A supercapacitor is a capacitor that has a higher capacity than traditional capacitors. It stores energy by polarizing electrolytes. It is an energy storage device between a capacitor and a battery. It not only has the characteristics of fast charging and discharging of a capacitor, but also has the energy storage characteristics of a battery. Its basic principle is the same as that of other types of electric double layer capacitors. It uses the electric double layer structure composed of activated carbon porous electrodes and electrolytes to obtain super large capacity.
Ⅰ. Electrode materials for supercapacitors
Electrode materials play a pivotal role in affecting the performance of supercapacitors. Therefore, the electrodes of supercapacitors should have excellent chemical stability and high electrical conductivity without causing great changes at high temperatures.
1. Conductive polymer material
Conductive polymers have become an important research direction of electrode materials because of their good conductivity, easy preparation, and low price. Polyaniline, polythiophene, and polypyrrole are currently the most commonly used conductive polymer materials, and polyaniline is very popular among researchers because of its simple preparation method, low cost, and good multi-redox performance. In recent years, people have made various improvements to it, including improving its surface morphology, structure, nanonization, etc., so as to combine with other electrode materials and conductive polymers. Therefore, combining with other structurally stable substances can reduce its expansion and contraction effects and improve its specific volume and cycle stability.
2. Metal oxide materials
Metal oxides have the characteristics of high energy density, excellent specific capacitance, stable physical and mechanical properties, low manufacturing cost, and environmental protection and pollution-free. What's more, it does not undergo any change in shape during charging and discharging, and does not expand or contract, which makes it a hot spot in the field of supercapacitor research.
Metal oxides that are often used as supercapacitor electrode materials include MnO2, Co3O4, RuO2, etc. RuO2 has excellent properties such as chemical stability, electrical conductivity, electrochemical properties, and specific cycle stability. It is a new type of material that was first used in supercapacitors. However, its application in energy storage devices is largely limited due to toxicity and high cost. The most commonly used electrode materials are metal oxides and their complexes, such as Fe, Co, Ni, Mn, and Mo. They are generally used to prepare supercapacitors with high energy density.
3. Carbon-based materials
Carbon-based materials have excellent physical and chemical properties, and have the advantages of low cost, various forms, strong practicability, easy processing, stable electrochemical performance, etc., and have been widely used in electrode materials.
(1) Carbon nanotubes
Carbon nanotube is a new type of seamless hollow tube. Because of its good electrical conductivity, good stability, large specific surface area, and large diameter ratio, it has become a good double-layer capacitor material. According to the carbon layer thickness of carbon nanotubes, it can be divided into multi-walled carbon nanotubes and single-walled carbon nanotubes.
(2) Activated carbon
Activated carbon is a specially processed porous carbon material that has the advantages of good pore structure, low cost, and the synthesis technology and methods of various raw materials. The surface size, physical and chemical properties of activated carbon depend on its precursor material and activation process. The carbonization process is to put a precursor containing a large amount of carbon into a high-temperature environment to obtain amorphous carbon. Under the action of the activator, a porous network structure will appear in most carbon particles.
(3) Carbon fiber
Carbon fiber is carbon with high strength and high modulus, with a carbon content of more than 90%. It has the characteristics of high temperature resistance, low density, heat conduction, and corrosion resistance. Because its surface is in the shape of fibers, it not only has the flexibility of fibers, but also has its excellent physical and chemical properties.
Since 1879, people have started research on carbon fiber. According to the literature, a new type of N-type porous carbon fiber electrode has been fabricated by electrospinning. Its electrochemical performance is excellent. After 10,000 times of charging and discharging, the electrochemical performance of the electrode material is still higher than 92%. However, the use of carbon fibers as electrode materials for supercapacitors also has many problems. Because carbon fiber cannot be used as an electrode alone, it must be activated and compounded by other materials in order to show good electrochemical properties.
Ⅱ. Advantages and disadvantages of supercapacitors
(1) Environmental protection and energy saving
Compared to traditional chemical batteries, supercapacitors do not involve chemical reactions. It does not emit environmental pollutants during storage and release, and has high energy conversion efficiency.
(2) Long charging and discharging life
The charge and discharge life of a supercapacitor can reach 500,000 times or 90,000 hours, while the charge and discharge life of a battery is difficult to exceed 1,000 times. Supercapacitors can provide very high discharge currents, but ordinary batteries usually cannot have such high discharge currents. The service life of some batteries with high discharge current will be greatly shortened under such high discharge current.
(3) Large electric capacity
Supercapacitors use activated carbon powder and activated carbon fibers as polarizable electrodes, which greatly increases the area in contact with the electrolyte. According to the calculation formula of capacitance, the larger the surface area of the two plates, the greater the capacitance. Therefore, the capacity of general electric double layer capacitors can easily exceed 1F. Its emergence suddenly increased the capacity range of ordinary capacitors by 3 to 4 orders of magnitude.
(4) Rapid charging and discharging
Supercapacitors can charge quickly from tens of seconds to minutes, while it would be impossible for batteries to fully charge in such a short time.
(5) High power density
Supercapacitors can provide high current instantaneously. Its low esr and power density are dozens of times that of lithium-ion batteries, making it suitable for high-current discharge.
(1) High cost
Compared with aluminum electrolytic capacitors, supercapacitors have larger internal resistance and cannot be used in AC circuits.
(2) Lower voltage resistance
Supercapacitor is an energy storage device whose cell voltage is limited by the decomposition voltage of the electrolytic solution. This means that if the voltage applied to the supercapacitor exceeds the decomposition voltage of the electrolytic solution, the electrolytic solution will undergo a decomposition reaction, causing the supercapacitor to fail.
(3) Low energy density
Compared with ordinary batteries, the energy density of supercapacitors is much lower than that of ordinary batteries.
Ⅲ. Applications of supercapacitors
The use of supercapacitors in DC power supply vehicles powered by internal combustion engines can solve the technical problem of insufficient power when the power supply vehicle starts the aircraft engine. At the same time, the supercapacitor has a great protective effect on the DC power supply vehicle power generation system, especially the internal combustion engine, at the moment of starting. As for the inverter power supply, it can also be combined with supercapacitors to form high-power special impact power supply equipment, which can provide thousands of amps of impact current for starting aircraft engines.
2. Wind power generation
At present, the main energy storage systems include batteries and supercapacitors. The charging and discharging characteristics of the storage battery are not good, and it needs maintenance. Its service life is short and its reliability is poor. However, the advantages of supercapacitors are very obvious. It has both good charge and discharge characteristics and a long cycle life, and is also environmentally friendly and energy-saving. Wind power generation has large current fluctuations, and supercapacitors do not have strict charging current limits, so they are very suitable for working environments such as wind turbines.
3. Military field
As the most important functional component in high-power pulse weapons and equipment, supercapacitor is a new type of energy storage device. It can burst out huge energy in a short period of time and can be called the "energy core" of many military equipment. It is widely used in armored vehicles, ships, spacecraft and other weapons and equipment manufacturers that require larger pulse discharge power.
(1) In tank applications, supercapacitors can be used as ignition power sources, dynamic aiming, etc. Because the supercapacitor can start at low temperatures between -40 degrees and +70 degrees, coupled with the stability of the supercapacitor, it can ensure the smooth operation of the tank and avoid failures on complex and changeable battlefields.
(2) The electromagnetic gun uses advanced electromagnetic launch technology, and it has become one of the most advanced combat weapon systems. Its key application lies in the instantaneous charge and discharge and large-capacity characteristics of supercapacitors. These characteristics can improve launch efficiency, enhance launch energy, shorten launch time, and meet the demand for improved maneuverability. In the application of aircraft carrier electromagnetic ejection, the role of electromagnetic gun is more important. Its precision strike capability and powerful energy output make the aircraft carrier more advantageous in war, and undoubtedly injects new vitality into military technology.
(3) In military small satellite systems, supercapacitors undergo special lightweight processing and can withstand extremely harsh environmental conditions, thus ensuring their service life. In addition, supercapacitors also play an important role in particle beam energy weapons and microwave weapon systems, providing critical energy support for these advanced weapon systems.
(4) In high-power radar applications, supercapacitors achieve pulse-type high power density output by virtue of their fast charging and high-current discharge characteristics, thus meeting the needs of radar use.
4. Industrial electronics
Supercapacitors are widely used in industrial fields, such as smart meters, communication modules, concentrators, vehicle-mounted equipment, electric toys, emergency lighting, elevators, tax control cash registers, electric forklifts, medical equipment, security equipment, X-ray machines, etc. . For example, a medical X-ray machine will be very powerful at the moment of exposure and photography. Mobile X-ray machines are usually equipped with backup batteries, but when used frequently, the battery may be subject to the impact of instantaneous large current discharge, resulting in shortened life and even affecting the clarity of photos. In addition, when the mains X-ray machine is started to take pictures, it will also have an instantaneous high current impact on the power grid, resulting in poor power quality of the power grid and other problems. The instantaneous power characteristics of supercapacitors can meet the current required by the X-ray machine to take pictures, effectively solving the above problems. In practical applications, we need to design corresponding capacitor solutions based on specific conditions.
5. Power system
Supercapacitors are widely used in power systems, mainly in distributed power generation and distribution network systems. Distributed power generation systems are mainly wind power generation and photovoltaic power generation, but these new energy power generation have the problem of unstable output power. When these distributed power generation are integrated into the power grid, they may have an impact on the power grid and affect the safe and stable operation of the power grid. Supercapacitors have the characteristics of high power density and high energy storage efficiency, so they can be used as energy storage devices for distributed power generation to provide instant compensation power for the power grid. When the power grid fails, supercapacitors can serve as the backup power supply of the power grid to ensure the continuity and stability of the power supply. The use of capacitors can not only buffer the instantaneous power of the power grid, but also compensate for active power or reactive power, smooth and stabilize the voltage fluctuations in the power grid, and solve problems such as sudden rise and fall of voltage and instantaneous power outages caused by power system faults.
6. Electric vehicles
Traditional batteries (such as lead-acid batteries) have low power density. During normal driving, the average power drawn by an electric vehicle from the battery is quite low, while the peak power during acceleration and climbing is quite high. The ratio of peak power to average power of a high-performance electric vehicle can reach 16:1. In fact, the energy consumed by electric vehicles during acceleration and hill climbing accounts for 2/3 of the total energy consumption. The emergence of battery and supercapacitor hybrid vehicles has solved the contradiction between the driving range and acceleration and climbing performance of electric vehicles.
The installation of supercapacitors not only solves the problem of difficult starting at low temperatures, but also greatly improves the performance of motor vehicles. Traditional lead-acid batteries have poor performance at low temperatures. At -40°C, their current output capacity is only about 1/10 of that at normal temperature, making it difficult to start motor vehicles at low temperatures. The normal operating temperature of supercapacitors is between -40°C and +70°C, allowing motor vehicles to start smoothly at -15°C and even at -30°C. This greatly improves the performance and service life of motor vehicles in various environments.
7. Urban rail transit
In urban rail transit, trains, as public transportation, need to switch between frequent starting and braking. This process often results in excessive inrush current and waste of braking energy. To solve this problem, supercapacitors, as an efficient energy storage and release device, are widely used in trains. It recovers and stores braking energy when the vehicle is braking, and releases the stored energy when the vehicle needs to accelerate. This effective use of energy not only improves the operating efficiency of the train, but also reduces energy waste.
Ⅳ. Precautions for using supercapacitors
1. Prevent high temperature
Supercapacitors should be protected from exposure to high temperatures. High temperatures can cause the electrolyte inside the supercapacitor to evaporate, causing the performance of the supercapacitor to degrade or causing safety hazards. Therefore, when using supercapacitors, we should place them in an environment with a suitable temperature to avoid the adverse effects of high temperatures.
2. Avoid overheating when welding
When soldering supercapacitors, we should pay attention to temperature control. Excessively high soldering temperatures may cause damage to the supercapacitor or present safety hazards. Therefore, when welding supercapacitors, we should choose the appropriate welding temperature and welding time to ensure the safety and stability of the supercapacitors.
3. Confirm polarity
Before using a supercapacitor, we must first confirm its polarity. The positive electrode of a supercapacitor is generally marked with a "+" symbol, while the negative electrode is marked with a "-" symbol. If the polarity is reversed, the supercapacitor will be damaged or leakage will occur. Therefore, when connecting the circuit, we must first confirm the polarity of the supercapacitor, and then wire it according to the correct connection method.
4. Pay attention to voltage balance when connecting in series
When multiple supercapacitors need to be used in series, we must pay attention to the voltage balance issue. Use in series can cause the voltage of some supercapacitors to be too high, damaging them or creating a safety hazard. Therefore, in order to prevent such problems from occurring, we need to take appropriate measures to ensure voltage balance when using multiple supercapacitors in series. In this way, we can ensure that supercapacitors can operate in a safe and reliable environment while extending their service life.
5. Do not expose to external environment
Supercapacitors should try to avoid exposure to the external environment, especially dust, water vapor and other harsh environments. Impurities such as dust and water vapor can affect the performance of supercapacitors, causing leakage or safety hazards. Therefore, when using supercapacitors, we should place them in a well-sealed environment to avoid adverse effects from the external environment.
6. Not used for high frequency charging and discharging
Supercapacitors are not suitable for long-term use under high-frequency charging and discharging conditions. High-frequency charging and discharging will cause the performance of supercapacitors to decrease and shorten their service life. Therefore, when using supercapacitors, we should choose the appropriate charge and discharge frequency based on its performance indicators and usage requirements to extend the service life of the supercapacitor.
7. Use at nominal voltage
Supercapacitors should be used at nominal voltage. Nominal voltage refers to the rated voltage of the supercapacitor. If the supercapacitor exceeds this voltage value, it may cause damage to the supercapacitor or cause safety hazards. Therefore, when using supercapacitors, we should perform reasonable voltage distribution according to its nominal voltage value to ensure that the supercapacitor works within a safe range.