Preparation And Capacitance Characteristics Of Nitrogen Doped Porous Carbon Based On Vinylidene Chloride Polymer

As a new type of energy storage device,supercapacitors combine the advantages of secondary batteries and conventional physical capacitors,such as high power density,long cycle life,high charge and discharge speeds,low maintenance cost,and environmental friendliness.As a result,they are used in mobile communications and information technology.Consumer electronics,electric vehicles and other aspects have important application prospects.Porous carbon materials have the advantages of large specific surface area,rich pore structure,stable physical and chemical properties,and wide source,and are one of the most commonly used electrode materials for supercapacitors.Therefore,the research and development of porous carbon materials with simple and inexpensive preparation processes and excellent electrical properties have become the focus of researchers.The vinylidene chloride polymer has unique pyrolysis characteristics and can be directly carbonized without a template to prepare a porous carbon material,which is expected to become an ideal supercapacitor electrode material.In view of the existing problems of polyvinylidene chloride based carbon in supercapacitors,such as the direct carbonization of vinylidene chloride obtained by direct carbonization,the surface area is low and the pore size distribution is single,and the process of template preparation of vinylidene chloride based carbon materials is complex and high in cost.In this paper,the porous polymer microspheres of vinylidene chloroethylene acrylonitrile（VDC-AN）are prepared from the porous carbon precursor of vinylidene chloride in the presence of porous carbon precursor,and the porous carbon electrode materials for supercapacitors are prepared by direct carbonization.In view of the poor hydrophilicity and low degree of graphitization of vinylidene chloride,the surface hydrophilic modification and catalytic graphitization modification of carbon materials have been carried out to prepare the electrode materials with excellent performance.The specific contents are as follows:（1）VDC-AN porous polymer microspheres were prepared by suspension polymerization to prepare in-situ nitrogen doped porous carbon with vinylidene chloride（VDC）and acrylonitrile（AN）as copolymerized monomers and pentane as a pore forming agent.The effect of pore composition on pore structure of porous carbon material was investigated.The morphology,graphitization degree,N element content and pore structure of the material were analyzed by means of SEM,XRD,Raman,XPS and nitrogen desorption.The electrochemical properties of the materials were tested under the three electrode system with 6 M KOH as electrolyte.The results show that the porous carbon material is spherical,the nitrogen content is up to 4.21%,and it has a part of graphitization structure.With the increase of the pore size,the specific surface area of carbon material increases first and then decreases.When the amount of pore agent is 25（based on the amount of monomers）,the specific surface area and pore volume of carbon materials（NPC-25）are up to 1560 m²/g and 0.76cm³/g respectively.And the carbon material under this condition has the best capacitance performance.At 1 A/g current density,the specific capacitance is as high as 243 F/g.（2）A nitrogen doped active carbon material（A-NPCs）was prepared by surface activation of nitric acid on NPC-25.The effect of activation modification on the structure and electrical properties of carbon materials was systematically studied.Through the design of orthogonal experiments,the optimal activation conditions for HNO₃ were determined as follows:HNO₃ concentration was 14 M,the time of activation was 4 h,and the activation temperature was 80℃.By means of FTIR,XRD,Raman,XPS and nitrogen adsorption,the structure of carbon materials and the changes of surface functional groups before and after the activation of HNO₃ have been analyzed.The results show that the graphite structure in the material is not destroyed after the activation of HNO₃,and the material has more pore structure and higher specific surface area,and the hydrophilic functional group of the material is increased.Further electrochemical measurements show that the activated carbon material has a specific capacitance of290 F/g at 1 A/g current density.（3）Using K₂FeO₄ solution to catalyze the activation of NPC-25,the graphitized nitrogen doped porous carbons（G-NPCs）were prepared.Through the orthogonal experiment design,the optimal catalytic activation conditions of K₂FeO₄ were determined as follows:the dosage of K₂FeO₄ is 0.01 mol,the activation temperature is 800℃,and the activation time is 1 h.The results of XRD and Raman showed that the carbon materials were changed from amorphous to stereotype and the degree of graphitization increased after the activation of K₂FeO₄,and nitrogen desorption showed that the specific surface area of the activated carbon material increased to2130 m²/g.Further electrochemical tests showed that the electrical properties of the materials were obviously improved and the current density was 1 A/g.The specific capacitance is increased from 243 to 340 F/g.