**3.1 Zero-dimensional (0-D) carbon nanoparticles**

They are round-shaped particles such as ultrafine activated carbon (AC), mesoporous carbon, carbon nanosphere, and carbon quantum dot, with a high specific area (AC: ⁓3000 m2 g−1) and an aspect ratio of nearly [10]. In addition, by tuning the pore size distribution and pore content, they can use as suitable supporting materials for composite electrodes.

### **3.2 One-dimensional (1-D) carbon nanostructures**

These are the high aspect ratio materials with fiber shaped and good electronic properties e.g. carbon nanotubes (CNT), carbon nanocoils, and carbon nanofibers (CNF), which facilitates the electrochemical reaction kinetics by 1-D charge transfer pathway.

*Novel Nanomaterials*

**Figure 1.**

in several orders of magnitude. Moreover, SCs provide higher specific power density

Based on different charge storage mechanisms, SCs are mainly divided into two categories, electrical double layer capacitors (EDLCs) and pseudocapacitors, as shown in **Figure 2**. EDLCs store the electrical charge by electrostatic force at the electrode-electrolyte interface, which is a physical process without involving electrochemical reactions on the electrode surface. In order to increase the capacitance and energy density of SCs, some electrochemically active materials, such as transition metal oxide and conducting polymers, have been explored as electrode materials for pseudocapacitors. The energy storage in pseudocapacitors originates from reversible surface faradaic redox reactions at the interface of electrolyte and

The capacitance of EDLCs is strongly dependent on effective surface area and the pore size distribution of the electrode [7, 8]. Typically, the carbon-based materials and their derivatives, including activated carbon, carbon nanotubes (CNTs) and graphene, with high conductivity, chemically-stability, and large surface area are widely utilized in EDLCs. Although the EDLCs possess high power density and excellent charge/discharge cycling stability, they suffer from low energy density owing to the relatively low capacitance of carbon-based materials. Pseudocapacitors

than batteries due to its unique charge storage mechanism.

**2.2 Factors affecting the performance of Supercapacitors**

**2. Theoretical background for supercapacitors**

**2.1 Principle and mechanism of supercapacitors**

*Ragone plot for various energy storage and conversion devices [6].*

**116**

electroactive materials.


 *Different carbon nanostructures used as electrode materials for EDLCs with onion-like carbon, carbon nanotubes, graphene, activated carbon, carbide-derived carbon, and templated carbon [16].*

**Table 1.**

**119**

*Carbon-Based Nanocomposite Materials for High-Performance Supercapacitors*

They are sheet like structures with high aspect ratio such as graphene, graphene oxide (GO) or reduced graphene oxide (rGO). In addition, they have high specific surface area, good mechanical strength, and excellent electrotonic conductivity, which helps them as promising electrode materials for SCs. For an example, single

These are the low dimensional building blocks such as carbon nanofoams or sponges with hierarchical porous channels, rich pore structures, higher electrical conductivity and better structural mechanical stability, which are extensively used in composite electrode materials for SCs. For an example, foam has high specific surface area with continuous electron transport path and large area of electrolyte-

**Table 1** shows some examples of different carbon nanostructured materials such as carbon onions, carbon nanotubes, graphene, and templated carbon, which are used as electrode materials for EDLCs. Each carbon nanostructured materials have its advantages and disadvantages. For example, carbon onions have high power per

formance due to excellent conductivity with high accessible ion adsorption capacity

density due to superior electrical properties and unique tubular structures for fast charge transportation but due to the high cost, their widespread applications are limited [13]. Recently, graphene has been attracted much attention as electrode materials for EDLC applications due to unique properties, like as ultrahigh specific surface area, unique conductivity, and exceptionally high mechanical strength [14]. However, the aggregation of sheets during electrode preparation limits the aspect of application. More recently, 3D porous carbon nanostructured materials are widely used for EDLCs because of rich pore structures and high surface areas but due to relative low conductivity and presence of micropores specific capacitance is insuffi

cient at a high current density [15]. Therefore, it is necessary to construct composite materials by coupling the advantages of different types of carbon nanostructured materials and high energy electrode materials such as transition metal oxides, metal hydroxides and metal dichalcogenides (TMDs) to enhance the energy density with

out the compromise of power density and also meet the requirement for fabrication of high energy storage devices. In the composite electrode material, different types of carbon nanostructured materials not only contribute to high capacitance but also provide an easy conductive path for charge transportation due to conductive nature.

**4. Carbon-metal oxide composite electrode materials**

2, MnO

2, Fe 3 O 4, V 2 O

been received significant attention and extensive studied as SC electrode materials due to Pseudo capacitance nature, which depends on the fast reversible redox reac

tion of electroactive species directly as well as in the vicinity of electrode surface [17–20]. The redox behavior is due to the multivalent property of the above oxides which changes their oxidation states by interaction with protons or hydroxide ions reversibly. In spite of their excellent specific capacitance, they still suffer from low conductivity, low rate capability, poor stability and durability during the process of charge/discharge. In contrast carbon materials shows excellent performance in

5, NiO, Co

3 O

4, and TiO

2, has


Many metal oxide such as RuO

⁓30 F g−1 [12]. On the other hand, CNTs have high energy

2

g−1 and charge mobility of




*DOI: http://dx.doi.org/10.5772/intechopen.95460*

**3.3 Two-dimensional (2-D) nanosheets**

V−1 s−1 [11].

200000 cm

2

electrode interface.

but low capacitance of

layered graphene has theoretical surface area of 2756 m

**3.4 Three-dimensional (3-D) porous nanostructures**

### **3.3 Two-dimensional (2-D) nanosheets**

*Novel Nanomaterials*

**118**

**Materials** Dimensionality

Conductivity Volumetric Capacitance

Cost Structure

**Table 1.**

*Different carbon nanostructures used as electrode materials for EDLCs with onion-like carbon, carbon nanotubes, graphene, activated carbon, carbide-derived carbon, and templated carbon [16].*

**Carbon onions**

0D High Low High

**Carbon nanotubes**

1D High

Low High

**Graphene**

2D High Moderate Moderate

**Templated carbon**

3D

Low

Low

High

They are sheet like structures with high aspect ratio such as graphene, graphene oxide (GO) or reduced graphene oxide (rGO). In addition, they have high specific surface area, good mechanical strength, and excellent electrotonic conductivity, which helps them as promising electrode materials for SCs. For an example, single layered graphene has theoretical surface area of 2756 m2 g−1 and charge mobility of 200000 cm<sup>2</sup> V−1 s−1 [11].

### **3.4 Three-dimensional (3-D) porous nanostructures**

These are the low dimensional building blocks such as carbon nanofoams or sponges with hierarchical porous channels, rich pore structures, higher electrical conductivity and better structural mechanical stability, which are extensively used in composite electrode materials for SCs. For an example, foam has high specific surface area with continuous electron transport path and large area of electrolyteelectrode interface.

**Table 1** shows some examples of different carbon nanostructured materials such as carbon onions, carbon nanotubes, graphene, and templated carbon, which are used as electrode materials for EDLCs. Each carbon nanostructured materials have its advantages and disadvantages. For example, carbon onions have high power performance due to excellent conductivity with high accessible ion adsorption capacity but low capacitance of ⁓30 F g−1 [12]. On the other hand, CNTs have high energy density due to superior electrical properties and unique tubular structures for fast charge transportation but due to the high cost, their widespread applications are limited [13]. Recently, graphene has been attracted much attention as electrode materials for EDLC applications due to unique properties, like as ultrahigh specific surface area, unique conductivity, and exceptionally high mechanical strength [14]. However, the aggregation of sheets during electrode preparation limits the aspect of application. More recently, 3D porous carbon nanostructured materials are widely used for EDLCs because of rich pore structures and high surface areas but due to relative low conductivity and presence of micropores specific capacitance is insufficient at a high current density [15]. Therefore, it is necessary to construct composite materials by coupling the advantages of different types of carbon nanostructured materials and high energy electrode materials such as transition metal oxides, metal hydroxides and metal dichalcogenides (TMDs) to enhance the energy density without the compromise of power density and also meet the requirement for fabrication of high energy storage devices. In the composite electrode material, different types of carbon nanostructured materials not only contribute to high capacitance but also provide an easy conductive path for charge transportation due to conductive nature.
