Fabrication Of Reduced Graphene Oxide And Cobalt-nickel Based Composites And Their Applications In Supercapacitor

Supercapacitor（also known as electrochemical capacitors）is a novel and green energy storage device which has the performance of high charge-discharge rate,long cycle life,high power density,safety and reliability,it has attracted wide attention from researcher.The electrode is one of the core components of the supercapacitor,and the improvement of its performance is a key for supercapacitor.Cobalt-nickel metal acted as the electrode has the excellent electrochemical properties.It is the main electrode material for researching supercapacitors at the present stage for its high stability,high theoretical capacity and low cost.Graphene,as a new type of 2D carbon material has excellent properties such as high specific surface area,excellent electrical conductivity,and good chemical stability.These properties make the graphene have a great application in the energy storage field.In this paper,we studied the effect of composites of graphene and transition metals on the electrochemical performance.The graphene was used to modify the transition metals to improve the synergistic effect of the conductivity and specific surface area of the composites to improve the electrochemical performance of the electrodes.The main research content and progress made are as follows:（1）The composites were synthesized by hydrothermal method when the urea,ammonia and hydrazine acted as nitrogen sources and reducing agents in N-doping graphene based cobalt oxide.Results showed that the composites showed a high performance when the urea acted as the nitrogen source and reductant.The high content of type of pyrrolic nitrogen and pyridine-N-oxide nitrogen is beneficial to for electrochemical process and the high degree of graphene is also express the excellent performance.When the urea is used as the nitrogen source,the electrochemical performance was the best among the nitrogen-doped（N-doped）graphene based cobalt oxide composites(334.8 F g^-1 at a current density of 5 A g^-1).The amorphous cobalt was acted as a material which provided the faradaic pseudocapacitance.The two material acted together to improve the properties.（2）In this work,a type of rGO-modified Ni-Co phosphates（Ni（Co）NH₄PO₄@rGO）microplates were fabricated for supercapacitors electrodes via a one-step hydrothermal method.The morphology of Ni（Co）NH₄PO₄ are microplates,while the microplate of Ni（Co）NH₄PO₄/rGO has been broken and combined with rGO.The rGO-modified microplates exhibit a high specific capacitance of 1020 F g^-1 at 1 A g^-1,which is 16.3%enhanced compared to that of the untreated Ni（Co）NH₄PO₄)ones(877 F g^-1).Furthermore,it is observed that after 700-cycles,the specific capacitance can increase to their maximum value of 1451 F g^-1 at current density of 10 A g^-1,and can still remain 1275 F g^-1 after 5000 cycles,showing a high cycling stability.The Ni（Co）NH₄PO₄showed the same tendency but the low values.These excellent electrochemical performances could be due to rGO modification and self-assembled well-aligned nanosheets.This type of structures can not only facilitate the transfer electrons,but also offer high electrochemical activity sites and short transport path length for electrolyte ions.（3）A hydrothermal method and a simple heat treatment process were used to prepare a nanowire-shaped urchin-like NiCo₂O₄,the NiCo₂O₄/rGO was combined with nanowires and reduced graphene oxide sheet.After modification of reduced graphene oxide,the sea urchin-like structure of NiCo₂O₄ was broken up,the NiCo₂O₄ of NiCo₂O₄/rGO formed a dispersed nanowire combined with rGO.The NiCo₂O₄/rGO exhibited a combined form of nanowire and nanosheets rGO.After the modification of rGO,the electrochemical performance of NiCo₂O₄/rGO was improved,and the specific capacitance at the current density of 1A/g could reach 1003F/g.At a current density of 10 A/g,after 10,000 cycles,the specific capacitance can be maintained at 501F/g,and the capacity retention rate is 57%.It can be seen that the graphene-modified NiCo₂O₄increases the specific surface area and conductivity of NiCo₂O₄/rGO,accelerates the ion migration rate in the electrochemical reaction,and also shortens the electron transport pathway,thereby improving the electrochemical performance and higher specific capacitance.