Preparation And Capacitive Properties Of Nickel-Cobalt Sulfide-Hydroxide/Expanded Graphite Composite

Supercapacitors have attracted extensively attention of researchers due to their advantages such as high power density,fast charge and discharge rates,and long cycle life.However,the commercial supercapacitors are limited by the low energy density and high price.Electrode material plays an important role in determining the electrochemical performance and price of supercapacitors.Therefore,it is urgent to develop an electrode materials with excellent electrochemical performance and low price.Double-layer electrode materials such as carbon material with high specific surface area have high stability and cycle life,but low energy density.Pseudo-capacitor materials such as metal oxides/hydroxides have excellent initial capacitance,but poor rate performance and cycle life.In view of the above-mentioned problem of electrode materials for supercapacitors,this thesis intends to choose expended graphite as sustain and prepare electrode materials with high specific capacitance via a green and environmentally friendly electrochemical deposition method.However,large-sized particles of Ni-S have poor electrical conductivity and tend to agglomerate during the charging and discharging process,resulting in poor initial specificcapacitance and cycle stability.In order to obtain smaller particle and improve conductivity,cobalt will be considered to be introduced.Finally,Ni-Co-S DHs/EG electrode material with hetero-structure is prepared by one-step electrodeposition method.The composite electrode material has excellent initial specific capacitance,rate performance and cycle stability.The detailed contents and results are as follows:(1)Firstly,the Ni-S/EG composite is investigated,the optimal experimental conditions are as follows:The deposition potential was-0.8 V(vs SCE),deposition time was 1500 s and electrolyte solution was 0.1 mol L-1 NiCl2·6H2O in a mixed solution of N,N-dimethylformamide and water(v/v,10:1).The composite displays a high specific capacitance of 1280.4 F g-1 at 1 A g-1,and the capacitance retention was 59.4%when the current density increased from 1 A g-1 to 20 A g-1.Furthermore,the capacitance retention was 69.6%after 1000 cycles at 10 A g-1.(2)Subsequently,the Co-S/EG composite is investigated,the optimal experimental conditions are as follows:The deposition potential was-0.8 V,deposition time was 1500 s and the electrolyte solution was 0.1 M CoCl2.The optimal electrode delivered a specific capacitance of 186.9 F g-1 at 1 A g-1,a rate performance of 74.5%as the discharging current density increasing from 1 A g-1 to 20 A g-1,and excellent capacitance retention of 95.5%after 1000 cycles charged/discharged at 10 A g-1.(3)The Ni-Co-S/EG composite are obtained via electrochemical co-deposition on expended graphite in the electrolyte which added required amount of CoCl2·6H2O in.When the co-deposition potential is set as-0.8 V,the co-deposition time is set as 1500 s and the Ni2+/Co2+ molar ratios are adjusted to be 1:1,the obtained Ni-Co-S/EG composite exhibits optimal electrochemical performance.As shown in SEM and TEM pictures,the Ni-Co-S/EG nanosheet randomly distributed at the surface of EG layer,the three-dimensional electronic conductive network consisted of Ni-Co-S nanosheet,expended graphite layer and CNT is beneficial to improve electrochemical performances.At the current density of 1 A g-1,the composite exhibits excellent specific capacitance of 1516.5 F g-1,while the specific capacitance remains 74.2%at 20 A g-1.The capacitance remains at 84.4%after 1000 cycles when the current density of 10 A g-1.Obviously,the electrochemical performances and cycle stability of Ni-S/EG are effectively improved by adding Co2+.(4)The nickel-cobalt-sulfide double hydroxide(Ni-Co-S DHs)composite with hetero-structure is prepared via one-step electrodeposition by adjusting the ion species of electrolyte by introducing NO3-.Electrochemical tests prove that Ni-Co-S DHs/EG has excellent electrochemical performance.The composite achieves a high specific capacitance of 1461.8 F g-1 at 2 A g-1,retains a high rate capability of 79.5%at 50 A g-1 and also exhibits a good cycling stability of 93.7%after 1000 cycles at 10 A g-1.The hetero interface of Ni-Co-S DHs/EG composite with a special electronic or atomic structure will significantly improve ionic and electronic conduction,reduce the charge transfer resistance of the electrode material,and effectively improve the rate performance and cycle stability.In summary,this thesis has adopted low-cost expended graphite as carbon supports,and Ni-S/EG with excellent electrochemical performances was prepared via a green and environmentally friendly potentiostatic deposition method.Nickel-based sulfide composite exhibits excellent capacitance and rate performance by adding Co2+.Ni-Co-S DHs/EG with hetero-structure was prepared by co-depositing sulfide and hydroxide via one-step deposition method,which has better cycle stability.Therefore these results demonstrate Ni-Co-S DHs/EG composite with excellent electrochemical performance has great potential for supercapacitors.