Electrochemical-assisted Synthesis Of Modified Graphite Paper And Its Composite For Supercapacitor

Supercapacitor is a kind of energy storage device with high power density but limited energy density,compared with lithium ion battery.Improving the energy density of supercapacitor while maintaining its excellent power density is an important goal of supercapacitor research.One way to achieve this goal is to develop electrode materials with high specific capacitance and high-rate performance.Nickel-cobalt layered double hydroxide（Ni Co-LDH）has excellent energy storage capacity based on multi-electron reaction,but its poor conductivity results in poor rate performance when directly used as electrode material.The three-dimensional current collector has open aperture and good electrical conductivity,which can promote electron and ion transfer.The electrochemical performance of Ni Co-LDH electrode can be improved effectively by using three-dimensional current collector.In this thesis,the construction method of three-dimensional current collector based on graphite paper is systematically studied.The application of composite based on modified graphite paper and Ni Co-LDH for supercapacitors is studied.The main contents are as listed as follows:（1）Modified graphite paper and its composite were prepared by one-step method.In a three-electrode system,using graphite paper as the working electrode and the aqueous solution based on Ni（NO₃）₂·6H₂O and Co（NO₃）₂·6H₂O as the electrolyte,by the dual role of nitrate ions（intercalation and reduction to hydroxide ions）,only by changing the electrode potential,the surface of graphite paper was partially exfoliated and the deposition of Ni Co-LDH was realized.It was found that Ni Co-LDH was uniformly combined with the surface of the graphite flake adhered on the base of graphite paper,and the resulting composite electrode shows high specific capacitance(1234 F g^-1,1 A g^-1)and high-rate performance(50 A g^-1,69.04%).（2）The expanded graphite paper with low structural defects was prepared by electrochemical-assisted method.The intercalated graphite paper was prepared in a two-electrode system using graphite paper as the working electrode and concentrated sulfuric acid as the electrolyte under a constant voltage.The intercalated graphite paper was reacted with hydrogen peroxide in water bath（50℃）and thus the van der Waals force between the tightly stacked graphite sheets was destroyed by gas release reaction between the graphite layers,and finally the expanded graphite paper was obtained.It was found that the electrochemical intercalation treatment for graphite paper has a significant effect on the structure of expanded graphite paper:the intercalated ions ensure the expansion of the inner layer of graphite paper and improve the loading sites for active materials.The result also shows that the EGP introduces fewer defects.Ni Co-LDH was supported on EGP by electrochemical deposition method.The composite electrode shows a high specific capacitance of 1650 F g^-1 at the current density of 1 A g^-1and high capacitance retention of 84.8%at the current density of 20 A g^-1.The excellent electrochemical performance of EGP@Ni Co-LDH is attributed to the fact that EGP provides lots of loading sites to promote the uniform deposition of Ni Co-LDH and thus exposing lots of active sites.Moreover,the EGP also provides the pathway for rapid electron and ion transport.（3）The activated graphite paper was prepared by electrochemical-assisted method at room temperature.The intercalated graphite paper was used as a precursor to react with NH₄HCO₃ solution at room temperature.The surface of graphite paper was exfoliated by gas release reaction induced by sulfuric acid and NH₄HCO₃.Finally,a three-dimensional structure for electron and ion transfer was constructed on the surface of graphite paper.It was found that the voltage used to prepare the intercalated graphite paper has a significant effect on the exfoliation effect of the graphite paper surface.Choosing the moderate intercalation voltage can partially exfoliate the surface while maintaining the macroscopic integrity of the graphite paper.Ni Co-LDH was deposited on the activated graphite paper（AGP）,and the obtained AGP@Ni Co-LDH exhibits a high specific capacitance of 1750 F g^-1 at the current density of 1 A g^-1and a high capacitance retention of 87.3%at the current density of 20 A g^-1.