The Supercapacitors: its Basic Principles, Classification, and its Electrical Performance

 In these days, the energy storage systems are playing an increasingly important role in different fields, and the relatively typical fields are like electric vehicles, power systems and some other fields. In this context, the super capacitors, as an energy storage technology, possesses excellent performances such as high power density, maintenance-free, and long life, and it have become the focus of attention in academia and industry.

 

How do ultracapacitors work

 

This section mainly will introduce the electrochemical mechanism of supercapacitors. The energy storage in supercapacitors is mainly in the interface between electrodes and electrolytes. This energy storage method has a great relationship with the electrode materials used. When the two electrodes of a supercapacitor are made from different types of materials, in this case, a comprehensive analysis of the energy storage mechanism of the product will not fully understand the working principle of the super capacitor. Based on this point, this section will briefly introduce the working principle of the super capacitor first; then elaborate the energy storage mechanism of different electrode-electrolyte interfaces, classify supercapacitors according to different electrodes and electrolytes, and introduce some electrical performance characteristics of supercapacitors.

 

1. The storage mechanism of the electric double layer capacitor

 

The electric double layer effect is that the positive and negative charges should be separative, which are generated by the accumulation at the electrode-electrolyte interface. It is the main mechanism for energy storage of supercapacitors such as activated carbon, carbon fiber, carbon felt and other carbon materials. The formation of the electric double layer effect is mainly caused by the increase or decrease of high-energy conduction band electrons on the electrode surface, which causes the movement of positive and negative charges in the electrolyte solution on the interface side, which is used to balance the charge imbalance caused by the change of the high-energy conduction band electrons on the electrode surface.

 

Considering the charge density on the electrode surface is depended by the applied voltage, the electric double layer capacitance differs depending on the voltage. The electrochemical reaction in the electric double layer capacitor mainly occurs on the electrode surface, and it is usually the adsorption and desorption behavior of anions and cations. The cyclic voltammetry curve of the electric double layer capacitor has a rectangular shape as shown in Fig. 2 (a), and the constant current discharge curve of this type of material has a linear relationship, as shown in Fig. 2 (c).

 

The electric double layer effect occurs at the interface between the electronic conductor and the ionic conductor, which is present in almost all electrochemical energy storage systems. However, it is generally regarded as a side reaction in electrolyzers, fuel cells, and batteries, and it will not be regarded as the main energy storage mechanism. On the contrary, the working principle of supercapacitors is based on this effect, which requires supercapacitors to maximize this effect in the design and development process.

 

2. The pseudocapacitance storage mechanism

 

The pseudocapacitance, also known as Faraday pseudocapacitance, are the capacitance that related to the electrode charging potential, when it is two-dimensional or quasi-two-dimensional spaces in the electrode surface or bulk phase, and when the electroactive substances undergo underpotential deposition, and highly reversible chemical adsorption, desorption or oxidation, and reduction reactions occur. It is the main mechanism for energy storage of metal oxides, metal carbides, and conductive polymer supercapacitors. Although these reactions are very similar to those in batteries, because both their charges pass through the electric double layer capacitor, the difference is that the formation of pseudocapacitance is more like caused by special thermodynamic behavior. The cyclic voltammetry curve and constant current discharge curve of the pseudocapacitance are similar to the electric double layer capacitor. Unlike the electric double layer capacitor, the pseudo-capacitor has a higher energy density, but is limited by the electrochemical reaction kinetics and the irreversibility of the reaction. As a result, the charge and discharge power and cycle life of the pseudo capacitor are smaller than the electric double-layer capacitor. It should be pointed out that due to the presence of active functional groups, most supercapacitor electrodes have pseudocapacitance. For example, the electrochemical response of electric double layer capacitors composed of nanomaterials such as graphene is mainly formed by redox reaction caused by defects in carbon materials.

 

3. The Faraday reaction storage mechanism

 

This storage mechanism is mainly based on the redox reaction of metal cations in the electrode, usually accompanied by the redox reaction of metal cations. The extraction and insertion of metal cations in the electrode material phase extraction will cause the gain and loss of electrons in the material and then stores energy. It mainly includes two ways of material phase transformation or alloying reaction. When these electrodes are charged and discharged, a plateau voltage will appear, which corresponds to the redox peak voltage in the cyclic voltammogram, as shown in Figure 2 (b) and 2 (d). Compared with the other two types of capacitors, Faraday capacitance have higher stored energy, which is generally 10-100 times that of electric double layer capacitors.

 

Some electrode materials that exhibit Faraday effect, such as Ni (OH) 2 or similar battery electrode materials, are considered to be pseudocapacitive materials in many literatures, which is confusing to the readers. Although this type of material has a higher energy storage energy density and is limited by the solid phase diffusion of the material ions, the high-power charge-discharge performance is far worse than the pseudocapacitance material.

 

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