Preparation And Capacitance Properties Of Nitrogen Doped Graphene Hydrogels

As a novel electrode material for supercapacitor,nitrogen-doped graphene hydrogel has attracted considerable attention owing to its large specific surface area,high conductivity and three-dimensional porous network structure.Nitrogen-doping in the graphene layer not only improves the electrical conductivity,but also increases the pseudo-capacitance to enhance the specific capacitance of the graphene-based capacitor.In the presence of modifier,nitrogen-doped graphene hydrogels had been synthesized via chemical reduction of graphene oxide using hydrothermal /solvothermal method.By optimizing the preparation conditions?reactant mass ratio,temperature,time,etc.?,the pore structure and specific surface area of the graphene hydrogels have been improved and the ability of storing the charges is further increased,thereby the capacitive performances of hydrogels are enhanced.?1?To further improve the capacitive performances of reduced graphene oxide-based materials is still crucial although N-doped reduced graphene oxide hydrogels?Nr GOHs?exhibit fairly good performance.Here the purified Nr GOHs?Nr GOHs-SA?are obtained by treating the Nr GOHs which are hydrothermally synthesized in the presence of o-phenylenediamine?o PDA?with concentrated sulfuric acid.The effect of preparation conditions on the hydrogels' electrochemical behavior has been systematically investigated.The results reveal that the capacitive performances of Nr GOHs-SA are remarkably enhanced compared with the samples of Nr GOHs.The optimal Nr GOH1:1-SA shows specific capacitance of about 519.8 F g^-1 at 1 m V s^-1 and excellent rate capability.The cells assembled by the optimal hydrogels exhibit the maximum energy density of 11.1 Wh kg^-1 and power density of 14.1 k W kg^-1,and can retain 93.3 % of its initial capacitance after 15000 cycles.This method can be developed to prepare high-performance Nr GOHs materials for energy storage applications.?2?The reduced graphene oxide hydrogel modified by o-phenylenediamine shows excellent capacitance performance after being purified by sulfuric acid.So we selected the isomers of aminophenol with the similarly structure,which contain amino and hydroxyl groups on the benzene ring are employed as structure regulator and modifier to prepare the reduced graphene oxide hydrogels?r GOHAPs?.The influences of preparation conditions including ratios of graphene oxide to aminophenol isomers,temperatures,times on the hydrogels' structure and properties were researched.The positions of the-NH₂ and-OH are different in the isomers,and the properties of synthetic materials are also significantly different.The results show that the oligomer of o-aminophenol is formed on reduced graphene oxide sheets while the molecules of p,m-aminophenol are bond to the sheets.Using two-electrode system,the optimal o-r GOHAP has a specific capacitance of about 338.8 F g^-1,and the assembled capacitor exhibits the maximum energy density of 11.4 Wh kg^-1 and power density of 7.26 k W kg^-1,and can retain 85.2 % of its initial capacitance after 20000 cycles.The further research discovers that the hydrogel prepared in the presence of p-aminophenol has larger specific surface area,total pore volume,specific capacitance and good rate capability(74.0% of 427.0 F g^-1 retains at 100 m V s^-1)than other.The cells assembled by the optimal p-r GOHAP hydrogels exhibit the energy density of 14.07 Wh kg^-1 at power density of 124.3 W kg^-1,and 4.96 Wh kg^-1 at 6.78 KW kg^-1.Moreover,92.2% of initial capacitance can be retained after 20,000 cycles in the cells.The p-r GOHAP hydrogel may have great application prospects for high performance supercapacitors.