Design, Preparation And Optimization Of Cobalt-based Bimetallic Hydroxide For Supercapacitors

With the increasing demand of energy,intense research has focused on the advanced energy storage devices.Supercapacitors have attracted worldwide interest and were considered as the three main promising devices of electric energy storage,due to their fast recharge capability,superior power density,as well as long cycle life.Cobalt-based double hydroxides can offer excellent supercapacitor performance due to its unique spatial structure.But the existing synthesis techniques easily cause electrode materials with serious agglomeration,which cause bad electrochemical performance.And high rate performance and cycle stability have always been the weakness.In this paper,cobalt-based double hydroxides with 3D spatial structure on graphene films are built to make these materials possess smaller transfer resistance,lager specific surface and excellent structural stability,in order to obtain outstanding supercapacitor property.Fe-Co hydroxides with different Fe/Co atomic ratios grown on nickel foams were synthesized by one-step electrochemical deposition.It was found that the influence of initial Fe/Co ratios in the precursor solutions on the structure and electrochemical performance of electrodeposited products was significant.Fe(OH)3 showed particle shape with average diameter of 200 nm.With addition of Co ions,frame-like structure consisting of smaller particles was formed for Fe-Co hydroxides.Based on the morphology of Co(OH)2,it was deduced that Co(OH)2 served as a network former constructing a tridimensional frame network structure.Fe-Co hydroxide with Fe/Co ratio of 1:1 exhibited two types of structure features: nanoflake-like network structure overall and nanoparticle structure with numerous mesoporous microscopically.As the supercapacitor electrode materials,the as prepared Fe-Co hydroxide electrode with Fe/Co ratio of 1:1 exhibited highest specific capacitance of 2255.6 F?g-1 at the current density of 1 A?g-1 and also showed good cycling performance of 73.5% capacity retention at current density of 10 A?g-1 after 2000 cycles.Ultrathin reduced graphene oxide films on nickel foam were fabricated via a facile dip-coating method combined with thermal reduction.Hierarchical Ni-Co layered double hydroxide nanosheets with network structure were electrodeposited on the ultrathin reduced graphene oxide films.The thickness of Ni-Co layered double hydroxide(LDH)nanosheets can be controlled through adjusting the deposition temperature.The as-prepared electrode exhibited excellent electrochemical performance with specific capacitance of 1454.2 F?g-1 at a current density of 1 A?g-1.An asymmetric supercapacitor device was designed with the as-prepared composites as positive electrode material and Nitrogen-doped reduced graphene oxide as negative electrode material.This device could be operated in a working voltage range of 0-1.8 V in 1 M KOH aqueous electrolyte,delivering a high energy density of 56.4 Wh?kg-1 at a power density of 882.5 W?kg-1.After 10000 consecutive charge-discharge tests at 10 A?g-1,this asymmetric supercapacitor revealed an excellent cycle life with 98.3% specific capacitance retention.These excellent electrochemical performances make it become one of most promising candidates for high energy supercapacitor device.Furthermore,we improved the preparation method of the graphene on conductive fabric(PCF@RGO)using ascorbic acid.We designed and fabricated an all-solid-state flexible asymmetric supercapacitor device consisting of hierarchical Ni-Co LDH electro-deposited on the PCF@RGO as a promising positive electrode and nitrogen doped graphene/ iron oxide(Fe2O3/N-RGO)as negative electrode in the presence of PVA/KOH gel electrolyte.Owing to outstanding electrochemical behavior and ultrahigh specific capacitance of PCF@RGO/Ni Co-LDH(1220.5 F?g-1 at 1 A?g-1)and Fe2O3/N-RGO(282 F?g-1 at 1 A?g-1)along with their excellent volume stability,the assembled all-solid-state flexible supercapacitor device exhibited an outstanding energy density of 61.1 Wh/kg accompanied by a remarkable long-term cycle stability with 90.6% specific capacitance retention even after 5000 charge-discharge cycles.The extraordinary performances of as-fabricated device make it become one of most promising candidates for high energy supercapacitor device.