Research Status and Development of Supercapacitor Carbon Materials

Supercapacitors are an ideal new energy storage device. In order to develop supercapacitors with excellent performance, from a material point of view, it is crucial to research and develop carbon electrode materials with high specific capacity in different electrolytes for supercapacitor applications. Currently focused on carbon-based materials such as activated carbon, glassy carbon, fiber, gel, high-density graphite, pyrolyzed polymer matrix foam, carbon nanotubes, high-activity mesophase carbon microspheres and honeycombs with nanopores Research on diamonds and other materials, as well as materials such as rare metal oxides and conductive polymers. Among these materials, carbon materials have been valued for their low cost, excellent performance, and have been successfully commercialized. Therefore, this paper intends to elaborate on the development and research of new carbon materials for supercapacitors.

1. Principle of supercapacitors

According to the energy storage mechanism, supercapacitors are generally classified into electric double layer capacitors and Faraday quasi-capacitors. The electric double layer capacitor is based on the theory of electric double layer, and the electrode material is activated carbon with large specific surface area. Faraday quasi-capacitors can be divided into metal oxides and conductive polymers according to different electrode materials. These capacitors mainly use a highly reversible rapid redox reaction occurring on the surface of the active material and the bulk phase to store energy. The electric double layer capacitor is based on the electric double layer theory, and uses the interface electric double layer capacitance formed between the electrode and the electrolyte to store energy. The Faraday quasi-capacitor is based on the Faraday process, which occurs during the electrochemical change of the Faraday charge transfer. Not only occurs on the surface of the electrode, but also deep inside the electrode, so that higher capacitance and energy density than the electric double layer capacitor can be obtained. Recently developed is a hybrid supercapacitor that combines the advantages of both. At present, a new asymmetric supercapacitor has been developed. The two electrode materials of the supercapacitor are different, which can better improve the performance of the supercapacitor.

Regardless of the principle, supercapacitors can be divided into four major parts: electrodes, electrolytes, current collectors, and spacers, as shown in Figure 1. At present, the research hotspots mainly focus on four aspects: carbon electrode materials, metal oxides and their hydrate electrode materials, conductive polymer electrode materials, and hybrid supercapacitors. The electrolyte needs to have high electrical conductivity and sufficient electrochemical stability so that the supercapacitor can operate at the highest possible voltage. The existing electrolyte materials mainly include a solid electrolyte, an organic electrolyte, and an aqueous electrolyte.

Research status and development of supercapacitor carbon materials:

Among all the electrochemical supercapacitor electrode materials, the most mature technology is the carbon material. The research started from the related patents published by Beck in 1957. The carbon electrode material has a large specific surface and low raw materials, which is advantageous for industrial large-scale production, but the specific capacity is relatively low, and the specific surface area of the material needs to be increased to increase the specific capacity. At present, the main research is porous carbon materials with high specific surface area and small internal resistance, (activated) carbon nanotubes, and carbon-containing composite materials modified with carbon-based materials (for example, composite materials such as activated carbon black). .

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 first carbon electrode material used in supercapacitors. The raw materials for preparing activated carbon are abundant, and petroleum, coal, wood, nut shell, resin and the like can be used for preparing activated carbon powder. The raw materials are different and the production process is slightly different. The raw material is prepared and activated by carbonization, and the activation method is physically activated (using CO2 and H20 vapor as an activator) and chemically activated (ZnC12, KOH or the like as an activator). The raw materials and preparation process determine the physical and chemical properties of the activated carbon. Ultra-high specific surface area activated carbon has been used to increase the capacity of double-layer capacitors. Japan has reported the use of petroleum asphalt as raw material to develop ultra-high specific surface area (2500-3000 m2/g) activated carbon for double-layer capacitors, but this material is not ideal. Therefore, a large amount of activated carbon has been developed in consideration of both the pore size distribution and the apparent density, and the mass specific capacity and volume specific capacity have been considered, and the overall performance of the capacitor has been improved.

2.2  Activated carbon fiber

Activated carbon fiber (ACF) is a highly efficient active adsorbent material and environmentally friendly engineering material with superior performance to activated carbon. The preparation of ACF is generally carried out by stabilizing the organic precursor fiber at a low temperature (200 to 400 ° C), followed by carbonization activation (700 to 1000 ° C). The organic fibers used as the ACF precursor mainly include cellulose-based, polyacrylonitrile-based, pitch-based, phenolic-based, polyvinyl alcohol, and the like. Commercialization is mainly the first four. The application of activated carbon fiber in double-layer capacitors has received more and more attention.

Japan Matsushita Electric Co., Ltd. used activated carbon powder as raw material to prepare the electrode of electric double layer capacitor. The model developed later was a phenolic activated carbon fiber with excellent conductivity and average pore diameter of 2 to 5 nm and a specific surface area of 1500-3000 m2/g. The advantage of the fiber is that the mass ratio is high, the conductivity is good, but the apparent density is low. H. Nakagawa developed a high-density activated carbon fiber (HD-ACF) by hot pressing, and made a supercapacitor electrode using this HD-ACF. For a unit capacitor of the same size, the capacitance of a capacitor using HD-ACF as an electrode is remarkably improved.

2.3  Carbon aerogel

Carbon aerogel is a new type of lightweight nanoporous amorphous carbon material by Pekala. PW and so on first discovered. It is characterized by high specific surface area, wide range of density variation, adjustable structure, special properties in electrical, thermal and optical fields, and has wide application prospects, especially its large specific surface area and high electrical conductivity make it a supercapacitor. Ideal electrode material. Carbon aerogels generally use resorcinol and formaldehyde as raw materials, and they undergo polycondensation reaction under sodium carbonate to form resorcinol formaldehyde gel. The solvent in the pores is removed by supercritical drying to form RF gas condensation. The gel, RF aerogel is charred under an inert atmosphere to obtain a carbon aerogel that maintains its network structure. The network structure of the carbon aerogel can be controlled by adjusting the ratio of resorcin to the catalyst and the concentration of the gel. The Lorenz National Laboratory of Lorenz, USA, researched and developed carbon aerogel carbon electrode capacitors with the support of the US Department of Energy. PowerStor has commercialized supercapacitors with carbon aerogels as electrodes.

2.4  Carbon nanotubes

From the perspective of the storage principle of capacitors, carbon nanotubes are ideal electrode materials. First, the carbon nanotubes are hollow tubes with large specific surface area, especially single-walled nanotubes, which is favorable for the formation of electric double layer capacitors. In addition, the carbon formed in the carbon nanotubes is sp hybridized, and is connected to each other by three hybrid bonds, generally forming a six-membered ring, and a hybrid bond is left, and the hybrid bond can be connected to a Faraday reaction. Functional groups (such as hydroxyl, carboxyl, etc.). Therefore, carbon nanotubes can not only form electric double layer capacitors, but also ideal materials that can fully utilize the principle of pseudo capacitor storage. Jiang Qi et al. of Chengdu Institute of Organic Chemistry of the Chinese Academy of Sciences conducted a preliminary study on the performance of electrochemical supercapacitors of carbon nanotubes. It is believed that multi-walled nanotubes with small diameter, short tube length and low degree of graphitization of carbon nanotubes are more suitable for double The electrode material of the electric layer capacitor. At present, the industrial production technology of carbon nanotubes is still immature and the price is very high. Its application on capacitors is also in the research stage, and there is still a long distance from the actual application.

3 . Outlook

Although carbon-based supercapacitors have been successfully commercialized, in order to further improve the performance of the capacitor, there are still many problems with the carbon electrode material, which needs further improvement. At present, porous carbon materials with large mass-to-capacity are relatively easy to obtain, but carbon materials with large volumetric capacity and long-term application stability are difficult to obtain. It is necessary to coordinate and solve the two contradictory problems of capacitance and long-term stability. Performance indicators, also need to develop some new activated carbon raw materials, activation technology. The pore size distribution of carbon materials is a key factor affecting the performance of supercapacitors. The well-controlled pore structure of carbon electrode materials needs further discussion. In addition, the internal resistance of the carbon-based capacitor is relatively large, and it is also necessary to improve the structure of the material itself.