Study on Thermal Chemical Vapor Deposition and Electrochemical Properties of Vertical Graphene Nanosheets

Author:Zeng Jie

Supervisor:yu jie

Database:Doctor

Degree Year:2018

Download:15

Pages:145

Size:10232K

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Graphene possesses excellent optical,electrical and mechanical properties etc,which has important prospects of application in energy storage,adsorption,electromagnetic shielding,and so forth.The application of graphene in most fields requires its high dispersion.However,due to the atomic layer structure of the graphene and the van der Waals force andπ-πinteraction between the layers,the graphene in powder form is easy to agglomerate and loses its structural characteristics and superior properties.A major route to coping with this problem is to prepare three-dimensional(3D)graphene network.Although much progress has been made along this route,currently,the 3D graphene still suffers from the problems of too large pore size,low electrical conductivity,and less exposure of the edges.The research in this dissertation is based on the above problems.The main research content and results are as follows:Three-dimensional graphene fibers(3DGFs)are prepared by combining electrospinning and thermal chemical vapor deposition(CVD).The vertical growth of the graphene nanosheets is achieved.The graphene sheets on the 3DGFs are interconnected with the edges fully exposed on the surface to form a novel 3D structure,which have solved the problem of graphene agglomeration.It is found that with increasing the growth time the edge thickness of the GSs is stable at 1-3atomic layers while the thickness of the inner part increases gradually.Moreover,the 3DGFs could be prepared in different growth atmospheres(CH4/H2,C2H2/H2,C2H5OH/H2 and CH4/NH3).The growth rate of the graphene sheets has increased for over 4 times by using the copper foam as the catalyst.The 3DGFs self-supported membrane has exhibited superior electrical conductivity(3.4×104-1.2×105 S/m),superhydrophobicity,superoleophilicity,and electromagnetic shielding performance.The contact angle for water is up to 173o.A membrane of 3μm has an specific electromagnetic shielding effectiveness of 60932 dB cm2/g.To further explore the application of 3DGFs,the effects of subsequent treatments with NH3,acid,air,and the combination of these methods on the structure and oxygen reduction reaction(ORR)properties of 3DGFs were investigated.It is found that the higher the doping temperature,the less the doping contents in 3DGFs,and the lower the ORR properties.With NH3 treatment,only0.62 at.%N are doped in the 3DGFs,which are all the pyridine N.The combination of air activation and NH3 treatment can significantly increase the doping content of N in 3DGFs.After treating in NH3 at 500 oC,the 3DGFs have obtained the highest doping content of O and N,the contents are 2.62 at.%and 8.68 at.%,respectively.The doping form of N is mainly graphite N.The combination of aicd and NH3treatment can further increase the doping content of N in 3DGFs.After treating in NH3 at 500 oC,the highest contents of nitrogen and oxygen have been obtained for the 3DGFs,which are 4.61 at.%and 4.41 at.%,respectively.The doping form of N is mainly pyridine N(2.36 at.%).With this treatment,the catalyst has the best ORR performance.The half-wave potential and limiting current density of the catalyst were 0.88 V and 5.86 mA/cm2,respectively.Moreover,the catalyst has exhibited a good methanol tolerance and superior cycle stability with 90%of its initial current retained after 50 h test.In addition,the ORR active sites of 3DGFs were analyzed.It is found that dopant,defects,and graphene edges have an important influence on the ORR performance of 3DGFs,especially the pyridine N.In order to further expand the application of vertical graphene sheets,their growth on conventional carbon fibers by thermal CVD are investigated.The material of vertical graphene nanosheets on carbon fibers(VG/CFs)is used as scaffold of Li metal for Li ion battery anode,which are prepared by infiltrating molten Li.The structure of the VG/CFs on the Li loading and the electrochemical property of Li on VG/CFs(Li/VG/CFs)is investigated.The longer the growth time of the VG/CFs,the better the electrochemical performance of the Li/VG/CFs composite anode.For the VG/CFs grown for 10 h(VG/CFs10),the sym metrical cell based on the Li/VG/CFs10 anode,have shown an overpotential of only 78 mV after1200 cycles at a current density of 5 mA/cm2.By SEM,electrochemical impedance spectroscopy(EIS),and N2 adsorption/desorption characterization,it is found that the excellent electrochemical performance of the Li/VG/CFs10 anode is related to the structure of 3D graphene,high conductivity,and large specific surface area of VG/CFs.Furthermore,the electrochemical performance of a full cell based on the Li/VG/CFs10 composite anode was investigated.When assembled into a full cell with lithium titanate(LTO)cathode,Li/VG/CFs10 composite anode have exhibited a capacity of 140 mAh/g at 3 C after 500 cycles(1 C=170 mAh/g).Moreover,when assembled into a full cell with lithium iron phosphate(LFP)cathode,the Li/VG/CFs10 composite anode have exhibited a capacity of 101 mAh/g at 3 C after1500 cycles and 90 mAh/g at 5 C,indicating superior rate performance.