Structure Changes And Electrochemical Performance Of Activated Carbon Fibers During Heat-treatment

Supercapacitors have been widely studied and applied in hybrid vehicles,new energy generation,aerospace and other fields due to their high power density,high charge/discharge speed,long cycle life and environmental friendliness.Carbon materials have become a research hotspot in the field of supercapacitor electrode materials,because of their high conductivity,stable chemical properties and low cost.As a kind of microporous carbon,activated carbon fiber has large specific surface area,high adsorption rate and concentrated pore size distribution.It is mainly produced after a certain procedure of carbonization and activation process by using viscose,pitch and polyacrylonitrile as precursors.The fiber diameter varieties from 5 ?m to 20 ?m.A large number of microporoes are formed during the preparation process,and directly open from the fiber surface,which makes ACF a good electrode material for supercapacitors.Like most carbon materials,the pore structure and microcrystalline structure of ACF will change significantly after heat-treatment,which will affect the performance of supercapacitor.At present,there are few studies on the changes of surface structure and properties of ACF during heat-treatment.Therefore,a series of ACFs were treated at different temperatures from 700? to 2800?.The effects of temperature on pore structure,micro-structure,elemental composition and conductivity of activated carbon fibers(ACFs)were investigated by nitrogen adsorption,X-ray diffraction,elemental analysis and resistivity test.In the meantime,ACFs were used as supercapacitor electrode materials to study the electrochemical performance in inorganic and organic systems.The major conclusions are summarized as follows:(1)With the increase of heat-treatment temperature,the oxygen content of ACF decreased,the carbon content increased,the electrical conductivity was improved and the specific surface area first increased and then decreased.The maximum specific surface area is reached when the temperature is 900? or 1100?.When the temperature is higher than 1100?,the interlayer spacing of ACF decreases,the crystallite size increases,and the structure tends to be like graphite structure.Compared with viscose-based and pitch-based ACF,PAN-based ACF prohibits the highest graphitization degree(88%)at 2800?.(2)When activated carbon fiber is used as supercapacitor electrode material,the heat treatment enhances its capacitance performance in organic system,and the performance of the capacitor in inorganic system is not significantly improved.In organic system,micropores are the main factors affecting the specific capacitance.When R-ACF is heat-treated at 900?,the microporous specific surface area and the capacitance(96 F·g-1)are both largest.The mesopores improved the rate capability in organic system,for instance,FE400 PAN-ACF has the largest mesopore specific surface area after heat-treated at 900?,and its rate capability at the current density of 20 A·g-1 can reach 88%.(3)In inorganic system,with the increase of heat treatment temperature,the oxygen-containing functional groups on the surface of ACF gradually decomposed and the wettability of the electrolyte deteriorates,which increases the ion transmission resistance.As a result,the specific capacitance gradually decreases.On the other hand,heat-treatment improves the supercapacitor's rate capability to some extent.