Influence Of Electrolyte Concentration On Performance Of The Carbon-based Supercapacitors Under The Same Conductivity

As a new type of energy storage devices,supercapacitors with the advantages of fast charging and discharging,high power density,good cycle performance,high safety and efficiency can be potentially used in wind power generation,industrial control,new energy vehicles,electronic equipment,national defense and military.Electrolyte is an important part of supercapacitorand greatly determines the performance of supercapacitor.With the concentration of the electrolyte increases,the conductivity increases first and then decreases.Therefore,there exists a case where the conductivity is equal for different concentration of electrolytes.At present,the influence of electrolyte concentration with the same conductivity on the performance of supercapacitors is not discussed.In this paper,supercapacitors are fabricated experimentally by using electrolytes with different concentrations but equal conductivity,and their performance are systematically tested.At the same time,the double electric layer model is established,and the current density in the discharge process of supercapacitor is deduced by Nernst-Planck equation,and explaining why the supercapacitor with high concentration of electrolyte has better rate performance.The main contents of this thesis are as follows:（1）The development prospects,working principle,classification and application of supercapacitors are briefly describe.The progress of the influence of electrolytes on the performance of supercapacitors are summarized,The research purpose and significance of this paper is proposed.（2）The materials needed for the experiment and their preparation methods are introduced,and the physical and chemical properties of potassium hydroxide aqueous solution,sodium perchlorate aqueous solution and EMImBF₄/acetonitrile solution including conductivity and viscosity are measured.The results show that the conductivity of these electrolytes increases first and then decreases,while the viscosity increases gradually with the increase of concentration.Then the commercial activated carbon electrode material（YP-50F）used in the experiment are characterized,It is found that YP-50F is a typical porous material with specific surface area of1371.40 m²/g,and pore volume of 0.78 cm³/g.Meanwhile,the fabrication process of the supercapacitor electrode and the assembly process of the supercapacitor used in the experiment in this paper are described.（3）Supercapacitors are assembled by electrolytes with different concentrations but equal conductivity and other components,measured with systematic electrochemical testing including cyclic voltammetry test,constant current charge-discharge test,AC impedance spectroscopy test,energy density and power density test.For KOH solution,NaClO₄ solution,EMImBF₄/acetonitrile solution,the experimental results show that the electrochemical stable voltage window of supercapacitors with high concentration electrolyte is respectively 0.1,0.2 and 0.2 V higher than those with low concentration electrolyte with the same conductivity.At the scanning speed of 200 mV/s,the capacity retention of supercapacitor with high concentrationKOHaqueoussolution,NaClO₄aqueoussolutionand EMImBF₄/acetonitrile solution is respectively 5.3%,2.8%and 6.1%higher than those with low concentration solution.Compared with the low concentration electrolyte but equal conductivity,the charge transfer resistance of supercapacitor with high concentration electrolyte is smaller.And the supercapacitor with high concentration solution has higher power density and higher energy density.Therefore,as the conductivity of electrolyte is equal,the supercapacitors with high concentration electrolyte for aqueous electrolyte and organic electrolyte have wider electrochemical stable voltage window,better rate performance,smaller charge transfer resistance,higher power and energy density.（4）The concentration distribution and potential distribution of electrolyte in the pipeline is obtained through establishing a double electric layer model of one dimensional pipeline.By combining the mass transfer equation with the established model,it can be concluded that the diffusion current density is proportional to 3/2power of concentration and inversely proportional to absolute viscosity in the supercapacitor assembled by electrolyte with equal conductivity.According to the theoretical analysis of the model,the supercapacitor with high concentration electrolyte has higher current density during the discharging process,when the conductivity of electrolyte is the same.It has higher capacity retention rate under the condition of high current charging and discharging.Raman spectroscopy and first-principle molecular dynamics simulations of electrolyte with different concentration show that most water molecules in high concentration electrolyte are coordinated with sodium ions,and there exists a little of free water.On the one hand,the hydrogen bonds are not formed,owing to the interaction between free water.On the other hand,the nature of water is changed by the coordination of oxygen atoms and sodium ions,as well as the activity of water is inhibited,and thus supercapacitors with high concentration electrolyte have wider voltage window.（5）The main research conclusions are summarized.And combining with this work,the future research is proposed.