**2. Mechanism of supercapacitors**

The reason behind the overview of SCs energy storage system is that SCs weigh less than that of battery with same energy storage capacity, fast access to stored energy, charging very fast than battery, charge/discharge cycle is 106 times, storage capacity is independent of number of charging discharging cycles, negligible environmental concerns, and energy density of SC is 10–100 times larger than that of traditional capacitors [12]. SCs can store substantially more energy than conventional capacitors because the charge separation takes place across a very small distance in the electrical double layer that constitutes the interface between an electrode and the adjacent electrolyte and an increased amount of charge can be stored on the highly extended surface area electrode materials. Electrochemical capacitors also known as supercapacitors exhibit high specific capacitance, high specific power, long cycle life, and fast charge/discharge rate. Theoretical capacitance values of some TMOs are represented in a bar diagram (**Figure 2**).

Supercapacitors are classified according to the energy storage mechanism as electrical double-layer capacitors (EDLCs), pseudocapacitors (PCs), and hybrid capacitors. In EDLCs, charge storage is based on reversible adsorption desorption mechanism at the electrode-electrolyte interface and not involving any faradaic reaction. Electric double layers are formed with the accumulation of charge over the opposite electrodes. Here, the electrode's exposed surface area to the electrolyte determines the capacitance. In other words, pore size of the electrode material should match with the ion size of the electrolyte to avoid capacitance drop. On the other hand, in pseudocapacitors or redox capacitors, charge storage is based on rapid

**Figure 2.** *Theoretical specific capacitance of some TMOs.*

faradaic reaction at the surface of electrode material. Generally, pseudocapacitors provide capacitance value higher than EDLCs. In hybrid systems, materials for EDLCs (capacitor-like power sources) and pseudocapacitors (energy sources that resemble batteries) are combined on a single electrode substrate [13].

Electrode is the key factor that determines the performance of any supercapacitor. Depending on the anode and cathode materials in SC device fabrication, it is classified into symmetric, asymmetric, and hybrid device. If two electrodes of the SC are of same material, it is called symmetric device and includes EDLC, pseudocapacitive, and hybrid-type material electrodes. If cathode and anode electrodes are of different types, then the combination is called asymmetric devices. Another classification is based upon the electrolyte. Water-based electrolyte refers as aqueous electrolyte devices, and solvent-based electrolytes come under organic electrolyte-based devices [14].

The electrode material of a supercapacitor is chosen such that it should possess some unique characteristics such as high conductivity, better resistance toward temperature change, large specific surface area, and environmental compatibility. Performance of a supercapacitor relies upon the ability of electrode material for the smooth conduct of faradaic charge transfer [15]. Porosity of the electrode material should be well tuned according to the application where it is used. Small pores yield better surface area, which in turn enhances the specific capacitance and energy density. The better surface area of a porous material enables much more reactive sites and promotes the transfer of electrons and ions. However, this small pore increases the equivalent series resistance (ESR) and hence reduces the specific *Review on Transition Metal Oxides and Their Composites for Energy Storage Application DOI: http://dx.doi.org/10.5772/intechopen.108781*

**Figure 3.** *Schematic representation of supercapacitor device model.*

power. Therefore, less porous materials are preferred for applications where high peak current is demanded [16]. Schematic representation of supercapacitor device model is shown in **Figure 3**.
