A Study On Preparation Of Three Dimensional Graphene Based On Carbon Foam And Its Structure And Properties

With the rapid development of electric vehicles(EVs),lithium ion batteries(LIBs)which serve as the power sources for EVs can not meet the demand of development.Graphene,a new type of material,is a promising candidate for the anode of LIBs due to its excellent properties like high conductivity,high specific area and flexibility.However,graphene sheets with two dimensional structure have to face the problem of agglomeration,thus diminishing its specific area and active sites for reaction.The preparation of three dimensional graphene is an effective way to solve this problem.It has general characteristic of high porosity,which could prevent the agglomeration and facilitate charge transfer in the reaction.Herein,we prepare the carbon foam with graphene structure on its surface,which also shows good lithium storage properties.In the preparation of precursor,the process cycle of fabricating foam could be reduced by modifying traditional technique.Polyacrylonitrile was selected as solute and maleic anhydride was selected as solvent.The solid mixture could be obtained by thermally induced phase separation.Subsequently,distilled water was used as extraction agent to remove original solvent.Finally,the distilled water could be removed by freeze drying and polyacrylonitrile foam with high porosity could be obtained.The polyacrylonitrile foam was then carbonized.The graphene structure can be generated on the surface of the carbon foam by adjusting the carbonization technique.In this work,the influence of temperature,atmosphere and time on carbon foam with graphene structure have been studied in detail.The study shows that with the increase of temperature,gas flow rate and reaction time,the crystallization of material can be significantly improved.Meanwhile,the roughness of the surface and the degree of generating graphene also increases.However,when the above conditions were beyond extent,the integrity of the structure would be destroyed.Under appropriate conditions,the carbon foam with highly directional pore structure could be obtained,and meanwhile,vertical graphene sheets with their edges exposed have formed on the carbon foam.The obtained carbon foam had doped nitrogen(1.51 %)and ultrahigh specific surface area(2149 m2/g).The study shows that the ammonia etching technique can generate surface graphene structure on polymer-derived carbon foam.The electrochemical tests have been carried out on the obtained materials andtheir lithium storage properties have been studied.The study shows that with the increase of temperature,gas flow rate and reaction time,the first irreversible capacity and the final reversible capacity would be significantly improved,and meanwhile,the charge transfer resistance would be decreased.The capacity of carbon foam with surface graphene structure would start to continuously increase after the decrease in the first few dozens of cycles.For capacity,carbon foam with graphene structure could deliver a reversible capacity of 2228 m Ah/g(0.1 A/g).With respect to the cycling performance,a stable specific capacity 1840 m Ah/g(1A/g)can be achieved after 600 cycles.For rate performance,a reversible capacity of137 m Ah/g can still be obtained even at a large current density(20 A/g).