What is the difference between supercapacitors and ordinary capacitors?

What is the difference between supercapacitors and ordinary capacitors?

Supercapacitors, also known as electrochemical capacitors, double-layer capacitors, gold capacitors, and Faraday capacitors, are a type of electrochemical component developed in the 1970s and 1980s for energy storage through polarized electrolytes.

It is different from traditional chemical power sources and is a power source with special performance that lies between traditional capacitors and batteries. It mainly relies on double layers and redox pseudocapacitive charges to store electrical energy. But there is no chemical reaction during its energy storage process, which is reversible, and it is precisely because this supercapacitor can be charged and discharged hundreds of thousands of times repeatedly.

The specific details of the structure of supercapacitors depend on their application and usage. Due to manufacturer or specific application requirements, these materials may vary slightly. The commonality of all supercapacitors is that they all contain a positive electrode, a negative electrode, and a separator between these two electrodes, with the electrolyte filling the pores separated by these two electrodes and the separator.

The structure of supercapacitors is composed of porous electrode materials with high specific surface area, porous battery separators, and electrolytes. The diaphragm should meet the conditions of having the highest possible ion conductivity and the lowest possible electronic conductivity, and is generally an electronic insulation material with a fiber structure, such as polypropylene film. The type of electrolyte is selected based on the properties of the electrode material. According to the different energy storage mechanisms, it can be divided into the following two categories:

1. Double layer capacitance: It is generated by the directed arrangement of electrons or ions at the electrode/solution interface, resulting in charge opposition. For an electrode/solution system, a double layer will be formed at the interface between the electron conducting electrode and the ion conducting electrolyte solution. When an electric field is applied to both electrodes, the anions and cations in the solution migrate to the positive and negative electrodes respectively, forming a double layer on the electrode surface; After removing the electric field, the positive and negative charges on the electrode attract ions with opposite charges in the solution, stabilizing the double layer and generating a relatively stable potential difference between the positive and negative electrodes.

At this point, for a certain electrode, an opposite ion charge equal to the charge on the electrode will be generated within a certain distance (dispersion layer), keeping it electrically neutral; When the two poles are connected to the external circuit, the charge on the electrode migrates and generates a current in the external circuit. The ions in the solution migrate to the solution and become electrically neutral, which is the charging and discharging principle of double-layer capacitors.

2. Faraday quasi capacitance: Its theoretical model was first proposed by Conway, which refers to the under potential deposition of electroactive substances on the surface of electrodes and in the two-dimensional or quasi two-dimensional space near the surface or bulk phase, resulting in highly reversible chemical adsorption desorption and redox reactions, and the generation of capacitance related to electrode charging potential. For Faraday quasi capacitors, the process of storing charges includes not only the storage on the double layer, but also the redox reaction between electrolyte ions and electrode active substances.

When ions (such as H+, OH -, K+, or Li+) in the electrolyte diffuse from the solution to the electrode/solution interface under the action of an external electric field, they enter the bulk phase of the active oxide on the electrode surface through the oxidation-reduction reaction at the interface, resulting in a large amount of charge being stored in the electrode. When discharging, these ions that enter the oxide will return to the electrolyte through the reverse reaction of the above oxidation-reduction reaction, and the stored charges will be released through the external circuit, which is the charging and discharging mechanism of Faraday quasi capacitors.

Advantages of supercapacitors:

1. Reaching Faraday level capacitance in a very small volume;

2. No need for special charging circuits and control discharge circuits;

3. Compared to batteries, overcharging and discharging do not have a negative impact on their lifespan;

4. From an environmental perspective, it is a green energy source;

5. Supercapacitors can be soldered, so there are no issues such as weak battery contact;

Disadvantages of supercapacitors:

1. Improper use can cause electrolyte leakage and other phenomena;

2. Compared with aluminum electrolytic capacitors, it has a higher internal resistance and therefore cannot be used in AC circuits;

The reason why supercapacitors are called "super" is:

1. Supercapacitors can be seen as two non reactive porous electrode plates suspended in the electrolyte. When charged on the plates, the positive electrode plate attracts the negative ions in the electrolyte, and the negative electrode plate attracts the positive ions, forming two capacitive storage layers. The separated positive ions are located near the negative electrode plate, and the negative ions are located near the positive electrode plate.

2. Supercapacitors store energy in the separated charges, and the larger the area used to store the charges and the denser the separated charges, the greater their capacitance.

3. The area of traditional capacitors is the flat surface area of the conductor. In order to obtain a larger capacity, the conductor material is rolled very long, and sometimes a special organizational structure is used to increase its surface area. Traditional capacitors use insulating materials to separate their bipolar plates, usually plastic film, paper, etc. These materials usually require the thinnest possible thickness.

4. The area of supercapacitors is based on porous carbon materials, whose porous structure allows for an area of up to 2000m2/g, and larger surface areas can be achieved through some measures. The distance of charge separation in supercapacitors is determined by the size of electrolyte ions attracted to the charged electrode. This distance (<10Å) is smaller than what traditional capacitor thin film materials can achieve.

5. The large surface area combined with a very small charge separation distance gives supercapacitors an astonishing electrostatic capacity compared to traditional capacitors, which is also their "super".