Preparation And Energy Storage Performance Of Vanadium-Based Metal Oxide@Ni-Co-S Composite Electrodes

Supercapacitors have attracted a great deal of interest because of their faster charging/discharging speed,higher power density and longer cycling stability than batteries.Especially for the supercapacitors employing aqueous electrolytes,the merits including good environmental friendliness,high safety and low cost further stimulate the research interest.Theoretically,it is viable to improve energy density(E)of an aqueous supercapacitor by increasing the working voltage(V)and/or the specific capacitance(C).However,as limited by the poor electrochemical stability of the components,improving E by increasing V is normally difficult especially for the aqueous supercapacitors.Developing the electrodes with a high C is thus a feasible approach of increasing E.Lots of transition metal oxides(TMOs)and transition metal sulfides(TMSs)have high theoretical specific capacitance due to the multiple oxidation states of these transition metal elements.Besides,the TMOs normally have superior stability and adhesion with the substrates/current collectors,and however,the TMSs generally have a narrower bandgap and thus a higher electrical conductivity than the corresponding TMOs.Accordingly,combining the corresponding advantages of TMOs and TMSs efficiently is a possible solution to realize an excellent charge storage capability.In addition,carbon nanotubes(CNTs)can achieve fast electron transport due to the unique tubular structure and good electrical conductivity.Therefore,in this paper,the vanadium-based metal oxides Ni Co₂V₂O₈(NCVO),Ni₃V₂O₈(NVO)and TMS Ni Co₂S₄(NCS)are studied.And by adding an appropriate amount of CNTs,the composite electrodes with good comprehensive performance are prepared by optimizing the experimental conditions,and finally assembled into an aqueous asymmetric supercapacitor(AASC)for the study of energy storage performance.The specific studies are as follows:(1)The Ni Co₂V₂O₈ samples were prepared at different hydrothermal temperatures,followed by the electrode slurry preparation and coating to obtain Ni Co₂V₂O₈/nickel foam(NCVO/NF),and 120°C was explored as the optimal hydrothermal temperature.On the basis of NCVO/NF,NCS nanosheets were then electrodeposited at different electrodeposition rates.The best composite cathode Ni Co₂V₂O₈@Ni Co₂S₄/NF(NCVO@NCS/NF)was finally prepared at a deposition rate of 5 m V s^-1.A high specific capacitance of up to 8964.0 m F cm^-2 at 1 m A cm^-2and good rate performance of 4788.0 m F cm^-2 at 60 m A cm^-2 can be delivered.The AASC assembled using the NCVO@NCS/NF cathode,the anode of activated carbon(AC)on carbon cloth(CC),the 2 M KOH electrolyte delivers a high energy density of0.376 m Wh cm^-2 at 0.800 m W cm^-2 and good cycle stability with?84.3%capacitance retention after 10000 cycles at 20 m A cm^-2.(2)The study continued the exploration of energy storage performance based on the optimal hydrothermal and deposition conditions obtained from the previous study.The CNTs@Ni₃V₂O₈/NF(CNTs@NVO/NF)was prepared by adding an appropriate amount of CNTs on the basis of retaining vanadium-nickel-based oxide in the hydrothermal reaction and the subsequent preparation and coating of electrode slurry.Then the CNTs@Ni₃V₂O₈@Ni Co₂S₄/NF(CNTs@NVO@NCS/NF)composite cathode was prepared by the electrodeposition of NCS on CNTs@NVO/NF.A specific capacitance as high as 8722.0 m F cm^–2 at 1 m A cm^–2 together with good rate performance,i.e.,4560.0 m F cm^-2 at 60 m A cm^-2 is achieved by the electrode.The cycling test demonstrates that?81.8%of its initial capacitance and?100%Coulombic efficiency over 10000 cycles at 10 m A cm^-2 can be maintained.The AASC assembled employing CNTs@NVO@NCS/NF cathode,AC/CC anode and the2 M KOH delivers an energy density as high as 0.414 m Wh cm^–2 at 0.800 m W cm^–2and good cycle performance with?14.1%total capacitance decay over the 10000cycles at 20 m A cm^–2.Overall,based on the previous experiment,this study has improved the cycling stability,Coulombic efficiency of the cathode and the energy density of the AASC,which also provides a valuable exploration for the development of the high-performance AASC based on vanadium-based metal oxides and TMSs composite electrodes.