Study The Preparation Of Pitch-based Hierarchical Porous Carbon And Its Application In EDLCs

Electric double layer capacitors（EDLCs）have attracted much attention due to fast charge and discharge rate,good stability and environmental-friendly.Porous carbons have been widely used as electrode material for EDLCs,and their specific surface area,pore structure,surface chemical property,microcrystalline structure,electrical conductivity,etc are closely related to the electrochemical performance.However,the low energy density of porous carbon-based EDLCs has greatly limited their application.In this study,we focus on the development of the porous carbons with high specific capacity and high voltage window by optimizing their pore structure and surface chemical properties,so as to improve the energy density of EDLCs.Nitrogen-doped hierarchical porous carbons were prepared by an“auxiliary activation”method with low activator dosage,in which low cost water soluble sulfonated asphalt（SP）as carbon source,urea as"functional additive",and potassium hydroxide as activator.By discussing the activation mechanism,it is found that the pore structure and surface chemical property of the porous carbons are related to the alkali-carbon ratio,the amount of urea,and the activation temperature.The higher urea used,the further developed of pore structure and the higher nitrogen content of the samples.With the rising of activation temperature,the oxygen content of the samples decreases and the mesopore ratio increases.When used as EDLCs electrode material,the C-0.5-1-900（the mass ratio of KOH/SP and Urea/SP is 0.5 and 1,the activation temperature is 900℃）exhibits the high specific capacity(158 F g^-1),wide voltage window（0-3.5 V）,and excellent rate performance(C_50/0.05=56%)in EMIMBF₄electrolyte.In addition,the relationship among the rate performance and the pore structure,electrical conductivity and the specific surface area was studied.The results showed that proper mesopore ratio and high electrical conductivity were beneficial to the rate performance of EDLCs.The porous carbons were heat-treated in ammonia atmosphere at different temperatures,and then various tools were used to analysis the changes of structure and surface element content.As the reduction temperature increases,the surface morphology of the porous carbons has no obvious change,the microcrystalline structure becomes more ordered,and the surface oxygen content reduces gradually;while the specific surface area of the porous carbon increases before 600℃and then decreases above 600℃.In TEABF₄/PC electrolyte,the sample treated at 800℃exhibits the optimized specific capacity(144 F g^-1),wide voltage window（from 0-2.7V to 0-3.3 V）,enhanced stability,and improved energy density(from 35.2 Wh kg^-1to52.5 Wh kg^-1).Therefore,heat-treatment in ammonia atmosphere is an effective method to improve the high voltage resistance characteristic of porous carbon-based EDLCs.Meanwhile,by studing the relationship between the voltage window and the material conductivity,it was found that high conductivity was beneficial to the rate performance of high voltage EDLCs.