The supercapacitor is a new type of energy storage device between the traditional capacitor and rechargeable battery. It not only has the characteristics of fast charge and discharge of the capacitor, but also has the characteristics of storing energy of the battery. The supercapacitor is a new type of component that stores energy through an interface double layer formed between electrode and electrolyte. When the electrode is in contact with the electrolyte, a stable double-layer charge with opposite polarity appears at the solid-liquid interface due to the action of Coulomb force, intermolecular force and interatomic force, and this solid-liquid interface is called an interfacial double layer. A double-layer supercapacitor is regarded as two inactive porous plates suspended in the electrolyte, and and these two plates are loaded with voltage. The potential added to the positive plate attracts the negative ions in the electrolyte, while the potential added to the negative plate attracts positive ions in the electrolyte, thus forming an electric double-layer capacitor on the surfaces of the two electrodes. According to the different electrode materials, electric double-layer capacitors can be divided into carbon electrode double-layer supercapacitors, metal oxide electrode supercapacitors and organic polymer electrode supercapacitors.
Figure 1. Supercapacitor structure.
- Carbon electrode capacitor: Carbon electrode capacitors have long been used in the motor starting system of electric vehicles. Today's research mainly focuses on improving the surface area of carbon materials and controlling the pore size and pore size distribution of carbon materials. Many different types of carbon materials have been developed, such as activated carbon powder, activated carbon fiber, carbon aerogel, carbon nanotube and so on.
- Precious metal oxide electrode capacitors: In the study of precious metal oxide electrode capacitors, precious metal oxides such as RuO2 and IrO2 are mainly used as electrode materials. Because the conductivity of the precious metal oxide electrode is better than that of the carbon electrode, and the precious metal oxide electrode is stable in sulfuric acid, a higher specific energy can be obtained. Therefore, the prepared precious metal oxide electrode capacitor has better performance than the carbon electrode capacitor, and it has a good development prospect. Using RuO2·nH2O amorphous hydrate as the electrode and 5.3mol/L H2SO4 as the electrolyte, the specific capacitance of the capacitor can reach 700 F/g. The specific capacitance of the capacitor prepared with amorphous hydrate MnO2·nH2O as the electrode and 2mol/L KCl aqueous solution as the electrolyte can reach 200 F/g.
Figure 2. SEM image of ruthenium oxide used as the electrode material of supercapacitor
- Conductive polymer electrode capacitors: As a new type of electrochemical capacitor, conductive polymer electrode capacitors have higher performance and superior electrical performance than precious metal supercapacitors. The structure of the corresponding polymer can be selected by design to further improve the performance of the polymer, thereby improving the performance of the capacitor. Conductive polymer electrode capacitors can be divided into three types: (1) symmetrical structure: the two electrodes in the capacitor are the same p-type doped conductive polymer (such as polythiophene) (2) asymmetrical structure: the two electrodes are different P-type doped polymer materials (such as polypyrrole and polythiophene) (3) contain conductive polymers that can be doped with p-type and n-type at the same time. This conductive polymer electrode capacitor can increase the capacitance-voltage and can make full use of the anions and cations in the solution during charging and discharging. Its discharge characteristics are very similar to that of a battery, and it has a good development prospect.
Figure 3. Structure diagram of polythiophene materials commonly used in conductive polymer electrode capacitors
- Wang H X, Zhang W, Drewett N E, et al. (2017). "Unifying miscellaneous performance criteria for a prototype supercapacitor via Co(OH)2 active material and current collector interactions." Journal of Microscopy, 267(1):34-48.
- Zhang Q Z, Zhang D, Miao Z C, et al. (2018). "Research Progress in MnO2-Carbon Based Supercapacitor Electrode Materials." Small, 14(24):e1702883.