Energy storage has an equal importance as energy production. To face the global challenges, recently, our modern society demands lightweight, flexible, inexpensive, and environmentally friendly energy storage systems (Meng et al., 2010; Chodankar et al., 2015). Battery and supercapacitor are the major energy storage devices. But, slow charge–discharge rate, short life cycles, and high weight of battery limit its applications in portable and wearable devises (Meng et al., 2010). At present, supercapacitors have been receiving a great attention, because of their important features such as high energy density, high power density, light weight, fast charging–discharging rate, secure operation, and long life span (Jayalakshmi and Balasubramanian, 2008; Chodankar et al., 2015). The supercapacitor is also called electrochemical capacitor. This is used in various applications such as hybrid vehicles, power backup, military services, and portable electronic devises like laptops, mobile phones, wrist watches, wearable devised, roll-up displays electronic papers, etc. (Lee et al., 2011; Wang et al., 2012).

On the basis of charge storage mechanism and material used as the electrode, the supercapacitors are divided into two categories: electrochemical double layer supercapacitors (EDLCs) and pseudocapacitor (Jayalakshmi and Balasubramanian, 2008). In EDLCs, the specific capacitance arises from the non-Faradaic charge storage mechanism between electrode and electrolyte interface (Jayalakshmi and Balasubramanian, 2008; Wang et al., 2012). The materials that have been used as electrode for EDLCs are porous carbon (Kang et al., 2015), SWNT (Liu et al., 2006), MWNT (Huang et al., 2014a), reduce graphine oxide (Zhang and Zhao, 2012), aerogel (Faraji and Ani, 2015), etc. In pseudocapacitor, the specific capacitance arises from Faradaic reaction at the electrode interface. The materials that have been studied as electrode for pseudocapacitors are transition metal oxides and conducting polymers (Wang et al., 2012).

In particular, the specific capacitance of the supercapacitors depends on the surface area and the pore size distribution of the electrode material. Compared with the transition metal oxides and conducting polymers, carbon and its different types have high surface area (3.270 m²g⁻¹) (Kang et al., 2015). However, this high surface area of carbon is not completely accessible for the electrolyte (Faraji and Ani, 2015). To overcome this shortcoming, the composites of carbon with transition metal oxides or conducting polymer have received great attention. These composite are also called hybrid materials. The use of hybrid material as an electrode in supercapacitors result in the third category of supercapacitors called hybrid supercapacitors. In hybrid supercapacitors, the specific capacitance arises from Faradic as well as non-Faradic charge storage mechanism at the electrode and electrolyte interface (Zhang et al., 2013; Pardieu et al., 2015).

The specific capacitance (C_s) (Fg⁻¹), energy density E (Wh kg⁻¹), power density P (kW kg⁻¹), and retention capacity or coulomb efficiency (η) are the crucial characteristics of the supercapacitor device. The (C_s) (Fg⁻¹) at the single electrode of the device is calculated given by, (1)Cs=1mV(Vc−Va)∫vavcI(v)dV where m is the mass (g cm⁻¹) deposited, I(v) is the response current (mA) of the electrode material for unit area, V is the scan rate, V_c−V_a is the operational potential window in (V), V_a anodic current, and V_c cathodic current. Energy density E (Wh kg⁻¹) and power density P (W kg⁻¹) of supercapacitor are calculated using following relations as, (2)E=0.5×Cs×(Vmax2−Vmin2)3.6 (3)P=EtD where C_s is specific capacitance (Fg⁻¹), V_max and V_min are the maximum and minimum voltage achieved during charging and discharging process, respectively, in volt (V), and t_D is the discharging time (s) for a cycle of the supercapacitor. The retention of specific capacitance is calculated using the relation, (4)η=tDtC where t_C and t_D are the charge and discharge time (s), respectively, for a cycle of the supercapacitor (Wang et al., 2010; Dubal et al., 2012).

The transition metal oxides have a great scientific significance. These are the basis of a variety of functional materials (Shinde et al., 2015). Among the various supercapacitor electrode materials, transition metal oxides offer high electronegativity, rich redox reactions, low cost, environmental friendliness, and excellent electrochemical performance. Different transition-metal oxides, such as IrO₂, RuO₂, Co₃O₄, MnO₂, Fe₂O₃, SnO₂, NiO, etc., have been extensively studied as the electrode material for supercapacitor (Luo et al., 2014). Among these, RuO₂ has been identified as a dominant candidate because it has high theoretical specific capacitance (1,358 Fg⁻¹), high electrical conductivity (300 S cm⁻¹), and high electrochemical stability (Yu et al., 2013). However, the high cost and toxicity associated with the RuO₂ limits its commercial applications (Deng et al., 2014).

Furthermore, the cobalt oxides have received significant interest in recent years because of their low cost, non-toxic, easy synthesis, and environmental friendly nature. The cobalt oxides have high theoretical capacitance (CoO: 4.292 Fg⁻¹, Co₂O₄: 3.560 Fg⁻¹) (Cheng et al., 2010; He et al., 2012). Additionally, cobalt oxides show excellent electrochemical behavior in alkaline as well as organic electrolyte. These have the ability to interact with the ions of the electrolyte at the surface as well as through the bulk of the material (Vijayakumar et al., 2013). The features of cobalt oxides such as morphology, structures, and dimension can be easily controlled via adjusting the preparative parameters such as, reaction temperature, reaction time, concentration of matrix solution, complexing agent, etc. (Wei et al., 2015a).

An optimize microstructure and controlled morphology of the material will enhance the specific surface area and pore size distribution, which facilitate the electrolyte ion transport in the material (Meher and Rao, 2011). Recently, many new approaches have been successfully in use to synthesize the meso and microporous nanostructure cobalt oxide materials such as hydrothermal method (Meher and Rao, 2011), chemical bath deposition method (Xu et al., 2010), hydrothermal precipitation method (Yu et al., 2009), solvothermal synthesis method (Yang et al., 2013), combustion synthesis method (Deng et al., 2014), microwave-assisted synthesis method (Vijayakumar et al., 2013), etc.

The specific capacitance of the cobalt oxide strongly depends on morphology, surface area, and pore size distribution. Recently, use of new synthesis approaches, surface modifying agents, complexing, and structure directing agent results in high-specific capacitance, which is equal the theoretical specific capacitance cobalt oxide. In this review paper, we have focused the recent advancements in the cobalt oxides and their composites as the electrode material. Table 1 shows the preparation and supercapacitive performance of cobalt oxide and their composites based supercapacitors.

Manganese (Mn) has different oxidation states. Out of these, the most stable oxidation states are Mn (II) and Mn (IV). The Mn (II) forms MnO, on the other hand, Mn (IV) forms MnO₂ and Mn₂O₃. The MnO₂ has α, β, γ, and δ -type polymorph (Chen et al., 2014; Salunkhe et al., 2015). The advantages of manganese-based metal oxides include low cost, low toxicity, natural abundance, and environmental friendly in nature (Sui et al., 2015; Wei et al., 2015b). In aqueous and organic electrolyte, the MnO, MnO₂, and Mn₂O₃ can form the different oxidation states. Thus, it results in the high-specific capacitance. The highest reported theoretical specific capacitance of MnO₂ is 1.370 Fg⁻¹ (Guo et al., 2015a; Wei et al., 2015b). However, the low electrical conductivity and large volume change during the charge–discharge process result in the unsatisfactory rate performance and cyclic stability. In consequence, this reduces the specific capacitance of the manganese oxides-based supercapacitors (Cabana et al., 2010; Chen et al., 2010). To overcome such hindrances, recently, the researchers have been executing many new strategies, such as use of carbon containing materials for increasing the electrical conductivity and adopt the volume buffers for relaxing internal stresses (Yao et al., 2008; Sui et al., 2015). Manganese oxides have been prepared by various synthesis methods, such as pulse laser deposition method (Xia et al., 2011), hydrothermal method (Zhang et al., 2014), electrochemical synthesis method (Jiang and Kucernak, 2002), redox deposition method (Bordjiba and Bélanger, 2009), successive hydrolysis–condensation method (Sawangphruk and Limtrakul, 2012), etc. Further, the detail of MnO₂ synthesis and their supercapacitive performance are shown in Table 2.

Recently, cobalt- and manganese-based metal oxide as the electrode materials for supercapacitor have been receiving the great attention. From the recent reports, it has concluded that, (1)

Advanced chemical method such as hydrothermal, pulse laser deposition, reverse microemulsion, microwave-assisted, etc., has been assisted to synthesize cobalt- and manganese-based metal oxide material.

All authors listed have made a substantial, direct, and intellectual contribution to the work and approved it for publication.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.