [81] of doped CNTs improves the specific surface area and, as a consequence, the capacitance due to the tube end opening process. Coaxial supercapacitors based on hollow carbon nanotubes synthesized by chemical vapor deposition (CVD) exhibit high rate capacitance, long cycle life, and good flexibility [93].

The combination of synergetic effects in boron carbonitride nanotubes (BCNs) resulting from the combined co-doping of B and N in CNTs and their well-aligned vertical structure (VA) also provides a significant enhancement of specific capacitance performance. Vertically aligned boron carbonitride nanotubes (VA-BCNs) were synthesized on a Ni-Fe-coated SiO2/Si substrate using the chemical vapor deposition method with a melamine diborate precursor. In this case, nitrogen atoms are bonded to carbons in both graphitic and pyridinic forms, and the resultant VA-BCNs show high specific capacitance (321 F g−1) with an excellent rate capability and high durability compared to nonaligned BCNs (167 F g−1) and undoped multi-walled carbon nanotubes (117 F g−1) [94].

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5. Hybrid materials of carbon nanotubes for energy storage

Carbon nanotubes are combined with carbon nanostructures, conducting polymers or metal oxides to form nanocomposite materials that display favorable electronic and mechanical properties [95]. Composites containing carbon nanotubes and an electroactive phase exhibit pseudocapacitive properties like metal oxide or conducting polymers and represent an important breakthrough for developing a new generation of supercapacitors based on three reasons: (i) the percolation of the electroactive particles is more efficient with nanotubes than with other carbon materials; (ii) the open mesoporous network formed by the entanglement of nanotubes allows ions to diffuse easily to the active surface of the composite components; and (iii) since the nanotubular materials are characterized by a high resiliency, the composite electrodes can easily adapt to the volumetric changes during the charging/discharging process, which improves drastically the cycling performance [22]. However, capacitors fabricated from inorganic pseudocapacitive materials typically suffer from higher internal resistance and lower lifetimes. The combination of carbon nanotubes with other carbon nanostructures results in an enhancement of energy density and lowering of the internal resistance [96].

5.1. Capacitors based on carbon nanotubes and different carbon nanostructures

The continued technological progress has led to the miniaturization of electronic devices with large volumetric energy densities. Most fabricated micro-supercapacitors based on carbon nanostructures exhibit excellent rate capabilities and stability, but low volumetric energy density. In general, increased volumetric energy density is obtained by the application of porous conductive electrode materials with sufficiently high packing density. The most popular electrode material is graphene sheets because of their ultrahigh surface area and excellent conductivity as well as high mechanical and chemical stability. However, various interesting structures of graphene, such as one-dimensional fibers, two-dimensional films, and three-dimensional foams, in spite of high gravimetric capacitance exhibit poor volumetric performance due to strong intersheet π-π interactions, which, while increasing the packing density, do not allow high ion accessibility. To solve this problem, the synergistic effect of graphene and carbon nanotubes is utilized by the preparation of graphene-carbon nanotubes composites. In most cases, graphene oxide is used due to its hydrophilic character. However, its conductivity is low, and hence, it is doped by carbon nanotubes with very high conductivity [97]. Numerous synthesis methods have been used and a large family of hybrid composites based on graphene oxide and carbon nanotubes with the specific capacitance in the range of 120–222 F g−1 depending on the preparation procedure has been discovered [77, 98, 99, 100, 101, 102, 103, 104].

Ternary carbon composites containing carbon nanotubes, graphene, and activated carbon exhibit much better capacitance performance. Such systems have specific capacitances several times higher than those of their components (Figure 10a). The excellent electrochemical properties of ternary composites can be attributed to the high surface area and low equivalent series resistance, demonstrating that they improve the electrochemical performance for supercapacitor applications [97]. Among the multicomponent composites, a novel type of highly flexible and all-solid state supercapacitor utilizing hybrid aerogels exhibits promising properties (Figure 10b) [105].

Figure 10.

(a) Cyclic voltammograms of different carbon particles (GO, CNT, AC) and their composite recorded in 1 M H2SO4. Reproduced with permission from Ref. [97]. Copyright 2017 Elsevier. (b) Schematic diagram of the all-solid-state supercapacitor where the polymer-gel electrolyte serves as the electrolyte and the separator. Insets show the flexibility of the device and SEM image of aerogel hybrid film. Reproduced with permission from Ref. [105]. Copyright 2015 American Chemical Society.

Recently, much attention has been paid to 3D pillared vertically aligned carbon nanotubes (VACNTs)—graphene architectures with a controllable nanotube length (PL)/intertube distance (MIPD) as electrode materials for energy-related devices (Figure 11). Theoretical studies have indicated that 3D pillared architectures, consisting of parallel graphene layers supported by vertically aligned carbon nanotubes (VACNTs) in between, possess desirable out-of-plane transport and mechanical properties while maintaining the excellent properties of their building blocks [102].

Figure 11.

(a) Schematic diagram of a 3D pillared VACNT graphene nanostructure. (b) SEM images of the 3D pillared VACNT graphene architectures. Reproduced with permission from Ref. [102]. Copyright 2011 American Chemical Society.

An ultrafast compact capacitor based on free-standing, flexible, and highly conducting films consisting of stacked nanoporous graphene layers pillared with single-walled carbon nanotubes (SWCNTs) was obtained by Pham and co-workers [109]. Figure 12 shows the carbon nanotube (CNT)-bridged graphene 3D building blocks via the Coulombic interaction between positively charged CNTs grafted by cationic surfactants and negatively charged graphene oxide sheets, followed by KOH activation. Such a structure enhances the accessible surface area and allows for fast ion diffusion. Due to this unique 3D porous structure, a remarkable electrochemical performance with a maximum capacitance as high as 199 F g−1 was achieved [99].

Figure 12.

Schematic for fabricating the ac-Gr/SWCNT hybrid nanostructure. (a) The CTAB-grafted SWCNTs are positively charged, and the GO layers are negatively charged due to their functional groups. (b) Schematic of the 3D SWCNT-bridged graphene block. Reproduced with permission from Ref. [99]. Copyright 2015 American Chemical Society.

The electrodeposition of nickel hydroxide on such hybrid nanostructures results in pseudocapacitance due to the Faradaic reaction associated with the Ni(OH)2 coating. The specific capacitance is increased from 110 F g−1 obtained for VACNT-graphene architectures to 1384 F g−1 for Ni(OH)2-coated VACNT graphene electrode [102].

Pristine carbon nanotubes usually provide unsatisfactory specific capacitance due to their relatively low surface area, which still needs to be enhanced. Composites containing hollow carbon nanospheres anchored to the surface of carbon nanotubes, CNT-HCS, (Figure 13) are synthesized via the hard template method following this trend. Disordered pores (~2 nm) observed in the shells of the carbon spheres facilitate the penetration of electrolyte ions and favor the rapid charge propagation during the charge/discharge process [106]. The electrochemical performance of capacitors based on the composites of carbon nanotubes and different carbon nanostructures are collected in Table 2.

Figure 13.

TEM images of CNT-HCS composite with different magnifications. Reproduced with permission from Ref. [106]. Copyright 2013 Elsevier.

Composite Specific capacitance (F g−1) Capacitance Power density Energy density Capacitance retention Reference
CNF/rGO/CNT 252 216 mF cm−2 9.5 mW cm−2 28.4 mW cm−2 99.5% [105]
HCS/CNT 201.5 30.1 F cm−3 11.8 kW kg−1 11.3 Wh kg−1 90% [106]
SWCNT/GO 305 F cm−3 1.085 mW cm−3 6.3 mWh cm−3 93% [98]
SWCNT/GO 199 211 mF cm−3 400 kW kg−1 110.6 Wh kg−1 98.2% [99]
GO/CNT/AC 636 550 W kg−1 16 Wh kg−1 99.8% [97]
Graphene/CNT 3.93 mF cm−3 115 W cm−3 2.42 mWh cm−3 86% [107]
SWCNT/CNH 43 3.50 kW kg−1 6.03 Wh kg−1 [96]

Table 2.

Selected values of electrochemical performance of capacitors based on composites of carbon nanotubes and different carbon nanostructures.

CNF, carbon nanofibril; rGO, reduced graphene oxide; HCS, hollow carbon spheres; AC, activated carbon; CNH, carbon nanohorns.

5.2. Capacitors based on carbon nanotubes and conducting polymers

Composites of conducting polymers with carbon nanotubes are promising electrode materials as supercapacitors because of their good conductivity, high surface area, and excellent ability to store energy [7, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117]. The composites combine the large pseudocapacitance of conducting polymers with the fast charging/discharging double-layer capacitance and excellent mechanical properties of carbon nanotubes [118]. In Scheme 1, the properties of carbon nanostructures, conducting polymers, and the composite materials are compared.

Scheme 1.

Properties of carbon nanostructures, conducting polymers and the composite materials.

CNTs/conducting polymer composites can be prepared by chemical [7, 13, 108, 113, 114, 118, 119, 120, 121] or electrochemical [14, 16, 95, 110, 111, 112, 115, 118, 119, 121] polymerization. This process can be carried out on pristine CNTs or CNTs modified with functional groups or heteroatoms as a non-covalent deposition polymeric layer onto the nanotubular surface or covalent functionalization of carbon walls by polymeric chains. In the case of covalent functionalization, two approaches can be used: first, when the monomer is attached to CNTs and, in the next step, when it is polymerized via chemical or electrochemical methods [122, 123], and second, in situ chemical polymerization in the presence of dopant and self-organizing agent [7, 124]. For noncovalently modified CNTs with polymers, several strategies have been developed. Currently, they involve physical mixing in solution, in situ polymerization of monomers in the presence of CNTs, surfactant- and template-assisted processing of composites, chemical functionalization of the incorporated nanotubular material, and electrochemical polymerization carried out on an electrode surface modified with carbon nanostructures or electrochemical co-polymerization [118, 125, 126]. In Figure 14, exemplary procedures of nanocomposites formation are schematically presented. In most cases, electrochemical synthesis provides homogenous films [16, 118]. However, chemical polymerization generates polymers with a more porous morphology [118, 127].

Figure 14.

A schematic representation of exemplary procedures of nanocomposites formation for electrochemical measurements. (A) in situ chemical polymerization, (B) chemical polymerization of monomers attached to carbon nanostructure, (C) electrochemical polymerization carried out on an electrode surface modified with carbon nanostructures.

Figure 15 shows the SEM images of SWCNTs/PPy composites prepared by deposition of PPy on pristine SWCNTs (Figure 15a) and on functionalized SWCNTs (Figure 15b). In the case of functionalized SWCNTs, an incomplete coverage of SWCNTs by PPy is observed [14].

Figure 15.

SEM images of (a) SWCNTs/PPy and (b) functionalized SWCNTs/PPy. Reproduced with permission from Ref. [14]. Copyright 2007 Elsevier.

A comparison of the electrochemical behavior of PPy, SWCNTs/PPy, and PPy/functionalized SWCNTs composites is shown in Figure 16. PPy films at a low scan rate exhibits a rectangular shape, which indicates an ideal capacitance behavior, while for higher scan rates, the curves are not rectangle-shaped, which indicates resistance-like electrochemical behavior due to the slow doping/dedoping process of the compact PPy layer. However, a pseudorectangular shape of the recorded cyclic voltammograms in all of the scan rates (up to 200 mV·s-1) is observed for the composites based on pristine SWCNTs (Figure 16b) and functionalized SWCNTs (Figure 16c). The PPy/functionalized SWCNTs composite exhibits also better capacitance properties compared to PPy/SWCNTs and pristine PPy (Figure 17).

Figure 16.

Cyclic voltammograms recorded for pristine (a) PPy, (b) SWCNTs/PPy and (c) functionalized SWCNTs/PPy composite films at different scan rates: (1) 10, (2) 20, (3) 50, (4) 100 (5) 200 mV s−1 in 1M KCl solution. Reproduced with permission from Ref. [14]. Copyright 2007 Elsevier.

Figure 17.

The specific capacitance of (1) pristine PPy film, (2) PPy/SWCNTs composite film and (3) PPy/functionalized SWCNTs composite film at various scanning rates solution. Reproduced with permission from Ref. [14]. Copyright 2007 Elsevier.

Another way to form composites based on CNTs and conducting polymers is covalent functionalization of CNTs with monomeric units. In the next step, the composite is obtained by copolymerization of the monomer with monomer units grafted on the CNTs surface using controlled potential electrolysis. Figure 18a shows the electrochemical behavior of polypyrrole/CNTs (PPY/CNTs) composites obtained at different charge densities. The broad peaks observed in the voltammograms are related to the redox process within the polypyrrole component. The composites exhibit higher currents than the pure polymeric material due to higher porosity (Figure 18c) compared to pristine PPy (Figure 18b) and as a consequence of the higher capacitance of PPy/CNTs [122].

Figure 18.

(a) Cyclic voltammograms recorded in propylene carbonate containing 0.1 M LiClO4 for the PPY/CNTs composite films and pure polypyrrole obtained in galvanostatic conditions at different charge densities. SEM images of (b) PPY and (c) PPY/CNTs composite. Reproduced with permission from Ref. [122]. Copyright 2013 Hindawi.

The presence of CNT components results in an increase in the practical range of electrical conductivity of the material. For example, C60-Pd polymer demonstrates pseudocapacitance behavior due to faradaic reduction of C60 in the negative potential range (Figure 19a); at potentials lower than this threshold, the material exhibits very high resistance. In the CNTs/C60Pd composite, the potential range of electrochemical activity increases. At negative potentials, the pseudocapacitive behavior of the polymeric component is still observed. However, the material also shows double-layer capacitance, mainly attributed to the conducting CNTs, in a less negative potential range (Figure 19b). Composites based on C60-Pd polymers and SWCNTs or MWCNTs exhibit specific capacitance equal to 994 F g−1 or 758 F g−1 for SWCNTs/ C60-Pd and MWCNTs/C60-Pd composites, respectively [119].

Figure 19.

Cyclic voltammogram of (a) C60-Pd polymer and (b) MWCNT/C60-Pd composite at 100 mV/s in acetonitrile containing 0.1 M (n-C4H9)4NClO4. Reproduced with permission from Ref. [119]. Copyright 2013 Elsevier.

A very promising energy storage behavior was reported for 3D structure composite based on aligned carbon nanotubes (ACNTs) offering a large specific surface area, superior electronic transfer ability through individual nanotubes, and chemical inertness [95]. Three-dimensional hybrid composite material composed of 2D fish scale-like polyaniline nanosheet arrays on graphene oxide sheets and carbon nanotubes exhibiting high specific capacitance of 589 F g−1 compared to 397 F g−1 for pristine PANI was also investigated [18]. Moreover, ternary composite systems containing carbon nanotubes, graphene, and conducting polymers were also investigated [128, 131]. The capacitance properties of selected composites of carbon nanotubes and conducting polymers are reported in Table 3.

Carbon nanostructure Composite Specific capacitance (F g−1) Reference
SWCNT SWCNT/PPy 200 [14]
SWCNT/PPy 305 [132]
SWCNT/PANI 247 [129]
SWCNT/PANI 485 [111]
SWCNT/PANI 707 [133]
SWCNT/PANI 1000 [134]
SWCNT/C60-Pd 994 [119]
MWCNT MWCNT/PANI 50 [120]
MWCNT/PANI 500 [112]
MWCNT/PANI 670 [130]
MWCNT/PPy 70 [120]
MWCNT/PPy 268 [7]
MWCNT/PPy 243 [16]
MWCNT/PPy 506 [130]
MWCNT/PPy 554 [126]
MWCNT/C60-Pd 758 [119]
MWCNT/PEDOT:PSS 30 [120]
MWCNT/PEDOT 237 [126]
Graphene/CNT Graphene/CNT/PPy 453 [132]
GO/CNT GO/CNT/PANI 413 [18]

Table 3.

Capacitance properties of selected composites containing carbon nanostructure and conducting polymer.

5.3. Composites containing carbon nanotubes and metal oxides

To overcome the low energy density of supercapacitors, pseudocapacitors based on transition metal oxides have been developed [135]. The most promising among them is manganese dioxide MnO2 because of its low cost, environmental compatibility, natural abundance, high energy density, and excellent capacitive performance in aqueous electrolytes [32, 135, 136, 137, 138, 139, 140, 141, 142, 143]. In aqueous electrolytes, the charging mechanism of MnO2 may be described by the following reaction:

E10

where M represents protons (H+) and/or alkali cations such as K+, Na+, and Li+. The charge storage is based either on the adsorption of cations at the surface of the electrode material or on the intercalation of cations in the bulk of the electrode material. However, the reported specific capacitance values for the various structures of MnO2 electrodes are still far from the theoretical one [144], which may be attributed to the intrinsically poor electronic conductivity of MnO2. To improve the capacitive performance of MnO2, composites with carbon nanotubes characterized by high conductivity and high surface area are formed [143]. The surface morphology of CNTs/MnO2 composite and its components is shown in Figure 20. Figure 21a shows the comparison of the specific capacitances of pristine CNTs, pure MnO2 and a composite where CNTs are decorated by MnO2 nanoflakes. CNTs exhibit the best rate capability, but their capacitance is the lowest due to the charge storage mechanism typical for double layer capacitors. The low rate capability of MnO2 is associated with its poor electronic conductivity and low specific surface area. The combination of MnO2 and CNTs provides the formation of a MnO2/CNT nanocomposite exhibiting good rate capability and high specific capacitance. The results obtained by electrochemical impedance spectroscopy (Figure 21b) show that the MnO2/CNT composite has a much lower diffusive resistance compared to pure MnO2 because the slope of the low-frequency straight line representing the diffusive resistance of electrolyte in the electrode pores and cation diffusion in the host materials is similar to the line obtained for the CNTs but much larger than that of the pure MnO2. Additionally, the charge transfer resistance, Rct, localized between Rct observed for CNTs and MnO2 suggests high specific surface area for the unique hierarchy architecture of the CNTs/MnO2 composite where the 3D electron path network constructed by the CNT cores and the nanoporous sheath composed of tiny MnO2 nanoflakes facilitate fast electron and ion transport [143].

Figure 20.

SEM images of (a) pristine CNTs, (b) flower-like MnO2, (c and d) MnO2/CNT nanocomposite. Reproduced with permission from Ref. [143]. Copyright 2012 Springer.

Figure 21.

(a) Dependence of specific capacitance on the sweep rate and (b) Nyquist plots for pristine CNTs, pure MnO2, and MnO2/CNT nanocomposite electrodes. Reproduced with permission from Ref. [143]. Copyright 2012 Springer.

Apart from MnO2 [9, 17, 135, 136, 143, 145, 146, 147, 148, 149, 150], other metal oxides such as hydrous RuO2 [151, 152, 153, 154], NiO [155, 156], Fe2O3 [157], Co3O4 [158], MoO3 [10, 135, 159], V2O5 [160], CeO2 [161], and NiCo2O4 [162], In2O3 [149], TiO2 [153], SnO2 [153], and (Sn + Mn)Ox [163] are also utilized for the formation of composite electrodes. In the case of the expensive hydrous ruthenium oxide (RuO2·xH2O) exhibiting excellent capacitance performance [164], relatively high conductivity and exceptional electrochemical reversibility, composites based on carbon nanotubes are formed to reduce its cost and enhance its capacitance properties [151, 152, 153, 154].

So far, most studies have been devoted to the deposition of the pseudocapacitance phase onto carbon nanostructures. However, it is also possible to encapsulate metal oxides with CNTs. The MnO2@CNTs material exhibits a significantly higher specific capacitance compared to MnO2 outside of carbon nanotubes (Table 4). The difference in the electrochemical behaviors of MnO2 enclosed in CNTs (MnO2-in-CNT) and MnO2 deposited on CNTs surface (MnO2-out-CNT) is shown in Figure 22. In such a system, the former stores charge electrostatically, and the latter stores charge in virtue of highly reversible redox reactions of MnO2. The broad anodic peak at 0.72–0.84 and 0.59–0.80 V for the samples MnO2-in-CNT and MnO2-out-CNT, respectively, is associated with the oxidation process of Mn(III) to Mn(IV) according to the reaction:

E11
Carbon nanostructure Composite Specific capacitance (F g−1) Reference
SWCNT SWCNT/In2O3 201 [149]
SWCNT/MnO2 253 [149]
SWCNT/NiC2O4 1642 [162]
MWCNT MWCNT/MoO3 70 [159]
MWCNT/MoO3 178 [10]
MWCNT/MnO2 250 [147]
MWCNT/MnO2 944 [145]
MWCNT/RuO2 138 [153]
MWCNT/RuO2 953 [151]
MWCNT/RuO2 1050 [154]
MWCNT/SnO2 93 [153]
MWCNT/NiO 160 [155]
MWCNT/NiO 523 [156]
MWCNT/TiO2 160 [153]
MWCNT/Co3O4 201 [158]
MWCNT/(Sn+Mn)Ox 337 [163]
MWCNT/MC/MnO2 351 [165]
MWCNT@MnO2@PPy 273 [12]
CNT MnO2-out-CNT 790 [9]
MnO2-in-CNT 1250 [9]
CNT/MnO2 199 [150]
CNT/MnO2 214 [148]

Table 4.

Capacitance properties of selected composites containing carbon nanostructure and metal oxide.

MC, mesoporous carbon.

Figure 22.

Cyclic voltammogram of blank cCNTs (CNTS with closed tips), oCNTs (CNTs with open tips), MnO2-out-CNT and MnO2-in-CNT in 1 M Na2SO4. Reproduced with permission from Ref. [9]. Copyright 2010 The Royal Society of Chemistry.

The oxidation potential of MnO2-in-CNT is shifted to higher values compared to that of MnO2-out-CNT because CNT-confined manganese oxide particles can easily form lower oxidation-state species compared to exterior particles. The double-layer capacitance of MnO2-in-CNT is increased due to both the formation of Mn(IV)/Mn(III) redox couple and the desolvation of electrolyte ions inside the channels of CNTs blocked by MnO2 nanoparticles [9].

Moreover, hybrid materials such as carbon nanotubes/mesoporous carbon/MnO2 [165], carbon nanotubes/3D graphene/MnO2 [11], and carbon nanotube@MnO2@polypyrrole [12] have been developed. Table 4 shows the capacitance of exemplary composites based on carbon nanostructures and metal oxides.

5.4. Capacitors based on carbon nanotubes and carbon gels

Recently, much attention has been paid to novel ultralight mesoporous carbon materials named carbon aerogels (CAGs) [15, 166, 167, 168, 169, 170, 171, 172, 173] or carbon xerogels [8, 174, 175, 176, 177, 178]. Carbon aerogels exhibit many interesting properties such as low mass density, continuous porosity, high surface area, and high electrical conductivity. These properties are characteristic of the aerogel microstructure, which is a network of interconnected primary particles with diameters between 3 and 25 nm (Figure 23a). However, aerogels consist of agglomerate particles linked by covalent bridges with a ladder structure. The contact between these particles and the space due to the pores unfortunately introduces a high internal resistance within the aerogel which limits their practical applications. To overcome this problem, composites of carbon aerogels with carbon nanotubes are formed (Figure 23b). CNTs with their high electrical conductivity work as nanopathways for charges and thus improve the intrinsic conductivity of the CAGs and their mechanical integrity as well [166].

Figure 23.

(a) Representation of the carbon aerogel internal structure and (b) targeted carbon nanotube aerogel composite. Reproduced with permission from Ref. [166]. Copyright 2011 Springer.

The main problem in the case of composite formation is the agglomeration of carbon nanotubes within the CAG’s pore network. A very useful approach for the formation of binderless carbon nanotube aerogel (CNAG) composites proposed by Bordjiba and co-workers [166, 172] provides a significant improvement in the dispersion of carbon nanotubes within the aerogel matrix and better interfacial coating. The CNAG material was prepared by a molding procedure, i.e., synthesis by a chemical vapor deposition method to grow carbon nanotubes directly onto a microfibrous carbon paper substrate. In the next step, the carbon aerogel was synthesized on the carbon nanotubes. The key feature of this method is eliminating the need to control the carbon nanotube concentration, which permits optimized dispersion processes to reinforce the aerogel’s networks. The CNAG electrode delivered very high specific capacitances equal to 524 F g−1. Furthermore, better integration of carbon nanotubes in the matrix of carbon aerogel improves its resistance to attack by the electrolyte and assures excellent cycle life. Such a system exhibits significantly higher capacitance and stability compared to the composite processed from the dispersion of its components in liquid phase, CAG-3%MWNT (Figure 24). Apart from the capacitance performance, binderless nanostructured electrodes also reduce the cost of manufacturing and avoid complicated interferences of the binders and conductivity enhancers used in practical electrodes [166, 172].

Figure 24.

Cycle life of CAG, CAG-3%MWNT and CNAG composites in 5 M KOH. Reproduced with permission from Ref. [166]. Copyright 2011 Springer.

The research on CNTs and carbon aerogel composites is relatively new because their very promising properties are still being developed. The composites that have been prepared so far exhibit excellent capacitance properties [8, 166, 170, 171, 172, 177, 178]. A large progress in this area of study can be expected in the near future.

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6. Conclusion and outlook

Supercapacitors have been developed to close the gap between conventional capacitors and batteries because of their high energy density and power density. In the development of electrochemical capacitors, carbon nanotubes and their composites have been widely used as electrode materials. The specific capacitance of pristine carbon nanotubes is relatively low and depends on many factors such as the kind of carbon nanotubular material, i.e., single- or multi-walled, its orientation, i.e., open or closed tips, surface area, synthesis method, solvent and supporting electrolyte. Compared with pristine carbon nanotubes, functionalized carbon nanotubes by heteroatoms or functional groups attached to nanotubes walls are expected to display improved capacitance performance. The formation of composites based on carbon nanotubes provides especially high surface area due to the presence of CNTs which is very important in the case of storage systems. Moreover, it enhances the properties of both the carbon nanotubes and the second component. Apart from the improvement in capacitance performance, the addition of CNTs reduces cost compared to metal oxide, improves stability compared to conducting polymers which exhibit rapid degradation in performance after repetitive cycles because of their swelling and shrinking, and improves the poor volumetric performance of supercapacitors based on other carbon nanomaterials. The capacitance properties strongly depend on the localization of the redox system. It was found that encapsulation of the redox phase inside a nanotubular material provides higher specific capacitance compared to a redox system situated outside of carbon nanotubes. Recently, a new generation of cheap storage systems based on mesoporous carbon aerogels was discovered. However, in this case, there is a problem with the non-homogeneous spread of carbon nanotubes within the whole network of carbon aerogel. Hence, increasing attention is needed to solve this problem because this system could be the future for storage devices. A very promising system seems to be the attachment of redox-active nanoparticles to carbon nanotubes. Compared to bulk materials, they exhibit unique properties arising from their nanoscale sizes, such as high electrical conductivity, large surface area, short path lengths for the transport of ions and high electrochemical activity. Ultrafast compact capacitors based on 3D hybrid structures that increase the accessible surface area and allow fast ion diffusion are introducing a new class of electrode materials for storage devices. A comparison of the properties of composites based on carbon nanotubes and their components is summarized in Scheme 2.

Scheme 2.

Comparison of properties of composites based on carbon nanotubes and their components.

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Acknowledgments

The authors acknowledge the National Center of Science (project No. 2016/21/B/ST5/02496 to KW) for a financial support.

References

  1. 1. Kandalkar SG, Gunjakar JL, Lokhande CD. Preparation of cobalt oxide thin films and its use in supercapacitor application. Applied Surface Science. 2008;254:5540-5544. DOI: 10.1016/j.apsusc.2008.02.163
  2. 2. Chen GZ. Supercapacitor and supercapattery as emerging electrochemical energy stores. International Materials Reviews. 2017;62:173-202. DOI: 10.1080/09506608.2016.1240914
  3. 3. Garcia P, Torreglosa JP, Fernandez LM, Jurado F. Control strategies for high-power electric vehicles powered by hydrogen fuel cell, battery and supercapacitor. Expert Systems with Applications. 2013;40:4791-4804. DOI: 10.1016/j.eswa.2013.02.028
  4. 4. Li J, Cheng X, Sashurin A, Keidar M. Review of electrochemical capacitors based on carbon nanotubes and graphene. Graphene. 2012;1:1-13. DOI: 10.4236/graphene.2012.11001
  5. 5. Du R, Zhao Q, Zhang N, Zhang J. Marcoscopic carbon nanotube-based 3D monoliths. Small. 2015;11:3263-3289. DOI: 10.1002/smll.201403170
  6. 6. In JB, Grigoropoulos CP, Chernov AA, Noy A. Growth kinetics of vertically aligned carbon nanotube arrays in clean oxygen-free conditions. ACS Nano. 2011;5:9602-9610. DOI: 10.1021/nn2028715
  7. 7. Reddy RK, Alonso-Marroquin F. Polypyrrole functionalized with carbon nanotubes as an efficient and new electrodes for electrochemical supercapacitors. AIP Conference Proceedings. 2017;1856:020002-1-020002-4. DOI: 10.1063/1.4985553
  8. 8. Fathy NA, Annamalai KP, Tao Y. Effects of phosphoric acid activation on the nanopore structures of carbon xerogel/carbon nanotubes hybrids and their capacitance storage. Adsorption. 2017;23:355-360. DOI: 10.1007/s10450-017-9860-y
  9. 9. Chen W, Fan Z, Gu L, Bao X, Wang C. Enhanced capacitance of manganese oxide via confinement inside carbon nanotubes. Chemical Communication. 2010;46:3905-3907. DOI: 10.1039/c000517g
  10. 10. Shakir I, Sarfraz M. Evaluation of electrochemical charge storage mechanism and structural changes in interwined MoO3-MWCNTs composites for supercapacitor applications. Electrochimica Acta. 2014;147:380-384. DOI: 10.1016/j.electacta.2014.09.073
  11. 11. Pan Z, Liu M, Yang J, Qiu Y, Li W, Xu Y, Zhang X, Zhang Y. High electroactive material loading on a carbon nanotube@graphene aerogel for high-performance flexible all-solid-state asymmetric supercapacitors. Advanced Functional Materials. DOI: 10.1002/adfm.201701122
  12. 12. Pequeno de Oliveira AH, Ferreira Nascimento ML, Pequeno de Oliveira H. Carbon nanotube@MnO2@polypyrrole composites: Chemical synthesis, characterization and application in supercapacitors. Materials Research. 2016;19(5):1080-1087. DOI: 10.1590/1980-5373-MR-2016-0347
  13. 13. Mi H, Zhang X, Xu Y, Xiao F. Synthesis, characterization and electrochemical behavior of polypyrrole/carbon nanotube composites using organometallic-functionalized carbon nanotubes. Applied Surface Science. 2010;256:2284-2288. DOI: 10.1016/j.apsusc.2009.10.053
  14. 14. Wang J, Xu Y, Chen X, Sun X. Capacitance properties of single wall carbon nanotube/polypyrrole composite films. Composites Science and Technology. 2007;67:2981-2985. DOI: 10.1016/j.compscitech.2007.05.015
  15. 15. Worsley MA, Satcher JH, Baumann T. Synthesis and characterization of monolithic carbon aerogel nanocomposites containing double-walled carbon nanotubes. Langmuir. 2008;24:9763-9766. DOI: 10.1021/la8011684
  16. 16. Sun X, Xu Y, Wang J, Mao S. The composite film of polypyrrole and functionalized multi-walled carbon nanotubes as an electrode material for supercapacitors. International Journal of Electrochemical Science. 2012;7:3205-3214
  17. 17. Cheng Q, Ma J, Zhang H, Shinya N, Qin LC, Tang J. Electrodeposition of MnO2 on carbon nanotube thin films as flexible electrodes for supercapacitors. Transactions of the Materials Research Society of Japan. 2010;35(2):369-372
  18. 18. Ning G, Li T, Yan J, Xu C, Wei T, Fan Z. Three-dimensional hybrid materials of fish scale-like polyaniline nanosheet arrays on graphene oxide and carbon nanotube for high-performance ultracapacitors. Carbon. 2013;54:241-248. DOI: 10.1016/j.carbon.2012.11.035
  19. 19. Kotz R, Carlen M. Principles and applications of electrochemical capacitors. Electrochimica Acta. 2000;45:2483-2498. DOI: 10.1016/S0013-4686(00)00354-6
  20. 20. Zhang Y, Feng H, Wu X, Wang L, Zhang A, Xia T, Dong H, Li X, Zhang L. Progress of electrochemical capacitor electrode materials: A review. International Journal of Hydrogen Energy. 2009;34:4889-4899. DOI: 10.1016/j.ijhydene.2009.04.005
  21. 21. Yamada H, Bandaru PR. Limits to the magnitude of capacitance in carbon nanotube array electrode based electrochemical capacitors. Applied Physics Letters. 2013;102:173113-1-173113-4. DOI: 10.1063/1.4803925
  22. 22. Pan H, Li J, Feng YP. Carbon nanotubes for supercapacitor. Nanoscale Research Letters. 2010;5:654-668. DOI: 10.1007/s11671-009-9508-2
  23. 23. Forse AC, Merlet C, Griffin JM, Grey CP. New perspectives on the charging mechanisms of supercapacitors. Journal of the American Chemical Society. 2016;138:5731-5744. DOI: 10.1021/jacs.6b02115
  24. 24. Karden E, Ploumen S, Fricke B, Miller T, Snyder K. Energy storage devices for future hybrid electric vehicles. Journal of Power Sources. 2007;168:2-11. DOI: 10.1016/j.jpowsour.2006.10.090
  25. 25. Thounthong P, Rael S, Davat B. Control strategy of fuel cell/supercapacitors hybrid power sources for electric vehicle. Journal of Power Sources. 2006;158:806-814. DOI: 10.1016/j.jpowsour.2005.09.014
  26. 26. Ghosh S, Inganas O. Conducting polymers hydrogels as 3D electrodes: Applications for supercapacitors. Advanced Materials. 1999;11:1214-1218. DOI: 10.1002/(SICI)1521-4095(199910)11:14<1214::AID-ADMA1214>3.0.CO;2-3
  27. 27. Fic K, Lota G, Meller M, Frackowiak E. Novel insight into neutral medium as electrolyte for high-voltage supercapacitors. Energy & Environmental Science. 2012;5:5842-5850. DOI: 10.1039/c1ee02262h
  28. 28. Reddy ALM, Amitha FE, Jafri I, Ramaprabhu S. Asymmetric flexible supercapacitor stack. Nanoscale Research Letters. 2008;3:145-151. DOI: 10.1007/s11671-008-9127-3
  29. 29. Chen T, Dai L. Carbon nanomaterials for high-performance supercapacitors. Materials Today. 2013;16:272-280. DOI: 10.1016/j.mattod.2013.07.002
  30. 30. Zhang LL, Zhao XS. Carbon-based materials as supercapacitor electrodes. Chemical Society Reviews. 2009;38:2520-2531. DOI: 10.1039/b813846j
  31. 31. Zhang LL, Zhou R, Zhao XS. Graphene-based materials as supercapacitor electrodes. Journal of Materials Chemistry. 2010;20:5983-5992. DOI: 10.1039/c000417k
  32. 32. Shi F, Li L, Wang XL, Gu CD, Tu JP. Metal oxide/hydroxide-based materials for supercapacitors. RSC Advances. 2014;4:41910-41921. DOI: 10.1039/c4ra06136e
  33. 33. Lu W, Qu L, Henry K, Dai L. High performance electrochemical capacitors from aligned carbon nanotube electrodes and ionic liquid electrolytes. Journal of Power Sources. 2009;189:1270-1277. DOI: 10.1016/j.jpowsour.2009.01.009
  34. 34. Huang J, Sumpter BG, Meunier V. A universal model for nanoporous carbon supercapacitors applicable to diverse pore regimes, carbon materials, and electrolytes. Chemistry A European Journal. 2008;14:6614-6626. DOI: 10.1002/chem.200800639
  35. 35. Barisci JN, Wallace GG, Baughman RH. Electrochemical studies of single-wall carbon nanotubes in aqueous solutions. Journal of Electroanalytical Chemistry. 2000;488:92-98. DOI: 10.1016/S0022-0728(00)00179-0
  36. 36. Li J, Cassell A, Delzeit L, Meyyappan M. Novel three-dimensional electrodes: Electrochemical properties of carbon nanotube ensembles. Journal of Physical Chemistry B. 2002;106:9299-9305. DOI: 10.1021/jp021201n
  37. 37. Pumera M. Electrochemical properties of double wall carbon nanotube electrodes. Nanoscale Research Letters. 2007;2:87-93. DOI: 10.1007/s11671-006-9035-3
  38. 38. Frackowiak E, Jurewicz K, Delpeux S, Beguin F. Nanotubular materials for supercapacitors. Journal of Power Sources. 2001;97-98:822-825. DOI: 10.1016/S0378-7753(01)00736-4
  39. 39. Krivenko AG, Matyushenko VI, Stenina EV, Sviridova LN, Krestinin AV, Zvareva GI, Kurmaz VA, Ryabenko AG, Dmitriev SN, Skuratov VA. Peculiarities of the electrochemical behavior of modified electrodes containing single-wall carbon nanotubes. Elecrochemistry Communications. 2005;7:199-204. DOI: 10.1016/j.elecom.2004.12.009
  40. 40. Oh J, Kozlov ME, Kim BG, Kim H-K, Baughman RH, Hwang YH. Preparation and electrochemical characterization of porous SWNT–PPy nanocomposite sheets for supercapacitor applications. Synthetic Metals. 2008;158:638-641. DOI: 10.1016/j.synthmet.2008.04.007
  41. 41. Liu CG, Fang HT, Li F, Liu M, Cheng HM. Single-walled carbon nanotubes modified by electrochemical treatment for application in electrochemical capacitors. Journal of Power Sources. 2006;160:758-761. DOI: 10.1016/j.jpowsour.2006.01.072
  42. 42. Vivekchand SRC, Rout CS, Subrahmanyam KS, Govindaraj A, Rao CNR. Graphene-based electrochemical supercapacitors. Journal of Chemical Sciences. 2008;120:9-13. DOI: 10.1007/s12039-008-0002-7
  43. 43. Jurewicz K, Babeł K, Pietrzak R, Delpeux S, Wachowska H. Capacitance properties of multi-walled carbon nanotubes modified by activation and ammoxidation. Carbon. 2006;44:2638-2375. DOI: 10.1016/j.carbon.2006.05.044
  44. 44. Deng L, Hao Z, Wang J, Zhu G, Kang L, Liu Z-H, Yang Z, Wang Z. Preparation and capacitance of graphene/multiwall carbon nanotubes/MnO2 hybrid material for high-performance asymmetrical electrochemical capacitor. Electrochimica Acta. 2013;89:191-198. DOI: 10.1016/j.electacta.2012.10.106
  45. 45. Valentini L, Armentano I, Kenny JM. Sensors for sub-ppm NO2 gas detection based on carbon nanotube thin films. Applied Physics Letters. 2003;82:961-963. DOI: 10.1063/1.1545166
  46. 46. Rueckes T, Kim K, Joselevich E, Tseng GY, Cheung C-L, Lieber CM. Carbon nanotube—Based nonvolatile random access memory for molecular computing. Science. 2000;289:94-97. DOI: 10.1126/science.289.5476.94
  47. 47. Martel R, Schmidt T, Shea HR, Hertel T, Avouris P. Single- and multi-wall carbon nanotube field-effect transistors. Applied Physics Letters. 1998;73:2447-2449. DOI: 10.1063/1.122477
  48. 48. Kamat PV, Thomas KG, Barazzouk S, Girishkumar G, Vinodgopal MD. Self-assembled linear bundles of single wall carbon nanotubes and their alignment and deposition as a film in a dc field. Journal of the American Chemical Society. 2004;126:10757-10762. DOI: 10.1021/ja0479888
  49. 49. Xin H, Woolley AT. Directional orientation of carbon nanotubes on surfaces using a gas flow cell. Nano Letters. 2004;4:1481-1484. DOI: 10.1021/nl049192c
  50. 50. Wang D, Song P, Liu C, Wu W, Fan S. Highly oriented carbon nanotube papers made of aligned carbon nanotubes. Nanotechnology. 2008;19:1-6. DOI: 10.1088/0957-4484/19/7/075609
  51. 51. Wang Y, Iqbal Z. Vertically oriented single-wall carbon nanotube/enzyme on silicon as biosensor electrode. JOM. 2005;34:27-29. DOI: 10.1007/s11837-005-0132-z
  52. 52. Cao A, Ajayan PM, Ramanath G, Baskaran R, Turner K. Silicon oxide thickness-dependent growth of carbon nanotubes. Applied Physics Letters. 2004;84:109-111. DOI: 10.1063/1.1636826
  53. 53. Liu Z, Shen Z, Zhu T, Hou S, Ying L. Organizing single-walled carbon nanotubes on gold using a wet chemical self-assembling technique. Langmuir. 2000;16:3569-3573. DOI: 10.1021/la9914110
  54. 54. Taurino I, Carrara S, Giorcelli M, Tagliaferro A, De Micheli G. Comparison of two different carbon nanotube-based surfaces with respect to potassium ferricyanide electrochemistry. Surface Science. 2012;606:156-160. DOI: 10.1016/j.susc.2011.09.001
  55. 55. Azam MA, Fujiwara A, Shimoda T. Significant capacitance performance of vertically aligned single-walled carbon nanotube supercapacitor by varying potassium hydroxide concentration. International Journal of Electrochemical Science. 2013;8:3902-3911
  56. 56. Luo Y, Li X, Gong Z, Sheng Z, Peng X, Mou Q, He M, Li X, Chen H. Aligned carbon nanotubes array by DC glow plasma etching for supercapacitor. Journal of Nanomaterials. DOI: 10.1155/2013/270289
  57. 57. Izadi-Najafabadi A, Futaba DN, Iijima S, Hata K. Ion diffusion and electrochemical capacitance in aligned and packed single-walled carbon nanotubes. Journal of the American Chemical Society. 2010;132:18017-18019. DOI: 10.1021/ja108766y
  58. 58. Park SK, Mahmood Q, Park HS. Surface functional groups of carbon nanotubes to manipulate capacitive behaviors. Nanoscale. 2013;5:12304-12309. DOI: 10.1039/c3nr04858f
  59. 59. Balasubramanian K, Burghard M. Electrochemically functionalized carbon nanotubes for device applications. Journal of Materials Chemistry. 2008;18:3071-3083. DOI: 10.1039/b718262g
  60. 60. Tasis D, Tagmatachris N, Bianco A, Prato M. Chemistry of carbon nanotubes. Chemical Reviews. 2006;106:1105-1136. DOI: 10.1021/cr050569o
  61. 61. Zhou Z, Zhang Z, Peng H, Qin Y, Li G, Chen K. Nitrogen- and oxygen-containing activated carbon nanotubes with improved capacitive properties. RSC Advances. 2014;4:5524-5530. DOI: 10.1039/c3ra45076g
  62. 62. Wang G, Liang R, Liu L, Zhong B. Improving the specific capacitance of carbon nanotubes-based supercapacitors by combining introducing functional groups on carbon nanotubes with using redox-active electrolyte. Electrochimica Acta. 2014;115:183-188. DOI: 10.1016/j.electacta.2013.10.165
  63. 63. Mombeshora ET, Ndungu PG, Jarvis ALL, Nyamori VO. Oxygen-modified multiwalled carbon nanotubes: Physicochemical properties and capacitor functionality. International Journal of Energy Research. 2017;41:1182-1201. DOI: 10.1002/er.3702
  64. 64. Tian Y, Amal R, Wang D-W. An aqueous metal-ion capacitor with oxidized carbon nanotubes and metallic zinc electrodes. Frontiers in Energy Research. 2016;4:1-14. DOI: 10.3389/fenrg.2016.00034
  65. 65. Smith B, Wepasnick K, Schrote KE, Bertele AR, Ball WP, O’melia C, Fairbrother DH. Colloidal properties of aqueous of acid-treated multi-walled carbon nanotubes. Environmental Science & Technology. 2009;43:819-825. DOI: 10.1021/es802011e
  66. 66. Wepasnick KA, Smith BA, Schrote KE, Wilson KH, Diegelmann SR, Fairbrother DH. Surface and structural characterization of multi-walled carbon nanotubes following different oxidative treatments. Carbon. 2011;49:24-36. DOI: 10.1016/j.carbon.2010.08.034
  67. 67. Wepasnick KA, Smith BA, Botter JL, Fairbrother DH. Chemical and structural characterization of carbon nanotube surfaces. Analytical and Bioanalytical Chemistry. 2010;396:1003-1014. DOI: 10.1007/s00216-009-3332-5
  68. 68. Smith B, Wepasnick K, Schrote KE, Cho H-H, Ball WP, Fairbrother DH. Influence of surface oxides on the colloidal stability of multi-walled carbon nanotubes: A structure-property relationship. Langmuir. 2009;25(17):9767-9776. DOI: 10.1021/la901128k
  69. 69. Datsyuk V, Kaylva M, Papagelis K, Parthenios J, Tasis D, Siokou A, Kallitsis I, Galiotis C. Chemical oxidation of multiwalled carbon nanotubes. Carbon. 2008;46:833-840. DOI: 10.1016/j.carbon.2008.02.012
  70. 70. Li M, Boggs M, Beebe TP, Huang CP. Oxidation of single-walled carbon nanotubes in dilute aqueous solutions by ozone as affected by ultrasound. Carbon. 2008;46:466-475. DOI: 10.1016/j.carbon.2007.12.012
  71. 71. Komarova NS, Krivenko AG, Stenina EV, Sviridova LN, Mironovich KV, Shulga YM, Krivchenko VA. Enhancement of the carbon nanowall film capacitance. Electron transfer kinetics on functionalized surfaces. Langmuir. 2015;31:7129-7137. DOI: 10.1021/acs.langmuir.5b00391
  72. 72. Ye JS, Liu X, Cui HF, Zhang W-D, Sheu F-S, Lim TM. Electrochemical oxidation of multi-walled carbon nanotubes and its application to electrochemical double layer capacitors. Electrochemistry Comunications. 2005;7:249-255. DOI: 10.1016/j.elecom.2005.01.008
  73. 73. Jang IY, Lee SH, Park KC, Wongwiriyapan W, Kim C, Teshima K, Oishi S, Kim YJ, Endo M. Effect of photochemically oxidized carbon nanotubes on the deposition of platinum nanoparticles for fuel cells. Electrochemistry Comunications. 2009;11:1472-1475. DOI: 10.1016/j.elecom.2009.05.036
  74. 74. Lee SH, Jung YC, Kim YA, Muramatsu H, Teshima K, Oishi S, Endo M. Optical spectroscopic studies of photochemically oxidized single-walled carbon nanotubes. Nanotechnology. 2009;20:1-5. DOI: 10.1088/0957-4484/20/10/105708
  75. 75. Zschoerper NP, Kotzenamier V, Vohrer U, Haupt M, Oehr C, Hirth T. Analytical investigation of the composition of plasma-induced functional groups on carbon nanotube sheets. Carbon. 2009;47:2174-2185. DOI: 10.1016/j.carbon.2009.03.059
  76. 76. Yang D-Q, Sacher E. Strongly enhanced interaction between evaporated Pt nanoparticles and functionalized multiwalled carbon nanotubes via plasma surface modifications: Effects of physical and chemical defects. Journal of Physical Chemistry C. 2008;112:4075-4082. DOI: 10.1021/jp076531s
  77. 77. Jung N, Kwon S, Lee D, Yoon D-M, Park YM, Benayad A, Choi J-Y, Park JS. Synthesis of chemically bonded graphene/carbon nanotube composites and their application in large volumetric capacitance supercapacitors. Advanced Materials. 2013;25:6854-6858. DOI: 10.1002/adma.201302788
  78. 78. Xu D, Ding B, Nie P, Shen L, Wang J, Zhang X. Porous nitrogen-doped carbon nanotubes derived from tubular polypyrrole for energy-storage applications. Chemistry A European Journal. 2013;19:12306-12312. DOI: 10.1002/chem.201301352
  79. 79. Gueon D, Moon JH. Nitrogen-doped carbon nanotube spherical particles for supercapacitor applications: Emulsion-assisted compact packing and capacitance enhancement. ACS Applied Materials & Interfaces. 2015;7:20083-20089. DOI: 10.1021/acsami.5b05231
  80. 80. Karakaya M, Zhu J, Raghavendra AJ, Podila R, Parler SG, Kaplan JP, Rao AM. Roll-to-roll production of spray coated N-doped carbon nanotube electrodes for supercapacitors. Applied Physics Letters. 2014;105:263103-1-263103-4. DOI: 10.1063/1.4905153
  81. 81. John AR, Arumugam P. Open ended nitrogen-doped carbon nanotubes for the electrochemical storage of energy in a supercapacitor electrode. Journal of Power Sources. 2015;277:387-392. DOI: 10.1016/j.jpowsour.2014.11.151
  82. 82. Liao L, Pan C. Enhanced electrochemical capacitance of nitrogen-doped carbon nanotubes synthesized from amine flames. Soft Nanoscience Letters. 2011;1:16-23. DOI: 10.4236/snl.2011.11004
  83. 83. Yun YS, Park HH, Jin H-J. Pseudocapacitive effects of N-doped carbon nanotube electrodes in supercapacitors. Materials. 2012;5:1258-1266. DOI: 10.3390/ma5071258
  84. 84. Dubal DP, Chodankar NR, Caban-Huertas Z, Wolfart F, Vidotti M, Holze R, Lokhande CD, Gomez-Romero P. Synthetic approach from polypyrrole nanotubes to nitrogen doped pyrolyzed carbon nanotubes for asymmetric supercapacitors. Journal of Power Sources. 2016;308:158-165. DOI: 10.1016/j.jpowsour.2016.01.074
  85. 85. Ayala P, Arenal R, Rummeli M, Rubio A, Pichler T. The doping of carbon nanotubes with nitrogen and their potential applications. Carbon. 2010;48:575-586. DOI: 10.1016/j.carbon.2009.10.009
  86. 86. Mousavi-Khosedel SM, Jahanbakhsh-bonab P, Targholi E. Structural, electronic properties, and quantum capacitance of B, N and P-doped armchair carbon nanotubes. Physics Letters A. 2016;380:3378-3383. DOI: 10.1016/j.physleta.2016.07.067
  87. 87. Terrones M, Souza Filho AG, Rao AM. Doped Carbon Nanotubes: Synthesis, Characterization and Applications. In: Jorio A, Dresselhaus G, Dresselhaus MS, editors. Topics in Applied Physics. Berlin, Heidelberg: Springer-Verlag Berlin Heidelberg; 2008. pp. 531-566. DOI: 10.1007/978-3-540-72865-8_17
  88. 88. Siraishi S, Kibe M, Yokoyama T, Kurihara H, Patel N, Oya A, Kaburagi Y, Hishiyama Y. Electric double layer capacitance of multi-walled carbon nanotubes and B-doping effect. Applied Physics A. 2006;82:585-591. DOI: 10.1007/s00339-005-3399-6
  89. 89. Yeh M-H, Lin L-Y, Li T-J, Leu Y-A, Chen G-L, Tien T-C, Hsieh C-Y, Lo S-C, Huang S-J, Chiang W-H, Ho K-C. Synthesis of boron-doped multi-walled carbon nanotubes by an ammonia-assisted substitution reaction for applying in supercapacitors. Energy Procedia. 2014;61:1764-1767. DOI: 10.1016/j.egypro.2014.12.207
  90. 90. Patino J, Lopez-Salas N, Gutierrez MC, Carriazo D, Ferrer ML, del Monte F. Phosphorous-doped carbon-nanotube hierarchical monoliths as true three-dimensional electrodes in supercapacitor cells. Journal of Materials Chemistry A. 2016;4:1251-1263. DOI: 1039/C5TA09210H
  91. 91. Lee JY, An KH, Heo JK, Lee YH. Fabrication of supercapacitor electrodes using fluorinated single-walled carbon nanotubes. Journal of Physical Chemistry B. 2003;107:8812-8815. DOI: 10.1021/jp034546u
  92. 92. Majeed S, Zhao J, Zhang L, Anjum S, Liu Z, Xu G. Synthesis and electrochemical applications of nitrogen-doped carbon nanomaterials. Nanotechnology Reviews. 2013;2(6):615-635. DOI: 10.1515/ntrev-2013-0007
  93. 93. Zang X, Xu R, Zhang Y, Li X, Zhang L, Wei J, Wang K, Zhu H. All carbon coaxial supercapacitors based on hollow carbon nanotube sleeve structure. Nanotechnology. 2015;26:1-8. DOI: 10.1088/0957-4484/26/4/045401
  94. 94. Iyyamperumai E, Wang S, Dai L. Vertically aligned BCN nanotubes with high capacitance. ACS Nano. 2012;6:5259-5265. DOI: 10.1021/nn301044v
  95. 95. Xu Y, Zhyang SQ, Zhang XY, He PG, Fang YZ. Configuration and capacitance properties of polypyrrole/aligned carbon nanotubes synthesized by electropolymerization. Chinese Science Bulletin. 2011;56:3823-3828. DOI: 10.1007/s11434-011-4745-z
  96. 96. Hiralal P, Wang H, Unalan HE, Liu Y, Rouvala M, Wei D, Andrew P, Amaratunga AJ. Enhanced supercapacitors from hierarchical carbon nanotube and nanohorn architectures. Journal of Materials Chemistry. 2011;21:17810-17815. DOI: 10.1039/c1jm12156a
  97. 97. Tran M-H, Jeong HK. Ternary carbon composite films for supercapacitor applications. Chemical Physics Letters. 2017;684:1-7. DOI: 10.1016/j.cplett.2017.06.025
  98. 98. Yu D, Goh K, Wang H, Wei L, Jiang W, Zhang Q, Dai L, Chen Y. Scalable synthesis of hierarchically structured carbon nanotube–graphene fibers for capacitive energy storage. Nature Nanotechnology. 2014;9:555-562. DOI: 10.1038/NNANO.2014.93
  99. 99. Pham DT, Lee TH, Luong DH, Yao F, Ghosh A, Le VT, Kim TH, Li B, Chang J, Lee YH. Carbon nanotube-bridged graphene 3D building blocks for ultrafast compact supercapacitors. ACS Nano. 2015;9:2018-2027. DOI: 10.1021/nn507079x
  100. 100. Yu D, Dai L. Self-assembled graphene/carbon nanotube hybrid films for supercapacitors. The Journal of Physical Chemistry Letters. 2010;1:467-470. DOI: 10.1021/jz9003137
  101. 101. Jha N, Ramesh P, Bekyarova E, Itkis ME, Haddon RC. High energy density supercapacitor based on a hybrid carbon nanotube-reduced graphite oxide architecture. Advanced Energy Materials. 2012;2:438-444. DOI: 10.1002/aenm.201100697
  102. 102. Du F, Yu D, Dai L, Ganguli S, Varshney V, Roy AK. Preparation of tunable 3D pillared carbon nanotube-graphene networks for high-performance capacitance. Chemistry of Materials. 2011;23:4810-4816. DOI: 10.1021/cm2021214
  103. 103. Huang Z-D, Zhang B, S-W O, Zheng Q-B, Lin X-Y, Yousefi N, Kim J-K. Self-assembled reduced graphene oxide/carbon nanotube thin films as electrodes for supercapacitors. Journal of Materials Chemistry. 2012;22:3591-3599. DOI: 10.1039/c2jm15048d
  104. 104. Byon HR, Gallant BM, Lee SW, Shao-Horn Y. Role of oxygen functional groups in carbon nanotube/graphene freestanding electrodes for high performance lithium batteries. Advanced Functional Materials. 2013;23:1037-1045. DOI: 10.1002/adfm.201200697
  105. 105. Zheng Q, Cai Z, Ma Z, Gong S. Cellulose nanofibril/reduced graphene oxide/carbon nanotube hybrid aerogels for highly flexible and all-solid-state supercapacitors. ACS Applied Materials & Interfaces. 2015;7:3263-3271. DOI: 10.1021/am507999s
  106. 106. Wang Q, Yan J, Wang Y, Ning G, Fan Z, Wei T, Cheng J, Zhang M, Jing X. Template synthesis of hollow carbon spheres anchored on carbon nanotubes for high rate performance supercapacitors. Carbon. 2013;52:209-218. DOI: 10.1016/j.carbon.2012.09.022
  107. 107. Lin J, Zhang C, Yan Z, Zhu Y, Peng Z, Hauge RH, Natelson D, Tour JM. 3-dimensional graphene carbon nanotube carpet-based microsupercapacitors with high electrochemical performance. Nano Letters. 2013;13:72-78. DOI: 10.1021/nl3034976
  108. 108. Sivakkumar SR, Kim WJ, Choi J-A, MacFarlane DR, Forsyth M, Kim D-W. Electrochemical performance of polyaniline nanofibres and polyaniline/multi-walled carbon nanotube composite as an electrode material for aqueous redox supercapacitors. Journal of Power Sources. 2007;171:1062-1068. DOI: 10.1016/j.jpowsour.2007.05.103
  109. 109. Ghosh A, Lee YH. Carbon-based electrochemical capacitors. Chemistry & Sustainability Energy & Materials. 2012;5:480-499. DOI: 10.1002/cssc.201100645
  110. 110. Peng C, Jin J, Chen GZ. A comparative study on electrochemical co-deposition and capacitance of composite films of conducting polymers and carbon nanotubes. Electrochimica Acta. 2007;53:525-537. DOI: 10.1016/j.electacta.2007.07.004
  111. 111. Gupta V, Miura N. Polyaniline/single-wall carbon nanotube (PANI/SWCNT) composites for high performance supercapacitors. Electrochimica Acta. 2006;52:1721-1726. DOI: 10.1016/j.electacta.2006.01.074
  112. 112. Zhang J, Kong L-B, Wang B, Luo Y-C, Kang L. In-situ electrochemical polymerization of multi-walled carbon nanotube/polyaniline composite films for electrochemical supercapacitors. Synthetic Metals. 2009;159:260-266. DOI: 10.1016/j.synthmet.2008.09.018
  113. 113. Canobre SC, Almeida DAL, Fonseca CP, Neves S. Synthesis and characterization of hybrid composites based on carbon nanotubes. Electrochimica Acta. 2009;54:5383-6388. DOI: 10.1016/j.electacta.2009.06.002
  114. 114. Paul S, Lee Y-S, Choi J-A, Kang YC, Kim D-W. Synthesis and electrochemical characterization of polypyrrole/multi-walled carbon nanotube composite electrodes for supercapacitor applications. Bulletin of the Korean Chemical Society. 2010;31:1228-1323. DOI: 10.5012/bkcs.2010.31.5.1228
  115. 115. Branzoi V, Branzoi F, Pilan L. Electrochemical fabrication and capacitance of composite films of carbon nanotubes and polyaniline. Surface & Interface Analysis. 2009;42:1266-1270. DOI: 10.1002/sia.3387
  116. 116. Chen X, Paul R, Dai L. Carbon-based supercapacitors for efficient energy storage. National Science Review. 2017;4:1-37. DOI: 10.1093/nsr/nwx009
  117. 117. Teresawa N, Asaka K. High-performance PEDOT:PSS/single-walled carbon nanotube/ionic liquid actuators combining electrostatic double-layer and faradaic capacitors. Langmuir. 2016;32:7210-7218. DOI: 10.1021/acs.langmuir.6b01148
  118. 118. Peng C, Zhang S, Jewell D, Chen GZ. Carbon nanotube and conducting polymer composites for supercapacitors. Progress in Natural Science. 2008;18:777-788. DOI: 10.1016/j.pnsc.2008.03.002
  119. 119. Grądzka E, Winkler K, Borowska M, Plonska-Brzezinka ME, Echegoyen L. Comparison of the electrochemical properties of thin films of MWCNTs/C60-Pd, SWCNTs/C60-Pd and ox-CNOs/C60-Pd. Electrochimica Acta. 2013;96:274-284. DOI: 10.1016/j.electacta.2013.02.035
  120. 120. Lee KYT, Shi HTH, Lian K, Naguib HE. Flexible multiwalled carbon nanotubes/conductive polymer composite electrode for supercapacitor applications. Smart Materials and Structures. 2015;24:1-17. DOI: 10.1088/0964-1726/24/11/115008
  121. 121. Kausar A. Performance of polyaniline doped carbon nanotube composite. American Journal of Polymer Science and Engineering. 2017;5:43-54
  122. 122. Raicopol M, Pruna A, Pilan L. Supercapacitance of single-walled carbon nanotubes-polypyrrole composites. Journal of Chemistry. 2013:1-7. DOI: 10.1155/2013/367473
  123. 123. Maubane M, Mamo MA, Nxumalo EN, van Otterlo WAL, Coville NJ. Tubular shaped composites made from polythiophene covalently linked to Prato functionalized N-doped carbon nanotubes. Synthetic Metals. 2012;162:2307-2315. DOI: 10.1016/j.synthmet.2012.10.020
  124. 124. Wei D, Kvarnstrom C, Lindfors T, Ivaska A. Electrochemical functionalization of single walled carbon nanotubes with polyaniline in ionic liquids. Electrochemistry Communications. 2007;9:206-210. DOI: 10.1016/j.elecom.2006.09.008
  125. 125. Angeles Herranz M, Martin N. Noncovalent functionalization of carbon nanotubes. In: Guldi DM, Martin N, editors. Carbon Nanotubes and Related Structures. Synthesis, Characterization, Functionalization, and Application. Weinheim: Wiley-VCH Verlag GmbH & Co. KgaA 2010. P. 103-134. DOI: 10.1002/9783527629930.ch5
  126. 126. Liu L, Yoo S-H, Park S. Composite materials with MWCNTs and conducting polymer nanorods and their application as supercapacitors. Journal of Electrochemical Science and Technology. 2010;1:25-30. DOI: 10.5229/JECST.2010.1.1.025
  127. 127. Frackowiak E, Khomenko V, Jurewicz K, Lota K, Beguin F. Supercapacitors based on conducting polymers/nanotubes composites. Journal of Power Sources. 2006;153:413-418. DOI: 10.1016/j.jpowsour.2005.05.030
  128. 128. Cheng Q, Tang J, Shinya N, Qin L-C. Polyaniline modified graphene and carbon nanotube composite electrode for asymmetric supercapacitors of high energy density. Journal of Power Sources. 2013;241:423-428. DOI: 10.1016/j.jpowsour.2013.04.105
  129. 129. Plonska-Brzezinska ME, Breczko J, Palys B, Echegoyen L. The electrochemical properties of nanocomposite films obtained by chemical in situ polymerization of aniline and carbon nanostructures. A European Journal of Chemical Physics and Phyical Chemistry. 2013;14:116-124. DOI: 10.1002/cphc.201200759
  130. 130. Khomenko V, Frackowiak E, Beguin F. Determination of the specific capacitance of conducting polymer/nanotubes composite electrodes using different cell configurations. Electrochimica Acta. 2005;50:2499-2506. DOI: 10.1016/j.electacta.2004.10.078
  131. 131. Aphale A, Maisuria K, Mahapatra MK, Santiago A, Singh P, Patra P. Hybrid electrodes by in-situ integration of graphene and carbon-nanotubes in polypyrrole for supercapacitors. Scientific Reports. 2015;5(14445):1-8. DOI: 10.1038/srep14445
  132. 132. Zhou C, Kumar S, Doyle CD, Tour JM. Functionalized single wall carbon nanotubes treated with pyrrole for electrochemical supercapacitor membranes. Chemistry of Materials. 2005;17:1997-2002. DOI: 10.1021/cm047882b
  133. 133. Liu J, Sun J, Gao L. A promising way to enhance the electrochemical behavior of flexible single-walled carbon nanotube/polyaniline composite films. Journal of Physical Chemistry C. 2010;114:19614-19620. DOI: 10.1021/jp1092042
  134. 134. Mikhaylova AA, Tusseeva EK, Mayrova NA, Rychagov AY, Volfkovich YM, Krestinin AV, Khazova O. Single-walled carbon nanotubes and their composites with polyaniline. Structure, catalytic and capacitive properties as applied to fuel cells and supercapacitors. Electrochimica Acta. 2011;56:3656-3665. DOI: 10.1016/j.electacta.2010.07.021
  135. 135. Yang P, Chen Y, Yu X, Qiang P, Wang K, Cai X, Tan S, Liu P, Song J, Mai W. Reciprocal alternate deposition strategy using metal oxide/carbon nanotube for positive and negative electrodes of high-performance supercapacitors. Nano Energy. 2014;10:108-116. DOI: 10.1016/j.nanoen.2014.08.018
  136. 136. Wu M-S. Electrochemical capacitance from manganese oxide nanowire structure synthesized by cyclic voltammetric electrodeposition. Applied Physics Letters. 2005;87:153102-1-153102-3. DOI: 10.1063/1.2089169
  137. 137. Devaraj S, Munichandraiah N. Effect of crystallographic structure of MnO2 on its electrochemical capacitance properties. Journal of Physical Chemistry C. 2008;112:4406-4417. DOI: 10.1021/jp7108785
  138. 138. Jiang H, Zhao T, Ma J, Yan C, Li C. Ultrafine manganese dioxide nanowire network for high-performance supercapacitors. Chemical Communications. 2011;47:1264-1266. DOI: 10.1039/c0cc04134c
  139. 139. Yan J, Khoo E, Sumboja A, Lee PS. Facile coating of manganese oxide on tin oxide nanowires with high-performance capacitive behavior. ACS Nano. 2010;4:4247-4255. DOI: 10.1021/nn100592d
  140. 140. Jiang R, Huang T, Liu J, Zhuang J, Yu A. A novel method to prepare nanostructured manganese dioxide and its electrochemical properties as a supercapacitor electrode. Electrochimica Acta. 2009;54:3047-3052. DOI: 10.1016/j.electacta.2008.12.007
  141. 141. Beadrouet E, Le Gal La Salle A, Guyomard D. Nanostructured manganese dioxides: Synthesis and properties as supercapacitor electrode materials. Electrochimica Acta. 2009;54:1240-1248. DOI: 10.1016/j.electacta.2008.08.072
  142. 142. Wei W, Cui X, Chen W, Ivey DG. Electrochemical cyclability mechanism for MnO2 electrodes utilized as electrochemical supercapacitors. Journal of Power Sources. 2009;186:843-550. DOI: 10.1016/j.jpowsour.2008.10.058
  143. 143. Xia H, Wang Y, Lin J, Lu L. Hydrothermal synthesis of MnO2/CNT nanocomposite with a CNT core/porous MnO2 sheath hierarchy architecture for supercapacitors. Nanoscale Research Letters. 2012;7(33):1-10. DOI: 10.1186/1556-276X-7-33
  144. 144. Kang J, Hirata A, Kang L, Zhang X, Hou Y, Chen L, Li C, Fujita T, Akagai K, Chen M. Enhanced supercapacitor performance of MnO2 by atomic doping. Angewandte Chemie. 2013;125:1708-1711. DOI: 10.1002/ange.201208993
  145. 145. Yan J, Fan Z, Wei T, Cheng J, Shao B, Wang K, Song L, Zhang M. Carbon nanotube/MnO2 composites synthesized by microwave-assisted method for supercapacitors with high power and energy densities. Journal of Power Sources. 2009;194:1202-1207. DOI: 10.1016/j.jpowsour.2009.06.006
  146. 146. Ma S-B, Nam K-W, Yoon W-S, Yang X-Q, Ahn K-Y, Oh K-H, Kim K-B. A novel concept of hybrid capacitor based on manganese oxide materials. Electrochemistry Communications. 2007;9:2807-2811. DOI: 10.1016/j.elecom.2007.09.015
  147. 147. Xie X, Gao L. Characterization of a manganese dioxide/carbon nanotube composite fabricated using an in situ coating method. Carbon. 2007;45:2365-2373. DOI: 10.1016/j.carbon.2007.07.014
  148. 148. Jin X, Zhou W, Zhang S, Chen GZ. Nanoscale microelectrochemical cells on carbon nanotubes. Small. 2007;3:1513-1517. DOI: 10.1002/smll.200700139
  149. 149. Chen P-C, Shen G, Shi Y, Zhou C. Preparation and characterization of flexible asymmetric supercapacitors based on transition-metal-oxide nanowire/single-walled carbon nanotube hybrid thin-film electrodes. ACS Nano. 2010;4:4403-4411. DOI: 10.1021/nn100856y
  150. 150. Zhang H, Gao G, Wang Z, Yang Y, Shi Z, Gu Z. Growth of manganese oxide nanoflowers on vertically-aligned carbon nanotube arrays for high-rate electrochemical capacitive energy storage. Nano Letters. 2008;8:2664-2668. DOI: 10.1021/nl800925j
  151. 151. Bi R-R, X-L W, Cao F-F, Jiang L-Y, Guo Y-G, Wan L-J. Highly dispersed RuO2 nanoparticles on carbon nanotubes: Facile synthesis and enhanced supercapacitance performance. Journal of Physical Chemistry C. 2010;114:2448-2451. DOI: 10.1021/jp9116563
  152. 152. Chen P, Chen H, Qiu J, Zhou C. Inkjet printing of single-walled carbon nanotube/RuO2 nanowire supercapacitors on cloth fabrics and flexible substrates. Nano Research. 2010;3:594-603. DOI: 10.1007/s12274-010-0020-x
  153. 153. Reddy ALM, Ramaprabhu S. Nanocrystalline metal oxides dispersed multiwalled carbon nanotubes as supercapacitor electrodes. Journal of Physical Chemistry C. 2007;111:7727-7734. DOI: 10.1021/jp069006m
  154. 154. Yan S, Wang HW, Qu P, Zhang Y, Xiao Z. RuO2/carbon nanotubes composites synthesized by microwave-assisted method for electrochemical supercapacitor. Synthetic Metals. 2009;159:158-161. DOI: 10.1016/j.synthmet.2008.07.024
  155. 155. Lee JY, Liang K, An KH, Lee YH. Nickel oxide/carbon nanotubes nanocomposite for electrochemical capacitance. Synthetic Metals. 2005;150:153-157. DOI: 10.1016/j.synthmet.2005.01.016
  156. 156. Gao B, Yuan B-Z, L-H S, Chen L, Zhang X-G. Nickel oxide coated on ultrasonically pretreated carbon nanotubes for supercapacitor. Journal of Solid State Electrochemistry. 2009;13:1251-1257. DOI: 10.1007/s10008-008-0658-4
  157. 157. Zhao X, Johnston C, Grant PS. A novel hybrid supercapacitor with a carbon nanotube cathode and an iron oxide/carbon nanotube composite anode. Journal of Materials Chemistry. 2009;19:8755-8760. DOI: 10.1039/b909779a
  158. 158. Shan Y, Gao L. Formation and characterization of multi-walled carbon nanotubes/Co3O4 nanocomposites for supercapacitors. Materials Chemistry and Physics. 2007;103:206-210. DOI: 10.1016/j.matchemphys.2007.02.038
  159. 159. Aravinda LS, Nagaraja KK, Udaya Bhat K, Bhat BR. Magnetron sputtered MoO3/carbon nanotube composite electrodes for electrochemical supercapacitor. Journal of Electroanalytical Chemistry. 2013;699:28-32. DOI: 10.1016/j.jelechem.2013.03.022
  160. 160. Sathiya M, Prakash AS, Ramesha K, Tarascon J-M, Shukla AK. V2O5-anchored carbon nanotubes for enhanced electrochemical energy storage. Journal of the American Chemical Society. 2011;133:16291-16299. DOI: 10.1021/ja207285b
  161. 161. Enterria M, Goncalves AG, Pereira MFR, Martins JI, Figueiredo JL. Electrochemical storage mechanisms in non-stoichiometric cerium oxide/multiwalled carbon nanotube composites. Electrochimica Acta. 2016;209:25-35. DOI: 10.1016/j.electacta.2016.05.036
  162. 162. Wang X, Han X, Lim M, Singh N, Gan NS, Jan M, Lee PS. Nickel cobalt oxide-single wall carbon nanotube composite material for superior cycling stability and high-performance supercapacitor application. Journal of Physical Chemistry C. 2012;116:12448-12454. DOI: 10.1021/jp3028353
  163. 163. Chiang DNK, Lim SS, Peng C. Novel bimetallic tin-manganese oxides/carbon nanotube nanocomposite and their charge storage properties. Journal—the Institution of Engineers, Malysia. 2014;75(1):24-39
  164. 164. Jang JH, Kato A, Machida K, Naoi K. Supercapacitor performance of hydrous ruthenium oxide electrodes prepared by electrophoretic deposition. Journal of the Electrochemical Society. 2006;153:A321-A328. DOI: 10.1149/1.2138672
  165. 165. Tao T, Zhang L, Jiang H, Li C. Functional carbon nanotube/mesoporous carbon/MnO2 hybrid network for high-performance supercapacitors. Journal of Nanomaterials. 2014;2014:1-6. DOI: 10.1155/2014/568561
  166. 166. Bordjiba T, Mohamedi M. Molding versus dispersion: Effect of the preparation procedure on the capacitive and cycle life of carbon nanotubes aerogel composites. Journal of Solid State Electrochemistery. 2011;15:765-771. DOI: 10.1007/s10008-010-1155-0
  167. 167. Kim SJ, Hwang SW, Hyun SH. Preparation of carbon aerogel electrodes for supercapacitor and their electrochemical characteristics. Journal of Materials Science. 2005;40:725-731. DOI: 10.1007/s10853-005-6313-x
  168. 168. Kim C-Y, Jang AR, Cho Y. Optimization of pore structures and supercapacitor properties of carbon aerogel electrodes. Asian Journal of Chemistry. 2012;24:4205-4212
  169. 169. Macias C, Rasines G, Garcia TE, Zafra MC, Lavela P, Tirado JL, Ania CO. Synthesis of porous and mechanically compliant carbon aerogels using conductive and structural additives. Gels. 2016;2:1-16. DOI: 10.3390/gels2010004
  170. 170. Bordjiba T, Mohamedi M, Dao LH. Novel binderless nanostructured carbon nanotubes—carbon aerogel composites for electrochemical double layer capacitors. ECS Transactions. 2008;6:183-189. DOI: 10.1149/1.2943237
  171. 171. Bordjiba T, Mohamedi M, Dao LH. Charge storage mechanism of binderless nanocomposite electrodes formed by dispersion of CNTs and carbon aerogels. Journal of the Electrochemical Society. 2008;155:A115-A124
  172. 172. Bordjiba T, Mohamedi M, Dao LH. Synthesis and electrochemical capacitance of binderless nanocomposite electrodes formed by dispersion of carbon nanotubes and carbon aerogels. Journal of Power Sources. 2007;172:991-998. DOI: 10.1016/j.jpowsour.2007.05.011
  173. 173. Hao P, Zhao Z, Li L, Tuan C-C, Li H, Sang Y, Jiang H, Wong PC, Liu H. The hybrid nanostructure of MnCo2O4.5 nanoneedle/carbon aerogel for symmetric supercapacitors with high energy density. Nanoscale. 2015;7:14401-14412. DOI: 10.1039/c5nr04421a
  174. 174. Annamalai KP, Fathy NA, Tao Y. Synthesis and capacitance performance of phosphorous-enriched carbon xerogel. Journal of Sol-Gel Science and Technology. DOI: 10.1007/s10971-017-4452-6
  175. 175. Lufrano F, Staiti P, Calvo EG, Juarez-Perez EJ, Menendez JA, Arenillas A. Carbon xerogel and manganese oxide capacitive materials for advanced supercapacitors. International Journal of Electrochemical Science. 2011;6:596-612
  176. 176. Liu X, Li S, Mi R, Liu L-M, Cao H, Lau W-M, Liu H. Porous structure design of carbon xerogels for advanced supercapacitor. Applied Energy. 2015;153:32-40. DOI: 10.1016/j.apenergy.2015.01.141
  177. 177. Ordenanan-Martinez AS, Rincon ME, Vargas M, Estrada-Vargas A, Casillas N, Barcena-Sato M, Ramos E. Carbon nanotubes/carbon xerogel-nafion electrodes: A comparative study of preparation methods. Journal of Solid State Electrochemistry. 2012;16:3777-3782. DOI: 10.1007/s10008-012-1819-z
  178. 178. Fernandez PS, Castro EB, Real SG, Visintin A, Arenillas A, Calvo EG, Juarez-Perez EJ, Menendez AJ, Martins ME. Electrochemical behavior and capacitance properties of carbon xerogel/multiwalled carbon nanotubes composites. Journal of Solid State Electrochemistry. 2012;16:1067-1076. DOI: 10.1007/s10008-011-1487-4

Written By

Emilia Grądzka and Krzysztof Winkler

Submitted: April 25th, 2017 Reviewed: October 13th, 2017 Published: December 20th, 2017