What Is The Structure of a Supercapacitor and How Does It Work?

Ⅰ. History of supercapacitors

In 1879, German physicist Helmholtz proposed an ultra-large capacitor with a farad level. This ultra-large capacitor is an electrochemical component that stores energy by polarizing an electrolyte.

In 1957, H. Becker applied for a patent for electrochemical capacitors using high specific surface area activated carbon as electrode material. The research and development of supercapacitors can be traced back to 1962. SOHIO produced a capacitor with an operating voltage of 6V and carbon materials as electrodes.

In 1966, while working on an experimental fuel cell design, researchers at Standard Oil of Ohio developed another component called an "electrical energy storage device."

In 1970, the electrochemical capacitor patent applied by Donald L. Boos was eventually registered as "Activated Carbon Electrode Electrolytic Capacitor".

In 1979, Japan applied the developed supercapacitor to the battery starting system of electric vehicles, launching the first commercial application of electrochemical capacitors.

In 1994, David A. Evans developed an "electrolytic-hybrid electrochemical capacitor" using the anode of a 200V high-voltage tantalum electrolytic capacitor.

Ⅱ. The structure of supercapacitors

The structure of supercapacitor includes the following parts:

1. Diaphragm

The separator in a supercapacitor is located between the positive and negative electrodes to isolate the positive and negative electrodes and maintain the purity of the electrolyte. The separator is usually made of a fibrous structure of an electronically insulating material, such as a polypropylene membrane.

2. Positive electrode

The positive electrode of a supercapacitor is usually made of a porous electrode material with a high specific surface area.

3. Negative electrode

The negative electrode of a supercapacitor is also usually made of a porous electrode material with a high specific surface area.

4. Electrolyte

The electrolyte in a supercapacitor is a solution with high ionic conductivity that is used to provide ion migration between the positive and negative electrodes. The type of electrolyte is selected based on the properties of the electrode material.

Ⅲ. Working principle of supercapacitors

Supercapacitors are capacitors that utilize the electric double layer principle. When an external voltage is applied to the two plates of a supercapacitor, the positive and negative electrodes store charges of different polarities respectively, which is the same as the principle of ordinary capacitors. The movement of supercapacitor charges creates an electric field on the bipolar plates. Due to the electric field force, it forms an opposite electric field at the interface between the electrode and the electrolyte, thereby balancing the internal electric field of the electrolyte.

Positive charges and negative charges are on the short gap contact surface between two different polarities. The charge distribution layer is called an electric double layer, and the capacitance is very large. When the redox electrode potential of the electrolyte is higher than the potential between the two plates, the electrolyte will not separate from the charges on the electrolyte interface, and the supercapacitor is in a normal working state. If the redox electrode potential of the electrolyte is lower than the voltage across the capacitor, the electrolyte will decompose with the charge at the interface, which will be an abnormal state. If the charge stored on the positive and negative plates is discharged through an external circuit, the amount of charge at the supercapacitor electrolyte interface gradually decreases. It can be seen that the charging and discharging of supercapacitors is a physical process without complex chemical reactions. Therefore, compared with chemical reaction batteries, its performance is more stable.

Ⅳ. Energy storage principle of supercapacitors

Supercapacitors use the electric double layer effect to store electrical energy, eliminating the need for a solid dielectric to separate charges like traditional capacitors. In the electric double layer of an electrode, electrical energy storage is mainly based on two different principles.

1. Hybrid capacitors, such as lithium-ion capacitors, use electrodes with two different properties: one is mainly electrostatic capacitor, and the other is mainly electrochemical capacitor.

2. The electrodes of electrochemical pseudocapacitors are usually metal oxides and conductive polymers. In addition to double-layer capacitors, there are also a large number of electrochemical pseudocapacitors. Pseudocapacitance is achieved by faradaic electron charge transfer, coupled with redox reactions, intercalation, or electrosorption. In an electrochemical capacitor, the amount of charge that can be stored per unit voltage is essentially a function of the electrode size. The capacitance values of capacitors of different storage principles can vary greatly.

When a voltage is applied to one end of an electrochemical capacitor, electrolyte ions move to oppositely polarized electrodes, forming an electrical double layer in which a single layer of solvent molecules acts as a separator. Pseudocapacitance occurs when specific adsorbed ions in the dielectric penetrate into the electric double layer. Pseudocapacitors store electrical energy on the surface of electrochemical capacitor electrodes with electric double layers through reversible faradaic redox reactions. Pseudocapacitance is a form of electrochemical energy storage through fast and reversible electron transfer between electrolytes and electrodes. This process is mainly achieved through a series of fast and reversible redox reactions, intercalation or electrosorption. No chemical reaction (no chemical bond formation) can take place between the adsorbed ions and the atoms on the electrode due to the mere charge transfer.

The electrons participating in the Faraday process travel to and from the valence electron states (atomic orbitals) of the redox electrode reactants. They enter the negative electrode and flow through an external circuit to the positive electrode, where they form a second electrical double layer with the same number of anions. The electrons arriving at the positive electrode are not transferred to the anions in the electric double layer, but remain next to the transition metal ions on the electrode surface. Therefore, the storage capacity of Faraday pseudocapacitance is limited by the number of reactants on the electrode surface.

Faraday pseudocapacitance occurs only with electric double layer capacitors, and its size may exceed the electric double layer capacitance of the same surface area by a factor of 100, depending on the nature and structure of the electrode, because all pseudocapacitance reactions occur on desolvated ions, which is much larger than the electric double layer capacitance of the same surface area. There are far fewer solute ions with solvent shells. The value of the pseudocapacitance follows a linear function within a narrow range, determined by the number of potential-dependent adsorbed anions.

The ability of an electrode to form a pseudocapacitance through redox reactions, intercalation, or electroadsorption depends largely on the chemical affinity of the electrode material for ions adsorbed on the electrode surface, as well as the structure and size of the electrode pores. Materials that can exhibit redox behavior in pseudocapacitors are mainly transition metal oxides, such as RuO2, IrO2 or MnO2. We can insert them into electrodes by doping them. The amount of charge stored in the pseudocapacitor is proportional to the voltage across it. The unit of pseudocapacitance is Farad.

3. Electrostatic double-layer capacitors (EDLC) generally use carbon electrodes or their derivatives. The capacitance value is much higher than the electrochemical pseudocapacitance. The charge is realized in the Helmholtz double layer at the interface between the electrode surface and the electrolyte. separation. The charge separation is on the order of a few angstroms (0.3–0.8nm), which is much smaller than conventional capacitors.

Excess charges with opposite signs will appear on both sides of the surface of the metal electrode inserted into the electrolyte solution and the liquid surface, resulting in a potential difference between the phases. Then, if two electrodes are inserted into the electrolyte at the same time, and a voltage smaller than the decomposition voltage of the electrolyte solution is applied between them, then the positive and negative ions in the electrolyte will quickly move to the two poles under the action of the electric field, and will move to the two poles respectively. A dense charge layer is formed on the surface of the upper electrode, that is, an electric double layer. The electric double layer formed by it is similar to the polarization charge generated by the dielectric in a traditional capacitor under the action of an electric field, thus producing a capacitive effect. The tight electrical double layer is similar to the dielectric layer in a traditional capacitor, but is only one molecule thick and therefore has a much larger capacity than an ordinary capacitor. Therefore, the calculation formula of traditional plate capacitors can also be used to calculate its capacitance:

The higher the permittivity ε, the larger the electrode plate surface area A, the smaller the plate spacing d, and the larger the capacitance C. Therefore, the capacitance of a double-layer capacitor is much higher than that of a conventional capacitor because the surface of the activated carbon electrode is extremely large and the electric double-layer distance is extremely small, only a few angstroms (0.3-0.8nm), which is equivalent to the order of magnitude of the Debye length.

Electric double layer capacitors do not have a traditional dielectric, but are separated by an insulator. This insulating layer allows the positive and negative ions in the electrolyte to pass through. The electrolyte itself cannot conduct electrons. Therefore, when charging is completed, no leakage will occur inside the capacitor (electrons will not flow from one pole to the other pole). When discharging, electrons on the electrodes flow from one pole to the other through an external circuit. The result is that the ion adsorption between the electrode and the electrolyte is significantly reduced, thereby redistributing the positive and negative ions in the electrolyte evenly.

Electric double layer capacitors have much higher power density than batteries. Therefore, although the energy density of existing electric double layer capacitors is 1/10 that of traditional batteries, their power density is 10 to 100 times that of the latter. They are suitable for applications between electrochemical cells (sustained energy release) and electrostatic capacitors (instantaneous energy release).

In an electrochemical capacitor, the amount of charge that can be stored per unit voltage is basically a function of the electrode size. The electrostatic energy storage in the electric double layer is linearly related to the stored charge and corresponds to the concentration of adsorbed ions. Since no chemical changes take place within the electrodes or electrolyte, the charge and discharge capacity of the electric double layer is theoretically infinite. The lifetime of a true supercapacitor is limited only by the evaporation of the electrolyte.

Ⅴ. The difference between supercapacitors and traditional capacitors

Capacitors store energy by separating charges. The larger the area where charge is stored, the smaller the distance over which the charge is isolated, and the greater the capacitance.

Traditional capacitors obtain their charge storage area from flat conductive materials. Only by winding a very long material can a large area be obtained, thereby obtaining a large capacitance. In addition, traditional capacitors use plastic film, paper or ceramics to separate the charge plates. The thickness of this type of insulating material cannot be made very thin.

Supercapacitors derive their charge-storage area from porous carbon-based electrode materials. The material's porous structure gives it a surface area of up to 2,000 square meters per gram of weight. The distance between charges in a supercapacitor is determined by the size of the ions in the electrolyte, and its value is less than 1o Angstroms. The huge surface area combined with the very small distance between the charges gives supercapacitors great capacitance. The capacitance value of a supercapacitor unit can range from one farad to several thousand farads.

Ⅵ. The difference between battery, capacitor and supercapacitors

Both capacitors and batteries are used to store energy. The difference is that batteries use chemical reactions to store and release energy. They are composed of positive and negative electrodes, which are filled with electrolyte and separated by a micro-perforated separator. Only ions can be allowed to pass through. When the battery is charged and discharged, ions shift in different directions between the two metal plates. During this movement, the battery heat rises, expands, and finally contracts. This chemical reaction continuously reduces the battery's life. One advantage of batteries is that they have a very high energy density and therefore can store large amounts of energy.

But capacitance is different in that it does not rely on chemical reactions, so it stores energy in the form of electrostatic charges. A dielectric, or insulator, is placed between the two metal plates in the capacitor to separate the positive and negative charges formed in the positive and negative electrode areas. It is this structure that allows it to store and quickly release energy, known as electrostatic charge. One of its advantages is that if a 3V capacitor is placed for 15-20 years, it still has a voltage of 3V. The voltage of the battery will decrease year by year. Another advantage is that capacitors have higher energy output capabilities than batteries, so they can be charged and discharged quickly in a short period of time. The disadvantage is that their energy density is very low. Therefore, it is more suitable for instantaneous power supply.

Supercapacitors make up for the shortcomings of batteries and capacitors. First of all, both sides of the dielectric in the supercapacitor are filled with electrolyte. When electricity is applied, ions gather on both sides of the dielectric to form a double-layer electronic structure. The distance between the two metal plates determines the performance of supercapacitors better than ordinary capacitors and even batteries. In ordinary capacitors, the distance between two metal plates is about 10-100 microns, while in supercapacitors, the distance between two metal plates is one ten thousandth of a micron. The reduction in distance means a larger electric field, which means more space for energy storage. At the same time, the carbon coating of the metal plate in the supercapacitor increases the surface area required for energy storage by 100,000 times. Therefore, compared with ordinary capacitors, the energy that supercapacitors can store is greatly increased.

Batteries store energy in watt hours, and capacitors store power in watts. The battery provides electrical energy through a long-term constant chemical reaction, the charging time is relatively long, and the characteristics of the charging current are relatively stringent. Instead, the capacitor is charged by applying a voltage across it, and the rate of charge depends largely on the external resistance. Batteries are capable of delivering power at a substantially constant voltage over an extended period of time. However, the discharge speed of the capacitor is very fast, and the output voltage decays exponentially.

Batteries are only good for a limited number of charges and discharges, depending on how much they are discharged. Capacitors, especially ultracapacitors, can be charged and discharged tens of millions of times. (This is also an important aspect of supercapacitors that are different from electrolytic chemistry. They do not have the limitation of electrode plate charge and discharge times like the working process of electrolytic chemistry.)

Ⅶ. Supercapacitor maintenance

1. Regular cleaning

We need to check whether there is dust or dirt on the supercapacitor housing, mounting bracket and other parts. If there is, we must clean and remove dirt in time to prevent static electricity from causing short circuits.

2. Storage environment

The correct storage environment is key to protecting the performance and service life of supercapacitors. During use, we store the supercapacitor in a suitable environment. The temperature should be kept between minus 40 and 50 degrees Celsius, and the humidity should be kept below 60%. The storage environment should avoid toxic, harmful or corrosive gas and liquid environments, otherwise the supercapacitor shell will be corroded and cause circuit breakage.

3. Capacitor polarity

In addition to voltage standards, supercapacitors also have fixed polarity. Before use, we must first confirm the polarity of the supercapacitor to avoid connecting the wrong polarity and affecting the performance of the supercapacitor. After completing the connection, we must not forcefully tilt or twist the capacitor to avoid loosening of the supercapacitor leads, resulting in performance degradation.

4. Pay attention to voltage

Supercapacitors have higher voltage requirements. During use, if the rated voltage of the supercapacitor is exceeded, the electrolyte of the supercapacitor will decompose. The electrolyte decomposes, the supercapacitor generates heat, and the capacity decreases, which affects the use time of the supercapacitor, thereby shortening the use time of the supercapacitor. Therefore, when performing maintenance, we must pay attention to the voltage of the supercapacitor. Once we find that the voltage does not meet the requirements, we must make adjustments in time.