Study On Preparation And Electric Energy Storage Performance Of Additional Element Doped Coal Tar Pitch-based Porous Carbon

Supercapacitors have been widely used in the field of energy,showing superior performance such as high energy density,high power and strong applicability.Therefore,they have attracted wide attention from scholars in related fields.Electrode material is a decisive factor to determine the electrochemical performance of capacitor.Porous carbon is considered as an important electrode material.However,porous carbon electrode materials also have many shortcomings in the process of application.Doping nitrogen and sulfur and other miscellaneous elements onto the surface of porous carbon can improve the pore structure of porous carbon materials on the one hand.On the other hand,by increasing the content of surface functional groups,the surface wettability and conductivity of materials can be effectively improved,thus effectively improving the energy storage performance of porous carbon.In this paper,a series of different N-rich coal Pitch-based porous carbon materials were prepared with melamine,ammonium sulfate and ammonium polyphosphate as dopants by taking the by-product coal Pitch of local charcoal enterprises in Yulin city as raw materials.The physical structures of different series of porous carbons were systematically characterized and used as capacitor electrodes to test their electrochemical properties and perform some theoretical calculations The specific research contents and conclusions are as follows:(1)The influence of coal pitch refining treatment and porous carbon preparation technology was studied.For refining coal bitumen by solvent extraction,the extraction of crude coal Pitch with aromatic alkane ratio of 1.2:1 and agent oil ratio of 1.6:1 can remove impurities such as quinoline insoluble substance in the Pitch.Dry chemical activation method is an effective way to activate porous carbon from coal Pitch,especially when the alkali-carbon ratio is less than 2:1,the effect of dry activation is not inferior to that of wet method.Preoxidation has little effect on the capacitive properties of the porous carbon materials,and the one-step or distributed activation process depends on the dopant.(2)Using melamine as nitrogen dopant and coal Pitch as precursor,a series of N-doped coal Pitch based porous carbons were prepared by a one-step method,and their structures were characterized and properties tested.It is found that when the ratio of melamine to coal pitch is 1:1 and KOH:CTP is 3:1,the surface area of the porous carbon electrode material can reach 2831.22 m~2/g,and the average pore size is 2.94 nm.The maximum specific capacitance value is 244.6 F/g when the current density is 0.2A/g.(3)Using ammonium sulfate and ammonium polyphosphate as NS and NP co-dopants,a series of porous carbon electrode materials were prepared by step method,and their structure and properties were tested.Using ammonium sulfate as dopant,the electrochemical performance of porous carbon electrode materials prepared by stepwise method is better than that prepared by one-step method.Under the stepwise method,when ammonium sulfate:coal pitch is 3:1 and KOH:CTP is 2:1,the specific surface area of porous carbon electrode materials can reach 1252 m~2/g,and the average pore diameter is 2.5 nm.The maximum specific capacitance is 164.09 F/g when the current density is 0.2 A/g.When ammonium polyphosphate is used as codopant,the introduction of nitrogen and phosphorus heteroatoms will enlarge the carbon layer spacing.When the mass ratio of ammonium polyphosphate and coal asphalt is 2:1,the specific surface area of much more porous carbon is only 1153.36 m~2/g,and the capacitance performance is relatively good.When the current density is 0.2 A/g,the specific capacitance reaches 289 F/g.(4)The structure models of different nitrogen,sulfur and phosphorus doped carbon materials were constructed approximately,and the effects of impurity doping on the intrinsic electronic properties of carbon materials and the adsorption properties of electrolyte were studied by using DFT theory.The calculation results show that different elements of nitrogen,sulfur and phosphorus have different charge transfer abilities to neighboring carbon atoms,which can change the electron redistribution of porous carbon,and thus affect the transport ability of carbon atoms themselves.On the other hand,the introduction of miscellaneous elements will modulate the adsorption capacity of electrode materials to ions in electrolyte.The same element introduced in the form of different functional groups to adjust the adsorption capacity of ions in solution is not the same,it can promote adsorption,can also weaken adsorption.These two factors combined to improve the material’s electrical storage performance.