2. Brief history of supercapacitor

Electrical charge storage by a surface was first discovered from the phenomena of rubbing amber with fur which attracts dust in ancient age. Invention of the Leiden jar in 1757 is the first developed technology for capacitor. This Leiden jar was further improved to flat capacitor by Banjamin Franklin. This resulted in the reduction of volume as well as increase in reliability and convenience. In the late nineteenth century Helmholtz solved the electrical charge storage by a capacitor by using Faraday's law. He proved the existence of two parallel sheets of opposite charges on the surface of metal and the solution side. He proposed the model of charge/ion distribution near metal surface. It is the foundation stone for the development of fundamental aspects of capacitive technology as well as the quantitative science which describe the nature of electrostatic behavior. In the General Electric Laboratories (1957), they have developed a capacitor by using two porous carbon electrodes and aqueous electrolyte. Later it is known as electrochemical double layer capacitor. They got the U.S. Patent for this (US Patent 2,800,616). Almost a decade later (1966), Standard Oil of Ohio's (SOHIO) scientists and engineers have developed the modern electrochemical supercapacitor (SC) capacitor using porous carbon and non-aqueous electrolyte [9–12]. The non-aqueous electrolyte enables to have wide potential window for SC, which results in the increase in storage capacity. SOHIO sold this technology to Nippon Electric Company (NCE, Japan) and they used it commercially for the first time as the backup power for computer memory and also named as the supercapacitor. Presently, NEC/TOKIN, ELNA, Maxwell Technologies, Panasonic and several other companies invest in the development of supercapacitors. In 2004, the worldwide market of supercapacitor was 100 million US dollars, while the worldwide sales of supercapacitor reached to 400 million US dollars by the end of 2010. It was estimated that the market of supercapacitor will rise to 2 billion US dollars by 2020 [13–15].

## 3. Different types of supercapacitor

The importance of supercapacitor as an energy storage was significantly increased in the last decade of twentieth century and it witnessed significant advances in the field. Several works by Conway et al. and others [United States Pat., 1996] have investigated and identified the underlying chemistry and developed the

Transition Metal Oxide-Based Nano-materials for Energy Storage Application DOI: http://dx.doi.org/10.5772/intechopen.80298

model for charge storage. Due to the charge storage mechanism, supercapacitors are categorized into two different types, electrochemical double layer capacitors (EDLCs, non-Faradic electrostatic storage) and pseudocapacitors (Faradic, redox reaction based capacitors). In addition, there is another class of supercapacitors known as hybrid supercapacitors which is the combination of both storage mechanisms. In this chapter, the storage mechanism, electrode materials, electrolytes of different supercapacitors will be discussed.

#### 3.1 Electrochemical double layer capacitor (EDLC)

EDLCs have a similar structure to that of conventional capacitors except the dielectric is being replaced by electrolyte. Two highly porous carbon electrodes are separated by a porous separator and electrolyte. The energy storage mechanism of EDLC relies on the non-Faradic process i.e. electrostatic adsorption ions at the electrode/electrolyte interface. During the charging, the positive and negative ions of the electrolyte are separated and adsorbed by negative and positive electrodes, respectively. The energy storage mechanism is based on the formation of double layers of electrolyte ions at the interface of electrode and electrolyte. This is similar to the parallel plate capacitor and the capacitance of EDLC can be calculated by Eq. (1)

$$\mathbf{C} = \frac{e\varepsilon\_0 \mathbf{A}}{d} \tag{1}$$

where, C is the capacitance, ε<sup>0</sup> is the dielectric constant in vacuum, ε is the dielectric constant of the double layer, A is the area of the electrode and d is the thickness of double layer. Various models have been proposed to explain the formation of double layer. In 1853, Helmholtz first introduced the idea of double layer. When a charged conductor is placed in contact with electrolyte, the distribution of electric charges will be modified. Two layers of opposite charges will be formed at the interface of electrode and electrolyte. These two layers are separated by molecular dimensions but there is no exchange of ions between the layers. Hence the capacitance of the double layer can be obtained from the aforementioned Eq. (1). This model is widely used to explain the storage of supercapacitor. But this model did not taken care of the effects of ions behind the first layer of the ions at the electrode/electrolyte interface. Various carbonaceous materials (activated carbons, graphene, CNT etc.) store charges via EDLC mechanism. Carbon based materials were the first choice for the commercial applications because of their rapid response, good electrical conductivity, high chemical stability, non-toxicity, high abundance and simplicity of design. Carbonaceous materials have very high specific surface area (1000–3500 m<sup>2</sup> g�<sup>1</sup> ) which is very useful for the charge storage since EDLC is a surface dependent phenomenon. But with the increase of specific surface area and porosity the stability and conductivity of the material decreases. In spite of this, mesoporous nature with high specific surface area is very much important for its application as an active electrode [9, 16–18].

#### 3.2 Pseudocapacitor

Another class of supercapacitor is pseudocapacitors which rely on the reversible redox reaction or Faradic reaction to store energy. Mainly transition metal oxides (e.g. ruthenium oxide, nickel oxide, manganese oxide, vanadium pent oxide etc.) and conducting polymer (polyaniline, polypyrrole, PEDOT:PSS, etc.) belongs to this group. Close surface to the electrolyte take part in redox reactions and this process can be classified into three distinct types which are underpotential deposition (adsorption pseudocapacitance), redox pseudocapacitance and intercalation

more increase in the global temperature will cause the polar ice caps and glaciers to melt, causing the sea level to rise flooding the costal lines [3]. In order to sustain human growth these issues have to address as soon as possible. To reduce the world's hunger for fossil fuels while maintaining the same life standards we have to focus on the alternative green energy sources like solar, wind, tidal etc. Although these sources have the ability to meet the world's energy requirements but the intermittent nature of these energy sources is an unavoidable problem which significantly stimulates the motivation of research on the energy storage systems. Today a variety of energy storage and conversion devices are available such as batteries, conventional capacitors, fuel cell and supercapacitors etc. But among such energy storage systems electrochemical capacitors or supercapacitors have drawn attention as one of the most promising energy storage systems because of their high power density, short charging time and long life span although having moderate energy density 10–15 mWh/g which is still very less compared to batteries. Different research groups in the world are trying to improve the energy density and overall life span of the device by suitably

Science,Technology and Advanced Application of Supercapacitors

Electrical charge storage by a surface was first discovered from the phenomena

of rubbing amber with fur which attracts dust in ancient age. Invention of the Leiden jar in 1757 is the first developed technology for capacitor. This Leiden jar was further improved to flat capacitor by Banjamin Franklin. This resulted in the reduction of volume as well as increase in reliability and convenience. In the late nineteenth century Helmholtz solved the electrical charge storage by a capacitor by using Faraday's law. He proved the existence of two parallel sheets of opposite charges on the surface of metal and the solution side. He proposed the model of charge/ion distribution near metal surface. It is the foundation stone for the development of fundamental aspects of capacitive technology as well as the quantitative science which describe the nature of electrostatic behavior. In the General Electric Laboratories (1957), they have developed a capacitor by using two porous carbon electrodes and aqueous electrolyte. Later it is known as electrochemical double layer capacitor. They got the U.S. Patent for this (US Patent 2,800,616). Almost a decade later (1966), Standard Oil of Ohio's (SOHIO) scientists and engineers have developed the modern electrochemical supercapacitor (SC) capacitor using porous carbon and non-aqueous electrolyte [9–12]. The non-aqueous electrolyte enables to have wide potential window for SC, which results in the increase in storage capacity. SOHIO sold this technology to Nippon Electric Company (NCE, Japan) and they used it commercially for the first time as the backup power for computer memory and also named as the supercapacitor. Presently, NEC/TOKIN, ELNA, Maxwell Technologies, Panasonic and several other companies invest in the development of supercapacitors. In 2004, the worldwide market of supercapacitor was 100 million US dollars, while the worldwide sales of supercapacitor reached to 400

million US dollars by the end of 2010. It was estimated that the market of

The importance of supercapacitor as an energy storage was significantly increased in the last decade of twentieth century and it witnessed significant advances in the field. Several works by Conway et al. and others [United States Pat., 1996] have investigated and identified the underlying chemistry and developed the

supercapacitor will rise to 2 billion US dollars by 2020 [13–15].

3. Different types of supercapacitor

90

choosing different electrode materials [4–8].

2. Brief history of supercapacitor

pseudocapacitance. Underpotential deposition arises when reversible adsorptions as well as removal of atoms occur at metal surface in two dimensional Faradic reactions. Redox pseudocapacitance exists when reversible redox reactions taken place at the electrode surface. In case of intercalation pseudocapacitance, ions are electrochemically intercalated into the structure of redox materials.

Although these three mechanisms are physically different from each other but they can be electrochemically governed by the Nernst equation. According to this equation, if the reaction potential E can be approximated by a linear function of (1 + Qr)/Qr, the specific capacitance can be obtained from Eq. (2)

$$\mathbf{C}\_{m} = \frac{nF}{mE} \left(\frac{\mathbf{1} + \mathbf{Q}\_{r}}{\mathbf{Q}\_{r}}\right) \tag{2}$$

potential window. To overcome this drawback now-a-days researchers are interested in using organic electrolytes as the main advantage of an organic electrolyte is

The cyclic voltammetry (CV) curve of active working electrode materials measured in any suitable electrolyte solution (organic, ionic and aqueous electrolyte) epitomizes the total stored charge which ascends from both Faradic and non-Faradic process. The presence of oxidation/reduction peaks in CV curve represents the Faradic charge transfer reaction between electrolyte and electrode material. The

> Cm <sup>¼</sup> <sup>i</sup> 2mv

where m and ν are the mass of the electroactive material and potential scan rate, respectively. Current (i) can be obtained by integrating the area of the curves

> Ð Vc Va i vð Þdv Vc � Va

where Va and Vc are the lowest and highest voltage of the potential range,

The specific capacitance value of the material can also be calculated from

Cm <sup>¼</sup> <sup>i</sup>

6. TiO2-V2O5 nanocomposites as supercapacitor applications

<sup>m</sup> � dV

where 'i' is the current applied, dV/dt is the average slope of the discharge curve

Among various transition metal oxides, vanadium oxides (V2O5) also known as vanadium pentoxides have already been studies as a promising supercapacitor electrode material for energy storage application due to its excellent physical properties,

dt � � (5)

(3)

(4)

total charge stored in electrode material can be calculated using Eq. (3):

i ¼

Galvanostatic charge discharge (GCD) profiles by using Eq. (5)

and m is the mass of active electrode materials [22, 23].

it can provide wide potential window �3.5 V. Since the energy density of a supercapacitor is directly proportional to the square of the cell voltage thus the organic electrolyte is much more suitable compared to other electrolytes. Among various organic electrolytes, acetonitrile and propylene carbonate (PC) are commonly used solvents. Propylene carbonate (PC)-based electrolytes are eco-friendly and can offer a wide electrochemical window, a wide range of operating temperature, as well as good electrical conductivity. In addition to promising advantages, the high cost, safety concern and toxic in nature still limit their commercial applications. Ionic electrolyte also known as room temperature molten salts-ionic liquids is one of the most promising electrolytes for the next generation energy storage application. Ionic electrolyte have lots of advantages such as high thermal stability, non-toxicity, non-flammability, high electrochemical stability and various combination of choices of cations and anions. But the low ionic conductivity at room temperature of ionic liquids is a great issue for practical application [20, 21].

Transition Metal Oxide-Based Nano-materials for Energy Storage Application

DOI: http://dx.doi.org/10.5772/intechopen.80298

5. Method for calculation of specific capacitance

using Eq. (4)

respectively.

93

where, n is the number of electron, F is the Faraday constant, m is the molecular weight of active electrode and Qr is the reaction quotient. Transition metal oxides are chosen as the active materials for supercapacitor electrode and they store charge via Faradic or redox mechanism. They exhibit large theoretical specific capacitance with multiple valence states which enables them one of the most studied materials group in the field of supercapacitor [1, 19].

#### 3.3 Hybrid supercapacitor

Hybrid supercapacitors are third type of supercapacitors which combine the features of both EDLCs and pseudocapacitors. The electrodes of hybrid supercapacitors are made with composite materials that include EDLC materials (carbonaceous materials such as activated carbon, graphene, CNT etc.) and pseudocapacitive materials (transition metal oxides and conducting polymers). There can also be asymmetric supercapacitor with one pseudocapacitive electrode and another EDLC electrode or hybrid electrode or vice-versa. Several binary and ternary composite based on polymer and CNTs have been prepared for electrochemical capacitive energy storage application. They offer large specific capacitance compare to individual one, which is due to the strong interaction between polymer and CNTs. Gupta and Miural were the first to propose that SWNT/PANI composite can be effectively used as the electrodes for supercapacitors. The highest specific capacitance value of 463 F g�<sup>1</sup> was obtained for 27 wt% CNT.

#### 4. Electrolytes

Besides the electrodes, another most important factor which can expressively influence the electrochemical performance of supercapacitor device is electrolyte. Generally, electrolyte exists in inside the separator as well as inside the active material layers. The important factors for an electrolyte are one wide potential window which is key factors to achieve higher energy density and the other is the high ionic concentration, low resistivity, low viscosity etc. which can also influence the power density of the supercapacitor device. There are three types of electrolyte usually used in supercapacitors: aqueous electrolyte, organic electrolytes and ionic electrolytes. Aqueous electrolytes (such as H2SO4, KOH, Na2SO4, HCl, NaCl and NH4Cl aqueous solution and so on) limit the cell voltage window of supercapacitor to typically 0–1 V due to their low electrochemical stability, which effectively reduces the energy density of the cell. It can also provide a higher ionic concentration with conductivity up to 1.0 S cm�<sup>1</sup> . Supercapacitors containing aqueous electrolyte may exhibit higher charge storage capacity but the main drawback is in terms of improving both energy and power densities due to their narrow working

Transition Metal Oxide-Based Nano-materials for Energy Storage Application DOI: http://dx.doi.org/10.5772/intechopen.80298

potential window. To overcome this drawback now-a-days researchers are interested in using organic electrolytes as the main advantage of an organic electrolyte is it can provide wide potential window �3.5 V. Since the energy density of a supercapacitor is directly proportional to the square of the cell voltage thus the organic electrolyte is much more suitable compared to other electrolytes. Among various organic electrolytes, acetonitrile and propylene carbonate (PC) are commonly used solvents. Propylene carbonate (PC)-based electrolytes are eco-friendly and can offer a wide electrochemical window, a wide range of operating temperature, as well as good electrical conductivity. In addition to promising advantages, the high cost, safety concern and toxic in nature still limit their commercial applications. Ionic electrolyte also known as room temperature molten salts-ionic liquids is one of the most promising electrolytes for the next generation energy storage application. Ionic electrolyte have lots of advantages such as high thermal stability, non-toxicity, non-flammability, high electrochemical stability and various combination of choices of cations and anions. But the low ionic conductivity at room temperature of ionic liquids is a great issue for practical application [20, 21].

#### 5. Method for calculation of specific capacitance

The cyclic voltammetry (CV) curve of active working electrode materials measured in any suitable electrolyte solution (organic, ionic and aqueous electrolyte) epitomizes the total stored charge which ascends from both Faradic and non-Faradic process. The presence of oxidation/reduction peaks in CV curve represents the Faradic charge transfer reaction between electrolyte and electrode material. The total charge stored in electrode material can be calculated using Eq. (3):

$$C\_m = \frac{i}{2mv} \tag{3}$$

where m and ν are the mass of the electroactive material and potential scan rate, respectively. Current (i) can be obtained by integrating the area of the curves using Eq. (4)

$$i = \frac{\int\_{V\_a}^{V\_c} i(v)dv}{V\_c - V\_a} \tag{4}$$

where Va and Vc are the lowest and highest voltage of the potential range, respectively.

The specific capacitance value of the material can also be calculated from Galvanostatic charge discharge (GCD) profiles by using Eq. (5)

$$C\_m = \frac{i}{m\left(-\frac{dV}{dt}\right)}\tag{5}$$

where 'i' is the current applied, dV/dt is the average slope of the discharge curve and m is the mass of active electrode materials [22, 23].
