Preparation And Electrochemical Properties Of Porous Graphene-based Electrode Materials

Supercapacitors,also known as double-layer capacitors,are a new type of energy storage device between conventional capacitors and secondary batteries.Electrode material as an important part of the super capacitor,its performance directly determines the use of capacitors.Electrode material as an important part of the super capacitor is one of the main factors that determine the performance of the capacitor.Development of electrode materials with excellent performance,low cost,and environmental friendliness has attracted the attention of researchers.MnO₂ is often used as a supercapacitor electrode material because of its low cost,abundant resources,high oxidation state,environmental friendliness,and high theoretical specific capacity.Due to its good electrical conductivity,excellent chemical stability and high theoretical surface area,graphite thin film is considered as an ideal supercapacitor electrode material.However,due to the interaction of van der Waals forces between carbon atoms,graphene thin films generated by chemical methods are prone to stacking between layers of graphene,so that the specific surface area cannot be compared with the theoretical value,and graphene is used as an electrode material.At that time,it is impossible to exhibit sufficient defect sites to store capacity,and therefore its specific capacitance is lower than the theoretical value.Based on this,in this thesis,porous graphene was prepared by activating graphene with different chemical etching methods,and then it was composited with manganese dioxide and melamine to obtain porous graphene/manganese dioxide composite material and nitrogen-doped porous graphene composite.materials,and their electrochemical performance as electrode materials for supercapacitors.The major findings are summarized as follows:1.GO formed a holey graphene after KOH etching and then integrated it with MnO₂ to prepare a series of 2DPG/MnO₂ composites.The structure of the composites is characterized by TEM,XPS,XRD and Raman.Subsequent electrochemical performance tests show that2DPG/MnO₂-3 composites have high specific capacitance and good cycle stability.The specific capacitance of 2DPG/MnO₂-3 is 214.7 F·g^-1when the current density is 0.5 A·g^-1,and the specific capacitance can still maintain at 97.5%after 2000 cycles of charge and discharge.The excellent performance is mainly attributed to the pore structure on the graphene sheet in the 2DPG/MnO₂-3composite and the synergistic effect between the two.It is superior to single group performance in terms of specific capacitance,cycle stability and other electrochemical performance.2.Graphene with multi-hole structure was prepared by mixed acid reflux-hydrothermal method,then 3DHG/MnO₂ composite material was prepared by redox reaction with KMnO₄ in a water bath at 30°C.Electrochemical studies show that the prepared 3DHG/MnO₂-3 composite has good electrochemical capacitance behavior,when the current density is 0.5 A·g^-1,the specific capacitance of 3DHG/MnO₂-3 is 192.2 F·g^-1,which can maintain good stability after 5000 cycles of charge and discharge.This good performance is closely related to its own structure,for example,the three-dimensional porous structure are conducive to the maximum contact between the electrolyte and the active material,while the in-plane porous structure shortens the transmission path of the electrolyte ions and electrons,thereby improving the electrochemical properties of the material.3.Since the nitrogen atom radius is similar to the carbon atom radius and the electronegativity is larger than carbon atoms,when an appropriate amount of nitrogen atoms are substituted to replace the carbon atoms in the carbon material,the nitrogen atoms will become electron donors,improve the charge mobility of negative charges on the carbon surface,thereby improve the electrochemical performance of materials.Graphene oxide as a raw material,first GO was activated with mixed acid to prepare pore graphene oxide,and then melamine was used as a reducing agent and nitrogen source,different amounts of melamine were added to preparethree-dimensional nitrogen doping porousgraphenecontaining different nitrogen doping amountsby hydrothermal method.SEM,TEM and N₂ adsorption and desorption tests show that the nitrogen-doped graphene material has rich micropore structure on the two-dimensional surface,and these pores shorten the transmission path of electrolyte ions and electrons.The galvanostatic charge-discharge test shows that 3DNHG-40 has the best supercapacitor performance,the specific capacitance of 3DNHG-40 at the current density of 0.5 A·g^-1is 230.1 F·g^-1,and after 1500 cycles,it still shows good cyclic stability.