Research progress of preparation of coal-based carbon foam and factors affecting its electrochemical performance
Carbon foam has broad application in the field of supercapacitor electrode materials due to its large specific surface area,high porosity and controllable structure.The carbon foam prepared from coal,coal liquefaction residue,coal liquefaction pitch,coal tar pitch and other coal-based materials has high conductivity,simple preparation process,wide source and low price of raw materials,which can be used as electrode materials for supercapacitors.Dif-ferent precursors have different ways,different the the properties and application are,therefore,summarizing and analy-zing the preparation and factors affecting electrochemical properties of coal-based carbon foam is necessary.The struc-ture of coal-based carbon foam is briefly introduced and the methods of separation coking,mixed precursor,high pres-sure nitriding,limited expansion,supercritical foaming,spontaneous foaming and template are analyzed.The influ-ence mechanism of specific surface area,pore size distribution,surface chemical group and degree of graphitization on electrochemical performance is expounded,which provides a reference for reasonably regulating the relationship be-tween various factors and electrochemical performance.The results show that higher effective specific surface area is beneficial to the specific capacitance and energy density,but the ultra-high specific surface area is mainly contributed by micropores,which will cause the electrolyte ions and solvation ions inaccessible and accumulate at the micropore or-ifice,affecting formation of the electric double layer.The ideal carbon electrode materials have three-dimensional lay-ered porous structure,the interconnected micro-meso-macropores will benefit to ion diffusion and electron transfer,and the advanced pore structure will profit to improve rate performance,power density and capacity retention.The con-ductivity depends on the degree of graphitization,the higher the carbonization temperature,the higher graphitiza-tion degree is,and the stronger the conductivity will be.However,high temperature will cause the decrease of structural integrity,destroying the three-dimensional layered porous structure,and the heteroatoms and functional groups of the materials will be separated out,reducing the wettability and Faraday pseudocapacitance of the materials,resul-ting in a significant decrease in specific capacitance.Heteroatom doping helps to improve the wettability and total spe-cific capacitance of the material,which will contribute pseudocapacitance,regulate the structure and improve the cycle stability and capacitance retention rate of the materials,but it also has adverse effect on the degree of graphitization and specific surface area.The four factors of specific surface area,pore size distribution,surface chemical groups and degree of graphitization interact and restrict each other,there is a optimal experimental condition,which may effec-tively regulate its specific capacitance,conductivity,cycle stability,rate performance,energy density,power density,but it is difficult to take into account all of them in practical research simultaneously.Therefore,other methods are necessa-ry in order to optimize the electrochemical performance.With template replication,physical and chemical activation,heteroatom doping,binary/ternary composite and other measures to design the structure of carbon foam and optimize its electrochemical performance will be a research highlights.
coal-based carbon foamelectrochemical performancepore size distributionsurface chemical groupsheteroatom dopingpreparation methoddegree of graphitizationspecific surface area
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