## 1 Introduction  
The rapid growth of the human population and the development of the global economy has caused the increasing demand for energy. The global energy need is predicted to be doubled by the mid-century and more than triple by the end of this century [1]. Therefore, an energy storage system is needed to store excess energy generated and to supply it to electrical devices effectively. Among energy storage devices, supercapacitors are always preferred due to their high performance in energy storage compared to batteries, flywheels, and traditional capacitors. This is because batteries have high energy density, but low power density and short life cycles. In addition to this, flywheels are cost-effective and have a long cycle life, but they have installment and safety issues due to their large size. Traditional capacitors can be charged and discharged in a short time, but they have low energy density. A supercapacitor is an electrical component that can store a large amount of electrical energy. It is a multicomponent system comprised of two electrodes connected to current collectors and a separator immersed in an electrolyte, and was first used in 1978 as a backup power source in order to maintain computer memory system [2]. Over the years, due to the development of technology, supercapacitors have been widely used in portable electronic devices and hybrid electric vehicles because of their rapid charging-discharging rates, long cycle life, and high specific power. Based on their charge-storage mechanism, supercapacitors are classified into three types: Electrochemical double layer capacitors (EDLCs), pseudocapacitors, and hybrid capacitors. EDLCs, which are made up of carbon-based materials, store charges electrostatically or non-Faradaically in an electrical double layer formed at the interface between an electrode and electrolyte [3]. Conversely, pseudocapacitors, which consist of transition metal oxides or conducting polymers, accumulate charges through faradic processes in which the charge is transferred across the electrode and electrolyte by reversible redox reaction occurring at the interface [4]. The hybrid capacitor, which is a combination of a EDLC and pseudocapacitor, where both electrical double layer and electrochemical processes are involved in charge storage, enable hybrid capacitors to have the advantages of both EDLCs and pseudocapacitors, i.e., high specific energy and power with good cyclic stability [2]. The materials used in supercapacitor electrodes need to have high conductivity, large surface area, good cyclic stability, and a high theoretical specific capacitance [5]. N-doped graphene (Figure 1) is a good carbon-based electrode material for supercapacitors because it has a large surface area, good cyclic stability, and high electrical conductivity, however, it has low specific capacitance. On the other hand, transition metal oxides such as manganese oxides and iron oxides have high specific capacitance, together with a minimal impact on the environment and low cost, but both of these two oxides have poor cyclic stability and conductivity. Therefore, graphene or N-doped graphene is usually used as a support for transition metal oxides, which provides high electrical conductivity and excellent cyclic stability [6]. Recently, composites of NDG with mixed metal oxides are extensively studied because the incorporation of two metal oxides or bimetallic oxides is believed to possess superior specific capacitance due to the synergistic effect of two types of metal oxides that offer richer redox reactions compared to single metal oxide or spinel oxides, which can give better electrochemical performance. [PERSON], et al. [7] synthesized nanocomposites of NDG/ZnO/NiO with a specific capacitance of 1834.9 F/g, which is higher than composites of NDG with single metal oxide because ZnO improved the conductivity of NiO. The nanocomposite has a capacity retention of 93% after 6000 cycles. [PERSON],  
Figure 1: Three common bonding configurations of N atoms in graphene. et al.