Facile Synthesis Of Transition Metal Sulfides/Graphene Composites And Electrochemical Performance For Electrochemical Energy Storage

Supercapacitors,which are new type energy storage devices that use the electrolyte double layer adsorption or redox reaction on or near the surface of the electrode materials or the intercalation/deintercalation ions in the electrode materials to achieve energy storage.They have many advantages such as excellent cycle life,fast charging/discharging rate and high power density,but their energy density is relatively lower.Therefore,the main point of developing supercapacitors is to create excellent electrode materials.There are many kinds of electrode materials.Transition metal sulfides,as one of electrode materials,which hold pseudocapacitance characteristics and high theoretical capacitance are the ideal electrode materials.In this thesis,we prepared the three dimensional,high specific surface area and porous electrode materials which consist of transition metal sulfides and graphene.The obtained materials were tested to represent the properties of structures,morphologies and electrochemical characteristics.By using 3D PPY/Ni-Co-S/GE/NF as a positive electrode and commercial activated carbon as a negative electrode,an asymmetric capacitor was successfully assembled.More details are as follow:1)The three dimensional graphene/nickel foam?3D GE/NF?substrate was prepared by chemical vapor deposition.Then a three-dimensional polypyrrole/cobalt sulfide/graphene/nickel foam?3D PPY/Co S/GE/NF?electrode was prepared by electrodeposition and chemical polymerization.The obtained 3D PPY/Co S/GE/NF electrode exhibits a specific capacitance of 3450 F g-1 at the current density of 1 A g-1,and still remains 2160 F g-1 at the current density up to 20 A g-1.It also demonstrated a high capacitance retention of 72.98% after 1000 cycles at 3 A g-1.2)On the basis of previous research system,using 3D GE/NF as a substrate and changing the concentration of Ni²⁺ and Co²⁺ of electrolyte to synthesize different three dimensional nickel cobalt sulfides/graphene/nickel foam?3D Ni-Co-S/GE/NF?electrodes.Results suggested that,among all prepared composite electrodes,the Ni-Co-S/GE/NF electrode with Ni/Co ratio of 1.68 showed the best electrochemical performance.Based on these results,we further created 3D PPY/Ni-Co-S-4/GE/NF electrode.The developed 3D PPY/Ni-Co-S-4/GE/NF delivered an extremely high specific capacitance of3994 F g-1at 1 A g-1.Even as the current density was increased up to 20 A g-1,it still remained a high specific capacitance of 2608 F g-1.Long term cycling test suggested that the 3D PPY/Ni-Co-S/GE/NF electrode can demonstrate a high capacitance retention of 82% after 2000 cycles.3)Based on above results,the asymmetric capacitor?ASC?device is assembled by using 3D PPY/Ni-Co-S/GE/NF as a positive electrode and activated carbon as a negative electrode.The results suggested that the potential window of 3D PPY/Ni-Co-S/GE/NF//AC ASC can be extended to1.55 V,and remained 112 F g-1 at a current density of 0.5 A g-1.The ASC device exhibited a maximum energy density of 37.37 Wh kg-1at power density of 387 W kg-1 with a capacitance retention of 79.40% after 3000 cycles at 3 A g-1.In addition,the ASC device can light a 3 V LED bulb served as a power supply,demonstrating its value of practical application.