1. Introduction

Rapidly depleting level of fuel reservoir along with the increasing effect of environmental pollution are the two most important concerns of twenty-first century. The rate at which the fossil fuel is being consumed today, it will take around 40 more years to run all the known oil deposits dry leaving the whole world into complete darkness. Fossil fuel is a very rich form of energy containing around 30– 50 MJ of energy per kilogram. Combustion of fossil fuels results in the emission of CO2, CH4, N2O etc. in the atmosphere which trap the solar radiation in the atmosphere [1, 2]. Although the natural trapping of solar radiation is vital for all the lives on the earth but due to excessive emissions of these gases the earth is getting hotter. According to the study conducted by NASA's Goddard institute, the Earth's average temperature has risen by 0.8°C since the beginning of the industrial revolution. Although this increment may seem very small but the alarming fact is that a little

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 choosing different electrode materials [4–8].

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

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

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)

<sup>C</sup> <sup>¼</sup> εε0<sup>A</sup>

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

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

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

<sup>d</sup> (1)

) which is very useful for the charge storage since

different supercapacitors will be discussed.

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

surface area (1000–3500 m<sup>2</sup> g�<sup>1</sup>

3.2 Pseudocapacitor

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its application as an active electrode [9, 16–18].

3.1 Electrochemical double layer capacitor (EDLC)
