Preparation And Electrochemical Properties Of Graphene Aerogel Modified By Heteroatoms And Its Composites

The physical structure and elemental composition of graphene materials have an important impact on their performance.Graphene aerogels,as a kind of graphene material with 3D network structure has received much attention these years.The 3D network structure with a large number of pores,provides channels for charge transfer and huge specific surface area for charge storage.The introduction of heteroatoms can improve the wettability of graphene,improve the charge transfer between the electrode material and the electrolyte,and provide pseudocapacitance to inprove the specific capacitance of the electrode material.Moreover,heteroatom doped graphene aerogels can inhabit the stacking of graphene sheets and expand the scope of application of graphene materials.In this dissertation,heteroatom-doped graphene aerogels were prepared by a simple bottom-up self-assembly hydrothermal method in the presence of three different modifiers.The relationship between structure and electrochemical performances was discussed.Meanwhile,the effect of oxygen-containing functional groups on the electrochemical properties of graphene aerogels was also explored by carbonizaiton of pure graphene aerogels at different temperatures.The details are as follows:(1)The modified graphene aerogel was first prepared into a hydrogel through hydrothermal method in the presence of three different modifiers(ethylenediamine,hydroxyethylidene diphosphate and L-cysteine),and then freeze-dried to an aerogel.The effect of different modifiers on the structure and supercapacitor performance of graphene aerogels was investigated.The relationship of doped heteroatoms and electrochemical performance of graphene aerogels was analyzed.The obtained graphene aerogels have different microscopic morphologies and the specific surface area.Among them,graphene aerogel modified by L-cysteine has the largest specific surface area of 397 m2 g-1,while the graphene aerogel modified by hydroxyethylidene diphosphate has the smallest one.In terms of supercapacitor performance,hydroxyethylidene diphosphate modified graphene aerogel exhibits the maximum specific capacitance of 248 F g-1.The symmetrical device was assembled using the modified graphene aerogel as the electrode,showing a specific capacitance of 45 F g-1 and high energy density of 6.45 Wh kg-1.(2)The graphene oxide prepared by the modified Hummers method was used to prepare pure graphene hydrogel through hydrothermal method.The GA was carbonized at different temperatures(200-700℃)to control the type and content of oxygen-containing functional groups.Furthermore,the effect of vacuum carbonization on the structure and supercapacitor performance of graphene aerogel was investigated.As carbonizaiton temperature increases,the specific surface area gradually increases,and the maximum specific surface area of the aerogel reaches 519 m2 g-1 at 600℃,but further increase in temperature lead to collapse of the framework structure and decrease of the specific surface area.Then,the samples carbonized at different temperatures under vacuum were subjected to electrochemical tests.At a carbonization temperature of 200℃,a maximum specific capacitance of 268 F g-1 was obtained.Through data and comparison of element composition and capacitance performance,it is inferred that the oxygen-containing carbon hydroxy group provides pseudo-capacitance for graphene aerogel and improves the electrode specific capacitance.(3)Hollow carbon spheres co-doped with nitrogen and sulfur obtained by using MF spheres as templates were employed as scaffold materials to fabricate hollow carbon sphere/graphene aerogel composites through hydrothermal method.The structure and supercapacitance performance of hollow carbon sphere/graphene aerogels composites were investigated by adjusting the ratio of hollow carbon spheres to graphene.When the ratio of resin spheres to graphene oxide is 1:3,the obtained hollow carbon sphere/graphene aerogel composites has a better hierarchical structure and good electrical capacity,reaching 292.8 F g-1 at 0.2 A g-1.It is inferred from the structure-effect analysis that the doping of the N,P,and S elements in the composite material increases the capacitance of the electrode material through the pseudocapacitance,and the synergistic effect of the hollow carbon sphere and graphene makes the capacitance of the composite material higher than that of single material.