Research On Preparation Of Activated Carbon By Freezing Method And Its Composite With MnO₂

Supercapacitor(also called electrochemical capacitor,electric double layer capacitor) is a new kind of device for energy storage between traditional capacitors and batteries.Supercapacitor has many advantages such as high energy density,high power density,long cycle life and fast charging speed.Supercapacitors are used in many fields such as mobile communication,electronic equipment,power vehicles and military.The broad application prospects of supercapacitor have attracted great attention in the whole world.The key to preparing high-performance supercapacitors is to prepare electrode materials with excellent electrochemical performance and low cost.In this paper,the preparation and characterization of high performance activated carbon(AC) and its composite with MnO2 and their application as electrode materials for supercapacitors were carried out.The main results of this paper are as follows:(1)AC was prepared by chemical activation with coconut shell as precursor and KOH as activator.The effects of the ratio of KOH to carbon,activation time and activation temperature on the pore structure and electrochemical chemical performance of AC were discussed.The optimized parameters of ratio of KOH to carbon,activation time and activation temperature were 5:1,3h and 800 ℃,respectively.AC prepared under condition mentioned above had an iodine adsorption value of 2857mg/g,a specific surface area of 2217m2/g,an average pore diameter of 2.96 nm.The specific capacitance of the AC at scanning speed of 2mV/s was 343.4F/g.When the scanning speed was increased from 2mV/s to 50 mV/s,the retention rate of the specific capacitance was 90.6%,which means the power performance of the AC was excellent.(2)Coconut shell was frozen before using.AC was prepared by chemical method with frozen coconut shell as precursor and KOH as activator.The effects of the ratio of KOH to carbon and freezing times on specific surface area,specific capacitance and pore structure of the frozen activated carbon were studied.The iodine adsorption value of the unfrozen carbonized coconut shell and the frozen carbonized coconut shell were 247.5 mg/g and 338.1 mg/g,respectively,which means the iodine adsorption value was increased by 36.6%,indicating that the freezing method can effectively increase the initial pore of the carbonized coconut shell.AC prepared with coconut shell frozen for one time and ratio of KOH to carbon of 3:1 had a specific surface area of 2410 m2/g,an average pore diameter of 3.35 nm,a total pore volume of 2.02 cm3/g and a high mesopore content of 62%.When the scanning speed was increased from 2 mV/s to 50 mV/s,the prepared AC achieved a specific capacity retention of 85.0%,showing good power performance.The study shows that pretreatment of coconut shell by freezing method can effectively reduce the amount of activator used.The resulted AC had high specific capacitance,good pore structure and excellent power performance.(3)The frozen AC-manganese oxide composite was prepared by combining frozen activated carbon with manganese oxide.The experimental results show that manganese oxide grew on the surface of AC pores with relatively larger diameter.The specific capacitance of the composites electrode at the scanning speed of 2m V/s was 413.47F/g.When the scanning speed was increased from 2m V/s to 20 m V/s,the retention rate of specific capacitance was as high as 90.94%.Compared with pure AC electrodes,the composite electrode had a specific capacitance increase of 20.5% at a scanning speed of 2m V/s.The composite material can effectively increase the specific capacitance of manganese oxide and improve its power performance.Compared with pure manganese oxide,the composite had a 120% increase in specific capacitance and a 10% increase in specific capacitance retention rate.