**1. Introduction**

It is a matter of great concern that the conventional energy sources on our planet are getting exhausted day by day. As the energy consumption demand is constantly increasing, new alternative energy sources are being developed across the world [1–5]. The highly threatening increase in pollution level and global warming invites our attention to the necessity of developing a clean energy portfolio. Special focus should be given in the development of alternative energy sources as well as its storage [6]. The most well-known energy production and storage technologies are batteries, fuel cells, and supercapacitors. In contrast to fuel cells and batteries, supercapacitors use a

different energy generating technique. Even though these three systems have distinct energy storage and conversion processes, there exists some electrochemical similarities between them. Common characteristics include the separation of electron and ion transport and the fact that the energy-producing activities occur at the electrode/ electrolyte interface's phase boundary. Also, the basic structure of these three systems consists of two electrodes in contact with an electrolyte solution [7].

Coming to the differences, in fuel cells and batteries, energy production occurs from chemical reaction via redox reaction, whereas in supercapacitors, energy is liberated through the diffusion of ions at the interface between the electrolyte and the inner side of the capacitor electrode plates forming electrostatic double layers. Supercapacitors are also called ultracapacitors. They act as a bridge between conventional capacitors and batteries. Conventional capacitors deliver high power density with low energy density and the batteries possess high energy density with low power density. Supercapacitors are generally preferred over batteries because they enable quick charging and can deliver energy at a pace that is comparably faster. Also, they are acceptable in terms of durability, stability, and life span [8]. Comparatively lower energy density of this system can be enhanced by wisely choosing the electrolyte and electrode material. **Figure 1** demonstrates the Ragone plot representing various energy storage systems.

Devices fabricated with TMOs are sophisticatedly important since they exhibit excellent performance in energy storage, wastewater treatment, gas sensing, photovoltaics, etc. In chemical industries, TMOs find application in dye degradation and for the conversion of various hydrocarbons. In energy storage devices, TMO acts as an efficient electrode material especially in supercapacitors and solar cell application. The incomplete d shell corresponding to TMOs resulted in these properties

**Figure 1.** *Ragone plot representing various energy storage systems.*

#### *Review on Transition Metal Oxides and Their Composites for Energy Storage Application DOI: http://dx.doi.org/10.5772/intechopen.108781*

including wide band gap, enhanced chemical reactivity, electrical conductivity, stability, and anti-corrosiveness [9]. To enhance the efficiency of materials, researchers are working on combining TMOs with other transition metals, metal oxides, carbon-based materials, etc. This can modify the surface area, pore characteristics, ion intercalation/deintercalation, conductivity, etc. Many works extending from ZnO/activated carbon to ternary composites such as ZnO/rGO/RuO2 are still under research study [5].

Graphene, the wonder material with single-atom thickness is one of the most wellliked carbon-based materials because of its high surface area (around 2630 m**2**/g), high conductivity, and chemical stability. The existence of Vander Waals force in graphene results in sudden agglomeration and causes a reduction in surface area and capacity [10]. Carbon-based materials including carbon nanotube (CNT), graphene oxide (GO), activated carbon, etc., are much attracted due to the highly porous structure and enhanced surface area. A combination of TMO with these materials can thus make better electrochemical performance. The size and shape of the nanoparticles can able to tune even the band gap energy of the material; thus, it can control various properties of the nanomaterial including the surface reactivity [11]. These characteristic features of nanostructures can be controlled by various synthesis methods and thus can control the band gap, pore features, etc.

In this chapter, we are focused on the synthesis of some selected TMOs and their composites and discuss the effect of synthesis procedure on the structural and optical characteristics of the material. Also, the article incorporates the device application of the proposed materials.
