Preparation Of Coal-based Porous Carbon Materials By Salt Template Method And Application To Supercapacitors

With the advantages of ultra-high cycle life and superior power density,supercapacitors occupy an important position in the electrochemical energy storage system.The influence of electrode material on the performance of capacitors is more important.Porous carbon materials are widely used because of the advantages of large specific surface area and stable performance.Coal,which is rich in resources and high in carbon content,is widely used for the preparation of porous carbon.But when coal is used as the carbon precursor,a large amount of chemical activator is often required for etching and it is difficult to achieve the regulation of pores.To achieve the precise regulation of the pores,we propose the preparation of coal-based hierarchical porous carbon by salt template method in this paper.Taixi anthracite is used as the carbon precursor,and the solid coal is made soluble by a stepwise oxidation method（K₂Fe O₄pre-oxidation and H₂O₂oxidation）.The soluble salts of zinc chloride（Zn Cl₂,283°C）,sodium chloride（Na Cl,801°C）and manganese chloride（Mn Cl₂,650°C）with different melting points are selected as template agents,respectively.The morphology,pore structure and surface chemical composition of the prepared materials are systematically characterized,and their electrochemical performance as supercapacitor electrodes are investigated.When ZnCl₂is used as the template,three-dimensional lumpy coal-based hierarchical porous carbon is prepared.Zn Cl₂forms interconnected pores during carbonization by“in situ occupancy”at low temperatures and“occupancy”at high temperatures.At the same time,it exhibits the activation effect.When the carbonization temperature is 700°C and the ratio of coal to Zn Cl₂is 1:5,the specific surface area of the porous carbon is 1581 m²g^-1and the mesoporosity is 45%.The material is used as the supercapacitor electrode with a high specific capacitance of 470.7 F g^-1at 0.5 A g^-1in 6 M KOH electrolyte and an energy density of 95.31 W kg^-1in the EMIMBF₄electrolyte when the power density is 213.13 Wh kg^-1.When NaCl is used as the template,the resulting materials have three-dimensional network-like hierarchical pore structures.The method of liquid nitrogen freezing can effectively shorten the crystallization time of Na Cl and control the size of Na Cl nanocrystals.During the carbonization process,the Na Cl nanocrystals act as in situ templates to form pores,and also have a catalytic effect at 600-700℃.The large amount of gas produced by the catalytic pyrolysis reaction interconnects the formed pores during the release.When the mass ratio of coal to Na Cl is 1:5,the microporosity and mesoporosity of the material are 58.2%and 31.3%,respectively.The unique pore structure enables it to exhibit excellent rate performance as supercapacitor electrode.When MnCl₂ is used as the template,honeycomb coal-based hierarchical porous carbon with high electrical conductivity is obtained.During the carbonization process,Mn Cl₂exhibits the template pore-forming effect,and at the same time,Mn Cl₂undergoes a redox reaction to eventually form the metal oxide Mn O,which has the catalytic graphitization effect improving the electrical conductivity of the material.When the carbonization temperature is 800°C and the mass ratio of coal to Mn Cl₂is 1:1.3,the material exhibits a hierarchical porous structure of“macropore（20%）-mesopore（31.1%）-micropore（48.9%）”.In addition,the material shows a capacity retention rate of 62%when the current density is increased from 1 A g^-1to 300A g^-1,and has good cycling stability in 6 M KOH electrolyte as the supercapacitor electrode.Low melting point salt（around 300℃）has strong fluidity and permeability in the process of carbon skeleton formation.It is easy to form micropores and mesopores with small pore size,and the specific surface area of the material is larger.High melting point salts（around 800℃）are easy to prepare porous carbon materials with specific morphology by adjusting the grain size.The salt with melting point around 600℃can play the role of a template in the carbonization process at low temperature conditions,but also has a certain mobility at high temperature.It is easy to prepare porous carbon with hierarchical pore structure.There are 37 figures,14 tables and 178 references.