**2.3 Carbon nanotubes**


Carbon nanotubes (CNTs) are quasi 1D nanomaterial formed by rolling one or more sheets of graphene concentrically. They have a unique cylindrical structure

#### **Table 2.**

*Some graphene and conducting polymer based composites and their capacitances.*

with few micrometer length and diameter in the range of nanometers. CNTs are typically classified as single-walled (SW), double-walled (DW), or multi-wall (MW), corresponding to the number of graphene layers forming CNTs. The very first CNT was formally reported in 1991 by Iijima, when he closely observed the structure of carbon-soot obtained by an arc-discharge method using TEM technology [71, 72]. Since then, both SWCNTs and MWCNTs have been extensively studied for their numerous possible applications. The presence of hexagonal lattice structure of graphene with sp2 bonded carbon atoms in CNTs contributes to its excellent properties like electrical and thermal conductivity, high mechanical strength, optimum chemical stability and low mass per unit volume [73]. In terms of tensile strength, CNTs are hundred times tougher than steel. They have Young's modulus of about 1.2 TPa (1 TPa for SWCNTs and 1.28 TPa for MWCNTs) and can withstand large strains before mechanical failure. The electrical conductivity of CNTs depends on their structure, i.e., MWCNTs with concentric tubular structure having inter-layer distance of about 0.34 nm shows metallic conductivity. Whereas, the SWCNTs have shown both metallic or semiconducting behavior depending on their chirality and diameter size. CNTs have been successfully synthesized and employed in various application namely chemical sensors, field emission sources, nanotweezer, scanning microscope probe tip, electro-mechanical actuators, etc. CNTs have a large crosswise dimension (<1, 1–2, 2–5, 5–10, and >10 μm), high specific surface area (SWCNT > 1600 m2 /g, MWCNT > 430 m<sup>2</sup> /g), and excellent carrier mobility for both ions and holes (15,000 cm<sup>2</sup> /Vs) and are being widely used as the active electrode in supercapacitors [74]. This can be attributed to the fact that CNTs have high aspect ratio so they tend to get entangled and form a porous structure with nanotube network skeleton. It also creates a mesoporous open central canal in case of MWCNTs. This provides an easy pathway for the electrolyte ions to move freely between electrode/electrolyte during charge and discharge cycles. In order to minimize the size of supercapacitor-based power cell for its real-world application, it is important to work towards high power density electrodes. Niu et al. [75] have fabricated a supercapacitor based on MWCNTs, which showed a capacitance value of 102 F/g, high power density >8 kW/kg and an energy density of ~1 Wh/kg in H2SO4 electrolyte. They also showed that such electrode material did not require any binder and was self-sufficient. A supercapacitor based on SWCNT electrode as reported by An et al. [76] showed comparatively higher specific capacitance value of 180 F/g in KOH (7.5 M) electrolyte solution with power density of 20 kW/kg and energy density in the range 7–6.5 Wh/kg. Similar to other carbon materials, CNTs are also being more commonly used as its composite. CNT/ conductive polymer composite have attained a lot of attention in terms of its capacitive applications as it combines high pseudocapacitance of conductive polymers with excellent mechanical properties of CNTs. They can be synthesized chemically or electrochemically. The electrochemical method involves either deposition of polymer on a CNT electrode, or co-deposition of polymer and CNT on electrode. The composites formed by electrochemical co-deposition are found to be most homogeneous. They show enhanced electron delocalization due to the presence of conjugated carbon chain and an unusual interaction between the polymer and CNTs. As a result, they exhibit excellent electrochemical charge storage properties and fast charge/discharge switching, making them promising electrode materials for high power supercapacitors.

#### **2.4 Hybrid carbon materials**

More recently, several attempts have been made to merge the unique properties of several carbon-based nanomaterials to form a hybrid material. For instance, Li

**13**

supercapacitors.

*Carbon-Based Composites for Supercapacitor DOI: http://dx.doi.org/10.5772/intechopen.80393*

of 330 mF/cm2

et al. [77] have reported synthesis of a flexible, light weight and self-standing film by combining activated carbon, CNT and reduced graphene oxide. The hybrid film is prepared by weaving 3D porous framework using CNTs and graphene that was further used to accommodate activated carbon particles using their intrinsic van der Waals force. In such a material, each component has an important role to play like carbon particles block the restacking of graphene structure and the CNTs improve electronic conductivity. The AC/CNT/rGO electrode thus formed showed a specific capacitance of 101 F/g in organic electrolyte at 0.2 A/g current density with a maximum energy density of 30 Wh/kg. Supercapacitors, are generally known to work only in a narrow temperature range. However, a flexible hybrid film consisting of CNTs and rGO has been reported to be able to operate between the temperature range −40 and 200°C. The electrode material exhibited a maximum area specific capacitance

100,000 cycles [78]. More interestingly, CNTs were intercalated between graphene sheets to retain high specific surface area by minimizing its aggregation [79], where π–π interaction between graphene sheets and CNTs also improve the electrical

conductivity and mechanical strength. Similarly, Yu and Dai produced hybrid films of CNT and graphene interconnected network with well-defined nanoporous structure [80], which exhibited a specific capacitance of 120 F/g in 1 M H2SO4 electrolyte and an almost rectangular cyclic voltammogram even at high scan rate of 1 V/s. Yu et al*.* developed a continued CNT and graphene hybrid fiber with well-defined mesopo-

electrical conductivity of 102 S/cm. The corresponding fiber-shaped supercapacitor

Carbon has already made a revolution in the world. Now its surface engineering with variety of functional materials and nanoporous structure are the fascinating parts of the research. In the light of the aforementioned studies, a broad range of carbon nanomaterials with various dimensions and unique morphological structure designs have been made and successfully implemented in energy storage device fabrication technologies. The incorporation of heteroatoms into the carbon network provides a new class of carbon materials with unique properties unmatched with parental carbon materials. Compositing carbon nanomaterials with metal oxides/ conducting polymers having pseudocapacitance increases energy density largely but compromise with rate capability and cycling life. In this chapter, we have thoroughly conferred the recent progress of carbon nanomaterials based supercapacitors. The potential of carbon nanomaterials and its composites for supercapacitors is in resolving the foreseen of clean energy mitigation. Various materials, methods and technologies have been employed in finding a solution for an energy storing device with high capacitance along with high energy/power density. No doubt, carbon materials are potential candidates for supercapacitor electrode materials but they need a supporting active material to enhance their performance. Nevertheless, increase in specific capacitance value, power density and energy density in certain cases gives a hope that if the research continues in right direction then one-day carbon material based supercapacitor can bring a new revolution in the electronic world. We are confident that the information presented in this chapter would definitely help in the research and development of carbon nanomaterials based

and 90% retention even after

/g with an

, which is comparable to the

current

with energy density of 1.7 mWh/cm3

rous structure [81], which showed specific surface area as high as 396 m2

density and a volumetric energy density of 6.3 mWh/cm3

**3. Summary and perspective**

energy density of a 4 V–0.5 mAh thin-film lithium ion battery.

demonstrate a volumetric specific capacitance of 305 F/cm3 at 26.7 mA/cm3

#### *Carbon-Based Composites for Supercapacitor DOI: http://dx.doi.org/10.5772/intechopen.80393*

*Science, Technology and Advanced Application of Supercapacitors*

structure of graphene with sp2

specific surface area (SWCNT > 1600 m2

carrier mobility for both ions and holes (15,000 cm<sup>2</sup>

with few micrometer length and diameter in the range of nanometers. CNTs are typically classified as single-walled (SW), double-walled (DW), or multi-wall (MW), corresponding to the number of graphene layers forming CNTs. The very first CNT was formally reported in 1991 by Iijima, when he closely observed the structure of carbon-soot obtained by an arc-discharge method using TEM technology [71, 72]. Since then, both SWCNTs and MWCNTs have been extensively studied for their numerous possible applications. The presence of hexagonal lattice

excellent properties like electrical and thermal conductivity, high mechanical strength, optimum chemical stability and low mass per unit volume [73]. In terms of tensile strength, CNTs are hundred times tougher than steel. They have Young's modulus of about 1.2 TPa (1 TPa for SWCNTs and 1.28 TPa for MWCNTs) and can withstand large strains before mechanical failure. The electrical conductivity of CNTs depends on their structure, i.e., MWCNTs with concentric tubular structure having inter-layer distance of about 0.34 nm shows metallic conductivity. Whereas, the SWCNTs have shown both metallic or semiconducting behavior depending on their chirality and diameter size. CNTs have been successfully synthesized and employed in various application namely chemical sensors, field emission sources, nanotweezer, scanning microscope probe tip, electro-mechanical actuators, etc. CNTs have a large crosswise dimension (<1, 1–2, 2–5, 5–10, and >10 μm), high

as the active electrode in supercapacitors [74]. This can be attributed to the fact that CNTs have high aspect ratio so they tend to get entangled and form a porous structure with nanotube network skeleton. It also creates a mesoporous open central canal in case of MWCNTs. This provides an easy pathway for the electrolyte ions to move freely between electrode/electrolyte during charge and discharge cycles. In order to minimize the size of supercapacitor-based power cell for its real-world application, it is important to work towards high power density electrodes. Niu et al. [75] have fabricated a supercapacitor based on MWCNTs, which showed a capacitance value of 102 F/g, high power density >8 kW/kg and an energy density of ~1 Wh/kg in H2SO4 electrolyte. They also showed that such electrode material did not require any binder and was self-sufficient. A supercapacitor based on SWCNT electrode as reported by An et al. [76] showed comparatively higher specific capacitance value of 180 F/g in KOH (7.5 M) electrolyte solution with power density of 20 kW/kg and energy density in the range 7–6.5 Wh/kg. Similar to other carbon materials, CNTs are also being more commonly used as its composite. CNT/ conductive polymer composite have attained a lot of attention in terms of its capacitive applications as it combines high pseudocapacitance of conductive polymers with excellent mechanical properties of CNTs. They can be synthesized chemically or electrochemically. The electrochemical method involves either deposition of polymer on a CNT electrode, or co-deposition of polymer and CNT on electrode. The composites formed by electrochemical co-deposition are found to be most homogeneous. They show enhanced electron delocalization due to the presence of conjugated carbon chain and an unusual interaction between the polymer and CNTs. As a result, they exhibit excellent electrochemical charge storage properties and fast charge/discharge switching, making them promising electrode materials

More recently, several attempts have been made to merge the unique properties of several carbon-based nanomaterials to form a hybrid material. For instance, Li

bonded carbon atoms in CNTs contributes to its

/g, MWCNT > 430 m<sup>2</sup>

/g), and excellent

/Vs) and are being widely used

**12**

for high power supercapacitors.

**2.4 Hybrid carbon materials**

et al. [77] have reported synthesis of a flexible, light weight and self-standing film by combining activated carbon, CNT and reduced graphene oxide. The hybrid film is prepared by weaving 3D porous framework using CNTs and graphene that was further used to accommodate activated carbon particles using their intrinsic van der Waals force. In such a material, each component has an important role to play like carbon particles block the restacking of graphene structure and the CNTs improve electronic conductivity. The AC/CNT/rGO electrode thus formed showed a specific capacitance of 101 F/g in organic electrolyte at 0.2 A/g current density with a maximum energy density of 30 Wh/kg. Supercapacitors, are generally known to work only in a narrow temperature range. However, a flexible hybrid film consisting of CNTs and rGO has been reported to be able to operate between the temperature range −40 and 200°C. The electrode material exhibited a maximum area specific capacitance of 330 mF/cm2 with energy density of 1.7 mWh/cm3 and 90% retention even after 100,000 cycles [78]. More interestingly, CNTs were intercalated between graphene sheets to retain high specific surface area by minimizing its aggregation [79], where π–π interaction between graphene sheets and CNTs also improve the electrical conductivity and mechanical strength. Similarly, Yu and Dai produced hybrid films of CNT and graphene interconnected network with well-defined nanoporous structure [80], which exhibited a specific capacitance of 120 F/g in 1 M H2SO4 electrolyte and an almost rectangular cyclic voltammogram even at high scan rate of 1 V/s. Yu et al*.* developed a continued CNT and graphene hybrid fiber with well-defined mesoporous structure [81], which showed specific surface area as high as 396 m2 /g with an electrical conductivity of 102 S/cm. The corresponding fiber-shaped supercapacitor demonstrate a volumetric specific capacitance of 305 F/cm3 at 26.7 mA/cm3 current density and a volumetric energy density of 6.3 mWh/cm3 , which is comparable to the energy density of a 4 V–0.5 mAh thin-film lithium ion battery.
