Studies On Chemical Functionalization Of Graphite Fluoride And The Application In Oxygen Reduction Reaction And Supercapacitors

As a carbon nanomaterial with the unique two-dimensional structure and excellent electrical,optical and mechanical properties,graphene has great application prospects in so many frontier fields such as energy conversion and storage devices（e.g.fuel cells,supercapacitors）,cancer diagnosis and treatment,water desalination and distillation.The intrinsic limitations of pristine graphene,such as poor solution–processability and zero band gap,which will hinder its real–life applications,can be addressed by chemical functionalization towards graphene.In most cases,graphene functionalization was achieved through the various reactions between the versatile oxygen–containing groups amongst graphene oxide precursor and numerous functional compounds.Unfortunately,the notoriously inferior electrical properties of functionalized graphene materials available from GO chemical derivatization obstruct further developments of high–performance graphene–based electrical devices,in light of many hexagonal lattice defects and holes including in GO.To address these issues,this dissertation focuses the defect–free graphene chemical functionalization starting from commercialized bulk graphite fluoride（bGF）,and encouragingly,the as–prepared graphene derivatives are well reachable in oxygen reduction reaction（ORR）electrocatalysis and supercapacitors.This dissertation mainly includes the following two parts:1.Studies on the preparation and ORR electrocatalysis of purely out–of–plane NH₂ and F cofunctionalized graphene materials.As the important reaction on cathode in fuel cells,ORR is usually catalyzed by Pt/C material.Apart from the high cost,its more disadvantages,such as easy to be poisoned,poor long–term stability,hinder the practical application in fuel cells.Therefore,it becomes the research focus in electrocatalytic fields to develop a high–efficiency,low–cost alternative to Pt/C catalyst.Presently,graphene doped by heteroatoms has been widely investigated in ORR electrocatalysis,and becomes the promising Pt/C alternative.As the most widely accepted doping configuration of heteroatoms,they are sp²–hybridized and cyclized into graphene hexagonal lattice using the synthetic strategies of chemical vapor deposition or GO calcination in the chemical environments containing heteroatoms.Because it still remains a great difficulty to precisely program heteroatoms on out–plane of graphene,purely out–of–plane NH₂ and F cofunctionalized graphene materials are rarely reported in ORR electrocatalysis.In this end,bGF,within which carbons are sp³–hybridized and fluorines are accordingly standing on graphene plane,is employed as the precursor and reacted with NaNH₂ through nucleophilic substitution.As a result,graphene materials cofunctionalized with NH₂ and F groups both in out–of–plane mode（NH₂–G–F）were successfully prepared.Elemental analysis data indicated that F and NH₂ contents in NH₂–G–F products can be flexibly regulated by simply adjusting the reaction conditions（such as reaction temperature,NaNH₂/bGF mass ratio）.Raman spectroscopy and X-ray photoelectron spectroscopy（XPS）results demonstrated that C–F_x groups in bGF were also removed through reductive elimination,which favorably led to partial recovery inπ–conjugation of graphene skeleton,in concomitant with nucleophilic substitution.scanning electron microscope（SEM）,high-resolution transmission electron microscopy（HR-TEM）and Atomic force microscopy（AFM）morphological characterizations showed that NH₂–G–F products remained sheets structure,and possessed considerably beautiful hexagonal honeycomb lattice.Finally,their ORR electrocatalysis of NH₂–G–F samples with variable amounts of F and NH₂ groups was investigated by cyclic voltammetry and linear sweep voltammetry.The electrochemical results unraveled that NH₂–G–F products indeed exhibited excellent ORR electrocatalytic activity,even outperformed commercial Pt/C,when F and NH₂ groups coexisted in NH₂–G–F products and they possessed an appropriate NH₂/F molar ratio.This fact implied a valuable synergistic effect between F and NH₂ groups.Additionally,NH₂–G–F electrocatalysts also behaved remarkable resistance to methanol toxicity and cycling stability.2.Studies on the preparation and supercapacitive properties of high–density diamine pillared graphene electrode materialsSupercapacitors（SC）are considered to be an ideal energy storage device due to their fast charging and discharging rate,excellent cycling stability and high power density.The electrode material is the central component of a supercapacitor and largely dictates its ultimate energy-storage performance.An ideal electrode requires a large ion-accessible specific surface area,high electrical/ionic conductivity and outstanding chemical/electrochemical stability.Due to be materialized with these above characteristics,graphene is expected to be the very suitable electrode material.Unfortunately,an unfavorable trade-off relationship between the gravimetric capacitance(C_wt)and the volumetric capacitance C_vol(C_vol=C_wt?ρ,andρis the electrode material density)is usually encountered because most graphene electrodes that yield high C_wt values are generally quite fluffy and lightweight(i.e.ρ<<1 g cm^-3),hardly resulting in both high in C_wt and C_vol.As one material with the highest intrinsic density among graphene family(ρ:².6 g cm^-3),bGF delivers very inferior capacitive performances due to its ultralow conductivity and tightly–stacked nature.It is envisioned that if graphene layers can be molecularly pillared by appropriate spacers,and meanwhile,the neighboring sheets are covalently linked through the reactions between bGF and binary functional compounds,the bGF-derived materials probably display both excellences in C_wt and C_vol,as result of having large ion-accessible specific surface areas as well as high bulk densities.Along this line,ethylenediamine（EDA）,p-phenylenediamine（PDA）and 4,4’-diaminodiphenyl ether（ODA）with different molecular configurations and sizes were employed as binary functional pillars to covalently support and link graphene plates through their nucleophilic substitution towards C-F units.Along with the increases in molecular rigidity and size（EDA→PDA→ODA）,X-ray diffraction（XRD）results showed that the interlayer spacing of the corresponding diamine pillared graphene products enlarged from 0.36nm for EDA-G to0.49 nm for PDA-G,and further to 0.62 nm for ODA-G.SEM observations and nitrogen adsorption-desorption technique measurements indicated that ODA-G was more hierarchically porous,and had a larger specific surface area than EDA-G and PDA-G.Moreover,EDA-G,PDA-G and ODA-G samples were determined to have bulk densities of 0.79,0.92 and 1.08 g cm^-3,and have electrical conductivities of 1375,1026 and 1264 S m^-1,respectively.Obviously,the diamine pillared graphene materials possess abundant porous structures,high conductivities together with large densities.Finally,the energy-storage performances of diamine pillared graphene materials were investigated by cyclic voltammetry and galvanostatic charge/discharge tests.Electrochemical results showed EDA-G mainly behaved electrochemical double-layer capacitance（EDLC）characteristic,whereas PDA-G and ODA-G featured the coexistence of EDLC and Faradaic pseudocapacitance.EDA-G and PDA-G delivered C_wt values of 103.6 and 243.6 F g^-1at the current density of 0.5 A g^-1,respectively,but yielded shrunk C_vol values of 81.8and 224.1 F cm^-3 when taking their bulk densities into consideration.Encouragingly,ODA-G exhibited a C_wt value as large as 328.5 F g^-1 and a further maximized C_volol value of 354.8 F cm^-3 thanks to its heavyweight nature,achieving the desired both excellences in C_wt as well as C_vol.In addition,ODA-G also exhibited outstanding rate performance and cycle stability.