CR927 battery.Research status and development of carbon materials for supercapacitors

　　Research status and development of carbon materials for supercapacitors

　　Supercapacitor is an ideal new energy storage device. In order to develop supercapacitors with excellent performance, from a material perspective, it is crucial to research and develop carbon electrode materials that are suitable for supercapacitor applications and have high specific capacities in different electrolytes. At present, the main focus is on carbon-based materials, such as activated carbon, glassy carbon, fibers, gels, high-density graphite, foam obtained by pyrolyzing polymer matrix, carbon nanotubes, highly active mesophase carbon microspheres and honeycombs with nanopores Shape diamond, etc., and research on materials such as rare metal oxides and conductive polymers [1]. Among these materials, carbon materials have attracted attention due to their low price, easy availability, and excellent performance, and have been successfully commercialized. Therefore, this article intends to elaborate on the development and research of new carbon materials for supercapacitors.

　　1 Principle of supercapacitor

　　According to the energy storage mechanism, supercapacitors are generally divided into electric double layer capacitors and Faraday quasi-capacitors. The electric double layer capacitor is based on the electric double layer theory, and its electrode material is activated carbon with a large specific surface area. Faraday quasi-capacitors can be divided into two types based on different electrode materials: metal oxides and conductive polymers. This type of capacitor mainly uses highly reversible rapid redox reactions that occur on the surface of active materials and bulk interfaces to store energy. Electric double layer capacitors are based on the electric double layer theory and use the interfacial double layer capacitance formed between electrodes and electrolytes to store energy. Faraday quasicapacitors are based on the Faraday process, which is produced during the electrochemical change of Faraday charge transfer. It not only occurs on the surface of the electrode, but also can penetrate deep into the interior of the electrode, so higher capacitance and energy density can be obtained than electric double layer capacitors. Recently, hybrid supercapacitors that combine the advantages of both have been vigorously developed. At present, new asymmetric supercapacitors have been developed [2]. The two electrode materials of this supercapacitor are different, which can better improve the relevant performance of the supercapacitor.

　　No matter what principle it is based on, supercapacitors can be divided into four parts: electrodes, electrolytes, current collectors and separators, as shown in Figure 1. At present, research focuses on four aspects: carbon electrode materials, metal oxides and their hydrate electrode materials, conductive polymer electrode materials, and hybrid supercapacitors. The electrolyte needs to be highly conductive and sufficiently electrochemically stable so that the supercapacitor can operate at the highest possible voltages. Existing electrolyte materials mainly include solid electrolytes, organic electrolytes and aqueous solution electrolytes.

　　Among all electrochemical supercapacitor electrode materials, carbon materials are the earliest and most mature technology, and their research began with the related patent published by Beck in 1957. Carbon electrode materials have a large specific surface and low raw materials, which are conducive to industrial large-scale production. However, the specific capacity is relatively low. It is necessary to increase the specific capacity by increasing the specific surface area of the material. At present, the main research is on porous carbon materials with high specific surface area and small internal resistance, (activated) carbon nanotubes, and carbon-containing composite materials that modify carbon-based materials (such as activated carbon black and other composite materials) .

　　2 Development of carbon electrode materials

　　2.1 Activated carbon powder

　　Activated carbon has a long history of industrial production and application. It is also the earliest carbon electrode material used in supercapacitors. There are rich sources of raw materials for preparing activated carbon. Petroleum, coal, wood, nut shells, resins, etc. can all be used to prepare activated carbon powder. The raw materials are different and the production process is also slightly different. The raw materials are carbonized and activated after being prepared. The activation methods are divided into physical activation (using CO2 and H20 steam as activators) and chemical activation (ZnC12, KOH, etc. are used as activators). The raw materials and preparation process determine the physical and chemical properties of activated carbon. Ultra-high specific surface area activated carbon began to be used to increase the capacity of double-layer capacitors. Japan has reported that ultra-high specific surface area (2500~3000m2/g) activated carbon was developed using petroleum asphalt as raw material for use as double-layer capacitors, but this material is not ideal. Therefore, many activated carbons have been developed taking into account the pore size distribution, apparent density and other properties. At the same time, the mass specific capacity and volume specific capacity have been taken into consideration to improve the overall performance of the capacitor.

　　2.2 Activated carbon fiber

　　Activated carbon fiber (ACF) is an 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 low temperature (200-400°C), followed by carbonization activation (700-1000°C). Organic fibers used as ACF precursors mainly include cellulose-based, polyacrylonitrile-based, asphalt-based, phenolic-based, polyvinyl alcohol, etc. The first four types are mainly commercialized. The application of activated carbon fibers in double-layer capacitors has attracted more and more attention.

　　In the early days, Panasonic Electric Company of Japan used activated carbon powder as raw material to prepare electrodes for electric double layer capacitors. Later developed models used phenolic activated carbon fibers with excellent conductivity, average pore diameter of 2 to 5nm, and a specific surface area of 1500 to 3000m2/g. Activated carbon The advantages of fiber are high mass specific capacity and good electrical conductivity, but low apparent density. H. Nakagawa developed high-density activated carbon fiber (HD-ACF) using a hot pressing method, and used this HD-ACF to make supercapacitor electrodes. For unit capacitors of the same size, the capacitance of capacitors using HD-ACF as electrodes is significantly improved.

　　2.3 Carbon aerogel

　　Carbon aerogel is a new type of lightweight nanoporous amorphous carbon material developed by Pekala. PW et al. first discovered [3]. It is characterized by high specific surface area, wide range of density changes, adjustable structure, special properties in electrical, thermal, optical and other aspects, and has broad application prospects. In particular, its large specific surface area and high conductivity make it a supercapacitor. Ideal electrode material. Carbon aerogels generally use resorcin and formaldehyde as raw materials. The two undergo a polycondensation reaction under the catalysis of sodium carbonate to form resorcinol-formaldehyde gel. The solvent in the pores is removed by supercritical drying to form RF air condensation. Glue, RF aerogel is carbonized in an inert atmosphere to obtain a carbon aerogel that maintains its network structure. By adjusting the ratio of resorcinol to catalyst and the concentration of the gel, the network structure of the carbon aerogel can be controlled. The Lorentz Livermore National Laboratory in the United States, with the support of the U.S. Department of Energy, researched and developed carbon airgel carbon electrode capacitors. PowerStor has commercialized supercapacitors with carbon aerogel electrodes.

　　2.4 Carbon nanotubes

　　From the perspective of the electricity storage principle of capacitors, carbon nanotubes are ideal electrode materials. First of all, carbon nanotubes are hollow tubes with a large specific surface area, especially single-wall nanotubes, which are conducive to the formation of electric double layer capacitance. In addition, the carbon in the carbon nanotube is sp hybridized, and three hybrid bonds are used to form a ring. Generally, a six-membered ring is formed, and there is one hybrid bond left. This hybrid bond can be connected to a faradaic reaction. Functional groups (such as hydroxyl, carboxyl, etc.). Therefore, carbon nanotubes can not only form an electric double layer capacitor, but are also an ideal material that can fully utilize the principle of pseudo-capacitance energy storage.

　　3 Outlook

　　Although carbon-based supercapacitors have been successfully commercialized, there are still many problems in carbon electrode materials that need to be further improved in order to further improve the performance of the capacitor. At present, porous carbon materials with large mass specific capacity are relatively easy to obtain, but carbon materials with large volume specific capacity and good long-term application stability are difficult to obtain. In order to coordinately solve the two contradictory issues of electric capacity and long-term application stability, In terms of performance indicators, some new activated carbon raw materials and activation technologies need to be developed. The pore size distribution of carbon materials is a key factor affecting the performance of supercapacitors. Better control of the pore structure of carbon electrode materials requires further exploration. In addition, the internal resistance of carbon-based capacitors is relatively large, and the structure of the material itself also needs to be improved.

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