**2. Theoretical background for supercapacitors**

## **2.1 Principle and mechanism of supercapacitors**

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 electroactive materials.

### **2.2 Factors affecting the performance of Supercapacitors**

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

**117**

**Figure 2.**

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

can achieve significantly higher energy density, as compared to EDLCs, because they have a variety of oxidation states for redox charge transfer reactions. However, relatively low electrical conductivity and poor rate capability and cycle stability of pseudocapacitive materials limit their widespread commercial applications [9]. Therefore, carbon-based materials with high conductivity and distinct structures can be combined with pesudocapacitive materials to exhibit synergistic effects for

Carbon material is EDLCs type for supercapacitor. In section 2.1, EDLCs has introduced their property, which store the electrical charge by electrostatic force at the electrode-electrolyte interface, as shown in **Figure 2**. It is not involving electrochemical reactions on the electrode surface. There are different types of carbon nanostructured materials, which can be used as single electrode materials due to

They are round-shaped particles such as ultrafine activated carbon (AC), mesoporous carbon, carbon nanosphere, and carbon quantum dot, with a high specific

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

the pore size distribution and pore content, they can use as suitable supporting

g−1) and an aspect ratio of nearly [10]. In addition, by tuning

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

supercapacitive performance, known as hybrid SCs.

**3. Carbon based composite electrode materials**

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

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

*Schematic diagram of (a) an electrical double layer capacitor and (b) a pseudocapacitor.*

area (AC: ⁓3000 m2

transfer pathway.

materials for composite electrodes.

their unique structural, mechanical, and electrical properties.

*Carbon-Based Nanocomposite Materials for High-Performance Supercapacitors DOI: http://dx.doi.org/10.5772/intechopen.95460*

can achieve significantly higher energy density, as compared to EDLCs, because they have a variety of oxidation states for redox charge transfer reactions. However, relatively low electrical conductivity and poor rate capability and cycle stability of pseudocapacitive materials limit their widespread commercial applications [9]. Therefore, carbon-based materials with high conductivity and distinct structures can be combined with pesudocapacitive materials to exhibit synergistic effects for supercapacitive performance, known as hybrid SCs.
