An in-depth understanding of the charge storage mechanism is essential to further improve the performance of supercapacitors. How does the charge storage method affect the performance of supercapacitors?
First of all, the charging mechanism has an impact on the power density, and adjusting the energy storage mechanism can undoubtedly improve the power performance of the device. Theoretical simulation studies have shown that the hydrophobic empty channel has a faster charging rate than the hydrophilic ion-filled channel. Since the ion diffusion and migration can quickly migrate into the carbon nanopores, the adsorption mechanism of the opposite-sign ion is conducive to fast charging. On the contrary, 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, it is different. The mechanism results in a change in the number of ions in the pores.
For example, the adsorption mechanism of the opposite sign ion can increase the ion concentration in the carbon pores, and the denser the ions are accumulated 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 in the pore. It can be seen that different ion adsorption, ion exchange and ion desorption mechanisms lead to different device power performance, which should be screened Come out an energy storage mechanism suitable for fast charging.
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 small free energy increase, that is, reducing the increase in voltage per unit of stored charge (equivalent to increasing the specific capacitance). Kondrat and Kornyshev pointed out that the adsorption of opposite-sign ions is disadvantageous in terms of energy. Because the ions entering the pores will reduce the entropy of the system. When ions of the same charge accumulate in the carbon pores, the enthalpy change is also disadvantageous. In the energy storage process dominated by the ion exchange mechanism, the total ion concentration in the pores remains unchanged, which reduces the enthalpy change caused by the dense accumulation of ions, and at the same time reduces the entropy change, and is in a favorable position in terms of energy. This may be the thermodynamic reason why the ion exchange mechanism clarified by the in-situ experimental technique is universal.
Due to the interaction between the charges, the desorption mechanism of the same ion reduces the enthalpy change, and at the same time increases the entropy change, thus increasing the capacitance. However, the same ion desorption mechanism has not been observed in reality, indicating that there are other important factors At work. Obviously, it is necessary to deeply understand the relationship between supercapacitor charging mechanism and capacitance. Under kinetic conditions, the charging mechanism depends on the relative movement rate of anions and cations in the pores. Experimental and theoretical research on these mechanisms will further promote the effective control of ion diffusion rates in different pores, thereby controlling the kinetic mechanism of energy storage and increasing the energy density of supercapacitor devices. The understanding of ion adsorption mechanisms is of great significance to the development of science and technology. Benefits, not only in the field of energy storage, the adsorption process of porous carbon is a key scientific issue in the fields of flow batteries, biofuel cells, biosensors, gas-sensitive materials, and capacitive deionization and desalination.