Studies On Functionalization Of Graphene By Amino Group/Indanthrone Molecules And Application In Flexible Supercapacitors

As a new type of electrochemical energy-storage device,supercapacitors attracted a lot of attention due to the characteristics of high power density,short charge time and long cycling life.According to energy-storage mechanism,electrochemical double-layer capacitors（EDLCs）and pseudocapacitors can be classified.Graphene is a representative electrode material of EDLCs.Generally,energy density of EDLCs is dictated by the surface area of electrode materials.However,the surface area of graphene materials will greatly shrink due to restacking in the process of processing or handling,resulting in a low specific capacitance and energy density.Pseudocapacitors usually deliver much larger capacitances as they store energy through redox reactions occurring on the electrode materials.However,these materials are nonconductive and their structure will degrade during electrochemical cycles.It still remains a challenge to develop supercapacitors with high energy density and good cycle stability.Although graphene oxide（GO）is a common precursor for graphene functionalization,the electrochemical performances of GO-converted materials are usually poor due to their many lattice defects and low conductivity.In contrast,bulk graphite fluoride（b GF）has a more integrated hexagonal lattice of graphene,and it only has C-F functional groups.Therefore,b GF is a promising precursor for graphene functionalization.In view of high C-F bond energy,b GF conversion usually requires strongly-basic nucleophilic reagents.However,under strong alkaline conditions,b GF is more likely to take place reductive defluorination,leading to a very small degree of functionalization.Therefore,we assume that b GF may achieve a high functionalization degree if using weak basic nucleophiles and optimizing the reaction conditions.In this part,we perform the reactions between b GF and NH₃·H₂O,an extremely weak alkaline nucleophile.Successfully,b GF is functionalized by amine and hydroxyl groups,yielding NH₂-G-OH products.Elemental analysis shows that the functionalization degree of-NH₂ and-OH can reach as high as 0.34 and reductive defluorination is greatly suppressed.X-ray photoelectron spectroscopy further demonstrates the-NH₂and-OH functionalization toward graphene.Raman spectroscopy shows that I_D/I_Gvalue pronouncedly decreases after functionalization.The scanning Raman mapping unravels the uniform distribution of-NH₂ and-OH groups.Energy dispersive spectroscopy further displays the existence of-NH₂ and-OH groups and their regular distribution.Finally,flexible all-solid-state supercapacitors in symmetrical configuration are assembled using the NH₂-G-OH products as electrode materials.The electrochemical tests determine that the device has a specific capacitance of 318 F g^-1at 0.2 A g^-1 and an energy density of 7.1 W h kg^-1.Its electrochemical performances nearly maintain unchangeable even when the device is bent at different states.As mentioned above,the electrode material of sole graphene generally gives small specific capacitance.The specific capacitance and energy density will be likely to be dramatically boosted with the coupling effect of EDLCs and pseudocapacitor mechanisms if the organic conjugated molecules having multi-electron redox reactions are introduced into the graphene skeleton.To this end,we select the indanthrone molecule of largeπ-conjugation structure and conceptualize the graphene-π-indanthrone heterojunction based on theπ-πinteraction between them.In this part,GO and commercially-available indanthrone are used as the precursors.Driven by the hydrogen bond andπ-πinteraction,indanthrone molecules are well supported on graphene scaffolds.After a hydrothermal reduction,the reduced GO-π-indanthrone heterojunction material（r GO-π-IDT）is constructed.Thermogravimetric analysis shows that GO is fully reduced.X-ray diffraction and UV-Vis diffuse reflectance spectra demonstrate the formation of heterojunction structure.Scanning electron microscope shows that r GO-π-IDT heterojunctions have layered structure.Finally,symmetrical flexible all-solid-state supercapacitors are assembled.This device has a capacitance retention of 87%even after 20000 electrochemical cycles,indicative of a remarkable cycling stability.It delivers an energy density of 19.6 W h kg^-1 even at a high power density of 30123 W kg^-1.In addition,the performance of the supercapacitor is almost the same when bent at different angles,which indicates that the device has an excellent flexibility and anti-bendability.