With the expansion of the application field of supercapacitors , some of its own shortcomings have also been exposed to the public eye. The first one is self-discharge, which has a significant impact on the energy storage capacity and cycle stability of supercapacitors . When the supercapacitor is independent of the external circuit, the self-discharge is caused by the parasitic current passing through one or a pair of independent electrodes in the positive and negative electrodes. Its generation can be mainly attributed to the following reasons:
1) When the electrode/electrolyte surface voltage is higher than the electrolyte decomposition voltage, a redox reaction occurs, which produces a self-discharge caused by a voltage-controlled Faradaic impedance that is proportional to the reaction rate The rate of electron transfer during the redox reaction is related to the overpotential,
Among them, V represents the working potential of the supercapacitor, and E0 represents the redox reaction potential. According to the Tafel formula, the rate of the redox reaction has an exponential relationship with the overpotential, which means that the self-discharge increases exponentially with the increase of the working voltage. At present, the most mature product commercialized is the organic electrolyte carbon-based supercapacitor. When the production process is poorly controlled, the system contains more water, and the product operating voltage is higher than the water splitting voltage of 1.23V, the following hydrolysis side reactions will occur. Cause the system to produce self-discharge.
2) When there are defects, impurities or the surface voltage of the electrode/electrolyte reaches the critical voltage, the electrolyte ion concentration will be high in/close to the electrode surface. Part of the ions diffuses back to the electrolyte, and the other part diffuses to the surface of the electrode and takes away part of the charge, so that the open-circuit voltage of the supercapacitor decreases. This process is greatly affected by temperature and initial open-circuit voltage. Studies have shown that the open-circuit voltage of supercapacitors and time obey the following laws:
V—the open circuit voltage across the capacitor;
V0—the initial voltage of the open circuit across the capacitor;
cR0—initial ion concentration in the high region;
D - ion diffusion coefficient;
C12—capacitance value after two electrode interface capacitances are connected in series.
3) When the supercapacitor has an intrinsic ohmic self-discharge resistance Rsd, its open circuit voltage and RsdC obey the following exponential relationship:
Compared with chemical batteries, supercapacitors have a much larger self-discharge characteristic. When the supercapacitor is disconnected from the charging circuit, its open circuit voltage will gradually decrease due to the existence of self-discharge, so that its energy storage can only last for a few days or more. For short periods of time, chemical batteries can store energy for months or even years. In addition, when the supercapacitor is excited by external factors (such as light, heating, vibration, etc.), its self-discharge problem is particularly prominent, and this excitation energy usually only requires 10-1000 μW, which greatly limits the application of supercapacitors. Taking carbon electrodes as an example, the self-discharge is exponentially related to the working voltage. When the supercapacitor works normally, high self-discharge will cause a large amount of energy loss, resulting in a decrease in Coulombic efficiency. When working at low temperature, the supercapacitor shows a rapid capacity decay, which is mainly due to the decomposition of the electrolyte on the electrode surface, especially when the surface functional groups exist, this process is more obvious.
Self-discharge is a key indicator that determines the performance of supercapacitors. Compared with chemical energy storage, the large self-discharge has greatly hindered the further promotion of supercapacitor market applications. Therefore, improving the self-discharge of supercapacitors is an urgent need to solve The traditional methods for improving supercapacitors mainly start with raw materials and production processes, including pore structure control and surface functional group treatment of activated carbon, control of separator thickness and pore size, and optimization of drying and aging processes; However, these methods for improving supercapacitors are not only complicated in process, but also have poor stability during use of supercapacitors.