Electrochemical capacitors (or supercapacitors) have become a new type of capacitor between ordinary capacitors and batteries due to their high power density, wide operating temperature range (-20~60℃), no pollution, long life and fast charging. Energy storage devices. In recent years, supercapacitors have attracted widespread attention from academia and industry as a new energy device in the 21st century. They can not only be used as hybrid power systems of electric vehicles, mobile communication devices and high-power high-charge output devices, but also in wastewater deionization. It has important application prospects in areas such as desalination treatment, solar power generation and energy storage devices for wind turbines.

　　Source of this article: Microcomputing Cloud Platform WeChat public account ID: v-suan

　　An electrochemical capacitor is an energy storage device that stores charges on the surface of a porous electrode with a high specific surface area through electroadsorption of ions or a fast Faradaic reaction. In the past decade, various performance indicators of supercapacitors have been greatly improved through nanoscale control of electrode materials. A thorough understanding of the charge storage mechanism is crucial to further improve the performance of supercapacitors.

　　Two supercapacitor energy storage mechanisms

　　As early as the 1960s, Conway proposed the concept of supercapacitors. A supercapacitor is an energy storage system composed of a pair of electrodes, a diaphragm, and an electrolyte solution. The diaphragm plays the role of electrical insulation and ion conduction, while the porous electrode plays the role of energy storage.

　　There are two electric energy storage mechanisms of supercapacitors. One is to store charge in the double electric layer at the interface between the electrode and the electrolyte. Typically, a high specific surface area carbon material is used as the porous electrode material. It is like an electronic sponge. It absorbs ions during charging and desorbs ions during discharging to form a double-layer electrochemical capacitor; the other is to use the fast Faradaic reaction that occurs on the electrode surface to store charge, usually using transition metal oxides or conductive polymers as electrodes. A capacitor made of materials, also known as a quasi-capacitor or pseudocapacitor.

　　In recent years, studies have shown that porous carbon with pore diameters less than 1 nm exhibits unusually high capacitance, subverting the traditional concept that micropores do not contribute much to capacitance. According to the electric double layer theory, there is a reciprocal relationship between capacity and electric double layer distance. When the carbon pore diameter d<1 nm, ions are desolvated in the pore channel, allowing charges to be packed more closely at the interface between the electrode and the electrolyte, shortening the distance between the ions and the double electric layer on the electrode surface, thereby increasing the capacitance.

　　Both experiments and theoretical simulations show that the maximum capacitance value occurs when the pore size and the diameter of the ion match. However, there are also studies claiming that there is no correlation between pore size and specific capacitance. The academic community is currently inconclusive on this and requires further research.

　　Pseudocapacitors are based on redox reactions that occur on the surface of oxide or conductive polymer electrodes. With the development of nano-oxide electrodes and carbon/oxide hybrid electrodes, the understanding of pseudocapacitive energy storage mechanisms is gradually changing. Due to the Details such as the oxide surface and charge transfer mechanism cannot be accurately characterized, and people's understanding of the mechanism of pseudocapacitors is far from sufficient.

　　Analyze and explore the mechanism of charge storage

　　1. Electric double layer capacitor

　　Electric double layer capacitors (EDLCs) store charge through the electroadsorption of ions on the electrode surface. Charge separation occurs at the electrode/electrolyte interface during polarization, which is called electric double layer capacitance C:

　　Among them, εr is the dielectric constant of the electrolyte, ε0 is the vacuum dielectric constant, d is the effective distance of the double electric layer, and A is the electrode specific surface area.

　　This theory was later further improved by Gouy, Chapman, Stern and Geary, etc., who believed that there are tight layers and diffusion layers in the electrolyte. Depending on the electrolyte used, the electric double layer capacitance is 5~20 μFcm-2. The specific capacitance of commercial activated carbon in aqueous electrolyte (150~300Fg−1) is generally higher than that in organic electrolyte (100~120Fg−1 ), but the organic system can work under a wider electrochemical window (more than 2.7 V under symmetrical assembly), while the electrochemical window of the aqueous system is limited by the water decomposition voltage and is less than 1.2 V, so the organic electrolyte system has Higher energy density E:

　　E=1/2CV2 (2)

　　2. Wettability of carbon channels and electrolyte

　　A large number of in-situ characterization experiments and computer simulation studies have shown that even when the applied voltage is zero, there are a large number of ions and solvent molecules in the porous carbon electrode pores due to the wettability of the carbon pores and electrolyte.

　　In-situ nuclear magnetic (NMR) studies also show that there are a large number of anions and cations in the carbon pores. When ionic liquids are used as electrolytes, the ion concentration is higher. When the ionic liquids are diluted with organic solvents, the ions in the pores are replaced by solvent molecules, and the ion concentration is greatly reduced.

　　NMR technology can not only study organic electrolyte systems, but is also suitable for the study of aqueous electrolyte systems. In the porous carbon (0.58 nm) and NaF systems, since the ionic radius is larger than the pore diameter of the porous carbon, even though the pores are filled with water, ions cannot enter the pores without voltage. Porous carbon with an average pore channel of 1.55 nm can allow ions to enter, indicating that steric hindrance is the main reason for preventing ions from entering the pore channels.

　　X-ray scattering (SAXS) and small-angle neutron scattering (SANS) were also used as probes to study the wettability of carbon pores without external voltage. Adding acetonitrile solvent to activated carbon fibers changes the intensity of neutron scattering, indicating that acetonitrile infiltrates the carbon pores. This method can study the wettability of different pore sizes and confirms that the smallest nanopores are not completely infiltrated. However, currently this method cannot Quantitatively study the number of ions and solvent molecules in the pores.

　　3. Quantitative description of charge storage mechanism

　　Supercapacitors store charges on the surface of the carbon electrode, with the electrode/electrolyte as the boundary. On the electrode side, electron migration and charge transfer mainly occur, while on the solution side, ion diffusion and charge transfer mainly occur.

　　Whether it is the formation of an electric double layer or pseudocapacitive energy storage, electroadsorption and fast Faradaic reactions mainly occur at the electrode/solution interface, and the process of charge transfer and transfer occurs between the interfaces. Only by fully understanding the charge transfer process between the electrode/electrolyte interface. To effectively understand the process of electrochemical energy storage.

　　The migration of electrons at the interface must not only span the distance in space, but also meet energy conditions. There are two ways of electron transfer. One is the classic activation transition, which needs to overcome a certain energy barrier; the other is quantum tunneling. Since the experimentally observed current density is much greater than the current in the classical activation transition, it can be It is believed that the mechanism of electron transfer in electrode reactions is tunneling.

　　The electrosorption process exchanges electrons through the electric double layer and can achieve rapid electron transfer, while the Faradaic reaction requires ions to cross the electric double layer for electron transfer. The electron transfer rate of the former is much higher than that of the latter, so the double-layer capacitor has better rate and cycle stability than the pseudocapacitive capacitor.

　　The energy storage mechanism is closely related to the electrode material, electrolyte solution and electrode polarization process. There may be multiple charge storage mechanisms when charging with an external voltage. The charging process is not only limited to the adsorption of ions of different signs on the electrode surface, but also includes the exchange mechanism of different ions and the desorption mechanism of ions of the same sign (Figure 1)

　　Figure 1 Different energy storage mechanisms when the carbon channels are completely filled with electrolyte. Different-sign ion adsorption, ion exchange and same-sign ion desorption mechanisms

　　(1) First of all, from a traditional point of view, the charged electrode surface usually reaches equilibrium by adsorbing ions of different signs, which is called the adsorption mechanism of ions of different signs;

　　(2) The second possibility is that the adsorption of ions of different signs is accompanied by the desorption of ions of the same sign from the pores (ions of the same sign refer to ions with the same charge as the electrode), that is, an ion exchange mechanism;

　　(3) The third possibility is the desorption mechanism of ions with the same sign.

　　In each case, the excess ionic charge in the carbon pores is balanced by the charge stored in the carbon electrode. In fact, the charging process mixes different mechanisms, such as ion exchange and adsorption of different ions often occurring simultaneously. By introducing the representative charging mechanism parameter

　　Where N(V) is the total number of ions in the channel at voltage V, N(V0) is the total number of ions in the channel at voltage V0 (V0=0), Ner(V) and Nco(V) are respectively at voltage V The number of ions of different signs and ions of the same sign in the channel. Qionic(V) and Qionic(V0) are the net ion charges in the channel under the start and stop voltages, and e is the charge of the electrons.

　　The X values represent different charging mechanisms: adsorption of opposite-sign ions, desorption of same-sign ions, and ion exchange mechanisms. X=+1 represents the adsorption mechanism of ions of different signs, X=0 represents the ion exchange mechanism, and X=−1 represents the desorption mechanism of the same ion.

　　When X is at an intermediate value, it indicates that there are two energy storage mechanisms at the same time. For example, X=0.3, there are both ion exchange and opposite-sign ion adsorption mechanisms during charging. Because 0.3 is closer to 0, the ion exchange mechanism dominates.

　　When V>V0, it indicates the charging process, and vice versa. The X values at the positive and negative poles are different, related to the selected voltage range, so they need to be calculated separately. For the case where the pores are completely empty at the initial voltage, there must be adsorption of ions of different signs when charging begins, because no ions of the same sign are desorbed in the pores at this time. For hydrophobic channels, ion pairs enter the channels during the charging process, and X>1 at this time.

　　How do energy storage mechanisms affect performance?

　　A thorough understanding of the charge storage mechanism is crucial to further improve the performance of supercapacitors.

　　How does the charge storage mechanism affect the performance of supercapacitors?

　　First of all, the charging mechanism has an impact on power density, and regulating the energy storage mechanism can undoubtedly improve the power performance of the device.

　　Theoretical simulation studies show that hydrophobic empty pores have a faster charging rate than hydrophilic ion-filled pores. Since ion diffusion migration can quickly migrate into carbon nanopores, the different-sign ion adsorption mechanism is conducive to fast charging. In contrast, the ion exchange mechanism requires ions to migrate in the opposite direction, which is not conducive to improving the power density of the device.

　　At the same time, different machine characteristics cause changes in the number of ions in the pores. For example, the heterosign ion adsorption mechanism can increase the ion concentration in the carbon pores, and the denser the ions are packed in the pores, the slower the diffusion rate. In addition to the stacking effect, the interaction between different ions and the electrode surface also affects the ion transport process within the pore.

　　It can be seen that different ion adsorption, ion exchange and ion desorption machine characteristics lead to different device power performance, and energy storage mechanisms suitable for fast charging should be screened out.

　　In principle, the energy storage mechanism directly affects the specific capacitance and therefore the energy density of the supercapacitor.

　　Under thermodynamic conditions, the charging process will follow the principle of minimum free energy increase, that is, reducing the voltage increase per unit stored charge (equivalent to increasing the specific capacitance). Kondrat and Kornyshev pointed out that the adsorption of ions with different signs is energetically unfavorable because the entry of ions into the pores will reduce the entropy of the system. When ions with the same charge accumulate in carbon pores, the enthalpy change is also unfavorable.

　　In the energy storage process where the ion exchange mechanism dominates. The total ion concentration in the pore channel remains unchanged, which reduces the enthalpy change caused by the dense accumulation of ions, and at the same time reduces the entropy change, leaving an energy advantageous position. This may be the thermodynamic reason for the ubiquity of ion exchange mechanisms elucidated by in situ experimental techniques.

　　The desorption mechanism of ions of the same sign reduces the enthalpy change due to the interaction between charges. At the same time the entropy change is increased and therefore the capacitance is increased. However, the co-ion desorption mechanism has not been observed in reality, indicating that there are other important factors at play.

　　Obviously, the relationship between the charging mechanism and capacitance of supercapacitors must be deeply understood. Under kinetic conditions, the charging mechanism depends on the relative movement rates of anions and cations in the pores. Experimental and theoretical studies on these mechanisms will further promote the effective control of ion diffusion rates in different pores, thereby controlling the kinetic mechanism of energy storage. Improve the energy density of supercapacitor devices. The understanding of the ion adsorption mechanism is of great benefit to the development of science and technology. Not only in the field of energy storage, the adsorption process of porous carbon is also used in the fields of flow batteries, biofuel cells, biosensors, gas-sensitive materials, and capacitive deionization and desalination. key scientific issues among them.

　　How in situ characterization techniques reveal electrode surface processes

　　Most current research on supercapacitors focuses on electrode materials and electrolyte solutions, lacking in-depth research on the energy storage mechanism. Supercapacitors store charge by adsorbing ions on the surface of microporous electrodes. Although there are many studies devoted to understanding this phenomenon, there is a lack of effective in-situ detection technology to selectively obtain various properties of the electrode/electrolyte solution interface and electrode surface. Information makes it impossible for people to understand the microscopic images of the charge and discharge process at the molecular level.

　　As people realize the importance of in-depth study of energy storage mechanisms in improving the performance of electrochemical capacitors, a variety of in-situ characterization techniques have been established to reveal electrode surface processes at the microscopic level.

　　In-situ technology includes in-situ spectroscopy and diffraction technology, as well as advanced computer simulation technology, which can conduct experimental and simulation studies of complex interface processes (Table 1).

　　Table 1 Principles, advantages and disadvantages of different in situ characterization techniques

　　In-situ nuclear magnetic resonance (NMR) can quantitatively study the adsorption process of different types of anions, cations and solvent molecules in microporous electrodes in real time. However, in some systems it is difficult to accurately distinguish the spectral peak positions of ions inside and outside the pores.

　　The electrochemical quartz crystal microbalance (EQCM) can measure the frequency change of the quartz crystal surface during the electrochemical process in real time, convert it into the mass change of the electrode, and then infer the charge storage mechanism from the total mass change of the electrode. But its disadvantage is that it is difficult to distinguish whether the mass change comes from anions, cations or solvent molecules.

　　In-situ infrared spectroscopy can measure the intensity changes of the ion chemical bond absorption spectrum, and can separately study the changes of anions and cations during the charging process. However, the penetration depth of infrared is only 0.4~1μm, so when the ions enter the pore, it is impossible to detect the infrared signal of the ions inside the pore. It must rely on measuring the concentration changes of the bulk electrolyte on the surface of the carbon particles for indirect testing, so it is difficult to completely quantify it. To study the ultra-electric energy storage mechanism.

　　In-situ scattering methods include X-ray and neutron ray scattering. The changes in ion scattering intensity during the charging process can be correlated with the energy storage mechanism to obtain information on the adsorption process in pores of different shapes. However, it is difficult to quantitatively study the scattering of different ions.

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