# Application of the Supercapacitor for Energy Storage in China: Role and Strategy
## 2 Technology Characterization
### Double-Layer supercapacitor  
A double-layer supercapacitor is a kind of supercapacitor based on the interface double-layer theory put forward by German physicist [PERSON] [6]. It is well known that a metal electrode inserted in an electrolyte solution has an excess charge with the opposite sign on both sides of the liquid surface, resulting in the potential difference between phases. The positive and negative ions in the electrolyte rapidly move toward the poles under the action of the electric field and form a tight charge layer (double electric layer) on the surface of the two upper electrodes, respectively. The formed double electric layer is similar to the polarized charge generated by the dielectric in the traditional capacitor under the action of the electric field, resulting in the capacitance effect. Because the distance between layers of charge in supercapacitors is much smaller than that in traditional capacitors, its capacity is much larger than that of traditional capacitors. Compared with the rechargeable battery, the double-layer capacitor can be charged more than 10\({}^{6}\) times. Therefore, the double-layer capacitor not only has the characteristics of a capacitor, but also has the properties of a battery. So, they are sometimes called battery-type capacitors. [PERSON] et al. used ZMA-LDH@Fe2O3/3 DHPCNF as an independent electrode material for supercapacitors [7], which has high capacitance under both positive and negative operating potentials. The composite electrode has an areal capacitance of 3437 F cm\({}^{-2}\) at 1 mA cm\({}^{-2}\), providing ultra-high cycle stability. [PERSON] et al. used the optimized ZnO/Co3O4/NiO-2 (ZCN-2) as a double-layer supercapacitor electrode, and the electrode showed excellent electrochemical performance [8]. When the current density is 1 Ag\({}^{-1}\), the specific capacitance of ZCN-2 is 1119.11 C g\({}^{-1}\). Under the condition of 10 Ag\({}^{-1}\), the capacitance can still maintain 93.75% after 5000 charge-discharge cycles. Its maximum energy density can reach 95.82 Wh kg\({}^{-1}\), and the cycle stability can reach 94.53% of the initial value after 10,000 charge-discharge cycles at the current density of 10 A g\({}^{-1}\). After charging, the device connected by two supercapacitors can successfully keep light-emitting diodes (LED) glowing for up to 30 min. [PERSON] et al. directly synthesized Co2 Mn bimetallic hydroxide nanowire on nickel foam by the hydrothermal method and then prepared Co2 Al LDH nanosheets on Co2 Mn nanowire by the hydrothermal method to obtain heterogeneous nanocomposites (Co2 Al/Co2 Mn) [9]. Using Co2 Al/Co2 Mn/NF/AC/NF as nanocomposite material, the performance of the supercapacitor is greatly improved. When the current density is 1 Ag\({}^{-1}\), the specific capacitance of the nanocomposites can reach 1125.9 Cg\({}^{-1}\) (2502 Fg\({}^{-1}\)). At a current density of 20 Ag\({}^{-1}\), its specific capacitance remains 82.53% of the initial specific capacitance after  
Figure 1: Schematic representation of (**a**) electrical double-layer capacitor (EDLC), (**b**) pseudocapacictor (PC) and (**c**) hybrid supercapacitor (HSC).  
6000 cycles. When the power density is 412.73 W kg\({}^{-1}\), the energy density of the hybrid supercapacitor can reach 64.58 Wh kg\({}^{-1}\).