Study On Electrochemical Properties Of Nitrogen-doped Graphene Films And Functionalized Porous Carbon Nanospheres

As a new type of green energy storage element,supercapacitors have shown great application prospect in many fields because of their high power density,fast charging and discharging,long service life and so on.Electrode material is one of the key factors to determine the performance of supercapacitors,among them,graphene has become a hot topic in recent years because of its unique structure,large specific surface area,good stability and high conductivity.However,graphene prepared by graphene oxide(GO)would easily get agglomerated because of the ?-? stacking,which limits its use in supercapacitors.And Porous carbon materials,the most widely used in supercapacitors,possess excellent physical and chemical stability and high specific surface area,but they always suffer from low bulking density(less than 0.5 g cm-3)due to their porous structure,accordingly leading to a relatively low volumetric performance,which is not beneficial to practical application.Based on the above discussion,compact nitrogen-doped graphene thin film was prepared by introducing the barrier between the graphene layers and nitrogen atom doping,and functionalized porous carbon nanospheres with high density were prepared by functionalization of porous carbon nanospheres,respectively.The main research contents are as follows:(1)The porous carbon nanospheres(PCS)were prepared by carbonization and activation of the polypyrrole(PPy)at high temperature,which was synthesized by emulsion polymerization,nitrogen-enriched carbon nanospheres(N PCS)were prepared through post-treatment of porous carbon nanospheres(PCS)using p-phenylenediamine(PPD),nitrogen-doped graphene film using PPD as barrier(NG)was prepared by the introduction of PPD as a molecular barrier between graphene oxide(GO)sheets and then the evaporation of excess PPD,thermal reduction and nitrogen doping process at 350?,and combining the advantages of the above two materials,nitrogen-doped carbon nanospheres/graphene(NPCSG)flexible film using PPD and nitrogen-enriched carbon nanospheres as barrier was prepared.As-prepared NPCS,NG and NPCSG contain high nitrogen content of 15.74,13.46 and 14.47 at%,and high density of 1.48,1.36 and 1.48 g cm-3,respectively.These materials show excellent electrochemical properties as electrode materials,the gravimetric and volumetric capacitance of NPCS and NG are 430 F g-1 and 636.4 F cm-3,439.2 F g-1 and 597.3 F cm-3 at 1 A g-1,respectively;NPCSG which combines the advantages of NPCS and NG can be directly used as electrodes and exhibit a remarkable gravimetric and volumetric capacitance(491.5 F g-1 and 727.4 F cm-3).(2)Using PPy as precursor,the porous carbon nanospheres(PCS)were prepared by carbonization at 1000? and activation of KOH at 750?.Functionalized porous carbon nanospheres were prepared(PPD/PCS,PAP/PCS,PNA/PCS and PATP/PCS)by functional treatment of PCS with p-phenylenediamine(PPD),p-aminophenol(PAP),p-nitroaniline(PNA)and p-aminothiopheno 1(PATP)through the solvent thermal reaction.The effects of feed ratio,reaction temperature and reaction time on the electrochemical properties of the materials were investigated.The results show that:PPD-PCS and PAP-PCS have the best electrochemical performance under the condition of the mass ratio 1:3,120? solvent thermal reaction for 12 h,and their gravimetric capacitances can reach 368 and 339.7 F g-1,while volumetric capacitances are up to 515.2 and 445 F cm-3,respectively.The optimum reaction conditions of PNA-PCS were the mass ratio PCS/PNA = 1:5,the reaction temperature and time were 120? and 12 h,and its gravimetric and volumetric capacitance are 273 F g-1 and 204.75 F cm-3,respectively.The best plan for preparation of PATP-PCS is 90?solvent thermal reaction for 12 h at mass ratio of 1:3,and its shows gravimetric capacitance of 195.35 F g-1 and volumetric capacitance of 183.63 F cm-3.Furthermore,PPD-PCS,PAP-PCS,PNA-PCS and PATP-PCS possess 94.7%,85.2%,97.4%and 94.7%capacitance retention after 3000 cycles of charging and discharging at 10 A g-1,indicating excellent cyclic performance.In addition,the electrochemical properties of the functionalized PCS were also studied in the acidic or alkaline electrolyte which added the corresponding organic molecules as the redox active medium,and the results show that the capacitance can be significantly improved in active electrolyte.The capacitance of PPD-PCS in the electrolyte of 2 M KOH + 50 mM PPD can reach 722.1 F g-1(0.5 A g-1);the capacitance of PAP-PCS was increased to 440.55 F g-1 in 1 M H2SO4 + 50 mM PAP electrolyte;PNA-PCS's capacitance in the electrolyte of 2 M KOH + 2 mM PNA can be as high as 1129 F g-1;capacitance of PATP-PCS in the 1 M H2SO4 + 50 mM PATP electrolyte also increased to 375.4 F g-1.