Study On Ion Diffusion In Porous Carbon Electrode And Performance Of Supercapacitor

Electric double layer capacitors as a new type of energy storage device mainly base on physical adsorption that stores charge in active materials to complete electrical energy storage.It has the characteristics of fast charge and discharge rate,high power density,and long cycle life.However,the problems of low energy density and severe capacity loss at high current density limit the practical application of electric double layer capacitors,which is still one of the issues that need to be solved urgently.In response to the above key scientific issues,this article achieves the dual effect of mass transfer kinetics and energy density by adjusting the microscale of the porous carbon electrode,optimizing the pore structure,strengthening the ion transport in the electrode material.The specific research contents are as follows:Using dopamine and layered bimetallic hydroxide MgAl-LDH as the precursor of the carbon source and template,using carbonization coupled KOH activation technology,two-dimensional porous carbon nanosheet materials with different thicknesses were prepared.The effect of dopamine reaction polymerization time on the microstructure and morphology of the material was investigated.Taking this two-dimensional porous carbon nanosheet as an example,the influence of the thickness of the two-dimensional nanosheet on the electrolyte ion diffusion behavior in the electrode was studied;the key factors affecting the ion diffusion were investigated.As a result,it was found that the thickness of the two-dimensional porous carbon nanosheets gradually increased as the polymerization time of dopamine reaction increased.The thickness of the two-dimensional porous carbon nanosheets,the electrode surface area and the Warburg coefficient all affect the diffusion and transport of electrolyte ions in the electrode.There is a balanced relationship between the thickness and diffusion of two-dimensional porous carbon nanosheets.Two-dimensional porous carbon nanosheets with a thickness of 99±3 nm have the largest ion diffusion coefficient.Electrochemical performance test results show that,under the three-electrode test system,in 6 M KOH electrolyte,a superior capacitance value of 280 F g^-1 can be obtained at 0.5 A g^-1,with a retention rate of 81%even at a large current density of 100 A g^-1.Based on this material,a symmetric supercapacitor was assembled,using ionic liquid EMIMBF4 as the electrolyte.When the power density was 1.8 kW kg^-1,the energy density was as high as 94 Wh kg^-1.Using starch and melamine as carbon and nitrogen sources,respectively,using pre-carbonization and KOH activation techniques,high specific surface area porous carbon with different mesopore volume ratios was prepared.The effect of the mass ratio of starch to melamine on the microstructure and electrochemical performance of porous carbon was investigated;the effect of the pore volume of mesoporous on the ion diffusion behavior of the electrolyte inside the porous carbon electrode was investigated.It was found that the specific surface area of the prepared porous carbon was up to 3828 m² g^-1.As the ratio of mesopore pore volume gradually increased from 0.23 to 0.70,the ion diffusion coefficient in the electrode gradually increased and the mass transfer rate in the electrode gradually increased.The electrochemical performance test results showed that,under the three-electrode test system,in the 6 M KOH electrolyte,at a current density of 1 A g^-1,the specific capacitance of porous carbon was as high as 351 F g^-1,at high current density of 100 A g^-1,the capacitance retention rate was as high as 80.4%.In 1 M H2SO4 electrolyte,at a current density of 1 A g^-1,the specific capacitance of porous carbon was as high as 323 F g^-1,at a large current of 100 A g^-1,the capacitance retention rate was as high as 77.4%.Assembled into a symmetrical capacitor,using ionic liquid EMIMBF4 as the electrolyte,when the power density was 2.0kW kg^-1,the energy density was as high as 115 Wh kg^-1,and when the power density was47.8 kW kg^-1,the energy density was 16 Wh kg^-1.After a stable cycle of 50,000 cycles,the capacitance retention rate was as high as 81.8%.