From the 1870s to the present, the development of super capacitors has gone through many important processes: In the late 1950s, some scientists proposed replacing double-layer electrochemical capacitors made of metal sheets with capacitors made of porous carbon materials, and It has been proven by practice. In other words, electrochemical capacitors have made rapid progress at this time. The world’s first commercial super capacitor came out in 1971, which marked that super capacitors have begun to enter the market operation stage; in the 1980s, In the 1990s, due to the introduction of pseudocapacitive electrode materials, the energy density of super capacitors has been greatly improved, reaching a farad level that has never been reached before. Only then did the so-called electrochemical capacitors be called true super capacitors. Name; In the 1990s, the development prospects of super capacitors were valued by Western developed countries, and they have proposed major projects related to it.

In 1879, Helmholz discovered the properties of double-layer capacitance and proposed the concept of electric double layer. However, the use of double-layer super capacitors for energy storage has only been in recent decades. In 1957, Becker (General Electric Co.. GE) proposed using capacitors with a specific capacity close to the battery as energy storage components. In 1968, Sohio (The Standard Oil Company) used high specific surface area carbon materials to produce electric double layer capacitors. In 1978, Japan’s Osaka Company produced gold capacitors, which were the earliest commercialized and mass-produced carbon electric double layer capacitors. In 1979, Nippon Electric Company, Limited began producing supercapacitors and used them in starting systems for electric vehicles. In 1980, Japan’s Panasonic Corporation studied super capacitors using activated carbon as electrode material and organic solution as electrolyte. After this, super capacitors began to be industrialized on a large scale.

1. Advantages of super capacitors

(1) High capacitance: The capacity of supercapacitors can reach up to several thousand farads, which is thousands of times higher than the capacity of platinum electrolytic capacitors and aluminum electrolytic capacitors of the same volume.

(2) Long cycle life: The charging and discharging process of supercapacitor is divided into two types according to its energy storage mechanism: one is the physical process of electric double layer, that is, there is only the transfer of ions or charges during the charging and discharging process, and no chemical or electrochemical reaction occurs. Another situation that triggers electrode phase change is the electrochemical reaction process. This reaction process has good reversibility and is not prone to phenomena such as crystalline transformation and shedding of active materials that affect the service life. All in all, no matter which of the above processes occurs, the capacitance of the supercapacitor decreases very little, and the number of cycles can reach tens of thousands of times, which is 5 to 20 times the number of cycles of the battery.

(3) Short charging time: Supercapacitors use high current to charge and can be quickly charged in a few seconds to minutes, while batteries require dozens of minutes to charge quickly, and frequent rapid charging will also affect the service life.

(4) High power density and high energy density: While supercapacitors provide a power density of 1000~2000W/kg, they can also output an energy density of 1~10Wh/kg. For this reason, supercapacitors are suitable for applications where short-term high power output is required. The mixed use of supercapacitors and battery systems can form a system with both high power density and high energy density.
energy storage system.

(5) Wide operating temperature range: The operating temperature range of supercapacitors is -40~70°C, while the operating temperature range of general batteries is between -10~50C. (6) Reliable operation, maintenance-free and environmentally friendly: Supercapacitors have a certain ability to resist overcharge and will not have much impact on their work in a short period of time, ensuring the reliability of system operation.

2. Disadvantages of super capacitors

(1) Low monomer working voltage: The working voltage of aqueous electrolyte supercapacitor monomer is generally 0~1.0V. The high output voltage of supercapacitors is achieved by connecting multiple single capacitors in series, and the series capacitors are required to have good consistency. The working voltage of a non-aqueous electrolyte supercapacitor monomer can reach 3.5V, but in actual use the maximum is only 3.0V. At the same time, the purity of non-aqueous electrolytes is high, and it needs to be produced in an assembly environment such as anhydrous and vacuum conditions.

(2) Possible leakage: Although the materials used in supercapacitors are safe and harmless, if the installation location is unreasonable, electrolyte leakage may still occur, affecting the normal performance of the supercapacitor.

(3) Supercapacitors are generally used under DC conditions and are not suitable for use in AC situations.

(4) Higher price: The cost of supercapacitors is much higher than that of ordinary capacitors

Supercapacitors are a new type of energy storage device between traditional capacitors and batteries, with capacities ranging from hundreds to thousands of farads. Compared with traditional capacitors, it has larger capacity, higher energy, wider operating temperature range and extremely long service life; compared with batteries, it has higher power density and no pollution to the environment. . Therefore, supercapacitor is an efficient, practical and environmentally friendly energy storage device. The performance comparison of several energy storage devices is shown in Table 1-2.

Table 1-2 Performance comparison of several energy storage devices

Currently, the indicators used to describe the performance of supercapacitors are:

1) Rated capacity: refers to charging to the rated voltage according to the specified constant current (for example, the charging current specified for super capacitors above 1000F is 100A, and those below 200F is 3A) for 2~3 minutes, and then discharged under the specified constant current discharge conditions. The product of the time until the terminal voltage is zero and the current is divided by the rated voltage value, in farads (F).

2) Rated voltage: the highest safe terminal voltage that can be used. Breakdown voltage, its value is much higher than the rated voltage, about 1.5 to 3 times the rated voltage, the unit is volts (V) 3) Rated current: refers to the current discharged to half of the rated voltage within 5 seconds, the unit is Ampere A)

4) Maximum stored energy: refers to the energy released when discharging to zero at rated voltage, the unit is Joule (J) or Watt-hour (Wh).

5) Energy density: also called specific energy. Refers to the energy given by the capacitor per unit mass or unit volume, in Wh/kg or w·h/L.

6) Power density: also called specific power. It refers to the discharge power of a super capacitor per unit mass or unit volume when it produces half and half electrical/heating effects under matching loads. It represents the ability of the super capacitor to withstand current, and the unit is kw/kg or kW/L.

7) Equivalent Series Resistance (ESR): Its value is related to supercapacitor electrolyte and electrode materials, preparation process and other factors. Generally, AC ESR is smaller than DC ESR and decreases as the temperature rises. Units are ohms (Q).

8) Leakage current: refers to the static loss caused by the internal equivalent parallel impedance when the supercapacitor maintains a static energy storage state. It is usually the current measured after applying the rated voltage for 72 hours, and the unit is Ampere (A).

9) Service life: refers to the length of time when the capacitance of the super capacitor is less than 20% of the rated capacity or the ESR increases to 1.5 times the rated value.

10) Cycle life: A super capacitor experiences one charge and discharge, which is called one cycle or one cycle. The cycle life of supercapacitors is very long, up to more than 100,000 times.

In the development of supercapacitors, the current focus is on liquid electrolyte electric double layer capacitors and composite electrode material/conductive polymer electrochemical super capacitors. The development of foreign supercapacitors is shown in Table 1-3.

Table 1-3 Development of supercapacitors abroad

In terms of the industrialization of super capacitors, the earliest products were products from NEC and TOKIN in 1980 and Panasonic and Mitsubishi in 1987. The nominal voltage of these capacitors is 2.3~6V, with an annual output of millions. In the 1990s, Russian ECOND Company and ELIT produced SC brand electrochemical capacitors with nominal voltages from 12 to 450V and capacitances from 1F to several hundred F, which are suitable for occasions requiring high-power starting power. In general, products from the United States, Japan, and Russia currently occupy almost the entire super capacitor market, and industrialized super capacitors are basically electric double layer capacitors. Some performance parameters of some electric double layer supercapacitor products are listed in Table 1-4.

Table 1-4 Some performance parameters of electric double layer super capacitor products

In my country, Beijing Nonferrous Metal Research Institute, Jinzhou Power Capacitor Co., Ltd., University of Science and Technology Beijing, Beijing University of Chemical Technology, Beijing Institute of Technology, Beijing Jinzhengping Technology Co., Ltd., Army Chemical Defense College, Harbin Jurong New Energy Co., Ltd., Shanghai Ao Wei Technology Development Co., Ltd. and others are conducting research on super capacitors. In 2005, the 863 project “Research on Key Technologies of Supercapacitor Energy Storage Systems for Renewable Energy Power Generation” undertaken by the Institute of Electrical Engineering of the Chinese Academy of Sciences passed expert acceptance. This project completed the research and development of a 300W·h/kW super capacitor energy storage system for photovoltaic power generation systems. In addition, relevant research groups such as North China Electric Power University are studying the application of supercapacitor energy storage system (Super capacitor Energy Storge System, SESS) to the distribution network of distributed power generation systems. But overall, my country’s research and application level in the field of supercapacitors clearly lags behind the world’s advanced level.

When using supercapacitors, you should pay attention to the following issues: 1. Supercapacitors have fixed polarity, and the polarity should be confirmed before use; 2. Super capacitors should be used at nominal voltage because when the capacitor voltage exceeds the nominal voltage It will cause the electrolyte to decompose, and the capacitor will heat up, reduce the capacity, and increase the internal resistance, shortening its life; 3. Due to the existence of ESR, supercapacitors cannot be used in high-frequency charging and discharging circuits: when using supercapacitors in series When there is a voltage balance problem between cells, simple series connection will cause one or several cell capacitors to be damaged due to overvoltage, thus affecting their overall performance.

With the development of power systems, distributed power generation technology has attracted more and more attention. As a necessary energy buffer link in distributed power generation systems, energy storage systems play an increasingly important role. The supercapacitor energy storage system uses multiple groups of supercapacitors to store energy in the form of electric field energy. When energy is urgently lacking or needed, the stored energy is released through the control unit to accurately and quickly compensate for the active and reactive power required by the system. work, thereby achieving balance and stable control of electrical energy. In 2005, a 450kW supercapacitor energy storage device was built in California, USA, to reduce fluctuations in the power delivered by a 950kW wind turbine to the grid. In addition, energy storage systems can also play an important role in improving the power quality of power system distribution networks. Through the inverter control unit, the reactive power and active power provided by the supercapacitor energy storage system to users and the network can be adjusted to achieve the purpose of improving power quality.

In the 35kV substations and 10kV switch stations built in my country from the 1960s to the 1980s, the operating mechanisms of most high-voltage switches (circuit breakers) are electromagnetic operating mechanisms. The corresponding DC system is equipped in the power distribution room of the substation or distribution station, which is used as the DC power supply for opening and closing operation, control and protection. These DC power equipment are mainly capacitor energy storage silicon rectifier opening and closing devices and DC panels partially composed of batteries. Capacitor energy storage silicon rectifier opening and closing devices were widely used at that time due to their simple structure, low cost, and low maintenance. However, there was a fatal flaw in actual use: poor reliability of accident opening and closing. The reason is that the capacity of electrolytic capacitors for energy storage is limited and the leakage current is large. Although DC panels composed of batteries can store a large amount of electrical energy and have become necessary devices in some important transformation and distribution stations, due to their extremely high operating costs and short service life, these devices can only be used at the 110kV level. It is difficult to promote the use of substations.

Supercapacitors make it possible to solve the above problems with their ultra-long service life, frequent and fast charge and discharge characteristics, and cheap price. If two 0.85F, 240/280V supercapacitors are connected in parallel, they can completely replace the bulky, frequent maintenance, and polluting battery pack. Since the energy consumption of one closing is only equivalent to 3% of the energy stored in the supercapacitor (70kJ), and this energy can be quickly replenished in the floating charge circuit, it is fully adaptable to continuous and frequent operations, and has extremely high High reliability.

Although many users choose Uninterrupted Power Supply (UPS) as a rescue device for equipment power supply when the grid is out of power or the grid voltage drops instantaneously, UPS is overqualified for instantaneous voltage drops. UPS is powered by batteries and has a long working time. However, due to the shortcomings of the battery itself (requires regular maintenance, short life), the UPS needs to always pay attention to the status of the battery during operation. The duration of power system voltage drops is often very short (10ms ~ 60s), so the advantage of using supercapacitors in this case is obvious over UPS: its output current can rise to hundreds of amps with almost no delay, and the charging speed is very fast It is fast and can store energy within minutes to facilitate the next power failure. Therefore, although the energy storage of the supercapacitor can be maintained for a short time, when it is used for about 1 minute, it has incomparable advantages – 500,000 cycles, no care required, and economical. In Singapore, a dynamic voltage recovery device (DVR) produced by ABB that uses supercapacitor energy storage is installed in a 4MW semiconductor factory to achieve 160ms fault ride-through.

The static synchronous compensator (STATCOM) is one of the main devices of Flexible AC Transmission Technology (FACTS) and represents the new development direction of reactive power compensation technology in the power system at this stage. It can quickly and continuously provide capacitive and inductive reactive power, achieve appropriate voltage and reactive power control, and ensure stable, efficient and high-quality operation of the power system. STATCOM based on double-layer capacitor energy storage can be used to improve the voltage quality of distributed power generation systems. It will gradually replace traditional superconducting energy storage in distributed power generation systems with power levels of 300~500kW. In terms of economics, the cost of an electric double layer capacitor energy storage device of the same capacity is almost the same as that of a superconducting energy storage device, but the former requires almost no operating costs, while the latter requires considerable cooling costs.
For supercapacitors, the direction and focus of future research is: utilizing the high specific power characteristics and rapid discharge characteristics of super capacitors to further optimize the application of supercapacitors in power systems. In addition, under the guidance of my country’s policy of vigorously developing new energy, in the fields of photovoltaic power generation and wind power generation, super capacitors provide favorable conditions for the improvement and development of key equipment with their fast charging and fast discharging characteristics.

1.2 Classification of super capacitors

As a new green energy storage component, supercapacitor has broad application prospects in electric vehicles, distributed power generation systems and other fields. Supercapacitors can be divided into three categories: electric double layer supercapacitors, pseudocapacitors and hybrid supercapacitors according to different energy storage principles.

1. Electric double layer super capacitors

Electric double layer supercapacitors use the interface double layer formed between electrodes and electrolytes to store energy. Electric double layer supercapacitors have made changes in manufacturing materials, such as: activated carbon electrode materials, which are processed with high specific surface area activated carbon materials to make electrodes, carbon airgel electrode materials, which are combined with precursor materials to prepare gels, and then carbonized Activation treatment serves as an electrode. When the electrode and the electrolyte come into contact, due to the action of Coulomb force, intermolecular force or interatomic force, a stable double layer of charges with opposite signs appears at the solid and liquid interface, which is called the interface double electric layer.

2. Pseudocapacitance

Pseudocapacitance, also known as Faraday quasicapacitance in general, refers to the capacitance on the electrode surface or body.
On the two-dimensional or quasi-two-dimensional space in the phase, the active material undergoes under-potential deposition and highly reversible chemical adsorption/desorption or oxidation/reduction reactions occur, thereby generating Faradaic capacitance. Pseudocapacitive super capacitors generally use metal oxide electrode materials and polymer electrode materials. They can be divided into adsorption eagle capacitors and redox eagle capacitors.

3. Hybrid super capacitors

Hybrid super capacitors can be divided into the following 3 types (based on different types of electrode materials):
1) It consists of an electrode with both electric double layer capacitance characteristics and an electrode with pseudocapacitance characteristics, or is composed of two different types of pseudocapacitance electrode materials.

2) Composed of super capacitor electrodes and battery electrodes

3) It consists of the anode of the electrolytic capacitor and the cathode of the super capacitor. The two poles of hybrid supercapacitors generally use “battery-type” materials with high energy density as active materials and “capacitor-type” materials with high power density, so they have the advantages of both.

1.3 Application prospects of super capacitors

Supercapacitors are also called electrochemical capacitors. They have stable performance, specific capacity is 20 to 200 times that of traditional capacitors, and specific power is generally greater than 1000W/kg. The cycle life and storable energy are much higher than traditional capacitors, and they charge quickly. Due to their extremely long service life, they can be used throughout the entire life cycle of the end product. When high-energy batteries and fuel cells are combined with supercapacitor technology, high power density, high energy density characteristics and long operating life can be achieved.

In recent years, high-power supercapacitors have shown a rising industry trend in the fields of electric vehicles, solar energy devices, heavy machinery and other fields. Many developed countries have regarded supercapacitor projects as national key research and development projects. The domestic and foreign markets of supercapacitors are showing a trend. Unprecedented prosperity.

The application of supercapacitors has become increasingly mature and has been widely used in fields such as industry, communications, medical equipment, military equipment, and transportation [4]. From small-capacity emergency energy storage to large-scale power energy storage, from independent energy storage to hybrid energy storage systems with batteries or fuel cells, supercapacitors have shown unique advantages. To sum up, the application directions of supercapacitors can be divided into the following four fields:

1. Main power supply, replacement power supply or backup power supply for low-power electronic equipment

1) Main power supply: Supercapacitors are suitable for use in main power supplies. Typical applications include electric toys, which have the advantages of being small in size, light in weight, high in power density and able to start quickly as the main power supply.

2) Replacement power supply: Supercapacitors are also suitable for use in replacement power supplies. Typical applications include road signs, solar watches, traffic lights and bus stop timetable lights.

3) Backup power supply: Supercapacitors are widely used in backup power supplies. Typical applications include vehicle meters, vehicle fare meters, radio wave receivers and cameras, etc.

2. Hybrid electric vehicles and electric vehicles

The lifespan of supercapacitors is hundreds of times that of electrochemical batteries (storage batteries and potassium-ion batteries, etc.) and does not require maintenance. Therefore, the total cost of supercapacitors applied to electric vehicles is much lower than that of general electrochemical batteries. Currently, countries all over the world are investing the most in developing electric vehicles, of which hybrid electric vehicles (Hybrid Electric Vehicles) are the ones that invest the most. Hybrid electric vehicles use batteries to provide normal operating power for electric vehicles. Supercapacitors are used to supplement power when accelerating and climbing, and ultracapacitors are used to store regenerative energy generated during braking. Therefore, electric vehicles using supercapacitors have the advantages of fast starting, fast acceleration and strong climbing ability.

3.Renewable energy power generation systems and distributed power systems

Supercapacitors can give full play to the advantages of high energy storage density, high power density, long cycle life and no need for maintenance. They can store energy alone or be mixed with other energy storage systems to store energy. Supercapacitors can be combined with solar cells and used in street lights, traffic warning signs, traffic lights, etc. They can also be used in distributed power generation systems, such as wind power stations, hydropower stations, etc. Energy storage through supercapacitors can improve the system. It plays the role of instant power compensation to improve the stability and reliability of the power supply system. This power supply method can well compensate for the unstable and unpredictable output power of power generation equipment.

4. Energy buffer

The energy buffer consists of a supercapacitor and a power converter. It is mainly used in variable frequency drive systems such as elevators. When the elevator accelerates up, the energy buffer supplies power to the DC bus in the drive system to provide the peak power required by the motor. When the elevator decelerates and descends, the energy buffer absorbs the energy fed back by the motor.

Supercapacitors can replace batteries in portable instruments such as driving micromotors, relays, and solenoid valves. It can avoid misoperation due to instantaneous load changes. Supercapacitors can also be used to power camera flashes, allowing the flash to achieve continuous use performance, thereby improving the camera’s ability to continuously shoot. It is applied to camera phones, allowing camera phones to use high-power LEDs. Supercapacitor technology can also be used in mobile wireless communications equipment. These devices often use pulses to maintain communication. Because supercapacitors have strong instantaneous charge and discharge capabilities and can provide high power, they have a wide range of applications in this field. Supercapacitors are almost irreplaceable components in important power systems of many large petrochemical, electronic, textile and other enterprises, especially in transient voltage and current stabilization of high-power systems. In addition, it is also very important for chip companies to consider power fluctuations when selecting sites, and supercapacitor systems can completely solve this problem.

Supercapacitors also play an irreplaceable role in short-term UPS systems, electromagnetic operating mechanism power supplies, solar power car anti-theft, car audio and other systems. In wind power or solar power systems, due to the instability of wind and solar energy, the battery will be repeatedly charged frequently, resulting in shortened life. Supercapacitors can absorb or supplement the fluctuations in electrical energy to solve this problem. Supercapacitors also have huge application value and market potential in electric vehicles, hybrid fuel vehicles and special load-carrying vehicles. As a power source for electric and hybrid vehicles, supercapacitors can be used alone or in combination with batteries. In this way, when the supercapacitor is used as a short-term drive power supply for electric vehicles, it can quickly provide a large current when the vehicle starts and climbs a hill to obtain power to provide powerful power; it can be quickly charged by the battery during normal driving; and it can be quickly charged by the battery during braking. Quickly store the large warm-time current generated by the generator, thereby reducing the restrictions on high-current discharge of the battery by electric vehicles, extending the cycle life of the battery, and improving the practicality of electric vehicles. The application of supercapacitors in the electric moped market is also expanding. The battery on an electric moped has strict charging and discharging current requirements, making it difficult to recover energy instantaneously. Supercapacitors can easily meet these requirements. The supercapacitor supplements the energy of the system when the electric moped is starting, accelerating and climbing, and completely recovers energy during braking to improve system performance.

As one of the key new energy storage products developed in the 21st century, super capacitors are being developed and produced by more and more countries and companies, and their rapid progress is obvious to all. At the 1st International Annual Conference on Electric Double Layer Capacitors and Hybrid Energy Storage held in 1991, the large single capacitor was a capacitor with a capacity of 470F designed and developed by Panasonic, and its voltage was 2.3V. Today, the capacity of single capacitors of the same size produced by Panasonic has exceeded 2000F. At the same time, not only Panasonic, but many companies around the world have begun to enter this field. These companies are mainly engaged in the development of large-scale manufacturing technology and marketing so that capacitor products can be used with portable electronic equipment and pulse power appliances on the market. It can be said that today’s super capacitor market has entered an era of competition: Maxwell’s company in San Diego is the leading manufacturer of large-scale electrochemical capacitors in the United States; PowerStor was developed from the carbon aerogel technology of Lawrence Livermore’s laboratory , and is now quite large-scale; South Korea’s Ness Company has been interested in small energy storage devices since the beginning. Its products have spread throughout the market, from small ones to the largest ones, and have now developed into a company. The company that leads the way in the pulse power performance of electrochemical capacitors; the products of Germany’s Siemens Matsushita also greatly surpass all its previous products. It is a subsidiary of Maxwell and later became EPCOS; recently, as one of the important members of the world’s electrolytic capacitor industry One Japanese chemical company has now officially joined the super capacitor industry-Power System Company founded by Mr. Okamura now has a production line of large-scale products; certain products of Russia’s ECOND Company, ELIT Company and ESMA Company It is also a force that cannot be underestimated in the super capacitor team. Among them, Russia’s ESMA company is the representative of the production of inorganic hybrid super capacitors. In recent years, some companies in our country have also begun to actively get involved in this industry, and have already possessed certain technical strength and industrialization capabilities. Important companies include Jinzhou Fu Company, Beijing Jixing Company, Beijing Hezhong Huineng, Shanghai Aowei Company, Jinzhou Jinrong Company, Shijiazhuang Gaoda Company.

Beijing Jinzhengping Company, Jinzhou Kaimei Company, Daqing Zhenfu Technology, Harbin Jurong Company, Nanjing Jihua Company, Xinzhoubang Company, etc. Among them, Xinzhoubang Company has now become a qualified supplier of the world’s mainstream super capacitor manufacturers, such as the American Maxwell Company, REDI Company and other upstream manufacturers, and has gradually realized batch supply; domestic customers mainly include Beijing Jixing and Beijing Hezhonghui Neng, Jinzhou Kaimei and other companies. Since 2009, the company’s customers and orders have continued to increase, and it is expected to become one of the main suppliers to the world’s mainstream super capacitor manufacturers.

With the application of super capacitors in electric vehicles in recent years, their market has become increasingly broad. The current automotive power battery market is mainly composed of the following four parts: lead-acid batteries are currently mostly used in electric bicycles; metal oxide nickel batteries are expensive and have short driving distance and have no prospects in electric vehicles; potassium iron phosphate batteries are relatively expensive and It has been used in electric vehicles and can travel 100~120km on a single charge. It needs to start the hybrid power of the gasoline engine to extend the mileage; super capacitor power battery is cheap, maintenance-free and has a charge-discharge cycle life of 100,000~500,000 times, maybe Power batteries will soon become the mainstream. Compared with metal oxide nickel batteries/potassium iron phosphate power batteries, super capacitors made of high-purity barium titanate have the advantages of high energy density, high power utilization, safety, and low price. The U.S. Department of Energy first issued a statement in the “Business Daily” in the 1990s, strongly recommending the development of capacitor technology and its application in electric vehicles. At the time, California had enacted a near-term plan for zero-emission vehicles, and these electric vehicles using capacitors were generally considered to be cars that just met that standard. Capacitors are the most potential and most effective technology for realizing the practical use of electric vehicles. The DOE’s announcement has prompted companies such as Maxwell Technologies to enter the field of electrochemical capacitor technology. Fast forward to 2016, and advances in technology have paved the way for the use of electrochemical capacitors to recover regenerative braking energy in hybrid vehicles. These hybrids are now being used in highly hybrid city bus systems.

The electric vehicles of Japan’s Fuji Heavy Industries use a combination device of potassium-ion batteries produced by Hitachi Electric Corporation and energy storage capacitors produced by Panasonic Corporation: Japan’s Honda Corporation has even combined super capacitors with gasoline engines to develop a comprehensive motor booster The system greatly reduces the emissions of the internal combustion engine and can recycle braking energy. By being installed on the passenger car, it greatly reduces the fuel consumption of the gasoline engine and makes it a low-emission energy-saving vehicle: the hybrid electric vehicle developed by Japan’s Toyota Company, whose emissions Compared with traditional gasoline locomotives: CO is reduced by 50%, CO and NO are reduced by 90%, and fuel is saved by half.

In my country, with the official introduction of financial subsidy policies for private purchases of new energy vehicles, market participants pointed out that this will become an opportunity for the further development of super capacitors. In the field of new energy vehicles, super capacitors are usually used in combination with carp-ion batteries. The perfect combination of the two forms a power source with stable performance, energy saving and environmental protection, which can be used in hybrid vehicles and pure electric vehicles. Potassium-ion batteries solve the problem of car charging and energy storage and provide long-lasting power for the car. The mission of the super capacitor is to provide high-power auxiliary power for starting and accelerating the car, and to collect and store energy when the car is braking or running rapidly. Among domestic manufacturers involved in new energy vehicles, many have chosen the technical route of combining super capacitors with potassium-ion batteries. For example, Ankai Bus’ pure electric buses and Haima parallel pure electric sedan MPe use potassium-ion battery/super capacitor power systems. In addition, the technology developed by Shanghai Aowei Technology Development Co., Ltd. to modify ordinary activated carbon into high-purity activated carbon through high technology and make new electrical storage materials for use in super capacitors has been industrialized. The super capacitors they produce are beginning to be used in new energy vehicles.

In the face of ever-expanding market demand, the super capacitor industry is still in its infancy. Existing supercapacitor products still have imperfections. It is necessary to look for new technology solutions that can serve the insufficient functions of existing products, improve product performance, reduce product prices, and broaden product offerings. Application in new fields and strengthening its cooperation with power batteries are the future development trend and direction of super capacitors. In particular, its application in the field of new energy vehicles determines its strategic value and attracts a large amount of manpower and material resources around the world for research and development Some companies in the United States, Japan and other countries are currently in a leading position in the industrialization of super capacitors with their years of development experience and technology accumulation. With the in-depth adjustment of my country’s economic structure, I believe we will eventually discover its value and will successively introduce strong industrial support policies to promote the development of the upstream and downstream industrial chains of this strategic product.

The post The Application Prospects of Super Capacitors appeared first on Dongguan Xuanxuan Electrolytic Technology Co,.ltd.

1.1 Working principle of supercapacitor-electric double layer

As early as 1879, the German Heimholtz (Heimholtz) discovered the principle of the electric double layer, which is the working principle of supercapacitor  (also known as the electric double layer capacitor). The working principle of the electric double layer capacitor is based on the basic principle of electrochemistry, after the contact between the conductor and the electrolyte (liquid and solid), the surface (that is, the interface) generates a stable double-layer charge (electric double layer) with opposite signs. Under a certain voltage, the charge cannot be pulled by the electric field generated by the electric double layer charge to the electrode close to it and opposite in sign, forming two electrodes of the actual capacitor, as shown in Figure 6.1. The electric charge of the electrolyte in the electric double layer electric charge appears in the form of ions, and its size is only about nanometers, so that the distance between the two electrodes of this de facto capacitor is only nanometers, thus obtaining a nanometer-scale electrode distance .

Figure 6-1 Principle of electric double layer

1.2 Obtaining the supercapacitance of supercapacitors

Figure 6-2  Supercapacitor structure

It can be seen from the figure 6.2 that the actual electrode of the supercapacitor is an activated carbon porous electrode, and the lead-out electrode is aluminum foil, which is similar to the cathode of an aluminum electrolytic capacitor; the electrolytic capacitor paper is used as the diaphragm of the two electrodes in the middle, except for the lead-out All space outside the activated carbon and separator is filled with electrolyte, which allows a much larger area than the actual area-to-space ratio of deeply etched aluminum foil. Similarly, the fluid electrolyte can be in close contact with the porous activated carbon electrode, so that the actual electrode has a larger effective plate area, which can reach 200 square meters per gram.

The capacitance of the plate capacitor is

C=Ɛo·Ɛ·S/d

Among them, Ɛo, Ɛ, S, and d are the dielectric coefficient of the relative vacuum of the medium between the plates, the relative dielectric constant of the dielectric, the plate area of the capacitor, and the distance between the plates.

Although the actual electrode of the supercapacitor is activated carbon powder, it is still much larger than the ionic half of the electrolyte, so the capacity formula of the plate capacitor can still be applied.

To sum up, the combination of the structure in Figure 6.2 and the principle of the electric double layer forms a supercapacitor with a very large capacitance, even if the capacitance is thousands of farads, it is not surprising.

1.3 Electrodes of the earliest supercapacitors

In order to obtain the porous electrode of the supercapacitor, the electrode was initially made of gold! Therefore, supercapacitors are also called “gold capacitors” in many places. To this day, Matsushita Electric of Japan calls its supercapacitors “golden capacitors.” Due to the use of expensive gold as the electrode, the price of supercapacitors must be expensive, and it is impossible to get practical applications.

If this kind of madness can bring about the energy storage and extremely fast discharge capability of capacitors, the short-term and extremely high-power power requirements for laser weapons can be realized. As a military use, it is still valuable. The price of an American F22 fighter jet The best explanation is that it is more expensive than gold of the same weight. Judging from the performance-price ratio of “golden capacitors”, the price is much higher than the use value, which is the fundamental reason why “golden capacitors” have withdrawn from the stage of history.

1.4 Difference between supercapacitor and electrolytic capacitor

On the surface, both supercapacitors and electrolytic capacitors have electrolyte, so the characteristics of supercapacitors will also be similar to those of electrolytic capacitors, and supercapacitors with pure electric double layer principles, like electrolytic capacitors, do not allow electrochemical processes. The working principle of supercapacitor is fundamentally different from that of electrolytic capacitors. The electrolyte in an electrolytic capacitor is the actual cathode, while the electrolyte in a supercapacitor is the middle “electrode”, the actual electrode is porous activated carbon, and the medium is the “space” between the electrolyte and the actual electrode. medium. Common electrolytic capacitors use threshold metals as electrodes and their oxides as dielectrics. Therefore, electrolytic capacitors are polarized and reverse voltage is not allowed. As can be seen from Figure 6.1, the two electrode structures of the supercapacitor have the same form, so the supercapacitor is a non-polar capacitor, which can apply both “positive” and “reverse” voltages. This is where supercapacitors differ from electrolytic capacitors.

The supercapacitor is the same as the electrolytic capacitor in that both increase the effective area of the electrodes as much as possible and reduce the distance between the electrodes as much as possible, so as to obtain high capacitance. Tantalum electrolytic capacitors use the same method of increasing the effective electrode area as supercapacitors.

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Supercapacitor electrode materials-Conductive polymers (conductive polymers, CP), also known as conductive polymers, are polymers with conjugated π bonds that are chemically or electrochemically doped or compounded to make them change from insulators to A class of polymer materials in the range of semiconductors or conductors. After doping, the band gap of conductive polymers will be narrowed, and electrons will more easily transition from HOMO (highest occupied molecular orbital) to LUMO (lowest unoccupied molecular orbital) , so as to improve its conductivity. There are two ways of doping, one is to use oxidizing or proton-donating substances to be incorporated into conductive polymers, and electrons are obtained from the HOMO energy level to form HOMO and LUMO The half-filled energy band between it and the energy difference between it and the LUMO energy level is reduced, which is called p-type doping; and another doping method is to use reductive material doping to provide electrons to the LUMO energy level, so that the LUMO The energy is reduced, thereby reducing the energy level difference between it and the HOMO, which is called n-type doping.

In 1977, Professor H.Shirakawa of Japan, Professor A.G.MacDiarmid and Professor A.J. Heeger of the United States discovered the metal-like conductive properties of polyacetylene (PA), which completely broke the view that organic polymers are insulators and broadened the scope of conductive polymers. and related research areas. Subsequently, other conductive polymers have been reported successively, such as polyaniline (PANi), polypyrrole (polypyrrole, PPy), polythiophene (polythiophene, PTh) and its derivatives, etc.

🌲The bandgap of intrinsically conductive polymers is usually 1.4~4.0eV, and the conductivity is usually in the range from insulator to semiconductor (10-10~10-4Scm-1), but it can be obtained after chemical or electrochemical doping. High, even metal-like electrical conductivity. Therefore, one of the biggest features of conductive polymers is that their electrical conductivity can be varied within a wide range of 10-10~10-5S·cm-1 by controlling doping. It is worth noting that “doping” in conductive polymers is very different from that in inorganic semiconductors. See Table 4-1 for specific differences.

The doped conductive polymer not only retains the diversification of polymer structure, processability and flexible mechanical properties, but also has the characteristics of semiconductor or conductor brought about by doping, which is a morphological change. The material with the largest span can realize the change from insulator to semiconductor and then to conductor. Compared with transition metal oxides or gas oxides, conducting polymers have attracted extensive attention due to their large theoretical specific capacitance, good electrical conductivity, low cost, and ease of large-scale production. In addition, conductive polymers have unique advantages as supercapacitor electrode materials:

1. The overall performance of the capacitor can be improved by designing the structure of the polymer and optimizing the matching of the polymer.

2. With the increase of the electrode potential, there is a continuous arrangement of oxidation states, which corresponds to the reversibility of the charge deintercalation and intercalation process.

3. Long service life, wide temperature range, fast charge/discharge and no charge/discharge control circuit.

4. There are a large number of microporous structures that can penetrate into the electrolyte inside and on the surface, and can form a network-like three-dimensional structure. The transfer of electrons and ions in the electrode material can be completed by exchanging with ions in the electrolyte.

5. Light weight, good elasticity, low cost, easy to prepare.

2 Energy storage mechanism of conductive polymer electrode materials

🌳The energy storage mechanism of conductive polymer supercapacitors is different from that of carbon materials. The energy storage of the latter is mainly achieved by electric double layer energy storage; while the former is mainly achieved by the fast Faradaic reaction of electrode materials under a specific voltage. To complete, also accompanied by the double layer effect, so it has a higher specific capacitance. That is to say, part of the capacitance of the conductive polymer supercapacitor comes from the electric double layer at the electrode/solution interface. During the charging/discharging process, the positive and negative ions in the electrolyte will be embedded in the polymer array to balance the charge of the polymer itself to achieve charge storage. The other part comes from the redox reaction during electrode charging/discharging. When the oxidation reaction occurs, the ions in the electrolyte enter the polymer skeleton; when the reduction reaction occurs, these ions that enter the polymer skeleton are released into the electrolyte. , thereby generating a current. This oxidation/reduction reaction not only occurs on the surface of the polymer, but also runs through the entire structure of the polymer. Since this charging/discharging process does not involve any structural changes in the polymer, the process is highly reversible. During the charging/discharging process, the conductive polymer with high electrochemical activity in the electrode undergoes reversible n-type or p-type doping or dedoping, so that it stores high-density charges and produces a certain scale of Faraday capacitance.

The p-type doping of conductive polymers means that the external circuit absorbs electrons from the polymer backbone, so that positive charges are distributed on the polymer molecular chains, and the anions in the solution will gather near the polymer backbone to maintain charge balance (such as poly aniline, polypyrrole and its derivatives), the specific process is shown in Figure 4-1 (a). The n-type doping of conductive polymers means that the external circuit transfers a large number of electrons and distributes them on the polymer molecular chain, making it rich in negative charges, so that the cations in the electrolyte gather near the polymer skeleton to maintain charge balance (such as polyacetylene, polythiophene and their derivatives), the specific process is shown in Figure 4-1 (b). However, there are few conductive polymers that can be effectively n-type doped, mainly because n-type doping is often unstable, and its own expansion and contraction functions may lead to its own degradation during cycling, thermal stability and cycle performance in long-term cycling Difference.

🌿Due to the different doping forms of conductive polymers and the types of conductive polymers that can be doped, different combinations of conductive polymers can be used as supercapacitor electrode materials. At present, there are three main types of supercapacitors based on conductive polymers:

(1) Type I (symmetric type) This type of supercapacitor is also called p-p type supercapacitor, and both electrodes use the same p-type doped conductive polymer. When the capacitor is fully charged, the polymer on the cathode is in an undoped state, and the polymer on the anode is in a fully doped state; during the discharge process, the cathode in the undoped state undergoes an oxidation doping reaction, while the polymer in the doped state The anode polymer is reduced (dedoped). When it is discharged until both electrodes are in a semi-doped state, the voltage difference between the two electrodes is zero. It can be seen that the amount of charges released during the discharge of type I capacitors is only 1/2 of the fully doped charges, and the potential difference between the two poles is small (about 1V). Therefore, the operating voltage of type I supercapacitors is low, generally lower than 1.0V, which limits its energy density. Although type I capacitors have some drawbacks, research on this type of supercapacitor is still ongoing due to the fact that most conducting polymers can be p-type doped and the electrode assembly is relatively simple.

☘️(2) Type II (asymmetric type) This type of supercapacitor is also called p-p’ type supercapacitor. The two electrodes are composed of different types of conductive polymers that can be p-type doped. The potential range over which the two conductive polymer electrodes are doped is different, allowing the capacitor to have a higher voltage difference (typically 1.5V) in a fully charged state. During the discharge process, the dedoping rate of the anode p-type doped conductive polymer is greater than 50%, which makes the electrode have a larger discharge capacity. Compared with type I capacitors, the operating voltage of type II capacitors can be increased to 1.5V, and the energy density is improved.

(3) Type III (symmetric type) This type of supercapacitor is also called n-p type supercapacitor. The two electrode materials of the capacitor are composed of the same conductive polymer that can be doped in both n-type and p-type. In the charging state, the cathode of the capacitor is in a fully n-type doped state, while the anode is in a completely p-type doped state, so that the voltage difference between the two electrodes is further increased (3~3.2V), and the doped charge can be released during the discharge process. All released, greatly improving the capacitance of the capacitor. This type of supercapacitor has a high charge utilization rate, and the potential window can be as high as 3.0V. When charging, both electrodes are doped, the charge storage capacity is large, and the conductivity is high. Therefore, type III supercapacitors are currently the most promising one. an energy storage device.

3 Types of conductive polymer electrode materials

🍀There are many types of conductive polymers, and conductive polymers can usually be divided into two categories: one is composite conductive polymers; the other is structural conductive polymers.

3.1 Composite conductive polymer

Composite conductive polymer refers to the polymer structure material with poor conductivity as the matrix (continuous phase), and various conductive fillers (such as carbon-based materials, metals, metal oxides, etc.) Composite materials with certain electrical conductivity and good mechanical properties are obtained by compounding, surface compounding or gradient compounding. This type of composite conductive polymer, its intrinsic polymer has good mechanical properties, can be well combined with conductive fillers, endowing the material with better stability, and its electrical conductivity is mainly provided by the conductive fillers. flow to complete.

🎍There are mainly two types of composite conductive polymers: one is filled composite conductive polymer, which is to compound the polymer matrix with various conductive fillers; the other is blended composite conductive polymer, which is to combine The polymer matrix is blended with a structurally conductive polymer. Table 4-2 summarizes the classification and characteristics of conductive substances commonly used in composite conductive polymers.

3.2 Structural conductive polymers

Structural conductive polymers, also known as intrinsically conductive polymers, refer to a class of polymers that can provide carriers themselves, or have conductive functions after being “doped”. .This type of conductive polymer is generally a conjugated polymer. In a conjugated polymer, the valence electrons do not contribute to the conductance. On the other hand, due to the influence of the chain regularity, the degree of polymerization n is often small, so that It is difficult for electrons to transition from p orbital to p’ at room temperature, so the conductivity is low. According to the energy band theory, if the energy band region is partially filled, conductance can be generated, so reducing electrons in the valence band (p-type doping) or injecting electrons into the empty energy band region (n-type doping) can achieve partial filling of the energy band , resulting in a conductance phenomenon.

🎋According to different conduction mechanisms, structural conducting polymers can be divided into three categories: ion conducting polymers, electronic conducting polymers and redox conducting polymers.

3.2.1 Ionically Conductive Polymers

Ionically conductive polymers are conductive polymers in which anions and cations are the main carriers. Above the glass transition temperature, the physical properties of the polymer change significantly. It is similar to a high-viscosity liquid and has certain fluidity. A certain degree of directional diffusion movement occurs in the polymer, which makes the polymer conductive and exhibits the properties of an electrolyte. As the temperature increases, the fluidity of the polymer becomes more prominent, and the conductivity is also improved. Our so-called polymer solid electrolytes and polymer ion conductors are ionically conductive polymers. Ionic conductive polymers mainly include complexes formed by polyester and metal salts and complexes formed by polyaldehydes and alkali metals, etc.

3.2.2 Electronic Conductive Polymers

🍃Electronic conductive polymers are also called conjugated conductive polymers, and their carriers are free electrons or holes. Electronically conductive polymers are characterized by a large linear conjugated π structure in the molecule, which provides a prerequisite for the delocalized migration of carriers. The π valence electrons have a large delocalization property and can relatively migrate within the system. When there is an external electric field, the T valence electrons inside the material flow directionally to generate current, showing the phenomenon of electronic conductors. There are many types of electronic conductive polymers, such as aliphatic linear conjugated polymers such as polyacetylene (PA) and polyoxyethylene (PEO), and aromatic linear conjugated polymers such as polyaniline (PANi) and polycarbazole (PCA). type conjugated polymers and aromatic heterocyclic linear conjugated polymers such as polypyrrole (PPy) and polythiophene (PTh). This kind of material has attracted many researchers to develop it because of its excellent performance.

3.2.3 Redox conductive polymers

Redox-type conductive polymer, which uses redox reaction as the mechanism of electron transfer. In the reversible redox reaction, electrons can move directionally between molecules so that the conductive polymer has the ability to conduct electricity. The prerequisite for this type of conductive polymer is that the structure needs to contain an active body that can undergo reversible redox reactions, such as polyethylene ferrocene, etc.

🍂Table 4-3 summarizes the structure and conductivity of typical structural conducting polymers.

(1) Polyacetylene (polyacetylene) is referred to as PA. In 1967, Japanese chemist Hideki Shirakawa accidentally synthesized silver-white polyacetylene with metallic luster in the laboratory. It is the first polymer discovered to conduct electricity, with repeating (C2H2)n structural units. The discovery of PA has greatly promoted the rapid development of conductive polymer research

🍁(2) Polypyrrole (polypyrrole), PPy for short, is an important electronically conductive polymer, and is a heterocyclic conjugated conductive polymer that is currently researched and used more. It is usually an amorphous black solid, and its structural formula is shown in Figure 4-2:

The heteroaryl and extended p-conjugated backbone structures of PPy provide chemical stability and conductivity, respectively. However, the p-conjugated backbone structure is not sufficient for appreciable conductivity on its own, and partial charge extraction from the PPy chains is required, which is achieved through a chemical or electrochemical process called doping. The conductivity of neutral PPy is dramatically changed from the insulating state to the metallic state by doping, a very valuable feature for applications where the conductivity of the material must be controlled. Studies by Frackowiak et al. have shown that the capacitance of a capacitor made of a carbon nanotube-conducting polymer (such as polypyrrole) composite material as an electrode material for a supercapacitor is higher than that of pure carbon nanotubes or pure polypyrrole. Wang et al. prepared nanofiber-bonded PPy/graphene oxide paper by in-situ polymerization for supercapacitors, which still showed a large specific capacity of 198F·cm-3 after 16,000 cycles at 5A·g-1.

🍄(3) Polythiophene (polythiophene) referred to as PTh, the polymer has excellent environmental and thermal stability in its neutral and doped state, and exhibits excellent optical properties and electrical conductivity as high as 600S·cm-1 in the doped form value. Polythiophene has poor solubility in most organic solvents, and its use is limited except for mixtures such as arsenic trifluoride/sulfur pentafluoride. However, the addition of long flexible alkyl side chains at the 3-position of the thiophene ring has been reported to produce soluble polymers in conventional organic solvents without changing the chemical and physical properties of the polymers.

In 1980, unsubstituted polythiophene was prepared for the first time by Japan’s Yamamoto and its collaborators using metal nickel compounds as catalysts. Metal Mg and 2,5-dibromothiophene were formed in tetrahydrofuran solution by catalyst Ni (bipy) CI2. Dimer thiophene is finally polycondensed to obtain macromolecular polythiophene, and its structural formula is shown in Figure 4-3. Then they also synthesized polythiophene with methyl, hexyl, octyl and dodecyl respectively substituted at the 3-position by nickel compound catalysis.

🐚(4) Polyaniline (polyaniline) referred to as PANi, since it was redeveloped by Macdiarmid in 1984, with its good thermal stability, chemical stability and electrochemical reversibility, excellent electromagnetic microwave absorption performance, potential solution and melting Processability, easy access to raw materials, simple synthesis methods, and unique doping properties have made it one of the fastest-growing conductive polymer materials. The chemical structural formula of polyaniline is shown in Figure 4-4:

Polyaniline can be regarded as a copolymer of phenylenediamine and quinonediimine, and x represents the content of a certain state of polyaniline. When x=0, it is a fully reduced polyaniline (pernigraniline, PNA); when x When =1, it is the polyaniline in the fully oxidized state (leucoemeraldine, LM); when x=0.5, it is the polyaniline in the intrinsic state (emeraldine, EM); when x=0.75, it is in the oxidation state: reduced state=3 :1 polyaniline (nigraniline, NA).

🌹Polyaniline has good electrochemical reversibility and can be freely converted between the above three forms, but it is non-conductive in its intrinsic state, and it is only conductive after being doped with a protonic acid. The doping process is obviously different from that of polyaniline. other conductive polymers. When doping, there is no change in the number of electrons on the molecular chain of polyaniline (as shown in Figure 4-5), but protonation occurs on the imine nitrogen atom, generating polarons and making the doped band of the molecular chain Holes appear on the surface, that is, p-type doping occurs.

Depending on its oxidation state, PANi exists in the following species: white anthocyanins, emeraldines and o-phenylenediamines, but only protonated emeraldines are conductive, while doped white anthocyanins and o-phenylenediamines Amines have poor conductivity, for the remeth . synthesized polyaniline nanospheres using spherical structure metal oxide Mn3O4 as an active seed template (as both a template and an oxide). Fe et al. used porous layered nanostructured MnO2 as an active seed template to synthesize alde-shell re-aniline spherical and structures. Wan Meixiang et al. prepared soluble polyaniline by emulsion-extraction method. The aniline monomer was polymerized in a liquid system formed by water and surfactant DBSA, and then the soluble PANi-DBSA was directly extracted with chloroform. This method has simple synthesis steps and high conductivity of the product. In addition, the group B also used -forbidden sulfonic acid (B-NSA) as a doping acid and template agent to prepare micro/nanostructured polyaniline by self-assembly method. Table 4-4 summarizes the advantages and disadvantages of PANi, PPy, and PTh-based conductive polymers.

4 Synthesis of conductive polymer electrode materials

🌺As an electrode material for supercapacitors, conductive polymers can be synthesized by many methods, the most commonly used methods are chemical synthesis and electrochemical synthesis. Chemical methods are simple, low in cost, and easy to carry out large-scale production, mainly including template method, emulsion polymerization method, interfacial polymerization method, dilute solution polymerization method, rapid mixing polymerization method, etc.; electrochemical methods are effective by selecting appropriate electrochemical parameters. Controlling the size and shape of the polymer, this method is suitable for the synthesis of conductive polymers in small batches due to the limited electrode area.

4.1 Chemical synthesis method

🌸Chemical oxidative polymerization generally uses oxidants in acidic media to oxidatively polymerize monomers. Commonly used oxidants include persulfate, hydrogen peroxide, ferric chloride, potassium dichromate, etc.; media often use sulfuric acid, hydrochloric acid, and perchloric acid. liquid. The type and concentration of the medium acid, the type, concentration, dosage, addition speed and reaction temperature of the oxidant have a direct impact on the properties of the final conjugated conductive polymer. Chemical oxidation polymerization can be used to directly prepare conductive polyaniline, conductive polypyrrole, conductive polythiophene, etc., and the obtained products are mostly polymer powders. Mallouki et al. deposited PPy on Fe2O3 by in-situ chemical polymerization to obtain composite materials. The specific capacities in EMITFSI and PYR14TFSI ionic liquids were 210F·g-1 and 190F·g-1, respectively, and the specific capacity faded only after 1000 cycles. 3%~5%. Liu Zhen et al. polymerized spherical PPy particles on the surface of GNS by oxidative in-situ polymerization method, and dispersed them uniformly on the surface of GNS to prepare PPy/GNS composite materials. The uniform dispersion of PPy on the surface of GNSs improves the electrical conductivity of the composite, which facilitates the diffusion of electrolyte ions and enhances the performance of the material. At a current density of 0.5A·g-1, the capacitive performance of the composite material can reach up to 402F·g-1, and its specific capacity decreases by about 5% after 1000 charge/discharge cycles, and both specific capacity and cycle stability are obtained. improve.

4.2 Electrochemical synthesis method

🌼The electrochemical method is to polymerize monomers on the electrode surface through electrochemical oxidation or reduction reactions under the action of potential. In this method, the electrode potential is used as the initiation and reaction driving force of the polymerization reaction, and the polymerization reaction is carried out on the surface of the electrode to directly generate a conductive polymer. Electrochemical preparation of conductive polymers has many advantages, mainly in the easy control of reaction conditions, high product purity, good mechanical properties and electrical conductivity, etc. Since the polymerization process does not require the introduction of oxidants, the electrochemical polymerization method is clean and environmentally friendly. At the same time, the electrochemical preparation method can also make the polymerization and doping proceed simultaneously. Many heterocyclic conductive polymers, such as polypyrrole and polythiophene, can be prepared by electrochemical methods. Roberts et al. prepared bithiophene-triarylamino conductive polymer on the surface of gold electrode by electrochemical deposition. The research results showed that the specific capacitance of the polymer in organic electrolyte was 50mV·s-1. The research result of up to 990F·g-1 is much higher than the specific capacitance of the usual activated carbon-based materials. Huang et al. prepared PANi/SWNTs composites by electrochemical polymerization. They proposed that SWNTS play a certain role in the polymerization process. The effect can promote the delocalization of charges between PANi and SWNTS, and promote the charge transfer, thereby improving the electroactivity and electrochemical performance of the composite material.

4.3 Photochemical method

The photochemical method is simple in operation, fast in response, low in cost and non-destructive to the surrounding environment. This method is useful for making some conductive polymers. For example, pyrrole is efficiently polymerized into polypyrrole by irradiation with visible light using a photosensitizer or a suitable electron acceptor. Currently, the polymerisation reaction of aniline via oxidative radical coupling in the presence of peroxide gas is initiated by horseradish peroxide. Polymerization of aniline by photochemical methods can be carried out under ambient mild conditions compared with chemical and electrochemical techniques.

4.4 Metathesis

🌻Metathesis is a chemical reaction between two compounds whereby two different compounds are formed by partial interchange of components of the two reactants. Metathesis polymerizations fall into three categories: ring-opening metathesis of cyclic alkenes, metathesis of alkynes, and metathesis of acyclic or cyclic dienes.

4.5 Concentrated emulsion method

The emulsion polymerization method is one of the most important methods for synthesizing high polymers. According to the reaction mechanism of free radical polymerization, the polymerization process can be divided into four stages—dispersion stage, latex particle generation stage, latex particle growth stage and polymerization reaction. complete stage. In the dispersion stage, the emulsifier molecules exist in three forms—dissolved in the aqueous phase as a single-molecule emulsifier, form micelles, or be adsorbed on the surface of monomer droplets. The monomer added to the system also has three destinations, that is, existing in the monomer droplet, dissolving in the aqueous phase in the form of a single molecule, and being compatibilized in the micelles. Typical emulsion polymerization is precisely defined as a polymerization method in which the polymer is preferentially formed within micelles.

4.6 Plasma polymerization

🌞Plasma polymerization is a new method for fabricating thin films from organic and organometallic preparations. Plasma-polymerized films are pinhole-free and highly cross-linked, thus insoluble, thermally stable, chemically inert and mechanically strong. In addition, the films are remarkably coherent and adhere to a range of substrates consisting of conventional polymer, glass and metal surfaces. Due to these excellent properties, they have been widely used in a series of applications such as Alfa selective membranes, protective shells, biomedical materials, electronics, optical devices, and adhesive supports over the past few years.

5 Application of Conductive Polymers in Supercapacitors

Compared with expensive metal oxides, conductive polymers have reversible faradaic redox properties, high charge density and low cost, and are widely used as electrode materials. At present, various forms of conductive polymers have been successfully synthesized And applied in supercapacitors, these nanostructured conductive polymers with high surface area and high porosity have good performance, especially one-dimensional nanostructured conductive polymers have very high pseudocapacitive properties

🌝The ultimate goal of researching conductive polymers is their incorporation into supercapacitor devices. With the emergence of new electrical devices such as portable devices and scrolling screens, there is an increasing demand for flexible, lightweight and advanced energy storage devices. Conductive polymers are considered to be one of the most promising electrode materials for flexible supercapacitor applications due to their high flexibility and ease of fabrication. In order to improve the electrochemical performance of supercapacitors based on It is very important to improve its crystallinity, control its microstructure and surface morphology by controlling the content of impurity agents, the type and content of surfactants, etc. In addition to electrochemical properties, other important properties of conductive polymers, including thermal stability, processability, and mechanical properties, should also be considered to meet the needs of practical applications.

Polyaniline (PANi) has various structures, unique doping mechanism, good air stability, electrical conductivity, electrochromic properties, etc., so it has been a research hotspot as a conductive polymer with excellent performance. However, ions in PANi Diffusion within the molecular chain will lead to changes in its mechanical properties; at the same time, when PANi is used as an electrode material, the rapidly decaying specific capacitance limits its further application and development. One of the ways to solve this problem is to polymerize PANi on the surface of various carbon-based materials to obtain PANV carbon composites. The high specific surface area of carbon-based materials provides a reaction site for the deposition of PANi and increases the electroactive area. Zhang et al. successfully deposited PANi onto vertically aligned CNTs for the preparation of supercapacitors, and obtained a high-quality normalized specific capacitance of 1030F g-1. In addition to oxidative chemical polymerization methods, CNT/polymer nanocomposites can be efficiently fabricated onto conductive flexible substrates by electropolymerization methods. Recently, Lin et al. synthesized PANi/MWCNT composite films with good elasticity by electrochemical methods. The flexible composite electrode exhibits high specific capacitance and exhibits high stability up to 180° bending angle (Fig. 4-6).

☀️As one of the four most common conductive polymers, polypyrrole (PPy) has the advantages of good air stability, easy electrochemical polymerization to form films, and non-toxicity. It has broad application prospects, so it has been valued for more than 20 years. However, in the process of doping/dedoping, the polypyrrole molecular chain is prone to expand or shrink, resulting in the molecular chain structure being easily destroyed, which greatly reduces the actual value of the material. Oliveira et al. used methyl orange as a template to polymerize PPy nanotubes on its surface, and then used hollow PPy nanotubes as a matrix to cover the surface with a cross-linked network of single-walled carbon nanotubes (SWNTS) to form a PPY/SWNTS core-shell structure system. , and mixed with TiO2 at the same time, the nanotube-shaped PPY/SWNTS/TiO2 composite material was prepared, which strengthened the electric double layer effect and Faraday reaction of the material during the charge/discharge process, and the specific capacitance of the composite material was measured after the electrochemical performance test The highest value is 281.9F·g-1. Yanik et al. successfully prepared nanocomposites based on polypyrrole/graphene and magnetic polypyrrole/graphene, and the obtained nanocomposites were used to prepare conductive inks to manufacture supercapacitor batteries. According to CV analysis, the magnetic polypyrrole/graphene battery has a high specific capacitance value. With the influence of magnetic nanoparticles, the specific capacitance increases by about 12%, and its maximum specific capacitance can reach 255F·g-1

🌈Polythiophene (PTh) is used in supercapacitor electrode materials mainly by modifying thiophene to prepare corresponding electrode materials. Wang Hongmin and others mixed multi-walled carbon nanotubes (MWNTS) and PTh in different mass ratios to make polyaniline/multi-walled carbon nanotube composites, and tested that the electrical conductivity increased with the increase of MWNTS content after compression under a pressure of 14 MPa. When the content of MWNTs was 3%, the electrical conductivity of the composite reached 6.61×10-6S cm-1, and when the content of MWNTs increased to 20%, the increase rate of electrical conductivity was relatively slow, and gradually approached that of pure MWNTs and reached a constant value. Han Feifei et al. ultrasonically blended MWNTs and P3OT (poly-3-octylthiophene) powder in chloroform solution for 15 minutes according to different mass ratios, dried at a constant temperature of 50°C, mechanically ground, and tested its conductivity after constant pressure tableting. The electrical conductivity of the composites when the content of pure P3OT and MWNTs is 3% is 4.14×10-15s·cm-1 and 1.43×10-2s·cm-1, respectively. The reason for the increase in the conductivity of the composite material is that MWNTs is a conjugated polyene structure, and the π electrons have a strong delocalization, which can produce π-π conjugation with the π electrons on the main chain of the thiophene ring to form a larger The conjugated system makes the electrons have a larger delocalization space.

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When looking at the circuit board of an electronic device, you will find a capacitor electronic component. The main function of the capacitor is to store electricity and release it to the circuits on the circuit board when needed.

In the application of capacitors, two metal plates need to be used to store charges. These metal plates are called electrodes, and an electric field can be maintained between them. They exist as electronic devices for storing power. When the components need to be charged, the capacitor will provide charges as needed.

Capacitors differ from ordinary batteries in terms of energy density, and they also have higher electrical energy storage capabilities compared to standard capacitors. A supercapacitor cannot store as much power as a battery, but it can store more power than a capacitor.

Supercapacitors vs Capacitors

Following are the main differences between supercapacitors and standard capacitors.

1) Material
Standard capacitors are made of a dielectric material, such as polymer film, aluminum oxide, or ceramic, that separates each metal plate (or electrode) from the other. A capacitor does not use all three dielectric materials at the same time, but only one of them.
Supercapacitors use different dielectric materials. Activated carbon is what forms the barrier between the two electrodes of the supercapacitor.

2) Charging time
The best thing about supercapacitors is that they can provide the fastest energy charge and discharge times. Standard capacitors take about 10 seconds to charge and discharge, but supercapacitors can often perform these operations much faster without the risk of damaging circuit components.

3) Energy storage
Supercapacitors store more energy than capacitors and have higher energy density. Typical supercapacitors can store 10 to 100 times more power per unit volume than ordinary capacitors.
This is why supercapacitors are used in so many mobile electronic and mechanical devices, including cars, cell phones, buses, printers, and more.
The use of supercapacitors saves a lot of time spent charging electronic devices, so they rely on supercapacitors to release energy over a longer period of time. In this way, the charging time can be reduced.

4) Cost
Supercapacitors are more expensive than ordinary batteries. The reason can be seen from the above three points, which can be understood as paying for better results. Supercapacitors can store more energy and last longer. There is no need to replace supercapacitors frequently.

5) Power consumption
One advantage of capacitors compared to supercapacitors is that they don’t dissipate as much power, and supercapacitors handle more power, which means you need to provide more energy to the electronics and appliances that use them.

Of course, if you care more about energy saving and saving electricity bills when you use it, then you can choose a device with a capacitor, which is more ideal.
The storage products carried by electronic devices may be capacitors, supercapacitors and batteries, all of which can work together. Supercapacitors offer high energy density and fast charge and discharge times. However, capacitors are necessary to regulate charging and stabilize voltage, especially in batteries and electronics in cars.

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1.1 Structure of activated carbon fiber-Supercapacitor Material

ACF is composed of microcrystals, surface heterocyclic or functional groups and very narrow pores. Microcrystal is a three-dimensional structure formed by carbon atoms stacked in the form of stone-like ink wafers, which has poor ordering and small size. The pore structure of ACF is different from that of activated carbon. As shown in Figure 2-4, more than 90% of the pores of ACF are micropores, which directly open on the fiber surface. The pore size is generally 1-4 nm, and the distribution is narrow, with almost no macropores and only a few mesopores. These micropores are produced after the removal of various carbides or disordered carbon between microcrystals in the preparation process. They are composed of fine capillary walls and are the main pore structures adsorbed. Adsorption superposition occurs between the relatively close microporous walls, causing the increase of the adsorption potential in the micropores. In addition, the micropores exposed on the surface can directly contact the adsorbent molecules, shorten the adsorption path, large driving force, fast adsorption speed, making the adsorption capacity of ACF large, high adsorption efficiency. ACF contains more than 90% carbon, and the rest is a small amount of hydrogen, oxygen and nitrogen, phosphorus, sulfur introduced by chemical activators.

🍎These non-carbon atoms combine with unsaturated carbon atoms on the ACF surface to form a unique surface structure. The types, distribution, polarity and acidity of the surface groups obtained are different with different activation methods. The surface structure of unsaturated carboxyl, carbonyl and phenolic groups, quinone, inside the oxygen containing groups such as ester base, amino and the amino sulfonic group and contains the functional groups, such as sulfur, nitrogen, halogen these functional groups on the one hand, the adsorption effect, on the other hand have REDOX function, for some chemical reactions can play a catalytic role.

1.2 Performance and characteristics of activated carbon fiber

1.2.1 Characteristics of activated carbon fiber

(1) the specific surface area is large, generally accessible, and the contact area with the adsorbent is large. (2) Microporous structure developed, pore volume, adsorption capacity is 1.5 ~ 10 times of ordinary granular activated carbon, so it has a very high adsorption efficiency. (3) The surface contains a large number of active functional groups, so that it has a strong REDOX ability, can play a catalytic role in some reactions.(4) Good molding, not easy powder, can be made into felt, cloth, paper, line and other forms, convenient for different uses and needs. (5) Acid resistance, alkali resistance, good electrical conductivity and heat resistance.

1.2.2 Performance of activated carbon fiber

🍐(1) Adsorption performance The structure of ACF is a non-polar adsorbent, and its specific surface area and pore size structure are the key factors affecting the adsorption performance. Compared with the general activated carbon, ACF has many excellent adsorption properties. ① Adsorption capacity of large ACF adsorption capacity can reach several times or even dozens of times of the traditional granular activated carbon, not only for inorganic, organic gas has a good adsorption capacityThe adsorption capacity of inorganic compounds, organic dyes and organophosphorus compounds in aqueous solution can also reach 5 times that of activated carbon. In addition, it also has a good adsorption effect on some microorganisms and bacteria, such as the adsorption rate of Escherichia coli up to 94% ~ 99%. ② Strong adsorption force in surface adsorption, the smaller the pore size, the larger the adsorption force field. Due to the narrow micropore of activated carbon fiber, its adsorption force field has a large holding effect, so that the adsorption capacity is significantly enhanced, so it is stronger than the ordinary adsorption material for the adsorption of low concentration substances, even for the order of 10-6 low concentration adsorbent still has a high adsorption capacity. ③ The adsorption speed is fast for the adsorption of gas can generally reach equilibrium in tens of seconds or minutes, and the adsorption of liquid only takes a few minutes to dozens of minutes to reach equilibrium. Similarly, because the fiber is thin, the outer surface is easy to be heated, so the desorption speed is also very fast. 

(2) REDOX properties ACF surface has REDOX properties due to a series of oxygen-containing functional groupsThis property is manifested by its ability to reduce some metal ions with high electrode potential adsorbed from aqueous solution to zero or low valence and enrich on the surface of ACF. For example, the reduction in aqueous solution can be used for the enrichment, recovery and smelting of trace precious metals.

🍊(3) Conductivity ACF, like carbon fiber, has excellent conductivity and can be used as an electrode to degrade organic pollutants in wastewater. Because the enrichment of organic matter by ACF is beneficial to eliminate the concentration polarization effect, the electrolytic rate and degradation efficiency can be effectively improved. (4) Catalytic properties ACF also has gas-phase oxidation and catalytic reduction properties, which can reduce nitric oxide to nitrogen in the presence of ammonia gas. If the surface is loaded with other metal catalysts, the catalytic effect is more significant.

1.3 Preparation of activated carbon fiber

The main types of raw materials for ACF preparation are viscose, phenolic, asphalt, polyacrylonitrile, polyvinyl alcohol, polystyrene, lignin (coke wood) and natural plant fiber substrates (sisal, hemp, flax, jute), etc. Among them, the first four kinds of preparation technology are mature and have been large-scale production in industry. The characteristics of ACF prepared from various raw materials are shown in Table 2-2.

🍋Using different fiber materials, the specific conditions of ACF preparation are also different, but the basic process flow generally includes three main links: pretreatment, carbonization and activation.

1.3.1 Pretreatment

🍌Pretreatment includes salt or alkali impregnation and pre oxidation. Salt or alkali impregnation is to fully immerse raw fiber in salt or alkali (phosphate, sulfate, ammonium salt or alkali, etc.) solution, and then dry it. In the process of impregnation, salt or alkali molecules are immersed into the raw fiber, which can play a role in swelling, catalytic dehydration or cross-linking, preventing the fragmentation and escape of fiber molecules during heat treatment, thus improving the strength, yield and adsorption performance of activated carbon fibers. Pre oxidation is to place the raw fiber in an oxygen atmosphere and slowly pre oxidize it for a certain time or according to a certain heating program within a certain temperature range. After the raw fiber is pre oxidized, the linear polymer chain in the raw fiber will undergo oxidation, dehydrogenation, cyclization and other reactions to transform into a heat-resistant and stable trapezoidal structure, so that the fiber is not easy to melt and deform in the process of high-temperature carbonization, still maintains the shape of the fiber, and improves its yield after carbonization and activation.

1.3.2 Carbonization

🍉Carbonization is the most important link in the production of ACF. It mainly refers to the process of heating the raw fiber in an inert gas (such as nitrogen or argon) environment for a certain time, removing most of the non carbon components in the raw material, and rearranging the remaining carbon elements into carbon fibers similar to graphite microcrystalline structure by thermal condensation polymerization. After carbonization, the raw fiber becomes a carbon material with a certain mechanical strength and an initial pore structure suitable for activation, which plays a decisive role in the production of ACF, and also affects the subsequent activation reaction, thus directly affecting the structure and performance of ACF. The main influencing factors of carbonization process include carbonization temperature, heating rate, carbonization time, carbonization atmosphere and fiber tension control.

1.3.3 Activation

🍇Activation is to etch carbonized fibers with oxidizing gas at high temperature, and make ACF form developed microporous structure or expand pore diameter through surface treatment, thereby regulating its specific surface area and surface oxygen functional groups. The activation process is complex, but the basic principle is that the activated molecules react with the carbon atoms on the fibers to form rich micropores and surface oxygen-containing functional groups. The activation methods include physical activation, chemical activation, chemical physical joint activation, etc.

(1) Physical activation method

🍓The method mainly used in the pre industry uses oxygen, water vapor or carbon dioxide as the activator to etch and oxidize the disordered carbon part of the raw fiber into holes at a high temperature of 700~1000 ℃. Some studies suggest that physical activation can be divided into three basic processes: the gasification of non graphite carbon and heteroatoms; Reaction of graphite carbon; Reforming of graphite layer. First, the amorphous carbon blocking the hole is gasified and the hole is opened; Further activation, the carbon atoms on the surface lattice, dislocation and edge react at different rates to form gas leaving the surface, resulting in new holes; During deep activation, the pore is further widened 

(2) Chemical activation method

🍈Chemical activation method is a method that uses potassium hydroxide, sulfuric acid, phosphoric acid, ammonia or zinc chloride and other chemical substances as the activator to make raw fibers contact with the activator through immersion and mixing and living reaction to form pores. Compared with the physical activation method, the activator in the chemical activation method can make the hydrogen and oxygen in the raw fiber escape mainly in the form of water vapor, inhibit the generation of by-product tar, thus increasing the yield of ACF and Its porosity and specific surface area. In addition, the chemical activation method can also reduce the carbonization and activation temperature of the raw fiber. However, chemical activation method is easy to cause environmental pollution and the product strength is poor. 

(3) Chemical physical joint activation method

🍒Chemical physical joint activation method is an activation method that combines chemical activation with physical activation. Usually, chemical activation is carried out first and then physical activation is carried out. Because the two methods for preparing ACF are complementary in terms of process complexity, cost, and ability to control pore structure, the combination of the two methods can flexibly control the pore structure of ACF, and even prepare ACF containing only micropores or only mesopores. In addition, in addition to the activation method, the type of activator, activation temperature, activation time and concentration of activator are also key factors affecting the activation process. The structure and properties of ACF are directly affected by the activation conditions and degree.

1.4 Functionalization of activated carbon fiber

🍑The pore structure has a direct impact on the physical adsorption performance of ACF. Different pore structures will lead to different selective adsorption, adsorption capacity and adsorption rate. The surface oxygen-containing active functional groups of ACF will directly determine the catalytic performance. In order to give full play to the adsorption and catalytic properties of ACF, the pore structure, specific surface area and surface characteristics of ACF need to be adjusted and modified, that is, functionalization.

1.4.1 Aperture adjustment

🥭The pore structure adjustment is generally carried out in the carbonization and activation stages of ACF, including increasing the pore volume and specific surface area, increasing the proportion of micropores, and creating uniform pore size. The pore diameter of the carbon adsorbent and the molecular size of the adsorbate are adjusted to an appropriate proportion by adjusting the pore diameter to obtain the best adsorption effect. The commonly used adjustment methods are as follows: Due to the decomposition of hydrophilic oxygen-containing functional groupsACF has good hydrophobicity. 

(1) Under the heating condition of carbon deposition method, ACF is in contact with hydrocarbon gas. As the carbon generated from hydrocarbon pyrolysis is deposited on the pore wall, the pore diameter is reduced. Therefore, proper pore diameter can be obtained by controlling the process conditions. The hydrocarbon organics used include methane, acetylene, isobutene, benzene, toluene and other hydrocarbons.

🍍(2) The metal compound catalytic activation method adds metal compounds to the ACF to increase the internal active points of the ACF micropores. When activated, the metal atoms selectively vaporize the highly crystalline carbon atoms, so that the micropores expand into mesopores. Generally, carbon atoms around metal atoms preferentially oxidize to form mesopores in fiber materials. ACF has good hydrophobicity.

(3) Under the heating condition of carbon deposition method, ACF is in contact with hydrocarbon gas. As the carbon generated from hydrocarbon pyrolysis is deposited on the pore wall, the pore diameter is reduced. Therefore, proper pore diameter can be obtained by controlling the process conditions. The hydrocarbon organics used include methane, acetylene, isobutene, benzene, toluene and other hydrocarbons. 

🥝(4) The metal compound catalytic activation method adds metal compounds to the ACF to increase the internal active points of the ACF micropores. When activated, the metal atoms selectively vaporize the highly crystalline carbon atoms, so that the micropores expand into mesopores. Generally, carbon atoms around metal atoms preferentially oxidize to form mesopores in fiber materials.

In addition, there are also methods for adjusting the pore structure, such as evaporation and plating, catalytic activation of organic compounds, etc.

1.4.2 Surface modification

🎄(1) Oxidation method：The oxidation method mainly uses strong oxidants to oxidize the surface groups of ACF under appropriate conditions to increase the oxygen containing groups on its surfaceStrong surface polarity. There are three main methods of oxidative modification: gas phase method, liquid phase method and electrochemical method. The gas phase method is to increase the oxygen containing functional groups on the surface of ACF by reacting with oxidizing gases such as O2 or O3 at higher temperatures. Liquid phase method is to use strong oxidizing liquids (nitric acid, sulfuric acid, etc.) to react with ACF for oxidative modification, and nitric acid modification is the most studied. In electrochemical method, ACF is used as an electrode in the electrolyte to react with ions in the solution through its excellent adsorption performance and high surface catalytic oxidation performance. Basova et al. used ammonium persulfate as electrolyte solution. After electrochemical oxidation of polyacrylonitrile based ACF at 50 ℃, oxygen containing groups were produced on the surface of ACF, mainly including hydroxyl, carbonyl and carboxyl groups.

(2) The surface loading method is used to load metal compounds to adsorb oxidized metal ions (Au3+, Hg2+, etc.) on the surface of ACF, and then use the reducibility of ACF to reduce metal ions into simple or low valent ions, so as to increase the adsorption performance of ACF through the strong binding force of metal or ions on the adsorbate. Load removal In addition to the generic compounds, the surface of ACF can also be modified with organic compounds and inorganic molecules.

🌲(3)The plasma treatment method generates a large amount of plasma through corona discharge in the gas medium, and uses these high-energy plasma to impact the material surface, so as to change the physical and chemical properties of the material surface without damaging the material surface characteristics, thus improving the material specific surface area, pore diameter, pore volume, surface functional groups and other related properties.

(4)Microwave modification by microwave irradiation can decompose the oxygen-containing acidic groups (hydroxyl, carbonyl) on the surface of ACF in a short time, and introduce basic groups such as pyrrolidone on the surface of ACF to increase its surface pH value and enhance its chemical stability. This method is energy-saving, time-saving and efficient. In addition to the generic compounds, the surface of ACF can also be modified with organic compounds and inorganic molecules.

1.5 Application of activated carbon in supercapacitors

🌳Activated carbon is the earliest carbon electrode material used in supercapacitors. Due to its abundant raw materials, low price and high specific surface area, it is still the first choice for commercial supercapacitors. Since Beck proposed to use activated carbon as electrode of double-layer capacitor in 1954, the application of activated carbon in supercapacitor has attracted much attention. According to the different source of raw carbon, the application of activated carbon in supercapacitors is briefly introduced.

1.5.1 Phenolic activated carbon fiber

🌴Phenolic ACF has become an ideal electrode material for supercapacitors due to its high carbonization yield, large pore size, good conductivity and high strength. 

In 1985, Panasonic Electric Company of Japan used phenolic resin activated carbon fiber with an average pore diameter of 2.5 nm for the preparation of electric plate materials for electric double-layer capacitors, which greatly improved the quality of the capacitors produced by the company. Zhang Yuqin et al. prepared phenolic ACF from phenolic fiber by KOH activation. 

☘️The results show that 900 ℃ is the best temperature for KOH to activate phenolic fiber, and the sample has the best circulation, small internal resistance, specific surface area and specific capacitance. Although the products show different specific surface area and specific capacitance at different activation temperatures, their overall pore size distribution is basically the same. With the increase of activation temperature, the capacitive performance and power characteristics of the sample are improved,The internal resistance is reduced. 

Yoshida et al. studied the relationship between the surface acidic functional groups of phenolic ACF in organic electrolyte and the electrochemical performance of electric double-layer capacitors. The research shows that acidic functional groups such as surface carboxyls, lipids and phenolic hydroxyl groups can cause leakage current of capacitors. Heat treatment under nitrogen at 1000 ℃ can effectively reduce the surface acidic oxygen-containing functional groups. Compared with asphalt and cellulose based ACF, phenolic based ACF after heat treatment has the lowest content of acidic oxygen-containing functional groups, the highest capacitance and the minimum leakage current.

1.5.2 Polyacrylonitrile based activated carbon fiber

🍀Xu et al. carbonized polyacrylonitrile based carbon fiber cloth at different temperatures, and then activated it with CO2 at 900 ℃ to obtain a series of ACF with different specific surface area and pore size distribution. It is found that the specific surface area and pore structure of ACF obtained at 600 ℃ carbonization are most suitable for supercapacitors, and the specific capacitance can still be obtained at the current density of, with high power characteristics. Li Ying took polyacrylonitrile as the precursorACF with large/mesoporous structure of in situ nitrogen rich hierarchical three-dimensional network was prepared by a new method combining wet spinning with KOH activation. This hierarchical pore ACF has high specific surface area, large pore volume and (mass fraction) high nitrogen atom content. Used in supercapacitors, it shows high energy density of high specific capacitance and excellent multiplying performance.

1.5.3 Asphalt based activated carbon fiber

🍃Li Haiyan prepared mesoporous ACF from general pitch carbon fiber by different activation processes, and compared the effects of cobalt salt immersion, primary activation and secondary activation on its specific surface area and pore structure. The results showed that the secondary activation energy increased the specific surface area and the mesopore ratio. Cobalt salt impregnation plays an obvious catalytic role in the activation process, but when the activation degree is too large, the addition of drilling salt will reduce the specific surface area and pore diameter of ACF.

1.5.4 Plant fiber based activated carbon fiber

🍁Liu Fengdan et al. used natural plant fiber ramie as raw material and ZnCl2 chemical activation method to prepare ACF at different activation temperatures. It was found that the specific surface area decreased with the increase of activation temperature. ACF supercapacitor activated at 650 ℃ has a specific capacitance of up to at constant current discharge, and has lower internal resistance, better power characteristics and longer cycle life.

The post Supercapacitor Material – Activated Carbon Fiber appeared first on Dongguan Xuanxuan Electrolytic Technology Co,.ltd.

🌱As one of the most abundant elements on earth, carbon plays a vital role in human life and social development. Located in group ⅣA in the second period of the periodic table, the outermost four valence electrons can form single bonds by sp3 hybridization, double bonds by sp2 hybridization and triple bonds by sp hybridization. Various bonding properties give carbon diverse existence forms. 

In terms of simple matter, carbon allotropes include three-dimensional diamond, two-dimensional graphite lamellae and graphene, one-dimensional carbine and carbon nanotubes, zero-dimensional fullerenes and quantum dots, and other forms with completely different structures and properties. 

🌿Carbon materials have various structures and different properties. It can be said that carbon materials almost include the characteristics of all substances on the earth, which makes carbon materials widely used. Among the electrode materials for supercapacitors, the earliest and most mature research technology is carbon materials.

Its research started from the relevant patent published by Beck in 1957, and its development has been more than 60 years. The reason why carbon materials become the first choice for the preparation of supercapacitor electrodes is that they usually have the following characteristics: (1) chemically inert, stable in various acid and alkali solutions, and do not react with electrodes; (2) Large specific surface area, well-developed pore structure and high porosity, which can adsorb a large amount of electrolyte solution;（3）High purity, good conductivity, small leakage current; (4) High thermal stability, stable performance in a wide temperature range; (5)Еasy to process into various shapes of electrodes; (6)The price is low and the source is abundant. 

☘️At present, the most studied electrode materials for supercapacitors mainly include activated carbon, active carbon fiber, carbon aerogel, carbon nanotube and graphene. 

Activated charcoal 

Activated carbon The industrial production and application of activated carbon has a long historyDue to its rich pore structure and huge specific surface area, it has strong adsorption and catalytic properties, and is widely used in environmental protection, chemical industry, energy, medicine and other fields. Activated carbon is the earliest carbon electrode material used in supercapacitors. 

1 The structure of activated carbon-Graphene supercapacitors

🍀Structure of activated carbon Activated carbon is a kind of amorphous carbon, which is composed of graphitized activated carbon microcrystal and non-graphitized amorphous carbon. The graphite microcrystal, which is similar to the two-dimensional structure of graphite, is a planar network structure formed by the hexagonal ring formed by sp2 mixed with hybrid carbon. These graphite microcrystals have a small particle size (about 1 ~ 3nm) and are arranged in an irregular and loose arrangement, which is referred to as “spiral layer structure” or “chaotic layer structure”, as shown in Figure 2-1.

Because the arrangement of graphite microcrystals is irregular and disordered, the pores of different sizes and shapes are formed between microcrystals and amorphous carbonites and between microcrystals, including open pore shape, semi-obturator pore shape and mesenchymal cage shape. According to the size of pores, it can be divided into: large pores (pore diameter greater than 50nm) are called supply or transport pores, accounting for a small proportion, weak adsorption performance, mainly play a role in providing channels for the entry of adsorbent molecules, which plays an important role in the adsorption speed; Mesopore (pore diameter between 2 and 50nm) is also called mesopore. 

🎍On the one hand, mesopore has the same function as macropore, which can act as a channel for adsorbent molecules to enter the micropore. On the other hand, mesopore can adsorb macromolecular substances that cannot enter the micropore. Micropores (pore diameter is less than 2nm), also known as adsorption pores, activated carbon more than 90% of the specific surface area and pore capacity are from the contribution of micropores, micropores for gas and liquid small molecules have a strong adsorption effect, to a large extent determines the adsorption performance of the entire activated carbon. The three kinds of pores are staggered and crossed to form a tree structure, as shown in Figure 2-2.

In addition, the pore size structure of activated carbon has different relationship with the molecular scale of adsorbent, and the adsorption state is also different. When the pore size of activated carbon is much smaller than the molecular diameter of adsorbent,Molecules can not enter the pore, activated carbon does not adsorption; When the pore size of activated carbon and adsorbent molecular diameter is similar, activated carbon on the molecular adsorption capture ability is the strongest, even if the concentration of low molecules can also be adsorbed; When the pore size of activated carbon is larger than the molecular diameter of adsorbent, the molecular capillary condensation occurs in the pore, which can increase the adsorption capacity. When the pore size of activated carbon is much larger than the adsorbent diameter, the molecules are easy to be absorbed, but also easy to occur desorption, so that the final adsorption amount is small. Therefore, only when the pore size of activated carbon and adsorbent molecules match each other can the adsorption process be carried out effectively.

2 Performance characteristics of activated carbon

🎋Activated carbon, as the earliest and most widely used electrode material for supercapacitors, has the following performance advantages. （1）Large specific surface area, the theoretical specific surface area of activated carbon is 500 ~ 3000m2·g-1, which is made into a supercapacitor electrode. The theoretical specific capacitance of a single electrode can be as high as 500F·g-1, but the actual specific capacitance is far less than this A value. （2）The pore structure is developed, and the number of pores is about 1020 ·g-1, which can absorb a large number of electrolyte solution molecules. (3) High chemical stability, no chemical reactions, not easy to be acid, alkali and other solutions corrosion. (4) High purity, good electrical conductivity, good thermal stability. （5）Easy to process, good compatibility with other materials. （6）The price is low and the source is abundant.

3 Preparation of activated carbon

3.1 Prepare raw materials

🍃Raw materials for the preparation of activated carbon There are abundant sources of raw materials for the preparation of activated carbon. Generally, as long as the materials rich in carbon can be used as raw materials for the preparation of activated carbon. According to the source of raw materials can be divided into plant raw materials and mineral raw materials two categories. Among them, plants have a wide range of raw materials, in addition to traditional wood,Coconut shell, walnut shell, apricot kernel, olive kernel, rice husk, etc., as well as agricultural and forestry by-products and waste living carbon, such as wood chips, bark, bamboo, cotton stalk, peanut shell, waste plastic, urban garbage, etc. At present, it is generally believed that the fruit shell is the best raw material for the preparation of activated carbon, which has high strength and very fine micropores, but the fruit shell resources are limited and not easy to concentrate storage. Mineral raw materials include coal measures raw materials and petroleum raw materials. Due to the rich, cheap and easy availability of coal resources, the coal measures raw material is the main raw material for preparing activated carbon in a long period of time. Petroleum raw materials mainly refer to carbon-containing products and wastes in the process of petroleum refining, such as asphalt, oil residue, petroleum coke, etc. Petroleum coke has the advantages of high carbon content, low ash content and good electrical conductivity, which is suitable for the preparation of activated carbon raw materials.

3.2 Preparation Methods

The preparation of activated carbon is generally divided into two steps: carbonization and activation. Carbonization refers to the heating of raw materials to a certain extent under conditions of air isolation or protection from inert gasesTemperature, so that the volatile non-carbon components in the raw material decomposition discharge. The whole process can be divided into four stages according to the temperature change: 

🍂The first stage is the drying process, the temperature is 120 ~ 150℃, mainly to remove the water evaporation in the raw material, the temperature is not high, the chemical composition of the raw material does not change; The second stage is the pre-carbonization process, when the temperature rises to 150 ~ 275℃, the thermal decomposition of raw materials is obvious, the chemical composition of materials begins to change, the internal structure recombines, and some unstable components begin to decompose. The third stage is the carbonization process, which is the key link of the carbonization of activated carbon. The reaction temperature reaches about 400℃, and the rapid decomposition of raw materials produces a large number of gases and liquids. The fourth stage is calcination process, the system temperature reaches 500℃, the raw material is further calcined, a small amount of residual volatile substances are discharged, and the carbon material with increased fixed carbon content is obtained. The essence of carbonization is the process of thermal decomposition and thermal condensation of organic matter in raw materials, among which the carbonization temperature, carbonization time, heating rate and other parameters are important factors affecting the quality of carbonization products, and will also have a certain impact on the subsequent activation process.

Activation process is the most critical step in the preparation of activated carbon, which can effectively regulate the specific surface area and pore structure of activated carbon. The activation process is a complex chemical reaction between activator and carbon material. This process can be divided into three main stages: the first stage is at high temperature, the initial pores blocked by disordered carbon atoms and heteroatoms under the action of activator are opened and further expanded, which is called transverse pore expansion; In the second stage, the unsaturated carbon atoms at the edge of the newly opened pores further react with the activator, making the pores develop continuously to the depth, and achieve the merger or connection between pores. In the third stage, new unsaturated carbon atoms or active spots are exposed to the microcrystalline surface, and the uneven combustion of the microcrystalline surface leads to the formation of a large number of new pores. Changing the temperature, time, gaseous environment and other conditions of activation reaction can regulate the porosity, pore size distribution and inner surface properties of activated carbon to a certain extent. 

🍁At present, the commonly used activation methods include physical activation method, chemical activation method, physical-chemical combined activation method and other activation methods.

(1) Physical activation method physical activation, also known asFor gas activation or thermal activation, carbon materials and water vapor, carbon dioxide, oxygen, air and other gases with oxidation characteristics are heated at a high temperature of 600 ~ 1200℃ for activation. Its essence is the oxidation reaction between the unsaturated carbon atoms in the raw material located at the corner of the microcrystal or the defect of the base surface and the oxidizing gas, which eliminates the residual volatile pyrolysis products in the carbon material and greatly increases the pore volume and the specific surface area of the obtained activated carbon material. 

🍄The main advantages of the physical activation method are simple production process, less pollution, and the product can be directly used without cleaning. However, the activation temperature of the method is higher, the activation time is longer, the energy consumption is high, and the activated carbon pore is not developed enough, and the specific surface area is low. 

(2) Chemical activation method Chemical activation method is a method of mixing thick solutions containing carbon raw materials and chemical reagents, stirring them evenly, heating up, pyrolysis, cooling, and continuous washing with detergent to remove activators. FIG. 2-3 shows its technological process. Its essence is through the dehydration, expansion and skeleton of chemical reagents on raw materials, so that the two are gas activation or thermal activation. It is the method of heating carbon materials and water vapor, carbon dioxide, oxygen, air and other gases with oxidation characteristics at the high temperature of 600 ~ 1200℃ for activation. Its essence is the oxidation reaction between the unsaturated carbon atoms in the raw material located at the corner of the microcrystal or the defect of the base surface and the oxidizing gas, which eliminates the residual volatile pyrolysis products in the carbon material and greatly increases the pore volume and the specific surface area of the obtained activated carbon material. The main advantages of the physical activation method are simple production process, less pollution, and the product can be directly used without cleaning. However, the activation temperature of the method is higher, the activation time is longer, the energy consumption is high, and the activated carbon pore is not developed enough, and the specific surface area is low. 

💐(3) Chemical activation method Chemical activation method is a method of mixing thick solutions containing carbon raw materials and chemical reagents, stirring them evenly, heating up, pyrolysis, cooling, and continuous washing with detergent to remove activators. FIG. 2-3 shows its technological process. Its essence is through the chemical reagents to the raw material dehydration, moistening swelling and skeleton action, so that bothA series of polycondensation and cross-linking reactions take place, thereby releasing part of the carbon atoms in the raw material, and at the same time releasing hydrogen and oxygen in the form of water vapor, forming a large number of pores.

In the chemical activation method, common activators are zinc chloride (ZnCl2), phosphoric acid (H3PO4), potassium hydroxide (KOH), sodium hydroxide (NaOH), calcium chloride (CaCl2) and so on. The action process of these activators on carbon raw materials is different, but the action mechanism is similar. Through the addition of activators, part of carbon, hydrogen and oxygen contained in the raw materials are decomposed and separated in the form of carbon dioxide, carbon monoxide and water vapor, and the carbonization temperature is significantly reduced at the same time.

🌷（1）ZnCl2 activation method ZnCl2 activation method is one of the earliest chemical activation methods for the preparation of activated carbon, its strong dehydration effect makes the activation temperature significantly reduced, generally in 500 ~ 700℃. The activation mechanism is that ZnCl2, as a Lewis acid, interacts with oxygen-containing functional groups, releasing hydrogen and oxygen in the carbon raw material in the form of water vapor, leading to the aromatization of carbon chains to form pore structures, and changing the thermal decomposition process of raw materials to inhibit the generation of tar. Due to the reaction temperature lower than 700℃, ZnCl2 in the form of liquid evenly distributed in the activated carbon, when the water to remove ZnCl2 washing, the formation of developed micro pores, but also caused the removal of ZnCl2 consumption, high activation cost, and pollution to the environment. 

(2) KOH activation method KOH is one of the most representative alkali activators, AMOCO research found that adding KOH to coal or petroleum coke, activation can be obtained after the high specific surface area of 2500m2·g-1 activated carbon. The activated carbon product obtained by this methodThe distribution of micropores is concentrated, the pore structure is uniform and developed, and the specific surface area is large, which has attracted a lot of attention from scholars at home and abroad in recent years. The specific process is to add alkali to the raw material in accordance with a certain proportion of mixing, after grinding and mixing evenly, in inert gas or closed system heating to 700 ~ 800℃ carbonization, activation, that is, to get a large number of cage-like microporous structure of activated carbon. According to the temperature of the system, the whole activation process is divided into four stages [1] : in the first stage, low temperature dehydration (<300℃), the attached water on the surface of the raw material and the combined water generated by the reaction overflow in the form of steam; The second stage, pre-activation (300 ~ 500℃), the production of water vapor and carbon dioxide, carbon monoxide and other gases and volatilization; In the third stage, the molecules are activated at moderate temperature (500 ~ 600℃), crosslinking or polycondensation reaction occurs, and some non-carbon elements are volatilized. In the fourth stage, KOH is almost completely transformed into K2CO3 and K2O by high temperature activation (>600℃). These two compounds are further reacted with carbon materials to generate highly active potassium. When the temperature is higher than 762℃, potassium diffuses in gas state to form vertical The final activated carbon product has a large number of micropores and high specific surface area. The main chemical reactions that occur during the whole process are as follows:

🌹In the process of KOH activation, there are many factors that affect the performance of products, and the main factors are as follows. 

a. Agent to material ratio: that is, the selection of activator KOH and carbon containing raw material between the mass ratio, it on activated carbon products Performance has a significant impact. When the ratio of KOH to carbon material is low, the activation reaction is not sufficient, and the products formed have fewer pores and lower specific surface area.When the alkali-carbon ratio is too large, the excess KOH will cause the excessive reaction of activated carbon, so that some of the formed better pores collapse into series of large pores, so that the product’s specific surface area and pore volume are reduced. Therefore, according to the material and particle size of carbon raw materials and specific process to choose the appropriate ratio of agent to material.

🥀b. Particle size of raw material: particle size of raw material directly affects the full degree of contact between raw material and activator. Under the same conditions, the smaller the particle size of the raw material, the more developed the pore structure of the product, but too small the particle size will bring difficulties to sample preparation and filtration, and affect the yield. 

c. Addition method of KOH: The addition method of KOH mainly includes simple incorporation method and impregnation method. The simple mixing method is to mix KOH powder and carbon raw material, which is simple to operate but has low yield, small product specific surface area and pore capacity. The impregnation method starts with KOH After a certain concentration of solution is prepared, the carbon raw material is added to the solution, and KOH is adsorbed to the surface and internal pores of the raw material through impregnation. This addition method can make the activator contact with the carbon material more fully, with a low loss rate and a larger specific surface area of the product. 

🌺d. Activation temperature: Theoretically, the higher the temperature, the higher the molecular activation energy and the higher the reaction degree, the larger the specific surface area of the product, and the phenomenon of pore expansion will also occur, but the yield will also be reduced.

e. activation time: some conditions, as the activation time of rights, product yield decreased, and the specific surface area, pore volume increased, but when the activation time after reaching a certain value, due to the reaction of generated microporous was further collapse into the holes or big hole, no longer increases the specific surface area and pore volume, it has reduced. In addition, the operation of KOH active agent washing and product drying at the end of the process also has a great influence on the performance of activated carbon. The specific surface area of activated carbon prepared by alkaline activation method is relatively high Large, short activation time, more mature process; However, alkali itself is corrosive to equipment, difficult to recover, in addition to the high activation temperature, energy consumption and other shortcomings, so there are still many problems in large-scale industrial production. 

🌸(3) H3PO4 activation method H3PO4 activation method is one of the most commonly used methods for the preparation of activated carbon because of its light pollution to the environment and low production cost. Its activation mechanism is similar to that of ZnCl2. H3PO4 plays the following roles in the activation process.

a. Dehydration: in the absence of H3PO4, most of the hydrogen and oxygen volatilized by organic matter, while in the form of water vapor removal under the action of H3PO4, more carbon can be retained and the yield increased.

🌼b. Swelling effect: at low temperature, H3PO4 penetrates into the raw material, accelerates the swelling and dissolution of cellulose and lignin in the raw material through ionization, and promotes subsequent hydrolysis and oxidation reactions.

🌻c. Oxidation: H3PO4 has certain oxidation ability. Above 200℃, H3PO4 forms pyrophosphate network structure through cross-linking polycondensation reaction. Pyrophosphate has strong corrosion and oxidation properties, and further oxidizes carbon materials to form more micropores and mesoporous materials.

🌞d. Aromatic condensation: When the temperature continues to rise, polycondensation carbon structure is formed by polycondensation, cyclization and cross-linking of phospholipid bond with organic matter or other polymers, which can be activated and transformed into random layer microcrystalline structure of carbon at appropriate temperature. 

In short, in the whole activation process, H3PO4 can promote the pyrolysis reaction process, reduce the activation temperature, prevent the particle shrinkage under high temperature conditions, reduce the formation of tar. After washing and removing phosphate, activated carbon products with developed pore structure can be obtained. The products prepared by phosphoric acid activation method have wide pore size distribution and well-developed mesoporous pores. Phosphoric acid itself has low corrosion and less environmental pollution. The activated carbon produced is uniform and stable.Good settling performance, wide application areas. 

🌝(3) physical-chemical combined activation method Physical-chemical combined activation method is to combine the advantages of physical activation and chemical activation, the first use of simple physical activation and chemical activation of the secondary activation method. However, this method still cannot overcome some adverse factors, and additional steps are added. At present, the composite activation technology of chemical impregnation and physical activation is mostly adopted. The activated carbon materials with excellent adsorption performance and reasonable pore size distribution can be prepared by controlling factors such as the quality ratio of the activator raw material, impregnation time, activation temperature and activation time. Hu et al. impregnated with ZnCl2 and activated coconut husk with CO2 at high temperature to prepare a series of mesoporous activated carbon with controllable pore structure. Zhang Wenhui et al. took coal-based carbon source as raw material, impregnated the sample with KOH and activated it with water vapor, and obtained the product with a specific surface area greater than 1500m2·g-1 and good adsorption performance in a relatively short time.

4 Activated carbon modification

🌛With the increasingly high performance requirements for carbon-based materials, simple carbonization, activation process has been difficult to meet, therefore, the late regulation and modification of activated carbon technology has been paid more and more attention. The modification of activated carbon includes two aspects: the first is the surface structure modification, which refers to the physical or chemical method to increase the specific surface area of activated carbon material and adjust the pore structure and distribution of activated carbon in the preparation process, so that the pore structure of activated carbon is changed, so as to change its adsorption and energy storage performance. Second, the surface chemical properties of activated carbon modification, is through a certain method to change the type and number of functional groups on the surface, the surface of the heteroatom and its surrounding atmosphere structure, so that the active site increase, so as to control its binding ability with the adsorbed. At present, the research on the late preparation technology of activated carbon modification and modification has attracted much attention. According to the different principles and characteristics of the technical treatment basis, the modification technology can be divided into the following types.

4.1 Heat treatment method

🌟Heat treatment refers to the process of heating activated carbon at high temperature under certain conditions. Through heat treatment, the initial pore size, pore volume and functional groups on the surface of the original carbon material can be changed, so as to obtain the activated carbon material with developed pores, low oxygen and oxidation resistance. Kim heat-treated activated carbon in high temperature and nitrogen environment to obtain activated carbon containing pyrrole nitrogen on the surface, which greatly improved the hydrophilicity and wettability of activated carbon. In addition to the ordinary heating method, the method of microwave heating modified activated carbon has many advantages, and has been paid more and more attention by researchers. Microwave heating mainly causes the shrinkage of carbon skeleton through rapid and efficient thermal action, which leads to the change of pore size and pore volume. In addition, the use of microwave heat treatment of activated carbon under different atmospheres will affect the properties of its surface groups, such as oxidizing atmosphere is conducive to the formation of acidic groups, reducing atmosphere is conducive to the formation of basic groups.

4.2 Surface oxidation method

✨Under appropriate conditions, the surface of activated carbon is oxidized by using oxidants to remove some impurities on the surface and improve the content of oxygen-containing functional groups (such as carboxyl group, phenolic hydroxyl group, ester group, etc.) on the surface, which can improve the infiltration of carbon surface and also play a certain role in pore reaming. Oxidizing modification of the commonly used oxidants are nitric acid, hydrogen peroxide, sulfuric acid, ozone, ammonium persulfate, etc., the use of oxidants are different, the number and type of oxygen-containing functional groups of the products are different. In addition, the pore structure, specific surface area, volume and pore size of the modified activated carbon will also change. Nitric acid is the most commonly used strong oxidizing agent, and related studies have been widely reported. Dubi et al. used coconut shell activated carbon and apricot shell activated carbon as raw materials and used concentrated nitric acid surface modification to make electrodes for supercapacitors, and the discharge specific capacitance increased significantly. Ran Longguo et al. treated the activated carbon with nitric acid at different concentrations, and the specific surface area of the activated carbon after 10% nitric acid treatment was as high as that of the activated carbon

4.3 Surface reduction method

⚡️Surface reduction modification refers to the reduction of functional groups on the surface of activated carbon with appropriate reducing agent at appropriate temperature, so as to increase the content of basic functional groups on the surface of activated carbon and enhance its non-polar surface, so as to improve its adsorption capacity for non-polar substances. The commonly used reducing agents are hydrogen, nitrogen, sodium hydroxide, potassium hydroxide and ammonia. Hydrogen or ammonia is the most commonly used method to prepare alkaline activated carbon. At 400-900 ℃, amides and aromatic amines can be generated on the surface of activated carbon after ammonia is added, and pyridine substances can be generated at higher temperatures. These nitrogenous functional groups will enhance the alkalinity of activated carbon surface. In addition, higher content of nitrogenous functional groups can be obtained by impregnating activated carbon in ammonia. Huang et al. found that the nitrogen content in the samples increased significantly after the modification of activated carbon by ammonia infiltration.

4.4 Load atom method

☄️The supported atom modification method is to impregnate the activated carbon in a certain solution, make use of the huge specific surface area and pore volume of activated carbon, and introduce metal ions or other heteroatoms into the activated carbon pore by liquid deposition method, so as to increase the adsorption effect of activated carbon on the adsorbent. The metal ions commonly used for loading are copper ions, iron ions, aluminum ions and silver ions, etc. The activated carbon after loading shows good potential in adsorption of fluoride ions, cyanide and heavy metals and other pollutants

4.5 Low-temperature plasma method

🌈Plasma modification is a process of using non-cohesive plasma gas to modify the material surface. A plasma is an aggregate state of matter with a sufficient number of positive and negative charged particles of approximately equal charge. The low temperature plasma used for activated carbon modification is mainly produced by corona discharge, glow discharge and microwave discharge. The most commonly used is oxygen plasma, which has strong oxidation, when the plasma hits the surface of carbon materials, it can oxidize the crystal Angle, crystal edge and other defects or double bond structure into oxygen-containing functional groups. Modified by low-pressure oxygen and nitrogen plasma, activated carbon with a surface rich in nitro, amino and acyl amino groups can be obtained. Low temperature plasma modification technology is easy to operate, mild reaction conditions, low price, good environmental safety, and the treatment effect is only limited to the surface without affecting the properties of the material. In addition to the above modification methods, activated carbon modification methods also include surface acid-base modification method, ozone oxidation method, microwave radiation method, organic matter grafting method and so on. These methods have their own characteristics, and can be used alone or combined to modify activated carbon, so as to achieve better modification effect. In the process of use, suitable modification methods should be purposefully selected according to the properties of adsorbent.

5 Application of activated carbon in supercapacitors-Graphene supercapacitor

☀️Activated carbon is the earliest carbon electrode material used in supercapacitors. Due to its abundant raw materials, low price and high specific surface area, it is still the first choice for commercial supercapacitors. Since Beck proposed to use activated carbon as electrode of double-layer capacitor in 1954, the application of activated carbon in supercapacitor has attracted much attention. According to the different source of raw carbon, the application of activated carbon in supercapacitors is briefly introduced.

5.1 Fruit shell-based activated carbon

🍏Hou min, etc by the coconut shell charcoal material as raw material, KOH as activator, discussed the basic carbon ratio, activation temperature and activation time on the structure and properties of the activated carbon (table 2- 1), in carbon ratio, alkali activation temperature of 800 ℃, activation time, can prepare the specific surface area, total pore volume and pore diameter are mainly distributed in, porosity, average pore diameter of high quality activated carbon materials. The activated carbon is used as electrode material, the specific capacitance in the electrolyte can be reached, has excellent electrochemical performance; Yang Jing et al. with walnut shell as raw material, using the secondary activation method to prepare activated carbon, charge/discharge, mass ratio capacitance as high as, capacitor energy density as high. Chen Xiaomei et al. prepared activated carbon electrode material by chemical activation method with walnut shell as precursor and KOH as electrolyte The test results show that the prepared activated carbon electrode material has ideal electrochemical capacitance behavior, the specific capacitance is as high as 0.25mA, the leakage current and the equivalent series resistance are only 0.39Ω, respectively. After 5000 cycles of charging/discharging, the capacitance still remains above 88%.

5.2 Rice husk-based activated carbon

🍎Song Xiaolan et al. used rice husk as raw material and NaOH as activator to obtain activated carbon with specific surface area at 800℃. In KOH electrolyte, the specific capacitance of capacitor was up to, and after 5000 cycles, the capacitance retention rate was still there. Using rice husk as raw material and NaOH as activator, He et al. obtained activated carbon with specific surface area as microwave heating, and its specific capacitance was still up to after the second cycle. After carbonizing rice husk under nitrogen atmosphere, impregnating with HF and activating with KOH, the activated carbon with graded pore structure containing large mesopore and rich micropore was finally obtained, with a high specific surface area and energy density

5.3 Bamboo charcoal based activated carbon

🍐Bamboo as a kind of renewable biomass raw material has the characteristics of fast growth, rapid renewal and strong regeneration ability, and bamboo resources are rich and widely distributed in our country, which provide convenience for us on raw materials. Bamboo charcoal is a solid product obtained by carbonization of bamboo at high temperature and without oxygen. It has the characteristics of high mechanical strength, developed pore structure, large specific surface area and good electrical conductivity. Lee et al. found that bamboo activated carbon has good micropore structure and high mesoporous rate, and can significantly improve the permeability of electrolyte when used as a fast ion channel in the electrode of supercapacitor. Bai Xiang et al. with bamboo as raw material, the carbonization of bamboo charcoal first physical activation, and then in the activation, the specific surface area of bamboo charcoal based activated carbon, pore size distribution between, specific capacity up to, and showed a good high current charge/discharge performance. Yang Shengjie et al. prepared activated carbon at high temperature under the protection of inert atmosphere with Moso bamboo as raw material, sodium hydroxide as activator, and obtained the optimal process conditions: activation time 2h, activation temperature, alkalo-carbon ratio (mass ratio), the first discharge ratio capacitance of the prepared material, the second cycle ratio capacitance retention rate, and the leakage current of the prepared material

5.4 Coal-based activated carbon

🍊Coal based activated carbon yield accounts for more than 70% of total activated carbon production in China, is the main variety of activated carbon production in China. Coal-based activated carbon has the characteristics of low production cost, wide source, simple preparation method, good electrical conductivity, high specific surface area, strong corrosion resistance, controllable pore structure and stable electrochemical performance. Therefore, coal-based activated carbon as electrode material has broad application prospects. Zhang Chuanxiang et al. Taixi anthracite as raw material, as activator, under the condition of carboniferous ratio, activation of activated carbon for specific surface area, mesoporous rate is, in the electrolyte capacitance is as high as, and under the high current density charge/discharge ratio capacitance retention rate is high, leakage current is only. Xing Baolin et al Henan Yongcheng anthracite as raw material, coal based activated carbon with high specific surface area, the capacitance is as high as, after the cycle, the ratio of capacitance to maintain the rate. Using bituminous coal as raw material, Zhang et al. prepared a kind of oxygen-rich activated carbon with medium specific surface area by rapid activation method. The oxygen-rich activated carbon as electrode material has higher energy density and power density, and its specific capacitance is as high as and at and current densities, respectively.

5.5 Petroleum-based activated carbon

🍋Petroleum coke is a by-product of petrochemical refining industry, which has abundant resources, wide distribution, low price, high carbon content, high yield of activated carbon and large specific surface area. In addition, the ash and volatile matter of petroleum coke are low, and the activated carbon produced has low impurity content and excellent performance. Song Yan et al. investigated the effects of different influencing factors on the properties of activated carbon products through orthogonal experiments, and the order of the effects was as follows: microstructure of raw coke > activation temperature > alkali-carbon ratio > particle size of raw material > activation time (” > “means” better “). Under the optimal combination conditions, Panjin petroleum coke was used as raw material and activated by KOH to prepare activated carbon with specific surface area. At the same time, it was found that the coke with small grain and fine Mosaic optical structure in the raw material had the highest reactivity with the activator, and the prepared activated carbon had the largest specific surface area. Tan Minghui et al. obtained porous carbon with petroleum coke as raw material and KOH activation, impregnated it in ferric nitrate solution after carbonization and high temperature activation of carbon dioxide, and prepared activated carbon with specific surface area and total pore volume. After the second activation by metal salt impregnation, the mesoporous ratio of the porous carbon increases from to, which significantly increases the charging/discharging rate of the electrode. By introducing the active gas hydrogen, Xiao Ronglin et al. obtained activated carbon with high specific surface area under the condition of reducing the amount of alkali, and showed better performance than the activated carbon prepared without introducing hydrogen. The added hydrogen can react with the functional groups on the surface of activated carbon, provide more active points, promote the development of the pore structure of activated carbon, and adjust the distribution of pore structure.

5.6 Bitum-based activated carbon

🍌Asphalt is a complex mixture of dark brown and high viscosity composed of different molecular weight hydrocarbon compounds and their nonmetallic derivatives, which can be mainly divided into coal tar asphalt, petroleum asphalt and natural asphalt. Among them, coal tar pitch is a by-product of coking, which has the advantages of not being disturbed by season, low price and high carbonization yield, and has been used by a large number of researchers as electrode materials for supercapacitors. Using magnesium oxide as template and coal tar pitch as carbon source, the mesoporous activated carbon material was prepared by one-step heating. The material has high specific capacitance, and the specific capacitance under and current density is and respectively. Using carbonized meso-phase asphalt as precursor and activator, Jing Ulrich prepared activated carbon electrode materials for supercapacitors. The effects of activation temperature, alkali-carbon ratio and process conditions on the pore structure and electrochemical behavior of activated carbon were investigated. The results show that the maximum specific capacitance of the activated carbon electrode prepared under the conditions of activation temperature and alkali-carbon ratio can be reached at time, the changes of the pore structure and specific capacitance of the activated carbon depend on the specific treatment process, and the mesoporous content has an important influence on the specific capacitance of the activated carbon electrode. He et al.] took coal tar pitch as raw material, only a small amount of KOH activator was used, and heated under the assistance of microwave to obtain activated carbon with a specific surface area. The performance of supercapacitor electrode material in different electrolytes is studied. It is found that the ratio of capacitor in the electrolyte is higher than that in the capacitor, and the energy density in the solution is higher.

5.7 Phenolic resin-based activated carbon

🍉Phenolic resin is obtained by polycondensation of phenolic compounds and aldehydes, among which the resin obtained by polycondensation of phenol and formaldehyde is the most important. As the earliest synthetic polymer, phenolic resin has attracted more and more attention because of its mature production process, low price, high carbonization yield, easy activation of pores and large specific surface area. Teng et al. used phenolic resin as raw material and as activator to prepare activated carbon with specific surface area and medium specific capacity. Geng Xin et al. prepared activated carbon with high specific surface area for supercapacitors by activation method using water-soluble phenolic resin as raw material. The results show that the activated carbon prepared at 650℃ has the largest specific surface area and the smallest micropore ratio. However, the micropores of activated carbon prepared at 700℃ and 750℃ are interconnected, and the conductivity and ion migration resistance are better than those of the former. Using silica sol as template and phenolic resin as carbon source, Wang Renqing et al. prepared activated carbon with specific surface area by template method. Under the current density, the specific capacitance could be reached. In addition, the pore size and pore size distribution of the product carbon can be controlled by adding materials such as polyvinyl alcohol or polyvinyl butyral, which are easy to crack and have low carbon residue, into the phenolic resin.

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