Tailoring Pore Size And Surface Modification Of Activated Carbon As Electrode Materials For Supercapacitor

Supercapacitor(also called electrochemical capacitor,electric double layer capacitor)is a new device for electric energy storage.It has not only much more energy than conventional capacitors,but also much higher power density than batteries.Electrode material is one of the most important components of a supercapacitor.It is a key factor influencing the performance of supercapacitor.The research and development of electrode materials have been a hot topic in both academe and industry field in recent years.In this dissertation,high performance activated carbons as electrode material for supercapacitor were prepared and characterized.The surface area and pore size of the activated carbon were tailored by using anvaced activation technique;besides,in order to improve the electrochemical performance,the conventional activated carbon, was modified by surface treatment with nitric acid,hydrogen peroxide,ammonia and metal oxdide,respectively.The properties of the modified carbons were investigated by moder instrements.The performance of the resultant carbon electrodes and testing supercapsitors was tested by electrochemical measurements through charge-discharge of constant current,cyclic voltammogram,impedence spectrum etc. Indeed,the relationship between the properties of carbon and the performance of the capacitors was studied.Main work was done as following:Activated carbons with controlled surface area and pore size were prepared by a simultaneous physical and chemical activating method using Coconut shell as precursor and ZnCl₂ and CO₂ as activating agents.The influences of activation parameters on the properties of carbon samples were systematically studied in order to porduce high performance electrode materials for supercapacitor.The results showed that the mass ratio of ZnCl₂ to coconut shell,activation temperature, activation time and CO₂ flow had significant influence.Activated carbons with special pore size and surface area cuold be tailored by adjusting activation parameters.As a result,the carbon electrod materials with suitable pore size and pore size distribution and surface area to electrolyte ions were obtained.The relationship between the pore structure,surface properties of the activated carbons and the performances of supercapatior has been investigated.It was found that there's no direct relationship between specific capacitance and specific surface area of carbon,however,the pore size distribution played an important role.In KOH electrolyte systrem,carbon surface of pores in the range of 1.5-2.0nm was favorable to the formation of electric double layer.The tesing capacitor exhibited good electrochemical reversibility and capacitive behavior.The specific capacitance reached a high value from 230F·g^-1to 360F·g^-1.A commercial activated carbon was modified by surface treatment using three chemicals,nitric acid,hydrogen peroxide and ammonia,respectively.The influence of surface function groups on the performace of carbon electrodes were investigated in details.The results showed that the modification caused a slight decrease in specific surface area and average pore size,the higher the concentration of chemical agent is,the larger decrease.The average pore size had few changes.The modification caused the obvious changes in surface chemistry and introduced new functional groups,so that improve the wettability of carbon surface.Besides,acid treatment removed impurity from carbon matrix,so that significantly improved the electrochemical behaviours of the mordified carbon,The specific capacitance increased form 145F·g^-1to 250F·g^-1,by 72.4%.The equivalent series resistance of the capacitor based on carbons treated by nitric acid and hydrogen peroxide decreased significantly,while It was worthy to note that the leakage current of the capacitor decreased extraordinarily,the smallest 3μA,1%of that of original carbon.A commercial activated carbon was modified by means of soaking in Ni(NO₃)₂ solution and subsequently pyrolysis.The results showed that nano NiO particles were deposited on the carbon surface,but the pore structure had minor changes only.The performance of asymmetrical electrochemical capacitors using modified carbons as anode material and activated carbons as cathode material was investigated. The results suggested that the asymmetrical electrochemical capacitors have super electrochemical performance.The specific capacitance of a modified carbon could be as high as 301.6F·g^-1,increased by 61.3%compared with 187F·g^-1of origional carbon electrode in symmetrical capacitor.Furthermore,the asymmetrical electrochemical capacitors had high stability,lower leakage current,high power.