Study On High Voltage Characteristic Of Electric Double Layer Capacitor Based On Carbon Material

Electric double layer capacitors have attracted much attention due to high power density, good stability, long cycle life. However, its energy density is very low, which limits its large-scale application. Thus, it is urgent to increase the energy density. In order to obtain high energy density, electrode materials should not only have high specific capacitance, but also have high voltage characteristic. Based on the equation E = 1/2 CV2, increasing the operating voltage is more effective due to the quadratic relationship. Activated carbon has been widely used as electrode material for Electric double layer capacitors in commercial, due to their low cost, readily availability and controllable pore size.In this paper, two methods to improve the working voltage of electric double layer capacitor based on activated carbon in commercial are studied. One is investigating the effect of heat treatment in H2 atmosphere on structure and electrochemical properties of activated carbon to find the optimal temperature of improving the working voltage. Another is comparing the different matching ratio of positive/negative electrode to find the optimal ratio, thus to improve the working voltage of electric double layer capacitor.Activated carbon material is heat-treated in H2 atmosphere at different temperatures, and then various tools are used to study the changes of structure and surface features. The results showed that surface morphology has no obvious change.The specific surface area and pore volume of sample treated at 600?have a slightly increase, while those of samples treated at higher temperature decrease. Oxygen containing functional groups on the sample are significantly reduced after treatment.The sample treated at 600 °C shows the optimized electrochemical performance with increase capacitance, enhanced stability, and improved energy density. At 3.0 V, its energy density reaches 19.5 Wh/kg while that of untreated sample is only 16.2 Wh/kg.Therefore, heat-treatment under H2 atmosphere is an effective method to improve the cycle life and energy density at high voltage of Electric double layer capacitors based on activated carbon materials.Through assembling electric double layer capacitors with positive/negative electrode matching ratio of 0.6:1,0.8:1, 1:1,1.2:1,1.4:1, it can find that capacitors with the mass ratio of 1.2:1 have the best electrochemical performance. At a currentdensity of 10 A/g, its retention rate of capacitance is the highest with 76.4% at 2.7 V,68.4% at 3.0 V, 59.4% at 3.25 V, respectively. By studying three electrodes cell, it could see clearly that the potential of positive and negative electrodes are asymmetry in the charging/discharging process, which provides a good explanation for different electrochemical properties with different mass ratio. Therefore, it can improve the electrochemical properties of electric double layer capacitors through adjusting the positive/negative electrode matching ratio.