This section mainly explains the working principle of supercapacitors. Its energy is mainly stored at the contact interface between the electrode and the electrolyte. This storage method is greatly affected by the selected electrode material. If the two electrodes of the supercapacitor are made of different types of materials, it is difficult to fully understand the working principle of the supercapacitor. Therefore, we will first briefly introduce the working principle of supercapacitors, and then explain in detail the energy storage mechanism between different electrodes and electrolytes. We will classify them according to the differences in electrodes and electrolytes, and introduce some electrical performance characteristics of supercapacitors.

1. Working principle of supercapacitors

As shown in Figure 1, the supercapacitor container is mainly composed of several parts such as current collectors, electrodes, electrolytes and diaphragms. The role of the diaphragm is the same as that of the diaphragm in the battery, which separates the two electrodes, prevents short circuits between the electrodes, and allows ions to pass through. The working principle of supercapacitors energy storage is to store electrical energy through the double-layer capacitor formed by charge separation at the interface between the electrolyte and the electrolyte.

Schematic diagram of supercapacitor structure and working principle

Schematic diagram of supercapacitor structure and working principle

2. Energy storage mechanism of supercapacitors

There are many materials used in the manufacture and production of supercapacitor electrodes and electrolytes. In order to gain a deeper understanding of the energy storage mechanism and working principle of supercapacitors and optimize the performance of supercapacitors, two experiments, cyclic voltammetry and constant current discharge, are usually required 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 pseudocapacitance storage mechanisms, respectively; b and d represent the cyclic voltammetry and constant current discharge curves of supercapacitor electrodes under Faraday capacitance storage mechanism, respectively.

working principle of supercapacitors

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

2.1 Energy Storage Mechanism of Double-layer Capacitors

The double-layer effect is a key aspect of the working principle of supercapacitors. The double-layer effect is the separation of positive and negative charges, which aggregate at the electrode-electrolyte interface and is the main mechanism for energy storage in carbon material supercapacitors such as activated carbon, carbon fiber, and carbon felt. The formation of the double-layer effect is mainly due to the increase or decrease of the surface charge of the electrode, which causes the movement of positive and negative charges in the electrolyte solution on the interface side to balance the charge imbalance caused by the change in the surface charge of the electrode.

Considering that the charge density on the electrode surface depends on the applied voltage, the double-layer capacitance varies with different voltages. The electrochemical reaction in the double-layer capacitor mainly occurs on the electrode surface, and is usually the adsorption and desorption behavior of anions and cations. The cyclic voltammetry curve of the double-layer capacitor is rectangular as shown in Figure 2 (a), and the constant current discharge curve of this type of material is linear, as shown in Figure 2 (c).

The double-layer effect occurs at the interface between the electron conductor and the ion conductor, and this phenomenon exists in almost all electrochemical energy storage systems. However, it is usually considered as a side reaction in electrolyzers, fuel cells, and batteries, and is not 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 during design and development.

2.2 Pseudocapacitor storage mechanism

Pseudocapacitors, also known as Faraday pseudocapacitors, are underpotential deposits of electroactive substances on the two-dimensional or quasi-two-dimensional space on the electrode surface or in the bulk phase, and highly reversible chemical adsorption, desorption or oxidation, reduction reactions occur, producing capacitance related to the electrode charging potential. It is the main mechanism for energy storage in metal oxide, metal carbide, and conductive polymer supercapacitors. Although these reactions are very similar to those in batteries, both charges pass through the double-layer capacitor. The difference is that the formation of pseudocapacitors is more caused by special thermodynamic behavior. 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 electrochemical reaction kinetics and the irreversibility of the reaction, resulting in pseudocapacitors with smaller charge and discharge power and cycle life than 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 redox reactions caused by defects in carbon materials.

2.3 Faraday reaction storage mechanism

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

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

3. Classification of supercapacitors

There are many classification standards for supercapacitors. This article mainly introduces two classification methods. The first is to classify according to the different energy storage mechanisms of electrode materials, and the second is to classify according to different electrolytes.

3.1 Classification based on different energy storage mechanisms

According to different energy storage mechanisms, supercapacitors can be divided into symmetrical supercapacitors, asymmetrical supercapacitors and hybrid supercapacitors. The performance of the three types of supercapacitors is shown in Table 1.

ProjectSymmetric supercapacitorAsymmetric supercapacitorHybrid supercapacitor
Main mechanismDouble layerDouble layer + PseudocapacitorDouble layer + Faraday
Energy density5Wh/Kg30Wh/Kg100Wh/Kg
Power density9KW/Kg5KW/Kg4KW/kg
Operating temperature-40C-80℃-25℃-60℃-40C-80℃
Typical electrodeCarbon materialsCarbon materials, metal oxides, conductive polymersCarbon materials, deintercalable materials, etc.
Typical electrolyteOrganic systemWater systemOrganic system
AdvantagesHigh power densityHigh power density, high energy densityHigh energy density
DisadvantagesLow energy densityHigh price, poor lifeLow power density

Table 1: Performance indicators of different types of supercapacitors

3.2 Classification based on different electrolytes

According to the type of electrolyte, it can be conventionally divided into aqueous electrolyte and organic electrolyte. Aqueous electrolytes include 1. Acidic electrolytes, which mostly use 36% H2SO4 aqueous solution as electrolyte, 2. Alkaline electrolytes, which usually use strong bases such as KOH and NaOH as electrolytes and water as solvent, 3. Neutral electrolytes, which usually use salts such as KCl and NaCl as electrolytes and water as solvents, and are mostly used as electrolytes for manganese oxide electrode materials; organic electrolytes usually use lithium salts such as LiClO4 as a typical representative, quaternary ammonium salts such as TEABF4 as a typical representative as electrolytes, organic solvents such as PC, ACN, GBL, THL and other organic solvents as solvents, and the electrolyte is close to saturated solubility in the solvent. In addition, solid electrolytes are also included. With the continuous breakthroughs in solid electrolytes for lithium-ion batteries, this type of electrolyte has become a research hotspot in the field of supercapacitor electrolytes.

4.Electrochemical properties of supercapacitors

This section will briefly discuss the electrical properties of supercapacitors, hoping to reveal the causes of some special phenomena in supercapacitors through analysis, and analyze the impact of these phenomena on capacitor performance. The following will also discuss how to correctly select supercapacitors that match the requirements of different application fields.

1. Relationship between voltage and capacity

Variability of capacity is one of the characteristics of supercapacitors, although this characteristic is not the most critical performance of supercapacitors. However, it needs to be considered when SC is part of an energy system. This is because the capacity variation of supercapacitors over the entire voltage range is between 15% and 20% of the rated capacity, which cannot be ignored in most designs of energy systems. The capacitance of supercapacitors can be measured by formula (1). The formula links the charge stored between the double layers with the voltage, indicating that the amount of charge stored between the double layers is proportional to the voltage. As the voltage increases, the charge distribution density near the double layers will increase.

C=Q/U(U is voltage, Q is charge)

In addition to the influence of voltage on the capacitance of supercapacitors, ambient temperature also affects the capacitance of supercapacitors. Although supercapacitors have a wide operating temperature range, wide temperature changes will have a certain impact on the capacitance of supercapacitors. As part of the energy storage tool, the impact of ambient temperature on the capacitance of supercapacitors should be fully considered when designing the system. Temperature mainly affects the capacitance of supercapacitors by affecting the Brownian motion of anions and cations on both sides of the double electric layer. Generally speaking, temperature has different effects on the Brownian motion of different ions, which means that when the temperature rises, the Brownian motion speed of different anions and cations increases greatly, which will reduce the capacitance of the capacitor. Some studies have reported that when the temperature changes by 1°C, the capacitance of the supercapacitor will change by 0.1%, indicating that when the operating temperature of the supercapacitor changes by 80°C, there will be an 8% capacity change. Although temperature changes have little effect on the capacitance of supercapacitors, they should also be given enough attention when designing the system.

2. Distribution of charge on the electrode surface

Usually, the matching of the electrode pore size and the electrolyte size has been considered in the design process of supercapacitors. It should be pointed out that the electrolyte solvent of supercapacitors is usually composed of polar molecules, such as water molecules, acetonitrile, etc. These polar molecules will produce solvation reactions with ions and combine into larger and more stable units. When the diameter of the electrode pore is smaller than the diameter of the free ion, the solvated ions and free ions will not pass through the pore and will not affect the double-layer capacitance; when the pore is larger than the solvated ion, the solvated ion will pass through the pore; when the pore is between the diameter of the ion and the solvated ion, the free ion will pass through the pore, and the solvated ion will be desolvated to form free ions to enter the pore. This process requires energy consumption. The existence of the latter two types of pores will affect the charge distribution of the double layer. The charge distribution has a certain relationship with the fast charge and discharge rate and life of the capacitor.

Research has found that it is precisely because of the existence of this solvation phenomenon that supercapacitors cannot achieve full charge and discharge under high power conditions. It is reported that the desolvated polarized ions and dissolved polarized ions in commonly used capacitors account for about 20% of the electrode surface area.

3. Ohmic polarization

During the charging and discharging process, ions and electrons in supercapacitors will move. On the one hand, due to the Joule heat generated by the movement of electrons, the kinetic energy of electrons is converted into heat energy and dissipated through the conductor. On the other hand, the ions will generate heat and be dissipated by friction with other ions during the movement of the electrolyte. The voltage required to overcome these two energy dissipations is proportional to the transported ions or current. This phenomenon is called the ohmic polarization phenomenon of supercapacitors.

Usually, temperature changes have opposite effects on electrons and ions. For solid electrodes, when the temperature rises, the atomic vibrations in the solid molecules become more intense, which will generate greater Joule heat. For ions, the temperature rises, accelerates the molecular movement, reduces the viscosity, and is conducive to reducing the energy loss caused by ion movement. Studies have shown that ion movement is more significantly affected by temperature, indicating that when the temperature rises, it is conducive to reducing the energy loss of supercapacitors.

Compared with other electrochemical energy storage technologies, ohmic polarization has less effect on supercapacitors. Since the typical application scenario of supercapacitors is high power, two aspects need to be paid attention to in this application scenario. On the one hand, the ohmic polarization of supercapacitors will cause significant changes in voltage, which will affect efficiency. Since the most significant feature of supercapacitors is high efficiency, ohmic polarization can be used as an important indicator to judge product performance; on the other hand, dangers caused by excessive temperature should be avoided as much as possible during use, and good thermal management is required during product design.

4. Self-discharge

High self-discharge is one of the main disadvantages of supercapacitors, which greatly limits the application of supercapacitors. In practice, the energy retention time of the product is short. Some researchers have found that the capacity loss rate is as high as 36% after being shelved for 2 hours. The capacity loss of supercapacitors is mainly caused by the leakage point flow formed by ions passing through the electrolyte membrane in the supercapacitor. The self-discharge rate of supercapacitors is linearly related to the storage time. Researchers reduce the self-discharge performance of supercapacitors by electrode coating, but sacrifice the energy density of supercapacitors.

Summary

This article explores the energy storage mechanism and working principle of supercapacitors in depth, mainly through double-layer capacitance and pseudocapacitance to achieve energy storage. Double-layer capacitance relies on charge separation at the electrode-electrolyte interface, while pseudocapacitance involves highly reversible chemical reactions of electroactive substances on the electrode surface. The article details the effects of different electrode and electrolyte materials on supercapacitor performance, and discusses electrochemical performance characteristics and optimization strategies.