Mechanism Of Formation And Supercapacitive Performance Of Carbide-derived Carbon

Carbide-derived carbon（CDC） is a new type of nanoporous carbon obtained by selective removal of the non-carbon atoms from carbide lattice layer by layer. It owns high specific surface area, developed porosity and some other properties（e.g. tunable pore size and distribution）. These make CDCs own potential applications in many fields, such as gas storage of hydrogen, electrode material for supercapacitors, catalyst supports, self–lubrication coatings and many more. In terms of the not well-understanding for the formation process of CDC, we take carbide/CDC interfaces as the object of research to disclose the formation mechanism of the microstructure in CDCs. Both wettability and supercapacitive performance of CDCs with different microstructure are investigated in KOH electrolyte. In addition, several methods are explored to improve the wettability of CDCs and consequently the supercapacitive performance. The effects of adding Fe（by ball milling） to carbide precursor on microstructure and supercapacitive performance are investigated. The main research works and results are listed as following:（1） Precursors（VC, TiC and NbC） were chlorinated only 10 minutes to ensure the existence of the carbide/CDC interface. TEM technique is used to observe the situation of the interfaces directly. The formation mechanism of CDC is analysed, the electrochemical performance of CDC with different microstructure in KOH aqueous electrolyte is investigated.Results show that: CDC is conformal conversion. The carbide-carbon transformation is always accompanied by internal stress, which has big influence on the types of CDC microstructure. For carbides with same structure, the lower volumetric concentration of carbon atom is, the larger internal stress and disordered structure formed at interface will be. Excessive stress among the graphite layers lead to a formation of avulsions which contribute to the pores in CDCs. The pores in CDCs can be regard as a result that the space of original carbide particle is divided by graphene layers, the thicker layer is, the bigger pore size and lower specific surface area will be. As CDC synthesized from carbides with low carbon volume fraction（e.g. NbC）, large internal stress formed at interface leads the structure in NbC-CDC is mainly composed by 1² layered graphene. Higher specific capacitance(147Fg^-1) obtained is caused by highly microporous surface area(2200 m² g^-1) in NbC-CDC.（2） Take different CDCs as research object to investigate the effects of the degree of order on the wettability and supercapacitive performance. In order to solve the hydrophobicity of graphitized CDC in KOH electrolyte, adding ethanol to electrolyte and activating CDC with HNO₃ is investigated.Results show that: The hydrophilicity of the CDCs in KOH electrolyte decreases with increasing degree of order, which has big influence on the supercapacitive performance. Adding ethanol can improved the wettability of graphitized CDC in KOH electrolyte greatly and consequently the supercapacitive performance, the optimal content of ethanol in KOH electrolyte is 10 wt.% and the specific capacitance increases from 6 to 65 Fg^-1.HNO₃ activation can introduce oxygen groups into CDC, it improves the wettability of graphitized CDC greatly and enhanced the supercapacitive performance notably. When TiC- CDC-1000 is activated by 12 mol L^-1 HNO₃, the specific capacitance can be increased from 6 to 130 Fg^-1.（3） Fe is introduced into carbides（VC, TiC and SiC） by ball-milling before chlorination. The effects of Fe on the microstructure and electrochemical performance are investigated.Results show that: During the ball-milling, the TiC powers are shattered and soldered continually. Some Fe from the milling balls and jar was mixed into the inner of the powder and surrounded by the powders. This distribution can prevent Fe from rapid reaction with pure chlorine and facilitates its catalytic effect during the whole chlorination. Due to the conformal reaction, the crack formed in carbides during ball-milling can be reserved in the form of pores. At lower chlorination temperature, Fe promotes the graphitization of CDCs obtained from TiC and VC greatly; when the temperature increases to a certain value, large amount of nano-diamonds appear in the structure, the degree of order decrease sharply at the same time, accompanied with the mesopores introduced by ball-milling, CDC shows a good supercapacitive performance. The introduction of Fe by ball-milling leads the formation of hollow carbon onions with larger lattice spacing.