Preparation And Controllable Defects Modulation Of Vertical Graphene Via High-flux PECVD And Its Application In Electrochemical Energy Devices

The development of high-performance electrochemical energy devices have been an important subject to effectively solve the current energy crisis and environmental pollution,during which,the design and preparation of electrode materials have been of vital importance in the electrochemical energy devices.Graphene-based electrode material possesses excellent electrical and mechanical performance.Nevertheless,the huge specific surface area and strong Van der Waals’force between each single graphene layer drive the layer-layer aggregation.As a result,the actual electrochemical specific surface of graphene-based electrode material is far less than the theoretical value,reduces the electrochemical active sites,and consequently limits performance of electrochemical devices.Therefore,it is of great research significance to develop a new type of graphene material system,and further systematically study its preparation process and morphology,defect control,finally find the internal relationship between the material itself and electrochemical performance.At the same time,it also has an important practical value and practical significance to develop a preparation route that can simply and effectively control the morphology and microstructure of this graphene material to meet the needs of renewable energy storage and conversion devices.Vertically standing graphene（VG）has been a new kind of graphene material,in which several layers of graphene sheets arranged in an orderly direction perpendicular to the substrate,with open structure and exposed graphene edge.However,the preparation and application of VG are still in the basic research stage.There are many critical preparation parameters to be studied,such as growth temperature,substrate choice,morphology-structure regulation,and morphology/structure-function relationship.In this doctoral research work,a high-flux plasma enhanced chemical vapor deposition system（HPECVD）was assembled by us with our unique talent for the preparation of VG.The growth mechanism of VG was systematically analyzed and the structure regulation process of VG was also studied in detail.Noteworthy,the morphology and structure were also rationally designed to achieve high electrochemical performance,for the applications in sodium ion battery,oxygen reduction electrocatalysis and rechargeable zinc-air battery.The main research content of this dissertation are as follows:（1）A HPECVD system was systematically studied through modeling the remote plasma system using the COMSOL Multiphysics.In this system,the magnetic mirror configuration was introduced to restrain the plasma and realize the long-distance propagation of plasma in the vacuum chamber without attenuation.Based on the model calculation,the density and thickness of VG can be orderly regulated by adjusting the plasma power,and substrate temperature.In addition,we realized the large-areal fabrication of VG on copper foil substrate below 300℃without extra joule heating as well as on oxide substrate（Si/Si O₂）,single crystal material substrate（Si<100>）,fiber（carbon fiber）,metal（stainless steel）and flexible substrates（polyimide）.We also analyzed the growing mechanism with respect to the thermodynamics and kinetics of VG growth,which is helpful for us to proceed following work.（2）On the basis of the previous work,one-step nitrogen doping（NVG）preparation of vertical graphene was proposed.The microstructures and porous macrostructure of NVG samples can be readily realized by changing the nitrogen doping content to optimize the active sites and mass transfer structures.The sample named NVG-30 possesses the optimized nitrogen doping content,abundant defect and edge structure,which can provide abundant active sites,fast electrons and mass transfer path with interconnected vertical structure for oxygen reduction reaction（ORR）.Electrochemical tests showed that NVG-30 catalyst exhibits excellent ORR activity,which is comparable to that of commercial Pt/C catalyst.When applied to Zinc-air batteries,the NVG-30 showed a high peak discharge energy density(167.9 m W cm^-2),which are superior to the performance of commercial Pt/C.（3）In situ nitrogen and oxygen co-doping of VG（NOVG）was obtained with the introduction of N₂ and O₂ during the material growth.An in-situ plasma diagnostic system was introduced in HPECVD to analyze the effects of introducing different precursor gases on plasma density,composition and concentration of active groups.Combined with EELS,XPS and SEM characterization,it was found that the C₂ active group in plasma radicals was more likely to contribute to sp² bonding in NOVG,while the introduction of O₂ and N₂ increased active groups such as CN and O,which promoted the sp³ bonding and lead to the doping of N and O.As a result,the introduction of heteroatom also increased the edge structure of graphene,which further affects the defect level of NOVG.The most thermodynamically stable structures of doped pyridine nitrogen and oxygen were found by DFT calculation.The change of free energy in ORR and OER processes showed that NOVG had lower reaction overpotential than NVG（ORR:0.62 V vs.1.03 V,OER:0.50 V vs.0.66 V）.Electrochemical tests also confirmed the superior performance of NOVG in ORR and OER.The NOVG based liquid rechargeable Zn-air battery exhibited an open circuit voltage of 1.48 V,a specific capacity of 740.2 m Ah/g and an energy density of 876.7Wh/kg,which is close to the theoretical value of Zn-air battery.The quasi-solid state/flexible rechargeable Zn-air battery also showed excellent cycle stability and bending resistance.（4）In this part,the preparation method of NVG with relatively-ordered morphology and moderate defect on copper foam substrate was proposed.Meanwhile,NVG-Al₂O₃ composite was also prepared by coating ultra-thin amorphous Al₂O₃ on the surface of NVG by ALD deposition technology.Theoretical and experimental results show that the surface coated Al₂O₃ helps to form a good 3D reaction interface between electrode and electrolyte in sodium ion battery and the formed favorable interface inhibits the side reaction of electrolyte,provides an accessible ion diffusion path,and further improves the electrochemical reaction kinetics.Electrochemical tests showed that the NVG-Al₂O₃ electrode exhibit a high reversible capacity of 835.0m Ah/g at a current density of 0.1 A/g.After 5000 cycles of charge and discharge,its capacity retention is up to 92.3%,showing excellent cycling stability.