Working principle of supercapacitors

Working principle of supercapacitors

The supercapacitor container mainly consists of several parts such as current collector, electrode, electrolyte, and separator. The function of the separator is the same as that of the separator in the battery, which separates the two electrodes, prevents short circuits between the electrodes, and allows ions to pass through. The basic principle of supercapacitor energy storage is to store electrical energy through a double layer capacitor formed by charge separation at the interface between the electrolyte and electrolyte.

Figure 1: Schematic diagram of the structure and working principle of supercapacitors

2、 Energy storage mechanism

There are many materials used for the manufacturing and production of electrodes and electrolytes in supercapacitors. In order to gain a deeper understanding of the energy storage mechanism of supercapacitors and optimize their performance, it is usually necessary to use cyclic voltammetry curves and constant current discharge experiments to characterize the performance of different supercapacitor electrodes. Figure 2 shows the cyclic voltammetry and constant current discharge curves of supercapacitor electrodes under different energy storage mechanisms, where a and c represent the cyclic voltammetry and constant current discharge curves of supercapacitor electrodes under double layer capacitance and pseudocapacitive storage mechanisms, respectively; b. D represents the cyclic voltammetry and constant current discharge curves of the supercapacitor electrodes under the Faraday capacitor storage mechanism.

Figure 2: Cyclic voltammetry and constant current discharge curves of double-layer capacitors under different storage mechanisms

1. Double layer capacitance storage mechanism

The double layer effect is formed by the separation of positive and negative charges, which accumulate at the electrode electrolyte interface. It is the main mechanism for energy storage of carbon materials such as activated carbon, carbon fiber, and carbon felt in supercapacitors. The formation of the double layer effect is mainly caused by the increase or decrease of high-energy conduction band electrons on the electrode surface, causing the movement of positive and negative charges in the electrolyte solution on the interface side, in order to balance the charge imbalance caused by the changes in high-energy conduction band electrons on the electrode surface.

Considering the surface charge density of the electrode, which depends on the applied voltage, the double-layer capacitance varies depending on the voltage. Electrochemical reactions in double-layer capacitance mainly occur on the electrode surface, and usually involve the adsorption and desorption behavior of anions and cations. The cyclic voltammetry curve of the double-layer capacitor shows a rectangular shape as shown in Figure 2 (a), and the constant current discharge curve of this type of material shows a linear relationship, as shown in Figure 2 (c).

The double layer effect occurs at the interface between electronic and ionic conductors, and is present in almost all electrochemical energy storage systems. However, in electrolyzers, fuel cells, and batteries, it is often considered a side reaction and is not considered a primary energy storage mechanism. On the contrary, the working principle of supercapacitors is based on this effect, which requires supercapacitors to quantify this effect as much as possible in the design and development process.

2. Pseudocapacitive storage mechanism

Pseudo capacitance, also known as Faraday quasi capacitance, is a two-dimensional or quasi two-dimensional space on the electrode surface or bulk phase where electroactive substances undergo under potential deposition, undergo highly reversible chemical adsorption, desorption or oxidation, reduction reactions, and produce capacitance related to the electrode charging potential. It is the main mechanism for energy storage in metal oxides, metal carbides, and conductive polymer supercapacitors. Although these reactions are similar to those in batteries, both charges pass through the double layer capacitance. The difference is that the formation of pseudocapacitance is more caused by special thermodynamic behaviors. The cyclic voltammetry curve and constant current discharge curve of pseudocapacitors are similar to those of double-layer capacitors. Unlike double-layer capacitors, pseudocapacitors have a higher energy density, but are limited by the kinetics of electrochemical reactions and the irreversibility of reactions, resulting in lower charging and discharging power and cycle life compared to double-layer capacitors. 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 double-layer capacitors composed of nanomaterials such as graphene is mainly formed by the oxidation-reduction reaction caused by carbon material defects.

3. Faraday reaction storage mechanism

This storage mechanism is mainly based on the oxidation-reduction reaction of metal cations in the electrode, usually accompanied by the oxidation-reduction reaction of metal cations. The extraction and insertion of metal cations in the electrode material phase cause the gain and loss of electrons in the material, thereby storing energy. It mainly includes two methods: material phase transformation or alloying reaction. These electrodes will exhibit a plateau voltage during charging and discharging, which corresponds to the oxidation-reduction peak voltage in the cyclic voltammetry curve, as shown in Figures 2 (b) and 2 (d). Compared to the other two types of capacitors, Faraday capacitors have higher energy storage, generally 10-100 times that of double-layer capacitors.

Often, electrode materials that exhibit Faraday effects, such as Ni (OH) 2 or similar electrode materials with battery properties, are considered pseudocapacitive materials in many literature, causing confusion for readers. Although these materials have higher energy storage density, their high-power charge and discharge performance is far inferior to that of pseudocapacitive materials due to the limitation of ion solid-state diffusion.