The Structure Design And Modify Of Graphene Based Film For Electrochemical Energy Storage

In recent years,graphene with its tremendous properties plays a major role in electrochemical energy device applications.Meanwhile,the graphene-based electrodes were fabricated by various coating techniques on existing electrodes that were utilized for energy device fabrication.Apart from these,it is also fabricated in a paper like a form(self-supported or free-standing)with its remarkable high conductivity,flexibility,robustness and electrochemical behavior which have been nominated for fabrication of flexible energy devices.Paper-like electrodes become an emerging field to fabricate lightweight and flexible electronic devices for future prospects.However,when graphene sheets are stacked in the form of"face to face",they will be tightly stacked due to“?-?”attraction,which not only reduces the effective active area,but also hinders the transmission of ions.In addition,the pure graphene film can only use electrostatic adsorption to store ions during the energy storage process due to its ordered graphite structure and few defects,and finally resulting in lower capacity storage.Therefore,how to effectively prevent the stacking of graphene sheets and introduce effective active sites is key process to enhance the performance of the graphene film-based electrode materials.From the perspective of graphene film modification,different types of modified graphene films were designed and prepared.And their electrochemical properties in potassium ion batteries,supercapacitors and zinc ion capacitors directly as electrodes are studied.The relationship between multi-directional pore structure,thermal reduction temperature,covalent grafting of small organic molecules,etc.and performance is explored,which provides ideas and directions for the design and development of high-performance graphene-base film electrode materials.The main research contents are as follows:(1)A method that combines freeze-drying technology,chemical etching and organic small molecule covalent grafting was proposed to prepare a N rich reduce graphene oxide film with cross-coupled porous network structure,and served as a self-supporting electrode for high-performance supercapacitors.The chemical etching time and thermal reduction temperature were systematically explored.Under the optimized conditions,the graphene-based composite film electrode exhibits a higher specific capacity(528 F g^-1 at a current density of 1 A g^-1).When assembling a symmetrical two-electrode device,it also has a high specific capacity(100.6 F g^-1)and energy density(14.0 Wh kg^-1).(2)A redox-active engineered holey reduced graphene oxide(HRGO)film anode was prepared by using the carboxylic acid functionalized polystyrene(PS-COOH)spheres as the template,the graphene oxide sheet is uniformly wrapped with PS-COOH spheres through electrostatic attraction process.PS-COOH spheres acted both as a hard template and a functionalizing agent during the process of thermal reduction.And the holey ion diffusion network channels and the oxygen functional groups were optimized by changing the annealing temperature.Under the best optimized conditions,the porous reduced graphene oxide film exhibits a high area capacity,which can reach 0.80m Ah cm^-2 at a current density of 0.1 m A cm^-2.(3)The porous graphene film was prepared by chemical etching and water intercalation strategy.The graphene-based composite film not only has multi-directional ion transmission channels,but the carbon nanotubes can also improve electronic conductivity and the mechanical strength of the film.In addition,freeze-drying technology is used to make H₂O molecules as a small molecule spacer to prevent the stacking of graphene sheets and increase the ion accessible surface area.When used as an anode for potassium ion batteries,it exhibits higher capacity(307.0 m A h g^-1)and cycle stability.When it is assembled the full potassium ion battery,with KVPO₄F as cathode,the voltage window can be extended to 4.9 V,and the energy density can reach 139.65 Wh kg^-1.(4)PPD molecule functionalized RGO film were synthesized through simple hydrothermal reduction using PPD as reducing and functional agent.In the graphene-based composite film,PPD not only acts as a functionalizing agent to increase active sites,but also acts as a small molecule spacer to increase the interlayer spacing of the graphene sheet,and at the same time can improve the wettability of the graphene film.By optimizing the mass ratio of graphene oxide to PPD,the best thin-film electrode obtains a high areal capacitance of 3012.5 m F cm^-2,especially a high energy density1.1 m Wh cm^-2 at a power density of 0.8 m W cm^-2.