Preparation And Electrochemical Performance Of Nitrogen And Sulfur Co-doped Graphene Aerogel For Supercapacitors

Supercapacitors,which have the advantage of high power density,fast charge and discharge rate,long cycle life,etc.,are one of the advanced energy storage devices for portable devices,electric vehicles,and stationary energy storage equipments.Depending on the energy storage mechanism,supercapacitors can be classified into electric double-layer capacitors?EDLCs?,Faraday pseudocapacitors,and hybrid supercapacitors.Carbon-based EDLCs are one of the most widely studied supercapacitors due to their low cost and good cycle stability.Graphene has become an attractive carbon material,and is widely used for supercapacitors because of its high specific surface area,excellent electrical conductivity,chemical stability,and mechanical flexibility.However,graphene sheets are prone to agglomeration,which makes it difficult to reach the theoretical specific surface area and conductivity values,resulting in the inability to emerge their excellent performance.The doping of heteroatoms can effectively change the surface structure of graphene,thereby inhibiting the agglomeration of graphene and ultimately improving its electrochemical properties.Ionic liquids have good thermal stability,high chemical stability,and wide electrochemical window when they are used as electrolyte,the energy density of supercapacitors can be imported greatly.Based on the above,a series of heteroatom co-doped graphene aerogels were designed and prepared by selecting a variety of heteroatom dopants with graphene oxide?GO?as the precursor.The morphology and chemical composition of the heteroatom co-doped graphene aerogels were investigated.And they were used as the electrode materials for symmetrical supercapacitors to study their electrochemical performance with the ionic liquid?1-ethyl-3-methylimidazolium tetrafluoroborate,EMIMBF₄?as the electrolyte.The main research contents are as follows:1.Nitrogen/sulfur co-doped reduced graphene oxide aerogel?N/S-rGA-1?with a cotton-like structure were prepared using a mild hydrothermal method with a heteroatom-rich trithiocyanate?TTCA?as the dopant.Results show that the obtained N/S-rGA-1 has an interconnected porous structure and high sulfur content?4.1%?.In addition,N/S-rGA-1 also has a high electrical conductivity(11.5 S cm^-1).Electrochemical test results display that N/S-rGA-1 has high specific capacitance of 180.5 F g^-1 at a current density of 1 A g^-1,and the capacitance retention of the material is 74%with the increases of current density from 1 to 10 A g^-1,indicating that the material has good rate performance.When the power density is 0.9kW kg^-1,the energy density of the N/S-rGA-1 based supercapacitor is 75.0 Wh kg^-1.2.Nitrogen and sulfur co-doped graphene aerogel?NS-GA?with 3D porous structure was synthesized using one-pot hydrothermal method with mercaptoethylamine as the dopant.Under two-electrode test system,the specific capacitance of NS-GA can reach 191.7 F g^-1.Compared with graphene aerogel(GA,117.2 F g^-1),sulfur-doped graphene aerogel(S-GA,152.0 F g^-1),and nitrogen-doped graphene aerogel(N-GA,171.5 F g^-1),NS-GA has better electrochemical property.As the current density increases,NS-GA exhibits excellent rate performance.Besides,supercapacitor based on NS-GA also has higher energy density.The excellent electrochemical performance of NS-GA highlights the synergistic effect between co-doped heteroatoms to effectively improve the electrochemical performance of graphene-based supercapacitors.3.Amino-functionalized nitrogen and sulfur co-doped reduced graphene oxide aerogel?F-NS-RGA?with hierarchical pore structure was prepared via two-step hydrothermal method using urea as a functional reagent and thiourea as the dopant.The nitrogen adsorption/desorption analysis shows that F-NS-RGA has a large specific surface area(447.4 m² g^-1)and average pore width?4.3 nm?.The electrochemical tests were carried out with F-NS-RGA as electrode material.The specific capacitance of F-NS-RGA at a current density of 1 A g^-1 can be up to 215.5 F g^-1.F-NS-RGA-based supercapacitor has an energy density of 118.6Wh kg^-1 at a power density of 1 kW kg^-1.After 2000 cycles,the capacitance retention of the material reaches 94.4%,indicating that the F-NS-RGA-based supercapacitor has good cycle stability.The modification of amino group together with the co-doping of the heteroatoms changes surface properties of the graphene and thus greatly improves the electrochemical performance of the graphene-based supercapacitors.The excellent electrochemical performance of F-NS-RGA indicates that it is a potential energy storage material.