A supercapacitor is an electrochemical device that is a transitional component between a battery and a common capacitor. The charging and discharging process is highly reversible, and it can perform high-efficiency (0.85 to 0.98) rapid (secondary) charge and discharge. The advantages include high specific power, long cycle life, and maintenance-free.
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In the past, due to the low specific energy of the supercapacitor, the discharge time was too short to be used in the automotive field. With the rapid development of supercapacitor technology, it has become a new hotspot in the research and application of the automotive field. Supercapacitor is not only suitable as an auxiliary energy source for subsystems such as automobile engine starting and power steering, but also can be used as an auxiliary energy source for electric vehicles in combination with batteries and fuel cells, thereby improving battery life and making up for insufficient fuel cell specific power. Limit the recovery of braking energy and so on. In short, it has a very broad application prospect in the automotive field.
Principle and classification of super capacitor
To be precise, the supercapacitor should be called an electrochemical capacitor (Electrochemical Capacitor). It provides higher specific energy than electrolytic capacitors, higher specific power and longer life than batteries.
Supercapacitors can be divided into three categories depending on the material used:
1. Double Layer Capacitor (DLC) using carbon electrode As shown in Figure 1, the double layer supercapacitor can be regarded as two inactive porous plates suspended in an electrolyte, and the voltage is applied to two On the board. The potential applied to the positive plate attracts negative ions in the electrolyte, and the negative plate attracts positive ions. Thereby an electric double layer capacitor is formed on the surface of the two electrodes.
Figure 1 Double layer supercapacitor
DLC is essentially an electrostatic energy storage method. Therefore, the size of the electric double layer capacitor is related to the electrode potential and the specific surface area. Therefore, activated carbon having a high specific surface area is often used as an electrode material of an electric double layer capacitor, thereby increasing the capacitance. For example, activated carbon can have a surface area of ​​up to 1000 m2/g after a specific chemical treatment, so that the unit weight can reach 100 F/g, and the internal resistance of the capacitor can be kept at a very low level. Carbon materials also have the advantages of low cost and mature technology. This type of supercapacitor is also the most widely used in automobiles.
2. Supercapacitors using metal oxide electrodes originally referred to capacitors with noble metal oxides RuO2 and IrO2 as electrodes. The adsorption capacitance is generated while the charge is transferred on the two electrodes by a reversible oxidation/reduction reaction. It differs from the electric double layer capacitor in that it is called Faradaic pseudocapacitance. Compared with the electrostatic capacity of the electric double layer capacitor, the capacity of the ultracapacitor under the same surface area is 10 to 100 times larger, so that a capacitor having a very small volume and a large capacity can be obtained. However, due to the high price of precious metals, it is mainly used in the military field.
3. Use the capacitance of the organic polymer electrode. At present, the technology is not very mature, the price is relatively expensive, and it is still in the laboratory research stage.
Research progress on supercapacitors for automobiles
At present, the United States, Europe and Japan are actively carrying out research and development work on supercapacitors for electric vehicles. Since 1992, the US Department of Energy and USABC have organized national laboratories (Lawrence Livermore, Los Alamos, etc.) and industry (Maxwell, GE, etc.) to jointly develop electric double-layer supercapacitors using carbon materials. The initial goal of the study was to increase the energy density of the supercapacitor to 5 Wh/kg while maintaining a power density of 1 kW/kg. This goal has been basically achieved, but the targets set by PNGV have not been completed on schedule. Relevant information shows that if the specific energy of the supercapacitor reaches 20Wh/kg, it will be ideal for hybrid vehicles.
In 1996, the European Community developed a plan for the development of electric vehicle supercapacitors. Led by SAFT, members include Alcatel-Asthom, Fiat, etc. The goal is: the specific energy reaches 6Wh/kg, the specific power reaches 1500W/kg, and the cycle life exceeds 100,000 times, meeting the requirements of electrochemical cells and fuel cell electric vehicles.
Japan has also established the "New Capacitor Research Society" and the NEW SUNSHINE development organization.
Currently, the leading countries in this technology field are Russia, Japan, Germany and the United States. Russia has focused on the research of capacitive car technology and electric vehicle braking energy recovery, and has made remarkable progress. Its startup supercapacitor has a specific power of 3000W/kg and a cycle life of more than 100,000 times, which is ahead of other countries. In Russia, there used to be a 950kg super capacitor to drive an electric bus carrying 50 passengers. Although its driving range is only 8-10km, its charging time is only 15 minutes.
Maxwell predicts that the price of its product PowerCacheTM will reach $30/cell in 2003. By 2003, the demand for supercapacitor cells in the automotive market will reach one million, and will increase rapidly to 100 million in 2008. Now, Full Power Technologies of the United States is developing low-cost supercapacitors.
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