- Energy storage mechanism of supercapacitors
A supercapacitor is a new type of energy storage element between electrolytic capacitors (capacitors using a thin oxide film as a dielectric) and electrochemical batteries, and its energy storage method is different from that of traditional capacitors. Traditional capacitors are composed of electrodes and a dielectric. The dielectric between the electrodes produces a polarization effect under the action of an electric field to store energy. An electrochemical capacitor does not have a medium, and relies on a unique electric double layer structure formed on the interface between the electrolyte and the electrode. Layers) store energy. The capacity of electrochemical capacitors is much larger than that of traditional capacitors, reaching the level of 103~104 Farads.
The development of supercapacitors has gone through a long time. The German Helmholtz discovered at the end of the 19th century that when the conductor electrode is inserted into the electrolyte, the conductor electrode is in contact with the electrolyte. Due to Coulomb, intermolecular (van der Waals) or intermolecular (covalent) forces, the net charge on its surface will attract some irregularly distributed heterogeneously charged ions from the solution, making them The solution side of the electrode/electrolyte solution interface is arranged in a row at a certain distance from the electrode, forming an interface layer with the same number of charges as the number of remaining charges on the electrode surface and the opposite sign. Thereby forming a layer on the electrode, the other layer in the solution of two charge layers, known as the electric double layer.
As shown in Figure 1. Due to a potential barrier at the interface, neither layer of charges can cross the boundary and neutralize each other. The electric double layer structure will form a plate capacitor.
If C is used to represent the capacitance of the electric double layer, its calculation formula is as shown in Figure 2:
In the formula, Vw is the maximum working voltage of the capacitor.
According to the calculation formula of capacitance, the electric double layer capacitance is proportional to the surface area of the electrode and inversely proportional to the thickness of the electric double layer. In the concentrated solution of the strong electrolyte, the thickness of the electric double layer is on the order of 0.1 nm. The electrode material can obtain a large capacitance, thereby improving the energy density of the supercapacitor. As a result, attention has been focused on carbon-based materials with large surface areas. Since Beck first applied for a patent for an electric double layer capacitor using activated carbon as an electrode material in 1954, the research on carbon-based supercapacitor electrode materials has been carried out for more than 40 years, mainly focusing on the preparation of high specific surface area and small internal resistance. Conduct modification research on porous carbon-based materials, including activated carbon, carbon black, carbon nanofibers, carbon aerogels, carbon nanotubes, glassy carbon, network-structured activated carbon, and carbonization products of certain organic compounds.
The capacitance of carbon electrode electrochemical capacitors mainly comes from the interface electric double layer. Theoretically, the higher the specific surface area of the activated carbon material, the corresponding specific capacity should also be larger, but in fact the measured capacity and specific surface area are not completely linear relationship, some carbon materials with smaller specific surface area instead Has a larger specific capacity. At the same time, complete electric double-layer capacitance is difficult to achieve, and some redox processes often occur on the surface of the electrode. For carbon electrodes made of fine particles with high specific surface area, the electrochemical Faradaic redox process on the electrode surface is often accompanied by the generation of electric double layer capacitance. This is partly due to the sp2, sp3 hybrid bonds on the carbon electrode surface.
In addition, the surface of the carbon electrode usually has a wake-type structure, and these redox functional groups will chemically react to provide a Faraday pseudocapacitor (Faraday Pseudocapacitor). one of the reasons. Later, people consciously used the Faraday quasi-capacitor energy storage principle to design other types of supercapacitors, such as metal oxide electrode supercapacitors, organic conductive polymer electrode supercapacitors, etc., using the redox reaction on the electrode to store energy, which can improve capacitors. The operating voltage of the supercapacitor greatly improves the specific energy of the supercapacitor. This capacitor will be introduced in more detail later. Another difference between Faraday quasi-capacitors and electric double-layer capacitors is that the electric double-layer capacitor needs to consume electrolyte during the charging process, while the concentration of the electrolyte in the Faraday capacitor remains relatively stable throughout the charging and discharging process. Faraday quasi-capacitance is not only generated on the surface of the electrode, but also inside the entire electrode. Its maximum charge and discharge performance is controlled by the ion orientation and charge transfer speed on the surface of the electro-alcoholic substance, so the charge transfer can be performed in a short time, just Get higher specific power. At the same time, during the entire charge and discharge process, there is no phase change on the electrode that determines the reaction rate and limits the life of the electrode, resulting in a long cycle life (more than 100,000 times).
- Classification of super capacitors
Generally, supercapacitors can be classified according to their electrode materials and electrolyte materials, and different supercapacitors have different characteristics.
2.1 Classification according to electrode material
According to the different electrode materials, supercapacitors can be divided into three categories: carbon electrode supercapacitors, metal oxide electrode supercapacitors and organic polymer material electrode supercapacitors.
(1) Carbon electrode electric double layer supercapacitor (DLC)
The full name of DLC is Double Layer Capacitor. The electrode of this capacitor mainly uses porous carbon materials as electrodes, such as activated carbon or silica carbon cloth, carbon powder and carbon fiber. The processing technology is mature and the active area is large. As the carbon powder, carbon cloth, carbon fiber and other materials of the electrode, the active area can reach 2500m2/g. In recent years, with the further research on carbon nanotubes, the active area of carbon electrodes has been further increased. ~90.4F/cm3), although the carbon electrode supercapacitor has the above advantages, it also has the disadvantage that its stability and conductivity decrease with the increase of the active area. Figure3 is the charge-discharge curve of the carbon electrode supercapacitor.
(2) Metal oxide electrode supercapacitor
Supercapacitors using metal oxides as electrode materials utilize the Faraday effect to store energy. This kind of capacitor uses metal oxides such as RuO2 and IrO2 as electrodes, and a series of redox reactions will occur on the electrodes during charging and discharging. Among them, the valence of Ru(Ir) will change between 3-6. Figure 4 is the charge-discharge curve of the metal oxide supercapacitor.
It can be seen from the charge-discharge curve that this capacitor has the charge-discharge characteristics of some batteries.
Compared with carbon electrodes, the electrical conductivity of metal oxide electrodes is 2 orders of magnitude larger than that of carbon, so metal oxide electrode supercapacitors can achieve very high mass specific capacity. The RuO2 electrode can reach 750F/g, while the carbon electrode is 100F/g. Moreover, the cycle life and charge-discharge performance of metal oxide supercapacitors are also quite good. The disadvantages of such supercapacitors are that the cost of electrode materials is too high, the electrolyte is limited, and the rated voltage of the capacitor is low. Hybrid Supercapacitor is a hybrid product of metal oxide supercapacitor and carbon electrode supercapacitor. On the one hand, it solves the problem of small specific energy of carbon electrode capacitors, and on the other hand, it can reduce the cost of supercapacitors. In recent years, Russian research institutions have made great progress in the study of supercapacitors with carbon-nickel electrode systems.
(3) Organic polymer material electrode supercapacitor
This capacitor uses organic polymers as electrode materials, which are hybridized to store energy using the Faraday quasi-capacitive effect. The mechanism of action is that the polymer achieves a very high stored charge density through rapid reversible n- or p-type doping and de-doping redox reactions in the polymer film on the electrode, resulting in a high Faraday A quasi-capacitor to store energy. Its higher working potential is due to the wider energy gap between the conduction band and valence band of the polymer. The discharge curve of the organic polymer electrode supercapacitor is shown in Figure5.
The use of organic polymer electrode supercapacitors can simultaneously improve the two indicators of mass specific energy and mass specific power of supercapacitors, and it has gradually become a research hotspot. The disadvantage of this capacitor is that the organic polymer material is prone to expansion and deformation, and the performance deteriorates and the stability is poor during the long-term cycle charge and discharge process.
2.2. Classification according to the type of electrolyte
According to the different electrolytes, supercapacitors can be divided into two categories: organic electrolytes and water-based solutions.
(1) Organic electrolyte supercapacitor
The biggest advantage of using organic electrolytes in supercapacitors is that it can increase the voltage of the supercapacitor monomer to reach more than 2V, the capacitor voltage can be stabilized at 2.3V, and can even reach 2.7V instantaneously. Therefore, the specific energy of supercapacitors using organic electrolytes is relatively high, which can reach 18 W·h/kg. The disadvantage of this type of capacitor is that the use of organic electrolytes requires special purification processes, and the electrodes must be covered with a specific coating to avoid corrosion of the electrodes. Its other disadvantage is that the ionization of the electrolyte is difficult, so the equivalent internal resistance is large, usually more than 20 times or even 50 times that of the aqueous solution, so the specific power index is lower.
(2) Water-based solution supercapacitor
The biggest advantages of water-based electrolytes are low internal resistance and high conductivity, which enable supercapacitors to obtain high specific power indicators. The second advantage of water-based solutions is that the purification and drying process is simple and inexpensive, thereby reducing the overall cost of supercapacitors. The disadvantage of water-based solution supercapacitors is that the monomer voltage is low and generally cannot exceed 2v, which limits the improvement of the specific energy of such supercapacitors.
At present, there are two main types of supercapacitors that can be used in electric vehicles: one is a “carbon-based supercapacitor” with activated carbon as the positive and negative electrode materials, and the other is a “hybrid supercapacitor” with nickel oxide as the positive electrode and activated carbon as the negative electrode Supercapacitors”. Carbon-based supercapacitors are currently the most technologically advanced and commercially successful supercapacitors. The nickel oxide/activated carbon hybrid supercapacitor technology is unique to Russia’s ESMA company.
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