Supercapacitors have both large capacity and high power density, so they have extremely wide application prospects in backup power supply, alternative power supply, high-power output, etc. The performance of supercapacitors mainly depends on electrode materials. In recent years, scholars from various countries have conducted a lot of research on electrode materials of supercapacitors.

　　The types of supercapacitors can be divided into electric double-layer capacitors, Faraday quasi-capacitors (also known as pseudo-capacitance capacitors) and hybrid capacitors based on their working principles. Electric double-layer capacitors are based on the electric double-layer theory and use the electric double-layer capacitance at the interface formed between the electrode and the electrolyte to store energy. The Faraday quasi-capacitor is based on the Faraday process, which is generated during the electrochemical change process of Faraday charge transfer, which not only occurs on the surface of the electrode, but also can penetrate deep into the electrode. According to these two principles, the current electrode materials of supercapacitors are mainly divided into three categories [1]: carbon materials, metal oxide and hydrate materials, and conductive polymer materials.

　　2Carbon material-based electrode material

　　Among all electrochemical supercapacitor electrode materials, the earliest research and the most mature technology is carbon material. The research started from related patents published by Beck in 1957. The research of carbon electrodes mainly focuses on the preparation of porous electrode materials with large specific surface area and small internal resistance. The carbon materials that can be used as supercapacitor electrodes mainly include: activated carbon, nano-carbon fiber, glassy carbon, carbon aerogel, nano-carbon Tube and so on.

　　Activated carbon (AC) is the earliest carbon electrode material used in supercapacitors [2]. It is mainly carbon, combined with hydrogen, oxygen, nitrogen, etc., has a good adsorption effect. Its characteristic is that its specific surface area is particularly large, and its specific capacity is more than five times higher than platinum black and palladium black [3]. J. Gamby [4] tested several activated carbon supercapacitors with different specific surface areas. Among them, the sample with the largest specific surface area of 2315m2˙g had the highest specific capacity, reaching 125F/g. At the same time, it was found that the specific surface area and pore structure of activated carbon have the highest specific capacity. The specific capacity and internal resistance of the electrode have a great influence.

　　Activated Carbon Fiber (ACF) is a highly efficient active adsorption material and environmentally friendly engineering material with better performance than activated carbon. The preparation of ACF generally involves stabilizing the organic precursor fiber at a low temperature (200°C~400)°C, followed by carbonization and activation (700°C~1000)°C. Japan’s Matsushita Electric Company used activated carbon powder as a raw material to prepare electrodes for electric double-layer capacitors in the early days. 1500~3000m2/g phenolic activated carbon fiber [5], the advantages of activated carbon fiber are high mass specific capacity, good conductivity, but low apparent density. H.Nakagawa developed high-density activated carbon fiber (HD-ACF) [6] by hot pressing, with a density of 0.2~0.8g/m3 without any adhesive. The electronic conductivity of this material is much higher than that of activated carbon powder electrodes, and the capacitance value increases with the increase of the density of activated carbon fibers. It is a promising electrode material. Using this HD-ACF to make supercapacitor electrodes [7], the results show that for unit capacitors of the same size, the capacitance of the capacitor using HD-ACF as the electrode is significantly improved.

　　Carbon aerogel is a new type of lightweight nanoporous amorphous carbon material. Its porosity is as high as 80%~98%, the typical pore size is <50nm, the network colloid particle size is 3~20nm, and the specific surface area is as high as 60~1000m2/ g, with a density of 0.05~0.80g/m3, it is a new type of material with many excellent properties (such as conductivity, optical conductivity and mechanical properties) and broad application prospects [8]. Meng Qinghan, Liu Ling [9] and others used low-molecular phenolic resin-furfural as the raw material to successfully synthesize carbon aerogels through a solution-sol-gel route. The results show that the carbon aerogel electrode is charged and discharged at 0.5 mA. When the electrode has a specific capacitance of 121F/g and a charge-discharge efficiency of 95%, it has excellent performance such as stable performance and high charge-discharge efficiency.

　　With the first discovery of carbon nanotubes in 1991, due to their unique structural properties, they have widely attracted the attention of people from all walks of life. Carbon nanotubes (CNTs) as electrode materials for supercapacitors have their advantages: high crystallinity, good conductivity, large specific surface area, and micropores are concentrated in a certain range (and the micropore size is controllable), theoretically It should be an ideal material for supercapacitors. Niu et al. [10] used carbon nanotubes with an average diameter of about 8nm to make sheet electrodes, which have an open-pore structure that is difficult to obtain with activated carbon and other carbon fibers. When 38% sulfuric acid is used as the electrolyte, the specific capacity at 1Hz frequency is 102F/g, the specific capacity at 100Hz frequency is 49F/g, and the specific power exceeds 8000W/kg.

　　There is still a lot of work to be done on the use of carbon nanotubes as electrode materials for electrochemical supercapacitors, such as the degree of graphitization of carbon nanotubes, the size of carbon nanotube diameters, the length of carbon nanotubes, and the degree of bending of carbon nanotubes. And the difference in the groups connected to the carbon nanotubes caused by different treatment methods will have a great impact on the performance of the electrochemical supercapacitor composed of it.

　　3 metal oxide and hydrate material

　　Introduction to common metal oxide and hydrate materials Some metal oxides and hydrates are very good materials for supercapacitor electrodes. The Faraday quasi-capacitance produced by the metal oxide electrode in the supercapacitor is much larger than the electric double layer capacitance on the surface of the carbon material electrode. Because a fast and reversible electrode reaction occurs on the metal oxide electrode, and the electrode reaction can penetrate deep into the electrode, the energy is stored in a three-dimensional space, which increases the energy density [11].　　 There are many research reports on Ru oxides and hydrates as electrode materials for supercapacitors, and their performance is relatively good. However, Ru is a noble metal and has a high cost, which is not conducive to large-scale industrial production. Therefore, people began to look for other cheap metal materials to replace Ru. Now, the research of manganese oxide electrode materials for supercapacitors has made great progress. Manganese dioxide with high surface area is a new type of electrode material with low price and good performance. Sol-gel method and electrochemical deposition method were used to prepare MnO2. Through comparison, it was found that the specific capacitance of MnO2 prepared by sol-gel method was 1/3 higher than that of MnO2 prepared by deposition method, reaching 698F/g, and After 1500 cycles, the capacity attenuation is less than 10% [12].

　　In addition to manganese oxide, nickel oxide and cobalt oxide materials are also very promising electrode materials for supercapacitors. Some researchers have also experimented with other metal oxides as supercapacitor materials. If the porous V2O5 hydrate is used as the active material of the electrode, the specific capacity can reach 350F/g.

　　The current research focus of metal composite electrode materials is to find a suitable metal or oxide to replace Ru, reduce the amount of Ru, reduce costs, and increase the specific capacitance of the electrode material. Cheng Jie et al. [13] used ultra-thin sintered composite nickel-cobalt electrode (Co:Ni about 1:4, thickness 0.31mm) as the positive electrode, activated carbon electrode with a specific capacitance of 250F/g as the negative electrode, and 7mol/LKOH solution as the negative electrode. The supercapacitor assembled by the electrolyte has high constant current charge and discharge efficiency, good rate performance, low self-discharge, specific energy up to 16Wh/kg, and maximum specific power up to 10kW/kg (based on the sum of the masses of the positive and negative electrodes ).

　　Zhang Baohong et al. [14] added PbO to MnO2 to inhibit the generation and accumulation of electrochemically inert material Mn3O4, thereby improving the performance of the electrode. From the 2000 cycle performance, when the current density is 50mA/cm, the MnO2 electrode added with -PbO still has good cycle performance, and the capacity decay is less than 10%.

　　There are also researchers [15] electrochemically depositing porous nanostructured SnO2 on a low-priced stainless steel electrode to prepare a composite electrode, using a 0.1M Na2SO4 solution as the electrolyte to form a Faraday quasi-supercapacitor that undergoes a redox reaction. . Then it was characterized by cyclic voltammetry, and the maximum characteristic impedance measured was 285F/g. Moreover, as the quality of SnO2 increases, the characteristic impedance also shows an increasing trend.

　　4 Conductive polymer electrode material

　　Because conductive polymer materials lack effective long-range order, the movement of their internal free charges is restricted, so most conductive polymers have poor conductivity, but when conductive polymer materials are in a doped state, their conductivity is significant Increase and increase electrical activity. Although the electrical conductivity of the polymer material has some restrictions on its application, when it is applied to the electrode material of supercapacitors, because the surface and inside of the material are distributed with a large number of micropores that can be filled with electrolyte, and can form a network of three-dimensional Structure, the migration of electrons and ions in the electrode can be completed by the exchange with ions in the electrolyte, so the conductive polymer as the electrode material of the supercapacitor does not need high conductivity.

　　A. Rudge et al. used poly(3-(4-fluorophenyl)thiophene) which can be doped and de-doped with p-type and n-type in 1mol/L tetramethylammonium trifluorosulfonate electrolyte in acetonitrile. (PF-PT) as the electrode material, a supercapacitor with a fully charged voltage of 3V, an energy density of 39W˙h/kg, and a power density of 35kw/kg was obtained.

　　Kat-suhikoNaoi et al. studied the application of poly(1,5-diaminoanthraquinone) in supercapacitors [16]. The electrochemically synthesized poly(1,5-diaminoanthraquinone) can form a type III supercapacitor in an electrolyte of 1.0mol/L tetraethylammonium perchlorate/propylene carbonate. The energy density of the capacitor is 25~46w/ h˙kg, the power density is 10.2~30.5kw/kg.　　5 Conclusion 　　 Since various single electrode materials have their own shortcomings, composite electrode materials will be used as a new type of supercapacitor electrode material. The supercapacitor composite electrode materials currently being developed include: C/RuO2, C/lrO2, C/V2O5, Co3O4/RuO2, SnO2/RuO2, C/PANI, etc. These materials can achieve a reasonable balance of material performance and cost, and It has excellent performance that a single electrode material does not have, and its application prospects are very broad.