The Modified Rice Husk-Based Activated Carbon By A Thermal Treatment Method And Its Electrochemical Performances For The Electrochemical Doublelayer Capacitance

Supercapacitor is a new type of high-efficiency energy storage device between the secondary battery and the traditional capacitor with the excellent characteristics of power density,high charge/discharge efficiency,and long cycle life.It has been widely recognized and developed rapidly in recent years.Activated carbon is an important active material for supercapacitors.It has the advantages of large specific surface area,abundant source of raw materials and stable performance.However,the surface chemistry and porous textures of activated carbon are different.For example,the specific surface area of micropore-based activated carbon materials is very high,but they are difficult to be fully utilized under the high charging/discharging currents,which limits their power characteristics for supercapacitors.Although the pore structure of the mesoporous activated carbon material is favorable for the transport of electrolyte ions,it can exhibit excellent power characteristics,the specific surface area of mesoporous carbon is low,resulting in a low specific capacitance,which limits the energy density of the supercapacitor.This indicates that the activated carbon material of a single pore structure is difficult to meet the increasingly high-performance requirements of supercapacitors.Rice husk-based activated carbon is a biomass activated porous carbon with excellent specific capacitance and power density characteristics that can be used in aqueous or organic supercapacitors.However,when the rice husk-based activated carbon is applied in supercapacitors,serious self-discharge problem occurs in the devices.The self-discharge of supercapacitors is related to the electrode active material and external factors,such as impurities,surface functional groups,ambient temperature,charge duration,initial voltage,etc.,which originates from side reactions or redox reactions inside the supercapacitor.In this paper,the pore structure characteristics of rice husk-based activated carbon and the types and contents of surface oxygen-containing functional groups are investigated,the factors affecting the self-discharge performance of super capacitors are studied.Herein,a simple modification method of rice husk-based activated carbon is proposed,which can reduce the self-discharge of rice husk-based activated carbon supercapacitor and improve the electrochemical performance of rice husk-based activated carbon supercapacitor.The specific research results are as follows:?1?We studied the pore structure characteristics of rice husk-based activated carbon and the type and content of oxygen-containing functional groups on the surface.Rice husk-based activated carbon is a kind of biomass activated carbon material with hierarchical porous,which micropore ratio is high,pore size distribution is within 2 nm,specific surface area(S_BET)and pore volume are 1919 m² g^-1 and 1.07 cm³ g^-1,respectively.XPS and Boehm titration test show that the surface of the rice husk-based activated carbon material contains abundant oxygen containing functional groups.However,when the rice husk-based activated carbon is applied to supercapacitors,serious self-discharge problem occurs in the devices Comparing with the YP-50?A commercial microporous carbon?.?2?We study the factors that affect the self-discharge performance of supercapacitors.Rice husk-based activated carbon was applied to the spiral supercapacitor.In the test of different initial voltage,we can see that under the same conditions,when the supercapacitor was charged to a higher initial voltage,the open circuit voltage dropped faster,which may be related to pore structure and ion distribution.In the test of different ambient temperatures,the spiral supercapacitor was charged to 2.5V at constant voltage and maintained for 8 h.The result showed that at higher temperature the self-discharge was much faster,owing to the higher ion mobility with the increase of temperature,which will increase the rate of charge leakage in the double layer.In the test of charging duration,the spiral supercapacitor was charges to2.5 V under 22?.The voltage of supercapacitor with a charging duration of 24 h was more stable.The supercapacitor was conducted a fully constant voltage charging process until the voltage reached a stable state,so that the ions were evenly distributed.At the first 6 hours,the dominant self-discharge seems to be the diffusion process.Some ionic substances in the electrolyte can undergo Faraday redox reactions to establish a concentration gradient of ion species in the electrolyte near the carbon surface during the charging of the capacitor.?3?In view of the characteristics of rice husk-based activated carbon materials,we mainly modify the rice husk-based activated carbon by a simple thermal treatment method for the oxygen-containing functional groups on the surface of the material,which can reduce the self-discharge behavior of rice husk-based activated carbon supercapacitor and improve the electrochemical performance of rice husk-based activated carbon supercapacitor.The material obtained by thermal treatment shows a high specific capacitance of 147 F g^-1 at a current density of 0.5 A g^-1,a value much higher than the capacitance of the pristine material of(116 F g^-1).After 24 hours of self-discharge experiment,the voltage of the material after thermal treatment maintained84.5%of its initial value?voltage retention for the rice husk-based activated carbon is75.2%?.After 10000 galvanostatic charge/discharge cycles at 1 A g^-1,the capacitance retention ratio of the prepared material is 92%while capacitance retention of rice husk-based activated carbon is 85%,indicating an excellent self-discharge endurance after the thermal treatment.