Preparations And Capacitive Properties Of GQDs@MnO₂ And GQDs-Co（OH）₂/Ni（OH）₂

Supercapacitors are widely regarded as a promising energy storage device because of their large capacity,fast charging,safety,environmental protection and good cycle stability.According to the electrochemical responses and design of electrodes,supercapacitor devices can be classed as follows three types:electrical double layer capacitors,pseudocapacitors,and hybrid supercapacitors.The electrode material of double-layer capacitor is usually carbon-based material,and the electrode material of pseudocapacitor is usually transition metal oxide or some hydroxide,etc.Hybrid supercapacitor is mainly assembled by two different types of electrode materials.As an emerging quasi-zero-dimensional graphene material with size typically among several tens of nanometers,graphene quantum dots（GQDs）have sparked attractive attention for improving the electrochemical performances of supercapacitors owing to their unique conductivity,nanosized effect and quantum confinement.This work mainly discusses the preparation,characterization and electrochemical performance of GQDs@Mn O₂and GQDs-Co（OH）₂/Ni（OH）₂.The research results include the following two parts:1、Chapter 3 report a highly efficient one-step method to synthesize the Mn O₂nanosheets anchored with GQDs to form the GQDs@Mn O₂composite.KMn O₄and graphene oxide（GO）were reacted by a mild hydrothermal method in a short reaction time to form the composite.The GQD@Mn O₂electrode presents high specific capacitance of 246 F g^-1at a scan rate of 1 m V s^-1.Furthermore,an hybrid supercapacitors based on the obtained GQDs@Mn O₂as anode and the active carbon（AC）as cathode is constructed,which exhibits a superior energy density of 29.9 Wh kg^-1at a power density of 538.0 W kg^-1as well as good cycling performance（81.3%retention after 8000 cycles）.This is mainly due to the fact that the graphene quantum dots have the intrinsic characteristics of graphene,quantum confinement and new properties of edge effects,so the resulting GQDs@Mn O₂composite material has enhanced electrical conductivity,increased wettability and available contact sites.2、The work mentioned in Chapter 4 is that GQDs-Co（OH）₂/Ni（OH）₂heterostructures are prepared through a mild and cost-effective homogeneous precipitation approach.Owing to the enhanced electron transport ability and increasing the contact of the active sites with electrolyte by introducing GQDs,the GQDs-Co（OH）₂/Ni（OH）₂electrode shows improved electrochemical performances compared with Co（OH）₂/Ni（OH）₂.Specifically,the G10-Co（OH）₂/Ni（OH）₂exhibits preeminent electrochemical performance with specific capacity of 1680 F g^-1at the current densities of 1 A g^-1,and retains 87%of the initial capacity after 5000 cycles at10 A g^-1.Furthermore,an HSCs was assembled with GQDs-Co（OH）₂/Ni（OH）₂and active carbon as the positive and negative electrodes,respectively.The fabricated hybrid supercapacitors presents a higher energy density of 26.9 Wh kg^-1at a power density of 750.7 W kg^-1with a wide operating potential window of 0-1.5 V.