| Graphene is considered to be an ideal flexible supercapacitor electrode material due to its large specific surface area,ultra-fast carrier mobility and superior mechanical strength.However,because the strong van der Waals force between the graphene sheets,the graphene is easily agglomerated,and the specific surface area is far lower than the theoretical value,resulting in low capacitance performance.By introducing pseudo-capacitive material between graphene sheets,the capacitance of graphene can be improved greatly.In this paper,the aqueous of graphene oxide(GO)is mixed with urea firstly,and then a certain concentration of p-phenylenediamine(PPD)is slowly added drop by drop to.The addition of urea can overcome the aggregation of GO sheets induced by the PPD.Flexible nitrogen-doped graphene(Nr GO)films electrode materials were prepared by using assembly,freeze drying-hydrothermal reduction method or freeze drying-thermal reduction methods,and their electrochemical capacitance properties were studied in detail.The specific work includes the following three aspects:(1)The flexible(Nr GO)films are prepared by treating the graphene oxide(GO)films assembled in the presence of urea and p-phenylenediamine(PPD)with hydroiodic acid firstly and then heating at 165 °C.The addition of urea can overcome the aggregation of GO sheets induced by the PPD,and the formed microcrystallines of urea and PPD can act as the templates to regulate the structure of GO films during the drying process.Compared with the r GO films obtained from pure GO films,measurements based on the twoelectrode system reveal that the capacitive performance of the films can be dramatically improved when the GO films are assembled in the presence of urea and PPD.The selected flexible Nr GO film exhibits a specific capacitance of about 430 F g-1 in 1.0 mol L-1 H2SO4 electrolyte,and the cells fabricated by the films exhibit a maximum energy density of 8.96 Wh kg-1 at the power density of 100.42 W kg-1,and maximum power density of 1.70 k W kg-1 at the energy density of 7.92 Wh kg-1 based on the active materials,andcan retain about 85% of its initial capacitance after 10000 cycles.This novel strategy may be developed as a route to prepare flexible N-doped r GO electrode materials with high-performance.(2)A certain amount of mixed mixture of GO,urea and PPD aqueous solution were mixed by ultrasonic,and then it was freeze-dried,and pressed into films before that the films were placed into Teflon-sealed autoclave and then heated at at 140°C for 12 hours,then the obtained films were washed completely,and dried in room temperature,the flexible N-doped graphene films were obtained.The addition of urea can overcome the aggregation of GO sheets induced by the PPD.Urea and PPD microcrystals formed during the freeze-drying process were used as templates to regulate the microstructure of the composite membrane.In order to compare with pure graphene film,measurements based on the two-electrode test show the flexible film exhibits a specific capacitance of about 352.1 F g-1 at a sweep rate of 0.5 m V s-1 in 1.0 M H2SO4.The maximum energy density can reach7.21 Wh kg-1 and the maximum power density can reach 1.70 k W kg-1.And an initial capacitance of about 92.4% can be retained after 10,000 cycles.(3)The mixture of GO,urea and PPD werewere mixed by ultrasonic,and then the mixture was freeze-dried,pressed into a film and heated at400°C for 2 hours in a muffle furnace.Finaly,the flexible Nr GO film was prepared.Freeze-drying maintains the integrity and dispersion uniformity of the internal structure of the mixture.The structure of the flexible Nr GO film was characterized in detail and used as electrode material to assemble the capacitor.The capacitive performance of the device assembled by flexible Nr GO film was tested in 1 M sulfuric acid electrolyte by two electrode method.The results show that the mass ratio capacitance of the optimal film can reach 273.3 F g-1 at a sweep rate of 0.5 m V s-1,and also has excellent cycle stability,can retain about 87.6% of its initial capacitance after 10000 cycles. |
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