Preparation And Supercapacitor Properties Of Graphene/Cobalt-based Bi-metal-oxides Composite Materials

With the growing crisis of fossil energy and environmental pollution,it is particularly important to develop new energy and efficient energy storage technologies.Supercapacitors possess the advantages of high specific capacity,high power density,good cycling stability,fast charge/discharge rate and environmental friendliness,but their low energy density limits the application and development.Electrode materials play an important role in the performance of supercapacitors.Therefore,research of supercapacitor electrode materials with high power density and high energy density has attracted enormous interest at home and abroad.Graphene has large surface area and excellent electrical conductivity,which is an ideal electrode material for electric double-layer capacitors.However,the low theoretical capacitance and agglomerate or restack of graphene affect its supercapacitor performance.Cobalt-based bi-metal-oxides have high specific capacitance and low cost.Nevertheless,their poor conductivity and structural collapse happened lead to lower specific capacitance than the theoretical capacitance.Therefore,we integrated cobalt-based bi-metal-oxides and graphene to form nanocomposites for improving high energy density and power density.Based on optimizing the preparation process and the mechanism of energy storage,the main research contents and results are as follows:(1)GO on Ni foam has been successfully reduced as low as 90℃ by using ammonia solution as the reducing agent.The nitrogen doping occurs simultaneously during the reduction process.As a result,nitrogen atomic ratio of rGO is about 3.60 at%.Nitrogen-doped makes rGO/NF has good electrical conductivity and electrochemical stability.And rGO films are directly coated on the framework of Ni foam without any binders,reducing the contact resistance.When used as binder free electrode,rGO/NF shows high specific capacitance(230 F g-1)and good cycling stability(the specific capacitance retention ratio is 96.7%after 2000 cycles).(2)By combination of Ni-Co bi-metal-oxides and graphene,NiCoO2/rGO/NF nanoarrays with porous architecture of flowers were directly grown on Ni foam by a simple hydrothermal synthesis method.The introduction of graphene into the ultrathin NiCoO2 nanoflakes improves the electrode conductivity of nanocomposites,which provides fast ion and electron transfer.As a supercapacitor electrode material,the specific capacity of NiCoO2/rGO/NF is high up to 2572 F g-1.The energy density of asymmetric supercapacitor assembled with NiCoO2/rGO/NF and rGO/NF is maintained up to 26.1 Wh kg-1 at high power density of 2784 W kg-1.(3)Then,using nickel foam as template and conductive substrate,3D(Co,Mn)3O4-rGO nanowire composites were prepared by hydrothermal method and annealing treatment,which effectively improved the electrochemical performance of the pure Co-Mn bi-metal-oxides.The 3D structure of nickel foam increases the effective specific surface area of the composites.The introduction of graphene inhibits the collapse of Co-Mn bi-metal-oxides structure.As a supercapacitor electrode,the composites have a high specific capacitance of 1582 F g-1 and excellent cycle stability(at a high current density of 10 A g-1,the retention is up to 87.4%after 8000 cycles).Similarly,MnCo/rGO/NF and rGO/NF were assembled into an asymmetric supercapacitor.The power density is 800 W kg-1 and the energy density is 28.2 Wh kg-1.The results show that Co-Mn bi-metal-oxides are also suitable as supercapacitor electrode materials,especially at high current densities.